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Cautionary Example of Incorrect Structural Assignment Due to Crystallographic Disorder.

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Cautionary Example of
Incorrect Structural Assignment Due to
Crystallographic Disorder**
By Otto Ermer*
In the course of crystal structure analyses, disorder phenomena often prove to be perturbations of unclear origin,
which lead to unreliable structural parameters or, in the
extreme case, can even prevent the derivation of a qualitatively correct structural model. As a spectacular example,
the crystal structure analysis of the secododecahedrene l"]
at room temperature is reported here. The present study
was stimulated by our interest in non-planar double
bonds[2a1.According to empirical calculations using our
force field'2b1,1 has marked pyramidal distortions at the
Csp2atoms (out-of-plane bending; cf. the calculated double
bond torsion angles in the formula diagram). Furthermore,
its short calculated H. . . H distance of 1.906 A is noteworthy.
Crystals of 1 were optically isotropic and gave X-ray
photographs with Laue symmetry m3 (Th), corresponding
to a cubic crystal system. An all-face centered lattice and
the space groups F23 or Fm3 followed from th? systematic
extinctions. The cell constant a = 10.895(5) A was measured on a four-circle diffractometer. There are four
CzoHzoformula units per unit cell resulting in a plausible
density pcalc=1.337 g cm-3 (due to lack of material a macroscopic density measurement was not performed). For
an ordered structure, these crystallographic data impose
molecular symmetry T o r Th.Clearly, this condition cannot
be met by the unsymmetrical secododecahedrene 1, and
hence a disordered structure must be concluded: the high
crystallographic symmetries apply only for a molecular
structure averaged over time and/or space.
1.9068
H
~
Y
H,
6
1
2
An ordered crystal structure would in the present case
be possible if it were assumed that the crystals examined
did not consist of the secododecahedrene 1 but of the
highly symmetrical dodecahedrane 2[']. Gaseous 2 has Ih
~ymmetry[~".~],
whose highest possible crystallographically
allowed subgroup is Th. Originally, we had indeed assumed an ordered structure and, based on simple structural-chemical and geometrical criteria, constructed a dodecahedrane crystal structure model in the centrosymmetric
space group Fm3. Subsequently, this model was optimally
fitted by least-squares to a set of 68 reflection intensities
measured on a diffractometer at room temperature (MoKa
radiation; sin6/;1<0.48 A-I). The final R value was 0.048
[*I
["I
Prw-Doz. Dr. 0. Ermer
Abteilung fur Chemie der Universitat
Postfach 102148, D-4630 Bochum 1 (Germany)
This work was supported by the Fonds der Chemischen Industrie and
the Deutsche Forschungsgemeinschaft.
Angew. Chem. Inr. Ed. Engl. 22 (1983)No.3
(R,=0.051; w = l/02(Fo);C refined anisotropically, H isotropically)[4! It should be noted that in later test calculations direct methods also produced appropriate dodecahedrane structure models. The very plausible results of the
refinement are shown in Figure 1 and correspond entirely
to expectations for the crystal structure of an almost spherical hydrocarbon: highly symmetrical space group, high
crystallographic molecular symmetry, (cubic) closest packing, relatively high thermal motion in the crystal and,
hence, somewhat shortened CC bond lengths[41.
Cb,
n
I
'
Fig. 1. Geometry (above; estimated staadard deviations for distances, angles,
and torsion angles, formally ca. 0.004 A and 0.3", respectively) and thermal
motion (below; 10% ellipsoids) of the dodecahedrane model refined on the
room temperature intensities of the secododecahedrene 1. The C atoms on
the threefold axes are designated by C(3) and those on the mirror planes by
C(m). The C(m)-C(m) bonds are dashed in the top drawing in order to illustrate the Th symmetry[".
The crystallographic identification of the material studied as dodecahedrane raised doubts based on the chemistry of the substance"]. Therefore, a 'H-NMR spectrum
was recorded of a fragment of the crystal which had been
used for the X-ray intensity measurements. This spectrum
showed unequivocally that the crystal consisted of the secododecahedrene 1, as originally presumed[s1.Accordingly, the crystal structure of 1 at room temperature is disordered in such a way that it can be described well by an ordered structural model of 2. Probably the still rather
spherical secododecahedrene 1 adopts 30 statistically distributed preferred orientations in the crystal, in each of
which the line interconnecting the methylene C atoms is
parallel to a dodecahedra1 edge. A structure averaged over
these orientations differs only slightly from that of dodecahedrane 2.
The present experiences may serve as a cautionary example that a satisfactory R value and even physically
highly plausible structural results do not always safeguard
against incorrect assignments in crystal structure analyses.
Received: October 28, 1982 [ Z 189 IE]
German version: Angew. Chem. 95 (1983) 251
CAS registry numbers:
5,2,1,6,3,4-[2,3]ButanediyI[1,4]-diylidenedipentaleno[2, I ,6-cde: 2', I ',6'
ghalpentalene, 53 129-39-8.
0 Verlag Chemie GmbH, 6940 Weinheim, 1983
OS70-0833/83/0303-02SI$02.50/0
25 1
[I] R. J. Ternansky, D. W. Balogh, L. A. Paquette, J. Am. Chem. Soc. 104
(1982) 4503. Prof. L. A. Paquette, Columbus, is thanked for crystals of
1.
[2] a) 0. Ermer: Aspekfe uon Kruffeeldrechnungen, Wolfgang Baur Verlag,
Miinchen 1981; b) 0. Ermer, S. Lifson, J . Am. Chem. Soc. 95 (1973)
4121.
[3] 0. Ermer, Angew. Chem. 89 (1977) 431; Angew. Chem. I n f . Ed. Engl. 16
(1977)411.
[4] The refined parameters of the dodecahedrane model (symmetrie Th) fitted to the intensity data of the secododecahedrene 1 are for the C atom
on a threefold axis C(3): x~y=z=O.1131(2); Ul,=U22=U3,,,=0.145(2),
UI2=U , , = U z 3 = -0.017(2) A2, for the C atom on a mirror plane C(m):
x=0.1848(3),
y=O,
~=0.0698(3); U I ,=!.119(3),
U22=0.173(4),
U3,=0.150(3), UI2= U2,=0, U13=-0.020(2) A2. The corresponding results for H(3) are: x=y=z=0.155(3); U,,,=0.20(2)
for H(m):
x=O.267(5), y=O, z=0.090(4); U,,,=O.22(2) A2. Deviations from I,, symmetry of this (averaged) structure are small, as would be expected for the
relatively rigid dodecahedrane molecule [I, 2a, 31. Further details on the
crystal structure investigation can be obtained from the Fachinformationszentrum Energie, Physik, Mathematik, D-7514 Eggenstein-Leopoldshafen, by quoting the depository number CSD 50312, the name of the
author, and the journal citation.
[S] L. A. Paquette, personal communication.
A2,
Table 1. Physical data and yields for the compounds 9, 10, and 11.
IR
v [cm - '1
"C-NMR [Svalues] [c]
C=N-N-GO-CH2-
8a
8b
9a
1640, 1690[a]
1620, 1650[b]
1585, 1700 @I
164.11
184.83
167.41
179.19
168.33/173.31 [d]
71.65
9b
1585, 1695 @I
169.671176.22 [d]
73.32
10a
1580, 1758 [b]
189.68
170.00
-
10b
1600, 1735 [b]
188.20
190.44
169.81 [el
166.63
-
11
1585, 1725[b]
188.92
171.36
55.33
Cpd.
-
M.p.["C]
43.5-44
153.5-154.5
97-102
(dec.)
158-159.5
(dec.)
113-117
(dec.)
109-114
(dec.)
126-129
(dec.)
Yield
[%]
71
63
66
45
70
63
64
fa] In CHCI3. [bI In Nujol. [c] 25.15 MHz, 25"C, solvent: CDC13 (%a, b, 90);
CD~NOZ
(9b, 10a, b, 11). [d] Assignment uncertain. [el Solvent: SO2, -7O"C,
CF3S03H-excess.
Reactions of N-Methylenecarboxamides with
Electrophiles: 1-Alkoxy-2-azaallenium Salts and
N-Acyl(methylene)ammonium Salts**
By Ernst-Ulrich Wurthwein *, Ruiner Kupfer, and
Clam Kaliba
Like the simpler amides, N-methylenecarboxamides 4[41
are ambident nucleophiles which can be attacked by electrophiles at oxygen or at nitrogen. Alkylation at the oxygen
atom is expected to yield I-alkoxy-2-azaaflenium salts 5 or
I-alkoxy-2-azaallyl cations 6 ; attack at the nitrogen atom
should Iead to N~acyl(methy1ene)ammoniumsalts 7['].
a, R = CH, b, R = p-CH,C,H,
monate. The I3C-chemical shift of its methylene group
(6 = 55.33) is typical for an N-ethyl(methylene)ammonium
salt. The two phenyl groups give rise to eight signals, in accordance with the planar C,-structure 11. These spectroscopic data indirectly support the favored alkylation of N methylenecarboxamides at the oxygen atom.
;c=o
I
4
5
6
The N-methylenecarboxamides Sa, b were synthesized
by acylation of diphenylmethaneimine. These were then
alkylated with oxonium salts or protonated with strong
acids. All products are stable, hygroscopic, crystalline solids, which can be purified and fully characterized (Table
1). According to their I3C-NMR-spectra, the alkylation
products 9a, b have I-ethoxy-2-azaallenium structures of
type 5.This structural assignment is supported, in particular by the '3C-chemical shifts of the methylene groups (9b :
6=73.32; ether-region). Both phenyl groups give rise to
only four signals, even at low temperatures (e.g. 9a:
-70 "C, CDC13, 25.15 MHz), which is consistent with the
formulated allene-type structure (C, symmetry). Strong IR
absorptions at 1695 and 1700 cm-', respectively, are ascribed to C=N=C- heteroallene stretching vibrations"].
The N-acylated methyleneammonium salt 11, which is
isomeric to 9b, was synthesized from N-diphenylmethylene(ethy1)amine and p-methylbenzoylium hexachloroanti-
The I3C-NMR data of the protonated derivatives 10a, b
are very similar to the data of 11, but they differ signifcantly from those of the 0-alkylated compounds 9a, b. In
particular, the low temperature I3C-NMR spectra (liquid
SO2, excess of trifluoromethanesulfonic acid) indicate the
favored nitrogen protonation: the C=N-signal is found at
6 = 189.99, the C=O signal at 6= 170.00; eight signals for
the two phenyl groups and the IR-absorptions (at 1758 and
1735 cm-', respectively) provide further evidence for attack at the nitrogen atom (c. f. also[*]).
8a,b
HSbC16
0
ii
/C-R
C=N?
Ph'
H
Ph\
SbCl?
10a, b
[*I Dr.
E.-U. Wiirthwein, R. Kupfer, C. Kaliba
Institut fur Organische Chemie der Universitat Erlangen-Niirnberg
Henkestrasse 42, D-8520 Erlangen (Germany)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
252
0 Verlag Chemie GmbH, 6940 Weinheim, 1983
Quantum mechanical calculations (ab initio, 3-21 GI9])
support the experimental results. A twisted cisoid conformation (dihedral angle C=N-C=O: 25.4") is predicted as
OS70-0833/83/0303-02S2 $02.50/0
Angew. Chem. I n f . Ed. Engl. 22 (1983) No. 3
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