TRIMETHYLSILYLOXO COMPLEXES OF OXOMOLYBDENUM(V) COUPLED ELECTRON-ELECTROPHILE TRANSFER-REACTIONS

The complexes LMo(VI)O(2)X [L = hydrotris(3,5-dimethylpyrazol-1-yl)bor ate; X = Cl, Pr, NCS, OPh, SPh, SCH(2)Ph] are converted to air-stable complexes LMo(V)O(OSiMe(3))X by one-electron coupled electron-electrop hile transfer (CEET) reactions involving cobaltocene and the electroph ilic reagent Me(3)SiCl. These complexes may also be obtained from LMo( V)O(OH)X by reaction with Me(3)SiCl in the presence of base. LMo(V)O(O SiMe(3))(SCH(2)Ph) crystallises in space group P2(1)/n, with a = 8.526 (1) Angstrom, b = 23.141 (3) Angstrom, c = 16.499 (2) Angstrom, beta = 103.75 (12)degrees and Z = 4. The complex exhibits a distorted octah edral structure with a facially tridentate L ligand and mutually cis o xo [Mo = O = 1.675 (4) Angstrom], silyloxo [Mo-O = 1.932 (4) A] and th iolato [Mo-S = 2.398 (2) Angstrom] ligands. The detailed redox propert ies of LMo(V)O(OR)X (R = SiMe(3), alkyl, aryl) differ from those of LM o(V)O(OH)X. Centres [(MoO)-O-V(OR)] are candidates for the stable ''in hibited'' forms of certain molybdenum enzymes formed under conditions which apparently disfavour the catalytically active [(MoO)-O-V(OH)] ce ntres. In the coordinating solvent pyridine (py), both LMo(VI)O(2)(SPh ) and LMo(V)O(OSiMe(3))(SPh) are reduced in one-electron steps to stab le LMo(IV)O(py)(SPh). LMo(IV)O(py)(SR) complexes are also obtained fro m LMo(VI)O(2)(SR) (R = Ph, CH(2)Ph, CHMe(2)) via a two-electron oxygen atom transfer reaction with tertiary phosphines in pyridine. Conseque ntly, the Mo(IV) product is accessible via a concerted two-electron st ep or via two one-electron steps.