The Origin of Relative Stability of Di-µ-oxo M–M Chiral Salen Complexes [M–M = Ti(IV)–Ti(IV), V(IV)–V(IV), Cr(IV)–Cr(IV), and Mn(IV)–Mn(IV)]: A Quantum-Chemical Analysis

Abstract Di-µ-oxo dimetal cores can form dinuclear complexes with definite stereochemistries. Recent studies describing the synthesis and evaluation of titanium(IV) and manganese(IV) di-µ-oxo dimetal chiral salen complexes prompted us to explore the corresponding vanadium(IV) and chromium(IV) complexes. We therefore modeled a series of di-µ-oxo dimetal complexes of titanium(IV), vanadium(IV), chromium(IV), and manganese(IV) with tetradentate salen ligands bearing different degrees of steric bulk. Quantum mechanical calculations were conducted to compare the stabilities and molecular structures of these complexes. The results of natural population analysis gave an insight into the charge distribution characteristics of the di-µ-oxo dimetal cores of these complexes. Our calculations showed that the di-µ-oxo dimetal complexes bearing the least sterically encumbered model ligand 1 preferred the M-helical form over the P-helical one. In the titanium(IV) and chromium(IV) complexes containing a practical chiral salen ligand 2, the P-helical form was found to be more stable than the corresponding M-form. In contrast, the corresponding vanadium(IV) and manganese(IV) complexes preferred to exist in the M-helical form. The degree of distortion of the di-µ-oxo dimetal core and the inter ligand interactions were key factors for determining the stability of these complexes.