NMR SPECTRA OF GUEST–HOST COMPLEXES
151
to acetone in their proton-accepting capacity. Such an
Hence, based on the spectral data obtained, one can
assignment is indirectly confirmed by the observed dif- conclude that molecular complexes I and II undergo
ferences between the concentrations of the partial solvolysis in acetone, probably, because of the
H3O[BF3OH] complex in the solutions of I and II. The breakdown of the H-bonding systems stabilizing these
greater number of water molecules contained in com- complexes, while the ionic complex III remains virtu-
pound I favors the formation of this complex. Each 19F ally unaffected by solvolytic transformations.
signal is split into two components spaced by 0.07 ppm
with an intensity ratio of ~1 : 4. This is a result of a 10B–
REFERENCES
11B isotopic shift (the natural abundance of 10B isotopes
is 18.83%, while that of 11B is 81.17%) [9]; the signal
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from the 19F nuclei bound to 11B appears in the higher
19
magnetic field. The character of the F NMR spectra
excludes BF3-catalyzed aldol condensation of acetone
in the solutions of compounds I and II [10], which
gives, apart from other products, a BF3 complex with
mesitylene oxide (δ(19F) = –145.5 ppm).
The 19F NMR spectrum of a solution of complex III
shows a singlet at δ(19F) = –138.4 ppm corresponding
to the pentafluorosilicate anion [11]. In the available lit-
erature, the δ(29Si) values for SiF–5 anion are lacking;
however, the chemical shift of a singlet in the 29Si NMR
spectrum (–112.9 ppm) is very close to the published
δ(29Si) values for pentacoordinated alkyl- and aryltet-
rafluorosilicates [RSiF4]– (–109.0 to –126.1 ppm) [12].
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tions containing salts of rather large cations [13], and in
this connection, our spectral data clearly indicate the
presence of the initial pentafluorosilicate III in the
solution, rather than its decomposition product, e.g.,
acetone-solvated ionic pair (H3O)SiF5. Note that the
observed 19F, B, Si signals show no fine structure
caused by 19F–11B and 19F–29Si spin–spin coupling,
which suggests the intermolecular exchange of fluoride
ligands in the solutions of I–III which is catalyzed by
water molecules and hydroxonium ions [14].
10. Gillespie, R.J. and Hartman, J.S., Can. J. Chem., 1968,
vol. 46, no. 24, p. 3799.
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Chem., 1989, vol. 28, no. 16, p. 3182.
11
29
13. Gel’mbol’dt, V.O. and Ennan, A.A., Usp. Khim., 1989,
vol. 58, no. 4, p. 626.
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RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27
No. 3
2001