An isopropyl substituted trispyrazolylborate ligand (Tpipr) was synthesized from sodium borohydride and 3-isopropylpyrazole using the melt method in a 65% yield. This TpiPr ligand was reacted in a 1:1 ratio with nickel nitrate or nickel chloride hexahydrate in methanol to generate TpiprNiNO3 and TpiprNiCl respectively in a good yield. These complexes were exposed to 5 eq. of NaX (X = Br-, I-, NCS-, and NO2-) in methanol to generate, in good yields, the corresponding TpiprNiX complex giving a series of complexes with varying metal environments. These complexes were characterized using 1H-NMR spectroscopy, infrared spectroscopy, UV-visible spectroscopy and small molecule X-ray crystallography. Results of NMR spectroscopy indicated paramagnetic behavior with peaks ranging from -15 to 85 ppm with the pyrazolyl hydrogen atoms being shifted the most downfield. The infrared spectra display a characteristic B-H stretch near 2500 cm-1, which is a higher energy than that of the free ligand. UV-visible spectroscopy indicated both d to d and charge transfer absorbances which varied greatly between the complexes. Small molecule X-ray analysis of TpiPrNiNCS revealed a bridging thiocyanate dimer in the solid state. Subsequent FTIR analysis, focusing on the thiocyanate ligand, suggested both monomer and dimer when dissolved in methylene chloride. Existence of a dimer supports the intermediate steric bulk of the isopropyl substituents on the ligand when compared to similar complexes with bulky t-butyl substituents that do not form dimers. The electrochemical properties of each complex were analyzed using cyclic voltammetry and the complexes exhibited similar voltammograms across the series in methylene chloride, which is surprising considering the difference in donor properties of the different X ligands. When the electrochemistry was performed in tetrahydrofuran, a coordinating solvent, the redox properties were more consistent with the donor properties of the X- ligand.

Synthesis, characterization, and electrochemical properties of tris(3-isopropylpyrazolyl)borate nickel complexes

249th National Meeting (2015)

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