Framework-catenation isomerism in metal-organic frameworks and its impact on hydrogen uptake.

Recent studies have focused on metal-organic frameworks (MOFs) with high hydrogen uptake1 in order to reach the 2010 DOE targets for on-board vehicular hydrogen storage.2 Strategies such as using pore sizes comparable to hydrogen molecules3 and introducing coordinatively unsaturated metal centers (UMCs)3b,c,4 have been explored. Recently, we have reported a biomimetic approach to UMCs utilizing entatic metal centers (EMCs).5 We have also demonstrated in both PCN-63c (porous coordination networks) and PCN-95 that interpenetration is an important factor contributing to their respective hydrogen uptake. However, interpenetration (or framework catenation) as an independent criterion has never been resolved from other factors. Normally in a given metal-ligand combination, either an interpenetrated network or a non-interpenetrated network is favored, not both. Conceptually, an interpenetrated MOF and its non-interpenetrated counterpart can be viewed as a supramolecular pair of stereoisomers. In reality, however, such framework-catenation isomerism has never been deliberately explored prior to the present report. To study the precise role of catenation in hydrogen uptake, using oxalate as a template, we have made the non-interpenetrated counterpart of PCN-6 (PCN-6′). At solvothermal conditions, a reaction between Cu(NO3)2‚2.5H2O and H3TATB in the presence of oxalic acid afforded turquoise octahedral crystals of PCN-6′ (yield: 60%). TATB represents 4,4′,4′′-s-triazine-2,4,6-triyl-tribenzoate. The reaction was run in dimethylacetamide (DMA) at 75 °C. The overall formula of PCN6′ is Cu6(H2O)6(TATB)4‚DMA‚12H2O, determined by X-ray crystallography, elemental analysis, and thermogravimetric analysis (TGA). Singe-crystal X-ray6 diffraction studies reveal that PCN-6′ crystallizes in cubic space group Fm-3m. It is isostructural with HKUST-1,7 mesoMOF-1,8 and with a single net of the interpenetrated PCN-6.3c In PCN-6′, dicopper tetracarboxylate paddlewheel SBUs (secondary building units) are linked by TATB bridges. Each SBU connects four TATB ligands, and each TATB binds three SBUs to form a Td-octahedron (Figure 1b), which has idealized Td symmetry with four ligands covering alternating triangular faces of the octahedron and an SBU occupying each vertex. Eight such Td-octahedra occupy the eight vertices of a cube to form a cuboctahedron through corner sharing with idealized Oh symmetry (Figure 1a).3c The average diameter of the void inside the cuboctahedron is 30.32 Å. Every cuboctahedron connects eight Tdoctahedra via face-sharing, and each Td-octahedron links four cuboctahedra to form a three-dimensional framework with a twisted boracite net topology (Figure 1c). Open square channels from all three orthogonal directions are identical in size and are 15.16 × 15.16 Å2 or 21.44 × 21.44 Å2 along the edges or diagonals, respectively (atom to atom distance). Alternatively the structure can be described as four honeycomb nets connected by the SBUs at the center of the hexagonal-edges; the TATB ligands occupy the corners of each hexagon and each hexagon is in a chair conformation (Figure 1d). The structure of PCN-6 (space group R-3m) can be reproduced by two identical interpenetrated nets of PCN-6′, the second being generated by translation of the first by c/5 (c represents the c-axis in PCN-6) along [111] direction of PCN-6′. PCN-6′ and PCN-6 are thus catenation isomers (Figure 1e,f). To the best of our knowledge, this is among the first pairs of such catenation isomers.3b Variables in the synthetic procedures of PCN-6 and PCN-6′ include temperature, solvent, and template addition. Controlled experiments were performed to confirm that template addition is the only factor determining the final topology. Without the addition of oxalic acid, the reaction between Cu(NO3)2‚2.5H2O and H3TATB in DMA, dimethylformamide (DMF), or diethylformamide (DEF) at 75 °C or 120 °C (with DMSO as solvent) led to the formation of PCN-6. In contrast, with oxalic-acid addition the same reaction in DMA, DMF, or DEF at 75 °C or 120 °C (DMSO) gives PCN6′. Thus, only template addition can account for the presence or absence of catenation. To test the general pertinence of this finding, another large trigonal-planar ligand HTB (for s-heptazine tribenzoate)9 was employed to react with Cu(NO3)2‚2.5H2O under reaction conditions similar to those of PCN-6 and PCN-6′. As expected, an interpenetrated MOF isostructural with PCN-6 (MOF-HTB) was obtained without the addition of oxalic acid, while a noninterpenetrated MOF isostructural with PCN-6′ (MOF-HTB′) was † Miami University. ‡ University of Kentucky. Figure 1. (a) Cuboctahedral cage; (b) Td-octahedral cage; (c) a view of the packing of PCN-6′ from the [001] direction; (d) a view of the cuboctahedral net from the [111] direction of PCN-6′; (e) space-filling model of the non-interpenetrated net in PCN-6′; (f) two interpenetrated nets in PCN-6. The large spheres shown in graphics a, b, and d represent void inside the cages. Published on Web 01/26/2007