Photocatalytic Reduction of CO₂ with Re-Pyridyl-NHCs

Abstract

A series of Re(I) pyridyl N-heterocyclic carbene (NHC) complexes have been synthesized and examined in the photocatalytic reduction of CO2 using a simulated solar spectrum. The catalysts were characterized through NMR, UV-vis, cyclic voltammetry under nitrogen, and cyclic voltammetry under carbon dioxide. The complexes were compared directly with a known benchmark catalyst, Re(bpy) (CO)3Br. An electron-deficient NHC substituent (PhCF3) was found to promote catalytic activity when compared with electron-neutral and -rich substituents. Re(PyNHC-PhCF3) (CO)3Br was found to exceed the CO production of the benchmark Re(bpy) (CO)3Br catalyst (51 vs 33 TON) in the presence of electron donor BIH and photosensitizer fac-Ir(ppy)3. Importantly, Re(PyNHC-PhCF3) (CO)3Br was found to function without a photosensitizer (32 TON) at substantially higher turnovers than the benchmark catalyst Re(bpy) (CO)3Br (14 TON) under a solar simulated spectrum.

A Robust Pyridyl-NHC-Ligated Rhenium Photocatalyst for CO2 Reduction in the Presence of Water and Oxygen

“…This decreased time scale, however, did not correlate to diminished reactivity which suggests that this catalyst is remarkably active toward CO 2 addition. Energetically, a slightly larger ∆G exists for the transfer of an electron from 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH; the SED used in our study) to the Re(bpy)(CO) 3 Br catalyst (1.03 V versus 1.14 V) [10]. The outer sphere electron transfer from the SED to the photoexcited catalyst (cat*) is presumably governed by Markus theory kinetics, with the ∆G values not being large enough to typically lie conclusively within the inverted region [28].…”

“…The Re-Br bond was found to substantially lengthen upon reduction by 0.08 Å when the neutral catalyst is compared with the singly reduced catalyst. The remaining Re-ligand The computational results also aid in understanding the unique irreversibility of the NHC catalysts toward single-electron reduction when compared with the reversible first reduction of Re(bpy)(CO) 3 Br [10]. The Re-Br bond was found to substantially lengthen upon reduction by 0.08 Å when the neutral catalyst is compared with the singly reduced catalyst.…”

“…Among known NHC-ligated photocatalysts, the phenyl-CF3 NHC substituent was previously found to give the most reactive catalyst when compared with phenyl, p-hexyloxyphenyl, or methyl substituents on the NHC ring [10]. Here, we reasoned that this group allows better delocalization of the added electron on the singly reduced catalyst across the ligand framework since the lowest energy excitation is thought to be a metal-to-ligand charge transfer (MLCT) followed by addition of an electron [24][25][26].…”

“…Despite more than 30 years of exploration, very few catalysts meet these criteria with a single metal center, and nearly all of them are based on one of five frameworks: Fe-p-TMA, Ir(tpy)(ppy)X, Ir(tpy)(bpy)X, Re(bpy)(CO) 3 X, or Re(pyNHC-R)(CO) 3 X (Figure 1) [7][8][9][10][11][12]. Among these catalysts, the Re(pyNHC-R)(CO) 3 X catalyst is unique in allowing the reduction of CO 2 to occur at the first reduction potential of the neutral catalyst ( Figure 2) [10,13]. Typically, CO 2 reduction photocatalysts first undergo photoexcitation followed by electron transfer from a sacrificial electron donor (SED) to give the anionic catalyst (Figure 2a).…”

“…Alternatively, after the first reduction sequence, the catalyst can In addition to a key mechanistic difference during photocatalytic CO2 reduction reactions, NHCligated catalysts have long been heralded as robust, highly reactive systems in general [20][21][22][23]. Concerning photocatalytic CO2 reduction with sunlight, only one catalyst framework has been put forward utilizing an NHC ligand [10]. Interestingly, when an electron-deficient aryl-CF3 group was used at the NHC wingtip, the catalyst was found to be faster reacting and more durable than Lehn's bipyridyl analogue under anhydrous conditions ( Figure 2).…”