Initiation of the Electrochemical Reduction of CO2 by a Singly

Jun 22, 2017 - This shoulder feature along with the oxidative return wave was found to correspond to the cyclic voltammetry (CV) signature of the [Ru-...
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Initiation of the Electrochemical Reduction of CO2 by a Singly Reduced Ruthenium(II) Bipyridine Complex Srinivasan Ramakrishnan and Christopher E. D. Chidsey* Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: The one-electron reduction of [CpRu(bpy)NCCH3]PF6 (Cp = cyclopentadienyl; bpy = 2,2′-bipyridine), abbreviated as [Ru-S]+, where S = CH3CN, in CO2-saturated acetonitrile initiates a cascade of rapid electrochemical and chemical steps (ECEC) at an electrode potential of ca. 100 mV positive of the first reduction of the ruthenium complex. The overall two-electron process leads to the generation of a CO-bound ruthenium complex, [Ru-CO]+, and carbonate, as independently confirmed by NMR spectroscopy. Simulations of the cyclic voltammograms using DigiElch together with density functional theory based calculations reveal that the singly reduced ruthenium complex [Ru-S]0 binds CO2 at a rate of ca. 105 M−1 s−1 at almost zero driving force. Subsequent to CO2 binding, all of the steps leading up to deoxygenation are highly exergonic and rapid. A model of the potential energy profile of the CO2 approach to the Ru center in the singly reduced manifold reveals a direct correlation between the reactivity toward CO2 and the nucleophilicity at the metal center influenced by different ligand environments. Through the binding of CO2 after the first reduction, overpotentials associated with consecutive electrochemical reductions are avoided. This work therefore provides an important design principle for engineering transition-metal complexes to activate CO2 under low driving forces.

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INTRODUCTION The electrocatalytic reduction of CO2 to a liquid fuel such as formic acid or methanol is an attractive strategy to use CO2 as a feedstock for storing renewable energy in chemical bonds.1,2 The several individual electron- and proton-transfer steps involved present a significant challenge for achieving high selectivity and turnovers at low overpotentials. Transitionmetal-based complexes are excellent electrocatalyst candidates to achieve the above goals because they offer control of the catalytic reaction coordinate through the tuning of metal− ligand−substrate interactions.3−5 In the past 3 decades, several homogeneous transition-metal complexes have been shown to be active electrocatalysts for the reduction of CO2, specifically the two-electron reduction of CO2 to CO.6−13 CO production is a good first step to producing desirable liquid fuels from CO2 because it can be subsequently hydrogenated to methanol.14,15 However, strong binding of CO to transition-metal centers often inhibits catalysis.16−18 For the two-electron electrochemical reduction of CO2 (Scheme 1), selectivity is determined by the relative thermodynamic and kinetic preference for CO, H2, and formate in the catalytic cycle mediated by a generic transition-metal complex, [M-S]+, where M is a transition-metal complex and S © 2017 American Chemical Society

Scheme 1. Pathways for the Two-Electron Electrochemical Reduction of CO2 Mediated by the Metal Complex [M-S]+

is a labile solvent ligand (active site). Two successive electrochemical reductions lead to a highly reducing metal center with an open coordination site, [M]−, that can either be protonated to generate the metal hydride intermediate on the path to H219 or formate20,21 or react with CO2 directly to generate the metal carboxylate intermediate on the path to CO.22 In either case, the overpotentials are dictated by the standard potential of the second electrochemical reduction because reactivity with CO2 and subsequent catalysis is achieved only at the doubly reduced metal complex. The Received: April 21, 2017 Published: June 22, 2017 8326

DOI: 10.1021/acs.inorgchem.7b01004 Inorg. Chem. 2017, 56, 8326−8333

Article

Inorganic Chemistry electrostatic repulsion experienced by the electron during the second reduction with the first added electron makes this reduction potential highly negative, resulting in low energy efficiencies. One strategy to circumvent this constraint is potential inversion,23 wherein the second reduction is thermodynamically easier than the first because of favorable inner-sphere ligand reorganization or spin-state change after the first reduction, resulting in a net two-electron reduction. However, reorganization will slow down electron transfer24 and, if significant, can lead to side reactions. Moreover, only precious metals such as iridium(III) and rhodium(III) are known to undergo such two-electron reductions,25 and very few cases are reported for first-row metals with no general guiding principles.26−31 Another interesting strategy to avoid high overpotentials, which is the subject of the present work, is engineering the electrocatalyst to undergo an ECE′ pathway,32 where E and E′ represent electron-transfer events and C represents CO2 binding8,33,34 (Scheme 1, in red). If E′ > E and C is sufficiently facile, then the two-electron process is achieved near the first reduction potential. Additionally, E should be sufficiently negative to confer optimal nucleophilicity to react with CO2. In this work, we report the extremely facile reduction of CO2 by the complex [CpRu(bpy)S]PF6 ([Ru-S]+, where S = CH3CN, Cp = cyclopentadienyl, and bpy = 2,2′-bipyridine) involving a rapid cascade of electrochemical and chemical steps near the first reduction potential of [Ru-S]+ in an ECEC mechanism, leading all the way to a CO-bound ruthenium complex, [CpRu(bpy)(CO)]+ ([Ru-CO]+), and free carbonate (eq 1). The products as well as the electron stoichiometry of the multistep process are determined by NMR spectroscopy. Through a combination of DigiElch35 simulations of our electrochemical data and density functional theory (DFT)based calculations, we model the intriguing reactivity of [RuS] + and highlight the central role of ligand-induced nucleophilicity at the metal center in influencing CO2 binding in an ECEC cascade. We believe our work will guide optimal strategies for reducing CO2 under low driving forces.

Figure 1. Cyclic voltammograms of 5 mM solutions of [Ru-S]+ without CO2 (in black, solid), under saturated CO2 (in red, dashed), and of [Ru-CO]+ (blue, dotted) in acetonitrile at 0.1 V/s. [TBAPF6] = 0.1 M (TBA = tetra-N-butylammonium); Fc = ferrocene.

voltammogram (Figure 1, red dashed line). The peak cathodic currents in both the presence and absence of CO2 vary linearly with the square root of the scan rate (Figure S2). At scan rates lower than 200 mV/s, a shoulder on the reduction peak becomes apparent. This shoulder feature along with the oxidative return wave was found to correspond to the cyclic voltammetry (CV) signature of the [Ru-CO]+ species generated in situ as a result of the electrochemical reduction events. [Ru-CO]+ was separately synthesized and characterized by NMR spectroscopy (Figure S3), and the cyclic voltammogram of its reversible reduction is overlaid in Figure 1 (blue dotted line). These results suggest that the one-electron reduction of [Ru-S]+ leads to a rapid reaction with dissolved CO2, consuming another electron from the electrode and leading to the generation of [Ru-CO]+, which is reduced to [Ru-CO]0 at this potential. The addition of Brønsted acids such as methanol or phenol did not have any effect on the cyclic voltammograms. In order to quantify the electron stoichiometry of this process, 1 equiv of a chemical reducing agent, Cp*2Co [Cp* = bis(pentamethyl)cyclopentadienyl; E° = −1.95 V vs Fc+/0],37 was added to 0.005 mmol of [Ru-S]+ in a sealed J. Young NMR tube, and the changes were monitored by 1 H NMR spectroscopy (Figure 2). Upon reaction of [Ru-S]+ with reducing equivalents in the presence of CO2, it is evident from Figure 2b that there is a mixture of [Ru-S]+ and a new diamagnetic species with an equivalent number of peaks corresponding to bpy and Cp protons. The chemical shifts of the latter protons in the new species are shifted downfield (5.26 ppm) from those in [Ru-S]+ (4.41 ppm), indicative of a stronger electron-withdrawing substituent on ruthenium. Indeed, this new species was confirmed to be the CO-bound ruthenium(II) complex [Ru-CO]+, based on comparisons with the NMR of a separately synthesized sample of [Ru-CO]+ (Figures S3 and S4). From integration of the old and new peaks in the 1H NMR spectrum (Figure 2b), we found that half of the [Ru-S]+ equivalents reacted with 1 equiv of Cp*2Co, thereby undergoing a two-electron reduction to produce 0.5 equiv of [Ru-CO]+. No loss of ligand was detected based on calibration against an internal integration standard, p-xylene (Figure 2). Two-electron deoxygenation of CO2 to a ruthenium-bound CO complex must be accompanied by the production of a carbonate species (eq 1) presumably ion-paired with either one of the cationic ruthenium species or Cp*2Co+. The same chemical reduction experiment was repeated with 13CO2, and a

[Ru‐S]+ + 2CO2 + 2e− → [Ru‐CO]+ + CO32 − + S

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RESULTS AND DISCUSSION One-electron reduction (E1) of [Ru-S]+ in acetonitrile shows irreversible behavior in the cyclic voltammogram (Figure 1), which is ascribed to CH3CN ligand loss (vide infra). Reoxidation of the resulting five-coordinate [CpRu(bpy)]0 species (E1′) and CH3CN rebinding (C1′) occur at a potential that is ca. 700 mV more positive compared to E1. At scan rates greater than 2 V/s, E1 becomes quasi-reversible (Figure S1) and thereby provides an upper bound for the rate constant for reductive solvent dissociation (vide infra). In CO2-saturated acetonitrile (0.28 M),36 the onset of the first reduction wave is shifted ca. 100 mV positive of E1, suggesting a high driving force for subsequent chemical and/or electrochemical steps involving CO2. In addition to the early onset, the peak reduction current at slow scan rates (Figure 1) is over 2 times greater than the corresponding current in the case of pure E1, suggesting a multielectron cascade following the first reduction step. Notably, while E1 was irreversible due to rapid solvent loss, under saturated CO2 conditions, the cascade of following steps results in an immediate oxidative return wave in the cyclic 8327

DOI: 10.1021/acs.inorgchem.7b01004 Inorg. Chem. 2017, 56, 8326−8333

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Inorganic Chemistry

Figure 2. 1H NMR (500 MHz) experiment in CD3CN with (a) 0.005 mmol of [Ru-S]+ + saturated CO2 and (b) 0.005 mmol of Cp*2Co added. pxylene is an internal calibrant. Peaks corresponding to [Ru-S]+ are denoted by asterisks and those corresponding to [Ru-CO]+ by circles.

Scheme 2. Chemical and Electrochemical Steps Following Initial Electron Transfer in the Absence (Left) and Presence (Right) of Saturated CO2

small peak at 163.52 ppm was observed in the 13C NMR spectrum, which was assigned to sparingly soluble carbonate. D2O (10% by volume) was added to this NMR tube to further solubilize any carbonate species through hydrolysis. Two new peaks at 159.42 and 167.37 ppm were observed and assigned to bicarbonate and carbonate, respectively, based on control experiments with KHCO3 and K2CO3 in D2O/CD3CN mixtures (Figure S5). To estimate the rates of the chemical steps following the initial reduction, we simulated our CV data based on the reaction scheme shown in Scheme 2 using DigiElch. The diffusion coefficient of all of the chemical species was set to 10−5 cm2 s−1, and the heterogeneous standard electron-transfer rate constant ks° for all electron-transfer processes was set to 0.05 cm s−1, the value reported for ferrocene under similar conditions.38−41 The E° value for E3 (−1.78 V vs Fc+/0) was obtained from the cyclic voltammogram of [Ru-CO]+ (Figure 1, blue dotted), and that for E2 (−1.25 V vs Fc+/0) was taken from DFT calculations benchmarked to the redox potential value for E3 (Table S1). The DFT-derived E° value for E2, as long as it is sufficiently positive of E1, does not affect the fits. The cyclic voltammograms at scan rates of 100 mV/s and 8 V/s were simulated to ensure that the fits were robust. First, we fit the cyclic voltammogram of [Ru-S]+ in the absence of CO2 (Figure 1, black solid) according to Scheme 2 (left) involving CH3CN dissociation (C1′) from the Ru center upon one-electron reduction and rebinding (C2′) after oxidation of the five-coordinate Ru cation (E1′). The best fits of this simulation are shown in Figure 3 (inset) and the fitted parameters listed in Table 1. From the optimal fits to the voltammograms, the first-order rate constant for the dissociation of CH3CN (C1′) was estimated to be ca. 35 s−1, and E° for the one-electron reduction of [Ru-S]+ without ligand loss (E1)

Figure 3. DigiElch simulations of the cyclic voltammogram of 4 mM [Ru-S]+ in the presence of 10 mM CO2 at scan rates of (a) 0.1 and (b) 8 V/s. The insets show the fits in the absence of CO2.

Table 1. Fitted Parameters from the DigiElch Simulation of the Cyclic Voltammogram of [Ru-S]+ in the Absence of CO2a E1 E1′ C1′ C2′ a

quantity

fitted value

E°/V E°/V Keq kf/s−1 Keq kf/s−1

−1.90 −1.15 >103 35 1010 104

Reduction potentials are reported versus the Fc+/0 couple.

was estimated to be −1.90 V versus Fc+/0. Sensitivity analyses were performed for all simulations to quantify the effects and interdependencies of the parameters being fit (Figure S6). On the basis of the results from the previous sections and precedent from the literature,34,42 we fit the cyclic voltammogram of [Ru-S]+ in the presence of CO2 to the mechanism 8328

DOI: 10.1021/acs.inorgchem.7b01004 Inorg. Chem. 2017, 56, 8326−8333

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Inorganic Chemistry

all cases, the associative equilibria are largely complete and do not affect the simulation (Figure S9a,c). In order to better understand the reactivity of this system with CO2 under reducing conditions, we used DFT to model the reaction coordinate at the standard states of all of the reactants and products using a polarizable continuum solvent model for acetonitrile (see the Computational Details) with appropriate experimental benchmarking. In Figure 4, the free energy of the reactants, including the electrons consumed in the reactions, at the standard potential of [Ru-S]+/[Ru-S]0 (E1) is set to zero. It is evident from the results that all of the intermediates leading to the products are energetically downhill from the reactants. After the one-electron reduction of [Ru-S]+, it is only negligibly downhill to exchange CO2 for the solvent ligand in [Ru-S]0. The subsequent electrochemical reduction of the CO2-bound neutral intermediate [Ru-CO2]0 to the anionic [Ru-CO2]− intermediate is exergonic by −15.3 kcal/mol. The chemical step following this reduction leading to [Ru-CO]+ and carbonate is further downhill by −7.5 kcal/mol. When considering this value, it is important to keep in mind that the continuum solvation model used for the calculation does not capture ion pairing of carbonate with [Ru-CO]+ and/or weak hydrogen bonding by residual water in the nominally dry acetonitrile used in the experiments. The doubly reduced intermediate [Ru]− (Figure 4, green, uphill by ca. 9 kcal/mol) is completely avoided because of the exergonic reactivity of the singly reduced complex [Ru-S]0 with CO2. The formation of [Ru-CO]+ from the reactants is therefore exergonic by ca. −24 kcal/mol. The reductive release of CO from [Ru-CO]0 to regenerate [Ru-S]0 would be uphill by ca. 31 kcal/mol, yielding an overall ΔG° of 2.4 kcal/mol to yield free CO and carbonate from 2 equiv of CO2. The calculations therefore suggest that the highly exergonic nature of the reaction profile results in a stable ruthenium-bound CO intermediate [Ru-CO]+ that does not evolve free CO at this potential. Consistent with this prediction, an electrocatalytic current is observed only at an extremely negative potential of ca. −2.70 V versus Fc+/0 (Figure S10). The DFT-derived spin-density plots of [Ru-S]0 clearly indicate that the one-electron reduction is predominantly bpy-based (Figure 5). Upon binding CO2, the unpaired spin

shown in Scheme 2 (right). According to this model, the singly reduced [Ru-S]0 complex reacts with CO2, before or after losing the bound acetonitrile ligand, to form the corresponding CO2-bound intermediate [Ru-CO2]0 (C1). This intermediate undergoes a one-electron reduction (E2) to form [Ru-CO2]−. Subsequent reaction with another molecule of CO2 (C2) leads to generation of the products, [Ru-CO]+ and carbonate. E3 represents the final reduction of [Ru-CO]+ to [Ru-CO]0, as observed experimentally (Figure 1). The overall process leading up to [Ru-CO]+ can therefore be represented as E1C1E2C2. Using this model, we were able to obtain good fits to the cyclic voltammograms as the concentration of CO2 was varied (Figures 3 and S7). The fitted thermodynamic and kinetic parameters are listed in Table 2. Under CO2-saturated Table 2. Fitted Parameters from the DigiElch Simulation of the Cyclic Voltammogram of [Ru-S]+ in the Presence of Saturated CO2 C1 C2

quantity

fitted value

Keq kf/M−1 s−1 Keq kf/M−1 s−1

103 105 250 5000

conditions, the simulations fit the data less well (Figure S8). However, the reoxidation peak current corresponding to [RuCO]0 is well captured over the range of scan rates. Notably, we found that the previous fitted rate of CH3CN dissociation from [Ru-S]0 [kf(C1′) = 35 s−1] is too low to account for the rapid generation of the [Ru-CO]+ species. Performing a sensitivity analysis (Figure S9) around the fitted parameters revealed that in order to simulate the CV features shown in Figure 3, the rate constant of CO2 binding to the reduced ruthenium complex, kf(C1), has to be on the order of ca. 105 M−1 s−1. Therefore, CO2 must bind in an associative fashion to the Ru center with concomitant or subsequent CH3CN dissociation. The subsequent reaction of the doubly reduced anionic intermediate [Ru-CO2]− with another 1 equiv of CO2 to generate carbonate and [Ru-CO]+ (C2) has a rate constant of ca. 5000 M−1 s−1. The simulation predicts this to be the rate-determining step. In

Figure 4. Reaction coordinate diagram for the deoxygenation of CO2 by [Ru-S]+. The free energies of all of the reactants at the [Ru-S]+/[Ru-S]0 potential (E1) are set to zero. Level of theory: BP86/LANL2DZ(Ru)/6-31+G*(C,H,N,O)/COSMO(CH3CN). 8329

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Table 3. DFT-Calculated Standard Reduction Potentials (E°/V vs Fc+/0) for the Three Electrochemical Steps, E1, E2, and E3, Outlined in Scheme 2 E1 E2 E3 a

Figure 5. Change in the unpaired spin density of [Ru-S]0 upon reaction with CO2 (isovalue = 0.01 electrons/Å3).

density shifts significantly from being bpy-based to being localized on a Ru d orbital and on the O atoms of the bound CO2. The ∠OCO angle in [Ru-CO2]0 is calculated to be 146.8°, consistent with the description of a CO2 radical anion bound to the RuII center. To gain further insight into the surprising reactivity of [RuS]0 with CO2, we considered a related ruthenium(II) complex reported recently by the Kubiak group,9 Ru(mesbpy)(CO)2Cl2 (mesby = 6,6′- dimesityl-2,2′-bipyridine), which they have found to be an active electrocatalyst for CO2 reduction. CO2 reduction was initiated only beyond the second reduction potential of this catalyst. For our calculations, we truncated the mesbpy ligand to bpy in our calculations and started from the acetonitrile-bound complex, [RuKub-S]+ (Scheme 3). Further, we considered its simplest bisphosphine analogue, [RuPMe3-S]+, as a perturbation to the ligand field to probe the difference in reactivity. The E° values for the E1, E2, and E3 processes (Table 3) indicate that, in all three complexes considered here, the reduction potentials of E3 and E2 are more positive compared to E1, suggesting that a multielectron pathway is feasible if the intermediary chemical step, CO2 binding, is feasible after E1. We performed a relaxed-scan optimization, lowering the Ru− C distance in 0.2 Å decrements to estimate the electronic energy profile as CO2 approaches the metal center in the singly reduced manifold of these systems (Figure 6). The results reveal a stark contrast among the three cases. In the case of [RuKub]0, CO2 is not bound. The energy keeps rising as CO2 gets closer to the Ru center, whereas in the case of [Ru]0, the association reaction goes over a barrier and falls into a welldefined stationary point. This suggests that the metal center in [RuKub]0 is not nucleophilic enough to bind CO2. In fact, if an energy minimization is forced to start from a CO2-bound state, CO2 is pushed out of the coordination sphere to a Ru−C distance beyond 4 Å. This is in contrast to [Ru-CO2]0, which is a well-defined stationary point on the potential energy surface. By substituting the CO groups cis to the incoming CO2 with more electron-donating PMe3 groups, we find that there is a significant shift in the reactivity as CO2 approaches the metal center. The relaxed-scan optimization reveals that the reactivity

[Ru-S]+

[RuKub-S]+

[RuPMe3 -S]+

−1.91 −1.25 −1.72

−1.59 a −1.25

−1.99 −1.11 −1.79

[RuKub-CO2]0 not a stationary point.

Figure 6. Relaxed-scan optimization of the CO2 approach to the Ru center. The Ru−C distance was lowered in nine decrements of 0.2 Å each. The gas-phase electronic energies for each step are reported relative to the starting point.

profile of [RuPMe3-S]+ closely mimics that of [Ru-S]+ (Figure 6). The spin densities of the singly reduced species for all three systems are shown in Figure S11. For the acetonitrile-bound species, the unpaired spin density is bpy-based. However, upon the loss of acetonitrile, the spin density moves toward the metal significantly in the case of [RuKub]0 and [RuPMe3-S]0, and to a lesser extent in [Ru]0, presumably because of the presence of stronger electron-withdrawing groups in the former two cases. Despite this localization of the spin density on the metal, CO2 is predicted to bind only to [Ru]0 and [RuPMe3-S]0 (Figure 6). The lack of reactivity toward CO2 exhibited by the singly reduced complex [RuKub-S]0 can therefore be reversed through the subtle tuning of the ligand environment to induce higher nucleophilicity at the RuII center. In order to activate CO2 at the first reduction potential of the molecular mediator, as in the case of [Ru-S]+, it is important not only to have the subsequent reduction potentials positive of the first reduction but also to have sufficient nucleophilicity at the singly reduced metal center to react with CO2.

Scheme 3. Comparison of Similar Bipyridineruthenium(II) Complexes

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DOI: 10.1021/acs.inorgchem.7b01004 Inorg. Chem. 2017, 56, 8326−8333

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S1). Intermediates were optimized in the gas phase, and a subsequent harmonic analysis was performed on the stationary points to obtain thermochemical corrections to the ground-state electronic energy to yield absolute free energies at the standard state. The free energies of solvation of the reaction partners ΔsolG(reac) and ΔsolG(prod) are computed for the gas-phase minimum-energy configurations using the COSMO54 solvation model for acetonitrile. The reaction free-energy change in solution is given by eq 2.55 For reduction potentials, the reaction free energies for the redox partners were converted to Volt units followed by the subtraction of 4.988 V, which is the absolute reduction potential of the Fc+/0 couple in a polarizable continuum solvation model for acetonitrile reported.56 The Cartesian coordinates of all of the optimized intermediates are listed in the Supporting Information. Spin-density plots were generated using GaussView 5.0.9.57

CONCLUSIONS We have shown that the one-electron reduction of a bipyridineruthenium(II) complex, [Ru-S]+, triggers a twoelectron process, viz., the rapid and highly exergonic deoxygenation of CO2. In the singly reduced manifold, the added electron, initially stored in the redox-active bipyridine ligand, transfers over to generate the anion radical of CO2 bound to the RuII center. The proposed mechanism involves an E1C1E2C2 cascade of steps, all extremely facile, leading to carbonate and a CO-bound ruthenium complex, [Ru-CO]+. The products as well as the electron stoichiometry were independently confirmed by NMR spectroscopy through the chemical reduction of [Ru-S]+ with Cp*2Co in CO2-saturated CD3CN. Electrochemical simulations of the voltammograms show that the rate of CO2 binding to the singly reduced ruthenium complex is on the order of 105 M−1 s−1. The slow step of the cascade was the reaction of [Ru-CO2]− with another molecule of CO2 to produce [Ru-CO]+ and carbonate. A comparison with similar bipyridineruthenium(II) complexes highlighted the importance of sufficient nucleophilicity at the singly reduced metal complex to bind CO2 and initiate subsequent reductions at less negative potentials.

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ΔG°COSMO(MeCN) = ΔG°COSMO(MeCN) − ΔG°COSMO(MeCN) reaction products reactants

All electrochemical simulations were performed using DigiElch using a planar semiinfinite 1D diffusion model (see the Supporting Information, section S3).

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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.7b01004. NMR spectra, electrochemical data, DigiEch simulations, and DFT data (PDF)

EXPERIMENTAL DETAILS

Synthesis of [Ru-S]PF6. CpRu(CH3CN)3]PF6 (Strem Chemicals, 434.28 g/mol, 0.115 mmol) and 2,2′-bipyridine (TCI Chemicals, 156.18 g/mol, 0.115 mmol) were stirred in dry acetonitrile under N2 for 24 h in an airtight flask. The light-yellow solution turns deep red because of the formation of [Ru-S]+. The solvent was evaporated, and the crude red solid was recrystallized with a CH3CN/diethyl ether mixture (yield = 64%). The compound was characterized by NMR spectroscopy, and the chemical shifts matched literature-reported values.43 Synthesis of [Ru-CO]PF6. A 9.2 mM solution of [Ru-S]+ in CD3CN in an NMR tube was saturated with CO gas from a tank under air-free conditions. The red solution slowly turned yellow overnight to yield [Ru-CO]PF6 (Figure S3). 1H NMR (500 MHz, CD3CN): δ 8.99 (d, 2H, J = 5 Hz), 8.37 (d, 2H, J = 8.5 Hz), 8.14 (t, 2H, J = 8 Hz), 7.50 (t, 2H, J = 7 Hz), 5.25 (s, 5H). 13C{1H} (101 MHz, CD3CN): δ 198.40, 157.95, 156.83, 139.53, 126.65, 124.25, 83.80. 19F (376 MHz, CD3CN): δ −73.3 (d). 31P{1H} (162 MHz, CD3CN): δ −143.53 (septet).43 Electrochemistry. CV experiments were performed with a BioLogic SP-200 potentiostat. A glassy-carbon-disk (3-mm-diameter) electrode from BASi Inc. was used as the working electrode, a platinum wire as the counter electrode, and a Ag/AgNO3(10 mM)/ TBAPF6 (0.1 M)/MeCN electrode as the reference electrode. All potentials are reported versus the Fc+/0 couple. The CV experiments were performed in a N2 glovebox in a 5 mL airtight glass cell fitted with a Teflon seal. For [CO2]-dependent cyclic voltammograms, suitable aliquots from an airtight Schlenk flask under 1 atm of CO2 were added to the cell with a microliter syringe. For complete saturation, the CV solution was charged with CO2 at 1 atm of pressure directly from a gas tank, through a direct feed into the N2 glovebox for about 10 min. Dry acetonitrile was obtained from an Innovative Technology PS-400-7 solvent purification system and stored under N2. TBAPF6 from Alfa Aesar Co. was twice recrystallized from absolute ethanol, dried in a vacuum oven, and stored under N2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Srinivasan Ramakrishnan: 0000-0003-3204-8095 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant titled “Electrohydrogenation: Enabling Science for Renewable Fuels” from The Global Climate and Energy Project at Stanford University. S.R. thanks the Center for Molecular Analysis and Design, Stanford University, for a graduate fellowship.

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REFERENCES

(1) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and Homogeneous Approaches to Conversion of CO2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38 (1), 89−99. (2) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.; Peden, C. H. F.; Portis, A. R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek, J. N. H.; Seefeldt, L. C.; Thauer, R. K.; Waldrop, G. L. Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation. Chem. Rev. 2013, 113 (8), 6621−6658. (3) Rakowski Dubois, M.; Dubois, D. L. Development of Molecular Electrocatalysts for CO2 Reduction and H2 Production/Oxidation. Acc. Chem. Res. 2009, 42 (12), 1974−1982. (4) Cokoja, M.; Bruckmeier, C.; Rieger, B.; Herrmann, W. A.; Kühn, F. E. Transformation of Carbon Dioxide with Homogeneous Transition-Metal Catalysts: A Molecular Solution to a Global Challenge. Angew. Chem., Int. Ed. 2011, 50 (37), 8510−8537. (5) Riplinger, C.; Sampson, M. D.; Ritzmann, A. M.; Kubiak, C. P.; Carter, E. A. Mechanistic Contrasts between Manganese and Rhenium

COMPUTATIONAL DETAILS

All Kohn−Sham DFT44−46 calculations were performed using the quantum chemistry software package Gaussian 09, revision D01.47 The pure BP8648,49 functional in its unrestricted form was used with a LANL2DZ50 effective core potential basis set on the Ru atom and the 6-31+G*51−53 basis set on all other atoms. The level of theory was used based on suitable benchmarking against experimental data (Table 8331

DOI: 10.1021/acs.inorgchem.7b01004 Inorg. Chem. 2017, 56, 8326−8333

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Inorganic Chemistry Bipyridine Electrocatalysts for the Reduction of Carbon Dioxide. J. Am. Chem. Soc. 2014, 136 (46), 16285−16298. (6) Costentin, C.; Robert, M.; Savéant, J.-M.; Tatin, A. Efficient and Selective Molecular Catalyst for the CO2-to-CO Electrochemical Conversion in Water. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (22), 6882−6886. (7) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M. A Local Proton Source Enhances CO2 Electroreduction to CO by a Molecular Fe Catalyst. Science 2012, 338 (6103), 90−94. (8) Froehlich, J. D.; Kubiak, C. P. The Homogeneous Reduction of CO2 by [Ni(cyclam)]+: Increased Catalytic Rates with the Addition of a CO Scavenger. J. Am. Chem. Soc. 2015, 137 (10), 3565−3573. (9) Machan, C. W.; Sampson, M. D.; Kubiak, C. P. A Molecular Ruthenium Electrocatalyst for the Reduction of Carbon Dioxide to CO and Formate. J. Am. Chem. Soc. 2015, 137 (26), 8564−8571. (10) Costentin, C.; Robert, M.; Savéant, J. M. Current Issues in Molecular Catalysis Illustrated by Iron Porphyrins as Catalysts of the CO2-to-CO Electrochemical Conversion. Acc. Chem. Res. 2015, 48 (12), 2996−3006. (11) Bhugun, I.; Lexa, D.; Savéant, J.-M. Catalysis of the Electrochemical Reduction of Carbon Dioxide by Iron(0) Porphyrins: Synergystic Effect of Weak Brönsted Acids. J. Am. Chem. Soc. 1996, 118 (7), 1769−1776. (12) Costentin, C.; Passard, G.; Robert, M.; Savéant, J.-M. Ultraefficient Homogeneous Catalyst for the CO2-to-CO Electrochemical Conversion. Proc. Natl. Acad. Sci. U. S. A. 2014, 111 (42), 14990−14994. (13) Sampson, M. D.; Froehlich, J. D.; Smieja, J. M.; Benson, E. E.; Sharp, I. D.; Kubiak, C. P. Direct Observation of the Reduction of Carbon Dioxide by Rhenium Bipyridine Catalysts. Energy Environ. Sci. 2013, 6 (12), 3748−3755. (14) Olah, G. A. Beyond Oil and Gas: The Methanol Economy. Angew. Chem., Int. Ed. 2005, 44 (18), 2636−2639. (15) Lee, S. Methanol Synthesis Technology; CRC Press: Boca Raton, FL, 1989. (16) Peterson, A. A.; Nørskov, J. K. Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3 (2), 251−258. (17) Taguchi, S.; Aramata, A.; Enyo, M. Reduced CO2 on Polycrystalline Pd and Pt Electrodes in Neutral Solution: Electrochemical and in Situ Fourier Transform IR Studies. J. Electroanal. Chem. 1994, 372 (1−2), 161−169. (18) Akhade, S. A.; Luo, W.; Nie, X.; Bernstein, N. J.; Asthagiri, A.; Janik, M. J. Poisoning Effect of Adsorbed CO during CO 2 Electroreduction on Late Transition Metals. Phys. Chem. Chem. Phys. 2014, 16 (38), 20429−20435. (19) Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. A Synthetic Nickel Electrocatalyst with a Turnover Frequency Above 100,000 s−1 for H2 Production. Science 2011, 333 (6044), 863−866. (20) 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 (12), 5500−5503. (21) Pugh, J. R.; Bruce, M. R. M.; Sullivan, B. P.; Meyer, T. J. Formation of a Metal-Hydride Bond and the Insertion of Carbon Dioxide. Key Steps in the Electrocatalytic Reduction of Carbon Dioxide to Formate Anion. Inorg. Chem. 1991, 30 (1), 86−91. (22) Chen, Z.; Chen, C.; Weinberg, D. R.; Kang, P.; Concepcion, J. J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J. Electrocatalytic Reduction of CO2 to CO by Polypyridyl Ruthenium Complexes. Chem. Commun. 2011, 47 (47), 12607. (23) Evans, D. H. One-Electron and Two-Electron Transfers in Electrochemistry and Homogeneous Solution Reactions. Chem. Rev. 2008, 108 (7), 2113−2144. (24) Roth, J. P.; Lovell, S.; Mayer, J. M. Intrinsic Barriers for Electron and Hydrogen Atom Transfer Reactions of Biomimetic Iron Complexes. J. Am. Chem. Soc. 2000, 122 (23), 5486−5498.

(25) Chirik, P. J.; Wieghardt, K. Radical Ligands Confer Nobility on Base-Metal Catalysts. Science 2010, 327 (5967), 794−795. (26) Lee, S.; Lovelace, S. R.; Arford, D. J.; Geib, S. J.; Weber, S. G.; Cooper, N. J. Reductively Induced Dimerization of the Ligated Benzene in [Mn(η6 -C6H6) (CO)3]+: Formation of the Initial C−C Bond by Anion/Cation Addition. J. Am. Chem. Soc. 1996, 118 (17), 4190−4191. (27) Reingold, J. A.; Virkaitis, K. L.; Carpenter, G. B.; Sun, S.; Sweigart, D. A.; Czech, P. T.; Overly, K. R. Chemical and Electrochemical Reduction of Polyarene Manganese Tricarbonyl Cations: Hapticity Changes and Generation of Syn- and Anti-Facial Bimetallic η4, η6-Naphthalene Complexes. J. Am. Chem. Soc. 2005, 127 (31), 11146−11158. (28) Sampson, M. D.; Nguyen, A. D.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C. P. Correction to “Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis. J. Am. Chem. Soc. 2015, 137 (10), 3718−3718. (29) Bartlett, P. N.; Eastwick-Field, V. A Reinvestigation of the Electrochemistry of [Ni(II) (bpy)3(ClO4)2] in Acetonitrile Using Rotating Disc and Rotating Ring-Disc Electrodes. Electrochim. Acta 1993, 38 (17), 2515−2523. (30) Richert, S. A.; Tsang, P. K. S.; Sawyer, D. T. Ligand-Centered Redox Processes for Manganese, Iron and Cobalt, MnL3, FeL3, and CoL3, Complexes (L = Acetylacetonate, 8-Quinolinate, Picolinate, 2,2′-bipyridyl, 1,10-Phenanthroline) and for Their tetrakis(2,6Dichlorophenyl)porphinato complexes[M(Por)]. Inorg. Chem. 1989, 28 (12), 2471−2475. (31) Bodini, M. E.; Willis, L. A.; Riechel, T. L.; Sawyer, D. T. Electrochemical and Spectroscopic Studies of manganese(II), -(III), and -(IV) Gluconate Complexes. 1. Formulas and OxidationReduction Stoichiometry. Inorg. Chem. 1976, 15 (7), 1538−1543. (32) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001. (33) 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 (14), 7500−7507. (34) Machan, C. W.; Chabolla, S. A.; Yin, J.; Gilson, M. K.; Tezcan, F. A.; Kubiak, C. P. Supramolecular Assembly Promotes the Electrocatalytic Reduction of Carbon Dioxide by Re(I) Bipyridine Catalysts at a Lower Overpotential. J. Am. Chem. Soc. 2014, 136 (41), 14598−14607. (35) Rudolph, M. Digital Simulations on Unequally Spaced Grids: Part 2. Using the Box Method by Discretisation on a Transformed Equally Spaced Grid. J. Electroanal. Chem. 2003, 543 (1), 23−39. (36) Fujita, E.; Creutz, C.; Sutin, N.; Szalda, D. J. Carbon Dioxide Activation by Cobalt(I) Macrocycles: Factors Affecting Carbon Dioxide and Carbon Monoxide Binding. J. Am. Chem. Soc. 1991, 113 (1), 343−353. (37) Connelly, N. G.; Geiger, W. E. Chemical Redox Agents for Organometallic Chemistry. Chem. Rev. 1996, 96 (2), 877−910. (38) Diggle, J. W.; Parker, A. J. Solvation of Ions-XX. The FerroceneFerricinium Couple and Its Role in the Estimation of Free Energies of Transfer of Single Ions. Electrochim. Acta 1973, 18 (12), 975−979. (39) Kadish, K. M.; Ding, J. Q.; Malinski, T. Resistance of Nonaqueous Solvent Systems Containing Tetraalkylammonium Salts. Evaluation of Heterogeneous Electron Transfer Rate Constants for the Ferrocene/Ferrocenium Couple. Anal. Chem. 1984, 56 (12), 1741− 1744. (40) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Mann, T. F.; Thormann, W.; Zoski, C. G. A Fast Electron Transfer Rate for the Oxidation of Ferrocene in Acetonitrile or Dichloromethane at Platinum Disk Ultramicroelectrodes. Anal. Chem. 1988, 60 (18), 1878−1882. (41) Kim, D. Y.; Yang, J. C.; Kim, H. W.; Swain, G. M. Heterogeneous Electron-Transfer Rate Constants for Ferrocene and 8332

DOI: 10.1021/acs.inorgchem.7b01004 Inorg. Chem. 2017, 56, 8326−8333

Article

Inorganic Chemistry Ferrocene Carboxylic Acid at Boron-Doped Diamond Electrodes in a Room Temperature Ionic Liquid. Electrochim. Acta 2013, 94, 49−56. (42) Clark, M. L.; Rudshteyn, B.; Ge, A.; Chabolla, S. A.; MacHan, C. W.; Psciuk, B. T.; Song, J.; Canzi, G.; Lian, T.; Batista, V. S.; Kubiak, C. P. Orientation of Cyano-Substituted Bipyridine Re(I) Fac-Tricarbonyl Electrocatalysts Bound to Conducting Au Surfaces. J. Phys. Chem. C 2016, 120 (3), 1657−1665. (43) Côrte-Real, L.; Robalo, M. P.; Marques, F.; Nogueira, G.; Avecilla, F.; Silva, T. J. L.; Santos, F. C.; Tomaz, A. I.; Garcia, M. H.; Valente, A. The Key Role of Coligands in Novel Ruthenium(II)Cyclopentadienyl Bipyridine Derivatives: Ranging from Non-Cytotoxic to Highly Cytotoxic Compounds. J. Inorg. Biochem. 2015, 150, 148−159. (44) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136 (3B), B864−B871. (45) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140 (4A), A1133− A1138. (46) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (47) 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.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas; Foresman, J. B.; Ortiz, J. V; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT. 2009. (48) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38 (6), 3098−3100. (49) Perdew, J. P. Density-Functional Approximation for the Correlation Energy of the Inhomogeneous Electron Gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33 (12), 8822−8824. (50) 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 (1), 270. (51) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54 (2), 724. (52) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor. Chim. Acta 1973, 28 (3), 213−222. (53) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A. 6-31G* Basis Set for Third-Row Atoms. J. Comput. Chem. 2001, 22 (9), 976−984. (54) Klamt, A.; Schüürmann, G. COSMO: A New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and Its Gradient. J. Chem. Soc., Perkin Trans. 2 1993, No. 5, 799−805. (55) Ho, J.; Klamt, A.; Coote, M. L. Comment on the Correct Use of Continuum Solvent Models. J. Phys. Chem. A 2010, 114 (51), 13442− 13444. (56) Namazian, M.; Lin, C. Y.; Coote, M. L. Benchmark Calculations of Absolute Reduction Potential of Ferricinium/Ferrocene Couple in Nonaqueous Solutions. J. Chem. Theory Comput. 2010, 6 (9), 2721− 2725. (57) Dennington, R.; Keith, T.; Millam, J. GaussView, version 5.0.9; Semichem Inc.: Shawnee Mission, KS, 2009.

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