Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO₂ to Formate by Ruthenium Complexes

Abstract

A series of 4,4′-dimethyl-2,2′-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru­(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways.

Promoting photocatalytic CO2 reduction with a molecular copper purpurin chromophore

“…The TCD detector and injection port were kept at 100 and 200 o C, respectively. 13 C isotopic labeling experiments were carried out in a 13 CO 2 atmosphere and gas products were detected by GC-MS (Agilent 7890A-5975C). HPLC (HP1100, Hewlett Packard) was used to analyze the photolysis products in the solution.…”

“…To achieve a high activity in photocatalytic CO 2 conversion, it is essential to develop satisfactory photosensitizers that have large absorption cross sections in the visible region and are capable of promoting efficient multiple electron transfer processes. Recent studies on precious metal-based PS (such as Ru, Ir, and Re) have shown good activity for CO 2 reduction [12][13][14][15][16][17][18][19][20][21][22][23][24][25] . However, molecular chromophores comprising earth-abundant elements usually give relatively low turnover numbers (TONs), which is a significant limitation in developing inexpensive AP systems [26][27][28][29] .…”

Water‐Stable DMASnBr3 Lead‐Free Perovskite for Effective Solar‐Driven Photocatalysis

“…Ref. [36]) through the vacuum level. Values are referred to the vacuum level (black) and to the standard hydrogen electrode (SHE, grey) using the computational alignment achieved in ref.…”

Are Amines the Holy Grail for Facilitating CO2 Reduction?

“…In photochemistry, the imidazoles or amines added as electron donors also end up serving as proton donors after the initial electron transfer. Fujita and co‐workers proposed for the photocatalytic reduction of CO 2 with Ru(CO) 2 (dmb) 2 complexes ( I.4 ) that the added triethanolamine assisted the formation of metal hydride species through proton transfer [18] . This finding was contradictory to the previous assumption that the main role of triethanolamine was to act as a Brøndsted base to deprotonate the radial cation of 1,3‐dimethyl‐2‐phenyl‐2,3‐dihydro‐1 H ‐benzoimidazole serving as a sacrificial electron donor.…”