Monitoring of the pre-equilibrium step in the alkyne hydration reaction catalyzed by au(Iii) complexes: A computational study based on experimental evidences

The coordination ability of the [(ppy)Au(IPr)]2+ fragment [ppy = 2-phenylpyridine, IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl 12, BF4 12, OTf 12, H2 O, 2-butyne, 3-hexyne) commonly involved in the crucial pre-equilibrium step of the alkyne hydration reaction is computationally investigated to shed light on unexpected experimental observations on its catalytic activity. Experiment reveals that BF4 12 and OTf 12 have very similar coordination ability towards [(ppy)Au(IPr)]2+ and slightly less than water, whereas the alkyne complex could not be observed in solution at least at the NMR sensitivity. Due to the steric hindrance/dispersion interaction balance between X and IPr, the [(ppy)Au(IPr)]2+ fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)]2+ (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf 12 and BF4 12, in agreement with experiment. Effect of the ancillary ligand substitution demonstrates that the coordination ability of Au(III) is quantitatively strongly affected by the nature of the ligands (even more than the net charge of the complex) and that all the investigated gold fragments coordinate to alkynes more strongly than H2 O. Remarkably, a stabilization of the water-coordinating species with respect to the alkyne-coordinating one can only be achieved within a microsolvation model, which reconciles theory with experiment. All the results reported here suggest that both the Au(III) fragment coordination ability and its proper computational modelling in the experimental conditions are fundamental issues for the design of efficient catalysts

Relativistic calculation of hyperfine and electron spin resonance parameters in diatomic molecules

C1 - Journal Articles Referee

Relativistic density functional theory using Gaussian basis sets

C1 - Journal Articles Referee

Understanding the Catalase-Like Activity of a Bioinspired Manganese(II) Complex with a Pentadentate NSNSN Ligand Framework. A Computational Insight into the Mechanism

The mechanism of H2O2 dismutation catalyzed by the recently reported 2,6-bis[((2-pyridylmethyl)thio)methyl]pyridine-Mn(II) complex ([MnS2Py3(OTf)2]) has been investigated by density functional theory using the S12g functional. The complex has been analyzed in terms of its coordination properties and the reaction of [MnS2Py3]2+ in a distorted square pyramidal coordination geometry with two hydrogen peroxide molecules has been investigated in our calculations. The sextet, quartet, and doublet potential energy profiles of the catalytic reaction have been explored. In the first dismutation process, the rate-determining step (RDS) is found to be the asymmetric O-O bond cleavage, which occurs on the sextet potential energy profile. A subsequent spin crossover from sextet to quartet, associated with a coordination rearrangement around the metal, can take place to generate a stable Mn(IV) dihydroxo intermediate. This could disfavor the ping-pong mechanism commonly considered to describe the H2O2 dismutation reaction, where the binding of the first H2O2 substrate leads to the release of one H2O product and the conversion of the catalyst into a Mn(IV) oxo complex. The formation of this stable intermediate, featuring a peculiar trigonal prismatic coordination geometry, paves the way for an alternative reaction pathway for the second dismutation process, termed the dihydroxo mechanism, where two water molecules and dioxygen are easily and simultaneously formed. The competing channels have different spin states: the sextet reaction pathway corresponds to the ping-pong mechanism, whereas the quartet reaction follows preferably the dihydroxo mechanism. The doublet reaction path is energetically disfavored for both channels. For the ping-pong mechanism, the RDS in the second dismutation process is represented by the second hydrogen-abstraction from H2O2, with a calculated energy barrier very close to that of the RDS in the first dismutation reaction. Explicit solvent molecules, counterions, and trace amounts of water are found to further support the preference for the asymmetric O-O bond breaking by favoring the end-on coordination mode of the first H2O2 to the catalys

The uranyl ion revisited: the electric field gradient at U as a probe of environmental effects

The experimental electric field gradient (EFG) at the U nucleus in uranyl is positive. It has been pointed out by Pyykkö that this could be a signature of a hole in the 6p shell induced by the strong bonding to the axial O atoms. We have revisited this issue with the help of relativistic density functional calculations, including accurate ZORA-4 calculations of the EFG. We confirm the existence of a 6p hole, with a positive contribution to the EFG, but we still find the EFG in the free uranyl ion to be negative due to the non-spherical electron distribution in the valence 5f shell caused by the bonding to the oxygens. A positive EFG only results in our calculations from the effect of the crystal environment of the uranyl ion, i.e. the coordination of three nitrate groups in the equatorial plane. Again the extended nature of 6p plays a key role, with an important positive contribution to the EFG coming from 6p tails in the high-lying electron pair orbitals of the closed shell nitrate ligands due to the orthogonality requirement. A further contribution comes from electron donation by the nitrate groups into the U 5

π Activation of Alkynes in Homogeneous and Heterogeneous Gold Catalysis

The activation of alkynes toward nucleophilic attack upon coordination to gold-based catalysts (neutral and positively charged gold clusters and gold complexes commonly used in homogeneous catalysis) is investigated to elucidate the role of the σ donation and π back-donation components of the Au–C bond (where we consider ethyne as prototype substrate). Charge displacement (CD) analysis is used to obtain a well-defined measure of σ donation and π back-donation and to find out how the corresponding charge flows affect the electron density at the electrophilic carbon undergoing the nucleophilic attack. This information is used to rationalize the activity of a series of catalysts in the nucleophilic attack step of a model hydroamination reaction. For the first time, the components of the Dewar–Chatt–Duncanson model, donation and back-donation, are put in quantitative correlation with the kinetic parameters of a chemical reaction