When is a polymorph not a polymorph? Helical trimeric O–H···O synthons in
trans-1,4-diethynylcyclohexane-1,4-diol
Clair Bilton,a Judith A. K. Howard,*a N. N. Laxmi Madhavi,b Ashwini Nangia,b Gautam R. Desiraju,*b
Frank H. Allenc and Chick C. Wilsond
a Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE.
E-mail: j.a.k.howard@durham.ac.uk
b School of Chemistry, University of Hyderabad, Hyderabad 500 046, India. E-mail: grdch@uohyd.ernet.in
c Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, UK CB2 1EZ
d ISIS Department, CLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK OX11 0QX
Received (in Oxford, UK) 18th June 1999, Accepted 22nd July 1999
Two polymorphs (A and B) of trans-1,4-diethynylcyclohex-
ane-1,4-diol represent a unique example of the simultaneous
occurrence of both conformational polymorphism and
conformational isomorphism, while a pseudopolymorphic
monohydrate is closely related.
unwanted cis-isomer was separated by repeated recrystallisa-
tions from EtOAc. Further recrystallisation of pure trans-1
yielded crystals of two modifications, A and B, in the same
¯
flask. We find that both forms crystallise in space group P1 with
Z = 3,† both structures have three symmetry-independent half-
molecules occupying distinct inversion centres, and both
structures assemble around a spine of helical, cooperatively
assisted, trimeric motifs formed via O–H···O bonds [A: Fig.
1(a), B: Fig. 1(b); d(O···H) = 1.70–1.76 Å, q (O–H···O) =
163–177° over both structures, H-atom positions neutron-
normalised). However, Fig. 1(a) shows that form A contains
two molecules of conformer 1a in which the hydroxy groups are
diequatorial, while the third molecule has the diaxial OH
conformation (1b). Fig. 1(b) shows that the reverse is true in
form B: two molecules have diaxial OH groups (1b) and the
third has the diequatorial OH conformation (1a). Interestingly,
form B is 3% more efficiently packed than form A but this is
compensated for by the better O–H···O hydrogen bonds in form
A (mean d = 1.715 A vs. 1.745 in form B).
Definitions of polymorphism, the existence of two or more
different crystal structures for the same compound,1 and
pseudopolymorphism, the existence of one or more solvated
crystalline forms of the same compound,2 are in common usage.
Similarly, the existence of different conformations of the same
molecule in crystals have been distinguished3 by the terms
conformational polymorphism, the occurrence of different
conformers in different polymorphic structural modifications,
and conformational isomorphism, the occurrence of different
conformers in the same crystal structure. Here we describe two
structural polymorphs and one pseudopolymorph of trans-
Amazingly, a third crystalline form was isolated from the
same flask and characterised by both low-temperature X-ray
and neutron diffraction.† Form C (P21/c, Z = 4) was shown to
be a 1:1 hydrate of the diequatorial OH conformer 1a, and the
crystal structure maintains the helical O–H···O trimer via the
assembly of two symmetry-independent half-molecules of 1a
and one O–H donor from a water molecule [Fig. 1(c), d =
1.64–1.80 Å, q = 164–177°]. The second O–H donor of the
water molecule interlinks inversion-related trimers (d = 1.91 Å,
q = 174°). In effect, the water molecule replaces the axial OH
group in form A so that the trimeric O-H···O hydrogen-bonded
helical spine is the dominant recurring pattern in all three
crystalline forms of 1.
1,4-diethynylcyclohexane-1,4-diol 1, in which the simultaneous
occurrence of both conformational polymorphism and con-
formational isomorphism are the key distinguishing features of
the crystal structures, and also provide a unique test of existing
definitions.
Our studies of 1 were prompted by recent interest4 in
It would appear that the presence of both conformers is
required for the formation of the robust helical trimeric synthon
in the unsolvated polymorphs, a supposition that is reinforced
by the crystal structure of trans-cyclohexane-1,4-diol 3.6 The
structures containing the gem-hydroxyethynyl fragment 2. As
well as the expected O–H···O hydrogen bonds, arrays of C–
H···O, O–H···p and C-H···p interactions (p = ethynyl, phenyl),
sometimes stabilised by cooperativity, frequently dominate the
crystal packing. This variety of interactions leads to consider-
able structural diversity amongst the > 90 structures containing
2 that are already available in the Cambridge Structural
Database (CSD).5 Simple mono-alcohols exhibit two types of
O–H···O aggregates (chains and rings), but even here there is no
predominant motif. Thus, the available evidence suggests that
structures containing 2 are strongly influenced by the remaining
portion of the molecule, prompting us to investigate the crystal
structure of compound 1, in which competitive effects are
minimised so that the intrinsic hydrogen-bonding preferences of
2 may be more clearly discerned.
diol 3 also contains the helical O–H···O trimer and crystallises
in space group P21/n with 1.5 molecules per asymmetric unit.
The molecule in general positions has diequatorial OH groups,
while the molecule on an inversion centre has diaxial OH
groups. The O–H···O trimer is then formed by two diequatorial
and one diaxial conformer, as in form A of 1.
Why is it that both conformers are required for trimer
formation in forms A and B of 1, and in the diol 3? It may be that
the simultaneous presence of inversion centres (arising from the
molecular structures of 1 and 3), and the 3- or 31-axes that could
arise if the trimer formed from three identical conformers of 1
Diol 1 was synthesised by adding TMSC·C-Li to cyclohex-
ane-1,4-dione and hydrolysis of the TMS groups by KOH. The
Chem. Commun., 1999, 1675–1676
1675
Bilton, Clair
