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

Abstract

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. © 2011 The Royal Society of Chemistry.