Chiral‐at‐Metal Racemization Unraveled for MX₂(a‐chel)₂ by means of a Computational Analysis of MoO₂(acnac)₂

Abstract

Octahedral chiral‐at‐metal complexes MX2(a‐chel)2 (a‑chel = asymmetric chelate) can rearrange their ligands by four mechanisms known as the Bailar (B), Ray‐Dutt (RD), Conte‐Hippler (CH), and Dhimba‐Muller‐Lammertsma (DML) twists. Racemization involves their interconnections, which were computed for MoO2(acnac)2 (acnac = β‐ketoiminate) using density functional theory at ωB97x‐D with the 6‐31G(d,p) and 6‐311G(2d,p) basis sets and LANL2DZ for molybdenum. Racemizing the cis(NN) isomer, being the global energy minimum with trans oriented imine groups, is a three step process (DML‐CH‐DML) that requires 17.4 kcal/mol, while all three cis isomers (cis(NN), cis(NO), and cis(OO)) interconvert at ≤ 17.9 kcal/mol. The B and RD twists are energetically not competitive and neither are the trans isomers. The interconnection of all enantiomeric minima and transition structures is summarized in a graph that also visualizes valley ridge inflection points for two of the three CH twists. Geometrical features of the minima and twists are given. Lastly, the influence of N‐substitution on the favored racemization pathway is evaluated. The present comprehensive study serves as a template for designing chiral‐at‐metal MX2(a‐chel)2 catalysts that may retain their chiral integrity.