The fundamental challenge of reducing CO2 into more valuable energy-containing compounds depends on revealing new catalysts for this process. By removal of the long-standing limitation of α-diimine ligation, which is dominant in photocatalytic complexes in this area, new visible-light, CO2-reducing photocatalysts based on Mn and Re supported by κ2-PN phosphinoaminopyridine ligands were identified. These catalysts, [M­{κ2-(Ph2P)­NH­(NC5H4)}­(CO)3Br], displayed excellent product selectivity and, by a change of only the metal center, gave a dramatic product switch from CO with M = Mn to HCO2H with M = Re. The catalyst systems were explored with variation of the ligand, electron donor, solvent, and photosensitizer. The products were definitively traced using 13CO2 as a substrate. Both complexes quenched the excited-state photosensitizer Ru­(bpy)3 2+*, suggesting oxidative quenching as a potential entry into the catalytic cycle.

cis-[Re(bpy)2(CO)2]+OTf− is a new integrated photosensitizer/catalyst for the selective visible light promoted photocatalytic reduction of CO2 to yield formic acid.

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 ...

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