Systematic Investigation of Relationship between Strength of NH•••S Hydrogen Bond and Reactivity of Molybdoenzyme Models

A series of monooxomolybdenum(IV) and dioxomolybdenum(VI) complexes containing two intramolecular NH center dot center dot center dot S hydrogen bonds were synthesized and characterized by H-1 NMR, UV-visible, IR, and Raman spectroscopies, and electrochemical measurements. Elimination of the steric effects enabled the accurate and quantitative evaluation of NH center dot center dot center dot S hydrogen bonds. Strong correlations among the strength of the hydrogen bond, the strength of the Mo-VI=O bond, and the redox potential were clearly shown. The hydrogen bonds stabilize the intermediate in the reaction between the monooxomolybdenum(IV) and Me3NO, resulting in acceleration of the reaction or retardation of trans-cis rearrangement. The proposed intermediate and the reaction mechanism, discussed on the basis of theoretical calculations, provided a unified explanation of the reaction.