Mónica Giménez-Marqués

Giant Crown-Shaped Polytungstate Formed by Self-Assembly of CeIII-Stabilized Dilacunary Keggin Fragments†

Polyoxometalates (POMs) are anionic metal–oxygen clusters with a remarkable variety of structures and potential applications (catalysis, medicine, materials science, and nanotechnology). The combination of their bifunctional activity as H/e reservoirs with the unique features of 4f ions (luminescence, magnetism, Lewis acid catalysis) is of particular interest and could afford species with enhanced properties owing to synergistic effects. Because of their oxophilicity and high coordination numbers, 4f ions display a powerful ability to link lacunary polytungstates (POTs) to build unprecedented architectures. The larger size of 4f ions compared to 3d metals prevents their full incorporation in lacunary frameworks as addendum atoms, and therefore additional sites are available for further derivatization. For monolacunary POTs, this mainly results in Peacock–Weakley sandwich POTs or in dimers and extended arrangements of 1:1 monomers. The use of diand trilacunary building blocks has led to a dramatic increase in the number and size of 4f-containing POTs in recent years. In addition to several medium to large assemblies, this approach has also resulted in a few giant species with more than 100 W atoms, which are amongst the largest POTs known. These are [As12Ce16W148O524(H2O)36] 76 , which has remained the largest POT since 1997; [Gd8As12W124O432(H2O)36] 60 , recently reported as the longest and second-largest POT; and [Ce20Ge10W100O376(OH)4(H2O)30] 56 , which contains the largest number of 4f ions besides being the fourth-largest POT. However, these assemblies are still small compared to Moblue POMs, for which a cluster as large as Mo368 is known. [5] Therefore, the search for giant POTs comparable to Mo blues is an attractive challenge, and supramolecular chemistry of 4f ions and lacunary fragments appears to be a suitable strategy that could allow rational design of tailored assemblies. Here we report [K K7Ce24Ge12W120O456(OH)12(H2O)64] (1) as the largest tungstogermanate and third-largest POT to date. Besides containing the largest number of 4f ions in a POM, 1 can be considered as the first giant POTwith a crown shape, that is, with a ring structure displaying a central cavity available for ion encapsulation in an inorganic analogue of the crown ethers, and thus a new type of topology is added to this still limited family of POMs. Recently, we reported the first 3d–4f heterometallic POM derived from the Weakley-type structure. With the aim of systematically incorporating 4f ions into this sandwich structure, we replaced half of the 3d precursor by a source of 4f ions in the direct synthesis of Weakley tungstogermanates. Instead of the expected 3d–4f POT, 1 was obtained as Na40K6[Ni(H2O)6]3[1]·nH2O (Ni-1, n 178) in moderate yield from this simple one-pot procedure. Polyanion 1 (Figure 1 a) can be viewed as the product of the K-directed self-assembly of twelve [Ce2GeW10O38] 6 subunits ({Ce2GeW10}) formed in situ, each of which is composed of a dilacunary Keggin fragment stabilized by coordination of two Ce ions on the vacant sites through four Ce O bonds (Figure 1b). As many as three distinct types of {GeW10} skeletons are observed: the enantiomeric forms b(1,8) and b(1,5) and the g(3,4) fragment, and all of them are of the anti-Lipscomb type. The vacant sites in the b forms are located at the W3O15 group and the central belt, whereas the Cs-symmetric g(3,4) form is obtained by removal of one WO6 octahedron from each rotated W3O13 triad (Figure S1, Supporting Information). Among the reported {bXW10}-containing POTs, [4c,10] only one b form displays vacancies at the belt and a W3O15 group to the best of our knowledge, namely, b(1,9). On the other hand, a single gdilacunary species was known to date: the C2v-symmetric g(1,2) form with vacancies at the two edge-sharing octahedra of the rotated triads. Therefore, all three {GeW10} fragments in 1 are completely unprecedented and they represent additional, interesting building blocks in POM chemistry. The fact that {Ce20W100}, composed of {b(4,11)-Ce2GeW10} subunits, is obtained from the [A-a-GeW9O34] 10 precursor at a similar pH suggests that a diversity of {GeW10} isomers may be formed under moderately acidic conditions regardless of the tungstogermanate source. Most likely, they are rapidly interexchangeable, highly reactive intermediates in the formation of the predominant {GeW11} species. In both cases, they could be stabilized by Ce coordination, which allowed [*] Dr. S. Reinoso, M. Gim nez-Marqu s Instituto de Ciencia Molecular (ICMol) Universidad de Valencia Catedr tico J. Beltr n 2, 46980 Paterna, Valencia (Spain) E-mail: santiago.reinoso@uv.es

Spin-Crossover Modification through Selective CO2 Sorption

We present a spin-crossover Fe(II) coordination polymer with no permanent channels that selectively sorbs CO2 over N2. The one-dimensional chains display internal voids of ∼9 Å diameter, each being capable to accept one molecule of CO2 at 1 bar and 273 K. X-ray diffraction provides direct structural evidence of the location of the gas molecules and reveals the formation of O═C═O(δ(-))···π interactions. This physisorption modifies the spin transition, producing a 9 K increase in T1/2.

A SIM‐MOF: Three‐Dimensional Organisation of Single‐Ion Magnets with Anion‐Exchange Capabilities

The formation of a metal-organic framework (MOF) with nodes that have single-molecule magnet (SMM) behaviour has been achieved by using mononuclear lanthanoid analogues, also known as single-ion magnets (SIMs), which enormously simplifies the challenging issue of making SMM-MOFs. Here we present a rational design of a family of MOFs, [Ln(bipyNO)4](TfO)3⋅x solvent (Ln=Tb (1); Dy (2); Ho (3); Er (4); TfO=triflate), in which the lanthanoid centres have an square-antiprismatic coordination environment suitable for SIM behaviour. Magnetic measurements confirm the existence of slow magnetic relaxation typical of SMMs, which has been rationalised by means of a radial effective charge model. In addition, we have explored the incorporation of bulky polyoxometalates (POMs) into the cavities of the SIM-MOF by anion exchange, finding that they do not interfere with the slow magnetic relaxation. This demonstrates the robustness of the frameworks and opens the possibility of incorporating non-innocent anions.

Tuning the magneto-structural properties of non-porous coordination polymers by HCl chemisorption

Responsive materials for which physical or chemical properties can be tuned by applying an external stimulus are attracting considerable interest in view of their potential applications as chemical switches or molecular sensors. A potential source of such materials is metal-organic frameworks. These porous coordination polymers permit the physisorption of guest molecules that can provoke subtle changes in their porous structure, thus affecting their physical properties. Here we show that the chemisorption of gaseous HCl molecules by a non-porous one-dimensional coordination polymer instigates drastic modifications in the magnetic properties of the material. These changes result from profound structural changes, involving cleavage and formation of covalent bonds, but with no disruption of crystallinity.

Spin switching in electronic devices based on 2D assemblies of spin-crossover nanoparticles.

Two-dimensional assemblies of triazole-based spin-crossover nanoparticles (SCO NPs) presenting different morphologies are prepared and electrically characterized. The thermal hysteresis loop in the electrical conductance near room temperature correlates with the NP morphologies and their 2D organization. The unprecedentedly large difference - up to two orders of magnitude - in the electrical conductance of the two spin states is of interest for applications.

Combination of Magnetic Susceptibility and Electron Paramagnetic Resonance to Monitor the 1D to 2D Solid State Transformation in Flexible Metal–Organic Frameworks of Co(II) and Zn(II) with 1,4-Bis(triazol-1-ylmethyl)benzene

Two families of coordination polymers, {[M(btix)(2)(OH(2))(2)]·2NO(3)·2H(2)O}(n) [M = Co (1), Zn (2), Co-Zn (3); btix = 1,4-bis(triazol-1-ylmethyl)benzene] and {[M(btix)(2)(NO(3))(2)]}(n) [M = Co (4), Zn (5), Co-Zn (6)], have been synthesized and characterized. The two conformations of the ligand, syn and anti, lead to one-dimensional (1D) cationic chains or two-dimensional (2D) neutral grids. Extrusion of the water molecules of the 1D compounds results in an irreversible transformation into the 2D compounds, which involves a change in conformation of the btix ligands and a rearrangement in the metal environment with cleavage and reformation of covalent bonds. This structural transformation has been followed by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements to monitor the minor modifications that the metal centers suffer.

Unravelling the chemical design of spin-crossover nanoparticles based on iron(II)–triazole coordination polymers: towards a control of the spin transition

We present a systematic study of the key synthetic parameters that control the growth of Fe–triazole spin-crossover nanoparticles and the effect of this size modulation on the spin transition.

Phase Transitions in Spin-Crossover Thin Films Probed by Graphene Transport Measurements

Future multifunctional hybrid devices might combine switchable molecules and 2D material-based devices. Spin-crossover compounds are of particular interest in this context since they exhibit bistability and memory effects at room temperature while responding to numerous external stimuli. Atomically thin 2D materials such as graphene attract a lot of attention for their fascinating electrical, optical, and mechanical properties, but also for their reliability for room-temperature operations. Here, we demonstrate that thermally induced spin-state switching of spin-crossover nanoparticle thin films can be monitored through the electrical transport properties of graphene lying underneath the films. Model calculations indicate that the charge carrier scattering mechanism in graphene is sensitive to the spin-state dependence of the relative dielectric constants of the spin-crossover nanoparticles. This graphene sensor approach can be applied to a wide class of (molecular) systems with tunable electronic polarizabilities.

Dynamic Magnetic Materials Based on the Cationic Coordination Polymer [Cu(btix)2]n2n+ [btix = 1,4-Bis(triazol-1-ylmethyl)benzene]: Tuning the Structural and Magnetic Properties through Anion Exchange

A three-dimensional coordination polymer, [Cu(btix)(2)(BF(4))(2)](n) [btix = 1,4-bis(triazol-1-ylmethyl)benzene], with antiferromagnetic interactions occurring via the organic ligand, has been prepared and characterized. It has been shown to permit the exchange of anionic species in the crystalline network with modification of the magnetic properties. Coordinated BF(4)(-) can be reversibly exchanged by different anions with (NO(3)(-) and Cl(-)) or without (PF(6)(-) and ClO(4)(-)) dynamic response of the organic ligand, which acts as the only linker between the metal centers. Interestingly, an irreversible exchange occurs with N(3)(-) anions to generate a new coordination polymer, [Cu(btix)(N(3))(2)](n), whose structure has been determined ab initio by powder X-ray diffraction, revealing a totally different connectivity between the Cu(II) centers. These structural transformations are accompanied by a change of the magnetic properties, which have been detected by electron paramagnetic resonance and magnetic susceptibility measurements.

Near Room-Temperature Memory Devices Based on Hybrid Spin-Crossover@SiO2 Nanoparticles Coupled to Single-Layer Graphene Nanoelectrodes.

The charge transport properties of SCO [Fe(Htrz)2 (trz)](BF4 ) NPs covered with a silica shell placed in between single-layer graphene electrodes are reported. A reproducible thermal hysteresis loop in the conductance above room-temperature is evidenced. This bistability combined with the versatility of graphene represents a promising scenario for a variety of technological applications but also for future sophisticated fundamental studies.