Boosting Visible‐Light Carbon Dioxide Reduction with Imidazolium‐Based Ionic Liquids

Efficiently generating C1 building blocks from environmentally friendly carbon sources, such as through photocatalytic CO2 reduction, is essential for fostering a sustainable circular economy. The pursuit of mild catalytic activation methods has yielded powerful catalysts that can be synergistically employed alongside various reaction media to enhance overall performance. Herein, we elucidate the influence of diverse imidazolium‐based ionic liquids as additives for visible‐light‐driven CO2 reduction with ruthenium(II)‐ and rhenium(I)‐bipyridine complexes. Our investigation reveals that incorporating ionic liquids into traditional solvents at concentrations below 10 % can markedly boost CO production while suppressing H2 generation. The best results were obtained for the highly basic ionic liquid [C2mim][OAc], resulting in a substantial rise in CO formation from 0.3 μmol/h to 5.4 μmol/h and an increase in turnover number from 3 to 59. This study underscores the cooperative influence of imidazolium‐based ionic liquids on CO2 photoreduction while circumventing their use as primary solvents, thus offering a promising avenue for sustainable chemical synthesis.


Introduction
Carbon fixation chemistry is a critical component of building a circular economy, especially in the face of rising atmospheric CO 2 levels and a shortage of natural resources.To make change happen, processes for converting CO 2 to chemical feedstocks must be developed, transforming chemical industries from petrol-based to renewable.This shift is essential for reducing our carbon footprint, mitigating climate change, and ensuring a more environmentally responsible future.3] One potential pathway for CO 2 utilization follows the formation of C 1 building blocks, e. g.CO that can be used as a resource for making more complex molecules.Until today, the most extensive catalytic processes, e. g.Fischer-Tropsch synthesis, hydroformylations or methanol carbonylations, rely on CO as reactive and versatile feedstock.The replacement of CO with CO 2 as the prime feedstock in carbonylative transformations for fine chemical synthesis would thus be highly desirable.6] The biggest challenge in CO 2 conversion is its activation, which typically requires harsh conditions and long reaction times owing to its high thermodynamic and kinetic stability.9][10] The search for mild catalytic activation strategies has led to a number of potent catalysts, mostly based on transition metal or organocatalytic species that can be optionally combined with different reaction media for a synergistic effect.[13][14][15] Moreover, the chemical interaction of many ionic liquids with CO 2 resembles the first step of CO 2 activation in the plant photosynthesis cycle.In particular, the chemisorption of CO 2 in ionic liquids has been shown to facilitate the bending of initial linear CO 2 . [16]The interactions of ionic liquid and CO 2 can further lower the overpotential of CO 2 reduction, as proven in electrochemical experiments. [17,18]s an alternative to purely chemical or electrochemical reductions, the exploitation of ubiquitous energy sources such as sunlight opens novel possibilities for the transformation of CO 2 , stimulating research in non-conventional catalytic activation.In photocatalysis, the metal catalyst is paired with a photosensitizer (PS) that can be excited by light, generating an electron-hole pair, followed by reductive quenching with the help of an electron-donating agent to pass an electron to the catalyst, causing a low valent unsaturated state of the catalyst that can interact with CO 2 .[21] As shown by Dupont and co-workers, ionic liquids possess the ability to stabilize localized charges and holes that can be generated electrochemically or by direct radiation.In case of suitable absorption properties, radical species are formed by homolytic cleavage of the imidazolium-CO 2 adduct even in the absence of an external photosensitizer, thus forming CO 2 À * and imidazolium + * radicals in aqueous media. [15,22]However, as reported by Daniele et al., this mechanism is highly dependent on the pH value, since a HCO 3 À driven reaction is competing. [23]he synergistic role of ionic liquids as co-catalysts for photocatalytic CO 2 reduction via formation of the imidazolium-CO 2 adduct was also demonstrated in aprotic solvents.Different imidazolium-based ionic liquids were added to the photocatalytic tandem system [Ru(bpy) 3 ]Cl 2 /CoCl 2 • 6 H 2 O in different co-solvents by Lin et al. [24] The best results were observed for a mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate ([C 2 mim]BF 4 ) with acetonitrile in a ratio of 75 % (v/v), resulting in an almost 3-fold increase in the reaction rate. [24]Furthermore, Asai et al. investigated the mechanism of CO 2 photoreduction in the presence of ionic liquids in more detail, using an Ir-based sensitizer and [Re(bpy)(CO) 3 ]Cl as a catalyst.The ionic liquids were used in pure form without co-solvent, and a strong influence of their molecular structure on photosensitizing and catalytic cycles was observed.Ultimately, triflate (OTf À ) based ionic liquids were identified as most efficient species, thus highlighting their potential as valuable solvents for photocatalytic CO 2 reduction [7] In here, we aim for a mild protocol for photocatalytic CO 2 reduction that benefits from the cooperative effect of imidazolium-based ionic liquids in combination with [Ru(bpy) 3 ](PF 6 ) 2 sensitiser and [Re(bpy)(CO) 3 ]Cl.In contrast to previous studies, we focused on the use of ionic liquids as a low-level additive (0.5-20 % (w/v)) in three different aprotic solvents rather than using the ionic liquid as a bulk solvent, thus aiming for a systematic investigation of the cooperative effects of ionic liquids on visible light-driven CO 2 photoreduction and an understanding of the counterion effect.

Results and Discussion
The design of the catalyst plays a crucial role in controlling selectivity and efficiency.[Re(bpy)(CO) 3 ]Cl, first characterized by Lehn et al., is known as a highly selective catalyst for CO formation and has been extensively investigated and established as a model catalyst in combination with visible light photosensitizer [Ru(bpy) 3 ](PF 6 ) 2 .27][28] As a starting point, a set of imidazolium-based ionic liquids with variable counterions was examined in combination with the catalytic model system using the polar and aprotic solvents dimethylformamide (DMF), acetonitrile (MeCN), and dimethyl-sulfoxide (DMSO) that are often used solvents in photocatalysis with [Re(bpy)(CO) 3 ]Cl (Figure 1).
To avoid issues from viscosity and miscibility and to keep the ionic liquid as low as possible, experiments were conducted with an ionic liquid content of 0.5-20 % (w/v).The initial screening of the ionic liquid content was performed in MeCN with [C 2 mim][OAc] 1 d (Table 1).Results indicate that the formation of CO is increasing when the ionic liquid content is increased up to 10 % (w/v); however, a further increase to 20 % (w/v) is not beneficial anymore.Moreover, with increasing ionic liquid content, the formation of H 2 is stronger suppressed.Consequently, we selected a concentration of 10 % (w/v) ionic liquid as an additive for all future experiment, corresponding to a molar fraction of 1.0-5.0• 10 À 5 , depending on the ionic liquid species.
For further insight in the role of the anion, various imidazolium-based ionic liquids were studied in MeCN (Fig-  Numbers indicate cations and a-g (letters) correspond to counteranions. [29]a] Parameter β reported for the corresponding ionic liquid with [C 3 mim] + cation.[30] Table 1.Photocatalytic CO 2 reduction in the presence of various amounts of  À based species 1 f, all ionic liquids made the photocatalytic reduction more selective for CO formation and suppressed the formation of by-products. In tcase of the best-performing ionic liquid, 1 d, the formation of H 2 was reduced by a factor of 13 from 5.8• 10 À 2 μmol to 4.3• 10 À 3 μmol in MeCN.Furthermore, CH 4 was only detected in negligible amounts in all samples.
The observed impact of the best performing ionic liquid 1 d is not limited to MeCN as a solvent, and experiments were alternatively performed in the apolar solvents DMF and DMSO (ESI Table S1, Figure S2 & S3).In fact, the positive effect is even more pronounced in DMF, with a 20-fold increase in CO yield compared to the reference system without ionic liquid 1 d.This observation might be attributed to differences in reaction mechanism. [31]The chosen solvents possess different ability to coordinate with the rhenium complex, resulting in a solvochromatic shift of the absorption spectra. [25]In general, MeCN is considered as the strongest coordinating solvent due to its additional ability for ligand exchange with Cl, thus stabilizing the 18-electron radical [Re(bpy)(CO) 3 (solvent)] as intermediate. [25,28]On the other hand the weaker coordination ability of DMF without ligand exchange was reported as beneficial aspect for the formation of the Re-TEOA-CO 2 adducts, which might be responsible for the increased reactivity. [20,32]In comparison, DMSO exhibits comparable polarity than MeCN, but showed less tendency for direct coordination with the Re complex, thus the observed enhancements in the reaction rate are less explicit.
Different factors and properties of ionic liquids may cause the observed enhancement in CO formation and influence the photocatalytic reduction of CO 2 .The outstanding performance of the carboxylate-based ionic liquid 1 d is not surprising, given its basic nature and known potential for interaction with CO 2 .[35][36] Corresponding C-2 protons are highlighted in Figure 1.The findings from photoreduction can, to some extent, be correlated with the parameter β, showing a relationship with photocatalytic yields for CO.Ionic liquids with high nucleophilicity and, therefore, high β values such as 1 d or 1 f increased CO formation compared to the reference reaction (Figure 1).The low yields obtained after the introduction of fluorinated alkyl groups, as in the cases of N(Tf) 2 anions (1 f), also correlates with lower values of the parameter β.It is interesting to note that an amino group in the side chain of the cation cannot compensate for the absence of a basic anion.This is evident from the comparison of the 1-ethyl-3methylimidazolium-based ionic liquid 1 f with the diisopropylamine-functionalized ionic liquid [iPr 2 N(CH 2 ) 2 mim][N(Tf) 2 ] 2 f, which both gave comparable yields for CO formation (ESI Table S2).The trend of anion basicity is also visible for the halide series (1 a-c) that follows the order Cl À > Br À > I À .This is in accordance with the anions' affinity for interaction with the C-2 protons according to their charge per surface area ratio, as well as with the Kamlet-Taft parameter β.
The unique ability of the acetate-based ionic liquid in CO 2 capture and utilization may also be related to the spontaneous N-heterocyclic carbene formation. [37]Apart from several NHCcatalyzed reactions, e. g. benzoin condensations, that are known to occur in carboxylate-based ionic liquids, the interaction of the free carbene with CO 2 has been reported by several authors and is a key player in its activation. [16,38]To study the interaction of CO 2 with [C 2 mim][OAc] 1 d as co-catalytic effect in the photocatalytic reduction, Fourier transform infrared spectroscopy (FT-IR) studies in transition mode were performed (Figure S3).A strong C=O stretching vibration at 1668 cm À 1 was observed after purging the reaction mixture with CO 2 that corresponds to [C 2 mim-CO 2 ] À , thus proving the chemisorption of CO 2 as a dominating interaction accompanied by bending of the CO 2 bond angle.Similarly, the formation of the CO 2 -carbene complex is evident in 1 H NMR (Figure S5a), as indicated by the doubling of imidazole core proton signals due to the formation of [C 2 mim-CO 2 ] À as the second species.The carbene complex formation is also evident from the appearance of an additional peak at 155 ppm that can be assigned to the CO 2 À carbon, along with a doubling of the remaining signals (ESI Figure S4). [39]Interestingly, the addition of TEOA as a sacrificial proton donor leads to the weakening or complete disappearance of the doubling of imidazole core proton signals (Figure S5), implying that TEOA as a protic agent interferes with the chemisorption mechanism.This is in line with literature studies, confirming that coordination of CO 2 with TEOA forming CO 2 -TEOA or Re-TEOA-CO 2 intermediates plays a key role in the reaction mechanism of this reaction. [20,32] similar effect is found for the addition of 5 % water (corresponding to molar ratio approx.200 : 1 for ionic liquid to water) to the photoreduction, where a decrease in CO formation and an increase in H 2 formation was observed (Table 2).As evident in 13 C NMR, the addition of water leads to disappearance of the 13 C NMR signal at 155 ppm (Figure S6).An additional signal at 163 ppm is indicating the formation HCO 3 À and CO 3 À supporting the hypothesis that in protic reaction media the reaction might proceed via HCO 3 À intermediate [40] Still, the CO formation in 1 d with 5 % H 2 O reached 3.96 μmol and TON of 44, which is higher compared to the reference value.
For further insight into the reaction mechanism, a set of control experiments was performed with the ionic liquid 1 d in DMF as best-performing combination (Table 3).Peng et al. and Qadir et al. reported CO formation via the direct photoexcitation of imidazolium-based ionic liquids. [22,23]For this reason, the photocatalytic activity was also studied in the absence of [Ru(bpy) 3 ](PF 6 ) 2 and [Re(bpy)(CO) 3 ]Cl; however, the formation of CO was not observed under otherwise similar conditions.This observation is in accordance with UV-vis spectroscopy data of ionic liquid and catalyst (ESI Figure S8), indicating that absorption of the ionic liquid is very low in comparison to the catalyst at the chosen conditions.On the other hand, photoreduction using just [Ru(bpy) 3 ](PF 6 ) 2 resulted in low CO formation in the range of 0.06 μmol with a low selectivity of 90.3 %, indicating H 2 formation to be much more prominent.As reported in literature, the presence of [Re(bpy)(CO) 3 ]Cl as cocatalysts favors the formation of CO.It is interesting to note that a similar effect on selectivity can be observed with 1 d instead of [Re(bpy)(CO) 3 ]Cl -in both cases, the selectivity for CO reaches > 99 %.However, both Ru and Re-based species are required to observe the substantial increase in CO formation caused by the addition of 1 d.
Photoluminescence (PL) studies were performed with 1 d to verify the role of ionic liquid addition.In line with the observed improvements in CO 2 photoreduction yields, we observe pronounced PL quenching of the excited [Ru(bpy) 3 ] 2À * state when in the presence of 1 d, which speaks for a more rapid charge transfer between the components of the photocatalytic system (Figure 3).Table 2. Photocatalytic CO 2 reduction in the presence and absence of water for three selected ionic liquids (1 a, 1 b and 1 d).
TON [b] Selectivity [c] Reference   Finally, the performance of the photocatalytic system was further investigated for up to 2 hours of reaction time (Figure 4).The reference system showed a low turnover frequency of around 0.05, which increased only slightly to about seven after 120 minutes.In contrast, the TON of the photosystem with 1 d increased rapidly up to 46 within only 45 minutes.
The rate dropped significantly afterwards to reach a stable rate of about 40, indicating that the higher reaction rate may impact the long-term stability of the catalytic system.Instant TOF indicates a complete inactivation of the catalytic system.This might be caused by the photolysis of [Ru(bpy) 3 ](PF 6 ) 2 , which has been previously reported in literature for different biphenyl sensitizer systems. [41,42]Eventually, the increases in reaction rate are paid off by reduced stability of the photosensitizer, suggesting that other catalytic systems -such as those based on redox-stable solid-state absorbers [43,44] -are required to fully take advantage of the beneficial effect of ionic liquids in photocatalytic CO 2 reduction.

Conclusions
In this study, we explored the use of various imidazolium-based ionic liquids as additives in photocatalytic reduction of CO 2 .We employed [Re(bpy)(CO) 3 ]Cl as catalyst and [Ru(bpy) 2 ](PF 6 ) 2 as sensitizer.In comparison to literature that focuses on reactions in pure ionic liquids or their mixtures with low amount cosolvents, we focused on the addition of ionic liquids as lowlevel additive, thus avoiding viscosity or mass transfer issues.For a comprehensive understanding the model reaction was tested in solvents MeCN, DMF and DMSO with a set of imidazolium based ionic liquids.
The results demonstrate that incorporating small amounts of 10 % (w/v) of ionic liquids to conventional solvents could significantly enhance the production of CO, eliminating the need to use ionic liquids as the primary solvents.A gradual increase of 1 d ionic liquid content from 0 to 10 % (w/v) in MeCN lead to stepwise increase of CO formation and successively improved H 2 suppression, whereas a higher content was not beneficial.Notably, the addition of [C 2 mim][OAc] 1 d in DMF led to nearly a 20-fold increase in CO yield.The impact of different counterions could be correlated with the Kamlet-Taft parameter β for solvent interactions in DMF and MeCN; however, this trend was less explicit in DMSO.Moreover, the inherent basicity of the C-2 proton in acetatebased ionic liquids allows for the chemisorption of CO 2 via formation of the carbene complex that goes along with bending the CO 2 bond angle to 138°, and thus pre-activation of CO 2 .The addition of water favours a change of the mechanism towards the presence of the [HCO 3 À ] species, which is evident from 13 C NMR and a reduced selectivity for CO formation.Mechanistic investigations also revealed that all components of the system were crucial for efficient CO reduction, and photoluminescence measurements further proved that ionic liquids positively influence the quenching behavior of the excited state of the ruthenium sensitizer.
However, the long-term reaction rate of the photosystem appeared to decrease after an initial rapid increase, suggesting accelerated aging of the photosensitizer in the presence of ionic liquids.To fully harness the benefits of ionic liquids in photocatalytic CO 2 reduction, it seems that alternative catalytic systems may be necessary, and our ongoing research is focused on exploring these possibilities.

Experimental section
Materials: Solvents for photocatalytic experiments, including N, Ndimethylformamide (DMF, anhydrous, 99.8 %), dimethylsulfoxide (DMSO, anhydrous, 99.9 %), and acetonitrile (MeCN, anhydrous, 99.8 %) were purchased from Sigma Aldrich Co. and stored over molecular sieve.CO 2 for application in all photocatalytic reactions was purchased from Messer Austria (> 99.995 %).Catalyst and catalyst precursors have been purchased from commercial suppliers if otherwise stated.The ionic liquids were synthesised according to standard literature procedure and analytical data was in accordance with literature . [45,46]Ionic liquids were dried under high vacuum and stored under argon.The water content was checked to be lower than < 3000 ppm by Karl Fischer titration before use.All reactions were conducted in an argon atmosphere unless explicitly stated as carried out under ambient conditions.
Sensitizer synthesis: Tris(2,2'-bipyridine)ruthenium(II) hexafluorophosphate was synthesised by dissolving tris(2,2'-bipyridine) ruthenium(II) chloride hexahydrate (0.30 g), 0.4 mmol in 150 mL water and adding 0.16 g (0.84 mmol) KPF 6 .The solution turned immediately from red to bright orange.After sitting overnight, the solid material was filtrated, washed with water, and dried in vacuum to yield 0.28 g (81 %) tris(2,2'-bipyridine)ruthenium(II) hexafluorophosphate. 1   Methods: FTIR spectra (transmission mode) were recorded on a PerkinElmer spectrum 65 FTIR spectrometer.Diffusive reflectance spectroscopy in solution was measured using a JASCO 670 spectrometer. 1H NMR spectra were recorded with a Bruker Advance UltraShield 600 MHz spectrometer, and chemical shifts were reported in ppm from TMS with a solvent resonance as the internal standard.Photoluminescence measurements (PL) were measured with a PicoQuant FluoTime 300 spectrophotometer using an Xe arc lamp (300 W power) as an excitation source and a double-grating monochromator.The detection system comprised a PMAHybrid 07 detector and a high-resolution double monochromator.The excitation wavelength utilised for steady-state measurements was 445 nm.The data was collected and later fitted using EasyTau2 software.GC analysis was performed with the SHIMADZU Nexis™ GC-2030 gas chromatograph equipped with a dielectric barrier discharge ionisation detector (BID) and 1 m ShinCarbon ST column (Restek Co.) General procedure for photocatalytic CO 2 reduction: Photo experiments were carried out in a glass reactor with a total capacity of 3.7 mL, comprising 1.5 mL for the solution and 2.2 mL for the headspace (ESI Figure S7).The glass reactor is equipped with a septum, water cooling, and a stirring bar.A Solis high-power 445 nm LED, operated with a DC2200 -High-Power 1-Channel LED Driver at a brightness setting of 5 %, served as the light source.The emission spectra of the lamp is depicted in ESI Figure S8, together with the absorption spectra of all relevant components of the system, Prior to the reactions, the reaction mixtures were purged with CO 2 for 3 minutes at a flow rate of 10 ml/min.The reaction temperature was set to 22 °C.Samples of gas aliquots were taken prior to reaction and in consecutive intervals.Drops of TON might be related to consecutive sampling of aliquots from the reactor headspace.The detection limit of the GC with regard to CO, which can be seen as a characteristic of the reliability of the reported data (both TONs and TOFs), is around 10 ppm.This value corresponds to TON of 0.01.
ure 2).The most prominent and distinct influence on CO 2 reduction performance was found for [C 2 mim][OAc] 1 d, which increased the yield of CO from 1.45 μmol to 5.19 μmol in MeCN.The observed CO formation corresponds to a rise in turnover number (TON) from 3 to 59. Within the halide series 1a-c, [C 2 mim][Cl] 1 a performed best and substantially raised the CO yield.Additionally, there is a trend for a decrease in activity in the halide series (1 a-c) in the order Cl À > Br À > I À .In case of [C 2 mim][MeSO 4 ] 1 e, results are difficult to compare since the

Figure 1 .
Figure 1.Structures and properties of the ionic liquids used in this study.The proton relevant for counterion interactions is highlighted in blue.Numbers indicate cations and a-g (letters) correspond to counteranions.[29] [a] Parameter β reported for the corresponding ionic liquid with [C 3 mim] + cation.[30] reaction became biphasic when performed in MeCN.In contrast, no positive effect could be observed for the ionic liquids [C 2 mim][NTf 2 ] 1 f and [C 2 mim][N(CN) 2 ] 1 g.In fact, CO formation decreased slightly in comparison to the reference values.The addition of imidazolium-based ionic liquids also affects the selectivity towards CO formation.Except for the N-(Tf) 2