Visible-Light Photocatalytic Conversion of Carbon Dioxide by Ni(II) Complexes with N4S2 Coordination: Highly Efficient and Selective Production of Formate

Abstract

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.

Noble metal-free bis-tridentate benzimidazole zinc(ii) and iron(ii) complexes for selective CO2photoreduction

“…5 For example, noble metal complexes such as Re, 6–8 Ru, 9–11 and Ir 12 used as catalysts for CO 2 photoreduction have been investigated extensively, but the high cost of noble metals is often not conducive to large-scale industrial applications. Therefore, in recent years, more and more noble metal-free complexes such as Fe, 13–16 Co, 17–20 Ni, 21–24 Mn, 25,26 Zn 27,28 and Cu 29,30 have been also employed for CO 2 reduction. Additionally, a photocatalytic system composed of non-precious metal catalysts and photosensitizers is strongly attractive in view of cost-effective and sustainable development.…”

“…31–35 For instance, Ni( ii ) complexes with N 4 S 2 coordination were developed for light-driven reduction of CO 2 with generation of HCOOH in the presence of the organic photosensitizer eosin Y. 22 Very recently, water-soluble Cu( i ) photosensitizer and Co( ii ) catalyst were combined for efficient CO 2 photoreduction in water. 36…”

Photochemical reduction of carbon dioxide to formic acid

“…Interestingly, complexes Ni( pbi)( pyS) 2 (Table 5; 69, pbi = 2-(2-pyridyl)-benzimidazole; pyS = pyridine-2-thiolate) and Ni( pbt)( pyS) 2 (Table 5; 70, pbt = 2-(2-pyridyl)benzothiazole) photoreduced CO 2 to formate in a selective way, with a TON FA of 14 000 and 13 100, respectively. 85 In general, the use of organic molecules instead of metalbased compounds increased the sustainability of these reactions. Moreover, organic molecules are often cheaper compared to the transition metal complexes.…”

Eosin: a versatile organic dye whose synthetic uses keep expanding

“…63 An important application of this concept within the critical fields of green chemistry and clean energy chemistry (hydrogen production) was recently reported by the group of Kim. 64 In this investigation, a bio-inspired nickel catalyst with eosin as cooxidant was used to fix carbon dioxide to produce formate ions. The same catalyst system was examined for hydrogen production.…”