Computational, Spectroscopic, and Electrochemical Studies of Molybdoenzyme and Hydrogenase Active Site Inspired Complexes

Abstract

Production of H2 as an alternate fuel source requires the discovery of new, efficient, and cheap catalysts. Extensive studies on the development of active site analogues of the [FeFe] hydrogenase enzyme have been carried out for this reason. In our studies, electrochemistry, photoelectron spectroscopy and density functional theory calculations were utilized to develop and study the catalytic mechanisms for many catalysts that produce molecular hydrogen. These techniques have also been used to determine the factors that influence the correlation between gas-phase ionizations and solution-phase oxidations for a series of Tp*Mo(V) complexes.Mechanistic studies were carried out on two hydrogenase-inspired catalysts: [(eta5-C5H5)Fe(CO)2]2 and (mu-1,2-benzenedithiolato)[Fe(CO)3]2. Electrochemistry and calculations indicate that both molecules enter into catalytic cycles from the reduction of the neutral procatalyst complexes. The molecules, after reduction, catalytically produce H2 from acids such as acetic acid and 4-tert-butylphenol through CECE mechanisms. During one of the studies, it was found that calculations and electrochemical simulations did not agree with the liturature pKa value for (eta5 C5H5)Fe(CO)2H, which is one of the proposed species in the catalytic mechanism. The pKa of (eta5-C5H5)Fe(CO)2H was redetermined experimentally and found to be 26.7.The aim of the third chapter is to make improvements on the catalytic activity of (mu-1,2-C6H4S2)[Fe(CO),3]2 by perturbing the electronic structure through ligand exchanges. Two different phosphine ligands, triphenyl phosphine and 1,3,5-triaza-7-phosphaadamantane, were successfully substituted for CO ligands on the (mu-1,2-benzenedithiolato)Fe2(CO)6 complex. The 1,2-benzenedithiolate ligand was also exchanged for 1,4-dimethyoxy-2,3-benzenedithiolate.The last chapter focuses on complexes of the general form Tp*MoO(OX)2 (where Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and (OX)2 = (OMe)2, (OEt)2, and (OnPr)2 for alkoxide ligands, and (OX)2 = O(CH2)3O, O(CH2)4O, and O[CH(CH3)CH2CH(CH3)]O for diolato ligands). Oxidation potentials and first ionization energies are shown to be highly sensitive to the number of carbon atoms present in the diolato and alkoxide ligands. A linear correlation between the solution-phase oxidation potentials and the gas-phase ionization energies resulted in an unexpected slope of greater than unity. Density functional theory calculations indicated that cation reorganization energies ranged from 0.15 - 0.51 eV, and this unique correlation was a result of the large differences in the cation reorganization energies.