Xiao
Wu
,
Zhihua
Lin
,
Cheng
He
* and
Chunying
Duan
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 1160, 24, China. E-mail: hecheng@dlut.edu.cn
First published on 15th November 2011
Channel-like and cage-like porous molecular materials Tb–PT1 and Sm–PT1 were constructed through packing and stacking of the amide-containing lanthanide-based octahedra [Tb6(H3L)4 (NO3)9-3H+]6+ and [Sm6(H3L)4(NO3)12]6+, respectively, where H3L is N′,N′,N′′-tris(pyridin-2-ylmethylene)benzene-1,3,5-tricarbohydrazide. Catalytic experiments on cyanosilylation reactions exhibited that the loading of only 1 mol% of Tb–PT1 leads to >90% conversion of the products with all the three nitrobenzaldehydes (NBA), but less than 10% conversion in the case of large size 3-formyl-1-phenylene-3 and 5-di-tert-butylbenzoate (BA) substrates. Adsorption of the substrates in the catalyst demonstrated a 3:
1 mole ratio (per octahedron) of these nitrobenzaldehydes, but no adsorption of BA could be observed. Luminescent detection of the suspension of Tb–PT1 in CH2Cl2 solution exhibited that the presence of nitrobenzaldehydes quenched the emissions efficiently, whereas the addition of BA did not cause any significant spectra variations. All these results revealed that the catalytic performance of Tb–PT1 was size-selective and the interactions corresponding to the Tb3+ sites were dominant in activating the aldehydes. The catalytic experiments on aldol reactions between cyclohexanone and these aldehydes exhibited that the smallest 4-NBA substrate has the largest conversion (80%) over the others. The presence of cyclohexanone led to absorption spectral changes and luminescence enhancements of the catalyst. These results suggested that interactions between cyclohexanone and the amide groups were dominant in activating the cyclohexanone and the aldol reactions possibly occurred within the cavities of the octahedron. Evaluation of catalytic performance of Sm–PT1 in which those channels in Tb–PT1 were blocked upon cyanosilylation and aldol reactions as well as the adsorption experiments were also carried out for a comparison.
The immobilization of homogeneous catalysts on solid supports represents an established field that is on the verge of being applied in industry,10 and the emerging young field of microporous metal–organic frameworks (MOFs) has attracted wide attention for their superior functional properties and applications in heterogeneous catalysis.11 The use of metal–organic macrocycles as molecular building blocks for the creation of extesively porous materials has implications for a whole generation of heterogeneous catalysts. Since the molecules defining the cavities conceivably align and stack in the solid state to create semi-infinite channel structures, these materials would be expected to typically display the combined advantages of homogeneous and heterogeneous catalyses,12 such as high activity and selectivity on the one hand, and easy separation and efficient recycling, on the other, that result in an overall lower cost compared to others.13 The ready tenability of such an approach is also expected leading to unique and useful heterogeneous catalysis, since the rigid and shape-persistent metal–organic macrocycles are versatile building blocks for supramolecular design and are possibly suited for the hierarchical assembly of porous materials with utilizable functional groups.
Through carefully incorporating amide groups as guest-accessible functional organic sites within the well-defined metal–organic architectures, researchers, including us, have reported a powerful approach to assemble metal-tunable Werner-type cages composed of transition metal and lanthanide ions for size- or shape-selective dynamic molecular sensing.14 In several recent discoveries, amide-containing metal–organic polyhedra (MOPs)13c,15 and MOFs16 were found to be capable of prompting chemical reactions efficiently. Since the coordinated unsaturated lanthanide ions are also able to function as active catalysis sites for acid promoting reactions,17 we envisioned that the assembled metal–organic polyhedra containing both potential unsaturated lanthanide ions and amide groups should exhibit efficient base-type and acid-type catalytic performances at the same time. To realize the above-mentioned postulation, herein we investigate the preparation of porous materials through the packing and stacking of the amide-containing lanthanide-based octahedra Tb–PT1 and Sm–PT1 in the crystalline solid14 and validate the catalytic properties in cyanosilylation reactions and aldol reactions, respectively.
Importantly, the packing patterns of the two octahedral cages in their crystals were quite different (Scheme 1). Tb–PT1 crystallized in a space group Pc1. Each octahedron positioned at a crystallographic C3 axis and connected with three neighbors through intermolecular π⋯π stacking interactions, featuring a two-dimensional (6,3) sheet having a cavity of a radius of about 1.5 nm. Adjacent sheets were further packed into three-dimensional networks, exhibiting one-dimensional channels. Sm–PT1 crystallized in a space group R
. The octahedron positioned at the crystallographic C3 axis also and connected with three neighbors through inter-molecular π⋯π interactions, featuring a 2D sheet similar to that of Tb–PT1. However, adjacent sheets in Sm–PT1 were packed in an ABAB fashion, blocking the 1D channels. In this case, the similar molecular structure but different packing patterns of Tb–PT1 and Sm–PT1 made the two compounds promising candidates to optimize the heterogeneous catalysis conditions of the given reactions. TGA analysis of crystalline samples Tb–PT1 and Sm–PT1 showed only 18% decrease in weight for both compounds at 210 °C, corresponding to the loss of the adsorbed solvents in the crystals. These results showed that both compounds were stable enough after losing the lattice solvent molecules and could be used as catalysts in a heterogeneous manner.
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Scheme 1 Structure of ligand PT1 and constructional fragments, showing the cationic lanthanide-based octahedral cages, and the packing diagram of the two dimensional layers consolidated by the octahedrons, showing the AAA fashion of the two-dimensional layers in the channel structure of Tb–PT1 and the ABAB fashion of the porous-like structure of Sm–PT1, respectively. |
While the smooth reaction between 2-nitrobenzaldehyde (2-NBA) (molecular size, 5.8 × 5.3 Å2), 3-nitrobenzaldehyde (3-NBA) (molecular size, 6.9 × 5.0 Å2) or 4-NBA (molecular size, 7.0 × 4.3 Å2) and Me3SiCN suggested that the window size of the catalyst is larger enough to allow such kind of substrates to pass through, just 13% of the cyanosilylation product was observed when the reaction of bulky aldehyde with larger size, 3-formyl-1-phenylene-(3, 5-di-tert-butylbenzoate) (BA) (molecular size, 12.9 × 9.1 Å2),20 was conducted after the same period of time, in the presence of Tb–PT1 catalyst. Given the expectation of comparable reactivity, it seems that the diameter of the channel (11.5 Å eliminating the van der Waals radii of about 3.6 Å) is too small to readily accommodate the transition state geometry required for activating the corresponding substrates. The potential size-selectivity of the cyanosilylation reactions suggested that the catalytic reactions mostly occurred in the channels of the catalysts, not on the external surface.
The investigation on the adsorption of the substrates in the catalyst also confirmed the size-selectivity. The desolvated catalyst was immersed in the solid catalyst in a CH2Cl2 solution containing the substrates for 3 h, and then filtrated and washed by CH2Cl2 several times to remove the substrate remaining on the surface. 1H NMR (DMSO-d6) of the sample immersed in the CH2Cl2 solution containing 4-NBA exhibited peaks at chemical shifts of 10.18, 8.43 and 8.17 ppm, assigned to the proton signals of the substrate, all of them being shifted downfield (δ = 0.1 ppm) (Fig. 1a–c). Detailed 1H NMR spectral analyses demonstrated the 3:
1 mole ratio of 4-NBA absorbed per polyhedron in the solid catalyst. However, 1H NMR (DMSO-d6) of Tb–PT1 immersed in a CH2Cl2 solution containing BA didn't exhibit any peaks of BA, which asserted that the BA was not incorporated by the catalyst (Fig. 1d).
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Fig. 1 1H NMR (DMSO-d6) spectra of Tb–PT1 that adsorbed different aldehydes: (a) 4-NBA, (b) 3-NBA, (c) 2-NBA and (d) BA. The peaks marked with a black rhombus indicate the protons in Tb–PT1, the others refer to the substrates. |
To further probe whether activation of the carbonyl species occurs inside the channels or on the surface of the solid catalyst, fluorescent titrations of the suspension of Tb–PT1 in CH2Cl2 upon addition of the substrates were investigated. When excited at 365 nm, free Tb–PT1 exhibited two intense emission bands of 465 nm (PT1 emission) and 525 nm (the Tb3+ emission assignable to the transitions of 5D4 → 7F4).21 The addition of 4-NBA, 3-NBA and 2-NBA reduced the luminescence of the Tb3+ emission (525 nm) and PT1 emission (465 nm) of the suspension of Tb–PT1 in CH2Cl2, while both luminescence of the Tb3+ emission and PT1 emission of the suspension of Tb–PT1 were almost unaltered with the addition of BA. This result suggested that only the molecules having suitable sizes had the potential to enter the channels or be encapsulated by cavities, and interact with the luminescence sites from which the luminescent responses exhibited size-selective performance.
The quenching effect was rationalized by the Stern–Volmer equation:22
I0/I = 1 + KSV [M] |
I 0 and I are the luminescence intensities of aldehyde-free Tb–PT1 and aldehyde-incorporated Tb–PT1, respectively, [M] is the molar concentration of the aldehydes added, and KSV is the quenching effect coefficient of the aldehyde. KSV was calculated to be ca. 5.4 M−1 from the experimental data for the examined aldehyde. The addition of 4-NBA to Tb–PT1 in an acetonitrile solution does not cause any obvious absorption spectroscopic changes, thus the fluorescence quenching by aldehydes should be ascribed to a PET (photo-induced energy transfer) mechanism between the guest molecules and Tb–PT1.23
Interestingly, KSV values referring to the intensities at 465 nm and at 525 nm are different (Fig. 2 inset). With the decrease of the intensities of the two emission bands upon incorporating 4-NBA, the I525/I465 ratio increased significantly. Since the two emission bands were isogenous, the characteristic Tb3+ emission was prompted by the excitation of the PT1 group. The quenching of the two emission bands was attributed to PET from the PT1 group to 4-NBA, whereas the increase of I525/I465 ratio should be attributed reasonably to the interactions between molecules of 4-NBA and the Tb3+ ions. Accordingly, the enhancement of the emitting efficiency at 525 nm (Tb3+ emission) was attributed to the replacement of the coordinated solvent molecules by the aldehydes (Fig. 3). These results demonstrated that the interactions corresponding to the Tb3+ sites were dominant in luminescent responses ratiometrically and in activating the aldehydes.
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Fig. 2 Family of luminescent spectra of Tb–PT1 emulsion in CH2Cl2 upon addition of various amounts of 4-NBA up to 0.5 mmol. The inset shows fluorescence responses of Tb–PT1 upon addition of 4-NBA in CH2Cl2 at 525 nm (red line) and at 465 nm (black line), excitation at 365 nm. |
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Fig. 3 Luminescent responses of Tb–PT1 upon addition of aldehydes (0.5 mmol) of interest. Intensities were recorded at 525 nm, excitation at 365 nm. |
To further validate the catalytic reactions occurring within the channels and in the surface of the catalyst, another lanthanide-based porous material Sm–PT1 having the same molecular structure but a different packing pattern was investigated. Molecules of Sm–PT1 octahedron in the crystals were packed into two-dimensional layers, but the layers stacked in an ABAB fashion to block the channels into cavities. The sorption experiments including the 1H NMR (d6-DMSO) of the immersed sample suggested the same absorption of substrate 4-NBA within Sm–PT1, and revealed the presence of 3 mole equiv. of 4-NBA per polyhedron. The loading of only 1 mol% ratio of Sm–PT1 (5 μmol) led to excellent conversion of the nitrobenzaldehyde substrates, whereas the conversions of the corresponding reactions catalyzed by Sm–PT1 were lower compared to the reactions prompted by Tb–PT1. And the conversion of cyanosilylation with BA catalyzed by Sm–PT1 was the lowest one and almost the same as that by Tb–PT1, while the conversion of all the entries was over 97% using Tb(NO3)3 and Sm(NO3)3 as catalysts under the same reaction conditions, respectively. Because the two compounds have similar molecular structures and almost the same coordination geometry, the difference in the reaction conversions might be one of the indicators that the reactions occurred within the channels or pores of the catalysts. The higher conversion of the channel structure Tb–PT1 compared to that of the porous structure Sm–PT1 suggested that the presence of nanometre channels might benefit the nitrobenzaldehydes diffusing swiftly through the channels and accommodating the transition state geometry required for activating these substrates.
Entry | Substrate | Tb–PT1 | Sm–PT1 | PT1 | |||
---|---|---|---|---|---|---|---|
Con.% | Dr(s/a) | Con.% | Dr(s/a) | Con.% | Dr(s/a) | ||
a The reaction was carried out at 37 °C for 5 d with cyclohexanone (5 mmol), aldehyde (0.5 mmol) and 0.01 mmol catalysts (2 mol%) in CH3OH/H2O, the conversion and diastereomer were determined by 1H NMR spectroscopy. | |||||||
5 | 4-NBA | 80.5 | 2.0![]() ![]() |
64.9 | 2.1![]() ![]() |
4.8 | syn |
6 | 3-NBA | 75.2 | 1.7![]() ![]() |
56.0 | 2.3![]() ![]() |
8.9 | syn |
7 | 2-NBA | 48.2 | 2.1![]() ![]() |
31.5 | 1.4![]() ![]() |
8.3 | syn |
8 | BA | 27.5 | 2.4![]() ![]() |
21.3 | 2.4![]() ![]() |
9.1 | syn |
Furthermore, in the heterogeneous manner the conversion of 4-NBA and cyclohexanone was just 4.8% with only cis isomer catalyzed by PT1 (8 mol% ratio) under the same conditions. Tb–PT1 solids could also be easily isolated from the reaction suspension by simple filtration alone and can be reused at least twice with a slight decrease in reactivity. Whereas the conversions of other two nitrobenzaldehydes with different dimensions (75.2% and 48.2%) were much lower than 4-NBA prompted by Tb–PT1, only 27.5% of the aldol product was observed in the case of the bulky aldehyde BA as substrate after the same period of time in the presence of Tb–PT1. While the sorption investigation and the catalytic cyanosilylation reactions of various aldehydes with the crystalline solids of Tb–PT1 suggested that molecules of all the three nitrobenzaldehydes could smoothly pass through the channels of the catalysts. The lower catalytic effect in the case of 2-, or 3- nitrobenzaldehydes as substrates should be likely attributed to that the size of the opening regular trigon within the octahedral molecule is too small for them to enter the inner cavities of the octahedra themselves, even considering the possible difference in their reactive activity. Accordingly, we think that the aldol reactions should take place within the octahedral cavities, and distinguish conversions rooted in the size of the opening regular trigon having an edge of about 10 Å2 within the octahedral molecule.
To further validate the potential catalytic mechanism and probe whether activation of the carbonyl species occurs inside the inner cavities of the polyhedron or in the channels/on the surface of the solid catalyst, fluorescent titration of the suspension of Tb–PT1 in CH2Cl2 upon addition of the cyclohexanone was investigated. As mentioned above, the Uv-vis spectra of Tb–PT1 do not have any significant absorption variation upon addition of 4-NBA even at high concentration. However, the addition of cyclohexanone led to the significant absorbance decrease at the peaks 330 nm and 365 nm, and caused the appearance of a new peak at about 415 nm (Fig. 4). The presence of sharp isosbestic points at about 400 nm and 350 nm indicates that only two species coexist in the equilibrium. Since the bands at 330 nm and 415 nm were mainly attributed to the ligand-based charge transfer bands,14 the absorption variations of the catalyst were mainly attributed to the potential interactions corresponding to the amide group. Importantly, the addition of cyclohexanone to the suspension of Tb–PT1 in CH2Cl2 caused luminescence enhancement of the Tb3+ emission (525 nm) and ligand-based emission (465 nm) significantly (Fig. 5). As the two bands were isogenous, and the characteristic Tb3+ emission was prompted by the excitation of the PT1 group, the luminescent enhancements of the two emission bands were reasonably attributed to interactions between the cyclohexanone and the amide groups in the ligand backbone. Clearly, these results revealed that the interactions between cyclohexanone and the solid of the catalyst were mainly taking place at the active sites on the ligand backbone, mainly attributed to the potential hydrogen bonding between the cyclohexanone molecules and amide groups.
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Fig. 4 Uv-vis spectra of the Tb–PT1 (5 × 10−6 M) upon addition of cyclohexanone (0.25 mM) to an acetontrile solution (a); Luminescent responses of Tb–PT1 upon addition of cyclohexanone (0.1 mL to 0.5 mL). |
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Fig. 5 Fluorescence responses of Tb–PT1 upon addition of cyclohexanone in CH2Cl2 at 525 nm (red line) and at 465 nm (black line). Excitation at 365 nm (inset). |
Interestingly, under the same experimental conditions, the loading of only 2 mol% ratio of Sm–PT1 (0.01 mmol) catalyst led to 65% conversion corresponding to 4-NBA with the diastereoselectivity of 2:
1 (syn
:
anti). The 21.3%/27.5% conversion (entry 8) in the presence of Sm–PT1/Tb–PT1 suggested that the narrow triangle opening of these molecular polyhedra was too small for BA to pass through. The almost same conversion and the same size-selective sequence of the two catalysts confirmed that the aldol reactions should take place within the octahedral cavities, given that the two compounds have similar molecular structures and almost the same coordination geometry, but different packing patterns. While the immobilization of unsaturated Lewis acid Ln ions within the relatively open frameworks having well-defined pores represents extensively investigated methodologies and enables researchers to target some unique porous LnMOFs with multifunctional properties and catalytic applications.28 The heterogeneous catalytic behavior of our materials is quite significant, it not only suggests that the porous molecular materials comprised of semi-infinite macrocycles have the potential to combine the excellent size discrimination properties of the macrocycles in solution and the advantage of the heterogeneous catalysts, but also demonstrates that the catalysts containing unsaturated lanthanide ions positioned at the intermolecular cavities of the crystals and amide groups within the inner cavities of the polyhedron have the potential to exhibit both base-type and acid-type catalytic driving forces directly. And owing to the different catalytic mechanisms, our systems also provide the possibility to tune the size-selectivity through controlling the catalytic reactions within the inner cavities of the discrete molecular polyhedron or within the intermolecular channels or pores of the materials consolidated.
The as-synthesized crystals of polyhedra lost part of their lattice molecules under ambient conditions. However, when the desolvated amorphous powders were reacted, they were restored to the original crystalline phase, which was detected by the XRPD patterns. Crystalline solids of the two compounds were also easily isolated from the reaction suspension by filtration alone and can be reused at least twice without loss of activity. The maintenance of the crystalline information of the solid compounds during the reaction processes suggested the possibility of the powder to be recycled with a comparable catalytic activity.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 675067. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1nj20736a |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 |