One-electron oxidized nickel(II) complexes of bis and tetra(salicylidene) phenylenediamine Schiff bases: from monoradical to interacting Ni(III) ions.

The nickel(II) complexes of the mono and di-nucleating Schiff base ligands H(2)L(OMe), H(2)L(NO2) and H(4)L(bis) respectively were synthesized and characterized. H(2)L(OMe) and H(2)L(NO2) differ from one another by the substituents of the phenylene spacer, electron-donating methoxy or electron-withdrawing nitro groups respectively. X-Ray crystal structure analysis shows that the nickel(II) ion(s) resides within a square planar geometry in each complex. Cyclic voltammetry curves reveal that the electrochemical communication is strongly influenced by the substituent and the solvent. The one-electron oxidized species Ni(L(OMe)) in CH(2)Cl(2) is a phenoxyl radical with partial delocalization of the spin density on a metal orbital (contribution of 6.8%), whereas Ni(L(NO2)) was found to disproportionate once it is generated. A shift of electronic hole is observed in the presence of pyridine: both Ni(L(OMe)) and the one-electron oxidation product of [Ni(L(NO2))] are converted into mononuclear octahedral nickel(III) complexes involving two axially bound pyridines. In the dinickel(II) complex of H(4)L(bis), namely [Ni(2)(L(bis))], the phenylene spacer mediates an electronic communication between the two metallic sites. Single oxidation of [Ni(2)(L(bis))] affords the delocalized phenoxyl radical Ni(2)(L(bis)), whose EPR signature is close to that of Ni(L(OMe)). Double oxidation affords the bis-{Ni(II)-delocalized radical} species Ni(2)(L(bis)). Each radical is located at a distinct metallic site and a weak but appreciable magnetic interaction exists between the paramagnetic centres. In the presence of pyridine, a complex involving two ferromagnetically coupled nickel(III) ions is obtained. The magnetic coupling has been estimated to 3.7 cm(-1), while the zero field splitting parameters are |D| = 0.012 cm(-1) and E = 0. They are weak, in agreement with the large intermetallic distance (7.7 A) observed in the neutral precursor [Ni(2)(L(bis))].