High-valent nickel oxamides. Part 2. Molecular and electronic structure of square-planar nickel(II), nickel(III) and nickel(III) pi-cation radical complexes with a tetradentate o-phenylenedioxamidate redox-active ligand

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 angstrom) than the parent nickel(II) complex (1.88-1.93 angstrom) and characteristic LMCT bands in the NIR region (lambda(max) = 794 and 829 nm) while the analogous MLCT bands for the nickel(II) complex are in the UV region (lambda(max) = 346 and 349 nm). The neutral species [NiL] is a nickel(III) o-benzosemiquinonediimine pi-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 pi(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 pb orbital, which are orthogonal to each other. The singlet and triplet states of the nickel(III) pi-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 (lambda(max) = 664 nm) and NIR (lambda(max) = 884 nm) regions, respectively.