title: Co(II), Ni(II), Cu(II) and Zn(II) complexes of a bipyridine bis-phenol conjugate: generation and properties of coordinated radical species.
abstract: Four bis-phenolate complexes [Zn(II)L], [Ni(II)L], [Cu(II)L] and [Co(II)L] (where [H(2)L = 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(II)L] consists of a single bi-electronic reversible wave at 0.06 V vs. Fc/Fc(+). The electrochemically generated dication is a diradical species [Zn(II)L˙˙](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(II)L] and [Cu(II)L] 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(II)L˙](+) exhibits a strong absorption band at 960 nm and an (S = ½) EPR signal centred at g(iso) = 2.02. [Cu(II)L˙](+) is EPR-silent, in agreement with a magnetic coupling between the metal and the radical spin. In contrast with the other complexes, [Co(II)L] was found to react with dioxygen (mostly in the presence of pyridine), giving rise to a stable (S = ½) superoxo radical complex [Co(III)L(Py)(O(2)˙)]. One-electron oxidation of [Co(II)L] at -0.01 V affords a diamagnetic cobalt(III) complex [Co(III)L](+) that is inert towards O(2) binding, whereas two-electron oxidation leads to the paramagnetic phenoxyl radical species [Co(III)L˙](+) whose EPR spectrum features an (S = ½) signal at g(iso) = 2.00.
authors.affiliation: Département de Chimie Moléculaire-Chimie Inorganique Redox Biomimétique-UMR CNRS 5250, Université J. Fourier, B. P. 53, 38041, Grenoble cedex 9, France.
authors.forename: Himanshu
authors.initials: H
authors.lastname: Arora
authors.forename: Christian
authors.initials: C
authors.lastname: Philouze
authors.forename: Olivier
authors.initials: O
authors.lastname: Jarjayes
authors.forename: Fabrice
authors.initials: F
authors.lastname: Thomas
mesh_terms: D003035:Cobalt / Q000737:chemistry; D003300:Copper / Q000737:chemistry; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D008024:Ligands; D019216:Metals, Heavy / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009532:Nickel / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010636:Phenols / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry
chemical_list: D005609:Free Radicals; D008024:Ligands; D019216:Metals, Heavy; D009942:Organometallic Compounds; D010636:Phenols; D003035:Cobalt; D003300:Copper; D009532:Nickel; D015032:Zinc