A Redox Series of Aluminum Complexes: Characterization of Four Oxidation States Including a Ligand Biradical State Stabilized via Exchange Coupling

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Abstract

Electrophilic activation and subsequent reduction of substrates is in general not possible because highly Lewis acidic metals lack access to multiple redox states. Herein, we demonstrate that transition metal-like redox processes and electronic structure and magnetic properties can be imparted to aluminum(III). Bis(iminopyridine) complexes containing neutral, monoanionic, and dianionic iminopyridine ligands (IP) have been characterized structurally and electronically; yellow (IP)AlCl(3) (1), deep green (IP(-))(2)AlCl (2) and (IP(-))(2)Al(CF(3)SO(3)) (3), and deep purple [(IP(2-))Al](-) (5) are presented. The mixed-valent, monoradical complex (IP(-))(IP(2-))Al is unstable toward C-C coupling, and [(IP(2-))Al](2-)(μ-IP-IP)(2-) (4) has been isolated. Variable-temperature magnetic susceptibility and EPR spectroscopy measurements indicate that the biradical character of the ligand-based triplet in 2 is stabilized by strong antiferromagnetic exchange coupling mediated by aluminum(III): J = -230 cm(-1) for Ĥ = -2J(Ŝ(L(1))·Ŝ(L(2))). Coordination geometry-dependent (IP(-))-(IP(-)) communication through aluminum(III) is observed electrochemically. The cyclic voltammogram of trigonal bipyramidal 2 displays successive ligand-based oxidation events for the two IP(1-/0) processes, at -0.86 and -1.20 V vs SCE. The 0.34 V spacing between redox couples corresponds to a conproportionation constant of K(c) = 10(5.8) for the process (IP(-))(2)AlCl + (IP)(2)AlCl → 2(IP(-))(IP)AlCl consistent with Robin and Day Class II mixed-valent behavior. Tetrahedral 5 displays localized, Class I behavior as indicated by closely spaced redox couples. Furthermore, CV's of 2 and 5 indicate that changes in the coordination environment of the aluminum center shift the potentials for the IP(1-/0) and IP(2-/1-) redox couples by up to 0.9 V.

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