Structural studies and computer simulation of the inclusion of aromatic hydrocarbons in a zinc 2,6-naphthalene dicarboxylate framework compound

We have examined the structural response of a zinc 2,6-naphthalene dicarboxylate framework solid, isostructural with MOF-105, to the inclusion during crystallization of dimethylformamide, benzene, toluene, and p-xylene. These compounds, which are made up of a stacked arrangement of four-connected layers, crystallize in the space group P2(1)/c with [Zn-2(ndc)(2)(DMF)(2)].1.6DMF (1), a = 8.075(1) Angstrom, b =16.891(2) Angstrom, c = 12.673(2) Angstrom, beta = 92.90(1)degrees, [Zn-2(ndc)(2)(DMF)(2)].C6H6 (2) a = 8.340(2)Angstrom, b = 15.660(4) Angstrom, c = 13.008(4) Angstrom, beta = 91.340(5)degrees, [Zn-2(ndc)(2)(DMF)(2)].C7H8 (3), a = 8.183(2) Angstrom, b = 16.245(3) Angstrom, c = 12.920(3) Angstrom, beta = 91.976(4)degrees, and [Zn-2(ndc)(2)(DMF)(2)].C8H10 (4), a = 7.973(2) Angstrom, b = 16.946(3) Angstrom, c = 12.922(3) Angstrom, beta = 92.798(4)degrees. The structure is found to include p-xylene with high selectivity from mixtures of xylene isomers. In the presence of only o- or m-xylene as an additive, the structure does not crystallize. The mobility of fully deuterated benzene and toluene within compounds 2 and 3 over the temperature ranges 123-294 and 173-294 K, respectively, has been measured by H-2 NMR using the quadrupole echo technique. Benzene is found to execute rapid hopping around its C-6 axis over the entire temperature range studied with an activation energy of 6(1) kJ mol(-1). The only motion of the toluene is rapid rotation of the -CD3 group. Computational modeling of the structures successfully reproduces the crystal structures and the changes in unit cell parameters and indicates that the binding energies of m-xylene within the structure are less favorable than those for p-xylene, predominantly as a result of the distortion of the framework required to accommodate the m-xylene. These lower binding energies explain the high selectivity for the uptake of p-xylene during crystallization.