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Inorganic–organic hybrid materials based on keggin type polyoxometalates and organic polyammonium cations
Journal of Molecular Structure 656 (2003) 27–35 www.elsevier.com/locate/molstruc
Inorganic –organic hybrid materials based on keggin type polyoxometalates and organic polyammonium cations Maxym Vasylyeva, Ronit Popovitz-Birob, Linda J.W. Shimonc, Ronny Neumanna,* a
b
Department of Organic Chemistry, Weizmann Institute of Science, Rehovat 76100, Israel Department of Materials and Interfaces, Weizmann Institute of Science, Rehovat 76100, Israel c Division of Chemical Services, Weizmann Institute of Science, Rehovat 76100, Israel Received 27 December 2002; revised 28 January 2003; accepted 28 January 2003 Dedicated to Professor Achim Mu¨ller on the occasion of his 65th birthday
Abstract Co-crystallization of a tri-ammonium cation with short and somewhat flexible ‘arms’, [N,N,N-tris[2-(dimethylamino)ethyl]1,3,5-benzenetricarboxamide]3þ, with a polyoxometalate trianion, PW12O32 40 , yielded an insoluble channeled or microporous structure. The polyoxometalate clusters are arranged in a layered and zig-zag fashion along the xy plane. Looking along the x˚ are observed. It was found that C– H· · ·O bonds aided in determining the axis, channels of a dimension of , 3.5 £ , 6.5 A crystal packing by providing directionality to the anion– cation interaction. On the other hand the co-crystallization of a tetraammonium cation with an extended and rigid tetrahedral configuration, 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4yl)vinyl]phenyl adamantane tetraiodide, with a polyoxometalate tetracation, SiW12 O42 40 ; yielded a lamellar structure with ˚ of the inorganic – organic hybrid material. alternating layers with spacing of 16.6 A q 2003 Elsevier B.V. All rights reserved. Keywords: Polyoxometalate; Ammonium cation; Microporous material; Lamellar structure
1. Introduction The design, synthesis and structural characterization of new hybrid materials, of which many applications can be predicted, through the assembly of organic and inorganic building blocks, is a highly visible research area [1]. The construction of such organic –inorganic hybrid compounds has established * Corresponding author. Tel.: þ972-8-934-3354; fax: þ 972-8934-4142. E-mail address: [email protected] (R. Neumann).
new areas of research in the chemistry of materials that is based upon a bridge between organic and inorganic chemistry and is useful in order to obtain multifunctional materials which exhibit coexistence of solid-state magnetic, electric and/or optical properties [2]. The synthesis of organic – inorganic hybrid compounds using polyoxometalate clusters as nanobuilding blocks is an attractive idea that can utilize the myriad and potentially important functions of polyoxometalates in areas as diverse as catalysis, molecular magnetism, photochemistry and medicine [3]. In the past, polyoxometalate based hybrid compounds have been constructed either by creation of
0022-2860/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-2860(03)00330-2
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electrostatic interactions between the inorganic and organic components or of by formation of covalent bonds between the organic and inorganic moieties. Examples of assemblies based upon electrostatic interactions are salts obtained from polyoxometalates and tetrathiafulvalenes [4] or planar aromatic donors such as anthracene and perylene derivatives [5], polyoxometalates immobilized on conductive polymers [6] or sol – gel matrices [7] and many others. [8] Polyoxometalates are also important as building units of supramolecular complexes since they can exhibit diverse self-assembly properties, thus controlling the formation of n-dimensional organic – inorganic hybrid networks in self-organization processes. This approach was successively applied to the fabrication of well-defined polyoxometalate containing films using Langmuir –Blodgett [9] and layer-bylayer deposition techniques [10]. As noted, polyoxometalates have potential self-assembly properties but in order to utilize these capabilities it is important to correctly design an organic component for the hybrid material synthesis. Unfortunately, polyoxometalates often precipitate quickly and non-selectively in the presence of organic multiply charged counter cations generally leading to the formation of insoluble amorphous solids. In addition, since the negative charge of the polyoxometalates is homogeneously distributed around the oxo cluster prediction of the directionality of the polyoxometalate –organic cation base interaction and thus prediction of the structure to be obtained is problematic. In this paper we show that the co-crystallization of polyammonium cations with polyoxometalate anions can lead to very different results depending on the structure of the polycation and its charge. In the first case, a tri-ammonium cation with a planar core and short and somewhat flexible ‘arms’ co-crystallized with a trianionic Keggin polyoxometalate to yield an insoluble two-dimensional microporous or channeled structure. The polyoxometalate clusters are arranged in a layered and zig-zag fashion along the xy plane. Looking along the x-axis, channels or micropores of a ˚ £ , 6.5 A ˚ are observed. In the dimension of , 3.5A second case, a tetraammonium cation with an extended, rigid tetrahedral structure yielded, upon crystallization with tetranionic Keggin polyoxometalate, a lamellar structure instead of a microporous one.
˚ , which is the approxiThe layer spacing was 16.6 A mate diameter of the tetracation. 2. Experimental 2.1. Materials and methods Reagents need for the synthesis of the polyammonium cation were obtained from Aldrich or Acros and used as received. The 1H NMR and 13C NMR spectra were measured on a Bruker DPX 250 spectrometer at 250 MHz in CDCl3 or DMSO-d6 and the chemical shifts are reported with reference to an internal tetramethylsilane standard. The IR spectra were measured on a Nicolet Prote´ge´ 460 FTIR; samples were deposited on a KBr disk or prepared as KBr based pellets. A Philips CM-120 transmission electron microscope, with accelerating voltage of 120 kV and tungsten filament was used to obtain the morphology and the electron diffraction pattern of the lamellar structures. The TEM samples were prepared by placing a drop of sonicated suspension of the hybrid material in EtOH on carbon/collodion coated copper grid. Under the experimental conditions used, there were no signs of crystal degradation. To obtain the onaxis diffraction pattern, crystallites with a proper orientation with respect to the TEM beam were chosen. 2.2. Crystallographic data collection and structure determination The data was collected on colorless monoclinic needles, cut to size 0.1 £ 0.1 £ 0.1 mm3 [3], using a Nonius-Kappa CCD diffractometer using graphite radiation. monochromated Mo Ka ðl ¼ 0:71073 AÞ 32 For the PW 12O 40 – N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex, 14,734 reflections and 12,735 independent reflections over a range of 2u ¼ 2:73 – 26:76 were collected with 0 # h # 16; 0 # k # 29; 228 # l # 28: The data were processed with Denzo-Scalepack [11]. The structures were solved by direct methods with SHELXS -97. Full-matrix least-squares refinement was based on F 2 with SHELX -97. Idealized hydrogen atoms were placed and refined in a riding model. The crystallographic data are presented in Table 1.
M. Vasylyev et al. / Journal of Molecular Structure 656 (2003) 27–35 Table 1 Crystal and structure refinement data for the PW12O32 40 – N,N,Ntris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex Empirical formula C33H51N12O43PW12 Formula weight 3541.03 Space group P2ð1Þ=n Temperature (K) 120 (2) ˚ l 0.71073 A ˚) a (A 12.954 (3) ˚) b (A 23.654 (5) ˚) c (A 22.845 (5) b (8) 93.18 (3) ˚ 3) V (A 6989 (2) Z 4 dcalcd (mg/cm3) 3.365 m (mm21) 19.782 Final Rindices R ¼ 0:0647 ½I . 2sðIÞ Rw ¼ 0:1770 P P P P R ¼ kF0 l 2 lFc k= lF0 l Rw ¼ ½lF0 2 Fc lÞw1=2 = ½F0 w1=2 :
2.3. N,N,N-Tris[2-(dimethylamino)ethyl]-1,3, 5-benzenetricarboxamide, 1 A solution of 0.5 g (1.88 mmol) of 1,3,5benzenetricarbonyl trichloride in 10 ml of methylene chloride was added to a solution of 0.5 g (0.62 ml, 5.63 mmol) of N,N-dimethylethylendiamine in 10 ml of methylene chloride over a period of 20 min with rapid stirring. The mixture was stirred for another 6 h and the solvent was removed at aspirator pressure (rotovap). The remaining solid was stirred with 15 ml of chloroform and 0.5 g of granular anhydrous sodium carbonate for 5 min. The mixture was filtered off and concentrated (rotovap) until only a few millilitres remained. Diethyl ether was then added to precipitate the product that was filtered off and recrystallized twice from a minimal amount of ethanol where diethyl ether was carefully added to initiate crystallization. The final yield was 0.4 g (51%) of a white solid, mp 115 –116 8C, 1H NMR (d6DMSO) d : 2.79 (18H, s), 3.28 (6H, t, J ¼ 5 Hz), 3.64 (6H, t, J ¼ 5 Hz), 8.69 (3H, s), 9.19 (3H, br t, J ¼ 5 Hz) 13C NMR (DMSO-d6) d : 35.08, 42.81, 55.98, 129.71, 134.51, 166.12 ppm. IR (KBr) n ¼ 3383; 3222, 3093, 3066, 1647, 1550, 1471, 1445, 1405, 1363, 1303, 1168, 988, 918 cm21.
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2.4. PW12O32 40 – N,N,N-tris[2-(dimethylamino) ethyl]-1,3,5-benzenetricarboxamidecomplex, 2 Ten millilitres of a 0.1 mM solution of the organic component in acetonitrile was slowly added to 10 ml of a 0.1 mM solution of phosphotungstic acid (H3PW12O40) in the same solvent. Within 3 days colorless monoclinic needles suitable for single crystal X-ray crystallography analysis were formed. IR (KBr) n ¼ 3529; 3134, 2961, 2927, 2761, 1652, 1465, 1417, 1384, 1366, 1331, 1236, 1062, 958, 879, 797, 736 cm21. 2.5. 1,3,5,7-Tetrakis{4-[(E)-2-pyridin-4yl-vinyl]phenyl}adamantane, 5 A mixture of 1.0 g (1.1 mmol) of 2, 0.02 g (0.028 mmol) palladium(II)chloride·bis(triphenylphosphane) were placed into a Schlenk flask and dissolved in 4 ml of N-methyl pyrrolidone (NMP). The flask was evacuated and refilled with Ar and 1 ml (9.25 mmol) of 4-vinylpyridine and 3 ml of triethylamine were added. The flask was sealed and heated (oil bath) at 80 8C over a period of 12 h with rapid stirring. The mixture was poured into 100 ml of water and the precipitate was filtered off, washed with methanol and ether, dissolved in 5 ml of chloroform and filtered. The solvent was removed from the filtrate at aspirator pressure and the remaining solid was stirred with 30 ml of ether, filtered off and dried under high vacuum for 2.5 h. The final yield was 0.57 g (63.1 %) of a pale-yellow solid, mp 282 –285 8C (decomp), 1H NMR (CDCl3) d : 2.22 (12H, br s), 7.01 (4H, d, JHH ¼ 16:3 Hz), 7.31 (4H, d, JHH ¼ 16:3 Hz), 7.37 (8H, d, JHH ¼ 6:2 Hz), 7.54 (16H, br s), 8.57 (8H, d, JHH ¼ 6:1 Hz) ppm. 13C NMR (CDCl3) d : 39.29, 46.96, 120.80, 125.52, 125.68, 127.11, 132.64, 134.25, 144, 61, 149.77, 150.09 ppm. 2.6. 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium4-yl)vinyl]phenyl adamantane tetraiodide, 3 To a solution of 0.140 g (0.16 mmol) 3 in 5 ml of chloroform and 1 ml of DMF, an excess of iodomethane (2.5 ml, 40.2 mmol) was added over 15 min with rapid stirring. Stirring was continued for 3 h. The yellow precipitate was filtered off, washed twice with ether and dried under high vacuum for 2.5 h.
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The yield was 0.21 g (93%) of a yellow solid, mp 290– 293 8C (decomp) 1H NMR (DMSO-d6) d: 2.18 (12H, br s), 4.24 (12H, s), 7.51 (4H, d, J ¼ 16:5 Hz), 7.75 (16H, br s), 8.03 (4H, d, J ¼ 16:5 Hz), 8.22 (4H, d, J ¼ 6:7 Hz), 8.85 (4H, d, J ¼ 6:7 Hz) ppm. 13C NMR (DMSO-d6) d : 41.53, 46.10, 47.31, 123.00, 123.80, 126.60, 128.53, 133.38, 140.88, 145.47, 152.53, 152.96 ppm. IR (KBr) n ¼ 3426; 3118, 3087, 3028, 2925, 2898, 2849, 1642, 1618, 1600, 1559, 1517, 1468, 1447, 1412, 1337, 1211, 1178, 975, 880, 835, 553, 521 cm21. 2.7. SiW12O42 40 – 1,3,5,7-tetrakis{4-[(E)-2(Nmethylpyridinium-4-yl)vinyl]phenyl adamantane complex 0.014 g (0.0099 mmol) of 1,3,5,7-tetrakis{4-[(E)2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane tetraiodide was dissolved in 10 ml of DMSO and added to a solution of [(C4H6)4Nþ]4(SiW12O40) (0.036 g, 0.0099 mmol) in 10 ml DMSO. IR (KBr) n ¼ 3128; 3049, 3006, 2915, 2852, 1643, 1619, 1599, 1559, 1518, 1470, 1434, 1413, 1384, 1337, 1186, 1015, 969, 923, 882, 792, 708, 667, 553, 531 cm21. 3. Results and discussion 3.1. General considerations and synthesis Polyoxometalates in general and the Keggin polyoxometalates used in this research are polyanions in which the negative charge is randomly distributed around the polyoxometalate cluster. This random distribution of the negative charge limits our ability to predict the topology attainable upon electrostatic interaction between a polyoxometalate (heteropolyanion) and a polyammonium cation. With this in mind, we thought despite this that the co-crystallization of a relatively compact or alternatively rigid ammonium based polycation with a polyanioic polyoxometalate may enable the directional control of the anion– cation packing arrangement. The trication precursor, N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide, 1, was synthesized from 1,3,5-benzenetricarbonyl trichloride and N,N-dimethylethylendiamine, Scheme 1. Crystallization of the inorganic trianionic –organic tricationic hybrid compound was accomplished using highly
Scheme 1. Synthesis of N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5benzenetricarboxamide, 1.
diluted solutions of phosphotungstic acid, H3 PW 12 O 40, and N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide, in a solvent, acetonitrile, with a relatively weak potential for crystal organization in order to minimize any solvent dependence on the packing arrangement obtained. As a result, thin colorless needles of PW12O32 40 –N,N,Ntris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex, 2, suitable for single crystal X-ray diffractometry were obtained after 3 days of slow precipitation. The IR spectrum of 2 showed that the polyoxometalate anion (1080, 982, 890, 806 cm21) and organic component (nCyO 1647(amide) cm21) were retained in the molecular structures. It has been found that in rigid polyphenylene tetrahedral molecules, the nonparallel arrangement of such a molecules and non-fluxional core inhibit crystallization as a result of weak interchain coupling [12]. With this in mind we designed and synthesized a tetrahedrally shaped quaternary ammonium salt based on an adamantane core, 1,3,5,7-tetrakis{4-[(E)-2(Nmethylpyridinium-4-yl)vinyl] phenyl adamantane tetraiodide, 3, Scheme 2. The design of a rigid tetrahedral shaped cation with relatively large distances between the nitrogen atoms of the pyridine moiety was predicted to prevent a close packing arrangement of the polyoxometalate anions, slow down the selfassembly process and also possibly impart directionality of anion –cation interaction. Therefore, 1,3,5,7tetrakis{4-[(E)-2-pyridin-4-yl-vinyl]phenyl} adamantane, 5, was synthesized from 1,3,5,7-tetrakis(4iodophenyl)adamantane, 4, and 4-vinyl pyridine by a palladium catalyzed Heck reaction. Methylation yielded the desired quaternary pyridinium salt, 3. Cocrystallization of the tetrabutylammonium salt of tungstosilicic acid (tetraanion, Keggin type polyoxometalate) with an equivalent amount of 3, preferably in DMSO, yielded bright yellow clusters of crystal like material after one and a half days of slow precipitation. The IR spectra of the complex shows that both the polyoxometalate anion [SiW12O40]4þ (1015, 969, 923,
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31
Scheme 2. Synthesis of 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane tetraiodide. 3.
882, 792 cm21) and 3 (nCyC 1643, 1619, 1599; nCyN 1559, 1518 cm21) were retained in the hybrid material. 3.2. Crystal structure of the PW12 O32 40 – N,N,N-tris[2(dimethylamino)ethyl]1,3,5-benzenetricarboxamide complex The structure of the polyoxometalate cluster, PW12O32 40 , has W –O, WyO, and PyO bond lengths
and bond angles in very close agreement to those described in the literature [13]. The network formed in the crystal structure, Fig. 1, may be described as follows. Along the x-axis there are alternating layers of the polyoxometalate and the N,N,N-tris[2(dimethylamino)ethyl] derivative where the aromatic ring is centered above and below the polyoxometalate, and the plane of the aromatic ring is parallel to the polyoxometalate moiety. The mean distance between
Fig. 1. View of the unit cell of the PW12O32 40 –N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex along the x-axis (the acetonitrile solvent molecules are omitted for clarity).
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Fig. 2. View of PW the PW12O32 40 – N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex along the y-axis (only the polyoxometalate cluster is shown for clarity).
Fig. 3. View of the channel or micropore in the PW12O32 40 –N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex.
M. Vasylyev et al. / Journal of Molecular Structure 656 (2003) 27–35
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Fig. 4. View of atoms Involved in the polyoxometalate–polyammonium cation interaction.
the plane of the aromatic ring and the surface of the ˚ indicating a polyoxometalate is approximately 3.6 A weak p – d interaction between the aromatic nucleus and the polyoxometalate. A view along the y-axis clearly reveals that the polyoxometalate molecules are in a zig-zag and layered arrangement in the x – y plane, Fig. 2. An extension of the view along the x-axis, Fig. 3, reveals a channeled structure along this axis, formed due to the undulation of polyoxometalate cluster and the short length of the cationic ammonium ‘arms’ required to electrostatically stabilize the structure. The dimensions based on space filling models with van der Waals radii of these channels or micropores (closest O – O distances between two polyoxometalate molecules) are approximately 3.5 ˚ . These channels contain disordered acetoby 6.5 A nitrile solvent molecules that were omitted for clarity. Another interesting observation, concerning the crystal structure, deals with the interaction of the ‘arm’ of the ammonium cation and the neighboring polyoxometalate cluster, Fig. 4. Although the proton
originating from the phosphotungstic acid is not resolved in the structure, other hydrogen atoms covalently bonded to carbon atoms adjacent to the ammonium center were resolved. In Table 2 are given bond distances between these hydrogen atoms and their associated carbon atoms and the nearest lying Table 2 Selected shortest intermolecular C – H· · ·O distances for the PW12O32 40 – N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex
H10B–O37 H10B–O20 H10A–O34 H10A–O20 H9C–O34 H9C–O15 H8B–O27 H8B–O15
H· · ·O ˚) distance (A
C· · ·O ˚) distance (A
C –H· · ·O angle (8)
2.965 2.881 2.730 2.633 2.565 2.656 2.463 2.739
3.673 3.174 3.402 3.174 3.429 3.374 3.410 3.437
131.57 98.88 127.56 116.07 149.96 131.97 165.19 129.33
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3.3. Lamellar structure of the SiW12O42 40 – 1,3,5,7tetrakis{4-[(E)-2(N-methylpyridinium-4yl)vinyl]phenyl adamantane complex [17] As noted above, co-crystallization of the tetrabutylammonium salt of tungstosilicic acid with an equivalent amount of 3, yielded bright yellow clusters, Fig. 5, of the SiW12O42 40 –1,3,5,7-tetrakis{4[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane complex after one and a half days of slow precipitation. Unfortunately all attempts to obtain large, defect free crystals suitable for X-ray diffraction analysis were unsuccessful, but the investigation of Fig. 5. Picture of microcrystals of the SiW12O42 40 – 1,3,5,7tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane material.
oxygen atoms of the polyoxometalate. Hydrogen – ˚ are measured. oxygen bond distances of 2.46 – 2.96 A The protonation of the tertiary amino group leads to the weak polarization of adjacent C – H bonds. This polarization appears to be sufficient for formation of C – H· · ·O where the H atoms are the donor atoms and O atoms are the acceptor atoms. This formation of weak hydrogen bonds is forced by the electrostatic interaction between the ammonium cation and polyoxometalate anion. In the hybrid crystal we have two types of C– H· · ·O contacts. All four hydrogen atoms are involved in a so-called bifurcated [14] hydrogen bridging. Each bifurcated hydrogen bond has major component with the shorter O –H bond distances and minor component with the longer O – H bond distances. The C – H· · ·O contacts differ one from another by the identity of the oxygen atom of the polyoxometalate involved. H10B, H9C and H8B atoms interact with both terminal and bridging oxygen atoms and H10 binds only to bridging oxygen atoms, Fig. 4. Following Jeffrey’s [15] classification of hydrogen bonds C – H· · ·O contacts are of a weak ˚ ) and weak directionality type (bond length . 2.2 A (bond angle . 908). It is also known that the shorter the C –H· · ·O hydrogen bonds the more likely its angle is to be close to 1808 [14 –16]. In our case the ˚ with u ¼ 1658 shortest contacts are 2.463 and 2.469 A which is close to 1808. Average C – H· · ·O bond length ˚ ðu . 908Þ and thus we can consider this as are , 3 A crystallographic evidence of C –H· · ·O bonding.
Fig. 6. Transmission electron micrograph of the SiW12O42 40 – 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane material. Insert: electron diffraction pattern.
M. Vasylyev et al. / Journal of Molecular Structure 656 (2003) 27–35
the morphology and two-dimensional electron diffraction pattern of the microcrystals was possible and used to determine the molecular packing arrangement, Fig. 6. As may be seen in the figure, clearly a lamellar structure for the polyoxometalate-3 hybrid material was obtained, with alternating layers o the polyoxometalate (dark lines) and the polycation 3 (light lines). The electron diffraction pattern along the principle axis of the crystallites was measured, Fig. 6 insert. The electron diffraction pattern confirms the lamellar structure and leads to a calculated layer ˚ . This value is in good separation of 16.6 ^ 0.2 A agreement with the calculated diameter of 3, lending strong proof to the formation of a lamellar structure in the SiW12O42 40 – 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane hybrid material with alternating polyoxometalate and tetracation 3 layers.
[3] [4] [5]
[6]
[7]
[8]
4. Conclusion Even though the negative charge of the polyoxometalates is homogeneously distributed around the oxo cluster, ordered structures can be obtained via co-crystallization with polyammonium counter cations. In one case, use of a cation with a planar core and short and somewhat flexible ‘arms’ yielded a channeled structure of high crystallinity. In another case, use of a polycation with an extended and rigid tetrahedral configuration led to a lamellar structure. Acknowledgements This research was supported by the Minerva Foundation and the Kimmel Center for Molecular Design. RN is the Israel and Rebecca Sieff Professor of Organic Chemistry.
[9]
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Inorganic –organic hybrid materials based on keggin type polyoxometalates and organic polyammonium cations Maxym Vasylyeva, Ronit Popovitz-Birob, Linda J.W. Shimonc, Ronny Neumanna,* a
b
Department of Organic Chemistry, Weizmann Institute of Science, Rehovat 76100, Israel Department of Materials and Interfaces, Weizmann Institute of Science, Rehovat 76100, Israel c Division of Chemical Services, Weizmann Institute of Science, Rehovat 76100, Israel Received 27 December 2002; revised 28 January 2003; accepted 28 January 2003 Dedicated to Professor Achim Mu¨ller on the occasion of his 65th birthday
Abstract Co-crystallization of a tri-ammonium cation with short and somewhat flexible ‘arms’, [N,N,N-tris[2-(dimethylamino)ethyl]1,3,5-benzenetricarboxamide]3þ, with a polyoxometalate trianion, PW12O32 40 , yielded an insoluble channeled or microporous structure. The polyoxometalate clusters are arranged in a layered and zig-zag fashion along the xy plane. Looking along the x˚ are observed. It was found that C– H· · ·O bonds aided in determining the axis, channels of a dimension of , 3.5 £ , 6.5 A crystal packing by providing directionality to the anion– cation interaction. On the other hand the co-crystallization of a tetraammonium cation with an extended and rigid tetrahedral configuration, 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4yl)vinyl]phenyl adamantane tetraiodide, with a polyoxometalate tetracation, SiW12 O42 40 ; yielded a lamellar structure with ˚ of the inorganic – organic hybrid material. alternating layers with spacing of 16.6 A q 2003 Elsevier B.V. All rights reserved. Keywords: Polyoxometalate; Ammonium cation; Microporous material; Lamellar structure
1. Introduction The design, synthesis and structural characterization of new hybrid materials, of which many applications can be predicted, through the assembly of organic and inorganic building blocks, is a highly visible research area [1]. The construction of such organic –inorganic hybrid compounds has established * Corresponding author. Tel.: þ972-8-934-3354; fax: þ 972-8934-4142. E-mail address: [email protected] (R. Neumann).
new areas of research in the chemistry of materials that is based upon a bridge between organic and inorganic chemistry and is useful in order to obtain multifunctional materials which exhibit coexistence of solid-state magnetic, electric and/or optical properties [2]. The synthesis of organic – inorganic hybrid compounds using polyoxometalate clusters as nanobuilding blocks is an attractive idea that can utilize the myriad and potentially important functions of polyoxometalates in areas as diverse as catalysis, molecular magnetism, photochemistry and medicine [3]. In the past, polyoxometalate based hybrid compounds have been constructed either by creation of
0022-2860/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-2860(03)00330-2
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electrostatic interactions between the inorganic and organic components or of by formation of covalent bonds between the organic and inorganic moieties. Examples of assemblies based upon electrostatic interactions are salts obtained from polyoxometalates and tetrathiafulvalenes [4] or planar aromatic donors such as anthracene and perylene derivatives [5], polyoxometalates immobilized on conductive polymers [6] or sol – gel matrices [7] and many others. [8] Polyoxometalates are also important as building units of supramolecular complexes since they can exhibit diverse self-assembly properties, thus controlling the formation of n-dimensional organic – inorganic hybrid networks in self-organization processes. This approach was successively applied to the fabrication of well-defined polyoxometalate containing films using Langmuir –Blodgett [9] and layer-bylayer deposition techniques [10]. As noted, polyoxometalates have potential self-assembly properties but in order to utilize these capabilities it is important to correctly design an organic component for the hybrid material synthesis. Unfortunately, polyoxometalates often precipitate quickly and non-selectively in the presence of organic multiply charged counter cations generally leading to the formation of insoluble amorphous solids. In addition, since the negative charge of the polyoxometalates is homogeneously distributed around the oxo cluster prediction of the directionality of the polyoxometalate –organic cation base interaction and thus prediction of the structure to be obtained is problematic. In this paper we show that the co-crystallization of polyammonium cations with polyoxometalate anions can lead to very different results depending on the structure of the polycation and its charge. In the first case, a tri-ammonium cation with a planar core and short and somewhat flexible ‘arms’ co-crystallized with a trianionic Keggin polyoxometalate to yield an insoluble two-dimensional microporous or channeled structure. The polyoxometalate clusters are arranged in a layered and zig-zag fashion along the xy plane. Looking along the x-axis, channels or micropores of a ˚ £ , 6.5 A ˚ are observed. In the dimension of , 3.5A second case, a tetraammonium cation with an extended, rigid tetrahedral structure yielded, upon crystallization with tetranionic Keggin polyoxometalate, a lamellar structure instead of a microporous one.
˚ , which is the approxiThe layer spacing was 16.6 A mate diameter of the tetracation. 2. Experimental 2.1. Materials and methods Reagents need for the synthesis of the polyammonium cation were obtained from Aldrich or Acros and used as received. The 1H NMR and 13C NMR spectra were measured on a Bruker DPX 250 spectrometer at 250 MHz in CDCl3 or DMSO-d6 and the chemical shifts are reported with reference to an internal tetramethylsilane standard. The IR spectra were measured on a Nicolet Prote´ge´ 460 FTIR; samples were deposited on a KBr disk or prepared as KBr based pellets. A Philips CM-120 transmission electron microscope, with accelerating voltage of 120 kV and tungsten filament was used to obtain the morphology and the electron diffraction pattern of the lamellar structures. The TEM samples were prepared by placing a drop of sonicated suspension of the hybrid material in EtOH on carbon/collodion coated copper grid. Under the experimental conditions used, there were no signs of crystal degradation. To obtain the onaxis diffraction pattern, crystallites with a proper orientation with respect to the TEM beam were chosen. 2.2. Crystallographic data collection and structure determination The data was collected on colorless monoclinic needles, cut to size 0.1 £ 0.1 £ 0.1 mm3 [3], using a Nonius-Kappa CCD diffractometer using graphite radiation. monochromated Mo Ka ðl ¼ 0:71073 AÞ 32 For the PW 12O 40 – N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex, 14,734 reflections and 12,735 independent reflections over a range of 2u ¼ 2:73 – 26:76 were collected with 0 # h # 16; 0 # k # 29; 228 # l # 28: The data were processed with Denzo-Scalepack [11]. The structures were solved by direct methods with SHELXS -97. Full-matrix least-squares refinement was based on F 2 with SHELX -97. Idealized hydrogen atoms were placed and refined in a riding model. The crystallographic data are presented in Table 1.
M. Vasylyev et al. / Journal of Molecular Structure 656 (2003) 27–35 Table 1 Crystal and structure refinement data for the PW12O32 40 – N,N,Ntris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex Empirical formula C33H51N12O43PW12 Formula weight 3541.03 Space group P2ð1Þ=n Temperature (K) 120 (2) ˚ l 0.71073 A ˚) a (A 12.954 (3) ˚) b (A 23.654 (5) ˚) c (A 22.845 (5) b (8) 93.18 (3) ˚ 3) V (A 6989 (2) Z 4 dcalcd (mg/cm3) 3.365 m (mm21) 19.782 Final Rindices R ¼ 0:0647 ½I . 2sðIÞ Rw ¼ 0:1770 P P P P R ¼ kF0 l 2 lFc k= lF0 l Rw ¼ ½lF0 2 Fc lÞw1=2 = ½F0 w1=2 :
2.3. N,N,N-Tris[2-(dimethylamino)ethyl]-1,3, 5-benzenetricarboxamide, 1 A solution of 0.5 g (1.88 mmol) of 1,3,5benzenetricarbonyl trichloride in 10 ml of methylene chloride was added to a solution of 0.5 g (0.62 ml, 5.63 mmol) of N,N-dimethylethylendiamine in 10 ml of methylene chloride over a period of 20 min with rapid stirring. The mixture was stirred for another 6 h and the solvent was removed at aspirator pressure (rotovap). The remaining solid was stirred with 15 ml of chloroform and 0.5 g of granular anhydrous sodium carbonate for 5 min. The mixture was filtered off and concentrated (rotovap) until only a few millilitres remained. Diethyl ether was then added to precipitate the product that was filtered off and recrystallized twice from a minimal amount of ethanol where diethyl ether was carefully added to initiate crystallization. The final yield was 0.4 g (51%) of a white solid, mp 115 –116 8C, 1H NMR (d6DMSO) d : 2.79 (18H, s), 3.28 (6H, t, J ¼ 5 Hz), 3.64 (6H, t, J ¼ 5 Hz), 8.69 (3H, s), 9.19 (3H, br t, J ¼ 5 Hz) 13C NMR (DMSO-d6) d : 35.08, 42.81, 55.98, 129.71, 134.51, 166.12 ppm. IR (KBr) n ¼ 3383; 3222, 3093, 3066, 1647, 1550, 1471, 1445, 1405, 1363, 1303, 1168, 988, 918 cm21.
29
2.4. PW12O32 40 – N,N,N-tris[2-(dimethylamino) ethyl]-1,3,5-benzenetricarboxamidecomplex, 2 Ten millilitres of a 0.1 mM solution of the organic component in acetonitrile was slowly added to 10 ml of a 0.1 mM solution of phosphotungstic acid (H3PW12O40) in the same solvent. Within 3 days colorless monoclinic needles suitable for single crystal X-ray crystallography analysis were formed. IR (KBr) n ¼ 3529; 3134, 2961, 2927, 2761, 1652, 1465, 1417, 1384, 1366, 1331, 1236, 1062, 958, 879, 797, 736 cm21. 2.5. 1,3,5,7-Tetrakis{4-[(E)-2-pyridin-4yl-vinyl]phenyl}adamantane, 5 A mixture of 1.0 g (1.1 mmol) of 2, 0.02 g (0.028 mmol) palladium(II)chloride·bis(triphenylphosphane) were placed into a Schlenk flask and dissolved in 4 ml of N-methyl pyrrolidone (NMP). The flask was evacuated and refilled with Ar and 1 ml (9.25 mmol) of 4-vinylpyridine and 3 ml of triethylamine were added. The flask was sealed and heated (oil bath) at 80 8C over a period of 12 h with rapid stirring. The mixture was poured into 100 ml of water and the precipitate was filtered off, washed with methanol and ether, dissolved in 5 ml of chloroform and filtered. The solvent was removed from the filtrate at aspirator pressure and the remaining solid was stirred with 30 ml of ether, filtered off and dried under high vacuum for 2.5 h. The final yield was 0.57 g (63.1 %) of a pale-yellow solid, mp 282 –285 8C (decomp), 1H NMR (CDCl3) d : 2.22 (12H, br s), 7.01 (4H, d, JHH ¼ 16:3 Hz), 7.31 (4H, d, JHH ¼ 16:3 Hz), 7.37 (8H, d, JHH ¼ 6:2 Hz), 7.54 (16H, br s), 8.57 (8H, d, JHH ¼ 6:1 Hz) ppm. 13C NMR (CDCl3) d : 39.29, 46.96, 120.80, 125.52, 125.68, 127.11, 132.64, 134.25, 144, 61, 149.77, 150.09 ppm. 2.6. 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium4-yl)vinyl]phenyl adamantane tetraiodide, 3 To a solution of 0.140 g (0.16 mmol) 3 in 5 ml of chloroform and 1 ml of DMF, an excess of iodomethane (2.5 ml, 40.2 mmol) was added over 15 min with rapid stirring. Stirring was continued for 3 h. The yellow precipitate was filtered off, washed twice with ether and dried under high vacuum for 2.5 h.
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The yield was 0.21 g (93%) of a yellow solid, mp 290– 293 8C (decomp) 1H NMR (DMSO-d6) d: 2.18 (12H, br s), 4.24 (12H, s), 7.51 (4H, d, J ¼ 16:5 Hz), 7.75 (16H, br s), 8.03 (4H, d, J ¼ 16:5 Hz), 8.22 (4H, d, J ¼ 6:7 Hz), 8.85 (4H, d, J ¼ 6:7 Hz) ppm. 13C NMR (DMSO-d6) d : 41.53, 46.10, 47.31, 123.00, 123.80, 126.60, 128.53, 133.38, 140.88, 145.47, 152.53, 152.96 ppm. IR (KBr) n ¼ 3426; 3118, 3087, 3028, 2925, 2898, 2849, 1642, 1618, 1600, 1559, 1517, 1468, 1447, 1412, 1337, 1211, 1178, 975, 880, 835, 553, 521 cm21. 2.7. SiW12O42 40 – 1,3,5,7-tetrakis{4-[(E)-2(Nmethylpyridinium-4-yl)vinyl]phenyl adamantane complex 0.014 g (0.0099 mmol) of 1,3,5,7-tetrakis{4-[(E)2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane tetraiodide was dissolved in 10 ml of DMSO and added to a solution of [(C4H6)4Nþ]4(SiW12O40) (0.036 g, 0.0099 mmol) in 10 ml DMSO. IR (KBr) n ¼ 3128; 3049, 3006, 2915, 2852, 1643, 1619, 1599, 1559, 1518, 1470, 1434, 1413, 1384, 1337, 1186, 1015, 969, 923, 882, 792, 708, 667, 553, 531 cm21. 3. Results and discussion 3.1. General considerations and synthesis Polyoxometalates in general and the Keggin polyoxometalates used in this research are polyanions in which the negative charge is randomly distributed around the polyoxometalate cluster. This random distribution of the negative charge limits our ability to predict the topology attainable upon electrostatic interaction between a polyoxometalate (heteropolyanion) and a polyammonium cation. With this in mind, we thought despite this that the co-crystallization of a relatively compact or alternatively rigid ammonium based polycation with a polyanioic polyoxometalate may enable the directional control of the anion– cation packing arrangement. The trication precursor, N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide, 1, was synthesized from 1,3,5-benzenetricarbonyl trichloride and N,N-dimethylethylendiamine, Scheme 1. Crystallization of the inorganic trianionic –organic tricationic hybrid compound was accomplished using highly
Scheme 1. Synthesis of N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5benzenetricarboxamide, 1.
diluted solutions of phosphotungstic acid, H3 PW 12 O 40, and N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide, in a solvent, acetonitrile, with a relatively weak potential for crystal organization in order to minimize any solvent dependence on the packing arrangement obtained. As a result, thin colorless needles of PW12O32 40 –N,N,Ntris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex, 2, suitable for single crystal X-ray diffractometry were obtained after 3 days of slow precipitation. The IR spectrum of 2 showed that the polyoxometalate anion (1080, 982, 890, 806 cm21) and organic component (nCyO 1647(amide) cm21) were retained in the molecular structures. It has been found that in rigid polyphenylene tetrahedral molecules, the nonparallel arrangement of such a molecules and non-fluxional core inhibit crystallization as a result of weak interchain coupling [12]. With this in mind we designed and synthesized a tetrahedrally shaped quaternary ammonium salt based on an adamantane core, 1,3,5,7-tetrakis{4-[(E)-2(Nmethylpyridinium-4-yl)vinyl] phenyl adamantane tetraiodide, 3, Scheme 2. The design of a rigid tetrahedral shaped cation with relatively large distances between the nitrogen atoms of the pyridine moiety was predicted to prevent a close packing arrangement of the polyoxometalate anions, slow down the selfassembly process and also possibly impart directionality of anion –cation interaction. Therefore, 1,3,5,7tetrakis{4-[(E)-2-pyridin-4-yl-vinyl]phenyl} adamantane, 5, was synthesized from 1,3,5,7-tetrakis(4iodophenyl)adamantane, 4, and 4-vinyl pyridine by a palladium catalyzed Heck reaction. Methylation yielded the desired quaternary pyridinium salt, 3. Cocrystallization of the tetrabutylammonium salt of tungstosilicic acid (tetraanion, Keggin type polyoxometalate) with an equivalent amount of 3, preferably in DMSO, yielded bright yellow clusters of crystal like material after one and a half days of slow precipitation. The IR spectra of the complex shows that both the polyoxometalate anion [SiW12O40]4þ (1015, 969, 923,
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31
Scheme 2. Synthesis of 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane tetraiodide. 3.
882, 792 cm21) and 3 (nCyC 1643, 1619, 1599; nCyN 1559, 1518 cm21) were retained in the hybrid material. 3.2. Crystal structure of the PW12 O32 40 – N,N,N-tris[2(dimethylamino)ethyl]1,3,5-benzenetricarboxamide complex The structure of the polyoxometalate cluster, PW12O32 40 , has W –O, WyO, and PyO bond lengths
and bond angles in very close agreement to those described in the literature [13]. The network formed in the crystal structure, Fig. 1, may be described as follows. Along the x-axis there are alternating layers of the polyoxometalate and the N,N,N-tris[2(dimethylamino)ethyl] derivative where the aromatic ring is centered above and below the polyoxometalate, and the plane of the aromatic ring is parallel to the polyoxometalate moiety. The mean distance between
Fig. 1. View of the unit cell of the PW12O32 40 –N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex along the x-axis (the acetonitrile solvent molecules are omitted for clarity).
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Fig. 2. View of PW the PW12O32 40 – N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex along the y-axis (only the polyoxometalate cluster is shown for clarity).
Fig. 3. View of the channel or micropore in the PW12O32 40 –N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex.
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Fig. 4. View of atoms Involved in the polyoxometalate–polyammonium cation interaction.
the plane of the aromatic ring and the surface of the ˚ indicating a polyoxometalate is approximately 3.6 A weak p – d interaction between the aromatic nucleus and the polyoxometalate. A view along the y-axis clearly reveals that the polyoxometalate molecules are in a zig-zag and layered arrangement in the x – y plane, Fig. 2. An extension of the view along the x-axis, Fig. 3, reveals a channeled structure along this axis, formed due to the undulation of polyoxometalate cluster and the short length of the cationic ammonium ‘arms’ required to electrostatically stabilize the structure. The dimensions based on space filling models with van der Waals radii of these channels or micropores (closest O – O distances between two polyoxometalate molecules) are approximately 3.5 ˚ . These channels contain disordered acetoby 6.5 A nitrile solvent molecules that were omitted for clarity. Another interesting observation, concerning the crystal structure, deals with the interaction of the ‘arm’ of the ammonium cation and the neighboring polyoxometalate cluster, Fig. 4. Although the proton
originating from the phosphotungstic acid is not resolved in the structure, other hydrogen atoms covalently bonded to carbon atoms adjacent to the ammonium center were resolved. In Table 2 are given bond distances between these hydrogen atoms and their associated carbon atoms and the nearest lying Table 2 Selected shortest intermolecular C – H· · ·O distances for the PW12O32 40 – N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide complex
H10B–O37 H10B–O20 H10A–O34 H10A–O20 H9C–O34 H9C–O15 H8B–O27 H8B–O15
H· · ·O ˚) distance (A
C· · ·O ˚) distance (A
C –H· · ·O angle (8)
2.965 2.881 2.730 2.633 2.565 2.656 2.463 2.739
3.673 3.174 3.402 3.174 3.429 3.374 3.410 3.437
131.57 98.88 127.56 116.07 149.96 131.97 165.19 129.33
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3.3. Lamellar structure of the SiW12O42 40 – 1,3,5,7tetrakis{4-[(E)-2(N-methylpyridinium-4yl)vinyl]phenyl adamantane complex [17] As noted above, co-crystallization of the tetrabutylammonium salt of tungstosilicic acid with an equivalent amount of 3, yielded bright yellow clusters, Fig. 5, of the SiW12O42 40 –1,3,5,7-tetrakis{4[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane complex after one and a half days of slow precipitation. Unfortunately all attempts to obtain large, defect free crystals suitable for X-ray diffraction analysis were unsuccessful, but the investigation of Fig. 5. Picture of microcrystals of the SiW12O42 40 – 1,3,5,7tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane material.
oxygen atoms of the polyoxometalate. Hydrogen – ˚ are measured. oxygen bond distances of 2.46 – 2.96 A The protonation of the tertiary amino group leads to the weak polarization of adjacent C – H bonds. This polarization appears to be sufficient for formation of C – H· · ·O where the H atoms are the donor atoms and O atoms are the acceptor atoms. This formation of weak hydrogen bonds is forced by the electrostatic interaction between the ammonium cation and polyoxometalate anion. In the hybrid crystal we have two types of C– H· · ·O contacts. All four hydrogen atoms are involved in a so-called bifurcated [14] hydrogen bridging. Each bifurcated hydrogen bond has major component with the shorter O –H bond distances and minor component with the longer O – H bond distances. The C – H· · ·O contacts differ one from another by the identity of the oxygen atom of the polyoxometalate involved. H10B, H9C and H8B atoms interact with both terminal and bridging oxygen atoms and H10 binds only to bridging oxygen atoms, Fig. 4. Following Jeffrey’s [15] classification of hydrogen bonds C – H· · ·O contacts are of a weak ˚ ) and weak directionality type (bond length . 2.2 A (bond angle . 908). It is also known that the shorter the C –H· · ·O hydrogen bonds the more likely its angle is to be close to 1808 [14 –16]. In our case the ˚ with u ¼ 1658 shortest contacts are 2.463 and 2.469 A which is close to 1808. Average C – H· · ·O bond length ˚ ðu . 908Þ and thus we can consider this as are , 3 A crystallographic evidence of C –H· · ·O bonding.
Fig. 6. Transmission electron micrograph of the SiW12O42 40 – 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane material. Insert: electron diffraction pattern.
M. Vasylyev et al. / Journal of Molecular Structure 656 (2003) 27–35
the morphology and two-dimensional electron diffraction pattern of the microcrystals was possible and used to determine the molecular packing arrangement, Fig. 6. As may be seen in the figure, clearly a lamellar structure for the polyoxometalate-3 hybrid material was obtained, with alternating layers o the polyoxometalate (dark lines) and the polycation 3 (light lines). The electron diffraction pattern along the principle axis of the crystallites was measured, Fig. 6 insert. The electron diffraction pattern confirms the lamellar structure and leads to a calculated layer ˚ . This value is in good separation of 16.6 ^ 0.2 A agreement with the calculated diameter of 3, lending strong proof to the formation of a lamellar structure in the SiW12O42 40 – 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane hybrid material with alternating polyoxometalate and tetracation 3 layers.
[3] [4] [5]
[6]
[7]
[8]
4. Conclusion Even though the negative charge of the polyoxometalates is homogeneously distributed around the oxo cluster, ordered structures can be obtained via co-crystallization with polyammonium counter cations. In one case, use of a cation with a planar core and short and somewhat flexible ‘arms’ yielded a channeled structure of high crystallinity. In another case, use of a polycation with an extended and rigid tetrahedral configuration led to a lamellar structure. Acknowledgements This research was supported by the Minerva Foundation and the Kimmel Center for Molecular Design. RN is the Israel and Rebecca Sieff Professor of Organic Chemistry.
[9]
[10]
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[14]
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