Evaluation of the stereoselectivity for titanium(IV)-based coordination entities induced by the enantiopure diphenylethene-1,2-diamine ligand

Abstract

This article describes the formation of neutral TiO4N2-based coordination entities where the titanium centers are coordinated by oxygenated ligands incorporating one or two biphenolato units, i.e. L1 and L2 respectively. In these systems, the sixfold coordination spheres of each metallic center are completed by the enantiopure bidentate diphenylethene-1,2-diamine compound, abbreviated as dpeda. A solvent dependent diastereomeric ratio is evaluated by 1H NMR for the monomeric [Ti(L1)2((1R,2R)-dpeda)] or [Ti(L1)2((1S,2S)-dpeda)] (abbreviated as [R-Ti] or [S-Ti] respectively) complexes. The highest diastereomeric ratio for [Ti(L1)2(dpeda)] is obtained in chloroform (2 : 1). Energy calculation and circular dichroism spectra simulation, obtained by DFT, permit to assign the configuration of the stereoisomer formed in excess. The (1R,2R)-(+)-dpeda privileges the Δ form and (1S,2S)-(–)-dpeda the Λ form of the [Ti(L1)2(dpeda)] stereoisomers. The helicates formulated as [Ti2(L2)2((1S,2S)-dpeda)2] and [Ti2(L2)2(1R,2R)-dpeda)2] (abbreviated as [S-Ti2] or [R-Ti2] respectively) are obtained by following a multi-component self-assembly approach. In this case, the diastereomeric ratios evaluated by 1H NMR are much lower compared to those determined for the monomeric species, and a privileged P and M configuration for the [Ti2(L2)2(1R,2R)-dpeda)2] helicate and the [Ti2(L2)2((1S,2S)-dpeda)2] helicate respectively is assigned through theoretical calculations. Overall, this article describes a strategy to favour handedness in a helicate system where the chiral control is originated from a ligand that is not inscribed within the helical framework of the architecture.