Hydrogen-bonded polyrotaxane-like structure containing cyclic (H2O)4 in [Zn(OAc)2 (m-bpe)]•2 H2O

Meng Tack Ng, Theivanayagam Deivaraja and Jagadese Vittal (National University of Singapore), Wim Klooster (ANSTO) and Garry McIntyre (ILL)

 Figure 1. An ORTEP diagram (showing 50% probability ellipsoids for the non-H atoms) of the asymmetric unit of 1.

The reaction of 4,4'-bipyridylethane (bpe) with Zn(OAc)2·2H2O has led to the formation of a coordination polymer, [Zn(OAc)2(µ-bpe)]·2H2O. This compound has a zigzag coordination polymeric structure as revealed by X-ray crystallography. However the presence of two lattice water molecules results in an interesting crystal structures.

The carboxylate carbonyl oxygen atoms of the Zn(OAc)2 groups from two different adjacent zigzag polymers and four lattice water molecules form 24-membered hydrogen-bonded ring. One of the two bpe ligands associated with each Zn(II) passes through the centre of this ring to form 2D hydrogen-bonded coordination polymeric structure. In the solid state, the adjacent 24-membered hydrogen-bonded rings further fused together through O-H··O hydrogen bonds among four waters to form cyclic (H2O)4.

This resulted in one dimensional hydrogen-bonded ribbon-like polymer comprised of fused alternating 24- and 8- membered O-H···O hydrogen-bonded rings and hence produce 3D hydrogen-bonded network with polyrotaxane-like association. A neutron diffraction study provides a detailed description of the hydrogen bonds involved. Research in crystal engineering and construction of coordination polymers with specific topologies seems to be progressing in recent years by virtue of the possible design of materials with specific electronic, optic, magnetic and catalytic properties.

In spite of a few cases wherein the molecules are assembled in a pre-determined fashion, prediction of the crystal structure is largely considered to be serendipitous. This could be attributed to the poor understanding of the role played by various factors employed for the growth of the particular crystal, and other subtle attractive or repulsive forces, that prevail in the crystal lattice.

The structure of hydrogen-bonded water clusters and channels continues to attract interest since they play a crucial role in contributing to the stability and function of biological assemblies. Several theoretical calculations and experimental evidence confirm the presence of water trimer, tetramer, pentamer, hexamer, octamers, decamers, etc. Furthermore, many of these water oligomers and polymers have been stabilized by organic and inorganic hosts.

Interpenetrating network structures having polyrotaxane-like connectivities are not very common among hydrogen-bonded coordination polymers and are only found abundantly in coordination polymeric network structures. We have therefore decided to investigate the solid-state structure in more detail using neutron diffraction. Neutron diffraction experiments Neutron diffraction data were collected on VIVALDI, at the Institut Laue-Langevin (ILL), Grenoble, France.

VIVALDI uses the Laue diffraction technique on an unmonochromated thermal-neutron beam and with a large solid-angle cylindrical image-plate detector, to increase the detected diffracted intensity by one to two orders of magnitude compared with a conventional monochromatic experiment. A colourless crystal, with well-developed faces and maximum dimensions 1.7 x 0.9 x 0.6 mm3, volume ~0.5 mm3, was selected, wrapped in aluminium foil with some silicon grease, and mounted on a vanadium pin.

Seven Laue diffraction patterns were collected at room temperature, at 30° intervals around the vertical axis perpendicular to the incident neutron beam, each exposure lasting 4.5 hours. Another seven patterns were collected with a different vertical orientation of the crystal. At a later date the crystal was cooled to 20 K, and 18 patterns were collected at 10° intervals around the vertical axis with an average exposure of 2.2 hours. A summary of the crystal, data-collection and refinement parameters is given in Table 1.

Table 1 This experiment was also a good test for the capabilities of the new Koala Quasi-Laue diffractometer, being build for the OPAL reactor. Structure of [Zn(OAc)2(m-bpe)]·2H2O A portion of the polymeric segment showing the coordination environment around Zn(II) is shown in Figure 1. Each Zn(II) atom adopts a four coordinate tetrahedral geometry, being coordinated to the nitrogen atoms of two crystallographically different pyridyl groups and the oxygen atoms of the two acetate ligands.

Two lattice water molecules have been found in the asymmetric unit, which are hydrogen bonded to the other oxygen atoms of the two acetate anions as shown in Figure 1.

The Zn(II) atom is bonded to two bridging bpe spacer ligands, as shown in Figure 2. A crystallographic inversion center is present in the middle of each bpe spacer ligand. The interplanar angle between the pyridyl rings in bpe is 0º and the two pyridyl rings in bpe have anti conformation with C-CH2-CH2-C torsion angle of 180º. All the polymeric chains are aligned parallel to each other in the ab plane and propagate along the b-direction.

Figure 2. A segment of the zigzag coordination polymer in 1. Only relevant atoms are shown.

The oxygen atoms of the carboxylate carbonyl groups present in Zn(OAc)2 from two different adjacent polymeric chains are O-H···O hydrogen bonded to four lattice water molecules to form a 24-membered ring as illustrated in Figure 3. This ring has the shape of a distorted square.

Adjacent polymeric chains below the hydrogen-bonded rings are slip-stacked along the c-direction such that one of the two bpe ligands goes through the centre of the larger hydrogen-bonded ring as shown in Figure 4. The side view of this penetration of the coordination polymeric chain into the hydrogen-bonded ring is shown in Figure 5.

It is clear that only the bpe ligand containing N2 atom goes through the centre of the hydrogen-bonded ring. The centre of the two methylene groups of bpe coincides with the crystallographic inversion centre present at the centre of the hydrogen-bonded ring. However, the bond parameter found in this bpe ligand is very similar to that with the atom N1.

The hydrogen-bonded polyrotaxane schematically depicted in Scheme 1, forms a sheet-like structure approximately in ab-plane. These hydrogen-bonded rings are further fused with two 8-membered rings formed by four water molecules through O-H···O hydrogen bonds to generate a 1D hydrogen-bonded polymer which propagates approximately along the c-direction as depicted in Figure 6. The 2D hydrogen-bonded polyrotaxane structures are mended through cyclic quasi-planar water tetramer to provide a 3D hydrogen-bonded coordination polymeric structure.

Figure 3. Diagram showing the 24-membered hydrogen-bonded ring in 1. Only relevant atoms are shown.

Figure 4. The hydrogen-bonded wheel and bpe axle in a rotaxane structure. Only selected atoms are shown for clarity.

Figure 5. A section of the hydrogen-bonded polyrotaxane sheet in 1.

Figure 6. Perspective view of the 1D hydrogen-bonded polymer formed between the (H2O)4 and Zn(OAc)2. Only relevant atoms are shown for clarity.

Scheme 1. Schematic diagram illustrating 2D sheet containing the hydrogen-bonded polyrotaxane structure in 1.

The distances and angles involving non-hydrogen atoms were more accurately obtained by the X-ray data, but none of the neutron values is significantly different, and the trends observed from the X-ray results are also obtained from the neutron data. Where neutron diffraction can make its contribution is when looking at the hydrogen atoms.

Looking at the 20 K data, all hydrogen-bond distances are in the range 1.06(1)-1.11(1) ? for the main structure, and in the range 0.93(2)-1.00(2) ? for the water molecules. These are normal values. The angles involving H1 through H10 are in the range 115.8(8)-122.3(8)°, those involving H11 are through H16 in the range 107(1)-113(1)°, and those within the water molecules are 102(1)° and 108(1)°, all as to be expected. Of interest is the hydrogen bonding.

The O···O distances are very similar to those obtained by X-ray, and those involving the water molecules are in a narrow range: 2.73(1) - 2.80(1) ? and the H···O distances are in the range 1.77(1) - 1.87(1) ?. These are fairly strong hydrogen bonds. The two hydrogen bonds which are almost linear, are 172(1)° and 175(2)°, and are those to the oxalate oxygen atoms, and the other angles are not far from being linear, are 157(1)° and 168(1)°, so this configuration is close to optimal.

There are no other strong hydrogen bonds in the structure. There are some weaker interactions: O2 forms bonds with H4 [2.43(2) ?] and H6 [2.45(1) ?], and O4 forms a bond with H7 [2.47(1) ?]. The only other potential hydrogen-bond acceptors are O1 and O3, which have a weak intramolecular interaction with one of the methyl hydrogens, O1…H14B [2.56(2) ?] and O3…H16C [2.50(2) ?].

Scheme 2. Schematic representation of the crystal structure of 1.

Reference

Hydrogen-bonded Polyrotaxane-like Structure containing Cyclic (H2O)4 in [Zn(OAc)2(m-bpe)].2H2O: X-Ray and Neutron Diffraction Studies. M.T. Ng, T.C. Deivaraj, W.T. Klooster, G.J. McIntyre and J.J. Vittal. Chem. Eur. J. 10, 5853-5859, 2004.