Why alkali metals and alkaline earth metals are not loaded the metal-organic frameworks

Different transition metal loaded metal-organic frameworks (MOFs) have been extensively studied. However, little literature has been written about alkali and alkali-earth metal-loaded MOFs. In this study, we take the DPyDFP linker used for the construction of MOFs as an example. The structures and electronic properties of alkali metals and alkaline earth metals loaded DPyDFP clusters have been investigated using density functional theory. The results indicate that the structural stability of alkali metals and alkaline earth metals loaded DPyDFP clusters is less than that of transition metals loaded DPyDFP clusters. The interior of the DPyDFP clusters has not enough space to locate a large alkali metal or alkaline earth metal atom except for Li, Be and Mg. It indicates that if the IAM and IIAM atoms are loaded on the linker to build a MOF framework, Li, Be and Mg are worth considering. The DPyDFP-IIAM clusters exhibit more structural stability and chemically stable than corresponding DPyDFP-IAM clusters. The HOMOs and LUMOs of the DPyDFP-IAM and DPyDFP-IIAM clusters mostly surround N and C atoms. The IAM-N and IIAM-N bonds exhibit certain covalent character. Due to the limited charge transfer between the IAM (or IIAM) atoms and the DPyDFP clusters, the IAM (or IIAM) atoms are not a good choice to design the novel linker to build a MOF framework.