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

Abstract

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 129 Xe nuclear magnetic resonance spectroscopy using hyperpolarized 129 Xe 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 129 Xe 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 129 Xe 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.

Hyperpolarized Xe NMR signal advancement by metal-organic framework entrapment in aqueous solution

“…4D) and a negative slope, which was consistent with previous studies of MOFs (37,43) but distinctively different from those in other molecular hosts (42,(44)(45)(46). Since there can be multiple Xe atoms in MOF pores, Xe-Xe interactions take place, which cannot occur relative to in the other molecular hosts with only one Xe atom per cavity (37,(44)(45)(46)(47)(48)(49). At higher temperatures, the chaotic motion of Xe is more vigorous, which shortens the residence time of Xe in ZIF-8 pores and decreases the Xe-Xe interactions, leading to the upfield change in the chemical shift of entrapped 129 Xe in comparison to lower temperatures.…”

129Xe NMR on Porous Materials: Basic Principles and Recent Applications

“…Kemnitzer et al investigated this disorder effect in the MOF CAU‐1, where one out of the four free positions at the aromatic linkers was modified with amino, acetamide, and methyl urea side chains. [ 104 ] The chemical shifts of adsorbed xenon calculated by the Fraissard approach were not expected to be suitable here due to the influence of the charged metal nodes. Existing DFT kernels for determining pore dimensions and linker distribution using argon physisorption were also found unsuitable.…”

Correlated disorder in metal–organic frameworks

“…Such is the case in the CAU-1 family, for example, for which hyperpolarised 129 Xe NMR spectroscopy has been used to identify correlations in side-chain orientations, in turn reflected in the experimental Ar sorption isotherms. 70 So too in the IRMOF-3-AMPh system (AMPh denotes a phenylamide substituent), where linker orientations can be ordered or disordered according to a clever choice of synthesis approach; the two polymorphs have profoundly different H 2 sorption profiles [ Fig. 4(a)].…”