Facile activation of H-H and Si-H bonds by boranes

The borane B(C(6)F(5))(3) is a precatalyst for H/Dexchange between H(2) and deuterium-labeled silanes (D(3)SiPh, D(2)SiMePh, DSiMe(2)Ph, DSiEt(3)). Experimental and DFT studies reveal that B(C(6)F(5))(3) itself cannot activate dihydrogen but converts to HB(C(6)F(5))(2) under the action of hydrosilane. The latter species easily activates H-H and Si-H bonds by a σ-bond metathesis mechanism, which was further confirmed by the reactions of BD(3)·THF with H(2).     

The addition of cyclic siloxanes containing Si-H bonds to unsaturated compounds

Summary We have obtained four new addition products of cyclic siloxanes containing the Si-H bond to unsaturated compounds of different classes. In the presence of chloroplatinic acid addition takes place according to the Farmer rule with formation of one adduct in each case.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12 p. 2230–2232, December, 1964     

Vibration frequencies of Si-H and Si-D bonds and the electronegativity of silyl groups

Summary A simple empirical relation is put forward for relating the vibration frequencies of Si-H and Si-D and also Ge-H and Ge-D bonds to the electronegativities of silyl and germyl groups.     

Dynamic NMR study of the rotation around "biphenyl-type" bonds in polycyclic sulfoxides

In a recent paper, we reported on the base-catalyzed rearrangement of bis-propargylic sulfoxides that eventually leads to polycyclic products featuring an unsaturated, cyclic substituent such as cyclohexenyl or phenyl. Due to steric constraints, the latter is positioned roughly perpendicularly to the tricyclic core, and in most cases, two rotamers can be observed in the ground state. In the present work, we report on the synthesis and the products of both symmetrical and asymmetrical starting materials. We also measure, by NMR techniques, the rotation rate of the side chain for several such polycyclic sulfoxides. The barriers for this process, which is similar to a biphenyl rotation, are very strongly dependent on the nature of the substrate, ranging between <7 and 21.0 kcal.mol(-1) for sulfoxides with two five-membered rings and two seven-membered rings, respectively. These barriers can be successfully simulated by molecular-mechanics calculations, and the geometries of the transition states are discussed.     

Cationic hafnium silyl complexes and their enhanced reactivity in sigma-bond metathesis processes with Si-H and C-H bonds

Reaction of the mixed-ring silyl methyl complex CpCp*Hf[Si(SiMe3)3]Me (4) with B(C6F5)3 in bromobenzene-d5 yielded the zwitterionic hafnium silyl complex [CpCpHfSi(SiMe3)3][MeB(C6F5)3] (7), which is stable for at least 12 h in solution. Addition of PhSiH3 to 7 rapidly produced HSi(SiMe3)3, CpCp*HfH(mu-H)B(C6F5)3, and oligomeric silane products. Reactions of CpCp*Hf(SiR3)Me (SiR3 = SitBuPh2, SiHMes2) with B(C6F5)3 rapidly produced HSiR3 in quantitative yield along with unidentified hafnium-containing species. However, reactions of Cp2Hf(SiR3)Me (SiR3 = Si(SiMe3)3 (8), SitBuPh2 (9), SiPh3 (10)) with B(C6F5)3 quantitatively produced the corresponding cationic hafnium silyl complexes 12-14. The complex Cp2Hf(SitBuPh2)(mu-Me)B(C6F5)3 (13) was isolated by crystallization from toluene at -30 degrees C and fully characterized, and its spectroscopic properties and crystal structure are compared to those of its neutral precursor 9. The sigma-bond metathesis reaction of 13 with Mes2SiH2 yielded HSitBuPh2 and the reactive species Cp2Hf(eta(2)-SiHMes2)(mu-Me)B(C6F5)3 (16, benzene-d6), which was also generated by reaction of Cp2Hf(SiMes2H)Me (11) with B(C6F5)3. Spectroscopic data provide evidence for an unusual alpha-agostic Si-H interaction in 16. At room temperature, 16 reacts with benzene to form Cp2Hf(Ph)(mu-Me)B(C6F5)3 (17), and with toluene to give isomers of Cp2Hf(C6H4Me)(mu-Me)B(C6F5)3 (18-20) and Cp2Hf(CH2Ph)(mu-Me)B(C6F5)3 (21). The reaction with benzene is first order in both 16 and benzene. Kinetic data including activation parameters (deltaH = 19(1) kcal/mol; deltaS = -17(3) eu), a large primary isotope effect (kH/kD = 6.9(7)), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.     

Second-order properties of bonds

By defining a localized hamiltonian for a saturated molecule its second order molecular properties may be regarded as a sum of bond contributions. Static and dynamic polarizabilities are calculated for the CH bond in methane and the latter one used to calculate the van der Waals interaction between two CH₄ molecules.     

Bond properties of metal-ligand bonds

An empirical relation has been developed to compute the bond order of ionic systems. The computed values have been compared with those calculated by using the experimental values of bond polarizability derivatives and are found to be in fair agreement. This shows that the proposed empirical relation for computing bond orders is quite suitable for ions.

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Bindungseigenschaften von Metall-Ligand-Bindungen

Zusammenfassung Es wurde eine empirische Beziehung entwickelt, die die Berechnung von Bindungsordnungen für ionische Systeme erlaubt. Ein Vergleich der berechneten Werte mit solchen, die aus experimentellen Daten für Differentiale von Bindungspolarisierbarkeiten erhalten wurden, zeigte die Brauchbarkeit der Methode für Ionen.

     

1H NMR spectroscopic study of hydrogen bonds and mobile NH protons in aqueous solutions of porphyrin ions

The concentration and temperature dependence of the chemical shifts of aromatic and -protons of tetra-meso substituted porphyrin ions in aqueous solution have been investigated. A considerable increase in the localization temperatures of interior NH protons of porphyrins dissolved in water compared to solutions of porphyrins in organic solvents was observed. Types of association of the porphyrins studied and the possibility of intra- and intermolecular hydrogen bonds are discussed.State Science Center, Institute of Biophysics, Ministry of Public Health, Moscow 123182. Institute of Biological and Medicinal Chemistry, Russian Academy of Medical Science, Moscow 119832, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 535–541, April, 1998.     

Bond functions and the topological properties of the bonds

The effect of bond functions on the topological properties of the bonds is studied by performing Hartree-Fock calculations for O₂, Cl₂, F₂, CN^–, N₂ and LiCl molecules. The results show that bond functions increse the degree of covalency of the bonds and the electronic density at the (3, –1) bond critical point. This effect is quite noticeable for intermediate interactions molecules (F₂ and CN^–) where a is obtained instead of a positive value. The optimal position of the bond functions seems to be at or near the (3, –1) bond critical point.     

The polar properties of valence bonds

Arguments based on bond-stretching frequencies, nuclear quadrupole coupling constants and bond lengths indicate that bonds having the same strength factor can have different electronic structures. It is then argued that a more useful probe for obtaining an insight into bond character is provided by infrared band intensities which can be analyzed to yield values for the constants occurring in bond moment functions. A new bond moment theory, in which the concept of “rehybridization moments” is introduced, is described. Using this theory, self-consistent values are obtained for the bond moment constants governing the band intensities for the molecules HCN, C₆H₆ and C₂H₄.