Nouvelles syntheses de Me2SiCl2 et Me3SiCl

Partial reduction of MeSiCl₃ and Me₂SiCl₂ using CaH₂ or (TiH₂)_n at high temperature (300°C) leads to MeSiHCl₂ and Me₂SiHCl, respectively, in good yields but in low proportion. In the presence of AlCl₃ as catalyst the reaction affords Me₂SiCl₂ and Me₃SiCl, in yields higher than those previously observed in the absence of a reducing agent. These redistribution reactions involve MeSiHCl₂ and Me₂SiHCl as intermediates. Consequently Me₂SiHCl with or without Me₂SiCl₂ and Alcl₃ deposited on carbon black as catalyst can undergo disproportionation to give Me₃SiCl.     

Ligands of low electronegativity in the VSEPR model: The structures of singlet carbenes

Equilibrium structures and force constants for skeletal bending from linearity have been calculated, in the MNDO approximation, for twenty five singlet carbenes CX₂. When the substituent X bears neither vacant orbitals nor lone pairs, the force constant becomes steadily more negative as the electronegativity of X increases; when X bears vacant orbitals, the C→ X π bond order and the force constant both increase with the electronegativity of X. When X bears lone pairs, the force constant parallels the HOMO—LUMO gap at linearity. Previous discussions of the structures of singlet carbenes are shown to be inadequate: the reported results support the interpretation in terms of the second-order Jahn-Teller effect of the observed stereochemical inactivity of lone pairs in the presence of ligands of low electronegativity.     

The crystal and molecular structure of oxo-bis[tribenzylsilicon(IV)]

Crystals of oxo-bis[tribenzylsilicon(IV)], O[(PhCH₂)₃Si]₂, are triclinic, space group P$\text{1}$, with a = 9.780(7), b = 9.938(9), c = 10.283(6) Å, α = 93.35(1), β = overall symmetry close to $\text{3}$: the SiOSi skeleton is strictly linear with SiO = 1.613(4) Å. The conformations of molecules and ions isoelectronic with O[(PhCH₂)₃Si]₂ are compared and discussed.     

The crystal and molecular structure of 1,1,1,3,3,3-hexaphenyl-1,3-disilapropane

Crystals of the title compound H₂C(SiPh₃)₂ are triclinic, space group P$\text{1}$, with a = 9.290(2), b = 12.128(4), c = 16.882(4) Å, α = 62.08(1), β = 106.88(1), γ = 117.28(2)° and Z = 2. The central skeletal angle SiCSi is 128.8(7)°. The structures of the molecules H₂C(SiR₃)₂ (R = H, CH₃, and Ph) are compared and discussed: a simple model for the skeletal geometry of species H₂C(MR₃)₂ is proposed, and tested against experimental data and theoretical calculations.     

Hexaorgano-substituted triatomics R3XYZR3: the configurations of organometallic sulphides (R3M)2S and the crystal and molecular structure of thio-bis[triphenyltin(IV)]

Crystals of thio-bis[triphenyltin(IV)], S(Ph₃Sn)₂, are orthorhombic, space-group P2₁2₁2₁, with a = 18.469(5), b = 17.648(5), c = 9.848(6) Å and Z = 4. The SnS distances are 2.405(9) and 2.417(7) Å and SnSSn angle is 107.3(2)°: there are no anomalous features in the structure analogous to those in O(Ph₃Sn)₂-MNDO calculations for a series of organometallic sulphides (Me₃M)₂S⁺ⁿ (M = Be, B, C, N, Si, P) indicate that the inversion barrier to linearity increases monotonically, and the skeletal bending force constant at linearity decreases correspondingly as the group Me₃M becomes more electro-negative.     

On the topotactic dehydration of VOHPO4 · 0.5 H2O into vanadyl pyrophosphate

The thermal analysis (DTA, TGA) of the dehydration under inert atmosphere of the vanadyl hydrogen phosphate hydrate VOHPO₄ · 0.5 H₂O recently studied, together with the characterization of both starting and final compounds (XRD, ir, and uv-visible spectroscopies) show that pseudomorphic vanadyl pyrophosphate is obtained in these conditions. This form γ-(VO)₂P₂O₇ is the active and selective catalyst in butane oxidation to maleic anhydride whereas β-(VO)₂P₂O₇ is selective in butene oxidation only. Pseudomorphic relations between VOHPO₄ · 0.5 H₂O and γ-(VO)₂P₂O₇ are evidenced by the comparison of XRD patterns, SEM and TEM experiments, and justified with the help of both structures. The low values of the activation energies calculated from kinetic data, E₁ = 21.4 and E₂ = 24.7 kcal · mole^?1 show that the dehydration of the hydrate and the formation of γ are topotactic, this hypothesis is confirmed and the mechanism stated precisely in view of the preceding results.     

In-plane force field of pyridine: Further refinement

The in-plane Urey—Bradley type force field of pyridine has been refined using frequencies of new deuterated species and revised assignments.     

Synthese et etude par RMN 119Sn de nouveaux composes benzyliques organostanniques

Preparations of new benzylic organotin compounds are presented. ¹¹⁹Sn NMR studies show long range coupling constants ¹¹⁹Sn¹¹⁹Sn and intramolecular coordination of tin.     

The vibrational spectrum of (maleic anhydride)iron tetracarbonyl

On the basis of solid-phase IR and Raman spectra, with some solution data for the IR, a reasonably complete vibrational assignment has been made for the modes of maleic anhydride in (maleic anhydride)iron tetracarbonyl. Shifts in v(C=C) and δ(CH) are consistent with a strong interaction with the metal, but relatively little coupling between the modes. More restricted assignments were made for modes associated with the (maleic anhydride)iron and Fe(CO)₄ fragments.     

Normalisation of standard electrode potential series,and evaluation of liquid junction potentials,between different solvents

An experimental procedure for the interrelation of standard electrode potential series in different solvents is described. The standard molar potentials at 25°C of the cesium amalgam electrode in water, ethylene glycol, and methanol are ?1.950, ?1.881, and ?1.800 V, respectively, with ultimate reference to NHE in water. The net contribution of the solvent to the liquid junction potential at the junctions water-ethylene glycol, water-methanol, and methanol-ethylene glycol is 0.028, 0.007, and 0.021 V, respectively (the positive side of the “solvent” junction potential being indicated by the first solvent mentioned for each pair).     

Cyclometallation. Palladium 2-arylpyridine complexes

A number of new cyclometallated compounds of palladium with 2-arylpyridines have been synthesized and characterized. Palladium acetate has been proven to be a more useful starting material than Li₂PdCl₄, since the resulting acetato-bridged dimers, [Pd(OAc)(2-arylpyridine)]₂, unlike the chloro-bridged dimers, are conveniently soluble in common organic solvents. The effect of varying substituents on the aryl nucleus supports the concept that after initial N-complexation, the 2-position of the aryl nucleus undergoes electrophilic attack by the palladium atom. Detailed NMR studies of the soluble acetato compounds showed that the 6-heteroaryl and “ortho” to the Pd-C bond protons in the complexes are shifted (≈0.75 and 0.5 ppm, respectively) upfield from the ligand position. These shielding effects are believed to be due primarily to through-space interactions of overlying aromatic rings and secondarily to through-bond (Pd-to-ligand) effects. Both acetato- and chloro-bridged dimers will react with certain ligands to give mononuclear species containing the intact cyclometallated ligand.     

Stereochemistry of reactions in the 1,2-dimethylsilacyclopentane ring systems. I. Stereospecific transformations

Preparations, separations of geometric isomers, and structural assignments based on nmr and on chemical evidence are described for a number of 1-substituted 1,2-dimethylsilacyclopentanes. A number of stereospecific reactions have been observed, and the stereochemistry is in all cases the same as that observed for acyclic silanes. A discussion of the role of ring strain in determining stereochemical outcome and reaction rates is presented.     

The electrochemical reduction of some fused ring systems

The electrochemical reduction of some fused ring systems containing four carbonyl moieties has been studied in alcoholic aqueous media. A mechanism has been proposed and clarified in which one carbonyl is reduced to a methylene group via an uptake of four electrons.     

Phase distribution in the HfO2Er2O3Ta2O5 system

Phase relationships obtained by heating coprecipitated oxide powders in the HfO₂Er₂O₃Ta₂O₅ system were investigated by X-ray diffractometry. Partial isothermal sections at 1100 and 1500°C are presented.     

Strength of the PtCS2 bond in Pt(PPh3)2(CS2)

The enthalpy of the reaction: Pt(PPh₃)₂(CH₂CH₂)(cryst.) + CS₂(g) → Pt(PPh₃)₂(CS₂)(cryst.) + CH₂CH₂(g) has been determined as ΔH = ? 4.40 ± 2.2 kJ mol^?1 from solution calorimetry, and the bond dissociation energy D(PtCS₂) shown to be slightly greater than D(PtC₂H₄).