Fe4S4 Cubane Type Cluster Immobilized on a Graphene Support: A High Performance H2 Evolution Catalysis in Acidic Water

The development of alternate catalysts that utilize non-precious metal based electrode materials such as the first row transition metal complexes is an important goal for economic fuel cell design. In this direction, a new Fe4S4 cubane type cluster, [PPh4]2[Fe4S4(DMET)4] (1) (DMET = cis-1,2-dicarbomethoxyethylene dithiolate) and its composite with functionalized graphene, (1@graphene) have been synthesized and characterized. The presence of nanocrystalline structures on graphene matrix in TEM and SEM images of 1@graphene indicate that the cluster (1) has been immobilized. The composite, 1@graphene evolves H2 gas from p-toluene sulfonic acid (TsOH) in a mixture of H2O and CH3CN under ambient conditions with a significant turnover number of 3200. 1@graphene electro-catalyzes H2 evolution at Ep, −1.2 V with remarkable throughput, catalytic efficiency and stability in only H2O or in only CH3CN. The Fe4S4 cluster (1) alone electro-catalyzes hydrogen evolution at Ep, −0.75 V from TsOH in CH3CN. The X-ray crystal structure of the Fe4S4 cluster (1) (λmax, CH2Cl2, 823 nm; ε, 2200 mol−1 cm−1) shows that it is dianionic with a cumulative oxidation state of +2.5 for the iron centers and short C-S bond distances (ca., 1.712 Å & 1.727 Å) indicating the presence of sulfur based radicals.

might be enhanced if they can be embedded or immobilized on a graphene support. Two dimensional nanomaterials of various compositions including graphene have been studied as catalysts in electrochemical reactions such as hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and carbon dioxide reduction 36 . Recently 2D nanomaterials including functionalized graphene, their hybrids have gained interests in electrochemical energy conversion and storage devices 37 . Defects existing in the 2D nanomaterials play an important role in tailoring of optical and electric properties. Functionalized graphene obtained by oxidation contains -COOH, -C=O, and -OH functional groups which can be used to immobilize metal ions and organic groups. Several graphene-photocatalyst composites have been developed recently for HER 38 . Graphene as a matrix would enhance the catalytic performance of the electrocatalysts embedded on it due to its large surface area, excellent conductivity and indefinite durability. Graphene oxide gets folded to close like fisted form and entraps a large molecule such as tetraphenylporphyrin (TPP) 39 . Several graphene based hybrid electrocatalysts as anodic and cathodic fuel cell electrode materials have been reviewed 40
The dianionic 1,2-dithiolenes coordinated to metal complexes are known to undergo oxidation resulting in monoanionic ligands with S-based radicals [41][42][43][44][45][46][47] . The considerably shorter C−S bond distances in a previously reported [Fe 4 (μ-S) 4 (S 2 C 2 (CF 3 ) 2 ) 4 ] 2− dianion and in cluster (1) indicate that the four monoanionic dithiolates provide only four negative charges to the cluster whereas the four inorganic sulfurs (S 2− ) provide eight negative charges making up a total of 12 negative charges which are satisfied by the four iron ions and two [AsPh 4 ]/[PPh 4 ] cations 48,49 . This confirms that the four iron centers are in a comprehensive oxidation state of +2.5 and the four dithiolene ligands have been oxidized to form monoanionic ligands with four S-based radicals. The appearance of a peak at m/z, 1514 in electro spray ionization mass spectrum (Fig. 3) indicates the stability of {[PPh 4 ][Fe 4 (μ 3 -S) 4 (DMET) 4 ]} − complex ion in CH 3 CN. The complex (1) is EPR silent both in the solid state and in CH 2 Cl 2 solution. The diamagnetic nature of the cluster (1) is understood from the room temperature magnetic moment (μ eff ) of <1 μ B . The presence of a band around λ, 823 nm (ε, 2200 M −1 cm −1 ) in the electronic spectrum of (1) in CH 2 Cl 2 (Fig. 3) can be assigned to an intervalence-charge transfer (IVCT) band which is observed in sulphur based radical containing complexes. This indicates the presence of S-based radicals and since the complex (1) is diamagnetic, the S-based radicals are expected to be coupled.
Synthesis and characterization of the composite, 1@graphene. The composite, 1@graphene was prepared by the ultrasonication of Fe 4 S 4 cubane type cluster, (1) and functionalized graphene for 10 h in CH 3 CN-H2O under an argon atmosphere. It was then evapourated to dryness and the black residue was labelled as 1@graphene. The IR spectral peaks of 1@graphene show significant shifts to lower frequencies compared with the Fe 4 S 4 cluster (1). The doublet at 1718 cm −1 and 1690 cm −1 observed in (1) due to -COO stretching of dithiolenes is shifted to 1616 cm −1 and 1610 cm −1 in 1@graphene and the doublet is also broadened (Supplementary information). This indicates the existence of the H-bonding interactions with -COOH of the f-graphene as shown in Fig. 1c. Furthermore, in 1@graphene, a broad band is observed at 3324 cm −1 indicating the presence of hydrogen bonded -OH groups. Functionalized graphene and the composite, 1@graphene were further analyzed by scanning electron microscopy (SEM) and results are displayed in Fig. 4. The sample containing functionalized graphene displayed transparent and thin filmy structures as shown in the Fig. 4A and B. The sample containing (mnt) 1−· ] 4−· (2) which displays nanocubes on recrystallization from CH 3 CN on a brass matrix and as nanotriangles on recrystallization from ethanol on a brass matrix. The EDX analysis of these nanocubes and nanotriangles confirmed the presence of carbon, iron, sulphur, phosphorous in the expected ranges as described in our unpublished results.  Cyclic voltammetric data and proton reduction. The Fe 4 S 4 cubane type cluster (1) undergoes three reversible one electron redox processes around E 1/2 , +0.45 V, +0.11 V and −0.49 V (ΔE, 60 mV) and two quasi-reversible redox processes around E, −1.01 V and −1.22 V in CH 2 Cl 2 (Fig. 6). The waves at E, −0.49 V, −1.01 V and −1.22 V can be assigned to the DMET ligand based redox processes since these were observed also in other related iron(III) dithiolene complexes and a nickel(II) complex of the DMET ligand [30][31][32] . The reversible redox process occurring near zero (E 1/2 , +0.107 V) can be assigned to Fe 3+ /Fe 2+ and the one occurring at E 1/2 , +0.45 V can be assigned to Fe 2+ /Fe 3+ redox process. These assignments of the redox waves of the cluster (1) have been done based on comparison of cyclic voltammetric profiles of various Fe 4 S 4 clusters, iron(II)/iron(III) and nickel(II)complexes of similar dithiolene ligands including classical tetrahedral Fe 4 S 4 clusters of Holm et al. [50][51][52][53][54][55][56][57][58][59] .
The complex (1) electro-catalyzes hydrogen evolution from p-toluene sulfonic acid (TsOH) in CH 3 CN. On addition of TSOH (0.25 M, 0.05 ml) to the cluster (1) in CH 3 CN, the negative current at potential E, −0.72 V was increasing as shown in Fig. 6B. But the reversible redox couples around E p , +0.445 V and Ep, +0.107 V were unaffected by the addition of TsOH. This indicates that the preferential sites of protonation could be the sulfur donors of the radical containing monoanionic dithiolate. The increase in current is due to the reduction of TsOH protons followed by the evolution of hydrogen gas. Controlled potential electrolysis of a mixture of the cluster (1) (0.025 mmol) and TsOH (0.25 mmol) was carried out in CH 3 CN at E p , −0.8 V. A net charge of 29 mC passed over a period of 2 minutes. Head space analysis of the electrochemical cell by gas chromatography confirmed the presence of H 2 gas. The cluster (1, 0.025 mmol) consumed 0.25 mmol of TsOH and the TON (turnover number) of (1) in CH 3 CN is 400. The H 2 evolution occurring at E p , −0.8 V using TsOH is proposed to be promoted by a S-radical based process. The iron bound monoanionic dithiolate type S-donor sites of DMET can be protonated upon addition of TSOH and reduce protons to H 2 on application of electric potential. In the process, the monoanionic dithiolate S-donors can in turn get oxidized to a fully oxidized, neutral di-radical ligand. The reduction of protons coupled to the oxidation of monoanionic dithiolate to neutral di-radical ligand is modulated by the iron center. Because similar DMET complexes of several other transition metal ions do not catalyze proton reduction at such a low reduction potential, viz., E p , 0.72 V.
The cyclic voltammogram of the composite, 1@graphene is similar to that of the cluster (1) as shown in Fig. 7A. It displays three reversible one electron redox processes at E p , −0.41 V, +0.23 V and +0.57 V in CH 3 CN which are assigned to S-radical based Fe 2+ /Fe 3+ redox processes as described above for the pure cluster in CH 2 Cl 2 (vide supra).  Fig. 7B and for clarity only the forward scans are displayed in Fig. 7C. The composite electrocatalyzes the above mentioned reduction to H 2 at the same potential, E p , −1.2 V in water as a solvent with better catalytic efficiency and higher current output. On addition of 0.1 g of TsOH (0.5 mmol in 1 mL H 2 O), overall current of −3.15 mA passed at E, −1.2 V as shown in Fig. 7D and for clarity only the forward scans are displayed in Fig. 7E. The composite (0.05 g, 0.025 mmol with respect to Fe 4 S 4 cubane cluster, 1) consumed 2 mmol of TsOH with a turnover number of 3200. Formation of bubbles and brisk effervescence were observed upon addition CH 3 CN (1mL) to the electrochemical cell containing the composite 1@graphene and TsOH in water. This was confirmed to be H 2 gas by head space analysis by gas chromatography 30 . The stability of the Fe 4 S 4 cubane type cluster and its composite, 1@graphene are confirmed in de-aerated water under argon due to the insolubility of (1) in water. The cluster (1) was extracted from the composite after cyclic voltammetric experiments by ultrasonication in CH 3 CN followed by centrifugation and evapouration to dryness. The elemental analysis of the residue indicated the presence of carbon, hydrogen, nitrogen and sulphur according to the percentage elemental composition of the cluster (1). The composite, 1@graphene isolated from water medium displayed an ESI-MS (−ve) signal at m/z, 1514. The IR spectrum of 1@graphene after catalysis is dominated by peaks due to ν NO3 (KNO 3 , supporting electrolyte) and ν S=O (tosic acid) stretching at 1371 cm −1 , 1177 cm −1 and 1120 cm −1 . But weak signals at 1720 & 1644 cm −1 (-COO of dithiolene) and at 1528 & 1490 cm −1 (-C=C-of dithiolene) coupled with the elemental analytical data indicate that the Fe 4 S 4 cluster is intact. The cluster@graphene is confirmed to be stable in the solid state in water where as it is prone for attack only when it is dissolved in CH 3 CN or even in a mixture of H 2 O-CH 3 CN. This is due to the fact that in CH 3 CN, the composite functions as an emulsion of graphene in the CH 3 CN solution of (1) where as in water, it is an emulsion in graphene solution. The composite remains intact with indefinite durability and it is well behaved in pure water. But in a mixture of H 2 O-CH 3 CN, both the cluster (1) and graphene get into solution. The composite, 1@ graphene shows an enhanced catalytic activity (TON, 3200) as compared to the pure Fe 4 S 4 cluster (1) alone (TON, 400). This could be due to in situ reduction of the oxidized Fe 4 S 4 cluster by graphene. As much as the cluster (1) gets oxidized after proton reduction, that much can be reduced on the graphene matrix immediately. The graphene matrix can function similar to an external sacrificial electron donor 60 . Only graphene in the absence of the Fe 4 S 4 cubane type cluster electrocatalyzes proton reduction at a higher negative potential, ca., −1.7 V as shown in Fig. 7F. But the catalysis onsets at the reduction potential, ca. Ep, −0.41 V itself. Under similar experimental conditions, only TsOH, did not show any response on a GCE in the potential range, E, 0.0 V to −1.8 V (supporting information).
In summary, an ideal catalytic material for the reduction of TsOH protons to H 2 gas in water has been achieved by immobilizing the Fe 4 S 4 cubane type cluster, [PPh 4 ] 2 [Fe 4 S 4 (DMET) 4 ] (1) on a functionalized graphene support. A high current output (3200 TON) and an extreme stability of the catalytic material in water was concluded from the cyclic voltammetric and ESI-MS experiments. Only Fe 4 S 4 cluster, (1) electrocatalyzes the same reduction reaction with low catalytic efficiency (400 TON) and concomitant decomposition to dimeric compound, [PPh 4 ] 2 [Fe III (DMET) 2 ] 2 . The Fe 4 S 4 cubane type cluster was synthesized in a novel synthetic route and structurally characterized. The cluster (1) is EPR silent and it displays reversible, one electron redox waves around E 1/2 , +0.107 V and at +0.45 V) which could be assigned to the Fe 3+ /Fe 2+ and Fe 2+ /Fe 3+ redox processes.  argon. The black colored reaction mixture was stirred for 8h at room temperature and then diethylether (100 ml) was added. The reaction flask was closed tightly and allowed to stand at 10 °C for two days under argon. Dark green crystals were formed which were filtered and stored under argon. Dark green single crystals suitable for X-ray diffraction were obtained by layering of diethylether onto acetonitrile solution of the complex under argon. Data for (1) Preparation of functionalized grapheme. Graphite powder (0.5 g) was taken in THF (80 mL) and water (20 mL), stirred at 38 C for 2 h and ultra-sonicated for 10 h. The solvents were decanted after centrifugation and the residue was dried thoroughly. This residue was treated carefully dropwise with concentrated H 2 SO 4 (30 mL) and fuming nitric acid (10 mL) at 0 °C. The reaction mixture was heated under reflux for 12 h and allowed to stand at 38 °C for 10 h. The supernatant acid layer was decanted and the residue was washed thoroughly with water by centrifugation and dried. FT-IR (ν, cm −1 ): 1718 (br.), 1600 (w). The intensity data for single crystals of (1) was collected at 120 K on a Bruker AXS Smart APEX CCD diffractometer with graphite monochromated MoK α radiation (0.71073 Ǻ). Data reduction and absorption corrections were done using SAINTPLUS program package. The structure was solved by direct and conventional Fourier methods and refined on F 2 by full-matrix