Design, synthesis and structural characterization of functional coordination polymers for storage and separation of gases of energetic and environmental relevance.

The research about new fuels, as an alternative to traditional fossil fuels, is one of the most important and pressing topics of the last decades. With this purpose, several strategies have been proposed: the more promising ones involve the use of hydrogen and methane as energy carriers. Even if, in principle, these gases seem to be the best alternative to liquid traditional fuels, their application in stationary and mobile systems, from a practical point of view, shows some di_culties. The most important obstacle to the development of an economy based on hydrogen is the di_culty to store this gas, both because of its physical characteristics and because of its low volumetric energy density, especially if compared to that of gasoline. In this sense, the Department of Energy of the US government has stated precise targets, for hydrogen storage and transportation parameters, that ful_ll safety and economic requirements. Among the great number of materials and methods proposed to store hydrogen and other gases of environmental interest, one of the more recent and promising one is based on MOFs (Metalorganic Open Frameworks), that is porous coordination polymers. This kind of systems is constituted by metal ions or metallic clusters connected with each other by organic ligands in order to build an open network with channels and cavities capable to include, in a selective way, guest molecules. The work presented in this doctoral thesis _ts in a wider project of synthesis and characterization of porous coordination polymers, with the aim of obtaining materials suitable for the adsorption and separation of environmentally relevant gases. Particularly, we have studied compounds of divalent metallic ions of the _rst transition period, with polyazaaromatic ligands functionalized with carbonylic and carboxylic groups. The synthetic methods used led, in most cases, to the formation of polycrystalline products that, for this reason, have been structurally characterized by means of X-ray powder di_raction (XRPD). Therefore, the adopted structural solution method from powder di_raction data is described in details in Chapter 2, as well as other analysis techniques that have supported the de_nition of a correct structural model. With this intent, elemental and thermal analysis (TGA and DSC) have been performed on the obtained products, in order to evaluate the materials composition and their thermal stability range. Besides, the thermal behavior of the materials has been studied through thermodi_ractometric measurements, thus obtaining information about the formation of polymers and about possible phase transitions induced by the gradual heating of the studied products. After the structural characterization of the compounds, the magnetic and adsorptive properties of the obtained materials have been studied; we have paid particular attention to the adsorption processes of nitrogen and hydrogen at 77 K and carbon dioxide and methane at 273 K. The _rst group of studied compounds includes three species, with general formula [fM(F-pymo)2gn]_2:5nH2O (M = Co, Zn, and Co0:21Zn0:79), based on the 5-uoropyrimidin-2-olate (F-pymo) ligand. In the three compounds the F-pymo ligands adopt an N,N'-exobidentate coordination mode, bridging the M(II) ions within a tridimensional sodalitic structure. Heating the species [fM(F-pymo)2gn]_2:5nH2O (M = Co, Zn), as described in detail in Chapter 3, at _rst leads to the formation of an anhydrous sodalitic phase; further heating induces the polymorphic transformation of this phase into a bidimensional layered structure. Through this chapter the adsorption measurements results toward H2 and N2 at 77 K and CH4 et CO2 at 273 K of the thermally activated sodalitic species will be also illustrated. A \gate-opening" mechanism, together with the polar nature of the pores and with exclusion e_ects due to the di_erent dimensions of the adsorbate molecules, lead to the exclusive adsorption of CO2, starting from a speci_c \gate-pressure". Mn(II) compounds with the pyrimidine-4,6-dicarboxylate (pmdc) ligand have been examined in Chapter 4: the variation of the synthetic conditions led to the isolation of four di_erent species, two monodimensional, one bidimensional and one porous tridimensional; a mixed iron(II) and manganese(II) compound has been also prepared, with a monodimensional polymeric structure. The 1D compounds with formula f[Mn(_-pmdc)(H2O)3] _ 2H2Ogn, f[Mn2(_-pmdc)2(H2O)5] _ 2H2Ogn,f[FeMn(_-pmdc)2(H2O)5] _ 2H2Ogn, and the bidimensional compound f[Mn(_3-pmdc)(H2O)] _ H2Ogn, have been characterized from the point of view of the magnetic properties, showing in general an antiferromagnetic behavior; N2 adsorption measurements have con_rmed the lack of permanent porosity in these materials. From the point of view of porous materials preparation for gas adsorption, the most relevant compound is the tridimensional one. This material, that displays a zeomimetic sodalitic structure, shows permanent porosity. Worthy of note, the anhydrous phase contains coordinatively unsaturated (square planar) Mn2+ centers, that are very unusual and could be interesting in adsorption processes and in catalysis. Adsorption measurements of N2 at 77 K and CO2 at 273 K has been interpreted in the light of this structure. The pmdc ligand has been also used in the preparation of Fe(II), Co(II), Ni(II) and Cu(II) compounds, described in Chapter 5. The obtained products, of general formula f[M(_-pmdc)(H2O)2] _ H2Ogn, at room temperature are monodimensional polymers. The study of the thermal behavior of these species showed that, when heated, at _rst they lose a water molecule, a_ording monodimensional bishydrated polymers. A further thermal treatment lead to the formation of the anhydrous phases, that are characterized by a low crystallinity. Only in the case of compound f[Cu(pmdc)]gn we have obtained information about the structural change that takes place during the thermal dehydration. During this process, the carboxylate groups deviate from coplanarity with the pyrimidinic ring, bridging the metal centers of adjacent chains and giving rise to the formation of bidimensional ordered layers. The observed conformational change of the ligands can be justi_ed by considering a Jahn-Teller distortion of the Cu(II) metal centers. The third and last ligand presented in this thesis is the 1,3,5-triazine- 4,6-dione-2-carboxylate (Hoxonate) dianion. The compounds formed by this ligand with the Cu(II) ion are discussed in Chapter 6. The _rst compound presented, f[Cu(Hoxonato)(H2O)]gn, is a monodimensional polymer characterized by a high thermal stability. Indeed, thermodi_ractometric studies have demonstrated that this material remains unchanged until the decomposition temperature (513 K), without losing the coordination water. The second copper(II) compound, f[Cu(Hoxonato)(4; 40-bpy)0:5] _ 1:5H2Ogn, is a tridimensional coordination polymer in which layers of [Cu(Hoxonato)]n are connected by 4,4'-bpy ligands, that act as pillars. Clathrated water molecules are hosted in isolated cavities. Even if the structural characteristics are not those of a porous compound, adsorption measurements of CO2 at 273 K have demonstrated that the system is capable to adsorb this gas in an e_cient and reversible way. This result is distinct from that obtained with CH4, of which a minimum quantity is retained at high pressure. The selectivity shown by this material motivated us to investigate its behavior in the separation of mixtures of CH4/CO2. These results are described at the end of this Chapter. The last Chapter of the thesis is dedicated to the description of Mn(II), Co(II), Ni(II) and Zn(II) compounds with the Hoxonato ligand. Also in this case the compounds containing only the Hoxonic ligand are 1D polymers, with formula f[M(Hoxonato)(H2O)2] _ H2Ogn (M = Mn, Co, Zn) and f[Ni(Hoxonato)(H2O)4]gn. By using also the 4,4'-bipyridine ligand we have instead obtained two crystalline compounds, one of Zn(II) and the other one of Ni(II): in the case of zinc the compound is bidimensional, while it was not possible to structurally characterize the species containing Ni, even if the gas adsorption and the magnetic data show that we are dealing with an extended ultramicroporous system. The latter is the only one, among the compounds presented in Chapter 7, with permanent porosity. The results presented show that the ligands used in this thesis work had an extremely versatile behavior, as it is reected in the structural diversity and in the properties of the obtained products. The symmetric and anionic nature of the employed ligands favors the formation of extended robust systems that do not need the presence of counterions hosted in the hollows of the structure. This characteristic permits that the compounds obtained with a tridimensional framework show, in all cases, a porous structure suitable for the adsorption of probe gases of environmental interest. The high selectivity shown in some adsorption processes opens possibilities in the use of these systems in processes of separation and catalysis of gases of industrial and environmental interest. Furthermore, we could demonstrate that it is possible to use non conventional synthetic strategies in coordination chemistry, that lead to the formation of the described compounds. The results described in this thesis work have given rise to _ve sci-16 enti_c papers. Four of these have been already published and are collected in Appendix A while a _fth is in preparation