Synthesis, dynamics and redox properties of eight-coordinate zirconium catecholate complexes

Abstract

Reaction of the 9,9-dimethylxanthene-bis(imine)-bis(catechol) ligand XbicH₄ with half an equivalent of Zr(acac)₄ affords the neutral tetracatecholate complex (XbicH₂)₂Zr, containing four iminium ions hydrogen bonded to the catecholates. The heteroleptic bis(catecholate)-tetraphenylporphyrin complex (TPP)Zr(XbicH₂) is formed from reaction of (TPP)Zr(OAc)₂ with XbicH₄ in the presence of base. Both compounds adopt an eight-coordinate square antiprismatic geometry around the zirconium center. NMR spectra of (TPP)Zr(XbicH₂) show that it is fluxional at room temperature, with homoleptic (XbicH₂)₂Zr showing fluxionality at higher temperatures. Calculations and kinetic isotope effect measurements suggest that the motions involve dissociation of a single catecholate oxygen and subsequent twisting of the seven-coordinate species. The compounds show reversible one-electron oxidations of each of the bound catecholates to bound semiquinones.