PAPER www.rsc.org/dalton | Dalton Transactions

Systematic exploration of a rutile-type zinc(II)–phosphonocarboxylate openframework: the factors that influence the structure†

Yun Ling,a,b Teng-Biao Liao,a Zhen-Xia Chen,a,b Ya-Ming Zhou*a and Lin-Hong Wenga

Received 22nd May 2010, Accepted 24th August 2010DOI: 10.1039/c0dt00525h

To systematically explore the assembly mechanism of a rutile-type open framework of {[Zn3(pbdc)2]·2H3O}n (3) (H4pbdc = 5-phosphonobenzene-1,3-dicarboxylic acid) constructed by 3-connected pbdcligands and 6-connected Zn3(CO2)4(PO3)2 secondary building units (Zn3-SBUs), three major factorsincluding solvothermal procedures, types of solvents and amines, are taken into consideration. Sevennovel structures, namely {[Zn5(pbdc)2(OH)2(H2O)4]·4H2O}n (1), {[Zn3(pbdc)2·H2O]·(Htea)·H3O·2–5(H2O)}n (2), {[Zn3(pbdc)2](H3O)2(dma)}n (4), {[Zn2(pbdc)(taea)]·3H2O}n (5), {[Zn3(pbdc)2(Hpda)2]·2H2O}n (6), {[Zn5(pbdc)2(Hpbdc)2]·2H2pz·9H2O}n (7), {[Zn3(pbdc)2]·Hpd·H3O·4H2O}n (8) are obtained.The results indicate that the layered-solvothermal method and the isopropanol solvent play crucialroles in the construction of the special anionic open framework of [Zn3(pbdc)2]2-. Changing these twofactors led molecular assembly away from the rutile-type open framework. However, amines play avariable role in the framework, which means that by using appropriate amines, molecular assemblycould generate the open framework of [Zn3(pbdc)2]2- with pores decorated by amines. These resultssuggest a different approach towards decorating pores in anionic frameworks with precise structuralinformation.

Introduction

Metal–organic frameworks (MOFs), as a new class of porousmaterials comprising organic ligands as links and metal cen-ters/clusters as nodes through coordination bonds, have receivedconsiderable interest due to their fascinating structural featuresas well as potential applications in gas storage/separation,1,2

sensors,3,4 magnetism,5 catalysis6,7 and so on.8–10 The rationaldesign and synthesis of MOFs are of great importance toachieve the desired crystalline porous materials, and rigid ligandshave been widely used and investigated for their predictablebridging and coordination fashions.9,11 Among the various MOFsconstructed using rigid ligands, systematic synthesis and struc-tural exploration of carboxylate ligands have been facilitatingthe development of rational design and synthesis of metal–carboxylate materials,12–14 especially following the propositionof reticular synthesis based on building-block methodology.15–18

However, frameworks based on phosphonates are less studiedthan those based on carboxylates.19–21 This may be related totheir properties,20,22 namely (i) they tend to generate chain or layerstructures due to their multidentate coordination features (as wellas their less predictable coordination fashions), and (ii) it is difficultto obtain the crystalline materials suitable for single-crystal

aShanghai Key Laboratory of Molecular Catalysis and Innovative Materials,Department of Chemistry, Fudan University, Shanghai, 200433, China.E-mail: ymzhou@fudan.edu.cn; Fax: +86 21 65643925; Tel: +86 2165642261bLaboratory of Advanced Materials, Fudan University, Shanghai, 200433,China† Electronic supplementary information (ESI) available: Detailed struc-tural information, TG analysis data and PXRD data. CCDC ref-erence numbers 751707, 776920, and 776921–776925. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c0dt00525h

diffraction. Therefore, overcoming these problems will acceleratethe development of metal phosphonate frameworks. Recently, alarge number of phosphonates modified by organic groups suchas –pyridyl,23–25 –NH2,26,27 and –COOH,28–33 have been used asligands. These hybrid multi-functional ligands not only make iteasier to isolate crystalline structures containing phosphonategroups, but also enable construction of new frameworks beyondtwo-dimensional (2D) layer structures, as well as introducing novelfunctions to the desired three-dimensional (3D) MOFs.

Motivated by these factors, our interest is to construct3D porous MOFs based on phosphonocarboxylates andexplore the relationship between synthetic conditions and finalstructures.34–36 Recently, we reported an anionic open framework of{[Zn3(pbdc)2]·2H3O}n (3) with a rutile-type topology comprisingtritopic 5-phosphonobenzene-1,3-dicarboxylic acid (H4pbdc, withsimilar 3-connected node to BTC, Scheme 1), which shows athermodynamic ‘breathing’ property and good shape-selectivecatalysis ability for Friedel–Crafts-type reactions.37 From astructural viewpoint, the 6-connected Zn3(CO2)4(PO3)2 secondarybuilding unit (Zn3-SBU) in 3 is very rare in metal–phosphono-carboxylate frameworks due to the multidentate coordinationfeature of phosphonate group.20 In this paper, we

Scheme 1 The tritopic connection model of pbdc, which is similar to thatof BTC.

10712 | Dalton Trans., 2010, 39, 10712–10718 This journal is © The Royal Society of Chemistry 2010

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systematically explore the assembly mechanism of{[Zn3(pbdc)2]·2H3O}n (3), and isolate seven new Zn(II)–phosphonocarboxylate frameworks, namely {[Zn5(pbdc)2(OH)2-(H2O)4]·4H2O}n (1), {[Zn3(pbdc)2·H2O]·(Htea)·H3O·2–5(H2O)}n

(2), {[Zn3(pbdc)2](H3O)2(dma)}n (4), {[Zn2(pbdc)(taea)]·3H2O}n

(5), {[Zn3(pbdc)2(Hpda)2]·2H2O}n (6), {[Zn5(pbdc)2 (Hpbdc)2]·2H2pz·9H2O}n (7) and {[Zn3(pbdc)2]·Hpd·H3O·4H2O}n (8). Theresults indicate that the synthetic method and the isopropanolsolvent play crucial roles for the open framework, as changeof these two factors generated a different framework. However,amines play a variable role to the framework, which means usingappropriate amines, the open framework of [Zn3(pbdc)2]2- couldbe reproduced with pores decorated by amines.

Experimental

Materials and physical measurements

All reagents were purchased from commercial sources and usedas received, except for the H4pbdc, which was synthesized by themethod described previously.37 Different types of amines in theexperiment are given in Scheme 2. IR spectra were measured ona Nicolet 470 FT-IR spectrometer in the range 4000–400 cm-1

with KBr pellets. Elemental analyses were carried out on theElementar Vario EL III microanalyzer after the samples weredried in vacuum. Powder X-ray diffraction (PXRD) patterns weremeasured using a Bruker D8 powder diffractometer with CuKa radiation (l = 1.5406 A). Thermogravimetric analysis (TGA)experiments were carried out on the TGA/SDTA 851 analyzerin the temperature range 30–800 ◦C under nitrogen flow with aheating rate of 10 ◦C min-1.

Scheme 2 Different types of amines used in the experiment.

Preparation of Zn(II)–phosphonocarboxylates 1–8

{[Zn5(pbdc)2(OH)2(H2O)4]·4H2O}n (1). A solution of H4pbdc(0.025 g, 0.1 mmol) in 5 mL n-butanol was added to a solutionof Zn(CH3COO)2·2H2O (0.032 g, 0.15 mmol) in 5 mL water, andthen triethylamine (0.04 g, 0.4 mmol) was added to the mixtureand stirred at room temperature for another 10 min. The mixturewas then moved into a 15 mL teflon-lined stainless steel autoclaveand heated at 140 ◦C for 3 days, followed by cooling to roomtemperature. Colorless lamellar crystals of 1 were collected byfiltration (yield: 48% based on H4pbdc). Elemental analysis calcd.

for 1 C16H24O24P2Zn5 (989.14): C, 19.43; H, 2.45%. Found: C,19.39; H, 2.47%. IR (cm-1): 3444m, 3068w, 1612s, 1564m, 1433m,1364m, 1211m, 1114m, 1010m, 989m, 775m, 731m, 571m.

{[Zn3(pbdc)2·H2O]·(Htea)·H3O·2–5(H2O)}n (2). The proce-dure was similar to that of 1 except replacing n-butanol byisopropanol. Colorless blocks of 2 were collected by filtration(yield: 10% based on H4pbdc). The crystals were not stable at roomtemperature due to the loss of lattice waters after being taken awayfrom the mother liquid, while the framework was stable accordingto the PXRD data. The number of lattice waters are determinedbased on single-crystal X-ray diffraction and TGA data. Elementalanalysis calcd. for 2 C22H35P2O19NZn3 (875.7): C, 30.18; H, 4.03;N, 1.60%. Found: C, 30.46; H, 3.69, N, 1.61%. IR (cm-1): 3446m, 3117w, 3066w, 2983w, 2946w, 1614s, 1568m, 1436m, 1369m,1308w, 1211m, 1139m, 1109m, 1012m, 990m, 779m, 732w, 689w,574m, 455m.

{[Zn3(pbdc)2]·2H3O}n (3)37. A solution of H4pbdc (0.025 g,0.1 mmol) and triethylamine (0.041 g, 0.4 mmol) in iso-propanol (5 mL) was carefully layered onto a solution ofZn(CH3COO)2·2H2O (0.032 g, 0.15 mmol) in deionized water(5 mL) in a 15 mL teflon-lined stainless steel autoclave, andthen heated at 140 ◦C for 3 days, followed by cooling to roomtemperature. Colorless rod-shape crystals of 3 were collected byfiltration (yield: 17% based on H4pbdc). Elemental analysis calcdfor 3 C16H12P2O16Zn3 (718.3): C, 26.75; H, 1.68%. Found: C, 26.71;H, 1.73%. IR (cm-1): 3436 m, 3068w, 2984w, 1614s, 1567m, 1436m,1368m, 1310w, 1211m, 1110m, 1012m, 990m, 777m, 729m, 574m,457m.

{[Zn3(pbdc)2](H3O)2(dma)}n (4). The procedure for prepara-tion of 4 was similar to 3 except replacing tea by methanamine(ma) or dimethylamine (dma). Colorless blocks were isolated.

{[Zn2(pbdc)(taea)]·3H2O}n (5). The procedure for preparationof 5 was similar to that of 3 except replacing tea by tris(2-aminoethyl)amine (taea). Colorless blocks were isolated.

{[Zn3(pbdc)2(Hpda)2]·2H2O}n (6). The procedure for prepara-tion of 6 was similar to that of 3 except replacing tea by propane-1,3-diamine (pda). Colorless blocks were isolated.

{[Zn5(pbdc)4(H2pz)2]·3H2O}n (7). The procedure for prepara-tion of 7 was similar to that of 3 except replacing tea by piperazine(pz). Colorless blocks were isolated.

{[Zn3(pbdc)2]·Hpd·H3O·4H2O}n (8). The procedure forpreparation of 8 was similar to that of 3 except replacing teaby pyrrolidine (pd). Colorless rod-like crystals of 8 were collectedby filtration (Yield: 22% based on H4pbdc). Elemental analysiscalcd. for 8 C20H25NO19P2Zn3 (841.5): C, 28.55; H, 2.99; N, 1.66%.Found: C, 28.52; H, 3.01, N, 1.64%. IR (cm-1): 3434m, 3065w,2959w, 2862w, 1614s, 1568m, 1435m, 1380m, 1209w, 1114m,1011m, 989m, 777m, 729w, 687w, 573m, 455m.

Crystallographic studies

Suitable single crystals of 1–8 were mounted on glass capillariesand data collection were carried out on a Bruker Apex CCDdiffractometer with graphite-monochromated Mo Ka radiation(l = 0.71073 A) at 293 K. Data reduction was performedwith SAINT, and semi-empirical absorption corrections were

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applied by SADABS program.38 Structures were solved by di-rect methods using the SHELXS program and refined withthe SHELXL program.39 Heavy atoms and other non-hydrogenatoms are directly obtained from difference Fourier map. Finalrefinements were performed by full-matrix least-squares methodswith anisotropic thermal parameters for all non-hydrogen atomson F 2. Generally, C-bonded H atoms were placed geometricallyand refined in the riding modes, while some of hte O-bonded Hatoms of water were first located in difference Fourier maps andthen refined in the riding mode. Crystallographic data are listed inTable S1†.

Results and discussion

Synthesis

In order to make it more reasonable to extensively explore theeffects of solvents and amines to the assembly of the rutile-typeanionic open framework of {[Zn3(pbdc)2]·2H3O}n, the experimentscarried out here were restricted to the same crystallizationtemperature. Solvent effects were studied by replacing isopropanolby different alcohols, and the roles of amines were investigated byreplacing tea with other amines, all procedures being kept as for3. Crystals of 4 could also be obtained in a mixed solution ofwater and DMF, in which the dma molecule was formed by in situdecomposition of DMF.

Crystal structure descriptions of 1–8

{[Zn5(pbdc)2(OH)2(H2O)4]·4H2O}n (1). 1 was isolated aslamellar crystals. Structure 1 crystallizes in triclinic system, P1space group (see CCDC 776920). In the structure of 1, thereis a Zn5-SBU with an inversion center located on Zn(2). Thephosphonate groups of pbdc ligands bridge the Zn5-SBUs togenerate the 1D inorganic chain (Fig. S1†), and then the inorganicchains are further cross-linked by tritopic pbdc ligands to generatethe dense 3D structure of 1.

{[Zn3(pbdc)2·H2O]·(Htea)·H3O·2–5(H2O)}n (2). 2 is a 3Dstructure crystallizing in the orthorhombic system, Pbcaspace group (see CCDC 751707). The structure consists of[Zn3(CO2)4(PO3)2H2O] (Zn3(H2O)-SBU), totally deprotonatedpbdc ligand, Htea and disordered lattice water molecules. Asshown in Fig. 1a, each Zn(II) ion in Zn3(H2O)-SBU adoptsa severely distorted tetrahedral coordination geometry. ThreeZn(II) ions in the Zn3(H2O)-SBU construct a scalene triangle,Zn(1) ◊ ◊ ◊ Zn(2), Zn(1) ◊ ◊ ◊ Zn(3) and Zn(2) ◊ ◊ ◊ Zn(3), with distancesof 3.56, 4.64 and 3.98 A respectively. The pbdc ligands in 2exhibit two different tritopic coordination modes which couldbe considered as two kinds of 3-connected nodes (Fig. 1a and1b). The Zn3(H2O)-SBU unit could be described as a decorated6-connected node (four C atoms of four carboxylate groups andtwo P atoms of two phosphonate groups), so the 3D framework of2 could be described as a (3,6)-connected framework (Fig. 1c) witha previously unreported Schlafli topology of (4.62)(63)(4.611.83).

{[Zn3(pbdc)2]·2H3O}n (3). Compound 3 was reportedin our previous work.37 3 consists of trimeric zinc–phosphonocarboxylate clusters (Zn3(CO2)4(PO3)2, Zn3-SBU) andpbdc linkers (Fig. 2, see CCDC 746203). The three Zn(II) ionsin the Zn3-SBU form an isosceles triangle with Zn(1) ◊ ◊ ◊ Zn(1)i

Fig. 1 (a) The coordination geometry of Zn(II) ions in Zn3(H2O)-SBU(left) and the molecular motif of 2 (right); (b) the simplified Zn3(H2O)-SBUin the 6-connected node and the simplified pbdc ligand in the 3-connectednode; (c) the dense (3,6)-connected framework of 2 with Schlafli topologyof (4.62)(63)(4.611.83).

and Zn(1) ◊ ◊ ◊ Zn(2) distances of 4.61 A and 3.56 A, respectively.The oxygen atoms from the phosphonate of pbdc are tridentate,capturing the isosceles triangle, and the carboxylates of pbdc adopttwo different coordination modes. One bridges Zn(1) and Zn(2)ions with O(6) and O(7) atoms in a syn–syn mode, while the other ismonocoordinated to the Zn(1) ion through O(4). Considering Zn3-SBU as a 6-connected node (four C atoms from four carboxylatesand two P atoms from two phosphonates as the vertices) and thepbdc as 3-connected node, the infinite 3D framework of 3 thencould be described as a (3,6)-connected rutile-type network withSchlafli topology of (4.62)2(42.610.83) (Fig. 2).

{[Zn3(pbdc)2](H3O)2(dma)}n (4). Compound 4 crystallizes inthe monoclinic system, C2/c space group (see CCDC 776921).In the structure of 4, the ZnO4 tetrahedron is connected byphosphonate and carboxylate groups, generating 1D inorganicchain. Then this inorganic chain is cross-linked by tritopic pbdcligands to construct the 3D structure of 4 (Fig. S2†).

{[Zn2(pbdc)(taea)]·3H2O}n (5). Compound 5 is a 3D struc-ture crystallizing in the monoclinic system, P21/c spacegroup (see CCDC 776922). The structure consists of[Zn4(COO)4(PO3)2(taea)2] (Zn4-SBU), totally deprotonated pbdcligands, and lattice water molecules. As shown in Fig. 3, twoZn(II) ions in the tetrahedral coordination geometry and twophosphonate groups form an eight-membered ring with an

10714 | Dalton Trans., 2010, 39, 10712–10718 This journal is © The Royal Society of Chemistry 2010

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Fig. 2 (a) The simplified Zn3-SBU in 6-connected node and the simplifiedpbdc ligand in 3-connected node, (b) the network of 3 showing a(3,6)-connected rutile net along the c axis.

Fig. 3 (a) The simplified Zn4-SBU in 6-connected node and the simplifiedpbdc ligand in 3-connected node (the taea coordinates to the Zn(II) ion asa terminal group); (b) the dense (3,6)-connected rutile-type net of 5.

inversion center. The taea ligand in the Zn4-SBU acts as a terminalchelating group coordinating to the trigonal bipyramidal Zn(2)ion. Considering the Zn4-SBU as a decorated 6-connected node(four C atoms of four carboxylate groups and two P atoms of twophosphonate groups) and the totally deprotonated pbdc ligand as a3-connected node, the 3D frameworks of 5 could then be describedas a (3,6)-connected network with Schlafli toplogy (4.62)2(4.610.83).

The topology is the same as that of 3, belonging to the rutile net.However, there is no available porosity in 5 due to the presence ofterminal coordinated taea ligands.

{[Zn3(pbdc)2(Hpda)2]·2H2O}n (6). Compound 6 crystallizes inthe monoclinic system, C2/c space group (see CCDC 776923).In the structure, Hpda ligands are monocoordinated to Zn(1)and Zn(3) ions. The phosphonate groups bridge tetrahedrallycoordinated Zn(1) and Zn(2) ions, generating an inorganic zigzagchain, on which the Zn(3) ions (in tetrahedral coordinationgeometry) are attached. These chains are then cross-linkedby tritopic pbdc ligands to generate the 3D structure of 6(Fig. S3†).

{[Zn5(pbdc)2(Hpbdc)2]·2H2pz·9H2O}n (7). Compound 7 crys-tallizes in the monoclinic system, C2/c space group (see CCDC776924). The Zn(2) ion, in a distorted square-pyramidal coordi-nation geometry, and the Zn(3) ion, in a tetrahedral coordinationgeometry, are connected by one phosphonate group, formingZn4-SBU. This SBU is then connected to Zn(1) ion throughanother phosphonate group, generating a 1D chain. This chainis further connected by the pbdc and Hpbdc ligands, formingthe 3D dense structure of 7. The guest molecule of piperazineacts as a counter-anion and a template for the construction of 7(Fig. S4†).

{[Zn3(pbdc)2]·Hpd·H3O·4H2O}n (8). Single-crystal X-raydiffraction study of 8 reveals that it crystallizes in the tetragonalsystem, I 42d space group, with similar cell parameters to 3 (seeCCDC 776925). The framework of 8 consists of trimeric zinc–phosphonocarboxylate clusters (Zn3(CO2)4(PO3)2, Zn3-SBU) and

Fig. 4 (a) The simplified Zn3-SBU in 6-connected node and the simplifiedpbdc ligand in 3-connected node, (b) the network of 8 showing the(3,6)-connected rutile net along the c axis; the Hpd filling the small channelsalong the a and b axes.

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pbdc linkers (Fig. 4), and could be described as (3,6)-connectedrutile-type network with Schlafli topology (4.62)2(42.610.83), thesame as that of 3. The Hpd guests are located in small channels (ca.6 ¥ 4 A, along the a and b axis) and interact with the oxygen atomof the mono-coordinated carboxylate group through hydrogenbonds. The large channels (ca. 6 ¥ 6 A, along the c axis) of 8 arefilled with lattice water and hydrated protons. According to theTGA curve of 8 (Fig. S5†), the first weight loss step is of 10.7%,assigned to the lost of lattice water (ca. 10.9%). The second stepweight loss is not observed till the decomposition of the frameworkafter 300 ◦C, suggesting the Hpd guests are strongly fixed in thesmall channels.

Mixed-solvothermal and layered-solvothermal methods

In this paper, we used two synthetic methods, the mixed-solvothermal and layered-solvothermal method. Here, the mixed-solvothermal method means that the organic solvent containingH4pbdc ligands and the aqueous solution of Zn(II) salts are mixed,the pH adjusted by organic amines, and the mixture sealed andheated at 140 ◦C. The layered-solvothermal method means thatthe ligands and amines are first dissolved in organic solution, andthen the organic solution is layered on the aqueous solution ofZn(II) salts in a teflon-lined stainless steel autoclave and heated at140 ◦C. This process has also been called biphasic solvothermalsynthesis in the literature.40,41

With the mixed-solvothermal method, and using n-butanol–water as solvent, a dense framework of 1 was isolated (see the ESIfor the PXRD pattern†). However, with the layered-solvothermalmethod in n-butanol–water, a new compound was isolated, whichis completely different from structure 1 (Fig. 6c). Replacing n-butanol by isopropanol, a 3D node-type framework of 2 wasisolated, which aroused our interest, because (3,6)-connectedframeworks based on phosphonocarboxylate have, to the best ofour knowledge, rarely been reported.29–36 Further slightly changingthe procedure from mixed-solvothermal to layered-solvothermalmethod, a rutile-type open porous framework of 3 was isolated.On going from 2 to 3, the coordinated water molecule in the6-connected vertex was removed and replaced by a syn–synbridged carboxylate group (Fig. 5), therefore the scalene triangleof Zn3(H2O)-SBU was turned into the isosceles triangle of Zn3-SBU, finally generating a (3,6)-connected rutile topology. Thestructural differences indicate that the assembly mechanism of2 and 3 might be related to the synthetic method. In the caseof 3, H4pbdc was first mixed with four equivalents of tea inisopropanol solution, generating uniform organic precursors. Theorganic solvent was then mixed with water under solvothermalconditions, which makes the pbdc ligands coordinate to Zn(II) ionsto form unique precursors for the open framework of 3. However,all the raw materials were mixed together in the procedure of 2. So,the deprotonation and coordination processes occur at the sametime, resulting in a dense framework. Therefore, the crucial steps tocause structural assembly away from 2 to 3 may be related to (i) theformation of the organic precursors in isopropanol, and (ii) thelayered-solvothermal synthetic method, which probably reducesthe rate of molecular assembly, offering an opportunity for Zn(II)ions and pbdc ligands to adjust their coordination fashions. So,based on the above results, synthetic methods greatly influence theassembly of Zn–pbdc.

Fig. 5 The different (3,6)-connected frameworks of 2 and 3 obtainedthrough different synthetic method, showing slight differences in the6-connected SBU, Zn3(H2O)-SBU for 2 and Zn3-SBU for 3.

Fig. 6 The PXRD patterns of the products obtained in isopropanol–wa-ter (a), 1-propanol–water (b), n-butanol–water (c), 2-butanol–water (d)compared with simulated data of 3 and 1 (the simulated data of 3 in the 2qrange 5–7◦ was magnified to make the peak at 6.12◦ clear for comparison).

Role of organic solvents

To further explore the effects of organic solvents in the layered-solvothermal method, isopropanol was replaced by 1-propanol,n-butanol, and 2-butanol, and microcrystalline products wereobtained in each case. The PXRD pattens (Fig. 6c and Fig.6d) proved that products obtained from n-butanol–water and 2-butanol–water are the same compounds. However, the patternsare greatly different from the simulated data of 3, though the mainpeak around 2q = 9.7◦ is similar. These results suggest a new ormixed phase when the organic solvent is n-butanol or 2-butanol.As for the product isolated in 1-propanol, the PXRD patteren (Fig.6b) is slightly different from the simulated data of 3, suggesting animpure phase of the product. So, the isopropanol is very importantfor isolating pure phases of the rutile-type open framework.

Taking the above results into consideration, the layered-solvothermal synthetic method and isopropanol solvent play

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Table 1 General pKa values of aminesa

Pyrrole Pyridine pz tma ma dma tea pd

0.0 5.2 pK1 9.8, pK2 5.7 9.8 10.6 10.7 10.7 11.3

a The pKa data were directly obtained from the website http://www.cem.msu.edu/~reusch/VirtualText/amine1.htm .

crucial roles in the assembly of the rutile-type open framework of3, any change of the two factors leading molecular assembly awayfrom the special open framework. So, in the following investigationto reveal the role of tea, all procedures were kept the same as thatfor the synthesis of 3.

Role of amines

Organic amines are widely used in the synthesis of zeolites, metal–phosphate molecular sieves and MOFs.42–44 Generally, the roles ofamines include as deprotonation agents, structure-directing agents(SDAs), templates, and nitrogen-donor ligands. In the cases of 2and 3, tea possibly plays different roles. In 2, tea is included inthe structure, possibly serving as a template, but the role of teafor generating 3 is not very clear. In order to study the role of teafor structure 3, various amines, including (i) chain-like amines; (ii)dendritic amines and (iii) cyclo-amines (Scheme 2) were used.

Except tma, pyridine and 1H-pyrrole, detectable crystals wereisolated under the conditions of other amines. Using taea asorganic base, the same rutile-type network (5) was isolated.However, taea is coordinated to the SBU in 5, greatly differentfrom the role of tea in 3. The pda plays the same role as taeain the structure of 6, serving as a ligand. The role of dma andpz is possibly as a template for constructing 4 and 7, respectively.Using pd as amine, 8 was isolated, showing exactly the same rutile-type framework as that of 3. In structure 8, Hpd filled the smallchannels (calculated to be at least 6 ¥ 4 A, taking van der Waalsradii into consideration) along the a and b axes (Fig. 7). The sameframework of 3 and 8 suggests some common role of tea and pd.

We supposed that the pKa value might act as one criterion.Under the low pKa values offered by amines such as 1H-pyrrole,pyridine and tma (Table 1), no product was isolated. With thehigher pKa values offered by ma, dma, tea and pd, structures 4 (ma,dma), 3 (tea) and 8 (pd) were obtained. The pKa values of ma anddma are quite similar to that of tea, while the strong template effectof dma makes the assembly deviate away from the open frameworkto form structure 4. The pKa value of pd is only slightly higher(11.3) than that of tea, possibly offering similarly basic conditionsrequired by the open framework, therefore making it feasible togenerate structure 8 with pores decorated by Hpd. However, asfor pz, neither the pKa1 value nor the value of (pKa1 + pKa2)is close to the value of tea, so framework of 7 was isolated.In addition, based on careful structural analysis, the specialsymmetry and geometry feature of pd (Fig. 7c) possibly suggestsanother matching requirement between the host framework andguest amines.

Based on the above analysis, tea and pd possibly offer ap-propriately basic conditions in isopropanol solution, formingsuitable precursors for the construction of special anionic rutile-type framework. For this framework, guests should have the rightsymmetry and geometry features (besides the size requirement) in

Fig. 7 (a) The structure of 3, looking along the c and a axis respectively;(b) the structure of 8, looking along the c and a axis respectively; (c) thesymmetry and the conformation of Hpd, showing the 2-fold rotation axis(symmetry code i: x, 1.5 - y, 0.25 - z).

order to be able to fit in during the crystallization procedure, forexample pd (calculated size 5.8 ¥ 5.6 ¥ 4.0 A3).

Perspective. The cation-exchange procedure is known as ageneral method to decorate anionic host frameworks.45–48 However,the exact positions and conformations of exchanged cation aminesin host frameworks are ambiguous, and sometimes amines couldblock up the pores and make it difficult for further comparativestudies, such as gas sorption and catalysis. We have shown herethat structure 8 could be described as an amine-functionalizedopen framework obtained by total synthesis, which shows anexact position and geometry of Hpd in the host framework aswell as leaving accessible pores along the c axis. Therefore, itshould be helpful for studying the active sites inside the pores,and investigations into this are now underway in our group.

Conclusions

For porous metal–phosphonate frameworks, the problem ofovercoming the formation of dense inorganic layer structuresis considered to be a primary issue. 5-Phosphonobenzene-1,3-dicarboxylic acid (H4pbdc) is a ligand with triconnected nodeinformation encoded, increasing the possibility to obtain node-type frameworks. By taking advantage of this well-designed ligand,eight Zn(II)–phosphonocarboxylate frameworks, including four(3,6)-connected frameworks, have been isolated. The formation

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of a special anionic open framework of {[Zn3(pbdc)2]·2H3O}n

(3) has been extensively investigated in this paper. The resultssuggest that the synthetic method and organic solvents playimportant roles in the assembly of the special open framework, anychange of these two factors leading to the formation of differentframeworks. However, organic amines are variable, and so byusing an amine of appropriate pKa value and suitable symmetry,molecular assembly can generate the special (3,6)-connected openframework of [Zn3(pbdc)2]2- with pores decorated by amines.

We anticipate that the iso-structures obtained through thisapproach will be better for studying the active sites inside poresbecause of the precise structural information.

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10718 | Dalton Trans., 2010, 39, 10712–10718 This journal is © The Royal Society of Chemistry 2010

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