Acidic ionic liquid/water solution as both mediumand proton source for electrocatalytic H2evolution by ½NiðP2N2Þ2�2þ complexesDouglas H. Pool, Michael P. Stewart, Molly O’Hagan, Wendy J. Shaw, John A. S. Roberts1, R. Morris Bullock, andDaniel L. DuBois1

Center for Molecular Electrocatalysis, Chemical and Materials Sciences Division, P.O. Box 999, K2-57, Pacific Northwest National Laboratory, Richland,WA 99352

Edited by Thomas J. Meyer, University of North Carolina, Chapel Hill, NC, and approved April 4, 2012 (received for review December 7, 2011)

The electrocatalytic reduction of protons to H2 by½NiðPPh

2NC6H4-hex

2Þ2�ðBF4Þ2 (where PPh2N

C6H4-hex2 ¼ 1,5-dið4-n-

hexylphenylÞ-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctaneÞ inthe highly acidic ionic liquid dibutylformamidium bis(trifluorome-thanesulfonyl)amide shows a strong dependence on addedwater. A turnover frequency of 43,000–53,000 s−1 has been mea-sured for hydrogen production at 25 °C when the mole fractionof water (χH2O) is 0.72. The same catalyst in acetonitrile withadded dimethylformamidium trifluoromethanesulfonate andwater has a turnover frequency of 720 s−1. Thus, the use of anionic liquid/aqueous solution enhances the observed catalyticrate by more than a factor of 50, compared to a similar acid in atraditional organic solvent. Complexes ½NiðPPh

2NC6H4X

2Þ2�ðBF4Þ2ðX ¼ H,OMe,CH2PðOÞðOEtÞ2,BrÞ are also catalysts in the ionicliquid/water mixture, and the observed catalytic rates correlatewith the hydrophobicity of X.

electrocatalysis ∣ homogeneous ∣ proton relay ∣ hydrogenase ∣ renewableenergy

Hydrogenase enzymes are active and efficient catalysts for H2

oxidation and production; for example, [FeFe] hydrogenaseexhibits a turnover frequency (TOF) as high as 9;000 s−1 at 30 °C,and operates at significantly lower overpotentials than most syn-thetic catalysts (1). In the proposed active site (Fig. 1, structure1), the noncoordinating amine near the vacant coordination siteof the distal iron atom Fed is thought to assist both the heterolyticcleavage of H2 and the movement of protons between the activesite and the bulk solution (2). This functionality has inspired theexploration of the role of pendant amines in the electrocatalyticproduction and oxidation of H2 (3–7).

The cyclic diphosphine ligands shown in structure 2 (Fig. 2)have positioned pendant amines in proximity to the Ni. Thesefunctional mimics of hydogenase have turnover frequencies ofup to 1;850 s−1 for the production of hydrogen, faster than somenaturally occurring hydrogenases (2, 4, 8–11). While variants of2 exhibit rapid turnover in acetonitrile, computational resultsindicate that much faster turnover is possible. The rate constantfor release of H2 from intermediate 4 has been estimated toexceed 106 s−1 at 25 °C (12). However, 4 can only form followingtwo endo protonation events (Fig. 2), and exo protonation limitsthe rate at which this intermediate is produced. The turnover fre-quency is then determined by the relative rates of formation andinterconversion of protonated isomers 3. Experimental observa-tions support this model: Smaller acids and the addition of waterresult in higher rates, allowing more ready access to endo proto-nation and more facile interconversion from the exo isomers(9, 13). These considerations led to the development of a Ni cat-alyst with cyclic diphosphine ligands having one pendant aminerather than two, precluding the N-H-N pinching interactionshown in structures 3b and 3c and affording a turnover frequencyof 106;000 s−1 at 22 °C in acetonitrile, albeit with a larger over-potential (0.6 V) than seen for variants of 2 (0.28–0.37 V) (14).

These complexes are designed to control the movement ofprotons between the metal and the pendant amines of the secondcoordination sphere. Control of proton movement beyond thesecond coordination sphere, as manifested in the proton conduc-tion channels of the hydrogenase enzymes, constitutes a naturalextension of this approach. In this work, we use a protic ionicliquid with pKa values closely matching those of the catalyst toaccomplish this control. Observations of rapid catalysis with½ðDMFÞH�OTf (DMF, dimethylformamide; Otf, trifluorometha-nesulfonate) in acetonitrile led us to select ½ðDBFÞH�NTf2 (DBF,di--nbutylformamide; NTf2, bis(trifluoromethanesulfonyl)amide)(Fig. 3), which serves as electrolyte, acid, and solvent (15). Cat-alytic currents observed in this medium increase dramaticallywith added water, and the highest rates are obtained with a sa-turated aqueous solution.

ResultsHere, we report a variant of complex 2 developed specifically tofavor interaction with hydrophobic media, presenting first thecharacterization of this complex and its electrocatalytic responsein conventional media. This is followed by an examination of itschemical behavior in the protic ionic liquid as a function of waterconcentration. Lastly, we describe the electrocatalytic resultswith this and related complexes in ionic liquid/water mixturesand present the methods employed for quantifying both turnoverfrequencies and overpotentials.

Synthesis and Characterization of P2PhN2

C6H4-hex and ½NiðP2 PhN2C6H4-hexÞ2�

ðBF4Þ2 (5).The ligand 1,5-di(4-n-hexylphenyl)-3,7-diphenyl-1,5-diaza-

Fig. 1. The proposed active site of [FeFe] hydrogenase (2).

Author contributions: D.H.P., M.P.S., M.O., W.J.S., J.A.S.R., R.M.B., and D.L.D. designedresearch; D.H.P., M.P.S., M.O., W.J.S., and J.A.S.R. performed research; D.H.P., M.P.S., M.O.,W.J.S., and J.A.S.R. contributed new reagents/analytic tools; D.H.P., M.P.S., M.O., W.J.S.,J.A.S.R., R.M.B., and D.L.D. analyzed data; and D.H.P., M.P.S., M.O., W.J.S., J.A.S.R., R.M.B.,and D.L.D. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence may be addressed. E-mail: john.roberts@pnnl.gov ordaniel.dubois@pnnl.gov.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1120208109/-/DCSupplemental.

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3,7-diphosphacyclooctane (P2PhN2

C6H4-hex) and theNi(II) complex,½NiðP2

PhN2C6H4-hexÞ2�ðBF4Þ2 (5), were prepared as shown in Fig. 4

(9, 10). The 31Pf1Hg NMR spectrum of 5 in CD3CN features asingle peak at 5.3 ppm, consistent with similar ½NiðP2N2Þ2�2þ com-plexes (9). Spectroscopic data and elemental analysis are consistentwith the structure shown; details are given in the SI Text.

Cyclic voltammetry of 5 in benzonitrile (PhCN; 0.1 MNBu4PF6) shows reversible Ni(II/I) and Ni(I/0) redox couples(E1∕2 ¼ −0.81 and −1.04 V vs. the ferrocenium/ferrocene cou-ple, Fcþ∕Fc; SI Appendix, Fig. S1A), similar to those reportedfor ½NiðP2

PhN2C6H4MeÞ2�ðBF4Þ2 (9). Voltammetry in acetonitrile

reveals a desorption wave on the oxidative scan following reduc-tion to Ni(0), indicating electrodeposition (SI Appendix, Table S1and Fig. S1B (16).

Hydrogen Production Electrocatalysis by 5 in Acetonitrile. Addingthe acid [(DMF)H]OTf to 5 in acetonitrile (0.1 M NBu4PF6)affords a sigmoidal trace consistent with catalytic reduction ofprotons to form H2 (Eq. 1 with B ¼ DMF, Fig. 5). The catalyticcurrent icat increases with added acid until substrate saturation isattained at approximately 0.16 M. Under these conditions, theturnover frequency (TOF) is equal to kobs (200 s−1), whichcan be calculated using Eq. 2 from icat∕ip where ip is the peakcurrent in the absence of substrate, n is the number of electronsper turnover (two), υ is the scan rate, and F, R, and T are Fara-day’s constant, the gas constant, and the temperature (10).

BHþ þ e− → Bþ 1

2H2 [1]

kobs ¼ 0.1992ðn2Fυ∕RTÞðicat∕ipÞ2 [2]

The half-peak potential Ep∕2 for the catalytic wave is −0.73 V,corresponding to an overpotential of 0.23 V, using the method ofEvans and coworkers (17). Adding water gives a maximum turn-over frequency of 740 s−1 with a Ep∕2 of −0.79 V and an over-potential of 0.28 Vat a water concentration of 1.4 M. These ratesare comparable to those observed for ½NiðPPh

2NPh

2Þ2�2þ underthe same conditions (720 s−1 with water) (9).

NMR Spectroscopy of 5 in ½ðDBFÞH�NTf2 and Its Mixtures with Water.Compound 5 gives a viscous orange-red solution in½ðDBFÞH�NTf2 which darkens with added water, up to a molefraction (χH2O) of 0.78, beyond which two phases are observed.31Pf1Hg NMR spectra (Fig. 6) show a broad singlet at−14 ppm and smaller peaks at 0 and 7 ppm. With addedwater, these resonances diminish and a singlet at 3 ppm emergeswith increasing intensity. Adding DBF to 5 in neat½ðDBFÞH�NTf2 produces a similar effect, with the 31P NMRresonance of 5 approaching its value in free DBF (6 ppm) (SIAppendix, Fig. S2).

Adding water results in the removal of one or both protons fromthe diprotic Ni(II) species (Fig. 6). The singlet at −14 ppm isassigned to diprotic 5. For comparison, in CD3CN, the closelyrelated doubly pinched diprotic isomer of ½NiIIðP2

PhNBn2Þ2�2þ

(where Bn is benzyl) appears at −15.3 ppm in the 31P NMR spec-trum (8). The two equally intense singlets are assigned to the in-equivalent P environments of a monoprotic species, and the singletat 3 ppm is from the aprotic Ni(II) species. Cyclic voltammetrysubstantiates deprotonation with added water, and open circuitpotential measurements show a decrease in solution acidity uponaddition of water.

31P NMR spectra obtained over one month show that complex5 is moderately stable in neat ½ðDBFÞH�NTf2 (a 25% decrease insignal was observed). Complex 5 in ½ðDBFÞH�NTf2 (χH2O ¼ 0.72)decomposes with t1∕2 ≈ 1 week. With HNTf2 in CD3CN, 5 de-composes rapidly to afford unbound, protonated ligand, and inneat DBF, 5 undergoes slow ligand displacement (t1∕2 ≈ 2 wk).

Cyclic Voltammetry of 5 in ½ðDBFÞH�NTf2.Fig. 7 shows voltammogramswith ½5� ¼ ½ferrocene� ¼ 1.1 mM in DBF (0.1 M NBu4PF6; redtrace) and in ½ðDBFÞH�NTf2 (blue trace). The ionic liquid is aproton source for hydrogen production, so only a catalytic waveis observed. This wave has Ep∕2 ¼ −0.38 V, considerably positiveof the Ni(II/I) couple in DBF (E1∕2 ¼ −0.82 V) reflecting proto-nation of 5 (8, 18), consistent with the NMR results discussedabove. Diffusion is slowed in the more viscous ionic liquid, attenu-ating both the Fcþ∕Fc wave and the catalytic current (19).

Cyclic Voltammetry of 5 in ½ðDBFÞH�NTf2 with AddedWater.As water isadded to 5 in ½ðDBFÞH�NTf2, icat increases and Ep∕2 shifts tomore negative values (Table 1). Fig. 8 shows the cyclic voltammo-

Fig. 2. Catalytic cycle for hydrogen production mediated by complexes with structure 2.

Fig. 3. Dibutylformamidium bis(trifluoromethanesulfonyl)amide. Fig. 4. Synthesis of complex 5.

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gram of 5 in ½ðDBFÞH�NTf2 with χH2O ranging from 0 to 0.75.At χH2O ¼ 0.57, the conversion of protic Ni(II) to aprotic 5 isessentially complete (Fig. 6). Ultimately, icat increases by a factorof 90, and Ep∕2 shifts by −0.25 V. Controlled-potential coulome-try of 5 in ½ðDBFÞH�NTf2 (χH2O ¼ 0.72) confirms the productionof hydrogen, giving a current efficiency of 92� 5% with a mea-sured turnover number 13. Without added catalyst, onset of hy-drogen production is observed at −1.2 V (SI Appendix, Fig. S3).Viscosity decreases with added water, increasing Fcþ∕Fc redoxcurrents (SI Appendix, Fig. S4). However, the increase in icat ismuch larger than expected due to viscosity changes alone. Thisis discussed in detail in the SI Text.

Cyclic Voltammetry of 5 in ½ðDBFÞH�NTf2 (χH2O ¼ 0.72). Catalyst Concen-tration Effects. Catalytic currents for hydrogen productionmediated by ½NiðPR

2NR 0

2Þ2�2þ catalysts typically show a lineardependence on [catalyst] (9, 11). The dependence of icat on [5]in ½ðDBFÞH�NTf2 (χH2O ¼ 0.72) is linear over 3 orders of mag-nitude (SI Appendix, Fig. S7), with ½5� < 1 mM, with currentsfalling off at higher [5]. Data collected with low catalyst concen-trations were employed to obtain turnover frequencies.

Voltammetry of Other ½NiðP2N2Þ2�ðBF4Þ2 Complexes in Anhydrous½ðDBFÞH�NTf2 and ½ðDBFÞH�NTf2/Water Mixtures. The½NiðP2

PhN2C6H4XÞ2�2þ complexes having X ¼ H (6), OMe (7),

CH2PðOÞðOEtÞ2 (8), Br (9), and CF3 (10), all catalysts in MeCN,(9) also show catalytic waves in dry ½ðDBFÞH�NTf2 that increase

with added water, and show a linear dependence of icat on [cat](SI Appendix, Figs. S8 and S9). With 10, low catalytic currents andpoorly defined plateaus obscured icat (SI Appendix, Fig. S10). Aswith 5, the catalytic wave for 6 shifts negative with added water.The CF3 substituted complex 10 shows little variation inEp∕2 withχH2O, as expected from NMR evidence that 10 is not protonatedin neat ½ðDBFÞH�NTf2 (SI Appendix, Fig. S11).

Determination of Turnover Frequencies. Steady-state catalyticcurrents icat can be used to determine turnover frequencies(kobs; s−1) using Eq. 3, where n is the number of electronsconsumed per turnover, A is the electrode area, and Dcat is thecatalyst diffusion coefficient (20, 21). The peak current ip of ann 0-electron wave for the same catalyst without substrate, whenobservable, will also depend on A, [cat], and Dcat (Eq. 4) (19),and kobs is given by icat∕ip, as shown in Eq. 2.(9–11, 14) Since½ðDBFÞH�NTf2 is a substrate for electrocatalytic H2 evolution,ip cannot be measured, and Eq. 2 cannot be used. Determinationof kobs using Eq. 3 requires the measurement of A and D.Determination of A is described in the SI Text.

icat ¼ nFA½cat�ðDcatkobsÞ1∕2 [3]

ip ¼ 0.4463n 0FA½cat�ðn 0FυDcat∕RTÞ1∕2 [4]

Catalysts were evaluated using ½ðDBFÞH�NTf2 (χH2O ¼ 0.72)to ensure solution homogeneity. Catalyst diffusion coefficientsin this medium were estimated voltammetrically using a nonca-talytic model complex ½NiðdppbÞ2�ðBF4Þ2 (11; dppb ¼ 1;2-bisðdiphenylphosphinylÞbenzeneÞ, and by 19F pulsed gradientspin-echo (PGSE) NMR spectroscopy using½NiðP2

PhN2C6H4CF3Þ2�ðBF4Þ2 (10). Steady-state linear sweep

voltammetry and chronoamperometry experiments of 5and 11 in MeCN (0.1 M NBu4PF6) gave D5 ¼ 6 × 10−6 andD11 ¼ 9 × 10−6 cm2 s−1. Chronoamperometry experiments of

Fig. 5. Cyclic voltammograms of 5 (0.9 mM) in acetonitrile (0.1 M NBu4PF6)without added acid (red trace, SI Appendix, Fig. S1) and with added acid(0.28 M [(dimethylformamide)H]OTf; blue trace). One millimeter glassy car-bon working electrode, scan rate υ ¼ 0.05 V s−1. Referenced to the Fcþ∕Fccouple.

Fig. 6. 31Pf1Hg NMR spectra of 5 in ½ðDBFÞH�NTf2 with χH2O ranging from 0to 0.75.

Fig. 7. Cyclic voltammograms of 5 (1.1 mM) and ferrocene (1.1 mM) inDBF (0.2 M NBu4PF6, red trace) and in ½ðDBFÞH�NTf2, (blue trace). 1 mm glassycarbon working electrode, scan rate υ ¼ 0.05 V s−1. Referenced to the Fcþ∕Fccouple.

Table 1. Voltammetric data for 5 in ½ðDBFÞH�NTf2 with water added

½Ni2þ� (mM) χH2O (½H2O�, M) icat (μA) Ep∕2 (V)* OCP (V)*† OP (V)

0.66 0 (0) 0.4 −0.38 0.041 0.420.64 0.30 (1.4) 0.8 −0.40 −0.012 0.390.63 0.47 (2.6) 3.4 −0.49 −0.058 0.430.62 0.57 (3.9) 5.6 −0.55 −0.107 0.440.60 0.64 (5.1) 15.2 −0.59 −0.152 0.440.59 0.69 (6.2) 28.0 −0.61 −0.184 0.430.58 0.72 (7.2) 29.5 −0.62 −0.207 0.410.56 0.75 (7.8) 35.8 −0.63 −0.219 0.41

Voltammograms are shown in Fig. 8. Overpotentials (OP) are based onopen circuit potential (OCP) measurements (SI Appendix, Fig. S5) and Ep∕2.*Referenced to Fcþ∕Fc.†Interpolated from ½H2O� vs. OCP data (SI Appendix, Fig. S6).

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11 in ½ðDBFÞH�NTf2 (χH2O ¼ 0.72) gave D11 ¼ 2.4 ×10−7 cm2 s−1. Assuming D5∕D11 is medium-independent, D5 is1.6 × 10−7 cm2 s−1 in this medium. The value of D10 in½ðDBFÞH�NTf2 (χH2O ¼ 0.72) is 1.3 × 10−7 cm2 s−1 by 19F PGSE,in agreement with D5 in ½ðDBFÞH�NTf2 (χH2O ¼ 0.72) estimatedusing electrochemical methods. Turnover frequencies were calcu-lated for each catalyst using Eq. 3 and D5 and D10 as lower andupper limits for the catalyst diffusion coefficients (Table 2).

Determination of Equilibrium Potentials for Hydrogen Production.The thermodynamic potentials for the interconversion of protonsand electrons with H2 and base (Eq. 1), required for the deter-mination of overpotentials, were obtained for ½ðDBFÞH�NTf2/water mixtures with χH2O from 0 to 0.72 under 1 atm H2. Foreach solution, the open circuit potential (OCP) (SI Appendix,Fig. S5) between a platinum electrode and a frit-separatedAgCl/Ag reference electrode was recorded (22) and referencedto the Fcþ∕Fc couple, placing the OCP on the scale used forthe catalytic experiments and eliminating the unknown junctionpotential between the reference and analyte compartments.Adding water to the ionic liquid shifts the OCP to more negativevalues, as expected for addition of a base (SI Appendix, Fig. S6).Experiments using 5% H2 in N2 rather than pure H2 shifted theOCP to more positive values, also as expected. Table 1 providesOCP values and overpotentials jEp∕2 −OCPj for catalyst 5,corresponding to the cyclic voltammograms shown in Fig. 8.Although Ep∕2 values shift negative with added water, the

overpotential for 5 stays fairly consistent across the range ofwater concentrations.

DiscussionControl of the movement of protons over different distance scalesis accomplished in hydrogenase enzymes by coupling the activesite of the catalyst with the reaction medium through protonchannels, and in polymer electrolyte membrane fuel cells byplacing the catalyst in contact with a proton-conducting mem-brane. The pendant amines built into ½NiðPR

2NR 0

2Þ2�2þ electro-catalysts with structure 2 shuttle protons between the metal andthe catalyst periphery, enabling rapid hydrogen production andoxidation in acetonitrile (4, 11). Fast proton movement in proticionic liquids (23, 24) has guided the development of media forhydrogen production at conventional platinum and other metallicelectrodes (22, 23, 25, 26).

These considerations prompted us to study catalysts with struc-ture 2, designed to control the intra- and intermolecular move-ment of protons, in acidic ionic liquid media exhibiting highproton mobility. Our principal findings are these: ½ðDBFÞH�NTf2is an excellent electrolyte and proton source for electrocatalyticproduction of H2. In this medium, complex 5 exhibits a strongrate enhancement upon the addition of water, showing that waterinfluences the kinetics of protonation and deprotonation; how-ever, the overpotentials are invariant with added water. Turnoveris substantially faster with this complex in ½ðDBFÞH�NTf2 (χH2O ¼0.72) than observed in acetonitrile:water with ½ðDMFÞH�þ. TheTOF values for complexes 5–9 appear to track the hydrophobicityof X, indicating that specific catalyst-solvent interactions maybe important in controlling proton movement.

Complex 5 dissolves in ½ðDBFÞH�NTf2 to produce species hav-ing 31Pf1Hg NMR resonances consistent with the diprotic andmonoprotic Ni(II) complexes shown in Fig. 6. The N-H-N pinch-ing interaction shown in these structures has been characterizedfor a similar Ni(II) system by isotopic labeling (8). The cyclic vol-tammogram of 5 in the neat ionic liquid shows a catalytic wavehaving a half-peak potential Ep∕2 ¼ −0.38 V vs. Fcþ∕Fc (Fig. 8).

The rate enhancements with added water shown in previousstudies of these complexes in acetonitrile are also observed inthe ionic liquid system. With complex 5 in particular, addingwater affords a 90-fold enhancement in the catalytic current(Fig. 8). As icat increases, the catalytic wave shifts from −0.38to −0.63 V vs. Fcþ∕Fc. Aprotic Ni(II) should be reduced at morenegative potentials than the protonated Ni(II) species shown inFig. 6, so the shift of Ep∕2 with added water to more negativepotentials is consistent with the deprotonation observed by NMRspectroscopy. These findings suggest that exo protonation of Ni(II) is slowing catalysis by 5 under dry conditions, and that theincrease in pH on addition of water precludes this preprotona-tion. A medium of sufficient acidity to protonate 5 in the Ni(II)oxidation state is unlikely to promote isomerization among proticintermediates in reduced states (intermediates 3, Fig. 2), sincethese states will be much less acidic than their Ni(II) congeners.

The deprotonation of exo-protonated Ni(II) species is onefactor in the substantial water effect observed with 5 in ½ðDBFÞH�NTf2. However, currents continue to increase significantly evenafter this deprotonation is essentially complete (χH2O ≥ 0.64,Table 1 and Fig. 6). Solution viscosity decreases with added water,increasing diffusion rates and thus catalytic currents [Eq. 3]; thismay largely account for the moderate increase in current with 10,however the magnitude of the increase in current with complex 5is greater than can be attributed to viscosity effects alone. Theeffect of water on the kinetics of proton transfer is perhapsthe most important contributing factor. This rate enhancement isattributed to increased rates of endo protonation of reduced Nispecies during catalysis, leading to faster production of isomer4 and thus faster H2 evolution (Fig. 2) (9, 13).

Fig. 8. Cyclic voltammograms of 5 (0.6 mM) in 0.83 mL of ½ðDBFÞH�NTf2,1 mm glassy carbon working electrode, υ ¼ 0.1 V s−1, initially and after eachof seven additions of 20 μL water. Referenced to Fcþ∕Fc.

Table 2. Comparison of catalytic turnover frequencies (TOF) andoverpotentials (OP) for ½NiðP2 PhN2

C6H4XÞ2�ðBF4Þ2 in ½ðDBFÞH�NTf2(χH2O ¼ 0.72) and MeCN (0.1 M NBu4PF6) with added[(dimethylformamide)H]OTf and water

½ðDBFÞH�NTf2 ðχH2O ¼0.72Þ

MeCN*

X TOF† (s−1) OP (V) TOF (s−1) OP (V)‡

n-hexyl (5) 4.3–5.3 × 104 0.40 7.4 × 102 0.28H (6) 5.5–6.8 × 103 0.42 7.2 × 102 0.32OMe (7) 6.3–7.8 × 102 0.37 4.8 × 102 0.33CH2PðOÞðOEtÞ2(8) 1.0–1.3 × 102 0.41 1.9 × 103 0.37Br (9) 4.4–5.4 × 103 0.44 1.0 × 103 0.29CF3 (10) — — 1.2 × 102 0.30

In amounts affording maximum catalytic currents.*See ref. 12 for 6–9.†Lower and upper limits calculated using D5 and D10, respectively.‡Calculated according to ref. 20. See Discussion for comparison ofoverpotentials for the different media.

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The turnover frequency of 5 in ½ðDBFÞH�NTf2 (χH2O ¼ 0.72)is 4.3–5.3 × 105 s−1, 60–70 times faster than observed for thiscomplex in acetonitrile using ½ðDMFÞH�þ with water added.TOF values for [NiðP2

PhN2C6H4XÞ2�ðBF4Þ2 complexes in ½ðDBFÞ

H�NTf2 (χH2O ¼ 0.72) are highest for X ¼ hexyl (5) and lowestfor X ¼ CH2PðOÞðOEtÞ2 (8) and span two orders of magnitude.These values were calculated for each catalyst using Eq. 3 andD5

and D10 as bracketing estimates for the catalyst diffusion coeffi-cients. Larger solutes diffuse more slowly, and with the exceptionof ½NiðP2

PhN2PhÞ2�ðBF4Þ2 (6), all of the catalysts are at least as

large as complex 10. Since overestimating diffusion coefficientswill lead to smaller estimates for TOF [Eq. 3], the upper limitslisted in Table 2 are conservative.

The turnover frequencies for complexes 5–9 do not track thedriving force for H2 release from intermediate 4 (Fig. 2) as ob-served for these complexes in acetonitrile (9), but instead appearto track the hydrophobicity of X, suggesting that ligand interac-tions with DBF or ½ðDBFÞH�þ are important. Self-assembly viainteraction of similarly hydrophobic moieties is well-known forionic liquids (27), and may play a role in directing proton delivery.In particular, interactions between the hexyl tails of 5 and thebutyl groups of dibutylformamide or ½ðDBFÞH�þ may provide ahigh local concentration of protic and basic sites, possibly facil-itating isomerization or favoring endo over exo protonation.Water could promote this self-assembly, and also furnish a con-duit for rapid proton movement between the bulk solution andthe catalyst proton relays. This structuring would likely be moresignificant for 5 than for CF3-substituted 10 and would slow dif-fusion, so the upper limit to turnover frequency for 5 calculatedusing D10 may be underestimated.

The overpotentials for complexes 5–9 in ½ðDBFÞH�NTf2(χH2O ¼ 0.72) are all near 0.4 V (Table 2, column 4), somewhatlarger than the values listed for acetonitrile (column 6), due inpart to the difference in the ðBHþÞ∕ðH2 þ BÞ equilibrium poten-tials measured directly for ½ðDBFÞH�NTf2 (χH2O ¼ 0.72), com-pared to those calculated for acetonitrile with ½ðDMFÞH�þ bya different method (17). The large increases in icat as water isadded to 5 in ½ðDBFÞH�NTf2 are not accompanied by changesin overpotential (Table 1), indicating that faster turnover frequen-cies can be achieved without increasing overpotentials, in thiscase by optimization of the reaction medium and its interactionswith the catalyst.

ConclusionsThe results presented here demonstrate rapid, efficientproduction of H2 mediated by a molecular electrocatalyst in amedium consisting of water and acid. The catalyst½NiðPPh

2NC6H4-hex

2Þ2�ðBF4Þ2 (5) dissolved in the highly acidicprotic ionic liquid dibutylformamidium bis(trifluoromethanesul-fonyl)amide and water (χH2O ¼ 0.72) exhibits a TOF of at least4 × 104 s−1, far greater than observed for related systems in acet-onitrile. Water increases rates without significantly increasingoverpotentials, a finding of particular significance in the contextof electrocatalysis for energy conversion. Of the catalysts studiedhere, rates correlate with ligand hydrophobicity, suggesting thatcatalyst-medium interactions are essential in determining cataly-tic properties. Interactions between the hexyl tails of 5 and thebutyl substituents of DBF and ½ðDBFÞH�þ are thought to pro-mote rapid protonation of the catalyst at endo positions, leadingto faster turnover. These results illustrate that control of protonmovement as a design principle extends beyond the catalyst itselfto encompass the catalyst, the medium, and their interactions.

Materials and MethodsThe sourcing and purification of commercially available chemicals, referencesfor known compounds, synthetic details for 5, and the instrumentation andmethods for routine NMR experiments, elemental analysis, and electrochem-istry are detailed in the SI Text. All voltammetry experiments were conductedat ambient temperatures (23–26 °C).

Cyclic Voltammetry of 5–10 in ½ðDBFÞH�NTf2. Addition of H2O. The Ni complexeswere dissolved in 1 mL of ½ðDBFÞH�NTf2 with stirring. In a representativeexperiment, a cyclic voltammogram was recorded (υ ¼ 0.1 V s−1) with 5(0.66 mM) and ferrocene (<5 mM) in ½ðDBFÞH�NTf2. Water was added in20–25 μL aliquots. After each addition, the solution was stirred briefly,and a cyclic voltammogram was recorded.

Diffusion NMR Experiments. The standard Varian stimulated echo PGSE pulsesequence was used (28). Experimental parameters included 4–64 scans, 4–7 ms gradient pulse lengths and diffusion delays of 0.01–1.0 s. Longer diffu-sion delays and larger gradient strengths were needed for the ionic liquidsamples due to the increased viscosity. The gradient strengths were variedfrom 0–20 Gcm−1. Diffusion measurements on the sample of 10 in½ðDBFÞH�NTf2 (χH2O ¼ 0.72) employed 19F nuclei due to the increased sensitiv-ity compared to 31P nuclei, which showed poor signal-to-noise ratios due tothe fast relaxation times and increased internal gradients (29). The 19F 90°pulse was 7.5 μs and relaxation delays of 20 s (10 × T1) were used. The tem-perature was maintained at 26 °C using an XRII852 Sample Cooler (FTSSystems).

Electrochemical Determination of D for 5 and 11 in MeCN (0.1 M NBu4PF6)and ½ðDBFÞH�NTf2 (χH2O ¼ 0.72). ½NiðdppbÞ2�ðBF4Þ2 (11; dppb ¼ 1; 2-bisðdiphenylphosphinylÞbenzeneÞ showed reversible Ni(II/I) and Ni(I/0) cou-ples in MeCN (0.1 M NBu4PF6) (30). In ½ðDBFÞH�NTf2 (χH2O ¼ 0.72), 11 showeda diffusion-controlled Ni(II/I) couple with E1∕2 ¼ −0.52 V (SI Appendix,Fig. S12). The Ni(I/0) couple was irreversible (Ep;red ¼ −0.8 V), but well sepa-rated from the Ni(II/I) couple. Steady-state linear sweep voltammetry andchronoamperometry experiments using solutions of 5 and 11 in MeCN(0.1 M NBu4PF6) with known concentrations gave D ¼ 6 × 10−6 and9 × 10−6 cm2 s−1, respectively. Chronoamperometry of 11 using a potentialstep from −0.3 to −0.7 V vs. Fcþ∕Fc gave D ¼ 2.4 × 10−7 cm2 s−1 for 11 in½ðDBFÞH�NTf2 (χH2O ¼ 0.72).

Open Circuit Potential Determinations in ½ðDBFÞH�NTf2 (χH2O ¼ 0 to 0.75). A1 mm platinum wire electrode was immersed in aqua regia for 30 min, rinsedin flowing 18 MΩ H2O for several minutes, then heated to an orange glow ina hydrogen/air flame, cooled in a stream of hydrogen, and transferred undernitrogen to the glovebox. Open circuit potentials were measured betweenthis electrode and a AgCl/Ag pseudoreference electrode containing MeCN(0.1 M NBu4PF6), separated from the analyte compartment by a Vycor frit.The analyte solution consisted of ½ðDBFÞH�NTf2 (0.6732 g, 0.498 mL) andferrocenium tetrafluoroborate (approximately 1 mg), sparged with H2 for10 min prior to measurement. The open circuit potential was recordedwith stirring for 30 s; the stirring was turned off, and the potential of thepseudoreference electrode vs. the Fcþ∕Fc couple was established voltamme-trically, using glassy carbon working and counterelectrodes in a three-elec-trode configuration. Hydrogen-sparged water was added in 10 μL aliquots,and the measurement sequence was performed twice after each addition. Ingeneral, the open circuit potential changed by less than 1 mV over 30 s.Chronopotentiograms are given in SI Appendix, Fig. S11, and plots of theopen circuit potential vs. [H2O] and lnð½H2O�Þ are given in SI Appendix,Fig. S12.

ACKNOWLEDGMENTS. This research was supported as part of the Center forMolecular Electrocatalysis, an Energy Frontier Research Center funded by theUS Department of Energy, Office of Science, Office of Basic Energy Sciences.Pacific Northwest National Laboratory is operated by Battelle for the USDepartment of Energy.

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