Trimethylsilyloxo complexes of oxomolybdenum(V) coupled electron-electrophile transfer reactions

The complexes LMo(VI)O(2)X [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; 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-electrophile transfer (CEET) reactions involving cobaltocene and the electrophilic 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(OSiMe(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 octahedral structure with a facially tridentate L ligand and mutually cis oxo [Mo = O = 1.675 (4) Angstrom], silyloxo [Mo-O = 1.932 (4) A] and thiolato [Mo-S = 2.398 (2) Angstrom] ligands. The detailed redox properties of LMo(V)O(OR)X (R = SiMe(3), alkyl, aryl) differ from those of LMo(V)O(OH)X. Centres [(MoO)-O-V(OR)] are candidates for the stable ''inhibited'' forms of certain molybdenum enzymes formed under conditions which apparently disfavour the catalytically active [(MoO)-O-V(OH)] centres. 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 stable LMo(IV)O(py)(SPh). LMo(IV)O(py)(SR) complexes are also obtained from 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. Consequently, the Mo(IV) product is accessible via a concerted two-electron step or via two one-electron steps.