Dynamic binuclear enediolato- and catecholato-zirconocene complexes relevant to the structural problem associated with sharpless oxidation catalysts

Abstract

Dimethylzirconocene complexes [ZrMe₂(C₅H₄R)₂](R = H or CH₃) were treated with catechol to give the (catecholato)zirconocene complexes. Low-temperature NMR spectra indicated dimeric metallatricyclic structures for these complexes and the corresponding (ethene-1,2-diolato)- and (but-2-ene-2,3-diolato)-zirconocenes. Dynamic temperature-dependent NMR spectroscopy revealed that all these complexes underwent a thermally induced degenerate rearrangement process, similar to the reorganization reaction of some Sharpless catalyst precursors, with Gibbs activation energies close to 50 kJ mol^–1. A labelling experiment showed the intramolecular nature of this automerization. (4-tert-Butylcatecholato)zirconocene formed two diastereoisomeric dimers which did not interconvert on the NMR time-scale (ΔG^‡_inter >90 kJ mol^–1) although the intramolecular rearrangement was observed [ΔG^‡_intra(241 K)= 52 ± 2 kJ mol^–1]. The intermolecular equilibration process probably proceeding via monomeric (catecholato)zirconocene was monitored in the case of the bulky (4-tert-butyl-catecholato)bis(tert-butylcyclopentadienyl)zirconium system by dynamic NMR spectroscopy at high temperature [ΔG^‡_inter(364 K)= 78 ± 5 kJ mol^–1].