An Organic Hydride Transfer Reaction of a Ruthenium NAD Model Complex Leading to Carbon Dioxide Reduction

Abstract

These are not the final page numbers! Ü Ü this is the first example of CO 2 reduction involving a renewable organic hydride.

Remarkable accelerating and decelerating effects of the bases on CO2 reduction using a ruthenium NADH model complex

“…1, and the rate constant is determined to be 2.2 Â 10 À2 s À1 from the slope of the plot. It should be noted that this rate constant (k obs(530 nm, MeCOO -) = 2.2 Â 10 À2 s À1 ) is nearly ten times larger than that in the PhCOO À system reported previously (k obs(530 nm, PhCOO À ) = 2.6 Â 10 À3 s À1 ) [36]. Such drastic differences in the CO 2 reduction rates of MeCOO À , PhCOO À , and CF 3 COO À systems can be ascribed to the basicity of the bases used, following the order MeCOO À (pK a = 23.51) > PhCOO À (pK a = 21.51) > CF 3 COO À (pK a = 12.65) [38].…”

“…According to previous results of absorption spectral changes in the PhCOO À system, CO 2 reduction to produce HCO 2 À consists of two distinct steps; the first is rapid formation of the base adduct of 1ÁHH with the base, PhCOO À , and the second is to afford the organic hydride transfer to CO 2 for the HCO 2 -production [36]. To confirm the formation of the base adduct in the MeCOO À system, Scheme 1.…”

“…Association of the base, PhCOO -, to the N-H moiety of the pbnHH ligand in 1ÁHH has recently been found to be indispensable for the organic hydride transfer reaction producing HCO 2 -from CO 2 [36]. This finding inspired us to research the effect of other bases, such as MeCOO -and CF 3 COO -, not only to clarify the base effect on the CO 2 reduction system, but also to control the hydride donor ability of 1ÁHH.…”

“…5, and the K value is determined as 1.1 Â10 5 M À1 from the slope of the plot. The larger K value of the MeCOO À system compared with that of the PhCOO À system (K = 7.8 Â 10 4 M À1 ) [36] is most likely due to the difference in the basicity between the bases (vide supra). We have also investigated the redox behavior of the base adduct, [Ru(bpy) 2 (pbnHH) (MeCOO -)] + , using cyclic voltammetry measurements.…”

“…We have previously demonstrated that a ruthenium complex possessing a newly designed NAD + model ligand, [Ru(bpy) 2 (pbn)](PF 6 ) 2 (1) (bpy = 2,2 0 -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)](PF 6 ) 2 (1ÁHH) (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 CO 2 to the corresponding formate (HCO 2 -) by CÀ ÀH bond dissociation of the NADH model ligand, pbnHH, in 1ÁHH, 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 CO 2 reduction system in 1ÁHH.…”