Di-μ-oxo Dimetal Core of MnIV and TiIV as a Linker Between Two Chiral Salen Complexes Leading to the Stereoselective Formation of Different M- and P‑Helical Structures

Because of restricted rotational freedom along the metal–metal axis, a di-μ-oxo dimetal core could be an excellent building block to create dinuclear compounds with well-defined stereochemistry, but their stereoselective synthesis remains a challenge. We herein report the formation of di-μ-oxo dimanganese­(IV) complexes with tetradentate salen ligands bearing different degrees of steric bulk, in order to study stereochemical aspects of the dimerization reaction that potentially generates multiple stereoisomers. X-ray crystallography shows that the di-μ-oxo dimanganese­(IV) complex with salen, where salen is (R,R)-N,N′-bis­(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine, adopts a unique structure in which two salen complexes are arranged in an M-helical fashion. According to the solution study using 1H, 2H NMR, and circular dichroism spectroscopies, the dimerization reaction is highly diastereoselective in the presence of the tert-butyl group at the 3/3′ position as a determinant steric factor. In contrast, the di-μ-oxo dititanium­(IV) complex with the same salen ligand was previously reported to afford an opposite P-helical dimer. The present DFT study clarifies that a less-covalent Ti–O bonding causes a distortion of the di-μ-oxo dititanium­(IV) core structure, generating a completely different framework for interligand interaction. The present study provides a solid basis to understand the stereochemistry for the formation of the di-μ-oxo dimetal core.