(Spectro)electrochemical and Electrocatalytic Investigation of 1,1

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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(Spectro)electrochemical and Electrocatalytic Investigation of 1,1′Dithiolatoferrocene−Hexacarbonyldiiron Manuel Häßner,† Jan Fiedler,‡ and Mark R. Ringenberg*,† †

Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany J. Heyrovský Institute of Physical Chemistry, The Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic

‡

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S Supporting Information *

an n-hexane solution, and the molecular structure is shown in Figure 1. The coordination geometry of 1 has two bridging

ABSTRACT: Hexacarbonyldiiron bridged by a 1,1′dithiolatoferrocene, [Fe(C5H4S)2{Fe(CO)3}2] (1), was synthesized, and the electrochemistry showed reversible oxidation at the Fe(C5H4S)2 site and quasi-reversible reduction at the hexacarbonyldiiron site. Spectroelectrochemical techniques showed reduction-induced ligand isomerization, where the thiolate ligand went from bridging to terminal and one carbon monoxide ligand moved to a quasi-bridging position; this mechanism was further supported by cyclic voltammetry simulation and density functional theory calculations. Complex 1 showed electrocatalytic activity toward hydrogen-evolving reaction.

Figure 1. Molecular structure of 1 ellipsoids shown at 50% probability. H atoms are omitted for clarity.

M

etalloenzymes tune the redox and spin states of transition metals by controlling the ligand environment to enable the rapid separation or combination of different charge states.1−7 Hydrogenase (H2-ase) enzymes facilitate the interchange of H2 with H+ and vice versa using only first-row transition-metal atoms.1,8−10 Hexacarbonyldiiron dithiolate complexes, which show structures similar to those of H2-ase active sites, are often used as an easily modified platform for the development of electrocatalysts although they often suffer from low catalytic performance.11 H2-ase-like complexes containing ferrocene moieties are often active electrocatalysts for hydrogen oxidation or proton reduction, possibly due to the ferrocene behaving like the Fe4S4 sites in the enzyme.12−16 Herein, we incorporate Fe(C5H4S)2 as the bridging moiety in a hexacarbonyldiiron cluster. The ruthenium and osmium homologues were previously reported, the first-row [Fe(C5H4S)2{Fe(CO)3}2] (1) was not described.17 The synthesis, electrochemistry, spectroelectrochemistry (SEC), and electrocatalytic proton reduction reactivity of 1, as well as theoretical calculations, are presented here. Starting from the polysulfide, 1,2, 3-t rithia[3]ferrocenophane18 and Fe3(CO)12 in refluxing hexane afforded 1 (Scheme 1; see the Supporting Information, SI, for synthetic details).19 Crystals suitable for X-ray analysis were obtained from

thiolate ligands, and the C5H4S ligands are in an eclipsed configuration. A geometry-optimized model based on the molecular structure of 1 was generated using density functional theory (DFT) calculations. The frontier Kohn−Sham orbitals for 1 are depicted in Figure S1. The highest occupied molecular orbital resides at the Fe(C5H4S)2 moiety, and the lowest unoccupied molecular orbital is an antibonding orbital between the two iron atoms in the hexacarbonyldiiron moiety (Figure S1; see the SI for computational details). The IR spectrum of 1 shows several bands in the νCO region: 2075m, 2038s, 2005m, and 1997m cm−1. The UV−vis spectrum shows an absorption band at 334 nm and two weak bands at 470 and 590 nm. The transitions found in the UV−vis spectrum of 1 were assigned according to time-dependent DFT (TD-DFT) calculations. The transition at 334 nm was assigned as a hexacarbonyldiiron-to-Fe(C5H4S)2 charge transfer, the transition at 470 nm is a d−d transition at the hexacarbonyldiiron moiety, and the lower-energy transition around 590 nm is a transfer from Fe(C5H4S)2 to the hexacarbonyldiiron moiety. The electron density difference maps are depicted in Figure S19 and described in Table S2. The transition at 590 nm is an inverse charge transfer from FeII to two FeI atoms. This transition has been described for other metallocene ligands bound to metal carbonyl complexes with the structure {Fe[C5H4(donor)]2}M(CO)n.20−22 The cyclic voltammetry (CV) of 1 (Figure 2 and

Scheme 1. Synthesis of 1

Received: October 22, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.8b02971 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. Proposed electrochemical mechanism (A) and simulated CV (red) of 1 mM 1 in 0.1 M Bu4NPF6/MeCN at 100 mV s−1. (B) and CV of 1 mM 1 in 0.1 M Bu4NPF6/MeCN with 10, 20, 30, 40, 50, 60, and 70 equiv of TFA (C).

Table 1) shows a reversible oxidation E1/2 = 0.3 V versus Fc0/+ [Fc = Fe(η5C5H5)2 is the reference for all potentials reported

Fe(C5H4S)2 is not as flexible as pdt−2 and is likely behaving like bdt2− = 1,2-benzendithiolate.28,29

Table 1. CV Parameters from Simulation

Scheme 2. Possible Dimerization of [1]−

E/Va −

[1] + e → 1 1 + e− → [1]− +

0.251(2) −1.441 (2)

[1]− ⇌ [1μ-CO]−

αb 0.453 0.618 Ka 2.23(7)

ks/cm s−1 b [calcd]a 1.19 × 10−2[2.9(3) × 10−2] 8.25 × 10−3[1.5(1) × 10−2] kf/s−1 a 11.0(7)

a

Determined from DigiElch simulations; see the SI for details. bSee the SI for calculations.

herein] with ia/ic = 0.9 and a quasi-reversible reduction at E1/2 = −1.44 V with ia/ic = 1.4 in 0.1 M Bu4NPF6/acetonitrile (MeCN). Additional information and figures from the electrochemistry as well as experiments performed in 0.1 M Bu4NPF6/CH2Cl2 can be found in the SI. Both redox couples were studied by IR and UV−vis SEC in 0.1 M Bu4NPF6/MeCN.23 The UV−vis and IR SEC spectra for [1]0/+ and computational analysis showed that oxidation occurred at the Fe(C5H4S)2 moiety; see the SI for further details. The IR SEC spectrum showed that reduction resulted in a bathochromic shift of the carbonyl bands (νCO) by 50 cm−1, consistent with increased electron density at the hexacarbonyldiiron sites. Additionally a band at 1715 cm−1 emerged and is typical of a bridging carbonyl (Figure 3).24−28 The UV−vis SEC

Unrestricted DFT calculations were performed on the openshell structure of [1]−.30 The energy minimum structure obtained for [1]− showed the spin density to be located in the antibonding orbital of the hexacarbonyldiiron moiety (Figure S2), causing elongation of dFe−Fe = 2.872 Å (calcd) (for 1, dFe−Fe = 2.506 Å), although both thiolate ligands remained in the bridging configuration. The transitions found in the UV−vis spectrum of [1]− could not be assigned according to TD-DFT calculations on this configuration (Figure S18). Further DFT analysis was performed on several possible open structures, and a second energy minimum was found containing a semibridging carbonyl, [1(μ-CO)]− (Figure 4). The spin density of the open

Figure 3. IR and UV−vis SEC spectra of [1]0/− in MeCN/0.1 M Bu4NPF6.

spectrum for the process of [1]0/− resulted in several new bands appearing at 470, 570, and 700 nm (Figure 3). Reduction of Fe− Fe H2-ase models can form μ-CO, and this is the case for a variety of [Fe2(SR)2(CO)nLm], where −SR is a bridging thiolate and L represents various ligands.24−26 Alternatively, reduction can result in dimerization; for example, reduction of [Fe2(pdt)(CO)6] (pdt−2 = 1,3-propanedithiolate) induced dimerization to form [(OC)3Fe(pdt)Fe(CO)2S(CH2)3S(Fe2(CO)7)]2−.28,29 The IR SEC spectrum of [1]− cannot be unambiguously assigned to a monomeric structure, and reduction may cause formation of the dimeric systems shown in Scheme 2, although,

Figure 4. Intrinsic bond orbital31 generated from open-shell calculations of [1(μ-CO)]− showing a bridging CO bonding orbital (left) and a π bond from terminal thiolate (right).

structure was localized at one of the iron carbonyl sites (Figure S3). [1(μ-CO)]− contained a partial bridging CO ligand on the Fe0 site pointed toward the FeI site, and a π orbital on the terminal S ligand further stabilizes the open geometry (Figure 4). Furthermore, the transitions found in the UV−vis spectrum of B

DOI: 10.1021/acs.inorgchem.8b02971 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry the open structure of [1]− could be assigned according to TDDFT calculations (Figure S18 and Table S4). On the basis of SEC observation, an electrochemical mechanism was simulated using DigiElch.32 The diffusion coefficient was determined from the Randles−Sevcik plot and D0[1] = 1.099 × 10−5 cm2 s−1 [1.5(1) × 10−5 (simulated)]. The Randles−Sevcik plot shows a linear relationships between [scan rate]1/2 and the current response, compounds are freely diffusional in solution in all forms (see the SI).33 The mechanism presented in Figure 2 shows that upon reduction one bridging thiolate ligand converts to a terminal thiolate; equilibrium constant Keq = 2.23(7) and rate constant kf = 11.0(7) s−1. A small anodic wave was observed at −0.694 V, which was simulated as the oxidation of the open structure [1(μ-CO)]−/0. A similar mechanism was previously described for other diiron dithiolate systems.25,27 The electrocatalytic properties of 1 were investigated for hydrogen-evolving reaction (HER), i.e., 2H+ + 2e− → H2, using trifluoroacetic acid (TFA) in MeCN (pKa = 12.65).34 The resulting CV curves of 1 upon the addition of 0−70 equiv of TFA are shown in Figure 2. A catalytic current grows in at Ep = −1.71 V (the catalytic current for TFA at glassy carbon electrode is at −2.06 V), and this cathodic current reaches a plateau because steady-state equilibrium is achieved, allowing for the rate constant k = 47 ± 9 s−1 to be determined using eq 1.35−38 Important parameters used in the application of eq 1 are the number of electrons (n = 2 for H2 formation), scan rate (100 mV s−1), and TFA concentration. icat /i p =

2 0.4463

CO)]; the former was −14 kcal mol−1 and the latter −8.7 kcal mol−1, relative to [1SH(μ-CO)], based on DFT calculations. The second path (path 2 in Scheme 3) shows protonation occurring at [1]−, where the hydride adopts a bridging configuration. The terminal hydride in [1H(μ-CO)] is typically more active toward the release of H2 than a bridging hydride.24 While the rate of this opening was found to be slow, the HER activity of 1 was faster; this would be due to the higher activity of a terminal hydride over a bridging hydride. The formation of a dimer (Scheme 2), such as with reduction of [(pdt)Fe2CO6],28,29 cannot be fully discounted with 1; however, the flexibility of the Fe(C5H4)2 moiety is likely to play an important role, as it does in the reduction chemistry, and is thought to behave similar to the reported [(bdt)Fe2(CO)6] and not [(pdt)Fe2(CO)6].24−26 1 represents a new example of a hexacarbonyldiiriondithiolate system that is inspired by the H2-ase active sites. The [Fe(C5H4S)2] ligand is a redox-active moiety in close proximity to the active hexacarbonyldiirion site, although it appears to provide structural rather than redox components concerning the reduction chemistry. A proposed electrocatalytic mechanism build on the results of the SEC and HER studies is presented in Scheme 3 and was further supported by DFT calculations. The terminal thiolate is the proposed site of protonation, acting as a proton relay. We are currently investigating the substitution of one or more of the CO ligands in order to optimize the HER rate.

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RTkcat[H ] Fν

ASSOCIATED CONTENT

S Supporting Information *

+ 2

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02971. Experimental and computational details, information concerning oxidation, CV and IR/UV−vis SEC in CH2Cl2, TD-DFT, and EDDM, crystallographic tables, and coordinates determined by DFT (PDF)

(1)

A proposed electrocatalytic mechanism based on the results of the SEC and HER studies is presented in Scheme 3. Two Scheme 3. Proposed Electrochemical Mechanism for HER Facilitated by 1

Accession Codes

CCDC 1872509 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mark R. Ringenberg: 0000-0001-7585-5757 Notes

The authors declare no competing financial interest.

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possible pathways were considered; path 1 in Scheme 3 operates with reduction, inducing one of the thiolate ligands to change from bridging in [1]− to terminal in [1(μ-CO)]−, with one of the CO ligands in a quasi-bridging position. A π orbital on the terminal thiolate ligand projects out from the metal and is likely the site of protonation in [1(μ-CO)]− (Figure 4). DFT calculations on [1H(μ-CO)] showed that protonation of the bridging position was +20 kcal mol−1 over thiolate protonation. The thiol (−SH) ligand in [1SH(μ-CO)] can undergo oxidative addition to form a terminal or bridging iron hydride [1H(μ-

ACKNOWLEDGMENTS The authors thank Prof. Wolfgang Kaim for support and advice and Dr. Wolfgang Frey for crystallographic measurements. M.R.R. gratefully acknowledges support by the state of BadenWürttemberg through bwHPC and the German Research Foundation (DFG) through Grant INST 40/467-1 FUGG for access to the Justus cluster. Funding was provided by the Institut für Anorganische Chemie, Universität Stuttgart, Stuttgart, Germany. C

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DOI: 10.1021/acs.inorgchem.8b02971 Inorg. Chem. XXXX, XXX, XXX−XXX