Pirazolatos metálicos porosos. Propiedades adsorbentes selectivas. (porous metal pyrazolates. Selective adsorption properties)

Resumen

Since many decades, classical inorganic porous materials (i.e. zeolites and activated carbons) are widely used in industrial processes, such as gas purification and separation, ion exchange, selective heterogeneous catalysis, biomedical applications, etc. Zeolites are, by far, the most employed materials not only because of their robustness and exceptional thermal stability but also for the regular distribution of their pores¿ size and shape that allow the interaction with atoms, ions and molecules throughout the bulk of the material. Nevertheless, the application of zeolites is limited by the pore size restriction (typically smaller than 1 nm) and by the lack of flexibility and transformability. In this context, a new class of hybrid organic-inorganic materials, firstly approached in 1978, gained renewed interest in the 1990s as a much more versatile alternative to zeolitic materials. The first big challenge of the research dealing with these Porous Coordination Polymers (PCPs, also called MOFs: Metal Organic Frameworks) was the achievement of materials with permanent porosity (i.e. materials robust enough not to collapse upon activation of the porous structure). Soon after, a huge number of structures have been synthesized envisaging an incredible range of applications. In general, PCPs are unlikely to compete with zeolites and other oxide-based porous materials in high-temperature applications owing to their limited long-term stability. In spite of that, these materials present many advantages that should be pointed out: i) an unlimited number of combinations of organic and inorganic bricks can be explored giving rise to a virtually infinite number of possible structures; ii) pores¿ size and shape can be systematically tuned with, in principle, no limits; iii) it is possible to functionalize the organic linker, or incorporate functional organic groups directly into the framework, so that porous solids, containing chemical groups capable of binding guests and/or catalysing reactions involving adsorbed guests, may be obtained; iv) the metal component and/or its interactions with guest molecules can be exploited to design porous materials with unusual physicochemical properties, such as redox activity, light absorption properties or magnetic effects; v) it is possible to use enantiopure organic ligands to obtain chiral porous materials for applications requiring enantioselective adsorption and catalysis. Anyway, so far, the low thermal and chemical stability as well as high sensitivity to moisture are the major limitations of PCPs materials for practical applications and, consequently, many efforts still must be done to improve their robustness. The work presented in this PhD thesis is part of a wide investigation concerning the synthesis and characterization of porous coordination polymers for the adsorption/desorption and separation of gases and small molecules of industrial, environmental or biomedical interest. A special focus is addressed to the achievement of thermally and chemically stable solids. To achieve this goal, a careful choice of the organic linkers¿ chemical nature is of paramount importance. Therefore, we decided to employ a set of symmetric organic ligands containing the pyrazole ring, eventually combined with the carboxylic acid functional group. The reason of this choice is the much more robust nature of the M-N(pyrazolate) coordinative bond compared to the quite labile M-O(carboxylate) bond, generally found in the vast majority of published PCPs structures. In this study, we have chosen the late first row transition metal ions (Co2+, Ni2+, Cu2+ and Zn2+) taking into account their borderline soft/hard acidic nature, their coordination versatility and labile nature, features that are adequate for the formation of robust and ordered polymeric structures with small or middle size N,O ligands. All the compounds presented in this work have been synthesized in our laboratories and characterized from the structural and textural points of view. In most cases, the robust nature of the M-N coordination bonds leads to the formation of polycrystalline products that have been characterized by means of X-ray powder diffraction. In few cases, the achievement of single crystal of suitable size (particularly in the case of mixed N,O-ligands) for X-ray diffraction measurements has been possible, allowing to solve the structure in a conventional way. After the preliminary structural characterization, the adsorptive properties of those solids exhibiting permanent porosity have been investigated in detail, with a further insight into some specific applications envisaged for each promising structure. The isolated compounds have been classified in four categories, focusing either on the local structure around the metal centre or on the properties of the bulk material. Therefore, the materials have been categorized as i) zeomimetic frameworks (Chapter 2); ii) soft porous coordination compounds (Chapter 3); iii) porous coordination polymers with structural analogies to MOF-5 material (Chapter 4) and iv) a series of isoreticular nickel based PCPs (Chapter 5). The assembly of 4-carboxypyrazolate ligand molecules (L12-) with Cu2+ cations in NH3/NH4+ buffer solution gives rise to the anionic network of formulation [NH4@Cu3(¿3-OH)(¿3-L1)3]n¿solv (NH4@Cu3(OH)L13), which is presented in Chapter 2. Unlike most zeotypic coordination networks (that do not usually permit cation-exchange processes since they have neutral or cationic nets) the anionic nature of this material, as well as zeolites, can give rise to exchange of the extraframework cations with the possibility to modulate the porosity of the network depending on the charge and size of the exchanged cations. Indeed, we have demonstrated that the ion-exchange processes lead to profound changes in the textural properties of A@Cu3(OH)L13 (A = extraframework cation) porous surface and in the adsorption selectivity for different separation processes of gases and vapours. Therefore, we studied the effect of extraframework cation exchange on the efficiency of the A@Cu3(OH)L13 (A = extraframework cation) series for the separation process of complex mixtures of gases (acetylene, carbon dioxide, methane, dinitrogen) and vapours (benzene, cyclohexane). Noteworthy, the substitution of Cu2+ by Cd2+ cations in the reaction medium affords a neutral layered material ([Cd(H2O)2L1]¿H2O) that does not maintain its crystallinity upon thermal activation. As a matter of fact, the removal of crystallization water molecules from the structure causes the irreversible collapse of the framework. Despite its lack of permanent porosity, this material is a nice example of first generation porous framework. Another class of porous coordination polymers of interest comprises those materials which are bi-stable and are able to respond to external stimuli. These flexible porous materials, classified by Kitagawa as third-generation materials, exhibit unusual gas separation properties as a consequence of guest responsive behaviour. In this regard, the research presented in Chapter 3 is devoted to the synthesis and properties of a novel flexible-PCP of formulation ([Cu2(L2)2(OH2)]¿DMF1.5)n ([Cu2(H2O)(L2)2]), obtained in the reaction of the 3,5-dimethyl-4-carboxypyrazole ligand (H2L2) with CuX2 (X = Cl, Br, NO3). This porous material shows intriguing structural transformations taking place upon guest removal/uptake. These structural transformations are accompanied by evident colour changes, proving that a spatial rearrangement of the ligands around Cu2+ metal centres takes place. Interestingly, [Cu2(H2O)(L2)2] system has an unprecedented acua bridge between the two copper atoms in the dimeric building unit, the (reversible) removal of which gives rise to unusual coordinatively unsaturated convergent adsorption active sites as well as to structural bi-stability. Indeed, the metastable evacuated phase (¿-[Cu2(L2)2]) transforms into an extremely stable porous material (¿-[Cu2(L2)2]) after freezing at liquid nitrogen temperature. Although ¿-[Cu2(L2)2] is inert to ambient moisture, it recovers the acua bridge upon soaking in water. The gas adsorption properties of both materials with permanent porosity (¿-[Cu2(L2)2] and ¿-[Cu2(L2)2]) have been studied by the measurement of single component gas adsorption isotherms and variable temperature pulse gas chromatography. The results reveal intriguing gas adsorption properties with guest triggered breathing phenomena as well as very high selectivity coefficients for binary gas mixtures of industrial/environmental interest (e.g. CO2/CO or CO2/N2). Under similar reaction conditions and with the same building units, another coordination polymer of formulation {[Cu(HL2)2]¿4H2O¿2DMF}n (Cu(HL2)2) has been obtained. In this material, the organic ligand is partially deprotonated (just at the carboxylic acid moiety, HL2-) therefore, the resulting net has reduced dimensionality and consists of 2D sheets where water guest molecules lye in the space between two adjacent layers while DMF molecules are located in the same plane of the sheets. This material has been studied just by the structural point of view, as a consequence of the unavailability of a phase pure bulk solid. Going back to Kitagawa¿s classification of coordination polymers, the milestone of rigid-PCPs with permanent porosity (second generation materials) is the [Zn4O(1,4-benzene-dicrboxylate)3] system, also known as MOF-5. This material exhibits very high permanent porosity. Despite its tremendous notoriety, MOF-5 has a considerable drawback for possible practical applications: its low stability to air moisture, a direct consequence of the M-O(carboxylate) bond hydrolysis. So, when MOF-5 material is envisaged for any practical application, this feature becomes its Achilles¿ heel since ambient moisture (that can not be generally completely removed) relentlessly destroy the framework. Consequently, Chapter 4 is devoted to the search of robust MOF-5 type materials. Thus, three novel materials with the [M4O(L)6] stoichiometry analogous to MOF-5, where M = Zn and Co, L = L2 for [Zn4O(L2)3] and [Co4O(L2)3]; M = Co, L = 4,4'-buta-1,3-diyne-1,4-diylbis(3,5-dimethyl-pyrazolate) (L6) for [Co4O(L6)3], are presented in which the hydrolysis sensitive M-O(carboxylate) coordination bonds are replaced (partially or completely) by the robust M-N(pyrazolate) coordination bonds. Another important contribution to the stability of these frameworks is provided by the methyl groups on L2 and L6 ligands that stabilize and protect the M4O moieties from hydrolisis. The resulting materials are highly hydrophobic and show enhanced thermal and chemical stabilities that open the way to the investigation of adsorptive properties in the presence of moisture. Therefore, the behaviour of [Zn4O(L2)3] MOF for the selective capture of harmful volatile organic compounds (VOCs, i.e. models of chemical warfare agents such as Sarin nerve gas and Mustard vesicant gas) as well as for the separation of benzene/cyclohexane mixtures has been investigated. The extraordinary selectivity for VOCs compared to water shown by [Zn4O(L2)3] material would be the key feature to open the way to a possible use under real conditions: eventually [Zn4O(L2)3] powders could be incorporated into air filters and protective textiles. Finally, Chapter 5 is devoted to a series of isoreticular MOFs based on Ni8(OH)4(H2O)2 octanuclear clusters connected by both bis-pyrazolate and carboxy-pyrazolate linear ligands. These materials, of general formulation [Ni8(OH)4(OH2)2(L)6]n (Ni8(L)6), show permanent porosity that increases with the ligand length, with very high values of BET specific surface areas for the materials with the longest organic linkers. The cubic close packing of the Ni8(OH)4(H2O)2 clusters give rise to n octahedral and 2n tetrahedral cavities per MOF formula unit. These cavities result to be highly hydrophobic especially for those materials containing bis-pyrazolate ligands and, therefore, their performances in the capture of diethylsulfide (Mustard gas, chemical warfare agent model) have been evaluated under dry and humid conditions. The results are comparable to those exhibited by two selected activated carbons exhibiting the highest capacity and hydrophobicity available in the market. Moreover, Ni8(L5)6 compound containing 4,4'-buta-1,3-diyne-1,4-diylbis(pyrazolate) ligand (L5) has been proved to be highly stable in Simulated Body Fluid (SBF) and therefore it has been used as a scaffold system for the incorporation and release of the non-conventional metallodrug [Ru(p-cymene)Cl2(pta)] (RAPTA-C). This system results to be a proof of concept of the suitability of MOF materials for metallodrug delivery purposes. Summarizing, a wide range of rigid as well as flexible porous coordination networks exhibiting high chemical, mechanical and thermal stabilities have been synthesized by the combination of metal ions and ligands under the appropriate conditions. Moreover, their adsorptive properties have been evaluated towards relevant applications such as the separation of complex mixtures of gases and vapours as well as the release of non-conventional metallodrugs. All these results are of interest for the future design of MOF materials suitable for applications of high social impact.