Rhodium(I) complexes of 8-methyl-, 8-ethyl-, and 8-isopropyl-quinolines and related 2-substituted derivatives

Abstract

The complexes [Rh(CO)(PPh₃)₂L][ClO₄] have been prepared from [Rh(CO)(PPh₃)₂(CH₃COCH₃)][ClO₄] on adding unidentate L (8-methyl-, -ethyl-, or -isopropyl-quinoline) or bidentate L (1,10-phenanthroline or quinoline-2-carboxaldehyde-N-methylimine). 8-Alkyl-substitution of the latter ligand gives the complexes [Rh(CO)-(PPh₃)₂L][ClO₄] as isomers with uni- and bi-dentate L respectively with relative stabilities of the unidentate form depending on the 8-substituent in the order Prⁱ > Et > Me. Unidentate L is also favoured by replacing PPh₃ by P(C₆H₁₁)₃, but bidentate co-ordination is found using AsPh₃ or PMe₂Ph. Equilibrium constants for chlorobridge cleavage of [Rh₂Cl₂(CO)₄] by 8-substituted and 2,8-disubstituted quinolines are reduced by increasing the size of the 8-substituent and even more by further introduction of a 2-methyl group. The contributions of distortions in the co-ordination to these changes in equilibrium constants are discussed.