Synthesis,spectroscopic and redox properties of nickel(II) salicylaldimine complexes containing sterically hindered phenols

The preparation and spectroscopic characterization of a series of new bis[N-(2,6-di-t-butyl-1-hydroxyphenyl)salicylaldiminato]nickel(II) complexes, [Ni(L^X)₂], bearing one or two OH and MeO substituents on the salicylaldehyde moiety, as well as radical species generated from these compounds by the oxidation with PbO₂, are reported. The [Ni(L^X)₂] chelates, which appear to be tetrahedral in the solid state and in dioxane solution, are converted into a square-planar configuration in non-donor solvents. The OH-substituted complexes, unlike their MeO analogues, form six-coordinate adducts in pyridine, DMF and DMSO. These new compounds, unlike their analogues with electron-withdrawing substituents (Cl, Br, NO₂), are easily oxidized by PbO₂ to produce Ni^II-stabilized phenoxy radicals in which the unpaired electrons are delocalized over the ligand and do not couple with the second radical center. No e.s.r. signals were observed that could be attributed to a M = ±2 transition of the triplet state biradicals.