Steric and Electronic Consequences of Flexibility in a Tetradentate Redox-Active Ligand: Ti(IV) and Zr(IV) Complexes

By Szigethy, GeÌ?za & Heyduk, Alan F.
Published in Inorganic Chemistry NULL 2011

Abstract

A redox-active, tetradentate ligand, N,N'-bis-(3-dimethylamino-propyl)-4,5-dimethoxy-benzene-1,2-diamide ([N2N2cat]2-), was developed, and the six-coordinate metal complexes [N2N2cat]TiCl2 (3) and [N2N2cat]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 [N2N2cat]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 [N2N2cat]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 [N2N2cat]Ti(μ-p-NC6H4Me) 2, in which the metal centers are five-coordinate because of dissociation of one amine arm of the [N2N2cat]2- ligand. While the bis-azide complex [N2N2cat]Ti(N3)2 was stable toward elimination of N2, the bis-phenylacetylide complex [N2N2cat]Ti(C≡CPh)2 could be oxidized by PhICl2, resulting in subsequent reductive elimination of 1,4-diphenylbutadiyne.

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