Role of Counterions in the Structural Stabilisation of Redox-Active Metal-Organic Frameworks

The crystal structures of metal-organic frameworks (MOFs) are typically determined by the strong chemical bonds formed between the organic and inorganic building units. However, the latest generation of redox-active frameworks often rely on counterions in the pores to access specific charge states of the components. Here, we model the crystal structures of three layered MOFs based on the redox-active ligand 2,5-dihyroxybenzoquinone (dhbq): Ti$_2$(Cl$_2$dhbq)$_3$, V$_2$(Cl$_2$dhbq)$_3$ and Fe$_2$(Cl$_2$dhbq)$_3$ with implicit and explicit counterions. Our full-potential first-principles calculations indicate that while the reported hexagonal structure is readily obtained for Ti and V, the Fe framework is stabilised only by the presence of explicit counterions. For high counterion concentrations, we observe the formation of an electride-like pocket in the pore center. An outlook is provided on the implications of solvent and counterion control for engineering the structures and properties of porous solids.