Tailoring the Dimensionality of Metal–Organic Frameworks Incorporating Pt and Pd. From Molecular Complexes to 3D Networks Jasmina Hafizovic´,†,‡ Alexander Krivokapic´,*,† Kai C. Szeto,†,‡ Søren Jakobsen,†,‡ Karl Petter Lillerud,†,‡ Unni Olsbye,†,‡ and Mats Tilset§
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2302–2304
Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway, Centre for Materials Science and Nanotechnology, UniVersity of Oslo, P.O. Box 1126 Blindern, N-0318 Oslo, Norway, and Centre for Theoretical and Computational Chemistry, Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway ReceiVed August 27, 2007; ReVised Manuscript ReceiVed September 28, 2007
ABSTRACT: Metal–organic frameworks (MOFs) incorporating Pt(II) or Pd(II) are formed in one-pot reactions from pyridine-3,5dicarboxylic acid with Pt(II) or Pd(II) and a cornerstone metal in DMF or water. Depending on the cornerstone metal and solvent, two- or three-dimensional networks are formed. The structures are determined by single-crystal x-ray diffraction and powder diffraction data, which show the isostructural nature of the Pd and Pt compounds formed. State-of-the-art crystal engineering involves the design and synthesis of new crystalline materials with specific properties that are dictated by their desired applications.1–3 Metal–organic frameworks (MOFs)4 constitute a recently discovered class of crystalline, porous materials that have attracted considerable attention. Their interchangeable linkers and coordinating metal ions offer great flexibility in framework design.5–9 This allows judicious manipulation of the pore or channel sizes,10 surface area, and type of metal sites in the MOFs. Most frequently, MOFs are investigated with respect to their potential as gas storage and separation materials.11–13 Catalysis is another emerging application of MOFs.14–18 Catalysts that utilize Pt and Pd are extensively used in largeand small-scale industrial processes.19 We have recently described PtII complexes that are capable of activating hydrocarbon C–H bonds in solution.20,21 Metal–organic Pt complexes have been demonstrated to act as homogeneous catalysts for selective functionalizaton of alkanes at moderate temperatures compared to those of existing heterogeneously catalyzed processes.21–24 Single-site heterogeneous catalysts may offer the selectivity and specificity seen in homogeneous systems combined with easy catalyst separation from reactants and products.25 One of our current approaches to catalyst design is to heterogenize molecular metal–organic systems by incorporating their structural motifs into metal–organic frameworks. We have recently prepared and characterized the first heterobimetallic Pt-lanthanide MOFs, in which PtII centers (as potential catalytic sites) are linked via bifunctional ligands and lanthanide cornerstone metals.26,27 One particular advantage of heterobimetallic systems, compared to the more extensively studied monometallic MOFs,5–9 is that the organic linkers surrounding the potentially active metal site can be selected to strongly resemble the local environment of a molecular catalyst, whereas the second metal, not necessarily catalytically active, constitutes the cornerstones of the network. It is a tremendous challenge to create stable networks with pore dimensions that allow reactants to enter the structure and access the catalytic sites. In this contribution, we describe an investigation of how network topology of heterobimetallic MOFs that contain PtII and PdII sites is influenced by the nature of the cornerstone metal ion and the solvent employed for the solvothermal * Corresponding author.
[email protected] † Department of Chemistry, University of Oslo. ‡ Centre for Materials Science and Nanotechnology, University of Oslo. § Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo.
Figure 1. Crystal structure of 1 (Pt/Co combination). The partly transparent atoms illustrate the disorder of the cobalt atoms and their coordinated water molecules. Hydrogen atoms and uncoordinated water molecules are omitted for clarity.
synthesis. These results constitute an important first step toward crystal engineering in heterobimetallic MOF synthesis. The new MOF materials were synthesized in Teflon liners by dissolving 0.10 mmol of K2PtCl4 or K2PdCl4 in 10 mL of water or DMF, followed by addition of 0.20 mmol of pyridine-3,5dicarboxylic acid (3,5-PDA) and 0.40 mmol of the respective cornerstone metal nitrate, M(NO3)n · XH2O (M ) Mg2+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, La3+, Ce3+, Nd3+). The Teflon liners were sealed inside steel autoclaves and placed in a preheated oven at 100 or 120 °C for 12 h. After being cooled in air to room temperature, the product was filtered and washed with the respective solvent. Variation of Mn+ and solvent resulted in three different structure types (1, 2, and 3). Structure type 1 is a molecular complex, whereas 2 and 3 represent 2D and 3D MOFs, respectively. The structures of these materials were determined by single-crystal X-ray diffraction analysis, and the crystalline purity of each sample was examined with powder X-ray diffraction (see the Supporting Information). Single-crystal XRD showed that the unit cells of the structures that belong to a distinct structure type are nearly identical, and powder XRD confirmed the isostructural nature of the products. In all structures, the PtII or PdII centers have the typical squareplanar geometry with two mutually trans-coordinated N atoms from two 3,5-PDA ligands; this coordination geometry is thermodynamically preferred for PtCl2(pyr)2 species.28 In the solid state, transPdCl2(3,5-PDA)2 is assembled to network structures held together by hydrogen bonds.29,30 Structure type 1 was formed in water with PtII and divalent M2+ cornerstone metal ions (M ) Mg, Mn, Co, Ni, Zn, Cd). A typical example of structure 1 is shown in Figure 1 (Pt/Co combination). The Pt–organic unit is connected to a Co2+ ion through coordination
10.1021/cg7008094 CCC: $37.00 2007 American Chemical Society Published on Web 10/12/2007
Communications
Figure 2. 2D framework of structure type 2 (Pt/Ce combination) viewed along the [100] direction. Hydrogen atoms and uncoordinated water molecules are omitted for clarity.
Crystal Growth & Design, Vol. 7, No. 11, 2007 2303 MOF materials potentially suitable for catalytic applications. The thermal stability and solvent desorption/adsorption characteristics of the structures are under investigation. In the layered structure types 1 and 2, the layers are separated by 3.29–3.38 Å and the shortest distance between Pt or Pd atoms is only 3.77–3.89 Å. However, in structures of type 3, the Pt or Pd centers are much further separated (spanning 9.82–10.01 Å), which bodes well for single-site catalyst activity. These results establish clear structural tendencies for heterobimetallic MOFs based on the pyridine-3,5-dicarboxylate ligand system. The resulting structural arrangement is primarily dependent on the applied solvent and the identity of the cornerstone metal ion. The M2+ cornerstones resulted in a series of molecular complexes from water, the M3+ lanthanide cornerstones gave 2D MOFs from water, and M2+ cornerstones furnished 3D MOFs from DMF. Moreover, PtII or PdII centers were selectively coordinated to the N atoms, whereas the cornerstone metal ions were always coordinated to the carboxylate groups of the 3,5-PDA units. This demonstrates that it is possible to selectively bind two different metals to the different sites of a heterobifunctional ligand to produce well-defined heterobimetallic MOFs.
Acknowledgment. We gratefully acknowledge generous financial support from the Norwegian Research Council and the FUNMAT@UIO program. We thank Carl-Henrik Görbitz for helpful discussions. The authors are grateful to SNBL (ESRF, Grenoble) for synchrotron beamtime (experiment 01-02-762). Supporting Information Available: Experimental details and powder XRD patterns (PDF); crystallographic information in CIF format. This material is available free of charge via the Internet at http:// pubs.acs.org.
References
Figure 3. 3D framework of structure type 3 (Pt/Zn combination) viewed along the [100] direction. Hydrogen atoms and DMF molecules have been omitted for clarity.
of an oxygen atom from one of the carboxylic acid groups. The M2+ ion and its two coordinated water molecules are disordered in all structures of type 1.31 No counteranions are observed in the structure, so the Co2+ charge is balanced by deprotonated carboxylate groups in the 3,5-PDA ligand.32,33 The layers that are depicted in Figure 1 are stacked along the [001] direction with an interlayer separation of ca. 3.38 Å and a shortest Pt. . .Pt distance of 3.7766(10) Å. These units are further held together by hydrogen bonds via confined water molecules. Structure type 2 was obtained in water from PtII or PdII with 3+ La , Ce3+, and Nd3+ cornerstone ions. The lanthanide ion is coordinated to four carboxylate groups and four water molecules, which gives a 2D layered structure. The layers shown in Figure 2 (Pt/Ce combination) are stacked along the [100] direction. They are separated by ca. 3.33 Å and held together by hydrogen bonds constituting a shortest Pt. . .Pt distance of 3.8516(6) Å. Structure type 3 was obtained in DMF from PtII or PdII with Mg2+, Mn2+, Co2+, Zn2+, and Cd2+ cornerstone ions. This structure type is shown in Figure 3 (Pt/Zn combination) and has a 3D covalently bonded framework. The pores are occupied by coordinated and uncoordinated DMF molecules. The pore walls have integrated Pt atoms with a shortest Pt. . .Pt distance of 9.816(21) Å. In all three structure types, the presence of channels that incorporate single Pt or Pd sites make the obtained heterobimetallic
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