Homogeneous Photocatalytic CO 2 Reduction to CO

In homogeneously catalyzed photochemical CO2 reduction, a distinction between photosensitized, photoinduced and photoassisted catalysis has to be made.^[201] In case of photosensitized catalysis, the catalytic system consists of a photosensitizer, which undergoes excited state reduction by a sacrificial electron/proton donor after light absorption. Electron transfer to a molecular catalyst then gives the species active in CO2 reduction. Accordingly, homogeneous catalysts for electrochemical CO2 reduction are in principle also suitable for photosensitized catalysis. In photoinduced carbon dioxide reduction by a catalyst, the CO2 reduction is performed by a compound which is produced from a precursor in a photochemical reaction. Photoassisted catalysis means that the photochemical reaction is part of the actual catalytic circle.

Accordingly, it is a prerequisite for performing thermodynamically uphill reactions. The mechanisms for electrochemical and photochemical CO2 reduction can be closely related not only in photosensitized CO2

reduction. Consequently, the photocatalysts discussed in the following are also active in electrochemical CO2 reduction, which can proof valuable concerning mechanistic studies.^[202]

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Following seminal work by Ziessel, complexes of the general structure [ReX(CO)3(bpy)] (X = Cl, Br) are active in visible light photochemical reduction of CO2 giving CO in high selectivity.^[203] Triethanolamine (TEAO) or triethylamine (TEA) is commonly employed as sacrificial electron and proton donor.

Photoexcitation results in population of a strongly oxidizing excited state which forms formal Re⁰ [Re(CO)3(bpy)] by reduction of the complex and halide loss. Electronic structure investigation suggests intramolecular electron transfer from the metal to the diimine ligand rather than a Re⁰ metalloradical.^[204] In case of electrocatalytic CO2 reduction by [ReX(CO)3(bpy)] type catalysts, initial two electron reduction opens up a second mechanism for substrate activation, which is not discussed here.^[205]

Scheme 30: Reactivity of [ReCl(CO)3(bpy)] upon photolysis in the presence of CO2.

Productive carbon dioxide activation by [Re(CO)3(bpy)] can proceed via a dimeric or a monomeric pathway.

As shown in Scheme 30, a carboxylate bridged Re^I-Re^I dimer or a Re^I hydroxycarbonyl is formed, respectively.^[204] The dimeric complex has been shown to liberate CO upon reaction with an additional equivalent CO2 giving a bridged carbonate. Based on computational analysis, Fujita proposes CO2 insertion into the rhenium oxygen bond followed by decarbonylation rather than deoxygenation of coordinated CO2.^[206] The monomeric metallacarboxylic acid [Re(CO2H)(CO)3(bpy)] forms hydrocarbonate [Re(OCO2H)(CO)3(bpy)] under CO2 atmosphere when irradiated. As in the dimeric pathway, CO is liberated and computational analysis suggests initial formation of tetracarbonyl [Re(CO)4(bpy)]⁺ by hydroxide abstraction.^[207,208] Fujita proposes a minor role of the monomeric path based on the low concentration of [Re(CO2H)(CO)3(bpy)] in stoichiometric CO2 reduction by in situ generated [Re(CO)3(bpy)].^[204] Re^I hydride complexes are not directly involved in CO formation, however photochemical decarboxylation of the hydroxycarbonyl is shown by Gibson.^[209] Alternatively, net hydrogen atom transfer from TEA/TEAO to [Re(CO)3(bpy)] is a plausible route for hydride formation. While the

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formed hydride can insert CO2 thermally or photochemically to give a formate, the lack of catalytic formate production suggests this to be an unproductive reaction.^[210,211]

Scheme 31: Mechanistic picture of photochemical CO2 reduction by [IrCl(tpy)(ppy)]PF6.

In homogeneous thermal rWGS catalysis, metal hydrides play a role in CO formation. Photo- and electrocatalytic systems discussed so far coordinate CO2 on a reduced metal center, instead. Hydrides are rather seen as intermediates in formation of side products like H2 and formate. Turning to iridium based photocatalysts, the situation changes. In 2013, Ishitani reported selective photocatalytic CO2 reduction to CO in the presence of TEAO by Ir^III precatalyst [IrCl(tpy)(ppy)]PF6 (tpy: terpyridine, ppy:

2-phenylpyridinyl).^[212] Hydride N-trans-[IrH(tpy)(ppy)]⁺ was identified as active catalyst and photochemical reactivity of the Ir^III hydride was assumed due to the lack of thermal reactivity of N-trans-[IrH(tpy)(ppy)]PF6 with CO2. Mechanistic work by Fujita attributes this stability in the presence of CO2 to the low hydricity of the complex.^[213] Small quantities of formate produced in the catalytic experiment are formed by isomerization to the unfavorable isomer C-trans-[IrH(tpy)(ppy)]PF6 which readily inserts CO2

due to a more labile hydride ligand. Importantly, isomerization is relevant in photocatalytic CO2 reduction as well. Once formed, C-trans-[IrH(tpy)(ppy)]PF6 acts as a photoacid, forming low-valent C-trans-[Ir(tpy)(ppy)] upon irradiation.¹⁶ Alternatively, Ir^I formation can take place from the thermodynamically favored isomer N-trans-[IrH(tpy)(ppy)]PF6 by photoexcitation and subsequent reduction by TEAO.

Notably, reduction of N-trans-[IrH(tpy)(ppy)]PF6 results in formation of C-trans-[Ir(tpy)(ppy)] as well,

16 C-trans-[Ir(tpy)(ppy)] shows a free coordination site trans to the carbon donor of the ppy ligand.

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showing that this isomer is favored on the pentacoordinate species. Computational analysis reveals that C-trans-[Ir(tpy)(ppy)] binds CO2 to yield a carboxylate and subsequent PCET steps result in formation of CO (Scheme 31).

Ishitani’s [IrCl(tpy)(ppy)]PF6 based system involves hydrides as intermediates in CO selective CO2

reduction. While hydrides are generated by oxidation of H2 in rWGS catalysis, the active Ir^III hydride is formed by oxidation of sacrificial TEAO, here. Photochemical rWGS catalysis, i.e. using H2 as reductant instead of sacrificial donors, is a largely undeveloped field of research. Heterogeneous materials for photocatalytic CO2 conversion to CO using H2O as reductant suffer from proton reduction as side reaction, motivating the use of H2 instead. Magnesium oxide was used by Tanaka as rWGS photocatalyst.^[214]

Interestingly, mechanistic work shows that a surface-bound formate is formed by CO2 adsorption, photoinduced electron transfer and subsequent reaction with H2. The formate is photochemically active and reduced another CO2 molecule upon photoexcitation.^[215] Tahir investigated rWGS catalysis in a TiO2 based monolith photoreactor and could show higher activity compared to the use of H2O as reductant.^[216] The mechanism of CO2 activation is assumed to be mostly identical to the photoelectrochemical approach discussed previously.

Homogeneous rWGS photocatalysis is limited to a notable example by Neumann.^[217,218] Here, a three-component system is used consisting of Pt/C for H2 activation and a tungsten based polyoxometallate [PW12O40]^3- covalently bound to the already discussed rhenium tricarbonyl photocatalyst for CO2 reduction.

Activation of H2 takes place heterogeneously on Pt/C and the activated hydrogen is transferred to the polyoxometallate giving separated protons and electrons. Photoinduced intramolecular electron transfer from the reduced polyoxometallate fragment to the rhenium catalyst results in formation of the crucial formal Re⁰ active species. The following CO2 reduction mechanism closely resembles the monomeric path discussed previously for [ReX(CO)3(bpy)]. However, in this example, all protons and electrons necessary are provided by the reduced polyoxometallate fragment. Connecting the polyoxometallate and the rhenium catalyst is crucial, as combination of the unmodified species does not give a catalytically active mixture.