Phenoxyl radicals: H-bonded and coordinated to Cu(ii) and Zn(ii)

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Abstract
Two pro-ligands (RLH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole (BzLH) or a 4,5-di-p-methoxyphenyl substituted imidazole (PhOMeLH), have been structurally characterised. Each possesses an intramolecular O–H⋯N hydrogen bond between the phenolic O–H group and an imidazole nitrogen atom and 1H NMR studies show that this bond is retained in solution. Each RLH undergoes an electrochemically reversible, one-electron, oxidation to form the [RLH]˙+ radical cation that is considered to be stabilised by an intramolecular O⋯H–N hydrogen bond. The RLH pro-ligands react with M(BF4)2·H2O (M = Cu or Zn) in the presence of Et3N to form the corresponding [M(RL)2] compound. [Cu(BzL)2] (1), [Cu(PhOMeL)2] (2), [Zn(BzL)2] (3) and [Zn(PhOMeL)2] (4) have been isolated and the structures of 1·4MeCN, 2·2MeOH, 3·2MeCN and 4·2MeCN determined by X-ray crystallography. In each compound the metal possesses an N2O2-coordination sphere: in 1·4MeCN and 2·2MeOH the {CuN2O2} centre has a distorted square planar geometry; in 3·2MeCN and 4·2MeCN the {ZnN2O2} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both 1 and 2, in CH2Cl2–DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(II) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that 1, 2, 3 and 4 each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)(RL)(RL˙)]+, comprising a M(II) centre bound to a phenoxide (RL) and a phenoxyl radical (RL˙) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF4]. The second oxidation produces [M(II)(RL˙)2]2+. The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [RLH]˙+ (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)(RL)(RL˙)]+ (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(II) is slightly more effective than Zn(II) in this respect.

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