Synthesis of oxomolybdenum bis(dithiolene) complexes related to the cofactor of the oxomolybdoenzymes†

Abstract

A synthetic strategy is reported for a general route to asymmetric dithiolenes. This has been used for the generation of [MoO(dithiolene)₂]^2– complexes [dithiolene = ^–SC(H)CS^–, R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl, quinoxalin-2-yl or 2-(dimethylaminomethyleneamino)-3-methyl-4-oxopteridin-6-yl], which have been characterised by analysis, spectroscopy and electrochemistry. The prototypical compound [PPh₄]₂[MoO(sdt)₂]·EtOH, where sdt = α,β-styrenedithiolate (R = phenyl), crystallises in the space group P2₁/c with a = 13.217(4), b = 31.820(8), c = 14.534(7) Å, β = 113.8(2)° and Z = 4. The MoOS₄ moiety is square-based pyramidal with the O atom at the apex [MoO, 1.700(5) Å] and contains a cis geometry of the phenyl groups. The physical properties of all the compounds are consistent with a retention of the MoOS₄ centre and the variation in the dithiolene ¹H NMR resonances, infrared ν(MoO) and ν(CC) stretching frequencies and the E_½ values for the Mo^V–Mo^IV couple are rationalised by a consideration of the nature of the R substituent. The complexes with the pterin [2-amino-4(1H)-pteridinone] substituent have a particular relevance to the Mo centre in oxomolybdoenzymes, and are the closest structural models to date for these enzymes containing two molybdopterin ligands per metal.