Interaction of molecular hydrogen with open transition metal centers for enhanced binding in metal-organic frameworks: a computational study

Molecular hydrogen is known to form stable, "nonclassical" sigma complexes with transition metal centers that are stabilized by donor-acceptor interactions and electrostatics. In this computational study, we establish that strong H2 sorption sites can be obtained in metal-organic frameworks by incorporating open transition metal sites on the organic linkers. Using density functional theory and energy decomposition analysis, we investigate the nature and characteristics of the H2 interaction with models of exposed open metal binding sites {half-sandwich piano-stool shaped complexes of the form (Arene)ML(3- n)(H2)n [M=Cr, Mo, V(-), Mn(+); Arene = C6H5X (X=H, F, Cl, OCH3, NH2, CH3, CF3) or C6H3Y2X (Y=COOH, X=CF3, Cl; L=CO; n=1-3]}. The metal-H2 bond dissociation energy of the studied complexes is calculated to be between 48 and 84 kJ/mol, based on the introduction of arene substituents, changes to the metal core, and of charge-balancing ligands. Thus, design of the binding site controls the H2 binding affinity and could be potentially used to control the magnitude of the H2 interaction energy to achieve reversible sorption characteristics at ambient conditions. Energy decomposition analysis illuminates both the possibilities and present challenges associated with rational materials design.