Hydrogenolysis and Hydroisomerization of Neopentane on Titanium and Zirconium Hydrides Stabilized on the Surface of SiO2: A Theoretical Study by Density Functional Theory

The model reactions of neopentane hydrogenolysis and hydroisomerization on transition metal hydrides (≡ Si?O)₃M^IVH (1), (=eqSi?O)₂MIVH² (2), and (≡Si?O)₂M^IIIH (3) (M = Zr, Ti) immobilized on the surface of silica were studied by density functional theory. It was shown that the hydrogenolysis of neopentane could occur on all the three types of metal hydrides, the catalytic activity of reaction centers increasing along the series M(IV) monohydrides 1 < M(IV) dihydrides 2 < M(III) hydrides 3. At the same time, the isomerization of the hydrocarbon skeleton of neopentane observed experimentally on titanium-based systems could only be explained by the participation of Ti(III) hydrides.     

Quantitative 2H NMR spectroscopy. 1. Thermodynamic H/D isotope effects in N,N-dimethylformamide and cyclopentadiene molecules

A new method for quantification of the relative distribution of deuterium in molecules is proposed. The technique is based on the lineshape analysis in the ²H NMR spectra obtained at the natural abundance level of deuterium with allowance for inhomogeneity of the magnetic field. The equilibrium thermodynamic H/D isotope effects for hindered rotation about the C—N bond in the N,N-dimethylformamide molecule and for prototropic exchange in the cyclopentadiene molecule were determined. The results obtained agree with those of DFT calculations of the vibrational energies.     

Indenyl and fluorenyl transition metal complexes: XV.Reactions of 1,1-dimethylindene,spirocyclopropane-1,1-indene,and spirocyclopropane-9,9-fluorene with L3M(CO)3 (L = NH3, Py; M = Cr,Mo, W)

Reaction of indene, 1,1-dimethylindene, spirocyclopropane-1,1-indene, and spirocyclopropane-9,9-fluorene with Py₃M(CO)₃/BF₃ · OEt₂ (M = Cr, Mo, W) involves two competing processes: (i) the formation of the ₆-arene complexes (A) and (ii) the oxidative addition at the C(1)R and C(9)R bonds (R = alkyl) with the formation of chelated σ,π-complex (B). The reaction pathway (A or B) is determined both by the metal and the ligand.     

Fine tuning the anion binding properties of 2,6-diamidopyridine dipyrromethane hybrid macrocycles

The synthesis, characterization, and anion-binding properties of a series of 2,6-diamidopyridine dipyrromethane hybrid macrocycles is presented. As part of this work, a new method for effecting the oxidation of dipyrromethane-based macrocycles in organic solvents has been developed. The macrocyclic frameworks presented here stand out because of their ease of synthesis and tunable anion-binding properties. Evidence for anion binding was obtained from UV-vis spectroscopic titrations carried out in acetonitrile. The results clearly indicate that by changing the flexibility, cavity size, and directionality of anion-binding moieties in the macrocyclic framework the anion selectivity may be changed dramatically. These results are in accord with density functional theory molecular modeling calculations performed on one member of the series.     

Protonation of ferrocene,ruthenocene, and osmocene: a density functional study

Protonated forms of the ferrocene, ruthenocene, and osmocene molecules in the gas phase were calculated using the density functional approach with the Becke—Lee—Young—Parr functional. The proton affinity energies of ferrocene, ruthenocene, and osmocene were estimated at 214.2, 220.3, and 229.7 kcal mol^–1, respectively. The addition of a proton to carbon atoms of the cyclopentadienyl ring in the ferrocene molecule and to the metal atom in the ruthenocene and osmocene molecules is more energetically favorable. No minimum corresponding to ring protonation was located on the potential energy surface of protonated osmocene. The C—H _endo bond in the ring-protonated [C₁₀H₁₁M]⁺ (M = Fe, Ru) cations is involved in agostic interaction with the metal atom. Transition states of interconversions between the ring-protonated and metal-protonated forms were identified. A specific group of protonated forms of the ferrocene and ruthenocene molecules includes four types of structures, viz., ring-protonated (1a,b) and metal-protonated (2a,b) structures, transition states of the 1 2 interconversion (3a,b), as well as ring-protonated structures with the cyclopentadiene ring folded along the C(2)—C(5) line so that the M—H _endo interaction is virtually negligible. The latter structures are required for [1,5]-sigmatropic shift of the exo-hydrogen atom in the Cp ring to occur. The results obtained were used for the interpretation of the available schemes of electrophilic substitution reactions in metallocenes and of the sigmatropic shift mechanisms.