Di-μ-oxo Dimetal Core of Mn~(IV) and Ti~(IV) as a Linker Between Two Chiral Salen Complexes Leading to the Stereoselective Formation of Different M- and P?Helical Structures

摘要

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. Xray 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.