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Reactivity and structure of gas phase olefin-transition metal ions by Fourier transform ion cyclotron… Kan, Ziti 1994

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REACTIVITY AND STRUCTURE OF GAS PHASE OLEFIN-TRANSITIONMETAL IONS BY FOURIER TRANSFORM ION CYCLOTRON RESONANCEMASS SPECTROMETRYbyzm KANB. Sc. Jilin University, 1984M. Sc. Jilin University, 1987A THESIS SUBMITthD IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIESDepartment of ChemistryWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIANovember, 1993© Ziyi Kan, 1993In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)_________________________Departmentof_____________The University of British ColumbiaVancouver, CanadaDate ° “ /95DE-6 (2)88)—n—ABSTRACTThe gas phase ion-molecule chemistry of the following organometallic complexes hasbeen examined using Fourier transform ion cyclotron resonance (Fr-ICR) mass spectrometrictechniques.OFe(CO)3Fe(CO)3Cr(CO)3 Fe(CO)31 2 3 4= -CH=CH2Fe-CH3 FeFe(CO)3 -CH(OH)CH5 6-9 10Positive and negative fragment ions generated by electron impact ionization from each of thecomplexes were allowed to react with the neutral parent molecule or other selected neutralmolecules. Monometallic ions typically reacted with the parent molecule to form multimetallicproducts such as (c-C7H8)Cr2CO),(c-C7H82CrCO)3(c-C7H82Cr3(CO)2 and (cC7H8)3r4(CO).The ion-molecule reaction pathways were determined by Fr-ICR multipleresonance ion ejection techniques. Abundances of ionic reactants and products weretemporally monitored to obtain kinetic data. A general trend that ion reactivity towards theparent molecule increased with increasing electron deficiency on the metal atom was observed.Some ions exhibited unexpected reactivity patterns, despite their large formal electrondeficiency, suggesting the presence of unusual bonding configurations in the ions. Forcomplexes 1 to 3, positive fragment ions formed by losing all three CO groups showedanomalously low reactivities. These low reactivities were rationalized by a gas phase ionrearrangement in which a f3-hydride intramolecularly transferred from the olefin ligand to themetal center to yield a metal hydride ion with a less-electron-deficient central metal atom. Themetal hydride ions were detected by a specific ion-molecule addition-dehydrogenation reaction—111—with deuterated methanol that was discovered in this research. The addition-dehydrogenationreaction appeared to be a definitive test for metal-hydrogen bonding in gas phaseorganometallic ions as ions lacking metal-hydrogen bonds underwent either substitutionreactions or adduct-formation reactions with methanol. The major driving forces for the 3-hydride transfer appeared to be the high degree of electron unsaturation at the metal center andthe formation of aromatic or other highly delocalized organic ligands. The low observedreactivities of some ions from complex 4 were explained by a coordination expansion (ri4 to116) process that also lowered the electron deficiency on the Fe atom. In some of thesubstituted ferrocene systems, a ring expansion process was inferred. For the negative ionchemistry of complex 3, an a-proton transfer from the cyclohexadienone ligand to thenegatively charged metal center (via ketone/enolate tautomerization) was suggested to explainthe observed negative ion reactivities. The ion processes observed in this research are ofimportance for a better understanding of the mass spectra of organometallic complexes andinferentially for a better understanding of the mechanism of C-H and C-C bond activation bygas phase transition metal ions.— iv —TABLE OF CONTENTSAbstract iiTable of contents ivList of Tables viiList of Figures viiiList of Abbreviations xiiiAcknowledgment xivChapter 1 Introduction 11.1 Mass Spectrometric Studies of Transition Metal-Containing Ions 11.2 Research Interests and Thesis Format 81.3 Experimental Section 91.3.1 Operating Principles of FT-ICR 91.3.2 Performance of FT-ICR 151.3.3 Experimental Techniques of FT-ICR 171.3.4 Experimental Parameters and Procedures 211.3.5 Basic Kinetics of Gas-Phase Ion-Molecule Reactions 23References 25Chapter 2 Gas-Phase Ion-Molecule Chemistry of CycloheptatrieneChromium Tricarbonyl-7H8Cr(CO)3 362.1 Time-Resolved Mass Spectra and Ion-Molecule Reaction Products 372.2 Ion-Molecule Reaction Pathways 412.3 Reactivities and Structures of Positive Ions 462.4 Detection of Hydrido-ic-Tropylium and Hydrido-it-CyclopentadienylStructures 532.4.1 FT-ICR-CID and Ion-Molecule Reaction with D2 542.4.2 Ion-Molecule Reaction with Methanol 582.5 Summary and Conclusions 65—v—References 68Chapter 3 Gas-Phase Ion-Molecule Chemistry of CyclohexadienoneIron Tricarbonyl(r14-c-C6H0)Fe(CO)3 713.1 FT-ICR Positive Ion Mass Spectra of(ri4-c-C6H0)Fe(CO)3 723.2 Self Ion-Molecule Clustering Reaction Pathways 763.3 Ion-Molecule Reactions with Methanol 883.4 Ion Reactivities and Structures 993.5 Cyclohexadiene Iron Tricarbonyl and Other Related Iron Complexes 1083.5.1 FT-ICR Mass Spectrum of(fl4-c-C6H8)Fe(CO)3 1083.5.2 Ion-Molecule Reaction with Methanol 1103.6 Summary and Conclusions 113References 116Chapter 4 Gas-Phase Ion-Molecule Chemistry of CyclooctatetraeneIron Tricarbonyl - (fl4-c-C8HS)Fe(CO)3 1194.1 Ion-Molecule Reactions and Ion Reactivities 1194.2 Reaction Pathways and Ion Structures 1234.2.1 Structures of Fragment Ions 1244.2.2 Reaction Pathways of C8HFe 1264.2.3 Reaction Pathways of C6HFe 1314.2.4 Reaction Pathways of Fe 1334.3 Summary and Conclusions 136References 138Chapter 5 Gas-Phase Ion-Molecule Chemistry of SubstitutedFerrocenes - (5H4R)Fe(CR’) 1405.1 Ferrocene- Cp2Fe 1405.2 1,1’-Dimethylfrrrocene - (Cp’CH3)2Fe 1435.2.1 FT-ICR Positive Ion Mass Spectrum 143— vi —5.2.2 Ion-Molecule Reactions 1475.3 Butylferrocene - CpFeCp’(CH2)3H 1515.3.1 FT-ICR positive ion mass spectrum 1515.3.2 Ion-Molecule Reactions 1535.4 a-Methylferrocenemethanol- CpFeCp’CH(OH)CH3 1565.4.1 FT-ICR positive ion mass spectrum 1565.4.2 Ion-Molecule Reactions 1605.5 Vinylferrocene - CpFeCp’CHCH2 1665.5.1 FT-ICR positive ion mass spectrum 1665.5.2 Ion-Molecule Reactions 1685.6 Summary and Conclusions 169References 170Chapter 6 Formation and Reactivity of Negative Organometallic Ions. 1736.1 Introduction 1736.2 Negative ion mass spectra of(rj4-CH6)Fe(CO)3,(r4-c-C6H8)Fe(CO)3and (r16-c-C7H8)C (CO)3 1776.3 Negative ion-molecule reactions of(14-c-C8H)Fe(CO)3 1786.3.1 Results 1786.3.2 Discussions 1826.4 Negative ion-molecule reactions of(14-c-C6H0)Fe(CO)3 1886.4.1 Results 1886.4.2 Discussions 1936.4.3 Reactions ofC6HOFe with CD3H and CH3I 1986.5 Summary and Conclusions 200References 202— vii —LIST OF TABLESTable 2.1 Distribution of ion abundances in the Fr-ICR positive ion massspectrum of(T6-c-C7H8)Cr(CO)3 39Table 2.2 Ion identification in the6-c-C7H8)Cr(CO)3system 40Table 2.3 Rate constants and electron deficiencies of positive ions in the(q6-c-C7H8)Cr(CO) system 47Table 3.1 Distribution of ion abundances in the Fr-ICR positive ion massspectrum of(fl4-c-C6H60)Fe(CO)3 73Table 3.2 Ion identification in the(14-c-C0)Fe(CO)3system 75Table 3.3 Ion masses and isotopic distributions of(C6HO)Fe(CO)2and C6HOFe2 87Table 3.4 Rate constants and electron deficiencies of positive ions in the(r1-c-C6H0)Fe(CO)3system 100Table 4.1 Distribution of ion abundances in the Fr-ICR positive ion massspectrum of(ri4-c-C8H)Fe(CO)3 120Table 4.2 Ion identification in the(r14-c-C8H)Fe(CO)3system 121Table 4.3 Rate constants and electron deficiencies of positive ions in the(ri4-c-C8H)Fe(CO) system 123Table 5.1 Distribution of ion abundances in the Fr-ICR positive ion massspectrum of (Cp’CH3)2Fe 144Table 5.2 Distribution of ion abundances in the Fr-ICR positive ion massspectrum of CpFeCp’(CH3)H 152Table 5.3 Distribution of ion abundances in the Fr-ICR positive ion massspectrum of CpFeCp’CH(OH)CH3 157Table 5.4 Distribution of ion abundances in the Fr-ICR positive ion massspectrum of CpFeCp’CHCH2 167Table 6.1 Rate constants and electron deficiencies of negative ions in the(T14-c-C6HO)Fe( O)3system 191— viii —LIST OF FIGURESCubic ion trap (cubic cell) of FT-ICRFT-ICR ion excitation and ion detectionFr-ICR sequence of events for single resonance (SGL) experimentFT-ICR sequence of events for multiple resonance and collision-induced dissociation experimentsThe vacuum system of the FT-ICRFT-ICR single resonance spectra of(r16-c-C7H8)C (CO)3taken atdelay times (DL3) of 0 (A), 100 (B), and 800 (C) millisecondsFT-ICR double resonance mass spectra of theCr I(C7H8)Cr(CO)3 systemTemporal variation of ion intensities for Cr reactingwith (C7H8)Cr(CO)3Fr-ICR triple resonance mass spectrum of theC5H6r I(C7H8)Cr(CO)3 systemTriple resonance mass spectra of theC7H8r I(C7H8)Cr(CO)3 systemTemporal variation of ion intensities forC7H8rreacting with (C7H8)Cr(CO)3Temporal variation of ion abundances in the(C7H8)Cr(CO)3systemmonitored by FT-ICR single resonance techniqueIon reactivity vs. electron deficiency of the(C7H8)Cr(CO)3systemFT-ICR-CID spectrum of C7H8rFT-ICR triple resonance spectra ofC7H8rtaken in the presenceof deuterium at delay times of 0 ms (A) and 800 ms (B)FT-ICR-CID spectra ofC7H8r(CO) andC7Hr(CO)2Partial Fr-ICR triple resonance mass spectra illustrating thereaction of7H8Cr(CO) with methanolPartial FT-ICR triple resonance mass spectra illustrating thereaction of C7H8r with methanolFigure 1.1Figure 1.2Figure 1.3Figure 1.4Figure 1.5Figure 2.1Figure 2.2Figure 2.3Figure 2.4Figure 2.5Figure 2.6Figure 2.7Figure 2.8Figure 2.9Figure 2.10Figure 2.11Figure 2.12Figure 2.13111314202138424344454546535556575860— ix —Figure 2.14 Partial Fr-ICR triple resonance mass spectra illustrating thereaction of C5H6r with methanol 63Figure 3.1 Fr-ICR single resonance spectra of(r4-c-C6HO)Fe( O)3taken atdelay time (DL3) = 0 (A), 300 (B), and 800 (C) milliseconds 74Figure 3.2 Triple resonance mass spectra of theC6H6OFe(CO) /(C6H0)Fe(CO)3system 76Figure 3.3 Temporal variation of ion intensities forC6HOFe(CO)reacting with (C6H60)Fe(CO)3 77Figure 3.4 Triple resonance mass spectra of theC6H6OFe(CO)2I (C6H60)Fe(CO)3system 78Figure 3.5 Temporal variation of ion intensities forC6HOFe(CO)2reacting with (C6H0)Fe(CO)3 79Figure 3.6 Triple resonance mass spectra of theC6HOFe I(C6H0)Fe(CO)3system 80Figure 3.7 Triple resonance mass spectrum of theC5HFe/C5H6Fe/(C6H0)Fe(CO)3system at a delay time of 200 milliseconds 81Figure 3.8 Triple resonance mass spectra of theC5H6Fe/(C6H0)Fe(CO)3system 82Figure 3.9 Triple resonance mass spectra of theC5HFe/(C6H0)Fe(CO)3/(C5H)2Fe system 83Figure 3.10 Triple resonance mass spectra of the Fe /(C6H0)Fe(CO)3system. 86Figure 3.11 Partial triple resonance mass spectra illustrating the reactions ofC6HOFe(CO) with methanol 89Figure 3.12 Partial triple resonance mass spectra illustrating the reactions ofC6HOFe(CO) with methanol 90Figure 3.13 Partial triple resonance mass spectra illustrating the reactions ofC6HOFe with methanol 92Figure 3.14 Partial triple resonance mass spectra illustrating the reactions ofC6H6OFe(OH) with methanol 93Figure 3.15 Partial triple resonance mass spectra illustrating the reactions ofC5HFe with methanol 94—x-Figure 3.16 Partial triple resonance mass spectrum that selectsC5H6FeandC5HFe in the presence of methanol, DL3 = 0 ms 95Figure 3.17 Partial triple resonance mass spectra illustrating the reactions ofC5H6Fe with methanol 96Figure 3.18 Partial triple resonance mass spectra illustrating the reactions ofC6HOFe2with methanol 97Figure 3.19 Temporal variation of ion abundances when fragment ions from(r14-c-C6H60)Fe( O)3undergo self clustering reactions with theneutral parent molecule 99Figure 3.20 Ion reactivity vs. electron deficiency of the(C6H0)Fe(CO)3system . 107Figure 3.21 FT-ICR positive ion mass spectrum of(T4-c-C8)Fe(CO) 108Figure 3.22 Partial triple resonance mass spectrum illustrating the reaction ofC6HFe(CO) with methanol 110Figure 3.23 Partial triple resonance mass spectra illustrating the reactions ofC6H8Fe(CO) with methanol 111Figure 4.1 FT-ICR positive ion mass spectrum of the(14-c-C8H)Fe(CO)3 120Figure 4.2 Temporal variation of ion abundances in the(r1-c-C)Fe(CO)system 122Figure 4.3 Triple resonance mass spectra of theC8HFe/(ri4-c-C8H)Fe(CO)3system 127Figure 4.4 Triple resonance mass spectra of theC8HFe2(CO) /(r4-c-CgH8)Fe(CO)3system 128Figure 4.5 Temporal variation of ion intensities forC8HFereacting with (r14-c-C8H)Fe(CO)3 129Figure 4.6 Triple resonance mass spectra of theC6HFe/(T4-c-C8H)Fe(CO)3system 131Figure 4.7 Temporal variation of ion intensities forC6HFereacting with (114-c-C8H)Fe(CO)3 132Figure 4.8 Triple resonance mass spectra of the Fe /(fl4-c-C8H8)Fe( O)3system 134Figure 4.9 Temporal variation of ion intensities for Fereacting with (r14-c-C8H)Fe(CO)3 135-xi-Figure 5.1 FT-ICR single resonance mass spectra of Cp2Fe taken at delay timesof 0 (A) and 300 (B) milliseconds 141Figure 5.2 Temporal variation of ion abundances in the Cp2Fe systemmonitored by FT-ICR single resonance technique 142Figure 5.3 FT-ICR positive ion mass spectrum of (Cp’CH3)2Fe 144Figure 5.4 FT-ICR double resonance mass spectra of (Cp’CH3)2Fe 145Figure 5.5 Variation of ion abundances with time of fragment ions andtheir ion - molecule reaction products in the (Cp’CH3)2Fesystem.... 146Figure 5.6 Triple resonance mass spectra of the FeCp’CH2/ (Cp’CH3)2Fesystem 148Figure 5.7 FT-ICR positive ion mass spectrum of CpFeCp’(CH2)3H 152Figure 5.8 Triple resonance mass spectra of the CpFe I CpFeCp’C4H9system. 154Figure 5.9 Temporal variation of ion intensities for CpFe reactingwith CpFeCpC4H9 155Figure 5.10 FT-ICR positive ion mass spectrum of CpFeCp’CH(OH)CH3 157Figure 5.11 Variation of ion abundances with time of the major fragment ionsand product ions in the CpFeCp’CH(OH)CH3system 159Figure 5.12 FT-ICR triple resonance mass spectrum of CpFeCp’C23/CpFeCp’CH(OH)CH3system 160Figure 5.13 Temporal variation of ion abundances for CpFeCp’C2H3reacting with CpFeCp’CH(OH)CH3 161Figure 5.14 FT-ICR double resonance spectrum of the Fe / CpFeCp’CH(OH)CH3system 161Figure 5.15 Temporal variation of ion abundances for Fe+ reactingwith CpFeCp’CH(OH)CH3 162Figure 5.16 FT-ICR double resonance mass spectrum of the CpFe /CpFeCp’CH(OH)CH3system 163Figure 5.17 Temporal variation of ion abundances for CpFe+ reactingwith CpFeCp’CH(OH)CH3 163Figure 5.18 FT-ICR positive ion mass spectrum of CpFeCp’CHCH2 166Figure 5.19 Variation of ion abundances with time of the major fragment ions-xli-and product ions in the CpFeCp’CHCH2system 167Figure 5.20 F1’-ICR double resonance mass spectrum of the Fe /CpFeCp’CHCH system 168Figure 5.21 Fr-ICR double resonance mass spectrum of the CpFe /CpFeCp’CHCH system 169Figure 6.1 FT-ICR negative ion mass spectra of(T14-c-C8H)Fe(CO)3 179Figure 6.2 Temporal abundance variation of major negative ions in the(r14-c-C8H)Fe(CO)3system monitored by FT-ICR singleresonance technique 180Figure 6.3 Double resonance mass spectra of theC8HFe(CO) /C8HFe(CO)3 system 181Figure 6.4 Double resonance mass spectra of theC8HFe(CO)2 /C8HFe(CO)3 system 182Figure 6.5 FT-ICR negative ion mass spectra of(fl4-c-C6H60)Fe(CO)3 189Figure 6.6 Temporal variation of abundances of the negative fragment ionsof(14-c-C6H0)Fe(CO)3and their secondary product ions 190-xlii-LIST OF ABBREVIATIONSA.M.U. Atomic Mass UnitCD Collision-Induced DissociationCp Cyclopentadienyl (C5H)Cp’ Substituted cyclopentadienyl (-C5H4)DBL DouBLe resonanceEl Electron Impact ionizationEqn. EquationeV electron VoltFT-ICR Fourier Transform Ion Cyclotron Resonance mass spectrometryICR Ion Cyclotron Resonance mass spectrometryJ Joulek kilokHz kiloHertzMe Methyl (-CH3)MHz MegaHertzmlz Mass-to-charge ratio of ionsms millisecondppm parts per millionrf racliofrequencySec. SecondSGL SinGLe resonanceTPL TriPLe resonanceV Voltco Cyclotron frequency— xiv —ACKNOWLEDGMENTI would like to express my sincere gratitude to my research supervisor,Dr. Melvin B. Comisarow, for his guidance, encouragement and patienceduring the course of this work; to my guidance committee members Drs. M.Blades, G. Bates and G. Patey; to my colleagues Shuping Chen, SandraTaylor and Zamas Lam for helpful discussions and suggestions, to Dr. G.Eigendorf for the opportunity to learn the mass spectrometric techniques otherthan Fourier transform mass spectrometry; to Dr. Irene Waller for herencouragement, to the staffs of the electronics shop and the mechanical shop inthis department for their frequent assistance in repairing the instrument; to Drs.G. Meng, H. Zhang and T. Zheng for many fruitful discussions in condensedphase organometallic chemistry.I extend special thanks to my wife Amy H. Que who always supportsme with her love.—1—CHAPTER 1IntroductionIn 1964, Schmacher and Taubenest1 first reported the gas-phase ion-moleculereactions of the ferrocene and nickelocene systems. Since then, there has been an increasinginterest in the gas-phase ion chemistry of transition metal carbonyls and organo-transitionmetal complexes2.In 1973, Muller4summarized that the results from ion-molecule reactionstudies of organometallic complexes were important in two aspects:1. Gas-phase ion-molecule reaction studies provide the possibility of investigatingvery elementary processes such as collisions between ions and molecules withoutcomplicating solvent phenomena.2. From ion-molecule reaction studies we get valuable information relating to theformation and stability of metal-metal and metal-ligand bonds.We also believe that the results are of significance in understanding fundamentalchemical processes and guiding condensed-phase synthesis. In addition, as different ionsmay interact differently with the same neutral precursor, ion reactivity and ion-moleculereaction product analysis can provide structural information about gas-phase organometallicions. The research work described in this thesis focuses on gas-phase ion-molecule clusteringreactions of organo-transition metal complexes by using Fourier transform ion cyclotronresonance522 (FT-ICR) mass spectrometric techniques. Since its invention in 1974,5.6 FTICR theoretical aspects,2345 FT-ICR techniques and FT-ICR applications4685 have beenreviewed almost every year.1.1 Mass Spectrometric Studies of Transition Metal-Containing IonsMost early studies on organo-transition metal complexes were performed onconventional magnetic sector mass spectrometers at high sample pressures (1 o- Torr)8688.Since gas-phase ions could not be trapped to react with neutral molecules before detection, the—2—reaction yields in these studies were limited. Since the 1970s, new mass spectrometrictechniques such as, guided ion beam89’90,flowing afterglow91’2,ion cyclotron resonance(ICR)93’4 and Fourier transform ion cyclotron resonance (FT-ICR)5’6have been developed,and the gas-phase ion chemistry of transition metal complexes has been studied widely byusing these techniques95’16 Among these techniques, FT-ICR mass spectrometry featuredwith wide mass range, high resolution, high speed, efficient ion trapping and ion ejectioncapability is most widely used nowadays. The FT-ICR multiple resonance technique13will beused extensively in this thesis as it is the technique of choice for establishing ion-moleculereaction pathways.In 1964, Schmacher and Taubenest’ observed ion-molecule reaction productsCp3Fe2 and Cp3Ni2twhen studying the mass spectra of ferrocene (Cp2Fe) and nickelocene(Cp2Ni) at low ionization energy (20eV) and high pressure(105Torr). A ‘triple-deckersandwich’ structure was suggested for these ‘ligand-rich’ ionic products. Werner117, in1972, prepared complex [Cp3Ni2]BF4in solution and confirmed a ‘triple-decker sandwich’structure for its cation portion, Cp3Ni2. Schmacher and Taubenest87 in 1966, studying themass spectra of [CpFe(CO)2],also observed cluster ions such as Cp4Fe4(CO)Cp3Fe(CO)4, andCp3Fe(CO)4 formed by ion-molecule clustering reactions. King88prepared the tetrameric cyclopentadienyl iron carbonyl and its mass spectrum correlated withSchumacher and Taubenest’s ion-molecule reaction observations.In another early study, MUller118 noted that the fragment ions and neutral molecules of‘r5-cyclopentadienyl transition metal carbonyl complexes of chromium, manganese, cobaltand vanadium interacted to produce secondary, bimetallic ion clusters consisting of variousamounts of Cp and CO units, for example,Cp2V(CO)3.MUller and Go114 also examinedion-molecule interactions of CpNiNO and its fragment ions. Again, cluster cation productscontaining two nickel atoms (Cp2Ni2 portion) were produced.Using ICR in 1973, Dunbar119 observed anion-molecule condensations in the metalcarbonyl systems Cr(CO)6,Ni(CO)4 and Fe(CO)5,that formed bimetallic cluster anions.Foster and Beauchamp’20’12examined positive fragment ions and ionic cluster products in—3—the Fe(CO)5 system, observing cluster fragments containing up to four iron atoms. UsingICR ion ejection technique, Foster and Beauchamp’22also unambiguously identified that theCp3Fe observed in the ferrocene system was formed by the reaction between CpFe and theneutral ferrocene molecule. In 1977, Corderman and Beauchamp’23 postulated that themarked non-reactivity of the cluster anion Cp2Co2(CO)2 in the negative ion-molecule regimeof CpCo(CO)2 probably reflected partial double bonding between cobalt atoms within theionic cluster. Detailed characterization and analysis of negative ion-molecule clusteringreactions and the ionic products in Fe(CO)5were described by Wronka and Ridge’24 whoused a frequency-swept ICR and ion ejection technique to establish product ion precursors.The authors related the reaction rate constant of an ion to the electron deficiency on its centralmetal atom. The general ion reactivity trend observed in the Fe(CO)5 system was that rateconstants increased with increasing electron deficiencies. Single metal-metal bonds wereassigned to the multi-metallic product ions having normal reactivities. For those dimetallicproduct ion having anomalously low reactivities such as Fe2(CO)5 andFe2(CO)6,doublemetal-metal bonds were assigned to their structures to adjust the electron deficiencies on thecentral metal atoms to match the observed ion reactivities. Such a reactivity / electron-deficiency trend was also observed in the later FT-ICR studies of positive ion chemistry ofRe2(CO)0,’5 Fe(CO)526 Cr(CO)6126 and Ni(CO)4 127 Deviations from the generalreactivity trend in these studies were explained by invoking unique gas-phase ion structuresformed by multiple metal-metal bonds and/or 4-electron CO donors.FT-ICR mass spectrometry was first used to study the ion-molecule chemistry oforganometallic complexes by Parisod and Comisarow in 1980.128 They successfullydemonstrated that complex reaction pathways could be resolved by FT-ICR double resonancetechnique, in which individual ions or groups of ions could be removed from the reaction cell,thus simplifying kinetic measurements on the remaining reactants. As a result of this ionejection technique, reaction pathways of fragment ions with their neutral parent molecules inCpMn(CO)3,CpCr(CO)2N0 and CpCr(CO)2NS systems were unambiguously determined.The cyclopentadienyl and carbonyl ligands in these systems were observed to retain their—4—structural integrity throughout entire ion-molecule reaction processes. Neutral elimination ofnitrogen and sulfur abstraction reactions causing cleavage of N-O and N-S bonds wereobserved in some ion-molecule processes.Although this very first FT-ICR examination of ion-molecule clustering reactions wason hydrocarbon-metal carbonyls, most subsequent FT-ICR studies have been on lesscomplicated transition metal carbonyl systems, such asRe2(CO)10,25’930Mn2(CO)10,’5H20s3(CO)10,31 ReMn(CO)10,32 Cr(CO)6,126 Fe(CO)5,126 Ni(CO)4,’27Co(CO)(NO),’27Fe(CO)5/Co(C(NO and Fe(CO)/Ni(4133 Fe orCoIFe(CO)5,’34Fe/Co2(CO)8,’35Co or V/Fe(CO)5,’36Fe(’CO)/Fe CO)5,’37FefFe(CO)5,CoICo2(CO)8and MofMo(CO)6,’38Fe or Co/Fe(CO)5orCo2(CO)8,139La orRhIFe(CO)5and RhICo(CO)3(NO),140Sc, Ti, V, Cr, Fe, Co, Ni, Cut, Nb orT&JFe(CO),14’CufFe(CO)5,142 and La2 with Fe(CO)5.’43 A general feature of thesestudies is that the bare metal ions or metal carbonyl ions associate with the neutral metalcarbonyl complexes to form multi-metallic carbonyl ions that are believed to have metal-metalbonds. Indeed, when accelerated and collided with an inert gas (collision-induceddissociation, CID), these multi-metallic ions lose their carbonyls to form bare homonuclearand heteronuclear ions such as Co2t139””Fe2+,139 CoFe,134’945 VFe+,136 ScFe,146and FeCo2.135 The metal-metal bond energies (D°) of these ions were obtained by liganddisplacement ion-molecule reactions (bracketing), photodissociation, or collision-induceddissociation techniques.147 For some MFe+ (M = a transition metal) ions, metal-metal bondenergies were measured to be in the range of 48 kcaL/mol to 75 kcaL/mol.141 D°(Fe+.Fe) ofFe2 , for example, was measured to be 62±5 kcallmol by photodissociation which is higherthan that of Fe-C6H6(C6H6was presumed to be a benzene), 55 ± 5 kcallmol, obtained bythe same technique.’41’148 These measured, relatively large bond energies are consistent withthe tendency of forming metal-metal bonds in the ion-molecule clustering reactions.In contrast to the CpCr and CpMn systems studied by Parisod and Comisarow,’28Chen149 noted that in the positive ion-molecule chemistry of CpCr(NO)2H3,Cp-metalbonds also tended to remain intact during the electron ionization process, as well as—5—throughout subsequent ion-molecule reactions, and retained 1: 1 Cp-to-metal ratios in mostproduct ions. Exceptions to this, where 1: 1 Cp-to-metal stoichiometry was not maintained,were found only where a bare metal ion underwent condensation with the parent neutralmolecule, or in highly oxygenated ionic cluster products such as Cp2r3O andCp4Cr5(NO)O3. These oxygenated clusters were postulated to possess either multiple metal-metal bonds or chromium-oxygen multiple bonding.Ion-molecule clustering reactions of a series of(r16-arene)Cr(CO)3 complexes werestudied by Freiser and co-workers in 1991.150 Except for Cr which formed (ri6-arene)Cr2(CO)1,2 ions, other arene-ligated chromium ions produced(fl6-arene)2Cr2(CO)ions in their initial reactions with the neutral parent molecules. Further cluster product ionswere formed by either expulsion of the arene ligand or preserving the 1:1 arene-to-Crstoichiometry. Bond energies of Cr to arenes were qualitatively estimated from their CIDresults. Another series of aryl transition metal carbonyl complexes including CpV(CO)4,CpMn(CO)3,CpCo(CO)2,(r16-c-CH)C (CO)3r14-CH6Fe(CO)3,were studied recently byTaylor.’51 Similar reaction routes were observed in each of these systems. As well, thereactivity of their positive ions were observed to vary directly with electron deficiency andcoordination site availability of central metal core.While the ion-molecule clustering reactions have been studied intensively, theactivation of C-C and C-H bonds of hydrocarbon molecules by bare or ligated transition metalions in the gas phase has also received much attention. In 1979, Allison, Freas and Ridge152first evidenced the oxidative addition of the gas-phase ion, Fe+, to isomeric butanes. Themetal ion reacted with the hydrocarbons to give elimination of molecular methane or molecularhydrogen via both C-C and C-H bond insertion. Since 1979, C-C and C-H bond activation ofhydrocarbons by gas-phase bare or ligated metal ions has been intensively studied.153474The activation process involves the cleavage of C-C or C-H bond of the hydrocarbon-metalions formed by the initial ion-molecule reactions between metal ions and hydrocarbons. Themechanism that has been generally accepted involves a f3-hydride transfer process yielding ahydrido-n-hydrocarbon intermediate as generalized in reaction 1.—6—(1)Allison and Ridge175 also demonstrated that the gas-phase metal ion, Ti, couldpromote elimination of two molecular hydrogens from isomeric methylpentenes,C5H9Me, togive a product ionC5HMeTi. It was suggested that the product ion was a metalmethylcyclopentadiene complex, 1, that rearranged to form a methylcyclopentadienyl metalhydride, 2.cc(2)TiH1 2Similarly, ions M-c-C5H6(M=Fe, Co, Ni and Rh), MC3H6(M=Fe, Co, Ni and Rh) andM-c-C7H8(Co and Rh) have been produced by reacting metal ions with cyclopentene,propane and cycloheptene, respectively.’58’176”77 It was suggested that each of these finalproduct ions underwent a further p-hydride transfer from the olefin ligand to the metal centerto form hydrocarbon metal hydride structures.158 The existence of metal-hydrogen bonds inthese products support the f-hydride transfer mechanism proposed for the C-C and C-H bondactivation processes.While the hydrido-ir-allyl structural moiety in equation 1 has been observed directly inthe condensed phase,’78”9the gas phase metal hydride character has mainly been probed byHID exchange with deuterium157613780 or ethene-d4.’58 Jacobson andFreiser158 observed that both Fe-c-C5H6and Co-c-C5H6underwent six HID exchangeswith deuterium, the first exchange was slow with the second exchange occurred much moreslowly. These observations concluded that the above two ions were in an equilibriumbetween a cyclopentadiene complex and a cyclopentadienyl metal hydride complex. The sameauthors did not observe H/D exchange in the presence of excess deuterium for both Ni-cC5H6and Rh-c-C5H6.This did not rule out the presence of metal-hydrogen bonding of—7—these two ions since metal hydride FeD+’8’did not undergo DJH exchange in the presence ofhydrogen. The structures of ions Ni-c-C5H6,Rh-c-C5H6and M-c-C7H8(M = Co andRh) were then tested by using ethene-d4(C2D4).’58 A single H/D exchange was observed forthese ions, which supports the metal hydride structures of these ions. Similarly, ionsMC3H6(M=Fe, Co, Ni and Rh) were tested with deuterium and C2D4and a hydrido-it-allylstructure was concluded for each of these ions. It has also been reported that someorganometallic ions having no metal hydride character undergo H/D exchange with deuteriumand C2D4 as well.158’80 These observations along with the fact that FeD does not reactwith H2 indicate that the HID exchange method is not always a definitive test for the metal-hydrogen bonds of gas-phase organometallic ions.Buckner and Freiser182’3 observed the reactions of ions McC5H6+(M=Fe, Co, Niand Rh) and RhC3H6with NH3. The reactions of Fe-c-C5H6,Co-c-C5H6and RhC3H6gave elimination of molecular hydrogen, yielding(C)M(NH2and(C3H5)Rh(NH2.The authors concluded that these three ions had structures 3 - 5. Elimination of molecularhydrogen was not observed in Ni-c-C5H6and Rh-c-C5H6,ruling out the hydrido-ndyclopentadienyl structures of these two ions. However, results obtained via reacting Ni-cC5H6and Rh-c-C5H6with C2D4contradicts this.’58 Therefore, a more definitive methodto detect metal-hydrogen bonds is needed.c7 c7I IFeH CoH3 41.2 Research Interests and Thesis formatTo summarize the above introduction, self ion-molecule clustering reactions andactivation of C-C and C-H bonds are the two major aspects in the study of gas-phasetransition metal ion chemistry. For self ion-molecule clustering reactions, most of the studieshave focused on transition metal carbonyls or transition metal complexes bearing aromatic5—8—cyclopentadienyl and benzene ligands. These ligands typically retain their structural integritythroughout the entire ion-molecule regime. For C-C and C-H bond activation study, bare andligated metal ions react with hydrocarbon to form olefin-metal product ions by eliminatingmolecular hydrogen andlor alkane. Hydrocarbon-metal ions formed without elimination ofany neutrals are generally not observed. Therefore, the reaction mechanism leading to theformation of the olefin-metal products are either proposed by analyzing the structures of thefinal product ions or elucidated by using isotopic-labeled hydrocarbons as neutralreactants.152’576084187In this research, ion-molecule chemistry of several known organometallic complexes isexamined. The complexes studied are: cycloheptatriene chromium tricarbonyl - (r6-c-C7H8)Cr(CO)3(6), cyclohexadiene iron tricarbonyl - (r14-c-C6H8)Fe( O)3(7),cyclohexadienone iron tricarbonyl - (r14-c-C6H0)Fe(CO)3(8), cyclooctatetraene irontricarbonyl-(14-c-C8H)Fe(CO)3(9), butadiene iron tricarbonyl -(14CH6)Fe(CO)3(10)and a series of substituted ferrocenes -(rI5c-CR)Fe(rIH.)(11, 12).Cr(CO)3 Fe(CO)36 9= -CH=CH2r-CH3I \\ Fe -CH3 FeF:(CO)3 -CH(OH)CHThe organic ligands of complexes 6-8 and some of the ferrocenes have saturatedcarbon(s) in their structures. 9 has an uncoordinated diene portion in its ligand. If and howthese ligands change their structures or coordination hapticities to the central metal atoms in the7 8—9--self ion-molecule clustering reactions and how the change affects ion reactivity created ourinitial interest in the ion chemistry of these compounds.As mentioned in the last section, a gas-phase olefin-metal ion, where the olefin hassaturated carbons, can undergo a p-hydride transfer reaction. Our ion reactivity resultsindicate the presence of the transfer in some of the ions formed from complexes 6-8, yieldinghydrocarbon-metal hydride ions. Testing the presence of these hydride ions and investigationon new probing methods are also the objectives of this research. In addition, since the ligandmetal linkages in these complexes have already been characterized in the condensed-phase,probing the presence of hydrocarbon-metal hydride ions would provide further information to3-hydride transfer mechanism of the C-C and C-H activation processes. Studying thesechanged ion structures is also of importance for a better understanding of the mass spectra oforganometallic complexes.The thesis is formatted in the following way: the rest of this introductory chapter willcover the fundamental operating principles of FT-ICR and its experimental techniques.Chapters 2, 3, 4, and 5 deal with the positive ion-molecule chemistry of compounds 6, 7 and8, 9, and the ferrocene series, respectively. The negative ion-molecule chemistry ofcompounds 6-10 is discussed together in Chapter 6. Some small neutral molecules such as,D2,CD3N/CHN,CD3JJCHIand deuterium-labeled methanol, were allowed to react withsome selected ions to probe the chemical behavior of metal hydride ions. Ion-moleculereactions with methanol are observed to be an effective detector of metal-hydrogen bonds.This will be discussed mainly in Chapters 2 and 3.1.3 Experimental Section1.3.1 Operating Principle of FT-ICRIt has long been known that, in an uniform static magnetic field, a moving chargedparticle will be subjected to a centripetal force (Lorentz force) which is perpendicular to the-— 10—direction of the particle movement. The movement of the particle will be constrained to acircular orbit perpendicular to direction of the magnetic field. The angular frequency of thecircular motion of the particle, co, is given byco=qB/m (1.1)where B is the magnetic field strength, q is the electronic charge, m is the mass of the particle.The angular frequency co is called the cyclotron frequency of the particle.In early 1930’s, Lawrence1889 demonstrated that a charged particle in a magneticfield can be accelerated by applying an oscillating electric field having the same frequency asthe cyclotron frequency of the particle. The accelerated particle will keep its intrinsic cyclotronfrequency and move in an expanded circular orbit. The absorption of energy by the particlefrom the alternative electric field is called cyclotron resonance. Equation 1.1 is known ascyclotron equation and relates the mass of a charged particle to its cyclotron frequency.Based on Lawrence’s cyclotron resonance principle, Hipple and co-workers94developed an ion cyclotron resonance (ICR) spectrometer in 1949. In this instrument, ionswere formed by electron impact and detected in an one-region cell by translational excitation oftheir cyclotron motion until they strike an ion collector. Restriction of the ion motion in thedirection of the magnetic field was achieved by applying a trapping voltage to the appropriateplates of the cell. In the first commercial ICR-MS instrument made by Varian Associates in1965,190 a three section ICR cell was employed. Ions were formed in an ion source regionand allowed to drift by the action of perpendicular electric and magnetic fields through ananalyzer region into the ion collector part of the cell. This drift-cell ICR-MS suffered fromlow sensitivity, low resolution, limited mass range and long spectrum recording time. Themethod, however, had been used quite extensively by a number of research groups mainlybecause its double resonance technique allowed establishing the relationship between reactantand product ions.19193Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR), invented byComisarow and Marshall in 1974,5,6 did not employ the same type of detection methods asconventional ICR mass spectrometers. The cyclotron resonance equation, w = qB/m, relates—11—an ion mass to its cyclotron frequency. In Fr-ICR, to measure an ion mass is to measure itscyclotron frequency. Figure 1.1 depicts a cubic ion-trap (cubic cell) used in FT-ICR.’7”8Figure 1.1 A cubic ion trap (cubic cell) of FT-ICR.’7”8The cell is composed of six squareplates. Two Vs are trapping plates to which is applied a direct voltage, typically, + 1 V forpositive ions and — 1 V for negative ions. N and S are receiver plates for detection of ioncyclotron motion. E and W are transmitter plates for RF excitation. C is an electron collectorfor attracting and monitoring the electron current. G is a grid for shielding the interior of thecell from the strong ionization voltage. F is a filament. P is a preamplifier for ion detection.A is a transmitter amplifier for the RF field. B is a magnetic field applied across the cubiccell.The cell is constructed of three pairs of parallel plates: trapping plates, transmitterplates, and receiver plates. The cell is enclosed in a vacuum chamber which is placed in anuniform magnetic field. Samples to be analyzed are normally introduced into the cubic cell vialeak valves. The samples are usually kept at low pressures (10-8 - 10-6 Torr). Positive andnegative ions are generated by a pulsed electron beam (electron impact). Electrons emittedfrom the filament are guided by the grid and collector to form a beam. Molecules in the path—12—of this beam are ionized. After ions are formed, ion motions parallel to the magnetic field (zdirection) are restricted by applying a same direct voltage to the two trapping plates. Initially,the trapped ions are centered around the middle of the cell and move in small circular orbitsperpendicular to the magnetic field at their cyclotron frequency. In order to determine the ioncyclotron frequency and thus determine the ion mass, two steps are needed - FT-ICRexcitation and F1’-ICR detection.FT-ICR excitation coherently excites the thermal cyclotron motions of the ions withradii in the sub millimeter range to greater cyclotron orbits. This is achieved by applying abroadband radio frequency (oscillating electric field) across the pair of transmitter plates. Anensemble of ions with same mass-to-charge ratio will absorb energy from the electric field attheir intrinsic cyclotron frequency and travel in a spiral path (Figure 1 .2a).After the frequency generator is turned off, all the ions are excited to an enlargedcircular orbit (Figure 1 .2b). The radius of the orbit is given by equation 1.2,12— V0(rms) t 2r- Bd (.)whereV0(rms) is the root mean square value of the excitation voltage, t is the duration of thepulse, d is the separation between the two transmitter plates. The equation shows that theradius of the ion orbit is independent of the ion mass.The movements of the excited ions will induce electric charges on the two receiverplates and a time domain signal current in the circuit connecting the two plates (Figure1.2b).” The signal current is converted to a voltage signal, Vm(t). In theory, Vm(t), the ICRsignal of a particular ensemble of ions with same mass-to-charge ratio, can be expressed byequation 1.3.12Vm(t) Nqr (1.3)dC COS((Omt)where N is the number of ions, C is the circuit capacitance, o is the cyclotron frequency of aparticular ensemble of ions with same mass-to-charge ratio, and t is time. Note that the ICRsignal voltage is proportional to the number of ions and the radius of the excited cyclotronmotion.—13—V(t)(a) (b)The radio frequency is on The radio frequency is offFigure 1.2 FT-ICR ion excitation and ion detection. (a). Ion excitation. When a radiofrequency field is applied an ICR cell, the ion motion is excited and its cyclotron radiusincreases as the ion follows a spiral orbit. (b). Ion detection. When the RF field is off, theion cyclotron radius is constant, and the ion cyclotron motion induces a signal current 7(t) thatis converted to an alternating voltage signal (t) and amplified by the preamplifier. Thevoltage signal V(t) is collected by a computer where it is Fourier transformed to givefrequency components.Ensembles of ions with different mass-to-charge ratios will be simultaneously excitedto the same enlarged orbit at different positions - simultaneous excitation. The movements ofthe ions will induce a composite ICR voltage signal, V(t), which is a sum of signals, each ofwhich comes from the excited cyclotron motion at a particular ion mass , equation 1.4.V(t)ZVm(t) (1.4)This composite time domain signal is collected by a computer and Fourier transformed to giveboth the intensity and frequency of each component signal at the same time - simultaneous— 14—detection. The final mass spectrum is derived by a mass I frequency unit conversion (Eqn.1.1) of the Fourier transformed signals.The above discussed elemental FT-ICR sequence of events is referred as an FT-ICRSinGLe (SGL) resonance experiment and is summarized in Figure 1.3. SGL denotes thatonly one RF sweep is applied for the purpose of ion excitation.Figure 1.3 Fr-ICR sequence of events for single resonance (SGL) experiment. Samplemolecules are ionized in the ion formation pulse (electron beam pulse in the research). Ionsformed are trapped in the ICR cell and simultaneously excited by a broad-range RF sweep inthe ion excitation event. The cyclotron motions of ions induce time domain signal in the iondetection event. A new cycle starts after all the ions in the cubic cell are cleared out in the ionquench event by application of a large voltage pulse on one of the trapping plates. The timedomain signal from each cycle is collected by a computer where it is Fourier transformed toshow both intensity and frequency of each component signal. The whole mass spectrum isfinally displayed by converting these analyzed frequencies into mass unit using the cyclotronequation (coo = qB/m). The variable delay time allows primary fragment ions to react withneutral molecules. If the delay time is set to zero (DL3=O), the FT-ICR detects the ordinarymass spectrum of the sample.Ion Ion IonFormation Excitation Detection(rf sweep)f]Variabl,_Time RepeatIonQuenchAccumulatedSignal OutputMass SpectrumFrequency/Mass UnitConversion ( co = qB/m)EEFourier TransformTime DomainSignal—15—1.3.2 Performance of FT-ICR-MSAny mass spectroscopic technique can be characterized by the performance criteria ofspeed, sensitivity, resolution, mass range, convenience and versatility.2’ These individualcriteria of FT-ICR are discussed in this section.Speed. Fr-ICR instruments simultaneously detect all ions in the whole massspectrum. This takes the time that a scanning ICR spectrometer would require to scan acrossjust a single ion peak in the mass spectrum. For routine mass spectral analysis, the reactiondelay time period is set to zero. Typical times for electron impact ionization, excitation,detection, quench and Fourier transformation would be 5 ms, 5 ms, 10 ms, and 1 ms,repeated 100 times which, with a Fourier transform time of 1000 ms, gives a total time of 3seconds.2’ This is comparable to the fastest scanning mass spectrometers and is severalthousand times faster than conventional scanning ICR spectrometers.Sensitivity. The signal-to-noise ratio of FT-ICR-MS is directly proportional to N112,with N being the number of times the signal is sampled.2’ A small ion peak can be enhanceddramatically with respect to the noise base line by simply accumulating its time-domain signal.Sensitivity can be defined relatively to conventional ICR instruments, absolutely in terms ofthe minimum number of ions detected or absolutely in terms of the partial pressure of neutralmolecules. Instruments for FT-ICR are, via addition of time signals prior to Fouriertransformation, up to 100 times as sensitive as conventional ICR instruments. About ten ionsis the minimum theoretical number which can be detected with current generation ofinstruments.20 A partial pressure of i014 atmospheres of sample has been observed17by FrICR. Sensitivity measured in terms of minimum amount of sample required to give a signal,the most important practical definition of sensitivity, indicates that low picogram quantities ofsample can be detected.Resolution and mass range. The ultrahigh mass resolution and very wide mass rangeof the Fr-ICR instrument are the two major features of the instrument which have mostinterested the mass spectrometry community. These features were predicted and demonstrated—16—very early in the development of the technique.8’9”17 Equation 1.1 (o = qB/m) indicatesthat there is no upper mass limit for FT-ICR instruments. Increasing the magnetic fieldstrength increases both the mass range and the mass resolution. For example, a theoreticalanalysis concluded that the actual upper mass limit would be mlz 50,000 in a 3-T magnet, m/z300,000 in a 7-T magnet, and > m/z 1,000,000 in a 14-T magnet.’94 When combined withthe electrospray ionization (ESI) technique’95 that generates multiply-charged ions, thecapability of FT-ICR to measure molecules with large molecular weights is even greater.An FT-ICR resolution of 4 x 108 for 40Ar has been achieved using a 7 Teslamagnetic field.196 This ultrahigh resolution of the FT-ICR instrument has been used tomeasure the precise mass difference between 3H+ and 3He+ with an accuracy of 3 parts inio’, for purposes of determining the mass of electron antineutrino)97’198 Mass accuracy ofup to 4 x 10-10 has been obtained for ions of CO, N2, 1H, 2H, 3H, and 12C.’99’2Frequency, more so than most physical parameters, can be measured to very highaccuracy with great ease, with the error of measurement limited only by the inherentuncertainty of the frequency itself. In part, this explains why FT-ICR achieves ultrahigh massresolution. The accuracy with which any frequency can be determined is limited by theclassical uncertainty principle, which states that the uncertainty in frequency measurement isinversely proportional to the observation time (T).2’Uncertainty in frequency measurement (zCo) = 1/ T (1.5)Thus, for a 1-sec. observation of a 1-KHz signal, the frequency uncertainty is 1 Hz or 1 partin 1000. For a lOs observation, the uncertainty is 0.1 Hz or 1 part in 10,000, and so on. Theuncertainty in frequency is manifested in the frequency domain by the finite line width of apeak in the mass spectrum.At its most fundamental level, an FT-ICR spectrometer is a “frequency meter”, withthe mass meter feature of the instrument only arising via Equation 1.1. The uncertainty inmeasuring cyclotron frequency is ultimately limited by the duration of FT-ICR time domainsignal. The uncertainty (i.e., the line width) can be greater if the signal is observed for less—17—than its duration. The duration of the time signal is limited principally by the backgroundpressure of neutral molecules with the duration being given approximately by2’Duration(s) = 2x108/ pressure(Torr) (1.6)Thus, at i0 Torr, a typical operating pressure, the F1’-ICR time signal will last for 0.2 s.This gives a frequency uncertainty or frequency line width of a few Hz.The FT-ICR mass line width Am, and the mass resolution Am I m are given,respectively, by,9’17Am=m2&o/qB (1.7)m/Am=qBIm&.o (1.8)These equations show that mass resolution is directly proportional to the magnetic field andinversely proportional to the ion mass, as well as inversely proportional to the frequencyuncertainty which can be lowered by lengthening the duration of the time domain signal.1.3.3 FT-ICR Experimental TechniquesIonization techniques other than El such as laser desorption I ionization (LDI), matrix-assisted laser desorption I ionization (MALDI), multi-photon ionization (MPI), fast atombombardment (FAB) and electrospray ionization (ESI) have all been adapted for use in FTICR, particularly for the analysis of large biomolecules.85 They have been well discussedpreviously and are not covered here. The following paragraphs introduce the ion ejection’3and collision-induced dissociation (CID)20’ techniques of FT-ICR.FT-ICR ion ejection achieves elimination of any ion or groups of ions from a massspectrum and is a powerful technique for studying gas phase ion-molecule reactions. Takingthe following general reactions as examples,A ÷ M —* C + NB + M —* D + Nwhere A+ and B+ are the primary reactant ions and C+ and D+ are the secondary product ions.To determine whether C+ is produced by A+ or B+, we can simply select one of the primary—18—ions (A+ or B+) at a time using FT-ICR multiple resonance techniques and monitoring itssubsequent ion-molecule reactions to see if the reactions form C+ or D+.As discussed in the FT-ICR excitation process, while the ions of a given mass areexcited at their resonant frequency in the FT-ICR cell, they absorb energy and move in a spiralpath into a larger orbit. If the amplitude or the duration of the radio frequency is large or longenough, the radius of the orbit increases enough to cause the ions to collide with one of thetransmitter or receiver plates and the ions will be absorbed by the plates. This forms the basisfor the ion ejection techniques in FT-ICR.’3’193In the actual FT-ICR ion ejection sequence, a high power radio frequency sweepcontaining the cyclotron frequencies of ions in a particular mass range is applied to the cellinstead of applying an RF field equal to the frequency of one particular ion. In such a case,the technique is titled as DouBLe resonance (DBL), meaning that one more RF sweep is usedin addition to the ion excitation RF sweep. This double resonance sequence is used to ejectone particular ion or ions in one particular mass range in the mass spectrum. Analogous to thedouble resonance, TriPLe resonance (TPL) experiments use two more RF sweeps to ejecttwo ions with different masses or ions in two different mass ranges. This allows the selectionof a particular ion of interest.Collision-induced dissociation (CD) is a widely used mass spectrometric techniquefor structural determination. In general, the CD technique consists of accelerating a given ioninto a collision gas thereby imparting energy to the ion and inducing fragmentation. CID inFT-ICR is achieved by using the following experimental arrangement. The ion of interest isfirst isolated using the ion ejection techniques to eject other ions (fixed RF or swept RF). Theselected ion is then accelerated by applying an extra RF pulse (fixed frequency) thatcorresponds to the cyclotron frequency of the selected ion. This excitation step is followed bya suitable interaction time during which the accelerated ion collide with a collision gas andfragment into smaller daughter ion(s). To ensure efficient collisional dissociation, thecollision gas is maintained at a higher pressure than that of the parent neutral molecule fromwhich the selected ion is generated. One important aspect of the FT-ICR-CID experiment is—19—that ion isolation and dissociation occur in the same spatial region. The ability to performsequential CID in FT-ICR does not require any hardware modification. Our instrument alsohas a particular ion ejection feature, Frequency During Beam (FDB), that can be coupled intoany of these sequences. In FDB, a fixed RF whose duration is equal to that of the beam pulseis applied during the ion formation event. This makes it possible to continuously eject one ofthe fragment ions formed in this time period. Indeed, versatile FT-ICR sequences of eventscan be arranged depending on the setups of the instrument hardware and the computersoftware. The sequences described here are only those used in this research.FT-ICR sequences of events for single resonance (SGL), multiple resonance (DBLand TPL), and collision-induced dissociation (CID) experiments are summarized in Figure1.4. The time delays (DL 1, DL2 and DL3) in these sequences can be set at different lengthsfor different purposes. For example, DL3 is usually set to a much higher value than thosetaken by the FT-ICR events of pulses and other delay times (DL1, DL2) and used as reactiontime of a particular reactant ion. In TPL, if we want to select a fragment ion to study itsreaction pathways, DL1 is set to zero. If we need to select a product ion in TPL or CD, DL1is set to a certain value for the product to form before the ion ejection.—20—Ion Ion Ion Ionformation excitation detection quench(rf sweep)J] Variable delay time (DL3) [] [] ] RepeatSingle ResonanceIon Ion Ion Ion Ion Ionformation ejection I ejection 2 excitation detection quench(rf sweep) (rf sweep) (if sweep)J1Lf1J1L3 r—[ r—i [1 RepeatTriple resonanceIon Ion Ion Ion Ion Ion Ionformation ejection 1 ejection 2 acceleration excitation detection quench(if swee ) (if sweep) (fixed rf) (if sweep)fLJ1JL_El_DL!J1_fl_flRePeatCollision-Induced DissociationSignal output Fr-ICR event controlComputerFigure 1.4 FT-ICR sequences of events for single resonance (SGL), triple resonance(TPL), and collision-induced dissociation (CD) experiments. SGL allows reactant ions withdifferent ion masses to react with a neutral molecule under same ICR conditions. TPL allowsthe selection of a single ion species to study its reaction pathways. CD) allows the selection ofa single ion species to study its fragmentation daughter ions on colliding with a target gas. Ifone of the ion ejection events in TPL is omitted, the sequence becomes that of a doubleresonance (DBL) which allows ejection of ions with a particular ion mass or ions in a massrange.—21—1.3.4 Experimental Parameters and ProceduresAll experiments were performed on a home-built FT-ICR mass spectrometer equippedwith a Nicolet FTMS-1000 console and a 1.9 Tesla electromagnet. A 2.54 cm cubic cell wasused as the ion trap and reaction cell. The trapping voltage was set to +1 V for positive ionsand -l V for negative ions. 20 - 40 eV and 1 - 5 eV electron beam pulses were used togenerate positive and negative fragment ions, respectively. A schematic diagram of thevacuum system of the instrument is shown in Figure 1. 5.Figure 1.5 Schematic diagram of the vacuum system of the FT-ICR. A sample contained ina sampling tube is introduced by means of a roughing valve, then passes through a leak valveinto the cell region. Sample pressure is maintained at a low, constant dynamic pressure bycontinuous ion pumping.Background pressure of the system was kept below 5 x iO Torr by a diffusion pumpand an ion pump. Samples were introduced into the vacuum system through a leak valve byusing their vapor pressures at room temperature. Sample pressures were maintained at a rangeof 10-8 - 10-6 Torr and were monitored by an ionization gauge (Varian model 971-1008).CubicCellMagnetIon Gauge—22—All the organometallic complexes used in the research were obtained commercially inhigh purity and was used as supplied. No impurities were found in their mass spectra. In theFr-ICR-CD experiments, nitrogen at ten times of the sample pressure was used as collisiongas. In the cases that a neutral reactant other than parent molecule such as methanol, was usedto react with ions, a separate inlet system was used to introduce the sample. Multiple freeze-pump-thaw cycles were used to remove non-condensable gases from liquid samples.A complex was generally examined by FT-ICR single resonance sequence first toobtain time-resolved mass spectra. These spectra were then calibrated to obtain exact ionmasses to determine the ion formulae. Unlike conventional mass spectrometry where standardmass calibration reagents are used to calibrate the mass scale, we used fragment ions of thepure compound as mass calibrants to calibrate our Fr-ICR mass scale. The calibration wasusually started with the mass spectrum with no delay time using the major isotopic peaks ofbare metal ions. They are: 54Fe+, 56Fe+, 57Fe+ and5OCr+, 52Cr+, 53Cr+, whose ion massesare certain. This calibration allowed the determination of the masses of those ions whosepeaks appeared near the calibrant ions. Mass spectra with longer delay times were thencalibrated one at a time by including the ion mass determined from the last calibration into themass calibrants. In order to compare reactivities of ions towards the same neutral parentmolecule under identical ICR conditions, the kinetic data obtained from these single resonancemass spectra were used to calculate the total reaction rate constant of each ion in the self ion-molecule clustering reactions. That is, the ion intensities in the mass spectra without any ionejection detected at measured reaction times (delay time, DL3) were used to plot curves ofintensity vs time. To minimize the errors caused by intensity measurement fluctuations, therelative intensities of ion peaks (Ii. ‘2, ... I) in a spectrum were processed to normalizedintensities given byNormalized intensity of ion i = 1The slopes of the linear decay portions of these curves could be measured to calculate thereaction rate constants.— 23 —The ion ejection techniques were then used to select a single ion species to study itsreaction pathways leading to the formation of the product ions in those mass spectra taken atlonger delay times. To obtain ion structure information, FT-ICR-CID fragmentation or ion-molecule reactions of the ions with a second neutral that were of interest were examined.Particular experimental arrangements will be discussed in detail in each chapter.1.3.5 Basic Kinetics of Gas Phase Ion-Molecule ReactionsSince a Fr-ICR permits convenient, rapid, and accurate measurement of the intensitiesof both reactant and product ions, it is a convenient device for studying the rates of ion-molecule reactions and mass spectrometrists have devoted a great deal of their investigations tosuch rate studies. It can easily be shown that even at very low pressures, the concentration ofions in the ion trap is much smaller than that of the neutral molecules present. For example,the molecular concentration at a pressure of 1.0 x io- Torr (P = 1.3 x iO Pa) and atemperature of 25 °C (T = 298 K) is about 3 x io molecules/cm3,as calculated below usingthe ideal gas equation.N PN0 9 3Concentration of neutral molecule ( v ) = R T 3 x 10 molecules/cmwhere N0 and R are Avogadro’s number and gas constant, respectively. The ion density in aFT-ICR cubic cell is around 3 x 106 molecules/cm3,which is 1000 times less than themolecular concentration. Consequently, the probability of reaction between an ion and aneutral molecule is very large compared to that for reaction of the ion with any neutralfragments formed in the course of either the primary fragmentation processes or the collisionalreactions. It thus follows that reaction of the ion will be entirely with the relatively abundantstable neutrals and will cause no change in the concentration of the neutral species. Therefore,the reactions can be considered to be pseudo-first-order.For the following simple first order ion-molecule reaction, the rate equations are:A + M—B+ C—24—- d[A] I dt = k [At] [M]Ln [Ak] = Ln [Mj0 + k [M] tk — slope of the curve of Ln [A] vs. t-[M]Where, [A+] is the concentration of the reactant ion, [M] is the concentration of the neutralreactant, t is the reaction time. It is immediately apparent that determination of the rateconstants requires a knowledge of the partial pressure of the neutral species and of theretention time of the primary ions in the reaction chamber. In FT-ICR, desirable reactiontimes are simply equal to the ion storage times, i.e., the time (DL3) between Fr-ICR ionformation and FT-ICR ion detection. In this work, the pressure of the neutral moleculemonitored by the ion gauge is used since only the relative ion reactivity is of importance inillustrating ion structures.—25—References(1) Schumacher, E.; Taubenest, R. Helv. Chim. 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Chem. 1982, 54, 96-101.- 36-CHAPTER 2Gas-Phase Ion-Molecule Chemistry of Cycloheptatriene ChromiumTricarbonyl(6.C’rH8)C r(CO)The mass spectrum of Cycloheptatriene chromium tricarbonyl (1) has been examinedpreviously on conventional mass spectrometers.’5 Ionization and fragmentation of themolecule gave a molecular ion and fragment ions formed by stepwise expulsion of carbonylsand the organic ligand. AC5H6r ion was also formed via ejection of neutral C2H fromC7H8r. MUller and Fenderl2 also observed a metal hydride ion, CrH, in their massspectrum.Taylor6 has recently studied the ion-molecule clustering reactions of an analogouscomplex, benzene chromium tricarbonyl (2). Those of a series of(r6-arene)Cr(CO)3complexes were examined by Operti.7 Both studies showed that the fragment ions of thesecomplexes underwent rich ion-molecule reactions with their neutral parent molecules, yieldingcluster ions containing up to 5 chromium atoms. Both symmetrical core (LCr) product ionssuch as(C6H)3CrCO)and (arene)3Cr3(CO)3 and unsymmetrical core (LiCr) productions such as (C6H2Cr3(CO)3 and (arene)2Cr3(CO were formed by the self ion-moleculeclustering reactions. The aromatic ligands of these complexes retained their structural integritythroughout the whole ion-molecule reactions./ co OC” j coCo Co1 2Unlike the benzene ligand, cycloheptatriene ring has a saturated carbon which is notcoordinated to the metal center. The present research focuses on whether, and by what- 37 -mechanism the ligand changes throughout the ion-molecule clustering reactions. The resultslead to a direct comparison with the systems studied by Taylor6and Operti7.2.1 FT-ICR Time-Resolved Mass Spectra and Ion-Molecule ReactionProductsAt a pressure of 1.7 x i0- Torr, a 25 eV x 20 ms Fr-ICR electron beam pulse on (Ti6-c-C7H8)Cr( O)3generated six abundant metal-containing positive ions: Cr,C5H6r,C7H8r,7H8Cr(CO),C7H8r(CO)2and the molecular ionC7H8r(CO)3.The massspectrum detected immediately after FT-ICR ion fonnation is shown in Figure 2.1(A) and thedistribution of ion abundances in this spectrum is listed in Table 2.1. This normal (DL3 =0)FT-ICR mass spectrum matches that reported by King’ using 70 eV electron impact, exceptthat the intensities of the hydrocarbon ions are much lower. By monitoring metastable ions,King’ also showed that fragment ionsC7H8r(CO)0.2were formed by stepwise ejection ofCO from the molecular ion,C5H6r was formed fromC7H8r by the ejection of C2H,and Cr is formed fromC7H8r’ by the ejection of the ligand. MUller and Fenderl2 alsoobserved the metastable ions for the above transitions, as well as the metastable ion for thetransition fromC5H6r to Cr via ejection ofC5H6.After the fragment ions were formed, measured reaction time delay periods wereapplied before Fr-ICR ion detection. The gas-phase ions reacted with the neutral parentmolecule(r16-c-C7H8)C (CO)3in these time periods to form product ions with higher mass-to-charge ratios, that were detected at the end of the delay. Figure 2.1(B) and Figure 2.1(C)show two of these time-resolved FT-ICR mass spectra taken at intermediate and longer delaytimes. All ions, except the molecular ion, are highly reactive with the neutral parent molecule,yielding secondary product ions such as C7H8r2(CO), (C7H8Cr)2 and(C7H8Cr)2(CO)3.Some of these secondary product ions such asCr(CO) andCr2(CO)react with the neutral parent molecule further to eventually form final productions containing up to 4 chromium atoms.-38-Mass in A.M.U.Figure 2.1 F1’-ICR single resonance spectra of(16-c-C7H8)Cr( O)3taken at delay times(DL3) of 0 (A), 100 (B), and 800 (C) milliseconds. (Pressure = 1.7 x io’ Torr; Electron10080-60 -40 -20-0100Cr+C5H6rIi’C7H8r+riC7H8r(CO)3—C7H8r(CO)—C7H8r(CO)2.——II,A. Delay time = 0 msHHNI/Cr‘‘•‘ \:— COCOI IlIJI IIIIII •I• 1111111 11111111111111111111111111111100 200 300 400 500 600-C7H8r(CO)3 B. Delay time = 100 ms80 - C7H8r2(CO)—(C7H8Cr)60-40 - (C7Hg)2Cr3CO20 -(C7H8Cr)2CO)30... 1•L’ 111111 1111111 I 111111111 II Iliii, d’’’ 11100 200 300 400 500 60100C. Delay time = 800 ms80 -C7H8r(CO)3t.111.— (CHgCr)60 -(C7H8Cr)2CO)3 +40- (C7H8)3Cr4CO)20- (C7H8)3Cr4CO)2-..- I i•• i •Ii ij I•i0100 200 300 400 500 600beam pulse = 25 eV x 20 ms; Trapping voltage = 1.0 V).- 39 -Table 2.1 Distribution of ion abundances in the FT-ICR positive ionmass spectrum of(ri6-c-C7H8)Cr(CO)3(delay time =0 millisecond)Fragment Ion m/z Relative Intensity(%)aCr 52 90.2CH2r 66 7.5C5H6r 118 12.4C7H8r’- 144 100.0Cr(CO) 172 37.97H8Cr(CO)2 200 22.5Cr(CO)3 228 62.9C5H 65 2.8C6H7 79 2.2C7H 91 18.9C7H8 92 6.3a. Relative to the base peak in the spectrum.The ion formulae indicated in these spectra were obtained from FT-ICR highresolution exact mass measurements. In order to obtain the ion masses, the time-resolvedmass spectra were mass calibrated. The calibration was started using the isotopic peaks ofchromium ion then went up to the ions with higher masses in the longer time spectra. Oncethe ion masses were obtained, their formulae were determined by comparing the measured ionmasses with the theoretically calculated ion masses. Product ions formed in this system arelisted in Table 2.2.-40-Table 2.2 Ion identification in the(q6-c-C7H8)Cr(CO)3systemIon formula Calculated m/za Measured m/zb Error(ppm)Fragment ions:CH2Cr 65.95616 65.95684 10.3C5H6r 117.98746 117.98515 -19.6C7HgCi 144.00311 143.99945 -25.48Cr(CO) 171.99803 171.99594 -12.27HCr(CO)2 199.99294 199.99209 -4.38Cr(CO)3 227.98785 227.98404 -16.7C5H5 65.039125 65.03763 -23.0C6H7 79.054775 79.05573 12.1C7H 91.054775 91.05241 -26.0C7H8 92.062600 92.05665 -64.6Product ions:(C7Hg)Cr2CO) 223.93854 223.93045 -36.1(Hg)CrCO) 251.93346 251.92139 -46.6(C78)2Cr3 339.94673 339.94369 -8.9(HrCO) 367.94164 367.94605 -1.8(7Hg)2r3CO 395.93656 395.90347 -83.6(8)rCO) 423.93148 423.94468 31.1(7Hg)3r4CO 511.94476 511.95748 24.8(8)rCO)2 539.93967 540.3065 1 79.4(7H3r4CO) 567.93458 568.08617 96.9(C)Cr2(C5 259.97492 259.98205 27.4(7Hg)Cr26 261.99057 261.99807 28.6(C8)Cr 288.00622 227.99026 -55.4(7H2Cr2(CO) 316.00114 316.00452 10.7(8)rCO) 343.99605 343.96988 -76.1(7H2rCO)3 371.99097 371.96402 -72.4a. Masses are calculated using the most abundant isotopes of C, H, 0 and Cr.b. Calibrations of FT-ICR mass spectra were initially started with the 3 majorisotopic peaks of Cr+ and went up step wisely to ions with higher masses.-41 -2.2 Ion-Molecule Reaction PathwaysReaction pathways leading to the formation of product ions were determined byisolating one ion at a time, by using FT-ICR multiple resonance ion ejection techniques, andmonitoring the subsequent ion-molecule reactions.Cr+ reactions. Figure 2.2 shows three FT-ICR double resonance mass spectra thatshow the reactivity of Cr4- ion towards the parent molecule. Spectrum A was obtained byapplying a radio frequency sweep corresponding to ion masses higher than 52 (mass-tocharge ratio of Cr4-) after the FT-ICR ion formation event. This RF sweep ejected all ions butCr4- from the reaction cell. Pure Cr+ was thus selected. After this ion selection, measuredreaction time delay periods were applied to allow Cr4- to react with the neutral parent molecule,(r16-c-C7H8)C (CO)3.Product ions, together with Cr4-, were detected in the FT-ICR iondetection event that was defined at the end of the time delays. Spectrum B, taken at a 100 msdelay time, clearly shows that the initial clustering reactions of Cr4- with the parent molecule+ +yield major secondary product ionsC7H8r2(CO) (mlz 252) andC7H8r2(CO) (mlz224). As shown in Spectrum C, which was taken at a 500 ms delay time, these secondaryproduct ions react further to form tertiary ions (C7H8)2Cr3CO)4- (mlz 396),(C7H8)2Cr3CO) (mlz 268), (C7H2Cr3(mlz 340), and(C7H8)2Cr(mlz 288). The finalproduct ions containing 4 chromium atoms are formed by the further reactions of the tertiaryions. It is interesting to note that ions (C7H8)2Cr34-(mlz 340), and(C7H8)2Cr4-(mlz 288),with no CO groups attached to them, are accompanied by dehydrogenated ions (C7H)2Cr34-(mlz 338) and(C7H)2Crj4-(mlz 286). As will be introduced in the following, these productions are believed to be formed via a n-hydride transfer from the cycloheptatriene ligand tochromium atom which is followed by a reductive elimination of hydrogen from metal centers.Figure 2.3 shows the temporal variation of ion abundances of Cr+ and its majorproduct ions. Clearly, product ions appear and reach their highest intensities at differentreaction times. This also indicates the sequential reaction pattern from primary ions to quarticions discussed above.-42-10080-60-40-20-A. Reaction time = 0 msCr+HHNI/Cr+ + Cr•OC’ j COCO10j..d—• I I—I I I I IlIjLII • I I l1IJI 1 I I 1j I I I I I I I I I I I I I I I I I100 200 300 400 500 600IIIB. Reaction time = 100 ms80- C7H8r2(CO)60- +4- Cr (C1H8)2Cr3CO)40 -C7H8r2(CO)20- / (C7H8)2Cr3CO)4£—Iii I I0 II IllIllIlIlIll III.100 200I I300I’ll400 500 600100806040200C. Reaction time = 500 ms (C7H8)3Cr4CO)—.+(C7H8)2Cr3CO) — +(C7H8)3r4(CO)2-+ +—C7H8r2(CO) (C7H)2Cr3 +‘l (C7H8)2Cr3CO)Cri.I.!... .. ..1. ..,,.. II•1100 200 300Mass in A.M.U.400 500 600Figure 2.2 FT-ICR double resonance mass spectra of the Cr I(C7H8)Cr(CO)3system.-43 -0.40.2.—ECz 0.10.00.4 0.6 0.8Reaction Time (Sec.)Figure 2.3 Temporal variation of ion intensities for Cr reacting with(C7H8)Cr(CO)3.Reactions 2.1 to 2.5 summarize the self ion-molecule clustering reactions initiated byCr ion that were identified by FT-ICR multiple resonance experiments.Cr +C7H8r(CO)3 —* C7H8r2(CO) + (3-n) CO (2.1)C7H8r2(CO) +C7H8r(CO)3— (C)Cr3CO)m + (3+n-m) CO (2.2)—4 (C7H82CrCO) + Cr(CO)3 (2.3)(C7H8)2Cr3CO)m +C7H8r(CO)3 —> (C)3CrCO) + (3+m-y) CO (2.4)(C7H8)2Cr,3—4 (C7H)2Cr,3 + H2 (2.5)where n = 1, 2; m = 0 - 3; x =0 -3; y = 1 -4. Similar ion-molecule reactions of Cr, exceptfor the dehydrogenation reactions, have also been observed previously in the (T16arene)Cr(CO)3 and(‘r16-benzene)Cr(CO) systems.6’7C5H6F+ reactions. Figure 2.4 shows a FT-ICR triple resonance mass spectrumofC5H6r (m/z 118) taken at a 500 millisecond delay time. It shows thatC5H6r reactswith78Cr(CO)3 to form two major ion-molecule clustering products, ion-44-(C7H8) r256(mlz 262) and a dehydrogenated ion(C7H) r25(m/z 260).Product ions containing more than two Cr atoms were not observed, indicating that each of thetwo product ions shows very low reactivity towards the neutral parent molecule.C5H6r +C7H8r(CO)3 -4 (C7H8) r256) + 3 CO (2.6)—> (C) r + 3 CO + H2 (2.7)100-Reaction Time = 500 ms80— +0 QH6Cr—+C7H8r256. 40-.—+—C7Hr25s20-___________L I k ‘ k—I I I I p • • • i i i I I I I I I I I I I I I I I I I50 100 150 200 250 300 350 400Mass in A. M. U.Figure 2.4 FT-ICR triple resonance mass spectrum of the C5H6r /(C7H8)Cr(CO)3system.C7H8r reactions. Reactions of ionC7H8r are illustrated in Figure 2.5 and2.6. C7H8r(mlz 144) undergoes clustering reactions with the parent neutral to form the• . + +major product ions(C7H8Cr)2CO)3 (m/z 372) and (C7H8Cr)2 (mlz 288). Product ionscontaining more than two chromium atoms were not observed due to the low reactivities ofthese secondary product ions. Similar to the reactions of Cr andC5H6r, an associationdehydrogenation product ion, (C7HCr)2is formed, along with the formation of(C7HgCr)2.C7H8r +C7H8r(CO)3 -4 (C7H8Cr)2CO)0..3+ (3-0) CO (2.8)—4 (C7H’Cr)2 + 3 CO + H2 (2.9)-45 -100-Reaction time = 500 ms80-+(C7H8Cr)2-S.—-— +40- (C7HCr)2+(C7H8Cr)2CO)3C7H8r20-0____________Ii_________1111111 11111 liii IiI 11111 liii III I 1111111111111111111..100 200 300 400 500 600Mass in A.M.U.Figure 2.5 Triple resonance mass spectra of theC7H8r I(C7H8)Cr(CO)3system.0.0-‘ C7H8r +‘ J (C7H8Cr)2•‘-0.5- ••-..0(C7I18Cr)2CO)3z 1.5--2.0-I I I I I I0.0 0.2 0.4 0.6 0.8 1.0 1.2Reaction Time (Sec.)Figure 2.6 Temporal variation of ion intensities for C7H8Cr’ reacting with(C7H8)Cr(CO)3.C7H8r(CO)1..3reactions. The reactions of (C7H8)Cr(CO) and(C7H8)Cr(CO)2with(16-c-C7H8)Cr(CO)3are summarized in Eqn. 2.10. The molecular ion-46 -C7H8r(CO)3is unreactive towards the neutral parent molecule as no product ions appearedin its triple resonance spectrum with the reaction time increased up to 1500 milliseconds.C7H8r(CO)1,j +C7H8r(CO)3 —> (C7HgCr)2CO03 + (Ito 5)CO (2.10)2.3 Reactivities and Structures of Positive IonsRate constants for the reactions of ions were determined from the known pressure ofthe neutral complex and the decay slopes of the curves of ion intensity (I I E I) vs. reactiontimes (DL3). Figure 2.7 shows typical data for major ions in the(16-c-C7H8)Cr( O)3system.-0.5._ -—CzC-2.0-2.51.00.0 0.2 0.4 0.6 0.8Reaction Time (Sec.)Figure 2.7 FT-ICR temporal variation of ion abundances in the(C7H8)Cr(CO)3system.-47 -The abundance of an ion typically increases to a maximum and then levels off ordiminishes depending on its reactivity. The decay portions of these curves are linear since theneutral concentration is much higher than the ion concentration and the kinetics of all thereactions are pseudo first order. The pseudo-first-order rate constants are then calculatedusing the known pressure of the neutral parent molecule and the decay slopes of these curves.The measured rate constants are listed in Table 2.3.IonTable 2.3 Rate constants and electron deficienciesRate Constanta Rel. Reactivityb ED ED*Cr 4.6 1.00 13C5H6r 3.9 0.85 9 7C7H8r 3.2 0.70 7 5CCr(CO) 3.4 0.74 57H8Cr(CO)2 2.3 0.50 3Cr(CO)3 0.0 0.00 17H8Cr2(CO) 2.6 0.57 7.5 45c,dCr(CO) 1.5 0.33 6.5 35c,d(C7H8Cr)2 0.079 0.017 5.5 15c,d(Cr)CO) 0.42 0.091 45(7H8Cr)2CO) 0.19 0.041 3.5(Cr)CO)3 0.00 0.00 2.5 0.Sd(7H8)2rCO) 1.1 0.24 5.7 37d( r3CO) 0.52 0.11 5.0 3.Oda. All rate constants are in the units of i0 cm3 molecule-1s.b. Relative reactivity = rate constant of an ion / rate constant of Cr.ED refers to the calculated electron deficiency assuming single metal-metal bondsand thatC7H8retains the 6 coordination mode andC5H6donates 4 electrons.ED* refers to the adjusted electron deficiency assuming (c) cyclopentadienyl- ortropylium-metal-hydrogen bonding and / or (d) multiple metal-metal bonds.The ED column in Table 2.3 gives calculated electron deficiencies per chromium atom,relative to 18 valence electrons per metal atom, assuming single metal - metal bonds between-48 -two chromium atoms and assuming C7H8 to be a 6-electron donor, as it is in the neutral parentmolecule. Adjusted electron deficiency values in the ED* column are calculated using theproposed ion structures discussed below. We believe that some ions in the table have formedmultiple metal-metal bonds and I or undergone an intramolecular 3-hydride transfer process toform cyclopentadienyl or tropylium-metal-hydrogen bonding.As given in the Table 2.3 and shown in Figure 2.7, ions in the(6-c-C7H8)Cr(CO)3system shows a reactivity trend: Cr >C7H8r CHgCr(CO) >Cr2(CO)>(C7Hg)2Cr3CO)>C7H8r(CO)3 (CCr)2CO)3 0. This decreasing trend of ionreactivity is apparently caused by the decreasing electron deficiencies on the central metalatoms of ions. The same kind of correlation between ion reactivity and electron deficiency ofcentral metal has been repeatedly observed previously.8”Note that the reactivity of ionC7HgCr (ED=7) is similar to that of ionC7HgCr(CO) (ED=5), even though the former isseemingly more electron deficient as it does not have a CO ligand. This contradicts the generaltrend that rate constants increase with increasing electron deficiencies. Another exceptionalcase is that ions (C7H8Cr)2and (C7H8Cr)2CO)3,with ED values of 5.5 and 2.5respectively, are essentially unreactive.The 17-electron molecular ions (ED =1) of both(16-benzene)Cr(CO)3and (16...arene)Cr(CO)3 have been observed to be unreactive towards their neutral parent molecules.6’7As the molecular ion,(C7H8)Cr(CO)3+,is also an unreactive ion in the present system, wesuggest that the unreactive dimetallic ion, (C7H8Cr)2CO)3has structure 3 and is themolecular ion of the as-yet-unprepared 18-electron compoundbis(6-cycloheptatriene)dichromium tricarbonyL 3 would have a triple metal - metal bond, by analogy to a knownC CC-49 -18-electron complex, bis(15 - cyclopentadienyl) dichromium tetracarbonyl, 412,13 It is thenappropriate to consider other dichromium product ions in Table 2.3 to be the fragment ions of3, and triple metal-metal bonds will be assumed for their structures as well.In an early study, Schumacher and Taubenest14observed similar associated ions suchas(C5H)3Fe2CO)4,(C5H3FeCO)4(C5H3FeCO),and(C5H)4FeCO)4,formed by ion-molecule reactions in the [(C)Fe( O)2]system. (CFeCO)wassuggested to have a tetrameric structure. Fragment ions in the mass spectrum ofcyclopentadienyl iron carbonyl tetramer prepared in the condensed phase matched the aboveion-molecule reaction products.15 A similar idea has also been used by Corderman andBeauchamp’6for the unreactive product ion, [CpCo(CO)]2in the CpCo(CO)2system andpartial double metal - metal bond was suggested to exist in this gas-phase ion by analogy withthe condensed-phase structural data.To explain the similar reactivities ofC7H8r (ED=7) andC7H8r(CO) (ED=5), wepropose thatC7H8Cr undergoes an intramolecular p3-hydride transfer reaction,(2.11)iH CrThat is, after loss of three CO groups, the central metal atom inC7H8r+ becomes so electrondeficient that it abstracts a hydride from the C7H8ring to become less electron deficient. Thetransfer leads to the formation of a hydrido-it-tropylium iOn and the conversion of thecoordination hapticity from fl6 to T7+fl1 Thus, the electron deficiency on Cr inC7HrWdrops by two. Therefore, the Cr atom in each ofC7HrH and (TI6 -C7H8)Cr(CO) has thesame electron deficiency (ED*=5), explaining why these two ions have the similar reactivity.An additional driving force for the above rearrangement appears to be the aromatizationof the seven-membered ring. Consistent with this analysis is the observation that the fragmentions from benzene chromium tricarbonyl, (fl6-c-C6H)Cr(CO)32, have the normalcondensation reactivity, Cr >C6Hr >C6H6CrCO >HCr(CO)2,with their parent- 50-molecule.6 The fragment ions from cyclopentadienyl vanadium tetracarbonyl,6(15-c-C5H)V(CO)4,and cyclopentadienyl manganese tricarbonyl,(fl5-c-C5H5)Mn(CO)3”7alsohave the normal reactivity order, V >C5HV>C5HV(CO) >C5HV(CO)2and Mn>C5HMn >C5HMn(CO) >C5HMn(CO)2with each of the parent neutrals. Thesesystems have no possible corresponding aromatization reaction as each of these complexesalready contains an aromatic ring.The C-H bond dissociation energy and the spatial position of hydrogen atom may alsobe relevant to the n-hydride transfer process. Compared to the cycloheptatriene ligand whosesp3 C-H bond dissociation energy is 73 kcal/mol, benzene has a greater C-H bonddissociation energy, 110 kcallmol. The hydrogen atoms of benzene are in the same planeformed by the six carbon atoms and can not stretch to the metal center. The distance betweenmetal atom and hydrogen atom in ionC6Hr is apparently longer than that in ion (p6...C7H8)Cr, 5, as schematically illustrated below.Cr Cr H(rj6-CH) r (16-C7H8) r, 5Freiser18 has recently shown that only the hydrogen atoms in the same side ofcyclohexane ring can be eliminated by metal ions. This may be interpreted that only ahydrogen atom at the endo position to the metal center in ions such as 5 is susceptible to 3-hydride transfer.To compare with condensed-phase data, a f-hydride transfer process has beenrecognized as the major step in the decomposition of metal-alkyl complexes.20’1 The transferis considered to involve vacant coordination sites on the metal centers. The r6 coordinationhapticity of(fl6-c-C7H8)Cr(CO)3 can be converted to an T7 hapticity via an intermolecularhydride abstraction by trityl cation (Ph3Cj (reaction. 2. 12).2224-51-+ [Ph3C]BF4 { t: jBF4E + Ph3CH (2.12)Cr(cO)3 Cr(CO)3Another analogous case to the hydrido-ic-tropylium chromium structure is that both cC7H8o and c-C7H8Rlr undergo slow H/D exchange with the deuterated reagent, C2D4inthe gas phase.’9 Jacobson19 rationalized these observations in terms of a conversion from acycloheptatriene-metal structure to a hydrido-it-tropylium structure. But the formation ofhydrido-it-tropylium ions was not very favorable for ionsC7H8o and c-C7H8Rh sinceonly a single slow H/D exchange was observed. Notice that chromium has only six electronsin its outer shell orbitals but both cobalt and rhodium have nine outer shell electrons.Therefore, the chromium atom inC7H8r+ is more electron deficient from an 18-electronnoble gas configuration. The intramolecular f3-hydride abstraction may thus becomefavorable.As we attribute the major driving force of the 3-hydride transfer to the electrondeficiency of the central metal atom, the more electron deficient fragment ion, C5H6r+(ED=9), presumably has an initial cyclopentadiene structure, 7, and is therefore expected toundergo a 3-hydride transfer as well to form a hydrido-7r-cyclopentadienyl structure, 8. Thesmall amount of metal hydride ion, CrH, observed by Muller2might be formed by cleavageof the metal-tropylium and metal-cyclopentadienyl bonds of 6 and 8.- (2.13)Cr CrH7 8IonsC7H8r2(CO) (ED=7.5) andC7H8r2(CO) (ED=6.5) are proposed to havestructures 9 and 10, respectively. It is also possible that each of the C7H ligands in ion 9and 10 is bridging to the two chromium atoms via r+r bonding, so the two metal centershave balanced electron deficiencies. The hydrogen and carbonyl may also migrate to the otherchromium atom or form hydrogen bridge and carbonyl bridge.- 52 -+ c_,coH” Nco Nco9 10Similarly, the secondary product ions, (C7H8Cr)2andC5H6r278,are proposedto have double hydrido structures, 11 and 12, to explain their unexpected low reactivities.— / H / HCr=C CrCrV H”12As introduced in Section 2.2, the formation of ions 11 and 12 were accompanied by theformation of the dehydrogenated products, (C7HCr)2andC5Hr7,reactions 2.7and 2.9. We believe that the two hydrogen atoms in each of the ions 11 and 12 can migrateto a single metal center and be reductively eliminated to yield (C7HCr)2 andC5Hr27.Quadruple metal- metal bonds may consequently be present in these twodehydrogenated ions.The product ions containing three chromium atoms, such as(C7H8)2Cr3CO and(C7H8)2Cr3CO),are proposed to have a triangular metal core constructed by double metal-metal bonds, as suggested by Fredeen10in the Cr(CO)6system. (C7H8)2Cr3CO) may haveformed a tropylium-metal-hydride structural moiety since the dehydrogenated ion(C7H)2Cr3was observed to accompany the formation of ion (C7H8)2r3.Adjusted electron deficiencies of ions calculated using structures formed by tropyliummetal-hydrogen bonding and multiple metal- metal bonding are listed in the ED* column inTable 2.3. Figure 2.8 shows a monotonic increase of rate constant with increasing electrondeficiency (ED*). The outlined points, which correspond to non-rearranged ion structures,are apparently off the curve.-53-5-4-3--.o 2-C-)-1—0-—1 —I I I I I0 2 4 6 8Electron DeficiencyFigure 2.8 Ion reactivity versus electron deficiency in the (C7H8)Cr(CO)3system. Thedata are from Table 2.3. Each ion whose reactivity is anomalously low is indicated in outlinefont. The solid dots correspond to the ED* values and the outline dots correspond to the EDvalues in Table 2.3. The bold face numerals correspond to the structures in the text.2.4 Probing the Hydrido-it-Tropylium and Hydrido-ic-CyclopentadienylStructuresWhile a hydrido-n-allyl structural moiety has been observed directly in the condensed-phase,25’6the gas-phase metal hydride character has been probed mainly by H/D exchangewith D2 orC2D4.7-32 Unfortunately, these techniques are not always definitive for detectingmetal-hydrogen bonds. Typical examples are the reactions of ions 13 and 14 withdeuterium.27 While 13 has metal hydride character and 14 does not, both ions undergo HIDexchanges with deuterium, reactions 2.14 and 2.15.C5HrH 8C7H8r(CO)C7HrW 6C7H8r(CO)2+CrC5)HI6ir 7C7H8r 5C7Hr2(CO)H 9 C71Ht8r2(CO)C7Hr2(CO) 10.C7]HI8r2(CO)(C7HCrH)211(C7H8Cr)2CO)33(C7]H[8Cr)210I I12 14-54-H DI ICpRh + D2 CpRh + HD (2.14)Co Co13+ 4HD (2.15)Similarly, while some metal hydride ions were observed to undergo H/D exchange withdeuterated reagent C2D4,ion 15, which was not considered to have metal hydride character,also undergoes H/D exchange withC2D4.31CH2Rh—IICH215In order to test the hydrido-ir-tropylium and the hydrido-it-cyclopentadienyl structuresinferred from the ion reactivity results, both FT-ICR-CID ion fragmentation and ion-moleculereactions with a second neutral molecule were examined. Nitrogen was used as collision gasin the CID experiments. Its pressure was kept at about 1.0 x 10-6 Torr. Deuterium andmethanol were selected as second neutral reactants in order to observe H/D ordehydrogenation reactions. Their pressures were also kept in the above range, that is, tentimes higher than that of(r6-c-C7H8)Cr(CO)3,to suppress self ion-molecule clusteringreactions. For the ion-molecule reactions with methanol, deuterium-labeled methanolsCH3OD, CD3OH, and CD3O were used to verify the compositions of the product ions.2.4.1 FT-ICR-CID and Ion-Molecule Reactions with D2.Figure 2.9 illustrates the Fr-ICR-CD experiments onC7H8r. For FT-ICR-CDexperiments, C7H8r was first isolated by two RF sweeps that ejected other ions thenaccelerated by applying a fixed radio frequency that corresponds to the cyclotron frequency ofCH214CD2- 55 -C7H8r. With application of a delay time after ion selection, the ion underwent multiplecollisions with the target gas to dissociate into fragment daughter ions. As shown in Figure2.9, Cr’ is the only Cl]) fragment ion formed fromC7H8r’. CrH orC7Hr ions, thatwould give a positive indication of a hydrido-ir-tropylium structure, were not notablyobserved.LL_____f1 C7H8r—’7Lrm1 ‘r5fl flMi5.5 1N P,MLJ.Figure 2.9 FT-ICR-CID spectrum ofC7H8rh C7H8r was accelerated on resonance bya 0.5 ms RF pulse and allowed to collide with N2 for 100 milliseconds.It has been demonstrated previously that FT-ICR-CID on olefin metal hydride ionsdoes not always give meaningful results. Ion cC5H6Fe+, for example, shows only a singlefragment ion, Few, in its FT-ICR-CID spectrum, but undergoes H/D exchange with D2.32The H/D exchange reaction indicates the presence of a hydrido-it-cyclopentadienyl structure.Deuterium gas (D2) was allowed to react withC7HgCr. As shown in Figure2.10(B), only a small amount ofC7HDCr is formed by a slow H/D exchange reaction.This vaguely indicates the presence of a hydrido-n-cyclopentadienyl structure. However,some organometallic ions such as FeH+ that has obvious metal hydride character have beenshown not to undergo H/D exchange in the presence of deuterium.29- 56 -Although both FT-ICR-CID and ion-molecule reaction with deuterium do not give aclear indication of a tropylium metal hydride structure of ionC7H8Cr, the results do not ruleout the possibility.FT-ICR-CID spectra ofC7H8r(CO) andC7H8r(CO)2are shown in Figure 2.11.Typical CD fragmentation of these carbonyl-containing ions gives stepwise elimination of COand the organic ligand, yielding the smaller fragment ions of(16-c-C7H8)Cr(CO)3.That is,I—zIij-rI-..>-IaI1jtiE-BC7HrH001In11LuI—z—4LjjI—a-JIiitiEC7HrD/• A0—[Tr,FI—1i441j4IIr II!r4IIII 4 4III I rIlIIIIrI.4I Ir 4 r150 155 1-jO l’j.S 150 155 160HCISS IN P,M,U.Figure 2.10 FT-ICR triple resonance spectra ofC7H8r taken in the presence ofdeuterium (1.0 x 10-6 Torr) at delay times of 0 ms (A) and 800 ins (B).- 57 -C7H8r(CO) CID C7H8r(CO)0(n-i) + CrBothC7HgCr(CO) andC7H8r(CO)2 do not undergo H/D exchange with D2. Fl’ICR-CD) could not be performed on ionC5H6rdue to its small ion intensity.EEL___>—flAC7HCr(CO)—..Li3LI [‘1>d CLi L.I I I I II I I5 100 150M5.5 IN M,Li.H - B C7H8r(CO)2—-C7H8r(CO)F-3Li Li>F- -FTrr50 100 150H0S.5 IN P,M.LJ.Figure 2.11 FT-ICR-CD) spectra ofC7H8r(CO) (A) andC7H8r(CO)2(B). The ionswere accelerated on resonance by a 0.5 ms RF pulse and allowed to collide with N2 for 100ms collision time.-58-2.4.2 Ion-Molecule Reactions with Methanol.C7H8r(CO). Figure 2.12 shows two FT-ICR triple resonance spectra ofC7H8r(CO) taken after a 200 milliseconds delay time in the presence of CH3OD andCD3O . The ion was first isolated by two RF sweeps then allowed to react with methanol byapplying a time delay before the ion detection.Figure 2.12 Partial FT-ICR triple resonance mass spectra illustrating the reaction ofC7H8r(CO) with methanol (Reaction time = 300 milliseconds).The spectra show thatC7H8r(CO) reacts with methanol to form a substitutionproduct ion, C7H8Cr(methanol). With a longer delay time, this product ion does not- 59 -undergo further reactions and does not self dehydrogenate. Using CH3OD as example, thereaction is given in reaction 2.16. A reaction mechanism is proposed in Scheme 2.1. The Dshift from the hydroxyl group of methanol to the metal center is analogous to Allison’ssuggestion for the reaction between Fe+ and methanol.33 Similar substitution reactions inwhich a methanol molecule displaces a CO group have been observed previously in somemetal carbonyl systems.33’4C7H8r(CO) + CH3OD —> (C7H8)Cr(CH3OD) + CO (2.16)m/z 172 m/z 177CH3OD D-ShiftI ICr- CO Crt... CrCO OCH3 D’ OCH3m/z 172 mlz 177Scheme 2.1 Substitution reaction ofC7H8r(CO) with methanol.C7H8r. Fr-ICR triple resonance mass spectra that illustrate the reactions ofC7H8r with methanol are shown in Figure 2.13. The ion was first isolated by FT-ICRtriple resonance sequence and then allowed to react with CH3O , CH3OD, or CD3O for 700milliseconds. As shown in Figure 2.13(A) and (B), whenC7H8rreacts with CH3O andCH3OD,C7r(OCH3)(mlz 174) is formed as the major product ion. The product ion isformed byC7H8rassociating with methanol and elimination of neutral H2 or HD from theassociated ion. This indicates that one of the two hydrogen atoms eliminated is from thehydroxyl group of methanol. As shown in Figure 2.13(C),C7H8r reacts with CD3O toform the major product ionC7Hr(OCD3)(mlz 177) by elimination of HD. This indicatesthat the other hydrogen atom eliminated is from the reactant ion,C7H8r, rather than fromthe methyl group of methanol. ReactingC7H8rwith CD3OH eliminates H2. This furtherconfirms that ionC7HgCr undergoes an addition-dehydrogenation reaction with methanol,one of the two hydrogen atoms eliminated is fromC7H8r, and the other is from thehydroxyl group of methanol.-60-CC,H8r A. CH3OF m1z144U]zUFz—C7Hr(OCH3)m/z 174>C5Hr(OCH3ia m1z148-JUNO 1.50 160 170 160 1.90 200aC.,H8r B. CH3ODF m)z144U]zUI—z—C7Hr(OCH3)Urn mlz 174>—C5Hr(OCH3)F inlz 148aIUDlLi0 1.50 1.60 170 160 1.90CCC. CD3OFI—4C7H8rU. m1z144Fz—C7Hr(OCD3)m/z 177C5Hr(OCD3)amIz 151U1 +.,NO 160 170 1.60 1.90 200MP55 iN P MU,Figure 2.13 Partial FT-ICR triple resonance mass spectra illustrating the reaction ofC7H8rwith methanol (Reaction time = 700 milliseconds).61-Using CH3OD as an example, the addition-dehydrogenation reaction ofC7H8rwithmethanol is summarized in reaction 2.17. The 700 millisecond reaction time for product ionsto form in Figure 2.13 indicates that the dehydrogenation reaction is slower than the+substitution reaction ofC7H8r(CO)C7H8r + CH3OD —* (C7H)Cr(OCH3+ HD (2.17)mlz 144 mfz 174Compared with the substitution reaction ofC7H8Cr(CO), reaction 2.16 where adehydrogenated product ion was not observed, the addition-dehydrogenation reaction ofC7H8rtends to confirm the hydrido-ir-tropylium structure of the ion, 6. A mechanism forthe addition-dehydrogenation reaction is proposed in Scheme 2.2.CH3ODCrt HD6,m/z144 16NH3c__Cr .. CrOCH3 H’’d OCH3D18,m1z144 17Scheme 2.2 Addition-dehydrogenation reaction ofC7H8r with methanol.Fragment ionC7HrH, 6, reacts with methanol to form a double hydride ion, 17, by eitherdirect ion insertion into the O-D bond of methanol (6 to 17) or byC7HrH associating withmethanol followed by a D-shift33 to the metal center (6 to 17 via 16). The double hydrideion, 17, undergoes a rapid reductive elimination of hydrogen to form the final metal - oxygenbonded product ion, 18. The reductive elimination of hydrogen via a double hydride ion hasbeen demonstrated repeatedly in the gas phase.31’545-62-The substitution product ofC7H8r(CO),C7H8r(methano1), does not selfdehydrogenate at longer reaction time. This indicates that the cycloheptatriene ligand retainsits structural integrity inC7H8r(CO).Ion 18 undergoes a facile fragmentation process that ejects C2H from the tropyliumring to form ionC5Hr(OCH3)(mlz 148) in Figure 2.13, reaction 2.18.I I + C2H (2.18)Crc CrOCH3 OCH318, mlz 174 19, m/z 148Uncoordinated tropylium ion, C7H has been observed to lose C2H to formC5H.4648Consistent with the presence of the tropylium ligand, the organometallic ion 18 undergoes ananalogous fragmentation to give a fairly large amount of ion 19. In contrast, the product ofC7H8r(CO),C7H8r(CH3OD), does not undergo this fragmentation. This also indicatesthat the C7H8r moiety is not the same in the fragment ionsC7H8r andC7H8r(CO)ti.e., the former has a hydrido-rt-tropylium structure and the latter has a cycloheptatrienechromium structure.C6Hr. C6H6Crt 20, generated by El on(i6-benzene)Cr(CO)3,was alsoreacted with methanol. Only an adduct-formation product (reaction 2.19) is observed as thehydrogen atom in benzene could not easily undergo 3-hydride transfer to form metalhydrogen linkage.+ CH3OD (C6H)Cr(CH3OD) (2.19)Cr20CsH6r. LikeC7H8r, fragment ionC5H6r, 8, reacts with methanol to forman addition-dehydrogenation product, reaction 2.20.-63 -C5H6r + CH3OD —* (C5H)Cr(OCH3+ + 1-ID (2.20)mlz 118The relevant mass spectra are illustrated in Figure 2.14 and the reaction mechanism isproposed in Scheme 2.3. This addition-dehydrogenation reaction again tends to confirm thehydrido-ic-cyclopentadienyl structure ofC5H6rinferred from the ion reactivity results.C________________________________ _________CA. CH3OD C5Hr(OCH3)-,H1-4I-nzC5H6r + CH3OD-,C5Hr(OCH3)+ HDZ m/z118 m/z148turn>C5HCr—ri4iii”414110 120 1’Ei0 1LjQ 150ci._________ ___ciB. CD1O C5Hr(OCD3)H4-InzUC5H6r + CD3O -C5Hr(OCD3)+ HDmJzllS mIzl5lüjLfl>C5HCrrrr—4110 120 l0 li0 150 160MPSS IN P.M.U.Figure 2.14 Partial FT-ICR triple resonance mass spectra illustrating the reaction ofC5H6r with methanol (Reaction time = 300 milliseconds).-64-CH3ODI IH CrOCHNCH3ODD-shift-IDI ICr .. CrtOCH3 H” A OCHm1z148 DScheme 2.3 Addition-dehydrogenation reaction ofC5H6rwith methanol.Cr andC7H8r2(CO). While Cr was not observed to react with methanol, itssecondary product ion,7H8Cr2(CO) (9, reaction 2.1), was found to react with methanol,yielding both substitution and substitution-dehydrogenation products. Using CH3OD as theexample, the reactions are summarized in reactions 2.21 and 2.22.C7HgCr2(CO) + CH3OD —* (C7H) r2OCH3Y’(78%) + CO + HD (2.21)m/z 224 m/z 226—> (C8) rCHOD)(22%) ÷ CO (2.22)m/z 229The reactions are explained by the mechanism shown in Scheme 2.4. Thesubstitution-dehydrogenation reaction tends to support a D-shift step in the additiondehydrogenation reactions such as reactions 2.17 and 2.20. The mechanism does not rule outthe possibility that the hydrogen atom is initially located on the other chromium atom and theD-shift and reductive elimination of hydrogen actually took place on the other chromium.Nevertheless, the dehydrogenation reaction confirms a metal-hydrogen linkage in the ionC7H8r2(CO), 9, and supports the tropylium-metal-hydrogen structural moiety proposed forthe ion.-65 -Cr+_CO 3-hydride transfer Cr, 9CH3OD CH3ODCrZ — ‘OCH3OCH3 Cr=C( 21U HI D-Shift-RDCrrOCH3 CrCrOCH3mlz 229 mlz 226Scheme 2.4 Substitution and substitution-dehydrogenation reactions ofC7HgCr2(CO)with methanol.2.6 Summary and ConclusionsExcept for the molecular ion, all the metal-containing fragment ions of (16-c-C7H8)Cr(CO)3are chemically reactive with the parent neutral molecule. Self ion-moleculeclustering reactions dominate all the reaction pathways in this system. The reactions lead tothe formation of large cluster ions of the form (C7H8Cr)2(CO) and (C7H8)lCr(CO),where x = 0-3, n = 2-4, y = 0-4. Unlike(16-benzene)Cr(CO)3 and(r16-arene)Cr(CO)3systems where symmetrical core (LCr) cluster ions containing four chromium atoms werealso formed, clustering in the present system dwindles with formation of (C7H8Cr)2-’- and(C7H8Cr)2CO)3.The latter ion is the largest symmetrical core product observed.The reactivities of ions towards the neutral parent molecule are observed to bedependent upon electron deficiencies of the central metal atoms. A general trend that ionreactivities increase with increasing electron deficiency is observed (Figure 2.8). Thisreactivity-electron deficiency trend has previously been observed in metal carbonyl systems.- 66-Unlike the(r6-benzene)Cr(CO)3 and other complexes with aromatic ligands, a normalreactivity order of fragment ions, M > LM > LM(CO) > LM(CO)2,is not observed.Instead, fragment ions, C7H8r andC7H8r(CO), are observed to have a similarreactivity. This indicates that when the central metal becomes highly electron deficient, thecycloheptatriene can change its structure to provide more electrons to lower the electrondeficiency of ionC7H8r+. An intramolecular f3-hydride transfer process that decreases theelectron deficiency on the central metal is proposed forC7H8r. ThusC7H8r has ahydrido-it-tropylium structure whereas the cycloheptatriene ligand inC7HgCr(CO) retains itsstructural integrity. As a result, the two ions have the same electron deficiency, explainingtheir comparable reactivity. The unreactive secondary product,(C7H8Cr)2CO)3is believedto be the molecular ion of an as-yet-unprepared 18-electron complex,bis(6-cycloheptatriene)dichromium tricarbonyl. This complex would have a triple metal-metal bond by analogy withknown organometallic compounds. Structure portions formed by the tropylium metal hydrideand/or triple metal-metal bond are also assigned to the product ions which have anomalouslylow reactivities.A specific ion-molecule reaction with methanol is used to probe the hydride structuresproposed in this study. IonsC7H8r(CO),C6Hr’ andC7H8r’ undergo differentreactions with methanol. The first two ions undergo substitution reaction and adductformation reaction, respectively, yielding(C7Hg)Cr(methanol) and (C6H6)Cr(methanol)ions which do not self dehydrogenate. C7H8r undergoes an addition-dehydrogenationreaction, yielding the product ion(C7H)Cr(OCH3.The addition-dehydrogenation reactionprovides strong proof for the presence of hydrido-7t-tropylum chromium and is suggested touse as a definitive test for gas-phase organometallic ions bearing metal-hydrogen linkages.Similarly, both C5H6r andC7H8r2(CO) undergo dehydrogenation reactions withmethanol, which indicates a hydrido-ir-cyclopentadienyl chromium structure of the former ionand a hydrido-it-tropylium chromium structural portion in the latter ion. As will be introducedin the next chapter, this addition-dehydrogenation reaction is also effective for detecting otherolefin-metal ions that have a probable metal-hydrogen linkage in their structures.- 67 -Since only these highly electron deficient ions are observed to have metal-hydrogenlinkages, it appears that the major driving forces for the 3-hydride transfer are the high degreeof electron unsaturation at the metal center and the formation of the aromatic tropylium ligand.That is, when the central metal atom becomes highly electron deficient, an organometallic iontends to change its metal-ligand coordination to become less reactive. As will be introduced inChapter 5, coordination expansion (ri4 to r16) of cyclooctatetraene ring may also stabilize anion.In the studies of activation of C-H and C-C bonds by gas-phase metal ions, ahydride transfer is also proposed to explain the elimination of molecular hydrogen fromhydrocarbons.31’545 Together with the results of these studies, the present study stronglysuggests that 3-hydride transfer plays an important role in many gas-phase ion-moleculereactions. The presence of tropylium and cyclopentadienyl chromium hydrides in the massspectrum of(16-c-C7H8)Cr(CO)3also suggests that the present study is of interest andimportance to better understanding of the mass spectra of organometallic complexes.- 68 -References(1) King, R. B. Appi. Spectrosc. 1969, 23, 536-546.(2) Muller, J.; Fenderl, K. Chem. Ber. 1970, 103, 3128 - 3140.(3) Davis, R.; Ojo, I. A.; Webb, M. L. Org. Mass Spectrom. 1978, 13, 547-548.(4) Games, D. E.; Gower, J. L.; Gower, M.; Kane-Maguire, L. A. P. J. Organomet.Chem. 1980, 193, 229-234.(5) Tibbetts, F. B.; Bursey, M. M.; Little, W. F.; Willeford, B. R.; Benézra, S. A.;Hoffman, M. K.; Jennings, P. W. Org. Mass Spectrom. 1972, 6, 475- 478.(6) Taylor, S. M. Ph.D. Thesis, The University of British Columbia, 1992.(7) Operti, L.; Vaglio, G. A.; Gord, J. R.; Freiser, B. S. Organometallics 1991, 10,104-111.(8) Wronka, J.; Ridge, D. P. J. Am. Chem. Soc. 1984, 106, 67 - 71.(9) Meckstroth, W. K.; Ridge, D. P.; Reents, W. D., Jr. J. Phys. Chem. 1985, 89, 612-617.(10) Fredeen, D. A.; Russell, D. H. J. Am. Chem. Soc. 1985, 107, 3762-3768.(11) Fredeen, D. A.; Russell, D. H. J. Am. Chem. Soc. 1986, 108, 1860-1867.(12) Curtis, M. D.; Butler, W. M. J. Organomet. 1978, 155, 131.(13) Hackett, P.; O’Neill, P. S.; Manning, A. R. Dalton Trans. 1974, 1625.(14) Schumacher, E.; Taubenest, R. Helv. Chim. Acta 1966,49, 1447.(15) King, R. B. Inorganic Chemistiy 1966, 5, 2227-2230.(16) Corderman, R. R.; Beauchamp, J. L. Inorg. Chem. 1977, 16, 3135-3139.(17) Parisod, G.; Comisarow, M. B. In Advances in Mass Spectrometry; A. Quayle, Ed.;Heyden & Son.: London, 1980; Vol. 8A; pp 212-223.(18) Huang, Y.; Freiser, B. S. In The 41st ASMS Conference on Mass Spectrometry andAllied Topics; San Francisco, California, 1993; pp 305a-305b.(19) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 72-80.- 69 -(20) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 4th ed.; John Wiley &Son: 1980, pp 1120-1255.(21) Powell, P. Principles of Organometallic Chemistry; 2nd ed.; Chapman and Hall: NewYork, 1988, pp 217 - 224.(22) Baer, S.; Brinkman, E. A.; Brauman, 3. I. J. Am. Chem. Soc. 1991, 113, 805-812.(23) Connor, J. A.; Rasburn, E. J. J. Organometal. Chem. 1970, 24, 441.(24) Munro, J. D.; Pauson, P. L. J. Chem. Soc., London, 1961, 3475.(25) Scherman, B. 0.; Schreiner, P. R. J. Chem. Soc., Chem. Commun. 1978, 223.(26) Tulip, T. H.; Ibers, 3. A. J. Am. Chem. Soc. 1979, 101, 4201.(27) Beauchamp, J. L.; Stevens, A. E.; Corderman, R. R. Pure & Appi. Chem. 1979, 51,967-978.(28) Byrd, G. D.; Freiser, B. S. J. Am. Chem. Soc. 1982, 104, 5944-5950.(29) Carlin, T. I.; Sallans, L.; Cassady, C. J.; Jacobson, D. B.; Freiser, B. S. J. Am.Chem. Soc. 1983, 105, 6320-6321.(30) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1983, 105, 7492-7500.(31) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 5876-5883.(32) Jacobson, D. B. Organometallics 1988, 7, 568-570.(33) Allison , J.; Ridge, D. P. J. Am. Chem. Soc. 1976, 98, 7445-7447.(34) Sellers-Hahn, L.; Russell, D. H. J. Am. Chem. Soc. 1990, 112, 5953-5959.(35) Allison, J.; Ridge, D. P. J. Am. Chem. Soc. 1977, 99, 35.(36) Allison, J.; Ridge, D. P. J. Am. Chem. Soc. 1979, 101, 4998-5009.(37) Allison, J. Prog. Inorg. Chem. 1986, 34, 627-677.(38) Eller, K.; Karrass, S.; Schwarz, H. Organometallics 1992, 11, 1637-1646.(39) Freiser, B. S. Chemtracts-Analyt. Phys. Chem. 1989, 1, 65-109.(40) Buckner, S. W.; Freiser, B. S. 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Soc. 1979, 101, 1783-1786.-71-CHAPTER 3Gas-Phase Ion-Molecule Chemistry of Cyclohexadienone IronTricarbonyl(r14-c-C6H0)Fe(CO)3Cyclohexadienone iron tricarbonyl is a yellow crystalline, air and moisture sensitivesolid, melting at 104 °C and subliming at 80 ‘C (0.2 mm Hg).1 The structural and chemicalproperties of the complex and some related r14-diene complexes were widely studied by Birchand co-workers.26A common reaction of these complexes in solution is the abstraction of ahydride by trityl cation (Ph3C), yielding pentahapto (r15) - dienyl ionic complexes.4’5Themass spectrum of(14-c-C6H0)Fe(CO) has been reported previously by Davis and coworkers.7 The fragmentation patterns proposed by the authors are shown in Scheme 3.1.The numbers in brackets are the relative ion intensities as a percentage of the base peak. Forthose processes marked with *, metastable ions were observed.T3_Fe(CO)1 T3—Fc(Co21_ T’—Fe(CO)nile 234(7) nile 206(9) rn/c 178(17).col*-nile 66(8) / rn/c 94(30) / \ rn/c 150(43)-14C0/ %\co-Fern/c 65(7)Fe Fe[Fe) nile 121(13) nile 122(27)rn/c 56(100)Scheme 3.1 Fragmentation of cyclohexadienone iron tricarbonyl,(T14-c-C6H0)Fe(CO)3As far as structures are concerned, Freiser and co-workers8’3have shown repeatedlythat gas-phase cyclopentadiene transition metal ions such as, C5H6Fe (mlz 122) andC5H6o, can rearrange to form hydrido-it-cyclopentadienyl structures(C5HMW). Allison-72 -and Ridge’4have also suggested the same kind of rearrangement in ionC5HMeTi(C5HMe= methylcyclopentadiene). Therefore, Scheme 3.1 provides ion compositions rather thanprecise ion structures.In this study, self ion-molecule clustering reactions of the fragment ions from (r14-c-C6H0)Fe(CO)3and ion-molecule reactions of these ions with a second neutral molecule,methanol, are examined. The reactivities and structures of ions are discussed.3.1 FT-ICR Positive Ion Mass Spectra of(rj4-c-C6H0)Fe(CO)3At a pressure of 1.7 x i0- Torr, a 40 eV x 15 millisecond FT-ICR electron beampulse on(114-c-C6H0)Fe(CO)3generates seven abundant metal-containing fragment ions:Fe,C5HFe,C5H6Fe,C6HOFe,C6HOFe(CO),C6HOFe(CO)2and the molecularion6OFe(CO)3.The FT-ICR spectrum detected immediately after ion formation isshown in Figure 3.1(A) and the ion abundances in this spectrum are listed in Table 3.1. Thisstandard FT-ICR spectrum of(r14-c-C6H0)Fe(CO)3is in accord with that reported byDavis,7 except for some differences in relative intensities. Smaller fragments have largerintensities in Davis’ spectrum since the authors used a higher electron impact ionizationenergy, 70 eV. In this study, the electron ionization energy was set to 40 eV to obtainrelatively strong intensities of most fragment ions.After fragment ions were formed, measured delay time periods were applied betweenFT-ICR ion formation and ion detection to allow these primary ions to react with the neutralparent molecule,(T14-c-C6H0)Fe(CO)3. Sample spectra taken at 300 and 800 millisecondsdelay times are shown in Figure 3.1(B) and (C). As shown by the spectra, primary ionsmainly undergo clustering reactions with the parent molecules to form product ions withhigher masses. Some of these product ions can react further to form product ions with evenhigher masses. Spectra taken at a certain delay times were calibrated in order to obtain exactmass measurements of product ions. Unlike conventional mass spectrometry where externalmass standard reagents are used, the fragment ions of pure(14-c-C6H0)Fe(CO)3were usedas mass calibrants in this FT-ICR study. Calibration of the time-resolved spectra (i.e., spectra- 73 -at Oms, 5Oms, lOOms, ..., etc.) in this system was initially started with the spectrum at zerodelay time using the three major isotopic peaks of Fe (i.e., 54Fe, 56Fe, and 57Fe) ascalibrants. This calibration allowed the determination of the masses of those ions whosepeaks were near the calibrants, such as C5H4 (mlz = 65) and C5H6(mlz = 66). Chemicalformulae of these ions were then determined by comparing their measured ion masses with thetheoretically calculated masses. The calibration then went on to higher masses in the spectrawith longer delay times, each time including ions determined in the last calibration intocalibrants. Once the measured ion masses were obtained, chemical formulae of the ions werederived. Ionic clustering products detected in this(r14-c-C6H0)Fe(CO)3system are listed inTable 3.2. The composition of product ions in the table were also supported by ion ejectionexperiments (Section 3.1.2).Table 3.1 Distribution of ion abundances in the FT-ICR mass spectrum of(T14-c-C6H0)Fe(CO)3 (Pressure = 1.7 x iO Torr; Electron beam pulse =40 eV x 15 milliseconds; Trapping voltage = 1.0 V; Delay time = 0).Fragment Ion Nominal Mass Relative Intensity(%)aFe 56 100Fe(CO) 84 11.2C3HFe 95 5.2C5HFe 121 27.6C5H6F& 122 90.3C6H6OFe 150 98.0HOFe(CO) 178 67.8C6H6OFe(CO)2 206 86.8HOFe(CO)3 234 27.4C5H 65 3.0C5H6 66 4.9C6HO 94 5.0a. Relative to the base peak (Fe) in the spectrum.-74-100- —C6HOFe A. Delay time = 0 msFe-.—60- — •C6HOFe(CO)’C?C5H6Fe—’40 I C6HOFe(CO)2 O-%.• I—Fe(CO)3I I20- C5H6Fe—” L_C6HOFe(CO)3C?T1 rT •T)0— . .100 200 300 400 500 600100-B. Delay time = 300 ms‘— 80-________v €— Primary Ions ii Ionic Products60-C?-••— 40-C? C6HOFe(CO)320-C?__________________________I0—I I •11111111100 200 300 400 500 600100-C. Delay time = 800 ms (C6HO)3FeCO)oOC?(C6HO)2Fe3CO)5— 40-C?.— C6HOFe(H)(CO)3C?ii ‘Ii. . • . . • • . L t — - — •0-100 200 300 400 500 600Mass in A.M.U.Figure 3.1 FT-ICR single resonance mass spectra of(r14-c-C6H0)Fe(CO)3taken at delaytime (DL3) = 0 (A), 300 (B), and 800 (C) milliseconds. (Pressure = 1.7 x i0 Torr; Electronbeam pulse = 40eV x 15 milliseconds; Trapping voltage = 1.0 V).- 75 -Table 3.2 Ion identification in the(q4-c-C6H60)Fe( O)3system.Ion formula Calculated mlza Measured mM’ Error(ppm)C5HFe 120.97406 120.97137 -22.2C5H6Fe 121.98188 121.97801 -31.7C6HOFe 149.97680 149.97 106 -38.3OFe(CO) 177.97171 177.97601 24.2C6H6OFe(CO)2 205.96663 205.95622 -50.5OFe(CO)3 233.96154 233.97397 53.1C6fl6OFe2 205.91173 205.90543 -30.65HFeC 215.01593 215.01058 -24.9C6OFe(CO)3 234.96937 234.96959 0.95HFe2 269.94303 269.95275 36.0CFe6O 270.95086 270.95369 10.55HFe2 271.95868 271.95881 0.5(C6O)Fe 297.93795 297.94109 10.5(CHO)2Fe 299.95360 299.94767 -19.8(6)FeCO) 327.94851 327.95545 21.2(HO)2eCO) 355.94343 355.92342 -55.3(5)(CO)Fe3CO) 381.86780 381.86563 -5.7(6HO)2Fe2(CO)3 383.93835 383.92703 -29.5(C5)Fe3CO) 409.86270 409.85481 -19.2()(CO)Fe) 419.91984 419.91246 -17.6(C6H)2Fe 447.91475 447.92004 11.8(CO)3Fe 449.93040 449.94461 31.6(5)eO)(CO) 475.90967 475.91512 11.4(C6H)3FeCO) 477.92532 477.93002 9.8(O)eCO)2 505.92024 505.93607 31.3(6H)Fe3CO)4 467.92309 467.91283 -21.9(O)2eCO)5 495.91801 495.9 1753 -1.0( )FeO)(CO) 531.89949 531.90524 10.8(C6H)3eCO) 533.91514 533.90855 -12.3(O)FeCO)4 561.91006 561.92253 22.2a. Masses are calculated using the most abundant isotopes of C, H, 0 and Fe.b. Calibrations of FT-ICR mass spectra were initially started with the 3 majorisotopic peaks of Fe+ and went up step wisely to ions with higher masses.-76 -3.2 Ion-Molecule Clustering Reaction PathwaysC6HOFe(CO). Figure 3.2 shows two FI’-ICR triple resonance mass spectra ofC6H6OFe(CO). Spectrum A shows that pureC6HOFe(CO) ion is isolated at zero delaytime (ion ejection to ion detection, DL3). Spectrum B shows that with a reaction time (delaytime) applied, product ions are formed by the ion-molecule reactions of this selected ion with(114-c-C6H0)Fe(CO)3. Figure 3.3 shows the temporal intensity variations of the major ionsin this6HOFe(CO) I(fl4-c-C6H6O)Fe( O)3 system.100-Figure 3.2 Triple resonance mass spectra of the C6H6OFe(CO) /(C6H0)Fe(CO)3system.A. Reaction Time = 0 msC6HOFe(CO)U60-40-.——2001008060G? 40cr:200liii0 200 300 400 500 600Mass in A.M.U.- 77 -1.00.8.—N0.40.2Figure 3.3 Temporal variation of ion intensities forC6HOFe(CO) reactingwith(C6H0)Fe(CO)The two figures clearly show thatC6HOFe(CO) reacts with the parent neutral toform product ion,(C6HO)2FeCO).This product ion reacts further to form cluster ionscontaining three iron atoms. The reaction sequence is summarized in Scheme 3.2. M in thescheme stands for the neutral parent molecule,(14-c-C6H0)Fe(CO)3.C6HOFe(CO) +C6HOFe(CO)3(C6HO)2FeCO) + 2C0-C6H0M-(5-n)CO M 2C0(C6HO)2Fe3CO)11 (C6HO)3FeCO)n =4, 5Scheme 3.2 Reaction pathways in theC6H6OFe(CO) /(r4-c-C6H6O)Fe(CO)3system.Reaction Time (Sec.)-78-C6HOFe(CO)2. Sample triple resonance spectra and the kinetic behavior of ionsare shown in Figure 3.4 and Figure 3.5, respectively. The reactions of this ion with theparent molecule are rather similar to those ofC6HOFe(CO), except(C6H6O)2FeC3isformed as one of the secondary product ions as well. Triple resonance spectra selecting(C6HO)2FeCO)3show that the ion gives same products as(C6HO)2FeCO).Thereaction pathways of thisC6HOFe(CO)2I(r14-c-C6H60)Fe(CO)3 system are summarized inScheme 3.3.I40201004020100806008060300 400Mass in A.M.U.Figure 3.4 Triple resonance mass spectra of theC6H6OFe(CO)2/(H)Fe(3system.- 79 -—C?C?N—C:EzFigure 3.5 Temporal variation of ion intensities forC6HOFe(CO)2reacting with(C6H0)Fe(CO)3.C6HOFe(CO)2 +C6HOFe(CO)3I(C6HO)2FeCO),3M -C6H0-(5 or 6- n)CO(C6HO)2Fe3CO)11n=4,5+ (3 or 2)COMI(2or3)CO(C6HO)3FeCO)Reaction Time (Sec.)Scheme 3.3 Reaction pathways in the C6H6OFe(CO)2/(T4-c-C6H6O)Fe(CO)3system.- 80-C6HOFe. Sample triple resonance spectra of this ion taken at 0 and 500milliseconds delay times after the ion selection are shown in Figure 3.6. The initial clusteringreactions of this ion with its parent neutral form the major secondary product ions(C6HO)2Feand(C6HO)2FeCO). In contrast to the reactions ofC6HOFe(CO)2andOFe(CO), a dehydrogenation product ion,(C6H5O)2Feis also formed in addition to(C6H0)2Fe+. The major further reactions of these secondary ions are summarized inScheme 3.4.I40100406006001008060200100 200 300 400806050020300 400Mass in A.M.U.Figure 3.6 Triple resonance mass spectra of theC6HOFe I(C6H0)Fe(CO)3system.- 81 -C6HOFe + C6HOFe(CO)3(C6HO)2FeCO) + 2C0 (C6H5O)2Fe+ 3C0 + H2 (C6HO)2Fe + 3C0M -C6H0MM -(3-n)CO(C6HO)2Fe3CO)4 (C6HO)3FeCO)n=0-3-C6H0-(3-m)CO(C6H5O)2Fe3CO) (C6H5O)2)Fe3CO)mm=0-3Scheme 3.4 Reaction pathways in theC6HOF& /(T4-c-C6H6O)Fe(CO)3system.C5HFe+ andC5H6Fe+. Since these two ions are only separated by one mass unit,it is difficult to use triple resonance techniques to select one ion from the other. Figure 3.7shows a triple resonance spectrum that selects these two ions after a 200 millisecond timedelay. The three major secondary products formed are also separated by only one mass unit.100-C5HFe26O (m/z270)‘ 80 Fe (m/z271)C5H6Fe2O (m1z272)60- +C5HFe (mlzl2l)— ::C5H6Fe (m/z122)0— — .‘ I‘‘ I‘ • I, • I i • i100 150 200 250 300MAss in A.M.U.Figure 3,7 Triple resonance mass spectrum of theC5HFe/C5H6Fe / (C6H60)Fe(CO)3system at a delay time of 200 milliseconds.- 82 -The reaction pathways of the two ions were determined by the following experimentalprocedure. Since the intensity of the fragment ionC5HFe(mlz 121) is much lower than thatofC5H6Fe(mlz 122),C5HFe (mlz 121) was partially ejected with the sacrifice of someC5H6Fe(mlz 122) intensity. Mass spectra at different reaction times after this ejection werethen monitored. Sample triple resonance spectra are shown in Figure 3.8.100-In the case ofC5H6Fe (m/z 122) ejection, allC5HFe (m/z 121) ions are ejected aswell, due to its smaller intensity. To selectC5HFe (mlz 121), a second neutral, (C5H)2Fe(ferrocene,C5HFe+ is one of its fragment ions) was introduced into the vacuum system toA. Reaction time = 0 ms— C5H6Fe (m/z122)10 150 200 250 30080-60-40-.—-.S—200-10080‘6040.—200MAss in A.M.U.Figure 3.8 Triple resonance mass spectra of theC5H6Fe/(C6H0)Fe(CO)3system.- 83 -enhance its intensity. Figure 3.9 shows typical triple resonance spectra with ejection ofC5H6Fe. The ion peaks at mJzl86 and m1z307 come from the ion-molecule reactions ofC5HFewith ferrocene (see Chapter 5).100-8060.——A. Reaction time = 0 ms8O-60-40-.—-200100rn— C5HFe (m/z121)C5H6Fe (m/z122)150 2001I1111t1 I250 30040200MAss in A.M.U.Figure 3.9 Triple resonance mass spectra of theC5HFe / (C6H60)Fe(CO)3I(C5H)2Fesystem. Ion peaks marked with X are formed by the reactions ofC5HFewith(C5H2Fe.By comparing the ion intensity changes in the triple resonance mass spectra of thesystems ofC5HFe /C5H6Fe I(C6H0)Fe(CO)3,C5H6Fe I(C60)Fe(CO)3,andC5HFe / (C6H60)Fe(CO)3I (C5H)2Fe systems, reaction pathways ofC5HFe andC5H6Fe are determined. For example, by looking at Figure 3.7, 3.8(B), and 3.9(B), we can- 84 -unambiguously see that the initial reactions of C5HFe give the major product ions(C5H)(6O)Fe2(m1s271) and(5H)(C6O)Fe2CO) (m1z299), and the initialreactions ofC5H6Fegive product ions(C)(CO)Fe(m1z270),(C5H6)(O)Fe(m1z272), and(C)(CO)FeCO)(mIz300). These are summarized in the followingreactions.C5HFe +C6HOFe(CO)3 —* (C5H)(6O)Fe2 + 3C0 (3.1)mIzl2l m/z271—* (C)(CO)FeCO) + 2C0 (3.2)m/z 299—* (C5H)Fe(C6O) ÷ Fe(CO)3 (3.3)m/z 21 5(unreactive)C5H6Fe + C6HOFe(CO)3 - (C5H)(6O)Fe2+ 3C0 + H2 (3.4)m/z122 m1z270—* (C5H6)(O)Fe2 + 3C0 (3.5)m/z272—> (CH6)(CO)FeCO) + 2C0 (3.6)m/z300By comparing the spectra at even longer times, further reaction pathways are determined.These are summarized in the following equations.(C5H)(6O)Fe2+ C6H6OFe(CO)3—* (C6HO)Fe(CO)3 + Neutrals (3.7)m/z27 1 m/z 235-4(C5)(O)2FeCO) ÷ CO (3.8)m/z 477—* (C5H)(6O)2Fe3+ 3C0 (3.9)m/z 421(C5H)(6O)Fe2 + M —>(C5H)(6O)(CFe3CO) + 2C0 (3.10)m/z270 m/z 448—>(C)(O)(CFe + 3C0 (3.11)m/z 420—*(C5H)(6O)Fe3CO)2+ C6H0 + CO (3.12)m/z 382- 85 -(C5H6)(0)Fe2 + M —* (C6H60)Fe(CO)3W + Neutrals (3.13)m/z272 m/z 235—*(C5)(O)2FeCO) + CO (3.14)m/z 478—*(CH6)(O)Fe3CO) + C6H0 + CO (3.15)m/z384Product ions containing four iron atoms have also been observed, which indicates thatsome of the tertiary ions are reactive towards the neutral iron compound.Fe. As shown by the triple resonance spectra in Figure 3.10, the initial reaction ofFe with(ri4-c-C6H0)Fe(CO)3predominately produces a product ion with m/z = 206. Thefurther reactions of this product ion with the parent molecule do not follow the simple reactionsequence ofC6HOFe(CO)2(m1z206) The triple resonance mass spectra of Fe at longertimes are rather similar to those ofC6HOFe andC5H6Fe, in that, some dehydrogenatedproducts are formed. This product ion also reacts with methanol differently than doesC6HOFe(CO)2(Section 3.3). These observations indicate that this product ion has adifferent composition or a different structure from that ofC6HOFe(CO)2.As mentioned inChapter 2, Cr reacts with the neutral parent molecule, (r1-c-C78)C (CO)3to form adimetallic cluster product ion,C7H8r2(CO). In other analogous organo-metal-carbonylsystems such as,(r16-c-C6H)Cr( O)3,(16-arene)Cr(CO)3, CpV(CO)4CpMn(CO)3, and(T14-CH6)Fe(CO), reactions of the bare metal ions with their neutral parent molecules alwaysgive dimetallic products. These observations suggest that the product ion formed at m1z206 inthis system may actually be CH6OFe2(m1z206), despite of the mass ambiguity arisen fromconfusion of 56Fe with 2C0 ligands (i.e., 2x28 amu = 56 amu). In order to measure ionmasses and isotopic distributions of (C6HO)Fe2and(CHO)Fe(CO)2,the followingexperiments were performed . A radio frequency (FDB) that corresponds to m1z56 wasapplied continuously in the ion formation event and no delay time was allowed between theshort beam (5 ms) pulse and the excitation sweep. This arrangement allowed continuousejection of Fe+ during the fragmentation of the compound and therefore minimized the chance- 86-60--..-C6HOFe24—,..m1z206Mass in A.M.U.for the formation of(C6HO)Fe2.The mass spectrum obtained under this ICR conditionwas then calibrated to measure the isotopic ion masses of(C6HO)Fe(CO)2and the isotopicpeak intensities (Table 3.3). The FDB (frequency during beam) radio frequency was then setto(C6HO)Fe(CO)2to eject this ion and delay times were applied, which ensured theformation of the product ion of Fe+ in high purity. The measured isotopic ion masses andabundances of product ion(C6HO)Fe2are summarized in Table 3.3.100- -Fe+A. Reaction Time = 0 ms40-20-liii I I’ II III II,100 200 300 400 5000-100-80-60-40-20-0-IB. Reaction Time = 400 ms(C6H0)2Fe3(CO) (C6HO)3FeCO)(C6H50)2Fe3(CO) (C6H50)2C6H0)Fe3(CO)NI L+FeF,100 200 300 400 30Figure 3.10 Triple resonance mass spectra of the Fe /(C6H0)Fe(CO)3system.- 87 -Both isotopic distributions and measured ion masses (Table 3.3) suggestC6HOFe2to bethe product of the reaction Fe +(114-c-C6H0)Fe(CO)3(Eqn. 3.16). The further reactions of(C6HO)Fe2with neutral parent are summarized in Scheme 3.5.Fe + C6HOFe(CO)3 —* (C6HO)Fe2 + 3C0m/z 56 m/z 206(3.16)Table 3.3 Ion masses and isotopic distributions of(C6HO)Fe(CO)2andC6HOFe2mfz rel. abundance (%)Ioncalcd. measd. calcd. measd.C6HO54Fe(CO)2 203.97 1 203.972 6 5.356Fe(CO) 205.966 205.956 100 100C6HO57Fe(CO)2 206.967 205.964 11 9.8C6HO56’4Fe2 203.916 203.930 13 9.956Fe 205.911 205.905 100 100C6HO56’7Fe2 206.912 206.948 11 11.5C6HOFe2 + C6HOFe(CO)3-C6H5OFe2(C6HO)Fe(CO)3-C6H0(C6H5O)2Fe3CO)-Fe(CO)(C6HO)2FeCO)-(5-n)CO(C6HO)3FeCO)n=2-4-2C0(C6HO)2Fe3CO(C6H5O)2Fe3COM -FeCO(C6H5O)2)Fe3CO)MScheme 3.5 Reaction pathways in theC6HOFe2I(q4-c-C6H60)Fe(CO)3system.- 88 -3.3 Ion-Molecule Reactions with MethanolFragment ions from(fl4-c-C6H60)Fe(CO)3 were also selected to react with methanol.In these experiments,(fl-c-C6H60)Fe(CO)3 was first introduced into the vacuum system at apressure of 1.7 x iO Torr. Methanol was introduced afterwards and the total pressure of thetwo neutrals was maintained at about 7.0 x iO Torn When a 40 eV x 15 ms ICR electronbeam pulse was applied, fragment ions from both(ri4-c-C6H0)Fe(CO)3and methanol weregenerated. Then a short delay time (typically 50 milliseconds) between ion formation eventand ion ejection events was applied to allow these ions to be energetically rçlaxed in themethanol environment. After the triple resonance ion ejection, the ICR reaction cell (the cubicion trap) contained the selected ion, neutral methanol, and neutral(r14-c-C6H0)Fe(CO)3.The triple resonance mass spectra show more products than those in the Section 3.1.2 sincethe selected ion reacts with both neutrals. Product ions formed by the ion-molecule reactionswith methanol could then be distinguished by comparing the spectra obtained with and withoutmethanol. Ion-molecule reactions with deuterium-labeled methanols CD3OH, CH3OD, andCD3O were also examined in order to verify the ion masses of these product ions and toelucidate reaction mechanisms.C6HOFe(CO). Figure 3.11 shows three partial FT-ICR triple resonance massspectra that illustrate the reactions ofC6HOFe(CO) with deuterium-labeled methanols.C6H6OFe(CO) reacts with methanol to form an abundant ligand substitution product ion,C6H6OFe(methanol) (reaction 3.17). This kind of substitution reaction has been previouslyobserved when metal carbonyl ions such asCr2(CO)4+,’5react with methanol.C6HOFe(CO) + CH3OD —>6HOFe(CH3D) + CO (3.17)m/z 178 m/z 183IonC6HOFe in the spectra is considered to be formed by collision-induced dissociation(CID) ofC6HOFe(CO) as its intensity did not increase with increasing delay times afterC6H6OFe(CO) disappeared from the spectra. As will be shown in Figure 3.13,C6HOF&reacts with CH3OD to form C6H5DOFe, C6H6OFe(OD) andC6HOFe(OCH3).Ions- 89 -CA. CH3OD C6HOFe(CH3D)—F4U]C6HOFe(cO)C6HOFe /C6H5DOFe C6HOFe(OD)Fa-jUCl0 150 160 170 100 190 200C_B. CD3OH C6HOFe(CD3H)—FU]z C6HOFe(COYUFzC.- C6HOFeUrn>I— C6HOFe(OH)a-jkJ J150 160 170 160 190 200• C. CD3O D C6HOFe(CD3D)—FU]z C6HOFe(cO)UFz‘Ijin> C6HOFe IC6H5DOFe C6HOFe(OD)Fci-JUll i 4I I 110 150 160 170 160 190 200MPSS iN P.M.U.Figure 3.11 Partial triple resonance spectra illustrating the reactions ofC6HOFe(CO)with methanol. Delay time (DL3) = 200 milliseconds.-90-F!—1j1zAJFz‘-4tLj>Fu-IA. CH3ODC6H5DOFe andC6HOFe(OD) in Figure 3.1 1A and C are therefore believed to be formedby HID exchange reaction and hydroxyl abstraction reaction ofC6HOFe.CL0-40-mC6HOFe IC6H5DOFeIIC6HOFe(CH3D)6OFe(CO)(CH3D)hAdC6HOFe(CO)Cl-i011111111160 160A A.200 221 80MOSS IN P MU,Figure 3.12 Partial triple resonance spectra illustrating the reactions ofC6HOFe(C0)2with methanol. Delay time (DL3) = 200 milliseconds.C6HOFe(CO)2.The partial triple resonance mass spectra of this ion taken at thepresence of CD3OH or CH3OD are shown in Figure 3.12. Methanol replaces one CO groupofC5H6OFe(CO)2to form6H6OFe(CO)(methanol) (reaction 3.18). Instead of secondCO substitution by methanol, C6H6OFe(CO)(methanol) rapidly ejects the remaining CO-91-group to form the final product ion, C6H6OFe(methanol), (reaction 3.19). CIDfragmentation also occurs, yielding daughter ionsCHOFe(CO) andC6H6OFe.C6H6OFe(CO)2 + CH3OD—>6H6OFe(CO)(CH3OD) + CO (3.18)m/z206 m/z211OFe(CO)(CH3D)—* C6H6OFe(CH3OD) + CO (3.19)mJz2ll mlz 183C6HOFe. The reactions ofC6HOFe with methanol are different from those ofC6HOFe(CO) and C6H6OFe(CO)2The spectra in Figure 3.13 show that the ion reactswith CD3OH to form two products,6HOFe(OH) and C6H6OFe(OCD). In the caseswhere CH3OD or CD3O is used, HID exchange product,C6H5DOFe, is also formed.Figure 3.14 shows the triple resonance mass spectra of product ionC6HOFe(OH).In order to selectC6HOFe(OH), a delay time before FT-ICR ion ejection was applied.C6HOFe(OH) was formed in this time period and was then selected (Spectrum A). Anotherdelay time was applied after the ion ejection to allow this product ion to react with CD3OH.C6HOFe(OCD3)is the only product ion formed. This proves thatC6H6OFe(OH) is anintermediate ion that reacts with methanol further to form the final product ion,C6HOFe(OCH3).Using CH3OD as example, reactions of ionC6HOFe with methanol are summarizedbelow.C6HOFe + CH3OD —* C6H5DOFe + CH3O (3.20)m/z 150 m/z 151—> C6HOFe(OD) + CH3 (3.21)m/z 168C6HOFe(OD) + CH3OD —* C6HOFe(OCH3) + D20 (3.22)m/z 168 m/z 181As these reactions are fairly rapid, secondary reaction products ofC6HOFe with theparent molecule such as (C6HOFe)2and(C6HOFe)2CO), could not form in sufficientamounts and their reactions with methanol were not observed.-92-HIUizUUrn>-4I—a-Jb-iliECiIHOFigure 3.13 Partial triple resonance mass spectra illustrating the reactions ofC6HOFeA. CH3OD C6HOFe(OD)CI‘—4Uizli-iH2:‘-IUI’>-4Ha-JLiC6HOFe /C6H5DOFe/C6HOFe(OCD3)CB. CD3OHC6HOFe(OH)C6HOFeC5H6OFe(OCH3)160 1?0 B0 190MOSS ]N P.M Li.with methanol. Delay time (DL3) = 300 milliseconds.-93 -CA. Reaction time = 0 ins—C6HOFe(O}{)If)liiI—zThLI11T>I.ciIlUCI I160 170 160 1.90 200 210 220C—________________________________________________CB.Reactiontime=ZOOmSIC$OFe(OH)zlil C6HOFe(OCD3)zThhJm>.4Ici-J160 170 180 1.90 200 210 220MPSS IN ftM.U.Figure 314 Partial triple resonance mass spectra illustrating the reactions ofC6HOFe(OH) with CD3OH. First delay time (DL 1) = 300 milliseconds (for both SpectrumA and B); Second delay time (DL3) =0 milliseconds (A), 200 milliseconds (B).-94-C5HFe andC5HFe26O. As shown by the triple resonance spectrum inFigure 3.15,CHFe slowly reacts with methanol to form an adduct-formation product ion,C5HFe(methanol), reaction 3.23.C5HFe + CH3OD —* (C5H)Fe(CH3OD) (3.23)mIzl2l mlz 154‘-411]zz‘U>‘-4I-aJUFigure 3.15 Partial triple resonance mass spectrum illustrating the reaction ofC5HFewith CH3OD. Delay time (DL3) = 600 milliseconds.A product ion,(C5H)(6O)Fe2OCH(mIz3Ol), was also observed in the highmass region of the above spectrum. As noted in Section 3.2,C5HFe+ reacts with the neutralparent molecule to form an abundant secondary product ion,Fe6O(mJz27 1)(reaction 3.1). Therefore, the product ion(C5H)(6O)Fe2OCD3(m1z304) is formedby the reaction ofC5HFe26Owith methanol, reaction 3.24. The reaction is anaddition-dehydrogenation reaction. That is, whenC5HFe26Oassociates withmethanol, a neutral hydrogen molecule is eliminated. By using deuterium-labeled methanolsas neutral reactants, the sources of the two hydrogen atoms eliminated can be unambiguouslylocated. One is from(C5H)(C6O)Fe2the other is from the hydroxyl group of methanol.(C5H)(6O)Fe2+ CH3OD —> (C5H)(6O)Fe2OCH3+ HD (3.24)m/z271 m/z301-95 -CsH6Fe. Since this ion differs in mass fromC5HFe by only one mass unit, thetwo ions were reacted together with methanol. The initial triple resonance spectrum is shownin Figure 3.16. Note that the abundance ofC5H6F& is much higher than that ofC5HFe.Typical triple resonance mass spectra with reaction time delay are shown in Figure3.17. As the adduct-formation product, C5H5Fe(methanol), has been proved to be formedfrom the reaction ofC5HFe+ with methanol, the remaining products are therefore formedfrom the reactions of C5H6Fe+ with methanol. Mass peaks corresponding toC5H6Fe(methanol) do not show in the spectra, indicating that ion C5H6Fe does notundergo an aâduct-formation reaction with methanol. Instead, the ion reacts with methanol toform addition-dehydrogenation products, reactions 3.25 and 3. 26. One of the two hydrogenatoms eliminated is fromC5H6Fe, the other one can be either the hydroxyl hydrogen or themethyl hydrogen of methanol. The ion intensities in Figure 3.17 indicate that the additiondehydrogenation reaction ofC5H6Fe with methanol is more rapid than the adduct-formationreaction ofC5HFewith methanol.C5H6Fe + CH3OD —* C5HFe(OCH3) + HD (3.25)mlz 122 mlz 152—* Fe(CH2OD) + H2 (3.26)mlz 153lOMfl5 IN PM.U,Figure 3.16 Partial triple resonance mass spectrum that selectsC5H6Fe andC5HFe inthe presence of methanol. Delay time (DL3) =0 milliseconds.-96-II-flzUFzIl-I>‘-4I—a-jU>—I—zUFzI-’,U>-4a-J(iIrrCii lO 15OMP5. IN PJ-1,U.Figure 3.17 Partial triple resonance mass spectra illustrating the reactions ofC5H6Fe withmethanol. Delay time (DL3) = 500 milliseconds.Similar to ion(C5H)(6O)Fe2,the secondary product ofC5H6Fewith theparent molecule,(C)(0)Fealso undergoes an addition-dehydrogenation reactionwith methanol, reaction 3. 27. The hydrogen atom eliminated from methanol is on thehydroxyl group.(C5H6)(O)Fe2+ CH3OD —> (C5H)(6O)Fe2OCH3+ HD (3.27)m1z272 m/z 302Fe andC6HOFe2.Allison and Ridge16 have demonstrated that the only production of the reaction of Fe with methanol is FeOW. This hydroxyl abstraction reaction of FeC]C‘--4B. CD3OC5H6FeVC5llFe(CD2OD) I C,H5Fe(OCDC5HFe(CD3OD)/60 I 7’-97 -was not observed in the present system. But the secondary product ion of Fe+ with the parentmolecule,C6HOFe2(m1z206), was found to react with methanol to form an additiondehydrogenation product, reaction 28. The relevant mass spectra are shown in Figure 3.18.C6HOFe2+ CD3OHm/z 206—> C6H5OFe2(OCD3) + H2m/z239(3.28)C‘-4I—I-.zUIz‘-4 CUrnI—aIU(1JLfl>-4FaIUtIE220 20MOSS IN P.M.U.A. Reaction time = 0 ins—C6HOFeci rirfill, , 11I;I,I200 210 220 250‘—aI—‘—aUizLiiI—z—C6HOFeI r rjrrI I I240 2SB. Reaction time 400 msCFe2(OCD3YC200 210 240Figure 3.18 Partial triple resonance mass spectra illustrating the reaction ofC6HOFe2with CD3OH. A fixed ion ejection frequency during beam (FDB) was set to m1z206 toeliminate ionC6OFe(CO)2”from the reaction cell. An 150 millisecond delay time (DL 1)was applied after Fr-ICR ion formation to allow the formation ofC6HOFe2.Two morefrequency sweeps were then applied two eject all ions butC6HOFe2,spectrum A. Afterthis ion selection, a second delay time (DL3), 400 milliseconds, was applied to allowC6HOFe2to react with CD3OH, spectrum B.- 98 -Since the isobaric ion mass ofC6HOFe2overlaps with that ofC6HOFe(CO)2,thefollowing experimental sequence was arranged to obtain pureC6HOFe2+shown in Figure3.18(A). An ion ejection frequency (FDB) corresponding to ion mass unit 206 wascontinuously applied during the beam pulse event. This allowed complete ejection ofC6HOFe(CO)2from the ion trap while fragment ions were formed. Then an 150milliseconds delay time (DL 1) was added to allow the formation ofC6HOFe2from thereaction of Fe withC6HOFe(CO)3.Following this time delay, two more frequency sweepswere applied to eject all the other ions butC6HOFe2.Another reaction time delay (DL3)was then added after this ion selection to allowC6HOFe2to react with methanol. Thespectra in Figure 3.18 were taken at 0 and 400 milliseconds (second delay, DL3) under theabove ICR conditions. Clearly, C6HOFe2 (m1z206) reacts with CD3OH to formC6H5OFe2(OCD3)(m1z239). The two eliminated hydrogen atoms are fromC6HOFe2andthe hydroxyl group of methanol, respectively. In contrast, fragment ion OFe(CO)(m/z206) reacts with CD3OH to form the abundant product ionsC6HOFe(CO)(CD3H +(mIz2 13) (reaction 3.18) andC6OFe(CD3H)(m/z 185) (reaction 3.19). Therefore, theaddition-dehydrogenation reaction ofC6HOFe2rules out the possibility that the reaction ofFe with the parent molecule formed OFe(CO) (m/z206).-99 --0.6-0.8-1.0-1.2-1.4-1.8-2.00.83.4 Ion Reactivities and Ion StructuresThe kinetic data obtained from FT-ICR single resonance experiment series (Section3.1) were used to determine the total reaction rate constant of each ion in the self clusteringreactions. Figure 3.19 shows typical intensity vs. time curves of some primary, secondary,and tertiary ions of the present system. The intensity of an ion typically increases to amaximum and then levels off or diminishes depending on its reactivity. The decay portions ofthese intensity curves are linear since the neutral concentration is much higher than the ionconcentration and all the reactions are pseudo first order. The pseudo-first-order rateconstants are then calculated using the known pressure of the neutral parent molecule and thedecay slopes of these curves. The measured rate constants are listed in Table 3.4..—Cl)CN—CzL -1.6C0.0 0.2 0.4 0.6Reaction Time (Sec.)Figure 3.19 FT-ICR temporal variation of ion abundances in the(ri4-c-C6H0)Fe(CO)3system.-100-Table 3.4. Rate constants and electron deficienciesIon Rate Constanta Rel. Reactivityb ED ED*Fe 16.8 1.00 11C5HFe 11.8 0.71 6CsH6Fe 10.3 0.61 7 56OFe 10.0 0.60 7 5CHOFe(CO) 10.4 0.62 56OFe(CO)2 4.33 0.26 3CHOFe(CO)3 2.31 0.14 1C6HOFe2 12.1 0.72 7.5 6.55Fe 8.06 0.48 4.5CHFe26O 6.38 0.38 5 45Fe 6.57 0.39 5.5 3.5(C6HOFe)2 6.64 0.39 5.5 3.5(OFe)CO 7.29 0.43 4.5(6HOFe)2CO 5.62 0.33 3.5(OFe)CO3 3.36 0.20 2.5a. All rate constants are in the units of 10-10 cm3 molecule4s1.b. Relative to the rate of Fe+ (= rate constant of an ion / rate constant of Fej.ED refers to the calculated electron deficiency assuming single metal-metal bondsand that cyclopentadiene (C5H6) and cyclohexadienone (C6H0)ligands donate 4electrons, and cyclopentadienyl (C5H)and cyclohexadienonyl (C6H50)ligandsdonate 5 electrons.ED* refers to the adjusted electron deficiency calculated using hydrido-iccyclopentadienyl and hydrido-it-cyclohexadienonyl structures formed viaintramolecular 3-hydride transfer.As shown in Figure 3.19, the curve marked with —.— has two slopes. It is plottedusing the intensities of the ion peaks (in the time-resolved mass spectra) at the mass unit of206. The two slopes of this curve indicate that the ion peak at m1z206 includes two ionspecies, and one is more reactive than the other. As discussed in the last two sections, the twoion species are C6H6OFe2andC6HOFe(CO)2.The smaller slope corresponds to- 101 -C6HOFe(CO)2as it is much less electron deficient. To plot the intensity vs. time curve forC6HOFe2, the ion intensity ofC6HOFe(CO)2,therefore, needs to be subtracted from thetotal intensity of these two ions. It is considered thatC6HOFe2is all gone at longer reactiontimes (>0.5 seconds) and the intensities at this time region comes fromC6HOFe(CO)2only. We can then derive the mathematical function for the curve at this region, which is: theintensity ofC6HOFe(CO)2= I ekt. The intensities ofC6H6OFe(CO)2at correspondingshorter times are then calculated by using this equation and subtracted from the total peakintensities at m1z206. The difference gives the intensity of C6H6OFe2.The intensity vs.reaction time curve for ionC6H6OFe2is then plotted for the determination of its rateconstant.The electron deficiency from 18 electrons of the central metal atom in each ion is listedin the ED and ED* columns. The values in ED column are calculated using the ion structuresshown in Scheme 3.1. That is, theC6H0ligand retains the same r4 coordination form as ithas in the neutral parent molecule. C5H6and C5H are cyclopentadiene and cyclopentadienylthat donate 4 and 5 electrons, respectively. The C6H50ligand inC5HFe26Oand(C6H5OFe)2+is considered to be a cyclohexadienonyl which donates 5 electrons to centralmetals. Single metal-metal bonds are assigned to dimetallic ions in analogy to previous gas-phase and condensed-phase observations. The values in ED* column are calculated usinghydrido-it-cyclopentadienyl and hydrido-n-cyclohexadienyl structures formed by [3-hydridetransfer reactions that are discussed in the following text.Previous study on ion-molecule clustering reactions of 18-electron organometalliccomplexes has shown that the reactivity of an ion is typically governed by the electrondeficiency from 18 electrons of the central metal ion: the greater the electron deficiency, thegreater the reactivity. The ion reactivity order of ions shown in the Figure 3.19 is: Fe>C6HOFe>(C6HOFe)2(C6HOFe)2CO>C6HOFe(CO)3.This reactivity trendis apparently the result of the decreasing electron deficiencies of these ions. Ion structuresdiscussed below are based on ion reactivities and ion-molecule reactions with methanol.-102-C5HFe andC5H6Fe. In Scheme 3.1, the organic ligands of ionsC5HFe andC5H6Fe have been proposed to be cyclopentadienyl and cyclopentadiene by Davis and coworkers.7 If the two ions have(fl5-c-C5H5)Fe and(rI4-c-C5H6)Festructures as suggested,the latter ion has a greater electron deficiency. However, as listed in Table 3.4, the reactivityofC5H6Fe(ED =7) is smaller than that ofC5HFe (ED = 6). This indicates that the C5H6ligand may have changed to provide more than 4 electrons to the central metal atom ofC5H6Fe+. Freiser and co-workers83 have demonstrated that when Fe+ reacts withcyclopentene,C5H6Fe is formed by elimination of a molecular hydrogen. This product ionwas suggested to be a cyclopentadiene complex (1) that rearranged to form hydrido-itcyclopentadienyl complex (2), reaction 3.29. Ion 2 undergoes HID exchange with D2, whichindicates its metal hydride character.10<Hexo 3-hydride transfer (3.29)endo— IFe FeLH1 2To explain our reactivity observations, we believe that theC5H6Feion formed in thepresent system undergoes the above intramolecular 3-hydride transfer reaction. The hydridoit-cyclopentadienyl structure (2) has an electron deficiency (ED*) of 5, explaining its lowerreactivity than that of(rl5-c-CH)Fe(ED = 6). The driving forces for this rearrangement areconsidered to be the electron deficiency of the central metal atom and the formation of a highlydelocalized aromatic ligand. That is, when the central metal becomes highly electronunsaturated, it abstracts a f3-hydride intramolecularly to form an aromatic cyclopentadienylligand. Huang26 has recently demonstrated that metal ions can only eliminate the hydrogenatoms in the same side of cyclohexane and tetralin rings. Since the elimination of molecularhydrogen from hydrocarbons by metal ions also involves a [i-hydrogen transfer from ligand tometal, this observation indicates that only the hydrogen atom at the endo position inC5H6Fecan be abstracted to the metal center.- 103 -When reacting with methanol, (T15-c-CH)Feforms a methanol adduct-formationproduct (Figure 3.15), butC5H6Fe undergoes addition-dehydrogenation reactions (Figure3.17). This also suggest the presence of a metal-hydrogen bond inC5H6Fe. The reactionmechanism is proposed in Scheme 3.6. The metal hydride ion may initially insert into eitherC-H bond or O-D bond of methanol to form double hydride ions which undergo rapidreductive elimination of neutral hydrogen molecules. In the case ofC5HFe+, after ioninsertions, hydrogen molecule could not be eliminated since metal-hydrogen bond did notexist originally.C-H insertion -H2CH3OD H CH2ODFet.....CH2ODI H m1z153FO-D insertion -HD cçyH” OCH3 OCH3D m/z152Scheme 3.6 Addition-dehydrogenation reaction ofC5H6Fe with methanol.Ions C5HFe andC5H6Fe were also allowed to react with ammonia. While theformer ion predominantly forms an adduct product,C5HFeNH3(reaction 3.30), the latterion forms a major addition-dehydrogenation product,C5HFeNH2(reaction 3.31).C5HFe + NH3 —> C5HFeNH3 (3.30)mlz 121 mlz 138C5H6Fe + NH3 -4 C5HFeNH2 + H2 (3.31)m/z 122 m/z 137The dehydrogenation reaction 3.31 is similar to the addition-dehydrogenation reaction withmethanol and indicates a metal-hydrogen linkage in ionC5H6Fe.-104-C6HOFe andC6HOFe(CO). If the C6H0 ligand retained its structuralintegrity and the same coordination pattern as in the neutral complex,C6HOFe (ED=7) isapparently more electron deficient thanC6HOFe(CO), 3 (ED=5). The former ion istherefore expected to show greater reactivity. As listed in Table 3.4, these two ions havesimilar observed reactivity. This indicates that the central metal atoms in the two ions have thesame electron deficiency. As confirmed by metastable ion monitoring (Scheme 3.1),C6HOFe is formed by stepwise ejection of three CO groups fromC6H0Fe(C0)3.Thesimilar reactivity of the two ions may be explained by the following fragmentation process thatyields(rI4-c-C5H6)Fe(CO), 4 (ED=5). That is, the third CO might be ejected from theorganic ligand rather than the metal-bonded CO.OV%-COI Fe(CO) K rFe(CO)3 4The possibility is ruled out by the reaction of the ion with methanol. IfC6HOFe hasstructure 4, it is expected to undergo a rapid ligand substitution reaction as does ion 3.Instead, the ion undergoes hydroxyl abstraction and H/D exchange reactions with CH3OD.IonC6HOFe, likeC5H6Fe, is proposed to have undergone a f’-hydride transfer reaction toform a hydrido-it-cyclohexadienonyliron structure, 6 (ED* = 5). As a result, the metal centerin each of C6H6OFe, 6 (ED* = 5) andC6HOFe(CO), 3 (ED = 5), has the same electrondeficiency, explaining their similar reactivities.p-hydride transfer°TJ-Fe— HC6HOFe was found to react with NH3. Product ions formed by displacement of ahydrogen atom (reaction 3.32) and addition-dehydrogenation (reaction 3.33) were observed inconsiderable amounts. This also suggests that fragment ionC6HOFe+ has structure 6.C6H6OF& + NH3 —* C6H5OFeNH3 (3.32)—* OFeNH2 (3.33)- 105 -C6HOFe2.As we attribute the driving force of the f-hydride transfer to the highdegree of electron deficiency of the central metal atom, ionC6HOFe2(ED = 7.5) is alsoexpected to undergo the similar rearrangement to form structure 7. The hydrogen atom in thestructure may be on the other iron atom or be a bridging hydrogen.OlFe+Ion 7 undergoes an addition-dehydrogenation reaction with CD3OH to form product ionC6H5OFe2(OCD3)(reaction 3.28), which indicates the metal-hydrogen linkage in thestructure. As only the hydroxyl hydrogen of methanol can be eliminated, the product ionC6H5OFe2(OCD3)is believed to be a metal-oxygen bonded ion (Fe-OCD3structure moiety).C5HFe26O. The secondary product ion ofC5HFe,C5HFe26O,also undergoes an addition-dehydrogenation reaction with methanol (reaction 3.24). Thisindicates the presence of metal-hydrogen bond in its structure which is proposed to be 8.0eCC5H6Fe2OandC5HFe26O. In the self clustering reactions ofC5H6Fe with(r14-c-CH60)Fe( O)3,a dehydrogenated product ion,C5HFe26O,isformed to accompany the formation of the associated product,C5H6Fe2O(reactions3.4 and 3.5). This is also explained by a 3-hydride transfer process. As the reactant ion,C5H6F&, is in a metal hydride form, the associative product, C5H6Fe26H6O,is believedto have structure 9 which undergoes a further p-hydride transfer to form a double hydridestructure 10. The two hydrogen atoms in 10 may migrate to a single metal atom and bereductively eliminated, yielding the dehydrogenated productC5HFe26O(11). Theaddition-dehydrogenation reaction ofC5H6Fe26H6O with methanol (reaction 3.27)-106-suggests that the ion has at least one metal-hydrogen bond in its structure (9 or 10, or amixture of 9 and 10).-H2°1Fe—Fe-’/Z12 1(C6HOFe)2and(C6H5OFe)2. Similarly, the structures of the associatedproduct, (C6HOFe)2(12 or 13, or both), and the dehydrogenated product, (C6H5OFe)2(14), ofC6HOFe are proposed in the following reaction scheme. As the reactant ion,C6HOFe+, undergoes rapid reaction with methanol, these products could not be tested for ametal-hydrogen bond by reacting with methanol.°Ti- Fe-Fe-1 -H2 °T;Ej- Fe-Fe1112 4°]QFeFeC0All the other ions in Table 3.4 are believed to have similar structures as the parentneutral except that a single metal-metal bond exists in the polymetallic ions.The adjusted electron deficiencies of ions calculated using the above proposedstructures are listed in the ED* column in Table 3.4. Figure 3.20 depicts a monotonic increase- 107 -of ion reactivity with the increasing electron deficiency (ED*). The outlined points whichcorrespond to non-rearranged ion structures are apparently off the curve.1.2-LI0.6-0.4-I I I0 1 2I I I3 4 5ElectronI I I I9 10 11 12Figure 3.20 Ion reactivity versus electron deficiency. The data are from Table 3.4. Eachion whose reactivity is anomalously low is indicated in outline font. The solid dotscorrespond to the ED* values and the outline dots correspond to the ED values. The bold facenumerals correspond to the structures in the text.++ C6H5OFe2 7C5H5Fe3++C6H6OFe(CO)+C5HFeH 2+C5H5Fe26HO 11+C5HFe2(H)6H010+(COFeH)2 13(CHOFe)2(CO)3+Fe+++ (CH6OFe)2+++C6HFe(CO)2+OFe(CO)3I I I6 7 8Deficiency (ED*)- 108 -3.5 Cyclohexadiene Iron Tricarbonyl and Other Related Iron ComplexesIn order to compare with the cyclohexadienone iron tricarbonyl system, ion-moleculereactions with methanol of the fragment ions formed by El on cyclohexadiene iron tricarbonyl[(r4-c-C6H8)Fe(CO)3], butadiene iron tricarbonyl [(r14-CH6)Fe(CO)3]and cyclooctatetraeneiron tricarbonyl [(r14-c-C8H)Fe(CO) were examined.3.5.1 FT-ICR Positive Ion Mass Spectrum of(14-c-C6H8)Fe(CO)3Figure 3.21 shows the normal FT-ICR positive ion mass spectrum of the complex.Electron impact on(14-c-C6H8)Fe(CO) generates six metal-containing positive fragmentions: Fe (mlz 56, 26.4%),C6HFe (mlz 134, 100.0%),C6HFe(CO) (mlz 162, 12.9%),C6H8Fe(CO) (mlz 162, 11.3%),C6H8Fe(CO)2(mlz 192, 35.4%), and the molecular ionFe(CO)3(mlz 220, 11.5%). This Fr-ICR El fragmentation is generally in accord withprior observations.6’2710080-.-C?0Figure 3.21 FT-ICR positive ion mass spectrum of(r14-c-C6H8)Fe(CO)3(Pressure = 1.7 xiO Torr, Electron beam pulse =40 x 15 milliseconds, Delay time =0 milliseconds)The molecular ion fragments by the typical sequential loss of carbonyl groups.Dehydrogenation is also a major fragmentation route that gives ionsC6HFe(CO) andC6HFe+. The loss of molecular hydrogen from the iron complex is in direct contrast to the6040C6HFe(CO)+20 C6HDelay Time = 0 ms+ I ——Fe(CO)3C6H8Fe(CO)2+50 100 150‘-C6H8Fe(CO)3200 250Mass in A.M.U.300 350-109-mass spectrum of cyclohexadiene in which the base peak corresponds to the loss of a singlehydrogen atom from the molecular ion. In addition, dehydrogenation becomes enoughprominent only after the loss of two carbonyl groups, i.e., when the iron atom becomeselectron deficient. By analogy to the last system, we propose the following process for thedehydrogenation fragmentation. After the loss of two CO groups, the metal center of thefragment ionC6H8Fe(CO)+, 15, becomes enough electron deficient to intramolecularlyabstract a 3-hydride from the cyclohexadiene ligand and form the metal hydride ion, 16. Ion16 undergoes a further n-hydride transfer to form the double hydride ion, 17, whichundergoes rapid reductive elimination of hydrogen to form fragment ionC6HFe(CO), 18.Ion 18 fragments further to form theC6HFeion in the mass spectrum.OC-Fe Fe..18Scheme 3.7 Self dehydrogenation of ionC6H8Fe(CO).The scheme suggests that the fragment ionC6H8Fe(CO) has a structure 15 or 16, ora mixture of each. The initial intramolecular hydride abstraction process (15 to 16) is similarto the condensed-phase intermolecular hydride abstraction reaction of the molecule,3’289reaction 3.34. The hydrogen elimination process via a double metal hydride ion (17 to 18)has been suggested repeatedly in the ion-molecule reactions between metal ions andhydrocarbon molecules.3048116-110-+ [Ph3CIBE( [7 JF4 + Ph3CH (3.34)Fe(CO)3 Fe(CO)33.5.2 Ion-Molecule Reaction with MethanolC6H8Fe(CO)2.Similar to the ionC6HOFe(CO)2of(fl4-c-C6H60)Fe(CO)3, ionC6HgFe(C0)2was observed to undergo a substitution reaction with methanol, reaction 3.35.C6H8Fe(CO)2 + CH3OD —* C6HgFe(CO)(CH3OD) ÷ CO (3.35)m/z 192 m/z 197C6HFe(CO) andC6H8Fe(CO). Figure 3.22 shows a triple resonancespectrum in which ionC6HFe(CO) was selected to react with CD3OH. The spectrumshows that the ion also undergoes a substitution reaction with methanol, reaction 3.36C6HFe(CO) + CD3OH —* C6HFe(CD3OH) + CO (3.36)m/z 162 m/z 169o-4C6HFe(CO) —C4r,J16Fe(CD3OH)>F-a.-JU1IT1II41 1 r ‘[? ‘b?jtf .jT I I1110 1.50 160 170 160 190MP55 IN fl,M.U.Figure 3.22 Partial triple resonance mass spectrum illustrating the reaction ofC6HFe(CO) with methanol. (Reaction time = 200 milliseconds).Figure 3.23 shows FT-ICR triple resonance mass spectra with bothC6H8Fe(CO)andC6HFe(CO) selected to react with deuterium-labeled methanol.—111—0A. CD3OHC6HFe(CO)UC8Fe(CO)>I_jUI . 1T I j1‘_‘____1__T____tr Il0 150 160 170 100 1900C6IIFe(CD3OH) + CJ17Fe(OCD3) B. CD3OHC6HFe(CO).i—C6H8Fe(CD3OH)ti:_IIII III ti 1111111150 160 170 100 190CC. CH3OD.m—C6I1Fe(CH3OD)C6IIFe(CO)1Iz>..—C6H8Fe(CH3OD)CJI7Fe(OCH3)’—150 160 170 190MPSS ]N P.M.UFigure 3.23 Partial triple resonance mass spectra illustrating the reaction of C6H8Fe(CO)with methanol. (A. Reaction time =0 millisecond; B, C. Reaction time =300 milliseconds).-112-The base peaks (mlz 169 and mlz 167) in the Spectra B and C correspond to the substitutionproduct of ionC6H6Fe(CO), C6H6Fe(methano1), (see Figure 3.22). Spectrum C showsthat ionC6H8Fe(CO) reacts with CH3OD to give product ionsC6H7Fe(OCH (mlz 166)and Fe(CH3OD) (mlz 169). C6H7Fe(OCH (m/z 166) is formed by an additiondehydrogenation reaction ofC6H8Fe(CO) with CH3OD, reaction 3.37.C6H8Fe(CHOD)(m/z 169) is formed by the substitution reaction, reaction 3.38. The addition-dehydrogenationproduct ofC6H8Fe(CO) with CD3OH,C6H7Fe(OCD3)(m/z 169), does not appear in theSpectrum B since its ion peak overlaps with the peak of the product ion ofC6HFe(CO),C6HFe(CD3OH)(m/z 169).C6H8Fe(CO) + CH3OD —* C6H7Fe(OCH3)+ CO + HD (3.37)m/z 164 m/z 166C6H8Fe(CO) + CH3OD —* C6H8Fe(CH3OD) + CO (3.38)m/z164 m/z169The addition-dehydrogenation reaction 3.37 and the substitution reaction 3.38 indicate that thefragment ion,C6H8Fe(CO), of(r4-c-C6H8)Fe(CO) is a mixture of ion 15 and ion 16(Scheme 3.7). This, in turn, supports the ion process proposed for the dehydrogenationfragmentation of the(r14-c-C6H8)Fe( O)3.C6HFe. Similar to ion C6Hr (Chapter 2, reaction 2.19), fragment ionC6H6Fe of(ri4-c-C8)Fe(CO)3,presumably a benzene-Fe complex, 19, undergoes anadduct-fonnation reaction with methanol, reaction 3.39.1”—Fe(CO)3Fe19 20C6HFe + CH3OD —+ C6HFe(CH3OD)+ (3.39)m/z 134 m/z 167The same ion generated by El on cyclooctatetraene iron tricarbonyl,(fl4-c-C8H8)Fe(CO)3, 20(Chapter 4), undergoes the same reaction. Reaction 3.39 is also similar to the reaction of ion- 113 -C5HFe+ with methanol where only an adduct-formation product was formed as well(reaction 3.23). These reactions indicate that the hydrogen atoms on the sp2 carbons ofalready-aromatic benzene and cyclopentadienyl ligands do not transfer to the metal center. Thefact that the sp2 C-H bond dissociation energy ofC6H (110 kcal I mol.) is much larger thanthe sp3 C-H bond dissociation energy ofC6H8 (70 kcal I mol.) is relevant. That the hydrogenatoms on the sp2 carbons of benzene and cyclopentadienyl are in the same plane of the carbonatoms and not stretching to the metal center also tends to be a factor that affects the 3-hydridetransfer.C4H6Fe. Consistent with reactions 3.39, 2.19 and 3.23, ion C4H6Fe, 21,generated by El on butadiene iron tricarbonyl(14-CH6)Fe(CO),22, also undergoes only anadduct-formation reaction with methanol, reaction 3.40.*Fe Fe(CO)321 22C4H6Fe + CH3OD - C4H6Fe(CH3OD) (3.40)m/zllO m/z 1433.6 Summary and ConclusionsAll the metal-containing fragment ions initially formed by electron impact oncyclohexadienone iron tricarbonyl,(i14-c-C6H0)Fe(CO)3are chemically reactive toward theneutral parent molecule. Self ion-molecule clustering reactions dominate all the reactionpathways. The clustering reactions are generally accompanied by the ejection of one to threeCO groups from the ion-molecule complexes. The loss of CO groups removes excess energythat allows the formation of large core cluster ions containing up to 3 iron atoms. The productions are typically of the form, (LFe)n(CO)m (n=2 or 3, m=0 - 4) andL2Fe3(CO)m (m=2 -5), where the ligand L can be a cyclopentadienyl, a cyclohexadienone or a cyclohexadienonyl.-114-Ion-molecule association-dehydrogenation is also a major reaction route for the particularfragment ions Fe,C5H6Fe andC6HOFe, and the secondary product ion C6H6OFe2.Ion reactivity analysis and ion-molecule reactions with methanol suggest that these highlyelectron deficient ions have metal-hydrogen bonds in their structures formed by a 3-hydridetransfer from the ligand to the metal center. These metal-hydrogen linkages are responsiblefor the association-dehydrogenation processes in the self ion-molecule reactions.Reactivities of ions towards their neutral parent molecule are observed to be dependentupon electron deficiencies of the central metal atoms. A general trend that ion reactivitiesincrease with increasing electron deficiency is observed. This reactivity-electron deficiencytrend has previously observed in metal carbonyl systems.’ 7-23 A similar reactivity trend, M+> LM > LM(CO) > LM(CO)2> LM(CO)3,has also been observed previously in someorganometallic systems bearing cyclopentadienyl or benzene ligands (L),2325 where M standsfor a metal atom. The fragment ions of(r14-c-C6H0)Fe(CO)3,C5H6Fe,C6HOFe andC6HOFe(CO) are observed to have similar reactivities. The anomalously low reactivities ofC5H6Fe andC6HOFe are explained by the intramolecular f3-hydride transfer reactiondiscussed in Chapter 2. We propose that each of ions (T14-C5H6)Fe and(fl4-C6H6O)Ferearrange via an intramolecular p-hydride transfer from the ligand to the central metal to form ahydrido-it-cyclopentadienyl structure and a hydrido-n-cyclopentadienonyl structure,respectively. The rearranged ions,(r5-CH)FeH and(fl5-C6H5O)FeH, have a sameelectron deficiency as ion(i4-C6HO)Fe(CO)t which explains the compatible reactivities ofthese three ions. Similarly, the highly electron deficient (ED > 5.5) secondary ions,C6HOFe2,C5HFe26H6OtC5H6Fe2Oand(C6HOFe)2,are also believed tohave undergone the p-hydride transfer to form hydrido-7c-cyclopentadienyl or hydrido-iccyclopentadienonyl structural portions.The specific ion-molecule reaction with deuterated methanol discussed in the lastchapter is again used to probe the hydride structures proposed in this chapter. The typicaladdition-dehydrogenation reactions or HID exchange reactions are observed for the ions thathave probable metal-hydrogen linkages, which proves the proposed structures.- 115-The closely related complex cyclohexadiene iron tricarbonyl,(fl4-c-C6H8)Fe(C0)3isobserved to fragment by ejection of CO groups and a neutral hydrogen. The loss of hydrogenonly occurs after two CO groups are ejected from the molecular ion,(r14-c-C6H8)Fe(CO)3.Using the methanol reaction method, the fragment ionC6H8Fe(CO) is probed to be a mixtureof a cyclohexadiene-iron complex,(fl4-c-C6H8)Fe(CO), and a cyclohexadienyl-iron hydridecomplex,(15-c-C6H7)Fe(H)(CO). In contrast, the fragment ionC6H8Fe(CO)2is detectedto be a pure cyclohexadiene-iron complex,(T4-c-C6H8)Fe(CO)2. These observationsindicate that the dehydrogenation fragmentation from C6HSFe(CO) toC6HFe(CO) is via a3-hydride transfer process. That is, the two hydrogen atoms on the two sp3 carbons in ionC6H8Fe(CO) can be intramolecularly abstracted to a single metal center and reductivelyeliminated. This compares to the single n-hydride transfer in ionC6HOFe whose ligandhas only one sp3 carbon.In Chapter 2, we concluded that the major driving forces of the f3-hydride transfer isthe high degree of electron deficiency of the central metal atom and the formation of thearomatic tropylium ligand. In this chapter, only the highly electron deficient ions are probedto have metal-hydrogen linkages, which is consistent with the results of Chapter 2. Inaddition, the formation of aromatic cyclopentadienyl ligand or other highly delocalized organicligands such as cyclohexadienonyl and cyclohexadienyl also contributes to the driving force.The bond dissociation energy of a C-H bond and the spatial position of a hydrogen atom alsotend to be the factors that affect the transfer. Highly electron deficient ions such asC6Hr,C6H6Fe,C5HFe andC4H6Fe (C6H = benzene, C5H = cyclopentadienyl, C4H6 =butadiene), do not undergo n-hydride transfer reactions. The ligands of these ions arealready-aromatic. The sp2 C-H bond dissociation energies of these aromatic ligands are largerthan the sp3 C-H bond dissociation energies of cycloheptatriene (C7H8), cyclopentadiene(C5H6)and cyclohexadiene (C6H8). The hydrogen atoms of these ligands are in the sameplane formed by the carbon atoms and are not stretching to the metal center. These tend to bethe reasons why to these ions do not undergo the n-hydride transfer reaction.-116-References(1) Birch, A. J.; Chamberlain, K. B. Org. Synth. 1977, 57, 107-112.(2) Birch, A. J.; Cross, P. E.; Lewis, J.; White, D. A.; Wild, S. B. J. Chem. Soc. (A)1968, 332-340.(3) Birch, A. J.; Jenkins, I., D. In Transition Metal Organometallics in Organic Synthesis;H. Alper, Ed.; Academic: New York, 1976; pp 1-82.(4) Birch, A. J.; Raverty, W. D.; Stephenson, G. R. J. Chem. Soc. Chem. Comm.1980, 857-859.(5) Birch, A. J.; Raverty, W. D.; Stephenson, G. R. J. Org. Chem. 1981, 46, 5166-5172.(6) Ratnayake Bandara, B. M.; Birch, A. J.; Raverty, W. D. J. Chem. Soc. Perkin Trans.11982, 1745-1753.(7) Calleja, H.; Davis, R.; Ojo, I. S. 0. Org. Mass Spec. 1977, 12, 109-110.(8) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1983, 105, 7492-7500.(9) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1984, 106, 4623-4624.(10) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 72-80.(11) Jacobson, D. B. J. Am. Chem. Soc. 1989, 111, 1626-1634.(12) Buckner, S. W.; Freiser, B. S. J. Am. Chem. Soc. 1987, 109, 4715-4716.(13) Buckner, S. W.; Freiser, B. S. Polyhedron 1989, 8, 1401-1406.(14) Allison, J.; Ridge, D. P. J. Am. Chem. Soc. 1977, 99, 35.(15) Sellers-Hahn, L.; Russell, D. H. .1. Am. Chem. Soc. 1990, 112, 5953-5959.(16) Allison, J.; Ridge, D. P. J. Am. Chem. Soc. 1976, 98, 7445-7447.(17) Wronka, J.; Ridge, D. P. .1. Am. Chem. Soc. 1984, 106, 67 - 71.(18) Meckstroth, W. K.; Ridge., D. P.; Reents, W. D. J. J. Phys. Chem. 1985, 89, 612-617.(19) Fredeen, D. A.; Russell, D. H. J. Am. Chem. Soc. 1985, 107, 3762 - 3768.(20) Fredeen, D. A.; H., R. D. J. Am. Chem. Soc. 1986, 108, 1860-1867.- 117 -(21) Fredeen, D. A.; Russell, D. H. .1. Am. Chem. Soc. 1987, 109, 3903-3909.(22) Ridge, D. P.; Mechstroth, W. K. In Gas Phase Inorganic Chemistry; D. H. Russell,Ed.; Plenum: New York and London, 1989; pp 93-113.(23) Russell, D. H.; Fredeen, D. A.; Tecldenburg, R. E. In Gas Phase InorganicChemistry; D. H. Russell, Ed.; Plenum: New York and London, 1989; pp 115-135.(24) Taylor, S. M. Ph.D. Thesis, The university of British Columbia, 1992.(25) Chen, S. Ph.D. Thesis, The university of British Columbia, 1992.(26) Huang, Y.; Ng, J.; Profilet, R.; Rothwell, I.; Fieiser, B. In The 41st ASMSConference on Mass Spectrometry and Allied Topics; San Francisco, California,1993.(27) Winters, R. E.; Kiser, R. W. J. Phys. Chem. 1965, 69, 3 198-3200.(28) Pearson, A. J. Acc. Chem. Res. 1980, 13, 463.(29) Lukehart, G. M. Fundamental Transition Metal Organometallic Chemistry;Books/Cole.: Belmont, 1985, pp 171.(30) Allison, 3.; Freas, R. B.; Ridge, D. P. J. Am. Chem. Soc. 1979, 101, 1332-1333.(31) Allison, J. Prog. Inorg. Chem. 1986, 34, 627-677.(32) Armentrout, P. B.; Beauchamp, 3. L. J. Am. Chem. Soc. 1981, 103, 6628-6632.(33) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. J. Am. Chem. Soc. 1981, 103,6624-6628.(34) Beauchamp, 3. L.; Stevens, A. E.; Corderman, R. R. Pure & Appl. Chem. 1979, 51,967-978.(35) Bjarnason, A.; Taylor, J. W. Organometallics 1990, 9, 1493 - 1499.(36) Allison, J.; Ridge, D. P. J. Am. Chem. Soc. 1976, 98, 7445-7447.(37) Allison, J.; Ridge, D. P. J. Am. Chem. Soc. 1977, 99, 35-39.(38) Blum, 0.; O’Bannon, P.; Schroder, D.; Schwarz, H. Organometallics 1993, 12,980-981.(39) Byrd, G. D.; Freiser, B. S. J. Am. Chem. Soc. 1982, 104, 5944-5950.-118-(40) Christ, J. C. S.; Eyler, J. R.; Richardson, D. E. J. Am. Chem. Soc. 1990, 112, 596- 607.(41) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1983, 105, 7492- 7500.(42) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1983, 105, 7484- 7491.(43) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 72 -80.(44) Huang, Y.; Freiser, B. S. J. Am. Chem. Soc. 1990, 112, 5085-5089.(45) Peake, D. A.; Gross, M. L.; Ridge, D. P. J. Am. Chem. Soc. 1984, 106, 4307-4316.(46) Peake, D. A.; Gross, M. L. Organometallics 1986, 5, 1236-1243.(47) Freiser, B. S. Chemtracts-Analyt. Phys. Chem. 1989, 1, 65-109.(48) Eller, K.; Schwarz, H. Chem. Rev. 1991, 91, 1121-1177.-119-CHAPTER 4Gas-Phase Ion-Molecule Chemistry of Cyclooctatetraene IronTricarbonyl- (114-c-C8H)Fe(CO)3Cyclooctatetraene iron tricarbonyl is a red, crystalline, air-stable solid, melting at 93-95 O1 X-ray results showed that the Fe(CO)3 group attaches to a “butadiene” part of theC8H ring and the C8H ring was in a dihedral form with the other butadiene partuncoordinated.2NMR studies show that the compound has.a fluxional behavior3’4.At roomtemperature, the compound in solution shows only a single proton resonance. With their lowtemperature 13C NMR results, Cotton and Hunter3 concluded a 1,2 shift mechanism for thefluxion. The fluxional behavior of the compound was also observed in the solid state4.As discussed in Chapters 2 and 3, (3-hydride transfer in certain ions has been observedas a gas-phase ion process that lowers ion reactivity. Our ion reactivity results on (r14-c-C8H)Fe(CO)3indicate that the uncoordinated free diene portion ofC8H ring can also lowerion reactivity by fully or partially coordinating to metal center(s).4.1 Ion-Molecule Reactions and Ion ReactivitiesAt a pressure of 6.0 x iO Torr, a 30 eV x 20 ms Fr-ICR electron beam pulse onc-C8H8)Fe(CO)3 generated six metal containing fragment ions: Fe, C6H6Fe,C8HFe,CHFe(CO), CgH8Fe(CO)2and the molecular ionC8HFe(CO)3.The positive ion FrICR mass spectrum of(ri4-c-C8)Fe(CO)3detected immediately after the beam pulse isshown in Figure 4.1 and the distribution of ion abundances in the spectrum is listed in Table4.1. This normal (DL3=0) FT-ICR mass spectrum matches that reported by King5 using 70eV electron impact, except that the intensities of hydrocarbon ions are much lower. Bymonitoring metastable ions, King5 has shown that the ionsC8HFe(CO)o..2are formed bystepwise ejection of CO groups from the molecular ion. The ion C6H6Feis formed fromC8HFeby ejection ofC2H and Fe is formed from C6H6Feby ejection ofC6H.-120-600Figure 4.1 FT-ICR positive ion mass spectrum of(ri4-c-C8H)Fe(CO)3(Pressure = 6.0 xi0 Torr; Electron beam pulse =30 eV x 20 ms; Trapping voltage = 1.0 V).Table 4.1 Ion abundances in the FT-ICR positive ion mass spectrum of (iac-C8H)Fe( O)3.Fragment Ion m/z Relative Intensity(%)aFe 56 56.7C6HFe 134 100.0C8HFe 160 72.5Fe(CO) 188 24.7C8HFe(CO)2 216 65.0Fe(CO)3 244 10.0C2H 28 3.1C6H5 77 4.6C6H 78 8.9FeC2W 81 4.8FeC2H 82 9.7C8H6 102 6.1C8H7 103 6.7C8H 104 6.6100 200 300 400 500Mass in A. M. U.a. Relative to the base peak in the spectrum.- 121 -After the fragment ions were generated, measured reaction time delay periods wereapplied before Fr-ICR ion detection. Gas-phase ions reacted with the parent molecule,(4-c-C8H)Fe(CO)3,in these time periods to form product ions with higher mass-to-charge ratios,which were detected at the end of the time delay. The time-resolved FT-ICR mass spectrawere then calibrated to obtain FT-ICR exact ion mass measurements using the knownfragment ions as calibrants. The formulae of product ions were determined by comparing themeasured ion masses with theoretically calculated ion masses. Product ions detected in thepresent system at longer reaction times are listed in Table 4.2. Unlike the (ri4-c-C6H0)Fe(CO)3system, fewer product ions containing three iron atoms are formed sincemost of the secondary ions are unreactive towards the neutral parent molecule, (rj4-c-C8H)Fe(CO)3.Table 4.2 Ion identification in the(rj4-c-C8H)Fe(CO)3system.Ion Formula Calculated m/za Measured m/zb Error(ppm)C6HFe 133.98188 133.98008 -13.4C8HFe 159.99753 159.99997 15.2CgHFe(CO) 187.99245 187.99303 3.18HFe(CO)2 215.98736 216.97174 -72.3CFe(CO)3 243.98228 243.9840 1 7.18HFe2(CO) 271.92229 271.92246 0.6Fe6 293.97941 293.97765 -6.0C8HFe2(CO)3 299.91721 299.93247 50.9(C)Fe 3 19.99507 320.00561 32.9(8H2FeCO) 375.98490 375.98615 3.3(Hg)Fe3CO 431.91983 431.92407 9.8(8)2FeCO) 459.91475 459.9268 1 26.2a. Ion masses are calculated using the most abundant isotopes of C, H, 0 and Fe.b. Calibrations of FT-ICR mass spectra were initially started with the 3 majorisotopic peaks of Fe+ and went up step wisely to ions with higher masses.-122-.—.—EICzCKinetic data obtained from the above temporal ion intensity monitoring and the knownpressure of neutral parent molecule were used to calculate the total reaction rate constant ofeach ion. Figure 4.2 shows curves of intensity vs reaction time of some primary andsecondary ions.-0.6-0.8-1.2-1.4-1.6-1.8-2.0-2.20.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4Reaction Time (Sec.)Figure 4.2 Temporal variation of ion abundances in the (T14-c-C8H)Fe(CO)3systemmonitored by FT-ICR single resonance.The linear decay of the ion intensities indicates that all the ion-molecule reactions arepseudo-first order. The apparent ion reactivities shown in the figure are: Fe+>C6HFe+C8HFe > C8H8Fe2(CO) The secondary ionsC8HFe26and (C8H8Fe)2do notreact further. Table 4.3 lists the rates measured by the FT-ICR single resonance experimentand calculated electron deficiencies (ED and ED*) on the metal atom(s) of each ionic species.The ED column in the table is calculated using the14-C8Hbonding mode as is in the neutralparent molecule. The ED* column is calculated assuming that the C8H ring donates 6 or 8- 123 -electrons to metal center(s) and possible partial double metal-metal bonding in multimetallicions.Table 4.3 Rate constants and electron deficiencies of the positive ions in the(fl4-c-C8H)Fe(CO)3 system.Species Rate Constanta Rel. Reactivityb El) ED*Fe 3.7 1.00 11C6HFe 1.9 0.51 5C8HFe 1.9 0.51 7 5Fe(CO) 1.5 0.41 5 3C8HFe(CO)2 0.0 0.00 3 1Fe(CO)3 0.2 0.05 1C8HFe2 1.9 0.51 7.5 5Fe(CO) 1.3 0.35 5.5 38HFe2(CO)3 0.7 0.19 5.5 1(C)Fe 0.0 0.00 5.5 1(8H)Fe26 0.0 0.00 4.5 1(CFeCO) 0.0 0.00 3.5 1(8H)2Fe3CO) 0.0 0.00 4.3 1.3(FeCO) 0.0 0.00 3.7 0.7a. All rate constants are in the unit of 10-10 cm3 molecu1e s1.b. Relative reactivity = rate constant of an ion / rate constant of Fe+.ED refers to the calculated electron deficiency assuming single metal-metal bondsand that C8H retains the r4 coordination mode and C6H donates 6 electrons.ED* refers to the adjusted electron deficiency assuming that C8H ring donates 6 or8 electrons to metal center(s).4.2 Reaction Pathways and Ion StructuresReaction pathways leading to the formation of the product ions listed in the Table 4.2were determined by isolating one ion at a time, by using FT-ICR multiple resonance ionejection techniques, and monitoring the subsequent reactions.-124-As introduced previously, the reactivity of an organometallic ion is generally governedby the electron deficiency from eighteen electrons of its central metal: the greater the electrondeficiency, the greater the reactivity’1.As indicated in Table 4.3, some positive ions in this(T14-c-C8H)Fe(CO)3 system show unexpectedly low reactivity towards a same neutralprecursor - the parent molecule. (C8H)2Fe2+, for example, has an ED value of 5.5 and doesnot show as high a reactivity as expected. The low reactivities of certain ions indicate that thecentral metal atoms have received more than 4 electrons from the cyclooctatetraene ligand.Proposed ion structures will be introduced along with the ion-molecule reaction pathways inthe following section. Most of the proposed C8H - Fe bonding modes have been previouslyobserved in some neutral cyclooctatetraene-iron complexes.4.2.1 Structures of Fragment IonsAs indicated in Table 4.3, the fragment ionC8HFe(CO)2is non reactive towards theparent neutral,(ri4-c-C8H)Fe(CO)3.Using Fr-ICR triple resonance mode, the ion wasisolated and allowed to interact with the parent molecule. With a 1500 millisecond reactiontime applied, no product ions were observed in the triple resonance mass spectrum. If theC8H ring retains the same r4 coordination mode as in the neutral complex, the ion will be a15-electron species. All the 15-electron fragment ions of analogous 18-electron compounds,such as (T5-Cp)V(CO)4’2,(fl-Cp)Mn(CO)3123,(4-CH6)Fe(CO)12,(r6-c-C6H)Cr(CO)312,(r16-arene)Cr(CO)3,(‘q6-c-C7H8)Cr(CO) (Chapter 2), (14-c-8)Fe(CO) (Chapter 3) and(r14-c-C6H60)Fe(CO) (Chapter 3), have previously beenobserved to be reactive with their neutral parent molecules, and their 17-electron molecularions are typically unreactive. Therefore, the non reactivity ofC8HFe(CO)2indicates thecoordination expansion shown in the reaction 4.1.4NFe(CO)i Fe(CO)24 (4.1)- 125 -That is, after a CO is ejected from the molecular ion, instead of retaining its i4 coordinationmode, 1, the C8H ring donates 6 electrons to the metal atom inC8HFe(CO)2to form an(fl6-c-C8Hg)Fe(CO)2 ion, 2. Ion 2 thus becomes a 17-electron species and unreactive. Thiscoordination expansion reaction may also be understood by noticing the fluxional behavior oftheC8H-Fe bonding in the parent neutral molecule, i.e., theC8H-Fe bond has a tendency toundergo 1,2-shift3 shown in the reaction 4.2.1,2-shiftFe(C0)3Fe(CO)3 (4.2)On becoming electron deficient, the iron atom can hold two more electrons from the C8Hring. In addition, metals such as chromium and molybdenum that have one pair valenceelectrons less than iron atom form(fl6-c-C8H8)M(CO)3 type molecules with cyclooctatetraeneligand14. In this case, the shortage of one electron pair has the equivalent effect as the ejectionof a CO from the molecular ion,(Ti4-c-C8H)Fe(CO)3.The 6 coordination mode ofC8Hring proposed here for(T16-c-C8H)Fe(CO)2also compares with that in the neutral complex,(rj6-c-C8H)(i4Fe’5.As the fragment ions,C8HFe(CO) andC8HFe, are formed by stepwise ejectionof CO from(r16-c-C8H8)Fe(CO)2t 2, the TI6 coordination mode will remain in the formertwo more electron deficient ions. That is, ions CgH8Fe(CO) andC8HFehave structures 3and 4.0: Fe(CO) Fe-126-The(fl6-c-C8H8)Fe structure, 4, is also indicated by its same measured reactivity as that ofC6HFe+, 5. That is, the metal atom in each of the ion 4 and ion 5 receives six electronsfrom their organic ligands and have the same electron deficiency (ED* = 5).The molecular ion,C8HFe(CO)y, is considered to retain ther4-C8H8Febondingand have the similar structure to the neutral parent molecule, i.e.,(rl-c-CH8)Fe( O)3Itssmall reactivity may be caused by the slow ejection of a CO that forms stable ion (16-c-C8H)Fe(CO)2,2 (reaction 4.3).Fe(CO)3 Fe(CO)2 + CO (4.3)This was not verified by the ion ejection experiment permitted by the present instrumentalconditions due to the low ion intensity of(14-c-CgH8)Fe(CO)3.4.2.2 Reaction Pathways of C8HFeFigure 4.3 shows three FT-ICR triple resonance mass spectra that illustrate thereactions of (r16-c-C8H8)Fe with the neutral parent molecule, (114-c-C8H)Fe(CO)3.Spectrum A was obtained by applying two radio frequency sweeps after the FT-ICR ionformation event that ejected all the other ions in the reaction cell but(fl6-c-C8H8)Fe. Afterthis ion selection, measured delay time periods were applied to allow(ri-c-C8H)Feto reactwith the parent molecule. Product ions formed, together with the reactant ion (i16-c-C8H)Fe, were detected in the FT-ICR ion detection event that was defined at the end of thetime delay. Spectrum B, taken at a short delay time, shows that the first step reactions ofC8HFe withC8HFe(CO)3 form secondary product ionsC8HFe2(CO),Fe2(CO)3,and(C8HFe)2.With the reaction times increased (Spectrum C), tertiaryions(C8H)FeCO) and(C8Hg)2Fe3CO.4are formed by the further reactions of thesecondary ions.-127-100-‘— 80-—.60-40-20-0100‘—‘ 80.—0100‘—‘ 80--.—.—20-0604020I I I IMass in A. M. U.Figure 4.3 Triple resonance mass spectra of theC8HFe/(r14-c-C8H)Fe(CO)3system.As shown in Figure 4.2, the secondary product ion (C8HFe)2is unreactive. TheC8HFeA. Reaction Time = 0 msI I100 200 300 400 500B. Reaction Time = 150 ms+C8HFe -C8HFe2(CO)3+ (C8H)2Fe3CO)C8HFe2(CO) —(C8H)2Fe+I I I I I i II I I I J I I I I I II I I • I I I II I II I I I I I I I I I)0 200 300 400 501(60-C. Reaction Time = 500 msC8HFe2(CO)3—i’40- C8HgFe1-. C8HFe2(CO)—..I I I(C8Hg)2Fe3CO(CHg)Fe2+h1000500.200 300 400‘ Imajor reactive secondary ion, C8H8Fe(CO) was isolated to react withC8HFe(CO)3by- 128 -applying an 150 ms delay time (DL 1) before the ion ejection sweeps (Figure 4.4, SpectrumA). Spectrum B in Figure 4.4 was taken at 300 ms (DL3) after the ion selection.100-A. Reaction Time = 0 ms‘—‘ 80-60 CgH8Fe2(CO)— 40-20-0—. , . I I I I I I I I I I I - I I I I I I I I I I I I I I I I I • I I I200 300 400 500100-B. Reaction Time = 300 ms (C8H)2Fe3CO)4—’80-+(C8H)2FeCO)60- +C8HFe2(CO) —40--20-111i[Lii III I100 200 300 400 500Mass in A. M. U.Figure 4.4 Triple resonance mass spectra of theC8HFe2(CO)/( gH)Fe(CO) system.Delay times: DL1 = 150 ms (A and B); DL3 =0 ms (A), DL3 = 300 ms (B).Spectrum B shows that the secondary ion,C8HFe2(CO) reacts with the parent molecule toform major tertiary product ions(C8Hg)2FeCO and(C8H2Fe3CO).Therefore, thetertiary ions,(C8H)2Fe3CO)4in the Spectrum C of Figure 4.3 are formed mainly fromthe slow (see Table 4.2) reaction ofC8HFe2(CO)3.Figure 4.5 shows the temporalvariation of the ion abundances ofC8HFe+ and its major product ions. The product ionsappear and reach their highest intensities at different reaction time. This also indicates thereaction sequence discussed above.-129-0.0.—-= -1.0ICzC-2.00.6Figure 4.5 Temporal variation of ion abundances in theC8HFeIC8HFe(CO)3system.Reactions 4.4 to 4.10 summarize the reaction pathways identified by FT-ICR multipleresonance ion ejection experiments.C8HFe2(CO)3 + C8H—> Fe(CO) + C8H + CO—> (C8HFe) + 3C0(C)2FeCO) + Fe(CO)3— (C8HFe3CO) + 3C0—* (C)2FeCO) + 2C0(C8HFe3CO),4+ (3, 2) COThe secondary ion, (C8HFe)2(Figure 4.2), and all the tertiary ions are unreactive.This indicates that the metal atoms in these ions have received more than 4 electrons from the0.0 0.1 0.2 0.3 0.4 0.5Reaction Time (Sec.)C8HFe + C8HFe(CO)3 —4C8HFe2(CO) + C8HFe(CO)3 —4C8HFe2(CO)3 + C8HFe(CO)3 —(4.4)(4.5)(4.6)(4.7)(4.8)(4.9)(4.10)-130-C8H ring to have 17 or close to 17 electrons structures. Scheme 4.1 gives possiblestructures of these ions.+ fFe(CO)3Co-3C0Fe — FeScheme 4.1 Proposed structures of the product ions ofC8HFe.Ion (fl6-C8H8)Fe associates with the parent neutral to form an intermediate ioniccomplex 6. The iron atom in 6 rearranges to coordinate to a single C8H ring in form of T +r4 followed by an ejection of a C8H ring to form 7, or by an ejection of a C8H ring and a6Fe — FeC6 CO7C8HFe(CO)3-3CO- Fe(CO)3c3FFe*D10 11- 131 -CO to form 8. The ri4,ri4-C8Hcoordination mode in ion 7 and 8 compares to that in theneutral complex (cis-r14,-C)Fe2(CO)56”7.The intermediate ion, 6, may also ejectthree CO groups to form ion 9. Previous gas-phase results have shown that dimetallic ionshaving fewer ligands may form multiple metal-metal bonds6’1189.Ion 9 is a stablesecondary product in the system. This tends to reflect that the ion would have a partial doublemetal-metal bond to accommodate a 17-electron configuration. Accordingly, partial metal-metal bonds are also assigned to ions 7 and 8. Tertiary product ions(C8HFe)2CO)and(C8H)2Fe3CO) are proposed to have structures 10 and 11, respectively. Ion 10compares to the “piano-stool” structure of the known complex (CpFe)2O40’1.The metalcore in ion 11 is analogous to the triangular metal core of ion Fe3(CO)12 observed by ESR inthe condensed phase22. The electron deficiencies of ion 10 and 11, ED* = 1 and ED* = 1.3,are consistent with their low reactivities (Table 4.3).4.2.3 Reaction Pathways of C6HFeReactions ofC6H6Fe are illustrated by the triple resonance spectrum in Figure 4.6and the temporal variation of ion abundances shown in Figure 4.7.100-Reaction time = 400 ms (C8H)2Fe3CO)80-C8HFe2(CO) C6HFe2860-40 C6HFe’(C8H)2FeCO)20-0— II,,IIIIi1IIIfIII._..lI.._hII ‘‘I’’’’’’100 200 300 400 500Mass in A. M. U.Figure 4.6 Triple resonance mass spectra of theC6HFe/C8HFe(CO)3system.-132-0.0.—C.)-—ECz•‘‘ -1.5C-2.0Figure 4.7 Temporal variation of ion abundances in theC6HFe/C8HFe(CO)3system.C6HFe reacts likeC8HFe with the neutral parent molecule to form two majorsecondary product ions,Fe2(CO) (reaction 4.11) andC6HFe28(reaction 4.12).C6HFe + C8HFe(CO)3 —* C8HFe2(CO) + CO + C6H (4.11)—>6Fe 3C0 (4.12)The further reactions ofC8HFe2(CO)that form the tertiary product ions in Figure 4.6 havebeen discussed above.6Fe+,as shown in Figure 4.7, is an unreactive secondaryproduct in the system. Structure 12 below is proposed for this ion. Ion 12 has an electrondeficiency of I (ED*) which is compatible with the low reactivity of the ion. Similar to theformation of ion 9,C6HFe+ reacts with parent neutral to form an ion-molecule associationintermediate. On ejection of three CO groups, the central metal atoms become so electronunsaturated that they accept all the 14 t-e1ectrons of C6H (i” + 12) and C8H (1 +0.8 1.0Reaction Time (Sec.)- 133 -Furthermore, a partial triple metal-metal bond allows the ion to have 17-electron structure andthat is consistent with its low reactivity. Triple metal-metal bonds in dimetallic complex, (iic-C4H)2Fe(CO)3, are known23. The q4 and 112 coordination forms of benzene are alsoknown in some arene complexes30.4.2.4 Reaction Pathways of FeFigure 4.8 shows two FT-ICR double resonance mass spectra that illustrate thereactions of Fe with the neutral parent molecule,(14-c-C8H)Fe(CO)3.As shown in theSpectrum B, Fe reacts withC8HFe(CO)3to form a product ion peak at m/z 216. Note thatthe fragment ion of the neutral parent complex,(16-c-C8H)Fe(CO)2t 2, has a mass-to-charge ratio of 216, but the ion peak at m/z 216 in the Spectrum B is assigned to a mixture of(r16-c-C8H)Fe(CO)2 andC8HFe2 formed via a charge transfer reaction (Eqn.4.13) and anion-molecule clustering reaction (Eqn.4. 14), respectively.Fe + C8HFe(CO)3 -4 C8HFe(CO)2 + Fe + CO (4.13)m/z 216—4 C8HFe + 3 CO (4.14)m/z 216The mass-to-charge ratios of the two ions are 215.987 and 215.932. The resolution power ofthe present instrument is not high enough to separate the two peaks of these two ions, but theisotopic peak intensity measurements indicate that ionC8HFe2has formed. That is, theisotopic peak intensity at m/z 214 is higher than that expected for ion 2.12-134-100-—A. Reaction Time = 0 ms80- Fec 60-C?-..— 40-C?20-C?i ii I liii IIi I I I I I I I i1 liii Jill I100 200 300 400100-B. Reaction Time = 1500 ms‘-‘ 80-C H Fe(CO) 1 C H Fe60- (C8H)2FeCO)— 40-(C8H)2Fe3CO)20-C?(1________III,_ I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I100 200 300 400Mass in A. M. U.Figure 4.8 Triple resonance mass spectra of the Fe IC8HFe(CO)3system.That the ion peak at mlz 216 is the mixture of(rI6-c-C8H)Fe(CO)2andC8HFe2isalso indicated by the products shown in the Spectrum B in Figure 4.8 and the temporal totalintensity change of the mixture shown in Figure 4.9. As mentioned previously, (rj6-c-C8H)Fe(CO)2+,2, is an unreactive ion and does not form any product ions at long reactiontime. The temporal intensity curve plotted using the total ion intensity at m/z 216 decreasesand then levels off. The decreased portion is caused by the reactivity of ionC8HFe2thatyields product ions (CgH8)2FeO)and(C8H)2Fe3CO),reactions 4.15 and 4.16.C8HFe2 + C8HFe(CO)3 —> (CFeCO) + Fe(CO) (4.15)—> (C8H)2Fe3CO) + CO (4.16)- 135 -.—-SS—ECzCFigure 4.9 Temporal variation of ion abundances in the Fe IC8HFe(CO)3system.The curve becomes flat at longer reaction times since all the ions ofC8HFe2+have reactedand only the unreactive ion, 2, contributes to the peak intensity at mlz 216.In both aromatic-hydrocarbon (cyclopentadienyl or benzene) metal carbonyl systemsand metal carbonyl systems, it has been previously observed that the first step reactions of abare metal ion with its parent neutral generally produce a molecular ion and an abundantdimetallic cluster ion with expulsion of a neutral CO. as generalized in the following reactions.Reaction 4. 17 is a charge transfer reaction.M + LM(CO) — LM(CO) + M (4.17)— LM2(CO).1 + CO (4.18)In the present system, the charge transfer reaction (Eqn. 4.17) of Fe withC8HFe(CO)3gives CgHgFe(CO)2,rather than the molecular ion(r1-c-C8H)Fe(CO)3This is consistentwith the low observed reactivity of the fragment ionCFe(CO)2.That is, the molecular0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4Reaction Time (Sec.)-136-ion formed via charge transfer ejects a CO to form stable ion(fl6-c-C8H8)Fe(C0)2, 2(reaction 4.3). On forming LM2(CO)..1type ion, i.e.,C8HFe2(CO) 13, the free dienepart of the C8H ring can provide four electrons to metal centers to eject the two CO groups(reaction 4.19). Therefore, the clustering production in reaction 4.14,C8HFe2is believedto have structure 14 which actually falls into the type, LM2(CO)n..1.+ V +2C0 (4.19)FeFço Fe—FeCO13 14Ion 14 has a similar structure to ion 8 and reacts similarly to give the analogous products,(C8H)2FeCO)and(C8H)2Fe3CO) (reactions 4.15 and 4.16).Structure 14 for ionC8HFe2is also compatible with its same measured ionreactivity as ion(rI-c-C8H8)Fe, 4 (Table 4.3). That is, the central metal atoms in ion 14and ion (T14-c-C8H)Fe,4, have the same adjusted electron deficiency (ED* = 5).Therefore, the two ions show the same reactivity towards the parent neutral.4.3 Summaries and ConclusionsSimilar to the effect of 3-hydride transfer observed in the(116-c-C7H8)Cr(CO)3,(T4-c-C6H0)Fe(CO)3and(r4-c-C6H8)Fe(CO) systems (Chapters 2 and 3), a coordinationexpansion process (4 6) in the(14-c-C8H)Fe(CO)3system can lower ion reactivity aswell. The uncoordinated butadiene part ofC8H ring can also lower the reactivity of a production by fully coordinating to two metal centers.The 15-electron fragment ionC8HFe(CO)2does not react with the parent molecule,which contradicts previous ion reactivity observations. The non reactivity of the ion indicatesthat its metal center has received two more electrons from the C8H ring, via a —coordination expansion process, to form a 17-electron species. Accordingly, more electrondeficient fragment ionsC8HFe(CO) andC8HFe are considered to have (T)6-c-C8H)Fe-137-structures. The r4 — q6 coordination expansion process is also supported by the samemeasured ion reactivities of ions(q6-c-CgH8)FeandC6HFe. That is, the metal center ineach of the two ions receives six electrons from its ligand.Product ions such asC8HFe2(CO)3,C8HFe2(CO) andC8HFe2(CO) areformed as secondary ions in the(14-c-C)Fe(CO) system. Their reactivities are lowerthan those expected by using afl4-c-C8H8 ligand. By analogy to the structures of knowncomplexes, these product ions are proposed to have a(T14,1-c-C8H)Fe2structural core. Theproduct ions (C8H)2FeandC8HFe26are not reacfive towards the parent molecule.They are proposed to have(rl4,1-c-C)Feand(13,TI5-c-C8H)Fe1-C6structures, respectively. In addition, the former ion is believed to have a partial metal-metalbond and the later ion has a partial triple metal-metal bond.All the proposed structures are consistent with previous gas-phase and condensedphase data.- 138 -References(1) Manuel, T. A.; Stone, F. G. J. Am. Chem. Soc. 1960, 82, 366-372.(2) Dickens, B.; Lipscomb, W. N. .1. Am. Chem. Soc. 1961, 83, 4862.(3) Cotton, F. A.; Hunter, D. L. .1. Am. Chem. Soc. 1976, 98, 1413-1417.(4) Campbell, A. J.; Fyfe, C. A.; Maslowsky, J. E. J. Am. Chem. Soc. 1972, 94, 2690-2692.(5) King, R. B. App!. Spectrosc. 1969, 23, 536-546.(6) Wronka, 3.; Ridge, D. P. .1. Am. Chem. Soc. 1984, 106, 67 - 71.(7) Meckstroth, W. K.; Ridge., D. P.; Reents, W. D. J. J. Phys. Chem. 1985, 89, 612-617.(8) Fredeen, D. A.; Russell, D. H. J. Am. Chem. Soc. 1985, 107, 3762 - 3768.(9) Fredeen, D. A.; H., R. D. J. Am. Chem. Soc. 1986, 108, 1860-1867.(10) Russell, D. H.; Fredeen, D. A.; Tecklenburg, R. E. In Gas Phase InorganicChemistry; D. H. Russell, Ed.; Plenum: New York and London, 1989; pp 115-135.(11) Ridge, D. P.; Mechstroth, W. K. In Gas Phase Inorganic Chemistry; D. H. Russell,Ed.; Plenum: New York and London, 1989; pp 93-113.(12) Taylor, S. M. Ph.D. Thesis, The university of British Columbia, 1992.(13) Parisod, G.; Comisarow, M. B. In Advances in Mass Spectrometry; A. Quayle, Ed.;Heyden & Son.: London, 1980; Vol. 8A; pp 212-223.(14) Cotton, F. A.; Hunter, D. L.; Lahuerta, P. J. Am. Chem. Soc. 1974, 96, 7926.(15) Mann, B. E. J. Chem. Soc., Dalton 1978, 1761.(16) Schwartz, 3. J. Chem. Soc., Chem. Comm. 1972, 814-815.(17) Fleicher, E. B.; Stone, A. L.; Dewar, R. B. K.; Wright, I. D. J. Am. Chem. Soc.1966, 88, 3158-3 159.(18) Corderman, R. R.; Beauchamp, J. L. Inorg. Chem. 1977, 16, 3135-3139.(19) Sellers-Hahn, L.; Russell, D. H. J. Am. Chem. Soc. 1990, 112, 5953-5959.(20) Mills, 0. S. Acta Crystallogr. 1958, 11,-139-(21) Adams, R. D.; Cotton, F. A. J. Am. Chem. Soc. 1973, 95, 6589-6594.(22) Krusic, P. J.; San Filippo, J.; Hutchinson, B.; Hance, R. L.; Daniels, L. M. J. Am.Chem. Soc. 1981, 103, 2129.(23) Fishier, I.; Hiidenbrand, K.; Koerner von Gustorf, E. Angew. Chem., mt. Ed. Engi.1975, 14, 54-55.(24) Churchill, M. R.; Mason, R. Proc. Roy. Sr. Ser. A 1966, 292, 61.(25) Hunter, G.; Lange, S.; Fischer, E. 0. Angew. Chem., mt. Ed. Engi. 1971, 10, 556.(26) Hunter, G.; Lange, S. Acta. Crystallogr., Sect B 1972, 28, 2049.(27) Fischer, E. 0.; Elschenbroich, C. Chem. Ber. 1970, 103, 162.(28) Davidson, I. M.; Triggs, C. J. J. Chem. Soc. A 1968, 1324.(29) Davis, R. E.; Pettit, R. J. J. Am. Chem. Soc. 1970, 92, 716.(30) Browning, J.; Green, M.; Penfold, B. R.; Spencer, J. L.; Stone, F. G. A. J. Chem.Soc. Chem. Comm. 1973, 31.-140-CHAPTER 5Gas-Phase Ion-Molecule Chemistry of SubstitutedFerrocenes - (5H4R)Fe(C’)Ferrocene complexes have been studied intensively in the condensed phase over thepast 30 years. Gas-phase studies of this class of compounds have mainly concernedexamination of their mass spectra1-’6.Under special operating conditions, Schumacher andTaubenest observed the ion Cp3Fe2+ in the mass spectrum of ferrocene1’2.The ion wasassigned to a “triple-decker sandwich” structure and considered to be formed by an ion-molecule clustering reaction between the fragment ions of ferrocene and ferrocene molecule.Similarly, the ion Cp3Ni2+was also observed in the mass spectrum of nickelocene’. UsingICR ejection techniques, Foster and Beauchamp9unambiguously identified the condensationreaction of the fragment ion CpFe+ with ferrocene molecule that forms the ion Cp3Fe2+.Initiated by these early mass spectrometric results, some remarkable complexes that have the“triple-decker sandwich” structures have been prepared and characterized’7-24.In this study, the gas-phase ion-molecule reactions of some alkylferrocenes areexamined by the FT-ICR technique. Structures of product ions are discussed by analogy tocondensed-phase observations. In our experiments, a 50 eV x 20 ms electron beam was usedto generate fragment ions from the neutral complexes. Sample pressures were maintained at4.0 x iO- Torr. For simplicity in writing, Cp is used for an unsubstituted cyclopentadienylligand (C5H)and Cp’ is used for a substituted cyclopentadienyl ligand(C5FL).5.1 Ferrocene - Cp2FeFigure 5.1 shows two FT-ICR single resonance mass spectra of Cp2Fe. Spectrum Awas detected immediately after ion formation. Electron impact on Cp2Fe generated threefragment ions: Fe, (mlz 56, 12%), CpFe (mlz 121, 32%) and the molecular ion Cp2Fe(m1z186, 100%). Spectrum B was taken at a delay time of 300 milliseconds after the ion- 141 -formation. Only one ion-molecule clustering reaction product, Cp3Fe2+(m/z 307), appears inthe spectrum.100-A. Reaction time = 0 ms‘-‘80- +— CpFe60-:40-CpFe— Fe. 20- Fe0.— , I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I100 200 300 400100-B. Reaction time = 300 ms‘—‘80- +Cp2Fe60-— 40-+Cp3Fe2!2: CpFeI I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I100 200 300 400Mass in A.M.U.Figure 5.1 FT-ICR single resonance mass spectra of Cp2Fe taken at delay times of 0 (A)and 300 (B) milliseconds. (Pressure = 4.0 x iO Torr; electron beam pulse = 50 eV x 20 ms;Trapping voltage = 1.0 V).FT-ICR multiple resonance ion ejection experiments unambiguously identified thecharge transfer reactions 5.1 and 5.2 that form the ferrocene molecular ion, and the clusteringreaction 5.3 that leads to the formation of Cp3Fe2. The molecular ion does not react with theneutral parent molecule.Fe + Cp2Fe -4 Cp2Fe + Fe (5.1)m1z56 m/z186CpFe + Cp2Fe —> Cp2Fe + CpFe (5.2)m/z 121 in/z 186-142-CpFe + Cp2Fe — Cp3Fe2mlz 121 m1z307(5.3)The product ion, Cp3Fe2+, has long been considered to have a “triple-decker sandwich”structure, 1. Structure 1 compares to the “triple-decker sandwich” cation, Cp3Ni2+,preparedin solution18.1The variation of intensity with time of the three fragment ions and the product ion,Cp3Fe2, is shown in Figure 5.2. The different slopes of the curves indicate a total ionreactivity order: Fe> CpF& > Cp2Fe.0.0.—-C.)C?Na— -2CzC-2.00.0 0.1 0.2 0.3 0.4Reaction Time ( Sec. )0.5 0.6Figure 5.2 Temporal variation of ion abundances in the Cp2Fe system monitored by theFT-ICR single resonance technique.1- 143 -The ion intensities of Fe and CpFe decay exponentially with reaction time, which indicatespseudo-first order reactions of these ions with the ferrocene molecule. From the slopes of thelinear decay curves and the known pressure, rate constants for the disappearance of Fe+ andCpFe are determined to be 5.8 x 10 and 3.7 x i0 cm3 molecule-1s. The former is therate constant of the reaction 5.1, the latter is the sum of the rate constants for reactions 5.2 and5.3. The exponential increase of ion Cp2Fe3 provides an independent determination of therate constant of reaction 5.3, which is measured to be 1.0 x iO cm3 molecule-1s1.Subtracting this from the sum of rate constants of reactions 3.2 and 5.3 gives a rate constantof 2.7 x iO cm3 molecule-1s1 for reaction 5.2. All these results generally agree with theICR examination of ferrocene previously performed by Foster and Beauchamp9.5.2 1,1 ‘-Dimethylferrocene - (Cp’CH3)2Fe5.2.1 FT-ICR Positive Ion Mass Spectrum of 1,1-DimethylferroceneFigure 5.3 shows the FT-ICR positive ion mass spectrum of l,1’-dimethylferrocene.The distribution of ion abundances in the spectrum is listed in Table 5.1. Electron impact onthe neutral molecule (Cp’CH3)2Fegenerates five major metal-containing ions: Fe+ (m1z56),CpFe (mIzl2l), FeCp’CH2(m/z134), CpFeCp’CH2 (m1z199) and the molecular ion(Cp’CH3)2F& (m1z214). As shown in the mass spectrum, the molecular ion dominates. Asa result, other fragment ions only show small relative intensities. Figure 5.4 shows two Fl’ICR double resonance mass spectra in which the molecular ion was initially ejected. Asshown in the Spectrum B of Figure 5.4, fragment ions predominantly undergo charge transferreaction with the neutral parent molecule to form the molecular ion, (Cp’CH3)2Fe+. Someionic products whose mass-to-charge ratios are higher than that of the molecular ion are alsoproduced through ion-molecule clustering reactions. In the ferrocene system, only aL3M2+type product (Cp3Fe2)was observed. In the present system, bothL3M2type andL2Mtype products are formed (L = ligand).-144-100--x 5 Delay time = 0 ms80- I60- CpFeCp’CH2rn— (Cp’CH3)2Fe40- CpFeç. + - FeCpCH Fe20- Fe Iliii_______—CH3-I I I I I I I I I •• -l I I I I 1.1 I I I I I I I I I I50 100 150 200 250 300 350 400Mass in A.M.U.Figure 5.3 FT-ICR positive ion mass spectrum of (Cp’CH3)2Fe. (Pressure = 4.0 x iOTorr; Electron beam pulse = 50eV x 2Oms; Trapping voltage = 1.0 V).Table 5.1 Ion abundances in the Fr-ICR positive ion mass spectrum dataof (Cp’CH3)2Fe.Fragment Ion m/z Relative Jntensity(%)aFe 56 7.5FeH 57 1.5C6H5 77 1.9C6H7 79 2.4C2HFe 81 0.9C2H4Fe 84 0.8CpFe 121 7.4FeCp’CH2 134 11.2FeCp’CH3 135 2.8Cp’CH2FeC 148 1.6CpFeCp’CH2 199 11.5CpFeCp’CH3 200 3.6Cp’CH3FeCp’CH2 213 10.0(Cp’CH)Fe 214 100.0a. Relative to the base peak, (Cp’CH3)2Fe.- 145 -II100-80—60-40-20-1008060400400Figure 5.4 FT-ICR double resonance mass spectra of (Cp’CH3)2Fe. The molecular ion,(Cp’CH3)2Fe,was initially ejected.Figure 5.5 shows the temporal variation of ion abundances monitored by the FT-ICRdouble resonance sequence. The slopes of the linear decay curves indicate the reactivity order,Fe> CpFe> FeCp’CH2CpFeCp’CH2>(Cp’CH3)2Fet for the fragment ions. Thiscompares to the ion reactivity order, Fe+> CpFe+> Cp2Fe+, observed in the ferrocenesystem. Each of the curves for FeCp’CH2and CpFeCpCH2 shows two reactivity slopes,which indicates two isomeric structures for each of these two ions.— FeCp’CHA. Reaction time = 0 msCpFe’Fe+I Ii— CpFeCp’CH20-r’, it’ll . . .50 100 150 2 )0 250. 300 350 4(20200 250Mass in A.M.U.-146--..C?C?N—Cz-1.50.6Figure 5.5 Variation of ion abundances with time of fragment ions and the ion-moleculereaction products in the (Cp’CH3)2Fesystem. The molecular ion (Cp’CH3)2Fewas ejectedinitially.Ions FeCp’CH2 and CpFeCp’CH2 have been noted in the previous massspectrometric studies of (CpCH3)2Fe and other ferrocene derivatives4’7.The Cp’CH2moiety in both ions was assumed to be a six-carbon ring in comparison with the stablecyclopentadienyl-iron-benzene cation (C5HFeC6)in its salts25. That is, ionsFeCp’CH2 and CpFeCp’CH2 were considered to have structures 2 and 4.÷ + .1+ c—CH1ç-CHleFe Fe0.0 0.1 0.2 0.3 0.4 0.5Reaction Time ( Sec. )2 3 4 5-147-To explain the two ion reactivities observed for each of the two ions in Figure 5.5, wepropose that ion FeCp’CH2 is a mixture of 2 and 3, and ion CpFeCp’CH2is a mixture of 4and 5. Ions 2 and 4 would be the less reactive isomers of the two ions since their centralmetal atoms are less electron deficient than the central metal atoms in their counterparts, ions 3and 5. Therefore the intensity curves of FeCpCH2 and CpFeCp’CH2at longer reactiontimes correspond to ions 2 and 4. In addition, while the intensities of ions 2 and 4 decrease,only the intensity of the molecular ion increases (Figure 5.5). This indicates that ions 2 and 4only undergo charge transfer reaction with the parent molecule to give the molecular ion.By extending the curves for FeC6H6, 2, and CpFeC6H6, 4, to zero reaction time,the ratios FeC5H4’CH2/ FeC6Hand CpFeC54’CH2/ CpFeC6Hwere estimated to be0.8 and 0.6, respectively.5.2.2 Ion-Molecule ReactionsFe and CpFeCp’CH2.FT-ICR multiple resonance experiments identify that thetwo ions only undergo charge transfer with the parent molecule, reactions 5.4 and 5.5. Thereactions form the molecular ion, (Cp?CH3)2Fe+, which does not react further.Fe + (Cp’CH3)2Fe — (Cp’CH3)2Fe + Fe (5.4)CpFeCp’CH2 + (Cp’CH3)2Fe — (Cp’CH3)2Fe+ + CpFeCpTH2 (5.5)CpFe. Unlike ions Fe and CpFeCpCH2,CpFe reacts with the neutral parentmolecule to give both a charge transfer product and a condensation product,CpFe2(C ’CH3),reactions 5.6 and 5.7.CpFe + (Cp’CH3)2Fe —* (Cp’CH3)2Fe + CpFe (5.6)m/z 214-4 CpFe2(Cp’CH3)2 (5.7)mlz 335- 148 -10060-40-20-1By analogy to the product ion observed in the ferrocene system, we propose a “triple-decker sandwich” structure, 6, for the product ion CpFe2(C ’CH3).6Cp’CH2Fe. The reactions of FeCp’CH2 with the parent molecule are illustratedby the FT-ICR triple resonance mass spectra shown in Figure 5.6.‘—‘ 80-.—-.—-.—FeCpCH2A. Reaction time = 0 ms0100 200 300 400 500300Mass in A.M.UFigure 5.6 Triple resonance mass spectra of the FeCp’CH2/ (Cp’CH3)2Fesystem.-149-Ion FeCp’CH2+also undergoes a charge transfer reaction with the neutral parentmolecule to form the molecular ion, reaction 5.8.FeCp’CH2 + (Cp’CH3)2Fe —* (Cp’CH3)2Fe + FeCptCH2 (5.8)m/z214Unlike the reaction of CpFe, FeCp’CH2reacts with the parent molecule to give majorproduct ions of bothL3Fe2andL2Fe types (reactions 5.9 to 5.11). These ionic productsare believed to be formed by the reaction of ion 3 since the intensities of the product ionsremained constant at longer reaction times (Figure 5.5). The L3Fe2 type product,(Cp’CH3)2FeCp’tis believed to have a “triple-decker sandwich” structure formed byan ion attack of the methylcyclopentadienyl ligand of the parent molecule.FeCp’CH2 + (Cp’CH3)2Fe — (CpCH3)2FeCp’ (5.9)mlz 134 mlz 348—> (Cp’CH2)3Fe + H2 (5.10)mlz 346—* (Cp’CH2)Fe + C6H + H2 (5.11)m/z 268Elimination of a molecular hydrogen occurs in the latter two reactions, which alsodiffers from the simple association reactions of CpFe+ with ferrocene and 1,1’-dimethylferrocene molecules, reactions 5.3 and 5.7. This indicates that the CH2moiety ofFeCp’CH2+is responsible for the dehydrogenation processes. Scheme 5.1 provides possiblegas-phase ion processes that explain the formations of the ions (Cp’CH2)3Fe and(Cp’CH2)Fe.-150-Scheme 5.1 Formation of ions (Cp’CH2)3Feand (Cp’CH2)Fe.As noted in the earlier chapters, a ligated metal ion can attack the metal center of anorganometallic molecule that bears carbonyl ligands. Loss of carbonyl groups in the processcarries away excess internal energy of the associated ion. Thus a relatively long-lived production containing metal-metal bond can be observed. Elimination of neutral hydrogen may havethe the same energetic effect as ejection of CO groups. Therefore, we believe that the productions (Cp’CH2)3Feand (Cp’CH2)Feformed in the reactions of FeCp’CH2have metal-metal bonding. That is, elimination of hydrogen removes energy and metal-metal bondformation becomes favorable. In the Scheme 5.1, the metal center of ion FeCp’CH2+-H2ç-CH2—CH21Fe—Fe Q-CH29 m/z 346—CH2 1)c:;—CH2—CH21Fe—Fe - ‘p1I HH\)8tCH3710 m/z346CH2\ 1FeFe1 1 m/z 268- 151 -approaches the metal center of dimethylferrocene molecule to form a metal-metal bond, ion 7.The CH2 moiety that has radical character attacks the methyl group to form a carbon-carbonbond, with a hydrogen atom transferring to a metal center simultaneously. The process formsion 8 in which the hydrogen atom bound to iron atom has hydride character and the hydrogenatom of the other methyl group has certain acidity. Therefore, the two hydrogen atomsneutralize to form a neutral hydrogen molecule. After the elimination of a neutral hydrogen,the newly formed CH2 moiety may associate with the metal center to form ion 9, or rearrangewith the Cp ring to form a C6H ring, ion 10. Ion 10 can lose its C6H ring to give ion 11[(Cp’CH2)2Fe2, reaction 5.11] due to the small bond energy ofC6H-Fe bond [D°(FeC6H)= 55 ± 5 kcal / mol.26,D°(Fe-C5H)> 87 ± 6 kcal I mol.2729]. Product ion(Cp’CH2)3Fe2 in reaction 5.10 may have a structure of 9 or 10, or a mixture of each. Thestructural units, 12 and 13, contained in the structures in Scheme 5.1, have beencharacterized in the condensed phase.30’___CH2-C12Fe—Fe5.3 Butylferrocene - CpFeCp’(CH2)3H35.3.1 FT-ICR positive ion mass spectrumFigure 5.7 shows the FT-ICR positive ion mass spectrum of CpFeCp’(CH2)3H.The distribution of ion abundances in the spectrum is listed in Table 5.2. Electron impact onthe neutral molecule CpFeCp’(CH2)3Hgenerates five major metal-containing ions: Fe(m1z56), CpFe (mIzl2l), Cp2Fe (m/z186), CpFeCp’CH2(m/z 199) and the molecular ionCpFeCp’(CH2)3CH3(m/z230).-152-I— 40.——C.)Figure 5.7 FT-ICR positive ion mass spectrum of CpFeCp(CH2)3H.(Pressure = 4.0 xiO Torr; Electron beam pulse = 50eV x 20 ms; Trapping voltage = 1.0 V).Table 5.2 Ion abundances in the FT-ICR positive ion mass spectrum ofCpFeCp’(CH2)3H.Fragment Ion m/z Relative Intensity(%)bC4H7 55 5.7Fe 56 10.1C6H7 79 3.3C7H 91 3.8CpFe 121 45.1FeCp’CH2 134 5.0CpF& 186 34.5CpFeCp’CH2 199 100CpFeCp’CH3 212 3.1CpFeCp’C25 214 3.1CpFeCp’C3H4 225 4.9CpFeCp’C4H7 240 6.5CpFeCp’C4H9 242 67.48060Delay time = 0 ms+CpFe20 +Fe0CpFeCpC4H9II I I+Cp2Fe100‘ç7- (CHCH3150 200 250.Mass in A.M.U.300 350 400a Relative to the base peak, CpFeCp’CH2.- 153 -The major fragment ions are formed from a cleavage of the butyl chain (a and Ii), and from acleavage of the unsubstituted-cyclopentadienyl iron bond. The a cleavage involves ahydrogen transfer from the butyl chain to its Cp ring that forms a molecular ion of ferrocene,Cp2Fe. Unlike the mass spectrum of dimethylferrocene, ion FeCp’CH2is only formed in asmall amount relative to the intensity of CpFe. In addition, ion FeCp’C4H9(mlz 177) isnot formed. These data tend to indicate that the Fe-Cp bond is stronger than the Fe-Cp’C4H9bond or the Fe-Cp’CH2 bond in ion CpFeCp’CH2.a(185) 13(199)CH- CH2CH2-CH31211 Fe 1775.3.2 Ion-Molecule ReactionsCp2Fe and CpFeCp’CH2. Fr-ICR multiple resonance experiments identify thatthe two ions undergo charge transfer reaction only; reactions 5.12 and 5.13, to form themolecular ion, CpFeCp’C4H9.Cp2Fe + CpFeCpC4H9 — CpFeCp’C4H9 + Cp2Fe (5.12)CpFeCp’CH2 + CpFeCp’C4H9 —* CpFeCp’C4H9 + CpFeCp’CH2 (5.13)CpFe. Figure 5.8 shows two FT-ICR triple resonance mass spectra that illustratethe reactions of CpFe withCp2Fe2CpC4H9Similar to its reactions in the ferrocene systemand the dimethylferrocene system, ion CpFe reacts with CpFeCp’C4H9to form a chargetransfer product, CpFeCp’C4H9and a condensation product,Cp2Fep’C reactions5.14 and 5.15. The molecular ion, CpFeCp’C4H9undergoes further fragmentation to formCpFe and CpFeCpCH2ions.-154-‘- 80-: 60-C40-20-0-Figure 5.8 Triple resonance mass spectra of the CpFe I CpFeCp’C4H9system.A dehydrogenated ion, FeCp’C4H7(mlz 175), is also formed in the system, reaction 5.16.Its curve of intensity versus time in Figure 5.9 shows that the ion reacts further withbutylferrocene molecule to form a condensation product ion, CpFe2(Cp’C4H9)(Cp’C7(mlz 417); reaction 5.17.CpFe + CpFeCpC4H9 —* FeCp’C4H7 + Cp2Fe + H2mlz 175100CpFe + CpFeCpC4H9 -4 CpFeCp’CH9+ CpFe (5.14)m/z 242—* Cp2Fe2’C4H9 (5.15)m/z 363CpFeA. Reaction time = 0 ms100100200I I [1 I I I300 400IMass in A.M.U.(5.16)- 155 -.—C?-.C?—EzFeCp’C4Hr + CpFeCpC4H9 -4 CpFe2(Cp’C4H9)(CpH7 (5.17)m)z4170.0-0.5-1.0-1.5-2.0.Figure 5.9 Temporal variation of ion intensities for CpFe reacting with CpFeCpC4H9.The L3Fe2 type product,Cp2FeCp’C4H9t is proposed to have a “triple-deckersandwich” structure 14 or 15, or a mixture of each. The dehydrogenated product ion,FeCp’C4H7+,is proposed to have structure 16, formed by the ion processes shown inScheme 5.2.0.0 0.1 0.2 0.3 0.4 0.5 0.6Reaction Time (Sec.)114 15- 156-H2 H2H2C7H H2CCHA \\ \\/r’\ / CH2 /r\ / CH2U 4I—Fe U 4i—FeMScheme 5.2 Formation of ion FeCp’C4H7.Scheme 5.2 shows that assisted by the butyl chain carbon, a neutral ferrocene can beeliminated from ion 15. The resultant ion undergoes a 3-hydride transfer and reductiveelimination of a molecular hydrogen to form ion 16, FeCp’C4H7.This reaction scheme isanalogous to the reaction mechanisms that have been well established for gas-phase reactionsbetween bare and ligated metal ions and hydrocarbon molecules3237.5.4 ct-Methylferrocenemethanol - CpFeCp’CH(OH)CH35.4.1 FT-ICR positive ion mass spectrum of CpFeCp’ CH(OH)CH3Figure 5.10 shows the positive ion mass spectrum of CpFeCp’CH(OH)CH3.Thedistribution of ion abundances is listed in Table 5.3. Electron impact on the neutral moleculeCpFeCp’CH(OH)CH3generates seven major metal-containing ions: Fe (m1z56), CpFe(mIzl2l), CpFeOH (mlz 138), FeCp’C2H3(mlz 147), Cp2Fe (m1z186), CpFeCp’C2H3(mlz 199) and the molecular ion CpFeCp’(CHH(m1z230). These fragment ions areidentical to those measured by Egger3.H2H2C’CH2FiOFe1 - Cp2Fe-H216- 157 -100-Delay time = 0 ms80- CpFeCp’C2H3-CpFeCp’CH(OH)CH360- /7— 40- CpFe C+ ç?— (H— CH3PFe(OH)_Fe OH.—-I. 20- +Fe Ii i I IiI II I______________________________I_,— I I I I i I I I I I I I I J ••l• I I I I I I I I I I I I I I50 100 150 200 250. 300 350 400Mass in A.M.U.Figure 5.10 FT-ICR positive ion mass spectrum of CpFeCp’CH(OH)CH3(Pressure =4.0 x iO Torr; Electron beam pulse = 50eV x 2Oms; Trapping voltage = 1.0 V).Table 5.3 Ion abundances in the FT-ICR positive ion mass spectrum ofCpFeCp’CH(OH)CH3.Fragment ion m/z Relative Intensity (%)Fe 56 17.8FeOW 73 3.3C7H 91 4.9CpF& 121 37.4FeCp’CH2 134 5.5CpFeOW 138 23.8FeCp’C2H3 147 16.3FeCp’CH(OH)CH 165 3.1Cp2Fe 186 10.9CpFeCp’CH3 212 100CpFeCp’CH(OH)CH3 230 56.7a. Relative to the base peak, CpFeCp’C2H3.As shown in Figure 5.10 and illustrated in Scheme 5.3, the major fragmentation of themolecular ion is loss of water giving CpFeCp’C2H3.Other important fragmentations includeloss of the unsubstituted ring, cleavage of the substituted ring leaving CpFe, and cleavage of-158-the substituted ring accompanied by a migration of the hydroxyl group to the iron atom givingthe fragment ion CpFeOH.÷ cc-CH=CHl1 2—-iI____—CH-CH2Fe i OHHmlz 147 Fe________m1z165 m/z212Imlz 147 + +- C2H m/z 230m/z121 OHm/z 138Scheme 5.3 Fragmentation of CpFeCp’ CH(OH)CH3The dehydration process has been previously confirmed by metastable ion monitoringin both hydroxyl-alkyl ferrocene derivatives and x-hydroxyl-alkyl biferrocene derivatives3’10.Deuteration studies have shown that the water is eliminated by a 1,2-mechanism3.By analogyto the classical dehydration reactions of organic alcohols when treated with an acid, we believethat the dehydration produces a vinyl group. That is, ion CpFeCp’C2H3is a stablemolecular ion of vinylferrocene. This explains its large intensity in the mass spectrum.Cleavage of the unsubstituted ring forms ion FeCp’CH(OH)CH3(mlz 165) which undergoesthe the same dehydration reaction to form ion FeCpCHCH2,i.e.,C7HFe. Spilners7hadassigned the structure of theC7HFe+ ion found in the fragmentation of alkylferrocenes to atropylium-iron structure. Metastable ion monitoring indicated thatC7HFeejected C2H toform CpFe, which is consistent with the typical fragmentation of uncoordinated tropyliumj3839• This also compares to the fragmentation processes,C7Hr —C5Hr+ C2H2-159--.—SzCandC7Hr(OCH3)—+C5Hr(OCH3)+ C2H,observed in the(ri6-c-C7H8)Cr(CO)3system (Chapter 2). TheC7HFe ion was also observed in the mass spectrum of 1,1’-divinylferrocene (Cp’CHCHFeCp . Using deuterium-labeled 1,1’-divinylferrocene-d2, Roberts6confirmed the presence of a tropylium-iron structure for the ion.Therefore, a tropylium-iron structure is assigned to the ion FeCp’C2H3+in the presentsystem. This structure assignment is also consistent with the ring expansion reactionobserved in the (Cp’CH3)2Fesystem (Section 5.2) and the coordination expansion in the (q6-c-C8H)Fe( O)3(Chapter 4). That is, when the central metal atom becomes electrondeficient, its organic ligand tends to rearrange to decrease the electron deficiency. Figure 5.11shows temporal intensity variation of major fragment ions.-0.4-0.6-0.8-1.0-1.2-1.4-1.6-1.8-2.0Figure 5.11 Variation of ion abundances with time of the major fragment ions and productions in the CpFeCp’CH(OH)CH3system. The molecular ion CpFeCp’CH(OH)CH3and ionCpFeCp’C2H3were initially ejected from the reaction cell.0.4 0.6Reaction Time (Sec.)-160-The ion reactivity ofC7HFe is lower than that of CpFe. This also indicates that ionC7HFehas a tropylium-iron structure since an ion with a(15-c-C5H42H3)Festructure isexpected to show the the same reactivity as ion CpFe. Figure 5.11 also shows the ionreactivities of the two major L2Fe2 type products CpFe2’CH3and CpFe2’CHThelatter ion is more reactive than the former ion.5.4.2 Ion-Molecule ReactionsCpFeCp’C2H3.Figure 5.12 shows a FT-ICR triple resonance mass spectrum thatshows the reactions of ion CpFeCp’C2H3(mlz 212) with the neutral parent molecule,CpFeCp’CH(OH)CH3.Charge transfer is a dominate reaction that forms the molecular ion.An ion with m/z 213 (CpFeCp’C + H) is also formed. It is not possible to determine ifthe ion is formed by a hydrogen atom abstraction from the neutral parent molecule byCpFeCp’C2H3or by an intermolecular dehydration reaction between CpFeCp’C2H3andthe parent molecule. As shown in the Figure 5.13, ion CpFeCp’C2H3(H)does not reactfurther. This tends to support the hydrogen abstraction reaction that forms a protonatedvinylferrocene molecular ion.100-Reaction time = 1000 ms80- CpFeCp’CH(OH)CH360- CpFeCpC2H3— +CpFeCp’C2H3(H)40-20-0— I I I I I I I I I — I I I I I I I I50 100 150 200 250 300 350Mass in A.M.U.Figure 5.12 FT-ICR triple resonance mass spectrum of the CpFeCp’C2H3/CpFeCp’CH(OH)CH3system.- 161 -0.0.—-..E -1.0CzC-1.5Figure 5.13 Temporal variation of ion abundances for CpFeCp’C2H3reacting withCpFeCp’CH(OH)CH3.Fe+. Figure 5.14 shows the double resonance mass spectra of the ion taken at areaction time of 800 milliseconds. Figure 5.15 shows the temporal intensity variation of Feand its major ion-molecule reaction product ions.Reaction Time ( Sec. )100-SCC—.—-S- Reaction time = 800 ms— CpFeCp’CH(OH)CH380 -60- CpFeCp’C2H3—CpFeCp’C2H3(H)40 -CpFe2’CH3+‘V....20-Fe0- i ‘ ‘ * , I I I I I I50 100 150 200I I I250 300Mass in A.M.U.Figure 5.14 FT-ICR double resonance spectrum of the Fe / CpFeCp’CH(OH)CH3system.350-162-.—CC.——2CzC3.0Figure 5.15 Temporal variation of ion abundances for Fe+ reacting withCpFeCp’CH(OH)CH3.The charge transfer reaction of Fe+ (reaction 5.18) produces the molecular ion,CpFeCp’CH(OH)CH3,which eliminates water to form CpFeCp’C2H3(reaction 5.19).The later ion interact with the parent neutral to form the ion peak at 213. A clustering production, CpFe2p’C2H3 (mlz 268), is also formed (reaction 5.20).Fe + CpFeCp’CH(OH)CH3 -4 CpFeCp’CH(OH)CH + Fe (5.18)mlz 2301-CpFeCp’C2H3+ H20 (5.19)mlz 212—> CpFe2’CH3+ H20 (5.20)mlz 268CpFe+. Figure 5.16 shows the double resonance mass spectra of the ion taken at areaction time of 500 milliseconds. Figure 5.17 shows the temporal intensity variation ofCpFe+ and its major ion-molecule reaction product ions.0.0 0.5 1.0 1.5 2.0 2.5Reaction Time ( Sec. )- 163 -CpFe2pC2H/Reaction Time ( Sec. )Figure 5.17 Temporal variation of ion abundances for CpFe+ reacting withCpFeCp’CH(OH)CH3.I100806040200Reaction time = 500 msCpFe CpFeCp’C2H3CpFeCp’C2H3(H)mlz 33350 100 150 200 250 300Mass in A.M.U.Figure 5.16 FT-ICR double resonance mass spectrum of the CpFe /CpFeCp’CH(OH)CH3 system..—z—N.——CzC-164-In addition to the predominate charge transfer reaction (reaction 5.21), the ion alsoundergoes clustering reactions with the parent molecule to form bothL2Fe2 andL3Fe2typeproducts (reactions 5.23 and 5.24). As shown in the Figure 5.17, theL2Fe type product,CpFe2Cp’C2H2t undergoes further reaction with the parent molecule. Product ionscontaining three iron atoms are not formed. This indicates that the ion only undergoes chargetransfer reaction with the parent neutral.CpFe + CpFeCp’CH(OH)CH3 —* CpFeCp’CH(OH)CH3+ CpFe (5.21)m/z 230.1CpFeCp’C2H3+ H20 (5.22)m/z212—* CpFe2’CH + H20 + C5H6 (5.23)m/z 267-4 Cp2Fep’CH3+ H20 (5.24)m/z 333The L3Fe2 type product ion, Cp2Fep’CH3(reaction 5.24), is still proposed to havetriple-decker sandwich structure, 17 or 18.1In ferrocene, dimethylferrocene and butylferrocene systems, Fe+ only underwentcharge transfer reaction with the parent molecule and ion CpFe only formed L3Fe2 typeproducts. Compared to these observations, the vinyl group formed by dehydration tends to becrucial for the formation of theL2Fe type product ions CpFe2’CH3(reaction 5.20) andCpFe2’CH(reaction 5.23). In addition, Figure 5.11 and Figure 5.15 show that ionCpFe’C3+does not show apparent reactivity towards the parent molecule. In contrast,ion CpFe2’CHundergoes further reactions (Figure 5.11 and Figure 5.17). Structures17 18- 165 -19 and 22 are proposed for the product ions CpFe2’CH3(reaction 5.20) andCpFe2’CH(reaction 5.23), respectively. The central metal atoms in 22 are moreelectron deficient than those in 19. This explains the small reactivity observed for ionCpFeCp’C2H2t 22.7 CCH2 1Fe Fe19H- C5H6___Fe Fe22As shown in Figure 5.10 and Scheme 5.3, dehydration is the major fragmentationprocess of the parent molecule that forms a vinyl group. The molecular ions formed by thecharge transfer reactions of Fe+ and CpFe+ also undergo dehydration to form a molecular ionof vinylferrocene. We believe that the coordination capability of vinyl group formed after thedehydration holds Fe and CpF& to formL2Fe2 type products 19 and 22. In addition, oneof the iron atoms may also insert into a C-H bond of the Cp ring to form a bridgingi5,r—cyclopentadienyl ligand. The cyclopentadiene ligand in ion 21 is easily lost to give ion 22,due to a small bond energy ofC5H6-Fe bond [D°(F&-C5H6)= 55 ± 5 kcal / mol., D°(FeC5H)> 87 ± 6 kcal / mol.2729]. Multi-metallic complexes containing a bridgingr5,r ..cyclopentadienyl group have been characterized in the condensed phase40. The conversionfrom 20 to 21 is analogous to the following known condensed-phase process45.H 1+CH2I20 21-166-5.5.1 FT-ICR positive ion mass spectrum of CpFeCp’CHCH2Figure 5.18 shows the FT-ICR positive ion mass spectrum of CpFeCp’CHCH2.Thedistribution of ion abundances in the spectrum is listed in Table 5.4. Electron impact on theneutral molecule CpFeCp’CHCH2generates four major metal-containing ions: Fe+ (m1z56),CpFe (mIzl2l), Cp2Fe (m1z186) and the molecular ion CpFeCpC2H3(m1z199). Figure5.19 shows the temporal behavior of the major fragment ions and their ion-molecule reactionproducts.100.-‘—‘ 80--‘.—t 60-— 40-—Figure 5.18 FT-ICR positive ion mass spectrum of CpFeCpCHCH2.(Pressure = 4.0 xio Torr; Electron beam pulse = 50eV x 2Oms; Trapping voltage = 1.0 V).5.5 Vinylferrocene - CpFeCp’CHCH2CpFeCp’CHCH2—.-Cp2FeI I20-CpFe—+Fe0Delay time = 0 msCH CH2I—• J50 100 150 200 250 300 350 400Mass in A.M.U.-167-.—N.——ECzCTable 5.4 Ion abundances in the FT-ICR positive ion mass spectrumof CpFeCp’CHCH2.Fragment ion m/z Relative Intensity (%)Fe 56 13.2C5H 65 2.0C6H5 77 3.9C7H 91 5.3FeC3H2 95 3.2CpFe’ 121 50.0FeCp’CH2 134 8.4FeCp’C2H3 147 3.4Cp2Fe 186 12.4CpFeCp’CH3 212 1001.0Figure 5.19 Variation of ion abundances with time of the major fragment ions and productions in the CpFeCp’CHCH2system. The molecular ion CpFeCp’CHCH2was initiallyejected from the reaction cell.0.0 0.2 0.4 0.6 0.8Reaction Time (Sec.)-168-5.5.2 Ion-Molecule Reactions50 100 150‘I • I•200 250Mass in A.M.U.(5.25)(5.26)300 350 400Figure 5.20 FT-ICR double resonance mass spectrum of the Fe / CpFeCp’CHCH2system.CpFe+. Figure 5.21 shows the double resonance mass spectra of the ion taken at areaction time of 200 milliseconds. In addition to the predominate charge transfer reaction(reaction 5.27), the ion also undergoes clustering reactions with the parent molecule that formbothL2Fe and L3Fe2 type products (reactions 5.28 and 5.29).CpFeCp’CHCH2+ CpFem/z 212—> CpFe2Cp’C2H2 ÷ C5H6m/z 267—* Cp2Fe2Cp’CHCHm/z 333Fe+. Figure 5.20 shows the double resonance mass spectra of the ion taken at areaction time of 300 milliseconds. The charge transfer reaction of Fe+ with the parent neutralproduces the molecular ion, CpFeCpCHCH2(reaction 5.25). A clustering product ion,CpFe2’CH3(mlz 268), is also formed (reaction 5.26).Fe + CpFeCp’CHCH2—> CpFeCp’CHCH2 + Fem1z212-4 CpFe2p’CHCHm/z 268100IReaction time = 300 ms80- CpFeCp’C2H3—..60 -40 -20 - ,Fe CpFe2’CH30CpFe + CpFeCp’CHCH2—4 (5.17)(5.28)(5.29)-169-I400Figure 5.21 FT-ICR triple resonance mass spectrum of the CpFe/CpFeCp’CHCH2system.Except for the molecular ion of -methylfeffocenemethanol, all the product ionsobserved in the present system are exactly the the same as those in the amethylferrocenemethanol system. As shown in Figure 5.19, product ions also show similarreactivities as in the last system. Therefore, these product ions are believed to have the samestructures as the product ions in the a-methylferrocenemethanol system, i.e., ions 17 to 22.These similarities, in turn, support the structural analysis in the last system.6.6 Summary and ConclusionsFragment ions of substituted ferrocene mainly undergo charge transfer reactions withtheir neutral parent molecules to form molecular ions. They also undergo ion-moleculeclustering reactions that generate product ions of both L3Fe2andL2Fe types in smalleryields. Ion reactivities are once again observed to vary with the electron deficiencies of centralmetals. A larger reactivity corresponds to a higher electron deficiency. Each of ionsFeC5H4CH2(Section 5.2.1) and FeC5H4C23(Section 5.4.1) is observed to have asmaller reactivity than that of ionC5HFe, which is consistent with the benzene-iron andtropylium-iron structures previously assigned to these two ions. TheL3Fe2+type productsMass in A.M.U.-170-are generally proposed to have “triple-decker sandwich” structures by analogy to previousgas-phase and condensed-phase results. During the formations L2Fe+ type products,elimination of neutral molecules such as H2,C6H,H20, and C5H6 occurs. The neutralelimination removes excess energy from the ion-molecule associated intermediate, which issimilar to organo-metal carbonyl systems where neutral CO groups are generally eliminated.This is believed to be the reason why theL2Fe type products are sufficiently observed in thesubstituted ferrocene systems but not observed in the ferrocene system. In addition, reactionsof Fe with dimethylferrocene and butylferrocene do not produce theL2Fe type products.Whereas, theL2Fe+type products are formed in the (x-methylferrocenemethanol system andthe vinylferrocene system by the reactions of Fe+. This is explained by the coordinationcapability of the vinyl group that holds the metal atom in the cluster ions. Similar to theproducts formed in organo-metal carbonyl systems, the L2Fe2+ type products are believed tohave metal-metal bonds. This is supported by both the ion reactivity observations and knownstructures of multi-metallic complexes.References(1) Schumacher, E.; Taubenest, R. Helv. Chim. Acta 1964, 47, 1525.(2) Schumacher, E.; Taubenest, R. Helv. Chim. Acta 1966,49, 1447.(3) Egger, H. Monatsh. Chem. 1966, 97, 602-618.(4) Roberts, D. T. 3.; Little, W. F.; Bursey, M. M. J. Am. Chem. Soc. 1967, 89, 6156-6164.(5) Roberts, D. T. J.; Little, W. F.; Bursey, M. M. J. Am. Chem. Soc. 1967, 89, 4917-4923.(6) Roberts, D. T. J.; Little, W. F.; Bursey, M. M. J. Am. Chem. Soc. 1968, 90, 973-975.(7) Spilners, I. 3.; Larson, 3. G. Org. Mass Spectrom. 1970, 3, 915 - 924.- 171 -(8) Cais, M.; Lupin, M. S. In Advances in Organometallic Chemistry; F. G. A. Stone andR. West, Ed.; Academic Press: New York, 1970; Vol. 8; pp 211 - 334.(9) Foster, M. S.; Beauchamp, J. L. J. Am. Chem. Soc. 1975, 97, 4814-4817.(10) Hisatome, M.; Ichida, S.; Yamakawa, K. Org. Mass Spectrom. 1976, 11, 31- 39.(11) Muller, J. In The organic Chemistry of Iron; E. A. K. V. Gustorf, F. W. Grevels andI. Fisehier, Ed.; Academic Press: New York, 1978; Vol. Volume 1; pp 145 - 173.(12) Mysov, E. I.; Lyatifov, I. R.; Materikova, R. B.; Kochetkova, N. S. J. Organomet.Chem. 1979, 169, 301-308.(13) Zhuk, B. V.; Domrachev, G. A.; Semenov, N. M. J. Organomet. Chern. 1980, 184,231-236.(14) Rapic, V.; Filipovic-Marinic, N. Org. Mass Spectrom. 1985, 20, 688- 689.(15) Lay, J. 0. 3.; Allison, N. T.; Yongskulrote, W.; Ferede, R. Org. Mass Spectrom.1986, 21, 371 - 374.(16) Barfuss, S.; Hirschwald, K. H. E.; Dowben, P. A.; Boag, N. M. J. Organomet.Chem. 1990, 391, 209-2 18.(17) Maslowsky, J. E. J. Chem. Edu. 1978, 55, 276-280.(18) Werner, H.; Salzer, A. Sythn. Inorg. Met.-Org. Chem. 1972, 2, 239.(19) Saizer, A.; Werner, H. Angew. Chem., mt. Ed. Engi. 1972, 11, 930.(20) Werner, H. J. Organomet. Chem. 1980, 200, 335.(21) Stelle, S.; Floriani, C.; Chiesa-Villa, A.; Guastini, C. Angew. Chem. 1987, 99,(22) Stelle, S.; Floriani, C.; Chiesa-Villa, A.; Guastini, C. Angew. Chem., mt. Ed. Engi.1987, 26,(23) Tremel, W.; Hoffmann, R.; Kertesz, M. J. Am. Chem. Soc. 1989, 111, 2030-2039.(24) Chase, K. J.; Bryan, R. F.; Woode, M. K.; Grimes, R. N. Organometallics 1991,10, 2631-2642.(25) Nesmeyanov, A. N.; Volkenau, N. A.; Bolesova, I. N. Tetrahedron Letter 1963, 25,1725.-172-(26) Hettich, R. L.; Jackson, T. C.; Stanko, E. M.; Freiser, B. S. J. Am. Chem. Soc.1986, 1986, 5086-5093.(27) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 2605 - 2612.(28) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 5876 - 5883.(29) Huang, Y.; Freiser, B. S. J. Am. Chem. Soc. 1991, 112, 5085-5089.(30) Wiess, E.; Hubel, W.; Merenyi, R. Chem. Ber. 1962, 95, 1155.(31) Behrens, U.; Wiess, E. J. Organomet. Chem. 1975, 96, 399.(32) Freiser, B. S. Anal. Chim. Acta 1985, 178, 137-158.(33) Allison, J. Prog. Inorg. Chem. 1986, 34, 627-677.(34) Freiser, B. S. In Techniques for the Study of Ion Molecule Reactions; J. M. Farrarand W. H. Saunders, Ed.; Wiley: New York, 1988; Vol. 20; pp 61-118.(35) Freiser, B. S. Chemtracts-Analyt. Phys. Chem. 1989, 1, 65-109.(36) Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum: New York, 1989, p412.(37) Eller, K.; Schwarz, H. Chem. Rev. 1991, 91, 1121-1177.(38) Ausloos, P. J. Am. Chem. Soc. 1982, 104, 5259-5265.(39) McLafferty, F. W.; Bockhoff, F. M. J. Am. Chem. Soc. 1979, 101, 1783-1786.(40) Hoxmeier, R.; Deubzer, B.; Kaesz, H. D. J. Am. Chem. Soc. 1971, 93, 536-537.(41) Guggenberger, L. J. Inorg. Chem. 1973, 12, 294.(42) Baker, E. C.; Yaymond, K. N.; Marks, T. J.; Wachter, W. A. .1. Am. Chem. Soc.1974, 96, 7586-7588.(43) Pez, G. P. J. Am. Chem. Soc. 1976, 98, 8072.(44) Wielstra, Y.; Gambarotta, S.; Spek, A. L.; Smeets, W. J. J. Organometallics 1990,9, 2142-2148.(45) Powell, P. Principles of Organometallic Chemistry; 2nd ed.; Chapman and Hall: NewYork, 1988, p 283.- 173 -CHAPTER 6Formation and Reactivity of Negative Organometallic Ions6.1 IntroductionGas-phase transition-metal negative ion chemistry is studied widely by massspectrometric techniques. While the negative ions are typically less reactive than theirpositive-ion counterparts, they do exhibit both familiar reactivity patterns and new behaviorthat is uniquely their own.Formation of negative ions. Transition-metal negative ion fragments can bederived from a variety of mass spectrometric ion generation techniques. Low-energy (< 15eV) electron impact ionization (El) is the most commonly used method that can produce bothmolecular ions and smaller fragment ions via electron attachment and dissociative electronattachment processes (AB + e -4 K + B or B + A, where AB is a diatomic molecule).1’218-electron mononuclear metal carbonyl complexes generally yield 17-electron negativefragment ions M(CO) by losing a CO group, reaction 6.1. Molecular ions, M(CO), areusually not observed.M(CO) —> M(CO).f + CO (6.1)Similarly, transition-metal carbonyl complexes bearing other ligands such as hydrogen,halogen, or alkyl also ionize by a dissociative electron attachment process in which mainlyneutral CO groups are expelled. In contrast, some olefm-metal-carbonyl complexes can alsoundergo simple electron attachment processes to form molecular ions in relatively largeamounts. Typical examples are the molecular ions of butadiene iron tricarbonyl,3’4C4H6Fe(CO)3(1) and cycloheptatriene iron tricarbonyl,5fl4-c-C7H8Fe( O)3(2) observed byIT’Fe(CO)3Fe(CO)31 2-174-magnetic sector instruments. The molecular ion,C4H6Fe(CO)3of complex 1 was reportedto have a lifetime of 80 p.s with respect to autodetachment.3Such an ion was considered to belong-lived since its lifetime is comparable with those of gas-phase organic ions, and thus canbe detected by the mass spectrometers with short time scales.1 The long lifetime, i.e., why itwas formed and detected, was rationalized by Blake1’4in terms of a 17-electron structure, 3,with the negative charge localized on carbon atom rather than on the metal atom, together withan—q3 coordination reduction process, reaction 6.2.C’H2(6.2)Fe(CO)3 Fe(CO)31 (18-electron) 3 (17-electron)However, Squires and Wang2’6rationalized the same observation in terms of another 17-electron complex(1-C4H)Fe(CO)3,4, with the negative charge localized on the metalatom, by analogy with condensed phase7’8observations.CHCH2e CH2,=CH (6.3)Fe(CO)3 Fe(CO)31 (18-electron) 4 (17-electron)Here, coordination reduction from i to 2 occurs to accommodate the extra electron. Similarto the structure 3 in reaction 6.2, formation of the negative molecular ion of complex 2 wasrationalized by the structure 5 in reaction 6.4. Each of the negative charges of 3 and 5 wasproposed to be able to delocalize to metal center to form an 18-electron anion, leaving a radicalsite on the ligand.1’5e (6.4)Fe(CO)3 Fe(CO)32 (1 8-electron) 5 (17-electron)In an ICR study of the 18-electron complex -CpCo(CO)2,(Cp = cyclopentadienyl),- 175 -6, the molecular ion CpCo(CO)2 (84%) and the fragment ion CpCo(CO) (16%) (17-electron)were the only two fragment ions generated using electron beam energy as high as 70 eV.9Similarly to the ion 4 in reaction 6.3, the abundant molecular ion had been early formulated as(r13-C5H)Co(CO)2,7, formed by an T to ri3 coordination reduction process, reaction 6.5,rather than a 19-electron species(5-Cp)Co(CO)j.10QCo(CO)2 e Co(CO)j (6.5)Ion-molecule reactions of negative transition-metal ions. Most earlystudies on negative ion-molecule clustering reactions were on transition metal carbonylsystems. In 1973, Dunbar, Ennever and Fackler,11 who used an ICR instrument, firstreported the ion-molecule reactions in Fe(CO)5,Cr(CO)6and Ni(CO)4 systems. Reactions6.6 to 6.9 were identified by ICR double resonance experiments.Fe(CO)3 + Fe(CO)5 —> Fe2(CO)6 + 2C0 (6.6)Cr(CO)3 + Cr(CO)6 — Cr2(CO)6 + 3C0 (6.7)Cr(CO)4 + Cr(CO)6 —> Cr2(CO)8 + 2CC (6.8)Ni(CO)3 + Ni(CO)4 —* Ni2(CO)6 + CO (6.9)The authors suggested thatNi2(CO)6 had a single metal-metal bond in its structure by analogywith the known metal-metal bonded anionNi2(CO)6.12 The other three ions were suggestedto have multiple metal-metal bonds. Cr2(CO)8,for example, was suggested to have a CrCrbond so that each of the two chromium atoms has either 17-electrons or 18-electrons.Subsequent ICR studies13’4of Fe(CO)5 reaffirmed reaction 6.6 as the principle clusteringreaction in the system, as well as the apparent non-reactivity of Fe(CO)4 (17-electron species)towards clustering with the parent molecule noted earlier by Dunbar. Wronka and Ridge15measured the relative reactivity of each ion in the Fe(CO)5system. Their results revealed arelationship between ion reactivity and electron deficiency, which is: the greater the electron-176-deficiency, the greater the reactivity of an ion. Those dimetallic product ions showinganomalously low reactivities such as Fe2(CO)5,6,7were assigned to have double metal-metalbonds by the authors. Trimetallic and tetrametallic product ions were suggested to have singlemetal-metal bonds in triangular and tetrahedral frameworks.Clustering reactions in(fl5-Cp)Co(CO) system were described by Corderman andBeauchamp.9While the molecular ion CpCo(C0)2 does not react with the parent molecule,the monocarbonyl fragment ion CpCo(C0) reacts to form a dimetallic product ion[CpCo(CO)]2,reaction 6.10.CpCo(CO) + CpCo(CO)2 -4 [CpCo(CO)]2 + CO (6.10)This product ion was assigned to a symmetrical CO bridging, partial metal-metal bondedstructure, 8, in analogy with the same dimer anion prepared and characterized in solution.18With a partial double metal-metal bond in its structure, ion 8 is an 18-electron species anddoes not react further with(fl5-Cp)Co(CO)2.8 (18-electron)Most other studies focus on negative ion-molecule reactions with other small inorganicand organic molecules such as H2,’9D2,’9H2S,°‘3C0,2’NH3,22 NO,9 PF39 O2,2224CH3O ,2°and CH3N25. Reactions of negative transition-metal ions with organichydrocarbons were also reported recently.2’629As negative organometallic ions generally show fewer ion-molecule clusteringreactions than positive ions, the complexes examined by FT-ICR in this research are discussedtogether in this chapter. They are: butadiene iron tricarbonyl(i4-CH6)Fe(CO)3(1),cyclohexadiene iron tricarbonyl (i4-c-C6H8)Fe(CO)3(9), cycloheptatriene chromiumCII0- 177 -tricarbonyl (r16-c-C7H8)C (CO)3 (10), cyclooctatetraene iron tricarbonyl (rj4-c-C8H)Fe(CO)3 (11) and cyclohexadienone iron tricarbonyl(T14-c-C6H0)Fe(CO)3(12).Fe(CO)3 Fe(CO)3Cr(CO)3 Fe(CO)39 10 11 12To study negative ions, the hardware of FT-ICR does not need further modifications.Ions can be trapped in the reaction cell by simply switching the trapping voltage from +1 Voltto -1 Volt. Low-energy Fl-ICR electron beams (0- 5 eV) were used to produce negative ionsfrom the above complexes. Sample pressure was kept at —i0 Torr range.6.2 Negative Ion Mass Spectra of(i4-CH6)Fe(CO)3,(14-c-C6H8)Fe(CO)3and(‘ri6-c-C7H8)Cr(CO)3Low-energy electron impact on butadiene iron tricarbonyl, (T14-CH6)Fe(CO)31,only generated one negative ion, (T14-CH6)Fe(CO)2 (17-electron species), which is inaccordance with the observations of Wang and Squires6 who used a FA (flow afterglow)apparatus. The molecular ion Cr13 or -C4H6)Fe(CO)3 (see reactions 6.2 and 6.3) was notobserved. This is because the time scales of FT-ICR and FA are longer than those ofmagnetic sector instruments and the lifetime of the molecular ion is shorter (80 .ts)3Therefore, it can not be detected by FT-ICR or FA instruments but can be detected bymagnetic sector instruments.3’4The 17-electron fragment ion(4-CH6)Fe(CO)j was foundto be unreactive with the parent molecule.Low-energy electron impact on cyclohexadiene iron tricarbonyl,(fl4-c-C6H8)Fe(CO)39, generated the fragment ionC6H8Fe(CO)2 (100%) and the molecular ionC6H8Fe(CO)3(4.0%). This compares to the Fe(CO) (66%),C6H8Fe(CO)3 (33%)4 and theC6H8Fe(CO)2 (100%),C6H8Fe(CO)3 (6.0%) ion distributions observed previously byusing magnetic sector instruments. Both ions are nonreactive towards ion-molecule clustering- 178 -reactions with the parent molecule. By analogy with reactions 6.2 to 6.5, the molecular ionC6H8Fe(CO)3 is proposed to have an i3 structure (13) or an r2 structure (14), and thefragment ionC6H8Fe(CO)2 retains the ri coordination form (15) as in the neutral parentcomplex. Both C6H8Fe(CO)3 andC6H8Fe(CO)2 are 17-electron species and have lowelectron deficiencies, which is consistent with their observed non-reactivity.G—Fe(CO)3 Q_Fe(CO)3 . OFe(C0)213 (17-electron) 14 (17-electron) 15 (17-electron)Similarly, low-energy electron impact on cycloheptathene chromium thcarbonyl, (q6-c-C7H8)Cr(CO)3, 10, generated the fragment ionC7H8r(CO)2 (37.1%) and the molecularionC7H8r(CO)3 (100.0%). This compares to theC7H8r(CO)2 (13%)C7H8r(CO)3(67%) ion distributions reported by Blake.5 Again, both ions are unreactive towards ion-molecule clustering reactions with the parent molecule. The molecular ionC7H8r(CO)3 isproposed to have an(r15-C7H8)C (CO)3structure (16) or an(ri4-C7H8)Cr(CO)3structure(17), and the fragment ion7H8Cr(CO)j has an(ri-C7H8)Cr(CO)j structure (18).Cr(CO)3 Cr(CO)3 Cr(CO)j16 (17-electron) 17 (17-electron) 18 (17-electron)6.3 Ion-Molecule Reactions of the Negative Ions of Cyclooctatetraene IronTricarbonyl-(i4-c-C8H8)Fe(CO)36.3.1 ResultsThe negative FT-ICR mass spectrum ofC8HFe(CO)3,11, at 6.0 x iO Torr andwith a 2.5 eV x 10 millisecond electron beam is shown in Figure 6.1(A). A low-energy-179-electron impact onC8HFe(CO)3generates only two negative fragment ions,C8HFe(CO)(mlz 188, 100.0%) andC8HFe(CO)2 (mlz 216, 19.6%). The molecular ionC8HFe(CO)3(mlz 244) was not observed even with an electron beam energy as low as 0 eV. Unlike thelast three systems, ion-molecule clustering reactions in the present system were observed. Asshown in Figure 6.1(B), reactions between fragment ions and the parent molecule producetwo major ionic products,C8HFe2(CO)3and(C8H)2FeCO)3.100-A. Reaction Time = 0 ms-60-C8HFe(COi—.40-20-C8H5Fe(CO)2____Fe(CO)3— I I I I • • I I I I I I I I I I I I I I I I I I I I I I I100 150 200 250 300 350 400 450100-B. Reaction Time = 600 ms. 80- C8HFe2(CO)360-. 40- (C8H)2FeCO)320-0 I I I I—I I I I I I I I I • • • I I I I I I I I I I I I I I100 150 200 250 300 350 400 450Mass in A.M.U.Figure 6.1 FT-ICR negative ion mass spectra of(14-c-C8H8)Fe(CO)3(Pressure = 6.0 xi07 Torr, Electron beam pulse = 2.5 eV x 10 ms, Trapping voltage = -1 V).Figure 6.2 shows the temporal intensity variations of the two fragment ions and thetwo major product ions. The intensities of both fragment ions decrease with increasing-180-reaction times, which indicates that both ions react with the parent molecule. The two majorproduct ions are unreactive..—BCzFigure 6.2 Temporal variation of abundances of major negative ions in the (r14-c-C8H)Fe(CO)3system monitored by FT-ICR single resonance technique.From the known neutral pressure and the exponential decay curve ofC8HFe(CO), totaldisappearance rate ofC8HFe(CO) is calculated to be 2.2 x 10-10 cm3 molecule-1s1. IonC8HFe(CO)2 shows two reactivities since its intensity decreases at shorter reaction times(0.1- 0.6 sec.) and remains constant at longer reaction times (0.6- 1.0 sec.). Thedisappearance rate of the reactive species ofC8HFe(CO)2 is calculated to be 1.8 x 10-10 cm3molecule1s1.Reaction pathways that lead to the formation of the ionic cluster products from the twofragment ions were determined by FT-ICR double resonance experiments. Figure 6.3 showsthe double resonance spectra of ionC8HFe(CO) taken at zero and 800 milliseconds reactiontimes. Spectrum B shows that the ion reacts with the parent molecule to give a dissociativecharge transfer product,C8HFe(CO)2 (reaction 6.10), and two series of clustering products,0.0 0.2 0.4 0.6 0.8 1.0Reaction Time (Sec.)- 181 -(C8H)2FeCO)3(mlz 404, reaction 6.11) andC8HFe2(CO) (n = 4 to 2, m/z 328, 300,272, reactions 6.12 to 6.14), withC8HFe2(CO)3being the major ion in the second series.C8HFe(C0) +C8HFe(CO)3 —> C8HFe(CO)2 + C8HFe(CO) + CO (6.10)m/z188 m/z216C8HFe(CO) +C8H3Fe(CO) —* (C8H)2FeCO)3 + CO (6.11)m/z188 m/z404C8HFe(CO) +C8HFe(CO)3—* C8HFe2(CO)4 + C8H (6.12)m/z188 m/z328.1..C8HFe2(CO)3 + CO (6.13)m/z300•1-C8HFe2(CO) + CO (6.14)m1z272100 -__________________________________________________________A. Reaction Time = 0 ms60-C8HFe(COY— I I I I I I I I I I I I I • I I I I I I I I I I I I I I100 150 200 250 300 350 400 45100-B. Reaction Time = 800 ms C8HFe2(CO)31:::- C8HFe(CO)2 (C8H)2FeCO)I I I I—I I I I I I I I I I I I I I I I I100 150 200 250 300 350 400 450Mass in A.M.U.Figure 6.3 Double resonance mass spectra of the C8H8Fe(CO) /C8HFe(CO)3system.-182-Figure 6.4 shows the double resonance spectra of ionC8HFe(CO)2.Spectrum Bshows that the ion reacts with the parent molecule to form a single clustering product,C8HFe2(CO)5 (mlz 356, reaction 6.15), in small amounts.C8HFe(CO)2 + C8HFe(CO)3 —> C8HFe2(CO)5 + C8H (6.15)m1z216 m/z356100 -__________________ ______________________________________A. Reaction Time = 0 ms-.C8HFe(CO)2—0— . . I • • • I • I I I I I I I I I I I I I I I I I100 150 200 250 300 350 400 45100-B. Reaction Time = 900 msC8HFe(CO)2—40-C8HFe2(CO)520-A Ii.,— I I I I J I I I I J I I I I J I I I I I I I I I I I100 150 200 250 300 350 400 450Mass in A.M.U.Figure 6.4 Double resonance mass spectra of theC8HFe(CO)2 /C8HFe(CO)3system.6.3.2 DiscussionAs mentioned before, electron saturated (18-electron) mononuclear olefin-metalcarbonyls generally yield 17-electron negative ions under low electron energy conditions byexpulsion of a neutral CO. These I 7-electron species are generally inert toward their neutral- 183 -parent molecule and a variety of other neutrals in the low-pressure gas phase. Transition metalcarbonyl complexes bearing olefin ligands such asT14-butadiene iron tricarbonyl and q4-cyclohexadiene iron tricarbonyl may produce negative molecular ions under similarconditions. This is made possible by the ability of such complexes to accommodate the extraelectron through a partial reduction in the bond order between the metal and the organic ligand(4 . or ii). Similarly, we believe thatC8HFe(CO)2 is a mixture of structures 19 and20 and the monocarbonyl fragmentC8HFe(CO) has structure 21.2OUectron)oc” ‘co___19 (17-electron)- co‘VF&21 (15-electron)Scheme 6.1 Structures of ionsC8HFe(CO) andC8HFe(CO)2.After the first CO group is expelled from the molecular ion, the 17-electron species formed,19, can either lose one more CO to form 15-electron speciesC8HFe(CO) (21), or undergocleavage of one metal-olefin bond to form another 15-electron species(1-C8H)Fe(CO)(20). Therefore, 19 would be the unreactive species ofC8HFe(CO)2 whose intensityremains constant, and 20 would be the reactive species ofC8HFe(CO)2 whose intensitydecreases with time (Figure 6.2). The two 15-electron anions,(r14-C)Fe(CO) (21) and(r12-C8H)Fe(CO) (20), show basically the same reactivity towards the parent molecule,which is consistent with the electron deficiency model.’5-184-All the major negative product ions are unreactive in the system, which indicates theelectron count in each of these product ions should be closed to 17.11,30 The following arethe proposed mechanisms for the formation of the product ions.Reaction of(fl2-C8H8)Fe(CO)j (20). In the gas-phase ion-molecule clusteringreactions of metal carbonyl complexes where metal-metal bonds are formed, ejection of one ortwo CO groups are usually involved in the reactions. This is because the first two CO groupsare wealdy bonded to the central metal. In reaction 6.15, neutral CO is not expelled, whichindicates ion 20 does not attack the Fe(CO)3moiety of the parent molecule. In Scheme 6.2,the electron deficient ion, 20, attacks the cyclooctatetraene ring from the opposite direction ofFe(CO)3part, leaving the Fe(CO)3part of the molecule undisturbed. The weak(i2-o1efin)-metal bond in 20 cleaves upon the complexation between iron atom and the free diene moietyand the final product, 22, is formed. Each of iron atoms in ion 22 can have either 18 or 17electrons, explaining its stability.Fe— CO-C8HScheme 6.2 Reaction of fragment ionC8HFe(CO)2.The iron complex, 23,31,32 has been prepared in the condensed phase. Although itsnegative mass spectrum was not reported in the literature, a low-energy dissociative electronattachment process of complex 23 would be expected to form a (M-CO) stable fragment ion,22, with 17, 18-electron configuration.+20 22- 185 -OCN ....._1IIcoFeReactions of(ri4-C8H)Fe(COI (21). Clusteiing reactions ofC8HFe(CO)with its neutral parent molecule form two major products,(C8H)2FeCO)3(reaction 6.11)andC8HFe2(CO)3 (reaction 6.13). Similar to the reaction of(r1-C)Fe(CO) Scheme6.3 is proposed for reaction 6.11. On the ion association with the free diene part of the parentmolecule, CO is ejected to form a stable 18-electronll7-electron anionic complex 24.Although known analogy of 24 was not found in literature, the observation suggests that the18-electron complex 25 could be prepared in the condensed phase. Ion 24 or ion (24- CO)would be the dominant negative fragment ion of the complex.21+V - COOC”s• ICOV.23Fe24Scheme 6.3 Formation of(C8H)2FeCO)3from fragment ionC8HFe(CO.-186-Fe—COCO25Formation of the abundant product ionC8HFe2(CO)3 (reaction 6.13) is accompaniedby the formation of ionsC8HFe2(CO)4(reaction 6.12) andC8HFe2(CO) (reaction 6.14),which resembles the typical reaction pattern of metal carbonyl systems.’3’5Therefore, metal-metal bonding is believed to be present in these stable product ions. Scheme 6.4 providespossible ion processes that lead to the formation of these products. The reactant ionC8HFe(CO) (21) may have attacked the cis-side of the free diene (route A) or the metalcenter (route B) of the parent neutral to form intermediate ionic complexes 26 and 27,respectively. On further intramolecular complexation of the intermediates, a cyclooctatetraenering is expelled to form product ionC8HFe2(CO)4,29, which loses one CO to giveC8HFe2(CO)3,30, and two CO groups to giveC8HFe2(CO),28. In analogy with theprevious structural analysis made for iron pentacarbonyl systems,”4”5together with theknown multiple metal-metal bonded neutral complex(rj-c-CH)2FeCO)33and partiallymetal-metal bondedCp2o(CO)2,’6’8ions 28 and 30 are proposed to have multiple metal-metal bonds as shown in the scheme. The 18-electron complex, 31 shown below Scheme6.4, has been prepared in the condensed phase and characterized by X-ray diffraction.3436The negative ion mass spectrum of 31 has not been examined, but a low-energy dissociativeelectron attachment by the molecule would be expected to produce ions 28 to 30 as itsfragment ions.- 187 -\>Fe\’I ‘) I Fcç.isco26 CoFe(CO]28 (16-, 17-electron)-2C0(OC)2Fe Fe(CO) I29 (18-, 17-electron)-Co(OC)Fe Fe(CO)I30 (17-, 17-electron)Scheme 6.4 Formation ofC8HFe2(CO).4from fragment ionC8HFe(CO).(OC)e Fe(CO)2Co/- C8H2731 (1 8-electron)-188-6.4 Ion-Molecule Reactions of the Negative Ions of Cyclohexadienone IronTricarbonyl -(ri4-c-C6H0)Fe(CO)36.4.1 ResultsUpon 1.6 eV electron impact at 1.7 x 10 Torr,(r4-c-C6HO)Fe( O)3forms threenegative fragment ions,C6HOFe (mlz 150,95.7%),C6HOFe(CO) (mlz 178, 75.6%), andC6HOFe(CO)2(m/z 216, 100.0%). This standard (DL3 =.0 ms) Fr-ICR negative ion massspectrum of(fl4-c-C6H60)Fe(CO)3 is shown in Figure 6.5(A). The other two spectra in thefigure were taken with the individual delay times. Clustering reactions of the fragment ionswith the parent molecule form the secondary product ions(C6H0)2FeCO) (m/z 356),(C6H0)2FeCO) (mlz 328) and a dehydrogenated ion, (C50)FeCO) (mlz 326).These secondary ionic products react further to form products containing three iron atoms.Figure 6.6 shows the temporal variation of ion intensities of the three fragment ions and thethree major secondary product ions. The fragment ionsC6HOFe andC6HOFe(CO) showlarger reactivities than the fragment ionC6HOFe(CO)2While the secondary product ions(C6H0)2FeCO) and(C6H0)2FeCO) also show small reactivities, the dehydrogenatedsecondary ion,(C6H50)2FeCO), shows a fairly large reactivity. Each of the fragment ionsC6HOFe(CO)2 andC6H6OFe(CO) shows two reactivities, which tends to indicate twodifferent structures for each of these two ions. In Figure 6.6, the more reactive species of thetwo ions are labeled with (a) and the less reactive species are labeled with (b), i.e.,C6HOFe(CO)2 (a) andC6HOFe(CO)2 (b) for ionC6HOFe(CO)2 and C6H6OFe(CO)(a), OFe(CO) (b) for ion6H6OFe(CO). The curve marked with 0 ... for speciesC6HOFe(CO) (a) is plotted by the intensity differences between the total intensity ofOFe(CO) (a) andC6HOFe(CO) (b) and the intensity of C6H6OFe(CO) (b) at certainreaction times less than 0.6 seconds. Total reaction rate constants of the ions in Figure 6.6 arecalculated from their decay slopes and known pressure of the parent molecule and are listed inTable 6.1.- 189 -Mass in A.M.U.Figure 6.5 FT-ICR negative ion mass spectra of(r4-c-C6HO)Fe( O)3(Pressure = 1.7 xi0 Torr; Electron beam pulse = 1.6 eV x 10 milliseconds; Trapping voltage = -1.0 V).A. Delay time = 0 ms—C6HOFe(CO)2——C6HOFe(CO)—— C6HOFeiI_ iI200 300 400‘1500100‘—‘ 80-60-— 40-0100-‘— 80--.60-— 40-.—20-0-100-‘— 80-.—60-— 40-.—-.‘ 20-0B. Delay time = 400 msC6HOFe(CO)2(C6H5OFe)2CO—(C6HOFe)2CO(C6HOFe)2(COI -j II IIII 11111111111111 11111111111111w I100 200 300 400 500 6(600)0(C6HOFe)3CO4I532, 534504,5061i1111111 111111500 600C. Delay time = 1400 msC6HOFe(CO)2 (C6HOFe)2CO(C6H5OFe)2CO100—[---I I I I I -- I I I I I I I200 300(C6HOFe)2COI 11111111 I III400-190-C6IIOFe(CO)2 (a) +C6HOFe(CO)2 (b) C6HOFe(CO)2 (b)-0.5-C6HOFe(CO) (a)OFe(CO) (b) (C6HOFe)2CO.--1.0-(C6HOFe)2CO-‘0 C6HOFe(CO) (b)‘S._ SSo215_ C6HOFeo-2.0 - C6HOFe(CO) (a) (C6H5OFe)2CO0I ‘ I ‘ I ‘ I I ‘ I ‘ I ‘0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4Reaction Time (Sec.)Figure 6.6 Temporal variation of abundances of the negative fragment ions and the majorsecondary product ions in the(14-c-C6H0)Fe(CO)3system.- 191 -Table 6.1 Rate constants and electron deficienciesIon Rate Constanta Electron Countb Adjusted Ele. CountCC6HOFe 9.2 13 15OFe(CO) (a) 9.0 15C6H6OFe(CO) (b) 3.4 15 17OFe(CO)2 (a) 3.0 17C6H6OFe(CO)2 (b) 0.0 17(5OFe)2CO 2.6 15, 16(6HOFe)CO 0.57 14, 15 16, 17(OFe)2CO 0.59 15, 16 16, 17a. All rate constants are in the units of 10-10 cm3 molecule-1s’.b. Electron count on each iron atom calculated assuming single metal-metalbonding in dimetallic ions and thatC6H0andC6H50are cyclohexadienone andcyclohexadienonyl ligands and donate 4 and 5 electrons to metal center,respectively.c. Adjusted electron count on each iron atom calculated using the ion structuresdiscussed in the text.As listed in Table 6.1 and shown in Figure 6.6, ionC6HOFe has a similar reactivityto the more reactive form of ionC6HOFe(CO), the ion OFe(CO)(a). Thedehydrogenated ion, (6H50)2FeCO), would be less electron deficient than(C6H0)2FeCO) assumingC6H50to be an5-cyclohexadienony1 ligand andC6H0to beani4-cyclohexadienone ligand. (C0)2FeCO) is therefore expected to be more reactivethan(C6H50)2FeCO). But the latter ion is actually more reactive than the former ion.Similarly, ion (C6H0)2FeCO) is seemingly more electron deficient than ion(C6H0)2FeCO),but both have rather similar reactivities. Another exceptional case is thatthe fragment ionC6UOFe (a 13-electron ion assuming an4-cyclohexadienone ligand) wasformed at an intensity of 95.7% (Figure 6.5A) relative to the 17-electron speciesC6HOFe(CO)2.A 13-electron organometallic ion has never been reported to form in such alarge intensity relative to its 17-electron counterpart. All these observations are rationalized inthe discussion section 6.4.2.-192-Reaction pathways of fragment ionsC6HOF& and C6H6OFe(CO) identified by FICR multiple resonance experiments are summarized in reactions 6.16 to 6.23.C6H6OFe. Reactions ofC6HOFe with the parent molecule produce the secondaryion(COFe)2CO (m1z328) and the dehydrogenated ion (C6H5OFe)2O (m1z326).These product ions react further to form ions with mass-to-charge ratios 506, 504 and 532(Figure 6.5), corresponding to formulae(C6H0)3FeCO)2,(C6H50)2)Fe3CO)and(CH0)2)Fe3CO)respectively. The former ion is formed by the reaction of(C6OFe)CO), and the latter two ions are formed by the reactions of(C6H5OFe)2CO).C6HOF& +C6HOFe(CO)3 — (C6HOFe)2CO + 2C0 (6.16)m/z150 m1z328—> (C6H5OFe)2CO + 2C0 + H2 (6.17)m/z 326(C6HOFe)2CO +C6HOFe(CO)3 —* (C6HOFe)3CO2 + 2C0 (6.18)m/z328 m/z506(C6H5OFe)2CO +C6HOFe(CO)3 —>(C6H50)2)Fe3CO) + CO (6.19)m/z326 m/z532—*(C0))FeCO) + 2C0 (6.20)m/z 504C6HOFe(COi. C6HOFe(CO) reacts with the parent molecule to form thesecondary product(C6HOFe)2CO (mlz 356) which reacts further to form tertiary ions(C6HOFe)3CO4(mlz 562) and(C6HOFe)3CO (mlz 534).C6HOFe(CO) +C6HOFe(CO)3—* (C6HOFe)2CO + 2C0 (6.21)m/z 178 m/z356(C6H6OFe)2O + C6HOFe(CO)3—* (C6HOFe)3CO4 + CO (6.22)m/z356 m/z 562—* (C6HOFe)3CO + 2C0 (6.23)m/z534- 193 -6.4.2 DiscussionAs discussed in Section 6.1, fragmentation by low-energy electrons (< 15 eV) involvea dissociative electron attachment process. The process typically generates abundant 17-electron negative ions, LM(CO)..f, from 18-electron organo-transition metal carbonylcomplexes [LM(CO)]. 15-electron ions LM(CO)n.2 can also be generated. LM(CO).3 ions(13-electron if the L-M bonding does not change), ionC6HOF& (95.7%) in the presentsystem, generally do not appear or only appear at low intensity relative to the 17-electron ions.As noted in Sections 6.2 and 6.3, under low-energy electron conditions, 17-electronfragment ionsC4H6Fe(CO)2 andCH8Fe(CO)2 dominated the mass spectra of thecorresponding complexes(14-CH6)Fe(CO)3and(r14-c-C6H8)Fe(CO)3.The complex(6-c-C7H8)Cr(CO)3yieldsC78r(CO) andC7r(CO)2,and the complex (T14-c-)Fe(CO) yieldsC8HFe(CO)2 andC8HFe(CO). Similarly, cyclopentadienylcomplexes CpV(CO)4,CpMn(CO)3,and CpCr(NO)2H3predominately form (M-COINO)ions with our instrument.37’8 The negative fragment ions formed in these cyclopentadienylsystems were considered to be 17-electron [16-electron in the case of CpCr(NO)2H3]species due to their low reactivity. Further rupture of a metal-carbonyl bond occurs to form(M-2C0) 15-electron species in the above three systems. Nevertheless, 13-electron ionsformed by ejection of three carbonyls or two nitrosyls were not generated with electron impactenergies increased to as high as 70 eV. These observations are consistent with the previousmass spectrometric results. That is, 18-electron metal carbonyls and olefin-metal carbonylsyield abundant 17-electron negative fragment ions, LM(CO)..f. In the ICR study ofCpCo(CO)2 (Cp = cyclopentadienyl), CpCo(CO)2 (84%) and CpCo(CO) (16%) were theonly two fragment ions generated by using an electron energy as high as 70 eV.9 The sameresults were also observed for this compound by using a magnetic sector instrument10and ourFT-ICR instrument.37 Similarly, (M-2C0) and (M-3C0) ions in the negative mass spectraof the following hydrocarbon-metal carbonyl complexes were either not observed or onlyobserved in very small amounts.4-194-ITFe(CO)3 Fe(CO)3 Fe(CO)3icFe(CO)3Compared to all these closely-related r14-diene andfl5-cyclopentadienyl systems, theoxygen atom in the cyclohexadienone ligand must have played an important role in theformation of the abundantC6HOFe in the present(14-c-C6H6O)Fe(CO)3system.Similar to structures 3 and 4 (reactions 6.2 and 6.4) whose negative charges wereassumed to localize on carbon atoms, the formation of the molecular ion of methylbenzonatechromium tricarbonyl(r16-c-CH5COOMe)Cr(CO)3was rationalized by localizing the chargeon the organic oxygen atom,‘:5.: OMeCr(CO)332 (1 8-electron)By analogy with these suggestions, we propose the following fragmentation scheme toexplain the formation of the abundant negative ion C6HfOFe from(fl4-c-C6H6O)Fe(CO)3.—. Ketone/Enolate V T1—* 6 H—Fe(CO)2 I —Fe(CO)2 I33 (17-electron) 34 (17-electron) 35 (19-electron)i-Co- Co—e(CO)38 (17-electron)°T—Fe(C0)36 (15-electron)-2C0°i3-Fe---H37 (15-electron)Scheme 6.5 Formation of negative fragment ions of(14-c-C6H60)Fe(CO)3.- 195 -The 17-electron (fl4-C6H6O)Fe(CO)2 (mlz 206, ion 33) and the 15-electron (iaC6H0)Fe(CO) (mlz 178, ion 36) are considered to have been formed by the normaldissociative electron attachment process with charge localized on metal. C6HOFe isconsidered to be formed by rearrangement of the negative charge to the organic oxygen atomvia, a ketone/enolate conversion. The conversion compares to the well-known organicchemical reaction 6.24, i.e., when a ketone is treated with a strong base, its a-proton can beabstracted by the base to form an enolate anion.+ W + HB (6.24)The F& unit in 33 can be considered as a strong Lewis base which abstracts an a-protonintramolecularly to form a 17-electron metal hydride ion(fl4-C6H50)Fe(CO)2H(34). Thedouble bond of the enolate conjugates with the diene portion to form benzene ring that donatessix ‘‘t electrons to the metal center to form a 19-electron intermediate ion 35. It has beenverified by metastable ion monitoring that decarbonylation of an electron-rich ion occurs bothsimultaneously and consecutively.5”0’39 Therefore, the 19-electron intermediate 35 wouldrapidly eject one or two CO groups to form 17-electron(r16-CH50)Fe(CO)W (38) and 15-electron (r16-C6HSO)FeW (37). The former ion (17-electron) would be the less reactivespecies of OFe(CO) (mlz 178) and is only formed in small amounts in thisdecarbonylation step. The negative charge of 37 (fragment ionC6HOF&) localizes on anoxygen atom whose electronegativity is much larger than that of carbon and metal atoms. Thismay make the ion long-lived and explain why C6H6OFe is formed and detected with such alarge abundance (95.7% relative to the 17-electron species) by a long-time scaled instrumentlike FT-ICR. Compared with the oxygen-containing complex(q6-c-CH5COOMe)Cr(CO)3where ions (M-3C0) and (M-2C0) were only observed to be 1.8% and 3.2% relative to (MCO) ion,39 the ketone / enolate conversion that assists the charge migration to oxygen atomand provides two more It electrons for the metal center tends to be crucial for the formation ofabundant(ri6-CH5O)FeH ion, 37, in the present system.-196-As indicated by the fragmentation scheme, each of the fragment ionsC6HOFe(CO)2(33 and 34) andC6HOFe(CO) (36 and 38) has two different structures. This is consistentwith the two reactivities observed for each of these two ions. In addition, the similarreactivities of the fragment ion C6HOFe (37) and the more reactive species ofC6HOFe(CO) (36) [ion C6H6OFe(CO) (a) in Figure 6.6 and Table 6.11 are consistent withtheir same electron deficiency. A hydride structure forC6HOF& (37) is also consistent withits association-dehydrogenation reaction (reaction 6.17) as explained in the following.Reactions ofC6HOFe (37). Scheme 6.6 is proposed to explain the reactions 6.16and 6.17 that form the secondary product ions (C6H0)2FeCO) (40) and(CH5O)2FeCO(41).0°1Fe-H + T—Fe(CO)3 -2C0041 (16, 15-electron) 40 (17, 16-electron)Scheme 6.6 Formation of the secondary product ions of the fragment ionC6HOFe.Metal hydride ionC6HOFe, 37, associates with the parent molecule to form a diiron hydrideion 39 (with loss of two CO groups). Ion 39 undergoes a hydrogen shift from ligand tometal to form the double hydride ion 40. The two hydrogen atoms on the two metal centerscan be partial eliminated to form a considerable amount (Figure 6.5B) of ion 41. Each of theiron atoms in dehydrogenated ion 41 can have 15 or 16 electrons (Average electron deficiency‘IIH-197-= 2.5). In contrast, each of the iron atoms in ion 40 can have 16 or 17 electrons (Averageelectron deficiency = 1.5). This is consistent with the greater reactivity observed for ion(C6H50)2Fe2(CO) (41) and the lower reactivity observed for(C6H0)2FeCO) (40)(Figure 6.6, Table 6.1)By analogy with the triangular metal core suggested by Wronka and Ridge15 and someknown condensed-phase triangular metal core clusters, structures 42, 43 and 44 could bepossible for the tertiary product ions (C6H0)3FeCO) (reaction 6.18),(C6H20))Fe3CO) (reaction 6.19) and(2)0)FeCO) (reaction6.20). It is also possible that two of the carbonyls in each of these structures are triplybridging with three metal centers (J.t3-CO), structure 45, as the carbonyls in complexesCp4Fe4(CO)4,Cp3o(CO) andCpNi(CO)2.4°0CoThe bridging hydrogen atoms in structure 39 to 42 are analogous to those in the knowndimetallic clusters HFe2(CO)8,H2Re(CO)g,HW(CO)8HCr2(NEt4)(CO)10HW2(CO)10 and the triangular metal core clusters HFe3(CO)1f,Re3(CO)12 andH20s3(CO)l 41Co42CO4344-198-Reactions ofC6HOFe(CO) (36). Fragment ion C6H6OFe(CO) reacts with theparent molecule to form(C0)2FeCO) (reaction 6.21) which has a very small reactivity(Figure 6.6, Table 6.1). This product is proposed to have structure 46. Here this low-reactivity ion is proposed to have a double metal-metal bond rather than a metal hydrogenlinkage since the ion does not dehydrogenate./I ‘Fe=Fe_!_ I.Co46 (16, 17-electron)The tertiary product ions(C6HOFe)3CO4(reaction 6.22) and(C6HOFe)3CO (reaction6.23) of 46 are proposed to have structures 47 and 48, respectively.ol6.4.3 Reactions ofC6HOFe with CD3I and CH3IAll three negative fragment ions of(14-c-C6H0)Fe(CO),(C6H0)Fe(CO)2,(C6H0)Fe(CO) and (C6H0)Fe, do not react with CH3OD, CD3OH, D20, CD3N, NH3and CH3Br. (C60)Fe(CO)2 and(C6H0)Fe(CO) do not react with CD3I either.(C6H0)Fe was found to react with CD3I to form a major product ion (C6H50)Fe1 (mlz276) and a product ion (C6H0)FeP (mlz 277) in small amounts; reactions 6.25 and 6.26. Asample mass spectrum that illustrate these reactions is shown in Figure 6.7. ReactingolCo047 48-199-(C6H0)F& with CH3I gives the same products, which rules out the possibility that ionmlz 277 is formed by a H/D exchange.C6HOFe + CD3I —* (C6H50)Fef + CD3H (6.25)m/z150 m1z226,71%—* (C6H0)Fe1 + CD3 (6.26)m/z 227, 29%C-4C6H5OFeI-* C6HOF&C6HOFer —>‘—4a-JIII 1 I “i150 200 250 900MPSS IN P.MU.Figure 6.7 Partial triple resonance mass spectrum ofC6HOF& taken in the presence ofCD3I. Total pressure = 1.0 x 10-6 Torr; Reaction delay time (DL3) = 400 milliseconds.Reaction 6.25 supports the cyclohexadienonyl-iron-hydrogen structure, 37, proposedfor the ionC6HOFe. A reaction mechanism is proposed below.—. I—.____O—*I CD3- CD3H1j—Fe—H + CD3I - — - -Fe—I37, rnlz 150 49 50, mlz 276Metal hydride ionC6HOFe, 37, inserts into the C-I bond of CD3H to form an intermediateion 49 which loses a neutral methane to form the ion(C6H50)Fef, 50, in reaction 6.25.- 200 -6.5 Summary and ConclusionsCompared with the positive ions discussed in Chapters 2 to 5, much fewer negativeproduct ions were observed due to the lower reactivities of negative organo-transition metalions.Low-energy Fr-ICR electron impact on complexes butadiene iron tricarbonyl (iaC4H6)Fe(CO)3(1), cyclohexadiene iron tricarbonyl (ri4-c-C6H8)Fe(CO)3(9) andcycloheptatriene chromium tricarbonyl(q6-c-C7H8)Cr(CO)3(10) generated abundant negativeions LM(CO)3 and LM(CO)2 (L = the organic ligand, M = metal atom). Each of thesenegative ions shows no reactivity towards their corresponding parent molecule. Theseobservations are typically parallel to the negative ion chemistry of analogous complexesdescribed previously and are rationalized accordingly, i.e., low-energy electron impact on 18-electron organometallic complexes typically generates abundant 17-electron fragment ions thatare typically inert towards clustering reaction with the parent molecule.Cyclooctatetraene iron tricarbonyl(q4-c-C8H)Fe(CO)3 (11) yields the fragment ionsC8HFe(CO) andC8HFe(CO)2.The two ions react with their parent molecule to formstable product ions (C)FeCO)3 andC8HFe2(CO).5The reactivities ofC8HFe(CO) andC8HFe(CO)2 towards the cyclooctatetraene iron tricarbonyl (114-c-C8H)Fe(CO)3(11) are believed to be caused by the uncoordinated butadiene portion of theC8H ring in the neutral complex. That is, the uncoordinated butadiene portion can complexwith the electron deficient metal atoms in the fragment ions to form the dimetaffic product ions(C8H)2FeCO)3andC8HFe2(CO)2..5. The structures of the product ions are proposedaccordingly, with the C8H ring providing all its eight 7t electrons to two iron atoms. Withthis coordination form of the C8H8 ring, all the product ions can have 16 to 18-electronstructures, which is consistent with their low reactivities observed.Unlike the complexes 1 and 9- 11, cyclohexadienone iron tricarbonyl (rj4-c-C6H0)Fe(CO)3(12) yields anC6HOF& ion in large amounts relative to the 17-electron ionOFe(CO)2.This is also dramatically different from the negative ion mass spectra of a-201-large number of closely-related hydrocarbon r14-diene iron complexes. We believe that theoxygen atom in the organic ligand is responsible for the formation of the large amount of ionC6HOF&. With the negative charge on the iron atom, the metal center in the 17-electron ionCHOFe(CO)2 acts as a strong Lewis base and can abstract a a-proton from thecyclohexadienone ligand. As a result, the negative charge migrates to the oxygen atom whoseelectron affinity is much larger than those of metal atoms and a carbon atom. We believe thisis why the ion becomes long-lived and can be detected by a Fr-ICR instrument whose time-scale is much longer than magnetic sector mass spectrometers. The above conversion is verysimilar to the ketone to enolate organic chemical process that forms carbon-carbon doublebond. The 15-electron fragment ionC6HOFe(CO) shows a reactivity similar to that of ionC6HOFe. This indicates that the carbon-carbon double bond formed has conjugated withthe other two double carbon-carbon bonds to form a benzene ring that provides all its six Itelectrons to the iron atom inC6HOFe. Thus, bothC6HOFe(CO) andC6HOF& areactually 15-electron species, which is compatible with their similar observed reactivities (Table6.1). The intramolecular a-proton abstraction reaction ofC6HOFe gives a metal hydridestructure of ionC6HOF&. 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