CoII, NiII, CuII and ZnII complexes of a bipyridine bis-phenol conjugate: Generation and properties of coordinated radical species

Abstract

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_1/2¹ = 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 = ½) 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 = ½) 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 = ½) signal at g_iso = 2.00.