Remarkable accelerating and decelerating effects of the bases on CO2 reduction using a ruthenium NADH model complex
Introduction
Development of innovative compounds having the ability to reduce carbon dioxide (CO2), an ultimate oxidation product of organic compounds, toward value-added chemical compounds [1], [2], [3], [4], [5], [6], [7], [8], [9], such as methanol [10], [11], [12], [13] and formic acid [14], [15], [16], [17], [18], [19], [20], [21], has become one of the most researched topics related to global warming and the depletion of fossil fuels [22]. Since the one electron reduction of CO2 to CO2
– is highly unfavorable, NAD+/NADH redox function in NAD (nicotinamide adenine dinucleotide) acting as a reservoir/source of hydride ion (equivalent to two electrons and one proton) in biological systems [23], [24], [25] has great potential as a renewable hydride donor system for innovative functions regarding CO2 conversion to value-added compounds.
There have been many studies with regard to organic NAD model compounds [26], [27] and transition metal complexes having NAD model ligands [28], [29], [30] to understand their unique redox and photophysical properties as well as photochemical and thermal reactivity. We have previously demonstrated that a ruthenium complex possessing a newly designed NAD+ model ligand, [Ru(bpy)2(pbn)](PF6)2 (1) (bpy = 2,2′-bipyridine, pbn = 2-(pyridin-2-yl)benzo[b][1,5]naphthyridine) [31], is efficiently reduced to the corresponding NADH-type two-electron reduced complex, [Ru(bpy)2(pbnHH)](PF6)2 (1·HH) (pbnHH = 2-(pyridin-2-yl)-5,10-dihydrobenzo[b][1,5]naphthyridine), when irradiated with visible light in the presence of a sacrificial reagent [32], [33], [34], [35]. Furthermore, we have recently accomplished the organic hydride reduction of CO2 to the corresponding formate (HCO2–) by C
H bond dissociation of the NADH model ligand, pbnHH, in 1·HH, driven by the association of the base, benzoate anion (PhCOO–), for the first time [36]. However, neither experiment has provided valuable insight into the effects of other bases on this CO2 reduction system in 1·HH. Moreover, no studies have been reported on the control over hydride donor ability by variation of the base.
Herein, we report that a drastic difference in the organic hydride transfer reaction converting CO2 to HCO2− using the ruthenium complex containing the NADH model ligand (1·HH) is observed by changing the base to either acetate anion (MeCOO−) or trifluoroacetate anion (CF3COO−). In addition, the base adduct formed from the reaction between 1·HH and the base has been fully characterized by absorption spectroscopy, 1H-NMR spectroscopy, ESI-Mass spectrometry, and electrochemical analyses. The present study has demonstrated that the choice of the base plays a key role in the CO2 reduction system utilizing 1·HH through the association of the base to the N–H moiety of the NADH model ligand pbnHH (Scheme 1); the difference in basicity between MeCOO− and CF3COO− lead to notable accelerating and decelerating effects on the rate of the organic hydride transfer reaction, as compared to PhCOO−.
Section snippets
Materials
All chemicals used for synthesis of the ligand and complex were purchased at the highest available purity and further purified by standard methods [37]. All solvents were purified by standard methods prior to use [37]. Tetra-n-butylammonium acetate and tetra-n-butylammonium trifluoroacetate were used as bases. The pbn ligand was prepared according to a literature procedure [31]. The NADH model complex, [Ru(bpy)2(pbnHH)](PF6)2 (1·HH), was obtained by chemical reduction of the NAD+ model complex,
Results and discussion
Association of the base, PhCOO–, to the N–H moiety of the pbnHH ligand in 1·HH has recently been found to be indispensable for the organic hydride transfer reaction producing HCO2– from CO2 [36]. This finding inspired us to research the effect of other bases, such as MeCOO– and CF3COO–, not only to clarify the base effect on the CO2 reduction system, but also to control the hydride donor ability of 1·HH. Addition of an excess amount (10 equivalents) of MeCOO− to a CO2 saturated acetonitrile (CH3
Conclusions
This study has clearly shown that the difference in basicity of MeCOO−, PhCOO−, and CF3COO− enables us to control the rate of organic hydride reduction of CO2 producing the corresponding HCO2–, which is driven by the association of the base to the N–H moiety of the NADH model ligand, pbnHH, in 1·HH. In particular, the addition of MeCOO– to the 1·HH solution under a saturated CO2 atmosphere results in the organic hydride transfer reaction to produce HCO2– from CO2, whereas no reaction proceeds
Acknowledgments
This work was supported in part by Grant-in-Aids for Scientific Research B (No. 26288024, K.T.) and for Scientific Research C (No. 25410067, H.O.) from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT).
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