The synthesis of different (tris(3-phenylpyrazolyl)borato)M(semiquinonate) complexes, TpPhM(SQ) [M = Zn(II) or Ni(II)], was achieved through independent, one-pot reactions of Na[TpPh], M(BF4)2 ● xH2O, NaOH and either catechol or 3,5-di-t-butylcatechol in methanol. Ambient dioxygen was used to oxidize the catecholate substituent to the semiquinonate radical in solution. With both metals, the unsubstituted semiquinonate radical dimerized in situ resulting in a [TpPh]2M(m-bis-semiquinonate) complex [Zn (1), Ni (2)]. The 3,5-di-tert-butylsemiquinonate radical (3,5-DBSQ) is stabilized by the additional bulk of the two t-butyl substituents and was isolated, intact, as blue TpPhZn(3,5-DBSQ) (3) and olive TpPhNi(3,5-DBSQ) (4). Small molecule X-ray diffraction data of 1, 3, and 4 confirms the presence of the k3-TpPh ligand and bidentate binding of the semiquinonate ligand giving rise to 5-coordinate metal complexes in each case. UV-visible spectroscopy of 1-4 confirm the presence of charge transfer bands with absorptivity values ranging from 20,000 to 423 cm-1 M-1 across all complexes giving rise to their intensely colored solutions. The electrochemical properties of 1-4 were analyzed by cyclic voltammetry (CV) in methylene chloride. The voltammograms of 1 and 3 show ligand-based electrochemistry only consistent with zinc(II) compounds whereas 2 and 4 indicate both metal- and ligand-based processes. There are no consistent redox trends across the four complexes. Computational analysis suggests the best electronic description of 4 is Ni(II) bound to a ligand radical with antiferromagnetic coupling between unpaired spins on the metal and ligand.

Synthesis, characterization and spectroscopic comparisons of tris(3-phenylpyrazolyl)borato zinc(II) and nickel(II) semiquinonate complexes derived from in situ oxidation of parent catecholate complexes

255th National Meeting (2018)

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