Oxo–tungsten bis-dithiolene complexes relevant to tungsten centres in enzymes

Abstract

The reaction of [WO₂(CN)₄]^4– with a series of dimethylamino- or thione-protected asymmetric dithiolenes (ene-1,2-dithiolates) has been used to synthesise [WO(dithiolene)₂]^2– complexes (dithiolene = ^–SC(H)C(R)S^–, R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl or quinoxalin-2-yl), which have been characterised by elemental analysis, spectroscopy and electrochemistry. The prototypical compound [PPh₄]₂[WO(sdt)₂]·EtOH [sdt = styrenedithiolate, ^–SC(H)C(Ph)S^–], crystallises in space group P2₁/c with a = 13.242(4), b = 31.894(8), c = 14.589(5) Å, β = 113.80(2)°, and Z = 4. The WOS₄ moiety is square-based pyramidal with the O atom at the apex [WO 1.724(7) Å, W–S_av 2.37 Å, O–W–S_av 108.7°] and contains a cis geometry of the phenyl groups, although ¹H NMR studies indicate that both cis and trans isomers are present in the isolated solid. The physical properties of all of the [WO(dithiolene)₂]^2– complexes are consistent with a retention of the W^IVOS₄ centre and each system undergoes a reversible electrochemical one-electron oxidation to the corresponding W^V system. The variation in the dithiolene ¹H NMR resonances, ν(WO) and ν(CC) (dithiolene) IR stretching frequencies, and the E_½ values for the W^V–W^IV couple can be rationalised by a consideration of the nature of the aryl substituent. These data are compared to those of the analogous Mo^IV complexes and to those of the catalytic centres of the tungstoenzymes and their oxomolybdoenzyme counterparts.