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

作者：Myers Thomas W; Kazem Nasrin; Stoll Stefan; Britt R David; Shanmugam Maheswaran; Berben Louise A^*

来源：Journal of the American Chemical Society, 2011, 133(22): 8662-8672.

DOI：10.1021/ja2015718

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)AlCl3 (1), deep green (IP-)(2)AlCl (2) and (IP-)(2)Al(CF3SO3) (3), and deep purple [(IP2-)Al](-) (5) are presented. The mixed-valent, monoradical complex (IP-)(IP2-)Al is unstable toward C-C coupling and [(IP2-)Al](2-) (mu-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 (H) over cap = -2J((S) over cap (L(1)) center dot (S) over cap (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 IP1-/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 H 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 IP1-/0 and IP2-/1- redox couples by up to 0.9 V.

• 出版日期2011-6-8