Abstract
The synthesis and characterisation of the new N,O-phenol-pyrazole pro-ligand, (LH)-L-pz, comprising a pyrazole covalently linked to an o,p-di-tert-butyl-substituted phenol, are herein reported. In CH2Cl2 at room temperature, the cyclic voltammogram (CV) of (LH)-L-pz 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 [(LH)-L-pz](center dot+). (LH)-L-pz reacts with M-II(BF4)(2) (M = Cu, Co) in a 2 : 1 ratio to afford the bis-(CuL2)-L-pz (1) and tris-(CoL3)-L-pz (2) complexes respectively. The X-ray structure of 1 reveals a Cu(II) ion in a square-planar trans-Cu-II-N2O2 coordination environment whereas that of 2 consists of a Co(III) ion with an octahedral mer-N3O3 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 CH2Cl2 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 CH2Cl2. 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(L-pz(center dot))(L-pz)]+ and [Co-III(L-pz(center dot))(L-pz)(2)](+) respectively.
