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

摘要

The complexes Cp*Rh(beapy)Cl (5) and Cp*Ir(beapy)Cl (6) were synthesized by reaction of [CP*MCl2](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 CH2 group of the benzyl substituent in the ligand forms an AB spectrum which was used for H-1 NMR coalescence measurements (T-c = 115.1 degreesC). The half-life for the racemization of Cp*Rh(beapy)Cl (5) was 46 ms at 115.1 degreesC. 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 degreesC in CD2Cl2 solution (obtained by time dependent integration of suitable H-1 NMR signals). Reaction of [CpRhCl2](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 H-1 NMR coalescence experiments, were 31 and 9.2 ms at 11.3 degreesC (CD2Cl2) for the forward and back reactions, respectively. (C) 2003 Elsevier Ltd. All rights reserved. [References: 23]