Synthesis and characterization of a hydro tris(3-phenylpyrazolyl)borato nickel(II) semiquinonate adduct

The in situ oxidation of 3,5-di-tert-butylcatecholate (3,5-DBC) by ambient 02 in the presence of nickel(II) tetrafluoroborate and sodium hydrotris-(3-phenylpyrazol-1-yl)borate (Na[TPph]) results in the formation of a square pyramidal Ni-II complex bearing a kappa(3)-[Tp(Ph)](-) and a bidentate, 3,5-di-tert-butylsemiquinonate ligand radical ([3,5-DBSQ](center dot-) characterized by X-ray crystallography, NMR spectroscopy and cyclic voltammetry (CV), which all support the presence of a bound semiquinonate ligand. Density-functional -theory (DFT) was used to calculate the electronic and magnetic structure of [Tp(Ph)]Ni(3,5-DBSQ) predicting strong, antiferromagnetic coupling (J = 364 cm(-1)) between the high-spin Ni-II center and the [3,5-DBSQ](-) radical consistent with the room-temperature magnetic moment of 2.4 mu(B) determined by H-1 NMR. Time-dependent DFT (TD-DFT) enables assignment of the experimental absorption spectrum and suggest electronic communication between the [Tp(Ph)](-) and (3,5-DBSQ)(-) ligands. (C) 2019 Elsevier Ltd. All rights reserved.