Recent progress in ligand-centered homogeneous electrocatalysts for hydrogen evolution reaction

Abstract

Electrocatalytic hydrogen evolution reaction (HER) is a critical process in a sustainable energy conversion system. Many metal complexes as small-molecule catalysts for electrochemical H₂ evolution have been reported over recent decades, which have made a great contribution to a detailed understanding of their catalytic mechanisms. In most reported mechanistic pathways, metal hydride species are always considered as key intermediates during H₂ formation, with the metal centre at the active site docking both electrons and protons. These metal-centred HER routes need to employ metal ions with open coordination sites, which can accommodate multiple redox states to involve two-electron hydride transfer for H₂ production. By comparison, an alternative approach, specifically focusing on redox-active ligand-centred electrocatalytic HER, has garnered increasing attention over the last six years. This review highlights recent progress in metal and metal-free ligand-centred homogeneous electrocatalytic HER since 2012 and discusses it according to two categories: organic ligand-centred and inorganic ligand-centred HER. In addition, some existing issues and suggestions for future directions in this catalytic area have been provided.

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Geng-Geng Luo

Geng-Geng Luo received his Ph.D. in Chemistry from Xiamen University in China in 2009. He studied for one year at the University of Rochester (USA) with Prof. Richard Eisenberg in the area of artificial light-harvesting and catalytic systems. Later on, he visited Prof. Zhang's group as a visiting scholar and conducted his research there for one year at Nanyang Technological University (Singapore). Currently, he is an associate professor at Huaqiao University and his research interests focus on coordination chemistry, supramolecular assembly and renewable energy catalysis.

Hai-Lin Zhang

Hai-Lin Zhang obtained her Bachelor of Engineering degree in Applied Chemistry from Huaqiao University in 2017. She is currently pursuing her Master of Science degree under the guidance of associate Prof. Geng-Geng Luo. Her current research interests are focused on molecular catalysis and photochemistry.

Yun-Wen Tao

Yun-Wen Tao obtained his B.S. in Biotechnology at the University of Electronic Science and Technology of China in 2014 and completed his Ph.D. on theoretical and computational chemistry in 2018 at Southern Methodist University, USA, under the supervision of Prof. Dieter Cremer and Prof. Elfi Kraka. He worked as a post-doc with Prof. Tamar Schlick at New York University on RNA design from June to December in 2018. His research interest covers method development and application of computational chemistry for deeper insight of chemical bonding and similarity.

Qiao-Yu Wu

Qiao-Yu Wu is a senior undergraduate student at Huaqiao University and is currently pursuing his Bachelor of Engineering degree in Applied Chemistry. Under the guidance of Dr Geng-Geng Luo, his main research interests are in the incorporation of water and CO2 reduction photo/electro-catalysts into molecular containers for solar fuel generation.

Dan Tian

Dan Tian received her Ph.D. in the field of Chemistry from Nankai University (China) in 2015. She then studied for two years at Nanjing Tech University in China with Prof. Ling Huang and Prof. Wei Huang in the area of lanthanide-doped upconversion MOFs and photophysical properties of solid-state boron-dipyrromethene dyes. She is currently a postdoctoral researcher working in collaboration with Prof. Qichun Zhang at Nanyang Technological University (Singapore). Her current interests are focused on the coordination chemistry and aggregation induced emission of small molecules.

Qichun Zhang

Qichun Zhang obtained his B.S. at Nanjing University in China in 1992, M.S. in physical organic chemistry (organic solid lab) at the Institute of Chemistry, Chinese Academy of Sciences in 1998, M.S. in organic chemistry at the University of California, Los Angeles (USA), and completed his Ph.D. in inorganic chemistry at University of California Riverside in 2007. Then, he joined Prof. Kanatzidis's group at Northwestern University as a Postdoctoral Fellow (2007–2008). In 2009, he joined the School of Materials Science and Engineering at Nanyang Technological University (NTU, Singapore) as an assistant professor. In 2014, he was promoted to associate professor with tenure, and later that year, he became an adjunct associate professor in the Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University. He received a TCT fellowship in 2013 and a lectureship from National Taiwan University in 2014. Currently, he is an associate editor of J. Solid State Chemistry, the International Advisory Board member of Chemistry – An Asian Journal, an Advisory board member of Journal of Materials Chemistry C, an Advisory board member of Materials Chemistry Frontiers, and an Advisory board member of Inorganic Chemistry Frontiers. Also, he has been a Guest Editor of Inorganic Chemistry Frontiers (2016–2017), Guest Editor of Journal of Materials Chemistry C (2017–2018), and Guest Editor of Inorganic Chemistry Frontiers (2017–2018). Currently, he is a fellow of the Royal Society of Chemistry and a highly-cited researcher for 2018. He has published more than 300 papers and 4 patents (H-index: 62).

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1. Introduction

The energy crisis and anthropogenic climate change related to the use of fossil fuels have signalled an urgent need to find carbon-neutral, clean, and renewable energy alternatives.¹ Hydrogen (H₂) has been considered as an ideal and environment-friendly molecular fuel due to its high energy density and its sole combustion product being water.² Although there are many ways to produce H₂, electrochemical hydrogen evolution reaction (HER) is known as a key enabling technology for large-scale H₂ production. In this regard, HER is deceptively a very simple reaction (2H⁺ + 2e⁻ → H₂), however, an efficient and durable catalyst is required to increase the reaction kinetics by lowering the high activation energy of H₂ generation, i.e., reducing the electrochemical overpotential.³

Besides natural biocatalysts in the form of hydrogenases (H₂ases), current efficient catalytic systems for electrochemical H₂ production include solid/heterogeneous and molecular/homogeneous ones. Heterogeneous catalytic systems are more difficult to study because the properties of catalysts highly rely on local variations in their surface morphology. By contrast, homogeneous molecule-type catalysts have become a widely-pursued research topic because their catalytic behaviours can be tuned at the molecular level to provide detailed mechanistic insights, which are difficult to obtain from the heterogeneous ones.

During the past twenty years, intense efforts have been put into the development of earth-abundant-metal based small molecule electrocatalysts for H₂ evolution, and several review articles with a particular emphasis on the structure–property relationship relevance to catalysis have been published.^4–6 Among all research on catalysed H₂ production, the mechanistic investigation on proton reduction catalysis is very important because it can provide us with some guidelines to design better molecular catalysts in the future.

In most cases, H₂ production catalysed by metal complexes is widely believed to happen through a common metal hydride intermediate with metal ([Mⁿ⁺]) ions at the reactive centre, formed by either consecutive or coupled proton and electron transfer, followed by two possible mechanisms for the formation of the subsequent H–H bond (Scheme 1). The first mechanism involves a homolytic or bimetallic route, where a metal–hydride species ([Mⁿ⁺–H]) can react with another metal hydride to release one H₂via reductive elimination. Alternatively, a heterolytic or monometallic pathway can also occur, where the metal hydride [Mⁿ⁺–H] is further reduced and protonated to evolve one H₂. In the latter pathway, two protons and two electrons are delivered to one single metal centre and a putative metal hydride complex ([M⁽ⁿ⁻¹⁾⁺–H]) is formed.⁴

Scheme 1 Proposed homolytic and heterolytic mechanisms for H₂ evolution via the formation of a metal–hydride intermediate. Note that M is designated as an active metal centre.

While deeply understanding a rich variety of metal–hydride-based catalytic mechanisms has led to significant advances in the design of efficient H₂-evolving molecular catalysts, these traditional metal-centred HER routes require the use of metal ions that should have available open coordination sites and are capable of accommodating multiple redox states for the two-electron hydride transfer for H₂ production.⁴ Thus, it is highly desirable to explore new alternative pathways in which metal hydrides are not necessary to enter the catalytic cycle.

In this respect, redox-active ligands beyond the reactivity of metals have received continuous attention. Initial interest in ligand hydride systems was partially inspired by biological reduction processes, where nicotinamide adenine dinucleotide phosphate or flavin adenine dinucleotide transfers ligand hydrides to an unsaturated substrate bound in a natural enzyme active site.⁷ Recent scientific efforts have been put into investigating redox-active ligand hydride systems and their utilization in catalytic reduction processes of C–H activation, water oxidation, alcohol oxidation, H₂ oxidation, and CO₂ activation, and in helping to mediate multielectron reactions.^8–11 However, to date, there are relatively few reports on the redox-active ligand-centred homogeneous catalysis of electrochemical H₂ production.

This review summarizes the research progress in the field of metal and metal-free ligand-centred electrocatalytic generation of H₂ within the past six years. We will describe two classes of ligand-centred H₂-evolving catalysis: organic ligand-centred and inorganic ligand-centred HER. Their electrocatalytic properties as well as structural and mechanistic features will be summarized and discussed in detail. Although the development of ligand-centred electrocatalytic H₂ evolution is not yet in the mainstream of current research in HER, we believe that this alternative approach might lead to more interesting discoveries and creative ideas in the design of better H₂-evolving molecular catalysts in the coming years.

2. Metal and metal-free organic ligand-centred electrocatalytic HER

2.1 Metal complexes with organic ligand-centred electrocatalytic HER

A rare-earth metal rhenium(III) complex with dithiolate ligand-centred electrocatalytic HER. The organic ligand 2-diphenylphosphino-benzenethiolate (dppbt) containing both phosphorus and sulfur as the donor atoms, initially reported by Dilworth et al.,¹² represents an archetypal class of redox-active, non-innocent ligands like dithiolenes. The group of Grapperhaus subsequently synthesized a neutral Re(III)-tris(thiolate) complex (1, in Fig. 1) containing three P,S-coordinated dppbt ligands.¹³ The coordination-saturated rare-earth Re(III) complex 1 exists in distinctly different oxidation and reduction states with two reversible non-innocent ligand oxidations and a reversible metal-centred reduction. Through the facile control of oxidation state by chemical or electrochemical methods, 1 was oxidized to form metal-stabilized thiyl radicals [1]⁺ and [1]²⁺. The cationic radicals [1]⁺ and [1]²⁺ make the sulfur centres more nucleophilic than the non-oxidized form 1. Thus, ethylene can be in a “locked on” or “locked off” state, or in dynamic equilibrium with 1 or its oxidized derivatives [1]⁺ and [1]²⁺via C–S bond formation/cleavage (Fig. 1).¹³

Fig. 1 Representation of 1 with different oxidation states and the corresponding ethylene (C₂H₄) binding adducts.

In view of the similarities of the frontier molecular orbitals between ethylene and molecular hydrogen, 1 was later studied by Grapperhaus et al. for electrocatalytic H₂ activation and formation.¹⁴ In the presence of base (triethylamine), 1 oxidizes H₂ in CH₂Cl₂ with a turnover frequency (TOF) of 4 ± 1 s⁻¹ and an overpotential of 0.67 V. When using weak acid (CH₃COOH) or strong acid (H₂SO₄) as the proton source, 1 reduces the protons to H₂ in CH₂Cl₂ with acid-independent rate constant of 32 ± 3 s⁻¹ and an overpotential of 0.38 V (Table 1). The rate constant (32 ± 3 s⁻¹) and overpotential are relatively low and remind us that catalysis at low overpotentials usually leads to low TOFs. A bulk electrolysis experiment of CH₃COOH (0.05 M) solution in CH₂Cl₂ at an applied potential of −1.8 V vs. Fc/Fc⁺ in the presence of 1 afforded a turnover number (TON) of H₂ evolution of 54 in a 6 h experiment with 73% faradaic efficiency. The rate of catalysis with a TON of 6 h⁻¹ is sustained over the 6 h electrolysis experiment, indicating the robustness of this electrocatalyst.

Table 1 Metal and metal-free redox organic ligand-based electrocatalytic HER^a

Catalyst	Redox-active organic ligand	Catalytic potential (Ep)^b	Proton source	Solvent	OP^c	k obs ^d	Ref.
a Conditions: GC working electrode. b Catalytic potential (Ep) of 1, 4 and 6–9vs. Fc/Fc^+; Ep of 2, 3 and 5vs. SCE. c For 1, 6–8, OP = overpotential = |Ep − E^θHA|; for 2–3, OP was determined from a bulk electrolysis experiment; for 5, OP = overpotential = |Ecat/2vs. NHE – 0.059pH|. d The observed rate constant (kobs) = TOF for 1, 6–8 was calculated in the acid-independent region according to the equation: ic/ip = {(2/0.4463) × [(RTkobs)/(Fv)]^1/2}, where ic is the steady state catalytic peak current in the presence of an acid and ip is the peak current measured in the absence of an acid. For 2–3, kobs = TOF was determined from a bulk electrolysis experiment; for 9, kobs was obtained from foot-of-the-wave analysis. e Not determined.
1	Diphenylphosphino-benzenethiolate	−1.7 V	CH3COOH	CH2Cl2	0.38 V	32 s^−1	14
2	Bis(imino)pyridine	−1.2 V	LutH^+	THF	0.78 V	19.4 s^−1	17
2	Bis(imino)pyridine	−1.2 V	DMAPH^+	THF	0.54 V	2.1 s^−1	17
3	Bis(pyrazolyl)pyridine	−1.26 V	[HNEt3]BF4	THF		4.85 s^−1	18
4	Porphyrin	−1.8 V	Benzoic acid	THF			19
4	Porphyrin	−1.37 V	Tosic acid	THF			19
5	Pyrazinedithiolate	−0.8 to −1.0 V	Acidic water	H2O	0.330–0.400 V		23
6	Bis(thiosemicarbazone)	−1.7 V	CH3COOH	CH3OH	0.756 V	1170 s^−1	25
6	Bis(thiosemicarbazone)	−2.3 V	CH3COOH	CH3CN	1.074 V	11 700 s^−1	25
7	Bis(thiosemicarbazone)	−1.7 V	CH3COOH	CH3CN	0.80 V	10 000 s^−1	26
7	Bis(thiosemicarbazone)	−1.9 V	CH3COOH	DMF	0.76 V	5100 s^−1	26
8	Bis(thiosemicarbazone)	−2.1 V	CH3COOH	CH3CN	1.43 V	1320 s^−1	25
9	Tetra(pentafluorophenyl) porphyrin	−1.3 V	Tosic acid	THF	1.02 V	0.528–0.891 s^−1	30

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To evaluate the catalytic reaction kinetics of 1, the rate law and H/D kinetic isotope effect (KIE) were measured. The rate law of the reaction displays a first-order dependence on the catalyst concentration and a second-order dependence on acid concentration, resulting in an overall third-order rate constant of 184 M⁻² s⁻¹. Reactions with deuterated acids display a large KIE of 9 ± 1, irrespective of deuterated acid (CD₃CO₂D and CF₃SO₃D). The similarities of the TON and KIE values in the strong and weak acids further support a common rate-determining step. The as-determined large KIE value suggests that the rate-determining step may be related to the release of H₂ with significant catalyst–hydrogen bond breaking occurring at the transition state. Metal hydrides are always formed or proposed in HER when catalysed by H₂ase enzymes and synthetic electrocatalysts. An inverse KIE with the values ranging from 0.54 to 0.57 was previously reported by Gray et al.,¹⁵ which was attributed to metal hydride formation via proton-coupled electron transfer in a rate-determining step. Compared with the inverse KIE values observed for metal hydrides, the relatively-high KIE value for 1 indicated a clearly different HER mechanism for H₂ evolution. Two hypothetical catalytic cycles have been proposed by Grapperhaus et al. for this electrocatalytic H₂ production by 1, namely the [ECCE] and [CECE] mechanism (with E standing for an electron transfer process and C relating to a chemical step, in this case protonation) (Scheme 2). Both mechanisms involve a key ligand–hydride as the hydrogen-evolving complex, which does not require metal hydride species and is suggested based on the similarities to the redox-regulated, ligand-centred binding of ethylene to 1.^14,16

Scheme 2 Proposed [ECCE] and [CECE] mechanisms for proton reduction catalysed by 1 = [ReL₃] (where L = dppbt (2-diphenylphosphino-benzenethiolate)) with a common ligand–hydride as a key species.

Main-group metal aluminium(III) complexes with bis(imino)pyridine and bis(pyrazolyl)pyridine ligand-centred electrocatalytic HER. In 2015, Berben et al. reported a molecular Al(III) bis(imino)pyridine complex [(^PhI₂P²⁻)Al(THF)H] (2 in Scheme 3, where ^PhI₂P²⁻ is a phenyl-substituted bis(imino)pyridine), which is found to promote H₂ evolution from protons.¹⁷ Complex 2 was an efficient solution electrocatalyst for H₂ production, with a TOF of 19.4 s⁻¹ and an overpotential of 0.78 V using 2,6-lutidinium acid (LutH⁺) as the proton source in THF solution. The authors noted that by using dimethylaminopyridinium (DMAPH⁺) as the proton source under identical conditions, the overpotential of electrocatalysis for 2 is decreased to 0.54 V with a TOF value ca. 3 times as low as that observed in the presence of LutH⁺. Bulk electrolysis on a glassy carbon electrode (GCDE) at −1.16 V vs. SCE in THF with 2 (0.3 mM) as the electrocatalyst and LutH⁺ (6 mM) or DMAPH⁺ (6 mM) as the proton source gives 70% and 85% faradaic yields with ∼5 and 0.6 TONs in 1.5 h, respectively. A [CCEE] mechanism for the reduction of protons to H₂ catalysed by 2 has been proposed (Scheme 3). The [CCEE] mechanism likely involves two protonation events at the ligand platform, followed by a two-electron reduction of the protonated ligand to liberate H₂ and regenerate 2. Notably, complex 2 is the first example of a non-transition-metal H₂-evolving molecular electrocatalyst via ligand-centred reduction and protonation events.

Scheme 3 A proposed [CCEE] mechanism for proton reduction catalysed by 2 with a doubly protonated intermediate.

In order to further design new Al complexes with redox-active ligands for HER, in another recent report, Berben's group used another Al(III) complex [(^iPrPz₂P)AlCl₂(THF)][AlCl₄] (3 in Scheme 4, where ^iPrPz₂P = bis(pyrazolyl)pyridine) with a redox non-innocent ligand ^iPrPz₂P as a molecular catalyst for the electrochemical production of H₂.¹⁸ Compound 3 exhibits a catalytic peak at −1.26 V vs. SCE in THF with salicylic acid or [HNEt₃]⁺ as the source of proton. Compound 3 was proposed to produce H₂ catalytically via the [EECC] reaction mechanism through ligand-centred HER (Scheme 4). The pathway involving two-electron reduction and single-ligand protonation of 3 affords a key intermediate [(^iPrHPz₂P⁻)AlCl₂], where each of the electron- and proton-transfer events is ligand-centred. Further protonation of the ligand–hydride [(^iPrHPz₂P⁻)AlCl₂] would formally close the catalytic cycle for H₂ production.

Scheme 4 A proposed [EECC] mechanism for proton reduction catalysed by 3 with a ligand–hydride as a key intermediate.

Although the above-proposed HER mechanisms by Al(III) complexes of ^iPrPz₂P and ^PhI₂P are based on ligand-centred proton and electron transfer, the former facilitates reduction before ligand protonation, while the latter is protonated twice before the reduction happens to liberate H₂. These results indicated that the control of ligand donor atom pK_a values can tune the HER mechanism of ligand-based hydride generation and transfer.

Transition-metal nickel(II) complexes with phlorin and pyrazinedithiolate ligand-centred electrocatalytic HER. In 2016, Nocera's group described a hangman Ni(II) metalloporphyrin (4, in Scheme 5), which has been investigated as a homogeneous electrocatalyst for HER from acidic solutions.¹⁹ In the absence of acid, the CV of 4 exhibited two reversible waves with E_1/2 = −1.37 V and E_1/2 = −1.99 V vs. Fc/Fc⁺, as well as a central irreversible couple with the peak potential E_p = −1.8 V. When titrated with a weak acid (benzoic acid, pK_a = 20.7),^20a the central irreversible wave became catalytic. In the presence of a strong acid (tosic acid, pK_a = 8.0),^20b a current enhancement at the first reversible wave was observed, which was indicative of an EC (reduction–protonation) sequence. On the basis of DFT calculations and simulated fits of electrochemical data, they revealed that the ligand could play a similar role to the metal centre, with electrons and protons stored in chemical C–H and O–H bonds of the ligand. The authors assumed that the key intermediate in H₂ evolution for 4 was not a traditional nickel hydride, but a nickel phlorin storing a formal hydride equivalent. Upon further ligand-based reduction, both the neutral and anionic phlorin species reacted with either weak or strong acids, respectively, to release H₂ (Schemes 5 and 6 for the weak and strong acid pathway, respectively). In both reactions, the C–H bond of the phlorin could be likened to the metal–hydride bond as a more conventional step to understand the H₂ release mechanism. It is worth noting that different metal contents in the hangman metalloporphyrin complexes might affect the catalytic HER mechanisms. A typical example is a cobalt analogue 4. The results from experimental and theoretical examination suggested that a traditional cobalt hydride was energetically more favourable than phlorin formation for electrocatalytic H₂ evolution by the hangman Co(II) metalloporphyrin.²¹

Scheme 5 Proposed mechanisms catalysed by 4 with weak benzoic acid.

Scheme 6 A proposed mechanism catalysed by 4 with strong Tosic acid.

Results from protein crystallography²² revealed that the active centre of [NiFe]-H₂ase contains a NiS₄ core (Fig. 2a), which has inspired the synthesis of many [NiFe]-H₂ase mimic complexes. Among them, Yamauchi and Sakai et al. recently reported that a Ni(II) pyrazinedithiolate complex [Ni(dcpdt)₂]²⁻ (5 in Fig. 2b, dcpdt = 5,6-dicyanopyrazine-2,3-dithiolate) having a NiS₄ core similar to the active site of [NiFe]-H₂ase can serve as an efficient molecular electrocatalyst for water reduction to generate H₂.²³ This bio-inspired molecular catalyst 5 was shown to effectively reduce protons in pH 4–6 buffered water with relatively low overpotentials (0.33–0.4 V) for HER. Moreover, in a 24 h controlled potential electrolysis at −0.95 V vs. SCE in an aqueous acetate buffer solution at pH 5, 5 was reported to catalyse H₂ evolution with a faradaic yield of ca. 100% and a TON of >2 × 10⁴ mol H₂ per mol cat on a GCDE, indicating its excellent durability for long-term use in electrochemical HER. A combined electrochemical and theoretical study suggested a possible catalytic mechanism involving a diprotonated one-electron-reduced species (i.e., [Ni^II(dcpdt)(dcpdtH₂)]⁻ or [Ni^II(dcpdtH)₂]⁻), which was a ligand-based reduction accompanied by protonation of the pyrazine donors formed at pH < 6.4, proposed as critical intermediates for electrocatalytic HER. The ligand-based proton-coupled electron-transfer pathway leads to electrocatalytic HER without applying the highly negative potential required to generate low-valent nickel intermediates.

Fig. 2 Chemical structures of the active site of [NiFe]-H₂ase (a) and nickel(II)-dithiolate complex 5 containing redox-active pyrazinedithiolate ligands (b) discussed in the text.

Transition-metal zinc(II) and copper(II) complexes with bis(thiosemicarbazone) ligand-centred electrocatalytic HER. The molecules known as bis(thiosemicarbazones) derived from 1,2-diones have been considered as a kind of versatile redox non-innocent ligand, and their tetradentate soft N₂S₂ coordination sphere is able to chelate low-valent transition metals, forming stable neutral complexes.²⁴ These complexes have proven appealing in HER since the non-innocent bis(thiosemicarbazone) ligands are easy to synthesize and their structure can be modified in multiple ways, allowing a rational design of the electronic structures and redox potentials of metal complexes. A zinc(II) complex (6, in Fig. 3), containing a redox active diacetyl-bis(N-4-methyl-3-thiosemicarbazide) moiety, was recently reported by Grapperhaus et al. to exhibit the homogeneous catalysis of electro-driven H₂ production.²⁵ Complex 6 electrocatalyzed proton reduction to form H₂ with a maximum TOF of 1170 s⁻¹ in CH₃OH and 11 700 s⁻¹ in CH₃CN at an overpotential of 0.756 V and 1.074 V by using CH₃COOH as the proton source. A bulk electrolysis experiment showed a TON of H₂ evolution of 37 in a 2.5 h experiment with a faradaic yield of 85% at a controlled potential of −1.7 V vs. Fc/Fc⁺ in a CH₃OH solution of 6 and CH₃CO₂H (12 mM) on a GCDE. Further mechanistic insights were obtained by the reduction of deuterated acid (CD₃CO₂D) substrates. Reactions with CD₃CO₂D display a small KIE of 1.2, which is distinct from the inverse KIE found by Nocera et al. and the large KIE observed by Grapperhaus et al.,^14,15 indicating a clearly different mechanism for H₂ evolution. The simulated kinetic and thermodynamic parameters revealed that the catalysis proceeds via ligand-centred processes involving the homocoupling of two neutral Zn(HL˙) radicals and the hetero-coupling of a neutral Zn(HL˙) radical with the cationic radical [Zn(H₂L˙)]⁺. Further DFT calculations elucidated protonation at the hydrazino nitrogen, which is favoured relative to other potential basic sites within 6. The formation of H₂ is realized via ligand hydride–proton coupling while avoiding traditional metal–hydride intermediates (Scheme 7).

Fig. 3 Chemical structures of zinc(II) and copper(II) complexes 6–7 with redox non-innocent bis(thiosemicarbazone) ligands discussed in the text.

Scheme 7 Possible catalytic pathways for HER by 6 = [ZnL] (where L = diacetyl-bis(N-4-methyl-3-thiosemicarbazide)) through hetero-coupling of a neutral Zn(HL˙) and cationic [Zn(H₂L˙)] + and homocoupling of neutral Zn(HL˙) radicals (blue arrows).

In addition to [ZnL], another Cu(II) complex containing the same diacetyl-bis(N-4-methyl-3-thiosemicarbazide) ligand (7, in Fig. 3) was also subsequently synthesized and assessed by Grapperhaus et al. as a H₂-evolving electrocatalyst.²⁶ Complex 7 exhibits a maximum TOF of 10 000 s⁻¹ in CH₃CN and 5100 s⁻¹ in DMF at an overpotential of 0.80 and 0.76 V, respectively. Controlled potential electrolysis confirmed 7 as a good electrocatalyst to produce H₂ with a minimum faradaic efficiency of 81% and TON as high as 73 while showing no sign of degradation over 23 h. The HER kinetics was probed using deuterated acid (CD₃CO₂D), demonstrating a KIE of 7.54, which is different from that of the Zn^II catalyst (KIE = 1.2).²⁵ A possible HER mechanism catalysed by 7 was examined by DFT computational studies. In the proposed mechanism, an initial chemical event involves the protonation of the hydrazino nitrogen on the ligand. This is followed by an electrochemical step assigned as a copper-centred reduction resulting in a d¹⁰ Cu(I) species [Cu(HL)] that is isoelectronic with the Zn(II) complex of bis(thiosemicarbazone) [Zn(HL)]⁺.²⁵ A second protonolysis of [Cu(HL)] occurs at the opposing hydrazino nitrogen of the ligand to yield [Cu(H₂L)]⁺. Further one-electron reduction of [Cu(H₂L)]⁺ leads to the formation of the H₂-evolving complex [Cu(H₂L)], through rapid intramolecular rearrangement. The overall [CECE] catalytic cycle for this process is given in Scheme 8.

Scheme 8 Plausible [CECE] mechanism for proton reduction by 7.

It has to be mentioned that the identity of metal ions may affect the type of HER mechanism. The impact of the metal is manifested in the initial protonation and reduction sites. For example, when the metal source was changed to Ni(II) or Co(II) ion, the corresponding metal complexes were also found to be efficient molecular electrocatalysts for H₂ production. However, in these cases, the HER mechanisms were proposed to be metal-centred, i.e. in the formation of metal hydrides, there is no ligand hydride as a key step in the proposed HER mechanism.^27,28

2.2 Metal-free organic molecules as molecular electrocatalysts for HER

In recent years, most research efforts on significant homogeneous catalysis have been concentrated on the development of catalysts using non-precious metals such as iron, cobalt, nickel, or molybdenum to produce H₂.^4–6 A HER catalytic system utilizing metal-free organic molecules as electrocatalysts is relatively uncommon. However, organic compounds as molecular catalysts may provide lower manufacturing costs, superior synthetic flexibility, and greater chemical stability. In fact, some simple organic molecules have been shown to be competent mediators and catalysts for electrochemical CO₂ reduction.²⁹ Thus, it is expected that organic compounds can also be employed as molecular catalysts for electrochemical H₂ production.

In 2016, Grapperhaus et al. investigated the free ligand diacetyl-bis(N-4-methyl-3-thiosemicarbazone) (8, in Fig. 4) towards its applicability for electrocatalytic HER.²⁵ This organic compound shows a catalytic peak at −2.10 V (vs. Fc/Fc⁺) in anhydrous CH₃CN with CH₃COOH as the proton source. Under acid-saturated conditions, 8 displayed the highest TOF of 1320 s⁻¹ with a high overpotential of 1.430 V. The high TOF of catalyst 8 comes at the price of a high overpotential (more than 1.4 V). In addition, the TOF value (1320 s⁻¹) of 8 is apparently lower compared to that of the corresponding Cu(II)-complex (10 000 s⁻¹) and Zn(II)-complex (11 700 s⁻¹).^25,26 A potential explanation for this behaviour was partially attributed to the Lewis acidities of Cu(II) and Zn(II), which can balance the charge of the anionic ligand, promote the protonation, and lower the reduction potential. Notably, this is the first example of a homogeneous and metal-free organic molecule acting as an efficient electrocatalyst for HER, which may provide a new direction towards the development of abundant organic compounds as H₂-evolving catalysts for the replacement of metals.

Fig. 4 Chemical structures of metal-free organic molecule-based H₂-evolving electrocatalysts (8–10) discussed in the text.

It is well-known that metal-free porphyrin compounds can exhibit rich multi-electron redox chemistry. Two basic nitrogen atoms of imine in the porphyrin core can potentially produce a diprotonated porphyrin species, indicating that free-base porphyrins can be electrochemically active. In 2018, Villagrán et al. reported a metal-free porphyrin catalyst, which is a similar and comparable electrocatalytic HER complex.³⁰ The free-base meso-tetra(pentafluorophenyl) porphyrin (9, in Fig. 4) was employed to electrocatalyze H₂ evolution using p-toluenesulfonic acid (HOTs) as the proton source in THF. The cyclic voltammogram (CV) on a GCDE of 9 in THF showed two reversible reduction waves with midpoint potentials of E_1/2 = −1.14 V and E_1/2 = −1.54 V vs. Fc/Fc⁺, assigned to the first and second reductions to produce the porphyrin radical anion [9]˙⁻ and the dianion species [9]²⁻, respectively. Upon successive addition of strong acid HOTs (pK_a (HOTs) = 8.7 in CH₃CN)³¹ as the proton source, the first reduction wave of 9 remains unchanged, while a new electrocatalytic current for the proton reduction appeared at the onset of the catalytic wave (−1.31 V), from which the overpotential for HER with 9 was determined to be 1.02 V. Controlled potential electrolysis of 9 at −1.31 V (vs. Fc/Fc⁺) on a GCDE was conducted over the course of 40 min, and more than 3.2 equiv. of charge was passed, which yielded H₂ at 90% faradaic efficiency. Foot-of-the-wave analysis was employed to measure the HER kinetics. The as-observed rate constant k_obs values of 0.528, 0.742 and 0.891 s⁻¹ at an acid concentration [H⁺] of 0.4, 0.9 and 1.2 mM were calculated from the slope of the linear region near the foot of the wave. Plotting k_obsversus [H⁺] gives a linear relationship, suggesting a first order dependence on [H⁺]. A KIE of 1.16 was observed when using deuterated HOTs. Interestingly, as a control, a second metal-free meso-tetraphenylporphyrin (10, in Fig. 4) was also evaluated as a HER electrocatalyst under the same experimental conditions. However, the achieved electrocatalytic property based on this porphyrin 10 was very low. The analyses of the shape of the CVs, spectroelectrochemical experiments, together with DFT computational studies, are consistent with the most favourable mechanistic process for HER in an [ECEC] sequence (Scheme 9). Although complex 9 is the first metal-free porphyrin molecule serving as an efficient H₂-evolving electrocatalyst, the disadvantages are that the HER occurs with a large overpotential (>1.0 V) and must be carried out using strong acid as the proton source.³²

Scheme 9 Proposed catalytic cycle by 9 for H₂ generation, following an [ECEC] mechanism.

3. Molybdenum-disulfide complexes for inorganic ligand-centred electrocatalytic HER

3.1 A molybdenum(IV)-disulfide complex for disulfide inorganic ligand-centred electrocatalytic HER in aqueous solution

In recent years, as one of the two-dimensional (2D) inorganic materials, molybdenum sulfide (MoS₂) has been identified as a promising material for non-precious heterogeneous electrocatalysis in the HER under acidic conditions.³³ However, the desired performance of MoS₂ is often limited by the density of active sites for the HER, which are thought to be localized to rare edges, where disulfide linkages or triangular MoS₂ units are exposed and can absorb H species with a small free energy while the bulk form is inert.³⁴ The use of biomimetic molecular electrocatalysts mimicking these rare edges is an attractive approach because these catalysts may retain elecrocatalytic activity at a small size and maximize the amount of active sites. Additionally, some major advantages of well-defined biomimetic catalysts include: (1) traditional molecular characterization techniques, such as MS, NMR spectroscopy, and single-crystal X-ray diffraction, can be employed to probe the local structures of catalysts; (2) they allow for a fine tuning of their catalytic properties through ligand modifications; and (3) these bio-inspired catalysts may provide detailed mechanistic insights into the active sites, which is difficult to achieve in nanoparticles.

Long et al. previously identified a Mo(IV)-oxo pentapyridine complex [(PY5Me₂)MoO]²⁺ (where PY5Me₂ = [2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine]) and its ability to electrocatalytically generate gaseous H₂ either from neutral pH water or from sea water.³⁵ A Mo^II/Mo^IV mechanism for HER catalyzed by the Mo^IV-oxo complex was proposed, in which a [(PY5Me₂)Mo(H₂O)]²⁺ species is responsible for the water splitting to release OH⁻ and H₂. Along these lines, the same group subsequently reported a well-defined Mo^IV-disulfide(S₂²⁻) pentapyridine complex [(PY5Me₂)MoS₂]²⁺ (11, in Fig. 5a and b), which is a highly-active yet stable molecular electrocatalyst for HER in acidic aqueous solution.³⁶ In an acetate buffer (pH = 3) on a Hg pool electrode, the CV of complex 11 shows an irreversible reductive process at −0.58 V vs. NHE corresponding to an overpotential of 0.4 V, which is consistent with catalytic proton reduction. A 23 h controlled potential electrolysis experiment of 11 at an overpotential of 0.78 V in aqueous acetate buffer (pH = 3) in a double-compartment cell generated H₂ at 100% faradaic efficiency. The effectiveness of this system is impressive: the lower-bound value for the TON calculated by using the bulk solution of electrocatalyst molecules afforded ∼3.5 × 10³ moles of H₂ per mole of catalyst, and the calculated upper-limit TON value reached 1.9×10⁷ moles of H₂ per mole of electrocatalyst, assuming a constant monolayer on the electrode. The molecular electrocatalyst 11 remains stable under these experimental conditions and its catalytic efficiency is comparable to those of the biocatalysts H₂ases.

Fig. 5 (a) Chemical structure of a molybdenum(IV)-disulfide pentapyridine complex 11 with an inorganic ligand platform (S₂²⁻) discussed in the text. (b) The single-crystal X-ray structure of 11. (c) A model of the 2D inorganic material MoS₂ highlighting the exposed triangular MoS₂ units on the edge sites.

It is important to note that complex 11 represents the first example of a side-on bound S₂²⁻ on Mo(IV) that mimics the structure of the proposed triangular MoS₂ active surface sites (Fig. 5c). It was expected that the findings from Mo^IV-S₂²⁻11 are encouraging because they have important implications for the further design of structural and functional biomimetic molecular catalysts of other layered transition-metal dichalcogenides (MoSe, WS₂, WSe, etc.), in the same way that bioinorganic chemists distil the structure and the reactivity of H₂ase enzymes by resembling the active core groups in metalloenzymes.

To provide further insight into the HER mechanism catalysed by 11, Dang et al. performed detailed DFT calculations to explore possible reaction intermediates and various proton reduction pathways.³⁷ For the classical HER mechanisms catalysed by well-defined molecular catalysts in acidic water, metal hydride intermediates are often invoked as key species in the context of H₂-evolving catalysts. However, the results from the theoretical calculations revealed that H₂ evolved through the traditional protonation of the metal centre to generate a molybdenum hydride is thermodynamically unfavourable. The preferred pathways are attributed to the formation of active sulfur hydrides via two- or three-electron reduction from the molybdenum(IV)-disulfide mimic 11 (Scheme 10). Moreover, theoretical calculations also suggested that two-electron and three-electron reduced sulfur hydride intermediates may react with a hydrated proton to generate H₂ with the help of water acting as a bridge for H₂ evolution from the intermolecular H⁺/H⁻ coupling between sulfur hydride complexes and hydrated protons.³⁷

Scheme 10 Two-electron reduction mechanism for 11-catalyzed HER.

3.2 Molybdenum(VI)-disulfide complexes with disulfide inorganic ligand-centred electrocatalytic HER in organic media

Inspired by the original MoS₂ biomimetic work of Long and Chang,³⁶ Wu et al. later successfully prepared and well-characterized a molybdenum(VI)-disulfide complex (12, in Fig. 6a) containing two redox-active S₂²⁻ inorganic ligands, which has been employed as a solution electrocatalyst for HER using CF₃CO₂H as the proton source.³⁸ The O=Mo(S₂)₂ moiety in 12 (see Fig. 6b for the crystal structure) mimics the terminal edges in MoS₂ that are active sites for HER. Different from the Mo^IV ion in 11, the oxidation state of the metal centre in 12 is +6. In the absence of acid, the CV of 12 in DMF showed an irreversible voltammetric response at −1.59 V vs. Fc/Fc⁺, assigned to two one-electron reduction processes, as suggested by constant-potential coulometry. Calculated charge density plots indicated that the reduction process is primarily S–S ligand-centred. The addition of the strong acid CF₃CO₂H (pK_a(CF₃CO₂H) = 6.0 in DMF)³⁹ to the solution of 12 in DMF resulted in an increase of the cathodic current at the near potential of the S–S reduction. In the acid-saturated regime, 12 exhibited a TOF of 95 s⁻¹ with an overpotential of 0.617 V. In a controlled potential electrolysis, complex 12 catalysed H₂ production at 83% faradaic efficiency on a GCDE. Further studies on two other high-valance Mo(VI)-disulfide derivatives (13 and 14, in Fig. 6a) revealed that simply varying substitutions at the para-position of 2,2′-bipyridine ligand affects the basicity of the S–S ligand, thereby changing the catalytic activities of H₂ production including the TOFs and overpotentials.³⁸ The TOF values of H₂ evolution increased from 227 to 246 s⁻¹ when the electron-donating group at the para-position of the 2,2′-bipyridine ligand was changed from MeO to t-butyl, with overpotentials varying from 0.806 to 0.86 V using CF₃CO₂H as the proton source in DMF solution. The electrocatalytic results are summarized in Table 2. The basicity of the S₂²⁻ ligands was affected by varying substitutions on the 2,2′-bipyridine, indicating that during the electrocatalytic reaction, the S–S inorganic ligand may be the initial site of protonation.

Fig. 6 (a) Chemical structures of molybdenum(VI)-sulfur complexes 12–14 with redox non-innocent inorganic sulfide scaffolds discussed in the text. (b) Representation of the single-crystal X-ray structure of 12 (all H atoms omitted for clarity).

Table 2 Molybdenum-disulfide complexes showing inorganic ligand-centred electrocatalytic H₂ production^a

Catalyst	Redox-active inorganic ligand	Catalytic potential (Ep)^b	Proton source	Solvent	OP^c	k obs ^d	Ref.
a Conditions: Hg pool working electrode for 11, and GC working electrode for 12–14. b Catalytic potential (Ep) of 11vs. NHE and Ep of 12–14vs. Fc/Fc^+. c For 11, OP = overpotential = |Eonvs. NHE – 0.059pH|; for 12–14, OP = overpotential = |Ep − E^θHA|. d k obs = TOF for 12–14 was calculated in the acid-independent region according to the equation: ic/ip = {(2/0.4463) × [(RTkobs)/(Fv)]^1/2}. ic is the peak current in the presence of an acid and ip is the peak current in the absence of an acid. For 11, kobs = TOF was determined from a bulk electrolysis experiment.
11	Disulfide	−0.58 V	pH 3, acetate buffer	H2O	0.40 V	280 s^−1	36
12	Disulfide	−1.63 V	CF3COOH	DMF	0.62 V	95 s^−1	38
13	Disulfide	−1.71 V	CF3COOH	DMF	0.86 V	246 s^−1	38
14	Disulfide	−1.77 V	CF3COOH	DMF	0.81 V	227 s^−1	38

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From the consideration of electrochemical measurements coupled with DFT calculations,^38,40 the electrocatalytic HER mechanism of the molecular MoS₂-mimicking complex 12 shown in Scheme 11 could be described as follows: (1) the reaction mechanism initially involves two one-electron reductions of the inorganic ligand S₂²⁻ in 12 to generate a subsulfide (S₂³⁻) species. The S₂³⁻ species is hypothesized to contain a special 2c/3e S–S σ half bond. Of note, the term ‘subsulfide’ and the corresponding S–S 2c/3e σ half bond were firstly coined by Berry.⁴¹ (2) The next step is the protonation of the S₂³⁻ ligand. The theoretical pathway showed that the second protonolysis occurs preferentially at the Mo=O moiety acting as a proton relay. (3) Protonated S₂³⁻ acts as the hydride and the protonated Mo=O acts as the proton source. The combination of the hydride and proton affords H₂ and the regeneration of 12. The overall catalytic cycle closes with an [EECC] mechanism (Scheme 11).

Scheme 11 A proposed catalytic cycle for H₂ generation by 12, following an [ECEC] mechanism.

4. Conclusions and outlook

In this review, we have summarized the fundamental aspects and recent advances of the metal and metal-free ligand-centred homogeneous electrocatalytic HER. Although reasonable progress has been made in this field of electrocatalysis, there still exist several issues to be addressed. For example, low catalytic turnover rates and large overpotentials required by most H₂-evolving molecular electrocatalysts (see Tables 1 and 2) are independent of the strength of the proton sources used. Typical examples are the metal-free organic molecular catalysts for electrocatalytic H₂ formation, which show relatively large overpotentials (exceeding 1.0 V). Another crucial problem is that most ligand-centred HER processes can only be conducted in organic media with the requirement of extra moderate and strong acids as proton sources. The use of organic solvents and extra acids for the electrochemical reduction of protons will impede these catalyst systems in future large scale applications.

While current research in catalytic H₂-evolving mechanisms is focused on the development of metal-centred HER, we believe that redox-active ligand-centred HER should not be overlooked. A few important directions in this field to be followed for the future design of HER catalysts are provided as follows: (i) homogeneous catalytic production of H₂ has long been dominated by transition-metal hydrides. In very recent advances, partly inspired by naturally biological systems, chemists have reported two purely organic molecules, which are potentially useful as electrocatalysts for H₂ production.^25,30 It is clear that there is a huge amount of unexplored potential for metal-free organic compounds in H₂ production catalysis. These metal-free hydride catalyst systems may be viable alternatives to transition-metal hydride catalysts because the catalytic activity and overpotential of organic molecules can be further tuned and improved by varying the electronic and/or structural parameters of organic compounds. Thus, the development of H₂-evolving catalysis without metals to allow the reaction to operate at a low overpotential might be an active topic in the coming years. (ii) As in the examples outlined above, metal and metal-free ligand-centred electrocatalytic HER is currently the subject of intensive investigations. By contrast, systems involving photochemical ligand-centred HER have received much less attention,⁴² and therefore it is an important issue to be investigated in the context of ligand-centred HER. (iii) Biomimetic molybdenum disulphide complexes have been performed as electrocatalysts for the HER. Therefore, another suggestion is to continue to design and synthesize structural and functional biomimetic molecular catalysts of other layered transition-metal dichalcogenides (MoSe, WS₂, WSe, etc.), in much the same way that one mimics the rare edges of MoS₂. (iv) Current mechanistic investigations of these ligand-centered proton reduction catalysis systems heavily rely on computational quantum chemical methods. However, the precise determination of mechanistic routes requires the isolation and characterization of some ligand hydride species. Therefore, apart from single-crystal X-ray structure determination of stable intermediates isolated from catalytic systems, a combination of spectroscopy and catalysis measurements should be performed for in situ identification of some unstable reactive ligand–hydride intermediates to gain more knowledge on the catalytic mechanism of these systems. It is our view that a better understanding of the mechanisms of H₂ formation and deep insight into the basic structure–function relationship of the ligand catalytic centre will aid the effective design of highly efficient catalysts based on ligand-centred HER.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to thank the National Natural Science Foundation of China (Grant No. 21641011, 21601086), the Natural Science Foundation of Fujian Province (Grant No. 2015J01053, 2015H0024) and the Natural Science Foundation of Jiangsu Province (BK20160994) for financial support of this work. Q. Z. acknowledges financial support from AcRF Tier 1 (RG 111/17, RG 2/17, RG 114/16, RG 8/16) and Tier 2 (MOE 2017-T2-1-021), Singapore. Q. Z. is also grateful for the support from the Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201833), Jilin University, China. G. L. also thanks the Provincial Innovative Foundation Project for undergraduates (201810385072).

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