Chiral hexarhodium carbonyl clusters containing heterobidentate phosphine ligands; a structural and reactivity study

Some intrinsically chiral [Rh6(CO)14((mu),(kappa)^2-PX)] clusters have been synthesized, beginning with reactions of [Rh6(CO)16–x(NCMe)x](x= 1, 2) with PX, where PX represents the bidentate bridging ligands diphenyl(benzothienyl)phosphine (1), diphenyl(2-thienyl)phosphine (2), di(2-thienyl)phenylphosphine (3), tris (2-thienyl)phosphine (4), diphenyl(2-pyridyl)phosphine (5) and diphenylvinylphosphine (6). The ligand tris(2furyl)phosphine (7) shows no bridging capability. When x= 1 the initial products are the clusters [Rh6(CO) 15((kappa)^1-PX)] which undergo spontaneous CO loss to form [Rh6(CO)14((mu),(kappa)^2-PX)]. The structures of the [Rh6(CO)15((kappa)^1-PX)] clusters have been elucidated by IR, NMR spectroscopy and FAB-MS spectrometry, and have been found to involve phosphorus atom coordination to a rhodium atom. In addition, the solid state structures of the [Rh6(CO)14((mu),(kappa)^2-Ph2P(2-benzothienyl))](8), [Rh6 (CO)14((mu),(kappa)^2-Ph2P(2-thienyl))](9), [Rh6(CO)14((mu),(kappa)^2-PhP(2-thienyl)2)](10) and [Rh6 (CO)14((mu),(kappa)^2-Ph2P(pyridyl))](12) clusters have been determined by X-ray crystallography. The various types of chirality exhibited by these clusters are discussed. A simple model is proposed to account for the ratios of stereochemical isomers found in the Rh6(CO)14((mu),(kappa)^2-PhP(2-thienyl)2 cluster. The kinetics of formation of the bridged clusters from the monosubstituted [Rh6(CO)15((kappa)^1-PX)] clusters have been studied.