Solvent-free polymorphism control in a covalent mechanochemical reaction

Article abstract of DOI:10.1021/cg2013705

Four crystal modifications of a Schiff base derived from 2-hydroxy-1-naphthaldehyde and 2-aminobenzonitrile have been obtained by conventional solution-based methods. Three polymorphs out of four can be synthesized under solvent-free conditions. Herein we

Full text of DOI:10.1021/cg2013705

Communication
pubs.acs.org/crystal
Solvent-Free Polymorphism Control in a Covalent Mechanochemical
Reaction
Dominik Cincic,* Ivana Brekalo, and Branko Kaitner
̌
́
Laboratory of General and Inorganic Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac
102a, HR-10000 Zagreb, Croatia
S
* Supporting Information
ABSTRACT: Four crystal modifications of a Schiff base derived from
2-hydroxy-1-naphthaldehyde and 2-aminobenzonitrile have been
obtained by conventional solution-based methods. Three polymorphs
out of four can be synthesized under solvent-free conditions. Herein we
report simultaneous solvent-free covalent synthesis and polymorphism
control of a single component system using mechanochemistry via neat
grinding and seeding-assisted grinding, i.e. neat grinding in the presence
of seed crystals. We describe the important role of seed crystals in
directing the supramolecular organization of the product of covalent
solvent-free reaction toward the intended polymorphic outcome. Single
crystal X-ray analysis of the four crystal forms revealed conformational
differences in molecules. Polymorphs display interesting and remarkably
different molecular packing features via C−H···N and C−H···O
interactions: 1D chains in form I, a 2D-network in forms III and IV,
and a 3D-network in form II.
or the design and synthesis of functional solid materials,
the ability to control the solid-state assembly of molecules
base8−10 derived from 2-hydroxy-1-naphthaldehyde (napht)
and 2-aminobenzonitrile (abn), namely N-(2-cyanophenyl)-2-
hydroxy-1-naphthaldimine (1), by a conventional solvent-based
method (Figure 1a).11 We also report that 1 can be synthesized
under solvent-free conditions12 (by melting or grinding) and
that the polymorphic outcome of 1 can be controlled in the
solid state by different mechanochemical methods: neat
grinding and seeding-assisted grinding, i.e. grinding in the
presence of seed crystals. Herein, we report the relationship
between all four crystal forms that was investigated by thermal
analysis (DSC) and powder X-ray diffraction (PXRD) (Figure
1d). Crystals of forms I and IV, if left standing in acetonitrile
vapor, convert spontaneously into form II in the period of a
day. Also, grinding, i.e. kneading, of these two forms, I or IV,
with a drop of acetonitrile affords once more a rapid and
complete conversion again into form II. We also observed that
heating of crystals of forms I or IV led to their transformation
into form III (Figure 1d). Thus formed crystals of form III can
be cooled to room temperature, and they are detectable by
PXRD. Melting of forms II and III is observed at 192 and 197
°C, respectively (see the Supporting Information).
F
into crystals is of considerable importance.1 In that context,
extensive effort has been dedicated to the study of poly-
morphism:2 the phenomenon of identical molecular species
occupying different crystalline arrangements. The conventional
approaches to controlling polymorphism, as well as the
screening of different crystal forms of a compound, are still
mainly based on a systematic exploration of all possible
crystallization conditions, such as by varying solvent, temper-
ature, supersaturation, etc.3 An overview of currently available
literature on controlling polymorphism by solvent-free methods
reveals that an overwhelming majority of reported research
studies have been based on melt crystallization or sublimation.4
Also in that context, many studies have focused on polymorphic
transformations and their control by compression, neat
grinding, liquid-assisted grinding, or grinding in the presence
of seed crystals.5 For example, we found a report by Otsuka and
Kaneniwa,6 who describe that the transformation among three
polymorphs of chloramphenicol palmitate in the solid state
proceeds several times faster in the presence of seed crystals.
Also, a recent study by Toda and co-workers7 described the
important role of seed crystals in the solid state inclusion
complexation of 2,2′-dihydroxy-1,1′-binaphthyl. To the best of
our knowledge, there is no report concerning the role of seed
crystals in directing the supramolecular organization of the
product of a covalent solvent-free reaction toward the intended
polymorphic outcome.
We were inspired to pursue the solvent-free mechanochem-
ical experiments in our study by a recent report from Dolotko
and co-workers,13 who described the solvent-free synthesis14 of
the Schiff base derived from o-vanilin and p-toluidine. They
Received: October 14, 2011
Revised: December 12, 2011
Published: December 13, 2011
Herein, we report the preparation of four crystal
modifications (termed forms I, II, III, and IV) of the Schiff
© 2011 American Chemical Society
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Figure 1. (a) Reaction scheme of preparation of 1. Polymorphic forms of 1: (b) preparation by a conventional solution-based method; (c) two-
dimensional fingerprint plots; and (d) transformation conditions.
established that at room temperature solid reactants react in a
liquid mixture formed upon grinding. We were also inspired by
an earlier report from Jones and co-workers,15 who describe
”green” polymorph control of cocrystallization. They were able
to control the polymorphic outcome of the mechanochemical
synthesis of caffeine cocrystals by grinding solid reactants with
minimal addition of a solvent of appropriate polarity. Over the
past decade, mechanochemical synthesis16 has shown a great
potential for clean and environmentally friendly preparation of
new materials.17 Methods such as liquid-assisted grinding
(LAG)18 or ion- and liquid-assisted grinding (ILAG)19 were
recently demonstrated as efficient synthetic methods in both
supramolecular and covalent synthesis.
We first attempted the solvent-free synthesis of 1 by manual
grinding of napht and abn in the stoichiometric ratio 1:1. After
10 min of grinding at room temperature and heating up to ca.
45 °C, the reaction mixture melts and forms a liquid mixture.
As revealed by PXRD and DSC, that liquid mixture, when left
standing at room temperature for 20 min, yields 1. The
polymorphic outcome of the product, form I, was identified by
comparing its PXRD pattern with those calculated from single
crystal data. Then we turned to explore the seeding of the
liquid mixture with the crystals of the other forms. In the
presence of a small amount of form II, the reaction resulted in
the formation of form II. Equivalent attempts of seeding the
melt with forms III and IV did not lead to forms III and IV,
respectively. On the other hand, after neat grinding (NG) for
20 min at 20 to 25 °C, the same starting materials appear to
react without the formation of a liquid phase. As revealed by
PXRD, this mechanochemical reaction also leads to the
formation of 1 but this time as form II (Figure 2). However,
we established that the performed mechanochemical experi-
ment led to only partial formation of 1. A quantitative reaction
of the solid reactants and pure form II was observed after 3
days standing at room temperature, based on PXRD.20 Once
formed, pure form II was characterized by FT-IR spectroscopy,
PXRD, and DSC (see the Supporting Information). These
observed results, that no liquid phase or extensive amorphous
intermediate phase was identified and that the reaction
proceeds upon standing of the milled mixture, can be related
with a recent report from Blagden and co-workers, who
describe the important role of particle size and intimate mixing
of reactants in the solid state reaction.21 However, water
generated in the reaction can also play a significant role, since
many organic reactions in the solid state proceed very
efficiently and selectively in the presence of even a small
amount of solvent vapor.22
Encouraged by the fact that a small amount of crystals of
form II can direct the polymorphic outcome of 1 and by the
fact that the reactants react in the solid state relatively slowly,
we turned to grinding experiments in the presence of seed
crystals. We expected that, by adding a small amount of seed
crystals of a desired polymorph, we can direct the polymorphic
outcome of a mechanochemical covalent synthesis toward that
same form. Indeed, seeding-assisted grinding of napht and abn
for 20 min at 20 °C in the presence of a small quantity of seed
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Crystal Growth & Design
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Figure 3. PXRD patterns of the product of the seeding-assisted
grinding experiment for a period of 8 days.
Figure 2. (a) Solvent-free synthesis of the three polymorphic forms of
1. (b) PXRD patterns of napht, abn, and polymorphs of 1 prepared by
the conventional solvent-based crystallization method and by solvent-
free methods and simulated patterns.
crystals of form III led to the formation of 1 as form III. As in
the NG synthesis of form II, mentioned above, this experiment
also led to the partial formation of 1 and the reaction of solid
reactants without the formation of a liquid phase. Quantitative
reaction and pure form III were observed after 8 days standing
at room temperature, based on PXRD (Figure 3). The obtained
pure form III was characterized by FT-IR spectroscopy, PXRD,
and DSC (see the Supporting Information). Attempts to
prepare forms I and IV by seeding-assisted grinding experi-
ments have been unsuccessful. As revealed by PXRD, these
experiments provided the same polymorphic outcome of 1,
form II.
Figure 4. Polymorphic forms I−IV of 1: (a) crystals, (b) overlapped
molecular structures, and (c) structural motifs.
The enol−imino tautomer was detected for all four crystal
forms from the molecular geometry consideration where the
C2O1 and C11N1 bond distances are in good agreement
with analogous values of corresponding Schiff bases derived
from 2-hydroxy-1-naphthaldehyde.10,24 The polymorphs dis-
play interesting and remarkably different molecular packing
governed by C−H···N and C−H···O interactions leading to
one-dimensional chains in form I, two-dimensional networks in
forms III and IV, and a three-dimensional network in form II
Single crystal X-ray analysis of the four crystal forms23 of 1
revealed conformational differences in molecules. They are not
planar due to the twist of the phenyl ring bonded to the amino
nitrogen out of the naphthalene moiety plane. Molecules are
the most planar in form I with a corresponding dihedral angle
of ca. 1.8°, while the most nonplanar molecules are in form II
with a corresponding dihedral angle of ca. 35.1° (Figure 4b).
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organization of the product of the solvent-free reaction toward
the intended polymorphic outcome. To the best of our
knowledge, we have presented the first example of simulta-
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control of a single component system using mechanochemistry
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details for solution-based synthesis, instrumental
experimental details, and PXRD, FTIR, and DSC data. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC 832329−832332 contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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L., Ed.; Pergamon Press: Oxford, 1985; Vol. 2, p 715 and references
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AUTHOR INFORMATION
Corresponding Author
*E-mail: dominik@chem.pmf.hr.
■
ACKNOWLEDGMENTS
This research was supported by grants from the Ministry of
Science and Technology of the Republic of Croatia (Grant No.
■
(9) (a) Schiff, H. Ann. Chim. 1864, 131, 118−119. (b) Tidwell, T. T.
Angew. Chem., Int. Ed. 2008, 47, 1016−1020.
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(10) Blagus, A.; Cincic, D.; Friscic, T.; Kaitner, B.; Stilinovic, V.
119-1193079-3069). We are grateful to Prof. Tomislav Frisc
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Maced. J. Chem. Chem. Eng. 2010, 29, 117−138.
and Dr. Vladimir Stilinovic for discussions and comments at
́
(11) The crystal structure of one polymorph out of four, form III,
was recently reported, but without any additional characterisation:
Zhou, J.-C.; Zhang, C.-M.; Li, N.-X.; Zhang, Z.-Y. Acta Crystallogr.,
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(20) The mechanochemical reactivity of solid reactants is expected to
improve upon ball milling. But, neat grinding for 20 min provided a
dark green slurry material which upon standing at room temperature
for 30 min leads to the formation of a solid product. As revealed by
PXRD, this experiment also resulted in partial reaction to form
compound 1. We have identified a mixture of form II and reactants
even after eight days standing at room temperature. Formation of a
liquid mixture during ball-milling is not surprising, and it is expected,
as manual grinding of napht and abn at room temperature, mentioned
above, and heating up to cca 45 °C provides a liquid mixture.
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(23) Crystal data for form I: C18H12N2O, Mr = 272.3, orthorhombic,
Pna21, a = 12.7255(11) Å, b = 26.833(2) Å, c = 3.8938(3) Å, V =
1329.58(18) Å3, Z = 4, Dcalc = 1.36 g cm−3, T = 293 K, unique refl =
1651, R = 0.065, wR = 0.097, S = 1.058. Crystal data for form II:
C18H12N2O, Mr = 272.3, orthorhombic, P212121, a = 7.6425(9) Å, b =
12.5079(11) Å, c = 13.9147(12) Å, V = 1330.1(2) Å3, Z = 4, Dcalc
=
1.36 g cm−3, T = 293 K, unique refl = 1666, R = 0.034, wR = 0.087, S =
1.083. Crystal data for form III: C18H12N2O, Mr = 272.3, monoclinic,
P21/n, a = 13.3537(4) Å, b = 7.2920(2) Å, c = 15.1552(5) Å, β =
114.823(4)°, V = 1339.39(7) Å3, Z = 4, Dcalc = 1.35 g cm−3, T = 293 K,
unique refl = 2905, R = 0.035, wR = 0.101, S = 1.089. Crystal data for
form IV: C18H12N2O, Mr = 272.3, monoclinic, P21/n, a = 4.9647(8) Å,
b = 18.738(3) Å, c = 15.175(2) Å, β = 96.223(14)°, V = 1403.4(4) Å3,
Z = 4, Dcalc = 1.29 g cm−3, T = 293 K, unique refl = 2450, R = 0.068,
wR = 0.123, S = 0.958.
(24) Cruz-Cabeza, A. J.; Groom, C. R. CrystEngComm 2011, 13, 93−
98.
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