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 [κ P ,κ C ,κ N ,μ N -(2,6-(i-Pr2POC6H3CH2NBn)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 (ΔE 0 1/2 = −0.07 V vs Cp2Fe/[Cp2Fe] + ), and spectroelectrochemical measurements conducted on the product of the electrolytic oxidation, [1]+•, 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 1, provided the electronic structure description, and helped interpret the experimentally obtained absorption spectra for 1 and [1] +·. These calculations indicate that the radical cation [1] +· is a dimeric, mixed-valent species (class III) wherein most of the spin density is delocalized over the two nickel centers (Ni+2.5 2N2), but some spin density is also present over the two nitrogen atoms (Ni2+ 2N2·). 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 κ P ,κ C ,κ 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 κ P ,κ C ,κ N -{2,6-(i-Pr2PO)(C6H3)(CHNBn)}NiBr (3) and κ P ,κ C ,κ 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.