Ligand-Bound CO₂ as a Nonclassical Route toward Efficient Photocatalytic CO₂ Reduction with a Ni N-Confused Porphyrin

Implementing the synergistic effects between the metal and the ligand has successfully streamlined the energetics for CO₂ activation and gained high catalytic activities, establishing the important breakthroughs in photocatalytic CO₂ reduction. Herein, we describe a Ni(II) N-confused porphyrin complex (NiNCP) featuring an acidic N-H group. It is readily deprotonated and exists in an anion form during catalysis. Owing to this functional site, NiNCP gave rise to an outstanding turnover number (TON) as high as 217,000 with a 98% selectivity for CO₂ reduction to CO, while the parent Ni(II) porphyrin (NiTPP) was found to be nearly inactive. Our mechanistic analysis revealed a nonclassical reaction pattern where CO₂ was effectively activated via the attack of the Lewis-basic ligand. The resulting ligand-bound CO₂ adduct could be further reduced to produce CO. This new metal-ligand synergistic effect is anticipated to inspire the design of highly active catalysts for small molecule activations.