Influence of aromatic ring substituents and co-ligand on the binding mode of a phosphine–borane-stabilized carbanion; crystal structures of [[(Me3Si)2{Ph2P(BH3)}C]KLn]2[Ln = (OEt2)2, pmdeta; pmdeta = N,N,N′,N″,N″-pentamethyldiethylenetriamine]

The phosphine–borane adduct (Me3Si)2CHPPh2(BH3) (1) is readily prepared from the reaction between (Me3Si)2CHPCl2 and 2 equiv of PhMgBr in refluxing THF, followed by treatment with BH3·SMe2. The reaction between 1 and MeK in cold diethyl ether proceeds cleanly to give [[(Me3Si)2{Ph2P(BH3)}C]K] (2), which may be crystallized in the presence of pmdeta to give the corresponding adduct [[(Me3Si)2{Ph2P(BH3)}C]K(pmdeta)]2 (2a) [pmdeta = N,N,N′,N″,N″-pentamethyldiethylenetriamine]. Treatment of 1 with MeK, followed by crystallization from cold diethyl ether, gives [[(Me3Si)2{Ph2P(BH3)}C]K(OEt2)2]2 (2b). X-ray crystallography reveals that in both 2a and 2b the phosphine–borane-stabilized carbanion ligand binds to the metal centers via its BH3 hydrogen atoms; there are no short contacts between the potassium ions and the carbanion centers. In 2b the ligand also binds the potassium ion through an η5-aryl interaction; in 2a this contact is absent.