organic compounds
1
2
3
2
at (1 � x, + y, � z), viz. (ii). The base molecule (x, y, z)
best of our knowledge, the latter is the only example in
addition to the diacid (II) of the present study in which the co-
existence of both dimeric and catemeric motifs has been
found.
accepts an equivalent hydrogen bond from its 2 -related
1
1
3
neighbour at (1 � x, � + y, � z).
2
2
Given the rarity of the concurrence of the two aggregation
patterns observed for (II), a search of the CSD was conducted
to shed light on other factors that might play an important
role. We retrieved 353 structures for 1,2-dicarboxylic acids. A
total of 21 hits were distilled from the initial set for 1,2-cis-
dicarboxylic acids, with subsequent elimination of cases for
which the two carboxylic acids are geometrically anti, as well
as those cases for which there is potential interference by
interactions due to other functional groups present in the
molecules. In Table 2 are shown the most relevant cases and
the pattern of association observed for each; Fig. 3 de®nes
each compound. As can be seen, the predominant pattern
observed for 1,2-dicarboxylic acids, with the exception of 3,3-
dimethylcyclopropane-1,2-dicarboxylic acid, is dimeric; to the
In (II), the OÐHÁ Á ÁO hydrogen-bonded catemeric
assembly is supported by very weak CÐHÁ Á ÁO contacts
(Fig. 3), in this case involving atom C3 and the C15 carboxyl O
iii
1
2
3
2
atom, C3ÐH3Á Á ÁO1 [symmetry code: (iii) 1 � x, � + y, � z].
It has been proposed that the catemeric motif arises due to
auxiliary/supporting weak interactions such as CÐHÁ Á ÁO
hydrogen bonds (Duchamp & Marsh, 1969; Das & Desiraju,
2006; Kuduva et al., 1999). Evidently, an increase in hydro-
phobicity through further TMS substitution in (II) compared
with diacid (I) (Begum et al., 2004), as well as weak CÐHÁ Á ÁO
hydrogen bonds, lead to the appearance of a catemeric motif
in diacid (II), a motif not present in (I) or in the other simple
cis-norbornene-1,2-carboxylic acids (Table 2). That the
hydrophobic factor itself does appear to play a role in the
generation of the catemeric motif can be clearly inferred from
the comparison of the crystal packings of cis-cyclopropane-
1
,2-dicarboxylic acid (CSD refcode FOJRAX) and its 3,3-
dimethyl analogue (KOJZEO; Table 2). While the dimeric
motif is common to both carboxylic acids, it is the dimethyl
substitution in cis-cyclopropane-1,2-dicarboxylic acid that
causes the co-existence of two patterns. The tendency for
increased hydrophobic aggregation in the crystal structures
appears to have a decisive effect in the overall crystal packing,
leading to the observation of the co-existence of the two
patterns in (II).
The co-existence of dimeric and catemeric motifs observed
for the title compound, for which we found just one precedent
in the CSD, thus appears to be directed by the hydrophobic
aggregation of the TMS groups. Though not uncommon, this
aggregation in¯uences the usual dimeric motif in favour of the
further formation of the chain. This is another instance of how
weak interactions may potentially in¯uence the molecular
association based on strongly directional supramolecular
synthons/motifs.
Figure 2
The two modes of self-assembly in (II), showing both dimeric and
catemeric motifs.
Experimental
To a suspension of small pieces (1 mm) of sodium (6.1 g, 0.265 mol) in
dry tetrahydrofuran (100 ml) in a 500 ml three-necked round-
bottomed ¯ask ®tted with a mechanical stirrer, a dropping funnel and
a condenser, under a nitrogen atmosphere, was added freshly distilled
1,3-cyclopentadiene (16.5 g, 0.25 mol) over a period of 45 min. The
dark-red reaction mixture was stirred for 2 h at room temperature.
Chlorotrimethylsilane (27.25 g, 0.25 mol) was then added dropwise
over a period of 1 h, during which time the mixture became warm and
changed colour from dark-red to blue to white with copious preci-
pitation of NaCl. Stirring was continued for another 3 h, and then the
mixture was ®ltered through glass wool and the precipitate washed
with tetrahydrofuran (2 Â 10 ml). The combined ®ltrates were cooled
in an ice±water bath and carefully treated with water (75 ml). The
layers were then separated. The aqueous layer was washed with ether
Figure 3
(
3 Â 50 ml), and the organic layers were combined, washed with
The catemeric associations of the diacid (II). Note that auxiliary CÐ
HÁ Á ÁO hydrogen bonds compliment the strongly directional OÐHÁ Á ÁO
hydrogen bonds.
water (3 Â 75 ml), dried (Na
2
SO ) and concentrated. The residue was
4
distilled under vacuum on a spinning band column. The fraction
ꢁ
o424 Begum et al.
C H
15 26
4
O Si
2
Acta Cryst. (2008). C64, o423±o425
Begum, Noor Shahina
