Hydrogen adsorption in the metal-organic frameworks Fe2(dobdc) and Fe2(O2)(dobdc)

The hydrogen storage properties of Fe-2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and an oxidized analog, Fe-2(O-2)(dobdc), have been examined using several complementary techniques, including low-pressure gas adsorption, neutron powder diffraction, and inelastic neutron scattering. These two metal-organic frameworks, which possess one-dimensional hexagonal channels decorated with unsaturated iron coordination sites, exhibit high initial isosteric heats of adsorption of -9.7(1) and -10.0 (1) kJ mol(-1), respectively. Neutron powder diffraction has allowed the identification of three D-2 binding sites within the two frameworks, with the closest contacts corresponding to Fe-D-2 separations of 2.47(3) and 2.53(5) angstrom, respectively. Inelastic neutron scattering spectra, obtained from p-H-2 (para-H-2) and D-2-pH(2) mixtures adsorbed in Fe-2(dobdc), reveal weak interactions between two neighboring adsorption sites, a finding that is in opposition to a previous report of possible 'pairing' between neighboring H-2 molecules.