The synthesis of three different (tris(3-phenylpyrazolyl)borato)Zn(oxocyclohexadienolate) complexes, TpPhZn(OCHD), was achieved through independent, one-pot reactions of Na[TpPh], Zn(BF4)2 ● xH2O, NaOH and either catechol, 4-t-butylcatechol or 3,5-di-t-butylcatechol in methanol. Ambient dioxygen was used to oxidize the catechol substituent to the semiquinonate radical anion in solution. The substitution pattern on the semiquinonate determined the fate of the radical anion in each case. The unsubstituted semiquinonate (SQ) produced a green [TpPhZn(OCHD)]2 dimer (1) through in situ, radical coupling of two TpPhZn(SQ) complexes. The 4-tert-butylsemiquinonate radical complex was unable to dimerize due to the bulky t-butyl substituent and decomposed via solvent interaction producing a red, dimethoxy substituted TpPhZn(OCHD-(OMe)2) complex (2). The 3,5-di-tert-butylsemiquinonate radical (3,5-DBSQ) is stabilized by the additional bulk of the second t-butyl substituent and was isolated, intact, as blue, TpPhZn(3,5-DBSQ) (3). All complexes were characterized by 1H-NMR spectroscopy, single crystal X-ray diffraction, infrared spectroscopy and UV-visible spectroscopy. Complexes 1 and 2 exhibited diamagnetic 1H-NMR spectrum consistent with radical decomposition while the spectrum of 3 was significantly broadened supporting the presence of the stabilized semiquinonate radical. Structural analysis of 1-3 revealed bidentate oxocyclohexadienolate ligand coordination to zinc within the pocket formed by the planar phenyl substituents on the TpPh ligand. Two different C-O bond lengths averaging 1.248(3) and 1.314(3) Å, across the series, is consistent with partial oxidation of the catechol starting material. The electrochemical properties of each complex were analyzed using cyclic voltammetry and 1-3 exhibited similar voltammograms in methylene chloride. The voltammogram of 1-3 contained a pseudo reversible one-electron reduction and an irreversible one-electron oxidation assigned to the redox activity of the oxocyclohexadienolate moiety. There were slight changes in the redox potentials across the three complexes consistent with the inductive effects of substitution on the oxocyclohexadienolate ligand.

Synthesis and characterization of a series of tris(3-phenylpyrazolyl)borato zinc(II) oxocyclohexadienolate complexes derived from in situ oxidation of parent catecholate complexes

253rd National Meeting (2017)

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