Cocrystallising rotamers of a pentacoordinate silicon complex with a chiral aminodiol ligand

Article abstract of DOI:10.1002/ejic.201201173

Crystal structure analysis reveals the cocrystallysation of two turnstyle rotamers of a pentacoordinate silicon complex with a chiral backbone derived from 2-amino-1,1,2-triphenylethanol. The structures of the two epimers are well reproduced by DFT calculations. In addition, the coexistence of two rotamers, which readily interconvert in solution, is confirmed by their solid-state NMR spectroscopic data. Possible mechanisms of their interconversion are discussed, and preference is given to a turnstile-like rotation of three ligands at the silicon atom about 120°. Copyright

Full text of DOI:10.1002/ejic.201201173

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DOI:10.1002/ejic.201201173
Cocrystallising Rotamers of a Pentacoordinate Silicon
Complex with a Chiral Aminodiol Ligand
Sebastian Schlecht,[a][‡] Maik Finze,[b] Rüdiger Bertermann,[b]
Walter Frank,[c] Anke Domann,[a] and Manfred Braun*[a]
Keywords: Chirality / Crystal structure / Density functional calculations / Solid-state NMR spectroscopy / Silicon /
Stereomutation
Crystal structure analysis reveals the cocrystallysation of two
turnstyle rotamers of a pentacoordinate silicon complex with
a chiral backbone derived from 2-amino-1,1,2-triphenyl-
ethanol. The structures of the two epimers are well repro-
duced by DFT calculations. In addition, the coexistence of
two rotamers, which readily interconvert in solution, is con-
firmed by their solid-state NMR spectroscopic data. Possible
mechanisms of their interconversion are discussed, and pref-
erence is given to a turnstile-like rotation of three ligands at
the silicon atom about 120°.
In this article, the synthesis of a pentacoordinate silicon
complex with an amine backbone derived from enantiomer-
ically pure 2-amino-1,1,2-triphenylethanol is described. The
crystal structure reveals the first cocrystallisation of rota-
mers at silicon. For reasons of confirmation, their struc-
tures were calculated, and the crystallographic study is sup-
ported by solid-state NMR spectroscopic measurements.
Introduction
Stereomutation at pentavalent centres has been studied
most intensively for phosphorus adopting a trigonal-bipy-
ramidal geometry.[1] Among the different mechanisms that
have been postulated in order to rationalise this phenome-
non[2] the Berry pseudorotation,[3] Ugi’s turnstile path[4] and
permutation mechanisms proposed by Muetterties[5] be-
came valuable concepts. In pentacoordinate silicon com-
plexes,[6] which have been investigated only to a minor ex-
tent, analogous stereomutation processes apply in many
cases.[7] As an alternative path of stereomutation, consecu-
tive fission and formation of a coordinate bond has been
taken into account in the case of silicon complexes.[8] The
incorporation of two pseudorotamers in a single crystal was
reported in the case of phosphoranes,[9] but to the be best of
our knowledge cocrystallising rotamers of pentacoordinate
silicon complexes have not so far been reported.
Results and Discussion
After penta- and hexacoordinate silicon complexes of
O,N,O-ligands with an imine moiety had been described re-
cently,[10,11] access to the amine counterpart was striven for.
Thus, the imine 1, readily available from a condensation of
1-formyl-2-naphthol with (R)-2-amino-1,1,2-triphenyl-
ethanol, was reduced with cyanoborohydride to give the
aminodiol ligand 2, which was isolated as the hydro-
chloride.[12] In order to avoid decomposition, which was ob-
served during storage, the crude product 2 was immediately
treated with dichlorodiphenylsilane in the presence of tri-
ethylamine to give the silicon complex 3 (Scheme 1), which
was obtained as a crystalline trichloromethane solvate. The
NMR spectra (1H, 13C, 29Si) measured in CDCl3 not only
at ambient temperature but also at –30 °C point to the exis-
tence of a single species. The silicon resonance (δ =
–82.7 ppm) indicates pentacoordination.[10]
The molecular structure, shown in Figure 1, surprisingly
revealed the coexistence of two epimers in a 1:1 ratio, both
of which feature a distorted trigonal-bipyramidal structure.
The TB-5-34-A configuration (anticlockwise) was assigned
to diastereomer 3a, whereas 3b adopts a TB-5-34-C (clock-
wise) configuration.[13] The selected bond lengths given in
Figure 1, in particular the Si–O and Si–N distances, differ
significantly in both epimers. The configuration at the ni-
[a] Heinrich-Heine-Universität Düsseldorf, Institut für Organische
Chemie und Makromolekulare Chemie,
Universitätsstr. 1, 40225 Düsseldorf, Germany
Fax: +49-211-8115079
E-mail: anke.domann@hhu.de
braunm@hhu.de
Homepage: http://www.chemie.uni-duesseldorf.de/Faecher/
Organische_Chemie/OC1/Braun
[b] Julius-Maximilians-Universität Würzburg, Institut für
Anorganische Chemie,
Am Hubland, 97074 Würzburg, Germany
E-mail: maik.finze@uni-wuerzburg.de
ruediger.bertermann@mail.uni-wuerzburg.de
[c] Heinrich-Heine Universität Düsseldorf, Institut für
Anorganische Chemie und Strukturchemie,
40225 Düsseldorf, Germany
E-mail: wfrank@uni-duesseldorf.de
[‡] New address: Bayer MaterialScience,
51368 Leverkusen, Germany
E-mail: sebastian.schlecht@bayer.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201201173.
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Scheme 1. Synthesis of the pentacoordinate silicon complex 3. Rea-
gents and conditions: (a) ref.[12]; (b) Cl2SiPh2, Et3N, CH2Cl2, –78
to +20 °C, 15 h; 49%.
trogen atom (S) that becomes stereogenic in the course of
the complexation is identical in both 3a and 3b. The geome-
tries of both epimers observed in the crystal structure of 1
were well reproduced by density functional calculations and
confirm the structure of a distorted trigonal bipyramid
(Figure 1 vs. Figure 2). A comparison of selected experi-
mental and calculated bond parameters is presented in
Table 1. According to the calculations, both diastereomers
are very close in energy with ΔH Ͻ 1 kJmol–1 [ΔG(25 °C)
Ͻ 2 kJmol–1].
It is evident that the epimers 3a and 3b originate from a
stereomutation at silicon. This interconversion may pro-
ceed, as depicted in Figure 2, by means of a 120° cyclic
permutation, thus following one of the mechanisms pos-
tulated by Muetterties.[5] In the course of this cyclic permu-
tation, one axial and one equatorial position are kept fixed,
whereas the remaining three substituents rotate about
120°.[14] An alternative isomerisation pathway is composed
of two consecutive Berry pseudorotations as shown in
Scheme S1 in the Supporting Information. This isomeri-
sation sequence would result in a trigonal bipyramid with
both oxygen atoms in axial positions and the nitrogen atom
in an equatorial position as intermediate between epimer
3a and 3b. Furthermore, a dissociative mechanism from
temporary cleavage of the Si–N bond, a reorientation of
the backbones and a reclosure of the Si–N bond could be
considered. Differentiation between these different iso-
merisation pathways based on the results of the experimen-
tal structural data is not possible. One should take into ac-
Figure 1. Molecular structure of complex 3a (a)/3b (b) in the crys-
tal; displacement ellipsoids are drawn at the 25% probability level;
H atoms are drawn with an arbitrary radius. Only one orientation
of the disordered phenyl group containing atoms C81 to C86 is
shown.
In contrast to the NMR spectra in solution, two signals
count, however, that both the consecutive Berry pseudoro- are present in the solid-state 29Si and 15N NMR spectra
tations and the dissociative pathway require severe reorien- (Table 2), which is in excellent agreement with the crystal
tation of the backbones and will probably encounter con- structure analysis that shows the presence of two epimers.
siderable steric hindrance and repulsion. We therefore pre- The δ(29Si) and δ(15N) values that were obtained from the
fer, as a rationale for the interconversion of epimers 3a and study in solution are in between those from the solid-state
3b, the 120° cyclic permutation as shown in Figure 2, par- spectra. Furthermore, the experimental chemical shifts are
ticularly since more recent calculations have revealed the in good agreement with calculated δ(29Si) and δ(15N) values,
turnstile rotation[15] as “the most viable alternative”[14] to which enabled the assignment of both epimers 3a and 3b.
the Berry pseudorotation.
In the solid-state 13C NMR spectrum, for some of the sig-
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Figure 2. Possible isomerisation pathway of epimer 3a to result in epimer 3b by a 120° cyclic permutation (highlighted by red arrows)
and calculated structures of both epimers [B3LYP/6-31++G(d)].
Table 1. Comparison of selected experimental and calculated bond lengths [Å] and angles [°] of epimers 3a and 3b.[a]
Parameter
Epimer 3a
Epimer 3b
Exptl.[b]
Calcd.
Exptl.[b]
Calcd.
Si–N
Si–O
Si–O2
Si–C1Ph
Si–C2Ph
(N–Si–CPh,trans ax
(O1–Si–O2)eq
(O1–Si–CPh eq
(O2–Si–CPh eq
Si(1)–N(1)
Si(1)–O(2)
Si(1)–O(1)
Si(1)–C(38)
2.105(4)
1.674(4)
1.678(4)
1.837(6)
1.870(5)
162.4(2)
132.6(2)
113.6(3)
111.1(2)
2.259
1.701
1.715
1.910
1.890
161.0
129.6
113.5
111.0
Si(2)–N(2)
Si(2)–O(4)
Si(2)–O(3)
Si(2)–C(75)
2.321(4)
1.634(4)
1.648(5)
1.842(6)
1.903(5)
173.7(3)
115.7(2)
126.2(3)
110.2(3)
2.438
1.684
1.691
1.895
1.903
172.2
116.5
123.8
110.4
Si(1)–C(32)
Si(2)–C(81)
)
N(1)–Si(1)–C(38)
O(1)–Si(1)–O(2)
O(2)–Si(1)–C(32)
O(1)–Si(1)–C(32)
N(2)–Si(2)–C(81)
O(3)–Si(2)–O(4)
O(4)–Si(2)–C(75)
O(3)–Si(2)–C(75)
)
)
[a] Calculated at the B3LYP/6-31++G(d) level of theory. [b] Labelling used for the X-ray structure analysis (Figure 1).
nals, two sets are found as well. In contrast to the X-ray in the NMR spectra, which are only rough estimates be-
diffraction study that indicates equal amounts of both rota- cause of the cross-polarisation applied during acquisition,
mers, in the solid state the relative intensities of the signals indicate a slightly higher ratio for epimer 3a.
Table 2. Comparison of selected experimental and calculated 29Si, 15N and 13C chemical shifts of epimers 3a and 3b.[a,b]
Parameter
Solution in CDCl3
Solid state[c]
3a
Calculation
3a
3b
Ratio[d]
3b
δ(29Si)
–82.7
n.o.[e]
42.6
67.8
153.6
–88.1
–313.1
43.6
83.9
154.8
–72.0
–326.3
40.8
78.8
153.4
2.3:1.0
1.7:1.0
1.3:1.0
1.3:1.0
1.2:1.0
–83.0
–290.9
47.9
74.3
163.4
–68.9
–306.2
42.3
74.8
163.1
δ(15N)
δ(13C) C-11
δ(13C) C-12
δ(13C) C-2
[a] Chemical shifts in ppm. [b] Calculated at the B3LYP/6-311++G(2d,p)//B3LYP/6-31++G(d) level of theory. [c] 29Si VACP/MAS NMR
(79.5 MHz); 15N VACP/MAS NMR (40.6 MHz); 13C VACP/MAS NMR (100.6 MHz); additional 13C NMR signals that were not assigned
to either epimer: δ = 66.6, 70.4, 125.0–135.7, 138.4, 143.9, 146.1, 149.0 ppm; in the29Si NMR spectrum a third weak signal can be found
at δ = –68.6 ppm. [d] The ratio is a rough approximation, since cross-polarisation was employed during acquisition. [e] n.o. = not
observed.
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operating at 500.13, 125.76, or 99.48 MHz for 1H, 13C and 29Si
nuclei, respectively. The NMR signals were referenced against TMS
(1H, 13C, 29Si) as internal standard. Solid-state 13C, 15N and 29Si
VACP/MAS-NMR spectra were recorded at 22 °C with a Bruker
DSX-400 NMR spectrometer with bottom layer rotors of ZrO2
(diameter 7 mm) containing approximately 100–150 mg of sample
(13C, 100.15 MHz; 15N, 40.55 MHz; 29Si, 79.50 MHz). The mea-
surements were conducted with a spinning rate of 6–7 kHz, a con-
tact time of 2 ms (13C), 4 ms (15N) or 5 ms (29Si), a 90° 1H transmit-
ter pulse length of 3.6 μs and a repetition time of 10 s for 29Si and
15N and 6 s for 13C. All chemical shifts were calibrated by setting
the 13C low-field signal of adamantane to δ = 38.48 ppm according
to the IUPAC recommendations[22] resulting in chemical shifts of
δ = 0 ppm for δ(13C) and δ(29Si) for TMS. For δ(15N) glycine was
applied as a secondary reference with δ = –342.0 ppm with respect
to MeNO2 (δ = 0 ppm).
Conclusions
For the first time the turnstile rotamers of a pentacoordi-
nate silicon complex have been demonstrated to cocrystal-
lise as shown not only by a crystal structure analysis but
also by a solid-state NMR spectroscopic study. Experimen-
tal and calculated data for the rotamers, which readily in-
terconvert in solution, are in excellent agreement. A 120°
turnstile rotation is assumed as the preferred path of the
observed stereomutation.
Experimental Section
Synthesis
(RC,SN)-3,4,4,6,6-Pentaphenyl-1,2,3,4-tetrahydronaphtho[1,2-h]-
[1,3,6,2]dioxazasilonin–Trichloromethane (3a and 3b): A 250 mL
two-necked flask equipped with a magnetic stirrer and a connection
to a combined nitrogen/vacuum line was charged with the amine
hydrochloride 2 (482 mg, 1.00 mmol) and closed with a septum.
The air in the flask was replaced by nitrogen, and dry dichloro-
methane (75 mL) was injected by syringe through the septum in
order to dissolve the solid. The solution was cooled to –78 °C, and
dichlorodiphenylsilane (0.21 mL, 1.0 mmol) and triethylamine
(0.28 mL, 2.0 mmol) were added in succession by syringe. After
stirring at –78 °C for 10 min, the solution was allowed to reach
room temperature over 12 h and poured into water. The organic
layer was separated, and the aqueous phase was extracted three
times with dichloromethane (30 mL each). The combined organic
layers were dried with sodium sulfate. The solvent was removed
under reduced pressure, and the residue was submitted to column
chromatography (silica gel; chloroform). The fractions containing
the product, as indicated by TLC control, were combined and con-
centrated by using a rotary evaporator, while the water bath was
kept at 40 °C. In the course of this, the crystalline silicon complex
3a/3b formed, which was collected and dried. Yield: 305 mg (49%).
Rf = 0.73 (chloroform). M.p. 138 °C. 1H NMR (500 MHz, CDCl3):
δ = 3.0 (br. s, 1 H), 3.70 (dd, J = 14.6, J = 6 Hz, 1 H), 3.88 (d, J
= 14.2 Hz, 1 H), 4.69 (d, J = 11.9 Hz, 1 H), 6.47 (d, J = 7.4 Hz),
7.01 (t, J = 7.4 Hz, 2 H), 7.06 (d, J = 8.7 Hz, 1 H), 7.10–7.25 (m,
16 H), 7.35 (d, J = 8.9 Hz, 1 H), 7.47 (d, J = 6.9 Hz, 2 H), 7.56 (d,
J = 7.2 Hz, 2 H), 7.69 (d, J = 8.5 Hz, 2 H), 7.22 (d, J = 7.2 Hz, 2
H) ppm. 13C NMR (125 MHz, CDCl3): δ = 42.6, 67.8, 83.2, 113.4,
121.1, 123.2, 123.5–134.3, 127.7, 129.1, 131.8, 142.3, 146.4,
153.6 ppm. MS (ESI): m/z (%) = 626 (15) [M + H]+, 471 (100).
HRMS: calcd. for C43H36SiNO2 626.2506; found 626.2509.
Crystal Structure Determination for Compound 3·CHCl3: Only rela-
tively weakly diffracting crystals of limited quality were available.
Some were selected by means of a polarisation microscope and
investigated with a Stoe Imaging Plate Diffraction System by using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Unit-
cell parameters were determined by a least-squares refinement on
the positions of 5968 reflections, distributed equally in reciprocal
space. Metric and systematic absences were consistent with mono-
clinic space group types P21/m and P21. The latter was confirmed
in the course of the structure refinement. Crystal data and further
details: C44H36Cl3NO2Si, Mr = 745.18, a = 18.2894(16) Å, b =
11.8876(5) Å, c = 19.4153(16) Å, β = 112.928(9)°, V = 3887.7(6) Å3,
Z = 4, ρ = 1.273 gcm–3, μ = 0.304 mm–1, T = 291 K, crystal dimen-
sions: 0.19 mmϫ0.19 mmϫ0.20 mm, 21362 measured, 12812
unique and 4770 observed reflections with I Ͼ 2σ(I), θmax = 25.00°,
LP correction, direct methods[23] and ΔF syntheses,[24] minimisation
2 2
2
of Σw(Fo2 – Fc ) , 920 refined parameters, (Δ/σ)max = 0.002, R1[Fo
Ͼ 2σ(Fo2)] = 0.054, wR2 = 0.085 (all data), w = 1/σ2(Fo2), S =
0.854, Δρmax/Δρmin +0.217 eÅ–3 and –0.289 e/Å3. Refined param-
eters include anisotropic displacement parameters for all non-hy-
drogen atoms with the exception of those of the disordered phenyl
group including atom C81 and C81A, respectively. With idealised
bonds lengths and angles assumed for all the NH, CH and CH2
groups, the riding model was applied for the corresponding H
atoms, and their isotropic displacement parameters were con-
strained to 120% of the equivalent isotropic displacement param-
eters of the parent nitrogen and carbon atoms. The disordered tri-
chloromethane solvate molecules were treated as rigid with ideal-
ised geometries. Appropriate anisotropic displacement restraints
had to be applied for their chlorine and carbon atoms, as well as
for selected atoms of the silicon complexes. The absolute structure
parameter[25] [x = 0.09(9)] indicates the choice of the correct enan-
tiomorphic structure model. CCDC-901493 contains the supple-
mentary crystallographic data. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Quantum Chemical Calculations: DFT calculations[16] were carried
out by using Becke’s three-parameter hybrid functional and the
Lee–Yang–Parr correlation functional (B3LYP)[17–19] with the
Gaussian 03 program suite.[20] Geometries were optimised, and en-
ergies were calculated with the 6-31++G(d) basis set. All structures
represent true minima on the respective hypersurface (no imaginary
frequency). DFT-GIAO[21] NMR shielding constants were calcu-
lated at the B3LYP/6-311++G(2d,p) level of theory by using the
geometries derived at the B3LYP/6-31++G(d) level of theory. The
13C and 29Si NMR shielding constants were calibrated to the re-
spective chemical shift scale by using predictions for Me4Si with a
chemical shifts of δ = 0 ppm, and for the calibration of the 15N
NMR shielding constants a prediction for liquid ammonia of δ =
–380.3 ppm was used.
Supporting Information (see footnote on the first page of this arti-
cle): Alternative isomerization pathway of epimer 3a to epimer 3b
by two consecutive Berry pseudorotations; geometries and energies
of epimer 3a and epimer 3b calculated at the B3LYP/6-31++G(d)
level of theory.
Acknowledgments
1
NMR Spectroscopy: H, 13C and 29Si NMR spectra were recorded
This work was supported by the Deutsche Forschungsgemein-
schaft. (Br 604/15-1,2).
at 25 °C in CDCl3 with a Bruker Avance DRX-500 spectrometer
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