Characterization of Divalent and Trivalent Species Generated in the Chemical and Electrochemical Oxidation of a Dimeric Pincer Complex of Nickel

The electrolytic and chemical oxidation of the dimeric pincer complex [kappa(P),kappa(C),kappa(N),mu(N) -(2,6-(i-Pr2POC6H3-CH2NBn)Ni](2) (1; Bn = CH2Ph) has been investigated by various analytic techniques. Cyclic voltammetry measurements have shown that 1 undergoes a quasi-reversible, one electron, Ni-based redox process (Delta E-1/2(0) = -0.07 V vs Cp2Fe/[Cp2Fe](+)), and spectroelectrochemical measurements conducted on the product of the electrolytic oxidation, [1](+center dot), have shown multiple low-energy electronic transitions in the range of 10000-15000 cm(-1). Computational studies using Density Functional Theory (B3LYP) have corroborated the experimentally obtained structure of!, provided the electronic structure description, and helped interpret the experimentally obtained absorption spectra for 1 and [1](+center dot). These calculations indicate that the radical cation [1](+center dot) is a dimeric, mixed-valent species (class III) wherein most of the spin density is delocalized over the two nickel centers (Ni2+2.5N2), but some spin density is also present over the two nitrogen atoms (Ni22+N2 center dot.). Examination of alternative structures for open shell; species generated from 1 has shown that the spin density distribution is highly sensitive toward changes in the ligand environment of the Ni ions. NMR, UV-vis, electron paramagnetic resonance (EPR), and single crystal X-ray diffraction analyses have shown that chemical oxidation of 1 with N-Bromosuccinimide (NBS) follows a complex process that gives multiple products, including the monomeric trivalent species kappa(P),kappa(C),kappa(N)-{2,6-(i-Pr2PO)(C6H3)(CH=NBn)}NiBr2 (2). These studies also indicate that oxidation of 1 with 1 equiv of NBS gives an unstable, paramagnetic intermediate that decomposes to a number of divalent species, including succinimide and the monomeric divalent complexes kappa(P),kappa(C),kappa(N)-{2,6-(i-Pr2PO)(C6H3)(CH=NBn)}NiBr (3) and kappa(P),kappa(C),kappa(N)-{2,6-(i-Pr2PO)(C6H3)(CH2N(H)Bn)}NiBr2 (4); a second equivalent of NBS then oxidizes 3 and 4 to 2 and other unidentified products. The divalent complex 3 was synthesized independently and shown to react with NBS or bromine to form its trivalent homologue 2. The new complexes 2 and 3 have been characterized fully.