Simultaneous tuning of activity and water solubility of complex catalysts by acid-base equilibrium of ligands for conversion of carbon dioxide

New strategy for the simultaneous tuning of catalytic activity and water solubility of complex catalysts is described on the basis of an acid-base equilibrium between pyridinol and pyridinolate as the catalyst ligands. Herein, half-sandwich complexes with 4,4'-dihydroxy-2,2'-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) served as highly efficient and recyclable catalysts for the hydrogenation of bicarbonate in water. The oxyanion generated from the phenolic hydroxy group shows strong electronic donation and polarity, which play significant roles in the catalytic activity and water solubility, respectively. As a result, turnover frequencies (TOF) up to 42 000 h(-1) and turnover numbers (TON) up to 222 000 have been obtained by using iridium catalysts under 6 MPa at 120 degrees C. Furthermore, an iridium DHPT catalyst was spontaneously precipitated at the end of the reaction. Iridium leaching was found to be 0.11 ppm (1.2% of the loaded catalyst), and the added base was completely consumed. The recovered catalyst could be recycled for four cycles with high catalytic activity. Consequently, the catalyst was homogeneous and highly activated at the beginning of the reaction, whereas it was heterogeneous and deactivated at the end. The catalytic system offers an environmentally benign process with high efficiency, easy separation, catalyst recycling, waste-free process, and aqueous catalysis.