Topochemical nitridation of Sr2FeMoO6.

The topotactic nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to cubic, Fm3[combining macron]m double perovskite oxynitride Sr2FeMoO4.9N1.1 where double-exchange interactions determine ferromagnetic order with TC ≈ 100 K. Substitution of oxide by nitride induces bond asymmetries and local electronically driven structural distortions, which combined with Fermi level lowering restricts charge itinerancy to confined regions and preclude spontaneous long-range magnetic order. Under a magnetic field, ferromagnetic correlations expand, favoring charge delocalization and a negative magnetoresistance is observed.

Energetic, Topological and Electric Field Analyses of Cation-Cation Nucleic Acid Interactions in Watson-Crick Disposition.

A theoretical study of the effect of the diprotonation on the nucleic acid bases (A : U, A : T and G : C) in Watson-Crick conformation has been carried out by means of DFT computational methods in vacuum. In addition, the corresponding neutral and monoprotonated binary complexes have been considered. Most of the diprotonated species studied are stable, even though the binding energy is positive due to the overall repulsive electrostatic term. Local electrostatic attractive forces in the regions of hydrogen bonds (HBs) are responsible for equilibrium geometries, as shown by the electric field lines connecting the electrophilic and nucleophilic sites involved in the HB interactions. Secondary electrostatic effects also affect the assembling of the nucleic acid complexes in either neutral or cationic form. In particular, the electric field lines flowing from electrophilic sites in one base to nucleophilic sites in the other reinforce the linking between them. Hence, when the nucleophilic site concerns the free lone pair of the heteroatom involved in the HB interaction as acceptor, the HB distance shortens. However, if the free lone pair of the HB acceptor interacts with an electrophilic site in the same molecule, the HB distance elongates, weakening the HB interaction. The topological analysis of the electron density distribution in HB regions indicates that neutral, monoprotonated and diprotonated complexes show no differences in the nature of their HB's.

Synthesis of Au Nanoparticles Assisted by Linker-Modified TiO2 Nanoparticles.

Plasmonic nanoparticles, especially gold ones, have been widely employed as photosensitizers in photoelectrovoltaic or photocatalytic systems. To improve the system's performance, a greater interaction of the nanoparticles with the semiconductor, generally TiO2, is desired. Moreover, this performance is enhanced when an efficient covering of TiO2 surface by the sensitizer is achieved. The Brust-Schiffrin-like methods are of the most employed approaches for nanoparticles synthesis. In a traditional approach, the reduction of the gold precursor is performed in the presence of a stabilizer (typically a thiol molecule) free in solution. A second step in which the obtained nanoparticles are anchored to the semiconductor surface is necessary in the case of photosensitive applications. Drawbacks like steric hindrance turn more difficult the covering of the semiconductor's surface by nanoparticles. In this paper, we report a variation of this methodology, where the linker is previously anchored to the TiO2 nanoparticles surface. The resulting system is employed as the stabilizer in the gold reduction step. This strategy is carried out in aqueous media in two simple steps. A great covering of the titania surface by gold nanoparticles is achieved in all cases and the gold nanoparticles in the resulting nanoaggregate might be useful for photoelectrovoltaic or photocatalytic applications.

Topochemical synthesis of cation ordered double perovskite oxynitrides.

Topochemical nitridation in ammonia at moderate temperatures of cation ordered Sr2FeWO6 produces new antiferromagnetic double perovskite oxynitrides Sr2FeWO6-xNx with 0 < x ≤ 1. Nitrogen introduction induces the oxidation of Fe2+ to Fe3+ and decreases TN from 38 K (x = 0) to 13 K for Sr2FeWO5N which represents the first example of a double perovskite oxynitride with both high cationic order and nitrogen content. This synthetic approach can be extended to other cation combinations expanding the possibility of new materials in the large group of double perovskites.

Charged versus Neutral Hydrogen-Bonded Complexes: Is There a Difference in the Nature of the Hydrogen Bonds?

A theoretical study on some carboxylic acid dimers formed by positively or negatively charged molecules has been carried out by using DFT methods. The resulting dimers possess either a charge of +2 or -2. In addition, the corresponding neutral complexes have also been considered. The electron density distribution described by the atoms in molecules and the natural bond orbital methods, as well as the electric field maps of the systems, have been analyzed and compared without finding significant differences between the neutral and ionic complexes. The interaction energy along the dissociation path of the charged dimers shows both a local minimum and a local maximum, defining a stability region between them. When this energetic profile is recalculated by removing the repulsion between the charged groups, it resembles to those of the neutral molecules. Hence, the characteristics of the charged dimers are similar to those of the neutral ones: the addition of a repulsion term for the charged groups permits to retrieve the energetic profiles dependence with the distance in the charged system. The interacting quantum atom (IQA) method has been used to calculate the interaction energy terms, including the classic Coulombic term between the whole molecules and the corresponding of the carboxylic acid groups. The IQA results show repulsive electrostatic interactions when the whole molecules are considered in the ionic complexes, but attractive ones between the carboxylic groups in both neutral and ionic complexes.

The paradox of hydrogen-bonded anion-anion aggregates in oxoanions: a fundamental electrostatic problem explained in terms of electrophilic···nucleophilic interactions.

A theoretical study of anionic complexes formed by two partly deprotonated oxoacids joined by hydrogen bonds has been carried out at the MP2 computational level. In spite of the ionic repulsion, local energy minima are found both in the gas phase and in aqueous solution. Electrostatic potential and electron density topologies, and the comparison with neutral complexes formed by oxoacids, reveal that the ionization has no significant effect on the properties of the hydrogen bonds. The stability of the complexes in the gas phase is explained by attractive forces localized in a volume situated in the hydrogen bond and defined as the electrostatic attraction region (EAR) and determined by the topological analyses of the electron density and the electrostatic potential, and by the electric field lines. In solution, the strong anionic repulsion is mostly screened by the effect of the surrounding polar solvent, which only leads to a weak destabilizing interaction in the hydrogen bond region and finally favors the overall stability of the complexes. The anion-anion complexes have been compared with the corresponding neutral ones (as salts or protonated forms), showing that EAR remains unchanged along the series.

Crystal structures and DFT calculations of new chlorido-dimethylsulfoxide-M(III) (M = Ir, Ru, Rh) complexes with the N-pyrazolyl pyrimidine donor ligand: kinetic vs. thermodynamic isomers.

New chlorido-dimethylsulfoxide-iridium(III), ruthenium(III) and rhodium(III) complexes with the 2-(1H-pyrazol-1-yl)-pyrimidine (pyrapyr) ligand (OC-6-N1)-[Rh(III)Cl3(DMSO-κS)(pyrapyr)] (1a, N = 3 and 1b, N = 4); (OC-6-N1)-[Ru(III)Cl3(DMSO-κS)(pyrapyr)] (2a, N = 3 and 2b, N = 4) and (OC-6-N1)-[Ir(III)Cl3(DMSO-κS)(pyrapyr)] (3a, N = 3 and 3b, N = 4) have been synthesized and characterized by spectroscopic techniques and by single crystal X-ray diffraction studies (1a, 1b, 2a, 2b, a disordered crystal 3a/3b and a cocrystal 3a·3b). In all cases, the metal centers show octahedral geometry coordinated to three chloride ligands and one S coordinated dimethylsulfoxide (DMSO-κS). The coordination sphere of the metal is completed by the pyrapyr molecule. Two different coordination modes are observed: (i) the DMSO-κS is opposite to the pyrimidinic N atom (IUPAC nomenclature is OC-6-31 denoted herein as trans); (ii) DMSO-κS is opposite to the pyrazolic N atom (IUPAC nomenclature is OC-6-41 denoted as cis). For Rh(III) the kinetic product (cis) yields the thermodynamic (trans) upon heating a solution of the kinetic product and both isomers have been X-ray characterized. Conversely for Ru(III), both kinetic and thermodynamic complexes have been obtained by using different procedures. Both isomers have been characterized by X-ray crystallography and the kinetic product does not yield the thermodynamic upon heating a solution of the former. Furthermore, the Ir(III) behaves differently, since both isomers are energetically equivalent and both isomers co-crystallize in the solid state. The kinetic/thermodynamic mechanism that yields the different isomers has been studied by using theoretical DFT calculations for each metal. Finally, two Ru(II) complexes (OC-6-N1)-[Ru(II)Cl2(DMSO-κS)2(pyrapyr)] (1a, N = 3 and 4b, N = 4) are also described and X-ray characterized. They were obtained as minor products during the synthesis of 2a.

Experimental and theoretical studies on the coordination chemistry of the N1-hexyl substituted pyrimidines (uracil, 5-fluorouracil and cytosine).

N(1)-Hexyl substituted pyrimidines were shown to present solubility properties closer to the real bases than the commonly used methyl and ethyl derivatives, yielding bi-layered structures in the solid state. The study of their coordination capabilities, mainly with Ag(I) and Hg(II), is presented in order to prove their reactivity. A series of coordination complexes, namely, [Hg(N(1)-hexyl-5-fluorouracilate)2]4·6H2O (1), (Ag(+))·[Ag(N(1)-hexyl-5-fluorouracilate)2](-) (2), [Ag(NO3)(N(1)-hexyluracil-κO(4))4] (3), [ZnBr2(N(1)-hexylcytosine)2] (4), [CdBr2(N(1)-hexylcytosine)2] (5), [HgBr2(N(1)-hexylcytosine)2] (6) and [CoBr2(N(1)-hexylcytosine)2] (7), have been synthesized in good yields and X-ray characterized. The presence of the hexyl chains and the fluorine atoms causes the formation of interesting 3D architectures in the solid state. Their structures have been further characterized by infrared spectra (IR) and elemental analyses. In addition, DFT-D3 calculations are used to study interesting noncovalent interactions observed in the solid state, like fluorine-fluorine, fluorine-π and hydrophobic interactions.

Synthesis, X-ray characterization and computational studies of Cu(II) complexes of N-pyrazolyl pyrimidine.

In this manuscript we report the synthesis and X-ray characterization of several complexes of Cu(II) with a 2-(1H-pyrazol-1-yl)-pyrimidine (L) ligand. Complexes CuLCl(2) (1), [CuL(2)(H(2)O)(2)](NO(3))(2) (2) and [CuL(2)H(2)O](NO(3))(2) (3) are mononuclear systems and [CuL(NO(3))(2)](n) (4) is polymeric. In the solid state, complexes 2 and 3 are characterized by the presence of anion-π interactions that are relevant for the final 3D architecture and packing. In complexes 1 and 4, where the counterion is directly bonded to the metal, anion-π interactions are not observed. High level ab initio calculations (RI-MP2/def2-TZVP) have been used to evaluate the noncovalent interactions observed in the solid state and the interplay between them. We also demonstrate that the presence of anions above the aromatic ligand is not due only to strong electrostatic interactions between the counterparts.

Electrostatics at the origin of the stability of phosphate-phosphate complexes locked by hydrogen bonds.

Stronger than you'd think: Ab initio calculations reveal that hydrogen bonds can lock phosphates into stable gas-phase complexes, showing that hydrogen bonding can overcome anion-anion repulsion. These complexes present a large energetic barrier of dissociation. The stability of the complexes can be explained in terms of the electrostatic interaction in the hydrogen-bond region.

C-H···X (X = O, N or π) interactions in benzyl carbamate.

The crystal packing and interaction energy of benzyl carbamate, C(8)H(9)NO(2), have been analysed in detail by the PIXEL method. Benzyl carbamate forms layers of hydrogen-bonded molecules, with the layers connected by weaker C-H...π interactions. According to the PIXEL analysis, combinations of C-H...X (X = O, N or π) interactions are comparable in energy with hydrogen bonding. These interactions are necessary for explaining the geometry and the assembly of the layers.

Tuning the interaction energy of hydrogen bonds: the effect of the substituent.

The effect of the substituent R in the hydrogen bonding properties of FH···FR (R = H, Al, Li, Cl and CCH) complexes has been studied by theoretical calculations. The dependency of the interaction energy with the hydrogen bond distance and R is explained in terms of the topologies of the electron density and the electrostatic potential. A simple model of the hydrogen bond interaction energy, which can be assimilated to an interaction potential, is defined in terms of a stabilizing mutual polarization of the monomers and an overall destabilizing contribution associated with the electron density reorganization when the overlap of the closed shells is large enough. This model shows an excellent agreement with the ab initio interaction energies and is common for all the analyzed complexes. The substituent effect is represented in the model by a single parameter that can be calculated from the electron distribution in the acceptor atom region. The perturbation in the hydrogen bonding interaction induced by the change of R presents a close similarity with that produced by an external electric field of the same order of magnitude than those found in crystalline solids, indicating that both perturbations should play a significant and similar role on the properties of hydrogen bonds in condensed matter.

Universal features of the electron density distribution in hydrogen-bonding regions: a comprehensive study involving H...X (X=H, C, N, O, F, S, Cl, pi) interactions.

Topological analyses of the theoretically calculated electron densities for a large set of 163 hydrogen-bonded complexes show that HX interactions can be classified in families according to X (X=atom or pi orbital). Each family is characterised by a set of intrinsic dependencies between the topological and energetic properties of the electron density at the hydrogen-bond critical point, as well as between each of them and the bonding distance. Comparing different atom-acceptor families, these dependencies are classified as a function of the van der Waals radius r(X) or the electronegativity chi(X), which can be explained in terms of the molecular orbitals involved in the interaction. According to this ordering, the increase of chi(X) leads to a larger range of HX distances for which the interaction is of pure closed-shell type. Same dependencies observed for HO interactions experimentally characterised by means of high-resolution X-ray diffraction data show a good agreement with those obtained from theoretical calculations, in spite of a larger dispersion of values around the expected fitting functions in the experimental case. Theoretical dependencies can thus be applied to the analysis of the experimental electron density for detecting either unconventional hydrogen bonds or problems in the modelling of the experimental electron density.

Effect of an external electric field on the dissociation energy and the electron density properties: The case of the hydrogen bonded dimer HF...HF.

The effect of a homogeneous external electric field parallel to the hydrogen bond in the FH...FH dimer has been studied by theoretical methods. The quantum theory of atoms in molecules methodology has been used for analyzing the electron distribution of the dimer, calculated with different hydrogen bond distances and external field magnitudes. It is shown that an electric field in the opposite direction to the dipole moment of the system strengthens the interaction due to a larger mutual polarization between both molecules and increases the covalent character of the hydrogen bond, while an external field in the opposite direction has the inverse effect. The properties of the complex at its equilibrium geometry with applied field have been calculated, showing that dependencies between hydrogen bond distance, dissociation energy, and properties derived from the topological analysis of the electron distribution are analogous to those observed in families of XDH...AY complexes. The application of an external field appears as a useful tool for studying the effect of the atomic environment on the hydrogen bond interaction. In the case of FH...FH, both the kinetic energy density and the curvature of the electron density along the hydrogen bond at the bond critical point present a surprisingly good linear dependence on the dissociation energy. The interaction energy can be modeled by the sum of two exponential terms that depend on both the hydrogen bond distance and the applied electric field. Moreover, as indicated by the resulting interaction energy observed upon application of different external fields, the equilibrium distance varies linearly with the external field, and the dependence of the dissociation energy on either the hydrogen bond distance or the external electric field is demonstrated to be exponential.

Synthesis and investigation of the boron cluster anion [7-(2'-pyridyl)-7,8-nido-dicarbaundecaborate] and its protonated form.

The structurally chiral [7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)](-), [](-), anion was prepared by a partial degradation reaction of 1-(2'-pyridyl)-1,2-closo-C(2)B(10)H(11). From this anion a protonated specie, H[7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)] , and a tetramethylammonium salt, [NMe(4)][7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)], [NMe(4)][] can be obtained. The (1)H{(11)B} DNMR study on in the temperature range from 298 to 203 K identified the weakly basic nitrogen atom in the pyridine ring as the proton accepting site in solid state and low temperature and revealed pronounced weakening of the nitrogen-proton interaction while the temperature increases. Capillary electrophoresis and X-ray diffraction confirmed the pyridine nitrogen atom as the proton binding site. Separation of the electrophoretically pure racemic [7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)](-) ion into two peaks by the chiral selector beta-cyclodextrine has been achieved.

Zero-flux surfaces of the electrostatic potential: the border of influence zones of nucleophilic and electrophilic sites in crystalline environment.

The topology of the electrostatic potential varphi(r) has been studied for single molecules using geometries and electron distributions rho(r) determined from high-resolution single-crystal X-ray diffraction data. The electrostatic potential gradient nablavarphi(r), which is the negative of the electric field E = -nablavarphi, has been represented, revealing the position of zero-flux surfaces and critical points. Local maxima and minima of the electrostatic potential are interpreted in terms of electrophilic and nucleophilic sites, which present influence zones delimited by zero-flux surfaces containing saddle points. The influence zones of the nucleophilic and electrophilic sites define two alternative partitions of the space in disjoint volumes, the completeness of these partitions depending on either the neutral or ionic character of the molecule. The results obtained by using this methodology are useful for the interpretation of the saddle points of the electrostatic potential, which are related to the limits of the influence zones and reveal the path for preferred attack on reactive sites with finite influence zones.

Topological properties of the electrostatic potential in weak and moderate N...H hydrogen bonds.

The topological analyses of the electrostatic potential phi(r) and the electron density distribution rho(r) have been performed for a set of 20 neutral complexes with weak and moderate N...H bonds. In all cases, a zero flux surface of the electrostatic potential containing a saddle point analogous to the bond critical point of the electron density distribution is observed. These surfaces define an equivalent of the atomic basin of rho(r) for the electrostatic potential, which exhibits zero net charge and can be regarded as an electrostatically isolated region if its volume is finite. The phi(r) and rho(r) zero flux surfaces divide the hydrogen-bonding region in three parts, being the central one related to the electrostatic interaction between donor and acceptor. This central region exhibits a relative size of approximately 13-14% of the N...H distance dNH, it belongs to the outermost shell of the nitrogen and is mainly associated with its lone pair. Topological properties of both rho(r) and phi(r), as well as the electron kinetic (G) and potential (V) energy densities, show similar dependences with dNH at both bond critical points (phi-BCP and rho-BCP). Phenomenological proportionalities between the rho(r) curvatures and G and V are also found at the electrostatic potential critical point. The curvatures of the electrostatic potential, which are interpreted in terms of the electrostatic forces in the bonding region, present the same exponential dependency as the electron density distribution, to which they are related by Poisson's equation.

Contributions to the application of the transferability principle and the multipolar modeling of H atoms: electron-density study of L-histidinium dihydrogen orthophosphate orthophosphoric acid. I.

The electron density of L-histidinium dihydrogen orthophosphate orthophosphoric acid has been determined from X-ray and neutron diffraction data at low temperature (120 K). Topological analysis of the electron density has been used to analyse the effect of the multipolar refinement strategy on the electron-density model in the hydrogen-bonding regions. The electron density at low temperature has also been used to acquire high-quality experimental thermal parameters at room temperature using the transferability principle. Molecular vibrations, TLS and normal mode analysis are discussed and studied at both temperatures.

Magneto-structural correlations in binuclear copper(II) compounds bridged by a ferrocenecarboxylato(-1) and an hydroxo- or methoxo-ligands.

The syntheses, characterization and X-ray crystal structures of the first two examples of asymmetrically bridged dinuclear copper(ii) complexes containing a ferrocenecarboxylato ligand and a methoxo group in [{Cu(dmen)}2(micro-OMe){micro-O2C(eta5-C5H4)Fe(eta5-C5H5)}](ClO4)2, 1, (dmen=N,N-dimethylethylenediamine) or an hydroxo group in [{Cu(tmen)}2(micro-OH){micro-O2C(eta5-C5H4)Fe(eta5-C5H5)}](ClO4)2, 2 , (tmen=N,N,N'N'-tetramethylethylenediamine) are reported. Magneto-chemical studies revealed that 1 and 2 exhibit opposite superexchange interactions between the two Cu(II) paramagnetic centers: an antiferromagnetic coupling (J = -11 cm(-1)) in 1 and a ferromagnetic interaction (J = +29 cm(-1)) in 2. The results obtained from these studies suggest that the weak interactions between the Cu(II) ions and the perchlorate anions detected in the crystal structures are important to introduce significant distortions in the heterobridged [Cu2(micro-OR){micro-O2C(eta5-C5H4)Fe(eta5-C5H5)}]2+ cores of 1 and 2, which clearly affect the nature and strength of the superexchange interactions. Computational studies based on density functional theory and ab initio multiconfigurational second-order perturbation theory (CASPT2) calculations have also been performed in order to rationalize the magnetic behaviour of 1 and 2. The magneto-structural correlations for complexes containing the [Cu(micro-OR)(micro-O2CR')Cu] core are discussed, and the relevance of the out-of-plane angle of the R group with the Cu(micro-OR)Cu plane established.

Preparation, solid-state characterization, and computational study of a crown ether attached to a squaramide.

[structure: see text] Crystals of a disecondary squaramide covalently linked to a crown ether presents a great variety of inter- and intramolecular nonbonded interactions including C-H/pi contacts, C-H...O and N-H...O hydrogen bonds, and pi-pi stacking between squaramide rings. Latter interaction, the stacking between squaramide rings, can be considered as an experimental evidence for the proposed aromaticity of squaramide when it is forming hydrogen bonds, either as acceptor or donor.