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Bioinorganic chemistry of aluminum, gallium. and indium complexes of 1-aryl-3-oxy-4-pyridinones Zhang, Zaihui 1991

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BIOINORGANIC CHEMISTRY OF ALUMINUM, GALLIUM, AND INDIUMCOMPLEXES OF 1 -ARYL-3-OXY-4-PYRIDINONESbyZaihui ZhangB.Sc., Wuhan University, People’s Republic of China, 1982M.Sc., Laurentian University, 1987A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THEREQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIESDepartment of ChemistryWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIADecember 1991© Zaihui Zhang, 1991In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.______________________________Department of CH&H(SVZYThe University of British ColumbiaVancouver, CanadaDate 14L. ie, ‘7?!DE-6 (2/88)ABSTRACTA series of 1 -aryl-3-hydroxy-2-methyl-4-pyridinones and tris( 1 -aryl-3 -oxy-2-methyl-4-pyridinonato)aluminum(III), gallium(III), and indium(III) complexes wereprepared and characterized. The 3-hydroxy-4-pyridinones prepared have a variety of arylsubstituents at the ring nitrogen atom (phenyl, p-tolyl, p-methoxyphenyl, and pnitrophenyl). The compounds were studied using a number of techniques including singlecrystal X-ray diffraction, variable temperature NMR spectroscopy, n-octanol/waterpartitioning, and potentiometric equilibrium measurements.The structures of 3-hydroxy-1-carboxymethyl-2-methyl-4-pyridinone (Hcmp), 3-hydroxy- 1 -(p-methoxyphenyl)-2-methyl-4-pyridinone (Hpap) and tris(3-oxy-2-methyl- 1-(p-tolyl)-4-pyridinonato)aluminum(III) (Al(ptp)3) and gallium(III) (Ga(ptp)3) complexeswere determined by single crystal X-ray diffraction. A zwitterionic structure is formed inHcmp as the ketone oxygen is protonated by the carboxyl proton. The metal complexesform rigidfac geometries incorporating hydrogen bonding water molecules which bridgethe metal chelating ligand oxygen atoms.Variable temperature proton NMR spectroscopy showed that the exchange processof the tris(ligand)metal complexes is the fac-mer geometric isomerization. The highlipophilicity of the 3-hydroxy-4-pyridinones and metal complexes is shown by the highpartition coefficients. Except for the p-nitrophenyl case, the pyridinones have logn-octanol/water partition coefficients (log P) greater than 1, and the complexes havesignificantly higher log P values (> 2). Complexes of 3-oxy-2-methyl- 1-phenyl-4-pyridinonate of aluminum(III), gallium(III) and indium(III) were characterized bypotentiometric titration. The overall stability constants f for the 3:1 complexes are 1030.7(M = A13j, 1036.3 (Gaj, and 1031.1 (In3)at 25 °C and the effective formation constants(f3e f) for the metal ions at physiological pH (7.4) are 1024.7 (M = Al3), 1030.3 (Gaj,and 1025.1 (In3). The high stability constants indicate that the ligands could compete withtransferrin for aluminum and gallium in blood plasma.11The highly lipophilic complexes of l-aryl-3-oxy-2-methyl-4-pyridinonates ofgallium(III)-67 were evaluated in vitro and in vivo as potential radiopharmaceuticals.Biodistribution studies of 67Ga complexes were carried out in rabbits, mice, rats and a dog.The results of these biodistribution studies showed high heart uptake in the rabbits anddog. This is the first report to show high heart uptake of 67Ga complexes by rabbits anddogs. The results suggest that these complexes have potential as myocardial imagingagents. The different biodistribution patterns found in mice and rats indicate a speciesdifference in the biodistribution of these complexes.Tris( 1 -alkyl-3-oxy-2-methyl-4-pyridinonato)metal(III) complexes and tris( 1 -alkyl3-oxy-6-hydroxymethyl-4-pyridinonato)metal(ffl) complexes were prepared in water usinga one-pot synthesis method directly from maltol and kojic acid, respectively. This methodbypasses the separate synthesis of ligand and metal complex, and has improved the yieldsof the tris(ligand)metal complexes.111TABLE OF CONTENTSABSTRACT iiTABLE OF CONTENTS ivLIST OFTABLES viiLIST OF FIGURES xLIST OF ABBREVIATIONS xiiiACKNOWLEDGEMENTS xviChapter 1 Introduction 1Chapter 2 Synthesis and Characterization 132.1. 3-Hydroxy-2-methyl-4-pyridinones 132.1.1. Introduction 132.1.2. Synthesis 3-Hydroxy-2-methyl- 1-phenyl-4-pyridinone 3-Hydroxy-2-methyl- 1 -(p-tolyl)-4-pyridinone 3-Hydroxy- 1 -(p-methoxyphenyl)-2-methyl-4-pyridinone 3-Hydroxy-2-methyl- 1 -(p-nitrophenyl)-4-pyridinone 3-Hydroxy- 1 -carboxymethyl-2-methyl-4-pyridinone 192.1.3. Characterization Elemental Analysis Proton NMR Spectroscopy Infrared Spectroscopy Mass Spectrometry 302.1.4. Discussion 332.2. Tris(3-oxy-2-methyl-4-pyridinonato)metal(Ill) Complexes 36iv2.2.1. Introduction .362.2.2. Synthesis 392.2.2.1. Tris(3-oxy-2-methyl-1-phenyl-4-pyridinonato)metal(III) Complexes 402.2.2.2. Tris(3-oxy-2-methyl- 1 -(p-tolyl)-4-pyridinonato)metal(Ill) Complexes 412.2.2.3. Tris(3-oxy- 1 -(p-methoxyphenyl)-2-methyl-4-pyridinonato)metal(III) Complexes 422.2.2.4. Tris(3-oxy-2-methyl- 1 -(p-nitrophenyl)-4-pyridinonato)metal(III) Complexes 422.2.2.5. Sodium Tris(3-oxy- 1-carboxymethyl-2-methyl-4-pyridinonato)metal(III) 432.2.3. Characterization 442.2.3.1. Elemental Analysis 442.2.3.2. Proton NMR Spectroscopy 462.2.3.3. Infrared Spectroscopy 482.2.3.4. Mass Spectrometry 542.2.4. Discussion 57Chapter 3 Solid State Studies 593.1. 3-Hydroxy-4-pyridinone Crystal Structures 593.1.1. Introduction 593.1.2. Results and Discussion 613.2. M(ptp)3 Crystal Structures 703.2.1. Introduction 703.2.2. Results and Discussion 72Chapter 4 Solution Studies 804.1. NMR Studies 804.1.1. Aluminum-27 NMR Spectroscopy 80V4.1.1.1. Introduction .804.1.1.2. Experimental 824.1.1.3. Results and Discussion 834.1.2. Variable-Temperature Proton NMR Spectroscopy 904.1.2.1. Introduction 904.1.2.2. Experimental 924.1.2.3. Results and Discussion 954.2. Partition Coefficient Determination 1024.2.1. Introduction 1024.2.2. Experimental 1044.2.3. Results and Discussion 1064.3. Stability Constant Determination 1114.3.1. Introduction 1114.3.2. Experimental 1124.3.3. Results andDiscussion 114Chapter 5 Biodistribution Studies of Tris(ligand) Gallium-67 Complexes 1275.1. Introduction 1275.2. Experimental 1285.3. Results and Discussion 135Chapter 6 One-pot Synthesis of 1-Alkyl-3-oxy-4-pyridinonate Chelate Complexesof Aluminum, Gallium, and Indium 1426.1. Introduction 1426.2. Experimental 1436.3. Results and Discussion 146Chapter 7 Conclusion and Suggestions for Future Work 150References 152Appendix 167viLIST OF TABLESThe effective ionic radii of some metal ions involved in biology 3Isotopes of gallium and indium useful in nuclear medicine 6Human serum transferrin binding constants for M3 8Results of elemental analyses for the 3-hydroxy-4-pyridinones 201H NMR spectral data for the 3-hydroxy-4-pyridinones 22Comparison of the chemical shifts of Hcmp and Hdpp 23Infrared absorption bands and their assignments for the 3-hydroxy-4-pyridinones 27El mass spectral data (m/z) for the 1-aryl-3-hydroxy-4-pyridinones 30El mass spectral data (m/z) for Hcmp 32Results of the elemental analyses of metal complexes 451H N1VIR spectral data of the metal complexes, ML3 471H NMR spectral data of the metal complexes,Na3M(cmp) 48Characteristic infrared absorption bands of the complexes 50Tentative assignments Of vM..O 52FAB mass spectral data (m/z) of the metal complexes, ML3 54El mass spectral data (m/z) of the complexes, ML3 55FAB mass spectral data (mlz) of Na3M(cmp)3 56Bond lengths (A) with estimated standard deviations 64Bond angles (deg) with estimated standard deviations 65A comparison of the hydrogen-bonding parameters and the lRstretching frequencies 67Comparison of the ring bond lengths (A) of several pyridinonesystems 69Table 3.2.1. The bond lengths (A) (in the chelate ring) with estimated standarddeviations for M(ptp)35.5H20 74Table 1.1.Table 1.2.Table 1.3.Table 2.1.1.Table 2.1.2.Table 2.1.3.Table 2.1.4.TableTableTableTableTableTableTableTableTableTableTableTableTable2. 3.1.4.viiTable 3.2.2. The bond angles (in the chelate ring) with estimated standarddeviations for M(ptp)35.5H2O 75Table 3.2.3. The bond lengths (A) (bonds forming the pyridinone ring) withestimated standard deviations for M(ptp)35.5H20 76Table 3.2.4. The bond angles (bonds forming the pyridinone ring) with estimatedstandard deviations for M(ptp)35.5H2O 77Table 3.2.5. Possible hydrogen bonding interactions in M(ptp)35.5H20 78Table 4.1. 27A1 NMR spectral data for aluminum complexes 84Table 4.2. Kinetic parameters for A1(ppp)3 and Ga(ppp)3 at the T 98Table 4.3. Ultraviolet spectral data, ?, nm (e x 10, M1•cm) 107Table 4.4. Partition coefficients (log F) values for the 3-hydroxy-4-pyridinonesand metal complexes 109Table 4.5. Log stepwise protonation constants (log K) for Hcmp, Hppp, andHpapat25and37°CinO.15MNaC1 116Table 4.6. Log metal-ligand stability constants (f3), and effective stabilityconstants (I3eff’ pH 7.4) for the equilibrium reactions of Al, Ga andIn with Hppp at 25 and 37 °C and 0.15 M NaC1 120Table 5.1. Biodistribution in % uptake in total organ of67Ga-citrate and67Ga(pap)3 in mice 137Table 5.2. Biodistribution in % uptake in total organ of67Ga(pap)3 in rats 137Table 6.1. The yields of tris(ligand)metal(ffl) complexes via the one-pot synthesis 148Table Al. Crystallographic data for Hcmp and Hpap 167Table A2. Final atomic coordinates (fractional) and Beq (A2) for Hcmp 169Table A3. Final atomic coordinates (fractional) and B (A2) for Hpap 170Table A4. Crystallographic data for M(ptp)3.5.5H20 171Table A5. Final atomic coordinates (fractional) and Beq (A2) forAl(ptp)35.5H20 173viiiTable A6. Final atomic coordinates (fractional) and B (A2) forGa(ptp)35.5H20 174Table A7. Bond lengths (A) with estimated standard deviations forM(ptp)35.5H20 175Table A8. Bond angles (deg) with estimated standard deviations forM(ptp)3•5.5H20 176ixLIST OF FIGURESThe speciation diagrams ofAl3 (top) and Ga3 (bottom) in aqueoussolutions 73-Hydroxy-4-pyrones and their metal complexes 101-Allcyl-3-hydroxy-4-pyridinones and their metal complexes 10MPTP and its analogues 12Mechanism of the conversion of a y-pyrone to a 4-pyridinone 14Commonly used 3-hydroxy-4-pyrones in synthesis 14The structures of 3-hydroxy-4-pyridinones and their abbreviations 17The structure of attempted glucosyl functionalized pyridinone 18Comparison of JR spectra of maltol and Hppp in two regions 25Infrared spectrum of Hppp between 1700 and 300 cm-’ 29Mass spectrum of Hppp (m/z) 31The tris(3-oxy-4-pyridinonato)metal complexes 39Infrared spectra of Al(ppp)3 (top) and Hppp (bottom) between 1700and 1400cm- 51Infrared spectra of Hpap (top), Al(pap)3 (middle) and Ga(pap)3(bottom) between 800 and 500 cm 534-Pyridinone tautomeric equilibrium and the resonance fomis 59ORTEP views of the Hcmp (left) and Hpap (right) molecules 62Stereodiagram of the unit cell packing in Hcmp 63Stereodiagram of the unit cell packing in Hpap 63ORTEP view down the c axis of the unit cell packing of theGa(dpp)312H20 complex 71ORTEP view of the Ga(ptp)3 molecule 73Stereoview along the c axis of the unit cell of Ga(ptp)35.5H20 79Stereoview along the a axis of the unit cell of Ga(ptp)3.5.5H20 79Figure 1.1.Figure 1.2.Figure 1.3.Figure 1.4.Figure 2.1.1.Figure 2.1.2.Figure 2.1.3.Figure 2.1.4.Figure 2.1.4.Figure 2.1.5.Figure 2.1.6.Figure 2.2.1.Figure 2.2.1.Figure 2.2.2.Figure 3.1.1.Figure 3.1.2.Figure 3.1.3.Figure 3.1.4.Figure 3.2.1.Figure 3.2.2.Figure 3.2.3.Figure 3.2.4.xFigure 4.1. 27A1 NMR spectra of Al(cmp)33 at pH 12.0 before and afterbackground correction 86Figure 4.2. Variable-pH 27A1 NMR spectra of A1(cmp)33 88Figure 4.3. Illustration of isomers of tris-ligand metal complexes withasymmetric bidentate ligands 90Figure 4.4. 1H NMR spectrum of Al(ppp)3 in CD3O at room temperature 93Figure 4.5. 1H NMR spectrum of Ga(ppp)3 in CD3O at room temperature 94Figure 4.6. 1H NMR spectra of CH3C for A1(ppp)3 (left) and Ga(ppp)3 (right)at -30°C 96Figure 4.7. Variable temperature 1H NMR spectra of CH3c of Al(ppp)3 in CD3O 100Figure 4.8. Variable temperature 1H NMR spectra of CH3C of Ga(ppp)3 in CD3O 101Figure 4.9. The protonation equilibria of the 3-hydroxy-4-pyridinones and theirconstants 115Figure 4.10. The equilibria between neutral Hcmp (H2L) and the zwitterion insolution, and protonation to yield the protonatedH3L 117Figure 4.11. Speciation diagrams for solutions containing a 1:3 ratio of Al3 toHppp 123Figure 4.12. Speciation diagrams for solutions containing 1 mM M3 and 3 mMHppp 124Figure 4.13. Plots of Al3 (top) and Ga3 (bottom) complexation (%) versus logof the total ligand concentration ([Cj) for several ligands 125Figure 4.14. Plot of log 33 for the aluminum, gallium, and indium complexesversus ligand 126Figure 4.15. Plot of log Kai versus the 3-hydroxy-4-pyridinones 126Figure 5.1. The structure of tris(ligand) gallium-67 complexes 128Figure 5.2. Scintigraphic images of rabbits 1 hour post-injection of67GaL3 131Figure 5.3. Schematic representation of the regions of interest in a rabbit 132xiFigure 5.4. Plots of activity versus time for the heart, liver and brain of rabbitsinjected with 67GaL3 and67Ga-citrate 133Figure 5.5. Scintigraphic image of a dog heart 30 minutes post-injection of67Ga(pap)3 134Figure 5.6. Percent uptake per organ for the blood, liver, kidney, muscle, and heart24 hours post-injection of67Ga(pap)3 and67Ga-citrate solutions inmice 136Figure 5.7. Plot of activity versus time for the heart of a dog injected with67Ga(pap)3 138Figure 5.8. The speciation of Ga3 in mice before and after blood dilution 140Figure 6.1. Scheme for the one-pot synthesis of tris(3-oxy-4-pyridinonato)-metal(III) complexes 147xiiLIST OF ABBREVIATIONSAbbreviation Meaitingacac acetylacetonateo-RT cx-substituted tropolonateAMS accelerator mass spectrometryATP adenosine triphosphatef3 overall stability constantcAMP cyclic adenosine monophosphateDFO desferrioxamineDNA deoxyribonucleic acidAGTC free energy of activation at coalescence temperature (Kcal/mole)frequency shift between peaks in absence of exchange (Hz)6x-y vibrational in-plane bending modeC molar absorptivity (M1•cm)fac facial geometric isomerfree induction decayabsolute hardness (eV)Hdpp 3-hydroxy- 1 ,2-dimethyl-4-pyridmoneHck chiorokojic acid, 5-hydroxy-2-chloromethyl-4-pyroneHcmp 3-hydroxy- 1 -carboxymethyl-2-methyl-4-pyridinoneH5DTPA diethylenetriaminepentaacetic acidH4EDTA ethylenediaminetetraacetic acidHka kojic acid, 5-hydroxy-2-hydroxymethyl-4-pyroneHma maltol, 3-hydroxy-2-methyl-4-pyroneHmepp 3-hydroxy- 1 -ethyl-2-methyl-4-pyridinoneHmhpp 3-hydroxy-2-methyl- 1 -hexyl-4-pyridinonexliiHmpp 3-hydroxy-2-methyl-4( 1H)-.pyridinoneHpa pyromeconic acid, 3-hydroxy-4-pyroneHpap 3-hydroxy- 1-(p-methoxyphenyl)-2-methyl-4-pyridinoneHpnp 3-hydroxy-2-methyl- 1 -(p-nitrophenyl)-4-pyridinoneHppp 3-hydroxy-2-methyl- 1 -phenyl-4-pyridinoneHptp 3-hydroxy-2-methyl- 1-(p-tolyl)-4-pyridinoneJR infraredK stepwise protonation constantstepwise stability constantkTc rate of exchange at coalescence temperature (second1)absorbance wavelength (nm)LFER linear free energy relationshipmCi milliCurie; equals 3.7 x 1 o disintegrations/secondmer meridional geometric isomerM(ck)3 tris(chlorokojato)metal complexM(dpp)3 tris(3-oxy- 1 ,2-dimethyl-4-pyridinonato)metal complexM(ka)3 tris(kojato)metal complexM(ma)3 tris(maltolato)metal complexM(mepp)3 tris(3-oxy- 1 -ethyl-2-methyl-4-pyridinonato)metal complexM(mhpp)3 tris(3-oxy-2-methyl- 1-hexyl-4-pyridinonato)metal complexM(mpp)3 tris(3-oxy-2-methyl-4-pyridinonato)metal complexM(pa)3 ths(pyromeconato)metal complexM(pap)3 tris(3-oxy- 1 -(p-methoxyphenyl)-2-methyl-4-pyridinonato)metalcomplexM(pnp)3 tris(3-oxy-2-methyl- 1-(p-nitrophenyl)-4-pyridinonato)metalcomplexM(ppp)3 iris(3-oxy-2-methyl- 1 -phenyl-4-pyridinonato)metal complexM(ptp)3 tris(3-oxy-2-methyl- 1-(p-tolyl)-4-pyridinonato)metal complexxivMPTP 1 -methyl-4-phenyl- 1 ,2,3,6-tetrahydropyridineNa3M(cmp)3 Sodium tris(3-oxy- 1 -carboxymethyl-2-methyl-4-pyridinonato)meta(ffl)NMR nuclear magnetic resonancevibrational stretching modeP n-octanol/water partition coefficientvibrational out-of-plane bending modetfac trifluoroacetylacetonateT coalescence temperature (K)T1 spin-lattice (longitudinal) relaxation time (second)T2 spin-spin (transverse) relaxation time (second)TQ nuclear quadrupole relaxation time (second)correlation time (second)UV ultravioletWj12 peak width at half height (Hz)xvACKNOWLEDGEMENTSFirst of all, I would like to thank my supervisor, Dr. Chris Orvig, for his guidanceand encouragement for the last four years and in the preparation of this thesis. My specialthanks go to Dr. Don Lyster who has done so much for me and to Dr. W. R. Cullen for hisconstructive suggestions during the final preparation of this thesis.Financial support from the University of British Columbia in the forms of theUniversity Graduate Fellowship and teaching assistantship is greatly acknowledged.I would like to extend my thanks to Dr. Dave Clevette (Speciation Dave) for hishelp with the potentiometric titrations; to Tom Hui for his synthetic efforts; and to Gord,and Dr. Can Vo and Tern Rihela for their assistance in the biodistribution studies.I would like to thank the technical staff in the Mass Spectrometry lab and the staffof the Department of Chemistry for their help. I would also like to thank Dr. Steve Rettigwhose help has been invaluable; Dr. S. 0. Chan and Mrs. M. Austria for their help withthe 27A1 NMR; and Mr. Peter Borda for his excellent work with the elemental analysis.Thanks also to the Orvig team members, past and present, from whom I havelearned a lot. I would like to give my special thanks to “Iron Lady” Martha, Jane and“Extra Large” Gord for their patience in reading through this thesis and their valuablesuggestions.Finally, to Xiaomei who always supports me with her love and to my parents whoalways want me to accomplish something. Without their encouragement and confidence inme this would not have been possible.xviChapter 1IntroductionAluminum is ubiquitous in the environment. It is the most abundant metal and thethird most abundant element in the earth’s crust. In biological systems, however, it ispresent only in trace amounts and has no accepted role in normal biological processes.1This suggests that aluminum may possess properties incompatible with fundamental lifeprocesses.In nature, aluminum exists only in the oxidized form, Al(Ill), occuring primarily inalumina and aluminosilicate minerals. At neutral pH these species are extremely insoluble,and therefore the concentration of dissolved A13+ is low in surface water. However, acidprecipitation has facilitated the transport of aluminum from soil to surface water.2 Elevatedconcentrations of aluminum have been reported for lakes and rivers in regions throughoutthe northern temperate zone including eastern Canada and the northeastern United States.3Until the early 1970’s, most forms of inorganic aluminum were considered to benontoxic.4 However, in the last two decades, a large body of evidence has accumulatedagainst the fact that aluminum is benign and several toxic effects of aluminum have beendiscovered.48 There are a number of books and extensive reviews on the chemistry andtoxicology of aluminum in biological systems.94Humans are exposed to aluminum through many sources in daily life. Aluminum isfound at relatively high concentrations in drinking water, in some prescription drugs and inmany processed foods.15 Aluminum cooking utensils and food and beverage packingmaterials are also main sources of aluminum.16 On average, 20- 50 mg/day of aluminum1is ingested but the total body burden in normal persons is about 30 mg.17 Normally, thebody maintains low aluminum concentrations through a combination of low intestinalabsorption and effective renal clearance. Toxicity is often observed when the intestinalbarrier to aluminum uptake is somehow bypassed. Osteomalacic osteodystrophy4’67”8and dialysis encephalopathy5’6”922have developed among patients on long-term dialysisdue to chronic kidney failure, and the high aluminum concentrations in the dialysate havebeen proven as an important etiologic factor for these diseases. Natives of the island ofGuam have a high incidence of amyotrophic lateral sclerosis (ALS) and it has beenhypothesized that this could be related to the high level of aluminum found in the soils andsurface waters of the regions where the disease is endemic.23The potential association between aluminum and Alzheimer’s disease (AD) beganwith the reports24’5describing the induction of a progressive encephalopathy characterizedby neurofibrillary degeneration in rabbits following direct exposure of the central nervoussystem to aluminum salts. Levels of aluminum concentration in certain brain areas ofpatients with AD were also found to approach those which induce brain diseases inanimals.26’7Aluminum is localized in the neurons containing neurofibrillary tangles28 andis present as an aluminosiicate body in the core of the senile plaques that are characteristicof AD.29 Although it is controversial at present to say that aluminum plays an etiologicalrole in AD, the evidence obtained is consistent with the hypothesis that aluminum isinvolved in the pathogenesis of this disease; that is, aluminum acts as a secondary factorcontributing to neuronal dysfunction. A very recent report has claimed that intramuscularadministration of desferrioxamine (DFO), the trivalent metal ion specific binding agent,may slow the clinical progression of the dementia associated with AD.3° The slowprogression of AD with the treatment of DFO may be the result of the removal of aluminumfrom the patients’ bodies, and this discovery supports the association of aluminum withAD.2Aluminum is now widely regarded as a neurotoxin. A very large number of toxicbiological effects of aluminum have been documented;3’however, its in vivo chemistryremains unclear. It is assumed that aluminum follows the iron pathway in vivo.32 Inextracellular fluid and plasma, aluminum is bound to the iron transport protein transferrin33and enters the cell with ferric ions via the transferrin receptor and coexists with iron in thecytosolic labile iron pooi (e.g., with citrate as a common chelator); the iron pathways are nolonger available to the intracellular aluminum (because of the subtle differences in theirchemistry) and it is eventually bound to phosphate ligands, including ATP,3437 inositolphosphates,32’8the phosphate groups of membrane lipophosphate,39 phosphorylatedproteins,40’1 and silicic acid.32’843 This indiscriminate preference of aluminum forthe phosphate ligands in vivo can affect or even inhibit some important enzymatic processesand protein activities.Table 1.1. The effective ionic radii of some metal ions involved in biology (from ref. 44).Ion Radius (A)aAl3 0.54Fe3 0.65bMg2 0.72Zn2 0.74Ca2 1.00a corresponding to 6 coordinate.b high-spin state.The aluminum cation is small and highly charged, binding to oxygen-containingligands strongly. It can also compete with, and replace, any of the common biologically3relevant cations (see Table 1.1) and this substitution can cause inhibition. In addition, theslow rate of ligand exchange offered by Al3 (e.g. 1O times slower than Mg2 and iOtimes slower than Ca2+)45 renders any complexes formed biologically inactive. Inparticular, the competition of Al3 with Mg2 in biological systems for the binding sites inenzymes would inhibit important processes and therefore give rise to different biologicalconsequences. Some of the biological effects of A13+ are listed as follows:1. The A1-ATP complex inhibits the action of hexokinase — amagnesium-dependent enzyme responsible for glucose phosphorylation, thefirst stage in the production of metabolic energy— by blocking the ability ofthe enzyme to transfer the terminal phosphate group to glucose.362. The binding of aluminum to phosphate groups of inositoltriphosphate— a second messenger molecule in cellular signaltransduction46’7with a vital function in areas of the brain42— may influenceits ability to release calcium ions and stimulate protein kinases.32383. Aluminum has marked effects upon the biochemistry of 3’, 5’-cyclic adenosine monophosphate (cAMP)— the magnesium-dependentsecondary messenger produced within the cell in response to a primarymessenger, i.e. a hormone or neurotransmitter, binding to a receptor at thecell membrane, activating protein kinases to phosphorylate other enzymes.4. Aluminum also inhibits the activation of adenosine triphosphatases(ATPase)— the enzymes responsible for the functioning of the ion pumpswhich maintain and correct the electrochemical environment in neurons andother cells— by replacing either sodium/potassium or magnesium.5. The formation of aluminum complexes with the phosphoryl headgroup in phospholipid membrane influences membrane fluidity which in turncan influence protein (receptor) mobility and receptor activity.3946. Aluminum has been proposed to bind to both phosphate groupsand bases in DNA. Three different complexes are assumed to form betweenaluminum and DNA. It is suggested that aluminum cross-links could lead todeleterious biological effects.40In order to understand the neurotoxic effects of aluminum, it is crucial to understandits metabolism in vivo, especially in the brain. The lack of suitable radionuclides of thiselement has hindered its development in biological studies. (Recently, it was reported thataccelerator mass spectrometry (AMS) was used to study aluminum kinetics in rats with26Al (tl/2 = 7 105 year) and the results demonstrate the potential of this technique forisotope tracer studies in animals as well as in humans.48) Isotopes of chemically similargroup 13 (lilA) elements, gallium and indium, are commercially available and they can beused as the markers of aluminum in biological studies if care is taken. It has been shownthat gallium was a suitable marker to replace aluminum in some physiologicalexperiments.4951Another important factor which has stimulated the rapid development of thecoordination chemistry of gallium and indium is the application of their radioactive isotopesin the field of diagnostic nuclear medicine. For each element, there are two isotopes whichare suitable for the detection methods in nuclear medicine (Table 1.2). The discovery in the1960’s that67Ga-citrate localized in soft tissue tumors52 engendered considerable clinicalresearch and the citrate complex (the 211j3commercially available radiopharmaceutical of67Ga) is now widely used in oncological nuclear medicine.5355 In spite of its availabilityand easy detection, the use of 67Ga in nuclear medicine has been limited to the detection oftumors and inflammatory lesions.5658 The useful forms of indium that have achieved themost interest in nuclear medicine are 111In - chloride, 111n - DTPA and 111In oxinelabelled blood cells.59 Another important application of indium radiopharmaceuticals is thecovalent attachment of bifunctional chelates to specific monoclonal antibodies directedagainst tumor associated antigens.60635Table 1.2. Isotopes of gallium and indium useful in nuclear medicine.Isotope 67Ga 68Ga 1lljj 113mjtia 78.1 h 68.3 mm 67.9 h 1.66 hyDecay Energy 93.3, 185 511 171 392(KeV) 300, 394 (1 annihilation) 245The group 13 (lilA) metal ions are classified as hard acids.64 Their aqueoussolutions are acidic, undergoing a series of extensive and complex hydrolysis processes toproduce monomeric and polymeric hydroxy compounds upon the increase of pH.65 Theseprocesses are pH, concentration and time dependent and under physiological conditions(pH = 7.4) these metal ions form insoluble neutral hydroxide species (Figure 1.1 from ref.65). To chelate these metal ions in an aqueous environment a ligand must be able tocompete with the hydroxide ion.For a gallium or indium complex to be useful as a potential radiopharmaceutical inbiological studies, it must be thermodynamically stable not only towards hydrolysis atphysiological pH but also towards competition with the blood serum protein transferrin fortrivalent metal ions. Transferrin is an iron transport protein in blood plasma with amolecular weight of —77000 Dalton. It contains two similar metal binding sites66’7 withhigh affinities for metal ions similar in size to Fe3. The binding constants (K1 and K2) oftransferrin for Al, Ga, In and Fe are listed in Table 1.3. The large binding constants andthe high concentration of vacant transferrin binding sites in human blood (—50 .tM68)maketransferrin a powerful scavenger of trivalent metal ions. It has been shown thataluminum33and gallium69’70in serum are bound to transferrin.6(a) 0.lm Al (Ill)100806040202 4 6 8 10 12pH(b) 105m Ga(]U)100-”.80Figure 1.1. The speciation diagrams of Al3 (top) and Ga3 (bottom) in aqueous solutions(from ref. 65). The species are expressed in the figures by the coefficients,(x, y), in which x is the number of M3 and y the number of OH-, or 0Hequivalents. 13,32 =[Al13O4(OH)4)2]7.(b) 10 m Al(flI)100H1)713,323,4r‘iT I I I‘I11,4II —IIiiI’IIIII,III, —1..’ .1:- l /: :—• 1,2’5 1I,’, •‘— ,III IIIi’• I,, III, I I— 1$.”• i’;:... I8060H4020H0C,2 4 6 8 10 12pH(a)0.lm GaIt)100‘ I‘,, i”26,65”80- I0iiIIIII: ‘I60-40-20-A’5II’’,‘‘ i’’ 122 4 6 8pH0604020• III. I II I;1/I1,0— ,‘1,4 —1,312’ ‘Itl /\ t’ V“I gl‘‘l—I.,•I’I’ III ‘‘‘ I: ‘III— II ‘ —g, I’J %S Ik ‘/‘I2 4 6 8pH.7Table 1.3. Human serum transferrin binding constantsa for M3+.M3 pjb Gac Feelog K1 ([HCO3-}, mM) 13.5 (5) 20.3 (27) 30.5 (40) 22.7 (27)logK2 12.5 19.3 25.5 22.1a pH 7.4, 25°C. b from ref. 71. C from ref. 72. d from ref. 73.e from ref. 68.When Ga3or In3 is injected in the form of the commonly administered Ga-citrateor In-chloride, the metal ions are rapidly bound by transferrin.7477 The radioisotopes arethen found in areas of high iron uptake: bone marrow, liver, spleen, kidney, soft tissuetumor, and inflammatory lesions. Gallium is also concentrated and secreted by themammary and salivary glands, and into the bowel; about two-thirds of the dose is retainedin the body over an extended period while the rest is excreted via kidneys and bowel. Toretain the metal complexes intact in vivo, a ligand must be able to bind Ga3+ and J3+strongly and compete with transferrin. With nuclear medicine in mind, the aqueouscoordination chemistry of gallium and indium with multidentate ligands incorporatinghydroxy, amino, carboxylate and catechol moieties has been explored.7896 Much of thework has been aimed at preventing these complex decomposition and demetalationprocesses in order to direct the radionuclide to a target organ, and also at developing newchelating agents for conjugation to monoclonal antibodies.There have been few attempts to develop the bidentate ligand chemistry with watersoluble chelates of aluminum, gallium and indium. 8-Hydroxyquinoline (oxine), whichchelates both 68Ga and 11 lJ in radiopharmaceutically active tris(ligand) complexes,79’978is one example in this area. This bidentate ligand has high formation constants for both8gallium99 and indium100 and can be used in the labeling of red blood cells, leukocytes andblood platelets with radioisotopes of either ion.78’997101 The tris(oxinato) complexesare neutral and lipophilic and cross cell membranes easily, but labeling must be done invitro to avoid transferrin competition. Subsequently, the labeled blood cells are thenreturned in vivo for the scanning procedure. Tropolone, acetylacetone, and 2-mercaptopyridine N-oxide have also been investigated as alternatives in the same transportsystem.98”°2For the last several years, our research group has been investigating thecoordination chemistry and in vivo properties of Al, Ga and In complexes with somebidentate ligands. We have extensively studied several classes of heterocyclic compoundscontaining the c-hydroxyketone moiety including the 3-hydroxy-4-pyrones (Figure 1.2)and the 1-alkyl-3-hydroxy-4-pyridinones (Figure 1.3). We have determined that theseneutral tris(ligand)metal complexes have a unique combination of properties: watersolubility, hydrolytic stability, variable lipophilicity and promising in vivo biologicalactivity. 103-110 Our collaborators have determined that maltol is a highly efficient, naturallyoccurring ligand capable of facilitating transport of aluminum.111 A toxicological study hasalso shown that Al(ma)3 and Al(ka)3 are potent neurotoxins in rabbits when injectedintracranially, and that Al(ma)3 is 20 times more neurotoxic than the Al-lactate complex.”2Now Al(ma)3 is becoming widely used in the study of aluminum neurotoxicity andaluminum mobility,113’5and it is commercially available (Aldrich). It has also beenevaluated in vitro that 3-hydroxy-4-pyridinones could be used as potential aluminumchelators to replace desferrioxamine in the treatment of Al accumulation and toxicity.6Our 67Ga biodistribution studies showed that, at appropriate concentrations, the 1-alkyl-3-hydroxy-4-pyridinones could redirect 67Ga from transferrin; however, rapid renal excretionof the67Ga-ligand complexes in mice and rabbits was also observed.1109= CH3,R6 = H= H, R6 = CH2OR2=R6HR2 = H, R6 = CH21R2=CH3,R6=R2 = H, R6 = CH2O=R6HR2 = H, R6 = CH2IM= Al, GaMaltol (Hma)Kojic acid (Hka)Pyromeconic Acid (Hpa)Chiorokojic acid (Hck)M(ma)3M(ka)3M(pa)3M(ck)3Figure 1.2. 3-Hydroxy-4-pyrones and their metal complexes.R=HR=CH3R=C2H5R = n-C6H13HmppHdppHmeppHmhppR=HR=CH3R=C2H5R = n-C6H13M = Al, Ga, InM(mpp)3M(dpp)3M(mepp)M(mhpp)3Figure 1.3. 1-Alkyl-3-hydroxy-4-pyridinones and their metal complexes./ R6310As part of this continuing project to explore the coordination chemistry ofaluminum, gallium and indium with bidentate ligands and to develop newradiopharmaceuticals for heart and brain imaging, the research project involved in thisthesis consists of three parts:a. design and synthesis of new lipophilic ligands and preparation of their metalcomplexes with aluminum, gallium and indium;b. a study of their coordination chemistry in solid and solution states;c. in vivo evaluation of their potential application as radiopharmaceuticals.From previous work with 1-alkyl-3-hydroxy-4-pyridinones and their metalcomplexes, it was found that the l-alkyl-3-oxy-4-pyridinone complexes of aluminum andgallium have significant enough thermodynamic stability to compete with transferrin atappropriate concentrations but that their lipophilicities are probably significantly lower thanideal. It was therefore necessary to develop new ligands that have higher lipophilicitywhile maintaining the desired thermodynamic properties. Lipophilicity of this class ofligands can be altered by changing the substituents at the ring nitrogen atom. Arylsubstitutents are an ideal choice, as a number of aromatically functionalized pyridinones ofconsiderable, but variable, lipophilicity can be easily obtained by the insertion of differentsubstituted anilines into maltol without altering the stability constants for the metal ions.Aryl substituents at the ring nitrogen are also of interest to the continuing solid-statestudy of the exoclathrate matrix. This array incorporates extensive hydrogen bondingthrough water molecules between tris(3-oxy-4-pyridinonato)metal(III) complexes andhexagonal water channels, and it has been found in complexes where the metal ion is Al3+,Ga3, and In3 and the nitrogen substituent is CH3 or C2H5.’°’7”9In addition, these pyridinones are analogs of the dopaminergic neurotoxin MPTP(1 -methyl-4-phenyl- 1 ,2,3,6-tetrahydropyridine, Figure 1.4). MPTP is neurotoxic tohumans,117 subhuman primates, and mice,8’9and it produces an irreversible11Parkinson’s disease syndrome. The research work aimed at elucidating the molecularmechanisms responsible for this effect is vigorous.120O__CN_CH3 0MPTPFigure 1.4. MPTP and its analogues.The preparation and characterization of 3-hydroxy-4-pyridinones andtris(ligand)metal complexes are described (Chapter 2). X-ray crystallography has beenused to study the solid state coordination chemistry of several pyridinone compounds andmetal complexes (Chapter 3). In the solution state studies (Chapter 4), potentiometricequilibrium measurements have been applied to study the thermodynamic properties of thepyridinones and metal complexes; variable-temperature proton NMR spectroscopy has beenused to investigate the fluxionality of the metal complexes while n-octanol/water partitioncoefficients of the compounds have been determined to evaluate their lipophilicity. Basedon the results of these studies, the potential utilization of these metal complexes in nuclearmedicine has been evaluated in vivo by bioclistribution studies of 67Ga tris(ligand)metalcomplexes (Chapter 5). In addition, one-pot synthesis of metal complexes of aluminum,gallium and indium of 1-alkyl-3-oxy-4-pyridinonates is described (Chapter 6); thissynthetic method allows the preparation of selected tris(ligand)metal complexes in one stepfrom purchased starting materials and allows the difficulties encountered previously in theligand synthesis to be overcome.12Chapter 2Synthesis and Characterization2.1. 3-Hydroxy-2-methyl-4-pyridinones2.1.1. IntroductionPyridinones, also known as pyridones or hydroxypyridines, are a well-knownclass of compounds. Reports on the synthesis of 4-pyridinones date back as early as1884, when the preparation of 4-pyridinone and 1-phenyl-4-pyridinone was described. 121Since then a considerable number of reports have accumulated in the literature on this classof compounds.122’24 The methods for preparing pyridinones can be classified into threemain categories: (1) ring closure of acyclic compounds; (2) conversion of otherheterocyclic ring systems; (3) substitution and replacement on pyridine and its derivatives.The conversion of - and 7-pyrones by ammonolysis or aminolysis is an importantand widely used method for the preparation of 2- and 4-pyridinones. It provides one of themost efficient routes for the synthesis of this type of nitrogen heterocycle, generallyproceeds in satisfactory yields under mild conditions and often represents the preferredpreparative method when the necessary pyrone is readily available.This type of conversion reaction proceeds by nucleophilic attack of a pyrone by aprimary amine, followed by ring opening, elimination of water, and ring closure to give thedesired pyridinone. The general mechanism of the conversion of a y-pyrone to a 4-pyridinone is schematically shown in figure 2.1.1 (from ref. 124).130- H20R1H2 LNR211Figure 2.1.1. Mechanism of the conversion of a y-pyrone to a 4-pyridinone.Nucleophilic attack of a ‘y—pyrone by ammonia and amines occurs at position 2 or 6.Electron withdrawing groups at position 3 or 5 can activate the y-pyrone toward thenucleophile, while electron donating groups at these positions can reduce the reactivity ofthe ‘y-pyrone toward the nucleophilic reagent. Groups at positions 2 or 6 can inhibit theattack, by steric obstruction.Although the electron-donating hydroxyl substituent can reduce the effectiveness ofthe conversion reaction, 3-hydroxy-4-pyrones have been used as the starting materials tosynthesize the corresponding 3-hydroxy-4-pyridinones. The most commonly used 3-hydroxy-4-pyrones are listed in Figure R2 = CH3,R5 = R6 = H Maltol( R6 = CH2O , R2 R5 H Kojic acidR = = COOH, R5 H Meconic acid0 R2 R2 = = R6 = H Pyromeconic acidFigure 2.1.2. Commonly used 3-hydroxy-4-pyrones in synthesis.Meconic acid is a valuable starting material. It can be treated with concentratedammonia,125 alkylamines (R = CH3,C2H5,i-C3H7),’26”7 and glycine’27 to give the0514corresponding N-substituted comenic acids, one carboxyl group being lost. Pyromeconicacid, the pyrolysis product of meconic acid, was used in the first total synthesis of 3-hydroxy-1-alanine-4-pyridinone, usually known as mimosine, as the starting material.128A series of N-alkyl substituted-4-pyridinones was made from meconic acid andpyromeconic acid.127 Maltol and kojic acid are commonly used in preparing a wide varietyof 3-hydroxy-4-pyridinones.’29’130The efficiency of the conversion reaction, however, depends on starting pyronesand amines. Inconsistency has been noted in reactions of this type. Pyromeconic acidreacts with methyl-,’27 ethyl-,’27 n-propyl-’27 and ipropylamines27 to give thecorresponding N-substituted-3-hydroxy-4-pyridinones. However, products could not beobtained from n-butylamine, cc (or 3)-pheny1-ethylaniine or cc,-diaminopropionic acidwith pyromeconic acid and meconic acid. Further evidence showing the inconsistency ofthe reaction is that meconic acid gives a pyridinone with glycine, but pyromeconic aciddoes not.’27 Even with the same pyrone and amines, the results could be different; forexample, Heyns and Vogelsang’3’reported 30% yield for the ammonolysis of kojic acidbut others found this reaction to be unproductive.132 Generally, more basic and lesshindered amines give higher yields of 4-pyridinones.It was found that the efficiency of the conversion reaction could be improved byblocking the 3-hydroxyl group with a methyl’33 or benzyl’34 group. However, theblocking and deblocking sequence is very time consuming; the reaction conditions for thedeblocking procedure are very rigorous, and this can be the major experimentalimpediment. Therefore, the direct synthesis of N-substituted 3-hydroxy-4-pyridinonesfrom y-pyrones is always of great interest. Kontoghiorghes reported a direct one-steppreparation of 1 -alkyl-3-hydroxy-2-methyl-4-pyridinones.’0Several 1 -aryl-3-hydroxy-4-pyridinones were also prepared by heating the mixture of maltol and the appropriate aminesin a sealed tube to 150 DC.135 Nelson prepared a series of 1-alkyl-3-hydroxy-2-methyl..4-pyridinones with improved synthetic procedures by controlling the pH of the reaction15mixture.136 This improved method gives better consistency in the conversion of maltol tothe pyridinones.In the work described in this thesis, maltol was used as the starting pyrone for theconversion reaction. Although the electron-donating methyl group at the 2 position couldreduce the efficacy of the conversion reaction, the decreased acidity of the hydroxyl proton(pKa value of 8.36 for maltol versus 7.69 for pyromeconic acid and 7.66 for kojic acid)due to the ortho effect137 of the methyl group would provide a stronger base, therefore, abetter ligand for Lewis acid metal ions. The higher stability constants of aluminum withthe l-alkyl-3-oxy-2-methyl-4-pyridinonate ligands (log 3 are .32b08.hb0) than that ofaluminum with mimosine (log is 29138) show that the 3-oxy-2-methyl-4-pyridinonatesare stronger bases than mimosine. Maltol is also commercially available.A number of 1 -aryl-3-hydroxy-4-pyridinones35”39and 1 -aryl-5-hydroxy-2-hydroxymethyl-4-pyridinones’40are known. They are reported to be suitable extractantsfrom aqueous solution for many metal ions, e.g., gallium(III),’41”2iron(HI))435vanadium(V),146’48 uranium(VI),149 thorium(IV),’50 protactinium(V))5°zirconium(IV),151 niobium(V),151 and tantalum(V).151 However, the synthesis of thesecompounds from maltol or benzylated maltol involved a sealed tube procedure utilizingreaction temperatures of 150 0C135,9 and use of a sealed tube is not applicable to largescale synthesis. Although a more convenient preparative method to synthesize 1-arylsubstituted-4-pyridinones was developed by Looker and his co-workers,’40in which thereaction was carried out in dilute aqueous hydrochloric acid at reflux temperature, themethylated kojic acid was employed. In the preparation described in the following section,l-aryl-3-hydroxy-4-pyridinones were directly obtained from maltol in a dilute hydrochloricacid medium at reflux temperature.A series of 3-hydroxy-2-methyl-4-pyridinones was synthesized from maltol and thefollowing amines: aniline, p-toluidine, p-anisidine, p-nitroaniline and glycine. Thestructures of these pyridinones are shown in Figure 2.1.3. They were named to emphasize16the hydroxyl group and the N-substituents, e.g., 3-Hydroxy-2-methyl- 1-(p-tolyl)-4-pyridinone, Hptp. The substituents were ordered to emphasize the acidic proton so thatthe conjugated base of the pyridinone could be readily identified, e.g., ptp.O=4N_c_R R=HHO C113 NO2 HpnpO N—CH2—COO— HcmpHO CH3Figure 2.1.3. The structures of 3-hydroxy-4-pyridinones and their abbreviations.The initial objective in synthesizing 3-hydroxy-1-carboxymethyl-2-methyl-4-pyridinone (Hcmp) was the utilization of this compound as the precursor to prepare aglucosyl functionalized pyridinone system (Figure 2.1.4). The synthesis of this specialpyridinone was attempted via different routes, and no product was obtained. However, theinteresting solid state chemistry (Chapter 3) of Hemp led us to investigate the coordinationchemistry of aluminum, gallium and indium complexes with this compound.17CH2OOH0:Figure 2.1.4. The structure of attempted glucosyl functionalized pyridinone.2.1.2. SynthesisAll chemicals were reagent grade and were used as received without furtherpurification: maltol (3-hydroxy-2-methyl-4-pyrone, Sigma), aniline (Fisher), p-toluidineand p-anisidine (Eastman Organic Chemicals), and p-nitroaniline (MCB). Benzylprotected maltol (3-benzyloxy-2-methyl-4-pyrone, Bzma) was prepared according to themethod of Harris152 and was used without further purification. Water was deionized(Barnstead D8902 and D8904 cartridges) and distilled (Corning MP-1 Megapure still).The compounds were prepared as described below. The progress of the reactions wasmonitored by thin layer chromatography on silica gel plates with 10% methanol in CH21as the eluent. As many of the synthetic procedures were similar only representativepreparations are given. The melting points were measured with a Mel-Temp apparatus andare uncorrected. The yields are for purified compounds and they are calculated based onmaltol. 3-Hydroxy-2-methyl-1-phenyl-4-pyridinone, Hppp. Maltol(4.03 g, 32.2 mmol) and aniline (6.35 g, 68.2 mmol) were suspended in 100 mL diluteHC1 solution (3 mL concentrated HC1 in 100 mL water). This mixture was refluxed for 72hours and then cooled to room temperature. A light yellow solid was collected by18filtration. It was recrystallized from hot methanol (after being decolorized with activatedcharcoal for 30 minutes) yielding an off-white solid (3.52 g, 55 % yield). m.p. 222 °C. 3-Hydroxy-2-methyl-1-(p-tolyl)-4-pyridinone, Hptp. Asuspension of maltol (4.00 g, 31.8 mmol) and p-toluidine (6.95 g, 65.0 mmol) in 100 mLof dilute acidic solution was refluxed for 72 hours and yielded 3.56 g of recrystallizedproduct (52 % yield). m.p. 253 °C. 3-Hydroxy-1-(p-methoxyphenyl)-2-methyl-4-pyridinone,Hpap. Maltol (4.02 g, 31.9 mmol) and p-anisidine (7.93 g, 64.4 mmol) gave 4.27 g ofthe product (58 % yield). m.p. 249 °C. 3-Hydroxy-2-methyl-1-(p-nitrophenyl)-4-pyridinone, Hpnp.3-Benzyloxy-2-methyl-4-pyrone (5.14 g, 23.8 mmol) and p-nitroaniline were suspendedin 120 mL of 5:1 dilute HC1:methanol. This suspension was refluxed for 73 hours. Anoily layer was separated from the aqueous layer while hot and was discarded. The aqueouslayer was cooled to yield a yellow solid which was collected by filtration and washed withdiethyl ether. The resulting bright yellow solid was dissolved in hot acidified water andrecrystallized by adding 8N NaOH solution dropwise to raise the pH of the solution to 7.The final product yield was 1.62 g (28 %). m.p. 293 - 295 °C. 3-Hydroxy- 1-carboxymethyl-2-methyl-4-pyridinone,Hcmp. Maltol (4.95 g, 39.2 mmol) and glycine (5.92 g, 78.8 mmol) were dissolved in100 mL hot distilled water and the pH of this solution was adjusted to about 9.1. Thissolution was refluxed for 20 hours and decolorized with activated charcol. The volume ofthe solvent was reduced to about 50 ml and 6 N HC1 solution was added to this solution tolower the pH to about 3 and a yellowish solid was obtained. Recrystallization of this solidfrom hot water twice yielded an off-white crystalline product (3.09 g, 43%). m.p. 258 -260 °C.192.1.3. CharacterizationThe 3-hydroxy-4-pyridinones were characterized by elemental analysis, protonNMR spectroscopy, infrared (IR) spectroscopy and electron impact mass spectrometry (ElMS). The results were completely consistent with the structures as shown in Figure 2.1.3.The elemental analyses (C, H, N) were performed by Mr. Peter Borda of theMicroanalytical Laboratory of this department. The JR spectra were recorded as KBrpellets in the range of 4000 - 200 cm-1 with a Perkin-Elmer PE 783 spectrophotometer andwere referenced to polystyrene film. Proton NMR spectra were recorded with a BrukerAC-200E or a Varian XL-300 spectrometer. Mass spectra were obtained with a KratosMS5O (electron impact ionization, El) instrument. Elemental AnalysisAll samples were purified by recrystallization in appropriate solvents as stated in thesection 2.1.2 and were dried in vacuo overnight. The calculated results and the actualresults are listed in Table 2.1.1 and are in a good agreement.Table 2.1.1. Results of elemental analyses for the 3-hydroxy-4-pyridinones(calculated[foundj).Compound Formula % C % H % NHppp C12H1N02 71.63 [71.661 5.51 [5.561 6.96 [6.96]Hptp 13N0 72.54 [72.70] 6.09 [6.10] 6.51 [6.52]Hpap C13HN03 67.52 [67.51] 5.67 [5.71] 6.06 [6.13]Hpnp 120204 58.54 [58.36] 4.09 [4.07] 11.38 [11.36]Hcmp C8H9N04 52.46 [52.57] 4.95 [4.98] 7.65 [7.63] Proton NMR SpectroscopyThe spectra of Hppp, Hptp and Hpap were recorded in CDC13 and referenced toTMS. The spectra of Hpnp and Hemp were recorded in d-DMSO and D20, respectively,and they were internally referenced tod5-DMSO and HOD. The chemical shifts and thecoupling constants are listed in Table 2.1.2. The methyl group on the pyridinone ring wasseen as a singlet. The hydroxyl protons were detectable in aprotic solvents: CDC13 forHppp, Hptp and Hpap, and6-DMSO for Hpnp. In all cases, peak integrations wereconsistent with the assignments as given in Table 2.1.2.The characteristic doublet of the ring proton doublets, HcHd, was observed inHemp, with a coupling constant of 8.0 Hz. In the Hppp, Hptp and Hpap spectra, only onering proton doublet (I-It) with a coupling constant of 7 - 8 Hz was resolved; the other one(Hd) appeared in the envelope of phenyl protons. For Hpap, two sets of doublets withdifferent coupling constants were observed: one doublet arose from the HcHd protons onthe pyridinone ring (coupling constant of 7.4 Hz) and the other from the two equivalentpairs of protons HeHf on the phenyl ring (coupling constant of 8.9 Hz). In the HcHddoublet, the signal from the protons on the carbon directly attached to the ring nitrogen isshifted downfield from that of the ring methyl group. Deshielding by the electronegativenitrogen is the reason for this and for assigning the lowfield doublet to Hj.15321Table 2.1.2. 1H NMR spectral dataa for the 3-hydroxy-4-pyridinones.O==(N.__()-_R and O=zçN_CH2COOHOH CH3 OH CH3g a g aHpppb Hptpb Hpapb Hpnpc HcmpdR= H CH3 OCH3 N02a 2.11 (s, 3H) 2.11 (s, 3H) 2.11 (s, 3H) 1.99 (s, 311) 2.44 (s, 311)b e 2.45 (s, 3H) 3.90 (s, 3H) - 4.87 (s, 2H)c 6.48 (d, 1H) 6.46 (d, 1H) 6.45 (d, 1H) 6.25 (d, 1H) 7.02 (d, 111)(J = 7.6 Hz) (J = 7.2 Hz) (J = 7.8 Hz) (J = 7.4 Hz) (J = 8.0 Hz)d+e+f 7.2- 7.6 7.1 - 7.4 6.9- 7.3 7.61 (d, 1H) 7.93 (d, 1H)(m, 6H) (m, 5H) (m, 5H) (J = 7.4 Hz) (J = 8.0 Hz)7.79 (d, 2H)(J= 8.9Hz)8.38 (d, 211)(J= 8.9Hz)g 5.76 (b) 5.23 (b) 5.66 (b) 3.60 (b)a Abbreviations used in this table: s = singlet, d = doublet, m = multiplet.b in CDCl3 C ind6-DMSO. d in D20, g is not observed.e Protonbislistedwithd+e+f.proton d only.22In the solid state (Chapter 3), the formation of zwitterions in Hcmp structure isobserved as the carboxyl group is deprotonated and the ketone oxygen atom on thepyridinone ring is protonated. This zwitterionic structure persists in the solution state asshown by comparison of the proton NMR spectra of Hcmp and Hdpp, a structuralanalogue of Hcmp, in D20. In the Hcmp spectrum, the chemical shifts of all the protonswere shifted downfield from those in the Hdpp spectrum. The downfield shifts of theprotons directly attached to the pyridinone ring, H and Hd, were more pronounced (—0.5ppm) than the methyl group. The chemical shifts of the protons of [-Icmp and Hdpp arelisted in Table 2.1.3.Table 2.1.3. Comparison of the chemical shiftsa of Hcmp and Hdpp.O=cNROH CH3aHcmp HdppbR = CH2OOH CH3a 2.44 2.36 0.08b 4.87 3.73 1.14dc 7.02 6.46 0.56d 7.93 7.58 0.45a in D20.b from ref. 129.C SHcmp -6Hdpp.d also affected by COOH group.23As zwitterions are formed in solution, a positive charge is introduced to thepyridinone ring. This positive charge is also delocalized over the ring system (see Chapter3) and changes the chemical environments of the protons related to the ring. Therefore, thechemical shifts of these protons should be affected. The deshielding effect of the positivecharge on the ring should be accounted for the downfield shifts of the proton signals.Also, the effect should be more pronounced for the protons attached directly to thepyridinone ring than the others (see Table 2.1.3). Infrared SpectroscopyInfrared spectroscopy offers quick information in confirming the outcome of theconversion reaction and in characterizing the pyridinone compounds. The ring skeletonvibrational modes of the pyrones are related to those found in This typicallyresults in four ring stretching modes between 1650 and 1400 cm-1 and the pattern changespredictably when the cyclic oxygen is replaced by nitrogen (Figure 2.1.4).In the VOH region, a broad band is observed at 3200 cm1,indicating that hydrogenbonds are formed in these compounds. In the region of 1700 - 1400 cm1, seven bands areobserved. Thev0and Vring(cc) stretching bands appear in this region as a typical four-band pattern. The skeletal stretchings of the benzene ring also appear in this region.155 Itis impossible to separate the v0 and the higher energy Vnng(cc) stretches in any of thecompounds. Numerous authors have come to this same conclusion in their considerationsof 4-pyridinone and related compounds.’56”7 The bands which do appear areextensively coupled and there is no mode localizing in any of these frequencies.’58 Thereis a reproducible series of four ring stretching modes which appears in manypyridinones’59and the fact that some of these bands are coupled to V0 makes individualassignments impossible. The appearance of the skeletal stretchings of the benzene ringcomplicates the spectra in this region. It is difficult to separate the bands from the skeletal24stretchings of the benzene ring from those of the pyridinone ring. The highest wavenumber band has the most C=O character and the relatively low energy of this band (below1650 cm1 in all spectra) indicates it is acting as a hydrogen bond acceptor.I I-3500 3000 — iooo 1400WAVEN UMBER (cw’)Figure 2.1.4. Comparison of JR spectra of maltol (top) and Hppp (bottom) in two regions.25In the 6CH region, weak to medium intensity bands are observed with additionalbands for the compounds with N-substitution consistent with the presence of additionalgroups. There is extensive mechanical coupling of the öOH andv modes in the regionof 1325 to 1235 cm-1,60162 so that a definite assignment of each of these modes is notachievable.The öring and7tring assignments correlate well with the literature and the sharp out-of-plane bending mode (—830 cm-1) is the most distinctive feature of the lower energyregion in all of the spectra. The sharp out-of-plane bending bands of the hydrogen atomsof the benzene ring also appear around 800 cm-1 and the band position and number ofbands are dependent on the number of substituents on the benzene ring.155 The sharpbands observed at 780 cm’ and 700 cm in the Hppp spectrum are the characteristic bandsfor the monosubstituted benzene. For 1,4-disubstituted benzenes, at —-820 cm’ there isone sharp characteristic band which is observed in the spectra of Hptp, Hpap, and Hpnp.The infrared spectrum of Hcmp shows a very broad band in the range of 3300 to2500 cm1, a result of the strong intermolecular hydrogen bonding of the carboxylgroups.’63 The characteristic four-band pattern between 1650 to 1500 cm1 for pyridinonering stretchings is obscured in Hcmp because of the formation of zwitterions and thedelocalization of the formal double bonds. The wide strong band from 1650 to 1600 cmis assigned to a mixture of stretchings of the carboxylate anion and the pyridinone ring.The infrared spectrum of Hppp in the region of 1700 to 700 cm is shown in Figure 2.1.5as an example of the JR spectra of these compounds.26Table 2.1.4. Infrared absorption bands and their assignments for the 3-hydroxy-4-pyridinones (cm).aAssignment Hppp Hptp Hpap Hpnp HempVOH 3200 (b) 3180 (b) 3180 (b) 3200 (b) 3300-2500 (s, b)VC..H(ring) 3045 (w) 3090 3030 31103050 (w) 3000 (w) 3080 (w)VC..H 2940 (w) 2960 (w) 2960 (w) 2930 (w)(CH3 or CH2) 2850 (w) 2930 (w) 2940 (w)2840 (w)1630 (s) 1630 (s) 1630 (s) 1635 (s) 16501600c(s,b)and 1585 1590 1580(sh)1570 (s) 1575 (s) 1575 (s) 1590 (s, b)1530 1535 1530 1530 15451515(s) 1510(s) 1515 15201490 (s) 1500 1490 14951400 (w) 1400 (w) 1400 (w) 1405 (w)öasCH 1455 1455(s) 1465 1495(s) 1450syCH 1390(w) 1380(w) 1380(w) 1350’(s) 1380-1350(s,b)1320(m) 1310(s) 1320(w)vcoand 1300(s) 1285 1300(s) 1310(s)öOH 1240 (b, s) 1250 (sp, b) 1250 1260 (b) 1270276ring 1210(s) 1210(s) 1205 1210 1210(w)1180 1190(s) 1170(s)1100(w) 1100(w) 1105(w) 11051040 (s) 1040 (s) 1040 (s) 1040 (w) 1075860itring 830 (s) 830 845 845 (b) 830ltring 780 820 825 835(sh)(benzene ring) 700a Abbreviations are as follows: v, stretching; 6, in-plane bend; it, out-of-planebend; as, asymmetric mode; sy, symmetric mode; s, strong absorption; m, mediumabsorption; w, weak absorption; b, broadened band; v, very; sp, split band; sh, shoulderband.b also VsyNoC alsov00-.d Benzene ring skeletal stretching bands are included in this group for Hppp, Hptp,Hpap, and Flpnp. For Hpnp, VaSNO is also appears in this region.28I .,.4 Ct u31400WAVENUMBERtM’) Mass SpectrometryThe molecular ions, base peaks and selected fragment ions for these compounds aresummarized in Table 2.1.5 for the 1-aryl-3-hydroxy-4-pyridinones and Table 2.1.6 forHcmp. The mass spectrum of Hppp is reproduced in Figure 2.1.6.The mass spectra of all 1-aryl-3-hydroxy-4-pyridinones show the [M- H1 ion asthe base peak. Loss of CO or HCO, which is a common process observed in 1-alkyl-3-.hydroxy-4-pyridinones,129is also observed in these compounds. The stable radical cationof m/z 125 present in the spectra of the alkyl pyridinones129is not present in the spectra ofthe aryl pyridinones. Loss of the substitutents at the para-position of the phenyl ringaccounts for the m/z 199 peak. The fragmentation of the aryl substituents results in thepeaks of m/z91, 77,65 and 51.Table 2.1.5. El mass spectral data (mlz) for the 1-aryl-3-hydroxy-4-pyridinones. Relativeintensities are included in parentheses.M [M - HJ C7H C6H5 other fragmentsHppp 201 (50) 200 (100) 77 (16) 172 (4), 51 (10)Hptp 215(72) 214(100) 91(61) 77(25) 199(15), 186(7),, 107 (29), 65 (70),51 (31)Hpap 231(88) 230(100) 91(12) 77(28) 202(11), 216(73),215(79), 199(31),188 (33), 108 (25),65 (12), 51 (12),32(67)Hpnp 246(87) 245(100) 77(8) 216(10), 199 (91)30100 90 80 70 60 50 41 31 21 11 S 110 91 80 70 61 50 40 30 25 10 0200172184I11111111180.— ‘4.’‘IIIIIII1111IIIII11111IIIIIIIIIIIIIIIIIIjIIIIIIII11111112002202402H280301154The fragmentation process for Hcmp is different from those of the 1-aryl-3-hydroxy-4-pyridinones. The base peak observed in this spectrum is the molecular radicalcation M. The peak at mlz 165 is due to the loss of H20 which is not usually observed in3 -hydroxy-4-pyridinones’29but commonly in 3-hydroxy-2-pyridinones.’64 Thisfragmentation process is probably due to the formation of zwitterions in the Hcmp structureor the presence of the carboxyl group. Like the alkyl pyridinones, the peak at mlz 125which is due to the Hmpp radical cation is also present in the spectrum of Hcmp. It is nodoubt a result of the stability of a dihydroxypyridinium moiety which should be favored inthe gas phase.’65 The ring fragments exclusively by the loss of CO and HCO (peaks ofm/z 97 and 96). The peak at m/z 44 is due to the radical cation of CO2.Table 2.1.6. El mass spectral data (m/z) for Hcmp. Relative intensities are included inparentheses.[M - H2OJ [M - COOH] Hmpp Other Fragments183 (100) 165 (55) 138 (34) 125 (13) 124 (15), 110 (47),97 (17), 96 (16),82(14), 69 (23),44 (50)322.1.4. DiscussionA series of 1-aryl-3-hydroxy-2-methyl-4-pyridinones and 3-hydroxy-1-carboxymethyl-2-methyl-4-pyridinone were prepared with fair yields. The preparation ofdifferent pyridinones had to be accomplished under different reaction conditions asdescribed in equations (1) to (3):O+H2N_K_R (1)HO CH3 HO CH3R = H, CH3 and OCH3O=(>O+H2N_K_NOHO=tN_i_NO2(2)C6H5H2O CH3 HO CH3O=O + HN—CH COOHp 9.1ON—H2-COO (3)HO CH3 HO CH3The syntheses of Hppp, Hptp and Hpap were carried out in acidic media, asattempts under basic conditions failed to yield any product. The one-pot synthetic methodin basic solution (see Chapter 6) was also attempted in order to prepare the tris(ligand)metalcomplexes directly, but yielded no products. In the preparation of Hpnp, benzylated maltolwas used as the starting material, however, the final product obtained was the desired 3-hydroxy-4-pyridinone. The attempt to produce this compound from maltol and pnitroaniline directly in acidic medium failed. For Hemp, reaction conditions similar to33those for the synthesis of 1-alkyl-3-hydroxy-4-pyridinones were maintained,136 i.e., thepH of the reaction mixture was kept between 9.1 to 9.3.The different conditions required for the preparation of the different pyridinonesmay be due to the varied basicity of the amines. The basicity of the amino group decreasesin the order of glycine (pKa 9.78) > p-anisidine (5.34) > p-toluidine (5.08) > aniline(4.63) > p-nitroaniline (1.0). Glycine is the strongest base among these amines and itsbasicity is similar to that of ailcylamines (pKa —10), therefore, the reaction of maltol withglycine would proceed under the same conditions as those of maltol with alkylaminesunder controlled pH.The basicities of aniline, p-toluidine, and p-anisidine are very close and the reactionof these amines with maltol would proceed under the same conditions. It was reported thatthe reaction of y-pyrone with aniline and other aromatic amines in approximately 6 Nhydrochloric acid produced N-aryl-4-pyridinones in excellent yields.’66 N-aryl-4-pyridinones were also obtained when methylated kojic acid reacted with aniline and itsderivatives in very dilute acidic soultion.140 The following equilibria may occur under thereaction conditions employed in the pyridinone synthesis described here; the formation ofpyroxonium salt is important in facilitating the nucleophiic attack by weakly basic aromaticamines during the reaction:1RNH2 + HC1 - RNH3C1 (4)Pyrone + HC1 - Pyroxonium salt (5)or Pyrone + RNfI3Cr - - Pyroxonium salt + RNH2 (5’)Pyroxonium salt’ + RNH2 - Pyridinone + H20 (6)Presumably, resonance structures of the pyroxonium salt of the following type areof importance (methylated kojic acid shown as an example’40):34OH OHOCH3 (L(OCH3 OCH3HOCH2 0 HOCH2 0 HThe nitrogen atom of the weakly basic aromatic amine would be sufficientlynucleophiic to attack the positive center, the 2 or 6 position of the pyroxonium salt. Attackat similar positions of the free pyrone is not precluded, however.p-Nitroaniline is the weakest base among the amines used in this study. Reactionsof maltol with p-nitroaniline at reflux in basic or neutral aqueous solution gave no product.The one-pot synthetic method was also attempted and no product was obtained. Asbenzylated maltol is more reactive than maltol, it could be used in reaction with pnitroaniline to prepare the desired pyridinone. The final product obtained from this reactionwas the corresponding debenzylated 3-hydroxy-4-pyridinone. The debenzylation processwas accomplished in the same procedure as the conversion reaction and no separatedebenzylation procedure was needed. It is known that the benzyl group can be removedunder less strenuous conditions than those for the methyl group.’34 Also, the introductionof the electron-withdrawing p-nitrophenyl group at the pyridinone ring nitrogen atom couldmake the benzyl group more labile. Therefore, under the reaction conditions the acidity ofthe reaction mixture would be strong enough to hydrolyze the benzyl blocking group.However, under similar conditions, reactions of aromatic amines with methylated kojicacid gave only the corresponding methylated pyridinones.’4°A separate demethylationprocedure was required to produce the final pyridinone products.HOCH2352.2. Tris(3-oxy-2-methyl-4-pyridinonato)metal (III) Complexes2.2.1. IntroductionAccording to the Hard and Soft Acid and Base (HSAB) principle, the group 13(hA) metal ions, Al3Ga3 and In, are classified as hard acidsM as they possess highpositive charge and small size and have no easily excited outer electrons. They prefer tocoordinate to hard bases, such as H20, OH- and other oxygen atom containing ligands.The order of hardness of these metal ions can be demonstrated by the absolutehardness,167 T*, of each ion:168 Al3: 45.77 eV > Ga3: 17 eV > In3: 13 eV. Theexceptionally large i value for Al3 is due to its very large fourth ionization potential.However, the chemical relevance of this large fourth ionization potential is questionable’68and therefore the absolute hardness of Al3 ion might be exaggerated.In aqueous solution, the octahedral M(H2O)63ions are quite acidic. The acidity ofthese hexaaquo species can be illustrated by their first dissociation constant which isdefined as:M(H2O)63 Ka - M(H2O)5(OH) + HK— [M(H2O)5(OH)J[H]a— [M(H2O)63]The equilibrium constants have been determined in the presence of noncomplexing anionsand the constants Ka for Al, Ga, and In are —1O, —1O, and —i04, respectively.’69* Absolute hardness, r, has an operational (and approximate) definition167 as:= (I - A)/2, where I is ionization potential and A is electron affinity. It determines howeasily the number of electrons can be changed.36Based on these Ka values, the order of acid strength is as Ga> In > Al. This unusualorder of the acidity of these group 13 (lilA) metal ions is due to d-block contraction inatomic size and a higher effective nuclear charge for Ga and, to a lesser extent, for In.170The ten added d electrons do not completely shield the extra ten positive charges on thenucleus. When this is contrasted with the shielding offered by the Ne core of Al3, theenhanced acidity of Ga3 and In3 is understandable.Due to the high affinity of the group 13 (lilA) metal ions for the hard bases H20and OW, their chemistry in aqueous solution is dominated by extensive and complicatedhydrolysis. The hydrolysis processes yield mono- or polynuclear hydroxo species. Theformation of these hydroxo species is dependent on time, pH, and concentration.65 Forexample, in solution with pH lower than 5, Al3 exists as the octahedral hexahydrate,Al(H2O)63.As the pH of a solution increases, Al(H2O)63undergoes successivedeprotonations to yield Al(H2O)5(OH) andAl(H2O)4(OH).Neutral solutions give anAl(OH)3 precipitate that redissolves in basic solutions, owing to the formation oftetrahedral Al(OH)4. Polynuclear species may also form, their composition being timedependent. Similar processes also occur with Ga3+ and 1n3+.The development of the coordination chemistry of the group 13 (lilA) metal ionshas been hindered by these complicated hydrolysis processes. Except for certain amino-and hydroxy-carboxylate ligands, most solution studies of metal ion-ligand interactionshave been performed in acid at pH values 3 in order to suppress the formation of thehydrolyzed species.’°°”7’Consequently, these studies only reveal part of the chelationstory of these metal ions. The appropriate coordination chemistry has recently become ofconcern to inorganic chemists due to recent interest in environmental and biological roles ofthese group 13 (ifiA) metal ions.There have been a number of aqueous solution studies of these metal ions withbidentate oxygen containing ligands, e.g., Al3 with salicylate ions’72 and hydroxycarboxylic acids99 and Ga3with hydroxyaromatic ligands.’7’With radiopharmaceutical37applications in mind, the coordination chemistry of oxine (8-hydroxyquinoline) withgallium and indium was studied.79’978 The synthesis of gallium complexes withcatecholate and benzohydroxamate ligands was reported by Raymond.’73 The speciationin aqueous solution of aluminum with the potentially tridentate ligand citrate wasinvestigated with potentiometric equilibrium measurements.174’5 The results obtained inthese studies indicate that citric acid is a good binder of aluminum, however, the identity ofthe species formed in solution remain controversial.’74’175In the last decade, there has been considerable interest in the various isomers (1,2;3,2; 3,4) of the hydroxypyridinones as metal ion binding groups. Kontoghiorghes and coworkers have used 3-hydroxy-1,2-dimethyl-4-pyridinone for iron overload treatment inpreliminary clinical trials.’76”7 Hider and his group have also used 3-hydroxy-4-pyridinone ligands to study iron mobilization from ferritin. 178 Raymond and co-workershave been examining the 1,2- and 3,2-isomers as sequestering agents for the actinides andiron.’79 There has also been some interest in mimosine (an N-(alanin-3-yl)-3-hydroxy-4-pyridinone with depilatory properties) and its isomers as chelating agents for variousdivalent180’1 and trivalent metal ions.138 Our group has been studying the conjugatebases of a series of 3-hydroxy-4-pyrones’°”°4and 1-alkyl-3-hydroxy-4-pyridinones’05107,109 as chelators for aluminum, gallium, and indium. They have been proven to beefficient chelators and the synthesis of the tris(ligand)metal complexes with aluminum,gallium, and indium has been shown to be quite easy and straight forward. Thesetris(ligand)metal complexes have also shown very high stability in neutral solutions and nohydrolyzed products have been detected. Based on these results, the complexes of thesemetal ions with 1-aryl-3-hydroxy-4-pyridinones were prepared.The conjugate bases of the 3-hydroxy-4-pyridinones, potentially, are anionicbidentate ligands that can chelate a metal ion via the deprotonated 3-hydroxyl and thecarbonyl oxygens. Each ligand would form a five-membered chelate ring with the metalcenter and the tris(ligand)metal complexes would be of neutral charge. In Figure 2.2.1 is38shown the tris(1-aryl-3-oxy-4-pyridinonato)metal complexes that were successfullyprepared and characterized in this study. This is the first report of the synthesis andcharacterization of Na3M(cmp)3 and the others have been reported by our group.’82JoR = H M(ppp)3) 0CH NO2 M(pnp)3Na3Na3[M(cmp)]M A1,Ga,andlnFigure 2.2.1. The tris(3-oxy-4-pyridinonato)metal complexes.2.2.2. SynthesisThe preparation procedures were straight forward and were similar for the synthesisof all metal complexes with a given ligand. In the text, only typical procedures aredescribed. In the preparations, the ligand to metal ion ratio was closely maintained at 3:1,the stoichiometric ratio of the tris(3-oxy-4-pyridinonato)metal complexes. As the39formation of the metal complexes is thermodynamically favored, it is not necessary to usean excess of ligand to push the reaction to completion.The ligands were prepared and purified as described in section 2.1.2. The otherchemicals were reagent grade and were used as received unless specified: Al(N03)9H0(Mallinckrodt), Ga(N039H2 and In(N03)5H2 (Alfa). As these metal complexesare very hygroscopic, the reported yields were obtained from the analytically purehydrates. The complexes are all non-volatile, charring and decomposing above 260 °C.Therefore, no melting point data could be obtained. For five of the aryl substitutedcomplexes, fast atom bombardment (FAB) ionization had to be used because of theirnonvolatility. The spectra were recorded with an AEI MS 9 and the samples wereintroduced on a copper tipped probe in a p-nitrobenzyl alcohol matrix. For theNa3M(cmp)3 complexes, the FAB-MS spectra were obtained in a p-nitrobenzyl alcohol +5% HC1 matrix. With this exception, the instrumentation and conditions were the same asthose reported for the characterization of the pyridinone compounds (see Section 2.1.3). Tris(3-oxy-2-methyl- 1-phenyl-4-pyridinonato)metal(III)Complexes, M(ppp)3Tris(3-oxy-2 -methyl-i -phenyl-4 -pyridinonato)aluminum(III) sesquihydrate,Al(ppp)31.5H20. Al(NO3)9HO(0.232 g, 0.617 mmol) and Hppp (0.400 g, 1.99mmol) were dissolved in 40 mL water with heating. The pH of this solution was raised to9.5 with 2M NaOH and a white solid precipitated. This was collected by filtration whilehot and recrystallized from hot MeOH to yield 0.367 g (91 %) product.Tris(3 -oxy-2 -methyl-i -phenyl-4 -pyridinonato)gallium(III) dihydrate,Ga(ppp)32H2O. The preparation was as for Al(ppp)3. Hppp (0.834 g, 4.15 mmol) andGa(N0)9H2 (0.601 g, 1.44 mmol) yielded 0.846 g (83 %) of the complex.40Tris(3 -oxy-2 -methyl-i -phenyl-4 -pyridinonato) indium(III) mo nohydrate,In(ppp)3H20. The preparation was as for Al(ppp)3. In(NO5H2O(0.267 g, 0.683mmol) and Hppp (0.427 g, 2.12 mmol) were used. Crystallization from hot methanolyielded 0.480 g (95 %) of the product. Tris(3-oxy-2-methyl-1-(p-tolyl)-4- pyridinonato)metal(III)Complexes, M(ptp)3Tris(3-oxy-2 -methyl-i -(p-tolyl)-4-pyridinonato)aluminum(III) hydrate,Al(ptp)3.55H20. A suspension of Hptp (0.598 g, 2.78 mmol) in 50 mL water was heatedto boiling and 6N HC1 (2 ml) was added to dissolve the Hptp. Al(N03.9H2(0.332 g,0.886 mmol) was dissolved in this solution and the pH was raised to 9.85 with 2M NaOH.The reaction mixture was cooled to room temperature and a white solid was obtained. Thiswas recrystallized from hot ethanol to yield 0.624 g (92 %) of the product.Tris(3 -oxy-2 -methyl-i -(p.. tolyl) -4-pyridinonato)gallium(III) monohydrate,Ga(ptp)3H2O. To a suspension of Hptp (1.015 g, 4.71 mmol) in 90 mL distilledwater/methanol (2:1) was added 2 mL of 6N HC1 solution. Ga(NO39H2O(0.612 g,1.46 mmol) was then added. The solution was heated to —60 OC and a white gummy solidwas precipitated by raising the pH of the solution to 7 with 2M NaOH solution. Thisprecipitate was extracted with CH21 (4 x 25 mL) when the solution was cool. Theaqueous layer was separated from the organic layer and discarded. Evaporation of CH21yielded a light pink solid which was recrystallized from hot ethanol to give 0.950 g (89 %)of product.Tris(3 -oxy-2 -methyl-i -(p - tolyl) -4-pyridinonato)indium(III) sesquihydrate,In(ptp)3i.5H2O. The preparation was as for Ga(ptp)3. Hptp (1.077 g, 5.01 mmol) andIn(NO3)5H2O(0.632 g, 1.62 mmol) yielded 1.12 g of product (90%).412.2.2.3. Tris(3-oxy-1-(p - met ho x y ph en y I)- 2-methyl -4-pyridinonato)metal (III) Complexes, M(pap)3Tris(3-oxy-1 -(p-methoxyphenyl)-2 -methyl-4-pyridinonato)aluminum(III)monohydrate, Al(pap)3H20. Hpap (0.993 g, 4.30 mmol) was dissolved in 40 mL hotdistilled water and Al(N0.9H20(0.504 g, 1.34 mmol) was added. The pH was raisedslowly to 7 with 8M NaOH. A white gummy solid was obtained after cooling to roomtemperature; this was then extracted with CH21 (4 x 20 mL). The aqueous layer wasseparated and discarded. The CH212 was removed under reduced pressure to yield apinkish solid. The product was collected by filtration, washed with diethyl ether and driedin vacuo producing 0.671 g (68 %).Tris(3 -oxy-i -(p -methoxyphenyl)-2 -methyl-4 -pyridinonato)gallium(III) mono -hydrate, Ga(pap)3H2O. The synthetic procedure was as for Al(pap)3. Hpap (0.504 g,2.18 mmol) and Ga(N0•9H2 (0.292 g, 0.698 mmol) yielded 0.456 g (84 %) of thecomplex.Tris(3 -oxy-i -(p-methoxyphenyl) -2 -methyl-4-pyridinonato)indium(III) dihydrate,In(pap)32H20. The procedure was as for Al(pap)3. Hpap (0.993 g, 4.30 mmol) andIn(N03)35H20 (0.539 g, 1.38 mmol) gave 1.04 g (90 %) of product. Tris(3-oxy-2-methyl-1-(p - nit r op hen yl ) -4-pyriclinonato)metal(III) Complexes, M(pnp)3Tris(3-oxy-2 -methyl-i -(p-nitrophenyl)-4-pyridinonato)aluminum(III) dihydrate,Al(pnp)32H20. The preparation was as for Al(pap)3. Recrystallization was carried out inhot ethanol. Hpnp (0.503 g, 2.04 mmol) and Al(NO9H2O(0.251 g, 0.670 mmol)yielded 0.522 g (98 %) of the recrystallized product.42Tris(3 -oxy-2 -methyl-i -(p-nitrophenyl) -4 -pyridinonato)gallium(III) hydrate,Ga(pnp)3•2.5H20. The procedure was as for Al(pnp)3. Hpnp (0.916 g, 3.72 mmol) andGa(N03)39H2 (0.501 g, 1.20 mmol) gave 0.616 g of product (60%).Tris(3-oxy-2-methyl-i -(p-nitrophenyl)-4-pyridinonato)indium(III) hemihydrate,In(pnp)3•O.5H20. The procedure was as for Al(pnp)3. Hpnp (0.517 g, 2.10 mmol) andIn(N03)3•5H20 (0.265 g, 0.677 mmol) gave 0.229 g of product (40 %). Sodium Tris(3-oxy-1-carboxymethyl-2-methyl-4-pyridinonato)metal(III), Na3M(cmp) 3Sodium Tris(3-oxy-i - carboxymethyl-2 -methyl-4-pyridinonato)aluminum(III),Na3A1(cmp)32H20. Hcmp (1.02 g, 5.51 mmol) was suspended in 40 mL hot distilledwater and 8N NaOH solution was added dropwise to dissolve it. Al(N03)9H2(0.680g, 1.81 mmol) was then added to this solution followed by addition of NaOH (8 M) toraise the pH to —9. The volume of this solution was reduced to —10 mL. The addition ofacetone to this solution yielded an orange oily layer which was separated from the aqueouslayer. A light pink solid was obtained from this oil upon the addition of more acetone. Thesolid was dried in vacuo at —80 °C overnight. A yield of 1.04g (85%) product wasobtained.Sodium Tris(3 -oxy-i -carboxymethyl-2 -methyl-4 -pyridinonato)gallium(III),Na3Ga(cmp)32.5H20. The synthetic procedure was as for Al(cmp)3. Ga(N0•9H20(0.504 g, 1.22mmol) and Hcmp (0.691g, 3.78 mmol) gave 0.77 g of product (87%).Sodium Tris(3 -oxy- I -carboxymethyl-2 -methyl-4-pyridinonato)indium(III),Na3In(cmp)4H2O. The synthetic procedure was as for Al(cmp)3. In(N03•5H20(0.508 g, 1.3Ommol) and Hcmp (0.734g. 4.01 mmol) gave 0.87 g of product (84%).43I2.2.3. CharacterizationAll the complexes were characterized by elemental analysis, proton NMRspectroscopy, infrared spectroscopy, and El-MS or fast atom bombardment massspectrometry (FAB-MS).In the JR and proton NMR spectra of the metal complexes, the changes from thoseof the free pyridinones due to chelation to the metal center are clearly shown. The changesare consistent with metal chelation via the deprotonated hydroxyl and the carbonyl oxygenatoms. The elemental analyses and the molecular ions observed in mass spectra areconsistent with an ML3 formulation. Elemental AnalysisPrior to analysis, each metal complex sample was purified by recrystallization anddned at -P85 °C in vacuo for at least 24 hours Analyses for C, H, and N in each complexwere consistent with the formulations of tris(ligand)metal species, however, thecompounds were very hygroscopic and formed reproducibly analyzable hydrates. Theresults are shown in Table 2.2.1. Analytical results corresponding to the anhydrous ML3formulation could not be obtained even by drying at 85 °C in vacuo for 24 hours withsubsequent storage and analysis under a nitrogen atmosphere.44Table 2.2.1. Results of the elemental analyses of metal complexes(calculated[found]).Compound Formula % C % H % NAl(ppp)31.5H20 C36H3A1N07.5 66.05 [65.97] 5.08 [5.19] 6.42 [6.33]Ga(ppp)32H20 C36H34GaN3O8 61.21 [61.17] 4.85 [4.89] 5.95 [6.02]In(ppp)3H20 36H2InNO7 58.95 [59.16] 4.40 [4.41] 5.73 [5.65]Al(ptp)35.5H20 C39H47AlNO115 60.93 [61.04] 6.16 [6.11] 5.47 [5.50]Ga(ptp)3H2O C39H38GaN7 64.13 [63.93] 5.24 [5.20] 5.75 [5.65]In(ptp)31.5H20 397InNO75 59.70 [59.67] 5.01 [4.91] 5.36 [5.28]Al(pap)3H2O C39H8A1NO10 63.67 [63.81] 5.21 [5.32] 5.71 [5.58]Ga(pap)3H20 398GaN10 60.17 [60.08] 4.92 [4.92] 5.40 [5.38]In(pap)32H20 C39H4OInN11 55.66 [56.00] 4.79 [4.58] 4.99 [4.97]Al(pnp)32H20 C39H1A1N6014 54.14 [54.14] 3.91 [3.91] 10.52[10.51jGa(pnp)32.5H20 392GaNO145 50.85 [50.85] 3.79 [3.701 9.88 [9.85]In(pnp)3.0.5HO C39H28InN6125 50.31 [50.04] 3.28 [3.35] 9.78 [9.78]Na3A1(cmp)32H20C245A1N3O1Na 42.68 [42.84] 3.73 [3.63] 6.22 [6.01]NaGa(cmp)32.5H20246GaN145Na 39.64 [39.791 3.60 [3.85] 5.78 [5.71]Na3In(cmp)34H20 C24H29InN3O16Na3 36.06 [36.20] 3.66 [3.64] 5.26 [5.37]452.2.3.2. Proton NMR SpectroscopyProton NIVIR spectral data of the tris(ligand)metal complexes are listed in Tables2.2.2 and 2.2.3. The spectral changes from the pyridinone compounds upon formation ofthe metal complexes are small, but there is a diagnostic shift in the chemical shifts of thering protons. The chemical shifts are closer to each other and the HJ signal is shiftedslightly upfield while the H signal is shifted down field (usually about 0.2 ppm). Thechemical shift of the ring methyl proton is also changed but to a lesser extent. Under thesame conditions as described previously for the pyridinone compounds, the peaks due tothe proton of the OH group are absent in the tris(ligand)metal complexes. Chelation to themetal center also affects the coupling constants (See Tables 2.2.2 and 2.2.3).The characteristic ring proton doublets, HHj, were resolved in the metal complexesof Hppp and Hpnp, with a coupling constant of 7.2 Hz for M(ppp)3 and 6.8 Hz forM(pnp)3. In the M(ptp)3 and M(pap)3 spectra, the Hd doublets appeared in the envelope ofphenyl proton signals, as observed in the spectra of these free ligands. For the M(pnp)3complexes, two doublets with different coupling constants were observed: the HcHdprotons on the pyridinone ring and HeHf protons on the phenyl ring. The couplingconstants are 6.7 Hz for the former and 8.8 Hz for the latter.Proton NMR spectra show that the tris(ligand)aluminum(llI) complexes undergo anexchange process at room temperature, while the tris(ligand)gallium(Ill) and indium(llI)complexes are not fluxional under the same conditions. The fluxionality of thesecomplexes will be discussed in Chapter 4.46Table 2.2.2. 1H NMR spectral data of the metal complexes, ML3.aMR a b c d+e+fAl H 2.18(s, 9H) - b 6.67(d, 3H, J=7.2Hz) 7.18(d,3H,J=7.2Hz)7.2 - 7.5(m, 15H)Ga H 2.19(s, 9H) - b 6.72(d, 3H, J=7.2Hz) 7.17(d,3H,J=7.2Hz)7.2 - 7.6(m, 15H)In H 2.19(s, 9H) - b 6.72(d, 3H, J=7.2Hz) 7.15(d,3H,J=7.2Hz)7.2 - 7.6(m, 15H)Al CH3 2.18(s, 9H) 2.43(s, 9H) 6.66(d, 3H, J=6.9Hz) 7.0 - 7.4(m, 15H)Ga CH3 2.19(s, 9H) 2.43(s, 9H) 6.70(d, 3H, J=6.9Hz) 7.1 - 7.4(m, 15H)In CH3 2.23(s, 9H) 2.44(s, 9H) 6.73(d, 3H, J=6.9Hz) 7.0 - 7.4(m, 15H)Al 0C113 2.13(s, 9H) 3.84(s, 9H) 6.61(d, 3H, J=6.8Hz) 6.8 - 7.3(m, 15H)Ga OCH3 2.15(s, 9H) 3.83(s, 9H) 6.64(d, 3H, J=6.8Hz) 6.9 - 7.3(m, 15H)In OCH3 2.17(s, 9H) 3.84(s, 9H) 6.67(d, 3H, J=6.9Hz) 6.9 - 7.3(m, 15H)A1C NO2 2.01(s, 9H) - 6.49(d, 3H, J=6.7Hz) 7.63(d,3H,J6.7Hz)7. 85 (d,6H,J=8 . 6Hz)8.41 (d,6H,J= 8.6Hz)Gac NO2 2.04(s, 9H) 6.53(d, 3H, J=6.8Hz) 7.63(d,3H,J=6.8Hz)7.85(d,6H,J=8.7Hz)8.4 1(d,6H,J=8.7Hz)InC NO2 2.08(s, 9H) 6.58(d, 3H, J=6.9Hz) 7.63(d,3H,J=6.9Hz)7.83(d,6H,J=8.8Hz)8.42(d,6H,J=8.8Hz)a in CDC13. b Protons b are listed with d + e + f. C ind6-DMSO.47Table 2.2.3. 1H NMR spectral data of the metal complexes,Na3M(cmp).aM a b c dAl 2.35(s, 9H) 4.70(s, 6H) 6.66(d, 3H, J=8.OHz) 7.51(d,3H,J=8.OHz)Ga 2.35(s, 9H) 4.70(s, 6H) 6.65(d, 3H, J=8.OHz) 7.52(d,3H,J=8.OHz)In 2.28(s, 9H) 4.70(s, 6H) 6.55(d, 3H, J=8.OHz) 7.52(d,3H,J=8.OHz)a inD2O. Infrared SpectroscopyEven though the general features of the IR spectra of the pyridinone compounds andthe corresponding metal complexes are similar, there are three diagnostic differencesbetween them: the disappearance of the hydroxyl stretching band at —3200 cm’, thebathochromic shift of V0 and Vcc and the appearance of some new bands below 800cm-1.The disappearance of the voH band at 3200 cm is the most noticeable change inthe spectra of all the metal complexes. A broad band due to the water 0-H stretchesappears in the region of 3500 to 3400 cm4 in the spectra of all these hydrated metalcomplexes.In the v0 and Vring(cc) stretching region, the characteristic four-band infraredspectral pattern is preserved in the complexes with a general bathochromic shift and apossible reordering upon formation of the tris(3-oxy-4-pyridinonato)metal complexescda 348(Table 2.2.4). The most pronounced shift occurs for the highest energy band with thechange of about 25 cm1. The superimposed spectra of Al(ppp)3 (top) and Hppp (bottom)in Figure 2.2.1 illustrate the changes typically observed. The skeletal vibration bands ofthe benzene ring also appear in this region and this is responsible for the extra bandsobserved. As it is difficult to separate the different bands, no specific assignment wasattempted. These bands are collectively assigned asv0 and Vrjng(cc). In the spectra ofM(cmp)33,a broad band between 1630 to 1600 cm1 is observed and this is due to thestretching of the COO- group.New bands appeared below 800 cm1 (Table 2.2.5) and are tentatively assigned asVM..O although there may be coupling to ring deformation modes in this frequency range.The IR spectra of the Al, Ga and In complexes with any given ligand are virtually identicalabove 800 cm1. In the region below 800 cm1, the difference between them is observableas at least one of the bands is consistently at lower energy in the Ga and In complexes. Acomparison of the spectra of Hpap (top), Al(pap)3 and Ga(pap)3 is illustrated in Figure2.2.2. The peaks tentatively assigned as VMO are indicated by asterisks.49Table 2.2.4. Characteristic infrared absorption bands of the complexes (cm-1). All arestrong and sharp bands.Complex v0and Vnng(cc)Al(ppp)3 1605 1590 1555 1520 1495 (w) 1470Ga(ppp)3 1600 1590 1550 1515 1490 (w) 1470In(ppp)3 1605 1590 1540 1510 1495 1470Al(ptp)3 1600 1590(w) 1550 1520 1510 1475Ga(ptp)3 1600 1590(w) 1550 1520 1505 1470In(ptp)3 1590 1540 1515 1500 1465Al(pap)3 1610 1600 (sh) 1555 1515 1470Ga(pap)3 1610 1595 1550 1520 1470In(pap)3 1610 1595 1540 1510 1470Al(pnp)3 1610 1595 1560 1525 1500 (sh) 1475Ga(pnp)3 1605 1590 1550 1525 1495 (w) 1470In(pnp)3 1610 1590 1540 1520(sh) 1500(w) 1470Na3Al(cmp)3 1615(b) 1560 1515 1490Na3Ga(cmp)3 1620(b) 1560 1510 1490Na3In(cmp)3 1620(b) 1555 1505 149050WAVENUMBER M.1)Figure 2.2.1. Infrared spectra of A1(ppp) (top) and Hppp (bottom) between 1700 andl400cm1.140051Table 2.2.5. Tentative assignments Of VM.O (cm1).Complex VM.(JAl(ppp)3 770 (m) 650 480 (b)Ga(ppp)3 765 650 400 (m, b)In(ppp)3 765 640 (w) 390 (m, b)Al(ptp)3 740 650 470 (b)Ga(ptp)3 740 650 320 (m, b)In(ptp)3 740 650 300 (m, b)Al(pap)3 740 690 (m) 490Ga(pap)3 735 (m) 685 (m) 380 (m)In(pap)3 735 (m) 680 (m) 380 (w)Al(pnp)3 720 (m) 665 (w) 480 (w, b)Ga(pnp)3 715 (m) 660 (w) 400 (w, b)In(pnp)3 715 (m) 600 (w) 390 (w,b)Na3A1(cmp)3 730 (m) 650 (w) 470 (b)Na3Ga(cmp)3 725 (m) 650 (m)Na3In(cmp)3 725 (m) 640 (m)52800WAVENUMBER M1Figure 2.2.2. Infrared spectra of Hpap (top), Al(pap)3 (middle) and Ga(pap)3 (bottom)between 800 and 300 cm1. The peaks tentatively assigned as VM..O areindicated by asterisks.53600 4002.2.3.4. Mass SpectrometryThe mass spectra of the complexes showed the expected HML3+,ML2+ and HMLfragmentation pattern when recorded in the FAB mode or the ?v1L3 ML2+ Ivn+ and L+fragmentation in the El mode. For the Ga complexes, the peaks are in the natural isotopicratio of 3:2 of 69Ga to 71Ga. The results are listed in Table 2.2.6 for FAB mode and inTable 2.2.7 for the El mode.Table 2.2.6. FAB mass spectral data (m/z) of the metal complexes, ML3.m/zHML3 ML2 HML H2LGa(ppp)3 670 469* 270 200 (Lj672 471Ga(pap)3 760* 529 300 232762 531 302Al(pnp)3 763 517* 273Ga(pnp)3 805 559*807 561In(pnp)3 851 605* 360 (ML) 247* indicates the base peak.54Table 2.2.7. El mass spectral data (m/z) of the complexes, ML3. The relative intensities ofthe peaks are included in parentheses.m/zN’fl.3 ML2 IV1L LAl(ppp)3 627 (2) 427 (100) 227 (2) 200 (77)ln(ppp)3 715 (0.1) 515 (5) 315 (3) 200 (100)Al(ptp)3 669(1) 455(100) 241(1) 214(37)Ga(ptp)3 711 (0.1) 497 (100) 283 (9) 214 (70)499 (67)In(ptp)3 757 (1) 543(23) 329 (27) 214 (100)Al(pap)3 717(1) 487 (100) 230(40)In(pap)3 805 (1) 575 (27) 345 (28) 230 (90)For Na3M(cmp)3 complexes a group of peaks with mass difference of 22 wereobserved, corresponding to each of the tris- and bis-ligand moieties. These peaks are dueto the successive loss of Na+ ions accompanying the protonation of the carboxylate group.The H2L+ cation was also observed in all spectra. The mass spectral data are summarizedin Table 2.2.8. For the Ga complex, the peaks are in the natural isotopic ratio of 3:2 of69Ga to 71Ga. The peaks corresponding to Na3M(cmp)3 parent ions were not detected.55Table 2.2.8. FAB mass spectral data (mlz) ofNa3M(cmp).am/zNa3A1(cmp)3 Na3Ga(cmp)3 Na3In(cmp)3[Na2M(cmp)3H2] 618 662 706660[NaM(cmp)3H3] 596 640 684638[M(cmp)3H4] 574 618 662616[Na2M(cmp)2] 435 479477[NaM(cmp)2H] 413 457 501455[M(cmp)2H2] 391 435 479433H2L 184 184 184a cmp2 O=cN-CH2COO-562.2.4. DiscussionThe tris(3-oxy-4-pyridinonato)metal complexes were prepared in a simple wayilusirated as follows:0HO3 I3 NCH2-COOH/O3÷ NaOH 1+ M M$NCH3 COONaThe acidic conditions required for the preparation of the pyridinones obviated theone-pot synthesis (see Chapter 6) of the metal complexes, and the pyridinones and theiraluminum, gallium and indium complexes had to be prepared separately and the yieldswere reasonably high.Due to the solubility limitation of the free pyridinones, the synthesis of tris(l-aryl3-oxy-2-methyl-4-pyridinonato)metal(HI) complexes had to be carried out in acidic mediawith heating. For M(ptp)3 complexes, a mixture of 2:1 water to methanol was used. Toavoid co-precipitation of the undissolved pyridinone compound and the metal complexes,3 + M3NaOH57the metal salt was added to a solution of the completely dissolved pyridinone compound.The pH of the solution was adjusted to 7 and the product precipitated. As the ligands havea very strong affinity for aluminum, gallium and indium, the complexing reaction happenedvery quickly as soon as the pH of the solution was raised to the 3 - 4 region. To insurecompleteness of the reaction, the pH of the solution was adjusted to neutral or slightlybasic. No side reaction occurred in this procedure.Extraction and recrystallization techniques were employed in the purification of themetal complexes. These complexes are very lipophilic and form an intractable gummysolid in aqueous solutions. However, they readily dissolve in some organic solvents, suchas chloroform and methylene chloride, and extraction into methylene chloride was used topurify the products. Recrystallization from methanol or ethanol separates the excess ligandfrom the product.The procedures used for the preparation of the other complexes were not suitablefor the preparation ofNa3M(cmp). Extraction with organic solvents did not work, nor didcooling as these complexes were too water soluble. Good results were obtained by usingacetone as a second solvent to precipitate the complexes.58Chapter 3Solid State Studies3.1. 3-Hydroxy-4-pyridinone Crystal Structures3.1.1. IntroductionOne of the important features of the chemistry of heterocyclic compounds istautomerism. There are several possible tautomers for 4-pyridinones; the individual formsall possess resonance structures and are stabilized by contributions from charge-separatedforms as shown in Figure OH_0_0Figure 3.1.1. 4-Pyridinone tautomeric equilibrium and the resonance forms.59In the tautomeric equilibrium between 4-hydroxypyridine b and 4-pyridinone c, thepredominant tautomer in polar solvents is the pyridinone form184 as the intermolecularhydrogen bonds are formed through N-H...OtC units. However, the hydroxypyridinetautomer appears to be the more stable one in the gas phase.’85”6 The differencesbetween the hydroxypyridine and the pyridinone tautomers are due to differences in thedegree of aromatic character. The hydroxypyridine form is clearly an aromatic system witha 6it-electron structure. The pyridinone tautomer also retains a considerable degree ofaromatic character, however, because it also has a cyclic array of six p-orbitals as part ofthe it-bonding system.’87The ring systems of x-pyrones and 7-pyrones show little evidence in aromaticcharacter.188’9 The crystal structures of 3-hydroxy-2-methyl-4-pyridinone (Hmpp),1293-hydroxy- 1 ,2-dimethyl-4-pyridinone (Hdpp)’29 and 3-hydroxy- 1 -ethyl-2-methyl-4-pyridinone (Hmepp)’36show only partial delocalization of formal double bonds which canbe perturbed by introducing larger alkyl groups (e.g. the ethyl group) onto the ringnitrogen atom.136 However, these compounds show more aromatic character than thepyrone systems, existing as the 4-pyridinone form in the solid state and as centrosymmetricO-HO=C hydrogen bonded dimeric units in their structures.The resonance of 4-pyridinones is of some importance when considering thepotential stability of the ths(4-pyridinonato)metal complexes. A significant contributionfrom the resonance forms with a partial negative charge on the carbonyl oxygen mightincrease the affinity of the ligand for strong Lewis acids, such as A13,Ga3 and In3ions. Delocalization of the ring double bonds and a lengthening of the carbonyl bondobserved in the 3-hydroxy-4-pyridinone structures would be evidence of the resonancehybridization in the 4-pyridinones.As phenyl groups are electron-withdrawing, the introduction of these substituents atthe ring nitrogen atom would decrease its ability to accommodate a positive charge therebydecreasing the contribution from the pyridinium form and therefore decreasing the tendency60toward delocalization of the formal double bonds of the pyridinone ring. On the otherhand, the ionizable proton on the carboxyl substituent could protonate the pyridinone ringto form zwitterionic dihydroxypyridinium species. Based on these assumptions, thestructures of Hcmp and Hpap were established by single crystal X-ray diffraction. Theeffect of different substituents at the ring nitrogen atom on the delocalization of the formaldouble bonds was investigated and compared among the three groups of substituentswhich have been studied by our group: alkyl, aryl and carboxyl.Single crystals of Hemp suitable for X-ray diffraction were obtained from asaturated aqueous solution by slow evaporation. Crystals of Hpap were grown by slowevaporation from an ethanol solution. All the crystal structures in this thesis weredetermined with a Rigaku AFC6S diffractometer by Dr. Steven J. Rettig of the U.B.C.Structural Chemistry Laboratory. In all cases the crystals were stable under theexperimental conditions.3.1.2. Results and DiscussionThe structures of Hemp and Hpap were established by single crystal X-raydiffraction. Crystallographic data and final atomic coordinates and equivalent isotropicthermal parameters Beq for Hemp and Hpap appear in Appendix as Tables Al, A2, and A3,respectively. ORTEP views of the molecules are shown in Figure 3.1.2. Stereodiagramsof the unit cell packing are shown for Hemp in Figure 3.1.3 and for Hpap in Figure 3.1.4.In Hemp molecules, the carboxyl group is deprotonated and the ketone oxygen atom on thepyridinone ring is protonated thus forming a zwitterion. There is an extraordinarily strongO-H 0 hydrogen bond (O(2)-H(2) 0(4) (l/2-i-, 1/2-, l/2+), .0 1.43 (2) A,.0 = 2.475 (1) A, O-H . .0 = 175 (2)°) between two molecules. Three-dimensionalnetworks of Hemp molecules are formed by CHO. O hydrogen bonds (C(6)-H(7) . O(4)(3/2-, l/2+, 1/2-i), H . O 2.46 (1) A, 0 = 3.333 (2) A, C-HO .. = 156 (l)° and61C(7)-H(8)• •O(3) (3/2-i, 1/2+, 1/2-.), H = 2.41 (2) A, C•• = 3.322 (2) A, C-H•= 162 (1)°) plus an additional O-H• 0 hydrogen bond (O(1)-H(1) 0(3) (1-x, -, 1-i),H-O = 1.94 (2) A, O•• 0 = 2.719 (1) A, O-H-O = 152 (2)°). The bridging carboxylgroups link Hemp molecules together (Figure 3.1.3).ozHZ0 oz 0H1H5 C4H 6 I ‘%.. HI 1 01C4 ThTC5 WC3 czCZ H5 b%fcr 4% H6 TH1 ‘C1H? ‘fHiCc H74 113 HZ118 IC? 6 114 IP 1110113 ce C1203J ‘-‘HSc9Cli118cio 1190311121111C131113Figure 3.1.2. ORTEP views of the Hemp (left) and Hpap (right) molecules; 50%probability thermal ellipsoids are shown. Hydrogen atoms have beenassigned artificially small thermal parameters for the sake of clarity.62Figure 3.1.3. Stereodiagram of the unit cell packing in Hcmp.JNFigure 3.1.4. Stereodiagram of the unit cell packing in Hpap.63Table 3.1.1. Bond lengths (A) with estimated standard deviations.CompoundAtomsHcmp HpapO(1)-C(3) 1.351 (1) 1.357 (2)O(2)-C(4) 1.320 (1) 1.254 (2)O(3)-C(8) 1.230 (1)O(3)-C(10) 1.363 (2)O(3)-C(13) 1.423 (3)O(4)-C(8) 1.269 (1)N(1)-C(2) 1.371 (1) 1.383 (2)N(1)-C(6) 1.348 (2) 1.354 (2)N(1)-C(7) 1.474 (1) 1.452 (2)C(1)-C(2) 1.489 (2) 1.492 (2)C(2)-C(3) 1.383 (1) 1.361 (2)C(3)-C(4) 1.409 (2) 1.438 (2)C(4)-C(5) 1.393 (2) 1.420 (2)C(5)-C(6) 1.363 (2) 1.351 (2)C(7)-C(8) 1.527 (2) 1.381 (2)C(7)-C(12) 1.380 (2)C(8)-C(9) 1.379 (2)C(9)-C(10) 1.382 (2)C(10)-C(11) 1.378 (2)C(11)-C(12) 1.383 (2)64Table 3.1.2. Bond angles (deg) with estimated standard deviations.CompoundAtomsHemp HpapC(1O)-O(3)-C(13) 117.9 (2)C(2)-N(1)-C(6) 121.61 (9) 120.3 (1)C(2)-N(1)-C(7) 119.91 (9) 121.0 (1)C(6)-N(1)-C(7) 118.42 (9) 118.7 (1)N(1)-C(2)-C(1) 119.74 (9) 119.2 (1)N(1)-C(2)-C(3) 118.5 (1) 118.8 (1)C(1)-C(2)-C(3) 121.8 (1) 122.1 (1)O(1)-C(3)-C(2) 118.0 (1) 118.3 (1)O(1)-C(3)-C(4) 121.29 (9) 118.7 (1)C(2)-C(3)-C(4) 120.7 (1) 123.0 (1)O(2)-C(4)-C(3) 117.4 (1) 121.5 (1)O(2)-C(4)-C(5) 124.4 (1) 124.1 (1)C(3)-C(4)-C(5) 118.1 (1) 114.4 (1)C(4)-C(5)-C(6) 119.9 (1) 121.4 (1)N(1)-C(6)-C(5) 121.2 (1) 122.1 (2)N(1)-C(7)-C(8) 113.01 (9) 119.5 (1)N(1)-C(7)-C(12) 119.9 (1)O(3)-C(8)-O(4) 124.8 (1)O(3)-C(8)-O(7) 119.9 (1)O(4)-C(8)-C(7) 115.2 (1)C(8)-C(7)-C(12) 120.7 (1)C(7)-C(8)-C(9) 119.2 (2)65C(8)-C(9)-C(10) 120.6 (2)0(3)-C(10)-C(9) 115.8 (2)0(3)-C(1O)-C(11) 124.4 (2)C(9)-C(10)-C(11) 119.8 (1)C(10)-C(ll)-C(12) 120.1 (2)C(7)-C(12)-C(11) 119.6 (2)The Hpap structure contains centrosymetric CHO. 0-C hydrogen bonded dimericunits (C(8)-H(7)• •0(2) (1/2-i, l/2-y-i), H0 = 2.34 (2) A, C 0 = 3.279 (2) A,C-H-0 = 160(1)°). Each dimer is linked to two others by 0-H0 hydrogen bonds(0(1)-H(1)•••0(2) -1/2-fl), H•••0 = 1.82 (2) A, 0•0 = 2.662 (2) A, 0-H•0 = 152(2)°) to form the three dimensional network (Figure 3.1.4).Both structures contain legitimate intermolecular (see above) and intramolecularC-H . .0 hydrogen bonds (C(1)-H(5)• O(2), H• .0 = 2.35 (2) A, C 0 = 2.803 (2) A,C-H “0 = 110 (1)° in Hcmp and C(1)-H(4)•••0(1), H••0 = 2.37 (3) A, C”0 = 2.798 (2)A, C-H 0 = 107 (2)° in Hpap). The formation of legitimate C-H”0 bonds has not beenobserved in the structures of reported 1-alkyl-3-hydroxy-2-methyl-4-pyridinones.’29”36In addition, there is an intramolecular 0-H0 bond observed both in Hemp (0(1)-H(1)0(2), H0 = 2.29 (2) A, 00 = 2.7 19 (2) A, 0-H0 = 112 (1)°) and in Hpap(0(1)-H(1)•••0(2), H•••0 2.36 (2) A, 00 = 2.749 (2) A, 0-H••0 = 106 (2)°)molecules.The strength of the hydrogen bonds can be judged by examining both the hydrogenbonding parameters and the JR stretching frequencies (Table 3.1.3). In the Hempstructure, the 0(2)-H(2)•”0(4) (carboxyl) distance and voH indicate strong hydrogenbonds.163 A weak hydrogen bond (00 distance > 2.7 A is classified as a weakhydrogen bond163) is formed involving the 3-.hydroxyl OH group. The difference in66strengths of the two hydrogen bonds can also be seen from the different bond lengths ofthe two C-O bonds in the anionic carboxyl group; C(8)-O(4) bond length (1.269 A) islonger than that of C(8)-O(3) (1.230 A). In the Hpap structrure, the O-HO bond isintermediate (between 2.7 - 2.6 A).163 All the angles are within the range typical forhydrogen bonds (140 - 1800).190Table 3.1.3. A comparison of the hydrogen-bonding parameters and the JR stretchingfrequencies (cm).Hcmp HpapO(2)-H(2)•••O(4) O(1)-H(1)•••O(3) O(1)-H(1)•O(2)fl••O (A) 1.43 (2) 1.94 (2) 1.82 (2)(A) 2.475 (1) 2.719 (1) 2.662 (2)O-H•O 175 (2) 152 (2) 152 (2)voH(cm’) 3300-2500 3180The six-membered pyridinone ring systems in both Hpap and Hcmp are slightly,but significantly, non-planar (maximum deviations from the mean planes being 0.0203 (19)A and 0.0099 (13) A, respectively), as noted earlier for 1-alkyl-3-hydroxy-2-methyl-4-pyridinones.129”36 The deviation from planarity is towards a N(1), C(4) boat in bothmolecules. In the Hpap molecule, the phenyl ring is planar to within 0.0063 (15) A andoriented roughly at a right angle to the pyridinone ring (the dihedral angle between the twoplanes is 7370) to minimize steric interaction with the adjacent C(1) methyl group. InHcmp, the plane formed by the carboxymethyl substituent on N(1) atom is planar to within670.0 148 (11) A and it is also perpendicular to the pyridinone ring (the dihedral angle is9 1.0°).The pattern of bond lengths within the six-membered pyridinone ring in Hpap isdifferent from those observed in Hmpp and Hdpp’29 but similar to that of Hmepp’36(Table 3.1.4). The most noticeable feature is the more pronounced localization of formaldouble bonds in Hpap vs that in Hmpp and Hdpp. Shorter C(2)-C(3), C(5)-C(6) andketone C(4)-O(2) bonds, and longer N(1)-C(2) and C(4)-C(5) bonds are observed inHpap. Also, the C(3)-C(4)-C(5) bond angle in Hpap is about 1° smaller than that in Hmppand Hdpp’29 and 4° smaller than in Hcmp. This compression indicates the strength of theC=O bond (shortest in Hpap, see Table 3.1.4) and is due in part to the lone pairs ofelectrons on the oxygen. In the Hemp structure, a different pattern of bond lengths andangles in the pyridinone systems is observed and the intra-annular bond angles are allwithin 1° of 120°. A significantly longer ketone O(2)-C(4) bond (1.320 A) is also observedthan in Hpap (1.254 A), Hmpp (1.280 A) or Hdpp (1.272 A). These bond lengths andangles taken together indicate that a delocalized pyridinone ring is obtained in Hemp andthat the positive charge of the zwitterion is distributed throughout the ring rather thanconcentrated at the nitrogen. In the pyridinone systems which we have studiedcrystallographically, the extent of the delocalization of the formal double bonds in thepyridinone ring follows the order: Hemp >> Hmpp Hdpp > Hmepp Hpap. It had beenpreviously sought in our laboratory to isolate crystals of the ring-based cations byacidifying Hdpp (and other non-carboxyl substituted 3-hydroxy-4-pyridinones) to makeHdpp+ or analogues without success.In conclusion, the structural studies of Hemp and Hpap reveal that there is anextensive hydrogen bonding network in both structures; hydroxyl and ketonefunctionalities contribute a number of hydrogen-bonding sites to each of the molecules.The protonated and delocalized pyridinone ring system was finally obtained in the solid68state in Hcmp by formation of a zwitterion, and it does indeed show highly averaged bonddistances.Table 3.1.4. Comparison of the ring bond lengths (A) of several pyridinone systems.RNOH3C OHCompoundHmppa Hmeppb Hpap HcmpR = H C2H5 -C6H4O H3 CH2OOHC=O 1.280 1.258 1.254 1.320N-C(2) 1.356 1.378 1.383 1.371C(2)-C(3) 1.371 1.370 1.361 1.383C(3)-C(4) 1.431 1.433 1.438 1.409C(4)-C(5) 1.411 1.410 1.420 1.393C(5)-C(6) 1.365 1.350 1.351 1.363N-C(6) 1.345 1.351 1.354 1.348a Data from ref. 129.b Data from ref. 136.693.2. M(ptp)3 Crystal Structures3.2.1. IntroductionAs part of the continuing solid-state study of the metal complexes with theexoclathrate matrix*, it is of interest to further explore the effect of the aryl substituents atthe ring nitrogen on the unusual hexagonal water channels. In the structures of tris(Nalkyl-3-oxy-2-methyl-4-pyridinonato)metal complexes, where the metal ion is Al3,Ga3tIn3 or Fe3 and the nitrogen-substituent is methyl or ethyl, an unusual hydrogen bondingsystem was observed. 105-107,109,191 The tris(ligand)metal complexes crystallized asdodecahydrates and the water molecules formed hydrogen bonded hexagonal channelswhich were in turn linked by other water molecules to the chelating oxygen atoms (Figure3.2. 1).b05 This extensive network bears a closer resemblance to one of the crystallineforms of ice (ih)’92 than to the typical structure of inorganic hydrates.Increasing the size of the metal atom in these ML312H0complexes fromaluminum to gallium to indium seemed to have little effect upon the intricate hydrogenbonding network.106’7 Furthermore, increasing the size of the nitrogen substituent Rfrom methyl to ethyl left the extensive hexagonal water channel network exoclathrateintact;109 however, changing the methyl or ethyl group to n-propyl group resulted in thedisruption of the hydrogen bonding network.193 Tris(1-(n-propyl)- or l-(n-butyl)-3-oxy-2-methyl-4-pyridinonato)metal complexes crystallized as trihydrates with no hexagonalwater channels and no extensive hydrogen bonding network.193An increase in ligand lipophilicity by introducing an aryl group at the ring nitrogenshould affect the hydrogen bonding network of the metal complexes even more. Also, the* An exoclathrate is a structure wherein the guest is held outside the host water structureinstead of being enclosed by it.70large aryl substituents at the ring nitrogen atom would be too sterically demanding for theformation of the water channels as in exoclathrates. Therefore, different structures of thetris(ligand)metal complexes of l-aryl-3-hydroxy-4-pyridinones in the solid state should beexpected.Figure 3.2.1. ORTEP view down the c axis of the unit cell packing of the Ga(dpp)312H20complex (from ref. 106).71The structures of Al(ptp)3 and Ga(ptp)3 were established by X-ray crystallographyand the results show that these complexes crystallize as hydrates with 5.5 water molecules.Single crystals suitable for X-ray analysis for both complexes were obtained fromacetone/2-propanol solutions by slow evaporation. Crystallographic data and final atomiccoordinates and equivalent isotropic thermal parameters Beq for M(ptp)35.5H20 (where M= Al and Ga) appear in Appendix as Tables A4, A5, and A6, respectively. The completelists of bond lengths and bond angles are summarized in Tables A7 and A8 in Appendix.3.2.2. Results and DiscussionThe crystal structures of the M(ptp)3.5.5H20 complexes were solved and found tobe isostructural. The complexes crystallize as thefac isomers (Figure 3.2.2) in the trigonalspace group of P31c. The bond lengths and bond angles (Tables 3.2.1 to 3.2.4) areconsistent with the previously solved structures of Al and Ga tris(3-oxy-4-pyridinonato)structures.105’6993 The distances and angles in the five-membered chelate rings arealso very close to those for the structurally characterized complexes of A13+ or Ga3+ withthe other four different ligands (N-CH3,N-C2H5,N-(n-propyl), and N-(n-butyl)).In the M(ptp)35.5H2O structures the hexagonal channels and the chains of watermolecules observed in tris(N-substituted-3-oxy-2-methyl-4-pyridinonato)metal complexeswith small alkyl substituents at the ring nitrogen are absent. But the hydrogen bondinginteractions between water molecules and the metal-chelating oxygen atoms remain (Table3.2.5).Distortions from true octahedral geometry around the metal atom are clearly shownby the metal-oxygen bond lengths and oxygen-metal-oxygen bond angles (Table 3.2.1).The M-oxy 0(1) bond lengths are consistently shorter than the M-ketone 0(2) bondlengths. Not surprisingly, despite partial delocalization of the carbonyl formal double bond72at 0(2), the deprotonated 3-hydroxy oxygen atom 0(1) tends to form a stronger bond withthe central metal atom.Figure 3.2.2. ORTEP view of the Ga(ptp)3 molecule. 50% probability thermal ellipsoidsare shown for the non-hydrogen atooms. Labeled non-hydrogen atomscomprise the asymmetric unit.C’sC”C12C6C5tCl73Table 3.2.1. The bond lengths (A) (in the chelate ring) with estimated standard deviationsfor M(ptp)35.5H20.DistanceAtomsM=Ga MA1M(1)-0(1) 1.962 (2) 1.886 (2)M(l)-0(2) 1.996 (2) 1.922 (2)0(1)-C(3) 1.323 (3) 1.321 (3)0(2)-C(4) 1.302 (3) 1.306 (3)C(3)-C(4) 1.419 (4) 1.418 (3)Chelation causes a compression in the interior angles of the chelate ring. Thestructure of Hptp was not determined crystallographically, but the structure of a verysimilar pyridinone, Hpap, has been established. Compared to the values of Hpap, the0(1)-C(3)-C(4) and 0(2)-C(4)-C(3) bond angles are decreased in the metal complexes(Table 3.2.2).The size of the metal ion affects the chelating ring strain and ring planarity. Thedecreased size of the metal ion is accompanied by an increase in chelating ring strain, asevidenced by the degree of compression of the 0-C-C bond angles. These angles aredecreased from 118.7 (1)° of 0(1)-C(3)-C(4) angle and 121.5 (1)° of 0(2)-C(4)-C(3) angleto 117.4 (2)° in the Ga complex and to 115.5 (2) and 115.9 (2) in the Al complex,respectively. Further evidence of this is seen in the (CH5)2B(ptp) complex when theseangles are further compressed to 111.6 (2)° and 112.1 (3)°, respectively.’94 The increaseof the size of the metal ion increases the deviation of the chelate ring from planarity. In theGa complex, the mean deviation from the mean plane is 0.0554 A, while a deviation of0.050 1 A is observed in the Al complex.74Table 3.2.2. The bond angles (in the chelate ring) with estimated standard deviations forM(ptp)3.5.5H0.aAngleAtomsM=Ga M=A1O(1)-M(1)-O(1)’ 90.65 (8) 90.42 (7)O(1)-M(1)-O(2) 83.15 (7) 84.41 (6)O(1)-M(1)-O(2)’ 95.04 (8) 94.67 (6)O(l)-M(1)-O(2)” 171.62 (8) 172.77 (6)O(2)-M(l)-O(2)’ 91.76 (8) 90.95 (7)M(1)-O(1)-C(3) 110.6 (2) 112.0 (1)M(1)-O(2)-C(4) 110.3 (2) 111.2 (1)O(1)-C(3)-C(2) 122.2 (2) 123.7 (2)O(1)-C(3)-C(4) 117.4 (2) 115.5 (2)O(2)-C(4)-C(3) 117.4 (2) 115.9 (2)O(2)-C(4)-C(5) 124.2 (3) 125.5 (2)a Here and elsewhere in this thesis, primed and double-primed atoms have coordinatesrelated to those in Tables AS and A6 in the Appendix by the symmetry operations: y - a, 1 -a, ; and l-y, 1 + a - , z; respectively.The delocalization in the C-O bonds is greater in the complexes than in the freeligand: the difference between O(1)-C(3) and O(2)-C(4) bond lengths decreases from0.103 A in Hpap to 0.015 A in the Al complex and 0.021 A in the Ga complex, indicatingthat the delocalization in the C-O bonds is slightly greater in the Al complex than in the Gacomplex.75In each complex, the pyridinone ring is slightly nonpianar, the distortion beingtoward an N(l)-C(4) boat for both structures. The maximum deviation from the meanplane is 0.0188 (25) A and 0.0163 (30) A from Al and Ga complexes, respectively. Themean deviation from the mean plane is larger in the Al complex (0.0109 A) than in the Gacomplex (0.0093 A). There are no significant differences in the ring bond lengths andbond angles between these two complexes.Table 3.2.3. The bond lengths (A) (bonds forming the pyridinone ring) with estimatedstandard deviations for M(ptp)35.5H20.DistanceAtomsM=Ga M=A1N(l)-C(2) 1.382 (3) 1.38 1 (3)N(1)-C(6) 1.352 (4) 1.350 (3)C(2)-C(3) 1.372 (4) 1.374 (3)C(3)-C(4) 1.419 (4) 1.418 (3)C(4)-C(5) 1.394 (4) 1.399 (3)C(5)-C(6) 1.359 (4) 1.355 (3)Chelation has a significant effect on the extent of delocalization of the pyridinonering formal double bonds (Tables 3.2.3 and 3.2.4). In the complexes, shorter C(3)-C(4)and C(4)-C(5) bonds are observed, also the intra-annular bond angles are all within 1.6° of120° and the C(3)-C(4)-C(5) bond angle increased from 114.4° in the uncomplexedpyridinone to 118.4° in the complexes. The comparison of these bond lengths and bondangles together with those of the delocalized pyridinone ring in Hcmp illustrates that theextent of the delocalization of the formal double bonds in the chelated pyridinone ring is76similar to that in the Hcmp structure. Upon the chelation of the ligand to the metal ion, amore delocalized pyridinone ring system is obtained. The same trend has been found in the(C6H5)2B(pip) complex.’94Table 3.2.4. The bond angles (bonds forming the pyridinone ring) with estimated standarddeviations for M(ptp)35.5H2O.AngleAtomsM=Ga M=AlC(2)-N(1)-C(6) 121.0 (2) 121.5 (2)N(1)-C(2)-C(3) 118.9 (3) 118.1 (2)C(2)-C(3)-C(4) 120.4 (2) 120.7 (2)C(3)-C(4)-C(5) 118.4 (2) 118.5 (2)C(4)-C(5)-C(6) 119.6 (3) 119.1 (2)N(1)-C(6)-C(5) 121.6 (3) 121.9 (2)There are four tris-ligand metal units in the unit cell of both complexes, and 5.5lattice water molecules per metal complex molecule. The complexes crystallized such thathydrophobic layers of N-substituted pyridinone rings alternated with hydrophilic layers ofwater molecules. The complete picture is accessible by examination of the two stereoviewsin concert (Figures 3.2.3 and 3.2.4). Comparison of the unit cell stereoview along the caxis (Figure 3.2.3) with those for the exoclathrate compounds shows that the p-tolylsubstituents are too sterically demanding to allow the necessary space near the corners ofthe unit cell for the hexagonal channels. The water molecules which hydrogen bond to thechelating oxygen atoms are clearly seen in this view (Figure 3.2.4) between neighboringGa(ptp)3 units down the c axis. The two-fold disordered lattice waters are also clearly seen77in the center of the unit cell and in each face. The replacement of N-CH3or N-C2H5withN-C6H4CH3on the pyridinone ring causes the disappearance of the exoclathrate structurebut does not completely remove all the hydrogen bonding interactions (Table 3.2.5) despitethe greatly increased lipophilicity of the ligand.Table 3.2.5. Possible hydrogen bonding interactions in M(ptp)35.5H20 (M = Al, Ga).Distance (A)Atoms Symmetry OperationM Al M = Ga (second 0 atom)O(1)”0(3A) 2.876 (4) 2.872 (5) 1-y, 1-x, 3/2-zO(1)”O(3B) 2.93 (4) 2.96 (2) x, 1-i-x-y, 3/2-zO(l)O(3B) 3.08 (6) 2.84 (2) 1-y, 1-x, 3/2-zO(2)”0(4B) 2.90 (2) 2.94 (3) l-y, 1-x, 1/2-zO(2)O(4A) 2.902 (4) 2.893 (4) x, l+y, zO(2)”O(4B) 2.93 (2) 2.84 (3) x, y, zO(3A)”0(4C) 2.68 (1) 2.60 (1) 1-x, 1-x-i-y, 1/2+zO(3A)O(4C) 2.89 (1) 2.78 (1) y, 1-x+y, 1-z78Figure 3.2.3. Stereoview along the c axis of the unit cell of Ga(ptp)35.5H20.Figure 3.2.4. Stereoview along the a axis of the unit cell of Ga(ptp)35.5H20.79Chapter 4Solution Studies4.1. NMR Studies4.1.1. Aluminum-27 NMR Spectroscopy4.1.1.1. IntroductionThe 27A1 nucleus is quadrupolar with a nuclear spin of 5/2. It has a quadrupolemoment of 0.149 x 10-24 cm2which interacts with local electric field gradients that couplethe nucleus to molecular motions. This interaction results in an efficient magneticrelaxation mechanism.’95 In the limit of fast motion, the nuclear spin quadrupolarrelaxation follows the equation:196/ 2’ / \21 1 1 3 21+3 1 X (eqQ-j— — —j 12(21- 1)+ ) h/27t) ‘where TQ, T1, and T2 are the quadrupolar, spin-lattice, and spin-spin relaxation times,respectively, I is the nuclear spin, eQ is the electric quadrupole moment, eq is the electricfield gradient, gives the deviation of the electric field gradient from axial symmetry, h isPlanck’s constant, and t is the rotational correlation time.It is well known that the relaxation rate of a nucleus with I> 1/2 is determined bythe strength of the interaction between the nuclear electric quadrupole moment and the80electric field gradient at a nucleus, modulated by the rate of tumbling of the molecule in thesolution. The electric field gradient (EFG) at a nucleus depends upon the arrangement andnature of the ligands bonded to its atom. Relaxation is least effective where the EFG issmall. Low EFGs, and therefore narrow lines occur for atoms surrounded by ligandswhich take up a regular cubic symmetry such as in tetrahedral or octahedral geometries.’97To allow full interpretation and assignment of an 27A1 spectrum, variouscontributions to the NMR signal parameters must be considered. The chemical shift andline width of quadrupolar nuclei are primarily dependent on the following factors: ligandtype, coordination number, ligand field symmetry, and the rate of chemical exchangeamong the various species in equilibrium.a. Ligand. A wide range of chemical shifts has been observed in aqueous solutionsfor aluminum complexes with a variety of ligands. Anions such as phosphate and sulfateproduce upfield shifts in the 27A1 spectra (-3 to -20 ppm).’98”9 Hydroxide complexesappear downfield; for example, l13(OH)2404)2at 62.5 ppm and Al(OH)4at 80ppm.200b. Coordination geometry. Aluminum exists mostly in two coordination geometries:octahedral and tetrahedral. Usually, hexacoordinate Al(III) compounds afford signalscovering approximately 60 ppm, from -40 to +20 ppm, most of which are located upfieldof the Al(1120)63+reference signal. Tetracoordinate species appear within two separateregions: 60 to 110 ppm and 140 to 180 ppm, depending on whether alkyl substitution isabsent or present, respectively.20’Normally, the chemical shift range for the 27A1 nucleusis approximately 450 ppm.202c. Symmetry. The 27A1 line width in the NMR spectrum reflects the degree ofsymmetry and arrangement of ligands around the Al(III) nucleus, that is, the moresymmetric the complex, the smaller the resultant line width. For example, the hexaaquoion [Al(H2O)6J3has a perfect cubic symmetry and it produces a sharp peak of line widthHz. Under similar conditions, Al(acac)3 and [Al(C204)3]exhibit line widths of about81100 and 125 Hz, reflecting a less than perfect cubic field and suggesting that thesix-membered ring of the acac complex better accommodates octahedral coordination thandoes the more constrained five-membered oxalate ring.203 The overall range of line widthsvary from 3 Hz (for Al(H2O)63,the commonly used reference for 27A1 NMRspectroscopy) to over 6000 Hz, depending on the geometry of the substituents about thenucleus.202 The sensitivity of aluminum spectra to ligand field alterations has also beenemployed to examine variations in symmetry for aluminum-substituted ferrichromes.204d. Exchange rates. The variation in line width for this quadrupolar nucleus as afunction of chemical exchange rate is a phenomenon common to all NMR nuclei.The 27A1 isotope is 100% naturally abundant and 20% as receptive as protons.Even though the 27A1 NMR spectra may contain relatively broad lines, they are easy tocollect and contain considerable information. Furthermore, the combination of line widthand chemical shift determination can provide an extremely powerful probe for studying thechemical and physical environment about an aluminum atom.There have been a number of studies using 27A1 NMR line widths as a probe todetermine the coordination number of organoaluminum complexes in organic solvents.201Aluminum complexes with several biologically active ligands have been studied by 27A1NMR spectroscopy in aqueous solution.34’204207 It has also been used extensively instudying the hydrolysis of aluminum complexes and in examining the pH dependentspeciation of aluminum)04”68’208 The hydrolysis of the tris-ligand aluminumcomplexes of acetohydroxamate,204oxalate,209 lactate,206 citrate,206 and a number ofhydroxy carboxylate ligands207 have been investigated by this technique. ExperimentalThe 27A1 NMR spectra were recorded at 18 to 20 °C with a Varian XL-300spectrometer operating at 78.16 MHz and accumulating 3500 transients with a pulse width82of 15 ps and a window of 37 kHz. The acquisition time was 0.21 s. All spectra werereferenced to the Al(H2O)63signal (as zero ppm) from 0.20 M Al(C104)3in 0.10 MHC1O4 with D20 added as a lock signal, and downfield chemical shifts are positive. Thebackground correction was done for each spectrum by subtracting a solvent spectrum rununder identical conditions. The spectra were obtained by the author.For the tris(l-aryl-3-oxy-4-pyridinonato)aluminum(llI) complexes, the 27A1 NMRspectra were obtained from CDC13 solutions, as the solubiity of these complexes in wateris very limited. The tris-ligand complex of aluminum with 3-hydroxy-1-carboxymethyl-2-methyl-4-pyridinone was used for the pH-dependent experiments. The complex waspurified, and dissolved in distilled and deionized water. The pH of the solution wasmonitored with a Fisher Accumet model 805 pH meter (calibrated with pH 4 and 10 buffersolutions). A solution of this complex was made in 10 mL of distilled water with —2 mLD20 added as an internal lock signal. The initial pH was adjusted to < 2 by the addition of8 M HC1. The pH was raised by the addition of 8 M NaOH and the solution wasequilibrated for about 10 minutes between pH changes. Aliquots of this solution weredrawn and filtered through glass wool into NMR tubes. The spectra were recorded fromlow pH and repeated at intervals of approximately one pH unit up to pH = 12 to assume theformation of Al(OH)4,the completely hydrolyzed species of the tris(ligand)metal complex.Between pH 2 and 3, more spectra were recorded as the mono- and bis-ligand speciesappeared in this region. The hydrolysis was completely reversible within the 2 - 3 hourtime frame of the experiment. Results and DiscussionThe 27A1 NMR spectral data, chemical shifts and line widths, of thetris(ligand)aluminum(III) complexes are listed in Table 4.1. The values for Na3A1(cmp)are obtained from the spectrum at pH 7.0 of the variable-pH experiment.83Table 4.1. 27Al NMR spectral data for aluminum complexes.aComplex chemical shift (ppm) line width (Wi12,Hz)Al(ppp)3 37 1270Al(ptp)3 38 1355Al(pap)3 37 1743Al(pnp)3 38 1368Na3Al(cmp)b 37 940a inCDCl3.b inH2O,pH=7.O.27Al NMR chemical shifts were consistent with those observed previously for 3-oxy-4-pyridinonate complexes of aluminum and the line widths at half-height (W112)wereconsistent with the increased size of the ligands from those reported previously. Thechemical shifts were 37 to 38 ppm, typical of the tris(3-oxy-4-pyridinonato)aluminum(Ill)chromophore,’°4”°68and the line widths of these N-arylpyridinone complexes werewider than those of N-alkylpyridinone complexes (400 - 900 Hz).106’8 As expected, theW112 values increase with the steric bulk of the para-substituted aryl substituent: H < CH3-N0<OCH3.For a hexacoordinate aluminum nucleus, the chemical shift occurs in the range of 20to -40 ppm and with the A106 species close to 0 ppm.202 There are exceptions to this rule,however. The tris(hydroxamato)aluminum complexes and the alumichrome trihydroxamatepeptides appear at 36 - 42 ppm204 and a dimeric acetate species gives a signal at 38ppm.208 These exceptions have led to the proposal that downfield shifts near 38 ppm84could be characteristic of octahedral chelation by small-ring-forming bidentate ligands.208The tris(3-oxy-4-pyronato) and the tris( 1-allcyl-3-oxy-4-pyridinonato)aluminum complexeshave chemical shifts of 37 - 39 ppm.104’68 The chemical shifts of the tris-ligandaluminum complexes studied in this thesis project were also observed at about 38 ppm.The downfield shifts are due to the inequivalence of the chelating oxy and carbonyl oxygenatoms and the formation of the five-membered chelating ring around the metal center. Thenonsymmetric arrangement of the coordinating atoms around the aluminum nuclei is alsoreflected by the relatively broad peaks of these tris-ligand aluminum complexes. This nonrigid cubic coordination sphere around the aluminum atom is also observed in the solidstate (Chapter 3).The variation in the line widths of the 27A1 NMR signal of these aluminumcomplexes could result from the variation in the size of the ligands. An increase of the sizeof the ligands could decrease the molecular tumbling rate, and therefore increase thecorrelation time, t. The relaxation rate (and therefore the line width, W112) for the largertris-ligand aluminum complexes would increase. In addition, the exchange processeswhich have been shown by proton NMR at room temperature for these aluminumcomplexes may contribute to the large line widths and the variations in line widths.Background correction was performed because of interference from the aluminumin the ceramics of the NMR probe. The aluminum in the probe has a broad signal (-- 6000Hz) centered at about 60 ppm and its interference has been documented.210’Thebackground signal from the probe overlaps with the 38 ppm signal from the tris-ligandaluminum complexes. The probe signal is also out of phase with the signal from thealuminum in the complexes which makes it difficult to phase the spectrum properly. It ispossible to do a background correction by subtracting the free induction decay (Fm) of asolvent blank from the sample FID and taking a fourier transform of the resulting signal.The solvent blank must be acquired under the same conditions as those for the samples.The effect of the background correction is shown in Figure 4.1 in which the spectra of85Na3A1(cmp)3 at pH 12.0 before and after background correction. The sharp signal at 80ppm is from A1(OH)4 and the peak at 37 ppm is from the tris-ligand aluminum complex.III1IIIIIIIIIl)IIII--I-lI--I1llJIIIIIIIIIIIIIIIIIII200 100 0 -100 -200ppmFigure 4.1. 27A1 NMR spectra of Al(cmp)3 at pH 12.0 before and after backgroundcorrection. The background signal is shown at the bottom for reference.AfterBeforeBackground86The variable-pH 27A1 NMR spectra of Na3A1(cmp)3 are shown in Figure 4.2.Hydrolytic stability similar to that of the N-alkylpyridinone complexes’°6”°8is observed.In the region of pH 4 to 10, there is only one peak at 37 ppm which corresponds to the trisligand complex. The acidic hydrolysis of the complex is evinced by the formation ofseveral aquo species in the acidic region. This results from the partial protonation of thecoordinated ligands to the aluminum and their replacement with water. The species[Al(cmp)2(H20)2] (at 28 ppm) and [Al(cmp)(H2O)4] (at 17 ppm) are formed between pH2 and 3. As the line widths of these species are broad, it is impossible to differentiatethem. The completely hydrated species Al(H2O)63is observed at 0 ppm as the pH dropsto 2.3. At a pH of 1.9, there is still a signal from the mono-ligand species[Al(cmp)(H2O)4J which is in equilibrium with Al(H2O)63,suggesting that the ligand hasa strong coordinating ability to the aluminum center. When the pH is raised, the ligands arereplaced by the hydroxide anion to form Al(OH)4 at pH 11(80 ppm, Wl/2 =90 Hz).The variable-pH study was also attempted with Al(ppp)3. However, it was notfeasible because a precipitate formed at pH 2.5 - 3.0 and the concentration was too low foran acceptable signal to noise ratio to be obtained.A comparison of the variable-pH spectra of Al(cmp)33 with those of Al(ma)3104indicates that the ths(3-oxy-4-pyridinonato)aluminum complex is more stable to both acidicand basic hydrolysis. In the spectra of Al(ma)3, the signal from Al(H20)3+first appearsat pH 3.2 and Al(OH)4 is observed at pH 9; the spectra of Al(cmp)33 show no signalsfrom either of these species at similar pH values.87ri Jill 111111 1[1TITTT1TTTTFTTTTTTFFm11T1TTTIJ200 100 0 —±00 PPM —200Figure 4.2. Variable-pH 27A1 NMR spectra of A1(cmp)33.887.0 Ez-From this variable-pH study, it is obvious that Al(cmp)33 is hydrolytically stable ina wide pH range, from pH < 4 to > 9. These results indicate that this aluminum complexshould resist in vivo hydrolysis at most physiological pHs except the highly acidicconditions of the stomach. The hydrolytic properties of the aluminum complexes with the1-aryl-3-oxy-4-pyridinonate ligands cannot be evaluated with this method because of thelimited water solubility. The determination of their stability constants and the pHdependent speciation of the aluminum complexes should provide an alternative way tostudy their hydrolytic properties and therefore to evaluate the possibility of in vivo stabilitystudies.894.1.2. Variable-Temperature Proton NMR Spectroscopy4.1.2.1. IntroductionTris-ligand complexes derived from unsymmetrical bidentate ligands can exist intwo geometrically isomeric forms,facial (fac) and meridional (mer)*, each of which has anenantiomeric pair of A and A stereoisomers as illustrated in Figure 4.3.Figure 4.3. Illustration of isomers of tris-ligand metal complexes with asymmetricbidentate ligands.The mechanism of stereochemical rearrangements of these metal complexes is asubject of considerable interest and importance. Both intermolecular and intramolecularmechanisms have been suggested for rearrangements of tris-ligand complexes. Theintramolecular rearrangement reactions result in two types of stereochemical change:geometric isomerization (fac ‘ mer) and enantiomerization (A A). These reactions canoccur separately or simultaneously depending on the mechanism of the rearrangement.Three types of mechanisms have been proposed: (1) the complete dissociation of one ligandto give a four-coordinate intermediate (intermolecular); (2) the rupture of one metal-ligandbond to give a five coordinate intermediate (intramolecular), and (3) intramolecular twisting* They are also frequently refered to as cis and trans isomers, respectively.fac-A fac- A mer- A mer- A90processes (Bailar or trigonal twist, and Rây-Dutt or rhombic twist) which involve nocleavage of any metal-ligand bonds.212 The majority of research work on therearrangement reactions of tris-ligand metal complexes has been directed towardsdifferentiating between the two intramolecular mechanisms.Rearrangement rates frequently exhibit a significant dependence on the nature of thechelate ligand and on the size and electronic features of the coordinated metal ion. Twolimiting types of tris-ligand systems have been defined based on the rate of therearrangement reactions; they were designated as “slow” and “fast”.213 Slow complexsystems are those whose geometrical isomers can be completely separated or partiallyresolved under ordinary conditions and are usually formed by inert metal ions such as Cr3+or Co3+. Fast or stereochemically nonrigid complex systems are those whoserearrangement rates are too fast to allow isomer separation or resolution but do permitisomer detection and kinetic studies by NMR. Typical fast complexes are formed fromlabile ions such as A13tGa3+, Jn3+, etc.If the rates of rearrangement reactions of a tris-ligand complex are within the NMRtime scale, NMR spectroscopy is very useful to study these processes because thefac andmer isomers have different symmetries. Thefac isomer has a threefold rotational symmetryand the mer isomer is asymmetric. The two isomers may be detected in the presence ofeach other as the three ligands of the fac isomer are magnetically equivalent and thechemical shifts of the nuclei on these ligands are different from those of their nonequivalentcounterparts in the mer isomer. If the chemical shift differences are large enough, theisomers may be identified and it is possible to measure the rates of isomerization andenantiomerization. In the early 60’s, variable temperature 19F NMR spectroscopy was firstused to follow the fluxional behavior of tris(trifluroacetylacetonate) (tfac) complexes ofaluminum(III), gallium(III), and indium(III) by Fay and Piper.214’5 This classic studyestablished the utility of nuclear magnetic resonance spectroscopy as a useful technique forthe study of stereochemically nonrigid inorganic complexes. The NMR technique has now91been used extensively to study the rearrangement reactions of tris-ligand complexes. Therearrangement reactions of tris(13-diketonato) metal complexes of aluminum(III),gallium(llI), and scandium(IH) have been studied by proton NMR.216-9 The fluxionalityof tris-ligand aluminum(III) and gallium(III) complexes with other ligands such asethylenediaminetetraacetate (EDTA4)22°and a-substituted tropolonates (a-RT)221’2 hasalso been examined by NMR techniques.The fluxional behavior of the tris(3-oxy-4-pyridinonato)aluminum(Ill) complexeswas clearly demonstrated at room temperature when they were characterized by protonNMR spectroscopy (Figure 4.4). The gallium and indium analogous complexes showedno such kind of fluxional behavior under the same conditions (Figure 4.5). The differentfluxional behaviors shown by these complexes prompted us to investigate therearrangement processes by proton NMR spectroscopy. ExperimentalThe variable temperature proton NMR spectra were recorded on a Bruker WH-400NMR spectrometer equipped with a variable temperature probe. The thermocouple wascalibrated using a methanol calibration standard and was accurate within ± 1 °C over arange of -70 to 60°C. The spectra were obtained in CD3O by the author and referencedto the solvent peak. The complexes, Al(ppp)3 and Ga(ppp)3, were prepared and purifiedas stated in Chapter 2.92:LNNNoJ-oC,Figure 4.4. 1H NMR spectrum of A1(ppp)3 in CD3O at room temperature.93HaHbC)23c/130e.E(Z0_________________________________________________EC.)2C.)LJLhZ2 JI-II—’IIIII8. Results and DiscussionThe coalescence of NMR signals due to rapid exchange of nuclei between twomagnetically nonequivalent sites has been observed in both organic and inorganicsystems.214’52 223 The characteristic spectra show the gradual merging of two ormore sharp resonance lines into a single broad line which eventually sharpens as thetemperature is raised. The spectra of Al(ppp)3 and Ga(ppp)3 exhibit these samecharacteristics, and therefore the coalescence of the four resonance lines of the CH3Cgroups in these complexes (vide infra) is attributed to a rapid exchange of the methylgroups between the four possible non-equivalent sites of the fac and mer isomers. Thisexchange may be attributed to very rapid isomerization and/or enantiomerization.The chemical shift differences in the downfield doublets are quite small and the useof Ha and Hb ring protons is limited. The protons on the phenyl ring show a complicatedpeak pattern and it is impossible to resolve them. The kinetic parameters, therefore, weredetermined from the temperature-dependent CH3c spectra. The fac- and mer-tris(ligand)complexes may be distinguished by means of proton NMR spectra. The CH3c groups inthefac isomer are magnetically equivalent, giving one resonance signal. The three CH3groups in the mer isomer are nonequivalent and three resonance signals therefore areobserved. The signals of CH3c groups of Al(ppp)3 and Ga(ppp)3 at -30 °C are shown inFigure 4.6. For both of the complexes, only three of the four signals were resolved; one ofthe mer isomer signals appeared under the signal of the fac isomer. Signals from the facisomer are marked with an asterisk in Figure 4.6. This assignment was used by Piper andFay for 19F NMR spectra of (tfac) complexes of aluminum(III), gallium(III), andindium(III)214’5 and Nelson for H NMR spectra of tris(1-alkyl-3-oxy-4-pyridinonato)metal complexes of aluminum(ffl) and gallium(Ill).’3695*Figure 4.6. 1H NMR spectra of CH3c for Al(ppp)3 (left) and Ga(ppp)3 (right) at -30 °C.The equilibrium distribution of the geometrical isomers and the electronic and stericfactors which affect it are important properties to consider when investigating therearrangement reactions of tris-ligand complex systems. A statistical distribution of theisomers would produce four peaks of equal intensity, that is, thefac isomer would be 25%of the total concentration. The statistical or nearly statistical distribution of isomers wouldusually be formed in solution if the complexes are not sterically constrained. This situationoccurs for the tris(3-diketonates) of cobalt(III)213’4 and other trivalent metalions.213’42 It has been stated that the mer isomer is the more stable isomer due to itslower dipole moment and this was used to explain the smaller than statistical equilibriumdistribution (18%) forfac isomer of the kinetically fast Al(tfac)3 complex.214’5 For somecomplexes, however, more fac isomer than the statistical distribution was found atequilibrium. The Al- and Ga-(cc-RT)3 complexes222 and Al(dpp)3 complex136 areexamples which have a slight excess of thefac isomer at equilibrium. For the Al(dpp)3complex, a 32% distribution of thefac isomer gave the best fit to the spectral simulation in*2. 10I I •2.20 2.10PPM96the absence of exchange. It was explained that the fac isomer had at least a smallthermodynamic advantage independent of the H-bonded water network.136 Simulations ofthe spectra of CH3c of Al(ppp)3 and Ga(ppp)3 in the absence of exchange with DNMR4program2indicate that the best fits are obtained with a 20% distribution offac isomer forboth Al(ppp)3 and Ga(ppp)3. The lower than the equilibrium distribution of thefac isomermay be due to the bulky phenyl substituent at the ring nitrogen and therefore the mer isomermay be more stable than thefac isomer in solution.For the exchange of nuclei between two nonequivalent sites, the rate of exchange atthe temperature of coalescence (kTc) is given by:ItAVkTCtwhere Av is the frequency separation (in Hertz) between the resonance components in theabsence of exchange.226 At the coalescence temperature, T, the free energy of activation,LGTC, can be calculated from the Eyring equation227 (assuming the transmissioncoefficient to be 1):KBkTC = j T exp(-AGT/RTC)where KB is Boltzmann’s constant, h is Planck’s constant and R is the gas constant.For the calculations with the CH3c spectra of A1(ppp)3 and Ga(ppp)3, i\v was takenas the difference between the signal for thefac isomer and the furthest signal from the merisomer. A source of error in using these equations is the temperature dependence of thechemical shifts presumably due to solvent-solute interactions.215 To correct for thistemperature dependence, Av is plotted against temperature at several points in the region ofslow exchange, and the line is extrapolated to the region of fast exchange. The Av couldthen be obtained from the extrapolated line at the temperature, T. For the A1(ppp)3 andGa(ppp)3 complexes, the values of Av were small (<5 Hz) and the variations due to thetemperature effect were within the resolution error of the instrument; therefore, the required97adjustments in Av were meaningless in these cases. The kTc and AGTC of Al(ppp)3 andGa(ppp)3 (in Table 4.2) were obtained without Av corrections.Table 4.2. Kinetic parameters for A1(ppp)3 and Ga(ppp)3 at the T.Complex T kTc(Hz) (K) (si) (Kcal/mol)Al(ppp)3 4.5 305 10.0 16.5 ± 1.3Ga(ppp)3 3.8 273 8.4 14.8 ± 1.2a Errors were estimated to be ± 2.b Errors were estimated assuming an order of magnitude error in the rate constants.The variable temperature 1H NMR spectra of CH3 for Al(ppp)3 and Ga(ppp)3 areshown in Figures 4.7 and 4.8, respectively. The M(ppp)3 complexes appeared to undergotwo exchange processes, one at low temperature and the other at high temperature. TheM(cc-RT)3complexes221’2were reported to undergo similar processes: a low temperatureexchange process (LTP) and a higher temperature exchange process (HTP). The LTP wasidentified as enantiomerization by means of a trigonal twist and the HTP as afac-merisomerization. These complexes have the similar a-hydroxyketone ligating moiety as the3-hydroxy-4-pyridinones to the metal center. By comparison with these complexes, theexchange processes observed at higher temperatures for Al(ppp)3 and Ga(ppp)3 may alsobe due to this fac-mer isomerization process. The M(ppp)3 complexes also exhibited thepredicted kinetic order of Al <Ga <In. The coalescence process of Ga(ppp)3 happened ata lower temperature and had a lower AGTC than Al(ppp)3 and the In analogues gave onlyan averaged spectrum in CD3O at -70 °C.98The best method to estimate the rate constants and to elucidate the mechanisms ofthe exchange reactions is the simulation of the experimental spectra. The spectralsimulations were attempted with DNMR3228 and DNMR4225 simulation programs. Theseprograms allow both mutual and non-mutual exchange routines. The non-mutual exchangeroutine has the capacity to handle as many as four or five configurations with nonequalpopulations. The mutual exchange routine allows the exchange between chemicalconfigurations, however, it cannot directly accommodate exchange between chemicalconfigurations of different symmetry and the population difference between thefac and merisomers cannot be incorporated. The simulated spectra did not match the experimentalspectra well enough to produce any useful information. Also, it is impossible to elucidatethe mechanisms of the exchange reactions based on the obtained results. An intermolecularmechanism has been proposed for the exchange process of the M(dpp)3 complexes.’6Based on these variable temperature NMR studies, it can be concluded thatfac-merisomerization is responsible for the fluxional behavior of the tris(3-oxy-4-pyridinonato)aluminum(Ill) and gallium(Ill) complexes of HTP. The mechanisms of therearrangement processes of these complexes could not be determined because of the smallseparations of the resonance peaks of the isomers and the lack of an appropriate simulationprogram.9935°C L32I I Iz.zg 2.10PPMFigure 4.7. Variable temperature 1H NMR spectra of CH3of A1(ppp)3 in CD3O .100_Gc==ZNd0 13Sec-2•10.30)I I • I2.20 2.10PPMFigure 4.8. Variable temperature 1H NMR spectra of CH3C of Ga(ppp)3 in CD3O .1014.2. Partition Coefficient Determination4.2.1. IntroductionThe most important factor governing the absorption of a drug in vivo is itslipophilicity, which is directly related to its partition coefficient. The partition coefficient,P, is a measure of the extent to which a solute is distributed between water and a water-immiscible liquid phase. It is defined as the equilibrium concentration of the monomericspecies of a compound in the non-aqueous phase, [D]11 divided by that of the neutral formin the aqueous phase, [D]a:229[D]R]aThe partition coefficient P, or its logarithm (log P), can have important use in predicting theproperties of molecules in transmembrane transport, protein binding, receptor affinity,pharmacological activity, etc.The distribution of a solute between two phases in which it is soluble has beenstudied for many years. The early investigations showed that the ratio of the concentrationsof solute distributed between two immiscible solvents was constant and independent of therelative volume of the solution used.230 A number of organic solvents have been used asthe model non-aqueous phase: diethyl ether, chloroform, olive oil, n-octanol, etc. Then-octanol/water system is a good model for the lipoidal biophase in living systems andtherefore could be useful in studying the distribution of solutes between blood and lipid inliving organism.1 In pharmacological research, the n-octanol/water partition coefficienthas been accepted as the operational definition of lipophilicity and is now widely employedin the design and development of new bioactive compounds.102The classic method to determine log P is the shake-flask method in which thedistribution of a solute in two immiscible solvents is determined by shaking the solute inthese solvents followed by analyzing the solute concentration in one or both of the phases.However, this method is experimentally difficult sometimes, and is very time consuming.Furthermore, the difficulties encountered in separating the immiscible phases and analyzingthe solute concentration in the phases have resulted in wide variation of the reported log Pvalues. Therefore, alternative methods for determining log P values have been sought.232The most widely used alternative is reverse phase high performance liquid chromatographywith methanol/water as the eluent.232’3 However, this method has been criticized asbeing unreliable, especially for hydrophobic compounds that require high proportions ofmethanol (>50%) in the eluent.234It was proposed that the log P of a compound is an additive-constitutive property ofthe substance. This made it possible to estimate this parameter using the additivityassumption. There are two widely used, essentially empirical ways for estimation of log F:Rekke?s f constant method235 and Leo and Hansch’s fragment approach.236 Both ofthese methods are based on the assumed additivity and have no scientific basis. They areonly applicable to a small number of simple compounds. Some computer programs havebeen developed to predict log P values.237 However, the computations are non-trival (andexpensive) and the estimated log P values are often quite different from the experimentalresults. Despite the problems encountered in the experiment, the shake-flask method isconsidered the most accurate and reliable method for determining log P values.236The determination of the log P values of gallium and indium complexes with the 1-aryl-3-hydroxy-4-pyridinone ligands is very useful in evaluating the potential asradiopharmaceutical imaging agents. The increasing interest in aluminum chelators for thetreatment of various neurological disorders linked to aluminum has made it worthwhile todetermine the log P values for the pyridinones and their tris-ligand aluminum complexes.A log P of 2 has been proposed as ideal for the design of barbiturates, but the complexity103of brain uptake makes it impossible to set a lower log P limit in neurological drugdesign.238 Levin reported a structure-activity relationship between the permeability of theblood-brain barrier and the log P and molecular weight of a substance.239 Although thelipophilicity of the tris-ligand complexes of aluminum, gallium and indium with a series of1-alkyl-3-hydroxy-4-pyridinones varies in a wide range, from log P <-1.75 for Al(mpp)3and In(mpp)3 to log P of 1.38 for Ga(mhpp)3,’36 the biodistribution studies of 67GaL3complexes showed similar distributions. They were excreted fast via renal clearance.110The log P values for these complexes might be significantly lower than ideal and the lowlog P values might have contributed to the rapid elimination of the67Ga-ligand complexes.The determination of the n-octanol/water partition coefficients for the 1-aryl-3-hydroxy-4-pyridinones and their metal complexes of aluminum, gallium and indium is described in thefollowing section and the potential of these complexes as radiopharmaceuticals is alsodiscussed.4.2.2. ExperimentalUltraviolet spectroscopy. As the solubility of these compounds in water is verylimited, the ultraviolet spectra were recorded in MeOH (spectroscopically pure, BDH) asthe solvent. The spectra were recorded from 400 to 200 nm with a Shimadzu UV-2100spectrophotometer.Partition coefficient determination. The partition coefficients were determined withn-octanollwater using the shake-flask method. Based on the previous studies carried out inour laboratory, UV spectroscopy was chosen as the analytical technique. Reagent graden-octanol was distilled and the first and last quarters were discarded. The pH 7.4 buffersolution was used as the aqueous phase. The two solvents were mutually saturated bystirring a 1:1 mixture overnight and the subsequent determinations were carried out in thesesaturated solvents. The 3-hydroxy-4-pyridinones and their metal complexes were purified104by recrystallization. A Shimadzu UV-2 100 spectrophotometer was used and the soluteconcentration in the aqueous phase was determined by monitoring the absorbance of the Bband at —284 nm for the 3-hydroxy-4-pyridinones or the absorbance of the E band at —230nm (292 nm for Al(pnp)3 and Ga(pnp)3) for the metal complexes (25 °C).All trial solutions were prepared by saturating the buffer solution with thecompounds. The initial solution was either prepared by proper dilution of the trial solutionor the trial solution was used directly depending on the solubility of the compound in pH7.4 buffer solution. Solutions with an absorbance between 0.5 - 1.5 prior to extractionwith n-octanol were considered to be ideal. 25 mL of the initial solution was prepared withbuffer solution. A 4 mL aliquot of the initial solution was withdrawn and placed in a 15mL plastic centrifuge tube labelled as the initial solution. Two 10.0 mL aliquots of initialsolution were placed in centrifuge tubes labelled as extraction tubes. An appropriatevolume of n-octanol (ranging from 0.020 mL to 2.0 niL depending on the lipophilicity ofthe compound) was then added to the extraction tubes and they were inverted 100 times (>2 minutes contact time). After an equilibration time of at least 15 minute, the tubes werecentrifuged for 15 minutes. The n-octanol layer was then removed with a disposablepipette. An absorbance reading was taken of the aqueous layer and one reading was madefor each tube. The initial solution and the reference buffer solutions were also treatedsimultaneously. Two readings were made for each initial solution tube. The aboveprocedure was repeated three times for each compound thus producing six measurementsof the initial absorbance and six post-extraction values. The P values were calculated fromthe absorbance of the aqueous phase as follows and the mean log P (r = 6) values and theirstandard deviations were reported.(initial absorbance) - (post-extraction absorbance) volume of bufferxPost-extraction absorbance volume of n-octanol105Ideally, samples of both phases should be analyzed to check for the material balanceas a guard against unforeseen losses. This requires an analytical procedure which can beused for both phases. As the volume of the n-octanol used for the extraction was so small,it was difficult to handle and analyze with the method used. It has been shown that if careis taken to ensure that no special solute interactions occur, reliable results can be obtainedby analyzing only one phase.24°4.2.3. Results and DiscussionUltraviolet Spectroscopy. The ultraviolet spectra of pyridine and its derivatives aresimilar to that of benzene. There are three absorption bands which originate from theit — it transitions in the UV spectrum of benzene: the B-band (benzenoid band, 256 nm),and two E-bands (ethylenic bands, 204 and 184 nm). In the UV spectrum of pyridine, theB-band is somewhat more intense than that of benzene as this transition is allowed forpyridine and forbidden for the more symmetrical benzene molecule. Due to the lone pair ofelectrons on the nitrogen, there is a weak n — it transition (R-band) that is observable inthe vapor phase and is generally swamped by the more intense B-band when the spectrumis determined in solution. The B-band in pyridine is located at 257 nm (in 95% ethanol,E =; conjugated and/or electron-donating substituents cause this band toshift to lower energy.’55 For the pyridinone compounds (except Hpnp, which iscomplicated by the nitro group), E and B bands were observed at —220 nm and 289 nm,respectively. For Hpnp, several bands appeared as shoulders. The complexation of theligands with the metal ions resulted in bathochromic and hyperchromic shifts of allabsorptions. Compared to their respective ligands, the B-bands in the metal-ligandcomplexes show bathochromic shifts of 13 - 20 nm and this shift follows the trend: In>Ga > Al (Table 4.3).106Table 4.3. Ultraviolet spectral data, ?, nm ( x 10-s, M.cml).aML3HL free ligandAl Ga InHppp 289 (19.2) 307 (34.3) 308 (32.7) 309 (34.7)221 (sh, 19.4) 232 (79.5) 233 (79.4) 234 (75.6)206 (28.5) 208 (53.2) 207 (54.0) 208 (52.6)Hptp 289 (19.4) 306 (34.3) 307 (35.4) 308 (33.9)220(sh, 18.6) 232 (85.2) 233 (76.3) 235 (82.9)205 (27.0) 209 (62.5) 209 (56.3) 211 (62.7)Hpap 289 (20.1) 306 (34.5) 307 (33.4) 308 (35.9)219 (22.0) 229 (79.0) 230 (77.5) 229 (82.4)205 (28.2) 207 (53.6) 205 (53.9)Hpnp 310 (sh) 345 (sh) 345 (sh) 345 (sh)284 (13.7) 297 (29.0) 298 (32.9) 298 (sh, 30.0)260 (sh) 255 (41.6) 255 (48.0) 253 (34.6)215 (sh) 227 (72.9) 227 (85.6) 225 (75.7)a inMeOH,25°C.107n-Octanol/water Partition coefficients. The results are shown in Table 4.4 and thelog P values of Hdpp and its metal complexes are also listed as references. With thevolume for partitioning used in the shake-flask method in this study, only a lower limit forthe indium complexes and Ga(ptp)3 could be estimated because their solubility in water atneutral pH was so low. The partition coefficients for Hcmp and Na3M(cmp)3 were notdetemiined because the solubility of these compounds in the pH 7.4 buffer solution is veryhigh and the partitioning with n-octanol under the conditions used was not feasible.The partitioning should be done at the lowest solute concentrations possible, sincelog P is concentration dependent and only theoretically valid at infmite dilution; however,concentrations of 10-1 M are considered sufficiently dilute for neutral molecules that havelittle tendency to associate in solution.240 In the experiments carried out in this study, asaturated metal complex solution was used as the initial solution in some cases in order toachieve a reasonable absorbance reading after partitioning with n-octanol, and theconcentration of these solutions did not exceed the 1 mM level.Due to the wide range of the lipophilicity of these compounds, the ratio of n-octanolto buffer solution had to be changed in the range from 5:1 to 500:1 depending on thelipophilicity of the compound. In the cases of a high ratio great care had to be taken toavoid producing an inseparable emulsion. It has been noted that the failure to completelyremove the emulsion would produce large errors in log P values.241 The n-octanol tobuffer ratio was also controlled so that the absorbance readings were higher than 0.2absorbance unit after partitioning and that the difference of the absorbance readings beforeand after partitioning with n-octanol was sufficiently large. For the pyridinonecompounds, the absorption peak at -‘220 nm appears as a shoulder, therefore, theabsorption peak at —284 nm was used to determine the log P values of these compounds.108Table 4.4. Partition coefficients (log P) values for the 3-hydroxy-4-pyridinones and metalcomplexes (standard deviations are in parentheses).L3HLAl Ga InHdppa-0.74 (8) <-1.75 -1.51 (6) <-1.75Hppp 1.11 (2) 2.01 (3) 2.11 (4) >3Hptp 1.72 (4) 2.65 (8) >3 >3Hpap 1.27 (2) 2.46 (2) 2.51 (2) >3Hpnp 0.77 (1) 1.46 (1) 1.49 (1) >3a From ref. 136.Compared to the lipophilicity of 1 -alkyl-3-hydroxy-4-pyridinones and their metalcomplexes (see Table 4.4), it is clearly shown that this series of pyridinones and their metalcomplexes are much more lipophilic. The order of the lipophilicity of the 1-aryl-3-hydroxy-4-pyridinones is: Hptp > Hpap > Hppp > Hpnp, as predicted on the basis of theN-substituents. According to Leo and Hansch’s fragment method, the hydrophobicconstants of the para-substituents (fragments) are in the order of CH3,0.56> OCH3,-0.02> NO, 0.28.241 The introduction of a methyl group onto the phenyl ring should increasethe log P value by 0.56 units and therefore the para-tolyl substituted pyridinone should bemore lipophilic than the phenyl substituted pyridinone. On the other hand, the paranitrophenyl substituted pyridinone should be less lipophilic than the pyridinone with thephenyl substituent. The log P value should be decreased by 0.28 log unit.The metal complexes are more lipophilic than the corresponding pyridinone;obviously the lipophilicity imparted by three aryl groups dominates the partitioning109process. The relative order of the lipophilicity of the melal-ligand complexes is: In > Ga >Al. A similar trend has been observed for M(mhpp)3 complexes)In summary, the high partition coefficients (log P) confirmed that the desired1.1 pophilicity had been introduced into the 3—hydroxy—4—pyridinones and the complexes.This should increase their lipid soluhility and membrane permeability.1104.3. Stability Constant Determination4.3.1. IntroductionThe chelation of metal ions in biological systems has recently been of great interestin coordination chemistry. In particular, thermodynamic data allow an unprecedentedpredictive capacity, which can be used in ligand design for the specific chelation of variousmetal ions.242’3 The speciation and chelation of metal ions such as Al3,Ga3,and In3in vivo requires a basic understanding of the properties of potential ligands. If the purposeof the ligand is to remove the metal ion then considerable water solubiity is needed inaddition to high thermodynamic stability of the metal-ligand complex. If a complex is to bea radioactive imaging agent, high lipophilicity is usually necessary to prevent washout andthe stability should be such that the complex is not demetallated before it reaches its ultimatebiological target. Thus, the determination of thermodynamic stability constants is a criticalcomponent of the design of complexes which will interact with biological systems.For a tris-ligand metal complex, a series of equilibria are established involving themetal ion and the ligand. The stability constants of the metal-ligand complex defined bythese equilibria are important thermodynamic properties. Frequently, the stability constantsof a metal complex are determined by the potentiometric equilibrium measurement method.An overall stability constant 133 for AlL3 of 1.5 x 1029 was found for aluminum-mimosinechelates.138 The stability constants of the group 13 (hA) metal complexes with 1 -alkyl-3-oxy-4-pyridinonate ligands have been determined in our laboratory.’°8”° It has beenreported that these oxypyridinonate ligands have strong affinities for these trivalent metalions and the overall stability constants range from 1032 (for Al and In complexes) to 1038(for Ga complexes). Based on the methodology established for these studies, theprotonation constants of Hcmp, Hppp, and Hpap and the stability constants of the group13 (lilA) metal complexes with ppp were determined. In this part of the thesis, the111thermodynamic characterization of metal-ligand systems by potentiometric equilibriummeasurements is presented, and some of the predictive capability of these data, whenapplied to a simple blood plasma model, is demonstrated. The computer model also allowsdirect comparison of ligand affinities for M3 regardless of differing denticities.4.3.2. ExperimentalPotentiometric Equilibrium Measurements. Potentiometric equilibriummeasurements of the 3-hydroxy-4-pyridinones in the absence (Hppp, Hpap, Hemp), andpresence of metal ions (Hppp) were performed with an Orion Research EA 920 pH meterequipped with Orion Ross research grade glass and reference electrodes. The electrodeswere standardized using standard HC1 - NaOH titrations to read -log [H4j directly. TheNernst equation was used to calibrate the system within the range 2.0 -log [Wj 4.0and a constant value of E° was found.NaOH solutions (0.15 M) were prepared from dilutions of 50% NaOH (less than0.1% Na2CO3) with freshly boiled, distilled, deionized water, and standardizedpotentiometrically against potassium hydrogen phthalate (BDH certified). A Metrohmautomatic buret (Dosimat 665) was used to add the standard NaOH. The temperature wasmaintained at 25.0 or 37.0 ± 0.1 °C throughout with water-jacketed beakers and a Julabocirculating bath, and the ionic strength was adjusted to 0.15 M (isotonic) by the addition ofNaC1. All solutions were continuously degassed with prepurified argon during the courseof a titration.All 3-hydroxy-4-pyridinones were recrystallized twice from the appropriatesolvents; their concentrations were obtained by weight. All metal-containing solutionswere obtained from appropriate dilution of atomic absorption standard solutions of Al, Gaor In (Sigma or Aldrich). The exact amount of excess acid present in the metal ionsolutions was determined by a plot of (V0 + V) x 10pH versus Vt (a Gran plot245’6),112where V0 is the initial volume of 1:1 metal-Na2HEDTA solution, and Vt is the volume ofadded standard NaOH. The base consumed is equal to the excess acid plus theNa2HEDTA protons. The total ligand to metal ratio in the titration was kept at just greaterthan 3:1 at mM concentrations.As an initial study, the metal-ligand titrations were performed in the range 2.0-log [WI 4.0 with a 3 to 1 excess of ligand at mM levels. The average L- coordinatedper metal, ñ, was plotted against -log [Li directly from the titration data. At the highestacidity measured (-log [Hj = 2.0), ñ was found to have a value near 1.5 for the Gasystems, indicating that the major species present are the mono- and bis-ligand Gacomplexes. With the Al and In systems, ñ was found to be near 0.5 at this point. In allsystems, ñ increased to above 2.5 at -log [WI = 4.0.The accurate determination of the stability constant for the mono-ligand Ga complex(which forms under highly acidic conditions) was troublesome. (Titration data to aminimum of ñ = 0.5 is desirable to measure log K1 accurately.) Standardization of theelectrode response under high acid concentrations (to -log [Wj = 1.0) was determined assuggested by Rossotti.244 A linear response of E° to [H+] was found under more acidicconditions. The Nernst equation becomes:E = E° - 2.303(RTIF)log [H] + C [Hflwhere RT/F is the Nernstian slope and C a measured constant. Titration data werecollected from the Ga-ligand systems to ñ = 1.0 (approximately to -log [W] = 1.3, thelowest practical limit). The addition of HC1 to these solutions gave an approximate ionicstrength of 0.2 M. The error associated with log K1 was rather large because of thefollowing: 1) imprecise measurement of changes in [H+] under these conditions; 2)extrapolation of the data to ñ = 0.5; and 3) small changes in ionic strength during thecourse of the titration. Due to all of these factors, the accurate log K1 could not be obtainedby titration. The reported values here for Ga are estimated by the linear free energy113relationship (LFER). The stability constants of M(pap)3 could not be obtained due tosolubiity limitations.Computations. The overall stability constants defined according to the equilibriumM3+ + nL = ML(3n)+, I’ were calculated using the least-square Fortran computerprogram BEST247 and the proton association constants (mH+ + L = HmL(m)+ (m =1, 2,3)) were determined by the calculation with the Fortran computer program PKAS.248 Inall M(III) systems, the computations allowed for the presence of M(OH)2,M(OH)2,M(OH)3,and M(OH)4. In addition, A12(OH)24tA13(OH)45tInCl2tInCl2,InCl3,and In(OH)Cl+ were included. Stability constants for these various metal species wereobtained from ref. Results and DiscussionThe 3-hydroxy-4-pyridinones are amphoteric (Figure 4.9). The two stepwiseprotonation constants* (Kai and Ka2) for Hppp and Hpap and the three stepwiseprotonation constants (Kai, Ka2, and Ka3) for Hcmp are given in Table 4.5. Theprotonation equilibria for these amphoteric compounds are described by equations (1) to (5)in Figure 4.9, where L = cmp, ppp and pap.Comparison with the analogous values for the 3-hydroxy-4-pyrones shows that thepyrones are stronger acids.100’2495 The protonation constants (log Kai) are 8.38 and7.61 for maltol and kojic acid249, respectively. The protonation constants (log Kai) ofcmp, ppp, and pap- are 9.70, 9.40, and 9.42, respectively. In all the hydroxypyronesand hydroxypyridinones there is an additional protonation constant (log Ka2; log Ka3 forHcmp) of about -1 in the former250 and 3.0 to 3.4 in the latter. This* The protonation constant Ka defined here is the reverse of the the ionization constant, Ka,as normally defined.114O 0 OH_____CIXOH LXOHLNCH- CH3 N CH3_ ____ _Ka2R R RL HL HLR = H, Hppp; R = OCH3,Hpap[HL] (1)H + L HL Kai= [Hj[Li___[H2L] (2)H + HL H2L Ka2 [Hj[HL]O 0 0 OHOHJIO OH OHKai Ka2-Ka3LNCH- NCH3 NCH3 - N CH3CH2OO CH2OO CH2OOH CH2OOHL2 HL H2L H3L[HL]H + L2 HL Kai= [Hj[L2](3)_[H2L] (4)+ HL H2L Ka2= [H][HU]_[H3Lj+ H2L - H3L K = (5)[HJ[H2L]Figure 4.9. The protonation equilibria of the 3-hydroxy-4-pyridinones and their constants.115difference is most likely an effect of the ring nitrogen atom, which is better able todelocalize positive charge into the ring than a ring oxygen, thereby stabilizing adihydroxypyridinium cation in acidic solution.Table 4.5. Log stepwise protonation constants (log K) for Hcmp, Hppp, and Hpap at 25and 37 °C in 0.15 M NaC1.aHcmp Hppp Hpap25°C 25°C 37°C 25°ClogKai 9.76(1) 9.40(1) 9.61(1) 9.42(4)log Ka2 3.43 (1) 3.03 (1) 3.18 (1) 3.16 (8)log Ka3 2.78 (3)a Numbers in parentheses represent standard deviations between successive runs.The log Kai constant of Hcmp is close to those for N-allcyl-3-hydroxy-2-methyl-4-pyridinones (log Kai = 9.76 versus 9.8b08hb0). This suggests that the deprotonatedcarboxymethyl group at the ring nitrogen in solution does not affect the electron density ofthe pyridinone ring to any significant extent versus the N-alkyl substituted pyridinones. Itis possible for Hcmp to form a zwitterion in solution; therefore, an equilibrium would beestablished between the neutral H2L and the zwitterionic species (Figure 4.10). Theprotonation processes of the neutral H2L (a in Figure 4.10) and the zwitterion (b in Figure4.10) would then contribute to both of the determined protonation constants log K andlog Ka3 for equilibria (4) and (5) in this study.116o OHCX: C5CH3CH2OOH CH2OO-Eneutral H2LI I ZWjttIiflj H2L\\H+OHCH2OOHH3LFigure 4.10. The equilibria between neutral Hemp (H2L) and the zwitterion in solution,and protonation to yield the protonatedH3L.The constants of Hppp and Hpap at 25 °C are similar and lower than those for theN-alkyl-3-hydroxy-2-methyl-4-pyridinones108(log Kai = 9.42 versus —9.8 and log K =3.16 versus —3.7), consistent with the electron-withdrawing effect of the aryl substituentsversus the electron donating effect of the alkyl groups. The similar protonation constantsfor both Hppp and Hpap show that the substituents at the para position of the phenyl ringdo not affect the protonation constants to a significant extent. The larger standarddeviations for the constants of Hpap are the result of the low concentration (—0.5 mM) usedin the determination due to the low solubility in water of this compound.On the basis of previous titration studies of aluminum, gallium, and indium with the1-alkyl-3-hydroxy-4-pyridinone ligands’°8”°and from the ñ plots described in the117Experimental Section, the metal-ligand equilibria may be adequately described by equations(6) to (8):M3 +L - wi2 f3 (6)M3 + 2L - - IvIL 132 (7)M3 + 3L ML3 13 (8)— [ML3]— [M31[Li”Hppp was found to have a very high affinity for the group 13 (lilA) trivalent metalions, particularly for Ga3. This is evinced by the high overall (f33) and conditional (logI3eff = log 133 - 3(log Kai - pH)’74) stability constants for Ga(ppp)3 in Table 4.6. Theoverall stability constant is lower than those found for the Ga complexes of N-alkylatedpyridinones (log 133 36 versus -38); 110 however, the lower Kai value for this ligand meansthat the conditional stability constant at pH 7.4 is only slightly smaller than that for the Nalkylated ligands (log 133eff 30.3 versus —P30.8). The same holds true for the Al and Incomplexes of ppp; the overall stability constants are lower than those found for Al’°8 orIn0with the N-alkylated pyridinones, but the conditional stability constants are only veryslightly smaller for ppp (for Al log P3eff 24.7 versus 24.9, for In log f3eff 25.1 versus25.6). Clearly, changes in the substituent on the ring nitrogen of the pyridinone ring makeminor alterations in the overall stability constants (log f33), and these variations may beattributed to the changes in the pKa5 of the ligands. The effective stability formationconstant at pH 7.4 (logI3eff) demonstrates this thermodynamic indifference when f33 isnormalized to blood plasma conditions (pH 7.4 and 0.15 M NaCl). The predominance ofthe ML3 species at physiological pH is nicely demonstrated both in the log 13eff values in118Table 4.6 and in the speciation diagrams of Figures 4.11 and 4.12. Based on these results,it can be assumed that the stability constants of the metal complexes with Hptp, Hpap andHpnp ligands are similar to those of M(ppp)3. The higher log 133 and log I3eff values forM = Ga versus M = In and the suspiciously greater affinity of transferrin for In73 versusfor Ga72 led us to examine the biodistribution of the gallium complexes (Chapter 5) ratherthan the indium complexes.The great stability of five-membered oxygen containing metallocycles incorporatinggroup 13 (lilA) metal ions has been previously documented.99’100 The functionalizablering nitrogen in the hydroxypyridinones allows a number of properties to be varied (watersolubiity, lipophilicity) without affecting the thermodynamic binding constants. The metalcomplexes of 1-aryl-3-hydroxy-4-pyridinone ligands are highly lipophilic while the metalcomplexes of 3 -hydroxy-2-methyl-4-pyridinone (Hmpp) and 3-hydroxy- 1 ,2-dimethyl-4-pyridinone (Hdpp) ligands are not lipophilic; however, their thermodynamic bindingconstants are close to each other.The increased thermodynamic stability of the metal complexes ofhydroxypyridinones compared with that of hydroxypyrones249 must be a result of thepoorer ability of the nitrogen-containing ring to delocalize negative charge in the formationof complexes. The N-containing heterocycle is poorly equipped (relative to the 0-containing hydroxypyrone) to delocalize negative charge, so the hydroxyl oxygen is harder(i.e. more polarized C()-O(&)) than in hydroxypyrones. Comparison of the Ga-0(hydroxyl) distances in Ga(ptp)3 (1.962 (2) A) and the tris(catecholato)gallate trianion(average 1.986 (6) A)’73 shows the strength of this electrostatic interaction.119Table 4.6. Log metal-ligand stability constants (f3), and effective stability constants(I3eff, pH 7.4) for the equilibrium reactions of Al, Ga and In with Hppp at 25and 37 °C and 0.15 M NaC1.a,bConstant Metal 25°C 37°Clog 13i Al 11.36(3) 11.86(10)Ga 17.5(2)CIn 13.34(1)log.32 Al 21.78(8) 23.13(21)Ga 28.8(2)cIn 22.66(2)log 133 Al 30.74(1 1) 32.44(23)Ga 36.3(2)cIn 31.12(3)logl3eff Al 24.74(14) 25.8 1(26)Ga 30.3(2)CIn 25.12(6)a The Ga constants are reported for solutions containing —0.2 M NaCl.b Numbers in parentheses represent standard deviations between successive runs.C Estimated by LFER.120Speciation diagrams calculated for aluminum, gallium, and indium are shown inFigures 4.11 and 4.12. The concentration dependence of the speciation of Al(ppp)3 isshown in Figure 4.11. At both mM and p.M levels of a 1:3 ratio ofAl3:Hppp, the metalligand species predominate at physiological pH (> 60%), however, hydroxide speciescoexist to a significant degree with the metal-ligand species under same conditions at thep.M level (Fig. 4.11). Ga and In complexes show concentration dependent propertiessimilar to that of the Al complex. The complete formation of [Ga(ppp)(H2O)4J2at a pHas low as 1 renders it difficult to accurately obtain i, and this affects the standarddeviations for the Ga complex constants. In fact, a reasonable value of f3 i had to beestimated by LFER. For In, the chioro species formed at low pH (Fig. 4.12). Mixedligand chioro or hydroxo species were not sought in the data analysis, as it was reportedpreviously that these species were negligible factors to the equilibria.’08 These diagramsalso show that the tris-ligand complexes, M(ppp)3, become dominant at a pH of about 4.5and hydrolysis does not occur until pH 9.Computer models have been used to simulate the interactions between metal ionsand blood components for many years.25’ The blood plasma model chosen for thespeciation of Al3 and Ga3 has been constructed based on the stability constants takenfrom the literature for Al and Ga complexes of citrate’°0”75and transferrin;71’2it contains1 p.M Al3 or Ga3, 100 p.M citrate, and 50 p.M empty sites of transferrin at pH 7.4,0.15 M NaCl (isotonic) and 25 °C (This is in order to make comparisons with otherconstants that are reported at 25 °C. The results of the stability of the metal-pyridinonecomplexes at 37 °C indicate no large variation in the stability constants (Table 4.6)). Inspite of the presence of phosphates (as H2P04 and HP042)in plasma of about 1.5 mM,they were excluded from the model because of a lack of reliable stability constant data.Figure 4.13 presents plots that can be used to compare metal-binding affinities of variousligands, regardless of denticity. Hppp has similar affinities for Al3 and Ga3 to those ofHdpp, the representative alkyihydroxypyridinone. Both of these pyridinones are more121efficient (low log[C] required) at complexing 100% of Al and Ga in this model than isH4EDTA10°(which is hexadentate and tetraprotic), and maltol for Al249 (bidentate andmonoprotic), or catechol for A12 (bidentate and diprotic). It should be noted that Fe hasnot been included in the model and that neither temperature nor ionic strength changes havebeen taken into account. These factors should not affect the results unduly, however. It isemphasized that the model’s predictive powers are limited in an absolute sense but are verypowerful in a comparative sense.The stability constants of the metal complexes follow the order Ga > In > Al, whichis the same order as the acidity of the hexaaquo ions, M(H2O)63. This trend isschematically shown in Figure 4.14 in which M(mpp)3 and M(dpp)3 are used asreferences. Using the log Kal of the 3-hydroxyl-4-pyridinones as a measure of basicity, aplot of log Kai versus the 3-hydroxy-4-pyridinones (Figure 4.15) correlates exactly withthe relative stabilities of the metal complexes. The dpp is the strongest base and it formsthe most stable metal complexes. These two graphs illustrate that the interactions betweenthe 3-hydroxy-4-pyridinone ligands and the group 13 (hA) metal ions do comply with theHSAB principle: harder acids prefers harder bases.The stability constants for the metal-ligand complexes show that the 3-hydroxy-4-pyridinones are very good chelators for the group 13 (lilA) metals. These quantitativeresults are in complete agreement with our experimental observations. The predictivecapability that these data afford is particularly useful towards the goal of using in vitrotechniques to assess the suitability of a ligand for in vivo studies.122‘::\a db— 60_______Ii mM AlmMHpppJ02:23456789W11.Iog[H+]100a e80Cb d— 60_______403LMHpppJk20i j0•.I I I • I I1 2 3 4 5 6 7 8 9 10 11-Iog[H+]Figure 4.11. Speciation diagrams for solutions containing a 1:3 ratio of A13 to Hppp.Top: mM and bottom: iiM (j.t = 0.15 M NaC1, 25 °C).a, {A1(H2O)6j3;b, [Al(ppp)(H2O)4]2;c, [A1(ppp)2(H2O)2]; d, A1(ppp)3;e, [A1(OH)4]; i, [A1(OH)(H05+;j, [A1(OH)(HO);and k, A1(OH)3.123100)OE-log[H+]1 (Ifl -db80-—— 60 CCC 40-2:--log[H÷]Figure 4.12. Speciation diagrams for solutions containing 1 mM M3 and 3 mM Hppp(t = 0.15 M NaC1, 25 °C). Top: M = Ga; and bottom: M = In.b, [M(ppp)(HO)4]2;c, [M(ppp)(HO)2]; d, M(ppp)3; e, [M(OH)4j;f, [InC1(HO)5]g, [InCl2(HO)];h, mCi3.124100-a b c d/e80 -1:::____aHdppb Hpppc H4EDTA20- d maltole catechol0- • . I • I • I • I-7 -6 -5 -4 -3 -2 -1log[C]100a b c80S0401aHp20 lb HpppH4EDTA0• • I • I • I • I-6 -5 -4 -3 -2 -1Iog[C]Figure 4.13. Plots of A13 (top) and Ga3 (bottom) complexation (%) versus log of thetotal ligand concentration ([C]) for several ligands at 1 ).tM M3, pH 7.4,25 °C, 100 iM citrate, and 50 j.LM empty transferrin-binding sites.125403836e.C— 34.A A32- 0A030-Hppp Hmpp Hdpp0 A1L3 A GaL3 A JnL3Figure 4.14. Plot of log f for the aluminum, gallium, and indium complexes versusligand. Data for M(mpp)3 and M(dpp)3are from refs. 108, 110.9.98009.78 -C9.58 -09.38-Hppp Hmpp HdppFigure 4.15. Plot of log Kal versus the 3-hydroxy-4-pyridinones (25 °C, 0.15 M NaC1).Data for Hmpp and Hdpp are from refs. 108, 110.126Chapter 5Biodistribution Studies ofTris(ligand)Gallium-67 Complexes5.1. IntroductionThe potential utilization of 67Ga and 68Ga in diagnostic nuclear medicine hasresulted in the rapid development of the chemistry of gallium and of ligands speciallydesigned to coordinate these radionuclides.78’982 5904110As part of the project to develop the coordination chemistry of gallium for use innuclear medicine, the potential application of the tris-ligand gallium-67 complexes with thel-aryl-3-oxy-4-pyridinonate ligands as imaging agents has been evaluated by thebiodistribution studies of 67GaL3. As described earlier in this thesis, it has beendetermined that the 1-aryl-3-hydroxy-4-pyridinone ligands have the thermodynamic abilityto compete with transferrin for Ga3 in vivo. From our work with 1-alkyl-3-hydroxy-4-pyridinones, the GaL3 complexes have potential as nuclear medicine imaging agentsbecause of their great stability in vivo.110 67Ga biodistribution studies110 showed that, atappropriate concentrations, the 1 -alkyl-3-hydroxy-2-methyl-4-pyridinones could protect67Ga from transferrin, the main scavenger for trivalent metal ions in the blood.66’7 Rapidrenal excretion of these 67Ga-ligand complexes in mice and rabbits was observed,however. This result suggested that the l-alkyl-3-hydroxy-4-pyridinones had significantthermodynamic stability required to compete with transferrin at appropriate concentrations,but that their lipophilicity as originally constituted might have been lower than ideal. Thiscould have resulted in rapid renal clearance. An increase in lipophilicity of gallium-67127complexes should change the biodistribution of these metal complexes and result in thelocalization of them in organs. As it has been shown earlier, the lipophilicity of thetris(ligand)metal complexes has been increased significantly by introducing aryi groupsonto the pyridinone ring.In this chapter, the results of in vivo biodistribution studies obtained withtris( 1 -aryl-3-hydroxy-4-pyridinonato)gallium-67 complexes (Figure 5.1) in rabbits, mice,rats and a dog is presented and the potential application of these complexes as nuclearimaging agents is commented on. The stability constants determined in vitro are used topredict the stability of the67Ga-containing molecules in vivo.JoR = H 67Ga(ppp)367G,J> ) OCH3CH ççR / NO2 67Ga(pnp)3\ /3Figure 5.1. The structure of tris(ligand)gallium-67 complexes.5.2. ExperimentalThe compounds Hppp, Hptp, Hpap and Hpnp were prepared and recrystallized asdescribed in Chapter 2. The67Ga-citrate starting material was commercially purchasedfrom Dupont and Trizma-7.4 buffer reagent was purchased from Sigma. Water waspurified, deionized (Barnstead D8904 and D8902 cartridges, respectively) and distilled(Coming MP-1 Megapure Still).Preparation of67Ga complexes. Stock solutions of the pyridinones were preparedin 0.05 M isotonic Trizma pH 7.4 buffer solution. For labelling, appropriate volumes of128the pyridinone stock solution and67Ga-citrate were dispensed and mixed in a 10 ml sterile,pyrogen-free, nitrogen-purged vial. The ligand concentration used was always higher than50 .tM (as required). The pH of the resulting solution was 7.4 at 25 °C. A radiochemicalpurity of greater than 99% was ascertained by thin-layer chromatography using IThCTMSG paper (Gelman Sciences Inc.) with 0.9% NaC1 solution as the eluant. The Rf valuesfor 67GaL3 and67Ga-citrate are 0 and 1, respectively. When using a C18 reverse phaseTLC plate (Whatman) with 2% MeOH in ethyl acetate as the eluent, the Rf values are about0.5 for 67GaL3.Imaging studies. Imaging studies were perfomied in rabbits and a dog. A 1 mL67GaL3 solution containing 1 mCi 67Ga was injected through the ear vein of anesthetizedWistar rabbits and whole body imaging was carried out on a Siemens large field of view ycamera. Five-minute-accumulation static images were obtained at 1 hour intervals for 6hours post-injection and a 24-hour image was also obtained. The images of rabbits 1 hourpost-injection are shown in Figure 5.2. These images were stored on an ADAC 3300computer. The regions of interest (heart, liver, brain and blood, gut in the citrate case,Figure 5.3) were then drawn and the counts and total pixel numbers were obtained. Theregion of the superior vena cava was selected for the blood component. The activities inthe organs were normalized to counts per pixel and then the activity in the blood componentwas substracted from each organ. Correction was also made for decay. Plots of activity(counts/pixel) versus time are shown in Figure 5.4. A comparative study was also carriedout, after injection of 1 mCi67Ga-citrate (Figure 5.4). For the dog (Mongrel, shepherdcross, 23 kg) imaging study, 4 mL67Ga(pap)3 solution containing 2 mCi 67Ga and 73iM Hpap was injected via the anticubital vein. Five-minute-accumulation static images ofthe left lateral thoracic region were obtained using an Elscint Apex 409 MA Camera. Thisimaging was done at 30 minute intervals for 3 hours. The image at 30 minutes postinjection is shown in Figure 5.5. Activities in the heart and the superior vena cava were129determined and the value in the heart was normalized as described above. The activity inthe heart versus time was then plotted (Figure 5.7).Biodistribution studies. Biodistribution studies were carried out in BALB/C mice(UBC, 37 - 46 g) and male Wistar rats (—175 g) using the 67Ga(pap)3 complex. Allinjections were standardized such that a 100-pt injection volume contained 1 jiCi of 67Ga.Mice were injected through the tail vein and sacrificed by exsanguination at 15 minutes, 1hour, 4 hours and 24 hours post injection, respectively. The blood, liver, kidney, spleen,lung, heart, muscle and brain were excised and their activities were determined in a well ‘ycounter. The total blood volume and muscle mass were calculated according to the reportedmethods.253’4 The results in percent uptake per total organ (mean ± standard deviationfor five mice) are summarized in Table 5.1. A comparative study with67Ga-citrate wasalso performed and the results are displayed in Table 5.1. Biodistribution of67Ga(pap)3 inrats was performed as above except that the animals were sacrificed by chemical overdoseand the results determined only at 15 minutes and 1 hour post injection (Table 5.2).130(a) (b)*‘I(c) (d)Figure 5.2. Scintigraphic images of rabbits 1 hour post-injection of67GaL3.(a)67Ga(ppp)3; (b)67Ga(ptp)3; (c)67Ga(pap)3; (d)67Ga(pnp)3.131HPAP 8APR900001 HPAP 1HRcElabCdeB 161S 008—APR—905. 00 /Figure 5.3. Schematic representation of the regions of interest in a rabbit. (a) brain;(b) blood; (c) heart; (d) liver, and (e) gut area.132:::900— d.•0.0.—V ..b..0600’ • • •0 5 10 15 20 251200& I —.I800‘ _________r-m..___._____- -400’ : •ir.h /Brain0’ • I • I • I0 5 10 15 20 25Time (hour)Ga(pap)3- Ga(ppp)3 -. - 0-• Ga(ptp)3—— b— Ga(pnp) S Ga-citrateFigure 5.4. Plots of activity versus time for the heart, liver and brain of rabbits injectedwith 67GaL3 and67Ga-citrate.133Figure 5.5. Scintigraphic image of a dog heart 30 minutes post-injection of67Ga(pap)3.1/2 HR DOG1345.3. Results and DiscussionThe rabbit studies show qualitatively that tris(1-aryl-3-oxy-4-pyridinonato) gallium-67 complexes localize in the heart, liver and brain. However, their kinetics are different.67Ga(pnp)3 is unique when compared with the other three complexes. It localizes in and iscleared from the heart and liver more quickly than the others (Fig. 5.4). Also, it is taken upby the brain quickly and remains in this organ for a long time (Fig. 5.4). This observationindicates that 67Ga(pnp)3 may have the potential for brain imaging. The partitioncoefficients of this series of complexes show that the lipophilicity increases in the order ofGa(pnp)3 (logp= 1.49) <Ga(ppp)3 (2.11) <Ga(pap)3 (2.51) < Ga(ptp)3 (>3). FromFigure 5.4, it can be seen that each complex has a different biodistribution pattern in therabbit further suggesting that lipophiicity of these complexes may be important in directinglocalization in different organs. 67Ga(pap)3 shows the highest heart activity (Fig. 5.4). Italso has the highest heart/liver ratio (1.27 for67Ga(pap)3, 1.20 for67Ga(ptp)3,1.13 for67Ga(ppp)3 and 0.97 for67Ga(pnp)3) at 24 hours post injection.The comparative67Ga-citrate image study was conducted in a rabbit to verify thedifferences in biodistribution of 67Ga as citrate and as 3-hydroxy-4-pyridinone complexes.The 67Ga-citrate biodistribution is similar to that of Ga-transferrin on injection.74Calculations based on the available stability constants in the literature72”show that all ofthe 67Ga after injection (as its citrate) should be complexed with transferrin. The uniquebiodistribution of these67Ga-complexes therefore suggests that there is a minimal amountof Ga-transferrin complex formed and that the Ga must have remained complexed to theligands long enough for localization to take place. For the67Ga-citrate imaging study, theactivity in the gut area increased dramatically with time; while there is no apparent increasein activity in the gut with injection of67GaL3.The results of biodistribution studies of67GaL3in rabbits show that67Ga(pap)3 hasthe most promising potential as a myocardial imaging agent and therefore, Hpap was135selected as the ligand for the biodistribution experiments in mice. Results listed in Table5.1 show that the biodistribution of67Ga(pap)3 in mice is very different from that of67Ga(pap)3 in rabbits. The results in mice show very low heart uptake and almost no brainuptake. A comparative study of67Ga-citrate biodistribution in mice was conducted in thesame way and the results of this experiment are also shown in Table 5.1. The percentuptake per organ into the blood, liver, kidney, muscle and heart after 24 hours is showngraphically in Figure 5.6; it demonstrates the difference in biodistribution of67Ga(pap)3versus67Ga-citrate. The uptake into the muscle and kidney was greater with citrate thanwith Hpap. This dramatic difference between the biodistribution of67Ga(pap)3 in rabbitsand in mice indicates that a species difference may also play a role in controlling the 67GaL3biodistribution.• Ga-citrate0 Ga(pap)310.00.0 i I —rliver heartFigure 5.6. Percent uptake per organ for the blood, liver, kidney, muscle, and heart 24hours post-injection of67Ga(pap)3 and67Ga-citrate solutions in mice.20.0 -‘Iblood kidney muscle136Table5.1.Biodistributionin%uptakeintotal organof 67Ga-citrateand67Ga(pap)3inmice(mean ±standarddeviationfor fivemice)15mm1hour4hours24hoursOrganGa-citrateGa(pap)3Ga-citrateGa(pap)3Ga-citrateGa(pap)3Ga-citrateGa(pap)3blood53.76(29.31)43.58(3.54)18.24(11.75)31.62(3.00)19.66(14.02)20.29(1.54)6.72(5.30)6.03(0.80)liver9.39(4.36)6.56(1.67)5.52(2.95)6.14(0.99)7.08(2.99)6.57(1.31)12.73(4.83)8.04(0.73)kidney3.20(1.18)1.80(0.26)2.03(0.88)1.77(0.11)3.28(0.96)1.71(0.18)4.33(1.12)1.87(0.07)spleen0.48(0.24)0.25(0.05)0.34(0.17)0.22(0.06)0.48(0.44)0.18(0.02)0.30(0.12)0.26(0.03)lung3.19(2.43)1.52(0.45)1.54(0.73)1.16(0.62)1.16(0.41)0.73(0.26)0.81(0.39)0.42(0.16)heart0.99(0.31)0.43(0.04)0.39(0.14)0.34(0.03)0.42(0.23)0.25(0.05)0.28(0.11)0.12(0.02)muscle49.08(6.76)17.82(3.84)42.16(8.58)16.66(1.57)37.18(14.35)15.02(1.57)14.25(2.17)5.74(0.98)brain0.25(0.13)0.16(0.06)0.14(0.08)0.15(0.03)0.15(0.09)0.10(0.03)0.11(0.05)0.06(0.02)Table5.2.Biodistributionin%uptakeintotal organof 67Ga(pap)3inrats(mean ±standarddeviationfor fiverats)Timebloodliverkidneyspleenlungheartmusclebrain15mm27.87(15.40)2.03(1.17)0.99(0.47)0.26(0.12)0.58(0.35)0.34(0.16)16.99(9.07)0.07(0.04)1hour22.41(11.71)2.33(1.68)1.00(0.52)0.24(0.07)0.47(0.29)0.29(0.17)16.49(9.52)0.07(0.02)(p-4.—-..0Ca• —To confirm this, a limited biodistribution experiment with67Ga(pap)3 in rats wasdone at two times post-injection (15 minutes and 1 hour). The organs studied were blood,liver, kidney, spleen, lung, heart, muscle and brain. The results are summarized in Table5.2. The biodistribution pattern of67Ga(pap)3 in rats is very similar to that in mice; verylow heart and brain uptake are demonstrated. As rats and mice are closely related, thebiodistributions of67Ga(pap)3 in each ought to be similar.In the dog study of67Ga(pap)3, the high heart uptake of the complex is clearly seen(Figure 5.5). The kinetics of67Ga(pap)3 in the heart of the dog (Figure 5.7) shows thatthe activity of67Ga(pap)3 in the heart remains relatively stable after 0.5 hours.600050004000__30002000100000.0 3.0Figure 5.7. Plot of activity versus time for the heart of a dog injected with67Ga(pap)3.The speciation of 67Ga in vivo can be roughly calculated using a simple bloodplasma computer model based on known concentration levels and available stabilityconstants for Ga3+ with human serum transferrin72and citrate.100 Hppp was selected as0.5 1.0 1.5 2.0 2.5Time (hour)138the representative 3-hydroxy-4-pyridinone for this calculation since its constants are knownaccurately. The protonation constants for the aryl-substituted pyridinones (log Kal = 9.40of ppp to Hppp versus 9.42 of pap- to Hpap and log Ka2 = 3.04 of Hppp to H2pppversus 3.16 of Hpap to H2pap) were found to be very similar, so similar results with theother ligands used in the biodistribution experiments (i.e. Hpap) can also be assumed.After the 67GaL3solution is injected into the vein of the animal, it is first carried to the heartand then washed out and gradually diluted in the blood stream. With an HL concentrationof 64 jiM (the concentration used in the rabbit image study) before the blood dilution, 99%of Ga should be equilibrated as GaL3, the balance as [GaL2(H2O)2] and Ga-transferrin.If the dilution of the injected solution in the blood is taken into account (1 mL 67GaL3intoabout 80 mL blood (normally, the blood volume is about 77.8 mL/kg)), then theconcentrations of the HL and Ga3 in the blood would be 7.9 x iO M and 3.1 x 10-10 M,respectively, and 100% of Ga would exist as Ga-transferrin complex. The initialcalculation result, without considering the dilution by the blood, reflects the uptake foundin vivo in rabbits; therefore, after bolus injection, first pass extraction by the heart reflectsthat of the injected complexes. After the equilibrium, the biodistribution would reflect thatof Ga-transferrin (kidney and liver).The same calculation of a 67Ga speciation in the dog can be done. Before taking thedilution of the injected solution by the blood (about 2 L) into account, 95% of Ga would beequilibrated as GaL3 complex, 4% as GaL2(H2O)2 and 1% as Ga-transferrin while 100%of Ga would be Ga-transferrin if dilution by the blood is considered. The results of thedog imaging study indicate that the Ga exists as the tris(ligand)complex in vivo longenough for it to localize in the heart. The localization must happen by first pass extraction.However, this will need further confimiation.The results of a Ga speciation calculation simulating the mice experiments areshown in Figure 5.8. The results show a similar trend to that of the rabbit and dog139speciation calculations before and after the blood dilution. The Ga should exist as a Gatransferrin complex in vivo after the blood dilution.— 5.8. The speciation of Ga3 in mice before and after blood dilution.From the results of the calculations, we conclude that the efficiency of the first passextraction of these complexes by the heart of the dog and rabbits is quite high, otherwise,the biodistribution of 67GaL3 complexes would be similar to that of67Ga-transferrin or67Ga-citrate. As soon as the injected complex solution is sufficiently diluted by the bloodstream, the ligand concentration is not high enough to compete with transferrin and thetris(ligand) complexes are demetallated by it; therefore, 67Ga in vivo should exist astransferrin complexes. As the experimental results show high heart uptake of thecomplexes, it means that the complexes exist in vivo mainly as tris(ligand) complexes andare extracted by the heart muscle when they first pass through the heart before the 67Ga ionis complexed ultimately to transferrin. When the first passage extraction efficiency of thecomplexes by the organs is not high or the complexes for some other reason do not localizein the heart as found in mice and rats, the injected solution is diluted by the blood streamGaL396%Ga-Tf I97%Ga-Tf1%+ GaL32%Before After140and the Ga is taken up by transferrin to form the Ga-transferrin complex. Also, the extractefficiency of67Ga-citrate and67Ga(pap)3 would be different and this difference should becounted for the difference in biodistribution of67Ga-citrate and67Ga(pap)3 in mice andrats. This would explain the slow clearance of the radionuclide from the blood of mice andrats (vide supra) and would also support the argument of the species difference.In conclusion, a series of radiolabelled tris(1-aryl-3-oxy-4-pyridinonato)-gallium(III) complexes have been prepared in high radiochemical purity and yield. Thesimple and rapid labelling procedure (with a high efficiency and excellent purity) yields aproduct which requires no further purification before biodistribution studies in animals.This formulation provides a kit amenable for clinical application. The lipophilicity of thecomplexes has been increased significantly by introducing aryl substituents on thepyridinone ring nitrogen to replace the alkyl substituents,07’136 while the highly stablechelate core has been maintained in these complexes. This increase in lipophilicity hasdramatically changed the biodistribution of these complexes in vivo. This change has beenshown by the in vivo studies. The rabbit and dog imaging studies show that thesecomplexes might be used as first pass extraction myocardial imaging agents. This is thefirst report of high uptake of 67Ga complexes in the heart of rabbits and dogs. Thecalculation of the Ga speciation and the mice and the rats biodistribution studies show that aspecies difference may play a role in the biodisthbution of67GaL3.141Chapter 6One-pot Synthesis of 1-Alkyl-3-oxy-4-pyridinonateChelate Complexes of Aluminum, Gallium, and Indium6.1. IntroductionAs described earlier, group 13 (lilA) metal complexes of a series of 1-alkyl-3-hydroxy-4-pyridinones are neutral, water soluble and lipophilic.’°6”°7’136 The pyridinonecomplexes are thermodynamically stable (overall formation constants 133 of 1032 to 1037)and they strongly resist hydrolysis because the tris(ligand) species predominate betweenpH 4 and pH 9•108,h10 Burgeoning interest in the bioinorganic chemistry of Al, Ga and Inhas necessitated the investigation of compounds of these elements (such as those with theN-substituted-3-hydroxy-4-pyridinone ligands) that are soluble and stable underphysiological conditions.”0The syntheses of these complexes have been reported previously;’06”73however, in these reports the metal complexes were prepared from the ligands, which weresynthesized separately in other work.129 It has become obvious that the development ofsimple methods to synthesize the pyridinone ligand complexes is of considerableimportance. The 3-hydroxy-4-pyridinones are usually prepared by amination of theanalogous 9130,133,136,140, 52,255-257 Two common methodshave been used to achieve this conversion process. The old method uses methyl-, benzyl-,or glucosyl-protected as the precursors; recently, in asecond procedure, workers have found that the hydroxypyrone can be converted142directly.130”36Three steps are involved in the first method: protection of the ring hydroxylgroup with methyl, benzyl, or glucosyl groups, conversion via the amine insertionreaction, and deprotection to give the pyridinone. The multistep reactions are, of course,time-consuming and rigorous. Control of the pH is vital to success with the secondmethod.136 A suitable and more general method was sought that would allow the directpreparation of tris(3-oxy-4-pyridinonato)metal(llI) complexes.Reactions of ligands coordinated to metal ions have been used frequently in thesynthesis of organic compounds and more frequently, in the preparation of their metalcomplexes.258-62 The preparation of macrocyclic metal complexes in high yield via themetal template effect is but one application of reactions of coordinated ligands. Thisknowledge of previous work on the reactions of coordinated ligands stimulated an interestin prepared tris(3-oxy-4-pyridinonato)metal(flI) complexes by this simple route. Describedin this chapter is the one-pot synthesis of group 13 (lilA) complexes of 1 and 2pyridinones derived from maltol and kojic acid, respectively.M)R = CH3,C2H56.2. ExperimentalMaterials and Methods. The materials were commercially purchased from Sigma:maltol, kojic acid, methylamine (40% aqueous solution), and ethylamine (70% aqueoussolution) and were used as received. 1H NMR spectra were recorded with a Bruker WH214380 NMR spectrometer; JR spectra were obtained on a Perkin-Elmer PE 783 spectrometerand were referenced to polystyrene. Characterization of the products was accomplished bycomparing the JR and 1H NMR spectral data with those of authentic samples prepared byliterature methods106’73263 as well as by elemental analysis where necessary. Theyields (%) are listed in Table 6.1.Tris(3-oxy-1 ,2 -dimethyl-4-pyridinonato)aluminum(III), Al(dpp)3. Maltol (2.51 g,19.9 mmol) and A1Cl36H2O (1.64 g, 6.79 mmol) were dissolved in 25 mL distilled waterand methylamine (40% aqueous, 25 mL) was added. The pH of this solution was adjustedto 11.0 and the reaction mixture was transferred to a 100-mL flask in which it was refluxedfor 6.5 hours. The excess amine was removed under reduced pressure and the volume ofthe solution was reduced to about 5 mL. The solution was stored in the refrigeratorovernight; the light pink crystals that formed were collected by filtration and recrystallizedfrom hot water. The recrystallized product was dried in vacuo and 2.63 g of product wasobtained.Tris(3 -oxy-1 ,2 -dimethyl-4 .pyridinonato)gallium(III), Ga(dpp)3. The syntheticprocedure was as for Al(dpp)3. Maltol (1.02 g, 8.09 mmol) and Ga(N03.9H20(1.12 g,2.68 mmol) with 10 mL of methylamine (40%) solution yielded 1.1 ig of pink product.Tris(3 -oxy-1 ,2 -dimethyl-4 -pyridinonato)indium(III), In(dpp)3. Maltol (1.05 g,8.32 mmol) and InCl3•4H20 (0.82 g, 2.80 mmol) in 25 mL of distilled water with 10 mLof methylamine aqueous solution (40%) gave 1.19 g of white product.Tris(3 -oxy-1 -ethyl-2 -inethyl-4 -pyridinonato)aluminum(III), Al(mepp)3. Maltol(1.03 g, 8.17 mmol) and AlCl36H2O(0.63 g, 2.58 mmol) were dissolved in waterfollowed by the addition of 5 mL aqueous ethylamine (70%). The pH of the solution wasadjusted to 11.1 and the solution was refluxed for 24 hours. The volume of the solutionwas reduced to about 5 mL. This solution was stored in the refrigerator overnight. Thesolid was collected by filtration and recrystallized from hot ethanol. The recrystallizedproduct was dried in vacuo, yielding 0.37g of off-white product.144Tris(3 -oxy-1 -ethyl-2 -methyl-4 -pyridinonato)gallium(III), Ga(mepp)3. Thepreparation procedure was as for Al(mepp)3. Ga(N0•9H20(1.14 g, 2.74 mmol),maltol (1.03 g, 8.17 mmol) and ethylamine (70%, 5 mL) gave 0.88 g off-white productwhich was recrystallized from hot water and dried in vacuo.Tris(3 -oxy -1 -ethyl-2 -methyl-4 -pyridinonato) indium(III), In(mepp)3. Thepreparation procedure was as for Al(mepp)3. 1.02 g of maltol and 0.82 g of InC134H2Owith 5 mL of 70% ethylamine aqueous solution yielded 1.22 g of product which wasrecrystallized from hot methanol and dried in vacuo.Tris(3 -oxy-6-hydroxymethyl-1 -methyl-4-pyridinonato)aluminum(III), Al(hmp)3.Kojic acid (1.98 g, 13.9 mmol) was dissolved in 60 mL distilled water, and methylamine(40% aqeous, 10.8 mL) was added. A1C136H20(1.05 g, 4.35 mmol) was then added tothis solution. The pH was adjusted to 11.0 and monitored regularly during two days ofrefluxing. The reaction mixture was subsequently cooled to room temperature, its volumewas reduced in vacuo, and it was then cooled in the refrigerator overnight. A yellowish solidwas collected by filtration. It was recrystallized from hot water and dried in vacuo, giving1.27 g of product.Tris(3 -oxy-6-hydroxymethyl-1 -methyl-4 -pyridinonato)gallium(III), Ga(hmp)3.The preparation procedure was as for Al(hmp)3. Kojic acid (1.10 g, 7.74 mmol),Ga(NO3)9H2 (1.01 g, 2.42 mmol) and aqueous methylamine (40%, 6 mL) gave 0.63gof yellowish product.Tris(3-oxy-6-hydroxymethyl-1 -methyl-4-pyridinonato)indium(III), In(hmp)3. To asolution of kojic acid (1.06 g, 7.45 mmol) and aqueous methylamine (40%, 6 mL) wasadded InC134H2O (0.674 g, 2.30 mmol). A preparation procedure similar to that forAl(hmp)3 yielded 0.72 g of white product.Tris(3 -oxy-6-hydroxymethyl-1 -ethyl-4 -pyridinonato)aluminum(III), A l(hep)3.Kojic acid (2.07 g, 14.6 mmol) and A1C13•6H20(1.09 g, 4.51 mmol) were dissolved in60 mL distilled water followed by the addition of 5 mL aqueous ethylamine (70%). The145pH was adjusted to 10.3 and maintained during 3 days of refluxing. The volume of thereaction mixture was reduced to about 10 mL; this solution was then cooled in arefrigerator overnight. A white solid (0.51 g) was obtained after recrystallization from hotwater and drying in vacuo for 12 hours.Tris(3-oxy-6-hydroxymethyl-1 -ethyl-4-pyridinonato)gallium(III), Ga(hep)3. Thesynthetic procedure was as for A1(hep)3. Kojic acid (1.13 g, 7.94 mmol), GaNO39H2O(1.04 g, 2.48 mmol) and 5 mL ethylarnine (70%, aqueous) gave 0.46g of white crystallineproduct.Tris(3 -oxy-6-hydroxymethyl-1 -ethyl-4-pyridinonato)indium(III), In(hep)3. Thepreparation procedure was as for Al(hep)3. A mixture of kojic acid (1.39 g, 9.78 mmol),InC134H2O (0.892 g, 3.04 mmol) and 5 mL ethylamine (70%, aqueous) gave 1.2 g ofwhite product.6.3. Results and DiscussionA series of N-substituted tris(3-oxy-4-pyridinonato)- and tris(3-oxy-6-hydroxymethyl-4-pyridinonato)complexes of aluminum(Ill), gallium(III) and indium(llI)were prepared in good yield by the one-pot synthesis method from commercially availablestarting materials. In this preparation, the metal-pyrone complex is first formed in situ andthen undergoes the insertion of the primary amine into the pyrone ring to form theappropriate 3-oxy-4-pyridinonate complex under controlled pH. The general syntheticscheme is outlined in Figure 6.1. Neither the metal-pyrone complexes nor the productligands were isolated because the aim of this work is the direct preparation; however, theconditions are completely compatible with those used in their synthesis.’°4”°67’263146-R2 = CH3,R6 = H maltol M = Al, Ga, In R1 = CH3,C2H5R2 = H, R6 = CH2O kojic acidFigure 6.1. Scheme for the one-pot synthesis of tris(3-oxy-4-pyridinonato)metal(III)complexes.The accepted mechanism for the preparation of a 4-pyridinone by conversion of anappropriate 4-pyrone is nucleophiic attack by a primary amine, followed by ring opening,elimination of water, and ring closure to give the 4-pyridinone.264 The nucleophilic attackis an important step in this process. Any factor which affects the electron density on thepyrone ring (especially next to the in-ring oxygen atom at C2 or C6) influences thenucleophiic attack step and therefore the whole conversion reaction. The reactivity of thepyrone ring towards the entering nucleophile varies with the substituents on the ring aswell. Electron-withdrawing groups should enhance the reactivity of the ring whileelectron-donating groups should have the opposite effect.Complex formation between a trivalent metal ion and the pyrone enhances thereactivity of the coordinated pyrone in two ways. In the pyrone amination reaction, thecharge localization on the ring plays a crucial role in determining the effectiveness of thereaction. Deprotonation of the hydroxyl group on the pyrone ring gives an anionichydroxyl oxygen which hinders the formation of the 4-pyrone resonance hybrid mostsusceptible to nucleophilic attack. Protonation of the primary amine will hinder thenucleophilic attack as well. Complexation of the hydroxyketone oxygen atom prevents theundesired deprotonation process; furthermore the metal ion causes polarization of the ligandthrough an electron withdrawing effect. This phenomenon results in increased acidity of3147the coordinated ligands upon conjugation and the coordinated pyrone will be moresusceptible to nucleophilic attack than the uncoordinated pyrone. Conversion reactions ofthe coordinated pyrones to the appropriate pyridinones should be easier. Group 13(IIIA)metals are strong Lewis acids and they form stable complexes with maltol and kojicacid.104’724965 They should strongly polarize the coordinated pyrone and make thenucleophilic attack by a primary amine much easier. The conditions of the conversionreactions should then be improved and the yield of the tris-ligand complexes should beincreased.Table 6.1. The yields of tris(ligand)metal(llI) complexes via the one-pot synthesis.R1 1 2Al CH3 90 57C2H5 46 21Ga CH3 85 49C2H5 61 32In CH3 82 54C2H5 63 63The results (Table 6.1) show that the yields of the complexes are quite reasonableby this one-pot synthesis method thereby verifying the hypotheses as outlined. The yieldfor the metal complexes 1 where R1 = CH3 compare very favorably with literature resultsfor the ligand (51% in three steps via protection;’2970%136 or 50%130 directly) followedby complex preparation (79% for Al, Ga’06). For 1 (Ri = C2H5), the one-pot synthesiscompares less favorably (ligand - 58%b09,136 or 24%130 directly; complex> 80%b09.136).148The syntheses using maltol and kojic acid were compared to investigate the effect ofdifferent substituents on the pyrone ring. Substituents influence the reactivity by changingthe electron density on the pyrone ring and, consequently, the basicity of pyrones.250Electron-donating groups, such as the methyl group on maltol, increase the electron densityon the pyrone ring and hinder nucleophilic attack while increasing the basicity of thepyrone. Accordingly, more stable complexes with metal ions are formed. Electron-withdrawing groups, such as the hydroxymethyl group on kojic acid, decrease the electrondensity on the pyrone ring facilitating nucleophilic attack. However, this decreasedelectron density decreases the basicity of the pyrone illustrated by the fact that kojic acid(pKa = 7.6) is a stronger acid than maltol (pKa 8.4).249250 These two effects arethought to compete during the reaction process. Upon complexation, the electron-withdrawing effect of the positively charged metal ions should be much more drastic thanthat of the ring substituents. The relative stability of the complexes would be important indetermining which effect is dominant during the reaction. The more stable the complex, themore pronounced the effect on the electron density should be. Even though electron-donating groups could impede the nucleophilic attack process, the enhanced stability of themetal complexes would compensate for this negative effect. The conversion reactionwould occur more easily for the pyrones with electron-donating substituents than thosewith electron-withdrawing substituents. The results in Table 6.1 show that the conversionreaction of maltol is a higher yield process than that of kojic acid.In conclusion, the one-pot synthesis method is a simple, efficient way to prepareths(3-oxy-4-pyridinonato)metal(III) complexes and it also has been proven to be a usefulexample of the reaction of a coordinated pyrone. The general applicability to bulkier alkylsubstitutents needs further investigation.149Chapter 7Conclusion and Suggestions for Future WorkThe objectives of this project were the synthesis and characterization of a new seriesof group 13 (lilA) metal complexes with the lipophilic 1-aryl-3-hydroxy-4-pyridinoneligands, the study of their coordination chemistry in solid and solution states, theapplication of this knowledge to biological systems, and the development of new galliumand indium radiopharmaceutical imaging agents with special interests in brain and heartimaging. As stated in the preceding chapters, these primary goals have been achieved.A series of 1-aryl-3-hydroxy-4-pyridinones and the metal complexes ofaluminum(llI), gallium(Ill), and indium(III) with these ligands were successfully preparedwith relatively high yields. These complexes have very interesting properties: highthermodynamic stability and high lipophilicity. The high effective stability constants atphysiological pH of these metal complexes show that this series of ligands can competewith transferrin for metal ions in blood plasma in vivo. The effective binding of the group13 (lilA) metal ions by these ligands is also demonstrated by the in vitro simulation of invivo speciation and the in vivo biodistribution studies. The lipophilicities of the 1-aryl-3-hydroxy-4-pyridinones and metal complexes have been increased significantly compared tothe first generation of 1-alkyl-3-hydroxy-4-pyridinones by introducing the aryl substituentsonto the pyridinone ring probably increasing their lipid solubility and membranepermeability. This has been proven by the results of the biodistribution studies of67GaL3.The promising results of the biodistribution studies of the tris(ligand)gallium-67complexes indicate that this series of complexes may have potential as myocardial imagingagents; this is the first example of the high uptake of 67Ga complexes in the heart of rabbits150and dogs. The dependence of the biodistribution pattern of the tris(ligand)gallium-67complexes on the species has also been demonstrated by these studies.One obstacle in using these complexes for in vivo studies is their limited solubilityin water; this has hindered our investigation in biodistribution studies. Future work,therefore, lies in the improvement of the water solubility of these complexes. Theintroduction of some hydrophilic substituents such as sulfonate on the phenyl ring shouldbe effective towards this goal. An alternative method is the development of multidentateligands. The stability of the complexes can be increased by the chelate effect, and thereforeless ligand would be needed to protect the demetalation of the complexes by transferrin.This is now being actively investigated in our group.Another area of study worth pursuing is kinetic studies of aluminum removal fromtransferrin by the hydroxypyridinone ligands or aluminum uptake by transferrin from thetris(ligand) complex. Initial studies have shown that the aluminum uptake by transferrinfrom Al(dpp)3 is a biphasic process which may correspond to the different affinity of themetal binding sites of transferrin for the trivalent metal ions. 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Rev. 1975, 4, 421.(260) Healy, M. D. S.; Rest, A. 3. Adv. Inorg. Chem. Radiochem. 1978, 21, 1.(261) Busch, D. H. Acc. Chem. Res. 1978, 11, 392.165(262) Black, D. S. C. In Comprehensive Coordination Chemistry; G. Wilkinson, R. D.Gillard and J. A. McCleverty, Eds.; Pergamon: Oxford, 1987; Vol. 1; pp 415-462.(263) Zhang, Z.; Hui, T. L. T.; Orvig, C. unpublished results.(264) Elkaschef, M. A.-F.; Nosseir, M. H. J. Am. Chem. Soc. 1960, 82, 4344.(265) Tuck, D. G.; Yang, M. K. J. Chem. Soc. A 1971, 3100.166AppendixTable Al. Crystallographic data for Hcmp and Hpap.aCompound Hcmp HpapFormula C8H9N04 C13HN03fw 183.16 231.25Crystal system monoclinic monocinicSpace group P21/n C2/ca, A 6.340(2) 10.8435(9)b, A 7.705(1) 16.680(2)c, A 16.226(1) 13.5665(6)94.74(1) 109.450(4)v, A3 789.9(3) 23 13.7(3)Z 4 8p,gIcm3 1.540 1.328F(000) 384 976Radiation Mo Cu0.71069 1.541781.17 7.43Crystal size , mm 0.12 x 0.35 x 0.45 0.25 x 0.25 x 0.40Transmission factors 0.93-1.04 0.74-1.00Scan type cx-20 co-20Scan range, deg in w 1.26+ 0.35 tan 0 0.94 + 0.30 tan 0Scan speed, deg/min 32 32Data collected ÷h, +k, ±1 +h, +k, ±120max. deg 60 155167Crystal decay negligible negligibleTotal reflections 2640 2477Total unique reflections 2443 2339Rmerge 0.020 0.010No. ofreflcnswithl3a(I) 1642 1819No. of variables 154 207R 0.030 0.04 1R 0.039 0.058gof 2.03 2.52Max N (final cycle) 0.01 0.02Residual density e/A3 -0.16 to +0.20 -0.16 to +0.24a Temperature 294 K, Rigaku AFC6S diffractometer, graphite monochromator, takeoffangle 6.0°, aperture 6.0 x 6.0 mm at a distance of 285 mm from the crystal, stationarybackground counts at each end of the scan (scan/background time ratio 2:1, up to 8rescans), a2(F) = [S2(C + 4B)+ (pF2]/Lp (S = scan rate, C = scan count, B =normalized background count, p = 0.02 for Hcmp and 0.03 for Hpap), function minimizedw(IF0I-IFI)2where w = 4F2/a(),R ZIIFI-IFII/EIF,R = (Ew(1F01-IFI)/EwTF)112,and gof = [Ew(IF0I-IF )/(m-n)]”.Values given for R, R, and gof arebased on those reflections with I 3G(I).168Table A2. Final atomic coordinates (fractional) and Beq (A2)* for Hcmp.Atom x y z B0 (1) 0.2550 (1) 0.1339 (1) 0.55699 (5) 3.30 (2) 0.6 127 (2) 0.2876 (1) 0.62640 (5) 3.30 (3) 0.6549 (1) -0.0052 (1) 0.28805 (5) 3.50 (4) 0.4348 (2) 0.0517 (1) 0.17817 (5) 3.4N (1) 0.4990 (1) 0.2696 (1) 0.37577 (5) 2.2C(1) 0.1619 (2) 0.1120(2) 0.38538 (8) 2.9C (2) 0.3566 (2) 0.1962 (1) 0.42452 (6) 2.1C (3) 0.3988 (2) 0.2029 (1) 0.50945 (6) 2.2C (4) 0.5850 (2) 0.2822 (2) 0.54494 (6) 2.4C (5) 0.7262 (2) 0.3505 (2) 0.49210 (7) 2.8C (6) 0.6794 (2) 0.3435 (2) 0.40863 (7) 2.7C (7) 0.4613 (2) 0.2614 (2) 0.28500 (7) 2.5C (8) 0.5226 (2) 0.0868 (2) 0.24938 (6) 2.4* B = (8/3)it2EUijai * aj * (ar aj)169Table A3. Final atomic coordinates (fractional) and B* (A2) for Hpap.Atom X y Z Beqo (1) -0.0381 (1) 0.09674 (8) -0.10482 (9) 4.50(2) 0.1571(1) 0.11469(7) -0.19037 (8) 4.30 (3) 0.3761 (1) 0.15245 (9) 0.55646 (9) 5.4N (1) 0.2409 (1) 0.13542 (8) 0.1235 (1) 3.7C(1) 0.0102(2) 0.1114(2) 0.1103(1) 5.1C(2) 0.1139(1) 0.1193(1) 0.0610(1) 3.6C(3) 0.0867 (1) 0.1132 (1) -0.0442 (1) 3.4C (4) 0.1842 (2) 0.1232 (1) -0.0937 (1) 3.4C (5) 0.3104 (2) 0.1434 (1) -0.0241 (1) 4.2C (6) 0.3351 (2) 0.1480 (1) 0.0801 (1) 4.2C (7) 0.2766 (1) 0.1381 (1) 0.2365 (1) 3.5C (8) 0.3022 (2) 0.2111 (1) 0.2872 (1) 4.1C (9) 0.3360 (2) 0.2133 (1) 0.3946 (1) 4.4C(10) 0.3436 (2) 0.1434 (1) 0.4510 (1) 3.9C(11) 0.3197 (2) 0.0707 (1) 0.3999 (1) 4.1C (12) 0.2855 (2) 0.0679 (1) 0.2922 (1) 4.1C (13) 0.3643 (3) 0.0845 (2) 0.6161 (2) 6.5* Beq = (8/3)it2EUja * aj * (a aj)170Table A4. Crystallographic data for M(ptp)35.5H20 (M = Al and Ga).acompound Al(ptp)3 5.5 HO Ga(ptp)3 5.5 HOformula C39H6A1N065.5 HO C39H6GaNO65.5 HOfw 768.79 811.53color, habit orange, prism yellow, prismcrystal size , mm 0.20 x 0.25 x 0.35 0.15 x 0.15 x 0.35crystal system trigonal trigonalspace group P31c P31ca,A 16.2990(8) 16.2817(8)c, A 16.784(2) 16.948 (2)v, A3 3861.6(6) 3890.9(6)Z 4 41.322 1.385F(000) 1628 1700p.(CuK), cm 9.77 14.77transmission factors 0.89-1.00 0.88-1.00scan type o-20 o-20scan range, deg in X 1.05+0.20 tan 0 1.00+0.30 tan 0scan speed, deg/min 32 16data collected ±h,+k,+l (IhIkI for h<0) ±h,+k,+l (IhIIkI for 4<0)20max’ deg 155 155cryst decay negligible negligibletotal no. of reflections 5972 6040no. of unique reflections 2839 2964Rmerge 0.029 0.03 1reflcnswithl>3cy(I) 1593 1665171no. of variables 190 185R 0.040 0.036R 0.054 0.048gof 2.04 1.56max A/G(fmalcycle) 0.09 0.50residual density e/A3 -0.20 to +0.16 -0.30 to +0.19a Temperature 294 K, Rigaku AFC6S diffractometer, Cu K radiation ( = 1.54178 A),graphite monochromator, takeoff angle 6.0°, aperture 6.0 x 6.0 mm at a distance of 285mm from the crystal, stationary background counts at each end of the scan(scan/background time ratio 2:1),a2(F)= [S2(C + 4B)+ (0.03F2j/Lp(S = scan speed,C = scan count, B = normalized background count), function minimized Ew(IF0I-lF )2where w =4F02/G(),R = EHF0I-IFH/EF,R = (Ew(IFoIIFcI)2/ Fol)’12andgof = [E(IFoI-IFcI)/(m-n)]112.Values given for R, R, and gof are based on thosereflections with I 3y(I).172Table A5. Final atomic coordinates (fractional) and B (A2) for A1(ptp)35.5H20.Atom x y z Bq 0CC.Al (1) 1/3 2/3 0.48 195 (7) 3.79 (3) 1/3o (1) 0.2850 (1) 0.5574 (1) 0.54634 (9) 4.18 (6)o (2) 0.3807 (1) 0.6024 (1) 0.41693 (9) 4.27 (6)O(3A) 0.4572 (3) 0.8360 (4) 0.8232 (3) 13.4 (3) 0.816o (3B) 0.35 (1) 0.7 10 (5) 0.7942 (7) 14 (3) 0.163o (4A) 0.4156 (6) -0.4156 1/4 13.0 (1) 0.264o (4B) 0.35 1 (2) 0.556 (2) 0.247 (1) 13 (1) 0.28 1o (4C) 0.4422 (9) 0.4314 (9) 0.2322 (6) 13.2 (6) 0.3 17N (1) 0.3345 (1) 0.3657 (1) 0.5344 (1) 4.48 (8)C (1) 0.2507 (2) 0.3967 (2) 0.6440 (2) 5.7 (1)C (2) 0.2994 (2) 0.4206 (2) 0.5652 (1) 4.13 (9)C (3) 0.3 159 (2) 0.5001 (2) 0.5235 (1) 3.80 (8)C (4) 0.3693 (2) 0.5258 (2) 0.4520 (1) 3.80 (8)C (5) 0.4068 (2) 0.4698 (2) 0.4257 (1) 4.5 (1)C (6) 0.387 1 (2) 0.3908 (2) 0.4673 (2) 4.8 (1)C (7) 0.3091 (2) 0.2755 (2) 0.57 10 (1) 4.7 (1)C (8) 0.3766 (2) 0.2630 (2) 0.6086 (2) 5.3 (1)C (9) 0.3497 (2) 0.1761 (2) 0.6443 (2) 5.9 (1)C (10) 0.2568 (3) 0.103 1 (2) 0.6429 (2) 5.9 (1)C(11) 0.1910 (2) 0.1179 (2) 0.6042 (2) 6.4(1)C (12) 0.2165 (2) 0.2035 (2) 0.5576 (2) 5.6 (1)C (13) 0.2282 (3) 0.0102 (2) 0.6827 (2) 8.4 (2)173Table A6. Final atomic coordinates (fractional) and B (A2) for Ga(ptp)3•5.5H20.Atom x y z B occ.Ga (1) 1/3 2/3 0.48079 (3) 3.95 (2) 1/3o (1) 0.2835 (1) 0.5527 (1) 0.5469 (1) 4.51 (8)0(2) 0.3811 (1) 0.5978 (1) 0.4149 (1) 4.36 (7)0 (3A) 0.458 1 (4) 0.8353 (4) 0.8237 (3) 12.5 (3) 0.7580 (3B) 0.367 (2) 0.7 12 (1) 0.804 (1) 12.4 (7) 0.1800(4A) 0.4174 (5) -0.4174 1/4 13.2 (1) 0.3150 (4B) 0.350 (2) 0.563 (2) 0.25 1 (2) 12 (1) 0.2550 (4C) 0.458 1 (9) 0.4455 (9) 0.2375 (8) 12.8 (7) 0.330N (1) 0.3362 (2) 0.363 1 (2) 0.5348 (1) 4.60 (9)C (1) 0.25 17 (3) 0.3950 (2) 0.6426 (2) 6.0 (1)C (2) 0.3009 (2) 0.4184 (2) 0.5643 (2) 4.5 (1)C (3) 0.3 164 (2) 0.4973 (2) 0.5228 (2) 3.9 (1)C (4) 0.3694 (2) 0.5224 (2) 0.45 17 (2) 4.0 (1)C (5) 0.4067 (2) 0.4664 (2) 0.4262 (2) 4.7 (1)C (6) 0.3883 (2) 0.3876 (2) 0.4678 (2) 5.0 (1)C (7) 0.3 106 (2) 0.2732 (2) 0.57 17 (2) 4.8 (1)C (8) 0.3783 (2) 0.2616 (2) 0.6095 (2) 5.3 (1)C (9) 0.35 10 (3) 0.1748 (3) 0.6457 (2) 6.1 (2)C (10) 0.2585 (3) 0.1020 (2) 0.6437 (2) 6.1 (2)C (11) 0. 1928 (3) 0.1158 (2) 0.6044 (2) 6.6 (2)C (12) 0.2 174 (2) 0.2009 (2) 0.5680 (2) 5.9 (1)C (13) 0.23 10 (4) 0.0088 (3) 0.6840 (3) 8.9 (2)174Table A7. Bond lengths (A) with estimated standard deviations for M(ptp)35.5H20.DistanceAtomsM=Ga M=A1M(1)-O(1) 1.962 (2) 1.886 (2)M(1)-O(2) 1.996 (2) 1.922 (2)O(l)-C(3) 1.323 (3) 1.321 (3)O(2)-C(4) 1.302 (3) 1.306 (3)N(l)-C(2) 1.382 (3) 1.38 1 (3)N(l)-C(6) 1.352 (4) 1.350 (3)N(1)-C(7) 1.448 (4) 1.450 (3)C(1)-C(2) 1.498 (4) 1.491 (3)C(2)-C(3) 1.372 (4) 1.374 (3)C(3)-C(4) 1.419 (4) 1.418 (3)C(4)-C(5) 1.394 (4) 1.399 (3)C(5)-C(6) 1.359 (4) 1.355 (3)C(7)-C(8) 1.367 (4) 1.369 (4)C(7)-C(12) 1.380 (4) 1.374 (4)C(8)-C(9) 1.394 (4) 1.392 (4)C(9)-C(10) 1.374 (5) 1.38 1 (4)C(10)-C(11) 1.372 (5) 1.374 (4)C(10)-C(13) 1.512 (5) 1.500 (4)C(11)-C(12) 1.380 (5) 1.386 (4)175Table A8. Bond angles (deg) with estimated standard deviations forAngleAtomsM=Ga M=A1O(1)-M(1)-O(ly 90.65 (8) 90.42 (7)O(1)-M(1)-O(2) 83.15 (7) 84.41 (6)O(1)-M(1)-O(2)’ 95.04 (8) 94.67 (6)O(1)-M(1)-O(2)” 171.62 (8) 172.77 (6)O(2)-M(1)-O(2)’ 91.76 (8) 90.95 (7)M(1)-O(1)-C(3) 110.6 (2) 112.0 (1)M(1)-O(2)-C(4) 110.3 (2) 111.2 (1)C(2)-N(l)-C(6) 121.0 (2) 121.5 (2)C(2)-N(l)-C(7) 119.9 (2) 119.9 (2)C(6)-N(1)-C(7) 118.8 (2) 118.4 (2)N(1)-C(2)-C(1) 119.9 (3) 120.1 (2)N(1)-C(2)-C(3) 118.9 (3) 118.1 (2)C(1)-C(2)-C(3) 121.1 (3) 121.7 (2)O(1)-C(3)-C(2) 122.2 (2) 123.7 (2)O(1)-C(3)-C(4) 117.4 (2) 115.5 (2)C(2)-C(3)-C(4) 120.4 (2) 120.7 (2)O(2)-C(4)-C(3) 117.4 (2) 115.9 (2)O(2)-C(4)-C(5) 124.2 (3) 125.5 (2)C(3)-C(4)-C(5) 118.4 (2) 118.5 (2)C(4)-C(5)-C(6) 119.6 (3) 119.1 (2)N(1)-C(6)-C(5) 121.6 (3) 121.9 (2)N(1)-C(7)-C(8) 119.8 (3) 120.3 (2)N(1)-C(7)-C(12) 118.7 (3) 118.9 (2)176C(8)-C(7)-C(12) 121.5 (3) 120.8 (2)C(7)-C(8)-C(9) 118.4 (3) 118.9 (3)C(8)-C(9)-C(10) 121.4 (3) 121.4 (3)C(9)-C(1O)-C(11) 118.5 (3) 118.2 (3)C(9)-C(1O)-C(13) 120.1 (4) 121.1 (3)C(11)-C(10)-C(13) 121.4 (4) 120.7 (3)C(1O)-C(11)-C(12) 121.6 (3) 121.3 (3)C(7)-C(12)-C(1 1) 118.6 (3) 119.4 (3)* Here and elsewhere in this chapter, primed and double-primed atoms have coordinatesrelated to those in Tables AS and A6 by the symmetry operations: - & 1 - , ; and l-y,1 + - ; respectively.177


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