Structural Studies and Computer Simulation of the Inclusion of Aromatic Hydrocarbons in a Zinc 2,6-Naphthalene Dicarboxylate Framework Compound

Abstract

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 P21/c with [Zn2(ndc)2(DMF)2]·1.6DMF (1), a = 8.075(1) Å, b = 16.891(2) Å, c = 12.673(2) Å, β = 92.90(1)°, [Zn2(ndc)2(DMF)2]·C6H6 (2) a = 8.340(2) Å, b = 15.660(4) Å, c = 13.008(4) Å, β = 91.340(5)°, [Zn2(ndc)2(DMF)2]·C7H8 (3), a = 8.183(2) Å, b = 16.245(3) Å, c = 12.920(3) Å, β = 91.976(4)°, and [Zn2(ndc)2(DMF)2]·C8H10 (4), a = 7.973(2) Å, b = 16.946(3) Å, c = 12.922(3) Å, β = 92.798(4)°. 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 2H NMR using the quadrupole echo technique. Benzene is found to execute rapid hopping around its C6 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.