REACTIONS OF WILKINSON CATALYST WITH PYRIDINE - OBSERVATION OF RHODIUM COMPLEXES CONTAINING BOTH PYRIDINE AND PHOSPHINE-LIGANDS

Dissolution of [RhCl(PPh3)3], Wilkinson's catalyst, in pyridine afford s an efficient catalyst for the selective reductive coupling of aldehy des. 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 re place 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]Cl (x = 1, 2 or 3). The reactions of thes e complexes with hydrogen have been shown to give cis-dihydride comple xes via oxidative addition. The complex trans-[Rh(CO)(py)(PPh3)2]ClO4 has been synthesised and characterised spectroscopically and found not to react with H-2 at room temperature. Preliminary in situ NMR measur ements have shown that under the conditions of the catalysis either ph osphine or pyridine can be replaced by a CO ligand. This results in a marked decrease in the activity of the rhodium centre towards oxidativ e addition of hydrogen and in the stability, with respect to eliminati on of H-2, of the dihydride.