Sensitized and Self‐Sensitized Photocatalytic Carbon Dioxide Reduction Under Visible Light with Ruthenium Catalysts Shows Enhancements with More Conjugated Pincer Ligands

Abstract

A new method to synthesize complexes of the type [(CNC)Ru II -(NN)L] n + has been introduced, where CNC is a tridentate pincer composed of two (benz)imidazole derived NHC rings and a pyridyl ring, NN is a bidentate aromatic diimine ligand, L = bromide or acetonitrile, and n = 1 or 2. Following this new method a series of six new complexes has been synthesized and characterized by spectroscopic, analytic, crystallographic, and computational methods. Their electrochemical properties have been studied via cyclic voltammetry under both N 2 and CO 2 atmospheres. Photocatalytic reduction of CO 2 to CO was performed using these complexes both in the presence (sensitized) and absence (self-sensitized) of an external photosensitizer. This study evaluates the effect of different CNC, NN, and L ligands in sensitized and self-sensitized photocatalysis. Catalysts bearing the benzimidazole derived CNC pincer show much better activity for both sensitized and self-sensitized photocatalysis as compared to catalysts bearing the imidazole derived CNC pincer. Furthermore, self-sensitized photocatalysis requires a diimine ligand for CO 2 reduction with catalyst 2 ACN being the most active catalyst in this series with TON = 85 and TOF = 22 h À 1 with an electron donating 4,4'-dimethyl-2,2'-bipyridyl (dmb) ligand and a benzimidazole derived CNC pincer.

Ruthenium (II) Complexes of CNC Pincers and Bipyridine in the Photocatalytic CO2 Reduction Reaction to CO Using Visible Light: Catalysis, Kinetics, and Computational Insights

“…Syntheses of the CNC pincer ligand precursors and the corresponding [(CNC)­RuCl­(MeCN) 2 ] + complexes were previously described. , Complex 1 OMe undergoes a ligand substitution reaction with 2,2′-bipyridine in refluxing methanol for several days to form complex 2A in 45% yield. This procedure is similar to our recently reported syntheses of related complexes . Complex 2A was then treated with silver triflate to replace the chloride with acetonitrile (from the solvent) to form complex 2B in high yield (86%) .…”

“…This procedure is similar to our recently reported syntheses of related complexes. 34 Complex 2A was then treated with silver triflate to replace the chloride with acetonitrile (from the solvent) to form complex 2B in high yield (86%). 51 Complexes 4A and 4B were synthesized in a similar manner, in 67 and 83% yields, respectively, but starting from complex 3 bearing benzimidazole-derived NHCs.…”

“…We have observed a similar trend in our past work with more highly conjugated CNC pincers generally favoring faster TOF for photocatalytic CO 2 reduction. 21,34 In contrast, for self-sensitized photocatalytic CO 2 reduction without [Ir(ppy) 3 ], catalyst 4C was the only catalyst to exhibit high activity (TOF init = 6.5 h −1 ) from the earliest observed time points (Table 3 and Figures 5 and 6). The advantages of benzimidazole-derived NHC ligands have been mentioned above, but the reasons for enhancements with bromide (4C) vs chloride (4A) and CH 3 CN (4B) monodentate ligands are unclear from the UV−vis and electrochemistry data (Table 2).…”

“…NHCs and multidentate ligands containing NHCs are emerging as a powerful class of ligands in organometallics and homogeneous catalysis due to a strong σ donor ability, straightforward synthesis, and several possible modifications. − For example, NHC ligand donor strengths can be altered by saturation of the heterocycle, benzannulation, varying the N -substituents, or introducing a third heteroatom in the heterocyclic ring. − The combination of pyridine rings and NHCs have been used to make bidentate, tridentate, and tetradentate ligated metal complexes, which have found application in catalytic CO 2 reduction. ,,,− Unlike NHCs, pyridine is a weaker σ donor but can serve as a π acceptor, and substitutions on the pyridyl ring can provide another handle to further tune the electronics at the metal center. Tridentate CNC pincers consisting of a pyridyl ring and two NHC rings have often been used along with bipyridine in ruthenium chemistry (Figure ).…”

Low-Valent Cobalt(I) CNC Pincer Complexes as Catalysts for Light-Driven Carbon Dioxide Reduction

“…PCO 2 RR studies were conducted using neutral white light LEDs as the photon source set to 1 Sun intensity from 400 to 700 nm (∼64 mW/cm 2 ). The photoreactions were undertaken in acetonitrile solutions saturated with CO 2 containing catalyst (1 × 10 –6 M), fac -Ir­(ppy 3 ) as the photosensitizer (PS, 1 × 10 –4 M), BIH as the sacrificial electron donor (SED, 1 × 10 –2 M), and triethylamine (TEA, 0.36 M) as a base to deprotonate the BIH cation after electron transfer to the excited PS which drives the irreversible electron transfer from BIH to the PS (Figures and ). , The PS ( fac -Ir­(ppy) 3 ) has a 160 mV downhill electron transfer from BIH when photoexcited and a minimum of a 410 mV downhill electron transfer driving force to the Co catalysts under CO 2 from the reduced PS. Aliquots of the headspace of the photocatalysis reaction were collected and analyzed by gas chromatography (GC) for the formation of products.…”