Angewandte
Chemie
precursor. Strikingly, 1 is capable of reacting with ammonia
ꢀ
under replacement of the dmap ligand and subsequent N H
=
addition to the Si O bond to give the corresponding sila-
hemiaminal L’Si(OH)NH2 and its silacarboxylic amide tau-
tomer C LSi( O)NH2 (Scheme 1).[9c] The remarkable forma-
=
tion of C is due to the pronounced basicity of the exocyclic
methylene group of the C3N2 chelate ligand L’.
The latter results prompted us to probe whether 1 is
capable of activating H2S and H2O to form the corresponding
silacarboxylic acids B (X = S, O; Scheme 1). Herein, we
report the facile synthesis and structural characterization of
the first stable silathiocarboxylic acid–base adduct and its
metalation to form a silathiocarboxylate. In addition, we
describe for the first time the formation of a donor–acceptor
stabilized silacarboxylic acid.
Treatment of a pale yellow suspension of 1 in THF with an
excess of dry hydrogen sulfide gas at room temperature led to
a clear yellow solution, from which complex 2 could be
obtained as yellow crystals in 90% yield (Scheme 2).
The mechanism is unknown but we assume that 2 is
formed via the intermediate 2’, which results from the
Figure 1. Molecular structure of 2. Thermal ellipsoids are drawn at
50% probability level. Hydrogen atoms, except that at O1, are omitted
for clarity. The bridging hydrogen atom at O1 could be localized in the
difference Fourier maps. Selected bond lengths [ꢂ] and angles [8]: Si1–
S1 1.993(1), Si1–O1 1.620(2), Si1–N2 1.809(2), Si1–N1 1.827(2), C1–
C2 1.514(3), C2–C3 1.394(3), C3–C4 1.389(3), C4–C5 1.499(3), O1–N3
2.703; O1-Si1-N2 102.63(9), O1-Si1-N1 102.27(8), N2-Si1-N1 97.46(8),
O1-Si1-S1 120.49(7), N2-Si1-S1 114.90(7), N1-Si1-S1 115.76(7), O1-
H1-N3 172.0.
=
addition of H2S to the Si O bond in 1. Subsequent isomer-
ization of 2’ by a 1,5-proton shift from the thiol group to the
exocyclic methylene group affords 2 as the final product. The
driving force for the tautomerization process originates both
from the pronounced Brønsted acidity of the SH group and
the basicity of the methylene group of the C3N2 ligand in 2’.
bond lengths of 1.827(2) and 1.809(2) ꢁ, respectively, are
similar to the corresponding values observed in the silathio-
carboxylic silylester LSi( S)OR (1.819(3), 1.833(4) ꢁ).[7b] The
=
ꢀ
ꢀ
bond lengths of C1 C2 (1.514(3) ꢁ) and C4 C5 (1.499(3) ꢁ)
confirm the presence of two exocyclic methyl groups.
ꢀ
The 1,5-proton shift is akin to that observed for the formation
Accordingly, the short Si S bond length of 1.993(1) ꢁ is
[8]
of C[9c] (Scheme 1) and the silathioformamide, LSi( S)H.
akin to those in the silathiocarboxylic silylester LSi( S)OR
=
=
[8]
(1.980(2) ꢁ)[7b] and LSi( S)H (1.985(1) ꢁ), and indicates a
=
Compound 2 is soluble in THF and dichloromethane,
marginally soluble in toluene but insoluble in n-hexane. The
constitution of 2 could be determined by multinuclear NMR
and IR spectroscopy, and its composition was proven by
electron-impact (EI) mass spectrometry and elemental anal-
ysis. The EI mass spectrum of 2 shows a base signal for the
=
significant Si S bond character. In contrast, the
ꢀ
Si1 O1 distance of 1.620(2) ꢁ in 2 is much larger than the
=
Si O length in the starting material 1 (1.545(2) ꢁ) and thus
ꢀ
represents a Si O single bond.
Notably, 2 does not undergo a 1,3-proton shift to give the
=
=
dmap-free species at m/z 495 [{LSi( S)OH} + H]. Mean-
possible LSi( O)SH···dmap isomer. It seems unlikely that the
while, the H and 13C NMR spectra of 2 (see the Supporting
Information) for the b-diketiminate ligand L exhibit similar
isomerization is prevented by the presence of a base because
1
base-free A remains stable in solution.[6d] In contrast,
=
chemical shifts to those reported for the germanium analogue
thiocarboxylic acids RC( S)OH are in equilibrium with the
=
A. The proton of the OH group in
2
resonates at
corresponding isomers RC( O)SH in solution through rapid
d = 6.35 ppm, that is, at a significantly lower field than that
proton migration between the oxygen and the sulfur atoms.[10]
The preference of LSi( S)OH 2 over LSi( O)SH is most
[6d]
in LGe( S)OH A (d = 2.0 ppm). The 29Si NMR spectrum
=
=
=
ꢀ
of 2 reveals a resonance signal at d = ꢀ30.0 ppm akin to the
likely a result of the weaker pp pp bond and the higher bond
[7c]
=
=
=
value observed for the silathiocarboxylic silylester LSi(
polarity of the Si O compared with the Si S subunit.
Compound 2 reacts easily with organometallic bases to
form metal silathiocarboxylates. Thus, treatment of 2 with one
molar equivalent of AlMe3 in toluene at ambient temperature
S)OR, which bears the same ß-diketiminate ligand L
(d=ꢀ41 ppm).[7b]
The molecular structure of 2 has been elucidated by
single-crystal X-ray diffraction analysis. The compound
crystallized in THF as a mono-THF solvate in the triclinic
=
furnished the silathiocarboxylate LSi( S){OAlMe2(dmap)} 3
with the dmap coordinated at the aluminum atom
(Scheme 2), as confirmed by 1H NMR spectroscopy. Reaction
of 2 with MeLi or LiN(SiMe3)2 resulted only in the formation
of inseparable mixtures. Complex 3 was isolated in high yield
(91%) in the form of colorless crystals and could be fully
characterized (1H, 13C, and 29Si NMR, IR, EI-MS, and
elemental analysis). Compound 3 is soluble in benzene,
toluene, and THF, but insoluble in n-hexane. In the 1H NMR
spectrum of 3, a typical high-field singlet can be observed for
the two methyl groups of the AlMe2 moiety at d = ꢀ0.57 ppm.
The 29Si NMR spectrum shows a slightly up-field-shifted
¯
space group P1. The compound bears one dmap ligand
ꢀ
connected to the silathiocarboxylic acid through an O H···N
hydrogen bond, and the O···N distance of 2.703 ꢁ falls into
ꢀ
the normal range of O H···N hydrogen bonds (Figure 1).
Obviously, the hydrogen bridge stabilizes the system and
prevents the dimerization process, which is usually observed
for carboxylic acids and has also been observed for the “base-
free” germanium analogues A.[6d] The six-membered SiN2C3-
ring in 2 is strongly puckered and the silicon atom possesses a
ꢀ
distorted tetrahedral coordination environment. The Si N
Angew. Chem. Int. Ed. 2010, 49, 6642 –6645
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim