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Induction of Liquid Crystalline Phases Formation of Discotic Systems by Doping Amorphous Polymers with Electron Acceptors.

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[5] J. C. Brewer, T. J. Collins, M. R. Smith, B. D. Santarsiero, J. Am. Chem.
SOC.110 (1988) 423.
[6] Crystal data for NaK,-Z.CH,OH, 3 H,O: space group P2,/c, a =
9.869(1), b = 17.323(2), c = 16.083(3) A, = 94.85(1)", V = 2739.7(7)
2 = 4 ; 1606 independent reflections with I > 30(I): four-circle diffractometer, Cu,., 3" I 2 6 I115". Lorentz, polarization, and adsorption corrections and corrections for linear decay were applied to the 3766 independent
reflections collected. Anomalous dispersion corrections were applied for
Co, K, and Na. The 35 non-hydrogen atoms were located using the
SHELXTL Direct Methods programs and the hydrogen atom positions
were located. Final cycles of least-squares refinement, which employed anisotropic thermal parameters for non-hydrogen atoms and isotropic
parameters for hydrogen atoms, gave R, = 0.053, R, = 0.058. Further details of the crystal structure investigation may be obtained from the Director
of the Cambridge Crystallographic Data Centre, University Chemical Laboratory, Lensfield Road, Cambridge CB2 1 EW (England). by quoting the
full literature citation.
17) The twist angle T and the pyramidalization terms x, and xN were obtained
from the primary torsion angles w l , w 2 ,and w , as follows: T = (wl w2)/2;
xN = (w2 - 0) n) mod 2 ~ xc; = (w, - IU, n) mod 2x. Here we use the
modified twist angle, ?: Z = (T) modn [3 a]. 7 maximizes at 2 90" and can be
interpreted as the angle between the idealized positions ofthe pn orbitals on
C and N. The pyramidalization terms maximize at 60".
% ' .
electron donor
low molecular weight
electron acceptor
columnar-nematic IN,)
Induction of Liquid Crystalline Phases:
Formation of Discotic Systems by Doping
Amorphous Polymers with Electron Acceptors
amorphous discoid p o l y m e r s
columnar-hexagonal (Dh.,)
liquid crystalline discotic polymer
Fig. 1. Schematic presentation of the induction of discotic mesophases in
amorphous discoid polymers via charge-transfer interactions with low molecular weight electron acceptors. Left: Side-group polymers. Right: Main-chain
polymers. TN F = 2.4,7-trinitrofluorenoneas acceptor. D,, = discotic hexagonal ordered (hexagonal long-range order of columns, whose discs are stacked
Doping of liquid crystals: Structure-property relationships
with regular spacings). N , = nematic columnar (nematic order, ie., exclusively
of low molecular weight and polymeric liquid crystals['] are
one long-range orientational order of the columns, whose discs are also stacked
of current interest, in particular those between molecular
with regular spacings). In contrast to the N, phase, the known discotic-nematic
structure and properties such as
ferroelectri~ity,[~~~ N, phases [3] do not consist of columns but rather of individual discs ordered
in nematic fashion.
photoreactivity,['"I and nonlinear optical activity.[2d1These
By Helmut Ringsdo$* Renate Wiistefeld, Elfiede Zerta,
Martina Ebert, and Joachim H. Wendorff
functionalities may be introduced into liquid crystals and
also varied not only by chemical synthesis but also by doping.['ayb'dlIt has been found that doping electron-rich discoid polymers with electron acceptors introduces new functional behavior and, moreover, even allows the induction of
mesophases in previously non-liquid-crystalline polymers.
Because of charge-transfer (CT) interactions, disc-shaped
parts of side-group or main-chain polymers can be arranged
in columns, which leads to discotic-columnar mesophases
(Fig. 1). Although the functionalization of liquid crystals by
doping has been extensively studied,"". b, dl induction of
mesophases by mixing of non-liquid-crystalline substances
has only been established for a few examples of rod-shaped,
low molecular weight
Induction of
mesophases in amorphous discoid polymers is of special importance since, although numerous amorphous discoid polymers are known,f51only a few liquid crystalline discotic polymers have been reported.[6*
Polyacrylates[81and polyesters['] containing disc-shaped,
electron-rich triphenylene units were employed in these induction experiments. The induction agents were fluorenone
derivatives["] with different acceptor strengths: 2,4,7-trinitrofluorenone (TNF) and 2,4,7-trinitrofluoren-9-ylidenemalonodinitrile (TNF-CN). Doping of the polymers was
['I Prof. Dr. H. Ringsdorf, Dip].-Chem. R. Wiistefeld, Dip].-Chem. E. Zerta
Institut fur Organische Chemie der Universitat
I-J.-Becher-Weg 18-20, D-6500 Mainz (FRG)
Dip].-Phys. M. Ebert, Prof. Dr. J. H. Wendorff
Deutsches K unststoff-Institut
Schlossgartenstrasse 6 R, D-6100 Darmstadt (FRG)
V2rlagsgeseiis~haftmbH, D-6940 Weinheiin,f 989
achieved by mixing in solution." The functionalized polymers were characterized by polarization microscopy, differential calorimetry, and X-ray diffraction.["]
induction of columnar mesophases in amorphous side-group
polymers: The polymethacrylate 1 (T, = 30 "C) and the polyacrylate 2 (T, = - 6 "C) were used as amorphous polymers
containing electron-rich triphenylene side groups.[5".81
When combined with the electron acceptor TNF," these
polymers form discotic phases due to CT interactions.
Mesophases are obtained after addition of only 25 mol%
TNF; they become isotropic at 170 "C (l/TNF) and 88 "C
(2iTNF) (Fig. 2).
Samples containing different amounts of TNF were prepared from 2 (Table 1). Only those with between 20 and
25 mol% TNF exhibit liquid crystalline phases. A mixture
containing 10 mol% TNF is amorphous, as is the neat polymer 2. Mixtures containing high amounts of TNF (molar
ratio 2/1 and l i l ) phase-separate during preparation.'"' The
enthalpies of clearing are very small ( < 0.4 J g- ') for the
induced liquid crystalline phases.
The structures of these mesophases were established by
X-ray diffraction studies (Fig. 3 and TabIe 1). The flat plate
photograph in Figure 3 a shows three reflections. A reguIar
spacing of 3.4 A between the molecules (triphenylene units,
TNF) within the columns can be derived from the sharp,
oriented reflection in the wide-angle region." The same
value was also found for the intracolumnar spacing in noninduced discotic phases of polysiloxanes containing tripheny-
Ange:ew. Chem. Int. Ed. Engl. 28 (1989) No. 7
X-ray beam
X-ray beam-(-0
g 30 i
g 50 N,
g 10 Nc 88
170 i
(P/TNF : 3/1)
: 3/1)
Fig. 2. Induction of nematic-columnar mesophases (N,) in the amorphous
discoid side-group polymers 1and 2 via CT interactions with the low molecular
weight electron acceptor TNF.
lene side groups.t6e1The weak-intensity halo at 4.2 8, is ascribed to the liquidlike short-range order of the alkyl
The strong-intensity small-angle reflection at
16.7 A is related to ordering of the columns. It is diffuse and
higher order reflections are absent. This is evidence against
a positional long-range order of the columns as present in,
for example, D,, phases. Orientational long-range order of
the columns, however, is present because the samples can be
macroscopically oriented. These results indicate the existence of nematic-columnar phases (Nc, Fig. I), which have
until now been found for l y o t r o p i ~ [ ' ~but
] not for thermotropic liquid crystals. Nematic N, phases align in such a
way that the column axes are parallel to the direction of
strain (Fig. 3c). The columns and the polymer chain of
higher-ordered columnar phases of undoped discotic sidegroup polymers have also been found to be oriented in the
same direction.[6e'
direction of
Fig. 3. Flat plate photograph of the nematic-columnar mesophase (N,)induced in the amorphous side-group polymer 2 for a 411 mol ratio of ZITNF.
X-ray beam impinges perpendicular (a) and parallel (h) to the direction of
strain. c) The columns are oriented with the column axes in the direction of
Induction of columnar mesophases in amorphous mainchain polymers: The induction of columnar mesophases in
discoid main-chain polymers was studied for the polyester
3c, which contains triphenylene units in the main chain. The
three polymers 3aJ5I 3b,[s*6'1and 3et51 differ only in the
length of the spacer: 3a and 3b, with shorter spacers, form
D,, phases; the polymer 3c is amorphous (T, = 35 "C).
Table 1. Induction of discotic phases in amorphous side-group polymers via
CT interactions. Structure and temperature range of mixtures of TNF and the
amorphous polyacrylates 1 and 2.
Polymer Poly- Temperature
merl range r C ]
within the between the Halo
columns [b] columns [c] [d]
3: I
g 10 Nc 47 i
g 10 N , 88 i
(g -7 i)
(g - S i)
g SO N , 170 i
[a] Mole ratio. [b] k0.03
amorphous two-phase
A. [c] f0.3 A. [d] i O . 1 A. [el Nonoriented, diffuse
AnKew. ChPm. In!. Ed. Engl. 28 (1989) No. 7
3a, x = 10 : g 50 D,
3b. x = 14 : g 60 ,D,
3 c , x = 20 : g 35 i
C5H1 1
220 i
150 i
When 3c is doped with TNF, however, CT complexation
increases the interaction between the discs in the main chain,
leading to the induction of discotic phases. The phase diagram (Fig. 4) reveals that mesophase induction starts at even
VerlagsgeseNschaft mbH. 0-6940 Weinheim. 1989
91 5
example, the values for inter- and intracolumnar spacings of
doped and undoped discotic polymers may be compared
(Table 2). Figure 5 shows X-ray flat plate photographs of the
Table 2. Induction of discotic phases in amorphous main-chain polymers via
CT interactions. Structure and temperature range of mixtures of TNF and the
amorphous polyester 3 c as well as comparison with the liquid crystalline
polyesters 3 a and 3b.
Fig. 4. Phase behavior of mixtures of the amorphous discoid main-chain polymer 3c and TNF. For amounts of TNF 2 25mol%. higher-ordered discotic
phases are induced. For 111 and 2/1 mixtures of 3c/TNF, D,, phases are present. For l S / l (40 mol% TNF) and 311 (25 mol% TNF) ratios, the mesophase
type could not be determined.
Polymer PoIy- Temperature
mer/ range [ ' C ]
g 35 i
g 32 i
g 25 I
g 20 D, 85 i
g 22 D,, 83 i
g 17 D,, 76 i
g 50 D,, 220 I
g 60 D,, 150 I
the colthe columns [b] umns [cl
amorphous [dl
amorphous [dl
amorphous [dl
25 mol% TNF. Clearing temperatures decrease slightly
(from 83 to 76 "C) with increasing amounts of TNF. Simultaneously, glass transition temperatures decrease from 35 "C
for the undoped polymer 3c to 17 "C,so that the phase width
remains constant over the entire doping range (to 50 mol%).
Enthalpies of clearing (0.6-1.3 J g-') are roughly a factor of
10 smaller than for undoped discotic polymers. If a stronger
electron acceptor, like TNF-CN, is used instead of TNF, the
clearing temperatures increase, for example, for a 2/1 ratio of
3c to electron acceptor, from 83 "C for T N F to 165 "C for
TNF-CN. This shows that the electron acceptor, in addition
to inducing mesophases, also strongIy influences the mesomorphic temperature range.
X-ray diffraction studies reveal that higher-ordered columnar phases (D,,,) are present for the TNF-containing
discotic main-chain polymers (Figs. 1 and 5). Such phases
are already known for undoped discotic main-chain polymers such as 3 a, b.16,I' Thus, the structural features - for
induced mesophases of the polyester 3c containing 50 and
33 mol % TNF. The sharp wide-angle reflection is assigned
to the intracolumnar disc spacings; the sharpness of the reflection indicates a regular arrangement with spacings of
between 3.38 and 3.40 A.
The hexagonal order in the induced D,, phase becomes
most evident in the hexagonal pattern of the X-ray photograph in Figure 5b. Values of approximately 27 8, are
[a] Mole ratio. [b] F0.03 [c] kO.1 [d] Nonoriented, diffuse reflections. [el
Only the Bragg spacings are known so far: 3.40, 21.6/15.5,4.4 the column
spacings cannot he given, since the phase type (D?) is still unknown.
X-ray beam
X-ray beam--
Fig. 5. Flat plate photographs of the hexagonal-columnar mesophase (DhJ induced in the amorphous main-chain polymer 3 c for a 131 (a and c ) and a 2il (b) mole
ratio of 3cjTNF. Impingement of the X-ray beam perpendicular (a, b) and parallel (c) to the direction of strain. d) In contrast to the side-group polymers 1 and 2.
but in analogy to the undoped discotic main-chain polymers [6 b, fl. the columns are oriented perpendicular to the direction of strain. This means that the polymer chains
and not the columns undergo extension upon stretching the polymer.
Vfr/agsgesetlschufl mhH, 0-6940 Weinhelm, 1989
0570-0833189/0707-0916 $OZ.SO/O
Angen. Chem. Int. Ed. EngI. 28 (1989) N o . 7
calculated for the spacing between the columns ( = hexagonal lattice constant). These intercolumnar spacings are
about 6-7 8, larger than those in noninduced discotic phases of 3a[5a1(20.4A) and 3bL6] (21.6 A). Strikingly,
they are even larger than the diameter of the hexasubstituted triphenylene discs with extended pentyloxy chains (23 -24 A). Using density measurements to explain this result,1161
a structural model seems likely in which the TNF molecules are not arranged exclusively within the columns
but also exist in the side-chain region between the columns.
Induction of columnar mesophases in polymer blends: CT
interactions may help to increase the miscibility of polymers,
resulting in the formation of one-phase polymer blends.[' 'I
This observation suggested the idea to combine the improved miscibility of polymers due to CT interactions with the
induction of discotic mesophases. In the first experiment,
TNF and two non-liquid-crystalline discoid polymers containing triphenylene side groups (polyacrylate 2Is1and polymalonate 4[18])were employed (Fig. 6). Polymers 2 and 4 are
immiscible. Addition of T N F results not only in miscibility
of the two incompatible polymers but also in the formation
of columnar mesophases. For a molar ratio of 1/1/1.2 for
2/4/TNF, the polymer blend 5, in analogy to the polyacrylate
2/TNF system, exhibits a nematic-columnar N, phase (temperature range - 13 to 89 "C).
Induction andfunctionalization of liquid crystalline phases:
Induction of mesophases by CT interactions opens a wide
range of possibilities for polymeric discotic liquid crystals.
The synthesis of polymeric as well as low molecular
g -10 k 20 i
1 rnol
1.2 rnol
g -13
89 i
disotic polymer blend 5
Fig. 6. Induction of discotic mesophases and miscibility in polymer blends via
CT interactions with the electron acceptor TNF.
Angew. Chem. Int. Ed. Engl. 28 (iY89) No. 7
weightr1'] discotic liquid crystals is no longer restricted to
the classical disc-shaped mesogens. The TNF derivatives
used here are nonmesogenic electron acceptors. Furthermore, a greater variety of structures is possible for discshaped electron-donor molecules; one possibility is the introduction of normally sterically perturbing functional groups.
The type of phase and the phase width may be varied over a
wide range by an appropriate choice of polymer system and
mixing ratio. Mesophase induction due to CT interactions also opens up a route to liquid crystalline polymer
blends. In addition to blends consisting of polymeric discoid
donors and low molecular weight acceptors, other combinations are conceivable, such as mixtures of main- and sidechain polymers or of discoid acceptor and donor polymers.
Noteworthy is the simultaneous induction of liquid crystalline phases and the introduction of functional behaviors
such as color, polarity, and photoreactivity. This approach to
functional discotic liquid crystals makes the induction of columnar phases via CT interactions of interest for technical
applications in such areas as electrical and photoconductive
Received: February 2. 1989 [Z 3185 IE]
German version: Angew. Chem. 101 (1989) 934
[l] a) H. Kelker, R. Hatz: Handbook of'LiquidCrystals, Verlag Chemie, Weinheim 1980; b) H. Finkelmann. Angew. Chem. Y9(1987) 840; Angm'. Chem.
Int. Ed. Engl. 26 (1987) 816.
[Z] a) A. V. Ivashchenko, V. G. Rumyantsev, Mol. Crysl. Liq. Crq'si. /SO
(1987) 1; b) L. A. Beresnev, L. M. Blinov, M. A. Osipov. S. A. Pikin ihid.
iS8 (1988) 979; c) I. Cabrera, V. Krongauz, H. Ringsdorf, An,qew. Chrm.
99 (1987) 1204; Angew. Chem. Int. Ed. Engl. 26 (1987) 1178; d j D. J.
Williams, ibid. 96 (1984) 637 and 23 (1984) 690.
[3] Nguyen Huu Tinh, C. Destrade, H. Gasparoux, Phys. L e t t . A 72(1979) 251.
[4] a) K. Araya, Y. Matsunaga, Bull. Chem. Soc. Jpn. 53 (1980) 3079; h) Y.
Matsunaga, N. Kamiyama, Y. Nakayasu, Mol. Crysr. Liq. Crysf. 147
(1987) 85.
[ S ] a) W. Kreuder, Dissertation, Universitzt Mainz 1986; b) 0. Karthaus.
Dip/omarbeil, Universitat Mainz 1988.
[6] Discotic polymers containing triphenylene mesogens: a) W. Kreuder, H.
Rmgsdorf, Makromol. Chem. Rapid. Comtnzrn. 4 (1983) 807; h) 0. Herrmann-Schonherr, J. H. Wendorff, W. Kreuder, H. Ringsdorf. [hid. 7(1985j
97; c) W Kreuder, H. Ringsdorf, P. Tschirner, ibid. 6 (1985) 97, d) G.
Wenz, ihid. 6 (1985) 577; e) B. Hiiser, H. Spiess, ibid. 9 (1988) 337; f) 0.
Herrmann-Schonherr, Dissertation, Technische Hochschule Darmstadt
[7] Discotic polymers containing benzene and phthalocyanine mesogens. a )
C. Sirlin, L. Bosio, J. Simon, .
Chem. Sot. Chem. Commun. 1987, 379; b j
A. Beck, H. Hanack, H. Lehmann, Synthesis 1987. 703; c) [6c].
[8] The discoid side-group polymers were obtahed by radical polymerization
of acrylates and methacrylates in solution. Relative molecular weights of
the discoid polymethacrylate 1 from GPC measurements: Al,,,, =
3800000 g mol-' (polystyrene (PS) standard, THF, measurement of the
refractive index (RI detection)), MGpc7= 4300000 g mol-' (polymethylmethacrylate standard, THF, RI detection): this corresponds to degrees of
polymerization P of 3900 and 5500, respectively. Relative molecular
weight of discoid polyacrylate 2 : M,, = 380000 g m o l - ' (PS standard,
CHCI,, UV detection) corresponds to P = 370.
[9] The main-chain polyesters 3a-3c were each obtained by melt condensation of an isomeric mixture [6a,c] of two triphenylenediacetates and the
corresponding 1,w-dicarboxylic acids (catalyst: p-toluenesulfonic acid).
3c: M,, = 13000 g mol-' (PS standard, CHCI,, UV detection); corresponds to P = 14.
[lo] J. E. Kuder. J. M. Pchan, S. R. Turner, D. L. F. Hinman, J Elec,rrochrm.
Six. 11 (1978) 1750.
Ill] Upon combination of the colorless and yellow solutions, respectively, of
the polymer and electron acceptor (in dichloromethane or THF), the color
turned red (3/TNF) or violet (I,Z/TNF). The mixtures were stirred for
several hours at room temperature. Evaporation of the solvent resulted in
separation of a black film, which was dried under oil-pump vacuum for
one to two days.
[12] Ortholux I1 POL-BK (Leitz).microscope used for polarization microscopy
investigations; DSC-4 (Perkin-Elmer) calorimeter used for differential
calorimetry investigations, heating and cooling rate = 20 K min-' (the
melting and clearing temperatures were taken from the peak maxima, the
glass transition temperatures were taken from the inflection point of the
VCH Verlag.rg~.seNst.hufi
mhH. D-6940 Weinheim. 1989
OS70-U833j89j0707-09178 02 SUjO
curve); X-ray diffraction measurements with nickel-filtered Cu,, radiation
(7. = 0.154 om) (oriented samples were obtained by stretching or by applying pressure at a given direction of flow [6b]).
1131 The interplanar separations of many low molecular weight CT complexes
containing TNF and polycyclic arenes lie between 3.1 and 3.5 A; see. for
example, L. J. E. Hofer, W. C. Peebles, Ana(pI. Chem. 24 (1952) 822.
[14] A M . Levelut, L Phys. Lert. 40 (1979) 81.
1151 N. Boden, R. J. Bushby, L. Ferris, C. Hardy, F. Six], Liy. Crysr. f(1986)
[I61 Taking the hexagonal lattice parameter and the intracolumnar distance
(see Table 2). it is possible to calculate the densities el and e2 for different
arrangements of the T N F molecules in the liquid crystalline phase (see
Fable 3). e, s all T N F molecules are arranged within the columns;
e2 G no TNF molecule is arranged within a column, that is, all TNF
molecules are located in the region between the columns. For the 111
The sodium and potassium aminoborates 2,[97'I which
are readily accessible from (E)-alkenylboranes 1[*I by reaction with NaNH, and KNH,, respectively, are intermediates
in the synthesis of the cyclic compounds 3 and are thus ideal
species for investigating intramolecular M-N interactions.
2a splits off methane already at room temperature, forming
an Si-N bond." '1
Y e 3
Table 3. Calculated and experimentally found densities of liquid crystalline
phases of polymer 3c with TNF.
l / l 0.50
211 0.57
lgcm - 'I
e2 [ g ~ m - ~ l
e3 [g c m - 7
I .04
mixture, the measured e3 value agrees with the g, value. For the 211
mixture, the measured density el is higher than the e2 value; in this case,
the structural arrangement appears to be more complex.
[17] a) T. Sulzberg, R. J. Cotter, 1 Polym. Sci. PurtA-f 8 (1970) 2747; b) J. M.
Rodriguez-Parada, V. Percec, Macromoletuks fY (1984) 55: c) C. Pugh, V.
Percec, ihid. 19 (1984) 65.
[18] The polymalonate 4 was obtained by Ti"-catalyzed transesterification of
a triphenylene malonate with dodecanediol [S b].
[19] Discotic-columnar phases can also be induced in low molecular weight
discoid systems through CT interactions, for example by addition of T N F
to non-liquid-crystalline triphenylene derivatives or to discotic-nematic
hexaphenylethynylene benzenes: M. Ebert, B. Kohne, K. Praefcke, H.
Ringsdorf, J. H. Wendorff, R. Wiistefeld, E. Zerta, unpublished results.
Pentacoordinated Elements of the 4th Main Group
with Four Organic Residues
By Roland Koster, Giinter Seidel, Bernd Wrackrneyer,*
Klaus Horchler and Dieter Schlosser
Hypervalent species of heavy elements of the fourth main
group of the periodic system, especially silicon,['] are of particular interest because of their importance as starting materials and reactive intermediates (e.g. silicon hydride"]) in
synthesis and because of their biological activity (e.g. silatranesr3]).A coordination number > 4 in the case of silicon,
however, has only been documented for compounds with at
least two electro-negative substituents on the silicon
51 We report here on the first NMR spectroscopic
detection of a long-lived species with a pentacoordinated
silicon atom, which has a coordinative Si-N bond besides
four organic residues. In addition, the results of NMR investigations on analogous tin and lead derivatives are reported.
Whereas an intramolecular Sn-N bond, analogous to that in
stannatrane, has been postulated for certain tetraorganostannanes,[61 and pentaorganostannates [R,Sn]@ have already been detected NMR spectr~scopically,[~~
an increase
in the coordination number of lead in tetraorganoplumbanes
was hitherto unknown.
a: M=Si,
b: M=Sn,
2b reacts similarly,~'2a1
whereas 2 c does not undergo any
simple secondary reaction. Although the cleavage of methane permits assumption of M-N interactions at least for
N = Si, Sn, proof of the existence of the anions derived from
2 [(b) in Eq. (2)J with a coordination number of 5 for M is
first furnished by the NMR spectra in T H F at several temperatures (Table 1).
In coordinating solvents such as THF or upon addition of
suitable crown ethers in THF or nonpolar solvents, the equilibrium (a) in Equation (2) can be shifted in such a way that
the alkali metal ion Ms prefers a coordination other than in
2. The position of the equilibria (a) and (b) in dilute solutions
depends on M' and M and on the temperature, as shown by
the 6(M) values measured under various conditions (cf.
Table 1). The difference of the 6(29Si)values in T H F (25 "C)
for 2a, M ' = Na (6 = - 18.6), and 2a, M ' = K
(6 = - 61 .0), typically demonstrates the dependence on the
alkali metal. The influence of M is e.g. immediately clear
from the fact that the 6 (' '9Sn) value for 2 b, M' = Na or for
2b, M' = K, in T H F at 25°C (6 = - 166.0) changes only
slightly compared to the value at - 50°C (6 = - 175.6).
Thus, in THF, the Sn-N interaction successfully competes
with the M'-N interaction at room temperature, i.e. the coordinative Sn-N bond is stronger than the coordinative Si-N
Prof. Dr. B. Wrackmeyer, Dip].-Chem. K. Horchler
Laboratorium fiir Anorganische Chemie der Universitat
Postfach 101251, D-8580 Bayreuth (FRG)
Prof. Dr. R. Koster. G. Seidel
Max-Planck-lnstitut fur Kohlenforschung
Kaiser-Wilhelm-Platz 1, D-4330 Miilheim a. d. Ruhr (FRG)
Dr. D. Schlosser
Chemische Fabrik Pfersee
Postfach 101409, D-8900 Augsburg (FRG)
Q VCH VerlagsgesellsrhaftmhH. 0-6940 Weinheim, I989
c : M=Pb; MI: Na,K
Angen. Chem. Int. Ed. Engl. 28 (1989) N o . 7
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