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

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)center dot-; 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 pi-delocal-ization while those of [TpPh]Zn(3,5-DBSQ) are consistent with the presence of a bound SQ radical ligand.

Automated summary

The structure and electrochemical properties of zinc(II) complexes with a face-capping ligand have been investigated, revealing the effect of the ligand on bound semiquinonate (SQ) radicals. The substitution pattern of the SQ ring plays a crucial role in determining the fate of the bound SQ radicals. Additionally, the protective pocket formed by the ligand's phenyl substituents is unable to prevent decomposition of SQ radicals with minimal steric bulk, suggesting that substitution of the SQ ring is the main contributor to their stabilization.