Optically active transition metal complexes. Part 133 . Preparation, epimerization and crystallization of chiral-at-metal rhodium(III) half-sandwich complexes

Abstract

The complexes Cp*Rh(beapy)Cl ( 5) and Cp*Ir(beapy)Cl ( 6) were synthesized by reaction of [Cp*MCl 2] 2 (M=Rh, Ir) and the deprotonated ligand Hbeapy, the condensation product of benzylamine and 2-pyrrolcarbaldehyde. In each case two enantiomers ( S M) and ( R M) arise, differing only in the metal chirality. The CH 2 group of the benzyl substituent in the ligand forms an AB spectrum which was used for 1H NMR coalescence measurements ( T c=115.1 °C). The half-life for the racemization of Cp*Rh(beapy)Cl ( 5) was 46 ms at 115.1 °C. Substitution of the chloride ligand in Cp*Rh(pepy)Cl ( 3), pepy anion of Schiff base (+)-2- N-[( S)-1-phenylethyl]pyrrolcarbaldimine, afforded the complexes Cp*Rh(pepy)Br ( 7) and Cp*Rh(pepy)I ( 8). Single crystals of the two compounds contained only one of the two diastereomers. In solution the compounds epimerized via a change of the metal configuration, the half-life for ( R Rh, S C)-Cp*Rh(NN*)I ( 8) being 40.8 min at −40 °C in CD 2Cl 2 solution (obtained by time dependent integration of suitable 1H NMR signals). Reaction of [CpRhCl 2] 2 with (+)- N-[( S)-1-phenylethyl]salicylaldimine gave CpRh(pesa)Cl ( 9). Surprisingly, single crystals of 9 contained both diastereomers ( R Rh, S C) and ( S Rh, S C) in a 1:1 ratio following the molecular recognition motif of the inverted pianostools. The half-lifes for the epimerization of 9 with respect to the metal center, derived from 1H NMR coalescence experiments, were 31 and 9.2 ms at 11.3 °C (CD 2Cl 2) for the forward and back reactions, respectively.