Influence of ligand backbone flexibility in group 4 metal complexes of tetradentate mixed tertiary amine/alkoxide ligands

Abstract

Simple epoxide ring opening chemistry using the cyclic secondary amine 1,4-diazacycloheptane or the related linear species N,N′-dimethylethylenediamine, and racemic (±)−3,3-dimethyl-1,2-epoxybutane gives access to the pendant alcohol functionalised ditertiary amine pro-ligands [HOCH(tBu)CH2N(R)CH2]2 (H2L1: R2 = CH2CH2CH2; H2L2: R2 = Me2). The contrasting reactions of H2L1 and H2L2 towards homoleptic group 4 alkoxides highlight the crucial role of ligand backbone flexibility in complex formation. Thus, the chemistry of the more conformationally rigid system (L1)2− appears to be constrained by the cyclic ligand core, such that it adopts a bridging (μ2:η2,η2) mode of coordination towards Ti(IV), leading to dinuclear metal systems [e.g.L1Ti2(OiPr)6]. By contrast, the more flexible linear system (L2)2− binds to both Ti(IV) and Zr(IV) in a chelating fashion leading, for example, to the synthesis of the C2 symmetric mononuclear complex rac-L2Ti(OiPr)2. Thus, a simple synthesis of diastereomerically pure, C2 symmetric, geometrically cis octahedral Ti(IV) complexes from racemic precursors is presented.