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. 2023 Feb 13;62(8):3409–3419. doi: 10.1021/acs.inorgchem.2c03355

Heteropolymetallic [FeFe]-Hydrogenase Mimics: Synthesis and Electrochemical Properties

Alejandro Torres †,‡, Alba Collado †,‡,*, Mar Gómez-Gallego †,‡, Carmen Ramírez de Arellano ‡,§, Miguel A Sierra †,‡,*
PMCID: PMC9976291  PMID: 36780261

Abstract

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The synthesis and electrochemical properties of tetranuclear [Fe2S2]-hydrogenase mimic species containing Pt(II), Ni(II), and Ru(II) complexes have been studied. To this end, a new tetranuclear [Fe2S2] complex containing a 5,5′-diisocyanide-2,2′-bipyridine bridging ligand has been designed and coordinated to the metal complexes through the bipyridine moiety. Thus, the tetranuclear [Fe2S2] complex (6) coordinates to Pt(II), Ni(II) and Ru(II) yielding the corresponding metal complexes. The new metal center in the bipyridine linker modulates the electronic communication between the redox-active [Fe2S2] units. Thus, electrochemical studies and DFT calculations have shown that the presence of metal complexes in the structure strongly affect the electronic communication between the [Fe2S2] centers. In the case of diphosphine platinum compounds 10, the structure of the phosphine ligand plays a crucial role to facilitate or to hinder the electronic communication between [Fe2S2] moieties. Compound 10a, bearing a dppe ligand, shows weak electronic communication (ΔE = 170 mV), whereas the interaction is much weaker in the Pt-dppp derivative 10b (ΔE = 80 mV) and virtually negligible in the Pt-dppf complex 10c. The electronic communication is facilitated by incorporation of a Ru-bis(bipyridine) complex, as observed in the BF4 salt 12 (ΔE = 210 mV) although the reduction of the [FeFe] centers occurs at more negative potentials. Overall, the experimental–computational procedure used in this work allows us to study the electronic interaction between the redox-active centers, which, in turn, can be modulated by a transition metal.

Short abstract

The synthesis and electrochemical properties of tetranuclear [Fe2S2]-hydrogenase mimic species bridged by bipy-Pt(II), Ni(II), and Ru(II) complexes have been studied. An experimental−computational approach has been used to determine the electronic interaction between the redox-active centers, which are modulated by the transition metal in the bridge.

Introduction

[FeFe]-Hydrogenases are enzymes capable of reversible conversion of H+ into H2.1−5 Due to the relevance of these transformations, a large number of studies on the [FeFe]-hydrogenase catalytic cycle have been carried out in the last decades, inspiring the efforts of synthetic chemists to design structural and functional models of these enzymes.6−8 Among the different types of [FeFe]-hydrogenase mimics, polynuclear iron–sulfur complexes (that is, complexes having several [Fe2S2] units), have attracted attention as they can exhibit electronic properties different from those of their mononuclear analogues.9−11 In these polynuclear systems, the connection of the [Fe2S2] moieties has been carried out either by covalent bonding through the substituents on the bridging sulfur ligands (1, 2, and 5 in Figure 1)10−12 or by incorporation of the linker as a ligand in the Fe atoms (3 and 4 in Figure 1).9,13

Figure 1.

Figure 1

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Examples of tetranuclear (1, 3, 4, and 5) and hexanuclear (2) iron–sulfur complexes.

Particularly interesting to us are those polynuclear systems having spacers that allow the electronic communication between the two [Fe2S2] centers as they can work as stable multi-electron relays. In this regard, the [FeFe] centers in complexes 1 and 2, bearing a rigid conjugate linker between the [Fe2S2] units, undergo two (1) or three (2) consecutive reversible two-electron metal-center based reductions, the reduced species being stabilized by delocalization of negative charges over the conjugate system. Additionally, complex 1 shows a good electrocatalytic behavior in the reduction of protons for ClCH2COOH.11 The electronic interaction between the [FeFe] centers is also possible through more flexible conjugate linkers, such as the diisonitrile bridging ligand in 3.9

However, despite the interest of polynuclear [Fe2S2] complexes as models to design new robust [FeFe]-hydrogenase mimics, it is remarkable that their potential had been little explored. In this context, the linkers reported to join the [Fe2S2] units are mere connectors between the electrochemically active diiron moieties, and, as far as we know, they have not been conceived for further modifications or reactivity studies. In our current research, we are interested in the development of methodologies for the incorporation of [FeFe]-hydrogenase mimics into diverse types of molecules.14−16 Here, our approach focuses on the design of a polynuclear complex 6 with an active linker, suitable to facilitate the electronic communication between the [FeFe] centers and also able to act as a ligand to incorporate transition metal complexes in the structure (Figure 2). The structure of 6 combines two [Fe2S2] units, known to be electrocatalytically active for hydrogen production, with a π-conjugated linker that also has the chelating properties of the 2,2′-bipyridine moiety, a ligand widely employed in transition-metal coordination chemistry. Complex 6 will be used as a scaffold to synthesize a series of tetranuclear [FeFe]-hydrogenase mimics built as a part of square planar (Pt, Ni) and octahedral (Ru) complexes (Figure 2). The influence of the incorporation of these metal cations in the linker on the electrochemical properties of the complexes and the effect on the electronic communication between the [FeFe]-centers will be presented. The results shown here are a first step into the development of new methodologies to incorporate polynuclear iron–sulfur complexes, mimetics of [FeFe]-hydrogenases, in a wide variety of metal complexes.

Figure 2.

Figure 2

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Strategy to incorporate metal complexes into a tetranuclear [Fe2S2]2-complex.

Results and Discussion

5,5’-Diisocyanide-2,2′-bipyridine (8) was prepared in 74% yield from 5,5′-diamino-2,2′-bipyridine (7)17 by formylation with formic acetic anhydride followed by dehydration with POCl3/NEt3 (Scheme S1). The coordination of 8 to [(μ-bdt)][Fe(CO)3]2 (9) (bdt = 1,2-benzenedithiolate) was achieved by a Me3NO·2H2O-promoted CO substitution reaction. The tetranuclear complex 6 was obtained as a red solid in 40% yield after purification through a silica gel flash chromatography (Scheme 1).

Scheme 1. Synthesis of the Tetranuclear [Fe2S2] Complex 6.

Scheme 1

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Complex 6 was structurally characterized by FTIR and NMR spectroscopy and mass spectrometry. The most relevant signals in the 13C{1H} NMR spectrum in CDCl3 were the singlets found at 210.6 and 209.0 ppm corresponding to the inequivalent CO ligands and the signal at 175.7 ppm corresponding to the coordinated CN-R moiety.18,19 The IR spectrum of 6 showed the stretching frequencies corresponding to the CN (2115 cm–1) and CO bonds (2077, 2038, and 1979 cm–1). The structure and connectivity of complex 6 was unambiguously determined by single-crystal X-ray diffraction analysis (Figure 3).

Figure 3.

Figure 3

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X-ray thermal ellipsoid plot of 6 (50% probability level) with the labeling scheme. Selected bond lengths (Å) and angles (°): Fe(1)-C(7) 1.786(16), Fe(1)-S(1) 2.2718(15), Fe(1)-S(2) 2.2768(13), Fe(1)-Fe(2) 2.4651(9), Fe(2)-S(1) 2.2625(13), Fe(2)-S(2) 2.2793(14), S(1)-C(21) 1.785(5), S(2)-C(22) 1.789(5), C(7)-N(7) 1.171(18), N(7)-C(5) 1.396(16), C(15)-N(17) 1.406(15), N(17)-C(17) 1.172(17), C(7)-Fe(1)-S(1) 90.9(7), C(7)-Fe(1)-S(2) 152.7(5), S(1)-Fe(1)-S(2) 80.39(5), C(7)-Fe(1)-Fe(2) 96.3(6), S(1)-Fe(2)-S(2) 80.54(5), Fe(2)-S(1)-Fe(1) 65.87(4), Fe(1)-S(2)-Fe(2) 65.51(4), N(7)-C(7)-Fe(1) 177(3), C(7)-N(7)-C(5) 172(3).

The structure shows a 5,5′-diisocyanide-2,2′-bipyridine ligand bridging two symmetry-related [(μ-SR)2Fe2(CO)5] units. The Fe-Fe bond length for 6 (2.4651(9) Å) lies in the range found for μ-benzene-1,2diothilate-dirion moieties (2.44–2.57 Å).20 The diisocyanide-bipyridine bridging ligand is bonded to the iron centers in a slightly distorted linear geometry with a Fe(1)-C(7) bond length of 1.786(16) Å and Fe(1)-C(7)-N(7) angle of 177(3)°.

The ability of the tetranuclear complex 6 to coordinate to Pt(II) and Ni(II) centers through the bipyridine moiety was evaluated next. Reaction of complex 6, NaBArF4, and the corresponding square-planar [Pt(P^P)Cl2] complexes [(P^P) = 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,1′-bis(diphenylphosphino)ferrocene (dppf)] in CH2Cl2, at room temperature, afforded compounds 10a–10c as BArF4 salts. These compounds were isolated as dark purple solids in quantitative yields (Scheme 2). Following the same protocol, employing [Ni(dppe)Cl2] as a reagent, compound 11 was isolated as a dark purple solid in quantitative yield (Scheme 2).

Scheme 2. Synthesis of [FeFe]-Pt (10) and [FeFe]-Ni (11) complexes.

Scheme 2

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Compounds 10a–10c and 11 were fully characterized by 1H, 13C{1H}, 31P{1H} and 19F{1H} NMR and IR spectroscopies and elemental analysis. The signal corresponding to the coordinated CN moiety was found in the 188.3–189.3 ppm range in the 13C{1H} NMR spectra in CDCl3 of compounds 10a–10c and 11, that is, significantly downfield shifted with respect to complex 6. Accordingly, the CN stretching frequency of 10a–10c and 11 in the IR spectra also shifts toward lower wavenumbers (2087–2090 cm–1 vs 2115 cm–1). One singlet was observed in the 13C{1H} NMR spectrum at ca. 208 ppm, assigned to the Fe(CO)3 groups. Additionally, broad signals were observed at a very similar chemical shift, which could be tentatively assigned to the CO ligands in the Fe(CO)2CNR-fragments. The broadness of these signals suggests restricted ligand rotation around the Fe center.18,19 The 31P{1H} spectra of compounds 10a–10c contained a singlet at 43.77, −2.62, and 12.87 ppm, respectively, with the expected satellites due to 31P-195Pt coupling (JP–Pt ≈ 3300 Hz). The coupling constant is in agreement with the expected cis-square planar geometry with the employed bisphosphines.21 The 31P{1H} spectra of 11 contained a singlet at 67.65 ppm.22

Additionally, the preparation of the octahedral ruthenium compound 12 was achieved by in situ generation of the bis-acetone-solvato salt [Ru(bpy)2(OCMe2)2][BF4]2 and subsequent reaction with 6 at rt. 12 was obtained as a BF4 salt, an orange solid in 86% yield, after flash chromatography on Al2O3 (Scheme 3), and it was characterized by 1H, 13C{1H}, and 19F{1H} NMR spectroscopies in CD3CN, IR spectroscopy, and HRMS. Two singlets were observed for the inequivalent CO-ligand C atoms at 211.3 and 209.9 ppm, and the signal corresponding to the CN ligand was found at 178.1 ppm. The 19F{1H} NMR spectrum showed the typical isotopic pattern corresponding to the presence of the BF4 anion at −152.12 (10BF4–) and −152.20 ppm (11BF4–). The presence of the CN, CO, and BF4 moieties was further confirmed by IR analysis. The CN stretching frequency appeared at 2109 cm–1, closer to that observed for complex 6 (2115 cm–1) than that obtained for the Pt and Ni complexes (ca. 2090 cm–1). These values suggest a very different electronic effect depending on the metal complex employed.

Scheme 3. Synthesis of the [FeFe]-Ru Compound (12).

Scheme 3

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Electrochemical and DFT Studies

Cyclic voltammetry (CV) was employed to study the electrochemical behavior of complexes 6, 10a–10c, 11, and 12. CVs were recorded in the cathodic direction at a scan rate of 100 mV s–1, and the data (versus Fc+/Fc) are collected in Table 1 and Figures 4, 6, 7, and 11 and Figures S1 and S2. Platinum compounds 10a–10c and 11 decomposed in CH3CN solution, and their electrochemistry was studied in CH2Cl2.

Table 1. Electrochemical Data of Compounds 6, 10a-c, 11, 12, 13a.

    reduction
entry compound Epc1 (E1/21) Epc2 (E1/22) Epc3 (E1/23) Epc4 (E1/24)
1 6b     –1.70  
  6c     –1.28 –1.38
2 10ab –0.91 (−0.88) –1.40 (−1.38) –1.75 –1.92
3 10bb –0.95 –1.42 –1.82 –1.90
4 10cb –0.80 –1.47 –1.77  
5 11b –1.18   –1.92  
6 12c –1.17 (−1.12) –1.37 –1.89 (−1.85) –2.10 (−2.07)
7 13b –1.26 (−1.22) –1.72    
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a

Potential given in V vs Fc+/Fc.

b

CV recorded in CH2Cl2.

c

CV recorded in MeCN.

Figure 4.

Figure 4

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Cyclic voltammograms of compound 6 (10–3 M) in CH2Cl2 (blue line) and MeCN (orange line; intensity multiplication factor = 4) solutions containing 10–1 M [NBu4]PF6 as supporting electrolyte at 25 °C. Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc+/Fc; scan rate: 100 mV/s.

Figure 6.

Figure 6

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Cyclic voltammograms of compounds 10a–10c (10–3 M) in CH2Cl2 solutions containing 10–1 M [NBu4]PF6 as supporting electrolyte at 25 °C. Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc+/Fc; scan rate: 100 mV/s.

Figure 7.

Figure 7

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Cyclic voltammogram of [Pt(dppe)(bpy)][PF6] (13) (10–3 M, CH2Cl2, 10–1 M [NBu4]PF6 as supporting electrolyte at 25 °C). Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Ag/AgCl; scan rate: 100 mV/s.

Figure 11.

Figure 11

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Electrochemical response of a CH2Cl2 solution of 6 (10 –3 M) in the presence of increasing amounts of acetic acid (0–20 equiv). Cyclic voltammograms registered at 25 °C. Supporting electrolyte: [NBu4]PF6 (10–1 M). Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc+/Fc; scan rate: 100 mV/s.

The CV of the tetranuclear [Fe2S2] complex 6 in CH2Cl2 exhibited a broad reduction wave at Epc = −1.70 V, but two oxidation processes were observed in the anodic back scan (Epa1 = −1.25 V, Epa2 = −1.33 V) (Figure 4). This observation suggests that two reduction events were occurring at an almost identical potential. In fact, two reversible processes were observed at −1.28 and −1.38 V when the CV was registered in CH3CN (ΔE = 100 mV) (Figure 4).

Since complex 6 contains three redox-active units, i.e., the [(μ-bdt)][Fe2(CO)5CN] fragments and the 2,2′-bipyridine spacer, and in order to obtain information about the reduction process, computational DFT studies (SMD(CH2Cl2)-B3LYP-D3/def2-SVP) were performed in this system.47 The study of the frontier orbitals confirmed that the LUMO of 6 connects the two [Fe2S2] fragments through the bis-isocyanide–bipyridine linker (Figure S42).

The computed successive electron uptakes (Figure 5) shows that the reduction of 6 involves the [Fe2S2] moieties in a sequential manner, as evidenced by the structural changes observed in these fragments along the process. The incorporation of the first two electrons particularly affects one of the [Fe2S2] centers23 and provokes the cleavage of the Fe(2)-S(4) bond [Fe(2)-S(4) distance changes from 2.34 Å in 60 to 3.80 Å in 62–] (Figure 5a). Accordingly, the orbital HOMO of 62– is located on the reduced [Fe2S2] fragment, whereas the LUMO is placed on the unaltered [Fe2S2] moiety (Figure 5b,c). The uptake of two more electrons (62–/64–) occurs on this unaltered [Fe2S2] unit, causing the breakage of the Fe(45)-S(47) bond (the distance changes from 2.34 Å in 62– to 3.94 Å in 64–) (Figure 5d).

Figure 5.

Figure 5

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Consecutive two-electron reductions of 6 showing the structural changes in the [Fe2S2] complexes: (a) dianion 62–; (b) and (c) HOMO and LUMO orbitals of 62–; (d) tetraanion 64– (computed at the SMD(CH2Cl2)-B3LYP-D3/def2-SVP level). Distances in Å. Isosurface value, 0.04.

The existence of two very close reduction events in the CV of 6 (ΔE = 100 mV in CH3CN, a single wave in CH2Cl2) (Figure 4) is compatible with the sequential process of the computational study but indicates that the electronic communication between the iron centers of 6 is very weak. Overall, the sequential reduction process can be summarized in eqs 1 and 2.

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The structural changes observed in Figure 5 are analogous to those reported in the computational studies on the reduction of the related [(μ-bdt)][Fe(CO)3]2 complex 9,24−26 although in this case, the breakage of the Fe–S bonds during the reduction was accompanied by the formation of a bridging CO. Although our calculations did not show bridging CO bonds in the broken [Fe2S2] fragments, there is a clear approaching of one of the CO ligands to both the Fe(2) and Fe(45) atoms in tetraanion 64– (final bond distances: 2.98 and 2.93 Å, respectively) (Figure 5). The remarkable structural changes observed in the [Fe2S2] moieties along the reduction process will be used in this article as probe of reduction processes centered on the [Fe2S2] fragments when studying the more complex systems 10a–10c, 11, and 12.

The effect of the coordination to different Pt(II) complexes through the bipyridine ligand in electronic communication between the two [Fe2S2] centers of 6 was addressed next. The CVs of 10a–10c contain up to four reduction processes in the −0.9 to −1.9 V range (data in Table 1 and Figure 6). Based on DFT studies (see Figures S52–S57), the first two waves at Epc1 ≈ −0.9 V and Epc2 ≈ −1.40 V can be assigned to the successive one-electron reductions of the bipyridine moiety (bipy0/1– and bipy1–/2–). This is in agreement with the behavior reported for other Pt(II)-bipyridine complexes.27 To support this asseveration, model compound [Pt(dppe)(bpy)][PF6] (13) was prepared, and its CV was recorded (Table 1 and Figure 7). It shows two reduction events at Epc1 = −1.26 V (reversible) and Epc2= −1.72 V (irreversible). These values are anodically shifted by 30 mV compared to those of 10a–10c, likely as a result of the electron withdrawing effect caused by the [Fe2S2] centers as substituents in the bipyridine skeleton.10,11

After the reduction of the bipyridine moiety (that would lead to species 10a–10c2–), the successive electron uptakes should necessarily involve the [Fe2S2] moieties. However, the CVs of complexes 10a–10c in Figure 6 show differences depending on the structure of the Pt(II) complex. Thus, two waves are clearly observed in the CV of 10a (Epc3 = −1.75 V; Epc4 = −1.92 V), but they approach in the CV of 10b (Epc3 = −1.82 V; Epc4 = −1.90 V) and turned into a single broad wave for 10c (Epc3 = −1.77 V).

The DFT calculations confirmed that the successive reductions of the [Fe2S2] centers of the species 10a–10c2– were accompanied by structural changes in the iron–sulfur moieties, as described above for complex 6. Interestingly, the cleavage of the Fe–S bonds and the reorganization of terminal CO ligands to a bridging position were not the only changes observed as the geometry of the Pt(II) complexes was also deeply affected during the reductions. For complex 10a,28 bearing a dppe ligand, the successive two electron uptakes of the reduced species 10a2– (10a2–/10a4– and 10a4–/10a6–) causes the progressive deviation of the Pt(II) center from the initial square-planar structure to finally reach a distorted-tetrahedral geometry in 10a6–, formally a Pt(0) complex (Figure 8).29−31 There, one of the P atoms (P1) is almost perpendicular to the equatorial plane that contains the three other donor atoms (P1–Pt–N angles: 106.5 and 106.2°; P2–Pt–N angles: 136.8 and 143.4°). The structural changes in the Pt center are compatible with an increase in electron density on the metal along the reduction process, which is in consonance with the existence of electronic communication between the [Fe2S2] units. Therefore, the two waves at Epc3 = −1.75 V and Epc4 = −1.92 V in the CV of 10a could be respectively assigned to the calculated 10a2–/10a4– and 10a4–/10a6– couples, which suggests that the reduction of the first [Fe2S2] unit increases the electronic density of the other through the Pt(dppe)-bipyridine linker. The difference between the Epc3 and Epc4 (ΔE = 170 mV) indicates a weak electronic interaction between the [Fe2S2] units in 10a. However, compared to complex 6 (a single wave in CH2Cl2), the effect of the incorporation of the Pt(dppe) complex on the electronic communication is highly remarkable.

Figure 8.

Figure 8

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Changes of the structure of the Pt(II) complex during the reduction of the [FeS2] moieties in species 10a2–: (a) 10a4–; (b) 10a6–. Computed at the SMD(CH2Cl2)-B3LYP-D3/def2-SVP level. Distances in Å. H atoms omitted for clarity.

The influence of the structure of the bisphosphine ligand was clearly manifested during the reduction of the [Fe2S2] moieties in the species 10b2– and 10c2–. A comparative computational study (Figures S54 and S56) reveals that the Pt-center of complex 10b, containing the flexible dppp ligand, was only slightly distorted during the reductions of the [Fe2S2] units (10b2–/10b4– and 10b4–/10b6–) (cis-P–Pt–N angles: 94.0 and 97.1°), whereas complex 10c, bearing a dppf ligand, suffered the elongation of one of the N–Pt bonds during the processes, allowing the distortion of the initially coplanar bipyridine ring. The effect of these changes in the electronic communication between the [Fe2S2] units is significant. The CV of complex 10b still shows two reduction waves (Epc3 = −1.82 V and Epc4 = −1.90 V) assignable to the 10b2–/10b4– and 10b4–/10b6– events, but the difference in potential values (ΔE = 80 mV) indicate that the electronic interaction is much weaker than in 10a. In turn, for complex 10c, there is only a broad reduction wave assignable to the 10c2–/10c4– and 10c4–/10c6– events (Epc3 = −1.77 V), revealing the lack of electronic communication in this complex. In a way, the decoordination of the Pt(dppf) during the reduction of the [Fe2S2] centers in 10c makes it comparable to complex 6, having a free bipyridine linker.

Overall, the sequential reductions of platinum complexes 10 can be summarized in eqs 3 to 6.

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The CV of compound 11, bearing a Ni(II)-dppe fragment, shows two irreversible reduction waves at Epc = −1.18 and −1.92 V (Figure S1). Despite the fact that this complex is stable in solution and could be fully characterized, we were unable to obtain reproducible successive CVs, suggesting that 11 and their reduced species are not stable under the experimental electrochemical conditions. Therefore, compound 11 was excluded from the rest of the study.

The electrochemical behavior of the octahedral Ru(II) species (12) was studied next. The CV of 12 in CH3CN shows four reduction events (Table 1 and Figure 9). The waves at Epc1 = −1.12 V (reversible) and Epc2 = −1.37 V (irreversible) can be assigned to the reductions of the bipyridine moieties, in agreement with the electrochemical behavior reported for other Ru-bipyridine complexes.32−35 According to the DFT calculations, the first sequential two-electron uptake involves the bipy0/1– and bipy1–/2– of the bis-isocyanide–bipyridine linker, while the next two-electron reduction can be assigned to the bipy0/1– and bipy1–/2– of one of the bipyridine–Ru ligands (see Figure 10a and Figures S58–S61).

Figure 9.

Figure 9

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Cyclic voltammogram of compound 12 (10–3 M in CH3CN, containing 10–1 M [NBu4]PF6 as supporting electrolyte at 25 °C). Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc+/Fc; scan rate: 100 mV/s. The CV of 12 was also recorded in CH2Cl2. However, the shorter solvent window associated with this solvent, prevented the observation of all the electrochemical processes. See Figure S2 for more details.

Figure 10.

Figure 10

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Computed structures of complex 12 (SMD(CH2Cl2)-B3LYP-D3/def2-SVP level) showing the changes in the structure of the [Fe2S2] complexes upon reduction. (a) 124–; (b) 126–; (c) 128–. Distances in Å. H atoms omitted for clarity.

After the reduction of the bipyridine moieties (that would lead to species 124–), the successive electron uptakes should necessarily involve the [Fe2S2] moieties (Figures S61 and S62). The CV of 12 also shows two reversible reduction waves at Epc3 = −1.85 and Epc4 = −2.07 V, compatible with the expected sequential two-electron reductions of the [Fe2S2] units. Accordingly to the computation, the first of them could be consistent with the 124–/126– event, a process that causes a noticeable structural change in both [Fe2S2] fragments, with the characteristic Fe–S bond cleavage associated to the FeIFeI/FeIFe0 reduction [(Fe(1)–S(4) and (Fe(45)–S(47) distances change from 2.34 Å in 124– to 3.89 Å in 126–]. In this case the reduction is accompanied by the formation of a bridging CO ligand between Fe(1) and Fe(2) (Figure 10b). The DFT calculations confirmed that the following reduction, 126–/128–, also involves the [Fe2S2] moieties, although now the additional structural changes in both units are subtle (Figure 10c).

It is well known that the reduction of [(μ-bdt)][Fe(CO)3]2 and structurally related complexes is an overall two-electron process, consisting on two successive monoelectronic events, the second one occurring at a less negative potential than the first one. As a result, only a single reduction wave is usually observed in their CVs.8,10,11,24,36−38 The electrochemical study of 12 (Figure 9) reveals a different behavior as the two successive reduction events of the [Fe2S2] units appear at very different potential values (ΔE = 210 mV). This result is undoubtedly related to the electronic connection between the [Fe2S2] fragments through the highly-reduced Ru-bipyridine spacer. In support, the DFT analysis of the anion-radical 127– formed by monoelectronic reduction of 126– reveals the spin density shared between the connected Fe atoms (0.06 e– and 0.11 e–, respectively) and the Ru-bipyridine spacer (Figure S63).

Electrocatalytic Hydrogen Evolution Reaction from Acetic Acid

The electrocatalytic behavior of compounds 6, 10a–10c and 12 in the reduction of protons from acetic acid was explored.39 Complex 6 showed the expected electrocatalytic behavior of a [(μ-bdt)][Fe(CO)3]2 derivative (Figure 11).11,24,37,38 The intensity of the first reduction event (−1.70 V) did not change upon addition of increasing amounts of acetic acid to a solution of complex 6 in CH3CN. However, this process becomes irreversible and the current intensity at −2.30 V increases with the acid concentration. The observed behavior is in agreement with an electrocatalytic process occurring at this latter potential.

The electrochemical behavior of the Pt complexes 10a–10c and the Ru complex 12 in the presence of acetic acid was also evaluated (Figure S3). As in 6, upon acid addition, the CVs of Pt complexes, 10a–10c, show the increase of current intensity at −2.30 V, but a current enhancement was also observed at less negative potentials, particularly in the waves assignable to the reduction of the [FeS2] fragments of complexes 10a and 10b. Both complexes have some degree of electronic interaction between the iron–sulfur centers through the Pt(II) spacer.

For complex 12 (Figure S3), the current intensity of the wave at −2.07 V increases upon the increase of AcOH concentration. The catalytic reduction of acetic acid occurs now at less negative potential than with complex 6 or Pt complexes 10a–10c (−2.30 V).

Conclusions

Herein, we report a tetranuclear [Fe2S2] complex (6) bearing a flexible conjugate linker that is able to act as a ligand to coordinate to Pt(II), Ni(II), and Ru(II) metal complexes. The incorporation of the metal complex in the linker is decisive for electronic communication between the redox-active [Fe2S2] units, likely because the metal coordination forces the bipyridine to be coplanar. Thus, diphosphine platinum compounds 10 enable the electronic interaction between the [FeFe] centers, but the structure of the phosphine ligand plays a crucial role to facilitate or to hinder the process. A weak electronic communication (ΔE = 170 mV) was observed for complex 10a, bearing a dppe ligand, whereas the interaction is much weaker in the Pt-dppp derivative 10b (ΔE = 80 mV) and virtually negligible in the Pt-dppf complex 10c. Alternatively, the electronic communication is facilitated by incorporation of a Ru-bis(bipyridine) complex, 12 (ΔE = 210 mV), although the reduction of the [FeFe] centers occur at more negative potentials. The results shown here offer a new approach to the design of polynuclear iron–sulfur complexes with flexible conjugated bridges, in which the electronic interaction between the redox-active centers can be modulated by a transition metal.

Experimental Section

General Methods

Unless stated otherwise, all the reactions were carried out under an Ar atmosphere using anhydrous solvents. The reaction work-ups were performed in air. [2,2′-Bipyridine]-5,5′-diamine, complexes 9 and 13, [Ni(dppe)Cl2], [Pt(dppe)Cl2], and [Ru(bipy)2Cl2]·2H2O were prepared according to reported protocols.17,22,40,41 Commercially available reagents (Me3NO·2H2O, POCl3, NaBArF4, [Pt(dppf)Cl2], and [Pt(dppp)Cl2]) were used as received without further purification. 1H, 13C{1H}, 31P{1H}, and 19F{1H} NMR spectra were recorded at ambient temperature in CDCl3 or DMSO-d6 on Bruker 500 or 300 MHz spectrometers. Chemical shifts are expressed in parts per million and referenced to residual solvent peaks (1H and 13C{1H}) or to an external reference (85% H3PO4 aqueous solution for 31P{1H} and C6H5CF3 for 19F{1H}). FTIR spectra (ATR) were recorded as solids or films (by slowly evaporating CHCl3 solutions of the compounds) on a Bruker Alpha spectrometer. ESI-HRMS was performed on an Agilent 6500 accurate mass spectrometer with a Q-TOF analyzer. Elemental analyses were carried out on an elemental microanalyzer LECO CHNS-932. Cyclic voltammograms were recorded using a Metrohm Autolab Potentiostat model PGSTAT302N with a glassy carbon working electrode, an Ag/AgCl 3 M as reference, and a Pt wire counter electrode. All the measurements were performed under Ar at room temperature from CH2Cl2 or CH3CN solutions containing 0.1 M [NBu4]PF6 as supporting electrolyte with analyte concentrations of 1 mM (scan rate 0.1 V/s).

Computational Details

All calculations were performed at the DFT level using the B3LYP functional42 as implemented in Gaussian0943 supplemented with Grimme’s dispersion correction D344 and the def2-SVP basis set.45 All minima were verified to have no negative frequencies. The geometries were fully optimized in vacuo and in CH2Cl2 or CH3CN using the continuum SMD model.46

Complex 6

In a 500 mL round-bottom flask, complex 9 (1.45 g, 3.45 mmol, 2 equiv) was dissolved in a CH2Cl2/CH3CN (2:1) mixture (105 mL). To this solution, a suspension of Me3NO·2H2O (384 mg, 3.45 mmol, 2 equiv) in CH3CN (105 mL) was added. After 5 min of stirring at room temperature, a solution of 8 (356 mg, 1.73 mmol, 1 equiv) in CH2Cl2 (35 mL) was added to the suspension. The reaction mixture was stirred at room temperature for 1.5 h. After this time, the solvent was removed under reduced pressure, and the reaction crude was purified by flash chromatography using a mixture of n-hexane/EtOAc (97:3) as eluent. Complex 6 was obtained as a red solid in 40% yield (831 mg). 1H NMR (500 MHz, CDCl3): δ 8.54 (br s, 2H, CHpy), 8.45 (d, J = 8.5 Hz, 2H, CHpy), 7.65 (d, J = 8.5 Hz, 2H, CHpy), 7.13 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr), 6.61 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr) ppm. 13C{1H} NMR (126 MHz, CDCl3): δ 210.6 (CO), 209.0 (CO), 175.7 (CN), 153.6 (Cpy), 148.8 (CSAr), 146.7 (CHpy), 134.0 (CHpy), 127.8 (CHSAr), 126.5 (Cpy), 126.4 (CHSAr), 121.9 (CHpy) ppm. IR (film): νC≡N 2115 (s); νC≡O 2077 (m), 2038 (vs) and 1979 (vs) cm–1. HRMS-ESI: m/z calcd. for C34H14Fe4N4O10S4 [M + H]+: 990.70687; found [M + H]+: 990.70488.

General Procedure for Synthesis of Complexes 10a–10c and 11

In a round-bottom flask, complex 6 (100 mg, 0.100 mmol, 1.01 equiv) and NaBArF4 (198 mg, 0.218 mmol, 2.2 equiv) were suspended in CH2Cl2 (15 mL). To this mixture, a solution of the corresponding [M(P^P)Cl2] (0.099 mmol, 1 equiv) in CH2Cl2 (10 mL) was added via a cannula The reaction mixture was stirred at room temperature for 2.5–24 h. After this time, the obtained suspension was filtered through Celite and the solvent was removed under reduced pressure. The residue was washed with a mixture of n-pentane/CH2Cl2 (9:1) (6 × 5 mL), affording the pure product as a solid.

Compound 10a

Following the general procedure (with 66 mg of [Pt(dppe)Cl2]), 10a was isolated after 24 h as a dark purple solid in quantitative yield (330 mg). 1H NMR (500 MHz, CDCl3): δ 7.75 (m, 12H + 2H, CHPPh + CHpy), 7.66 (br s, 16H, CHB(ArF)4), 7.61–7.55 (m, 8H + 2H, CHPPh + CHpy), 7.42 (s, 8H, CHB(ArF)4), 7.24 (d, J = 9.9 Hz, 2H, CHpy), 7.11 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr), 6.65 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr), 2.47 (m, 4H, PCH2) ppm. 13C{1H} NMR (126 MHz, CDCl3): δ 208.2 (CO), 189.2 (CN), 161.8 (q, J(C-F)= 49.9 Hz, CB(ArF)4), 151.7 (Cpy), 148.5 (CHpy), 147.3 (CSAr), 138.4 (CHpy), 136.1 (CHPPh), 134.8 (CHB(ArF)4), 133.5 (CHPPh), 131.6 (m, CHPPh), 131.5 (m, CPPh), 130.1 (Cpy), 129.1 (q, J(C–F) = 33.1 Hz, CB(ArF)4), 128.0 (CHSAr), 127.2 (CHSAr), 124.9 (CHpy), 124.5 (q, J(C–F) = 272.7 Hz, CB(ArF)4), 117.7 (CHB(ArF)4), 29.2 (d, J(C–P) = 50.4 Hz, PCH2) ppm. 31P{1H} NMR (202 MHz, CDCl3): δ 43.77 (J(P–Pt) = 3346.8 Hz) ppm.19F{1H} NMR (471 MHz, CDCl3): δ −62.72 ppm. IR (film): νC≡N 2087 (m); νC≡O 2041 (s) and 1993 (vs); νC–F 1354 (s) and 1276 (vs); νC–B 1123 (vs) cm–1. Anal. calcd (%) for C124H62B2F48Fe4N4O10P2PtS4: C 44.99; H, 1.89; N, 1.69; S, 3.87. Found: C, 44.86; H, 1.90; N, 1.72; S, 3.73.

Compound 10b

Following the general procedure (with 68 mg of [Pt(dppp)Cl2]), 10b was isolated after 24 h as a dark purple solid in quantitative yield (331 mg). 1H NMR (500 MHz, CDCl3): δ 7.79–7.73 (m, 12H, CHPPh), 7.67 (br s, 16H + 2H, CHB(ArF)4 + CHpy), 7.60 (br s, 8H, CHPPh), 7.51 (d, J = 8.4 Hz, 2H, CHpy), 7.44 (s, 8H, CHB(ArF)4), 7.15–7.11 (m, 4H + 2H, CHSAr + CHpy), 6.66 (br s, 4H, CHSAr), 2.49 (br s, 2H, PCH2CH2), 2.12 (m, 4H, PCH2CH2) ppm. 13C{1H} NMR (126 MHz, CDCl3): δ 208.2 (CO), 189.0 (CN), 161.8 (q, J(C-F)= 49.9 Hz, CB(ArF)4), 151.6 (Cpy), 148.8 (CHpy), 147.3 (CSAr), 137.9 (CHpy), 135.8 (CHPPh), 134.8 (CHB(ArF)4), 132.9 (CHPPh), 131.6 (m, CHPPh + CPPh), 129.6 (Cpy), 129.2 (q, J(C–F) = 32.6 Hz, CB(ArF)4), 128.0 (CHSAr), 127.2 (CHSAr), 124.7 (CHpy), 124.6 (q, J(C–F) = 272.7 Hz, CB(ArF)4), 117.7 (CHB(ArF)4), 21.4 (d, J(C–P) = 31.8 Hz, PCH2CH2), 16.4 (PCH2CH2) ppm. 31P{1H} NMR (202 MHz, CDCl3): δ -2.62 (J(P–Pt)= 3230.6 Hz) ppm. 19F{1H} NMR (471 MHz, CDCl3): δ −62.69 ppm. IR (film): νC≡N 2088 (m); νC≡O 2043 (s) and 1998 (vs); νC −F 1355 (s) and 1277 (vs); νC–B 1125 (vs) cm–1. Anal. calcd (%) for C125H64B2F48Fe4N4O10P2PtS4: C, 45.17; H, 1.94; N, 1.69; S, 3.86. Found: C, 44.79; H, 2.04; N, 1.65; S, 3.71.

Compound 10c

Following the general procedure (with 82 mg of [Pt(dppf)Cl2]), 10c was isolated after 24 h as a dark purple solid in quantitative yield (345 mg). 1H NMR (500 MHz, CDCl3): δ 7.79–7.72 (m, 8H, CHPPh), 7.69 (br s, 16H + 2H, CHB(ArF)4 + CHpy), 7.64 (t, J = 7.7 Hz, 4H, CHPPh), 7.50 (dd, J = 8.5, 2.1 Hz, 2H, CHpy), 7.47–7.43 (m, 8H + 8H, CHB(ArF)4 + CHPh), 7.26 (d, J = 8.5 Hz, 2H, CHpy), 7.11 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr), 6.64 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr), 4.61 (s, 4H, CHCp), 4.34 (s, 4H, CHCp) ppm. 13C{1H} NMR (126 MHz, CDCl3): δ 208.4 (CO), 188.8 (CN), 161.8 (q, J(C–F) = 49.9 Hz, CB(ArF)4), 151.9 (Cpy), 149.2 (CHpy), 147.4 (CSAr), 138.2 (CHpy), 135.1 (CHPPh), 134.9 (CHB(ArF)4), 133.6 (CHPPh), 130.7 (m, CHPPh + CPPh), 129.3 (Cpy), 129.1 (q, J(C–F) = 31.9 Hz, CB(ArF)4), 128.0 (CHSAr), 127.1 (CHSAr), 124.6 (q, J(C–F) = 272.7 Hz, CB(ArF)4), 124.5 (CHpy), 117.7 (CHB(ArF)4), 77.1 (m, CHCp + CCp) ppm. 31P{1H} NMR (202 MHz, CDCl3): δ 12.87 (J(P–Pt) = 3349.6 Hz) ppm. 19F{1H} NMR (471 MHz, CDCl3): δ −62.65 ppm. IR (film): νC≡N 2090 (m); νC≡O 2043 (s) and 1999 (s); νC–F 1355 (s) and 1277 (vs); νC–B 1126 (vs) cm–1. Anal. calcd (%) for C132H66B2F48Fe5N4O10P2PtS4: C, 45.74; H, 1.92; N, 1.62; S, 3.70. Found: C, 45.36; H, 2.03; N, 1.62; S, 3.43.

Compound 11

Following the general procedure (with 53 mg of [Ni(dppe)Cl2]), 11 was isolated after 2.5 h as a dark purple solid in quantitative yield (314 mg). 1H NMR (500 MHz, CDCl3) δ: 7.91 (br s, 8H, CHPPh), 7.77 (t, J = 7.7 Hz, 4H, CHPPh), 7.68 (br s, 16H, CHB(ArF)4), 7.60 (t, J = 7.7 Hz, 8H, CHPPh)7.56 (d, J = 2.2 Hz, 2H, CHpy) 7.46 (dd, J = 8.8, 2.2 Hz, 2H, CHPy), 7.42 (s, 8H, CHB(ArF)4), 7.12 (dd, J = 5.5, 3.3 Hz, 4H, CHSAr), 7.03 (d, J = 8.8 Hz, 2H, CHpy), 6.65 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr), 2.30 (m, 4H, PCH2) ppm. 13C{1H} NMR (126 MHz, CDCl3): δ 208.3 (CO), 188.3 (CN), 161.8 (q, JC–F = 49.7 Hz, CB(ArF)4), 151.2 (Cpy), 149.5 (CHpy), 147.3 (CSAr), 138.5 (CHpy), 136.2 (CHPPh), 134.8 (CHB(ArF)4), 133.1 (CHPPh), 131.8 (m, CHPPh + CPPh), 129.2 (q, JC-F = 33.6 Hz, CB(ArF)4), 129.0 (Cpy), 128.0 (CHSAr), 127.17 (CHSAr), 124.5 (q, JC–F = 272.9 Hz, CB(ArF)4), 123.5 (CHpy), 117.7 (CHB(ArF)4), 29.0 (t, JC–P = 25.0 Hz, PCH2) ppm. 31P{1H} NMR (202 MHz, CDCl3): δ 67.65 ppm. 19F{1H} NMR (471 MHz, CDCl3): δ −62.64 ppm. IR (film): νC≡N 2088 (m); νC≡O 2041 (s) and 1994 (vs); νC–F 1354 (s) and 1276 (vs); νC–B 1123 (vs) cm–1. Anal. calcd (%) for C124H62B2F48Fe4N4NiO10P2S4: C, 46.93; H, 1.97; N, 1.77; S, 4.04. Found: C, 46.66; H, 1.90; N, 1.74; S, 3.83.

Compound 12

In a 50 mL round-bottom flask, cis-[Ru(bpy)2Cl2]·2H2O (150 mg, 1 equiv) was dissolved in 18 mL of degassed acetone and AgBF4 (114.5 mg, 2 equiv) was added to the mixture. The reaction was stirred for 5 h at room temperature protected from the light. After this time, the suspension was filtered through Celite in order to remove AgCl. Then, 25 mL of CH2Cl2 and 316 mg (1.1 equiv) of the complex 6 were added to the filtrate. The reaction mixture was stirred for 18 h at room temperature. After this time, the solution was concentrated to 4–5 mL and Et2O was added until an orange precipitate was obtained. The solid was collected and purified by column chromatography over Al2O3 (neutral, activity I) using a mixture of CH2Cl2/MeOH (95:5) as eluent. Complex 12 was obtained as an orange solid in 86% yield (350 mg). 1H NMR (500 MHz, CD3CN): δ 8.53 (d, J = 8.2 Hz, 4H, CHbipy), 8.45 (d, J = 8.8 Hz, 2H, CHpy), 8.13–8.08 (m, 4H, CHbipy), 7.90 (d, J = 8.8 Hz, 2H, CHpy), 7.69 (d, J = 5.5 Hz, 2H, CHbipy), 7.63 (d, J = 5.5 Hz, 2H, CHbipy), 7.62 (s, 2H, CHpy), 7.48–7.38 (m, 4H, CHbipy), 7.17 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr), 6.68 (dd, J = 5.5, 3.2 Hz, 4H, CHSAr) ppm. 13C{1H} NMR (126 MHz, CD3CN): δ 211.3 (CO), 209.9 (CO), 178.1 (CN), 157.8(C), 155.9 (C), 153.1 (CH), 152.6 (CH), 150.9 (CH), 149.0 (C), 139.3 (CH), 135.2 (CH), 129.2 (C), 128.8 (CH), 128.7 (CH), 128.5 (CH), 127.7 (CH), 126.4 (CH), 125.4 (CH), 125.4 (CH) ppm. 19F{1H} NMR (471 MHz, CD3CN): δ −152.14 (10BF4–), −152.20 (11BF4–) ppm. IR (film): νC≡N 2109 (m); νC≡O 2040 (s) and 1979 (vs); νB–F 1052 (vs) cm–1. HRMS-ESI: m/z calcd. for C54H30Fe4N8O10RuS4 [M]2+: 701.86990; found [M]2+: 701.87625.

Acknowledgments

Support for this work under grants PID2019-108429RB-I00 and RED2018-102387-T from the MCINN (Spain) is gratefully acknowledged. M.A.S. thanks the Fundación Ramón Areces for a grant from the XVIII Concurso Nacional de Ayudas a la Investigación en Ciencias de la Vida y de la Materia (CIVP18A3938). A.C. thanks the MINECO (Spain) for a Juan de la Cierva-Incorporación Fellowship.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c03355.

  • Synthesis and characterization of compounds 7 and 8; electrochemical information; computational details; DFT coordinates; NMR and IR spectra of all compounds (PDF)

Author Present Address

∥ Dpto. de Química Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28,049 Madrid, Spain (A.C.)

The authors declare no competing financial interest.

Supplementary Material

ic2c03355_si_001.pdf (3.5MB, pdf)

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