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

Abstract

Re(pyNHC-PhCF 3 )(CO) 3 Br is a highly active photocatalyst for CO 2 reduction. The PhCF 3 derivative was previously empirically shown to be a robust catalyst. Here, the role of the PhCF 3 group is probed computationally and the robust nature of this catalyst is analyzed with regard to the presence of water and oxygen introduced in controlled amounts during the photocatalytic reduction of CO 2 to CO with visible light. This complex was found to work well from 0-1% water concentration reproducibly; however, trace amounts of water were required for benchmark Re(bpy)(CO) 3 Cl to give reproducible reactivity. When ambient air is added to the reaction mixture, the NHC complex was found to retain substantial performance (~50% of optimized reactivity) at up to 40% ambient atmosphere and 60% CO 2 while the Re(bpy)(CO) 3 Cl complex was found to give a dramatically reduced CO 2 reduction reactivity upon introduction of ambient atmosphere. Through the use of time-correlated single photon counting studies and prior electrochemical results, we reasoned that this enhanced catalyst resilience is due to a mechanistic difference between the NHC-and bpy-based catalysts. These results highlight an important feature of this NHC-ligated catalyst: substantially enhanced stability toward common reaction contaminates.

Durable Solar-Powered Systems with Ni-Catalysts for Conversion of CO2 or CO to CH4

“…A number of earth-abundant metal complexes have been reported that are capable of catalyzing CO 2 reduction to CO or HCO 2 – through visible light-driven reactions. − Very recently, molecular transition metal catalysts capable of producing methane from carbon dioxide and visible light have emerged; however, reports of this reaction are rare. , Even more rare is the light-driven catalytic reduction of carbon dioxide to methane in the presence of solvent quantities of water . In fact, the vast majority of molecular photocatalytic systems for CO 2 reduction utilize anhydrous solvents such as acetonitrile or N , N -dimethylformamide . In general, improvements to the durability of photocatalytic CO 2 reduction systems are greatly needed.…”

“…Tetraaza (N 4 ) macrocycles supporting earth-abundant transition metals have produced some of the highest turnover numbers in CO 2 reduction catalysis . Additionally, electron-rich N -heterocyclic carbenes (NHCs) have engendered more robust CO 2 reduction catalysts in recent examples because they provide strong metal–ligand bonding interactions. ,,− Combining these two stabilizing features into a single complex is highly desirable, and Jurss and coworkers have recently shown the electrocatalytic reduction of CO 2 to CO with two NHC-ligated macrocyclic nickel complexes …”

N‐Heterocyclic and Mesoionic Carbenes of Manganese and Rhenium in Catalysis

“…Later, the same authors probed the role of the criticalto-reactivity 4-CF 3 group computationally. [63] The NHC-based ligand on 31 a was shown to have single occupied molecular orbital (SOMO) delocalization onto the 4-CF 3 ring after the catalyst was reduced with a planarization of the ligand πsystem. Through time-correlated single photon counting studies, complex 31 a was found to have a significant shorter excited-state lifetime than [Re(bpy)(CO) 3 Br].…”

“…Through time-correlated single photon counting studies, complex 31 a was found to have a significant shorter excited-state lifetime than [Re(bpy)(CO) 3 Br]. [63] Further studies were performed to explore two different aspects: (i) the impact of several electron withdrawing substituents (CF 3 , bis CF 3 , NO 2 , CN) on the aryl group of the py-NHC-aryl ligand (complexes 31 a-31 d, Scheme 25) and (ii) the impact of extending conjugation along the backbone of the ligand (complex 35, Scheme 25). [64] Notably, all complexes acted as direct photocatalysts for the CO 2 reduction reaction in the presence of a sacrificial SD with different TON and TOF values (entries 5-8, Table 3).…”

“…The reason for this behavior is due to the easier dissociation of the bromide ligand in comparison with the chloride. Later, the same authors probed the role of the critical‐to‐reactivity 4‐CF 3 group computationally [63] . The NHC‐based ligand on 31 a was shown to have single occupied molecular orbital (SOMO) delocalization onto the 4‐CF 3 ring after the catalyst was reduced with a planarization of the ligand π‐system.…”