Photogeneration of Hydride Donors and Their Use Toward CO2 Reduction Page: 3 of 7
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B N L-90201-2009-CP
PHOTOGENERATION OF HYDRIDE DONORS AND THEIR USE TOWARD CO2
Etsuko Fujita, James T. Muckerman, Dmitry E. Polyansky
Brookhaven National Laboratory
Upton, NY 11973-5000
Despite substantial effort, no one has succeeded in efficiently producing methanol from CO2
using homogeneous photocatalytic systems. We are pursuing reaction schemes based on a
sequence of hydride-ion transfers to carry out stepwise reduction of CO2 to methanol. We are
using hydride-ion transfer from photoproduced C-H bonds in metal complexes with bio-inspired
ligands (i.e., NADH-like ligands) that are known to store one proton and two electrons.
1. Photogeneration of Hydride Donors: We have shown that a polypyridylruthenium complex
with an NAD+/NADH model ligand, [Ru(bpy)2(pbn)]2+ (2+, bpy = 2,2'-bipyridine, pbn = 2-(2-
N -2 zH 2. pyridyl)-benzo[b]-1,5-naphthyri-dine) in a
N N wet CH3CN/amine solution, undergoes
NRu] EN/HN H H proton-coupled two-electron reduction to
1.5 2 [1.HH]2+ give [Ru(bpy)2(pbnH)]2+ ([1HH]2+
After Irradiation pbnHH = 5,10-dihydro-2-(2-pyridyl)-
- benzo[b]-1,5-naphthyridine) upon irradiation
. Before Irradiation of visible light (300-600 nm). When
-~ triethylamine was used as a sacrificial
electron donor, the quantum yield for
U 40 500 6o goo soo [1-HH]2+ formation is 0.21 at 355 nm. This
Wavelength, nm result opens a new door to photoinduced
catalytic hydride-transfer reactions !
2. Kinetic and Mechanistic Investigation of Hydride Donor Generation by Pulse Radiolysis:
Using the pulse radiolysis technique, we determined the mechanism of formation of [1-HH]2+ in
water. Protonation of the one-electron- H
reduced species  takes place below N N - N N H N 2
pH 11 to form  (pKa -11). Species  -NtRu 2+ I -NfRI] 2 pa=1. N' u H 3
disproportionates through a 7-stacked 3+HI 3
dimer at low pH (see below), and the 2]. k=1.2X1O M s 22X1M
dimer dissociates to yield 2+ and H [Ru]-N 4+
[1HH]2+. The cross reaction between i 'N N 2+
 and  yields the same final -NjRJu H H 2 N-Hu
products at high pH, probably by [1-HH]2+
forming a N-H N hydrogen bonding interaction between the two species, the transfer of an
electron to , which subsequently acquires a proton.
3. Stereo-Specific Photochemical Formation of a C-H Hydride: Our photolysis experiments
with D20 and H20 solutions containing 2+/CH3CN/triethanolamine produced [1-DD]2+ (m/z
337.5838) and [1-HH]2+ (m/z 336.5725), respectively. 1H and 13C NMR indicate that stereo-
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Fujita,E.; Muckerman, J.T. & Polyansky, D.E. Photogeneration of Hydride Donors and Their Use Toward CO2 Reduction, article, June 7, 2009; United States. (digital.library.unt.edu/ark:/67531/metadc932077/m1/3/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.