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Li12Si7 a Compound Having a Trigonal Planar Si4 Cluster and Planar Si5 Rings.

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(8 ppm) compared to C-4 and C-6 as C-7 is deshielded due to
its allylic
An X-ray structure analysis of (6b) is in
progress.
The silver complex ( 6 4 reacts with dienes at room temperature to give the corresponding cycloadducts (3) and (10)(33). Gas chromatographically, the sole impurity found in all
cases was the cis-olefin (7) (2-13%). The following diastereomeric ratios a / b were determined by capillary gas chromatography: (3): 4.4:l; (11): 2.0:l; (12): 2.4:l. The ratio a/
b = 4.4 :1 of the furan adducts (3)differs drastically from that
found in the trapping of free trans-olefin (2) ( a / b = 0.95 :l)l3l
and suggests that part of the cycloaddition takes place in the
ligand sphere of the silver ion. The insolubility of ( 6 4 in solvents suitable for anthracene hinders cycloaddition of the
complex. However, (13) (m.p. 122°C) was obtained when
ammonia was passed into the stirred suspension of (6a) in
the saturated anthracene solution in dichloromethane. Reaction of (6a) with trans-cyclooctene (5) gave tris(trans-cyc1ooctene)silver perchlorate (4),m. p. 197 "C (decomp.). Preliminary experiments show that cis,trans-cycloheptadienes
can also be stabilized as silver complexes.
Received: January 4, 1980 IZ 645 IE]
supplemented: April 15, 1980
German version: Angew. Chem. 92, 1068 (1980)
[I] J. F Liebman. A. Greenberg, Chem. Rev. 76, 31 1 (1976); E. J. Carey, F A.
Carey, R. A. E. Winler, J. Am. Chem. SOC.87, 934 (1965).
[2] J. Inoue, S. Takamuku, H Sakurai. J. Chem. SOC.Perkin Trans. I1 1977,
1635.
[3] W. Kirmse, H . Jendralla, Chem. Ber. 111, 1857 (1978).
[4] N o cycloadducts can be detected if furan is added after 3 h.
[5] A similar stabilization of photochemically generated trans-cycloheptene by
copper(1) triflate was recently reported J. Th. M. Evers, A. Mackor, Recl.
Trav. Chim. Pays-Bas 98, 423 (1979); Tetrahedron Lett. 1980, 415.
161 16a) could not be crystallized. A rough purification was achieved by saltingout (6a) from a saturated solution with NaNO,.
[7] Whirham and Wright, J. Chem. SOC.C 1971, 883, prepared the diastereomers
of 3-methoxy-( I I?-cyclooctene via a stereochemically unambiguous route.
They found for the (2RS,3SR)-isomer J2.)= 2.1 Hz. for the (2RS,3RS)-isomer
J z 2 = 9 . 0Hz. They also quoted J, "=9.1 Hz, J,.8 =3.55 Hz.
181 0.Ermer. Angew. Chem. 86, 672 (1974); Angew. Chem. Int. Ed. Engl 13,
604 (1974)
191 All the new compounds gave satisfactory 'H-NMR spectra. (6a): m.p. 157159°C (dec.) 161, 'H-NMR (DzO. TMS ext.): S=5.80 (ddd, I-H, J,.z=17.9
Hz. J 1 7 = 9 2 Hz. J 1 . 7 =4.3 Hz). 5.50(dd, 2-H, Jl,2=t7.9 Hz. J2,,=7.9 Hz),
4.20 (rn, 3-H). 3.50 ( s . OCH,). 2.73-1.30 (m. 8H); "C-NMR (D20. TMS
ext.) 6 = 123.0 (d, C-l), 120.6 (d, C-2). 81.7 (d, C-3), 57.5 (q, OCH,), 39.2 (t.
C-7). 31.3 (I). and 31.1 (1) (C-4, C-6). 23.0 (1, C-5); essentially in agreement
with MS of (7). (66): m.p. 142-143 "C (dec.), 'H-NMR. as (6a); "C-NMR
(DIO/TMS ext.): 6 = 121.7 (d, JcH=161.6 Hz. C-I), 120.9 (9. JcF=317.7 Hz.
CF,). 119.9 (d, J c ~ = 1 6 1 . 6Hz, C-2). 81.3 (d. J0=150.4 Hz, C-3). 57.2 (4.
JCH
= 142.6 Hz. OCH,), 38 9 (1, JCH
= 129.4 Hz, C-7). 31.0 and 30.7 (t, C-4,
C-6). 22.6 (t, C-5).
Lii2Si7, a Compound Having a Trigonal Planar Si,
Cluster and Planar Si5 Rings
By Hans Georg von Schnering, Reinhard Nesper, Jan Curda,
and Karl-Friedrich Tebbe"'
In a renewed study of the lithium-silicon system we were
able to showlll that the long-known violet compound "Li2Si"
actually has the composition Lit4Si6(Li2.33Si).The metallic
gray compound immediately adjacent on the silicon-rich side
has now been identified as Lii2Si7.This is the silicon-richest
phase in the Li/Si system; it was formerly described as
[*] Prof. Dr. H. G . von Schnering ['I, Dr. R. Nesper, Dipl.-Ing. J. Curda
Max-Planck-Institut fur Festkorperforschung
Heisenbergstrasse I . D-7000 Stuttgart 80 (Germany)
Prof. Dr. K.-F. Tebbe
lnstitut fur Anorganische Chemie der Universitat
Greinstrasse 6, D-5000 Koln 41 (Germany)
Si
".I
si \
Fig. 1. Crystal structure of Lit2Si,, projection of the unit cell along the a axis (Li,
large circles; Si, small circles). Space group Pnrna (No. 62); a=861.0(2),
b = 1973.8(4), c = 1434.1(4) pm; 8 formula units in the unit cell; 2190 reflections
hkl, MoK<. radiation. R=0.035 (anisotropic). None of the atoms shows anomaor occupation densities. Subsequent A€ synthesis is wlthout
lous coefficients U,,
notable contours [6].
Lil3Si7I2l.Preparation of the pure compound from the elements was accomplished in well baked-out, sealed tantalum
ampoules at 1270 K. Syntheses under various conditions,
thermal analysis, and coulometric titration demonstrate that
Li12Si7coexists in equilibrium with Si and Lii4Si6.The compound has a almost negligible phase width (1.69sLi/
S i s 1.71); it melts congruently at 890 K and is extraordinarily sensitive to moisture and oxygen.
Li12Si7is unequivocally a semiconductor with a band gap
of EG = 0.6 eV and a specific conductivity at room temperature of ~ ( 2 9 8 ) = 1 0 -0 ~- l c m -' . The compound is diamagnetic with a considerably greater susceptibility than silicon
(xmol= - 23 x
cm3 mo1-l for Lit 7i4Si).These physical
properties are important for a n understanding of the bonding
because they are indicative of a normal valence compound.
The structure should therefore fulfil the rules of Zintl and
KZemm[31and of Mooser and Pearson141.
Li12Si7forms orthorhombic crystals (Fig. 1) with a surprising new kind of anionic Sin cluster: Planar Sis rings-analogues are already known in the germanide Li, ,Ge,['I-are
accompanied by hitherto unknown star-shaped trigonal planar
Si, clusters of 3/m-D3,, symmetry. The bond lengths d(Si-Si)
in the Si5 ring vary between 235.6 and 238.1 pm (d=236.8
pm) and those in the Si4 star between 236.5 and 239.3 pm
(8=238.0 pm); they are only slightly longer than the interatomic distance of a Si-Si single bond (235.1 pm).
Attempts to describe this structure in terms of the model of
formal ions run into difficulties: for Li24Si,42 (Li +)24(Si4x
-)
(Si5y-)*, the expression x + 2y= 24 would apply. The formal
charge assigned to silicon (Si2-) owing to the homonuclear
divalency of the Si atoms in the Sis ring gives y = 10 as solution. However, this leads directly to Si44-, a 20e system normally found for the tetrahedrane structure of the isoelectronic P4 molecule. Is the Si44- star a n excited tetrahedrane
state then? Assuming a n (sp' + p) configuration for all four
Si atoms, a plausible description can be formulated with
three three-center bonds and seven nonbonding electron
pairs. Another remarkable variant results with the solution
x = y = 8 . The polyanion Si4'- would be isoelectronic with
the carbonate anion C 0 3 2 - and the polyanion Sisx- with cyclopentene. We hope that this interesting structure will rouse
the interest of theoreticians. The isostructural compound
Li12Ge7has likewise been prepared.
Received. July 28. 1980 [Z 639 IE]
German version: Angew Chem 92. 1070 (1980)
[ '1 To whom correspondence thould be addressed.
Angew. Chem Inr. Ed. Engl. I9 (1980) No. 12
si
I
- Si
0 Verlag Chemre, GmbH. 6940 Wernherm, 1980
0570-0833/80/12l2-l033
$ 02.50/0
1033
[ I ] H. G. 0. Schnering, R. Nesper, K.-F Tebbe, J Curda, Z. Metallkde. 7 1 . 357
(1980)
[21 H . Axel. H Schafer, A. Werss, 2. Naturforsch. B 20, 1302 (1965).
[31 E. Zinrl, Angew. Chem. 52. I (1939); W Klemm- Festkorperprobleme, Vol.
111. Vieweg. Braunschweig 1963; cf. also H. .Schu/er, B. Ersenmann. W. Muller, Angew. Chem. 85, 742 (1973); Angew. Chem. Int. Ed. Engl. 12. 694
(1973).
I41 E. Mouser, W. B. Peurson, Prog. Semicond. 5. 103 (1960)
(51 U . Frank, W. Muller, Z. Naturforsch. B 30. 313 (1975)
161 H. G. u. Schnering et a/., Z. Kristallogr., in press.
[ I ] H. Meyer, G. Nagorsen, Angew. Chem. 91, 587 (1979). Angew. Chem. Int.
Ed Engl. 18. 551 (1979).
[2] J. M . Roberrson, H. M . M . Shearer, G. A. Sim, D. G. Watson, Acta Crystallogr. I S . 1 (1962). For some other examples see J . D. Dunitr, X-ray Analysis
and the Structure of Organic Molecules, Cornell University Press, 1979, pp.
103-106.
[3] H Wunderlich, D. Mootr. Acta Crystallogr. B 27, 1684 (1971): u = 10.082,
b=5.518, c = 10.943
p = 118.53" or after axis transformation, a'= 10.771
A, L3'= 124.68". Slightly different values are given by C J. Brown, Acta Crystallogr. 21, 170 (1966).
[41 E U. Wiirrhwein, P. u. R. Schleyer. Angew. Chern. 91, 588 (1979); Angew.
Chem. Int. Ed. Engl. 18, 553 (1979)
A,
Planar Four-Coordinated Silicon?
Planar Four-Coordinated Silicon-A
By Jack D. Duniiz"]
By Giinier Nagorsen and Heinrich Meyert'l
The announcement by Meyer and Nagorsen['] that the
bis(o-pheny1enedioxy)silane molecule (1) is planar in the
The criticism leveled by Dunitz'" at our communication[*]
is partly justified. However, we had ourselves already emphasized that the space group determination is based on the
absence of a few reflections and is hence not entirely compelling.
From 1975 to 1979 it proved impossible to again obtain
suitable single crystals of the extremely sensitive monomeric
bis(o-pheny1enedioxy)silane (I), and experimental results
were therefore published only after supporting evidence had
been obtained from model c a l ~ u l a t i o n s ~ ~ ~ ~ 1 .
The similarity between the lattice data of catechol and (1)
appears remarkable at first sight, although "a" is significantly different. On closer examination, though, it actually provides powerful support for our structural model: for the H
bond system linking two catechol molecules in planar fashion merely has to be replaced by silicon in a planar coordination, and the similarity in cell dimensions become entirely
plausible.
Pure catechol melts at a temperature 8 " C lower than our
substance and is stable to atmospheric moisture. Some of our
(few) single crystals were hydrolyzed; catechol was identified
alongside a gel of silicic acid. Within seconds of exposure to
air, the lustrous crystals had become covered with a gel. Our
identification of (1) was also based on mass spectrometry under the conditions of crystal growth.
We consider a possible disorder of the molecules in the
crystal of (1) to be hardly applicable, since most of the cases
cited by Dunztzf'l are (almost) planar, somewhat asymmetric
systems whose disorder is explicable. Rather, it was precisely
the lack of a n isostructural relationship with the orthocarbonic ester that drew our attention to this structural problem.
Received: September 25, 1980 [Z 640b IE]
crystalline state is sufficiently novel and provocative that it
can hardly avoid drawing critical attention to the evidence
on which it is based. In the absence of a detailed X-ray analysis this evidence consists of a crystallographic symmetry argument: X-ray rotation and Weissenberg photographs led to
a monoclinic unit cell with dimensions a= 10.56, b = 5.60,
c = 10.96 A, p= 122", space group P2,/c; with two molecules
Of (C,&O,),Si
per cell each silicon atom must occupy a crystallographic inversion center and hence must lie in a common plane with its four bonded oxygen atoms.
There are two weak points in this argument. In the first
place, with a short b axis it may be dangerous to eliminate
the alternative space group P2/c on the basis of a few systematically absent (OkO; k = 2n + 1) reflections. If this space
group is admitted as a possibility the silicon atoms could sit
on twofold rotation axes instead of inversion centers and the
symmetry argument for the planarity of the S i 0 4 grouping
would lose its force. In the second place, even if the space
group P2,/c is accepted, the method of assigning molecular
symmetry from space group arguments is not infallible because the molecular arrangement in the crystal may be disordered. There are many cases where molecular symmetries assigned in this way would obviously be in error. Perhaps the
most famous case is that of azulene (P2,/a, Z = 2)I21.
There is still another serious flaw in the experimental evidence: the chemical identity of the crystalline compound that
was actually examined by X-ray diffraction is by no means
proven. In fact the cell dimensions cited by Meyer and Nagorsen have a suspicious resemblance to those of catecholf3',
a compound that might well be produced from (1) in the
presence of traces of water.
In summary, the experimental evidence by Meyer and NagorsenI'1 is insufficient to establish (1) as the first compound
with planar four-coordinated silicon. The theoretical argum e n t ~ [ 'of
. ~course
~
retain whatever significance they had.
German version: Angew. Chem. 92, 1071 (1980)
[ I ] J . D Dunrtr, Angew. Chern. 92. 1070(1980); Angew. Chem. Int. Ed. Engl. 19,
1034 (1980).
[2] H . Meyer, G. Nugorsen, Angew. Chem. 91, 587 (1979); Angew. Chem. Int.
Ed. Engl. 18, 551 (1979).
[3] E. U. Wurrhwein. P. u. R. Schleyer. Angew. Chem. P I , 588 (1980): Angew.
Chem. Int. Ed. Engl. 18, 553 (1979).
Received: March 21, 1980 [Z 640a I€]
German version: Angew. Chem. 92, 1070 (1980)
['I
Prof. Dr. J. D. Dunitz
Organic Chemistry Laboratory
Swiss Federal Institute of Technology (ETH), ETH-Zentrum
CH-8092 Zurich (Switzerland)
I'[
Regmered numes, rrademarks. eic used m rhrspurnai, even wiihoui specfic rndrcorwn !hereo/, ore noi
Q Verlag Chemie. CmbH. D~6940Weinhelm. 1980 - Punted
8"
Reply
10
Prof. D r G. Nagorsen, Dr. H. Meyer
Institut fur Anorganische Chemie der Universitat
Meiserstrasse I . D-8000 Munchen 2 (Germany)
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