Molecular confinement in porous crystals

The leitmotiv of the present PhD thesis is the study of the molecular nanoconfinement inside crystalline Metal-Organic-Frameworks (MOFs), employing different types of guests, ranging from biomimetic organometallic Ni-complexes to small organic molecules featured by biological and pharmaceutical activity. The study of molecular encapsulation performed with already known mesoporous MOFs is presented in Chapters 1 and 2. The two MOFs employed derive from the solvothermal combination of a tritopic carboxylic linker, named TATB (4,4′,4″-triazine-2,4,6-triyl-tribenzoic acid) with two different metal ions: Cu2+ (PCN-6’) and Zr4+(PCN-777). The first research has investigated the application of PCN-6’ as suitable host for nicotine trapping. Our aim was to inspect, through an in-depth SC-XRD analysis, if the guest was partially ordered or organized in nano-clusters inside the pores of the host. We only partially succeeded in the guest modeling: only the nicotine molecules close to copper atoms showed a distribution of the electron density which could be modeled as molecules of nicotine coordinated to the metal centers. A second research line, in collaboration with Département de Chimie Moléculaire, Université Joseph Fourier-Grenoble, has considered the heterogenization of biomimetic Ni complexes, active in the homogeneous proton reduction to H2, by their inclusion inside a robust mesoporous Zr-based MOF, named PCN-777. We aimed at stabilizing the molecular catalyst inside the host simply taking advantage of supramolecular interactions, thus avoiding the need of complicate post-synthetic-modification approaches, usually followed for the encapsulation of organometallic species inside a MOF matrix. The choice of PCN-777 as host systems derives from its high thermal (Tdec> 330°C) and chemical (aqueous solution, pH=3) robustness, features which make the MOF well suited for electrocatalytic applications. The accomplishment of the complex loading, as well as the confirmation of its integrity, was confirmed by ESEM-EDS, ICP, SS-NMR e BET measurements. Hence, we moved our attention on the optimization of PCN-777@complex thin film deposition on conductive glasses (FTO). Preliminary cyclic voltammetries performed on FTO-PCN-777@complex outlined promising proprieties for the hydrogen evolution reaction (as described in Chapter 3). Later on, we directed our efforts to the synthesis of new MOFs. We designed ex-novo ligands exploiting the Pd(0) mediated Buchwald coupling reaction. A small library of amino-carboxylic and amido-pyridinic linkers were successfully synthesized in high yields. Hence, the final goal was the building of flexible frameworks, bearing amidic or aminic anchoring sites for the guest uptake. Then we combined these ligands with a broad series of metal salts, obtaining a small family of novel MOFs. In Chapters 4 and 5 the synthesis procedure, structural characterization and study of the new materials are provided. Hence, some of the new MOFs were tested both in the storage of gas (CO2 and N2) as well as in the uptake of small organic molecules. In particular, a series of APIs (Active Pharmaceutical Ingredients) characterized by a substituted phenol core, were chosen as suitable guests. The attention was mainly directed to carvacrol, thymol, eugenol and propofol. The mutual hydrogen bond donor/acceptor character of the chosen phenolic guests was believed to be well suited to facilitate the host-guest interactions with the amine or amide functionalities protruding from the MOFs walls. The soaking protocol in neat guest led to good results: we succeeded, thanks to SC-XRD analysis performed with synchrotron light radiation (XRD1, Electra Sincrotrone Trieste), to locate with high precision the guest structures and interactions responsible of the nanoconfined inclusion processes. It should be underlined that only very few examples of structurally defined guest@MOF systems are reported in literature, and that the structural definition of the nanoconfined guest∙∙∙guest interactions must be considered an absolute novelty. Some preliminary tests aimed at elucidating the possibility of having a controlled guest release by thermal treatment have then been carried out, results which are collected at the end of Chapter 4.