Visible‐Light Photocatalytic Reduction of CO₂ to Formic Acid with a Ru Catalyst Supported by N,N′‐Bis(diphenylphosphino)‐2,6‐diaminopyridine Ligands

Abstract

Visible-light photocatalytic CO 2 reduction is carried out by using aR u II complex supportedb yN,N'-bis(diphenylphosphino)-2,6-diaminopyridine( "PNP")l igands, an unprecedented molecular architecture for this reaction that breaks the longstanding domination of a-diimine ligands. These competent catalysts transform CO 2 into formic acid with high selectivity and turnover number.Aproposed mechanism, with combined electron transfer and catalytic cycles, models the experimental rate of formic acid production.Designing and assembling catalysts that can utilize the energy of visible light to overcome the barriers to reduce CO 2 is an important fundamentala nd technological challenge. Success in this endeavor requires confronting the inherent stabilitya nd low thermodynamic value of CO 2 and would transform this ubiquitous waste product into av aluable feedstock. For example, photocatalytic formation of formic acid, at wo-electron reductionp roduct of CO 2 ,w ould yield ac ommodity chemical and al iquid fuel. Furthermore, formic acid has been identified and explored as ap otential carrier of dihydrogen. [1,2] Homogeneous catalysts have been developed for CO 2 reduction under both electrochemical and photochemical conditions. [3][4][5][6] Photocatalysis is ap articularly appealing approacha s it relies on an essentially limitless and clean solar energy source.P hotocatalytic systemsc onsisto fi ntegrated components that include ap hotosensitizer (PS)f or harvesting the energy of the light, an electron donor (ED)t hat provides electrons for the reduction, and ac atalyst (CAT)t hat is as ite for the transformation of CO 2 .T oo ur knowledge,s ince their discovery in 1985, [7] all of the reported molecular photocatalysts based on Ru II have used a-diimine supporting ligandsa nd these speciesh ave fallen into two broad groups.O ne class are bis(a-diimine) speciesr epresentedb ycis-[Ru(N^N) 2 (CO) 2 ] 2 + [8][9][10][11] and the other are mono(a-diimine) catalysts represented by cis,trans-[Ru(N^N)(CO) 2 Cl 2 ]. [12][13][14] Althoughavariety of substituents on the a-diimine ligands have been productively explored to improvec atalystp erformance, given the maturity of this field, ab roader variationo fm olecular architecture is required to provide new insights and stimulate new concepts in this field. Ligand variation and discoverya re ac entral challenge in catalysis and expanding Ru-based photocatalysts beyond the restrictions of a-diimine support is certainly warranted. Support for this approachc omes from av ery recent report on the use of ap hotocatalytic phosphine-substituted Ru II terpyridine complex-trans-[Ru(tpy)(8-quinolyl(diphenyl)phosphine)-(MeCN)] 2 + -that functions as both ap hotosensitizer andacatalyst for CO 2 reduction. [15,16] Recently,p incer ligand-supported catalysts of Fe, Co, Ru, and Ir have been shown to have excellent activity and selectivity for hydrogenation of CO 2 . [17][18][19][20][21][22][23] Pincer-supported ruthenium complexes can catalyze hydrogenation of CO 2 to yield formate, as wella st he ...

Towards Sustainable Oxalic Acid from CO2 and Biomass

“…Hence, the most common systems for this process are homogeneous catalysts, which have been first used by Tanaka and co‐workers in the 1980s [228] . In recent years this topic gained new interest and many new systems were studied and developed [224,229–235] . The selectivity of the homogeneous systems depends on a myriad of parameters in the process design including the solvent for CO 2 solubilization, electron and proton sources, photosensitizers for light‐harvesting, and lastly the design of the catalysts itself [224] .…”

Photochemical reduction of carbon dioxide to formic acid

“…Similar results were obtained, however a slightly lower efficiency was observed. 49 In 2020, Arikawa et al used CNC pincer complexes since they were known to incorporate CO 2 . 50 In their experiment, CO 2 was reduced in the presence of the CNC Ru catalyst [Ru (dmb) 3 ](PF 6 ) 2 (23) (dmb = 4,4′-dimethylbipyridine) as a photosensitizer, BI(OH)H as the sacrificial electron donor and DMA/ TEOA as the solvent mixture.…”

Rational Construction of Light-Driven Catalysts for CO2 Reduction

“…Most of the molecular catalysts consist of noble-metal centers as active and adsorption sites, combining noble-metal centers with photoresponsive catalysts not only minimizes the cost but also accelerates electron transport. For example, Ru- and Re-based molecular catalysts can be incorporated into photoelectrode or photocatalysts for CO 2 reduction, showing high selectivity to CO production and robust catalytic sites. ,, Ishitani et al incorporated an Ru­(II)–Re­(I) supramolecular photocatalyst and Ru­(II) redox photosensitizer into an NiO electrode for PEC CO 2 reduction, and the well-controlled poly-RuRe/NiO showed 778 nmol of PEC production at −0.7 V vs Ag/AgCl for 5 h . Considering the cost of noble metal complexes, it is feasible to replace noble metals with transition metals for CO 2 reduction, such as Co-tetraaza­macro­cyclic complexes, Co-porphyrin, Co-phthalocyanine, derived cis -CoN 4 complexes, cobalt phthalocyanine (CoPc), − etc.…”

“…For example, Ru-and Re-based molecular catalysts can be incorporated into photoelectrode or photocatalysts for CO 2 reduction, showing high selectivity to CO production and robust catalytic sites. 60,121,122 Ishitani et al incorporated an Ru(II)− Re(I) supramolecular photocatalyst and Ru(II) redox photosensitizer into an NiO electrode for PEC CO 2 reduction, and the well-controlled poly-RuRe/NiO showed 778 nmol of PEC production at −0.7 V vs Ag/AgCl for 5 h. 123 Considering the cost of noble metal complexes, it is feasible to replace noble metals with transition metals for CO 2 reduction, such as Cotetraazamacrocyclic complexes, Co-porphyrin, Co-phthalocyanine, derived cis-CoN 4 complexes, cobalt phthalocyanine (CoPc), 124−126 etc. Recently, Reisner et al employed polymeric CoPc coupled with mesoporous C 3 N 4 (mpg-CN x ) as a photosensitizer for photocatalytic CO 2 reduction.…”