Communication pubs.acs.org/JACS
Metal?Organic Polymers Containing Discrete Single-Walled Nanotube as a Heterogeneous Catalyst for the Cycloaddition of Carbon Dioxide to Epoxides
Zhen Zhou,????,?,?
Cheng He, Jinghai Xiu, Lu Yang, and Chunying Duan*??
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China
*S Supporting Information
inexpensive and abundant renewable C1 building block and is
ABSTRACT: The cycloaddition of carbon dioxide to epoxides to produce cyclic carbonates is quite promising and recognized to be environmentally benign.11 ,12
According to does not result in any side products. A discrete single-walled green chemistry and atomic economy, the cycloaddition of carbon metal?organic nanotube was synthesized by incorporating a dioxide to an epoxide to produce cyclic carbonates is quite tetraphenyl-ethylene moiety as the four-point connected node. promising, as the latter compounds are used widely in industry The assembled complex has a large cross-section, with an and the incorporation of carbon dioxide into these chemicals does
exterior wall diameter of 3.6 nm and an interior channel not result in any side products.13?15
Inspired by advances in diameter of 2.1 nm. It features excellent activity toward the homogeneous catalysts with high activity and heterogeneous
cycloaddition of carbon dioxide, with a turnover number of catalysts with excellent selectivity,16 ?18
we herein report the 17,500 per mole of catalyst and an initial turnover frequency synthesis and catalytic properties of a single-walled as high as 1000 per mole of catalyst per hour. Only minimal metal?organic nanotube Ni?TCPE1 for the cycloaddition of decreases in the catalytic activity were observed after 70 h carbon dioxide by the incorporation of the tetrakis(4-under identical reaction conditions, and a total turnover carboxyphenyl)ethylene (H4TCPE) moiety as the four-point number as high as 35,000 was achieved. A simple connected node (comparison of relative porous MOFs suggested that the Scheme 1). We envisioned that the partially
cross-section of the channels is an important factor
Scheme 1. View of the Isolated Metal?Organic Nanotubes
influencing the transport of the substrates and products in the Crystals
a
through the channel.
C
atalysts based on permanently porous or channel frame-works
have the potential to unify the best features of homogeneous and
heterogeneous catalysts.1?3
Considered to be promising analogues
of carbon nanotubes, single-walled tubal metal?organic
frameworks (MOFs) constructed by organic linkers and metal ions or clusters have attracted a great deal of attention because of their
intriguing structural diversity and outstanding physical and
chemical properties.4?6
Their modular nature and facile
a
Constitutive/constructional fragments are shown.
tunability makes these materials ideal heterogeneous catalysts because they possess active sites and accessible channels for the
attraction and retention of substrates.7,8
However, from a
twisted ethyl core and multiple rotational phenyl rings would
synthetic point of view, the fabrication of well-defined single-cause a nonplanar conwalled metal?organic nano-tube requires relatively extreme formation of highly connected figuration of the ligand,19
frameworks. Relative benefiting the to the synthesis conditions, which leads to di?culties in the precise
phenyl rings, the weak stabilize π-stacking interactions are expected to control of their size and shape.9
An ongoing challenge for the and good structural transformability, favoring the formation of hierarchical structures with loose molecular packing development of e?cient metal? organic tubal catalysts includes
diverse the careful selection and incorporation of the catalytic sites obtained under the similar synthetic conditions, demonstrating structures. Besides, a (4,4)-network Ni?TCPE2 was within the original building blocks. Furthermore, strict control of that strict control of the synthetic conditions is of great the assembling processes is critical for the accurate positioning of
signi ficance during the self-assembly.
these catalytic sites within
The solvothermal reaction of Ni(NO3)2·6H2O, H4TCPE and the inner surface of the discrete nanotubal frameworks.
10
L-proline (L-Pro) in a mixture of DMF and H2O at 373 K for 3
dioxide In terms of catalysis, the e?attractive both industrially and academically because it is an
into useful chemicals cient under transformation mild conditions of is carbon very Received: July 28, 2015
? XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b07925
J. Am. Chem. Soc. XXXX, XXX, XXX?XXX
Journal of the American Chemical Society days gave the compound Ni?TCPE1 in 10% yield. Elemental and Communication equivalent to 8.0% of the MOFspowder X-ray di?raction (XRD) analyses indicated that the bulk laser scanning microscopy of the ’ weight (guest-adsorbed Figure S12acrystals ). Confocal gave a
sample consisted of a pure, single phase (Figure S5). Single-strong green fluorescence response20
that can be assigned to crystal X-ray structural analysis revealed that Ni?TCPE1 crystallizes in the P-3 space group. flL-Pro serves only as a molecules uorescein (template for the synthesis; it is not detected in the final, pure penetrated deeply into the channels, rather than remaining on the
throughout Figure S12f). the The crystals uniform suggests distribution that of the the dyes dye crystal products. Two carboxylate groups and one water molecule external surface.21
The results demonstrate the ability of Ni? bridge two octahedral and independent nickel ions. The Ni2 unit TCPE1 to adsorb organic substrates within its open channels. The that connects to four carboxylate groups from four di?erent tube-like structure with modified open metal sites provides ligands acts as a four-point connected node (Figure 1a).
adequate space for CO2 uptake and promotes highly selective and
e?cient chemical transformations (Table 1).22?24
It was that the CO2 uptake of Ni?TCPE1 was as high as 47.8 cm3found
g?1
at
273 K and 32.8 cm3 g?1
at 298 K (Figure 2c). Table 1. Ni-TCPE-Catalyzed Coupling of Epoxides with CO2
Figure 1. (a) Structure of Ni?TCPE1 showing the binuclear Ni2 unit; (b) the side view of the 1D nanotube with the pink column representing the channel; (c) the top view of the nanotube; (d) the packing pattern between them along the b axis. Color code: Ni, cyan; O, red; N, blue; C, gray. The H atoms and lattice solvents are omitted for clarity.
Each of this deprotonated TCPE ligand connects to four Ni2 units a
Reaction conditions: epoxide (20 mmol), catalyst (10 μmol, based on and also serves as a four-point connected node (Figure 1b). Ni), and TBABr (0.3 mmol) under carbon dioxide (1 MPa), 373 Accordingly, the skeleton of Ni?TCPE1 with this topology is and 12 h. The yields were determined by 1H NMR analysis. b
K
Under considered to consist of equivalent linkers and four-point common conditions, but without the epoxide (10 mmol).
connected nodes; a large open-ended, single-walled metal? organic nanotube is formed. The top view of the nanotube indicates that it is an undulated hexanuclear metallamacrocycle with a large 90-membered ring consisting of six nickel atoms and six TCPE ligands with S 6 symmetry. Notably, Ni?TCPE1 has a large cross-section, with an exterior wall diameter of 3.6 nm and an interior channel diameter of 2.1 nm (Figure 1c). To the best of our knowledge, it represents the largest discrete single-walled metal
?organic nanotube reported to date. Ni?TCPE1 can be regarded as a nanotube folded from a (4,4)-topological sheet. Analogous to other types of single- walled carbon nanotubes, it is instructive to consider how the (4,4) square is rolled to create the nanotubes. The tube axis is along the a direction diagonal to the (4,4)-square layer. The large nanotubes are anchored together by hydrogen bonds involving the coordinated water molecules around the Ni(2) ions and the lattice water molecules. The nanotubes are closely packed in a hexagonal manner to form a three-dimensional (3D) structure. The PLATON program was used to calculate
the void volume of Ni?TCPE1. A void volume of ～4600 ?3
per unit cell (～51% of the cell volume) was determined. For the structures, the contribution of heavily disordered solvent molecules was treated by the Squeeze procedure. The side
view of the framework reveals other quadrangular Figure 2. (a) Histogram of the yields of excessive styrene oxide using with eNi?TCPE under standard conditions (CO
?ective dimensions of 8.4 × 8.4 ?2
openings
along a and b axes. MPa when the pressure decreased to 0.4 2 pressure was increased to 1 MPa); (b) the structure of the inner surface of the tube. Dye-uptake studies were performed From the structure analysis view, Ni(1) atom is positioned on styrene-oxide-impregnated species Ni?TCPE1′; (c) CO2 adsorption by soaking Ni?TCPE1 in a methanol solution containing 2′,7′-isotherms of Ni?TCPE1 measured at 273 and 298 K; and (d) enlarged dichorofluorescein. These experiments gave a quantum uptake
view of the Ni?TCPE1′ showing the positions of the substrate and the interactions between the tube and the substrate.
B
DOI: 10.1021/jacs.5b07925
J. Am. Chem. Soc. XXXX, XXX, XXX?XXX
Journal of the American Chemical Society Communication Our catalytic experiments were focused on the cycloaddition of cooperative carbon dioxide and epoxides. In a typical experiment, the between the adsorbed substrates and the nickel ions. After the weak interactions enforced the spatial proximity reactions were conducted in an autoclave reactor using the coordinating epoxide (20 mmol) with carbon dioxide purged to 1 MPa under a sites in the pores of desolvated Niwater molecules are removed, open Ni active solvent-free environment at 373 K. In the presence of 0.3 mmol of and epoxy ring through the oxygen atom of epoxide and also can can serve as Lewis acid catalytic ?TCPE1 might be activated sites to activate the tetrabutylammonium bromide (TBABr), the loading of 0.5 mol‰ serve as charge-dense binding sites that capture carbon dioxide ratio of Ni?TCPE1 (based on Ni) a?orded an almost complete because of its compatible quadrupole moment and
conversion within 12 h. The TOF was ～165 per mole of catalyst polarizability.31,32
From a per hour. No significant change in the conversion was observed reaction is initiated by Br?
mechanistic point of view, the
generated from TBABr, which when the phenyl group in styrene oxide was substituted for a attacks the less-hindered methylene carbon atom of the
phen-oxymethyl group. The introduction of oxiran-2-ylmethoxy activated epoxide to open the epoxy ring.33,34
The activated or methoxy groups onto the phenyl ring gave ～96% and 94% of epoxide intermediate reacts with activated carbon dioxide to the respective products under the same reaction conditions. When yield a cyclic carbonate with high ethe reactant was the enantiopure R- or S-styrene oxide, the
reaction of carbon dioxide gave excellent enantioselectivity (with procedures Ni?TCPE2 was obtained with ?ciency and selectivity. the an ee value of ～92%) (Table S4). The retention of chirality crystallized in chiral space group when the amount of similar synthetic CL-Pro 2. This structure contains a was increased, and it demonstrated that the selective ring opening occurred disparate metal cluster of three Ni(II) ions with di?erent preferentially at the methylene C?O bond of the terminal
coordination modes. The linear Ni3 clusters are bridged by epoxides.25,26
The loading of excessive styrene oxide (87.5 mmol) three with the unchanged quantity of Ni?TCPE1 (5 μmol) gives an and sixμ2-oxygen atoms, one oxygen atom from LTCPE ligands ( oxygen atoms that belong to four carboxylic groups of -Pro molecule, Figure 3a). The Ni atoms are coordinated in an
initial TOF up to 1000 per mole of catalyst per hour. To the best of our knowledge, this value is greater than any previously reported value for MOF-based catalysts for the cycloaddition carbon dioxide to epoxides under the similar conditions,27,28
of
and Ni?TCPE1 also exhibited high e?ciency even under atmospheric
pressure of carbon dioxide, at room temperature (
Table S7).29,30
Recyclability is an essential feature of any catalyst considered for use in industrial applications. As for the small amount of catalyst in a reaction, the unavoidable loss of the catalyst would course inaccurate decrease of yields after each reaction (Table S3). In this regard, experiments were performed using a large excess of styrene oxide (87.5 mmol) and Ni?TCPE1 (5 μmol) and by maintaining the pressure of the system (from 0.4 to 1.0 MPa) by adding carbon dioxide (Figure 2a). No other reaction conditions
were altered. These time-course experiments gave a total TON value of 17,500 per mole of catalyst after 10 times of catalysis Figure 3. (a) The structure of the Ni3 unit in Ni?TCPE2; (b) the (32.5 h). Dye uptake studies on the recovered catalysts revealed side view of the channels displaying the environments of TCPE; (c and d) the truncated 3D structure and schematic representation that they exhibited 2′,7′-dichorofluorescein uptake ability (7.6%) of the PtS-type network. Color code: Ni, cyan; O, red; N, blue; C, almost identical to that of the original catalyst. Most important, gray. H atoms are omitted for clarity. only a slight decrease in the catalytic activity is observed after 70
h reaction, and a total TON value of 35,000 for Ni?TCPE1 was octahedral fashion; one of Ni atom from one side of cluster is achieved after 20 times of repeating catalytic reaction. It should coordinated to the N and O atoms within one be noted that such a high value of TON (>20,000) reveals that and one lattice water molecule, while the other side of Ni atom L-Pro molecule Ni?TCPE1 has the broad prospects for the practical application in is directly coordinated to two lattice water molecules. The Nithe chemical industry for the carbon dioxide cycloaddition to clusters cyclic carbonates.
TCPE to are generate cross-linked 3 a 3D through extended carboxylic network groups with from 1D
The quality of the styrene-oxide-impregnated crystals Ni? quadrilateral TCPE1 was su?cient for X-ray structural analysis (noted as Ni? the c direction (channels Figure 3of c). In the absence of guest molecules, 17.9 × 17.9 ?2
when viewed along TCPE1′). The same space group and almost identical cell the e?ective free volume of Ni?TCPE2 was calculated using dimensions between the impregnated crystals and the original PLATON to be 46.4% of the crystal volume. The Nicrystals confirmed that the Ni?TCPE1 framework was maintained are (Figure 2b). Multifold edge-to-face aromatic interactions were are considered to be the linkers. The overall framework can be
regarded as four-point-connected nodes, and the 3 clusters ligands observed between the phenyl groups of the TCPE ligands and regarded as a PtS topology with the Schlaflisymbol 424
those of the substrates, with the shortest interatomic separation CObeing 3.43 ? (Figure 2d). IR spectrum of Ni?TCPE1′2 uptake of Ni?38. The
?1
two epoxy vibration peaks at 982 and 871 cm?1 revealed
(Figure S10). 1
H 273 K and 29.9 cm
3 TCPE2 g?1
was declined to 44.7 cm g at
at 298 K (Figure S13). NMR of the crystals Ni?TCPE1′ in DMSO-d6/DCl exhibits the addition Ni?TCPE2 of carbon exhibits dioxide e?cient to epoxides. activity The toward activity the of cyclo-Ni? characteristic peaks of styrene oxide with significant downfield
TCPE2 is signishifts of free styrene oxide (Figure S11). The obvious shifts of 1
H the two catalysts contain the same amount of open metal sites. ficantly lower than that of Ni?TCPE1, though NMR and IR spectra compared with the free styrene oxide The recycling experiment was based on large excess of styrene suggested the adsorption and the activation of the styrene oxide in oxide (87.5 mmol) and Ni?TCPE2 (5 μmol). After 10 times of
the channels of the MOFs. The
C DOI: 10.1021/jacs.5b07925
J. Am. Chem. Soc. XXXX, XXX, XXX?XXX
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