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Feb 6, 2017 - *E-mail: [email protected]. ... This paper provides unique insight into the H2 evolution mechanism of a cobalt bisdithiolate cataly...
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Identification of an Electrode-Adsorbed Intermediate in the Catalytic Hydrogen Evolution Mechanism of a Cobalt Dithiolene Complex Katherine J. Lee, Brian D. McCarthy, Eric S. Rountree, and Jillian L. Dempsey* Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, United States S Supporting Information *

ABSTRACT: Analysis of a cobalt bis(dithiolate) complex reported to mediate hydrogen evolution under electrocatalytic conditions in acetonitrile revealed that the cobalt complex transforms into an electrode-adsorbed film upon addition of acid prior to application of a potential. Subsequent application of a reducing potential to the film results in desorption of the film and regeneration of the molecular cobalt complex in solution, suggesting that the adsorbed species is an intermediate in catalytic H2 evolution. The electroanalytical techniques used to examine the pathway by which H2 is generated, as well as the methods used to probe the electrode-adsorbed species, are discussed. Tentative mechanisms for catalytic H2 evolution via an electrode-adsorbed intermediate are proposed.

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1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane),13−15 and hangman porphyrins.16,17 In this work, we extended similar electrochemical methods to gain mechanistic insight into the pathways of hydrogen evolution catalyzed by [Co(bdt)2]− in CH3CN. These studies revealed that addition of acids to a CH3CN solution of [Co(bdt)2]− results in the rapid formation of black particulates and adsorption of a redox active material onto glassy carbon electrodes. Optical analysis showed that subsequent application of a reducing potential to the film results in loss of the adsorbed species and regeneration of [Co(bdt)2]− in solution. On the basis of this intriguing result, we propose that [Co(bdt)2]− catalyzes H2 evolution via an electrode-adsorbed intermediate that remains molecular in nature and discuss possible catalytic mechanisms incorporating an electrode-adsorbed species. While examples exist for adsorbed catalytic species, the proposed mechanism is one of the only clear examples of a homogeneous species undergoing catalysis via an electrode-adsorbed intermediate and could have implications for the development of graphene-interfaced heterogeneous catalytic systems for H2 generation.18,19

INTRODUCTION Efficient catalysts for the hydrogen evolution reaction (HER) are important for the development of technologies capable of solar-to-fuel energy conversion.1,2 Molecular electrocatalysts are attractive because their electronic structure, and thus their catalytic reactivity, can be tuned via ligand modifications.3−6 Importantly, mechanistic and kinetic information about molecular electrocatalysts can be gleaned through electrochemical7 and spectroscopic methods.8 The economic viability of homogeneous HER catalysts requires that they are based on earth-abundant, first-row transition metals. Toward this goal, an array of nickel, cobalt, and iron HER catalysts have been developed.9 [Co(bdt)2]− (where bdt = 1,2-benzenedithiolate) (Scheme 1) has been reported to evolve H2 in 1:1 CH3CN/H2O and dry Scheme 1. Structure of Co(bdt)2−

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organic solvents (CH3CN or DMF) upon the addition of either p-toluenesulfonic acid or trifluoroacetic acid.3,10 On the basis of experimental observations that the catalytic wave grows from the [Co(bdt)2]−/2− reversible redox couple,10 subsequent computational studies proposed that [Co(bdt)2]− evolves H2 via initial [Co(bdt)2]−/2− reduction followed by double protonation of the dithiolene ligands to form [Co(bdtH)2], reduction of [Co(bdtH)2] to [Co(bdtH)2]−, and intramolecular proton transfer to form a cobalt hydride.4 To our knowledge, this intriguing proposed mechanism has not been experimentally probed in either aqueous or organic solvents. Recently, our group and others have utilized electrochemical methods to investigate the elementary steps governing HER 2+ ph ph ph catalysts, including cobaloxime,11,12 [Ni(Pph 2 N2 )2] (P2 N2 = © XXXX American Chemical Society

RESULTS AND DISCUSSION Electrochemistry of [NBu4][Co(bdt)2] in CH3CN. The cyclic voltammogram of [NBu4][Co(bdt)2] in acetonitrile shows a reversible one-electron reduction with an E1/2 of −1.24 V versus Fc+/0 (Figure 1A). As electron delocalization is common for metal dithiolenes, the [Co(bdt)2]−/2− couple likely involves population of an orbital that has combination of metal and ligand character.20−22 Addition of 0−2 equiv of anilinium (pKa = 10.6223) results in the appearance of a prewave (A) on the cathodic trace and a Received: October 24, 2016

A

DOI: 10.1021/acs.inorgchem.6b02586 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (A) Cyclic voltammogram of 2.5 mM [Co(bdt)2]− recorded in 0.25 M [NBu4][PF6] acetonitrile solution at 200 mV s−1. The E1/2 of the [Co(bdt)2]−/2− couple is −1.24 V. (B) Cyclic voltammogram of 1 mM [Co(bdt)2]− in 0.25 M [NBu4][PF6] acetonitrile at 200 mV s−1 with 0−2 equiv anilinium. (C) Peak current of the prewave A observed for a solution of 1 mM [Co(bdt)2]− plotted versus added anilinium concentration gives a linear relationship indicating that substrate diffusion governs the catalytic response.

Figure 2. (A) Cyclic voltammograms of 0.5 mM [Co(bdt)2]− in the presence of 1 equiv of 4-tert-butylanilinium, pKa = 11.1 (red trace); anilinium, pKa = 10.62 (green trace); 4-chloroanilinium, pKa = 9.7 (blue trace). Voltammograms recorded at 200 mV/s in in 0.25 M [NBu4][PF6] acetonitrile. (B) Plot of the prewave peak potential versus acid pKa gives a line with a slope of 76 mV/pKa unit. Experimental data indicated by blue dots and experimental averages by red dots along with associated standard deviation. Acid identities: (1) anilinium, (2) 4-chloroanilinium, (3) 4bromoanilinium, (4) 4-trifluoromethoxyanilinium, (5) 4-iodoanilinium, (6) 4-methylbenzoateanilinium, and (7) 4-trifluoromethylanilinium.

second total catalysis zone, KT1, where the increasing magnitude of the prewave obscures the reversible couple.7,12 Between 0 and 2 equiv of anilinium, the prewave current increases linearly with C0A, indicating that substrate diffusion governs the voltammetric response which is consistent with total catalysis (Figure 1C).7,25 Addition of 2−3 equiv of anilinium causes the reversible wave to become increasingly obscured by the prewave, as is anticipated for transition from the KT2 to the KT1 zone. However, above 2 equiv of anilinium the prewave peak current is no longer linearly related to the acid concentration. This is inconsistent with total catalysis as the catalytic wave should still be controlled by substrate diffusion (SI Figures S1 and S2). Above 3 equiv of anilinium the waveform shape becomes increasingly irreproducible. The voltammograms observed exhibit two peaks at similar potentials, resulting in a large degree of overlap between the peaks. The current ratio of the two peaks was inconsistent between different experiments with no clearly identifiable variable dictating the ratio (SI Figures S3 and S4). While we were unable to assign these peaks, we postulated that they represent competing processes. These inconsistencies precluded rigorous quantitative kinetic analysis and suggested that catalysis proceeds through parallel pathways and/or species. Tied with the added complication of multiple peaks and general waveform inconsistency at higher acid concentrations, we focused our analysis on the betterbehaved 0−2 acid equiv region to examine the pathways of reactivity in this regime.

small peak (B) on the anodic trace that precede the [Co(bdt)2]−/2− couple, as well as a small feature (D) that appears negative of the [Co(bdt)2]−/2− couple on the cathodic trace (Figure 1B). We initially suspected that prewave A corresponded to the catalytic wave. A catalytic voltammogram with a prewave preceding the reversible redox wave in the absence of acid is suggestive of total catalysis (Zone KT2).7,24,25 In this regime, the active catalyst consumes all substrate local to the electrode at potentials positive of the reversible redox couple resulting in an irreversible peaked catalytic redox feature. Scanning further cathodically reduces the remaining catalyst and, as no substrate is available, the reduced catalyst is reoxidized during the anodic scan. The presence of redox features B and D are not expected in the total catalysis regime and suggest more complex reactivity than H2 evolution alone. Despite these complications, we were intrigued by the possibility that prewave A corresponded to total catalysis and decided to focus further studies on redox feature A to better understand the reactivity. The peak potential and peak current of total catalysis waveforms depend on scan rate (υ), substrate concentration (C0A), catalyst concentration (C0P), and rate constant (kobs).12,25 As stoichiometric amounts of anilinium are titrated into solution, the current of reductive feature A increases. In addition, increasing anilinium concentration results in a loss of reversibility of the [Co(bdt)2]−/2− couple (C) (Figure 1B). This behavior is consistent with transition from KT2 to the B

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observed between individual measurements (5−18 mV). Interestingly, the magnitude of variance appeared to increase with [Co(bdt)2]− concentration (Figure 3). These large

As noted above, total catalysis can be further diagnosed by varying the scan rate.12,25 Scan rate dependence was probed using 1 equiv of anilinium (SI Figures S5 and S6). Peak A shifts to more negative potentials with increasing scan rate, eventually overlapping with the [Co(bdt)2]−/2− peak. The convergence of a catalytic prewave and the reversible redox wave of the catalyst was observed for cobaloxime.12 On the basis of this precedent, it was postulated that increasing scan rate results in a transition from zone KT2 to zone KG or KG* where slow reaction kinetics result in a peak-like catalytic wave associated with buildup of the singly reduced complex, [Co(bdt)2]2−.12 If kobs is dependent on acid strength, as was previously observed in the investigation of cobaloxime,11 then the pKa of the acid used will influence the catalytic peak location. Cyclic voltammograms of [Co(bdt)2]− were recorded in the presence of a series of para-substituted aniliniums spanning 4 pKa units: 4-methoxyanilinium (pKa = 11.8623), 4-tertbutylanilinium (pKa = 11.126), anilinium (pKa = 10.6223), 4-chloroanilinium (pKa = 9.726), 4-bromoanilinium (pKa = 9.4323), 4-trifluoromethoxyanilinium (pKa = 9.2811), 4-iodoanilinium (pKa = 9.2713), 4(methylbenzoate)anilinium (pKa = 8.6211), and 4-trifluoromethylanilinium (pKa = 8.0323) (SI Figures S7−S15). This acid series was chosen to minimize structural variance and because anilinium itself has a small homoconjugation constant of ca. 4;27 we expect structurally similar aniliniums to also have small homoconjugation constants. A 1 equiv portion of acid was used in each case to ensure that all redox features were identifiable. The potential of redox feature A was found to shift positive with decreasing pKa (Figure 2A). For acids with pKa > 10.62, the prewave overlaps with the [Co(bdt)2]−/2− couple such that peak potential could not be accurately determined. The peak that formed when using anilinium (pKa = 10.62) appears 130 mV positive of E1/2([Co(bdt)2]−/2−). For acids with pKa < 10.62, decreasing acid pKa results in the prewave shifting further positive with the prewave for the strongest acid investigated falling 320 mV positive of the E1/2([Co(bdt)2]−/2−). Plotting the peak shift of feature A versus the acid pKa shows a linear trend with a slope of 76 mV per pKa unit (Figure 2B). Previous work with a Ni(II) bisphosphine ditholate compound found that the maximum, diffusion-limited kinetic peak shift for a stepwise ECE mechanism is approximately 150 mV positive, where a kinetic peak shift occurs when the magnitude of the peak shift directly corresponds to the rate constant of the chemical step.28 The 320 mV positive peak shift observed for [Co(bdt)2]− suggests that this mechanism is not kinetically controlled and that a thermodynamic effect is operative. However, a large amount of variance in both the location (10−40 mV at 0.5 mM [Co(bdt)2]−) and broadness of peak A was observed which precluded more rigorous investigation into the possibility of a thermodynamically controlled process. Catalyst concentration (CP0) should also control peak location for total catalysis. Relationships between the peak potential and C0P have been rigorously derived by Savéant and co-workers for one-electron, one-substrate (EC) reactions24 and have recently been reported for a multielectron, multisubstrate system under limited conditions.11 To determine the impact of C0P on the prewave, voltammetric responses were recorded for 0.25−1 mM solutions of [Co(bdt)2]− using 1 equiv of 4-chloroanilinium (SI Figures S16−S20). There is a weak correlation (R2 = 0.83) between the peak shift and the logarithm of [Co(bdt)2]− concentration, with a slope of 27 mV/decade. Once again, a large variance in peak potential is

Figure 3. Plot of the prewave peak potential versus logarithm of [Co(bdt)2−] concentration. Data collected using 1 equiv of 4chloroanilinium at 200 mV s−1. Experimental data are represented by blue dots, and average and standard deviation of experimental data are shown in red.

amounts of variation prevented us from performing quantitative kinetic analysis and suggested that the prewave is actually governed by more complex events than purely total catalysis kinetics. Rinse Tests Reveal an Electrode-Adsorbed Material. We postulated that the increasing variance in peak potential with increasing [Co(bdt)2]− concentration could arise from the in situ formation of an electroactive electrode-adsorbed species. This would be expected to accelerate with higher concentration of [Co(bdt)2]− and thus explain the larger variance with higher [Co(bdt)2−] concentration. Two rinse tests were conducted in the absence of an applied potential to investigate possible formation of an electrode-adsorbed electroactive species. First, a freshly polished electrode was soaked in a 2.5 mM [Co(bdt)2]− solution containing 1 equiv of 4-chloroanilinium for 30 min, rinsed with CH3CN, and then scanned in an electrolyte-only solution. Three major features are observed, two overlapping cathodic peaks (I and J) and an anodic feature (K) (Figure 4A). Second, freshly polished electrodes were soaked in a solution of 2.5 mM [Co(bdt)2]− with 1 equiv of 4-chloroanilinium for 15 s to 20 min, rinsed with CH3CN, and scanned in a fresh acid-only solution (Figure 4B and Figure S23). Multiple redox features (E, F, G, H) are observed; the relative magnitudes and number of features evolve as the presoak time is increased (Figure 4B). The potential and current range for feature E was comparable to that observed for the prewave A in the CVs of [Co(bdt)2]− with 1 equiv of acid (Figure 6), suggesting that variation in A is caused by the growth of E over time. Peak H, which is the feature with the most negative potential, falls between −1.55 and −1.57 V after pretreatment for between 15 s and 1 min and is no longer distinguishable after 3 min. This is more positive than the reduction potential for 4-chloroanilinium which has been reported by McCarthy et al.26 as −1.86 ± 0.11 V, suggesting that none of the features correspond to proton reduction of 4-chloroanilinium. Ultimately, none of these features could be assigned clearly to independent chemical phenomena of molecular species in solution and were all attributed to the newly formed film. The formation of the film can also be visualized over time as the working electrode surface becomes increasingly discolored (SI Figure S47). Further, rinse tests demonstrated that the film was stable C

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Figure 4. (A) Cyclic voltammograms recorded in an electrolyte-only solution using a freshly polished electrode (blue trace) and an electrode treated in a solution of 2.5 mM [Co(bdt)2]− and 2.5 mM 4-chloroanilinium for 30 min (black trace). (B) Cyclic voltammograms recorded in a 2.5 mM 4chloroanilinium electrolyte solution using electrodes treated with 2.5 mM [Co(bdt)2]− and 2.5 mM 4-chloroanilinium. Pretreatment times ranged from 15 s to 10 min.

Figure 5. (A) Peak potential of E versus acid pKa of the scanning solution formed using para-substituted anilinium acids under three conditions: pretreatment with 2.5 mM [Co(bdt)2]− and 2.5 mM 4-chloroanilinium (blue), 1 mM [Co(bdt)2]− and 1 mM 4-chloroanilinium (orange), and 1 mM [Co(bdt)2]− and 1 mM 4-bromoanilinium (gray). (B) Comparison of peak potential E versus acid pKa of the scanning solution for films formed under three conditions: pretreatment with 2.5 mM [Co(bdt)2]− and 1 equiv of either 4-chloroanilinium (blue), trifluoroacetic acid (green), or HCl (purple). The acids used for the scanning solutions (2.5 mM) include (1) anilinium, (2) 4-chloroanilinium, (3) 4-bromoanilinium, (4) 4trifluoromethoxyanilinium, (5) 4-methylbenzoateanilinium. Rinse tests with TFA and HCl were not performed with 2 or 4. Acid concentration for rinse test was identical to that used to form the film.

manner comparable to that observed for peak A (Figures 2B and 3). Rinse tests were repeated with a series of nonanilinium acids to determine whether film formation is specific to aniliniums. Working electrodes were pretreated with solutions containing either trifluoroacetic acid (pKa = 12.65),27 toluenesulfonic acid (pKa = 8.6),29 or trichloroacetic acid (pKa = 10.56).27 Rinse tests were then performed with the acid used to form the film in the scanning solution. Voltammetric responses could be observed in all rinse tests (SI Figures S29−31), confirming that film formation is not specific to anilinium acids. Next, rinse tests were used to probe whether the acid used during pretreatment influences the redox properties of the film. Electrodes were pretreated using either trifluoroacetic acid, HCl, or 4-bromoanilinium, and rinse tests were performed using a series of anilinium acids in the scanning solution (SI Figures S25, S26, and S28). In all cases, the observed voltammetric responses had the same general shape as was observed in the rinse tests with 4-chloroanilinium. In addition, the same general trend of the potential of peak E becoming more positive with decreasing acid pKa was observed (Figure 5). However, the actual peak potentials of E were notably different when the film was formed with TFA or HCl (Figure 5B) while the peak potentials were relatively similar bewteen the films formed with 4-chloroanilinium (pKa = 9.726) or 4bromoanilinium (pKa = 9.4323) (Figure 5A). This suggests that the redox properties, and likely the identity, of the film are acid-

under acidic (SI Figure S32) and basic conditions (SI Figures S33 and S34) but unstable when a reducing potential is applied in pure electrolyte (SI Figures S35 and S36). Crucially, no voltammetric response was observed if the electrode was soaked in 2.5 mM [Co(bdt)2]− without acid for 20 min, rinsed, and scanned in a 2.5 mM 4-chloroanilinium solution (SI Figure S22). This is similar to the result recently reported by Lehnert and co-workers which found low catalyst loading and activity for [Co(bdt)2]− on bulk graphite electrodes after soaking for 12 h in a 1 mM solution of [Co(bdt)2]− (experiments with [Co(bdt)2]− on glassy carbon electrodes were not explored by Lehnert).19 In addition, control experiments demonstrated that anilinium does not adsorb to the electrode under these conditions (SI Figure S21). These rinse tests clearly show that the treatment of [Co(bdt)2]− with acid results in deposition of an electrodeabsorbed redox active material. To test whether this film gave similar responses as the pKa and concentration dependence experiments performed for [Co(bdt)2]− with 1 equiv acid discussed above (Figures 2A,B and 3), electrodes presoaked with [Co(bdt)2]− and 1 equiv of 4-chloroanilinium were used to obtain CVs of solutions containing anilinium acids with a range of pKa values and acid concentrations. These tests confirmed that the peak location of redox feature E becomes more positive with decreasing acid pKa and increasing acid concentration (Figure 5A and SI Figures S24 and S27), in a D

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Figure 6. (A) Rinse test current (blue) and potential (red) of peak E recorded as a function of pretreatment time. Dashed lines indicate the current and potential of prewave A for the 2.5 mM [Co(bdt)2]− and 2.5 mM 4-chloroanilinium solution used to pretreat the electrodes. Cyclic voltammograms of pretreatment solutions obtained with an electrode that was submerged in the solution for less than 90 s. (B) Cyclic voltammogram of 2.5 mM [Co(bdt)2]− and 2.5 mM 4-chloroanilinium pretreatment solution (black trace) overlaid with cyclic voltammogram of 2.5 mM 4-chloroanilinium solution obtained with an electrode that had been pretreated 10 min (blue trace).

dependent. Two possible explanations for the similarities between the 4-chloroanilinium and 4-bromoanilinium rinse test data are that these acids are structurally similar and have similar pKa values. When the presoak time used when pretreating electrodes was varied, the current of redox feature E in 2.5 mM 4chloroanilinium was found to increase in a nonlinear manner over time, and the peak potential of E was found to vary (Figure 6A). For example, a variation of 25 mV was observed for peak E when electrodes were soaked for 0−3 min. Despite this inconsistency of peak potential with soak time, the average peak potential of prewave A observed in the CV of 2.5 mM [Co(bdt)2]− with 1 equiv of 4-chloroanilinium (Epc = −0.995 V) fell within the rinse test peak potential range (Epc = −0.994 V to −1.020 V) (Figure 6A). Taken together, the variance in prewave current and peak potential, even on the shortest utilized time scales, explains the larger variance seen in the CVs recorded in solutions of [Co(bdt)2]− and 1 equiv of acid with freshly polished electrodest (Figures 2B and 3). Rinse tests clearly indicate the in situ formation of an electroactive electrode-adsorbed species upon soaking a glassy carbon electrode in a solution of [Co(bdt)2]− solutions with 1 equiv of acid. The voltammetric response of a treated electrode in acidic solution contains peak E that grows in with increasing soak time. Though the peak location of E varies, the peak location range of E encompasses the average potential of prewave A observed in the CVs of [Co(bdt)2]− with 1 equiv of acid. This suggests that the variance in the location and broadness of redox feature A can be at least partially attributed to the rapid formation of the redox active film. Optically Monitoring the Reaction of [Co(bdt)2]− with Acid. UV−vis absorbance spectroscopy was used to gain insight into electrode film formation. In the absence of acid, [Co(bdt)2]− absorbs strongly in the visible and near UV with prominent features at 360 and 656 nm. Addition of pcyanoanilinium (pKa = 730) to a 0.25 M solution of [Co(bdt)2]− results in the formation of black particles and the loss of solution absorbance. The magnitude of the solution bleaching after 30 min is proportional to acid concentration; complete loss of UV−vis signal was observed upon addition of >1 equiv of p-cyanoanilinium under atmospheric conditions (Figure 7). Interestingly, when the same experiment is performed in a nitrogen-filled glovebox, UV−partial bleaching of the [Co(bdt)2]− UV−vis signal is observed, and black particles form, but complete conversion is not observed. Though the rate of degradation is slower with weaker acids, degradation is still observed on the shorter time scales relevant

Figure 7. UV−vis absorbance spectra of 0.25 mM [Co(bdt)2]− in acetonitrile 30 min after the addition of p-cyanoanilinium under atmospheric conditions. Solutions were filtered prior to obtaining spectra to reduce scattering arising from the formation of black particles.

to electrochemical experiments (SI Figures S37 and S38). The rate of particulate precipitation also increases with increasing acid concentration, as demonstrated by trifluoroacetic acid. A slow loss of [Co(bdt)2]− signal can be observed via UV−vis upon addition of 1 equiv of TFA (SI Figure S39). In contrast, addition of 100 equiv results in nearly complete solution bleaching after 30 min and the instantaneous precipitation of black particulates (SI Figure S40). Recent work by Marinescu and co-workers has reported the adsorption of cobalt dithiolene polymers to electrode surfaces to form metal−organic surfaces capable of electrocatalytic hydrogen production.18,31 In these examples, the cobalt dithiolene polymer is formed in situ as a black particulate material. Adsorption is then accomplished by immersing the electrode in the reaction mixture. On the basis of this precedent, it was postulated that the black particles observed in our experiments were the source of the film. To test this hypothesis, the black particles were formed via a reaction of 2.5 mM [Co(bdt)2]− with 30 equiv of pcyanoanilinium, isolated, and resuspended in acetonitrile. A glassy carbon working electrode was submerged in the black particle solution for either 30 or 60 min, rinsed, and used to record a CV in a 1 mM 4-chloroanilinium solution. After the electrode soaked for 60 min, the CV of the scanning solution showed a small peak in the range expected for E suggesting a small amount of film formed when the electrode was submerged in the black particle solution (SI Figure S41). Addition of [Co(bdt)2]− or 4-chloroanilinium to the black E

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Figure 8. (A) Normalized UV−vis absorbance spectrum of electrolyte solution after application of reducing potentials to a 10 cm × 20 cm × 2 cm glassy carbon plate that had been soaked in a solution of [Co(bdt)2]− and p-cyanoanilinium for 1 week (blue) overlaid with spectrum of 0.25 mM solution of [Co(bdt)2]− in CH3CN (red). (B) Ten sequential linear voltammograms of an electrolyte-only solution were taken with the treated glassy carbon plate. Shown are linear voltammogram 1 (blue), 2 (red), and 10 (green). Voltammograms are not referenced to Fc+/0 as internal standards would interfere with subsequent UV−vis studies. (C) UV−vis absorbance spectrum of a solution of benzophenone radical anion and black particles in CH3CN shows regeneration of [Co(bdt)2]− upon chemical reduction of black particles, formed with either 5 mM [Co(bdt)2]− and 2 equiv of p-cyanoanilinium (blue) or 0.5 mM [Co(bdt)2]− and 100 equiv of TFA.

features in subsequent linear voltammograms taken in the same electrolyte solution are likely associated with [Co(bdt)2]− in solution, where the decrease in current is the result of [Co(bdt)2]− diffusing away from the electrode. If the linear voltammogram of a treated glassy carbon plate is taken in a 2.5 mM p-cyanoanilinium solution, three features can be observed: a broad peak at −0.6174 V versus Ag/Ag+ and two overlapping peaks at −0.9068 and −0.9779 V versus Ag/Ag+ (SI Figure S46). Insoluble black particles are visible in the acidonly solution, indicating that [Co(bdt)2]− had formed in solution and immediately reacted to form black particulates. If the plate is then rinsed with CH3CN and a linear voltammogram is taken in a fresh electrolyte solution without acid, no features are observed. This indicates that complete desorption of the film is observed in acid-only solution. Though the black particles would be expected to readsorb to the electrodes once formed in solution, it is likely that they were not formed in a high enough concentration or close enough to the electrode to rapidly rebind, explaining why no feature is seen in the second electrolyte-only voltammogram. Because the electrolyte-only and acid solution used to take the first linear voltammograms of the treated glassy carbon plate did not contain an internal reference (which would interfere with subsequent UV−vis measurements), it is not possible to directly compare the observed redox features. In summary, addition of acid to a solution of [Co(bdt)2]− results in the formation of insoluble black particles and the loss of [Co(bdt)2]− in solution (Figure 7). When this reaction is run in acetonitrile in an inert atmosphere, complete consumption of [Co(bdt)2]− starting material is not observed, suggesting that this reaction is an equilibrium under inert conditions. Electrochemical rinse tests show that a redox active film forms on the electrode surface when a glassy carbon electrode is submerged in a solution of [Co(bdt)2]− with 1 equiv of acid without application of a potential. Further rinse tests confirmed that a redox active film also forms when the black particulates adsorb onto the electrode and that the presence of [Co(bdt)2]− or acid in solution increases the rate of adsorption (SI S41−43). Application of a reducing potential to the electrode-adsorbed film in the presence and absence of acid results in the desorption of the film and regeneration of homogeneous [Co(bdt)2]− (Figure 8A), which rapidly reacts in the presence of acid to form black particulates. Chemical reduction of the black particulates also results in regeneration of

particulate solution was found to accelerate the rate of adsorption (SI Figures S42 and S43), indicating that adsorption of the film in situ is more favorable than formation of the film from a solution of only the black particles. Reduction of Film and Black Particles Regenerates of [Co(bdt)2]−. To further probe the nature of the film, a 10 cm × 20 cm × 2 cm glassy carbon plate was submerged in a solution of [Co(bdt)2]− and p-cyanoanilinium for 1 week, rinsed, and then subjected to reducing conditions in the form of 10 linear voltammograms in a pure electrolyte solution (SI Figure 44). The UV−vis absorbance spectrum of the electrolyte after application of reducing potentials contained peaks located at nearly identical positions as the spectrum for [Co(bdt)2]−, suggesting that [Co(bdt)2]− was regenerated in its molecular form (Figure 8A). The relative absorbances of the UV features (366 nm) were disproportionately larger than those observed in the spectrum of pure [Co(bdt)2]−, possibly from contamination of residual conjugate base, which strongly absorbs below 330 nm. Chemical reduction of the isolated black particles, formed with either anilinium or nonanilinium acids, by benzophenone radical anion (Eo′ = −2.2 V vs Fc+/0)32 also results in the regeneration of [Co(bdt)2]−, as indicated by the nearly identical peak positions in the UV−vis absorbance spectrum of the resulting solution (Figure 8C). Again, the relative absorbance of the 366 nm peak was larger than observed in the spectrum of pure [Co(bdt)2]− which can be attributed to the presence of benzophenone which absorbs weakly between 380 and 310 nm and strongly below 310 nm. This provides further evidence that the black particles are the source of the electrode-adsorbed species. The linear voltammograms of the presoaked plate also supported stripping of an electrode-adsorbed material upon reduction to re-form a homogeneous molecular species. The first linear voltammogram contained a large, broad peak across the potential range scanned (ca. −0.3 to −1.1 V vs Ag/Ag+). The second linear voltammogram showed an immediate drop in current intensity and a sharpening of the peak, reminiscent of a pure electron transfer process (Figure 8B and SI Figure S44). Subsequent linear voltammograms had the same peak shape but decreased current. If the second linear voltammogram was taken in a fresh solution of pure electrolyte, no current was passed (SI Figure S45). Taken together, this suggested that the redox feature in the first scan is associated with stripping of the electrode-adsorbed material to regenerate [Co(bdt)2]−. Redox F

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Figure 9. High resolution XPS spectra of the (A) Co 2p region and (B) S 2p region for dropcast [Co(bdt)]− (blue trace), film formed with 1 equiv of p-cyanoanilinium (green trace), film formed with 2 equiv p-cyanoanilinium (black trace), and dropcast black particles (red trace).

the homogeneous [Co(bdt)2]− (Figure 8C). This indicates that the heterogeneous film and the black particles are composed of the same molecular cobalt species and that, under reducing conditions, the film and insoluble particles regenerate molecular [Co(bdt)2]−. Investigating the Structure of the Black Precipitate and Film. The insolubility of the particulates and films in a diverse array of solvents as well as the noncrystalline nature of the particulates precluded rigorous characterization of the intermediate. The observed formation of both the black particulates and electrode films upon the addition of acid suggests that the transformation of the molecular cobalt species first involves protonation. The increased rate of precipitation as acid strength is increased (Figure 7 and SI Figure S38) further supports this conclusion. We also considered the possibility that precipitation is promoted by exchange of the [NBu4]+ cation of [NBu4][Co(bdt)2] with cationic anilinium acids in solution. However, precipitation and film formation is observed with neutral acids such as HCl and TFA as well as cationic acids (SI Figures S25, S26, and S29). To further investigate whether protonation is necessary for particulate formation when using cationic acids, as opposed to solubility changes with cation exchange, N,N,N-trimethylbenzenaminium tetrafluoroborate ([An-(CH3)3][BF4]), the N-methylated analogue of anilinium, was synthesized. One would anticipate that [An-(CH3)3][BF4] would form the black particulates and film if precipitation was the result of simply cation exchange. However, no precipitation or bleaching of the [Co(bdt)2]− signal was observed via UV− vis upon addition of up to 25 equiv of [An-(CH3)3][BF4] (SI Figures S51 and S52), and rinse tests using electrodes pretreated in [Co(bdt)2]− and 1 equiv [An-(CH3)3][BF4] showed only a small increase in current (SI Figure 49), which control experiments demonstrate can be partially attributed to the deposition of [An-(CH3)3][BF4] (SI Figure S48). These rinse tests suggest that a small amount of electrode deposition is possible. However, this current increase is much smaller than the current observed when pretreating with acids, and no welldefined peaks are visible, indicating cation exchange is not the dominant mechanism associated with film/particulate formation and supporting protonation as the primary mechanism leading to the observed solubility changes. To further investigate the composition of the film and insoluble material formed, X-ray photoelectron spectroscopy was used to analyze the materials. In order to investigate the film, substrates were prepared by soaking a glassy carbon plate in a solution of [Co(bdt)2]− and 1 or 2 equiv of pcyanoanilinium for 1 week and then rinsing with acetonitrile. To analyze the particles, black particles from the reaction of

[Co(bdt)]− and 2 equiv of p-cyanoanilinium were isolated and dropcast onto a glassy carbon plate. XPS analysis showed that all three samples contained cobalt, sulfur, and nitrogen (SI Figures S58−S69 and Table S1). The peaks in the Co 2p, S 2p, and N 1s regions overlap for all three samples, providing further evidence that the film and black particles are made of the same molecular species. These samples were also compared to the XPS of dropcast [NBu4][Co(bdt)2] (SI Figures S54−S57). For all samples, the Co 2p region contains only one doublet indicating that only one type of molecular cobalt species is present (Figure 8A and SI Figures S55, S59, S63, and S67). The S 2p region of dropcast [NBu4][Co(bdt)2] (SI Figure S56) contains a single doublet indicating that only one type of sulfur is present, as expected given the symmetry of the molecule. By contrast, the S 2p region of the film and particle samples contains a broad doublet with a slightly higher binding energy than that seen for [Co(bdt)2]− (Figure 9B and SI Figures S60, S64, and S68). Further analysis of the S 2p region of the film and particle samples shows that the peak is composed of multiple overlapping doublets. This indicates a break in the symmetry of the molecule that suggests that one or more of the sulfur sites has been protonated. Unfortunately, the N 1s region of the XPS spectra was too low in intensity to conclusively determine whether p-cyanoanilinium had also been incorporated in the structure (SI Figures S57, S61, S65, and S69). Though these experiments provide some indirect insight into the nature of the insoluble materials formed upon acidification of solutions containing [NBu4][Co(bdt)2], the structure and identity of this putative reaction intermediate is still not known. For instance, it is unclear whether the identity of this intermediate is acid-dependent, though this is suggested by rinse tests described above. Further, protonation could promote formation of an extended molecular structure involving intermolecular interactions. This kind of behavior has been observed with cobalt-bis(diartyldithiolene) complexes which form Co−S bridged dimers.33 However, the formation of the intermediate upon protonation and the regeneration of the complex upon reduction allows us to postulate possible mechanisms, even without precise information about the structure of the reaction intermediate involved in H 2 production. Mechanistic Considerations. Eisenberg and co-workers reported that addition of trifluoroacetic acid or p-toluenesulfonic acid to a 1:1 CH3CN/H2O solution of [Co(bdt)2]− triggers the appearance of a catalytic wave that grows from the [Co(bdt)2]−/2− reversible couple at −1.01 V versus Fc+/0. Controlled potential coulometry experiments at −1.01 V confirmed that the current enhancement corresponds to G

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Inorganic Chemistry hydrogen production, demonstrating that [Co(bdt)2]− is an active electrocatalyst in aqueous media. Cyclic voltammograms in dry organic solvents, CH3CN or DMF, showed similar behavior, though smaller current enhancements were observed. In addition, [Co(bdt)2]− was reported to be an active catalyst for visible light driven hydrogen evolution when paired with a photosensitizer.10 Through experiments reported in this work, we found that addition of acids to a solution of [Co(bdt)2]− in CH3CN results in the formation of an electrode-adsorbed film. The regeneration of [Co(bdt)2]−, a known hydrogen evolution catalyst in acetonitrile, upon application of reducing potentials suggests that the electrode-adsorbed species, upon reduction, generates hydrogen. On the basis of these experiments, several catalytic mechanisms can be postulated. The formation of the electrode-adsorbed material upon acid addition suggests that the first step involves protonation of [Co(bdt)2]− to yield either a singly or doubly protonated species, [Co(bdt)2−n(bdtH)n]n−1 where n = 1 or 2, respectively. It should be emphasized that, though written as complete protonation, it is possible that the molecular acid is coordinated to the cobalt complex. Addition of acid also results in the precipitation of insoluble black particles, which may form in different sizes. UV−vis absorbance measurements have shown that this first protonation step is under equilibrium. Rinse tests performed with a glassy carbon electrode that had been submerged in a solution of black particulates showed that these particles can adsorb to the glassy carbon electrode. Adsorption of the particulates is accelerated by the presence of [Co(bdt)2]− or acid in solution, suggesting in situ adsorption is more favorable and that the particles may nucleate directly on the surface when the electrode is submerged in a solution of [Co(bdt)2]− with acid. Application of a potential to the electrode-adsorbed [Co(bdt)2−n(bdt-H)n]n−1 or chemical reduction of the [Co(bdt)2−n(bdt-H)n]n−1 particles in the absence of acid reduces the protonated intermediate to regenerate [Co(bdt)2]−. This evidence is highly suggestive that hydrogen is forming; however, we were not able to detect hydrogen via 1H NMR upon addition of benzophenone radical to a solution of black particles in CD3CN. This is not unexpected as the absence of acid meant that the system was not under catalytic conditions, and thus, the amount of H2 produced may have been too low to be detected. After formation of the protonated intermediate, two pathways for the H2 forming step in the absence of acid are possible, depending on whether [Co(bdt)2]− is initially singly or doubly protonated. If the doubly protonated species [Co(bdt-H)2]+ is formed, a monometallic pathway could occur where [Co(bdt-H)2]+ could be doubly reduced upon application of a potential to generate H2 via an intramolecular pathway (Scheme 2). If the singly protonated [Co(bdt)(bdtH)] species is formed, then a bimetallic pathway can be imagined in which two electrode-adsorbed species are reduced upon application of a potential and subsequently react

homolytically to release H2 and re-form 2 equiv of [Co(bdt)2]− (Scheme 2B). As noted above, it is also possible that intermolecular interactions between the protonated intermediate occur. Though this will impact the stoichiometry of the intermediate, H2 formation can still proceed via a monometallic or bimetallic pathway. It should be noted that when acid is present in solution, additional pathways may occur. For instance, it is possible that, upon application of a potential, the reduced electrode-adsorbed [Co(bdt)2−n(bdt-H)n]n−1 intermediate could react with excess acid in solution to generate H2. Computational studies have investigated the electrochemical reaction pathway for H2 evolution by [Co(bdt)2]− and its derivatives.4 These studies suggested that protonation of [Co(bdt)2]− was unlikely and that the relative pKa for protonation of the ligand in [Co(bdt)2]− was lower than for the reduced species. Instead, the first step in the mechanism for H2 evolution was assumed to involve reduction to [Co(bdt)2]2−. Experimental and computational studies investigating the H2 evolution mechanism of analogous cobalt-diaryldithiolene complexes in DMF have also reported that the first step involves reduction of the active catalyst.33 Our experimental work indicates that the unreduced species is sufficiently reactive in the presence of acid that protonation can indeed be the first step in the catalytic cycle. The discrepancy between experiment and theory could be explained by an equilibrium protonation step followed by irreversible precipitation. However, this does not preclude the mechanism postulated in the computational mechanistic study for [Co(bdt)2]−. Mechanistic calculations for [Co(bdt)2]− were performed in 1:1 CH3CN/H2O while this experimental work was performed in CH3CN. The presence of water often has a drastic impact on the electrochemistry of a complex, including influencing the mechanism of catalysis,7,13 and it is currently unclear how water would affect the rate and equilibrium of protonation of [Co(bdt)2]−. An additional consideration is acid strength. The mechanism of H2 evolution has been shown to be dependent on acid strength for other H2 evolution catalysts.14 The utilization of sufficiently weak acids could disfavor protonation of the unreduced form resulting in the dominance of the reduction initiated mechanism. Using stronger acids could then result in the pathways operating in parallel or result in the dominance of the protonation initiated pathway. Though the mechanistic hypotheses postulated in this work are intriguing, the adsorption and desorption of the redox active electrode-adsorbed species [Co(bdt)2−n(bdt-H)n]n−1 leads to complex electrochemistry which precludes clear mechanistic analysis via the methods typically utilized in analysis of homogeneous electrocatalysis, namely, scan rate, catalyst concentration, and acid concentration dependence studies. An additional complication is whether solution-based chemistry is simultaneously occurring, and, if it is occurring, whether these homogeneous processes corresponds to electrocatalytic H2 production. Our experiments have primarily probed the preformed film and its reactivity upon initial reduction. Once regenerated, does [Co(bdt)2]− immediately react to form an insoluble material, or can it react homogeneously? The broadness and variability of the prewave features observed in the cyclic voltammogram of [Co(bdt)2]− with acid suggest that two processes are occurring at similar potentials. As the rate of black particle formation was found to depend on time, acid source pKa, and acid concentration, it is possible that reactions involving electrode-

Scheme 2. Possible Mechanisms for H2 Evolution by [Co(bdt)2]− via an Electrode-Adsorbed Intermediate

H

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using a Pure Process Technology solvent system. Water for polishing was obtained from a Milli-Q system. Tetrabutylammonium hexafluorophosphate (TCI, >98%) was recrystallized from hot EtOH, washed with cold ethanol, and dried under vacuum for 8 h at 80 °C. The procedure for synthesis and recrystallization of [NBu4][Co(bdt)2] was adapted from literature methods where the only deviation involved substitution of potassium metal for sodium.35 Identity was confirmed by UV−vis absorbance spectroscopy. Absorbance measurements were taken using an Agilent Cary 60 UV−vis spectrometer under atmospheric conditions. Anilinium tetrafluoroborate,26 4methoxyanilinium tetrafluoroborate,26 4-tert-butylanilinium tetrafluoroborate,26 4-chloroanilinium tetrafluoroborate,26 4-trifluoromethoxyanilinium tetrafluoroborate,11 4-iodoanilinium tetrafluoroborate, 4(methylbenzoate)anilinium tetrafluoroborate,11 4-trifluoromethylanilinium tetrafluoroborate,11 and N,N,N-trimethylbenzenaminium tetrafluoroborate36 were prepared as reported. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer. Spectra were obtained with a monochromatic Al Kα X-ray source, and survey and high resolution scans were obtained with pass energies of 80 and 20 eV, respectively. All spectra were corrected to the C 1s peak at 284.6 eV. Electrochemical Methods. All electrochemical measurements were performed in a nitrogen-filled glovebox, using electrode leads that were fed through a custom port and connected to a Pine Instruments WaveDriver potentiostat. A three-electrode cell was used for all experiments, utilizing glassy carbon working and counter electrodes (CH Instruments, either 3 mm or 1 mm diameter) and a silver wire pseudoreference that had been immersed in a glass tubed fitted with a porous Vycor tip and filled with a 0.25 M [NBu4][PF6] acetonitrile solution. A 3 mm working electrode is used unless specifically noted. The glassy carbon electrodes were polished using a Milli-Q water slurry of 0.05 μm polishing powder (CH Instruments, containing no agglomerating agents), rinsed, and sonicated in Milli-Q water, and rinsed with acetone. Working electrodes were electrochemically pretreated with three cyclic scans between 0.5 and −2.5 V (approximately) at 200 mV s−1 in 0.25 M [NBu4][PF6] acetonitrile solution. For anilinium titrations, cyclic voltammograms were obtained using a clean, pretreated working electrode, and anilinium was sequentially titrated into the same solution of [Co(bdt)2]−. For all other experiments, cyclic voltammograms of [Co(bdt)2]− with acid were obtained using a clean, pretreated working electrode and a new solution for each scan, and the scan was taken 60 s after acid addition to ensure uniformity in degradation.

adsorbed intermediates will be in competition with purely solution-based mechanisms. Despite these challenges, the possibility of an electrode-adsorbed intermediate in the catalytic cycle for H2 evolution is an interesting hypothesis that warrants investigation.

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CONCLUSION [Co(bdt)2]−, known to mediate the electrocatalytic production of hydrogen, was investigated in CH3CN using a series of parasubstituted anilinium acids to ascertain the reaction pathway through which hydrogen is evolved. In the course of examining the reaction mechanism, large variation was seen in the observed catalytic potentials. The magnitude of variance increased with increasing [Co(bdt)2]− concentration, suggesting the rapid formation of a redox active electrode-adsorbed film. Rinse tests confirmed that a redox active species adsorbed to the electrode only in the presence of acid and that adsorption also occurs upon addition of non-anilinium acids, including neutral acids such as TFA and HCl. When an electrode coated with the adsorbed film was scanned in an acidcontaining solution, pronounced features were observed. The peak potential of this film-based redox feature was dependent on the pKa and concentration of the acid in the solution, making it a viable explanation for the observed peak variance. These studies revealed that addition of acids to a CH3CN solution of [Co(bdt)2]− results in the rapid formation of a black powder and adsorption of a redox active material onto glassy carbon electrodes, precluding rigorous mechanistic analysis using electrochemical techniques which are derived for purely homogeneous catalysts, cleanly surface-anchored catalysts, or purely heterogeneous catalysts. Applying a potential to the electrode-adsorbed film results in the loss of the adsorbed species, and optical analysis of the resulting solution indicates that the molecular species [Co(bdt)2]− has been regenerated in solution. On the basis of this interesting result, it is proposed that [Co(bdt)2]− catalyzes H2 evolution via an electrodeadsorbed intermediate which maintains its molecular nature in the film. Though the rapid redox active film formation precluded catalytic kinetic analysis, it opens up the possibility of other exciting future directions. The attachment of catalysts to electrode surfaces remains a significant challenge in the construction of water splitting devices. Recently, Lehnert and co-workers reported the electrode-adsorbed cobalt dithiolene complex [NBu4][Co(S2C6Cl2H2)2] as robust systems for dihydrogen production on graphitic electrodes.19,34 In these cases, the material was adsorbed to the electrodes in nonacidic conditions, taking up to 12 h,19,34 compared to the 15 s required for adsorption when a glassy carbon electrode is submerged in a solution of [Co(bdt)2]− and 1 equiv of 4chloroanilinium (Figure 4B). In addition, Lehnert found [Co(bdt)2]− to be only weakly absorbed to graphitic surfaces after soaking the surface in a solution containing only [Co(bdt)2]−.19 If acid addition does drastically enhance the rate of catalyst adsorption to the electrode surface, then this could provide a facile and inexpensive method to expand the number of catalysts and surfaces that can be used to form heterogeneous dihydrogen production manifolds.

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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.6b02586. Experimental procedures, additional spectroscopic data, additional electrochemical data, and electrode pictures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Katherine J. Lee: 0000-0003-0213-8030 Jillian L. Dempsey: 0000-0002-9459-4166 Notes

The authors declare no competing financial interest.

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EXPERIMENTAL SECTION

ACKNOWLEDGMENTS We thank Dr. Carrie L. Donley for assistance with XPS measurements. This work was supported by the U.S.

General Considerations. All reactions were performed using either standard Schlenk or glovebox techniques. Acetonitrile (Fisher Scientific, HPLC grade, >99.9%) was degassed with argon and dried I

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(16) Roubelakis, M. M.; Bediako, D. K.; Dogutan, D. K.; Nocera, D. G. Proton-Coupled Electron Transfer Kinetics for the Hydrogen Evolution Reaction of Hangman Porphyrins. Energy Environ. Sci. 2012, 5, 7737−7740. (17) Bediako, D. K.; Solis, B. H.; Dogutan, D. K.; Roubelakis, M. M.; Maher, A. G.; Lee, C. H.; Chambers, M. B.; Hammes-Schiffer, S.; Nocera, D. G. Role of Pendant Proton Relays and Proton-Coupled Electron Transfer on the Hydrogen Evolution Reaction by Nickel Hangman Porphyrins. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 15001− 15006. (18) Downes, C. A.; Marinescu, S. C. Efficient Electrochemical and Photoelectrochemical H2 Production from Water by a Cobalt Dithiolene One-Dimensional Metal-Organic Surface. J. Am. Chem. Soc. 2015, 137, 13740−13743. (19) Eady, S. C.; MacInnes, M. M.; Lehnert, N. A Smorgasbord of Carbon: Electrochemical Analysis of Cobalt−Bis(benzenedithiolate) Complex Adsorption and Electrocatalytic Activity on Diverse Graphitic Supports. ACS Appl. Mater. Interfaces 2016, 8, 23624− 23634. (20) Dithiolene Chemistry; Stiefel, E. I., Ed.; Progress in Inorganic Chemistry; John Wiley & Sons, Inc.: Hoboken, NJ, 2003; Vol. 52. (21) Machata, P.; Herich, P.; Lušpai, K.; Bucinsky, L.; Šoralová, S.; Breza, M.; Kozisek, J.; Rapta, P. Redox Reactions of Nickel, Copper, and Cobalt Complexes with “Noninnocent” Dithiolate Ligands: Combined in Situ Spectroelectrochemical and Theoretical Study. Organometallics 2014, 33, 4846−4859. (22) Eisenberg, R.; Gray, H. B. Noninnocence in Metal Complexes: A Dithiolene Dawn. Inorg. Chem. 2011, 50, 9741−9751. (23) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. Extension of the Self-Consistent Spectrophotometric Basicity Scale in Acetonitrile to a Full Span of 28 pKa Units: Unification of Different Basicity Scales. J. Org. Chem. 2005, 70, 1019− 1028. (24) Savéant, J.-M. Elements of Molecular and Biomolecular Electrochemistry; John Wiley & Sons, Inc.: Hoboken, NJ, 2006. (25) Savéant, J.-M.; Su, K. B. Homogeneous Redox Catalysis of Electrochemical Reaction: Part VI. Zone Diagram Representation of the Kinetic Regimes. J. Electroanal. Chem. Interfacial Electrochem. 1984, 171, 341−349. (26) McCarthy, B. D.; Martin, D. J.; Rountree, E. S.; Ullman, A. C.; Dempsey, J. L. Electrochemical Reduction of Brønsted Acids by Glassy Carbon in AcetonitrileImplications for Electrocatalytic Hydrogen Evolution. Inorg. Chem. 2014, 53, 8350−8361. (27) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic Solvents; IUPAC Chemical Data Series; Blackwell Science: Oxford, UK, 1990. (28) McCarthy, B. D.; Donley, C. L.; Dempsey, J. L. Electrode Initiated Proton-Coupled Electron Transfer to Promote Degradation of a Nickel(II) Coordination Complex. Chem. Sci. 2015, 6, 2827− 2834. (29) Kütt, A.; Leito, I.; Kaljurand, I.; Sooväli, L.; Vlasov, V. M.; Yagupolskii, L. M.; Koppel, I. A. A Comprehensive Self-Consistent Spectrophotometric Acidity Scale of Neutral Brønsted Acids in Acetonitrile. J. Org. Chem. 2006, 71, 2829−2838. (30) Appel, A. M.; Lee, S.; Franz, J. A.; DuBois, D. L.; Rakowski DuBois, M.; Twamley, B. Determination of S−H Bond Strengths in Dimolybdenum Tetrasulfide Complexes. Organometallics 2009, 28, 749−754. (31) Clough, A. J.; Yoo, J. W.; Mecklenburg, M. H.; Marinescu, S. C. Two-Dimensional Metal-Organic Surfaces for Efficient Hydrogen Evolution from Water. J. Am. Chem. Soc. 2015, 137, 118−121. (32) Connelly, N. G.; Geiger, W. E. Chemical Redox Agents for Organometallic Chemistry. Chem. Rev. 1996, 96, 877−910. (33) Letko, C. S.; Panetier, J. a; Head-Gordon, M.; Tilley, T. D. Mechanism of the Electrocatalytic Reduction of Protons with Diaryldithiolene Cobalt Complexes. J. Am. Chem. Soc. 2014, 136, 9364−9376. (34) Eady, S. C.; Peczonczyk, S. L.; Maldonado, S.; Lehnert, N. Facile Heterogenization of a Cobalt Catalyst via Graphene Adsorption:

Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0015303 and the University of North Carolina at Chapel Hill. J.L.D. acknowledges support from a Packard Fellowship for Science and Engineering and the Alfred P. Sloan Foundation. This work made use of instrumentation at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).

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