A Microporous Co2+ Metal Organic Framework with Single-Crystal to

Nov 21, 2014 - Department of Chemistry, University of Cyprus, Nicosia 1678, Cyprus. ‡ ... National and Kapodistrian University of Athens, Panepistim...
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A Microporous Co2+ Metal Organic Framework with Single-Crystal to Single-Crystal Transformation Properties and High CO2 Uptake Eleni E. Moushi,† Andreas Kourtellaris,† Ioannis Spanopoulos,∥ Manolis J. Manos,‡ Giannis S. Papaefstathiou,§ Pantelis N. Trikalitis,∥ and Anastasios J. Tasiopoulos*,† †

Department of Chemistry, University of Cyprus, Nicosia 1678, Cyprus Department of Chemistry, University of Ioannina, Ioannina 45110, Greece ∥ Department of Chemistry, University of Crete, Voutes 71003, Heraklion, Greece § Laboratory of Inorganic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou 157 71, Greece ‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of {[Co9(INA)18(H2O)6]·11DMF·15H2O}∞ (Co9-INA·11DMF· 15H2O) (INA− = the anion of isonicotinic acid) is reported. It exhibits a rigid 3D-porous structure with a Co9 repeating unit consisting of four [CoII2(μ-O2CR)2(μ-H2O)] subunits (two unique) linked through bridging INA− ligands to an isolated CoII ion (half unique). The [CoII2] dimers and the isolated CoII ion have assembled to create a trinodal (6,7,8)coordinated network with point symbol (32 .4 11.5 6.6 2) 2(32.418.54.64)2(34.44.54.63). Gas sorption studies revealed that Co9-INA exhibits 910 m2 g−1 BET area, 4.2 mmol g−1 CO2 uptake at 273 K/1 bar, and 6.7 CO2/CH4 selectivity at zero coverage. Furthermore, Co9-INA displays capability for exchange of the guest solvent molecules by various organic molecules in a single-crystal to single-crystal fashion. Direct and alternating current magnetic susceptibility studies revealed the existence of dominant antiferromagnetic interactions between the Co2+ ions that result in a paramagnetic ST = 3/2 spin ground state value. Overall, this work emphasizes the potential of relatively simple and inexpensive polytopic ligands, such as isonicotic acid, to stabilize microporous MOFs with significant CO2 sorption capacity.

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INTRODUCTION Metal organic frameworks (MOFs) are porous materials based on metal ions or clusters and bridging organic ligands.1−3 Over the past decade there has been a tremendous research interest in the construction of such materials mainly because of their intriguing architectures and novel physical properties that lead to potential applications in a series of areas including gas storage and separation,4−8 catalysis,9 magnetism,10−12 sensing,13 etc. Microporous MOFs containing an appreciable internal surface area and flexible chemical composition have attracted particular concern since they are strong candidates for use in the adsorption and storage of gases of environmental interest such as H2, CH4, and CO2 and they are also good candidates for use in single-crystal to single-crystal (SCSC) transformation reactions.14−33 Concerning the adsorption of gases with environmental interest, special attention is given to CO2 capture, since it is the primary anthropogenic greenhouse gas, and its sequestration from the environment has drawn a considerable amount of interest. Additionally, it has potential for the purification and storage of flue gases and fuel.17,18 Selective trapping of CO2 © XXXX American Chemical Society

from air or the emission of coal-fired power plants under ambient conditions is a main priority for many research groups. Currently, several methods are used for this purpose; however, many of them involve a high energy penalty and are inefficient in terms of releasing adsorbed CO2 molecules.34 Thus, alternative methods have been proposed, with many including a range of porous materials. Among the various classes of the latter, MOFs are possibly the most promising ones for this goal due to their high surface areas, adjustable and ordered structures, open metal sites, and low density.14−18,34 Studies in this field have focused on (a) the rational design and synthesis of novel frameworks and the understanding of the relationships between their structures and CO2 uptake and (b) the ability of MOFs to retain their open framework throughout cycles of removal and resorption of guest molecules. However, for practical applications, the cost of the materials synthesis is an issue of vital importance, and thus, MOFs/CO2 sorbents Received: July 30, 2014 Revised: October 16, 2014

A

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Table 1. Selected Crystal Data for Co9-INA and Its Exchanged Analogues empirical formula fw T (K) radiation cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (g/cm3) μ (mm−1) reflns collected unique reflns Rint R1a [I > 2σ(I)] wR2b (all data) GOF Δρmin/max (e Å3) a

Co9-INA

Co9-INA/Bz

Co9-INA/Tol

C108H72Co9N18O50 2952.21 100(2) Cu Kα (λ = 1.54180 Å) monoclinic C2/c 31.263(5) 25.383(5) 26.217(5) 116.261(5) 18 657(6) 4 1.051 6.661 35 569 16 587 0.0507 0.0592 0.1651 0.938 1.253/−0.759

C120H84Co9N18O42 2980.42 100(2) Cu Kα (λ = 1.54180 Å) monoclinic C2/m 31.112(2) 25.326(2) 12.902(2) 113.280(7) 9339(2) 2 1.060 6.625 17 729 8507 0.0480 0.0613 0.1816 1.005 0.937/−0.776

C121H92Co9N20O44 3060.52 100(2) Cu Kα (λ = 1.54 180 Å) monoclinic P2/n 12.653(5) 25.497(5) 28.551(5) 90.945(5) 9210(4) 2 1.104 6.740 32 436 16 312 0.0464 0.0827 0.2735 1.034 2.688/−1.334

R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = {Σ[w(|Fo|2 − |Fc|2)2]/Σ[w(|Fo|4)]}1/2.

coverage and 273 K). Moreover, Co9-INA displays a remarkable capability to undergo single-crystal to single-crystal (SCSC) transformations involving insertion/removal of guest solvent molecules.

with readily available and low cost polytopic ligands are more attractive and more likely to be commercialized. One quite inexpensive commercially available polytopic ligand that has been proven powerful for the construction of multidimensional metal−organic coordination networks is isonicotinic acid (HINA).35−56 This is because this ligand tends to bind metal centers with both pyridyl N and carboxylate O donor atoms to form extended networks with various structures, such as chains, ladders, bricks, square grids, wheels, cages, etc.54 Moreover, the carboxylate group helps to balance the metal charges resulting in neutral species containing in their pores only guest solvent molecules that could be potentially removed through various solvent exchange processes. Microporous metal organic frameworks containing pores of appreciable size are also excellent candidates for use in SCSC transformation reactions19−33 since guest molecules can fairly easily enter into their pores without the need to employ drastic conditions during the insertion process that would deteriorate the compound crystallinity. Such SCSC transformation reactions lead to mainly three different types of structural alterations: (a) modifications of the organic ligands,19−22 (b) insertion/removal of guest molecules,23 and (c) changes in the coordination environment of metal ions.23−26 Such postsynthetic modifications are particularly important, since they can afford MOFs with guest-induced properties and multiple functions (from the insertion of suitable guests, SCSC of type b) or MOFs with fine-tuned properties (from the introduction of proper functional groups, SCSC of types a and c).25,27−29 We herein report a new MOF {[Co9(INA)18(H2O)6]· 11DMF·15H2O}∞ denoted as Co9-INA that was prepared from the combination of HINA and Co(NO3)2 in DMF/H2O under solvothermal conditions. Co9-INA exhibits a rigid 3Dporous structure with interesting structural features. Co9-INA shows an appreciable BET area (910 m2 g−1), the highest among the known Co-INA MOFs, and significant CO2 uptake (4.2 at 273 K/1 bar) and selectivity over CH4 (6.7 at zero

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EXPERIMENTAL METHODS

Materials. Reagent grade chemicals were obtained from Aldrich and used without further purification. Water was distilled in-house Syntheses. Co9-INA. HINA (0.05 g, 0.41 mmol) was dissolved in DMF/H2O (5:1 mL) in a 20 mL glass vial, and to this solution was added solid [Co(NO3)2]·6H2O (0.10 g, 0.34 mmol). The mixture was sonicated for 2 min and, then, heated without stirring at 87 °C for 24 h. During this period, purple polyhedral-like crystals of Co9-INA were formed. They were isolated by filtration, washed several times with DMF and diethyl ether, and dried under vacuum. Yield: ∼67%. The dried crystalline product was analyzed as [Co9-INA·11DMF·15H2O] Anal. Calcd for C141H191O68N29Co9: C 43.31, H 4.92, N 10.39. Found: C 43.42, H 4.98, N 10.56. Co9-INA/X (X = Benzene, Toluene). Single crystals of Co9-INA· 11DMF·15H2O (0.01 g, 0.0026 mmol) and the solvent (5 mL) were mixed in a 23 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and placed in an oven operated at 60 °C for toluene or 100 °C for benzene, remained undisturbed at this temperature for 2 days, and then allowed to cool at room temperature. The crystals of the exchanged compound were isolated by filtration and dried in air. The dried crystalline products were analyzed as [Co9-INA·2Bz·24H2O] Anal. Calcd for C120H144O66N18Co9: C 42.08, H 4.24, N 7.36. Found: C 42.27, H 4.39, N 7.48. Also, [Co9-INA·2Tol·4DMF·22H2O] Anal. Calcd for C134H172O68N22Co9: C 43.39, H 4.67, N 8.31. Found: C 43.52, H 4.83, N 8.42. Single-Crystal X-ray Crystallography. Single-crystal X-ray diffraction data were collected on an Oxford-Diffraction Supernova diffractometer, equipped with a CCD area detector utilizing Cu Kα (λ = 1.5418 Å) radiation. Suitable crystals were attached to glass fibers using paratone-N oil and transferred to a goniostat where they were cooled for data collection. Empirical absorption corrections (multiscan based on symmetry-related measurements) were applied using CrysAlis RED software.57 The structures were solved by direct methods using SIR200458 and refined on F2 using full-matrix leastsquares with SHELXL97.59 Software packages used were as follows: B

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CrysAlis CCD for data collection,57 CrysAlis RED for cell refinement and data reduction,57 WINGX for geometric calculations,60 and DIAMOND61 for molecular graphics. The non-H atoms were treated anisotropically, whereas the aromatic H atoms were placed in calculated, ideal positions and refined as riding on their respective carbon atoms. The H atoms of water molecules could not be located. Electron density contributions from disordered guest molecules were handled using the SQUEEZE procedure from the PLATON software suit.62 Several restraints (DFIX, ISOR, DELU) have been applied in order to limit the disorder of the ligated and guest molecules of the three crystal structures. Selected crystal data for Co9-INA and the exchanged compounds are summarized in Table 1. CCDC 1015676− 1015678 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac. uk/data_request/cif. Physical Measurements. Elemental analyses (C, H, N) were performed by the in-house facilities of the University of Cyprus, Chemistry Department. IR spectra were recorded on KBr pellets in the 4000−400 cm−1 range using a Shimadzu Prestige −21 spectrometer. PXRD diffraction patterns were recorded on a Shimazdu 6000 Series X-ray diffractometer (Cu Kα radiation, λ = 1.5418 Å). Thermal stability studies were performed with a Shimadzu TGA 50 thermogravimetric analyzer. Variable-temperature dc magnetic susceptibility data down to 1.80 K were collected on a Quantum Design MPMS-XL SQUID magnetometer equipped with a 70 kG (7 T) dc magnet. Diamagnetic corrections were applied to the observed paramagnetic susceptibilities using Pascal’s constants. Samples were embedded in solid eicosane to prevent torquing in the dc field. Alternating current magnetic susceptibility data were collected on the same instrument employing a 3.5 G ac field oscillating at frequencies up to 1500 Hz. Gas Sorption Measurements. Low-pressure argon, hydrogen, carbon dioxide, and methane adsorption measurements were carried out on an Autosorb 1-MP instrument from Quantachrome equipped with multiple oil-free pressure transducers for highly accurate analyses and an oil-free vacuum system. Ultrahigh purity grade Ar (99.999%), He (99.999%), H2 (99.999%), CO2 (99.999%), and CH4 (99.9995%) were used for all adsorption measurements. Prior to analysis, as-made Co9-INA was soaked in CH2Cl2 at room temperature for three days during which the supernatant solution was replaced six times. The dichloromethane suspended samples were transferred inside the chamber of a supercritical CO2 dryer (Bal-Tec CPD 030), and CH2Cl2 was exchanged with liquid CO2 over a period of 5 h at 8 °C. During this period, liquid CO2 was vented under positive pressure every 5 min. The rate of CO2 venting was always kept below the rate of filling so as to maintain full drying conditions inside the chamber. Following venting, the temperature was raised to 40 °C (above the critical temperature of CO2), kept there for 1 h, and then slowly vented over the period of 1 h. The dried sample was transferred immediately inside a preweighted, argon-filled 9 mm cell and closed using CellSeal provided by Quantachrome to prevent intrusion of oxygen and atmospheric moisture during transfers and weighing. The cell was then transferred to the outgassing station where the sample was evacuated under dynamic vacuum at room temperature until the outgas rate was less than 2 mTorr/min. After evacuation, the sample and cell were reweighed to obtain the precise mass of the evacuated sample. Finally, the tube was transferred to the analysis port of the gas adsorption instrument.

Note that HINA is a very important and well-used ligand for MOFs chemistry since it can bind to the metal ions with both its pyridyl nitrogen atom and carboxylate oxygen atoms and result in multidimensional coordination polymers. The excellent bridging ability of HINA has been experimentally proven from the isolation of an important number of MOFs with several 3d metal ions such as Cu2+, Mn2+, Co2+, Zn2+, Ni2+, etc.54 Compound Co9-INA was initially formed together with the known [Co(INA)2]·xH2O40 from the reaction of [Co(NO3)2]· 6H2O and HINA in a molar ratio 1:1.2 in DMF at 100 °C. In order to isolate only the Co9-INA compound, several modifications were applied to the reaction mixture which included use of (i) various molar ratios of the starting materials, (ii) various reaction solvents [dimethylacetamide (DMA), Nmethyl-2-pyrrolidone (NMP)] or solvent mixtures, (iii) different reaction temperatures. One of these reaction variations that included use of a DMF/H2O solvent mixture instead of DMF resulted in the isolation of Co9-INA complex as the only product. The choice of the DMF/H2O solvent mixture was based on the observation that Co9-INA contained ligated H2O molecules whereas the known compound did not. Thus, it was anticipated that the use of H2O in the reaction mixture will direct the equilibrium toward the formation of the product that contains H2O, i.e., Co9-INA, a hypothesis that was proven to be correct. Thus, the reaction of [Co(NO3)2]·6H2O with HINA in a molar ratio 1:1.2 in DMF/H2O (5:1) at 87 °C afforded single crystals of Co9-INA in 67% yield. Crystal Structure. Representations of the Co9 repeating unit and the 3D structure of Co9-INA are shown in Figures 1 and 2, respectively. Compound Co9-INA crystallizes in the monoclinic space group C2/c and features a neutral threedimensional porous framework with guest DMF and water molecules. Its asymmetric unit consists of one-half of the Co9 repeating unit and several DMF and water lattice molecules.

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RESULTS AND DISCUSSION Synthesis. We have been systematically investigating over the past few years the use of polytopic organic ligands together with various amino-alcohols as a method for the synthesis of new MOFs.63 These investigations were extended to the use of simple, well-known N-donor/carboxylate ligands in the presence or not of amino-alcohols. In this work, we report a new MOF that was isolated from our investigations on the use of the well-known N-donor ligand HINA in Co2+ chemistry.

Figure 1. Representation of the repeating unit of Co9-INA. H atoms, lattice water, and DMF molecules have been omitted for clarity. Color code: Co, purple; O, red; N, blue; C, yellow. C

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PLATON (taking into account the van der Waals radii of the atoms).62 A representation of the pore network of Co9-INA with the structure visualization program MERCURY68 reveals a continuous and complex 3D pore network, where the cavities communicate through wide channels with a diameter ≥4 Å (Figure 3). When this study was in progress a Ni2+ and a Co2+INA MOFs displaying related structures to that of Co9-INA were reported.69,70

Figure 2. Representation of the 3D porous structure of Co9-INA along a axis. H atoms, lattice water, and DMF molecules have been omitted for clarity. Color code: Co, purple; O, red; N, blue; C, yellow.

There are three types of subunits (SBUs) in the structure that are linked together to form the nonanuclear repeating unit: (i) a mononuclear [Co(INA)6] unit consisting of an octahedral Co2+ ion (Co5) linked to two O atoms from two monodentate COO− groups of different INA− ligands and four pyridyl N atoms from four different INA− ligands and (ii) two dinuclear [Co2(μ2-H2O)(INA)x] units ([Co2]A, Co1−Co2; [Co2]B, Co3−Co4) where each Co ion has CoO4N2 (Co1, Co2, and Co4) or CoO5N (Co3) octahedral coordination geometry and the Co···Co separations in [Co2]A and [Co2]B units are 3.602 and 3.577 Å, respectively. Each [Co2(μ2-H2O)(INA)x] moiety is also doubly bridged by two syn−syn carboxylate groups from two INA− ligands. Although both types of dinuclear units have the same [CoII2(μH2O)(μ-O2CR)2] core as described above they differ in the peripheral ligation since the Co1/Co2 ions in [Co2]A are further coordinated to four pyridyl N atoms and two monodentate carboxylate O atoms from six different INA− ligands, whereas Co3/Co4 ions in [Co2]B are coordinated to three pyridyl N atoms, two monodentate carboxylate O atoms from five different INA− ligands, and a terminal water molecule. It is interesting to note that even though there are several coordination modes reported for INA− ligand, in Co9-INA only two of them appear. In particular, 10 INA− ligands adopt the μ2:η1:η1 bridging mode and the other 8 the μ3:η1:η1:η1 one (Scheme S1, f, h, Supporting Information).64 The mononuclear [Co(INA)6] unit and the dinuclear [Co2]A and [Co2]B units are interconnected through INA− ligands to create a 3D framework. The compound displays a trinodal (6,7,8)-coordinated network with stoichiometry of (6-c)(7-c)2(8-c)2 and point symbol (32.411.56.62)2(32.418.54.64)2(34.44.54.63) (Figure S1, Supporting Information).65−67 The framework pores accommodate guest solvent molecules (water and DMF) that are involved in hydrogen bonding interactions with carboxylic oxygen atoms of the INA− ligands as well as coordinated water molecules. The latter are also hydrogen bonded to carboxylic oxygen atoms of INA− ligands. The solvent-accessible volume of Co9-INA calculated by PLATON62 (excluding all solvents from the pores) is 18 657.0 Å3, corresponding to 50.9% of the unit cell volume. The 3D structure contains cavities with diameters ∼7 Å as found by

Figure 3. Representation of the pore network (shown in yellow) of Co9-INA (only pores and channels with diameter ≥4 Å are shown) along c axis. Color code: Co, purple; O, red; N, blue; C, gray.

Comparison of Co 9-INA with Reported Co-INA Complexes. At this point, it is worth comparing the structure of Co9-INA with those of other Co-INA complexes reported in the literature. A Cambridge database search revealed that there are 15 Co complexes containing the INA− ligand as the only organic bridging ligand, 13 of which display polymeric structures, and the other 2 are 0D compounds.40−54 Some complexes also contain OH−, OCH3−, HCO2−, and N3− bridging ligands. Selected structural data for all Co-INA complexes are provided in Table 2. This comparison revealed that Co9-INA possesses the highest nuclearity repeating unit among the Co-INA networks. Although the dinuclear SBUs [Co2]A and [Co2]B present in the Co9-INA have already been found in other Co-INA complexes,41 the mononuclear moiety [Co(INA)6], where the Co2+ ion is linked to two monodentate Ocarboxylate atoms and four pyridyl N atoms of six different INA− ligands, appears for the first time in the literature. Comparing the topological features of compound Co9-INA with those of known Co-INA complexes (Table 2), it was realized that Co9INA is the only Co-INA complex with a unique 3-nodal (6c)(7-c)2(8-c)2 network topology. The novel topological features of Co9-INA have probably resulted from the conjunction of the unprecedented mononuclear unit with the [Co2]A and [Co2]B dinuclear SBUs. We note that the two recently reported Ni2+ and Co2+ analogous MOFs display very similar topological features to those of Co9-INA.69,70 Solvent Exchange Single-Crystal to Single-Crystal Transformation Studies. The fact that Co9-INA has a relatively open structure and good stability in air (for several months) and various solvents in combination with the presence of highly disordered solvent molecules in its pores motivated us to investigate the solvent exchange properties of this compound with various solvents. Thus, single-crystal solvent exchange D

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Table 2. Selected Structural Data for Co-INA Compounds from the Literature and the Present Work compd 1 2 3 4 5 6 7 8 9 10 11 12 13 14 a

dimensionality

[Co2(INA)4(H2O)] [Co(INA)2] [Co4(INA)5(μ3-OH)2(H2O)(EtOH)]NO3] [Co(INA)2(H2O)2] [Co1.5(N3)(OH)(INA)] [{Co3(INA)7.5(μ3-O)}{Co3(INA)2.5(μ3-OH)(μ-OH)(C2H5OH) (H2O)2}3]NO3 [Co3(INA)4(OH)(C2H5OH)3](NO3) [Co(INA)2] [Co(INA)(HINA)(H2O)(NO3)] [Co(NA)(INA)]a [Co(INA)2(H2O)4] [Co8(OCH3)6(HCO2)4(INA)6] [Co(INA)2] [Co9(INA)18(H2O)6]

net

ref

3D 3D 3D 0D 3D 3D

ecu: 36.415.57 rtl: (4.62)2(42.610.83) vmr: 33.411.56.6 N/A 3,3,6,6T1: (4.82)2(42.6)4(45.64.85.10)2(46.82.107) (34.49.56.62)3(36.415.515)

41 40 42 43 44 45

3D 3D 1D 3D 0D 3D 3D 3D

hex: 36.418.53.6 sqc518: 46.64 N/A bcu: 424.64 N/A (3.6.7)4(32.4.52.64.74.82)2(34.42.54.614.74) dia: 66 (32.411.56.62)2(32.418.54.64)2(34.44.54.63) 3-nodal (6-c)(7c)2(8-c)2

46 48 49 50 51 52 53 this work

NA is the anion of nicotinic acid.

Figure 4. Representations of the 3D porous structure of (a) Co9-INA/Bz and (b) Co9-INA/Tol. H atoms and lattice water and DMF molecules have been omitted for clarity. Color code: Co, purple; O, red; N, blue; C, yellow.

benzene (2 per Co9-INA) and water molecules and no DMF (Figure 4a), whereas Co9-INA/Tol contains guest toluene (2 per Co9-INA, one of which was severely disordered and removed with SQUEEZE, see below), DMF, and water molecules (disordered molecules were handled using the SQUEEZE procedure from the PLATON software suite), Figure 4b. The solvent-accessible volumes of Co9-INA/Bz and Co9-INA/Tol calculated by PLATON62 (excluding all solvents from the pores) are 9338.7 and 9210.0 Å3 corresponding to 47.8% and 49.2% of their unit cell volumes, respectively. The 3D structures contain cavities with diameters ∼6.4 Å (for Co9INA/Bz) and 6.8 Å (for Co9-INA/Tol) as found by PLATON (taking into account the van der Waals radii of the atoms).62 A representation of the pore network of Co9-INA/Bz and Co9INA/Tol with the structure visualization program MERCURY68 reveals for both complexes a continuous and complex 3D pore network, where the cavities communicate through wide channels with a diameter ∼3.2 Å. The above values for the solvent accessible volumes pore and channels sizes for Co9INA/Bz and Co9-INA/Tol are very similar to those for the

experiments were carried out with environmentally hazardous organic molecules such as benzene and toluene. To undoubtedly confirm the replacement of DMF solvent molecules by the two solvents, single-crystal X-ray crystallography measurements were performed. Heterogeneous solventexchange reactions of single-crystals of Co9-INA with toluene at 60 °C and benzene at 100 °C resulted in single crystals that were macroscopically very similar in shape and size to those of the pristine Co9-INA compound. Both processes were proven to be SCSC transformations by the determination of the crystal structures of the exchanged compounds. Furthermore, the preservation of the crystallinity of the exchanged compounds is reflected on the good refinement of their crystal structures (all reported R1 values are