Coordination chemistry of a redox non-innocent NHC bis(phenolate) pincer ligand with nickel(II)

Abstract

The electrochemical properties of the neutral nickel(II) complexes of two NHC bis(di-tert-butylphenolate) pincer ligands, namely 1 and 2, are reported. Complex 1 shows a reversible redox wave at 0.10 V vs Fc+/Fc, which corresponds to the oxidation of the phenolate moiety. The complex 1+ SbF6− was prepared and structurally characterized. The nickel ion lies in a square planar geometry identical to that observed for 1, with contracted NiO bond distances. Complex 1+ exhibits a remarkable absorption band in the NIR region, 6370 cm−1 (13,450 M−1 cm−1), assigned to a charge transfer transition associated with the radical moiety. A band shape analysis supports its classification as a mixed-valent (Class II/III) compound. Complex 1+ exhibits a resonance in the EPR spectrum at giso = 2.056, as expected for a paramagnetic (S = ½) system. The shift from the free electron value suggests the participation of a nickel d-orbital in the SOMO. DFT calculations support the mainly radical character of 1+, but show that the spin density is shared between the metal center, which harbours a spin of 0.28, and both phenolic rings of the ligand. The carbene linker does not demonstrate any significant spin population.