J. Colin Young

Pentacoordinate silicon compounds: Stereochemical non-rigidity of chelates formed by intramolecular ring-closure. Crystal structure of 8-dimethylamino-1-trifluorosilylnaphthalene☆

The application of dynamic NMR spectroscopy to the study of stereochemical non-rigidity in pentacoordinate chelated organosilicon compounds is described. It is shown that in the compounds Me2NCH(Me)C6H4SiXYZ, non-dissociative ligand permutation at silicon can be distinguished unambiguously from processes associated with rupture of the chelate ring and nitrogen inversion. The crystal and molecular structure of 8-Me2NC10H6SiF3 has been determined. Pentacoordination of the silicon atom is confirmed, with the donor nitrogen atom and a fluorine atom occupying axial sites in an overall trigonal bipyramidal geometry. The N → Si separation is 2.3 A (average of two distinct but closely related molecular conformations), which is less than the C1C8 distance in the naphthalene nucleus, indicating a substantial bonding interaction. NMR studies of the dynamic behaviour of the Me2N group, and where possible (19F, 1H) of the monodentate ligands in 8-dimethylamino-1-silylnaphthalene compounds, together with the results for the chelated benzylaminosilicon compounds, confirm that inversion of the absolute configuration at the silicon atom is not achieved by this process. The free energies of activation for non-dissociative ligand permutation at a silicon range from less than 7 kcal mol−1 [SiH3, Si(OR)3], which is below the limit of direct measurement, to 13 kcal mol−1 for Me2NCH(Me)C6H4SiF3; difunctional silicon chelate compounds (Cl, F, OR) display values from 9–12 kcal mol−1. These are comparable with those determined for fluxional processes in acyclic pentacoordinate silicon compounds.

Pentacoordinated silicon compounds. Intramolecular ring closure, site preferences of substituents and the stability of the resulting chelates

Dynamic NMR studies have been made of chelation in 2-(dimethylaminomethyl)phenylsilanes and 2-[1-(dimethylamino)ethyl]phenylsilanes with a wide range of substituents on the silicon atom. Temperature-dependent 19F spectra of compounds of the type Me2NCH2C6H4SiMeFX in which the geometry about the silicon atom is trigonal bipyramidal with the donor nitrogen atom axial, have established the preference of the substituent X for the axial site trans to the donor nitrogen atom, relative to that of the fluorine atom. Combined with other structural data this leads to te apicophilicity series: H < alkyl < aryl < OR,NR2 < F ≈ SR < Cl,OCOR. It is concluded that the apicophilicity is closely correlated with the ability of the bond trans to the donor atom to be stretched by electron donation to the central atom. The stability of the chelates has been investigated by dynamic 1H NMR spectroscopy of the potentially diastereotopic NMe2 groups. When the ligand that occupies the site trans to the donor nitrogen atom is electronegative, the stability is largely determined by the nature of that ligand, and increases with the apicophilicity. Intramolecular coordination is barely detectable in the case of an alkoxy group. Compounds containing only alkyl and aryl substituents in addition to the bidentate ligand are also not chelated, but dihydrogeno-alkyl (or -aryl) derivatives, in which the hydrogen atoms occupy the equatorial sites, and trihydrogeno derivatives, are relatively strongly coordinated.

Synthesis, chemical behaviour and crystal structure of cobalt-stabilized carbenes, (Ph3Ge)(CO)3CoC(OEt)R

Abstract Cobalt stabilized carbenes are readily synthesized by reaction of organolithium compounds with (CO) 4 CoGePh 3 . The neutral carbenes are formed from the anion thus obtained by treatment with an ethylating reagent. Anions α to the carbene atom can be generated by treatment with a strong base (NaH, RLi) and trapped with Et 3 OBF 4 . However attempts to trap both types of anions with chlorosilanes failed, and instead the compound (CO) 4 CoGePh 3 was recovered in good yield. An X-ray structural determination of one of these complexes, (Ph 3 Ge)(CO) 3 Co(OEt)Et, shows unusually short bond lengths in the carbene moiety and the linkage trans to the triphenylgermyl group. The unusual chemical behaviour can be related to special features of the structure of the complex.

The chemical behaviour of cobalt-stabilized carbenes having a trisubstituted silyl or germyl ligand. Stereospecific formation of benzoylsilanes from the reaction of organosilyl-cobalt tetracarbonyl derivatives with phenyllithium

Abstract We report the chemical behavior of cobalt-stabilized carbenes, R 3 M(CO) 3 CoC(OEt)R′, and their parent anions, R 3 M(CO) 3 CoC(O − )R′, where M = Si or Ge. The anions where M = Si, R′ = Ph decompose thermally into the corresponding benzoylsilanes; when the silicon atom is chiral (R 3 = MePh-1-Np) optically active R 3 SiCOPh is obtained with complete retention of configuration.

Five-co-ordinated silicon compounds: geometry of formation by intramolecular co-ordination. Crystal structure of 2-(dimethylaminomethyl)phenyl-1-naphthylsilane

Studies of a wide range of intramolecularly five-co-ordinated silicon compounds with chelating nitrogen donors have been made, with a view to obtaining further information on the details of nucleophilic substitution reactions at silicon. The crystal structure of 2-(dimethylaminomethyl)-phenyl-1 -naphthylsilane has been determined by single-crystal X-ray diffraction (R= 0.074 for 564 observed reflections). Crystals are tetragonal, space group I, with Z= 8 in a unit cell of dimensions a= 21.845(5) and c= 7.119(2)A. The central silicon atom shows essentially trigonal-bipyramidal co-ordination, with the axial positions occupied by the naphthyl group and the donor nitrogen atom, and the two hydrogen atoms occupying equatorial positions. Comparison with other structures reported in the literature shows that this geometry, always with axial disposition of the donor atom, is generally adopted by molecules of this type. Silicon-29, 19F, and 1H n.m.r. spectroscopic data for similar compounds with varying substituents on the silicon atom show that the same geometry is adopted by these intramolecularly co-ordinated species in solution.