Reactions of Wilkinson's catalyst with pyridine; observation of rhodium complexes containing both pyridine and phosphine ligandst

Abstract

Dissolution of [RhCl(PPh₃)₃], Wilkinson's catalyst, in pyridine affords an efficient catalyst for the selective reductive coupling of aldehydes. The reaction requires a close balance of the hydrogenating power and alkyl coupling ability of the catalyst. The species present in the pyridine solution of Wilkinson's catalyst have been studied in order to probe the origin of this selectivity. Pyridine has been found to replace one or more of the phosphine ligands in the precursor complex to give complexes of the type [RhCl(py)_x(PPh₃)_3–x](py = pyridine, x = 1 or 2) and [Rh(py)_x,(PPh₃)_4–x]CI (x = 1, 2 or 3). The reactions of these complexes with hydrogen have been shown to give cis-dihydride complexes viaoxidative addition. The complex trans-[Rh(CO)(py)(PPh₃)₂] ClO₄has been synthesised and characterised spectroscopically and found not to react with H₂ at room temperature. Preliminary in situ NMR measurements have shown that under the conditions of the catalysis either phosphine or pyridine can be replaced by a CO ligand. This results in a marked decrease in the activity of the rhodium centre towards oxidative addition of hydrogen and in the stability, with respect to elimination of H₂, of the dihydride.