Time- and Temperature-Dependent Study in the Three-ComponentZinc-Triazolate-Oxybis(benzoate) System: Stabilization of NewTopologies

Partha Mahata, Manikanda Prabu, and Srinivasan Natarajan*

Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science,Bangalore 560 012, India

ReceiVed April 9, 2009; ReVised Manuscript ReceiVed June 8, 2009

ABSTRACT: Three new three-dimensional zinc-triazolate-oxybis(benzoate) compounds, [{Zn3(H2O)2}{C12H8O(COO)2}2-{C2H2N3}2] ·2H2O (I), [Zn7{C12H8O(COO)2}4{C2H2N3}6] ·H2O, (II), and [{Zn5(OH)2}{C12H8O(COO)2}3{C2H2N3}2] (III), have beensynthesized by a hydrothermal reaction of a mixture of Zn(OAc)2 ·2H2O, 4,4′-oxybis(benzoic acid), 1,2,4-triazole, NaOH, and water.Compound I has an interpenetrated diamond structure and II and III have pillared-layer related structures. The formation of ahydrated phase (I) at low temperature and a completely dehydrated phase (III) at high temperature suggests the importance ofthermodynamic factors in the formation of three compounds. Transformation studies of I in the presence of water shows the formationof a simple Zn-OBA compound, [Zn(OBA)(H2O)] (IV), at 150 and 180 °C and compound III at 200 °C. The compounds have beencharacterized by single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, IR, and photoluminescencestudies.

Introduction

Metal-organic framework (MOF) compounds are an impor-tant family and are being investigated for their many applica-tions, both actual as well as potential.1 The MOF compoundsexhibit considerable variety and diversity in their structures withmany having reasonably porous architectures.2 The MOFs, ingeneral, are formed by combining the coordination versatilityof the metal ion and the variable binding modes of the organicligands.3 There have been some attempts toward classifying andunderstanding the MOF structures using node and net basedapproaches.4 Persistent research over the years has shown thatthe combination of the metal ion and the organic ligand canresult in many phases, the formation of which would dependon a number of experimental variables such as pH,5 solvent,6

and temperature.7 In light of this, it may be noted that the roleof pH, Si/Al ratio, temperature, or the time of reaction has beenwell understood in the family of microporous aluminosilicatezeolites.8 Such studies in MOF compounds are beginning togain importance and prominence.9 Though there is no conver-gence yet with regard to the many experimental parameters,the time and temperature appear to have some effect, especiallytoward the control of the topology and the dimensionality ofthe MOF structures.

Of the many ligands that have been employed for thepreparation of MOF structures, the use of polycarboxylic acids,polypyridine ligands, and the combination of them appears tohave given rise to structures of importance.1-4 The use ofimidazole, recently, as the ligand has generated a considerablevariety of zeolite-like open structures with large interconnectingchannels.9b,10 This observation suggests that newer and interest-ing frameworks could be prepared by a judicious choice ofnitrogen containing ligands, which could either be neutral orcharged. Thus, the 1,2,4-triazole offers an excellent choice as aligand as it can bind with the metal centers through three typesof binding modes: µ1,2, µ2,4, and µ1,2,4. Predictably, the use oftriazole has given rise to many interesting layered structures

that are based on metal-triazolate connectivity.11 The use of onemore ligand such as the carboxylate can enhance the chancesof preparing interesting and higher-dimensional structures.12

One of the themes pursued in our laboratory is to investigatethe role of the time and the temperature of reaction in the formationof MOF structures.7e,9a A continuation of this theme for the three-component system involving Zn, oxy-bis(benzoic acid), andtriazole yielded three products. The compounds, [{Zn3(H2O)2}-{C12H8O(COO)2}2{C2H2N3}2] ·2H2O (I), [Zn7{C12H8O(COO)2}4-{C2H2N3}6] ·H2O (II), and [{Zn5(OH)2}{C12H8O(COO)2}3-{C2H2N3}2] (III), have been obtained as pure phases at differenttemperatures and times of the reactions. All the three compoundsexhibit three-dimensionally extended structures involving bond-ing between Zn2+, triazolate, and oxy bis(benzoate). In thispaper, we present the synthesis, structure, topological relation-ships, time- and temperature-dependent studies, and possibletransformation reactions.

Experimental Section

Materials. The reagents needed for the synthesis of the compounds,Zn(OAc)2 ·2H2O [Ranbaxy, India, 99%], 4,4′-oxybis(benzoic acid),[Lancaster, U.K, 99%], 1,2,4-triazole [Ranbaxy, India, 99%], and NaOH[CDH, India, 99%] are used as received and without any furtherpurifications. The water used was double distilled through a Milliporemembrane.

Synthesis and Initial Characterization. A typical reaction mixturecontaining Zn(OAc)2 ·2H2O (0.22 g, 1 mM), 4,4′-oxybis(benzoic acid)(0.26 g, 1 mM), 1,2,4-triazole (0.068 g, 1 mM), and NaOH (0.08 g, 2mM) and 8 mL of water was heated in a 23 mL PTFE-lined stainlesssteel autoclave at different times (1-5 days) and temperatures (100-220°C) under autogenous pressure. The synthesis conditions and otherrelated parameters are summarized in Table 1. The efforts resulted inthree different phases identified by single crystal X-ray diffraction(XRD). Pure phases of the compounds (I, II, and III) were obtained,respectively, at 100 °C/1 day, 150 °C/4 day, and 220 °C/1 day (basedon the smallest reaction time). Colorless rod-like crystals of I, colorlessrectangular crystal of II, and colorless block-like crystal of III wereobtained with 70-80% yield (Supporting Information, Figure S1).

Powder XRD patterns were recorded on well ground samples(produced in the three distinct conditions mentioned above) in the 2θrange 5-50° using Cu KR radiation (Philips X’pert) (SupportingInformation, Figures S2-S4). The XRD patterns of the above samples

* To whom correspondence should be addressed. E-mail: snatarajan@sscu.iisc.ernet.in.

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10.1021/cg900397r CCC: $40.75 2009 American Chemical SocietyPublished on Web 06/23/2009

indicated that the products are new materials, the pattern being entirelyconsistent with the simulated XRD pattern generated based on thestructure determined using the single-crystal XRD. In all otherconditions the products contain either the pure phase of one compoundor a mixture of the above phases, which were identified by the powderXRD (Supporting Information, Figures S5-S9). In most of the cases,we observed a reasonable yield for the products, which are typicallyin the range of 70-75% based on Zn. Elemental analysis - for I: calcd(%) C 41.89, H 3.05, N 9.16; found C 41.65, H 3.11, N 9.27; for II:calcd (%) C 42.75, H 2.41, N 13.20; found 42.6, H 2.45, N 13.37; forIII: calcd (%) C 43.62, H 2.37, N 6.64; found C 43.5, H2.45, N 6.7.

Thermogravimetric analysis (TGA) has been carried out in air (flowrate ) 20 mL min-1) in the temperature range 30-850 °C (heatingrate ) 5 °C min-1) (Supporting Information, Figure S10). The TGAstudies of I show a weight loss in two distinct steps. The first weightloss of 6% in the temperature range 70-160 °C may be due to thepartial removal of the water molecules, and the second weight loss inthe range of 280-460 °C leads to the decomposition of the framework.The total observed weight loss of 74% corresponds well with the lossof all the water molecules, the triazolate ligands, and the carboxylates(calc. 73.4%). Compound II shows a small weight loss of 1% in thetemperature range 150-200 °C, which is consistent with the loss ofthe extra framework water molecules (calc. 0.94%), and the next weightloss in the range of 270-450 °C corresponds to the decomposition ofthe framework. The total observed weight loss of 70% correspondswell with the loss of the water molecule, the triazolate ligands, and thecarboxylates (calc. 70.2%). Compound III shows only one weight lossof 68% in the range of 260-460 °C, which corresponds well with thedecomposition of the framework due to the loss of the triazolate ligandsand the carboxylates (calc. 67.9%). The final calcined product in allthe cases was found to be crystalline by powder XRD and correspondsto ZnO (JCPDS: 36-1451) for all the three compounds. The IR spectrumwas recorded on a KBr pellet (Perkin-Elmer, SPECTRUM 1000)(Supporting Information, Figure S11).

Single-Crystal Structure Determination. A suitable single crystalof each compound was carefully selected under a polarizing microscopeand glued carefully to a thin glass fiber. The single crystal data werecollected on a Bruker AXS smart Apex CCD diffractometer at 293(2)K. The X-ray generator was operated at 50 kV and 35 mA using Mo

KR (λ ) 0.71073 Å) radiation. Data were collected with ω scan widthof 0.3°. A total of 606 frames were collected in three different settingsof � (0, 90, 180°) keeping the sample-to-detector distance fixed at 6.03cm and the detector position (2θ) fixed at -25°. The data were reducedusing SAINTPLUS,13 and an empirical absorption correction wasapplied using the SADABS program.14 The structure was solved andrefined using SHELXL9715 present in the WinGx suite of programs(Version 1.63.04a).16 All the hydrogen atoms of the carboxylic acidsand the water molecules were initially located in the difference Fouriermaps, and for the final refinement, the hydrogen atoms were placed ingeometrically ideal positions and held in the riding mode. Finalrefinement included atomic positions for all the atoms, anisotropicthermal parameters for all the non-hydrogen atoms, and isotropicthermal parameters for all the hydrogen atoms. Full-matrix least-squaresrefinement against |F2| was carried out using the WinGx package ofprograms.16 Details of the structure solution and final refinements aregiven in Table 2. CCDC: 725569-725571 contain the crystallograph-ic data for this paper. These data can be obtained free of chargefrom The Cambridge Crystallographic Data Center (CCDC) via www.ccdc.cam.ac.uk/data_request/cif.

Results and Discussion

Structure of [{Zn3(H2O)2}{C12H8O(COO)2}2{C2H2N3}2] ·2H2O (I). The asymmetric unit of I consists of three crystal-lographically independent Zn2+ ions, two oxybis(benzoate)(OBA) anions, two triazolates (trz), two coordinated and twoextra-framework water molecules (Supporting Information,Figure S12). Both the triazolate ligands show µ1,2,4 bridgingmode with the three Zn2+ ions. The two OBA anions exhibitdifferences in their connectivity. Thus, OBA(1) has both thecarboxylate units with monodentate connectivity, and OBA(2)has one carboxylate with monodentate connectivity and the otherone with bidentate connectivity (Supporting Information, FigureS13). Of the three Zn Species, Zn(1) and Zn(2) is tetrahedrallycoordinated by two oxygen atoms and two nitrogen atoms fromtriazolate units. One of the oxygen atoms bound to Zn(2) is

Table 1. Synthesis and Related Parameters for the Three-Component System, Zinc Triazolate-oxybis(benzoate)

compoundsynthesis

temperature/timea H2O/Zn2+ b Zn2+/COO- M+2/103 Å3bridging COO-/nonbridging

COO-

[{Zn3(H2O)2}{C12H8O(COO)2}2{C2H2N3}2] ·2H2O (I) 100 °C /24 h 4/3(2/3) 1 3.39 0/4[Zn7{C12H8O(COO)2}4{C2H2N3}6] ·H2O (II) 150 °C /96 h 2/7(0) 1.25 3.71 1/3Zn5(OH)2}{C12H8O(COO)2}3{C2 H2N3}2] (III) 220 °C /24 h 0(0) 1.5 4.2 4/2

a On the basis of the first observation of phase (smallest reaction time and lowest temperature). b The first number in the H2O/Zn2+ columns refers tothe total water content and the second, given in parentheses, refers to coordinated water molecules.

Table 2. Crystal Data and Structure Refinement Parameters for [{Zn3(H2O)2}{C12H8O(COO)2}2{C2 H2N3}2] ·2H2O (I),[Zn7{C12H8O(COO)2}4{C2H2N3}6] ·H2O (II), and [{Zn5(OH)2}{C12H8O(COO)2}3{C2 H2N3}2] (III)a

structure parameter I II III

empirical formula C32H28N6O14Zn3 C68H46N18O21Zn7 C46H30N6O17Zn5

formula weight 916.71 1908.8 1265.61crystal system triclinic monoclinic monoclinicspace group P1j (No. 2) C2/c (No. 15) P21/c (No. 14)a (Å) 10.0121(16) 9.812(2) 15.0444(3)b (Å) 11.0949(18) 36.015(8) 19.1731(4)c (Å) 17.737(3) 21.799(5) 16.8680(3)R (deg) 104.988(3) 90.0 90.0� (deg) 99.792(3) 101.814(4) 102.4630(10)γ (deg) 105.608(3) 90.0 90.0volume (Å3) 1771.3(5) 7540(3) 4750.88(16)Z 2 4 4T (K) 273(2) 293(2) 293(2)Fcalc (g cm-3) 1.719 1.680 2.769µ (mm-1) 2.093 2.274 2.569θ range (deg) 1.99-28.04 1.48-28.03 1.75-25.73λ (Mo KR) (Å) 0.71073 0.71073 0.71073R indices [I > 2σ(I)] R1 ) 0.0402, wR2 ) 0.0759 R1 ) 0.0591, wR2 ) 0.1315 R1 ) 0.0296, wR2 ) 0.0764R indices (all data) R1 ) 0.0655, wR2 ) 0.0834 R1 ) 0.0915, wR2 ) 0.1496 R1 ) 0.0391, wR2 ) 0.0812

a R1 ) ∑|F0| - |Fc|/∑|F0|. wR2 ) {∑[w(F02 - Fc

2)2]/∑[w(F02)2]}1/2. w ) 1/[σ2(F0)2 + (aP)2 + bP], P ) [max(F0

2,0) + 2(Fc)2]/3, where a ) 0.0349and b ) 0.0353 for I, a ) 0.0583 and b ) 14.5639 for II, a ) 0.0403 and b ) 4.3139 for III.

3684 Crystal Growth & Design, Vol. 9, No. 8, 2009 Mahata et al.

terminal and is a water molecule. Zn(3) has a distorted trigonalbipyramidal coordination by two carboxylate oxygen atoms fromone OBA, two nitrogen atoms from two different triazolates,and one terminal water molecule. The average Zn-O and Zn-Ndistances of 2.025 and 2.018 Å, respectively, result from thisbonding. The O/N-Zn-O/N bond angles are in the range60.51(10)-151.91(10)°. The selected bond distances are listedin Table 3.

The linkages between Zn(1)O2N2, Zn(2)O(H2O)N2, Zn(3)-O2(H2O)N2 units with triazolate and OBA unit gives rise to anextended three-dimensional structure. The complex three-dimensional structure of I can be explained by consideringsimpler building units. Thus, the Zn(2) and Zn(3) ions areconnected by two trz ligand forming a dimetic unit,Zn2N4O3(H2O)2, which is further linked through a triazolate unitto Zn(1) forming a one-dimensional structure (Figure 1a). Theone-dimensional structure is connected by OBA through in-plane and out-of-plane connectivity to give rise to a three-dimensional structure with one-dimensional channels (Figure1b). A careful study of the overall structure shows that the three-dimensional structures are interpenetrated with another similarone in such a way that the second structure is shifted by half aunit cell. This completely blocks the open channels of the 3Dstructure. This can be visualized easily as shown in Figure 1c.In this representation, the OBA and the trz are represented as

diconnected and triconnected points, respectively, and the bondsof the same 3D network are presented with one unique color.

To understand this structure better, we used the descriptionbased on the node and net approach (Figure 2a). Thus, in I, thetrz ligands act as the triconnected node and the OBA units actas the diconnected node. Out of the three Zn2+ ions, Zn(1) actsas a four connected node, whereas the Zn(2) and the Zn(3) actas the triconnected node. The topological analysis were carriedout using the TOPOS4.0 program (Supporting Information,Fig-ure 14).17

To simplify this structure, the metal centered nodes can beconsidered exclusively. Thus, we find two 4-connected nodes.The dimeric unit of Zn(2) and Zn(3) [Zn2N4O3(H2O)2] isconnected with four Zn(1)O2N2 tetrahedra (through two OBAand two trz) and the Zn(1)O4 unit is bonded with four dimericunits (through two OBA and two trz). The connectivity betweenthe two four connected nodes gives rise to adamantane typeunits, which is the characteristic building unit for the diamondnet (Figure 2b) and I, indeed, has a diamondoid topology (Figure2c). The diamondoid topology of I, derived using TOPOS4.0,17

has a vertex symbol of 62.62.62.62.62.62 based entirely on themetal-centered nodes. This structure can be described as anexample of a decorated diamond network.4a

Table 3. Selected Bond Distances (Å) Observed in[{Zn3(H2O)2}{C12H8O(COO)2}2{C2 H2N3}2] ·2H2O (I),[Zn3.5{C12H8O(COO)2}2{C2H2N3}3] ·0.5H2O (II), and

[{Zn5(OH)2}{C12H8O(COO)2}3{C2 H2N3}2] (III)a

bond distances, Å bond distances, Å

I

Zn(1)-O(1)#1 1.9721(19) Zn(1)-N(6) 2.010(2)Zn(1)-O(2) 1.982(2) Zn(2)-O(3) 1.964(2)Zn(1)-N(3) 2.008(2) Zn(2)-O(4) 1.980(2)Zn(2)-N(2) 2.006(2) Zn(2)-N(4)#2 2.014(2)Zn(3)-O(5) 1.995(2) Zn(3)-N(1) 2.028(2)Zn(3)-N(5)#2 2.041(2) Zn(3)-O(6)#3 2.053(2)Zn(3)-O(7)#3 2.229(3)

II

Zn(1)-O(1)#1 1.989(5) Zn(2)-N(3) 1.995(4)Zn(1)-N(8)#2 2.044(4) Zn(2)-N(6) 2.003(4)Zn(1)-N(1) 2.044(4) Zn(2)-O(4) 2.010(3)Zn(1)-N(4)#3 2.047(4) Zn(3)-N(5)#3 2.031(4)Zn(1)-O(2)#1 2.293(7) Zn(3)-N(2) 2.037(4)Zn(2)-O(3) 1.920(4) Zn(3)-N(7)#2 2.045(4)Zn(3)-O(5)#4 2.083(5) Zn(3)-O(6)#4 2.165(6)Zn(4)-O(7) 1.964(3) Zn(4)-O(7)#5 1.964(3)Zn(4)-N(9)#5 1.990(4) Zn(4)-N(9) 1.990(4)

III

Zn(1)-O(1)#1 2.010(2) Zn(1)-O(2)#3 2.1051(19)Zn(1)-O(2) 2.0228(19) Zn(1)-O(4) 2.125(2)Zn(1)-O(3)#2 2.080(2) Zn(1)-O(5) 2.357(2)Zn(2)-O(6) 1.996(2) Zn(2)-O(7) 1.998(2)Zn(2)-N(3) 2.005(2) Zn(2)-N(6) 2.016(2)Zn(3)-O(8)#4 1.931(2) Zn(3)-O(2) 1.953(2)Zn(3)-N(5)#5 2.000(2) Zn(3)-N(1) 2.045(2)Zn(4)-O(9)#6 1.911(2) Zn(4)-O(10) 1.918(2)Zn(4)-N(2)#7 1.998(2) Zn(4)-N(4) 2.017(2)Zn(5)-O(10) 1.907(2) Zn(5)-O(11)#8 1.956(2)Zn(5)-O(12)#9 1.969(2) Zn(5)-O(5)#10 1.985(2)

a Symmetry operations used to generate equivalent atoms for I: #1 x,y + 1, z + 1. #2 x + 1, y, z. #3 x + 1, y, z + 1; for II: #1 x + 1, -y, z+ 1/2. #2 -x - 1/2, -y + 1/2, -z + 1. #3 x + 1, y, z. #4 x + 1/2, y+ 1/2, z. #5 -x - 1, y, -z + 1/2; for III: #1 x - 1, y, z. #2 -x, y + 1/2, -z + 1/2. #3 -x + 1, -y + 2, -z + 1. #4 -x + 2, -y + 2, -z +1. #5 -x + 1, y + 1/2, -z + 1/2. #6 -x + 2, -y + 1, -z + 1. #7 -x+ 1, y - 1/2, -z + 1/2. #8 -x, y - 1/2, -z + 1/2. #9 -x + 1, -y +1, -z + 1. #10 x, y - 1, z.

Figure 1. (a) Figure shows the connectivity between the Zn2+ ions andthe 1,2,4-triazolate in [{Zn3(H2O)2}{C12H8O(COO)2}2{C2 H2N3}2] ·2H2O (I). Note the formation of the one-dimensional chains. (b) Thethree-dimensional structure of I in the ab plane. (c) Figure shows theinterpenetration observed in I. The two layers are represented in twodifferent colors (pink and green) (see text).

Zinc-Triazolate-Oxybis(benzoate) System Crystal Growth & Design, Vol. 9, No. 8, 2009 3685

Structure of [Zn7{C12H8O(COO)2}4{C2H2N3}6] ·H2O (II).The asymmetric unit of II consists of four crystallographically

independent Zn2+ ions, of which one of them [Zn(4)] occupy aspecial position4e with a site multiplicity of 0.5. There are twooxybis(benzoate) (OBA) anions, three triazolates (trz), and oneextra-framework water molecule [occupying a special position4e

with a site multiplicity of 0.5] present in the asymmetric unit(Supporting Information, Figure S15). The triazolate ligandsexhibit µ1,2,4 bridging modes, similar to that observed in I. Thetwo OBA anions show differences in their connectivity. Thus,OBA(1) has one carboxylate with monodentate connectivity andthe other with bidentate connectivity with one Zn2+ ion andOBA(2) has one carboxylate with bidentate connectivity withone Zn2+ ion and the other carboxylate bidentate (bridging)connectivity with two Zn2+ ion (Supporting Information,Figure 16). Of the four Zn species, Zn(1) and Zn(3) have adistorted trigonal bipyramidal coordination by two carboxylateoxygen atoms of one OBA and three nitrogen atoms of threedifferent trz. Zn(2) and Zn(4) are tetrahedrally coordinated bytwo carboxylate oxygen atoms of two different OBA and twonitrogen atoms of two different trz. The Zn-O and Zn-N bondhave average distances of 2.049 and 2.023 Å, respectively. TheO/N-Zn-O/N bond angles are in the range 57.7(2)-150.0(2)°.The selected bond distances are listed in Table 3.

The three-dimensional structure of II can be explained byconsidering simpler building patterns. Thus, the Zn(1) and Zn(3)ions are connected by two triazolate anions forming a dimer,Zn2N6O4, which are further linked by a triazolate to Zn(2)forming a one-dimensional zigzag chain (Figure 3a). The chainis similar to that observed in I. Unlike I, here the chains areconnected to each other through a Zn(4)O2 tetrahedral unit viaa triazolate to form the one-dimensional ladder type structure(Figure 3b). The one-dimensional ladders are connected to eachother through the carboxylate group (bridging carboxylate) ofOBA(2) units to give rise to the two-dimensional connectivity(Figure 3c). The OBA units are cross-linked in II to form thethree-dimensional structure without any open channels (Figure3d).

The topological analysis of II (Figure 4a)17 reveals that thethree trz act as the triconnected node. Out of the two OBA units,OBA(1) acts as the diconnected node and the OBA(2) acts asthe triconnected node. Out of the four Zn2+ ions, Zn(1) andZn(3) are connected with one OBA and three trz. On the otherhand, Zn(2) and Zn(4) are connected with two OBA and twotrz. As a result, all the four Zn act as the 4-connected node.The topological analysis were carried out using the TOPOS4.0program (Supporting Information, Figure 17).17

Figure 2. (a) Figure shows the three-dimensional node based connectivity of Zn2+ ions and the ligands (OBA, trz) in I, where all the Zn, OBA, andtrz act as the nodes. Color code: Zn - cyan, trz - purple, OBA green. (b) Figure shows the adamantane type unit obtained based on the connectivityof [Zn2N4O3(H2O)2] and Zn(1)O2N2 through the OBA and the triazolate ligands. (c) Figure shows the diamondoid type topology based on theconnectivity between the metal center nodes [Zn2N4O3(H2O)2 and Zn(1)O2N2].

3686 Crystal Growth & Design, Vol. 9, No. 8, 2009 Mahata et al.

The topology of the layer arrangement of II can beexplained better by considering the metal centered nodedescription (Figure 4b). Accordingly, three different typesof nodes can be defined: (1) the dimeric unit of Zn(1) andZn(3) [Zn2N6O4]; (2) Zn(2)O2N2 tetrahedra; (3) Zn(4)O2N2

tetrahedra. In this scheme, the dimeric unit of Zn(1) and Zn(3)[Zn2N6O4] is connected with two Zn(2)O2N2 tetrahedra andone Zn(4)O2N2 tetrahedra through three trz and act as the3-connected node. Zn(2)O2N2 tetrahedra is connected withtwo dimeric units through two trz and with one Zn(4)O2N2

tetrahedra through the carboxylate group of OBA and act asa 3-connected node. The Zn(4)O2N2 tetrahedra, on other hand,is connected with two dimeric units [Zn(1), Zn(3)] throughtwo trz and with two Zn(2)O2N2 tetrahedra through twocarboxylate groups of OBA and acts as the 4-connected node.The connectivity of the three nodes gives rise to a binodal(two 3- and one 4-connected nodes) two-dimensional net-work. The Schlafli symbol for the two-dimensional networkis (62.84)(62.8)4. This type of structural arrangement has notbeen observed before in any of the earlier known binodalnets.18

Structure of [{Zn5(OH)2}{C12H8O(COO)2}3{C2H2N3}2] (III).The asymmetric unit of III consists of five crystallographi-

cally independent Zn2+ ions, three oxybis(benzoate) (OBA)

anions, and two triazolates (trz) units (Supporting Information,Figure S18). The triazolate ligands show µ1,2,4 bridging modeswith three Zn2+ ions as seen in I and II. The OBA anions showdifferences in their connectivity. In OBA(1), one carboxylatehas bidentate (bridging) connectivity with two Zn2+ ions andthe other one has tridentate (bridging) connectivity with twoZn2+ ions. OBA(2) and OBA(3) show similarities in the bondingwith one of the carboxylate having monodentate connectivity,while the other has bidentate connectivity (bridging) with twoZn2+ ions (Supporting Information, Figure S19). Of the fiveZn species, Zn(1) is octahedrally coordinated by four carboxylateoxygen atoms from three different OBA and two µ3-OH anions[O(2)]. Zn(2), Zn(3), Zn(4), and Zn(5) are all tetrahedrallycoordinated. Zn(2) has two carboxylate oxygen atoms from twodifferent OBA units and two nitrogen atoms from two differenttrz, Zn(3) and Zn(4) have one carboxylate oxygen atom, twonitrogen atoms from two different trz, and one µ3-(OH)- unit,and Zn(5) has three carboxylate oxygen atoms from threedifferent OBA and one µ2-(OH)- unit. The Zn-O and Zn-Nbond distances have average values of 2.014 Å and 2.0135 Å,respectively. The O/N-Zn-O/N bond angles are in the range58.35(8)-177.14(9)°. The selected bond distances are listed inTable 3.

Figure 3. (a) Figure shows the connectivity between Zn(1), Zn(2), Zn(3) and the triazolate units forming a one-dimensional chain in[Zn7{C12H8O(COO)2}4{C2H2N3}6] ·H2O, II. (b) Figure shows the connectivity between the Zn2+ ions and the 1,2,4-triazolate. Note the formationof a one-dimensional ladder-like unit. (c) The connectivity between the two ladders through the carboxylate groups to give rise to the two-dimensionallayer in II. Color code: Zn - cyan, C-gray, O - red, N - blue. Note the bridging carboxylate groups. (d) Figure shows the connectivity between thelayers through the OBA unit forming the three-dimensional structure.

Zinc-Triazolate-Oxybis(benzoate) System Crystal Growth & Design, Vol. 9, No. 8, 2009 3687

The three-dimensional structure of III can be explained byconsidering connectivities between the Zn, the triazolate, andthe hydroxyl anions, which form a two-dimensional layer. Thus,the Zn(3) and Zn(4) ions are connected by two trz to formdimetic units, Zn2N4O2(OH)2, which are further bonded to Zn(2)ions to give rise a one-dimensional zigzag chain (Figure 5a).This chain is similar to the Zn-triazolate chain structure observedin I and II. Two Zn(1) ions are bonded together through twoOH anions [O(2)] forming a different dimeric unit, Zn2O4(OH)2,which are bonded with Zn(5)O3(OH) tetrahedra through thecarboxylate oxygens forming a tetramer unit, Zn4O12(OH)4

(Figure 5b). The linkages between the one-dimensional chainsand the tetrameric units forms the two-dimensional layerstructure (Figure 5c). The two-dimensional layers are pillaredby the OBA units forming the three-dimensional structure(Figure 6a).

The overall topology of the structure of III is not simple asthe two-dimensional layers are more condensed, as they areformed by the connectivity between the Zn2+ ions, the triaz-olates, and the OH- anions. This framework can, then, beexplained as a pillared layered structure. The two-dimensionalzinc-triazolate layer can be described based on the metal centernodes. In this description, one can consider four different typesof nodes: (1) dimeric unit of Zn(3) and Zn(4) [Zn2N4O2(OH)2;dimer(1)]; (2) dimeric unit of two Zn(1) [Zn2O4(OH)2; dimer(2)];(3) Zn(2)O2N2; (4) Zn(5)O3(OH). Here the dimer(1) is connectedwith one Zn(5)O3(OH) [through µ2-OH group], two Zn(2)O2N2

[through two triazolate] and a dimer(2) [through µ3-OH group]to act as a 4-connected node. Dimer(2) is connected with twodimer(1) [through two µ3-OH] and two Zn(5)O3(OH) [throughcarboxylate groups] to act as a 4-connected node. On the otherhand, both the Zn(2) and Zn(5) act as the 3-connected nodes,where Zn(2)O2N2 is connected with two dimer(1) [through thetriazolate] and one Zn(5)O3(OH) [through the carboxylate group]and the Zn(5)O3(OH) is connected with dimer(1) [through µ2-OH], dimer(2) [through the carboxylate group] and Zn(2)O2N2

[through the carboxylate group]. The connectivity between thefour nodes is shown in Figure 6b. The layer structure of III isalso based on the binodal metal connectivity and has beenobserved for the first time. As mentioned before, the layersessentially comprise 4-, 5-, and 6- membered ring units andnot open (more condensed). The Schlafli symbol for the two-dimensional network is (4.5.6)4(4.52.72)2(42.52.72).

Figure 4. (a) Figure shows the node based connectivity between theZn2+ ions and the ligands (OBA, trz) in II. Note the cross-linking bythe OBA. Color code: Zn - cyan, trz - purple, OBA green. (b) Figureshows the connectivity based on the three metal center nodes in IIwithin the layer (see text).

Figure 5. (a) Figure shows the connectivity between the Zn(2)2+,Zn(3)2+, and Zn(4)2+ ions with 1,2,4-triazolate in Zn5(OH)2}-{C12H8O(COO)2}3{C2 H2N3}2], III. Note that the one-dimensionalstructure has a zigzag arrangement. (b) Figure shows the connectivitybetween Zn(1)2+ and Zn(5)2+ by the (OH)- ions and the carbxylategroup forming the tetrameric unit. (c) The connectivity between theone-dimensional chains and the tetramers forming the two-dimensionallayer in III.

3688 Crystal Growth & Design, Vol. 9, No. 8, 2009 Mahata et al.

Studies on the Variation of Time and Temperature. Wehave made a detailed study of the effect of time and temperatureon the reaction mixture in the system Zn-triazole-oxybis(benzoicacid). The products of each experiment (synthesis) weresubjected to powder XRD investigations, and the results arerepresented as a two-dimensional temperature vs time plot inFigure 7. In all the cases, we observed only the phases (pure ora mixture) that have been isolated in the present study(compounds I, II, and III) (Supporting Information, FiguresS5-S9). This conclusion was arrived at by a careful study ofthe powder XRD patterns of the products of the reaction andcomparing that with the simulated XRD patterns, of compoundsI, II, and III (Figure 7).

Generally, the formation of the more hydrated phase (I) wasobserved at lower temperatures (100-125 °C) followed by theother phases (II and III). Interestingly, the pure phases of IIand III were obtained at 150 °C (4-5 days) and at 220 °C,respectively. This observation suggests that the system under-goes dehydration at elevated temperatures - the total watercontent in the system decreases with an increase of time and

temperature of the reaction. This indicates that the formationof some of the phases is thermodynamically controlled. Similarobservations have also been made in the literature in metalcarboxylate systems.7b,9a

In this context, it may be of some importance to note thatthe formation different phases have been observed during thestudy of the variation of reactions parameters in amine templatedzinc phosphite phases.19 It was shown that five phases ofdifferent dimensionality and structure could be prepared froman essentially three-component reaction mixture - zinc salt,phosphoric acid, and 1,4-bis (3-aminopropyl) piperazine (AP-PIP). Studies of this nature have also been reported earlier duringthe formation of a family of uranium fluorides in the presenceof 2-methyl piperazine and piperazine.20

The time and temperature variation studies along with acareful scrutiny of the structures indicate that the bonding aroundthe Zn2+ ion gradually shows subtle differences (Table 1 andFigure 7). The triazolate ligands have the same coordinationmode in all three structures. The connectivity of the OBAligands, however, shows notable differences. In I, each car-boxylate group bonds with only one Zn2+ ions and no bridgingmodes were observed (Zn2+/COO- ) 1). In II, the carboxylategroups shows bridging modes as well as the monodentate modein the bonding with Zn2+ (Zn2+/COO- ) 1.25). In III, thebridging mode of the carboxylate group (Zn2+/COO- ) 1.5)increases and we find four out of the six carboxylate groupsbridges the Zn2+ centers. This increase in the bridging modealso brings about an increase in the number of Zn atoms per1000 Å3 [Zn2+/103 Å3 ) 3.39 (I), 3.71(II), and 4.2(III)]. Theincrease in the bridging mode of the carboxylate also reflectson the structural arrangement as well. Thus, in I the OBAligands show end-to-end bonding with the Zn2+ ions, whichgives rise to a larger separation between the four connectednodes, which results in the interpenetration of the diamondoidnetwork in I (see Figure 1c). In II, the bridging carbxylate groupgives rise to a moderately condensed layer based on the Zn-triazolate-carboxylate connectivity and also the cross bondingof the OBA gives rise to a more closed three-dimensional

Figure 6. (a) The three-dimensional structure of III. Note that the OBAunits pillar the layers. (b) The view of the two-dimensional layer basedon the connectivity of the metal centered nodes (see text).

Figure 7. Temperature vs time plot of the formation of the threecompounds (I-III). The various phases were identified by comparingthe powder XRD patterns with the simulated patterns generated fromthe single-crystal structure. Color code: I, white; II, gray; III, black.The approximate ratio of yields of the products where mixture of twophases are formed are 70:30 (II:III; 180 °C/1day), 40:60 (II:III; 200°C/1 day), 25:75 (II:III, 180 °C/2 day), 15:85 (II:III, 200 °C/2 day),40:60 (I:II, 150 °C/3 day), 20:80 (II:III, 180 °C/3 day), 15:85 (II:III,180 °C/4 day), and 10:90 (II:III, 180 °C/5 day).

Zinc-Triazolate-Oxybis(benzoate) System Crystal Growth & Design, Vol. 9, No. 8, 2009 3689

structure. In III, the increased bridging modes of the carboxylategroup with Zn2+ ions give rise to a condensed layer. Theformation of a condensed layer with hydroxyl bridging atelevated temperatures has been observed by us as part of thetemperature- and time-dependent studies on the manganeseoxybis(benzoate) systems.7e,9a Similar observations have alsobeen made by Cheetham et al. as well.7b

Transformation Studies. It may be noted that the one-dimensional chain structure formed between Zn and triazolateis common in all three structures. The connectivity between 1Dchains by OBA and triazolate ligands brings about the observeddifferences among the three structures. Thus, one can infer thatstructurally all three compounds are related. If the formation ofthese phases have thermodynamic control, then it may bepossible to convert at least one of the structures, viz I, into otherstructures. To this end, we have investigated the possibletransformation of the most hydrated phase (I) to the lesser ones(II and III). For this study, I (0.092 g, 0.25 mM) was heatedwith water (2 mL) in a 7 mL autoclave at 150, 180, and 200 °Cfor 24 h. The resulting products of the experiments wereanalyzed using powder XRD studies (see Supporting Informa-tion, Figure S20). The powder XRD indicated that at 150 °C anew compound (IV) is formed along with I. A mixture of thethree compounds I, III, and IV were formed at 180 °C, and at200 °C, we observed only the pure phase of III. The new phase(IV) was identified using the single crystal X-ray diffractionstudy. The transformation studies also indicate that I is stableup to 180 °C, above which it transforms to III. The structureof [Zn(OBA)(H2O)] (IV) has a simple two-dimensional con-nectivity between the Zn2+ ions and OBA units without anytriazolate units and has been reported earlier (see SupportingInformation, Figure S21).21

From the transformation study, it is clear that the formationof these phases is through a dissolution and recystalizationmechanism. The formation of IV at 150 and 180 °C shows thatthe triazolate is easily removed from the system and againneeded for the formation of III. It is quite clear that compoundIII is, probably, the stable phase in the three-component systemsinvolving Zn/triazolate/OBA. During the transformation studies,we never observed the formation of II. This suggests that IIcan form only on prolonged heating. It may be noted that IIwas prepared as a pure phase only after heating for 4 days at150 °C.

Transformation studies of this nature have also been carriedout in amine templated zinc phosphate and zinc arsenate familyof compounds.22 It has been established that the zero-dimensional molecular complexes of zinc phosphate/zinc ar-senate can be transformed to structures of higher dimensionalityby a suitable modification of the experimental conditions.Similarly, low-dimensional (one- and two-) zinc phosphatestructures have also been shown to transform under appropriateconditions.23,24 Transformation studies of this nature have beencarried out in the amine templated open-framework structures,but such studies are not many in the MOF compounds. In thissense, the transformation studies described above are importantas they provide important clues for our understanding of theformation of MOF compounds.

Luminescence Properties. Room temperature solid-statephotoluminescence studies were carried out on powderedsamples (Perkin-Elmer, UK) (Figure 8). All the three compounds(I-III), the sodium salt of OBA and the triazolate exhibitsphotoluminescence. Sodium salt of OBA and the triazolateshows emissions at 320 nm and 380 nm, respectively, whenexcited using a radiation of 250 nm. For all the compounds,

we observed a peak at 320 nm and a shoulder at 375 nm. Theobservation of a similar position and shape of the emission peaksof the three compounds with respect to the emission peaks ofthe two ligands indicates that the emission of the threecompounds may be due to the intraligand electronic transitions.The observed transitions could be due to the π* f n or π* fπ transitions of the two ligands (OBA and triazolate). Theemission at ∼320 nm can be assigned due to the intraligandemission from the OBA ligand and emission at ∼375 nm rangecan be assigned due to the intraligand emission from thetriazolate ligand. The small shift observed in the bands amongthe three compounds could be due to the slight coordinationdifferences of Zn2+ ions.25 The intraligand emission intensityof the three compounds appear to be higher than the ligands,which may be attributed to the chelation of the ligands to themetal center. The bonding of the ligands with the metal increasesthe rigidity of the ligands along with a reduction in the loss ofenergy due to possible decay through other radiationlessprocesses.26

Conclusions

The synthesis and structure characterization of three differentMOFs have been accomplished. To the best of our knowledge,this is the first study illustrating the role time and temperaturein a three-component system, which is much more complex thanthe two-component system investigated in the literature.7,9

Formation of a three-dimensional MOF with interpenetrateddiamondiod topology at low temperature and two noninterpen-etrated pillared layer structures at higher temperature and timeis noteworthy. The present study suggests that the variation ofsolvent and pH would exercise subtle effects on the formationof related structures. We are currently pursuing this theme.

Acknowledgment. S.N. thanks the Department of Scienceand Technology (DST), Government of India, for the award ofa research grant, and the authors thank the Council of Scientificand Industrial Research (CSIR), Government of India, for theaward of a fellowship (PM) and a research grant. S.N. alsothanks the Department of Science and Technology, Governmentof India, for the award of the RAMANNA fellowship.

Figure 8. Emission spectra: (a) Na salt of OBA, (b) Na salt of triazolate,(c) I, (d) II, and (e) III.

3690 Crystal Growth & Design, Vol. 9, No. 8, 2009 Mahata et al.

Supporting Information Available: X-ray crystallographic files inCIF format, image of single crystals, bond angle table, IR, TGA, PXRD,and addition figures. This material is available free of charge via theInternet at http://pubs.acs.org.

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