Article pubs.acs.org/IC
Photocatalytic Reduction of CO2 with Re-Pyridyl-NHCs Aron J. Huckaba, Emily Anne Sharpe, and Jared H. Delcamp* Department of Chemistry and Biochemistry, University of Mississippi, 405 Coulter Hall, University, Mississippi 38677, United States S Supporting Information *
ABSTRACT: A series of Re(I) pyridyl N-heterocyclic carbene (NHC) complexes have been synthesized and examined in the photocatalytic reduction of CO2 using a simulated solar spectrum. The catalysts were characterized through NMR, UV−vis, cyclic voltammetry under nitrogen, and cyclic voltammetry under carbon dioxide. The complexes were compared directly with a known benchmark catalyst, Re(bpy) (CO)3Br. An electron-deficient NHC substituent (PhCF3) was found to promote catalytic activity when compared with electron-neutral and -rich substituents. Re(PyNHC-PhCF3) (CO)3Br was found to exceed the CO production of the benchmark Re(bpy) (CO)3Br catalyst (51 vs 33 TON) in the presence of electron donor BIH and photosensitizer fac-Ir(ppy)3. Importantly, Re(PyNHC-PhCF3) (CO)3Br was found to function without a photosensitizer (32 TON) at substantially higher turnovers than the benchmark catalyst Re(bpy) (CO)3Br (14 TON) under a solar simulated spectrum.
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INTRODUCTION
CO2 is available in tremendous quantities. Solar powered conversion of CO2 to a fuel could contribute a valuable step toward closing the “carbon cycle” and provide fuel precursors from an under utilized waste.1,2 Electrochemically reducing CO2 through one electron reductions is not energetically efficient as the conversion of CO2 to CO2•− is energetically demanding (−1.97 V vs NHE in DMF). Multiple electron reduction products may be formed at significantly less negative potentials (i.e., CO at −0.53 V vs NHE or CH3OH at −0.38 V vs NHE). However, kinetically a catalyst is desirable to drive the reaction and control product selectivities.3−9 Ideally, the energy to drive these catalytic reductions would be derived directly (photocatalysis) or indirectly (solar-driven electrocatalysis) from the sun. Numerous reports on electrocatalysis since the 1970s have emerged3,10 using a host of transition metals including Re,11 Ru,12 Co,13 Fe,14 Ni,15,16 and others.17−21 Many of these electrocatalysts have been demonstrated as capable of reducing CO2 with a visible light photosensitizer (Figure 1).22−29 However, reports of only three catalyst series24 capable of both absorbing solar spectrum irradiation and reducing CO2 exist Re(bpy) (CO)3X,30,31 Ir(tpy) (ppy)Cl,32 and Ir[(thiazole)2 bipyridine] (Figure 1).33 While several systems have been set forward for immobilization of these catalysts which have dramatically boosted turnover number (TON) values, the homogeneous TONs of these catalysts have remained low (typically pKa 7 in MeCN).59,60 Assuming a pKa of 7 at the lowest, the standard reduction potential is near −0.3 V versus SCE for CO2 to CO and water in MeCN (Figure 5), applying the approximations of Saveant and co-workers.14 Carbonic acid is present when CO2 is taken up by H2O to give CO3H2. Since carbonic acid will be present in nondried solvents, a pKa value of ∼17 in MeCN for carbonic acid can be used to derive the upper limit of the standard reduction potential of CO2 to CO at approximately −0.9 V versus SCE.14 The first reduction potentials of 2−5 are all higher in energy than the two-electron reduction of CO2 to CO and water in acetonitrile (MeCN) by at least 600 mV at our highest estimated standard reduction potential (Figure 5). To investigate the viability of complexes 2−5 for the reduction of CO2, each of the electrocatalysts were evaluated by comparison of CV curves excluding and including CO2 (Figure 4 and Supporting Information). Upon dissolving 2−5 in MeCN under an atmosphere of N2, two irreversible reductions and an irreversible oxidation were observed. 43 Exchanging the atmosphere for CO2 by vigorous bubbling for 15 min lead to a catalytic current increase in the first reduction wave with a larger current increase of the second wave. To analyze the rate of electrochemical reduction of CO2, the peak current increases E
DOI: 10.1021/acs.inorgchem.5b02015 Inorg. Chem. XXXX, XXX, XXX−XXX
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photocatalytic reduction of CO2 with the fac-Ir(ppy)3 sensitizer using an AM 1.5 solar simulated light source. Toward the aim of solar driven photocatalysis, we utilized AM 1.5 spectrumsimulating filters on a 150 W Xe lamp to standardize irradiation intensity rather than employing cutoff-filter methods. As illustrated in Figure 1, Table 3, and Figures 7−8, the Table 3. Comparison of Catalyst Performance at One Hour entrya
catalyst
μmoles of CO
TON
1 2 3 4 5b 6
2 3 4 5 5 1
2.80 5.20 3.80 10.00 10.20 6.20
14 26 19 50 51 31
0.2 μmol catalyst/2 mL 5% TEA/MeCN solution with 0.2 μmol facIr(ppy)3 and 0.02 mmol BIH. Solutions irradiated for 1 h after vigorous bubbling with CO2 for 15 min with a solar simulator (150 W Xe lamp, AM1.5 filter). Each data point is the average of two runs. b Catalyst was irradiated for 4 h. a
Figure 6. UV−vis absorption (top) and emission (bottom) curves for catalysts 2−5, benchmark catalyst 1 and PS.
Table 2. Optical Properties of Catalysts 1−5a catalyst 1f 2 3 4 5 PSg
λmax (nm) 370 358, 354, 351, 356, 380,
402b 400b 402b 404b 480b
λonset (nm)c
ε (M−1 cm−1)
λem (nm)d
EMLCT‑GS (eV)e
450 440 440 440 440 500
2890 4850, 1200b 4750, 1100b 4600, 1010b 4800, 1010b 11000, 1050b
585 458 500 452 496 515
2.53 2.97 2.94 3.00 2.94 2.53
Figure 7. Catalyst turnover number versus time for each of the Re(I) catalyzed reactions. Reactions are run after vigorous bubbling with CO2 for 15 min with 0.2 μmol catalyst, 0.2 μmol fac-Ir(ppy)3 and 0.02 mmol BIH in 2 mL of a 5% TEA/MeCN solution. Solutions are irradiated with a solar simulated spectrum set to 1 sun (150 W Xe lamp, AM1.5 filter). Each data point is the average of two runs.
a Measured in acetonitrile. bIndicates shoulder. cTaken from the intercept of the baseline and a tangent line on the absorption spectrum on the low energy side of the longest wavelength transition. d Measured in MeCN using 390 nm excitation wavelength for 2−5 and 425 nm for 1 and PS. eEMLCT‑GS was estimated from the onset of the high energy side of the emission curve in MeCN. The absorption and emission spectrum are plotted on the same graph in the Supporting Information for each catalyst. EMLCT‑GS was converted from nanometers to electron volts through the eq 1240/EMLCT‑GS nm = EMLCT‑GS eV. fEmission information previously measured.63 gAdditional information available from prior publications on this PS.64,65
sacrificial donors such as TEA or BIH by a minimum of 570 and 1090 mV, respectively (Figure 5). As visible light absorption is possible with these catalysts (∼50 nm width), we sought to compare photocatalytic studies both without and with a PS to extend the visible light absorption window by approximately 80 nm (Figure 6). Electrochemically, the E(S/S−) values of catalysts 2−5 are wellpositioned for favorable electron transfers from a reduced facIr(ppy)3 sensitizer with driving forces ranging from 690 to 610 mV. Since the E(S/S−) values were found to be positioned favorably for both CO2 reduction and electron transfer from a fac-Ir(ppy)3 sensitizer, we first evaluated complexes 2−5 and the chloride analogues of these complexes as catalysts in the
Figure 8. Catalyst turnover number comparison for each of the Re(I) catalysts (0.2 μmol catalyst/2 mL 5% TEA/MeCN solution with 0.2 μmol fac-Ir(ppy)3 and 0.02 mmol BIH). Solutions irradiated with 150 W Xe lamp set to 1 sun and equipped with an AM1.5 filter after vigorous bubbling with CO2 for 15 min. Each data point is the average of two runs. F
DOI: 10.1021/acs.inorgchem.5b02015 Inorg. Chem. XXXX, XXX, XXX−XXX
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TON, entries 3 and 9). The performance of catalyst 5 without a PS matches that of benchmark catalyst 1 with a PS. Natural light during peak overhead sunlight hours in April at Oxford, MS led to similar TON values to our solar simulator (entries 3 and 4). Control experiments for removal of PS and BIH, removal of CO2, and removal of catalyst all led to no appreciable CO formation (entries 5, 7−8). Electron transfer from BIH is slightly downhill in energy while electron transfer from TEA is significantly uphill in energy relative to the estimated PS excitedstate reduction potential. Since the BIH oxidation potential is better matched than TEA in MeCN for reducing the PS, BIH may allow more efficient access to the PS•− state which accounts for the increase in catalyst performance using the PS with BIH and low reactivity with TEA only (ΔGBIH‑PS = −40 mV; ΔGTEA‑PS = +480 mV). Interestingly, in the absence of TEA, catalyst 5 demonstrates a similar TON value to experiments with TEA (entries 3 and 6) indicating an added base is not necessary in this system.35
Re(PyNHC) (CO)3X complexes 2−5 were found to function as CO2 reduction catalysts in the presence of a photosensitizer, BIH, TEA, and MeCN. A correlation between the electron density tuning substituent on the NHC ring and performance was observed with the most withdrawing substituent (PhCF3) giving higher performance as higher TONs were observed (Figure 7) and the most donating substituent (Me) giving the lowest TON values. This is in line with our original hypothesis that the ligand centered reduction would be problematic without the addition of electron deficient groups when replacing a pyridyl ring on a bipyridyl ligand with an NHC ring. The CF3-substituted 5 gave the highest TONs followed by electronically neutral arylated catalyst 3. Methyl-substituted catalyst 2 and hexyloxy-substituted 4 performed at the lowest TONs. Notably, catalyst 5 performs at substantially higher TONs (51) than benchmark catalyst 1 (33 TON). For the two highest performing NHC-catalysts (3 and 5) the bromide complexes were found to significantly outperform the chloride complexes (Figure 8). In general, the chloride complexes performed at similar TON to that of the bromide complexes when electron rich NHC substituents were employed and at substantially lower TON values when electron neutral or deficient ligand substituents were employed. This is explainable in that one of the early steps in the photocatalytic cycle likely involves halide dissociation after photoexcitation and electron transfer to the catalyst.39 Electron deficient substituents likely promote ligand coordination to the Re center allowing for only more labile ligands such as Br− to dissociate. Regardless of counterion, all catalysts were found to halt CO production after 4 h of irradiation. The evaluation of each reaction component on the highest performing catalyst, 5, is reported in Table 4. The effect of
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CONCLUSIONS We have found the ubiquitous Re(I) pyridyl-based photocatalysts may have improved catalyst turnover under photocatalytic reaction conditions through substitution of a pyridine with an NHC ligand. This led to a decrease in light absorption and destabilization of each complex’s reduction potential; however, the potentials of each complex were positioned suitably for the reduction of CO2 and the transfer of electrons from a reduced photosensitizer, fac-Ir(ppy)3. Each of the complexes performed as CO2 reduction catalysts in the presence of the PS and the strong electron donor BIH. Catalyst 5 performed much higher than Re(bpy) complex 1 in each set of reaction conditions employing a solar simulated spectrum. Both complexes 5 and 1 reached good TONs of (51 for 5 and 33 for standard 1) with PS and BIH. Catalyst 5 attained good TON with no PS (32 TON), making Re(PyNHC) (CO)3X just the fourth series of catalysts capable of nonphotosensitized CO2 catalytic reduction with visible light. Studies leading to further catalyst improvement, electron withdrawing group optimization, and optimizing electrocatalytic performance are currently ongoing.
Table 4. Experimental TON Values Under Various Conditions entrya
catalyst
PSb
e− donorc
1 2 3 4d 5 6 7e 8 9
5 5 5 5 5 5 5 none 1
yes yes no no no no yes yes no
TEA, TEA TEA, TEA, TEA BIH TEA, TEA, TEA,
BIH BIH BIH
BIH BIH BIH
μmoles of CO (TON) 10.2 0.10 6.40 6.64 0.04 6.43 0.08 0 2.8
(51) (0.5) (32) (33) (0.2) (32) (0.4) (0) (14)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02015. UV−vis absorption and emission data, cyclic voltammogram data showing oxidation and reductions, cyclic voltammogram data with N2 and CO2 atmospheres (PDF) NMR spectra of reported compounds (PDF)
a
Unless otherwise noted, reactions conditions are identical to those listed in Figure 4 with an irradiation time of 4 h. Each data point is the average of two runs. bPS = fac-Ir(ppy)3. cWhen two components are listed both are present. TEA as 5% of the solvent mixture by volume and BIH as 0.02 mmol. All experimental values are identical to those reported in the Experimental Section except where noted. dMeasured on a rooftop with natural sunlight on a sparsely cloudy day from 11 am −3 pm. eNitrogen atmosphere.
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AUTHOR INFORMATION
Corresponding Author
removing the strong electron donor material BIH is remarkable (51 TON with BIH versus 0.5 TON without BIH, entries 1 and 2). Notably, NHC-based catalyst 5 is able to efficiently reduce CO2 without a PS since the removal of the PS did not stop catalytic performance of 5 (51 TON vs 32 TON, entries 1 and 3). In the absence of PS, 5 was found to give substantially higher TON values than 1 under analogous conditions (32 vs 14
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval of the final version of the manuscript. Notes
The authors declare no competing financial interest. G
DOI: 10.1021/acs.inorgchem.5b02015 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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ACKNOWLEDGMENTS The authors would like to thank NSF (NSF OIA-1539035), the University of Mississippi, and the UM Sally McDonnell Barksdale Honors College for funding.
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DOI: 10.1021/acs.inorgchem.5b02015 Inorg. Chem. XXXX, XXX, XXX−XXX
