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 Mn 4 Cl−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 - σ* 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 Mn 4 Cl−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 H 2 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%.