Building a Better Model for the Molybdenum Cofactor: A New Class of Molybdenum Dithiolene Complexes

The molybdenum cofactor (Moco) is comprised of a molybdenum or tungsten center,at least one dithiolene chelate, and a tricyclic pyranopterin ligand. It is present in all living things, and it participates in global carbon, nitrogen, and sulfur cycles. The exact role the ligand plays in enzymatic turnover is unknown, as crystal structures capture Moco at just one point in time. Models are therefore relied upon to replicate its structure and the chemistry it may display as it moves through catalytic cycles. While there are models in existence possessing molybdenum centers and dithiolenechelates, there are no reports of pyranopterin-substituted dithiolene complexes until this work. Four groups of molybdenum dithiolene complexes were synthesized using four different N-heterocyclic ligands as R substituents: [Tp*Mo(S/O)(S2BMOQO)]-,[Tp*Mo(S/O)(S2BMOPP)]-, [Tp*Mo(S/O)(S2BOQO)]-, and [Tp*Mo(S/O)(S2BOPP)]-. They are comprised of quinoxaline- (BMOQO and BOQO) and pterin-substituted (BMOPPand BOPP) dithiolene ligands possessing tertiary and secondary R’-OH groups. Two kinds of ring structures were observed, as dehydration and intramolecular cyclization of the alcohol group resulted in pyrrole ring formation, and treatment with triphenylphosphine resulted in pyran ring closure on the pyrazine ring of the quinoxaline or pterin ligands, as observed in S2BOQO, S2BMOPP, and S2BOPP. The energetically-preferred pyrrole ring form of S2BMOQO was found to exhibit fascinatingelectronic flexibility, resulting in LLCT in absorption studies and a significant!!! dithiolene fold angle for a Mo4+ species. The electron-withdrawing nature of bothpyran- and pyrrole-cyclized forms of the complexes studied was evidenced by large, positive shifts in Mo4+/5+ redox potentials during CV experiments, and shifts to higher Mo=O stretching frequencies in IR. Proton and carbon NMR were used to structurally characterize the complexes where possible, and X-ray crystallography confirmed not only the placement of the atoms in the Tp*Mo(O)(pyrrolo-S2BMOQO)complex, but revealed electronic asymmetry across the dithiolene chelate. In total, these four families of complexes make up a new generation of Moco models with structural and reactivity properties relevant to the enzyme active site