CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 4 259-262
Communications Interconvertible Supramolecular Transformations Jagadese J. Vittal* and Xiandong Yang Department of Chemistry, National University of Singapore, Singapore 117543 Received April 18, 2002;
Revised Manuscript Received May 20, 2002
ABSTRACT: Interconversion between a 3D hydrogen bonded network structure in [Zn2(Rsala)2(H2O)2]‚2H2O, (H2(Rsala) ) N-(2-hydroxy-5-R-benzyl)-L-alanine; R ) Cl and Me) and a chiral 3D coordination polymeric network [Zn2(Rsala)2] is reported to take place by thermal dehydration and rehydration process. Transformation from one type of structure to another is not common in supramolecular chemistry.1 Solid-state supramolecular reactions involving transformation of different structures are very rare since they involve breaking and forming of bonds in more than one direction compared to solid-state organic photochemical reactions which mainly involve two molecules.2 Iordanidis and Kanatzidis have discovered single-crystal-to-single-crystal solid-state structural transformations based on the redox-induced zipper action of selenium atoms by iodine.3 Prior these reports in 1998, we found that a ZnII complex, [Zn2(sala)2(H2O)2]‚ 2H2O (1) (H2sala ) N-(2-hydroxybenzyl)-L-alanine) having a three-dimensional hydrogen bonded honeycomb-like network structure with chiral cavities in the solid-state undergoes irreversible structural transformation assisted by N-H‚‚‚O hydrogen bonds to another having threedimensional coordination polymeric network structure, [Zn2(sala)2], 2 upon thermal dehydration.4 A similar CuII complex, [Cu2(sala)2(H2O)], with a novel, single-stranded helical coordination polymer on heating loses water molecules irreversibly in the solid-state and cross-links to produce a chiral open network structure similar to the ZnII analogue.5 In this communication, we have shown that this structural conversion can be made reversible by suitably modifying the backbone of the H2sala ligand as shown in Figure 1. Such reversible structural transformations may find potential applications as molecular switches.6 Most of the reversible topotactic solid-state reactions involve removal of guest solvent molecules from cavities.7 The reduced Schiff base, H2Rsala (R ) Cl and Me), obtained from 5-substituted salicyladehyde and L-alanine, is a tridentate ligand with a flexible backbone that has a chiral center and a prochiral center on the nitrogen atom. When reacted with Zn(II), Clsala forms a dimeric compound, [Zn2(Clsala)2(H2O)2]‚2H2O, 3 similar to that reported for sala ligand, as revealed by X-ray crystallography.8 In 3, each ZnII center is bridged by two phenolic oxygen atoms and the base of the distorted square pyramid is completed by the nitrogen and an oxygen atom of the * To whom correspondence should be addressed. Tel: + (65) 6874-2975. E-mail:
[email protected].
Figure 1. Structural diagram of Rsala ligand.
carboxylate group. A water molecule occupies the apical site in each metal center. The dimer has 2-fold crystallographic symmetry at the center of the Zn2O2 ring, and hence both aqua ligands are on the same side as illustrated in Figure 2. Similar structural features have also been observed for [Zn2(Mesala)2(H2O)2]‚2H2O, 4.9 It may be noted that the structures of 3 and 4 are isomorphous and isostructural with 1 (with the exception of the substituents). The presence of N-H‚‚‚O and O-H‚‚‚O hydrogen bonds between H2O, NH groups, and “free” carbonyl oxygen atoms in the dimeric structure of 3 leads to the formation of an interesting three-dimensional network structure as shown in Figure 3.10 In this hydrogen bonded network structure, six dinuclear units form a honeycomb like chiral cavity (3.8 × 9.2 Å). A similar hydrogen bonding pattern was also observed in 4.11 The hydrogen bonding architecture present in 3 and 4 is very similar to that observed in 1. Hence, it is anticipated that these two hydrated samples would undergo thermal dehydration reactions accompanied by the formation of new Zn-O bonds from neighboring Rsala ligands, which would subsequently lead to the formation of the expected threedimensional coordination polymeric network structure. If this is true, then all the four water molecules (two in the lattice and two coordinated to the metals) are expected to be lost by 100 °C. The thermogravimetric curves in Figure 4 show that the dehydration is completed by 110 °C. However, the DTG curves reveal that only ∼90% of the expected water molecules have been removed around 110 °C and remaining by ∼180 °C in 3 and ∼220 °C in 4. On the contrary, 1 could be completely dehydrated below 110
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Figure 2. A perspective view showing the geometry of the dinuclear 3, numbering scheme and the hydrogen bonds. The hydrogen atoms attached to carbon atoms were omitted for clarity. Selected fragments of neighboring molecules involved in hydrogen bonding are shown with open bonds. Selected bond distances [Å] and angles [°]: Zn1‚‚‚Zn1a 3.0911(6), Zn1-O1 2.003(2), Zn1-O2 2.016(2), Zn1-O4 1.997(2), Zn1-N1 2.132(2), Zn1-O1a 2.234(2), C9-O2 1.267(4), C9-O3 1.238(3), Zn1-O1-Zn1a 100.27(8), N1Zn1-O1 92.48(8); N1-Zn1-O2 80.76(8), O4-Zn1-O1 110.75(9), O4-Zn-O2 123.8(1), O4-Zn1-N1 94.85(9), O2-C9-O3 124.4(3).
°C (Figure 4c) in 1, and the resultant dehydrated product 2 could be recrystallized from water or aqueous methanol to obtain single crystals suitable for X-ray diffraction analysis. However, to our surprise, we are unable to grow the single crystals of dehydrated compounds, since 3 and 4 are regenerated upon recrystallization from water or aqueous MeOH. This behavior is strikingly different from sala compounds. Normally, lattice water will be lost in the temperature region 50-100 °C, and the coordinated water molecules can only be removed above ca. 150 °C, depending on the nature of bond between water and the metal ions.12 Since ∼90% of the water is lost around 100 °C, it may be concluded that the driving force for this behavior is the formation of a new Zn-OCO bond leading to another 3D network structure similar to 2. Then why is it difficult to remove ∼10% of water? A closer look at the packing reveals that the chloro and methyl substituents at the para positions indeed occupy the channels in the lattice (Figure 4). The water molecules are naturally expected to use these chiral channels, which are partially blocked by these substituents Scheme 1.
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Figure 3. Packing diagram showing the hydrogen bonding network in the bc plane of 3.
Figure 4. The TG and DTG of (a) 3, (b) 4 and (c) 1.
in 3 and 4 during the dehydration process, and hence complete removal of water molecules is hindered by 100 °C. Then why do the anhydrous 3 and 4 become reversible in water? A plausible explanation may be obtained by analyzing the packing of the anhydrous 2. Close interactions were analyzed by placing Cl and Me groups artificially in the crystal structure of 2, which shows that some repulsive interactions indeed exist between the substitu-
Schematic Diagram Illustrating the Change from Hydrogen Bonding to Covalent Bonding Network Structures during Thermal Dehydration Process
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Figure 5. The images of the single crystals of 3 at (a) 30 °C and (b) at 150 °C showing that the morphology is preserved before and after the thermal dehydration.
ents and atoms in the other parts of the 3D coordination network. It is likely that these interactions are responsible for the observed interconvertibility. When the single crystals of 3 and 4 are carefully dehydrated thermally, they became opaque while maintaining their single-crystal shape and morphology (Figure 5). The retention of single-crystal morphology supports that the point group, 432 and the space group, P43212 are preserved after thermal dehydration, while the loss of crystallinity indicates that they are not isostructural.2b Similar observation of the single crystals of 1 in the solidstate thermal dehydration process appear to support that 3 and 4 in fact undergo supramolecular structural transformation in the solid-state.13 In summary, three-dimensional hydrogen bonded network structures having chiral channels can also be generated by suitably modifying the backbone of the multidentate ligand, sala. The dreams of crystal engineers include the rational design and control of the crystal structures, and fine-tuning of the structure by modifying the components of the ligands employed.14 When the sala ligand was employed, the supramolecular conversion was found be not interconvertible, i.e., could not be rehydrated once coordination polymeric structures were formed.4,5 We have now shown that such supramolecular structures can be interconverted by substituting the backbone of the ligand by suitable but relatively nonreactive functional groups. The substituent on the phenyl ring occupies the chiral channels and thus changes the irreversible solid-state supramolecular transformation into an interconvertible one as depicted in Scheme 1. Thermal dehydration reactions assisted by the complementary N-H‚‚‚O bonds in the solid-state have produced interesting solid-state supramolecular transformation. This work, we believe, is a step forward in the rationalization the solid-state supramolecular transformations. The knowledge gained in this study may help us to predict the transformations of the already reported compounds in the Cambridge Structural Database and to finetune the structures of the existing supramolecular architectures. Experimental Section The two ligands, N-(2-hydroxy-5-methyl-benzyl)-L-alanine (H2Mesala) and N-(2-hydroxy-5-chloride-benzyl)-L-alanine (H2Clsala],
were synthesized similar to their parent compound H2Sala as reported earlier.15 3: To a solution of H2Clsala (23 mg, 0.1 mmol) in water (10 mL) containing LiOH (4.8 mg, 0.1 mmol) was added the solution of Zn(CH3CO2) 2‚2H2O in water (10 mL). The mixture was stirred for 30 min, and then filtered and kept at room temperature to yield big colorless crystals that were separated by decantation and dried in air. Yield: 12 mg, (36%). Anal.: calcd for C20H28Cl2N2O10Zn2: C 36.2, H 4.0, N 4.3; found: C 36.5, H 4.2, N 4.5. IR (KBr, [cm-1]): 1596 (s, νas(CO2)); 1480 (s, νs(CO2)); 1281 (s, ν(C-Oarb). TG weight loss: 10.9% (calcd), 11.0% (observed) for 4H2O. 4: Zn(ClO4)2‚6H2O (37.6 mg, 0.1 mmol) solution in water (10 mL) was added to a solution of H2Mesala (21.5 mg, 0.1 mmol) in water (15 mL) and LiOH (2.4 mg, 0.1 mmol) dropwise with stirring. The mixture was filtered and left at room temperature to yield distorted octahedral-shaped colorless crystals, which were separated by decantation and dried in air. Yield: 14 mg (45%). Anal: calcd for C22H34N2O10Zn2: Calcd. C 42.7; H 5.6, N 4.5; found: C 42.9, H 5.3, N 5.2. IR (KBr, [cm-1]): 1602 (s, νas(CO2)); 1494 (s, νs(CO2)); 1276 (s, ν(C-Oarb). TG weight loss: 11.7% (calcd), 10.2% (observed) for 4H2O.
Acknowledgment. This research is supported by Grant R143-000-153-112 to J.J.V. from the National University of Singapore. Supporting Information Available: Figures of molecular structure and packing of 4, hot stage microscope photographs of 1 and 4, repulsive interactions in dehydrated 3 and 4, along with the CIF files for 3 and 4 are available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Ayi, A. A.; Choudhury, A.; Natarajan, S.; Neeraj, S.; Rao, C. N. R. J. Mater. Chem. 2001, 11, 1181-1191. (b) Choudhury, A.; Neeraj, S.; Natarajan, S.; Rao, C. N. R. J. Mater. Chem. 2001, 11, 1537-1546. (c) Lee, E.-W.; Kim, Y.J.; Jung, K.-Y. Inorg. Chem. 2002, 41, 501-506. (d) Onoda, A.; Yamada, Y.; Okamura, Y.; Doi, M.; Yamamoto, H.; Yeyama, N. J. Am. Chem. Soc. 2002, 124, 1052-1059. (2) (a) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647-678. (b) Theocharis, C. R.; Jones, W. In Organic Solid-state Chemistry; Desiraju, G. R., Ed.; Elsevier, Amsterdam, 1987; pp 47-68. (c) Tanaka, K.; Toda, F. Chem. Rev. 2000, 100, 1025-1077. (d) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky, E. B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641-3649. (e) MacGillivray, L. R. Cryst. Eng. Comm. 2002, 7.
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(3) (a) Iordanidis, L. Kanatzidis, M. G. Angew. Chem., Int. Ed. 2000, 39, 1928-1930. (b) Iordanidis, L.; Kanatzidis, M. G. J. Am. Chem. Soc. 2000, 122, 8319-8320. (4) Ranford, J. D.; Vittal, J. J.; Wu, D. Angew. Chem., Int. Ed. Engl. 1998, 37, 1114-1116. (5) Ranford, J. D.; Vittal, J. J.; Wu, D.; Yang, X. Angew. Chem., Int. Ed. Engl. 1999, 38, 3498-3501. (6) (a) Matsumoto, M.; Mizuguchi, Y.; Mago, G.; Eguchi, S.; Miyasaka, H.; Nakashima, T.; Tuchagues, J.-P. Angew. Chem., Int. Ed. Engl. 1997, 36, 1860-1862. (b) Collier, C. P.; Jeppesen, J. O.; Luo, Y.; Perkins, J.; Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2001, 123, 1263212641. (c) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P. Science, 1997, 278, 252-254. (d) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res. 2001, 36, 433-444. (7) See for example: (a) Rabe, S.; Mu¨ller, U. Z. Anorg. Allg. Chem. 1999, 625, 1367-1370. (b) Englert, U.; Ganter, B.; Wagner, T.; Kla¨ui, W. Z. Anorg. Allg. Chem. 1998, 624, 970974. (c) Kepert, C. J.; Rosseinsky, M. J. Chem. Commun. 1999, 375-376. (d) Li, H.; Eddaoudi, M.; O’Keefe, M.; Yaghi, O. M. Nature 1999, 402, 276-279. (8) X-ray structure determination of 3:l6 [Zn2(Clsala)2(H2O)2]‚ 2H2O crystal data: Tetragonal space group P43212, a ) 10.8912(3), c ) 23.4460(9) Å, V ) 2781.1(2) Å3, Z ) 4, Fcalcd ) 1.572 g cm-3. All the hydrogen atoms were located successfully. The positional and common isotropic thermal parameters were refined for the hydrogen atoms in the coordinated water molecule, and riding models were used for the rest. In the final least-squares refinement cycles on |F|2, the model converged at R1 ) 0.0422, wR2 ) 0.0816, and Gof ) 0.916 for 3519 reflections with Fo > 2σ(Fo) and 177 parameters, and R1 ) 0.0539 and wR2 ) 0.0846 for all 4418 data. Flack parameter, x was refined to -0.004(14). (9) X-ray structure determination of 4:l6 [Zn2(Mesala)2(H2O)2]‚ 2H2O crystal data: tetragonal space group P43212, a ) 10.9409(1), c ) 23.8980(5) Å, V ) 2860.67(7) Å3, Z ) 4, Fcalcd ) 1.433 g cm-3. All the hydrogen atoms were located successfully. The positional and common isotropic thermal parameters were refined for the hydrogen atoms in the coordinated water molecule, and riding models were used for the rest. In the final least-squares refinement cycles on |F|2, the model converged at R1 ) 0.0518, wR2 ) 0.0869, and Gof ) 1.133 for 2766 reflections with Fo > 2σ(Fo) and 185 parameters, and R1 ) 0.0834 and wR2 ) 0.0954 for all 3650 data. Flack parameter, x was refined to 0.01(2). (10) The close intermolecular hydrogen bonding contacts are
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H(1)‚‚‚O(2_$1),2.12(2)Å;H(4A)‚‚‚O(3_$1),1.90(2)Å;H(1SA)‚‚‚O(3_$2), 2.11(4) Å; H(4B)‚‚‚O(1S), 2.00(5) Å; N(1)‚‚‚O(2_$1), 3.029(3) Å; O(4)‚‚‚O(3_$1), 2.651(3) Å; O(1S)‚‚‚O(3_$2), 2.729(3) Å; O(4)‚‚‚O(1S), 2.747(4) Å; N(1)-H(1)‚‚‚O(2_$1), 170(4)°; O(4)-H(4A)-O(3_$1), 171(4)°; O(1S)-H(1SA)-O(3_$2), 143(4)°; O(4)-H(4B)-O(1S), 170(5)°; the N(1)-H(1), O(4)H(4A), O(4)-H(4B), O(1S)-H(1SA) and O(1S)-H(1SB) distances are 0.92, 0.76(2), 0.76(2), 0.73(2), and 0.79(2) Å, respectively. The H(4A)-O(4)-H(4B) and H(1SA)-O(1S)H(1SB) angles are 111(4) and 110(4)°, respectively. Operators for generating equivalent atoms: $1: x - 1/2, -y + 1/2, -z + 1/4 and $2: -y + 1/2, x - 1/2, z - 1/4. The close intermolecular hydrogen bonding contacts are H(1)‚‚‚O(2_$1), 2.26(5) Å; H(4B)‚‚‚O(3_$1), 1.99(4) Å; N(1)‚ ‚‚O(2_$1), 3.075(5) Å; O(4)‚‚‚O(3_$1), 2.639(5) Å; N(1)-H(1)‚ ‚‚O(2_$1), 171(5)°; O(4)-H(4B)-O(3_$1), 158(6)°; N(1)H(1), O(4)-H(4A), O(4)-H(4B), O(1S)-H(1SA) and O(1S)H(1SB) distances are 0.82, 0.69(3), 0.69(3), 0.69, and 0.69 Å, respectively. The H(4A)-O(4)-H(4B) and H(1SA)-O(1S)H(1SB) angles are 111(4) and 123(6)°, respectively. Operators for generating equivalent atoms: $1: x - 1/2, -y + 1/2, -z + 1/4. (a) Nicolaev, A. V.; Logvinenko, Y. A.; Myachia, L. I. Thermal Analysis, Academic: New York, 1969; Vol 2, p 779. (b) Berg, L. G. In Differential Thermal Analysis; Mackenzie, R. C., Ed. Academic: London, 1970; Vol. 1, p 343. (c) Paulik, F. Special Trends in Thermal Analysis, John Wiley: England, 1995; p 16. The crystal structure of the anhydrous 1 (i.e., 2) is known.4 Superimposed structures of the building blocks of 1 and 2 revealed that the ligand conformation has changed drastically so as to retain its point group in the crystal lattice. (a) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629-1658. (b) Zaworotko, M. J. Chem. Comm. 2001, 1-9. (c) Zaworotko, M. J. Angew. Chem., Int. Ed. Engl. 1998, 37, 1211-1213. Koh, L. L.; Ranford, J. D.; Robinson, W. T.; Svensson, J. O.; Tan, L. C.; Wu, D. Inorg. Chem. 1996, 35, 6466-6472. General crystallographic details: Data were collected on a Siemens SMART CCD system with graphite-monochromated Mo KR radiation and a sealed tube (2.4 kW) at 23 °C. Absorption corrections were made with the program SADABS (G. M. Sheldrick, Go¨ttingen, 1996), and the crystallographic software package SHELXTL (SHELXTL Reference Manual, Version 5.03, Wisconsin, 1996) was used for all calculations.
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