Bifunctional catalysts for the CO₂ fixation: Structural optimization to maximize the synergetic effect

We developed various metalloporphyrins as bifunctional catalysts for the synthesis of cyclic carbonates from epoxides and CO₂ under solvent-free conditions. The structures of the bifunctional catalysts were optimized step by step. As a result, a Zn porphyrin bearing eight nucleophiles at the meta positions showed a high turnover number (TON = 240,000) at 120°C at 1.7 MPa CO₂ and catalyzed the reaction even at atmospheric CO₂ pressure (balloon) at ambient temperature (20°C). Triporphyrin catalysts showed very high catalytic activity: TON = 220,000 and TOF = 46,000 h^-1 for the Mg triporphyrin, and TON = 310,000 and TOF = 40,000 h^-1 for the Zn triporphyrin. The high catalytic activity of these bifunctional catalysts was due to the cooperative action of Mg²⁺ or Zn²⁺ and Br^- and a conformational change (induced-fit) of the quaternary ammonium cation.