Metallacarborane-Based Metal−Organic Framework with a Complex Topology

The long, linear cobalt(III) bis(dicarbollide)based bis(isophthalic acid) anion was synthesized as a tetraphenylphosphonium salt in five steps from 8-iodo-closo1,2-C2B10H11. The solvothermal reaction between the anionic bis(isophthalic acid) linker and copper(II) nitrate in acidified DMF yielded single crystals. Despite the tendency for copper(II) and analogous linear tetraacids to form members of an isoreticular family of metal−organic frameworks (MOFs) with the fof topology, single-crystal X-ray diffraction analysis revealed the growth of three different frameworks. These MOFs, NU-150, NU-151, and NU-152, have three distinct topologies: fof, sty, and hbk, respectively. NU-152 has a novel quadrinodal topology in which cuboctahedral coordination polyhedra are each connected to 10 neighboring polyhedra via the cobalt bis(dicarbollide) portions of the linkers. The formation of these frameworks illustrates the limitations of structure prediction in MOF chemistry and the possibility of using flexible linkers to generate unexpected topologies. Furthermore, this work represents the first example of the incorporation of an anionic bis(dicarbollide) unit into a MOF. ■ INTRODUCTION Porous coordination polymers constitute a class of materials with potential in the areas of ion exchange, catalysis, gas separation, sensing, and storage. Crystalline porous coordination polymers, commonly known as metal−organic frameworks (MOFs), are unique as their properties can be precisely tailored through judicious choice of ligand and metal combination. The underlying topologies of MOFs are particularly important as they define the properties of the material. Topology is dictated by the geometries of the structural components of the MOF, which are typically polytopic organic ligands and metal clusters with defined geometries, herein referred to as “linkers” and “nodes”, respectively. Often, the relationships between the structural components are well-defined, which allows for structure prediction and the targeting of specific MOFs with desirable properties. This implementation of rational design is a particularly attractive aspect of MOF chemistry. Much attention has been focused on the investigation of isoreticular frameworks, i.e., families of MOFs with the same underlying topology, as they provide a unique opportunity to investigate structure−activity relationships in a systematic fashion. However, there are several important exceptions in which the expected topologies are not obtained. An example of this is the family of isoreticular zinc(II)-based MOFs, known as the IRMOF series. This family, which contains the archetypal MOF-5, has received much attention over the past several years. MOF-5, which contains octahedral six-connected Zn4O nodes, is synthesized cleanly under solvothermal conditions. However, if the material is synthesized via vapor diffusion at room temperature, a two-dimensional network (MOF-2) is formed in which the terphthalate linkers are connected via square-planar, four-coordinate biszinc(II) paddlewheels. The combination of a tetratopic bis(isophthalic acid) linker and a square planar four-coordinate biscopper(II) paddlewheel node reliably produces, with a few exceptions, MOFs with the fof topology (Figure 1). Since the first report of this topology in MOF-505, this family of isoreticular MOFs has been studied extensively and shows potential in the areas of hydrogen and methane storage. Of the 20 linear Received: December 5, 2013 Revised: February 6, 2014 Published: February 10, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 1324 dx.doi.org/10.1021/cg401817g | Cryst. Growth Des. 2014, 14, 1324−1330 bis(isophthalic acid) linkers that have been used in combination with biscopper(II) paddlewheels, 18 produce MOFs with the fof topology. The remaining two linkers, which are based on the naphth-1,4-diyl subunit or a porphyrin, produce MOFs that crystallize in tetragonal space groups with the stx and lvt topologies, respectively. In these cases, the steric bulk of the cores of the linkers renders the fof topology unfeasible. In a related example, a fluorene-based ligand with a slight curvature produced a MOF with the rare sty topology. Interestingly, it is not possible to generate the sty topology with rigid, linear linkers because of the orientation of opposing biscopper(II) paddlewheels (vide infra). As a part of our research into the synthesis and properties of carborane-based MOFs, we have been investigating a series of axially functionalized bis(dicarbollide) metal complexes as linkers. Bis(dicarbollide) complexes are attractive because they combine rich redox chemistry, comparable to that of cyclopentadienyl complexes, with a linear geometry that is perfectly suited to the construction of extended materials. MOFs that are based on these metallacarboranes are particularly interesting as they show potential in the area of stimuli responsive porous materials for gas storage and separation, sensing, and waste remediation applications. We were targeting a MOF with both high porosity and high phase purity, and thus prepared an anionic cobalt bis(dicarbollide) complex that is functionalized in the axial positions with 5-ethynylisophthalic acid groups [5]− (Scheme 1). This linker was chosen specifically because of the above-mentioned propensity of analogous tetraacids to form, reliably and in high purity, highly porous MOFs with the Figure 1. Structure of isoreticular MOFs with the fof topology. (a) Generalized structure viewed along the crystallographic a axis. Atom color code: C = small gray spheres; O = red; Cu = blue. The spacer subunit between the isophthalate groups is represented by a large gray sphere. (b) Simplified representations of the generalized tetraacid linker and biscopper(II) paddlewheel nodes. The tetraacid linker is composed of two three-coordinate (3-c) vertices, which are represented by red triangles, connected via the spacer group (gray spheres). (c) Several examples of organic and inorganic spacer unit. (d, e) Simplified extended structure, illustrating the topology of the framework. Scheme 1. (A) Synthesis of Linker [PPh4][5]. a (B) ORTEPType Representation of the Crystallographically Determined Molecular Structure of the [5]− Anion in Its Cesium Salt Reagents and conditions: (i) (trimethylsilyl)ethynylmagnesium bromide (3 equiv), PdCl2(PPh3)2 (5 mol %), THF/Et2O, 65 °C, 16 h, 92%. (ii) KOH (2 equiv), H2O/MeOH (1:4), rt, 2 h, 96%. (iii) diethyl 5-iodoisophthalate, Pd(PPh3)4 (3 mol %), CuI (10 mol %), iPr2NEt/THF (1:1), 65 °C, 24 h, 85%. (iv) CsF, EtOH, reflux, 6 h, 95%. (v) CoCl2, NaOH (40% aq), 105 °C, 1 h; Et2O, HCl (aq); [PPh4]Br, H2O. Thermal ellipsoids are drawn at 50% probability. Atom color code: Co = purple; C = gray; H = white; B = pink; O = red. Cesium cations are omitted for clarity. Crystal Growth & Design Article dx.doi.org/10.1021/cg401817g | Cryst. Growth Des. 2014, 14, 1324−133