Exploring chemical complexity in Group IV reticular solids

Laburpena

This thesis tackles the challenge of expanding the synthetic tools available to increase the chemical complexity of reticular materials based on group IV metals by targeting both the organic and inorganic part of these porous, hybrid materials. Chapter 1 gives an insight into the increase of chemical complexity in reticular solids, that provides a general context to the results developed during this thesis. Next, the introduction of chemical variance in organic connectors is described in Chapters 2, 3 and 4, whereas the chemical transformation of inorganic nodes is introduced in Chapter 5. All the work described is built upon the use of two sub-types of reticular solids, Metal-Organic Polyhedra (MOPs) and Metal-Organic Frameworks (MOFs), both based on Ti(IV) and Zr(IV) metals. Chapter 1 provides an overview of the rise of chemical complexity in reticular solids, especially in Metal-Organic Frameworks (MOFs). In the first part, the concept of reticular chemistry, topology and isoreticular expansion, as well as the necessary tools to design and prepare porous periodic structures are introduced. Then, the porous networks are presented by increasing order of complexity. The classification starts with the study of binary networks, continuing with the presentation of the networks of intermediate complexity, such as multivariate and multicomponent frameworks. And finally, it advances in complexity up to hybrid multicomponent frameworks. The discussion also includes the presentation of the characterization techniques used for deciphering the organic and inorganic variance. Moreover, it highlights the importance of the structural richness and intrinsic properties of complex MOFs to extend their applications in relevant research fields. On the following chapters, the experimental results are also presented in increasing order of chemical complexity. Chapter 2 proposes to extend the chemical diversity of Metal-Organic Polyhedra (MOPs) by combining the use of hydroxamic group as metal binders in the organic linker with titanium. Our approach enables the synthesis of the first example of a porous Titanium-Organic Polyhedra, cMUV-11 (cMUV = cage-type Material of Universitat de València), which displays permanent porosity with a value close from the highest reported so far. cMUV-11 crystallizes as discrete neutral cubes whose packing in the solid state is controlled by a H-bond network of N-H···O H-bonds involving complementary hydroxamic groups from adjacent cages. This new robust MOP is compatible with the combination of functionalized linkers that determine pore chemistry and tune the structural response to solvent evacuation. Chapter 3 reports the synthesis of single crystals of UiO-68 and its photoactive-derivate UiO-68-TZDC, which contains tetrazine moieties in the organic linker. The analysis of single-phase frameworks reveals the necessity of combining both linkers in the same structure for obtaining a robust platform for catalytic applications. UiO-68 crystals are amenable to post-synthetic linker exchange reactions which result in the combination of TPDC and TZDC linkers in multivariate UiO-68-TZDC%. The fine selection of synthetic conditions allows the control of linker spatial distributions over the crystal. The careful analysis of resultant microstructures (core-shell and homogeneous distributions) facilitates the understanding of the photocatalytic behaviour of solids. Chapter 4 explores the use of tetrazine moiety as a tag for general framework post-functionalization in one-step. The compatibility of tetrazine with inverse Electron-Demand Diels-Alder (iEDDA) reactivity assists the covalent modification of UiO-68-TZDC. The selected mild conditions guarantee quantitative transformations respecting the intrinsic structure and porosity of the original framework. Our protocol enables the introduction of a broad scope of dienophiles leading multiple pore environments with diverse chemical functionalities, such as hydroxyl, phenyl, succinimide, terminal carboxylic groups, aliphatic chains, chiral centres and even fullerenes, in the resultant pyridazine and dihydropyridazine frameworks. The effect of tetrazine reticulation on iEDDA reactivity is also analysed. Finally, Chapter 5 presents an unprecedented methodology to introduce metal variance in the inorganic nodes of heterometallic titanium-organic frameworks by metal-exchange reaction in MUV-10 crystals (MUV = Material of Universitat de València). This MOF presents Ti2Ca2 metal-oxo clusters which combine hard Ti4+ ions and soft Ca2+ centres more prone to metal exchange. The soaking of the MUV-10 crystals in different methanolic salt solutions (M = Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) in controllable times drives a topological transformation into heterometallic MUV-101 (Co, Fe, Ni, Zn) which is isostructural to MIL-100 and MUV-102 (Cu), an heterometallic analogue of HKUST. This methodology enables the formation of new heterometallic titanium frameworks, that are not accessible under solvothermal conditions, and opens the door to the synthesis of additional titanium heterometallic phases following the principles of reticular chemistry.