A new series of bis(ene-1,2-dithiolato)tungsten(IV), -(V), -(VI) complexes as reaction centre models of tungsten enzymes: Preparation, crystal structures and spectroscopic properties

The carbomethoxy substituted dithiolene ligand (L-COOMe) enabled us to develop a series of new bis(ene-1,2- dithiolato) tungsten complexes including (WO)-O-VI, W-IV(OSiBuPh2), (WO2)-O-VI, (WO)-O-VI(OSiBuPh2) and (WO)-O-VI(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 p-delocalization was observed between the (WO2)-O-VI 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 (WO)-O-VI(S) complex has indicated that the W=S bond of 2.2 angstrom 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 (WO)-O-VI(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 upsilon(C=C) stretches in the rR spectrum of aldehyde oxidoreductase. Sulfur atom transfer reaction from the (WO)-O-VI(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.