Article pubs.acs.org/IC
Activating Fe(I) Porphyrins for the Hydrogen Evolution Reaction Using Second-Sphere Proton Transfer Residues Atanu Rana,† Biswajit Mondal,† Pritha Sen, Subal Dey, and Abhishek Dey* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, India 700032 S Supporting Information *
ABSTRACT: Iron porphyrin complexes with second-sphere distal triazole residues show a hydrogen evolution reaction (HER) catalyzed by the Fe(I) state in both organic and aqueous media, whereas an analogous iron porphyrin complex without the distal residues catalyzes the HER in the formal Fe(0) state. This activation of the Fe(I) state by the secondsphere residues lowers the overpotential of the HER by these iron porphyrin complexes by 50%. Experimental data and theoretical calculations indicate that the distal triazole residues, once protonated, enhance the proton affinity of the iron center via formation of a dihydrogen bond with an Fe(III)−H− intermediate.
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INTRODUCTION The transition metal-catalyzed hydrogen evolution reaction (HER) normally proceeds via the protonation of an electron rich transition metal center.1−18 This general mechanism includes natural enzymes like hydrogenases19−24 and most molecule- and material-based catalysts.9,25 In these cases, the formation of a metal hydride species, as a reaction intermediate, has been established crystallographically26 or spectroscopically.5,27−32 Generation of sufficient electron density on the transition metal requires its reduction to low-valent states (M1+ or M0).10,33,34 Low-valent transition metal-containing systems (molecules or materials) are generally produced at very low potentials and often suffer from ligand dissociation25 and subsequent generation of metal nanoparticles.35 Several strategies have been developed to stabilize low-valent metal centers for efficient HER.4,16,36 Use of π-accepting and soft ligands21 and solvation4,21,37 have been shown to increase the thermodynamic potential required to generate these low-valent centers, and use of chelating ligands is known to prevent formation of metal nanoparticles.4 Metalloporphyrins are very stable and very amenable to structural modifications, giving them a distinct advantage over other porphyrinoids.12,17 While iron porphyrins are well-known to stabilize high-valent states of metals,18,38,39 the ability to sustain a metal center in its lowvalent states has recently been associated with corroles.4 However, a growing body of literature suggests that low-valent states of iron porphyrins are quite stable and have unique catalytic activities.13,14 An erstwhile effort that aimed to investigate the HER activity of iron porphyrins revealed that the iron center can be protonated only in its formal Fe(0) state at very low pH’s, which lead to substantial overpotential in HER. A protonation equilibrium (H+ + B = HB+) having an acid dissociation constant Ka can be expressed in terms of the © 2017 American Chemical Society
enthalpy (H) and entropy (S) of association (eq 1; note that the sign is changed to relate the ΔG° of protonation with Ka). pK a = −ΔG°/(2.303RT ) = −(ΔH ° − T ΔS°)/2.303RT (1)
Thus, factors that can either decrease the ΔH° (i.e., the proton affinity) or increase the ΔS° of this protonation process can lead to higher pKa values for species, the metal center in the case of HER. Changing the electronic structure of the metal center by changing the donor strength of the ligand changes the ΔH° of protonation and affects the pKa in a straightforward manner. However, an increase in the pKa of the metal center, affected by second-sphere residues (i.e., groups in the vicinity that do not directly interact with the metal), may also allow protonation at a higher pH, resulting in HER by the metal center with a lower overpotential. The ΔH of protonation can be made more favorable by stabilizing the metal hydride intermediate using a dihydrogen bond with a protonated basic residue. This investigation reports such a case. The ΔS° of this protonation process is always positive as the loss of translational entropy is compensated by an increase in the solvation entropy of the ions (conjugate acid and base).15 Protonation of a metal center, during HER, presents a unique situation in which the incoming proton is a hydrogen bond donor whereas, upon binding to the metal center, it is reduced to a hydride, which is a hydrogen bond acceptor.40 Over the past few years, we have reported a series of iron porphyrin complexes with triazole groups in the distal pocket that promote hydrogen bonding interactions (Figure 1). These hydrogen bonding interactions not only were found to form Received: July 19, 2016 Published: February 7, 2017 1783
DOI: 10.1021/acs.inorgchem.6b01707 Inorg. Chem. 2017, 56, 1783−1793
Article
Inorganic Chemistry
Figure 1. Model structures of investigated Fe porphyrin complexes, Blue for nitrogen, saffron for iron, brown for bromine, gold for carbon, and white for hydrogen.
HER in its Fe(0) state. This unique reactivity originates from a dihydrogen bonding interaction with the metal hydride species and results in a 50% reduction in the overpotential for HER.
with axial ligands, including dioxygen, but also affected the spin state of the iron center and reorganization energies and tuned the selectivity of catalytic O2 reduction.41−44 The presence of hydrophobic and bulky tBu and Fc substitutions on the triazoles offered a more organized hydrogen bonding environment relative to that seen with smaller hydrophilic substituents.44 In fact, the advantage of inclusion of secondsphere basic residues in HER has been reported.11,45−48 The basic residues resulted in lowering the overpotential of HER, i.e., a positive shift in the electrochemical potential of HER resulting in better kinetics. These shifts have been attributed to a positive shift in the thermodynamic formal potential (E°) required to reduce the metal center to its catalytically active redox state in the presence of charged protonated basic residues in the vicinity.47,48 In this report, we demonstrate that the presence of triazole residues in the second sphere of the Fe porphyrin catalyst [FeFc4, FeFc1, and Fe(tBu)4 and Fc and tBu represent bulky and hydrophobic ferrocene and tert-butyl substituents on the triazole ring, respectively] allows protonation of the iron center in the formally Fe(I) state, whereas in the absence of these second-sphere residues (FeTPP), the Fe can be protonated only in its formal Fe(0) state. As a result, an iron porphyrin complex with distal triazoles catalyzes HER in the Fe(I) state, whereas an iron porphyrin complex having the same coordination environment but no distal basic residue catalyzes
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RESULTS Two iron porphyrins with four triazole substituents in the distal site are considered. These complexes have been previously shown to allow strong hydrogen bonding interactions with axial ligands like O2 and OH−. A complex with a single triazole substituent and tetraphenyl porphyrins is also investigated to gain insight into the mechanism of action. Homogeneous Electrochemistry in Acetonitrile. Cyclic voltammetry (CV) of iron meso-tetraphenylporphyrin (FeTPP) in the absence (Figure 2 black line) and presence of p-toluenesulfonic acid (TsOH) shows that the hydrogen evolution (HER) current of FeTPP is much more negative than that of the FeII/I process (Figure 2 and Figure S23).33,34 α4Tetra-2-(3-ferrocenyl-1,2,3-triazolyl)phenylporphyrin (FeFc4), α 4 -tetra-2-(3-tert-butyl-1,2,3-triazolyl)phenylporphyrin (FetBu4), and (3-ferrocenyl-1,2,3-triazolyl)phenyltriphenylporphyrin (FeFc1) in degassed acetonitrile presumably show the Fe(III/II), Fe(II/I), and Fe(I/0) processes, respectively, whereas in the presence of TsOH, these complexes show HER (Figures 2 and 3A−D) with an earlier onset at −1.15 V that overlays with the Fe(II/I) process, 1784
DOI: 10.1021/acs.inorgchem.6b01707 Inorg. Chem. 2017, 56, 1783−1793
Article
Inorganic Chemistry
Fe(I) states of FeFc4 and FetBu4 yields rates of 9.8 ± 0.2 and 9.1 ± 0.2 s−1, respectively, in the presence of 25 equiv of acid (Figures S24 and S25). While the data clearly suggest that triazole groups in the distal site activate the Fe(I) state for HER, it is not clear if all four triazole groups are required. Hence, a compound with only one triazole group, FeFc1, is utilized. CV of the FeFc1 complexes shows Fe(III/II), Fe(II/I), and Fe(I/0) processes at potentials identical to those of the other complexes (Figure 3D, blue). HER by the Fe(I) state is observed for FeFc1 in the presence of TsOH (Figure 3C). However, the observation of two secondary processes in the CV response, one at −1.26 V and the other at −1.48 V, makes the analysis of its rate difficult. These additional processes arise because multiple species are present in solution in the presence of an acid (vide inf ra). Similar secondary processes are observed in hangman porphyrins that also bear one distal substituent.50 Formation of a protonated porphyrin species has been deemed responsible for such behavior.51 Spectroelectrochemistry has been utilized to analyze the details of this process (vide inf ra). Effect of PPh3 on the Electrochemistry of Iron Porphyrins. Fe porphyrins without any distal superstructure, such as the FeTPP used here, are known to catalyze proton reduction in the Fe(0) state.33 However, Nocera’s group has shown activation of Fe(I) toward hydrogen evolution by increasing the electron density of the metal center with an axial ligand like triphenylphosphine in hangman Fe porphyrins.50 PPh3 not only activates Fe(I) but also restricts the side reaction by high-spin Fe(II) species produced during HER by Fe(0) porphyrin along with exertion of a strong trans influence on the Fe(I) center.50 Thus, the HER by the Fe(I) state in the presence of PPh3 can provide a reference potential for the Fe(I)-catalyzed HER observed here. The E1/2 values of the iron-centered redox processes show only small shifts after addition of 100 mM PPh3, and this observation is similar to hangman Fe porphyrin (Figure 4A).53 HER activity in the
Figure 2. CV of FeTPP with increasing equivalents of TsOH in acetonitrile with 100 mM tetrabutylammonium perchlorate as the supporting electrolyte and ferrocene added as an internal standard. GCE is used as the working electrode, Pt as the counter electrode, and Ag/AgCl as the reference electrode. The scan rate was 100 mV/s. Potential with respect to Fc+/Fc.
suggesting that the Fe(I) state in the FeFc4 complex could catalyze reduction of a proton (Figure 3A). Note that the shift of the Fe(III/II) process at approximately −0.10 V in the presence of TsOH is likely due to dissociation of the axial ligand (bromide) to the iron in the presence of a strong acid. A similar activity of the Fe(I) state for HER is also obtained from another triazole containing Fe porphyrins [FetBu4 (Figure 3B)]. The Fe(II/I) and Fe(I/0) potentials are quite similar for these complexes (Figure 3D, green, purple, and red traces), suggesting that the electronic structures of the Fe centers in these four complexes, which vary only in the substitution of the meso-aryl position, are, expectedly, similar. Thus, the catalytic HER activity exhibited by the Fe(I) state in the triazole-bearing complexes must originate from the inclusion of the triazoles in the second sphere. The catalytic rates for HER with the Fe(I) state can be determined either using the foot-of-the-wave analysis49 (FOWA) of the kinetic region at acid concentrations at which the HER current depends on the acid concentration or by analyzing icat/iip at acid concentrations where the HER current plateaus. The FOWA of the electrocatalytic HER by
Figure 3. CV of (A) FeFc4, (B) FetBu4, and (C) FeFc1with respect to the number of equivalents of TsOH added. (D) CV data of 0.5 mM FeTPP, FeFc4, FetBu4, and FeFc1 in acetonitrile using glassy carbon as the working electrode (GCE), Ag/AgCl as the reference electrode, and a Pt wire as the counter electrode. TBAP (100 mM) was used as the supporting electrolyte. Ferrocene was used as an internal standard. The scan rate was 100 mV/s. Note that CV of the low-valent states is complicated by HER because of residual water and ligand dissociation. However, no degradation is observed (Figures S17−S20).52 Potential reported with respect to Fc+/Fc. 1785
DOI: 10.1021/acs.inorgchem.6b01707 Inorg. Chem. 2017, 56, 1783−1793
Article
Inorganic Chemistry
Figure 4. (A) CV of FeFc1 with 100 mM PPh3 (purple) and without PPh3 (saffron). (B) CV of FeTPP (blue) and FeFc1 (red) with 100 equiv of PPh3 and 10 equiv of TsOH. GCE is used as the working electrode, Pt as the counter electrode, and Ag/AgCl as the reference electrode. The scan rate was 100 mV/s. Potential with respect to Fc+/Fc used as an internal reference.
Figure 5. (A) Rotating disk electrochemistry (RDE) of FeFc4 physisorbed on an edge-plane pyrolytic graphite electrode at different pH’s. Pt is used as the counter electrode and Ag/AgCl as the reference electrode. The scan rate was 50 mV/s. The rotation speed was 300 rpm. The inset shows a plot of icat (−0.9 V) with respect to pH and (B) the RDE of FeFc4 at pH 1.02 (blue) and pH 2.13 (red) and that of FeTPP at pH 1.02 (green). The catalysts are physisorbed on EPG. The scan rate was 50 mV/s. The rotation speed was 300 rpm. (C) Bulk electrolysis of the catalyst physisorbed on the carbon rod at −0.7 V vs Ag/AgCl (blue line) and the same carbon rod under similar conditions without a catalyst (red line) in 0.5 M H2SO4 under an argon atmosphere. Pt is used as the counter electrode.
supporting the proposal that the Fe(I) state is activated for HER in these porphyrins. Heterogeneous Electrochemistry in Aqueous Medium. The FeFc4 complex is absorbed on an edge-plane graphite (EPG) electrode, and the resulting electrode is dipped in buffered aqueous solutions. CV at neutral pH and LSV at different pH’s indicate that above pH 2, HER proceeds with an onset potential of −0.9 V (>10 μA current). However, when the pH is
