Trapping of\nKey “Ate” Intermediates\nof NHC-Group IV Relevant to Catalyzing Copolymerization of Cyclohexene\nOxide with CO<sub>2</sub>

Together with bimetallic systems, metalates derived from\nanionic\nnucleophile-activated monometallic systems have shown very high catalytic\nperformances for polycarbonates in epoxide–CO<sub>2</sub> copolymerization.\nHowever, examples of isolated metalates are rather scarce. Lately,\na putative initiating hafnium “ate” species was isolated\nupon the addition of [PPN]Cl to the <i>N</i>-heterocyclic\ncarbene (NHC) complex of hafnium [PPN][({κ<sup>3</sup>-<i>O,C,O</i>}-NHC)HfCl<sub>3</sub>] <b>3-Hf</b>. Inspired\nby this lead, Ti and Zr “ate” analogues of <b>3-Hf</b>, <b>3-Ti</b> and <b>3-Zr</b>, respectively, were synthesized.\nAll the “ate” complexes exhibited high activity (TOF\n≈ 363 h<sup>–1</sup>) and polycarbonate selectivity\n(≥99%) in the copolymerization of cyclohexene oxide (CHO) and\nCO<sub>2</sub> under mild conditions. Monitoring the ring-opening\nof CHO at room temperature with <b>3-Hf</b> revealed the rapid\nformation of a rare metalate intermediate, [PPN][({κ<sup>3</sup>-O,C,O}-NHC)HfCl<sub>2</sub>(OC<sub>6</sub>H<sub>10</sub>Cl)] <b>5-Hf</b>. Under similar conditions, excess addition of CHO to <b>3-Hf</b> formed a CHO adduct of <b>5-Hf</b> species (<b>6-Hf</b>) and at 80 °C led further toward another metalate\nintermediate, [PPN][({κ<sup>3</sup>-O,C,O}-NHC)HfCl(OC<sub>6</sub>H<sub>10</sub>Cl)<sub>2</sub>] <b>7-Hf</b>. Kinetic studies\nrevealed the first-order dependence in both the catalyst and CHO concentrations\nand zero-order dependence in CO<sub>2</sub> with a Gibbs free energy\nof 24.4 kcal·mol<sup>–1</sup> at 80 °C. DFT calculations\nperformed on the catalytic system suggest <b>7-Hf</b> to be\none of the key active catalytic species favoring CO<sub>2</sub> insertion\nduring copolymerization.