Polímeros de coordinación porosos, magnéticos y luminiscentes basados en derivados policlorotrifenilmetílicos con iones de metales de transición y lantándos

Resumen

Summary Porous, magnetic and luminescent coordination polymers based on carboxylate-substituted polychlorotriphenylmethyl derivatives with transition and lanthanide metal ions In recent years, the construction of metal-organic coordination polymers has attracted considerable attention in supramolecular and materials chemistry due to the formation of fascinating structures and potential applications as optoelectronic, magnetic, porous materials, among others. Among these, the 3-D coordination polymers, known as metal-organic frameworks (MOF) are the most investigated. The interest for supramolecular frameworks is not only due to their possible applications as functional materials, but also for their intriguing and diverse molecular topologies and assembling processes. The approach towards the development of this kind of functional architectures consists in using molecular building blocks able to assemble via coordination chemistry and/or H-bonding. Until now, the most widely route used to form a metal-organic framework consisted in the direct assembly of metal ions with bridging ligands. However, the synthesis of robust MOFs with increasing pore size dimensions and simultaneous luminescence and long-range magnetic properties still remains a challenge. While the achievement of strong magnetic interactions requires short superexchange pathways between open-shell-metal units, the synthesis of porous MOFs usually requires long polytopic connectors. Therefore, the use of long organic ligands to connect open-shell units is expected to decrease or, in the worst of the cases, disrupt the superexchange magnetic pathways. A promising strategy to overcome such an inconvenience is based on the linkage of magnetically active metal ions by persistent polytopic organic radicals properly functionalized. In such cases organic radicals may act as a magnetic relay and, therefore, the resulting porous structures exhibit larger magnetic couplings and dimensionalities in comparison with those systems made up from diamagnetic polytopic coordinating ligands. In the present work, we studied carboxylate-substituted polychlorotriphenylmethyl radicals (PTM) as open-shell organic building-blocks to be used in this metal-radical approach. To date, the use of radicals as connectors of transition metal ions has allowed the fabrication of one- and two-dimensional magnetic metal-organic open-frameworks. Keeping in mind the remarkable structural and magnetic properties obtained with mono- bi- and three carboxylate-substituted polychlorotriphenylmethyl radicals, and with the idea of continuing to further increase both structural and magnetic dimensionalities, we report herein for the first time, the synthesis of a new PTM derivative (PTMmTC), that is an organic three-carboxylic acid derivative with - COOH groups substituted in one of the meta positions of each of the three phenyl rings. To the best of our knowledge, such a radical is the first example of a species with electronic open-shell with such characteristics. With the purpose to increase the dimensionality of the metal-organic frameworks we developed two methodologies: First, the use of the hexatopic radical ligand PTMHC with transition metal ions, taking into account that the proper selection of the number and location of carboxylic groups in this type of radical seems to be a powerful strategy to rationally extend the coordination of single metal ions creating metal-radical frameworks. The second strategy uses the tritopic radical ligand PTMTC with lanthanide ions that have higher coordination spheres. Using the first methodology, based on PTMHC radical and copper ions, we obtained a 3-D interpenetrated architecture of formula [Cu6(PTMHC)2(4,4'-bipy)3(H2O)12]n (31). We obtain also the isomorphic 3-D coordination polymer with formula [Cu6(¿H-PTMHC)2(4,4'-bipy)3(EtOH)6(H2O)6]n (32) using the non magnetic derivate ¿H-PTMHC. Both 3-D structures can be described as two interpenetrating primitive cubic nets connected through bipyridine linkers. The coordination polymer 32 shows weak antiferromagnetic interactions between Cu(II) ions at low temperature. A deeper study of the data allows the analysis of the magnetic behaviour of 31 as a 3-D antiferromagnet, with TC = 0.39 K, with ferromagnetic exchange interactions that do not propagate with equal strength along the three crystallographic directions above this critical temperature. Due to the interpenetrated structure obtained with the PTMHC radical we decided to use the second methodology based on lanthanide metal ions. To explore the reactivity of carboxylic-substituted PTM radicals with lanthanide metal ions we first studied the possibility to obtain polymers with the mono- and hexa- carboxylic radical derivatives of PTM. Thus, while the PTMMC radical yields the complex ([Ln(H2O)9](PTMMC)3(PTMMCH)3¿7H2O¿6EtOH) (33), Ln = Eu, Ce and La, which show a lamellar polar - apolar separated ion pair structure alternating PTM-based bilayers with nonaaquaeuropium cations, the PTMHC radical produces the complex [Eu2(PTMHC)(H2O)13]¿ 16H2O¿EtOH (34) which exhibits a 1-D chain-like structure. Magnetic properties of both complexes were studied in the 2 - 300 K range showing the presence of very weak inter-radical antiferromagnetic interactions below 5 K. The latter result was particularly promising for different aspects: (i) it clearly showed the possibility to create extended metal - organic frameworks using lanthanides and carboxylate PTM radicals; and (ii) since lanthanide (III) ions are well known to form isomorphous structures, it opened real opportunities to build similar magnetic materials working with lanthanide ions possessing a magnetic ground state. The 3-connecting PTMTC radical with Ln(III) ions forms a series of three-dimensional open-frameworks with the formula [Ln(PTMTC)(DMF)3] (35), (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm). Similarly, the isomorphic series of polymers [Ln(¿H-PTMTC)(DMF)¿2H2O] (36), (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) is obtained with the non-radical ¿H-PTMTC derivatives. Both series of polymers associate a lattice complex T topology with large channels (total potential void volume of 62% of the cell volume) and guest-induced reversible crystal to amorphous transformations. Changing the reaction conditions, the series of 3-D coordination polymers [Ln(PTMTC)(EtOH)2(H2O)]n (37), Ln = Ce, Eu, Gd, Tb and the isomorphous ones [Ln(¿H-PTMTC)(EtOH)2(H2O)]n (38), (Ln = Sm, Eu, Gd, Tb) are obtained. They are constructed around a paddle-wheel motive structure formed by Ln(III) ions and carboxylate groups, and show 40% void volumes. Magnetic and luminescence properties of all these series of polymers 35Ln, 36Ln, 37Ln and 38Ln were studied. From the magnetic characterizations performed with polymers 35Ln, 36Ln, 37Ln and 38Ln, it turns out that the magnetic {PTM ... Ln} interactions use to be very weak. With the information available up to now, it seems that the polymer 35Ce, with an f1 ion does not establish any magnetic interaction, while lanthanide f2 - f5 ions establish antiferromagnetic interactions with the PTM radicals. Similar type of interactions is observed for lanthanide f10 - f12 ions, whereas the coordination polymers of lanthanide f7 and f8 ions (35Gd and 37Tb) exhibit ferromagnetic interactions. Concerning the luminescence properties it has been observed that radical PTMTC based polymers 35Ln and 37Ln, due to the strong absorption in the red region, quench the transitions of lanthanide ions. For the non-radical ¿H-PTMTC based polymers 36Ln and 38Ln, the characteristic luminescence of lanthanides ions is observed. Therefore, in the latter polymers the ¿H-PTMTC ligand acts as an efficient antenna. In the case of Eu, Tb, Dy and Sm ions, a high intensity luminescence with relative high lifetime (¿) is observed: for instance the values of lifetime reach 0.34 ms and 0.84 ms for coordination polymers 36Eu and 36Tb, respectively.