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 [NiII(RL–)2] by AgSbF6 or [RuIII(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 (NiII) is retained, whereas one of the β-diketiminate
ligands is oxidized to give formally a mixed-valence complex, [NiII(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 [NiII(RL0.5•–)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, [NiI(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.