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

摘要

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~-)_2AlCl (2) and (IP~-)_2A1(CF_3SO_3) (3), and deep purple [(IP~(2-))A1]~- (5) are presented. The mixed-valent, monoradical complex (IP~-)(IP~(2-))A1 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 H = - 2J(S_(L(I)) ·S_(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 EP~(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~-)_2AlCl + (IP)_2AlCl - 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, CVs 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.