Spectral studies and crystal structures of molybdenum(VI) complexes containing pyridine or picoline as auxiliary ligands: interaction energy calculations and free radical scavenging studies

Article abstract of DOI:10.1007/s11243-020-00440-6

Three cis-MoO₂ complexes [MoO₂(CAB)(py)] (1), [MoO₂(CAB)(3-pic)] (2) and [MoO₂(CAB)(4-pic)] (3) which vary in?the nature of the?heterocyclic bases in the auxiliary coordination site derived from an ONO donor aroylhydrazone (H₂CAB) have been synthesized and characterized by various physicochemical methods. The single-crystal X-ray diffraction studies reveal that the complexes adopt a distorted octahedral N₂O₄ coordination sphere around the Mo(VI) center in which the ONO donor atoms of hydrazone moiety and one oxido oxygen constitute the NO₃ basal plane and the axial position by the other oxido oxygen and nitrogen atom of coordinated pyridine molecule in [MoO₂(CAB)(py)] (1) or picoline molecule in [MoO₂(CAB)(3-pic)] (2) and [MoO₂(CAB)(4-pic)] (3). The hydrogen bonding interaction generates a two-dimensional supramolecular sheet-like architecture in [MoO₂(CAB)(py)] (1) and [MoO₂(CAB)(3-pic)] (2), whereas a three-dimensional network?was observed in [MoO₂(CAB)(4-pic)] (3). The interaction energy calculations reveal that the dispersion energy component dominates over other components and [MoO₂(CAB)(3-pic)] (2) is found to be energetically more stable. Furthermore, the aroylhydrazone shows free radical scavenging activity, whereas the complexes are inactive.

Full text of DOI:10.1007/s11243-020-00440-6

Transition Metal Chemistry
https://doi.org/10.1007/s11243-020-00440-6
Spectral studies and crystal structures of molybdenum(VI) complexes
containing pyridine or picoline as auxiliary ligands: interaction energy
calculations and free radical scavenging studies
Daly Kuriakose · M. R. Prathapachandra Kurup1,2
1
Received: 17 September 2020 / Accepted: 4 November 2020
©
Springer Nature Switzerland AG 2020
Abstract
Three cis-MoO complexes [MoO (CAB)(py)] (1), [MoO (CAB)(3-pic)] (2) and [MoO (CAB)(4-pic)] (3) which vary in the
2
2
2
2
nature of the heterocyclic bases in the auxiliary coordination site derived from an ONO donor aroylhydrazone (H CAB) have
2
been synthesized and characterized by various physicochemical methods. The single-crystal X-ray diꢀraction studies reveal
that the complexes adopt a distorted octahedral N O coordination sphere around the Mo(VI) center in which the ONO donor
2
4
atoms of hydrazone moiety and one oxido oxygen constitute the NO basal plane and the axial position by the other oxido
3
oxygen and nitrogen atom of coordinated pyridine molecule in [MoO (CAB)(py)] (1) or picoline molecule in [MoO (CAB)
2
2
(
3-pic)] (2) and [MoO (CAB)(4-pic)] (3). The hydrogen bonding interaction generates a two-dimensional supramolecular
2
sheet-like architecture in [MoO (CAB)(py)] (1) and [MoO (CAB)(3-pic)] (2), whereas a three-dimensional network was
2
2
observed in [MoO (CAB)(4-pic)] (3). The interaction energy calculations reveal that the dispersion energy component
2
dominates over other components and [MoO (CAB)(3-pic)] (2) is found to be energetically more stable. Furthermore, the
2
aroylhydrazone shows free radical scavenging activity, whereas the complexes are inactive.
Introduction
responsible for diꢀerent biological activities, such as anti-
oxidant, anti-inꢂammatory, anti-hypertensive, antimicrobial
and anticancer properties [5–10]. Among transition met-
als, the chemistry of molybdenum has become an impor-
tant area due to its potential application in diverse ꢁelds
[11–15]. Molybdenum complexes have a unique place in the
advancement of coordination chemistry [16]. Due to their
extended applicability in biology and pharmacology, com-
plexes containing pyridine ring moieties are of great interest
[17–20]. The ꢁeld of supramolecular architecture is very
important in coordination chemistry. Various supramolecu-
lar interactions including conventional and non-conventional
hydrogen bonds and non-covalent interactions such as π–π
interactions, C–H⋯π, anion (halide)⋯π and even C–H⋯C
interactions gave rise to a number of 1D, 2D and 3D supra-
molecular architectures [21–23].
Aroylhydrazones are well-established class of molecules
with several possible structures including conꢁgurational
isomers, viz. E and Z, around the imine (C=N) bond and
amido/iminol tautomers. Due to the presence of multiple
donor sites such as protonated/deprotonated amide oxy-
gen, imine nitrogen and additional donor site (usually N
or O) provided from the carbonyl compound, they exhibit
diverse chelating modes with greater eꢀects in a wide vari-
ety of ꢁelds [1–4]. The presence of the azomethine group
(–NH–N=CH–) connected with carbonyl group makes them
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-020-00440-6) contains
supplementary material, which is available to authorized users.
Due to their facile synthesis, structural ꢂexibility, stabil-
ity and high potential as eꢃcient catalysts, a great deal of
work has been carried out on the synthesis and characteriza-
tion of cis-dioxidomolybdenum(VI) complexes containing
*
M. R. Prathapachandra Kurup
mrpcusat@gmail.com; mrp@cukerala.ac.in
1
2
Department of Applied Chemistry, Cochin University
of Science and Technology, Kochi, Kerala 682 022, India
2
+
the [MoO ] core [24–26]. The articles regarding the bio-
2
logical activities of molybdenum complexes are, however,
Department of Chemistry, School of Physical Sciences,
Central University of Kerala, Tejaswini Hills, Periye,
Kasaragod, Kerala 671 320, India
much more limited. The present study reports the synthe-
ses, spectral and structural characterization of cis-MoO
2
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
complexes of an ONO-donor aroylhydrazone ligand, which
diꢀer in the presence of neutral N-donor heterocyclic bases
in the auxiliary coordination site. We also report free radi-
cal scavenging studies of the ligands and complexes, herein.
and 4-picoline for complex 4) was added and reꢂuxed again
1
for 2 h. Single crystals suitable for X-ray analysis obtained
2
on slow evaporation of the resulting orange colored solutions
were ꢁltered oꢀ and washed with methanol and dried over
P O in vacuo (Scheme 1).
4
10
Analytical data for C H Cl MoN O ([MoO (CAB)
2
1
17
2
3
5
2
Experimental procedure
(py)] (1)): Yield: 0.34 g (61%); color: orange; M.W.:
−1
5
58.22 g mol ; elemental analysis calculated/found (%): C
1
Materials
45.2/45.3, H 3.1/3.1, N 7.5/7.5; H NMR (400 MHz, DMSO-
d ): δ 2.96 (3H, s, –CH ), 3.89 (3H, s, –OCH ), 7.23–8.64
6
3
3
−
1
The chemicals and solvents used in the syntheses were used
without further puriꢁcation. The chemicals, 3′,5′-dichloro-2′-
hydroxyacetophenone (Aldrich), 3-methoxybenzhydrazide
(11H, aromatic); FT–IR: ν (cm ) 1583 and 1545 ν(C=N),
1337 ν(C–O), 935 and 921 ν(Mo=O).
Analytical data for C H Cl MoN O ([MoO (CAB)
2
2
19
2
3
5
2
(
Alfa Aesar) and bis(acetylacetonato)dioxomolybdenum(VI)
Aldrich) used were of analar quality. Methanol, pyridine,
(3-pic)] (2)): Yield: 0.39 g (68%); color: orange; M.W.:
−1
(
572.24 g mol ; elemental analysis calculated/found (%):
1
3
-picoline and 4-picoline were purchased from Spectrochem.
C 46.2/46.1, H 3.4/3.3, N 7.3/7.3; H NMR (400 MHz,
The aroylhydrazone (H CAB·H O) was synthesized by a
DMSO-d ): δ 2.35 (3H, s, –CH (3-pic)), 2.96 (3H, s, –CH ),
2
2
6
3
3
previously reported procedure [27].
3.89 (3H, s, –OCH ), 7.22–8.47 (10H, aromatic); FT–IR: ν
3
−
1
(
cm ) 1590 and 1547 ν(C=N), 1335 ν(C–O), 925 and 853
ν(Mo=O).
Analytical data for C H Cl MoN O ([MoO (CAB)
Synthesis details
2
2
19
2
3
5
2
Synthesis of 3′,5′‑dichloro‑2′‑hydroxyacetophenone‑3‑me
(4-pic)] (3)): Yield: 0.34 g (59%); color: orange; M.W.:
−1
thoxybenzoylhydrazone monohydrate (H CAB·H O)
572.24 g mol ; elemental analysis calculated/found (%):
2
2
1
C 46.2/46.3, H 3.4/3.5, N 7.3/7.4; H NMR (400 MHz,
3
1
-Methoxybenzhydrazide (0.17 g, 1.00 mmol) dissolved in
DMSO-d ): δ 2.32 (3H, s, –CH (4-pic)), 2.91 (3H, s, –CH ),
6
3
3
0 mL methanol, 3′,5′-dichloro-2′-hydroxyacetophenone
3.84 (3H, s, –OCH ), 7.18–8.43 (10H, aromatic); FT–IR: ν
3
−
1
(
0.21 g, 1.00 mmol) dissolved in 10 mL methanol was added
(cm ) 1591 and 1545 ν(C=N), 1333 ν(C–O), 927 and 914
ν(Mo=O).
and reꢂuxed for 4 h. Single crystals suitable for X-ray analy-
sis obtained on evaporation of the resulting reaction mixture
were separated, washed and dried over P O in vacuo [27].
Physical measurements
4
10
Analytical data for C H Cl N O (H CAB·H O) Yield:
1
6
16
2
2
4
2
2
−
1
0
.29 g (77%); color: Yellow; M.W.: 371.21 g mol ; elemen-
The micro-analyses of carbon, hydrogen and nitrogen were
performed on a Vario EL III CHNS analyzer. IR spectra
of the compounds were recorded in KBr pellets with FTIR
tal analysis calculated/found (%): C 51.8/51.7, H 4.3/4.5,
1
N 7.6/7.8; H NMR (400 MHz, DMSO-d ): δ 2.86 (3H, s,
6
−
1
–
CH ), 3.85 (3H, s, –OCH ), 7.22 (1H, q, aromatic), 7.5 (3H,
spectrometer (JASCO-4100) in the region 4000–400 cm .
3
3
m, aromatic), 7.63 (1H, d, aromatic), 7.68 (1H, d, aromatic),
The proton NMR spectra were collected in DMSO-d on
6
−
1
1
3
1.55 (1H, br, OH), 14.43 (1H, br, NH); FT–IR: ν (cm )
a Bruker Avance III, 400 MHz spectrometer having 9.4 T
superconducting magnet using tetramethylsilane (TMS) as
an internal reference. Electronic spectra in DMF solutions
were recorded on a Thermo scientiꢁc evolution 201 UV–Vis
double beam spectrophotometer in the 200–800 nm range.
548 ν(O–H), 3439 ν(N–H), 1660 ν(C=O), 1592, ν(C=N).
Synthesis of molybdenum(VI) complexes
Synthesis of [MoO (CAB)(py)] (1), [MoO (CAB)(3‑pic)] (2)
2
2
and [MoO (CAB)(4‑pic)] (3)
Crystallographic data collection and reꢀnement
2
For 10 mL methanolic solution of 3-methoxybenzhy-
drazide (0.17 g, 1.00 mmol), 10 mL methanolic solution of
Single crystals of the Mo(VI) complexes, [MoO (CAB)
2
(py)] (1), [MoO (B)(3-pic)] (2) and [MoO (CAB)(4-pic)]
2
2
3
′,5′-dichloro-2′-hydroxyacetophenone (0.21 g, 1.00 mmol)
(3) with suitable dimensions were mounted on a Bruker
SMART APEXII charge-coupled device (CCD) diꢀrac-
tometer, equipped with a graphite crystal, incident-beam
monochromator with Mo Kα (λ=0.71073 Å) radiation as the
X-ray source. The data collection was performed at 293 K.
Data reduction was performed using the program Bruker
SAINT. APEX2 and SAINT were used for cell reꢁnement
is added and refluxed for 3 h. Bis(acetylacetonato)
dioxomolybdenum(VI) (0.33 g, 1.00 mmol) dissolved in
1
0 mL methanol was added slowly to the above reꢂuxing
solution, and reꢂuxation was continued for another 30 min.
For the reꢂuxing solution, 0.5 mL of respective heterocyclic
base (pyridine for complex 2 and 3-picoline for complex 3
1
3

Transition Metal Chemistry
Scheme 1 Synthetic route for complexes 1–3
[
28]. Absorption corrections were carried out using SAD-
ꢀdW
ꢀdW
di − ri
de − re
dnorm
=
+
ABS based on Laue symmetry using equivalent reꢂections
ꢀ
dW
ꢀdW
ri
r
e
[
29]. The structure was solved by using SHELXS-97 direct
methods and reꢁned by full-matrix least-squares reꢁnement
where d is the distance from any surface point to the
e
2
on F using SHELXL-2018/1 [30] as well as WinGX soft-
nearest nucleus external to the surface, and d is deꢁned as
i
ware package [31]. The molecular and crystal structures
were plotted using ORTEP-3 [31], DIAMOND version 3.2 g
the distance to the nearest nucleus internal to the surface,
ꢀdW
i
ꢀdW
e
respectively. r
atoms. The d
and r
are the van der Waals radii of the
[
32] and MERCURY 3.10.1 software [33]. Non-hydrogen
surfaces are displayed as red–white–blue
norm
atoms were reꢁned anisotropically. All H atoms on C were
repositioned in their calculated positions, guided by diꢀer-
ence maps, with C–H bond distances 0.93–0.96 Å. H atoms
(
shorter than vdW-equal to vdW-longer than vdW separa-
tion, respectively) color code. The particular atom pair con-
tacts are highlighted by deconstructing the pseudo-mirrored
2D ꢁngerprint plots [35–37]. The d vs d ꢁngerprint plots
were assigned as U =1.2U (1.5 for Me). Some reꢂections
iso
eq
i
e
were omitted owing to bad agreement. The crystallographic
data and structure reꢁnement parameters for complexes 1, 2
and 3 are given in Table 1.
were displayed in the range 0.6–2.8 Å including reciprocal
contact.
Interaction energy calculations and energy
framework analysis
Hirshfeld surface analysis
The Hirshfeld surface (HS) analysis and the associated 2D
The interaction energies were calculated for a cluster of
radius 3.8 Å generated around the molecule present in the
ꢁ
ngerprint plot were performed using the program Crystal-
Explorer 17.5 by inputting CIF (crystallographic informa-
asymmetric unit. The interaction energy calculations (E )
tot
tion ꢁle) [34]. The normalized contact distance (d
) was
norm
between the molecules are obtained using monomer wave-
functions calculated at the HF/3-21G level using symmetry-
independent molecule with scale factors to determine E tot:
mapped over a color scale of − 0.348 Å (red) to+2 Å (blue)
for complexes. The shape index was mapped in the ꢁxed
color range − 1 to 1 Å and curvedness in the range − 4 to 4
k
=1.019, k =0.651, k =0.901 and k =0.811 [38].
pol dis rep
′
ele
Å. The normalized contact distance (d
) was based on d
norm
e
The energies are expressed in terms of electrostatic (E ),
ele
and d and van der Waals radii of the atoms. It is given by
i
1
3

Transition Metal Chemistry
Table 1 Crystallographic data and structure reꢁnement for Mo(VI) complexes 1, 2 and 3
Parameters
1
2
3
Empirical formula
Formula weight
C H Cl MoN O
558.22
C H Cl MoN O
572.24
C H Cl MoN O
22 19 2 3 5
572.24
2
1
17
2
3
5
22 19
2
3
5
Crystal system, space group
Monoclinic, P2 ∕c
Monoclinic,P2 ∕n
Orthorhombic,Pbca
1
1
Cell parameters
a (Å)
b (Å)
c (Å)
α (°)
12.8549 (8)
11.7903 (7)
16.1183 (9)
90°
7.8667 (10)
14.344 (2)
20.433 (3)
90°
14.1119 (7)
16.6927 (9)
19.4198 (10)
90°
β (°)
γ (°)
Volume V (Å )
Z
112.774 (3)°
90°
2252.5 (2)
4
1.646
0.858
97.097 (5)°
90°
2287.9 (5)
4
1.661
0.847
90°
90°
4574.6 (4)
8
1.662
0.847
2304
3
−
3
Calculated density (ρ) (Mg m
)
)
Absorption coeꢃcient, μ (mm−
F(000)
1
1120
1152
Crystal size (mm)
0.3×0.25×0.2
0.4×0.35×0.2
0.35×0.25×0.2
Reꢂections collected/unique reꢂections 26,661/5697 [R(int)=0.0323]
27,964/5710 [R(int)=0.0339] 52,714/5712 [R(int)=0.0401]
(
R )
int
Completeness to θ
25.242 (99.9%)
0.842 and 0.814
5697/0/289
1.038
25.242 (99.9%)
0.776 and 0.72
5710/0/298
1.066
25.242 (99.9%)
0.844 and 0.816
5712/0/301
1.121
Maximum and minimum transmission
Data/restraints/parameters
Goodness of ꢁt on F2
Final R indices
R =0.0326,
R =0.0325,
R =0.0478,
1
1
1
[
I>2σ (I)]
wR =0.0768
wR =0.0850
wR =0.1173
2
2
2
R indices (all data)
R =0.0531,
R =0.0426,
R =0.0690,
1
1
1
wR =0.0871
wR =0.0899
wR =0.1342
2
2
2
−3
Largest diꢀerence peak and hole (e Ǻ ) 0.680 and − 0.495
0.518 and − 0.527
0.615 and − 0.631
∑
∑
�
∑ ∑
w(F − F ) ∕ w(F )
o c o
2ꢀ1∕2
�
�
∕
R1 =
Foꢀ − F
ꢀFoꢀ
2
2
2
2
�
c�
wR2
=
′
′
polarization (E ), dispersion (E ) and exchange repulsion
freshly prepared DPPH (3 mg/25 mL methanol) for 30 min in
dark. The absorbance was measured at 517 nm using UV–Vis
spectrophotometer. The assay was carried out in triplicate, and
the mean was reported. The concentration required to result
in 50% inhibition (IC value) was also calculated. The DPPH
pol
dis
′
(
E
) terms [39]. The total energy (E ) is represented as
rep tot
�
�
�
�
Etot = k E + kpol
E
+ k E + krepE ,
pol rep
dis dis
ele ele
5
0
where the k values are scale factors.
radical scavenging activity was calculated using the formula:
The energy frameworks are the graphical representations
of the magnitudes of the intermolecular interaction energies.
The molecular pair-wise interaction energies are represented
as cylinders joining the centroids of pairs of molecules.
Frameworks were constructed for E as red cylinders, E
ꢀ
ꢁ
A0 − A1
%
inhibition =
× 100.
A0
⋅
elec
dis
A and A =Absorbance at 517 nm of the radical (DPPH )
0
1
as green and E as blue.
in the absence and presence of antioxidant.
tot
Free radical scavenging studies
Results and discussion
The free radical scavenging studies of the aroylhydrazone and
the complexes were examined by DPPH ((2,2-diphenyl-1-pic-
rylhydrazyl) assay. Diꢀerent concentrations (10, 20, 30, 40 and
The cis-MoO complexes having the general formula
2
[
(
MoO (CAB)D] (D = pyridine (complex 1), 3-picoline
2
complex 2) and 4-picoline (complex 3) were synthesized by
5
0 µg/mL) of the test compounds (dissolved in DMSO) and
reꢂuxing solutions of [MoO (acac) ] with aroylhydrazone in
2
2
a methanolic blank were incubated at 37 °C with 187 µL of
1
3

Transition Metal Chemistry
0
the presence suitable heterocyclic bases. The complexes are
stable toward air and moisture in the solid state at room tem-
perature. The complexes are characterized using elemental
bands in the visible region conꢁrms the 4d electronic con-
ꢁguration of Mo(VI) moiety [40, 41].
1
analyses, IR, UV–Vis and H NMR spectra. The molecular
Crystal structure analysis
structures of complexes 1–3 are determined using SCXRD
studies.
Molecular structure descriptions of complexes 1–3
The asymmetric unit of monomeric complexes 1–3 consists
of only one molecule. In all the complexes, the molybdenum
center is octahedrally coordinated by phenolate oxygen (O1),
azomethine nitrogen (N1), iminolate oxygen (O2), two ter-
minal oxido oxygen atoms (O4 and O5) and nitrogen atom
(N3) of the monodentate N-donor auxiliary ligand (pyri-
dine in complex 1, 3-picoline in complex 2 or 4-picoline
in complex 3). The aroylhydrazone behaves as a dianionic
tridentate ligand and coordinates to the molybdenum center
in the iminolate form. Molecular structures and atom num-
bering scheme of complexes 1–3 are shown in Figs. 1, 2
and 3. Selected bond angles (°) and bond distances (Å) are
listed on Table S1. Complexes 1 and 2 crystallized in mono-
clinic space groups P2 /c and P2 /n, respectively, whereas
Spectral studies
The spectrum of aroylhydrazone was compared with the
spectra of Mo(VI) complexes 1–3, and the characteristic IR
bands are displayed in Table S1. The two medium inten-
−
1
sity broadbands at around 3548 and 3439 cm allocated
to stretching vibrations of O–H, N–H and a strong band at
−
1
1
660 cm due to ν(C=O) mode in the ligand disappear upon
complexation which indicate the coordination of phenolate
2
+
and iminolate oxygen to the [MoO ] core [40, 41]. Strong
2
−
1
band assignable to ν(C=N) observed at around 1592 cm
of the free aroylhydrazone was red shifted to 1583, 1590
−
1
and 1591 cm , respectively, in the corresponding com-
plexes indicating the coordination of azomethine nitrogen
to the metal ion [40, 41]. Besides, the appearance of a new
band assignable to asymmetric stretching vibration of newly
1
1
complex 3 crystallized in orthorhombic space group Pbca.
A comparison of the molecular structures of the metal free
ligand and molybdenum complexes reveals several interest-
ing features [30]. The Z conꢁguration of the aroylhydrazone
with respect to N2–C9 bond is retained in all complexes.
The C7–N1 bond length of the metal free ligand, 1.277(2)
Å, increases to 1.305(3), 1.299(3) and 1.298(5) Å in com-
plexes 1–3, respectively. The slight increase in N1–N2 bond
distances 1.393(3), 1.401(3) and 1.403(4) Å, respectively, in
complexes 1–3 from 1.369(2) Å conꢁrms increase in elec-
tron delocalization on complexation. Since the aroylhydra-
zone is coordinated in the iminolate form, the C9–O2 bond
is expected to assume a single bond character, but due to
electron delocalization in the coordinated ligand, the actual
bond distances lie between the distances expected for C–O
and C=O [43, 44]. The C9–O2 bond lengths are 1.326(3),
1.327(3) and 1.313(5) Å, respectively, in complexes 1–3.
In addition to this, the C9–N2 bond distances 1.292(3),
1.296(3) and 1.302(5) Å are closer to C=N distances due
to enolization on complexation. The oxido oxygen (O4/O5)
and N atom, N3, of heterocyclic base occupy the axial posi-
tions of the distorted octahedron. The central metal atom,
molybdenum, is displaced toward the axial oxido oxygen
(O4/O5) from the mean square plane deꢁned by equatorial
donor atoms (O1, N1, O2, O4/O5) by 0.3255, 0.3298 and
0.3348 Å for complexes 1–3, respectively.
−
1
formed C=N in the complexes in the range 1525–1550 cm
substantiates the existence of iminolate form of ligand in the
complexes. The two absorption bands corresponding to sym-
metric and asymmetric stretching vibrations of ν(Mo=O) of
−
1
cis-MoO were observed in the region 850–936 cm [40,
2
4
1]. The pyridine stretching vibrations were observed in the
−
1
range 1400–1500 cm in complexes 1–3.
1
The H NMR spectra of dioxidomolybdenum(VI) com-
plexes were recorded in DMSO-d (Figs. S1–S4). After com-
6
plexation, the resonance corresponding to protons attached
to the nitrogen (N2) and oxygen (O1) disappeared indicat-
ing that the aroylhydrazone coordinates through phenolate
and iminolate oxygens, O1 and O2. The aromatic protons
show signals in the 7.2–8.3 ppm range. The resonance peaks
assignable to –CH group of picoline in complexes 2 and 3
3
were observed at 2.35 and 2.32 ppm, respectively.
The electronic spectra of molybdenum(VI) complexes
–
5
were recorded in 1 × 10 M DMF solution, and the spec-
tral assignments are summarized in Table S2 (Fig. S5).
The electronic spectra of the complexes display absorption
band probably due to n→π* and π→π* transitions involv-
ing molecular orbitals located on aromatic ring, azome-
thine nitrogen, etc., in the range 270–330 nm [42–44]. The
O(pꢀ) → Mo(dꢀ) LMCT ligand-to-metal charge transfer
transitions involving the promotion of an electron from the
The aroylhydrazone ligand is coordinated in the meridi-
onal fashion forming ꢁve- and six-membered chelate rings
with intraligand bite angles O(1)–Mo(1)–N(1), 80.30(7)°,
80.14(7)° and 80.01(11)°, respectively, for complexes 1–3
and O(2)–Mo(1)–N(1), 72.83(7)°, 72.23(7)° and 72.84(11)°,
ꢁ
lled HOMO of the ligand to the empty LUMO of Mo were
observed at 399, 396 and 415 nm, respectively, for com-
plexes 1, 2 and 3 [40, 41]. The absence of d–d transition
1
3

Transition Metal Chemistry
Fig. 1 ORTEP plot of the
molecular structure of
[
MoO (CAB)(py)] (1) with
2
displacement ellipsoids at 30%
probability level along with
atom numbering scheme of the
non-hydrogen atoms. (Color
ꢁ
gure online)
Fig. 2 ORTEP plot of the
molecular structure of
[
MoO (CAB)(3-pic)] (2) with
2
displacement ellipsoids at 30%
probability level along with
atom labeling scheme of the
non-hydrogen atoms. (Color
ꢁ
gure online)
respectively, for complexes 1–3 [40, 41]. The dihedral angle
between ꢁve and six membered metallocycles fused along
Mo(1)–N(1) junction is found to be 9.83(4)°, 9.52(5)° and
tetragonality or tetragonal distortion parameter, T values of
0.9505, 0.9442 and 0.9449 Å, in complexes 1–3, respec-
tively, indicates slight distortion from regular octahedral
coordination geometry in these complexes. The bond dis-
7
.167(8)° for complexes 1–3, respectively. As a result of
trans inꢂuence of oxido oxygen, Mo(1)–N(3) bond distances
are considerably larger than Mo–N1 bond distances.
tances to molybdenum are in the order Mo–O
<Mo–
oxido)
(
O
<Mo–O
<Mo–N
<Mo–O
(azo) (solvent).
(
phenolic)
(iminolate)
Similar to other reported complexes, oxido oxygens (O4
and O5) are mutually cis and Mo(1)=O(4) and Mo(1)=O(5)
bond distances fall in the range 1.68–1.71 Å. This is also
conꢁrmed from O(4)–Mo(1)–O(5) bond angles [40, 41]. The
In all the complexes, the gathering of molecules in the
particular manner in the unit cell results in hydrogen bond-
ing and π–π interactions. In complex 1, three intermolecular
non-conventional hydrogen bonding interactions in which the
1
3

Transition Metal Chemistry
Fig. 3 ORTEP plot of the
molecular structure of
[
MoO (CAB)(4-pic)] (3) with
2
displacement ellipsoids at 30%
probability level along with
atom labeling scheme of the
non-hydrogen atoms. (Color
ꢁ
gure online)
oxido oxygens, O4 and O5, behave as acceptor generates a
two-dimensional supramolecular architecture (Table S4 and
Figs. 4 and 5). The bifurcated hydrogen bond between O5 and
hydrogens H5 and H8A gives rise to a R (6) motif (Fig. 5).
2
The hydrogen bonding interactions present in the system give
a two-dimensional supramolecular architecture (Table S4 and
Fig. 6) in complex 2, and the C3–H3···Cl2 hydrogen bonding
2
2
interaction resulted in a R (8) motif (Fig. 7). In complex 3,
similar to other complexes due to the presence of several oxy-
gens and chlorine atoms, four non-conventional intermolecular
hydrogen bonding interactions are present in the system. The
diꢀerent hydrogen bonding interactions present in the system
gave a three-dimensional supramolecular architecture to the
complex (Table S4 and Fig. 7). In complex 1, the centroids
Cg(1) and Cg(4) involved in π–π interaction with Cg(3) and
Cg(5), respectively, at a distance of 3.841(18) and 3.844(18)
Å contribute to the stability of packing (Table S4 and Fig. 8).
The π–π interactions observed in the crystal system resulted in
a one-dimensional supramolecular architecture. In complexes
Fig. 4 Hydrogen bonding interactions of complex 1. (Color ꢁgure
online)
2
and 3, the supramolecular arrangement is strengthened by
Cg(1)···Cg(3) and Cg(4)···Cg(5) interactions (Table S4 and
Fig. 9).
contacts are represented as red spots on the d
surface.
norm
In the HSs mapped with a shape index plot, the presence
of red and blue triangles (bow-tie pattern) highlighted by
red dotted circle in all complexes clearly indicates the
presence of π–π stacking interactions. The appearance of
ꢂat surfaces delineated by a blue outline in HS mapped
over curvedness is another evidence for π–π stacking
Hirshfeld surface analysis and the 2D ꢀngerprint
plot
The HSs mapped over d
, shape index and curved-
norm
ness are shown in Fig. 10. The predominant interatomic
1
3

Transition Metal Chemistry
Fig. 5 2D supramolecular
network formed by hydrogen
bonding interactions and the
ring motif formed is highlighted
in yellow for complex 1. (Color
ꢁ
gure online)
Fig. 6 a Hydrogen bonding interactions of complex 2. b 2D supramolecular network formed by hydrogen bonding interactions and the ring
motif formed is highlighted in red circles for complex 2. (Color ꢁgure online)
interactions (Fig. 10). The bow-tie pattern for complexes
in the HSs mapped with shape index function is displayed
in Fig. S6.
and 6.1%, respectively, for complexes 1 to 3. The absence
of halide–halide interaction is also evident from Hirshfeld
surface. The ꢁngerprint plot analysis shows that weak inter-
actions contribute signiꢁcantly to the packing of molecules.
The relative contributions of various signiꢁcant interactions
are represented as a histogram, and the contribution of low
percentage interactions is represented as others (Fig. S7).
The histogram reveals that low percentage interactions also
contribute to the packing of molecule.
The percentage contributions of various intermolecular
contacts represented by two-dimensional ꢁngerprint plots
with d and d ranging from 1.0 to 2.8 Å are presented in
i
e
Fig. 11. The H···H contacts occupy a major share of the HS
for all the complexes. The O···H/H···O interactions cover
the next largest region of the ꢁngerprint plot and appeared
as 21.3, 23.2 and 23.9%, respectively, for complexes 1–3.
Due to the presence of coordinated pyridine and picoline
molecule, the C···H/H···C interaction which is concentrated
highly at the edges contributes signiꢁcantly to the HS. The
N···H/H···N interactions contribute negligibly to the total
Hirshfeld surface. The C···C contacts mirrored at the diago-
nals represent π–π stacking interactions and occupy 5.7, 4.6
Interaction energy calculations and energy
framework analysis
The interaction energy calculations for complexes 1–3
presented in Table 2 reveal that the highest total interac-
tion energy (− 116.6, − 118.3 and − 119.6 kJ mol− for
1
1
3

Transition Metal Chemistry
Fig. 7 a H-bonding interactions and b 3D supramolecular network formed by hydrogen bonding interactions of complex 3. (Color ꢁgure online)
Fig. 8 1D supramolecular chain
formed by π–π interactions for
complex 1. (Color ꢁgure online)
−
1
complexes 1, 2 and 3, respectively) is shown by the green
for complex 1 and 3) or red (for complex 2) colored mol-
(− 1.4 kJ mol ) is located at a distance of 12.32 Å (sym-
metry operation − x, y + 1/2, − z + 1/2).
(
ecule with symmetry operation, (− x, − y, − z) located at
distances of 5.77, 5.61 and 5.99 Å from the centroid of
the selected molecule for complexes 1, 2 and 3, respec-
tively (Table S5 and Fig S4). For the complex 1, the
orange colored molecule with same symmetry opera-
tion located at a distance of 10.97 Å from the centroid of
the selected molecule shows the lowest total interaction
The average interaction energy values for complexes 1, 2
−
1
and 3 are found to be − 293.6, − 321.6 and − 305 kJ mol ,
respectively (Table 2). The interaction energy is dominated
by dispersion energy framework for all complexes (− 298.2,
−1
− 359.9 and − 328.4 kJ mol , respectively) [40]. The result
also establishes that the complex 2 is found to be energeti-
cally more stable. The stability follows the order complex
2>complex 3>complex 1.
−
1
energy (− 3.4 kJ mol ). In complex 2, the blue colored
molecule (x + 1/2, − y + 1/2, z + 1/2) at 13.46 Å has the
The energy framework analysis gives a three-dimen-
sional topological representation of the interaction
in which the radius of the cylinder is proportional to
the magnitude of the interaction energy (Fig S8). The
−
1
lowest interaction energy (− 2.5 kJ mol ). For complex 3,
the violet colored molecule with lowest interaction energy
1
3

Transition Metal Chemistry
Fig. 9 π···π interactions of complexes 2 and 3. (Color ꢁgure online)
Fig. 10 Hirshfeld surfaces
mapped over a d , b shape
norm
index and c curvedness for 1–3.
(
Color ꢁgure online)
thickness of the cylinder represents the relative strength
of molecular packing in different directions. The cylin-
ders corresponding to π–π stacking interactions are shown
as thick tubes or pillars in these crystal structures.
Free radical scavenging studies
The radical scavenging activity of aroylhydrazone, H CAB
2
and the molybdenum(VI) complexes are investigated using
1
3

Transition Metal Chemistry
Fig. 11 2D ꢁnger print plots for complexes 1, 2 and 3 with d and d ranging from 1 to 2.8 Å and their plots delineated into diꢀerent major con-
e
i
tact types. (Color ꢁgure online)
Table 2 Interaction
E′ele
E′pol
E′dis
E′rep
Etot
Average Etot
energies calculated from the
CrystalExplorer (kJ/mol)
Complex 1
Complex 2
Complex 3
− 98.7
− 94.4
− 98
− 39.4
− 43.2
− 45
− 298.2
− 359.9
− 328.4
142.7
175.9
166.4
− 279.17
− 305.93
− 290.1
− 293.6
− 321.6
− 305
·
DPPH assay. The absorbance of DPPH is found to be
decreased signiꢁcantly in the presence of aroylhydrazone
and does show a remarkable change in the case of complexes
(
Fig. S9). This shows that ligands show good free radical
scavenging activity compared to corresponding complexes.
The decrease in absorbance in the presence of aroylhydra-
zone may be due to the formation of stable DPPH-H com-
pound (Fig. 12). The IC value of H CAB·H O was found
5
0
2
2
to be 58.39 µg/mL. This can be explained by hydrogen atom
transfer (HAT) mechanism. Aroylhydrazone contains two
abstractable hydrogens (phenolic and imine hydrogen),
and no such hydrogens are present in complexes 1, 2 and 3.
Hydrogen abstraction depends on bond dissociation energy.
The lower the bond dissociation energy, the more favora-
ble will be the site for H-abstraction. The bond dissociation
energy was approximately 390 and 460 kJ/mol for N–H and
O–H, respectively. Based on this, we can conclude that imine
hydrogen was abstracted. In addition to this as evident from
single-crystal X-ray diꢀraction studies, the hydrogen atom
abstraction from phenolic ring is quite diꢃcult under exam-
ined conditions due to the formation of strong intramolecular
Fig. 12 Plot of percentage inhibition versus drug concentration for
aroylhydrazone H CAB. (Color ꢁgure online)
2
hydrogen bond with azomethine nitrogen [30]. The reaction
·
of DPPH radical may generate nitrogen (N )-centered radical
(hydrazonyl radical which is stabilized through conjugation
1
3

Transition Metal Chemistry
·
Scheme 2 Plausible reaction of DPPH with H CAB
2
(
Scheme 2)) [45, 46]. Due to the absence of such hydrogens,
Supplementary data
the complexes presented in this study do not show free radi-
cal scavenging activity.
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center
as CCDC 1972050, 1972051 and 1972058 for complexes
[
MoO (CAB)(py)] (1), [MoO (CAB)(3-pic)] (2) and
2
2
Conclusion
[MoO (CAB)(4-pic)] (3), respectively. Copies of this infor-
2
mation may be obtained free of charge via www.ccdc.cam.
ac.uk/conts/retrieving.html or from the Director, CCDC, 12
Union Road, Cambridge, CB2, 1EZ, UK (fax: + 44-1223-
336-033; e-mail: deposit@ccdc.cam.ac.uk.).
The present paper deals with the synthesis of a three cis-
MoO complexes derived from an ONO donor aroylhydra-
2
zone with the sixth coordination site occupied by diꢀer-
ent heterocyclic bases. The complexes were characterized
using various physicochemical techniques. All three com-
plexes are structurally characterized by single-crystal X-ray
diꢀraction studies. The aroylhydrazone behaves as a dian-
ionic tridentate ligand and coordinates to the metal center
in the tautomeric iminolic form. The complexes exhibit
distorted octahedral geometry. The pivotal role of hydrogen
bonding and weak non-covalent forces in the development
of supramolecular framework are investigated. Hirshfeld
surface analysis reproduces the upshots of crystal structure
studies, and the role of hydrogen bonding interactions in
the development of diverse supramolecular architecture is
also well established. The various intermolecular contacts
including low percentage interactions were identiꢁed and
quantiꢁed using associated 2D ꢁngerprint plot. The interac-
tion energy calculation reveals that the dispersion energy
component dominates over electrostatic, polarization and
repulsion energy component. Complex 2 is energetically
more stable than complexes 1 and 3. Due to the presence of
abstractable hydrogen, aroylhydrazones show free radical
scavenging activities, whereas the complexes are inactive.
Acknowledgements DK thanks the University Grants Commission,
New Delhi, India, for the award of a Senior Research Fellowship
(
F.17-45/2008(SA-1) dated 11.08.2014). The authors are thankful to
the Sophisticated Analytical Instrumentation Facility, Cochin Univer-
sity of Science and Technology, Kochi, India, for elemental analyses,
1
H-NMR spectra and single-crystal X-ray diꢀraction measurements.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conꢂict of
interest.
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