A cobalt molecular catalyst for hydrogen evolution reaction with remarkable activity in phosphate buffered water solution

Abstract

Herein, we show that [Cp*Co(2-ampy)I]I (2-ampy = 2-aminomethyl-pyridine) is an extremely active catalyst for HER, exhibiting a TOF of 109 000 s⁻¹ in phosphate buffered water solution (pH 7). The key to this remarkable activity stems from the establishment of a network of weak interactions in the second coordination sphere. As a matter of fact, both experimental and theoretical studies strongly suggest that the –NH₂ functionality of the 2-ampy ligand acts as an anchoring and orienting group for H₂PO₄⁻ through the establishment of an intermolecular hydrogen bonding with it that, in turn, intermolecularly donates a proton to Co–H liberating H₂.

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The production of green hydrogen by renewable energy-powered water electrolysis is considered a viable solution to the problems of fossil solar-fuels depletion and the dramatic environmental consequences of their massive use.^21–24 Catalysis plays an essential role in the realization of an efficient electrolysis apparatus making it possible to minimize the energy expenditure required, close to that established by thermodynamics, for both reductive and oxidative processes.^25–29 With regard to the hydrogen evolution reaction (HER) significant advances have been achieved in the identification of molecular catalysts based on earth-abundant metals, such as iron,^30–32 cobalt,^33–38 nickel,^39–42 copper^43,44 and molybdenum.^45,46 Several key factors contributing to improve HER catalysis have been identified.^47–55 It has been understood that having a hanging functionality acting as proton source in the proximity of the metal active site is essential to facilitate the protonation of the M–H intermediate, thus leading to H₂ evolution (D–H⋯H–M, Scheme 1a). This is exactly what occurs in [Fe,Fe] or [Ni,Fe] hydrogenases,^56–59 which efficiently catalyze the reduction of protons to H₂ [turnover frequency (TOF) values up to ca. 10⁴ s⁻¹], where an –NH moiety of the thiolate bridging ligand is responsible for intramolecularly relaying the proton from the external source to the M–H fragment. This teaching of Nature inspired the development of many mono- and dinuclear complexes, having a basic group in the second coordination sphere, as structural and functional models of hydrogenase.^39,60–68 For instance, Rauchfuss and co-workers demonstrated that the presence of a pendant amine, in a structural model of [Fe,Fe] hydrogenase, enhances the HER rate.⁶⁹ DuBois and co-workers reported a nickel complex [Ni(PPh₂NPh)₂](BF₄)₂, (PPh2NPh = 1,3,6-triphenyl-1-aza-3,6-diphosphacycloheptane) with remarkable activity (TOF = 106 000 s⁻¹, in acetonitrile with 1.2 M of water) in HER attributed to the key role played by a pendant amine acting as intramolecular proton relay.³⁹ Analogously, Nocera and co-workers showed that the presence of a carboxylic acid hanging group of a cobalt “hangman” porphyrin facilitates HER by mediating, intramolecularly, a proton-coupled electron transfer (PCET) process.^66–68

Scheme 1 Second coordination sphere proton relay processes (a–c) facilitating HER catalysis (M = transition metal, D–H = proton donor, A–H = anchoring group).

Interestingly, there are now some pieces of evidence clearly indicating that HER reaction can be also facilitated by intermolecular protonation (D–H⋯H–M, Scheme 1b). Particularly, Bren and co-workers recently reported insightful studies on the effects of pK_a and buffer nature on the activity of cobalt-based catalysts.^70–72 They found an acceleration of HER rates of 2 to 4 orders of magnitude moving from water to buffered water solution, at the same pH, demonstrating an active role of the buffer as a proton donor. Sakai and co-workers attributed the remarkable activity of a cobalt catalyst with a pentadentate macrocyclic ligand in HER to the key role played by H₂PO₄⁻ derived from the phosphate buffer as a proton source.⁷³

Herein, we show that [Cp*Co(2-ampy)I]I (2-ampy = 2-aminomethyl-pyridine; 1, Scheme 2) is an extremely active catalyst for HER exhibiting a remarkable TOF of 109 000 s⁻¹ in phosphate buffered water solution (PBS, pH 7). Mechanistic studies suggest that the key to success of 1 lies in the occurrence of a crucial hydrogen bond network in the second coordination sphere: the –NH₂ functionality of the 2-ampy ligand acts as an anchoring and orienting group for H₂PO₄⁻ through the establishment of an intermolecular hydrogen bonding with it, which, in turn, donates a proton to the M–H group (D–H⋯A–H⋯H–M, Scheme 1c).⁷⁴ This hypothesis has been supported by i) contrasting the electrocatalytic performances of 1 to those of selected members of the novel family of potentially active [Cp*CoL₃] catalysts (Scheme 2), deliberately turning of or altering the crucial intermolecular hydrogen bonds by a proper choice of the bidentate ligand; ii) in depth quantum DFT calculations aimed at understanding the ligand effect on the catalytic performance implicitly taking into account the interactions with the buffer.

Scheme 2 Structure of cobalt complexes.

Novel complexes 1 and 2 were synthesized by the reaction of Cp*Co(CO)I₂ (ref. 75) with commercially available inexpensive 2-ampy and characterized both in solution and in the solid state (ESI†) (Scheme 3). Complexes 3 (ref. 76) and 4 (ref. 77) were reported by one of us and prepared as per the procedure described.

Scheme 3 Synthesis of cobalt complexes 1 and 2 (ORTEP diagram of complexes with thermal ellipsoids are shown at the 50% probability level, H atoms are omitted for clarity).

Complexes 1–4 were characterized electrochemically by cyclic voltammetry (CV) and square wave voltammetry (SWV) (Fig. 1). Each compound displays a redox event in the −0.2 and −0.5 V potential range attributed to the Co^III/II reduction. Peak current versus square root of scan rate is always linear, indicating a diffusional nature of the process (Fig. S2–S5†).

Fig. 1 a) CVs of 1–4 (500 μM) at 100 mV s⁻¹ scan rate and b) SWVs of 1–4 (500 μM) recorded in 0.1 M PBS at pH 7 under N₂.

As shown in Table 1, the half-wave potential (E_1/2) and redox behavior of the Co^III/Co^II couple of 1–4 are sensitive to the nature of the ligand coordinated at the Cp*Co moiety. 1–2 show a quasi-reversible reduction as indicated by the large peak-to-peak separation (ΔE_p), with a peak current ratio (i_ox/i_red) nearly one. Notably, a positive shift of 58 mV is observed when comparing the E_1/2 of 1 (−0.350 V) to that of 2 (−0.292 V). Considering the greater electron donation ability of methyl relative to hydrogen, a shift in the opposite direction should be observed suggesting that a different ligand is coordinated at Cp*Co(N,N) moiety in 1 and 2.

Table 1 Cyclic voltammetry data for 1–4

Complex	E 1/2 (V)	ΔEp (mV)	i ox/ired
1	−0.350	95.2	0.93
2	−0.292	88.2	0.89
3	−0.345	65.9	1.49
4	−0.375	64.1	1.01

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To validate this possibility, CVs and SWVs of 1 and 2 were recorded in a 0.1 M NaClO₄ (pH 7) electrolyte solution instead that in PBS. This led to an anodic shift of 30 mV of the Co^III/II redox potential of 1, while had no (or a negligible) effect on that of 2 (Fig. S6 and S7†), confirming the hypothesis that 1 and 2 in PBS differ in the nature of the third ligand coordinated at Co, which is most likely phosphate in 1 and water in 2. Consistently, a cathodic shift of E_1/2 was observed for 1 upon increasing pH (i.e. increasing the HPO₄²⁻/H₂PO₄⁻ ratio, Fig. S8†).^78,79 The Co^III/II redox couple of 3 and 4 are observed at E_1/2 = −0.345 (ΔE_p = 65.9 mV) and −0.375 V (ΔE_p = 64.1 mV), respectively, indicating the higher electron donor properties of the carboxylate ligand.

At more cathodic potentials, an additional irreversible wave, characterized by a significant current increase, was observed (Fig. 2). It was ascribed to the electrocatalytic hydrogen evolution reaction: 2H⁺ + 2e⁻ → H₂ (HER). H₂ production was confirmed by rotating ring disk electrochemistry (ESI†). 1 and 2 show similar onset potentials (−1.13 V and −1.11 V, respectively). For 2 the catalytic wave is preceded by an irreversible wave that might be ascribed to the transformation of the precatalyst into the active species or the formation of a stable catalytic intermediate.^78,793 and 4 exhibit a considerably more anodic onset potential of −0.88 V and −0.95 V, respectively. As shown in Table 2, the largest current increase (i_cat/i_d = 136, where i_cat is the maximum catalytic current and i_d is the cathodic peak current of the Co^III/II redox couple) was observed for 1, suggesting an extremely fast catalytic reaction, followed by 2 (i_cat/i_d = 77), 4 (i_cat/i_d = 15) and 3 (i_cat/i_d = 10). Remarkably, moving from 1 to 2, i.e. substituting a hydrogen atom with a methyl group, strongly affects the catalytic activity, causing a decrease of approximately one-half of the i_cat/i_d ratio.

Fig. 2 a) CVs of 1–4 (500 μM) at 100 mV s⁻¹ scan rate recorded in 0.1 M PBS at pH 7 under N₂. b) Enlarged view of Co^III/II redox region of the first CV of 1–4 (500 μM) recorded at 1 V s⁻¹ from 0.19 to −1.45 V in 0.1 M PBS at pH 7 under N₂ (asterisks denote the new oxidation features observed on the return sweep ascribed to catalyst degradation products).

Table 2 HER electrocatalytic data for 1–4

Complex	i cat/id	η(Ecat/2) (V)	TOF (s^−1)
1	136	1.034	39 000 ± 3000
2	77	0.964	16 000 ± 2500
3	10	0.635	900 ± 50
4	15	0.689	2000 ± 250

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Before conducting further mechanistic studies, we investigated whether the electrocatalytic activity of 1–4 was indeed ascribed to the homogeneous catalyst in solution, rather than to any electrocatalytically active species bound to the surface. The possible formation of any deposited species under catalytic conditions was evaluated by continuously recording 50 CV scans from 0.19 to −1.45 V at 1 V s⁻¹ scan rate on a 500 μM solution of catalyst. In all cases, the catalytic current at −1.45 V gradually loses intensity as new oxidation waves appear in the −0.7 to 0.2 V potential range (Fig. S13–S16†). This behaviour is consistent with the formation of new deposited species derived from partial catalyst decomposition. The intensity of the Co^III/II redox couple of 1–4 also decreased over the same period, further indicating that the surface composition of the glassy carbon electrode was changing. However, the rinse test, performed after 50 scans, clearly shows a negligible activity of the catalyst decomposition products. Controlled-potential electrolysis (CPE) measurements carried out at −1.35 V confirmed the rather short live of the catalytic systems, as well as the substantial inactivity of deposited species, since the recorded current abruptly decreases, reaching the one obtained in the absence of catalyst in less than 3 minutes (Fig. S17†). Exception made for complex 4, whose degradation somehow leads to a slightly active species (Fig. S17†). It is worth noting that a significant difference was found between 1 and 2–4 in terms of catalyst stability. In particular, the degradation of 2–4 is more pronounced than that of 1. This can be deduced by the noticeable appearance of new oxidative waves in the first CV of the solution of 2–4. Unlike 1, for which additional redox events due to catalyst decomposition are almost negligible (Fig. 2b). This result demonstrates that 1, not only is the most active catalyst, but also the one that is less prone to degradation. More importantly, it was found that such degradation processes are only relevant in those experiments for which the catalyst is exposed to reductive potentials for a substantial period of time. As a matter of fact, in CV experiments of 1 recorded at high scan rates (>6 V s⁻¹), i.e. where the time scale of the experiment is considerably reduced, no sign of degradation is observed (Fig. S18†). Under such conditions, pure catalytic regime is reached, allowing the TOF to be determined.

Considering the above results, to prevent that any deposited species negatively influences the current response of the studied complexes, only the first scan, recorded on a freshly polished electrode, was considered. Moreover, this work focuses only on the evaluation of the activity of the molecular complexes in solution and the related HER mechanism, not on long-term activity since in that case the formation of heterogeneous deposits on the electrode surface cannot be neglected.

In order to benchmark our catalysts, the HER overpotential, η(E_cat/2),^80,81 and TOF^82,83 of 1–4 were determined (ESI†). As summarized in Table 2, 1–2 operates at higher overpotentials (1.034 and 0.964 V, respectively) than 3–4 (0.635 V and 0.689 V, respectively). Catalysts with somewhat lower overpotential have been reported in the literature.^63,84–86 For 1, a remarkable TOF of 39 000 s⁻¹ was observed (Fig. S20†). Whereas TOF values of 16 000 s⁻¹, 900 s⁻¹ and 2000 s⁻¹ were obtained for 2, 3 and 4, respectively (Fig. S21–S23†). It can be hypothesized that the superior performances of 1 is due to the presence of the –NH₂ functionality that binds H₂PO₄⁻ anion, properly orienting it in a way to facilitate the proton transfer to Co–H (vide infra).^87,88 The significant lower activity of 2 compared to 1 is also consistent with such interpretation since having a –NHMe in place of –NH₂ clearly disfavors hydrogen bonding with H₂PO₄⁻, at least statistically. The higher activity of 4 with respect to 3 might be attributed to the greater ability of the former to undergo hydrogen bonding with H₂PO₄⁻, using the lone pairs of the coordinated carboxylic oxygen atom.

Interestingly, the Log(TOF) versus overpotential plot for 1–4 exhibits a nice linear trend (Fig. 3) suggesting that the same reaction mechanism is reasonably active and the structural differences of complexes systematically tune the free energies of intermediates and reaction steps of the catalytic cycle.^89–94 Our DFT computational results are in line with this hypothesis (vide infra).

Fig. 3 Plot of Log(TOF) as a function of catalyst overpotential.

Additional electrochemical mechanistic studies were carried out for 1 exhibiting the highest catalytic performances. TOF remains constant (TOF_mean = 42 000 s⁻¹) changing catalyst concentration in the 100–500 μM range (Fig. 4a), consistently with a first order on 1. By varying the pH of PBS from 5 to 6 does not significantly affect the activity of 1 (TOF = 49 000 ± 2000 s⁻¹, pH 5; TOF = 55 000 ± 4000 s⁻¹, pH 6) (Fig. 4b). However, a further increase of pH leads to a significant decrease of TOF (42 000 ± 2000 s⁻¹, pH 7; 2400 ± 250 s⁻¹, pH 8). This peculiar behavior suggests that H₂ evolution rate might be also influenced by the concentration of H₂PO₄⁻, whose concentration decreases as pH increases.^38,95–97 To corroborate this hypothesis, electrocatalytic experiments were performed at constant pH varying buffer strength. It was observed a marked increase of TOF with increasing buffer strength (Fig. 4c), up to a value of 109 000 ± 7000 s⁻¹, obtaining another strong hint that H₂PO₄⁻ plays an active role in hydrogen evolution, likely acting as proton source. Consistently, a significantly lower current was observed using NaClO₄ or K₂SO₄ as supporting electrolytes instead of PBS (Fig. 4d).

Fig. 4 Dependence of TOF on a) [1] (0.1 M PBS at pH 7 under N₂), b) pH ([1] = 200 μM, 0.1 M PBS under N₂), and c) [PBS] ([1] = 200 μM, pH 7 under N₂). d) CVs of 1 (200 μM) at 100 mV s⁻¹ scan rate using PBS (black trace), NaClO₄ (red trace) and K₂SO₄ (blue trace) as supporting electrolyte at pH 7 (0.1 M).

The experimental results reported above were complemented by extensive DFT computational studies for 1 (Fig. 5) and 3 (as reference, ESI†) aimed at providing insights into the mechanistic aspects of HER catalysis, focusing on understanding the intimate reason why 1 exhibit such a remarkable activity. Our results indicate that HER catalysis by 1 and 3 follows the same reaction mechanism (Fig. 5a and S33†), which is consistent with the observed scaling relationships for these catalysts. In the first step, [Co(III)–H]⁺ⁿ [where Co = Cp*Co(N,N)] forms through a PCET pathway by far at lower energy (+0.95 eV for 1 and +0.78 eV for 3) compared to the two-step reduction-protonation counterpart (+1.66 eV for 1 and +1.58 eV for 3). Importantly, the calculated energy associated to the formation of the hydride species of 3 (0.78 eV), which is responsible for the measured overpotential, is significantly lower than that of 1 (0.95 eV). This trend nicely correlates with the experimentally measured overpotentials of 1 and 3, which amounts to 1.03 V and 0.63 V, respectively. Once [Co(III)–H]⁺ⁿ is formed, the presence of the phosphate becomes crucial for the HER catalytic mechanism. It was found that the most favorable pathway for H₂ generation involves the coordination of H₂PO₄⁻ in the second sphere of [Co(III)–H]⁺ⁿ resulting in the formation of the key intermediate [Co(III)–H]⁺ⁿ(H₂PO₄⁻). It is in this context that the difference between 1 and 3 appear the most. The optimized [Co(III)–H]⁺ⁿH₂PO₄⁻ structures shown in Fig. 5b, reveal that while a strong Co–H⋯HOPO₃H dihydrogen bond^98,99 is present both in 1 (distance = 1.59 Å) and 3 (distance = 1.56 Å), a strong –NH₂⋯OPO₃H⁻ establishes in 1 (distance = 1.78 Å). The cooperativity of those two types of H-bonding in 1 contributes to a more facile association of H₂PO₄⁻ in the second coordination sphere in 1 (+0.08 eV) than in 3 (+0.29 eV), thus favoring hydrogen donation. Once H₂PO₄⁻ is in the second coordination sphere, the kinetic barrier to transfer a proton to Co–H is almost zero for both 1 and 3, and the increase of energy passing from [Co(III)–H]⁺ⁿ(H₂PO₄⁻) to [Co(III)–OH₂]⁺ⁿ and H₂, through a PCET pathway, is similar (+0.15 eV for 1, +0.12 eV for 3; ESI†).

Fig. 5 a) Proposed catalytic HER cycle for 1 and energetics at 0.0 V (blue) and 0.95 V (red). At potentials higher than 0.95 V, all steps involving an electron transfer from the electrode become exergonic. b) DFT-optimized structures for [1(III)–H]⁺(H₂PO₄⁻) and [3(III)–H](H₂PO₄⁻). Hydrogen-bond distances are reported in Å.

In conclusion, we have reported some molecular cobalt catalysts highly active in hydrogen evolution reaction. Particularly, complex [Cp*Co(2-ampy)I]I 1, whose second coordination sphere was rationally designed for having a –NH₂ moiety that attracts and orients H₂PO₄⁻ of the buffer, which, in turn, acts as proton donor to the metal hydride functionality, exhibits extremely high TOF (109 000 s⁻¹), importantly, working in water exclusively. To the best of our knowledge, only an organometallic [2Fe–2S] mimic of the active site of an [FeFe]-hydrogenase enzyme shows considerably higher TOF in water solution (250 000 s⁻¹).⁶⁵ Considering that 1 belongs to a broad class of compounds, we believe that a rational design of the ancillary ligands, not only in terms of desired electronic and steric features but also of proper functionalities in the second coordination sphere, will likely lead to the discovery of other catalysts with even better performances.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been funded by the European Union NextGenerationEU under the Italian Ministry of University and Research (MUR) National Innovation Ecosystem grant ECS00000041 VITALITY. BS acknowledge SERB (CRG/2020/001282) for to support this re-search. PD and PC thank CSIR and IITK for their fellowship. BS thanks Dr. Manoj K. Gangwar for initial study in isolation of these complexes. We acknowledge Università degli Studi di Perugia and MUR for support within the project Vitality, Progetti Fondo Ricerca di Ateneo 2021 and PON Ricerca e Innovazione DM 1062 (J91B21003250006). We also thank the Fondazione Perugia (project 21101, 2022.0405) for supporting our research.

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Footnote

† Electronic supplementary information (ESI) available: The authors have cited additional references within the supporting information.^1–20 CCDC 1939352 and 2297771. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cy00209a

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