Selective formation of cis-diacyl, cis-PPh₂R rhodium(III) complexes by the reaction of rhodium(III) cis-diacyl, trans-PPh₂R complexes with aliphatic diamines

The cis-diacyl, trans-PPh₂R complex [RhCl(PPh₂(o-C₆H₄CO))₂(pyridine)] (1) reacts with substituted aliphatic diamines to afford selectively cationic cis-diacyl, cis-PPh₂R, diamine derivatives [Rh(PPh₂(o-C₆H₄CO))₂(NN′)]⁺ (NN′ = 1,2-diphenylethylenediamine, 2; 1,2-propanediamine, 3; N-methylethylenediamine, 4; N,N-dimethylethylenediamine, 5; N,N′-dimethylethylenediamine, 6; N,N,N′-trimethylethylenediamine, 7) with high stereoselectivity depending on the N-donor ligand employed. Complexes 2 and 3 contain a single isomer, while 4 is a mixture of two isomers, 4a and 4b. Formation of 4a occurs first and is followed by isomerisation to 4b until the equilibrium 4a:4b = 1:4 ratio is attained. In contrast, 5 and 6 contain a single isomer. More basic amino groups prefer positions trans to an acyl group while less basic amino groups are trans to a phosphine group. The preferred intramolecular N—HO hydrogen bond formation between an amino and an acyl coordinated ligands, trans to the phosphorus atoms, appears to be relevant to the selectivity observed. 7 is a mixture of two isomers 7a and 7b in a 7a:7b = 5.7:1 ratio. N,N,N′,N′-tetramethylethylenediamine or N,N′-diphenylethylenediamine led to the elimination of the N-donor ligands and the formation of a mixture of isomers of [Rh₂(μ-Cl)(μ-PPh₂(o-C₆H₄CO))₂(PPh₂(o-C₆H₄CO))₂]⁺ (8), where the Rh atoms are triply bridged by two acyl groups in a head-to-tail arrangement and by a chloride. The reaction of [Rh(PPh₂(o-C₆H₄CO))₂(ndmeen)]ClO₄ (5) with acids led to the displacement of the diamine and the formation of a [8a]⁺:[8b]⁺:[8c]⁺ = 1:1:3 mixture. 8c, containing the weakest σ-donor oxygen atoms trans to the strongest σ-donor acyl groups, represents the most electronically favourable geometry for 8. All the complexes were fully characterized spectroscopically. Single crystal X-ray diffraction analysis was performed on 5, 6, 8a and 8b.