DOI: 10.1021/cg101225j
Nonamer Water Cluster Encapsulated in a Heterometallic Supramolecular Complex
2010, Vol. 10 5031–5033
Di Sun, Dan-Feng Wang, Na Zhang, Rong-Bin Huang,* and Lan-Sun Zheng State Key Laboratory of Physical Chemistry of Solid Surface, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Received September 16, 2010; Revised Manuscript Received October 12, 2010
ABSTRACT: A crownlike nonamer water cluster has been observed in the heterometallic supramolecular complex {[Cu(eda)2(H2O)2]3 3 [Ag6(mna)6] 3 9H2O} (1), which is built from one anionic hexanuclear silver cluster [Ag6(mna)6]6- and three mononulear [Cu(eda)2(H2O)2]2þ cations (H2mna = 2-mercaptonicotinic acid, eda = ethylenediamine). Moreover, the thermogravimetric curve, UV-vis absorption, and photoluminescence spectra of 1 were discussed. Water is a major chemical constituent of the planet’s surface, and as such, it has been indispensable for the genesis of life on the earth.1 Small water clusters are the subject of considerable theoretical and experimental interest, as they can lead to an understanding of the structure of bulk water as well as its anomalous behavior.2 So far, a surge of publications dealing with the hydrogen-bonded water clusters and extended water aggregates has appeared; however, it is worthy that there are fewer odd-numbered water clusters than even-numbered ones.3 Among the evennumbered water clusters, tetramers,4 hexamers,5 octamers,6 decamers,7 dodecamers,8 tetradecamers,9 hexadecamers,10 octadecamers,11 and icosadecamer12 are well-known within the lattice of a crystal host. However, surprisingly little is known of the discrete odd-numbered water clusters,13 even though trimers14 and pentamers15 are familiar in crystal hydrates. Particularly, the higher nuclearity, odd-numbered nonamer cyclic water cluster seems to be a mysterious member and has rarely been captured in the cavity of a supramolecular complex.14b Herein, we present a discrete cyclic (H2O)9 water cluster, which is embedded in a heterometallic supramolecular complex, showing a similar structure to the simple bridged hydrocarbon bicyclo[3.3.1]nonane with a crownlike conformation. The complex {[Cu(eda)2(H2O)2]3 3 [Ag6(mna)6] 3 9H2O} (1) was synthesized by diffusion of two pre-prepared solutions (see Supporting Information). The composition of 1 was further deduced from X-ray single crystal diffraction, elemental analyses, and IR spectroscopy. The solid FT-IR spectrum (Figure S1 in the Supporting Information) of complex 1 shows a very intense broad band around 3400 cm-1 attributed to the water molecules. The disappearance of the S-H stretching band around 2560 cm-1 due to the free ligand suggests Ag-S bond formation in this complex. The asymmetric and symmetric stretching vibrations of the carboxyl group are at 1578 and 1385 cm-1, respectively. There is no band in the region 1690-1730 cm-1, indicating complete deprotonation of the carboxyl groups.16 The phase purity of 1 is sustained by its powder X-ray diffraction pattern, which is consistent with that simulated on the basis of the singlecrystal X-ray diffraction data (Figure S2). Energy dispersive X-ray spectroscopy proves the kinds of elements in 1 (Figure S3). These results are in good agreement with solid state crystal structures. X-ray single-crystal diffraction analysis reveals that 1 crystallizes in the monoclinic C2/c space group with an asymmetric unit that contains three crystallographically unique Ag(I) ions, one and a half Cu(II) ions, three mna ligands, four eda ligands, and seven and a half water molecules (Figure 1). The charge neutrality is achieved by three deprotonated -COOH and -SH of three *Correspondence e-mail:
[email protected]. Fax: 86-592-2183074. r 2010 American Chemical Society
Figure 1. Molecular structure of 1, showing the coordination environments of the Ag(I) and Cu(II) centers. All hydrogen atoms and lattice water molecules are omitted for clarity. (Symmetry codes: (ii) -x þ 1, y, -z þ 1/2; (vii) x, -y þ 2, z þ 1/2.)
Figure 2. Top view (a) and side view (b) of the nonamer water cluster. (Symmetry codes: (iii) -x þ 1/2, y þ 1/2, -z þ 1/2; (x) x - 1/2, y þ 1/2, z; (xi) -x, y, -z þ 1/2.)
mna ligands. The C2 axis passes through Cu1 and O5W, giving them half occupancy. The anionic [Ag6(mna)6]6- lies on the crystallographic inversion center. The detailed bond distances and angles are listed in Table S1. The cationic and anionic parts in complex 1 have been reported,17 so the structure of complex 1 is not discussed here and the emphasis will be on the novel nonamer water cluster. As shown in Figure 2, nine water molecules in 1 are hydrogen bonded to form a crownlike geometry which is constructed by a pair of fused boatlike six-membered rings sharing two hydrogen bonds. In detail, there are one O5W and two symmetry-related O1W, O2W, O3W, and O4W in this water cluster. Among them, only O1W is not isolated but weakly coordinates with Cu(II). Alternatively, this nonamer can also be seen as an eight-membered ring with a bridgehead O5W up. A C2 axis passes through O5W, giving the nonamer a C2 symmetry. In the nonamer, O5W acts as a double hydrogen bond acceptor and the other eight water molecules act as single hydrogen bond donor as well as Published on Web 10/20/2010
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Table 1. Geometrical Parameters of Hydrogen Bonds and Angles (A˚ and deg) for the Water Nonamera D-H 3 3 3 A D-H/A˚ H 3 3 3 A/A˚ D 3 3 3 A/A˚ D-H 3 3 3 A/deg O1W-H1WB 3 3 3 O5W 0.85 1.96 2.800(4) 169 O1W-H1WA 3 3 3 O4Wiii 0.85 1.96 2.793(4) 166 O2W-H2WA 3 3 3 O1W 0.85 2.14 2.954(5) 161 O3W-H3WB 3 3 3 O2W 0.85 2.32 2.970(6) 133 O4W-H4WB 3 3 3 O3Wix 0.85 2.07 2.885(5) 159 O2W-H2WB 3 3 3 O4viii 0.85 2.00 2.812(5) 160 O3W-H3WA 3 3 3 O1 0.85 2.00 2.817(4) 162 O4W-H4WA 3 3 3 O3 0.85 1.85 2.696(4) 177 O5W-H5WA 3 3 3 O1 0.85 1.95 2.722(3) 150 a Symmetry codes: (iii) -x þ 1/2, y þ 1/2, -z þ 1/2; (ix) x þ 1/2, y - 1/2, z; (x) x - 1/2, y þ 1/2, z; (xi) -x, y, -z þ 1/2.
acceptor. Within the nonamer water cluster, each water molecule is involved in three hydrogen bonds, except the O5W (four hydrogen bonds), giving a total of 18 hydrogen bonds. As we know, hitherto, only two crystallographically characterized discrete nonamer water aggregates have been reported; one exhibits a finite zigzag water chain,14b and another can be described as an S-shaped water hexamer connecting a near linear water trimer,18 both of which are not cyclic water aggregates and indicate the bicyclic nonamer water cluster in 1 is a novel one. The hydrogenbonded O 3 3 3 O separations in the nonamer (Table 1) span the range 2.793(4)-2.970(6) A˚, with an average value of 2.880(5) A˚, which is longer than the 2.758 A˚ generated from the ab initio calculations.19 As listed in Table S2, the average O 3 3 3 O 3 3 3 O bond angle in the nonamer is 84.57(13)°, which is different from the tetrahedral angle found in ice Ih and Ic,8b and the average torsion angle involving four neighboring water molecules is 97.56(15)°. The structural landscape associated with the nonamer is extensive, and theoretical calculations20 based on the HartreeFock level of theory as well as the DFT levels predict five cyclic structural isomers for the nonamer water cluster. Among them, the minimum-energy structure can be described as follows: one additional water molecule breaks one of the edges of the D2d cubic octamer water cluster and is two-coordinated to it. Alternatively, it also can be built from a pentamer and a tetramer, which are fused together by sharing four hydrogen bonds. This geometry had 13 hydrogen bonds and was also suggested by infrared depletion spectroscopy of size-selective clusters.21 Clearly, theoretical predictions of the geometry of water cluster have little bearing on the liquid or solid state, where there is a subtle interaction between the formation of a low-energy configuration and its interaction with its environments.6a Hence, although the bicyclic crownlike nonamer in 1 may be a high-energy configuration, the lattice of a supramolecular host may offer an additional stabilization energy for it. Even if considering the subtle interactions between water cluster and its host in the liquid or solid state, the cyclic structure is possibly more stable than a linear one, owing to the possibility of formation of a maximum number of hydrogen bonds within the cluster.14b The O1W in the nonamer weakly interacts with Cu(II) in the axial direction. Thus, in effect, the water nonamer, as a supramolecular entity, fulfills the role of an exo-bidentate metal bridging ligand. Overall, the nonamer water cluster is anchored by Owater-H 3 3 3 Omna hydrogen bonds (avg 2.824(4) A˚, Table S3) and a Cu-O1W weak interaction (2.794(3) A˚), which contribute to the stabilization of the overall structure (Figure S4). Complex 1 is purple in the solid state, and its UV-vis absorption spectra are shown in Figure S5. The low-energy broad peak at ca. 546 nm is mainly assigned to the metal-centered (MC) transition, which can deactivate via ultrafast nonradiative pathways and is typical for other [Cu(eda)2(H2O)2]2þ complexes,22 while intense absorption bands in the range 260-356 nm should be the transition of ligand involving π orbitals of the aromatic ring of the mna moiety.
Figure 3. Photoluminescence of free ligand and complex 1 in the solid state.
Recent interest in photoluminescent metal complexes has mostly focused on Cu(I), Ag(I), and Au(I) complexes;23 in contrast, the photoluminescent properties of heterometallic complexes, especially the Ag(I)-Cu(II) complexes, have received rare attention. The photoluminescences of complex 1 and Na6[Ag6(mna)6]17c as well as free H2mna ligand were measured in the solid state at room temperature (Figure 3). Under 365 nm excitation, the Na6[Ag6(mna)6] emission is at 551 nm, accompanying a shoulder peak at 469 nm. Considering the emission of free H2mna (λem = 470 nm, λex = 300 nm), the peaks at 469 and 551 nm can be attributed to π-π* ligand centered transitions of mna ligand and ligand-to-metal charge transfer (LMCT), mixed with metal-centered (d-s/d-p) transitions, respectively.24 Compared to Na6[Ag6(mna)6], the maximum of the emission band of complex 1 is blue-shifted to 421 nm (λex = 300 nm), mainly belonging to π-π* ligand centered transitions of the mna ligand. The large emission shift between 1 and Na6[Ag6(mna)6] (Δem = 130 nm) indicates the excited state is essentially changed by incorporation of Cu(II), which possesses nonemissive low-lying MC levels that would quench the LMCT excited state by energy transfer or thermal equilibration.23a These results also indicate that introducing the secondary metal ions into the hexanuclear Ag(I) cluster can realize the modulation of the emission behavior. In the thermal gravimetric (TGA) curve (Figure S6), a gradual weight loss of 1 occurred from 50-106 °C, which is attributed to the loss of both coordination and lattice water molecules. The weight loss is about 11.89%, in correspondence with the calculated value of 11.31%. From 110 to 398 °C, 54.22% weight loss occurred, which corresponds to the decomposition of the organic ligands eda and mna (calculated: 53.55%). In summary, we observed a novel crownlike nonamer water cluster encapsulated in a heterometallic supramolecular complex obtained by diffusion of two presynthesized solutions. Moreover, the UV-vis absorption and photoluminescence spectra as well as the thermal stability of 1 were discussed. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20721001 and 21071118), 973 Project (Grant 2007CB815301) from MSTC, and The National Science Fund of China for Fostering Talents in Basic Science (No. J0630429). Supporting Information Available: The synthesis of complex 1, selected bond lengths and angles, table of hydrogen bonds, additional pictures, X-ray crystallographic files in cif format for complex 1, simulated and experimental X-ray powder diffraction patterns, IR and UV-vis spectra, and TGA curve. This material is available free of charge via the Internet at http://pubs.acs.org.
Communication
Crystal Growth & Design, Vol. 10, No. 12, 2010
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