Valentina Vetere

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.

A theoretical study of linear beryllium chains: Full configuration interaction

We present a full configuration interaction study of Be(N) (N=2,3,4,5) linear chains. A comparative study of the basis-set effect on the reproduction of the energy profile has been reported. In particular, the 3s1p, 4s2p, 4s2p1d, 5s3p2d, and 5s3p2d1f bases were selected. For the smallest chains (i.e., Be(2) and Be(3)), smaller basis sets give dissociative energy profiles, so large basis set is demanded for the reproduction of equilibrium minima in the structures. For Be(4) and Be(5) linear chains, the energy profiles show a minimum also by using the smallest basis sets, but the largest ones give a much stronger stabilization energy. For all the structures, two spin states have been studied: the singlet and the triplet. It is shown that the energy separation of the two states, in the equilibrium region, is small and decays exponentially with respect to the number of atoms in the chain. Finally an interpolative technique allowing for the estimation of the long-chain parameters from shorter ones is presented.

Full configuration interaction study of the metal-insulator transition in model systems: LiN linear chains (N=2,4,6,8)

The precursor of the metal-insulator transition is studied at ab initio level in linear chains of equally spaced lithium atoms. In particular, full configuration interaction calculations (up to 1 x 10(9) determinants) are performed, in order to take into account the different nature of the wave function at different internuclear distances. Several indicators of the Metal-Insulator transition (minimum of the energy gap, maximum of the localization tensor or of the polarizability) are considered and discussed. It is shown that the different indicators give concordant results, showing a rapid change in the nature of the wave function at an internuclear distance of about 7 bohrs.

Toward Reliable DFT Investigations of Mn-Porphyrins through CASPT2/DFT Comparison

The low-energy spectroscopies of Mn(II) and Mn(III) porphyrin (P) complexes were investigated using complete active space and subsequent perturbative treatment (CASPT2) as well as DFT-based calculations. Starting from DFT optimizations of Mn(II)P and Mn(III)PCl using crystallographic data, the CASPT2 results show that whatever the relative position of the Mn(II) ion with respect to the porphyrin cavity, the high-spin state S = 5/2 of the [MnP] unit lies much lower in energy than the intermediate S = 3/2 state. Not only are these results in agreement with experimental observations but they also differ from previous theoretical conclusions. In the Mn(III) complexes, σ and π charge redistributions compete to result in a S = 2 ground state. The performances of different functionals have been tested in the reproduction of the CASPT2 spin gaps. Our results confirm that the Mn(II) system is very challenging, as GGA functionals fail in the spin states ordering and in the reproduction of the gaps, unless a high percentage of exact HF exchange (55%), as in KMLYP, is incorporated. This inspection demonstrates the need for specific active space functional to investigate the low-energy spectroscopy of [MnP] units.

New insights in the electrocatalytic proton reduction and hydrogen oxidation by bioinspired catalysts: a DFT investigation.

In this paper, we present a DFT study of the proton reduction mechanism catalyzed by the complex [Ni(P₂(H)N₂(H))₂](2+), bioinspired from the hydrogenases. A detailed analysis of the reactive isomers is discussed together with the localizations of the transitions states and energy minima. The reactive catalytic species is a biprotonated Ni(0) complex that can show different conformations and that can be protonated on different sites. The energies of the different conformations and biprotonated species have been calculated and discussed. Energy barriers for two different reaction mechanisms have been identified in solvent and in gas phase. Frequency calculations have been performed to check the nature of the energy minima and for the calculations of entropic energetic terms and zero point energies. We show that only one conformation is mostly reactive. All the others species are nonreactive in their original form, and they have to pass through conformational barriers in order to transform in the reactive species.

Full configuration-interaction study of the metal-insulator transition in model systems

The precursor of the Metal-Insulator transition is studied at ab-initio level in equally-spaced Lin and Ben linear chains. In particular, large-size Full Configuration-Interaction (FCI) calculations (up to one billion determinants for Li8 and half a billion for Be5) have been performed. At FCI level, it is possible to take into account the different nature of the electronic wavefunction at the different internuclear distances. Several indicators of the Metal-Insulator transition (minimum of the Koopmans energy gap, maximum of the localization and polarizability tensors) are considered and discussed. In is shown that the three considered properties indicate a Metal-Insulator transition at distances that are in remarkable agreement.

Covalent vs electrostatic interactions in rare earth systems: a comparative study of U(III), U(IV), and U(V) and Nd(III) bonding properties by DFT and CAS-PT2 approaches.

A description of the electronic structure of F(3)UCO, F(3)NdCO, F(4)UCO, and F(5)UCO has been obtained by Complete Active Space second-order perturbation theory CASPT2 calculations using a relativistic effective core potential. These multiconfigurational calculations have been compared to the DFT description combined with a quasi-relativistic ZORA scalar approach. Geometries have been optimized for both levels of calculations and frequencies computed in the DFT formalism. The bonding properties of U(III) have been compared to those of Nd(III) and of higher oxidation states of U(IV,V). Both methodologies are consistent and show a decrease of the covalent character of the U-CO bonding with a higher oxidation state, U(IV) or U(V), as well as its absence for for the isoelectronic Nd(III) species.

Regioselectivity of methyl radical addition to fluoroethenes: a quantum mechanical study

Abstract The addition of the methyl radical to mono-, di- and tri-substituted alkenes has been studied by both post-Hartree–Fock and hybrid Hartree–Fock/density functional (B3LYP) methods. The B3LYP model confirms its reliability concerning activation energies and reaction enthalpies, but fails to reproduce the inversion of regioselectivity along the fluoroethene series. Only coupled cluster computations are completely reliable, but single-point computations at B3LYP geometries provide results close to those issuing from much more expensive coupled cluster geometry optimizations. The origin of regioselectivity trends is analysed in terms of spin densities and deformation energies.