J Chem Crystallogr (2011) 41:1834–1838 DOI 10.1007/s10870-011-0180-x
ORIGINAL PAPER
Synthesis and Crystal Structure of an Expanded Square Grid Metal Organic Material, [Cu(L1)(DMF)]n(2.64 DMF) Patrick Nugent • Lukasz Wojtas Michael J. Zaworotko
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Received: 5 May 2011 / Accepted: 26 September 2011 / Published online: 5 October 2011 Ó Springer Science+Business Media, LLC 2011
Abstract A new square grid metal organic material, [Cu(L1)(DMF)]n(2.64 DMF) (1; L1 = N-(4-carboxyphenyl)-trimellitimide), has been prepared by solvothermal reaction of Cu(NO3)22.5H2O, H2L1, and caffeine in N,Ndimethylformamide (DMF). The title compound crystallizes in the monoclinic crystal system, space group C2/c, ˚ , b = 23.7473 (7) A ˚ , c = 18.1998 with a = 18.5176 (6) A ˚ ˚ 3. The (5) A, b = 107.527 (2)°, and V = 7631.7 (4) A undulating sheets, formed by linking of Cu2(l2-CO2)4 square paddlewheel nodes by the angular L1, stack to form ˚2 large square channels of approximately 11.9 9 11.9 A running along the c axis. The geometry and conformation of the linker result in slight distortion of the grids. Further, the 147° bend angle between coordinating moieties of L1 is amenable to forming a diverse array of paddlewheel-based supramolecular isomers. Keywords Paddlewheel MOM Coordination polymer Supramolecular isomer Square grid
Introduction Over the past two decades, judiciously selected combinations of metal ion or metal cluster nodes and multitopic
Electronic supplementary material The online version of this article (doi:10.1007/s10870-011-0180-x) contains supplementary material, which is available to authorized users. P. Nugent L. Wojtas M. J. Zaworotko (&) Department of Chemistry, University of South Florida, CHE 205, 4202 E. Fowler Avenue, Tampa, FL 33620-5205, USA e-mail:
[email protected] URL: http://chemistry.usf.edu/faculty/zaworotko/
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organic linkers have afforded a wide variety of crystalline polymeric materials that are net-like and can exhibit porosity coupled with additional functionality attributable to the nodes and/or linkers [1–8]. These metal organic materials, MOMs, exhibit properties such as selective gas sorption [9–13], catalysis [14–16], magnetism [17, 18] and small molecule separations [14, 19, 20]. A noteworthy subset of MOMs is exemplified by those sustained by dimetal tetracarboxylate ‘‘square paddlewheel’’ nodes, a ubiquitous structure in carboxylate coordination chemistry [21–25]. As its name suggests, the square paddlewheel can be simplified to a square planar node, which can then be linked to four neighboring nodes via four dicarboxylate linkers. Square grid or two-dimensional (4,4) networks constructed of square paddlewheel nodes and dicarboxylate linkers are very well studied and the dicarboxylate linkers may be linear or angular in nature. The partially rigid molecules 1,3-benzenedicarboxylate (1,3-bdc) and 1,4-benzenedicarboxylate (1,4-bdc) represent prototypal linkers that form the square grid motif. However, in addition to the square grid motif, 1,3-bdc and 1,4-bdc may link paddlewheels to form a number of other supramolecular isomers [26], all possessing 1:1 metal-tolinker stoichiometry. Indeed, there have been eight supramolecular isomers observed involving 1,3-bdc and four with 1,4-bdc [27]. The first square grid network sustained by this variety of linker and node, MOF-2, was reported in 1998 by Yaghi and coworkers [28]. MOF-2 is composed of divalent zinc paddlewheels and 1,4-bdc linkers and was one of the first MOMs shown to exhibit permanent porosity. The crystal structure of a copper-based analogue with a Langmuir surface area significantly higher than that of MOF-2 (752 vs. 270 m2/g) was reported in 2009 [29]. In this contribution we detail a new member of this class of two-dimensional (4,4)-network, [Cu(L1)(DMF)]n(2.64
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DMF) (1; H2L1 = N-(4-carboxyphenyl)-trimellitimide [30]). The bent shape and torsional angles present in L1 play an important role in determining the shape of the resulting grid layers. H2L1 (Fig. 1) can be synthesized in a solvent-free manner through condensation of the appropriate anhydride with the appropriate primary amine. Solvent-free synthesis is an environmentally, financially, and practically appealing alternative to methods that rely on toxic, expensive and/or wasteful solvents and has been used by us to prepare other imide based ligands [31–33].
powder (83.2% yield). Synthesis of H2L1 was supported by IR spectroscopy, which exhibited absorptions consistent with the conversion of anhydride to imide (Fig. S3). [Cu(L1) (DMF)]n(2.64 DMF), 1 15 mg of H2L1, 6.0 mg of Cu(NO3)22.5H2O, and 6.0 mg of caffeine were dissolved in 0.50 mL of N,N-dimethylformamide and heated at 115 °C for 24 h, affording blue plate-shaped crystals in ca. 98.1% yield (based on Cu). Crystal Structure Determination
Experimental Materials and Physical Measurements All starting materials are commercially available and were used as received without further purification. The linker H2L1 has been reported previously [30]. IR data was collected using diffuse reflectance on a Nicolet Avatar Smart MIRacle 320 FT-IR spectrometer. Powder x-ray diffraction (PXRD) was conducted on a Bruker D8 ADVANCE, h/2h ˚ ). 2h diffractometer using Cu-Ka radiation (k = 1.54056 A scans between 3 and 40° with a step size of 0.02° were performed for a duration of 92 min and 33 s. Thermogravimetric analysis (TGA) was carried out on a Perkin Elmer STA 6000 instrument under nitrogen atmosphere. The as-synthesized sample (14.91 mg) was heated from 28 to 600 °C at a rate of 5 °C/min. TGA (Fig. S1) indicates that the initial weight loss for 1 corresponds to the removal of coordinated and uncoordinated DMF (calculated 41.6%, observed 40.0%). The PXRD pattern of the bulk sample (Fig. S2) is in good agreement with the pattern calculated from single-crystal data. Synthesis
X-ray diffraction data were collected on Bruker-AXS SMART-APEXII CCD diffractometer using CuKa ˚ ) radiation. Indexing was performed using (k = 1.54178 A APEX2 [34] (difference vectors method) whereas data integration and reduction were performed using SaintPlus 6.01 [35]. An absorption correction was performed by multi-scan method implemented in SADABS [36]. Space groups were determined using XPREP implemented in APEX2 [34]. The structure was solved using SHELXS-97 (direct methods) and refined using SHELXL-97 (fullmatrix least-squares on F2) contained in APEX2 [34] and WinGX v1.70.01 [37–40] program packages. All nonhydrogen, framework atoms were refined anisotropically. Disordered molecules of DMF were refined using restraints with isotropic displacement parameters. The contribution of the remaining disordered, unidentifiable solvent (DMF or water) was treated as diffuse using Squeeze procedure as implemented by Platon [41, 42]. Hydrogen atoms attached to the framework atoms were placed in geometrically calculated positions and included in the refinement process using a riding model with isotropic thermal parameters: Uiso (H) = 1.2 Ueq (CH) and Uiso (H) = 1.5 Ueq (CH3). Crystal data and refinement parameters are presented in Table 1.
N-(4-carboxyphenyl)-trimellitimide, H2L1 [30] 4-aminobenzoic acid (5.00 mmol, 686 mg) and trimellitic anhydride (5.00 mmol, 961 mg) were ground with a mortar and pestle for several minutes to produce a powdered mixture of the starting materials. This mixture was then heated at 230 °C for 2.5 h, resulting in a light yellow
Fig. 1 N-(4-carboxyphenyl)-trimellitimide, ChemDraw Ultra 8.0)
H2L1
(created
with
Results and Discussion The title compound crystallizes in the monoclinic space group C2/c and consists of two-dimensional undulating square grid layers which stack in a slipped ABCDA fashion. Each layer is composed of 4-connected paddlewheel nodes that are linked by L1. The asymmetric unit of 1 is presented in Fig. 2. Each copper atom in 1 assumes a slightly distorted square pyramidal geometry, with the carboxylate oxygen atoms of four linkers coordinated at the basal positions and one DMF oxygen atom occupying the apical position. The Cu–Cu distance in each paddle˚ . Adjacent paddlewheels are linked by L1 wheel is 2.632 A to form a distorted square grid motif with void dimensions
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Table 1 Crystal data and structure refinement summary for 1 Empirical formula
C19H14CuN2O72.64(DMF)
Formula weight
639.95
Temperature (K) ˚) Wavelength (A
1.54178
Crystal system
Monoclinic
Space group ˚) a (A
18.5176 (6)
100 (2)
C2/c
˚) b (A ˚ c (A)
23.7473 (7)
b (°)
107.527 (2)
18.1998 (5)
˚ 3) Volume (A
7631.7 (4)
Z
8
Calculated density (g/cm3)
1.114
Absorption coefficient (mm-1)
1.222
F(000) Crystal size (mm3)
2666 0.10 9 0.08 9 0.06
Fig. 3 The distorted square grid motif in 1 (created with Materials Studio (MS Modeling) version 4.0)
h range for data collection (°)
3.12–65.87
Limiting indices (h, k, l)
(-20/21, -27/27, -21/21)
˚ 2, respectively while an ˚ 2 and 10.8 9 10.8 A 10.3 9 10.3 A analogous 1,3-bdc structure [43] contains alternating cav˚ 2 (ignoring apical pyridine ligands) ities of 10.1 9 10.4 A 2 ˚ . and 6.4 9 6.4 A Caffeine was included in the synthesis of 1 because its structure and molecular recognition features make it representative of natural products and pharmaceutically relevant compounds. In addition, caffeine is readily identifiable by a variety of instrumental methods including IR and UV/ Vis spectroscopy, gas chromatography, and x-ray crystallography. The study of caffeine sorption from solution by MOMs therefore serves as a benchmark and will aid in developing an understanding of specific host–guest interactions for the design of new MOM sorbents. The distorted hourglass shape of the cavities in the grids and the undulating nature of the grids in 1 can be attributed to the geometry of L1. If L1 is simplified to two vectors that extend from each carboxylate carbon atom and meet at the centroid of the phenyl ring which is fused to the imide ring, the approximate angle subtended between the vectors is 147°. The torsional angles present in L1 also play an important role in defining the structure of 1. While the torsional angle between the carboxylate group attached to the N-phenyl ring and the phenyl ring itself is nearly zero, that between the other carboxylate of L1 and the phenyl ring of the bicyclic portion to which it is attached is 19.8°. Also, the five-membered imide ring and the N-phenyl ring of L1 are oriented at a torsional angle of 43.0–53.7° with respect to one another. This angle cannot be precisely measured because the imide nitrogen atom is not ideally trigonal planar. The geometry and conformation of L1 result in adjacent paddlewheels being inclined at an angle of 23.8° relative to one another. As mentioned earlier, the sheets stack in an ABCDA arrangement in the c direction. The interlayer separation, as
Reflections collected/unique
25165/6503 [Rint = 0.1062]
Completeness to h = 65.87°
97.9%
Absorption correction
Semi-empirical from equivalents
Max and min transmission
0.9303 and 0.8876
Refinement method
Full-matrix least-squares on F2
Data/restraints/parameters
6503/34/371
Goodness-of-fit on F2
1.023
Final R indices [I [ 2r (I)]
R1 = 0.0775, wR2 = 0.2084
R indices (all data) Largest different peak and hole (e/ ˚ 3) A
R1 = 0.1053, wR2 = 0.2261 0.750 and -0.552
Fig. 2 ORTEP view of the asymmetric unit of [Cu(L1)(DMF)]n (2.64 DMF), 1, with solvent omitted and ellipsoids at 30% probability level (created with Ortep-3, version 2.2)
˚ 2 (excluding van der Waals radii) as of ca. 18.8 9 12.3 A depicted in Fig. 3. The void space within the squares of 1 is substantially greater than that in prototypal square grids containing 1,4- and 1,3-bdc. For comparison, MOF-2 and its copper analogue possess void dimensions of
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calculated by dividing the distance between paddlewheels ˚ . The distance in nearest ‘A’ layers by four, is 7.40 A ˚, between nearest copper atoms in adjacent layers is 7.59 A while the methyl groups of apically-coordinated DMF molecules extend nearly into the square voids of the layers above and below. Stacking of the layers results in square channels (Fig. 4) with dimensions of approximately ˚ 2 (not taking into account van der Waals 11.9 9 11.9 A ˚ 2 with van radii) extending along the c axis (8.7 9 8.7 A der Waals). Free DMF molecules occupy the void space of 1, which exhibits a solvent-accessible volume of 60.1% as determined by Platon [42]. An analysis of 11 square grid structures [44, 45] formed by the square paddlewheel and 1,4-bdc linkers reveals that the number of sheets before repetition of the stacking arrangement varies from 2 to 8 (mean = 4.4), with inter˚ (mean = 6.6 A ˚) layer distances ranging from 5.1 to 9.4 A [46]. It is apparent upon examination of these structures that, in general, an inverse relationship exists between stacking number and interlayer distance. In other words, a higher stacking number is consistent with more efficient packing between layers. Meanwhile, examination of 14 undulating grids containing 1,3-bdc linkers [47] reveals that the stacking number varies from 1 to 4 (mean = 2.8) ˚ while interlayer distances range from 7.7 to 10.6 A ˚ (mean = 9.0 A). Judging by mean interlayer distance, 1,4-bdc grids pack more efficiently than their 1,3-bdc counterparts. This observation is perhaps related to the undulating nature of 1,3-bdc grids and/or the nature of the axial ligands. The majority of known 1,3-bdc grids possess bulkier pyridyl-type axial ligands while 1,4-bdc structures contain smaller axial ligands like DMF, water, and methanol. As mentioned previously, there are eight supramolecular isomers (i.e. topologies) that have been observed when 1,3bdc links square paddlewheels and four when 1,4-bdc serves as the linker. In the case of 1,3-bdc, possible topologies include kagome´ [48], square grid [43, 49–52], nanoball-1 [53, 54], nanoball-2 [54], usf [55], CdSO4 [55], mot [50], and NbO. In the case of 1,4-bdc, the known topologies are kagome´ [56], square grid [28], CdSO4 [50], and NbO [57]. Linker geometry in these isomers can be quantified by three parameters: the bending angle between carboxylate moieties (1,3-bdc = 120°, 1,4-bdc = 180°), the twisting angle between carboxylate planes, and the bending angle between carboxylate planes and the benzene plane [50]. The 147° bend angle between carboxylates in L1 lies roughly midway between that in 1,3- and 1,4-bdc, meaning that L1 may have the potential to form isomers that have been observed with either of the prototypal linkers. In summary, we have prepared a MOM, 1, which is sustained by copper (II) paddlewheel nodes and an angular
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Fig. 4 View of the square channels running along the c axis in 1 (created with Materials Studio (MS Modeling) version 4.0)
dicarboxylate linker, L1. The linker, which can be synthesized in a solvent-free manner, creates a distortion of the grid motif due to its distinct geometry and conformation. The grids themselves have significantly expanded dimensions in comparison with those of prototypal square grids based upon 1,3- or 1,4-bdc. Stacking of the layers in 1 generates square channels which have the potential to accommodate a wide variety of guest species. Finally, we note that the intermediate bend angle of L1 may endow it with the potential to form paddlewheel-based supramolecular isomers that are accessible by either of the prototypal linkers.
Supplementary Data Supplementary crystallographic data for [Cu(L1)(DMF)]n (2.64 DMF) is contained in CCDC 823167. Copies of this information may be obtained free of charge via http://www. ccdc.cam.ac.uk/data_request/cif (or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax: ?44(0)1233-336033; e-mail:
[email protected]). Acknowledgment This work was generously supported by the Department of Defense—Defense Threat Reduction Agency (DoD-DTRA) through HDTRA1-08-C-0035.
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