Synthesis and crystal structures of some cobalt halide derivatives containing alkyl or silanolato groups

The alkylcobalt halide (2) and the lithium silanolatocobaltates Li(THF)2(μ-Br)2Co(μ-OSiMe3)2Co(μ-Br)2Li(THF)2 (4) and [LiBr{Li(THF)}2{CoBr(OSiMe3)3}]2 (5) form centrosymmetrical halogen-bridged dimers in the crystalline state. Compound 4 shows bromide bridges between lithium and cobalt, and silanolato bridges between the cobalt atoms. Below 200 K there are significant antiferromagnetic interactions between the cobalt centres. Compound 5 crystallises in a novel structure in which two cubane-like fragments containing silanolato bridges are linked together through a Li2Br2 ring.