Trapping of Key “Ate” Intermediates of NHC-Group IV Relevant to Catalyzing Copolymerization of Cyclohexene Oxide with CO₂

Auteurs

Suresh L., Lalrempuia R., Fjermestad T., Törnroos K.W., Bour J., Frache G., Nova A., Le Roux E.

Référence

Organometallics, 2024

Description

Together with bimetallic systems, metalates derived from anionic nucleophile-activated monometallic systems have shown very high catalytic performances for polycarbonates in epoxide-CO2 copolymerization. However, examples of isolated metalates are rather scarce. Lately, a putative initiating hafnium “ate” species was isolated upon the addition of [PPN]Cl to the N-heterocyclic carbene (NHC) complex of hafnium [PPN][({κ3-O,C,O}-NHC)HfCl3] 3-Hf. Inspired by this lead, Ti and Zr “ate” analogues of 3-Hf, 3-Ti and 3-Zr, respectively, were synthesized. All the “ate” complexes exhibited high activity (TOF ≈ 363 h-1) and polycarbonate selectivity (≥99%) in the copolymerization of cyclohexene oxide (CHO) and CO2 under mild conditions. Monitoring the ring-opening of CHO at room temperature with 3-Hf revealed the rapid formation of a rare metalate intermediate, [PPN][({κ3-O,C,O}-NHC)HfCl2(OC6H10Cl)] 5-Hf. Under similar conditions, excess addition of CHO to 3-Hf formed a CHO adduct of 5-Hf species (6-Hf) and at 80 °C led further toward another metalate intermediate, [PPN][({κ3-O,C,O}-NHC)HfCl(OC6H10Cl)2] 7-Hf. Kinetic studies revealed the first-order dependence in both the catalyst and CHO concentrations and zero-order dependence in CO2 with a Gibbs free energy of 24.4 kcal·mol-1 at 80 °C. DFT calculations performed on the catalytic system suggest 7-Hf to be one of the key active catalytic species favoring CO2 insertion during copolymerization.

Lien

doi:10.1021/acs.organomet.4c00371