Chiral‐at‐Metal Racemization of Tris‐Chelated Octahedral Complexes: A DFT Case Study of Al(acnac) 3

Abstract

ABSTRACT Configurationally stable chiral M(α‐chel) 3 complexes (α‐chel = asymmetric chelating ligand) play a central role in asymmetric catalysis, yet the fundamental processes governing their racemization remain poorly understood. Density functional theory calculations at the ωB97X‐D/def2tzvpp and ωB97X‐D/6‐311++G(d,p) levels were employed to investigate the racemization and isomerization mechanisms of Al(acnac) 3 as a prototypical M(α‐chel) 3 system. Of the mer and fac enantiomeric isomers the mer form is favored by 1.7 kcal mol −1 . Their isomerization is kinetically challenging with a 26.7 kcal mol −1 high‐energy d,l‐cis pathway. Racemization of mer ‐Al(acnac) 3 via either trans or d,l‐trans transition structures occur with slightly lower barriers of approximately 20 kcal mol −1 for both. Methyl substitution of all N‐chelate sites enhances the stabilization of the mer isomer over the fac isomer to 5.8 kcal mol −1 and inhibits access to the cis , trans , and d,l‐cis transition structures due to steric repulsion. This leaves the d,l‐trans pathway as the sole channel for Δ‐ mer ⇌ Λ‐ mer racemization, albeit with a very substantial barrier of 28.1 kcal mol −1 . These results establish a unified mechanistic and topological framework for racemization in M(α‐chel) 3 complexes and provide design principles for developing robust chiral‐at‐metal architectures.