Neutral penta- And hexacoordinate silicon(IV) complexes containing two bidentate ligands derived from the α-amino acids (S)-alanine, (S)-phenylalanine, and (S)-tert-leucine

Article abstract of DOI:10.1002/chem.201000294

The neutral hexacoordinate silicon(IV) complex 6 (SiO₂N ₄ skeleton) and the neutral pentacoordinate silicon(IV) complexes 7-11 (SiO₂N₂C skeletons) were synthesized from Si(NCO)₄ and RSi(NCO)₃ (R = Me, Ph), respectively. The compounds were structurally characterized by solid-state NMR spectroscopy (6-11), solution NMR spectroscopy (6 and 10), and single-crystal X-ray diffraction (8 and 11 were studied as the solvates 8·CH₁CN and 11·C₅H₁₂·0.5CH₃CN, respectively). The silicon(IV) complexes 6 (octahedral Si-coordination polyhedron) and 7-11 (trigonal-bipyramidal Si-coordination polyhedra) each con-tain two bidentate ligands derived from an a-amino acid: (S)-alanine, (S)-phenylalanine, or (S)-tert-leucine. The deprotonated amino acids act as monoanionic (6) or as mono- and dianionic ligands (7-11). The experimental investigations were complemented by computational studies of the stereoisomers of 6 and 7.

Full text of DOI:10.1002/chem.201000294

DOI: 10.1002/chem.201000294
Neutral Penta- and Hexacoordinate Silicon(IV) Complexes Containing Two
Bidentate Ligands Derived from the a-Amino Acids (S)-Alanine,
(S)-Phenylalanine, and (S)-tert-Leucine
Smaranda Cota, Matthias Beyer, Rꢀdiger Bertermann, Christian Burschka, Kathrin Gçtz,
Martin Kaupp, and Reinhold Tacke*[a]
Abstract: The neutral hexacoordinate
silicon(IV) complex 6 (SiO2N4 skele-
ton) and the neutral pentacoordinate
silicon(IV) complexes 7–11 (SiO2N2C
skeletons) were synthesized from
11 were studied as the solvates
8·CH3CN and 11·C5H12·0.5CH3CN, re-
spectively). The silicon(IV) complexes
tain two bidentate ligands derived from
an a-amino acid: (S)-alanine, (S)-phe-
nylalanine, or (S)-tert-leucine. The de-
protonated amino acids act as mono-
anionic (6) or as mono- and dianionic
ligands (7–11). The experimental inves-
tigations were complemented by com-
putational studies of the stereoisomers
of 6 and 7.
6
(octahedral Si-coordination poly-
hedron) and 7–11 (trigonal-bipyramidal
Si-coordination polyhedra) each con-
Si(NCO)4 and RSi
N
Ph), respectively. The compounds were
structurally characterized by solid-state
NMR spectroscopy (6–11), solution
NMR spectroscopy (6 and 10), and
single-crystal X-ray diffraction (8 and
Keywords: amino acids · coordina-
tion chemistry
·
coordination
modes · silicon · stereochemistry
Introduction
gands derived from a-amino acids, namely, compounds 6–
11. To the best of our knowledge, compound 6 is the first
hexacoordinate silicon(IV) complex with ligands derived
from an a-amino acid, (S)-alanine. Contrary to the penta-
coordinate silicon(IV) complexes 1–5 (two dianionic chelate
ligands), compound 6 contains two monoanionic chelate li-
gands and thus represents a neutral species. Quite remark-
ably, in the neutral pentacoordinate silicon(IV) complexes
7–11, both of these different coordination modes are realiz-
ed; one of the two chelate ligands acts as a bidentate di-
anionic ligand (analogous to compounds 1–5), whereas the
other one acts as a bidentate monoanionic ligand (analogous
to compound 6). The studies presented herein were per-
formed as part of our systematic investigations on higher-co-
ordinate silicon compounds (for recent publications, see
ref. [5]; for other recent publications dealing with higher-co-
ordinate silicon compounds, see ref. [6]).
In general, the chemistry of higher-coordinate silicon(IV)
complexes with bidentate mono- and dianionic ligands is
well explored.[1] Very little is known, however, about higher-
coordinate silicon compounds that contain bidentate ligands
derived from a-amino acids. Some years ago, we reported
the first examples of this type, compounds 1–5.[2,3] These
zwitterionic l5Si-silicates contain two identical bidentate di-
anionic ligands derived from glycine, (S)-alanine, (S)-valine,
(S)-tert-leucine, or (S)-phenylalanine. Compounds 1–5 have
been synthesized by using “the zwitterion trick”,[4] that is,
the pentacoordinate (formally negatively charged) silicon
atom has been incorporated into a molecular framework
that also contains a tetracoordinate (formally positively
charged) nitrogen atom. We have now succeeded in synthe-
sizing a series of structurally different neutral higher-coordi-
nate silicon(IV) complexes that also contain bidentate li-
[a] Dipl.-Chem. S. Cota, Dipl.-Chem. M. Beyer, Dr. R. Bertermann,
Dr. C. Burschka, Dr. K. Gçtz, Prof. Dr. M. Kaupp, 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
6582
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Chem. Eur. J. 2010, 16, 6582 – 6589

FULL PAPER
anionic O,N ligands of 6–11. It is of interest to note that the
oligopeptide formation from Me3SiOC(O)CHRN
(R=Me, Bn, tBu) only occurs upon addition of the (cyana-
to-N)silanes (Si(NCO)4, MeSi(NCO)3, and PhSi(NCO)3, re-
ACHTUNGRTEN(NUNG SiMe3)H
A
R
ACHTUNGTRENNUNG
spectively); NMR spectroscopy studies performed with tri-
methylsilyl (S)-N-(trimethylsilyl)alaninate that was stored
without solvent or as a solution in dichloromethane at room
temperature over a period of three weeks did not show any
evidence for oligopeptide formation.
Results and Discussion
The identities of 6–11 were established by elemental anal-
yses (C, H, N), single-crystal X-ray diffraction studies (8 and
11 were studied as the solvates 8·CH3CN and
11·C5H12·0.5CH3CN), and VACP/MAS NMR experiments
(13C, 15N, 29Si). Owing to their poor solubility in organic sol-
vents and/or decomposition upon dissolution, compounds 7–
9 and 11 could not be characterized by NMR spectroscopy
Syntheses: Compounds 6–11 were synthesized according to
Scheme 1 by the treatment of tetra(cyanato-N)silane,[7] tri-
(cyanato-N)methylsilane,[7] and tri(cyanato-N)phenylsilane[8]
with two molar equivalents of trimethylsilyl (S)-N-(trime-
thylsilyl)alaninate,[9] trimethylsilyl (S)-N-(trimethylsilyl)phe-
in solution, whereas studies of
([D6]DMSO) were possible.
6 and 10 in solution
Crystal structure analyses: Compounds 6, 7, 8·CH3CN, 9, 10,
and 11·C5H12·0.5CH3CN were structurally characterized by
single-crystal X-ray diffraction. The crystal data and the ex-
perimental parameters used for the crystal structure analy-
ses are given in Table 1. The molecular structures of 6–11 in
the crystal are shown in Figures 1–6; selected bond lengths
and angles are given in the figure legends. All compounds
ꢀ
studied form intermolecular N H···O hydrogen bonds that
lead to the formation of infinite three-dimensional (6, 7,
8·CH3CN) or two-dimensional (9) networks, or to infinite
one-dimensional chains (10, 11·C5H12·0.5CH3CN) in the
crystal.[10]
As can be seen from Figure 1, the Si-coordination poly-
hedron of 6 is a somewhat distorted octahedron. The two
monodentate cyanato-N ligands and the two oxygen ligand
atoms O1 and O1A each occupy cis positions, whereas the
two amino nitrogen atoms N1 and N1A are in trans posi-
tions. The maximum deviations from the ideal 90 and 1808
angles amount to 4.578 (N1-Si-N2, 94.57(5)8) and 7.928 (N1-
Scheme 1. Syntheses of compounds 6–11.
nylalaninate,[9] and trimethylsilyl (S)-N-(trimethylsilyl)-tert-
leucinate,[9] respectively. The syntheses were performed at
ꢀ70 to ꢀ208C (6–8, 10), at ꢀ30 to ꢀ208C (9), and at ꢀ45 to
ꢀ208C (11). All syntheses were carried out in acetonitrile.
Compound 6–11 were isolated as colorless crystalline solids
(yields: 51 (6), 56 (7), 34 (8), 48 (9), 57 (10), and 49% (11)).
The moderate yields of compounds 6–11 can be explained
by the occurrence of a side reaction of the respective disily-
Si-N1A, 172.08(8)8), respectively. The Si O distances
ꢀ
ꢀ
(1.7995(10) ꢃ) of 6 are similar to the axial Si O bond
lengths of the distorted trigonal-bipyramidal Si-coordination
polyhedra of compounds 1–5 (1.8058(14)–1.8524(13) ꢃ) and
ꢀ
ꢀ
7–11 (1.7871(14)–1.8038(11) ꢃ). The Si N1 and Si N1A
bond lengths of 6 (1.8857(11) ꢃ) are significantly shorter
ꢀ
than the axial Si N distances of compounds 7–10
(1.9658(17)–1.9932(14) ꢃ) (same coordination mode). The
ꢀ
ꢀ
lated amino acid [Me3SiOC(O)CHRN
Bn, tBu), which leads to the formation of an oligopeptide of
the type Me3SiOC(O)CHR[N(SiMe3)C(O)CHR]nN-
(SiMe3)H. The oligopeptide formation involves the elimina-
tion of trimethylsilanol (Me3SiOH), which can undergo a
condensation reaction to yield hexamethyldisiloxane
(Me3SiOSiMe3) and water. Both the oligopeptide and the
disiloxane could be detected as by-products in the synthesis
of 6–8 (NMR spectroscopy studies). The water formed in
this side reaction may be the proton source that could ex-
plain the existence of the NH2 group of the bidentate mono-
(SiMe3)H] (R=Me,
Si N distances of the Si NCO moieties of 6 are similar to
those of other hexacoordinate silicon(IV) complexes with
cyanato-N ligands.[5b,11] The same holds true for the N-C-O
AHCTUNGTRENNUNG
ꢀ
G
angles of the Si NCO groups, whereas the Si-N2-C1 and Si-
N2A-C1A angles (146.33(13)8) of 6 are significantly smaller
than the Si-N-C angles of the Si NCO groups of other hexa-
ꢀ
coordinate
silicon(IV)
complexes
(151.62(12)–
169.77(12)8).[5b,11]
As can be seen from Figures 2–5, the Si-coordination
polyhedra of 7, 8·CH3CN, 9 (two molecules in the asymmet-
ric unit), and 10 are distorted trigonal bipyramids, with
Chem. Eur. J. 2010, 16, 6582 – 6589
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6583

R. Tacke et al.
Table 1. Crystallographic data for compounds 6, 7, 8·CH3CN, 9, 10, and 11·C5H12·0.5CH3CN.
6
7
8·CH3CN
9
10
11·C5H12·0.5CH3CN
formula
Mr
C8H12N4O6Si
288.31
C7H14N2O4Si
218.29
C14H19N3O4Si
321.41
C19H22N2O4Si
370.48
C24H24N2O4Si
432.54
C24H41.5N2.5O4Si
457.19
T [K]
100(2)
193(2)
193(2)
193(2)
100(2)
100(2)
l (MoKa) [ꢃ]
crystal system
space group (no.)
a [ꢃ]
b [ꢃ]
c [ꢃ]
0.71073
tetragonal
P41212 (92)
7.69110(10)
7.69110(10)
20.1180(4)
90
1190.04(3)
4
1.609
0.71073
orthorhombic
P212121 (19)
7.6563(6)
10.9386(9)
12.4438(12)
90
1042.16(16)
4
1.391
0.71073
orthorhombic
P212121 (19)
8.5016(7)
11.1129(13)
17.5068(16)
90
1654.0(3)
4
1.291
0.71073
orthorhombic
P212121 (19)
6.5828(6)
23.3334(18)
24.524(2)
90
3766.9(6)
8
1.307
0.71073
monoclinic
P21 (4)
6.6931(4)
13.5206(9)
11.9450(8)
103.534(2)
1050.94(12)
2
0.71073
tetragonal
I41 (80)
17.9928(2)
17.9928(2)
16.1878(4)
90
5240.65(15)
8
1.159
b [8]
V [ꢃ3]
Z
1calcd [gcmꢀ3
]
1.367
0.147
456
m [mmꢀ1
(000)
]
0.230
600
0.218
464
0.162
680
0.151
1568
0.121
1992
F
A
crystal size [mm]
2q range [8]
index ranges
0.2ꢄ0.1ꢄ0.1
5.68–61.00
ꢀ10ꢁhꢁ10
ꢀ7ꢁkꢁ10
ꢀ25ꢁlꢁ28
9842
0.5ꢄ0.5ꢄ0.5
7.28–56.16
ꢀ10ꢁhꢁ9
ꢀ11ꢁkꢁ14
ꢀ16ꢁlꢁ16
6283
0.5ꢄ0.5ꢄ0.5
5.92–56.08
ꢀ11ꢁhꢁ10
ꢀ14ꢁkꢁ13
ꢀ23ꢁlꢁ18
8202
0.5ꢄ0.4ꢄ0.3
4.82–55.96
ꢀ8ꢁhꢁ8
ꢀ29ꢁkꢁ30
ꢀ26ꢁlꢁ32
27644
0.36ꢄ0.1ꢄ0.1
3.50–57.04
ꢀ8ꢁhꢁ8
ꢀ18ꢁkꢁ16
ꢀ16ꢁlꢁ15
30894
0.3ꢄ0.1ꢄ0.03
3.20–66.30
ꢀ27ꢁhꢁ27
ꢀ27ꢁkꢁ27
ꢀ24ꢁlꢁ24
102915
collected reflns
unique reflns
1812
2512
3983
8931
5062
10016
Rint
0.0357
0.0220
0.0349
0.0414
0.0520
0.0475
restraints
0
0
0
0
1
41
parameters
97
1.146
0.0405/0.3333
0.0307
0.0816
139
1.061
0.0530/0.0676
0.0301
0.0801
211
1.072
0.0630/0.0000
0.0445
0.1028
489
0.977
0.0455/0.0000
0.0328
0.0755
289
1.029
0.0264/0.3140
0.0269
0.0667
304
1.077
0.0604/1.9965
0.0373
0.1032
S[a]
weight parameters a/b[b]
R1[c] [I>2s(I)]
wR2[d] (all data)
Flack parameter
0.02(14)
0.00(12)
ꢀ0.01(13)
0.03(7)
0.05(7)
0.00(6)
max/min residual electron density [eꢃꢀ3
2
]
+0.358/ꢀ0.214 +0.235/ꢀ0.151 +0.345/ꢀ0.209 +0.276/ꢀ0.178 +0.266/ꢀ0.192 +0.494/ꢀ0.438
2
2
[a] S={S[w
A
E
; ACHTUNGTRENNUNG ACHTUNGRTENNGUN
n=number of reflections; p=number of parameters. [b] wꢀ1 =s2(Fo )+(aP)2 +bP, with P=[max(Fo ,0)+2Fc2]/3.
2
2
[c] R1=SjjFo jꢀjFc jj/SjFo j. [d] wR2={S[w
A
(Fo )2]}0.5
ACHTUNGTERNNUNG .
Berry distortions (transition trigonal bipyramid!square
pyramid; pivot atom C1) of 20.7 (7), 23.8 (8·CH3CN), 21.2
(9, molecule I), 12.0 (9, molecule II), and 19.6% (10).[10]
Figure 2. Molecular structure of 7 in the crystal (probability level of dis-
ꢀ
Figure 1. Molecular structure of 6 in the crystal (probability level of dis-
placement ellipsoids 50%). Selected bond lengths [ꢃ] and angles [8]: Si
ꢀ
ꢀ
ꢀ
ꢀ
placement ellipsoids 50%). Selected bond lengths [ꢃ] and angles [8]: Si
O1 1.7910(13), Si O2 1.7118(11), Si N1 1.6901(14), Si N2 1.9932(14),
ꢀ
ꢀ
ꢀ
O1 1.7995(10), Si N1 1.8857(11), Si N2 1.8026(12); O1-Si-O1A 88.27(7),
O1-Si-N1 86.51(5), O1-Si-N1A 87.80(5), O1-Si-N2 89.77(5), O1-Si-N2A
176.83(5), N1-Si-N1A 172.08(8), N1-Si-N2 94.57(5), N1-Si-N2A 90.92(5),
N2-Si-N2A 92.29(8), Si-N2-C1 146.33(13), N2-C1-O3 176.99(19).
Si C1 1.8401(18); O1-Si-O2 86.63(6), O1-Si-N1 87.69(6), O1-Si-N2
167.64(6), O1-Si-C1 95.29(8), O2-Si-N1 123.85(7), O2-Si-N2 83.57(6), O2-
Si-C1 111.63(8), N1-Si-N2 91.52(6), N1-Si-C1 124.52(9), N2-Si-C1
95.31(8).
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Silicon Complexes with Ligands Derived from a-Amino Acids
FULL PAPER
Figure 3. Molecular structure of 8 in the crystal of 8·CH3CN (probability
level of displacement ellipsoids 50%). Selected bond lengths [ꢃ] and
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: Si O1 1.7871(14), Si O2 1.7082(14), Si N1 1.6885(17), Si N2
ꢀ
1.9658(17), Si C1 1.854(2); O1-Si-O2 86.98(7), O1-Si-N1 87.68(7), O1-Si-
N2 168.25(7), O1-Si-C1 96.12(8), O2-Si-N1 125.90(9), O2-Si-N2 84.22(7),
O2-Si-C1 110.61(9), N1-Si-N2 91.18(8), N1-Si-C1 123.49(10), N2-Si-C1
94.27(8).
The two axial positions of 7–10 are occupied by the O1
oxygen atom and the N2 nitrogen atom (NH2 group), where-
as the O2 oxygen atom, the N1 nitrogen atom (NH group),
and the C1 carbon atom are found in the three equatorial
sites. Thus, compounds 7–10 have non-VSEPR (VSEPR =
valence-shell electron pair repulsion) structures because
only one of the two oxygen atoms occupies an axial position.
This phenomenon is not yet understood, but computational
studies of 7 also demonstrated the non-VSEPR structure to
be more stable than the isomeric structure with the two
oxygen ligand atoms in the axial sites (see the Computation-
al Studies section). The experimental finding, however,
should not be overestimated; the energy differences be-
tween the various isomers of 7 are rather small, and the dif-
ferent structures observed for compounds 7–10 (non-
VSEPR structures) and 11 (VSEPR structure) might be the
result of crystal-packing effects.
Figure 4. Molecular structures of the two crystallographically independ-
ent molecules (I and II) of 9 in the crystal (probability level of displace-
ment ellipsoids 50%). Selected bond lengths [ꢃ] and angles [8] of mole-
ꢀ
ꢀ
ꢀ
ꢀ
cule I: Si1 O1 1.8038(11), Si1 O2 1.7207(12), Si1 N1 1.7052(15), Si1 N2
ꢀ
1.9731(13), Si1 C1 1.8654(18); O1-Si1-O2 87.85(6), O1-Si1-N1 86.63(6),
O1-Si1-N2 169.01(6), O1-Si1-C1 96.42(7), O2-Si1-N1 124.53(7), O2-Si1-
N2 85.19(5), O2-Si1-C1 112.11(7), N1-Si1-N2 90.32(6), N1-Si1-C1
123.36(8), N2-Si1-C1 94.07(7). Selected bond lengths [ꢃ] and angles [8]
ꢀ
ꢀ
ꢀ
of molecule II: Si21 O21 1.7995(10), Si21 O22 1.7220(12), Si21 N21
ꢀ
ꢀ
1.7114(13), Si21 N22 1.9823(12), Si21 C21 1.8581(17); O21-Si21-O22
87.38(5), O21-Si21-N21 87.39(5), O21-Si21-N22 170.55(6), O21-Si21-C21
93.88(7), O22-Si21-N21 120.56(6), O22-Si21-N22 85.21(5), O22-Si21-C21
115.63(7), N21-Si21-N22 91.34(5), N21-Si21-C21 123.80(7), N22-Si21-C21
94.61(7).
ꢀ
The sums of the equatorial bond angles of 7–10 amount
to 3608 and the axial O1-Si-N2 angles are in the range
167.64(6)–170.55(6)8. The equatorial O2-Si-N1, O2-Si-C1,
and N1-Si-C1 angles are in the ranges 120.56(6)–125.90(9),
110.61(9)–115.63(7), and 123.36(8)–124.52(9)8, respectively.
Si C bond lengths of the zwitterions 1–5 (1.906(3)–
1.9274(18) ꢃ).
As in the case of compounds 7–10, the Si-coordination
polyhedron of 11·C5H12·0.5CH3CN is a distorted trigonal bi-
pyramid (Figure 6; Berry distortion=26.8%),[10] however,
the coordination mode of the bidentate monoanionic O,N
ligand of 11 is quite different. The oxygen atoms O1 and O2
of 11 occupy the two axial positions, whereas in the case of
compounds 7–10 only one of the two oxygen atoms of the
ligand are found in an axial position. Therefore, in contrast
to compounds 7–10, the structure of 11 is compatible with
the VSEPR concept. The O1-Si-O2 angle of 11 amounts to
164.88(4)8 and the sum of the equatorial bond angles is
ꢀ
The axial Si O1 distances of compounds 7–10
(1.7871(14)–1.8038(11) ꢃ) are somewhat longer than the
ꢀ
equatorial Si O2 bond lengths (1.7042(10)–1.7220(12) ꢃ)
ꢀ
and similar to the axial Si O distances of the zwitterionic
compounds 1–5 (1.8058(14)–1.8356(19) ꢃ). The equatorial
ꢀ
Si N1 bond lengths of 7–10 (1.6885(17)–1.7114(13) ꢃ) are
ꢀ
substantially shorter than the axial Si N2 distances
ꢀ
(1.9658(17)–1.9932(14) ꢃ) and similar to the equatorial Si
N bond lengths of 1–5 (1.7087(17)–1.725(3) ꢃ). The equato-
ꢀ
ꢀ
359.98. The axial Si O1 (1.8199(8) ꢃ) and Si O2
(1.7876(8) ꢃ) bond lengths are similar to the axial Si O dis-
tances of compounds 1–5 (1.8058(14)–1.8356(19) ꢃ) and 7–
ꢀ
ꢀ
rial Si C1 distances of compounds 7–10 (1.8401(18)–
1.8654(18) ꢃ) are significantly shorter than the equatorial
Chem. Eur. J. 2010, 16, 6582 – 6589
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6585

R. Tacke et al.
Figure 5. Molecular structure of 10 in the crystal (probability level of dis-
ꢀ
placement ellipsoids 50%). Selected bond lengths [ꢃ] and angles [8]: Si
ꢀ
ꢀ
ꢀ
ꢀ
O1 1.8004(9), Si O2 1.7042(10), Si N1 1.6991(12), Si N2 1.9677(11), Si
C1 1.8583(13); O1-Si-O2 88.35(5), O1-Si-N1 87.11(5), O1-Si-N2
169.43(5), O1-Si-C1 95.04(5), O2-Si-N1 123.98(5), O2-Si-N2 84.66(5), O2-
Si-C1 111.65(5), N1-Si-N2 90.24(5), N1-Si-C1 124.37(6), N2-Si-C1
94.90(5).
Figure 6. Molecular structure of 11 in the crystal of 11·C5H12·0.5CH3CN
(probability level of displacement ellipsoids 50%). Selected bond lengths
ꢀ
ꢀ
ꢀ
[ꢃ] and angles [8]: Si O1 1.8199(8), Si O2 1.7876(8), Si N1 1.7040(9),
ꢀ
ꢀ
Si N2 1.8842(9), Si C1 1.8659(11); O1-Si-O2 164.88(4), O1-Si-N1
86.48(4), O1-Si-N2 84.91(4), O1-Si-C1 96.70(4), O2-Si-N1 90.28(4), O2-
Si-N2 84.86(4), O2-Si-C1 97.59(4), N1-Si-N2 124.96(4), N1-Si-C1
121.45(5), N2-Si-C1 113.53(4).
ꢀ
10 (1.7871(14)–1.8038(11) ꢃ). The equatorial Si N1 (NH
ꢀ
group) bond length of 11 is similar to the equatorial Si N
(NH group) distances of compounds 1–5 (1.7087(17)–
1.725(3) ꢃ) and 7–10 (1.6885(17)–1.7114(13) ꢃ), and the
ꢀ
equatorial Si N2 (NH2 group) bond length (1.8842(9) ꢃ) is
ration of 6a corresponds to the experimentally established
structure of 6 in the crystal (Figure 1). Based on these re-
sults, it is assumed that the two most stable isomers 6a and
6b represent the two species (molar ratio, ca. 1:1) detected
by solution NMR spectroscopy (see below).
The configurations of the six possible stereoisomers of 7,
the diastereoisomers 7a–7 f, and their calculated relative en-
ergies are shown in Figure 8. The maximum energy differ-
ence amounts to 12.5 kJmolꢀ1. The configuration of 7a cor-
responds to the experimentally established structures of 7
ꢀ
somewhat shorter than the axial Si N (NH2 group) distan-
ces of 7–10 (1.9658(17)–1.9932(14) ꢃ).
Computational studies: To obtain more information about
the stereochemistry of compounds 6–11, structure optimiza-
tions and frequency calculations for the eight possible ste-
reoisomers of the hexacoordinate silicon(IV) complex 6 and
for the six possible stereoisomers of the pentacoordinate sili-
con(IV) complex 7 were performed. In these studies, com-
pound 7 also served as a model
system for the related penta-
coordinate silicon compounds
8–11. The computational stud-
ies were performed with the
TURBOMOLE 5.10[12] program
package at the BP86[13]/SVP[14]
level of theory.
The configurations of the
eight possible stereoisomers of
6, the diastereoisomers 6a–6h,
and their calculated relative en-
ergies are shown in Figure 7.
The maximum energy differ-
ence amounts to 42.5 kJmolꢀ1.
The three isomers with both
NH2 groups in the trans posi-
tions (2.5 (6a),
0 (6b) and
14.2 kJmolꢀ1 (6g)) are the most
Figure 7. Configurations and calculated relative energies for the eight possible stereoisomers of compound 6,
stable species, and the configu- the diastereoisomers 6a–6h.
6586
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 6582 – 6589

Silicon Complexes with Ligands Derived from a-Amino Acids
FULL PAPER
compounds 7–9 and 11 could not be characterized in solu-
tion.
Conclusion
With the synthesis of 6, we have succeeded in preparing the
first hexacoordinate silicon(IV) compound with ligands de-
rived from an a-amino acid. In this neutral hexacoordinate
silicon(IV) complex, (S)-alanine behaves as a bidentate
monoanionic O,N ligand. For the first time, this particular
coordination mode has also been realized for pentacoordi-
nate silicon compounds, namely, the neutral silicon(IV)
complexes 7–11. In these compounds, however, the respec-
tive a-amino acids ((S)-alanine, (S)-phenylalanine, and (S)-
tert-leucine) also act as bidentate dianionic O,N ligands. On
the basis of these results, the synthesis of a whole variety of
novel penta- and hexacoordinate silicon(IV) complexes with
mono- and/or dianionic bidentate O,N ligands that derive
from natural and unnatural a-amino acids should be possi-
ble. Future studies are needed to explore the potential of
this chemistry. In this context, it should be mentioned that
compounds 7 and 9 and other related silicon(IV) complexes
with an SiMe moiety and two bidentate ligands derived
from different a-amino acids might be of interest for practi-
cal applications. The hydrolysis of these compounds would
lead to the formation of the respective free a-amino acids
and methylsilanetriol (MeSi(OH)3), a silicon food supple-
ment of long-standing use in humans that is currently being
discussed as a potential drug for the improvement of bone
and other connective tissue health.[15] Compounds 7 and 9
and related silicon(IV) complexes might also serve as silicon
food supplements or as pro-drugs for methylsilanetriol.
Figure 8. Configurations and calculated relative energies for the six possi-
ble stereoisomers of 7, the diastereoisomers 7a–7 f.
and 8 (studied as 8·CH3CN) in the crystal, whereas 7c corre-
sponds to the structure of 11 in the crystal of
11·C5H12·0.5CH3CN (Figure 6). The configuration of 7b cor-
responds to the crystal structures of 9 and 10 (Figures 4 and
5). As the maximum energy difference observed for 7a–7 f
is rather small, it is not surprising that three different config-
urations, namely, 7a, 7b, and 7c, are realized in the crystal
structures of the pentacoordinate silicon(IV) complexes
studied. Due to this small energy difference, a configuration-
al assignment for the two species of 10 (molar ratio, ca.
1:1.3) detected by solution NMR spectroscopy is not possi-
ble (see below).
NMR spectroscopic studies in the solid state and in solution:
Compounds 6–11 were characterized by solid-state VACP/
MAS NMR spectroscopy (13C, 15N, 29Si). The spectra ob-
tained are compatible with the results of the crystal struc-
ture analyses. The isotropic 29Si chemical shifts (d=ꢀ187.4
(6), ꢀ84.4 (7), ꢀ96.0 (8), ꢀ87.9/ꢀ85.5 (9; two crystallo-
graphically independent molecules), ꢀ98.5 (10), ꢀ92.3 ppm
(11)) clearly indicate the presence of hexacoordinate (6)
and pentacoordinate (7–11) silicon atoms. The hexacoordi-
nate silicon(IV) complex 6 and the pentacoordinate silico-
n(IV) complex 10 were also studied by solution NMR spec-
troscopy (1H, 13C, 15N, 29Si) in [D6]DMSO. The data obtained
indicate the presence of more than one species in solution
in both cases. The isotropic 29Si chemical shifts clearly indi-
cate the existence of two hexacoordinate species in the case
of 6 (d=ꢀ191.9/ꢀ191.8 ppm; molar ratio, ca. 1:1 (c=
20 mm)) and the existence of two pentacoordinate species in
the case of 10 (d=ꢀ99.1/ꢀ97.7 ppm; molar ratio, ca. 1:1.3
(c=72 mm)). The two species of 6 in solution are most
likely to be the diastereoisomers 6a and 6b, whereas a con-
figurational assignment for the two isomers of 10 in solution
is not possible (see the Computational Studies section). Due
to their poor solubility and/or decomposition in [D6]DMSO,
Experimental Section
General procedures: All syntheses were carried out under dry argon. The
organic solvents used were dried and purified according to standard pro-
cedures and stored under dry nitrogen. Melting points were determined
with a Bꢁchi Melting Point B-540 apparatus using samples in sealed ca-
pillaries. The 1H, 13C, 15N, and 29Si NMR spectroscopy studies in solution
were performed at 238C on a Bruker Avance 500 (1H, 500.1 MHz; 13C,
125.8 MHz; 15N, 50.7 MHz; 29Si, 99.4 MHz) or on a Bruker DRX-300
NMR spectrometer (15N, 30.4 MHz; 6 only). [D6]DMSO was used as the
solvent. Chemical shifts (ppm) were determined relative to internal
[D5]DMSO (1H, d=2.49 ppm), internal [D6]DMSO (13C, d=39.5 ppm),
external formamide (15N, d=ꢀ268.0 ppm), or external TMS (29Si, d=
0 ppm). Analysis and assignment of the 1H NMR data was supported by
2D 1H–1H, 13C–1H, 15N–1H, and 29Si–1H correlation experiments. Assign-
ment of the 13C NMR data was supported by DEPT 135 and the above-
mentioned 13C–1H correlation experiments. 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 300 mg of sample (13C, 100.6 MHz; 15N,
40.6 MHz; 29Si, 79.5 MHz; external standard, TMS (13C and 29Si, d=
0 ppm) or glycine (15N, d=ꢀ342.0 ppm); spinning rate, 6–7 kHz; contact
time, 2 (13C), 1–3 (15N), or 3–5 ms (29Si); 908 1H transmitter pulse length,
3.6 ms; repetition time, 4 s).
Synthesis of 6: Trimethylsilyl (S)-N-(trimethylsilyl)alaninate (1.17 g,
5.01 mmol) was added in a single portion at ꢀ708C to a stirred solution
Chem. Eur. J. 2010, 16, 6582 – 6589
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6587

R. Tacke et al.
of tetra(cyanato-N)silane (490 mg, 2.50 mmol) in acetonitrile (15 mL),
and the mixture was then kept undisturbed at ꢀ208C for 3 d. The result-
ing colorless, crystalline solid was isolated by filtration, washed with cold
(08C) n-pentane (3ꢄ10 mL), and dried in vacuo (0.8 mbar, 208C, 6 h);
yield, 365 mg (1.27 mmol, 51%). M.p. >2308C (decomp). The following
solution NMR data refer to two isomers (molar ratio, ca. 1:1): 1H NMR
238C, 24 h); yield, 628 mg (1.45 mmol, 57%). M.p. >3208C (decomp).
The following solution NMR data refer to two isomers (molar ratio A:B,
ca. 1:1.3): 1H NMR (c=72 mm): d=2.51–3.24 (m, 4H; CH2, A and B),
2.89 (A) and 2.95 (B) (2d, 3Jꢂ1.7 Hz, 1H; NH, A and B), 3.53–3.94 (m,
2H; CH, A and B), 5.11 (t, J=11.0 Hz; B), 5.50 (dd, J=12.0, J=9.2 Hz,
A), and 6.05–6.11 (m, 2H; NH2, A and B), 7.14–7.43 ppm (m, 15H;
C6H5, A and B); 13C NMR (c=72 mm): d=34.9/41.54 (CH2, B), 36.1/
41.48 (CH2, A), 54.4/57.2 (CH, A), 55.4/56.7 (CH, B), 126.2, 126.3, 126.6,
126.7, 126.8, 127.5, 127.6, 128.07, 128.09, 128.26, 128.33, 128.36, 128.44,
128.6, 128.7, 129.1, 129.15, 129.18, 129.4, 129.5, 131.8, 132.6, 136.6, 138.4,
137.3, 138.3, 138.6, and 139.6 (C6H5, A and B), 170.5/174.4 (CO, B),
171.2/174.5 ppm (CO, A); 15N NMR (50.7 MHz; c, 72 mm): d=ꢀ340.5
(NH, B), ꢀ337.4 (NH, A), ꢀ327.9 (NH2, B), ꢀ326.8 ppm (NH2, A);
(c=20 mm): d=1.32/1.33 (d, 6H, 3J
ACTHNUGTRNE(NUG H,H)=7.3 Hz; CH3), 3.48–3.57/3.57–
3.66 (m, 2H; CH), 6.58–6.69 and 7.10–7.28 ppm (m, 4H; NH2); 13C NMR
(c=20 mm): d=16.4/16.8 (CH3), 49.81/49.83 (CH), 118.7 (br“s”; NCO),
171.2/171.4 ppm (CO); 15N NMR (30.4 MHz; c, 20 mm)): d=ꢀ322.0 ppm
(NH2, 15N signals for the two isomers not resolved), 15N signals for the
NCO moieties not detected; 29Si NMR (c
=
20 mm): d=ꢀ191.9/
ꢀ191.8 ppm; 13C VACP/MAS NMR: d=18.7 (CH3), 51.0 (asymmetric
“d”; CH),[16] 122.8 (br “d”; NCO),[16] 177.0 ppm (CO); 15N VACP/
MAS NMR: d=ꢀ315.9 (NCO), ꢀ314.2 ppm (NH2); 29Si VACP/
MAS NMR: d=ꢀ187.4 ppm (br“s”);[16] elemental analysis calcd (%) for
C8H12N4O6Si (288.29): C 33.33, H 4.20, N 19.43; found: C 33.5, H 4.3, N
19.3.
29Si NMR (c
=
72 mm): d=ꢀ99.1 (B), ꢀ97.7 ppm (A); 13C VACP/
MAS NMR: d=32.9 (CH2), 40.1 (CH2), 54.2 (br asymmetric “d”; CH),[16]
57.9 (asymmetric “d”; CH),[16] 127.3, 127.6, 127.8, 128.4, 129.2, 130.2,
130.6, 133.0, 134.7, and 137.7 (C6H5), 168.9 (CO), 181.2 ppm (CO); 15N
VACP/MAS NMR: d=ꢀ334.0 (NH), ꢀ324.4 ppm (NH2); 29Si VACP/
MAS NMR: d=ꢀ98.5 ppm (br asymmetric “d”);[16] elemental analysis
calcd (%) for C24H24N2O4Si (432.55): C 66.64, H 5.59, N 6.48; found: C
66.3, H 5.5, N 6.6.
Synthesis of 7: Trimethylsilyl (S)-N-(trimethylsilyl)alaninate (1.17 g,
5.01 mmol) was added in a single portion at ꢀ708C to a stirred solution
of tri(cyanato-N)methylsilane (423 mg, 2.50 mmol) in acetonitrile
(10 mL), and the mixture was then kept undisturbed at ꢀ208C for 3 d.
The resulting colorless crystalline solid was isolated by filtration, washed
with cold (08C) n-pentane (3ꢄ10 mL), and dried in vacuo (0.8 mbar,
208C, 24 h); yield, 306 mg (1.40 mmol, 56%). M.p. >2888C (decomp);
13C VACP/MAS NMR: d=4.6 (SiCH3), 16.9 (CCH3), 22.7 (CCH3), 50.1
(asymmetric “d”; CH),[16] 52.3 (asymmetric “d”; CH),[16] 176.1 (CO),
180.0 ppm (CO); 15N VACP/MAS NMR: d=ꢀ325.8 (NH or NH2),
ꢀ324.1 ppm (NH or NH2); 29Si VACP/MAS NMR: d=ꢀ84.4 ppm (asym-
metric “d”);[16] elemental analysis calcd (%) for C7H14N2O4Si (218.28): C
38.52, H 6.46, N 12.83; found: C 38.5, H 6.4, N 12.8.
Synthesis of 11: Trimethylsilyl (S)-N-trimethylsilyl)leucinate (1.50 g,
5.44 mmol) was added in a single portion at ꢀ458C to a stirred solution
of tri(cyanato-N)phenylsilane (610 mg, 2.64 mmol) in acetonitrile
(20 mL), and the mixture was held at ꢀ458C for 5 min, layered with n-
pentane (15 mL), and then kept undisturbed at ꢀ208C for 20 d. The re-
sulting colorless crystalline solid was isolated by filtration, washed with
cold (08C) n-pentane (2ꢄ10 mL), and dried in vacuo (0.8 mbar, 608C,
14 h); yield, 468 mg (1.28 mmol, 49%). M.p. 2418C (subl); 13C VACP/
MAS NMR: d=28.2 (CACTHUNGTRENNN(UG CH3)3), 34.8 (CACHUTNRTEGN(GUNN CH3)3), 37.4 (CCAHTNUGTREN(NUGN CH3)3), 65.8 (br
asymmetric “d”; CH),[16] 128.4, 129.0, 130.8, 137.3 and 138.7 (C6H5), 171.6
(CO), 179.4 ppm (CO), resonance signals for n-pentane and acetonitrile
not detected; 15N VACP/MAS NMR: d=ꢀ339.2 (NH or NH2),
ꢀ328.8 ppm (NH or NH2); 29Si VACP/MAS NMR: d=ꢀ92.3 ppm
(br“s”);[16] elemental analysis calcd (%) for C18H28N2O4Si (364.52): C
59.31, H 7.74, N 7.69; found: C 58.7, H 7.8, N 7.7.
Synthesis of 8: Trimethylsilyl (S)-N-(trimethylsilyl)alaninate (1.17 g,
5.01 mmol) was added in a single portion at ꢀ708C to a stirred solution
of tri(cyanato-N)phenylsilane (578 mg, 2.50 mmol) in acetonitrile
(10 mL), and the mixture was then kept undisturbed at ꢀ208C for 3 d.
The resulting colorless crystalline solid was isolated by filtration, washed
with cold (08C) n-pentane (2ꢄ10 mL), and dried in vacuo (0.1 mbar,
608C, 8 h); yield, 240 mg (856 mmol, 34%). M.p. >3608C (decomp); 13C
VACP/MAS NMR: d=15.5 (CH3), 22.1 (CH3), 50.3 (br asymmetric “d”;
CH),[16] 52.7 (asymmetric “d”; CH),[16] 128.2, 130.6, and 138.5 (C6H5),
173.9 (CO), 177.4 ppm (CO); 15N VACP/MAS NMR d=ꢀ323.2 (NH or
NH2), ꢀ320.3 ppm (NH or NH2); 29Si VACP/MAS NMR: d=ꢀ96.0 ppm
(br asymmetric “d”);[16] elemental analysis calcd (%) for C12H16N2O4Si
(280.36): C 51.41, H 5.75, N 9.99; found: C 50.7, H 5.7, N 9.7.
Crystal structure analyses: Suitable single crystals of 6, 7, 8·CH3CN, 9, 10,
and 11·C5H12·0.5CH3CN were obtained directly from the respective reac-
tion mixtures (see the Synthesis section). The crystals were mounted in
inert oil (perfluoropolyalkyl ether, ABCR) on a glass fiber and then
transferred to the cold nitrogen gas stream of the diffractometer (Stoe
IPDS (7, 8·CH3CN, 9; graphite-monochromated MoKa radiation, l=
0.71073 ꢃ); Bruker Nonius KAPPA APEX II (6, 10, and
11·C5H12·0.5CH3CN; Montel mirror, MoKa radiation, l=0.71073 ꢃ)). All
structures were solved by direct methods (SHELXS-97).[17] The non-hy-
drogen atoms were refined anisotropically (SHELXL-97).[17] For the CH
hydrogen atoms, a riding model was employed, whereas the NH hydro-
gen atoms were localized in the difference Fourier syntheses and refined
freely. CCDC-757738 (6), 757739 (7), 757740 (8·CH3CN), 757741 (9),
757742 (10), and 757743 (11·C5H12·0.5CH3CN) contain the supplementary
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.
Synthesis of 9: Trimethylsilyl (S)-N-(trimethylsilyl)phenylalaninate
(1.51 g, 4.88 mmol) was added in a single portion at ꢀ308C to a stirred
solution of tri(cyanato-N)methylsilane (413 mg, 2.44 mmol) in acetoni-
trile (10 mL), and the mixture was then kept undisturbed at ꢀ208C for
10 d. The resulting colorless crystalline solid was isolated by filtration,
washed with cold (08C) n-pentane (2ꢄ10 mL), and dried in vacuo
(0.1 mbar, 608C, 8 h); yield, 429 mg (1.16 mmol, 48%). M.p. >3688C
(decomp). The following NMR data refer to two crystallographically in-
dependent molecules: 13C VACP/MAS NMR: d=1.8 (SiCH3), 34.7
(CH2), 38.2 (CH2), 43.1 (CH2), 45.7 (CH2), 54.6 (br asymmetric “d”;
CH),[16] 56.0 (br asymmetric “d”; CH),[16] 57.1 (br asymmetric “d”;
CH),[16] 59.5 (br asymmetric “d”; CH),[16] 126.1, 126.5, 127.3, 127.6, 128.8,
129.5, 130.4, 131.4, 135.3, 135.7, 136.6, and 139.1 (C6H5), 172.4 (CO),
173.3 (CO), 179.0 ppm (2 CO); 15N VACP/MAS NMR: d=ꢀ332.2 (2N,
NH or NH2), ꢀ323.1 (NH or NH2), ꢀ321.2 ppm (NH or NH2); 29Si
VACP/MAS NMR: d=ꢀ87.9 (br asymmetric “d”),[16] ꢀ85.5 ppm (br
asymmetric “d”);[16] elemental analysis calcd (%) for C19H22N2O4Si
(370.48): C 61.60, H 5.99, N 7.56; found: C 61.3, H 6.0, N 7.7.
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(1.58 g, 5.10 mmol) was added in a single portion at ꢀ708C to a stirred
solution of tri(cyanato-N)phenylsilane (590 mg, 2.55 mmol) in acetonitrile
(11 mL), and the mixture was then kept undisturbed at ꢀ208C for 3 d.
The resulting colorless crystalline solid was isolated by filtration, washed
with cold (08C) n-pentane (3ꢄ10 mL), and dried in vacuo (0.8 mbar,
6588
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 6582 – 6589

Silicon Complexes with Ligands Derived from a-Amino Acids
FULL PAPER
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Received: February 3, 2010
Published online: April 29, 2010
Chem. Eur. J. 2010, 16, 6582 – 6589
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