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Inorganics 2019, 7(4), 50; https://doi.org/10.3390/inorganics7040050

Article
Electrochemical and Computational Insights into the Reduction of [Fe2(CO)6{µ-(SCH2)2GeMe2}] Hydrogenase H-Cluster Mimic
1
Department of Pharmacy, Al-Zaytoonah University of Jordan, P.O. Box 130, Amman 11733, Jordan
2
Institut für Integrierte Naturwissenschaften, Universität Koblenz-Landau, Universitätsstr. 1, D-56070 Koblenz, Germany
3
Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena, Humboldt Str. 8, 07743 Jena, Germany
4
ERCOSPLAN Ingenieurbüro Anlagentechnik GmbH, Arnstädter Straße 28, 99096 Erfurt, Germany
*
Authors to whom correspondence should be addressed.
Received: 8 February 2019 / Accepted: 26 March 2019 / Published: 10 April 2019

Abstract

:
The electrochemical reduction of the complex [Fe2(CO)6{µ-(SCH2)2GeMe2}] (1) under N2 and CO is reported applying cyclic voltammetry. Reduction of complex 1 in CO saturated solutions prevents the possible release of CO from the dianion 12−, while the latter reacts with additional CO forming a spectroscopically uncharacterized product P1. This product undergoes a reversible redox process at E1/2 = −0.70 V (0.2 V∙s−1). In this report, the structure of the neutral complex 1, isomers of dianionic form of 1, and P1 are described applying DFT computations. Furthermore, we propose reaction pathways for H2 production on the basis of the cyclic voltammetry of complex 1 in presence of the strong acid CF3SO3H.
Keywords:
[FeFe]-hydrogenase; cyclic voltammetry; catalysis; hydrogen production; DFT calculations

1. Introduction

Hydrogen (H2) has shown its potential to act as an alternative energy resource with a high energy density [1]. Moreover, hydrogen is an important starting material for the synthesis of fertilizers (e.g., potassium nitrate or ammonium nitrate production), where ammonia is produced via the Haber-Bosch process [2]. The cleanest way to produce H2 is via water electrolysis using platinum as catalyst [3]. The latter is both rare and expensive, limiting its usefulness in large-scale energy storage [4]. In microbes, production and oxidation of hydrogen are catalyzed under ambient conditions with high efficiency and low energy features through enzymes called [FeFe]-hydrogenases [5,6,7,8,9,10] containing a [Fe2S2] cluster exhibiting a bridging dithiolato ligand as well as cyanido and carbonyl ligands, the so-called H-cluster, which is the active site being responsible for the catalytic process (Figure 1a) [11,12,13,14,15,16]. A detailed understanding of the catalytic mechanism of these enzymes has been of great interest to date and is an important subject of research [17,18,19,20].
Until now, the catalytic mechanism for H2 evolution by the H-cluster remains a subject of discussions. Nevertheless, several mechanisms were proposed based on EPR, Mössbauer spectroscopy, pH dependent FTIR spectro-electrochemistry, and DFT calculations [17,18,19,20]. Moreover, numerous hexacarbonyl diiron dithiolato complexes showing structures similar to the H-cluster were synthesize and characterized, and their catalytic applications were studied in detail in order to obtain a deeper insight into the electronic and structural characteristics of the [Fe2S2] core of the H-cluster and to reach a better understanding of the factors stabilizing its rotated state [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40]. The rotated state of the H-cluster offers a vacant site (Figure 1a) at which protons or H2 interact in the catalytic proton reduction or H2 oxidation [17,18,19,20].
We are very keen to learn more about the influence of the nature of the dithiolato ligand on the physical and electrochemical properties of the diiron core in [FeFe]-hydrogenase mimics. In two recent papers, we found that the introduction of the heavier group 14 atoms at the bridgehead position (Y) of [‒XCH2YCH2X‒] (X = S or Se and Y = GeMe2 or SnMe2) ligands results in an almost planar structure of the –SCYCS– moiety for Y = GeMe2 (complex 1) or SnMe2 (Figure 1b) [35]. Our previous work showed that the electron density of the μ-S atoms increases (and, consequently, that of Fe–Fe bond) [22] on going from CMe2 to SiMe2 to GeMe2 to SnMe2. As a consequence, protonation of the Fe–Fe bond is already possible using the moderately strong acid CF3CO2H in the case of Y = SnMe2 and X = Se [38].
In continuation of our research on the influence of heteroatoms toward the structural and electrochemical properties of the model complexes [Fe2(CO)6{µ-(SCH2)2Y}], we describe herein the electrochemical behavior of complex 1 (Y = GeMe2) in the absence and presence of CF3SO3H. Furthermore, DFT computations on complex 1 and its reduction products are described.

2. Results and Discussion

The cyclic voltammogram of [Fe2(CO)6{µ-(SCH2)2GeMe2}] (1) in CH2Cl2/NBu4PF6 under N2 atmosphere exhibits two reduction processes at 0.2 V·s−1: A partially reversible wave at half-wave potential E1/2 ≈ −1.67 V and an irreversible (irr.) wave at −1.85 V [35]. We previously proposed that the first reduction event arises from an ECE process (E = Electron transfer and C = Chemical process), whereas the second cathodic wave is due to reduction of a follow-up reaction product [35]. The follow-up reaction may involve loss of a CO ligand from the reduced species of complex 1, as it was found for other diiron hexacarbonyl complexes [41]. Thus, we performed the experiments using a CO saturated solution to prevent CO loss from the reduced species. The corresponding results are depicted in Figure 2.
The second reduction wave at −1.85 V is not observed, even at slow scan rates, if the solution of complex 1 is CO saturated (Figure 2a,b). This observation clearly indicates that the CO saturated solution effectively inhibits the loss of CO ligands from the reduced species of complex 1, hence preventing the formation of the follow-up reaction product. As a consequence, it would be expected that the reduction of complex 1 should become reversible. However, we can notice in Figure 2a,b that the reversibility is enhanced only very slightly. In addition, only one oxidation wave occurs at −0.67 V (ν = 0.05 V∙s−1) or at −0.64 V (ν = 0.2 V∙s−1) when the experiment is performed using CO-saturated solutions of complex 1. This oxidation event is also detected when the cyclic voltammetry is performed under N2, but it co-occurs with other oxidation processes (see the insets of Figure 2a,b). Thus, the presence of CO in the solution of complex 1 inhibits the follow-up reaction occurring under N2, but a reaction between the dianion 12− and CO takes place leading to a spectroscopically uncharacterized product P1. The formation of P1 is responsible for the oxidation wave at −0.64 V (ν = 0.2 V∙s−1). Figure 2d shows that the oxidation of P1 (producing another product P2) is a reversible process with E1/2 = −0.70 V (Epc = −0.75 V and Epa = −0.64 V). This behavior was also described for the complex [Fe2(CO)5P(OEt)3{µ-(S2CH2)2(Ph)P = O}] [42]. Scheme 1 summarizes the electrochemical reactions of complex 1 under N2 as well as CO atmospheres. To gain insights into the structure of P2, we performed DFT calculations on the two-electron reduction of complex 1 and subsequently on the reaction between the dianionic species 12− and CO.

2.1. DFT Calculations on the Reduction of Complex 1

High-level DFT calculations were performed on the neutral dinuclear iron dithiolato complex 1 as well as on the corresponding doubly reduced species. Previous DFT computations on doubly reduced species of diiron dithiolato complexes considered two possible isomers: (1) A symmetrical dianion in which the Fe–Fe bond is broken, and (2) a rearranged dianion in which the dithiolato ligand is not symmetrically coordinated to the diiron hexacarbonyl moiety [29,42]. One major aspect of these calculations was the question of whether a rearranged structure of the dianion is thermodynamically more stable than the symmetrical isomer. In addition, we also calculated a structure of another dianion, where another CO ligand was added, and which might correspond to species P1 that is formed under CO atmosphere. Calculations were performed applying the B3LYP/6-311++G (d,p) functional and basis set as it is implemented in Gaussian09 [43,44,45,46,47]. Moreover, relativistic ECPs of the Stuttgart-Dresden group were used for iron and germanium atoms [46,47]. As the addition of CO represents a bimolecular reaction, thermal and entropic corrections were considered. It also turned out that the use of a continuum solvent model for CH2Cl2 was crucial to obtain results that correspond to the experimental outcome of the electrochemical investigations [48,49]. All energy values Ecorr as well as the results of frequency calculations are summarized in Table S1 (Supplementary material), where Ecorr corresponds to the abovementioned thermal and entropic corrections. Vibrational analysis for all calculated molecules shows that they represent minimum structures on the hypersurface (numbers of imaginary frequencies, NImag = 0).
Table 1 summarizes selected bond lengths of the calculated compounds. The molecular structures are depicted in Figure 3. From the data, it can be seen that in 1, 1, and 12−, the dithiolato ligand symmetrically coordinates the two iron atoms with both sulphur atoms acting as bridging atoms. As it is expected, the iron–iron bond distance is elongated upon reduction from 253.0 pm in 1 via 287.7 pm in 1 to 350.2 pm in 12−. In addition, the angle between the Fe–Fe–S planes is reduced from the typical butterfly geometry in 1 with the corresponding angle being calculated to 67.04° via 58.63° in 1 to 37.70° in 12−. According to our DFT calculations, the rearranged isomer 12−(isomer) is slightly stabilized with respect to 12− by 4.7 kJ/mol. In 12−(isomer), one of the sulphur atoms symmetrically bridges the two iron atoms, which now show a distance of 271.6 pm. The second sulfur atom coordinates only one of the iron atoms. In addition, there is one CO ligand that shows a semi-bridging coordination mode in the rearranged structure 12−(isomer). The addition of another CO ligand to 12−(isomer) is slightly endothermic (+3.0 kJ/mol) and leads to P1. Nevertheless, no excess of CO was considered in the calculation of the free energy of the addition of CO to 12−(isomer). In P1, a Fe(CO)4 moiety and a Fe(CO)3S2 subunit are connected by a weak iron–iron contact (306.0 pm). This interaction produces a distorted trigonal bipyramidal coordination sphere for Fe2 and a distorted octahedral environment for Fe1. Figure 4 shows the highest occupied molecular orbital, HOMO, of the respective compound, and it can be seen that a weak interaction of the two dz2 orbitals of the iron atoms is supported by three semi-bridging CO ligands and two lone pairs at the thiolato ligand. Indeed, an analogous structure to P1 has been proposed by DFT calculations for the product from the reaction between the dianion of [Fe2(CO)5P(OEt)3{µ-(S2CH2)2(Ph)P = O}] and CO [42].

2.2. Electrochemical Reduction of 1 in the Presence of CF3SO3H

The results of electrochemical investigations on the behavior of complex 1 in the presence of CF3SO3H (pKaDCE = −11.4, DCE = dichloroethane) [50] are shown in Figure 5. We only show results for 0.5–2 equiv. of CF3SO3H with respect to 1, since a significant reduction of this strong acid takes place at higher concentrations. In the presence of 0.5 equiv. of CF3SO3H, only a shoulder at the original reduction wave of 1 (Epc = −1.71 V at 0.2 V∙s−1) is observed at −1.60 V. This shoulder is shifted by 110 mV from the two-electron wave in the absence of CF3SO3H and it is attributed to the protonation of the reduced species of complex 1 affording 1H (Scheme 2). If the concentration of CF3SO3H is increased, the current of the shoulder is shifted to a value that is be comparable to that of the reduction wave in the absence of acid, but without a further increase of its intensity. This suggests that there is no catalytic H2-production at this potential. The cyclic voltammograms exhibit two additional waves (at −1.74 V and −2.07 V) in the presence of CF3SO3H, which show an increase in the current in response to increasing acid concentration. We attribute the wave at −2.07 V, which is observed at one or more equiv. of CF3SO3H, to the reduction of 1H giving 1H2−. The release of H2 at this potential would take place upon protonation of 1H2− (Scheme 2). The other process, which is observed at −1.74 V in the presence of 2 equiv. of CF3SO3H, may arise from reduction of the protonated form of 1H (i.e., 1H2). Similar mechanisms to those shown in Scheme 2 were previously reported for other diiron dithiolato complexes [29,51].

3. Experimental Section

3.1. Electrochemistry: Instrumentation and Procedures

These experiments do not involve corrections for the iR drop. Cyclic voltammetric experiments were performed in a three electrodes cell using a Radiometer potentiostat (µ-Autolab Type-III or an Autolab PGSTAT 12, Metrohm Autolab, Utrecht, Netherland) driven by the GPES software (4.9.005, Metrohm Autolab). The working electrode consisted of a vitreous carbon disk that was polished on a felt tissue with alumina before each CV scan. The Ag/Ag+ reference electrode was separated from the analyte by a CH2Cl2-[NBu4] [PF6] bridge. All potentials are quoted against the ferrocene–ferrocenium couple. Ferrocene was added as an internal standard at the end of the experiments.

3.2. Theoretical Calculations

Full geometry optimizations (i.e., without symmetry constraints) were carried out with the Gaussian09 program package using throughout the hybrid Hartree-Fock-DFT approach (B3LYP/6-311++G (d,p)) [43,46,47]. The B3LYP functional were previously found to be of suitable theoretical level for the study of iron and ruthenium carbonyl complexes [37,52,53,54,55]. For iron and germanium atoms, we used a relativistic ECP of the Stuttgart-Dresden group (SDD) [46,47]. Stationary points were rigorously characterized as minima according to the number of imaginary modes by applying a second-order derivative calculation (vibrational analysis). Zero-point energy (ZPE) corrections were applied. Solvent effects were addressed by performing a polarizable continuum model calculation for CH2Cl2 using the CPCM model [48,49].

4. Conclusions

The two-electron reduction of complex 1 is followed by a chemical process responsible for the irreversibility of the reduction at slow scan rates. The presence of free CO in the solution prevents this process, but CO reacts with the reduced species of 1 forming the spectroscopically uncharacterized product P1 that undergoes a reversible redox process at E1/2 = −0.70 V (0.2 V∙s−1) as shown in Scheme 1. DFT calculations show that the two-electron reduction of 1 leads to a dianionic compound 12− which still shows a symmetrical coordination mode of the dithiolato ligand toward the diiron hexacarbonyl unit. An isomeric form of 12−, 12−(isomer), is slightly more stable and, upon addition of CO, leads to a compound P1 in which a quite weak iron–iron interaction is established by the overlap of the dz2 orbitals at both iron atoms with the delocalized π-system of three semi-bridging CO ligands and two lone pairs at the sulphur atoms. In Scheme 2, the cyclic voltammetric behavior of complex 1 in the presence of CF3SO3H is described, showing proposed H2-releasing pathways.

Supplementary Materials

The following are available online at https://www.mdpi.com/2304-6740/7/4/50/s1, Table S1: Calculated energies and numbers of imaginary frequencies.

Author Contributions

H.A.-F., L.R.A. and W.W. conceived and designed the experiments; H.A.-F. and L.R.A. performed the experiments. All authors were involved in the analysis of the data; W.I. performed DFT calculations. All authors were involved in writing the paper.

Funding

This research received no external funding.

Acknowledgments

H.A.-F. and L.R.A. are acknowledged to the Deutsche Akademischer Austauschdienst for a scholarship.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) The H-cluster of the [FeFe]-Hydrogenase. (b) Mimics of the H-cluster containing heavier group 14 atoms at the bridgehead position (Y) of the dithiolato ligand [–SCH2YCH2S–] (Y = GeMe2 (complex 1) or SnMe2).
Figure 1. (a) The H-cluster of the [FeFe]-Hydrogenase. (b) Mimics of the H-cluster containing heavier group 14 atoms at the bridgehead position (Y) of the dithiolato ligand [–SCH2YCH2S–] (Y = GeMe2 (complex 1) or SnMe2).
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Figure 2. Cyclic voltammetry of 0.843 mM complex 1 in CH2Cl2/NBu4PF6 under conditions shown in the inset of each panels (ae). Part of the voltammograms (ac) are enlarged and shown in the inset for clarification. Glassy carbon electrode (diameter = 3 mm). Potential, E, is in volt (V) against the ferrocenium/ferrocene couple. The arrows indicate the scan direction.
Figure 2. Cyclic voltammetry of 0.843 mM complex 1 in CH2Cl2/NBu4PF6 under conditions shown in the inset of each panels (ae). Part of the voltammograms (ac) are enlarged and shown in the inset for clarification. Glassy carbon electrode (diameter = 3 mm). Potential, E, is in volt (V) against the ferrocenium/ferrocene couple. The arrows indicate the scan direction.
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Scheme 1. Electrochemistry of complex 1 under N2 and CO atmospheres.
Scheme 1. Electrochemistry of complex 1 under N2 and CO atmospheres.
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Figure 3. Calculated molecular structures of 1 (upper left), 1 (upper middle), 12− (upper right), 12−(isomer) (lower left), and P1 (lower right).
Figure 3. Calculated molecular structures of 1 (upper left), 1 (upper middle), 12− (upper right), 12−(isomer) (lower left), and P1 (lower right).
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Figure 4. Highest occupied molecular orbital, HOMO, of P1.
Figure 4. Highest occupied molecular orbital, HOMO, of P1.
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Figure 5. Cyclic voltammetry of complex 1.01 mM complex 1 (CH2Cl2/NBu4PF6) in the presence of 0.5–2 equiv. CF3SO3H. Glassy carbon electrode (diameter = 3 mm). Potential, E, is in volt (V) against the ferrocenium/ferrocene couple. The arrows indicate the scan direction.
Figure 5. Cyclic voltammetry of complex 1.01 mM complex 1 (CH2Cl2/NBu4PF6) in the presence of 0.5–2 equiv. CF3SO3H. Glassy carbon electrode (diameter = 3 mm). Potential, E, is in volt (V) against the ferrocenium/ferrocene couple. The arrows indicate the scan direction.
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Scheme 2. Proposed reactions for the reduction of CF3SO3H catalyzed by complex 1.
Scheme 2. Proposed reactions for the reduction of CF3SO3H catalyzed by complex 1.
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Table 1. Selected bond lengths and distances (pm) of calculated species.
Table 1. Selected bond lengths and distances (pm) of calculated species.
1112−12−(Isomer)P1
Fe1–Fe2235.0287.7350.2271.6306.0
Fe1–S1231.3236.4241.8233.2-
Fe1–S2231.3235.9239.9--
Fe2–S1231.8235.8240.0234.9245.1
Fe2–S2231.8236.9242.1243.6241.3

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