Inorganic Complexes Retain Diethyl Ether Well Above Its Boiling Point through OH2‚‚‚OEt2 Hydrogen Bonding Lu Tian and Jagadese J. Vittal* Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe, Singapore 117543
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 4 822-824
ReceiVed October 31, 2005; ReVised Manuscript ReceiVed February 17, 2006
ABSTRACT: The structures of [M(2,2′-bpy)(SC{O}Ph)3(H2O)]‚Et2O (M ) La (1), Pr (2), and Sm (3); 2,2′-bpy ) 2,2′-bipyridine) have an eight-membered hydrogen-bonded ring formed between two diethyl ether (Et2O) and two aqua ligands, and they retain Et2O above its boiling point despite the presence of channels in the lattice for Et2O to escape. The inception temperatures for the weight loss of Et2O, 82, 69, and 51 °C for 1, 2, and 3, respectively, appear to be controlled by the size of the metal ions. Et2O-free solid 1 is able to take back Et2O quantitatively. There has been a sustained interest in solids that hold onto their gaseous and very volatile guests due to their potential applications as hydrogen, methane, and other gas storage materials.1-5 Atwood, Barbour and co-workers have elegantly demonstrated that van der Waals interactions in nonporous organic crystals can be used to confine methane, Freon, and other highly volatile molecules well above their boiling points.6-8 The presence of low boiling liquids in the crystal lattice often poses problems in X-ray crystallography, especially if the guest molecules do not snugly fit into the cavity or channels and if there is very weak or no interaction between the guests and the host molecules. Data collection of crystals containing such low boiling solvents is often problematic as the crystals lose their single-crystal nature due to solvent loss. Diethyl ether (Et2O) with a relatively low boiling point (34.6 °C) is no exception as it is frequently used as a precipitant liquid to grow single crystals. Hence, it is not surprising that the Cambridge Structural Database (version 5.26, November 2004) search reveals that about 56% of the crystals containing ether solvate have been found to be disordered.9 To our surprise, we found Et2O solvate in the crystals of three lanthanide compounds to be stable well above its boiling point, and the details of our findings are discussed below. Reaction of NaSC{O}Ph, 2,2′-bipy (2,2′-bipyridine) and LaCl3‚ 7H2O afforded yellow crystals of [La(2,2′-bpy)2(SC{O}Ph)3] as the major product (56%) and [La(2,2′-bpy)(SC{O}Ph)3(H2O)]‚Et2O (1) as a minor product (10%). The solid-state structure of the latter reveals nine coordination geometry at the metal center with N2O4S3 donor set as shown in Figure 1.10 The corresponding praseodymium(III) and samarium(III) complexes, 2 and 3, respectively, have also been synthesized by a similar method, and their crystal structures are isomorphous and isostructural to 1.11 In the crystal structure, two La(III) complexes in 1 are strongly hydrogen bonded to two Et2O solvates through aqua ligands forming an eight-membered ring (graph set notation, R24(8))12 as shown in Figure 1. The O-H‚‚‚O bonds are medium strong.13 Surprisingly, these hydrogen bonds are strong enough for the single crystals to sustain the Et2O solvate in a vacuum (10-1-10-2 Torr) for several hours. In fact, the diffraction data for 2 were collected after the single crystals were subjected to a vacuum (0.1 Torr) for 12 h. The packing diagram of 1 without the Et2O solvate and hydrogen atoms in Figure 2 clearly indicates the presence of channels parallel to the a-axis created by the stacking of metal complexes to host the Et2O. Et2O could have escaped through these channels very easily at room temperature. The La(III) complex does not lose Et2O until 82 °C, as shown by the onset weight loss in thermogravimetry (TG) sudies which show the loss of one Et2O (observed, 9.0%; calculated, 9.3%) * To whom correspondence should be addressed. Tel: (65) 6516-2975. Fax: (65) 6779-1691. E-mail:
[email protected].
Figure 1. The triclinic unit cell contents of 1. All the C-H hydrogen atoms have been omitted for clarity. The hydrogen bond parameters: for O4H4a‚‚‚O5(1 - x, 1 - y, 1 - z); 0.74(2); 2.05(2), 2.786(2) Å and 169(2)° and for O4-H4b‚‚‚O5; 0.75(2), 2.14(3), 2.875(2) Å and 171(3)°.
Figure 2. Packing diagram of 1. The diethyl ether solvate and hydrogen atoms have been omitted to show the channels formed along the a-axis.
molecule in the temperature range 82-111 °C followed by the loss of a coordinated water in the temperature region 113-130 °C (Figure 3). A similar pattern for 2 is also observed from TG, but 2 is thermally stable only up to 69 °C. But the weight loss regions of Et2O and H2O are clearly resolved better than 1. The TG of the samarium compound [Sm(2,2′-bpy)(SC{O}Ph)3(H2O)]‚Et2O (3) has
10.1021/cg0505766 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/16/2006
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Figure 5. A plot of Et2O weight loss inception temperature versus the ionic radii of the metals in 1-3.
Figure 3. (a) The TG curves with a heating rate of 1 °C min-1 for 1; (b) TG curves for 2.
Figure 4. Selected IR spectra from TG-IR measurements for 1.
been found to lose Et2O and H2O in a single step in the temperature range 51-110 °C (weight loss observed, 11.0% and calculated, 11.4%). The evolved gas from TG experiments was monitored by IR spectral studies simultaneously for 1 and 2. Figure 4 shows the IR spectral data for 1 which complements the TG data. The evolution of Et2O was observed above 82 °C, as indicated by the increase in the intensity of the four main bands, 2980 (νs C-H), 2871 (νas C-H), 1378 (δC-H), 1130 (νC-O) cm-1. The maximum intensity was observed at 100 °C (which is in agreement with the derivative thermogravimetric (DTG) maxima observed for the loss of Et2O. Although this is followed by the loss of coordinated water in the temperature region 113-130 °C, the IR did not show the bands
due to water; perhaps the water is not truly vaporized in the sense that it may have adhered to the apparatus after liberation from the sample. The difference between the onset temperature, Ton, and the boiling point, Tb, has been employed as a useful parameter to compare the thermal stability of guest molecules in the solids.14 The Ton - Tb values observed in 1, 2, and 3 are 48.4, 34.4, and 16.4°, respectively. The differences between Ton - Tb values for 1-3 indicate that it is possible to control the release of Et2O in the solid state. Further, the increase in the ionic radii appears to increase the Ton - Tb value, and a plot of the ionic radius versus the weight loss inception temperature for Et2O shows a linear relationship as shown in Figure 5. A scandium complex has been reported to have similar centrosymmetric hydrogen-bonded (OH2‚‚‚OEt2)2.15 Compound [trans-VOCl2(H2O)2]‚2Et2O forms one-dimensional chains with the molecules of adjacent monomers held together by hydrogen bonded (OH2‚‚‚OEt2)2.16 However, the Et2O retention property has not been explored for these two compounds. The stabilization of volatile Et2O in a solid well above its boiling point by hydrogen bonding is not without precedent. An organic compound with OH functionality has been found to retain Et2O up to 64.2 °C, which is 29.6 °C above its normal boiling point.17 In our present study, the host-guest interactions are facilitated by metal-bound aqua ligands in a concerted fashion. The onset temperature for the release of Et2O determined by DSC was also found to be consistent with the TG results. The enthalpy changes for the Et2O loss has been found to be 98.2 kJ mol-1 for 1. After the removal of Et2O, the desolvated 1 was exposed to the vapors of Et2O again for 2 h and air-dried for 30 min at room temperature; the solid takes back the solvent as confirmed by TG weight loss and the IR spectrum of the sample. This property appears to be robust when this cycle was repeated for the second time. However, the crystallinity becomes amorphous after losing Et2O. In summary, this report describes the crystal structures and thermal dehydration properties of three unusual lanthanide complexes that retain Et2O well above its boiling point through O-H‚ ‚‚O hydrogen bonding despite the presence of channels in the lattice for Et2O to escape. Further, the onset temperature of Et2O loss from the solid could be controlled by the size of the metal ions in these complexes. Et2O-free solid 1 is able to take back Et2O quantitatively. Acknowledgment. We would like to thank the National University of Singapore for their generous funding for this project. Supporting Information Available: Details of syntheses, characterization, TG-DTG of 1-3, TG-IR of 1 and 2, DSC of 1, and stick diagrams of 2 and 3; CIFs for 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.
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