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

Year: 2024

Type: article

Abstract: High Resolution Image Download MS PowerPoint Slide Together with bimetallic systems, metalates derived from anionic nucleophile-activated monometallic systems have shown very high catalytic performances for polycarbonates in epoxide–CO 2 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][({κ 3 - O,C,O }-NHC)HfCl 3 ] 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 CO 2 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][({κ 3 -O,C,O}-NHC)HfCl 2 (OC 6 H 10 Cl)] 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][({κ 3 -O,C,O}-NHC)HfCl(OC 6 H 10 Cl) 2 ] 7-Hf . Kinetic studies revealed the first-order dependence in both the catalyst and CHO concentrations and zero-order dependence in CO 2 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 CO 2 insertion during copolymerization.

Source: Organometallics

DOI: 10.1021/acs.organomet.4c00371