Asymmetric Induction and Configurational Stability at the Metal Centre in Half‐Sandwich (η 6 ‐ p ‐Cymene)ruthenium(II) and (η 5 ‐C 5 Me 5 )rhodium(III) Complexes Containing Chiral N‐N* Ligands with Different Rigidity and Flexibility

The half‐sandwich chelate complexes [Ru(η 6 ‐ p ‐cymene)(N‐N*)Cl]X (X = Cl, PF 6 ) and [Rh(η 5 ‐C 5 Me 5 )(N‐N*)Cl]X (X = SbF 6 , PF 6 ; N‐N* = ( S a )‐ 1 and ( S a )‐ 2 ] have been prepared by treating the complexes [{Ru(η 6 ‐ p ‐cymene)Cl 2 } 2 ] and [{Rh(η 5 ‐C 5 Me 5 )Cl 2 } 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 C 2 ‐symmetric chiral framework trans ‐2,5‐dimethylpyrrolidinyl or (+)‐( S )‐2,2′‐(2‐azapropane‐1,3‐diyl)‐1,1′‐binaphthalene. The chelate complexes [Ru(η 6 ‐ p ‐cymene)(N‐N*)Cl]Cl and [Rh(η 5 ‐C 5 Me 5 )(N‐N*)Cl]SbF 6 [N‐N* = ( S a )‐ 1 and ( S a )‐ 2 ] were obtained in CH 3 OH or CHCl 3 , with 100 % diastereomeric excess, in the absolute configurations ( S a , S Ru ) and ( S a , R Rh ), 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 ( S a , R Rh ) absolute configuration of [Rh(η 5 ‐C 5 Me 5 )(N‐N*)Cl]PF 6 [N‐N* = ( S a )‐ 1 and ( S a )‐ 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(η 6 ‐ p ‐cymene)(N‐N*)Cl]PF 6 [N‐N* = ( S a )‐ 1 and ( S a )‐ 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(η 6 ‐ p ‐cymene)(N‐N*)Cl]PF 6 and [Rh(η 5 ‐C 5 Me 5 )(N‐N*)Cl]SbF 6 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)