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

Abstract

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.