Single-Crystal to Single-Crystal Transformation of Cyclic Water Heptamer to Another (H2O)7 Cluster Containing Cyclic Pentamer Mohammad Hedayetullah Mir and Jagadese J. Vittal* Department of Chemistry, Faculty of Science, 3 Science DriVe 3, National UniVersity of Singapore, Singapore 117543
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1478–1480
ReceiVed March 4, 2008
ABSTRACT: A discrete cyclic water heptamer trapped inside a three-dimensional coordination polymer undergoes single-crystal to singlecrystal transformation to a new type of water heptamer containing a pentamer ring buttressed by a dimer, accompanied by phase transition below 273 K. The structural differences between the heptamers in BF4- and ClO4- salts have been attributed to the influence of the anions in the channels. There is hardly a compound that has been thoroughly investigated more than water, and this is due to its importance in many biological, chemical, and physical processes.1 In the past several decades, considerable attention has been focused on theoretical2,3 and experimaental4,5 studies of small water clusters to understand the structures and characteristics of liquid water and ice.1,6,7 Water clusters can play an important role in the stabilization of supramolecular systems both in solution and in the solid state, and there is clearly a need for a better understanding of how such water aggregations are influenced by the overall structure of their surroundings.8–13 Metal-organic framework (MOF) structures with suitable organic ligands can provide void spaces where discrete water clusters can exist. Metal ions in such structures can also act as glues in holding the water clusters. Interactions between the water aggregates and the surroundings often play a key role in stabilizing the unusual water clusters in the crystal lattice. In this context, we recently reported a reversible structural transformation between a cyclic water heptamer and a bicyclic (H2O)7 cluster inside a threedimensional (3D) coordination polymer, and it has been found that the water cluster was sustained by additional weak interactions with BF4- anions in the channels.14 Since BF4- and ClO4- anions have a similar size and shape, changing the BF4- anions to a ClO4anion will provide a rare opportunity to study the influence of anions on the structure of the water cluster and structural transformation, if any. In this communication we report single-crystal to singlecrystal transformation of a discrete water heptamer to a new (H2O)7 cluster containing a cyclic pentamer accompanying phase transformation of [Cu3(phen)3(muco)2(H2O)2](ClO4)2 · 5H2O, 1 (phen ) 1,10-phenanthroline, H2muco ) trans,trans-muconic acid or 1,3butadiene-1,4-dicarboxylic acid). Compound 1 was synthesized by a procedure similar to the BF4salt, 2, reported.14 The blue single crystals were separated by hand picking from the colorless crystals of muconic acid. Fortunately 1 is isomorphous and isostructural to 2 at room temperature and crystallized15 in the monoclinic space group C2/c with Z ) 4. The connectivity between the muconate anion and Cu(II) atoms resembles diamondoid topology. The channels formed along the c-axis are occupied by the coordinated water, lattice-water molecules, and ClO4- ions. The presence of cyclic water heptamer very similar to that in 2 is recognized as shown in Figure 1a. Although both 1 and 2 form a cyclic water heptamer, there are differences in the way the hydrogen atoms are involved in the formation of cyclic heptamer especially O3S and O5. Both hydrogen atoms of O5 are involved in the ring formation, while only one of the hydrogen atoms of O3S is in the connectivity of the heptamer in 1 and vice-versa in 2. * To whom correspondence should be addressed. E-mail:
[email protected].
The interactions between the water ring with the anions of the two complexes at room temperature (296 K) are shown in Figure 1a,b. In 1, there is a hydrogen bond between O3S and the oxygen atom of the neighboring ClO4- ion, which is absent in 2 because of the unfavorable orientation of hydrogen atoms attached to O3S. Again, the differences between the O1S · · · O2S distances (O1S · · · O2S 3.510(6) Å in 1, 3.428(4) Å in 2) appear to dictate the differences in the low temperature structures of the water cluster when cooling the crystals to 223 K.
Figure 1. Perspective views showing the interactions between the water clusters and anions in 1 (a) and 2 (b). The crystallographically disordered hydrogen atoms are not shown for clarity. The atoms with superscript ‘a’ are related by -x + 1, y, -z + 1/2 symmetry.
10.1021/cg800244g CCC: $40.75 2008 American Chemical Society Published on Web 04/22/2008
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Figure 3. A schematic diagram showing the interconversion between the water clusters.
Figure 2. Perspective views showing the interactions of the cyclic water heptamers with anions in low temperature phase in 1 (a) and 2 (b).
On cooling to 223 K, the data crystal undergoes a phase transition from C2/c to P21/c, and the crystallographic 2-fold symmetry in the water heptamer is removed. The connectivity of the 3D coordination polymer is preserved, while the cyclic water heptamer is changed to a new (H2O)7 cluster containing cyclic pentamer as shown in Figure 2. The O2S · · · O5S distance, 3.107(8) Å indicates nonbonding,6b,16 and the structure of the low temperature water cluster is different from that observed in 2 where the O2S · · · O5S distance is 2.865(5) Å. At room temperature, ClO4- ion forms an O-H · · · O hydrogen bond with both O2S and neighboring O3S. When the sample is cooled, the distance between O1S and O2S is shortened from 3.510(2) Å to 2.850(5) Å, which is more than the shortening observed in 2, resulting in the cleavage of the hydrogen bond between O2S and O3S. In 1 O1S · · · O2S distance shortens from 3.428(4) Å to 2.997(6) Å. At room temperature, the BF4ion forms a hydrogen bond only to O2S due to preferential orientation of hydrogens and the neighboring O3Sa is not able to form a hydrogen bond to BF4- ion but is present at lower temperature. As a result, the distance between O1S and O2S increases and a new hydrogen bond is formed between O1S and O2Sa. Therefore, it appears that the flexibility of the hydrogen bonds allows the water cluster to adjust its geometry in response to the change demanded by the surrounding environment. During the phase transition from C2/c (high temperature form) to P21/c (low temperature form) a and c axes interchanged and the channels where the water clusters and anions previously occupied along the c-axis in the room temperature form is now along the a-axis in the low temperature form. The phase transition between
Figure 4. (a) A portion of the 1D coordination polymeric structure viewed from the a-axis. (b) Packing of the 1D polymer due to hydrogen-bonded interactions, viewed from the b-axis. (c) Helical strands formed by aqua ligand and nitrate anions.
1480 Crystal Growth & Design, Vol. 8, No. 5, 2008 the two monoclinc phases is reversible as confirmed by single crystal X-ray data and cell parameters, and hence the conversion of cyclic heptamer to a new (H2O)7 cluster containing a cyclic pentamer as shown in Figure 3. However, the DSC tracing shows that the conversion is sluggish, although reversible. In order to study the effect of semicoordinating anions on the water structure, we attempted to prepare a similar coordination polymer with NO3- anion. However under similar experimental conditions we were able to isolate [{Cu(phen)(H2O)}2(muco)](NO3)2, 3. X-ray crystallography15 reveals the formation of a onedimensional (1D) coordination polymer (Figure 4). Such structural diversity in MOFs influenced by anions is well-known. For example, a planar anion (D3 h) like NO3- can form two-dimensional structures, whereas tetrahedral (Td) BF4- and ClO4- generated 3D networks.17 The zigzag coordination polymer propagates along the c-axis. The aqua ligands are hydrogen bonded to the oxygen atoms of the nitrate anions thus producing a 3D hydrogen-bonded network structure as shown in Figure 4b, while Figure 4c shows that the hydrogen-bonded H2O-NO3- aggregates has helical conformation. Although there are few reports on the hydrogen-bonded H2O-NO3system forming discrete ring and zigzag clusters, the helical H2ONO3- strand appears to be scarce.18 In summary, we have investigated the influence of anions on the structural transformation of cyclic water heptamer. Although BF4- and ClO4- ions have similar size, shape, and hydrophilicity,19 subtle changes impinged on the room temperature structure of water cluster resulting in two new monocyclic water heptamers with slightly different hydrogen-bonded connectivity. Hence, the water heptamer with a cyclic pentamer buttressed by a dimer in the low temperature form of ClO4- salt is completely different from the bicyclic (H2O)7 cluster in BF4- salt. Although a large number of water aggregrates have been characterized in crystalline solids,4,6b the influences of ions, temperature, phase transition, etc., on the water cluster structures have rarely been investigated.14,20 The single-crystal to single-crystal transformation between the water clusters reported here is expected to be prevalent in other systems also. The formation of a 1D zigzag coordination polymeric structure for the NO3- salt with helical water-nitrate anion agglomeration highlights the dependency of the overall topology on the nature of the anion.
Acknowledgment. We are very thankful for financial support from the Ministry of Education, Singapore through the National University of Singapore Grant R-143-000-283-112. Supporting Information Available: Text giving experimental details, Tables S1-S4, Figures S1 and S2, and crystallographic CIF data for Crystals 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.
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