Functional molecular materialssupramolecular networks and coordination polymers with optical and/or magnetic properties

Resumen

The field of molecular materials attracts the interest of many scientists due to the huge potential of molecular chemistry in materials design and fine tuning of materials properties. The two different projects addressed in this thesis represent two topics that stand out in this area, spin crossover (SCO) compounds and coordination polymers with optical and/or magnetic properties, due to their significant progress and rapid development in the last years. The first part of this thesis is based on the use of hydrogen bonds in the design of multifunctional iron(II) supramolecular networks with abrupt spin transitions. Chapter 1 is an introduction to this topic from the perspective of [Fe(bpp)2]2+ complexes (bpp = 2,6-bis(pyrazol-3-yl)pyridine). Chapter 2 shows a comparative analysis of [Fe(bpp)2]2+ salts with the flexible adipate or the rigid terephthalate as hydrogen bond acceptors. This study shows the relevance of the rigidity of the anions in defining the magnetic properties. Moreover, two anhydrous materials with distinct magnetic behaviour and crystallinity were obtained for the adipate derivative depending on the desolvation treatment used. Chapter 3 describes the incorporation of acentric mononegative anions which enables the optimisation of the hydrogen bond connectivity. In particular, the use of the isonicotinate linker has allowed the construction of an acentric diamond-like crystal lattice. The anhydrous phase of this material exhibits a phase transition between two ferroelectric structures in the same temperature range in which the spin crossover is observed. The employment of the isonicotinate N-oxide anion has led to the isolation of two salts that exhibit abrupt spin transitions triggering novel structural rearrangements that respond towards light irradiation or temperature change: a supramolecular linkage (H-bond) isomerisation involving a carboxylate moiety and a proton displacement between two anions that form a short strong hydrogen bond. Isotopic experiments and neutron diffraction data reveal that hydrogen bonding influences the spin crossover properties. Specifically, hydrogen bonds in the 2nd iron coordination sphere are found to be shorter for the low-spin species in comparison to those in the high-spin state. The second part of this thesis relies on the synthesis and characterisation of coordination polymers based on a novel family of ethynyl-bridged polytopic ligands. The aim of this work is to improve the stability in water and at extreme pH values of current coordination compounds through the use of the chelating picolinate functional group. Chapter 4 contains a brief summary about synthetic approaches and applications of these materials. Chapter 5 describes the synthesis (Sonogashira-type cross-coupling reactions) and characterisation of the novel family of ligands. The synthetic methodology, thermal, structural, luminescent and magnetic properties of crystalline compounds constructed from these ligands and divalent ions are described in Chapter 6. Instead, Chapter 7 gathers the progress achieved with trivalent lanthanide ions. Sensing experiments reveal that Zn and Eu derivatives can be selectively used as luminescent probes in aqueous medium towards Fe3+ and nitrobenzene detection through luminescent quenching. All these polymers are remarkably robust in water within a wide pH range. Finally, Chapter 8 compiles the general conclusions of the present dissertation.