Synthesis and Electrocatalytic Property of Diiron Hydride Complexes

Oct 19, 2015 - K. Michael Schäfer , Leonie Reinders , Jan Fiedler , and Mark R. Ringenberg. Inorganic Chemistry 2017 56 (23), 14688-14696. Abstract |...
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Synthesis and Electrocatalytic Property of Diiron Hydride Complexes Derived from a Thiolate-Bridged Diiron Complex Dawei Yang, Yang Li, Baomin Wang, Xiangyu Zhao, Linan Su, Si Chen, Peng Tong, Yi Luo, and Jingping Qu* State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, 2 Linggong Road, Dalian 116024, P. R. China S Supporting Information *

ABSTRACT: Interaction of a diiron thiolate-bridged complex, [Cp*Fe(μη2:η4-bdt)FeCp*] (1) (Cp* = η5-C5Me5; bdt = benzene-1,2-dithiolate) with a proton gives an FeIIIFeIII hydride bridged complex, [Cp*Fe(μ-bdt)(μH)FeCp*][BF4] (3[BF4]). According to in situ variable temperature 1H NMR studies, the formation of 3[BF4] was evidenced to occur through a stepwise pathway: protonation occurring at an iron center to produce terminal hydride [Cp*Fe(μ-bdt)(t-H)FeCp*][BF4] (2) and subsequent intramolecular isomerization to bridging hydride 3[BF4]. A one-electron reduction of 3[BF4] by CoCp2 affords a paramagnetic mixed-valent FeIIFeIII hydride complex, [Cp*Fe(μ-η2:η2-bdt)(μ-H)FeCp*] (4). Further, studies on protonation processes of diruthenium and iron−ruthenium analogues of 1, [Cp*M1(μbdt)M2Cp*] (M1 = M2 = Ru, 5; M1 = Fe, M2 = Ru, 8), provide experimental evidence for terminal hydride species at these bdt systems. Importantly, diiron or diruthenium hydride bridged complexes 3[BF4], 7[BF4] and iron−ruthenium heterodinuclear complex 8[PF6] can realize electrocatalytic hydrogen evolution.

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INTRODUCTION Transition metal hydrides have received continuous attention in the field of organometallic chemistry, as they are reported to play important roles within hydrogen-related transformations in both biological conversions mediated by metalloenzymes and organic synthesis catalyzed by transition metal complexes.1 In this context, iron hydrides have been implicated as key reactive intermediates in the catalytic cycles of hydrogenase2 and nitrogenase3 enzymes. In the [FeFe]-hydrogenase mimic area, based on the structure of the active site called H-cluster,4 a number of butterfly Fe2S2 complexes with the coordination sphere complemented by CO, CN − , PR 3 , NHC (Nheterocyclic carbenes), and bidentate phosphine ligands have been synthesized as structural and functional models.5 However, using Cp* as an auxiliary ligand is rare, which may be formally regarded as a combination of two CO and one CN− ligands from the coordination number and overall charge. The protonation of diiron dithiolates is a central step in the hydrogen evolution process. According to biophysical and computational studies, it is proposed that the proton binds to the distal iron center to form a terminal hydride species.6 However, the bridging hydride species are involved as intermediates in the electrochemical proton reduction using most [FeFe]-hydrogenase mimics as catalysts.5,7 Only a few diiron model complexes undergo terminal hydride intermediates in the protonation and subsequent isomerization processes, which are evidenced by in situ spectroscopic or crystallographic characterizations.8 Hence, a fascinating challenge of mimicking © XXXX American Chemical Society

the hydride intermediates is the tendency of the terminal hydrides to isomerize to bridging hydrides. Among numerous reported [FeFe]-hydrogenase model complexes, [(CO)3Fe(μ-bdt)Fe(CO)3] (bdt = benzene-1,2dithiolate) has been extensively studied as a proton reduction electrocatalyst, which displays low overpotentials and less negative potentials for reduction events.9 In further research on the catalytic mechanism, terminal or bridging hydride complexes were proposed as essential intermediates by computational and experimental chemistry.10 The redox noninnocent bdt ligand supports the versatile coordination geometries, which can potentially provide different binding sites for hydride.11 To our knowledge, there is no bdt-bridged hydrogenase model complex with Cp* as an ancillary ligand being reported. Our group focuses on constructions of homonuclear12 or heteronuclear13 metal−sulfur clusters containing iron and their reactivity toward various small molecules.14 As part of our efforts toward chemically mimicking the biological nitrogen fixation, we have disclosed such diazene ligated thiolate diiron model complexes.12a,b Recently we reported a thiolate-bridged diiron nitrogenase model complex [Cp*Fe(μ-η2:η4-bdt)FeCp*] (1), which can realize the conversion of diazene to ammonia.12d These findings indicate a possible route to biological nitrogen fixation: N2 → HN = NH → HN−NH2 Received: July 6, 2015

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DOI: 10.1021/acs.inorgchem.5b01508 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry → NH2 → NH3. Herein, we describe the synthesis and characterization of two hydride bridged diiron complexes [Cp*Fe(μ-bdt)(μ-H)FeCp*][BF4] (3[BF4]) and [Cp*Fe(μη2:η2-bdt)(μ-H)FeCp*] (4) and provide some experimental evidence for the formation of terminal hydride intermediate 2 monitored by in situ 1H NMR. Studies on the protonation process of analogues of 1, [Cp*Ru(μ-η2:η4-bdt)RuCp*] (5) and [Cp*Fe(μ-bdt)RuCp*] (8), provided compelling evidence for a terminal hydride in these bdt systems. Importantly, hydride bridged complexes 3[BF4] and 7[BF4] were proven to be electrocatalysts for the proton reduction.

(3,5-bis(trifluoromethyl)phenyl)borate anion) and 3[BPh4] (BPh4 = tetraphenylborate anion) were also afforded with the same method using Lut·HBArF4 and Lut·HBPh4 (Lut = 2,6lutidine) as proton sources, respectively. Unfortunately, both complexes failed to afford single crystals suitable for the X-ray diffraction experiment. To provide further evidence for the existence of a bridging hydride in 3[BF4], one-electron reduction of 3[BF4] was carried out with CoCp2 as the reductant. The color of the solution changed gradually from violet to brownish green, whereupon the neutral complex [Cp*Fe(μ-η2:η2-bdt)(μ-H)FeCp*] (4) was obtained as green microcrystallines in 70% yield. The 1H NMR spectrum of 4 exhibits three broad singlets at −6.12, −2.91, and −1.33 ppm in the high field region, which are attributed to methyl protons of Cp* and two sets of protons of bdt. Analogous to 3[BF4], the above-mentioned three singlets of 4 also shift reversibly with the temperature change (Figure S17). Although the μ-hydride signal is still not detected due to the paramagnetism, the molecular structure of 4 confirmed the presence of the μ-hydride (Figure 1). The Fe−

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RESULTS AND DISCUSSION Treatment of 1 with 1 equiv of HBF4·Et2O in CH2Cl2 from −78 °C to room temperature afforded the μ-hydride diiron complex [Cp*Fe(μ-bdt)(μ-H)FeCp*][BF4] (3[BF4]) as a main product in 75% yield. At the same time, complex 1[BF4] was also obtained as a byproduct in 15% yield by oneelectron oxidation of 1 using HBF4, along with the formation of 0.5 equiv of H2. The ESI-HRMS spectrum exhibits the molecular ion peak [3]+ with an m/z of 523.0873 (calcd 523.0880), which confirms the existence of a hydride subunit in 3[BF4]. The room temperature 1H NMR spectrum of 3[BF4] in CD2Cl2 features a broad signal at −2.51 ppm for Cp* methyl protons and two resonances at 3.71 and 4.81 ppm for bdt protons. The signals of Cp* and bdt shift reversibly with the changes of temperature (Figure S13). No diagnostic μ-hydride signal is observed at room temperature, which is likely due to the paramagnetism of 3[BF4]. When the temperature dropped to −80 °C, the peaks broaden and shift to common regions of methyl and phenyl groups. This result suggests 3[BF4] should be a diagmagnetic species at low temperature similar to our previously reported formal FeIIIFeIII complexes.12b,d The solidstate SQUID magnetization data of 3[BF4] from 2 to 295 K are shown in Figure S30. The effective magnetic moment increases gradually from 0.63 μB at 2 K to 1.72 μB at 295 K, which is consistent with 1H NMR spectra of 3[BF4] at various temperatures. Although the introduction of a hydride to complex 1 formed an [Fe−H−Fe] fragment with a three-center two-electron bond, the electron density of the iron centers is not significantly increased. So we proposed the structure of 3[BF4] should be unsymmetric like its precursor complex 1, in which the bdt ligand inclines to one iron center (Scheme 1). Computational

Figure 1. ORTEP (ellipsoids at 50% probability) diagram of 4.

Fe distance of 2.4159(13) Å is indicative of a bonding interaction of the two iron atoms. The bdt ligand binds to the two iron centers in a μ-η2:η2 fashion. The μ-hydride was located at a position that was unsymmetrical with respect to the Fe−Fe vector. Fe1−H and Fe2−H distances are 1.58(6) and 1.62(5) Å, respectively. To the best of our knowledge, 4 is a rare example of a mixed valence diiron hydride bridged complex bearing a bdt ligand, whose two iron centers are in the formal oxidation states of +2 and +3. Generally, mixed valence diiron hydride complexes attract chemists’ attention because they are postulated as important intermediates during the catalytic turnover of [FeFe]-hydrogenase.15 The reaction of complex 4 with 1 equiv of HBF4 afforded 3[BF4] and 1/2 H2 (Scheme 2). In this hydrogen evolution reaction, the product is not the desired 1[BF4] but hydride bridged complex 3[BF4], and a similar reaction was reported by the Rauchfuss group.16 To probe the formation process of diiron hydride bridged complex 3[BF4], in situ variable temperature 1H NMR studies were performed (Figure 2). After adding 1 equiv of HBF4·Et2O to a CD2Cl2 solution of 1 at −78 °C, the proton signals of 1 instantly vanished and a series of new signals appeared. A broad peak at −1.15 ppm is assigned to a paramagnetically shifted Cp* of aforementioned 1[BF4]. Another set of peaks suggests the existence of an intermediate 2 during the formation process of 3[BF4] (Scheme 3A). Complex 2 is a diagmagnetic species,

Scheme 1. Synthesis of the Diiron Hydride Bridged Complex 3[BF4]

results also found that the energy is lowest when the bdt ligand is crooked (Table S9). The mobility of the bdt ligand between two iron centers in the solution can lead to the equalization of two Cp*; for this reason, only one resonance for two Cp* appears in the 1H NMR spectrum of 3[BF4]. However, no single crystal suitable for X-ray diffraction analysis was obtained owing to the slow decomposition of 3[BF4] in all available crystallization systems. Similarly, 3[BArF4] (BArF4 = tetrakisB

DOI: 10.1021/acs.inorgchem.5b01508 Inorg. Chem. XXXX, XXX, XXX−XXX

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have the same ligands and configuration except for central metals. First of all, under the same conditions as for the protonation of 1, hydride bridged complex [Cp*Ru(μ-η2:η4bdt)(μ-H)RuCp*] (7[BF4]) was synthesized upon protonation of diruthenium analogue 5 (Scheme 3B). The 1H NMR results indicate complex 7[BF4] is not a paramagnetic species as 3[BF4] but a diagmagnetic one. The 1H NMR of 7[BF4] exhibits two resonances at 2.09 and 1.51 ppm for two inequivalent Cp*, one resonance at 7.75 ppm for four equivalent protons of bdt, and a high field signal at −13.14 ppm for the bridging hydride. The structure of 7[BF4] was confirmed crystallographically, which is consistent with spectral characterizations (Figure 3). Crystal analysis reveals there is no difference in the coordination mode of the bdt ligand between protonated product 7[BF4] and unprotonated precursor 5 (Table S6). Since iron and ruthenium are kin metals, we speculate diiron hydride bridged complex 3[BF4] may also have a similar structure as 1 except for the bridging hydride ligand. When protonation of 5 was monitored by in situ 1H NMR at low temperatures, we also observed the terminal hydride signal at −18.39 ppm in the high field region, which is assigned to complex 6. Above 0 °C, this terminal hydride 6 converted quantitatively to the bridging hydride 7[BF4] (Figures S33, S34). One step further, a novel thiolate-bridged iron−ruthenium heterodinuclear complex [Cp*Fe(μ-bdt)RuCp*] (8) was synthesized through assembly of [Cp*Ru(MeCN)3][PF6]19 and the remodeling derivative of 1[BF4] following one-electron reduction with CoCp2. Complex 8 has two isomers (8a and 8b) in the solution (Scheme 3C). When treating 8 with HBF4 in CD2Cl2 at −78 °C, two signals appear at −15.79 and −20.07 ppm in the high field of 1H NMR spectroscopy, which are ascribed to the terminal hydride in iron and ruthenium, respectively. As far as we know, 8 is the first heterodinuclear complex with iron that protonation occurred at two different metals. This result is due to the mobility of the bdt ligand between iron and ruthenium. However, protonations of other reported heterodinuclear complexes containing iron with

Scheme 2. One-electron reduction of the diiron hydride complex 3[BF4]

and the two peaks at 1.35 and 1.77 ppm are ascribed to the methyl protons of two inequivalent Cp* and the two signals at 7.75, 7.49 ppm to protons of bdt. Noticeably, a resonance appears at −17.76 ppm in the high field region, which may be assigned to a terminal hydride. Recently, Rauchfuss and coworkers also reported a diiron dithiolate dihydride complex with a terminal hydride at −18.9 ppm.17 Usually, terminal hydrides of diiron dithiolates appear at relatively low field chemical shifts (−1.21 to −6.26 ppm).8a,b There was no change in the 1H NMR spectra when the temperature rose from −78 °C to −30 °C. When the temperature was −30 °C, the Cp* signal of 3[BF4] became appreciable, which indicates the isomerization has occurred. When the temperature rose from −10 to 0 °C, the signals of 3[BF4] significantly increased and shifted to the higher field. Meanwhile, the two Cp* signals of 2 obviously decreased. When the temperature increased to 20 °C, the signals of 2 completely disappeared, while the signals of 3[BF4] greatly intensified and shifted further upfield. When the temperature dropped again to −78 °C, complex 2 did not appear. This result suggests conversion of complex 2 to 3[BF4] is not a stereochemical rearrangement of the bdt ligand but isomerization of the hydride ligand. According to the research of in situ 1H NMR, we speculate that the terminal hydride complex 2 is a transient intermediate en route to the μ-hydride complex 3[BF4]. In order to provide stronger evidence for the existence of the terminal hydride species in this bdt system, we investigated the protonation of analogues of complex 1, [Cp*Ru(μ-η2:η4bdt)RuCp*] (5)18 and [Cp*Fe(μ-bdt)RuCp*] (8), which

Figure 2. In situ 1H NMR spectra of protonation of 1 in CD2Cl2 from −78 to 20 °C (dark blue, 2; dark purple, 3[BF4]; dark green, 1[BF4]). C

DOI: 10.1021/acs.inorgchem.5b01508 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 3. Protonation Reactions of 1, 5, and 8 and Isomerization Reactions of the Resulting Terminal Hydride Complexes 2, 6, and 9a

Reagents and conditions: (i) 1 equiv of HBF4·Et2O, CH2Cl2, −78 °C to −30 °C; (ii) CH2Cl2, −30 °C to rt, 75%; (iii) 1 equiv of HBF4·Et2O, CH2Cl2, −78 °C to −20 °C; (iv) CH2Cl2, −20 °C to rt, 88%; (v) 1 equiv of HBF4·Et2O, CH2Cl2, −78 °C to −10 °C; (vi) CH2Cl2, −10 °C to rt, 90%. a

The redox properties of two homodinuclear hydride bridged complexes 3[BF4] and 7[BF4] were studied by cyclic voltammetry (Figures S37, S38). Complex 3[BF4] displays a reversible one-electron reduction event at E1/2red = −0.91 V versus Fc+/0, which is assigned to the FeIIIFeIII/FeIIIFeII couple. This result is consistent with the aforementioned reduction of 3[BF4] with CoCp2 (E1/2red = −1.15 V).12d Unlike 3[BF4], diruthenium hydride complex 7[BF4] exhibits two reversible one-electron reductions at E1/2red = −1.39 V, −1.62 V, which are assigned to the RuIIIRuIII/RuIIIRuII couple and RuIIIRuII/ RuIIRuII couple, respectively. The cyclic voltammogram of 7[BF4] indicates 7[BF4] is more difficult to reduce than 3[BF4], which is consistent with the fact that 7[BF4] can not be reduced by CoCp2. The electrocatalytic properties of complexes 3[BF4], 7[BF4] for proton reduction were investigated by cyclic voltammetry (see Figures 4, S39). Upon addition of 1 equiv of HBF4·Et2O to a CH2Cl2 solution of 3[BF4], a new reduction peak appears at Epred = −1.21 V, which is attributed to the FeIIFeIII/FeIIFeII couple. The current height of the reduction peak at −1.21 V increases linearly as the acid concentration increases (Figure 4), and as expected the potential is shifted toward a more negative cathodic value, which are diagnostic for proton catalysis

Figure 3. ORTEP (ellipsoids at 50% probability) diagram of 7[BF4].

propanedithiolate ligand occurred at only one metal.20 Upon warming to room temperature, they all isomerize to the same complex [Cp*Fe(μ-η2:η4-bdt)(μ-H)RuCp*][BF4] (10[BF4]) with a characteristic bridging hydride signal at −13.53 ppm. According to the protonation studies of the above-mentioned iron and ruthenium homodinuclear or heterodinuclear complexes by low temperature in situ 1H NMR spectroscopies, we established the terminal hydride species as kinetic intermediates in the formation process of thermodynamically more stable bridging hydride complexes in these bdt systems. D

DOI: 10.1021/acs.inorgchem.5b01508 Inorg. Chem. XXXX, XXX, XXX−XXX

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us to develop new Fe−S clusters which can activate more inert dihydrogen and dinitrogen.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were routinely carried out under an argon atmosphere, using standard Schlenk-line techniques. All solvents were dried and distilled over an appropriate drying agent under argon. Complex [Cp*Fe(μ-η2:η4-bdt)FeCp*] (1),12d Lut· HBPh4,21 Lut·HBArF4,22 [Cp*Ru(μ3-Cl)]4,23 and [Cp*Ru(MeCN)3][PF6]19 were prepared according to the literature. HBF4·Et2O (Aldrich) and CoCp2 (Aldrich) were used without further purification. Spectroscopic Measurements. The 1H NMR spectra were recorded on a Brüker 400 Ultra Shield spectrometer. Infrared spectra were recorded on an NEXVSTM FT-IR spectrometer. Elemental analyses were performed on a Vario EL analyzer. ESI-MS were recorded on a UPLC/Q-Tof micro spectrometer. Magnetic measurements of the samples were performed using a Quantum Design SQUID (MPMS-XL-7) magnetometer. Computational Methods. DFT calculations were performed with a Gaussian 09 program. The diiron hydride complex was structurally optimized with the TPSSTPSS functional in a combination of basis sets 6-31G(d) for C, H, S atoms and 6-311G(d) for Fe. Electrochemistry. Electrochemical measurements were recorded using a BAS-100W electrochemical potentiostat at a scan rate of 100 mV/s. Electrochemical experiments were carried out in a threeelectrode cell under argon at room temperature. The working electrode was a glassy carbon disk (diameter 3 mm), the reference electrode was a nonaqueous Ag/Ag+ electrode, the auxiliary electrode was a platinum wire, and the supporting electrolyte was 0.1 M n Bu4NPF6 in CH2Cl2. All potentials reported herein are quoted relative to the FeCp2/FeCp2+ couple. Electrocatalysis studies were performed by the stepwise addition of different amounts of HBF4 with a microsyringe. Bulk electrolysis was performed under the same conditions as the cyclic voltammetry measurements, except that the solutions were vigorously stirred. Gas chromatography was performed with a Techcomp GC7900 gas chromatography instrument with argon as the carrier gas and a thermal conductivity detector. X-ray Crystallography. The data for complexes 1[BF4], 4, 5, 7[BF4], and 8[BF4] were afforded on a Brüker SMART APEX CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Empirical absorption corrections were performed using the SADABS program.24 Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F2 using Shelx97.25 Anisotropic thermal displacement coefficients were determined for all non-hydrogen atoms. Hydrogen atoms were placed at idealized positions and refined with fixed isotropic displacement parameters. Preparation of [Cp*Fe(μ-η2:η4-bdt)FeCp*][BF4] (1[BF4]). A solution of complex [Cp*Fe(μ-η2:η4-bdt)FeCp*] (1) (109 mg, 0.21 mmol) in 6 mL of CH2Cl2 at room temperature was treated with Fc· BF4 (57.3 mg, 0.21 mmol). After 3 h, Volatiles were removed in vacuo. The residue was washed with n-hexane three times and dried in vacuo. The violet-black solids were obtained in 95% yield (121.5 mg, 0.20 mmol). The crystals of 1[BF4] suitable for X-ray analysis were afforded by CH2Cl2 solution layered with n-hexane. 1[BF4] was also obtained as a byproduct in 15% yield during the synthesis of 3[BF4]. 1H NMR (400 MHz, CD2Cl2): δ −0.44 (br, 15H, Cp*-CH3), −22.02 (br, 15H, Cp*-CH3), 13.68 (s, 2H, bdt-H), 14.72 (s, 2H, bdt-H). IR (Film; cm−1): 2959, 2922, 2855, 1561, 1452, 1424, 1379, 1261, 1093, 1055, 800, 765, 736. Anal. Calcd for C26H34Fe2S2BF4: C, 51.26; H, 5.63; S, 10.53. Found: C, 51.31; H, 5.68; S, 10.47. Preparation of [Cp*Fe(μ-bdt)(μ-H)FeCp*][BF4] (3[BF4]). At − 78 °C, HBF4·Et2O (0.11 mmol, 20 μL, 50% w/w) was slowly added to a suspension of [Cp*Fe(μ-η2:η4-bdt)FeCp*] (1) (50.2 mg, 0.10 mmol) in 5 mL of CH2Cl2 with vigorous stirring, and then the solution color immediately changed from aubergine to violet. The reaction mixture was allowed to gradually warm to ambient temperature for about 4 h. The resulting dark purple solution was evaporated to dryness under vacuum, then the purplish black residues were washed

Figure 4. Cyclic voltammograms of 3[BF4] (1 mM in 0.1 M n Bu4NPF6 in CH2Cl2 under Ar) with increments of HBF4 (0, 1, 5, 10, 15, 20, 25, 30 mM).

demonstrated by cyclic voltammetry. According to the abovementioned protonation and reduction studies, we proposed the mechanism in this diiron system may be an ECEC process. In this process, complex 3[BF4] is first reduced to give a neutral hydride bridged complex 4, and then 4 may be protonated at the sulfur of the bdt ligand to form an unstable species,10 which subsequently underwent a one-electron reduction occurring at −1.21 V; finally, a protonation process regenerates complex 3[BF4] accompanied by hydrogen release. Further evidence for the electrocatalytic process of 3[BF4] was confirmed by bulk electrolysis of a CH2Cl2 solution of 3[BF4] (1 mM) with HBF4 (30 mM) at −1.30 V. After electrolysis over a 300 s period, about 0.71 μmol of H2 was detected by GC (Figure S46) and 0.158 C passed through the cell which suggests the theoretical production of hydrogen is 0.82 μmol. So the Faraday efficiency for H2 was calculated to be 87%. In addition, the current height of the reduction peak at E1/2red = −1.39 V for 7[BF4] grows with increasing concentration of HBF4, suggesting that 7[BF4] is also catalytically active for electrochemical proton reduction (Figure S39). However, electrochemical measurements of 3[BF4], 7[BF4] confirm that acids (such as AcOH) weaker than HBF4 do not protonate at the metal−metal bond so that no catalysis process is observable. Besides, we also investigated the electrocatalytic hydrogen evolution of iron−ruthenium complex 8[PF6], which can also realize this function and exhibit better stability than 3[BF4] in the presence of excess acids.

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CONCLUSION In summary, hydride bridged diiron complexes 3[BF4] and 4 were synthesized and characterized, and the solid-state structure of 4 was confirmed by crystallographic analysis. The formation process of 3[BF4] was monitored by in situ 1H NMR, which proposed terminal hydride species 2 was a key intermediate in this isomerization process. According to the protonation studies of diruthenium and iron−ruthenium analogues, more powerful evidence for the presence of the terminal hydride were provided. With 3[BF4] or 7[BF4] as a catalyst, electrocatalytic hydrogen production was realized. Compound 1 can activate not only hydrazine or its derivatives12d but also a proton (this work), showing the bifunctionality of this diiron complex. These results stimulate E

DOI: 10.1021/acs.inorgchem.5b01508 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry with Et2O (3 × 5 mL) and dried in reduced pressure to obtain a purplish crystalline powder 3[BF4] (44.0 mg, 0.07 mmol, 75%). 1H NMR (400 MHz, CD2Cl2): δ −2.51 (s, 30H, Cp*−CH3), 3.71 (s, 2H, bdt-H), 4.81 (s, 2H, bdt-H) (no peak corresponding to μ-hydride was identified). IR (Film; cm−1): 2963, 2918, 1477, 1427, 1408, 1379, 1282, 1220, 1093, 1054, 1033, 768, 520. ESI-HRMS: Calcd for [3]+ 523.0880; Found 523.0873. Anal. Calcd for C26H35Fe2S2BF4: C, 51.18; H, 5.78; S, 10.51. Found: C, 51.13; H, 5.80; S, 10.48. According to the same procedure, 3[BArF4] and 3[BPh4] were also obtained in 85% and 67% yields, respectively. 3[BArF4]: 1H NMR (400 MHz, CD2Cl2): δ −2.55 (s, 30H, Cp*−CH3), 3.64 (s, 2H, bdtH), 4.76 (s, 2H, bdt-H), 7.53 (s, 4H, Ph-H), 7.69 (s, 8H, Ph-H) (no peak corresponding to μ-hydride was observed). IR (Film; cm−1): 3070, 2966, 2922, 2860, 1610, 1448, 1427, 1355, 1278, 1127, 1019, 887, 839, 745, 713, 682, 670, 580. ESI-HRMS: Calcd for [3]+ 523.0880; Found 523.0879. Anal. Calcd for C58H47Fe2S2BF24: C, 50.24; H, 3.42; S, 4.63. Found: C, 50.19; H, 3.45; S, 4.58. 3[BPh4]: 1H NMR (400 MHz, CD2Cl2): δ −2.15 (s, 30H, Cp*-CH3), 4.04 (s, 2H, bdt-H), 5.07 (s, 2H, bdt-H), 6.81 (s, 4H, Ph-H), 6.98 (s, 8H, Ph-H), 7.35 (s, 8H, Ph-H) (no peak corresponding to μ-hydride was observed). ESI-HRMS: Calcd for [3]+ 523.0880; Found 523.0876. Anal. Calcd for C50H55Fe2S2B: C, 71.27; H, 6.58; S, 7.61. Found: C, 71.22; H, 6.62; S, 7.59. Preparation of [Cp*Fe(μ-η2:η2-bdt)(μ-H)FeCp*] (4). To a stirred solution of [Cp*Fe(μ-bdt)(μ-H)FeCp*][BF4] (3[BF4]) (71.2 mg, 0.12 mmol) in 7.5 mL of CH2Cl2 was added CoCp2 (22.1 mg, 0.12 mmol) at −78 °C, followed by slowly elevating the temperature to room temperature for 4 h. Volatiles were removed in vacuo, and the crude product was extracted with n-hexane (3 × 5 mL). The dark-green solids 4 (42.8 mg, 0.08 mmol) were achieved in 70% yield after drying in reduced pressure. Crystals of 4 suitable for the Xray diffraction experiment were grown from saturated n-hexane solution at −30 °C. 1H NMR (400 MHz, CD2Cl2): δ −6.12 (s, 30H, Cp*-CH3), −2.91 (s, 2H, bdt-H), −1.33 (s, 2H, bdt-H) (no hydride signal was found in the region). IR (Film; cm−1): 3052, 2970, 2906, 2853, 1475, 1427, 1374, 1243, 1093, 1070, 1024, 737. Anal. Calcd for C26H35Fe2S2: C, 59.67; H, 6.74; S, 12.25. Found: C, 59.63; H, 6.75; S, 12.19. Preparation of [Cp*Ru(μ-η2:η4-bdt)RuCp*] (5). This method is not the same as published procedures of previous literature.15 A suspension of disodium benzene-1,2-dithiolate in 20 mL of THF, prepared by the reaction of NaH (8.7 mg, 0.36 mmol) and benzene1,2-dithiol (25.7 mg, 0.18 mmol) at 0 °C, was transferred via a cannula to the cooled [Cp*Ru(μ3-Cl)]4 (98.4 mg, 0.09 mmol). The mixture was stirred overnight at room temperature. The resulting orange red solution was evaporated under reduced pressure. The residue was extracted by 30 mL of n-hexane three times. After being concentrated and refrigerated to −30 °C, the orange crystals of 5 (89.0 mg, 0.14 mmol) were obtained in 80% yield. 1H NMR (400 MHz, C6D6): δ 1.48 (s, 15H, Cp*−CH3), 2.00 (s, 15H, Cp*−CH3), 6.75 (m, 2H, bdtH), 7.56 (m, 2H, bdt-H). Anal. Calcd for C26H34Ru2S2: C, 50.96; H, 5.59; S, 10.46. Found: C, 50.90; H, 5.50; S, 10.39. Preparation of [Cp*Ru(μ-η2:η4-bdt)(μ-H)RuCp*][BF4] (7[BF4]). Complex [Cp*Ru(μ-η2:η4-bdt)RuCp*] (5) (62.4 mg, 0.10 mmol) was dissolved in 6 mL of CH2Cl2 with stirring, and then HBF4·Et2O (0.12 mmol, 21 μL, 50% w/w) was added dropwise which resulted in a rapid color variation from orangish red to turkey red. The reaction was then allowed to warm to room temperature for 4 h. Volatiles were removed under reduced pressure, and the crude solids were washed with diethyl ether (3 × 10 mL) and with n-hexane (3 × 10 mL) and dried in vacuo to yield 7[BF4] as a dark-red powder (62.8 mg, 0.09 mmol, 88%). Single crystals of 7[BF4] for X-ray crystallography analysis were afforded from a CH2Cl2 solution layered with n-hexane at ambient temperature. 1H NMR (400 MHz, CD2Cl2): δ 1.51 (s, 15H, Cp*CH3), 2.09 (s, 15H, Cp*-CH3), 7.75 (s, 4H, bdt-H), −13.14 (s, 1H, μH). IR (Film; cm−1): 3091, 2964, 2919, 2852, 2095, 1471, 1423, 1406, 1384, 1282, 1261, 1217, 1093, 1056, 1033, 770. ESI-HRMS: Calcd for [7]+ 615.0281; Found 615.0246. Anal. Calcd for C26H35Ru2S2BF4: C, 44.57; H, 5.04; S, 9.15. Found: C, 44.61; H, 4.99; S, 9.11.

Preparation of [Cp*Fe(μ-η2:η4-bdt)RuCp*][BF4] (8[BF4]). A solution of complex [Cp*Fe(μ-η2:η4-bdt)FeCp*][BF4] (1[BF4]) (173.9 mg, 0.29 mmol) in 5 mL of MeCN at room temperature was stirred for 2 h. The green precipitates were gradually obtained. After removing the red purple MeCN solution, [Cp*Ru(MeCN)3][BF4] (129.3 mg, 0.29 mmol) in 5 mL of CH2Cl2 was added to the reaction system. The solution color gradually darkened to a yellow black for 3 h. After removal of the solvents in vacuo, the residue was washed with n-hexane (3 × 10 mL). After drying under reduced pressure, a gray black powder 8[BF4] (165.3 mg, 0.25 mmol) was obtained in 87% yield. Slow diffusion of n-hexane into the concentrated CH2Cl2 solution of 8[BF4] afforded black crystals suitable for the X-ray diffraction experiment. 1H NMR (400 MHz, CD2Cl2): δ 2.04 (s, 15H, Cp*−CH3), −30.37 (br, 15H, Cp*−CH3), 11.64 (s, 2H, bdt-H), 13.01 (s, 2H, bdt-H). IR (KBr; cm−1): 3434, 3117, 2964, 2916, 1472, 1423, 1405, 1379, 1261, 1218, 1094, 1056, 1033, 802, 768, 520. ESI-HRMS: Calcd for [8]+ 568.0503; Found 568.0483. Anal. Calcd for C26H34RuFeS2BF4: C, 47.72; H, 5.24; S, 9.80. Found: C, 47.66; H, 5.21; S, 9.79. Preparation of [Cp*Fe(μ-bdt)RuCp*] (8). To a stirred solution of [Cp*Fe(μ-η2:η4-bdt)RuCp*][BF4] (8[BF4]) (85.8 mg, 0.13 mmol) in 8 mL of CH2Cl2 was added a precooled solution of CoCp2 (24.8 mg, 0.13 mmol) in 2 mL of CH2Cl2 at −78 °C. The reaction mixture was allowed to gradually warm to ambient temperature for about 4 h. The resulting yellow solution was evaporated to dryness under vacuum. The crude product was extracted with n-hexane (3 × 5 mL). After removing volatiles in vacuo, the yellow solids 8 (55.9 mg, 0.10 mmol) were achieved in 75% yield. The ratio for two isomers of complex 8 (8a and 8b) is 2:1 according to 1H NMR spectroscopy. 1H NMR (400 MHz, CD2Cl2): 8a δ 1.50 (s, 15H, Cp*-CH3), 1.91 (s, 15H, Cp*-CH3), 7.20 (q, 2H, bdt-H), 7.82 (q, 2H, bdt-H) 8b 1.43 (s, 15H, Cp*-CH3), 1.97 (s, 15H, Cp*-CH3), 6.86 (q, 2H, bdt-H), 7.32 (q, 2H, bdt-H). IR (KBr; cm−1): 3401, 3151, 3056, 2967, 2897, 1569, 1470, 1445, 1422, 1402, 1376, 1309, 1219, 1072, 1027, 941, 808, 740, 662, 586. Anal. Calcd for C26H34RuFeS2: C, 55.02; H, 6.04; S, 11.30. Found: C, 55.08; H, 6.01; S, 11.27. In Situ Protonation Studies. In an NMR tube, the diiron complex [Cp*Fe(μ-η2:η4-bdt)FeCp*] (1) (6 mg, 0.01 mmol) was dissolved in 0.4 mL of CD2Cl2 at room temperature. Then the sample was degassed after cooling to −78 °C. A slight excess of cold HBF4·Et2O (0.01 mmol, 2 μL, 50% w/w) was added to the NMR tube, which was quickly sealed off. The NMR tube was vigorously shaken to initiate the protonation at −78 °C and rapidly transferred into a precooled (−80 °C) NMR probe. The spectrum was recorded from −80 to 20 °C. Spectra were obtained following equilibration at the listed temperature for at least 5 min. According to the same procedure, in situ protonation of the diruthenium complex [Cp*Ru(μ-η2:η4-bdt)RuCp*] (5) and iron−ruthenium complex [Cp*Fe(μ-bdt)RuCp*] (8) also were manipulated by 1H NMR analyses.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01508. Spectroscopic data (PDF) X-ray crystallographic data for 1[BF4] (CIF) X-ray crystallographic data for 4 (CIF) X-ray crystallographic data for 5 (CIF) X-ray crystallographic data for 7[BF4] (CIF) X-ray crystallographic data for 8[BF4] (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.5b01508 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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A.; Peck, J. N. T.; Ibrahim, S. K.; Oganesyan, V. S.; Pickett, C. J. J. Am. Chem. Soc. 2011, 133, 18606. (16) Wang, W.; Nilges, M. J.; Rauchfuss, T. B.; Stein, M. J. Am. Chem. Soc. 2013, 135, 3633. (17) Wang, W.; Rauchfuss, T. B.; Zhu, L.; Zampella, G. J. Am. Chem. Soc. 2014, 136, 5773. (18) Hörnig, A.; Englert, U.; Kölle, U. J. Organomet. Chem. 1994, 464, C25. (19) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc. 1989, 111, 1698. (20) (a) Huynh, M. T.; Schilter, D.; Hammes-Schiffer, S.; Rauchfuss, T. B. J. Am. Chem. Soc. 2014, 136, 12385. (b) Carroll, M. E.; Chen, J.; Gray, D. E.; Lansing, J. C.; Rauchfuss, T. B.; Schilter, D.; Volkers, P. I.; Wilson, S. R. Organometallics 2014, 33, 858. (21) Ulmann, P. A.; Braunschweig, A. B.; Lee, O.-S.; Wiester, M. J.; Schatz, G. C.; Mirkin, C. A. Chem. Commun. 2009, 34, 5121. (22) Zuccaccia, C.; Bellachioma, G.; Cardaci, G.; Macchioni, A. J. Am. Chem. Soc. 2001, 123, 11020. (23) Fagan, P. J.; Mahoney, W. S.; Calabrese, J. C.; Williams, I. D. Organometallics 1990, 9, 1843. (24) Sheldrick, G. M. SADABS, Program for area detector adsorption correction, Institute for Inorganic Chemistry; University of Göttingen, Germany, 1996. (25) (a) Sheldrick, G. M. SHELXL-97, Program for refinement of crystal structures; University of Göttingen: Germany, 1997. (b) Sheldrick, G. M. SHELXS-97, Program for solution of crystal structures; University of Göttingen: Germany, 1997.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21231003), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13008), and the “111” project of the Ministry of Education of China.

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DOI: 10.1021/acs.inorgchem.5b01508 Inorg. Chem. XXXX, XXX, XXX−XXX