Synthesis and catalytic application of N-heterocyclic carbene copper complex functionalized conjugated microporous polymer

Abstract

A N-heterocyclic carbene copper(I) complex functionalized conjugated microporous polymer (CMP-NHC-CuCl) was synthesized by palladium-catalyzed Sonogashira cross-coupling chemistry. The resulting CMP-NHC-CuCl proved to be a good heterogeneous catalyst in the hydrosilylation of functionalized terminal alkynes with boryldisiloxane to afford (β,β)-(E)-vinyldisiloxane with high stereoselectivity, and the catalyst could be used four times without obvious loss in catalytic activity. Moreover, CMP-NHC-CuCl was also efficient in catalyzing the hydrosilylation of CO₂ with triethoxysilane to form silyl formate under mild conditions.

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Since a N-heterocyclic carbene (NHC) copper complex was first reported by Arduengo and co-workers in 1993,¹ its applications have attracted broad attention in homogeneous catalysis,² especially for the hydrosilylation of carbonyl compounds producing the corresponding silyl compounds,³ carbene transfer reactions generating a three-membered ring,⁴ [3 + 2] cycloaddition of azides and alkynes enabling new triazole ring reaction⁵ and miscellaneous reactions providing γ-selective allylic products.⁶ Recently, novel catalytic applications of NHC-Cu(I) complexes are emerging in various transformations of CO₂ (ref. 2e–g) such as the carboxylation of a variety of substrates,⁷ reduction of CO₂ into CO⁸ or formic acid.⁹

It is generally known that homogeneous catalytic processes usually suffer from some drawbacks, including difficulty in the separation of catalysts, the purification of the product and the recovery of the expensive catalysts. Additionally, the residual metal in the products probably causes serious problems for the applications of bioactive substrates, particularly for medical purpose. The immobilization of homogeneous catalysts onto solid supports is an effective method to solve the above problems. In 2010, Zhang and co-workers successfully synthesized the novel poly-(NHC)CuCl, in which the main-chains contain a great amount of (NHC)CuCl units. The resultant poly-(NHC)CuCl was used to catalyze the transformation of CO₂ to carboxylic acids through C–H bond activation of terminal alkynes.¹⁰ Shortly afterwards, Yang group developed the silica-supported NHC-Cu(II) catalyst for oxidative coupling of terminal alkynes with H-phosphonates, and no significant loss in the catalytic reactivity was observed after recycling six times.¹¹ More recently, a 3-D diamondoid metal organic framework using bis-NHC Cu(I) complex as a building block was successfully applied to catalyze the hydroboration of CO₂ to provide formamides.¹²

Conjugated microporous polymers,¹³ as a new class of porous materials with high synthetic diversification, have been used as promising solid supports in heterogeneous catalysis.¹⁴ Main method for creating catalytically active CMPs is to design and synthesize the bridging ligands containing orthogonal functional groups, as shown in Scheme 1. The primary functional groups could be linked by suitable nodes to form extended networks, whereas the orthogonal secondary functional groups can then be employed to generate catalytic sites. Through this synthetic strategies, many CMPs containing Fe,^14a Re,^14b Rh,^14b Co^14e and Zn^14f complexes have been synthesized and applied for catalytic applications.

Scheme 1 Synthetic strategy of CMP-NHC-CuCl.

In this study, we firstly report the synthesis of a copper-coordinated conjugated microporous polymer (CMP-NHC-CuCl) by linking NHC-CuCl with 1,3,5-triethynylbenzene. The CMP-NHC-CuCl exhibits high activity towards the hydrosilylation of functionalized terminal alkynes to selectively synthesize (β,β)-(E)-vinyldisiloxane, as well as the chemical transformation of CO₂ to form silyl formate.

Results and discussion

Due to its high air- and moisture-stability, IPrCuCl has the potential to be used as secondary functional group to construct copper complex functionalized CMPs.¹⁵ Detailed synthetic routes are showed in Scheme 2. CMP-NHC-CuCl was synthesized by the Sonogashira cross coupling polycondensation of an iodo-NHC-CuCl 1 and 1,3,5-triethynylbenzene in the presence of Pd(0) and CuI as catalyst under alkaline conditions. According to the literature procedures,^13,14 polycondensation in toluene in the presence of triethylamine as base and Pd tetrakis-(triphenylphosphine) palladium(0) as palladium source allows for the preparation of CMP-NHC-CuCl 2 in an isolated yield of 86.7%. The CMP-NHC-CuCl was insoluble in all solvents tested. After repeated rinse with water, CH₂Cl₂, methanol and acetone, CMP-NHC-CuCl was rigorously wash by Soxhlet extraction for 24 h with CH₂Cl₂, methanol and acetone, respectively, to remove any entrapped impurities, and then dried under vacuum for 24 h at 80 °C. The resultant CMP-IPr(CuCl) showed higher catalyst loading and more accessible catalytic centers based on the iodo-NHC-CuCl linkers.

Scheme 2 Synthetic route of CMP-NHC-CuCl.

The morphology of CMP-NHC-CuCl was investigated by scan electron microscopy (SEM) and transmittance electron microscopy (TEM) measurements. SEM image in Fig. 1A displayed that CMP-NHC-CuCl consists of submicro spheres, while TEM image in Fig. 1B revealed the presence of nanometer-scale pores (≈0.4 nm in diameter) on the polymer surface. The porous properties of the networks were investigated by nitrogen adsorption analyses at 77.3 K. The BET surface area was found to be 388 m² g⁻¹ (the Langmuir surface area is 580 m² g⁻¹) and the total volumes were 0.56 cm³ g⁻¹ at P/P₀ = 0.99. As shown in Fig. 1C, the adsorption isotherm displayed a notable nitrogen gas uptake at low relative pressure (P/P₀ < 0.01) reflecting an abundant micropore structure.¹⁶ The TGA curve in Fig. 1D reveals a stability of the materials at least up to 270 °C.

Fig. 1 Synthetic route of CMP-NHC-CuCl SEM (A) and TEM (B) images of CMP-NHC-CuCl. (C) N₂ sorption isotherm at 77.3 K ( = adsorption; = desorption). (D) TGA of CMP-NHC-CuCl under N₂ atmosphere. Ramp rate = 5 °C min⁻¹.

Considering the excellent thermal stability of CMP-IPr(CuCl) and the successful incorporation of IPr(CuCl) into this network, the catalytic behaviour of CMP-IPr(CuCl) was investigated in detail. Firstly, the hydrosilylation of terminal alkynes with boryldisiloxane was chosen as a model reaction to selectively synthesize functionalized vinylsilanes. Our previous study on NHC copper(I) catalyzed hydrosilylation had shown that IPr(CuCl) complex was effective catalyst system to transform terminal alkynes employing 1,1,3,3-tetramethyl-1,3-(pinacolboryl)disiloxane 3 as silicon source to (β,β)-(E)-vinyldisiloxane 4a as the sole detectable isomer with perfect regio- and stereoselectivity under mild conditions (room temperature, 2 h).¹⁷ CMP-IPr(CuCl) displayed excellent dispersion ability in THF. To out delight, CMP-IPr(CuCl) proved to be an effective catalyst for the hydrosilylation of phenylacetylene with 3, and all results are shown in Table 1. The reaction could proceed smoothly in the 1 mol% CMP-IPr(CuCl) at room temperature, affording (β,β)-(E)-4a with an isolated yield of 34% within 2 h (entry 1). It is worth noting that the stereoselectivity of CMP-IPr(CuCl) is comparable to those of homogeneous catalyst and only (β,β)-(E)-4a was produced with high regio- and site-selectivity, confirmed by the analysis of the crude mixture by ¹H-NMR spectroscopy. When the catalyst loading was increased 5 mol%, the yield of (β,β)-(E)-4a reached to 79% at the same conditions (entry 3). Moreover, the highest yield (92%) could be obtained by the prolonged reaction time of 6 h (entry 4).

Table 1 CMP-NHC-CuCl catalyzed hydrosilylation of phenylacetylene with boryldisiloxane 3^a

Entry	Cat. (mol%)	Time (h)	Yield^b (%)	Selectivity^c (β,β/α,β/α,α)
a Reaction conditions: Si–B reagent (77.2 mg, 0.2 mmol), phenylacetylene (40.8 mg, 0.4 mmol), CMP-IPr(CuCl), NaO^tBu (1.2 equiv. to the amount of [Cu]), MeOH (65 μL, 1.6 mmol, 4.0 equiv.), THF (1 mL), room temperature, unless otherwise noted.b Yield of isolated product.c Determined by analysis of the crude mixture.
1	1	2	34	>99 : 1 : <1
2	2	2	50	>99 : 1 : <1
3	5	2	79	>99 : 1 : <1
4	5	6	92	>99 : 1 : <1

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Furthermore, several types of terminal alkynes were subjected to the catalytic reaction with boryldisiloxane 3 in order to evaluate the scope of the present transformation, and the results are summarized in Table 2. Functional groups such as methyl-, bromo- and nitro-groups at the para position of the aromatic ring were tolerated in the reaction, although electron withdrawing substituents retarded the reaction rate (4b–4d). Other substrates with a variety of functional groups, such as alkyl, cyano, ether, ester, were tolerated in these reaction (4e–4i), giving the corresponding (β,β)-(E)-vinyldisiloxanes in good to excellent yields with no observable byproduct formation. The reaction with propargyl alcohol did not require the use of additional MeOH. The desired product 4j could be obtained by hydrolysis and SiO₂ flash chromatography purification.¹⁷

Table 2 Scope in terminal alkynes^a

Entry	Product	Yield^b (%)
a (β,β)-(E)-Vinyldisiloxanes 4b–4j obtained from their corresponding terminal alkynes using CMP-IPr(CuCl) as heterogeneous catalyst. General conditions: Si–B reagent (0.2 mmol, terminal alkyne (0.4 mmol), CMP-IPr(CuCl) (5 mol%), NaO^tBu (6 mol%), MeOH (1.6 mmol), THF (1 mL), RT, 6 h, unless otherwise noted.b Yield of isolated product; (β,β)/(α,β)/(α,α) >99 : 1 : <1, as determined by analysis of the crude mixture by ^1H-NMR spectroscopy.c No additional MeOH.
1		4b	90
2		4c	82
3		4d	75
4		4e	91
5		4f	90
6		4g	88
7		4h	75
8		4i	80
9^c		4j	87

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Since CMP-IPr(CuCl) is stable and insoluble in common organic solvents, the investigation of the recycle use of CMP-IPr(CuCl) is very convenient. After the hydrosilylation reaction was completed, the reaction mixture was filtered to collect the catalyst for the next round of hydrosilylation of phenylacetylene with boryldisiloxane 3. As shown in Table 3, CMP-NHC-CuCl exhibited excellent stability, and was recycled for four times, without obvious loss in catalytic activity.

Table 3 Recycle test of CMP-IPr(CuCl) in the hydrosilylation of phenylacetylene with boryldisiloxane 3

Run	Yield (%)	Selectivity (%)
		(β,β)-E	(α,β)-E	(α,α)-E
1st	92	>99	<1	<1
2nd	90	>99	<1	<1
3rd	90	>99	<1	<1
4th	89	>99	<1	<1

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Chemical transformation of CO₂, as an inexpensive, abundant, nontoxic and renewable C1 feedstock, always attract extensive attention worldwide of the scientists, especially in the areas of environment and chemistry.¹⁸ The NHC–copper alkoxide complex IPrCu(O^tBu) have recently been reported to catalyze the hydrosilylation of CO₂ with triethoxysilane, affording the silyl formate at room temperature under 1 atm CO₂.^5a IPrCu(O^tBu) complex is readily prepared from the corresponding chloride through the reaction with sodium tert-butoxide.¹⁹ Therefore, we further investigated the diverse activity of CMP-IPr(CuCl) to catalyze the hydrosilylation of CO₂ in the presence of catalytic amount of NaO^tBu, and the results are shown in Scheme 3. The hydrosilylation was performed with 1.67 mol% CMP-IPr(CuCl), although the product yield (A 26.3%) was lower than that when IPrCu(O^tBu) complex as homogeneous catalyst.^9a However, the product yield could be significantly increased to 51.5% (B) and 91.7% (C), respectively, by increasing CMP-IPrCuCl concentration to 5 mol% and 10 mol%, respectively. In order to clarify this process, the reaction order in [CMP-IPrCuCl] was further studied by means of in situ FTIR (see ESI, Fig. S4–S9†). The results showed that the reaction is first-order in [CMP-IPrCuCl], so the rate of the silyl formate was obviously affected by the catalyst concentration.

Scheme 3 CMP-IPrCuCl catalyzed hydrosilylation of CO₂.

Conclusions

In summary, we have reported the first example of IPr(CuCl) functionalized conjugated microporous polymer, CMP-IPr(CuCl). CMP-IPr(CuCl) has been found to be efficient heterogeneous catalysts for the hydrosilylation of terminal alkynes with good substrate tolerance and could be reused at least four times without a significant loss in catalytic efficiency. Further investigations of the catalytic diversity showed that CMP-IPr(CuCl) also was efficient in catalyzing the hydrosilylation of CO₂ to afford the silyl formate under mild conditions.

Experimental section

Unless otherwise stated, all manipulations were performed using standard Schlenk techniques under a dry nitrogen atmosphere or an Innovative Technology glovebox under Ar. NMR spectra were recorded on a Bruker Avance II 400M type (¹H NMR, 400 MHz; ¹³C NMR, 100 MHz) spectrometer. Infra-red spectra (IR) were recorded using a Nicolet NEXUS FT-IR spectrophotometer. THF and d⁶-benzene were purified by distilling from sodium/benzophenone under a N₂ atmosphere. Iodo-NHC-CuCl 1²⁰ and 1,1,3,3-tetramethyl-1,3-(pinacolboryl)disiloxane 3¹⁷ were synthesized according the responding literature. Commerically available terminal alkynes and triethoxysilane were used without further purification.

Synthesis of CMP-IPr(CuCl)

Iodo-IPr(CuCl) (368 mg, 0.5 mmol), 1,3,5-triethynylbenzene (75 mg, 0.50 mmol), tetrakis-(triphenylphosphine)palladium(0) (30 mg, 0.025 mmol) and CuI (20 mg, 0.10 mmol) were dissolved in a mixture of toluene (2.5 mL) and triethylamine (1.25 mL). The reaction mixture was heated to 80 °C and stirred for 72 h. The mixture was cooled to room temperature, and the insoluble precipitated network polymer was filtered and washed three times with dichloromethane, water and methanol (30 mL × 3) respectively to remove the unreacted substrates. Further purification of the polymer was carried out using a Soxhlet extraction with water, CH₂Cl₂, methanol and acetone (1 : 1 : 1 : 1) for 24 h. The product was dried under vacuum with the temperature of 80 °C for 24 h and isolated as a yellow powder (yield: 286 mg, 86.7%). IR (KBr): 2961, 2928, 2863, 1626, 1595, 1436, 1387, 882 cm⁻¹. Elemental combustion analysis (%) calcd for C₄₁H₃₇N₂Cu (based on 100% reaction of iodo group): C 74.64, H 6.11, N 4.25, Cu 9.63; found: C 71.43, H 5.77, N 4.05. The copper content of CMP-IPr(CuCl) was determined to be 1.4 mmol g⁻¹ based on ICP analysis.

General procedure for CMP-IPr(CuCl) catalyzed hydrosilylation of terminal alkynes with Si–B 3

In a glovebox, a vial was charged with CMP-IPr(CuCl) (14.2 mg, 5 mol%), NaO^tBu (2.3 mg, 6 mol%) and THF (1 mL). After 30 min stirring at room temperature, phenylacetylene (40.8 mg, 0.4 mmol), 1,1,3,3-tetramethyl-1,3-(pinacolboryl)disiloxane 3 (77.2 mg, 0.2 mmol) and methanol (65 μL, 1.6 mmol, 4.0 equiv.) were sequentially added to the solution, then continue to stir at room temperature for 6 hours. After removal of the solvents under reduced pressure, the crude product was purified by silica gel column chromatography (hexane) to afford 4a as a colourless liquid (92% isolated yield). ¹H NMR (400 MHz, CDCl₃): δ 7.51 (d, J = 7.4 Hz, 4H), 7.39 (t, J = 7.4 Hz, 4H), 7.33 (t, J = 7.4 Hz, 2H), 7.05 (d, J = 19.2 Hz, 2H), 6.54 (d, J = 19.2 Hz, 2H), 0.36 (s, 12H). ¹³C NMR (100 MHz, CDCl₃): δ 144.66, 138.45, 128.85, 128.50, 126.88, 1.23.

General procedure for CMP-IPr(CuCl) catalyzed hydrosilylation of CO₂ with (EtO)₃SiH

In a glovebox, a 5 mL vial was charged with CMP-IPr(CuCl) (21.5 mg, 10 mol%), NaO^tBu (3.5 mg, 12 mol%), (EtO)₃SiH (49.2 mg, 0.3 mmol) and C₆D₆ (1.5 mL) respectively. After 10 minutes stirring at room temperature, the resulting mixture was transferred from glovebox into a 100 mL Schlenk flask with a CO₂ balloon. The reaction was carried out at room temperature for 10 hours with continuous stirring. Then, the yield was determined by ¹H NMR of the crude reaction mixture. 5 (yield: 91.7%). ¹H NMR (400 MHz, C₆D₆): δ 7.71 (s, 1H), 3.84 (q, J = 6.9 Hz, 6H), 1.10 (t, J = 6.9 Hz, 9H). ¹³C NMR (100 MHz, CDCl₃): 158.21, 60.15, 18.02.

Acknowledgements

This work is supported by National Natural Science Foundation of China (Grant No. 21402021), the Program for Changjiang Scholars and Innovative Research Team in University (IRT13008). X.-B. Lu gratefully acknowledges the Chang Jiang Scholars Program (No. T2011056) from Ministry of Education, People's Republic of China.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C cross-polarization magic-angel spinning NMR spectroscopy of N-heterocyclic carbene copper complex functionalized conjugated microporous polymer, kinetic studies and the characterization data of products. See DOI: 10.1039/c6ra07786b

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