Steric and electronic consequences of flexibility in a tetradentate redox-active ligand: Ti(IV) and Zr(IV) complexes

A redox-active, tetradentate ligand, N,N'-bis-(3-dimethylamino-propyl)-4,5-dimethoxy-benzene-1,2-diamide ([N(2)N(2)(cat)](2-)), was developed, and the six-coordinate metal complexes [N(2)N(2)(cat)]TiCl(2) (3) and [N(2)N(2)(cat)]ZrCl(2) (4) were synthesized. The tetradentate ligand was determined to be fluxional in 3 and 4, enabled by reversible dissociation of the neutral amine groups of the [N(2)N(2)(cat)](2-) ligand. Both amine arms of 3 could be replaced by N,N-dimethylaminopyridine with an overall free energy change of -4.64(3) kcal mol(-1) at 298 K. Cyclic voltammetry experiments were used to probe the redox capabilities of the [N(2)N(2)(cat)](2-) ligand: complex 3 exhibited two one-electron oxidations at -0.19 and -0.52 V versus [Cp(2)Fe](+/0) while 4 exhibited a single two-electron oxidation at -0.55 V. Substitution of the chlorides in 3 for an imide afforded the dimer {[N(2)N(2)(cat)]Ti(μ-p-NC(6)H(4)Me)}(2), in which the metal centers are five-coordinate because of dissociation of one amine arm of the [N(2)N(2)(cat)](2-) ligand. While the bis-azide complex [N(2)N(2)(cat)]Ti(N(3))(2) was stable toward elimination of N(2), the bis-phenylacetylide complex [N(2)N(2)(cat)]Ti(C≡CPh)(2) could be oxidized by PhICl(2), resulting in subsequent reductive elimination of 1,4-diphenylbutadiyne.