Cobalt and nickel diimine-dioxime complexesas molecular electrocatalysts for hydrogenevolution with low overvoltagesPierre-Andre Jacques,a Vincent Artero,a,1 Jacques Pecautb, and Marc Fontecavea,c

aLaboratoire de Chimie et Biologie des Metaux, Universite Joseph Fourier, Grenoble, Centre National de la Recherche Scientifique, Unite Mixte de Recherche5249, CEA, DSV/iRTSV, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France; bLaboratoire de Reconnaissance Ionique et Chimie de Coordination; UniteMixte de Recherche Centre National de la Recherche Scientifique/Universite Joseph Fourier/CEA E3, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France;and cCollege de France, 11 place Marcelin-Berthelot, 75231 Paris cedex 05, France

Edited by Jean-Marie P. Lehn, Universite Louis Pasteur - ISIS, Strasbourg Cedex, France, and approved September 30, 2009 (received for review July 15, 2009)

Hydrogen production through the reduction of water appears tobe a convenient solution for the long-run storage of renewableenergies. However, economically viable hydrogen production re-quests platinum-free catalysts, because this expensive and scarce(only 37 ppb in the Earth’s crust) metal is not a sustainable resource[Gordon RB, Bertram M, Graedel TE (2006) Proc Natl Acad Sci USA103:1209–1214]. Here, we report on a new family of cobalt andnickel diimine-dioxime complexes as efficient and stable electro-catalysts for hydrogen evolution from acidic nonaqueous solutionswith slightly lower overvoltages and much larger stabilities to-wards hydrolysis as compared to previously reported cobaloximecatalysts. A mechanistic study allowed us to determine that hy-drogen evolution likely proceeds through a bimetallic homolyticpathway. The presence of a proton-exchanging site in the ligand,furthermore, provides an exquisite mechanism for tuning theelectrocatalytic potential for hydrogen evolution of these com-pounds in response to variations of the acidity of the solution, afeature only reported for native hydrogenase enzymes so far.

bio-inspired chemistry � cobaloxime � electrocatalysis �hydrogen evolution reaction � hydrogenase

Hydrogen production, through the reduction of water, ap-pears to be a solution to consider for the long-run storage

of renewable energetic resources (1). The hydrogen evolutionreaction (her) is apparently a very simple reaction but, as withmost multielectronic processes, it is a slow process that should becatalyzed. Nickel-based electrodes can be used for that aim instrongly alkaline media, but this technology results in averageenergetic yields (�50–70%), suffers from corrosion issues, andcan hardly be miniaturized due to the lack of polymer exchangemembrane (PEM) material that is stable under these conditions.Actually, technological research now focuses on PEM electro-lytic cells for which carbon-supported platinum nanoparticles aregenerally used as catalysts. This scarce and expensive noblemetal is, however, not a renewable resource. Thus, progress toa mature hydrogen economy depends on breakthroughs infinding new catalytic materials based on earth-abundant ele-ments (2). To that aim, the combination of a common electrodematerial with a coordination complex of a first-row transitionmetal, able to catalyze the reaction at a potential close tothe standard potential of the H�/H2 couple, seems attractive.Macrocyclic cobalt complexes are very competitive in thatrespect (3). In the past four years, we and others reported oncobaloximes—cobalt complexes containing bridged bidentateoximato ligands (see Scheme 1)—(4–7) as a new class of suchcatalysts with good catalytic stability and �250–300 mV over-voltage—the difference between the observed catalytic potentialand the thermodynamic H2/2H� � 2 e� equilibrium. To date,these catalysts are among the most efficient ones and, thus, arecurrently exploited for the construction of H2-evolving photo-catalytic devices that actually rival previously reported systems

using Pt nanoparticles as catalysts (8–12). However, despite thepseudomacrocyclic nature of their equatorial ligand, co-baloximes are prone to hydrolysis under acidic conditions:protonation at two oximato functions in the equatorial planeindeed results in the decomposition of the complex through thecleavage of the tetradentate macrocyclic ligand into two dioximeligands and suppression of the chelate effect. To date, only a fewof such catalysts have been reported (4–7), and there is an urgentneed to expand the series to facilitate their grafting on surfacesfor the construction of H2-evolving electrodes or to allow theirimplementation into original supramolecular light-harvestingdevices.

Here, we report on a new family of nickel and cobalt H2-evolving catalysts with stable tetradentate ligands providing adiimine-dioxime coordination sphere. Although the substitutionof imine for oxime residues allows keeping of the remarkableelectrocatalytic properties in terms of overvoltage and turnoverfrequencies (TOF), the tetradentate nature of the equatorialligand warrants a high stability against hydrolysis. Insights intothe catalytic mechanism are given with emphasis on the role ofproton relays in the second coordination sphere of the metalcenters.

Results and DiscussionScheme 1 shows the structures of the various compounds con-sidered in this study. A first series is based on the diimine-dioxime ligand, N2,N2�-propanediylbis(2,3-butandione 2-imine3-oxime), further abbreviated as (DOH)(DOH)pn (13, 14). Anew ligand, ((MOH)(MOH)pn), where the methyl oxime groupsin (DOH)(DOH)pn are replaced by hydrogen, has been pre-pared from E-pyruval-oxime and 1,3-diaminopropane accordingto the general procedure established by Marzilli et al. (14).Cobalt and nickel complexes of these two ligands with variousaxial ligands have been synthesized. We also prepared theBF2-linked analogues, [Co((DO)2BF2)pnBr2] and[Ni((DO)2BF2)pn](ClO4), starting from Co(OAc)2�4H2O andNi(OAc)2�4H2O, respectively, and carrying out the reaction indiethylether in the presence of excess BF3�Et2O. Alternatively,we observed that [Co((DO)2BF2)pnBr2] forms spontaneously

Author contributions: V.A. and M.F. designed research; P-A.J. and V.A. performed research;J.P. contributed new reagents/analytic tools; V.A. and M.F. analyzed data; and V.A. wrotethe paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Data deposition: The atomic coordinates have been deposited in the Cambridge StructuralDatabase, Cambridge Crystallographic Data Centre, Cambridge CB2 1EZ, United Kingdom(CSD reference nos. 738486 for compound [Co(MO)(MOH)pnCl2], 738487 for[Ni(DO)(DOH)pn] (ClO4), and 738485 for [Co(DO)(DOH)pnBr2]).

1To whom correspondence should be addressed. E-mail: vincent.artero@cea.fr.

This article contains supporting information online at www.pnas.org/cgi/content/full/0907775106/DCSupplemental.

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when [Co(DO)(DOH)pnBr2] is placed in the presence ofHBF4�Et2O in CH3CN. The X-ray structures of [Co(MO)(MO-H)pnCl2] (Fig. 1) and [Co(DO)(DOH)pnBr2] (Fig. S1) reveal anoctahedral environment around the cobalt ion with two axialhalide ligands located trans to each other. The same coordinationsphere is assumed for [Co((DO)2BF2)pnBr2], whereas the nickelion in [Ni(DO)(DOH)pn](ClO4) (Fig. S2) and[Ni((DO)2BF2)pn](ClO4) is in a square-planar configuration.

The nickel center in [Ni(MO)(MOH)pnCl] likely adopts asquare-pyramidal configuration as reported for the[Ni((DO)2BF2)pnI] analogue (15).

Table 1 summarizes the redox features of these diimine-dioxime compounds obtained from cyclic voltammetry measure-ments, together with those of two reported cobaloximes (4, 5).Cobalt complexes display two, successive, reversible, monoelec-tronic, electron-transfer steps assigned to CoIII/CoII and CoII/CoI

processes, respectively, as reported in ref. 14. The proton-linkeddiimine-dioxime cobalt derivatives are reduced to the CoI stateat potentials much more positive than that of H-linked co-baloximes, such as [Co(dmgH)2pyCl] and [Co(dmgH)2(H2O)2](dmg2� � dimethylglyoximato dianion). This shift indicates alower electron density on the metal center. This density alsodepends on the nature (H vs. Me) of the oxime substituent, and

Scheme 1. Molecular structures for the various Co and Ni diimine-dioxime complexes considered in this study. The structure of the cobaloxime complex[Co(dmgBF2)2L2] is represented in the Upper Right.

Fig. 1. X-ray structure of [Co(MO)(MOH)pnCl2] (50% thermal ellipsoids).Selected bond lengths (Å) and angles (°): Co-N(4) 1.8902(19), Co-N(1)1.8952(18), Co-N(3) 1.9140(17), Co-N(2) 1.9247(17), Co-Cl(1) 2.2373(6), Co-Cl(2)2.2400(6), O(1)-N(1) 1.331(2), O(2)-N(4) 1.324(2), O(1)-H(1O) 1.03(3), O(2)-H(1O) 1.43(3), O(1)…O(2) 2.443(2), N(4)-Co-N(1) 96.79(8), N(4)-Co-N(3)82.00(8), N(1)-Co-N(3) 178.77(8), N(4)-Co-N(2) 177.91(8), N(1)-Co-N(2) 82.04(7),N(3)-Co-N(2) 99.17(8), N(4)-Co-Cl(1) 90.78(6), N(1)-Co-Cl(1) 90.12(6), N(3)-Co-Cl(1) 89.65(6), N(2)-Co-Cl(1) 90.95(5), N(4)-Co-Cl(2) 88.64(6), N(1)-Co-Cl(2)89.58(6), N(3)-Co-Cl(2) 90.64(6), N(2)-Co-Cl(2) 89.62(5), Cl(1)-Co-Cl(2)179.32(3), O(1)-H(1O)…O(2) 169(3).

Table 1. Redox potentials of Co and Ni complexes (V vs. Fc�/0)determined from cyclic voltammetry measurements (100 mV�s�1)in CH3CN at a GC electrode in the presence of nBu4NBF4 (0.1mol�L�1)

Complexe E1/2 (MIII/MII) E1/2 (MII/MI)

�Co(DO)(DOH)pnBr2� �0.57 �1.11�Co(MO)(MOH)pnCl2� �0.65 �0.96�Co((DO)2BF2)pnBr2� �0.38 �0.84�Co(dmgBF2)2(CH3CN)2� (4)* �0.2 (irrev.cathodic) �0.93�Co(dmgH)2pyCl� �1.05 (irrev. anodic) �1.48�Co(dmgH)2(OH2)2� — �1.48�Ni(DO)(DOH)pn�(ClO4) — �1.22�Ni((DO)2BF2)pn�(ClO4) — �0.97�Ni(MO)(MOH)pnCl� — �1.04�Co(DO)(DOH)pn(PPh3)� �0.52 �0.82�Co(DO)(DOH)pnBr(PPh3)� �0.18 �0.94

*These values have been checked in the present study for correct referencetoward the Fc�/0 couple.

20628 � www.pnas.org�cgi�doi�10.1073�pnas.0907775106 Jacques et al.

[Co(MO)(MOH)pnCl2] displays the highest potential values.The introduction of one difluoroboryl bridge between the oximegroups results in a further 200–250-mV positive shift for bothCoIII/CoII and CoII/CoI processes.

The nickel derivatives only display one reversible monoelec-tronic redox process assigned to NiII/NiI. Redox potentials are

�100 mV more negative than the corresponding CoII/CoI de-rivatives, reflecting a higher electron density at the metal center.Here again, the introduction of a dif luoroboryl bridge in[Ni((DO)2BF2)pn](ClO4) is followed by a 250-mV positive shiftof the NiII/NiI wave.

Fig. 2 Top shows the cyclic voltammograms (CV) of[Co((DO)2BF2)pnBr2] recorded in CH3CN at a glassy carbonelectrode in the presence of increasing amounts of p-cyanoanilinium tetrafluoroborate (pKa � 7.6 in CH3CN) (16) asthe source of protons. A catalytic wave develops at potentialsclose to E°(CoII/I) (Tables 1 and 2) and corresponds to electro-catalytic hydrogen evolution with an overvoltage of �230 mV—the overvoltage can be simply obtained by subtracting thestandard potential for the H�/H2 couple (E0(H�/H2)) listed inTable 2 from the measured electrocatalytic potential (17). Thesame behavior, but with lower overvoltages (190 and 110 mV),is observed with weaker acids such as tosic acid monohydrate andanilinium tetrafluoroborate, respectively (see Table 2 and Figs.S3 and S4). In the case of trif luoroacetic acid (TFA) (Fig. S5),hydrogen apparently evolves with no overvoltage. Actually, theclassical determination of the standard potential of the H�/H2couple (17) does not take into consideration the homoconjuga-tion reaction (self-association of the acid with its conjugatedbase), leading to an underestimation of the overvoltage that canreach �100 mV under the conditions used. This mechanismindeed increases the effective acidity of these acids, in particularfor TFA (18).* In terms of overvoltages, [Co((DO)2BF2)pnBr2]is quite comparable to [Co(dmgBF2)2(CH3CN)2] (4, 19). Thecatalytic current enhancement is, however, slightly greater thanthat obtained with [Co(dmgBF2)2(CH3CN)2], for which an over-all catalytic rate constant of 770 mol�1�L�s�1 has been estimatedunder the same conditions (19).

Switching to [Co(DO)(DOH)pnBr2] as the catalyst (Table 2and Fig. 2 Middle and Bottom), hydrogen evolution occurs at apotential shifted anodically with regard to that of the CoII/CoI

couple—the stronger the acid, the larger the shift (Table 2, Fig.2 Middle and Bottom, and Figs. S6 and S7).

Regarding the nickel complexes, a similar anodic shift of thecatalytic wave with regard to the NiII/I process is observed, goingfrom BF2-linked to H-linked nickel complexes (Table 2 and Figs.S8–S11). Whatever the acid used, [Ni((DO)2BF2)pn](ClO4)catalyzes hydrogen evolution exactly at the potential of theNiII/NiI couple. Interestingly, [Ni(DO)(DOH)pn](ClO4) cata-lyzes H2 evolution from the weak acid, Et3NH� (pKa � 18.7), butat potentials more negative than that of the NiII/I couple(Fig. S8).

*However, a detailed analysis of this phenomenon, that renders E0(H�/H2) dependent onthe concentration of acid in solution, is beyond the scope of this article.

Fig. 2. Cyclic voltammograms of [Co((DO)2BF2)pnBr2] and[Co(DO)(DOH)pnBr2]. (Top) Cyclic voltammograms of [Co((DO)2BF2)pnBr2] (1mM) in the presence of p-cyano-anilinium tetrafluoroborate (0, 1, 3, 5, 10equiv). (Middle) Cyclic voltammograms of [Co(DO)(DOH)pnBr2] (1 mM) in thepresence of anilinium tetrafluoroborate. (Inset) Zoom on the CoIII/CoII process.(Bottom) Same catalyst, but p-cyanoanilinium tetrafluoroborate; glassy car-bon electrode–CH3CN solution containing 0.1 mol�L�1 [nBu4N][BF4]. Scan rate:100 mV�s�1.

Table 2. Electrocatalytic H2 evolution peak potentials (V vs. Fc�/0) in the presence of Co and Ni complexes measured at a GC electrodeby cyclic voltammetry (100 mV�s�1) in the presence of 3 equiv of various acids in CH3CN (nBu4NBF4 0.1 mol�L�1)

Acid p-Cyanoanilinium Tosic acid Anilinium TFA Et3NHCl

pKa 7.6 8.7 10.7 12.65 18.7E0(H�/H2) in CH3CN* �0.59 �0.66 �0.77 �0.89 �1.25�Co((DO)2BF2)pnBr2� �0.82 �0.85 �0.88 �0.89 —�Co(DO)(DOH)pnBr2� �0.78 �0.83 �0.97 �1.10 —�Ni(DO)(DOH)pn�(ClO4) �0.95 �1.01 �1.09 �1.20 �1.54�Ni((DO)2BF2)pn�(ClO4) �0.95 �0.94 �0.98 �1.00 —�Ni(MO)(MOH)pnCl� �0.97 † �1.0 �1.00 —�Co(dmgBF2)2(CH3CN)2� �0.80 �0.93 �0.89 �0.90�Co(dmgH)2pyCl� — — — — �1.48

*These calculated values do not take into account the homoconjugation reaction (self-association of the the acid with its conjugated base) that may increasethe effective acidity of these acids, in particular for tosic acid and TFA.

†The species is unstable in the presence of high concentrations of acid.

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Hydrogen evolution was confirmed by bulk electrolysis exper-iments performed in the presence of the various proton donorspreviously investigated. The results are reported in Table 3.Several 10ths of turnovers can be achieved with approximatelyquantitative faradaic yields in most cases using[Co(DO)(DOH)pnBr2] and [Co((DO)2BF2)pnBr2] as the cata-lysts, thus comparing well with the previously reported co-baloxime complexes (Table 3). The nickel compound [Ni(MO)-(MOH)pnCl] appears less stable with a lower faradaic yield andan obvious decomposition after 3 h in the presence of p-cyanoanilinium tetraf luoroborate. [Ni(DO)(DOH)pn](ClO4)decomposes more rapidly even in less-acidic solution. No hy-drogen could be detected in the course of electrolysis experi-ments when [Co(MO)(MOH)pnCl2] was used as the catalyst,although some modifications of its cyclic voltammograms couldbe observed in the presence of acid.

A striking difference of this series of diimine-dioxime cobaltcomplexes with the previously reported cobaloxime complexesresides in the large robustness of the former toward hydrolysisunder acidic conditions. The stability of millimolar solutions of[Co(DO)(DOH)pnBr2] and [Co(DO)2BF2)pnBr2] in the pres-ence of 30 equiv of p-cyanoanilinium tetrafluoroborate—thestronger acid in the above series—has been monitored spectro-scopically in CH3CN, and no evolution of either 1H NMR (inCD3CN) or UV-visible spectra could be noticed over two weeks.As a comparison, [Co(dmgBF2)2(CH3CN)2] was decomposedunder the same conditions with a half-life time of 15h (4).

Three issues can be addressed regarding the mechanism for H2evolution catalyzed by these compounds: (i) the oxidation stateof the active species initiating the catalytic cycle, (ii) the natureof the hydrogen evolution step (20), where two pathways, eitherheterolytic and monometallic or homolytic and bimetallic, mayaccount for hydrogen evolution (Scheme 2), and (iii) the specificrole of the bridging link, H vs. BF2, as far as the electrocatalyticpotential is concerned.

In the case of [Co((DO)2BF2)pnBr2], the observation of acatalytic wave developing very close to the CoII/CoI potential isan indication for hydrogen evolution from a CoIII-hydride in-

termediate formed by protonation of the electrochemicallygenerated CoI species (Scheme 2) (21). We calculated theequilibrium constants for both homolytic and heterolytic H2evolution routes from p-cyanoanilinium in CH3CN using themethodology developed by Spiro et al. (22) (SI Text and TableS1). As a conclusion of this analysis, heterolytic hydrogenevolution (rG0

hetero � �2 kJ�mol�1) is far less favorable andthus, less likely than the homolytic one (rG0

homo � �24kJ�mol�1). On that basis, the reactivity of this compound resem-bles that of [Co(dmgBF2)2(CH3CN)2] (rG0

hetero � 2.5 kJ�mol�1,rG0

homo � �33 kJ�mol�1).In the case of the H-linked complex [Co(DO)(DOH)pnBr2],

the situation is quite different because the electrocatalytic waveis observed at potentials more positive than that of the CoII/CoI

process. The thermodynamic constants determined in acetoni-trile indicate that the heterolytic route is much more favorable(rG0

hetero � �24 kJ�mol�1). Homolytic hydrogen evolution isalso thermodynamically possible, so that both mechanisms maycompete depending on the experimental conditions. This mech-anistic issue was addressed by studying the reaction of a cobalt(I)derivative with protons. For that purpose, the cobalt(I) com-pound [Co(DO)(DOH)pn(PPh3)] was prepared through thereduction of [Co(DO)(DOH)pnBr2] by NaBH4 in the presenceof one equivalent of PPh3 (23). The rotating-disk electrodesteady-state voltammogram of this blue cobalt(I) compound(�max � 650 nm in benzene) displays an anodic wave at �0.87 Vvs. Fc/Fc� (Epc � Epa � 90 mV) in CH3CN, corresponding to itsoxidation to the CoII state. Addition of an excess of p-cyanoanilinium bromide to a solution of this compound inCH3CN or CH2Cl2 was shown to generate one equiv of H2 percobalt complex within 15 min (Scheme 3), thus supporting thehypothesis that the CoI species is the active catalytic species.UV-vis, 1H NMR, CV, and rotating-disk electrode steady-statevoltammetry measurements show that the resulting solutioncontains the CoIII complex [Co(DO)(DOH)pnBr2], togetherwith free PPh3.

However, UV-visible spectroscopic monitoring of the reactionshows that a CoII species (�max � 470 in CH3CN) is quantitativelyformed during the first 30 s of the reaction and further evolvesto the final CoIII state. This observation thus supports a homo-lytic mechanism for H2 evolution yielding an intermediate CoII

species. We independently prepared this CoII compound byreacting (DOH)(DOH)pn, CoBr2�H2O, and PPh3 in acetone.The resulting compound displays reversible CoII/CoI and quasi-reversible CoII/CoIII processes in CH3CN, both at (Table 1)distinct from those observed for [Co(DO)(DOH)pnBr2] or[Co(DO)(DOH)pnPPh3], consistent with a [Co(DO)(DOH)-pnBr(PPh3)] composition. This species is stable in CH3CN, but

Table 3. Bulk electrolysis experiments for H2 evolution from various acids (50 mmol) catalyzed by Co and Ni complexes (0.5 mmol)carried out in CH3CN (nBu4NBF4 0.1 mol�L�1) at a graphite rod electrode

Catalyst Proton source Applied potential, V vs. Fc�/0 TON/3 h Faradaic yield, %

�Co(DO)(DOH)pnBr2� p-CN(C6H4)NH3(BF4) �0.78 40 92Tosic acid �0.83 4 50PhNH3(BF4) �0.97 27 90TFA �1.0 20 91

�Co((DO)2BF2)pnBr2� p-CN(C6H4)NH3(BF4) �0.82 20 100Tosic acid �0.85 15 100TFA �0.89 7 90

�Ni(DO)(DOH)pn�(ClO4) Et3NHCl �1.54 7 30�Ni((DO)2BF2)pn�(ClO4) p-CN(C6H4)NH3(BF4) �0.95 4 60�Ni(MO)(MOH)pnCl� p-CN(C6H4)NH3(BF4) �1.1 20 70�Co(dmgBF2)2(CH3CN)2� p-CN(C6H4)NH3(BF4) �0.92 50 �100�Co(dmgH)2pyCl� Et3NHCl �1.32 50 �85

The amount of evolved hydrogen has been determined through GC analysis.

Scheme 2. Possible mechanistic pathways for H2 evolution.

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readily reacts with p-cyanoanilinium, generating H2 (half anequiv) and the cobalt(III) complex [Co(DO)(DOH)pnBr2]. Wethus propose that the slow disproportionation of the CoII speciesgenerates the CoIII and CoI compounds, with the latter com-pound further reacting with protons and evolving H2 as de-scribed above.†

Finally, the anodic shift of the electrocatalytic wave withregard to the CoII/CoI potential in the presence of strong acidsis consistent with the formation of a protonated, and thus easierto reduce, catalytically active species. Protonation at theO–H…O bridge, yielding a species bearing two protonatedoxime ligands is likely. This hypothesis is further supported bythe following observations: (i) a careful examination of the cyclicvoltammograms of [Co(DO)(DOH)pnBr2] in Fig. 2 (see Inset)shows a slight shift of the CoIII/II system, when the acid concen-tration is increased. The shift of the CoIII/II couple is quite lowrelative to that of the electrocatalytic wave developing on theCoII/I process. This difference indicates that the more reducedthe species, the more displaced the protonation equilibrium,probably as a direct effect of the overall charge of the complex.(ii) The 1H NMR signal assigned to the oxime-bound protondisappears in the presence of anilinium, indicative of a fastexchange process. (iii) The {BF2}-linked cobalt complex cannotbe protonated at the oxime functions, engaged in covalent bondswith the boron atom. Accordingly this complex catalyzes hydro-gen evolution at the CoII/CoI potential strictly, with no effect ofthe strength of the acid used.

The presence of an H�-exchanging site in the catalyst providesthe catalyst with a mechanism to adjust its electrocatalyticpotential for hydrogen evolution to the acido-basic conditions ofthe solution and allows keeping the overvoltage for the reductionof acids within reasonable values over a wide range of pKas. Theprotonation of amine residues in the second coordination sphereof some H2-evolving catalysts has been used to lower theovervoltage (24–27) but, to the best of our knowledge, such aprogressive shift of the electrocatalytic potential, as a function ofthe strength of the proton source used, has never been observedfor a synthetic molecular catalyst. Similarly, hydrogenase en-zymes (28) have this ability to adapt their electrocatalyticpotential by modifying their surface charge through protonationand, thus, to catalyze H2/H� interconversion near the equilib-rium over a wide range of pH values (29). Interestingly, such aH�-exchanging site in close proximity to the catalytic metalcenter opens the possibility for intramolecular proton transfersteps along the catalytic cycle, enhancing turnover frequency.Noteworthy, DuBois and colleagues have shown how the pres-ence of a basic site in the second coordination sphere of acatalytic site, able to accommodate a hydride ligand, mayenhance its catalytic activity for hydrogen evolution (24, 25).

Whether such an open oxime-bridge is involved in the protontransfer steps along the catalytic cycle for hydrogen evolution(Scheme 4), promoting fast Co-H formation and/or acceleratingits protonation via a proton–hydride interaction, is a fascinatinghypothesis that needs to be further investigated.

ConclusionIn summary, we have identified a previously undescribed familyof molecular cobalt and nickel-based H2-evolving catalysts thatdisplays excellent catalytic properties (overvoltages, turnoverfrequencies) and greater stabilities than the previously reportedcobaloximes. These catalysts are thus promising candidates forthe molecular engineering of an H2-evolving cathode material orthe design of unique supramolecular H2-evolving photocatalyticsystems.

Materials and MethodsMethods and Instrumentation. NMR spectra were recorded at room tempera-ture in 5-mm tubes on a Bruker AC 300 spectrometer equipped with a QNPprobehead, operating at 300.0 MHz for 1H and 75.5 MHz for 13C. Solvent peaksare used as internal references relative to Me4Si for 1H and 13C chemical shifts.Elemental analyses were done at the service central d’analyse of the CentreNational de la Recherche Scientifique, Vernaison, France.

Electrochemical Measurements. Electrochemical analysis was done by using anEG&G potentiostat, model 273A.

The electrochemical experiments were carried out in a three-electrodeelectrochemical cell under nitrogen atmosphere, using glassy carbon workingelectrodes for cyclic voltammetry and a graphite rod for bulk electrolysisexperiments. The auxiliary electrode was a platinum grid. The referenceelectrode was based on the Ag/AgCl/KCl 3M couples. All potentials given inthis work are with respect to the ferricinium/ferrocene (Fc�/Fc) couple, whosepotential has been measured after each experiment by adding authentic Fc inthe cell. The experiments were conducted in acetonitrile with tetrabutylam-monium tetrafluoroborate, [Bu4N]BF4, as the supporting electrolyte. Addi-tions of acid were done by syringe from solutions of the acid in the sameelectrolyte.

For bulk electrolysis experiments, a tight cell was used, connected to aPerkin-Elmer Clarus 500 gas chromatograph equipped with a porapack Q80/100 column (6� 1/8) thermostated at 40 °C and a TCD detector thermo-stated at 100 °C. The platinum-grid counter electrode was placed in a separate

†As the CoIII-H/CoII-H couple should lie at a potential comparable and possibly more positivethan that of the CoII/Col couple, we cannot, however, definitely exclude that H2 evolutionoccurs in a heterolytic way. The reaction would proceed through protonation of a CoII-Hspecies, produced by reduction of the CoIII-H intermediate either at the electrode or fromreaction with bulk CoI.

Scheme 3. Stoichiometric hydrogen formation from a reduced CoI complex.

Scheme 4. Possible intramolecular proton transfer steps along the catalyticmechanism of H-linked dioxime H2-evolving catalysts.

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compartment connected by a glass-frit and filled with the electrolytic solution.During controlled-potential coulometry experiments, the cell was flushedwith nitrogen (5 ml�min�1), and the output gas was sampled (100 �l) every 2min and analyzed. Calibration was done by using a commercial electrodeloaded with platinum active layer (BASF LT140EWSI, platinum loading 0.5mg�cm�2).

Synthesis. (DOH)(DOH)pn, [Co(DO)(DOH)pnCl2] and [Ni(DO)(DOH)pn](ClO4)were prepared as described in refs. 13 and 30. [Co(DO)(DOH)pn(PPh3)] wasprepared in the glove box via a described procedure (23). Other chemicalswere used as received. Commercial acetonitrile for electrochemistry wasdegassed by bubbling nitrogen through it. The supporting electrolyte (n-Bu4N)BF4 was prepared from (n-Bu4N)HSO4 (Aldrich) and NaBF4 (Aldrich) anddried overnight at 80 °C under vacuum.

[Co(DO)(DOH)pnBr2]. [Co(DO)(DOH)pnCl2] was dissolved in water and NaBr (10equiv) was added. After 5 min, the green precipitate was filtered and washedwith Et2O.

[Co((DO)2BF2)pnBr2]. Co(OAc)2�4H2O (518 mg, 2.08 mmol) and BF3�OEt2 (2.63 ml,20.8 mmol) were added to a suspension of (DOH)(DOH)pn (500 mg, 2.08 mmol)in Et2O (30 ml). The mixture was stirred for 24 h at room temperature underargon. The brown precipitate was then collected by filtration, washed withEt2O, and dissolved in water (20 ml). Addition of excess KBr gave a greenprecipitate that was collected by filtration, washed with H2O and Et2O, anddried under vacuum (772 mg, 1.53 mmol). Yield: 73%. 1H NMR (300 MHz,acetone-d6): � 4.17 (m, 4H), 2.82 (s, 6H), 2.77 (s, 6H), 2.64 (m, 2H).

[Ni((DO)2BF2)pn](ClO4). Ni(OAc)�4H2O (207 mg, 0.83 mmol) and BF3�OEt2 (1.05ml, 8.03 mmol) were added to a suspension of (DOH)(DOH)pn (200 mg, 0.83mmol) in Et2O (20 ml). The mixture was stirred for 24 h at room temperatureunder argon. The precipitate was then collected by filtration, washed withEt2O, and dissolved in water (20 ml). Addition of excess NaClO4 gave an orangeprecipitate that was collected by filtration, washed with H2O (10 ml) and Et2O,and dried under vacuum (185 mg, 0.42 mmol). Yield: 50%. 1H NMR (300 MHz,acetone-d6): � 3.54 (t, 4H, J � 5.1), 2, 50 (s, 6H), 2.30 (s, 6H), 2.22 (m, 2H).

(MOH)(MOH)pn. Anti-pyruvic aldehyde-1-oxime (960 mg, 11 mmol) and 1,3-diaminopropane (460 �l, 5.5 mmol) were stirred in CH2Cl2 (30 ml) at 40 °C in

the presence of molecular sieves (4 Å). After 18 h, the molecular sieves wereremoved by filtration. The desired product (720 mg, 3.4 mmol, 43%) was thenprecipitated by addition of Et2O (50 ml), yielding a white powder collected byfiltration and dried under vacuum. 1H NMR (300 MHz, DMSO-d6): � � 11.57(broad, s, 2H), 7.54 (s, 2H), 3.44 (t, 4H), 1.96 (s, 6H), 1.92 ppm (m, 2H). 13C NMR(75 MHz, DMSO-d6): � � 163.61, 152.24, 49.57, 31.67, 13.58 ppm.

[Co(MO)(MOH)pnCl2]. CoCl2�6H2O (224 mg, 0.94 mmol) was added to a suspen-sion of (MOH)(MOH)pn (200 mg, 0.94 mmol) in acetone (20 ml). The mixturewas stirred for 3 h at room temperature and bubbled with air for 1 h. The greenprecipitate was then collected by filtration, washed with Et2O, and dried overvacuum, which gave 160 mg (0.47 mmol). Yield: 50%. 1H NMR (300 MHz,DMSO-d6): � 18.85 (s, 1H, Hox), 8.12 (s, 2H), 3.95 (t, 4H), 2, 61 (s, 6H), 2.35 (m, 2H).13C NMR (75 MHz, DMSO-d6): � 173.8, 148.8, 49.5, 27.2, 18.4. IR (KBr, cm�1):3441(broad), 1614, 1520.

[Ni(MO)(MOH)pnCl]. NiCl2�6H2O (112 mg, 0.47 mmol) was added to a suspensionof (MOH)(MOH)pn (100 mg, 0.47 mmol) in EtOH (25 ml). The mixture wasstirred for 3 h at room temperature. The brown precipitate was then collectedby filtration, washed with Et2O, and dried over vacuum (145 mg, 0.43 mmol).Yield: 91%. 1H NMR (300 MHz, D2O): � 8.11 (broad, s, 2H), 4.01 (bs, 4H), 2.00 (s,6H), 1.83 (bs, 2H). IR (KBr, cm�1): 3509, 3412, 2986, 1504, 1438.

Mechanistic Study. The blue [Co(DO)(DOH)pn(PPh3)] complex was dissolved inCH3CN (0.0123 mol�L�1) under argon. An excess of p-cyanoanilinium bromide (10equiv) was added and the reaction monitored using UV-visible spectroscopy.

The cobalt(II) complex formed within 30 s was synthesized independentlyin the glove box by reacting (DOH)(DOH)pn (200 mg, 0.88 mmol), CoBr2�H2O(288 mg, 0.88 mmol), and PPh3 (232 mg, 0.88 mmol) in acetone (20 ml). Thesolution was evaporated, and CH3CN was added. The resulting solution dis-plays an absorption maximum at 470 nm. An excess of p-cyanoaniliniumbromide (10 mmol) was added, and the reaction was monitored using UV-visible spectroscopy.

Crystallographic data (Table S2) and additional electrocatalytic data aregiven in the Supporting Information.

ACKNOWLEDGMENTS. The authors thank Carole Baffert and Mathieu Raza-vet for preliminary experimental studies, J. T. Muckerman (Brookhaven Na-tional Laboratory, NY) for stimulating discussions on catalytic mechanism, andacknowledge the French National Research Agency (ANR, Grant 07-BLAN-0298-01) for financial support.

1. Lewis NS, Nocera DG (2006) Powering the planet: Chemical challenges in solar energyutilization. Proc Natl Acad Sci USA 103:15729–15735.

2. Gordon RB, Bertram M, Graedel TE (2006) Metal stocks and sustainability. Proc NatlAcad Sci USA 103:1209–1214.

3. Artero V, Fontecave M (2005) Some general principles for designing electrocatalystswith hydrogenase activity. Coordin Chem Rev 249:1518–1535.

4. Baffert C, Artero V, Fontecave M (2007) Cobaloximes as functional models for hydro-genases. 2. Proton electroreduction catalyzed by difluoroborylbis(dimethylg-lyoximato)cobalt(II) complexes in organic media. Inorg Chem 46:1817–1824.

5. Razavet M, Artero V, Fontecave M (2005) Proton electroreduction catalyzed by co-baloximes: Functional models for hydrogenases. Inorg Chem 44:4786–4795.

6. Hu X, Brunschwig BS, Peters JC (2007) Electrocatalytic hydrogen evolution at lowoverpotentials by cobalt macrocyclic glyoxime and tetraimine complexes. J Am ChemSoc 129:8988–8998.

7. Pantani O, Anxolabehere-Mallart E, Aukauloo A, Millet P (2007) Electroactivity ofcobalt and nickel glyoximes with regard to the electro-reduction of protons intomolecular hydrogen in acidic media. Electrochem Commun 9:54–58.

8. Du PW, Knowles K, Eisenberg R (2008) A homogeneous system for the photogenera-tion of hydrogen from water based on a platinum(II) terpyridyl acetylide chromophoreand a molecular cobalt catalyst. J Am Chem Soc 130:12576–12577.

9. Probst B, Kolano C, Hamm P, Alberto R (2009) An efficient homogeneous intermolec-ular rhenium-based photocatalytic system for the production of H2. Inorg Chem48:1836–1843.

10. Fihri A, Artero V, Pereira A, Fontecave M (2008) Efficient H2-producing photocatalyticsystems based on cyclometalated iridium- and tricarbonylrhenium-diimine photosen-sitizers and cobaloxime catalysts. Dalton Trans 5567–5569.

11. Fihri A, et al. (2008) Cobaloxime-based photocatalytic devices for hydrogen produc-tion. Angew Chem Int Ed 47:564–567.

12. Li C, et al. (2009) Photochemical hydrogen production catalyzed by polypyridyl ruthe-nium–cobaloxime heterobinuclear complexes with different bridges. J OrganometChem 694:2814–2819.

13. Costa G, Mestroni G, de Savorgnani E (1969) Vitamin B12 model compounds – Cobaltchelates of bis(diacetylmonoxime-imino)propane 1–3. Inorg Chim Acta 3:323–328.

14. Gerli A, Marzilli LG (1992) Costa-type-B12 models – Synthesis, structural characteriza-tion, and electrochemistry of chloro derivatives. Inorg Chem 31:1152–1160.

15. Anderson OP (1981) Structural variations in macrocyclic complexes – The structure of(difluoro[3,3�-(trimethylenediimino)bis(2-butanone oximato)]borato)iodonickel(II),[Ni(cyclops)I]. Acta Crystallogr B 37:1194–1198.

16. Appel AM, DuBois DL, DuBois MR (2005) Molybdenum-sulfur dimers as electrocatalystsfor the production of hydrogen at low overpotentials. J Am Chem Soc 127:12717–12726.

17. Felton GAN, Glass RS, Lichtenberger DL, Evans DH (2006) Iron-only hydrogenasemimics. Thermodynamic aspects of the use of electrochemistry to evaluate catalyticefficiency for hydrogen generation. Inorg Chem 45:9181–9184.

18. Izutsu K (1990) Acid-Base Dissociation Constants in Dipolar Aprotic Solvents (BlackwellScientific, Oxford, UK).

19. Hu XL, Cossairt BM, Brunschwig BS, Lewis NS, Peters JC (2005) Electrocatalytic hydrogenevolution by cobalt difluoroboryl-diglyoximate complexes. Chem. Commun 4723–4725.

20. Chao TH, Espenson JH (1978) Mechanism of hydrogen evolution from hydridoco-baloxime. J Am Chem Soc 100:129–133.

21. Schrauzer GN, Holland RJ (1971) Hydridocobaloximes. J Am Chem Soc 93:1505–1506.22. Kellett RM, Spiro TG (1985) Cobalt(I) porphyrin catalysis of hydrogen-production from

water. Inorg Chem 24:2373–2377.23. Weakley TJR, Marks J, Finke RG (1994) The cobalt(I) coenzyme B12 model complex

[2,10-diethyl-3,9-dimethyl-1,4,8,11-tetraazaundeca-1,3,8,10-tetraene-1,11-diolato(1-)](triphenylphosphine)cobalt(I). Acta Crystallogr C 50:1690–1692.

24. Curtis CJ, et al. (2003) [Ni(Et2PCH2NMeCH2PEt2)2]2� as a functional model for hydro-genases. Inorg Chem 42:216–227.

25. Wilson AD, et al. (2006) Hydrogen oxidation and production using nickel-basedmolecular catalysts with positioned proton relays. J Am Chem Soc 128:358–366.

26. Capon J-F, Gloaguen F, Petillon FY, Schollhammer P, Talarmin J (2009) Electron andproton transfers at diiron dithiolate sites relevant to the catalysis of proton reductionby the [FeFe]-hydrogenases. Coordin Chem Rev 253:1476–1494.

27. Wang N, et al. (2008) A proton-hydride diiron complex with a base-containing diphos-phine ligand relevant to the [FeFe]-hydrogenase active site. Chem. Commun 5800–5802.

28. Fontecilla-Camps JC, Volbeda A, Cavazza C, Nicolet Y (2007) Structure/function rela-tionships of [NiFe]- and [FeFe]-hydrogenases. Chem Rev 107:4273–4303.

29. Vincent KA, Parkin A, Armstrong FA (2007) Investigating and exploiting the electro-catalytic properties of hydrogenases. Chem Rev 107:4366–4413.

30. Uhlig E, Friedrich M (1966) Nickelchelate des bis-(diacetylmonoxim-imino)-propan-1,3 und des bis-(diacetylmonoxym-imino)-athans-1,2. Z Anorg Allg Chem 343:299 –307.

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