Inelastic Neutron Scattering and Theoretical Studies of H<sub>2</sub> Sorption in a Dy(III)-Based Phosphine Coordination Material

A combined inelastic neutron scattering (INS) and theoretical study of H<sub>2</sub> sorption was performed in PCM-16, a phosphine coordination material (PCM) with the empirical formula [(CH<sub>3</sub>)<sub>2</sub>NH<sub>2</sub>]­[Dy<sub>2</sub>(tctpo)<sub>2</sub>(O<sub>2</sub>CH)] (tctpo = tris­(<i>p</i>-carboxylato)­triphenylphosphine oxide). INS measurements at different loadings of H<sub>2</sub> 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 H<sub>2</sub> sorption in PCM-16 revealed that the H<sub>2</sub> molecules sorbed at two main sites in the material: (1) the (CH<sub>3</sub>)<sub>2</sub>NH<sub>2</sub><sup>+</sup> 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 (CH<sub>3</sub>)<sub>2</sub>NH<sub>2</sub><sup>+</sup> ions. These counterions provide for the strongest H<sub>2</sub> sorption sites in the material, which corresponds to an isosteric heat of adsorption (<i>Q</i><sub>st</sub>) value of close to 8 kJ mol<sup>–1</sup>. The calculated rotational barrier for the (CH<sub>3</sub>)<sub>2</sub>NH<sub>2</sub><sup>+</sup>–H<sub>2</sub> 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 H<sub>2</sub> 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 H<sub>2</sub> sorption energetics in such materials.