Article pubs.acs.org/IC
Catalytic Hydride Transfer to CO2 Using Ru-NAD-Type Complexes under Electrochemical Conditions Debashis Ghosh,† Katsuaki Kobayashi,‡ Takashi Kajiwara,† Susumu Kitagawa,† and Koji Tanaka*,† †
Institute for Integrated Cell-Material Sciences (KUIAS/iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
‡
S Supporting Information *
ABSTRACT: The catalytic performance of Ru-NAD-type complexes [Ru(tpy)(pbn)(CO)]2+ ([1]2+; tpy = 2,2′;6′,2″terpyridine; pbn = 2-(pyridin-2-yl)benzo[b][1,5]naphthyridine) and the Ru-CO-bridged metallacycle [2]+ was investigated in the context of the electrochemical reduction of CO2 in H2O/CH3CN at room temperature. A controlled-potential electrolysis of [1]2+ and [2]+ afforded formate (HCOO−) as the main product, under concomitant formation of minor amounts of CO and H2. Metallacycle [2]+ showed a higher selectivity toward the formation of HCOO− than [1]2+ (HCOO−/CO for [1]2+, 2.7; HCOO−/CO for [2]+, 7). The generation of HCOO− via a catalytic hydride transfer from the NADH-type ligands of [1]2+ and [2]+ to CO2 was supported by the experimental results and a comparison with the reduction of CO2 catalyzed by [Ru(tpy)(bpy)(CO)]2+ under similar conditions. A mechanism for the catalytic reduction of CO2 by [1]2+ and [2]+ was proposed based on the experimental evidence. The thus-obtained results may help to expand the field of NADH-assisted reduction reactions.
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INTRODUCTION The increase of the concentration of CO2 in the atmosphere is one of the most serious causes for global warming,1 and establishing control over anthropogenic CO2 emissions has hence become of paramount importance.2 One strategy to reduce the release of anthropogenic CO2 into the atmosphere is to transfer it into value-added chemicals or fuels.3 Thus, it is hardly surprising that the development of new catalytic systems that use solar4 or electrical5 energy for the reduction of CO2 to, e.g., HCOOH, CO, or alcohols, has received increased attention from the global research community. Among the various conceivable CO2 reduction products, HCOOH (often obtained as HCOO−) is highly important because it can serve as a hydrogen-storage material,6 a reducing agent for organic compounds,7 a liquid fuel in formic acid fuel-cell applications,8 and as a feedstock for bacteria to produce higher alcohols as gasoline substitutes.9 Therefore, the selective formation of HCOO− from CO2 under photo- and/or electrochemical conditions is currently a major research target. One approach to develop such methods is based on the mimicking of natural photosystems (e.g., Photosystem I).10 In natural photosystems, the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) stores a hydride, i.e., the equivalent of two electrons and a proton, which are ultimately used to reduce CO2. Converting NAD+-like ligands in transition-metal complexes11 to NADH-type ligands should © XXXX American Chemical Society
accordingly generate species that are capable of transferring multiple redox equivalents in the form of H−, thus avoiding the high-energy pathways typically associated with single-electrontransfer steps. Previously, we have reported that [Ru(bpy)2(pbn)]2+ (bpy = 2,2′-bipyridine; pbn = 2-(pyridin-2yl)benzo[b][1,5]naphthyridine),12 which contains an NAD+type ligand undergoes electro- and photochemical two-electron reductions in aqueous solution to form the corresponding NADH-type two-electron-reduced complex [Ru(bpy) 2 (pbnHH)] 2+ (pbnHH = 2-(pyridin-2-yl)-5,10dihydrobenzo[b][1,5]naphthyridine).13−16 The treatment of [Ru(bpy)2(pbnHH)]2+ with RCOO− (R = CH3 or C6H5) in CH3CN under an atmosphere of CO2 afforded HCOO− under concomitant formation of [Ru(bpy)2(pbn)]2+.17 Large kinetic isotope values (k H /k D = 5−8) revealed that [Ru(bpy)2(pbnHH)]2+ acts as a hydride donor in the presence of a base.18 However, this method furnishes only stoichiometric amounts of HCOO− (maximum turnover number for HCOO− = 1.25) because of the absence of a proton donor. Recently, Dyer et al.10a have reported the electrochemical reduction of CO2 catalyzed by 6,7-dimethyl-4-hydroxy-2-mercaptopteridine (PTE), which exhibits hydride-transfer properties. However, Saveant and Tard have very recently10b reported that PTE does Received: June 5, 2017
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DOI: 10.1021/acs.inorgchem.7b01427 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
couple (Ep,c = −0.84 V) possibly because of the two-electron reduction of pbn to afford the Ru-pbnHH scaffold.14,16,19 The reversible redox couple at E1/2 = −1.26 V was thus assigned to the tpy/tpy•− redox couple, given that [Ru(tpy)(bpy)(CO)]2+ exhibits the tpy/tpy•− redox couple at E1/2 = −1.29 V in CH3CN (Figure 2c).20 The structure of the isolated [1.HH]2+ (Figure 3) has already been confirmed by single-crystal X-ray diffraction analysis.19
not catalyze the reduction of CO2 under the above-reported electrochemical conditions. To the best of our knowledge, other reports on the reduction of CO2 to obtain HCOO− using renewable hydride sources remain elusive. The development of efficient catalytic systems for the reduction of CO2 to HCOO− using renewable hydride sources should thus be highly desirable. In this context, we have reported the synthesis of [Ru(tpy)(pbn)(CO)]2+ ([1]2+) and the Ru-CO-bridged metallacycle [2]+, which contains a pbn moiety (Figure 1).19 Herein, we disclose the catalytic formation of HCOO− (major product) and CO (minor product) from the electrochemical reduction of CO2 mediated by [1]2+ and [2]+.
Figure 3. Chemical structure of [1.HH]2+.
Figure 4 shows the AcOH-dependent cyclic voltammograms of [2]+ in CH3CN (scan rate = 50 mV s−1; room temperature). A cathodic potential sweep exhibited two reversible waves at E1/2 = −1.07 and −1.57 V (Figure 4a; black) in the absence of AcOH. The reversible redox couple at E1/2 = −1.57 V was strongly influenced by the addition of AcOH: the peak current of this reversible redox couple decreased with increasing
Figure 1. Chemical structures of [1][PF6]2 and [2][PF6], in which the hexafluorophosphate [PF6] is a complex counterion.
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RESULTS AND DISCUSSION Electrochemical Behavior of [1]2+ and [2]+ at pH
