Modulation of coordination in pincer-type isonicotinohydrazone Schiff base ligands by proton transfer

Article abstract of DOI:10.1039/c8ce01580e

We present here two different coordination polyhedra of pincer type N₂O hydrazone based ligands supplemented with thiocyanate ions. The compounds namely [Hg(SCN)₂(HL¹)] (1) and [Hg(SCN)₂(HL²)] (2) hav

Full text of DOI:10.1039/c8ce01580e

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Modulation of coordination in pincer-type
isonicotinohydrazone Schiff base ligands by
proton transfer†
Cite this: CrystEngComm, 2019, 21,
08
1
a
b
b
Ghodrat Mahmoudi, c * Ali Akbar Khandar, Farhad Akbari Afkhami,
de
f
Barbara Miroslaw,
Atash V. Gurbanov,
Antonio Franconetti
Fedor I. Zubkov,
and Antonio Frontera *h
g
h
Alan Kennedy,
2
We present here two different coordination polyhedra of pincer type N O hydrazone based ligands
1
2
supplemented with thiocyanate ions. The compounds namely [HgIJSCN)
2 2
IJHL )] (1) and [HgIJSCN) IJHL )] (2)
have a common isonicotinohydrazone fragment and have been prepared by using a coordination driven
self-assembly of the HgIJSCN)
2
with two different ligands including 2-benzoylpyridine-
1
2
isonicotinoylhydrazone (HL ), and 2-acetylpyridine-isonicotinoylhydrazone (HL ). In compound 1 the ligand
coordinates to the mercury center in the keto form (N–NHCO) whereas, in compound 2, the proton
at the hydrazine group has been shifted to the uncoordinated pyridine ring and the ligand acted as a zwit-
terion. The structures provide a complementary system for proton transfer within the ligand molecule in-
volving the keto–enol tautomerization of the amide group and 4-pyridyl N protonation. As a result, the rel-
ative location of orbitals and ligands in the complexes are different as well as the bonding strength and the
coordination polyhedra. We have also studied electrostatically enhanced π⋯π (either conventional or in-
volving the chelate ring) interactions observed in the solid state of both compounds and analyzed them
using DFT calculations, molecular electrostatic potential surface and Bader's theory of atoms in molecules.
Received 14th September 2018,
Accepted 30th October 2018
DOI: 10.1039/c8ce01580e
rsc.li/crystengcomm
ing blocks in the synthesis of metal coordination compounds
is the unpredictability of the structure of the products which
Introduction
Crystal engineering is not a trivial problem and still many as-
pects of controlling crystal growth need to be studied and it is
not easy to predict the outcome of crystallization. In fact, a
may depend on several factors such as the nature of the li-
gands used, the oxidation state, geometry, and size of the
metal centers, the metal-to-ligand ratio, etc. Hydrazone–pyr-
1
–6
7–9
major challenge using polydentate chelating–bridging build-
idine based chelating ligands are known N,N,O pincer type li-
1
0
gands. They are usually synthesized by the one-pot Schiff-
1
1
a
base condensation reaction in high yields and it makes
them a good subject for studies because the geometries of the
ligands can have a great impact on the structural architecture
Department of Chemistry, Faculty of Science, University of Maragheh, P.O. Box
5
5181-83111, Maragheh, Iran
b
Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz,
1
2
P.O. Box 5166616471, Tabriz, Iran
of coordination compounds.
Pyridine based ligands
c
Department of Crystallography, Faculty of Chemistry, Maria Curie-Sklodowska
containing amide groups generally are coordinated to the
metal centers through their pyridyl nitrogen atoms and inter-
act with each other by hydrogen bonds involving the amide
groups, and the location of pyridyl substituent at the periph-
eral part of the main molecular skeleton in the iso-
nicotinohydrazone based ligand may result in interesting su-
University, Pl. Marii Curie-Sklodowskiej 3, 20-031 Lublin, Poland.
E-mail: barbara.miroslaw@poczta.umcs.lublin.pl
d
Department of Organic Chemistry, Baku State University, Z. Khalilov str. 23, AZ
1
148Baku, Azerbaijan
e
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de
Lisboa, Av. Rovisco Pais, 1049–001 Lisboa, Portugal
f
Organic Chemistry Department, Faculty of Science, Peoples' Friendship University
1
3
pramolecular motifs.
The chromophore hydrazone
of Russia (RUDN University), 6 Miklukho-Maklaya St., Moscow, 117198, Russian
Federation
functional group is conformationally flexible and it has nu-
merous hydrogen bonding sites including a molecular back-
bone. Moreover, isonicotinohydrazone based ligand possesses
an additional pyridine hydrogen bond acceptor region and
the ability to establish π⋯π stacking interactions taking ad-
vantage of the aromatic rings located on the two extremities
g
Department of Pure & Applied Chemistry, University of Strathclyde, 295
Cathedral Street, Glasgow G1 1XL, Scotland, UK
h
Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa
km 7.5, 07122 Palma de Mallorca (Baleares), Spain
†
Electronic supplementary information (ESI) available. CCDC 1867818 and
867819. For ESI and crystallographic data in CIF or other electronic format see
1
1
3
DOI: 10.1039/c8ce01580e
of the molecule. The strength of coordination bonding may,
108 | CrystEngComm, 2019, 21, 108–117
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however, be tuned by some changes in the electronic struc-
ture of the ligand when proton transfer occurs (Scheme 1). In
pyridyl groups, the electron lone pair does not delocalize,
therefore it is a good proton acceptor. On the other hand, the
Synthesis of the complexes
1
2
Compounds [HgIJSCN)
2
IJHL )] (1) and [HgIJSCN)
2
IJHL )] (2) were
synthesized using the same method mixing equimolar quan-
tities of mercuryIJII) salt and the ligand, as explained below. A
branched tube was used to obtain suitable crystals for X-ray
1
4
amide part may undergo keto–enol transformation.
By
tuning the crystallization conditions, the resulting complexes
may have different localization of protons within the ligand
determination, as detailed in Scheme S1.†
1
[
HgIJSCN)
2
IJHL )] (1). MercuryIJII) thiocyanate (0.158 g, 0.5
1
3c,14f
molecule and different coordination architectures.
In
1
mmol) and 2-benzoylpyridine-isonicotinoylhydrazone (HL )
0.151 g, 0.5 mmol) were placed in the main arm of a branched
the present work, we study the differences in the structure of
two mercuryIJII) coordination compounds of hydrazone Schiff
bases containing pyridine groups. The change in the
electronic structure of the ligand, due to proton transfer,
changes its ligating properties and the relative location of or-
bitals and ligands.
(
tube. Methanol (10 ml) was carefully added to fill the arms. The
tube was sealed and immersed in an oil bath at 60 °C while the
branched arm was kept at ambient temperature. After 3 days,
crystals of 1 isolated in the cooler arm were filtered off, washed
with acetone and ether, and dried in air. The isolated yield was
The geometric features and the coordination behavior of
the ligands in the crystal structure of compound 2 revealed a
preference for the formation of chelate ring stacking along
with more conventional pyridine⋯pyridine stacking interac-
tions. In contrast, the steric effect of the phenyl group in the
0
3
.217 g, 70%. m. p. 206 °C. Anal. calcd for C H HgN OS : C,
20 14 6 2
8.80; H, 2.28; N, 13.57%. Found: C, 38.49; H, 2.17; N, 13.94%.
−1
FT-IR (cm ) selected bands: 3452 (w), 3093 (w), 2117 (vs), 1663
(m), 1596 (m), 1538 (vs), 1466 (m), 1437 (m), 1282 (vs), 1136
(m), 965 (m), 839 (m), 772 (m) and 661 (m).
1
HL ligand (compound 1) prevents the formation of chelate
2
[
HgIJSCN)
2
IJHL )] (2). MercuryIJII) thiocyanate (0.158 g, 0.5
ring stacking. To gain insight into these structural features,
we employed Hirschfield surface analysis and DFT (M06-2X)
calculations of interaction energies, which confirm that che-
late ring stacking is a robust synthon to be considered in
crystal engineering.
2
mmol) and 2-acetylpyridine-isonicotinoylhydrazone (HL )
0.120 g, 0.5 mmol) were placed in the main arm of a
(
branched tube. Methanol (10 ml) was carefully added to fill
the arms. The tube was sealed and immersed in an oil bath
at 60 °C while the branched arm was kept at ambient temper-
ature. After 3 days, crystals of 2 isolated in the cooler arm
were filtered off, washed with acetone and ether, and dried
Experimental and theoretical methods in air. The isolated yield was 0.240 g, 84%. m. p. 180 °C.
6 2
Anal. calcd for C15H12HgN OS : C, 32.34; H, 2.17; N, 15.09%.
Found: C, 32.60; H, 2.22; N, 15.29%. FT-IR (cm ) selected
bands: 3423 (w), 3368 (w), 2993 (w), 2061 (vs), 1622 (m), 1593
Materials and methods
−
1
All chemicals were commercially available and the solvents
were used without further purification. The hydrazone–pyri-
dine based ligands were synthesized according to the
(
8
m), 1534 (s), 1481 (s), 1438 (m), 1288 (s), 1157 (m), 948 (m),
34 (m), 787 (m) and 699 (m).
8
1
reported method as described elsewhere. In short, pure HL
2
and HL were obtained by the condensation of ethanolic so-
lutions of isonicotinohydrazide, and 2-benzoylpyridine and
Hirshfeld surface analysis
2
-acetylpyridine, respectively. Elemental analyses were carried
The Hirshfeld surfaces and 2D fingerprints of the studied com-
pounds were generated with the use of the Crystal Explorer
package ver. 3.1. Crystal structures were imported from CIF
files. Hirshfeld surfaces were generated using high resolution
and mapped with the dnorm and shape-index functions. 2D fin-
gerprint plots were prepared with the use of the same software.
out using a Heraeus CHN-O-Rapid analyzer and FT-IR spectra
were recorded on a Bruker Tensor 27 FT-IR spectrometer with
KBr pellets in the range of 4000–400 cm . Melting points of
the prepared metal complexes were measured using an
Electrothermal 9100 apparatus.
15
−
1
Scheme 1 Possible proton transfer mechanisms in isonicotinohydrazone based ligands and the structure of complexes 1 and 2.
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2
0
X-ray diffraction
gram. For the Hg atom we have used the LanL2DZ basis
set. This level of theory is adequate for studying noncovalent
interactions dominated by dispersion effects like π-stacking.
The basis set superposition error for the calculation of inter-
action energies has been corrected using the counterpoise
The diffraction intensities for 1 and 2 were measured at room
temperature using the ω-scan technique on Bruker four-circle
diffractometers (APEX-II CCD) with graphite-monochromated
MoKα radiation (λ = 0.71073 Å). The detector frames were
2
1
22
1
6
method. The “atoms-in-molecules” (AIM) analysis of the
integrated by the use of the program SAINT and the empiri-
cal absorption corrections were performed using the SADABS
electron density has been performed at the same level of the-
2
3
1
7
ory using the AIMAll program. Molecular electrostatic po-
tential surfaces have been computed at the same level using
the GAUSSIAN-09 program.
program. The structures were solved and refined using the
SHELXS and olex2.refine refinement package using Gauss–
1
8,19
Newton minimization.
All non-hydrogen atoms were re-
fined anisotropically, and the positions of all hydrogen atoms
were placed in idealized positions except for the H atom at Results and discussion
pyridine in 2 which was found in the difference Fourier
maps. Then, all H-atoms were refined as the ‘riding model’
Molecular structure
Compound 1 crystallizes in the monoclinic C2/c space group
Table 1) with the amide fragment being in a keto tautomeric
form. A voluminous benzyl group is substituted at the C7
with isotropic displacement parameters set at 1.2 (1.5 for the
methyl group) times Ueq of the appropriate carrier atoms. De-
tails of the crystallographic data collection and refinement
parameters are given in Table 1.
CCDC 1867818 and 1867819 contain the supplementary
crystallographic data for this paper.
(
atom and it is rotated by ca. 60° in regard to the hydrazone
mean plane (Fig. 1). Compound 2 (P2 /c) has a methyl sub-
1
stituent instead of a benzyl one. Under the reaction condi-
tions, the amide group undergoes deprotonation. The proton
is transferred from the amide N1 to pyridyl N4 atom forming
a zwitterion (Fig. 2). The bond lengths (Table 2), as well as
the intermolecular hydrogen bond motifs (Table S1, Fig. S1
and S2†), confirm the keto form in 1 and the enol form of
the amide group in 2. The C1–N1 bond length in 2 is shorter
by 0.02 Å, whereas the C1–O1 and C1–C8 bonds are longer by
Theoretical methods
The geometries of the complexes included in this study were
computed at the M06-2X/def2-TZVP level of theory using the
crystallographic coordinates within the GAUSSIAN-09 pro-
0
.03 and 0.02 Å, respectively, than that in the keto form in 1.
Table 1 Crystal data and structure refinement data for 1 and 2
The electronic coupling in the molecule in crystal 2 results in
a flat structure (Table 3 and Fig. 3). Conversely, in compound
Crystal
1
2
1
the molecule is bent where the hydrazone fragment is not
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C
20
H
14
N
6
2
OS Hg
C
15 12 6 2
H N OS Hg
planar with the O1 and C8 atoms being below (0.21 Å) and
above (0.30 Å) the mean C1N2N1 plane, respectively. A com-
parison of both compounds is provided in Fig. 3.
The consequences of proton transfer are also pronounced
in the geometry of the coordination spheres. In both com-
619.11
296(2)
Monoclinic
C2/c
28.861(6)
9.191(2)
17.149(3)
111.76(3)
4224.8(17)
8
557.04
296(2)
Monoclinic
P2 /c
12.9176(3)
7.7756(2)
17.4321(4)
93.02(1)
1748.5(1)
4
1
b/Å
c/Å
plexes the organic ligands serve as tridentate N O pincer type
2
β/°
Volume/Å
3
Z
−3
ρcalc g cm
1.9465
7.510
2.1159
9.060
−
1
μ/mm
FIJ000)
Crystal size/mm
Radiation
2353.2
0.20 × 0.22 × 0.40
Mo Kα (λ =
1048.6
0.08 × 0.22 × 0.24
Mo Kα (λ =
3
0.71073)
0.71073)
2Θ range for data
4.68 to 50.48
3.16 to 65.12
collection/°
Reflections collected
Independent reflections
11 606
3711
40 853
6334
R
R
int = 0.0433
sigma = 0.0483
R
int = 0.0285
Rsigma = 0.0248
Data/restraints/parameters 3711/0/270
6334/0/226
0.990
2
Goodness-of-fit on F
0.829
= 0.0366, wR
.0830
= 0.0561, wR
.0880
Final R indexes [I ≥ 2σ(I)]
R
0
1
2
2
=
=
R
1
= 0.0256, wR
0.0441
1
R = 0.0460, wR
2
2
=
=
Final R indexes [all data]
R
1
0
0.0497
Largest diff. peak/hole/e
Å
1.85/−2.35
1.32/−1.55
−3
CCDC no.
1867818
1867819
Fig. 1 Thermal ellipsoids and coordination polyhedron in 1.
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Table 3 Selected bond angles and torsions in 1 and 2 in deg
Angle
1
2
S1–Hg1–S2
O1–Hg1–S1
O1–Hg1–S2
N2–Hg1–O1
N3–Hg1–O1
S1–Hg1–N2
S1–Hg1–N3
S2–Hg1–N2
152.1(1)
86.1(1)
85.5(1)
61.8(2)
126.7(2)
104.6(1)
101.4(1)
94.7(1)
105.0(2)
65.4(2)
100.2(4)
96.7(3)
171.1(5)
13.2(7)
169.0(5)
107.5(1)
108.6(1)
96.3(1)
69.1(1)
138.0(1)
148.8(1)
105.5(1)
103.6(1)
96.1(1)
69.0(1)
97.6(1)
99.7(1)
175.9(2)
2.6(3)
S2–Hg1–N3
N2–Hg1–N3
Hg1–S1–C19/or C14
Hg1–S2–C20/or C15
O1–Hg1–N3–C7
C1–N1–N2–Hg1
N1–N2–Hg1–N3
Fig. 2 Thermal ellipsoids and coordination polyhedron in 2.
179.0(2)
chelators, but in the zwitterionic structure 2 the distance be-
tween Hg1⋯O1 atoms is shorter (by 0.29 Å) than that in 1
(Table 2). Additionally, the ligand to metal orbital relative ori-
entation is different. In 1, the coordination polyhedron is a
deformed trigonal bipyramid (Fig. 1). The pincer ligand lies
in the base and two thiocyanate ions are in the apical posi-
tions at nearly the same distance of ca. 2.4 Å. In 2, the pincer
ligand is bonded strongly to the metal center with shorter
intramolecular N–Hg and O–Hg distances than that in 1. It
IR spectroscopy
The IR spectrum of 1, where the amide N-atom is not
−1
deprotonated, exhibits both the amide N–H (3452 cm ) and
−1
CO (1663 cm ) stretching vibrations that are typical for a
24
free ligand and there is no peak characteristic of the
deprotonated ligand. On the other hand, the free ligand
CO stretching vibrations are not observed in the IR spec-
trum of compound 2 and an absorption band response to the
forms
a
distorted square pyramid with the iso-
nicotinohydrazone part of the ligand and with one of the
−
SCN ions lying in the same plane. The second thiocyanate
−1
CN–NC–O moiety appearing around 1622 cm indicates
anion takes the apical position with a longer distance Hg1–
S2 of 2.640(1) Å (Fig. 2).
that the ligand has lost the amide proton during the
tautomerization, and the proton is attached to the pyridyl ni-
trogen of the isonicotine fragment making a zwitterion.
π⋯π interactions
Hirshfeld surface analysis
The flattening of the isonicotinohydrazone moiety in 2 is es-
sential for π⋯π intermolecular interactions. In 2, the
2
5,26
The Hirshfeld surface analysis
of 1 and 2 gives a clear dif-
2
-pyridyl and 4-pyridyl rings (R1 and R2 in Table S2†) interact
ference in the molecular structure and crystal packing. The
antennas related to the N⋯H contacts in 2 are longer and
narrower than those in 1 as a result of shorter N⋯H
with each other with nearly full molecular overlay (Fig. 4, Ta-
ble S2†). In 1, the π⋯π interaction is possible only between
two 2-pyridyl fragments of the molecules transformed by in-
version (R1 rings in Table S2;† Fig. 5). The second π⋯π con-
tact in 1 is between 4-pyridyl and benzyl rings (R2 and R3 in
Table S2†). In crystal of 2, the columns of stacked molecules
are the main packing motif, whereas in 1 a zig-zag arrange-
ment is present.
Table 2 Selected bond lengths for 1 and 2 in Å
Bond
1
2
Bond
1
2
Hg1–N2 2.491(5) 2.279(2) C7–N3
Hg1–N3 2.423(6) 2.459(2) C8–C9
Hg1–S1 2.399(2) 2.428(1) C8–C12
Hg1–S2 2.406(2) 2.640(1) C9–C10
Hg1–O1 2.687(5) 2.392(2) C10–N4
1.334(9)
1.332(4)
1.371(10) 1.389(4)
1.380(10) 1.385(4)
1.385(10) 1.372(4)
1.312(10) 1.324(4)
1.342(10) 1.327(4)
1.377(10) 1.373(4)
C1–O1
C1–N1
C1–C8
N1–N2
N2–C2
1.226(7) 1.257(3) C11–N4
1.346(9) 1.324(3) C11–C12
1.477(9) 1.499(4) N5–C19/or C14 1.143(13) 1.143(4)
1.382(7) 1.372(3) N6–C20/or C15 1.158(14) 1.148(4)
1.282(9) 1.288(3) S1–C19/or C14 1.643(11) 1.667(4)
C2–C13 1.467(9) 1.496(4) S2–C20/or C15 1.650(13) 1.649(3)
C3–N3 1.346(9) 1.338(3)
Fig. 3 Comparison of the planarity of the ligands in complexes 1 (top)
and 2 (bottom).
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cessible to the particular geometry around the Hg metal cen-
ter and the location of the SCN ligands.
In the crystal packing of compound 1, the mononuclear
HgIJII) complex forms self-assembled dimers in the solid state
governed by the formation of an antiparallel π–π interactions
involving the coordinated pyridine rings, assisted by
H-bonding interactions between the aromatic CH bonds and
the pseudohalide (see Fig. 8a). These CH⋯N/S hydrogen
bonds are expected to be energetically strong due to the en-
hanced acidity of the H atoms due to the coordination of the
pyridide to the HgIJII) metal center. The dimerization energy
−
1
of this self-assembled dimer is ΔE
1
= −27.9 kcal mol , which
Fig. 4 The π⋯π intermolecular interactions between 2-pyridyl and
4-pyridyl rings in 2 (nearly full overlay of the molecules).
is large due to the contribution of both interactions. In an ef-
fort to calculate the contribution of the different forces, we
have computed a theoretical model in which one of the
pseudoligands (SCN ) has been replaced by a hydrido ligand
(see small arrows in Fig. 8b) and consequently the H-bonding
interactions are not formed. Consequently, the interaction
energy is reduced to ΔE = −15.9 kcal mol that can be attrib-
uted to the contribution of the antiparallel π-stacking interac-
tion. This binding energy is stronger than that of more con-
ventional π-stacking interactions due to the presence of the
HgIJII) metal center that increases considerably the dipole mo-
ment of the π-system. The contribution of the H-bonding in-
teractions to the formation of the assembly can be evaluated
−
intermolecular interactions in 2 (Fig. 6). In 1, the additional
aromatic substituent shows that the H⋯H contacts are more
abundant (21.3 vs. 13.7%), but the aromatic character of the
molecule is more pronounced in the N⋯N contacts due to
the π⋯π stacks in 2 (0.9 vs. 0.1%). The Hg⋯H contacts show
that in 2 the metal core is more hindered than that in 1 (0.2
vs. 1.2%).
−
1
2
2
7
Theoretical study
−
1
The theoretical study reported herein is devoted to compar-
ing the energetic features of the two types of π-stacking inter-
actions (chelate ring–π and π–π) observed in the crystal pack-
ing of compounds 1–2 described above (see Fig. 4 and 5).
These interactions are crucial to understanding the crystal
packing of complexes 1–2. In addition to the classical
π-stacking interaction involving aromatic rings, other planar
moieties can also participate in more “unconventional” stack-
by difference (ΔE
1
−ΔE = −12.0 kcal mol ), thus revealing a
2
major contribution of the π-stacking.
In order to provide additional evidence for the existence
of the noncovalent interactions commented above we have
analyzed the self-assembled π-stacked dimer of compound 1
2
9
using Bader's theory of “atoms in molecules” (AIM). The ex-
istence of a bond critical point and bond path connecting
two atoms is an unambiguous evidence of interaction. In
Fig. 8c, we show the AIM analysis of the dimer of compound
1 and it can be observed that the antiparallel π–π interaction
is characterized by the presence of two bond critical points
2
7
ing interactions.
For instance, chelate rings with
2
7
delocalized π bonds establish stacking interactions similar
2
8
to those of aromatic organic molecules in transition-metal
complexes. Similar chelate ring π-stacking interactions have
also been reported by us in HgX and PbX (X = Cl, Br and I)
2
2
1
3,14
coordination compounds.
To study the donor–acceptor ability of HgL complexes, we
n
have computed the molecular electrostatic potential (MEP)
surfaces of compounds 1 and 2, which are shown in Fig. 7.
Expectedly, the most negative electrostatic potential corre-
sponds to the region of the SCN ligands and the most posi-
tive part is located in the region of the N–H groups. There-
fore, the H-bonding interaction between the N–H and SCN
groups should be energetically favored. Furthermore, perpen-
dicular to the molecular plane, in compound 2 each 5-mem-
bered chelate ring has almost negligible MEP values. Conse-
quently, the stacking interactions between chelate rings will
likely be dominated by dispersion effects. The MEP value is
large and positive over the protonated pyridine rings and it is
−
1
negative (−5 kcal mol ) in the coordinated pyridine ring.
Therefore, pyridine–pyridine interactions are expected to be
electrostatically greatly favored due to a significant electro-
static attraction. In compound 1, the chelate ring is not ac-
Fig. 5 The π⋯π intermolecular interactions between two 2-pyridyl
rings and between 4-pyridyl and benzyl rings in 1.
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Fig. 6 2D fingerprint plots and percentage contributions of contacts to the Hirshfeld surface in the structures of 1 and 2 calculated from crystal
structures.
(
green spheres) that interconnect two atoms of the pyridine
uncoordinated pyridine rings and chelate ring⋯chelate ring
(CR–CR) stacking interactions. Remarkably, opposite to com-
pound 1, the square planar pyramidal geometry of the Hg
atom in compound 2 facilitates the approximation of the che-
late rings, resulting in shorter CR⋯CR interactions (3.54 Å).
rings, thus confirming the interaction. Furthermore, the dis-
tribution of critical points reveals the existence of two sym-
metrically related C–H⋯N and C–H⋯S H-bonding interac-
tions since two bond critical points connect the N and S
atoms of the pseudohalide with the aromatic H-atoms. A ring
critical point (yellow sphere) also appears upon complexation
due to the formation of a supramolecular ring.
In compound 2, we have computed the interaction energy
of the self-assembled π-stacked dimer shown in Fig. 9a,
which is stabilized by an intricate combination of
noncovalent interactions, that is, H-bonds (red dashed lines),
π–π stacking interactions between the coordinated and
−
1
The dimerization energy (ΔE
3
= −64.6 kcal mol ) is very large
due to the contribution of the three interactions and the
strong dipole⋯dipole interactions due to the zwitterionic na-
ture of the ligand. In an effort to roughly estimate the contri-
bution of the different forces that govern the formation of
the self-assembled dimer, we have computed a theoretical
model in which the methyl groups have been replaced by H
atoms (see small arrows in Fig. 9b) and consequently the C–
Fig. 7 MEP surfaces plotted onto the van der Waals surfaces of compound 1 (left) and 2 (right) and the energies at selected points of the surface
−
1
are given in kcal mol
.
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Fig. 8 (a) Interaction energy of the self-assembled π-stacked dimer observed in the solid state of compound 1. Distances in Å. (b) Interaction en-
ergy in the theoretical model of 1. (c) AIM analysis of the self-assembled dimer retrieved from the X-ray structure of compound 1. Bond and ring
critical points are represented by green and yellow spheres, respectively. The bond paths connecting the bond critical points are also represented
by dashed lines.
Fig. 9 (a) Interaction energy of the self-assembled π-stacked dimer observed in the solid state of compound 2. Distances in Å. (b and c) Geome-
tries and interaction energies in the theoretical models of 2. (d) AIM analysis of the self-assembled dimer retrieved from the X-ray structure of
compound 1. Bond critical points are represented by green spheres. The bond paths connecting the bond critical points are also represented by
dashed lines. Ring and cage critical points have been omitted for clarity.
114 | CrystEngComm, 2019, 21, 108–117
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H⋯S H-bonds involving the methyl groups are not formed.
These reports along with the structures reported herein pro-
vide strong evidence for preferential formation of chelate ring
stacking, which can be considered as a synthon interaction
for (iso)nicotinohydrazide metal complexes. Moreover, the
interaction energies reported herein confirm that the che-
late–chelate interactions are stronger than those reported for
As a result, the interaction energy is reduced to ΔE = −57.6.
4
Therefore, the contribution of both symmetrically equivalent
−
1
H-bonds can be estimated by difference (−7 kcal mol ). Fur-
thermore, we have used an additional dimer, where the
pyridinium ring has been replaced by a hydrogen atom, and
consequently, the py⋯py interactions and H-bonds are not
formed. Since the pyridine rings are protonated, in this
model we have added a hydrogen atom to the hydrazone
2
8
classical π–π complexes.
group to keep the model neutral. The resulting interaction Conclusions
−
1
energy is further reduced to ΔE = −30.5 kcal mol which cor-
5
Herein, we reported the syntheses and characterization of
two mercuryIJII) complexes of two isonicotinohydrazone Schiff
responds to the contribution of the CR–CR π-stacking interac-
tion. The contribution of both py⋯py interactions and C–
−
1
2
base ligands. The ligands acted as pincer type tridentate N O-
H⋯S bonds can be estimated by difference (−27.1 kcal mol )
thus emphasizing the importance of these charge-assisted
π-stacking interactions. The CR–CR interaction is unexpect-
edly strong, which is likely due to the model used for the cal-
culations where an H-atom has been added to the hydrazone
group. This H-atom likely forms an extra H-bond with the S
atom of the pseudohalide ligand, thus also contributing to
the binding energy (see blue dashed line in Fig. 9c). There-
fore, the conventional py⋯py interaction is underestimated
and the CR–CR interaction is overestimated using this model.
Nevertheless, the strong binding energy of the whole assem-
bly indicates that this combination of two types of π-stacking
interactions and H-bonds is a robust binding motif in the
solid state of compound 2.
In Fig. 9d, we show the AIM analysis of compound 2. Each
conventional π–π interaction (pyridine rings) is characterized
by the presence of two bond critical points that interconnect
the two atoms of the protonated pyridine ring to the two
atoms of the coordinated one, thus confirming the interac-
tion. Furthermore, the distribution of critical points reveals
the existence of four symmetrically related C–H⋯S
H-bonding interactions. Each one is characterized by a bond
critical point and bond path connecting the H atom with the
S atom of the pseudohalide. Finally, the unconventional
CR⋯CR interaction is confirmed by the presence of two bond
critical points interconnecting the Hg atoms to the N atoms
and two additional bond critical points connecting the O
atoms to the C atoms of the hydrazone group. The value of
the Laplacian of the charge density at the bond critical points
is positive, as is common in closed-shell interactions.
donor ligands, in which the isonicotinamide pyridine nitro-
gen remained uncoordinated. By using thiocyanate ligands
along with the organic ligands, the coordination geometry
around each mercury center resulted in a distorted trigonal
bipyramid and a square-pyramid in 1 and 2, respectively.
Compounds 1 and 2 exhibit interesting π–π stacking interac-
tions in their solid state, which have been analyzed using
DFT calculations in detail. The combination of conventional
π-stacking with more unconventional chelate ring stacking
interactions is a robust binding motif in compound 2. The
protonation of 4-pyridyl substituent may increase the
strength of the pincer type coordination in hydrazone based
Schiff base ligands. The presented systems are complemen-
tary in the sense of proton donor and proton acceptor proper-
ties of the molecular subcomponents. The proton transfer
should not be overlooked when designing new structures
based on supplementary molecular fragments such as
hydrazone based ligands substituted with pyridines. It should
be emphasized that steric effects (bulky phenyl vs. methyl)
are the origin of the different behaviours of the complexes.
The proton transfer in the isonicotinohydrazone methyl
2
substituted ligand (HL ) enables the intramolecular coupling
and flattening of the molecule which increases the chances
for stronger pincer type coordination. It favors the conven-
tional (π–π) and unconventional chelate ring stacking interac-
tions, which are useful tools in crystal engineering.
Conflicts of interest
The binding energies and π-stacking distances (3.54 Å) de-
scribed above for the CR⋯CR interactions in compound 2
are comparable to those reported for HgIJII) complexes where
halide instead of pseudohalides were used as co-ligands (3.30
to 3.67 Å). CR⋯CR interactions have been also described
recently in ZnIJII) and CdIJII) complexes involving (iso)
nicotinohydrazide ligands that exhibited similar binding en-
There are no conflicts to declare.
Acknowledgements
3
0
We are grateful to the University of Maragheh for the finan-
cial support of this research. The publication was also pre-
pared with the support of the “RUDN University Program
5-100”. AF thanks the MINECO/AEI from Spain for a “Juan de
la Cierva” contract. We thank the MINECO/AEI from Spain for
financial support (project number CTQ2017-85821-R, FEDER
funds). We are grateful to the CTI (UIB) for free allocation of
computer time.
−
1
13a
ergies (≃28 kcal mol ) and distances (3.44–3.50 Å).
More-
over, a recent study has described CR⋯CR interactions in
CuIJII) polymers where polydentate (E)-2-(1-(pyridin-2-
yl)methyleneamino)terephthalic acid was used as the ligand.
3
1
The solid state X-ray architecture of the polymers was
governed by CR⋯CR interactions along with H-bonding.
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References
68; ( f ) D. Hean, T. Gelbrich, U. J. Griesser, J. P. Michael and
A. Lemmerer, CrystEngComm, 2015, 17, 5143–5153; (g) J. A.
Lessa, G. L. Parrilha and H. Beraldo, Inorg. Chim. Acta,
2012, 393, 53–63; (h) G. N. Babu, A. R. B. Rao, S. Keesara and
S. Pal, J. Organomet. Chem., 2017, 848, 243; (i) P. Doungdee,
S. Sarel, N. Wongvisetsirikul and S. Avramovici-Grisaru,
J. Chem. Soc., Perkin Trans. 2, 1995, 319.
1
2
3
M. O. Besenhard, P. Neugebauer, O. Scheibelhofer and J. G.
Khinast, Cryst. Growth Des., 2017, 17, 6432.
P. Erk, H. Hengelsberg, M. F. Haddow and R. van Gelder,
CrystEngComm, 2004, 6, 474.
N. Tumanova, N. Tumanov, K. Robeyns, F. Fischer, L.
Fusaro, F. Morelle, V. Ban, G. Hautier, Y. Filinchuk, J.
Wouters, T. Leyssens and F. Emmerling, Cryst. Growth Des.,
14 (a) S. Yumnam and L. Rajkumari, J. Chem. Eng. Data, 2009, 54,
28; (b) C. M. Armstrong, P. V. Bernhardt, P. Chin and D. R.
Richardson, Eur. J. Inorg. Chem., 2003, 1145; (c) C. B. Aakeröy,
S. Forbes and J. Desper, CrystEngComm, 2012, 14, 2435; (d) Z.
He, C. He, E. Q. Gao, Z. M. Wang, X. F. Yang, C. S. Liao and
C. H. Yan, Inorg. Chem., 2003, 42, 2206; (e) K. L. Abboud, R. C.
Palenik, G. J. Palenik and R. M. Wood, Inorg. Chim. Acta,
2007, 360, 3642; ( f ) F. A. Afkhami, A. Akbar Khandar, G.
Mahmoudi, W. Maniukiewicz, J. Lipkowski, J. M. White, R.
Waterman, S. G. Granda, E. Zangrando, A. Bauzái and A.
Frontera, CrystEngComm, 2016, 18, 4587.
2
018, 18, 954.
4
J. R. G. Sander, D.-K. Bučar, R. F. Henry, B. N. Giangiorgi,
G. G. Z. Zhang and L. R. MacGillivray, CrystEngComm,
2
013, 15, 4816.
5
6
7
D.-K. Bučar, Cryst. Growth Des., 2017, 17, 2913.
G. R. Desiraju, J. Am. Chem. Soc., 2013, 135, 9952.
A. J. Blake, N. R. Champness, P. Hubberstey, W.-S. Li, M. A.
Withersby and M. Schröder, Coord. Chem. Rev., 1999, 183,
1
17.
8
9
M. Sarkar and K. Biradha, Cryst. Growth Des., 2007, 7, 1318.
A. Beheshti, W. Clegg, V. Nobakht and R. W. Harrington,
Cryst. Growth Des., 2013, 13, 1023.
15 S. K. Wolff, D. J. Grimwood, J. J. McKinnon, M. J. Turner, D.
Jayatilaka and M. A. Spackman, Crystal Explorer package, ver.
3.1, University of Western Australia, Perth, Australia, 2013.
16 SAINT Plus, Data Reduction and Correction Program, v. 6.01,
Bruker AXS, Madison, Wisconsin, USA, 1998.
17 SADABS, v.2.01, Bruker/Siemens Area Detector Absorption
Correction Program, Bruker AXS, Madison, Wisconsin, USA,
1998.
1
0 K. J. Szabó and O. F. Wendt, Pincer and Pincer-Type Com-
plexes: Applications in Organic Synthesis and Catalysis, by
John Wiley & Sons, Inc, 2014.
1
1 (a) S. Yumnam and L. Rajkumari, J. Chem. Eng. Data,
2
009, 54, 28; (b) A. A. Khandar, B. K. Ghosh, C.
Lampropoulos, M. S. Gargari, V. T. Yilmaz, K. Bhar, S. A.
Hosseini-Yazdi, J. M. Cain and G. Mahmoudi, Polyhedron,
18 G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem.,
2015, 71, 3.
2
015, 85, 467; (c) M. Abedi, O. Z. Yesilel, G. Mahmoudi, A.
19 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard
and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339.
20 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F.
Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.
Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V.
Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Gaussian,
Inc., Wallingford, CT, 2009.
Bauza, S. E. Lofland, Y. Yerli, W. Kaminsky, P. Garczarek,
J. K. Zareba, A. Ienco, A. Frontera and M. S. Gargari, Inorg.
Chim. Acta, 2016, 443, 101; (d) G. Mahmoudi, A. Bauzá, A. V.
Gurbanov, F. I. Zubkov, W. Maniukiewicz, A. Rodríguez-
Diéguez, E. López-Torres and A. Frontera, CrystEngComm,
2
016, 18, 9056; (e) M. S. Gargari, V. Stilinović, A. Bauzá and
A. Frontera, Chem. – Eur. J., 2015, 21, 17951.
1
2 (a) M. Fujita, D. Oguro, M. Miyazawa, H. Oka, K. Yamaguchi
and K. Ogura, Nature, 1995, 378, 469; (b) J. Xie, S. Shen, R.
Chen, J. Xu, K. Dong, J. Huang, Q. Lu, W. Zhu, T. Ma, L. Jia,
H. Cai and T. Zhu, Org. Lett., 2017, 13, 4413–4419; (c) Z. Y.
Hao, Q. W. Liu, J. Xu, L. Jia and S. B. Li, Chem. Pharm. Bull.,
2
010, 58, 1306–1312.
1
3 (a) G. Mahmoudi, J. K. Zaręba, A. Bauza, M. Kubicki, A.
Bartyzel, A. Keramidas, L. Butusov, B. Miroslaw and A.
Frontera, CrystEngComm, 2018, 20, 1065; (b) A. A. Khandar,
F. A. Afkhami, S. A. Hosseini-Yazdi, J. M. White, S. Kassel,
W. G. Dougherty, J. Lipkowski, D. Van Derveer, G. Giester
and F. Costantino, Inorg. Chim. Acta, 2015, 427, 87; (c) A. A.
Khandar, F. A. Afkhami, S. A. Hosseini-Yazdi, J. Lipkowski,
W. G. Dougherty, W. S. Kassel, H. R. Prieto and S. G.
Granda, J. Inorg. Organomet. Polym., 2015, 25, 860; (d) F. A.
Afkhami, A. A. Khandar, J. M. White, A. Guerri, A. Ienco,
J. T. Bryant, N. Mhesn and C. Lampropoulos, Inorg. Chim.
Acta, 2017, 457, 150; (e) F. A. Afkhami, A. A. Khandar, G.
Mahmoudi, M. Amini, E. Molins, P. Garczarek, J. Lipkowski,
J. M. White and A. M. Kirillov, Inorg. Chim. Acta, 2017, 458,
21 S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553–566.
22 R. F. W. Bader, Chem. Rev., 1991, 91, 893–928.
23 T. A. Keith, AIMAll (Version 13.05.06), TK Gristmill Software,
Overland Park KS, USA, 2013.
24 T.-F. Zhu, R.-H. Chen, T.-L. Ma, Y. Wang and Z.-Q. Xu,
Transition Met. Chem., 2015, 40, 485.
25 J. J. McKinnon, D. Jayatilaka and M. A. Spackman, Chem.
Commun., 2007, 3814.
116 | CrystEngComm, 2019, 21, 108–117
This journal is © The Royal Society of Chemistry 2019

View Article Online
CrystEngComm
Paper
2
2
6 M. A. Spackman and D. Jayatilaka, CrystEngComm, 2009, 11, 19.
7 (a) B. D. Ostojić, G. V. Janjić and S. D. Zarić, Chem.
Commun., 2008, 6546–6548; (b) Z. D. Tomić, S. B. Novaković
and S. D. Zarić, Eur. J. Inorg. Chem., 2004, 11, 2215–2218; (c)
D. N. Sredojević, Z. D. Tomić and S. D. Zarić, Cent. Eur. J.
Chem., 2007, 5, 20–31; (d) Z. D. Tomić, D. N. Sredojević and
S. D. Zarić, Cryst. Growth Des., 2006, 6, 29–31; (e) D. N.
Sredojević, G. A. Bogdanović, Z. D. Tomić and S. D. Zarić,
CrystEngComm, 2007, 9, 793–798; ( f ) A. Castineiras, A. G.
Sicilia-Zafra, J. M. Gonzales-Perez, D. Choquesillo-Lazarte
and J. Niclos-Gutierrez, Inorg. Chem., 2002, 41, 6956–6958;
Geerlings, Nucleic Acids Res., 2005, 33, 1779–1789; (c) J.
Sponer, K. E. Riley and P. Hobza, Phys. Chem. Chem. Phys.,
2008, 10, 2595–2610; (d) X. J. Wang, L. C. Gui, Q. L. Ni, Y. F.
Liao, X. F. Jiang, L. H. Tang, L. H. Zhang and Q. Wu,
CrystEngComm, 2008, 10, 1003–1010; (e) S. L. Cockroft, C. A.
Hunter, K. R. Lawson, J. Perkins and C. J. Urch, J. Am. Chem.
Soc., 2005, 127, 8594–8595; ( f ) T. Sato, T. Tsuneda and K.
Hirao, J. Chem. Phys., 2005, 123, 104307–104317; (g) S.
Grimme, Angew. Chem., Int. Ed., 2008, 47, 3430–3434; (h) M.
Rubeš, O. Bludsky and P. Nachtigall, ChemPhysChem,
2008, 9, 1702–1708; (i) E. C. Lee, D. Kim, P. Jurecka, P.
Tarakeshwar, P. Hobza and K. S. Kim, J. Phys. Chem. A,
2007, 111, 3446–3457; ( j) M. O. Sinnokrot and C. D. Sherrill,
J. Phys. Chem. A, 2006, 110, 1065610668; (k) R. Podeszwa, R.
Bukowski and K. Szalewicz, J. Phys. Chem. A, 2006, 110,
10345–10354; (l) M. Pitonak, P. Neogrady, J. Rezac, P.
Jurecka, M. Urban and P. Hobza, J. Chem. Theory Comput.,
2008, 4, 1829–1834.
(
g) E. Craven, C. Zhang, C. Janiak, G. Rheinwald and H.
Lang, Z. Anorg. Allg. Chem., 2003, 629, 2282–2290; (h) U.
Mukhopadhyay, D. Choquesillo-Lazarte, J. Niclos-Gutierrez
and I. Bernal, CrystEngComm, 2004, 6, 627–632; (i) D. Pucci,
V. Albertini, R. Bloise, A. Bellusci, A. Cataldi, C. V. Catapano,
M. Ghedini and A. Crispini, J. Inorg. Biochem., 2006, 100,
1
575–1578; ( j) S. P. Mosae, E. Suresh and P. S.
Subramanian, Polyhedron, 2009, 28, 245–252; (k) X. J. Wang,
H. X. Jian, Z. P. Liu, Q. L. Ni, L. C. Gui and L. H. Tang,
Polyhedron, 2008, 27, 2634–2642; (l) S. Chowdhury, M. G. B.
Drew and D. Datta, Inorg. Chem. Commun., 2003, 6,
29 R. F. W. Bader, J. Phys. Chem. A, 1998, 102, 7314–7323.
30 F. A. Afkhami, A. A. Khandar, G. Mahmoudi, W.
Maniukiewicz, A. V. Gurbanov, F. I. Zubkov, O. Sahin, O. Z.
Yesilel and A. Frontera, CrystEngComm, 2017, 19, 1389.
31 S. Saha, N. Biswas, A. Sasmal, C. J. Gómez-García, E.
Garribba, A. Bauza, A. Frontera, G. Pilet, G. M. Rosair, S.
Mitra and C. Roy Choudhury, Dalton Trans., 2018, DOI:
10.1039/C8DT02417K.
1
014–1016; (m) X. Wang, O. V. Sarycheva, B. D. Koivisto,
A. H. McKie and F. Hof, Org. Lett., 2008, 100, 297–300.
2
8 (a) A. K. Tewari and R. Dubey, Bioorg. Med. Chem., 2008, 16,
1
26–143; (b) P. Mignon, S. Loverix, J. Steyaert and P.
This journal is © The Royal Society of Chemistry 2019
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