Identifying Selective Host–Guest Interactions Based on Hydrogen Bond Donor–Acceptor Pattern in Functionalized Al-MIL-53 Metal–Organic Frameworks

Abstract

We present a study analyzing the selectivity of host–guest interactions in a series of functionalized Al-MIL-53-X metal–organic frameworks with X = H, NH2, and NHCHO using acetone, ethanol, and water as probe molecules. While the amino group introduces additional hydrogen bond donor centers the NHCHO anchors function as donor and acceptor. The guests were chosen due to their ability to act solely as an acceptor in the case of acetone, whereas ethanol and water provide acceptor and donor qualities with a gradual decrease of the acceptor strength toward ethanol. The characterization of the host–guest interactions includes a comprehensive solid-state NMR spectroscopic study based on a full assignment of 1H and 13C high-resolution spectra using CRAMPS decoupling schemes to enhance 1H resolution combined with advanced 2D HETCOR (1H–13C, 1H–27Al, and 1H–14N) spectra at high magnetic fields. In spite of a pronounced dynamical disorder of the guests, we could identify a preferred binding of the acetone via a NH···OC hydrogen bond for the NH2 and the NHCHO anchor groups by analyzing trends in the 13C isotropic chemical shifts. At the same time 1H–1H through-space connectivities reveal a close vicinity of the acetone methyl groups to the benzene rings of the linkers. In contrast, for ethanol and water, the interaction with the anchor groups is too weak to compete with the thermal disorder at room temperature.