Synthesis, structure, spectroscopy and redox chemistry of square-planar nickel(ii) complexes with tetradentate o-phenylenedioxamidates and related ligands

Abstract

A series of four-coordinate square-planar nickel(II) complexes of o-phenylenebis(N′-methyloxamidate) (L¹) and related o-phenylene(N′-methyloxamidate)oxamate (L²) and o-phenylenebis(oxamate) (L³) tetradentate ligands have been synthesized and characterized structurally, spectroscopically and electrochemically. The parent nickel(II)–L¹ complex presents an intense MLCT band in the UV region (λ_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 Å) 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, [Ni^IILⁱ]²⁻ (i = 1–3), experience two redox processes in acetonitrile at 25 °C. The first redox process, at moderately low potentials (E₁ = 0.12–0.52 V vs. SCE), is a reversible one-electron metal-centered oxidation to the corresponding monoanionic nickel(III) complexes, [Ni^IIILⁱ]⁻. The second redox process, at relatively high potentials (E₂ = 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 π-cation radical ligand, [Ni^III(Lⁱ)˙⁺]. The singly and doubly oxidized species of the parent nickel(II)–L¹ complex have been prepared by chemical oxidation and characterized spectroscopically in acetonitrile at −40 °C. The stable singly oxidized nickel(III)–L¹ species presents an intense LMCT band in the NIR region (λ_max = 910 nm) and a rhombic X-band EPR spectrum (g₁ = 2.193, g₂ = 2.080 and g₃ = 2.006) characteristic of square-planar low spin (S = 1/2) Ni^III ions. The unstable double oxidized nickel(III)–L¹ π-cation radical species exhibits a rather intense visible band (λ_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.