Influence of aromatic ring substituents and co-ligand on the binding mode of a phosphine–borane-stabilized carbanion; crystal structures of [[(Me 3Si) 2{Ph 2P(BH 3)}C]KL n] 2[L n = (OEt 2) 2, pmdeta; pmdeta = N, N, N′, N″, N″-pentamethyldiethylenetriamine

Abstract

The phosphine–borane adduct (Me 3Si) 2CHPPh 2(BH 3) ( 1) is readily prepared from the reaction between (Me 3Si) 2CHPCl 2 and 2 equiv of PhMgBr in refluxing THF, followed by treatment with BH 3·SMe 2. The reaction between 1 and MeK in cold diethyl ether proceeds cleanly to give [[(Me 3Si) 2{Ph 2P(BH 3)}C]K] ( 2), which may be crystallized in the presence of pmdeta to give the corresponding adduct [[(Me 3Si) 2{Ph 2P(BH 3)}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 [[(Me 3Si) 2{Ph 2P(BH 3)}C]K(OEt 2) 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 BH 3 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 η 5-aryl interaction; in 2a this contact is absent.