LiquidâLiquid Interphase (Biphasic) - ACS Publications - American

DOI: 10.1021/cg101251a

Liquid-Liquid Interphase (Biphasic) as the Reaction Medium in the Assembly of a Hierarchy of Structures of 4,40 -Azodibenzoic Acid with Zinc and Cadmium

2011, Vol. 11 735–747

Saurav Bhattacharya,† Udishnu Sanyal,‡ and Srinivasan Natarajan*,† †

Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India, and ‡Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India Received September 23, 2010; Revised Manuscript Received December 1, 2010

ABSTRACT: The compounds [Zn(C12H8N2)]2[C12N2H8(COO)2]2 3 (C6H12O) 3 (H2O), I, [Zn(C12H8N2)][C12N2H8(COO)2], II, [Cd(C12H8N2)(H2O)][C12N2H8(COO)2] 3 (H2O), III, [Zn(C10N2H8)][C12N2H8(COO)2] 3 0.5(C10N2H8), IV, [Cd(C12N2H8(COO)2 3 H2O], V, and [Zn3(μ2-O)(μ3-O)3][C12N2H8(COO)2], VI, have been synthesized by using a biphasic approach (I, III, V, VI) or regular hydrothermal method (II, IV). The compounds exhibit one (I and II), two (III), and three dimensionally (IV, V, VI) extended structures. The flexible azodibenzoate ligand gives rise to a 3-fold interpenetration (IV) when the synthesis was carried out using normal hydrothermal methods. The biphasic approach forms structures without any interpenetrations, especially in the three-dimensional structures of V and VI. Formation of Cd2O2 dimers in V and extended M-O(H)-M two-dimensional layers in VI suggests the subtle structural control achieved by the biphasic method. Transformation studies indicate that it is possible to transform I to II. Lewis acid catalytic studies have been performed to evaluate the role of the coordination environment in such reactions. All the compounds have been characterized by a variety of techniques that includes powder X-ray diffraction, infrared, thermogravitric analysis, UV-vis, photoluminescence studies.

Introduction Inorganic-organic hybrid compounds based on metal carboxylates, popularly known as metal organic frameworks (MOFs) or inorganic coordination polymers, constitute an important area of research. The wide diversity in the structures along with the possibility of using them in important applications in the areas of sorption, separation, and catalysis are the driving forces for the continued interest in the study of these solids.1 Of the many compounds that are known in the family of MOFs, those based on aromatic carboxylates are the dominant ones.2,3 It has been shown that heterocyclic carboxylic acids also lead to interesting structures and properties.4-10 Recently, multifunctional organic ligands based on imidazole and related derivatives have been employed for the preparation of hybrid compounds with great success.10d-f The synthesis of metal-organic compounds has been carried out employing many different approaches. One such method is the biphasic solvothermal method. This method is advantageous as the metal salt is taken in the aqueous medium and the organic acid is taken in an organic solvent, which is generally immiscible with water such as cyclohexanol. The reaction is expected to progress at the interface between the two liquids. This method has been exploited for the synthesis of hybrid compounds,10g nanoparticles, nanocrystals, etc.10h The biphasic synthesis method appears to be advantageous for metals with low reduction potentials such as Cu2þ ions,10i and for organic acids that might be unstable under hydrothermal conditions. Modifications in the biphasic method have been employed profitably by Yaghi and co-workers to prepare a variety of interesting metal-organic compounds.3 This approach, though offering many advantages, did not receive the attention of the synthetic chemists. We have been *Corresponding author. E-mail: [email protected]. r 2011 American Chemical Society

interested in the use of newer techniques for the preparation of hybrid compounds. To this end, we have employed the biphasic mixture method for the synthesis of metal-organic compounds based on zinc and cadmium. We were also intrigued at the possibility of using 4,40 -azodibenzoic acid (ABA) as a linker between the metal centers. The use of ABA in the formation of MOFs has resulted in limited success only.11 It is likely that the ABA is not very stable under the synthesis conditions. We sought to use the biphasic conditions by taking ABA in cyclohexanol and performing the reaction under biphasic solvothermal conditions. During the course of this investigation, we have now prepared six compounds exhibiting a hierarchy of structures. The compounds [Zn(C12H8N2)]2[C12N2H8(COO)2]2 3 (C6H12O) 3 (H2O), I, and [Zn(C12H8N2)][C12N2H8(COO)2], II, have onedimensional (1D) structures; [Cd(C12H8N2)(H2O)][C12N2H8(COO)2] 3 (H2O), III, has a two-dimensional (2D) structure and [Zn(C10N2H8)][C12N2H8(COO)2] 3 0.5(C10N2H8), IV, [Cd(C12N2H8(COO)2.H2O], V, and [Zn3(μ2-O)(μ3-O)3][C12N2H8(COO)2], VI, have three-dimensional (3D) structures. In this paper, we present the synthesis, structure, and characterization of the hybrid compounds isolated using ABA. Experimental Section Synthesis and Initial Characterization. The ABA required for the syntheses was prepared employing well-established procedures.11i The ABA was purified and characterized before using it for the preparation of the compounds presented here. Most of the compounds were prepared employing biphasic solvothermal methods. The synthesis conditions employed during the present study are presented in Table 1. In a typical synthesis, for I, Zn(CH3COO)2 3 2H2O (0.22 g, 1 mmol) was dissolved in H2O (5 mL), and ABA (0.27 g, 1 mmol) and 1,10-phenanthroline (0.2 g, 1 mmol) were dissolved in cyclohexanol (5 mL) and layered above the aqueous solution. The reaction mixture was placed in a PTFE vessel and sealed in a Published on Web 01/19/2011

pubs.acs.org/crystal

CHN analysis for I: Calc(%) C 61.70%; H 3.90%; 9.93%; found: C 61.8%; H 3.7%; 10.1%; for II: Calc(%) C 60.72; H 3.11; N 10.89; found: C 61.13; H3.18; N 10.76; for III: Calc(%) C 50.9; H 2.9; N 9.13; found: C 51.3; H 3.2; N 9.5; for IV: Calc(%)C 61.28; H 3.52; N 12.32; found: C 60.98; H 3.44; N 12.30; for V: Calc(%) C 42.14; H 2.50; N 7.02; found: C 41.86; H 2.44; N 7.03; for VI: Calc(%)C 34.8; H 2.17; N 5.8; found: C 35.0; H 2.15; N 6.1. b Yields are calculated based on the respective metals. Compositions given are molar compositions. c To prepare a pure phase of VI, a modified composition was needed.

70 72 200 3.0 Zn(CH3COO)2 3 2H2O þ 4,40 -azodibenzoic acid þ pyrazine þ 42.0 cyclohexanol þ 278.0 H2O 7

a

c

75 70 65 80 60

[Zn(C12H8N2)]2[C12N2H8(COO)2]2 3 (C6H12O) 3 (H2O), I [Zn(C12H8N2)][C12N2H8(COO)2], II [Cd(C12H8N2)(H2O)][C12N2H8(COO)2] 3 (H2O), III [Zn(C10N2H8)][C12N2H8(COO)2] 3 0.5(C10N2H8), IV [Cd(C12N2H8(COO)2 3 H2O], V [Zn3(μ2-O)(μ3-O)3][C12N2H8(COO)2], VI þ unidentified phase [Figure S23(b), SI] [Zn3(μ2-O)(μ3-OH)3][C12N2H8(COO)2], VI 72 72 72 72 72 72 150 150 150 150 150 200 Zn(CH3COO)2 3 2H2O þ 4,4 -azodibenzoic acid þ 1,10-phenanthroline þ 42.0 cyclohexanol þ 278.0 H2O Zn(CH3COO)2 3 2H2O þ 4,40 -azodibenzoic acid þ 1,10-phenanthroline þ 3.0 NaOH þ 278.0 H2O Cd(CH3COO)2 3 2H2O þ 4,40 -azodibenzoic acid þ 1,10-phenanthroline þ 42.0 cyclohexanol þ 278.0 H2O Zn(CH3COO)2 3 2H2O þ 4,40 -azodibenzoic acid þ 4,40 -bipyridine þ 3.0 NaOH þ 278.0 H2O Cd(CH3COO)2 3 2H2O þ 4,40 -azodibenzoic acid þ pyrazine þ 42.0 cyclohexanol þ 278.0 H2O 1.0 Zn(CH3COO)2 3 2H2O þ 4,40 -azodibenzoic acid þ pyrazine þ 42.0 cyclohexanol þ 278.0 H2O 1 2 3 4 5 6

S. no.

0

composition

temp (°C) time (h)

product

yield (%)b

Crystal Growth & Design, Vol. 11, No. 3, 2011

Table 1. Synthesis Composition and Conditions Employed for the Preparation of Compounds I-VIa

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Bhattacharya et al. stainless steel autoclave, heated at 150 °C for 3 days, and cooled to room temperature in air. The final product contained large quantities of red crystals, which were vacuum filtered, washed with deionized water, and dried at ambient conditions. All the compounds were characterized by powder X-ray diffraction (PXRD), IR, UV-vis, and thermogravimetric analysis (TGA). The PXRD patterns were recorded in the 2θ range 5-50° using Cu-KR radiation (Philips, X’pert-Pro). The observed PXRD patterns were new and the patterns were entirely consistent with the simulated XRD patterns generated based on the structures determined using the single crystal XRD (Supporting Information, Figure S1). Infrared (IR) spectroscopic studies have been carried out in the mid-IR region (4000 to 400 cm-1) on KBr pellets (PerkinElmer, SPECTRUM 1000). IR spectra for all the compounds gave sharp bands. The observed bands are assigned to the respective vibration12 (Supporting Information, Table S1; Figure S2). The solid state UV-vis spectroscopic studies were carried out at room temperature (Perkin-Elmer model Lambda 35). The observed reflectance spectra were converted into a absorption-like spectra using the Kubelka-Munk function (Supporting Information, Figure S3). The diffuse reflectance UV-vis spectra at room temperature were recorded for all the prepared compounds, I-VI. The observed optical spectra of the compounds were also compared with the sodium salt of azodibenzoic acid, 1,10-phenanthroline, and 4,40 bipyridine molecules to ascertain the various possible optical transitions in the compound (Supporting Information, Figure S3). The higher dimensional compounds (III-VI) exhibit a main peak centered in the region 480-497 nm. This suggests that the observed main peak may be due to the electronic transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the ABA. Similar values have been observed earlier for compounds prepared with the azo group.13 The additional observed bands at 380 and 333 nm (I and II), 390 nm (III, V, and VI), 390 and 319 nm (IV), and 409 nm (Nasalt of ABA) may be due to the intraligand transition. Room temperature solid state photoluminescence studies were carried out on powdered samples (Perkin-Elmer, L.S.55) (Supporting Information, Figures S4 and S5). The photoluminescence (PL) spectra of 1,10-phenanthroline, 4,40 -bipyridine were recorded using the excitation wavelengths of 260 nm and 295 nm, respectively. The PL spectra of the Na-salt of ABA were recorded using excitation wavelengths of 300 and 390 nm. The PL spectra of the compounds were recorded using an excitation wavelength of 390 nm. The excitation and emission intensities were controlled by using a slitwidth of 10 nm. The photoluminescence spectra of the ligands exhibited distinct emission features (Supporting Information, Figure S4). Thus, for 1,10-phenanthroline, we observed three peaks centered around 364, 382, and 404 nm, which correspond to σ*- n, π*- π, and π*- n transitions, respectively. The Na-ABA ligand exhibited only two emissions centered around 533 and 598 nm, when excited using a wavelength of 390 nm. The emission bands are due to π*- π and π*-n transitions of the acid. Similar emissions have been observed earlier.14 In all the compounds, we observed a blue shift in the emission peaks observed for the Na-ABA (Supporting Information, Figure S5). The observed emission bands may be due to the intraligand charge transfer. TGA was carried out (Metler-Toledo) in an oxygen atmosphere (flow rate = 50 mL/min) in the temperature range of 30-800 °C (heating rate = 10 °C/min) (Supporting Information, Figure S6). The results of TGA analysis are summarized in Table S2, Supporting Information. Single-Crystal Structure Determination. A suitable single crystal of each compound was carefully selected under a polarizing microscope and glued to a thin glass fiber with a cyanoacrylate (superglue) adhesive. 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 A˚) radiation. Data were collected with ω scan width of 0.3°. A total of 606 frames were collected at three different settings of j (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,15 and an empirical absorption correction was applied using the SADABS program.16 The structure was solved and

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Table 2. Crystal Data and Structure Refinement Parameters for Compounds I-VIa empirical formula formula weight crystal system space group a (A˚) b (A˚) c (A˚) R (°) β (°) γ (°) volume (A˚3) Z temperature (K) Fcalcd (g cm-3) μ (mm-1) wavelength (A˚) θ range (deg) R index [I > 2σ(I)] R (all data)

I

II

III

IV

V

VI

C58H44N8O10Zn2 1143.75 monoclinic C2/c (No. 15) 29.084(5) 18.553(5) 20.463(5) 90 107.775(5) 90 10515(4) 8 100(2) 1.445 0.981 0.71073 1.47-26.00 R1 = 0.0534, wR2 = 0.1476 R1 = 0.0794, wR2 = 0.1572

C26H16 N4O4Zn 513.80 monoclinic P21/n (No. 14) 11.715(2) 16.470(3) 12.837(3) 90 107.714(3) 90 2359.5(8) 4 293(2) 1.446 1.081 0.71073 2.06-28.05 R1 = 0.0445, wR2 = 0.0981 R1 = 0.0771, wR2 = 0.1112

C26H16N4O6Cd 592.84 monoclinic P21/n (No. 14) 16.722(5) 6.451(5) 25.122(5) 90 104.131(5) 90 2628(2) 4 100(2) 1.498 0.877 0.71073 3.34-25.99 R1 = 0.0559 wR2 = 0.1495 R1 = 0.0650 wR2 = 0.1554

C29H20 N5O4Zn 567.87 monoclinic C2/c (No. 15) 24.175(6) 11.436(3) 18.386(5) 90 101.507(4) 90 4981(2) 8 293(2) 1.515 1.033 0.71073 1.72-28.00 R1 = 0.0575, wR2 = 0.1018 R1 = 0.0845, wR2 = 0.1107

C14H8N2O5Cd 398.64 monoclinic C2/c (No. 15) 14.8013(13) 6.3249(5) 28.897(2) 90 93.569(1) 90 2700.0(4) 8 293(2) 1.961 1.644 0.71073 2.76-26.36 R1 = 0.0561, wR2 = 0.1385 R1 = 0.0629, wR2 = 0.1439

C28H21N4O14Zn5 964.44 monoclinic C2/c (No. 15) 33.9988(10) 5.6638(1) 16.5382(5) 90 101.437(3) 90 3121.40(14) 4 293(2) 2.052 3.866 0.71073 2.44-26.00 R1 = 0.0376 wR2 = 0.1047 R1 = 0.0570 wR2 = 0.1083

)

)

a R1 = Σ Fo| - |Fc /Σ|Fo|; wR2 = {Σ[w(Fo2 - Fc2)]/Σ[w(Fo2)2]}1/2. w = 1/[F2(Fo)2 þ (aP)2 þ bP]. P = [max(Fo, O) þ 2(Fc)2]/3; for I, a = 0.0823 and b = 19.5616; for II, a = 0.0524 and b = 0.4233; for III a = 0.0843 and b = 14.7846; for IV, a = 0.0368 and b = 4.3890; for V, a = 0.0762 and b = 27.9419 and for VI, a = 0.0574 and b = 0.2911.

refined using SHELXL9717 present in the WinGx suit of programs (version 1.63.04a).18 The hydrogen positions of the lattice water molecule in I could not be located though the hydrogens of the cyclohexanol guest molecule could be located. The hydrogen positions for the coordinated water as well as the solvent water molecules in III could not be located. The hydrogen positions of the coordinated water molecule in V and μ3-oxygens in VI were placed in geometrically ideal positions along with all the organic molecules and held in the riding mode. Full-matrix least-squares refinement against |F2| was carried out using the WinGx package of programs.18 The 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. Details of the structure solution I-VI and final refinements for all the structures are given in Table 2. The crystallographic data for the compounds have been deposited at the Cambridge Crystallographic Data Center (CCDC) and can be obtained free of charge via www.ccdc.cam.ac.uk. The CCDC numbers for the compounds are 792130 for I, 792131 for II, 792132 for III, 792133 for IV, 792134 for V, and 792135 for VI.

Results and Discussion Structure of [Zn(C12H8N2)]2[C12N2H8(COO)2]2 3 (C6H12O) 3 (H2O), I. The asymmetric unit of I contains 78 non-hydrogen atoms. There are two crytallographically independent Zn2þ ions, two azodibenzoate and two 1,10-phenanthroline molecules, one cyclohexanol and one water molecule. Of the two Zn2þ ions, Zn(2) has a distorted trigonal bipyramidal geometry with three oxygens and two nitrogens, and Zn(1) has a distorted octahedral geometry with four oxygens and two nitrogens (Supporting Information, Figure S7). The Zn-O/N bonds have distances in the range 1.969(3)-2.326(3) A˚ (av. 2.148 A˚) and the O/N-Zn-O/N bond angles are in the range of 60.23(10)-165.53(11)° (Table 3; Supporting Information, Table S3). The structure of I is formed by the connectivity between [ZnO3N2] and [ZnO4N2] polyhedral units and the carboxylates to give rise to a 1D structure (Figure 1a). The two acids exhibit differences in the connectivity with the Zn2þ ions. While acid-1 has a bidentate connectivity through both the carboxylate groups, acid-2 has a bidentate and a monodentate connectivity with the Zn2þ ions [Supporting Information, Figure S7(c)]. The 1,10-phenanthroline molecules are

Table 3. Selected Bond Distances Observed in Compounds I-VIa distance (A˚)

bond

Compound I 2.257(3) Zn(2)-O(5) 2.071(3) Zn(2)-O(6) 2.319(3) Zn(2)-O(7) 2.109(3) Zn(2)-N(3) 2.109(3) Zn(2)-N(4) 2.039(3)

Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(3) Zn(1)-N(1) Zn(1)-N(2) Zn(1)-O(4) Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(3)

bond

Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(3)

Compound II Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(3)

Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(3)

Compound III 2.297(3) Cd(1)-O(4) 2.364(3) Cd(1)-N(1) 2.336(4) Cd(1)-N(2)

Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(3)

Compound IV 2.006(2) Zn(1)-N(1) 2.001(2) Zn(1)-N(2) 1.999(2)

Cd(1)-O(1) Cd(1)-O(2)#1 Cd(1)-O(3)

Compound V 2.323(5) Cd(1)-O(4) 2.172(5) Cd(1)-O(4)#2 2.337(5) Cd(1)-O(5)

Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(3) Zn(1)-O(3)#1 Zn(1)-O(4) Zn(2)-O(4) Zn(2)-O(4)#2 Zn(2)-O(5)

Compound VI 1.977(3) Zn(2)-O(5)#2 2.0168(5)Zn(2)-O(6) 2.012(3) Zn(3)-O(3) 2.084(3) Zn(3)-O(4)#3 2.086(3) Zn(3)-O(6) 1.964(3) Zn(3)-O(7)#4 1.964(3) Zn(3)-O(8) 2.199(3)

distance (A˚)

1.969(3) 2.326(3) 2.011(2) 2.090(3) 2.101(3)

Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(3) 2.299(4) 2.369(4) 2.373(5) 2.165(3) 2.208(3)

2.396(4) 2.426(5) 2.255(5) 2.199(3) 1.961(4) 1.994(3) 1.995(3) 2.0437(19) 2.082(3) 2.190(3)

a Symmetry transformations used to generate equivalent atoms: V: #1: -x, -y þ 2, -z þ 1, #2: -x, y, -z þ 1/2, #3: x, y þ 1, z, #4: -x, y þ 1, -z þ 1/2. VI: #1: -x þ 1, -y, -z þ 1, #2: -x þ 3/2, -y þ 1/2, -z þ 1/2.

bound to the Zn2þ ions and act as a terminal ligand. The two ABA molecules (acid-1 and acid-2) alternate in their bonding with the Zn2þ ions and give rise to 1D Zn-ABA chains which lie along the 2-fold axis. Because of this arrangement the

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Figure 1. (a) View of the arrangement of the 1D chains in I. Dashed lines show the possible π 3 3 3 π interactions (see text). (b) Space filled mode of two 1D chains in I. Note the formation of a helix-like arrangement.

chains are twisted around each other resembling a helical structure, which appears to maximize the favorable π 3 3 3 π interactions between the protruding 1,10-phenanthroline molecules (Figure 1b). Structure of [Zn(C12N2H8)][C12N2H8(COO)2], II. The asymmetric unit of II contains 35 non-hydrogen atoms of which one Zn2þ ion is crystallographically independent. The

asymmetric unit also has two different azodibenzoate units and one 1,10-phenanthroline ligand. The azodibenzoate molecules are related by a center of symmetry along the NdN bond. The Zn2þ ion has distorted trigonal bipyramdal geometry with three carboxylate oxygens and two nitrogen atoms from the phenanthroline ligand (Supporting Information, Figure S8). The Zn-O/N bonds have distances in the range

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Figure 2. (a) View of the 1D chains in II. (b) The arrangement of the 1D chains in II. Dashed lines show the possible π 3 3 3 π interactions (see text).

of 1.947(2)-2.246(2) A˚ (av. 2.097 A˚), and the O/N-ZnO/N bond angles are in the range of 59.82(8-149.63(9)° (Table 3; Supporting Information, Table S3). The structure of II has [ZnO3N2] trigonal bipyramidal units connected by the carboxylate units forming the 1D structure (Figure 2a). The two carboxylic acids in II exhibit differences in the bonding with the Zn2þ ions. Thus, acid-1 has both the carboxylate groups exhibiting a bidentate connectivity with the Zn2þ ions and acid-2 has both the carboxylate groups having a monodentate connectivity with with the Zn2þ ions [Supporting Information, Figure S8(c)]. As expected, the 1,10-phenanthroline molecules are bound to the Zn2þ ions and are terminal. The two different ABA molecules strictly alternate in the bonding with the Zn2þ ions to form a 1D zigzag chain (Figure 2a). The chains are arranged in such a way that the π 3 3 3 π interactions between the terminal phenanthroline molecules are maximized (Figure 2b). Structure of [Cd(C12H8N2)(H2O)][C12N2H8(COO)2] 3 (H2O), III. The asymmetric unit of III has 37 non-hydrogen atoms which contains one crystallographically independent Cd2þ ion, one azodibenzoate, one 1,10-phenanthroline molecule, one coordinated water molecule, and one lattice water molecule. The Cd2þ ion has a distorted octahedral geometry with three carboxylate oxygens, two nitrogens from 1,10-phenanthroline, and a terminal coordinated water molecule (Supporting Information, Figure S9). The Cd-O/N bond distances are in the range 2.297(3)-2.373(5) (av. 2.335 A˚) and the O/N-Cd-O/ N bond angles are in the range 70.25(16)-168.22(14) (Table 3; Supporting Information, Table S3). The structure of III has [CdO4N2] octahedral units connected by the azodibenzoate moiety to form a 2D layered structure. The carboxylic acid group of the ABA exhibits differences in the bonding with the Cd2þ ions. While one

Figure 3. (a) View of the layer in III. Note that the ABA molecules criss-cross connecting the Cd centers forming the bilayer like structure. Note that the Cd centers are arranged along the 21 axis (see text). (b) The bilayers and the layer arrangement. Note that the 1,10-phenanthroline molecules occupy the interlayer spaces.

carboxylate unit of the ABA binds with one Cd2þ ion in a monodentate mode, the other carboxylate group binds with two Cd2þ ions in a bis-monodentate mode [Supporting Information, Figure S9(c)]. The Cd2þ ions are positioned above and below a plane to give a helical arrangement due to the presence of the 21 axis (Figure 3a). The ABA units crisscross and connect with the Cd centers forming the 2D layer (Figure 3a). This connectivity, in fact, gives rise to a unique double layer arrangement for the layers. The layers are arranged in ABCABC... fashion. The 1,10-phenanthroline molecules, bound to the Cd centers, hang in the interlayer spaces (Figure 3b). Structure of [Zn(C10N2H8)][C12N2H8(COO)2] 3 0.5(C10N2H8), IV. The asymmetric unit of IV contains 39 non-hydrogen atoms. There is one crystallographically independent Zn2þ ion, two azodibenzoate anion (the two azodibenzoate molecules are related by center of symmetry along the NdN bond), one bound 4,40 -bipyridine molecule, and a free bipyridine molecule, which is related by the center of symmetry along the C-C bond, constituting the asymmetric unit. The Zn atom has a distorted trigonal bipyramidal geometry, with three carboxylate oxygen atoms and two nitrogen-atoms of the 4,40 -bipyridine (Supporting Information, Figure S10). The Zn-O/N bond distances are in the range of 1.999(2)-2.208(3) A˚ (av. 2.104 A˚), and the O/ N-Zn-O/N bond angles are in the range of 84.86(9)177.91(10) (Table 3; Supporting Information, Table S3). The 3D structure of IV consists of a network between [ZnO3N2] trigonal bipyramidal units, the azodibenzoate and

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Figure 4. (a) View of the layers in IV. Note the alternate connection between the eight-membered rings by the ABA units (see text). This connectivity gives rise to interpenetration in IV. (b) The connectivity between the eight-membered rings and the 4,40 -bipyridine ligands. (c) View of the 3D structure of IV. Note the presence of free 4,40 -bipyridine units in the channels. (d) The 3-fold interpenetration observed in IV.

4,40 -bipyridine moieties. The azodibenzoate molecules exhibit differences in the connectivity with the Zn2þ ion. Both the carboxylate groups of acid-1 are bound to two Zn-centers via a bis-monodentate coordination, while the carboxylate groups of acid-2 are bound to Zn-center via a monodentate coordination [Supporting Information, Figure S10(c)]. The Zn2þ ions are connected through the carboxylate units to form an eight-membered ring, which is the secondary building unit (SBU). The connectivity between the SBU and the ABA is such that alternate units are connected to form layers that interpenetrate (Figure 4a). The SBU units are also bonded with the 4,40 -bipyridine units to form a 1D ladderlike arrangement (Figure 4b). The interconnectivity between these units gives rise to a 3D structure with extra large voids (16.7 11.9 A˚). The free 4,40 -bipyridine molecules occupy the channels (Figure 4c). The structure of IV exhibits a 3-fold interpenetration (Figure 4d). The presence of both the bound as well as the free bipyridine was also independently confirmed by IR spectroscopic studies (Supporting Information, Table S1, Figure S2). Thus, the CdN stretching frequency appearing at ∼1590 cm-1 can be assigned for the free bipyridine and at ∼1620 cm-1 for the bound bipyridine. The characteristic “ring breathing” mode of the substituted pyridine occurs at ∼1009 cm-1 for the bound bipyridine and at ∼985 cm-1 for the free bipyridine. Also, the CdC stretching frequency occurred at ∼1533 cm-1 for the free bipyridine and at ∼1550 cm-1 for the bound bipyridine. These vibrations clearly suggest that there are two types of bipyridine units in IV. Structure of [Cd(C12N2H8(COO)2 3 H2O], V. The asymmetric unit of V contains 22 non-hydrogen atoms consisting of one crystallographically independent Cd2þ ion, one azodibenzoate ion, and one coordinated water molecule. The Cd ion has a distorted octahedral environment [CdO6] with five

carboxylate oxygen, two of which have μ3 connectivity [O(4)], and one coordinated water molecule (Supporting Information, Figure S11). The Cd-O bond distances are in the range of 2.172(5)-2.427(5) A˚ (av. 2.300 A˚), and the O-Cd-O bond angles are in the range of 84.75(19)-170.35(17)o (Table 3; Supporting Information, Table S3). The structure of V consists of a network between the CdO6 octahedral units and the carboxylate, ABA. The carboxylic acid group of the ABA exhibits differences in the bonding with the Cd2þ ions. Both the carboxylate units bind with two Cd2þ ions, but one of the oxygens of the carboxylate unit connects with two Cd centers [Supporting Information, Figure S11(c)]. The Cd2þ ions are bound together through the μ3-O, [O(4)], forming a simple dimer unit. The Cd centers also bond with the carboxylate units forming the eight-membered rings. The Cd2O2 dimers and the eight-membered rings alternate and connect together forming 1D chains (Figure 5a). The 1D chains are arranged angularly with an average angle of 23° (Figure 5b). The chain units are connected together by the ABA moiety to form a 2D layer (Figure 5c). The layers are further connected by another ABA moiety to give rise to the 3D structure (Figure 5d). The coordinated water molecule [O(5)] in V forms O(5)-H 3 3 3 O(3) hydrogen bonds, with an O-O distance of 2.754(7) A˚ and O-H 3 3 3 O angle of 165(12)o. Structure of [Zn3(μ2-O)(μ3-OH)3][C12N2H8(COO)2], VI. The asymmetric unit of VI contains 27 non-hydrogen atoms of which three Zn atoms are crystallographically independent. All the three Zn atoms exhibit 5-coordination with respect to the nearest neighbor oxygen atoms. Zn(1) and Zn(2) exhibit a distorted trigonal bipyramidal geometry, while Zn(3) has a distorted square pyramidal geometry. The three Zn atoms are connected through three μ3-oxygens [O(3), O(4), and O(6)] and one μ2-oxygen [O(2)]. The oxygen atom, O(2), occupies a special position (4b) with a site multiplicity

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Figure 5. (a) View of the 1D chains formed by the connectivity between Cd2O2 dimers and the eight-membered rings. (b) View of the arrangement of the 1D chains in V. Note that the two chains are not parallel (see text). (c) The connectivity between the chains and the ABA molecules. (d) The 3D structure of V in the ac plane.

of 0.5. The Zn-O bond distances are in the range 1.961(4)-2.199(3) [av. 2.032 A˚ for Zn(1), 2.08 A˚ for Zn(2), and 2.092 A˚ for Zn(3)]. Selected bond distances are listed in Table 3. Bond valence sum calculations show that the three μ3-oxygens, O(3), O(4), and O(6), are actually hydroxyl groups (-OH) (Supporting Information, Table S4). The structure of VI consists of a linkage between the Zn2þ ions and the ABA units forming a 3D structure. The Zn2þ ions are connected through μ2-O, μ3-OH, and the carboxylate oxygen vertices to form an infinite Zn-O-Zn 2D layers (Figure 6a). The Zn2þ ions are bonded together to form a fully connected 36 net (Figure 6b). Compounds based on fully connected 36 net are not many in the literature and the observation of such a network in V is noteworthy. The layers are connected through the ABA units forming the 3D structure (Figure 6c). This arrangement resembles the pillared layer structures reported earlier in the literature.19 Transformation Reactions The 1D compounds I and II appear to be closely related, which prompted us to investigate the possible interconversion between the two compounds. As can be noted from Table 1 (synthesis conditions), compound II was prepared using water, whereas I was prepared by the use of the biphasic route (cyclohexanol and water). In addition, NaOH was also added during the synthesis of II. In order to carry out the transformation reaction of II to I, compound II (0.045 g) was heated in a cyclohexanol/water (1:1 v/v) mixture at 180 °C for varying periods of time. The products were isolated and examined using PXRD (Supporting Information, Figure S15). From the

PXRD studies, it is clear that compound I starts to form after about 24 h of heating, and the entire transformation of II to I appears to be completed in 120 h [Supporting Information, Figure S15(f)]. We have also carried out attempts to transform compound I to II by adding NaOH in water at 180 °C. Our attempts were not successful as compound I remained unreacted even up to 72 h under these conditions as confirmed by PXRD (Supporting Information, Figure S16). We have also carried out an analysis of the strength of the π 3 3 3 π interactions in both the compounds. The centroidcentroid distance (d) and their interplanar angles (θ) between the 1,10-phenanthroline ligands have been measured. A value of d = 3.961 A˚ and θ=3° for I and d = 3.499 A˚ and θ=1° for II have been observed (Supporting Information, Figure S13). The low θ values indicate that the 1,10-phenanthroline rings are arranged one over the other, but stacked antiparallel to each other, which have been observed earlier20a (Supporting Information, Figure S13). To investigate this further, we have carried out AMI parametrized Hamiltonian available in the Gaussian Program Suite.20b Typically, we employed the B3LYP/6-31 þ G(d,p) as the basis set. AMI methods together with a semiclassical dipolar description have been employed to establish the relationship between the stability and geometries of the organic molecules. The individual molecules are dipolar (dipole moment of the 1,10-phenanthroline molecules calculated at AM1 level is ∼3.35 D), and the most favorable arrangement of the two dipolar molecules would involve antiparallel orientation, which would cancel the overall net dipole moment.20c-e In both compounds I and II, we observed antiparallel arrangement of the 1,10-phenanthroline molecules,

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weak interactions and in the range comparable with hydrogen bond energies, observed in many organic solids.20f Structural Comparison

Figure 6. (a) The 2D layer in the structure of VI. Note that the layer is formed by Zn-O(H)-Zn linkages. (b) The T-atom connectivity (T = Zn) in VI showing the 36 net. (c) View of the 3D structure of VI.

which suggests that there is no net overall dipole moment. We do observe a weak dipole moment of 0.4 D in the case of I, which could be due to the marginally higher angle of θ between the two aromatic rings. The arrangement of the individual 1D chains is such that there are no significant acid-acid or acid-phenanthroline interactions even though both have extensive π electrons (aromatic rings). The single point energy calculated without the symmetry constraints gave a π 3 3 3 π interaction energy of 2.24 kcal/mol for I and 3.38 kcal/mol for II. These are typical values associated with

New metal-organic hybrid compounds have been synthesized by the use of a liquid-liquid interface as the reaction medium. Of the six phases isolated in the present study, two compounds (II and IV) were prepared by employing the normal hydrothermal method. A careful examination of the synthesis conditions and the resulting structures indicated that the compounds isolated by the biphasic approach have interesting features. From the synthesis point of view, compounds II and IV were prepared under similar experimental conditions except for the presence of 4,40 -bipyridine in the reaction mixture. The 4,40 -bipyridine, expectedly, binds with the metal center and, in fact, helps enhance the dimensionality of compound IV. Compounds III and V, on the other hand, were prepared using 1,10-phenanthroline and pyrazine, respectively. While the 1,10-phenanthroline binds with the metal center in III, pyrazine appears to participate in a pH controlling role in V. A possible rationale for this behavior can be arrived at by comparing the pKa values of phenanthroline and pyrazine ligands. 1,10-Phenanthroline with a pKa of 4.91 appears to be a stronger base compared to pyrazine (pKa=1.1) and leads to binding with the metal center. In addition, the bulky nature of the phenanthroline ligand hanging from the Cd metal center interrupted the possible formation of a 3D structure in III but gives rise to a bilayer structure. The effect of the reaction temperature on the formation of Zn (VI) and Cd (V) compounds can be correlated with the structure. The role of temperature during the formation of MOFs has been a subject of study in recent years.21 It appears that the higher reaction temperature favors the formation of a fully dehydrated phase (devoid of water) giving rise to M-O(H)-M type of linkages in VI. In compound V, formed at 150 °C, we observed one coordinated water molecule in the structure along with the formation of Cd2O2 dimers. In VI, however, we observed a 2D arrangement of M-O(H)-M connectivity pillared by the ABA acid. This illustrates that the temperature of the reaction mixture does play an important role not only in the formation of a compound but also in the bonding. This observation is in agreement with the earlier ones reported in the literature.21 During the present study, we observed that the Zn atoms have five nearest neighbor coordination. We wanted to investigate the coordination geometry around Zn atoms in all the compounds based on the method described by Addison and Rao.22 Accordingly, the five coordinated geometry can be rationalized by the parameter “τ”, which is dependent on the angle between the ligands and the central metal atom (Scheme 1). In a system with a five coordinated metal center [M in Scheme 1], ideal square pyramidal geometry will be obtained if R = β = 180°, keeping A as the axial ligand (β =< BMD). For a perfect trigonal bipyramidal geometry, R becomes 120° and BMD becomes the main axis. But for square pyramidal geometry, M moves out of the plane BCDE so that R and β becomes less than 180° and can be rationalized by the value (β - R) which has a value of 0° for a C4v and 60° for a D3h coordination geometry. Thus, Addison and Rao defined a geometrical parameter τ = (β - R)/60 applicable to the five coordinated system as a measure of the square pyramidal or a trigonal bipyramidal geometry; τ should be 1 for a perfect trigonal bipyramidal geometry and 0 for a perfect square pyramidal

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recently23 (Figure 7). The copper compound, [Cu(OBA)(H2O)]2 3 0.5H2O, was formed by oxy-bisbenzoic acid, which is flexible around the central oxygen atom. Here 5-fold interpenetration has been observed (Supporting Information, Figure S19). Though a rigid linker, 4,40 -bipyridine, has been used in the preparation of IV, the flexible nature of the acid gives rise to interpenetration. It may be noted that both the compounds have been prepared by hydrothermal methods.

geometry, and any deviation from this may be considered as a distortion. The parameters (R, β, and τ) were calculated for the compounds with 5-coordinated geometry and the values are listed in Table 4. From the values, we observe that the compounds I, II, and IV have distorted trigonal bipyramidal geometry for the Zn atom. In compound VI, which has three different Zn atoms, the τ values indicate that Zn(1) has a distorted tbp geometry (τ = 0.76), Zn(2) is close to being a perfect tbp (τ = 0.94), and Zn(3) has a distorted squarepyramidal (sp) geometry (τ=0.3) (Supporting Information, Figure S17). Structurally, the present compounds can be compared with other carboxylate-based MOF compounds reported in the literature. The bilayered compound III is closely related to the compound, [Cd(4,40 -ABA)(phen)(H2O)]n, reported previously (Supporting Information, Figure S18).11b The difference between the two structures appears to be in the total number of water molecules. It is likely that the biphasic route employed in the present study might have resulted in the formation of III with different water molecules. The difference in the extra framework water molecules also gives rise to changes in the space group as well as in the lattice parameter between the two compounds. The Zn-ABA compound IV exhibits 3-fold interpenetration. Interpenetration of the structures has been routinely observed in MOF compounds, especially when flexible linkers are used. In the present study, the two benzoic acid groups are flexible about the -NdN- bond. It may be pertinent to compare the structure with other similar compounds synthesized in the presence of flexible carboxylic acids. Thus, the structure of IV can be compared with the copper compound [Cu(OBA)(H2O)]2 3 0.5H2O (OBA = 4,40 -oxybisbenzoic acid), reported Scheme 1. A Five Coordinate Metal Center (M) with the Coordinated Atoms Labeled as A-Ea

a

Figure 7. (a) The connectivity between copper paddlewheels via 4,40 -oxy-bisbenzoic acid. This connectivity gives rise to 5 fold interpenetration. (b) View of the layer observed in IV. Note the close similarity between the two connectivities. In IV, the layers are connected by 4,40 -bipyridine units giving rise to a 3-fold interpenetration (see text).

R is the < EMC.

Table 4. List of the τ Values and Other Parameters Observed for the 5-Coordinated Znþ2 Ions in the Structures, I, II, IV, and V compound I II IV VI

Zn(2) Zn(1) Zn(1) Zn(1) Zn(2) Zn(3)

R (angle CME) (°)

(angle BMD) (°)

τ = (β - R)/60

% tetragonal elongationa

% trigonal compressionb

coordination geometry adopted

118.3 126.6 135.3 129.9 119.9 154.4

157.9 149.6 177.9 175.4 176.1 173.6

0.66 0.38 0.71 0.76 0.94 0.3

2.13 -5.8 0.1 -0.247 0 2.5

15.34 8

distorted tbp distorted tbp distorted tbp distorted tbp tbp distorted sp

c

3.3 10.6 1.2

a A bond vs C (or E) bond, as 100(A - C)/C where A and C are distances of atoms A and C from M. b B (or D) bond vs C (or E) bond, as 100(B - C)/B where B and C are distances of B and C from M. c Not definable as different sets of atoms.

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Figure 8. (a) View of the 3D structure of [Zn(ADB)(H2O)]n.11e (b) The PtS net structure observed in [Zn(ADB)(H2O)]n.11e (c) The derived PtS net observed in V. Note the difference between the two nets.

The synthesis of compounds prepared using the liquidliquid interphase, on the other hand, forms structures without any interpenetration. One possible rationale for this observation could be that the formation of a particular structure is

facilitated only at the interphase, which is expected to have much slower kinetics. During the present synthesis, the formation of the 3D compounds V and VI without interpenetrations illustrates this view.

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The compound V appears to have a close structural relationship with the reported Zn-ABA compound, [Zn(ADB)(H2O)]n.11e In V, we find that the Cd centers form a dimer connected through a μ3-oxygen, which are further linked by the carboxylate group forming a 1D chain (Figure 5a). In [Zn(ADB)(H2O)]n, the Zn centers are connected through the carboxylates forming a 1D chain which are connected by the ABA units forming the layer and the 3D structure (Figure 8a, Supporting Information, Figure S20]. The difference between the two structures arises due to the formation of the Cd2O2 dimers and the 1D chains not being parallel to each other. The overall connectivity in both the structures, however, is comparable (Figures 5d and 8a). The Zn compound exhibits a 2-fold interpenetration, whereas no such interpenetration has been observed in the present compound, V. Another way to understand complex frameworks is to perform a structural analysis based on node and net connectivity. We have carried out a TOPOS analysis24 on compound V as well as the Zn compound, [Zn(ADB)(H2O)]n. Both compounds have the same Schafli symbol of 42.84. This suggests that the structure must be a derivative of PtS net. Compounds based on PtS net structures have been known in MOFs.25 The metal center is, in general, tetrahedral (4-connected), but the anion has a square planar geometry. In the Zn compound [Zn(ADB)(H2O)]n, we do find that the structure is a perfectly ordered PtS structure (Figure 8b), but in V, the PtS net is distorted giving rise to a new network (Figure 8c), NIMTAE.24 It is likely that the formation of Cd2O2 dimers in V would have created distortions in the structure giving rise to the new topology, which is closely related to PtS. According to TOPOS analysis, this is the first observation of this net. The 1D chain structure observed in V has been previously observed in Y2(C12N2H8)2(C8H4O4)3] 3 H2O26 (Supporting Information, Figure S22). The layer arrangement in the structure of VI has been reported before.27 From the topological point of view, the Zn-O(H)-Zn 2D layers have a 36 net, which indicates that the compound has a 6 connected node at the metal center. Similar connectivity has been observed before in [Zn3(1,4BDC)(DEF)2] 3 DEF,27 where the Zn3 trimers are connected with six 1,4-BDC units (six connected node) (Figure 9a). In compound VI, the Zn-centers are connected to six other Zncenters via the μ2 and the μ3 coordinated oxygen atoms forming the 36 connected layer (Figure 9b). In VI, the layers are further connected by the ABA unit giving rise to a pillared layered structure (Figure 6c), whereas the [Zn3(1,4-BDC)(DEF)2] 3 DEF has only a 2D arrangement. It may be noted that the 36 net in VI is formed purely by the M-O(H)-M linkages without involving the ABA units. Heterogeneous Catalytic Studies. It has been shown recently that compounds containing cadmium exhibits Lewis acidic properties.28 During the present study, two cadmium containing compounds have been isolated with two- (III) and three-dimensional (V) structures. We desired to explore the possible Lewis acid catalytic behavior of the cadmium compounds by carrying out a study of the cyanosilylation of imines. The heterogeneous catalysis studies were carried out following the procedures reported earlier in the literature.28 In a typical catalytic reaction, powdered catalyst material (0.1 mmol) was suspended in CH2Cl2 (10 mL) solution of imine (A, 0.09 g, 0.5 mmol). Trimethylsilyl cyanide (0.075 g, 0.75 mmol) was added at 0 °C, and the reaction was carried out for 4 h with continuous stirring. The product, (B), at the end of the reaction was separated and analyzed. It was observed that compound V produced the aminonitrile quantitatively (∼99%), while compound III formed only ∼50% of the


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Figure 9. (a) The 36 net observed in the layered structure of [Zn3(1,4-BDC)(DEF)2] 3 DEF.22 Note that the nodes are connected by 1,4 BDC units. (b) View of the layer observed in VI. Note that the layer is formed by the connectivity between the nodes by μ3-OH and μ2-O and carboxylate oxygens.

nitrile product as shown by 1H NMR spectra (Supporting Information, Figure S24). In order to prove the catalytic nature of the reaction, control experiments in the absence of the catalyst were also carried out, which gave a yield of ∼5-6% of the nitrile product. This indicates that the formation of the nitrile require a Lewis acid catalytic site.

The stability of the compounds, III and V, were examined after the catalytic study by PXRD, which indicated that the

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compounds are stable and exhibited the same XRD patterns observed for the as-synthesized phase. We have also performed the catalytic studies on the used catalyst, which also exhibited comparable catalytic behavior as that of the fresh catalyst. This study indicates that the cadmium containing compounds are stable under catalytic reaction conditions and also the catalyst can be reused repeatedly without loss of activity. Conclusions The synthesis, structure, and characterization of six new azodibenzoates of zinc and cadmium have been accomplished. The studies suggest that the biphasic method provides a viable option for preparing new compounds with interesting structures. The observation of non-interpenetrated structures in some of the compounds (V and VI) indicates that the formation is controlled at the interphase. The transformation of one of the 1D structures (II) to the other (I), by replicating the synthetic conditions, suggests that the energies of formation of such structures are likely to be similar. Lewis acid catalysis with the prepared compounds (III and V) indicates reasonable activity, which can be correlated to the coordination environment around the metal center. Though the present study establishes the usefulness of the biphasic approach for the preparation of new hybrid compounds, more examples are required to evaluate and understand the efficacy of this method. Acknowledgment. The authors thank Professor Balaji R. Jagirdar of Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore, India, for help with the catalytic studies. S.N. thanks the Department of Science and Technology (DST), Government of India, for the award of a research grant and also for RAMANNA Fellowship. The authors also thank the Council of Scientific and Industrial Research (CSIR), Government of India, for the award of a research grant (S.N.) and a fellowship (S.B.). Supporting Information Available: List of important IR bands observed in compounds I-VI (Table S1): The result of the TGA studies for the compounds I-VI (Table S2); Selected bond angles for the compounds I-VI (Table S3); powder X-ray patterns of (a) simulated and (b) experimental of compounds I-VI (Figure S1); IR spectra for compounds I-VI (Figure S2); UV-vis spectra of compounds I-VI (Figure S3), photoluminescence spectra of the ligands 4,40 -bipyridine, 1,10-phenanthroline, and ABA (Figure S4), photoluminescence spectra of the compounds I-VI (Figure S5), TGA of the compounds I-VI (Figure S6); asymmetric unit, coordination geometry around the zinc ions and the connectivity between ABA and Zn in I (Figure S7); asymmetric unit, coordination geometry around the zinc ions and the connectivity between ABA and Zn in II (Figure S8), asymmetric unit, coordination geometry around the cadmium ions and the connectivity between ABA and Cd in III (Figure S9); asymmetric unit, coordination geometry around the zinc ions and the connectivity between ABA and Zn in IV (Figure S10); asymmetric unit, coordination geometry around the cadmium ions and the connectivity between ABA and cadmium in V (Figure S11); asymmetric unit, coordination geometry around the zinc ions and the connectivity between ABA and Zn in VI (Figure S12); the π 3 3 3 π interactions in compounds I and II (Figure S13); the depiction of the stacking of the chains in the structure of I and II (Figure S14); the PXRD patterns as a function of time during the transformation of II-I (Figure S15); the PXRD patterns as a function of time during the transformation of I-II (Figure S16); coordination geometry around the zinc ions (Figure S17); structure of reported compound, [Cd(4,40 -ABA)(phen)(H2O)]n (Figure S18); five-fold interpenetration in reported compound [Cu(OBA)(H2O)]2 3 0.5H2O (Figure S19); structure of reported compound [Zn(ADB)(H2O)]n (Figure S20); triangular building unit (Zn3

Bhattacharya et al. cluster) in VI (Figure S21); 1D zigzag Y-O-Y chains in reported compound Y2(C12N2H8)2(C8H4O4)3] 3 H2O (Figure S22); the PXRD of the products during the synthesis of V and VI at different temperatures (Figure S23); NMR spectra of the product after heterogeneous catalysis (Figure S24); bond valence sum calculations (Table S4). This material is available free of charge via the Internet at http:// pubs.acs.org.

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