Gaining Insights on the H₂-Sorbent Interactions: Robust soc-MOF Platform as a Case Study

We report on the synthesis and gas adsorption properties (i.e., Ar and H₂) of four robust 3-periodic metal-organic frameworks (MOFs) having the targeted soc topology. These cationic MOFs are isostructural to the parent indium-based MOF, In-soc-MOF-1a (for NO₃^-), previously reported by us, and likewise are constructed from the assembly of rigid μ₃-oxygen-centered trinuclear metal carboxylate clusters, [M₃O(O₂Câ)₆], where M = In³⁺ or Fe³⁺. Each inorganic trinuclear molecular building block (MBB), generated in situ, is bridged by six 3,3′,5,5′-azobenzenetetracarboxylate (ABTC^4-) ligands to give the extended (4,6)-connected MOF, soc-MOF. In our previous work, we confirmed that the parent soc-MOF, i.e., In-soc-MOF-1a, possesses unique structural characteristics (e.g., vacant In binding sites and narrow pores with higher localized charge density), which led to exceptional hydrogen (H₂) storage capabilities. Therefore, charged MOFs with soc topology can be viewed collectively as an ideal prototypical platform to examine the impact of specific structural parameters on H₂-MOF interactions via systematic gas adsorption studies. We infer that enhanced binding of molecular H₂ is primarily governed by the presence and type of vacant metal centers (i.e., Fe was shown to exhibit stronger H₂-MOF interactions at low H₂ loading compared to the In analogues). These findings are evident from the associated isosteric heat of adsorption (Q_st) at low loadings and inelastic neutron scattering (INS) experiments of the rotational transitions of sorbed H₂, as well as, temperature-programmed desorption (TPD) studies (for a select compound). The importance of localized charge density is also highlighted, where the extra-framework nitrate anions in the Fe-soc-MOF-1a (for NO₃^-) facilitate enhanced binding affinities as compared to the chloride analogue.