Inelastic Neutron Scattering and Theoretical Studies of H₂ Sorption in a Dy(III)-Based Phosphine Coordination Material

A combined inelastic neutron scattering (INS) and theoretical study of H₂ sorption was performed in PCM-16, a phosphine coordination material (PCM) with the empirical formula [(CH₃)₂NH₂]­[Dy₂(tctpo)₂(O₂CH)] (tctpo = tris­(p-carboxylato)­triphenylphosphine oxide). INS measurements at different loadings of H₂ revealed a peak occurring at low rotational tunnelling energies (ca. 5–8 meV), which corresponds to a high barrier to rotation and, therefore, a strong interaction with the host. Molecular simulations of H₂ sorption in PCM-16 revealed that the H₂ molecules sorbed at two main sites in the material: (1) the (CH₃)₂NH₂⁺ counterions and (2) within the small pores of the framework. Two-dimensional quantum rotation calculations revealed that the peak occurring from approximately 5–8 meV in the INS spectra for PCM-16 is associated with sorption onto the (CH₃)₂NH₂⁺ ions. These counterions provide for the strongest H₂ sorption sites in the material, which corresponds to an isosteric heat of adsorption (Q_st) value of close to 8 kJ mol^–1. The calculated rotational barrier for the (CH₃)₂NH₂⁺–H₂ interaction in PCM-16 (45.60 meV) is higher than those for a number of extant metal–organic frameworks (MOFs), especially those that contain open-metal sites. This study provides insights into the H₂ sorption mechanism in a PCM for the first time and shows how the inclusion of counterions in porous materials is a promising method to increase the H₂ sorption energetics in such materials.