A mononuclear molybdenum(V) mono-oxo biphenyl-2,2′-dithiolate complex in which the metal resides within a cleft formed by the ligands and that exhibits N-H^...S hydrogen bonding in the solid state

Abstract

The reaction of Mo(O)₂(acac)₂, H₂L (2,2′-dimercaptobiphenyl), and NEt₃ produced the mononuclear Mo(V) complex Et₃NH[Mo(O)(L)₂] (1). Molybdenum mono-oxo tetrathiolate complexes such as 1 are studied as potential structural or functional models for pyranopterin-containing molybdoenzymes. Complex 1 has been crystallographically characterized. The solid-state structure reveals that the molybdenum ion sits within a cleft formed by the biphenyl backbone of the ligands, providing some steric protection. In addition, there is a hydrogen bond between the amine hydrogen of [Et₃NH]⁺ and one of the thiolate sulfur atoms. A difference in solution reactivity between 1 and a derivative without a hydrogen-bonding counterion suggests that hydrogen bonding occurs in solution also. There are two short S-S contacts and small S-Mo-S angles in the structure of 1 that may reflect a slight bonding interaction. Such short S-S distances and small angles have been found in a couple of other Mo-thiolate complexes and in many of the molybdoenzyme crystal structures. Further characterization of 1 by EPR, IR, and UV-vis spectroscopies, as well as by cyclic voltammetry, is discussed and compared to known Mo(V)-oxo-tetrathiolate complexes as well as to relevant molybdoenzyme data. Reactions to generate Mo(VI) complexes from 1 resulted in net oxidation at the ligand to form its disulfide derivative, which dissociated from the metal center. This result suggests that modifications to the ligand to prevent this process are needed.