Theoretical study of exchange coupling in 3d-Gd complexes: large magnetocaloric effect systems

Polynuclear 3d transition metal-Gd complexes are good candidates to present large magnetocaloric effect. This effect is favored by the presence of weak ferromagnetic exchange interactions that have been investigated using methods based on Density Functional Theory. The first part of the study is devoted to dinuclear complexes, focusing on the nature and mechanism of such exchange interactions. The presence of two bridging ligands is found more favorable for ferromagnetic coupling than a triple-bridged assembly, especially for complexes with small M-O···O-Gd hinge angles. Our results show the crucial role of the Gd 5d orbitals in the exchange interaction while the 6s orbital seems to have a negligible participation. The analysis of the atomic and orbital spin populations reveals that the presence of spin density in the Gd 5d orbital is mainly due to a spin polarization effect, while a delocalization mechanism from the 3d orbitals of the transition metal can be ruled out. We propose a numerical DFT approach using pseudopotentials to calculate the exchange coupling constants in four polynuclear first-row transition metal-Gd complexes. Despite the complexity of the studied systems, the numerical approach gives coupling constants in excellent agreement with the available experimental data and, in conjunction with exact diagonalization methods (or Monte Carlo simulations), it makes it possible to obtain theoretical estimates of the entropy change due to the magnetization/demagnetization process of the molecule.     

Room-temperature synthesis and crystal,magnetic, and electronic structure of the first silver copper oxide

Ag(2)Cu(2)O(3) is the first known silver copper oxide. It was prepared by coprecipitation at room temperature and ambient pressure and shows an increased thermal stability compared with silver oxides. The crystal structure (tetragonal, a = 5.8862(2) A, c = 10.6892(4) A, Z = 4, I4(1)/amd) was refined from neutron and X-ray powder diffraction data, and it is related to that of the mineral paramelaconite (Cu(4)O(3)). In addition to a thorough characterization (chemical and TG analyses, XPS, crystal structure, and electrochemical, magnetic, and transport properties), we have carried out band structure calculations [extended Hückel tight binding (EHTB) and spin polarized density functional (DFT) band calculations] for the title silver copper oxide and for the related paramelaconite structure (Cu(II)-Cu(I) mixed-valence system) with special incidence into the magnetic behavior and coupling constants in these magnetically novel 3-D compounds. This new oxide represents an important precedent in solid state inorganic chemistry but also has potential interest concerning its magnetic, electrochemical, and catalytic properties.     

Equilibria between metallosupramolecular squares and triangles with the new rigid linker 1,4-bis(4-pyridyl)tetrafluorobenzene. Experimental and theoretical study of the structural dependence of NMR data

The new fluorinated rigid ligand L, 1,4-bis(4-pyridyl)tetrafluorobenzene, was used in combination with different diphosphine Pd(II) and Pt(II) triflates to build metallosupramolecular assemblies. Complex equilibria between triangular and square entities were detected for all the cases. Characterization of the equilibria was accomplished by 1H, 31P(1H), 19F, and 195Pt(1H) NMR in combination with mass spectrometry. The square/triangle ratio was seen to depend on several factors, such as the nature of the metal corners, the concentration, and the solvent. The relative stability of the square and triangular complexes was explored by using force field methods. A GIAO-DFT study was carried out to analyze the changes of the 31P and 1H NMR data with the geometry of the complexes.     

Observation of spatial migration of excitation in Er-doped optical fibers by means of a population grating technique

The influence of spatial migration of excitation among neighboring Er3+ ions on the dynamics of population grating formation in Er-doped optical fibers is reported. The effect manifests itself in an additional increment in the grating formation rate compared with the fluorescence growth rate observed for the same spatially uniform average light power. The experiments, performed at lambda=1549 nm in the configuration of transient two-wave mixing with two similar single-mode Er-doped fibers with significantly different erbium concentrations (approximately 640 and approximately 5600 parts in 10(6)), demonstrated the essential contribution of this effect to the grating formation rate in the latter fiber. The evaluated diffusion coefficient proved to be approximately 2.3 x 10(-9) cm2/s, which ensures effective migration of the excitation by approximately 48 nm, i.e., by approximately 18 average inter-ion distances in this fiber.     

Exchange coupling in mu-aqua:mu-oxo vs. di-mu-hydroxo dinuclear Cu(II) compounds: a density functional study

Theoretical methods based on density functional theory have been applied to study the differences in exchange coupling between the di-mu-hydroxo and mu-aqua:mu-oxo tautomers of a dinuclear Cu(II) complex. The calculations indicate that the two compounds are totally different from a magnetic point of view. The transfer of one proton from one of the two hydroxo bridges to the other leaves an asymmetric structure with only one effective exchange pathway through the oxo bridge. The coordinated water molecule and an associated perchlorate counterion, although not directly involved in the exchange coupling, play an important role in the magnetic properties of this compound since their presence is crucial in the determination of the geometrical details of the Cu-O(ox)-Cu linkage and hence in the exchange coupling between the copper atoms.     

Magnetic structure of the large-spin Mn10 and Mn19 complexes: a theoretical complement to an experimental milestone

High-spin molecules have been proposed as candidates for the storage of information at the molecular level. The electronic structure of two complex magnetic molecular systems, Mn 10 and Mn 19, is characterized by means of a computational study based on density functional theory. All the exchange interactions in the recently reported Mn 19 complex with the highest known spin value of 83/2, and in its highly symmetric Mn 10 parent compound, are ferromagnetic. In these complexes, there are two kinds of ferromagnetic coupling: the first one corresponds to Mn (II)-Mn (III) interactions through a double mu 2-alkoxo-mu 4-oxo bridge where the high coordination number of the Mn (II) cations results in long Mn (II)-O bond distances, while the second one involves Mn (III)-Mn (III) interactions through mu 2-alkoxo-mu 3-eta (1):eta (1):eta (1) azido bridging ligands with long Mn (III)-N distances due to a Jahn-Teller effect.     

An NMR study of sequential intermediates and collateral products in the conversion of 1,3,6,8-tetraazatricyclo[4.4.1.1]dodecane (TATD) to 1,3,6,8-tetraazatricyclo[4.3.1.1]undecane (TATU)

In situ ¹H nuclear magnetic resonance spectroscopy was used to investigate the processes that occur during the synthesis of 1,3,6,8-tetraazatricyclo[4.3.1.1^3,8]undecane (TATU). NMR analysis showed a reaction mixture containing more than one compound. The production of these intermediates and collateral products was rationally supported by a careful ¹H NMR monitoring study. We characterized 1,3,5-triazabicyclo[3.2.1]octane (TABO, 4) and 3-(2-aminoethyl)-1,3,5-triazabicyclo[3.2.1]octane (AETABO, 7) by ¹H and ¹³C NMR in D₂O solution inside the NMR sample tube, as an intermediate and collateral product of the reaction, respectively. Further, a reaction of 1,3,6,8-tetraazatricyclo[4.4.1.1^3,8]dodecane (TATD) with ¹⁵N-labeled ammonium chloride was carried out. The ¹⁵N NMR and GC-MS experiments indicated that ¹⁵N was incorporated into TATU, TABO, and urotropine.     

Tuning Single-Molecule Conductance in Metalloporphyrin-Based Wires via Supramolecular Interactions

Nature has developed supramolecular constructs to deliver outstanding charge-transport capabilities using metalloporphyrin-based supramolecular arrays. Herein we incorporate simple, naturally inspired supramolecular interactions via the axial complexation of metalloporphyrins into the formation of a single-molecule wire in a nanoscale gap. Small structural changes in the axial coordinating linkers result in dramatic changes in the transport properties of the metalloporphyrin-based wire. The increased flexibility of a pyridine-4-yl-methanethiol ligand due to an extra methyl group, as compared to a more rigid 4-pyridinethiol linker, allows the pyridine-4-yl-methanethiol ligand to adopt an unexpected highly conductive stacked structure between the two junction electrodes and the metalloporphyrin ring. DFT calculations reveal a molecular junction structure composed of a shifted stack of the two pyridinic linkers and the metalloporphyrin ring. In contrast, the more rigid 4-mercaptopyridine ligand presents a more classical lifted octahedral coordination of the metalloporphyrin metal center, leading to a longer electron pathway of lower conductance. This works opens to supramolecular electronics, a concept already exploited in natural organisms.