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.

New dioxo–molybdenum(VI) complexes, (Et4N)(Ph4P)[MoVIO2(S2C2(CO2Me)2)(bdt)] (2) and (Et4N)(Ph4P)[MoVIO2(S2C2(CO2Me)2)(bdtCl2)] (4) (S2C2(CO2Me)2 = 1,2–dicarbomethoxyethylene–1,2–ditholate, bdt = 1,2–benzenedithiolate, bdtCl2 = 3,6–dichloro–1,2–benzenedithiolate), that possess at least one ene–1,2–dithiolate ligand were synthesized by the reaction of their mono–oxo–molybdenum(IV) derivatives, (Et4N)2[MoIVO(S2C2(CO2Me)2)(bdt)] (1) and (Et4N)2[MoIVO(S2C2(CO2Me)2)(bdtCl2)] (3), with Me3NO (Chart 1). Additionally, the bis(ene–1,2–dithiolate)MoVIO2 complex, (Et4N)(Ph4P)[MoVIO2(S2C2(CO2Me)2)2] (6), was isolated. Complexes 2, 4, and 6 were characterized by elemental analysis, negative–ion ESI mass spectrometry, and IR spectroscopy. X-ray analysis of 4 and 6 revealed a MoVI center that adopts a distorted octahedral geometry. Variable temperature 1H NMR spectra of (CD3)2CO solutions of the MoVIO2 complexes indicated that the Mo centers isomerize between Δ and Λ forms. The electronic structures of 2, 4, and 6 have been investigated by electronic absorption and resonance Raman spectroscopy and bonding calculations. The results indicate very similar electronic structures for the complexes and considerable π–delocalization between the MoVIO2 and ene–1,2–dithiolate units. The similar oxygen atom transfer kinetics for the complexes results from their similar electronic structures.

The carbomethoxy substituted dithiolene ligand (L(COOMe)) enabled us to develop a series of new bis(ene-1,2-dithiolato)tungsten complexes including W(IV)O, W(IV)(OSiBuPh(2)), W(VI)O(2), W(VI)O(OSiBuPh(2)) and W(VI)O(S) core structures. By using these tungsten complexes, a systematic study of the terminal monodentate ligand effects has been performed on the structural, spectroscopic properties and reactivity. The structure and spectroscopic properties of the tungsten complexes have also been compared to those of the molybdenum complexes coordinated by the same ligand to investigate the effects of the metal ion (W vs. Mo). X-ray crystallographic analyses of the tungsten(IV) complexes have revealed that the tungsten centres adopt a distorted square pyramidal geometry with a dithiolene ligand having an ene-1,2-dithiolate form. On the other hand, the dioxotungsten(VI) complex exhibits an octahedral structure consisting of the bidentate L(COOMe) and two oxo groups, in which π-delocalization was observed between the W(VI)O(2) and ene-1,2-dithiolate units. The tungsten(IV) and dioxotungsten(VI) complexes are isostructural with the molybdenum counter parts. DFT calculation study of the W(VI)O(S) complex has indicated that the W=S bond of 2.2 Å is close to the bond length between the tungsten centre and ambiguously assigned terminal monodentate atom in aldehyde oxidoreductase of the tungsten enzyme. Resonance Raman (rR) spectrum of the W(VI)O(S) complex has shown the two inequivalent L(COOMe) ligands with respect to their bonding interactions with the tungsten centre, reproducing the appearance of two ν(C=C) stretches in the rR spectrum of aldehyde oxidoreductase. Sulfur atom transfer reaction from the W(VI)O(S) complex to triphenylphosphines has also been studied kinetically to demonstrate that the tungsten complex has a lower reactivity by about one-order of magnitude, when compared with its molybdenum counterpart.

Oxo-sulfido- and oxo-selenido-molybdenum(VI) complexes with an ene-1,2-dithiolate ligand are generated as models of the active sites of molybdenum hydroxylases. The sulfide and selenide groups are highly reactive toward triphenylphosphine in the order of Se > S.

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