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Hafnium complexes stabilized by a macrocyclic ligand Corkin, James R. 2000

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Hafnium Complexes Stabilized by a Macrocyclic Ligand by James R. Corkin B.Sc. (Hon.), University of British Columbia, 1997 A thesis submitted in partial fulfillment of the requirement for the degree of MASTER OF SCIENCE in THE F A C U L T Y OF G R A D U A T E STUDIES Department of Chemistry We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH C O L U M B I A April 2000 © James R. Corkin, 2000 UBC Special Collections - Thesis Authorisation Form Page 1 of 1 In p resenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head of my department or by h i s or her re p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of C \)9> Vfi \ S The U n i v e r s i t y of B r i t i s h Columbia Vancouver, Canada Date Rpr.l 17/aOOQ http://www.library.ubc.ca/spcoll/thesauth.html 4/11/00 i i ABSTRACT The goal of this thesis is to investigate the chemistry of hafnium complexes of the macrocyclic ligand, PhP(CH2SiMe2NSiMe2CH2)2PPh ([P2N2]). The hafnium dichloride, [P2N2]HfCl2 (1) is used as a starting material for all other compounds synthesized. 1 is isostructural with its zirconium analogue, [P 2 N 2 ]ZrCl 2 , but does not exhibit identical chemistry. While reaction of [P2N2]ZrCl2 with 2 equivalents of CgK under N 2 resulted in formation of a zirconium dinitrogen compound, the same is not true for the hafnium analogue. Instead, the major product of the reaction of 1 with 2 equivalents of C«K is a hafnium dimer, {[P2N2]Hf}2 (2). [P2N2]HfI2 (3) is synthesized via the reaction of 1 with excess trimethylsilyliodide. Evidence will be presented that ([P2N2]Hf)2(u.- r|2:ri2-N2) (4) is the major product of the reaction between 3 and 2 equivalents of CgK under dinitrogen, while {[P2N2]Hf}2 (2) and [P2N2]Hf(C7H8) (5) appear to be minor products. Reaction of 1 with 2 equivalents of MeMgCl results in the formation of [P2N2]HfMe2 (6). The zirconium analogue of 6 reacts with toluene under an atmosphere of Ff2 to give [P2N2]Zr(CvH8), but this is not true for 6. Instead, reaction of 6 under H2 results in the formation of a hafnium tetrahydride, {[P2N2]Hf}2(n-H)4 (7). A metathesis reaction of one equivalent of 1 with one equivalent of Mg(C4H6)(THF)2 results in formation of [P2N2]Hf(C4H6) (8). Unlike most mononuclear [P2N2] compounds, this compound exhibits 2 inequivalent phosphorus atoms evidenced by an A B doublet pattern in the ^ Pj 1 !!} N M R spectrum. T A B L E O F C O N T E N T S A B S T R A C T ii T A B L E O F C O N T E N T S iii LIST O F T A B L E S vi LIST O F FIGURES vii LIST O F ABBREVIATIONS ix A C K N O W L E D G E M E N T S xiii D E D I C A T I O N xiv C H A P T E R 1 G E N E R A L I N T R O D U C T I O N 1.1 Comparison of Hafnium and Zirconium 1 1.2 Introduction to Dinitrogen 5 1.3 Nitrogen Fixation 7 1.4 Transition Metal Dinitrogen Complexes 9 1.5 Ligands and Dinitrogen Complexes from the Fryzuk Group 13 1.6 The Scope of this Thesis 19 1.7 References 20 C H A P T E R 2 T O W A R D S A N E W H A F N I U M DINITROGEN C O M P L E X 2.1 Introduction to [P2N2] Zirconium Complexes 25 2.2 Synthesis and Characterization of [P2N2]HfCl2(l) 26 2.3 Preliminary Attempts to Form a Hafnium Dinitrogen Complex . 31 2.4 Synthesis and Characterization of [P2N2]Hfl2 (3) 36 iv 2.5 Synthesis and N M R Characterization of 38 {[P2N2]Hf}2((a-Ti2:Ti2-N2) (4) 2.6 Further Characterization of {[P 2N 2]Hf} 2(|a-ri 2:ri 2-N 2) (4) 43 2.7 Preliminary Reactivity Studies on {[P 2N 2]Hf} 2(uvn 2:r| 2-N 2) (4). 45 2.8 Summary 47 2.9 References 48 C H A P T E R 3 O R G A N O M E T A L L I C A N D H Y D R I D E C O M P L E X E S O F H A F N I U M 3.1 Transition Metal Hydrides 50 3.2 Synthesis and Characterization of [P 2N 2]HfMe 2 (6) 54 3.3 Attempted Synthesis of [P 2N 2]Hf(C 7H 8) (5) 57 3.4 Synthesis and Characterization of {[P 2N 2]Hf} 2(>H)4 (7) 59 3.5 Synthesis and Characterization of [P2N2]Hf(C4H6) (8) 61 3.6 Summary 63 3.7 References 64 C H A P T E R 4 E X P E R I M E N T A L P R O C E D U R E S 4.1 General Procedures 66 4.2 Reagents and Starting Materials 66 4.3 Analytical Methods 67 4.4 Hydrazine Analysis 67 V 4.5 Synthesis of New Compounds 68 4.5.1 [P2N2]HfCl2 (1) 68 4.5.2 {[P 2N 2]Hf} 2 (2) 69 4.5.3 [P 2N 2]HfI 2 (3) 70 4.5.4 {[P2N2]Hf}2(p.-n2:ri2-N2) (4) 70 4.5.5 [P 2N 2]HfMe 2 (6) 71 4.5.6 {[P 2N 2]Hf} 2{u-H 4} (7) 71 4.5.7 [P 2N 2]Hf(C 4H 6) (8) 72 4.6 References 73 F U T U R E P R O S P E C T S 74 A P P E N D I X A.1 X-ray Crystallographic Analysis of [P 2 N 2 ]HfCl 2 (1) 76 A.2 X-ray Crystallographic Analysis of [P 2N 2]HfMe 2 (6) 80 vi LIST OF T A B L E S Table 1.1 Properties of Hafnium and Zirconium which 2 Differ Significantly Table 1.2 Reduction Potentials for Zirconium/Hafnium Analogues 4 Table 1.3 Summary of the Differences in Chemical Properties 13 between Nitrogen and Phosphorus Table 2.1 A Comparison of Selected Bond Lengths (A) and 29 Angles (deg) in [P 2N 2]MC1 2 M=Zr, Hf Table 2.2 The results of a study of the reaction of [P 2 N 2 ]HfCl 2 (1) 33 with 2 equivalents of CgK under 1 atm N 2 in toluene Table 2.3 The results of a study of the reaction of [P 2N 2]HfI 2 (3) 38 with 2 equivalents of CsK under 1 atm N 2 in toluene Table 2.4 The results of a study of the reaction of [P 2N 2]HJI 2 (3) 41 with 2 equivalents of CgK under 4 atm N 2 in toluene Table 3.1 Selected Bond Lengths (A) and Angles (deg) in [P 2N 2]HfMe 2 (6). 57 vii LIST OF F I G U R E S Figure 1.1 The only crystallographically characterized hafnium(III) 4 metallocene compound. Figure 1.2 A representation of the molecular orbitals of dinitrogen 6 and their relative energies. Figure 1.3 The structural model of the FeMo cofactor site determined 9 by single crystal X-ray crystallography. Figure 1.4 The binding modes of dinitrogen in mononuclear 10 and dinuclear transition metal compounds. Figure 1.5 The only known hafnium dinitrogen complex, 11 {Cp*2Hf(N2)}2(it-'n1:ri1-N2). Figure 1.6 Three weakly activated dinitrogen complexes 12 with type D bonding (mononuclear side-on). Figure 1.7 A pictorial representation of [N(SiMe 2CH 2PR 2) 2]" 1 ([PNP]) 14 Figure 1.8 The route to formation of ([PNP]ZrCl)2(n2-r|2:'n2-N2) 15 Figure 1.9 An orbital interaction diagram for the bonding molecular 16 orbitals of side-on and edge-on bridging of the dinitrogen ligand between two ML4 fragments. Figure 1.10 The route to formation of [P2N2]Li2(l,4-dioxane) 18 Figure 2.1 Some of the reactions of [P 2 N 2 ]ZrCl 2 which have relevance 25 to this thesis. Figure 2.2 An ORTEP diagram of [P 2 N 2 ]HfCl 2 (1) showing the solid 28 state molecular structure. Figure 2.3 The ] H and ^ P ^ H } N M R spectra of [P 2 N 2 ]HfCl 2 (1) 29 Figure 2.4 The process by which equilibration of [P 2N 2] silyl 30 methyls is believed to take place. Figure 2.5 The ' H and ^ P ^ H ) N M R spectra of {[P 2N 2]Hf} 2 (2) 32 viii Figure 2.6 The 3 1 P{ 'H} N M R spectrum of [P 2 N 2 ]HfCl 2 (1) + 2 CgK 33 under 1 atm N 2 after 25 hours. Figure 2.7 The J H and " P l / H } N M R spectra of [P 2N 2]HfI 2 (3) 37 Figure 2.8 The ^ P ^ H } N M R spectrum of the reaction between 39 [P 2N 2]HfI 2 (3) and 2 equivalents of CgK under 1 atm N 2 after 25 hours. Figure 2.9 The 3 1 P{'H} N M R spectrum of the reaction between 40 [P 2 N 2 ]Hfl 2 (3) and 2 equivalents of C 8 K under 4 atm N 2 after 8 days. Figure 2.10 The silyl methyl region of the ' H N M R spectrum 41 for the products from the reaction of 3 with 2 equivalents of CgK in toluene for 8 days. Figure 2.11 The phenyl region of ! H N M R spectrum for 42 the products from the reaction of 3 with 2 equivalents of CgK in toluene for 8 Days. Figure 2.12 Selected E.I. Mass Spectrometry data for ([P 2N 2]Hf) 2N 2 44 Figure 2.13 Theoretical isotope distribution pattern for E.I. Mass 44 Spectrometry analysis of ([P 2N 2]Hf) 2N 2 as determined by Isotope Pattern Calculator™ v. 1.6.6. Figure 2.14 The ^ and 3 1 P{'H} N M R spectra for the reaction 46 shown in Equation 2.5. Figure 3.1 Useful organic functional groups derived 51 from acylzirconium compounds. Figure 3.2 The lH and 3 1 P{ 1 H} N M R spectra of [P 2N 2]HfMe 2 (6). 55 Figure 3.3 An ORTEP Diagram showing the solid state 56 molecular structure of [P 2N 2]HfMe 2 (6). Figure 3.4 The ' H and 3 I P{ 'H} N M R spectra for the attempted 58 synthesis of [P 2N 2]Hf(C 7Hg). Figure 3.5 The 3 1 P{ 1 H} and lHNMR spectra for {[P 2N 2]Hf} 2(^-H) 4 (7). ... 61 Figure 3.6 The lH and 3 1 P{ 'H} N M R spectra of [P2N2]Hf(C4H6) (8) 63 LIST OF A B B R E V I A T I O N S H proton 'H{ 3 1 P} observe proton while decoupling phosphorus H or D deuterium 3 1 P{ 1 H} observe phosphorus while decoupling proton A angstrom, 10"10 m A45 8 absorbance at 458 nm wavelength of light Anal. analysis atm atmosphere A U atomic units br. broad Bz benzyl group, (-CH 2C6H 5) Calcd. calculated cis cisoidal cat. catalyst cm"1 wave number Cp cyclopentadienyl group, [C5H5]" Cp penta(methyl)cyclopentadienyl group, [CsMes] d doublet deg degrees D M E 1,2-dimethoxyethane, (CHsO^CyHU d" number of d electrons dd doublet of doublets coupling pattern E.I. electron impact Enz. enzyme eq. equivalent(s) etc. etcetera fac facial g grams HOMO highest occupied molecular orbital Hz Hertz, seconds"1 I nuclear spin quantum number, or iodine IR infrared n J A -B n-bond coupling constant between A and B atoms K degrees Kelvin kJ kiloJoule L U M O lowest unoccupied molecular orbital M central metal atom, or molar m multiplet M + parent ion Me methyl group, - C H 3 mg milligram(s) MHz megaHertz mL millilitre mm millimetre mmol millimole m-Ph meta-hydrogen in phenyl ring M O molecular orbital m s spin orientation quantum number OEPG octaethylporphyrinogen N M R nuclear magnetic resonance [PNP] [N(SiMe2CH2PR2)2]1" [P 2N 2] [PhP(CH2SiMe2NSiMe2CH2)2PPh]2" o-Ph ortho-hydrogen in phenyl ring Ph phenyl group, -C6H5 P-Ph phenyl group of [P2N2] phosphine /?-Ph para-hydrogen in phenyl ring ppm parts per million 'Pr isopropyl group R alkyl substituent RT room temperature S total spin quantum number s singlet or seconds t triplet trans transoidal THF tetrahydrofuran, C 4 H 8 O w 1/2 base width at half height 5 chemical shift, or a molecular orbital with 2 nodal pi r| x x = hapticity [i-X bridging X-ligand 71 molecular orbital with 1 nodal plane % percent %T percent transmittance o molecular orbital with no nodal planes ° degrees °C degrees Celsius A C K N O W L E D G E M E N T S xiii First, I must thank my supervisor, Dr. Mike Fryzuk. His guidance and support have been invaluable, particularly his editing of this thesis. His patience during the writing of this thesis has been nothing short of remarkable. To all of the members of the Fryzuk lab in general, thanks for the memories and forbearance. To Mike Shaver, for bringing laughter to the lab, to Mike Petrella, for his support, on the court and off, to Bruce McKay, for help with the hydrazine analysis and the use of his desk, to Lara Morello, for letting me deluge her with questions before her comprehensive, to Chris Kozak and Fran Kerton, for their help and general knowledge of chemistry, to Chris Carmichael, for letting me kick him off the PC so much, and especially to Sam Johnson, for answering my many questions with many more questions. I greatly appreciate the help of the U B C Department of Chemistry support staff, in particular Dr. Brian Patrick (X-ray crystallography), Brian Ditchburn (glassblowing), Peter Borda (elemental analysis), Marietta Austria and Liane Darge (NMR spectroscopy). Thanks also to all the people in stores for being so helpful when I was in desperate search of chemicals and equipment. Finally, a very special thanks to Teresa Jackson for always being there. xiv To my parents 1 CHAPTER 1 INTRODUCTION 1.1 Comparison of Hafnium and Zirconium The chemistry of hafnium is extremely similar to that of zirconium, more so than that of any other pair of elements. 1>2 The primary reason for this parallelism is the effect known as the lanthanide contraction, which accounts for the similarity in size between second and third row transition metals and can be rationalized as follows. There are four common types of orbitals: s,p,d, and f. Of these, f orbitals are the most diffuse and therefore the least penetrating. For this reason, f electrons do not shield valence electrons effectively in comparison with electrons in other orbitals.^ Therefore, the valence electrons of elements with f electrons are held more closely to the nucleus. However, this f electron effect is not sufficient by itself to explain the lanthanide contraction, somewhere between 10 and 30% of the effect can be contributed to relativistic effects,^ which will be discussed below. The effects of the lanthanide contraction are observed most strongly when comparing zirconium to hafnium. The result is that the radii of hafnium ions and atoms are nearly identical to those of zirconium. The M 4 + ionic radius is 0.74 A for zirconium versus 0.75 A for hafnium. The covalent radius is estimated at 1.45 A for zirconium versus 1.44 A for hafnium J Nearly every organometallic compound of hafnium for which X-ray crystal data are available is isostructural with its zirconium analogue, with Chapter 1 References begin on page 20 2 metal-ligand distances approximately 0.01 A to 0.02 A shorter for hafnium. ^ It should be noted that hafnium-ligand bonds are generally 5 to 10% stronger than their zirconium counterparts irrespective of the nature of the ligandA? jhe slightly smaller covalent radius may play a small part in this increased bond strength but it certainly does not completely account for it. In truth, the reason for this is not well understood, but the empirical evidence is unassailable. Only three bulk properties of these two metals are significantly different: density, neutron-absorbing abilities, and transition temperatures such as melting point and boiling point.^ These are summarized in Table 1.1. None of these three properties appears to have an effect on the reactivity of these two metals and are included only for completeness and to emphasize the similarity between hafnium and zirconium. Table 1.1 Properties of Hafnium and Zirconium which Differ Significantly Density Neutron Melting Point (°C) Boiling Point (°C) (gem"3) Absorptivity Zirconium 6.51 Very Low 1857 4200 Hafnium 13.28 600x higher 2222 4450 One notable exception to the chemical similarity between hafnium and zirconium is their reduction potentials. Interestingly, it is this difference in reduction potential that allows zirconium to be separated from hafnium. 8,9 An explanation for this difference Chapter 1 References begin on page 20 3 arises from an examination of the previously mentioned relativistic effects. For electrons in atoms or molecules, the velocity of the electron, v, will increase with increasing atomic number, Z. Thus, relativistic effects are more noticeable for heavier elements. As v increases, the relativistic rest mass of the electron, m, will increase and the relativistic average radius for s and p electrons will decrease. This results in a relativistic contraction of s and p orbitals. Since d and f orbitals are not as penetrating as s and p orbitals they will not be contracted. In fact, d and f orbitals will expand due to increased screening by the contracted s and p orbitals. ^  In addition, d and f orbitals are energetically destabilized due to this indirect effect. 1 M 2 In the case of zirconium/hafnium, the larger atomic number for hafnium leads to more contracted s and p shells and more diffuse and destabilized f and d orbitals. The contraction of the s and p shells explains the relativistic contribution to the lanthanide contraction mentioned earlier. The relativistic destabilization of the d orbitals on hafnium provides a theoretical explanation for the difficulty in reducing hafnium compared with zirconium. This destabilization discourages electron addition and thus makes reduction of hafnium more arduous. 13, 14 Due to the paucity of even partially, characterized hafnium(III) compounds, 7, 15 a comparison of reduction potentials with zirconium is difficult. However, Table 2.1 shows those compounds for which the reduction potentials of both a zirconium and hafnium analogue have been measured. The important fact to note is the increase in reduction potential in going from zirconium to hafnium, supporting the supposition that hafnium is more difficult to reduce than zirconium. Chapter 1 References begin on page 20 4 Table 1.2 Reduction Potentials for Zirconium/Hafnium Analogues Complex or Reaction Reduction Potential4 (V) Reference Hafnium Zirconium M 4 + ^ M -1.70 -1.55 16 MCl2{r|5-C5H3(SiMe3)2}2 -1.87 -1.55 17 MC1 6 2 " -3.1 -2.53 13 aReduction potentials are measured versus the saturated calomel electrode Another aspect of hafnium chemistry that demonstrates the difficulty in reducing hafnium(IV) is the almost complete lack of stable hafnium(III) compounds. Unlike titanium, for which an oxidation state of +3 is fairly common, zirconium(III) chemistry is comparatively rare5 and hafnium(III) chemistry is virtually non-existent. Only one hafnium(III) metallocene, shown in Figure 1.1, has been crystallographically characterized.^ A literature search did not reveal the existence of any other crystallographically characterized hafnium(III) compounds. Me3C^"M63 M e 3 C X H f _ , C M e ? Figure 1.1 The only crystallographically characterized hafnium(III) metallocene compound. Chapter 1 References begin on page 20 5 For a variety of reasons, the chemistry of hafnium is not considered very fashionable, at least in comparison with that of zirconium. 18 Besides the similarity of hafnium to zirconium, obtaining highly pure hafnium starting materials can be arduous because of the difficulty in separating it from zirconium. 15 In addition, despite the incredible similarity between the two elements, many of the practical applications of zirconium chemistry are less efficient when extended to hafnium chemistry. For example while certain zirconocenes have been shown to be fairly active polymerization catalysts, 19-21 n o highly active hafnium polymerization catalyst is yet known. As a result of these factors, the chemistry of hafnium is much less studied than that of zirconium. 1.2 I n t r o d u c t i o n to D i n i t r o g e n A major portion of this thesis deals with the activation of dinitrogen. As such, some background material is presented below. Dinitrogen comprises 78 percent of our atmosphere;22 however, unlike other small molecules such as C O 2 , O2, etc., which play an important role in metabolism, N2 does not. This seems surprising at first, because one would anticipate that such a ubiquitous and simple molecule would serve some function in such a complex lifeform as a human being. Furthermore, although N2 is isoelectronic with carbon monoxide, its reactivity is nowhere near as extensive as that of CO. A discussion of the bonding scheme of the N2 molecule may shed some light on this situation. Figure 1.2 shows the molecular orbital scheme of dinitrogen, as derived from Chapter 1 References begin on page 20 6 Schematic Description Energy Number of Electrons o N N O 17i g antibonding Degenerate pair in xy and xz planes (LUMO) -7 eV N C Z ^ N 3<jg weakly bonding (HOMO) -15.58 eV N N l 7 r g bonding Degenerate pair in xy and xz planes -17.1 eV ON N( 2au antibonding -18.7 eV 2ag bonding -35.5 eV (Estimated) Figure 1.2 A representation of the molecular orbitals of dinitrogen and their relative energies. photoelectron spectra^,24 md theoretical calculations.25,26 j h e r e are two aspects of this molecular orbital diagram that are worthy of discussion. First, the H O M O - L U M O gap of N 2 is very large (0.6 eV larger than that of carbon monoxide). Second, the HOMO energy is comparatively very low, meaning the large gap is at precisely the energy range Chapter 1 References begin on page 20 7 in which other molecules have frontier orbitals available for chemical reactions.27 These two features help to explain why dinitrogen is so unreactive compared with carbon monoxide. Of course, the lack of a dipole moment also contributes to N 2 ' s rather inert nature. 1.3 Nitrogen Fixation Despite its apparent inertness, nitrogen is an essential element in biological systems. It is present in many reduced forms in the molecules that form the building blocks of life: amino acids, nucleotides, etc. The abundance of nitrogen in the earth's solid crust is only 0.002% by mass.28 At one time, the only source of fixated nitrogen was from these scarce minerals (e.g. saltpeter (KNO3), and Chile saltpeter (NaNOa)) or from living sources. When land is extensively cultivated, fixed nitrogen is removed at a rate greater than that at which it can be naturally replenished. This requires that nitrogen-containing compounds be added to the soil as fertilizers, the most common of which is ammonia. In industry, the system discovered by Haber and made industrially feasible by Bosch dominates the nitrogen fixation landscape. The Haber process converts mixtures of dihydrogen and dinitrogen to ammonia using a heterogeneous iron catalyst and is shown in Equation 1.1.29 FG est N2(g) + 3 H2(g) • 2 NH3(g) 1.1 > 450 °C > 270 atm. Chapter 1 References begin on page 20 8 The ammonia produced may then be used as a fertilizer or converted into other nitrogen containing compounds, which may be used in explosives, plastics, fibers and industrial chemicals. Unfortunately, this process requires a large amount of energy to produce the high temperatures and pressures necessary. Recently, a ruthenium on graphite catalyst has been developed which allows for slightly milder conditions (70-105 atm, 350-470 °C), but this has not yet seen widespread use and is still under development.30 Nature, though, has no problems whatsoever fixing dinitrogen under ambient temperature and pressure. There are many bacteria that fix dinitrogen using enzymes known as nitrogenases.29 The commonly accepted reaction is shown in Equation 1.2. N 2 + 8H + + 8e~+16MgATP E " Z " » 2NH 3 + H 2 + 16MgADP + 16HP0 4 2" 1.2 It has long been known that the active site of many nitrogenase enzymes contains the transition metals iron and molybdenum. Recently, single-crystal X-ray analyses for FeMo proteins of Clostridium pasteurianum^^^2 and Azotobacter vinelandii^'^ have produced a model which contains cuboidal Fe4S3 and Fe3MoS3 units bridged by three sulfides as shown in Figure 1.3. While it is still not clear exactly how dinitrogen binds to this site, the six central Fe atoms are coordinatively unsaturated, possibly providing an N 2 coordination site. Also, nitrogenases have been discovered which contain vanadium-* 6,37 or iron atoms-^ in place of molybdenum. These discoveries indicate that many different transition metals may facilitate nitrogen fixation. Chapter 1 References begin on page 20 9 S T T — F e ^ — S ( C y s )—F e ^ - ; Fe' S - — F e ^ -S-F e — ; S / / "Fe s—Mo-—0C0 ^Fe—-s N(His) O O C C H 2 C — C H 2 C H 2 C O O " Figure 1.3 The structural model of the FeMo cofactor site determined by single crystal X-ray crystallography. 1.4 Transition Metal Dinitrogen Complexes For some time now, the creation of a catalytic nitrogen fixation cycle that takes place near room temperature and at low pressures has been considered one of the "holy grails" of chemistry. The presence of transition metals in nitrogenase and their use as catalysts in the Haber process have naturally led to an investigation of transition metal dinitrogen complexes. Allen and Senoff reported the first major breakthrough in this field in 1965 when they published their now famous paper-* 8 wherein they detailed the formation of [Ru(NH3)sN2]X2 where X = Br", I", BF4" and PF6". This compound is mononuclear and the bonding of dinitrogen is end-on, which is by far the most common mode of bonding in dinitrogen compounds.27,39,40 Figure 1.4 details the possible modes of coordination in transition metal dinitrogen compounds. Type A bonding is most often seen in 18-electron complexes with strong field ligands such as phosphines.27,39,40 Their N - N bond distances appear to be almost universally the same, around 1.12 A, no matter the nature of the metal or co-Chapter I References begin on page 20 10 ligands, while free N 2 has an N - N bond distance of 1.0975 A. 2 7> 3 9> 4 0 Thus, the dinitrogen ligand in the majority of these complexes is considered to be weakly activated. Crystallographically characterized complexes of type B have yet to been found. Such a binding mode has been proposed to occur as an intermediate in the end to end rotation of an r[l-N2 ligand in a rhenium complex, (n5-C5Me5)Re(CO)2(N2).41 The zirconium complex [Cp2Zr(CH2SiMe3)(N2)] was suggested to contain an n 2 - N 2 ligand on the basis of EPR and IR data, 4 2 but details are still uncertain. As well, an excited state of an osmium complex has been very recently shown to exhibit type B bonding using the photocrystallographic technique where a complex is excited while mounted on a diffractometer 4 3 End-on Mononuclear (A) M N N Side-on Mononuclear (B) M N ^ = N M M, N N M End-on Dinuclear (C) Side-on Dinuclear (D) M v .M X N End-on Side-on Dinuclear (E) Figure 1.4 The binding modes of dinitrogen in mononuclear and dinuclear transition metal compounds. Chapter 1 References begin on page 20 11 Type C is also fairly common and observed in transition metal dinitrogen metal complexes that span the periodic table.27,3 9 j n e o n i v k n o w n hafnium dinitrogen complex^ exhibits both type C and type A bonding. The route to the formation of this complex is shown in Figure 1.5. Cp* Cp* \ / Na/K, N 2 N % 2Cp* 2Hfl 2 N II N DME (-41 °C) \ V \ I Hf / \ Cp* Cp* F i g u r e 1.5 The only known hafnium dinitrogen complex, {Cp 2Hf(N2)}2(|-i-Although no molecular structure has been determined, the complex is believed to be isostructural with the zirconium analogue for which a molecular structure has been determined employing single crystal X-ray crystallography.45,46 Very careful control of reaction conditions was necessary to isolate any hafnium dinitrogen compound. Only when sodium/potassium alloy was used as the reducing agent, dimethoxyethane as the solvent and a hafnium iodide as the starting material, did the researchers isolate appreciable amounts of the dinitrogen compound. Attempts to prepare this compound using other reducing agents, solvents, and hafnium halides often resulted in a change of solution color to the appropriate color but it was not possible to isolate any dinitrogen complex in appreciable quantity. In contrast, the zirconium analogue was produced from Cp^ZrCb, not Cp*2ZrI 2, choice of solvent and reducing agent did not prove to be vital, Chapter 1 References begin on page 20 12 and yields of the zirconium dinitrogen compound were higher. The zirconium analogue has a bridging N 2 with an N - N bond distance of 1.182(5) A and terminal N 2 ' s with N - N bond distances of 1.116(8) A and 1.114(7) A, indicating a low level of activation of the dinitrogen ligands.45,46 Presumably, this also holds true for the hafnium dinitrogen compound. Examples of type D dinitrogen bonding are rare in comparison with types A and C . A nickel-lithium complex, [{(C 6H5Li)3Ni} 2N 2-2(C 2H 5) 20] 2, has been isolated in which the dinitrogen molecule is bound side-on with an unusually long N - N bond length of 1.35 A.47 As well, two samarium complexes,48,49 a n c j a u r a n i u m complex^ have been reported to exhibit this mode of bonding and are shown in Figure 1.6. (THF)2Li(OEPG) Cp* Cp* R R F i g u r e 1.6 Three dinitrogen complexes with type D bonding. In contrast to the nickel-lithium compound and the samarium complex on the left of Figure 1.6, the N - N bond length in the latter two complexes is comparable with free dinitrogen (1.09 A). Examples of type D bonding from this laboratory will be discussed below. A tantalum complex, {[NPN]TaH} 2N 2 ([NPN] = PhP(CH 2SiMe 2NPh) 2), which exhibits the very rare type E bonding^1 has recently been synthesized in the Fryzuk lab. Chapter 1 References begin on page 20 13 1.5 Ligands and Dinitrogen Complexes from the Fryzuk Group Ligands with a nitrogen donor atom are well known, as are ligands with a phosphorus donor atom. However, despite sharing the same group in the periodic table, nitrogen and phosphorus are certainly not identical in their chemical behaviour. Some differences between the chemistries of nitrogen and phosphorus are summarized in Table 1.3 Table 1.3 . Summary of the Differences in Chemical Properties between Nitrogen and Phosphorus5^ Nitrogen Phosphorus Very strong pn-pn bonds Weak pn-pn bonds Example: N 2 Example: P 4 pn-dn bonding is rare Weak to moderate but important d7c-p7r and d%-d% bonding No valence expansion Valence expansion Example: N F 3 Example: PF 5 It is the second property listed in Table 1.3 that has the most importance when considering ligands of each element. The inability of nitrogen to act as an electron density acceptor makes nitrogen ligands more suitable for electropositive transition metals and high oxidation states. Phosphorus ligands, on the other hand, are more suited to late metals and low oxidation states where electron density is available for backbonding. A particularly useful aspect of phosphine donors in ligands is the ability to 31 utilize P N M R spectroscopy; the phosphorus-31 nucleus (1=1/2) is 100% abundant and Chapter I References begin on page 20 14 possesses reasonable sensitivity (6.6% that of 'H) for spectroscopic study.53 Chemical shift values span a wide range (>600 ppm) and thus provide general information regarding coordination of a phosphine to a metal center. Spin coupling to other nuclei is quite sensitive to the stereochemistry of a complex and may supply a more detailed analysis of the coordination sphere. 'Hybrid' ligands have been designed which combine phosphine and amide donors. The first such ligand created in the Fryzuk lab was named [PNP] and is shown in Figure 1.7. Me 2 Me 2 R 2P M PR 2 R = Me, Ph, Pr1, Bu* F i g u r e 1.7 A pictorial representation of [N(SiMe 2CH 2PR2)2]" 1 ([PNP]). As an anionic, formal six-electron donor, the [PNP] ligand is somewhat comparable to the isoelectronic Cp ligand. Due to the presence of both amide and phosphine donors in one chelating array, [PNP] forms complexes with both early and late transition metals. 54-60 As w e \ ^ j t j s possible to change the chemical properties of the ligand by altering the substituents on the phosphines.^l Of particular interest to this thesis is the formation of ([PNP]ZrCl)2(u-r| 2:r| 2-N2) from a zirconium(IV) trichloride starting material as shown in Figure 1.8.62 Chapter 1 References begin on page 20 15 Me 2 F i g u r e 1.8 The route to formation of ([PNP]ZrCl) 2(u^r| 2:r | 2-N2). The N - N bond distance of 1.548(7) A in this complex is the longest discovered to date and is much longer than that observed for the N - N single bond of hydrazine, 1.46 A. This side-on bridging mode can be rationalized using qualitative molecular orbital arguments as presented in the literature^ and briefly summarized below. Different M O schemes can be generated for end-on and side-on bridging modes of the N 2 ligand between two M L 4 fragments. Three d orbitals on each metal (dxy,dXz and d y z) which point between the ligands on each of the M L 4 fragments and the N 2 unit are available for n or 5 bonding.63 in the case of end-on bridging, two % molecular orbitals are generated. In the side-on case, one % orbital and one 8 orbital are generated. These two cases are illustrated in Figure 1.9. Chapter 1 References begin on page 20 16 End-on Bonding Side-on Bonding F i g u r e 1.9 An orbital interaction diagram for the bonding molecular orbitals of side-on and edge-on bridging of the dinitrogen ligand between two M L 4 fragments. In general, end-on bridging is a more favourable coordination mode because § orbitals are intrinsically less stabilized than % orbitals. The reason for this is that overlap for 5 orbitals should be less than that for n orbitals in analogy to metal-metal quadruple bond arguments.64 So the question arises, why does side-on bridging arise at all? In the case of ([PNP]ZrCl)2(n-ri2:r|2-N2), it is believed that the amide donor is held by the phosphine ligands in such a conformation that it overlaps with the appropriate d orbital to form a % bond. This prevents that particular metal d orbital from n bonding with the N 2 unit, thus forcing the side on bridging conformation. 62 Interestingly, i f one of the chlorides in the starting material is replaced with a Cp ligand, the bonding mode of dinitrogen changes to end-on and the bond length decreases to 1.301(3) A. It appears that Chapter 1 References begin on page 20 17 this is due to % overlap between the Cp ligand and the d orbital that would be used for 8 overlap with the N2 unit. Clearly, the choice of ancillary ligands plays an important role in determining the availability of certain orbitals which, in turn, determines which bonding mode is lower in energy. Attempts have been made to form a [PNP] hafnium dinitrogen complex in a similar manner but they were unsuccessful. While [PNP] has proven to be an extremely versatile ligand, and much fascinating chemistry has been reported,54-58,60,65 o n e m a j o r 'pitfall' proved to be the dissociation of the phosphine donors from the metal. This dissociation of the phosphine donors led to uncontrolled reactivity, particularly for the early transition metals.56,61 j n a n attempt to circumvent this problem, an anionic macrocyclic ligand, [PhP(CH2SiMe2NSiMe2CH2)2PPh]2" ([P 2N 2]) was designed using lithium as a template.66 The synthesis of [P2N2] is shown in Figure 1.10 in a form slightly modified from that presented in the literature. The amide ligands will form strong bonds with the early transition metals and the macrocylic ligand design holds the phosphines more firmly in place than the previous chelating ligand, [PNP]. As well, conversion from a chelating ligand to a macrocyclic ligand allows for further stabilization of complexes due to the macrocyclic effect. 67 Chapter 1 References begin on page 20 18 Me 2 Me 2 / S i ^ Si CI CI 2 LiPHPh Et 20 O 0 C -2LiCI r Ph-PH Me 2 S i . Me 2 N ^ HP—Ph H CI k H -N. Si Me 2 CI . J Si Me 2 4 "BuLi Et 20 0 ° C - 4nBuH -2LiCI Me 2 Me 2 SL .Si S'- N Ph—P-f Si Si Me 2 Me 2 R-Ph 0 Me 2 .Si Me 2 Si Ph—P-<-x> N S Si Si R-Ph Me, Men S=Et2Q Figure 1.10 The route to formation of [P2N2]Li2(l,4-dioxane). The [P 2 N 2 ] ligand has been used in the preparation of many metal complexes.68-75 J^Q most relevant metal complexes for this thesis are those of zirconium, which will be discussed in the following chapter. Chapter 1 References begin on page 20 19 1.6 The Scope of this Thesis The objective of this thesis is to explore the chemistry that arises when the [P2N2] macrocyclic ligand is bound to the group 4 metal, hafnium. As detailed in the introduction to each of the following two chapters, the incorporation of [P2N2] onto zirconium led to some novel and intriguing chemistry. Due to the similarity between zirconium and hafnium it was hoped that much of this zirconium chemistry might be duplicated or perhaps expanded upon with hafnium. The work presented in Chapter 2 focuses on the synthesis and characterization of a new hafnium dinitrogen complex. Likely due to the difficulty in reducing hafnium, in comparison with zirconium, this synthesis was not as straightforward as originally hoped. The molecular structure of [P2N2]HfCl2 is also presented. Further investigation into organometallic hafnium complexes has been explored and is discussed in Chapter 3 along with a new hafnium hydride. Again, while some of the chemistry for hafnium is very similar to that of zirconium, there are appreciable differences in the synthetic routes to some of these complexes. Chapter 1 References begin on page 20 20 1.6 References 1) Cotton, F. A. ; Wilkinson, G. Advanced Inorganic Chemistry; 5 ed.; John Wiley & Sons: New York, 1988, pp 778. 2) Ibid., pp 957. 3) Greenwood, N . N . ; Earnshaw, A. Chemistry of the Elements; 2 ed.; Reed Educational and Professional Publishing Ltd, 1997, pp 1232. 4) Laerdahl, J. K.; KFaegri, J. J. Chem. Phys. 1998, 109, 10806. 5) Cardin, D. J.; Lappert, M . F.; Raston, C. L. Chemistry of Organo-Zirconium and -Hafnium Compounds; 1st ed.; Ellis Horwood Limited: West Sussex, 1986, pp 18. 6) Lappert, M . F.; Patil, D. S.; Pedley, J. B. J. Chem. Soc, Chem. Commun. 1975, 830. 7) Wayne, A. K. ; Bella, S. D.; Gulino, A.; Lanza, G.; Fragala, I. L. ; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 355. 8) Larsen, E. M . ; Moyer, J. W.; Gil-Arnao, F.; Camp, M . 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Soc. 2000,122, 532 44) Roddick, D. M . ; Fryzuk, M . D.; Seidler, P. F.; Hillhouse, G. L.; Bercaw, J. E. Organometallics 1985, 4, 97. 45) Manriquez, J. M . ; Bercaw, J. E. J. Am. Chem. Soc. 1974, 96, 6229. 46) Manriquez, J. M . ; Sanner, R. D.; Marsh, R. E.; Bercaw, J. E. J. Am. Chem. Soc. 1976, 98, 8351. 47) Jonas, K. Angew. Chem. Int. Ed. (English) 1973,12, 997. 48) Evans, W: J.; Ulibarri, T. A. ; Ziller, J. W. J. Am. Chem. Soc. 1988,110, 6877. 49) Jubb, J.; Gambarotta, S. J. J. Am. Chem. Soc. 1994,114, 4411. 50) Roussel, P.; Scott, P. J. Am. Chem. Soc. 1998, 120, 1070. Chapter 1 References begin on page 20 23 51) Fryzuk, M . D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc. 1998, 120, 11024. 52) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 5 ed.; John Wiley & Sons: New York, 1988, pp 384. 53) Abraham, R. J.; Fisher, J.; Lotus, P. Introduction to NMR Spectroscopy; John Wiley & Sons: New York, 1988, pp 4. 54) Fryzuk, M . D.; Macneil, P. A. ; Rettig, S. J.; Secco, A. S.; Trotter, J. Organometallics 1982, 1, 918. 55) Fryzuk, M . D.; Rettig, S. J.; Westerhaus, A. ; Williams, H . D. Inorg. Chem. 1985, 24, 4316. 56) Fryzuk, M . D ; Carter, A.; Westerhaus, A. Inorg. Chem. 1985, 24, 642. 57) Fryzuk, M . D.; Joshi, K. Organometallics 1989, 8, 722. 58) Fryzuk, M . D.; Mylvaganam, M ; Zawarotko, M . J.; MacGillivray, L. R. J. Am. Chem. Soc. 1993, 115, 10360. 59) Fryzuk, M . D.; Mylvaganam, M . ; Zawarotko, M . J.; MacGillivray, L. R. Organometallics 1996, 15, 1134. 60) Fryzuk, M . D.; Leznoff, D. B.; Ma, S. F.; S.J.Rettig; V .G . Young, J. Organometallics 1998 ,77,2313. 61) Fryzuk, M . D.; Carter, A. Organometallics 1992,11 , 469. 62) Fryzuk, M . D.; Haddad, T. S.; Murugesapilla, M . ; McConville, D. H . ; Rettig, S. J. J. Am. Chem. Soc. 1993,115, 2782. 63) Albright, T. A.; Burdett, J. K.; Whangbo, M . H. Orbital Interactions in Chemistry; Wiley-Interscience: New York, 1985. Chapter 1 References begin on page 20 24 64) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 5 ed.; John Wiley & Sons: New York, 1988, pp 1087. 65) Fryzuk, M . D.; Mylvaganam, M . ; Zawarotko, M . J.; MacGillivray, L. R. Organometallics 1996, 15, 1134. 66) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. J. Chem. Soc, Chem. Commun. 1996, 2783. 67) Cotton, F. A. ; Wilkinson, G. Advanced Inorganic Chemistry; 5 ed.; John Wiley & Sons: New York, 1988, pp 47. 68) Bowdridge, M . R. Niobium Phosphine Macrocyclic Complexes; UBC: Vancouver, 1998, pp 80. 69) Fryzuk, M . D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997, 275, 1445. 70) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997,119, 9071. 71) Fryzuk, M . D.; Giesbrecht, G. R.; Rettig, S. J. Inorg. Chem. 1998, 37, 6928. 72) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. Organometallics 1998,17, 846. 73) Fryzuk, M . D.; Johnson, S. A.; Rettig, S. J. Organometallics 1999, 18, 4059. 74) Jafarpour, L. New Ligands: Design and Coordination Chemistry with the Transition Metals andLanthanides; UBC: Vancouver, 1999. 75) Leznoff, D. B. Paramagnetic Organometallic Complexes; UBC: Vancouver, 1997, pp 318. Chapter 1 References begin on page 20 25 CHAPTER 2 TOWARDS A NEW HAFNIUM DINITROGEN COMPLEX 2.1 Introduction to [P2N2] Zirconium Complexes The inspiration for the hafnium chemistry which appears in this thesis arose from some very interesting reactions of [P2N2]ZrCl2 (A). As such a brief discussion of this zirconium chemistry is in order. A series of reactions arising from [P 2 N 2 ]ZrCl 2 is shown in Figure 2.1. C Figure 2.1 Some of the reactions of [P 2 N 2 ]ZrCl 2 which have relevance to this thesis. Chapter 2 References begin on page 48 26 Previous work from the Fryzuk lab has shown that reduction of [P2N2]ZrCl2 (A) under N2 results in the formation of a dinuclear zirconium dinitrogen complex, {[P 2N 2]Zr} 2(u-n 2:r| 2-N 2) (B).1 Similar to the [PNP] complex shown in Figure 1.8, this complex, also contains dinitrogen coordinated in a side-on mode. The X-ray crystal structure of this compound indicated an N - N bond length of 1.43 A. Also shown in Figure 2.1 is an unprecedented reaction whereby the dinitrogen complex reacts with H 2 to form {[P2N2]Zr}2(u-Ti2-N2HXu-Fl) (C), which has an N - H bond and a bridging hydride.1 Moreover, under an argon atmosphere, reduction of A yields the zirconium dimer, {[P2N2]Zr}2 (D).2 The phenyl groups coordinated to zirconium in this compound are 'activated', that is, they the metal center has donated electron density to the phenyl rings and they have lost their aromaticity. Evidence of this is supplied both in the X-ray crystal structure and in the ! H N M R spectrum. The goal of this chapter is to detail attempts to extend the chemistry shown in Figure 2.1 by altering the metal center from zirconium to hafnium. The first step towards completion of this goal was to synthesize the hafnium analogue of A. 2.2 Synthesis and Characterization of [P2N2]HfCl2 Reaction of the TFIF adduct of hafnium(I V) tetrachloride with one equivalent of [P 2N 2]Li 2(l,4-dioxane) resulted in the formation of [P2N2]HfCl2 (1). The optimum conditions for this reaction occurred when a suspension of starting materials was refluxed in toluene overnight at approximately 120 °C. This resulted in isolation of diamagnetic 1, as a white solid, in excellent yields; this is shown below in Equation 2.1. The reaction Chapter 2 References begin on page 48 27 also takes place in THF at room temperature but the yields are lower. Whether this is due to the lower temperature or the change in solvent is uncertain. .1 [P2N2]HfCl2 (1) Complex 1 was characterized by J H and 3 I P{ 1 H} N M R spectroscopy, microanalysis and X-ray crystal structure analysis. Crystals of 1 suitable for X-ray diffraction were grown from a saturated D M E solution. An ORTEP diagram of 1 is shown in Figure 2.2. The solid state molecular structure of 1 is best described as distorted octahedral. N(l) , N(2), Cl(l) and Cl(2) can be described as forming a square plane with a combined equatorial angle of 360.1°. P(l) and P(2) can then be described as adopting axial positions with a P(l)-Hf(l)-P(2) angle of 153.76(3)° which is drawn back from the optimum 180° due to the constraints of the macrocyclic ligand framework. The small Chapter 2 References begin on page 48 28 cavity size of the [P2N2] ligand forces the metal to sit atop the ligand, which places the two chlorides cis to each other. Si(3) F i g u r e 2.2 An ORTEP diagram of [P2N2]F£fCl2 (1) showing the solid state molecular structure. The silyl methyl groups have been omitted; only the ipso carbons of the phosphorus phenyl groups are shown. The [P2N2] framework is distorted, adopting a C2 twist in the solid state. This feature has been observed in many [P2N2] and [PNP] c o m p l e x e s . 3 T h e solid state structure of 1 is nearly identical to that of its zirconium analogue, [P2N2]ZrCl2 (A), and Table 2.1 below shows a comparison of selected bond lengths and angles for the two compounds. As with the zirconium analogue, the Hf-N, -P and -CI bond lengths are not unusual and compare with those of other Group 4 amido and phosphine complexes.4>9-12 Chapter 2 References begin on page 48 29 Table 2.1 A Comparison of Selected Bond Lengths (A) and Angles (deg) in [P 2N 2]MC1 2 M=Zr, Hf Bond Lengths M=Hf M=Zr Bond Angles M=Hf M=Zr M(l) -N(l ) 2.125(3) 2.136(4) N(l)-M(l)-N(2) 96.6(1) 96.8(2) M(l)-N(2) 2.134(3) 2.125(4) P(l)-M(l)-P(2) 153.76(3) 152.52(6) M(l)-P(l) 2.684(1) 2.694(2) C1(1)-M(1)-C1(2) 83.32(4) 82.57(7) M(l)-P(2) 2.673(1) 2.707(2) N(1)-M(1)-C1(2) 90.88(9) 89.5(1) M(1)-C1(1) 2.461(1) 2.455(2) N(2)-M(1)-C1(1) 89.22(9) 91.3(1) M(1)-C1(2) 2.461(1) 2.448(2) P{ 1H} —i—i—i—i—i—i—i—i—i—i i i i i i i i i i 8.0 7.6 7.2 2.0 1.0 0.0 (ppm) (ppm) Figure 2.3 The  lH and  3lP{ lri} N M R spectra of [P 2 N 2 ]HfCl 2 (1). The two regions of the ^ N M R spectrum are not shown on the same scale. Chapter 2 References begin on page 48 30 As part of an ongoing theme of this thesis, a comparison with the zirconium analogue of this compound is in order. Figure 2.3 shows the *H and 3 1 P{ 'H} N M R spectra of [P 2 N 2 ]HfCl 2 (1). The 3 1 P{ 1 H} N M R spectrum of 1 shows a singlet resonancewhich is shifted 8.5 ppm downfield from that of the zirconium analogue. The ' H N M R spectrum is virtually identical to that of the zirconium analogue with slight displacements of each characteristic signal. The solid state molecular structure of 1 does not appear to hold in solution. The solution structure is more compatible with a C 2 v symmetry. Only 2 resonances due to silyl methyl protons are observed reflecting the "top and bottom" asymmetry of 1. If the C 2 symmetry of the solid state held in solution, one would expect to observe 4 silyl methyl resonances. It seems, therefore, that the [P 2N 2] framework is quite flexible in solution and undergoes a "rocking" motion centered at the trigonal silyl amide groups as shown in Figure 2.4. CI ci 1 -Figure 2.4 The process by which equilibration of [P 2N 2] silyl methyls is believed to take place. Silyl methyl groups are omitted for clarity. Chapter 2 References begin on page 48 31 2.3 P r e l i m i n a r y A t t e m p t s to F o r m a H a f n i u m D i n i t r o g e n C o m p l e x Despite the knowledge that reduction of hafnium is not as easy as reduction of zirconium, it was still believed that attempting to synthesize a hafnium dinitrogen complex would be a worthy endeavor. The first attempt followed a procedure identical to the one that formed the zirconium dinitrogen complex, involving reduction of the starting chloride complex with 2 equivalents of potassium graphite 1 but the results were not as expected. Instead of the desired dinitrogen complex, the major product, as indicated by 3 1 P{'H} N M R spectroscopy, was a hafnium dimer, {[P 2N 2]Hf} 2 (2), with coordination of the activated phenyl rings on one phosphorus to the other metal atom as shown in Equation 2.2. CI c i V C 8 K 4 atm N 2 Hf P 8 Days Toluene Silyl methyl groups omitted for clarity 2.2 2 Chapter 2 References begin on page 48 32 As shown in Figure 2.1, this compound also has a zirconium analogue, {[P 2N 2]Zr} 2 (D), which is the major product when [P 2 N 2 ]ZrCl 2 (A) is reduced with CgK in the absence of N 2 , for example under Ar or vacuum.2. Complex D was first noticed as an impurity in the reaction that produces {[P 2N 2]Zr} 2(u.-ri 2-N 2) (B). For the zirconium system, under N 2 , the dimer was a fairly minor impurity. Unfortunately, when the metal center is changed to hafnium, the dimer appears to be the major product of this reaction, at least under these conditions. Figure 2.5 shows the *H and 3 1 P{ 1 H} N M R spectra of {[P 2N 2]Hf} 2 (2). r~si^N - S i "n s i P 7 ± - s.. U s Si N - S i - J 31 p { 1 H j L^JJ {[P2N2]Hf}2 (2) 1 H C 6 D 5 H Ph SiMe 2 Ring Activated C H 2 AAxV 15 10 - i — i — i — i — i i i r 5 0 (ppm) 4 (ppm) F i g u r e 2.5 The lU and 3 1 P{ 'H} N M R spectra of {[P 2N 2]Hf} 2 (2). A timed reaction of [P2N2]HfJCl2 (1) with 2 equivalents of CgK under 1 atmosphere of nitrogen was undertaken in order to carefully monitor the reaction by 3 1 P{ ] H} N M R spectroscopy. The results are shown below in Table 2.2 and Figure 2.6 indicates the signals listed in Table 2.2. Chapter 2 References begin on page 48 33 Table 2.2 The results of a study of the reaction of [P2N2]HfCl2 (1) with 2 equivalents of CgK under 1 atm N 2 in toluene Compound [P2N2]HfCl2 (1) {[P2N2]Hf}2 (2) Others 3 1 P Shift (ppm) -5.8 (s) 11.7(m), 1.9(m)a 8.6 (s) -8.2 (s) % after 1 Hour" 89% 8.5% 1.8% 0.0% 4 Hours 52% 38% 6.8% 1.6% 8 Hours 20% 63% 12% 3.3% 25 Hours 0% 80% 16% 3.3% 4 Days 0% 80% 17% 2.9% aSpin system appears to be A A ' B B ' . bPercentages of each compound determined from integration of their 3 1 P{ 1 H} N M R signals. 31P{1H} {[P2N2]Hf}2(2) " i — 1 r Postulated [P2N2]Hf(C7H8) (5) 10 ~ l — I — I 6 ~ i —1 — i — 1 r [P2N2]HfCI2(1) 2 (ppm) Postulated {[P2N2]Hf}2(n-Ti2:r,2-N2) (4) Figure 2.6 The 3 1 P{ 1 H} N M R spectrum of [P2N2]HfCl2 (1) + 2 CgK under 1 atm N 2 after 25 hours. * indicates unknown. Chapter 2 References begin on page 48 34 While this experiment was not particularly enlightening, there are two features that are worthy of discussion. First, the reaction that produces the dimer, 2, is much faster than initially anticipated, appearing to be complete after only 25 hours. Secondly, the signals at 8.6 ppm and -8.2 ppm are intriguing. From a comparison with the chemical shifts in analogous zirconium compounds, it can be speculated that the signal at 8.6 ppm is due to a toluene complex, 5, while the signal at -8.2 ppm is due to a dinitrogen complex. The formation of the toluene complex, 5, is interesting because once again, as with the dimer, much more is formed for hafnium than is observed for zirconium in the analogous reaction. In the case of zirconium, formation of the toluene complex in this reaction is practically negligible. In fact, characterization of the zirconium toluene complex came from a completely different reaction between [P2N2]ZrMe2, hydrogen and toluene,^ which will be described in greater detail in Chapter 3 While the signal assigned to the dinitrogen complex is very small, it is an encouraging sign, as it indicates that modification of this procedure may provide a route to {[P2N2]Hf}2(n-ri2:r|2-N2) (4). Equation 2.3 summarizes the compounds that appear to have formed at the end of this reaction as determined from 3 1 P{ 1 H} N M R spectroscopy. One disappointing aspect of this experiment was the color of the solution produced. While the majority of zirconium dinitrogen complexes exhibit an intense deep blue or blue-green color in solution, the solution in this case was a dark brown which matches the color of {[P2N2]Zr}2 (D) not { ^ ^ Z r H u - r i ' . - n 2 - ^ ) (B). As these reaction conditions proved to be unsuccessful in forming any appreciable quantity of the desired hafnium dinitrogen compound, the first modification to the Chapter 2 References begin on page 48 35 CI CI V Hf l \ ^-Si-— I Silyl methyl groups omitted for clarity S i N 1 2 C 8 K 1 atm N 2 Hf-U s i N-si—I 2 80 % 4 Days To luene Hf / \ N N \ / L _ S >N^s i J N - S i - f t 4 3.3 % 2.3 -Hf^ U s r N - s U 5 16 % synthetic plan was to change the reducing agent. Sodium-potassium alloy (Na/K) was used to prepare the only known hafnium dinitrogen compound, and so Na/K was the logical alternative. Unfortunately, using either toluene or D M E as solvents, an intractable gray paste was formed from which no N M R data were interpretable and attempts to grow crystals were also unsuccessful. Chapter 2 References begin on page 48 36 2.4 Synthesis and Characterization of [P 2N2]HfI 2 (3) As previously described, the only known hafnium dinitrogen complex, (Cp*2Hf(N2)}2(u.-N2), was prepared from a diiodide complex after attempts to use a dichloride complex as a starting material were unsuccessful. Thus, it seemed reasonable to investigate the effects of a change in the precursor to be reduced. Reaction of a toluene solution of [P2N 2 ]HfCi2 (1) with a slight excess of trimethylsilyliodide resulted in formation of [P2N2]HfI2 (3) as shown in Equation 2.4. In this case, there is no analogous zirconium compound with which to compare complex 3. Assuming an analogous structure for the diiodide as was found for the dichloride, one would expect little change in the N M R spectra upon replacement of chloride with iodide, and to a degree this is observed. The 3 1 P{ 1 H} N M R signal is still a singlet resonance and Chapter 2 References begin on page 48 37 the change in chemical shift from that of 2 is 0.4 ppm downfield. In the ' H N M R spectrum of the diiodide complex, the two silyl methyl signals are now separated by 0.3 ppm while for the dichloride complex the separation is less than 0.05 ppm. The 3 1 P{ 1 H} and ! H N M R spectra are shown below in Figure 2.7. 3 1P{1H} I I V J — S i I—S>'N ] H o-Ph 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -5 -6 -7 -8 (ppm) m,p-Ph C 6 D 5 H 8.0 7.6 7.2 (ppm) 2.0 [P2N2]Hfl2 (3) Ring C H 2 ^ A M \ AAAA\ 1.0 (ppm) SMe? 0.0 F i g u r e 2.7 The X H and 3 1 P{ 1 H) N M R spectra of [P2N2]Hfl2 (3). The two regions of the N M R spectrum are not shown on the same scale. To date, only a partially solved X-ray crystal structure is available for 3, as all crystals grown have exhibited twinning and refinement proved impossible. For this reason, an ORTEP diagram is unavailable; however, the X-ray crystal data available do show the Chapter 2 References begin on page 48 38 connectivity of the molecule is that postulated from the ' H and 3 I P{ 1 H} N M R data and shown in Figure 2.7. 2.5 Synthes is a n d N M R C h a r a c t e r i z a t i o n o f {[P 2N 2 ]Hf}2 (u - r| 2 :r i 2 -N2) (4) Reaction of [P 2N 2]HfI 2 (3) with 2 equivalents of CsK under 1 atmosphere of dinitrogen produced a mixture of products and a solution with an intense blue color, a color characteristic of the {[P2N2]Zr}((j,-ri2:ri2-N2) analogue. These products, their tentative assignments, and their percentages as determined by 3 1 P{'H} N M R spectroscopy are summarized in Table 2.3. T a b l e 2.3 The results of a study of the reaction of [P2N2]Fffl2 (3) with 2 equivalents of CgK under 1 atm N2 in toluene Compound [P2N2]Hfl2 (3) {[P2N2]Hf}2 (2) [P 2N 2]Hf(C 7H 8) (5) {[P2N2]Hf}2(!a-N2) (4) 3 1 P Shift (ppm) -5.4 (s) 11.7(m), 1.9(m)a 8.6 (s) -8.2 (s) % after 1 Hour" 100% 0% 0% 0% 4 Hours 77% 9.2% 8.4% 5.7% 8 Hours 55% 21% 14% 9.4% 25 Hours 31% 26% 21% 20% 72 Hours 4.6% 33% 32% 29% aSpin system appears to be A A ' B B ' . Percentages of each compound determined from integration of their 3 1 P{ 1 H} N M R signals. Chapter 2 References begin on page 48 39 Figure 2.8 shows the 3 1 P{ 1 H} N M R spectrum after 25 hours for the reaction of [P2N2]Hfl2 (3) with 2 equivalents of CgK under 1 atmosphere of nitrogen. In comparison with Figure 2.6, the signal at 8.6 and the signal at -8.2 ppm have increased in size (and integration sum) rather dramatically while the signals for the dimer, 2, have decreased. The 3 1P{ 1F£} N M R chemical shift of the speculated dinitrogen complex combined with the intense blue color was an excellent indication of dinitrogen complex formation, but it remained uncertain that the signal at -8.2 ppm was due to a dinitrogen complex. P{1H} Postulated [P2N2]Hfl2 (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 — 15 10 5 0 -5 -10 (ppm) Figure 2.8 The 3 1 P{ 1 H} N M R spectrum of the reaction between [P2N2]Hfl2 (3) and 2 equivalents of CgK under 1 atm N2 after 25 hours. Chapter 2 References begin on page 48 40 In an attempt to increase the yield of the dinitrogen complex, 4, the reaction was repeated in a sealed reactor, under 4 atmospheres of dinitrogen for a period of 8 days. With these changes in reaction conditions, the signal at -8.2 ppm became the major signal. The large increase in integration size of this signal, when the only change to the reaction is to increase the N 2 pressure, is further evidence that it can be assigned to a dinitrogen complex. Unfortunately, a higher percentage of starting material remains when compared to the same reaction using 1 atm of N 2 , and the other impurities are still present although in smaller amounts as shown in Figure 2.9 and listed in Table 2.4. Postulated 31P{1H} {[P2N2]Hf}2(u-T12:Ti2-N2) (4) Postulated [P2N2]Hf(C7H8) (5) "I 1 1 1 1 1 1 1 1 1 1 <~ ~l 1 I 1 I r 14 12 10 8 6 4 2 0 -2 -4 -6 -8 -10 (ppm) F i g u r e 2.9 The 3 1 P{ 'H} N M R Spectrum of the reaction between [P 2N 2]HfI 2 (3) and 2 equivalents of CgK under 4 atm N 2 after 8 days. Chapter 2 References begin on page 48 41 T a b l e 2.4 The results of a study of the reaction of [P 2N2]HfI 2 (3) with 2 equivalents of C g K under 4 atm N 2 in toluene Compound [P 2 N 2 ]Hfl 2 3 {[P 2N 2]Hf} 2 2 [P 2N 2]Hf(C 7H 8) 5 {[P 2N 2]Hf} 2(u-N 2) 4 J 1 P Shift (ppm) -5.4 (s) 11.7(m), 1.9(m)a 8.6 (s) -8.2 (s) % after 8 Days under 4 atm N 2 19% 16% 13% 52% Previous work from the Fryzuk laboratory has shown that at room temperature the ' H N M R spectrum for the analogous zirconium dinitrogen compound, {[P2N2]Zr}2(u,-r | 2:r | 2-N 2) (B) has the following characteristics. The phenyl region has a fairly broad 2 {[P 2N 2]Hf} 2 3 [ P 2 N 2 ]H f l 2 4 {[P 2N 2]Hf} 2(u-Ti 2:r| 2-N 2) 5 [ P 2 N 2 ]H f (C 7 H 8 ) 1 1 I I 1 1 1 1 | i i l n i i i l | l l l l i i i i i | i i i i i i i i i | I I | l i i M i i i i | I i [ l i i i i i i l l | l i 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 -0.0 -0.1 -0.2 -0.3 (ppm) F i g u r e 2.10 The silyl methyl region of the N M R spectrum for the products from the reaction of [P 2N 2]HfI 2 (3) with 2 equivalents of C 8 K in toluene for 8 days. Chapter 2 References begin on page 48 42 signal at 8.1 ppm assigned to the o-Ph proton, while the m-Ph and p-Ph protons are sharper at 6.9 ppm. The methylene region shows a characteristic multiplet. The silyl methyl region shows one sharp resonance at 0.4 ppm and a broad resonance at 0.2 ppm. At low temperatures (eg. -80°C) 4 silyl methyl resonances can be clearly seen, while at high temperatures, the broad signal becomes much sharper giving two sharp signals in total for the silyl methyl region. Figure 2.10 shows the silyl methyl region of the ] H N M R spectrum for the products from the reaction of 3 with 2 equivalents of CgK in 2 {[P2N2]Hf}2 3 [P 2 N 2 ]Hf l 2 , 4 4 {[P 2N 2]Hf} 2(n-T 1 2:T 1 2-N 2) 5 [P 2 N 2 ]Hf(C 7 H 8 ) i i i i i i i i i i i 1 1 1 1 1— 8.2 8.0 7.8 7.6 7.4 7.2 7.0 6.8 (ppm) F i g u r e 2.11 The phenyl region of the ' H N M R spectrum for the products from the reaction of 3 with 2 equivalents of CgK in toluene for 8 Days. A l l unmarked signals are indeterminable due to overlap. Chapter 2 References begin on page 48 43 toluene for eight days. Compounds 2 and 3 have been previously isolated so their silyl methyl signatures could be unambiguously assigned. 4 and 5 were assigned by comparing this ' H N M R spectrum with the ' H N M R spectra for the analogous zirconium complexes. Inspection of this lH N M R spectrum reveals that the broad resonance seen for {[P2N 2]Zr} 2(p.-r| 2:ri 2-N2) (B) at 0.2 ppm seems to be missing. It is possible that it is either buried under the impurity signals or broadened to a greater extent than observed for the zirconium analogue. The phenyl region of the ' H N M R spectrum is shown in Figure 2.11 where, once again, assignment of the product signals was done first by assigning previously isolated compounds 2 and 3 and then by comparison with the zirconium analogues. The three lowest field resonances in Figure 2.11 are due to the o-Ph protons for each of the marked compounds. The m/p-Ph proton assignments are naturally more tentative due to extensive overlap. The methylene region of this J H N M R spectrum is extremely complicated due to signal overlap among the 4 products. Attempts to isolate 4 have been unsuccessful to this point. Each of the reaction components appears to have very similar solubilities in all solvents used. 2.6 F u r t h e r C h a r a c t e r i z a t i o n o f {[P 2N 2 ]Hi7 2 (u-Ti 2 :r | 2 -N2) (4) Further evidence for the formation of 4 comes from E.I. mass spectrometry data. Shown in Figure 2.12 is a portion of the E.I. mass spectrum. Figure 2.13 shows the theoretical mass spectrometry signature one would expect from 4. This signature was created using Isotope™ v. 1.6.6, a program designed specifically to model mass Chapter 2 References begin on page 48 44 spectrometry signatures. The theoretical signature is a near exact match for the experimental one shown in Figure 2.12. In Figure 2.12, signatures for {[P 2N 2]Hf}2N where a single nitrogen molecule has been stripped from the parent compound and for {[P 2N 2]Hf} 2 (2), can also be seen and are labeled. 55 c o 100 , 80 60 40 ([P 2N 2]Hf) 2N 2 20 1350 ([P 2N 2]Hf) 2 1400 M/z ([P 2N 2]Hf) 2N 1450 F i g u r e 2.12 Selected E.I. mass spectrometry data for ([P 2N 2]Hf) 2N 2 100 80 -I t 60 J | 40 20 0 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 M/z F i g u r e 2.13 Theoretical isotope distribution pattern for E.I. mass spectrometry analysis of ([P 2N 2]Hf) 2N 2 as determined by Isotope Pattern Calculator™ v. 1.6.6. Chapter 2 References begin on page 48 45 Not shown here, but also observed in the E.I. mass spectrum, are the signatures for [P2N2]Hf(C7H8) (5) and for [P2N2]HfI and [P2N2]Hf. Interestingly, no signature is observed for [P2N2]HfI2 (3), likely due to the weakness of the Hf-I bond. To confirm the presence of a dinitrogen compound in the products from the reaction of [P2N2]HfI2 (3) with 2 equivalents of CgK , the mixture of products was degraded with H C 1 and the resultant products analyzed to detect hydrazine according to literature procedure. 1 3 Hydrazine was detected, confirming the presence of a dinitrogen compound, however it was not possible to determine the percent of hydrazine formed per moi of {[P2N2]Hf}2(n-r|2:ri2-N2) (4), due to the inability to isolate 4. 2.7 P r e l i m i n a r y R e a c t i v i t y S t u d y on {[P 2N 2]Hf}2(n-Ti 2:r | 2-N 2) (4) As discussed in the introduction to this chapter, the zirconium analogue of 4 shows unprecedented reactivity with H 2 , forming a compound with a bridging hydride and an N - H moiety as opposed to simply eliminating N 2 . While to this point it has not been possible to isolate 4, the mixture of products, which includes 4, was reacted with H 2 as shown in Equation 2.5. Figure 2.14 shows the 3 1 P{ 1 H} and *H N M R spectra for this reaction. 2 {[P 2N 2]Hf} 2 3 [ p 2N 2 ]H f l 2 4 a t m H 2 • 2.5 4 {[P2N2]Hf}2(H-ri2:r,2-N2) H e x a n e s 5 [P 2 N 2 ]Hf(C 7 H 8 ) Chapter 2 References begin on page 48 46 Figure 2.14 The *H and 3 1 P{ ! H} N M R spectra for the reaction shown in Equation 2.5. The 3 1 P{ 1 H} N M R spectrum shows a mixture of products with the two major signals labeled. The signal at 8.6 ppm is assigned to one of the starting materials in the reaction, [P2N2]Hf(C7H8) (5). The signal at -10.0 ppm has been very tentatively assigned as {[P2N 2]Hf} 2((i-ri 2-N2H)(|j,-H). The 'Ff N M R spectrum is too complicated to assign any of the normal signature signals for [P 2N 2] but one interesting signal to note is that at 5.65 ppm. This signal is broad and in the general region where N - H signals are observed. Chapter 2 References begin on page 48 47 The N-H signal for the zirconium analogue is observed at 5.53 ppm, extremely close to this signal. This reaction also provides further evidence that the 3 1 P { 1 H ) signal at -8.2 is due to {[P2N2]Hf}2(n-'n2:ri2-N2) (4) while the signal at 8.6 ppm is due to [P 2N 2]Hf(C 7H 8) (5). A look back at Figure 2.9 reveals the presence of four compounds. The signals assigned to {[P 2N 2]Hf} 2 (2) and [P 2N 2]HfI 2 (3) are unambiguous because these two compounds have been previously isolated. The signals at 8.6 ppm and -8.2 ppm are tentatively assigned to 5 and 4 respectively. Figure 2.14 reveals that following reaction of this mixture of compounds with H 2 , only the signal at 8.6 ppm remains unchanged. As well, the 'Fl NMR spectrum reveals the presence of activated phenyl signals that are assigned to 5 confirming that this compound has not changed under these conditions. Thus, it seems reasonable to assign the signal at 8.6 ppm to the toluene complex, 5, and by process of elimination the signal at -8.2 ppm is due to 4. As well, 4 is expected to react with H 2 while 5 is not, which is what is observed if the 3 1 P{ 1 H} signals are assigned as outlined above. 2.8 S u m m a r y The hafnium chemistry presented in this chapter is, in many ways, very similar to the zirconium chemistry discussed in the introduction to the chapter. The X-ray crystal structure of [P 2N 2]HfCl 2 (1) is isostructural with that of its zirconium analogue which is as expected. However, it is at this point that the chemistry of the two metals begins to show some subtle and not so subtle differences. It was not possible to synthesize a Chapter 2 References begin on page 48 48 hafnium dinitrogen compound in any appreciable amount from [P2N2]HfCl2 (1), instead a hafnium dimer, {[P2N2]Hf}2 (2) was the major product. However, alteration of the hafnium starting material from a chloride complex to an iodide complex did result in the formation of a hafnium dinitrogen complex, {[P2N2]Hf}2(|it-ri2:'n2-N2) (4), as shown by N M R spectroscopy, E.I. mass spectrometry, and hydrazine analysis. Unfortunately, at this point in time, it has not been possible to either isolate compound 4 or grow X-ray quality crystals. However, as best can be determined, this is only the second hafnium dinitrogen compound reported. Finally, the observation of an N - H signal in the ! H N M R spectrum for the reaction of 4 with H 2 is a promising sign. 2.9 References 1) Fryzuk, M . D.; Love, J. B.; Rettig, S. I ; Young, V. G. Science 1997, 275, 1445. 2) Fryzuk, M . D.; Love, J. B., Unpublished results. 3) Fryzuk, M . D.; Johnson, S. A. ; Rettig, S. J. Organometallics 1999,18, 4059. 4) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. Organometallics 1998, 17, 846. 5) Fryzuk, M . D.; Leznoff, D. B.; Ma, S. F.; S.J.Rettig; V .G . Young, J. Organometallics 1998, 17, 2313. 6) Fryzuk, M . D.; Mylvaganam, M . ; Zawarotko, M . J.; MacGillivray, L. R. J. Am. Chem. Soc. 1993, 115, 10360. 7) Fryzuk, M . D.; Haddad, T. S.; Murugesapilla, M . ; McConville, D. H.; Rettig, S. J. J. Am. Chem. Soc. 1993,115, 2782. Chapter 2 References begin on page 48 49 8) Fryzuk, M . D.; Macneil, P. A.; Rettig, S. J.; Secco, A. S.; Trotter, J. Organometallics 1982, 1, 918. 9) Black, D. G.; Swenson, D. C ; Jordan, R. F. Organometallics 1995,12, 3539. 10) Fryzuk, M . D.; Carter, A. Organometallics 1992, 11, 469. 1 l)Fryzuk, M . D.; Haddad, T. S.; Berg, D. J. Coord. Chem. Rev. 1990, 99, 137. 12) Brand, H ; Arnold, J. Organometallics 1993, 12, 3655. 13) Watt, G. W.; Chrisp, J. D. Analytical Chemistry 1952, 24, 2006. Chapter 2 References begin on page 48 50 CHAPTER 3 ORGANOMETALLIC AND HYDRIDE COMPLEXES OF HAFNIUM 3.1 Transition Metal Hydrides There is much current interest in the development of transition metal complexes as reagents for organic synthesis. 1 Transition metal hydrides constitute one important class of compounds currently under study. 2 - 4 They are utilized in both catalytic and stoichiometric processes.^ One process which employs a transition metal hydride is the process known as hydrozirconation. Hydrozirconation employs zirconium hydrides to form zirconium-carbon bonds by inserting an alkene or an alkyne into a zirconium-hydrogen bond. Many organometallic compounds of zirconium have been formed in this manner and this process also finds uses in organic synthesis. 6 As well, dihydridozirconocene will catalytically hydrogenate alkenes under an atmosphere of H 2 as shown in Equation 3.1.7 \ / {Z rH 2 ( t 1 -C 5 H 5 ) 2 } n | | ^ C = C ^ + 2 H 2 ^ 2 H C C H 3.1 Figure 3.1 shows some of the reactions of zirconium compounds that ultimately form useful organic functional groups. 9 Chapter 3 References begin on page 64 51 R-CH=CH, Cp2Zr(H)CI Hydrozirconation R-CH 2-CH 2-ZrCp 2CI CO 2571.5 atm R-CH 2 -CH 2 -C-ZrCp 2 CI HCI/H,0 Electrophilic abstraction NaOH H 2 0 2 R CH 2 C H 2 — C H R - C H 2 - C H 2 - C - O H Br2/MeOH R - C H 2 - C H 2 - C - O M e Figure 3.1 Useful organic functional groups derived from acylzirconium compounds. Hafnium hydrides are less studied than their zirconium counterparts and hydrohafnation is virtually unknown in organic synthesis. One study did show that hydrohafnation is more sensitive to steric effects than hydrozirconation. 10,11 There are two typical methods for preparing early transition metal hydrides. The first involves the use of a reducing agent and a hydride source to replace an anionic halide ligand. An example where L1AIH4 performs both functions1 2 is shown in Equation 3.2. Chapter 3 References begin on page 64 52 [{Cp 2ZrCI(0)}] Li[AIH 4] 1 / 2 [ {Cp 2 ZrH 2 } 2 ] 3.2 The second hydride preparation method entails hydrogenolysis of an organometallic compound with H2 gas. An example is shown in Equation 3.3 where temperatures of 80 °C and pressures of 60 atm are required. 13 (n - C 5 H 4 R ) 2 M M e 2 2 C H / 1 / 2 [ { (n 5 -C 5 H 4 R) 2 MH 2 } 2 ] 3.3 M = Z r o r Hf R = H , M e , P r i , B u t , P h C H 2 , or P h M e C H For [P2N2] metal complexes, both methods have been previously employed successfully in the Fryzuk lab. {[P2N2]Ta}2(p:-H)4 was prepared using a tandem hydrogenolysis reduction sequence 14 as shown in Equation 3.4. Me, = Me -TaC Silyl methyl groups omitted for clarity 5 H, -6 MeH I S L N ^ S i 7 H.H| Hi H \ - S i N p Si-|\ —Si—J 3.4 Chapter 3 References begin on page 64 53 On the other hand, {[P2N2]Zr}2(|a-H)4 was prepared 1 5 from [ P 2 N 2 ] Z r C l 2 using K C 8 as a reducing agent under 4 atmospheres of H2 as shown in Equation 3.5. CI CI V T-SOVOI~| V—S>NVlSi_J Si-& Silyl methyl groups omitted for clarity 4 H 2 4 C 8 K Toluene -4 MeH /^s>NK-r sH Si-'A r-sktrol""l U S J ^ S J J 3.5 Interestingly, it does not appear to be possible to form a zirconium hydride from [P2N2]ZrMe2. When this reaction was attempted, an unexpected toluene complex resulted 1 5 as shown in Equation 3.6. Me Me V -Zr. Si--& Silyl methyl groups omited for clarity 4 atm H 2 Toluene/THF -2 MeH U s r N ^ s i _ j 3.6 Chapter 3 References begin on page 64 54 To the best of our knowledge, this reaction is unprecedented. In the absence of H.2, no reaction takes place, indicating the likely presence of a hydride intermediate but this has not been experimentally confirmed. The question also arose as to whether or not this reaction was specific only to toluene, or i f other aromatic organometallic compounds could be made. Unfortunately, [P2N2]ZrMe2 is extremely thermally and light sensitive. As such, it is difficult to prepare in sufficient quantities to use in subsequent reactions. It is a well established precedent that certain organometallic hafnium(IV) compounds are more thermally stable than their zirconium analogues. 16-19 As well, the use of hafnium analogues can facilitate investigation of intermediates in cases where this proves difficult for zirconium compounds.20-22 jhe first step in investigating the hafnium chemistry of the [P2N2] ligand was to prepare the hafnium compound, [P2N2]HfMe2 (6). 3.2 Synthesis and Characterization of [P2N2]HfMe2 (6) CI CI 6 Chapter 3 References begin on page 64 55 The synthesis of [P2N2]HfMe2 (6) was accomplished by the metathesis reaction of [P 2 N 2 ]HfCl 2 (1) with two equivalents of the Grignard reagent, MeMgCl as shown in Equation 3.7. Compound 6 was characterized by  lH and 3 1 P{ 1 H} N M R spectroscopy, elemental analysis and X-ray crystal structure analysis. — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — r — i — i — i — i — i — i — i — i — i — i — i — i — i — i — [ — 8.0 7.5 7.0 6.5 1.2 0.8 0.4 0.0 (PPm) (ppm) F i g u r e 3.2 The ' H and 3 1 P{ ! H} N M R spectra of [P 2N 2]HfMe 2 (6). The two regions of the *H N M R spectrum are not shown on the same scale. Figure 3.2 shows the ' H and 3 1 P{ 'H} N M R spectra of 6. In a manner similar to most other mononuclear group 4 metal [P 2N 2] complexes known, the dimethyl compound adopts C 2 v symmetry in solution at ambient temperature as evidenced by N M R spectroscopy. As expected, the methyl protons are coupled to two equivalent phosphorus Chapter 3 References begin on page 64 56 nuclei, and appear as a triplet at 0.65 ppm. Unlike the case of the zirconium analogue, their signal is not masked by the [P2N2] ligand methylene protons, but is clearly visible. Interestingly, while it was not possible to grow crystals of the zirconium analogue of this compound, crystals of the hafnium analogue were grown easily from slow evaporation of a D M E solution. A n ORTEP diagram of complex 6 is shown in Figure 3.3. C(3) Figure 3.3 An ORTEP diagram showing the solid state molecular structure of [P2N 2]HfMe 2 (6). The substitution of methyl groups for chloride moieties when going from 1 to 6 has resulted in a substantial structural change. The geometry around the metal center is now best described as trigonal prismatic with the two trigonal planes described by: N(l) , P(l), and C(25); and N(2), P(2), and C(26). The methyl carbons are not located in a Chapter 3 References begin on page 64 57 square plane with the amido nitrogens, but are rotated out of this plane. As well, the phosphine and amide "bite angles", P(l)-Hf-P(2) and N(l)-Hf-N(2), have both changed considerably. The phosphine bite angle has decreased from 153.76(3)° to 135.79(3) ° while the amide bite angle has increased from 96.6(1) ° to 113.2(1) °. Table 3.1 shows selected bond distances and angles for complex 6. The Hf-C, - N and -P bond distances are once again, not unusual and compare with those of other Group 4 methyl complexes. 16,23 T a b l e 3.1 Selected Bond Lengths (A ) and Angles (deg) in [P 2N2]HfMe 2 (6) Bond Lengths Bond Angles Hf(l)-N(l) 2.147(3) N(l)-Hf(l)-N(2) 113.5(1) Hf(l)-N(2) 2.149(3) P(l)-Hf(l)-P(2) 135.79(3) Hf(l)-P(l) 2.781(1) C(25)-Hf(l)-C(26) 82.0(2) Hf(l)-P(2) 2.783(1) N(l)-Hf(l)-C(25) 97.2(2) Hf(l)-C(25) 2.263(1) N(2)-Hf(l)-C(25) 134.6(2) Hf(l)-C(26) 2.277(1) 3.3 A t t e m p t e d Synthes is o f [ P 2 N 2 ] H f ( C 7 H 8 ) (5) Having prepared [P 2N 2]HfMe 2 (6), the subsequent aim was the synthesis of a toluene complex, [P 2N 2]Hf(C 7H 8) (5), using methods similar to those which produced [P 2N 2]Zr(C 7H 8). Chapter 3 References begin on page 64 Postu la ted Hydride S igna l 31 P{ 1H} 2 Major Products No Peak at 8.6 6 (Starting Material) F i g u r e 3.4 The ' H and ^ P l ' H } N M R spectra for the attempted synthesis [ P 2 N 2 ] H f ( C 7 H 8 ) (5). Chapter 3 References begin on page 64 59 A reactor containing [P 2N2]HfMe 2 (6), in a mixture of toluene and THF, was sealed under 4 atmospheres of H 2 . The  lH and 3 1 P{ 1 H} N M R spectra of the results of this reaction are shown in Figure 3.4. If formation of 5 had resulted, as expected, the 3 1 P{ 1 H} N M R spectrum would exhibit a signal at 8.6 ppm as observed in the spectrum for the hafnium dinitrogen reaction. As well, the ! H N M R spectra of [P 2 N 2 ]Zr(C 7 H 8 ) has signals between 2 and 4 ppm which are attributed to activation of the toluene ring. One would expect to see similar signals for the hafnium analogue in the ! H N M R spectrum. However, neither of these features is observed in the N M R spectra shown in Figure 3.4. Integration of the 3 1 P{ ! H} N M R spectrum indicates 2 major products; the sharp signal at 4.9 ppm and the broad lump centered at -2.9 ppm. As well, a small amount of the starting material, 6, is present. The *H N M R spectrum indicates a multiplet centered at 10.7 ppm which looks identical to that for the hydride in {[P 2N 2]Zr} 2(u.-H) 4. Thus, it seemed that it may be possible to form a hafnium hydride using a variation of the reaction shown in Figure 3.4. In fact, the formation of {[P2N2]Hf}2(u.-H)4 by hydrogenation of 6 required only a small change in the synthetic plan. 3.4 Synthes is a n d C h a r a c t e r i z a t i o n o f {[P 2N 2]Hf}2(M--H)4 (7) By changing solvents from a toluene/THF mixture to neat benzene, the formation of {[P2N2]Hf}2(u.-H)4 (7) was possible as depicted in Equation 3.8. Complex 7 is sparingly soluble in benzene and crashes out of solution as yellow microcrystals, making separation from the parent compound a fairly simple matter. Chapter 3 References begin on page 64 60 The ^ P ^ H } and ' H N M R spectra are shown in Figure 3.5. In this case, the change in the chemical shift of the phosphorus nuclei is more drastic than in other [P2N2] compounds when switching from {[P2N2]Zr}2(p.-H)4 to its hafnium analogue. The signal is observed at 4.4 ppm, 10.3 ppm downfteld from the zirconium analogue signal, which is at -5.9 ppm. As well, the hydride quintet has moved downfteld 4.25 ppm from 5.0 ppm to 9.25 ppm. The other characteristic signals in the ' H N M R spectrum have not changed appreciably. Chapter 3 References begin on page 64 61 T 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 ' 1 1 1 1 1 1 1 1 1 1 1 -9.0 8.5 8.0 7.5 1.2 0.8 0.4 0.0 (PPm) (ppm) F i g u r e 3.5 The 3lP{ lH} and : H N M R spectra for {[P 2N 2]Hf} 2(^-H) 4 (7). * Indicates impurity or solvent. 3.5 Synthes is a n d C h a r a c t e r i z a t i o n o f [ P 2 N 2 ] H f ( C 4 H 6 ) (8) Can other chemistry of [P 2 N 2 ]ZrCl 2 be extended to hafnium analogues? Again, a metathesis reaction of one equivalent of [P 2N 2]FffCl 2 (1) with one equivalent of Mg(C4Ff6)(THF)2 results in the formation of [P2N2]Hf(C4H6) (8) in good yield as shown in Equation 3.9. Chapter 3 References begin on page 64 62 CI CI V Hf s i^ IN—SI-'^  N L - s T N ^ S i - J Silyl methyl groups omitted for clarity Mg(C 4 H 6 ) (THF) 2 THF 18 Hours _^ Hf •7-Si |.N U ^ S i ^ N 3.9 P Si-7\ ^ • S i JComplex 8 was characterized by ' H and 3 1 P{ 1 H} N M R spectroscopy and elemental analysis. The 3 1 P{ 1 H} and *H N M R spectra are shown in Figure 3.6. For this compound, the 3 1 P{ 1 H} N M R spectrum shows an AJ3 doublet pattern for non-equivalent phosphorus nuclei centered at 0 ppm with a large P-P coupling constant of 78.2 Hz. This coupling pattern is identical to that observed in the zirconium analogue. The *H N M R spectrum indicates that the solid state C 2 twist observed in so many [P 2N 2] compounds, is maintained in solution at room temperature, in contrast to most [P 2N 2] compounds where C 2 v symmetry is observed in solution. One silyl methyl resonance is quite broad, and broadened features are also observed in other characteristic signals. Chapter 3 References begin on page 64 63 31P{1H} T H„ "I—I—I 1—I—I—1 1—| 1 1—I 1—| 1—I—I—I—|—I—i—I—I—|—!~ 10 0 -10 r s i ^ ! r V S i i x [P2N2]Hf(C4H6) (8) (ppm) 1 H C 6 D 5 H SiMe, o-Ph m,p-Ph Ring CH 2 H 1 i 1 1 1 1 i 1 1 1 1 i 1 1 1 ' i 1 1 1 1 i 1 1 1 1 i 1 i i i | i i i i | i i i i | i i i i | i i i i | i i i 8.0 7.0 6.0 2.0 1.0 0.0 (ppm) (ppm) F i g u r e 3.6 The ' H and  3l?{ lH) N M R spectra of Pa^HfrcOk) (8). The two regions of the  lH N M R spectrum are not shown on the same scale. 3.6 S u m m a r y As with the chemistry presented in Chapter 2, the hafnium chemistry presented in this chapter begins with a new hafnium compound, [P2N2]HfMe2 (6), which is assumed to be isostructural with its zirconium analogue. Once again it is at this point that the chemistry of the two metals begins to diverge. It was hoped that a hafnium toluene Chapter 3 References begin on page 64 64 complex could be generated and its reaction pathway studied to elucidate features of the zirconium analogue, but this proved unfeasible. Instead, a new hafnium hydride compound, {[P2N2]Hf}2(u,-H)4 (7), has been prepared and characterized. Finally, a butadiene complex of hafnium, [P2N2]Hf(C4He) (8), has also been prepared and characterized. 3.7 References 1) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry; Corey, P. F., Ed.; Prentice-Hall, Inc.: New Jersey, 1997, pp 377. 2) Pearson, R. G. Chem. Rev. 1985, 85, 41. 3) Moore, D. S.; Robinson, S. D. Chem. Soc. Rev. 1983, 415452. 4) Hlatky, G. G.; Crabtree, R. H. Coord. Chem. Rev. 1985, 65, 1. 5) Collman, J. P.; Hegedus, L. S. Principles and Applications of Organotransition Metal Chemistry; 1st ed.; Kelly, A. , Ed.; University Science Books: M i l l Valley, California, 1980, pp 60. 6) Cardin, D. J.; Lappert, M . F.; Raston, C. L. Chemistry of Organo-Zirconium and -Hafnium Compounds; 1st ed.; Ellis Horwood Limited: West Sussex, 1986, pp 24. 7) Wailes, P. C ; Weigold, H. ; Bell, A. P. J. Organomet. Chem. 1972, 43, C32. 8) Negishi, E.; Takahashi, T. Aldrichimica Acta 1985, /<5, 31. 9) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry; Corey, P. F., Ed.; Prentice-Hall, Inc.: New Jersey, 1997, pp 408. Chapter 3 References begin on page 64 65 10) Cardin, D. J.; Lappert, M . F.; Raston, C. L. Chemistry of Organo-Zirconium and -Hafnium Compounds, 1 ed.; Ellis Horwood Limited: West Sussex, 1986, pp 401. 11) Tolstikov, G. A.; Miftakov, M . S.; Valeev, F. A. Izv. Akad. Nauk SSSR, Ser. Khim. 1979, 2576. 12) Kautzner, B.; Wailes, P. C ; Weigold, H. J. Chem. Soc, Chem. Commun. 1969, 1105. 13) Couturier, S.; Gautheron, B.J. Organomet. Chem. 1978, 157, C61. 14) Fryzuk, M . D.; Johnson, S. A ; Rettig, S. J. J. Am. Chem. Soc. 1998,120, 11024. 15) Fryzuk, M . D.; Love, J. B., Unpublished results. 16) Fryzuk, M . D.; Carter, A. Organometallics 1992, 11, 469. 17) Helmut, G. A ; Sanzo, F. P. D.; Rausch, M . D.; Uden, P. C. J. Organomet. Chem. 1976, 107, 257. 18) Razuvaev, G. A. ; Vyshinskaya, L. I.; Mar'in, V . P. Dokl. Akad. Nauk SSSR 1975, 225, 827. 19) Razuvaev, G. A. ; Mar'in, V . P.; Drushkov, O. N . ; Vyshinskaya, L. I. J. Organomet. Chem. 1982,231, 125. 20) Bercaw, J. E.; Moss, J. R. Organometallics 1992, 11, 639. 21) Roddick, D. M . ; Bercaw, J. E. Chem. Ber. 1989, 122, 1579. 22) Roddick, D. M . ; Fryzuk, M . D.; Seidler, P. F.; Hillhouse, G. L. ; Bercaw, J. E. Organometallics 1985, 4, 97. 23) Fryzuk, M . D.; Haddad, T. S.; Berg, D. J. Coord. Chem. Rev. 1990, 99, 137. Chapter 3 References begin on page 64 66 CHAPTER 4 Experimental Procedures 4.1 General Procedures A l l manipulations were performed under prepurified nitrogen in a Vacuum Atmospheres HE-553-2 workstation equipped with an MO-40-2H purification system or using Schlenk-type glassware. The term "reactor" refers to a cylindrical, thick-walled Pyrex vessel equipped with a 5 mm (or 10 mm, for larger bombs) Kontes Teflon needle valve and a ground glass joint for attachment to a vacuum line. These "reactors" allow pressures as high as 4 atm to be maintained inside the tube. 4.2 Reagents and Starting Materials THF was predried by refluxing over CaH 2 for at least 24 hours and dried by refluxing over sodium benzophenone ketyl, followed by distillation under argon. Toluene and hexanes were purchased in anhydrous form from Aldrich, sparged with N2, and deoxygenated by filtration through columns containing alumina and Q-5 catalyst under a positive pressure of N2 .I Benzene, diethyl ether, and dimethoxyethane were also refluxed over sodium benzophenone ketyl and distilled under argon. Deuterated THF (C+DgO) and deuterated benzene (C^De) were refluxed under vacuum with sodium potassium amalgam (Na/K), vacuum transferred to a clean vessel and "freeze-pump-thawed" three times prior to use. Chapter 4 References begin on page 73 67 The following compounds were prepared by published procedures: HfCl 4 (THF) 2 )2 [syn-P2N2]Li2(dioxane),3 C 8 K 4 , and Mg(C4H6)(THF)2. Me 3SiI, MeMgCl, BzMgCl, and H 2 were used as received. Diatomaceous earth (Celite) was dried overnight at 170 ° C before being taken inside the glovebox for use. 4.3 A n a l y t i c a l M e t h o d s *H and 3 1 P{ 1 H} N M R spectroscopy were performed on a Bruker AMX-500 instrument (500.135 M H z and 202.458 MHz) or a Bruker AC-200 machine (200.132 MHz and 81.015 MHz). *H N M R spectra were referenced to internal C 6 D 5 H (7.15 ppm) and 3 1 P{ 1 H} N M R spectra were referenced to external P(OMe 3 ) 3 (141.0 ppm, with respect to 85% H 3 P 0 4 at 0.00 ppm). C, H and N microanalyses were performed by Mr. P. Borda of this department. The X-ray crystal structures were determined at U B C Crystallographic Services in this department by Dr. B. Patrick. The crystals were loaded in a small glass vial and covered with paratone oil in the glovebox to prevent decomposition. Details of the structure determinations are given in the Appendices. 4.4 H y d r a z i n e A n a l y s i s Hydrazine was analyzed according to the procedure published by Watt and Chrisp. 5 The products from the reaction of [P 2N 2]HfI 2 (3) with 2 equivalents of C g K were weighed into a 200 mL Schlenk tube (0.280g) and dissolved in 50 mL of Et 2 0. 10 Chapter 4 References begin on page 73 68 mL of 2.0 M HC1 in Et20 was added to this solution. The solution immediately lost its blue color and a white precipitate formed. The clear solution and white precipitate were stirred for 1 hour and the solvents removed in vacuo. The flask was exposed to the air, extracted into 100 mL of H2O and filtered into a 500 mL volumetric flask which was filled with 1.0 M HC1. A 2.0 mL aliquot of this solution was added to 20 mL of a color developer (p-dimethylaminobenzaldehyde, 4.0 g; ethanol, 200 mL; 1.0 M HC1, 20 mL) and diluted to 25.0 mL with 1.0 M HC1. Absorbance readings of the sample were taken within 10 minutes. The spectrometer was set to 100% transmittance using a blank solution consisting of 1.0 M HC1. Absorbance readings of 0.33, 0.34 and 0.33 A U were obtained at A 4 5 8 . 4.5 Synthes is o f N e w C o m p o u n d s 4.5.1 [ P 2 N 2 ] H f C I 2 (1) Li2(dioxane)[P2N2] (6.83 g, 10.8 mmol) and HfCl 4 (THF) 2 (5.00 g, 10.8 mmol) were each weighed into a thick walled reactor. Addition of 150 mL toluene to this intimate mixture resulted in a cloudy white solution. The reactor was sealed and the mixture was refluxed for 18 hours at 120 °C. The solvents were then removed in vacuo, the off white residue extracted into toluene (2 x 50 mL) and filtered through Celite. The filtrate was evaporated to dryness and the residue washed with hexanes (2 x 10 mL) yielding 7.25 g, 86 % of 1 as a white powder. The product was crystallized by the slow evaporation of a D M E solution. Chapter 4 References begin on page 73 69 Anal. Calcd. For C 2 4 IL2Cl 2 HfN 2 P 2 Si 4 : C, 36.85; H, 5.41; N , 3.58. Found: C, 37.04; H, 5.48; N , 3.56. *H N M R (C 6 D 6 , 25 °C, 500.135 MHz) 5 7.87 (m, 4H, o-H phenyl), 7.05 (m, 6H, m/p-H phenyl), 1.41 ( A B X m, 4H, C H 2 ring), 1.32 ( A B X m , 4H, C H 2 ring), 0.38 (s, 12H, SiMe 2 ring), 0.34 (s, 12H, SiMe 2 ring). 3 1 P{ 1 H} (C 6 D 6 , 25 °C, 202.458 M H z ) 5 -5.8 (s) 4.5.2 { [ P 2 N 2 ] H f ) 2 (2) [P 2 N 2 ]HfCl 2 (1) (0.73g, 0.93 mmol) and C 8 K (0.25g, 1.86 mmol) were each weighed into a 200 mL thick-walled reactor, to which 50 mL of toluene was added, all in the glovebox. The reactor was removed from the glovebox and cooled to -198 °C in a liquid nitrogen bath while under a flow of N 2 . After 30 minutes, the reactor was sealed and allowed to warm to room temperature. The reaction mixture was stirred for 8 days and filtered through Celite. Toluene was removed in vacuo and the residues dissolved in hexanes and filtered. The hexanes solution was left in a -40 °C freezer overnight where 2 crashed out of solution as a yellow brown powder. Yield: 0.22g, 32.2%. Note: further studies showed that it does not matter what type of atmosphere (N 2 , Ar, or near vacuum) is inside the reactor. Anal. Calcd. For C48H84Hf2N4P4Si8: C, 40.52; H , 5.95; N , 3.95. Found: C, 40.39; H, 5.97; N , 3.75. ' H N M R (C 6 D 6 , 25 °C, 200.132 MHz) 6 7.40 (m, 4H, o-H phenyl), 7.10 (m, 6H, m/p-H phenyl), 4.19 (m, 8H, m/p-H activated phenyl), 3.39 (m, 4H, o-H activated phenyl), 1.49 ( A B X m, 8H, C H 2 ring), 1.15 ( A B X m, 8H, C H 2 ring), 0.52 (s, 12H, SiMe 2 ring), 0.50 (s, Chapter 4 References begin on page 73 70 12H, SiMe 2 ring), 0.31 (s, 12H, SiMe 2 ring), 0.04 (s, 12H, SiMe 2 ring). 3 1 P{'H} (C 6 D 6 , 25 °C, 81.015 MHz) 5 11.7 (m, 2P), 1.9 (m, 2P) 4.5.3 [P 2 N 2 ]HfI 2 (3) 10.0 mL (70 mmol, -10 equivalents) of MesSil was added to a stirred solution of [P 2 N 2 ]HfCl 2 (1) (5.21g, 6.66 mmol) in 80 mL of toluene at -78 °C. The reactor was sealed and the solution stirred for 18 hours, during which time the solution acquired a very pale yellow-green color. Toluene was removed in vacuo and the residues washed with hexanes (2x10 mL) yielding 2.8g, 88 % of 3 as a very pale green powder. Anal. Calcd. For C 2 4 H 4 2 I 2 H f N 2 P 2 S i 4 : C, 29.87; H, 4.39; N , 2.90. Found: C, 30.02; H, 4.43; N , 2.89. ' H N M R (C 6 D 6 , 25 °C, 200.132 MHz) 5 7.92 (m, 4H, o-H phenyl), 7.05 (m, 6H, m/p-H phenyl), 1.58 ( A B X m, 4H, C H 2 ring), 1.27 (AJ3X m, 4H, C H 2 ring), 0.48 (s, 12H, SiMe 2 ring), 0.39 (s, 12H, SiMe 2 ring). 3 1 P{ 1 H} (C 6 D 6 , 25 °C, 81.015 MHz) 5 -5.4 (s) 4.5.4 {[P 2N 2]H02(u-r | 2.Ti 2-N 2) (4) C 8 K (0.33g, 3.32 mmol) and [P 2N 2]HfI 2 (3) (1.21g, 1.26 mmol) were each weighed into a 200 mL thick-walled reactor, to which 50 mL of toluene was added, all in the glovebox. The reactor was removed from the glovebox and cooled to -198 °C in a liquid nitrogen bath while under a flow of N 2 . After 30 minutes, the reactor was sealed and allowed to warm to room temperature. The solution was stirred for 16 hours and a blue-purple color was observed. The reactor was then allowed to stir vigorously for another 7 days. The solution was then filtered through Celite and evaporated to dryness. Chapter 4 References begin on page 73 71 Washing with a minimal amount of hexanes (2 x 10 mL) yielded 0.85 g of a dark solid. 3 1 P{ 1 H} N M R indicates the composition of this solid to be approximately 52% 4, 19% 3, 16% 2, and 13% 5. Mass spec. (E.I.): M + 1450. 4.5.5 [ P 2 N 2 ] H f M e 2 (6) To a solution of [P 2 N 2 ]HfCl 2 (1) (3.02 g, 3.86 mmol) in THF was added 3.0 M MeMgCl in THF (2.57 mL, 7.77 mmol) at 0 °C. The solution was warmed to room temperature and stirred for lh after which the solvents were evaporated in vacuo. The off white residue was extracted into toluene (2 x 20 mL) and filtered through celite. The filtrate was evaporated to dryness and the residue washed with hexanes (2 x 10 mL) yielding 2.24 g, 77 % of 6 as a white powder. The product was crystallized by the slow evaporation of a D M E solution. Anal. Calcd. For C 2 6 H 4 8 H f N 2 P 2 S i 4 : C, 42.12; H, 6.53; N , 3.78. Found: C, 42.14; H, 6.52; N , 3.74. ' H N M R (C 6 D 6 , 25 °C, 500.135 MHz) 5 7.62 (m, 4H, o-H phenyl), 7.07 (m, 6H, m/p-H. phenyl), 1.03 (m, 8H, C H 2 ring), 0.72 (t, 6H, Zr-CH 3), 0.22 (s, 12H, SiMe 2 ring), 0.20 (s, 12H, SiMe 2 ring). ^ P ^ H } (C 6 D 6 , 25 °C, 202.458 MHz) 5 -9.6 (s) 4.5.6 { [P 2 N 2 ]Hf } 2 {u-H 4 } (7) 50 mL of C 6 H 6 was added to 1.02g (1.3 mmol) of [P 2N 2]HfMe 2 (6) in a reactor which was sealed. The reactor was "freeze-pump-thawed" three times to remove all gases. H 2 was introduced to the reactor which was cooled to -198 °C. After the solvent had thawed but prior to warming to room temperature, the reactor was shaken to promote Chapter 4 References begin on page 73 72 dissolution of H2 in the solution. The reactor was left overnight during which time a microcrystalline yellow solid precipitated. Removal of solvent by filtration resulted in 0.345 g, 37.2% of 7. Anal. Calcd. For C 4 8 H 8 8 H f N 2 P 2 S i 4 : C, 40.41; H, 6.22; N , 3.93. Found: C, 40.56; H, 6.21; N , 3.76. ' H N M R (C 6 D 6 , 25 °C, 500.135 MHz) 5 9.25 (m, 4H, Hf-H), 7.19 (m, 8H, o-H phenyl), 7.12 (m, 12H, m/p-H phenyl), 0.83 (m, 16H, C H 2 ring), 0.04 (s, 24H, SiMe 2 ring), -0.03 (s, 24H, SiMe 2 ring). 3 1 P{ 1 H} (C 6 D 6 , 25 °C, 202.458 MHz) 5 4.4 (s) 4.5.7 [ P 2 N 2 ] H f ( C 4 H 6 ) (8) To an intimate mixture of [P 2 N 2 ]HfCl 2 (1) (1.42g, 1.8 mmol) and Mg(C4H6)(THF)2 was added 15 mL of THF at 0 °C. When warmed to room temperature, the solution took on a deep red color. The solution was stirred overnight and THF removed in vacuo. The residues were dissolved in 15 mL of hexanes and the solution placed in the freezer at a temperature of - 40 °C. Overnight, a red powder precipitated giving 0.90 g, 65% of 8. Anal. Calcd. For C 2 8 H 4 8 H f N 2 P 2 S i 4 : C, 43.93; H , 6.32; N , 3.66. Found: C, 43.72; H, 6.28; N , 3.66. [ H N M R (C 6 D 6 , 25 °C, 200.132 MHz) 5 7.62 (m, 4H, o-H phenyl), 7.08 (m, 6H, m/p-H phenyl), 6.03 (dd, 2H, meso-H), 2.18 (dd, 2H, C A syn-H), 0.61 (dd, 2H, C A anti-H), 1.14 ( A B X m, 4H, C H 2 ring), 1.12 ( A B X m, 4H, C H 2 ring), 0.38 (s, 12H, SiMe 2 ring), 0.22 (s, 12H, SiMe 2 ring). ^ { ' H J N M R (C 6 D 6 , 25 °C, 80.015 MHz) 5 4.7 and -4.8 (d, 2 J P P = 78.2Hz) Chapter 4 References begin on page 73 4.6 Re fe rences 1) Pangborn, A. B.; Giardello, M . A.; Grubbs, R. H . ; Rosen, R. K. ; Timmers, F. Oganometallics 1996, 15, 1518. 2) Manzer, L. E. Inorg. Chem. 1982, 21, 135. 3) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. J. Chem. Soc., Chem. Commun. 1996, 2783. 4) Bergbreiter, D. E.; Killough, J. M . J. Am. Chem. Soc. 1978, 100, 2126. 5) Watt, G. W.; Chrisp, J. D. Analytical Chemistry 1952, 24, 2006. Chapter 4 References begin on page 73 74 FUTURE PROSPECTS A wide range of chemistry has been demonstrated for the new hafnium [P2N2] complexes presented in this thesis, but there is need for further research within this area. In general, many of the systems presented are in need of further physical characterization, while the synthesis of new products is always a worthy goal. Further work is certainly needed in terms of characterization of {[P2N2]HT}2(u.-r|2:r|2-N2) and perhaps alteration of the synthetic plan to improve yields and/or decrease impurity production. If this compound can be isolated, there are many possibilities for further reactivity. Theoretical studies have indicated that a {[P2N2]M}2((X-TI2-HNNH)(|LI-H) 2 complex may be accessible. 1 Perhaps the greater thermal stability of hafnium complexes compared with zirconium complexes may allow synthesis of this compound. [P2N2]HfCi2 (1) has proven to be a useful starting material for many of the compounds presented in this thesis. While the synthesis of [P2N2]HfMe2 (2) and its ability to form a hydride complex where the zirconium analogue did not was of interest, further work is necessary to determine why this compound differs in reactivity so drastically from the zirconium analogue. As well, many other alkyl systems of hafnium may be synthesized as only the methyl derivative has been examined to this point. Of particular interest is the reactivity of [P2N2]HfCi2 with LiCH.2SiMe3. Recent work from the Fryzuk laboratory with the analogous niobium(IV) complex, [P2N2]NbCl2 has shown that these reaction conditions can produce niobium alkylidenes, for example [P2N2]Nb=CHSiMe3.2 Perhaps this reaction could be extended to generate a rare example of a hafnium alkylidene. Finally the need for X-ray crystal structures can not be 75 overemphasized. It would be of great interest to compare the bonding of many of these hafnium compounds with their zirconium analogues. References 1) Basch, H. ; Musaev, D. G.; Morokuma, K. ; Fryzuk, M . D.; Love, J. B.; Seidel, W. W.; Albinati, A. ; Koetzle, T. F.; Klooster, W. T.; Mason, S. A. ; Eckert, J. J. Am. Chem. Soc. 1999, 121, 523. 2) Fryzuk, M . D.; Kozak, C.K., Unpublished results. 76 APPENDIX A.1 X-ray Crystallographic Analysis of [P2N2]HfCl2 (1) A.1 .1 C r y s t a l D a t a Empirical Formula Formula Weight Crystal Color, Habit Crystal Dimensions Crystal System Lattice Type Lattice Parameters Space Group Z value Dca/c Fooo H(MoKa) C24H42N2Cl2HfP2Si4 782.29 clear, chip 0.15 x 0.15 x 0.15 mm orthorhombic Primitive a = 9.3152(8) A b= 16.5962(5) A c = 21.3811(7) A V = 3305.5(2) A 3 P 2 i 2 i 2 i (#19) 4 1.572 g/cm3 1568.00 35.71 cm"1 A . l . 2 In tens i t y M e a s u r e m e n t s Diffractometer Radiation Detector Aperture Data Images Rigaku/ADSC CCD MoKct (A, = 0.71069 A) Graphite monochromated 94 mm x 94 mm 464 exposures @ 35.0 seconds cp oscillation Range (x = 0) © oscillation Range (x = 90.0) Detector Position Detector Swing Angle No. of Reflections Measured Corrections A . 1.3 Structure Solution and R( Structure Solution Refinement Function Minimized Least Squares Weights p-factor Anomalous Dispersion No. Observations (I>0.00o(I)) No. Variables Reflection/Parameter Ratio Residuals (on F 2 , all data): R; Rw Goodness of Fit Indicator Max Shift/Error in Final Cycle No. Observations (I>3a(I)) 77 0.0-190.0° -19.0-23.0° 40.62 mm -5.55° 55.8° Total: 28469 Unique: 3999 (R,„, = 0.066) Lorentz-polarization Absorption (trans, factors: 0.7796- 1.0000) Patterson Methods (DIRDIF92 PATTY) Full-matrix least-squares Sco(jFo2| - |Fc21 )2 co = l/[a2(Fo2)] 0.0000 A l l non-hydrogen atoms 6846 316 21.66 0.036; 0.055 0.51 0.00 5913 Residuals (on F, I>3a(I)): R; Rw Maximum peak in Final Diff. Map Minimum peak in Final Diff. Map 0.020; 0.025 0.68 elk1 -1.10 e/A3 Table A . l : Atomic Coordinates and B, Atom X y z B e g Hf(l) 0.08378(2) 0.97608(1) 0.16637 0.890(4) Cl(l) -0.1402(2) 0.94810(10) 0.10928(9) 2.77(3) Cl(2) 0.0421(2) 0.84100(10) 0.21020(10) 3.97(5) P(l) -0.031(10) 1.08273(8) 0.24618(7) 1.10(3) P(2) 0.2891(2) 0.92249(9) 0.08962(7) 1.38(3) Si(l) 0.2845(2) 1.06481(9) 0.27779(7) 1.19(3) Si(2) 0.4049(2) 0.91470(10) 0.22206(7) 1.69(3) Si(3) 0.2511(2) 1.10590(10) 0.06974(8) 1.72(3) Si(4) -0.0373(2) 1.15670(10) 0.11763(8) 1.62(3) N(l) 0.2724(5) 0.9895(3) 0.2213(2) 1.21(8) N(2) 0.1025(5) 1.0873(3) 0.1169(2) 1.39(8) C(l) 0.1310(6) 1.1375(3) 0.2661(3) 1.21(9) C(2) 0.4056(7) 0.8657(3) 0.1413(3) 1.8(1) C(3) 0.3793(6) 1.0166(4) 0.0711(3) 2.3(1) C(4) -0.1363(6) 1.1472(4) 0.1955(3) 1.7(1) C(5) 0.4503(6) 1.1290(4) 0.2742(3) 2.4(1) C(6) 0.2705(7) 1.0193(4) 0.3573(3) 2.3(1) C(7) 0.3787(8) 0.8358(4) 0.2832(3) 2.9 C(8) 0.5897(8) 0.9541(5) 0.2343(4) 3.6 C(9) 0.1921(9) 1.1226(5) -0.0131(3) 3.4 C(10) 0.3649(8) 1.1929(4) 0.0946(4) 2.8 C ( l l ) -0.1725(8) 1.1444(4) 0.0533(3) 2.8 C(12) 0.0260(8) 1.2639(4) 0.1142(3) 2.6 C(13) -0.1222(5) 1.0671(4) 0.3204(2) 1.4 C(14) -0.1755(6) 1.1332(4) 0.3539(3) 2.1 C(15) -0.2378(7) 1.1212(5) 0.4124(3) 2.9 C(16) -0.2492(9) 1.0448(5) 0.4370(3) 3.4 C(17) -0.1970(8) 0.9799(6) 0.4043(3) 3.4 C(18) -0.1333(7) 0.9908(4) 0.3454(3) 2.3 C(19) 0.2695(6) 0.8646(3) 0.0184(3) 1.5 C(20) 0.3915(6) 0.8336(4) -0.0127(3) 2.0 C(21) 0.3749(7) 0.7831(4) -0.0635(3) 2.1 C(22) 0.2399(7) 0.7645(4) -0.0857(3) 2.0 C(23) 0.1195(7) 0.7964(4) -0.0571(3) 2.3 C(24) 0.1341(6) 0.8465(4) -0.0049(3) 1.9 APPENDIX A.2 X-ray Crystallographic Analysis of [P2N2]HfMe2 (6) A.2 .1 C r y s t a l D a t a Empirical Formula Formula Weight Crystal Color, Habit Crystal Dimensions Crystal System Lattice Type Lattice Parameters Space Group Z value D c a / C Fooo p.(MoKa) C 2 6H48HfN 2 P 2 Si 4 741.46 clear, block 0.30 x 0.30 x 0.10 mm monoclinic Primitive a = 11.3209(7) A b = 10.8041(3) A c = 28.1451(9) A V = 3441.6(2) A 3 P2i/n (#14) 4 1.431 g/cm3 1504.00 32.75 cm"1 A .2 .2 In tens i ty M e a s u r e m e n t s Diffractometer Radiation Rigaku/ADSC CCD M o K a ( X = 0.71069 A) Graphite monochromated Detector Aperture 94 mm x 94 mm Data Images (j) oscillation Range (% = 0) Q oscillation Range (% = 90.0) Detector Position Detector Swing Angle No. of Reflections Measured Corrections A.2.3 Structure Solution and R< Structure Solution Refinement Function Minimized Least Squares Weights p-factor Anomalous Dispersion No. Observations (I>0.00a(I)) No. Variables Reflection/Parameter Ratio Residuals (on F 2 , all data): R; Rw Goodness of Fit Indicator Max Shift/Error in Final Cycle No. Observations (I>3a(I)) 81 772 exposures @ 16.0 seconds 0 .0 -189 .9° -19 .0 -23 .0° 40.47(1) mm -5.52(1)° 55.8° Total: 23903 Unique: 7571 (R m r = 0.041) Lorentz-polarization Absorption (trans, factors: 0 .6526- 1.0000) Direct Methods (SIR97) Full-matrix least-squares I Q ( | F O 2 | - | F C 2 | ) 2 ffl = l/[a2(Fo2)] 0.0000 A l l non-hydrogen atoms 7219 316 22.84 0.048; 0.107 1.47 0.06 6957 Residuals (on F, I>3o(I)): R; Rw Maximum peak in Final Diff. Map Minimum peak in Final Diff. Map T a b l e A . 2 : A t o m i c C o o r d i n a t e s a n d B, Atom X y Hf(l) 0.65750(1) 0.23340(1) P(l) 0.50370(1) 0.03890(9) P(2) 0.74380(9) 0.43630(9) Si(l) 0.38980(1) 0.28500(1) Si(2) 0.73980(1) -0.03330(1) Si(3) 0.79260(1) 0.19670(1) Si(4) 0.51990(1) 0.50130(1) N(l) 0.51010(3) 0.34560(3) N(2) 0.73090(3) 0.12540(3) C(l) 0.42520(4) 0.11840(4) C(2) 0.58270(4) -0.09030(4) C(3) 0.73360(3) 0.36020(4) C(4) 0.63670(4) 0.56280(4) C(5) 0.36070(6) 0.36310(6) C(6) 0.25150(5) .29060(6) C(7) 0.80190(5) -0.06790(5) 0.036; 0.063 1.35 elk3 -1.37 eVA 3 z Beq 0.65160(5) 1.428(4) 0.63100(4) 1.93(2) 0.60230(3) 1.56(2) 0.59870(5) 2.14(2) 0.60430(5) 2.23(2) 0.54750(4) 1.87(2) 0.64240(5) 2.51(3) 0.62890(1) 1.78(7) 0.59540(1) 1.92(7) 0.58290(1) 2.36(9) 0.60400(2) 2.45(9) 0.54440(1) 1.91(8) 0.60220(2) 3.0(1) 0.53980(2) 4.6(1) 0.63280(3) 4.1(1) 0.66420(2) 4.1(1) C(8) 0.82610(5) -0.12370(5) 0.56060(3) 4.9(2) C(9) 0.74710(5) 0.12780(5) 0.48850(2) 3.9(1) C(10) 0.95660(5) 0.20020(5) 0.55270(2) 3.5(1) C ( l l ) 0.57740(6) 0.52430(6) 0.70410(2) 4.7(1) C(12) 0.38170(6) 0.59320(5) 0.63480(3) 4.9(2) C(13) 0.39430(4) -0.03090(4) 0.66870(2) 2.44(9) C(14) 0.27350(4) -0.02800(5) 0.65940(2) 3.5(1) C(15) 0.19700(5) -0.08220(7) 0.69140(2) 4.8(2) C(16) 0.23790(6) -0.14090(7) 0.73160(2) 4.9(2) C(17) 0.35790(5) -0.14420(7) 0.74110(2) 4.8(2) C(18) 0.43600(4) -0.08990(5) 0.70970(2) 3.4(1) C(19) 0.88480(3) 0.51510(4) 0.60590(1) 1.76(8) C(20) 0.95490(4) 0.53830(5) 0.56660(2) 2.8(1) C(21) 1.05610(5) 0.60970(5) 0.57160(2) 3.4(1) C(22) 1.08800(4) 0.66040(5) 0.61520(2) 2.7(1) C(23) 1.02030(5) 0.63620(5) 0.65420(2) 3.3(1) C(24) 0.92130(4) 0.56220(5) 0.64960(2) 3.2(1) C(25) 0.59360(5) 0.19070(5) 0.72540(2) 3.5(1) C(26) 0.83120(5) 0.26530(5) 0.69210(2) 3.6(1) 

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