Assemblies of Two Mixed-Ligand Coordination Polymers with Two

Aug 8, 2007 - Chih-Chieh Wang , Meu-Ju Ke , Cheng-Hsiao Tsai , I-Hsuan Chen , Shin-I Lin , Tzuen-Yeuan Lin , Li-Mei Wu , Gene-Hsiang Lee , Hwo-Shuenn ...
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Assemblies of Two Mixed-Ligand Coordination Polymers with Two-Dimensional Metal-Organic Frameworks Constructed from M(II) Ions with Croconate and 1,2-Bis-(4-pyridyl)ethylene (M ) Cd and Zn)

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1783-1790

Chih-Chieh Wang,*,† Shih-Min Tseng,† Sue-Yi Lin,† Fang-Chen Liu,† Shuen-Chieh Dai,† Gene-Hsiang Lee,‡ Wei-Ju Shih,# and Hwo-Shuenn Sheu# Department of Chemistry, Soochow UniVersity, Taipei, Taiwan, Instrumentation Center, National Taiwan UniVersity, Taipei, Taiwan, and National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan ReceiVed February 13, 2007; ReVised Manuscript ReceiVed June 20, 2007

ABSTRACT: Two metal-croconate (C5O52-) complexes, {[Cd2(bpe)(C5O5)2(H2O)]‚2H2O}n (1) and {[Zn2(bpe)2(C5O5)2]}‚H2O}n (2) (bpe ) 1,2-bis(4-pyridyl)ethylene)) with two-dimensional (2-D) metal-organic frameworks (MOFs), have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction studies. Structural determination reveals that compound 1 possesses a 2-D layered MOF with two rectangular boxes as the basic building units through the connectivity of CdII with µ3-croconates, µ5-croconates, and bpe ligands. In compound 2, a 2-D brick-wall-like layered framework is formed by using a rectangular grid as the basic building unit through the connectivity of ZnII with hybrid µ3-croconates, µ4-croconates, and bpe ligands. Two layers are then cross-linked by the other bpe ligands to complete a 2-D bilayered MOF. Both of the 2-D MOFs are extended to three-dimensional supramolecular architectures through π-π and C-H‚‚‚O interactions between the croconate and the pyridyl rings of bpe, which exhibit high thermal stability and retain their crystalline forms up to 350 °C. Introduction The design and synthesis of coordination polymers with metal-organic networks1-5 (MOFs) via self-assembly of metal ions and multifunctional ligands have attracted much attention due to their potential application as functional porous materials6-9 for physisorption and chemisorption, catalytic action, and interesting physical properties. The versatile structural topologies of MOFs depend on both the selection of the coordination geometry of metal centers and the coordination behavior of the multifunctional organic ligands. During the past several years, the bonding characteristics of croconate (C5O52-, dianion of 4,5dihydroxycyclo-pent-4-1,2,3-trione) with first-row transition metal ions and some complexes of croconate associated with another co-ligands such as imidazole, histamine, 2,2′-bipyridine, and the bis(2-pyridylcarbonyl)amido anion, have been widely investigated and show that this cyclic oxocarbon ligand can be coordinated to metal ions using various bonding modes (Scheme 1) as a terminal bidentate (I),10-18 bridging bidentate/modentate (II),10c,11,19,20 and bridging bis-bidentate ligand through either four (III)10c,21,22 or three adjacent (IV)10b,10d,22,23 croconate oxygens. In recent years, four new bridging modes of croconate as a bidentate/monodentate plus bridging monodentate (V),24 bis-bidentate plus bridging bis-monodentate across four croconate-oxygens (VI),24 bridging bidentate/modentate (VII),25 and bidentate plus bridging bis-monodentate through three croconate-oxygens (VIII),26 have been used in the construction of two- or three-dimensional (2-D or 3-D) MOFs. We also have employed croconate associated with N,N′-donor rodlike spacer ligands, a rigid 4,4′-bipyridine and a flexible 1,2-bis(4-pyridyl)ethane (dpe), respectively, to create two mixed-ligands 2-D MOFs, where both of their extended networks are constructed * To whom correspondence should be addressed. Fax: 886-2-28811053. Tel.: 886-2-28819471 ext 6824. E-mail: [email protected]. † Soochow University. ‡ National Taiwan University. # National Synchrotron Radiation Research Center.

by related pyridyl-based ligands and croconate with two different bridging modes.27 With our continuous interest in the study of metal-croconate coordination polymers, we report here the syntheses and structures of two complexes, {[Cd2(bpe)(C5O5)2(H2O)]‚2H2O}n (1) and {[Zn2(bpe)2(C5O5)2]‚H2O}n (2), with another rigid N,N′-donor rodlike spacer ligand 1,2-bis(4pyridyl)ethylene (bpe), in which the croconate functions as hybrid bridging modes with bis-bidentate through three-adjacent (µ3), and tris-bidentate through the five croconate-oxygens (µ5) (IV and IX in Scheme 1) in 1 and bridging bis-bidentate through four-adjacent (µ4) and through three-adjacent (µ3) (III and IV in Scheme 1) in 2, respectively, in the construction of their 2-D MOFs. Both of them show high thermal stability mainly attributed to their stable extended 3-D supramolecular architectures by the π-π and C-H‚‚‚O interactions between the croconate and the pyridyl rings of bpe. Experimental Section Materials and Physical Techniques. All chemicals were of reagent grade and were used as commercially obtained without further purification. The infrared spectra was recorded on a Nicolet Fourier transform IR, MAGNA-IR 500 spectrometer in the range of 500-4000 cm-1 using the KBr disc technique. Thermogravimetric analyses (TGA) of compounds 1 and 2 were performed on a computer-controlled PerkinElmer 7 Series/UNIX TGA7 analyzer. Single-phased powder samples of 1 (1.893 mg) and 2 (1.344 mg) were loaded into alumina pans and heated with a ramp rate of 2 °C/min from room temperature to 800 °C under a nitrogen atmosphere. The in situ powder X-ray diffraction (PXRD) was performed at the beamline BL01C2 of the National Synchrotron Radiation Research Center (NSRRC) Taiwan. The synchrotron X-ray radiation was generated from the superconducting wavelength shifted magnet of 5.0 T with a ring energy of 1.5 GeV typical ring current of 200 to 120 mA. The X-ray wavelength was 0.77490 Å, which was delivered by a double crystal monochromator with two Si(111) crystals. Samples 1 and 2 were sealed in 1 mm quartz capillary and heated under a hot air stream from room temperature up to 600 °C during the XRD measurement. Two-dimensional PXRD patterns were recorded by using a Mar345 imaging plate detector with a pixel size of 100 µm and a typical exposure time of 5 min. The

10.1021/cg070158w CCC: $37.00 © 2007 American Chemical Society Published on Web 08/08/2007

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sample-to-imaging plate distance was ca. 260 mm, and the diffraction angle was calibrated by the standard powders of Ag-Behenate and Si powder (NBS640b). The one-dimensional (1-D) XRD profile was converted using the FIT2D program of a cake type integration. Synthesis of {[Cd2(C5O5)2(bpe)(H2O)]‚2H2O}n (1). The reaction of Cd(NO3)2‚4H2O (0.0308 g, 0.1 mmole), Na2C5O5 (0.0168 g, 0.1 mmole), bpe (0.100 g, 0.055 mmole), and deionized water (6 mL) in a mole ratio of 2:2:1.1 was sealed in a Teflon-lined acid digestion bomb and allowed to proceed at 180 °C for 3 days, and then the reactant mixture was cooled to room temperature at a rate of ca. 6 °C‚h-1. The bright-yellow platelike crystals were obtained in the solution at room temperature, and the resulting solution (pH ) 6.07) was left to stand for 2 days. The crystals (yield: 24.8 mg, 70.7%) were filtered off and washed with deionized water. Anal. Found: C, 35.35; H, 2.26; N, 3.97. Calcd for C22H16Cd2N2O13: C, 35.64; H, 2.16; N, 3.78. IR (KBr) ν (cm-1): 3441 (m), 1683 (m), 1607 (vs), 1520 (vs), 1505 (vs), 1480 (vs), 1430 (m), 1386 (m). Synthesis of {[Zn2(bpe)2(C5O5)2]}‚H2O}n (2). The reaction of ZnCl2 (0.0068 g, 0.05 mmol), Na2C5O5 (0.0093 g, 0.05 mmol), bpe (0.091 g, 0.05 mmol), and deionized water (6 mL) in a mole ratio of 1:1:1 was sealed in a Teflon-lined acid digestion bomb and allowed to proceed at 180 °C for 3 days, and then the reactant mixture was cooled to room temperature at a rate of ca. 6 °C‚h-1. The pale-yellow needle-like crystals were obtained in the solution at room temperature, and the resulting solution (pH ) 5.33) was left to stand for 2 days. The crystals (yield: 21.5 mg, 54.2%) were filtered off and washed with deionized water. Anal. Found: C, 51.01; H, 2.34; N, 6.97. Calcd for C34H22Zn2N4O11: C, 51.42; H, 2.77; N, 7.06. IR (KBr) ν (cm-1): 3058 (w), 1675 (m), 1613 (s), 1552 (s), 1515 (vs), 1476 (vs), 1429 (s), 1420 (s), 1344 (m), 1205 (m). Crystallographic Data Collection and Refinement. Single-crystal structure analyses of compounds 1 and 2 were performed on a Siemens

SMART diffractometer with a CCD detector with Mo radiation (λ ) 0.71073 Å) at room temperature. A preliminary orientation matrix and unit cell parameters were determined from three runs of 15 frames each, each frame corresponding to a 0.3° scan in 10 s, followed by spot integration and least-squares refinement. For each structure, data were measured using ω scans of 0.3° per frame for 20 s until a complete hemisphere had been collected. Cell parameters were retrived using SMART28 software and refined with SAINT29 on all observed reflections. Data reduction was performed with the SAINT29 software and corrected for Lorentz and polarization effects. Absorption corrections were applied with the program SADABS.30 Direct phase determination and subsequent difference Fourier map synthesis yielded the positions of all non-hydrogen atoms, which were subjected to anisotropic refinements. Hydrogen atoms were placed in their geometrically generated positions. The final full-matrix, least-squares refinement on F2 was applied for all observed reflections [I > 2σ(I)]. All calculations were performed using the SHELXTL-PC V 5.03 software package.31 Crystallographic data and details of data collections and structure refinements of 1 and 2 are listed in Table 1. CCDC-626048 and 626047 for 1 and 2 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 [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (Internet.) +44-1223/336-033; E-mail: [email protected]].

Results and Discussion Synthesis and IR Spectroscopy. Compounds 1 and 2 can be synthesized only under hydrothermal conditions by the reactions of Cd(NO3)2‚4H2O (1) and Zn(NO3)2‚4H2O (2), respectively, Na2C5O5 and bpe with stoichiometric molar ratios

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Crystal Growth & Design, Vol. 7, No. 9, 2007 1785

Table 1. Crystallographic Data and Refinement Parameters of {[Cd2(bpe)(C5O5)2(H2O)]‚2H2O}n (1) and {[Zn2(bpe)2(C5O5)2]}‚H2O}n (2) formula fw cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g/cm3) µ (mm-1) F(000) reflns collected unique reflns (I > 2σ(I)) max and min trans R1, wR2a (I > 2σ(I)) R1, wR2a (all data) GOF on F2 no. of variable a

1

2

C22H16Cd2N2O13 741.17 monoclinic P21/n 17.593(1) 7.5729(5) 18.224(1) 90.0 105.211(2) 90.0 2342.9(3) 4 2.101 1.892 1448 19042 4254 0.9113, 0.7644 0.0469, 0.1027 0.0647, 0.1193 1.077 353

C34H22Zn2N4O11 793.30 monoclinic P21/c 8.1000(5) 24.084(1) 16.1866(9) 90.0 92.014(1) 90.0 3155.8(3) 4 1.670 1.592 1608 30250 5243 0.8832, 0.6917 0.0444, 0.0935 0.0716, 0.1053 1.019 460

RF ) ∑||Fo - Fc||/∑|Fo|; Rw(F2) ) [∑w|Fo2 - Fc2|2/∑w(Fo4)]1/2.

of 2:2:1.1 and 1:1:1, respectively, in aqueous solution (6 mL) at 180 °C for 3 days. The synthetic procedures under conventional solution conditions with different molar ratios at room temperature have been tested but did not work for the syntheses of these two compounds. The resulting products are stable in air at ambient temperature and insoluble in common solvents such as water, alcohol, and acetonitrile. The synthetic reproducibility is excellent. The most relevant IR features are those associated with the chelating croconate ligands. Strong and broad absorptions occurring in the range 1610-1350 cm-1 are characteristic of the salts of CnOn2- ions,32 and in the present case they can be assigned to vibrational modes representing mixtures of C-O and C-C stretching motions. The presence of medium-strength absorptions at 1683 cm-1 for 1 and 1675 cm-1 for 2 may be assigned to the coordinated coordinated carbonyl group of croconate. The infrared spectrum shows a broad band in the region 3300-3500 cm-1, which can be assigned to the stretching vibration, ν(O-H) of the water molecules. Structure Description of {[Cd2(bpe)(C5O5)2(H2O)]‚2H2O}n (1). The X-ray structure determination reveals that compound 1 is isostructural with {[Cd2(C5O5)2(4,4′-bpy)(H2O)]‚H2O}n, and its 2-D layer framework contains two crystallographically independent cadmium(II) ions with different coordination geometries. The molecular structure (Supporting Figure 1) of 1 around Cd(1) and Cd(2) ions is deposited as Supporting Information. Cd(1) lies in a distorted pentagonal bipyramidal environment, consisting of one nitrogen atom from a bpe ligand with a bond distance of Cd(1)-N(1) ) 2.248(5) Å and six oxygen atoms from three bridging croconate ligands with bond distances of Cd(1)-O(1) ) 2.569(4), Cd(1)-O(2) ) 2.363(3), Cd(1)-O(3) ) 2.398(3), Cd(1)-O(4) ) 2.431(4), Cd(1)-O(6) ) 2.469(4), and Cd(1)-O(10) ) 2.292(4) Å, while the Cd(2) lies in a distorted octahedral environment, consisting of one nitrogen atom from a bpe ligand with a bond distance of Cd(2)-N(2) ) 2.268(5) Å and five oxygen atoms from two bridging croconate ligands and one water molecule with bond distances of Cd(2)-O(1) ) 2.418(4), Cd(2)-O(5) ) 2.339(4), Cd(2)-O(9) ) 2.325(4), Cd(2)-O(10) ) 2.319(4), and Cd(2)-O(11)w ) 2.257(4) Å. The bpe ligand acts as a bis-

Figure 1. (a) Two basic building units in compound 1 with the small rectangular box formed via the connectivity between Cd(II) ions and µ3-croconates, µ5-croconates, and the large rectangular box formed via the connectivity between the Cd(II) ions and bpe, µ5-croconates. Dotted lines represent the hydrogen bonding interactions (O(11)‚‚‚‚O(4)). (b) A schematic representation of the 2-D layered MOF formed by two rectangular boxes extended along the [010] and [001] directions. Red rods represent µ3-croconate bridges, blue rods are for bpe bridges, and red triangle are for µ5-croconate bridges.

Figure 2. Crystal packing of 1 along the b-axis showing the formation of a 3-D supramolecular structure by interlayer “sandwiched” π-π interactions among 2-D layered MOFs.

monodentate bridging ligand to connect Cd(1) and Cd(2) ions (Figure 2) with a Cd(1)‚‚‚Cd(2C) separation of 13.855 Å. It is noteworthy that the croconate plays an important role in the construction of the 2-D layer framework associated with coligand bpe. The croconate acts as a bridging ligand with two different bridging modes. The first one is a bridging bis-bidentate mode through three-adjacent22,23,27 (µ3) croconate oxygen atoms (IV in Scheme 1) (O(6), O(9), O(10)) to link Cd(1) and Cd(2) ions with the Cd(1)‚‚‚Cd(2) separation of 4.378 Å, which is in comparable to that of 4.381 Å in [Cd2(4,4′-bipy)(C5O5)2(H2O)].27a The other one adopts a tris-bidentate through the five

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Table 2. π-π Interactions (Face-to-Face) in 1a dihedral slip distance of distance between ring (i) angle angle centroid (i) the (i,j) ring fHardReturnring (j) (i,j)/° (i,j)/° form ring (j)/Å centroids/Å R(1) f R(2)i R(2) f R(1)ii R(1) f R(3)iii R(3) f R(1)iv

18.3 18.3 18.3 18.3

15.98 18.17 5.58 33.11

3.657 3.886 3.786 3.426

3.804 4.090 3.804 4.090

a Symmetry code: (i) ) x + 1, -y - 1/2, z + 1/2; (ii) ) x + 1, -y + 1/2, z + 1/2; (iii) ) x, -y - 1/2, z - 1/2; (iv) ) x, -y + 1/2, z - 1/2. R(i)/R(j) denotes the centroids of ith/jth of bpe/croconate; R(1) ) C(6)C(7)-C(8)-C(9)-C(10); R(2), R(3) ) N(2)-C(18)-C(19)-C(20)C(21)-C(22).

Table 3. The O-H‚‚‚O H-bonds and C-H‚‚‚O Interaction for 1a D-H‚‚‚A (Å) O(11)-H(11)‚‚‚O(4)i O(11)-H(11′)‚‚‚O(9)ii O(12)-H(12′)‚‚‚O(13) O(13)-H(13)‚‚‚O(12) O(13)-H(13′)‚‚‚O(7)iii C(14)-H(14A)‚‚‚O(7)ii C(15)-H(15A)‚‚‚O(8)ii C(16)-H(16A)‚‚‚O(3)iv C(18)-H(18A)‚‚‚O(12)ii C(19)-H(19A)‚‚‚O(13)ii C(21)-H(21A)‚‚‚O(3)iv

D-H (Å) H‚‚‚A (Å) D‚‚‚A (Å) ∠ D-H‚‚‚A (°) 0.90 0.90 0.91 0.91 0.90 0.93 0.93 0.93 0.93 0.93 0.93

1.92 1.92 1.97 2.18 2.28 2.58 2.33 2.48 2.45 2.47 2.50

2.791 2.810 2.764 2.764 3.067 3.510 3.120 3.401 3.230 3.310 3.404

163 172 145 121 146 174 143 172 142 151 166

a Symmetry code: (i) ) x, y + 1, z; (ii) ) x + 1/2, -y + 3/2, z + 1/2; (iii) ) -x, -y + 1, -z; (iv) ) -x + 1/2, y + 1/2, -z + 1/2.

croconate oxygens (µ5)27a bridging mode (IX in Scheme 1) (O(3),O(4)/(O(1),O(2),O(5)) to link two crystallographically identical Cd(1) and one Cd(2) ions as an isosceles triangle Cd3 unit (Figure 1a) with Cd(1)‚‚‚Cd(1A), Cd(1)‚‚‚Cd(2A), and Cd(1A)‚‚‚Cd(2A) separations of 7.573, 7.499, and 4.773 Å, respectively, which are also comparable to those of 7.584, 7.501, and 4.785 Å in [Cd2(4,4′-bipy)(C5O5)2(H2O)].27a Compound 1 is a 2-D layer MOF constructed by two rectangular boxes as the basic building blocks. The first rectangular box (Figure 1a) shows that four Cd(1) and four Cd(2) ions occupy the box corners and are linked by four µ3- and two µ5-croconate bridging ligands to complete the smaller box with a size of 7.573 × 4.773 × 4.378 Å. The strong intermolecular hydrogen bond between the coordinated oxygen atom of croconate and the coordinated water (black dashed line in Figure 1a; (O(4)i‚‚‚O(11) ) 2.791 Å; i ) x, 1 + y, z) is found to enhance the stabilization of the box. The other rectangular box (Figure 1a) shows that four Cd(1) and four Cd(2) ions also occupy at the box corners but are linked by four bpe and two µ5-croconate bridging ligands to complete the larger box with a size of 13.855 × 7.573 × 4.773 Å. Both of the two rectangle boxes share their faces with the neighboring ones forming a 2-D layered framework along [010] and [001] directions. A more clearly schematic representation is shown in Figure 1b, in which the same boxes are repeatedly arranged along the b-axis, and different boxes are then alternatively arranged along the c-axis. The adjacent independent 2-D layers are mutually interlocked by an interlayer “sandwiched” π-π interaction26,33 between the µ3-croconate and two pyridyl rings of bpe ligand (Table 2) and C-H‚‚‚O interactions between the croconates and the bpe ligands (Table 3), resulting in a stable 3-D extended architecture with an ABCABC‚‚‚ stacking pattern among these layered MOFs (Figure 2). The lattice water molecules (O(12), and O(13)) are encapsulated by 2-D layers and are stabilized by H-bonding with the oxygen atoms of croconate and coordinated water molecules (Table 3). The C-H‚ ‚‚O interaction present here is moderately strong due to the sp2

hybridization of the associated C atom, which necessarily increases the acidity of the C atom.34 It is worth noting that not only is the coordinated O(3) atom of µ5-croconate involved in bifurcated C-H‚‚‚O interaction but also two uncoordinated O(7) and O(8) of µ3-croconate also take part in the C-H‚‚‚O interaction. Structure Description of {[Zn2(bpe)2(C5O5)2]‚H2O}n (2). The 2-D bilayered framework of compound 2 having the formula [Zn2(bpe)2(C5O5)2]‚H2O reveals that two crystallographic independent Zn centers both have a distorted {MN2O4} octahedral coordination environment, bonded with two nitrogen donors of two bpe ligands at cis position and with four oxygen donors of two croconate. The molecular structure (Supporting Figure 2) of 2 around Zn(1) and Zn(2) is deposited as Supporting Information. The bpe ligands also act as bis-monodentate bridging ligands to link Zn(1) and Zn(2) ions with Zn(1)-Zn(2) separations of 13.565 and 13.442 Å. It is noteworthy that the croconate also adopts two different bridging bis-bidentate modes to construct the 2-D MOF associated with the coligand bpe. The first one is a bridging bis-bidentate mode through four21,22,27b (µ4) (III in Scheme 1) croconate oxygen atoms (O(1)/O(2) and O(4)/(O5)) to link Zn(1) and Zn(2) ions with a Zn(1)-Zn(2) separation of 7.028 Å. The other one adopts a bridging bis-bidentate mode but through three-adjacent22,23,27 (µ3) (IV in Scheme 1) croconate oxygen atoms (O(6), O(7), and O(10)) to link Zn(1) and Zn(2) ions with Zn(1)-Zn(2) separations of 4.799 Å. The 2-D brick-wall-like layer (Figure 3a, red and blue layers) is formed with a rectangle-grid as a basic building unit, which consists of six Zn ions bridged by two µ3-croconate, two µ4croconate, and two bis-monodentate bpe, giving rise to a 12menbered rectangular aperture with sizes of approximate 18.353 × 7.028, Å (Zn-Zn distances). Two layers are then parallel intercrossed and cross-linked by an out-of-plane bpe in a bismonodentate fashion to give a 2-D bilayered framework (Figure 3a), which is similar to that of [M(dpe)(C5O5)],27b but with different intercrossed angles of 0° and 55° between two layers by bpe and gauche dpe bridges, respectively. The schematic representations of 2 and [M(dpe)(C5O5)] shown in Figure 3, panels c and d, respectively, illustrate their differences in a more straightforward manner. More significantly, compound 2 can be distinguished by its unique topological feature, namely, selfinterpenetration, in which the shortest circuits are penetrated by rods of the same network (the red and blue circuits in Figure 3e). Although an increasing number of self-interpenetrated 3-D frameworks exist,35-38 such a phenomenon in 2 remains extremely rare for 2-D MOFs. Like compound 1, adjacent independent 2-D bilayered frameworks with identical connectivity are also mutually interlocked by an interlayer “sandwiched” π-π interaction26,33 between the µ3-croconate and pyridyl rings of bpe (Table 4) and C-H‚‚‚O interactions between the croconates and the bpe ligands (Table 5), resulting in a stable 3-D supramolecular architecture with an ABAB‚‚‚ stacking pattern among these bilayered MOFs (Figure 4). The lattice water molecules (O(11)) are encapsulated by 1-D channels formed from the 1-D alternatively trapezoid, inverse-trapezoid chain of the bilayered MOF (Figure 3b) and are stabilized by C-H‚‚‚O interactions between the bpe ligands and the coordinated water molecules (Table 5). Molecular Symmetry and π-Electron Delocalization of Croconate. The correlations between molecular symmetry and π-electron delocalization around the five-membered ring of croconate have received much attention.39-41 Charge-localized (C2V) and charge-delocalized (D5h) forms can coexist in a

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Figure 3. (a) Clear view (along the b-axis) of the 2-D bilayered MOF of compound 2 with two layers (red, upper layer and blue, lower layer) cross-linkage by bpe ligands (pink and green) (b) 1-D alternative trapezoid, inverse-trapezoid chain in the bilayer viewed along the a-axis. (c) A schematic representation of 2. (d) A schematic representation of [M(dpe)(C5O5)]. (e) Self-interpenetration in 2 (see the red and blue circuits). Table 4. π-π Interactions (Face-to-Face) in 2a ring (i) fHardReturnring(j) R(1) f R(2)i R(2) f R(1)i R(1) f R(3)ii R(3) f R(1)ii

Table 5. The C-H‚‚‚O Interaction for 2a

dihedral slip distance of distance between angle angle centroid (i) the (i,j) ring (i,j)/° (i,j)/° form ring (j)/Å centroids/Å 20.6 25.8 20.6 9.0

18.85 27.52 34.20 36.02

3.833 3.577 3.347 3.262

4.050 4.033 4.050 4.033

a Symmetry code: (i) ) -x + 1, y + 3/2, -z + 3/2; (ii) ) -x + 1, y + 3/2, -z + 1/2. R(i)/R(j) denotes the centroids of ith/jth of bpe/croconate; R(1) ) C(6)-C(7)-C(8)-C(9)-C(10); R(2) ) N(1)-C(11)-C(12)C(13)-C(14)-C(15); R(3) ) N(2)-C(18)-C(19)-C(20)-C(21)-C(22).

hydrogen-bonded host lattice.39 In this study, for the bisbidentate mode through three-adjacent (µ3) (IV) of croconate, two uncoordinated oxygens of O(7), O(8), and O(8), O(9) in 1 and 2, respectively, both show a diketonic form with shorter bond distances of C(7)-O(7) 1.227(7), C(8)-O(8) 1.224(7) Å in 1 and C(8)-O(8) 1.215(4), C(9)-O(9) 1.218(4) Å in 2, respectively, compared to other three C-O distances of 1.238(7)-1.293(6) Å (1) and 1.257(3)-1.264(4) Å (2) of the metalcoordinated oxygen atoms (Tables 6 and 7). Notably, the C-C bond lengths between two nonbonded CdO groupss1.494(9) Å both for C(7)-C(8) and C(8)-C(9) in 1 and 2, respectivelys are close to being single bond in character, and they are also longer than other C-C bond lengths, 1.416(7)-1.481(8) Å and

D-H‚‚‚A (Å) C(15)-H(15A)‚‚‚O(7) C(17)-H(17A)‚‚‚O(1)i C(18)-H(18A)‚‚‚O(10)ii C(27)-H(27A)‚‚‚O(2) C(27)-H(27A)‚‚‚O(11)iii C(30)-H(30A)‚‚‚O(9)iv

D-H (Å) H‚‚‚A (Å) D‚‚‚A (Å) 0.93 0.93 0.93 0.93 0.93 0.93

2.59 2.58 2.47 2.56 2.52 2.54

3.084 3.464 3.031 3.053 3.278 3.075

∠ D-H‚‚‚A (°) 114 160 119 113 139 117

a Symmetry code: (i) ) -x, -y, -z; (ii) ) x - 1, y, 1 + z; (iii) ) x, -y + 1/2, z - 1/2; (iv) ) -x + 2, y + 1/2, -z - 1/2.

1.425(4)-1.471(4) Å in 1 and 2, respectively, indicating a partial π-electron localization form of croconate in the five-membered ring (Scheme 2 a) upon complexation.27 Such partial π-electron localization molecular geometry of C5O52- seems to be in accordance with a charge-localized form of croconate in a hydrogen-bonded host lattice,39 alkali metal croconate salts,40 and theoretical predictions.41,42 Unlike the former µ3-croconate, the differences among five C-O and C-C distances (Table 6) of tris-bidentate through the five croconate-oxygens (µ5) bridging mode (IX) in 1 are not so obvious and in the range of 1.238(6)-1.262(6) and 1.447(7)-1.462(7) Å, which indicate a more π-delocalization bridging mode (Scheme 2b) upon complexation, close to a free croconate dianion.27,39,40 For bis-bidentate mode through four-adjacent (µ4) (III) of croconate in 2, the

1788 Crystal Growth & Design, Vol. 7, No. 9, 2007

Wang et al.

Figure 4. Crystal packing of 2 along the b-axis showing the formation of a 3-D supramolecular structure by interlayer “sandwiched” π-π interactions (white dashed line) among 2-D bilayered MOFs. Table 6. C-O and C-C Bond Lengths (Å) of C5O52- for 1 O(1)-C(1) O(2)-C(2) O(3)-C(3) O(4)-C(4) O(5)-C(5) O(6)-C(6) O(7)-C(7) O(8)-C(8) O(9)-C(9) O(10)-C(10)

µ5-C5O521.262(6) C(1)-C(2) 1.238(6) C(2)-C(3) 1.245(6) C(3)-C(4) 1.253(6) C(4)-C(5) 1.248(6) C(5)-C(1) µ3-C5O521.238(7) C(6)-C(7) 1.227(7) C(7)-C(8) 1.224(7) C(8)-C(9) 1.250(7) C(9)-C(10) 1.293(7) C(10)-C(6)

1.451(7) 1.462(7) 1.451(7) 1.454(7) 1.447(7) 1.481(8) 1.494(9) 1.465(8) 1.417(7) 1.431(8)

Table 7. C-O and C-C Bond Lengths (Å) of C5O52- for 2 O(1)-C(1) O(2)-C(2) O(3)-C(3) O(4)-C(4) O(5)-C(5) O(6)-C(6) O(7)-C(7) O(8)-C(8) O(9)-C(9) O(10)-C(10)

µ4-C5O521.258(4) C(1)-C(2) 1.245(4) C(2)-C(3) 1.221(4) C(3)-C(4) 1.245(4) C(4)-C(5) 1.259(4) C(5)-C(1) µ3-C5O521.264(4) C(6)-C(7) 1.262(3) C(7)-C(8) 1.215(4) C(8)-C(9) 1.218(4) C(9)-C(10) 1.257(3) C(10)-C(6)

1.443(4) 1.482(4) 1.484(4) 1.445(4) 1.431(4) 1.425(4) 1.471(4) 1.494(4) 1.462(4) 1.425(4)

uncoordinated oxygen atom (O(3)) show a ketonic form with shorter bond lengths (1.221(4) Å for C(3)-O(3)) compared to other C-O bond lengths (1.245(4)-1.258(4) Å) of the metalcoordinated oxygen atoms (Table 7). Two C-C bond lengths Scheme 2

Figure 5. (a) Thermogravimetric analysis of compound 1. (b) Thermogravimetric analysis of compound 2.

of 1.482(4) for C(2)-C(3) and 1.484(4) for C(3)-C(4) around the CdO bond are obviously larger than other three C-C bond lengths in the range of 1.431(4)-1.445(4) Å (Table 7), indicating an intermediate pattern (Scheme 2c) between the charge-localized (C2V) and charge-delocalized (D5h) forms of croconate24 upon complexation. Thermogravimetric Analysis and in Situ PXRD Measurement. To study the thermal stability of these materials, TGA associated with in situ temperature-dependent XRD measurements were performed on polycrystalline samples of compounds 1 and 2, which are shown in Figure 5, panels a and b, respectively. During the heating process, the TGA analysis shows that the first weight loss of 5.84% (calc 4.86%) for 1 and 4.07% (calc 2.27%) for 2, respectively, corresponding to the loss of crystallized and coordinated water molecules occurred in the range of approximately 50-200 °C. In the temperature range of approximately 200-280 °C, both compounds are stable without any weight loss. On further heating, these samples decomposed at approximately 280-470 °C with losses of 57.64 and 75.69% for 1 and 2, respectively. The in situ temperaturedependent PXRD were applied to study the thermostabilities and phase transitions of 1 and 2 as shown in Figure 6a,b. Their simulation powder data extracted from single-crystal data were also listed for comparison. Both compounds were heated up to 800 °C during the XRD measurements. Compound 1 showed the thermal stability up to 350 °C, while a second phase was observed at 240 °C. The phase transition was complete at 350 °C. The crystalline then decomposed at 480 °C and finally

Mixed-Ligand Coordination Polymers

Crystal Growth & Design, Vol. 7, No. 9, 2007 1789

demonstrated that both of two 3-D supramolecular architectures have quite high thermostability and they retain their crystalline forms up to 300 °C. Acknowledgment. The authors acknowledge the National Science Council (NSC), Taiwan ROC, for financial support. Supporting Information Available: Bond lengths and angles for Cd(1) and Cd(2) coordinations spheres in complex 1 and for Zn(1) and Zn(2) coordination spheres in 2; ORTEP drawings of coordination spheres. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 6. In situ synchrotron powder X-ray diffraction patterns (λ ) 0.77490 Å) associated with the simulated diffractogram from the singlecrystal X-ray data. The temperature ramps from room temperature to 600 °C. (a) Compound 1; (b) compound 2. The baselines for each temperature were shifted for clarity.

became amorphous at 600 °C (see Figure 6a). Compound 2 showed no phase transition at a temperature up to 400 °C. Upon heating up to 500 °C, compound 2 decomposed, and the ZnO crystalline formed (PDF#653411, space group P63mc, a ) 3.249, c ) 5.206 Å) (Figure 6b). The XRD measurements are in accordance with the TG analyses, and both results indicate that compounds 1 and 2 have quite high thermal stability and retain their crystalline forms up to at least 350 °C. The high thermal stability is mainly attributed to mutual interlocking among the 2-D MOFs by the interlayer “sandwiched” π-π interactions between the µ3-croconate and the pyridyl rings of bpe. Conclusions In this paper, we have presented the syntheses and crystal structures of two coordination polymers, {[Cd2(bpe)(C5O5)2‚ (H2O)]‚2H2O}n (1) and {[Zn2(bpe)2(C5O5)2]‚(H2O)}n (2), with interesting 2-D layered and bilayered MOFs, respectively. Both of their 2-D MOFs are constructed by the metal centers and croconate ligands adopting two bridging modes associated with co-ligands bpe and are then extended to 3-D supramolecular architectures by unfamiliar interlayer “sandwiched” π-π interactions between croconate and bpe ligands and C-H‚‚‚O interactions. TGA associated with in situ PXRD measurements

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