Infrared Resonance Raman of Bilayer Graphene: Signatures of Massive Fermions and Band Structure on the 2D Peak.

Few-layer graphene possesses low-energy carriers that behave as massive Fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV, we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes that are resonant in the visible, and induces a clearer frequency-separation of the substructures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each substructure, and comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron-phonon matrix elements.

Tandem templating strategies facilitate the assembly of calix[8]arene-supported Ln18 clusters.

Calix[n]arenes offer ideal chemical functionality through the polyphenolic lower rim to construct nano-sized coordination clusters with lanthanide (Ln) metal ions (e.g., NdIII10, GdIII8). However, the number of metal centers they can accommodate is still limited compared to that achievable with smaller ligands (e.g., GdIII140, GdIII104). Here, we exploit a combination of the "anion template strategy" and "templating ligands" to synthesise three highly symmetric (D3h, trigonal planar) LnIII18 (Ln = La, Nd, and Gd) systems, representing the largest calix[n]arene-based coordination clusters yet. The LnIII18 fragment is templated by a chloride anion located at the center of the cluster, wherefrom twelve µ3-OH- ligands bind 'internally' to the eighteen LnIII ions. 'Externally' the metallic skeleton is connected by p-tert-butylcalix[8]arene, oxo, chloro and carbonate ligands. The crystal packing in the lattice reveals large cylindrical channels of ∼26 Å in diameter, whose pore volume corresponds to ∼50% of the unit cell volume (using a 1.2 Å spherical probe radius). Magnetic measurements reveal the predominance of weak antiferromagnetic exchange in the Gd analog. Heat capacity data of GdIII18 reveal a high magnetic entropy with -ΔSm = 23.7 J K-1 kg-1, indicating potential for engineering magnetic refrigerant materials with calix[8]arenes.

Coordination-triggered redox activity of early and late lanthanide calix[4]arene complexes.

The methylation of p-tert-butylcalix[4]arene in the distal 1,3-phenolic sites provides H2L = {p-tert-butylcalix[4](OMe)2(OH)2arene}. This unit acts as a rigid coordinating ligand to early and late lanthanide metal ions, enabling the construction of two families of mononuclear compounds featuring (N(nBu)4)[LnIIIL(acac)2]·CH3CN (Ln = Pr (1), Nd (2), Ho (3), and Er (4)) and (N(nBu)4)2[LnIIIL{Mo5O13(OMe)4(NO)}]·CH2Cl2 (Ln = Nd (5) and Er (6)). The metal ions adopt distorted bicapped trigonal prismatic coordination environments, resulting in slow relaxation of the magnetization for 4. These compounds exhibit reversible redox waves at positive potentials, centered within the calix[4]arene ligand, representing a new type of calix[n]arene-based electrochemical activity induced by coordination to the metal centers.

Organoarsonates Enable Single-Site Condensation of Hexalacunary {P2W12} Polyoxotungstates.

A systematic study of the condensation reactions of arylarsonic-functionalized [α-P2W12O48]14- units in acidic aqueous media identified that the specific presence of an amino group in the ortho position of the phenyl rings induces a dimerization process that allowed isolation of discrete dimeric polyanions [(o-H2N-C6H4-AsO3)4P4W24O85]14- (1) with an unprecedented polyoxometalate skeleton characterized by two seminal {P2W12} groups joined via a single W-O-W bridge. At the same time, addition of divalent transition metal ions (MnII, CoII, and NiII) in the reaction mixture directed a condensation process on a completely different pathway resulting in one-dimensional (1D) coordination polymers based on V-shaped [{M(H2O)4}P4W24O92(C6H6AsNO)2]14- polyanions (M = MnII (2), CoII (3), and NiII (4)). All polyanions were isolated as hydrated mixed potassium/dimethylammonium salts and thoroughly characterized in the solid state. 31P NMR studies showed that the discrete cluster 1 is comparatively stable in 1 M LiCl aqueous solution and thus represents a potential precursor for subsequent reactions.

Chemically detaching hBN crystals grown at atmospheric pressure and high temperature for high-performance graphene devices.

In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow to fabricate high carrier mobility graphene-hBN devices. By combining spatially-resolved confocal Raman spectroscopy and electrical transport measurements, we confirm the excellent quality of these crystals for high-performance hBN-graphene-based van der Waals heterostructures. The full width at half maximum of the graphene Raman 2D peak is as low as 16 cm-1, and the room temperature charge carrier mobilitiy is around 80 000 cm2/(Vs) at a carrier density 1 × 1012cm-12. This is fully comparable with devices of similar dimensions fabricated using crystalline hBN synthesized by the high pressure and high temperature method. Finally, we show that for exfoliated high-quality hBN flakes with a thickness between 20 and 40 nm the line width of the hBN Raman peak, in contrast to the graphene 2D line width, is not useful for benchmarking hBN in high mobility graphene devices.

Probing Enhanced Electron-Phonon Coupling in Graphene by Infrared Resonance Raman Spectroscopy.

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at K, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D^{'} peaks with respect to that measured in graphite. Comparing with fully ab initio theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two-dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

Tris-decorated multi-iron polyoxotungstates.

Solution-stable tris(hydroxymethyl)aminomethane-functionalized FeIII-containing polyoxotungstates exhibit an unusual anchoring mode of triol moieties, with one -NH2 and one -CH2OH group remaining accessible for post-functionalization or chemisorption. The redox-active title compounds have been isolated under unusually mild reaction conditions and characterized in the solid state and in aqueous solutions.

Controlled Hydrolysis of Phosphate Esters: A Route to Calixarene-Supported Rare-Earth Clusters.

Phosphate ester bonds are widely present in nature (e. g. DNA/RNA) and can be extremely stable against hydrolysis without the help of catalysts. Previously, we showed how the combination of phosphoryl and calix[4]arene moieties in the same organic framework (LPO ) allows isolation of single lanthanide (Ln) metal ions as [LnIII (LPO )2 ](O3 SCF3 )3 . Here we report how by controlling the reaction conditions a new hydrolyzed phosphoryl-calix[4]arene ligand (H3 LHPO ) is formed as a result of LnIII -mediated P-OEt bond cleavage in three out of the eight possible sites in LPO . The chelating nature of H3 LHPO traps the LnIII species in the form of [LnIII (LHPO )((EtO)2 P(O)OH)]2 dimers (Ln=La, Dy, Tb, Gd), where the Dy derivative shows slow magnetization relaxation. The strategy presented herein could be extended to access a broader library of hydrolyzed platforms (Hx LHPO ; x=1-8) that may represent mimics of nuclease enzymes.

Ag(II) as Spin Super-Polarizer in Molecular Spin Clusters.

Using quantum mechanical calculations, we examine magnetic (super)exchange interactions in hypothetical, chemically reasonable molecular coordination clusters containing fluoride-bridged late transition metals or selected lanthanides, as well as Ag(II). By referencing to analogous species comprising closed-shell Cd(II), we provide theoretical evidence that the presence of Ag(II) may modify the magnetic properties of such systems (including metal-metal superexchange) to a surprising degree, specifically both coupling sign and strength may markedly change. Remarkably, this happens in spite of the fact that the fluoride ligand is the least susceptible to spin polarization among all monoatomic ligands known in chemistry. In an extreme case of an oxo-bridged Ni(II)2 complex, the presence of Ag(II) leads to a nearly 17-fold increase of magnetic superexchange and switching from antiferro (AFM)- to ferromagnetic (FM) coupling. Ag(II)─with one hole in its d shell that may be shared with or transferred to ligands─effectively acts as spin super-polarizer, and this feature could be exploited in spintronics and diverse molecular devices.

FeII/III and MnII complexes based on 2,4,6-tris(2-pyridyl)-triazine: synthesis, structures, magnetic and biological properties.

Reaction of the polypyridyl ligand 2,4,6-tris(2-pyridyl)-s-triazine (tpt) with mono- and polynuclear Fe(iii) or Mn(ii) precursors, specifically tri- or hexanuclear Fe(iii) pivalates (piv-), [Mn6O2(piv)10(Hpiv)4] and Mn(ii) isobutyrate (ib-), in various solvents and under various reaction conditions showcase the ligand's surprising coordination characteristics. The reactions result in mononuclear [FeIII(tpt)(tptH)][FeIIICl4]4·2(thf)·0.23(H2O) (1), [FeIII(piv)(tpt)Cl2] (2), [FeII(tpt)Cl2]·2(H2O) (3a), dinuclear [FeIII 2O(tpt)2Cl4] (3), and heptanuclear [FeIII 7O4(piv)12(tpt-O)]·A [A = MeCN (4a) or 4(dioxane) (4b)] and [FeIII 7O4(piv)11(tpt-O)(i-PrO)(i-PrOH)]·0.75(i-PrOH) (5), as well as the mononuclear compounds [MnII(tpt)(NO3)(H2O)2](NO3) (6) and [MnII(tpt)(ib)(Cl)(MeOH)]·MeOH (7). Single-crystal X-ray diffraction analyses identify tpt as a tridentate NNN donor ligand that forms two five-membered metallocycles in 1-3, 6 and 7, whereas in 4 and 5 five tpt N atoms form coordinative bonds accompanied by an unusual metal-induced oxidation of one of the carbon atoms of the central triazine core. Magnetic properties of Fe(iii)-tpt (2-5), Fe(ii)-tpt (3a), and Mn(ii)-tpt (7) compounds show dominant antiferromagnetic coupling for polynuclear coordination cluster compounds. The Mn(ii)-tpt complexes 6 and 7 exhibit efficient catalytic properties in the production of enzymes by microorganisms, which concerns the synthesis of exocellular proteases in Fusarium gibbosum CNMN FD 12 or Trichoderma koningii Oudemans CNMN FD 15 fungi strains. Thus, compounds 6 and 7 can be used for producing proteolytic enzymes with wide applications including in the food, detergents and pharmaceutical industries.

Tetrahedral M4(µ4-O) Motifs Beyond Zn: Efficient One-Pot Synthesis of Oxido-Amidate Clusters via a Transmetalation/Hydrolysis Approach.

While zinc µ4-oxido-centered complexes are widely used as versatile precursors and building units of functional materials, the synthesis of their analogues based on other transition metals is highly underdeveloped. Herein, we present the first efficient systematic approach for the synthesis of homometallic [M4(µ4-O)L6]-type clusters incorporating divalent transition-metal centers, coated by bridging monoanionic organic ligands. As a proof of concept, we prepared a series of charge-neutral metal-oxido benzamidates, [M4(µ4-O) (NHCOPh)6] (M = Fe, Co, Zn), including iron(II) and cobalt(II) clusters not accessible before. The resulting complexes were characterized using elemental analysis, FTIR spectroscopy, magnetic measurements, and single-crystal X-ray diffraction. Detailed structural analysis showed interesting self-assembly of the tetrahedral clusters into 2D honeycomb-like supramolecular layers driven by hydrogen bonds in the proximal secondary coordination sphere. Moreover, we modeled the magnetic properties of new iron (II) and cobalt (II) clusters, which display a general tendency for antiferromagnetic coupling of the µ4-O/µ-benzamidate-bridged metal centers. The developed synthetic procedure is potentially easily extensible to other M(II)-oxido systems, which will likely pave the way to new oxido clusters with interesting optoelectronic and self-assembly properties and, as a result, will allow for the development of new functional materials not achievable before.

Structural Phase Transitions and Magnetic Superexchange in MI AgII F3 Perovskites at High Pressure.

Pressure-induced phase transitions of MI AgII F3 perovskites (M=K, Rb, Cs) have been predicted theoretically for the first time for pressures up to 100 GPa. The sequence of phase transitions for M=K and Rb consists of a transition from orthorhombic to monoclinic and back to orthorhombic, associated with progressive bending of infinite chains of corner-sharing [AgF6 ]4- octahedra and their mutual approach through secondary Ag⋅⋅⋅F contacts. In stark contrast, only a single phase transition (tetragonal→triclinic) is predicted for CsAgF3 ; this is associated with substantial deformation of the Jahn-Teller-distorted first coordination sphere of AgII and association of the infinite [AgF6 ]4- chains into a polymeric sublattice. The phase transitions markedly decrease the coupling strength of intra-chain antiferromagnetic superexchange in MAgF3 hosts lattices.

Hybrid lanthanide double-deckers based on calixarene and polyoxometalate units.

Complementarity of calixarene (H2L) and polyoxometalate ligands results in the first organic-inorganic [MIIIL{Mo5O13(OMe)4(NO)}]2- (M = Y, Gd and Dy) hybrid, directing the formation of a distorted square-antiprismatic MO8 ligand field and resulting in slow relaxation of the magnetisation for the Dy derivative.

Cyclodextrin-Templated Co(II) Grids: Symmetry Control over Supramolecular Topology and Magnetic Properties.

While inherent complexation properties and propensity for self-organization of cyclodextrins (CDs) render them potentially promising scaffolds of magnetic materials, this research area is still at an embryonic stage. We report on the synthesis and structure characterization of a new sandwich-type complex, [(α-CD)2Co3Li6(H2O)9] (α-1), which represents a smaller analogue of the previously characterized [(γ-CD)2Co4Li8(H2O)12] (γ-1) cluster. A comprehensive structural analysis of α-1 and a careful reinvestigation of γ-1 reveal how the symmetry of CD ligands determines the molecular composition and supramolecular arrangements of Co/Li sandwich-type complexes. Furthermore, the first comparative studies of the magnetic properties in this type of system point to subtle differences in the magnetic behavior of both compounds. The sandwich-type complexes α-1 and γ-1 exhibit field-induced slow magnetic relaxation, defining a new family of magnetic materials with a pillared grid-like supramolecular structure composed of weakly interacting CoII centers forming an SMM.

Cyclophane with eclipsed pyrene units enables construction of spin interfaces with chemical accuracy.

Advanced functionality in molecular electronics and spintronics is orchestrated by exact molecular arrangements at metal surfaces, but the strategies for constructing such arrangements remain limited. Here, we report the synthesis and surface hybridization of a cyclophane that comprises two pyrene groups fastened together by two ferrocene pillars. Crystallographic structure analysis revealed pyrene planes separated by ∼352 pm and stacked in an eclipsed geometry that approximates the rare configuration of AA-stacked bilayer graphene. We deposited this cyclophane onto surfaces of Cu(111) and Co(111) at submonolayer coverage and studied the resulting hybrid entities with scanning tunnelling microscopy (STM). We found distinct characteristics of this cyclophane on each metal surface: on non-magnetic Cu(111), physisorption occurred and the two pyrene groups remained electronically coupled to each other; on ferromagnetic Co(111) nanoislands, chemisorption occurred and the two pyrene groups became electronically decoupled. Spin-polarized STM measurements revealed that the ferrocene groups had spin polarization opposite to that of the surrounding Co metal, while the pyrene stack had no spin polarization. Comparisons to the non-stacked analogue comprising only one pyrene group bolster our interpretation of the cyclophane's STM features. The design strategy presented herein can be extended to realize versatile, three-dimensional platforms in single-molecule electronics and spintronics.

Trigonal Prismatic Coordination of Discrete Rare Earth Ions, Enforced by the Polyoxotungstate [P4 W27 O99 (H2 O)]16.

A family of solution-stable polyanions [Na⊂{LnIII (H2 O)}{WVI O(H2 O)}PV 4 WVI 26 O98 ]12- (Ln=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) represent the first examples of polyoxometalates comprising a single lanthanide(III) or yttrium(III) ion in a rare trigonal prismatic O6 environment. Their synthesis exploits the reactivity of the organophosphonate-functionalized precursor [P4 W24 O92 (C6 H5 PV O)2 ]16- with heterometal ions and yields hydrated potassium or mixed lithium/potassium salts of composition Kx Lny H12-x-y [Na⊂{Ln(H2 O)}{WO(H2 O)}P4 W26 O98 ]⋅nH2 O⋅mLiCl (x=8.5-11; y=0-2; n=24-34; m=0-1.5). The Dy, Ho, Er and Yb derivatives are characterized by slow magnetization relaxation.

Expansion of Zirconium Oxide Clusters by 3d/4f Ions.

Two series of charge-neutral coordination clusters featuring quasi-isostructural metal oxide cores, isolated as [Zr6Fe2Ln2O8(ib)14(bda)2(NO3)2]·xMeCN (Ln = La (1), Ce (2), Pr (3), and Nd (4); ib- = isobutyrate; H2bda = N-butyldiethanolamine) and [Zr6Fe2Ln2O8(ib)14(mda)2(NO3)2]·xMeCN (Ln = La (5), Ce (6), Pr (7), and Nd (8); H2mda = N-methyldiethanolamine), were obtained via one-pot reactions of [Fe3O(ib)6(H2O)3]NO3 as a critical precursor, Ln(NO3)3·6H2O (Ln = La, Ce, Pr, and Nd), the respective aminoalcohol, and [Zr6O4(OH)4(ib)12(H2O)]·3Hib in an acetonitrile solution. The coordination clusters in 1-8 feature {Zr6O8} cores that are structurally expanded by two 4f (Ln3+) and two 3d (Fe3+) metal ions, each individually coordinated to one of the eight oxide centers of {Zr6O8}, producing a metal skeleton where the 3d/4f positions cap four of the triangular faces of the central Zr6 octahedron. The coordination clusters differ in the chosen aminoalcohol coligands, N-butyldiethanolamine or N-methyldiethanolamine, which lead to a different isobutyrate coordination pattern in the two series, while the {Fe2Ln2Zr6O8} core structure remains virtually unaffected. All eight coordination clusters are obtained in moderate to good yields of 29-66% after only several days. Complexes 1-8 are stable against air and moisture; they are also surprisingly thermally stable up to 280 °C in air and in nitrogen atmosphere, and they represent the first reported examples of 3d/4f-functionalized zirconium oxide clusters.

Three intersecting {V12} rings: {V30Sb8}, an ultra-large polyoxovanadate cluster shell.

[VIV30SbIII8O78]12-, currently the largest known antimonato-polyoxovanadate (Sb-POV), features three perpendicular, intersecting 12-membered rings of edge-sharing O4V[double bond, length as m-dash]O square pyramids. While in two rings the apices of all O4V[double bond, length as m-dash]O pyramids point outwards, four apices of the third ring are directed into the cavity of the cluster shell, a concave structural motif not previously observed in polyoxovanadate chemistry. SbIII centers cap the eight niches defined by the octands of the {V30O78} cluster shell, resulting in discrete trigonal pyramidal SbO3 units, a second unprecedented feature. Within the resulting spin topology with numerous local geometrically frustrated motifs, the 30 spin-1/2 sites couple antiferromagnetically via a complex set of exchange pathways.

Phosphorylated-calix[4]arene double-deckers of single rare earth metal ions.

Combination of phosphoryl and calix[4]arene moieties in the same organic framework (LPO) directs the formation of homoleptic double-decker complexes [LnIII(LPO)2](OTf)3 for Ln = Tb and Dy, with the latter displaying slow relaxation of the magnetisation.

Fusing pyrene and ferrocene into a chiral, redox-active triangle.

A macrocycle that integrates three ferrocene-pyrene dyads in a triangular C2-symmetric arrangement is synthesised as a racemate in a simple one-pot approach. Crystal structural analysis reveals two enantiomeric conformers that pack alternatingly via π-π stacking and interconvert dynamically in solution. Electrochemical investigations indicate weak electrostatic interactions between Fc groups upon oxidation to a mixed valence triangle.