Conformational analysis of cationic (R,S)- and (R,R)-(salen)manganese complexes possessing axial chirality as a chiral element based on x-ray crystallography: an explanation of the effect of apical ligand on enantioselection by (salen)manganese catalyst

Abstract

The structures of (R,S)- and (R,R)-(salen)manganese(III) complexes [(R,S)-2a,b and (R,R)-2a,b] possessing axial chirality as one of their two chiral elements were unambiguously determined by X-ray crystallographic analysis. The basal salen ligands of the complexes were demonstrated to adopt a stepped conformation but the degree of their non-planarity varied with the apical ligands and the relative stereochemistry of two chiral elements of the complexes. From these structural analyses, it was indicated that two factors, the chirality of the conformation of the five-membered chelate ring and the OH-π interaction between the apical aqua ligands and the 2''-phenyl group in the C3-naphthyl substituent, dictates the ligand conformation and, in turn, influences the asymmetric induction by (R,S)- and (R,R)-Mn-salen complexes.