Mixed μ-phosphido or μ-thiolato μ-halo-dimolybdenum(III) compounds [Mo 2Cp 2(μ-SMe) 2(μ-X)(μ-Y)] (X = PPh 2, Y = Cl; X = SCH 3, Y = Br, I): Electrochemical and structural comparisons – The X-ray structure of [{Mo 2Cp 2Br(μ-O)(μ-SMe) 2} 2(μ-MoO 4)

Abstract

Mixed μ-phosphido or μ-thiolato μ-halo-dimolybdenum(III) compounds [Mo 2Cp 2(μ-SMe) 2(μ-X)(μ-Y)] (X = PPh 2, Y = Cl ( 1); X = SCH 3, Y = Br ( 3), I( 4)) have been studied. Syntheses and X-ray structures of the new bromo and iodo analogues of [Mo 2Cp 2(μ-SMe) 3(μ-Cl)] ( 2) are reported. While preparing 3 a side-product 5 was obtained and structurally characterised as the Mo 5 system [{Mo 2Cp 2Br(μ-O)(μ-SMe) 2} 2(μ-MoO 4)], containing a {Mo IV–Mo IV–O–(Mo VIO 2)–O–Mo IV–Mo IV} unit. The influence of the bridging groups on the structures and electrochemical behaviour of the complexes [Mo 2Cp 2(μ-SMe) 2(μ-X)(μ-Y)] 1– 4 has been investigated. In MeCN–[NBu 4][PF 6] the first oxidation of [Mo 2Cp 2(μ-SMe) 2(μ-PPh 2)(μ-Cl)] ( 1) is quasi-reversible, contrasting with the essentially irreversible first oxidation of the thiolate analogue [Mo 2Cp 2(μ-SMe) 3(μ-Cl)] ( 2) under similar conditions. The effects of lowering the temperature or increasing the scan rate on the oxidation of 1 were examined: the initial quasi-reversible oxidation at first became irreversible but at still higher scan rates or lower temperatures the oxidation was again quasi-reversible. This suggests that the oxidation of 1 in MeCN is followed by reversible coordination of acetonitrile to afford the species [Mo 2Cp 2(μ-SMe) 2(μ-PPh 2)(Cl)(MeCN)] + ( 7 +). Cyclic voltammetry of [Mo 2Cp 2(μ-SMe) 3(μ-Y)](Y = Br ( 3) or I ( 4)) showed two quasi-reversible diffusion-controlled oxidation steps in CH 2Cl 2–[NBu 4][PF 6] or thf–[NBu 4][PF 6]. In acetonitrile, the fast loss of the halide ligands results in the formation of [Mo 2Cp 2(μ-SMe) 3(MeCN) 2] + species.