Rational Analysis of Melting Point Behavior of Co-Crystals of 4-Nitrophenol with Some Aza-Compounds

Article abstract of DOI:10.1021/acs.cgd.6b01011

Various co-crystals of 4-nitrophenol with aza-compounds have been synthesized and characterized by single-crystal X-ray diffraction method. Thermal analysis of all the co-crystals infers higher melting point compared to their respective co-formers and the increase in melting point perhaps because of the cross-chain close-packed arrangement of the constituent molecules. The trend of increase in melting point among the co-crystals reported here is difficult to correlate because of the diverse nature of the aza-compounds. Therefore, we have grouped the co-crystals based on the position of the nitrogen atom of aza-compounds. Apart from structural features the melting point trend among the co-crystals has also been correlated by considering the parameters such as crystal density, packing efficiency, and number of strong hydrogen bonds. It has been noticed that within the group the co-crystal with higher value of above said parameters shows higher melting point compared to the co-crystals with lower values, inferring a linear correlation between any of these parameters and the corresponding melting point of the co-crystal.

Full text of DOI:10.1021/acs.cgd.6b01011

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Article
Rational Analysis of Melting Point Behaviour of Co-
crystals of 4-Nitrophenol with Some Aza-compounds
Amrita Nayak, and Venkateswara R. Pedireddi
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01011 • Publication Date (Web): 07 Sep 2016
Downloaded from http://pubs.acs.org on September 8, 2016
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Rational Analysis of Melting Point Behaviour of Co-
crystals of 4-Nitrophenol with Some Aza-compounds
Amrita Nayak* and V. R. Pedireddi
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Solid State and Supramolecular Structural Chemistry Laboratory, School of Basic Sciences,
Indian Institute of Technology Bhubaneswar, Bhubaneswar 751 007, India.
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Email: amrita.nayak003@gmail.com
ABSTRACT
Various coꢀcrystals of 4ꢀnitrophenol with azaꢀcompounds have been synthesized and
characterized by single crystal Xꢀray diffraction method. Thermal analysis of all the coꢀ
crystals infers higher melting point compared to their respective coꢀformers and the increase
in melting point perhaps because of the crossꢀchain close packed arrangement of the
constituent molecules. The trend of increase in melting point among the coꢀcrystals reported
here is difficult to correlate because of the diverse nature of the azaꢀcompounds. Therefore
we have grouped the coꢀcrystals based on the position of the nitrogen atom of azaꢀ
compounds. Apart from structural features the melting point trend among the coꢀcrystals has
also been correlated by considering the parameters such as crystal density, packing efficiency
and number of strong hydrogen bonds. It has been noticed that within the group, the coꢀ
crystal with higher value of above said parameters shows higher melting point compared to
the coꢀcrystals with lower values, inferring a linear correlation between any of these
parameters and the corresponding melting point of the coꢀcrystal.
INTRODUCTION
Coꢀcrystallization1ꢀ8 is a viable supramolecular synthetic method to bring multiple
molecular components into a crystal lattice and it is an amenable pathway for a molecular
solid to get altered physicochemical properties.9ꢀ17 It is well documented that such
physicochemical properties of molecular solids depend upon the type and strength of the
intermolecular interactions present in the molecular ensembles.18ꢀ27 Melting point is one of
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the important physical property of a molecular solid which also could be varied via coꢀ
crystallization process.28ꢀ31 For instance, Braga et al.,32 reported the oddꢀeven effect of
melting point behaviour of homologous aliphatic dicarboxylic acids in the coꢀcrystals with
coꢀformers 1,2ꢀbis(4ꢀpyridyl)ethane and 1,3ꢀbis(4ꢀpyridyl)propane. The oddꢀeven effect of
melting point is retained with respect to the total number of carbon atoms in the coꢀcrystal.
However, Bond et al.,33 showed reversed melting point behaviour in the coꢀcrystals of
aliphatic dicarboxylic acids with pyrazine and this anomalous behaviour is explained based
on the density of coꢀcrystals. Further, Veidis et al.,34 explained melting point behaviour of coꢀ
crystals of phenanthridine with various dicarboxylic acids based on the strength of the
intermolecular interactions apart from the crystal density.
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It is to note that, in general, most of the coꢀcrystals have melting point in between the
corresponding coꢀformers35ꢀ42 and very few coꢀcrystals have higher melting point than their
corresponding individual components43ꢀ44. In fact, the statistics given by Schultheiss et al.45,
corroborate this fact, wherein out of 50 pharmaceutical coꢀcrystals, only four have higher
melting point as compared to their coꢀformers. In addition, it is also interesting to note that
the coꢀcrystals are being observed with higher melting points are mostly the molecular
complexes with carboxylic acid functional moieties. Thus, towards thrust for coꢀcrystals with
higher melting points, engaging molecules with different functional groups other than
carboxylic acid, we have found seven coꢀcrystals of 4ꢀnitrophenol and different aza
compounds (Chart 1) with higher melting points.
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Chart 1
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reactants
complex
(composition)
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(1) + (a)
1a (2:1)
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4ꢀnitrophenol (1)
(1) + (b)
(1) + (c)
(1) + (d)
(1) + (e)
(1) + (f)
1b (2:1)
1c (2:1)
1d (2:1)
1e (2:1)
1f (1:1)
4,4'ꢀbipyridine (a)
phenazine (d)
1,2ꢀbis(4ꢀpyridyl)ethene (b)
1,2ꢀbis(4ꢀpyridyl)ethane (c)
1,10ꢀphenanthroline (f)
Pyrazine (e)
Acridine (g)
(1) + (g)
1g (1:1)
EXPERIMENTAL SECTION
All chemical reagents and solvents were obtained from commercial suppliers and
used as received without any further purification. Highly pure spectroscopic grade solvents
were used for all crystallization experiments. All molecular complexes 1a – 1g were prepared
by dissolving the respective reactants in MeOH and allowing the solvent to evaporate under
ambient conditions. In all cases good quality single crystals suitable for Xꢀray diffraction
analysis were obtained.
In a typical coꢀcrystallization experiment, 20 mg of 4ꢀnitrophenol (0.145 mmol) and
26 mg 1,2ꢀbis(4ꢀpyridyl)ethane (0.145 mmol) were dissolved in 10 ml of MeOH in a 25 ml
conical flask, by warming on a water bath. The resultant solution was allowed to evaporate
under ambient conditions and single crystals were obtained over a period of 48ꢀ72 hrs.
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Crystal structure determination
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Good quality single crystals of 1aꢀ1g were carefully chosen after they were viewed
under Leica microscope supported by a rotatable polarizing stage. The crystals were mounted
on a goniometer by gluing to a glass fiber and crystal data were collected on a Bruker singleꢀ
crystal Xꢀray diffractometer (D8 VENTURE) equipped with a PHOTON 100 CMOS
detector. All the crystals were stable throughout the data collection and data collection was
smooth. The structures were solved by intrinsic phasing method followed by full matrix least
squares refinement against F2 using SHELXTL and no anomalies were observed at any stage
of structure solutions. All nonhydrogen atoms were refined anisotropically and hydrogen
atoms were either refined or placed in calculated positions. The intermolecular interactions
were computed using PLATON46 or Mercury47 software. The packing diagrams were
generated using Diamond48.
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Differential scanning calorimetry (DSC)
Thermal analysis was carried on PerkinꢀElmer DSC 8000 instrument. Stainless steel
pans were used for the experiment, and the calibration of the instrument was done with the
standard sample indium. A sealed empty pan was used as reference pan and samples were
heated under a nitrogen atmosphere at a scan rate of 10o minꢀ1.
RESULTS AND DISCUSSION
Crystal structure analysis of the complexes 1aꢀ1g.
The obtained coꢀcrystals, 1aꢀ1g, were analysed by Single crystal Xꢀray diffraction
technique and the resultant asymmetric unit contents of all have been shown in Figure 1 in
the form of ORTEP.
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(a)
(c)
(b)
(d)
(f)
(e)
(g)
Figure 1. ORTEP representation of asymmetric unit contents of coꢀcrystals (a) 1a (b) 1b (c)
1c (d) 1d (e) 1e (f) 1f (g) 1g.
4ꢀnitrophenol and 4,4'ꢀbipyridine, 1a:
The coꢀcrystal of 4ꢀnitrophenol (1) and 4,4'ꢀbipyridine (a) were obtained from MeOH
by slow evaporation method. Structure determination by single crystal Xꢀray diffraction
method reveals that crystal lattice has an asymmetric unit of 1 and a in a 2:1 ratio. ORTEP of
the contents of asymmetric unit is shown in Figure 1a. Important and pertinent structure
determination parameters are listed in Table 5. Packing analysis reveals that the molecular
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recognition between the constituent components is established through OꢀH...N (H...N, 1.73
Å) hydrogen bonds, forming a trimeric unit comprised of one pyridyl molecule and two
phenolic moieties (Figure 2a). The trimeric units extend further in one dimension with the
help of CꢀH…O hydrogen bonds produces a zigꢀzag chain (Figure 2b). Such zigꢀzag chain
arises due to two different interaction modes of the nitro groups from two unique phenol
molecules present in the asymmetric unit. The one which is labelled as A, interact with other
molecule through centrosymmetric dimeric CꢀH…O hydrogen bonds with the corresponding
H...O distances being 2.56 Å (Table 6). In case of other which is labelled as B, the entities are
present in two planes, inclined at an angle of 55.39o. Such inclination prevents the formation
of centrosymmetric dimeric hydrogen bonds. Thus, a single CꢀH…O (H...O, 2.63 Å) hydrogen
bond is formed between the nitrophenol molecules (Figure 2c). Such one dimensional chains
are connected to others through CꢀH…O hydrogen bonds (H...O, 2.32 Å). It is to note that the
chains, interact each other through CꢀH...O hydrogen bonds as mentioned above, are arranged
orthogonal to each other to yield a crossꢀchain structure in three dimension as shown in
Figure 2d.
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(b)
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(d)
Figure 2. (a) Molecular interaction between 1 and a through OꢀH...N hydrogen bond. (b) The
zigꢀzag arrangement of chain in 1a. (c) Dimeric and catemeric CꢀH...O hydrogen bonds
between the nitrophenol molecules in the chain of 1a. (d) The crossꢀchain structure of 1a
connected by CꢀH...O hydrogen bonds.
4ꢀnitrophenol, 1 and 1,2ꢀbis(4ꢀpyridyl)ethene, 1b:
The molecular complex of 1 and 1,2ꢀbis(4ꢀpyridyl)ethene (b) obtained from MeOH
solution is characterized through single crystal Xꢀray diffraction method reveals that the coꢀ
formers are present in the lattice in a 2:1 ratio of 1 and b. The basic recognition between the
coꢀformers, 1 and b, has been established through OꢀH...N (H...N, 1.72 Å) hydrogen bonds
leading to a trimeric unit (Figure 3a). These units propagate further through centrosymmetric
dimeric CꢀH…O (H...O, 2.77 Å) hydrogen bonds between the nitro groups to produce the 1ꢀD
chains as observed in 1a (Figure 3b). Such a correlation between 1a and 1b may be related to
the similarity of the coꢀformers in both the complexes except for the increased dimension of
azaꢀdonor. However, difference in chain arrangements arises due to the twisting of pyridyl
group of a by 25° with respect to other pyridine unit, whereas, b is planar. The chains are
crossed as observed in 1a and form a crossꢀchain structure in three dimensional arrangement
(Figure 3c) with the aid of CꢀH…O (H...O, 2.60 Å) hydrogen bonds.
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(a)
(b)
(c)
Figure 3. (a) Molecular recognition between 1 and b through OꢀH...N hydrogen bonds. (b)
Extension of the trimeric unit through centrosymmetric CꢀH...O hydrogen bonds. (c) Crossꢀ
chain structure of 1b in three dimension connected by CꢀH...O hydrogen bonds.
4ꢀnitrophenol, 1 and 1,2ꢀbis(4ꢀpyridyl)ethane (c), 1c:
A mixture of 1 and 1,2ꢀbis(4ꢀpyridyl)ethane (c) in MeOH on slow evaporation gave
good quality crystals. Structure determination by single crystal Xꢀray diffraction reveals that
the coꢀformers are present in the crystal lattice in a 2:1 ratio of 1 and c, respectively. The
structural analysis of 1c shows that the constituent molecules 1 and c establish the primary
recognition through the OꢀH…N (H…N, 1.73 Å) hydrogen bonds to yield a trimeric unit as
observed in case 1a and 1b (Figure 4a). Participation of one of the nitro group oxygen atom
in CꢀH…O (H…O, 2.34 Å) hydrogen bonding with the pyridyl hydrogen atom, results into
formation of a tetrameric unit as shown in Figure 4b. Extension of these tetrameric units in
one dimension produces molecular tape (Figure 4c) which is different from 1a and 1b where
the CꢀH...O hydrogen bond formed between the phenolic units. The interesting feature in three
dimensional arrangement of these molecular tapes is that, the pyridyl moieties of the adjacent
tapes are flipped by angle of 54.02o. Such deviation from planarity enables the phenolic
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components to utilize the remaining nitro group oxygen atom (which is not participated in
tetramer formation) in the formation of CꢀH…O (H…O, 2.41 Å) hydrogen bonding with the
hydrogen atom of the pyridyl group present in the adjacent tape. Such interconnection of
tapes gives rise to a crossed tape network (Figure 4d).
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(b)
(a)
(c)
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Figure 4. (a) Molecular interaction between 1 and c through OꢀH...N hydrogen bond. (b)
Tetrameric unit forrmed between 1 and c through OꢀH...N and CꢀH....O hydrogen bonds. (c)
Molecule tape constituting tetrameric units of 1c. (d) The crossꢀchain structure of 1c
connected by CꢀH...O hydrogen bonds.
4ꢀnitrophenol 1, and phenazine (d), 1d:
Towards variation in finding different azaꢀdonor compounds for the preparation of coꢀ
crystals with 1, phenazine has been chosen as the coꢀformer where the nitrogen atom are in
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the same ring opposite to each other. The coꢀcrystals prepared between 1 and d shows a 2:1
molecular complex as represented in form of ORTEP in the Figure 1d. The sterically free
nitrogen atoms of the phenazine molecule provide an opportunity of easy approach of
phenolic groups to participate in molecular recognition process. The basic recognition
between the components is established through OꢀH...N (H...N, 1.83 Å) hydrogen bonds,
resulting into a trimeric unit as shown in Figure 5a. Packing analysis of molecules in the
lattice discloses that twoꢀdimensionally the components are arranged in a layer fashion. The
analysis of the layers reveals that, the trimeric units enable the phenolic groups to interact
with each other through centrosymmetric CꢀH…O (H...O, 2.40 Å) hydrogen bonds, involving
both the oxygen atoms of the –NO2 group. As a result the phenolic groups are connected to
each other and form a chain which are held together by the phenazine molecules (Figure 5b).
The intriguing feature of this layer arrangement is that the phenazine moieties are not in the
same plane as that of the phenolic chains, rather they inclined to the plane by an angle of
56.68o (Figure 5c). Such arrangement of constituent moieties lead to a crossed layer network
in three dimension as depicted in Figure 5d.
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(d)
Figure 5. (a) Molecular interaction between 1 and d through OꢀH...N hydrogen bond. (b)
Molecular arrangement of 1 and c in 1c in a layer structure. (c) Inclined phenazine
molecules to the layer arrangement. (d) The crossꢀlayer structure of 1c.
4ꢀnitrophenol, 1, and pyrazine (e), 1e:
In continuation with azaꢀdonor having the nitrogen opposite in the same ring, we have
coꢀcrystallized pyrazine (e) with 1 and the single crystal Xꢀray structural analysis of the coꢀ
crystal reveals the components are crystallized in 2:1 ratio of 1 and e respectively. The
asymmetric unit content of the coꢀcrystal has been illustrated in form of ORTEP as
mentioned in Figure 1e. The basic recognition between the components has been achieved
through OꢀH...N (H…N, 1.76 Å) hydrogen bonds, producing a supramolecular motif
constituting of two phenolic groups and one pyrazine molecule (Figure 6a). The potentiality
of phenolic oxygen to act as hydrogen bond acceptor apart from its donor capability for
establishing OꢀH…N hydrogen bond results into the formation of dimers of phenolic moieties
through centrosymmetric CꢀH…O (H…O, 2.41Å) hydrogen bonds. The dimers thus obtained
further propagate through CꢀH…O (H…O, 2.42Å) hydrogen bonds, established between
oxygen atom of the nitro group of 1 and hydrogen atom of e, to produce one dimensional
chain (Figure 6b). In three dimensions such chains are crossed each other as observed in 1a,
1b and 1c (Figure 6c). However, in case of 1e the crossꢀchain interaction is observed through
the OꢀH...N interaction rather than CꢀH...O hydrogen bonds as noted in 1a, 1b and 1c.
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(b)
(a)
(c)
Figure 6. (a) Molecular recognition between 1 and e through OꢀH...N hydrogen bonds. (b)
Chain structure obtained for 1e stabilized through CꢀH...O hydrogen bonds. (c) Crossꢀchain
structure obtained for 1e connected by OꢀH...N hydrogen bonds.
4ꢀnitrophenol, 1 and 1,10ꢀphenanthroline (f), 1f:
The coꢀcrystal of 1 and 1,10ꢀphenanthroline (f) were obtained in the form of single
crystals from MeOH. Structure determination reveals formation of a molecular complex of 1
and f in a 1:1 ratio unlike in 1a – 1e, where it is 2:1. The molecules of 1 interact with f
through a pairꢀwise, OꢀH...N/CꢀH...N hydrogen bonds with the corresponding H...N distances
of 1.75 and 2.64 Å, respectively, producing a dimeric unit (Figure 7a). This is due to the
segregation of nitrogen donors on one side f which imposes a severe steric hindrance for the
approach of two phenolic components. The dimeric ensembles thus produced are further held
together by CꢀH...O (H...O, 2.31 Å) hydrogen bonds, established between pyridyl hydrogen
atom and nitro oxygen atom, leading to the formation of a helical arrangement (Figure 7b).
Such helices are crossed further with the aid of CꢀH...O (H...O, 2.54 and 2.67 Å) hydrogen
bond by utilizing both nitro and phenolic oxygen atoms to yield a crossꢀchain structure as
observed in the case of 1a, 1b and 1e (Figure 7c).
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(a)
(b)
(c)
Figure 7. (a) A dimeric unit of 1 and f connected by OꢀH...N/CꢀH...O pairwise hydrogen bonds.
(b) The helical chain of 1d obtained by the CꢀH...O hydrogen bond between the dimeric units.
(c) The crossed helices of 1d in three dimension.
4ꢀnitrophenol, 1 and acridine (g), 1g:
The structural analysis of coꢀcrystal of 1 and acridine (g) crystallized from methanol
shows a 1:1 molecular complex (Figure 1g). The composition is seems to be obvious as the
acceptor pyridyl molecule, acridine, has only one binding site nitrogen atom. The constituent
molecules participate in molecular recognition phenomena through single OꢀH…N (H…N,
1.73 Å) hydrogen bond by utilizing the donor phenolic OH group and acceptor pyridyl
nitrogen. The basic supramolecular motif thus formed has been illustrated in Figure 8a. The
phenolic oxygen atom can accommodate both OꢀH…N and CꢀH…O hydrogen bonds by acting
as hydrogen bond donor in former and as acceptor in later case. This potentiality of phenolic
oxygen atom enables the formation of a tetrameric unit as illustrated in Figure 8b. The nitro
group oxygen atom involves in the formation of a centrosymmetric CꢀH…O (H…O, 2.53 Å),
which facilitate propagation of tetrameric units in one dimension in form of molecular tape as
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shown in Figure 8c. Such tapes are crossed each other in three dimension to produce a
crossed tape arrangement as represented in Figure 8d. However, the crossed tapes are
connected by weak van der Waals interactions.
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(a)
(c)
(b)
(d)
Figure 8. (a) Interaction of 1 and g through OꢀH...N hydrogen bond. (b) Tetrameric unit of 1 and
g obtained through CꢀH...O hydrogen bonds. (c) Extended tetrameric unit yielded a tape
structure of 1g. (d) Crossꢀtape structure of 1g.
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Supramolecular features and melting point behaviour of coꢀcrystals
5
6
7
8
It is interesting to note that all the coꢀcrystals show a 3ꢀD crossꢀchain close packed
network structure which might reflect in their melting point behaviour. Thus, melting point is
determined by Differential Scanning Calorimetry (DSC) and the observations are listed in
Table 1. All the coꢀcrystals, 1aꢀ1g, show higher melting point compared to their respective
coꢀformers. This may be due the strong heteromeric interactions, established between the coꢀ
formers in the coꢀcrystals in comparison to their homomeric aggregation in the pure form
(supporting information, Figure S2). However solely based on interaction strength the
melting point elevation phenomena could not be concluded.
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Table 1. Melting point of the coꢀcrystals, 1a – 1g
CoꢀCrystal CoꢀCrystal Coformersa Difference
m.p. (oC)
m.p. (oC) in m.p. (oC)b
164
114/114
153/114
110/114
177/114
52/114
50
11
30
6
1a
1b
1c
1d
1e
1f
164
153
183
117
154
132
3
117/114
110/114
37
18
1g
aFirst value is the m.p. of the respective azaꢀdonor in the coꢀcrystal, while the second value is
b
for the 4ꢀnitrophenol. Difference is calculated between the coꢀcrystal and higher melting
point coꢀformer.
Correlation of melting point trend among the coꢀcrystals reported here would be
difficult due to the structural diversity among the azaꢀcompounds. However, we have
attempted to corroborate the melting point behaviour by grouping the azaꢀcompounds
based on the positions of nitrogen atoms. Thus,
a
,
b
and
c
are clustered in one group
and are
(I) wherein the pyridyl units are separated by covalent bond(s). While
d
e
placed in second group (II) because of rigid conformation of the azaꢀcompounds apart
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from the nitrogen atoms are placed in same ring opposite to each other. Although
f
and
are
g
are rigid like
d
and
e
, the acceptor(s) is placed in one side as a result
f
and
g
7
8
classified as third group (III) entities. In group I, the aza compounds,
a
, b and c,
9
exhibit crossed chain network structures of varied degree of entanglement as depicted
in Figure 2d, 3c and 4d. Such variation in structure can be explained based on the 1ꢀD
structural features of the molecular components which in turn depend upon the
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possibility of twisting of pyridyl groups of azaꢀcompounds. However only
twisting of pyridyl group with the angle of 25° whereas and are collinear and
parallel respectively. This differentiation in the geometrical features of and lead
a shows
b
c
a
,
b
c
to the formation of 1ꢀD zigꢀzag chain (1a) (Figure 2b), 1ꢀD planar chain (1b) (Figure
3b) and 1ꢀD planar tape (1c) (Figure 4c), which ultimately induce variation in their
respective 3ꢀD arrangement. 1a and 1b with crossꢀchain structures with close packing
arrangement show higher melting point of 164°C. However, 1c shows the melting
point of 154°C perhaps because of less efficient crossꢀchain packing arrangement.
Apart from structural correlation melting point behaviour of the coꢀcrystals, 1a
is further corroborated using packing efficiency and crystal density as listed in Table
2. The increase in packing efficiency and crystal density is in the order of 1a≈1b 1c
ꢀ1c,
>
,
clearly indicates the higher melting point for 1a and 1b compared to 1c. Apart from
this total number of possible “strong hydrogen bonds” in the coꢀcrystals is calculated
because intermolecular interactions are the key component for the aggregation of
molecular entities in supramolecular assemblies. Thus calculation has been done by
following the assumption of Veldis et. al. by considering three CꢀH…O bonds are
equivalent to one strong OꢀH…N hydrogen bond34 (Table 6). 1a and 1b with almost
equal “strong hydrogen bonds” shares the melting point; however 1e with low number
of strong hydrogen bond shows less melting point compared to 1a and 1b
.
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Table 2. The number of hydrogen bonds, density and packing coꢀefficient in correlation with
melting point of the coꢀcrystals, 1a – 1c.
6
7
8
9
CoꢀCrystal CoꢀCrystal Density
Total
Packing
m.p.
(gcmꢀ1) Strong hydrogen Coꢀefficient (%)
bonds
164
164
153
1.467
1.409
1.345
3.33
73.4
71.6
67.8
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1a
1b
1c
3
1.66
The three dimensional aggregation of complexes 1d (Figure 5d) and 1e (Figure 6c) in
group II also exhibit distinguished features with respect to their packing. Phenazine being
equipped with extra aromatic rings forms layer arrangement whereas the involvement of
ortho hydrogen atoms of pyrazine, devoid of fused aromatic rings forms CꢀH…O hydrogen
bonds results in to chain structure. Such differences in structural features are also reflected in
their melting point as phenazine shows higher melting point value compared to that of
pyrazine. The melting point behaviour further quantified with consideration of total possible
strong hydrogen bonds, packing coꢀefficiency and coꢀcrystal density as listed in Table 3.
Table 3. Correlation between coꢀcrystal melting point, density, hydrogen bond efficiency and
packing coꢀefficiency of complexes 1d and 1e.
CoꢀCrystal CoꢀCrystal Density
Total
Packing
m.p.
(gcmꢀ1) Strong hydrogen Coꢀefficient (%)
bonds
1.461
183
117
3
71.9
1d
1e
1.390
2.33
67.6
Coꢀcrystals 1f and 1g are grouped together because of their 1:1 stoichiometry. Though
in both the cases cross chain networks have been obtained but features of such networks are
completely distinguished from each other due to their striking difference in the 1D structure.
In case of 1f 1D helical chain has been obtained whereas 1D molecular tape has been
encountered for 1g (Figure 7b and 8c). These complexes also show linear variation of their
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melting points with respect to any of the discussed parameters such as crystal density,
packing coefficiency and number of strong hydrogen bond (Table 4). Higher the value of any
considered parameter results into elevated melting point of the coꢀcrystal. The higher melting
point for 1f could be understood from its structural features as the crossed helical chains of 1f
protrude each other for efficient packing through series of CꢀH...O hydrogen bonds. Thus,
enhances the density as well as packing coefficient of the complex. In case of 1g, the
molecules are arranged as planar tapes and undergo crossꢀchain structure. It is to note that,
although 1g forms a crossꢀchain structure, the chains are connected only by weak van der
Waals interactions. Thus, the molecular complex, 1g, exhibit lower density and packing
efficiency hence the complex has lower melting point as compared to 1f.
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Table 4. Correlation between coꢀcrystal melting point, density, total number of strong
hydrogen bonds and packing coꢀefficiency of group III complexes.
CoꢀCrystal CoꢀCrystal Density
Total
Packing
m.p.
(gcmꢀ1) Strong hydrogen Coꢀefficient (%)
bonds
154
132
1.424
3
71.3
1f
1.389
2
70.4
1g
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7
1a
1b
1c
1d
1e
1f
1g
8
9
formula
2(C6H5NO:
(C10H8N2)
2(C6H5N3): 2(C6H5NO: 2(C6H5N3:
2(C6H5N3):
(C4H4N2)
(C6H5NO3)
:
(C12H8N2)
(C6H5NO):
(C13H9N)
(C12H10N2)
(C12H12N2)
(C12H8N2)
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11
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13
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22
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29
30
31
32
33
34
35
36
37
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formula
weight
434.40
needle
460.44
462.46
458.42
358.31
needle
319.31
318.32
plate
crystal
shape
needle
plate
Needle
plate
crystal
colour
pale yellow pale yellow
colourless pale yellow
colourless
monoclinic
P21/c
colourless
colourless
monoclinic
P21/n
crystal
system
monoclinic
monoclinic
monoclinic
Triclinic
orthorhomb
ic
space group
P21/c
P21/c
P21/c
P212121
P
1
a/Å
b/Å
19.090(4)
3.808(1)
27.347(1)
90
12.760(3)
3.880(1)
22.520(1)
90
11.642(2)
7.386(2)
13.509(3)
90
3.923(2)
9.681(5)
14.337(8)
76.99(2)
86.28(2)
79.09(2)
520.9(5)
1
10.421(4)
7.043(2)
12.206(4)
90
3.916(2)
15.758(1)
24.138(2)
90
3.992(1)
16.269(5)
23.581(6)
90
c/Å
α/0
β/0
98.38(3)
90
103.28(3)
90
100.55(1)
90
107.19(1)
90
90
91.26(1)
90
γ/0
90
V/Å3
1966.8(7)
4
1085.1(4)
2
1142.0(4)
2
855.8(5)
2
1489.8(2)
4
1531.1(7)
4
Z
Dc /g cmꢀ3
T(K)
λ (Mo Kα)
µ, mmꢀ1
2θ [0]
F(000)
1.467
296
1.409
296
1.345
296
1.461
1.390
296
1.424
296
1.381
296
296
0.71073
0.109
52.74
904
0.71073
0.104
52.74
480
0.71073
0.099
54.99
484
0.71073
0.108
0.71073
0.109
53.12
372
0.71073
0.100
56.49
664
0.71073
0.095
52.66
664
56.69
238
total
reflections
15620
4521
22135
19724
21481
34525
10983
unique
reflections
5295
355
2483
194
2614
156
2596
190
1758
147
3681
269
2817
218
no.of
parameters
R1, I>2σ (I)
0.0567
0.1787
0.0454
0.1170
0.0481
0.1457
0.0452
0.1137
0.0493
0.1582
0.0442
0.1011
0.0532
0.1230
WR2, I>2σ
(I)
CCDC No.
994011
994013
994012
994015
1426432
994014
1426431
Table 5. Crystallographic information for the coꢀcrystals of 1aꢀ1g
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Table 6. Characterization of hydrogen bond distances (Å) and angles ( °) observed in the structures 1aꢀ1g*
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8
9
HꢀBond
OꢀH…N
1a(bpy)
1b (bpyee)
1c (bpyea)
1d (phenz)
1e (pyrazine)
1f (1,10)
1.75 2.69 161 1.73 2.69
1g (acri)
1.74 2.72 170 1.72 2.69 175 1.73 2.69 166 1.83 2.80 168 1.76
1.73 2.71 174
2.73
167
164
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2.56 3.28 123 2.75 3.45 122 2.41 3.47 167 2.80 3.85 165 2.60
2.54 3.25 122 2.60 3.53 143 2.34 3.41 169 2.73 3.36 116 2.41
3.51
3.44
3.44
3.47
2.73
142
157
176
161
167
2.31 3.37 165 2.53 3.45
2.54 3.35 130 2.61 3.51
2.78 3.65 138 2.60 3.36
2.69 3.32 117
143
140
127
CꢀH…O
2.32 3.37 163 2.54 3.22 120
2.46 3.26 130 2.61 3.26 118
2.60 3.34 124
2.74 3.51 127 2.36
2.63 3.43 130 2.42
2.40 3.31 140 1.76
2.39 3.29 139
2.67 3.34 120
2.71 3.40 121
2.64 3.43 131
CꢀH…N
* In each row the three numbers for every structure represent H...A and D...A distances and ˂DꢀH...A
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CONCLUSIONS
Coꢀcrystals of 4ꢀnitrophenol with various azaꢀcompounds are synthesized and
characterized through single crystal Xꢀray diffraction method. The structural analysis of all
the coꢀcrystals shows a close packed arrangement due to crossꢀchain network structures of
the constituent molecules. Thermal analysis of all coꢀcrystals, 1aꢀ1g, shows higher melting
point compared to their respective coꢀformers and such elevated melting point of the coꢀ
crystals may be due to the crossꢀchain close packed arrangement of the constituent molecules.
For ease of understanding of the melting point behaviour, the coꢀcrystals are categorized into
three groups based on the conformation and positions of the nitrogen atoms of azaꢀ
compounds. Apart from the structural features, the melting point properties of the coꢀcrystals
are corroborated with crystal density, packing coꢀefficient and number of strong hydrogen
bonds which infers a linear relationship between the coꢀcrystal melting point and any of these
parameters. Thus, in group I 1a and 1b show higher melting point compared to 1c, in group II
1d is higher than 1e and in group III 1f shows higher melting point than 1g. This study of
melting point behaviour of coꢀcrystals of 4ꢀnitrophenol with various azaꢀcompounds would
assist the organic material chemist to synthesize materials with high thermal stability.
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ASSOCIATED CONTENT
Supporting Information
DSC plots of the complexes
Hirshfeld surface analysis
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: amrita.nayak003@gmail.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge MHRD for financial support.
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For Table of Contents Use Only
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Rational Analysis of Melting Point Behaviour of Co-
crystals of Nitrophenol with Some Aza-compounds
Amrita Nayak* and V. R. Pedireddi
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Solid State and Supramolecular Structural Chemistry Laboratory, School of Basic Sciences,
Indian Institute of Technology Bhubaneswar, Bhubaneswar 751 007, India.
Email: amrita.nayak003@gmail.com
Coꢀcrystals of 4ꢀnitrophenol with various azaꢀcompounds exhibits higher melting point
compared to their respective coꢀformers. Such thermal behaviour linearly correlated with
various parameters such as the crossꢀchain supramolecular architectures, coꢀcrystal density,
packing coꢀefficiency and number of strong hydrogen bonds.
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