L. I. Gubanova

(Aroyloxymethyl)trifluorosilanes: a new class of pentacoordinate silicon compounds

Abstract A new class of organosilicon compounds containing a pentacoordinate silicon atom, (aroyloxymethyl)trifluorosilanes (AFS), has been prepared. The presence of an intramolecular coordinate F3Si ← OC bond is supported by X-ray diffraction, IR spectroscopy and dipole moment data. Si ← O coordination in AFS has been shown to remain intact in the gaseous and liquid states below 420 K as well as in solution in most organic solvents. The dielectric constant of the medium significantly affects the nature of the coordinate Si ← O bond and that of the SiF, CO, COC bonds. In pyridine the intramolecular coordinate Si ← O interaction is disturbed or greatly weakened. At 420–500 K a reversible reaction takes place: ArC OOCH 2 Si F3 α ArCOOCH2SiF3 The changes in enthalpy and entropy are equal in this case: −△H  8.1 ± 0.7 kcal/mol; −°S  15.3 ± 0.8 e.u. T  450 K An assignment of bands in the IR spectra of AFS resulting from vibrations of atoms in the SiF3, >CO and COC groups is given. The electron impact-induced primary disintegration of the molecular ion 4-XC6H4COOCH2Si mainly involves abstraction of a fluorine atom.

Intramolecular coordination in methyl(aroyloxymethyl)fluorosilanes

Abstract The IR spectra of solutions of methyl(aroyloxymethyl)fluorosilanes, C 6 H 5 -COOCH 2 Si(CH 3 )F 2 (I), 4-CH 3 C 6 H 4 COOCH 2 Si(CH 3 )F 2 (II), C 6 H 5 -COOCH 2 Si(CH 3 ) 2 F (III) in 11 solvents in the temperature range 223–363 K, and in the gaseous state have been studied. The molecules of the above compounds display an intramolecular coordination between silicon and oxygen of the carbonyl group. Unlike (aroyloxymethyl)trifluorosilanes [1], there exists the equilibrium between the molecules with and without Si ← O bonds in the solutions of I–III. The proportion of intramolecularly coordinated molecules is greater in solvents of low polarity. This is determined not by enthalpy (Δ H ) values of the Si ← O bond, but a change in the entropy (Δ S ) of the system upon coordination. The coordinate Si ← O bond becomes stronger with increasing number of fluorine atoms in the molecule. A strong dependence of the spectroscopic parameters characterizing the intramolecular coordination on the dielectric constant and the solvent refractive index has been established.

Intramolecular coordination in molecules of C6H5COOCH2CH2SiX3 (X = F, Cl)

Silicon-substituted 2-benzoyloxyethylsilanes, C6H5COOCH2CH2SiX3 with X = F, Cl, OCH3 or 13(OCH2CH2)3N, have been synthesized for the first time. The molecules of compounds with X = F and Cl may exist in two forms, acyclic with a tetracoordinated silicon atom and cyclic with a six-membered ring and a pentacoordinated silicon atom. The cyclic form of the molecules C6H5COOCH2CH2SiX3, with X = F, Cl, are present in solutions of these compounds in non-polar solvents, the proportion increasing with decreasing temperature.

Vibrational spectra of (O → Si)-(aroyloxymethyl)trifluorosilanes

Abstract The IR spectra of (aroyloxymethyl)trigfluorosilanes 4-X-C 6 H 4 COOCH 2)SiF(in3 (AFS) containing an intramolecular O → Si coordinate bond, have been examined in the 500-200 cm −1 region. The band near 330 cm −1 is assigned to the O → Si bond stretching vibration because of its λ sensitivity to changes in the polarity of the medium and because of its response to the nature of substituent X as well as to the formation of AFS-pyridine complexes. Normal-coordinate analysis of a model compound HCOOCH 2 SiF 3 has been carried out. By use of a set of frequency partial derivatives by force constants a force field corresponding to the experimental frequencies and equilibrium geometry of 4-F-C 6 H 4 COOCH 2 SiF 3 has been selected. The v (O → Si) stretching vibration is delocalized by three low-frequency motions 360, 330 and 218 cm −1 . The effect of electronic and kinematic factors on the AFS vibrational spectra has been evaluated.

The interactions of (=O→Si)-(benzoyloxymethyl)trifluoro-and -(benzoyloxymethyl) methyldifluorosilanes with a solvent

The IR spectra of solutions of (=O→Si)-(benzoyloxymethyl)trifluorosilane (1),-(benzoyloxymethyl)methyldifluorosilane (2), and butyl benzoate (3) are examined in the region of thev(C=O) stretching vibrations in 24 solvents. The ability of compounds1—3 to undergo specific intermolecular interactions is evaluated from the dependence ofv(C=O) on the Kamlet-Taft (π*,α, β) parameters, which was obtained for the carbonyl groups involved in the intramolecular coordinate (=O→Si) bond and for free carbonyl groups. The corresponding values of the coefficients in the Kamlet-Taft equations are indicative of a weak ability of pentacoordinate silicon compounds1 and2 to undergo acid-base interactions.

Rotational isomerism in ortho-substituted (benzoyloxymethyl)trifluorosilanes containing a pentacoordinated silicon atom

Conclusions1.In the gaseous state and in an organic solvent ortho-substituted (benzoyloxymethyl)-trifluorosilanes 2-X-C6H4COO·CH2SiF3 (X=Cl, OMe) exist as mixtures of rotational isomers with the cis and trans orientation of the substituent X and the carbonyl group. In solution both isomers contain an intramolecular coordination bond.2.The value of the parameter B, which characterizes the dipole-dipole interaction between the compounds and the medium, is larger for the cis isomer of the methoxy derivative than for the less polar trans isomer. The contribution from the induction and orientation components to the energy of the van der Waals interaction between the investigated molecules and the medium was estimated on the basis of the temperature dependence of the parameter B.

New fluorine-containing pentacoordinate silicon compounds

Abstract Dimethyl(trifluoroacetoxymethyl)acyloxysilanes, F 3 CCOOCH 2 Me 2 SiOCOR (R = Me, CF 3 ) with a pentacoordinate Si atom (according to 29 Si NMR data) have been synthesized. A new type of pentacoordinate silicon derivatives, organyltetra- acyloxysilicates, M + [XCH 2 Si(OCOF 3 ) 4 ] − and dimethyltris(trifluoroacyloxy)silicates, M + [Me 2 Si(OCOCF 3 ) 3 ] − were revealed for the first time by 29 Si NMR spectroscopic titration of XCH 2  Me 3−n Si(OCOF 3 ) n (X = H, Cl, Br, I; n = 2, 3) with trifluoroacetic and acetic salts, RCOOM (M = NH(C 2 H 5 ) 3 , N(C 2 H 5 ) 4 , Na). The reaction of XC 6 H 4 COOCH 2 SiMe 3−n (OR) n (n = 1−3) with SF 4 , HF, BF 3 affords XC 6 H 4 COOCH 2 SiMe 3−n F n . These compounds can also be prepared by the reaction of ClCH 2 SiMe 3−n F n (n = 1−3) with XC 6 H 4 K. Judging by X-ray diffraction, the silicon atom in XC 6 H 4 COOCH 2 SiMe 3−n F n (n = 2, 3) is pentacoordinate. The presence of intramolecular coordinate Si←O bond is clearly manifested in the IR and 13 C, 17 O, 19 F and 29 Si NMR spectra of these compounds. In non-polar solvents C 6 H 5 COOCH 2 SiF 3 forms l:1 complexes with organic bases such as pyridine, triethylamine, 2,2′-bipyridine, 2-phenanthroline, N,N,N′,N′-tetra- methylethylenediamine. The reaction of (aroyloxymethyl)tri- fluorosilanes with KF, CsF and NH 4 F gives the corresponding (aroyloxymethyl)tetra- and pentafluorosilicates, M + n [XC 6 H 4 COOCH 2 SiF 3+n ] n− (M = K, CsF, NH 4 ; n = 1,2). The IR and UV spectra suggest the silicon atom in (aroyloxymethyl)- pentafluorosilicates to be heptacoordinate.

INFLUENCE OF SOLVENT ON IR SPECTRA OF AROYL(HYDROXYMETHYL)TRIFLUOROSILANES

ConclusionsDipole-dipole interaction of the solvent with molecules of aroyl (hydroxymethyl) trifluorosilanes 4-XC6H4COOCH2SiF3 is responsible for changes in frequency of the stretching vibrations of groups included in a five-membered ring, containing an intramolecular coordination bond Si ← 0 (the frequencies νC=O and νSiFa, are lowered, and the frequencies νasCOC are raised). The value of the orientation-induction parameter in the Buckingham equation, reflecting the dependence of νC=O and νSiFa on the parameters of the medium (ɛ, n), increases with increasing electron-donor capability of the substituent X, indicating an increase in the strength of the coordination bond. The coefficient ρ in the Hammett equation is determined by the polar properties of the medium.

Dipole moments and molecular structure of (aroyloxymethyl)-trifluorosilanes and methyl (aroyloxymethyl) fluorosilanes

Conclusions1.The dipole moments of the Si ← O=bond in (aroyloxymethyl)trifluorosilanes and the heats of its formation (−ΔH) for halo-substituted molecules were determined.2.It was shown that methyl(aroyloxymethyl)fluorosilanes are an equilibrium mixture of two forms of molecules with a tetra- and a pentacoordinated silicon atom.

Spectroscopic manifestations of the electronic effect of the substituent X in the molecules of (aroyloxymethyl)-trifluorosilanes and methyl (aroyloxymethyl) fluorosilanes 4-X-C6H4COOCH2Si(Me)3−nFn

Conclusions1.In the spectra of (aroyloxymethyl) trifluorosilanes and methyl (aroyloxymethyl) fluorosilanes with the general formula 4-X-C6H4COOCH2Si(Me)3−nFn (n=1–3) the variation of the ΝC=O frequencies with variation of the substituent X in the aromatic ring is significantly larger in the spectra of the cyclic molecules with an intramolecular Si ↞- O coordination bond than for the acyclic molecules containing a free C=O group and a tetracoordinated silicon atom.2.The frequencies for the stretching vibrations of the COC and SiF groups in the spectra of the molecules with the pentacoordinated silicon atom depend on the nature of the substituent X. Alternation between positive and negative values of the coefficient ρ is observed in the Hammett equations relating the frequencies and the σP values: ρ > 0 (ΝC=O, SiFax), ρ < 0 (ΝCOC).