Hydrogen uptake by {H[Mg(HCOO)₃]⊃NHMe₂} _∞ and determination of its H₂ adsorption sites through monte carlo simulations

A detailed analysis of the solvothermal synthesis in DMF of the polymeric magnesium formate {H[Mg(HCOO)₃]⊃NHMe₂} _∞ (1) from Mg(ClO₄)₂•6H₂O revealed that the final crystalline product is formed after an acid-catalyzed DMF hydrolysis, producing formic acid and dimethylamine. The former bridges magnesium(II) centers, creating the 3D scaffold, while the latter is trapped inside the cubic cavities of the polymer, engaging in strong hydrogen bonding with the formate ions of the cage. After thermal activation and guest removal, the material was tested for hydrogen uptake at T = 77 K over the 0-80 bar pressure range, and the existence of preferred H₂ adsorption sites was assessed through grand canonical Monte Carlo (GCMC) simulations. No specific low-energy site was found, and the H₂ molecules positions within the framework cavities are mainly dependent on packing effects. Thus, at low H ₂ loadings the most populated site is the center of the cubic cavities, even though, at higher pressures, two more "localized" positions have been found by the simulation because of the reduced freedom of movement. The maximum experimental H₂ uptake corresponds to 8.8 mg/g or 13.5 mg/cm³.