Density Functional Theory Calculation on the Racemization Mechanism of Metal-Induced Axial Chirality: Axial Rotation vs Coordination Geometry Change

Axially chiral molecules exhibit atropisomerism, with racemization energy barriers typically governed by steric hindrance. This study investigates the racemization mechanism of metal-induced axial chirality in four-coordinate, square-planar binuclear Pt(II) complexes bearing one cyclometalated sym-tetraacetylethane bridging ligand and two cyclometalated 2-phenylpyridine ligands. Notably, the experimental racemization barrier (34.4 kcal/mol) of the complex featuring four bulky methyl groups at the ortho-positions of its chiral axis is lower than that of the classical axial chiral reference, 1,1'-binaphthol (40.5 kcal/mol), suggesting an alternative racemization pathway beyond a simple axial rotation. Given the relatively low bond dissociation energies of coordination bonds, we employed density functional theory to simulate a novel pathway involving a change in coordination geometry from four- to three-coordinate. The resulting three-coordinate intermediate, which contains a monodentate bridging ligand, can readily undergo single-bond rotation to form its enantiomer. The calculated energy barriers for this process range from 34.0 to 40.4 kcal/mol using various functionals and basis sets. Although these values are somewhat higher than most reported racemization barriers of chiral-at-metal complexes (<32 kcal/mol), they substantiate the feasibility of the proposed mechanism. Consequently, this study offers valuable insights for the rational design of chiral-at-metal complexes.