Synthesis and Reactivity of Copper ( I ) and Iron ( II ) Carboxylate - Bridged Dimetallic Complexes

Chapter 1. Synthesis and Characterization of a Novel Class of Dicopper(I) Bis(carboxylate)-Bridged Complexes The syntheses, spectroscopic properties, crystal and molecular structures, and bonding of several dicopper(I) bis(carboxylate)-bridged complexes are described in which the bridging dicarboxylate ligand is the dianion of m-xylylenediamine bis(Kemp's triacid imide) (H2XDK, 1). A sterically demanding benzyl derivative of H2XDK was prepared, H 2BXDK (6). Reaction of these two ligands as well as the propyl analog, H2PXDK (2), with thallium ethoxide provided dithallium salts T12L (L = XDK, 7; PXDK, 8; and BXDK, 9). Reaction of 7, 8, or 9 with excess CuBr(Me 2S) in CH 2Cl2 /MeCN afforded dinuclear acetonitrile adducts [Cu 2L(MeCN)] (L = XDK, 10; PXDK, 11; and BXDK, 12) in high yield and multigram quantities. The coordination chemistry of the Cu 2(XDK) platform was explored with a range of ancillary ligands. Treatment of 10, 11, or 12 with an excess of the specified neutral donor ligand provided complexes [Cu 2(XDK)(PPh 3)2] (13), [Cu 2(XDK)(2,6-Me 2PhNC) 3] (14a), [Cu2(XDK)(g-2,6-Me 2PhNC)(2,6-Me 2PhNC) 2] (14b), [Cu 2(XDK)(NB) 2] (15, NB = norbornene), [Cu 2(XDK)(tmeda)] (17), [Cu 2(PXDK)(tmeda)] (18), [Cu 2(BXDK)(tmeda)] (19), and [Cu(4,4'-Me 2bpy) 2][Cu(XDK)] (20). Reaction of 10 with an excess of cyclohexene resulted in the loss of the acetonitrile ligand, affording the parent unsubstituted complex [Cu 2(XDK)] (16). Attempts to prepare anionic carbon-bridged dicopper(I) complexes with alkyl or aryl lithium compounds or cyanide reagents resulted instead in extraction of one of the Cu(I) ions, affording (Et 4N)[Cu(PXDK)] (21) and [CuLi(XDK)(THF) 2] (22). Crystallographic chemical analysis of the complexes revealed linear two-coordinate, trigonal three-coordinate, and pseudotetrahedral four-coordinate copper(I), depending upon the composition, and variable degrees of Cu-Cu bonding (dcu-cu range, 2.5697(8)-3.4211(6) A). Chapter 2. Synthesis and Characterization of Cu(I)-Cu(II) Mixed-Valence and Cu(I)M(II) Heterodimetallic Bis(carboxylate-bridged) Complexes: Structural, Electrochemical, and Spectroscopic Investigations The synthesis, spectroscopic, electrochemical, and structural properties of a series of Cu(I)Cu(II) bis(carboxylate-bridged) complexes are described, as well as related investigations with Cu(I)M(II) (M = Fe and Zn) analogs, utilizing the dianionic m-xylylenediamine bis(Kemp's triacid imide) ligand system. Treatment of previously reported [Cu 2(XDK)(MeCN)] (1) or [Cu2(PXDK)(MeCN)] (2, PXDK = the propyl derivative of XDK) with one equivalent of silver(I) triflate, trifluoroacetate, or tetrafluoroborate in THF afforded mixed valence complexes [Cu2L(A-X)(THF) 2] (X = triflate, L = XDK, 4, or PXDK, 5; X = trifluoroacetate and L = XDK, 6); or [Cu 2L(THF) 4]X (X = tetrafluoroborate, L = XDK, 7, or PXDK, 8). Complex 8 was also prepared from equimolar (Et4N)[Cu(PXDK)] (3) and copper(II) triflate. Solid state structural investigations on 4, 6, and 8 revealed symmetric square pyramidal coordination environments about each copper atom and short Cu-Cu distances ranging from 2.3988(8)-2.4246(12) A., features which taken together imply significant metal-metal bonding character. The nature of the Cu-Cu bonding in this series of complexes was further interrogated by (a) comparative structural and ligand exchange studies with mixed-metal analogs [CuZn(PXDK)(OTf)(THF) 2(H20)] (9), [CuFe(PXDK)(OTf)(NB)MeCN)] 2 (10a, NB = norbornene), and CuZn(PXDK)(OTf)(NB)(H 20) (11), all of which exhibited longer metal-metal distances ranging from 3.294(2)-3.732(2) A and monodentate, terminal triflate ligation; (b) by variable temperature and field EPR studies, which showed that complexes 4-8 possess a fully delocalized electronic structure in the solid state and solution down to liquid helium temperatures; and (c) by molecular orbital calculations on simplified models of 4-8, which revealed a Cu-Cu bonding interaction in the SHOMO and SOMO, comprised mainly of a-type overlap between the dx2-y2 orbitals. In addition, complex 4 was shown by cyclic voltammetry to under go a chemically reversible and electrochemically quasireversible one-electron reduction that is markedly positive for a Cu(I)Cu(II) complex with an all-oxygen, dianionic donor set. Overall, we provide definitive evidence that, utilizing the XDK scaffold, an unusual series of class m mixed-valence dicopper complexes has been accessed. These studies should aid in understanding an electronically similar Cu-Cu bonded system at the biological CuA site. Chapter 3. Preparation of Sterically Hindered Diiron(II) Tetracarboxylate Complexes, and Their Reactivity Toward Dioxygen A family of diferric peroxo-bridged complexes has been prepared which matches exactly the combination of carboxylate and imidazole donors of the Hperoxo intermediate in sMMO. A variety of methods including UV-Vis, EPR, Resonance Raman, and M6ssbauer spectroscopies have been employed to elucidate their structures. These adducts have inequivalent iron coordination environments, a feature which is attributed either to an asymmetric peroxide bridge, a disproportionate arrangement of the ancillary carboxylate and nitrogen donor ligands about each iron center, or a combination of the two. Kinetic studies revealed that the rate of peroxo formation is first order in diiron(II) complex and 02, implicating a rate-determining bimolecular collision between the reaction components. Moreover, the rate of peroxo formation tracks inversely with the steric demands of the carboxylate substituents, implying that a carboxylate shift is associated with the rate-determining step. The oxygenation rate is affected much more dramatically by the basicity of the N-donor ligand, however, with a difference of >10 5 s-1 noted for the pseudo-first-order rate constants with 1-alkylimidazole versus pyridine ligation. Chapter 4. Biomimetic Oxidation Studies With Discrete Diiron(III) Bis(carboxylate)Bridged Peroxo Complexes: Toward Functional Models of Oxygen Intermediates of Soluble Methane Monooxygenase The reactivity properties of two diiron(III) peroxo complexes have been fully elucidated. These adducts behave as nucleophilic/basic peroxides, exhibiting no propensity to serve as oxygen atom donors for even potent acceptors such as triphenylphosphine. The 1-butylimidazole ligands in 1 enhance the nucleophilicity/basicity of the peroxo compared to the pyridine analog, probably because the former ligand is a superior a-donor. This feature imparts more electron density to the iron centers, which in turn is transmitted to the peroxo ligand. Thermolysis of the peroxo adducts in a variety of media results in solvent oxidation, providing ketone and alcohol products in modest yield based on the complex. A detailed analysis of cyclopentane oxidation provided evidence for a radical chain autoxidation mechanism, a reactivity manifold which does not mimic the hydrocarbon oxidation chemistry of sMMO. Thesis Supervisor: Stephen J. Lippard Title: Arthur Amos Noyes Professor of Chemistry This thesis is dedicated to my family for all your love, support and patience, and in memory of my grandfather, Roland C. LeCloux.