José Ramón Galán-Mascarós

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 Probes Confer Multistability to Organic Conducting Polymers

Switchable organic conductors can be readily obtained by combining organic conducting polymers (CPs), with the unparalleled bistability of spin crossover (SCO) complexes. Here it is reported how CPs with embedded SCO components exhibit synergic multistability. Upon acting on the SCO probes by external stimuli (thermal activation in this case), the spin transition induces up to a 300% difference in the electrical conductivity of the CP component between the low-spin and high-spin regimes, and with a wide hysteresis at technologically relevant temperatures. These results pave the way for the exploitation of the unique SCO switching capabilities in electronic devices.

Cation-Directed Dimeric versus Tetrameric Assemblies of Lanthanide-Stabilized Dilacunary Keggin Tungstogermanates

Reaction of mid- to late lanthanide ions with GeO2 and Na2WO4 in NaOAc buffer results in a library of [Ln2 (GeW10O38)](6-) clusters (Ln2), which consist of dilacunary Keggin fragments stabilized by the insertion of 4f atoms in the vacant sites and show the ability to undergo cation-directed self-assembly processes. In the presence of Na(+), two β-Ln2 subunits assemble by means of Ln-O(WO5)-Ln bridges to form the chiral [Ln4(H2O)6(β-GeW10O38)2](12-) dimeric anions (ββ-Ln4, Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). When Cs(+) is present, two Ln4-like dimers further assemble into the [{Ln4(H2O)5(GeW10O38)2}2](24-) species (Ln8, Ln = Ho, Er, Tm, Yb, Lu). Two types of tetramers coexist in the solid state: One shows a full ββ-Ln8 architecture, whereas the other one is a mixed αβ-Ln8 assembly in which each β-subunit is linked to its corresponding α-Ln2 derivative. Regardless of differences in isomeric forms and the relative arrangement of Ln2 subunits, all anions display virtually identical {Ln4} cores as a common structural feature. A combination of ESI mass spectrometry and (183)W NMR spectroscopy experiments indicates that Ln8 tetramers fragment into Ln4 dimers upon dissolution, which undergo partial dissociation into Ln2 monomers and slow dimer/monomer equilibration. This is most likely followed by β-to-α isomerization of Ln2 clusters with consequent reassembly, as indicated by isolation of three additional αα-Ln4 derivatives. Magnetic and photoluminescence properties in the Na-ββ-Ln4 series are also discussed.

Hysteretic Spin Crossover above Room Temperature and Magnetic Coupling in Trinuclear Transition-Metal Complexes with Anionic 1,2,4-Triazole Ligands

The reaction of 4-(1,2,4-triazol-4-yl)ethanesulfonate (L) with Zn(2+), Cu(2+), Ni(2+), Co(2+), and Fe(2+) gave a series of analogous neutral trinuclear complexes with the formula [M3(μ-L)6(H2O)6] (1-5). These compounds were characterized by single-crystal X-ray diffraction, thermogravimetry, and elemental analysis. The magnetic properties of compounds 2-5 were studied. Complexes 2-4 show weak antiferromagnetic superexchange, with J values of -0.33 (2), -9.56 (3), and -4.50 cm(-1) (4) (exchange Hamiltonian H = -2 J (S1S2+S2S3)). Compound 5 shows two additional crystallographic phases (5 b and 5 c) that can be obtained by dehydration and/or thermal treatment. These three phases exhibit distinct magnetic behavior. The Fe(2+) centers in 5 are in high-spin (HS) configuration at room temperature, with the central one exhibiting a non-cooperative gradual spin transition below 250 K with T1/2 = 150 K. In 5 b, the central Fe(2+) stays in its low-spin (LS) state at room temperature, and cooperative spin transition occurs at higher temperatures and with the appearance of memory effect (T1/2↑ = 357 K and T1/2↓ = 343 K). In the case of 5 c, all iron centers remain in their HS configuration down to very low temperatures, with weak antiferromagnetic coupling (J = -1.16 cm(-1)). Compound 5 b exhibits spin transition with memory effect at the highest temperature reported, which matches the remarkable features of coordination polymers.

Polyoxometalate electrocatalysts based on earth-abundant metals for efficient water oxidation in acidic media

Water splitting is a promising approach to the efficient and cost-effective production of renewable fuels, but water oxidation remains a bottleneck in its technological development because it largely relies on noble-metal catalysts. Although inexpensive transition-metal oxides are competitive water oxidation catalysts in alkaline media, they cannot compete with noble metals in acidic media, in which hydrogen production is easier and faster. Here, we report a water oxidation catalyst based on earth-abundant metals that performs well in acidic conditions. Specifically, we report the enhanced catalytic activity of insoluble salts of polyoxometalates with caesium or barium counter-cations for oxygen evolution. In particular, the barium salt of a cobalt-phosphotungstate polyanion outperforms the state-of-the-art IrO2 catalyst even at pH < 1, with an overpotential of 189 mV at 1 mA cm-2. In addition, we find that a carbon-paste conducting support with a hydrocarbon binder can improve the stability of metal-oxide catalysts in acidic media by providing a hydrophobic environment.

Thermal selectivity of intermolecular versus intramolecular reactions on surfaces.

On-surface synthesis is a promising strategy for engineering heteroatomic covalent nanoarchitectures with prospects in electronics, optoelectronics and photovoltaics. Here we report the thermal tunability of reaction pathways of a molecular precursor in order to select intramolecular versus intermolecular reactions, yielding monomeric or polymeric phthalocyanine derivatives, respectively. Deposition of tetra-aza-porphyrin species bearing ethyl termini on Au(111) held at room temperature results in a close-packed assembly. Upon annealing from room temperature to 275 °C, the molecular precursors undergo a series of covalent reactions via their ethyl termini, giving rise to phthalocyanine tapes. However, deposition of the tetra-aza-porphyrin derivatives on Au(111) held at 300 °C results in the formation and self-assembly of monomeric phthalocyanines. A systematic scanning tunnelling microscopy study of reaction intermediates, combined with density functional calculations, suggests a [2+2] cycloaddition as responsible for the initial linkage between molecular precursors, whereas the monomeric reaction is rationalized as an electrocyclic ring closure.

Ln12 -Containing 60-Tungstogermanates: Synthesis, Structure, Luminescence, and Magnetic Studies.

A new class of hexameric Ln12 -containing 60-tungstogermanates, [Na(H2 O)6 ⊂Eu12 (OH)12 (H2 O)18 Ge2 (GeW10 O38 )6 ](39-) (Eu12 ), [Na(H2 O)6 ⊂Gd12 (OH)6 (H2 O)24 Ge(GeW10 O38 )6 ](37-) (Gd12 ), and [(H2 O)6 ⊂Dy12 (H2 O)24 (GeW10 O38 )6 ](36-) (Dy12 ), comprising six di-Ln-embedded {β(4,11)-GeW10 } subunits was prepared by reaction of [α-GeW9 O34 ](10-) with Ln(III) ions in weakly acidic (pH 5) aqueous medium. Depending on the size of the Ln(III) ion, the assemblies feature selective capture of two (for Eu12 ), one (for Gd12 ), or zero (for Dy12 ) extra Ge(IV) ions. The selective encapsulation of a cationic sodium hexaaqua complex [Na(H2 O)6 ](+) was observed for Eu12 and Gd12 , whereas Dy12 incorporates a neutral, distorted-octahedral (H2 O)6 cluster. The three compounds were characterized by single-crystal XRD, ESI-MS, photoluminescence, and magnetic studies. Dy12 was shown to be a single-molecule magnet.

Single‐Molecule‐Magnet Behavior in the Family of [Ln(OETAP)2] Double‐Decker Complexes (Ln=Lanthanide, OETAP=Octa(ethyl)tetraazaporphyrin)

Double-decker complexes of lanthanide cations can be readily prepared with tetraazaporphyrins (porphyrazines). We have synthesized and characterized a series of neutral double-decker complexes [Ln(OETAP)2 ] (Ln=Tb(3+), Dy(3+), Gd(3+), Y(3+); OETAP=octa(ethyl)tetraazaporphyrin). Some of these complexes show analogous magnetic features to their phthalocyanine (Pc) counterparts. The Tb(3+) and Dy(3+) derivatives exhibit single-molecule magnet (SMM) behavior with high blocking temperatures over 50 and 10 K, respectively. These results confirm that, in double-decker complexes that involve Tb or Dy, the (N4)2 square antiprism coordination mode has an important role in inducing very large activation energies for magnetization reversal. In contrast with their Pc counterparts, the use of tetraazaporphyrin ligands endows the presented [Ln(OETAP)2] complexes with extraordinary chemical versatility. The double-decker complexes that exhibit SMM behavior are highly soluble in common organic solvents, and easily processable even through sublimation.

Strong hard X-ray magnetochiral dichroism in paramagnetic enantiopure molecules.

Fil: Ceolin, Marcelo Raul. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas; Argentina

Photoinduced Oxygen Evolution Catalysis Promoted by Polyoxometalate Salts of Cationic Photosensitizers

The insoluble salt Cs15K[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (CsCo9) is tested as heterogeneous oxygen evolution catalyst in light-induced experiments, when combined with the homogeneous photosensitizer [Ru(bpy)3]2+ and the oxidant Na2S2O8 in neutral pH. Oxygen evolution occurs in parallel to a solid transformation. Post-catalytic essays indicate that the CsCo9 salt is transformed into the corresponding [Ru(bpy)3]2+ salt, upon cesium loss. Remarkably, analogous photoactivated oxygen evolution experiments starting with the [Ru(bpy)3](5+x)K(6−2x)[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]·(39+x)H2O (RuCo9) salt demonstrate much higher efficiency and kinetics. The origin of this improved performance is at the cation-anion, photosensitizer-catalyst pairing in the solid state. This is beneficial for the electron transfer event, and for the long-term stability of the photosensitizer. The latter was confirmed as the limiting process during these oxygen evolution reactions, with the polyoxometalate catalyst exhibiting robust performance in multiple cycles, upon addition of photosensitizer, and/or oxidant to the reaction mixture.