Communication pubs.acs.org/IC
Heterometallic Metal−Organic Frameworks That Catalyze Two Different Reactions Sequentially Debraj Saha,†,‡ Dipak K. Hazra,§ Tanmoy Maity,† and Subratanath Koner*,† †
Department of Chemistry, Jadavpur University, Kolkata 700032, India Department of Physics, Uluberia College, Howrah 711315, India
§
S Supporting Information *
Herein, we report a series of metal carboxylate compounds, {[CuMg(pdc)2(H2O) 4]·2H2O}n (1), [CuCa(pdc) 2]n (2), [CuSr(pdc)2(H2O)3]n (3), and {[CuBa(pdc)2(H2O)5]·H2O}n (4), where H2Pdc is pyridine-2,5-dicarboxylic acid. While 1 is a 2D framework compound, the rest are of the 3D variety. Catalytic efficacy of these framework compounds in olefin epoxidation reactions, and subsequently in situ epoxide ringopening reactions, has been investigated. To our knowledge, these are the first examples of hetero-MOFs where two different metal centers catalyze two different chemical reactions sequentially under heterogeneous conditions. The phase-pure crystals 1−4 were grown under hydrothermal conditions and were characterized by single-crystal X-ray diffraction [see the Supporting Information (SI), Tables S1 and S2]. Compound 1 features a 2D structure constructed by a ribbonlike 1D chain of hexacoordinated CuII centers, CuO4N2, which is connected through hexacoordinated tetraaqua MgII centers of a MgO6 unit (Figure 1a). Each carboxylate ligand is coordinated to two Cu centers and one Mg center through three carboxylate O atoms and one pyridyl N atom to give rise to a 2D framework (Figure 1a). To gain deeper insight into the structures of 1−4, topological analysis was performed by reducing the multidimensional structure to a simple node-and-linker net (see the SI, Figure S2). X-ray structure analysis of 2 revealed the
ABSTRACT: A series of copper- and alkaline-earthmetal-based multidimensional metal−organic frameworks, {[CuMg(pdc)2(H2O)4]·2H2O}n (1), [CuCa(pdc)2]n (2), [CuSr(pdc)2(H2O)3]n (3), and {[CuBa(pdc)2(H2O)5]· H2O}n (4), where H2Pdc = pyridine-2,5-dicarboxylic acid, were hydrothermally synthesized and characterized. Two different metals act as the active center to catalyze two kinds of reactions, viz., olefin to its epoxide followed by epoxide ring opening to afford the corresponding vicinal diol in a sequential manner.
M
etal−organic frameworks (MOFs), polymers in which metal centers are connected through organic linkers into extended one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) networks via coordinate bonding, attracted immense attention in heterogeneous catalysis.1 Metal centers of the network, especially coordinatively unsaturated metal centers present in the catalyst or such a center generated in situ in reaction conditions, act as active sites of catalysis.2 With the advent of growing knowledge on the synthesis of framework compounds, diverse predictable architectures have been realized.3 MOFs reported so far mostly contain only one type of metal (homometallic); hence, these are capable of catalyzing one type of reaction when employed in catalytic reactions. Recently, attempts were made to incorporate metal centers in MOFs by postsynthetic functionalization of frameworks to design heterogeneous catalysts.4 The incorporation of metal nanoparticles in MOF materials has also been attempted to create catalytically active centers.5 In the latter case, the metal nanoparticles clinging to the MOF surfaces tended to migrate and aggregate into larger particles to minimize their surface energy. As a result, MOF materials often suffered deactivation during continuous reactions. It is, therefore, desirable that the prospective catalytically active metal centers remain in the network of the MOFs. However, reports on heterometallic MOFs (hetero-MOFs), i.e., framework compounds that contain two different metals, are limited.6 With this in mind, we explored the possibility of designing heterometallic frameworks connecting copper-based low-dimensional (for example, 0D or 1D) networks to alkaline-earth metals to afford 2D frameworks, which could be further functionalized to 3D networks. While copper(II) is an efficient catalyst for epoxidation reaction,7 alkaline-earth metals, being Lewis acidic, are capable of catalyzing acid-catalyzed reactions subsequently.8 © XXXX American Chemical Society
Figure 1. Network connectivity in 1−4 (a−d, respectively). Received: February 3, 2016
A
DOI: 10.1021/acs.inorgchem.6b00292 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry formation of a 3D network constructed by tetracoordinated CuII centers of a CuO2N2 unit, which is connected to four carboxylato-bridged 1D chains of hexacoordinated CaII centers of a CaO6 unit (Figure 1b). In this case, a carboxylate ligand is coordinated to one Cu center and three Ca centers through four carboxylate O atoms and one pyridyl N atom to give rise to a 3D arrangement (Figure 1b). In compound 3, a 3D framework is constructed by pentacoordinated CuII centers of a CuO3N2 unit, which is connected to heptacoordinated triaqua SrII centers of a SrO7 unit (Figure 1c). The carboxylate ligand in 3 features two types of connectivity. In one, the ligand is coordinated to two Cu centers and two Sr centers through four carboxylate O atoms and one N atom. In another, the ligand is coordinated to one Cu center and two Sr centers through three carboxylate O atoms and one N atom. Network connectivity in 3 can be easily visualized because four corners of the basal plane of an octahedron are occupied by Sr centers and two axial positions by Cu centers, while the corners are connected through two pairs of carboxylate ligands of different connectivity (Figure 1c). These octahedra are further linked to each other in eight different directions to construct a 3D network (see the SI, Figure S3a). Compound 4 forms a 3D framework constructed by pentacoordinated CuII centers of a CuO3N2 unit, which is connected to nonacoordinated pentaaqua BaII centers of a BaO9 unit (Figure 1d). The overall network connectivity of 4 is almost similar to that of 3 (see the SI, Figure S3b). Thermogravimetric analysis of these compounds reveals that frameworks are thermally stable up to ∼300 °C after removal of water molecules (see the SI, Figures S4−S7). Catalytic epoxidation of alkenes and subsequent ring-opening reactions using 1−4 were performed in an acetonitrile medium at 60 °C in the presence of hydrogen peroxide under heterogeneous conditions (Table 1; see also the SI, Figures S8−S11). In every case, diol was the major product. The overall conversion from olefin to diol increases from 1 to 4. In the case of 1 and 2, olefin-to-epoxide conversion initially increases and reaches a maximum, and then it decreases gradually (Figure 2; see also SI Figure S12). Subsequently, epoxide converts to the corresponding diol, and in most of the cases, conversion was 100%. However, in some cases, the formation of side products was also noticed. For example, in the case of cyclohexene, 2-cyclohexane1-ol, 2-cyclohexane-1-one, and 2-hydroxycyclohexanone were detected, while in the case of styrene, benzaldehyde and benzoic acid were detected in minor amounts. Therefore, it is clear that, in the case of 1 and 2, olefins convert to diols through the formation of the corresponding epoxides. However, in 3 and 4, no epoxide product was detected. This difference may be attributed to the higher catalytic activity of Sr/Ba than of Mg/Ca. The size of the alkaline-earth-metal ions increases from Mg2+ to Ba2+, which leads to the accommodation of a greater number of ligands in the coordination sphere of heavier alkaline-earth-metal ions in comparison to the lighter ones. Thereby, the possibility of having open metal sites increased in the case of heavier alkalineearth metals, which, in turn, enhanced the catalytic efficiency in the case of Sr/Ba. It is well established that open metal sites have direct relevance in the enhancement of the catalytic activity or gas adsorption in MOFs.9 Evidently, epoxides that were formed during reactions catalyzed by 3 and 4 immediately converted to the corresponding diols. To ascertain the role of the CuII and MgII centers in catalysis, we undertook a few control experiments. It is well established that CuII acts as an efficient catalyst in epoxidation reactions.7 To this end, we selected a 1D copper(II) complex, {[Cu(pdc)-
Table 1. Olefin-to-Diol Conversion of Olefins Catalyzed by 1−4a % yield of product substrateb A
B
C
D
E
conversion (wt %)
reaction time (h)
(i) 94 (ii) 100 (iii) 100 (iv) 100 (i) 90 (ii) 100 (iii) 100 (iv) 100 (i) 83 (ii) 100 (iii) 100 (iv) 100 (i) 70 (ii) 100 (iii) 100 (iv) 100 (i) 100 (ii) 100 (iii) 100 (iv) 100
24 12 6 6 24 12 6 6 24 12 6 6 24 12 6 6 24 12 6 6
epoxide
diol
others
28
28 82 88 90 80 100 100 100 61 100 100 100 51 100 100 100 46 86 92 94
38d 18d 12d 10d
10
22
19
22
32e 14e 8e 6e
TOFc (h−1) 103 181 446 533 99 181 446 533 91 181 446 533 77 181 446 533 110 181 446 533
a
Reaction condition: see the SI. bA = cyclohexene, B = cyclooctene, C = 1-hexene, D = 1-octene, and E = styrene. cTurnover frequency (TOF) = moles converted per moles of active site per hour. d2Cyclohexane-1-ol, 2-cyclohexane-1-one, and 2-hydroxycyclohexanone. e Benzaldehyde, benzoic acid. (i)−(iv) denote the catalytic activity of compounds 1−4, respectively.
Figure 2. Conversion of olefin to diol through epoxide, catalyzed by 1.
(im)2}·2H2O]}n (im = imidazole), which catalyzes epoxidation reactions, converting olefins to their corresponding epoxides as major products under heterogeneous conditions (see the SI, Table S3 and Figure S13). Recently, we explored alkaline-earthmetal-based MOF catalysts in aldol-type reactions, exploiting the Lewis acid character of the metal centers.10 One such compound, [Mg(pdc)]n,11 catalyzes epoxide-to-diol conversion smoothly under acid/base-free heterogeneous conditions at 40 °C (see the SI, Table S4 and Figure S14). Besides, epoxide-to-diol conversions were also investigated using compounds 1−4 as catalysts (see the SI, Table S5 and Figures S15−S18). B
DOI: 10.1021/acs.inorgchem.6b00292 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Interestingly, in the latter case, diols were formed comparatively in a shorter reaction time (except for 2) and afforded lower amounts of side products even from epoxycyclohexane. It is worth noting that {[Cu(pdc)(im)2}·2H2O]}n does not catalyze epoxide-to-diol conversion and [Mg(pdc)]n does not catalyze epoxidation of olefins. From the above results, it may be concluded that the Cu and alkaline-earth-metal centers act as active sites for epoxidation and epoxide-to-diol conversion reactions, respectively. The catalytic efficacy of compounds 1−4 is not the same. A closer look at the structural features of the active centers afforded a plausible structure−activity correlation. In 1, the Cu centers are hexacoordinated, while the Mg centers have the lowest Lewis basicity in the series of alkaline-earth metals present in the catalysts. These two factors may be cooperatively influenced by the lowering of the efficiency of the catalyst in both epoxidation and diol formation reactions. In 2, the Cu centers are in coordinatively unsaturated square-planar geometry, which may facilitate the full conversion of olefins to epoxides in only 2 h (see the SI, Figure S9). However, the diol formation rate is not as fast as the epoxide formation for 2, and it takes 12 h for 100% of epoxide to convert to its corresponding diol. Barium, being the heaviest alkaline-earth metal reported here, shows the highest diol formation rate for catalyst 4. Nevertheless, the overall time required for 100% conversion of olefins is the same as that for 3 and 4 (6 h). It may be realized that the formation of diol from epoxide (intermediate) happens at a faster rate compares to the olefin-to-epoxide conversion, which, in fact, depends on the similar active site (Cu) environment in 3 and 4. To confirm the heterogeneous behavior of the catalysts, a hot filtration experiment was conducted that clearly demonstrated that the metal was not leached out from solid catalysts during the catalytic reactions.11 The catalysts could easily be recovered after completion of the reaction by simple centrifugation, washed thoroughly with acetonitrile, and dried at room temperature. The recovered catalyst showed almost the same catalytic activity in successive runs (see the SI, Tables S6−S8). The powder X-ray diffraction patterns of the recovered catalysts clearly suggest that their structures are well maintained after several cycles of reactions (see the SI, Figures S19−S24). In essence, we successfully prepared and structurally characterized four new copper and alkaline-earth-metal-based bimetallic framework compounds. Framework compounds act as multifunctional catalysts, where copper acts as an active center for epoxidation of olefins and, subsequently, alkaline-earth metals act as active centers for epoxide ring-opening reactions, in tandem, to afford vicinal diol. With an increase in the size of the alkaline-earth metals, the diol formation rate from epoxides rapidly enhanced because of the increase of the open metal sites. Frameworks behaved as heterogeneous catalysts and can easily be recovered and reused without significant catalyst deactivation due to either leaching of the active species or degradation of the structure.
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for compounds 1−4, and additional tables and figures (PDF) X-ray crystallographic data in CIF format (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address
‡ D.S.: Department of Chemistry, Darjeeling Government College, Darjeeling 734101, India.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial assistance received from UGC through a project (to S.K.) is gratefully acknowledged. D.K.H. is grateful to the Department of Inorganic Chemistry, IACS, India, for lowtemperature data collection.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00292. Experimental details, crystallographic data and structure refinement parameters, selected bond lengths and angles C
DOI: 10.1021/acs.inorgchem.6b00292 Inorg. Chem. XXXX, XXX, XXX−XXX
