Asymmetric Induction and Configurational Stability at the Metal Centre in Half-Sandwich (η6-p-Cymene)ruthenium(II) and (η5-C5Me5)rhodium(III) Complexes Containing Chiral N-N* Ligands with Different Rigidity and Flexibility

The half-sandwich chelate complexes [Ru(η6-p-cymene)(N-N*)Cl]X (X = Cl, PF6) and [Rh(η5-C5Me5)(N-N*)Cl]X (X = SbF6, PF6; N-N* = (Sa)-1 and (Sa)-2] have been prepared by treating the complexes [{Ru(η6-p-cymene)Cl2}2] and [{Rh(η5-C5Me5)Cl2}2] with the bidentate N-N* chiral ligands in their enantiomerically pure form. The ligands contain rigid 2-pyridinyl or 8-quinolinyl skeletons and the C2-symmetric chiral framework trans-2,5-dimethylpyrrolidinyl or (+)-(S)-2,2′-(2-azapropane-1,3-diyl)-1,1′-binaphthalene. The chelate complexes [Ru(η6-p-cymene)(N-N*)Cl]Cl and [Rh(η5-C5Me5)(N-N*)Cl]SbF6 [N-N* = (Sa)-1 and (Sa)-2] were obtained in CH3OH or CHCl3, with 100 % diastereomeric excess, in the absolute configurations (Sa,SRu) and (Sa,RRh), respectively; the ligands (R,R)-3 and (R,R)-4 gave the corresponding products as pairs of diastereomers with high de. A kinetic effect is present during the formation of the ruthenium and rhodium chelate complexes. The (Sa,RRh) absolute configuration of [Rh(η5-C5Me5)(N-N*)Cl]PF6 [N-N* = (Sa)-1 and (Sa)-2], which were obtained as a single diastereomer, was assigned by X-ray diffraction determination of their molecular structures. The nucleophilic substitution reaction of chloride by iodide in [Ru(η6-p-cymene)(N-N*)Cl]PF6 [N-N* = (Sa)-1 and (Sa)-2], in methanol at 328 K, occurs with retention of configuration at the metal centre, and a possible mechanism is proposed on the basis of kinetic measurements. The results indicate a striking analogy between the isoelectronic complexes [Ru(η6-p-cymene)(N-N*)Cl]PF6 and [Rh(η5-C5Me5)(N-N*)Cl]SbF6 containing the same N-N* chiral ligand. A rationalisation of the results is proposed on the basis of X-ray diffraction analysis and density functional calculations.