Exploring Local Disorder within CAU-1 Frameworks Using Hyperpolarized ¹²⁹Xe NMR Spectroscopy

The sorption properties of metal-organic frameworks (MOFs) can be influenced by introducing covalently attached functional side chains, which make this subclass of porous materials promising for applications as diverse as gas storage and separation, catalysis, and drug delivery. The incorporation of side groups usually comes along with disorder, as the synthesis procedures rarely allow for one specific position among a larger group of equivalent sites to be selected. For a series of isoreticular CAU-1 frameworks, chosen as model compounds, one out of four positions at every linker is modified with equal probability. Here, we investigate the influence of this disorder on Ar sorption and ¹²⁹Xe nuclear magnetic resonance spectroscopy using hyperpolarized ¹²⁹Xe gas. Models used for predicting the pore dimensions as well as their distributions were derived from the unfunctionalized framework by replacing one proton at every linker with either an amino, an acetamide, or a methyl urea functionality. The resulting structures were optimized using density functional theory (DFT) calculations. Results from void analyses and Monte Carlo force field simulations suggest that for available Ar nonlocal DFT (NLDFT) kernels, neither the pore dimensions nor the distributions induced by the side-chain disorder are well-reproduced. By contrast, we found the ¹²⁹Xe chemical shift analysis for the shift observed at high temperature to be well-suited to develop a detailed fingerprint of the porosity and side-chain disorder within the isoreticular CAU-1 series. After calibrating the ¹²⁹Xe limiting shift of the amino-functionalized framework with DFT calculations, the downfield shifts for the other two derivatives are an excellent measure for the reduction of the accessible pore space and reveal a strong preference for the side chains toward the octahedral voids for both cases. We expect that the strategy presented here can be commonly applied to disorder phenomena within MOFs in the future.