Large (H2O)56(OH)6 and (H

Large (H2O)56(OH)6 and (H2O)20 Clusters inside a Nanometer-Sized M6L8 Cage Constructed by Five-Coordinated Copper(II) and Flexible Carboxamide-Containing Tripodal Ligand Yan Wang,† Taka-aki Okamura,‡ Wei-Yin Sun,*,† and Norikazu Ueyama‡

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 802–804

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China, and Department of Macromolecular Science, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed December 18, 2007; ReVised Manuscript ReceiVed January 23, 2008

ABSTRACT: N′,N″,N′′′-Tris(pyrid-4-ylmethyl)-1,3,5-benzenetricarboxamide (TPMBA-4) reacts with Cu(BF4)2 · 6H2O to form a zerodimensional cagelike complex [CuII6(OH)6(TPMBA-4)8](BF4)3(OH)3 · 85H2O (1). Interestingly, X-ray crystallographic analysis revealed that large (H2O)76(OH)6 clusters with outer-shell (H2O)56(OH)6 and inner-hosted (H2O)20 ones are encapsulated in the cavity of 1. It presents a new mode of association of water molecules for research on water or ice. Water is believed to be the most important part of life. However, the physical and chemical properties of water are anomalous because of the different arrangements of hydrogen bonds and their fluctuations,1 and the correct description of the nature of hydrogen-bonds among water clusters is a major difficulty.2 Up to now, many groups have devoted great efforts on the study of water or ice, and results including the structures and properties of water clusters, oligomers, and low-dimensional water polymers in supramolecular solids have been reported.3–9 On the other hand, the discrete cages have attracted great interest from chemists for a long time due to the existence of the inner cavity of the cage, which can have abilities to host guest molecules or ions, to stabilize labile species, to promote or catalyze some specific reactions, etc.10 However, cages that encapsulate water cluster are rarely reported. In this work, we report a M6L8 cagelike complex with novel (H2O)56(OH)6 and (H2O)20 clusters encapsulated in its cavity. The reaction between N′,N″,N′′′-tris(pyrid-4-ylmethyl)-1,3,5-benzenetricarboxamide (TPMBA-4) and Cu(BF4)2 · 6H2O in CH3NO2/ CH3OH solution gave a blue solid [CuII6(OH)6(TPMBA-4)8](BF4)3(OH)3 · 85H2O (1).11 X-ray crystallographic analysis showed that complex 1 crystallized in space group of I432 and has a zerodimensional cagelike structure.12 In 1, each copper(II) atoms is fivecoordinated by four pyridyl N and one hydroxyl O atoms, while in the reported Cu(II) cages the Cu(II) atoms are coordinated by dimethyl sulfoxide (DMSO) molecules.10h The difference between our cage and the reported ones is caused by different coordination environments of central ions predominated by synthetic conditions. In 1, each TPMBA-4 ligand adopts a cis,cis,cis-conformation to connect three different Cu(II) atoms. As shown in Figure 1a, six Cu(II) and eight TPMBA-4 ligands form the cagelike structure of 1 with a diameter of the cavity of 15.75 Å, which is slight shorter than that in [Pd6(TPMBA)8](NO3)12; however, the pores at the edges of the octahedron in 1 with dimensions of 6.59 × 14.53 Å2 are larger than those in [Pd6(TPMBA)8](NO3)12.10g The remarkable difference between complex 1 and the reported ones is the encapsulation of novel (H2O)56(OH)6 and (H2O)20 clusters in the cavity of 1 (Figure 1b,c).10g,h Hydrogen bonding interactions were inferred by considering the O · · · O close contacts between the oxygen atoms involved (the O · · · O distance less than the sum of van der Waals radii of 3.04 Å).13 The hydroxyl groups were verified by the charge balance and the difference in the coordination ability between the hydroxyl group and the water molecule. It is noteworthy that such novel clusters can be regarded * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 86-25-83314502. † Nanjing University. ‡ Osaka University.

as two parts. The inner one is a (H2O)20 water cluster with a pentagonal dodecahedral shape, which is one of the most stable water clusters (Figure 2a).14 The (H2O)20 cluster is formed by O5 and O8 atoms with O · · · O distances of 2.723(20) (O5 · · · O5#1, #1 ) -x, y, -z), 2.753(15) (O5 · · · O8#2, #2 ) -x + 1, z, y), and 2.817(15) (O5 · · · O8#3, #3 ) y, z, x - 1) Å, respectively, which is close to the corresponding value of 2.759 Å in ice Ih.15 The average O-O-O angle of 108.0° in (H2O)20 cluster is close to the tetrahedral geometry, in agreement with the preferred tetrahedral arrangements of hydrogen-bonds in ice Ih.16 On the other hand, the outer shell is a (H2O)56(OH)6 cluster, which is formed by 6 hydroxyl O atoms and 56 water molecules (Figure 2b). As shown in Figure 2c, the (H2O)56(OH)6 cluster is formed by a (H2O)10(OH)2 unit consisting of two O2, four O4, four O6, and two O7 by a lateral-sharing mode. In the (H2O)10(OH)2 unit, the O · · · O distances vary from 2.788(23) Å to 2.998(23) Å with an average value of 2.875(23) Å, which is slightly longer than the corresponding value in ice Ih, while the average O-O-O angle is 106.7°, which is comparable to the value in ice Ih.16 It is interesting that the outer (H2O)56(OH)6 cluster hosts the inner (H2O)20 one as a guest to achieve the final (H2O)76(OH)6 motif (Figure 2d). Such encapsulation structure is stablized by O5 · · · O4 and O8 · · · O7 interactions with O · · · O distances of 2.728(24) and 2.705(10) Å, respectively. The (H2O)76(OH)6 clusters are associated with the M6L8 cage through coordinated hydroxyl groups (Figure 1b). Furthermore, the connection modes of O atoms of water molecules are different in the clusters. Each of the O5, O4, O8, and O7 atoms is four interacted with other O atoms, while the O6 atom is not. Such hydrogen-bonding deficient water molecules are also found at the surface of ice, and the recent X-ray absorption spectroscopy and Raman scattering studies of liquid water also show the fact that significant numbers of O atoms show less than four interaction in liquid water.17 To the best of our knowledge, the present study provides the first example of water clusters with encapsulation modes containing hydroxyl groups, although water clusters formed by outer host and inner guest have been reported.14c,18 The thermogravimetric analysis (TGA) data of 1 showed that the water molecules are lost below 200 °C. The easy removal of water molecules confirms that the cluster is weakly interacted with the host cage, and the water cluster shows liquid water properties.19 In summary, an M6L8 cage formed by a flexible carboxamidecontaining tripodal ligand and Cu(II) salt was obtained, in which interesting (H2O)56(OH)6 and (H2O)20 clusters were hosted and stabilized. The characterization of such water clusters containing hydroxyl groups gives us a further insight into the studies on water clusters. It can be considered that the coordinated hydroxyl groups

10.1021/cg701242z CCC: $40.75  2008 American Chemical Society Published on Web 02/08/2008

Communications

Crystal Growth & Design, Vol. 8, No. 3, 2008 803

Figure 1. (a) The M6L8 cagelike structure of 1. (b) Crystal structure of 1 with the (H2O)56(OH)6 and (H2O)20 clusters occupying the cavity of cage. (c) Schematic structure of 1, in which the M6L8 cage is represented by the octahedron (pale yellow) and the (H2O)56(OH)6 and (H2O)20 clusters are shown in red and blue, respectively.

Acknowledgment. This work was financially supported by the National Science Fund for Distinguished Young Scholars (Grant No. 20425101), the National Natural Science Foundation of China (Grant Nos. 20731004 and 20721002), and the National Basic Research Program of China (Grant No. 2007CB925103). Supporting Information Available: X-ray crystallographic data in CIF format. This material is available free of charge via Internet at http://pubs.acs.org.

References

Figure 2. (a) The inner (H2O)20 cluster formed by O5 and O8 atoms. (b) The outer (H2O)56(OH)6 cluster formed by O2, O4, O6, and O7 with a large inner cavity, in which the O atoms of OH- are indicated by green balls. (c) Perspective view of (H2O)10(OH)2 unit in the outer (H2O)56(OH)6 cluster. (d) The encapsulated (H2O)56(OH)6 and (H2O)20 clusters in 1.

on Cu(II) atoms form hydrogen bonds with the water molecules and make the whole clusters stably hosted in the cavity of 1.

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