Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes

Conspectus

Mimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a first-ranking target for artificial photosynthesis. Pursuing that goal requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. Cobalt complexes based on glyoxime ligands, called cobaloximes, emerged ten years ago as a first generation of such catalysts. They are now widely utilized for the construction of photocatalytic systems for hydrogen evolution.

In this Account, we describe our contribution to the development of a second generation of catalysts, cobalt diimine-dioxime complexes. While displaying similar catalytic activities as cobaloximes, these catalysts prove more stable against hydrolysis under strongly acidic conditions thanks to the tetradentate nature of the diimine-dioxime ligand. Importantly, H₂ evolution proceeds via proton-coupled electron transfer steps involving the oxime bridge as a protonation site, reproducing the mechanism at play in the active sites of hydrogenase enzymes. This feature allows H₂ to be evolved at modest overpotentials, i.e. close to the thermodynamic equilibrium over a wide range of acid-base conditions in non-aqueous solutions.

Derivatization of the diimine-dioxime ligand at the hydrocarbon chain linking the two imine functions enables the covalent grafting of the complex onto electrode surfaces in a more convenient manner than for the parent bis-bidentate cobaloximes. Accordingly we attached diimine-dioxime cobalt catalysts onto carbon nanotubes and demonstrated the catalytic activity of the resulting molecular-based electrode for hydrogen evolution from aqueous acetate buffer. The stability of immobilized catalysts was found to be orders of magnitude higher than that of catalysts in the bulk. It led us to evidence that these cobalt complexes, as cobaloximes and other cobalt salts do, decompose under turnover conditions where they are free in solution. Of note this process generates in aqueous phosphate buffer a nanoparticulate film consisting of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte. This novel material, H₂-CoCat, mediates H₂ evolution from neutral aqueous buffer at low overpotentials.

Finally, the potential of diimine-dioxime cobalt complexes for light-driven H₂ generation has been attested both in water/acetonitrile mixtures and in fully aqueous solutions. All together, these studies hold promises for the construction of molecular-based photoelectrodes for H₂ evolution and further integration in dye-sensitized photo-electrochemical cells (DS-PECs) able to achieve overall water splitting.

Introduction

The amount of solar energy reaching the Earth exceeds our societal needs by several orders of magnitude.¹ However, worldwide energy demand does not correlate with the availability of sunlight. Trapping energy in chemical bonds, by producing fuels, is the only way to storing at the terawatt scale. Such a solution has already been massively implemented by photosynthetic organisms which use sunlight to sustain their metabolism and produce biomass. Mimicking this natural process to produce solar fuels is the founding principle of a large field of research called “artificial photosynthesis”.² Solar-driven water-splitting and production of molecular hydrogen has been set as a first target in this context, in line with the promises held by H₂ as an energy vector. A related key challenge is the finding of new efficient and robust catalysts based on earth abundant elements for the reduction of protons into H₂.³ To design such catalysts, inspiration naturally arises from the dinuclear FeFe and NiFe active sites of hydrogenases (Figure 1),⁴ the metallo-enzymes achieving H⁺/H₂ interconversion both at fast rate and close to the thermodynamic equilibrium. A number of promising catalytic systems derived from this approach.⁵ Actually mimics of another important enzyme, the cobalt-containing vitamin B₁₂ (Figure 1), also called cobalamin, proved among the most efficient molecular electrocatalysts for H₂ evolution. In its super-reduced state (B_12sr) featuring a Co^I center, cobalamin is the most powerful nucleophile in Nature.⁶ Accordingly, cobalt bis-glyoximato complexes, largely developed by Schrauzer as B₁₂ mimics and known as cobaloximes (Figure 1),⁶ can be protonated in their reduced form to yield hydridocobaloximes or tautomeric forms, the structure of which is still under investigation.⁷ Such species actually turn to be the key intermediates in H₂ evolution catalysed by this class of compounds.⁸ Catalytic activity, initially reported in 1983 by Ziessel and coworkers in the context of light-driven H₂ evolution,⁹ was confirmed from 2005 by two independent electrochemical studies carried out in our laboratory¹⁰ and in the one of J. C. Peters at Caltech.¹¹ Further studies involved the groups of H. B. Gray^8b and R. Eisenberg.^8c For now, cobaloximes appear as one of the most active series of molecular catalysts when considering turnover frequencies and overpotential requirement, as evidenced by a recent benchmarking study in non-aqueous solutions.¹² However, these compounds suffer from degradation in acidic solutions, which limits their stability under turnover conditions. This prompted us to investigate the reactivity of another class of B₁₂ mimics. Cobalt diimine-dioxime complexes (Figure 1), mainly developed by Marzilli and coworkers in the 80’s,¹³ indeed contain single tetradentate equatorial ligands which are hardly displaced whereas hydrolysis of both oxime bridges in cobaloximes leads to the displacement (or exchange)¹⁴ of both glyoxime ligands. In this Account, we highlight the potential of this second generation of cobalt catalysts for H₂ evolution as an opportunity to design molecular-engineered electrodes and light-driven systems working in aqueous media.

Figure 1.

Structures of the active sites of NiFe and FeFe hydrogenases and of vitamin B₁₂ (R = H₂O, CN⁻, CH₃, adenosyl…), proton-bridged and difluoroboryl-annulated cobaloximes (L = solvent (H₂O, CH₃CN, DMF) and cobalt diimine-dioxime complexes 1X₂ and 2X₂ (X = Br, Cl…)

Synthesis

Diimine-dioxime ligands are synthetized in one step through Schiff base condensation of butanedione-monoxime on diamine compounds. These ligands are abbreviated as (DOH)₂xx where xx indicates the length of the hydrocarbon chain separating the two imine functions. Hence (DOH)₂pn (Figure 1) corresponds to N²,N²′-propanediylbis-butan-2-imine 3-oxime. Metalation readily occurs when the ligand is reacted with a metallic salt.¹⁵ Specifically, cobalt complexes rapidly react with oxygen from the air to yield octahedral Co^III derivatives such as [Co(DO)(DOH)pnX₂] (1X₂, Figure 1), in which the diimine-dioxime ligand occupies all four equatorial positions. The oximato oxygen atoms are engaged in a hydrogen bond. The bridging proton can be replaced by a difluoroboryl group, as in [Co((DO)₂BF₂)pnX₂] (2X₂, Figure 1), through reaction with BF₃·Et₂O.

Electrocatalytic activity for H₂ evolution

Black traces in figure 2 correspond to the cyclic voltammograms (CV) of 1Br₂ and 2Br₂ recorded in acetonitrile at a glassy carbon electrode.¹⁶ Two quasi-reversible monoelectronic processes are observed corresponding to the stepwise reduction of the Co^III complex to the Co^II and Co^I derivatives. X-ray absorption spectroscopies have shown that halide ligands are fully displaced by acetonitrile in the reduced Co^II and Co^I states.¹⁷ Upon reduction, the coordination number also decreases from 6 in the Co^III state to 5 in both the Co^II and Co^I state, a feature supported by DFT calculations.¹⁸

Figure 2.

Cyclic voltammograms of 1Br₂ (top) and 2Br₂ (bottom) (1 mM, black traces) recorded in CH₃CN (0.1 M nBu₄NBF₄) at a glassy carbon electrode (100 mV.s⁻¹) in the presence of p-cyanoanilinium tetrafluoroborate (red traces; 1, 3, 5 and 10 equiv.) and anilinium tetrafluoroborate (blue traces; 1, 3, 5 and 10 equiv.).

Electrocatalytic hydrogen evolution is evidenced in the CVs of 1Br₂ and 2Br₂ by the appearance of an irreversible wave that raises in height with increasing concentrations of proton sources such as anilinium or para-cyanoanilinium. H₂ production was quantified by gas chromatography coupled to coulometric monitoring, during bulk electrolysis experiments carried out at constant potential values corresponding to those of the electrocatalytic waves. In the case of the BF₂-annulated complex 2Br₂, this electrocatalytic wave develops at potentials corresponding to the Co^II/Co^I process, in line with the behavior of cobaloximes (Figure 2).^10-11

By contrast, in the case of the H-bridged complex 1Br₂ electrocatalytic H₂ evolution occurs at potentials positively shifted with regards to the Co^II/Co^I couple. The extent of this shift depends on the strength of the acid used as proton source as shown in figure 2. We hypothesized that proton transfer at the oxime bridge is involved in the rate determining step of the catalytic reaction. To further support this hypothesis, we consequently carried out computational studies at the DFT level in collaboration with M. J. Field at the Institute of Structural Biology in Grenoble.^18a Figure 3 displays a part of the mechanistic scheme that resulted from calculations in good agreement, within methodological errors,¹⁹ with electrochemically determined potentials of the redox processes (both measured and computed versus the Fc⁺/Fc couple), and reproducing the electrocatalytic behavior of H-bridged diimine-dioxime complexes. Point of entry of the catalytic cycle is a Co^II derivative. A subsequent proton-coupled electron transfer process leads to a Co^I compound containing a protonated oxime bridge, which turns to be the initial active state, denoted as Co^ILH. Hammes-Schiffer and coworkers reached the same conclusion on the basis of similar DFT studies.^18b The computed potential of the Co^II/Co^ILH couple appears significantly more positive than that of the Co^II/Co^I couple, consistent with the observed positive shift of the electrocatalytic potential. A second PCET step is then required to form a Co^II-hydride species bearing a protonated ligand, Co^IIHLH. Finally H₂ is produced, either through external protonation or through an intramolecular mechanism that would reproduce the formation of a dihydrogen bond,²⁰ as suggested to occur at the active sites of hydrogenases.^5a We have been able to locate a transition state for such an intramolecular process whose activation energy value is ~90 kJ.mol⁻¹, a value in agreement with other studies regarding H₂-evolving catalysts²¹ or enzymes.²² Formation of a H₂-bound species is suggested by theoretical computations. However, given the fact that H₂ elimination is entropically favored, it may not exist as a true intermediate in the catalytic mechanism.

Figure 3.

Possible pathways for catalytic hydrogen evolution, involving PCET processes, with DFT-optimized structures of the key intermediates. Axial ancillary ligands have been omitted for the sake of clarity.

Nonetheless, these calculations confirm that a Co(II)-hydride species is the active intermediate for H₂ evolution, in agreement with the mechanism initially proposed by Eisenberg,²³ and further supported by DFT studies by Muckerman²⁴ and Hammes-Schiffer²⁵ for parent cobaloximes. They also support a bridge-protonation pathway. Proton relays have already been shown crucial for promoting heterolytic H⁺/H₂ interconversion. The most popular examples are probably DuBois’s nickel bisdiphosphine complexes containing basic amino sites in the second coordination sphere.^5c Whether bridge protonation takes place in H₂ evolution mediated by cobaloximes is unclear and still under investigation.^{7b, 7d, e} In the present case, protonation of the oxime bridge allows diimine-dioxime complexes to adapt their electrocatalytic potential to the acid-base conditions of the medium, as shown in Table 1. Table 1 compares the potentials measured at half the catalytic wave and the equilibrium potential of the H⁺/H₂ couple in acetonitrile, computed from tabulated values and taking into account the homoconjugation effect as previously reported.²⁶ From these data and in first approximation, it was possible to estimate the overpotential value at the middle of the catalytic wave (such an estimation is more accurate with S-shaped voltammograms recorded at rotating electrodes),²⁶ by subtracting the standard potential of the H⁺/H₂ couple from the measured value. It follows that the overpotential requirement for H₂ evolution catalysis is kept within a 220-290 mV window over a wide range of acid-base conditions - i.e. in the presence of acids with pKas ranging from 7.6 to 12.6. To the best of our knowledge, such a behavior reproducing that of hydrogenases and platinum surfaces has never been reported for synthetic catalysts. In addition, these values compare well with those previously obtained for cobaloximes.^10-11 Current work in progress in our laboratory is devoted to the accurate determination of turnover frequencies, with the aim to establish “catalytic Tafel plots”, displaying the relationship between turnover frequency and overpotential, for a rational benchmarking of the catalytic activity.¹²

Table 1.

pKas and equilibrium potentials of the H⁺/H₂ couple in CH₃CN for various acids (10 mM), taking into account homoconjugation when applicable,²⁶ together with the half-wave potentials of the electrocatalytic wave corresponding to H₂ evolution mediated by 1Br₂ (data reported in refs ¹⁶ and ²⁶).

	p-cyanoanilinium	tosic acid	anilinium	trifluoroacetic acid
pKa	7.6	8.7	10.7	12.6
Eo(H+/H2)	−0.46 V vs Fc+/0	−0.48 V vs Fc+/0	−0.68 V vs Fc+/0	−0.68 V vs Fc+/0
E cat1/2	−0.68 V vs Fc+/0	−0.73 V vs Fc+/0	−0.92 V vs Fc+/0	−0.96 V vs Fc+/0

Assessment of catalytic activity in aqueous media

We found that diimine-dioxime complexes decompose under aqueous catalytic conditions. Actually, in pH 7 aqueous phosphate buffer they experience an electroreductive transformation into a nanoparticulate material which is deposited on the electrode as confirmed by SEM analysis (Figure 4A).²⁷ Small nanoparticles with average diameter of 10 nm (Figure 4A) are initially formed. Upon prolonged deposition time or when reduction is carried out at a more negative potential, a ~2 μm thick film made of larger particles (100 nm) is obtained. The same coating can be obtained starting from simple cobalt salts. On the basis of advanced spectroscopic measurements (XPS – Figure 4B, EDX, EXAFS and XANES) we could show that this novel material, called H₂-CoCat, consists of metallic cobalt coated with a cobalt-oxo/hydroxo/phosphate layer in contact with the electrolyte. Interestingly, H₂-CoCat mediates H₂ evolution from neutral aqueous buffer with onset potential, i.e. the potential at which we could analytically detect H₂ evolution using continuous-flow gas chromatography, of 50 mV vs RHE (Figure 4C). Remarkably, it can be converted upon anodic equilibration into the cobalt-oxide film catalyzing O₂ evolution described by Kanan and Nocera.²⁸ The switch between the two catalytic forms is fully reversible and corresponds to a local interconversion between two morphologies and compositions at the surface of the electrode.

Figure 4.

SEM image of an ITO electrode modified by electrolysis at −0.9 V vs Ag/AgCl of a solution containing 1Cl₂ (0.5 mM) in aqueous phosphate buffer (0.5 M, pH 7) for 1 h (0.1 C.cm⁻²_geometric) (A), XPS data (Co_2p, O_1s and P_2p cores) of a H₂-CoCat material deposited onto FTO substrate (blue traces) versus commercial Co₃(PO₄)₂·xH₂O (black traces) (B), and polarization of a FTO electrode modified with H₂-CoCat (blue dots) compared with that of a Co foil electrode (black dots) in phosphate buffer (0.5 M, pH 7) at 0.05 mV.s⁻¹ (C). Reproduced with permission from ref ²⁷. Copyright (2012) Nature Publishing Group.

This raises the general and important question of distinguishing true homogeneous catalysis by metal complexes from heterogeneous catalysis by metal or metal-oxide particles derived from in situ transformations. This situation, much more prevalent than currently appreciated, sometimes hard to detect and quite ubiquitous in catalysis, has been recently reviewed in the context of electro or photochemical processes.²⁹ As an example, H₃PO₄/H₂PO₄⁻ aqueous solution (pH 2.2) was previously reported suitable for the benchmarking of the catalytic activity for H₂ evolution of molecular cobalt catalysts including 1(H₂O)^{2+ 30a}. However, recent investigations in our laboratory evidenced that decomposition occurs under these conditions, accompanied with the deposition of an electrocatalytically active coating at the electrode.^30b Similar observations have also been made for cobalt complexes in non-aqueous media in the presence of strong acids.³¹

Molecular-based electrode materials

The results described above may seem disappointing, at least in the context of molecular chemistry. Ligand design allows for a fine tuning of the catalytic properties, but the inherent fragility of the resulting catalysts, when exposed to reducing/oxidizing and/or hydrolytic conditions, may condemn such an approach in the context of photoelectrochemical water splitting. In other words, is it worth developing molecular catalysts if they transform under turnover conditions into catalytically active species that can alternatively be obtained from simple metal salts? The answer is “yes” since we could evidence that immobilization onto electrode surface is a very efficient way to stabilize such molecular catalysts and avoid their degradation/transformation under catalytic conditions. To achieve this goal, we functionalized the hydrocarbon chain bridging the two imine functions to create a covalent link between these catalysts and multi-wall carbon nanotubes (MWCNTs) used as electrode material. We first prepared an azido derivative starting from 2-hydroxy-1,3-diaminopropane (Figure 5).³² Alternatively, malonic synthesis strategies proved fruitful to generate a series of ligands derivatized with different functional groups.³³ In our case, azide-alkyne coupling (AAC) allowed preparing 3Cl₂ (Figure 5) containing a terminal activated ester group. For that purpose, we used a strained cyclooctyne precursor enabling the use of copper-free AAC conditions³⁴ and thus avoiding substitution of copper for cobalt in the complex. In parallel, MWCNTs, deposited as a mat, were decorated with amino functions through the electroreduction of 4-(2-aminoethyl)benzene diazonium. This results in the coating of the electrode with a film of aminoethylphenylene oligomers as represented in figure 5.³⁵ Finally 3Cl₂ was covalently, thus durably, anchored on MWCNTs through the formation of amide linkages, as confirmed by XPS.

Figure 5.

Synthetic strategy for the preparation of 3Cl₂-functionnalized MWCNTs electrodes (DCC = dicyclohexylcarbodiimide).

Cyclic voltammograms measured on the modified electrode displayed two redox systems corresponding to the Co^III/Co^II and Co^II/Co^I couples (Figure 6) assigned by analogy with 3Cl₂. The first system is ill-defined but the Co^II/Co^I wave is reversible with peak intensities directly proportional to scan rate, confirming covalent immobilization of the cobalt complexes on the electrode surface. Surface concentrations as high as 4.5 nmol.cm⁻² could be determined from the integration of this monoelectronic system.

Figure 6.

Cyclic voltammogram of 3Cl₂ recorded in CH₃CN (0.1 M nBu₄NBF₄) at a MWCNT electrode (purple trace) and cyclic voltammogram of a 3Cl₂-functionalized MWCNTs electrode (blue trace) recorded in CH₃CN (0.1 M Et₄NCl). The response of the unmodified MWCNTs electrode is represented with a dashed black trace. Scan rate: 100 mV.s⁻¹.

The resulting material proved active for electrocatalytic H₂ generation at relatively low overpotentials from pure aqueous solutions ranging from pH 2.2 to 5, whereas no catalytic activity could be detected in pH 7 phosphate buffer solutions.³⁶ The onset potential for H₂ evolution was found to be 350 mV vs RHE in pH 4.5 acetate buffer (Figure 7).³² This material is also remarkably stable, allowing extensive cycling without degradation. This is underlined by a 7-hour electrolysis experiment, during which each immobilized cobalt center achieves more than 50 000 turnovers without significant deactivation. Electrochemical and XPS measurements confirmed preservation of the molecular structure (except for axial ligands that are rapidly exchanged in reduced states)¹⁷ of the grafted complex after catalytic turnover. SEM was also used to exclude the formation of any deposit, such as the H₂-CoCat material,²⁷ at the surface of MWCNTs.

Figure 7.

Linear sweep voltammograms (solid traces; left scale) of a 3Cl₂-functionalized MWCNTs electrode (blue trace) compared to unmodified MWCNTs material (black trace) in a pH 4.5 acetate buffer (0.1 M) recorded at 0.1 mV.s⁻¹ with simultaneous monitoring of H₂ production rate (dashed traces; right scale) using constant-flow continuous GC measurements.

Previous studies of 1Br₂ solubilized in non-aqueous media showed significant degradation after 50 turnovers.¹⁶ This clearly indicates that grafting provides a largely increased protection of these cobalt catalysts against modification during extensive cycling. A similar example has been reported by our group with DuBois’ nickel bisdiphosphine catalysts. Once immobilized on carbon nanotubes, they can turn over more than 100 000 times without loss of activity, nor formation of heterogeneous active metal species.^{37a, 37b} Though no detailed “post-mortem” analysis has been reported for cobaloxime or cobalt diimine-dioxime catalysts yet, a recent work on related diimine-dipyridine metal complexes³⁸ showed insightful details on possible deactivation pathways. Reduction at the metal site may generate transient carbon-based α-imino radical species subjected to reductive homocoupling, that finally yields dimeric complexes with new metal coordination spheres likely not suitable for catalysis. Our hypothesis is that once grafted onto electrode materials the molecular complexes are isolated one from each other and cannot react in such bimolecular manner. Another mechanism for deactivation includes ligand hydrogenation through hydride transfer from the Co^IIIHL species, either directly formed from the Co^I state or through isomerism of the Co^ILH species (Figure 3).³⁹ For complexes grafted onto electrodes, immediate reduction of this species is likely to compete with hydride transfer and to form the catalytically competent Co^II-hydride species, resulting in enhanced activity and stability.

We note however that covalent immobilization of a catalyst is not always as straightforward as a way to generate efficient electrocatalytic materials. This point arises from, at least, two reasons. Firstly, the linker introduced between the catalytic moiety and the surface often acts as an insulator, which results in the requirement of increased overpotentials for catalysis to occur.^{3, 40} In the course of a long-standing and fruitful collaboration with B. Jousselme and S. Palacin from the Matter Science Division of CEA (Saclay, France),^{27, 32, 35, 37} we could show that, when performed onto MWCNTs, the electroreduction of diazonium salts provides a quite unique way to graft a significant amount of redox probes on the electrode surface without introducing such electron transfer barrier.^35b Secondly, the structure of a molecular catalyst is significantly reorganized on completion of the catalytic cycle. This statement is particularly true for redox catalysts transitioning through various oxidation states. The immobilization mode must thus enable easy conformational changes. Grafting of diimine-dioxime complexes onto oxide surfaces have been described by J. C. Peters⁴¹ and E. Reisner.^{33a, 42} However, these immobilization modes, involving two anchoring groups, do not allow for H₂ evolution catalysis to occur at significant rate, which probably originates from constrains generated at the cobalt center upon grafting.

Photochemical systems

Cobalt diimine-dioxime catalysts also proved to be active for H₂ evolution under light-driven conditions through their association with molecular Ru-, Ir- or Re-based photosensitizers. Such homogeneous systems rely on sacrificial electron donors such as triethanolamine, triethylamine (TEA) or ascorbic acid. In the context of a collaboration with the group of M. Wang and L. Sun at Dalian Institute of Technology (China), we assessed the photocatalytic activity of 1Br₂ in mixed H₂O/CH₃CN conditions in the presence of TEA and a cyclometalated iridium-based photosensitizer.⁴³ Turnover numbers as high as 300 could be obtained after 4 h of visible light irradiation. Under the same conditions, the BF₂-annulated complex 2Br₂ only led to 50 TON, consistent with the inability of this complex to be protonated at the oxime bridge. The addition of triphenylphosphine resulted in a significant increase of the stability of the system with up to 700 TON achieved within 10 h (Figure 8). Phosphine ligands are known to stabilize low oxidation states of cobalt complexes that are the key intermediates in the H₂ evolution mechanism. Indeed, stabilization of the Co^I state of 1Br₂ by coordination of PPh₃ has been confirmed by cyclic voltammetry measurements with a ~280 mV anodic shift of the electrochemical potential of the Co^II/Co^I couple. In order to get more information on the reduced species generated during photocatalysis, we monitored the reaction using UV-visible spectroscopy: starting from the Co^III complex, irradiation leads after 10 – 20 s to a first absorption band at 472 nm, characteristic of a Co^II species. When PPh₃ is present in solution, a second band at 638 nm then appears after 70 s, corresponding to the exact signature of the Co^I complex [Co(DO)(DOH)pn(PPh₃)] (Figure 8). By contrast, in the absence of PPh₃, the resting state of the catalyst displayed another signature, characterized by two new bands centered at 602 and 678 nm. Similar spectroscopic signatures have been reported for reduced cobaloxime [Co(dmgBF₂)₂(H₂O)₂] ^{7e, 11b, 23, 44} and have been assigned to a spectroscopic Co^I state, a statement supported by ab-initio calculations.⁴⁵ However, a similar spectrum has also been calculated for a Co^II-H intermediate.²⁴ While more studies are still needed to fully characterize these intermediates, it seems that coordination of the π-acceptor phosphine ligand stabilizes the system by preventing side reactions, such as the above mentioned hydride transfer processes,^{39b, c} to occur. Similarly, a derivative of 1Br₂ containing a pendant methylpyridine ligand^33b showed increased activity under photochemical conditions.⁴⁶ These results highlight the relevance of ligand tuning for the optimization of such systems.

Figure 8.

Photocatalytic system based on 1Br₂ (top), H₂ evolution versus time (bottom - left) and spectroscopic signature of the resting state bottom - right) during continuous irradiation (in the absence (black traces) and the presence (blue traces) of 2 equivalents of PPh₃. The dashed blue trace shows the spectrum of authentic [Co(DO)(DOH)pn(PPh₃)].

Cobalt diimine-dioxime catalysts so far display lower activity under light driven conditions as compared to parent cobaloximes.^{8a, 8c} They however prove quite versatile as H₂ evolution catalysts as far as the reaction conditions are concerned: they function in pure organic solutions⁴⁷ as well as in fully aqueous conditions.^{46, 48} Not surprisingly, the optimal acid/base conditions in terms of activity in water (up to 90 TON) correspond to a pH of 4,^48a thus matching the optimal value determined for the molecular-based cathode material containing the same cobalt diimine-dioxime catalytic moiety.³²

Summary and Outlook

In the past five years, we have developed cobalt diimine-dioxime complexes as molecular catalysts for H₂ evolution. The tetradentate nature of the diimine-dioxime ligand warrants stability against hydrolysis, while letting an oxime bridge available to act as a proton relay. In addition, derivatization of this ligand at the hydrocarbon chain bridging the two imine functions enables its coupling with a surface, or possibly a photosensitizer, in a much more convenient manner than for the parent cobaloximes. Cobalt diimine-dioxime catalysts finally prove active for H₂ evolution in fully aqueous conditions both after immobilization onto electrode materials and in light-driven homogeneous conditions. These two properties hold promises for the construction of molecular-based photocathodes as key components of dye-sensitized photo-electrochemical cells.

Biographies

BIOGRAPHICAL INFORMATION

Nicolas Kaeffer received both a Chimie ParisTech engineer diploma and a master level in molecular chemistry from the Université Pierre et Marie Curie (Paris) in 2012. Since there, he works as a graduate student in CEA-Grenoble, on the construction of molecular cathodes and photocathodes for hydrogen evolution based on cobalt diimine-dioxime complexes, under the supervision of Dr Vincent Artero.

Murielle Chavarot-Kerlidou studied organic chemistry at the University of Grenoble (PhD 1998) and obtained a CNRS position (2002) to develop organometallic chemistry in Paris, from where she moved in 2009 to join CEA-Grenoble and work on hydrogen photoproduction.

Vincent Artero graduated from the Ecole Normale Supérieure (Ulm) and studied inorganic chemistry at the University Pierre et Marie Curie in Paris (PhD 2000). He moved to the CEA center in Grenoble (Life Science Division) in 2001 to develop bio-inspired chemistry related to hydrogen production and artificial photosynthesis. He received the “Grand Prix Mergier-Bourdeix de l’Académie des Sciences” (2011) and was granted in 2012 with a Starting Grant from the European Research Council (ERC).