Synthesis and molecular structures of 2-trimethylsilyl-, 2-trimethylgermyl-, and 2-trimethylstannyl-pyridines

Article abstract of DOI:10.1515/znb-1999-0105

5-Methyl-2-trimethylsilyl-pyridine (1) has recently been prepared via the "in situ" Grignard reaction of 1-bromo-5-methyl-pyridine with magnesium and trimethylchlorosilane in refluxing tetrahydrofuran (thf) and structurally characterized. 2-Trimethylgermyl-(2) and 2-trimethylstannyl-pyridine (3) were now obtained from 2-bromo-pyridine through metallation (with n-BuLi) and treatment of the intermediates with Me₃GeBr and Me₃SnC1, respectively, in diethylether/ thf at -70°C. The crystal and molecular structure of compound 2 has been determined by low temperature (in situ) single crystal X-ray diffraction methods. There is a significant bending of the Me₃Ge substituent towards the nitrogen heteroatom [Ge-C-N = 114.7(2)°]. This phenomenon is known from previous studies of the silicon analogue 1 to be not due to intramolecular (peripheral) Si/Ge←N coordination, but to be rather an intrinsic property of the heteroarene skeleton, as also confirmed by quantum-chemical calculations. Furthermore, there is no evidence for intermolecular coordination in the crystals. Such interactions could also be ruled out for the solution state of 2 and 3 through variable temperature multinuclear NMR investigations.

Full text of DOI:10.1515/znb-1999-0105

Synthesis and Molecular Structures of 2-Trimethylsilyl-, 2-Trimethylgermyl-,
and 2-Trimethylstannyl-pyridines
Frank Riedm iller, A lexander Jockisch, H ubert Schm idbaur*
Anorganisch-chemisches Institut der Technischen Universität München,
Lichtenbergstrasse 4, D-85747 Garching, Germany
Z. Naturforsch. 54 b, 13-17 (1999); received June 1, 1998
Pyridine, 2-Silyl-pyridine, 2-Germyl-pyridine, 2-Stannyl-pyridine, Molecular Structure
5-Methyl-2-trimethylsilyl-pyridine (1) has recently been prepared via the “in situ” Grig-
nard reaction of 1-bromo-5-methyl-pyridine with magnesium and trimethylchlorosilane in
refluxing tetrahydrofuran (thf) and structurally characterized. 2-Trimethylgermyl- (2) and 2-
trimethylstannyl-pyridine (3) were now obtained from 2-bromo-pyridine through metallation
(with /?-BuLi) and treatment of the intermediates with Me.^GeBr and Me.^SnCl, respectively,
in diethylether/ thf at -70°C. The crystal and molecular structure of compound 2 has been
determined by low temperature (in situ) single crystal X-ray diffraction methods. There is
a significant bending of the Me.^Ge substituent towards the nitrogen heteroatom [Ge-C-N =
114.7(2)°]. This phenomenon is known from previous studies of the silicon analogue 1 to be
not due to intramolecular (peripheral) Si/Ge—N coordination, but to be rather an intrinsic
property of the heteroarene skeleton, as also confirmed by quantum-chemical calculations.
Furthermore, there is no evidence for mtermolecular coordination in the crystals. Such inter-
actions could also be ruled out for the solution state of 2 and 3 through variable temperature
multinuclear NMR investigations.
Introduction
substitution pattern. Structural and spectroscopic in-
vestigations have been carried out to exam ine both
the crystalline and the solution state.
O rganom etallic derivatives of nitrogen hetero-
cycles are useful synthons in preparative organic,
organom etallic and coordination chem istry [1 - 9].
In the pyridine series the orr/io-substituted species
are particularly im portant ow ing to potential chelat-
ing and neighbouring group assistance capabilities
[10 - 12]. The m olecular structures of these com -
pounds have attracted considerable attention be-
cause of alleged evidence for intra- or interm olecu-
lar interactions betw een the heteroatom (E = Si, Ge,
Sn etc.) and the nitrogen donor function [13 - 17]. In
a recent paper from this laboratory [18] it has been
dem onstrated that the structural peculiarities do not
have their origin in peripheral E -N donor/acceptor
interactions (E = Si), but are rather based on intrin-
sic structural param eters o f the pyridine heterocy-
cle. Both experim ental and theoretical results are
consistent w ith this interpretation [18],
Preparative Results
The synthesis of 5-methyl-2-trimethylsz7y/-pyri-
dine (1) has recently been reported via a m ethod
established for other trim ethylsilyl-pyridines [13,
18 - 20]. For this preparation the corresponding
brom o-pyridine is treated w ith m agnesium and
trim ethylchlorosilane in refluxing tetrahydrofuran.
Standard work-up of the reaction m ixture gives a
colourless, distillable liquid (b.p. 60 °C at 5 mbar,
m.p. -23°C) in 48% yield (eq. (1)).
For the preparation of 2-trim ethylgerray/- (2) and
2-trim ethyl5ranny/-pyridine (3) 2-brom o-pyridine
is lithiated using «-butyllithium in diethylether /
tetrahydrofuran at -78°C (-70°C). 2-Lithio-pyridine
is then treated in situ w ith trim ethylbrom ogerm ane
or -chlorostannane, respectively. The products are
colourless distillable liquids (2: b.p. 135 °C/300
Torr, m.p. -25°C, 45% yield; 3: b.p. 124-127°C/100
Torr, m.p. -37°C, 35% yield) (eq. (2), (3)).
We have extended these studies now to include
representative exam ples w ith E = Ge, Sn, together
with a silicon com pound w ith a slightly different
The analytical and spectroscopic data o f all
three com pounds are consistent w ith the proposed
* Reprint requests to Prof. Dr. H. Schmidbaur.
0932-0776/99/0100-0013 $ 06.00 © 1999 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com
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F. Riedmiller et al. ■Trimethylsilyl-pyridines
14
H,C
H,C
Mg, Me3SiCl, -MgBrCl
THF, RT(66 °C)
(1)
SiMe-,
"BuLi, -nBuBr
Me3GeBr, - LiBr
(2)
Et20 , -78 °C
Li Et20, -78 °C (RT)
GeMer,
Br
Br
"BuLi, -"BuBr
Me3SnCl, -LiCl
(3)
Et20/THF, -70 °C
Lj Et20/THF, -70 °C
SnMe-j
com position and structure (Experim ental Part). The
m ultinuclear NM R studies included the isotopes
73Ge and ll9Sn, w hich show standard chem ical
shift data. The values for Sn are in good agree-
m ent w ith data for other trialkylaryltin com pounds
w ith tetracoordinate tin atom s. The chem ical shift
value is not tem perature dependent betw een +25 and
-85 °C, and therefore association through S n-^N
interaction (inte/m olecular) can be ruled out for d8-
toluene solutions. In the low tem perature spectra the
1l9Sn-*H spin-spin couplings are well resolved.
W hile N M R spectroscopy with the 73Ge nuclei
is generally very difficult, there is no problem to
obtain a well defined 73Ge signal for com pound 2
at 6 = -17.6 (rel. GeCU). In the absence of a suit-
able collection of reference data, this value must go
w ithout com m ent. The spin-spin couplings 73G e-' H
are not fully resolved.
As reported previously [18], the structure of 5-
m ethyl-2-trim ethylsilyl-pyridine (1) shows a tilting
of the trim ethylsilyl substituent tow ards the nitro-
gen atom: The angles S i-C l-N and S i-C l-C 2 are
115.8(2) and 124.6(2)°, respectively, m eaning for
both a deviation from the 120° standard by about 5°
in opposite directions. The positioning of the m ethyl
group has no such anom alies, w ith the angles C 6-
C4-C3/C5 close to 120° (Fig. 1).
The rotatory position of the M e^Si group is such
that the N atom is staggered relative to the m ethyl
groups with C 8 and C9, w hile the m ethyl group at
C7 is eclipsed relative to the group C2-H1. Note
that it is very tem pting again to attribute this con-
form ation to a peripheral donor/acceptor interaction
N^>Si. It is only after a com parison w ith the (anal-
ogous) structure of 2-m ethyl-pyridine (2-picoline)
and pyridine itself, and also w ith results from the-
oretical calculations (M P2/6-31 G* level of theory)
[18], that it becom es clear that the observed effect
is an intrinsic property of the pyridine heterocycle,
and has nothing to do w ith peripheral coordina-
tive bonding, {ortho-hydrogen atom s and -m ethyl
groups show the sam e tilt tow ards the nitrogen
atom s.) This correlation qualifies the S i-N coordi-
nation as a phantom , w hich should be rationalized
in a different way.
The 14N resonances of the three hom ologues 1-3
and the corresponding M e^C-pyridine have chem -
ical shifts of b = -37.0 (3), -43.7 (2), -50.7 (1),
and -64.7 for 2-'butyl-pyridine. There is thus a sys-
tem atic decrease of the shielding of the nitrogen
nucleus as the group IV elem ent (C, Si, Ge, Sn)
becom es heavier, larger, and less electronegative.
This regression is com m on for the chem ical shift of
nuclei close to the elem ents of a given group of the
Periodic Table.
2-Trim ethylgerm yl-pyridine (2) crystallizes or-
thorhom bically, space group Pbca, Z = 8 (at 133 K).
The lattice contains isolated m olecules with no
crystallographically im posed sym m etry. The ger-
m anium atom is in the plane of the heterocycle, but
the G e-bound m ethyl groups are not displaced sym -
m etrically about this m olecular plane (Fig. 2, 3).
Structural Results
Single crystals could be grown from com pounds
1 and 2 at low tem perature using “in situ” crystal
growth techniques. The tin analogue 3 did not give
suitable crystals (m onoedric twins).
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F. Riedmiller et al. • Trimethylsilyl-pyridines
15
Fig. 2. Molecular structure of 2 (ORTEP drawing with
50% probability ellipsoids). Projection parallel to the
plane of the pyridine ring.
Fig. 3. Molecular structure of
2
(ORTEP drawing
The dihedral angle C 8-G e-C l-C 2 = 9.1° shows a
deviation from a staggered/eclipsed conform ation.
The projection down on to the m olecular plane
show s again a pronounced tilt of the Ge atom to-
w ards the N atom with angles G e-C l-N = 114.7(2)
and G e-C 1-C2 124.0(2)°. The G e-N distance is thus
reduced to 2.808 A. In summary, the disposition is
sim ilar to that in 1 , and sim ilar argum ents can be
used to interpret the results.
with 50% probability ellipsoids). Selected bond lengths
[A] and angles [°]: Ge-Cl 1.960(2), Ge-C6 1.939(3),
Ge-C7 1.948(3), Ge-C8 1.947(3), N-Cl 1.351(3), N-
C5 1.338(4), C1-C2 1.393(3), C2-C3 1.380(4), C3-
C4 1.369(4), C4-C5 1.391(4); Cl-Ge-C6 107.92(11),
C 1-Ge-C7 109.38(12), Cl-G e-C8 107.91(12), C6-
Ge-C7 108.82(14), C6-Ge-C8 111.40(14), C7-Ge-C8
111.3(2), N-Cl-Ge 114.7(2), Ge-Cl-C2 124.0(2), N-Cl-
C2 121.2(2), C1-C2-C3 120.1(2), C2-C3-C4 118.9(2),
C3-C4-C5 118.2(3), C4-C5-N 123.9(3).
The packing of the m olecules in the lattice gives
no evidence for m term olecular contacts other than
standard van-der-W aals distances (Fig. 4).
priately dried, distilled and saturated with dry nitrogen;
glassware was dried in an oven and filled with nitrogen.
Starting materials were commercially available. 1H-, 13C-,
29Si-, 73Ge-, ll9Sn- and l4N-NMR spectra were recorded
with a Jeol JNM-GX 270 spectrometer ('H at 270.17
MHz, l3C at 67.94 MHz, 29Si at 53.67 MHz) or a Jeol
JNM-LA 400 spectrometer ('H at 400.05 MHz, l3C at
100.50 MHz, 29Si at 79.38 MHz, 14N at 28.90 MHz,
73Ge at 13.83 MHz, and ll9Sn at 149.10 MHz, the lat-
ter with SnMe4 and GeCU as external references). Mass
spectra were recorded with a mass-selective detector HP
MS 5971 A (EI-MS 70eV) of an analytical GLC-MS
Hewlett Packard 5890 Series II chromatograph (column
HP1, crosslinked methylsilicon gum 12 m/0.2 mm, thick-
ness of film 0.33 n m). IR spectra were recorded on a
Midac FT-IR Prospect spectrometer. Elemental analyses
The present results are in full agreem ent with
recent findings for the fully hydrogen-substituted
prototypes of the silyl-pyridines [18]. Even for
the heavier (and larger) group IV elem ents, where
donor-acceptor interactions are more likely, the
structural data are com parable and show no conse-
quences of coordination interactions beyond stan-
dard tetracoordination.
Experimental Section
General methods: All experiments were carried out
under a dry nitrogen atmosphere. Solvents were appro-
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16
F. Riedmiller et al. ■Trimethylsilyl-pyridines
6 = -2.20 (GeMe3), 122.33 (C5), 127.75 (C3), 133.69
(C4), 150.42 (C6), 170.73 (C2 ipso). -73Ge-NMR (C6D6,
23 °C): 6 = -17.6 [m]; -16.8 (-85 °C). -,4N-NMR (C6D6,
23 °C): 6 = -43.7 [bs], -MS (El, 70 eV): ni/z = 196 [M+],
194 [M+-2H], 182 [100%, M+-CH3], 166 [M+-2CH3], 152
[M+-3CH3], 118 [GeMe3+], 103 [GeMe2+], 88 [GeMe+],
78 [C5H4N+], 139, 126, 65,51.
C8H 13NGe (195.7)
Found C 48.8 H 6.8 N7.1 %,
Calcd C 49.1 H 6.7 N 7.2 %.
2-Trimethylstannyl-pyridine (3)
To a solution of 2-bromopyridine (5.0 g, 0.032 mol)
in 30 ml of diethylether/thf a solution of n-butyllithium
in hexane (22.0 ml, 1.6 M, 0.035 mol) is added dropwise
with cooling at -70 °C. After stirring for 30 min a pale
brown solid is formed. A solution of trimethylchloro-
stannane (6.6 g, 0.033 mol) in 25 ml of diethylether is
added with vigorous stirring. After stirring for lh at -70
°C the mixture is allowed to warm to room temperature,
and stirring is continued for about lh. After removing the
solvent, the remaining oil is extracted with 30 ml of pen-
tane. The extract is fractionally distilled to give a colour-
less liquid product. Yield 2.68 g (35%; b.p. 124-127 °C
at 100 mbar; m.p. ca. -37 °C). -YH-NMR (C6D6, 23 °C):
6 = 0.38 (s, 1/snH = 53 Hz, 9H, SnMe3), 6.78 (dd, V HH
= 7.5/5 Hz, 1H, pyridine-Hs), 7.14 (ipt, Vhh = 7.5 Hz,
1H, pyridine-H4), 7.27 (dm, Vhh = 7.5 Hz, 1H, pyridine-
H3), 8.77 (d, V Hh = 5 Hz, 1H, pyridine-H6).-,3C{,H}-
NMR (C6D6, 23°C): 6 = -9.54 (SnMe3), 122.30 (C5),
131.52 (C3), 133.34 (C4), 150.71 (C6), 173.53 (C2 ipso). -
ll9Sn-VT-NMR (C6D5CD3, 20 °C): «5 = -52.3 [m,VSnH
= 53 Hz], 6 = -51.2 (-50 °C), 6 = -50.2 (-85 °C). -
14N-NMR (C6D6, 23 °C): 6 = -37.0 [bs], -MS (El, 70
eV): tn/z = 243 [M++l], 242 [M+], 228 [100%, M+-CH3],
198 [M+-3CH3], 196 [M+-2H-3CH3], 163 [SnMe3+], 144
[SnMe2+], 130 [SnMe+], 78 [C5H4N+], 65, 51.
Fig. 4. Packing diagram of 2.
were performed by the Mikroanalytisches Laboratorium
des Anorganisch-chemischen Instituts der Technischen
Universität München.
5-Methyl-2-trimethyM\y\-pyridine (\)
The synthesis followed the literature procedure for
other trimethylsilyl-pyridines[13, 18]. A colourless liq-
uid is obtained in 48% yield (b.p. 60 °C at 5 mbar, m.p.
-23 °C). -29Si-NMR (C6D6, 20 °C): 6 = -6.21 [s], -,4N-
NMR (C6D6, 19 °C): 6 = -50.7 [bs],
2-Trimethylgermyl-pyridine (2)
To a solution of 2-bromopyridine (4.0 g, 0.025 mol)
in 20 ml of diethylether a solution of n-butyllithium in
hexane (15.8 ml, 1.6 M, 0.025 mol) is added dropwise
with cooling (-78 °C). A pale brown solid is formed.
After stirring at -78 °C for 30 min a solution of trimethyl-
bromogermane (5.0 g, 0.025 mol) in 15 ml of diethylether
is added with vigorous stirring. After 1h of stirring at this
temperature the mixture is allowed to warm to room tem-
perature and stirring is continued for about lh. 30 ml
of pentane are added to the mixture and the precipitate
is filtered off. After removing the solvent from the fil-
trate, the product is separated by fractional distillation. A
colourless liquid is obtained. Yield 2.23 g (45%; b.p. 135
°C at 300 mbar; m.p. -25 °C). -‘H-NMR (C6D6, 23 °C):
6 = 0.41 (s, 9H, GeMe3), 6.68 (dd, 3/ Hh = 8/5 Hz, 1H,
pyridine-H.s), 7.06 (i/’t, Vhh = 8 Hz, 1H, pyridine-H4),
7.15 (dm, Vhh = 8 Hz, 1H, pyridine-H3), 8.65 (d, Vhh =
5 Hz, 1H, pyridine-H6).-l3C{'H}-NM R (C6D6, 23°C):
C8H l3NSn (241.8)
Found C 39.3 H 5.2 N 5.6 %,
Calcd C 39.7 H 5.4 N 5.8 %.
Crystal structure determinations
2-Trimethylgermyl-pyridine 2 was condensed into a
glass capillary. Single crystals were prepared “in situ”
on the diffractometer unit by cooling at 1 K h_ l.The
specimen was used for measurement of precise cell con-
stants and intensity data collection on an Enraf Non-
ius CAD4 diffractometer (Mo-/CQ radiation, A(Mo-ArQ)
= 0.71073 A). During data collection, three standard re-
flections were measured periodically as a general check
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17
F. Riedmiller et al. ■Trimethylsilyl-pyridines
electron densities: +0.56/-0.60 eA-3. Anisotropic thermal
parameters, tables of distances and angles, and atomic
coordinates have been deposited at Fachinformationszen-
trum Karlsruhe, Gesellschaft für wissenschaftlich-techni-
sche Information mbH, D-76344 Eggenstein-Leopolds-
hafen. The data are available on request on quoting CSD
No. 410417.
of crystal and instrument stability. No significant changes
were observed.
Crystal data for 2: CsH^GeN; Mr = 195.78; or-
thorhombic; space group Pbca; a = 8.1284(7); b =
12.1793(7); c = 18.727(2) A; V = 1853.9(3) A3; Z = 8;
Pcaic = 1.403 Mg/m3; F(000) = 800; p(M o-K J = 3.24
mm-1 , T = 133 K; 2331 reflections measured, 2017
unique. No absorption correction was applied. The struc-
ture was solved by direct methods (SHELXS-86) [21 ] and
completed by full-matrix least-squares techniques against
F2 (SHELXL-93) [22], The thermal motion of all non-
hydrogen atoms was treated anisotropically. All hydrogen
atoms of 2 were found and refined with isotropic con-
tributions. Refinement of 143 parameters for 2016 data
converged to a wR2 (R 1) of 0.0785 (0.0310). Residual
Acknowledgements
This work has been supported by Deutsche
Forschungsgemeinschaft and Fonds der Chemischen In-
dustrie. The authors are indebted to Mr. J. Riede for care-
fully establishing the crystallographic data sets.
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