A review of exotic quadrupolar metal NMR in MOFs

The rapidly expanding field of metal-organic frameworks (MOFs) and their numerous associated applications has captured the interest of researchers worldwide. These materials are often difficult to fully characterize through one specific avenue due to their intricate and occasionally complicated structures, highlighting the need for a multi-pronged characterization strategy. Solid-state NMR is an increasingly popular tool for MOF investigations due to the rich local information it can provide about the target nucleus, which is complementary to long-range structural characterization methods such as X-ray crystallography. The metal centers in MOFs often play an integral role in guest adsorption, catalysis, and other applications; however, their local environment can also be frustratingly difficult to probe through spectroscopic methods. There has been tremendous progress in the characterization of MOFs by directly interrogating the metal center using solid-state NMR experiments, particularly those targeting exotic quadrupolar metals such as 25 Mg, 39 K, 43 Ca, 45 Sc, 47/49 Ti, 67 Zn, 69/71 Ga, 91 Zr, 115 In, and 139 La. In this contribution, we summarize basic principles of exotic quadrupolar metal NMR along with general acquisition strategies, review significant works in the field in the past 8 years, and highlight the types of information researchers can expect to uncover in MOF systems using this approach.