Probing the Effects of Electron Deficient Aryl Substituents and a π‐System Extended NHC Ring on the Photocatalytic CO₂ Reduction Reaction with Re‐pyNHC‐Aryl Complexes**

Abstract

The ever-expanding need for renewable energy can be addressed in part by photocatalytic CO 2 reduction to give fuels via an artificial photosynthetic process driven by sunlight. A series of rhenium photocatalysts are evaluated in the photocatalytic CO 2 reduction reaction and via photophysical, electrochemical, and computational studies. The impact of various electron withdrawing substituents on the aryl group of the pyNHC-aryl ligand along with the impact of extending con-jugation along the backbone of the ligand is analyzed. A strong correlation between excited-state lifetimes, photocatalytic rates, and computationally determined dissociation energy of the labile ligand of these complexes is observed. Additionally, computed orbital analysis provides an added understanding, which allows for prediction of the potential impact of an electron withdrawing substituent on photocatalysis.

Ligand−Structure Effects on N−Heterocyclic Carbene Rhenium Photo− and Electrocatalysts of CO2 Reduction

“…The observed photocatalytic performance of 26 TONs of CO offered by Re−NHC−3 is remarkably similar to a plateau TON of approximately 25 reported for a complex similar to that of Figure 1 a, except for the replacement of the imidazole portion of the ligand with benzimidazole [ 29 ]. In that case, a solar simulator was used as the irradiation source.…”

“…All three complexes investigated, Re−NHC−1−3 ( Scheme 1 B), were synthesized using similar methods to those previously reported for this class of Re tricarbonyl complexes [ 29 ] (see the Materials section in Experimental). All compounds were characterized using 1 H and 13 C NMR spectroscopy, including the precursor ligands (see Figures S18−S30, Supporting Information ), elemental analysis, mass spectrometry and IR spectroscopy.…”

“…Hence, the potential energy diagram of Figure 7 is based on the measured E o (onset potential) values for the reduction of the parent complexes, E (A/A − ) , and the estimated values of the first reduction potential of the excited species, E* (A/A − ) , were calculated using Equation (1), where E MLCT is the energy (in eV) derived from the high energy onset of the emission spectra of Figure 2 b [ 10 ]. To estimate the standard reduction potential, E 0 (CO2/CO), for the two−electron reduction of CO 2 to CO under the applied experimental conditions, similar considerations to those adopted in the works of Delcamp et al [ 10 , 29 ] were applied, where an upper (i.e., most negative) limit for E 0 (CO2/CO) was determined by considering the p K a of the most acidic species in solution (which is likely to be carbonic acid formed due to the presence of residual moisture). The value for E 0 (CO2/CO) in DMF has been reported as −0.73–0.0592 p K a (V vs. Fc + /Fc) [ 60 ].…”

“…To optimize the effect, some counter balancing of the electron density at the NHC ring was found to be useful, as demonstrated by the improved catalytic activity arising from the introduction of an electron−withdrawing group at the phenyl ring attached to the NHC ring [ 10 , 26 ]. Delcamp and co−workers followed up their initial report with a series of systematic studies in which they replaced Br − with other axial ligands, such as Cl − and P(OEt) 3 [ 28 ], and −CF 3 with other electron−withdrawing groups (such as −NO 2 or −CN), and additionally investigated the impact of experimental conditions on the photocatalytic performance [ 10 , 29 ]. The utilization of Re−NHC dicarbonyl complexes as photocatalysts for CO 2 reduction has been reported by other researchers [ 30 , 31 ].…”

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

“…[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). Re complexes bearing inductively electron withdrawing substituents (31 a-31 d) exhibited higher TOF numbers compared to complex 35 bearing π-electron withdrawing groups via resonance.…”