Redox chemistry of nickel(II) complexes supported by a series of noninnocent β-diketiminate ligands

Nickel complexes of a series of β-diketiminate ligands ( ^RL^-, deprotonated form of 2-substituted N-[3-(phenylamino)allylidene]aniline derivatives ^RLH, R = Me, H, Br, CN, and NO₂) have been synthesized and structurally characterized. One-electron oxidation of the neutral complexes [Ni^II( ^RL^-)₂] by AgSbF₆ or [Ru ^III(bpy)₃](PF₆)₃ (bpy = 2,2′-bipyridine) gave the corresponding metastable cationic complexes, which exhibit an EPR spectrum due to a doublet species (S = 1/2) and a characteristic absorption band in near IR region ascribable to a ligand-to-ligand intervalence charge-transfer (LLIVCT) transition. DFT calculations have indicated that the divalent oxidation state of nickel ion (Ni^II) is retained, whereas one of the β-diketiminate ligands is oxidized to give formally a mixed-valence complex, [Ni^II( ^RL^-)(^RL^•)]⁺. Thus, the doublet spin state of the oxidized cationic complex can be explained by taking account of the antiferromagnetic interaction between the high-spin nickel(II) ion (S = 1) and the organic radical (S = 1/2) of supporting ligand. A single-crystal structure of one of the cationic complexes (R = H) has been successfully determined to show that both ligands in the cationic complex are structurally equivalent. On the basis of theoretical analysis of the LLIVCT band and DFT calculations as well as the crystal structure, the mixed-valence complexes have been assigned to Robin-Day class III species, where the radical spin is equally delocalized between the two ligands to give the cationic complex, which is best described as [Ni^II(^RL ^0.5•-)₂]⁺. One-electron reduction of the neutral complexes with decamethylcobaltocene gave the anionic complexes when the ligand has the electron-withdrawing substituent (R = CN, NO₂, Br). The generated anionic complexes exhibited EPR spectra due to a doublet species (S = 1/2) but showed no LLIVCT band in the near-IR region. Thus, the reduced complexes are best described as the d⁹ nickel(I) complexes supported by two anionic β-diketiminate ligands, [Ni^I(^RL ^-)₂]^-. This conclusion was also supported by DFT calculations. Substituent effects on the electronic structures of the three oxidation states (neutral, cationic, and anionic) of the complexes are systematically evaluated on the basis of DFT calculations.