High-valent nickel oxamides. Part 1. Synthesis, structure, spectroscopy and redox chemistry of square-planar nickel(II) complexes with tetradentate o-phenylenedioxamidates and related ligands

A series of four-coordinate square-planar nickel(II) complexes of o-phenylenebis(N'-methyloxamidate) (L-1) and related o-phenylene(N'-methyloxamidate) oxamate (L-2) and o-phenylenebis(oxamate) (L-3) tetradentate ligands have been synthesized and characterized structurally, spectroscopically and electrochemically. The parent nickel(II)-L-1 complex presents an intense MLCT band in the UV region (lambda(max) = 357 nm) and a distinctive 1 s -> 4p CT satellite in the Ni K-edge XANES spectrum (E = 8339.2 eV). These features together with the short Ni-N(amidate) bond lengths (1.85-1.93 angstrom) as revealed by the analysis of the Ni K-edge EXAFS spectrum and confirmed by single-crystal X-ray diffraction are typical of square-planar low spin (S = 0) Ni-II ions. The dianionic nickel(II) complexes, [(NiLi)-L-II](2-) (i = 1-3), experience two redox processes in acetonitrile at 25 degrees C. The first redox process, at moderately low potentials (E-1 = 0.12-0.52 V vs. SCE), is a reversible one-electron metal-centered oxidation to the corresponding monoanionic nickel(III) complexes, [(NiLi)-L-III](-). The second redox process, at relatively high potentials (E-2 = 0.86-1.04 V vs. SCE), is a quasireversible to irreversible one-electron oxidation largely centered on the o-benzenediamidate fragment of the non-innocent ligand, yielding the corresponding neutral nickel(III) complexes with a o-benzosemiquinonediimine pi-cation radical ligand, [Ni-III(L-i)(center dot+)]. The singly and doubly oxidized species of the parent nickel(II)-L-1 complex have been prepared by chemical oxidation and characterized spectroscopically in acetonitrile at -40 degrees C. The stable singly oxidized nickel(III)-L-1 species presents an intense LMCT band in the NIR region (lambda(max) = 910 nm) and a rhombic X-band EPR spectrum (g(1) = 2.193, g(2) = 2.080 and g(3) = 2.006) characteristic of square-planar low spin (S = 1/2) Ni-III ions. The unstable double oxidized nickel((III))-L-1 pi-cation radical species exhibits a rather intense visible band (lambda(max) = 645 nm) that is tentatively assigned as a MLCT transition from the Ni-III-benzosemiquinone type ground state to the Ni-IV excited state.