A DFT study using a “discrete-continuum” modeling of the reaction medium, employed to unravel the role of water in the activation of dihydrogen by [CpRu(PTA)2Cl] (PTA = 1,3,5-triaza-7-phosphaadamantane), has revealed that the solvent takes an active part in lowering the energy barriers of the overall heterolytic splitting of the H−H bond. PTA itself promotes the heterolytic activation of the Ru(η2-H2) bond through one of its nitrogen atoms via a solvent-mediated intramolecular proton transfer.

The isomerization of complex [Cp*Fe(dppe)(eta2-H2)]+, generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CF3COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)2]+ is irreversible and follows first-order kinetics in the -10 to +15 degrees C range with Delta H double dagger = 21.6 +/- 0.8 kcal mol(-1) and DeltaS double dagger = 5 +/- 3 eu. The isomerization rate constant is essentially independent of the nature and quantity of a strong acid. Density functional theory (DFT) calculations on various models, including the complete system at both the quantum mechanics/molecular mechanics (QM/MM) and full QM levels, probe the relative importance of steric and electronic effects for the relative stability of the nonclassical and classical isomers and identify two likely isomerization mechanisms: a "direct" pathway involving simultaneous H-H bond breaking and cis-trans isomerization and a "via Cp" pathway involving agostic C5Me5H intermediates. Both pathways are characterized by activation energies in close correspondence with the experimental value (21.3 and 22.2 kcal mol(-1), respectively). Further kinetic studies were carried out for the Cp*Fe(dppe)H + CF3COOD and Cp*Fe(dppe)D + CF3COOD systems at 273 K. The [Cp*Fe(dppe)(eta2-HD)]+ complex establishes a very rapid isotope redistribution equilibrium with the eta2-H2 and eta2-D2 analogues. The equilibrium constant value (K = 3.3 +/- 0.3) indicates a significant equilibrium isotope effect. Simulation of the rate data provides access to the individual isomerization rate constants kHH, kHD, and kDD for the three isotopomers, yielding kinetic isotope effects: kHH/kHD = 1.24 +/- 0.01 and kHD/kDD = 1.58 +/- 0.01 (and, consequently, kHH/kDD = 1.96 +/- 0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H2]+ model system, for the [Cp*Fe(dhpe)(eta2-XY)]+ isotopomers as well as transition states for the "direct" (TSdir) and "via Cp" (TSrot) pathways (X = H, D) allowed the computation of the expected isotope effects. A comparison with the experiment strongly suggests that the mechanism occurs via the "direct" pathway for the present system, although the small difference in the calculated energy barriers suggests that the "via Cp" pathway may be preferred in other cases.

We report on self-assembly of calix[4]arenes deposited on Au(110)-(1x2). The molecules consist of a macrocycle with four phenol rings linked by methylene groups in the ortho-position. The scanning tunneling microscopy images together with the low-energy electron diffraction patterns allow us to identify the molecular conformer and the unit cell. The intermolecular interactions are rather weak and are governed by pi stacking and van der Waals-type interactions as deduced from molecular mechanics calculations. The two-dimensional molecular self-ordering is due to a strong interaction with the gold substrate resulting in the replacement of the initial (1x2) missing row reconstruction by a (1x3) structure upon molecular adsorption.

The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.

The dihydride-dichloro complex OsH2Cl2((PPr3)-Pr-i)(2) (1) reacts with 4.0 equiv of HSnPh3 to afford 2.0 equiv of ClSnPh3 and the tetrahydride-bis(stannyl)osmium(VI) derivative OsH4(SnPh3)(2)((PPr3)-Pr-i)(2) (2), which has been characterized by X-ray diffraction analysis. DFT calculations in a model system of 2, in which the bulky ligands have been replaced by small models, followed by QM/MM optimizations with the real ligands have allowed the complete determination of the hydride positions and the role played by the steric effects on the experimental structure.

The hybrid quantum mechanics/molecular mechanics (QM/MM) method IMOMM is applied to the calculation of the reaction of thiophene with Cp*RhPMe3, which is a good experimental model for homogeneous catalytic hydrodesulfurization (HDS). The validity of the results is checked by comparison with previously reported full QM calculations on the same system. The geometries obtained with IMOMM are in good agreement with those obtained with full QM. Concerning the energies of the reaction path, differences from full QM results are found for steps involving a change in the oxidation state of the Rh atom. The effect of the methyl substituents on each reaction step is quantified and separated into steric and electronic contributions.

Soutenue le 20 octobre 2000 devant le jury composé de