Unusual cocrystals made of a Schiff base metal complex and an organic molecule - Close-packing vs. hydrogen bond interactions

Article abstract of DOI:10.1016/j.molstruc.2014.04.082

The mononuclear complexes [ML₂]⁰ (M = Co, Ni, Zn; HL - Schiff base ligand formed in situ from 2-pyridinecarbaldehyde and anthranilic acid, AA) can efficiently interact with unreacted AA molecules to produce CoL₂·AA·H₂

Full text of DOI:10.1016/j.molstruc.2014.04.082

Accepted Manuscript
Unusual cocrystals made of a Schiff base metal complex and an organic mole-
cule – close-packing vs. hydrogen bond interactions
Elena A. Buvaylo, Vladimir N. Kokozay, Katia Rubini, Olga Yu. Vassilyeva,
Brian W. Skelton
PII:
S0022-2860(14)00449-9
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.04.082
MOLSTR 20589
Reference:
Journal of Molecular Structure
To appear in:
Received Date:
Revised Date:
Accepted Date:
6 March 2014
25 April 2014
28 April 2014
Please cite this article as: E.A. Buvaylo, V.N. Kokozay, K. Rubini, O.Y. Vassilyeva, B.W. Skelton, Unusual
cocrystals made of a Schiff base metal complex and an organic molecule – close-packing vs. hydrogen bond
interactions, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.04.082
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Unusual cocrystals made of a Schiff base metal complex and an organic molecule – close-
packing vs. hydrogen bond interactions
Elena A. Buvayloa, Vladimir N. Kokozaya, Katia Rubinib, Olga Yu. Vassilyevaa,*, and Brian W.
Skeltonc
a Department of Chemistry, Taras Shevchenko National University of Kyiv, 64/13 Volodymyrska
str., Kyiv 01601, Ukraine. E-mail: vassilyeva@univ.kiev.ua
b Dipartimento di Chimica G. Ciamician, Universita` degli Studi di Bologna, via Selmi 2, 40126
Bologna, Italy
c Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley,
Western Australia 6009, Australia
Highlights
- Cocrystals of Schiff base Co, Ni and Zn complexes and anthranilic acid were reported.
- The organic molecules are inserted between the layers of ML2 units.
- The two independent AA molecules in 1–3 show different H-bonding preferences.
- The solid-state architectures of 1–3 and a parent complex are contrasted.
- Thermal behaviour of 1 and 3 was investigated.
Keywords: transition metals; Schiff bases; Hydrogen bonding; crystal structure; thermal analysis;
cocrystals
Abstract
The mononuclear complexes [ML2]0 (M = Co, Ni, Zn; HL – Schiff base ligand formed in situ from
2-pyridinecarbaldehide and anthranilic acid, AA) can efficiently interact with unreacted AA
molecules to produce CoL2·AA·H2O (1), NiL2·AA·H2O (2) and ZnL2·AA·0.25CH3OH·0.5H2O (3)
cocrystals. Compounds 1–3 have been obtained as single crystals and characterized by elemental
analysis, IR spectroscopy, thermal analysis, and single-crystal X-ray diffraction techniques. The
compounds crystallize in the triclinic space group P1 with 1 and 2 being isomorphous. Neutral ML2
,
molecules in 1–3 show no crystallographically imposed symmetry with the metal atoms
octahedrally surrounded by two anionic ligands in a mer configuration. Of the two
crystallographically distinct AA molecules, one molecule only is engaged in H-bonding N/O–H···O
interactions with ML2 units. The solid-state organization of the cocrystals is described as an
* Corresponding author. E-mail address: vassilyeva@univ.kiev.ua.

2
insertion of the organic molecules between the layers of ML2 complexes as they occur in the
reported native NiL2·H2O structure.
Introduction
The metal complexes containing Schiff bases are the most fundamental chelating systems in
coordination chemistry. Their attractive chemical and physical properties and their wide-ranging
applications in numerous scientific areas have been widely explored [1]. Lately, the interest in
Schiff bases chemistry has shifted to their use as molecular building blocks in the construction of
multicomponent supramolecular assemblies. Salen frameworks have shown to be valuable motifs in
the design of novel (bio-inspired) materials constructed by noncovalent assembly or by using
covalent linkages to other macromolecules, such as DNA and cavitand-based macrocycles [2].
Zn(II)salphen (phenyl-bridged salen) building blocks have demonstrated to be highly useful in
supramolecular chemistry and nanotechnology [3]. Recently, mononuclear metal Schiff base
complexes were found to produce cocrystals with ammonium salts or complexes containing
ammonia ligands, diprotonated diamines or dicarboxylic acids as hydrogen bond donors [4]. The
packing of the components in the resultant crystals is thought to be driven by the ability of the
empty O2O′2 compartment to act as a hydrogen bond acceptor [4].
The phenomenon of cocrystallization is a hot topic that has attracted considerable attention from
an unusually broad spectrum of scientists. There has been a growing number of reports on the
crystal engineering [5], nomenclature [6], syntheses [7], and properties [8] of cocrystals. The great
potential of cocrystals is believed to reside in their different bulk material and physico-chemical
properties with respect to those of the co-formers. This is why cocrystallization is emerging as an
attractive solid state alternative to the usual salts and solvates in the pharmaceutical field [9].
The term cocrystal is controversial in the literature as the field of crystal engineering, which
focuses on targeted attempts to bring different molecules together into a crystalline solid in a
definite stoichiometric ratio, is still evolving [10]. Bond’s definition of a cocrystal as a multi-
component molecular crystal is generally recognized as one of the most encompassing.6c
Historically, the area of cocrystals has been dominated by organic compounds, for which the
concept of attractive non-covalent interactions – ‘‘supramolecular synthons’’ [11] – helped
establish a framework for systematic explorations. Current views of cocrystals seem to be
broadened to the extent that a variety of multi-component crystalline assemblies composed of
organic and organometallic molecules [12], inorganic ions/salts [13], an ionic liquid [14], purely
metal complexes [15], or organic-inorganic hybrid structures formed by organic cations and anionic
metal complexes [16] are all referred to as cocrystals. In this context, cocrystallization offers access

3
to a wide range of supramolecular solid-state architectures by means of combining inorganic and
organic moieties, and therefore Schiff-base metal complexes appear good candidates in the quest.
During the last few years, we have been exploring the chemistry of 3d metal complexes of
Schiff base ligands with the aim to prepare mono- and heterometallic polynuclear compounds.
Open-air reactions of metal powder and another metal salt with a solution containing a condensation
product of aldehyde and primary amine, yielded a number of novel Co/Fe, Cu/Fe, Fe, Co and Mn
compounds. Details of the used synthetic approach as well as its applications are given in [17].
In this report, an example of unusual aggregation of a mononuclear Schiff base metal complex
with an organic molecule is described. The tridentate carboxylate ligand 2-N-(2’-
pyridylimine)benzoic acid (HL) results from the condensation between 2-pyridinecarbaldehide and
anthranilic acid (AA) and in the deprotonated state coordinates a metal ion via the pyridine-N, the
imine-N and the carboxylate-O to form one six- and one five-membered chelate rings (Scheme 1).
Scheme 1. Chemical structures of 2-N-(2’-pyridylimine)benzoic acid, HL, (left) and CoL2·AA·H2O
(1) (right).
CoL2·AA·H2O (1), NiL2·AA·H2O (2) and ZnL2·AA·0.25CH3OH·0.5H2O (3) cocrystals demonstrate
an interesting case in which two crystallographically non-equivalent molecules of AA are also
chemically distinct in the way they participate in H-bonding. The reason for cocrystallization in
terms of supramolecular interactions is not easy to rationalize in these organic-inorganic systems.
Experimental section
Materials and general methods
2-pyridinecarbaldehide was purchased from Merck and used as received. All other chemicals were
purchased from local suppliers and used without further purification. All solvents were of AP-
grade. All the experiments were carried out in air. Elemental analyses were performed with ICP
spectrometer (Fisons Instruments, ARL Model 3410+) for Co, Ni, Mn, and Zn. CHN-analysis was
performed with a Perkin–Elmer 2400 analyzer. Infrared (IR) spectra were recorded on a Perkin-
Elmer 1600 FT-IR spectrometer from KBr pellets in the 400–4000 cm–1 region. The solid-state EPR
spectrum was recorded on a Radiopan PS100.X spectrometer operating at X-band.

4
Thermogravimetric analysis (TGA)
TGA was performed on a Perkin Elmer TGA7 instrument. Approximately 3 mg of the sample was
heated over the temperature range of 40 to 910 °C at a heating rate of 10 °C min–1. The samples
were purged with a stream of flowing nitrogen throughout the experiment at 20 ml min–1.
Differential scanning calorimetry (DSC)
DSC was performed with a Perkin Elmer Pyris Diamond differential scanning calorimeter,
equipped with an ULPS90 intracooler. The samples (5–10 mg) were placed in open Al-pans.
Temperature and enthalpy calibrations were performed using high purity standards (n-decane,
benzene and indium). The samples were heated in the DSC cell under inert nitrogen conditions
(flow rate of 20 mL min–1) from 25 to 250–350 °C at a rate of 10 °C min–1.
Single crystal structure determination
Crystallographic data for the structures were collected at 100(2) K on an Oxford Diffraction Gemini
(for 1 and 3) or Xcalibur (for 2) diffractometer fitted with Mo-Kα (λ = 0.71073 Å) radiation.
Following analytical absorption corrections and solution by direct methods, the structures were
refined against F2 with full-matrix least-squares using the program SHELXL-97 [18]. The solvent
in 1 and 2 consists of two molecules of anthranilic acid, disordered about crystallographic inversion
centers, plus a water molecule. The water molecule hydrogen atoms were not located. The solvent
in 3 was modelled as a water molecule disordered over two sites each with site occupancy factors
constrained to 0.5, together with a methanol molecule refined with site occupancy constrained to
0.5 after trial refinement. Solvent hydrogen atoms were not located. The hydrogen atoms of the
amine groups in 3 were refined with N–H distances restrained to ideal values (0.88 Å with an
uncertainty of 0.02 Å). All remaining hydrogen atoms in 1–3 were added at calculated positions and
refined by use of a riding model with isotropic displacement parameters based on those of the
parent atom. Anisotropic displacement parameters were employed for the non-hydrogen atoms.
Details of the data collection and processing, structure solution and refinement for 1–3 are
summarized in Table 1 while the selected bond lengths and angles data are presented in Tables S1–
S2 (see the Supporting Information).
Table 1. Crystallographic data for 1–3.
1
2
3
Empirical Formula
C26H18CoN4O4· C26H18N4NiO4·
·C7H7NO2·H2O ·C7H7NO2·H2O
C26H18N4O4Zn·C7H7NO2·
·0.25(CH4O)·0.5(H2O)

5
Formula weight
664.53
664.31
669.97
Crystal system
Triclinic
Triclinic
Triclinic
Space group
P1
P1
P1
T/K
100(2)
100(2)
100(2)
a/Å
11.0021(6)
11.2003(6)
12.9773(7)
111.052(5)
99.251(4)
96.138(4)
2
10.9945(7)
11.1895(6)
13.0203(8)
110.866(5)
99.766(5)
96.221(5)
2
11.2823(3)
12.9457(5)
22.0016(7)
98.058(3)
97.449(3)
111.112(3)
4
b/Å
c/Å
α/°
β/°
γ/°
Z
V/Å3
Dcal/g cm–3
1449.27(14)
1.523
1450.07(15)
1.521
2911.21(16)
1.529
No. of reflections measured
14074
15750
50103
No. of independent reflections 7520
8551
22941
Rint
0.0548
0.0306
0.0348
Final R1 values (I>2σ(I))
Final wR(F2) values (I>2σ(I))
Final R1 values (all data)
Final wR(F2) values (all data)
CCDC No.
0.0624
0.1197
0.0999
0.1336
984089
0.0514
0.0618
0.1089
0.1478
0.0698
0.0863
0.1164
0.1604
984090
984091
Synthesis of the Schiff base ligand
The ligand HL was prepared by refluxing 2-pyridinecarbaldehide (0.38 ml, 4 mmol) with
anthranilic acid (0.55 g, 4 mmol) in 20 ml methanol for half an hour. The resultant yellow solution
was left in open air overnight and used as ligand without further purification.
Synthesis of CoL2·AA·H2O (1)
To a stirred methanol solution (10 ml) of CoCl2·6H2O (0.24 g, 1 mmol) in a 50 ml conic flask, Mn
powder (0.06 g, 1 mmol) and HL in methanol from the previous preparation (0.21 g, 4 mmol) were
added. The mixture was heated to 50 °C and magnetically stirred for 50 minutes until total
dissolution of the manganese powder was observed. The resulting red-brown solution was filtered
and allowed to stand at room temperature. Dark-red block-shaped microcrystals of 1 were formed
after 4 days. They were collected by filter-suction, washed with dry PriOH and finally dried in

6
vacuo (yield: 58%). Elemental analysis calcd (%) for C33H27Co1N5O7 (664.53): C, 59.64; H, 4.10;
N, 10.54; Co, 8.87; found: C, 59.33; H, 3.92; N, 10.67, Co 9.1%.
Synthesis of NiL2·AA·H2O (2)
To a stirred methanol solution (10 ml) of NiCl2·6H2O (0.24 g, 1 mmol) in a 50 ml conic flask, HL in
methanol from the previous preparation (0.21 g, 4 mmol) was added. The solution was magnetically
stirred for 30 minutes. The resulting red-brown solution was filtered and allowed to stand at room
temperature. Red block-shaped microcrystals of 2 were formed next day. They were collected by
filter-suction, washed with dry PriOH and finally dried in vacuo (yield: 63%). Elemental analysis
calcd (%) for C33H27N5Ni1O7 (664.29): C, 59.67; H, 4.10; N, 10.54; Ni, 8.84; found: C, 59.16; H,
3.97; N, 10.33; Ni 8.7%.
Synthesis of ZnL2·AA·0.25CH3OH·0.5H2O (3)
To a stirred dmf solution (5 ml) of Zn(CH3COO)2 (0.37 g, 2 mmol) in a 50 ml conic flask, HL in
methanol from the previous preparation (0.21 g, 4 mmol) was added. The solution was magnetically
stirred for 30 minutes. The resulting red-brown solution was filtered and allowed to stand at room
temperature. Yellow block-shaped microcrystals of 3 were formed by the following day. They were
collected by filter-suction, washed with dry PriOH and finally dried in vacuo (yield: 48%).
Elemental analysis calcd (%) for C33.25H27N5O6.75Zn (669.97): C, 59.61; H, 4.06; N, 10.45; Zn,
9.76; found: C, 59.29; H, 4.02; N, 10.08; Zn 10.2%.
Results and discussion
Synthesis and Spectral Characterization
The Schiff base ligand HL was prepared by the condensation of 2-pyridinecarbaldehide and AA
in methanol and the resulting solution was used for the preparation of complexes without further
purification as described elsewhere [19]. 1–3 were formed in methanolic (1, 2) or methanolic/dmf
(3) medium from the HL solution and the corresponding metal salt (4:1 mole ratio for 1 , 2 and 2:1 -
for 3) as red-brown (1, 2) and yellow (3) microcrystalline solids in good yield. In the case of 1,
there was also Mn metal powder present in the reaction mixture as the intent was to prepare a
heterometallic compound. Analytical data provide the evidence that compounds have the formulae
CoL2·AA·H2O (1), NiL2·AA·H2O (2) and ZnL2·AA·0.25CH3OH·0.5H2O (3).
The solid-state X-band frequency EPR spectrum of 1 measured at room temperature exhibited a
well resolved sextet of the characteristic 55Mn hyperfine lines (I = 5/2) at g ≈2 (Fig. S1), which is
consistent with a high-spin S = 5/2 Mn(II) mononuclear species. This indicates that 1 appears to be

7
doped with a negligible amount of Mn(II) ions diluted in an isostructural host as no manganese
content was detected by ICP analyses.
A Cambridge Structural Database (CSD) search did not reveal the crystal structure of the pure
Schiff base ligand, as opposed to its analogues based on m- and p-aminobenzoic acids.20 Known
metal complexes containing deprotonated HL [19] were prepared by in situ Schiff base synthesis –
a synthetic approach utilized in the present work as well. That accounts for the origin of AA in the
composition of 1–3. It should be noted that use of preformed Schiff bases does not necessarily
guarantee their “safety” during coordination reactions with metal ions [21].
The IR spectra of 1–3 (Fig. S2) are very rich and assignment of the relative bands was based on
comparison with the literature data. In the spectra of the three compounds, several peaks and
shoulders (3475, 3434, 3393sh, 3360sh, 3107sh, 3061, 3024 for 1; 3443, 3400, 3334, 3179, 3062,
3031 for 2; and 3446, 3397, 3383, 3284sh, 3223sh, 3174sh, 3097sh, 3059, 3026 cm–1 for 3) appear
on a broad band observed in the N/O–H stretching region. The greatly increased intensity and very
broad nature of this band seems to be indicative of the presence of OH and NH functional groups
along with lattice and adsorbed water molecules. Weaker bands centered at 2940 cm–1 are ascribed
to C–H stretching of the –HC=N– group of the ligand [22].
Two strong bands observed at 1591, 1360 (1) and 1590, 1360 cm–1 (2, 3) are due to υas(COO)
and υs(COO) vibrations of the metal coordinated carboxylate functionalities. The coordination
mode of the carboxylate can be assigned on the basis of the difference of these two frequencies [Δυ
= 231 (1); 230 (2, 3) cm–1], which indicates the presence of monodentate coordination [23]. A
strong band that appears at 1616 (1) and 1607 cm–1 (3) as a shoulder or merged into the higher
energy COO– band is attributed to the imine bond [19a]. Medium to weak sharp bands at 1562,
1488, 1442 (1) and 1563, 1490, 1441 (3) as well as a strong band observed in the spectra at 777 cm–
1
correspond to the pyridine ring [22, 24]. The spectra also display sharp bands at 1680 (1) and
1656, 1680 cm–1 (3) merged into the higher energy COO– band which can be assigned to the C=O
stretching vibration of the ‘free’ AA [25].
Crystal structures of 1 and 2
The Co and Ni compounds are isomorphous and crystallize in the triclinic space group P1; the
asymmetric unit consists of one neutral ML2 molecule, two molecules of anthranilic acid with
occupancy 0.5 and disordered about crystallographic inversion centers, and a solvent water
molecule. ML2 molecule has no crystallographically imposed symmetry. The molecular diagram
and numbering scheme of 1 is shown in Fig. 1, which illustrates the general geometry of the two
compounds.

8
Fig. 1. Molecular structure and labeling of 1 with ellipsoids at the 50% probability level. The
hydrogen atoms are represented by spheres of arbitrary radii.
The ligand molecules are deprotonated at the carboxylato oxygen atom and coordinate to the M(II)
atom through the azomethine, pyridine-N and carboxylato-O atoms in such a way that the metal
atom is octahedrally surrounded by two anionic ligands in a mer configuration. The M–N/O
distances (Tables S1, S2) fall in the range 1.990(2)–2.088(18) Å, the trans angles at the metal atom
lie in the range 167.96(6)–176.95(7)°, the cis angles vary from 80.93(7) to 98.81(7)°. The
coordination geometry around the metal centre is similar to that reported for NiL2·H2O [22a]. The
deprotonated ligand molecules adopt an almost planar conformation, the dihedral angles between
the phenyl and pyridyl planes of each ligand are 19.1(1), 13.0(1)° (1) and 20.83(8), 13.80(8)° (2).
Two crystallographically non-equivalent molecules of anthranilic acid have similar structural
configurations with small differences in bond lengths and angles. The latter are strictly comparable
to those of the crystal structures of various polymorphic forms of AA [26]. The carboxylic
hydrogen atom positions were placed in calculated positions and refined. The lengths of the two
carboxylic C–O bonds of 1.228(14), 1.342(15) and 1.233(13), 1.326(12) Å for 1, 1.228(9),
1.307(10) and 1.249(11), 1.318(10) Å in the case of 2 unequivocally confirm a molecular form of
both molecules of the acid. The intramolecular N–H···O bonds of 2.702(9) and 2.727(8) (1),
2.701(7) and 2.714(5) Å (2) (Table 2) are similar to those found in the crystal packing of the acid
itself [26]. Remarkably, two independent AA molecules show different H-bonding preferences.
While one of them serves as a bridge between two complex molecules through N/O–H···O bonds

9
(Fig. 2, Table 2), another acid molecule is H-bonded to water molecule of crystallization (O1) only
(Fig. 2). In its turn O1 appears to act as a donor to carboxyl oxygen atom of ML2, O42 {x–1, y–1,
z}, as evidenced by the O···O distance of 2.698(7) (1) and 2.718(5) Å (2).
Table 2. Geometry of hydrogen bonds for 1–3 (Å and °).a
D-H...A
d(D-H)
d(H...A) d(D...A)
<(DHA)
CoL2·AA·H2O (1)
intramolecular
N(52)-H(52A)...O(51)
N(62)-H(62A)...O(61)
intermolecular
0.88
0.88
2.07
2.10
2.702(9)
2.727(8)
128.0
127.9
O(62)-H(61)...O(42)1
N(62)-H(62B)...O(22)
O(52)-H(52)...O(1)
O(1)···O(42)2
0.84
0.88
0.84
2.05
2.17
1.66
2.835(6)
2.904(6)
2.481(6)
2.698(7)
155.9
140.6
165.0
NiL2·AA·H2O (2)
intramolecular
N(52)-H(52A)...O(51)
N(62)-H(62A)...O(61)
intermolecular
0.88
0.88
2.07
2.08
2.701(7)
2.714(5)
128.4
127.8
O(62)-H(61)...O(42)1
N(62)-H(62B)...O(22)
O(52)-H(52)...O(1)
O(1)···O(42)2
0.84
0.88
0.84
2.02
2.17
1.69
2.836(5)
2.902(4)
2.474(4)
2.718(5)
162.0
139.7
153.3
ZnL2·AA·0.25CH3OH·0.5H2O (3)
intramolecular
N(102)-H(10B)...O(101)
N(202)-H(20A)...O(201)
intermolecular
0.910(18) 1.97(3)
0.898(16) 2.07(2)
2.722(3)
2.687(4)
138(3)
124(2)
O(102)-H(102)...O(82)
N(102)-H(10A)...O(22)3
O(202)-H(202)...O(1)
0.84
0.887(18) 2.05(2)
0.84 1.76
1.82
2.637(3)
2.903(3)
2.588(6)
165.2
161(3)
167.4

10
O(202)-H(202)...O(2)
O(42)···O(1)4
O(42)···O(2)4
0.84
1.79
2.605(7)
2.684(8)
2.679(10)
164.6
a Symmetry transformations used to generate equivalent atoms: 1 2-x, 2-y, 1-z; 2 x–1,
y–1, z; 3 1-x, 1-y, 2-z; 4 1-x,-y,2-z
Fig. 2. The hydrogen bonded connectivity in 1 (CH hydrogen atoms are omitted). One of the two
orientations of the disordered AA molecules has been omitted for clarity.
The overall packing of 1 and 2 shows alternating layers of ML2 molecules and anthranilic acid
plus water molecules which can be seen when the structure is viewed down the b axis (Fig. 3). The
neighbouring ML2 molecules in a layer are related by an inversion centre with M···M separations
being about 6.56 and 6.58 Å. The complex system of hydrogen bonds helps to stabilize the
structure.

11
Fig. 3. The alternation of ML2 and AA plus water molecules layers in the isomorphous cocrystals 1
and 2. One of the two orientations of the disordered AA molecules has been omitted for clarity. H
atoms are not shown.
Crystal structure of ZnL2·AA·0.25CH3OH·0.5H2O (3)
The Zn compound also crystallizes in the triclinic space group P1. However, it is not isomorphous
with 1 and 2. There are two independent ZnL2 molecules in the asymmetric unit. The differences
between them are minor as shown by the dihedral angles between the phenyl and pyridyl planes of
each ligand. These are 14.42(8) and 10.64(8)° for molecule 1 and 21.84(8) and 13.12(7)° for
molecule 2. The asymmetric unit also contains two crystallographically non-equivalent molecules
of anthranilic acid, a water molecule of crystallization disordered over two sites (O1 and O3) and a
methanol molecule (O2) with occupancy 0.5. Both Zn atoms with no crystallographically imposed
symmetry (Figs. 4, S3) are in a distorted octahedral environment that is chemically equivalent to
that for Co and Ni centres in 1 and 2. The Zn–N/O distances (Table S2) fall in the range
1.9979(15)–2.1710(15) Å, the trans angles at the metal atoms are equal to 161.03(7)–173.96(6)°,
the cis angles vary from 78.02(6) to 102.97(6)°.
Fig. 4. Molecular structure and labeling of 3 (molecule 1) with ellipsoids at the 50% probability
level. The hydrogen atoms are represented by spheres of arbitrary radii.

12
Considering the intra- and intermolecular H-bonding interactions, the two non-equivalent
molecules of anthranilic acid in 1, 2 and 3 demonstrate the same behaviour. The structural data on
the C–O and C=O distances [1.220(3), 1.319(3) and 1.202(3), 1.331(3) Å for the two molecules]
indicate that anthranilic acid in the Zn compound is present in the neutral form. The parameters of
the intramolecular N–H···O hydrogen bonds (Table 2) are similar to those for the Co and Ni
compounds. The pattern of the intermolecular interactions is similar to that in 1 and 2. One of AA
molecules bridges two ZnL2 complex molecules through its carboxyl and amino groups, the
N/O···O distances being 2.903(3) and 2.637(3) Å, respectively (Figure 5, Table 2). Another AA
molecule is H-bonded to water/methanol molecule of crystallization as indicated by the O···O
distances of 2.588(6) and 2.605(7) Å (Table 2). Similarly to the previous compounds, this solvent
molecule is also involved in H-bonding to a carboxylate oxygen atom of further ZnL2, O42, with
the corresponding O···O distances being 2.684(8) and 2.679(10) Å.
Fig. 5. The hydrogen bonded connectivity in 3 (CH hydrogen atoms are not shown).
The AA, water and methanol molecules occupy the spaces between the layers of the complex (Fig.
6). The adjacent ZnL2 molecules in a layer show a packing mode similar to that in 1 and 2 with
Zn···Zn separations approximating 6.37 and 6.67 Å.

13
Fig. 6. Crystal packing of 3 viewed along a. H atoms are not shown.
Analysis of the three structures indicates that CoL2·AA·H2O, NiL2·AA·H2O and
ZnL2·AA·0.25CH3OH·0.5H2O can be considered as multi-component solids assembled through the
use of hydrogen bonds. A stable hydrogen bonding motif can be identified that remains intact across
the three related structures (Figs. 2, 5). The ability of AA to cocrystallize with various organic and
metal-organic compounds due to the presence of a number of hydrogen-bond donor and acceptor
sites is well documented [9c, 12, 27]. Cocrystals 1–3 demonstrate an interesting case in which two
crystallographically non-equivalent molecules of AA are also chemically distinct in the way they
participate in H-bonding. The presence of AA molecule H-bonded to water molecule of
crystallization only in all the three structures suggests that it is not an accidental guest. Remarkably,
adjacent ML2 molecules of 1–3 retain a mutual orientation and intermolecular distances found in the
structure of the native Schiff base metal complex represented by NiL2·H2O [19a] (refcode
KAZGOI). For the latter, neighbouring NiL2 molecules in the ac plane are related by an inversion
centre (Ni···Ni separations of about 6.60 and 6.70 Å) and via intermolecular hydrogen bonding to
water molecules form a two-dimensional network. 1, 2 and 3 can be described as an insertion of AA
molecules in between the layers of ML2 molecules (Fig. 7).
Fig. 7. Crystal packing of NiL2·H2O[19a] viewed along b. H atoms are not shown.
Thermal analysis of 1 and 3
The decomposition of 1 in the nitrogen flow proceeds in four steps with major weight losses
occurring above 260 °C (Fig. S4, Table 3). The first weight loss below 100 °C suggests that the
compound is hygroscopic. Further multicomponent weight losses that occur at different rates are
ascribed to the beginning of decomposition and final destruction of the organic components with
removal of carbon at high temperatures. Presumably, CoO is the final product and the difference
between the theoretical weight loss (88.72%) and the experimental one (86.03%) can be explained

14
by the presence of a small amount of Co2O3. The DCS curve of 1 (Fig. S5) shows a “large bump”
profile typical of a sample with several processes occurring simultaneously. The multiplicity of
processes having different energetic effects reflects the multiplicity of losses in TGA. 3 exhibits
three decomposition steps which occur in the temperature intervals 100–200, 240–480 and 500–800
°C (Fig. S6). The first stage shows a gradual decrease in weight due to the loss of solvent molecules
which produces a weak endothermic effect (a very flat peak in the range 100–160 °C in the DCS
curve, Fig. S7). The second endothermic peak at 203 °C (∆H = 26.50 Jg–1) is attributed to melting
in the presence of the decomposition process. The second stage with a weight loss of 36% is
described by the peak at 325 °C in the DTG curve. The major weight loss takes place in the
subsequent stage which is characterized by the peak at 610 °C in the DTG curve. 11.95% residue
remaining at the end corresponds to ZnO (calc. 12.5%).
Table 3. Thermal decomposition data of 1 and 3.
Cocrystal Temperature Temperature
Temperature peak Weight
Residue exp.
(calc.) (%)
CoO
range (°C)
37–162
peak in DTG (°C) in DSC (°C) loss (%)
1
3
62
-
7.17
162–262
262–463
463–738
37–240
200, 235
325, 420
495, 625
140, 215
325
6.24
13.97 (11.28)
35.72
36.87
6.29
100–160 (endo),
203 (endo)
ZnO
240–476
476–820
36.33
45.38
11.95 (12.50)
610
Concluding remarks
We have reported the formation of a new series of cocrystals between mononuclear Co, Ni and Zn
Schiff-base complexes and the organic molecule of anthranilic acid built through hydrogen
bonding. ML2 molecules in CoL2·AA·H2O, NiL2·AA·H2O and ZnL2·AA·0.25CH3OH·0.5H2O show
no crystallographically imposed symmetry with the metal atoms octahedrally surrounded by two
anionic ligands in a mer configuration. The cocrystal structures can be seen as an insertion of the
acid molecules in between the layers of ML2 complexes as they occur in the native NiL2·H2O[19a]
structure. A remarkable feature of the structures of 1, 2 and 3 is the presence of two
crystallographically non-equivalent molecules of anthranilic acid that show different H-bonding
preferences. While one serves as a bridge between two ML2 molecules through N/O–H···O bonds,
another acid is H-bonded to solvent molecule only.

15
Analysis of existing crystal structures represents the first step in a crystal engineering
experiment. General trends observed in a series of relevant crystal structures, in terms of the
prevalence of specific supramolecular synthons as compared to others, is believed to provide
valuable insight for crystal engineering strategies toward the generation of novel multiple-
component crystals. However, from the synthon theory point of view the structures described in this
paper are not easy to rationalize. If one half of the anthranilic acid molecules demonstrates
anticipated hydrogen-bonding patterns why the other does not? It can be inferred by contrasting the
packing arrangements observed in 1–3 and the parent complex NiL2·H2O [19a] that the cocrystal
structures are dictated by close packing rules and those allow potential hydrogen bonds to manifest
themselves. Then a question arises on a driving force for cocrystallization in the present case. So the
structures of 1–3 appear to be a good example of the dichotomy between chemical interactions and
geometrical close-packing [5a].
Acknowledgments
This work was partly supported by the State Fund for Fundamental Researches of Ukraine (project
54.3/005). The authors acknowledge the facilities, scientific and technical assistance of the
Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy,
Characterisation & Analysis, the University of Western Australia, a facility funded by the
University, State and Commonwealth Governments.
References
[1] P.A. Vigato, V. Peruzzo, S. Tamburini, Coord. Chem. Rev. 2012, 256, 953–1114.
[2] S. J. Wezenberg, A. W. Kleij, Angew. Chem. 2008, 120, 2388–2399; Angew. Chem. Int. Ed.
2008, 47, 2354–2364 and references therein.
[3] C. J. Whiteoak, G. Salassa, A. W. Kleij, Chem. Soc. Rev, 2012, 41, 622–631 and references
therein.
[4] a) A. Cucos, A. Ursu, A. M. Madalan, C. Duhayon, J.-P. Sutter, M. Andruh, CrystEngComm
2011, 13, 3756–3766.; b) M. Nayak, S. Sarkar, P. Lemoine, S. Sasmal, R. Koner, H. A. Sparkes,
J. A. K. Howard, S. Mohanta, Eur. J. Inorg. Chem. 2010, 744–752; c) E. C. Constable, G.
Zhang, C. E. Housecroft, M. Neuburger, S. Schaffner, CrystEngComm 2009, 11, 657–662.
[5] a) G. R. Desiraju, J. Am. Chem. Soc. 2013, 135, 9952–9967; b) N. Blagden, D. J. Berry, A.
Parkin, H. Javed, A. Ibrahim, P. T. Gavan, L. L. De Matos, C. C. Seaton, New J. Chem. 2008,
32, 1659–1672; c) C. C. Seaton, CrystEngComm 2011, 13, 6583–6592; d) G. He, P. S. Chow, R.
B. H Tan, Cryst. Growth Des. 2009, 9, 4529–4532.

16
[6] a) G. R. Desiraju, CrystEngComm 2003, 5, 466–467; b) J. D. Dunitz, CrystEngComm 2003, 5,
506; c) A. D. Bond, CrystEngComm 2007, 9, 833–834; d) S. Aitipamula, R. Banerjee, A. K.
Bansal, K. Biradha, M. L. Cheney, A. R. Choudhury, G. R. Desiraju, A. G. Dikundwar, R.
Dubey, N. Duggirala, P. P. Ghogale, S. Ghosh, P. K. Goswami, N. R. Goud, R. R. K. R. Jetti, P.
Karpinski, P. Kaushik, D. Kumar, V. Kumar, B. Moulton, A. Mukherjee, G. Mukherjee, A. S.
Myerson, V. Puri, A. Ramanan, T. Rajamannar, C. M. Reddy, N. Rodriguez-Hornedo, R. D.
Rogers, T. N. G. Row, P. Sanphui, N. Shan, G. Shete, A. Singh, C. C. Sun, J. A. Swift, R.
Thaimattam, T. S. Thakur, R. K. Thaper, S. P. Thomas, S. Tothadi, V. R. Vangala, N.
Variankaval, P. Vishweshwar, D. R. Weyna, M. J. Zaworotko, Cryst. Growth Des. 2012, 12,
2147−2152.
[7] a) C. B. Aakeröy, J. Desper, M. Fasulo, I. Hussain, B. Levin, N. Schultheiss, CrystEngComm
2008, 10, 1816–1821; b) D. R. Weyna, T. Shattock, P. Vishweshwar, M. J. Zaworotko, Cryst.
Growth Des. 2009, 9, 1106–1123.
[8] a) D. I. A. Millar, H. E. Maynard-Casely, D. R. Allan, A. S. Cumming, A. R. Lennie, A. J.
Mackay, I. D. H. Oswald, C. C. Tang, C. R. Pulham, CrystEngComm 2012, 14, 3742– 3749; b)
S. Ghosh, C. M. Reddy, CrystEngComm 2012, 14, 2444–2453; c) G.-G. Hou, H.-J. Zhao, J.-F.
Sun, D. Lin, X.-P. Dai, J.-T. Han, H. Zhao, CrystEngComm 2013, 15, 577–585.
[9] a) L. Rajput, P. Sanphui, G. R. Desiraju, Cryst. Growth Des. 2013, 13, 3681–3690; b) V. K.
Srirambhatla, A. Kraft, S. Watt, A. V. Powell, Cryst. Growth Des. 2012, 12, 4870–4879; c)
Pharmaceutical Salts and Co-crystals, Editors: J. Wouters, L. Quéré, D. E. Thurston, A.
Martinez, The Royal Society of Chemistry, 2011, 406 p.
[10] G. R. Desiraju, in Pharmaceutical Salts and Co-crystals, ed. J. Wouters, L. Quéré, D. E.
Thurston and A. Martinez, The Royal Society of Chemistry, 2011, pp. 1–8.
[11] a) G. R. Desiraju, Angew. Chem. 1995, 107, 2541–2558; Angew. Chem., Int. Ed. 1995, 34,
2311–2327; b) G. R. Desiraju, J. J. Vittal, A. Ramanan, Crystal Engineering: A Textbook, World
Scientific Pub Co Inc, 2011.
[12] D. Braga, S. L. Giaffreda, K. Rubini, F. Grepioni, M. R. Chierotti, R. Gobetto, CrystEngComm
2007, 9, 39–45.
[13] a) H. G. Brittain, J. Pharm. Sci., 2013, 102, 311–317; b) S. Bukenya, T. Munshi, I. J. Scowen,
R. Skyner, D. A. Whitaker, C. C. Seaton, CrystEngComm 2013, 15, 2241–2250.
[14] D. G. Golovanov, K. A. Lyssenko, M. Yu. Antipin, Y. S. Vygodskii, E. I. Lozinskaya, A. S.
Shaplov, Cryst. Growth Des. 2005, 5, 337–340.
[15] a) L. K. Das, A. Biswas, C. J. Gómez-García, M. G. B. Drew, A. Ghosh, Inorg. Chem. 2014,
53, 434–445; b) C. C. Chou, C. C. Su, H. L. Tsai, K. H. Lii, Inorg. Chem. 2005, 44, 628–632; c)

17
C. Ma, J. Wang, L. Zhao, Y. Shi, W. Luo, Y. Yan, M. Fang, X. Wang, H.-K. Liu,
CrystEngComm 2013, 15, 10311–10315.
[16] F. Guo, M. Zhang, N. Lu, H. Guan, J. Tong, B. Wang, CrystEngComm, 2011, 13, 6753–6758.
[17] a) E. N. Chygorin, O. V. Nesterova, J. A. Rusanova, V. N. Kokozay, V. V. Bon, R. Boča, A.
Ozarowski, Inorg. Chem. 2012, 51, 386−396; b) D. S. Nesterov, E. N. Chygorin, V. N.
Kokozay, V. V. Bon, R. Boča, Y. N. Kozlov, L. S. Shul’pina, J. Jezierska, A. Ozarowski, A. J.
L. Pombeiro, G. B. Shul’pin, Inorg. Chem. 2012, 51, 9110–9122; c) O. V. Nesterova, E. N.
Chygorin, V. N. Kokozay, V. V. Bon, I. V. Omelchenko, O. V. Shishkin, J. Titiš, R. Boča, A. J.
L. Pombeiro, A. Ozarowski, Dalton Trans. 2013, 42, 16909–16919; d) E. N. Chygorin, J. A.
Rusanova, R. I. Zubatyuk, O. V. Shishkin, Acta Crystallogr. Sect. E 2012, 68, m668; e) S. R.
Petrusenko, O. M. Stetsyuk, I. V. Omelchenko, Acta Crystallogr. Sect. E 2013, 69, m326–m327;
f) E. A. Buvaylo, V. N. Kokozay, O. Yu.Vassilyeva, B. W. Skelton, Acta Crystallogr. Sect. E
2013, 69, m165–m166.
[18] G. M. Sheldrick, Acta Cryst. 2008, A64, 112–122.
[19] a) A. Mukhopadhyay, S. Pal, J.Chem.Cryst. 2005, 35, 737–744; b) B. Bag, N. Mondal, S.
Mitra, V. Gramlich, J. Ribas, M. S. El Fallah, Polyhedron 2001, 20, 2113–2116; c) S. Sen, S.
Mitra, D. Luneau, M. S. El Fallah, J. Ribas, Polyhedron 2006, 25, 2737–2744; d) S. K. Dey, B.
Bag, K. M. A. Malik, M. S. El Fallah, J. Ribas, S. Mitra, Inorg. Chem. 2003, 42, 4029–4035.
[20] CSD (version 5.35 December 2013): F. H. Allen, Acta Crystallogr. Sect. B: Struct. Sci. 2002,
58, 380–388.
[21] H.-Y. Wu, Y.-F. Feng, S.-R. Wang, W.-P. Huang, Acta Cryst. 2006, C62, m358–m359.
[22] K. Ohno, T. Itoh, C. Yokota, Y. Katsumoto, J. Mol. Struct. 2006, 825, 143–150.
[23] a) G. B. Deacon, R. J. Phillips, Coord. Chem. Rev. 1980, 33, 227–250; b) K. Nakamoto,
Infrared and Raman Spectra of Inorganic and Coordination Compounds, fifth ed., John Wiley,
New York, 1997 (Part A and B).
[24] M. Bala, A. I. P. Sinha, Asian J. Chem. 1989, 1, 392–400.
[25] T. G. Ros, M. K. van der Lee, A. J. van Dillen, J. Geus, D. C. Koningsberger, J. Mol. Catalysis
A-Chemical 2002, 186, 13–24.
[26] a) H. Takazawa, S. Ohba, Y. Saito, Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 1986,
42, 1880–1881; b) C. J. Brown, M. Ehrenberg, Acta Crystallogr. Sect. C: Cryst. Struct.
Commun. 1985, 41, 441–443; c) T. H. Lu, P. Chattopadhyay, F. L. Liao and J. M. Lo, Anal. Sci.
2001, 17, 905–906.
[27] S. Banerjee, R. Murugavel, Cryst. Growth Des. 2004, 4, 545–552 and references therein.

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Graphical abstract
+
→

19
Unusual cocrystals made of a Schiff base metal complex and an organic molecule – close-
packing vs. hydrogen bond interactions
Elena A. Buvayloa, Vladimir N. Kokozaya, Katia Rubinib, Olga Yu. Vassilyevaa,*, and Brian W.
Skeltonc
Highlights
- Cocrystals of Schiff base Co, Ni and Zn complexes and anthranilic acid were reported.
- The organic molecules are inserted between the layers of ML2 units.
- The two independent AA molecules in 1–3 show different H-bonding preferences.
- The solid-state architectures of 1–3 and a parent complex are contrasted.
- Thermal behaviour of 1 and 3 was investigated.
* Corresponding author. E-mail address: vassilyeva@univ.kiev.ua.