Zinc 4-Carboxyphenylphosphonates with Pillared Layered Framework

Jul 4, 2008 - Synopsis. Pillared layered compounds [Zn(H2O)6][Zn8(OOCC6H4PO3)6(4,4′-bipy)] (1) and (dabcoH)2[Zn8(OOCC6H4PO3)6]·6H2O (2) (dabco = 1,...
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Zinc 4-Carboxyphenylphosphonates with Pillared Layered Framework Structures Containing Large 12-Membered Rings Built Up from Tetranuclear Zn4 Clusters and CPO3 Linkages Jin-Tang Li, Deng-Ke Cao, Bin Liu, Yi-Zhi Li, and Li-Min Zheng*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2950–2953

State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed January 17, 2008; ReVised Manuscript ReceiVed May 15, 2008

ABSTRACT: Twoisostructuralzincphosphonates[Zn(H2O)6][Zn8(OOCC6H4PO3)6(4,4′-bipy)](1)and(dabcoH)2[Zn8(OOCC6H4PO3)6] · 6H2O (2) (dabco ) 1,4-diazabicyclo[2.2.2]octane) have been obtained by hydrothermal reactions of zinc sulfate and 4-carboxyphenylphosphonic acid in the presence of additional organic ligands 4,4′-bipyridine and 1,4-diazabicyclo[2.2.2]octane, respectively. Both show pillared layered open framework structures. The inorganic layers are of particular interest in which the highly symmetric tetranuclear clusters, composed of corner-sharing {ZnO4} and {ZnO3N} tetrahedra, are linked by {CPO3} tetrahedra through cornersharing, forming an inorganic layer containing large 12-membered windows. The [Zn(H2O)6]2+ cations (for 1) or lattice water (for 2) reside in the cavities generated by the 12-membered windows and the organic pillars. Introduction The assembly of metal phosphonates with open-framework or porous structures has been of growing interest owing to their potential applications in catalysis, molecular separation, proton conductivity and others.1 There are at least three approaches that have been developed to construct such materials. One is to increase the interlayer distances in the pillared layered metal phosphonates by using the organic linkers with large lengths. Compounds such as Zr(O3PC6H4C6H4PO3) and Zn(O3PC6H4C6H4PO3)(H2O)2 are obtained for this purpose but their pore sizes tend to be fairly small due to the close packing of the pillars.2,3 Spacer ligands such as methylphosphonic or phosphate acid are thus introduced in order to overcome this close pillar disposition. This approach proves to be successful for compound Zr(HPO3)1.33(O3P-R-PO3)0.33 (R ) 3,3,5,5-tetramethylbipheny1) which shows a regular pillar separation.4 More recently, it is reported that porous networks may be achieved by the direct reaction of the bisphosphonic acid with zirconium(IV) or tin(IV) salts in excess of the stoichiometric ratio and without spacers.5 The second approach is to expand the pore sizes through the introduction of templates.6 Numerous inorganic porous materials have been synthesized by this approach, and the incorporated organic templates can be removed easily by heat treatment or other methods.7 Although many metal phosphonates with openframework structures have been obtained via template-assisted technique, there are only a few exceptions that the organic templates could be removed with the framework structure maintained.8 In the third approach, multidentate building blocks are employed and thus modular structures with controlled pore size distribution are expected.9 On the basis of N,N′-piperazinebismethylenephosphonate ligand, for instance, Serre et al. prepared the corresponding aluminum and titanium compounds with Langmuir surface areas close to 500 m2/g and pore volumes close to 0.19 cm3/g.10 By using a tetra-phosphonate ligand, tetrakis-1,3,5,7-(4-phosphonatophenyl)adamantane, as the building block, porous titanium and copper phosphonates have been obtained with surface areas being ∼500 m2/g and 182 m2/g, * To whom correspondence should be addressed. E-mail: lmzheng@ netra.nju.edu.cn. Fax: +86-25-83314502.

respectively.11 For phosphonate ligands embedded with carboxylate groups, compounds Zn(O3PCH2COOH) · H2O,12 Co2(H2O)2[O3PCH2(C6H4)CH2PO3],13 Na[Zn(O3PC2H4COO)] · H2O14 and [Zn7L6][Zn(H2O)6]2 · 16H2O15 with porous structures have also been reported. In this paper, we use a rigid 4-carboxyphenylphosphonic acid (4-cppH3) to react with zinc sulfate in the presence of organic bases such as 4,4′-bipyridine and 1,4-diazabicyclo[2.2.2]octane (dabco). It is found that both 4,4′-bipy and dabco are involved in the coordination with Zn atoms. Subsequently, compounds [Zn(H2O)6][Zn8(OOCC6H4PO3)6(4,4′-bipy)](1)and(dabcoH)2[Zn8(OOCC6H4PO3)6] · 6H2O (2) (dabco ) 1,4-diazabicyclo[2.2.2]octane) are obtained. Both show pillared layered structures where the inorganic layers contain large 12-membered windows. Experimental Section Materials and Methods. All starting materials were of reagent quality and were obtained from commercial sources without further purification. The 4-carboxyphenylphosphonic acid (4-cppH3) was synthesized by an Arbuzov-type reaction using methyl 4-bromobenzoate as the precursor and anhydrous nickel chloride as catalyst.16 Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. The IR spectra were obtained as KBr disks on a VECTOR 22 spectrometer. Thermal analyses were performed in nitrogen with a heating rate of 20 °C/min on a TGA V5.1A Dupont 2100 instrument. XRD were performed on an XRD-6000 X-ray diffractometer. N2 adsorption measurements were carried out in a TriStar 3000 V6.03 A Surface Area and Porosity Analyzer of Micrometritics Instrument Corporation. Synthesis of [Zn(H2O)6][Zn8(OOCC6H4PO3)6(4,4′-bipy)] (1). A mixture of ZnSO4 · 7H2O (0.3 mmol, 0.0862 g), 4,4′-bipy (0.2 mmol, 0.0312 g) and 4-cppH3 (0.2 mmol, 0.0404 g) in 8 mL of H2O (pH ) 3.04), stirred for 10 min at room temperature, was transferred to a Teflon-lined autoclave and heated at 160 °C for 2 days. After the autoclave was slowly cooled to room temperature, colorless blocky crystals of 1 were obtained as a monophasic product, as judged by the XRD measurements. Yield: 0.0600 g (88% based on Zn). Anal. found (calcd) for C52H44N2O36P6Zn9: C, 30.43 (30.51); H, 2.12(2.17); N, 1.25 (1.37)%. IR (KBr, cm-1): 3444(s), 1607(vs), 1554(s), 1499(w), 1398(s), 1385(s), 1303(w), 1282(w), 1225(m), 1167(s), 1149(s), 1126(vs), 1081(vs), 1021(w), 953(vs), 854(w), 813(w), 775(m), 741(m), 703(w), 626(s), 601(m), 567(m), 477(m). Synthesis of (dabcoH)2[Zn8(OOCC6H4PO3)6] · 6H2O (2). A mixture of ZnSO4 · 7H2O (0.3 mmol, 0.0862 g), 1,4-diazabicyclo[2.2.2]octane (dabco) (0.2 mmol, 0.0224 g) and 4-cppH3 (0.2 mmol, 0.0404 g) in 8

10.1021/cg8000653 CCC: $40.75  2008 American Chemical Society Published on Web 07/04/2008

Zinc 4-Carboxyphenylphosphonates

Crystal Growth & Design, Vol. 8, No. 8, 2008 2951

Table 1. Crystallographic Data and Refinement Parameters for 1-2 compound

1

2

empirical formula fw crystal system space group a (Å) b (Å) c (Å) V (Å3) R, β, γ (°) Z Fcalcd (g · cm-3) F (000) goodness-of-fit on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (All data)a (∆F)max, (∆F)min (e Å-3)

C52H44N2O36P6Zn9 2047.04 rhombohedral R3j 13.4702(10) 13.4702(10) 31.214(2) 4904.9(6) 90, 90, 120 3 2.079 3054 1.029 0.0431, 0.0969 0.0592, 0.1008 -0.796, 0.971

C54H62N4O36P6Zn8 2051.86 rhombohedral R3j 13.3374(10) 13.3374(10) 31.652(5) 4876.1(9) 90, 90, 120 3 2.096 3096 1.062 0.0572, 0.1581 0.0753, 0.1660 -0.888, 0.605

a

Figure 1. Building unit of [Zn8(OOCC6H4PO3)6(4,4′-bipy)]2- in 1 with atomic labeling scheme (50% probability). All H atoms are omitted for clarity.

R1 ) Σ|Fo| - |Fc|/Σ|Fo|. wR2 ) [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2. Table 2. Selected Bond Lengths [Å] and Angles [o] for 1a

Zn1-O1 Zn1-O2B Zn2-O1 Zn2-N1 P1-O1 P1-O3 C7-O5 O3D-Zn1-O4F O3D-Zn1-O2B O4F-Zn1-O2B O3D-Zn1-O1 O4F-Zn1-O1 O2B-Zn1-O1 P1-O1-Zn1 P1-O2-Zn1A C7-O4-Zn1E P1-O3-Zn1D O1B-Zn2-O1 O1A-Zn2-O1

2.055(3) 1.953(3) 1.964(3) 1.999(6) 1.549(3) 1.500(3) 1.239(5) 130.57(13) 104.01(12) 115.94(13) 102.62(11) 100.40(12) 96.75(12) 119.79(18) 134.6(2) 110.3(3) 142.19(19) 107.69(8) 107.69(8)

Zn1-O3D Zn1-O4F Zn3-O2W Zn3-O1W P1-O2 C7-O4 O1B-Zn2-N1 O1-Zn2-N1 O1B-Zn2-O1A Zn2-O1-Zn1 C8-N1-Zn2 C8A-N1-Zn2 C8B-N1-Zn2 C10A-N1-Zn2 C10-N1-Zn2 P1-O1-Zn2 C10B-N1-Zn2

1.898(3) 1.910(3) 2.033(5) 2.039(5) 1.502(3) 1.274(5) 111.20(8) 111.20(8) 107.69(8) 114.87(13) 122.9(6) 122.9(6) 122.9(6) 122.7(7) 122.7(7) 123.24(17) 122.7(7)

a Symmetry codes: A: -y + 1, x - y, z; B: -x + y + 1, -x + 1, z; C: -x + 4/3, -y + 2/3, -z - 1/3; D: -x + 1, -y + 1, -z; E: y - 1/ 3, -x + y + 1/3, -z + 1/3; F: x - y + 2/3, x + 1/3, -z + 1/3.

Table 3. Selected Bond Lengths [Å] and Angles [°] for 2a Zn1-O1 Zn1-O2A Zn2-O1 P1-O1 P1-O3 C7-O5 O1-Zn1-O2A O1-Zn1-O3C O1-Zn1-O4G O2A-Zn1-O3C O2A-Zn1-O4G O3C-Zn1-O4G P1-O2-Zn1B P1-O3-Zn1C

2.013(6) 1.921(7) 1.965(5) 1.535(5) 1.490(5) 1.236(8) 102.8 (2) 103.4(2) 99.9(2) 102.1(2) 113.0 (2) 131.9(2) 134.6(3) 142.3(3)

Zn1-O3C Zn1-O4G Zn2-N1 P1-O2 C7-O4 O1-Zn2-N1 O1-Zn2-O1A Zn1-O1-Zn2 C7-O4-Zn1D P1-O1-Zn1 P1-O1-Zn2 C8-N1-Zn2

1.914(4) 1.925(5) 1.951(10) 1.514(6) 1.269(8) 107.3(1) 111.6(2) 111.9(2) 113.2(4) 122.2(3) 124.7(3) 110.6(4)

a Symmetry codes: A: -y + 1, x - y, z; B: -x + y + 1, -x + 1, z; C: -x + 1, -y + 1, -z; D: x - y + 2/3, x + 1/3, -z + 1/3; E: y + 1/ 3, -x + y + 2/3, -z + 2/3; F: x - y + 1/3, x - 1/3, -z + 2/3; G: y 1/3, -x + y + 1/3, -z + 1/3.

mL of H2O (pH ) 3.14), stirred for 10 min at room temperature, was transferred to a Teflon-lined autoclave and heated at 160 °C for 3 days. After the autoclave is slowly cooled to room temperature, colorless blocky crystals of 2 were obtained as a monophasic product, judged by the XRD measurements. Yield: 0.0608 g (79% based on Zn). Anal. found (calcd) for C54H61N4O36P6Zn8: C, 31.58 (31.62); H, 2.95 (3.00); N, 2.65 (2.73)%; IR (KBr, cm-1): 3440(s), 1618(s), 1602(s), 1551(m), 1495(w), 1468(w), 1439(w), 1393(s), 1362(m), 1300(w), 1186(m),

Figure 2. One inorganic layer of structure 1 viewed along the c-axis. The [Zn(H2O)6]2+ ions are located within the 12-membered rings. 1146(m), 1121(s), 1099(vs), 1031(w), 1006(m), 943(m), 867(w), 848(w), 777(m), 734(m), 705(m), 619(m), 586(m), 541(m), 487(m), 466(m). Crystallographic Studies. Single crystals of dimensions 0.28 × 0.26 × 0.24 mm3 for 1 and 0.32 × 0.26 × 0.24 mm3 for 2 were used for structural determinations on a Bruker SMART APEX CCD diffractometer using graphite-monochromatized Mo KR radiation (λ ) 0.71073 Å) at room temperature. A hemisphere of data was collected in the θ range 1.86-26.00° for 1 and 1.88-26.00°for 2 using a narrow-frame method with scan widths of 0.30° in ω and an exposure time of 5 s/frame. Numbers of observed and unique [I > 2σ (I)] reflections are 8813 and 2144 (Rint ) 0.0363) for 1 and 8858 and 2142 (Rint ) 0.0420) for 2, respectively. The data were integrated using the Siemens SAINT program17 with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Empirical absorption corrections were applied. The structures were solved by direct methods and were refined on F2 by full matrix least-squares using SHELXTL.18 All the non-hydrogen atoms were located from the Fourier maps, and were refined anisotropically. For compound 1, the C(8), C(9), C(10) and C(11) atoms of 4,4′-bpy are disordered over three sites, each with occupancy 1/3. The coordinated water molecules O(1w) and O(2w) are also disordered, with occupancies of 2/3 and 1/3 respectively. All H atoms were refined isotropically, with the isotropic vibration parameters related to the non-H atom to which they are bonded. Crystallographic and refinement details of 1-2 are listed in Table 1. Selected bond lengths and angles are given in Tables 2 and 3. CCDC 668500-668501 contain the supplementary crystallographic data for this paper.

Results and Discussion Description of the Structures. Compound 1 crystallizes in the rhombohedral space group R3j. The asymmetric unit consists of 3/2 Zn atoms, one 4-cpp3-, 1/6 4,4′-bipy and one water molecule (Figure 1). There are three crystallographically distinguishable Zn atoms. The Zn(1) atom locates at a general position, and has a distorted tetrahedral environment. Three sites are occupied by phosphonate oxygen atoms [O(1), O(2B),

2952 Crystal Growth & Design, Vol. 8, No. 8, 2008

Li et al.

Figure 3. Crystal packing of structure 1.

Figure 4. Building unit of structure 2 with atomic labeling scheme (50% probability). All H atoms except for H2B attached to N2 atom are omitted for clarity.

Figure 6. Thermal analyses of compounds 1 and 2.

Figure 5. One inorganic layer of structure 2 viewed along the c-axis. The lattice water molecules within the ring are shown as black solid circles.

O(3D)] from three equivalent phosphonate ligands. The carboxylate oxygen atom [O(4F)] from the fourth phosphonate ligand fills in the remaining site. The Zn(2) atom resides on the C3 axis, and also shows a distorted tetrahedral geometry. The four positions are filled with three phosphonate oxygen atoms [O(1), O(1A), O(1B)] from three equivalent phosphonate ligands and one pyridine nitrogen [N(1)] from 4,4′-bipy. The Zn(3) atom locates on the C3 axis, and is surrounded by six water molecules. The Zn-O bond lengths fall in the range of 1.898(3)-2.055(3) Å. The Zn(2)-N bond length is 1.999(6) Å. These are in agreement with other zinc carboxy phosphonates such as Zn3[O3P(CH2)2CO2]2 [Zn-O: 1.896(4)-2.051(3) Å].10 The 4-cpp3- serves as a penta-dentate ligand by using its three phosphonate and one carboxylate oxygens. The phospho-

nate oxygen O(1) acts as a µ3-O bridge and links Zn(1) and Zn(2) atoms. The remaining two phosphonate oxygens [O(2), (O3)] are each coordinated to a single Zn(1) atom. Hence each {CPO3} tetrahedron is corner-shared with three {Zn(1)O4} and one {Zn(2)O3N} tetrahedra. Each {Zn(2)O3N} is corner-shared with three {Zn(1)O4} tetrahedra via phosphonate oxygens, forming a tetrameric cluster of Zn4. Subsequently, the Zn4 clusters are connected by phosphonate groups into a twodimensional inorganic layer which contains large 12-membered Zn6P6O12 windows besides 3-membered Zn2PO3 and 4-membered Zn2P2O4 rings (Figure 2). The longest Zn · · · Zn distance within the 12-membered window is 10.695 Å. The layers are cross-linked by both 4-cpp3- and 4,4′-bipy ligands through the coordination of the carboxylate oxygen O(4) with Zn(1) and pyridyl nitrogen N(1) with Zn(2) atoms. Therefore, a pillared layered structure with composition {Zn8(OOCC6H4PO3)6(4,4′bipy)}n2n- is constructed (Figure 3). The anionic charge of the framework is compensated by the discrete [Zn(H2O)6]2+ cations which reside in the cavities generated by the 12-membered windows within the inorganic layer and the organic pillars (Figures 2 and 3). Weak hydrogen bond interactions are found between the coordinated water and carboxylate oxygen O(5) atoms [O(1w) · · · O(2i): 3.031(5) Å, O(2w) · · · O(5): 3.031(3) Å, symmetry code: i, 2/3 - x, 1/3 - y, 1/3 - z]. Compound 2 is isostructural to 1. In this case, it is dabco that is coordinated to the Zn(2) atom by using one of its nitrogen atoms [N(1)] (Figure 4). The other nitrogen atom [N(2)] is protonated and hydrogen-bonded to the equivalent dabcoH+ with N(2) · · · N(2ii) distance 2.656(13) Å (symmetry code: ii, 4/3 -

Zinc 4-Carboxyphenylphosphonates

x, 2/3 - y, -1/3 - z). Therefore, a dimer of (dabcoH)2 is formed which behaves as a pillar to link the neighboring inorganic layers, similar to the 4,4′-bipy in compound 1. However, since the overall charge of the {Zn8(OOCC6H4PO3)6(dabcoH)2}n framework has already been balanced by the protonated dabcoH+, only lattice water molecules fill in the spaces generated by the 12-membered windows and the organic linkages (Figure 5). As a result, the cell volume of compound 2 is slightly reduced compared with that of compound 1 (Table 1). Thermal Analyses. The TGA trace of 1 shows a mass loss of 5.6% between 190 and 380 °C, attributed to the loss of the coordinated water molecules (calcd. 5.3%) within the 12membered window (Figure 6). The mass loss between 380 and 735 °C (35.4%) is due to the decomposition of the organic groups in the compound. For 2, the mass loss of 5.6% is observed in the range 50 - 100 °C, corresponding to the release of the lattice water molecules (calcd. 5.3%). Between 100 and 400 °C, there appears a plateau. XRD measurements reveal that the pillared layered structure of 2 is maintained after the dehydration process (Figure S2, Supporting Information). The total mass loss is 35.8% upon heating to 735 °C in air (Figure 6). The N2 absorption experiments are conducted on the dehydrated sample, after treating 2 at 110 °C for 6 h under the protection of superpure N2. Unfortunately, the BET surface area obtained is very small (6.1318 m2/g for 2). Conclusion In this paper we report the hydrothermal syntheses of two novel zinc 4-carboxyphenylphosphonates [Zn(H2O)6][Zn8(OOCC6H4PO3)6(4,4′-bipy)] (1), (dabcoH)2[Zn8(OOCC6H4PO3)6] · 6H2O (2) in the presence of different second organic ligands. Compounds 1 and 2 are isostructural where both the phenyl groups of the phosphonate ligand and the second organic ligand [4,4′-bipy or 1,4-diazabicyclo[2.2.2]octane (dabco)] serve as pillars to link the inorganic layers. The highly symmetric tetranuclear clusters, composed of corner-sharing {ZnO4} tetrahedra, are linked by {CPO3} tetrahedra through cornersharing, forming an inorganic layer containing large 12membered windows. The Zn(H2O)62+ cations or lattice water reside within these windows. Acknowledgment. This work is supported by the NSFC (Nos. 20325103, 20631030, 20721002), the National Basic Research

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Program of China (2007CB925102) and 111 Project under Grant No. B07026. Supporting Information Available: Crystallographic files in CIF format; two figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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