Time- and Temperature-Dependent Study in the Three-Component Zinc-Triazolate-Oxybis(benzoate) System: Stabilization of New Topologies Partha Mahata, Manikanda Prabu, and Srinivasan Natarajan*
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3683–3691
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 been synthesized 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 a hydrated phase (I) at low temperature and a completely dehydrated phase (III) at high temperature suggests the importance of thermodynamic factors in the formation of three compounds. Transformation studies of I in the presence of water shows the formation of a simple Zn-OBA compound, [Zn(OBA)(H2O)] (IV), at 150 and 180 °C and compound III at 200 °C. The compounds have been characterized by single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, IR, and photoluminescence studies. Introduction Metal-organic framework (MOF) compounds are an important family and are being investigated for their many applications, both actual as well as potential.1 The MOF compounds exhibit considerable variety and diversity in their structures with many having reasonably porous architectures.2 The MOFs, in general, are formed by combining the coordination versatility of the metal ion and the variable binding modes of the organic ligands.3 There have been some attempts toward classifying and understanding the MOF structures using node and net based approaches.4 Persistent research over the years has shown that the combination of the metal ion and the organic ligand can result in many phases, the formation of which would depend on a number of experimental variables such as pH,5 solvent,6 and temperature.7 In light of this, it may be noted that the role of pH, Si/Al ratio, temperature, or the time of reaction has been well understood in the family of microporous aluminosilicate zeolites.8 Such studies in MOF compounds are beginning to gain importance and prominence.9 Though there is no convergence yet with regard to the many experimental parameters, the time and temperature appear to have some effect, especially toward the control of the topology and the dimensionality of the MOF structures. Of the many ligands that have been employed for the preparation of MOF structures, the use of polycarboxylic acids, polypyridine ligands, and the combination of them appears to have given rise to structures of importance.1-4 The use of imidazole, recently, as the ligand has generated a considerable variety of zeolite-like open structures with large interconnecting channels.9b,10 This observation suggests that newer and interesting frameworks could be prepared by a judicious choice of nitrogen containing ligands, which could either be neutral or charged. Thus, the 1,2,4-triazole offers an excellent choice as a ligand as it can bind with the metal centers through three types of binding modes: µ1,2, µ2,4, and µ1,2,4. Predictably, the use of triazole has given rise to many interesting layered structures * To whom correspondence should be addressed. E-mail: snatarajan@ sscu.iisc.ernet.in.
that are based on metal-triazolate connectivity.11 The use of one more ligand such as the carboxylate can enhance the chances of preparing interesting and higher-dimensional structures.12 One of the themes pursued in our laboratory is to investigate the role of the time and the temperature of reaction in the formation of MOF structures.7e,9a A continuation of this theme for the threecomponent system involving Zn, oxy-bis(benzoic acid), and triazole 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 different temperatures and times of the reactions. All the three compounds exhibit three-dimensionally extended structures involving bonding between Zn2+, triazolate, and oxy bis(benzoate). In this paper, we present the synthesis, structure, topological relationships, time- and temperature-dependent studies, and possible transformation 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 further purifications. The water used was double distilled through a Millipore membrane. Synthesis and Initial Characterization. A typical reaction mixture containing 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, 2 mM) and 8 mL of water was heated in a 23 mL PTFE-lined stainless steel autoclave at different times (1-5 days) and temperatures (100-220 °C) under autogenous pressure. The synthesis conditions and other related parameters are summarized in Table 1. The efforts resulted in three 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 (based on the smallest reaction time). Colorless rod-like crystals of I, colorless rectangular crystal of II, and colorless block-like crystal of III were obtained 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 2θ range 5-50° using Cu KR radiation (Philips X’pert) (Supporting Information, Figures S2-S4). The XRD patterns of the above samples
10.1021/cg900397r CCC: $40.75 2009 American Chemical Society Published on Web 06/23/2009
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Table 1. Synthesis and Related Parameters for the Three-Component System, Zinc Triazolate-oxybis(benzoate) compound
synthesis temperature/timea
H2O/Zn2+ b
Zn2+/COO-
M+2/103 Å3
bridging COO-/nonbridging COO-
[{Zn3(H2O)2}{C12H8O(COO)2}2{C2H2N3}2] · 2H2O (I) [Zn7{C12H8O(COO)2}4{C2H2N3}6] · H2O (II) Zn5(OH)2}{C12H8O(COO)2}3{C2 H2N3}2] (III)
100 °C /24 h 150 °C /96 h 220 °C /24 h
4/3(2/3) 2/7(0) 0(0)
1 1.25 1.5
3.39 3.71 4.2
0/4 1/3 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 to the 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 formula weight crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) volume (Å3) Z T (K) Fcalc (g cm-3) µ (mm-1) θ range (deg) λ (Mo KR) (Å) R indices [I > 2σ(I)] R indices (all data)
C32H28N6O14Zn3 916.71 triclinic P1j (No. 2) 10.0121(16) 11.0949(18) 17.737(3) 104.988(3) 99.792(3) 105.608(3) 1771.3(5) 2 273(2) 1.719 2.093 1.99-28.04 0.71073 R1 ) 0.0402, wR2 ) 0.0759 R1 ) 0.0655, wR2 ) 0.0834
C68H46N18O21Zn7 1908.8 monoclinic C2/c (No. 15) 9.812(2) 36.015(8) 21.799(5) 90.0 101.814(4) 90.0 7540(3) 4 293(2) 1.680 2.274 1.48-28.03 0.71073 R1 ) 0.0591, wR2 ) 0.1315 R1 ) 0.0915, wR2 ) 0.1496
C46H30N6O17Zn5 1265.61 monoclinic P21/c (No. 14) 15.0444(3) 19.1731(4) 16.8680(3) 90.0 102.4630(10) 90.0 4750.88(16) 4 293(2) 2.769 2.569 1.75-25.73 0.71073 R1 ) 0.0296, wR2 ) 0.0764 R1 ) 0.0391, wR2 ) 0.0812
R1 ) ∑|F0| - |Fc|/∑|F0|. wR2 ) {∑[w(F02 - Fc2)2]/∑[w(F02)2]}1/2. w ) 1/[σ2(F0)2 + (aP)2 + bP], P ) [max(F02,0) + 2(Fc)2]/3, where a ) 0.0349 and b ) 0.0353 for I, a ) 0.0583 and b ) 14.5639 for II, a ) 0.0403 and b ) 4.3139 for III. a
indicated that the products are new materials, the pattern being entirely consistent with the simulated XRD pattern generated based on the structure determined using the single-crystal XRD. In all other conditions the products contain either the pure phase of one compound or a mixture of the above phases, which were identified by the powder XRD (Supporting Information, Figures S5-S9). In most of the cases, we observed a reasonable yield for the products, which are typically in 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; for III: 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 (flow rate ) 20 mL min-1) in the temperature range 30-850 °C (heating rate ) 5 °C min-1) (Supporting Information, Figure S10). The TGA studies of I show a weight loss in two distinct steps. The first weight loss of 6% in the temperature range 70-160 °C may be due to the partial removal of the water molecules, and the second weight loss in the range of 280-460 °C leads to the decomposition of the framework. The total observed weight loss of 74% corresponds well with the loss of all the water molecules, the triazolate ligands, and the carboxylates (calc. 73.4%). Compound II shows a small weight loss of 1% in the temperature range 150-200 °C, which is consistent with the loss of the extra framework water molecules (calc. 0.94%), and the next weight loss in the range of 270-450 °C corresponds to the decomposition of the framework. The total observed weight loss of 70% corresponds well with the loss of the water molecule, the triazolate ligands, and the carboxylates (calc. 70.2%). Compound III shows only one weight loss of 68% in the range of 260-460 °C, which corresponds well with the decomposition of the framework due to the loss of the triazolate ligands and the carboxylates (calc. 67.9%). The final calcined product in all the cases was found to be crystalline by powder XRD and corresponds to ZnO (JCPDS: 36-1451) for all the three compounds. The IR spectrum was recorded on a KBr pellet (Perkin-Elmer, SPECTRUM 1000) (Supporting Information, Figure S11). Single-Crystal Structure Determination. A suitable single crystal of each compound was carefully selected under a polarizing microscope and glued carefully to a thin glass fiber. The single crystal data were collected 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 width of 0.3°. A total of 606 frames were collected in three different settings of φ (0, 90, 180°) keeping the sample-to-detector distance fixed at 6.03 cm and the detector position (2θ) fixed at -25°. The data were reduced using SAINTPLUS,13 and an empirical absorption correction was applied using the SADABS program.14 The structure was solved and refined using SHELXL9715 present in the WinGx suite of programs (Version 1.63.04a).16 All the hydrogen atoms of the carboxylic acids and the water molecules were initially located in the difference Fourier maps, and for the final refinement, the hydrogen atoms were placed in geometrically ideal positions and held in the riding mode. Final refinement included atomic positions for all the atoms, anisotropic thermal parameters for all the non-hydrogen atoms, and isotropic thermal parameters for all the hydrogen atoms. Full-matrix least-squares refinement against |F2| was carried out using the WinGx package of programs.16 Details of the structure solution and final refinements are given in Table 2. CCDC: 725569-725571 contain the crystallographic data for this paper. These data can be obtained free of charge from 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 crystallographically independent Zn2+ ions, two oxybis(benzoate) (OBA) anions, two triazolates (trz), two coordinated and two extra-framework water molecules (Supporting Information, Figure S12). Both the triazolate ligands show µ1,2,4 bridging mode with the three Zn2+ ions. The two OBA anions exhibit differences in their connectivity. Thus, OBA(1) has both the carboxylate units with monodentate connectivity, and OBA(2) has one carboxylate with monodentate connectivity and the other one with bidentate connectivity (Supporting Information, Figure S13). Of the three Zn Species, Zn(1) and Zn(2) is tetrahedrally coordinated by two oxygen atoms and two nitrogen atoms from triazolate units. One of the oxygen atoms bound to Zn(2) is
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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 Zn(1)-O(2) Zn(1)-N(3) Zn(2)-N(2) Zn(3)-O(5) Zn(3)-N(5)#2 Zn(3)-O(7)#3
1.9721(19) 1.982(2) 2.008(2) 2.006(2) 1.995(2) 2.041(2) 2.229(3)
Zn(1)-N(6) Zn(2)-O(3) Zn(2)-O(4) Zn(2)-N(4)#2 Zn(3)-N(1) Zn(3)-O(6)#3
2.010(2) 1.964(2) 1.980(2) 2.014(2) 2.028(2) 2.053(2)
Zn(2)-N(3) Zn(2)-N(6) Zn(2)-O(4) Zn(3)-N(5)#3 Zn(3)-N(2) Zn(3)-N(7)#2 Zn(3)-O(6)#4 Zn(4)-O(7)#5 Zn(4)-N(9)
1.995(4) 2.003(4) 2.010(3) 2.031(4) 2.037(4) 2.045(4) 2.165(6) 1.964(3) 1.990(4)
Zn(1)-O(2)#3 Zn(1)-O(4) Zn(1)-O(5) Zn(2)-O(7) Zn(2)-N(6) Zn(3)-O(2) Zn(3)-N(1) Zn(4)-O(10) Zn(4)-N(4) Zn(5)-O(11)#8 Zn(5)-O(5)#10
2.1051(19) 2.125(2) 2.357(2) 1.998(2) 2.016(2) 1.953(2) 2.045(2) 1.918(2) 2.017(2) 1.956(2) 1.985(2)
II Zn(1)-O(1)#1 Zn(1)-N(8)#2 Zn(1)-N(1) Zn(1)-N(4)#3 Zn(1)-O(2)#1 Zn(2)-O(3) Zn(3)-O(5)#4 Zn(4)-O(7) Zn(4)-N(9)#5
1.989(5) 2.044(4) 2.044(4) 2.047(4) 2.293(7) 1.920(4) 2.083(5) 1.964(3) 1.990(4)
Zn(1)-O(1)#1 Zn(1)-O(2) Zn(1)-O(3)#2 Zn(2)-O(6) Zn(2)-N(3) Zn(3)-O(8)#4 Zn(3)-N(5)#5 Zn(4)-O(9)#6 Zn(4)-N(2)#7 Zn(5)-O(10) Zn(5)-O(12)#9
2.010(2) 2.0228(19) 2.080(2) 1.996(2) 2.005(2) 1.931(2) 2.000(2) 1.911(2) 1.998(2) 1.907(2) 1.969(2)
III
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.
terminal and is a water molecule. Zn(3) has a distorted trigonal bipyramidal coordination by two carboxylate oxygen atoms from one OBA, two nitrogen atoms from two different triazolates, and one terminal water molecule. The average Zn-O and Zn-N distances of 2.025 and 2.018 Å, respectively, result from this bonding. The O/N-Zn-O/N bond angles are in the range 60.51(10)-151.91(10)°. The selected bond distances are listed in 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 an extended three-dimensional structure. The complex threedimensional structure of I can be explained by considering simpler building units. Thus, the Zn(2) and Zn(3) ions are connected by two trz ligand forming a dimetic unit, Zn2N4O3(H2O)2, which is further linked through a triazolate unit to Zn(1) forming a one-dimensional structure (Figure 1a). The one-dimensional structure is connected by OBA through inplane and out-of-plane connectivity to give rise to a threedimensional structure with one-dimensional channels (Figure 1b). A careful study of the overall structure shows that the threedimensional structures are interpenetrated with another similar one in such a way that the second structure is shifted by half a unit cell. This completely blocks the open channels of the 3D structure. This can be visualized easily as shown in Figure 1c. In this representation, the OBA and the trz are represented as
Figure 1. (a) Figure shows the connectivity between the Zn2+ ions and the 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) The three-dimensional structure of I in the ab plane. (c) Figure shows the interpenetration observed in I. The two layers are represented in two different colors (pink and green) (see text).
diconnected and triconnected points, respectively, and the bonds of the same 3D network are presented with one unique color. To understand this structure better, we used the description based on the node and net approach (Figure 2a). Thus, in I, the trz ligands act as the triconnected node and the OBA units act as the diconnected node. Out of the three Zn2+ ions, Zn(1) acts as a four connected node, whereas the Zn(2) and the Zn(3) act as the triconnected node. The topological analysis were carried out using the TOPOS4.0 program (Supporting Information,Figure 14).17 To simplify this structure, the metal centered nodes can be considered exclusively. Thus, we find two 4-connected nodes. The dimeric unit of Zn(2) and Zn(3) [Zn2N4O3(H2O)2] is connected with four Zn(1)O2N2 tetrahedra (through two OBA and two trz) and the Zn(1)O4 unit is bonded with four dimeric units (through two OBA and two trz). The connectivity between the two four connected nodes gives rise to adamantane type units, which is the characteristic building unit for the diamond net (Figure 2b) and I, indeed, has a diamondoid topology (Figure 2c). The diamondoid topology of I, derived using TOPOS4.0,17 has a vertex symbol of 62.62.62.62.62.62 based entirely on the metal-centered nodes. This structure can be described as an example of a decorated diamond network.4a
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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, and trz act as the nodes. Color code: Zn - cyan, trz - purple, OBA green. (b) Figure shows the adamantane type unit obtained based on the connectivity of [Zn2N4O3(H2O)2] and Zn(1)O2N2 through the OBA and the triazolate ligands. (c) Figure shows the diamondoid type topology based on the connectivity between the metal center nodes [Zn2N4O3(H2O)2 and Zn(1)O2N2].
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 a special position4e with a site multiplicity of 0.5. There are two oxybis(benzoate) (OBA) anions, three triazolates (trz), and one extra-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 ligands exhibit µ1,2,4 bridging modes, similar to that observed in I. The two OBA anions show differences in their connectivity. Thus, OBA(1) has one carboxylate with monodentate connectivity and the other with bidentate connectivity with one Zn2+ ion and OBA(2) has one carboxylate with bidentate connectivity with one 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 a distorted trigonal bipyramidal coordination by two carboxylate oxygen atoms of one OBA and three nitrogen atoms of three different trz. Zn(2) and Zn(4) are tetrahedrally coordinated by two carboxylate oxygen atoms of two different OBA and two nitrogen atoms of two different trz. The Zn-O and Zn-N bond have average distances of 2.049 and 2.023 Å, respectively. The O/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 by considering 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 chain is similar to that observed in I. Unlike I, here the chains are connected to each other through a Zn(4)O2 tetrahedral unit via a triazolate to form the one-dimensional ladder type structure (Figure 3b). The one-dimensional ladders are connected to each other through the carboxylate group (bridging carboxylate) of OBA(2) units to give rise to the two-dimensional connectivity (Figure 3c). The OBA units are cross-linked in II to form the three-dimensional structure without any open channels (Figure 3d). The topological analysis of II (Figure 4a)17 reveals that the three trz act as the triconnected node. Out of the two OBA units, OBA(1) acts as the diconnected node and the OBA(2) acts as the triconnected node. Out of the four Zn2+ ions, Zn(1) and Zn(3) are connected with one OBA and three trz. On the other hand, Zn(2) and Zn(4) are connected with two OBA and two trz. As a result, all the four Zn act as the 4-connected node. The topological analysis were carried out using the TOPOS4.0 program (Supporting Information, Figure 17).17
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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 formation of a one-dimensional ladder-like unit. (c) The connectivity between the two ladders through the carboxylate groups to give rise to the two-dimensional layer in II. Color code: Zn - cyan, C-gray, O - red, N - blue. Note the bridging carboxylate groups. (d) Figure shows the connectivity between the layers through the OBA unit forming the three-dimensional structure.
The topology of the layer arrangement of II can be explained better by considering the metal centered node description (Figure 4b). Accordingly, three different types of nodes can be defined: (1) the dimeric unit of Zn(1) and Zn(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 and one Zn(4)O2N2 tetrahedra through three trz and act as the 3-connected node. Zn(2)O2N2 tetrahedra is connected with two dimeric units through two trz and with one Zn(4)O2N2 tetrahedra through the carboxylate group of OBA and act as a 3-connected node. The Zn(4)O2N2 tetrahedra, on other hand, is connected with two dimeric units [Zn(1), Zn(3)] through two trz and with two Zn(2)O2N2 tetrahedra through two carboxylate 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 network. The Schla¨fli symbol for the two-dimensional network is (62.84)(62.8)4. This type of structural arrangement has not been observed before in any of the earlier known binodal nets.18 Structure of [{Zn5(OH)2}{C12H8O(COO)2}3{C2H2N3}2] (III). The asymmetric unit of III consists of five crystallographically 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 modes with three Zn2+ ions as seen in I and II. The OBA anions show differences in their connectivity. In OBA(1), one carboxylate has bidentate (bridging) connectivity with two Zn2+ ions and the other one has tridentate (bridging) connectivity with two Zn2+ ions. OBA(2) and OBA(3) show similarities in the bonding with one of the carboxylate having monodentate connectivity, while the other has bidentate connectivity (bridging) with two Zn2+ ions (Supporting Information, Figure S19). Of the five Zn species, Zn(1) is octahedrally coordinated by four carboxylate oxygen atoms from three different OBA and two µ3-OH anions [O(2)]. Zn(2), Zn(3), Zn(4), and Zn(5) are all tetrahedrally coordinated. Zn(2) has two carboxylate oxygen atoms from two different OBA units and two nitrogen atoms from two different trz, Zn(3) and Zn(4) have one carboxylate oxygen atom, two nitrogen atoms from two different trz, and one µ3-(OH)- unit, and Zn(5) has three carboxylate oxygen atoms from three different OBA and one µ2-(OH)- unit. The Zn-O and Zn-N bond distances have average values of 2.014 Å and 2.0135 Å, respectively. The O/N-Zn-O/N bond angles are in the range 58.35(8)-177.14(9)°. The selected bond distances are listed in Table 3.
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Figure 4. (a) Figure shows the node based connectivity between the Zn2+ ions and the ligands (OBA, trz) in II. Note the cross-linking by the OBA. Color code: Zn - cyan, trz - purple, OBA green. (b) Figure shows the connectivity based on the three metal center nodes in II within the layer (see text).
The three-dimensional structure of III can be explained by considering connectivities between the Zn, the triazolate, and the hydroxyl anions, which form a two-dimensional layer. Thus, the Zn(3) and Zn(4) ions are connected by two trz to form dimetic 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 observed in I and II. Two Zn(1) ions are bonded together through two OH anions [O(2)] forming a different dimeric unit, Zn2O4(OH)2, which are bonded with Zn(5)O3(OH) tetrahedra through the carboxylate oxygens forming a tetramer unit, Zn4O12(OH)4 (Figure 5b). The linkages between the one-dimensional chains and the tetrameric units forms the two-dimensional layer structure (Figure 5c). The two-dimensional layers are pillared by the OBA units forming the three-dimensional structure (Figure 6a). The overall topology of the structure of III is not simple as the two-dimensional layers are more condensed, as they are formed by the connectivity between the Zn2+ ions, the triazolates, and the OH- anions. This framework can, then, be explained as a pillared layered structure. The two-dimensional zinc-triazolate layer can be described based on the metal center nodes. In this description, one can consider four different types of 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 connected with one Zn(5)O3(OH) [through µ2-OH group], two Zn(2)O2N2
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-dimensional structure has a zigzag arrangement. (b) Figure shows the connectivity between Zn(1)2+ and Zn(5)2+ by the (OH)- ions and the carbxylate group forming the tetrameric unit. (c) The connectivity between the one-dimensional chains and the tetramers forming the two-dimensional layer in III.
[through two triazolate] and a dimer(2) [through µ3-OH group] to act as a 4-connected node. Dimer(2) is connected with two dimer(1) [through two µ3-OH] and two Zn(5)O3(OH) [through carboxylate groups] to act as a 4-connected node. On the other hand, both the Zn(2) and Zn(5) act as the 3-connected nodes, where Zn(2)O2N2 is connected with two dimer(1) [through the triazolate] and one Zn(5)O3(OH) [through the carboxylate group] and the Zn(5)O3(OH) is connected with dimer(1) [through µ2OH], dimer(2) [through the carboxylate group] and Zn(2)O2N2 [through the carboxylate group]. The connectivity between the four nodes is shown in Figure 6b. The layer structure of III is also based on the binodal metal connectivity and has been observed for the first time. As mentioned before, the layers essentially comprise 4-, 5-, and 6- membered ring units and not open (more condensed). The Schlafli symbol for the twodimensional network is (4.5.6)4(4.52.72)2(42.52.72).
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Figure 7. Temperature vs time plot of the formation of the three compounds (I-III). The various phases were identified by comparing the powder XRD patterns with the simulated patterns generated from the single-crystal structure. Color code: I, white; II, gray; III, black. The approximate ratio of yields of the products where mixture of two phases 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).
Figure 6. (a) The three-dimensional structure of III. Note that the OBA units pillar the layers. (b) The view of the two-dimensional layer based on the connectivity of the metal centered nodes (see text).
Studies on the Variation of Time and Temperature. We have made a detailed study of the effect of time and temperature on the reaction mixture in the system Zn-triazole-oxybis(benzoic acid). The products of each experiment (synthesis) were subjected to powder XRD investigations, and the results are represented as a two-dimensional temperature vs time plot in Figure 7. In all the cases, we observed only the phases (pure or a mixture) that have been isolated in the present study (compounds I, II, and III) (Supporting Information, Figures S5-S9). This conclusion was arrived at by a careful study of the powder XRD patterns of the products of the reaction and comparing that with the simulated XRD patterns, of compounds I, II, and III (Figure 7). Generally, the formation of the more hydrated phase (I) was observed at lower temperatures (100-125 °C) followed by the other phases (II and III). Interestingly, the pure phases of II and III were obtained at 150 °C (4-5 days) and at 220 °C, respectively. This observation suggests that the system undergoes dehydration at elevated temperatures - the total water content in the system decreases with an increase of time and
temperature of the reaction. This indicates that the formation of some of the phases is thermodynamically controlled. Similar observations have also been made in the literature in metal carboxylate systems.7b,9a In this context, it may be of some importance to note that the formation different phases have been observed during the study of the variation of reactions parameters in amine templated zinc phosphite phases.19 It was shown that five phases of different dimensionality and structure could be prepared from an essentially three-component reaction mixture - zinc salt, phosphoric acid, and 1,4-bis (3-aminopropyl) piperazine (APPIP). Studies of this nature have also been reported earlier during the formation of a family of uranium fluorides in the presence of 2-methyl piperazine and piperazine.20 The time and temperature variation studies along with a careful scrutiny of the structures indicate that the bonding around the Zn2+ ion gradually shows subtle differences (Table 1 and Figure 7). The triazolate ligands have the same coordination mode in all three structures. The connectivity of the OBA ligands, however, shows notable differences. In I, each carboxylate group bonds with only one Zn2+ ions and no bridging modes were observed (Zn2+/COO- ) 1). In II, the carboxylate groups shows bridging modes as well as the monodentate mode in the bonding with Zn2+ (Zn2+/COO- ) 1.25). In III, the bridging mode of the carboxylate group (Zn2+/COO- ) 1.5) increases and we find four out of the six carboxylate groups bridges the Zn2+ centers. This increase in the bridging mode also brings about an increase in the number of Zn atoms per 1000 Å3 [Zn2+/103 Å3 ) 3.39 (I), 3.71(II), and 4.2(III)]. The increase in the bridging mode of the carboxylate also reflects on the structural arrangement as well. Thus, in I the OBA ligands show end-to-end bonding with the Zn2+ ions, which gives rise to a larger separation between the four connected nodes, which results in the interpenetration of the diamondoid network in I (see Figure 1c). In II, the bridging carbxylate group gives rise to a moderately condensed layer based on the Zntriazolate-carboxylate connectivity and also the cross bonding of the OBA gives rise to a more closed three-dimensional
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structure. In III, the increased bridging modes of the carboxylate group with Zn2+ ions give rise to a condensed layer. The formation of a condensed layer with hydroxyl bridging at elevated temperatures has been observed by us as part of the temperature- and time-dependent studies on the manganese oxybis(benzoate) systems.7e,9a Similar observations have also been made by Cheetham et al. as well.7b Transformation Studies. It may be noted that the onedimensional chain structure formed between Zn and triazolate is common in all three structures. The connectivity between 1D chains by OBA and triazolate ligands brings about the observed differences among the three structures. Thus, one can infer that structurally all three compounds are related. If the formation of these phases have thermodynamic control, then it may be possible to convert at least one of the structures, viz I, into other structures. To this end, we have investigated the possible transformation of the most hydrated phase (I) to the lesser ones (II and III). For this study, I (0.092 g, 0.25 mM) was heated with water (2 mL) in a 7 mL autoclave at 150, 180, and 200 °C for 24 h. The resulting products of the experiments were analyzed using powder XRD studies (see Supporting Information, Figure S20). The powder XRD indicated that at 150 °C a new compound (IV) is formed along with I. A mixture of the three compounds I, III, and IV were formed at 180 °C, and at 200 °C, we observed only the pure phase of III. The new phase (IV) was identified using the single crystal X-ray diffraction study. The transformation studies also indicate that I is stable up to 180 °C, above which it transforms to III. The structure of [Zn(OBA)(H2O)] (IV) has a simple two-dimensional connectivity between the Zn2+ ions and OBA units without any triazolate units and has been reported earlier (see Supporting Information, Figure S21).21 From the transformation study, it is clear that the formation of these phases is through a dissolution and recystalization mechanism. The formation of IV at 150 and 180 °C shows that the triazolate is easily removed from the system and again needed for the formation of III. It is quite clear that compound III is, probably, the stable phase in the three-component systems involving Zn/triazolate/OBA. During the transformation studies, we never observed the formation of II. This suggests that II can form only on prolonged heating. It may be noted that II was prepared as a pure phase only after heating for 4 days at 150 °C. Transformation studies of this nature have also been carried out in amine templated zinc phosphate and zinc arsenate family of compounds.22 It has been established that the zerodimensional molecular complexes of zinc phosphate/zinc arsenate can be transformed to structures of higher dimensionality by a suitable modification of the experimental conditions. Similarly, low-dimensional (one- and two-) zinc phosphate structures have also been shown to transform under appropriate conditions.23,24 Transformation studies of this nature have been carried out in the amine templated open-framework structures, but such studies are not many in the MOF compounds. In this sense, the transformation studies described above are important as they provide important clues for our understanding of the formation of MOF compounds. Luminescence Properties. Room temperature solid-state photoluminescence studies were carried out on powdered samples (Perkin-Elmer, UK) (Figure 8). All the three compounds (I-III), the sodium salt of OBA and the triazolate exhibits photoluminescence. Sodium salt of OBA and the triazolate shows emissions at 320 nm and 380 nm, respectively, when excited using a radiation of 250 nm. For all the compounds,
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Figure 8. Emission spectra: (a) Na salt of OBA, (b) Na salt of triazolate, (c) I, (d) II, and (e) III.
we observed a peak at 320 nm and a shoulder at 375 nm. The observation of a similar position and shape of the emission peaks of the three compounds with respect to the emission peaks of the two ligands indicates that the emission of the three compounds may be due to the intraligand electronic transitions. The observed transitions could be due to the π* f n or π* f π transitions of the two ligands (OBA and triazolate). The emission at ∼320 nm can be assigned due to the intraligand emission from the OBA ligand and emission at ∼375 nm range can be assigned due to the intraligand emission from the triazolate ligand. The small shift observed in the bands among the three compounds could be due to the slight coordination differences of Zn2+ ions.25 The intraligand emission intensity of the three compounds appear to be higher than the ligands, which may be attributed to the chelation of the ligands to the metal center. The bonding of the ligands with the metal increases the rigidity of the ligands along with a reduction in the loss of energy due to possible decay through other radiationless processes.26 Conclusions The synthesis and structure characterization of three different MOFs have been accomplished. To the best of our knowledge, this is the first study illustrating the role time and temperature in a three-component system, which is much more complex than the two-component system investigated in the literature.7,9 Formation of a three-dimensional MOF with interpenetrated diamondiod topology at low temperature and two noninterpenetrated pillared layer structures at higher temperature and time is noteworthy. The present study suggests that the variation of solvent and pH would exercise subtle effects on the formation of related structures. We are currently pursuing this theme. Acknowledgment. S.N. thanks the Department of Science and Technology (DST), Government of India, for the award of a research grant, and the authors thank the Council of Scientific and Industrial Research (CSIR), Government of India, for the award of a fellowship (PM) and a research grant. S.N. also thanks the Department of Science and Technology, Government of India, for the award of the RAMANNA fellowship.
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Supporting Information Available: X-ray crystallographic files in CIF format, image of single crystals, bond angle table, IR, TGA, PXRD, and addition figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG900397R
