Molecular and electronic structure of square-planar nickel(ii), nickel(iii) and nickel(iii) π-cation radical complexes with a tetradentate o-phenylenedioxamidate redox-active ligand

Abstract

The molecular and electronic structures of the electron transfer series of four-coordinate square-planar nickel complexes with the ligand o-phenylenebis(N′-methyloxamidate), [NiL]^z (z = 2−, 1−, 0), have been evaluated by DFT and TDDFT calculations, and most of their experimentally available structural and spectroscopic properties (X. Ottenwaelder et al., Dalton Trans., 2005, DOI: 10.1039/b502478a) have been reasonably reproduced at the B3LYP level of theory. The anionic species [NiL]²⁻ and [NiL]⁻ are genuine low-spin nickel(II) and nickel(III) complexes with diamagnetic singlet (S = 0) and paramagnetic doublet (S = 1/2) states, respectively. The nickel(III) complex presents shorter Ni–N(amidate) bond distances (1.85–1.90 Å) than the parent nickel(II) complex (1.88–1.93 Å) and characteristic LMCT bands in the NIR region (λ_max = 794 and 829 nm) while the analogous MLCT bands for the nickel(II) complex are in the UV region (λ_max = 346 and 349 nm). The neutral species [NiL] is a nickel(III) o-benzosemiquinonediimine π-cation radical complex with a diamagnetic singlet (S = 0) and a paramagnetic triplet (S = 1) states fairly close in energy but fundamentally different in orbital configuration. The singlet metal–radical ground state results from the antiferromagnetic coupling between the 3d_yz orbital of the Ni^III ion (S_M = 1/2) and the π_b orbital of the benzosemiquinone-type radical ligand (S_L = 1/2), which have a large overlap and thus strong covalent bonding. The triplet metal–radical excited state involves the ferromagnetic coupling between the Ni^III 3d_zx orbital and the benzosemiquinone-type π_b orbital, which are orthogonal to each other. The singlet and triplet states of the nickel(III) π-cation radical complex possess characteristic quinoid-type short–long–short alternating sequence of C–C bonds in the benzene ring, as well as intense MLCT transitions in the VIS (λ_max = 664 nm) and NIR (λ_max = 884 nm) regions, respectively.