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

Article abstract of DOI:10.1021/ic101468p

A redox-active, tetradentate ligand, N,N′-bis-(3-dimethylamino- propyl)-4,5-dimethoxy-benzene-1,2-diamide ([N₂N₂ ^cat]^2-), was developed, and the six-coordinate metal complexes [N₂N₂^cat]TiCl₂ (3) and [N₂N₂^cat]ZrCl₂ (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 [N₂N 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 [N₂N₂ ^cat]^2- ligand: complex 3 exhibited two one-electron oxidations at -0.19 and -0.52 V versus [Cp₂Fe]^+/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 {[N₂N₂ ^cat]Ti(μ-p-NC₆H₄Me)}₂, in which the metal centers are five-coordinate because of dissociation of one amine arm of the [N₂N₂^cat]^2- ligand. While the bis-azide complex [N₂N₂^cat]Ti(N ₃)₂ was stable toward elimination of N₂, the bis-phenylacetylide complex [N₂N₂^cat] Ti(C≡CPh)₂ could be oxidized by PhICl₂, resulting in subsequent reductive elimination of 1,4-diphenylbutadiyne.