Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel N-Heterocyclic Carbene–Isoquinoline Complex

Abstract

ABSTRACT:The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO 2 fixation, several key considerations must be balanced, including: (1) catalyst selectivity for promoting CO 2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earthabundant nickel complexes supported by N-heterocyclic carbene-amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO 2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni( Pr bimiq1)] 2+ (where Pr bimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane, 1c) emerges as a catalyst for electrochemical reduction of CO 2 with the lowest cathodic onset potential (E cat = −1.2 V vs. SCE). Using this earthabundant catalyst with Ir(ppy) 3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO 2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s -1 , respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO 2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion. 3 IntroductionThe search for sustainable resources has attracted broad interest in the potential use of carbon dioxide as a feedstock for fuels and fine chemicals. [1][2][3][4][5][6][7][8][9][10] In this context, the photocatalytic reduction of CO 2 is an attractive route that can take advantage of the renewable and abundant energy of the sun for longterm CO 2 utilization, 6,11-13 with the eventual target of coupling the reductive half-reaction of CO 2 fixation with a matched oxidative half-reaction such as water oxidation to achieve a carbon-neutral artificial photosynthesis cycle. 14-21 Before this ultimate goal can be realized, however, a host of basic scientific challenges must be addressed, including developing systems that balance selectivity, efficiency, and cost. With regard to selectivity, it is critical to minimize the competitive reduction of water to hydrogen that is typically kinetically favored over CO 2 reduction, as well as selectively convert CO 2 to one carbon product. 22,23 Another primary consideration is the use of visible-light excitation, which more effectively harvests the solar spectrum and avoids deleterious high-energy photochemical pathways. Semiconductors such as TiO 2 and SiC have been widely employed as heterogeneous catalysts for photochemical and photoe...

Rhenium(i) trinuclear rings as highly efficient redox photosensitizers for photocatalytic CO2 reduction

“…, Ru( ii ), 23–26 Co( ii ), 27 Ni( ii ), 28,29 Re( i ), 30–33 Fe( ii ), 34,35 Os( ii ), 36 Ir( iii ) 37 and Mn( i ), 38 have been reported mostly with Ru( ii )–trisdiimine complexes, typically [Ru(bpy) 3 ] 2+ or [Ru(4dmb) 3 ] 2+ (4dmb = 4,4′-dimethyl-2,2′-bipyridine) as PSs. We selected three efficient and widely studied catalysts, i.e.…”

Nickel N-heterocyclic carbene complexes and their utility in homogeneous catalysis

“…The first approach utilizes the transmetallation route (Scheme 13) for preparing the (665), [201] (666−667), [202] (675), [203] (676), [161] (677−678), [204] (679), [203] (682), [205] (683), [206] (684−685), [207] (686−687), [208] (689), [209] (690), [210] (691), [211] (692−695), [203] (697−702), [203] (703−706), [212] (707), [213] (708) [214] and (709−710) [215] complexes from the respective Ag(I)−NHC complexes by the treatment with various Ni(II) precursors namely, NiCl 2 (PPh 3 ) 2 , (DME)NiX 2 (X = Cl, Br) and…”

“…The following nickel complexes of the chelated NHC ligands, (685−687) [208] and (703) [212] have been employed in the electro-and photo-catalytic conversion of CO 2 to CO. …”

Hybrid bioinorganic approach to solar-to-chemical conversion

“…Elegant advances in traditional approaches to CO 2 reduction driven by electrical and/or solar inputs using homogeneous (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16), heterogeneous (17)(18)(19)(20)(21)(22)(23)(24)(25)(26), and biological (7,(27)(28)(29)(30)(31) catalysts point out key challenges in this area, namely (i) the chemoselective conversion of CO 2 to a single product while minimizing the competitive reduction of protons to hydrogen, (ii) long-term stability under environmentally friendly aqueous conditions, and (iii) unassisted light-driven CO 2 reduction that does not require external electrical bias and/or sacrificial chemical quenchers. Indeed, synthetic homogeneous and heterogeneous CO 2 catalysts are often limited by product selectivity and/or aqueous compatibility, whereas enzymes show exquisite specificity but are generally less robust outside of their protective cellular environment.…”