Pascale Maldivi

Synthesis, Structure, and Bonding of Stable Complexes of Pentavalent Uranyl

Stable complexes of pentavalent uranyl [UO(2)(salan-(t)Bu(2))(py)K](n) (3), [UO(2)(salan-(t)Bu(2))(py)K(18C6)] (4), and [UO(2)(salophen-(t)Bu(2))(thf)]K(thf)(2)}(n) (8) have been synthesized from the reaction of the complex {[UO(2)py(5)][KI(2)py(2)]}(n) (1) with the bulky amine-phenolate ligand potassium salt K(2)(salan-(t)Bu(2)) or the Schiff base ligand potassium salt K(2)(salophen-(t)Bu(2)) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [U(V)O(2)(salan-(t)Bu(2))(py)][Cp*(2)Co], 6, were also isolated from the reduction of [U(VI)O(2)(salan-(t)Bu(2))(py)], 5, with Cp*(2)Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO(2)py(5)]I.py (2). The measured redox potentials E(1/2) (U(VI)/U(V)) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give mu(eff) of 2.20 and 2.23 mu(B) at 300 K respectively, which is compared with a mu(eff) of 2.6(1) mu(B) (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g(x) = 1.98; g(y) = 1.25; g(z) = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the pi(u)*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO(2)(salan-(t)Bu(2))(py)K] and [UO(2)(salophen-(t)Bu(2))(py)K] complexes with respect to disproportionation and hydrolysis.

Performance of the `parameter free' PBE0 functional for the modeling of molecular properties of heavy metals

Abstract A comparative density functional study of a new functional (PBE) and of the corresponding hybrid version (PBE0) has been performed on LnX 3 species (X=F, Cl, Br, I and Ln=La, Gd, Lu), including a relativistic effective core potential. Geometry optimizations and harmonic frequency calculations were carried out. Both PBE and PBE0 give better results than previous B3LYP computations, and the hybrid PBE0 was found to perform better than the PBE functional for the reproduction of geometrical features, while both give similar good agreement for frequencies. Finally, the addition of polarization functions in the halogen basis set greatly improves both geometric parameters and frequencies.

Comparative studies of quasi‐relativistic density functional methods for the description of lanthanide and actinide complexes

We present a comparative Density Functional Theory (DFT) study based on two different implementations of relativistic effects within the Kohn–Sham (KS) approach, to describe the metal–ligand interaction in I3M‐L complexes (L = NH3, NCCH3, CO and M = La, Nd, U). In the first model, the scalar corrections were included by a quasi‐relativistic approach (QR) via the so‐called ZORA or Pauli Hamiltonians, while in the second, these effects are taken into account in a quasi‐Relativistic Effective Core Potential (RECP). These relativistic approaches were used in conjunction with various gradient corrected (GGA) or hybrid (SCH) functionals. The structural parameters obtained from geometry optimizations have been compared to experimental structural trends, and rationalized by a KS orbital analysis. Both approaches provide similar results for mainly ionic metal–ligand bonds (e.g., for the σ‐donor ligand L = NH3). For the π‐acceptor ligands (NCCH3, CO), the QR approach is in agreement with experimental trends and consistent with the presence of a backbonding interaction between U(III) and the neutral ligand, which does not exist in the lanthanide homologues. The GGA/RECP methods also reproduce this phenomenon, while the SCH/RECP scheme fails to describe this interaction. The role of the RECP, of its size, and of additional polarization functions has also been examined. Finally, the failure of the SCH/RECP approach was interpreted as a consequence of a bad estimation of frontier orbital energy levels in the uranium and ligand species.

Predictions of Optical Excitations in Transition-Metal Complexes with Time Dependent-Density Functional Theory: Influence of Basis Sets.

The calculation of the absorption spectra of four families of transition-metal complexes (Ni(CO)4, MnO4(-), MF6 (M = Cr, Mo, W) and CpM(CO)2 (M = Rh, Ir)) has been undertaken to unravel the influence of basis sets onto excitation energies, oscillator strengths, and assignments. Three among the most common pseudopotentials, with the corresponding valence basis sets, and two all-electron basis sets have been used for the metal center description in the framework of the time dependent Density Functional Theory (TD-DFT). Our results show that this approach does not particularly depend on the basis set used on the metal atoms. Furthermore, the chosen functional PBE0 provides transitions in good agreement with experiments, and it provides an accuracy of about 0.3 eV, comparable to that of refined post-Hartree-Fock methods.

IONIC VERSUS COVALENT CHARACTER IN LANTHANIDE COMPLEXES. A HYBRID DENSITY FUNCTIONAL STUDY

Abstract The geometric structures and harmonic vibrational frequencies of La, Gd and Lu trihalides have been investigated by a hybrid density functional/Hartree-Fock approach coupled with a relativistic effective core potential. The adequacy of this electronic protocol is confirmed by the fairly good agreement with experimental data and with those obtained by more expensive post-HF computations. A detailed analysis of the electronic density has been performed using the natural bond localization procedure and its recent extension to natural electronic resonance theory to understand the role of charge transfer in the bonding of lanthanide complexes. This approach underlines the role of electrostatic interactions in the lanthanide-halogen bond, even if a charge transfer mechanism plays a role, especially in the more weakly bonded bromide and iodide derivatives.

Structural study of crystalline and columnar copper (II) soaps

Abstract A homologous series of binuclear copper (II) linear chain alkanoates together with two branched chain and one aromatic substituted copper (II) alkanoates have been synthesized and studied by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. All of these are crystalline at room temperature, they are mesomorphic in nature above c. 100°C, with the exception of copper propionate which remains crystalline up to its thermal decomposition above 200°C. A systematic study has shown that the linear chain alkanoates, starting from the pentanoic derivative, produce columnar mesophases with hexagonal symmetry. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two dimensional hexagonal lattice. The repeat unit in a column is a binuclear dicopper tetracarboxylate complex. Two transition regimes have been detected leading from the crystal to the columnar mesophase: one dominated by the interactions between the polar heads, t...

Ab Initio Molecular Dynamics Study of a Highly Concentrated LiCl Aqueous Solution

The properties of a highly concentrated aqueous lithium chloride solution (|LiCl| = 14 mol L(-1)) are investigated using Car-Parrinello molecular dynamics. The coordination spheres of lithium ions, chloride ions, and water molecules are described successively. On the whole, our simulation provides results-distances and coordination numbers-in very good agreement with experimental data. The lithium solvation shell is found to exhibit a tetrahedral configuration on average, with three stable clusters observed during the simulation: Li(+)-4H2O, Li(+)(H2O)3Cl(-), and Li(+)(H2O)2(Cl(-))2. The chloride coordination sphere is logically formed by strong Cl-H hydrogen bonds with neighboring water molecules, for a mean coordination number of 4.4. The structuring of water molecules is strongly affected by the high concentration in LiCl. The hydrogen bond network is globally broken down, but little variation is calculated on water dipoles (μ = 3.07 D) because of the strong polarization from Li(+) and Cl(-) ions. We also point out some of the characteristic features of such a highly concentrated solution: water bridging between Li(+) and Cl(-) hydration spheres, Li(+)-Cl(-) ion-pairing, and intermediate behavior between dilute solutions and molten salts. Finally, the reliability of our simulation to describe ion-pairing is discussed.

Diruthenium(II,II) tetra-μ-alkylcarboxylates: Magnetic susceptibility studies of their electronic configuration and thermotropic liquid crystalline mesophase

Abstract Diruthenium(II,II) complexes of several fatty acids have been isolated and the temperature dependence of their magnetic susceptibility studied. The data are consistent with the existence of an M s =0 ground state and of a thermally accessible M s =±1 excited state, both derived from a degenerate σ 2 π 4 δ 2 π* 2 δ* 2 or σ 2 π 4 δ 2 π* 3 δ* 1 electronic configuration by a large zero-field splitting ( D ≈300 cm − ). The formation of a columnar liquid-crystalline phase is reflected by a sharp increase in the magnetic moment of these complexes at the phase transition.

Vibrational studies of some dicopper tetracarboxylates which exhibit a thermotropic columnar mesophase

Abstract Infrared spectra have been obtained for a series of copper alkanoate dimers which include the caproate (dicopper tetrahexanoate), pelargonate (nonanoate), undecanoate, laurate (dodecanoate), palmitate (hexadecanoate) and docosanoate. Band assignments have been made wherever possible through the use of isotopic substitution, temperature studies and, in part, by Raman spectra. The results indicate that it should be possible to follow phase changes to the respective columnar mesophases of these complexes by monitoring the position of the methylene stretching frequency.

Investigation of Hybrid Molecular/Silicon Memories With Redox-Active Molecules Acting as Storage Media

In this paper, a physical investigation of hybrid molecular/Si memory capacitor structures is proposed, where redox-active molecules act as storage medium. Fc and ZnAB3P porphyrin were grafted on (100) Si with both a direct bond and a chemical linker in order to investigate the electron transfer properties of the molecule/Si system. The chemical structures of the molecular layers were analyzed with X-ray photoelectron spectroscopy. Cyclic voltammetry and impedance spectroscopy were also performed on capacitor structures in order to characterize the charge transfer between the redox molecules and the Si and the effect of an organic linker on its rate. To explain our results, an original electrical model of molecule/Si memory structures is proposed. Calculated data are compared to impedance results for the cases of Fc either directly grafted or with linker. The model allows us to give a theoretical confirmation of the influence of the linker over the redox energy and on the time constant of the molecular capacitor. Finally, density functional theory calculations provide an in-depth physical insight of the effect of the linker over the Fc redox energy. The results obtained in this paper show the strong impact of the engineering of the redox molecules and their linker on the electron transfer properties.