Assemblies of Two New Metal−Organic Frameworks Constructed from

Assemblies of Two New Metal-Organic Frameworks Constructed from Cd(II) with 2,2′-Bipyrimidine and Cyclic Oxocarbon Dianions CnOn2- (n ) 4, 5) Chih-Chieh Wang,*,† Chen-Tsung Kuo,† Jing-Chun Yang,† Gene-Hsiang Lee,‡ Wei-Ju Shih,# and Hwo-Shuenn Sheu#

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 8 1476-1482

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 23, 2007; ReVised Manuscript ReceiVed May 17, 2007

ABSTRACT: Two extended networks of Cd(II) with 2,2′-bipyrimidine (bpym) and cyclic oxocarbon dianions CnOn2- (n ) 4, 5) with formulas [Cd(C4O4)(bpym)0.5(H2O)] (1) and [Cd(C5O5)(bpym)0.5(H2O)] (2) have been synthesized and characterized by singlecrystal X-ray diffraction studies. Structural determination reveals that, in compounds 1, each Cd center lies in a seven-coordinate environment bonded to two bpym nitrogen atoms and five oxygen donors from four squarate and one water molecules. The squarate acts as a bridging ligand with two different binding modes, a tetramonodentate (µ4) and a bis-bridging (µ4) coordination mode, linking the Cd(II) ions to form a two-dimensional (2D) metal-squarate layer. Adjacent layers are then mutually linked via bridges of bis-bidentate bpym ligands constructing a three-dimensional (3D) triangular metal-organic framework (MOF). In compound 2, each Cd center lies in an eight-coordinate environment bonded to two bpym nitrogen atoms and six oxygen donors from three croconate ligands and one water molecule. Each croconate (C5O52-) adopts a new bis-bidentate/monodentate (µ5) bridging mode and links crystallographically identical Cd ions, forming a one-dimensional (1D) bichain. Adjacent bichains are then mutually linked via the bridges of bis-bidentate bpym ligands constructing a 2D layered MOF, which is then extended to a 3D supramolecular architecture by two intermolecular π-π interactions between the pyrimidyl rings of bpyms and between cyclic five-membered rings of croconates. Both MOFs are thermally stable, as evidenced by thermogravimetric analysis and in-situ powder X-ray diffraction measurements. Introduction The introduction of the historical significance and interesting chemistry of monocyclic oxocarbon dianions CnOn2- (n ) 3, deltate; n ) 4, squarate; n ) 5, croconate; n ) 6, rhodizonate), which is a nonbenzenoid aromatic system, are well described in several books and reviews.1-4 Over the past four decades, interest in the whole family of the oxocarbon dianions has focused not only on their aromaticity and molecular symmetry but also on their coordination chemistry with transition metal ions. The theoretical studies indicated that the aromaticity decreases with increasing ring size and that rhodizonate (C6O62-) possesses only a small degree of aromatic character.5 In this regard, Mak and co-workers were able to resolve both chargedelocalized (Dnh) and charge-localized (C2V) forms of oxocarbon dianions stabilized in a hydrogen-bonded host lattice.6 Much research effort has concentrated on the exploitation of squarate (C4O42-) and croconate (C5O52-) ligands in the construction of versatile coordination polymeric architectures by using various binding modes. The squarate, C4O42-, has been widely used as a polyfunctional ligand, such as hydrogen bonding or π-π interactions, for the construction of extended supramolecular architecture and also used as a bridging ligand with various bonding modes (µ1 to µ6 bridges) to build up many novel extended networks.7-17 The planar D4h structures of squarate have been well established for their metal salts or complexes. The bonding characteristics of croconate (C5O52-) with transition-metal ions have also received much attention, and the results

revealed that the cyclic oxocarbon ligand can be coordinated to the metal ions with various binding modes.17-26 The variation of both C-C and C-O bond-lengths indicates that croconate possesses either an enediolate form in a terminal bidentate mode19,20 or an intermediate pattern between the chargelocalized (C2V) and charge-delocalized (D5h) forms in multidentate modes.21-25 Recently, we have reported the first metal-rhodizonate coordination framework, [M(C6O6)(bpym)(H2O)]‚nH2O (M ) Cd, n ) 1; M ) Mn, n ) 2; bpym ) 2,2′bipyrimidine), in which rhodizonate and bpym both act as the bis-bidentate ligands connecting the metal ions to form a novel two-dimensional (2D) boatlike M6 metal-organic framework (MOF).26 The molecular structure of rhodizonate shows a nonplanar C2 symmetry. With our continuous effort on the study of metal coordination polymers27 with monocyclic oxocarbon dianion, the structural topology of a Cd ion with bpym and the other two monocyclic oxocarbon dianions, squarate and croconate, seems to be interesting and worth investigating. Focusing on this approach, we report here two coordination polymers, [Cd(C4O4)(bpym)0.5(H2O)] (1) and [Cd(C5O5)(bpym)0.5(H2O)] (2), in which the squarate acts as the bridging ligand with two coordination modes, a tetra-monodentate µ4 and a new 1,3-bisbridging µ4 (I and II in Scheme 1) and the croconate act as the bridging ligand with a new bis-bidentate/monodentate µ5coordination mode (III) to form two new three-dimensional (3D) extended networks. Their thermal stability and molecular symmetry of croconate and squarate are also reported. Experimental Section

* 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.

Materials and Physical Techniques. All chemicals were of reagent grade and were used as commercially obtained without further purification. The infrared spectra were recorded on a Nicolet Fourier Transform IR, MAGNA-IR 500 spectrometer in the range of 500-

10.1021/cg070189r CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

MOFs from the Cd(II) with bpym Scheme 1

4000 cm-1 using the KBr disc technique. Thermogravimetric analyses (TGA) of the title compounds were performed on a computer-controlled Perkin-Elmer 7 Series/UNIX TGA7 analyzer. Single-phased powder samples of 1 (4.470 mg) and 2 (1.002 mg) were loaded into alumina pans and heated with a ramp rate of 5 °C/min from room temperature to 800 °C under nitrogen atmosphere. The in situ powder X-ray diffraction was performed at the beamline BL17A1 of the National Synchrotron Radiation Research Center (NSRRC) Taiwan. The synchrotron ring energy is 1.5 GeV with a typical current of 200 to 120 mA. The X-ray wavelength was 1.3323 Å, which is delivered by a triangular Si(111) bent crystal monochromator. The title samples were sealed in 1-mm quartz capillary and heated under a hot air stream from room temperature up to 600 °C during the in situ X-ray diffraction (XRD) measurements. 2D powder X-ray diffraction patterns were recorded by using Fuji BAS2500 imaging plate with the pixel size of 100 µm and typical exposure time of 10 min. The sample to imaging plate distance was ca. 280 mm, and the diffraction angle was calibrated by the standard powders of Ag-behenate and Si powder (NBS640b). The one-dimensional (1D) XRD profile was converted using FIT2D program. Synthesis of [Cd(C4O4)(bpym)0.5(H2O)] (1). A solution (2 mL) of squaric acid (H2C4O4, 11.4 mg, 0.1 mmol) was added to a solution (4 mL) of Cd(NO3)2‚4H2O (30.8 mg, 0.1 mmol) and 2,2′-bipyrimidine (15.8 mg, 0.1 mmol) at room temperature to give a colorless solution (pH ) 1.54). Yellow needle-like crystals were obtained after several days in 35% yield. Anal. Calc. for C8H5CdN2O5: C 28.88, N 8.71, H 1.56. Found: C 28.61, N 8.46, H 1.32. IR (KBr pellet): ν ) 3277 (m), 3054 (m), 1711 (w), 1569 (s), 1527 (vs), 1410 (s), 1385 (s), 1272 (m), 1230 (m) cm-1. Synthesis of [Cd(C5O5)(bpym)0.5(H2O)]‚H2O (2). A solution (3 mL) of croconic acid (H2C5O5, 14.2 mg, 0.1 mmol) was added to a solution (6 mL) of Cd(NO3)2‚4H2O (30.8 mg, 0.1 mmol) and 2,2′-bipyrimidine (7.9 mg, 0.05 mmol) at room temperature to give a colorless solution (pH ) 2.15). Yellow block crystals were obtained after several days in 32% yield. Anal. Calc. for C9H5CdN2O6: C 30.96, N 8.02, H 1.44. Found: C 30.63, N 8.34, H 1.25. IR (KBr pellet): ν ) 3421 (m), 3054 (w), 1663 (m), 1558 (vs), 1539 (vs), 1411 (s), 1217 (w) cm-1. Crystallographic Data Collection and Refinement. Single-crystal structure analyses of compounds 1 and 2 were performed on a Siemens SMART diffractomer 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 3 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 compounds 1 and 2 are listed in Table 1. CCDC-651745 and 256897 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, Cam-

Crystal Growth & Design, Vol. 7, No. 8, 2007 1477 Table 1. Crystallographic Data and Refinement Parameters of [Cd(C4O4)(bpym)0.5(H2O)] (1) and [Cd(C5O5)(bpym)0.5(H2O)] (2) chemical formula molecular weight crystal system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z Fcalc/g cm-3 T/K µ/mm-1 crystal size/cm 2θ range/deg refls collected refls unique refls with [I > 2σ(I)] NV RF, Rw(F2) (all data)a RF, Rw (F2) [I > 2σ(I)]a GOF ∆Fmax/min./e Å-3 a

1

2

C8H5N2O2Cd 273.54 triclinic P1h 7.5532(6) 7.6420(6) 8.4674(7) 71.386(1) 73.065(1) 74.956(1) 435.56(6) 2 2.086 295(2) 2.470 0.15 × 0.15 × 0.05 5.22-54.96 5740 1991 1940 153 0.0195, 0.0473 0.0188, 0.0468 1.010 0.705/-0.421

C9H5N2O6Cd 349.55 monoclinic C2/m 11.3146(6) 7.6357(4) 11.8779(6) 90.0 107.616(1) 90.0 978.07(9) 4 2.374 295(2) 2.258 0.12 × 0.12 × 0.03 3.60-54.98 4881 1207 1134 100 0.0257, 0.0557 0.0231, 0.0544 1.026 0.421/-0.558

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

bridge CB2 1EZ, UK; fax: (Internet) +44-1223/336-033; E-mail: [email protected]].

Results and Discussion Synthesis and IR Spectroscopy of Compounds 1 and 2. The preparations of compounds 1 and 2 were carried out via the reactions of Cd(NO3)2‚4H2O and bpym with H2C4O4 and H2C5O5, respectively, in aqueous solution under acidic conditions for a period of several days to yield yellow crystals. Both of them are very stable in air at ambient temperature and insoluble in common solvents such as water, alcohol, and acetonitrile. The synthetic reproducibility is high. The most relevant IR features are those associated with the chelating oxocarbon dianions. The IR spectrum of 1 presents a strong broad absorption at 1569 and 1527 cm-1, which correspond to the strong broad band at ca. 1530 cm-1 found in the spectrum of Na2C4O4 and which has been tentatively assigned to a combination of C-O and C-C stretching vibrations of squarate.32 The weak band at 1711 cm-1 has been assigned to the localized CdO and are consistent with the 1,3-bis-bidentate coordination of the squarate. In 2, the characteristic, strong C-O and C-C stretching vibrations of croconate appear at 1558 and 1539 cm-1. The presence of medium-strength absorption at 1663 cm-1 may be assigned to the coordinated carbonyl groups.26 Structural Description of [Cd(C4O4)(bpym)0.5(H2O)] (1). The fundamental building unit of compound 1 is shown in Figure 1, composed of a dinuclear Cd(II) unit separated by 3.784 Å with a N2O5 chromophore about the Cd(II) ions. Each Cd(II) center is bonded to two nitrogen donors of one bpym ligand and five oxygen atoms from four bridging squarate ligands and one water molecule. The corresponding bond lengths and bond angles around the Cd center are listed in Table 2. It is important to note that the squarate ligand acts as both a tetramonodentate µ4 (I in Scheme 1) and a new 1,3-bis-bridging µ4 (II in Scheme 1) coordination modes to bind two cadmium atoms forming a Cd2O6 dimer (Figure 1). Such dimeric units form an infinite 2D cadmium-squarate layer running along the a-axis and b-axis by the linkers of bis-bridging squarate and tetramonodentate

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Figure 1. Centrosymmetric arrangement of the dimeric Cd(II) unit in compound 1 with atom labeling scheme (ORTEP drawing, 50% thermal ellipsoids). Table 2. Selected Bond Lengths (Å) and Angles (°) for Compound 1a Cd(1)-O(4) Cd(1)-O(3) Cd(1)-N(1) Cd(1)-O(1)ii O(1)-C(1) O(2)-C(2) O(3)-C(3) O(4)-C(4) O(4)-Cd(1)-O(5) O(5)-Cd(1)-O(3) O(5)-Cd(1)-O(1) O(4)-Cd(1)-N(1) O(3)-Cd(1)-N(1) O(4)-Cd(1)-N(2)i O(3)-Cd(1)-N(2)i N(1)-Cd(1)-N(2)i O(5)-Cd(1)-O(1)ii O(1)-Cd(1)-O(1)ii N(2)i-Cd(1)-O(1)ii

2.289(2) 2.321(2) 2.405(2) 2.528(2) 1.270(3) 1.242(3) 1.257(3) 1.245(3) 85.42(7) 85.11(7) 160.58(7) 82.18(6) 116.99(6) 128.58(7) 74.71(6) 66.38(6) 84.27(6) 77.80(6) 137.52(6)

Cd(1)-O(5) Cd(1)-O(1) Cd(1)-N(2)i

2.299(2) 2.333(2) 2.511(2)

C(1)-C(2) C(1)-C(2)iii C(3)-C(4)iv C(3)-C(4)ii

1.468(3) 1.467(3) 1.469(3) 1.476(3)

O(4)-Cd(1)-O(3) O(4)-Cd(1)-O(1) O(3)i-Cd(1)-O(1) O(5)-Cd(1)-N(1) O(1)-Cd(1)-N(1) O(5)-Cd(1)-N(2)i O(1)-Cd(1)-N(2)i O(4)-Cd(1)-O(1)ii O(3)-Cd(1)-O(1)ii N(1)-Cd(1)-O(1)ii

155.98(7) 84.51(6) 97.58(6) 77.28(7) 117.59(6) 122.66(7) 76.36(6) 81.15(6) 75.97(6) 156.00(6)

a Symmetry operations used to generate equivalent atoms: i ) -x - 1, -y + 2, -z + 1; ii ) -x + 1, -y + 2, -z; iii ) -x, -y + 2, -z; iv ) x, y + 1, z.

squarate, respectively (Figure 2a). In this dimer, the cadmium atoms are connected via bpym units through a bis-bidentate bridging mode, linking the cadmium-squarate layers to give rise to a 3D triangular MOF as shown in Figure 2b. The coordination modes exhibited by squarate in 1 is quite different from that found generally in metal-squarate. Here, the tetradentate µ4-bridging mode is well-known in metal-squarate complexes, but dimer formation by squarate moieties is not common and mostly used on the construction of coordination polymers.7a,11a,14b,16b The other 1,3-bis-bridging µ4-coordination mode is rarely found in metal-squarate complexes, but there are a few reports in the literature, where the squarate unit exhibits a similar coordination.12b,13 So, to the best of our knowledge, compound 1 is the first 3D coordination polymer consisting of a Cd/squarate system with both tetramonodentate and bis-bridging coordination modes. It is also noted that, independent of tetramonodentate and bis-bridging squarates, the differences among four C-C distances are not obvious, with the averaged C-C distances of 1.468(3) Å for tetramonodentatesquarate and 1.473(3) Å for bis-bridging-squarate, respectively

Figure 2. (a) 2D layered architecture formed by the Cd(II) ions and squarate in the bc plane. bpym and water molecules are omitted for clarity. (b) Representation of the 3D triangular network.

(Table 2). The results indicate a more π-delocalization bridging mode (D4h) close to a free squarate ring.6 TGA associated with in situ temperature-dependent XRD measurements agree well with the crystallographic observations. During the heating process, TGA (Figure 3a) revealed that compound 1 underwent a two-step weight loss and thermally stable up to 148 °C, and the first weight loss of 5.81% (calc 5.60%) corresponding to the loss of coordinated water molecules occurred in the range of approximate 148-173 °C. In the temperature range of approximate 173-250 °C, 1 was stable without any weight lost. Upon further heating, these samples decomposed at approximately 250-540 °C with a weight loss of 53.51%. Further, the stability of the materials was investigated by in situ powder X-ray diffractometry (Figure 4), which indicated that 1 maintained its crystal form from room temperature to 160 °C. A phase transition was observed, starting from 210 °C and completed at 300 °C. After 400 °C, 1 was decomposed and formed CdO crystalline. Of particular note, the TGA studies revealed that the coordinated water molecules in 1 can be desorbed by heating a sample to 210 °C to a constant weight. In addition, the water can be reabsorbed by exposing the material to water vapor as the sample is cooled to room

MOFs from the Cd(II) with bpym

Figure 3. (a) TGA of compound 1. (b) TG measurements of cyclic dehydration and hydration processes were repeated three times. Solid line: the variation of weight loss with time; dashed line: the variation of temperature with time.

temperature. This procedure was repeated several times to demonstrate the reversibility of the process (Figure 3b). Structural Description of [Cd(C5O5)(bpym)0.5(H2O)] (2). As depicted in Figure 5, all cadimum centers in compound 2 are eight-coordinate bonded to two nitrogen atoms of bpym and six oxygen atoms of three croconate ligands and one water molecule. The corresponding bond lengths and bond angles around the Cd center are listed in Table 3. Each croconate adopts a new bridging mode, in which two pairs of oxygen atoms (O(1)/ O(3A)) and O(1B)/O(3B)) act as chelating atoms bonded to crystallographically independent cadimum ions forming a 1D chainlike framework (Figure 6a). Two chains are mutually connected by the linkage of the Cd ions and the fifth oxygen atoms (O(2)) of croconate, which adopts a unusual bonding manner with the Cd-O bond being perpendicular to the croconate plane, to complete the 1D bichain framework (Figure 6a). The Cd-O(2) bond with a bond distance of 2.646(3) Å is obviously longer than other Cd-O bonds with bond distances of 2.414(2) and 2.444(2) Å. To the best of our knowledge, the result in 2 demonstrates a new 1D bichain MOF, in which C5O52- acts as unprecedented bridging mode bonded with the metal ions. Adjacent bichains are further mutually linked via the bridges of bpym ligands that adopt a bis-bidentate bridging mode forming a 2D layered MOF (Figure 6b). The Cd-Cd separations are deduced to be 6.723 and 7.636 Å through the C5O52- bridges and 6.430 Å through the bpym bridges. In particular, adjacent independent 2D layers are mutually inter-

Crystal Growth & Design, Vol. 7, No. 8, 2007 1479

Figure 4. Various temperature powder diffraction patterns of [Cd(bpym)0.5(C4O4)‚H2O] (1) and its simulation from single-crystal data.

Figure 5. Coordination sphere about Cd(II) with atom labeling scheme (ORTEP drawing, 50% thermal ellipsoids).

locked by interlayer face-to-face π-π interactions24,33 between the pyrimidyl rings of bpym ligands and between five-membered rings of croconate ligands, resulting in a stable 3D supramolecular architecture shown in Figure 6c. The perpendicular

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Table 3. Selected Bond Lengths (Å) and Angles (°) for Compound 2a Cd(1)-O(4) Cd(1)-O(3) Cd(1)-O(1)i Cd(1)-N(1) O(1)-C(1) O(2)-C(2) O(3)-C(3) O(4)-Cd(1)-O(3)i O(3)-Cd(1)-O(3)i O(3)i-Cd(1)-N(2)ii O(4)-Cd(1)-O(1)i O(3)-Cd(1)-O(1)i O(4)-Cd(1)-O(1) O(3)-Cd(1)-O(1) O(1)-Cd(1)-O(1)i O(3)i-Cd(1)-N(1) N(2)ii-Cd(1)-N(1) O(1)-Cd(1)-N(1) O(3)i-Cd(1)-O(2)iii N(2)ii-Cd(1)-O(2)iii O(1)-Cd(1)-O(2)iii

2.400(3) 2.414(2) 2.444(2) 2.470(3) 1.243(3) 1.239(4) 1.244(3) 96.87(5) 146.03(9) 75.39(5) 75.24(6) 70.54(6) 75.24(6) 143.25(6) 72.75(8) 76.56(5) 67.53(9) 133.34(6) 95.04(5) 77.24(9) 71.47(6)

Cd(1)-O(3)i Cd(1)-N(2)ii Cd(1)-O(1) Cd(1)-O(2)iii C(1)-C(2) C(1)-C(3) C(3)-C(3)iv O(4)-Cd(1)-O(3) O(4)- Cd(1)-N(2)ii O(3)-Cd(1)-N(2)ii O(3)i-Cd(1)-O(1)i N(2)ii-Cd(1)-O(1)i O(3)i-Cd(1)-O(1) N(2)ii-Cd(1)-O(1) O(4)-Cd(1)-N(1) O(3)-Cd(1)-N(1) O(1)i-Cd(1)-N(1) O(4)-Cd(1)-O(2)iii O(3)-Cd(1)-O(2)iii O(1)i-Cd(1)-O(2)iii N(1)-Cd(1)-O(2)iii

2.414(2) 2.420(3) 2.444(2) 2.646(3) 1.468(3) 1.454(3) 1.457(5) 96.87(5) 144.46(9) 75.39(5) 143.25(6) 130.64(6) 70.54(6) 130.64(6) 76.93(9) 76.56(5) 133.34(6) 138.30(8) 95.04(5) 71.47(6) 144.77(9)

a Symmetry operations used to generate equivalent atoms: i ) x, -y + 1, z; ii ) -x, -y + 1, -z + 1; iii ) -x + 1/2, -y + 1/2, -z + 2; iv ) x, -y + 2, z.

Figure 7. (a) TGA of compound 2. (b) Various temperature powder diffraction patterns of [Cd(bpym)0.5(C5O5)‚H2O] (2) and its simulation from single-crystal data.

Figure 6. (a) The 1D bichain framework through the croconate bridges with bis-bidentate/monodentate mode. (b) 2D layered architecture formed by the Cd(II) ions and croconate and bpym ligands. (c) Crystal packing along the b-axis showing the formation of a 3D supramolecular architecture by intermolecular π-π interactions.

distances between two centroids of the interaction rings are 3.745 and 3.99 Å for pyrimidyl ring and croconate, respectively. As to a new type of monodentate/bis-bidentate (µ5) binding mode of croconate found in 2, it is of fundamental importance to examine the correlation between molecular symmetry and π-electron delocalization around the five-membered ring. The dimensions of the C5O52- species resolved in 2 conform a nearly π-electron delocalized structure. The deviations of five oxygen

atoms and cyclic five-membered ring in C5O52- are smaller than 0.05 Å, indicating a planar molecular geometry. Notably, the differences among five C-O and C-C distances of such monodentate/bis-bidentate (µ5) bridging modes in 2 are not so obvious in the range of 1.239(4)-1.243(3) and 1.454(3)-1.468(3) Å (Table 3), respectively, establishing a π-electron delocalized molecular symmetry. In sharp contrast to the intermediate pattern between charge-localized (C2V) and chargedelocalized (D5h) forms in multidentate modes,20-25 the results of 2 imply a π-electron delocalization in the five-membered ring when croconate acts as a bridging ligand with a monodentate/bis-bidentate (µ5) binding mode. It is also noteworthy that this case demonstrates for the first time that croconate, upon acting as a bridging ligand, can exist in a fully π-electron delocalized molecular symmetry, which is in accordance with the charge-delocalized form of croconate in a hydrogen-bonded

MOFs from the Cd(II) with bpym

host lattice,6 alkali metal croconate salts34 as well as theoretical predictions.35 The TGA studies revealed that compound 2 underwent a onestep weight loss and thermally stable up to 190 °C. The decomposition process was started at 190 °C and completed at 488 °C (Figure 7a). Unlike compound 1, the reversibility of desorbed and reabsorbed coordinated water did not happen in 2. The in situ powder diffraction patterns (Figure 7b) revealed that 2 maintained its crystal form from room temperature to 190 °C. A phase transition occurred at 190-275 °C. After 380 °C, compound 2 decomposed, and the decomposition process was completed at 488 °C, forming a CdO crystalline. This high thermal stability is mainly attributed to mutual interlocking among the 2D MOFs by the interlayer π-π interaction between the pyrimidyl rings of bpyms and between cyclic five-membered rings of croconate ligands. Conclusions In conclusion, a series of Cd(II) coordination polymers with monocyclic oxocarbon dianions CnOn2- and bpym ligands have been completely studied, including the present work of [Cd(C4O4)(bpym)0.5(H2O)] (1) and [Cd(C5O5)(bpym)0.5(H2O)] (2) and the previous work of [Cd(C6O6)(bpym)0.5(H2O)]‚(H2O).26 All of the MOFs are unique and quite interesting. In compound 1, an infinite 2D cadmium-squarate layer is formed via the connectivity between the dimeric Cd(II) connector and a squarate linker adopting tetramonodentate and bis-bridging coordination modes. Adjacent layers are connected by the bridges of bpym ligands and extended to a triangular 3D MOF. Compound 2 is a 2D layered framework formed via the bridges of the Cd ion with croconate adopting a new bis-bidentate/ monodentate coordination mode and bpym ligands. Adjacent independent 2D layers are orderly arranged by interlayer faceto-face π-π interaction of bpym’s pyrinidyl rings and croconate and then extended to a 3D supramolecular architecture. The TGA and in situ powder X-ray diffraction (PXRD) measurements are in accordance with the crystallographic observations, and both results provide evidence that 1 and 2 have quite high thermostabilities that keep their crystalline forms up to 140 and 190 °C, respectively. These structural thermostablility properties may be potential useful in the application of catalytic chemical reactions and gas molecule storage such as hydrogen containers. In comparing the molecular symmetry for three cyclic oxocarbon dianions, squarate (C4O42-) and croconate (C5O52-) exist in a fully π-electron delocalized D4h and D5h molecular symmetry, but rodizonate (C6O62-) exists in a nonplanar C2V molecular symmetry.26 Acknowledgment. The authors wish to thank the National Science Council (NSC), Taiwan, ROC, for financial support. Supporting Information Available: Crystallographic information files. This material is available free of charge via the Internet at http:// pubs.acs.org.

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