Conversion of a stannylphosphenium complex into a stannylenephosphine complex of Mo and W: double bond migration from MP to MSn

Four-legged piano-stool complexes, Cp(CO)2(SnR3)M{Me(OMe)}, containing a stannyl group (SnPh3, SnMe3, SnnBu3) and diamino-substituted phosphite (Me(OMe) which is abbreviated as PNN(OMe)) were prepared for Mo and W, and the reactions of these complexes with a Lewis acid (BF3·OEt2 and TMSOTf) were examined. For the SnPh3 and SnMe3 complexes, stannylene complexes [Cp(CO)2(SnR2)M{PNN(R)}]X (X=OTf− or BF4−) were formed with migration of an R group from Sn to P. X-ray analyses of [Cp(CO)2(SnPh2)W{PNN(Ph)}]OTf and [Cp(CO)2(SnMe2)W{PNN(Me)}]OTf reveal that these complexes are regarded as stannylene tungsten complexes stabilized by an oxygen of OTf−. In the reaction of the SnnBu3 complex with TMSOTf, the corresponding stannylene complex and an unexpected neutral phosphenium complex, Cp(CO)2M{PNN}, were formed. However, when BF3·OEt2 was used in place of TMSOTf, cationic phosphenium complexes in addition to a neutral phosphenium complex were observed. In any case, an OMe anion abstraction by a Lewis acid takes place at the first stage to give a cationic phosphenium complex, and then aryl or alkyl migration takes place to give a stannylene complex. A relatively strong SnnBu bond retards the alkyl migration, resulting in the observation of the cationic phosphenium complexes.