Modifying Cage Structures in Metal-Organic Polyhedral Frameworks for H2 Storage

Three isostructural metal-organic polyhedral cage based frameworks (denoted NOTT-113, NOTT-114 and NOTT-115) with (3,24)-connected topology have been synthesised by combining hexacarboxylate isophthalate linkers with {Cu-2(RCOO)(4)} paddlewheels. All three frameworks have the same cuboctahedral cage structure constructed from 24 isophthalates from the ligands and 12 {Cu-2(RCOO)(4)} paddlewheel moieties. The frameworks differ only in the functionality of the central core of the hexacarboxylate ligands with trimethylphenyl, phenylamine and triphenylamine moieties in NOTT-113, NOTT-114 and NOTT-115, respectively. Exchange of pore solvent with acetone followed by heating affords the corresponding desolvated framework materials, which show high BET surface areas of 2970, 3424 and 3394 m(2)g (1) for NOTT-113, NOTT-114 and NOTT-115, respectively. Desolvated NOTT-113 and NOTT-114 show high total H-2 adsorption capacities of 6.7 and 6.8 wt%, respectively, at 77 K and 60 bar. Desolvated NOTT-115 has a significantly higher total H-2 uptake of 7.5 wt% under the same conditions. Analysis of the heats of adsorption (Q(st)) for H-2 reveals that with a triphenylamine moiety in the cage wall, desolvated NOTT-115 shows the highest value of Q(st) for these three materials, indicating that functionalisation of the cage walls with more aromatic rings can enhance the H-2/framework interactions. In contrast, measurement of Q(st) reveals that the amine-substituted trisalkynylbenzene core used in NOTT-114 gives a notably lower H-2/framework binding energy.