Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Reactivity toward Unsaturated Small Molecules of Thiolate-Bridged Diiron Hydride Complexes Dawei Yang,† Sunlin Xu,† Yixin Zhang,† Ying Li,† Yang Li,† Baomin Wang,† and Jingping Qu*,†,‡ †
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China Key Laboratory for Advanced Materials, East China University of Science and Technology, Shanghai, 200237, People’s Republic of China
‡
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S Supporting Information *
ABSTRACT: In the presence of 1 equiv of tBuNC, the homolytic cleavage of the FeIII−H bond in the diiron terminal hydride complex [Cp*Fe(t-H)(μη2:η4-bdt)FeCp*][BF4] (1[BF4]) smoothly took place to release 1/2 H2, followed by binding of a tBuNC group to the unsaturated FeII center. Interestingly, upon exposure of 1[BF4] to 1 atm of acetylene, the isomerization process of the hydride ligand from the terminal to bridging coordination site was unaffected. Upon treatment of the diiron hydride bridged complex 2[BF4] with acetylene at 30 °C, two FeIII−H bonds were broken, and then an acetylene molecule was coordinated to the diiron centers in a novel μ-η2:η2 side-on fashion. In the above reaction system, the hydride ligands whether terminal or bridging all play a role as the electron donor for the reduction of the diiron centers from FeIIIFeIII to FeIIIFeII. These reaction patterns are reminiscent of the vital E4 state responsible for N2 binding and H2 liberation in the catalytic cycle of nitrogenase, which contains two {Fe−H−Fe} motifs as electron reservoirs for the reduction of the iron centers. Differently, when treating 1[BF4] with TMSN3, the terminal hydride ligand was inserted into the azide group to give a diiron amide complex 4[BF4] in moderate yield.
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INTRODUCTION
resonance (EPR) spectroscopy shows that the iron hydrides are paramagnetic species.5,6a These significant developments prompted bioinorganic chemists to construct suitable model systems for getting deep insight into the potential role of iron hydride species in the catalytic reductions of various substrates with double or triple bond by nitrogenase enzymes. This represents a challenging task since an ideal mimic should possess highspin iron centers supported by weak-field sulfur donors with low coordination number. In sharp contrast with the ubiquity of dithiolate-bridged diiron hydride complexes as important intermediate species in the catalytic cycle of [FeFe]-hydrogenase,8 only a few iron hydride systems were built based on key features of the nitrogenase hydride intermediates.9 Peters and co-workers utilized the strong-field tridentate phosphine ligand to synthesize a paramagnetic mixed-valence FeIIFeIII hydride bridged complex in an S = 1/2 spin state.10 Actually, among numerous diiron hydrides as intermediate mimics of hydrogenase, the examples of FeIIFeIII hydride complex are also rare,11 which have the tendency of homolytic cleavage of the FeIII−H bond to liberate 1/2 H2.12 Besides, Holland and coworkers reported low-coordinate iron complexes supported by a β-diketiminate weak-field ligand, which exhibit versatile reactivity patterns toward various nitrogenase related sub-
Nitrogenase enzymes catalyze the conversion of atmospheric dinitrogen (N2) into ammonia to sustain life on Earth, inevitably accompanied by the reduction of protons to release H2.1 In fact, this class of metalloenzyme can also accomplish the reductions of many other nonphysiological unsaturated small molecules.2 Although the active center called te FeMocofactor was unambiguously elucidated as an FeMo heteromultinuclear metal−sulfur cluster for the reduction of substrates,3 some key mechanistic questions remain unsolved, especially, where and how substrates bind to the FeMocofactor.4 In the Thorneley−Lowe kinetic scheme, interaction of N2 with the FeMo-cofactor is considered to occur in the E4 state, which represents the resting state accepts four electrons and four protons.5 Recently, biochemists and chemists have focused on the belt region of the FeMo-cofactor; especially, electron−nuclear double resonance (ENDOR) spectroscopic results clearly reveal that the E4 state contains two bridging hydrides between two belt iron centers and two protons bound to sulfide ligands.6 Subsequently, the two bridging hydrides in the {Fe−H−Fe} subunits convert into H2 accompanied by delivery of two electrons to the FeMo-cofactor for N2 binding.4,7 During this process, transient terminal hydrides may be also involved, which are derived from isomerization of the bridging hydrides.7 Furthermore, electron paramagnetic © XXXX American Chemical Society
Received: September 1, 2018
A
DOI: 10.1021/acs.inorgchem.8b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Scheme 1. Reactivity of Thiolate-Bridged Diiron Hydride Complexes 1[BF4] and 2[BF4] towards Unsaturated Small Molecules
demonstrated that the homolytic cleavage of the FeIII−H bond in 1[BF4] smoothly occurs followed by coordination of a t BuNC molecule to give a bdt-bridged diiron complex, [Cp*Fe(t-tBuNC)(μ-η2:η4-bdt)FeCp*][BF4] (3[BF4]) as a black microcrystalline powder in excellent yield. The terminal hydride ligand converted into H2 in 68% yield confirmed by GC analysis and simultaneously transferred one electron to iron center, which led to a change in formal valences of the two iron centers from FeIIIFeIII to FeIIIFeII. On the basis of the above understanding, the number of total d electrons of the two iron centers in 3[BF4] is odd, which suggests that 3[BF4] should be a paramagnetic species caused by unpaired d electron. As expected, there are several broad peaks located at the range from −8 to 8 ppm with an obvious paramagnetic shift, difficult to access clear assignment. In addition, the solution magnetic susceptibility of 3[BF4] at room temperature was measured by the Evans method to obtain the μeff value of 2.1 μB, which implies the existence of one unpaired electron. Consistently, the EPR spectrum of 3[BF4] in the solid state showed a diagnostic signal at g = 2.05. Commonly, the vibration frequency of the NC triple bond in the IR spectrum can provide useful structural information about the binding mode of the coordinated isonitrile ligand. In the IR spectrum of 3[BF4], a strong absorption peak at 2135 cm−1 is observed assignable to the terminal tBuNC group, which has a blue shift compared to the free tBuNC observed at 2121 cm−1. Besides, electrospray ionization-high resolution mass spectrometry (ESI-HRMS) exhibits the molecular ion peak [3]+ with an m/z of 605.1541 (calcd. 605.1537), which also confirms the existence of the tBuNC subunit in 3[BF4]. In sharp contrast, almost no reaction of hydride bridged complex 2[BF4] with 1 equiv of tBuNC at room temperature was observed by 1H NMR spectroscopy and ESI-MS analysis. The resulting residues are mainly unreacted complex 2[BF4] and a small amount of unknown species. When the the amounts of tBuNC are increased or the reaction temperature is elevated, complex 2[BF4] will be decomposed to some
strates.13 Recently, a novel diiron sulfide complex with a bridging hydride was successfully synthesized and exhibited good reactivity toward terminal alkyne and CO2.14 Our group has settled down to construct multinuclear metallic complexes adopting the weak-field thiolate ligand for structural and functional biomimics of various metalloenzymes, especially, mimicking the biological nitrogen fixation.15 Recently, we described a rare paramagnetic thiolate-bridged FeIIIFeIII hydride bridged complex [Cp*Fe(μ-η2:η4-bdt)(μH)FeCp*][BF4] (2[BF4], Cp* = η5-C5Me5, bdt = benzene1,2-dithiolate) derived from temperature-dependent isomerization of diiron terminal hydride complex [Cp*Fe(t-H)(μη2:η4-bdt)FeCp*][BF4] (1[BF4]). Besides, complex 2[BF4] exhibits excellent one-electron redox behavior for the formation of a mixed-valence FeIIFeIII hydride bridged complex [Cp*Fe(μ-η2:η2-bdt)(μ-H)FeCp*] (2).16 Among these complexes, the iron centers are in +3 or +2 formal oxidation states similar to those in the FeMo-cofactor.17 Herein, we investigate the reactivity of diiron terminal and bridging hydrides toward nitrogenase related substrates. When treating with alkynes and isocyanides, hydrides play as an electron storage to reduce the iron center for providing the coordination site of nitrogenase substrates, along with the formation of dihydrogen. Differently, treatment of terminal hydride with azide gave an amide complex through the hydrogen transfer insertion.
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RESULTS AND DISCUSSION Initially, we commenced our investigation by examining the reactivity of diiron hydride complex toward isocyanides, which represent an important class of substrate and inhibitor of nitrogenase.2 As shown in Scheme 1, we chose tBuNC as a representative example to understand its interaction pattern with the potential diiron active center of the FeMo-cofactor. After 1 equiv of tBuNC was added to a CH2Cl2 solution of temperature-sensitive 1[BF 4] at −78 °C, a series of complicated color changes were observed from −78 °C to room temperature. Various spectroscopic characterizations and X-ray crystallographic analysis of the resulting product B
DOI: 10.1021/acs.inorgchem.8b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
(1.155(5) Å),21 [Cl2Fe(μ-SEt)2(t-tBuCN)FeCp*] (1.149(5) Å),21 and some heteronuclear complexes with {Cp*Fe(tBuNC)} fragment.22 Next, we examined the reactivity of azide, which can also play as a substrate of nitrogenase to be transformed into N2, NH3, or N2H4.2,23 As illustrated in Scheme 1, the reactivity of hydride complexes toward organic substituted azide TMSN3 was investigated as a model reaction. Reaction of terminal hydride complex 1[BF4] with 1 equiv of TMSN3 from −78 °C to room temperature gave a diiron amido complex [Cp*Fe(μNH2)(μ-η2:η2-bdt)FeCp*][BF4] (4[BF4]) judged from various spectroscopic methods including 1H NMR, IR, and ESIHRMS. Different from the reactivity toward isonitriles, the hydride did not transform into hydrogen but transferred to the azido group and then release a N2 molecule to give an amido species. This result is completely different from the diiron reaction system with the β-diketiminate ligand built by Holland, which afforded a μ1,3-η1:η1 end-to-end azide bridged diiron complex.13b In our bdt system, the potential imido complex is unstable confirmed by density functional theory (DFT) calculation;15c therefore the final product is a welldefined amido species obtained in relatively low yield. Similar transfer hydrogenation of the azido group to amido bridged ligand was also observed in the dinuclear system.24 The molecular structure of 4[BF4] is also characterized by X-ray diffraction analysis. Except the discrepancy of the counterion, other structural data are almost consistent with the previously reported complex [Cp*Fe(μ-NH2 )(μ-η 2:η 2-bdt)FeCp*][BPh4] (4[BPh4]).15c The crystal structure of 4[BF4] clearly reveals that a bridging amido group binds to the two iron centers.25 The Fe−N distances of 1.917(4) and 1.919(4) Å are slightly shorter than those of 4[BPh4]; however, as expected for coordination of sp3-hybridized nitrogen atom to the iron center. The short Fe−Fe bond length of 2.4427(7) Å suggests the strong interaction of the two iron atoms. The spectroscopic results show that the solution structure is in good agreement with the solid-state structure. As acetylene (C2H2) is isoelectric with N2, it is usually chosen as an ideal candidate of N2 for exploring the nitrogen fixation detailed mechanism.26 First, we investigated the reactivity of temperature-sensitive terminal hydride complex 1[BF4] toward acetylene at low temperature. Exposure of 1 atm of acetylene to the CH2Cl2 solution of complex 1[BF4] from −78 °C to room temperature did not afford the desired product involving the HCCH group determined by the ESIHRMS analysis. The 1H NMR result shows only that the isomerization process occurred to give a stable hydride bridged complex 2[BF4]. In sharp contrast, after stirring of the CH2Cl2 solution of hydride bridged diiron complex 2[BF4] in the acetylene atmosphere at 30 °C for 8 h, a thiolate-bridged diiron complex possessing a side-on bound acetylene ligand [Cp*Fe(μ-η2:η2-bdt)(μ-η2:η2-HCCH)FeCp*] (5[BF4]) was obtained in 71% yield (Scheme 1). The homolytic cleavage of two FeIII−H bonds promoted by the acetylene molecule resulted in the formal reduction of the iron center. Surprisingly, a small amount of H2 was detected by gas chromatography; however, a similar phenomenon was also observed in the reaction of [(β-diketiminate)Fe(μ-H)]2 with TMSN3 reported by Holland et al.13b Different from previously reported monodentate thiolate system, there is no analogue eight-electron-donor allylidene anion detected, which is derived from carbon−carbon coupling of two HCCH molecules promoted by the {Fe(μ-SEt)2Fe} core.15b This
mononuclear tBuNC complexes confirmed by ESI-MS and IR spectra. Complex 3[BF4] exhibits excellent solubility in CH2Cl2, but no dissolution in low polar solvents such as n-hexane. Whether in the solid state or in solution, complex 3[BF4] is all particularly air-sensitive. This complex is not very robust in solution under an inert atmosphere for a long time, as it gradually decomposed to other unidentified species during recrystallization for getting single-crystals with X-ray quality at ambient temperature. However, single-crystals suitable for Xray diffraction analysis can be obtained by slow diffusion of nhexane into a CH2Cl2 solution of 3[BF4] at −40 °C. The ORTEP drawing of 3[BF4] is shown in Figure 1, and the most
Figure 1. ORTEP (ellipsoids at 50% probability) diagram of 3[BF4]. The BF4− counterion, one cocrystallized CH2Cl2 molecule, and hydrogen atoms on carbon atoms are omitted for clarity.
striking feature of this cluster is the existence of a terminal BuNC ligand compared with parent complex [Cp*Fe(μ-η2:η4bdt)FeCp*][BF4].16 The introduction of the tBuNC ligand into this diiron scaffold causes some obvious changes in the structural data due to the steric hindrance effect. The Fe···Fe distance of 3.191(2) Å is remarkably lengthened, which indicates the absence of iron−iron bond interaction. The Fe1− S1 and Fe1−S2 bonds are slightly elongated in contrast with the precursor complex, which are close to Fe2−S1 and Fe2−S2 bonds. Due to the occupancy of the vacant site of the Fe1 center with tBuNC, two Cp* rings rotate inwardly with a dihedral angle of 72.3(3)°. The core Fe2S2 framework is substantially puckered as a butterfly with a torsion angle of 145.5(1)°. The bdt group plays as a π-radical monoanionic bridging ligand linked to the two iron centers in a μ-η2:η4 manner, which can stabilize the electron-deficient diiron platform.15c The tBuNC ligand terminally σ-bonded to the Fe1 center with the Fe1−C27−N1−C28 fragment in a nearly linear arrangement. The Fe1−C27 bond length is 1.864(7) Å, and the Fe1−C27−N1 bond angle is 178.7(5)°, which are similar to reported iron complexes with an analogous slightly bent structural moiety.18 The C27N1 bond length of 1.143(7) Å is slightly shorter than that of the nitrogen−carbon triple bond length (1.157(5) Å),19 and close to those of thiolate-bridged diiron tBuNC complexes featuring Cp* [Cp*Fe(μ-1 κ 3 SSS’:2 κ 2 SS-tpdt){Cp*Fe-(t- t BuNC)}][PF 6 ] (1.148(8) Å),20 [Cp*Fe(t-SEt)(μ-SEt)2(t-tBuCN)FeCp*] t
C
DOI: 10.1021/acs.inorgchem.8b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
dihedral angle of Fe1−C27C28−Fe2 of 82.58(16)°. The two Cp* rings are nearly parallel with the dihedral angle of 13.6(2)°. Similarly, treatment of 2[BF4] with excess PhCCH in CH2Cl2 at 30 °C for 8 h also gave a structurally similar complex [Cp*Fe(μ-η2:η2-bdt)(μ-η2:η2-CHCPh)FeCp*][BF4] (6[BF4]) in moderate yield (Scheme 1). Like 5[BF4], the 1 H NMR spectrum of 6[BF4 ] also shows some paramagnetic shifted broad resonances, which are difficult to be unambiguously assigned. The solution μeff of 6[BF4] was determined to be 1.86 μB by the Evans method, which further proves complex 6[BF4] is a paramagnetic species. Furthermore, ESI-HRMS analysis exhibits the expected molecular ion peak of [6]+ with an m/z of 624.1284 (calcd 624.1271), which clearly confirmed the PhCCH fragment was embedded into the thiolate-bridged diiron cluster. Similarly, no appearance of corresponding absorption of ν(CC) suggests that the existence of remarkable π back-bonding interaction between the two metal centers and the phenylacetylene group. The molecular structure of complex 6[BF4] was confirmed by X-ray crystallography. Its ORTEP drawing is shown in Figure 3, and selected bond lengths and angles are given in
result may be attributed to the rigidity of the bdt ligand, which limits the sulfur ligand to take part in this reaction. Besides, steric hindrance of the bdt ligand cannot provide enough space for accommodating so big a bridging group. The 1H NMR spectrum of 5[BF4] shows several very broad peaks with obvious paramagnetic shifts, which makes the certain assignments difficult. The paramagnetism of 5[BF4] was further determined by solution effective magnetic susceptibility at room temperature with μeff = 1.69 μB. In addition, the rare side-on coordination mode led to the strong π back-bonding from the iron center to the HCCH ligand, which can obviously decrease the bond order of the acetylene group. This phenomenon is consistent with the result of the disappearance of the diagnostic CC stretching vibration absorption in the IR spectrum. A weak absorption peak at 3053 cm−1 is assignable to C−H stretching vibration of C2H2 group. Furthermore, the electrospray ionization high-resolution mass spectrometry (ESI-HRMS) provides direct evidence for the existence of acetylene group, in which there is a molecular ion peak [5]+ with an m/z of 548.0962 (calcd 548.0958). Unfortunately, no suitable crystals for X-ray diffraction experiment were obtained in all available double solvent recrystallization systems. After the counterion BF4− was exchanged by PF6− through salt metathesis, single crystals of complex 5[PF6] suitable for X-ray diffraction analysis were smoothly obtained. The geometric arrangement of the cationic portion in 5[PF6] is a symmetrical structure shown in Figure 2. The
Figure 3. ORTEP (ellipsoids at 50% probability) diagram of 6[BF4]. The BF4− counterion and hydrogen atoms on carbon atoms except the side-on phenylacetylene group are omitted for clarity.
Table S4. The geometric arrangement is very similar to that of complex 5[PF6]. The two iron centers are bridged by a bdt ligand in μ-η2:η2 mode and a PhCCH group via the πelectrons in a side-on pattern. This coordination mode is similar to previously reported monodentate thiolate-bridged PhCCH coordinated complex [Cp*Fe(μ-SEt)2(μ-η2:η2CHCPh)FeCp*][PF6].15b The Fe−S bonds and Fe−C bonds are slightly elongated compared to 5[PF6], while the Fe−Fe distance is almost not obviously changed. These tiny changes are all attributed to the substitution of a hydrogen atom of acetylene using the phenyl group. The C31−C32 bond length is 1.302(5) Å, which is slightly shorter than 1.319(13) Å of analogue with two SEt bridges.15b The significant bending of the C31−C32−C33 unit of 141.9(4)° suggests the carbons on the alkyne group are close to sp2 hybridization.
Figure 2. ORTEP (ellipsoids at 50% probability) diagram of 5[PF6]. The PF6− counterion and hydrogen atoms on carbon atoms except side-on acetylene group are omitted for clarity.
HCCH group binds to the two iron centers via the πelectrons of the triple bond in a rare μ-η2:η2 mode. The Fe−Fe bond length of 2.481(2) Å suggests the presence of a strong bonding interaction between the two iron centers. The average Fe−C bond in the {Fe(μ-η2:η2-C2H2)Fe} fragment is 1.985(5) Å, which is in the normal range.27 The C27−C28 bond distance is 1.277(8) Å, which is between C−C triple bond (1.20 Å) in acetylene and C−C double bond (1.35 Å) in ethylene. Relatively long Fe−C bonds and C−C bond indicate the two iron centers distinctly weaken the CC bond by strong π back-bonding from the metal center to the π* orbitals of acetylene. The carbon−carbon bond of C2H2 bridging ligand is nearly orthogonal to the Fe−Fe bond vector with the D
DOI: 10.1021/acs.inorgchem.8b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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with Et2O (3 × 5 mL). The blue-green solids 3[BF4] (109.6 mg, 0.16 mmol) were obtained in 89% yield after drying in reduced pressure. Recrystallization of 3[BF4] from a concentrated CH2Cl2 solution diffused by n-hexane at −30 °C afforded crystals suitable for X-ray diffraction experiment. 1H NMR (400 MHz, CD2Cl2): δ −4.09 (br), −2.21(s), −1.41(s), 0.23 (s), 0.84 (s), 1.25 (s), 1.54 (s), 4.04 (br), 6.44 (br). μeff (CD2Cl2, Evans method, 25 °C) = 2.1 μB. IR (KBr; cm−1): 3122, 2978, 2911, 2135 (νN≡C), 1376, 1053, 747. ESI-HRMS: Calcd for [3]+ 605.1537; Found 605.1541. Anal. Calcd for C32H45Fe2S2NCl2BF4: C, 49.45; H, 5.84; N, 1.80. Found: C, 49.75, H, 5.91, N, 1.62. Synthesis of [Cp*Fe(μ-η2:η2-bdt)(μ-NH2)FeCp*][BF4] (4[BF4]). This method is not the same as published procedures of previous literature reports.25 To a vigorously stirring solution of [Cp*Fe(μη2:η4-bdt)FeCp*] (62.6 mg, 0.12 mmol) in 6 mL of CH2Cl2 was dropwise added HBF4·Et2O (0.12 mmol, 21 μL, 50% w/w). After vigorously stirring for 30 min at −78 °C, the solution color changed completely from aubergine to violet, which suggested the formation of [Cp*Fe(t-H)(μ-η2:η4-bdt)FeCp*][BF4] (1[BF4]). Then TMSN3 (13.8 mg, 0.12 mmol) was dropwise added to the resulting solution at −78 °C. The color of the reaction mixture gradually changed from violet to bright yellow as the temperature rose from −78 °C to room temperature. After removal of the solvent, the residue was washed with n-hexane (5 mL × 3) to give yellow product 4[BF4] (37.5 mg, 0.06 mmol, 50%). Crystals suitable for X-ray diffraction were obtained from a CH2Cl2 solution layered with n-hexane at room temperature. 1 H NMR (400 MHz, CD2Cl2): δ 1.51 (s, 30H, Cp*-CH3), 6.45 (s, 2H, bdt-H), 6.84 (s, 2H, bdt-H), 8.35 (br, 2H, NH2). ESI-HRMS: Calcd For [4]+ 538.0989; Found 538.0986. IR (Film, cm−1): 3338 (νNH2), 3277 (νNH2), 2965, 2918, 2852, 1434, 1380, 1061, 1029, 768. Anal. Calcd For C26H36Fe2S2NBF4: C, 49.95; H, 5.80; N, 2.24. Found: C, 49.75; H, 5.75; N, 2.01. Synthesis of [Cp*Fe(μ-η2:η2-bdt)(μ-η2:η2-CHCH)FeCp*][BF4] (5[BF4]). The solution of [Cp*Fe(μ-η2:η4-bdt)(μ-H)FeCp*][BF4] (83 mg, 0.14 mmol) in CH2Cl2 (8 mL) was set under 1 atm of CHCH, and then stirred at 30 °C for 8 h. The color of the reaction mixture changed from dark violet-red to yellow-green. After removal of the solvent, the residue was washed with n-hexane (5 mL × 3) to give a yellow-green product 5[BF4] (64 mg, 0.10 mmol, 71%). The anion exchange reaction of 5[BF4] with NH4PF6 was performed to give complex 5[PF6] in good yield. Single-crystals of 5[PF6] suitable for X-ray diffraction were obtained from a CH2Cl2 solution layered with n-hexane at room temperature. Proton resonances in the 1H NMR spectrum of 5[BF4] are difficult to be accurately assigned due to its paramagnetism. μeff (CD2Cl2, Evans method, 25 °C) = 1.69 μB. IR (KBr, cm−1): 3434, 3053 (ν≡C−H), 2967, 2909, 1634, 1430, 1378, 1055, 762. ESI-HRMS: Calcd For [5]+ 548.0958; Found 548.0962. Anal. Calcd For C28H36Fe2S2PF6: C, 48.50; H, 5.23. Found: C, 48.81; H, 5.31. Synthesis of [Cp*Fe(μ-η2:η2-bdt)(μ-η2:η2-CHCPh)FeCp*][BF4] (6[BF4]). The solution of [Cp*Fe(μ-η2:η4-bdt)(μ-H)FeCp*][BF4] (90 mg, 0.15 mmol) and CHCPh (75 mg, 0.74 mmol) in CH2Cl2 (8 mL) was stirred at 30 °C for 8 h. After removal of the solvent, the residue was washed with n-hexane (5 mL × 3) to give yellow-green product 6[BF4] (74 mg, 0.10 mmol, 67%). Crystals suitable for X-ray diffraction were obtained from a CH2Cl2 solution layered with n-hexane at room temperature. Like 5[BF4], proton resonances in the 1H NMR spectrum of 6[BF4] are difficult to be accurately assigned due to its paramagnetism. μeff (CD2Cl2, Evans method, 25 °C) = 1.86 μB. IR (KBr, cm−1): 3433, 3062, 2979, 2910, 1633, 1431, 1378, 1083, 761. ESI-HRMS: Calcd For [6]+ 624.1271; Found 624.1284. Anal. Calcd For C34H40Fe2S2BF4: C, 57.41; H, 5.67. Found: C, 57.66; H, 5.76.
CONCLUSIONS In summary, we have exhibited interesting reaction patterns of thiolate-bridged FeIIIFeIII hydride complexes toward various unsaturated small molecules. Upon exposure to the acetylene atmosphere, the bridging hydride was eliminated as H2 accompanied by coordination of an acetylene molecule to the diiron centers in a novel μ-η2:η2 fashion. Similarly, the terminal hydride can also be converted into H2 derived from the homolytic cleavage of FeIII−H bond, followed by coordination of a tBuNC group. Unexpectedly, a tBuNC group terminally bound to an iron center, and no isomerization was observed from −78 °C to room temperature. The above reaction types are reminiscent of the hydrides in the proposed E4 inermediate state during the catalytic cycle of nitrogenase, which are eliminated to liberate H2 and meantime deliver electrons to the FeMo-cofactor for substrate binding. Differently, when interacting with TMSN3, insertion of the azido group into the Fe−H−Fe bond and then release of a N2 molecule facilely gave a diiron amide complex. This thiolatebridged diiron hydride system provides a new platform for getting deep insight into the genuine role of bridging hydrides for the multielectron biological reduction process from N2 to NH3.
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EXPERIMENTAL SECTION
General Consideration. All manipulations were performed under an argon atmosphere by standard Schlenk techniques unless otherwise specified. All solvents were dried and distilled over an appropriate drying agent under argon. Complexes 1[BF4] and 2[BF4] were prepared according to literature procedures.16 CHCH (Dalian GuangMing Special Gas Products Co., Ltd.), CHCPh (Energy Chemical), tBuNC (Energy Chemical), and TMSN3 (Energy Chemical) are commercial available and used as received without further purification. Spectroscopic Measurements. The 1H NMR spectra were recorded on a Brüker 400 Ultra Shield spectrometer. Infrared spectra were recorded on a NEXVSTM FT-IR spectrometer. ESI-HRMS spectra were recorded on a HPLC/Q-Tof microspectrometer. Elemental analyses were performed on a Vario EL analyzer. Solution phase magnetic measurements were performed by the Evans method.28 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 Procedures. Single-crystal X-ray diffraction studies were carried out 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.29 All structures were solved by direct methods and refined by full-matrix least-squares procedures on F2 using SHELXTL 97.30 All of the non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were generated and refined in ideal positions. Crystal data and collection details for complexes 3[BF4], 5[PF6], and 6[BF4] are given in Tables S1 in the Supporting Information. For 3[BF4], the “simu” restraint was used with the deviation being 0.005 on C28 to C31 atoms, and 0.02 on C11 to C20 atoms to avoid the Hirshfeld Test Diff on several C−C bonds. Synthesis of [Cp*Fe(t-tBuNC)(μ-η2:η4-bdt)FeCp*][BF4] (3[BF4]). To a vigorously stirring solution of [Cp*Fe(μ-η2:η4-bdt)FeCp*] (97.2 mg, 0.18 mmol) in 10 mL of CH2Cl2 was dropwise added HBF4·Et2O (0.18 mmol, 32 μL, 50% w/w). After vigorously stirring for 30 min at −78 °C, the solution color changed completely from aubergine to violet, which suggested the formation of [Cp*Fe(tH)(μ-η2:η4-bdt)FeCp*][BF4] (1[BF4]). Then tBuNC (15.5 mg, 0.18 mmol) was dropwise added to the resulting solution at −78 °C. After undergoing a series of complicated color changes from −78 °C to room temperature, the solution color finally became bluish green. Solvent was removed in vacuo, and the crude product was washed
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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.8b02459. E
DOI: 10.1021/acs.inorgchem.8b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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X-ray crystallographic data and spectroscopic data of all complexes (PDF) Accession Codes
CCDC 1033582, 1053873, and 1863916 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing
[email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Baomin Wang: 0000-0001-9058-4983 Jingping Qu: 0000-0002-7576-0798 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21231003, 21571026, 21690064), 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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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
