Solid-state NMR provides location-specific information on the chemical environment inside MOF pores

Abstract

Solid-state nuclear magnetic resonance (NMR) spectroscopy can capture the chemical inequivalence of differently shaped metal-organic framework (MOF) pores. We show that the chemical shifts of reporting substituents that extend a defined distance into a MOF pore reflect the pore shape. Comparison of porous coordination network (PCN)-222, PCN-224, and PCN-223 demonstrates that the same linker and node provide three distinct chemical environments in the three different types of pores present in the MOFs. The chemical inequivalence of differently positioned substituents ensures that NMR spectroscopy can reveal into which pore post-synthetically added ligands are facing, without a need for single crystals, and even if only 10% of nodes are functionalized. The operational facility with which location-resolved information can be obtained permits NMR to shed light on the thermodynamics and kinetics of mixed-linker MOF growth, the presence of defects, and the dynamic behavior of added ligands.