Two Iron Complexes as Homogeneous and Heterogeneous Catalysts

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Two Iron Complexes as Homogeneous and Heterogeneous Catalysts for the Chemical Fixation of Carbon Dioxide Chandan Kumar Karan and Manish Bhattacharjee* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *

ABSTRACT: Two new bimetallic iron−alkali metal complexes of amino acid (serine)based reduced Schiff base ligand were synthesized and structurally characterized. Their efficacy as catalysts for the chemical fixation of carbon dioxide was explored. The heterogeneous version of the catalytic reaction was developed by the immobilization of these homogeneous bimetallic iron−alkali metal complexes in an anion-exchange resin. The resin-bound complexes can be used as recyclable catalysts up to six cycles.

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INTRODUCTION The primary greenhouse gas CO2 is an abundant source for the nontoxic and cheapest C1 building block.1,2 The rapidly increasing rate of the carbon dioxide production due to increased industrial activities has triggered the global warming and climate changes.3 Therefore, the capture and conversion of the CO2 as a renewable4,5 building block is necessary and is a challenge. The transition-metal catalyzed6−10 fixation of CO2 is the most promising method. The synthesis of the fivemembered cyclic carbonates from the reaction of CO2 with the epoxide in the presence of metal complexes is one of the most effective ways of CO2 fixation. Depending on the reaction conditions and the Lewis acidity of the metal complex, the linear carbonates transform into either cyclic carbonates12,13 or polycarbonates.14−16 In recent years, many complexes of transition metals like copper,6,7 cobalt,8,9 zinc,10 nickel,11 etc. have been successfully used as catalysts for the formation of cyclic carbonates. However, the example of iron complexes for the synthesis of cyclic carbonates is rare.17−21 The catalysts capable of operating at the atmospheric pressure and room temperature are more desirable than the others,17−23 which operate at high temperature and high pressure. The main drawback of the metalcomplex catalyzed homogeneous reaction is the difficulty in recovery of the compound from the reaction mixture and its reusability.24,25 An alternate route is necessary for the conversion of these metal complexes from homogeneous26,27 nature to heterogeneity.28,29 Some metal−organic frameworks (MOFs)30,31 and the covalent−organic frameworks32 have been used as heterogeneous catalysts for chemical fixation of CO2. However, most of the reported methods require high temperature and pressure. Also, from the “green chemistry” point of view performing the reaction without using any solvent © XXXX American Chemical Society

is warranted. Recently we showed that a copper hydrogel is an efficient and reusable catalyst for the conversion of CO2 to cyclic carbonate.6 Herein we report the synthesis and structure of two new bimetallic iron−alkali metal complexes, namely, [FeK(H2L)2]· 2H2O (1) and [Fe2Na2(H2L)4(H2O)2]·3H2O (2) {H4L = 3hydroxy-2-(2-hydroxy-3-methoxy-benzylamino)propionic acid}. These compounds are efficient catalysts for chemical fixation of CO 2 . Interestingly, the complexes can be immobilized on an anion-exchange resin, and the immobilized complexes can be used as the heterogeneous and reusable catalyst for the fixation of CO2.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All the chemicals used were of reagent-grade products. Elemental analyses were performed using PerkinElmer CHN analyzer model 2400. The IR spectra were recorded on a Parkin-Elmer model UATR Two spectrometer. The ligand, 3-hydroxy-2-(2-hydroxy-3-methoxy-benzylamino)propionic acid (H4L), was synthesized by the reported method.33 The elemental analysis of the anion-exchanged resin was performed using a field emission scanning electron microscope (FESEM). The samples were prepared by crushing the resin material, dispersing in water, drop casting on the glass slide, and then drying in air. A thin layer of gold was coated on the sample to minimize sample charging before the experiment. The inductively coupled plasma-mass spectrometry (ICPMS) measurements were done using a Thermo Fisher Scientific iCAP Q. Synthesis of Metal Complexes, [FeK(H2L)2]·2H2O (1) and[Fe2Na2(H2L)4(H2O)2]·3H2O (2). The ligand H4L (0.241 g, 1.0 mmol) and KOH/NaOH (4.0 mmol) were dissolved in 10 mL of aqueous methanol (1:9), and to this was added methanolic solution (10 mL) of Received: February 10, 2018

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DOI: 10.1021/acs.inorgchem.8b00379 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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anhydrous FeCl3 (0.162 g, 1.0 mmol). The chocolate brown colored solution was stirred for 4 h and then filtered. When the solution stood for two weeks, thin brownish needlelike crystals, suitable for X-ray diffraction, were obtained. 1: yield: 39%. Anal. Calcd for C22H28FeKN2O12 (molecular wt 607.41): C, 43.50%; H, 4.65%; N, 4.61%. Found: C, 42.98%; H, 4.84%; N, 4.67%. IR (KBr) (ν, cm−1): 3410, 3190, 2930, 1620, 1480, 1240, 1080, 860, 740, 560. UV−visible: λmax nm (ε) = 300 (4877), 500 (1818). 2: yield: 36%. Anal. Calcd for C44H60Fe2N4Na2O25 (molecular wt 1202.63): C, 43.94%; H, 5.03%; N, 4.66%. Found: C, 44.12%; H, 4.84%; N, 4.75%. IR (KBr) (ν, cm−1): 3430, 3120, 2930, 1620, 1480, 1240, 1080, 860, 740, 590. UV−visible: λmax nm (ε) = 300 (10 570), 500 (3984). X-ray Crystallography. Single-crystal X-ray data of the complexes were collected on Bruker Smart APEX II CCD system that uses graphite monochromated Mo Kα radiation (λ = 0.710 73 Å). The structures were solved by the direct method and refined by the leastsquare method on F2 employing WinGX34 package and the relevant programs {SHELX-9735 and ORTEP-336} implemented therein. Nonhydrogen atoms were refined anisotropically, and hydrogen atoms on C atoms were fixed at calculated positions and refined using a riding model. The details of crystal data collection and refinement of complexes are summarized in Table 1. The important bond distances and bond angles are given in Tables S1 and S2 in the Supporting Information, respectively.

RESULTS AND DISCUSSION The complexes were prepared from the reaction of the ligand H4L with anhydrous FeCl3 in the presence of NaOH/KOH. The complexes were characterized by elemental analysis, IR, and UV−visible spectroscopy and structurally characterized by single-crystal X-ray crystallography. Crystal Structure of the Complexes [FeK(H2L)2]·2H2O (1). The complex crystallizes in orthorhombic space group P212121. The asymmetric unit of 1 consists of an Fe(III) center coordinated to two [H2L]2− ligands, one K atom, and two water molecules (Figure 1a). The Fe(III) center is in a distorted octahedral coordination environment and is coordinated to two carboxylate oxygens, O2 and O7, axially. The basal positions are occupied by two nitrogens, N1 and N2, and two phenolic oxygens, O1 and O6, from two ligands. The potassium center K1 is coordinated to phenolic oxygens, O1 and O6, methoxy oxygens O5 and O10, carboxylate oxygens, O2, O3, and O3# (# = x − 1, y, z), and an oxygen O4 of the −CH2OH side chain. Thus, the potassium is octa-coordinated and is in a bicapped octahedral coordination environment. The two potassium ions are bridged by the carboxyl oxygen O3. The Fe1 and the K1 centers are bridged by the phenoxy oxygens O1 and O6 (Figure 1b). In the solid state, there exists a onedimensional (1D) helical polymeric chain (Figure 1c), which extends along the crystallographic a axis with a pitch length equal to the length of the crystallographic a axis (8.401 Å). The absolute structure (Flack) parameter37 {0.02(4)} shows that the coordinates correspond to the absolute structure of the compound. Crystal Structure of the Complexes [Fe2(H2L)4Na2(H2O)2]·3H2O (2). The complex 2 crystallizes in monoclinic space group P21. The asymmetric unit consists of two iron centers, Fe1 and Fe2, each coordinated to two dinegatively charged [H2L]2−, two potassium ions, K1 and K2, two potassium-bound water molecules, and three water molecules of crystallization (Figure 2a). The Fe1 center is coordinated to two carboxylate oxygens O2 and O7, which occupy the axial positions. The equatorial positions are occupied by two nitrogens, N1 and N2, and two phenolate oxygens, O1 and O6, from two different ligands. Similarly, the Fe2 center is coordinated to carboxylate oxygens O12 and O17, ligand nitrogen atoms N3 and N4, and phenolate oxygens O11 and O16. Both the iron centers are in a highly distorted octahedral coordination environment (Figure 2b). The Na1 center is coordinated to phenolate oxygens O1 and O6, carboxylate oxygens O17 and O18, and methoxy oxygens O5 and O10. Also, the Na1 center is coordinated to the water oxygen O1W. Similarly, the Na2 center is coordinated to methoxy oxygens O15 and O20, phenolate oxygens O11 and O16, carboxylate oxygens O7 and O8, and the water oxygen O2W. Both the sodium ions are in highly distorted monocapped octahedral coordination environments. The Fe1 and the Na1 centers are bridged by phenolate oxygens O1 and O6, and the Na1 and Fe2 centers are bridged by carboxylate oxygen O17. Similarly, the Na2 and Fe1 centers are bridged by the carboxylate oxygen O7, and the Fe2 and Na2 centers are connected by the phenolate oxygens O11 and O16 (Figure 2b). In the solid state there exist intra-and intermolecular hydrogen bonding (H-bonding) interactions. Because of the presence of intermolecular H-bonding interactions H-bonded helical chains are formed along the crystallographic a, b, and c

Table 1. Crystallographic Data for 1 and 2 complex

1

2

empirical formula FW T, K crystal system space group a, Å b, Å c, Å β, deg V, Å3 Z, ρ, Mg m−3 μ, mm−1 F(000) refln collected ind refln data/restn/param GOF on F2 absolute structure parameter final R indices [I > 2σ(I)]

C22H28FeKN2O12 607.41 100(2) orthorhombic P212121 8.4011(13) 13.344(5) 21.710(3) 90.00° 2670.3(7) 4, 1.491 0.784 1228 32 074 5452 5452/0/346 1.007 0.02(5) 0.0728, 0.1764

C44H60Fe2N4Na2O25 1202.63 100(2) monoclinic P21 11.915(2) 20.411(4) 12.221(2) 117.648(5)° 2632.8(8) 2, 1.515 0.657 1248 30 853 8772 8772/1/726 1.024 −0.004(16) 0.0514, 0.0972

Article

General Procedure for the Cycloaddition of CO2 to Epoxides. The epoxide (20 mmol), tetrabutylammonium bromide (TBAB, 10 mol %), and the complex (0.01 mmol) were taken in a Schlenk tube, CO2 was purged at 1 bar pressure at room temperature, and the reaction mixture was stirred for 48 h. After that, an aliquot of the reaction mixture was dissolved in CDCl3, and the 1H NMR was recorded for the calculation of conversion. Preparation of 1@resin and 2@resin through AnionExchange Method. An aqueous solution (3 mL) of the complex (0.01 mmol) was taken in a vial, and to this, the anion-exchange resin {Amberlite IRA-400(Cl)} was added. The mixture was stirred for 12 h; after that, the brown color of the aqueous solution disappeared, and the color of the resin turned deep brown. Then the resin was filtered and dried over filter paper. ICP-MS Analysis of the 1@resin and 2@resin. The 1@resin and 2@resin (0. 0035 g) were treated with concentrated nitric acid (1 mL) and digested. After that, concentrated nitric acid (1 mL) was added, and 300 μL of the solution was taken and diluted to 10.5 mL. After that, 10 mL of the solution was used for the ICP-MS experiment. B


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Inorganic Chemistry

Figure 1. ORTEP views of the asymmetric unit of 1 (a). Ball-and-stick model of the coordination environment around the potassium and iron centers in 1 (b). View of the 1D coordination network along the a axis (c).

in the presence of 2 (0.005 mmol), the conversion was found to be very negligible (Table 2, entry 2). However, when the reaction was performed in the presence of 0.025 mol % of 2 and 5 mol % of TBAB, the conversion was found to be only 61% (Table 2, entry 3). In the presence of 0.05 mol % of 2 and 5 mol % of TBAB, the conversion was found to be moderate (72%; Table 2, entry 4). The maximum conversion (89%) was observed when 10 mol % of the TBAB and 0.05 mol % of 2 was used (Table 2, entry 5). In the present case comparatively higher amount of TBAB is required, as the reactions were done under an ambient condition, in contrast to the reported reactions, which were performed at high pressure and temperature. After the optimization of the reaction conditions, we performed the reaction using various types of epoxides containing short to long side-chain aliphatic and aromatic groups with both the catalysts. From the observed turn over number (TON; Table 3), it is clear that 2 is more efficient than 1. For the aliphatic hydrocarbon side chain containing epoxides, the conversion was found to be good. However, for the aryl

axes with pitch lengths equal to the corresponding lengths of the a, b, and c axes and thus forms a H-bonded 3D network (Figures S3 and S4, Supporting Information). The observed Flack parameter37 {−0.004(16)} shows that the coordinates correspond to the absolute structure of the complex. The difference between the solid-state structures of 1 and 2 can be attributed to the difference between the ionic size of Na+ and K+ ions. The comparatively smaller ionic radius of Na+ affords a discrete complex, and the larger size of the K+ ion produces a 1D polymeric chain, where the K+ ion acts as the node. It may be noted that earlier reports from this laboratory also showed the effect of the size of the alkali metal cation on the solid-state structures of bimetallic complexes.38−40 Catalytic Properties of 1 and 2 toward the Chemical Fixation of CO2. We were interested in the efficacy of the complexes as catalysts for CO2 fixation (Scheme 1). Initially, the reaction of propylene oxide was performed with CO2 at room temperature under solvent-free conditions for 48 h. In the presence of 5 mol % TBAB the conversion was found to be only 14% (Table 2, entry 1). When the reaction was performed C


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Inorganic Chemistry

Figure 2. ORTEP views of the asymmetric unit of 2 (a) and the coordination environment around the sodium and iron centers (b).

Table 3. Reactions of Epoxides with CO2 in the Presence of 1 and 2a

Scheme 1. Reaction of Epoxides with CO2 Catalyzed by 1 and 2 and Tetrabutylammonium Bromide

Table 2. Optimization of the Reaction of Propylene Oxide with CO2a entry

2 (mol %)

1 2 3 4 5

0.025 0.025 0.05 0.05

TBAB (mol %)

conversionb (%)

5

14 0 61 72 89

5 5 10

a

Reaction conditions: epoxide (20 mmol), catalyst (0.05 mol %), and TBAB (0.64 g, 10 mol %) under CO2 purged at 1 bar pressure at 300 K and 48 h. bFrom1H NMR. c24 h.

a

Epoxide (20 mmol) under CO2 (1 bar); temperature, 300 K; time, 48 h. bFrom1H NMR.

interested in immobilizing the complexes on an ion-exchange resin and use the resin-bound complexes as the recyclable catalysts. The complex 2 was dissolved in water, and to this, the anion-exchange resin, Amberlite IRA-400(Cl), was added and stirred for 12 hours (Scheme 2). The brown color of the solution disappeared, and the color of resin changed to deep brown (Figure 3). After that, the ion-exchanged material (2@ resin) was separated. The 23Na NMR (NaCl standard) of the complex in D2O shows a broad peak at δ 8.4 ppm. The marked shift and the broad nature of the signal are due to the presence of paramagnetic iron centers. The 23Na NMR of the solution after the exchange reaction shows a sharp peak at δ −0.5 ppm (Figure 3e). The NMR experiment shows that chloride anions are replaced by the anionic complex [Fe(H2L)2]−. UV−Visible spectra of the aqueous solution of 2 were recorded before and after the immobilization of 2 to confirm further the immobilization of 2 on the resin. The UV−visible spectrum of 2 shows a ligand-to-metal charge transfer (LMCT) transition at 500 nm along with the internal ligand transition

epoxides (Table 3, entry 4), the conversion was found to be low. The low conversion may be due to the electronwithdrawing effect41 of the phenyl ring in styrene epoxide. The reactions performed at high temperature and under high pressure are comparatively faster than the reactions done at room temperature but under high pressure.42−44 It may be noted that there are only a few reports1,7,9,45 on the fixation of CO2 under ambient condition, and the reactions were found to be much slower. Compared to these reports1,7,9,45 the catalytic activities of 1 and 2 are found to be quite good. Immobilization of the Complexes on an Anion Exchange Resin and Use of the Resin-Bound Complexes as Heterogeneous Catalysts. The main disadvantage in using the soluble metal complexes as catalysts is due to the homogeneous nature of the reaction, due to which separation and reusability of the catalysts are difficult. Thus, we were D


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Inorganic Chemistry Scheme 2. Representation of the Immobilization of the Complexes on Amberlite IRA-400(Cl)

UV−visible spectra of the aqueous solution of 2 before and after the reaction of the cation-exchange resin Amberlite IRA200(Na) did not show any change. Thus, it is confirmed that the complex 2 was not immobilized on the cation-exchange resin. Similarly, 1 was also immobilized (1@resin), and the immobilization was confirmed by UV−visible spectroscopy (Figure 4a). The resin materials after the immobilization were subjected to FESEM elemental analysis, which confirms the presence of iron but does not show the presence of sodium or potassium (Supporting Information, Figures S1 and S2). To know the exact amount of the catalyst immobilized in the resin, we performed the ICP-MS experiment. We found that, for the 1@resin, the iron loading is 1.25 wt %, and for the 2@resin, the iron loading is 1.71 wt %. From the ICP-MS experiment, we also found that the amount of sodium and potassium is negligible in the catalyst-immobilized resin material. In the case of complex 2 as opposed to the solid-state structure, the complex dissociates in aqueous solution into Na+ and [Fe(H2L)2]−. Thus, in both cases the anionic complex [Fe(H2L)2]− is immobilized in the resin and acts as the active center during the catalysis. The IR spectra of the resin after the immobilization of the complexes and those of the pure complexes are very similar (Figures S5 & S6, Supporting Information). This also support that the structural identity of [Fe(H2L)2]− anion remains intact.

Figure 3. Photograph of (a) aqueous solution of complex 2, (b) anionexchange resin, IRA-400(Cl) before adsorption, (c) anion-exchange resin after adsorption of complex 2, (d) after separation of the 2@ resin, and (e) 23Na NMR spectra of the complex 2 before and after adsorption in D2O solvent.

(Figure 4b). When the complex was loaded, the characteristic peak of the complex at 500 nm disappeared (Figure 4b). The

Figure 4. UV−Visible spectra of (a) aqueous solution of complex 1before and after the immobilization and (b) aqueous solution of complex 2 before and after the immobilization. E


Inorganic Chemistry

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CONCLUSION In summary, we have synthesized two new iron−potassium and iron−sodium bimetallic complexes using an amino acidcontaining reduced Schiff base. These two iron complexes are effective catalysts for chemical fixation of CO2 under 1 bar pressure and at room temperature having high TONs. Also, through an ion-exchange process, the complexes can be immobilized on an anion-exchange resin, and the resin-bound complexes can be used as reusable heterogeneous catalysts.

After the ion-exchange process, the exchanged resin materials, that is, 1@resin and 2@resin, were used as catalysts for the same reaction. We performed the reaction of three different derivatives of epoxide in the presence of 1@resin and 2@resin (Table 4). The observed TONs show that 2@resin is Table 4. Reactionsa of Epoxides with CO2 in the Presence of 1@resin and 2@resin

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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.inorgchem.8b00379. Materials and methods, X-ray structural studies, IR, and NMR spectra, and FESEM, analysis (PDF)

a

Reaction conditions: epoxide (20 mmol), catalyst (0.05 mol %), and TBAB (0.64 g, 10 mol %) under CO2 purged at 1 bar pressure at 300 K and 48 h. bFrom1H NMR.

Accession Codes

CCDC 1578515−1578516 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

comparatively more efficient. Also, in the case of 2-ethyloxirane, the TON of the 2@resin catalyzed reaction is slightly higher (Table 4, entry 2). However, in the reaction of styrene oxide with CO2 catalyzed by 2@resin, the TON is higher compared to that catalyzed by 1@resin (Table 4, entry 3). It may be noted that the TONs reported here are either higher1,42−44 (Supporting Information, Table S3) or comparable7,45 to those reported earlier. As mentioned above, we were interested in the recyclability of the resin-bound complexes as catalysts. Accordingly, the reaction of CO2 with propylene oxide was carried out in the presence of 1@resin and 2@resin. After the reaction, the resin was separated by filtration and washed thoroughly at first with ethyl acetate and then with distilled water. Then it was dried in air and reused for the next catalytic cycle. The immobilized complexes were used for the subsequent five cycles (Figure 5). The materials were found to exhibit excellent catalytic efficiency without much loss of activity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Manish Bhattacharjee: 0000-0002-2197-9524 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Dept. of Science & Technology, Government of India, New Delhi, for X-ray and NMR facilities (Grant No. SR/ FST/CSII-026/2013). C.K.K. thanks IIT Kharagpur for financial assistance. The authors thank the reviewers for their helpful comments.

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REFERENCES

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Figure 5. Recyclability experiment of 1@resin and 2@resin using propylene oxide as the substrate.

The reaction solution after the separation of the immobilized catalyst was subjected to ICP-MS measurement to investigate the extent of leaching. The ICP-MS measurement shows that only 270 ppb of iron is present in the solution after the reaction. Thus, it can be concluded that the amount of leaching is low, and because of this the catalytic efficiency remains high even after five cycles. F


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