Role of the Quantum Interactions in H₂ Adsorption on Late Transition Metal Chelated Linkers of Covalent Organic Frameworks

Transition metal (Tm) chelation is an effective strategy to achieve optimal binding enthalpy (▵H) of H₂-adsorption in the linkers of covalent organic frameworks (COFs). The first principle-based DFT method has been implemented to determine the H₂ adsorption in nine organic linkers chelated with transition metal atoms from Cr to Zn. The obtained range of binding enthalpy for single H₂ adsorbed on the pure and chelated complexes is -7 to -20 kJ/mol, which is required for onboard H₂ storage. The Linker-3 chelated with Ni (II) metal exhibits the most favorable binding enthalpy of approximately -18.72 kJ/mol for the single adsorbed H₂ molecule, which falls within the physisorption range. Some of the complexes have shown the binding enthalpy range between physisorption and chemisorption, i. e., in that case, H₂ binds via Kubas interactions. However, physisorption-based complexes are preferable to others because physisorption is a reversible process with rapid kinetics. This study reveals that the dispersion, polarization, and electrostatic interactions mainly contribute to the binding enthalpy of H₂ adsorption. Molecular surface potential analysis verifies the origin of induced dipole moment in the H₂ molecule, which enhances the hydrogen adsorption in transition metal chelated COFs.