Dynamic processes in Mixed-Ligand Metal Organic Frameworks

Metal-organic frameworks (MOFs) are versatile porous materials composed by organic linkers bridging metal nodes that find applications in gas storage, separation, and drug delivery, thanks to their tunable structures, intrinsic porosity and ability to interact with guest molecules. All these properties depend on the nature of the ligands and the metal nodes composing the structure. In details the use of amidic decorated ligands allows the insertion of hydrogen bond interacting moieties, providing excellent anchoring points for guest molecules encapsulation. In this thesis we explored the properties of the so-called pillared ligand metal organic frameworks (PL-MOFs) in terms of dynamicity and guests inclusion. In detail, in Chapter 2, the design and synthesis of flexible PL-MOFs with engineered topologies is treated. The chapter introduces the importance of pillaring in formation of amide containing heteroleptic MOFs, metal organic frameworks obtained from the combination of two or more organic linkers, analyzing the methodology involved for the synthesis of the pillars. All the pillars were synthesized through Buchwald-Hartwig amidation, combining dibromo derivatives with different aromatic scaffolds (benzene, naphthalene, biphenyl) with isonicotinamide to obtain ditopic bisamidic-dipyridinic ligands. These ligands were then reacted with common dicarboxylic acid as 1,4-benzene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, and 4,4’-biphenyl dicarboxylic acid resulting in different topologies, ranging from catenated to interpenetrated frameworks. This chapter is meant to provide information on some of the materials that are going to be used in the following chapters. In Chapter 3 the flexibility of PUM168, a triple-interpenetrated MOF, was investigated revealing its adaptability to various solvents and reversible structural changes during guest molecule exchanges. The ability to rearrange between solvates in single-crystal-to-single-crystal transformations permitted a structural determination of the deformation accompanying the guest exchange processes inside the MOF. In the second part of the chapter, PUM168 was exposed to some N-containing heterocycle compounds, like pyridine derivatives and quinoline, using soaking procedures in either pure liquid or solution, to evaluate its dynamicity. PUM168 demonstrated sensitivity toward pyridines alas tolerating quinoline and permitting a fine visualization of the inclusion of the guest in the channels. In Chapter 4 we compared the essential oil components storage and release capabilities of PUM168 and PUM210, two MOFs deriving from the same bisamidic-bispyridinic ligands but featuring different types of entanglement with PUM168 being characterized by flexible triple-interpenetrated framework and PUM210 featuring a more rigid four-fold catenated framework. The MOFs were studied in the uptake processes of a equimolar mixture of thymol and carvacrol, following the internalization of the guest via 1H NMR and TGA analyses. The release profiles of the loaded materials were compared to the one of pure oils using head-space mass gas chromatography (HS-MS-GC). The study showed that PUM168, with its flexible structure, released essential oils gradually over 15 days, while the more rigid PUM210 released them more steadily. Chapter 5 evaluated the gas sorption properties for CO2 uptake of four MOFs, differentiated by their type of entanglement, two isoreticular doubly interpenetrated MOFs (named PUM420 and PUM520) and two catenated MOFs (named PUM410 and PUM510). Interpenetrated frameworks showed stable CO₂ sorption and catenated frameworks, particularly PUM410, demonstrating a dynamic gate-opening behavior and high CO₂ uptake at 40 bar. These results highlight the role of framework flexibility in various applications, including guest storage, release, and gas sorption.