Water versus Acetonitrile Coordination to Uranyl. Effect of Chloride Ligands

Optimizations at the BLYP and B3LYP levels are reported for the mixed uranyl chloro/water/acetonitrile complexes [UO₂Cl_<i>n</i>(H₂O)_<i>x</i>(MeCN)_{5<i>–n</i>−<i>x</i>}]^2–<i>n</i> (<i>n</i> = 1–3) and [UO₂Cl_<i>n</i>(H₂O)_<i>x</i>(MeCN)_{4<i>–n</i>−<i>x</i>}]^2–<i>n</i> (<i>n</i> = 2–4), in both the gas phase and a polarizable continuum modeling acetonitrile. Car–Parrinello molecular dynamics (CPMD) simulations have been performed for [UO₂Cl₂(H₂O)­(MeCN)₂] in the gas phase and in a periodic box of liquid acetonitrile. According to population analyses and dipole moments evaluated from maximally localized Wannier function centers, uranium is less Lewis acidic in the neutral UO₂Cl₂ than in the UO₂²⁺ moiety. In the gas phase the latter binds acetonitrile ligands more strongly than water, whereas in acetonitrile solution, the trend is reversed due to cooperative polarization effects. In the polarizable continuum the chloro complexes have a slight energetic preference for water over acetonitrile ligands, but several mixed complexes are so close in free energy Δ<i>G</i> that they should exist in equilibrium, in accord with previous interpretations of EXAFS data in solution. The binding strengths of the fifth neutral ligands decrease with increasing chloride content, to the extent that the trichlorides should be formulated as four-coordinate [UO₂Cl₃L]⁻ (L = H₂O, MeCN). Limitations to their accuracy notwithstanding, density functional calculations can offer insights into the speciation of a complex uranyl system in solution, a key feature in the context of nuclear waste partitioning by complexant molecules

Liquid Methanol from DFT and DFT/MM Molecular Dynamics Simulations

We present a comparative study of computational protocols for the description of liquid methanol from <i>ab initio</i> molecular dynamics simulations, in view of further applications directed at the modeling of chemical reactivity of organic and organometallic molecules in (explicit) methanol solution. We tested density functional theory molecular dynamics (DFT-MD) in its Car–Parrinello Molecular Dynamics (CPMD) and Quickstep/Born–Oppenheimer MD (CP2K) implementations, employing six popular density functionals with and without corrections for dispersion interactions (namely BLYP, BLYP-D2, BLYP-D3, BP86, BP86-D2, and B97-D2). Selected functionals were also tested within the two QM/MM frameworks implemented in CPMD and CP2K, considering one DFT molecule in a MM environment (described by the OPLS model of methanol). The accuracy of each of these methods at describing the bulk liquid phase under ambient conditions was evaluated by analyzing their ability to reproduce (<i>i</i>) the average structure of the liquid, (<i>ii</i>) the mean squared displacement of methanol molecules, (<i>iii</i>) the average molecular dipole moments, and (<i>iv</i>) the gas-to-liquid red-shift observed in their infrared spectra. We show that it is difficult to find a DFT functional that describes these four properties equally well within full DFT-MD simulations, despite a good overall performance of B97-D2. On the other hand, DFT/MM-MD provides a satisfactory description of the solvent–solute polarization effects with all functionals and thus represents a good alternative for the modeling of methanol solutions in the context of chemical reactivity in an explicit environment

Speciation of La(III) Chloride Complexes in Water and Acetonitrile: A Density Functional Study

Car–Parrinello molecular dynamics (CMPD) simulations and static computations are reported at the BLYP level of density functional theory (DFT) for mixed [LaCl_<i>x</i>(H₂O)_<i>y</i>(MeCN)_<i>z</i>]^3–<i>x</i> complexes in aqueous and nonaqueous solution (acetonitrile). Both methodologies predict coordination numbers (i.e., <i>x</i> +<i> y</i> +<i> z</i>) that are successively lower than nine as the Cl content increases from <i>x</i> = 0 to 3. While the static DFT method with implicit solvation through a polarizable continuum model overestimates the binding strength of chloride and erroneously predicts [LaCl₂(H₂O)₅]⁺ as global free-energy minimum, constrained CPMD simulations with explicit solvent and thermodynamic integration reproduce the weak binding of chloride in water reasonably well. Special attention is called to the dipole moments of coordinated water molecules as function of coligands and solvent, evaluated through maximally localized Wannier function centers along the CPMD trajectories. Cooperative polarization of these water ligands by the metal cation and the surrounding solvent is remarkably sensitive to fluctuations of the La–O distances and, to a lesser extent, on the La-water tilt angles. The mean dipole moment of water ligands is rather insensitive to the other coligands, oscillating around 3.2 D, 3.5 D, and 3.3 D in MeCN, water, and [dmim]­Cl solution, respectively, the latter being an archetypical ionic liquid

Evidence of a Donor–Acceptor (Ir–H)→SiR₃ Interaction in a Trapped Ir(III) Silane Catalytic Intermediate

The ionic iridacycle [(2-phenylenepyridine-κ<i>N</i>,κ<i>C</i>)­IrCp*­(NCMe)]­[BArF₂₄] ([<b>2</b>]­[BArF₂₄]) displays a remarkable capability to catalyze the O-dehydrosilylation of alcohols at room temperature (0.4 × 10³ < TON < 10³, 8 × 10³ < TOF_<i>i</i> < 1.9 × 10⁵ h^–1 for primary alcohols) that is explained by its exothermic reaction with Et₃SiH, which affords the new cationic hydrido-Ir­(III)-silylium species [<b>3</b>]­[BArF₂₄]. Isothermal calorimetric titration (ITC) indicates that the reaction of [<b>2</b>]­[BArF₂₄] with Et₃SiH requires 3 equiv of the latter and releases an enthalpy of −46 kcal/mol in chlorobenzene. Density functional theory (DFT) calculations indicate that the thermochemistry of this reaction is largely dominated by the concomitant bis-hydrosilylation of the released MeCN ligand. Attempts to produce [<b>3</b>]­[BF₄] and [<b>3</b>]­[OTf] salts resulted in the formation of a known neutral hydrido-iridium­(III) complex, i.e. <b>4</b>, and the release of Et₃SiF and Et₃SiOTf, respectively. In both cases formation of the cationic μ-hydrido-bridged bis-iridacyclic complexes [<b>5</b>]­[BF₄] and [<b>5</b>]­[OTf], respectively, was observed. The structure of [<b>5</b>]­[OTf] was established by X-ray diffraction analysis. Conversion of [<b>3</b>]­[BArF₂₄] into <b>4</b> upon reaction with either 4-<i>N</i>,<i>N</i>-dimethylaminopyridine or [<i>n</i>Bu₄]­[OTf] indicates that the Ir center holds a +III formal oxidation state and that the Et₃Si⁺ moiety behaves as a Z-type ligand according to Green’s formalism. [<b>3</b>]­[BArF₂₄], which was trapped and structurally characterized and its electronic structure investigated by state-of-the-art DFT methods (DFT-D, EDA, ETS-NOCV, QTAIM, ELF, NCI plots and NBO), displays the features of a cohesive hydridoiridium­(III)→silylium donor–acceptor complex. This study suggests that the fate of [<b>3</b>]⁺ in the O-dehydrosilylation of alcohols is conditioned by the nature of the associated counteranion and by the absence of Lewis base in the medium capable of irreversibly capturing the silylium species