Pyridine-M²⁺ [M = Mg, Ca]: A promising organometallic system for potential hydrogen storage: In silico study

Pyridine-M²⁺ [M = Mg, Ca] hosts have been identified as potent H₂ storage material, and this has been explored by a variety of functionals within the density functional theory (DFT) context. Around the linearly connected metal centre, the pyridine-Mg²⁺ and pyridine-Ca²⁺ building blocks can adsorb 6 and 7H₂ molecules, respectively. The stability of Pyridine-M²⁺ [M = Mg, Ca] hosts has been proven by chemical hardness, aromaticity, and molecular dynamic simulations. After H₂ adsorption, the aromaticity still remains. Adsorption of H₂ molecules at a quasi-molecular type is supported by the fact that their average adsorption energy/H₂ (E_ads/H₂) is between 0.22 and 0.72 eV. Pyridine-Mg²⁺@6H₂ and pyridine-Ca²⁺@7H₂ both have very impressive gravimetric wt % values (10.43 and 10.53 respectively). Average delocalization correction energy (ΔE_CT^av) supports the charge transfer type interaction between metal ions and adsorbed H₂ molecules. The topological analysis reflects that bonding between metals (Mg & Ca) and trapped H₂ molecules are mostly non-covalent types. Gibbs free energy changes (ΔG) [eV] indicate that the molecular hydrogen adsorption process will be spontaneous at 298 K for pyridine-Mg²⁺@nH₂ systems and at or below 205 K for pyridine-Ca²⁺@nH₂ systems and atom centered density matrix propagation (ADMP) study suggests the higher temperature for desorption.