Dicationic Ionic Liquid Composite as A Task-Specific

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Lignosulfonate/Dicationic Ionic Liquid Composite as A TaskSpecific Catalyst Support for Enabling Efficient Synthesis of Unsymmetrical 1,3-Diynes with A Low Substrate Ratio Bingbing Lai, Rongxian Bai, and Yanlong Gu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04451 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Sustainable Chemistry & Engineering

Lignosulfonate/Dicationic Ionic Liquid Composite as A TaskSpecific Catalyst Support for Enabling Efficient Synthesis of Unsymmetrical 1,3-Diynes with A Low Substrate Ratio Bingbing Lai,† Rongxian Bai†,* and Yanlong Gu†,‡,* †

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education. Hubei Key Laboratory of Material Chemistry

and Service Failure, School of Chemistry and Chemical Engineering Huazhong University of Science and Technology (HUST) 1037 Luoyu road, Hongshan District, Wuhan 430074 (China) ‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation Lanzhou Institute of Chemical Physics Chinese Academy of Sciences, Lanzhou, 730000 (P.R. China) * Corresponding authors. E-mail address: [email protected] ABSTRACT: In Glaser hetero-coupling reactions, one of the alkyne precursors have to be used in five to ten times of excess. This not only wasted the organic substrate, but also decreased the catalytic efficiency of the metal catalysts. To solve this problem, we prepared a lignosulfonate/dicationic ionic liquid composite via ion exchange process. The thereby obtained materials can be used as catalyst supports for preparing heterogeneous Cu-based catalysts. The supports and the catalysts were characterized by many physicochemical methods including FT-IR, elemental analysis (EA), FSEM, FTEM, XPS, and TG. Interestingly, the catalyst can enrich the alkynol component in the reaction system, thus enabling Glaser hetero-coupling reactions of alkynols and phenylacetylenes to proceed well with a low substrate ratio. Substrate scope of the Glaser hetero-coupling reaction, enriching effect of the composite support for the alkynol component, and the recyclability of the catalyst were also investigated. KEYWORDS Glaser Hetero-Coupling; Lignosulfonate; Heterogeneous Catalyst; Biomass

Introduction The immobilization of homogeneous metal complex catalysts has aroused considerable interest over the past few years because of the intrinsic advantages of solid catalyst, such as the easy separation and the good recyclability,1-4 thus a variety of immobilization methods have been reported up to now.5-8 However, Vittorio Farina and Johannes G. de Vries et al9 have critically reviewed the usage of immobilized homogeneous catalysts recently, and pointed out the reasons why the traditional immobilized homogeneous catalysts are difficult to be used in industry. The poor stability of metal complex and the limitation of mass transfer between substrates and the solid catalysts are the first two among all the deficient factors. Therefore, the suggestions to industrial chemists to use a solid-immobilized metal complex catalyst in their processes are usually met with quite underwhelming enthusiasm. On the basis of these facts, we proposed to use the immobilization of homogeneous metal complex catalyst as one of the technological methods to improve the performance of the catalyst. If some drawbacks of the homogeneous system other than the recyclability of the catalyst can be overcome with the aid of the immobilization, the necessity of implementing the immobilization will be demonstrated. Therefore, we attempted to develop some new methods to immobilize homogeneous metal complex catalyst, which enabled us to access a remarkable improvement in terms of either the catalytic activity or the reaction selectivity. Synergistic effect between the metal complex and the solid support is expected to be one of the important reasons to account for the improvement.

However, the conventional catalyst supports have been well studied and extensively used in the researches of immobilizing homogenous catalyst. In order to find a new synergistic effect to facilitate our study, a new type of support that has unique physicochemical property should be developed. Conjugated 1,3-diynes have been used as important building blocks in organic synthesis,10 and these kind of molecules also widely exist in natural products, pharmaceuticals and bioactive compounds.11 Therefore, especially for unsymmetrical diynes,1215 much attention has been paid to their synthesis.16-18 The present ways to construct conjugated 1,3-diynes include the following five methods: (i) homo-coupling of vinyl dibromides,19 (ii) decarboxylative coupling of vinyl dibromides and alkynyl carboxylates,20 (iii) elimination of (Z)-arylvinyl bromides under basic conditions,21 (iv) decarbonylation of diynones22 and (v) Glaser hetero-coupling of two different terminal alkynes.23-25 The last method has been widely used in the practical synthesis because of the following two reasons: (i) the starting materials are easily available; and (ii) the reaction conditions are quite mild. However, due to the easy formation of the homo-coupling product, one of the alkyne precursors has to be used in large excess amount in order to get a high yield of the hetero-coupling product. This not only increased the cost of the synthesis, but also generated some organic wastes. In some cases, the efficiency of the product isolation is also detrimentally affected. Some homogeneous catalytic systems, such as dppm(Au)Br2/Phen, AgOTF/Ni(OAc)2.4H2O and Cu0/TMEDA have been developed to solve this problem.26-28 Unfortunately, these catalytic systems are plagued by either the high cost of catalyst or the poor recyclability. Therefore, for the synthesis of unsymmetrical 1,3-dikynes with Glaser reaction, a cost-effective and recyclable catalyst is still needed. Particularly, efforts should be paid to how to avoid the use of large excess amount of the alkyne precursor. Alkynol is often used in Glaser hetero-coupling reaction. Because of the presence of a hydroxyl group, this component is able to interact with a hydrogen-bond donor or acceptor. When a hydrogen-bond-inert alkyne, e.g. phenylacetylene, was used as the counter-part reagent of alkynol, it can be speculated that a hydrogen-bond-active solid supporting material might be able to selectively absorb the alkynol component via hydrogen bond interaction. Thus, the concentration of the alkynol component near to the solid surface can be increased to some extent. This phenomenon, if happen, can be a potential tool for us to design a task-specific catalyst that enabled the Glaser hetero-coupling reaction of alkynol and phenylacetylene to proceed with a low substrate ratio. To realize this goal, it is necessary to find a hydrogen-bond-active solid supporting material. The solid is not only able to immobilize easily homogeneous catalyst but also

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capable of acting as either hydrogen-bond donors or acceptors, allowing thus a local amplification of the alkynol concentration. Considering the fact that phenylacetylene is a lipophilic species, to ensure a smooth hetero-coupling reaction, the supporting material should not be strongly hydrophilic. Otherwise, the reaction will encounter a mass transfer problem of phenylacetylene component, which may result in an extensive homo-coupling of alkynol, destroying thus the reaction selectivity. Recently, we have used sodium lignin sulfonate, which is a bulky waste generated in paper making industry (800,000 t/year), as an anionic supporting material to immobilize cationic metalbased catalyst.29 In the molecular skeleton of sodium lignosulfonate, there are many organic functionalities, such as aryl, hydroxy and carboxy groups. This allows sodium lignosulfonate to be a hydrogen-bond-active solid, and able to act as both hydrogen bond donor and acceptor. The lignin-based aryl groups in the structure endued also sodium lignosulfonate a lipophilic property. Therefore, lignosulfonate may be an appropriate supporting material to prepare a catalyst for Glaser hetero-coupling reaction of alkynol and phenylacetylene with a low substrate ratio. Sodium lignin sulfonate can react with dicationic or polycationic species to generate a hydrophobic polymer via ion exchange process in water (Flocculation Process). Inspired by this fact, in order to facilitate the anchoring of a metal complex catalyst, we decided to use a functionalized dicationic imidazolium salt to react with sodium lignosulfonate in an aqueous solution. The obtained hydrophobic material can then be used as a supporting material to immobilize a homogeneous Glaser coupling catalyst by means of post grafting procedure. With this strategy, in this work, we prepared a lignosulfonate/dicationic imidazolium salt composite via flocculation process, and then used it as a supporting material for the first time to immobilize a Cu-TMEDA catalyst (TMEDA = tetramethylethylenediamine). Interestingly, this solid catalyst displayed remarkable catalytic activity in the Glaser hetero-coupling reaction of alkynol and phenylacetylene to synthesis of unsymmetrical 1,3-diynes with a low substrate ratio.

Results and Discussion As we mentioned above, the poly-anionic material sodium lignin sulfonate tended to react easily with polycationic species in water to generate the hydrophobic solid. Then a water-soluble dicationic imidazolium salt (DIS) was synthesized by the quaternization reaction of N-methylimidazole and bis(2-chloroethyl)amine hydrochloride in alcohol under reflux condition.30-31 Afterwards, the DIS was added dropwise to the aqueous solution of sodium lignosulfonate, thus generating a hydrophobic dicationic imidazolium salt/lignosulfonate solid composite via ion exchange process (denoted as DIS@LS, Fig. 1). To immobilize TMEDA fragment, the as-prepared DIS@LS composite was then treated with divinyl sulfone and N1,N1,N2 -trimethylethane-1,2-diamine via a consecutive twice Michael addition reactions (Fig. 1).32 This gave a new composite (DIS@LS-TMEDA). Because of the robust

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coordination between Cu species and TMEDA moiety, DIS@LSTMEDA composite was subsequently utilized to immobilize homogeneous Cu species in alcohols under reflux condition.33-34 Finally the obtained catalyst was denoted as DIS@LS-TMEDACu, in which, the loading of Cu species was confirmed to be 0.95 mmol.g-1 by means of analysis with ICP-MS. A referential catalyst was also prepared. Merrifield resin (MR) was used as a supporting material for the referential catalyst, because MR had no hydroxy groups on its surface. Besides, its surface area is closed to that of sodium lignosulphonate, minimizing thus the possible effect of physical absorption of the support to the alkyne precursors.35 Chloromethyl groups in MR can be used as an anchor site to immobilize the TMEDA fragments. Thus, TMEDA fragment was firstly grafted onto the surface of Merrifield resin under basic conditions. The generated MR-TMEDA composite was then directly used as a support to immobilize copper species (ESI, Fig. S1).36 The obtained referential catalyst MR-TMEDA-Cu has a Cu loading of 0.42 mmol.g-1. In order to confirm the amount of grafted TMEDA fragment on DIS@LS-TMEDA composite, the prepared materials including DIS@LS and DIS@LS-TMEDA were submitted to elemental analysis (ESI, Table S1). Compared with DIS@LS, the nitrogen content of DIS@LS-TMEDA increased significantly, indicating occurrence of grafting TMEDA fragment (Table S1). By assuming the increase of the nitrogen content to the covalent anchoring of TMEDA fragment, the TMEDA loading can be calculated. It was 0.56 mmol.g−1. Besides element analysis, the successful grafting of TMEDA fragment was also evidenced by FT-IR spectroscopy. As shown in Fig. 2, compared with pristine lignosulfonate (a), the appearance of two new peaks at 3100 cm-1 (line 1) and 1170 cm1 (line 2) were respectively assigned to the stretching vibration of N–H and C–N in dicationic imidazolium salt and N1,N1,N2 trimethylethane-1,2-diamine (Fig. 2, b, c and d).37-39 Three distinctive bands at 920 cm-1, 836 cm-1 and 752 cm-1 were attributed to the C–C stretching vibration of –CH2 aliphatic chain (line 3, 4, 5).40 The peak approximately located at 619 cm-1 was corresponded to the =C–H deformation vibration from dicationic imidazolium salt (line 6). All of the above results forcibly proved that the TMEDA fragment was successfully grafted onto lignosulfonate. The chemical composition of referential catalyst MR-TMEDA-Cu was also investigated by FT-IR spectroscopy (ESI, Fig. S2). Compared with the spectrum of blank Merrifield resin (Fig. S2, a), the existence of grafted TMEDA could be authenticated by the emergence of two new peaks at 2778 cm-1 and 814 cm-1, which were assigned to the symmetry stretching vibration of –CH2– and frame vibration fingerprint spectrum of C– C from TMEDA fragment, respectively (Fig. S2, b).41-42 In addition, the stretching vibration of C–N bond appeared at 1214 cm-1. A noticeable change for the disappearance of the peak located at 1264 cm-1 was observed, implying that the chloromethyl groups in Merrifield resin was completely converted, and contributed to form the TMEDA fragments.43


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Fig. 1 Schematic procedure of preparing DIS@LS-TMEDA-Cu catalyst.

920836752 619

3100

a 1170

b

Transmittance [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c

Fig. S 3c, the Auger Cu LMM spectra differentiated the presence of Cu+ at 569.9 eV, while Cu0 was observed at 567.5 eV. In the N 1s spectrum, the corresponding peaks located at 402.4 eV (C-N), 400.6 eV (N,N,N coordinate N–Cu) and 399.4 eV (C=N/N–H) could also be observed.56-57

d

3 4

1

5

6

2 3300 3000 2700 1800

1600

1400 1200 -1 Wavenmber cm

1000

800

600

Fig. 2 IR spectra of a) LS, b) DIS@LS, c) DIS@LS-TMEDA and d) DIS@LSTMEDA-Cu.

XPS was employed to characterize the chemical states of elements on the surface of DIS@LS-TMEDA-Cu catalyst. The high-solution XPS spectra of Cu 2p in Fig. 3a showed there was no characteristic Cu2+ shakeup satellite peaks at 938−945 eV, suggesting that no Cu2+ species existed in DIS@LS-TMEDA-Cu4447 . The lower BE peak at ∼929 eV could be ascribed to Cu+ or Cu0 species.48 Because it was difficult to differentiate Cu+ and Cu0 by Cu 2p3/2 XPS spectrum, Auger Cu LMM spectra was further utilized. As shown by the inset in Fig. 3a, the presence of Cu+ was confirmed by the peak at ∼570 eV, while Cu0 was identified by the peak at 567.5 eV.49-50 This result indicated that the Cu+ species were partially reduced during the immobilization. In the XPS pattern of C 1s (Fig. 3b), contributions of the C=N (288.8 eV), C– O/C–S (286.8 eV), C–N/C–C (285.9 eV), C=C (284.9 eV) bonds could be distinctively found.51 In the N 1s spectrum (Fig. 3c), the peaks located at 399.7 eV, 400.4 eV and 401.9 eV were corresponded to C–N bond, N,N,N coordinate N–Cu and C=N/N– H bond, respectively.52-53 The O 1s spectrum in Fig. 3d clearly evidenced the presence of four kinds of chemical environments for oxygen atoms. In detail, the peak located at 533.9 eV, 533.3 eV and 532.6 eV were respectively characterized to the C–O bond from benzene ring and the –OH group in the skeleton of DIS@LSTMEDA-Cu,54 while the peak situated at 531.6 eV was attributed to the O=S bond.55 Similarly, the principal elements on the surface of MR-TMEDA-Cu were also investigated by XPS. As shown in

Fig. 3. XPS spectra of DIS@LS-TMEDA-Cu in the regions of (a) Cu 2p, (b) C 1s, (c) N 1s and (d) O 1s.

To investigate the surface morphologies and the Cu species of the synthesized catalysts, FSEM and FTEM were carried out and the results were given in Fig. 4, Fig. 5 and Fig. S4. As shown in Fig. 4, all of the obtained materials were shown similar morphology, mainly featured by block-shaped particles with uneven sizes ranging from 1.0 to 10 μm (Fig. 4, a-c). Additionally, from the EDX spectrum (Fig. 4d), the immobilized Cu species and other expected elements including O, S and N were clearly observed. Apart from EDX spectrum, the immobilized Cu species in DIS@LS-TMEDA-Cu catalyst were also investigated by FTEM. The particle size distribution in Fig. 5a (inset) showed that the immobilized Cu species existed in the form of nanoparticles with a size of about 2.0 nm. The HRTEM image in Fig 5b revealed that the fringe spacing of about 0.240 nm and 0.297 nm were respectively corresponded to the (111) and (110) lattice planes of Cu2O,58-59 while the fringe spacing of 0.228 nm was ascribed to



the (111) lattice plane of Cu0.60 These results also indicated that the Cu+ species were partially reduced to Cu0 during the immobilization, which are in good agreement with the XPS analysis. According to the literatures, apart from Cu+,61 metal Cu(0)-based nanocatalysts, such as Cu/C3N462 and Cu/TiO2,63 can also effectively catalyze the Glaser coupling reaction. Therefore, it could be speculated that the partial change of valence states in Cu species had no negative impact on the catalytic activity of DIS@LS-TMEDA-Cu catalyst for the heterocoupling reaction. As for the referential catalyst, the surface morphologies of MR, MR-TMEDA and MR-TMEDA-Cu were also characterized by FSEM and the results were shown in Fig. S4. The original MR exhibited smooth and porous spherical morphology with an average diameter of 100 μm (Fig. S4, a). After grafting of TMEDA and loading of Cu species, the obtained materials, MR-TMEDA and MR-TMEDA-Cu, still kept the spherical structure, but displayed a slight increase in diameter. The surfaces of these materials changed also a bit rough (Fig. S4, b-c).64 A part of pores were also sealed, perhaps due to the grafting of TMEDA and loading of Cu species. The BET surface analysis results also verified that the specific surface area and pore volume of MR-TMEDA-Cu were decreased after modifying MR material (Fig. S2, ESI). In the EDS spectrum of MR-TMEDACu in Fig. S 4d, all the expected elements could be clearly observed.

(a)

(b)

(c)

(d)

Fig. 4. FSEM images of a) DIS@LS; b) DIS@LS-TMEDA; c) DIS@LS-TMEDACu; d) EDX of DIS@LS-TMEDA-Cu.

(a)

(b) 1.0 1.5 2.0 2.5 3.0 3.5

Particle size (nm)

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TGA was used to determine the thermal stability of all prepared composites. As shown in Fig. S5, DIS@LS sample was found to be quite stable at temperature below 250 °C (Fig. S5, b). After grafting TMEDA moiety, although the organic component in DIS@LS-TMEDA was significantly increased compared with that in DIS@LS, the thermal stability was not altered obviously. And the weight loss came only slightly earlier, at about 200 °C (Fig. S5, c). Before and after loading Cu species onto the DIS@LSTMEDA, no apparent change in terms of thermal ability could be found. (Fig. S5, a). Such a thermostability of DIS@LS-TMEDACu is favorable for its catalytic uses. In addition, the thermostability of the referential catalyst was also investigated. As shown in Fig. S6, all of the referential materials showed excellent thermostability below 300 °C. Specially, for blank Merrifield resin, the decomposition did not occur until the temperature was over 400 °C. Accompanied by grafting organic fragment, MR-TMEDA sample started to decompose at 350 °C, showing a slightly inferior stability than Merrifield resin. After immobilizing Cu species, MRTMEDA-Cu also showed good stability, whose decomposed temperature was at 380 °C, only a little lower than Merrifield resin. Therefore, it could be concluded that the referential catalyst also had an outstanding thermostability. An oxidative hetero-coupling of phenylacetylene 1a and 2methyl-3-butyn-2-ol 2a was used as the model reaction to test their catalytic activities of the obtained catalysts. In literature, to maximize the reaction yield, one of the alkyne precursors was used in 5 to 10 times of excess. One of the main purposes of this work is to decrease the ratio of substrate. Therefore, the ratio of 1a/2a was fixed initially at 2/1. The reaction was performed at 40 °C by using DIS@LS-TMEDA-Cu as a catalyst. Firstly, the effect of solvents was investigated, and the results were summarized in Table 1. Among various solvents tested, tetrahydrofuran (THF) was found to be the best one (entries 1 to 6). The reaction proceeded smoothly, giving the cross-coupling product, conjugated diyne 3a, in moderate yield (55%) after 10 h of reaction under O2 atmosphere (entry 1). The reactions in dipolar and aprotic solvents, nitromethane and dichloromethane, proceeded also gently, but the yields were inferior compared with THF (entries 2 and 3). Strong polar protic solvents, such as water and ethanol, can act as hydrogen bond donors or acceptors. The reaction media offered a hydrogen-bond-rich environment that weakens the interaction between substrate 2a (2-methyl-3-butyn2-ol) and the solid support (DIS@LS-TMEDA-Cu). Therefore, protic solvents, such as water and ethanol, were inappropriate to this catalyst for enriching one of the reaction components (entries 4 and 5). Non-polar solvent, toluene, also showed poor performance in the model Glaser reaction (entry 6). These results imply also that the solvent may not be innocent, a weakly coordinating solvent like THF may facilitate the metal complex to maximize the catalytic activity. Table 1. Oxidative hetero-coupling reaction of 1a and 2a catalyzed by [email protected]

Fig. 5. FTEM images of a) DIS@LS-TMEDA-Cu; b) HRTEM of Cu species in DIS@LS-TMEDA-Cu. The inset in (a) HRTEM of DIS@LS-TMEDA-Cu, particle size distribution of Cu active species. b) the fast Fourier Transformation of the selected areas in red boxes.

Entry

Solvent

Yield (%)b

1 2

THF CH3NO2

55 50


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3 4 5 6 a

b


CH2Cl2 H2O EtOH Toluene

45 Trace 26 10

Reaction conditions: 1a: 0.4 mmol, 2a: 0.2 mmol, solvent: 1.0 ml, DIS@LSTMEDA-Cu: 20.0 mg, O2 atmosphere.

Entry 1 2c 3d 4d 5d 6d 7e 8f 9g 10g 11g,h 12g,i

Isolated yield.

Lei et al65 found that adding small amount of NiCl2.6H2O was helpful for increasing the yield of Glaser reaction. We thus added 5 mol% of NiCl2.6H2O into the reaction system. It indeed worked, and the yield of 3a was increased to 70% (Table 2, entry 1). NiCl2.6H2O alone cannot catalyze this reaction (entry 2). To further increase the reaction yield, one equivalent of triethylamine was added. In this case, the yield of 3a can be increased to 83% (entry 3). This result is quite remarkable as the reaction was performed with a low substrate ratio (1a/2a was 2/1). When equal amount of substrate was used, the yield of 3a dropped to 63% (entry 4). Increase of the ratio was helpful for the hetero-coupling reaction, and when 1a/2a was 5/1, yield of 3a reached 91% (entry 5). This observation is in a good accordance with the previous reports.66-68 To ensure a good synthetic yield and also maximize the utilizing efficiency of substrate and the solvent for the product isolation, we continued the reactions with a ratio of 1a/2a = 2/1. We suspected that, thanks to the formation of hydrogen bonds between alkynol and the solid support, alkynol component may be selectively enriched to some extent in the solid-liquid interface. Therefore, the catalyst works well with a low substrate ratio. The evidences to prove our hypothesis were given in the next part. When the ratio of 1a/2a was 1/5, the hetero-coupling product 3a was formed only in 14% yield (entry 6). In this case, homocoupling of 1a is predominant. It is conceivable that, with this ratio of substrate, the enriching effect of the lignosulfonate/ dicationic ionic liquid support to 2a is not sufficient. But, the Cu catalyst is so active that can be easily occupied by another alkyne component, 1a. Further investigations revealed that the reaction was also affected by temperature and reaction time (entries 7 and 8). Finally, the optimal conditions were confirmed to be as follows: 10 mol% of DIS@LS-TMEDA-Cu, 5 mol% of NiCl2.6H2O, 1.0 equiv. of Et3N, THF as solvent, 40 °C, and 10 h. A referential catalyst, MR-TMEDA-Cu, was also used in this reaction. The dosage of catalyst should be doubled because the Cu loading (0.42 mmol/g) of MR-TMEDA-Cu was nearly half of DIS@LS-TMEDA-Cu (0.95 mmol/g). When the substrate ratio of 1a/2a was 5:1, the reaction proceeded very well, and the yield of 3a reached 90% (entry 9). This indicated that MR-TMEDA-Cu had also a quite good catalytic active for promoting Glaser reaction. But, when the substrate ratio was decreased to 2/1, the yield of 3a dropped to 36% (entry 10). Increasing the temperature and elongating the time showed no obvious benefits to the yields (entries 11 and 12). The support of MR-TMEDA-Cu catalyst has no hydrogen bonding ability so that the alkynol component 2a cannot be absorbed to the catalyst surface. Therefore, a high substrate ratio is necessary to ensure the reaction with MRTMEDA-Cu a good yield. Table 2. Oxidative hetero-coupling reaction of 1a and 2a catalyzed by DIS@LSTMEDA-Cu/NiCl2.a

Ratio (1a:2a) 2:1 2:1 2:1 1:1 5:1 1:5 2:1 2:1 5:1 2:1 2:1 2:1

Yield (%)b 70 Trace 83 63 91 14 58 79 90 36 35 37

a

Reaction conditions: 1a: 0.4 mmol, 2a: 0.2 mmol, DIS@LS-TMEDA-Cu: 20.0 mg, NiCl2.6H2O: 0.01 mmol, THF: 1.0 ml, O2 atmosphere.

b

Isolated yield.

c

Only 0.01 mmol NiCl2.6H2O was used.

d

Triethylamine (0.2 mmol) was added.

e

Room temperature.

f

4 h.

g

Catalyst: MR-TMEDA-Cu (40.0 mg).

h

50 oC.

i

12 h.

The substrate scope of the DIS@LS-TMEDA-Cu-catalyzed Glaser hetero-coupling reaction was explored. The reactions were performed under the optimized conditions, and the results were listed in Table 3. Phenylacetylenes with different substituents can smoothly react with 2a, producing the heterocoupling products generally in good yields (entries 1 to 4). When 4-fluorophenylacetylene was used, H–F type of hydrogen bond might also be formed. Therefore, the support can enrich both of the alkynol and 4-fluorophenylacetylene components. As a result, the homo-coupling reactions occurred simultaneously, imposing thus a negative effect on the hetero-coupling reaction. Therefore, the expected products, 3f, 3n, and 3s, were isolated only in the yields ranging from 40% to 50% (entries 5, 13 and 18). For the similar reason, alkynes substituted by an amino or a pyridyl group also gave relatively low yields. Typical examples were 3g (35%), 3m (57%), 3o (45%) and 3ac (38%). These alkynes can interact with the supporting material via an N–H type of hydrogen bond, diminishing the specific selectivity of the support for enriching the alkynol component. However, the alkynes substituted with thienyl group proceeded well, giving the corresponding product 3h in 80% of yield. The normal phenylacetylenes can react smoothly with different kinds of alkynols, such as prop-2-yn-1-ol (2b), 2(prop-2-yn-1-yloxy)ethan-1-ol (2c), and 2-(prop-2-yn-1yloxy)propan-1-ol (2d), generating the hetero-coupling products in moderate to good yields. An alkynol with steric hindrance, 1ethynylcyclohexan-1-ol (2g), could also be converted to the expected product 3z in 80% of yield (entry 25). When the alkynol component was replaced with a terminal alkyne that has no hydroxy group, such as (prop-2-yn-1-yloxy)benzene (2h), the hetero-coupling products were formed only in modest yields (entries 26 to 28). It is reasonable because no hydrogen bond can be formed with this alkyne, and the support cannot enrich this component. As thus, with a low substrate ratio, the homo-coupling



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adsorption enthalpies were negative (∆H < 0), indicating that the adsorption was an exothermic process. In addition, all adsorption enthalpies were approximately at –(15–17) KJ/mol, which were in the range of hydrogen bonding force –(8–50 KJ/mol).69-71 Adsorption free energies ∆G < 0 suggested the alkynol component could spontaneously adsorb onto the surface of DIS@LS-TMEDA composite.72 Besides, the adsorption entropy ∆S < 0 manifested that the degree of freedom (DOF) decreased in the catalytic system, demonstrating the occurrence of hydrogen bonding adsorption process.73-74

reactions occurred dominantly, destroying the selectivity to hetero-coupling product. The adsorption thermodynamics experiments were applied to confirm the existence of hydrogen-bond adsorption (detail procedures are in ESI). The adsorption isotherm of alkynol component onto DIS@LS-TMEDA composite from THF solution was displayed in (Fig. 6a). The plots of adsorption isosters and adsorption capacity vs equilibrum concentration derived from Fig. 6a were respectively shown in Fig. 6b and Fig. 6c. The related thermodynamics parameters including ∆H, ∆G and ∆S were calculated according to the following thermodynamics equations (3-1, 3-2 and 3-3). As shown in Table 4, all the obtained

Table 3 The substrate scope of phenylacetylene and alkynol hetero-coupling reaction with low substrate ratioa

Entry

Product

Yield

Entry

Product

Yield

b

(%)b

(%) 1

69

15

3b 2

60 3p

80

16

71

17

85

18

40

19

35

20

80

21

95

22

86

23

67

24

71

25

63

3c 3q 3

57

3d 3r 4

3e 5

45

3s 76

3f 3t 6

52

3g 3u 7

60

3h 3v 8 3i 9

42

3w 50

3j 3x 10 3k 11

44

3y

3l

80 3z


Page 7 of 12

12

57

26

35

3aa

3m 13

50

27

45

28

49

3n

3ab

14

38

3o a

3ac

Phenylacetylenes: 0.4 mmol, alkynols: 0.2 mmol, THF: 1.0 ml, DIS@LS-TMEDA-Cu: 20.0 mg, O2 atmosphere. b Isolated Yield.

The adsorption enthalpy ∆H was calculated based on eq 1: lnC = ∆H/(RT)+K Adsorption free energy ∆G was calculated based on eq 2: ∆G = -nRT Adsorption entropy ∆S was calculated based on eq 3: ∆S = (∆H-∆G)/T -1.0 （b）

0.8 （a）

-1.2

0.4

(2) (3)

0.2

0.4 0.6 C0(mmol/ml)

303K 313K 0.8 1.0

q = (nalkyne original - nalkyne detected)/mcomposite 0.25mg/mmol

-1.3 -1.4

0.20 mg/mmol

-1.6 3.18 3.20 3.22 3.24 3.26 3.28 3.30 1000/T

0.0

(c) -0.5

lnq

-1.0 -1.5 -2.0

303K 313K -2.0-1.8-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.2 lnC’

Fig. 6. (a) Adsorption isotherms of 2a onto DIS@LS-TMEDA composite from THF solution; (b) Adsorption isosters of 2a onto DIS@LS-TMEDA composite from THF solution; (c) Plot of adsorption capacity vs equilibrum concentration.

Table 4 Thermodynamics parameters of 2a onto DIS@LS-TMEDA composite from THF solution. q (mg/mg) 0.30 0.25 0.20

- ∆H (kJ/mol) 16.31 15.68 16.55

- ∆G (KJ/mol) 303 K 313 K 2.47 2.51 2.47 2.51 2.47 2.51

amount of these two alkynes in the bulk liquid phase before and after adding a solid material. Before adding a solid, the mixture is a homogeneous system (entry 1). When lignosulfonate was added to the mixture, because the solid could absorb some amount of the alkynol or phenylacetylene component, the amount of 2a or 1a in the bulk liquid phase decreased to some extent (entry 2). The adsorbing capacity of a solid material can be defined as the changes of alkynol or phenylacetylene component in the bulk liquid phase divided by the weight of the solid.

0.30mg/mmol

-1.5

0.2 0.0 0.0

(1)

-1.1

0.6

lnC

q(mg/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


- ∆S (J/(mol.K)) 303 K 313 K 45.68 44.09 43.60 42.08 46.47 44.86

The enriching effect of the lignosulfonate/dicationic ionic liquid composites to these two reaction precursors, alkynol and phenylacetylene, was investigated. To mimic the real reaction conditions, all the experiments were carried out nearly with the same substrate ratio and the same solvent. Thus, 1a, (0.41 mmol) and 2a (0.199 mmol) were mixed in THF (1.0 mL). After heating to 40 °C, a solid support was added into the mixture. The concentrations of 1a and 2a in the bulk liquid phase were respectively detected by gas chromatography. Table 5 shows the

At 40 °C, the adsorbing capacity of LS for 2a was calculated to be 1.500 mmol g-1. Because there was no Cu species in LS, the concentration decrease of alkynol in the bulk liquid phase was mainly attributed to the absorption by LS via forming hydrogen bond rather than its involvement in reaction. The absorption phenomenon also occurred when DIS@LS-TMEDA was added into the mixture (entry 3). The adsorbing capacity of DIS@LSTMEDA for 2a was calculated to be 1.250 mmol/g. The two supports, LS and DIS@LS-TMEDA, can also absorb phenylacetylene component at 40 °C, but the magnitude is not as efficient as to the alkynol. It is conceivable that the intrinsic lipophilicity of lignosulfonate facilitate phenylacetylene to diffuse into the matrix of the organic composite. The DIS@LS-TMEDA support was made by combining LS and a dicationic ionic liquid followed by decoration with TMEDA. The adsorbing capacity of DIS@LS-TMEDA is also affected by the fragments of the ionic liquid and TMEDA. Because the dicationic ionic liquid and the TMEDA moiety are all organic components, the adsorbing capacity of DIS@LS-TMEDA for 1a is higher than that of LS (0.950 vs 0.800). By the same token, because the organic components introduced into the DIS@LS-TMEDA support decreased the loadings of hydrogen-bond-active groups, such as hydroxyl and carboxylic acid, the adsorbing capacity of DIS@LSTMEDA for 2a is lower than that of LS (1.250 vs 1.500). Due to the lack of –OH groups and the poor lipophilicity, the referential catalyst MR-TMEDA almost showed no affinity to substrate 1a or 2a. Based on these results, it could be deduced that on one hand, the lignosulfonate/dicationic ionic liquid support, DIS@LSTMEDA, was able to interact with alkynol via forming hydrogen bond, enriching the alkynol component in the solid-liquid interface, and on the other hand, the intrinsic lipophilicity of LS ensured also a good diffusion of the phenylacetylene component. With this kind of synergistic effect, the homo-coupling of phenylacetylene was inhibited. And therefore, the hetero-coupling reaction proceeded well with a low substrate ratio.



Table 5 The amount change of 1a and 2a in the bulk phase with different material. 1 2 3 4 Iterma,b DIS@LSMRBlank LS TMEDA TMEDA Original n1a (mmol) 0.410 0.410 0.410 0.410 Detected n1a 0.410 0.391 0.394 0.410 (mmol) q1a (mmol/g) 0 0.800 0.950 0.115 Original n2a (mmol) 0.199 0.199 0.199 0.199 Detected n2a 0.199 0.169 0.174 0.198 (mmol) q2a (mmol/g) 0 1.500 1.250 0.050 a

The amount of 1a and 2a were detected by gas chromatography in the bulk phase. GC column procedure: 50 °C, 4 min, 20 °C/min to 100 °C, 0.5 min. Internal standard substance: benzene propane.

b

Reaction conditions: material (20 mg), THF (1.0 ml), 40 °C, 10 h, O2 atmosphere.

In order to confirm heterogeneity, we also investigated Cu leaching during the reaction. The reaction mixture (including the catalyst) was allowed to stir for a period of time firstly, then the catalyst was isolated by a hot filtration. The liquid mixture was allowed to stir once again but no significant yield increase was observed. Besides, with the aid of ICP-MS analysis, we found that the Cu content of the catalyst did not change appreciably after the reaction, verifying heterogeneous property of the catalyst. It can be stated that the DIS@LS-TMEDA-Cu did not leach significantly into the reaction mixture during the reaction. The recyclability of DIS@LS-TMEDA-Cu catalyst for the hetero-coupling reaction of phenylacetylene and alkynol was evaluated. After the recovered solid catalyst was washed with ethanol and dried under vacuum, it was used again in the hetero-coupling reaction (the detail procedure see ESI). As shown in Fig. 7, after four consecutive runs, the catalyst was still capable of catalyzing the model reaction in 79% of yield, indicating that the present DIS@LSTMEDA-Cu catalyst was robust and stable under the reaction conditions and it could be recycled without obvious loss of its catalytic activity. After fifth run, the slight decrease in catalytic activity may be due to the unavoidable loss of solid catalyst during recovery and washing. The IR (Fig. S7) spectrum of the recovered DIS@LS-TMEDA-Cu showed no obvious structure changes.

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A lignosulfonate/dicationic ionic liquid composite was prepared via ion exchange process, and then used as catalyst support for preparing heterogenous Cu-based catalyst. The thereby obtained catalyst was proven to be able to enrich the alkynol component in the Glaser hetero-coupling reaction of an alkynol and phenylacetylene, thus enabling the reaction to proceed well with a low substrate ratio. The substrate distribution analysis and thermodynamic calculation demonstrated that the unique physicochemical property of the prepared catalyst enabled the formation of hydrogen bonds between alkynol and the composite support, increasing thus the alkynol concentration in the solidliquid interface. Therefore, this protocol successfully avoided the use of a large excess amount of aromatic alkyne. Many unsymmetrical 1,3-dikynes can be synthesized in high yields under mild reaction conditions with low substrate ratio. The catalyst was also recyclable in the Glaser hetero-coupling reaction.

Experimental Section The chemical compositions of samples were characterized by FT-IR spectra (EQUINOX 55, Bruker) in the wavenumber range of 4000–400 cm−1. Thermogravimetric analysis (TGA) was performed under a flow of nitrogen by heating the material from room temperature to 500 °C at a rate of 10 °C.min−1. The surface areas of catalyst and N2 adsorption isotherms (77.3 K) were measured by using surface area analyzer (Micromeritics ASAP 2020 M). Before testing, the samples were degassed at 110 °C for 8 h under vacuum (10−5 bar). Elemental analysis (EA) was determined by using a Vario Micro cube Elemental Analyser (Elementar, Germany). ICPMS data were recorded on an ELAN DRC-e. Gas chromatography was tested on FuLi 9750 instrument by using benzene propane as internal standard substrate with the following temperature programming: Keeping at 50 °C for 4 min; rising to 100 °C at the speed of 20 °C/min, keeping at 100 °C for 0.5 min. The surface morphologies of samples were observed by Field emission scanning electron microscopy (FSEM, Sirion 200, Holland) with an energy-dispersive X-ray (EDX) spectroscopy. The lattice fringe of loaded Cu species in the catalysts were observed by Field emission transmission electron microscope (FTEM, Tecnai G2 F30, Holland). X-ray photoelectron spectra (XPS) were recorded on the X-ray photoelectron spectrometer (SHIMADZU-Kratos AXIS-ULTRA DLD600W) at a base pressure of 2 × 10−9 Pa in the analysis chamber using Al Kα radiation. 1H and 13C NMR spectra were recorded on a Bruker AV-400. Chemical shifts are expressed in ppm relative to Me4Si in solvent and CH2Br2 was used as internal standard substance for NMR quantitative analysis. All chemicals were reagent grade and used as-received without further purification. Deionized water was used in all the experimental process.

Acknowledgements

Fig. 7 Recycling of DIS@LS-TMEDA-Cu in the hetero-coupling reaction of phenylacetylene and alkynol with low substrate ratio.

Conclusions

The authors thank the National Natural Science Foundation of China (21761132014, 21872060), the Fundamental Research Funds for the Central Universities of China (2016YXZD033) and Opening fund of Hubei Key Laboratory of Material Chemistry and Service Failure (No. 2017MCF01K) for the financial support. The Cooperative Innovation Center of Hubei province and the testing center of HUST are also acknowledged.

Associated Content


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The Supporting Information is available free of charge on the ACS Publications website at DOI:. Characterization of the referential catalyst, experimental sction, EA results, TG results and NMR results.

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For Table of Contents Use Only Reaction proceeding

Before reaction

After reaction

Synopsis: Biomass sodium lignosulfonate was used to prepare heterogenous Cu-based catalyst, enabling the Glaser hetero-coupling reaction to proceed well with a low substrate ratio.


Dicationic Ionic Liquid Composite as A Task-Specific

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