Synthesis, structure and electrochemical properties of tris(3-phenylpyrazolyl)borato zinc(II) adducts with o-semiquinonate ligands

Abstract

The structure and electrochemical properties of zinc(II) complexes bearing the face-capping, hydrotris-(3-phenylpyrazol-1-yl)borate, [TpPh]–, ligand are reported while investigating the effect of the [TpPh]– ligand on bound, o-semiquinonate(SQ) radicals. The o-SQ complexes were synthesized by in situ oxidation of parent catecholate complexes with the fate of the bound, SQ radical dependent on the substitution pattern of the SQ ring. A stable SQ radical was isolated using 3,5-di-tert-butylsemiquinonate, (3,5-DBSQ)–; however, the unsubstituted, [TpPh]ZnII(SQ) radical complex decomposed through oxidative coupling forming a dimer complex bridged by a fused bis-SQ ring system. The protective pocket formed by the phenyl substituents on [TpPh]– is not able to prevent decomposition of bound, SQ radicals bearing minimal steric bulk, implying SQ ring substitution is the major contributor towards stabilization of SQ radicals in this system. The redox properties of the {[TpPh]Zn}2(bis-SQ) determined through cyclic voltammetry are consistent with a bis-SQ assignment having inefficient π-delocalization while those of [TpPh]Zn(3,5-DBSQ) are consistent with the presence of a bound SQ radical ligand.