Imidazole and nitrogen-fulvalene derivatives as reversible hydrogen carriers: A thermodynamic DFT approach

This work employs temperature-dependent density functional theory (DFT) calculations at gas phase to evaluate the hydrogen carrier potential of heterocyclic compounds: 2-(1,3-dihydroimidazol-2-ylidene)-1,3-dihydroimidazole (TAF), 2-(2H-imidazole-2-ylidene)-2H-imidazole (2IMDZ), 2-(1H-imidazole-2-yl)-1H-imidazole (2BMDZ), and their fully hydrogenated derivative, 2-imidazolidin-2-ylimidazolidine (TAF SAT ). Thermodynamic parameters (ΔH, ΔS, and ΔG) revealed that hydrogenation is exergonic for all systems. Notably, 2BMDZ exhibits a temperature-dependent inversion of ΔG, thermodynamically enabling reversible hydrogen uptake and release under appropriate conditions. In contrast, 2IMDZ favors hydrogen storage but shows limited spontaneous dehydrogenation. Molecular orbital and electrostatic potential analyses reveal that the electron density in 2IMDZ is more symmetric distributed due to its planarity, while 2BMDZ displays asymmetric charge density and nodal HOMO features. Global reactivity descriptors support these trends, with 2IMDZ showing higher electrophilicity and 2BMDZ demonstrating characteristics conducive to reversibility. These insights and an additional benchmark comparison reveals 2IMDZ as a hydrogen storage-oriented system and 2BMDZ as a promising reversible hydrogen carrier. Further experimental and kinetic validation is necessary to assess physical properties and catalytic performance. • “2BMDZ shows a temperature-dependent inversion of ΔG in hydrogen storage.”. • “Planar 2IMDZ allows broad electron density distribution for reactivity.”. • “Resonance stabilization in 2BMDZ shifts favorability to dehydrogenation.”. • “2BMDZ enables reversible H 2 cycling, while 2IMDZ is a storage-oriented candidate.”. • “Benchmark validation against NEC and DBT systems.”.