Nonredox Metal Ions Promoted Olefin Epoxidation by Iron(II

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Cite This: Inorg. Chem. 2017, 56, 15138−15149

Nonredox Metal Ions Promoted Olefin Epoxidation by Iron(II) Complexes with H2O2: DFT Calculations Reveal Multiple Channels for Oxygen Transfer Jisheng Zhang,† Wen-Jie Wei,† Xiaoyan Lu,† Hang Yang, Zhuqi Chen, Rong-Zhen Liao,* and Guochuan Yin*

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School of Chemistry and Chemical Engineering, Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, P. R. China S Supporting Information *

ABSTRACT: Nonredox metal ions play significant roles in a wide range of biological and chemical oxidations in which they can modulate the oxidative reactivity of those redox metal ions. With environmentally benign H2O2 as oxidant, the influence of nonredox metal ions on an iron(II) complex mediated olefin epoxidation was investigated through experimental studies and theoretical calculations. It was found that adding nonredox metal ions like Sc3+ can substantially improve the oxygen transfer efficiency of the iron(II) complex toward cyclooctene epoxidation even in the presence of certain amount of water. In 18Olabeling experiments with 18O water, the presence of Sc3+ provided a higher 18O incorporation in epoxide. In UV−vis studies, it was found that the presence of Sc3+ makes both FeIII−OOH and FeIVO species unstable. Density functional theory calculations further disclosed that, in the presence of Sc(OTf)3, the Sc3+ adducts of FeIII−OOH and FeIVO species are capable of epoxidizing olefin as well as FeVO species, thus opening multiple channels for oxygenation. In particular, in the pathway of cyclooctene epoxidation, the FeIVO/Sc3+ adduct-mediated epoxidation is more energetically favorable than that of the FeVO species (12.2 vs 17.2 kcal/mol). This information may implicate that the presence of certain nonredox metal ions can facilitate these redox metal ions mediating biological and chemical oxidations happening at a relatively low oxidation state, which is more energetically accessible.

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INTRODUCTION Oxygen transfer represents one of the most important processes in biological and chemical oxidations, in which redox metal ions play significant roles.1−7 Generally, an active metal oxo species at high oxidation state is believed to serve as the key active intermediate for oxygenation in heme and nonheme enzymes, while in certain cases, its precursor, the metal hydroperoxide species, was also proposed to be responsible for oxygenation in synthetic models.8−20 In P450 enzymes, a “push−pull” mechanism was proposed for the heterolytic O−O bond cleavage of the FeIII−OOH species to generate the active FeVO species. In this event, a hydrogenbonding network near the distal oxygen atom of the FeIII− OOH moiety was proposed to facilitate the heterolytic O−O bond cleavage, that is, playing the role of the pull effect.21,22 Similarly, in nonheme synthetic models, a water or acetic acid binding to the iron(III) was also proposed to assist the heterolytic O−O bond cleavage of the FeIII−OOH moiety to generate the FeVO species that serves as a highly active species in alkane hydroxylation and olefin epoxidation.23−27 In the case of homolytic O−O bond cleavage, the resulting FeIV © 2017 American Chemical Society

O species at relatively low oxidation state was suspected to be a sluggish oxidant for oxygen transfer like olefin epoxidation.28,29 However, in certain cases, when the iron(V) state is not accessible, the iron(IV) becomes the highest state and exhibits the high reactivity in versatile oxidations.9 As well as water and acetic acid reported in the literature,23−27 Sun and co-workers recently reported that H2SO4 can accelerate asymmetric epoxidation of olefin with high enantiomeric excess (ee) values by their manganese catalyst with H2O2 oxidant.30 The role of these additives, including water, acetic acid, and H2SO4, has been assigned to build a hydrogen-bonding network for the heterolytic O−O bond cleavage of the Mn+−OOH moiety, a role of the pull effect. Even more, a second role of H2SO4 was proposed to increase the oxidizing capability of the MnVO species in olefin epoxidation,30 and a Brønsted acid-promoted oxygen transfer with MnIVO species was even reported by Nam and Fukuzumi.31 Received: September 25, 2017 Published: November 28, 2017 15138

DOI: 10.1021/acs.inorgchem.7b02463 Inorg. Chem. 2017, 56, 15138−15149

Article

Inorganic Chemistry These findings have inspired us to address whether Lewis acid can play a similar role of these Brønsted acids in H2O2based oxygen transfer process, since these nonredox metal ions have played significant roles in versatile biological oxidative events.32−35 Indeed, nonredox metal ions serving as Lewis acid to improve the oxidizing capability of high valent metal ions had been widely observed in different synthetic models by Lau,36,37 Collins,38 Nam and Fukuzumi,39−43 Goldberg,44−46 and us.47−51 It was found that the presence of Lewis acid can positively shift the redox potentials of the active metal ions, thus improving their capability in electron transfer, hydrogen transfer, and oxygenation. We further disclosed that, in a catalytic process using PhI(OAc)2 as oxidant, adding Lewis acid can dissociate the in situ generated μ-O-bridged dimeric MnIII/ MnIV species, a sluggish oxidant, to the monomeric manganese(IV) oxo species that interacts with Lewis acid and leads to efficient epoxidation.52,53 Similar improvement was also observed by adding Lewis acids to ruthenium(II) complex catalyzed epoxidation with PhI(OAc)2.54 However, a Lewis acid-promoted oxygenation, for example, in olefin epoxidation by redox metal ions, has not been reported in a catalytic process with H2O2, an oxidant commonly generated in biological metabolisms. The challenge possibly comes from the feasible solvation of Lewis acid by water, which may block its pull effect and/or its interaction with the active metal oxo species. Here, we presented the first example of Lewis acid-promoted olefin epoxidation by iron(II) complex with H2O2. Detailed density functional theory (DFT) calculations revealed that the presence of Lewis acid not only promotes the heterolysis of the FeIII− OOH moiety to generate the FeVO species but also facilitates the direct oxygenation by the FeIII−OOH species, and it even makes oxygenation by the FeIVO species more energetically favorable than the FeVO species in its epoxidation pathway, thus opening multiple channels for oxygen transfer.

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potentially explosive with little impurity, which should be caref ully stored in the HDPE container. In preparing 30% H2O2 solution f rom 90% H2O2 with 18O-water, one should wear protective clothes and make sure that the glass equipment used is clean enough and that the lab is kept with good ventilation. Instrumentation. UV−vis spectra were collected on Analytikjena specord 205 UV−vis spectrometer, and time-resolved spectra were acquired with SX20 stopped-flow spectrometer equipped with photodiode array. Gas chromatography−mass spectrometry (GCMS) analysis was performed on an Agilent 7890A/5975C spectrometer. Electron paramagnetic resonance (EPR) experiments were conducted on a Bruker A300 spectrometer. General Procedure for the Lewis Acid-Promoted Epoxidation by Fe(BPMEN)(OTf)2 Catalyst. In a typical experiment, Fe(BPMEN)(OTf)2 (0.0015 mmol), Lewis acid (0.0015 mmol), and cyclooctene (0.3 mmol) were first dissolved in 3 mL of acetonitrile, and then 0.05 mL of 30% H2O2 (0.45 mmol) was added into this solution. The reaction mixture was stirred in water bath at 298 K for 1 h. After that, the product was quantitatively analyzed by GC using the internal standard method. Control experiments using Fe(BPMEN)(OTf)2 or Lewis acid alone as catalyst were performed in parallel. Reactions were performed at least in duplicate, and average data were used in discussion. General Procedure for the Epoxidation Kinetics Catalyzed by Fe(BPMEN)(OTf)2 in the Presence of Sc(OTf)3. In a typical experiment, Fe(BPMEN)(OTf)2 (0.0015 mmol), Sc(OTf)3 (0.0015 mmol), and cyclooctene (0.3 mmol) were first dissolved in 3 mL of acetonitrile, and then 0.05 mL of 30% H2O2 (0.45 mmol) was added into this solution. The reaction solution was stirred in water bath at 298 K. After that, the product was quantitatively analyzed at set intervals by GC using the internal standard method. Control experiments using Fe(BPMEN)(OTf)2 alone as catalyst were performed in parallel. Reactions were performed at least in duplicate, and average data were used in discussion. General Procedure for the 18O-Labeling Experiments of Catalytic Epoxidation by Fe(BPMEN)(OTf)2 in the Presence of Sc(OTf)3. In a typical experiment, Fe(BPMEN)(OTf)2 (0.0015 mmol), Sc(OTf)3 (0.0015 mmol), and cyclooctene (0.3 mmol) were first dissolved in 0.45 mL of acetonitrile, and then 0.05 mL of freshly prepared 30% H2O2 (90% H2O2/H18O = 1:2, v/v) (0.45 mmol) was added into this solution. The reaction solution was stirred in water bath at 298 K for 1 h, and the product analysis was performed by GCMS. The 18O enrichments in oxygenation products were calculated based on the relative abundance of mass peaks at m/z = 111 and 113, respectively, in GC-MS graphs. General Procedures for the EPR Characterizations of Fe(BPMEN)(OTf)2 with H2O2 in the Absence/Presence of Sc(OTf)3. In a typical experiment, the solution was prepared directly in an EPR tube by mixing 0.2 mL of Fe(BPMEN)(OTf)2 (3 mM) in acetonitrile with 0.2 mL of H2O2 (30 mM), or 0.2 mL of Fe(BPMEN)(OTf)2 (3 mM) and Sc(OTf)3 (3 mM) in acetonitrile with 0.2 mL of H2O2 (30 mM) at room temperature. The EPR tube was frozen immediately with liquid nitrogen after hydrogen peroxide was injected, and all of the EPR experiments were conducted at 120 K.

EXPERIMENTAL SECTION

Materials. All chemical reagents were commercially available and used without further purification. Cyclooctene and 1,2-epoxycyclooctane were purchased from Aldrich. NaOTf, Mg(OTf)2, and Sc(OTf)3 came from Aldrich. Other trifluoromethanesulfonates including Zn(OTf)2, Ca(OTf)2, Al(OTf)3, Y(OTf)3, and Yb(OTf)3 came from local chemical companies. H218O (95% 18O atom) came from Acros. BPMEN and its iron complex FeII(BPMEN)(OTf)2 (Figure 1) were synthesized according to the known methods (BPMEN = N,N′dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane).55 H2 O2 (90%) was prepared by vacuum fractional distillation according to the literature.56 Caution! The highly concentrated hydrogen peroxide was

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RESULTS AND DISCUSSION Lewis Acid-Promoted Olefin Epoxidation by Nonheme Iron(II) Complex with H2O2. In literature, to achieve efficient epoxidation with environmentally benign H 2O2 oxidant, a series of nonheme iron and manganese complexes have been explored and applied in olefin epoxidation, and among them, Fe(BPMEN)(OTf)2 was proved to be an excellent catalyst in Jacobson and Que’s independent studies.57,58 In the present study, employing the Fe(BPMEN)(OTf)2 complex as a model catalyst, a series of nonredox metal ions were first scanned to investigate the influence of Lewis acids on the catalytic epoxidation of cyclooctene, and the results are summarized in Table 1. The selections of Lewis acid were based on their Lewis acidity, which are related to their net

Figure 1. Representation of the FeII(BPMEN)(OTf)2 structure used in this study. 15139


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Inorganic Chemistry Table 1. Lewis Acid-Promoted Catalytic Epoxidation of Cyclooctenea by Fe(BPMEN)(OTf)2 with H2O2 entry 1 2 3 4 5 6 7 8 9 10 11

Lewis acid

conv (%)

yield (%)

NaOTf Mg(OTf)2 Ca(OTf)2 Zn(OTf)2 Ba(OTf)2 Y(OTf)3 Yb(OTf)3 Al(OTf)3 Sc(OTf)3 HOTf

38.0 49.7(7.0) 44.1(3.3) 45.3(10.0) 70.1(4.0) 47.2(5.2) 63.1(9.6) 64.5(6.7) 97.5(4.0) 99.9(5.0) 58.5(6.0)

20.2 21.2(0.7) 24.1(0.7) 31.1(0.8) 42.3(0.7) 20.2(0.6) 33.1(0.6) 36.0(0.7) 62.4(0.9) 64.3(3.0) 31.6(0.5)

Figure 2. Time-course of cyclooctene epoxidation catalyzed by Fe(BPMEN)(OTf)2 in the absence (black line) and presence (red line) of Sc(OTf)3. Conditions: CH3CN 3 mL, cyclooctene 0.1 M, Fe(BPMEN)(OTf)2 0.5 mM, Sc(OTf)3 0.5 mM, 30% H2O2 0.05 mL at 298 K. (●) The conversion of cyclooctene. (▲) The yield of the epoxidation product.

a

Conditions: CH3CN 3 mL, cyclooctene 0.1 M, Fe(BPMEN)(OTf)2 0.5 mM, Lewis acid 0.5 mM, 30% H2O2 0.05 mL at 298 K for 1 h. The data in parentheses represent the control experimental results using Lewis acid alone.

charge and ionic radius, and redox metal ions like Cu2+ and Fe3+ were carefully avoided to eliminate the confused information from their redox properties. As shown in Table 1, in the absence of Lewis acids, Fe(BPMEN)(OTf)2 alone as catalyst provided 38.0% conversion of cyclooctene with 20.2% yield of the epoxide in 1 h at 298 K, while addition of Lewis acids improved the catalytic efficiency to different extent. Adding 1 equiv of Zn(OTf)2 (relative to iron(II) complex, same as below) gave 70.1% conversion with 42.3% yield of the epoxide, and 1 equiv of Y(OTf)3 gave 63.1% conversion with 33.1% yield of the epoxide. In particular, with 1 equiv of Al(OTf)3 or Sc(OTf)3, it provided 97.5% or 99.9% conversion with 62.4% or 64.3% yield of the epoxide, respectively. Other byproducts include hydrogen abstraction products of cyclooctene and hydrolysis product of epoxide as identified by GCMS analysis (vide infra). In a complementary experiment, adding 1 equiv of HOTf gave only 58.5% conversion with 31.6% yield of the epoxide, indicating that the potential hydrolysis of Sc(OTf)3 to release HOTf under epoxidation conditions did not affect the epoxidation reaction seriously, while in the control experiments, all these Lewis acids alone showed very poor epoxidation activity; thus, a Lewis acid directly catalyzed epoxidation, like TS-1 catalyst,59 is ignorable here. Clearly, the presence of these nonredox metal ions can improve the catalytic efficiency of Fe(BPMEN)(OTf)2 in epoxidation. The promotional effects of Lewis acids were further evidenced by the time-course of catalytic epoxidation as shown in Figure 2. As disclosed, addition of Sc(OTf)3 clearly accelerated the epoxidation rate of cyclooctene, leading to much higher conversion with much higher yield of epoxide when compared with that in the absence of Sc(OTf)3, thus improving the utilization efficiency of oxidant. In addition, it was found that the epoxidation reaction can almost complete in 15 min. Figure 3 displayed the influence of the ratio between Sc(OTf)3 and Fe(BPMEN)(OTf)2 on the olefin epoxidation. It disclosed that the catalytic efficiency was apparently improved when the ratio increased from 0 to 2, while further increasing the Sc(OTf)3 loading leads to a relatively low catalytic efficiency. Since using 30% aqueous H2O2 as oxidant unavoidably introduces plenty of water to the catalytic system, which may solvate the added Lewis acids, the influence of water content was next investigated for olefin epoxidation in this

Figure 3. Influence of the ratio between Sc(OTf)3 and Fe(BPMEN)(OTf)2 on the catalytic epoxidation of cyclooctene. Conditions: Fe(BPMEN)(OTf)2 0.25 mM, Sc(OTf)3 0−1 mM, cyclooctene 0.1 M, H2O2 0.05 mL at 298 K for 1 h. (●) The conversion of cyclooctene. (▲) The yield of the epoxidation product.

system. As shown in Table 2, the catalytic efficiency decreased gradually as the amount of extra water increased. However, in each case, the addition of Sc3+ promoted the epoxidation efficiency when compared with those in the absence of Sc(OTf)3. Even in the case of adding 0.1 mL of extra water Table 2. Influence of Water Content on the Catalytic Epoxidationa of Cyclooctene by Fe(BPMEN)(OTf)2 with H2O2 entry

H2O (mL)

conv (%)

yield (%)

1 2 3 4

0 0.025 0.05 0.1

99.9(38.0) 82.4(25.4) 54.6(10.8) 21.9(6.6)

64.3(20.2) 44.4(9.2) 31.9(5.7) 8.6(3.1)

a

Conditions: CH3CN 3 mL, cyclooctene 0.1 M, Fe(BPMEN)(OTf)2 0.5 mM, Lewis acid 0.5 mM, 30% H2O2 0.05 mL, H2O 0−0.1 mL at 298 K for 1 h. The data in parentheses represent using Fe(BPMEN)(OTf)2 alone as control experiments. 15140


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Figure 4. UV−vis change of Fe(BPMEN)(OTf)2 with H2O2 in the absence (a) and presence (b) of Sc(OTf)3. Conditions: CH3CN 3 mL, Fe(BPMEN)(OTf)2 1 mM, Sc(OTf)3 1 mM, 30% H2O2 10 mM, 293 K, interval time 20 s.

OOH and FeIVO species less stable and otherwise more active as disclosed by DFT calculations (vide infra). Certainly, the altered spectra could also be due to the changes in absorption maxima and extinction coefficients in the presence of Sc3+, or the presence of multiple species, which cannot be definitely excluded. In EPR studies, the presence of the iron(III) species is also evidenced at the beginning of adding H2O2 to the Fe(BPMEN)(OTf)2 in acetonitrile, and adding Sc3+ clearly caused the intensity of the iron(III) species to decrease. As shown in Figure 5, in the absence of Sc(OTf)3, the

in addition to 0.05 mL of 30% aqueous H2O2, the presence of Sc(OTf)3 still provided 21.9% conversion of cyclooctene with 8.6% yield of epoxide, whereas without Sc(OTf)3, it gave only 6.6% conversion with 3.1% yield. Clearly, the presence of a large amount of water would block the interaction of Lewis acid with the iron catalyst, leading to poor promotional effect. UV−Vis Detection of the Intermediates Generated by Nonheme Iron(II) Complex with H2O2. In the literature, formations of the FeIII−OOH, FeIVO, and FeVO species have been proposed when treating nonheme iron complexes with H2O2 under different conditions.23−27,60−64 In the case of Fe(BPMEN) complex, it may first react with H2O2 to form the FeIII−OOH species, which was generally believed as a sluggish oxidant.23 The homocleavage of the O−O bond in the FeIII− OOH moiety leads to the formation of the FeIVO species that is active for hydrogen abstraction but still sluggish for oxygen transfer.28 The FeVO species, generated by the heterocleavage of the O−O bond in the FeIII−OOH moiety, is generally proposed as the active species for oxygen transfer like olefin epoxidation.23−27,60,61,65−68 Here, the intermediate formations by treating Fe(BPMEN)(OTf)2 complex with H2O2 were investigated through UV−vis studies. As shown in Figure 4a, upon addition of H2O2, a new iron species having maximum absorbance around 560 nm appeared immediately, and then it gradually decayed to another species having absorbance above 700 nm. These phenomena are similar to those observations in Rybak-Akimova’s UV−vis studies with the same Fe(BPMEN) complex,69 in which the first iron species is assigned to the FeIII−OOH species, and the latter is the FeIVO species. As they reported, in the presence of acetic acid, the maximum absorbance of the FeIVO species was observed around 740 nm. In another case, using isopropyl ester of 2-iodoxybenzoic acid (IBX ester) as oxidant showed a slightly blue shift of the FeIVO species having the maximum around 720 nm, implicating its slight coordination environment change. In present studies, because of the instability of the FeIVO species at room temperature, although the formation of the FeIVO species is indicated with the decay of the FeIII− OOH species (Figure 4), the maximum absorbance of the FeIVO species was unclear. Alternatively, the UV-difference spectra disclosed a maximum absorbance around 726 nm, which can be assigned to the FeIVO species (Figure S1). Notably, in the presence of Sc(OTf)3, the absorbance of both FeIII−OOH and FeIVO species weakens clearly when compared with that in the absence of Sc(OTf)3 (Figure 4a vs 4b), implying that the presence of Sc(OTf)3 makes both FeIII−

Figure 5. EPR spectra of Fe(BPMEN)(OTf)2 with H2O2 in the absence (black line) and presence (red line) of Sc(OTf)3. Conditions: solvent CH3CN, H2O2 15 mM. (a) Fe(BPMEN)(OTf)2 1.5 mM; (b) Fe(BPMEN)(OTf)2 1.5 mM, Sc(OTf)3 1.5 mM, frozen by liquid nitrogen.

immediate frozen solution of Fe(BPMEN)(OTf)2 with H2O2 in acetonitrile revealed the g values of 2.21, 2.14, and 1.96 with high intensity, which can be assigned to the low-spin iron(III) species as well as that in Rybak-Akimova’s studies.69 In the presence of Sc(OTf)3, the intensity of the iron(III) species weakened apparently, which is consistent with those in UV−vis studies. That is, the presence of Sc3+ makes FeIII−OOH less stable and otherwise more active as disclosed by DFT calculations (vide infra). However, it is worth mentioning that the iron(IV) species is EPR silent, and we are not able to obtain the information on the iron(V) species from EPR studies. Isotopic Labeling Experiments for Olefin Epoxidation Using 18O Water. Although the FeVO species was generally proposed as the key active species for olefin epoxida15141


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Table 3. 18O Enrichments in the Products of Cyclooctene Epoxidationa by Fe(BPMEN)(OTf)2 Catalyst in the Absence/ Presence of Sc(OTf)3

catalyst

H2O 18O%

1 18O%

2 18O%

3 18O%

4 18O%

5 18O−16O(18O)%

6 18O−16O(18O)%

Fe(II) Fe(II) + Sc(III)

64.5 66.2

65.7 65.5

5.2 7.2

49.9 66.7

48.2 59.3

23.6(0) 57.6(5.3)

59.5(6.0)

a

Conditions: CH3CN 0.5 mL, Fe(BPMEN)(OTf)2 3 mM, Sc(OTf)3 3 mM, cyclooctene 0.6 M, and 30% H2O2 0.5 mL at 298 K for 1 h. 30% H2O2 was prepared by 90% H2O2 (0.5 mL) and 97% H218O (1.0 mL), and the product distribution was shown in Figure S3.

tion,23−27,60,61,65−68 its direct observation by UV−vis studies was not accessible. Alternatively, the involvement of the highvalent metal oxo species in oxygen transfer was popularly evidenced by 1 8 O-labeling experiments with 1 8 O water.19,20,52−54,70−74 In the literature, to conduct the efficient 18 O-water experiment with H2O2 as an oxidant source, a large excess of 18O water was generally employed to achieve enough 18 O enrichments in products.75 For example, 1000 equiv of 18O water was employed by Que and co-workers to investigate 18O enrichement in iron(II) complex catalyzed olefin epoxidation. In present studies, because the presence of bulky water leads to low catalytic efficiency for epoxidation (see Table 2), the employment of large excess of 18O water in addition to normal 30% aqueous H2O2 oxidant for 18O-labeling experiments was not operative. Therefore, to achieve enough catalytic activity for epoxidation and simultaneously compress the competition of H216O with H218O in oxygen exchange, 30% aqueous H2O2 oxidant was freshly prepared by using 90% concentrated aqueous H2O2 with H18O (97% 18O enrichment) (v/v = 1:2). The 16O and 18O enrichments in the epoxide were determined by the relative abundance of mass peaks at m/z = 111 and 113 in GC-MS analysis for epoxide, and the 18O labeling results in products are summarized in Table 3. In the presence of Sc(OTf)3, the 18O enrichment in epoxide is 7.2%, while it is 5.2% in the absence of Sc(OTf)3. The low 18 O enrichment in epoxide was likely due to the high reactivity of the FeVO species for epoxidation compared with its exchange with H 2 18 O, which was evidenced by DFT calculations (vide infra). In general, if the FeVO species was the sole active species for epoxidation, one may postulate that interaction of Sc3+ cation with the FeVO functional group may retard the 18O exchange, thus leading to low 18O enrichment in epoxide. Here, the higher 18O enrichments in the presence of Sc(OTf)3 may have implicated that alternative iron species have been involved and played significant roles in epoxidation. If only the FeIII−OOH species served as a second candidate for epoxidation in the presence of Sc(OTf)3, it would lead to a lower 18O enrichment in epoxide. Accordingly, in the presence of Sc(OTf)3, a third active species other than FeIII− OOH species may have been involved in olefin epoxidation, which caused the increased 18O enrichments in epoxide in 18O water-labeling experiments. In addition to epoxide as the major product, some hydrogen abstraction products were also identified by GC-MS analysis (Figure S3), which is responsible for the relatively low

selectivity of the epoxide. Unlike the low 18O enrichments in epoxide, the generated ketone products generally have high 18O enrichments. In the absence of Sc(OTf)3, the 18O enrichments in products 1, 3, and 4 were 65.7%, 49.9%, and 48.2%, respectively, while in the presence of Sc(OTf)3, they changed to 65.5%, 66.7%, and 59.3%, respectively. In the literature, both FeIVO and FeVO species have been reported to be capable of hydrogen abstraction.76−80 Here, higher 18O enrichments in ketone products may have indicated that the FeIVO species play the significant role in hydrogen abstraction. As it is more stable than the FeVO species, it makes 18O exchange between 18 O water and FeIVO species more feasible than FeVO species. In addition, the dihydroxy and its further oxidized αhydroxy ketone products were also identified by GC-MS analysis. Notably, the dihydroxy product was not detected in GC-MS analysis in the absence of Sc(OTf)3, while a relatively large GC-MS peak was observed for the dihydroxy product in the presence of Sc(OTf)3, suggesting a Lewis acid-catalyzed hydrolysis of epoxide (see Figure S3). DFT Calculations for Cyclooctene Epoxidation with FeIII−OOH, FeIVO, and FeVO Species in the Absence/ Presence of Sc3+. To understand the role of Lewis acids in the LFeIIIOOH-mediated (L = BPMEN) epoxidation of cyclooctene, density functional calculations were performed with and without Sc(OTf)3. First, the epoxidation of cyclooctene was calculated without the addition of Sc(OTf)3. It is pertinent to mention that Quinonero and co-workers even reported the mechanism of LFeIIIOOH-catalyzed (L = BPMEN) ethylene epoxidation,75 in which two different pathways have been investigated, starting from LFe III (OOH)(NCCH 3 ) and LFeIII(OOH)(OH2). For LFeIII(OOH)(NCCH3), the OOH group attacks the ethylene substrate directly, concomitant with the cleavage of the O−OH bond. For LFeIII(OOH)(OH2), the heterolytic cleavage of the O−O bond takes place first, which is coupled with a proton transfer from the ferric-bound water molecule to the hydroxide. This leads to the formation of a high-valent FeVO(OH) species, which can then oxidize ethylene to form the epoxide. Here, the epoxidation mechanism was reinvestigated using cyclooctene as the substrate, and Gibbs energies obtained using larger basis sets with dispersion corrections are presented. The overall mechanism is quite similar; however, the results also show some important differences from the previous studies. The starting complex is LFeIII(OOH)(NCCH3) (labeled as React, Figure S4), the Gibbs energy of which was set to zero 15142


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Inorganic Chemistry Scheme 1. Different Pathways for (BPMEN)FeOOH-Catalyzed Epoxidation of Cyclooctenea

a

Energies are given in kilocalories per mole (in red) relative to the React except for FeIVO pathway (in blue).

(Scheme 1). In Scheme 1, the spin states with the lowest energies are presented, and the energies for other spin states can be found in the Supporting Information (Figure S5). The exchange of acetonitrile by water is endergonic by 2.8 kcal/mol, while it was estimated to be endothermic by 9.0 kcal/mol in the previous studies.75 From the React, the epoxidation of cyclooctene takes place in a stepwise manner. First, the OOH group attacks the substrate double bond to form a C−O bond and a carbon-centered radical, and a similar pathway has been found for naphthalene 1,2-dioxygenase.81 In apocarotenoid oxygenase, an FeIII−OO·− (or an FeII−OO·− substrate radical) was found to be the reactive species to attack the substrate double bond.82 In present studies, this step was calculated to have a barrier of 24.4 kcal/mol in the sextet, and the resulting intermediate lies at +14.0 kcal/mol relative to the React. The preference of the sextet channel can be understood from the orbital diagram shown in Figure 6. During the reaction, one electron from the cyclooctene π orbital is transferred to the dxz* orbital of the ferric ion, which is evidenced by the σ- and σ*-like molecular orbitals between the substrate and the FeOOH moiety in TS1 shown in Figures S23−S25. At React, the Fe−O1 distance (1.90 Å) in the sextet is significantly longer than those in the doublet and quartet (1.79 Å for both states), and the reason is that one electron is populated in the dz2* orbital (Figure 6). The longer Fe−O1 distance results in weaker overlap between the dxz orbital of Fe and the px orbital of O1. Consequently, the antibonding orbital dxz* formed by interaction of these two orbitals has lower energy compared with those for doublet and quartet. The sextet thus favors the electron transfer from the substrate to the iron center via O1, which explains the much lower barrier for the sextet, and the doublet and quartet have similar but higher barriers. Subsequently, the O−O bond cleavage takes place, in concert

Figure 6. Orbital diagrams for cyclooctene oxidation (TS1) and O−O bond cleavage (TS1′) in different spin states.

with the formation of the second C−O bond, which is associated with a barrier of 18.4 kcal/mol relative to the React. In this process, both the sextet and the quartet might be involved, as the Gibbs energy difference is only 1.1 kcal/mol, which is also an indication of two-state reactivity.83 When water is coordinated to the metal ion, the heterolytic cleavage of the O−O bond takes place in the doublet state with a barrier of 17.9 kcal/mol. The quartet and sextet now have much higher barrier, which differs from that for the cyclooctene oxidation via TS1. An orbital diagram (Figure 6) can also be used to rationalize this observation. For the O−O bond cleavage, one electron is now shifted from the dyz* orbital of Fe 15143


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Inorganic Chemistry Scheme 2. Different Pathways for (BPMEN)FeOOH-Catalyzed Epoxidation of Cyclooctene Assisted by Sc(OTf)3a

a

Energies are given in kilocalories per mole relative to their reactant complexes.

to the σ* orbital of the peroxide O−O bond. The sextet has the highest barrier, as its dyz* orbital has higher energy, and also the electron transfer leads to exchange destabilization from five d electrons to four d electrons.84,85 The dyz* orbitals of the doublet and quartet have similar energies. However, for the doublet, a β electron is transferred to the O−O σ* orbital, which is evidenced by the increase of spin density on Fe from React′ (0.92) to TS1′ (1.42). Consequently, the exchange interaction in the doublet state increases, while it decreases in the quartet. These effects thus strongly favor O−O bond cleavage at the doublet state. This leads to the generation of an FeVO(OH) intermediate Int1′ (Figure S6), which is a quartet, and its Gibbs energy is 4.2 kcal/mol below the React′; consequently, a spin crossing takes place during the reaction. These results are similar to that for (TPA)FeIII(OOH)(OH2) (TPA = tris(2-pyridylmethyl)amine).86,87 The Int1′ is very reactive, and its epoxidation of cyclooctene has a barrier of only 4.5 kcal/mol in the doublet state. Also, the LFeII(OOH)(H2O) complex (React″) was considered as a potential starting complex (Gibbs energy shown in Figure S7 and structures shown in Figure S8). The heterolytic cleavage of the O−O bond takes place in the quintet state, with a very facile barrier, being only 7.6 kcal/mol. As a result, an FeIVO(OH) intermediate Int1″ is formed (Figure S8), in which the Int1″ is a quintet and lies at −29.1 kcal/mol relative to the React″. Compared with LFeIII(OOH)(H2O), the heterolytic cleavage of O−O bond at FeII is more feasible to take place, because it is much easier to transfer an electron from FeII to the O−O σ* orbital than that from FeIII. In addition, formation of the Int1″ may also proceed by oneelectron reduction of the Int1′, in which the electron may come from solvent or the Fe(II) species, and the Int1″ is believed to have much lower reactivity compared with FeVO(OH) in oxidations.28,29 The epoxidation of cyclooctene using FeIV

O(OH) (structures shown in Figure S8) proceeds via a concerted transition state 5TS2″ associated with a barrier of 24.8 kcal/mol in the quintet state relative to Int1″ (Scheme 1, blue data), and this step is exergonic by 7.1 kcal/mol (Figure S7). No carbon-centered intermediate could be located, which is different from the (TMCS)Fe IV O (TMCS = 1mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) catalyzed propene epoxidation.88 Wang and co-workers even investigated the epoxidation reaction mediated by [(TMC)LFeIVO] (TMC= 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and they found that the reactivity of the FeIVO species is significantly affected by the ligand trans to the oxo group. The complex with a neutral acetonitrile ligand is the most reactive, followed by trifluoroacetate, azide, and thiolate.89 Taken together, these calculation data clearly support that, in the absence of Sc3+, the FeVO species is much more active than the FeIII−OOH and FeIVO species in oxygen transfer.87,90 To rationalize the 18O isotope-labeling experiments, we calculated oxygen exchange at the Int1′ via a proton transfer. If this oxygen exchange has lower barrier than the following cyclooctene oxidation, then the 18O from water can be largely incorporated into the final epoxide product. The calculations showed that this tautomerization assisted by a water molecule (TS1*, see Figure S9) has a barrier of 12.5 kcal/mol relative to the Int1′ (Figure S10), and this barrier is much higher than that for cyclooctene epoxidation (4.5 kcal/mol, see Scheme 1). In addition, the 18O from water cannot be incorporated into the product by pathway from LFeIII(OOH)(NCCH3). Remarkably, oxygen exchange (TS2*, see Figure S11) starting from FeIV O(OH) (Int1″) has a barrier of 13.8 kcal/mol (Figure S12), which is much lower than that for cyclooctene epoxidation (24.8 kcal/mol, see Scheme 1). As a consequence, oxygen exchange takes place easily at the FeIV state. Taken together, 15144


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FeIII−OOH, and the barrier for the first step decreases to 18.7 kcal/mol (Scheme 2 and Figure S16), which is 5.7 kcal/mol lower than that without Sc(OTf)3 (Scheme 1). The following step has a barrier of 19.4 kcal/mol relative to the ReactSc, which is slightly higher than that for the first step. Notably, this pathway may compete with the heterolytic pathway, which involves the generation of the FeVO species (barrier of 17.2 kcal/mol). Thus, a second active species ReactSc, that is, the Sc3+ adduct of the FeIII−OOH species, is capable of epoxidizing cyclooctene, energetically comparable to that of the FeVO species, which leads to a higher efficiency for epoxidation in the presence of Sc(OTf)3 as demonstrated in experimental tests (Table 1, vide supra). It is worth mentioning that the reactivity of the FeIII−OOH is also affected by the ligand. On the one side, in cytochrome P450, DFT calculations by Shaik and coworkers showed that the porphyrin FeIII−OOH species is not capable of promoting epoxidation reactions.92 On the other side, de Visser and co-workers showed that the FeIII−OOH with a pentadentate nitrogen-based ligand (N-methyl-N,N′,N′tris(2-pyridyl-methyl)ethane-1,2-diamine) enables the hydroxylation of aromatic ring with a barrier of 23−26 kcal/mol,93,94 which is quite close to our value of 24.4 kcal/mol for the FeIII− OOH-mediated epoxidation reaction without Sc3+. Notably, in our system, in the absence of Sc3+, epoxidation by the FeVO has a much lower activation barrier (17.9 kcal/mol, see Scheme 1) than that by the FeIII−OOH (24.4 kcal/mol), which makes the FeIII−OOH become a sluggish oxidant in epoxidation when compared with FeVO. While, in the presence of Sc3+, it makes the activation barrier of the FeIII−OOH-mediated epoxidation reduce to 19.4 kcal/mol (see Scheme 2), which is comparable to that of epoxidation by the FeVO (17.9 kcal/ mol); thus, it becomes competitive with the Fe(V)O in epoxidation. Next, the effect of adding Sc(OTf)3 on the epoxidation reaction using FeIVO(OH) was calculated, in which FeIV O(OH) can be generated through the heterolytic cleavage of the O−O bond in the FeII(OOH)(H2O) complex or oneelectron reduction of the FeV(O)(OH) complex (vide supra). In the FeIV/Sc complex React′′Sc (Figure S17), a hydroxide is bridging the two metal ions, similarly to that in the ReactSc, and the Sc3+ cation is simultaneously coordinated to three OTf− anions and two water molecules, one of which further forms a hydrogen bond with the FeIVO moiety. Remarkably, the epoxidation of cyclooctene now (TS1″Sc, Figure 7) has a barrier of only 12.2 kcal/mol in the high-spin quintet state (Figure S18), which is 12.6 kcal/mol lower than that without the assistance of Sc3+ (Scheme 1). The significant effect of adding Sc3+ can be rationalized from the change of charge and energies of the four single occupied 3d orbitals. The natural bond orbital (NBO) charge on Fe is +1.05 and +1.14, respectively, for the complex without and with Sc3+. In addition, the total charge on the Sc(OTf)3(OH2)2 moiety is −0.19, suggesting significant charge transfer from the ferryl complex to the Sc(OTf)3(OH2)2 moiety. Furthermore, the single occupied 3d orbital energies decrease upon the binding of Sc3+ (Scheme S1), which also favors the electron transfer to these orbitals during the substrate oxidation. In this case, the epoxidation of cyclooctene can start from all three oxidation states, namely, FeIII−OOH, FeIVO(OH), and FeVO(OH), when Sc(OTf)3 is presented. Importantly, Sc(OTf)3 lowers the barrier significantly for the two pathways using FeIII−OOH and FeIV O(OH). Complimentarily, two other possible structures have also been considered, namely, a structure with the release of

these data rationalized that very small amount of the 18O from water can be incorporated into the final epoxide product in 18 O-labeling experiments as shown in Table 3. When Sc(OTf)3 is added into the solution, three water molecules can be ligated to Sc3+ to form a Sc(OTf)3(OH2)3 complex, which was calculated to be exergonic by as much as 27.4 kcal/mol (referred to Sc(OTf)3 plus three water molecules). The pKa of Sc(OTf)3(OH2)3 was calculated to be 3.1 in acetonitrile, suggesting that it is most likely protonated. However, the reported experimental pKa values of HCl and HBr in acetonitrile are 10.3 and 5.5, respectively,91 indicating that the calculation of pKa is quite challenging, as the total charge of the complex changes. In addition, further exchange of one OTf− ligand by a water molecule to form Sc(OTf)2(OH2)4 was found to be endergonic 13.4 kcal/mol. These results support the scenario that water can solvate the added Lewis acid, like Sc3+. The reaction of Sc(OTf)3(OH2)3 with the React′ to generate an Fe/Sc complex React′Sc, with the release of one water molecule, was calculated to be exergonic by 0.5 kcal/mol (Scheme 2, structures for two isomers shown in Figure S13). In the React′Sc, the Sc3+ cation coordinates to the peroxide anion. This coordination has a minor effect for the following O−O bond heterolytic cleavage (TS1′Sc, Figure 7),

Figure 7. Optimized structures for the TS1SC, TS2SC, TS1′SC, and TS1″SC. Distances are given in angstroms, and Mulliken spin densities are shown in red italic.

which has a barrier of 17.2 kcal/mol in the doublet state (Figure S14). In addition, the reaction now becomes endergonic by 6.0 kcal/mol. However, it should be pointed out that the structure of the Fe/Sc complex is mainly based on chemical intuition, and only a number of possible Sc3+ models were considered in the present study. The change of the ligand environment of Sc3+ may have some minor effect on the reactivity of the iron complex. The release of one water molecule and one proton from the React′Sc leads to the formation of a dinuclear five-membered ring structure ReactSc (Figure S15). As disclosed, the involvement of Sc(OTf)3 has a significant effect on the epoxidation of cyclooctene from FeIII−OOH. The optimized transition states for the two steps (TS1Sc and TS2Sc) are shown in Figure 7. The Lewis acid increases the oxidation power of 15145


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Inorganic Chemistry one water molecule to form a five-membered ring structure (AReact″sc, see Figure S19) and an isomer with an oxo bridging the two metal ions and with one water molecule hydrogen bonded to the hydroxide (B-React″sc, see Figure S19). The former has a similar barrier for the following epoxidation reaction, whereas the latter is associated with much higher barrier (Figure S20). Taken together, the presence of Lewis acid significantly reduced the energetic barriers of oxygen transfer from FeIII− OOH and FeIVO species to olefin, and it makes both of them as the candidates of oxygen transfer as well as the FeVO species, which is well-consistent with that adding Sc(OTf)3 to Fe(BPMEN) complex substantially promotes its efficiency in olefin epoxidation (vide supra). Meanwhile, as shown in Figure 2, the addition of Sc3+ apparently affects the epoxidation kinetics of the Fe(BPMEN) catalyst. In DFT calculations, it is disclosed that, in the absence of Sc(OTf)3, the heterocleavage of the O−O bond from the FeIII−OOH species has the highest barrier (17.9 kcal/mol) in the pathway of epoxidation by the FeVO species. While in the presence of Sc(OTf)3, Sc3+assisted heterocleavage of the O−O bond from the FeIII−OOH species to generate the FeVO species also has the highest activation Gibbs energy (17.2 kcal/mol) in the pathway of epoxidation by the FeVO or FeIVO/Sc3+ species, the epoxidation by the FeIII−OOH/Sc3+ species has the highest barrier of 19.4 kal/mol in the pathway of epoxidation by the FeIII−OOH species. However, it is worth highlighting that the Fe IV O species can be directly generated from the heterocleavage of the FeII−OOH species having the barrier of only 7.6 kcal/mol (see Scheme 1), which makes the epoxidation by the FeIVO/Sc3+ species having the highest barrier of only 12.2 kcal/mol, much lower than that through the pathways of the Sc3+ adducts of the FeIII−OOH and FeVO species (19.4 and 17.2 kcal/mol). Taken together, Sc3+ has been involved in the rate-determining step of all three epoxidation pathways, which is well-consistent with that adding nonredox metal ions like Sc3+ can substantially accelerate the epoxidation rate as displayed in Figure 2. In addition, these results are also well-consistent with that, in the 18O water labeling experiments, the presence of Sc3+ makes 18O incorporation in epoxide slightly higher than that in the absence of Sc3+ (see Table 3). On the one side, in DFT calculations, it disclosed that FeIII−OOH species is competitive for oxygen transfer to olefin in the presence of Sc3+, which would lead to a lower 18O incorporation in epoxide. On the other side, the FeIVO species is now capable of transferring oxygen to olefin, and its epoxidation pathway is even more energetically favorable than FeIII−OOH and FeVO species (12.2 vs 19.4 vs 17.2 kcal/mol), which leads to a higher 18O incorporation in epoxide. In other words, in the presence of Sc3+, the barriers for oxygen transferring from both FeVO (4.5 kcal/mol) and FeIVO species (12.2 kcal/mol) to olefin are lower than that for the FeVO species formation (17.2 kcal/mol). Once the FeVO species was generated by Sc3+assisted heterocleavage of the FeIII−OOH species, the oxo can be transferred to olefin by either the FeVO species directly or by the reduced FeIVO species under the assistance of Sc3+, thus improving the oxidant utilization efficiency as observed by experiments. Furthermore, the overall higher 18O incorporation in epoxide observed in experiments has clearly highlighted the significant contributions of the FeIVO species in olefin epoxidation in the presence of Sc3+, thus further supporting the crucial roles of Sc3+ in promoting the oxygenation capability of

the originally sluggish FeIVO species. In heterogeneous oxidation using metal oxide as catalyst, versatile nonredox metal ions are popularly added to the redox metal oxide as additives to modify their reactivity and/or stability in catalysis;95,96 however, the intrinsic roles of those additives have not been clearly elucidated yet due to the complicated reaction conditions that cause serious obstacles in in situ characterizations of catalysts and mechanistic studies. Here, the interaction of Sc3+ with the iron species through the oxygen linkage, which resembles the μ-oxo bridges among the redox metal ions and the additives in metal oxide catalysts, facilitating the oxygenation happening at an energetically more favorable lower oxidation state, that is, iron(V) versus iron(IV), provided the clear clues to rationalize the roles of those additives in metal oxide catalysts for heterogeneous oxidations.

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CONCLUSIONS This work demonstrated that adding nonredox metal ions like Sc3+ can significantly improve the olefin epoxidation efficiency of Fe(BPMEN) complex with aqueous H2O2 as oxidant, and it makes 18O incorporation in epoxide higher than that in the absence of Sc3+. In DFT calculations, it disclosed that, without Lewis acid, the FeVO species is much more energetically favorable for oxygen transfer than both FeIII−OOH and FeIV O species. In the presence of Lewis acid like Sc3+, it makes the oxygen transfer from FeIII−OOH and FeIVO species to olefin energetically accessible as well as that of FeVO species (19.4 vs 12.2 vs 17.2 kcal/mol), thus opening multiple channels for oxygen transfer. In particular, in the pathway of cyclooctene epoxidation, the highest barrier for the FeIVO/Sc3+ adductmediated epoxidation is lower than that for the FeVO species. The Lewis acid improved oxygen transfer capability of the FeIVO species disclosed here may have provided new clues to understand the crucial roles of these nonredox metal additives in heterogeneous oxidations as well as the role of Ca2+ in water oxidation by natural enzymes. As evidence, these nonredox metal ions can make the redox metal ions at relatively lower oxidation state become powerful oxidants, thus facilitating the biological and chemical oxidations happening at a more energetically accessible level.

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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.7b02463. UV−vis spectra, GC-MS graphs for 18O water-labeling experiments, computational details, structures, energy diagrams, and coordinates (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (G.Y.) *E-mail: [email protected]. (R.-Z.L.) ORCID

Zhuqi Chen: 0000-0002-0503-9671 Rong-Zhen Liao: 0000-0002-8989-6928 Guochuan Yin: 0000-0003-1003-8478 Author Contributions †

These authors equally contributed to this work.

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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. 21573082, 21303063, 21671072, and 21503083), the Fundamental Research Funds for the Central Universities (2017KFKJXX014), and Natural Science Foundation of Hubei Scientific Committee (2016CFA001). The GC-MS analysis was performed in the Analytical and Testing Center of Huazhong Univ. of Science and Technology.

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