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We report the synthesis and characterization of a family of mixed-valent dinickel complexes, formally Ni23+, featuring a naphthyridine-based PNNPiPr pincer ligand (PNNPiPr = 2,7-bis(di-iso-propylphosphino-methyl)-1,8-naphthyridine). The mixed valent Ni23+ complexes, Ni2(μ-Br)2PNNPiPr2NiBr4 (1), Ni2(κ2-OAc)3PNNPiPr (2), and Ni2(κ2-OPiv)3PNNPiPr (3), were synthesized from the addition of two equivalents of a nickel(II) salt followed by an additional equivalent of Ni(COD)2 (COD = cyclooctadiene) to the PNNPiPr ligand. The delocalized Ni23+ mixed valence assignment was corroborated by single-crystal X-ray diffraction, electron paramagnetic resonance, and magnetic susceptibility measurements. In addition to these measurements, the presence of the Ni–Ni bond and symmetrical structure qualifies these complexes as Class III mixed valence in the Robin-Day classification scheme. Cyclic voltammetry experiments used to characterize the redox properties of these complexes exhibited a metal-centered, quasi-reversible couple associated with the Ni2(II,II)/Ni2(I,II) reduction event, with E1/2 values ranging from −0.83 V to −1.05 V vs (C5H5)2Fe+/0 in acetonitrile for the series. Further tracking by UV–vis spectroscopy showed that the mixed-valent and (PNNPiPr)Ni2(II,II) complexes can be interconverted through either chemical oxidation or reduction using ferrocenium tetrafluoroborate ((C5H5)2FeBF4) or cobaltocene ((C5H5)2Co), respectively.
Chavez et al. (Thu,) studied this question.