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

Chemical Science EDGE ARTICLE Cite this: Chem. Sci., 2016, 7, 6728 Rhenium(I) trinuclear rings as highly efficient redox photosensitizers for photocatalytic CO2 reduction† Jana Rohacova and Osamu Ishitani* We developed new cyclic Re(I)-based trinuclear redox photosensitizers with both high oxidation power in the excited state and strong reduction power in the reduced form. These excellent properties were achieved by introducing electron-donating groups on the diimine ligand of the Re(I) metal centre and by connecting each Re(I) unit with polyphenyl–bisphosphine bridging ligands. These Re-rings were applied to homogenous visible light-driven photocatalytic CO2 reduction in conjunction with various mononuclear catalysts, such as Re(I), Ru(II) and Mn(I) metal complexes, employing a relatively weak sacrificial electron donor, triethanolamine. Each system showed good product selectivity (CO or HCOOH) and an excellent quantum yield of product formation FCO ¼ 0.60 to 0.74 using fac- Received 2nd May 2016 Accepted 4th July 2016 [ReI(bpy)(CO)3(CH3CN)]+, FHCOOH ¼ 0.58 using trans(Cl)–RuII(dtbb)(CO)2Cl2 and FHCOOH ¼ 0.48 using a fac-[MnI(dtbb)(CO)3(CH3CN)]+ catalyst. The high photocatalytic efficiencies for CO2 reduction are DOI: 10.1039/c6sc01913g attributed to efficient reductive quenching of the Re-ring by triethanolamine and fast electron transfer www.rsc.org/chemicalscience from the generated one-electron-reduced species of the ring to the catalyst. Introduction Redox photosensitizers (PSs) have been widely used in various photocatalytic reactions such as for organic synthesis, dyesensitized solar cells, photoinduced H2 or O2 production from water and reduction of CO2.1–8 The rst step of the photosensitization is the photoexcitation of the PS, followed by a reductive or oxidative quenching reaction with a substrate or semiconductor particles and electrodes. The produced oneelectron-reduced or one-electron-oxidized species (OERS or OEOS, respectively) donates an electron or hole, respectively, to another substrate in the nal process of the photosensitization. Therefore, PSs are required to have the following properties: (1) stability of the excited state, (2) stability of the OERS and/or OEOS, (3) strong oxidation and/or reduction power in the excited state and (4) strong reduction or oxidation power of the OERS or OEOS. Some transition-metal complexes are frequently used as PSs not only because they full the aforementioned requirements but also because they have a strong absorption in the visible region, which is an important feature for solar energy conversion. Most reported PSs are mononuclear metal complexes, and Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-NE-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan. E-mail: † Electronic supplementary information (ESI) available: Franck–Condon analysis; photochemical one-electron-reduced species formation and characterisation; photophysical, electrochemical and quenching properties of R(4$5) in DMA; photophysical and electrochemical properties of the catalysts; photocatalytic CO2 reduction experiments and additional data. See DOI: 10.1039/c6sc01913g 6728 | Chem. Sci., 2016, 7, 6728–6739 their types have been limited mostly to Ru(II)-diimine and cyclometalated-Ir(III) complexes and their derivatives. Although some metal-porphyrins and metal-phthalocyanines as well as Pt(II)-, Os(II)-, Re(I)- and Fe(II)-diimine complexes have also been investigated as PSs, they are used only in limited types of reactions.8–14 On the contrary, the possibilities of using multinuclear metal complexes as PSs have only been scarcely investigated to date.15 We recently reported the photochemical synthesis of ringshaped multinuclear Re(I) complexes with cis, trans[Re(bpy)(CO)2(P–P)2]+ (bpy ¼ 2,20 -bipyridine, P–P ¼ PPh2– (CnHm)–PPh2) as repeating units.16–18 They exhibited outstanding photophysical and electrochemical properties, such as high emission quantum yields, along with long lifetimes of the 3MLCT excited states, even in solution at room temperature, and stability in the excited state and stronger oxidation power in the excited state compared to the corresponding mononuclear Re complex. They can also photochemically accumulate multiple electrons in one molecule. In the rst report on these Re-rings, we also briey introduced the idea that the Re-ring could be used as an extremely efficient PS for photocatalytic CO2 reduction in tandem with a Re(I) catalyst under visible light irradiation. Herein, we report the potentialities of these Re-rings as PSs in detail. Newly designed and synthesized trinuclear Re-rings, R(X), where each Re(I) unit is connected with p-bis(diphenylphosphino)benzene (Chart 1), were applied to photocatalytic CO2 reduction with three kinds of typical catalysts, namely fac[Re(bpy)(CO)3(CH3CN)]+, trans(Cl)–Ru(dtbb)(CO)2Cl2 and fac[Mn(dtbb)(CO)3(CH3CN)]+ (dtbb ¼ 4,40 -di-tert-butyl-2,20 - This journal is © The Royal Society of Chemistry 2016 Edge Article Chart 1 Structure and abbreviations of the trinuclear Re(I) rings R(X). All complexes were synthesized as PF6 salts. bipyridine), whose structures are shown in Chart 2. In all cases, the quantum yields of CO2 reduction were very high. Chemical Science cyclization process can be completed simultaneously. A tricarbonyl Re(I)–diimine complex was reacted with Me3NO, which is reported to be an effective decarbonylation reagent for metal carbonyl complexes,19,20 affording the biscarbonyl-Re(I) mononuclear complex as a building block with a labile ligand (Re(X)ph-L, L ¼ solvent molecule, Scheme 1). These reactions were instantaneous even under mild conditions and proceeded quantitatively. Prolonged reux without any additional reagents led to both coupling and cyclization, affording the corresponding trinuclear Re-ring as the major product, with larger linear and eventually ring-shaped Re(I) multinuclear complexes as minor products (Fig. S1, ESI†). The Re-rings R(4$5), R(OMe) and R(5) were successfully isolated from the reaction mixtures using size exclusion chromatography in ca. 20% yield, which is highly comparable with or better than the total yields of the multi-step synthetic strategy, including the photochemical reaction (Scheme S1, ESI†). Photophysical properties Results and discussion Design and synthesis of the Re-rings The design of the new Re-rings was based on our previous work, which clearly demonstrated the relationship between the ring structures (i.e., the size of the ring and the type of the bridging bisphosphine ligand P–P) and their photophysical and electrochemical properties.16–18 The trinuclear Re-ring connected with a phenylene spacer in the bisphosphine ligand exhibited both a long lifetime and a strong oxidation power in the excited state. The corresponding OERS, which are important intermediates in the redox-photosensitized reactions, were relatively stable. In order to tune these propertie (...truncated)