Electrocatalytic Reduction of CO2 into Formate with Glassy Carbon

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Electrocatalytic Reduction of CO2 into Formate with Glassy Carbon Modified by [Fe4N(CO)11(PPh2Ph-linker)]− David B. Cluff, Amela Arnold, James C. Fettinger, and Louise A. Berben* Department of Chemistry, University of California, Davis, California 95616, United States

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S Supporting Information *

ABSTRACT: Immobilization of molecular electrocatalysts with retention of catalytic activity is necessary if they will be incorporated into functional photoelectrochemical devices. Most often, immobilization diminishes catalytic performance. Glassy-carbon electrodes covalently modified with [Fe4N(CO)12]− are active for electrocatalytic formate production from CO2 at −1.2 V vs SCE in aqueous solutions buffered at pH 5−9. The modified electrodes are stable for at least 4 days, as demonstrated using cyclic voltammetry experiments. Electrode modification was performed via cycloaddition of alkyne-functionalized [Fe4N(CO)12]− with azide-modified glassy-carbon electrodes.

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electrolysis experiments performed at −1.25 V in pH 9 buffered solutions (Chart 1).

INTRODUCTION The modification of heterogeneous electrodes or photoelectrodes with homogeneous electrocatalysts is often discussed as a necessary path forward if homogeneous catalysts will be used in solar-to-fuels devices.1 In many instances, however, the activity of homogeneous catalysts changes or diminishes upon attachment to the electrode,2 and so this area needs more attention if changes in reactivity will be better understood and strategies to improve performance achieved. Carbon electrodes, such as glassy carbon (GC), graphite, diamond, or porous carbons, are some of the most useful due to their cost effectiveness and high overpotential for background H+ reduction, and despite the interest and advantages that come from covalent attachment of molecular catalysts to glassy-carbon electrodes (GC), relatively few examples have been reported for their modification.3 Those reports include electrocatalysts that perform H2 evolution,4,5 O2 reduction,6 CO2 reduction to CO,7 or water oxidation.8 In this work we assess the CO2-to-formate reaction using [Fe4N(CO)11(PPh2Ph-triazole-GC)]− (1-TrGC), which comprises the catalyst [Fe4N(CO)12]− covalently attached to GC using Cu-catalyzed cycloaddition chemistry (Tr is triazole). There are very few efficient electrocatalysts known for CO2-toformate conversion, either homogeneous9 or heterogeneous,10 and we have previously reported that [Fe4N(CO)12]− is an electrocatalyst that reduces CO2 to formate selectively at −1.2 V vs SCE, in pH 6.5 buffered water and in MeCN/H2O (95/ 5).11 This first-row transition-element catalyst has long-term stability combined with selectivity and 97% Faradaic efficiency for formate formation in water, and so it is a good candidate with which to assess the effects of surface immobilization on the activity for formate production. We find that the GC electrodes modified with [Fe4N(CO)11(PPh2PhCH2-linker)]− are stable over days and that they maintain the ability of [Fe4N(CO)12]− to produce formate from CO2 during © XXXX American Chemical Society

Chart 1. Schematic of 1-TrGC

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RESULTS AND DISCUSSION Synthesis of [Fe4N(CO)11(PPh2PhCCH)]−. To obtain a derivative of [Fe4N(CO)12]− that could be covalently attached to a GC electrode, we targeted an alkynyl-functionalized cluster, since the alkynyl functional group has been used previously to enable a cycloaddition reaction with azidemodified glassy carbon.6 Reaction of [Na(diglyme)2][Fe4N(CO)12)] with PPh2PhCCH in refluxing THF over 20 h afforded [Na(diglyme)2][Fe4N(CO)11(PPh2PhCCH)] (Na1) as a black powder in 77% yield. The strongest of the carbonyl bands in the IR spectrum of 1 are observed at 1972 and 1987 cm−1 and are shifted to lower energy in comparison Special Issue: Organometallic Electrochemistry: Redox Catalysis Going the Smart Way Received: June 10, 2018

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DOI: 10.1021/acs.organomet.8b00396 Organometallics XXXX, XXX, XXX−XXX

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Organometallics with those in [Fe4N(CO)12]−, consistent with more electron density from the cluster core shared over fewer carbonyl ligands in 1 (Figure S1). Proton, carbon, and phosphorus NMR spectra are all consistent with incorporation of one phosphine ligand onto [Fe4N(CO)12]− (Figures S2−S4). To facilitate crystallization of 1−, we synthesized Et4N-1 (see the Experimental Section for details and Figures S5 and S6). The solid-state structure of Et4N-1 was obtained using singlecrystal X-ray diffraction performed on black block-shaped crystals grown from a concentrated solution of THF held at −20 °C, over 1 week (Tables S1 and S2 and Figure 1). The

electrodes were characterized using CV performed with 1TrGC as the working electrode in 0.1 M NaCl, 5 mM acetate buffered solution at pH 5 (Figure 2, blue trace). CVs performed with 1-TrGC show a reduction event at Ep = −1.3 V vs SCE (Figure 2, left).

Figure 2. (left) CVs of 1-TrGC recorded in 0.1 M NaCl, 4 mmol acetate buffer pH 5 solution, under N2 (blue), after rinse with water in air (green), and after storage in a vacuum desiccator for 3 days (red trace) and a scan of the polished GC plate (orange). (right) CV of 1TrGC in 0.3 M Bu4NBF4 THF solution (blue) and the baseline used to approximate the area under the CV scan (red). Figure 1. (left) Solid-state structure of [Fe4N(CO)11(PPh2PhC CH)]− in 1. Green, gray, red, purple, and blue colors represent iron, carbon, oxygen, phosphorus, and nitrogen, respectively. Ellipsoids are given at 50% probability, and H atoms are omitted. (right) CV of 0.1 mM 1 in 0.1 M solution of Bu4NBF4 in MeCN with a GC working electrode and scan rate 100 mV/s.

To assess the robustness of 1-TrGC, a series of experiments were performed over 4 days (Figure 2, left). Initial CVs were taken in a pH 5 solution, and then the modified electrode was removed and a CV of the solution was taken with a clean GC button electrode to confirm that no 1 had dissolved into the solution. The solution was then changed, the modified electrode was rinsed, and a new CV was taken in the fresh solution (Figure 2). In addition to the above experiment to test for the durability of 1-TrGC, the same electrode as used above was stored in a vacuum desiccator for 3 days and then tested again. Transfers of electrode between solutions, desiccators, and rinsing were all performed in air with no precautions taken to exclude oxygen and water. Solutions were sparged with N2 to collect the CVs. This series of experiments indicates that 1TrGC is stable after storage in a vacuum desiccator, is stable to repeated electrochemical experiments, and is stable after brief periods of exposure to air. An experimentally determined surface concentration for 1− arranged on the GC was calculated from eq 1:13

bond lengths and angles in the solid-state structure are similar to those in the previously reported Et 4 N[Fe 4 N(CO)11(PPh3)].12 Both the Fe−Fe bond lengths and Fe−N bond lengths remained relatively unchanged with the inclusion of the weakly electron donating alkynyl group. The Fe−P bond length of 2.2123(6) Å falls between those of Fe−P in Et4N[Fe4N(CO)11(PPh3)] (2.2206(6) Å) and Et4N[Fe4N(CO)11(PPh2EtOH)] (2.2028(6) Å), and these trends correlate well with a steric effect that we previously proposed.12 As the Tolman cone angle on the phosphine increases, where PPh3 (145°) > PPh2(C6H4-4-CCH) > Ph2P(CH2)2OH (approximately 140°), the Fe−P bond length also increases. Electrochemical measurements performed on 0.1 mM solutions of 1 using cyclic voltammetry were conducted in a 0.1 M Bu4NPF6 solution in MeCN (Figure 1, right). An irreversible reduction event that we assigned to the 1−/2− couple was observed at Ep = −1.46 V vs SCE. An oxidation event was observed at −1.21 V. This electrochemical signature agrees very well with our previous report on the electrochemistry of [Fe4N(CO)11(PPh3)]−. The insolubility of Et4N1 and Na-1 in aqueous solution prevented us from assessing CV data in buffered solutions. We have previously studied the sodium salt of [Fe4N(CO)12]− in aqueous solution,11b but attachment of the nonpolar phosphine ligand lowers aqueous solubility. Synthesis of 1-TrGC. Covalent attachment of Na-1 to a GC electrode was accomplished by methods slightly modified from those reported by Chidsey and co-workers.6 Using this method, a cycloaddition reaction between the alkynyl functional group on Na-1 and azide-terminated GC provides a triazole ring that serves as a covalent linkage between 1 and GC in the final product (1-TrGC; Chart 1). The modified

Γ = q/nFA

(1)

where Γ is surface coverage in mol/cm , q is charge passed in C, n is number of electrons, F is Faraday’s constant in C mol−1, and A is the area of the electrode in cm2. The value for q was determined from the area under the reduction peak in a CV experiment using 1-TrGC as the working electrode in a 0.3 M Bu4NBF4 solution in THF (Figure 2, right). The surface coverage was calculated from the experimental data as 7.74 × 10−11 mol/cm2 (Calculation S2). We also calculated the theoretical surface coverage of 1 on the basis of the size of 1− as determined from the solid-state structure (1.46 × 10−10 mol/cm2, Calculation S1). The experimentally measured surface area is approximately 50% of the theoretical maximum coverage, and this is attributed to incomplete coverage of the GC surface during either the azide attachment to GC or in the cycloaddition reaction, and this result is fairly typical in surface modification chemistry.6 2

B

DOI: 10.1021/acs.organomet.8b00396 Organometallics XXXX, XXX, XXX−XXX

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Organometallics The CV experiments also show that the full width at halfmaximum (fwhm) for the reduction peak is 160 mV. Theoretically the fwhm should be 90.3 mV.14 Broad redox events are very commonly observed for molecules attached to substrates, and two common explanations for that behavior are either that multiple orientations of the molecule are present on the surface or that packing of the molecule next to the surface impedes the molecular dynamics needed for efficient electron transfer.7,15,16 CO2 Reduction by 1-TrGC. CVs of 1-TrGC in aqueous solution were collected in acetate buffer (pH 5), bicarbonate buffer (pH 7), and borate buffer (pH 9), and the onset potentials of the reduction event under each of those conditions were −0.99, −1.07, and −1.21 V, respectively (Figure S7). We used two different approaches to determine the onset potential for catalysis, and these yielded plots of onset potential vs pH with slopes of 53 and 64 mV/pH unit, respectively. This is consistent with a concerted electron proton transfer (EPT) process for which the theoretical slope of the plot should 59 mV/pH unit, according to the Nernst equation (Figure 3). This result suggests that the first step in a

Figure 4. CPEs (left) under N2 in 0.1 M KHCO3 at pH 8 and (right) under CO2 in 0.2 M K2CO3 at pH 9. In both frames blue denotes 1TrGC and red a polished GC electrode.

Table 1. CPE Experiments Performed with 1-TrGC at −1.25 V vs SCE in 0.2 M KHCO3 Solution at pH 9 under 1 atm of CO2, over 50 mina q (C) N2 CO2 no cat.

2.9 ± 2 2.4 ± 0.8 0.5 ± 0.3

FE (%) HCO2−

FE (%) H2

TON HCO2−

TON H2

75 ± 20

67 ± 10 28 ± 5 60 ± 13

52500

139000 19000

a Experiments under N2 are at pH 8 with 0.1 M KHCO3. Each experiment was performed three times.

97% FE for formate production, and so a small drop in efficiency is observed here. As other examples, CO production by a porphyrin Fe system generated 95% CO consistently in homogeneous and heterogeneous arrangements,7b whereas (cyclam)Ni electrocatalysts lose all ability to generate CO upon heterogenization due to conformational changes of the cyclam ring.2a Summary of the Effects of Heterogenization on Catalysis. The foregoing results demonstrate that heterogenization of [Fe4N(CO)12]− provides a catalyst for CO2 reduction that operates at the same potential and overpotential as we previously reported in homogeneous solution and that the performances of the two systems differ only in a selection of the possible metrics. At −1.25 V vs SCE, both [Fe4N(CO)12]− and 1-TrGC produce formate from CO2, and the yields are 97 and 75%, respectively. The lowered yield of formate with the heterogeneous system could potentially be attributed to local pH gradients induced at the electrode surface, which will have more effect on product profile in the heterogeneous system, where the catalyst is confined to the electrode surface. Turnover number (TON), which describes the amount of product formed per mole of catalyst, is much higher for the heterogeneous system, since a greater fraction of catalyst molecules employed in a heterogeneous system participates in the reaction. Homogeneous electrocatalysts only enter the catalytic cycle when they contact the electrode and the amount of catalyst present is also greater. In addition to these minor changes in performance metrics, mechanistic changes are noted upon heterogenization. The initial electron and proton transfer reactions that generate the hydride intermediates [H-Fe4N(CO)12]− or H-1-TrGC are different. Experiments performed in the absence of CO2 indicate a concerted EPT mechanism for hydride formation using 1-TrGC, whereas sequential ET, PT has been established for [Fe4N(CO)12]−.11 The EPT mechanism for localization of the electron and proton simultaneously at the surface of a

Figure 3. (left) CVs of 1-TrGC in 0.1 M NaCl, with 5 mM acetate pH 5 (red), bicarbonate pH 7 (green), and borate pH 9 (blue) buffers. (right) Plot of onset potential vs pH: (orange) onset defined as current exceeds 0.1 mA/cm2; (blue) onset defined as where the current deviates from baseline (see Figure S7 for details).

catalytic cycle performed in buffered aqueous solution is an EPT event to form H-1-TrGC.17 Using [Fe4N(CO)12]− in homogeneous solution, it is known that formation of the intermediate hydride [H-Fe4N(CO)12]− proceeds by sequential ET and PT events, as observed in CV experiments where the onset potential and reductive peak potential are pH independent.18 To investigate the stability and reactivity of 1-TrGC controlled-potential electrolysis experiments (CPEs) were performed under both N2 and CO2 atmospheres (Figure 4). Under a N2 atmosphere at pH 8 in 0.1 M KHCO3 solution 1TrGC showed an FE of 67% for H2 production over a 50 min experiment. When the headspace was changed for CO2 in a 0.2 M K2CO3 solution with pH 9, the FE for H2 dropped to 28%, and the majority of the remaining charge was used in the production of formate, which had a 75% FE, on the basis of quantification by NMR spectroscopy (Table 1). No CO was detected by GC-TCD. In control experiments, when a polished GC plate was used, the charge passed during the experiment was significantly less than that with 1-TrGC. In addition, CVs taken before and after the CPE experiments showed only minor differences in comparison with those collected before CPE (Figure S8). CPE experiments performed with [Fe4N(CO)12]− in homogeneous solution have previously afforded C

DOI: 10.1021/acs.organomet.8b00396 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

reagents were purchased from commercial vendors and used without further purification. [Na(diglyme)2][Fe4N(CO)11PPh2(C6H4-4-CCH)] (Na-1). Na-1 was synthesized using a modification of a previously published literature method for phosphine substitution of [Fe4N(CO)12]−.12 In a nitrogen-filled glovebox, [Na(diglyme)2][Fe4N(CO)12] (100 mg, 0.116 mmol) was dissolved in 5 mL of THF, and 1.05 equiv (0.121 mmol) of Ph2PC6H4-4-CCH (34.8 mg) was added. The reaction mixture was heated to 55 °C overnight. By IR and CV, some starting Na[Fe4N(CO)12] remained. Another 0.5 equiv (17.4 mg) of the phosphine was added, and the reaction mixture was heated to 55 °C for a further 8 h. The solution was concentrated, and the black oil was triturated with hexanes to remove any unreacted phosphine. The oil was suspended in benzene and concentrated until a black solid formed in 77% yield. 1H NMR (300 MHz, C6D6): δ 8.05−7.31 (br, m, 12H, aryl), 7.10−7.05 (br, m, 2H, aryl), 3.07−3.00 (br, 12H, coordinated diglyme), 2.69 (br, 1H, CCH). 31P NMR (162 MHz, THF): 67.07 (s) ppm. 13C NMR (140 MHz, CDCl3): δ 59.2 (s, diglyme), 69.5 (s, diglyme), 70.9 (s, diglyme), 77.2 (s, alkyne), 78.2 (s, alkyne), 128.0 (d, aromatic C−H), 129.8 (d, aromatic C−H), 131.6 (d, aromatic C− H), 131.8 (d, aromatic C−H), 132 (d, aromatic C−H), 133.4 (d, aromatic C−H), 133.7 (d, aromatic C−H), 217.6 (s, CO). Anal. Calcd (found): C, 45.98 (45.78); H, 3.86 (3.89); N, 1.25 (1.35). IR (THF): νCO 2038 (w), 1966 (sh), 1972 (vs), 1987 (vs) cm−1. Et4N[Fe4N(CO)11PPh2(C6H4-4-CCH)] (Et4N-1). To facilitate crystallization, the tetraethylammonium salt of Na-1 (Et4N-1) was prepared. Beginning with [Et4N][Fe4N(CO)12], Et4N-1 was synthesized in a manner similar to that for Na-1: [Et4N][Fe4N(CO)12] (0.104 mmol, 71.5 mg) was dissolved in THF (3 mL), and Ph2PC6H4-4-CCH (0.104 mmol, 29.7 mg) was added. The dark green-yellow solution was stirred at 55 °C for 7 h, at which point 31P NMR, IR, and CV analysis showed complete consumption of the phosphine and formation of the substituted cluster. The solution was concentrated to an oil, dissolved in a minimum amount of THF, and stored at −20 °C over 1 week, after which black oily solids crashed out of solution (54 mg, 55% yield). 31P NMR (162 MHz, THF): 68.35 (s) ppm. IR (THF): νCO 2036 (w), 1964 (sh), 1971 (vs), 1985 (vs) cm−1. Single crystals suitable for X-ray diffraction were grown from a concentrated THF solution of Et4N-1 at −20 °C over 1 week, as black blocks. Preparation of Modified Carbon Materials. All manipulations were carried out using standard Schlenk or glovebox techniques under a N2 atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thorough sparging with Ar gas followed by passage through an activated alumina column. Glassy carbon (GC) plates were purchased from Tokai Carbon. Reduction of GC prior to functionalization was performed using hydrazine, as has been described for graphene and mesoporous carbon.22 All other reagents were purchased from commercial vendors and used as received. Modification of 1.1 cm2 GC plates was performed with Na-1 according to literature procedures.6 Two etched copper plates were submerged in glacial acetic acid for 1 min and then dried under a stream of N2 gas and placed in an oven at 110 °C for 60 min, before being taken into an N2-filled glovebox. A 5 μL drop of Na-1 dissolved in THF (typically 1 mM) was placed on one of the two Cu plates. An azide-modified GC electrode was then placed with one face in the solution. Another 5 μL drop of 1 dissolved in THF was placed on the upward face of the azide-modified GC electrode, and a second Cu plate was placed on top. The reaction sample sat for at least 600 s before it was rinsed with THF. Electrochemical Measurements. Cyclic voltammograms were recorded under a dinitrogen (Praxair, 99.998%) atmosphere using a CH Instruments Model 620D or 1100 Electrochemical Analyzer, a glassy-carbon working electrode (Tokai Carbon, nominal surface area of 1.86 cm2), a platinum-wire auxiliary electrode, and an Ag/AgCl (0.001M) aqueous reference electrode with a Vycor tip. CV measurements on modified GC were performed using the modified GC as the working electrode with a Pt counter electrode and an Ag/ AgCl reference electrode. All measurements were recorded in 1.0 M NaCl or 1.0 M KHCO3/H2CO3 buffered aqueous solution (pH 6.5)

heterogeneous electrocatalyst is also referred to as the Volmer reaction. The observation that ET and PT are concerted for 1TrGC could imply that the conjugated linker between the molecular catalyst and the heterogeneous electrode is efficiently mediating electronic coupling so that the kinetics of the EPT reaction mimic those of a more conventional heterogeneous electrode where discrete molecular orbitals do not dictate reactivity. Covalently immobilized molecular catalysts that promote an EPT mechanism have previously been reported.17,19 On the basis of the similarities between the 1-TrGC and previously reported [Fe4N(CO)12]−, we propose that the rate-determining step in formate formation is the transfer of hydride to CO2 to create formate. The cluster is then quickly reduced by one electron to close the catalytic cycle.

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CONCLUSIONS We have shown that a simple cycloaddition reaction can be used to modify GC with a substituted analogue of the formateproducing electrocatalyst, [Fe4N(CO)12]−. The activity of [Fe4N(CO)12]− for CO2 reduction to formate persists in the functionalized material, 1-TrGC, and after electrolysis over 50 min formate was detected with 75 ± 20% FE. H2 is also produced with ∼28% FE. Heterogenization of the molecular catalyst promotes a concerted electron proton transfer mechanism for formation of the intermediate hydride H-1TrGC, in contrast to the stepwise ET, PT mechanism previously reported for homogeneous [Fe4N(CO)12]−: this observation is support for an efficient electronic coupling of 1 to the electronic structure of the GC. The modified electrodes were tested for stability over 3 days and showed almost no degradation, which is promising for the use of [Fe4N(CO)12]− in device applications, including photoelectrochemical cells. Ongoing work will address reaction conditions to better control the product ratios of formate and H2.

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EXPERIMENTAL SECTION

X-ray Structure Determination. X-ray diffraction studies were carried out on either a Bruker Photon100 CMOS diffractometer equipped with a CCD detector.20 Measurements were carried out at −173 °C using Mo Kα radiation (0.71073 Å). Crystals were mounted on a Kaptan loop with Paratone-N oil. Initial lattice parameters were obtained from a least-squares analysis of more than 100 centered reflections; these parameters were later refined against all data. Data were integrated and corrected for Lorentz−polarization effects using SAINT and were corrected for absorption effects using SADABS2.3. Space group assignments were based upon systematic absences, E statistics, and successful refinement of the structures. Structures were solved by direct methods with the aid of successive difference Fourier maps and were refined against all data using the SHELXTL 5.0 software package. Thermal parameters for all non-hydrogen atoms were refined anisotropically. Hydrogen atoms, where added, were assigned to ideal positions and refined using a riding model with an isotropic thermal parameter 1.2 times that of the attached carbon atom (1.5 times for methyl hydrogens). Preparation of Compounds. All manipulations were carried out using standard Schlenk or glovebox techniques under a dinitrogen atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thorough sparging with argon (Praxair, 99.998%) gas followed by passage through an activated alumina column. Deuterated solvents were purchased from Cambridge Isotopes Laboratories, Inc., degassed, and stored over activated 3 Å molecular sieves prior to use. [Na(diglyme)2][Fe4N(CO)12]16 and Ph2PC6H4-4-CCH21 were prepared using modified syntheses from the literature. All other D

DOI: 10.1021/acs.organomet.8b00396 Organometallics XXXX, XXX, XXX−XXX

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or 0.1 M Bu4NBF4 in MeCN at a scan rate of 100 mV/s. Reported potentials are all referenced to the SCE couple and were determined using ferrocene as an internal standard, where the E1/2 value for ferrocene/ferrocenium is +0.16 V vs SCE in H2O.23 All other reagents were purchased from commercial vendors. pH measurements of aqueous solutions were performed using a VWR SympHony pH meter with a posi-pHlo glass electrode. The estimated error for reported pH values is ±0.1 pH unit. The meter was calibrated prior to use using a three-point calibration with standard buffers (BDH) of pH 4, 7, and 10. Controlled-Potential Bulk Electrolysis (CPE). Controlledpotential electrolysis (CPE) experiments were performed in a custom-designed gastight glass cell under 1 atm of static dinitrogen (Praxair, 99.998%) or CO2, as needed. Solutions were sparged with the gas of interest prior to the commencement of the experiment. The counter electrode compartment was separated from the working electrode compartment by a glass frit of fine porosity. In a typical experiment, 18 mL of electrolyte solution was used in the working electrode compartment and 25 mL of electrolyte was used in the counter electrode compartment. The working electrode was a glassycarbon plate (Tokai Carbon) with the nominal surface area immersed in solution of 1.86 cm2. The auxiliary electrode was a coiled Pt wire (BASi). CO2 was obtained from dry ice and transferred to experiments via cannula and tubing. Gas measurements were performed using a gastight syringe (Vici) to inject 100 μL gas samples into a Varian 3800 gas chromatograph equipped with a thermal conductivity detector. Gas samples were extracted from a sparged, septum-capped side arm on the working electrode compartment. No CO was detected. Between CPE experiments, the cell was sonicated in 0.1 M HCl, rinsed, sonicated in bicarbonate, rinsed, and sonicated in water for 3 × 10 min. Other Physical Measurements. 1H NMR spectra were recorded at ambient temperature using a Varian 600 MHz spectrometer. 13C NMR were recorded using a Bruker 800 MHz spectrometer. Chemical shifts were referenced to residual solvent. 31P NMR spectra were recorded on a Bruker 400 MHz spectrometer at ambient temperature and referenced using an external H3PO4 standard (chemical shift of H3PO4 0 ppm). Quantitative measurement of H2 was performed on a Varian 3800 GC equipped with a TCD detector and a Carboxen 1010 PLOT fused silica column (30 m × 0.53 mm) (Supelco) using dinitrogen (99.999%, Praxair) as the carrier gas. H2 concentration was determined using a previously prepared working curve. 1H NMR spectroscopy was used to assess solutions for formic acid, acetate, and methanol using an 800 MHz spectrometer. An internal standard of a known amount of DMF, as a dilute solution in 100% C6D6, was prepared and sealed in a glass capillary tube. A 500 μL portion of the CPE solution was injected into an NMR tube with the internal standard capillary. The integration of the 1H resonance at 7.65 ppm for DMF was used to quantify formic acid produced (8.48 ppm). Elemental analyses were conducted by the Science Centre at the London Metropolitan University.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail for L.A.B.: [email protected]. ORCID

James C. Fettinger: 0000-0002-6428-4909 Louise A. Berben: 0000-0001-6461-1829 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Energy, Office of Science, Basic Energy Sciences, for support from award number DESC0016395. The National Science Foundation provided the dual source X-ray diffractometer (CHE-1531193) and the NMR instrumentation (DBIO-722538 and CHE 443516).

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REFERENCES

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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.organomet.8b00396. Calculations of surface coverage, electrochemical data, and crystallographic data (PDF) Accession Codes

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DOI: 10.1021/acs.organomet.8b00396 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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DOI: 10.1021/acs.organomet.8b00396 Organometallics XXXX, XXX, XXX−XXX