Crystal-to-Crystal Transformation from a Weak Hydrogen-Bonded Two-Dimensional Network Structure to a Two-Dimensional Coordination Polymer on Heating Ghodrat Mahmoudi and Ali Morsali*
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 391–394
Department of Chemistry, Faculty of Sciences, Tarbiat Modares UniVersity, P.O. Box 14115-175, Tehran, Islamic Republic of Iran ReceiVed September 14, 2007; ReVised Manuscript ReceiVed NoVember 23, 2007
ABSTRACT: Single crystal to single crystal transformation of a new mercury(II) coordination polymer with ligand 1,4-bis(2-pyridyl)2,3-diaza-1,3-butadiene (bpdb), [Hg2(µ-bpdb)I4] (1r) to [Hg2(µ-bpdb)(µ-I)2I2]n (1β), has been reported and structures of 1r and 1β were determined by X-ray crystallography. The thermal stability of compounds 1r and 1βwere studied by thermal gravimetric (TG) and differential thermal analyses (DTA). Powder X-ray diffraction experiments showed that the phase transitions described for single crystals also occur in macroscopic powder samples and lead to monophasic products and the crystal-to-crystal transformation described is not reversible. Structural transformations with the retention of single-crystal character directly reflect the relationship between the solids involved. These conversions are rare and normally occur merely through minor movements of atoms in the crystal. During the last two decades, the rational design and syntheses of novel coordination polymers have made considerable progress in the field of supramolecular chemistry and crystal engineering,1–5 not only because of the intriguing structural motifs of the coordination polymers but also because of their potential applications in catalysis, molecular adsorption, magnetism, nonlinear optics, luminescence, and molecular sensing. Studies on crystal-to-crystal transformations involving coordination polymers and networks are more recent.6–26 There are several types of structural transformations, which are primarily influenced by the expansion of coordination numbers, thermal association, condensation, rearrangement of bonds, or the removal/ exchange of solvents.27 In this paper, we report the thermal conversion of a twodimensional network held together by weak CH · · · I hydrogen bonding into a covalent two-dimensional framework by heating, [Hg2(µ-bpdb)I4] (1r) to [Hg2(µ-bpdb)(µ-I)2I2]n (1β), bpdb ) 1,4bis(2-pyridyl)-2,3-diaza-1,3-butadiene. Materials and Physical Techniques. With the exception of the ligand 1,4-bis(2-pyridyl)-2,3-diaza-1,3-butadiene (bpdb), which were prepared according to the literature procedures,28 all reagents and solvents for the synthesis and analysis were commercially available and used as received. IR spectra were recorded using Perkin-Elmer 597 and Nicolet 510P spectrophotometers. Microanalyses were carried out using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. The thermal behavior was measured with a PL-STA 1500 apparatus. X-ray powder diffraction (XRD) measurements were performed using a Philips diffractometer of X’pert Company with monochromatized CuKR radiation. Crystallographic measurements were made at 298K for compounds 1r and 1β using a Bruker APEX area-detector diffractometer. The intensity data were collected using graphite-monochromataed Mo KR radiation. The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2. Structure solution and refinement was accomplished using SIR97, SHELXL97, and WinGX.30–32 Crystal data for 1r: monoclinic space group P21/c, a ) 8.6938(8) Å, b ) 17.4652(17) Å, c ) 16.5326(13) Å, β ) 117.759(3)°, V ) 2221.4(4) Å3, Z ) 4, T ) 298 K. The refinement of 199 parameters on the basis of 3447 independent reflections (of a total of 3924) converged at R1 ) 0.0599, wR2 ) 0.1424. Crystal data for 1β: monoclinic space group * Corresponding author. E-mail:
[email protected].
P21/c, a ) 7.5453(11) Å, b ) 7.4618(11) Å, c ) 18.556(3) Å, β ) 100.914(2)°, V ) 1025.8(3) Å3, Z ) 2, T ) 298 K. The refinement of 100 parameters on the basis of 1796 independent reflections (of a total of 4912) converged at R1) 0.0549, wR2)0.1341.
Synthesis of [Hg2(µ-bpdb)I4] (1r) and [Hg2(µ-bpdb)(µ-I)2I2]n (1β). Single crystals of 1r suitable for X-ray diffraction were prepared by a brunched tube method.29 1,4-Bis(2-pyridyl)2,3-diaza-1,3-butadiene (bpdb) (0.5 mmol, 0.105 g) and mercury(II) iodide (0.159 g, 0.5 mmol) were placed in the main arm of the branched tube. Methanol was carefully added to fill the arms, the tube was sealed, and the ligand-containing arm immersed in an oil bath at 60 °C, whereas the other arm was kept at ambient temperature. After 2–3 days, yellow crystals (d.p. ) 186 °C), which were deposited in the cooler arm, were isolated, filtered off, washed with acetone and ether, and air-dried (0.118 g, yield 85%). Found: C, 12.50; H, 0.80; N, 5.10. Calcd. for C12H10Hg2I4N4: C, 12.87; H, 0.90; N, 5.00. IR (cm-1) bands: 491(w), 627(w), 765(m), 1001 (w), 1149(m), 1213(w), 1410(m), 1568(m), 1619(m), 3040(w). A yellow monomeric [Hg2(µ-bpdb)I4] (1r) polymerizes on heating the solid at 50–60 °C to form a brown 2D coordination polymer, [Hg2(µ-bpdb)(µ-I2)I2]n (1β). Found: C, 12.50; H, 0.95; N, 5.05. Calcd for C12H10Hg2I4N4: C, 12.87; H, 0.90; N, 5.00. Figure 1 shows the structures of the basic dimeric and polymeric building block of compounds 1rand 1β. In compound 1r, the ligand bpdb chelates with one nitrogen atom each from the pyridyl and diaza groups on one side of the molecule coordinating to a mercury(II) ion and produces the dimer units. The Hg · · · Hg distances within the I2Hg-(bpdb)-HgI2 moieties are 6.390(1) Å. The individual dimeric units in compound 1r are almost parallel to each other and further linked by weak I · · · HCpy hydrogen bonding (Figure 3) with bond distances of 3.12(1) and 3.16(1) Å. These weak C-H · · · I interactions in 1r grow the dimeric units into a twodimensional network. On heating at temperatures 50–60 °C, the yellow [Hg2(µ-bpdb)I4] (1r) changes to a brown compound, [Hg2(µbpdb)(µ-I)2I2]n (1β), and the X-ray structure determination of the brown compound reveals a number of unique features. The structure of this solid-state product bears a close relationship to that of the reactant 1r; they have the same crystal and space group; however, although both Hg(II)-bpdb compounds, 1r and 1β, both crystallized in the monoclinic space group P21/c, they are not isostructural. The intermolecular interactions stabilizing the structure have changed from CH · · · I weak hydrogen bonding only in 1r to covalent bonding in 1β, as shown in Figures 2 and 3. The crystal structure of 1β consists of two-dimensional polymer (Figure 3b). In compound 1β the ligands bpdb are chelating with each one nitrogen atom of the pyridyl and diaza groups on one
10.1021/cg700883y CCC: $40.75 2008 American Chemical Society Published on Web 12/21/2007
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Figure 1. ORTEP diagram of the (a) compound [Hg2(µ-bpdb)I4] (1r) and (b) compound [Hg2(µ-bpdb)(µ-I)2I2]n (1β); 50% probability level ellipsoids. (i): -x, y + 1/2, -z + 1/2; ii: -x, -y, -z; iii: x, -y + 1/2, z + 1/2.
side of the molecule coordinating to a mercury(II) ion. The ligand is located on a crystallographic inversion center, thus forming I2Hg(µ-bpdb)-HgI2 moieties. Of the I- anions,each one acts as a monodentate ligand,whereas the other forms a full halogen bridge to a mercury atom of an adjacent I2Hg(µ-bpdb) moiety. In the compound 1β, two I atoms bridge between three different Hg atoms, thus building a Hg-µI-Hg-µI-Hg bridged two-dimensional polymer as shown in Figure 3b. The coordination number in this compound is five and the geometry can be regarded as a distorted tetragonal pyramid with two pyridyl nitrogen and two bridging iodide atoms in the basal positions and one unbridging iodide in the apical position. The Hg · · · Hg distances within the I2Hg-(µ-bpdb)-HgI2 moieties are 6.372(1), those bridged by the iodide atoms are 5.039(3) Å.
The solid-state transformation starts from the dimeric unit [Hg2(µbpdb)I4] (1r). The halogen ligands are oriented toward the neighboring dimeric units. A schematic representation of this topology is shown in Figure 4(top). Dashed lines indicate the shortest nonbonding contacts between the halogen substituents of one dimer and the mercury atoms of its neighbor; these interatomic distance is 3.86(2) Å. One may assume that these contacts shorten in the course of the solid-state reaction and turn into bonds. This approach between the dimers is accompanied by variations in the metal-centered angles and by expansion of the originally terminal mercury-iodine bonds. The rearrangement product [Hg2(µ-bpdb)(µI)2I2]n (1β) represents a two-dimensional network in which the one of iodo ligands unsymmetrically bridge metal cations. The additional Hg-I bonds increase the coordination number of the cations from
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Figure 2. Transformation of weak hydrogen-bonded 2D network structure to 2D coordination polymeric network structure.
4 to 5, leading to a distorted square pyramidal coordination. This change in network topology from dimer to two-dimensional connectivity is started when solid 1r is warmed at temperatures 50–60 °C and the atomic movement underlying the rearrangement apparently occurs under very mild conditions. The reaction is not reversible and polymer 1β is not converted back to 1r to temperatures 0 °C. The structures of the bulk materials for 1r and 1β were confirmed by matching their X-ray powder patterns with those generated from the corresponding single crystals (see the Supporting Information, Figure 1S). Acceptable matches were observed between the simulated single-crystal X-ray data pattern (see the Supporting Information, Figure 1Sa) and the experimental powder X-ray diffraction patterns for bulk crystalline sample as obtained from the synthesis of compound 1r (see the Supporting Information, Figure 1Sb). Transformation by thermal treatment at temperatures 50–60 °C (panels c and d in Figure 1S of the Supporting Information) results in a significant change in the powder pattern, but acceptable matches were observed between the simulated singlecrystal X-ray data pattern (see the Supporting Information, Figure 1Sc) and those from the experimental powder X-ray diffraction patterns for bulk crystalline sample as obtained by heating of complex 1r (see the Supporting Information, Figure 1Sd). These facts clearly indicate that the phase transitions described for single crystals also occur in macroscopic powder samples and lead to monophasic products and the crystal-to-crystal transformation described is not reversible. And the dimeric structure of compound 1r transforms to compound 1β by heating. To confirm the transformation of the compound 1r to compound 1β upon heating of the sample, we recorded TGA and DTA for
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Figure 3. (a) View of a section of the 2D network produced by CH · · · I weak hydrogen bonds in 1rand (b) view of a section of the 2D network in 1β.
the original sample and the same sample after 5 hours of heating at 50–60 °C (see the Supporting Information, Figures 2S and 3S). TG curve of compounds 1r and 1β indicate that this compound does not melt and is stable up to 186 °C, at which point it begins to decompose. The compound 1β decomposes at 186–600 °C with three exothermic effects at 195, 505, and 610 °C (see the Supporting Information, Figure 3S). As it has been shown from the Supporting Information, Figures 2S and 3S, the thermal behavior of compounds 1r and 1β is very similar and in spite of this, the transformation really take places before 50 °C and is complete by 60 °C. We performed a DSC of compound 1r at lower temperature (-100 °C to RT) too and there was not any change, indicating no transformation at lower temperature. Acceptable matches, with slight differences in ν for some of bands, were observed between the IR spectrum of compound 1r (see the Supporting Information, Figure 4Sa) and those from the compound 1β (see the Supporting Information, Figure 4Sb). The slightly different values of ν are a result of the structural transformations of the compound 1r to compound 1β. In summary, a yellow dimeric [Hg2(µ-bpdb)I4] (1r) polymerizes on heating the solid at 50–60 °C to form a brown 2D coordination polymer, [Hg2(µ-bpdb)(µ-I)2I2]n (1β). In the crystal lattice, one iodine atom mutually attacks the neighboring Hg(II) centers and forms new Hg-I bonds, thereby expanding the coordination geometry from distorted tetrahedral to distorted square pyramidal. Structural transformations of this complex are the first solid-state structural transformation involving the transformation of weak hydrogen-bonded 2D network structure to 2D coordination polymeric network structure that is influenced by the expansion of mercury(II) coordination number, four to five. Attempts to regener-
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References
Figure 4. Schematic representation of the solid-state conversion of two neighboring units in 1r to form the two-dimensional network in 1β.
ate 1β to give 1r have been unsuccessful; therefore, retransformation appears to be irreversible.
Acknowledgment. This work was supported by the Tarbiat Modares University. Supporting Information Available: Figures 1S-4S (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for the structure reported in the paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-655153 (1r) and CCDC-643432 (1β). Copies of the data can be obtained on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: 44–1223/336033; e-mail:
[email protected]).
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