INVESTIGATION OF THE ENERGY OF THE DITHIOLENE FOLDING INTERACTION IN METALLOCENE-­DITHIOLENE COMPLEXES USING SPECTROSCOPIC AND COMPUTATIONAL ANALYSIS

Abstract

The studies presented in this dissertation focus on elucidating key aspects of the interaction of metals and dithiolenes. The interaction is probed in a series of metallocene-dithiolene compounds (CpML where Cp = cyclopentadienyl, M = Mo, V, Ti and L = benezedithiolato and quinoxalinedithiolato) that have relatively simple electronic structures. The straightforward electronic structures were selected for assigning spectral features and correlating changes in electronic structure with changes in geometry, specifically the dithiolene fold angle. The experimental methods used to investigate the electronic structures include gas-phase photoelectron spectroscopy, X-ray absorption spectroscopy (XAS), resonance Raman (rR), and cyclic voltammetry (CV). Results from the experiment were supported by computational modeling with density functional theory.Results from the first part of the dissertation attempt to quantify the orbital interaction energy of the metal and dithiolene by comparing gas-phase X-ray and UV photoelectron (XPS/UPS) ionization energies. However, it was found that the metallocene compounds exhibit significant mixing of cyclopentadienyl orbital character with the frontier metal and dithiolene orbitals, which affects the orbital energies. Though unexpected, the implications for observing mixing between the dithiolene and other ligand orbitals can impact the understanding of Mo enezyme active sites as well as other synthetic systems.The next set of experiments looked at the effects of altering the electronic nature of the dithiolene ligand. The effect on the orbital energies of the molecules was probed by gas-phase UPS and CV. These results show no overall effect on the interaction between the dithiolene and the metal despite definite differences in the ligand electronic structure. Further experiments to probe the metal-ligand covalency using XAS also showed little change in the metal-ligand interaction.Finally, the relationship between geometric and electronic structure was investigated by comparing results from UPS vibrational structure with those from rR data. These data sets suggest that a metal-sulfur core breathing mode maybe active in electron transfer to and from the metal center in these types of compounds. The vibrational mode assigned to a dithiolene folding motion was shown to have a significant dependence on the metal electron count of the complexes as expected from basic molecular orbital theory.