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Redox Chemistry of Nickel(II) Complexes Supported by a Series of Noninnocent β-Diketiminate Ligands

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Redox Chemistry of Nickel(II) Complexes Supported by a Series of Noninnocent β-Diketiminate Ligands

† Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
‡ Department of Medical Biochemistry, Kurume University School of Medicine, Kurume, Fukuoka 830-0011, Japan
§ Department of Chemistry, Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512,Japan
 School of Environmental Science and Engineering, Kochi University of Technology, Tosayamada, Kami, Kochi, 782-8502, Japan
Inorg. Chem., Article ASAP
DOI: 10.1021/ic5006693
Publication Date (Web): June 2, 2014
Copyright © 2014 American Chemical Society

Synopsis

Nickel complexes of a series of β-diketiminate ligands (deprotonated form of 2-substituted N-[3-(phenylamino)allylidene]aniline derivatives, RL−, R = Me, H, Br, CN, and NO2) have been synthesized and structurally characterized. One-electron oxidation of the neutral complexes [NiII(RL−)2] gave the corresponding cationic complexes, [NiII(RL0.5•−)2]+, in which β-diketiminate was oxidized acting as a noninnocent ligand, and the generated radical spin equally delocalizes into the two ligands giving class III mixed-valence speceis, whereas one-electron reduction took place at the metal center to give the anionic nickel(I) complexes [NiI(RL−)2]−. Substituent effects on the electronic structures of the three oxidation states (neutral, cationic, and anionic) of the complexes are systematically evaluated using DFT calculations.

Abstract

Abstract Image
Nickel complexes of a series of β-diketiminate ligands (RL–, deprotonated form of 2-substitutedN-[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.
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