Formation, Structure and Reactivity of Boryloxycarbyne Complexes of Group 6 Metals

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Abstract
Reaction of the diborane(4) B2(NMe2)2I2 with two equivalents of K[(η5‐C5H5)M(CO)3] (M=Cr, Mo, W) yielded the dinuclear boryloxycarbyne complexes [{(η5‐C5H5)(OC)2MCO}2B2(NMe2)2] (4 a, M=Mo; b, M=W; c, M=Cr), which were fully characterised in solution by multinuclear NMR methods. The Mo and W complexes 4 a, b proved to be kinetically favoured products of this reaction and underwent quantitative rearrangement in solution to afford the complexes [{(η5‐C5H5)(OC)2MCO}B(NMe2)B(NMe2){M(CO)3(η5‐C5H5)}] (5 a, M=Mo; b, M=W); 5 a was characterised by X‐ray crystallography in the solid state. Corresponding reactions of B2(NMe2)2I2 with only one equivalent of K[(η5‐C5H5)M(CO)3] (M=Mo, W) initially afforded 1:1 mixtures of the boryloxycarbyne complexes 4 a, b and unconsumed B2(NMe2)2I2. This mixture, however, yielded finally the diborane(4)yl complexes [(η5‐C5H5)(OC)3M{B(NMe2)B(NMe2)I}] (6 a, M=Mo; b, M=W) by [(η5‐C5H5)(OC)3M] transfer and rearrangement. Density functional calculations were carried out for 4 c and 5 a, b.