Hybrid magnetic materials based on coordination chemistry: from switching magnetic molecules to 2D coordination polymers

As part of the research area in molecular materials, this thesis presents several strategies to prepare new multifunctional hybrid materials. Some relevant results have been obtained: The reaction of FeII with the carboxylate derivative of 1-bpp ligand (bppCOOH) has afforded a new spin-crossover complex. The compound [FeII(bppCOOH)2](ClO4)2 presents an abrupt spin transition at 383 K, due to strong intermolecular interactions via hydrogen bonding, and LIESST effect, with a TLIESST of 60 K. This value of TLIESST is much higher than expected for the family of FeII complexes based on the 1-bpp ligand. These results together with similar results obtained with the FeII complex of the carboxy-ester derivative of 1-bpp (bppCOOEt), [FeII(bppCOOEt)2](ClO4)2(H2O)2, seems to indicate that very high T1/2 may induce higher TLIESST than those calculated from the data of compounds with lower T1/2. More examples are needed to confirm this result. The presence of the carboxylate group at the pyridine ring allows the anchoring of the spin-crossover complex to metal-oxide surfaces, which is an essential step for the preparation of devices based on the spin-crossover phenomenon. Preliminary reflectivity IR and XPS results have confirmed that deposition of the complex on a surface is possible by simply immersing a glass substrate covered by Al2O3 into an acetonitrile solution of the complex. Synchrotron measurements are needed to calculate the HS/LS ratio with the temperature after deposition. On the other hand, deprotonation of the carboxylic acid and reaction with FeIII has allowed the formation of the nonanuclear cluster [FeIII3(µ3-O)(H2O)3(FeII(bppCOOH)(bppCOO))6](ClO4)13·((CH3)CO)6(H2O)12.25, in which six partially deprotonated [FeII(bppCOOH)2] are linked by one of their carboxylate to a trimer of FeIII ions. Finally, the deposition of [FeII(bppCOOH)2](ClO4)2 on surfaces, and the study of the LIESST effect (structure and relaxation kinetics of the photoinduced metastable state) in the compounds [FeII(bppCOOH)2](ClO4)2 and [FeII(bppCOOEt)2](ClO4)2(H2O)2 will be described in a future report. Derivatives of 1-bpp ligand have been incorporated into an Anderson POM in compound (C16H36N)3[MnMo6O24(C16H15N6O)2]·(C4H9NO)2(H2O)2.5 (1) by using the well-known tris-alkoxo-amide tripodal functionalization. The structure of 1, solved by single-crystal X-ray diffraction, consists of an Anderson POM with two terminal 1-bpp ligands. Direct reaction in a 1:1 Fe2+:POM ratio gives rise to a 1D “amorphous” polymer in compound (C16H36N)[Fe(MnMo6O24(C16H15N6O)2)]·(H2O)4 (3), whereas recrystallization in DMF of the precipitate obtained with a 2:1 Fe2+:POM ratio induces the formation of compound [FeII(H2O)(C3H7NO)]2[MnMo6O24(C16H15N6O)2](OH)·(H2O)(C3H7NO)3 (2). The structure of 2, solved by single crystal X-ray diffraction, is formed by a 2D network, in which each [MnMo6O24(C16H15N6O)2]3- POM is linked to two FeII ions through the two 1-bpp ligands, while the two FeII ions are linked to a neighboring POM through an oxo ligand and to one DMF and one water solvent molecules. This gives rise to layers of [FeII(H2O)(C3H7NO)]2[MnMo6O24(C16H15N6O)2]+ units with OH- counterions. These results confirm the versatility of the coordination chemistry of tris-alkoxo-amide tripodal functionalized POMs to obtain a great variety of structures ranging from 2D cationic networks in compound 2 to amorphous 1D polymers in compound 3. Attempts to crystallize the 1D polymeric compound, with FeII connected with two functionalized POMs through the tridentate 1-bpp ligand, have been unsuccessful as reactions with metals of this functionalized POM lead to insoluble polymers. The structure of 2 may indicate that the excess of metal lead to coordination with the oxo groups of the POM and the recrystallization in polar solvents such as DMF involves dissociation of the 1-bpp-metal coordination bond. More attempts to slow down the precipitation of the polymer in a 1:1 metal:POM ratio are needed by using slow diffusion techniques and other solvents. Magnetic properties of 1 have shown that it presents a field-induced relaxation of magnetization due to magnetic anisotropy of MnIII, as observed in other mononuclear MnIII complexes reported very recently. This is the first example of d-metal POM exhibiting SIM behavior reported in the literature. The similar behavior of 2, 3 and the reference compound (C16H36N)3[MnMo6O24{(OCH2)3CNH2}2] (4), one of the simplest functionalized Anderson POM reported so far, confirms that this type of behavior is general for this type of structures. This opens the way to the preparation of hybrid POMs combining this property with other magnetic properties of interest. For instance, spin-crossover behavior could be expected if two 1-bpp ligands were coordinated to FeII as observed in compound 3. However, the magnetic properties indicate that FeII complexes remain in the LS state. Possible strategies to enhance spin transition of the FeII ions are the use of other counterions or solvents, as the spin transition of this type of complexes is very sensitive to the packing and intermolecular interactions in the bulk solid, induced by the nature of anions and solvent molecules or the introduction of substituents in the 1-bpp derivative. A new family of compounds with spin-crossover complexes inserted into bimetallic anilate-based networks has been prepared and characterized. The insertion of [FeIII(sal2-trien)]+ and derivatives gave rise to extended 2D compounds of formula [FeIII(sal2-trien)][MnIICrIII(Cl2An)3]·(CH2Cl2)0.5(CH3OH)(H2O)0.5(CH3CN)5, [FeIII(4-OH-sal2-trien)][MnIICrIII(Cl2An)3]·(solvate), [FeIII(sal2-epe)][MnIICrIII(Br2An)3]·(CH3CN)4(solvate) and [FeIII(5-Cl-sal2-trien)][MnIICrIII(Br2An)3]·(CH2Cl2)(CH3OH)(H2O)4(CH3CN)1.5(solvate). These four compounds present the well-known honeycomb structure, similar to that found for other extended oxalate or anilate-based networks. It is formed by [MnIICrIII(X2An)3]- sheets in the ab plane alternating with layers of FeIII cationic complexes. The magnetic properties of these compounds indicate that the FeIII ions remain in their HS or LS state between 2 and 300 K. This absence of spin-crossover in the 2D network can be explained by the lack of intermolecular interactions between the FeIII complexes in the large anilate-network, and by the rigidity of the ligand around the FeIII ion, imposed by strong intermolecular contacts (π-π stacking) between the anilate ligand and the phenolate rings of the FeIII complex. At the same time, the anilate network presents a ferrimagnetic long-range ordering at ca. 10 K in the four compounds, which is significantly higher than the ordering temperatures obtained for oxalate networks with the same metal ions (ca. 5 K) or for 2D anilate networks with other templating cations (ca. 6 K for [NBu4]+ and [(H3O)(phz)3]+ salts). The use of the FeII complex [FeII(tren)imid3)]2+ has led to the compound [FeII(tren)imid3)]2[MnIICl2CrIII(Cl2An)3]Cl·(CH3OH)(CH2Cl2)3(CH3CN)0.5. It presents a 1D anionic network, which is surrounded by two [FeII(tren)imid3)]2+ complexes, one Cl- anion, and solvent molecules. In this compound, the intermolecular contacts between FeII cations, as well as the absence of π-π stacking between the cations and the anilate ligand, result in the spin transition of half of the FeII complexes between 280 and 100 K. Also, the bimetallic anilate network presents a magnetic long-range ordering below 2.6 K, as a result of the ferromagnetic interchain interactions. This is the first anilate-based compound combining spin-crossover and magnetic ordering. Photomagnetic measurements are in progress to study the effect of photoinduced spin-crossover on the magnetic properties of the anilate network. Insertion of the smaller cationic complex [FeIII(acac2-trien)]+ in the anilate network has afforded a new type of structure, in which the FeIII cation is inserted into the hexagonal cavities of the 2D anilate network, and not between the layers as in the previous 2D oxalate and anilate-based compounds. Magnetic characterization of the compounds [FeIII(acac2-trien)][MnIICrIII(Cl2An)3]·(CH3CN)2 and [FeIII(acac2-trien)][MnIICrIII(Br2An)3]·(CH3CN)2, and the diamagnetic reference [GaIII(acac2-trien)][MnIICrIII(Br2An)3]·(CH3CN)2, shows an increase of the ordering temperature of the network (ca. 11K), but also an absence of spin-crossover, as the FeIII cations remain in the HS state. This type of structures paves the way to the synthesis of a new type of multifunctional materials, in which new properties could be obtained thanks to the formation of isolated chains of cations in close contact with the magnetic network. Furthermore, the layered character of these compounds enables the exfoliation of the bulk crystals into nanosheets, by mechanical or solvent-assisted methods. The best results have been obtained with the micromechanical exfoliation (Scotch-tape method), which has not been used previously for the exfoliation of coordination polymers. This method has allowed the exfoliation of the compounds [FeIII(acac2-trien)][MnIICrIII(Br2An)3]·(CH3CN)2, and [FeIII(sal2-trien)][MnIICrIII(Cl2An)3]·(CH2Cl2)0.5(CH3OH)(H2O)0.5(CH3CN)5 previously described, which is formed by alternating cationic and anionic layers, into nanosheets of about 2 nm of height and hundreds of nm of size. These two compounds can also be exfoliated by a solvent-assisted method, which is the method previously used to exfoliate neutral coordination polymers. Nanosheets of about 5 nm of thickness have been obtained after immersion and sonication in ethanol. The obtention of nanosheets of these two compounds opens a way for the delamination of 2D materials with more sophisticated structures than those exfoliated so far, and for the study and applications of single 2D nanosheets with combination of properties, such as magnetic ordering and spin-crossover. Finally, an interesting possibility derived from these structures could be the study of the properties of a single complex confined in the hexagonal cavities of the anilate-based network. Finally, a new family of FeIII polynuclear complexes based on tetradentate Schiff-base ligands has been reported. The nuclearity of the FeIII cluster in the compounds [Fe8(µ-L1)8(µ-O)4(NCS)8](ClO4)5(NCS)3, [Fe6(µ-L2)6(µ-O)3(NCS)6](ClO4)2(NCS)4 and [Fe4(µ-L3)2(µ-OH)2(µ-OHO)(NCS)7(OH2)] have been tuned by small modification on the bis(bidentate) bridging ligand or the counter-ion. Magnetic characterization shows strong antiferromagnetic interactions and antiferromagnetic ground states in the three compounds, due to the presence of pairs of HS FeIII ions bridged by -oxo or -hydroxo ligands. This work is a first step in the search of high-nuclearity compounds with interesting properties such as spin-crossover, which could be rise with stronger ligand fields, or single-molecule magnet behavior, by the use of metal ions with a higher magnetic axial anisotropy.