A microscopic model for helical twisting power by the optical isomers of an octahedral metal complex

A computational approach to the evaluation of helical twisting powers (HTP) of chiral metal complexes of [Ru(blade)₂(backbone)] type is presented. The dopant contains helically attached "blade" ligands and an elongated "backbone" ligand, and some remarkably powerful examples have been reported. In this work, the observed HTP is interpreted in terms of a microscopic interaction of a dopant and host molecules with atomistic details. For this purpose, the stable structure of a triad system comprising a dopant and two host molecules was obtained by geometry optimization using Gaussian03. As a result, the host molecules interacted attractively with the dopant, being twisted in the same direction as observed experimentally. Interaction energy was assessed as a function of the dihedral angle between the two host molecules, leading to a quadratic dependence with a minimum at the equilibrium twisting angle of -32°. Based on this, the expression was derived, in which helical twisting power was given in terms of the equilibrium twisting angle of a pair of strongly interacting host molecules.