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

Abstract

The nickel(II) complexes of the mono and di-nucleating Schiff base ligands H₂L^OMe, H₂L^NO2 and H₄L^bis respectively were synthesized and characterized. H₂L^OMe and H₂L^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 CH₂Cl₂ 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 H₄L^bis, namely [Ni₂(L^bis)], the phenylene spacer mediates an electronic communication between the two metallic sites. Single oxidation of [Ni₂(L^bis)] affords the delocalized phenoxyl radical [Ni₂(L^bis)]⁺, whose EPR signature is close to that of [Ni(L^OMe)]⁺. Double oxidation affords the bis-{Ni^II-delocalized radical} species [Ni₂(L^bis)]²⁺. 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 Å) observed in the neutral precursor [Ni₂(L^bis)].