A Porous and Interpenetrated Metal–Organic Framework Comprising Tetranuclear IronIII–Oxo Clusters and Tripodal Organic Carboxylates and Its Implications for (3,8)-Coordinated Networks
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2290–2293
Sang Beom Choi,† Min Jeong Seo,† Miyoung Cho,† Yunjeong Kim,† Mi Kyung Jin,‡ Duk-Young Jung,‡ Jung-Sik Choi,§ Wha-Seung Ahn,§ Jesse L. C. Rowsell,| and Jaheon Kim*,† Department of Chemistry, Soongsil UniVersity, Seoul 156-743, Korea, Department of Chemistry, Sungkyunkwan UniVersity, Suwon 440-746, Korea, Department of Chemical Engineering, Inha UniVersity, Incheon 420-571, Korea, and Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074 ReceiVed July 12, 2007; ReVised Manuscript ReceiVed September 17, 2007
ABSTRACT: A porous metal–organic framework (MOF), Fe4(µ3-O)2(BTB)8/3(DMF)2(H2O)2 · (DMF)10(H2O)2 (1) (BTB ) benzene-
1,3,5-tribenzoate, DMF ) N,N-dimethylformamide) has two interpenetrating 3D frameworks composed of fused cages and shows a stronger affinity for CO2 than most other known highly porous MOFs. The unique tetranuclear iron–oxo clusters act as 8-connectors, which are linked by 3-connecting BTBs, to give a partially augmented the net (using the symbolism of O’Keeffe and co-workers). The underlying implications for (3,8)-coordinated networks are also discussed in relation to the framework structure of 1. Metal–organic frameworks (MOFs) are crystalline, extended structures composed of inorganic and organic secondary building units (SBUs), such as metal oxide clusters and aromatic polycarboxylates.1 They have potential applicability in such fields as magnetism,2 molecular separation,3 chiral catalysis,4 and fuel gas or CO2 storage.5,6 This versatility stems from their unique structures, which have sufficient rigidity to maintain nanosized pores and, in some cases, dynamic responses to guest species.7 It is expected that additional novel properties or niche applications will be discovered as our knowledge of the available MOF structure types is expanded; however, the common topologies (i.e., the nets describing the geometric connectivity of the SBUs) that MOFs adopt is small in number.8 To discover MOFs with greater structural diversity, it is a prerequisite to synthesize new types of SBUs. Already, square M2(O2C–)4, octahedral Zn4O(O2C–)6, and trigonal prismatic M3O(O2C–)6 clusters have been widely used to assemble highly porous materials.1,9,10 Here, we report the synthesis, structure, and properties of an interpenetrated, porous material assembled from 8-coordinated prismatoid iron–oxo clusters. When benzene-1,3,5-tribenzoic acid (H3BTB) (0.12 g, 0.27 mmol) and FeCl2 · 4H2O (0.22 g, 1.1 mmol) in 10.0 mL of N,Ndimethylformamide (DMF) were heated in a sealed tube at 130 °C for 1 day, dark brown cubic crystals were formed, of which bulkphase purity was confirmed by powder X-ray diffraction (PXRD). Using a combination of elemental analysis, thermogravimetric analysis (TGA), superconducting quantum interference device (SQUID) measurement, and single-crystal X-ray diffraction analysis,11 the material was formulated as Fe4O2(BTB)8/3(DMF)2(H2O)2 · (DMF)10(H2O)2 (1), with the structure illustrated in Figure 1. The crystal structure of 1 contains an iron–oxo cluster, wherein four octahedrally coordinated cations are bridged by two pyramidalized µ3-O (Figure 1a). The four Fe atoms are coplanar; Fe(1) and its symmetry equivalent, Fe(1a), are ligated by four * To whom correspondence should be addressed. Fax: +82-2-824-4383. E-mail:
[email protected]. † Soongsil University. ‡ Sungkyunkwan University. § Inha University. | Oberlin College.
carboxylate oxygens, a DMF oxygen atom, and a µ3-O, while Fe(2) and Fe(2a) are ligated by three carboxylate oxygens, a water molecule, and two µ3-O. The bonds between Fe and µ3-O are 1.837(3) and 1.927(3) Å for Fe(1) and Fe(2), respectively, and the Fe–O(carboxylate) bonds range in length from 1.958(3) to 2.054(3) Å, which are typical of FeIII–O bonds.12 Bond valence sum analysis and SQUID magnetic data support the assignment of oxidation state +3 to all Fe atoms, with high spin d5 configurations (see the Supporting Information). The two µ3-O atoms are located on opposite sides of the mean plane defined by the four Fe atoms, with a deviation of 0.274(2) Å. The structural features are similar to those in the molecular [Fe4(µ3O)2(O2CCMe3)8(2-pic)2] complex, where eight carboxylate groups ligate four Fe atoms in a bridging-monodentate fashion.12c In comparison to this molecular counterpart, 1 contains six bridging dimonodentate carboxylates and two monodentate ones, in which noncoordinated oxygen atoms form hydrogen bonds with the coordinated water molecules [O(10) · · · O(4) ) 2.816(5) Å]. This bonding arrangement is probably related to the geometric requirements for the underlying topology of the material, which is discussed in greater detail below. As in other MOFs, the connectivity of the inorganic clusters has important consequences for the porosity of the material. The organic link used to assemble 1 is a trigonal tricarboxylate, and eight of these are bound to each inorganic cluster. Following the conventional SBU analysis scheme,13 the eight carboxylate C atoms of the inorganic cluster are designated the points of extension. These define a distorted prismatoid or more precisely a solid facetted by six convex, skew quadrilaterals. A net that describes the topology of 1 can be constructed by connecting these vertices to each other and to the 3-coordinated vertices, which describe the organic links (Figure 1b). The resulting net is ideally cubic but rhombohedrally distorted in this material, defining two types of cages. Due to the relative size of the larger of these cages, its apertures, and the high symmetry of the topology, material 1 forms as two independent frameworks that are interpenetrated (Figure 1c). While interpenetration diminishes the total pore volume available in the material, the calculated solvent-accessible volume (using the calc solV routine in PLATON14) is 50.1% of the
10.1021/cg070640e CCC: $37.00 2007 American Chemical Society Published on Web 10/20/2007
Communications
Crystal Growth & Design, Vol. 7, No. 11, 2007 2291
Figure 1. Description of the crystal structure of 1. (a) Thermal ellipsoid plot of a structural fragment highlighting the organic and inorganic SBUs, drawn with 50% probabilities. An inversion center is located in the middle of the Fe4(µ3-O)2 cluster. O(1S) is a DMF oxygen atom, and O(10) is a coordinated water oxygen atom. Hydrogen bonds are drawn as dashed lines. (b) A BTB link can be schematized as a 3-connector; similarly, the Fe4(µ3-O)2(O2C–)8 inorganic SBU is drawn as a prism of vertices. (c) The material is composed of two interpenetrated frameworks, each constructed by linking the SBUs described in (b).
crystal structure, including the coordinated water and DMF molecules. Pores 10 Å in diameter reside within the second type of cage, defined by an approximate octahedron of Fe4(µ3O)2(O2C–)8 clusters connected through eight BTB links. These pores are accessible through elliptical apertures measuring 4.0 × 8.0 Å. TGAs indicated that the solvent molecules residing in the pores of 1 could be removed by heating to 150 °C (see the Supporting Information). Measured adsorption of N2 by 1 at 77 K yielded a reversible, nonhysteretic type-I isotherm. From these data, the Brunauer–Emmett–Teller (BET) and Langmuir apparent surface areas were calculated as 1121 and 1835 m2 g-1, respectively, and the pore volume was calculated as 0.69 cm3 g-1. The material also reversibly adsorbs up to 2.1 wt % of hydrogen gas at 77 K and 1 bar (see the Supporting Information). Material 1 also shows an enhanced affinity for CO2 compared to many other porous MOFs. Adsorption isotherm data measured over a range of temperature (see Figure 2a) were used to calculate the isosteric adsorption enthalpy, giving values of 28.3 kJ mol-1 by the Toth method15 and 26.4 kJ mol-1 using a variant of the Clausius–Clapeyron equation.16 At 273 K, under lowpressure ( 2σ(I), 4889 reflections], wR2 ) 0.1677 (all data). Cambridge Crystallographic Data Center (CCDC) 632182 contains the crystallographic data in CIF files.
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