Zwitterionic and anionic multinuclear pentacoordinate silicon(IV) complexes with bridging (R,R)-tartrato(4-) ligands and SiO5 skeletons: Synthesis and reactivity in aqueous solution

Article abstract of DOI:10.1002/chem.201102815

Two nutrients in one molecule: A zwitterionic λ⁵Si, λ⁵Si′-disilicate (1) was synthesized and characterized. It contains ligands that exclusively derive from natural products ((R,R)-tartaric acid, choline). Hydrolysis of 1 yields 2, which shows a remarkable kinetic stability in water. Upon dissolution of 1 and 2 in water, the nutrients choline and orthosilicic acid are formed by hydrolysis Copyright

Full text of DOI:10.1002/chem.201102815

DOI: 10.1002/chem.201102815
Zwitterionic and Anionic Multinuclear Pentacoordinate Silicon(IV)
Complexes with Bridging (R,R)-Tartrato(4ꢀ) Ligands and SiO5 Skeletons:
Synthesis and Reactivity in Aqueous Solution
Bastian Theis, Jçrg Weiß, W. Peter Lippert, Rꢀdiger Bertermann,
Christian Burschka, and Reinhold Tacke*[a]
Apart from a few exceptions, pentacoordinate silicon(IV)
compounds are very sensitive to water and undergo rapid
hydrolysis.[1] In continuation of our ongoing search for pen-
tacoordinate silicon(IV) complexes that exist in aqueous so-
lution (for selected publications, see ref. [2]), we have now
succeeded in synthesizing the dinuclear pentacoordinate sili-
con(IV) complexes 1 and 2 (Scheme 1). The zwitterionic
l5Si,l5Si’-disilicate 1 with its two SiO5 skeletons contains li-
gands that exclusively derive from natural products ((R,R)-
tartaric acid, choline). Upon dissolution in water at ambient
temperature (up to a concentration of ca. 1m), compound 1
undergoes rapid hydrolysis to form the dianionic l5Si,l5Si’-
disilicate 2, which shows a remarkable kinetic stability in
aqueous solution. At ambient temperature, it hydrolyzes
very slowly over a period of more than 10 days to afford
(R,R)-tartaric acid, choline, and orthosilicic acid.
Silicon is nowadays claimed to have beneficial effects on
several human diseases, for example, osteoporosis, ageing of
skin, hair, and nails, and arteriosclerosis.[3] Orthosilicic acid
is the major silicon species present in drinking water and is
the most readily available source of silicon to man. In this
context, compounds 1 and 2 may be of interest as potential
dietary and food supplements that upon hydrolysis yield the
nutrient orthosilicic acid. Choline is also known to be crucial
for normal functioning of the mammalian organism and is
an essential nutrient as well.[4] Therefore, compounds 1 and
2 may also be of interest as potential choline dietary and
food supplements.
Compound 1 was synthesized according to Scheme 1: tet-
ramethoxysilane was treated with choline (R,R)-trihydro-
gentartrate (molar ratio 1:1) in boiling N,N-dimethylforma-
mide and isolated in 83% yield as a colorless, high-melting
solid (m.p.>4008C). Dissolution of 1 in water at ambient
temperature and immediate freeze drying of the resultant
aqueous solution afforded 2 in 94% yield as a colorless
solid (purity ca. 98%). Attempts to remove the water by
using a rotary evaporator (458C, 70 mbar, 5.5 h) resulted in
a significantly reduced purity of 2.
The absolute configuration of the two (R,R)-tartrato(4ꢀ)
ligands controls the absolute stereochemistry at the two sili-
con atoms (both with D configuration) of compounds 1 and
2, which were obtained as diastereomerically and enantio-
merically pure products. Due to its zwitterionic nature, com-
pound 1 is almost insoluble in nonpolar organic solvents and
also exhibits a very poor solubility in most polar organic sol-
vents, whereas 2 is well soluble in DMSO. The identities of
1 and 2 were established by elemental analyses (C, H, N)
and NMR spectroscopic studies in the solid state (13C, 15N,
29Si) and in solution (1H, 13C, 29Si), and 1 was additionally
characterized by a crystal-structure analysis of the solvate
1·3DMSO.[5,6] The identity of 2 was further established by
cryoscopic studies and ESI-MS experiments (see below).
The zwitterionic (D,D,R,R,R,R)-configured l5Si,l5Si’-disili-
cate 1 (Figure 1) contains two SiO5 skeletons, with two pen-
Scheme 1. Synthesis of the dinuclear, pentacoordinate, silicon(IV) com-
plexes 1 and 2.
[a] Dr. B. Theis, J. Weiß, Dr. W. P. Lippert, Dr. R. Bertermann,
Dr. C. Burschka, Prof. Dr. R. Tacke
Universitꢀt Wꢁrzburg, Institut fꢁr Anorganische Chemie
Am Hubland, 97074 Wꢁrzburg (Germany)
Fax : (+49)931-31-84609
E-mail: r.tacke@uni-wuerzburg.de
2202
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2202 – 2206

COMMUNICATION
Figure 1. Molecular structure of 1 in the crystal of 1·3DMSO (probability
level of displacement ellipsoids 50%). Selected bond lengths [ꢃ] and
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: Si1 O1 1.789(2), Si1 O2 1.666(2), Si1 O3 1.813(2), Si1 O4
ꢀ
ꢀ
ꢀ
ꢀ
1.660(2), Si1 O5 1.636(2), Si2 O8 1.801(2), Si2 O9 1.658(2), Si2 O10
ꢀ
ꢀ
1.807(2), Si2 O11 1.666(2), Si2 O12 1.647(3); O1-Si1-O2 89.43(11), O1-
Si1-O3 177.36(12), O1-Si1-O4 90.53(12), O1-Si1-O5 88.15(13), O2-Si1-O3
88.54(11), O2-Si1-O4 121.76(12), O2-Si1-O5 119.91(15), O3-Si1-O4
89.11(10), O3-Si1-O5 94.32(13), O4-Si1-O5 118.30(14), O8-Si2-O9
89.42(12), O8-Si2-O10 176.59(12), O8-Si2-O11 88.29(11), O8-Si2-O12
89.04(13), O9-Si2-O10 90.20(12), O9-Si2-O11 122.48(12), O9-Si2-O12
119.57(15), O10-Si2-O11 89.06(11), O10-Si2-O12 94.09(13), O11-Si2-O12
117.84(15).
Figure 2. Kinetics of the hydrolysis of 2 in 1) D2O at two different con-
centrations (10 mm black circles and 100 mm blue circles) and 2) a 0.1m
solution of DCl in D2O (10 mm red circles) at 238C. The kinetic data
1
were extracted from H NMR spectra (500.1 MHz), in which the CH res-
onance signals of 2 and those of the free (R,R)-tartaric acid served as the
probe. The measurements were started immediately upon dissolution of
the precursor 1.
tacoordinate (formally negatively charged) silicon atoms
and two tetracoordinate (formally positively charged) nitro-
gen atoms. The two tetradentate (R,R)-tartrato(4ꢀ) ligands
bridge the two silicon(IV) coordination centers, and one 2-
(trimethylammonio)ethanolato ligand is bound to each of
the two silicon atoms. The two silicon coordination poly-
hedra of 1·3DMSO are best described as distorted trigonal
bipyramids (transition trigonal bipyramid ! square pyra-
mid: 6.6% (Si1, pivot atom O5) and 8.2% (Si2, pivot atom
O12)).[7] The axial sites (O1, O3; O8, O10) are occupied by
the carboxylato oxygen atoms of the two (R,R)-tartrato(4ꢀ)
ably high kinetic inertness of 2 against hydrolysis is probably
founded in the chelate effect of the (R,R)-tartrato(4ꢀ) li-
gands that bridge the two silicon(IV) coordination centers.
In this context, it is interesting to note that the kinetic sta-
bility of the pentacoordinate silicon complex 2 is significant-
ly higher than that of the tetracoordinate silicon compound
tetramethoxysilane (SiACTHNUTRGNE(UGN OMe)4). This has been demonstrated
by additional kinetic studies (1H NMR spectroscopic analy-
sis; 300.1 MHz; 238C) with an equimolar mixture of 2 and
SiACHTUNGTRENNUG
ꢀ
ligands. The axial Si O bond lengths (1.789(2)–1.813(2) ꢃ)
pends on the pH value (Figure 2). Studies of solutions with
are significantly longer than the equatorial ones (1.636(2)–
1.666(2) ꢃ). The maximum deviations from the ideal 908,
1208, and 1808 angles of the two SiO5 skeletons amount to
4.32(13)8, 2.48(12)8, and 3.41(12)8, respectively.
Cryoscopic studies of a freshly prepared 33 mm aqueous
solution of 1 revealed a rapid hydrolysis of the zwitterion.[8]
The freezing-point depression measured clearly demonstrat-
ed the presence of three particles (2; two cations and one
dianion) instead of one particle (1; one zwitterion). This
finding is supported by the results of ESI-MS studies of a
freshly prepared aqueous solution of 1 (detection of a clus-
ter consisting of one choline cation and the l5Si,l5Si’-disili-
cate dianion (m/z=486.04) and the monoprotonated dianion
(m/z=382.94).[9]
The kinetic stability of 2 in aqueous solution was studied
1
by H NMR spectroscopy by using the CH resonance signals
of the bound (R,R)-tartrato(4ꢀ) ligands of 2 and those of
the free tartaric acid (formed by hydrolysis) as the probe.
Integration of these resonance signals allowed for the deter-
mination of the hydrolysis kinetics. At concentrations of 10
and 100 mm (solvent, D2O), compound 2 (obtained upon
dissolution of the precursor 1 in D2O) was found to undergo
a slow hydrolysis at 238C over a period of more than
10 days (Figure 2), followed by gel formation. This remark-
Figure 3. Kinetics of the hydrolysis of an equimolar mixture of 2 and
SiACHTUNGRTNE(UNG OMe)4 in D2O/CD3CN (1:1 (v/v); c=50 mm) at 238C. The kinetic data
1
were extracted from H NMR spectra (300.1 MHz), in which the CH res-
onance signals of 2 and the CH3 resonance signals of (MeO)xSi(OD)4ꢀx
(x=1–4) and MeOD served as the probe. For the kinetic measurements,
compound 2 was generated in situ in the NMR tube by hydrolysis upon
dissolution of 1·3DMSO in the D2O/CD3CN mixture, followed by imme-
diate addition of SiACTHNUTRGNEUNG(OMe)4.
Chem. Eur. J. 2012, 18, 2202 – 2206
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2203

R. Tacke et al.
10 mm 2 and 0.1m DCl in D2O (obtained by dissolution of
the precursor 1) at 238C demonstrated 2 to be hydrolyzed
completely within a few minutes, followed by gel formation.
Under more basic conditions, compound 2 also undergoes a
rapid hydrolysis; studies of a solution with 10 mm 2 and
0.1m NaOD in D2O at 238C revealed a spontaneous gel for-
mation upon dissolution of 1. As the hydrolysis and gel for-
mation of 2 were too fast, the hydrolysis kinetics could not
1
be monitored by H NMR spectroscopy.
As can be seen from Scheme 2, the SiOH groups of the
dianion of 2 (A) can undergo condensation reactions. When
extending the duration of the above-mentioned freeze-
Figure 4. A) 29Si NMR spectrum of 2 measured directly after dissolution
of 1 in D2O; B) 29Si NMR spectrum of 2 obtained by freeze drying of an
aqueous solution of 2 for 16 h in [D6]DMSO; C) 29Si NMR spectrum of
the product mixture obtained by freeze drying of an aqueous solution of
2 for 96 h in [D6]DMSO; the spectrum shows discrete resonance signals
around ꢀ106 (SiOH), ꢀ110 (SiOCH2), and ꢀ113 ppm (SiOSi; for details,
see the Experimental Section). All 29Si NMR spectra were recorded at
99.4 MHz.
Scheme 2. Anionic di- and tetranuclear pentacoordinate silicon(IV) com-
plexes (A–D) detected in the product mixture obtained by freeze drying
of an aqueous solution of 2 (generated by dissolution of 1·3DMSO in
water) over a period of 96 h.
straightforward one-step synthesis, by starting from the
readily available, inexpensive starting materials tetrame-
thoxysilane and choline (R,R)-trihydrogentartrate. Com-
pound 1 can be readily dissolved in water at room tempera-
ture up to a concentration of approximately 1m and then
undergoes a rapid hydrolysis to form the l5Si,l5Si’-disilicate
2, which can be isolated by freeze drying. In aqueous solu-
tion, the dianion of 2 shows a remarkable kinetic inertness
against hydrolysis and hydrolyzes very slowly to afford
(R,R)-tartaric acid, choline, and orthosilicic acid. However,
under strongly acidic and basic conditions, rapid hydrolysis
is observed. Owing to their easy synthetic access and their
chemical behavior in aqueous solution, compounds 1 and 2
may be of interest as potential dietary and food supplements
that upon dissolution in water yield the two nutrients ortho-
silicic acid and choline. Future studies have to evaluate the
potential of 1 and 2 for such applications.
drying process from 16 h (preparation of 2—ca. 98%
purity—starting from 1) to 96 h, a product mixture was ob-
tained (see Figure 4, Scheme 2, and the Experimental Sec-
1
tion). Detailed H, 13C, and 29Si NMR spectroscopic studies
of this mixture, including 1H DOSY experiments, revealed
the presence of 1 (traces), choline, the dinuclear pentacoor-
dinate silicon(IV) complexes A (dianionic) and B (mono-
anionic), and the tetranuclear pentacoordinate silicon(IV)
complexes C (tetraanionic) and D (dianionic). The identity
of A–C was further supported by ESI-HRMS studies,[9]
whereas the mass-spectrometric detection of D failed (for
details, see the Experimental Section). In addition to the di-
or tetranuclear complexes A–D, the NMR studies also indi-
cated the presence of higher oligomers; however, the struc-
ture of these species could not be established unequivocally
(for details, see the Experimental Section).
In summary, the zwitterionic dinuclear pentacoordinate
silicon(IV) complex 1 can be prepared in good yield in a
The high kinetic stability of the dianion of 2 against hy-
drolysis might also be of interest for silicon biochemistry:
2204
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2202 – 2206

Multinuclear Pentacoordinate Silicon(IV) Complexes
COMMUNICATION
Data for 1 (0.8%): 1H NMR (500.1 MHz, [D6]DMSO): d=4.17 ppm (s,
4H; CH); 13C NMR (125.8 MHz, [D6]DMSO): d=75.2 (CH), 172.6 ppm
(C=O); 29Si NMR (99.4 MHz, [D6]DMSO): d=ꢀ109.9 ppm.
Already three decades ago it was speculated that silicon(IV)
complexes with chelate ligands derived from organic hy-
droxyl compounds may play a role in the biochemistry of sil-
icon, by controlling the transport of silicon, its concentration
in a soluble pool, and its deposition as silica.[10] In this con-
text, the existence and the chemical behavior of the dianion
of 2 in aqueous solution is quite remarkable.
Data for A (11.5%): 1H NMR (500.1 MHz, [D6]DMSO): d=4.01 (s, 4H;
CH), 4.63 ppm (s, 2H; SiOH); 13C NMR (125.8 MHz, [D6]DMSO): d=
75.3 (CH), 173.3 ppm (C=O); 29Si NMR (99.4 MHz, [D6]DMSO): d=
ꢀ105.96 ppm; ESI-HRMS: m/z calcd for [dianion
+ one choline
cation]ꢀ: 486.03770; found: 486.03704.
Data for B (6.6%): 1H NMR (500.1 MHz, [D6]DMSO): dA =4.04 (2H;
CHC(O)OSiOH) and dX =4.13 (2H; CHC(O)OSiOCH2) (AX system,
3J(AX)=2.1 Hz), 4.71 ppm (s, 1H; SiOH); 13C NMR (125.8 MHz,
[D6]DMSO): d=75.1 (CHC(O)OSiOH), 75.4 (CHC(O)OSiOCH2), 172.9
(CHC(O)OSiOCH2),
173.0 ppm
(CHC(O)OSiOH);
29Si NMR
Experimental Section
(99.4 MHz, [D6]DMSO): d=ꢀ109.8 (SiOCH2), ꢀ106.02 ppm (SiOH);
ESI-HRMS: m/z calcd: 468.02713; found: 468.02654.
Compound 1: Tetramethoxysilane (4.57 g, 30.0 mmol) was added drop-
wise within 30 min to a vigorously stirred suspension of choline (R,R)-tri-
hydrogentartrate (7.60 g, 30.0 mmol) in boiling N,N-dimethylformamide
(150 mL), and the reaction mixture was vigorously stirred for a further
24 h under reflux and then cooled to 208C. The precipitate was isolated
by filtration, washed with diethyl ether (3ꢄ30 mL), and dried in vacuo
(0.01 mbar, 208C, 3 h) to give 1 in 83% yield as a colorless solid (6.88 g,
12.4 mmol). M.p.>4008C; 1H NMR (500.1 MHz, [D6]DMSO): d=3.07
(s, 18H; NCH3), 3.27–3.42 (m, 4H; OCH2CH2N), 3.81–3.97 (m, 4H;
OCH2CH2N), 4.16 ppm (s, 4H; CH); 13C NMR (125.8 MHz, [D6]DMSO):
Data for C (40.9%): 1H NMR (500.1 MHz, [D6]DMSO): dA =3.98 (2H;
CHC(O)OSiOH) and dB =3.993 (2H; CHC(O)OSiOSi) (AB system,
3J(AB)=2.1 Hz), dA =3.98 (2H; CHC(O)OSiOH) and dB =3.994 (2H;
CHC(O)OSiOSi) (AB system, 3J(AB)=2.1 Hz), 4.611 (s, 1H; SiOH),
4.612 ppm (s, 1H; SiOH); 13C NMR (125.8 MHz, [D6]DMSO): d=75.1
(CHC(O)OSiOSi), 75.3 (CHC(O)OSiOH), 173.3 (CHC(O)OSiOSi),
173.4 ppm (CHC(O)OSiOH); 29Si NMR (99.4 MHz, [D6]DMSO): d=
ꢀ113.22 (SiOSi), ꢀ113.20 (SiOSi), ꢀ106.03 ppm (SiOH); ESI-HRMS:
m/z calcd for [tetraanion
+
3 choline cations]ꢀ: 1058.17182; found:
1058.17066.
d=53.1 (t, 1J
ACHTUNGTRENNUNG
(13C,14N)=3.5 Hz; NCH3), 57.6 (OCH2CH2N), 66.4 (t,
1
1J(13C,14N)=2.0 Hz; OCH2CH2N), 75.1 (CH), 172.5 ppm (C=O);
ACHTUNGTRENNUNG
Data for D (11.8%): H NMR (500.1 MHz, [D6]DMSO): dA =4.017 (2H;
CHC(O)OSiOSi) and dX =4.09 (2H; CHC(O)OSiOCH2) (AX system,
3J(AX)=2.1 Hz), dA =4.019 (2H, CHC(O)OSiOSi) and dX =4.09 ppm
(2H, CHC(O)OSiOCH2) (AX system, 3J(AX)=2.1 Hz); 13C NMR
(125.8 MHz, [D6]DMSO): d=75.0 (CHC(O)OSiOSi), 75.4 (CHC(O)O-
SiOCH2), 173.1 (CHC(O)OSiOCH2), 173.3 ppm (CHC(O)OSiOSi);
29Si NMR (99.4 MHz, [D6]DMSO): d=ꢀ113.3 (SiOSi), ꢀ113.2 (SiOSi),
ꢀ109.8 ppm (SiOCH2); ESI-HRMS: the detection of D failed, probably
29Si NMR (99.4 MHz, [D6]DMSO): d=ꢀ109.9 ppm; 13C VACP/MAS
NMR (100.6 MHz): d=54.3 (3C) (NCH3), 54.5 (3C) (NCH3), 60.1, 60.9,
and 64.3 (2C) (OCH2CH2N), 75.2, 75.9 (2C), and 76.7 (CH), 171.5, 172.8,
and 173.7 ppm (2C) (C=O); 15N VACP/MAS NMR (40.6 MHz): d=
ꢀ327.3 ppm (2N); 29Si VACP/MAS NMR (79.5 MHz): d=ꢀ110.5,
ꢀ110.1 ppm; elemental analysis (%) calcd for C18H30N2O14Si2 (Mr =
554.61): C 38.98, H 5.45, N 5.05; found: C 38.7, H 5.4, N 5.2.
ꢀ
due to rapid hydrolytic Si OC cleavage in the water matrix.
Compound 2: A freshly prepared solution of 1 (303 mg, 564 mmol) in
water (2 mL) was freeze dried (16 h) to give 2 in 94% yield as a colorless
solid (312 mg, 528 mmol). 1H NMR (500.1 MHz, D2O): d=3.12 (brs,
18H; NCH3), 3.42–3.46 (m, 4H; OCH2CH2N), 3.96–4.01 (m, 4H;
OCH2CH2N), 4.57 ppm (s, 4H; CH); 13C NMR (125.8 MHz, D2O): d=
Data for higher oligomers (28.4%): 1H NMR (500.1 MHz, [D6]DMSO):
d=3.96 (brs, CH); 13C NMR (125.8 MHz, [D6]DMSO): d=75.1 (CH),
173.5 ppm (C=O); 29Si NMR (99.4 MHz, [D6]DMSO): d=ꢀ113.35
(SiOSi), ꢀ113.32 (SiOSi), ꢀ113.30 ppm (SiOSi); no corresponding SiOH
signals could be detected. According to the 1H DOSY experiments, the
difference between the diffusion coefficients of A and C is similar to that
between the diffusion coefficients of C and the higher oligomers.
53.9 (t, 1J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
(13C,14N)=4.0 Hz; NCH3), 55.6 (OCH2CH2N), 67.4 (t,
1J(13C,14N)=3.0 Hz; OCH2CH2N), 75.3 (CH), 175.6 ppm (C=O);
29Si NMR (99.4 MHz, D2O): d=ꢀ106.6 ppm; 1H NMR (500.1 MHz,
[D6]DMSO): d=3.09 (brs, 18H; NCH3), 3.36–3.41 (m, 4H; OCH2CH2N),
3.78–3.86 (m, 4H; OCH2CH2N), 4.03 ppm (s, 4H; CH), 4.66 (brs, 2H;
SiOH), 5.23 (brs, 2H; CH2OH); 13C NMR (125.8 MHz, [D6]DMSO): d=
Keywords: bioinorganic
chemistry
·
choline
·
53.2 (t, 1J
ACHTUNGTRENNUNG
(13C,14N)=3.8 Hz; NCH3), 55.2 (OCH2CH2N), 66.9 (t,
pentacoordination · silicic acid · silicon
1J(13C,14N)=2.8 Hz; OCH2CH2N), 75.3 (CH), 173.3 ppm (C=O);
ACHTUNGTRENNUNG
29Si NMR (99.4 MHz, [D6]DMSO): d=ꢀ106.0 ppm; 13C VACP/MAS
NMR (100.6 MHz): d=54.8 (NCH3), 56.8 (OCH2CH2N), 68.4
(OCH2CH2N), 76.4 (CH), 175.6 ppm (C=O); 15N VACP/MAS NMR
(40.6 MHz): d=ꢀ327.0 ppm; 29Si VACP/MAS NMR (79.5 MHz): d=
ꢀ105.6 ppm; MS (ESIꢀ): m/z: 486.04 [dianion + one choline cation]ꢀ,
382.94 [monoprotonated dianion]ꢀ; elemental analysis (%) calcd for
C18H34N2O16Si2 (Mr =590.64): C 36.60, H 5.80, N 4.74; found: C 36.6, H
5.8, N 4.8.
[1] Selected reviews dealing with higher-coordinate silicon compounds:
a) R. R. Holmes, Chem. Rev. 1996, 96, 927–950; b) D. Kost, I. Ka-
likhman in The Chemistry of Organic Silicon Compounds, Vol. 2,
Part 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998,
pp. 1339–1445; c) V. Pestunovich, S. Kirpichenko, M. Voronkov in
The Chemistry of Organic Silicon Compounds, Vol. 2, Part 2 (Eds.:
Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998, pp. 1447–1537;
d) C. Chuit, R. J. P. Corriu, C. Reyꢅ in Chemistry of Hypervalent
Compounds (Ed.: K.-y. Akiba), Wiley-VCH, Weinheim, 1999,
pp. 81–146; e) R. Tacke, M. Pꢁlm, B. Wagner, Adv. Organomet.
Chem. 1999, 44, 221–273; f) M. A. Brook, Silicon in Organic, Or-
ganometallic, and Polymer Chemistry, Wiley, New York, 2000,
pp. 97–114; g) R. Tacke, O. Seiler in Silicon Chemistry: From the
Atom to Extended Systems (Eds.: P. Jutzi, U. Schubert), Wiley-VCH,
Weinheim, 2003, pp. 324–337; h) D. Kost, I. Kalikhman, Adv. Orga-
nomet. Chem. 2004, 50, 1–106; i) D. Kost, I. Kalikhman, Acc. Chem.
Res. 2009, 42, 303–314.
Mixture of condensation products: The mixture of condensation products
was prepared according to the synthesis of 2 (see above), however, the
duration of the freeze-drying process was extended to 96 h. The solid
product obtained was dissolved in [D6]DMSO, and the solution was ana-
1
1
lyzed by H, 13C, and 29Si NMR spectroscopy (including H DOSY experi-
ments)[6] and by ESI-HRMS spectrometry.[9] The results obtained for a
typical sample[11] are given below. For reasons of clarity, only the charac-
teristic 1H, 13C, and 29Si chemical shifts of the single components that
could be identified unequivocally are listed; the chemical shifts of the
bound 2-(trimethylammonio)ethanolato ligands and the free choline are
not given. The quantifications of the single components in the mixture
(given in %) are based on the integrals of the corresponding CH reso-
nance signals in the 1H NMR spectrum (integral of all CH signals=
100%).
[2] a) R. Tacke, R. Bertermann, C. Burschka, S. Dragota, Angew.
Chem. 2005, 117, 5426–5429; Angew. Chem. Int. Ed. 2005, 44, 5292–
5295; b) X. Kꢀstele, P. Klꢁfers, R. Tacke, Angew. Chem. 2006, 118,
Chem. Eur. J. 2012, 18, 2202 – 2206
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2205

R. Tacke et al.
3286–3288; Angew. Chem. Int. Ed. 2006, 45, 3212–3214; c) B. Theis,
C. Burschka, R. Tacke, Chem. Eur. J. 2008, 14, 4618–4630.
[6] NMR studies of 1 and 2: Solution 1 H, 13C, and 29Si NMR spectra
were recorded at 238C on a Bruker Avance 500 NMR spectrometer
(1H, 500.1 MHz; 13C, 125.8 MHz; 29Si, 99.4 MHz) with [D6]DMSO
or D2O as the solvent. Chemical shifts (ppm) were determined rela-
tive to internal [D5]DMSO (1H, d=2.49 ppm, [D6]DMSO), internal
HDO (1H, d=4.70 ppm, D2O), internal [D6]DMSO (13C, d=
39.5 ppm, [D6]DMSO), or external TMS standards (13C, d=0 ppm,
D2O; 29Si, d=0 ppm, [D6]DMSO, D2O). Assignment of the NMR
data was supported by 1H,1H COSY, 1H,1H EXSY, DEPT 135,
13C,1H HSQC, 13C,1H HMBC, and 29Si,1H HMQC experiments. Ad-
ditionally, the assignment to the components in the mixture of con-
densation products was further supported by 1H DOSY experiments
at 308C on a Bruker DMX 600 NMR spectrometer (1H, 600.1 MHz;
solvent, [D6]DMSO). The kinetic measurements of the hydrolysis of
[3] a) K. Van Dyck, R. Van Cauwenbergh, H. Robberecht, H. Deelstra,
Fresenius J. Anal. Chem. 1999, 363, 541–544; b) R. Jugdaohsingh,
S. H. C. Anderson, K. L. Tucker, H. Elliott, D. P. Kiel, R. P. H.
Thompson, J. J. Powell, Am. J. Clin. Nutr. 2002, 75, 887–893; c) R.
Jugdaohsingh, K. L. Tucker, N. Qiao, L. A. Cupples, D. P. Kiel, J. J.
Powell, J. Bone Miner. Res. 2004, 19, 297–307; d) A. Barel, M. Ca-
lomme, A. Timchenko, K. De Paepe, N. Demeester, V. Rogiers, P.
Clarys, D. Vanden Berghe, Arch. Dermatol. Res. 2005, 297, 147–
153;e) R. Jugdaohsingh, J. Nutr. Health Aging 2007, 11, 99–110;
f) T. D. Spector, M. R. Calomme, S. H. Anderson, G. Clement, L.
Bevan, N. Demeester, R. Swaminathan, R. Jugdaohsingh, D. A.
Vanden Berghe, J. J. Powell, BMC Musculoskel. Disord. 2008, 9: 85;
g) S. Sripanyakorn, R. Jugdaohsingh, W. Dissayabutr, S. H. C. An-
derson, R. P. H. Thompson, J. J. Powell, Br. J. Nutr. 2009, 102, 825–
834; h) Scientific Opinion of the Panel on Food Additives and Nu-
trient Sources added to Food on choline-stabilized orthosilicic acid
for nutritional purposes to food supplements following a request
from the European Commission; The EFSA Journal 2009, 948, 1–
23.
the equimolar mixture of 2 and SiACHTNUGTRENUNG(OMe)4 were performed at 238C
on a Bruker DRX-300 NMR spectrometer (1H, 300.1 MHz). Solid-
state 13C, 15N, and 29Si VACP/MAS NMR spectra were recorded at
228C on a Bruker DSX-400 NMR spectrometer with bottom layer
rotors of ZrO2 (diameter, 7 mm) containing approximately 250 mg
of sample (13C, 100.6 MHz; 15N, 40.6 MHz; 29Si, 79.5 MHz; external
standard, TMS (13C, 29Si; d=0 ppm) or glycine (15N, d=
ꢀ342.0 ppm); contact time, 2 ms (13C), 3 ms (15N), or 5 ms (29Si); 908
1H transmitter pulse length, 3.2–3.6 ms; repetition time, 4–5 s).
[7] The Berry distortions were analyzed by using the PLATON program
system; A. L. Spek, PLATON, Utrecht University, Utrecht, The
Netherlands, 2008.
[4] a) S. H. Zeisel, J. K. Blusztajn, Annu. Rev. Nutr. 1994, 14, 269–296;
b) S. H. Zeisel, Nutrition 2000, 16, 669–671.
[5] Crystal structure analysis of 1·3DMSO: X-ray diffraction experi-
ments at different temperatures showed that normal cooling of the
crystals resulted in low resolution, split, or apparently twinned crys-
tals and considerable variation of the cell parameters. The best data
were obtained near room temperature. The crystals were mounted
with adhesive oil on a MiTeGen-Microloop, and the diffraction ex-
periments were performed with a Bruker Nonius KAPPA APEX II
diffractometer (Montel mirror, MoKa radiation, l=0.71073 ꢃ). With
several data sets, the rigid part of the molecular structure could be
easily located by direct methods (XS, SHELXTL 2008/4 (G. M.
Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122), but one of
the two 2-(trimethylammonio)ethanolato ligands and all of the three
DMSO molecules were always found to be disordered among two
positions. During structure refinement by full matrix least squares
on F2 (XL), these parts of the structure were restrained to similar
geometry and temperature factors. For the CH hydrogen atoms, a
riding model was employed. CCDC-861239 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Selected data for
1·3DMSO: single crystal of dimensions 0.37ꢄ0.33ꢄ0.25 mm ob-
tained by slow cooling of a hot (1208C) saturated solution of 1 in
DMSO, C24H48N2O17S3Si2, Mr =789.00, analysis at 283(2) K, mono-
clinic, space group P21 (no. 4), a=13.8704(6), b=9.6628(4), c=
[8] The cryoscopic studies were performed with a freshly prepared
33 mm aqueous solution of 1 using a Beckmann apparatus.
[9] ESI-MS studies of 2 and ESI-HRMS studies of the mixture of con-
densation products: The measurements were performed on a Bruker
ESI-TOF (microTOF focus) spectrometer (1 pmolmLꢀ1 in water).
[10] a) R. K. Iler, The Chemistry of Silica: Solubility, Polymerization,
Colloid and Surface Properties, and Biochemistry, Wiley, New York,
1978, pp. 783–784; b) P. B. Kaufman, P. Dayanandan, Y. Takeoka,
W. C. Bigelow, J. D. Jones, R. Iler in Silicon and Siliceous Structures
in Biological Systems (Eds.: T. L. Simpson, B. E. Volcani), Springer-
Verlag, New York, 1981, pp. 442–446; c) C. W. Sullivan in Silicon
Biochemistry (Eds.: D. Evered, M. OꢆConnor), Wiley, Chichester,
UK, 1986, pp. 70–73; d) O. Seiler, C. Burschka, D. Schwahn, R.
Tacke, Inorg. Chem. 2005, 44, 2318–2325; and references cited
therein; e) J. B. Lambert, S. A. Gurusamy-Thangavelu, K. Ma, Sci-
ence 2010, 327, 984–986; f) T. Schmiederer, S. Rausch, M. Valdebe-
nito, Y. Mantri, E. Mçsker, T. Baramov, K. Stelmaszyk, P. Schmied-
er, D. Butz, S. I. Mꢁller, K. Schneider, M.-H. Baik, K. Hantke, R. D.
Sꢁssmuth, Angew. Chem. 2011, 123, 4317–4321; Angew. Chem. Int.
Ed. 2011, 50, 4230–4233.
[11] The composition of the various samples obtained was not always
identical and showed a certain degree of variation.
14.3432(7) ꢃ, b=103.122(2)8, V=1872.18(14) ꢃ3, Z=2, 1calcd
1.400 gcmꢀ3, m=0.333 mmꢀ1, F
(000)=836, 2qmax =61.068, 47902 col-
=
AHCTUNGTRENNUNG
lected reflections, 9958 unique reflections (Rint =0.0264), 583 param-
eters, 619 restraints, S=1.044, R1 =0.0556 [I>2s(I)], wR2 (all
data)=0.1700, absolute structure parameter=0.01(11), max./min.
Received: September 8, 2011
Published online: January 23, 2012
residual electron density=+0.579/ꢀ0.319 eꢃꢀ3
.
2206
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2202 – 2206