The efficient and selective light-driven conversion of carbon dioxide to formate is a scientific challenge for green chemistry and energy science, especially utilizing visible-light energy and earthabundant catalytic materials. In this report, two mononuclear Ni(II) complexes of pyridylbenzimidazole (pbi) and pyridylbenzothiazole (pbt), such as Ni(pbt)(pyS) 2 (1) and Ni(pbi)(pyS) 2 (2) (pyS = pyridine-2thiolate), were prepared and their reactivities studied. The two Ni complexes were examined for CO 2 conversion using eosin Y as a photosensitizer upon visible-light irradiation in a H 2 O/ethanol solvent. The photoreaction of CO 2 catalyzed by complexes 1 and 2 selectively affords formate with a high efficiency (14 000 turnover number) and a high catalytic selectivity of ∼99%. Undesirable proton reduction pathways were completely suppressed in the photocatalytic reactions with these sulfur-rich Ni catalysts under CO 2 . Hydrogen photoproduction was also studied under argon. Their kinetic isotope effects and influence of solution pH for formate and H 2 production in the photocatalytic reactions are described in relation to the reaction mechanisms. These bioinspired Ni(II) catalysts with N/S ligation in relation to [NiFe]-hydrogenases are the first examples of early transition metal complexes affording such high selectivity and efficiencies, providing a future path to design solarto-fuel processes for artificial photosynthesis.

Bioinspired photosynthetic systems composed of photocatalysts and enzymes are a notable framework for converting CO2 to high-value chemicals. However, catalyst/enzyme deactivation and poor electron transfer kinetics in multistep photochemical processes severely limit their catalytic efficiencies. In this study, Janus-type DNA nanosheets (NSs) presenting two different DNA sequences on each face were utilized as a support for the selective immobilization of a Rh complex and formate dehydrogenase (FDH) for concerted catalytic reactions for CO2 reduction. Based on the face selectivity, DNA-conjugated Rh complex and FDH were immobilized on NSs into four different configurations: Rh complex on NS (NS1), FDH on NS (NS2), Rh complex and FDH on opposite faces of NS (NS3), FDH and Rh complex on the same face of NS (NS4). The catalytic system exhibited CO2 conversion efficiencies highly dependent on the spatial organization of Rh complex and FDH, showing the reactivity for the formate production in the order of NS1 coupled with free FDH > NS3 > NS2 coupled with free Rh complex > NS4 > free Rh complex and FDH. The NS1 coupled with free FDH showed turnover number (TON) of 1360 for the formate production based on NAD+, which is the highest value reported thus far for Rh-based photocatalyst/enzyme coupled systems. The results demonstrate that the compartmentalization of photocatalysts and biological enzymes is a viable approach for improving the efficiency of CO2 conversion and provide important design rules for building efficient artificial photosynthetic systems.

Various (pentamethylcyclopentadienyl)-rhodium(III) complexes comprising aromatic bidentate ligands [(Cp*)Rh(L)Cl] + (Cp* = pentamethylcyclopentadienyl, L = 2,2 0bipyridine, 1,10-phenanthroline, and their derivatives) were prepared to compare their reactivities of chemical cofactor regeneration. When the catalytic NADH regeneration was performed with sodium formate À , the reaction rates were compared with six Rh(III) complexes. The kinetics of the reactions including kinetic isotope effects are also studied in the chemical NADH regeneration. The Rh(III) complexes react with formate efficiently to afford the intermediates [(Cp*)Rh(L)(H)] + and CO 2 . The electronic and steric effects of the ligands of the Rh complexes were observed on the reaction rates. The overall reaction rates of the NADH regeneration were also obtained using DCOO À in H 2 O. Such H/D exchange rates of [(Cp*)Rh(L)(H)] + and the observation of a deuterium kinetic isotope provide valuable mechanistic insight into the catalytic NADH regeneration.