Sulfur-Centered Reactivity of Oxidized Iron-Thiolate Complex toward

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Sulfur-Centered Reactivity of Oxidized Iron-Thiolate Complex toward Unsaturated Hydrocarbon Addition Yahui Zhang,† Dawei Yang,*,† Ying Li,† Xiangyu Zhao,† Baomin Wang,† and Jingping Qu*,†,‡ †

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, P. R. China Key Laboratory for Advanced Materials, East China University of Science and Technology, Shanghai 200237, P. R. China

‡

Organometallics Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/31/18. For personal use only.

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ABSTRACT: The sulfur-based reactivity of transition-metal thiolate complexes toward alkenes has received extensive attention as a significant proposal for a potential olefin purification scheme. The one-electron oxidation of halfsandwich iron thioether-dithiolate complex [Cp*Fe(η3-tpdt)] (1, Cp* = η5-C5Me5; tpdt = S(CH2CH2S−)2) resulted in the formation of iron-stabilized thiyl radicals, which can interact with unsaturated hydrocarbons such as alkenes or alkynes to give the corresponding trithioether-iron adducts. Interestingly, during this transformation process, the formal oxidation state of the iron center changed from +IV to +II, which suggests a two-electron transfer from unsaturated hydrocarbons to the iron center. Furthermore, the electrochemical measurements reveal this C−S bond formation is an irreversible process, during which ethylene cannot release under electrochemical reductive conditions. These adducts were unambiguously identified by various spectroscopic and X-ray crystallographic characterizations.

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INTRODUCTION Reactivity of transition-metal thiolates has attracted significant attention owing to their industrial and biological importance.1 Commonly, activation and transformation of substrates occur at the metal center, rather than at the sulfur atoms of the thiolate ligand.2 Although sulfur-centered reactivity was observed as early as in the 1960s,3 a renewed research upsurge was inspired by the potential application for separation and purification of olefins reported by Wang and Stiefel.4 In this system, complex Ni[S2C2(CF3)2]2 can realize reversible olefin binding and release through the two sulfur atoms in the ligand. Furthermore, Fekl and co-workers got deeper insight into this mononuclear nickel reaction system by spectroscopic, kinetic, and computational data.5 These experimental results showed the anionic reduced nickel complex plays a crucial role for the formation of the alkene adducts; however, in the presence of neutral complex dihydrodithiins and metal, decomposition products are preferred. Similarly, Grapperhaus reported a cationic complex [Ru(dppbt)3]+ (dppbt = 2-diphenylphosphino-benzenethiolate) was also responsible for sulfur-based addition of alkenes.6 Further, the reversible C−S bond formation/cleavage between ethylene and [Re(dppbt)3] and its oxidized derivatives can be realized through the facile control of oxidation state.7 Different from Fekl’s system, these metal trithiolate complexes promote the alkenes addition upon oxidation not reduction. Furthermore, density functional theory (DFT) calculation results indicate in Grapperhaus’s reaction scaffold one-electron oxidation of metal thiolates induces metal-stabilized thiyl radicals, which are essential to further addition of unsaturated organic compounds.8 The formation of the thiyl radicals is attributed to the existence of the noninnocent sulfur ligands © XXXX American Chemical Society

like dithiolenes, which have the ability to delocalize electron density. Similarly, Yan and co-workers adopted [S2C2(B10H10)]2− unit to synthesize Co complexes, which also exhibit sulfur-centered addition of alkynes and can release the alkyne disulfurated products.9 Unexpectedly, Goh utilized soft thioether-dithiolate tpdt (tpdt = S(CH 2CH2S−)2) tridentate ligand to generate the half-sandwich ruthenium complex, which displayed sulfur-based alkylation by electrophilic attack of alkylation agent.10 In addition, the addition of acrylonitrile on the adjacent two sulfur atoms in this system was also observed in the presence of I2.11 Although the thiyl radicals derived from iron−sulfur clusters are significant to catalytic transformation in the biological system, there is no report about iron-stabilized thiyl radicals for alkene and alkyne addition whether using a chemical or electrochemical method. In our preliminary work, we adopted different mono- or bidentate thiolates as bridging ligands to construct a series of novel diiron nitrogenase mimics, which exhibit excellent reactivity toward various nitrogenase related substrates.12−14 Recently, we utilized the flexible tridentate thioether-dithiolate tpdt ligand to synthesize several mono-, bi-, and trinuclear iron complexes.15 Among these complexes, the mononuclear iron complex [Cp*Fe(η3-tpdt)] (1, Cp* = η5-C5Me5) displays good capability to generate heteronuclear metal complexes by the assembly reaction,16 which is attributed to nucleophilicity of the thiolate ligand induced by the iron−sulfur d-p π interaction. These results prompt us to further study the potential rich sulfur-based reactivity. Herein, we explore whether the tpdt ligand has the ability to delocalize electron Received: July 15, 2018

A

DOI: 10.1021/acs.organomet.8b00493 Organometallics XXXX, XXX, XXX−XXX

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In order to get deep insight into this ethylene addition process of two adjacent sulfur atoms in the tpdt ligand of complex 1 in the presence of one-electron oxidant, the UV−vis absorbance spectra were performed (Figure 1). During

density like other redox noninnocent dithiolenes. In the presence of oxidant, complex 1 facilely undergoes sulfurcentered addition reactions with a variety of alkenes and alkynes, which is demonstrated to be an irreversible process by cyclic voltammetry.

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RESULTS AND DISCUSSION Synthesis and Characterization of Alkene and Alkyne Adducts. Generally, metallodithiolates are regarded as excellent nucleophilic ligands for the construction of multinuclear metal−sulfur clusters, with adjacent thiolate sulfurs in the cis-configuration serving as bidentate S-donor ligand.17 In our previous work, several heteronuclear iron−sulfur clusters were successfully established based on nucleophilic replacements of thiolate sulfur donors in precursor complexes,18 especially, using our recently reported mononuclear iron complex 1.15,16 These results prompted us to further investigate the sulfur-based nucleophilic reactivity of complex 1 toward electron-deficient substrates such as unsaturated hydrocarbons. Different from neutral molybdenum trisdithiolenes reported by Fekl, which can trap ethylene by the two sulfur atoms of the bdt ligand (bdt = benzene-1,2-thiolate),19 there was no similar sulfur-based addition reaction observed in our mononuclear iron system. However, similar to the oxidized ruthenium trithiolate reported by Grapperhaus,6,8a C−S bond formation by alkene addition was realized in the reaction scaffold of iron complex 1 as outlined in Scheme 1. Thus,

Figure 1. Absorbance spectra recorded during Fc·PF6 was added to the CH2Cl2 solution of complex 1 in the presence of ethylene at room temperature. Data is collected every 5 s.

oxidation of 1 in the ethylene atmosphere with Fc·PF6, two low-energy bands at 420 and 581 nm associated with complex 1 had an obvious decrease compared to initial intensity within 35 s. Band location and intensity change of these two absorbance peaks are very similar to the [Re(dppbt)3] reaction system (390 and 581 nm),7 which indicates our iron system should also exist transient metal-stabilized thiyl radicals. Distinctly, when purging inert gas argon into the reaction solution, the intensity of these two bands cannot be restored to the initial state. In the absence of ethylene, one band at 581 nm assigned for complex 1 gradually decreases, and simultaneously, two bands at 390 and 480 nm significantly increase. The locations of two new peaks are consistent with those in the UV−vis spectrum of [Cp*Fe(μ-1k3SSS’:2k2SS-tpdt)FeCp*][PF6] as shown in Figure S47. This assignment is further confirmed by ESI-MS analysis of reaction residue. Next, we explored the general applicability of this reaction for other alkenes. The results show that analogous alkene adduct complexes 2b[BPh4]−2d[BPh4] were smoothly obtained using diene or aromatic or cyclic olefins in a similar synthetic pathway (Scheme 1). Notably, the reaction selectivity is significantly affected by the reaction solvent and the amount of alkene. When THF or CH3CN was chosen as the solvent, no primary ion signal for the corresponding alkene adduct was observed in ESI-MS, which may be attributed to their coordination capacity to hinder the formation of stable intermediate. In addition, when stoichiometric alkene was added to the reaction system, mixed-valent diiron complex with bridging tpdt ligand15 was produced as a main byproduct, which could be derived from redox recombination of metastable one-electron oxidized FeIV complex. However, when the concentration of alkenes remarkably increased, complex 1 almost quantitatively converted to alkene adducts. Furthermore, these alkene adducts can also be synthesized in high yields by employing air as the oxidant, meanwhile, no sulfur-centered oxygenation like Grapperhaus’ ruthenium system occurred in our mononuclear iron reaction platform.20

Scheme 1. Synthesis of Alkene Adducts from Complex 1a

a

Reagents and conditions: (i) ex. alkene, 1 equiv of Fc·PF6, CH2Cl2, rt, 0.5 h; (ii) 1 equiv of NaBPh4, CH2Cl2, rt, 5 h.

several stable trithioether iron complexes 2a[BPh4]−2d[BPh4] were synthesized in high yields, upon one-electron oxidation of complex 1 in the presence of excess alkenes. These alkene addition products are insensitive to dioxygen and moisture whether in solution or in the solid-state, and they remain robust even after purging with inert gas for a long time or exposure to vacuum. This fact suggests these sulfur-based alkene additions are completely irreversible. Initially, we investigated the interaction of oxidized mononuclear iron complex 1 with ethylene. At room temperature, treatment of complex 1 with 1 equiv of Fc·PF6 in the presence of excess ethylene following by counteranion exchange using NaBPh4 generated a sulfur-centered ethylene adduct complex 2a[BPh4]. The electrospray ionization highresolution mass spectrometry (ESI-HRMS) displays an expected molecular ion peak 2a+ with an m/z of 371.0629 (calcd 371.0624) with the appropriate isotopic distribution, which unambiguously identifies the cationic composition of 2a[BPh4]. B

DOI: 10.1021/acs.organomet.8b00493 Organometallics XXXX, XXX, XXX−XXX

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NMR spectrum of 2a[BPh4] shows a resonance at 1.61 ppm for methyl protons of Cp* and two resonances at 1.63 and 2.06 ppm for two sets of protons of the methylene group, which clearly indicates complex 2a[BPh4] has a symmetrical geometry in solution. Similarly, the 1H NMR spectra of other complexes also show a diagnostic sharp peak assigned to a methyl group of Cp*, which all fall in the common range from 1.53 to 1.70 ppm. Notably, the proton signals of hydroxyl groups in 3b[BPh4] are not observed, which suggests the very broad signals of active hydrogen could be submerged in the baseline. Nevertheless, the proton signal of the methylene group adjacent to the hydroxyl in 3b[BPh4] appears at 3.60 ppm, which is remarkably downfield-shifted caused by the deshielding effect by the hydroxyl group. This similar deshielding effect is also observed in the corresponding 13C NMR spectra. In addition, the IR spectrum of 3b[BPh4] exhibit one characteristic absorption band at 3546 cm−1 assigned to the terminal hydroxyl stretching vibration, which provides direct evidence for the existence of the hydroxyl group and excludes the possibility of dehydration like Matsumoto’s group.23 Molecular Structures of Alkene and Alkyne Adducts. When using PF6 as the counteranion, no single-crystals with good quality were obtained from various available double solvent systems. In order to obtain single-crystals suitable for X-ray diffraction analysis, counteranion exchange reactions were performed at ambient conditions. Expectedly, the singlecrystals were all grown from CH2Cl2 solution layered with nhexane at ambient temperature except 3a[BPh4] and 3b[BPh4]. The solid-state structures of these mononuclear iron adducts were unambiguously determined by X-ray diffraction analysis, which are fully consistent with the above various spectroscopic data. Details of the data collection and refinement are presented in Table S1−S3 and selected bond lengths and angles are given in Table 1. The ORTEP drawings of representative complexes 2a[BPh4], 2b[BPh4], 2d[BPh4], 3c[BPh4], and 4[BPh4] are shown in Figure 2. The crystal structure of 2a[BPh4] reveals that the ethylene addition takes place at the two neighboring thiolate sulfur atoms of the tpdt ligand to generate a new five-membered FeSCCS ring. Moreover, the ethylene addition leads to a change of the six-coordinate iron atom from a 17e center to an 18e one, accompanied by the formal valence change of Fe from + III to + II. As a result, there is an obvious variation in the angle of S1−Fe1−S2, which changes from 102.09(9)° in 1 to 90.07(5)° in 2a[BPh4]. The three Fe−S bond lengths of 2.202(1), 2.212(1), and 2.185(1) Å in 2a[BPh4] are slightly shorter than those in 1 (2.228(2), 2.220(2), 2.241(2) Å),15 but the average of Fe−S length (2.200(1) Å) is very close to that reported in structurally similar mononuclear iron complexes [CpFe(ttcn)][BPh4] (Cp = η5-C5H5; ttcn = 1,4,7-trithiacyclononane) (2.208(2) Å)24 and [CpFe(ttcd)][PF6] (ttcd = 1,4,7trithiacyclodecane) (2.1823(7) Å).25 The enhanced ironthioether π bonding is responsible for this decrease of the Fe−S bond distance in this half-sandwich iron complex system. The distance between Cp* ring and the iron center is 1.6988(5) Å, which is bound tighter than complex 1 (Fe−Cp* = 1.756(9) Å).15 The C15−C16 distance in the ethylene group is 1.46(1) Å, which is very close to other two C−C single bond between two sulfur atoms (1.45(1) and 1.50(1) Å), which is obviously elongated compared with a double bond (1.33 Å).26 The molecular structures of complexes 2b[BPh4]-2d[BPh4] bear a resemblance to that of 2a[BPh4] except for the

In addition, we also studied the reactions of complex 1 with various alkynes under similar conditions. As shown in Scheme 2, the alkynes also added into the adjacent cis-dithiolate sulfur Scheme 2. Synthesis of Alkyne Adducts from Complex 1a

a Reagents and conditions: (i) ex. alkyne, 1 equiv of Fc·PF6, CH2Cl2, rt, 0.5 h; (ii) 1 equiv of NaBPh4, CH2Cl2, rt, 5 h.

atoms to give homologous trithiother-FeII complexes 3a[BPh4]−3c[BPh4]. Similar C−S bond formation by alkyne addition to a metal-stabilized thiyl radical was also found in the mononuclear ruthenium system.21 However, unlike Yan’s halfsandwich mononuclear cobalt complex system,9b the organic part derived thermodynamic stability of these complexes with favorable low-spin, d6 electronic configuration of the FeII center precluded disassociation of organic products from the iron center. The above results show interaction of complex 1 with alkenes or alkynes under the same conditions afforded similar sulfur-centered adducts, which prompts us to further explore the propensity of this iron thiolate complex to which kind of unsaturated hydrocarbons. Hence, we next examined the reaction of complex 1 with excess styrene and phenylacetylene at a ratio of 1:1 under the aforementioned conditions. Interestingly, complex 1 completely transformed to complex 2c[BPh4], and no 3c[BPh4] was detected. One step further, we investigated the reactivity of complex 1 toward 4-vinyl phenylacetylene, which possesses both C−C double and triple bonds. As expected, alkene adduct iron complex 4[BPh4] was also obtained, and no alkyne adduct was observed (Scheme 3). These results indicate iron-stabilized Scheme 3. Synthesis of 4[BPh4] from Complex 1a

a

Reagents and conditions: (i) ex. 4-vinyl phenylacetylene, 1 equiv of Fc·PF6, CH2Cl2, rt, 0.5 h; (ii) 1 equiv of NaBPh4, CH2Cl2, rt, 5 h.

thiyl radicals derived from the oxidation of complex 1 are prone to combine olefin rather than alkyne. This phenomenon is consistent with the fact that alkynes add around 100 times slower than alkenes to ruthenium-stabilized thiyl radicals determined by the rate constants using cyclic voltammetric methods.22 Due to the excellent solubility of the above complexes in common organic polar solvent such as CH2Cl2, 1H and 13C NMR spectroscopic data and ESI-HRMS were smoothly obtained in the solution state. As expected, all complexes are diamagnetic species judged by 1H NMR spectroscopy. The 1H C

DOI: 10.1021/acs.organomet.8b00493 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Selected Bond Distances (Å) and Angles (deg) in Alkene and Alkyne Adducts Fe1−S1 Fe1−S2 Fe1−S3 Fe1−S(ave) C11−C12 C13−C14 C15−C16 Fe1−Cp* S1−Fe1−S2 S1−Fe1−S3 S2−Fe1−S3 S−Fe1−S(ave)

2a[BPh4]

2b[BPh4]

2c[BPh4]

2d[BPh4]

3c[BPh4]

4[BPh4]

2.202(1) 2.212(1) 2.185(1) 2.200(1) 1.45(1) 1.50(1) 1.46(1) 1.6988(5) 90.07(5) 91.20(6) 89.96(5) 90.41(5)

2.198(3) 2.215(2) 2.205(2) 2.206(2) 1.51(1) 1.53(2) 1.51(2) 1.7058(7) 89.23(10) 90.85(12) 88.88(10) 89.65(11)

2.2052(9) 2.1826(9) 2.2061(10) 2.1980(9) 1.473(6) 1.478(5) 1.513(5) 1.6990 (5) 90.31(3) 90.04 (3) 90.94(4) 90.43(3)

2.1960(9) 2.2095(8) 2.1977(9) 2.2011(9) 1.495(5) 1.505(7) 1.524(4) 1.7087(4) 89.37(3) 90.70(4) 90.08(3) 90.05(3)

2.1973(8) 2.1735(8) 2.2139(9) 2.1949(8) 1.451(8) 1.446(9) 1.303(4) 1.6979(5) 88.63(3) 88.47(3) 89.29(4) 88.80(3)

2.044(5) 2.158(4) 2.305(5) 2.169(5) 1.540(5) 1.538(5) 1.557(5) 1.6946(3) 94.56(17) 89.9(2) 88.69(19) 91.05(19)

Figure 2. Molecular structures of complexes 2a[BPh4] (a), 2b[BPh4] (b), 2d[BPh4] (c), 3c[BPh4] (d), and 4[BPh4] (e). Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms and BPh4 anion are omitted for clarity.

molybdenum complex,28b but close to that (1.312(9) Å) in the alkyne addition products reported by Yan.29 Electrochemical Studies for Irreversible Ethylene Addition. Initially, the redox behaviors of complex 1 and its alkene and alkyne adducts were explored by cyclic voltammetry. Their redox peak potentials are listed in Table 2. As shown in Figure 3a, complex 1 undergoes a quasireversible redox event at E1/2 = −0.09 V versus Fc+/0 attributed to the formal FeIII/FeIV couple, which is obviously positively shifted compared with the ruthenium analogue (−0.55 V)

substituent group in the functionalized tpdt ligand. The iron centers are all in a piano-stool configuration with Cp* ring and functionalized tpdt ligand oriented on opposite sides. The original thiolate sulfur donors S1 and S2 are bridged by various alkenes, which caused the two sulfur atoms to change from thiolate to thioether and the double bond in alkenes to elongate to a normal single bond. As shown in Figure 2, this sulfur-based alkene addition is a highly selective reaction, and other potential active sites did not participate in this transformation. For example, when this reaction was performed in the presence of excess isoprene, the ORTEP drawing of 2b[BPh4] clearly reveals only one terminal CC double bond of isoprene added into the two sulfur atoms of the tpdt ligand, rather than isolating a [2 + 4] cycloaddition product like the Ru2S2 core complex.27 Besides, in the molecular structure of complex 2d[BPh4], the six carbon atoms of cyclohexene after the addition to the sulfur atoms already have a serious distortion and are no longer in a plane. In comparison with alkene adducts, it is difficult to obtain single-crystals of alkyne adducts suitable for X-ray diffraction analysis, and only complex 3c[BPh4] was confirmed by crystallographic characterization. In the solid-state structure of 3c[BPh4], the alkyne addition also takes place selectively at S1 and S2 sites to generate a five-membered ring and the S1− Fe1−S2 angle of 88.63(3)° is also smaller than that in 1. Moreover, the C15−C16 bond distance of 1.303(4) Å is typical of a double bond, which is somewhat shorter than those (1.35(1) Å) found in the reported tungsten complex28a and

Table 2. Electrochemical Data of 1, 2a[BPh4]−2d[BPh4], 3a[BPh4]−3c[BPh4], and 4[BPh4]a complex

Ea

Ec

E1/2

1 2a[BPh4] 2b[BPh4] 2c[BPh4] 2d[BPh4] 3a[BPh4] 3b[BPh4] 3c[BPh4] 4[BPh4]

−0.03 0.43 0.46 0.47 0.42 0.48 0.46 0.52 0.52

−0.15 0.30 0.31 0.33 0.28 0.37 0.34 0.41 0.34

−0.09 0.37 0.39 0.40 0.35 0.43 0.40 0.47 0.43

a

In CH2Cl2 (1 mM) recorded at scan rates of 100 mV/s with Bu4NPF6 (0.1 M) as the supporting electrolyte. Potentials are referred versus Fc+/Fc.

n

D

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on iron scaffold as described in Scheme 4. First, the oneelectron oxidation of complex 1 induces an increased formal Scheme 4. Possible Mechanism for the Addition of Ethylene to Complex 1 Involving Potential Intermediates A and B

Figure 3. Cyclic voltammograms of Complex 1 in the absence (a) and presence (b) of 1 atm of ethylene and 2a[BPh4] (c). Data were collected in CH2Cl2 with 0.1 M nBu4NPF6 as supporting electrolyte. Potentials are referenced versus Fc+/Fc.

oxidation state of the iron center from +III to +IV. Because of the existence of highly covalent Fe−S bonds in oxidized intermediate A, an unpaired electron can be delocalized over iron and sulfur atoms to generate iron-stabilized thiyl radicals B, which are responsible for ethylene addition to form new C− S bond to produce adduct 2a[BPh4]. Similarly, the generation of the metal-stabilized thiyl radical as a key intermediate for further C−S bond formation with alkenes or alkynes was also observed in mononuclear ruthenium6,8a and copper systems.32 Interestingly, in the absence of ethylene, iron-stabilized thiyl radical will self-assemble into reported diiron complex with a tpdt bridge in moderate yield.15 This result is different from other metal complex systems involving thiyl radicals, such as those of Goh’s10 and Wieghardt’s33 work, which all involve the generation of a coordinated thiyl radical and then intermolecular coupling to give the corresponding disulfide complexes. However, we cannot realize the oxidation of 1 to yield disulfide species in this iron system.

reported by Goh.10 Differently, the corresponding oxidation processes of alkene or alkyne adducts occur at positive potentials, for example, the redox couple of ethylene adduct appears at E1/2 = 0.37 V (Figure 3c). This phenomenon was attributed to the change in the formal oxidation state of the iron center, which is derived from the variation of the tpdt ligand from negative two valent thioether-dithiolate to neutral trithioether by sulfur-centered ethylene addition. As shown in Table 2, discrepancy of substituents on alkenes or alkynes has a minimal effect on the potentials of the redox couples. Furthermore, in order to get more insight into the sulfurbased alkene addition process in this mononuclear iron system, electrochemical experiments using cyclic voltammetry were performed. As shown in the Figure 3b, complex 1 can be easily one-electron oxidized at Epox = −0.06 V to give an extremely unstable FeIV species, which is also proposed in the similar Goh’s ruthenium system.10 Subsequently, this intermediate species can rapidly interact with ethylene to generate the corresponding FeII adduct. Next, this FeII adduct can facilely undergo one reversible one-electron redox process at E1/2 = 0.38 V. Afterward, unlike other reversible ethylene capture systems,4,7 there is no expected reduction process observed as FeIV/FeIII couple of complex 1, even when the scanning scope was expanded to solvent window.30 This result clearly indicates ethylene cannot release under electrochemical conditions consistent with aforementioned chemical conditions. In this iron reaction platform, no occurrence of the C−S bond cleavage may be caused by the thermodynamic stability of the coordinatively saturated FeII complexes. Differently, chemical or electrochemical reduction of the mononuclear rhenium and ruthenium complexes with tridentate 9S3 ligand (9S3 = 1,4,7trithiacyclononane) all resulted in instantaneous C−S bond cleavage to yield ethylene.31 Based on our above chemical and electrochemical experimental results and similar reported mononuclear ruthenium reaction system,8,10 we proposed a possible mechanism for this sulfur-centered ethylene addition based

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CONCLUSIONS In summary, a series of alkenes and alkynes can be added to complex 1 upon chemical oxidant Fc·PF6 to yield the corresponding stable trithioether-FeII complexes. Surprisingly, these complexes can also be obtained in excellent yields by convenient, environmentally friendly air oxidation in the presence of excess NH4PF6. By UV−vis spectroscopic measurements and electrochemical studies, this novel mononuclear iron reaction framework was confirmed to a suitable metal complex system for capture of alkenes and alkynes, however, reversible release of alkenes and alkynes by the C−S bond cleavage did not realize as the final adducts are thermodynamically stable. Further investigations on other sulfur-based reactivity and related catalytic transformation are in progress.

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EXPERIMENTAL SECTION

General Procedures. Unless otherwise specified, manipulations were carried out under a dry argon or nitrogen atmosphere by using standard Schlenk techniques. Solvents were dried by conventional procedures and distilled under nitrogen prior to use. Ethylene, isoprene, styrene, cyclohexene, 1-pentyne, 4-pentyn-1-ol, and phenylacetylene were available commercially and used without further E

DOI: 10.1021/acs.organomet.8b00493 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

of complex 1 (178 mg, 0.52 mmol) and alkene (2.60 mmol) at room temperature under a dry argon atmosphere, and the resulting solution was stirred for about 0.5 h. During the time, the solution changed gradually from green to orange. Then NaBPh4 (178 mg, 0.52 mmol) was added to the resultant solution, and the solution was stirred for another 5 h. All volatiles were removed under vacuum, the residue was washed with Et2O three times to remove excess alkenes and byproduct Fc, and then dried in vacuo. The products 2b[BPh4]− 2d[BPh4] were obtained as an orange powder. Method B: NH4PF6 (254 mg, 1.56 mmol) was added to a CH2Cl2 solution (10 mL) of complex 1 (178 mg, 0.52 mmol) and alkene (2.60 mmol) at room temperature, and the resulting solution was stirred for about 1 h in air. During the time, the solution changed gradually from green to orange. Then NaBPh4 (178 mg, 0.52 mmol) was added to the resultant solution, and the solution was stirred for another 5 h. After the solvent was filtered and removed in vacuo, the residue was washed with Et2O three times to remove excess alkene and then dried under vacuum. The products 2b[BPh4]−2d[BPh4] were obtained as an orange powder. Crystals of 2b[BPh4]−2d[BPh4] suitable for X-ray diffraction were obtained from the concentrated CH2Cl2 solution layered with n-hexane at room temperature. 2b[BPh4]. Method A: yield 87%; Method B: 80%. 1H NMR (400 MHz, CD2Cl2): δ 1.61 (s, 15H, Cp*−CH3), 1.63 (s, 3H, CH2CCH3), 1.84 (s, 2H, SCH2CHS), 1.98 (m, 1H, SCH2), 2.04 (s, 1H, SCH2CHS), 2.23 (m, 2H, SCH2), 2.37 (m, 2H, SCH2), 2.71 (m, 3H, SCH2), 4.89 (s, 1H, CH2CCH3), 5.04 (s, 1H, CH2CCH3), 6.89 (m, 4H, BPh4-H), 7.04 (m, 8H, BPh4-H), 7.33 (m, 8H, BPh4-H). 13C NMR (100 MHz, CD2Cl2): δ 9.93 (Cp*-CH3), 19.45 (CH2CCH3), 33.13 (SCH2), 33.47 (SCH2), 38.10 (SCH2), 39.11 (SCH2), 41.49 (SCH 2 CHS), 57.31 (SCH 2 CHS), 86.59 (Cp*-C), 116.92 (CH2CCH3), 122.23 (BPh4-C), 126.12 (BPh4-C), 136.34 (BPh4-C), 140.16 (CH2CCH3). ESI-HRMS: Calcd for 2b+ 411.0938; Found 411.0952. IR (film, cm−1): 3053, 2963, 2852, 1579, 800, 705, 612. Anal. Calcd for C43H51FeS3B: C, 70.68; H, 7.04. Found: C, 70.47; H, 6.73. 2c[BPh4]. Method A: 90%; Method B: 88%. 1H NMR (400 MHz, CD2Cl2): δ 1.54 (s, 15H, Cp*−CH3), 1.67 (s, 2H, SCH2CHS), 1.85 (m, 2H, SCH2), 1.95 (m, 2H, SCH2), 2.28 (m, 4H, SCH2), 2.90 (m, 1H, SCH2CHS), 6.90 (m, 4H, BPh4-H), 7.04 (m, 8H, BPh4-H), 7.21 (m, 2H, Ph-H), 7.35 (m, 8H, BPh4-H), 7.41 (m, 3H, Ph-H). 13C NMR (100 MHz, CD2Cl2): δ 9.83 (Cp*-CH3), 34.02 (SCH2), 34.63 (SCH2), 36.66 (SCH2), 38.29 (SCH2), 42.70 (SCH2CHS), 55.19 (SCH2CHS), 86.71 (Cp*-C), 122.27 (BPh4-C), 126.13 (BPh4-C), 128.17 (Ph-C), 129.30 (Ph-C), 129.62 (Ph-C), 136.35 (BPh4-C), 136.75 (Ph-C). ESI-HRMS: Calcd for 2c+ 447.0938; Found 447.0943. IR (film, cm−1): 3055, 2963, 2852, 1579, 1422, 1262, 801, 704, 612. Anal. Calcd for C46H51FeS3B·CH2Cl2: C, 66.28; H, 6.27. Found: C, 65.96; H, 6.62. 2d[BPh4]. Method A: yield 88%; Method B: 86%. 1H NMR (400 MHz, CD2Cl2): δ 1.51 (m, 8H, SCHCH2CH2), 1.60 (s, 15H, Cp*− CH3), 1.62 (s, 2H, SCHCHS), 1.76 (m, 2H, SCH2), 1.94 (m, 2H, SCH2), 2.11 (m, 4H, SCH2), 6.89 (m, 4H, BPh4-H), 7.04 (m, 8H, BPh4-H), 7.32 (m, 8H, BPh4-H). 13C NMR (100 MHz, CD2Cl2): δ 9.96 (Cp*-CH3), 23.09 (SCHCH2CH2), 27.76 (SCHCH2CH2), 35.80 (SCH2), 51.91 (SCHCHS), 85.98 (Cp*-C), 122.26 (BPh4-C), 126.10 (BPh4-C), 136.32 (BPh4-C). ESI-HRMS: Calcd for 2d+ 425.1094; Found 425.1085. IR (film, cm−1): 3054, 2963, 2854, 1579, 1479, 1262, 1096, 1026, 802, 705, 612. Anal. Calcd for C44H53FeS3B·CH2Cl2: C, 65.15; H, 6.68. Found: C, 65.21; H, 6.31. General Procedures for the Preparation of Complexes 3a[BPh4]−3c[BPh4]. Method A: A solution of Fc·PF6 (182 mg, 0.55 mmol) in CH2Cl2 (10 mL) was added to a CH2Cl2 solution (10 mL) of complex 1 (189 mg, 0.55 mmol) and alkyne (2.75 mmol) at room temperature under a dry argon atmosphere, and the resulting solution was stirred for about 0.5 h. During the time, the solution changed gradually from green to orange. Then NaBPh4 (188 mg, 0.55 mmol) was added to the resultant solution, and the solution was stirred for another 5 h. After the solvent was filtered and removed in vacuo, the residue was washed with Et2O three times to remove unreacted alkynes and byproduct Fc, and then dried under vacuum. The

purification. Anhydrous FeCl2 (Aldrich) and S(CH2 CH 2SH)2 (Aldrich) were used as received without further purification. Complex 115 and 4-vinyl phenylacetylene34 were prepared according to literature method. Infrared spectra were recorded on a NEXVSTM FT-IR spectrometer. Elemental analyses were performed using a Vario EL analyzer. The 1H and 13C NMR spectra were recorded on a Brüker 400 Ultra Shield spectrometer. ESI-HRMS were recorded on a HPLC/Q-Tof mass spectrometer. The UV−vis absorption spectra were recorded on a PerkinElmer Lambda 35 spectrophotometer. X-ray Crystallography Procedures. The data were obtained on a Brü ker SMART APEX CCD diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). Empirical absorption corrections were performed using the SADABS program.35 Structures were solved by direct methods and refined by full-matrix least-squares method based on all data using F2 using SHELX2014.36 All of the nonhydrogen atoms were refined anisotropically. All of the hydrogen atoms were generated and refined in ideal positions. For 2a[BPh4], the thermal parameters of CH2Cl2 were restrained to be similar with the adjacent atom and the C−Cl bond distances in the solvent CH2Cl2 molecule were fixed at 1.80 Å. The “delu” restrain was used with the deviation being 0.001 to avoid the Hirshfeld Test Diff on two Fe−S bonds. For 2b[BPh4], the “basf/twin” refinement was used to avoid the problem that the flack x is ≫0. The “delu” restrain was used with the deviation being 0.001 to avoid the Hirshfeld Test Diff on Fe−S bond. All C−C bond distances on the Cp* ring were fixed. For 3c[BPh4], the thermal parameters of carbon atom on the tpdt ligand were restrained to be similar with the adjacent atoms. The “delu” restrain was used with the deviation being 0.001 to avoid the Hirshfeld Test Diff on C−C bond. For 4[BPh4], the thermal parameters of carbon atoms on the tpdt ligand and all sulfur atoms were restrained to be similar with the adjacent atoms. Several S−C bond distances and C−C bond distances were fixed. The “delu” restrain was used with the deviation being 0.001 to avoid the Hirshfeld Test Diff on C−C bond. The “isor” restrain was used with the deviation being 0.001 on two carbon atoms. Crystal data and collection details for 2a[BPh4], 2b[BPh4], 2c[BPh4], 2d[BPh4], 3c[BPh4], and 4[BPh4] are listed in Tables S1, S2, and S3. Electrochemistry. Electrochemical measurements were performed using a BAS-100 W electrochemical potentiostat at a scan rate of 100 mV/s. Cyclic voltammetry experiments were carried out in a three-electrode 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 Fc+/Fc couple. Preparation of Complex 2a[BPh4]. A solution of Fc·PF6 (189 mg, 0.57 mmol) in CH2Cl2 (10 mL) was added to complex 1 (196 mg, 0.57 mmol) in CH2Cl2 (10 mL) in the atmosphere of ethylene (1 atm) at room temperature, the resulting solution was stirred for about 0.5 h. During the time, the solution changed gradually from green to orange. After nitrogen was replaced ethylene, NaBPh4 (195 mg, 0.57 mmol) was added to the resultant solution, and then the mixture was stirred for another 5 h. All volatiles were removed under vacuum, the residue was washed with Et2O three times to remove byproduct Fc and then dried in vacuo. The product 2a[BPh4] (359 mg, 0.52 mmol, 91%) (based on 1) was obtained as an orange powder. Crystals of 2a[BPh4] suitable for X-ray diffraction were obtained from the concentrated CH2Cl2 solution layered with n-hexane at room temperature. 1H NMR (400 MHz, CD2Cl2): δ 1.61 (s, 15H, Cp*− CH3), 1.63 (m, 6H, SCH2), 2.06 (m, 6H, SCH2), 6.89 (m, 4H, BPh4H), 7.04 (m, 8H, BPh4-H), 7.33 (m, 8H, BPh4-H). 13C NMR (100 MHz, CD2Cl2): δ 9.90 (Cp*-CH3), 35.36 (SCH2), 85.90 (Cp*-C), 122.27 (BPh4-C), 126.14 (BPh4-C), 136.30 (BPh4-C). ESI-HRMS: Calcd for 2a+ 371.0624; Found 371.0629. IR (film, cm−1): 3054, 2984, 1579, 1479, 1265, 1024, 738, 612. Anal. Calcd for C40H47FeS3B: C, 69.56; H, 6.86. Found: C, 69.82; H, 6.94. General Procedures for the Preparation of Complexes 2b[BPh4]−2d[BPh4]. Method A: A solution of Fc·PF6 (172 mg, 0.52 mmol) in CH2Cl2 (10 mL) was added to a CH2Cl2 solution (10 mL) F

DOI: 10.1021/acs.organomet.8b00493 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

After the solvent was filtered and removed in vacuo, the residue was washed with Et2O three times to remove excess 4-vinyl phenylacetylene, and then dried under vacuum. The product 4[BPh4] was obtained as an orange powder. Crystals of 4[BPh4] suitable for X-ray diffraction were obtained from the concentrated CH2Cl2 solution layered with n-hexane at room temperature. 4[BPh4]. Method A: yield 86%; Method B: 80%. 1H NMR (400 MHz, CD2Cl2): δ 1.53 (s, 15H, Cp*−CH3), 1.66 (s, 2H, SCH2CHS), 1.89 (m, 1H, SCH2), 2.03 (m, 2H, SCH2), 2.16 (m, 2H, SCH2), 2.32 (m, 3H, SCH2), 2.87 (m, 1H, SCH2CHS), 3.23 (m, 1H, PhCCH), 6.88 (m, 4H, BPh4-H), 7.02 (m, 8H, BPh4-H), 7.12 (m, 2H, Ph-H), 7.32 (m, 8H, BPh4-H), 7.52 (m, 2H, Ph-H). 13C NMR (100 MHz, CD2Cl2): δ 9.88 (Cp*-CH3), 34.08 (SCH2), 34.83 (SCH2), 36.67 (SCH2), 38.52 (SCH2), 42.57 (SCH2CHS), 55.10 (SCH2CHS), 79.01 (PhCCH), 82.86 (PhCCH), 86.91 (Cp*-C), 122.30 (BPh4-C), 123.23 (Ph-C), 126.15 (BPh4-C), 128.28 (Ph-C), 133.23 (Ph-C), 136.36 (BPh4-C), 137.54 (Ph-C). ESI-HRMS: Calcd for 4+ 471.0938; Found 471.0929. IR (film, cm−1): 3294, 3054, 2964, 2854, 1579, 1479, 1261, 1028, 734, 706, 612. Anal. Calcd for C48H51FeS3B·2CH2Cl2: C, 62.52; H, 5.77. Found: C, 62.81; H, 6.14.

products 3a[BPh4]−3c[BPh4] were obtained as an orange powder. Method B: NH4PF6 (269 mg, 1.65 mmol) was added to a CH2Cl2 solution (10 mL) of complex 1 (189 mg, 0.55 mmol) and alkyne (2.75 mmol) at room temperature and the resulting solution was stirred for about 1 h in air. During the time, the solution changed gradually from green to orange. Then NaBPh4 (188 mg, 0.55 mmol) was added to the resultant solution, and the solution was stirred for another 5 h. After the solvent was filtered and removed in vacuo, the residue was washed with Et2O three times to remove excess alkyne and then dried under vacuum. The products 3a[BPh4]−3c[BPh4] were obtained as an orange powder. Crystals of 3c[BPh4] suitable for X-ray diffraction were obtained from the concentrated CH2Cl2 solution layered with n-hexane at room temperature. 3a[BPh4]. Method A: yield 89%; Method B: 83%. 1H NMR (400 MHz, CD2Cl2): δ 0.89 (m, 3H, CH2CH2CH3), 1.33 (m, 2H, CH 2 CH 2 CH 3 ), 1.65 (s, 15H, Cp*−CH 3 ), 2.04 (m, 2H, CH2CH2CH3), 2.20 (m, 2H, SCH2), 2.50 (m, 6H, SCH2), 5.88 (s, 1H, SCHCS), 6.88 (m, 4H, BPh4-H), 7.03 (m, 8H, BPh4-H), 7.31 (m, 8H, BPh4-H). 13C NMR (100 MHz, CD2Cl2): δ 9.88 (Cp*-CH3), 13.40 (CH2CH2CH3), 22.55 (CH2CH2CH3), 34.41 (SCH2), 35.43 (SCH2), 36.21 (SCH2), 40.26 (SCH2), 41.22 (CH2CH2CH3), 86.56 (Cp*-C), 122.29 (BPh4-C), 126.15 (BPh4-C), 128.46 (SCHCS), 136.36 (BPh4-C), 152.88 (SCHCS). ESI-HRMS: Calcd for 3a+ 411.0938; Found 411.0931. IR (film, cm−1): 3055, 2963, 1579, 1480, 1261, 1026, 706, 612. Anal. Calcd for C43H51FeS3B·CH2Cl2: C, 64.79; H, 6.55. Found: C, 65.08; H, 6.31. 3b[BPh4]. Method A: yield 86%; Method B: 80%. 1H NMR (400 MHz, CD2Cl2): δ 1.62 (s, 2H, CH2CH2CH2OH), 1.66 (s, 15H, Cp*−CH3), 1.82 (m, 2H, CH2CH2CH2OH), 2.07 (m, 2H, SCH2), 2.30 (m, 2H, SCH2), 2.61 (m, 4H, SCH2), 3.60 (m, 2H, CH2CH2CH2OH), 5.95 (s, 1H, SCHCS), 6.89 (m, 4H, BPh4-H), 7.04 (m, 8H, BPh4-H), 7.33 (m, 8H, BPh4-H). 13C NMR (100 MHz, CD2Cl2): δ 9.90 (Cp*-CH3), 15.48 (CH2CH2CH2OH), 29.41 (CH2CH2CH2OH), 35.70 (SCH2), 36.48 (SCH2), 40.60 (SCH2), 41.53 (SCH2), 66.04 (CH2CH2CH2OH), 86.53 (Cp*-C), 122.23 (BPh4-C), 126.07 (BPh4-C), 128.56 (SCHCS), 136.31 (BPh4-C), 152.71 (SCHCS). ESI-HRMS: Calcd for 3b+ 427.0887; Found 427.0872. IR (film, cm−1): 3546 (νOH), 3054, 2964, 2854, 1720, 1581, 1417, 613. Anal. Calcd for C43H51FeS3BO: C, 69.17; H, 6.88. Found: C, 69.28; H, 6.54. 3c[BPh4]. Method A: yield 87%; Method B: 80%. 1H NMR (400 MHz, CD2Cl2): δ 1.70 (s, 15H, Cp*−CH3), 2.12 (m. 4H, SCH2), 2.50 (m. 4H, SCH2), 6.35 (s, 1H, SCHCS), 6.87 (m, 4H, BPh4-H), 7.01 (m, 8H, BPh4-H), 7.31 (m, 8H, BPh4-H), 7.45 (m, 3H, Ph-H), 7.52 (m, 2H, Ph-H). 13C NMR (100 MHz, CD2Cl2): δ 9.98 (Cp*CH3), 36.07 (SCH2), 36.22 (SCH2), 42.08 (SCH2), 42.51 (SCH2), 86.97 (Cp*-C), 122.28 (BPh4-C), 126.14 (BPh4-C), 128.13 (SCHCS), 128.54 (Ph-C), 129.78 (Ph-C), 131.43 (Ph-C), 136.36 (BPh4-C), 151.95 (SCHCS). ESI-HRMS: Calcd for 3c+ 445.0781; Found 445.0773. IR (film, cm−1): 3054, 2963, 2853, 1579, 1479, 1263, 1029, 800, 706, 612. Anal. Calcd for C46H49FeS3B: C, 72.25; H, 6.46. Found: C, 71.98; H, 6.42. Preparation of Complex 4[BPh4]. Method A: A solution of Fc· PF6 (175 mg, 0.53 mmol) in CH2Cl2 (10 mL) was added to a CH2Cl2 solution (10 mL) of complex 1 (182 mg, 0.53 mmol) and 4-vinyl phenylacetylene (339 mg, 2.65 mmol) at room temperature under a dry argon atmosphere, and the resulting solution was stirred for about 0.5 h. During the time, the solution changed gradually from green to orange. Then NaBPh4 (181 mg, 0.53 mmol) was added to the resultant solution, and the solution was stirred for another 5 h. After the solvent was filtered and removed in vacuo, the residue was washed with Et2O three times to remove excess 4-vinyl phenylacetylene and byproduct Fc, and then dried under vacuum. The product 4[BPh4] was obtained as an orange powder. Method B: NH4PF6 (259 mg, 1.59 mmol) was added to a CH2Cl2 solution (10 mL) of complex 1 (182 mg, 0.53 mmol) and 4-vinyl phenylacetylene (339 mg, 2.65 mmol) at room temperature, and the resulting solution was stirred for about 1 h in air. During the time, the solution changed gradually from green to orange. Then NaBPh4 (181 mg, 0.53 mmol) was added to the resultant solution, and the solution was stirred for another 5 h.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00493. Synthesis, characterization, and structure data (PDF) Accession Codes

CCDC 1568729−1568730, 1568733−1568734, 1568737, and 1849370 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 data_ [email protected], 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 Authors

*E-mail: [email protected] (J.Q.). *E-mail: [email protected] (D.Y.). 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. 21571026, 21690064, 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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REFERENCES

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DOI: 10.1021/acs.organomet.8b00493 Organometallics XXXX, XXX, XXX−XXX