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

Abstract

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.