Trimethylsilyloxo complexes of oxomolybdenum(V)

The complexes LMo^VIO₂X L?=?hydrotris(3,5-dimethylpyrazol-1-yl)borate; X?=?Cl, Br, NCS, OPh, SPh, SCH₂Ph] are converted to air-stable complexes LMo^VO(OSiMe₃)X by one-electron coupled electron-electrophile transfer (CEET) reactions involving cobaltocene and the electrophilic reagent Me₃SiCl. These complexes may also be obtained from LMo^VO(OH)X by reaction with Me₃SiCl in the presence of base. LMo^VO(OSiMe₃)(SCH₂Ph) crystallises in space group P2₁/n, with a?=?8.526 (1) Å, b?=?23.141 (3) Å, c?=?16.499 (2) Å, β?=?103.75 (12)° 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) Å], silyloxo Mo–O?=?1.932 (4) Å] and thiolato Mo–S?=?2.398 (2) Å] ligands. The detailed redox properties of LMo^VO(OR)X (R?=?SiMe₃, alkyl, aryl) differ from those of LMo^VO(OH)X. Centres Mo^VO(OR)] are candidates for the stable "inhibited" forms of certain molybdenum enzymes formed under conditions which apparently disfavour the catalytically active Mo^VO(OH)] centres. In the coordinating solvent pyridine (py), both LMo^VIO₂(SPh) and LMo^VO(OSiMe₃)(SPh) are reduced in one-electron steps to stable LMo^IVO(py)(SPh). LMo^IVO(py)(SR) complexes are also obtained from LMo^VIO₂(SR) (R?=?Ph, CH₂Ph, CHMe₂) 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.