Bis(cyclopentadienyl)nickel(II) μ-Thiolato Complexes as

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Bis(cyclopentadienyl)nickel(II) μ‑Thiolato Complexes as Proton Reduction Electrocatalysts Yong Xin Christel Goh, Hui Min Tang, Wen Liang James Loke, and Wai Yip Fan* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore

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S Supporting Information *

ABSTRACT: Thiolato-bridged cyclopentadienylnickel dimeric complexes have been prepared and found to be efficient and robust proton reduction electrocatalysts using acetic acid as the proton source. From cyclic voltammetry studies, moderate overpotentials of around 0.6 V and ic/ ip values from 7.8 to 12.2 have been determined for 20 equiv of acetic acid at a scan rate of 100 mV/s. A turnover number of around 7 has been determined for each of the nickel complexes. The thiolato substituent of the complex does not appear to influence the catalysis significantly. Each of the nickel complexes acts as a robust homogeneous catalyst that could sustain continuous proton reduction for hours. On the basis of the experimental data, an electrochemical− chemical−electrochemical−chemical mechanism describing the catalytic process has been proposed as well.

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INTRODUCTION In an effort to reduce the dependence on fossil fuels as energy sources, much research has been carried out to search for new clean fuels.1−3 The generation of dihydrogen (H2) gas as a carbon-free fuel from water splitting has attracted much attention.1,3,4 Water splitting consists of two parts; the oxidation of water, which generates dioxygen, and the reduction of protons, which produces H2.2,5 While platinum metal is able to catalyze proton reduction readily, its high cost and rarity prevent widespread applications.6,7 A lot of effort has thus been devoted to finding inexpensive, earth-abundant catalysts to drive the proton reduction process more efficiently.5,7 In nature, it has been discovered that some iron- and nickelcontaining enzymes called [FeFe] and [NiFe] hydrogenases are able to carry out proton reduction and its reverse process, hydrogen oxidation, very efficiently at ambient temperatures.3,6 Therefore, much attention has been turned to preparing iron and nickel complexes that are stable and yet able to mimic the performance of these hydrogenases.3 Over the years, there has been a wealth of nickel-only complexes synthesized and tested as proton reduction catalysts.5,8−12 Many of these complexes are coordinated to phosphines, thiolates, and nitrogen-based ligands such as amines and pyridines.5,8−12 However, to our surprise, nickel complexes containing metal−carbon bonds have rarely been used for such a purpose. In this work, we surmise that organonickel complexes, which can support low oxidation states (0 to 2+) of the nickel center, may have the potential to act as efficient proton reduction catalysts. In this work, we have prepared thiolato-bridged cyclopentadienylnickel(II) complexes and evaluated their performance as electrocatalysts in the reduction of a weak acid, such as acetic acid (CH3COOH), acting as the proton source in acetonitrile (CH3CN). Because of the bridging thiolates, the complexes also bear some resemblance to the active site of the [NiFe] hydrogenase. Three such organonickel © XXXX American Chemical Society

complexes have been synthesized (Scheme 1), and two important parameters, the catalytic efficiency and overpotential Scheme 1. Molecular Structures of Complexes 1−3

toward proton reduction, have been evaluated using cyclic voltammetry for each of the complexes. In addition, we have also proposed a mechanism, based on the experimental data, on how these organonickel complexes are able to catalyze proton reduction.

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RESULTS AND DISCUSSION Complexes 1−3 were synthesized following a slight modification of the published method.13 Briefly, equimolar amounts of nickelocene and the respective thiol dissolved in toluene were allowed to react over a few hours at room temperature to afford each of the black complexes as the main product in high yields. The 1H NMR and electrospray ionization mass spectrometry (ESI-MS) spectra obtained for 1−3 were in excellent agreement with the literature data (see Figures S1−S6).13,14 Because the structures of these nickel complexes have only been previously inferred via spectroscopic studies, we have now managed to obtain the single-crystal X-ray structure for complex 1.13,14 The crystals were grown in a chloroform/ hexane mixed solvent and stored at low temperatures for days before being subjected to X-ray diffraction (Figure 1). Received: May 23, 2019

A

DOI: 10.1021/acs.inorgchem.9b01507 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Displacement ellipsoid (50% probability level) plot of the solid-state structure of 1. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Ni1−S1 2.1805, Ni1−S1A 2.1831, S1−C1 1.8441, Ni1−Ni1A 3.314. Selected bond angles (deg): S1−Ni1−S1A 88.189, Ni1−S1−S1A 91.814, Ni1−S1−C1 104.16, S1−C1−C2 113.44.

Complex 1 crystallizes in the orthorhombic space group Pbca, with unit cell dimensions a = 8.0419(7) Å, b = 10.5953(10) Å, c = 24.305(2) Å, and α = β = γ = 90°. There are two nickel(II) centers in the complex, with each nickel coordinated to a cyclopentadienyl ring and bridged by two thiolate groups. The Ni2S2 moiety is planar with an Ni−S−Ni angle of 91.814(13)° and an S−Ni−S angle of 88.189(13)°. The benzyl groups are oriented trans to each other, presumably because of steric factors. The four Ni−S bond distances are similar at around 2.18 Å. However, the long Ni−Ni distance of 3.134 Å indicates the absence of a metal−metal bond,15 consistent with the 18electron rule counting, which does not require such a bond to be formed. The features of the cyclic voltammograms of 1−3 in the absence of an organic acid are first described. All potential values were calibrated against the standard Fc+/Fc couple (set to 0.0 V) in an CH3CN solvent. Three molecular peaks are observed for each of the complexes (Figure 2 and Table 1). A reversible molecular peak located in the −0.2 to −0.4 V region is attributed to the Ni3+/Ni2+ couple, where one of the nickel centers of the complex undergoes oxidation to form a cationic complex. On the other hand, the reversible peak observed around the −1.5 to −1.7 V range is assigned to reduction of the complex to its anion. It is interesting that reversible peaks are observed, suggesting that the anionic nickel complex probably remains intact as a dimeric structure instead of undergoing dissociation to form some monometallic species. We also note that, for reversible redox processes, the ideal peak-to-peak width is inversely proportional to the number of electrons transferred.16 Because the widths of the two processes are very similar to that of the one-electron Fc+/Fc couple, we assigned each of them as a one-electron process as well. Further into the negative potential region, an irreversible peak was also recorded in the −2.3 to −2.7 V region for each of the complexes. We have attributed this redox process to a further one-electron reduction of the anion. The electrochemical behavior of 1 was then investigated in the presence of CH3COOH, which was used as the proton source in CH3CN. When 2 equiv of CH3COOH (relative to the catalyst) was introduced to the solution, a broad signal with a peak at −2.1 V was observed (Figure 3). The intensity of this signal continued to increase as more acid was added, which we attributed to the catalytic proton reduction wave.

Figure 2. Cyclic voltammograms recorded at a scan rate of 0.1 V/s in a CH3CN solution containing 1 mM (a) 1, (b) 2, and (c) 3 with 0.1 M NBu4PF6 as the electrolyte. The reversible peak at 0 V is due to the addition of ferrocene as the standard. The cathodic scan was first performed from −0.5 to −2.8 V. Then the scan (toward anode) was reversed from −2.8 to +0.5 V and reversed again to reach the starting point −0.5 V.

Table 1. Electrochemistry Data for 1−3 molecular peaks (V)a

catalytic Ecat/2 (V) at 20 equiv of acid overpotential (V) ic/ip (100 mV/s) (%)b TON

1

2

3

E1/2 = −0.34 E1/2 = −1.62 Epc = −2.66 −1.95 0.57 7.8 6.6

−0.41 −1.70 −2.60 −1.99 0.61 8.8 7.0

−0.28 −1.50 −2.36 −1.99 0.61 12.2 7.6

a

E1/2 = reversible peak, and Epc = irreversible reduction peak. bThe current ratio ic/ip, where ic = peak catalytic current and ip = peak current of the Ni 2+/Ni+ peak, is given for a concentration of acid of 20 mM and a concentration of the catalyst of 1 mM.

The appearance of a peak is indicative of a molecular homogeneous process, where there is competition between diffusion of the substrate to electrode and its consumption at the electrode.17 We have also ascertained through control experiments that acid reduction by the glassy carbon (GC) electrode itself only began to occur at potentials more negative than −2.4 V, consistent with the value reported in the literature.18 In addition, when the proton source was switched to a stronger acid such as trifluoroacetic acid, the nickel complexes were observed to decompose gradually. Thus, the instability of the B

DOI: 10.1021/acs.inorgchem.9b01507 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

25 equiv of acid) and 200 μA (for 100 equiv of acid) when 1 acted as the catalyst (Figure 3a). This observation may indicate that the nickel catalysts are not stable under reducing conditions at high acid concentration, hence leading to greater catalyst decomposition. We used Dempsey’s definition of the overpotential necessary for catalysis, as given by the following equation:17 overpotential necessary for catalysis = |Ecat/2 − E HA °|

where Ecat/2 is the catalytic half-wave potential and i 2.303RT zy zzpK a,HA E HA ° = E H+° − jjj F k {

Because the values for EH+° and pKa of CH3COOH have already been determined to be −0.028 V and 22.3, respectively,4,17 the thermodynamic potential EHA° can be calculated to be −1.35 V. Following the guidelines of Appel and Helm, the values of Ecat/2 were determined based on the half-wave potential, the potential at which half of the maximum current is obtained.19 The potential value offers a higher precision compared to the peak or onset potential because it results in a smaller variance upon determination of Ecat/2 for a nonideal catalytic wave.19 We standardized the calculation of the overpotentials as 20 equiv of acid. Thus, the overpotential for the catalytic wave at −1.95 V for complex 1 was determined to be 0.57 V. The ic/ip current ratio (see the definition in Table 1) was also estimated for the catalytic peak (at 20 equiv of acid) to be 7.8. Similar calculations were made for complexes 2 and 3, and the results are given in Table 1. Complexes 1−3 appear to show similar catalytic behavior albeit at slightly different catalytic efficiencies and overpotentials. It appears that whether an aliphatic or aromatic thiolato ligand was used does not influence the proton reduction process in a significant manner. In addition, the intensity of the first molecular reduction reversible peak at −1.62 V for complex 1 increased linearly with the square root of the scan rate (see Figures S7 and S8). This is consistent with reversible electron-transfer processes of freely diffusing species, as described by the Randles−Sevcik equation.16 Conversely, a linear relationship between the peak current and scan rate was expected if the analyte was adsorbed onto the electrode.16 Hence, this result suggests that the nickel complexes are acting as homogeneous catalysts. In order to test the robustness of the nickel complexes as catalysts for sustained proton reduction, constant potential electrolysis of 100 mM CH3COOH was first carried out for 1 at −2.1 V close to the peak potential of the catalytic wave. Indeed, the catalytic current (charge/time) could be maintained for several hours (see Figure S9). It was also possible to observe continuous bubbling at the tip of the electrode. These results show that complex 1 is able to sustain its performance continuously with little decomposition. In addition, the gas in the headspace was collected in the same experiment and identified to be H2 by mass spectrometric analysis. In the bulk electrolysis experiment, we were also able to calculate the turnover number (TON) from the faradaic yield and electron charge passing through the process over a few hours (see the calculations in the Supporting Information). Average TONs of 6.6, 7.0, and 7.6 have been estimated for complexes 1−3, respectively. Similar to their overpotential and catalytic efficiency values, the TONs of each of the complexes

Figure 3. Cyclic voltammogram recorded at a scan rate of 0.1 V/s in a CH3CN solution containing 1 mM (a) 1, (b) 2, and (c) 3 with 0.1 M NBu4PF6 and CH3COOH (0−100 mM). The directions of the scans were the same as those carried out in Figure 2. The blank cyclic voltammogram, which shows glassy carbon electrode-catalyzed proton reduction beyond −2.4 V, is given in Figure S13.

complexes prevented us from making reliable measurements under strong acid conditions. We also note that the first molecular reduction peak was still observed at the same potential (−1.62 V) and similar intensity in the presence of acid during the reduction cycle (going toward more negative potential). However, as more acid was added, the peak gradually became smaller during the oxidation cycle (going toward more positive potential) until it disappeared at very high acid concentrations. This observation suggests that the initial reduction of complex 1 to its anionic form was not affected by the acid. However, the anion was gradually consumed or removed via protonation in the catalytic process and gave rise to a decreasing signal at −1.62 V as the potential swept back to the positive region. Interestingly, oxidation of complex 1 was still observed at −0.34 V even under high acid concentrations, indicating that the concentration of 1 in the diffusion layer was replenished by diffusion from the bulk solution. Interestingly, we note a difference in the current between the peak and switching potentials (at −2.4 V) of about 100 μA (for C

DOI: 10.1021/acs.inorgchem.9b01507 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry did not show a significant dependence on the thiolato substituents. The UV−visible absorption spectra of complex 1 before and after 5 h of bulk electrolysis have been recorded. As observed in Figure S10, the intensity of the spectrum showed only a slight decrease after 2 h of electrolysis but remained at that constant value for the next 3 h. Importantly, no new bands appeared throughout the electrolysis period. We carried out further experiments to shed light on the mechanism of the proton reduction process. In order to examine its effect on the catalytic wave, the scan rate was varied from 10 mV/s to 1 V/s. Interestingly, curve crossing was observed when the scan rate was lowered to 40 mV/s and below (Figure 4). The appearance of curve crossing under

Scheme 2. Proposed Proton Reduction Mechanism Catalyzed by 1

for all three complexes in the presence of CH3COOH appeared to be similar, whereby a reversible one-electron molecular reduction peak was first observed before the advent of the catalytic wave at a more negative potential. The potential of the molecular reduction peak also did not shift in the presence of acid, which suggests that the nickel complex remained unprotonated at least until it underwent a oneelectron reduction (see Figure S12). Thus, it is most likely that proton reduction was initiated by a reversible electrochemical step (E). Although the reduced species was not characterized experimentally, it is, nevertheless, interesting to speculate the structure of the reduced species. From the reversibility of its redox process, we believe that a dimeric nickel structure remained intact upon reduction so that greater delocalization of the negative charge could take place. Furthermore, as described previously, complex 1 appeared to have been regenerated as the potential swept back to the positive region such that its oxidation peak was observed at almost full intensity at −0.34 V again. This observation may also suggest that the subsequent intermediates generated upon further reduction and protonation also retained dimeric structures. In all three complexes, the second molecular reduction peak observed from −2.3 to −2.7 V occurred at a more negative potential than the catalytic wave. Hence, the second step of the proton reduction catalysis could not be electrochemical as well. Instead, we propose that protonation (C) occurred at the complex so that the combination of these first two steps (EC) would afford a neutral intermediate species containing a Ni−H bond. In Scheme 2, we proposed a bridging hydride complex as a possibility for the intermediate. The next two steps are difficult to ascertain independently. The voltammetry data are unable to be distinguished between the EC or CE step because any fine features would have been masked by the broad catalytic wave. However, as discussed, our scan rate experiments (Figure 4) lend support to an ECE mechanism,20 which in this case would favor an electrochemical step as the third step. Thus, the fourth (final) step in the proton reduction process would have to involve a chemical pathway; for example, a nickel intermediate generated after the ECE steps may recombine with a free proton to generate hydrogen and reform complex 1.

Figure 4. Cyclic voltammogram recorded with varying scan rates in a CH3CN solution containing 0.1 mM complex 1 with 0.1 M NBu4PF6 and 20 mM CH3COOH. Note that the voltammograms at 40 and 50 mV/s are magnified in order to display the curve crossings clearly. The actual peak catalytic current values (μA) are 130 (40 mV/s), 150 (50 mV/s), and 210 (100 mV/s).

these conditions is usually ascribed to the generation of a relatively stable intermediate, which is easier to reduce than the parent species (complex 1).20,21 The chemical environment would have changed during the anodic scan and usually leads to a crossing between the cathodic and anodic cycles. An electrochemical−chemical−electrochemical (ECE) mechanism is associated with this process.20 Although curve crossing could result from catalyst decomposition, our UV−visible spectral monitoring results indicated only slight decomposition of the catalyst over a period of a few hours, which is far longer than the time required to carry out a cyclic voltammogram scan. A rinse test was also conducted in which the electrodes were removed after bulk electrolysis and rinsed several times with CH3CN (see Figure S11). When the electrodes were placed in a fresh solution of CH3COOH in CH3CN (without any of the catalysts), no catalytic peak was recorded at the operating potential. This observation again suggests that proton reduction is indeed homogeneous in nature rather than originating from nickel deposits on the electrodes. The reaction pathways taken for the reduction of two protons to generate a H2 molecule involve four steps in the form of either chemical (C) or electrochemical (E) reactions (Scheme 2).22,23 We note that the voltammograms recorded D

DOI: 10.1021/acs.inorgchem.9b01507 Inorg. Chem. XXXX, XXX, XXX−XXX

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black crystals (yield: 89%). ESI-MS: m/z (+)491.8 ([M]+). 1H NMR (CDCl3, 500 MHz): δ 7.30 (m, 10H, phenyl H), 4.50 (s, 10H, C5H5), 3.82 (s, 4H, CH2). Anal. Calcd: C, 58.36; H, 4.90. Found: C, 58.12; H, 4.94. 2 and 3 were synthesized in a fashion similar to that of 1 by simply changing the thiol precursor to 1-octanethiol and p-thiocresol (4methylbenzenethiol), respectively. The complexes were collected via recrystallization with hexane. Complex 2. Yield: 53% (black oily substance). ESI-MS: m/z (+)536.1 ([M]+). 1H NMR (CDCl3, 500 MHz): δ 4.14 (s, 10H, C5H5), 2.68 (t, 4H, CH2 next to S), 0.88−1.64 (m, 30H, remaining H on the octyl chain). Anal. Calcd: C, 58.03; H, 8.24. Found C, 57.82; H, 8.37. Complex 3. Yield: 83% (black solid). ESI-MS: m/z (+)491.8 ([M]+). 1H NMR (CDCl3, 500 MHz): δ 7.93 (d, 4H, phenyl H ortho to S), 7.00 (d, 4H, phenyl H meta to S), 4.53 (s, 10H, C5H5), 2.34 (s, 6H, CH3). Anal. Calcd: C, 58.36; H, 4.90. Found: C, 58.08; H, 4.93. Electrochemical Methods. (a) Electrochemistry was performed with a Princeton Applied Research potentiostat model 263A using a three-electrode system. The working electrode was a 3-mm-diameter planar GC disk, used in conjunction with a platinum counter electrode and a pseudoreference silver wire electrode submerged in a glass tube fitted with a porous glass Vycor tip containing Bu4NPF6 in CH3CN. Prior to each scan, the 0.1 M electrolyte Bu4NPF6 solution with a 1 mM catalyst in CH3CN (7 mL) was deoxygenated via continuous purging of nitrogen gas for 15 min. GC electrodes were polished with 1 μm diamond and 0.05 μm alumina slurries, rinsed with Milli-Q water, and sonicated in ultrapure water to remove the residual polishing powder. All scans were referenced to the ferrocene added prior to each measurement set. (b) For controlled potential electrolysis, a 1 mM solution of each of the nickel complexes with 100 mM CH3COOH in 0.1 M Bu4NPF6 in CH3CN was electrolyzed at the respective catalytic peak potential (vs Fc+/Fc) in a gastight electrochemical cell. The experiment was carried out three times, with the periods of electrolysis and current monitored so that the average values could be calculated. The gaseous content of the reaction vessel was removed with a syringe of 1.0 cm3 volume and injected into a mass spectrometer tuned to m/z 2, corresponding to the detection of H2. The signal intensity is calibrated with the signal produced by the injection of a known pressure of H2 gas taken from a H2 cylinder (Soxal, 99.99%). The faradaic yield was then determined by the average of three readings of the [H2 detected]/[electrons used for electrolysis] ratio.

We emphasize that the mechanism shown here is proposed based on our experimental data. There are, of course, many other possible pathways. For example, protonation could have occurred on one of the sulfur atoms of our nickel complexes. The dimeric structures of the nickel complexes would still be preserved. The sulfur protonation pathway has been proposed for thiolato-bridged [NiFe] hydrogenase mimics.24−26 In addition, the last two steps of the proton reduction process could also be combined into a single proton-coupled electrontransfer step.27 As part of our further work, we are interested in providing more insight into the mechanism. When computational calculations were carried out, the energies of the proposed nickel intermediates and activation energies of the various reactions were obtained and may lend more support to our proposed mechanism. Work is also underway to further investigate and optimize the catalytic performance of these nickel complexes by synthesizing derivatives containing nitrogen-based functional groups such as amines, imines, and pyridines on the thiol linkage.

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CONCLUSION

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EXPERIMENTAL SECTION

We prepared and characterized three thiolato-bridged cyclopentadienylnickel dimers that are able to catalyze proton reduction. The cyclic voltammetry data showed that reasonable overpotentials of 0.57−0.61 V and ic/ip values of 7.8−12.2 could be achieved in the presence of 20 equiv of CH3COOH acting as the proton source. From the average TON of 7.0 estimated for each of the nickel complexes, it appeared that the thiolato substituent does not significantly affect the catalysis. The electrochemical studies also lent support to these nickel complexes acting as recoverable and robust electrocatalysts. An electrochemical proton reduction mechanism was proposed to account for the experimental observations. We hope that our findings here may lead to more research being carried out on organonickel complexes as robust proton reduction catalysts.

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Chemicals. Nickelocene (Sigma-Aldrich), benzyl mercaptan (Sigma-Aldrich, 99%), 1-octanethiol (Sigma-Aldrich, 98.5%), pthiocresol (Alfa Aesar, 98%), tetrabutylammonium hexafluorophosphate (NBu4PF6; Tokyo Chemical Industry Co. Ltd., >98.0%), and glacial acetic acid (CH3COOH; Merck, 100%) were used as received. Toluene (VWR Chemicals, reagent grade, >99%), n-hexane (Fisher Chemical, certified ACS, >65%), chloroform (VWR Chemicals, HPLC grade, >99.5%), and acetonitrile (CH3CN; Fulltime Chemical, HPLC/Spectro grade, >99%) were dried with 4 Å molecular sieves (Alfa Aesar). All syntheses were carried out under a nitrogen environment. 1 H NMR spectra were recorded in CDCl3 using a Bruker Avance 500 (AV500) Fourier transform spectrometer operating at 500 MHz at room temperature. ESI-MS was conducted using a Finnigan MAT LCQ spectrometer. Single-crystal X-ray structural studies were performed on a Bruker-AXS Smart Apex CCD single-crystal diffractometer. Data were collected at 100(2) K using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). Preparation of [CpNi(μ-SX)]2 [X = CH2Ph (1), C8H17 (2), and p-C6H4CH3 (3)]. In the preparation of 1, benzyl mercaptan (0.3 mmol, 35.2 μL) was added to a toluene solution (8 mL) containing nickelocene Ni(C5H5)2 (0.3 mmol, 56.7 mg). The solution gradually darkened from green to dark brown upon stirring for 5 h at room temperature. Following completion of the reaction, the solvent was removed under vacuum, and the residual solids were dissolved in a minimal amount of chloroform. Complex 1 was crystallized from a saturated chloroform solution, via the slow diffusion of hexane, giving

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01507. NMR and mass spectra of 1−3, relationship between the molecular peak current and scan rate, bulk electrolysis, rinse test, potential shifts of the molecular peaks with and without acid, and crystallographic data for 1 (PDF) Accession Codes

CCDC 1908379 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wai Yip Fan: 0000-0002-9963-0218 E

DOI: 10.1021/acs.inorgchem.9b01507 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

carbon in acetonitrile-implications for electrocatalytic hydrogen evolution. Inorg. Chem. 2014, 53 (16), 8350−8361. (19) Appel, A. M.; Helm, M. L. Determining the Overpotential for a Molecular Electrocatalyst. ACS Catal. 2014, 4 (2), 630−633. (20) Fox, M. A.; Akaba, R. Curve Crossing in the Cyclic Voltammetric Oxidation of 2-Phenylnorbornene. Evidence for an ECE Reaction Pathway. J. Am. Chem. Soc. 1983, 105 (11), 3460− 3463. (21) Houmam, A.; Hamed, E. M. Application of the dissociative electron transfer theory and its extension to the case of in-cage interactions in the electrochemical reduction of arene sulfonyl chlorides. Phys. Chem. Chem. Phys. 2012, 14 (1), 113−124. (22) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2001. (23) Elgrishi, N.; McCarthy, B. D.; Rountree, E. S.; Dempsey, J. L. Reaction Pathways of Hydrogen-Evolving Electrocatalysts: Electrochemical and Spectroscopic Studies of Proton-Coupled Electron Transfer Processes. ACS Catal. 2016, 6 (6), 3644−3659. (24) Brazzolotto, D.; Wang, L.; Tang, H.; Gennari, M.; Queyriaux, N.; Philouze, C.; Demeshko, S.; Meyer, F.; Orio, M.; Artero, V.; Hall, M. B.; Duboc, C. Tuning Reactivity of Bioinspired [NiFe]Hydrogenase Models by Ligand Design and Modeling the CO Inhibition Process. ACS Catal. 2018, 8 (11), 10658−10667. (25) Tang, H.; Hall, M. B. Biomimetics of [NiFe]-Hydrogenase: Nickel- or Iron-Centered Proton Reduction Catalysis? J. Am. Chem. Soc. 2017, 139 (49), 18065−18070. (26) Tai, H.; Higuchi, Y.; Hirota, S. Comprehensive reaction mechanisms at and near the Ni-Fe active sites of [NiFe] hydrogenases. Dalton Trans. 2018, 47 (13), 4408−4423. (27) Elgrishi, N.; McCarthy, B. D.; Rountree, E. S.; Dempsey, J. L. Reaction Pathways of Hydrogen-Evolving Electrocatalysts: Electrochemical and Spectroscopic Studies of Proton-Coupled Electron Transfer Processes. ACS Catal. 2016, 6 (6), 3644−3659.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication was made possible by funding through a National University of Singapore research grant (Grant 143000-641-112).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b01507 Inorg. Chem. XXXX, XXX, XXX−XXX