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

Abstract

Some intrinsically chiral [Rh₆(CO)₁₄(μ,κ²-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(2-furyl)phosphine (7) shows no bridging capability. When x = 1 the initial products are the clusters [Rh₆(CO)₁₅(κ¹-PX)] which undergo spontaneous CO loss to form [Rh₆(CO)₁₄(μ,κ²-PX)]. The structures of the [Rh₆(CO)₁₅(κ¹-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(μ,κ²-Ph₂P(2-benzothienyl))] (8), [Rh₆(CO)₁₄(μ,κ²-Ph₂P(2-thienyl))] (9), [Rh₆(CO)₁₄(μ,κ²-PhP(2-thienyl)₂)] (10) and [Rh₆(CO)₁₄(μ,κ²-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(μ,κ²-PhP(2-thienyl)₂ cluster. The kinetics of formation of the bridged clusters from the monosubstituted [Rh₆(CO)₁₅(κ¹-PX)] clusters have been studied.