Phenolate and phenoxyl radical complexes of Cu(ii) and Co(iii), bearing a new redox active N,O-phenol-pyrazole ligand

The synthesis and characterisation of the new N,O-phenol-pyrazole pro-ligand, ^pzLH, comprising a pyrazole covalently linked to an o,p-di-tert-butyl-substituted phenol, are herein reported. In CH ₂Cl ₂ at room temperature, the cyclic voltammogram (CV) of ^pzLH exhibits a quasi-reversible one-electron oxidation process (at E _1/2 = 0.66 V vs. Fc ⁺/Fc) attributed to the formation of the phenoxyl radical cation [ ^pzLH] ⁺. ^pzLH reacts with M ^II(BF ₄) ₂ (M = Cu, Co) in a 2:1 ratio to afford the bis-Cu ^pzL ₂ (1) and tris-Co ^pzL ₃ (2) complexes respectively. The X-ray structure of 1 reveals a Cu(ii) ion in a square-planar trans-Cu ^II-N ₂O ₂ coordination environment whereas that of 2 consists of a Co(iii) ion with an octahedral mer-N ₃O ₃ coordination sphere; formed by the chelation of two (in 1) or three (in 2) N,O-bidentate phenolate ligands respectively. Both structures are preserved in CH ₂Cl ₂ solution, as revealed by their NMR (for 2) and EPR (for 1) data. The CVs of 1 and 2 consist of two (at E _1/2: 0.43 and 0.58 V vs. Fc ⁺/Fc) and three (E _1/2 = 0.12, 0.54 and 0.89 V vs. Fc ⁺/Fc) reversible one-electron oxidation processes, respectively. The one-electron electrochemical oxidation of 1 and 2 produces the oxidised species, 1 ⁺ and 2 ⁺, which are stable for several hours at room temperature under inert atmosphere in CH ₂Cl ₂. The UV/vis and EPR data obtained for 1 ⁺ and 2 ⁺ are unambiguously consistent with the latter being formulated as Cu(ii)- and Co(iii)-phenoxyl radical complexes, as [Cu ^II( ^pzL)( ^pzL)] ⁺ and [Co ^III( ^pzL)( ^pzL) ₂] ⁺ respectively.