Self-Assembled Hexameric Water Clusters Stabilized by a Cyano-Bridged Trimetallic Complex Uday Mukhopadhyay and Ivan Bernal* Department of Chemistry, University of Houston, Houston, Texas 77204 Received April 29, 2005;
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1687-1689
Revised Manuscript Received July 12, 2005
ABSTRACT: An icelike hexameric water cluster, stabilized by a cyano-bridged trimetallic complex, was characterized by X-ray diffraction studies, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The stereochemical role of water in proteins is well established; however, detailed positional information for the water molecules and their nature (e.g., do they form clusters? If so, what size and shape?) is still preliminary. Nonetheless, there is mounting evidence of the presence of ordered water clusters in the active clefts of proteins such as Scarpharca dimeric hemoglobin,1 crambin,2 actidin,3 and carbonic anhydrase C.4 Water can also play an important role in biological self-assembly processes, for example, in stabilizing the conformation of native biopolymers.5,6 For all those reasons, water clusters have been extensively studied, both theoretically and experimentally, in attempts to provide insight into the structure and properties of liquid water or ice.7-12 A variety of water clusters, such as trimers, tetramers, and pentamers, have cyclic and quasiplanar minimum energy structures, and water hexamers are particularly interesting among the possible clusters. Recently, it has been asserted that this cluster is the smallest possible unit that can exhibit some of the properties found in bulk water.13 Theoretical studies have shown that the energetically favorable isomers of water hexamers are cage, book, prism, boat, and cyclicsthe most stable one being the cage isomer, whose existence has been proven experimentally.14 The higher energy cyclic hexamer was isolated, and its structure was characterized by IR spectroscopy.15 Recently, chair16,17 and boat18 cyclic hexamers, included in host lattices, have been characterized using X-crystallographic studies. A very thorough examination of the stereochemistry of water molecules present in crystals of organic compounds was recently published.19 Recent studies of the stereochemistry of one-, two-, and three-dimensional assemblies of transition metal complexes, bridged by metallic hexa- and octa-cyanide linkages, have revealed an interesting set of water clusters, stabilized within the crystals by hydrogen-bonded linkages to the cyano nitrogen nonbonded pairs. Here, we report the structure of self-assembled cyclic water hexamers (Figure 2a) with an icelike chair conformation (Figure 2b) stabilized by several cyano ligands of a trimetallic complex. Altogether, six water clusters are strongly hydrogen bonded to six different cyano nitrogens of [Mo(CN)8]4-, whose remaining two cyanides are the bridges to two pancakelike amino-Cu(II) moieties. In the process of synthesizing different cyano-bridged metal complexes, we isolated a trimetallic, cyano-bridged complex that stabilized icelike hexameric, chair form water clusters in a heretofore never seen manner.20 A recent report described an infinite, hexagonal, twodimensional, water sheet stabilized by the solid-state structure of CdNi(CN)4 layers.18 Here, the hexameric water cluster is unique and unlike the previously published * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. The space filling hexameric water ring is surrounded by six cyano-bridged complexes and connected each other by hydrogen bonding.
hexamer clusters.16-18 In most cases, water hexamers form water ribbons residing in channels and are connected to the host lattices via very weak hydrogen bonding. However, in our case, discrete water hexameric clusters are connected to the host lattices via very strong hydrogen bonding. In our synthesis, an aqueous solution of [Cu(L)Cl2]21 (L ) 1,4-bis(3-aminopropyl)piperazine) was slowly added, in total darkness, to an aqueous solution of K4[Mo(CN)8] until a 2(Cu):1(Mo) mole ratio was achieved at 21 °C (Scheme 1). After several days, beautifully shaped dark blue crystals were formed, suitable in size for single-crystal X-ray diffraction.22 Two [Cu(L)Cl2] cations are connected by two cyanides to a [Mo(CN)8]4- anion, as is shown in Scheme 1. An extension into a polymeric assembly was purposely prevented by the steric hindrance introduced by the amine backbone (L). The remaining six cyanides are connected through hydrogen bonding with six hexameric water clusters. Every water cluster is also connected with six [Mo(CN)8]4- anions (Figure 1) and form the three-dimensional (3D) polymeric arrays. The two copper ions exist in a distorted square pyramidal geometry, connected to five nitrogens of which four are from the amine ligand (L) and a fifth one is from a cyanide bridge. Present in the asymmetric unit are four cyanides and three water molecules, while the Mo atom, with half occupancy, is at a special position. Three additional molecules of water are generated by a 2-fold axis, to form the hexameric water cluster. In each hexamer, the average distance between the oxygen atoms is 2.74 Å, which is comparable with the O- - -O distance in ice Ic (2.75 Å)23 and Ih (2.759 Å)24 at -130 and -90 °C, respectively. The bond angles between Ow1Ow2-Ow3, Ow2-Ow3-Ow1A and Ow3-Ow1A-Ow2A
10.1021/cg0501940 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/30/2005
1688
Crystal Growth & Design, Vol. 5, No. 5, 2005
Communications
Figure 3. 3D packing diagram of the molecule along the bprojection. Ligand molecules are not included for clarity.
Figure 2. (a) Ball and stick representation of the hexameric water cluster. Dotted turquoise lines show the hydrogen bonding between water molecules. Bond distances for the hydrogen bonding present are shown. (b) Ball and stick representation of the chair conformation of the hexameric water cluster. Bold turquoise lines show the hydrogen bonding between the water molecules.
Scheme 1. Diagramatic Description of the Reaction Leading to the Trimetallic Assembly, and the Crystal Structure of the Trinuclear Cyano-Bridged Complex
are 120.77, 118.78, and 104.45°, respectively. The water hexamer clusters are formed with the help of a O-H- - -O hydrogen bond (range of the bonds between 1.845 and 2.792 Å). The hydrogen bonding between waters and cyanide
Figure 4. The FTIR spectra of the complex before (a) and after (b) heating of the sample at 120 °C.
nitrogens are relatively strong (average distance 1.94 Å). In the crystal packing Cu(app)Cl2 molecules are surrounded by waters. Water clusters remain stable at room temperature (22 °C) for an indefinite time. Thermogravimetric (TG) analysis analysis shows that the total water loss occurs in one step within 100 °C. The Fourier transform infrared (FTIR) spectroscopy results show the O-H stretching frequency of the water cluster as a broad band, around 3300 cm-1 (Figure 4a), which disappeared (Figure 4b) when the compound was heated to 120 °C for 2 h. The remaining spectrum is more or less the same due to the unchanged nature of the complex at that temperature. In summary, we have characterized an icelike selfassembled hexameric water cluster trapped within a metal complex molecular host. We feel that these kinds of studies may be important in understanding the hydrogen-bonding motifs responsible for the anomalous character of water in living systems, as well as in the study of synthetic models, useful in theoretical and computational studies. Acknowledgment. We thank the Robert A. Welch Foundation for support (Grant E-592 to I.B.) and for a postdoctoral appointment awarded to U.M. Supporting Information Available: X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
Communications
Crystal Growth & Design, Vol. 5, No. 5, 2005 1689
References (1) Royer, W. E.; Pardanani, A.; Gibson, Q. H.; Peterson, E. S.; Feldman, J. M. Proc. Natl Acad. Sci. U.S.A. 1996, 93, 14526. (2) Teeter, M. M. Proc. Natl Acad. Sci. U.S.A. 1984, 81, 6014. (3) Baker, E. N. J. Mol. Biol. 1980, 141, 441. (4) Liljas, A.; Kannan, K. K.; Bergsten, P. C.; Waara, I.; Fridborg, K.; Standberg, K.; CarlbomL. U.; Jarup, Lovgren, S.; Petef, M. Nat. New Biol. 1972, 235, 131. (5) Dehl, R. E.; Hoeve, C. A. J. Chem. Phys. 1969, 50, 3245. (6) Migchelsen, C.; Berendsen, H. J. C.; Rupprecht, A. J. Mol. Biol. 1968, 37, 235. (7) Ugalde, J. M.; Alkorta, I.; Elguero, J. Angew. Chem. 2000, 112, 733; Angew. Chem. Int. Ed. 2000, 39, 717. (8) Ludwig, R. ChemPhysChem 2000, 1, 53. (9) Ludwig, R. Angew. Chem. 2001, 113, 1856; Angew. Chem. Int. Ed. 2001, 40, 1808. (10) Liu, K.; Krujan, J. D.; Saykally, R. J. Science 1996, 271, 929. (11) Colson, S. D.; Dunning, T. H., Jr. Science 1994, 265, 43. (12) Matsumoto, K.; Saito, S.; Ohmine, I. Nature 2002, 416, 409. (13) Gregory, J. K.; Clary, D. C.; Liu, K.; Brown, M. G.; Saykally, R. J. Science 1997, 275, 814. (14) Liu, K.; Brown, M. G.; Carter, C.; Saykally, R. J.; Gregory, J. K.; Clary, D. C. Nature, 1996, 381, 501. (15) Nauta, K.; Miller, R. E. Science 2000, 287, 293. (16) Custelcean, R.; Afloroaei, C.; Vlassa, M.; Polverejan, M. Angew. Chem. 2000, 112, 3224; Angew. Chem. Int. Ed. 2000, 39, 3094. (17) Foces-Foses, C.; Cano, F. H.; Martinez-Ripoll, M.; Faure, R.; Roussel, C.; Claramunt, R. M.; Lopez, C.; Sanz, D.; Elguero, J. Tetrahedron Asymmetry 1990, 1, 65. (18) Park, K.-M.; Kuroda, R.; Iwamoto, T. Angew. Chem. 1993, 105, 939; Angew. Chem., Int. Ed. Engl. 1993, 32, 884. (19) Infantes, L.; Chisholm, J.; Motherwell, S. CrystEngComm 2003, 5, 480.
(20) We have searched the Cambridge File (Conquest release of November 2004) for all crystalline hydrates using the drawn motif {[(Anymetal)(CN)8] + H2O} and found no examples of hexameric water clusters attached to the cyano nitrogens. (21) CuCl2 (1.70 g, 10 mmol) in methanol (20 mL) was slowly added to a stirring solution of ligand (L) (2.00 g, 10 mmol) in methanol (25 mL). The resultant blue solution was stirred for 15 min, and its solvent was removed with a rotary evaporator, whereupon a blue solid was formed. (22) X-ray crystallography: Data were collected with an EnrafNonius CAD-4 diffractometer, using monochromatized Mo KR radiation. They were collected over the range 4° e 2θ e 52° and corrected for absorption. Data were processed using the WinGX package (Farrugia, L. J. Appl. Crystallogr. 1999, 32, 837). Solution and refinement used the programs SHELXS97 (Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467) and SHELXL97 (Sheldrick, G. M. SHELXL-97. Program for the Solution and Refinement of Crystal structures; University of Go¨ttingen: Germany, 1997). Crystal data: C14H30CuMo0.5N8O3, M ) 469.97, monoclinic, a ) 20.735(5) Å, b ) 9.323(3) Å, c ) 20.978(5) Å, β ) 95.80(4)°, V ) 4034.7(19) Å3, T ) 293(2) K, space group C2/c, Z ) 8, µ(Mo KR) ) 1.411 mm-1. 3964 reflections measured, 3268 unique. R1 ) 0.0269, wR2 ) 0.062. Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 265556. Copies of the data can be obtained free of charge on application to CCDC, 12 union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223336-033; e-mail:
[email protected]). (23) Ko¨nig, H. A. Z. Kristallogr. 1944, 105, 279. (24) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: Oxford, 1969.
CG0501940
