Racemization versus retention of chiral information during the formation of silicon and tin complexes with chiral Schiff base ligands

Article abstract of DOI:10.1016/j.poly.2012.08.027

Chiral Schiff base ligands with O,N,O′-coordination ability have been prepared with amino acid esters from the chiral pool. Unfortunately the chiral information is lost during the formation of complexes with these chiral ligands with silicon tetrachloride

Full text of DOI:10.1016/j.poly.2012.08.027

Polyhedron 47 (2012) 46–52
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Polyhedron
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Racemization versus retention of chiral information during the formation
of silicon and tin complexes with chiral Schiff base ligands
⇑
Gisela Warncke, Uwe Böhme , Betty Günther, Martin Kronstein
Institut für Anorganische Chemie, Technische Universität Bergakademie Freiberg, Leipziger Str. 29, D-09596 Freiberg, Germany
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Chiral Schiff base ligands with O,N,O -coordination ability have been prepared with amino acid esters
from the chiral pool. Unfortunately the chiral information is lost during the formation of complexes with
these chiral ligands with silicon tetrachloride. This was demonstrated with the help of two X-ray
structures which show different reaction products depending on chosen reaction conditions. The reaction
Received 20 July 2012
Accepted 8 August 2012
Available online 18 August 2012
of N-(salicylidene)-
planarization of the amino acid ester and formation of a ketene acetal 2a. The reaction of N-(salicylid-
ene)- -valinmethylester (1b) with SiCl without triethylamine leads to racemization of the amino acid
ester and formation of the complex 2b. A mechanism for the formation of both products has been
proposed. It is possible to transfer the chiral information present in the ligand system into the complex
4
D-phenylglycine methylester (1a) with SiCl in presence of triethylamine leads to
Keywords:
Schiff base ligands
Chiral ligands
Coordination compounds
Silicon
L
4
by using dichlorodimethylstannane for the complex formation. Dimethyltin[N-(salicylidene)-
D
-phenyl-
Tin
glycine methylester] (3a) crystallizes in the chiral space group P2 2 2 with one diasteromer in the
1 1 1
asymmetric unit. 1H NMR analysis of 3a with the help of a lanthanide shift reagent proves the presence
of only one diasteromer in the bulk material.
Ó 2012 Elsevier Ltd. All rights reserved.
1
. Introduction
silicon complexes. We will discuss possible reaction mechanisms
towards these products. The preservation of chiral information
Silicon and tin complexes with Schiff base ligands have received
considerable attention due to their bacteriostatic [1], antimicrobial
2], biocidal [3], bactericidal [4] and fungicidal [5,6] properties or
was possible with a tin complex.
[
2
. Experimental
their possible application as antitumor-reagents [6]. Numerous
publications about silicon complexes with salen-type ligands
The necessary chemicals were used as commercially available.
0
[
7–10] and with tridentate O,N,O -ligands [11–14] have been
Reactions with air- or moisture-sensitive reagents were carried
out under dry Argon. THF, DME, and triethylamine were distilled
from sodium/benzophenone under Argon prior to use. Dichloro-
published recently. Now our interest is shifted towards hypercoor-
dinated silicon and tin compounds with chiral ligands. We have
recently published the results of penta- and hexacoordinated
silicon complexes with chiral Schiff base ligands derived from
salicylaldehyde and chiral amino alcohols [15]. Chiral Schiff bases
are of certain biological importance as they occur in pyridoxal-
depending enzyme-catalyzed biochemical reactions [16,17]. The
system salicylaldehyde–amino acid could be considered as a sim-
ple model for these complex enzyme systems. Further possible
points of interest are the chirality transfer from silicon to carbon
2 4
methane was distilled under Argon from CaH , hexane from LiAlH .
NMR: BRUKER DPX 400, TMS as internal standard – elemental
analyses: Foss Heraeus CHN-O-Rapid – Polarimetry: Perkin Elmer
Polarimeter 241.
2
.1. Preparation of Schiff bases
Ligands 1a and 1b were prepared from salicylaldehyde and
a-
[
18,19] and the preparation of silanes with silicon centered
2 2
amino acid methylester hydrochlorides in CH Cl according to a
chirality as reagents or substrates [20,21].
previously published method [22]. NMR spectroscopic data and
elemental analyses for 1a and 1b agree well with data which have
been published before for these ligands.
Herein we report the syntheses and structures of silicon and tin
complexes of N-(salicylidene)-
a-amino acids. The X-ray crystal
structures surprisingly show the loss of chiral information for the
2
.1.1. Ligand 1a
-Phenylglycine methylester hydrochloride (10.0 g, 49.6 mmol)
was dissolved in 120 ml of dichloromethane with triethylamine
⇑
Corresponding author. Tel.: +49 3731 39 2050; fax: +49 3731 39 4058.
E-mail address: Uwe.Boehme@chemie.tu-freiberg.de (U. Böhme).
D
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.027

G. Warncke et al. / Polyhedron 47 (2012) 46–52
47
(
6.9 ml, 49.6 mmol). Salicylaldehyde (4.9 ml, 47.2 mmol) was
(N@CH); 171.5 (COO); Elemental Anal. Calc. for C13
H
17NO
3
: C,
slowly added, whereby the color of the solution changed slowly
to yellow. The reaction mixture was stirred overnight at room tem-
perature before it was extracted twice with approx. 50 ml water.
66.36; H, 7.28; N, 5.95. Found: C, 66.01; H, 7.27; N, 5.75%.
2.2. Preparation of silicon and tin complexes
2 4
The organic layer was dried with Na SO , filtered and concentrated
in vacuo. The remaining yellow oil crystallized while standing at
The syntheses of the silicon and tin complexes 2a, 2b and 3a
were carried out as previously published for titanium complexes
with amino acid Schiff bases [23].
20
room temperature, giving 12.33 g 1a (91% yield), [
a
]
D
= + 59.3°
, TMS, 298 K,
); 5.19 (s, 1H, N–CH); 6.86–7.47 (m,
1
(
c = 0.3 g/100 ml CHCl
ppm) d = 3.76 (s, 3H, OCH
H, Ar–H); 8.38 (s, 1H, N@CH); 13.12 (s, 1H, OH). C NMR (CDCl
TMS, 298 K, ppm) d = 52.7 (OCH ); 74.9 (N–CH); 117.2–137.0 (Ar–
C); 161.0 (C–OH); 166.9 (COO); 170.8 (N@CH); Elemental Anal.
Calc. for C16 : C, 71.36; H, 5.61; N, 5.20. Found: C, 71.30;
H, 5.59; N, 5.06%.
3 3
). m.p. = 64 °C. H NMR (CDCl
3
13
9
3
,
2
.2.1. Complex 2a
a (4.0 g, 14.9 mmol) was dissolved in 150 ml THF and triethyl-
amine (4.3 ml, 30.9 mmol) was added. The mixture was cooled to
°C and slowly silicon tetrachloride (1.1 ml, 7.4 mmol) was added.
3
1
H15NO
3
0
The reaction mixture instantly turned orange and a white precipi-
tate formed. The mixture was stirred magnetically for 2.5 h before
filtering. The solvent was removed by reduced pressure from the
orange colored filtrate. Single crystals for the X-ray structure anal-
ysis where obtained from DME/n-hexane (<1% yield). m.p. = 190–
2
.1.2. Ligand 1b
Triethylamine (4.1 ml, 29.6 mmol) was added to as suspension
-valine methylester hydrochloride (5.0 g, 29.6 mmol) in 90 ml
2
9
1
92 °C. Si NMR (CDCl
3
, TMS 298 K, ppm): ꢀ175.5, ꢀ177.1.
of
L
dichloromethane. As salicylaldehyde (2.8 ml, 26.9 mmol) was
added slowly the color of the reaction mixture turned yellow.
The suspension was stirred at room temperature over night. The
solution was washed twice with destilled water and the organic
layer was dried with Na SO . The drying agent was filtered of
2 4
and the filtrate concentrated in vacuo. The obtained yellow oil
2.2.2. Complex 2b
1b (1.0 g, 4.29 mmol) was dissolved in 40 ml of dichlorometh-
ane and silicon tetrachloride (0.5 ml, 4.29 mmol) was added. The
color of the reaction mixture immediately turned red and after a
while yellow, a white precipitate formed. After 1.5 h of stirring
the solvent was removed in vacuo, the residue was stirred at reflux
in dichloromethane, and then filtered. Single crystals could be
gained from the filtrate stored at 8 °C (0.44 g, 22% yield).
was crystallized from ethanol/hexane, giving 2.12 g 1b (33% yield),
2
0
1
[
(
a
]
D
= ꢀ38.7° (c = 0.3 g/100 ml CHCl
3
). m.p. = 30 °C. H NMR
3
CDCl
3
,
TMS, 298 K, ppm) d = 0.96 (m, 6H, CH
3
,
i-Pr,
J
H–
3
29
1
H
= 6.7 Hz); 2.37 (m, 1H, CH, i-Pr,
N–CH,
J
H–H = 6.1/6.7 Hz); 3.72 (d, 1H,
H–H = 6.1 Hz); 3.80 (s, 3H, O–CH ); 6.86–7.34 (m, 4H, Ar–
m.p. > 350 °C. Si NMR (CD
NMR (CD Cl , TMS, 298 K, ppm) d = 1.14–1.27 (m, 12H, CH
H–H = 7.0 Hz), 2.43–2.57 (m, 2H, CH, i-Pr,
2
Cl
2
, TMS, 298 K, ppm) d = ꢀ179.2.
, i-Pr,
H–H = 6.4/7.0 Hz);
H–H = 6.4 Hz); 6.67–7.53 (m, 8H, Ar);
H
3
J
3
2
2
3
13
3
3
H); 8.30 (s, 1H, N@CH); 13.15 (s, 1H, OH). C NMR (CDCl
3
, TMS,
, i-Pr); 31.7 (CH, i-Pr); 52.0 (O–
); 77.8 (N–CH); 117.0–132.7 (Ar–C); 161.0 (C–OH); 166.5
J
J
3
2
CH
98 K, ppm) d = 18.2/19.4 (CH
3
4.03, 4.08 (2d, 2H, N–CH,
8.35, 8.39 (2s, 2H, HC@N). C NMR (CD
J
1
3
3
2
Cl
2
, TMS, 298 K, ppm)
Table 1
Crystal data and structure refinement for compounds 2a, 2b, and 3a.
Compound
2a
2b
3a
Empirical formula
Formula weight
T (K)
C
32
H
26
N
2
O
6
Si
C
24
H
26
N
2
O
6
Si
C
17
H
17NO
3
Sn
562.64
100(2)
466.56
153(2)
402.01
153(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P2 /c
13.7469(3)
24.2066(5)
8.1047(1)
90
93.483(1)
90
2691.98(9)
4
triclinic
P1ꢀ
10.2958(3)
11.7003(4)
19.2626(6)
97.9300(10)
91.926(2)
93.090(2)
2292.94(13)
4
1.352
0.146
orthorhombic
P2
9.2503(2)
11.0468(2)
15.3382(3)
90
1
1 1 1
2 2
a
(°)
b (°)
(°)
90
90
c
3
V (Å )
1567.35(5)
4
1.704
1.642
800
Z
D
calc (Mg/m3)
1.388
0.138
1176
Absorption coefficient (mmꢀ1)
F(000)
984
Crystal size (mm)
Theta range for data collection (°)
Limiting indices
0.44 ꢁ 0.20 ꢁ 0.15
0.53 ꢁ 0.40 ꢁ 0.26
0.39 ꢁ 0.35 ꢁ 0.20
1.68–28.00
1.76–27.00
2.27–28.00
ꢀ18 6 h 6 18, ꢀ31 6 k 6 31,
ꢀ13 6 h 6 12, ꢀ14 6 k 6 14,
ꢀ24 6 l 6 18
31145/9956 [Rint = 0.0231]
(27.00°) 99.4%
ꢀ12 6 h 6 12, ꢀ14 6 k 6 14,
ꢀ20 6 l 6 20
ꢀ
10 6 l 6 10
44942/6507 [Rint = 0.0387]
(28.00°) 100.0%
Reflections collected/unique
Completeness to theta
Absorption correction
40502/3783 [Rint = 0.0225]
(28.00°) 100.0%
Semi-empirical from equivalents
Maximum and minimum transmissions 0.9796 and 0.9418
Refinement method
0.9630 and 0.9267
0.7348 and 0.5669
Full-matrix least-squares on F2
Data/restraints/parameters
Goodness-of-fit on F
6507/0/380
1.023
9956/129/690
1.098
3783/0/209
1.103
2
Final R indices [I > 2
R indices (all data)
Absolute structure parameter
r
(I)]
R
R
–
1
= 0.0358, wR
= 0.0505, wR
2
= 0.0833
= 0.0918
R
R
–
1
= 0.0476, wR
= 0.0638, wR
2
2
= 0.1306
= 0.1372
R
R
1
= 0.0140, wR
= 0.0146, wR
2
= 0.0370
= 0.0373
1
2
1
1
2
0.034(14)
0.589 and ꢀ0.227
Largest difference in peak and hole
0.336 and ꢀ0.321
0.873 and ꢀ0.413
ꢀ
3
(
e A )

4
8
G. Warncke et al. / Polyhedron 47 (2012) 46–52
d = 18.1, 18.9, 20.2, 20.4 (CH
6
1
3
, i-Pr); 33.3, 33.9, 33.9, 34.1 (CH, i-Pr);
3.7, 69.5 (N–CH), 117.3, 117.4, 119.6, 119.8, 120.6, 120.9, 133.9,
34.1, 138.5, 138.6 (CAr); 161.2, 161.5, 166.9, 167.7, 173.4 (CAr
Si: C, 61.78; H,
icon. Furthermore, the absence of the methyl group implies the
cleavage of the methyl ester group during the reaction.
,
Complex 3a shows a chemical shift of ꢀ156.0 ppm in the 119Sn
NMR spectrum. This implies a hypercoordinated tin compound.
C@N, COO). Elemental Anal. Calc. for C24
26 2 6
H N O
1
3
5
.62; N, 6.00. Found: C, 60.80; H, 4.63; N, 5.33%.
The C NMR spectra of the complex and of the free ligand are very
similar. The carbonyl carbon, the aromatic carbon with the hydro-
xyl group and the imin carbon atom show the highest chemical
shift due to their neighborhood to the electronegative oxygen or
nitrogen atoms (174.0, 171.3, 187.0 ppm). These and the shifts of
the aromatic carbon atoms (d 138.2–117.1 ppm) show no signifi-
cant differences between the free and the complex bound state.
This is to be expected as the aromatic ligand moieties do not take
part in the coordination of the central tin ion. The complex has no
signal for a methyl ester carbon atom around 52.7 ppm, which is
the greatest difference of the two spectra next to the fact, that
the two tin bound methyl functions show a shift of 59.0 ppm. After
the addition of 10% of europium tris[3-(trifluoromethylhydroxy-
2.2.3. Complex 3a
1
a (3.0 g, 11.1 mmol) was dissolved in 150 ml THF with trieth-
ylamine (3.3 ml, 23.9 mmol) and cooled down to 0 °C. When dim-
ethyltindichloride (2.4 g, 11.1 mmol) was added slowly, the color
of the mixture turned orange and a white precipitate formed.
The reaction mixture was stirred at room temperature over night
and its color turned red. The precipitate was filtered and the sol-
vent was removed by reduced pressure. Single crystals for the X-
ray structure analysis could be gained from DME/n-hexane, giving
20
0
1
.45 g (10% yield). m.p. = 203–205 °C, [
a]
D
= +144.5° (c = 0.2 g/
, TMS, 298 K, ppm) d = ꢀ156.0.
3
, TMS, 298 K, ppm) d = 0.79 (6H, Me–Sn); 5.19 (s,
1
19
00 ml CHCl
3
).
Sn NMR (CDCl
3
1
methylene)-(+)-camphorate], Eu(tfc) , as shift reagent to the com-
3
H NMR (CDCl
1
1
3
plex diluted in CDCl , the H NMR signal of the proton bound to the
3
1
H, N–CH); 6.73–7.39 (m, 9H, Ar–H); 8.31 (s, 1H, N@CH).
C
asymmetric carbon atom appears shifted towards lower field (now
d = 5.63 ppm instead of 5.20 ppm). Furthermore, it is still a singlet,
what shows the enantiomerical purity of the bulk material.
3
NMR (CDCl , TMS, 298 K, ppm) d = 8.5 (Sn–Me); 71.1 (N–CH);
1
17.1–138.2 (Ar–C); 169.0 (C–O–Sn); 171.3 (C@N); 174.0 (C@O).
Elemental Anal. Calc. for C17 Sn: C, 50.79; H, 4.26; N, 3.48.
Found: C, 50.00; H, 4.62; N, 3.81%.
H
17NO
3
3.2. X-ray structures
2.3. X-ray data collection and solution
Single crystals of complex 2a could be gained from a mixture of
DME/n-hexane. The molecular structure is shown in Fig. 1, selected
bond lengths and angles in Tables 2 and 3. The compound crystal-
Single crystal X-ray diffraction data of 2a, 2b, and 3b were col-
lected on a BRUKER NONIUS X8 APEX2 CCD diffractometer using
graphite monochromated Mo K radiation (k = 0.71073 Å). Struc-
lizes in the monoclinic crystal system in the space group P2
1
/c and
a
0
has four molecules in the unit cell. Two tridentate O,N,O -ligands
are coordinated at the silicon atom yielding an octahedral coordi-
nation geometry. For crystallographic data see experimental sec-
tion. The bond angles between neighboring atoms at silicon vary
tures were solved with direct methods and refined with full-matrix
least-squares methods. All non-hydrogen atoms were anisotropi-
cally refined. Hydrogen atoms were placed in idealized positions
and refined isotropically (riding model). Structure solution and
2
refinement of F against all reflections were carried out with the
software SHELXS-97 and SHELXL-97, G.M. Sheldrick, Universität Göttin-
gen (1986–1997). Data of structure determination and refinement
are summarized in Table 1.
3
. Results and discussion
3.1. Syntheses and spectral characterizations
The enantiomerically pure Schiff base ligands 1a and 1b could
be gained in good yields from the condensation of the amino acid
methyl ester hydrochlorides with salicylaldehyde. The complexes
2
a, 2b and 3a were prepared in quite low yields through the reac-
tion of the ligands with silicon tetrachoride or dimethyltindichlo-
ride, respectively.
1
13
The H and C NMR spectra of the complexes were interpreted
2
9
by comparing them with the spectra of the free ligands. Si,
1
19
respectively
Sn NMR spectroscopy gives first information about
the presence of higher coordinated central atoms in the complexes
29
under investigation. Complex 2a shows two signals at Si NMR
shifts of ꢀ175.5 and ꢀ177.1 ppm, which imply the existence of
two isomers. More detailed NMR spectroscopic analyses could
not be realized due to the fact, that the yield of the complex was
extraordinary low (<1%).
Complex 2b has a chemical shift of ꢀ179.2 ppm in the 29Si NMR
spectrum, which implies hexacoordination. The 13C and 1H NMR
spectra show two signals with a slightly different shift for one car-
bon atom or hydrogen atom respectively, indicating racemization.
There are no significant differences between the chemical shifts of
the carbon atoms of the free ligand in comparison to that of the
1
complex. In the H NMR spectrum the shifts for the hydroxyl
Fig. 1. ORTEP plot of Si(OC
6 4 2
H CH@NC(Ph)COOMe) (2a). The non-hydrogen atoms
hydrogen atom cannot be found because of the complexation of sil-
are drawn with 50% probability ellipsoids.

G. Warncke et al. / Polyhedron 47 (2012) 46–52
49
Table 2
distances, which can be explained by the coordinative character
of these bonds. The angles between 85.23° and 95.09° between
neighboring atoms at silicon and nearly 180° between the opposite
atoms represent a distorted octahedral coordination geometry. The
distortion arises from the shorter bond from silicon to the phenolic
oxygen atoms O3 and O6. As a result the silicon atom is not located
in the same plane as the atoms O1, O3, N1 and N2 and O4, O6, N1
and N2, respectively. In contrast to this O1, O3, O4 and O6 share
one plane with silicon as the angular sum is 360°.
Selected bond lengths of 2a in Å.
Si1–O1
Si1–O3
Si1–O4
Si1–O6
Si1–N1
Si1–N2
1.818(1)
1.728(1)
1.797(1)
1.720(1)
1.898(1)
1.901(1)
N1–C2
N1–C9
C1–C2
N2–C25
N2–C18
C17–C18
1.401(2)
1.303(2)
1.368(2)
1.300(2)
1.409(2)
1.363(2)
In the coordination sphere of silicon the two ligands (O1, O3,
Si1, N1 and O4, O6, Si1, N2) are perpendicular with an angle of
9.74(±0.04)°. If both ligand systems are considered as planar
plane C10 to C15, C9, N1, C2, C1, O1, O2 and second ligand, respec-
tively) their arrangement is also nearly perpendicular with an an-
gle of 78.12° (0.02). Furthermore, the aromatic group (C10–C15) is
tilted in comparison to the plane of Si1, O1, O3, and N1 by
6.78(±0.06)°. In the second ligand molecule of the complex the
angle between the phenyl group (C26–C31) and the plane Si1,
O4, O6, and N2 is only 5.28(±0.05)°. The complex crystallizes in
1
the centrosymmetric space group P2 /c, therefore the compound
is not chiral and has no optical activity as it is found in the free
enantiomerically pure ligand. The chirality was lost during com-
plex formation. This can be explained by a planarization of the
Table 3
Selected bond angles of 2a in °.
8
(
O6–Si1–O3
O3–Si1–O4
O6–Si1–N1
O4–Si1–N1
O6–Si1–N2
O4–Si1–N2
O3–Si1–O1
N1–Si1–N2
C1–C2–N1
C1–C2–C3
N1–C2–C3
93.77(5)
O6–Si1–O1
O4–Si1–O1
O3–Si1–N1
O1–Si1–N1
O3–Si1–N2
O1–Si1–N2
O6–Si1–O4
91.54(5)
85.90(5)
93.99(5)
85.23(5)
91.56(5)
88.97(5)
177.22(5)
88.77(5)
87.44(5)
91.25(5)
95.09(5)
85.97(5)
174.59(5)
173.74(5)
109.8(1)
126.8(1)
123.2(1)
2
C17–C18–N2
C17–C18–C19
N2–C18–C19
109.9(1)
126.2(1)
123.8(1)
a
-carbon atoms C2 and C18 of the amino acid unit. The angular
between 85.23(5)° (O1–Si1–N1) and 95.09(5)° (O6–Si1–N2) indi-
cating the distortion of the coordination polyhedron (see Table 3).
The Si–O distances are comparable to similar octahedral silicon
complexes [11,15]. The Si–N distances are longer than the Si-O
sums around C2 and C18 are 359.8° and 359.9° which confirms
planarization of these atoms. This is supported by the bond dis-
tances N1-C9, N1–C2, and C1–C2 that imply an enlargement of
Ph
N
O
O
O
O
O
Si
O
N
+
-
SiCl4, + 4 NEt3
4 HNEt3Cl
Ph
2a
i-Pr
CH
O
R
N
*
O
O
N
O
Si
O
OMe
O
+
SiCl4
O
OH
-
2 HCl, - 2 MeCl
CH
N
1
1
a: R = Ph (D-phenylglycine derivative)
i-Pr
b: R = i-Pr (L-valine derivative)
2
b
+
Me2SnCl2
-
HCl, - MeCl
Ph
H
N
O
O
Sn
Me
O
Me
3
a
Scheme 1.

5
0
G. Warncke et al. / Polyhedron 47 (2012) 46–52
the conjugated aromatic system of the phenyl group (C10–C15).
The same effect is observed between the atoms N2-C25, N2–C18,
and C17–C18. Therefore a valence structure with silicon coordi-
nated ketene acetal ligands as shown in Scheme 1 can be assigned
to compound 2a.
tive bonding. Apparently the methyl ester function of the ligand
was transformed into a silyl ester during complex formation. That
means the methyl ester group was cleaved off. The octahedral
coordination geometry around the silicon atom is distorted with
angles between neighboring atoms of 84.62(6)–93.80(7)° and near
180° between axial positioned atoms. The carbon atoms C2 and
Single crystals of complex 2b were grown from dichlorometh-
ane. The compound crystallizes in the triclinic crystal system in
the space group Pbar1 with two crystallographically independent
molecules in the asymmetric unit. The discussion of the structural
data will be performed only with one of these molecules. The
structural features of the other molecule are very similar. The
molecular structure of one molecule is shown in Fig. 2, selected
bond lengths and angles in Tables 4 and 5. Two ligand dianions
are coordinated around silicon with octahedral coordination geom-
etry. The isopropyl group of one of the two complex ligands is dis-
ordered (C15, C16, C17) in this molecule. (One of the amino acid
units is disordered in the other crystallographic independent
molecule. See CIF-file for details.) The two Si–N-bonds Si1-N1
and Si1–N2 are longer than the Si–O bonds, that implies coordina-
3
C14 are tetrahedral (sp hybridization), which can be shown at
the sums of the bond angles of the non-hydrogen atoms around
these atoms. The sum of the bond angles N1–C2–C1, C1–C2–C3,
and N1–C2–C3 is 332.6°. The sum of the bond angles at C14 are
319.2° with C15A and 342.3° with C15B. The compound crystal-
ꢀ
lizes in the centrosymmetric space group P1, which is an inversion
center as symmetry element. Chiral compounds that crystallize in
this space group are bound to exist as their racemic form (R and S)
in the crystal structure. In the literature several explanations and
possibilities for the racemization of amino acids have already been
discussed [24–26]. In contrast to 2a, where the chiral information
was lost due to planarization of the amino acid
a carbon atom,
racemization is the reason for the loss of the optical activity in
complex 2b.
In contrast to the experiments with organosilanes, an enantio-
merically pure tin Schiff base complex was isolated from the
6 4 2
Fig. 2. ORTEP plot of Si(OC H CH@NCH(i-Pr)COO) (2b). The non-hydrogen atoms are
drawn with 50% probability ellipsoids.
Table 4
2 6 4
Fig. 3. ORTEP plot of Me Sn(OC H CH@NCH(Ph)COO) (3a). The non-hydrogen atoms
are drawn with 50% probability ellipsoids.
Selected bond lengths of 2b in Å.
Si1–O1
Si1–O3
Si1–N1
1.786(1)
1.736(1)
1.886(2)
Si1–O4
Si1–O6
Si1–N2
1.790(1)
1.724(1)
1.877(2)
Table 6
Selected bond lengths of 3a in Å.
Table 5
Selected bond angles of 2b in °.
Sn1–O1
Sn1–O3
Sn1–N1
2.203(1)
2.090(1)
2.190(1)
Sn1–C16
Sn1–C17
2.095(2)
2.099(2)
O6–Si1–O3
O6–Si1–O1
O3–Si1–O1
O6–Si1–N2
O3–Si1–N2
O1–Si1–N2
O4–Si1–N2
N1–Si1–N2
N1–C2–C1
C1–C2–C3
N1–C2–C3
90.37(7)
89.44(7)
178.52(7)
93.79(7)
91.22(7)
90.26(7)
84.62(6)
171.26(7)
105.5(2)
110.5(1)
116.4(2)
O6–Si1–O4
O3–Si1–O4
O1–Si1–O4
O6–Si1–N1
O3–Si1–N1
O1–Si1–N1
O4–Si1–N1
N2–C14–C13
N2–C14–C15A
N2–C14–C15B
C13–C14–C15A
C13–C14–C15B
178.28(7)
90.32(7)
89.91(7)
93.33(7)
93.80(7)
84.74(4)
88.20(6)
105.3(1))
111.3(3)
115.3(3)
102.6(2)
121.7(3)
Table 7
Selected bond angles of 3a in °.
O3–Sn1–C16
O3–Sn1–C17
C16–Sn1–C17
O3–Sn1–N1
N1–Sn1–O1
94.67(7)
O3–Sn1–O1
C16–Sn1–O1
C17–Sn1–O1
C16–Sn1–N1
C17–Sn1–N1
157.31(5)
89.79(8)
91.63(7)
112.15(7)
104.01(6)
98.05(7)
142.76(8)
83.41(5)
74.33(5)

G. Warncke et al. / Polyhedron 47 (2012) 46–52
51
reaction of 1b with dimethyltindichloride. Single crystals of 3a
3.3. Discussion of reaction mechanisms
were obtained from THF. The molecular structure is shown in
Fig. 3, selected bond lengths and angles in Tables 6 and 7. The com-
pound crystallizes as orthorhombic crystals in the space group
Scheme 1 shows the general outcome of the performed reac-
tions. Three different reaction pathways have been observed. The
reaction of 1a with silicon tetrachloride in presence of triethyl-
amine gives an octahedral complex 2a with a planarized ligand
system. The reaction of the ligand molecule 1b with silicon tetra-
chloride leads to an octahedral complex 2b with a racemized li-
gand system. The reaction of 1a with dimethyltindichloride
yields a pentacoordinate complex 3a showing retention of the chi-
ral information.
1 1 1
P2 2 2 with four molecules in the unit cell. Only one enantiomer
is present in this chiral space group. Inspection of the chirality at
C2 shows, that the R-configuration of the ligand system has been
retained during the synthesis. The pentacoordinated compound is
a 1:1 complex where the dimethyl tin unit is coordinated to one
tridentate Schiff base ligand. The coordination geometry around
the tin atom can be analyzed with the Addison parameter
s [27].
The value of is 0.24, which corresponds to a distorted square pyr-
s
A possible mechanism for the formation of 2a is shown in
Scheme 2 (left column). In a first step (A) the silicontetrachloride
reacts with the phenolic hydroxide group eliminating a molecule
of hydrogen chloride. This is bound by triethylamine as its hydro-
chloride which forms a precipitate. The second eq. triethylamine
amid with the atoms O1, O3, C16 and C17 forming the base and N1
the top of the pyramid. For crystallographic data see experimental
section. The phenylic amino acid group has a nearly perpendicular
orientation to the plane formed by the atoms C10 to C15 and O3,
C9, N1, C2, C1, O1 and O2. The two tin bound methyl groups at
abstracts the
a-proton of the amino acid moiety (B) resulting in
8
8.9° have a nearly perpendicular orientation to the ligand plane.
a planar ketene acetal structure. The second ligand molecule can
react in equal manner leading to the hexacoordinated silicon com-
plex 2a.
The tin atom is in the same plane with the atoms N1, C16 and
C17, which is proved by the angular sum of 358.9°.
H
H
R
N
OMe
O
R
N
OMe
A'
O
+ HCl
+
SiCl4
OH
OSiCl3
+
2 NEt3
B'
H
A
R
OMe
-
HNEt3Cl
+
NEt3
OMe
R
Cl-
H
N
OH
N
O
SiCl3
SiCl3
O
O
B
C'
R
-
2 HCl
-
HNEt3Cl
OMe
N
O
SiCl2
D'
+
R
O
HCl
+
-
1a, + 2 NEt3
2 HNEt3Cl
- MeCl
O
CH
N
O
SiCl2
O
R
+
1b
N
- HCl, - MeCl
R
O
O
CH
O
O
O
O
Si
N
O
O
O
N
Si
O
O
O
R
2a, R = Ph
CH
N
R
b, R = i-Pr
2
Scheme 2.

5
2
G. Warncke et al. / Polyhedron 47 (2012) 46–52
As the reaction of 1b with silicon tetrachloride was carried out
without triethylamine, racemization has taken place under acidic
conditions. During the racemization process there has to be an
tion of chirality in this reaction. The two silicon complexes 2a
and 2b with their surprising structural features allowed us to de-
velop a uniform concept explaining the racemization of the ligand
system during complex formation.
intermediate step which includes a planar coordination, i.e. a
2
sp -hybrid geometry around the
a carbon atom of the amino acid
moiety. A possible mechanism is shown in Scheme 2 (right col-
Appendix A. Supplementary data
umn). Again silicon tetrachloride reacts with the phenolic hydroxyl
0
function and a molecule hydrogen chloride is released (A ). In this
CCDC 876252, 876259, and 876286 contain the supplementary
crystallographic data for 2a, 2b, and 3a, respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
case, it is not neutralized by an equivalent of triethylamine but can
0
protonate the amino acid carbonyl oxygen (B ). The carbocation
neutralizes its positive charge through the formation of a double
0
bond with the
a
carbon atom (C ) under cleavage of the C–H-bond
at the chiral carbon atom. This results in an enlargement of the
conjugated system and a ketene acetal structure, which is the same
intermediate as in the mechanism discussed for the formation of
3
36-033; or e-mail: deposit@ccdc.cam.ac.uk.
References
2
a. In contrast to the formation of 2a the reaction proceeds with
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a
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[
[
[
[
[
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4
. Conclusion
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0
amino acid esters and salicylaldehyde react with silicon tetrachlo-
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(
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