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Low melting molecular complexes. The structures of molecular complexes oftri- and di-chloromethanes with small ketones and 1,4-dioxane†‡
D. S. Yufit* and J. A. K. Howard
Received 14th November 2011, Accepted 9th December 2011
DOI: 10.1039/c2ce06526f
The crystals of nine new low-melting molecular complexes (LmMC) of chloroform and
dichloromethane with organic solvents have been grown in situ and structurally characterized by single-
crystal X-ray crystallography. In spite of seeming simplicity of the components, the structures display
a variety of types and motifs of intermolecular interactions. The obtained structures are compared with
the structures of pure components and previously known LmMC and some general tendencies are
outlined.
Introduction
Recent years have witnessed a surge of interest in cryo-crystal-
lization methods. This technique, which consists of in situ crys-
tallization of a compound, which is liquid or gaseous under
ambient conditions, followed by a single crystal X-ray structural
study of the obtained crystal, was developed about 50 years ago.2
It is a relatively well established but time consuming and
unpredictable procedure, which requires a great deal of patience.
In spite of all these difficulties the method was successfully used
recently for obtaining structural information not only for pure
compounds, but also for a variety of other purposes such as the
synthesis of co-crystals,3 search for new polymorph modifica-
tions4 and studying of isotopic effects on the molecular packing.5
These studies are also of interest for pharmaceutical and mate-
rials chemistry and provide excellent models for theoretical
calculations as the studied systems usually contain small mole-
cules with a limited number of intermolecular interactions.6
In continuation of our studies of low-melting molecular
complexes (LmMC)1 we performed further in situ co-crystalli-
zation experiments using common solvents such as chloroform
(CF) and dichloromethane (DCM) in combinations with small
cyclic and non-cyclic ketones, namely cyclobutanone (CB),
butanone-2 (MEK), acetone (AC), and with 1,4-dioxane (DO).
The crystal structure of a co-crystal of DCMwith cyclohexanone
(CH) was also determined. The reported molecular complexes
were synthesized in order to probe in detail the differences in
co-crystallization behaviour of these chlorinated solvents. These
LmMC also provided an opportunity to compare the packing
Department of Chemistry, Durham University, South Rd, Durham, DH13LE, UK. E-mail: [email protected]
† Part 2.1
‡ Electronic supplementary information (ESI) available. CCDCreference numbers [CCDC NUMBER(S)]. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c2ce06526f
This journal is ª The Royal Society of Chemistry 2012
arrangements in these closely related molecular complexes with
the results reported in ref. 1 and with the structures of pure
components of these co-crystals. The present study was also
expected to give more information on the relative importance of
various intermolecular interactions (CH/O, Cl/Cl, etc.) in
LmMC of CF and DCM. In addition the structural data of these
LmMC could be useful for modeling the properties of solvent
mixtures.
Here we report the crystallization and structural studies of
nine new LmMC (Scheme 1). The melting points, crystal data
and structural refinement parameters for compounds 1–9 are
given in Table 1, parameters of short intermolecular interactions
are listed in Table 2, while the tables of the geometrical param-
eters of the molecules are submitted as the ESI‡†.
Results
The structures of LmMC of CF and DCM with CB are shown in
Fig. 1 and 2. In structure 1 two CF and two CB molecules are
linked together in centrosymmetrical cyclic dimers.
Scheme 1 The studied LmMC.
CrystEngComm, 2012, 14, 2003–2008 | 2003
Table 1 Crystal data and structure refinement for LmMC 1–9
Compound 1 2 3 4 5 6 7 8 9
Empiricalformula
C4H6O �CHCl3
2C4H6O �CH2Cl2
C6H10O �CH2Cl2
C4H8O �2CHCl3
C4H8O �CH2Cl2
C3H6O �CHCl3
C3H6O �CH2Cl2
0.5C4H8O2 �CHCl3
0.5C4H8O2 �CH2Cl2
Melting point/K 200–203 185–186 195–196 175–179 168–170 164–166 175–178 216–217 205–206Data collectiontemperature/K
190.0(2) 180.0(2) 180.0(2) 165.0(2) 160.0(2) 150.0(2) 150.0(2) 200.0(2) 200.0(2)
Crystal system Monoclinic Orthorhombic Triclinic Monoclinic Monoclinic Monoclinic Monoclinic MonoclinicOrthorhombic Space group P21/n Pnma P�1 Pn P21/c P21/n C2/c P21/nPbcaa/�A, b/�A, c/�A 6.7004(7),
8.2245(8),15.4159(15)
10.3053(12),20.118(2),5.5119(4)
6.2893(6),7.8029(9),10.301(1)
6.054(2),6.965(3),16.346(6)
8.592(1),12.762(2),7.6903(9)
5.8895(4),13.6031(10),10.0396(8)
14.5833(12),8.2213(7),13.9240(10)
5.9481(5),9.3502(8),12.148(1)
7.829(1),10.025(2),14.764(3)
a/�, b/�, g/� 90.00, 101.00(3), 90.00
90.00, 90.00,90.00
103.76(1),103.15(1),98.40(1)
90.00,95.86(3),90.00
90.00, 102.77(2), 90.00
90.00,90.99(2),90.00
90.00, 117.21(1), 90.00
90.00,100.93(2),90.00
90.00,90.00,90.00
Volume/�A3 833.91(14) 1142.7(2) 467.28(9) 685.6(4) 822.39(18) 804.21(10) 1484.7(2) 663.36(10) 1158.8(4)Z 4 4 2 2 4 4 8 4 4rcalc/mg mm�3 1.509 1.308 1.301 1.506 1.268 1.466 1.280 1.636 1.479m/mm�1 1.021 0.537 0.632 1.218 0.706 1.053 0.776 1.269 0.985Reflectionscollected
6939 9197 4033 4192 6089 7047 6403 5682 8652
Independentreflections, Rint
2027, 0.0364 1360, 0.0677 2029, 0.0150 2599, 0.0830 1831, 0.0368 2268, 0.0381 1733, 0.0242 1507, 0.0379 1462,0.0316
Data/restraints/parameters
2027/0/110 1360/0/93 2029/0/139 2599/2/121 1831/0/113 2268/0/101 1733/0/96 1507/0/84 1462/0/80
Goodness-of-fiton F2
1.075 1.072 1.048 0.957 1.004 1.061 1.047 1.066 1.032
Final R1 indexes[I > 2s(I)]
0.0342 0.0325 0.0339 0.0497 0.0303 0.0289 0.0418 0.0364 0.0326
Final wR2 indexes[all data]
0.1041 0.0792 0.0977 0.1205 0.0696 0.0772 0.1394 0.1043 0.0848
Table 2 Geometrical parameter of intermolecular interactions instructures 1–9 (�A, �)a
Compound Contact C/O C–H H/O C–H/O
1 Cl3/O11 3.197(1)C1s/O1 3.252(2) 0.90(2) 2.41(2) 155(2)
2 C1s/O1 3.287(2) 0.96(2) 2.35(2) 168(1)C2/O12 3.635(2) 0.97(2) 2.68(2) 169(1)
3 C1s/O13 3.282(2) 0.96(2) 2.48(2) 142(1)C1s/O14 3.330(2) 0.95(2) 2.44(2) 155(2)
4 (av.) C1s/O1 3.14(1) 1.00(5) 2.27 144(4)C2s/O1 3.18(2) 1.00(5) 2.19 166(4)C4/O15 3.32(2) 0.98(5) 2.34 160.2
5 C1s/O1 3.355(2) 0.94(2) 2.42(2) 175(1)C1s/O16 3.249(2) 0.93(2) 2.44(2) 146(2)
6 C1s/O1 3.229(2) 0.90(2) 2.51(2) 138(2)C1s/O12 3.090(2) 0.90(2) 2.41(2) 133(2)
7 C1s/O1 3.388(2) 0.90(2) 2.49(2) 175(2)C1s/O14 3.181(2) 0.91(2) 2.39(2) 146(2)
8 C1s/O1 3.209(2) 0.95(2) 2.29(2) 162(2)C2/O12 3.553(2) 0.96(3) 2.73(2) 145(2)
9 C1s/O1 3.225(2) 0.93(2) 2.34(2) 157(2)C1s/O17 3.391(3) 0.96(2) 2.46(2) 166(2)
a Symm. codes: (1) (�x, 1 � y, 1 � z), (2) (1 � x, 1 � y, 1 � z), (3) (1 � x,�y, 1 � z), (4) (1 + x, 1 + y, z), (5) (1 + x, y, z), (6) (x, 3/2 � y, 1/2 + z) and(7) (1/2 + x, y, 1/2 � z).
Fig. 1 Centrosymmetrical dimer in structure 1.
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The molecules in dimer 1 are connected by Cl3CH/O contacts
and by not very commonly occurring Cl/O interactions (Cl/O
distance is 3.197 �A). However, similar Cl/O interactions were
observed previously in LmMC of CF with dimethylformamide.1
The carbonyl groups of adjacent CB molecules are anti-parallel
2004 | CrystEngComm, 2012, 14, 2003–2008
to each other and the C/O distance between them is equal to
3.370 �A. The shortest Cl/Cl contact in structure 1 is relatively
long (3.581�A). The CFmolecules are combined by these contacts
in zig-zag chains, parallel to the b-axis. There are no close CH/O intermolecular contacts in structure 1 (the shortest H/O
distance is equal to 3.026 �A) but additional weaker Cl/O
interactions (Cl/O 3.284 �A) exist between different dimers.
The packing of molecules in LmMC 2 is quite different from
that found in 1. There are two CB molecules per DCM and they
are linked by Cl2CH/O interactions (H/O 2.34 �A). Adjacent
CB molecules in structure 2 are bonded together by a pair of
CH/O contacts (H/O 2.68 �A) which join the dimers in chains,
This journal is ª The Royal Society of Chemistry 2012
Fig. 2 The chain of molecules in structure 2 (a); the packing of molecules in structure 2 (view along the c-axis) (b).
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parallel to the b-axis (Fig. 2a). There are weaker CH/C]O and
Cl/Cl contacts present between the chains. The resulting
arrangement is quite unusual—in structure 2 the double layers of
CB molecules alternate with a single layer of DCM molecules
(Fig. 2b).
Unlike dissimilar structures 1 and 2 the arrangement of
molecules in LmMC of DCM with CH (3) is analogous to that
found previously1 in the corresponding CF complex—the mole-
cules are linked by Cl2CH/O interactions and form centro-
symmetric dimers (Fig. 3). However, there are no CH/O
contacts between dimers in 3 and the molecules in adjacent
dimers are connected by dipole–dipole interactions: the carbonyl
groups of the ketones are anti-parallel to each other with cor-
responding C/O distances of 3.265 �A. All Cl/Cl distances in
the structure are longer than 3.68 �A.
A seemingly minor replacement of cyclic ketone (CB) by
a similar non-cyclic one (MEK) results in significant changes in
molecular arrangements. The CF and MEK crystallize as 2 : 1
LmMC (4) where disordered MEK molecules are linked by
CH/O interactions in chains, parallel to the a-axis (Fig. 4a).
Two CF molecules are connected to each MEK molecule by
Cl3CH/O contacts but oriented differently—one of the contacts
is in the plane of MEKmolecule while the second, shorter one, is
perpendicular to the plane. There are weak Cl/Cl contacts
(3.581 �A) between CF molecules of different chains. Zigzag
chains in the structure of LmMC 5 are formed by alternating
DCM and MEK molecules linked together by Cl2CH/O bonds
(Fig. 4b). There are no short Cl/Cl distances in the structure
Fig. 3 Centrosymmetric dimer in structure 3.
This journal is ª The Royal Society of Chemistry 2012
and the chains are connected by weak and no direction-specific C
(methyl)–H/Cl, O contacts.
While the main ‘‘building blocks’’ in structures 4 and 5 are
similar (chains), in structures 6 and 7 they differ significantly.
The centrosymmetric dimers, similar to those found earlier1 in
the structures of LmMC of CF with cyclopentanone (CP) and
CH, are formed in crystal 6 (Fig. 5a), while a familiar pattern of
zig-zag chains is observed in 7 (Fig. 5b). It should be noted that in
contrast to structure 1 halogen bonds Cl/O are not formed in 6
and instead the molecules are linked by bifurcated Cl3CH/O
interactions. The dimers in structure 6 are connected by pairs of
C–H/O interactions between AC molecules (H/O 2.82 �A) and
by relatively long (3.558 �A) Cl/Cl contacts. In structure 7
carbonyl groups of neighboring chains are anti-parallel with
corresponding C/O distances of 3.411 �A. No close Cl/Cl
contacts are found in structure 7.
The structures of 1,4-dioxane LmMC (1 : 2) with CF (8) and
DCM (9) are shown in Fig. 6. The DO molecules in structure 8
are arranged in chains linked by pairs of CH/O interactions
while CF molecules are situated on both sides of the chain and
bound to it by Cl3CH/O contacts. There are no close inter-
chain contacts in structure 8 and adjacent chains are interdigi-
tated. While CF molecules in crystal 8 connected only to the
main DO chains, DCMmolecules in LmMC 9 play an important
role in forming a motif and together with DO molecules produce
a network of CCl2H/O interactions, which combine the mole-
cules in 2D layers, perpendicular to the b-axis. The molecules in
layers form 22-membered macrocycles of R86(22) type.
7 Similar
to structure 8 there are no close and distinctively directed inter-
layer contacts in LmMC 9.
Discussion
Schematic representation of the main motifs of inter-component
interactions in structures of LmMC 1–9 is shown in Scheme 2. As
it was mentioned above one of the features of most LmMC is
a limited number of possible intermolecular interactions. The
absence of aromatic fragments and strong ‘‘classical’’ hydrogen
bonds in studied structures makes them look simple. However,
even these relatively simple systems are not intuitively predict-
able and show high versatility in bonding modes. Compounds
1–9 contain three types of potential H-donors (CF, DCM and
various methylene and methyl groups of ketones and DO) and
CrystEngComm, 2012, 14, 2003–2008 | 2005
Fig. 4 Chains of the molecules in structures 4 (a) (only one component of disordered MEK is shown), and 5 (b).
Fig. 5 Dimer in structure 6 and chains in structure 7.
Fig. 6 (a) Fragment of the chain in 8 and (b) a layer in 9.
Scheme 2 Main motifs of inter-component interactions in structures
1–9.
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two types of H-acceptor—carbonyl oxygen atoms of ketones and
heterocyclic ether oxygen atoms of 1,4-dioxane. It is also known
that in comparison with other halogens chlorine is less inclined to
participate in halogen bonds Hal/X.8 However, even in such
a restricted set of compounds all potential donors and acceptors
are not equivalent. Indeed, recent theoretical calculations9 show
once more that the polarity of the oxygen atom in carbonyl
groups of small cyclic ketones and the dipole moments of these
groups decrease in the order of CB > CP > CH. The acceptor
ability of these C]O groups decreases accordingly and ether
oxygen atoms have less acceptor character than carbonyl ones.
At the same time the acidity of hydrogen atom in CF is higher
than that in DCM molecule which makes it a better potential H-
donor.10 Obviously a DCM molecule may be involved in two
non-bifurcated CH/X interactions while that of CF in only one.
It should also be noted that both carbonyl and ether groups in
the compounds studied are acting as acceptors in two close
contacts simultaneously.
As it was already mentioned, the LmMC of CF with cyclo-
pentanone and cyclohexanone are built of cyclic dimers,1 where
the molecules are linked by bifurcated Cl3CH/O interactions.
So it was not really surprising to find a similar arrangement in
2006 | CrystEngComm, 2012, 14, 2003–2008
structure 3 where the only difference is the non-bifurcated
character of CH/O contacts. The H/O distances in dimers 3
are slightly longer than those in the corresponding CF co-crystal
(shortest ones are 2.44 and 2.41 �A in 3 and CF LmMC respec-
tively). It correlates well with the increased number of H-donors,
their lower acidity and change of the CH/O contact character.
However the structures of LmMC 1 and 2 have turned out to
be quite different. A slight increase in the polarity of the carbonyl
group in CB in comparison with CP and CH makes the combi-
nation of halogen Cl/O and hydrogen CH/O bonds in 1 more
favourable than bifurcated contacts in corresponding CP and
CH LmMC. The same increase in polarity of carbonyl group and
lower acidity of H-atoms in DCM in comparison to CF resulted
in emergence of double CH/O contacts between CB molecules
in 2. Interestingly enough the geometry of inter-component
interactions observed in LmMC 1 is totally different from that
calculated recently by DFT and MP2 methods for the same
system CF/CB.9 The calculated model shows CCO/H torsion
angles of about 0� (43.8� in 1), H/O distances of 2.12–2.13 �A
(2.41(2) �A in 1) and no Cl/O interactions, observed in 1.
Both the shape of the molecule and the polarity of carbonyl
group change in transition from CB to its non-cyclic analogue
MEK. It makes the corresponding LmMC 4 and 5 quite different
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from 1 and 2. It is not really possible to point out a single factor
responsible for such a radical change. Probably the shape of the
molecule plays a more important role here as in LmMC 6 with
smaller AC molecule the components form the same cyclic
dimeric motif, found in 1 and 2. On the other hand the packing of
components in 5 and 7 is remarkably similar and does not involve
ketone/ketone interactions in the chains.
The difference between dioxane LmMC 8 and 9 may be
explained by the difference in the number of H-atoms in CF and
DCM—indeed, the lack of donor atoms in structure 8 is
compensated by formation of dioxane/dioxane CH/O
contacts.
It is interesting to compare the interactions between molecules
of the same component in the studied LmMC and in the crystals
of pure components. Thus, the two independent CB molecules in
the structure of pure CB11 are linked together into anti-parallel
dimers by dipole–dipole interactions and into chains by pairs of
CH/O contacts. Similar CB dimers have been found in 1 while
in structure 2 CB molecules are linked together in chains by pairs
of CH/O contacts (Fig. 7). We consider that co-crystallization
has effectively ‘selected’ and thence ‘removed’ a fragment from
the observed structure of the pure component and that the
fragment ‘removed’ in this way depends on the co-crystallizing
partner.
The same interrelation may be traced between the structures of
stable low-temperature polymorphic modification of pure
acetone12 and LmMC 6 and 7. Interestingly in this case the
dimers of acetone molecules are found in LmMC 7 with DCM,
while in LmMC 6 with CF the double-connected chains of
acetone molecules are present. On the other hand only in the
structure of 8 the dioxane molecules form double-bonded chains,
similar to those in the structure of pure dioxane,13 while in 9 the
dioxane molecules are isolated and do not form any close
contacts. Also, in structures 4 and 5 the mutual arrangement of
the MEK molecules is totally different from that found in the
structure of pure MEK,14 which may indicate the possibility of
polymorphism of pure MEK.
Fig. 7 Interactions between CB molecules in structures of pure CB (a),
1 (b) and 2 (c).
This journal is ª The Royal Society of Chemistry 2012
Conclusions
The results presented demonstrate that not only chloroform but
also dichloromethane form low-melting molecular complexes
with small ketones, these ketones could be either cyclic or acyclic.
It also has been found that similar complexes are formed with
1,4-dioxane. In all cases the (Cl)CH/O contacts are present in
the structure. The important role of Cl/O halogen bonds in the
formation of LmMC is confirmed. In some cases the fragments
of motifs, found in structures of pure components, also have
been observed in LmMC. Interestingly, the change in co-crys-
tallization ‘‘partner’’ changes the selected motif. Implications of
this observation for the design of further co-crystals have to be
explored.
It should be noted that the structures described in the paper
most probably represent only a part of the total picture. It is
quite possible that under another crystallization condition other
polymorph modifications of presented LmMC and/or LmMC of
different stoichiometries could be obtained. The absence of some
compounds also does not necessarily mean that they could not be
crystallized at all—possibly the proper experimental conditions
for crystallization have not been yet found. For example, so far
all our attempts to crystallize LmMC of DCM with cyclo-
pentanone have failed, usually yielding the crystals of pure
components, while LmMC 2 has been obtained only after several
weeks of attempts.
General experimental procedure
A mixture of two liquids was sealed in a borosilicate capillary of
0.2–0.5 mm diameter and 15–30 mm length. The capillary was
fixed on a standard goniometer head which was mounted on the
goniometer of a Bruker SMART CCD 6000 diffractometer,
equipped with a Cryostream (Oxford Cryosystems) cooler and
a special attachment for collecting data using bulk equipment.15
The usual technique was applied for crystallization of mixtures:
slow cooling of the capillary accompanied (if necessary) by
occasional shock cooling eventually resulted in the formation of
a polycrystalline sample. Several partial meltings of the sample
and careful tuning of the temperature were normally required
after that in order to achieve an optimal crystal growth rate.
When a crystal of acceptable quality was finally obtained, the
temperature usually was lowered by 10–15 �C and the data were
collected in two 180� u-scans. Crystallographic information and
refinement parameters are shown in Table 1.
References
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