Investigation of molybdenum sulfur interactions via single crystal EPR and photoelectron spectroscopy: Implications to molybdoenzymes

Abstract

A series of MoS(n) complexes were studied to afford a better understanding of the factors which affect the electronic structures in these complexes. Molybdenum sulfur interactions are of interest because of their importance in biology, and because of the utility of such complexes as catalysts in the petroleum industry. The He I gas phase photoelectron spectra (PES) of the series LMo(E)(OR)₂ [(L=hydrotris(3,5-dimethylpyrazolyl)borate), (E=O, NO)] were explored as a point of reference to the analogous thiolate complexes. These studies show that the nitrosyl complexes (formally Mo(II)) are 0.8 eV more difficult to ionize than the oxo analogues (formally Mo(V)). This counterintuitive result is ascribed to the differing π interactions of the nitrosyl and oxo ligands. The first single crystal EPR study of an oxo-Mo(V) complex with a single diothiolene-type ligand, LMoO(bdt) [bdt = 1,2-benzenedithiol], is reported. The principal g tensor elements was found to be oriented in the MoS₂ plane nearly perpendicular to the Mo-O vector, with a magnitude of 2.004. This large g value is attributed to contributions from sulfur spin-orbit coupling. These findings suggest that the singly occupied molecular orbital (SOMO), which is expected to be primarily metal in character, contains a significant amount of S character. The HeI, HeII, and NeI PES of LMoE(SPh)₂ and LMoE(tdt) [E = O, NO, S; tdt = 3,4-toluenedithiol] show that the ligand E and the formal oxidation state of the metal have little effect on the position of the first ionization potential, suggesting that the Mo-S interaction is the dominant factor in the electronic structure. An empirical equation is developed to quantify the relative amount of sulfur character in the "metal" orbitals. The results of these studies suggest that thiolate donors act as "electronic buffers" to a Mo center because of the close energy matching between Mo 4d and S 3p orbitals. This energy matching can be invoked to explain the utility of MoS(n) complexes as effective biological and industrial catalysts.