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

Abstract

AbstractThe 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)