Porphyrin-related MOFs: structural design towards environmental challengues

Abstract

Development and improvement of multifunctional and environmentally friendly materials is a constant challenge for Materials Science and Technology. In this sense, MOF (Metal-Organic Framework) materials are of great interest, due to the fact that these porous solid coordination networks exhibit attractive properties, with potential applications in fields in which society manifests an increasing demand of knowledge as, for example, energy storage and transport, drug delivery, capture of pollutants and greenhouse effect gases, and heterogeneous catalysis, among others.

In this context, the work herein presented was designed with the aim of developing new porphyrin-based MOF materials. Thus, our goal is to mimic the functions that porphyrins play in biological systems, as catalysis, in order to reproduce them in the solid state, as well as applications with environmental interest, as gas sorption and separation. With this aim, TCPP porphyrin (meso-tetra-4-carboxyphenylporphyrin) and protoporphyrin IX have been combined with first transition metals, such as manganese, nickel and copper. In this way, we explored the different coordination modes that these ligands offer with different metals. In addition, carboxylic secondary ligands (1,2,4,5-benzenetetracarboxylic acid and isophthalic acid) have been occasionally used as modulating agents. The synthesis design has been focused on suitable combinations of metals and ligands and the selection of the proper conditions. In this way, even if quite a high amount of compounds were obtained, just five of them are presented in this thesis, synthesized under mild solvothermal conditions. Preliminary characterization has been carried out using quantitative elemental analysis, infrared spectroscopy, X-ray diffraction and fluorescence and density measurements. The structural study was performed using single-crystal X-ray diffractometry and the thermal study was carried out by means of thermogravimetry and thermodifractometry. In compounds with the appropriate structural features, heterogeneous catalytic experiments and gas and pollutants adsorption experiments have been performed, according to the aim of this thesis. In addition, in cases where it was considered appropriate, compounds have also been characterized by ultraviolet-visible (UV-Vis) and electron paramagnetic resonance (EPR) spectroscopy and by means of magnetic susceptibility measurements. Besides, occasionally, quantum-mechanical calculations based on the functional density theory (DFT) have been carried out. The first compound, with formula [Cu24(m-BDC)24(DMF)20(H2O)4]·24DMF·40H2O, (DMF = N,N-dimethylformamide) consists of cuboctahedral clusters of 24 CuII ions linked by isophthalic acid (m-BDC), where protoporphyrin IX plays an important role in the production of high quality crystals. By means of a crystallochemical study including other similar compounds existing in the literature, in this thesis two mother structures, of greater symmetry, have been identified, the rest of them deriving from them. On the other hand, structural features of this compound, able to host a large number of solvent molecules, correspond to its adsorption capability, confirmed by the study of pollutants adsorption in aqueous solution, with adsorption kinetics are among the most rapid in literature. Faced with the difficulty of obtaining new materials with protoporphyrin IX, the inclusion of porphyrin TCPP as the main ligand has allowed to obtain the 2D compound [Mn3(TCPP)(H2O)4]·nD (D=solvent), with a layered crystalline structure, which consists of MnTCPP units linked by dimers of pentacoordinated MnII. This arrangement gives rise to large structural voids. The use of diverse characterization techniques allowed to determine its high thermal stability and its solid state transformation to the 0D compound [Mn(H4TCPP)(H2O)2]·nD. This transformation takes place by means of a cationic exchange Mn2+↔2H+ of the non-dissolved compounds in the mother liquor. These solid state transformations are unusual in this type of compounds where common processes are induced by the solvent exchange. By adding the first transition metals Ni and Cu, combined with the porphyrinic ligand TCPP, two new compounds have been obtained with different dimensionality: the 3D compound [Ni5(H2TCPP)2O(H2O)4]·nS (S=solvent) and the 0D compound [Cu(H4TCPP)]·6DMF. The first one has a three-dimensional crystalline structure formed by TCPP units and NiII ions, joined by ferromagnetic NiII trinuclear complexes and it exhibits a large accessible pore volume. The crystal structure of the second one is based on TCPP and CuII monomers. Both have high thermal stability and high catalytic activity. The structural features of compound [Ni5(H2TCPP)2O(H2O)4]·nS provide significant robustness that makes its crystal lattice stable after several catalysis cycles, confirming its properties as heterogeneous and recyclable catalyst. In addition, due to its large surface area, this compound rises as a good adsorbent for pollutants in solution and in gaseous state, with some selectivity towards certain gases in mixtures with industrial interest.

Therefore, the work gathered in this report is an approach of three different topics: porphyrins of biological systems in the solid state, crystallography (and structural analysis), and the scientific vocation to contribute to the improvement of our society. All of them come together in this thesis in order to coexist with beautiful purposes: to face the environmental challenges demanded by our society and to expand the knowledge in the field of advanced materials as a paradigm of the evolution of our species.