Monooxomolybdenum(VI) Complexes Possessing Olefinic Dithiolene Ligands: Probing Mo−S Covalency Contributions to Electron Transfer in Dimethyl Sulfoxide Reductase Family Molybdoenzymes

Abstract

A monooxo Mo(VI) model complex for the oxidized active site in the DMSOR family of molybdoenzymes has been synthesized and structurally characterized. The compound was obtained from the desoxo Mo(IV) derivative by clean oxygen atom transfer from an amine N–oxide in a manner similar to that observed in the enzyme. A combination of electronic absorption and resonance Raman spectroscopies, coupled with the results of bonding and excited state calculations, has been used to provide strong support for a highly covalent Mo(dxy)-S(dithiolene) π* bonding interaction in the Mo(VI) complex. It is proposed that the resulting Mo-S covalency facilitates electron transfer regeneration of the catalytically competent DMSOR Mo(IV) active site.

A Model for the Active-Site Formation Process in DMSO Reductase Family Molybdenum Enzymes Involving Oxido−Alcoholato and Oxido−Thiolato Molybdenum(VI) Core Structures

“…The absorption bands at 360 and 423 nm due to [Mo VI O 2 L 2 ] 2 − decrease in intensity with the concomitant appearance of the new absorption bands at 539 and 742 nm with a clear isosbestic point at 473 nm. The final spectrum exhibits absorption bands at λ max = 539 nm ( ε = 2430 M −1 cm −1 ) and 742 nm (2130), which are close to those observed for the oxido-siloxido-molybdenum(VI) complex, [Mo VI O(OSi i Pr 3 )L 2 ] − ( λ max = 563 nm ( ε = 4301 M −1 cm −1 ) and 729 (1280) in acetonitrile), 47,48 suggesting the formation of the expected product [Mo VI O( O Et)L 2 ] − . The titration curves based on absorbance changes at 360, 539, and 742 nm are shown in the inset of Figure 2a, which clearly indicate the 1 : 1 stoichiometry of [Mo VI O 2 L 2 ] 2 − : CSA.…”

“…55,47 Our bonding calculations show that a cis S(p z ) orbital on L B and an O(p) orbital from the alcoholate form d-p π bonding interactions with the Mo(d xy ) redox orbital which, for the Mo(VI) state, is the acceptor orbital (LUMO) in these low-energy LMCT transitions. The relevant Kohn-Sham orbitals for [Mo VI O( O Et)L 2 ] − are given in Figure S8.…”

“…The geometry around the Mo center is more distorted than the molybdenum(VI) center of [MoO 2 L 2 ] 2 − and is similar to that of (Et 4 N)[Mo VI O(OSi i Pr 3 )L 2 ], which we have reported previously. 26, 47 However, the Mo1–O2–C13 bond angle of 134.6(10)° is significantly smaller than the Mo–O–Si bond angle of 164.04(16)°. Furthermore, the O1 oxygen atom interacts with one of the acetone methyl groups through a hydrogen bond (3.16(3) Å, Figure S7).…”

Spectroscopic and Electronic Structure Studies of a Dimethyl Sulfoxide Reductase Catalytic Intermediate: Implications for Electron- and Atom-Transfer Reactivity

“…With respect to pyranopterin mediated electronic coupling between the Mo site and exogenous redox partners, recent spectroscopic studies of a small molecule analogue of oxidized DMSOR have provided evidence that a single pterin may serve as a conduit for electron transfer in the Mo(VI)→Mo(V) step, and this results from a single dithiolene sulfur donor contributing approximately 18% sulfur character to the redox-active orbital. 16 In this regard, it is noteworthy that for all DMSOR family enzymes that have been examined crystallographically and possess endogenous redox-active centers in addition to the molybdenum center, the nearest redox partner to the molybdenum lies to the Q pterin side of the center. Assuming the Q-pterin is indeed the physiological conduit for electron transfer regeneration in DMSOR, the U+1 orbital may play a significant role in the Mo(V)→Mo(IV) electron transfer process by contributing to an overall increase in electronic coupling between the Mo ion and the dithiolene component of the Q-pterin since this orbital possesses approximately five times the Q-pterin dithiolene sulfur character as the S0 orbital.…”

Molybdenum(VI) Dioxo and Oxo-Imido Complexes of Fluorinated β-Ketiminato Ligands and Their Use in OAT Reactions

“…14,15 Although the imido ligand is not found in the active sites of the enzyme families, such compounds represent examples of monooxo molybdenum(VI) compounds related to xanthine oxidase and DMSO reductase, which are limited in the literature. 16 −19 We found them to be stable toward dimerization, presumably due to the steric demand at the tert-butyl imido nitrogen, and to be active in the OAT reaction to trimethyl phosphine. Comparison of the oxo-imido to corresponding dioxo compounds allowed us to investigate the influence of the second doubly bonded ligand at molybdenum.…”