Bimetallic complexes: The fundamental aspects of metal-metal interactions, ligand sterics and application

Metal containing complexes have been used to catalyze various organic transformations for the past few decades. The success of several mononuclear catalysts led to transition metal catalysts used in pharmaceuticals, environmental, and industrial processes. While mononuclear complexes have been used extensively, bimetallic systems have received far less attention. Bimetallic or polynuclear sites are commonly found in metalloenzymes that perform elegant transformation in biological systems, underlying their significance. Inorganic chemists take inspiration from nature and design model bimetallic complexes to further study this cooperativity effect. A bimetallic platform offers many structural and functional differences such as the identity of the metal atoms and the bonding interactions between metals, which have been reflected in their unique catalytic ability and reactivity. This dissertation encompasses work related to the computational study of metal-metal interactions of bimetallic systems, the 1H NMR study of stereochemical and conformational changes in solution of N,Nʹ-diarylformamidines, the synthesis of dizinc formamidinate complexes, and the synthesis and catalytic ability of dicopper formamidinate complexes. In the first part, DFT calculations are used to study factors that influence metal-metal bond lengths in various complexes. Several experimentally obtained X-ray crystal structures were used as the basis for the study. Differences in metal-metal separations were investigated through various functionals, indicating the importance of charge, orbital interactions, and formal bond order. BH&HLYP SDD/aug-CC-PVDZ geometry optimizations of octahalodimetalate anions Tc2X8n- (X = Cl, Br; n=2, 3), Re2X82- (X = Cl, Br), and Mo2Cl84- reproduced M-M bond distance trends observed experimentally. The study demonstrated that the increase in σ and π bond strength resulted in the shortening in Tc-Tc bond distance from Tc2X82- to Tc2X83-, which was further supported by the short Mo-Mo bond in the Mo2Cl84- ion. This study was expanded further through the inclusion of [M2Cl4(PMe3)4]n+ (M = Tc, Re, n = 0-2) and [Mo2E4]n- (E = HPO4 or SO4, n = 2-4), allowing a systematic study on the role of charge on the metal atoms. PBEO SDD/aug-CC-PVDZ calculations revealed that both formal bond order and formal charge on the metal atoms dictate the trends in M-M bond strength. The second half of this dissertation focuses on the synthesis and characterization of bimetallic Zn- and Cu-formamidinate complexes. The stereochemical exchange of substituted N,Nʹ-diarylformamidines were studied through 1H NMR in various solvents. Alkyl substituents placed on the ortho positions were found to shift the isomeric equilibrium in solution through destabilization of the hydrogen-bond dimer evident in X-ray crystal structures. The Z-isomer of substituted N,Nʹ-diarylformamidines is observed in CDCl3, C6D6, and DMSO-d6 when the ligands feature significant steric hinderance. Similar ortho substituted N,Nʹ-diarylformamidines were also used to enforce steric interactions to limit the nuclearity of Zn-formamidinate complexes. Various dizinc formamidinate complexes were synthesized through direct and transmetalation routes. NMR and mass spectrometry were used alongside X-ray crystal structures to fully characterize the dizinc complexes. Dicopper formamidinates formed through a transmetallation route were synthesized and feature distinct short Cu…Cu separations thought to be brought about by metalophillic interactions. Preliminary results suggest catalytic ability of dicopper formamidinates in cyclopropanation and aziridination of styrene with various diazo compounds. The catalytic activity suggests the formation of dicopper carbene and nitrene intermediates, of which only few published experimentally observed examples exist in the literature