CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1569-1574
Articles Crystallographic and Theoretical Evidence of Acetonitrile-π Interactions with the Electron-Deficient 1,3,5-Triazine Ring Tiddo J. Mooibroek,† Simon J. Teat,‡,⊥ Chiara Massera,§ Patrick Gamez,*,† and Jan Reedijk*,† Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Diamond Light Source Ltd., Diamond House, Chilton, Didcot, Oxfordshire OX11 0DE, U.K., Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, UniVersita´ degli Studi di Parma, Parco Area delle Scienze 17/A, 43100 Parma, Italy ReceiVed March 15, 2006; ReVised Manuscript ReceiVed April 26, 2006
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: Reaction of zinc(II) chloride with the ligand N,N′-{2,4-(di-2-pyridylamino)-1,3,5-triazin-6-yl}-1,4,10,13-tetraoxa7,16-diazacyclooctadecane (oxodendtriz) in acetonitrile/methanol results in the formation the tetranuclear complex [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5. Notably, its crystal structure reveals unique π-interactions with the first experimental evidence of electronrich/electron-poor molecule pairing, which is corroborated by computational studies. In addition, infinite helical water chains with both right- and left-handed configurations are present in the crystal lattice. Introduction Noncovalent interactions between molecules are weak intermolecular contacts that play a pivotal role in biological systems and govern the physicochemical properties of molecular systems in the condensed phase.1 Noncovalent binding interactions are nowadays commonly used for the self-assembly of large supramolecular aggregates in solution with specific chemical properties.2-4 Moreover, the molecular recognition of guest molecules by synthetic host receptors still remains an important application of supramolecular chemistry. In that context, recent theoretical5 and experimental6 reports have clearly shown that the 1,3,5-triazine ring can act as a Lewis acid, which favorably interacts with anions.7,8 Recently, we have undertaken research investigations on coordination compounds involving N-donor ligands containing the s-triazine ring. 1,3,5-Triazine derivatives have undoubtedly found outstanding applications in the field of supramolecular chemistry,9 where the electron-poor heteroaromatic moiety is implicated in key supramolecular interactions, that is, hydrogen bonds or π interactions.10 Thus, investigations on potential noncovalent interactions between a neutral molecule (Lewis base) and 1,3,5-triazine can be of general interest regarding the * To whom correspondence should be addressed. E-mail addresses:
[email protected] (P.G.);
[email protected] (J.R.). † Leiden University. ‡ Diamond Light Source Ltd. ⊥ Present address: Advanced Light Source, Berkeley Lab, 1 Cyclotron Rd, MS2-400, Berkeley, California 94720. § Universita ´ degli Studi di Parma.
Chart 1. N,N′-{2,4-(Di-2-pyridylamino)-1,3,5-triazin-6-yl}1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (Oxodendtriz)
role of aromatic rings in supramolecular chemistry. In addition, such studies may lead to the development of a novel approach for the design of effective systems exhibiting noncovalent molecular recognition properties. In the present study, the first coordination compound of the azacrown ligand N,N′-{2,4-(di-2-pyridylamino)-1,3,5-triazin-6yl}-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (Chart 1, oxodendtriz) is reported. Reaction of 4 equiv of zinc(II) chloride with 1 equiv of the ligand oxodendtriz in acetonitrile/methanol leads to the formation of a tetranuclear metal complex, which exhibits unique supramolecular features owing to the presence of acetonitrile-π interactions with the triazine ring and infinite helical water chains. Experimental Section General. The ligand precursor 2-chloro-4,6-(di-2-pyridylamino)1,3,5-triazine (Cl-dpyatriz) was prepared under argon following a literature method.11 Solvents and reagents were purchased from
10.1021/cg060144a CCC: $33.50 © 2006 American Chemical Society Published on Web 05/27/2006
1570 Crystal Growth & Design, Vol. 6, No. 7, 2006 commercial sources and used without further purification. The syntheses were performed under an inert atmosphere of argon, and the purifications were commonly performed in air. Elemental analyses (C, H, and N) were performed with a Perkin-Elmer 2400 analyzer. FTIR spectra were recorded with a Perkin-Elmer Paragon 1000 FTIR spectrophotometer, equipped with a Golden Gate ATR device, using the reflectance technique (4000-300 cm-1). 1H and 13C NMR spectra were recorded using a DPX 300 Bruker (300 MHz) instrument. Chemical shifts are reported in ppm (parts per million) relative to the solvent peak. Electrospray ionization (ESI) mass spectroscopy was carried out using a Finnigan Aqa mass spectrometer equipped with an ESI source. Sample solutions (10 µL of a 1 mg mL-1 solution) were introduced in the ESI source by using a Dionex ASI-100 automated sampler injector and an eluent running at 0.2 mL min-1. Preparation of N,N′-{2,4-(Di-2-pyridylamino)-1,3,5-triazin-6-yl}1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (Oxodendtriz). 1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane (4,13-diaza-18-crown-6) (0.26 g, 1.18 mmol) was dissolved in a tetrahydrofuran/acetonitrile solvent mixture (THF/CH3CN ) 5:3; 10 mL) under argon. Two equivalents of N,N-diisopropylethylamine (DIPEA) (0.23 g, 1.80 mmol) were added to the two-necked round-bottomed flask, after which Cl-dpyatriz was added portion-wise (0.87 g, 1.91 mmol). After completion of the addition, the reaction mixture was heated under reflux for 24 h. Thereafter, the reaction mixture was cooled to 4 °C, and the white precipitate was isolated on a glass filter. The precipitate was washed with methanol (2 × 8 mL) to remove the N-diisopropylethylamine hydrochloride salt. The pure product oxodendtriz was dried overnight at 100 °C under reduced pressure. Yield ) 89% (1.15 g). 1H NMR (300 MHz, MeOD, 25 °C): δ ) 3.34 (s, 8H, 2-crown-H), 3.36 (dd, 8H, 5-crown-H), 3.51 (dd, 8H, 6-crown-H), 7.18 (dd, 8H, 4-py-H), 7.51 (d, 8H, 6-py-H), 7.76 (dd, 8H, 5-py-H), 8.27 (d, 8H, 3-py-H) ppm. 13C NMR (300 MHz, CDCl3): 48.2 (6-crown-C), 69.2 (5-crown-C), 70.3 (2-crown-C), 120.4 (6-py-C), 122.8 (4-py-C), 136.8 (5-py-C), 148.4 (3-py-C), 155.6 (1-py-C), 164.5 (6-triaz-C), 165.6 (2-triaz-C) ppm. IR (neat): 2876, 1590, 1549, 1528, 1471, 1458, 1427, 1406, 1368, 1300, 1264, 1234, 1152, 1112, 1100, 1076, 1026, 996, 986, 938, 910, 849, 806, 780, 770, 752, 741, 671, 654, 634, 620, 576, 535, 512 cm-1. ESI mass spectroscopy, m/z found (calcd): M+ ) 1097.9 (1097.2); M2+ ) 549.1 (548.6). Anal. Calcd for C58H56N20O4: C, 63.49; H, 5.14; N, 25.53%. Found: C, 63.00; H, 5.59; N, 25.45%. Preparation of [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5. ZnCl2 (10.9 mg, 0.08 mmol) was added to a solution of oxodendtriz (21.8 mg, 0.02 mmol) in methanol/acetonitrile 1:1 (25 mL). The resulting reaction mixture was left unperturbed for the slow evaporation of the solvent. After 6 days, colorless single crystals suitable for X-ray crystallography were obtained. Yield ) 83.8% (27.5 mg). 1H NMR (300 MHz, MeOD, 25 °C): δ ) 3.42 (m, 16H, 2-crown-H and 5-crownH) 3.48, (dd, 8H, 6-crown-H), 7.31 (dd, 8H, 4-py-H), 7.60 (d, 8H, 6-pyH), 7.86 (dd, 8H, 5-py-H), 8.36 (d, 8H, 3-py-H) ppm. Anal. Calcd for C62H65Cl8N22O5.5Zn4: C, 42.52; H, 3.74; N, 17.59%. Found: C, 42.91; H, 4.11; N, 17.99%. Main IR absorption bands for [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5: 1605, 1582, 1569, 1533, 1480, 1466, 1417, 1386, 1368, 1307, 1273, 1119, 1104, 1084, 1054, 1028, 808, 790, 773, 760, 745, 689, 667, 654, 644, 630, 428 cm-1. X-ray Crystallographic Analysis and Data Collection. Intensity data for single crystals of [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5 were collected using station 9.812 Synchrotron Radiation Source at Daresbury laboratory with radiation (λ ) 0.6894 Å) on a Bruker APEX II CCD area-detector diffractometer. The crystal was mounted on the end of a two-stage glass fiber with perfluoropolyether oil, and placed in the cold flow produced with an Oxford Cryosystems 700 series cryostream cooler (150 K). The data nominally covered a sphere of reciprocal space by collecting a series of frames using a series of three 180° ω-rotations with φ angles of 0°, 120°, and 240°. Reflection intensities were integrated using SAINT,13 allowing for the plane-polarized nature of the primary synchrotron beam. Corrections were applied semiempirically for absorption and incident beam decay.14 Unit cell parameters were refined from the observed ω angles of all strong reflections in the complete data sets. Structure solution and refinement were performed with the SHELXTL package.15 The structure was solved by direct methods and completed by iterative cycles of ∆F syntheses and full-matrix least-squares refinement against F2. The number of parameters used were 460 with 173 restraints. CCDC-608271 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
Mooibroek et al.
Figure 1. ORTEP drawing (30% probability level) of [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5. Color code: N, blue; C, gray; O, red; Zn, brown; Cl, green. Hydrogen atoms, water, and acetonitrile molecules are omitted for clarity. [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. Fax: +44-1223/336-033. E-mail: deposit@ ccdc.cam.ac.uk]. Geometric calculations and molecular graphics were performed with the PLATON package.16 Computational Methods. Geometry optimizations and frequency calculations of the complex triazine-acetonitrile were carried out at the Hartree-Fock (HF) and Møller-Plesset second-order (MP2)17,18 level, as implemented in Gaussian0319 using the 6-311++G(3d,p) basis set. Default optimization procedures were used and symmetry was disabled in all calculations. The geometries of the isolated acetonitrile and 1,3,5-triazine fragments were optimized at the MP2/6-311G(d,p) level. Three different starting geometries for the complex were chosen, and all the resulting structures were characterized as minima on the MP2/ 6-311++G(3d,p) potential energy surface by frequency calculations. To obtain more reliable values of the binding energy between the two fragments, single-point calculations at the MP2 level were carried out on two of the optimized structures using the 6-311G(d,p), 6-311++G(d,p), 6-311++G(3df,p), cc-PVDZ, cc-PVTZ, and cc-PVQZ basis sets. The basis set superposition error (BSSE)20 was corrected in all calculations with the method of Simon21 as implemented in Gaussian03. Calculations have been carried out on an IBM SP5/512 supercomputer at CINECA.22
Results and Discussion [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5 was obtained by treatment of zinc(II) chloride with the ligand oxodendtriz (Chart 1) in methanol/acetonitrile at room temperature. The structure of the complex is represented in Figure 1. Upon coordination to the zinc ions, the NMR resonances of the pyridine protons are logically shifted downfield (by about 0.10 ppm), as compared to those of the free ligand (see Experimental Section). Crystallographic data for this coordination compound are listed in Table 1, while metric parameters around the metal centers are listed in Table 2. The neutral tetranuclear complex [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5 crystallizes on a crystallographic inversion center in the monoclinic space group P21/n (Table 1). As shown in Figure 1, the coordination compound comprises four tetracoordinated zinc ions, which are almost in perfect tetrahedral geometries. The angles around Zn(1) and Zn(2) are close to 120°, varying from 106.63(9)° to 119.85(4)° for Zn(1) and from 107.26(12)° to 117.95(10)° for Zn(2) (Table 2). However, one angle is significantly smaller than the ideal value corresponding to a perfect tetrahedron. Indeed, the N(20)-Zn(1)-N(30) angle is 90.03(12)° and the N(40)-Zn(2)-N(50) angle is only 89.29(18)°. These distortions most likely originate from the small bite angle of the dipyridylamino bidentate moiety. The chloride
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Table 1. Crystallographic Data and Refinement Details for [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5 chemical formula formula weight temp (K) radiation type radiation wavelength (Å) crystal system space group unit cell params a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) cell volume (Å3) Z calcd density (g/cm3) absorption coefficient, µ (mm-1) F(000) crystal color crystal size (mm3) reflns for cell refinement data collection method
C62H65Cl8N22O5.5Zn4 1751.44 150(2) synchrotron 0.6894 monoclinic P21/n
θ range for data collection index ranges completeness to θ ) 25.00° reflns collected independent reflns reflections with F2 > 2σ absorption correction min. and max. transmission structure solution refinement method weighting parameters a, b data/restraints/params final R indices [F2 > 2σ] R indices (all data) goodness-of-fit on F2 largest and mean shift (su) largest diff. peak and hole (e Å-3)
20.0353(11) 9.0755(5) 21.8692(12) 90 96.769(2) 90 3948.8(4) 2 1.473 1.531 1782 colorless 0.10 × 0.08 × 0.02 6985 (θ range 2.94° to 29.43°) Bruker APEX II CCD diffractometer ω rotation with narrow frames 1.82° to 29.52° h -28 to 28; k -8 to 12; l -31 to 31 96.0% 34 726 11 582 (Rint ) 0.0285) 9546 semiempirical from equivalents 0.81 and 0.93 direct methods full-matrix least-squares on F2 0.0837, 9.1080 11582/173/460 R1 ) 0.0706, wR2 ) 0.1985 R1 ) 0.0830, wR2 ) 0.2064 1.105 0.001 and 0.000 1.180 and -1.165
Table 2. Selected Bond Lengths [Å] and Angles [deg] for [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5a Zn(1)-Cl(1) Zn(1)-Cl(2) Zn(1)-N(20) Zn(1)-N(30)
2.1924(11) 2.2215(9) 2.067(3) 2.061(3)
Cl(1)-Zn(1)-Cl(2) Cl(1)-Zn(1)-N(20) Cl(1)-Zn(1)-N(30) Cl(2)-Zn(1)-N(20) Cl(2)-Zn(1)-N(30) N(20)-Zn(1)-N(30)
119.85(4) 114.76(9) 113.59(9) 107.76(8) 106.63(9) 90.03(12)
Zn(2)-Cl(3) Zn(2)-Cl(4) Zn(2)-N(40) Zn(2)-N(50)
2.2139(15) 2.205(2) 2.080(5) 2.038(4)
Cl(3)-Zn(2)-Cl(4) Cl(3)-Zn(2)-N(40) Cl(3)-Zn(2)-N(50) Cl(4)-Zn(2)-N(40) Cl(4)-Zn(2)-N(50) N(40)-Zn(2)-N(50)
117.95(10) 109.15(12) 113.12(12) 116.53(14) 107.26(12) 89.29(18)
a
Symmetry operations for equivalent atoms: A -x + 1, -y, -z + 1.
anions Cl(1), Cl(2), Cl(3), and Cl(4) are located at normal distances from the zinc centers with bond distances in the range 2.1924(11)-2.2215(9) Å. The coordinated pyridine nitrogen atoms bind to Zn(1) and Zn(2) with typical bond lengths of 2.038(4)-2.080(5) Å. Both the Zn-Cl and the Zn-N bond distances observed in the two metal coordination spheres can be considered as normal.23,24 One of the most striking features of this coordination material is the remarkable acetonitrile-π interaction with the ligand (Figure 2). Interactions between an anion and the s-triazine ring
Figure 2. Acetonitrile-π interaction in [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5 between the nitrogen atom of a disordered solvent molecule and the s-triazine ring: centroid‚‚‚N(1s) ) 3.249(5) Å; triazine plane-centroid-N(1s) axis ) 75.48(3)°; centroid‚‚‚N(1s′) ) 3.087(5) Å; triazine plane-centroid-N(1s′) axis ) 82.84(3)°. W An interactive 3D image is available in CSD Mercury format (http:// www.ccdc.cam.ac.uk/products/csd_system/mercury/).
have been favorably predicted by several research groups using computational methods,5,25,26 and the first crystallographic evidence of such noncovalent binding has been recently published.6 Very recently, Ugozzoli et al.27 carried out theoretical investigations on the intermolecular interactions between the electron-rich surface of benzene and the electron-poor surface of 1,3,5-triazine, which appeared to be energetically possible. Lately, Tarakeshwar and Brutschy28 reported computational studies that clearly demonstrate favorable interactions between ammonia and various aromatic π-systems, such as toluene or chlorobenzene. The present case therefore represents the first experimental proof for such interactions between a neutral electron-rich molecule and an electron-deficient aromatic ring. Thus, a disordered acetonitrile molecule is located on top of a triazine ring with its nitrogen atoms N(1s) and N(1s′) [with occupancy factors of 50% for both, N(1s) and N(1s′)] pointing to the center of the heteroaromatic ring. The distance between the nitrogen atom N(1s) of the acetonitrile molecule and the π-cloud of the 1,3,5-triazine ring is 3.249(5) Å and the angle of the triazine plane-centroid-N(1s) axis to the plane is 75.2(3)° (Figure 4). For N(1s′), the N(1s′)-centroid bond length is shorter, namely, 3.087(5) Å, and the angle triazine planecentroid-N(1s′) axis amounts to 82.84(3)° (Figure 2). The above-mentioned interaction was also studied from the theoretical point of view. First of all, a geometry optimization of the complex acetonitrile-triazine was carried out, starting from three different geometries (complex 1, 2, and 3, see Figure 3, panels a, b, and c, respectively). The choice of such initial geometries was suggested by the pattern of the molecular electrostatic potential (MEP), plotted onto the isodensity surface calculated at 0.0004 e Å-3 for the two isolated fragments (see Figure 4): in complex 1, the acetonitrile molecule is perpendicular to the triazine moiety with the nitrogen atom pointing to the center of the heteroaromatic ring; in complexes 2 and 3, the reciprocal orientation of the two fragments was that obtained by the X-ray analysis (see Figure 2 and Table 2). The result of all the geometry optimizations led to the structures of complex 4 (Figure 5a) and complex 5 (Figure 5b) (see Supporting Information for the details). In complex 4, the equilibrium
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Figure 3. Starting geometry for complexes 1 (N-centroid 3.150 Å) (a), 2 (N-centroid 3.249 Å) (b), and 3 (N-centroid 3.087 Å) (c). Color code: N, blue; C, gray; H, white, centroid, green.
Figure 4. Molecular electrostatic potential (MEP) for acetonitrile (a) and 1,3,5-triazine (b). MEP values in kJ/mol are color-coded as follows: from -100 (red) to +100 (light blue) for triazine and from -150 (red) to +150 (light blue) for acetonitrile.
N-centroid distance is 2.949 Å, while the initial geometries 2 and 3 both converged to the same equilibrium structure (complex
Figure 5. Optimized geometries at the MP2/6-311G(d,p) level. Complex 1 converged to complex 4 (N-centroid 2.949 Å) (a) and complexes 2 and 3 to complex 5 (N-centroid 3.116 Å) (b). Color code: N, blue; C, gray; H, white, centroid, green.
5) in which the N-centroid distance is 3.116 Å, the angle between the triazine plane and the N-centroid axis is 90°, and the C-CtN fragment is parallel with respect to the aromatic
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Table 3. Calculated HF and MP2 Interaction Energies of Complexes 4 and 5 Using Different Basis Setsa complex 4
6-311G(d,p)
6-311++G(d,p)
6-311++G(3df,p)
cc-PVDZ
cc-PVTZ
cc-PVQZ
EHF EMP2 Ecorrb
3.55 -7.47 -11.02
3.06 -9.82 -12.88
4.71 -12.32 -17.03
4.82 -6.03 -10.85
4.79 -10.10 -14.89
4.73 -12.02 -16.75
complex 5
6-311G(d,p)
6-311++G(d,p)
6-311++G(3df,p)
cc-PVDZ
cc-PVTZ
cc-PVQZ
EHF EMP2 Ecorrb
5.30 -10.05 -15.35
5.38 -11.89 -17.27
6.19 -16.52 -22.71
6.14 -8.92 -15.06
6.16 -14.61 -20.77
6.20 -16.95 -23.15
a
The energies are in kJ/mol, and all the values are corrected for BSSE. b MP2 correlation interaction energies calculated as difference between EMP2 and
EHF.
Figure 7. Racemic (left- (L) and right- (R) handed) hydrogen bonded helixes with characteristic dimensions, as found in the crystal lattice of [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5.
Figure 6. View of two infinite helical water chains along the crystallographic c axis. W An interactive 3D image is availible in CSD Mercury format (http:// www.ccdc.cam.ac.uk/products/csd_system/mercury/).
ring. Single-point calculations with different basis sets were then carried out on the equilibrium geometries of complexes 4 and 5 to evaluate their interaction energies. The results, summarized in Table 3, show that complex 5 is the more stable system with a MP2/cc-PVQZ energy that is more negative by 4.93 kJ/mol compared to that of complex 4. Moreover the high value of the correlation energy (Ecorr ) EMP2 - EHF) in both complexes indicates that the major source of attraction between the fragments comes from dispersion interactions. In conclusion, this theoretical study shows that a strong attraction occurs between the two fragments triazine and acetonitrile and that the optimized geometry when a CH3CN molecule is free to approach the heteroaromatic ring is quite similar to that observed in the crystal structure analysis. Furthermore, this crystal structure reveals a remarkable helical hydrogen-bonding network present in the lattice (Figure 6). These supramolecular helices involve the oxygen atoms O(1w), O(2w), and O(3w) of water molecules. The O-O distances range from 2.58(2) to 2.85(2) Å, indicating moderate to strong hydrogen bonding interactions.29 In addition, the oxygen atom O(3w) is H bonded to the chloride ion Cl(4), further stabilizing the unusual architecture [Cl(4)‚‚‚O(3w) ) 3.018(17) Å]. The hydrogen bonded water chains cocrystallize as chiral left and right helices, resulting in a racemic mixture (see Figures 6 and 7). The main geometric characteristics of these water strands are depicted in Figure 7. The internal diameter of the aqueous fiber is about 4.05 Å. Each loop of the helix is formed by six
molecules of water, corresponding to a distance of approximately 9.08 Å. The left- and right-handed supramolecules are separated by a distance of 11.7 Å. To the best of our knowledge, this particular infinite hydrogen bonded helicate assembly represents the first example of such water chain. Especially, the involvement of molecular water chains as “proton wires” in biological systems is of eminent importance and is receiving great attention from the scientific community.30-32 Conclusions The occurrence of acetonitrile-π interactions within the complex reported herein is crucial, because it illustrates for the first time the possibility to have interaction between a neutral molecule and an electron-deficient aromatic ring. This significant experimental result confirms computational studies earlier published, and thus supports an electron-rich/electron-poor synthetic approach to design and prepare triazine-based receptors for benzene, acetonitrile, and so on. The study of water clusters is an important and challenging topic of contemporary supramolecular chemistry since it may help to get a better insight into the mechanism of proton transfer in biological systems. In addition, such water molecular self-assemblies are essential to understand how hydrogen-bonded channels can be purposely generated, since the formation of noncovalent bonding interactions is difficult to control. Acknowledgment. This work has been supported financially by the Graduate Research School Combination “Catalysis”, a joint activity of the graduate research schools NIOK, HRSMC, and PTN. Financial support from COST Action D21/003/01, EET, and MURST through COFIN and CINECA are thankfully acknowledged. Prof. Ugozzoli is kindly acknowledged for his advice on the theoretical calculations. We acknowledge the provision of time on the Small Molecule Crystallography Service at the CCLRC Daresbury Laboratory via support by the European Community - Research Infrastructure Action under the FP6 “Structuring the European Research Area” Program
1574 Crystal Growth & Design, Vol. 6, No. 7, 2006
(through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”). Note Added after ASAP Publication. An earlier version of this paper posted ASAP on the web on May 27, 2006, did not include links to web-enhanced objects. This content has been added in this new version posted June 21, 2006. Supporting Information Available: X-ray crystallographic data for [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5 (CIF format) and geometry coordinates for complexes 4 and 5 (Table SI1). This material is available free of charge via the Internet at http://pubs.acs.org.
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CG060144A
