NMR of Organic Linkers in MOFs and COFs

Metal–organic frameworks (MOFs) and covalent-organic frameworks (COFs) are excellent candidates for many practical applications due to their advantageous features, such as high porosities and selective guest adsorption. Organic ligands play a crucial role in both MOFs and COFs as versatile structural linkers and as sites for introducing or tailoring functionalities. Comprehensive characterization of the environments of organic ligands in MOFs and COFs is necessary for further understanding of structure–property relationships and for the rational design of future materials. Solid-state nuclear magnetic resonance (SSNMR) can provide detailed molecular-level information regarding the chemical environment around a target linker atom. Given that organic linkers in MOFs and COFs typically feature many NMR-active nuclei such as 1H, 11B, 13C, 15N, 17O, 19F, and 31P, SSNMR can yield detailed insights into these systems. SSNMR of MOFs and COFs can be used to ascertain the number of unique atoms in the crystal structure, map the distribution of organic linkers, investigate structural defects, probe ligand dynamics, and study guest binding locations. In this chapter, we review reports regarding SSNMR characterization of organic linkers in MOFs and COFs, along with the effects of paramagnetic centers near the surrounding organic linkers on SSNMR spectra. The advantages and drawbacks of various SSNMR methods and approaches for linker characterization in MOFs and COFs are also discussed.