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.

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The crystal structures of two trisiodide octacoordinated uranium(III) complexes of tris[(2-pyrazinyl)methyl]amine (tpza), which differ only by the ligand occupying the eighth coordination site (thf or MeCN), and of their lanthanum(III) analogues have been determined. In the acetonitrile adducts the M-N(pyrazine) distances are very similar for U(III) and La(III), while the U-N(acetonitrile) distance is 0.05 A shorter than the La-N(acetonitrile) distance. In the [M(tpza)I(3)(thf)] complexes in which the monodentate acetonitrile ligand, a weak pi-acceptor ligand, is replaced by a thf molecule, a sigma-donor only, the mean value of the distance U-N(pyrazine) is 0.05 A shorter than the mean value of the La-N(pyrazine) distance. Since we are comparing isostructural compounds of ions with very similar ionic radii, these differences indicate the presence of a stronger M-N interaction in the U(III) complexes and therefore suggest the presence of a covalent contribution to the U-N bonding. The selectivity of the tpza ligand toward U(III) complexation (with respect to that of La(III)) in the presence of sigma-donor-only ligands has been quantified by the value of K(U(tpza))/K(La(tpza)) measured to be 3.3 +/- 0.5. The analysis of the metal-N-donor ligand bonding was carried out by a quasi-relativistic density functional theory study on small model compounds, of formula I(3)M-L (M = La, Nd, U; L = acetonitrile, pyrazine) and I(3)M-(pyrazine)(3) (M = La, U). The structural data obtained from geometry optimizations on these systems reproduce experimental trends, i.e., a decrease in the M-N distance from La to U, combined with an increase of the C-N distance in the acetonitrile derivatives. A detailed orbital analysis carried out on the resulting optimized complexes did not reveal any orbital interaction between the trivalent lanthanide cations (Ln(3+)) and the N-donor ligands. In contrast, a back-donation electron transfer from 5f U(3+) orbitals to the pi* virtual orbital of the ligand was observed for both acetonitrile and pyrazine. Evaluation of the total bonding energy between the MI(3) and L fragments shows that this orbital interaction leads to a stabilization of the uranium(III) system compared to the lanthanide species.

μ-Nitrido-bridged diiron porphyrins and phthalocyanines are known to be efficient oxidants that are able to oxidize some of the strongest C−H bonds in nature, such as the one in methane. The origin of their catalytic efficiency is poorly understood, and in order to gain insight into the structural and electronic features of this chemical system, we performed a detailed and systematic study into their chemical properties and reactivities using density functional theory. Our work shows that μ-nitrido-bridged diiron porphyrins and phthalocyanines are highly active catalytic oxidants, which react with methane with very low reaction barriers and a rate-determining hydrogen-atom-abstraction step. Furthermore, the μ-nitridobridged diiron porphyrin and phthalocyanine complexes react with a free energy of activation that is more than 10 kcal mol −1 lower in energy than that found for cytochrome P450 Compound I, which is known to be one of the most efficient C−H hydroxylating agents in Nature. We have analyzed the electronic configuration of reactants and transition states in detail and have identified the key properties of the oxidants that lead to this rate enhancement. In particular, the potency of the oxidant is related to the orbital mixing patterns along the Fe−O axis, whereby the axial iron(IV)-nitrido group donates sufficient electron density to affect the pK a of the oxo group as well as the strength of the O−H bond formed in the iron(IV)-hydroxo complex. The studies confirm that μ-nitrido diiron-oxo complexes should react via oxygen atom transfer readily even with strong C−H bonds as in methane. The results are analyzed with orbital diagrams, valence bond, and thermochemical cycles and explain the intricate details of the mechanism and the properties of the oxidant.

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.

The magnetic properties of several mixed-valent diruthenium long-chain carboxylates of general formula Ru(2)(RCO(2))(4)X (X = Cl, DOS, or RCO(2), DOS = dodecyl sulfate, and RCO(2) = linear aliphatic carboxylate or dialkoxy- or trialkoxybenzoate) were studied in the temperature range 6-400 K. All of the compounds exhibit a strong zero-field splitting (D = ca. 75 cm(-)(1)), independent of the nature of the axial anion X or of the equatorial substituent R. In the X = RCO(2) series an intermolecular antiferromagnetic (AF) interaction zJ = ca. -2 cm(-)(1) was found, whereas in the case of an X = DOS analogue, this interaction is very weak (-0.2 cm(-)(1)). The X = Cl series shows three distinct types of interdimer magnetic exchange: very weak, moderate (|zJ| approximately 2 cm(-)(1)), or strong (|zJ| > 10 cm(-)(1)). One representative complex in this series, Ru(2)(C(4)H(9)CO(2))(4)Cl, has been structurally characterized by X-ray crystallography. Crystal data: tetragonal system, space group I&fourmacr;2d, a = 14.137(3) Å, c = 26.246(5) Å, and Z = 8. Examination of the structures and magnetic behaviors suggests that the AF exchange in this series correlates with the Ru-Cl-Ru intermolecular angle; a qualitative explanation in terms of overlap of magnetic orbitals is proposed. Magnetic susceptibility measurements in the columnar mesophase of the mesomorphic congeners indicate that no significant structural change occurs at the crystal-liquid crystal transition.

Although BTP (2,6-di(1,2,4-triazin-3-yl)pyridine) has been widely evidenced as the most effective nitrogen ligand for the selective complexation of trivalent actinides from lanthanide counterparts, the origin of its selectivity is still an open question. Neither experimental data nor theoretical calculations have been able to rationalize the role of covalency in real experimental BTP complexes. We show herein with DFT calculations on [M(BTP)3]3+ (M = La, U, Cm, Gd) that, even if back-bonding effects are significant in the U-BTP bond, it is the contrast of donation on 6d and 5f Cm(III) orbitals that explains, at least in part, its selective complexation to BTP.

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 I(3)M-L complexes (L = NH(3), NCCH(3), 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 sigma-donor ligand L = NH(3)). For the pi-acceptor ligands (NCCH(3), 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.