Article pubs.acs.org/Organometallics
Hydration of Nitriles to Amides by Thiolate-Bridged Diiron Complexes Peng Tong, Dawei Yang, Yang Li, Baomin Wang, 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: A series of nitrile-coordinating complexes [Cp*Fe(μ-SEt)RCN]2[PF6]2 (1, R = alkyl, aryl, vinyl, amine) have been obtained by the reaction of [Cp*Fe(μ-SEt)MeCN] 2 [PF 6 ] 2 (1a) with various nitriles in acetone. Complexes 1 can realize the hydration of a nitrile ligand under ambient conditions. Complexes [Cp*Fe(μ-SEt)2(μη1:η1-NH(O)CR)FeCp*][PF6] (2) were successfully isolated as intermediates during the hydration process, with 2b and 2e (R = CH2CH and Et2N) being characterized by spectrometry and X-ray crystallography. Treatment of 2 with HBF4·Et2O in the presence of nitriles released corresponding amides 3. At the same time, the structural features of the [Fe2S2] scaffold were retained. These results confirmed that the hydration of nitriles was realized by cooperative interaction on diiron centers.
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INTRODUCTION Hydration of nitriles to corresponding amides is a valuable reaction in academia and industry. Amides are used not only as intermediates in organic synthesis but also for a wide variety of applications in the production of polymers, drug stabilizers, detergents, and herbicides.1 In industry, hydration of nitriles is conventionally performed with strong acids and bases. However, these processes present several disadvantages, such as high-energy demands, harsh conditions, and the generation of wasteful byproducts. To overcome these limitations, a number of mononuclear transition-metal complexes have been synthesized as catalysts for conversion of nitriles to corresponding amides.2 Most of these catalysts have been performed using noble metals such as ruthenium, rhodium, and iridium.3 However, the high cost of these metals has severely limited large-scale applications. Therefore, chemists are focusing their attention on the use of first-row transition metals, especially iron and cobalt. The first example of hydration of a nitrile carried out by a ferrous center to corresponding amide was reported by Mandon and coworkers.4 However, this system is only efficient for the hydration of the cyanide-α-substituted tripod (6-cyano 2pyridylmethyl)bis(2-pyridylmethyl)amide. The conversion of other nitriles to corresponding amides was not recorded in this work. Compared to monometallic centers, dimetallic centers widely exist in metalloenzymes (e.g., ureas 5 or purple acid phosphatase6), which can realize the catalytic transformation of small molecules by a synergetic effect. Plenty of dimetallic complexes were reported as structural or functional mimics for © 2015 American Chemical Society
capturing intermediate species or achieving the enzymatic catalytic cycle.7 Over the past decades, some diiron hydrolase functional model complexes were obtained and can promote nitrile hydration by the cooperative activation of bimetallic centers.8 The first example that a diiron complex can hydrate acetonitrile to acetamide was reported by Que and coworkers.8a They suggested that hydration of coordinating acetonitrile was realized by nucleophilic attack of the hydroxide bonded to the other iron center. However, release of the acetamide product was not documented in this work. In our previous work, we reported a series of thiolate-bridged diiron complexes, which provide bimetallic reaction sites to small molecules. 9 In that study, [Cp*Fe(μ-SEt)(MeCN)]2[PF6]2 (1a) stands out as a versatile precursor toward the construction of nitrogenase mimics through ligand substitution of the weakly coordinated acetonitrile by nitrogenous ligands.9d In view of the existence of the acetonitrile ligands in 1a, we questioned whether this compound could serve as a nitrile hydratase functional model complex for the hydration of nitriles. Along this line, we report the synthesis of several nitrile-coordinating complexes [Cp*Fe(μ-SEt)RCN]2[PF6]2 (1) from [Cp*Fe(μ-SEt)MeCN]2[PF6]2 (1a) through ligand exchange with various nitriles or cyanamide derivatives (for example, acrylonitrile and cyanamide), together with their conversions to diiron−amide complexes [Cp*Fe(μSEt)2(μ-η1:η1-NH(O)CR)FeCp*][PF6] (2) under ambient conditions (Scheme 1). Among complexes 2, 2b (R = Received: May 7, 2015 Published: July 13, 2015 3571
DOI: 10.1021/acs.organomet.5b00387 Organometallics 2015, 34, 3571−3576
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Organometallics
[Cp*Fe(μ-SEt)]2 and two terminal CH2CHCN ligands, which are σ-bonded to the metal centers through their nitrogen atoms (Fe1−N1 = 1.908(5) Å). The N1−C25 and C25−C26 bond lengths (1.135(8) and 1.56(7) Å) and the C25−C26− C27 angle (114.0(5)°) are close to those of free acrylonitrile. To get further insight into nitrile ligand exchange in our diiron dithiolate system, we investigated the reactivity of 1a toward some aromatic nitriles. For example, when 1a was treated with excess benzonitrile at ambient temperature for 24 h, 1g was obtained as dark brown microcrystalline solids in 88% yield. An ORTEP drawing of 1g is shown in Figure 2. The structure of
Scheme 1. Nitrile Hydration on Diiron Centers
CH2CH) and 2e (R = Et2N) were characterized by spectroscopy and X-ray crystallography. Treatment of complexes 2 with HBF4·Et2O afforded the corresponding amides. At the same time, the diiron core structure still remained.
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RESULTS AND DISCUSSION We initiated our studies by investigating the ligand exchange of complex 1a with other nitriles. To our delight, the reaction of 1a with excess CH2CHCN in acetone at room temperature for 24 h resulted in the diiron thiolate-bridged cluster 1b as brown microcrystalline solids in 90% yield. The 1H NMR spectrum of 1b in CD2Cl2 shows resonances at 5.76−6.11 ppm for coordinating acrylonitrile protons (Supporting Information Figure S31). The electrospray ionization high-resolution mass spectrometry (ESI-HRMS) exhibits the expected molecular ion peak [1b−(CH 2 CHCN)−(PF 6 ) 2 ] 2+ with an m/z of 278.5992 (calcd 278.5991), which confirms the existence of the one coordinating acrylonitrile subunit in 1b. The CN stretching vibration (2241 cm−1) of 1b is similar to that of free acrylonitrile (2231−2248 cm−1).10 The solid-state structure of 1b is shown in Figure 1. The Fe− Fe distance of 2.748(2) Å is indicative of an intermetallic bond that falls in the Fe−Fe single bond distance range between 2.5 and 2.8 Å.11 Complex 1b consists of a di(μ-thiolate)diiron unit
Figure 2. Molecular structure of complex 1g. ORTEP drawing with 30% probability ellipsoids. All hydrogen atoms and counteranion PF6− are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−Fe1A 2.739(2), Fe1−S1 2.220(3), Fe1−S1A 2.220(3), Fe1−N1 1.897(6), N1−C25 1.144(8), C25−C26 1.427(9); Fe1−S1−Fe1A 76.1(2), Fe1A−Fe1−N1 94.7(3), Fe1−N1−C25 176.6(6), Cp*1− Cp*2 65.3.
1g resembles that of 1b. The two Cp* ligands are also in mutually cis orientation with a dihedral angle of 65.3°. The positions of Fe1, Fe1A, S1, and S1A are in the same plane, with a dihedral angle of 178.9(2)°. The benzonitrile ligands are σbonded to the metal centers, and the dihedral angle between two benzene groups of benzonitriles is only 0.17°. Analogous complexes 1c−1l were obtained in a similar manner (Scheme 1). The coordinating nitriles of 1 are listed in Table 1. Metal centers help to promote activation and hydration of nitrile ligands. To probe the hydration process of the coordinating nitrile by diiron centers, the UV−vis spectra were obtained (Figure 3). Titration of Et3N (in 0.1 equiv aliquots) to an aqueous (H2O/MeCN = 1:1) solution of 1a (allowing 0.5 h for equilibration between each reaction) results in the loss of the bands at 306, 364, and 440 nm and the formation of a new species 2a, with a band at 348 nm (Figure 3). One equivalent of Et3N is required for the reaction to reach completion. A similar result was observed when we replaced 1a with 1b. The synthesis and characterization of these new complexes 2 will be discussed later. In order to investigate further the hydration process of nitrile on metal centers, the following experiments were performed. Treatment of 1b with 1 equiv of Et3N in aqueous acetone at ambient temperature for 2 h afforded [Cp*Fe(μ-SEt)2(μ-η1:η1NH(O)CCHCH2)FeCp*][PF6] (2b) in 88% yield (Scheme 1). The choice of base is critical to the reaction since no reaction was observed when changing Et3N for the weaker base pyridine even at 80 °C. The 1H NMR spectrum of 2b in CD2Cl2 shows two peaks at 1.45 and 1.34 ppm, which are ascribed to the methyl protons of two inequivalent Cp*. The IR spectrum of 2b shows the ν(N−H) band at 3353 cm−1, which
Figure 1. Molecular structure of complex 1b. ORTEP drawing with 30% probability ellipsoids. All hydrogen atoms and counteranion PF6− are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−Fe1A 2.748(2), Fe1−S1 2.224(2), Fe1−S1A 2.232(2), Fe1−N1 1.908(5), N1−C25 1.135(8), C25−C26 1.56(7), C26−C27 1.26(7); Fe1−S1−Fe1A 76.2(1), Fe1A−Fe1−N1 95.4(2), Fe1−N1−C25 173.7(5), C25−C26−C27 114.0(5), Cp*1−Cp*2 69.2(2). 3572
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Organometallics Table 1. Hydration of Nitriles into Amides Activated by Thiolate-Bridged Diiron Complexes
Figure 3. Monitoring the hydration reaction between 1a and Et3N in H2O/MeCN (1:1) at room temperature by UV−vis spectroscopy, showing that 1 equiv of Et3N is required for the reaction to reach completion.
Figure 4. Molecular structure of complex 2b. ORTEP drawing with 30% probability ellipsoids. All hydrogen atoms and counteranion PF6− except those on N1, C26, and C27 are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−Fe2 2.641(2), Fe1−N1 1.959(3), Fe2−O1 1.977(2), N1−C25 1.302(4), O1−C25 1.288(4), C25−C26 1.484(4), C26−C27 1.304(5); Fe1−N1−C25 125.1(2), Fe2−O1−C25 122.9(2), N1−C25−O1 122.7(3), N1−C25−C26 119.3(3), O1−C25−C26 118.0(3).
C−O bond angle from 120° was observed. As expected, 2b has a N−C−O bond angle of 122.7(3)°. In comparison of the structural data for 1b and 2b, the Fe−Fe distance of 2b is significantly shorter than that of 1b (2.748(2) Å) because of the replacement of two acetonitrile ligands with a bridging amidate ligand.14 Noticeably, the distance of N1−C25 (1.135(8) Å) in 1b lengthened compared with that in 2b (1.302(4) Å). This is likely due to the conversion of C25 and N1 from sp to sp2 hybridization. In addition, we also studied hydration of cyanamide ligands on the diiron centers. However, we did not obtain the crystal of complex 2c with a bridging deprotonated urea ligand. To obtain deep insight into the hydration of the cyanamide ligand, synthesis of [Cp*Fe(μ-SEt)Et2NCN]2[PF6]2 (1e) was carried out with Et2NCN as the substrate. In the same way, treatment of 1e with 1 equiv of Et3N in aqueous acetone at ambient temperature produces [Cp*Fe(μ-SEt) 2 (μ-η 1 :η 1 -NH(O)CNEt2)FeCp*][PF6] (2e) in 88% yield. In the 1H NMR spectrum of 2e, two resonances for Cp* methyl protons did not appear in the usual range (1.0−2.0 ppm) but appeared at 0.504 and 0.686 ppm (Supporting Information Figure S46). This result suggests that 2e should be a paramagnetic complex, which may be caused by antiferromagnetic coupling of two low-
Reaction conditions: 2 (0.1 mmol), nitrile (0.3 mmol), HBF4·Et2O (0.1 mmol), acetone (2 mL), rt for 2 h. Yields are based on isolated amides. bReaction conditions: 1 (0.1 mmol), H2O (1 mL), Et3N (0.1 mmol), acetone (2 mL), rt for 2 h. cQuantitative analyses are based on HPLC-MS (Supporting Information). a
is consistent with reported complexes in the literature (3300− 3365 cm−1).12d The ESI-HRMS exhibits the molecular ion peak [2b − PF6]+ with an m/z of 574.1569 (calcd 574.1564), which confirms the existence of the deprotonated acrylamide ligand in 2b. The molecular structure of 2b is further confirmed by X-ray crystallographic analysis (Figure 4). The overall structure consists of two Cp*Fe units bridged by two syn-axial SEt ligands, in which the Fe−Fe distance of 2.641(2) Å is indicative of the presence of a bonding interaction between the two Fe centers. The N1−C25 and O1−C25 bond lengths of 2b are 1.302(4) and 1.288(4) Å, respectively. These values are close to the average values for such bonds in amidate-bridged transitionmetal complexes (1.27−1.32 Å).13 In complex 2b, the amidate ligand retains an sp2 hybridization, so little deviation in the N− 3573
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Organometallics spin FeIII centers. The ESI-HRMS shows the molecular ion peak [2e − PF6]+ with an m/z of 619.2145 (calcd 619.2142). An ORTEP diagram of 2e is shown in Figure 5. The structure of 2e resembles that of 2b, which is bridged by a
characterized by 1H NMR. In the presence of 3 equiv of cinnamonitrile, complex 1j′ was obtained again. Hydration of the cinnamonitrile proceeded exclusively at the cyano group to afford the corresponding α,β-unsaturated amide (entry 10). The yields of [Cp*Fe(μ-SEt)2(μ-η1:η1-NH(O)CR)FeCp*][PF6] (2g−2l) were not influenced by the electronic effects of the substituents on the aromatic ring of benzonitriles. Encouraged by these results, we further investigated the hydration reaction of cyanamide compounds (entries 3−6). We can obtain the deprotonated urea-coordinating complex 2c in 85% yield. Subsequently, urea can be prepared independently by treatment of 2c with HBF4·Et2O. The identification of urea was confirmed by HPLC-MS. In this system, Me2NCN, Et2NCN, and Bn2NCN were also hydrated to give corresponding products in high yields. As mentioned above, cyano groups were converted into acylamino groups on diiron active centers, while the dinuclear system was still retained. The proposed mechanism of this hydration reaction is shown in Scheme 2. First, the deprotonation of H2O leads to the
Figure 5. Molecular structure of complex 2e. ORTEP drawing with 30% probability ellipsoids. All hydrogen atoms and counteranion PF6− except that on N1 are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−Fe1A 3.223(1), Fe1−N1 1.916(2), Fe1A−O1 1.916(2), N1−C25 1.300(3), O1−C25 1.300(3), C25−N2 1.353(5); Fe1−N1−C25 132.6(2), Fe1A−O1−C25 132.6(2), N1−C25−O1 123.1(3), N1−C25−N2 118.5(2), O1−C25−N2 118.5(2).
Scheme 2. Possible Reaction Pathway for Hydration of Nitriles
bidentate deprotonated amide ligand. However, the Fe−Fe bond length of 3.223(1) Å is longer than that in 2b (2.641(2) Å), falling out of the range of the Fe−Fe bonding lengths. Meanwhile, the nonplanar Fe2S2 core has a dihedral angle of 135.6(1)°. The solid-state structure of 2e has a two-fold axis through the C25 atom, and the center of two iron atoms cause a disorder in crystallography (Figure 5). The positions of N1 and O1 were not determined by single-crystal X-ray diffraction analysis, so the average bond length of N1−C25 and O1−C25 is 1.300(3) Å. Even though a number of amidate-bridged dinuclear complexes have been reported,12,15,16 rare examples delivered amides in the presence of a proton source.17 This may be largely due to the tight binding of the bridged amidate ligand to metal centers. This deficiency prompted us to evaluate the reactivity of our diiron complexes 2 with HBF4·Et2O to see whether amides could be delivered. Gratifyingly, treatment of an acetone solution of 2 with 1 equiv of HBF4·Et2O in the presence of 3 equiv of nitriles gave the corresponding amides 3 that were identified by their 1H NMR spectra. At the same time, the RCN-coordinated complexes [Cp*Fe(μ-SEt)RCN]2[PF6][BF4] (1′) were regenerated. Amides were not obtained when changing HBF4·Et2O for other weak acids (such as Lut·HBPh4 and Lut·HCl, Lut = 2,6-lutidine, BPh4 = tetraphenylborate anion) under similar conditions. Subsequently, the scope of this diiron hydration system with regard to various kinds of nitriles was examined. The hydration showed substrate generality with respect to aliphatic, aromatic, α,β-unsaturated, heteroaromatic, and cyano nitriles (Table 1). For example, treatment of 1a with cinnamonitrile for 24 h at room temperature in acetone resulted in [Cp*Fe(μ-SEt)(PhCHCHCN)]2[PF6]2 (1j). After 1 equiv of Et3N was added to the solution of 1j in aqueous acetone, the reaction mixture was stirred for about 2 h at ambient temperature. [Cp*Fe(μ-SEt)2(μ-η1:η1-NH(O)CCHCHPh)FeCp*][PF6] (2j) was successfully isolated as green microcrystalline solids. When 2j was treated with 1 equiv of HBF4·Et2O in acetone, the release of cinnamamide occurred. The residue was extracted with diethyl ether to obtain cinnamamide as white solids
formation of intermediates A and nucleophilic attack of the hydroxide to the nitrile carbon. Then, the formation of complexes 2 was realized by intramolecular proton transfer of intermediates B. Both stages have been explained by the reaction of dinuclear complexes.18 Finally, the corresponding amides were gained in the presence of HBF4·Et2O and nitriles, along with the regeneration of 1. In order to determine the proton source of the N−H bond, we replace water with deuteroxide. Under similar conditions, 2a-D was obtained in 88% yield (see the Supporting Information). The ESI-HRMS shows the molecular ion peak [2a-D − PF6]+ with an m/z of 563.1605 (calcd 563.1595). We also performed an experiment to substitute H218O for water under similar conditions. Complex 2a-18O was gained in 87% yield. The ESI-HRMS shows the molecular ion peak [2a-18O − PF6]+ with an m/z of 564.1605 (calcd 564.1606). To exclude the effect of solvent, the hydration of acetonitrile was performed with deuterated acetone solvent. However, 2a-D was not detected by ESI-MS, which explains that the proton 3574
DOI: 10.1021/acs.organomet.5b00387 Organometallics 2015, 34, 3571−3576
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acrylonitrile. After 24 h, the resulting solution was evaporated to dryness at reduced pressure. The residue was washed with diethyl ether three times and dried under vacuum. Complex 1b (0.31 g, 0.35 mmol) was obtained as a brown crystalline powder in 90% yield. The crystals of 1b, suitable for X-ray analysis, were obtained by a saturated acetone solution layered with diethyl ether: 1H NMR (400 MHz, CD2Cl2) δ 5.756−6.106 (m, 6H), 2.950 (q, 4H), 1.970 (t, 6H), 1.454 (s, 30H); IR (film; cm−1) 2963, 2924, 2241, 1381, 1055, 838, 557. Anal. Calcd for C30H46F12Fe2N2P2S2: C, 40.02; H, 5.15; N, 3.11; S, 7.12. Found: C, 40.07; H, 5.21; N, 3.09; S, 7.07. Analogous complexes 1c−1l were obtained similarly in 80−90% yields. The spectra of these complexes are shown in the Supporting Information. [Cp*Fe(μ-SEt)2(μ-η1:η1-NH(O)CR)FeCp*][PF6] (2a, R = Me). A solution of 1a (0.317 g, 0.36 mmol) in acetone/water (8 mL/2 mL) at room temperature was treated with 1 equiv if Et3N (52 μL, 0.36 mmol). After 2 h, the resulting yellow-green solution was evaporated to dryness at reduced pressure. The residue was washed with 5 mL of diethyl ether and extracted with dichloromethane to obtain a yellowgreen solution of 2a. Green crystalline powder 2a (0.245 g, 0.32 mmol, 89%) was isolated after the dichloromethane was removed in vacuo. The crystals of 2a, suitable for X-ray analysis, were obtained by a saturated dichloromethane solution layered with diethyl ether: 1H NMR (400 MHz, CD2Cl2) δ 1.890 (t, 6H), 1.751 (q, 4H), 1.420 (s, 15H), 1.341 (s, 3H), 1.310 (s, 15H), the proton signal of N−H group was not detected; IR (film; cm−1) 3365, 2964, 2927, 1731, 1561, 1471, 1226, 1023, 842, 557; ESI-HRMS calcd for [2a − PF6]+ 562.1564; found 562.1567. Anal. Calcd for C26H44F6Fe2NOPS2: C, 44.14; H, 6.27; O, 2.26; N, 1.98; S, 9.07. Found: C, 44.19; H, 6.22; O, 2.32; N, 1.92; S, 9.02. According to the same procedure, 2b−2l were also obtained in 70−93% yields. The spectra of these complexes are shown in the Supporting Information. [Cp*2Fe2(μ-SEt)2(μ-η1:η1-NDC(O)Me)][PF6] (2a-D). Using a procedure similar to that used for 2a, 2a-D was synthesized using D2O: yield 88%; ESI-HRMS calcd for [2a-D − PF6]+ 563.1595; found 563.1605. Anal. Calcd for C26H43DF6Fe2NOPS2: C, 44.08; H, 6.40; O, 2.26; N, 1.98; S, 9.05. Found: C, 44.15; H, 6.45; O, 2.20; N, 1.92; S, 9.15. [Cp*2Fe2(μ-SEt)2(μ-η1:η1-NHC(18O)Me)][PF6] (2a-18O). Using a procedure similar to that used for 2a, 2a-18O was synthesized using H218O: yield 87%; ESI-HRMS calcd for [2a-18O − PF6]+ 564.1606; found 564.1605. Anal. Calcd for C26H44F6Fe2N18OPS2: C, 44.02; H, 6.25; O, 2.54; N, 1.97; S, 9.04. Found: C, 44.14; H, 6.36; O, 2.44; N, 1.93; S, 9.13. Reaction of 2a with HBF 4·Et 2O in the Presence of Acetonitrile. To a stirred solution of 2a (0.153 g, 0.2 mmol) in 2 mL of acetone were added HBF4·Et2O (40.0 μL, 0.2 mmol) and acetonitrile (0.6 mmol) at ambient temperature via a microsyringe, followed by stirring for 2 h. The resulting brown solution was evaporated to dryness at reduced pressure. The dark-brown residue was extracted with Et2O to obtain a brown crystalline powder [Cp*Fe(μ-SEt)RCN]2[PF6][BF4] (1a′, R = Me) in 90% yield. After the solvent was removed in vacuo, the white solid acetamide (3a) was obtained in 87% yield. Analytically pure acetamides 3b and 3d−3l were also obtained in 74−91% yields. 3a: 1H NMR (400 MHz, CDCl3) δ 6.281 (br, 1H), 6.111 (br, 1H), 2.010 (s, 3H). Anal. Calcd for C2H5NO: C, 40.67; H, 8.53; N, 23.71; O, 27.09. Found: C, 40.75; H, 8.59; N, 23.78; O, 27.18. 3b: 1H NMR (400 MHz, CDCl3) δ 6.321 (d, 1H), 6.242 (d, 1H), 5.817 (m, 1H). Anal. Calcd for C3H5NO: C, 40.67; H, 8.53; N, 23.71; O, 27.09. Found: C, 40.75; H, 8.59; N, 23.78; O, 27.16. 3c: Anal. Calcd for CH4N2O: C, 20.00; H, 6.71; N, 46.65; O, 26.64. Found: C, 20.10; H, 6.78; N, 46.72; O, 26.72. 3d: 1H NMR (400 MHz, CDCl3) δ 6.913 (br, 2H), 3.064 (br, 6H). Anal. Calcd for C3H8N2O: C, 40.90; H, 9.15; N, 31.79; O, 18.16. Found: C, 40.97; H, 9.21; N, 31.87; O, 18.22. 3e: 1H NMR (400 MHz, CDCl3) δ 5.991 (br, 2H), 3.404 (q, 4H), 1.254 (t, 6H). Anal. Calcd for C5H12N2O: C, 51.70; H, 10.41; N, 24.12; O, 13.77. Found: C, 51.76; H, 10.48; N, 24.18; O, 13.85. 3f: 1H NMR (400 MHz, CDCl3) δ 7.220−7.360 (m, 10H), 5.580 (br, 2H), 4.460 (s, 4H). Anal. Calcd for C15H16N2O: C, 74.97; H, 6.71; N, 11.66; O, 6.66. Found: C, 74.91; H, 6.77; N, 11.60; O, 6.62. 3g: 1H NMR (400 MHz, CDCl3) δ 7.816 (d, 2H), 7.530 (t,
does not originate from the solvent. These results indicate that the proton source and oxygen source of the μ-amidate-N,O bridge stem from the water ligand.
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CONCLUSION In summary, thiolate-bridged diiron complex [Cp*Fe(μSEt)MeCN]2[PF6]2 (1a) reacted readily with different nitriles, yielding [Cp*Fe(μ-SEt)RCN]2[PF6]2 (1). Complexes 1 can realize the hydration of the nitrile ligand in ambient conditions. Simultaneously, [Cp*Fe(μ-SEt)2(μ-η1:η1-NH(O)CR)FeCp*][PF6] (2) were successfully isolated as intermediates during the hydration process. Treatment of 2 with HBF4·Et2O in the presence of nitriles delivered corresponding amides 3. At the same time, the structural features of the [Fe2S2] scaffold were retained. These results indicated that the hydration of nitriles was accomplished by cooperative interaction on diiron centers. Further studies on the catalytic hydration of nitriles and activation of other small molecules at diiron sites are underway.
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EXPERIMENTAL SECTION
General Procedures. All manipulations were routinely carried out under an argon atmosphere, using standard Schlenk-line techniques. All organic solvents and Et3N were dried and distilled over an appropriate drying agent under argon. Complex [Cp*Fe(μ-SEt)MeCN]2[PF6]2 was prepared according to the literature.9d NH4PF6 (Aldrich), organonitriles (Aldrich), and HBF4·Et2O (Aldrich) were used without further purification. Spectroscopic Measurements. The 1H NMR spectra were recorded on a Bruker 400 Ultra Shield spectrometer. Infrared spectra were recorded on a NEXVSTM FT-IR spectrometer. Elemental analyses were performed on a Vario EL analyzer. ESI-MS were recorded on a UPLC/Q-ToF microspectrometer. The UV−vis absorption spectra measurement was performed on a PerkinElmer Lambda 35 spectrophotometer. Chromatographic Conditions. HPLC separation was achieved on an ACCELA high-performance liquid chromatograph (HPLC) system, which consisted of an ACCELA 1250 pump and an ACCELA autosampler. A Click Mal column (150 mm × 4.6 mm i.d., 5 μm, Acchrom Co. Ltd., China) was used in the experiment with the mobile phase of 95/5 (v/v) acetonitrile/CH3COONH4 (5 mM). Eluent flow rate was set at 200 μL/min, and the column was kept at ambient temperature. The injection volume was 1 μL. Mass Spectrometry. Product monitoring was achieved using a TSQ Quantum Ultra mass spectrometer (Thermo Scientific, San Jose, CA) equipped with a heated electrospray ionization source (HESI). Optimization of the precursor ion, product ion, and collision energy was carried out via direct injection of the urea standard solution at a flow rate of 10 μL/min into the mass spectrometer. The instrument was operated in positive ionization mode and selective reaction monitoring with m/z between 61.2 and 44.5, CE of 17, and T lens of 63. The MS source conditions were as follows: capillary voltage of 3000 V, vaporizer temperature of 250 °C, capillary temperature of 300 °C, sheath gas pressure of 35, and aux gas pressure of 10. Xcalibur 2.2 software (Thermo) was used for instrument control, data acquisition, and processing. X-ray Crystallography. The data for complexes 1b, 1g, 2a, 2b, and 2e were obtained on a Bruker SMART APEX CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Empirical absorption corrections were performed using the SADABS program.19 Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F2 using SHELX97.20 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. [Cp*Fe(μ-SEt)RCN]2[PF6]2 (1b, R = CHCH2). A solution of complex [Cp*Fe(μ-SEt)MeCN]2[PF6]2 (1a, 0.33 g, 0.39 mmol) in 10 mL of acetone at room temperature was treated with 10 equiv of 3575
DOI: 10.1021/acs.organomet.5b00387 Organometallics 2015, 34, 3571−3576
Article
Organometallics 1H), 7.446 (t, 2H), 6.126 (br, 2H). Anal. Calcd for C7H7NO: C, 69.41; H, 5.82; N, 11.56; O, 13.21. Found: C, 69.48; H, 5.89; N, 11.63; O, 13.27. 3h: 1H NMR (400 MHz, CDCl3) δ 7.710 (d, 2H), 7.241 (d, 2H), 6.032 (br, 2H), 2.400 (s, 3H). Anal. Calcd for C8H9NO: C, 71.09; H, 6.71; N, 10.36; O, 11.84. Found: C, 71.17; H, 6.78; N, 10.42; O, 11.90. 3i: 1H NMR (400 MHz, CDCl3) δ 7.756 (d, 2H), 7.427 (d, 2H), 5.990 (br, 2H). Anal. Calcd for C7H6ClNO: C, 54.04; H, 3.89; Cl, 22.79; N, 9.00; O, 10.28. Found: C, 54.12; H, 3.97; Cl, 22.72; N, 9.09; O, 10.35. 3j: 1H NMR (400 MHz, CDCl3) δ 7.640 (d, 1H), 7.504 (dd, 2H), 7.360 (t, 3H), 6.472 (d, 1H), 5.820 (br, 2H). Anal. Calcd for C9H9NO: C, 73.45; H, 6.16; N, 9.52; O, 10.87. Found: C, 73.52; H, 6.24; N, 9.60; O, 10.94. 3k: 1H NMR (400 MHz, CDCl3) δ 7.536 (m, 2H), 7.100 (br, 1H), 5.838 (br, 2H). Anal. Calcd for C5H5NOS: C, 47.23; H, 3.96; N, 11.01; O, 12.58; S, 25.22. Found: C, 47.29; H, 3.90; N, 11.11; O, 12.63; S, 25.27. 3l: 1H NMR (400 MHz, CDCl3) δ 8.356 (s, 1H), 7.929−7.876 (m, 4H), 7.589−7.553 (m, 2H), 6.220 (s, 1H), 5.790 (s, 1H). Anal. Calcd for C11H9NO: C, 77.17; H, 5.30; N, 8.18; O, 9.35. Found: C, 77.23; H, 5.35; N, 8.24; O, 9.41. Analysis of Urea (3c). To a stirred solution of 2c (72.4 mg, 0.1 mmol) in 2 mL of acetonitrile was added HBF4·Et2O (20.0 μL, 0.1 mmol) at ambient temperature via a microsyringe, followed by stirring for 2 h. The resulting brown solution was evaporated to dryness at reduced pressure. The mixture was extracted with 20 mL of deionized water. Then, a 1 mL mixture of the aqueous solution was diluted with 9 mL of acetonitrile to obtain the mixed solution that was ready for injection. The yield of urea was obtained by the calibration curve.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis, characterization, structure, and spectroscopic data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00387.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 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 (No. 21231003) and 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.organomet.5b00387 Organometallics 2015, 34, 3571−3576
