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Mixed-Valent Dinuclear Nickel Complexes Enabled by an Expanded PNNP Pincer Ligand
Anthony J Chavez1, William Buratto1, Phan N Phu2
1Department of Chemistry & Biochemistry, University of California, Santa Barbara, California 93106, United States.
None:
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)2PNNPiPr]2[NiBr4] (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)2Fe][BF4]) or cobaltocene ((C5H5)2Co), respectively.
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