Nature and tunability of enhanced hydrogen binding in metal-organic frameworks with exposed transition metal sites

Metal-organic framework (MOF) compounds with exposed transition-metal (TM) sites were recently found to exhibit significantly larger experimental heats of adsorption of H-2 than classical MOFs, thus attracting greater attention. Here we show that the hydrogen binding in Mn4Cl-MOF is not of the expected Kubas type because there is (a) no significant charge transfer from TM to H-2, (b) no evidence of any H-2-sigma* Mn-d orbital hybridization, (c) no significant H-H bond elongation, and (d) no significant shift in H-H stretching mode frequency. We make predictions for the magnetic superexchange interactions in Mn4Cl-MOF and determined low- and high-spin states of the Mn ion as local minima with very different hydrogen binding energies. We show that, by replacing Cl with F or Br, one can tune the H2 binding energy. We further reveal that the major contribution to the overall binding comes from the classical Coulomb interaction which is not screened due to the open-metal site and explains the relatively high binding energies and short H-2-TM distances observed in MOFs with exposed metal sites compared to traditional ones. Finally, we show that the orientation of H-2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%.