Co-II, Ni-II, Cu-II and Zn-II complexes of a bipyridine bis-phenol conjugate: Generation and properties of coordinated radical species

Four bis-phenolate complexes [(ZnL)-L-II], [(NiL)-L-II], [(CuL)-L-II] and [(CoL)-L-II] (where [H2L = 2,2'-[2,2'] bipyridinyl-6-yl-bis-4,6-di-tert-butylphenol] have been synthesized. The copper(II) and nickel(II) complexes have been characterized by X-ray diffraction, showing a metal ion within a square planar geometry, slightly distorted towards tetrahedral. The cyclic voltametry (CV) curve of [(ZnL)-L-II] consists of a single bi-electronic reversible wave at 0.06 V vs. Fc/Fc+. The electrochemically generated dication is a diradical species [(ZnL center dot center dot)-L-II](2+) that exhibits the typical phenoxyl pi-pi* band at 395 nm. It is EPR-silent due to magnetic interactions between the phenoxyl moieties. The CV curves of [(NiL)-L-II] and [(CuL)-L-II] exhibit two distinct ligand-centred one-electron oxidation waves. The first one is observed at E-1/2(1) = 0.38 and 0.40 V for the nickel and copper complex, respectively, and corresponds to the formation of M(II)-coordinated phenoxyl radicals. Accordingly, [(NiLS center dot)-L-II](+) exhibits a strong absorption band at 960 nm and an (S = 1/2) EPR signal centred at g(iso) = 2.02. [(CuLS center dot)-L-II](+) is EPR-silent, in agreement with a magnetic coupling between the metal and the radical spin. In contrast with the other complexes, [(CoL)-L-II] was found to react with dioxygen (mostly in the presence of pyridine), giving rise to a stable (S = 1/2) superoxo radical complex [(CoL)-L-III(Py)(O-2(center dot))]. One-electron oxidation of [(CoL)-L-II] at -0.01 V affords a diamagnetic cobalt(III) complex [(CoL)-L-III](+) that is inert towards O-2 binding, whereas two-electron oxidation leads to the paramagnetic phenoxyl radical species [(CoLS)-L-III](+) whose EPR spectrum features an (S = 1/2) signal at giso = 2.00.