Optically active transition metal complexes: Part 125. Preparation and epimerization of chiral-at-metal pentamethylcyclopentadienyl-rhodium(III) and iridium(III) half-sandwich complexes

Abstract

The optically active bidentate Schiff base ligands (+)-2- N-[( S)-1-phenylethyl]pyrrolcarbaldimine NN*H and (+)- N-[( S)-1-phenylethyl]salicylaldimine ON*H react with [Cp*MCl 2] 2 (M=Rh, Ir) to give the half-sandwich complexes Cp*Rh(NN*)Cl ( 1), Cp*Ir(NN*)Cl ( 2), Cp*Rh(ON*)Cl ( 3) and Cp*Ir(ON*)Cl ( 4). In all four cases two diastereomers ( R M, S C) and ( S M, S C) arise, differing only in the metal configuration. However, in each of the systems 1– 4, only the ( R M, S C) diastereomer crystallized. X-ray structure analyses established the absolute configuration in the crystals of all four complexes. In solution the compounds epimerize via a change of the metal configuration and approach equilibria ( R M, S C) ( S M, S C) in which for 1 and 2 the ( S M, S C) diastereomers dominate. Time dependent integration of 1H NMR signals of the complexes 1 and 2 revealed that the epimerization is a clean first-order reaction. The half-lives in CD 2Cl 2 solution at −50°C were 19.4 min for 1 and 30.6 min for 2. From the temperature dependence Δ H ≠ was determined as 62.4 kJ mol −1 for 1 and 44.3 kJ mol −1 for 2 and Δ S ≠ as 148 J K −1 mol −1 for 1 and 65 J K −1 mol −1 for 2. For the compounds 3 and 4, the kinetic data for the epimerization with respect to the metal center were derived from 1H NMR coalescence experiments: half lives 35 ms for 3 at 21±3° (CH 2Cl 2 solution) and 58 ms for 4 at −1.8±3°C (CHCl 3 solution). These epimerization studies show that the metal configuration is labile in complexes 1 and 2 and extremely labile in complexes 3 and 4.