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.由于沿金属-金属轴的旋转自由度受到限制，双μ-氧双核金属核心（di-μ-oxo dimetal core）可作为制备具有明确立体化学结构双核化合物的优良构筑基元，但其立体选择性合成仍为一项挑战性课题。本文报道了带有不同空间位阻程度的四齿salen配体（salen）的双μ-氧二锰(IV)配合物的合成，旨在研究可能生成多种立体异构体的二聚反应的立体化学特征。X射线晶体学表征结果显示，采用(R,R)-N,N′-双(3,5-二叔丁基水杨醛亚胺基)-1,2-环己二胺（即salen配体）的双μ-氧二锰(IV)配合物呈现独特结构：两个salen配合单元以M螺旋构型排布。通过氢-1、氘核磁共振（1H、2H NMR）与圆二色谱开展的溶液相研究表明，当3/3′位引入叔丁基作为关键空间位阻调控因子时，该二聚反应具有极高的非对映选择性。与之相对，此前已有研究报道，采用相同salen配体的双μ-氧二钛(IV)配合物会生成构型相反的P螺旋二聚体。本研究通过密度泛函理论（DFT）计算阐明，钛-氧键合作用的共价性较弱，会导致双μ-氧二钛(IV)核心结构发生畸变，进而形成完全不同的配体间相互作用框架。本研究为理解双μ-氧双核金属核心形成过程中的立体化学机制提供了坚实的理论基础。