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

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)C(R)S-, 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 = alpha,beta-styrenedithiolate (R = phenyl), crystallises in the space group P2(1)/c with a = 13.217(4), b = 31.820(8), c = 14.534(7) Angstrom, beta = 113.8(2)degrees and Z = 4. The MoOS4 moiety is square-based pyramidal with the O atom at the apex [Mo = O, 1.700(5) Angstrom] 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 H-1 NMR resonances, infrared v(Mo=O) and v(C=C) stretching frequencies and the E-1/2 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.