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

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.