Structural Diversity in Partially Fluorinated Metal ... - ACS Publications

Jan 21, 2010 - Pradip Pachfule, Chandan Dey, Tamas Panda, Kumar Vanka, and Rahul ... Chemistry Division, National Chemical Laboratory, Dr. Homi Bhaba ...
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DOI: 10.1021/cg9013812

Structural Diversity in Partially Fluorinated Metal Organic Frameworks (F-MOFs) Composed of Divalent Transition Metals, 1,10-Phenanthroline, and Fluorinated Carboxylic Acid

2010, Vol. 10 1351–1363

Pradip Pachfule, Chandan Dey, Tamas Panda, Kumar Vanka, and Rahul Banerjee* Physical/Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhaba Road Pune 411008, India Received November 4, 2009; Revised Manuscript Received December 30, 2009

ABSTRACT: Seven new partially fluorinated metal-organic frameworks (F-MOFs) have been synthesized under different solvothermal conditions (H2O or N,N-dimethylformamide (DMF)) from transition metal cations [Zn(II), Co(II), and Mn(II)], 1,10-phenanthroline (phen), and 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) (C17H10F6O4, H2hfbba) to determine the influence of reaction conditions on the formation of metal-organic frameworks. This family of materials displays a striking degree of structural similarity depending on the solvent of synthesis. Of the seven materials, two contain three-dimensional connectivity: Co3(hfbba)6(phen)2 (F-MOF-6) and Mn3(hfbba)6(phen)2 (F-MOF-10). Three materials are one-dimensional structures: Zn(hfbba)0.5(phen)(HCO2) (F-MOF-8), Mn(Hhfbba)2(phen) (F-MOF-11), and [Mn(hfbba)2(dm-phen)] 3 (H2O) (F-MOF-11A). Lastly, Co(hfbba)(phen)2 3 2(H2hfbba)(H2O)(HCO2) (F-MOF-7) and Zn(hfbba)(phen)2 3 2(H2hfbba) (H2O)(HCO2) (F-MOF-9) are discrete zero-dimensional molecular complexes. F-MOF-6 and -10, which feature a three-dimensional (3D) framework with pcu topolgy are formed in DMF like F-MOF-8 and at 85 °C. The remaining F-MOFs are formed by a solvothermal reaction at 120 °C in water. F-MOF-7 and -9 are isostructural discrete zero-dimensional molecular complexes (a ∼ 19.5; b ∼13.7; c ∼ 26.2/29.6 A˚; β ∼ 99.9/120.04; V ∼ 6840 A˚3). All these F-MOFs were structurally determined by singlecrystal X-ray diffraction. Solid-state properties such as UV-vis and the thermal stability of F-MOF-6 to -11A have also been studied. Insight into the factors influencing the preferred crystallization of a specific complex over others has been obtained from full quantum chemical (QM) calculations using density functional theory (DFT).

*To whom correspondence should be addressed. E-mail: r.banerjee@ ncl.res.in. Fax: þ 91-20-25902636. Tel: þ 91-20-25902535.

reports of interesting H2-storage properties in materials containing porous surfaces with exposed fluorine atoms.11 However, there are only a few reports of MOFs containing fluorinated carboxylates as perfluorinated carboxylates12 are significantly more acidic than nonfluorinated carboxylates and subsequently less soluble in common organic solvents. Fluorinated acids are also less stable than their nonfluorinated analogues, and it has been found that they often decompose at temperatures commonly used to form MOFs of higher dimensionality (125-180 °C). In our previous attempt to investigate the flexibility of the ligands on the resulting framework, we synthesized a number of F-MOFs with a flexible fluorinated dicarboxylate building block and various heterocyclic coligands.13 In this contribution, we report seven new F-MOFs synthesized from a flexible fluorinated dicarboxylate building block, 1,10-phenanthroline and transition metal ions [Zn(II), Co(II), and Mn(II)] as metal centers in aqueous and N,N-dimethylformamide (DMF) media. It should be noted that the fluorination referred to in the rest of the paper is only partial, as not all the hydrogens were substituted with fluorines in the obtained crystal structures. The influence of the solvent (H2O/DMF) on the formation of the F-MOFs has been investigated by means of density functional theory (DFT) calculations. We attempt to rationalize the influence of the solvent (H2O/DMF) on the formation of the F-MOFs. These F-MOFs formulated as Co3(hfbba)6(phen)2 (F-MOF-6), Co(hfbba)(phen)2 3 2(H2hfbba)(H2O)(HCO2) (FMOF-7), Zn(hfbba)0.5(phen)(HCO2) (F-MOF-8), Zn(hfbba)(phen)2 3 2(H2hfbba)(H2O)(HCO2) (F-MOF-9), Mn3(hfbba)6(phen)2 (F-MOF-10), Mn(Hhfbba)2(phen) (F-MOF-11), [Mn(hfbba)2(dm-phen)] 3 (H2O) (F-MOF-11A) (hfbba=4, 40 (hexafluoroisopropylidene) bis-benzoate, phen=1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline = dm-phen

r 2010 American Chemical Society

Published on Web 01/21/2010

Introduction In recent years, the design and synthesis of novel metal-organic framework (MOFs) materials have provoked significant interest because of their intriguing architectures1 and potential applications in the areas of gas storage and sequestration,2 sensors,3 nonlinear optics (NLO),4 ion exchange,5 magnetism,6 catalysis,7 and porosity or zeolitic behaviors.8 Carboxylic acids among many other ligands have been proven to be excellent building blocks for the construction of MOFs as they can adopt versatile coordination conformations, which can generate zero-, one-, two-, and three-dimensional (0D, 1D, 2D, and 3D) structures. Modification of the size and shape of the ligands often leads to a modification in the structures and properties of the resulting MOFs. In general, MOFs synthesis occurs in solvothermal media (water, organic solvents, or their mixtures, recently ionic liquids),9 yet none of the synthetic details described in the literature explain the cause behind the solvent choice, despite it being an important parameter in the phase-pure synthesis of a desired phase. As a result, it is still a challenge to predict the resulting structure of an MOF beforehand as its formation is not only influenced by the geometrical and electronic factors of metal ions but is also dependent on other factors such as the rigidity or flexibility of the ligands, choice of solvent and solvent polarity, temperature, metal/ligand ratio, and pH.10 As a result, any correlations between synthesis conditions and structures are drawn only after the materials have already been synthesized. Our interest in the use of fluorinated links in synthesizing fluorinated metal-organic frameworks (F-MOFs) is based on

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Scheme 1. Schematic Diagram Showing Solvent Dependence on the Synthesis of F-MOFs Reported in This Paper

(Scheme 1) display interesting H-bonded (in H2O) and 3D (in DMF) structural features depending on the solvent of synthesis. These F-MOFs have been determined by single crystal X-ray diffraction analyses and further characterized by IR spectra, powder X-ray diffraction (PXRD), and thermogravimetric analyses (TGA). Experimental Section Materials and Physical Measurements. All reagents and solvents for synthesis and analysis were commercially available and used as received. The Fourier transform (FT) IR spectra (KBr pellet) were taken on a Perkin Elmer FT-IR Spectrum (Nicolet) spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded on a Phillips PNAlytical diffractometer for Cu KR radiation (λ= 1.5406 A˚), with a scan speed of 2° min-1 and a step size of 0.02° in 2θ. TGA experiments were carried out in the temperature range of 25-700 °C on a SDT Q600 TG-DTA analyzer under N2 atmosphere at a heating rate of 10 °C min-1. Solid-state UV spectra were measured at room temperature on a Perkin-Elmer Precisely Lambda 650 spectrometer. General Synthesis of F-MOF-6 to -11A. Direct solution reactions at room temperature give rise to microcrystalline precipitate in most MOF synthesis.14 The hydrothermal method has been demonstrated to be a very promising technique for growing MOF crystals. Thus, hydrothermal methods were applied to obtain larger single crystals suitable for X-ray diffraction. H2hfbba is sparingly soluble in water at room temperature. To obtain pure crystals of F-MOF11, -11A, -7, and -9, a strong stirring of the mixture in deionized water is necessary before it is transferred to an acid-digestion bomb for hydrothermal reaction. In this work, two synthetic pathways were applied to prepare these F-MOFs. F-MOF-11, -11A, -7, and -9 were obtained by hydrothermal reaction of the ligand with M(NO3)2 3 xH2O in water in a acid digestion bomb at higher temperature (120 °C), whereas F-MOF-6, -8, and -10 were synthesized by a hydrothermal reaction in a glass vial at lower temperature (85 °C). In each synthetic case, an approximate M/H2hfbba/phen composition of 1/5/0.6 was found to produce the best yield of the product. All these complexes are insoluble in common organic solvents. The compositions of F-MOF-6 to -11A were determined by IR and X-ray single-crystal diffraction techniques. The phase purities of the bulk samples were further identified by PXRD patterns, which display an essential similarity to those of the calculated ones (see Figure 11). Synthesis of Co3(hfbba)6(phen)2 (F-MOF-6). 0.5 mL of 1,10phenanthroline stock solution (0.20 M) and 1.5 mL of 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) stock solution (0.20 M) were mixed in a 5 mL vial. To this solution was added 0.5 mL of Co(NO3)2 3 6H2O stock solution (0.20 M). The vial was capped and heated to 85 °C for 72 h. The mother liquor was decanted and the products were washed with DMF (15 mL) three times. Pink-colored crystals of F-MOF-6 were collected by filtration and dried in air (10 min) (yield: 70%). Crystals suitable for X-ray diffraction were grown by heating the reaction mixture for 96 h. However, heating the reaction mixture for a longer period decreases the yield.

Pachfule et al. Synthesis of Co(hfbba)(phen)2 3 2(H2hfbba)(H2O)(HCO2) (FMOF-7). Hydrothermal reaction of Co(NO3)2 3 6H2O (0.035 g, 0.12 mmol) with 1,10-phenanthroline (0.023 g, 0.12 mmol) and excess 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) (0.196 g, 0.50 mmol) in a 25 mL acid-digestion bomb using deionized water (7 mL) at 120 °C for 3 days produced pink-colored crystals of F-MOF-7 in quantitative yield. Crystals were collected by filtration, washed with ethanol, and dried in air (10 min). Synthesis of Zn(hfbba)0.5(phen)(HCO2) (F-MOF-8). 0.5 mL of 1,10-phenanthroline stock solution (0.20 M) and 1.5 mL of 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) stock solution (0.20 M) were mixed in a 5 mL vial. To this solution was added 0.5 mL of Zn(NO3)2 3 6H2O stock solution (0.20 M). The vial was capped and heated to 85 °C for 72 h. The mother liquor was decanted and the products were washed with DMF (15 mL) three times. Colorless crystals of F-MOF-8 were collected by filtration and dried in air (10 min) (yield: 55%). Crystals suitable for X-ray diffraction were grown by heating the reaction mixture for 96 h. However, heating the reaction mixture for a longer period decreases the yield. Synthesis of [Zn(hfbba)(phen)2] 3 2(H2hfbba)(H2O)(HCO2) (FMOF-9). Hydrothermal reaction of Zn(NO3)2 3 6H2O (0.036, 0.12 mmol) with 1,10-phenanthroline (0.023 g, 0.12 mmol) and excess 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) (0.196 g, 0.50 mmol) in a 25 mL acid-digestion bomb using deionized water (7 mL) at 120 °C for 3 days produced colorless crystals of F-MOF-9 in quantitative yield. Crystals were collected by filtration, washed with ethanol, and dried in air (10 min). Synthesis of Mn3(hfbba)6(phen)2 (F-MOF-10). 0.5 mL of 1,10phenanthroline stock solution (0.20 M) and 1.5 mL of 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) stock solution (0.20 M) were mixed in a 5 mL vial. To this solution was added 0.5 mL of Mn(NO3)2 3 xH2O stock solution (0.20 M). The vial was capped and heated to 85 °C for 72 h. The mother liquor was decanted and the products were washed with DMF (15 mL) three times. Colorless crystals of F-MOF-10 were collected by filtration and dried in air (10 min) (yield: 72%). Crystals suitable for X-ray diffraction were grown by heating the reaction mixture for 96 h. However, heating the reaction mixture for a longer period decreases the yield. Synthesis of Mn(Hhfbba)2(phen) (F-MOF-11). Hydrothermal reaction of Mn(NO3)2 3 xH2O (0.035, 0.12 mmol) with 1,10-phenanthroline (0.023 g, 0.12 mmol) and excess 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) (0.196 g, 0.50 mmol) in a 25 mL acid-digestion bomb using deionized water (7 mL) at 120 °C for 3 days produced colorless crystals of F-MOF-11 in quantitative yield. Crystals were collected by filtration, washed with ethanol, and dried in air (10 min). Synthesis of [Mn (hfbba)2(dm-phen)] 3 (H2O) (F-MOF-11A). Hydrothermal reaction of Mn(NO3)2 3 6H2O (0.035, 0.12 mmol) with 2,9dimethyl-1,10-phenanthroline (0.025 g, 0.12 mmol) and excess 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) (0.196 g, 0.50 mmol) in a 25 mL acid-digestion bomb using deionized water (7 mL) at 120 °C for 3 days produced colorless crystals of F-MOF11A in quantitative yield. Crystals were collected by filtration, washed with ethanol, and dried in air (10 min). X-ray Crystallography. All single crystal data were collected on a Bruker SMART APEX three circle diffractometer equipped with a CCD area detector (Bruker Systems Inc., 1999a)15 and operated at 1500 W power (50 kV, 30 mA) to generate Mo KR radiation (λ= 0.71073 A˚). The incident X-ray beam was focused and monochromated using Bruker Excalibur Gobel mirror optics. Crystals of the F-MOFs reported in the paper were mounted on nylon CryoLoops (Hampton Research) with Paraton-N (Hampton Research). Crystals were flash frozen to 100(2) K in a liquid nitrogen cooled stream of nitrogen. Data were integrated using Bruker SAINT software.16 Data were subsequently corrected for absorption by the program SADABS.17 The space group determinations and tests for merohedral twinning were carried out using XPREP.18 In all cases, the highest possible space group was chosen. All structures were solved by direct methods and refined using the SHELXTL 97 software suite.19 Atoms were located from an iterative examination of difference F-maps following least-squares refinements of the earlier models. Hydrogen atoms were placed in calculated positions and

C46 H23 F12 N2 O8 Mn 1014.60 293(2) 0.71073 monoclinic C2/c a = 45.75(4)A˚ b = 10.703(9) A˚ c = 8.848(7)A˚ β = 90.348(15)° 4332(6) 4 1.556 0.977 R1 = 0.0686, wR2 = 0.1667 R1 0.0949, wR2 = 0.1934 C25.5 H14 F6 N1.33 O4 Mn 571.98 100(2) 0.71073 monoclinic C2/c a = 37.09(2)A˚ b = 13.643(7) A˚ c = 29.671(12) A˚ β = 135.04(2)° 10609(9) 2 1.074 0.877 R1 = 0.1119, wR2 = 0.2597 R1 = 0.1465, wR2 = 0.2990 392.25 298(2) 0.71073 monoclinic P2/c a = 30.257(3) A˚ b = 7.5419(6) A˚ c = 15.2513(13) A˚ β = 104.405(2)° 3370.8(5) 8 1.546 0.863 R1 = 0.0469, wR2 = 0.0823 R1 = 0.1350, wR2 = 0.1088 formula weight temperature (K) wavelength (A˚) crystal system space group unit cell dimensions

volume Z density (calculated) goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data)

C17 H10 F6 O4 empirical formula

C25 H13.33 F6 N1.33 O12 Co 569.3 100(2) 0.71073 monoclinic C2/c a = 36.616(6) A˚ b = 13.551(2) A˚ c = 26.373(4) A˚ β = 127.80(2)° 10340(3) 4 1.097 1.041 R1 = 0.0858, wR2 = 0.1915 R1 = 0.1202, wR2 = 0.2070

C76 H45 F18 N4 O15 Co 1655.09 293(2) 0.71073 monoclinic P21/c a = 19.453(5) A˚ b = 13.737(4) A˚ c = 29.563(6) A˚ β = 120.04(13)° 6839(3) 4 1.607 1.153 R1 = 0.0896, wR2 = 0.1706 R1 = 0.1251, wR2 = 0.1866

C21.5 H13 F3 N2 O4 Zn 485.71 293(2) 0.71073 monoclinic C2/c a = 41.929(8) A˚ b = 9.9010(15) A˚ c = 42.941(8) A˚ β = 166.939(3)° 4028.6(12) 4 1.602 0.994 R1 = 0.0412, wR2 = 0.1067 R1 = 0.0512, wR2 = 0.1176

C76 H47 F18 N4 O15 Zn 1663.56 100(2) 0.71073 monoclinic P21/c a = 19.409(8) A˚ b = 13.750(6) A˚ c = 29.496(10) A˚ β = 120.04(2)° 6814(5) 4 1.622 1.009 R1 = 0.0429, wR2 = 0.1165 R1 = 0.0609, wR2 = 0.1263

F-MOF-11 F-MOF-10 F-MOF-9 F-MOF-8 F-MOF-7

Table 1. Crystal Data and Structure Refinement for F-MOFs Reported in This Paper

F-MOF-6 F-ACID

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C31 H20 F6 N2 O4.5 Mn 661.43 293(2) 0.71073 monoclinic P2/n a = 14.661(19) A˚ b = 9.5836(13) A˚ c = 21.999(3) A˚ β = 107.042(2)° 2955.3(7) 2 1.487 1.043 R1 = 0.0589, wR2 = 0.1171 R1 = 0.0979, wR2 = 0.1384

Crystal Growth & Design, Vol. 10, No. 3, 2010 F-MOF-11A

Article

Figure 1. (a) The twisted conformation of (hexafluoroisopropylidene) bis(benzoic acid). (b) Helical packing of (hexafluoroisopropylidene) bis(benzoic acid). included as riding atoms with isotropic displacement parameters 1.2-1.5 times Ueq of the attached C atoms. Data were collected at 298(2) K for F-MOF-7, -8, -11, -11A. For the other four F-MOFs presented in this paper, data collection took place at 100(2) K. This lower temperature was considered to be optimal for obtaining the best data. All structures were examined using the Adsym subroutine of PLATON20 to ensure that no additional symmetry could be applied to the models. All ellipsoids in ORTEP diagrams are displayed (see Supporting Information) at the 50% probability level unless noted otherwise. In F-MOF-6 Co1 is disordered about a center of symmetry, which would mean that the Co 3 3 3 Co 3 3 3 Co arrangement would not have to be perfectly linear. Co1 has a rather small sphere when refined on the special position and refined isotropically. In order to treat the Co1 better it has been placed in a position off the special position, as two half atoms, giving a slightly bent Co 3 3 3 Co 3 3 3 Co. The two half atoms are too close together and were refined anisotropically. O4 has also been refined isotropically over two positions. Data collection at higher temperature did not solve these refinement problems. Supporting Information contains a detailed data collection strategy and crystallographic data for the seven F-MOFs reported in this paper. Crystal data and details of data collection, structure solution, and refinement are summarized in Table 1. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the CCDC as deposition No CCDC 752226-752233 (see also Table 1). Copies of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 lEZ UK (fax: þ 44 (1223) 336 033; e-mail: [email protected]). Computational Details. All the calculations were carried out using DFT as implemented in the Turbomole suite of programs.21 Geometry optimizations were performed using the B-P 86 functional.22 The electronic configuration of the atoms was described by a triple-ζ basis set augmented by a polarization function (TURBOMOLE basis set TZVP).23 This basis set has been shown24 to provide results that are very close in equilibrium geometries and energies to what can be obtained with the DFT basis set limit. The choice of the BP-86 functional was validated by doing the gas phase geometry optimization calculations of the two structures ; referred to as Case I and Case II in the article ; with the Perdew, Burke, and Erzenhof (PBE) density functional.25 The results indicated that there was only a marginal difference (0.3 kcal/mol) for the difference in energy between the two structures using the B-P 86 (4.4 kcal.mol) and the PBE (4.7 kcal/mol) functional, thereby validating our basis set/functional choice. The resolution of identity (RI),26 along with the multipole accelerated resolution of identity (marij)27 approximations

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Figure 2. (a) Polyhedral representation of the SBU of F-MOF-6 showing the coordination environment of Co. (b) [Co3(CO2)6] SBUs with three octahedral Co2þ ions. (c) Isolated cage in F-MOF-6. (d) 3D pcu network in F-MOF-6. (e) One-dimensional channels of F-MOF-6 shown in spacefill model. Color code: Co (pink), N (blue), O (red), C (black), F (green). were employed for an accurate and efficient treatment of the electronic Coulomb term in the density functional calculations. The energy determined is the Gibbs free energy (ΔG), which was obtained by (i) first doing a geometry optimization calculation to determine the electronic energy and then (ii) doing frequency calculations to determine the zero-point energy and the entropic contributions. Along with the gas phase calculations, full geometry optimizations were also done incorporating solvent effects with the COSMO28 model, using DMF (ε=38.5) and water (ε=80.0) as solvents.

Results and Discussion Recently, the use of perfluorinated succinic and terephthalic acid has received considerable attention as these ligands have afforded numerous hybrid structures with many different transition metals and coligands.29 It has also been noticed that the reactions of perfluorinated dicarboxylates with metal cations under hydrothermal conditions do not afford materials containing these ligands and that a second ligand is necessary to incorporate these perfluorinated ligands into MOF structures. As a result, most of the reports to date of

materials containing perfluorinated dicarboxylates involve a second ligand, which is typically a simple, nonfluorinated, nitrogen-containing molecule. As part of our ongoing investigations of different synthetic approaches for design of hybrid materials incorporating perfluorinated ligands, we studied the hydrothermal chemistry of 4,40 -(hexafluoroisopropylidene) bis-benzoate (C17H8F6O4, hfbba) with transition metal cations [Zn(II), Co(II), Mn(II), and Cu(II)] to understand the influence of the solvent (H2O/DMF) on the formation of the F-MOFs. We used 4,40 -(hexafluoroisopropylidene) bis(benzoic acid) because (a) twisted conformation of this ligand could lead to a possible helical structure (Figure 1), (b) the different deprotonated degrees of the ligands under different reaction conditions may result in variable coordination modes in the products, (c) long molecular structure of this primary building unit may lead to the formation of microporous coordination frameworks with channels. The influence of the solvent (H2O/DMF) on the formation of the F-MOFs was also reflected in the structures of the materials. For example, F-MOF-6 and F-MOF-10, synthesized from

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Figure 3. (a) ZnN2O4 SBU in F-MOF-8. (b) The hfbba, phen, and formate ligands bridge metal centers in two directions, forming a grid-like 1-D sheet. (c) Packing diagram of F-MOF-8 (view down the b axis).

DMF, contain 3D connectivity, whereas F-MOF-11, -11A, -7, -9, and -1, synthesized from H2O, are 1D structures or discrete 0D molecular complexes (Scheme 1). F-MOF-8 and -5 have a 1D structure although they were synthesized from DMF. However, they have a higher structural dimensionality compared to the counterparts (F-MOF-9 and -1) that had been synthesized from water. We will first discuss the crystal structure of F-MOFs that have been synthesized in DMF and follow that by a discussion of the structures synthesized in H2O. Structure of Co3(hfbba)6(phen)2 (F-MOF-6). The crystallographic analysis reveals that F-MOF-6 crystallizes in the monoclinic space group C2/c and has a noninterpenetrated pcu topology.30 The asymmetric unit contains one and a half Co ions and hfbba liands. The structure of F-MOF-6 contains [Co3(CO2)6] SBUs with three octahedral Co2þ ions. Co(1) and Co(3) atoms are coordinated to two bidentate 1,10-phenanthroline ligands [Co-N distances, 2.227(2) and 2.245(2) A˚] and six different hfbba ligands [Co-O distances range from 2.084(2) to 2.264(2) A˚] (Figure 2a). Coordination sphere of the central octahedral Co2þ ion include six oxygen atoms [Co-O distances range from 2.084(2) to 2.264(2) A˚] from the bridging hfbba ligands (Figure 2b). This [Co3(CO2)6] SBUs connects to six hfbba ligands. Each hfbba ligand binds through both carboxylate oxygens to two adjacent metal atoms of the trimer and thus forming a shared octahedral edge (Figure 2c). These six hfbba ligands are arranged in a roughly octahedral manner around the trimer, bridging to six other trimers to form a 3D pcu network (Figure 2d). The framework in F-MOF-6 is highly open,

containing 1D channels of 4.7  4.0 A˚ dimension (the channel size is measured by considering the van der Waals radii of the constituting atoms)31 along the crystallographic a axis (Figure 2e). These channels are occupied by solvent molecules. Calculation with PLATON20 shows that the effective volume for the inclusion is 44% of the crystal volume. Appearance of strong peaks at 1415 and 1609 cm-1 in the IR spectrum confirms the coordinated carboxylates. Structure of Zn(hfbba)0.5(phen)(HCO2) (F-MOF-8). The structure of F-MOF-8 contains ZnN2O4 trigonal prisms where the Zn atom is coordinated to a bidentate phenanthroline ligand [Zn-N distances, 2.119(2) and 2.183(2) A˚] and one hfbba and two formate ligands32 [Zn-O distances range from 2.001(4) to 2.082(2) A˚; Figure 3a]. These two formate and one hfbba ligands bind to three adjacent Zn2þ ions to form a ladder-like 1D network (Zn 3 3 3 Zn distances are 5.704(2) and 14.951(4) A˚, respectively) along crystallographic a axis (Figure 3b). The overall structure is only a 1D layer, as the capping nature of the phenanthroline ligands prevents any additional connectivity. These 1D ladders stack on top of each other forming a columnlike arrangement (Figure 3c). -CF3 groups in the 1D ladders interact with each other to form a type I F2C-F 3 3 3 FCF2 synthon (D, 3.258(2) A˚; d, 2.873 (3) A˚; θ1 ≈ θ2, 93.5°).33 Structure of Mn3(hfbba)6(phen)2 (F-MOF-10). F-MOF-6 and -10 have very similar structures (a = 36.62; b = 13.55; c = 26.37; β = 127.8 vs a = 37.09; b = 13.64; c = 29.67; β = 135.0) containing similar [M3(CO2)6] secondary building units (SBUs) with three octahedral Mþ2 ions. As in the case

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Figure 4. (a) Polyhedral representation of the SBU of F-MOF-10 showing the coordination environment around Mn. (b) [Mn3(CO2)6] SBUs with three octahedral Mn2þ ions. (c) Isolated cage in F-MOF-10. (d) 3D pcu network in F-MOF-10. (e) One-dimensional channels of F-MOF10 shown in spacefill model. Color code: Mn (dark green), N (blue), O (red), C (black), F (green).

of F-MOF-6 in F-MOF-10, Mn(1) and Mn(3) atoms are coordinated to two bidentate phen ligands [Mn-N distances, 2.227(2) and 2.245(3) A˚] and six different hfbba ligands [Mn-O distances range from 2.084(5) to 2.264(2) A˚] (Figure 4a). The coordination sphere of the central octahedral Mn2þ ion includes six oxygen atoms [Mn-O distances range from 2.084(1) to 2.236(4) A˚] from the bridging hfbba ligands (Figure 4b). The [M3(CO2)6] SBUs connect to six hfbba ligands which are arranged in a roughly octahedral manner around the trimer, bridging to six other trimers to form a 3D pcu network (Figure 4c). The framework in F-MOF-10 is also open, containing 1D channels of

4.4  4.0 A˚ dimension (the channel size is measured by considering the van der Waals radii of the constituting atoms) along the crystallographic a axis (Figure 4e). These channels are occupied by solvent molecules. However, calculation with PLATON20 shows that the effective volume for the inclusion is 33% of the crystal volume. Appearance of strong peaks at 1405 and 1606 cm-1 in the IR spectrum confirms the coordinated carboxylates. Structures of Co(hfbba)(phen)2 3 2(H2hfbba)(H2O)(HCO2) (F-MOF-7) and [Zn(hfbba) (phen)2]cdt2(H2hfbba)(H2O)(HCO2) (F-MOF-9). F-MOF-1,12 -7, -9, -11, and -11A have been synthesized from H2O in a 25 mL acid-digestion bomb

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Figure 5. Crystal structure of F-MOF-7. (a) Polyhedral representation of the SBU showing the coordination environment of Co(II). (b) Hydrogen-bonded (COOH)4 cluster in F-MOF-7. (c) Hydrogen bonding between hfbba and octahedral Cu(phen)2(Hhfbba) units. (d) Hydrogen-bond motifs of H2hfbba between the coordination layers. (e) Packing diagram of F-MOF-7 in spacefill model (view down the b axis). Color code: Co (pink), N (blue), O (red), C (black), F (green). Diagrams representing the crystal structure of F-MOF-9 were not shown to avoid repetition.

at a temperature range 120-150 °C. F-MOF-7 and -9 are isostructural containing Co and Zn, respectively. The asymmetric unit of F-MOF-7 and -9 (space group P21/c) consists of one crystallographically independent Co2þ/Zn2þ ion, one Hhfbba (i.e., singly protonated H2hfbba), two free H2hfbba, two phen, one formic acid, and one lattice water molecule. Each Mþ2 (Zn/Co) ion is surrounded by four nitrogen atoms from two chelating phen ligands and two oxygen atoms from one Hhfbba anion, composing a slightly distorted octahedral geometry with M;O distances ranging from 2.073(1) to 2.385(4) A˚ and M;N distances ranging from 2.112(3) to 2.152(4) A˚ (Figure 5a). One carboxylate of the H2hfbba adopts a bidentate chelating mode and the other remains uncoordinated. The 1D structure of F-MOF-7 and 9 consists of layers of Mþ2 ions coordinated to the H2hfbba ligands and phen molecules. Two of these octahedral M(phen)2(Hhfbba) units are attached by hydrogen bonds between the terminal Hhfbba and uncoordinated H2hfbba groups to form a 1D hydrogen-bonding framework (Figure 5c,d). In F-MOF-7 and -9, the free carboxylic acids

(four of them) are H-bonded in a catemeric fashion to form a (COOH)4 cluster (Figure 5b). Hydrogen bonds involving the lattice water molecules act as donors to join both kinds of carboxylic acids to generate a 2D organization in the ab plane. The average O-H 3 3 3 O distance in this finite hydrogen bonded network is 1.700(2) A˚ and the average angle is 172.2 A˚. It is noteworthy that the synthetic conditions and structure for F-MOF-7 and -9 are very similar to previously reported F-MOF-1 (a ∼ 19.5; b = 13.7; c = 29.6 A˚; β = 120.04; V = 6840 A˚3 vs a = 19.35; b = 13.75; c = 26.01 A˚; β = 99.95; V = 6822 A˚3). All these F-MOFs contain similar [MN4O2] SBUs with octahedral Mþ2 ions. The residual positive charge on the metal ion remains as it binds to only one CO2- functionality. Structures of Mn(Hhfbba)2(phen) (F-MOF-11). The 1D structure of F-MOF-11 (space group C2/c) consists of layers of Mn2þ ions coordinated to H2hfbba ligands, phen molecules. As shown in Figure 6a, the asymmetric unit of F-MOF-11 comprises one-half of a Mn2þ ion, one Hhfbba ligand (i.e., singly protonated H2hfbba), and a half molecule

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Figure 6. Crystal structure of F-MOF-11. (a) Polyhedral representation of the SBU showing the coordination environment of Mn (II). (b) 1D helical chain of Mn2þ ions in F-MOF-11. (c) The 1D structure of F-MOF-11 consists of layers of Mn2þ ions coordinated to hfbba ligands, phen molecules. (e) Packing diagram of F-MOF-11 (view down the b axis). Color code: Mn (dark green), N (blue), O (red), C (black), F (green).

of phen (Figure 6a). Each Mn2þ ion is surrounded by four oxygen atoms from four Hhfbba anions and two nitrogen atoms from one chelating phen ligand, composing a slightly distorted octahedral geometry with Mn-O distances ranging from 2.066(4) to 2.217(5) A˚ and Cu-N distances ranging from 2.208(1) to 2.402(3) A˚ (Figure 6b). Four Hhfbba ligands bind through both deprotonated carboxylate oxygens to two adjacent Mn atoms thus forming a carboxylate bridging 1D helical chain, extending along the crystallographic a directions (Figure 6b). Hhfbba ligands are arranged in a roughly tetrahedral manner around the MnO4N2 helix bridging to four other helixes via (COOH)4 dimers (Figure 6d). Structures of [Mn(hfbba)2(dm-phen)] 3 (H2O) (F-MOF11A). In an attempt to perturb the MnO4N2 helix by adding some substitutions in the phen ligand, we used 2,9-dimethyl1,10-phenanthroline in place of phenanthroline. In F-MOF11A we found that the octahedral MnO4N2 SBU is robust enough to hold this geometric perturbation. However, the helical structure in F-MOF-11 changes to a 1D grid due to this. The 1D grid structure of F-MOF-11A (space group P2/n) consists of layers of Mn2þ ions coordinated to hfbba

ligands and phen molecules. As shown in Figure 7a, the asymmetric unit of F-MOF-11 comprises one crystallographically independent Mn2þ ion, one hfbba ligand, one molecule of phen, and one lattice water molecule. Each Mn2þ ion is surrounded by four oxygen atoms, from three hfbba ligands and two nitrogen atoms from one chelating dm-phen ligand, composing a slightly distorted octahedral geometry with Mn-O distances ranging from 2.081(4) to 2.485(5) A˚ and Cu-N distances ranging from 2.264(1) to 2.324(3) A˚ (Figure 7a). Two of the four oxygens with which each Mn atom is coordinated belong to one hfbba ligand, while the other two hfbba ligands bind through both deprotonated carboxylate oxygens to two adjacent Mn atoms thus forming a carboxylate bridging 1D grid structure, extending along the crystallographic c axis (Figure 7c). The dm-phen ligands attached to Mn2þ ions interdigitate between two adjacent layers forming a C-H 3 3 3 O bond34 (D, 3.457(2) A˚; d, 2.593(3) A˚; θ1, 154.1°) with the lattice water molecules (Figure 7d). We tried to investigate the influence of the solvent and temperature on the formation and changes from 1D/0D to 3D when we change the solvent from H2O to DMF.

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Figure 7. Crystal structure of F-MOF-11A. (a) Polyhedral representation of the SBU showing the coordination environment of Mn (II). (b) 1D chain of Mn2þ ions in F-MOF-11A. (c) The 1D structure of F-MOF-11A consists of layers of Mn2þ ions coordinated to hfbba ligands, phen molecules, H2O molecules occupy the void space between the layers. (d) Packing diagram of F-MOF-11A (view down the b axis). Color code: Mn (dark green), N (blue), O (red), C (black), F (green).

However, for Zn(II) and Cu(II), a similar solvent change results in a 0D to 1D structural change. It is noteworthy that as soon as we use DMF as a solvent there is a dimensionality increase for the resulting F-MOFs. A possible reason behind this could be the solubility of the H2hfbba ligand in water. The poor solubility of the fluorinated dicarboxylic acid over the heterocyclic coligands may play an important role in the formation of the H-bonded/1D structures over 2D/ 3D structures. As H2hfbba is the major structural building block, its limited abundance during the formation/ crystallization due to its poor solubility in water may lead to the formation of the H-bonded/1D structures over 2D/3D frameworks (Figure 8). We also tried to computationally verify that the 0D F-MOF structures are more stable in H2O than in DMF as they are cationic-anionic complexes or salts, whereas F-MOFs with 2D/3D frameworks are stable in DMF as they are neutral frameworks. Table 2 summarizes the synthetic conditions of all F-MOFs reported in this paper. Indeed, from the results discussed above, we have come to the conclusion that the choice of solvent

Figure 8. Summary of the structural characteristics of F-MOFs reported in this paper. Of the nine materials, two contain 3D connectivity: (F-MOF-6 and -10). Four materials are 1D structure: (F-MOF-5, -8, -11, -11A), and three (F-MOF-1, -7, -9) are discrete 0D molecular complexes.

and reaction condition is critical in determining the molecular structures of the resultant F-MOFs. Our results indicate that for metals such as Co(II) and Mn(II) the structure.

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Table 2. Summary of the Synthetic Conditions of F-MOFs Reported in This Paper F-MOF

solvent

temperature (°C)

time (h)

6 7 8 9 10 11 11A

DMF H2O DMF H2O DMF H2O H2O

85 120 85 120 85 120 120

72 72 72 72 72 72 72

Pachfule et al. Table 3. Relative Energies, Obtained from DFT Calculations, for the Structures Denoted As Case I and Case II in Figure 2

ratio (M: L1: L2) 1: 4.17: 0.67 1: 5.61: 0.65 1: 4.03: 0.65 1: 5.44: 0.63 1: 4.03: 0.65 1: 5.61: 0.65 1: 5.61: 0.71

Figure 9. Top: The two copper complexes, crystal structures for which have been obtained in different solvents (DMF and water). Bottom: The two different structures that were investigated using DFT: in one case, the phen ligand is displaced by the H2hfbba ligand (Case I), while in Case II, the H2hfbba ligand is displaced by the neutral phen ligand.

DFT Studies on F-MOF-1 and -5. The crystal structures obtained for [Cu(hfbba)(phen)2 3 2(H2hfbba)(H2O) (HCO2)] (F-MOF-1) and [Cu(hfbba)2(phen)2 3 0.5(DMF)] (F-MOF-5) indicate that there are two possible complexes that can form during the crystallization process, shown in Figure 9. F-MOF-5 is obtained when the solvent employed is DMF (ε = 38.5) and F-MOF-1 is crystallized out from water (ε = 80.0). We decided to investigate, using DFT, the possible reasons as to why changing the solvent results in a change in the complex formed. The computational approach that was adopted is as follows: we looked at two possibilities, shown as Case I and Case II in Figure 9 below. In Case I, the structure of F-MOF-5 was optimized along with an extra phen moiety that was not bound to the complex. This was done so that an exact comparison could be done with the other complex, shown as Case II in Figure 9. In this structure, the structure of F-MOF-1 was optimized with an extra anion of the acid also included, nor coordinated to the complex. Thus, the two eventual structures in the Cases I and II contain the same number of atoms, and an energy comparison between these two structures would give us insight into the relative abilities of the two ligands: the neutral phen and hfbba, to compete for the copper center. Table 3 shows the results obtained from the DFT calculations: the ΔG values show the stabilities of the structures shown in Figure 9: Cases I and II, relative to each other. The geometry optimizations were done in the gas phase, as well as in the two different solvents DMF and H2O. In the gas phase, as the values in Table 1 indicate, the anionic acidic species is

gas phase solvent DMF solvent water

relative ΔG values (kcal/mol) Case I

relative ΔG values (kcal/mol) Case II

0.00 0.00 0.00

þ7.1 þ0.5 -2.8

not stabilized, leading to the structure shown as Case I to be significantly more stable, by 7.1 kcal/mol. It is interesting to note that the relative stability of this complex is significantly reduced to only 0.5 kcal/mol, when the geometry optimization is done in DMF. The reason for this is that the polarity of DMF is also able to stabilize the separated anionic species (constituent of the F-MOF-1), making it almost as stable as the Case I complex (constituent of the F-MOF-5). However, since the Case I complex remains more stable, albeit by a much smaller margin, it is F-MOF-5 that is eventually obtained upon crystallization in DMF. Now, as Table 1 indicates, it becomes clear that the significantly greater polarity of water (ε = 80.0 as compared to DMF’s ε = 38.5) acts to stabilize the separated anionic species to a greater extent, and hence the Case II complex (constituent of the F-MOF-1) turns out to be more stable than the Case I complex, by 2.8 kcal/mol. Therefore, it is this F-MOF-1 that is obtained experimentally when the solvent used is water. A look at the optimized geometries obtained from the DFT calculations further underlines this fact. Figure 10 shows the three complexes obtained for Case II (constituent of the F-MOF-1) when optimized in the gas phase and in the two solvents DMF and H2O. It is seen that the separation between the copper complex and the anionic acidic species increases as the polarity of the solvent is increased: the separation between one of the oxygens in the anionic ligand and the copper center is 4.753 A˚ when the structure is optimized in DMF, while it is 4.925 A˚ when the structure is optimized in the more polar solvent water. This is because Coulombic interactions are decreased in solvents of higher dielectric, thereby stabilizing ionic species in solution. This also allows the entropic effects, included in the calculations, to favor the formation of F-MOF-1 in water. This effect is not observed in Case I, because here the separated ligand is the neutral phenanthroline ligand. As Figure 10 shows, the separation of the ligand from the copper complex remains almost constant. The results obtained indicate that the increase in the polarity of the solvent will tend to favor the crystallization of the F-MOF-5 (1D) structure over the FMOF-1 (H-bonded) structure.35 Thermal Stability and PXRD. To examine the architectural and thermal stability of F-MOFs reported in this paper, we prepared them at the gram scale to allow detailed investigation of the aforementioned properties. TGA performed on as-synthesized F-MOF-6 to -11A revealed that these compounds have high thermal stability (see Section S3 in Supporting Information, for all data and interpretations regarding guest mobility and thermal stability of F-MOF-6 to -11A). The TGA of compounds F-MOF-7, -8, -9, -11, and -11A (Figure 12h) demonstrates that the compound has an excellent thermal stability as no strictly clean weight loss step occurred below 300 °C. This indicates that the coordinated water/solvent molecules leave the framework at the point of decomposition. A sharp weight loss at 300 °C indicates decomposition of the host framework. TGA trace for F-MOF-6 and -10 is totally different from the other F-MOFs

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Figure 10. Six geometry optimized structures for Case I and II. (a) Case I structure optimized in gas phase; (b) Case I structure optimized in DMF; (c) Case I structure optimized in water. (d) Case II structure optimized in gas phase; (e) Case II structure optimized in DMF; (f) Case II structure optimized in water. The color code: Cu (light blue), N (blue), O (red), C (gray), F (green), H (black).

reported in this paper as it possess 1D channels (5.5 A˚ in diameter). The TGA trace for F-MOF-6 showed a gradual weight-loss step of 20.3% (25-200 °C), corresponding to escape of all DMF and water molecules trapped in the pores (4 DMF; calcd. 18.3% and one H2O) followed by a plateau (200-350 °C). A similar TGA has been observed for F-MOF-10 where all DMF and water molecules trapped in the pores escaped below 200 °C (loss of 2 DMF; calcd. 19.3% and one water molecules). In order to confirm the phase purity of the bulk materials, PXRD experiments were carried out on all complexes. The PXRD experimental and computer-simulated patterns of all of them are shown in Supporting Information (Figures S1-S7). As shown in Figure 11, all major peaks of experimental PXRD patterns of compounds F-MOF-6 to -11A match quite well with that of simulated PXRDs, indicating their reasonable crystalline phase purity. The experimental

patterns of F-MOF-7 and -9 have a few diffraction lines that are unindexed and some that are slightly broadened in comparison with those simulated patterns. This is probably due to the loss of H-bonded water molecules from the lattice which can produce a different phase than that obtained in the single crystalline form. Incidentally, investigations with F-MOF-7 and -9 showed that no weight loss step occurred below 300 °C, which also indicates that the loss of H-bonded water occurs before the TGA experiment. Adsorption Properties of F-MOF-6. The sample of F-MOF-6 was prepared by a thorough washing of crystals with CHCl3 followed by an evacuation at 120 °C for 3 h. The BET surface area of F-MOF-6 was found to be 20 m2/ g. This is quite low and may therefore indicate a constriction of the channels after solvent removal. Hydrogen adsorption isotherms for F-MOF-6 are shown in Figure 12. While the volume of hydrogen adsorbed by F-MOF-6 is

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from the combination of metal carboxylates with a heterocyclic coligand. We successfully synthesized seven new F-MOF structures with a flexible fluorinated dicarboxylate building block, phenanthroline, and different transition metal cations [Zn(II), Co(II), Mn(II), and Cu(II)]. Fluorinated dicarboxylates have been largely neglected to date in MOF synthesis and can be used to obtain diverse MOF structures. These materials could possibly emerge as hydrogen storage materials, as they have a promising H2 uptake and strong binding properties. The structural differences between F-MOF-5, -6, -8, and -10 and -7, -9, -11, and -11A clearly demonstrate the critical role of conditions in determining the product structure. These materials exhibit a great degree of structural diversity despite similar structural components, that is, the dicarboxylate building block, phenanthroline and Cu. These F-MOFs therefore provide an excellent reminder that structure and dimensionality in MOF synthesis are often highly dependent on subtle differences, irrespective of the choice of metal and ligand, and that the resulting structures as a whole are difficult to predict. A rationalization of the influence of the solvent was attempted with sophisticated quantum chemical calculations with a good basis set using DFT, and it was found that the polarity of the solvent played a major role in determining the relative stabilities of complexes in solution, and therefore, in deciding the eventual structure that was crystallized out of solution. We are continuing to utilize other substituted, neutral, nitrogen-donor ligands to rationally design and synthesize new MOFs with specific structure and properties.

Figure 11. (Top) Comparison of PXRD patterns of the as-synthesized F-MOF-6 (red) with the simulated pattern from the singlecrystal structure (blue). (Bottom) Overlay of TGA traces of assynthesized F-MOFs indicating their stability reported in the paper.

Acknowledgment. P.P. acknowledges CSIR for a project assistantship (PA-II) from CSIR’s XIth Five Year Plan Project (NWP0022-H). C.D. and T.P. acknowledges CSIR, New Delhi, India, for fellowship support. K.V. acknowledges the Centre for Excellence in Scientific Computing (COESC), NCL. R.B. and K.V. acknowledge Dr. S. Sivaram, Director NCL for start-up grants and CSIR’s XIth Five Year Plan Project (Grant No: NWP0022-H) for funding and Dr. S. Pal and Dr. K. Vijaymohanan for their encouragement. Supporting Information Available: Description of experimental details, including synthetic methods, crystallography, supplementary figures, including TGA, infrared spectroscopy, powder XRD profiles, tables of crystallographic data and CIF files, and anisotropic thermal ellipsoids for F-MOFs reported in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 12. H2 isotherm of the F-MOF-6 shows an uptake of 0.9 wt % at 77 K. Filled and open symbols represent adsorption and desorption branches, respectively.

rather modest (0.9 wt % H2) at 77 K, this report once again proves that hybrid materials containing a per-fluorinated ligand can immerge as a hydrogen storage materials. Conclusion In this contribution, we have tried to understand the influences of solvent variables on the resulting MOF structure

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