Reactivity of quadruply bonded dimolybdenum(II) complexes with alkyl isocyanides: Mo-Mo bond cleavage or retention?

Abstract

The quadruply-bonded dimolybdenum(II) complexes Mo 2(μ-dppm) 2X 4 (X = Br or I) can be prepared in high yield from Mo 2(μ-dppm) 2Cl 4 (dppm = Ph 2PCH 2PPh 2) via simple halide-exchange reactions (using NaX in acetone). The complexes Mo 2(μ-dppm) 2X 4 (X = Cl, Br or I) react with one equivalent of RNC (R = i-Pr or t-Bu) in the presence of TlPF 6 (in THF) or KPF 6 (in acetone) to give [Mo 2X 3(dppm) 2(CNR)]PF 6, the first examples of multiply bonded Mo 4+ 2 complexes that contain coordinated π-acceptor RNC ligands. The reactions of Mo 2(μ-dppm) 2X 4 with an excess of RNC lead to seven-coordinate mononuclear cations [MoX(dppm)(CNR) 4] +, which are in turn converted into [Mo(CNR) 5(dppm)] 2+ and thence [Mo(CNR) 7] 2+. The significance of these results, insofar as they relate to the cleavage of mixed halide-phosphine complexes of dimolybdenum(II) by isocyanides and the substitution chemistry of [Mo(CNR) 7](PF 6) 2 by phosphine ligands, are discussed. Comparisons are made with related chemistry of dinuclear and mononuclear molybdenum(II) complexes that contain bidentate nitrogen donors such as 2,2′-bipyridine, 1,10-phenanthroline and 1,4-disubstituted diaza-1,3-butadienes. The reactions between cis-Mo 2(mhp) 2Cl 2(PMePh 2) 2 (mhp is the anion of 2-hydroxy-6-methylpyridine) and t-BuNC (in the presence of KPF 6) result in the formation of mixtures of Mo 2(mhp) 4 and [Mo(CN- t-Bu) 5(PMePh 2) 2](PF 6) 2