Proton reduction using cyclopentadienyl Fe(II) (benzene-1,2-dithiolato) carbonyl complexes as electrocatalysts

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Highlights

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    Cyclopentadienyl diiron thiolate complexes as proton reduction catalysts.

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    CV studies showed sustainable proton reduction with TFA as the proton source.

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    DFT calculations were used to describe a plausible proton reduction mechanism.

Abstract

The UV photolysis of cyclopentadienyl iron dicarbonyl dimer [CpFe(CO)2]2 and benzene-1,2-dithiol or 3,6-dichloro-1,2-benzenedithiol afforded dinuclear iron complexes [Cp2Fe2(CO)(bdt)(μ-CO)] (1) and [Cp2Fe2(CO)(Cl2-bdt)(μ-CO)] (2) respectively (bdt = benzene-1,2-dithiolato, Cl2-bdt = dichloro-1,2-benzenedithiolato). Further oxidation of the two complexes resulted in the release of CO and generated [CpFe(bdt)]2 (3) and [CpFe(Cl2-bdt)]2 (4). All four complexes were found to catalyse proton reduction at a similar overpotential and rate when trifluoroacetic acid (TFA) was used as a proton source. Both experimental and computational studies lent support to a mononuclear iron intermediate species carrying the CpFe(bdt) or CpFe(Cl2-bdt) moiety acting as the catalyst in the proton reduction process.

Graphical abstract

Four diiron complexes [Cp2Fe2(CO)(bdt)(μ-CO)], [Cp2Fe2(CO)(Cl2-bdt)(μ-CO)], [CpFe(bdt)]2 and [CpFe(Cl2-bdt)]2 have been prepared and found to catalyse proton reduction from trifluoroacetic acid.

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Introduction

The generation of hydrogen from water is often regarded to be the ideal clean energy source as opposed to hydrocarbon fuels [[1], [2], [3]]. The catalytic electrochemical reduction of protons, one of the two key steps in water-splitting [[4], [5], [6]] has been the subject of much investigation. Although platinum has been found to be a highly efficient catalyst [7,8], extensive studies are still being conducted with the aim of discovering much lower cost catalysts containing earth-abundant metals such as iron and nickel [[8], [9], [10]].

Many studies have been carried out on iron complexes as the catalyst since it was discovered that the [FeFe] hydrogenase [11,12] catalyzes proton reduction close to the thermodynamic potential. Many mimics or model complexes, including a series of dithiolato diiron complexes [Fe2(SR)2L6] (L = CO, CN, phosphines, or a combination of these ligands) have been developed with varying proton reduction performance in terms of the overpotential and catalytic efficiency [[13], [14], [15], [16], [17], [18]]. However it is also worth exploring the performance of other classes of iron complexes in an effort to search for an even more efficient and robust catalyst [[22], [23], [24], [25], [26]].

As part of our interest in using iron complexes as proton reduction catalysts, we have prepared and characterised cyclopentadienyl iron (CpFe) systems containing benzenedithiol (bdtH2) ligands in this work. The benzenedithiol ligand has often been used as a chelating ligand to form stable mononuclear and dinuclear metal complexes [[27], [28], [29], [30], [31]]. One of the main functions of the ligand is to bring two iron centers together to form a dinuclear iron complex which bear some resemblance to the [FeFe] hydrogenase. Simple mononuclear iron dicarbonyl complexes bearing cyclopentadienyl ligands such as CpFe(CO)2 (thf)+BF4− have also been reported as functional proton reduction catalysts [[32], [33], [34], [35]]. However to the best of our knowledge, we have not found examples of CpFe complexes containing the benzenethiol ligands acting as proton reduction catalysts.

Therefore two diferrous complexes [Cp2Fe2(CO)(bdt)(μ-CO)] (1) and [Cp2Fe2(CO)(bdt)(μ-CO)] (2), and two diferric complexes [CpFe (bdt)]2 (3) and [CpFe(Cl2-bdt)]2 (4) have been synthesized via UV–Vis photolysis of [CpFe(CO)2]2 and the corresponding benzenedithiol ligand. The performance of these complexes as proton reduction catalysts have been investigated using cyclic voltammetry (CV), in the presence of trifluoroacetic acid (TFA) as the proton source in acetonitrile (MeCN). A mechanism has also been suggested based on our experimental data and supported by density functional theory calculations.

Section snippets

Results and discussion

Complex 1 was synthesized via UV–Vis photolysis of [CpFe(CO)2]2 with one equivalent of 1,2-benzenedithiol in toluene. Complex 2 was prepared using 3,6-dichloro-1,2-benzenedithiol as the ligand instead. Interestingly, complexes 3 and 4 could be prepared using the same method but using two equivalents of the ligands instead. We propose that the reactions proceeded via a radical mechanism initiated by the cleavage of the precursor dimer [CpFe(CO)2]2, upon irradiation of light [[36], [37], [38]].

Conclusions

Four diiron complexes 1–4 containing cyclopentadienyl and benzenedithiolato ligands have been prepared and characterised by infrared and NMR spectroscopy and mass spectrometry. Single crystal X-ray diffraction studies have been carried out on complexes 1 and 4. All four complexes have been found to exhibit catalytic activity for proton reduction with TFA acting as the proton source in acetonitrile. The overpotential for each complex has been determined to be in the range of 0.6 V. Using

Chemicals

Cyclopentadienyl iron (II) dicarbonyl dimer [CpFe(CO)2]2, 1,2-benzenedithiol C6H4(SH)2, 3,6-dichloro-1,2-benzenedithiol C6H2Cl2(SH)2, tetrabutylammonium hexafluorophosphate Bu4N+PF6− were purchased from Aldrich. Ferrocene Fc was purchased from Riedel-de Haën. Solvents such as toluene (PhMe) and dichloromethane (DCM) was purchased from VWR Chemicals, while acetonitrile (MeCN) was purchased from Avantor and hexane (C6H14) was purchased from Fisher Scientific. Deuterated solvents such as

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We acknowledge the National University of Singapore for participating through research grant 143-000-B49-114.

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