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

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.