Double oxidation localizes spin in a Ni bis-phenoxyl radical complex.

The electronic structure of a doubly oxidized Ni salen complex NiSal(tBu) (Sal(tBu) = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) has been investigated by both experimental and theoretical methods. The doubly oxidized product was probed by resonance Raman spectroscopy, UV-vis-NIR, and EPR to determine the locus of oxidation as well as the spectroscopic signature of the complex. It was determined that double oxidation of NiSal(tBu) affords a bis-ligand radical species in solution via the presence of phenoxyl radical bands at ν(7a) (1504 cm(-1)) and ν(8a) (1579 cm(-1)) in the Raman spectrum, and the loss of the intense NIR transition reported for the mono-radical complex (Angew. Chem., Int. Ed., 2007, 46, 5198). Spectroscopic experiments, complemented by DFT calculations, show that the two radical spins are predominantly localized on the phenolate moieties, in opposition to the extensive delocalization over the ligand framework observed for the mono-radical analogue.