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 [Zn^IIL], [Ni^IIL], [Cu^IIL] and [Co^IIL] (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 [Zn ^IIL] consists of a single bi-electronic reversible wave at 0.06 V vs. Fc/Fc⁺. The electrochemically generated dication is a diradical species [Zn^IIL]²⁺ that exhibits the typical phenoxyl π-π* band at 395 nm. It is EPR-silent due to magnetic interactions between the phenoxyl moieties. The CV curves of [Ni^IIL] and [Cu ^IIL] 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, [Ni ^IIL]⁺ exhibits a strong absorption band at 960 nm and an (S = ) EPR signal centred at g_iso = 2.02. [Cu^IIL] ⁺ is EPR-silent, in agreement with a magnetic coupling between the metal and the radical spin. In contrast with the other complexes, [Co ^IIL] was found to react with dioxygen (mostly in the presence of pyridine), giving rise to a stable (S = ) superoxo radical complex [Co ^IIIL(Py)(O₂)]. One-electron oxidation of [Co ^IIL] at -0.01 V affords a diamagnetic cobalt(iii) complex [Co ^IIIL]⁺ that is inert towards O₂ binding, whereas two-electron oxidation leads to the paramagnetic phenoxyl radical species [Co^IIIL]⁺ whose EPR spectrum features an (S = ) signal at g_iso = 2.00.