N-Heterocylic Carbene-Based Mn Electrocatalyst ... - ACS Publications

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N‑Heterocylic Carbene-Based Mn Electrocatalyst for Two-Electron CO2 Reduction over Proton Reduction Kuber Singh Rawat,† Arup Mahata,† Indrani Choudhuri,† and Biswarup Pathak*,†,‡ †

Discipline of Chemistry, School of Basic Sciences and ‡Center for Material Science and Engineering, Indian Institute of Technology (IIT) Indore, Indore, Madhya Pradesh 452020, India S Supporting Information *

ABSTRACT: A bifunctional, chelating N-heterocyclic carbene-pyridine (NHC-pyridine) containing Mn(I) complex [MnBr(NHC-pyridine)(CO)3] displays a strong selectivity for CO2 reduction over proton reduction. Interestingly, the two-electron reduction of this complex occurs at a single potential, as opposed to MnBr(bpy)(CO)3, which is reduced by two electrons in two separate one-electron reductions. Here, the Gibbs free energy barriers, reduction potentials, rate constants, and pKa values are predicted with theory to understand the one- vs two-electron reduction mechanism. The effects of weak and strong Brønsted acids [HCl, TFE (2,2,2-trifluoroethanol), PhOH, CH3OH, and H2O] are studied to gauge the preference for CO2 vs proton reduction; water is found to be an ideal proton donor that allows for strong CO2 selectivity.

[Re(bpy)Cl(CO)3] and [Mn(bpy)Br(CO)3] complexes.33,34 They demonstrated the role of strong and weak Brønsted acids for such reduction reactions.35 Re-based complexes can selectively reduce CO2 in the presence of weak Brønsted acids,32 but they reduce CO2 at higher over potentials. Recently, Mn-based complexes have been studied as alternatives for Re-based complexes36 as Mn is cheap and abundant in nature and these compounds reduce CO2 at lower over potentials.37 Mn-based complexes Mn(R2-bpy)(CO)3Br [R = H or Me] have been reported38 for electrochemical CO2 reduction in the presence of the weak Brønsted acids, whereas Re-based complexes reduce CO2 in the absence of Brønsted acids though their catalytic activity increases in the presence of Brønsted acids.39,40 Similarly, Smieja et al. reported a Mn−bpy complex [Mn(bpy-tBu)(CO)3Br] that shows high selectivity and stability for CO2 reduction in the presence of weak Brønsted acids [water, methanol, and TFE (2,2,2-trifluoroethanol)].36 Sampson et al. used a bulky bpy ligand (6,6′dimesityl-2,2′-bipyridine) for Mn-based complexes and showed that the reduction proceeds through a two-electron transfer process.41 F. Hartl et al. recently studied a nonaromatic αdiimine ligand in place of the traditional bipyridine ligand for the manganese complex and demonstrated that the Mn complex has a strong π electron delocalization which stabilizes the five-coordinated anionic species formed after two-electron reduction.42 Recently, Agarwal et al.43 reported the reduction of CO2 to CO using N-heterocyclic carbene (NHC-pyridine) based Mn complexes [MnBr(N-methyl-N′-2-pyridylbenzimida-

1. INTRODUCTION An appreciable amount of research is into the development of alternative energy sources,1,2 as the current dependence on fossil fuel-based energy has a negative effect on our environment.3 CO2 concentrations are increasing steadily; thus, the biggest challenge is to reduce the level of carbon dioxide in the atmosphere. Conversion of CO2 into useful organic products is the key step as carbon dioxide is an important feedstock for organic chemicals due to its low cost, high abundance, and lower toxicity.4 Reduction of CO2 to CO is a crucial step for methanol synthesis5−8 and the hydroformylation reaction.9 Such conversion leads to the synthesis of high energy density fuels (CH3OH, HCOOH, CH4). Alternative energy sources such as solar and wind are very promising but depend on the geographical position. Therefore, storage plays a key role for sustainability. Conversion of electrical to chemical energy is an important concept for energy storage. There are many possible ways such as photocatalysis10−12 and electrocatalysis13−20 that electrical energy can be converted into chemical energy by reducing CO2 to important organic products. Metal-complexbased electrocatalysts have mainly two advantages over others. First, metal complexes can be tuned easily by ligand modifications. Second, metal complexes can have a vacant site available initially or during the course of the reaction.21 Many transition metal-based complexes (Ni, Fe, Re, Cr, Ir, Mo, etc.) have been studied extensively for the homogeneous electrocatalytic reduction of carbon dioxide,22−28 but Re-based complexes [Re(bpy-R)X(CO)3] (X = Cl−, Br−; bpy-R = 4,4′dimethyl-2,2′-bipyridine when R = Me; R = H, Me, OMe, tBu) have been vastly studied and reported to be promising catalysts for CO2 reduction.29−32 Carter et al. studied the mechanistic pathways of the electrocatalytic reduction of CO2 to CO using © XXXX American Chemical Society

Received: March 2, 2016 Revised: April 8, 2016

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DOI: 10.1021/acs.jpcc.6b02209 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

(UB3LYP) as implemented in the Gaussian 09 package.46−49 Geometry optimizations are carried out using the 6-31+G** basis sets50,51 for nonmetals (C, H, O, N, Cl, F, and Br) and LANL2DZ with effective core potentials (ECPs) for Mn.52 In all calculations, the D3 version of Grimme’s dispersion corrections is applied.53 Single-point energy calculations are performed by the UB3LYP method using the aug-cc-pVDZ basis set54 for nonmetals and LANL2DZ ECP for the Mn atom. Acetonitrile (ε = 35.69) solvent is used for all calculations as an implicit solvent using the polarizable continuum model (CPCM)55,56 as implemented in Gaussian 09. Harmonic vibrational frequencies are calculated to characterize the nature of the stationary points. Zero-point and entropy corrections are included in the Gibbs free energy calculations. The reduction potential (E) is calculated57 as follows

zol-2-ylidene)(CO)3] where two-electron reduction of these species happens at a single potential. In another study44 they showed the role of axial ligands toward the CO2 reduction with MnX(N-ethyl-N′-2-pyridylimidazol-2-ylidene)(CO)3 (X = Br, NCS, and CN) complexes and reported that NCS and CN ligands favor the proton reduction over CO2 reduction.44 However, Br favors the CO2 reduction over proton reduction.44 Thus, the role of Br as an axial ligand is very important for the CO2 reduction reaction. In the present study, we performed a systematic computational study in inert and CO2 atmospheres to understand the whole electrocatalytic reduction mechanism of CO2 using a bifunctional, chelating N-heterocyclic carbene-pyridine (NHCpyridine) containing Mn complex 1 [1 = MnBr(N-methyl-N′2-pyridylbenzimidazol-2-ylidene)(CO)3]. Here, Br is used as an axial ligand to improve the CO2 reduction over proton reduction. Previous studies show that CO2 can be electrochemically reduced via two pathways: (1) a relatively slow oneelectron pathway and (2) a more rapid two-electron pathway.45 However, the one-electron pathway is a very slow process. Thus, the more rapid two-electron pathway is very important for the CO2 reduction reaction. However, the two-electron reduction can happen at the same potential or via two separate one-electron reductions. Unlike the one-electron reduction, the two-electron reduction is proton dependent and highly affected in the presence of Brønsted acids. Interestingly, the NHCpyridine containing Mn complex favors the two-electron pathway, and the two-electron reduction occurs at a single potential rather than in two separate potentials. Thus, this is a very interesting finding in comparison to the previous reports on two-electron CO2 reduction reactions.22−42 The main motivation of this work is to understand the role of the NHC-pyridine ligand toward two-electron CO2 reduction at a single potential. This could be something to do with the redoxactive nature of the NHC-pyridine ligand as the similar MnBr(bpy)(CO)3 complex reduces CO2 in two separate potentials.34 On the other hand, the two-electron reduction occurs at a single potential in inert and CO2 environments. However, in the inert environment, the metal complex is reduced at a less negative potential (−1.35 V) than in the CO2 environment (−1.48 V). Thus, it will be interesting to know why the two-electron reduction occurs at a higher potential in the CO2 environment than in the inert environment. We believe our reaction mechanism study of this system will help in understanding the role of the redox-active ligand toward twoelectron CO2 reduction over proton reduction, which in turn will be very helpful for designing highly active and selective NHC-pyridine-based electrocatalysts. We modeled the complete electrocatalytic cycles for the CO2 to CO reduction catalyzed by complex 1. The reaction free energies, reduction potentials, and activation barriers are calculated for the whole reduction reaction to evaluate the most favorable reduction pathway for the electrocatalytic process. We calculated pKa values for weak and strong Brønsted acids (HCl, TFE, PhOH, CH3OH, and H2O) and for some important intermediates to understand the proton reduction process. The effects of Brønsted acids are also investigated to understand their roles for CO2 vs proton reduction reactions.

⎛ ΔG(soln) ⎞ 0 E = ⎜− ⎟ − Eref ⎝ ⎠ nF

(1)

where ΔG(soln) (Figure S1; see details in the Supporting Information) is the reduction free energy change of the redox reaction in the solution phase (here acetonitrile is the solvent), n is number of electrons, F is Faraday’s constant, and E0ref is the reduction potential of the reference electrode. Here the reference electrode is the saturated calomel electrode (SCE) and the potential is 4.67 V in acetonitrile solution.58,59 All reduction potentials (E) reported here are referenced to the SCE. We calculated pKa values for different Brønsted acids based on the thermodynamic cycle (Supporting Information, Figure S2).60 Though the dispersion- and nondispersioncorrected results show a similar trend, the nondispersioncorrected results are more in agreement with the experimental findings.43 Recently, Carter et al. used the same method and basis sets for CO2 reduction reactions, and their calculated results are very much in agreement with their experimental data.33 Thus, the nondispersion-corrected results are given in the manuscript, and the dispersion-corrected results are given in the Supporting Information. As the electronic energies obtained using the aug-cc-pVDZ basis set (except for the Mn atom where LANL2DZ ECP is used) are very much in agreement with our nondispersion-corrected results; thus, we calculated them only for scheme 1, and these data are given in the Supporting Information. The Gibbs free energy of a proton in the gas phase is −6.3 kcal/mol61 and in acetonitrile solvent 260.2 kcal/mol.62 Here, the 1.89 kcal/mol correction is included in the proton solvation energy in acetonitrile solvent.63 Reaction rate constants (k) are calculated as ⎛ −ΔG‡ , ° ⎞ ⎛k T ⎞ k = ⎜ B ⎟ × K ° × exp⎜ ⎟ ⎝ h ⎠ ⎝ RT ⎠

(2)

where kB is the Boltzmann constant (1.38 × 10−23 m2·kg·s−2· K−1), T is the temperature (298.15 K), h is Planck’s constant (6.626 × 10−34 m2·kg/s), K0 is the inverse of the standard state concentration (1 M), R is the universal gas constant (8.314 J K−1 mol−1), and ΔG‡,° is the Gibbs free energy of the activation barrier (obtained in the standard state of 1 M, 298.15 K, and 1 atm for all species). The natural bond orbital (NBO) charges are calculated to understand the charge distribution on the Mn complexes.64

2. COMPUTATIONAL DETAILS All calculations are carried out using Becke’s three-parameter exchange and the Lee−Yang−Parr correlation functional B


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3. RESULTS AND DISCUSSION We begin the analysis by studying the most favorable reduction pathway for complex 1 ([MnBr(NHC-pyridine)(CO)3]) in inert and CO2 environments. The CO2 vs proton reduction selectivity is then discussed in the presence of strong and weak Brønsted acids. The Mn−CO(OH) bond formation followed by C−OH bond dissociation is discussed at the end. 3.1. Formation of Active Catalytic Species. 3.1.1. In the Absence of CO2 Environment. The X-ray crystal structure of complex43 1 (Figure 1) is optimized and compared with

Figure 1. Optimized structure of complex 1. Values in parentheses denote experimental bond distances (in Angstroms). Figure 2. Calculated reaction free energies (ΔG in kcal/mol) and reduction potentials (E) are reported for the possible products after one- and two-electron reductions of complex 1 (in inert environment). Values in parentheses are experimentally reported reduction potentials.43 (a) Reduction potential is calculated using the equation 1/2(3-Dimer) + e− → 4.

experimental structural parameters (such as bond lengths and bond angles). Our calculated structural parameters are in agreement with the available experimental data (see Supporting Information, Figure S3 and Table S1). In inert atmosphere, all reactions are modeled in acetonitrile solvent. Two possible pathways (1 → 2 → 3 and 1 → 3) are modeled (Figure 2 for GD3, see Supporting Information, Figure S4) for the first reduction of complex 1 [MnBr(NHCpyridine)(CO)3]. The first reduction can proceed through oneelectron reduction [1 + e− → 2] [MnBr(NHC-pyridine)(CO)3]¯ followed by a ligand to metal charge transfer (LMCT) resulting in a Br− loss and formation of a five-coordinated complex 3 [Mn(NHC-pyridine)(CO)3]. The calculated first reduction potential of 1 → 2 is E = −1.38 V, which agrees well with the experimental value of −1.35 V. The halide (Br−) ion dissociation (2 → 3 + Br−) is calculated to be a spontaneous process.34 The activation barrier (1.9 kcal/mol, Figure S5 in Supporting Information) for the Br− ion dissociation is low (2 → 3 + Br−). We calculated the rate constant at room temperature for the halide dissociation, and we find the rate constant (2.6 × 1011 M−1 s−1) is very high. Thus, the oneelectron reduction (1 → 2) followed by a Br− ion loss (2 → 3 + Br−) is thermodynamically (at an applied potential of −1.40 V) and kinetically very favorable. Thus, these two independent steps could be a result of the first reduction of complex 1. Such bromide anion dissociation has been reported in the Mn−byp complex [Mn(bpy)Br(CO)3],38 and the formed neutral moiety can then react with itself to the formation of a dimer.34 Thus, we also considered the possibility of dimer formation (3 → 3-Dimer), and the staggered (staggered vs eclipsed conformers) one is calculated to be the most stable dimer. The dimer formation (3 → 3-Dimer) is calculated to be endergonic by 7.4 kcal/mol, which (3-Dimer) could be then reduced to complex 4 (calculated E = −1.42 V, experimental E = −1.64 V). Therefore, dimer formation is not thermodynami-

cally favorable, and such a finding agrees well with the experimental report,43 where they reported the formation of a limited quantity of dimer. On the other hand, complex 1 can be directly reduced to 3 (1 → 3). However, in that case the reaction should be modeled as two independent steps (1 → 2 and 2 → 3 + Br−). Thus, in this case the first step is an electrochemical step (1 → 2) followed by a chemical step (2 → 3 + Br−). Generally the electron transfer step is faster than the ligand dissociation step. Thus, we believe the reaction is proceeding through reduction (1 → 2, E = −1.38 V) followed by a halide dissociation (2 → 3) as predicted earlier. Similarly, complex 2 can be directly reduced to 4. Again, this reaction should be modeled as two independent steps (2 + e−→ 2′ and 2′ → 4 + Br−). However, 2′ [{MnBr(NHCpyridine)(CO)3}−2] is not a minimum in the potential energy surface. Thus, in this case the reaction might be proceeding via a chemical step followed by an electrochemical step (2 → 3 + Br− and 3 + e− → 4) or via an electrochemical step followed by a chemical step. We find all three possible steps are endergonic by 26.6 (1 → 2), 22.2 (1 → 3), and 26.6 kcal/mol (2 → 4). However, at an applied potential of −1.40 V, all electrochemical reactions are feasible. Another possibility is complex 3 reduced to 4 [Mn(NHC-pyridine)(CO)3]¯ (calculated E = −1.58 V), but in that case, the reaction is endergonic by 30.7 kcal/mol, which is more negative than the applied potential of −1.40 V. The ideal theory story would be that the first and second reduction potentials appear at very close voltages, since the experimental report finds a single two-electron reduction C


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Figure 3. Calculated reaction free energies (ΔG in kcal/mol) and reduction potentials (E) are listed for possible products after one- and twoelectron reductions of complex 1 (in CO2 environment).

wave.43 Our results show closest agreement with experiment43 when considering the first electron reduction is 1 + e− → 2 (calculated E = −1.38 V, experimental E = −1.35 V) and followed by second electron reduction is 2 + e− → 4 (calculated E = −1.40 V, experimental E = −1.35 V). However, formation of 3 is also favorable as Br− has a low dissociation barrier. The neutral complex (3) can then be reduced to 4. We believe the second electron reduction wave in the experiment could be the reduction of 3 + e− → 4 (calculated E = −1.58 V). Thus, we believe, at an applied potential of −1.40 V, the reaction might be proceeding through both pathways (1 → 2 → 3 and 1 → 2 → 4). However, the second electron reduction potential of the experiment (−1.64 V) could be actually the reduction of complex 3 (3 → 4, −1.58 V).43 Therefore, we can say the reaction might be going through via an ECE mechanism (electron transfer followed by a chemical reaction followed by an electron transfer) as predicted by the experimentalists43 or via an EEC mechanism (electron transfer followed by an electron transfer followed by a chemical reaction). However, at an applied potential of −1.40 V, the reaction might be occurring through an EEC mechanism. 3.1.2. In Addition of CO2: ECE Mechanism. In CO2 environment, the reactions are modeled using a single explicit CO2 molecule in the continuum model of acetonitrile solvent. Under an atmosphere of CO2 the reaction must be proceeding through different intermediates compared to the reaction in inert atmosphere, as the two-electron reduction wave occurs at a different potential (−1.48 V) than in inert environment (−1.35 V). Thus, we believe CO2 is interacting with the metal complex (1) through either the metal center or the NHCpyridine ligand. However, complex 1 is an 18-electron species. Thus, CO2 does not interact with the metal center. However, we find the nitrogen-based NHC-pyridine ligand (1) could interact (Figure 3) with the CO2 to form a stable CO2-bonded complex (1-CO2, Figure 3). The reaction free energy is endergonic by 4.5 kcal/mol for the CO2 binding to 1 (for details see Supporting Information, Figure S6 and S7 and Table S2). The dispersion-corrected reaction free energy is improved (by 0.8 kcal/mol) to 3.7 kcal/mol. The CO2 binding barrier (dispersion corrected) is calculated to be 5.1 kcal/mol. The room-temperature rate constant was calculated to be 1.2 × 109 M−1 s−1 for the CO2 binding to 1. Thus, CO2 binding to 1 is kinetically favorable. Similarly, CO2 interacts with the NHCpyridine ligand of 2 and 3 to form 2-CO2 and 3-CO2,

respectively. However, complex 4 strongly interacts with CO2 to form 4-CO2 (Figure 3). The CO2 binding energies are calculated and given in Table S2 (see Supporting Information), and we find such interactions are very weak in nature. However, at an electrode potential of −1.40 V, such interactions are thermodynamically favorable. Under an atmosphere of CO2 (Figure 3) we assume the first reduction reaction is 1-CO2 + e− → 2-CO2 (calculated E = −1.48 V, experimental E = −1.35 V) followed by a halide ion dissociation (2-CO2 → 3-CO2 + Br¯) takes place. The halide ion dissociation is more exergonic (−4.3 kcal/mol) in CO2 atmosphere than in inert atmosphere (−3.9 kcal/mol). We could not locate the transition state for halide ion dissociation. However, the activation barrier could be lower (not necessarily) in CO2 atmosphere than in inert atmosphere as the reaction free energy is more exergonic in a CO2 atmosphere than in inert atmosphere. Thus, we believe halide ion dissociation is much more favorable in CO2 environment. The second reduction potential is (3-CO2 + e− → 4-CO2) calculated to be −1.51 V, which is very close to the first reduction potential of −1.48 V. Thus, we believe the two-electron reduction reaction occurs at the same potential as both reduction reactions are happening in close voltages (−1.48 and −1.51 V). Another possibility is that complex 1-CO2 can be reduced to 3-CO2. Thus, reduction of 1-CO2 and Br− dissociation could happen at the same time. The reaction energy will be the same whether complex 1-CO2 is directly reduced to 3-CO2 (1-CO2 → 3-CO2; E = −1.29 V) or via 2-CO2 (1-CO2 → 2-CO2 → 3CO2). Thus, this reaction is modeled as two independent steps. We assume the reaction is proceeding through reduction of 1CO2 (1-CO2 → 2-CO2) followed by a halide ion dissociation (2-CO2 → 3-CO2) reaction, as the electron transfer reactions are faster compared to chemical reactions. Similarly, complex 2-CO2 can be reduced to 4-CO2. Again, this reduction might be proceeding through two independent steps. The reaction might be happening through either reduction followed by a chemical reaction or chemical reaction followed by a reduction. However, the reduced 2-CO2, i.e., [{MnBr(NHC-pyridine·CO2)(CO)3}−2], is not calculated to be a minimum in the potential energy surface. Thus, we believe the reaction takes place via a chemical reaction (2-CO2 → 3CO2 + Br−) followed by a reduction reaction (3-CO2 + e− → 4-CO2). D


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Figure 4. Calculated reaction free energies (ΔG in kcal/mol) and reduction potentials (E) are listed for possible products after one- and twoelectron reductions of complex 1 (in H2O environment).

Figure 5. (a) Protonation of 4 for formation of 5 (pKa value is calculated in acetonitrile solvent). Brønsted acid-bonded transition state structures (TS4→5) are presented as follows: (b) HCl, (c) PhOH, (d) TFE, (e) methanol, and (f) water bonded. Bond distances are in Angstroms.

is used for the CO2 reduction reaction.43 Thus, there could be a competition between CO2 vs proton reduction depending on the concentration of water present in solution. Thus, like CO2, H2O could interact with the metal complex (1) through the NHC-pyridine ligand (1-H2O, Figure 4). The reaction free energy is endergonic by 4.5 kcal/mol for H2O binding to 1 (see Supporting Information, Figures S8 and S9). The reaction free energy is improved by 1.0 kcal/mol (3.5 kcal/mol) after inclusion of dispersion correction. As we could not locate the transition state for this step, we believe the barrier could be very similar to CO2 binding to 1 and thus kinetically and thermodynamically favorable. Similarly, H2O can interact with

Thus, under CO2 atmosphere too the reduction mechanism is proceeding through an ECE mechanism as previously learned under inert atmosphere. Here, first, complex 1-CO2 is reduced to 2-CO2 (calculated E = −1.48 V, experimental E = −1.35 V), followed by a Br− ion dissociation. This leads to formation of 3CO2, which is then reduced to 4-CO2 (calculated E = −1.51 V). 3.1.3. In Addition of Water: Proton Reduction. Similarly in the water medium, the reactions are modeled using a single explicit H2O molecule in the continuum model of acetonitrile solvent. The possibility of proton reduction is modeled in the water medium. Even under an atmosphere of CO2, 5% of water E


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Figure 6. (a) Reaction free energy (ΔG in kcal/mol) and activation barriers (ΔG‡ in kcal/mol) for the formation of 4-CO2 and 6. Transition state structures of CO2 binding to 4 through (b) O (TS−OCO) and (c) C (TS-CO2) atoms. Bond distances and angels are in Angstroms and degrees.

transfer in the presence of strong to weak protonating agents [HCl, PhOH, TFE (2,2,2-trifluoroethanol), H 2 O, and CH3OH]. The effects of different Brønsted acids (HCl, PhOH, TFE, CH3OH, and H2O) as protonating agents are modeled using Brønsted acid as a single explicit molecule and acetonitrile as a continuum solvent. The reaction free energies and activation barriers for protonation (TS4→5) are given in Table 1(for GD3

the NHC-pyridine ligand of 2 and 3 complexes to form 2-H2O, and 3-H2O, respectively. However, complex 4 can strongly interact with H2O to form 4-H2O (Figure 3), as complex 4 has a vacant site for binding. The H2O binding energies are given in Table S3 (see Supporting Information), and these interactions are very weak in nature. However, at an electrode potential of −1.40 V vs SCE, such interactions could be thermodynamically favorable. Interestingly, the calculated electrochemical potentials (Figure 4) of the H2O-bonded complexes are very similar and comparable with the CO2-bonded complexes (Figure 3). In fact, the calculated reduction potentials are less negative for the H2O-bonded complexes than the CO2-bonded complexes. Thus, we believe formation of 4-H2O and 4-CO2 is thermodynamically favorable at an applied potential of −1.50 V. In the following section we discuss the CO2 vs proton reduction selectivity. 3.2. Selective Reactivity of the Active Catalytic Species. The presence of protonating agents is very important for the CO2 reduction. However, in that case there is a competition between CO2 vs proton reduction. Hence, there is a high chance a proton will bind to the active catalytic species (4), and in that case proton reduction will be favored over CO2 reduction. Thus, the reaction selectivity of complex 4 toward CO2 vs H+ reduction is studied in the presence of various Brønsted acids as they play a major role toward reaction selectivity. We considered strong to weak Brønsted acids [HCl, PhOH, TFE (2,2,2-trifluoroethanol), H2O and CH3OH], as such protonating agents are reported to improve product selectivity.35,36 More importantly, addition of Brønsted acids leads to the stable and efficient production of CO from CO2.36 Intermediate 4 is an active catalyst, which can either react with CO2 or H+. The protonation (H+) on complex 4 (Figure 5) will lead to the formation of a neutral intermediate 5 (5 = [Mn(NHC-pyridine)(CO)3]), and binding (Figure 6) to CO2 will lead to the formation of 6 (6 = [Mn(N NHCpyridine)(CO)3(OCO)]¯) and 4-CO2. In the following section we investigated the proton reduction process, i.e., proton

Table 1. Reaction Free Energies (ΔG in kcal/mol) and Activation Barriers (ΔG‡ in kcal/mol) Are Listed for the Protonation of Complex 4 in Different Brønsted Acidsa A−H bond lengths proton source

gas phase

TS

difference

Mn−H

ΔG

(ΔG‡)

HCl PhOH TFE H2O CH3OH

1.29 0.97 0.97 0.97 0.97

1.56 1.47 1.62 1.73 1.75

0.27 0.50 0.65 0.76 0.78

2.13 1.78 1.72 1.70 1.69

−36.3 −6.0 0.9 12.3 12.7

0 21.5 23.7 29.5 31.5

a

All bond distances (A−-H, Mn−H) are in Angstroms.

see Supporting Information Table S4). The reaction free energies are −36.3, −6.0, 0.9, 12.3, and 12.7 kcal/mol for HCl, PhOH, TFE, H2O, and CH3OH, respectively. The calculated activation barriers for protonation are 0.0, 21.5, 23.7, 29.5, and 31.5 kcal/mol in HCl, PhOH, TFE, H2O, and CH3OH, respectively. Hence, the proton transfer from HCl (strong Brønsted acid) to 4 is a barrierless process with a reaction free energy of −36.3 kcal/mol. On the other hand, proton transfer is difficult (Table 1) in the presence of weak Brønsted acids. The free energy barriers for protonation (Figure 5) in water and methanol (Table 1) are very much comparable compared to protonation in other mediums. Moreover, in the case of methanol, the highly unstable CH3O¯ conjugate base forms a chemical bond with the ligand moiety, thus affecting the overall reaction.33 Therefore, we believe water is the best proton donor for selective CO2 reduction as in the presence of water proton F


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The Journal of Physical Chemistry C Table 2. NBO Charges on Complex 4 and Brønsted Acids-Bonded TS4→5 Transition State Structures species

TS-HCl

4

TS-CO2

TS-PhOH

TS-TFE

TS-H2O

TS-CH3OH

Mn NHC-pyridine

−1.52 0.13

−1.57 0.28

−1.58 0.29

−1.70 0.57

−1.72 0.61

−1.71 0.62

−1.72 0.63

for CO2 reduction. Hence, the noninnocent nature of the NHC-pyridine is really interesting and could be the reason for the selective CO2 reduction. Hence, N-heterocyclic carbene (NHC-pyridine) has an important role in the electrocatalytic reaction due to the nature of the redox-active (noninnocent) ligand. Once complex 4 accepts an electon, the NHC-pyridine ligand delocalizes the negative charge over the π orbitals. As a result the negative charge gets stabilized. Similarly, NHC-pyridine ligand behaves as a noninnocent ligand when 4 binds to CO2. Thus, the metal transfers the charge to the C atom of CO2. Our frontier orbitals (HOMO) study shows that the maximum electron density is delocalized over the Mn atom (Figure 7) rather than on the

transfer is not kinetically favorable. Thus, the metal site will be available for the CO2 reduction reaction. As the protonation barriers (Table 1) are varying from strong to weak Brønsted acids, we believe such trend can be explained from their transition state (TS) structures (Figure 5). The Mn−H bond distances are very much comparable (1.69−1.78 Å, Table 1) in the weak Brønsted acids-based (CH3OH, H2O, TFE, PhOH) TS structures (TS4→5) but different in the strong Brønsted acid-based (HCl; Mn−H = 2.13 Å) TS structure. Thus, the trend in protonation barriers can be explained from their structural parameters (Mn−H bond distances) too (Table 1). We find the protonation barrier is lower for the higher Mn− H distances. We further investigated the A−H (A = O or Cl) bond lengths in the TS structures with respect to their gas-phase geometries. The A−H bond length increases by 0.27, 0.50, 0.65, 0.76, and 0.78 Å in the HCl, PhOH, TFE, H2O, and CH3OH acids-based TS structures, respectively. In the case of weak Brønsted acids-based TS structures, the protonation barriers can be explained from their A···H bond distances. The activation barrier is higher (Table 1) for the long A···H bond distances. Binding of CO2 to 4 is possible through (Figure 6) either oxygen (6) or carbon atoms (4-CO2). The formation of complex 6 and 4-CO2 is endergonic by 29.6 and 4.9 kcal/mol, respectively. Their activation barriers (Figure 6) are 29.6 (4 → 6; TS−OCO) and 11.1 kcal/mol (4 → 4-CO2; TS-CO2), respectively. Thus, CO2 binding through the C atom (4-CO2) is thermodynamically and kinetically more favorable (by 24.7 kcal/mol) over binding through the O atom (6). Interestingly, in the water medium, CO2 binding to 4 (4 → 4-CO2; barrier = 11.1 kcal/mol) is favorable over protonation (4 → 5; barrier = 29.5 kcal/mol). Therefore, 4 shows a great selectivity for CO2 (4-CO2) reduction over proton reduction (5). We did NBO analysis (Table 2) to understand the selectivity of CO2 binding to 4 over protonation in the presence of different Brønsted acids. The NBO charges of the transition state structures are given in Table 2. We also summarized (Table 2) the summed up charge of the NHC-pyridine ligand to understand its role as a redox-active ligand toward reaction (CO2 vs proton reduction) selectivity. Table 2 shows the summed up charge on the NHC-pyridine ligand is 0.28 in complex 4, and it decreases to 0.29, 0.57, 0.61, 0.62, and 0.63 in the TS-CO2, TS-PhOH, TS-TFE, TS-H2O, and TS-CH3OH structures, respectively, where as the NBO charge increases to 0.13 in the TS-HCl structure. This shows that the NHCpyridine ligand has both the donor and the acceptor properties in the Mn−NHC-pyridine complex. Therefore, the donor/ acceptor properties of the redox-active noninnocent NHCpyridine ligand65 should be considered while designing electrocatalysts for CO2 vs proton reduction. Interestingly, the charges (Table 2) on the NHC-pyridine and Mn atom are not changed in the TS-CO2 structure. However, NHC-pyridine acts as a donating ligand for the CO2 binding. On the contrary, it acts as an acceptor for the proton reduction reaction. Therefore, NHC-pyridine acts as a noninnocent (redox) ligand

Figure 7. Highest occupied molecular orbitals of (a) 4 and (b) 4-CO2. Electron density difference maps of (c) 4 and (d) 4-CO2 (turquoiseand purple-colored areas indicate depletion and accumulation of electron density). Electrostatic potential of (e) 4 and (f) 4-CO2. Isosurface value is set to 0.02 e·Å−3.

NHC-pyridine ligand. As the NHC-pyridine ligand increases the σ donor property of the pyridine ligand reduces the overall electron density over the π-ligand system. Therefore, it is expected that the HOMO of 4 can easily interact with the π* orbital of the CO2. We derived the electron density difference maps (EDDM) between the ground and the excited states of 4 and 4-CO2 (Figure 7c and 7d) to explicitly show precisely where frontier electrons reside within different intermediates (4 and 4-CO2). The turquoise-colored regions in 4 suggests that the electrons are localized on the metal center. However, after binding to CO2, the frontier electrons (in 4-CO2) are delocalized. G


Article


Figure 8. All possible reaction pathways are presented for CO2 to CO reduction. Reaction energies (ΔG in kcal/mol), reduction potentials (E), activation barriers (ΔG‡ in kcal/mol), and pKa values are given for their respective steps. Here H+ is modeled using a explicit water molecule.

Table 3. Calculated pKa Values for Various Brønsted Acids and Intermediates in Acetonitrile Solventa

a

species

5

7

HCl

water

methanol

acetonitrile

PhOH

TFE

pKa

27.7

23.4

5.6 (11)33

35.2 (33.7)35

35.4 (34.1)35

37.2 (37.5)35

24.03 (22.3)35

28.2 (26.7)35

pKa values listed in parentheses are taken from earlier studies (refs 33 and 35).

Figure 9. Transition state (TS) structures of proton transfer from water to (a) 4-CO2 and (b) 7. Bond distances are in Angstroms.

binds to 4) will be preferred over proton reduction (i.e., H+ binding to 4). Thus, the CO2 reduction should be carried out in a medium where the proton transfer is not thermodynamically/ kinetically favorable. The proton transfer is difficult if a medium has a higher pKa value than 4/4-CO2. Thus, we calculated pKa values in acetonitrile solvent for different Brønsted acids (Table 3) to find the suitable medium for CO2 reduction reaction.33,35 We find the proton transfer will not be favorable in the presence of weak Brønsted acids as weak Brønsted acids have higher pKa values than 4/4-CO2. Thus, we used water as a proton source to stabilize the intermediate (4-CO2).66,67 We find the protonation of 4-CO2 (4-CO2 + H+ → 7; 7 = [Mn(NHC-pyridine)(CO)3(CO2H)]) is exergonic by −28.3 kcal/mol with a free energy barrier of 3.8 kcal/mol (Figure 9). Thus, the CO2 protonation barrier is lower in comparison to the protonation barrier of 4 in the presence of weak Brønsted acids. The room-temperature rate constant for protonation of 4-CO2 is 1.1 × 1010 M−1 s−1. Thus,

Electrostatic potential (ESP) surfaces are plotted (Figure 7e and 7f) to show the charge localization/delocalization over the metal complexes. In the case of complex 4, the negative charge (red color) is more concentrated on the metal center (Figure 7e). As a result, complex 4 strongly binds to CO2. On the other hand, the ESP of 4-CO2 (Figure 7f) shows the negative charge is highly localized over the Mn-CO2 bond. 3.3. Formation and Breaking of C−OH bond. In case the proton is not available within the ligand moieties then an external proton source is required for the CO2 reduction. The ideal proton source for CO2 reduction can also be assessed from the pKa values of the intermediates present in the solution. Thus, we calculated pKa values for 5 (4 + H+ → 5, Figure 4) and 7 (4-CO2 + H+ → 7, Figure 8 and for GD3 see Supporting Figure S10) present in solution. The calculated pKa values (see Figure S3; Supporting Information) are 27.7 and 23.4 for 5 and 7, respectively. Our pKa study shows that intermediate 4 has more proton affinity than 4-CO2. Thus, in such cases, it is very unlikely that CO2 reduction (i.e., CO2 H


Article

The Journal of Physical Chemistry C Table 4. Calculated Rate Constant (M−1 s−1) for the Important Electrocatalytic Steps reaction

1 → 1-CO2

2→3

4→5

4→6

4 → 4-CO2

4→7

7→8

K

1.2 × 109

2.6 × 1011

2.0 × 10−9

1.7 × 10−9

5.1 × 104

1.1 × 1010

1.8 × 10−4

The reaction rate constants are calculated (see computational section) for the whole electrocatalytic cycle and are given in Table 4. The high activation barrier step is the C−OH bond cleavage step (7 → 8, Figure 7) with a rate constant value of 1.5 × 10−4 M−1 s−1. Therefore, we believe this is the ratedetermining step for the whole electrocatalytic process. On the other hand, protonation of 4-CO2 is the fastest (Figure 8) step with a rate constant value of 1.0 × 1010 M−1 s−1.

this is a very fast process compared to the protonation on 4 (rate constant in water = 2.0 × 10−9 M−1 s−1). This is because CO2− is an unstable species and ready to react. Hence, the CO2 binding to 4 is favorable over H+ binding in the presence of water. Acids with lower pKa values reverse the reaction selectivity and thus favor H2 production over CO2 reduction.68 The overall catalytic cycle for CO2 reduction is presented in Figure 8. The hydroxycarbonyl complex (7) is formed upon protonation to 4-CO2. The reduction mechanism can proceed via two different pathways. The reaction will proceed via either (i) protonation followed by reduction (P−R) or (ii) reduction followed by protonation (R−P). In the P−R pathway, protonation of 7 initiates (Figure 7) the C−OH bond dissociation for the formation of 8 and water. The cationic carbonyl complex (8) produces 10 upon reduction. In the R−P pathway, 7 is reduced to 9, which undergoes C−OH bond dissociation upon protonation to form 10. Protonation on 7 was calculated (Figure 8) to be more exergonic than reduction of 7. However, the activation barrier is very high for the protonation of 7. Thus, the rate constant (Table 4) is very slow for the formation of 7. The calculated pKa values of the intermediates 7 and 9 are 23.4 and 28.7, respectively. Thus, complex 9 has a higher affinity for proton than 7. The calculated reaction energies are −28.1 and −38.0 kcal/mol for protonation on 7 and 9, respectively. The preference of the R−P vs P−R pathways can be predicted from the 18-electron counting rule of the complexes 7, 8, and 9. Complex 7 (18-valence electrons) upon reduction will form a 19-valence electron species (9). Our results show such reduction is not thermodynamically favorable due to the high reduction potential (−2.4 V). Therefore, 7 favors protonation (7 + H+ → 8 + H2O) over reduction (7 + e− → 9). However, this is the slowest step for the CO2 reduction reaction. Thus, this could be the rate-determining step for the CO2 reduction reaction. NHC-pyridine ligand plays an important role as it favors protonation (7 → 8) over reduction. The previous reports show that the bpy ligand-based Re(bpy)(CO)3Cl complex favors a reduction reaction, whereas the Mn(bpy)(CO)3Br complex favors both pathways when a potential is applied.34 Therefore, the ligands and metals play an important role for reaction selectivity. In the next step, the cationic 8 ([Mn(NHC-pyridine)(CO)4]+) species can undergo a reduction reaction via two pathways as shown in Figure 7. If 8 is reduced to 10 then the calculated reduction potential is −1.97 V (8 + e−→ 10), which is much higher than the reduction potential of 3 + e− → 4 (E = −1.58 V; Figure 2). Thus, the reduction of 8 (8 → 10) is not feasible under the reaction potential. This can be explained from the total electron counting rule. 10 is a 19-electron species and thus not a stable intermediate. On the other hand, 8 could be reduced to 3 (8 + e− → 3 + CO, E = −1.15 V), where CO will be released as a byproduct, and this pathway is favorable (−1.25 V) under the reaction potential. Therefore, the whole electrocatalytic CO2 reduction process can be cyclic under the experimental potential. The other probable steps are also considered as presented in Figure 7. For example, 10 can be reduced to 4 (E = −0.76 V) under the reaction potential.

4. CONCLUSIONS Two-electron reduction of the N-heterocyclic carbene-pyridine (NHC-pyridine) containing Mn complex is computationally investigated under inert and CO2 gas environments. We find Nheterocyclic carbene-containing Mn complex (NHC-pyridine) shows strong selectivity for CO2 reduction over proton reduction. Our reaction mechanism study shows that the reduction reaction proceeds through different intermediates under inert and CO2 gas environments. Under an atmosphere of CO2, the sigma donor ligand (NHC-pyridine) binds to CO2 before reduction. As a result, the first reduction potential shifts toward cathodic and the second reduction potential shifts toward anodic. Thus, both reduction reactions happen at the same potential. Therefore, the interactions between the ligand (NHC-pyridine) and the CO2 play an important role for the two-electron reduction reaction happening at the same potential. The NHC-pyridine-based complex shows a different reduction mechanism (ECE mechanism) compared to Mn− bpy complexes where halide ion dissociation is more preferred over reduction. Thus, the remarkable selectivity of the Mn complex for CO2 reduction is credited to the presence of the NHC-pyridine ligand.

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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.jpcc.6b02209. Full Gaussian reference; calculations of reduction potentials and pKa values; comparisons between geometrical parameters of optimized and X-ray data of complex 1; comparison of calculated reduction potentials, free energy changes, and free energy barriers between without GD3 and with GD3; calculations of binding (EB) and reaction free energies (ΔG); optimized coordinates of all intermediates and transition states (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Phone: +91-731-2438-772. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank IIT Indore for the lab and computing facilities. This work was supported by DST-SERB [Grant no. EMR/2015/ 002057], Government of India, New Delhi. K.S.R. thanks UGC I


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(21) Doherty, M. D.; Grills, D. C.; Muckerman, J. T.; Polyansky, D. E.; Fujita, E. Towards More Efficient Photochemical CO2 Reduction: Use of ScCO2 or Photogenerated Hydrides. Coord. Chem. Rev. 2010, 254, 2472−2482. (22) Beley, M.; Collin, J.; Ruppert, R.; Sauvage, J.-P. Electrocatalytic Reduction of CO2 by Ni Cyclam+2 in Water: Study of the Factors Affecting the Efficiency and the Selectivity of the Process. J. Am. Chem. Soc. 1986, 108, 7461−7467. (23) Fujita, E.; Haff, J.; Sanzenbacher, R.; Elias, H. High Electrocatalytic Activity of RRSS-[NiIIHTIM](C104)2 and [NIIDMC](C104) 2 for Carbon Dioxide Reduction (HTIM = 2,3,9,l0Tetramethyl-l,4,8,11-tetraazacyclotetradecane; DMC = C-meso5,12Dimethyll,4,8,11-tetraazacyclotetradecane). Inorg. Chem. 1994, 33, 4627−4628. (24) Clark, M. L.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C. P. Electrocatalytic CO2 Reduction by M(bpy-R)(CO)4 (M = Mo, W; R = H, tBu) Complexes: Electrochemical, Spectroscopic, and Computational Studies and Comparison with Group 7 Catalysts. Chem. Sci. 2014, 5, 1894−1900. (25) He, C.; Tian, G.; Liu, Z.; Feng, S. A Mild Hydrothermal Route to Fix Carbon Dioxide to Simple Carboxylic Acids. Org. Lett. 2010, 12, 649−651. (26) Kang, P.; Cheng, C.; Chen, Z.; Schauer, C. K.; Meyer, T. J.; Brookhart, M. Selective Electrocatalytic Reduction of CO2 to Formate by Water-Stable Iridium Dihydride Pincer Complexes. J. Am. Chem. Soc. 2012, 134, 5500−5503. (27) Portenkirchner, E.; Kianfar; Sariciftci, N. S.; Knör, G. TwoElectron Carbon Dioxide Reduction Catalyzed by Rhenium(I) Bis(imino)acenaphthene Carbonyl Complexes. ChemSusChem 2014, 7, 1347−1351. (28) Song, J.; Klein, E. L.; Neese, F.; Ye, S. The Mechanism of Homogeneous CO2 Reduction by Ni(cyclam): Product Selectivity, Concerted Proton−Electron Transfer and C−O Bond Cleavage. Inorg. Chem. 2014, 53, 7500−7507. (29) Agarwal, J.; Fujita, E.; Schaefer, H. F., III; Muckerman, J. T. Mechanisms for CO Production from CO2 Using Reduced Rhenium Tricarbonyl Catalysts. J. Am. Chem. Soc. 2012, 134, 5180−5186. (30) Agarwal, J.; Johnson, R. P.; Li, G. Reduction of CO2 on a Tricarbonyl Rhenium(I) Complex: Modeling a Catalytic Cycle. J. Phys. Chem. A 2011, 115, 2877−2881. (31) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Thermochemical Insight into the Reduction of CO to CH3OH with [Re(CO)]+ and [Mn(CO)]+ Complexes. J. Chem. Soc., Chem. Commun. 1984, 328− 330. (32) Smieja, J. M.; Kubiak, C. P. Elucidation of the Selectivity of Proton-Dependent Electrocatalytic CO2 Reduction by fac-Re(bpy)(CO)3Cl. Inorg. Chem. 2010, 49, 9283−9289. (33) Keith, J. A.; Grice, K. A.; Kubiak, C. P.; Carter, E. A. Elucidation of the Selectivity of Proton-Dependent Electrocatalytic CO2 Reduction by fac-Re(bpy)(CO)3Cl. J. Am. Chem. Soc. 2013, 135, 15823−15829. (34) Riplinger, C.; Sampson, M. D.; Ritzmann, A. M.; Kubiak, C. P.; Carter, E. A. Mechanistic Contrasts between Manganese and Rhenium Bipyridine Electrocatalysts for the Reduction of Carbon Dioxide. J. Am. Chem. Soc. 2014, 136, 16285−16298. (35) Riplinger, C.; Carter, E. A. Influence of Weak Brønsted Acids on Electrocatalytic CO2 Reduction by Manganese and Rhenium Bipyridine Catalysts. ACS Catal. 2015, 5, 900−908. (36) Smieja, J. M.; Sampson, M. D.; Grice, K. A.; Benson, E. E.; Froehlich, J. D.; Kubiak, C. P. Manganese as a Substitute for Rhenium in CO2 Reduction Catalysts: The Importance of Acids. Inorg. Chem. 2013, 52, 2484−2491. (37) Haynes, W. M. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2011. (38) Bourrez, M.; Molton, F.; Chardon-Noblat, S.; Deronzier, A. [Mn(bipyridyl)(CO)3Br]: An Abundant Metal Carbonyl Complex as Efficient Electrocatalyst for CO2 Reduction. Angew. Chem., Int. Ed. 2011, 50, 9903−9906. (39) Smieja, J. M.; Benson, E. E.; Kumar, B.; Grice, K. A.; Seu, C. S.; Miller, A. J. M.; Mayer, J. M.; Kubiak, C. P. Kinetic and Structural

for the research fellowship, and A.M. and I.C. thank MHRD for the research fellowships.

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REFERENCES

(1) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and Homogenous to Conversion of CO2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38, 89−99. (2) Roy, S. C.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Towards Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons. ACS Nano 2010, 4, 1259−1278. (3) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, F.; Hille, R.; Kenis, P. J. A.; et al. Opportunities and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation. Chem. Rev. 2013, 113, 6621−6658. (4) Aresta, M. Carbon Dioxide as Chemical Feedstock; Wiley-VCH Verlag GmbH & Co.: Weinheim, Germany, 2010; pp 9−11. (5) Lange, J. P. Methanol Synthesis: a Short Review of Technology Improvements. Catal. Today 2001, 64, 3−8. (6) Martin, O.; Perez-Ramirez, J. New and Revisited Insights into the Production of Methanol Synthesis Catalysts by CO2. Catal. Sci. Technol. 2013, 3, 3343−3352. (7) Frenzel, J.; Kiss, J.; Nair, N. N.; Meyer, B.; Marx, D. Methanol Synthesis on ZnO from Molecular Dynamics. Phys. Status Solidi B 2013, 250, 1174−1190. (8) Wiedner, E. S.; Appel, A. M. Thermochemical Insight into the Reduction of CO to CH3OH with [Re(CO)]+ and [Mn(CO)]+ Complexes. J. Am. Chem. Soc. 2014, 136, 8661−8668. (9) Franke, R.; Selent, D.; Borner, E. Applied Hydroformylation. Chem. Rev. 2012, 112, 5675−5732. (10) Suzuki, T. M.; Tanaka, M.; Morikawa, T.; Iwaki, M.; Sato, S.; Saeki, S.; Inoue, M.; Kajino, T.; Motohiro, T. Direct Assembly Synthesis of Metal Complex-semiconductor Hybrid Photocatalysts Anchored By Phosphonate for Highly Efficient CO2 Reduction. Chem. Commun. 2011, 47, 8673−8675. (11) Sekizawa, K.; Maeda, K.; Domen, K.; Koike, K.; Ishitani, O. Artificial Z-Scheme Constructed with a Supramolecular Metal Complex and Semiconductor for the Photocatalytic Reduction of CO2. J. Am. Chem. Soc. 2013, 135, 4596−4599. (12) Yamanaka, K.; Sato, S.; Iwaki, M.; Kajino, T.; Morikawa, T. Photoinduced Electron Transfer from Nitrogen-Doped Tantalum Oxide to Adsorbed Ruthenium Complex. J. Phys. Chem. C 2011, 115, 18348−18353. (13) Peterson, A. A.; Nørskov, J. K. Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3, 251−258. (14) Reske, R.; Duca, M.; Oezaslan, M.; Schouten, K. J. P.; Koper, M. T. M.; Strasser, P. Controlling Catalytic Selectivities CO2 Electroreduction on Thin Cu Metal Overlayers. J. Phys. Chem. Lett. 2013, 4, 2410−2413. (15) Lim, D. H.; Jo, H. J.; Shin, D. Y.; Wilcox, J.; Ham, H. C.; Nam, S. W. Carbon Dioxide into Hydrocarbon Fuels on Defective Graphene-Supported Cu Nanoparticles from First Principles. Nanoscale 2014, 6, 5087−5092. (16) Xiao, J.; Frauenheim, T. Activity and Synergy Effects on a Cu/ ZnO(0001) Surface Studied Using First-Principle Thermodynamics. J. Phys. Chem. Lett. 2012, 3, 2638−2642. (17) Bernstein, N. J.; Akhade, S. A.; Janik, M. Density Functional Theory Study of Carbon Dioxide Electrochemical Reduction on the Fe(100) Surface. J. Phys. Chem. Chem. Phys. 2014, 16, 13708−13717. (18) Keith, J. A.; Carter, E. A. Theoretical Insights into PyridiniumBased Photoelectrocatalytic Reduction of CO2. J. Am. Chem. Soc. 2012, 134, 7580−7583. (19) Keith, J. A.; Carter, E. A. Electrochemical Reactivities of Pyridinium in Solution: Consequences for CO2 Reduction Mechanism. Chem. Sci. 2013, 4, 1490−1496. (20) Keith, J. A.; Carter, E. A. Theoretical Insights into Electrochemical CO2 Reduction Mechanisms Catalyzed by Surface-Bound Nitrogen Heterocycles. J. Phys. Chem. Lett. 2013, 4, 4058−4063. J


Article


(58) Namazian, M.; Coote, M. L. Accurate Calculation of Absolute One-Electron Redox Potentials of Some para-Quinone Derivatives in Acetonitrile. J. Phys. Chem. A 2007, 111, 7227−7232. (59) Konezny, S. J.; Doherty, M. D.; Luca, O. R.; Crabtree, R. H.; Soloveichik, G. L.; Batista, V. S. Reduction of Systematic Uncertainty in DFT Redox Potentials of Transition-Metal Complexes. J. Phys. Chem. C 2012, 116, 6349−6356. (60) Ho, J.; Coote, M. L. pKa Calculation of Some Biologically Important Carbon Acids - An Assessment of Contemporary Theoretical Procedures. J. Chem. Theory Comput. 2009, 5, 295−306. (61) Tawa, G. J.; Topol, I. A.; Burt, S. K.; Caldwell, R. A.; Rashin, A. A. Calculation of the Aqueous Solvation Free Energy of the Proton. J. Chem. Phys. 1998, 109, 4852−4863. (62) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide. J. Phys. Chem. B 2007, 111, 408−422. (63) Pratt, L. M.; Merry, S.; Nguyen, S. C.; Quan, P.; Thanh, B. T. A Computational Study of Halomethyllithium Carbenoid Mixed Aggregates with Lithium Halides and Lithium Methoxide. Tetrahedron 2006, 62, 10821−10828. (64) Glendening, F. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, Version 3.1. (65) Lyaskovskyy, V.; de Bruin, B. D. Redox Non-Innocent Ligands: Versatile New Tools to Control Catalytic Reactions. ACS Catal. 2012, 2, 270−279. (66) Franco, F.; Cometto, C.; Ferrero Vallana, F.; Sordello, F.; Priola, E.; Minero, C.; Nervi, C.; Gobetto, R. A Local Proton Source in a [Mn(bpy-R)(CO)3Br]- Type Redox Catalyst Enables CO2 Reduction Even in the Absence of Brønsted Acids. Chem. Commun. 2014, 50, 14670−14673. (67) Agarwal, J.; Shaw, T. W.; Schaefer, H. F.; Bocarsly, A. B. Design of a Catalytic Active Site for Electrochemical CO2 Reduction with Mn(I)-Tricarbonyl Species. Inorg. Chem. 2015, 54, 5285−5294. (68) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′-Bipyridine) tricarbonyl chloro rhenium(I) and Related Complexes as Homogeneous Catalysts). Helv. Chim. Acta 1986, 69, 1990−2012.

Studies, Origins of Selectivity, and Interfacial Charge Transfer in the Artificial Photosynthesis of CO. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15646−15650. (40) Wong, K.-Y.; Chung, W.-H.; Lau, C.-P. The Effect of Weak Brønsted Acids on the Electrocatalytic Reduction of Carbon Dioxide by a Rhenium Tricarbonyl Bipyridyl Complex. J. Electroanal. Chem. 1998, 453, 161−170. (41) Sampson, M. D.; Nguyen, A. D.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C. P. Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis. J. Am. Chem. Soc. 2014, 136, 5460−5471. (42) Zeng, Q.; Tory, J.; Hartl, F. Electrocatalytic Reduction of Carbon Dioxide with a Manganese(I) Tricarbonyl Complex Containing a Nonaromatic α-Diimine Ligand. Organometallics 2014, 33, 5002−5008. (43) Agarwal, J.; Shaw, T. W.; Stanton, C. J.; Majetich, G. F.; Bocarsly, A. B.; Schaefer, H. F. NHC-Containing Manganese(I) Electrocatalysts for the Two-Electron Reduction of CO2. Angew. Chem., Int. Ed. 2014, 53, 5152−5155. (44) Agarwal, J.; Stanton, C. J., III; Shaw, T. W.; Vandezande, J. E.; Majetich, G. F.; Bocarsly, A. B.; Schaefer, H. F., III Exploring the Effect of Axial Ligand Substitution (X = Br, NCS, CN) on the Photodecomposition and Electrochemical Activity of [MnX(N− C)(CO)3] Complexes. Dalton Trans. 2015, 44, 2122−2131. (45) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.; Meyer, T. J. One- and Two-Electron Pathways in the Electrocatalytic Reduction of CO2 by fac-Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine). J. Chem. Soc., Chem. Commun. 1985, 1414−1416. (46) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision B.01; Gaussian Inc.: Wallingford, CT, 2009. (47) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (48) Becke, A. D. Density-functional Thermochemistry. IV. A New Dynamical Correlation Functional and Implications for ExactExchange Mixing. J. Chem. Phys. 1996, 104, 1040−1046. (49) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the ColicSalvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (50) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V.R. Efficient Diffuse Function-Augmented Basis Sets for Anion Calculations. III. The 3-21+G Basis Set for First-Row Elements, Li−F. J. Comput. Chem. 1983, 4, 294−301. (51) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor. Chim. Acta 1973, 28, 213−222. (52) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (53) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (54) Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. the Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007. (55) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995−2001. (56) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies, Structures, and Electronic Properties of Molecules in Solution with the C-PCM Solvation Model. J. Comput. Chem. 2003, 24, 669−681. (57) Marenich, A. V.; Ho, J.; Coote, M.; Cramer, C. J.; Truhlar, D. G. Computational Electrochemistry: Prediction of Liquid-Phase Reduction Potentials. Phys. Chem. Chem. Phys. 2014, 16, 15068−15106. K


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