Synthesis, Spectral Characterization and Crystal Structures of Dioxidomolybdenum(VI) Complexes Derived from Nicotinoylhydrazones

Article abstract of DOI:10.1007/s10870-018-0756-9

Abstract: Cis-dioxidomolybdenum(VI) complexes [MoO₂L¹(DMSO)] (1), [MoO₂L²(H₂O)]·DMF (2) and [MoO₂L³(DMF)] (3) were synthesized by solution based reactions of aroylhydrazones (H

Full text of DOI:10.1007/s10870-018-0756-9

Journal of Chemical Crystallography
https://doi.org/10.1007/s10870-018-0756-9
ORIGINAL PAPER
Synthesis, Spectral Characterization and Crystal
Structures of Dioxidomolybdenum(VI) Complexes Derived
from Nicotinoylhydrazones
T. M. Asha1 · M. R. P. Kurup1,2
Received: 12 July 2018 / Accepted: 29 November 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Cis-dioxidomolybdenum(VI) complexes [MoO2L1(DMSO)] (1), [MoO2L2(H2O)]·DMF (2) and [MoO2L3(DMF)] (3) were
synthesized by solution based reactions of aroylhydrazones (H2L1 =5-chloro-2-hydroxybenzaldehyde nicotinoylhydrazone,
H2L2 =2-hydroxy-5-iodobenzaldehyde nicotinoylhydrazone and H2L3 =3,5-dichloro-2-hydroxybenzaldehyde nicotinoylhy-
drazone) with bis(acetylacetonato)dioxidomolybdenum(VI) complex, [MoO2(acac)2] in presence of DMSO or DMF. The
synthesized aroylhydrazones and their molybdenum complexes were characterized by elemental analysis, spectroscopic
1
techniques (FT-IR, UV–Vis, H NMR) and conductivity measurements. Finally the three dimensional structures of the
complexes were confirmed by single crystal X-ray diffraction studies. Crystal structures of 1, 2 and 3 were solved by direct
methods and refined with full-matrix least-squares calculations using the SHELXS97 and SHELXL2014 software programs
respectively. Complex 1 got crystallized in monoclinic space group, P21/c with Z=4, whereas complexes 2 and 3 in triclinic
space group, P1 with Z=2. The ligands, H2L1−3 showed coordination to the metal ion in a dibasic tridentate manner through
deprotonated phenolate oxygen, azomethine nitrogen and enolate oxygen.
Graphical Abstract
All the three dioxidomolybdenum(VI) complexes exhibit a distorted octahedral geometry around molybdenum atom. ORTEP plot of
[MoO2L2(H2O)]·DMF. (Displacement ellipsoids are drawn at 50% probability)
Keywords Aroylhydrazones · Molybdenum complexes · X-ray diffraction studies · Crystal structure · Azomethinenitrogen
Introduction
Aroylhydrazones are significant class of ONO donor ligands
which have played an important role in the development of
* M. R. P. Kurup
coordination chemistry of molybdenum. Furthermore the
study of oxidomolybdenum complexes coordinated with
mrpcusat@gmail.com; mrp@cukerala.ac.in
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

Journal of Chemical Crystallography
such tridentate ligands have drawn considerable atten-
tion due to their similarity in the active site of majority of
molybdoenzymes [1–3] as well as for the involvement of
molybdenum(VI) compounds as catalyst in several industrial
processes, such as epoxidation of olefin [4], olefin metathesis
[5] and isomerization of allylic alcohol [6]. In this context,
dioxidomolybdenum(VI) complexes have been extremely
well investigated [7–14], particularly with respect to the
catalytic role of transferase enzymes like nitrate reductase in
which their active sites consist of a cis-dioxidomolybdenum
moiety [15–17]. In recent times, a number of aroylhydra-
zones obtained by the reactions of aromatic acidhydrazides
with o-hydroxycarbonyl compounds have been used in
molybdenum chemistry. These type of ligands are of particu-
lar interest because the MoO2L or MoOL type of complexes
possess one or two “open” coordination sites that can be
utilized for substrate binding properties [18–21] and hence
these species have frequently been considered as models for
enzymatic reactions and catalytic sites. As a part of our stud-
ies on aroylhydrazone complexes [22–24], this article reports
the synthesis and characterization of three new mononuclear
cis-dioxidomolybdenum(VI) complexes, [MoO2L1(DMSO)]
(1), [MoO2L2(H2O)]·DMF (2) and [MoO2L3(DMF)] (3) with
aroylhydrazones derived from nicotinic acid hydrazide and
substituted salicylaldehydes as primary ligands with special
reference to their crystal structures. In order to investigate
the effect of substituents on the stabilization of the overall
crystal structure, salicylaldehydes substituted with different
halogens were selected.
Microanalysis studies were done on a Vario EL III CHNS
analyzer.
Synthesis
Synthesis of the Aroylhydrazones
The aroylhydrazones were prepared by the condenzation
reaction of the nicotinic acid hydrazide with the correspond-
ing substituted 2-hydroxybenzaldehydes in a 1:1 molar ratio
(Fig. 1).
A hot methanol solution (30 mL) of nicotinic acid
hydrazide (1 mmol, 0.137 g) and 1 mmol of the correspond-
ing substituted 2-hydroxybenzaldehydes (5-chloro-2-hy-
droxybenzaldehyde (H2L1), 2-hydroxy-5-iodobenzaldehyde
(H2L2) or 3,5-dichloro-2-hydroxybenzaldehyde (H2L3)) were
mixed and refluxed for 3 h. The resulting yellow solution
was allowed to cool to room temperature. The compounds
were separated as pale yellow crystalline solids.
H2L1: Total yield = 0.179 g (65%). Anal. Calc. for
C13H10ClN3O2: C, 56.64; H, 3.66; N, 15.24. Found: C,
56.51; H, 3.72; N, 15.10%. IR (νmax/cm−1):ν(C=O) 1658s;
ν(N–H) 3051 m; ν(O–H) 3345 m. 1H NMR (δ ppm): 8.79
(1H, HC=N), 13.32 (1H, N–NH), 9.10 (1H, phenolic OH),
6.96–8.64 (7H, aromatic).
H2L2: Total yield = 0.264 g (72%). Anal. Calc. for
C13H10IN3O2: C, 42.53; H, 2.75; N, 11.45. Found: C,
42.57; H, 2.72; N, 11.10%. IR (νmax/cm−1):ν(C=O) 1659s;
ν(N–H) 3045 m; ν(O–H) 3027 m. 1H NMR (δ ppm): 8.78
(1H, HC=N), 12.294 (1H, N–NH), 9.09 (1H, phenolic OH),
6.79–8.78 (7H, aromatic).
H2L3: Total yield = 0.245 g (79%). Anal. Calc. for
C13H9Cl2N3O2: C, 50.35; H, 2.92; N, 13.55. Found: C,
50.12; H, 2.86; N, 13.51%. IR (νmax/cm−1):ν(C=O) 1653s;
ν(N–H) 3027 m; ν(O–H) 3399 m. 1H NMR (δ ppm): 8.81
(1H, HC=N), 12.542 (1H, N–NH), 9.10 (1H, phenolic OH),
7.59–8.79 (6H, aromatic).
Experimental
General Procedures
The starting materials, 5-chloro-2-hydroxybenzaldehyde,
2-hydroxy-5-iodobenzaldehyde, 3,5-dichloro-2-hydroxy-
benzaldehyde, nicotinic acidhydrazide, bis(acetylacetonato)
dioxomolybdenum(VI) complex were supplied by Sigma
Aldrich chemical company. Reagent grade solvents were
dried and distilled prior to use. All other chemicals were
of reagent grade, available commercially and used without
further purification.
Synthesis of Dioxidomolybdenum(VI) Complexes
All the dioxidomolybdenum(VI) complexes were prepared
by the reaction of [MoO2(acac)2] with the corresponding
aroylhydrazones in the presence of donor solvents DMSO
and DMF. The complexes were prepared by the following
methods.
IR spectra were recorded on a JASCO FT-IR-5300 spec-
trometer in the 4000–400 cm−1 range using KBr discs. 1H
NMR spectra were recorded using a Bruker AMX 400 FT-
NMR Spectrometer in DMSO-d6 by using TMS as the ref-
erence. UV–Vis spectroscopic data were recorded in DMF
using Thermo Scientific Evolution 220 model UV–Vis spec-
trophotometer in the 200–900 nm range. Molar conductiv-
ity measurements were carried out at room temperature on
a Systronic model 303 direct reading conductivity meter.
[MoO2L1(DMSO)] (1)
To a methanolic solution of the aroylhydrazone, H2L1
(1.0 mmol, 0.275 g), methanolic solution of [MoO2(acac)2]
(1.0 mmol, 0.326 g) was added. The solution mixture was
refluxed with vigorous stirring for half an hour to get an
orange colored precipitate. To this solution, few drops of
1 3

Journal of Chemical Crystallography
O
O
Fig. 1 Reaction scheme for the
synthesis of the aroylhydra-
zones and their molybdenum
complexes
H
N
NH2 R2
+
MeOH
Reflux
N
H
H
N
R2
N
+
H2O
N
OH
O
HO
H2Ln
R1
R1
Nicotinic acid
hydrazide
Substituted
2-hydroxybenzaldehyde
H2L1 = R1=H, R2= Cl
H2L2 = R1=H, R2= I
H2L3 = R1=R2= Cl
H
N
N
N
R2
N
N
N
R2
N
R2
N
N
[MoO2(acac)2]
[MoO2(acac)2]
O
O
O
O
O
O
HO
DMF
Mo
O
DMSO
Reflux
Mo
R1
R1
Reflux
R1
H2Ln
(n=1-3)
O
O
O
R1=R2= Cl
-2Hacac
R1=H, R2= Cl
O
S
-2Hacac
N
[MoO2(acac)2]
H2O
(from
above
reaction)
DMF
complex 3
complex 1
Reflux
R1=H, R2= I
-2Hacac
N
N
R2
N
O
O
O
DMF
Mo
O
R1
O
H
H
complex 2
Hacac = acetylacetone
DMSO was added and refluxed for another 3 h. An orange
colored solution thus obtained was allowed to stand at room
temperature. Orange block shaped crystals were isolated
after 3 days. The crystals were separated, washed with
methanol and dried in air. Total yield=0.258 g (54%). Anal.
Calc. for C14H12ClMoN5O4S: C, 37.59; H, 2.89; N, 8.69;
S, 6.56 Found: C, 37.55; H, 2.94; N, 8.76; S, 6.68%. IR
(νmax/cm−1):ν(C=N) 1550; ν(Caro–O) 1422; ν(N–N) 1143;
ν(C–O) 1244; ν(Mo=O) 902, 932. 1H NMR (δ ppm): 9.15
(1H, HC=N), 6.96–8.64 (7H, aromatic). Conductivity (10−3
Anal. Calc. for C16H17IMoN4O6: C, 32.70; H, 2.86; N, 9.45.
Found: C, 32.75; H, 2.81; N, 9.50%. IR (νmax/cm−1):ν(C=N)
1569; ν(Caro–O) 1412; ν(N–N) 1197; ν(C–O) 1256;
ν(Mo=O) 920, 939. 1H NMR (δ ppm): 8.941 (1H, HC=N),
6.79–8.77 (7H, aromatic). Conductivity (10− 3 M, DMF):
5.1 mho cm2 mol−1
.
[MoO2L3(DMF)] (3)
To a methanolic solution of the aroylhydrazone, H2L3
(1.0 mmol, 0.310 g), methanolic solution of [MoO2(acac)2]
(1.0 mmol, 0.326 g) was added and refluxed with stirring
for half an hour to get clear yellow colored solution. To
this solution, few drops of DMF was added and reflux was
continued for another 2 h. A yellow colored solution thus
obtained was allowed to stand at room temperature. Yellow
block shaped crystals were isolated after a week. The crystals
were separated, washed with methanol and dried in air. Total
yield=0.300 g (59%). Anal. Calc. for C16H14Cl2MoN4O5:
C, 37.79; H, 2.75; N, 11.24. Found: C, 37.75; H, 2.71; N,
11.10%. IR (νmax/cm− 1):ν(C=N) 1580; ν(Caro–O) 1442;
ν(N–N) 1198; ν(C–O) 1263; ν(Mo=O) 903, 927. 1H NMR
(δ ppm): 8.936 (1H, HC=N), 7.61–8.79 (7H, aromatic).
M, DMF): 4.9 mho cm2 mol−1
.
[MoO2L2(H2O)]·DMF (2)
Methanolic solution of the aroylhydrazone, H2L2
(1.0 mmol, 0.367 g) was added to a methanolic solution of
[MoO2(acac)2] (1.0 mmol, 0.326 g) and the resultant mixture
was refluxed with vigorous stirring for about half an hour to
get clear yellow colored solution. To this solution, few drops
of DMF was added and refluxed for another 3 h resulting
in a yellow colored solution which was allowed to stand at
room temperature. Yellow block shaped crystals were iso-
lated on the following day. The mixture was filtered, washed
with methanol and dried in air. Total yield=0.304 g(52%).
Conductivity (10−3 M, DMF): 3.7 mho cm2 mol−1
.
1 3

Journal of Chemical Crystallography
interactions of molecular crystals. Hirshfeld surface analyses
were carried out and finger print plots were plotted using
the software CrystalExplorer 17.5 [30–33]. Surfaces in 3D
(dnorm) that have been mapped over a range of − 0.328 to
1.5807 Å for 1, −0.682 to 1.3509 Å for 2 and −0.1624 to
1.2845 Å for 3, shape index (−1.0 to 1.0 Å) and curvedness
(− 4.0 to 0.4 Å). The asphericity (Ω) [34] and globularity
values [35] signifying the degree of anisotropy of objects
and deviation from spherical surface, respectively are also
tabulated in Table 1. The expanded 2D fingerprint plots [36]
are displayed in the range of 0.6–2.6 Å viewed with the de
and di distance scales displayed on the graph axes where, de
and di are the nearest distances to the nuclei from outside
and inside the surface to the Hirshfeld surface, respectively.
The red spots over the surface indicate the intercontacts
involved in hydrogen bond [37]. The dark-red spots on the
dnorm surface arise as a result of the short interatomic con-
tacts, i.e., strong hydrogen bonds, while the other intermo-
lecular interactions appear as light-red spots. The intermo-
lecular and hydrogen bond interactions like H⋯H, O⋯H and
C⋯H appear as two distinct spikes of almost equal lengths
overlaid on grey color 2D plots of all atoms appear as blue
patches over grey color 3D plots.
Structure Determination Procedures
Single Crystal Data Analysis
Single crystals of the compounds 1–3 with suitable dimen-
sions were selected for X-ray diffraction measurements
and mounted on a Bruker SMART APEXII CCD diffrac-
tometer, equipped with a graphite crystal, incident-beam
monochromator and a fine focus sealed tube with Mo Kα
(λ = 0.71073 Å) radiation as the X-ray source. The unit
cell dimensions were measured and the data collection was
performed. The programs SAINT and XPREP were used
for data reduction and APEX2 and SAINT were used for
cell refinement [25]. Absorption corrections were carried
out using SADABS based on Laue symmetry using equiva-
lent reflections [26]. The structure was solved by using
SHELXS-97 direct methods and refined by full matrix least-
squares refinement on F2 using SHELXL-2014/7 [27] on a
WinGX software package [28]. The molecular and crystal
structures were plotted using ORTEP-3 [28] and DIAMOND
version 3.2 g [29].
In all the complexes, anisotropic refinements were per-
formed for all non-hydrogen atoms and all H atoms on C
atoms were placed in calculated positions, guided by dif-
ference maps, with C–H bond distances of 0.93–0.96 Å.
H atoms were assigned as Uiso =1.2 Ueq (1.5 for Me). The
hydrogen atoms, H(5A) and H(5B) attached to the oxygen
atom of the coordinated water molecule in 2 were located
from difference maps and their distances were restrained
using DFIX and DANG instructions with distance restraint
of O–H=0.86 0.01 and H⋯H=1.36 0.02 Å followed by
refinement of their displacement parameters.
Results and Discussion
The reaction of [MoO2(acac)2] with three ONO donor aroyl-
hydrazone ligands yielded three new dioxidomolybdenum
complexes. The complexes are stable at room temperature
and are insoluble in most of the organic solvents, but are
readily soluble in DMF and DMSO. Molar conductivity
values suggest the non-conducting nature of the complexes.
The sulfur atom of the coordinated DMSO molecule in
complex 1 was disordered over two closely positioned sets
of sites (C14 and C15) with site occupation factor of about
0.570(3) for the major occupied and 0.430(3) for the minor
occupied sites. The hydrogen atoms attached to the afore-
mentioned carbon atoms and their disordered counterparts
were fixed using HFIX instructions. The distance restrained
to be equal to SADI instructions. The connectivity array was
restrained using DELU instructions. Similarly the coordi-
nated DMF molecule of complex 3 was also disordered over
its two carbon atoms (C(14) and C(15)) with a site occupa-
tion factor of 0.51(4) for the major occupied and 0.49(4) for
the minor occupied sites. The reflections (1 0 0) in 1, (0 1 1),
(0 1 0) and (1 0 1) in 2 and (0 1 0), (0 0 1) in 3 were omitted
owing to bad agreement. The details of the crystal structure
and data refinement are given in Table 1.
Spectral Analysis
Infrared Spectra
IR spectral analysis gave an insight into the structure of the
compounds by providing valuable information regarding the
various functional groups present. The prominent IR bands
in the aroylhydrazones and their respective complexes are
depicted in Table 2.
The IR spectra of the aroylhydrazones exhibited ν(O–H)
bands corresponding to phenolic group in the range
3244–3399 cm− 1 and ν(N–H) bands around 3050 cm− 1
.
All the aroylhydrazones exhibited a strong band around
1650 cm− 1 indicating the presence of a C=O group. This
band confirms the existence of the ligand in the amido
form in the solid state [38]. The C–O stretching band was
observed around 1275 cm−1, whereas the ν(N–N) stretching
Hirshfeld Surface Calculations
Hirshfeld surface analysis serves as a powerful tool for
gaining additional insight into the various intermolecular
vibrations were found between the region 1130–1185 cm−1
.
1 3

Journal of Chemical Crystallography
Table 1 Crystal data and refinement details of 1–3
Parameters
[MoO2L1(DMSO)] (1)
[MoO2L2(H2O)]·DMF (2)
[MoO2L3(DMF)] (3)
Empirical formula
Formula weight
Crystal system
Space group
C14H12ClMoN5O4S
477.74
Monoclinic
P21/c
C16H17I MoN4O6
584.18
Triclinic
C16H14Cl2MoN4O5
509.15
Triclinic
P1
P1
Cell parameters
a (Å)
b (Å)
c (Å)
α (°)
β (°)
10.5241(8)
11.6924(7)
14.7696(10)
90
91.030(2)
90
1817.1(2)
4
1.746
1.014
952
0.35×0.30×0.20
2.222–28.342
−14≤h≤12
−15≤k≤15
−16≤l≤19
9.9769(4)
10.4976(4)
10.7660(5)
70.141(2)
68.268(2)
85.075(2)
984.10(7)
2
1.972
2.276
568
0.35×0.30×0.30
2.785° to 25.049°
−11≤h≤11
−12≤k≤9
−12≤l≤12
8.1333(6)
8.2838(6)
15.4942(12)
81.116(4)
76.704(4)
72.278(3)
963.70(12)
2
1.755
0.994
508
0.35×0.30×0.30
2.68° to 25.00°
−9≤h≤9
−9≤k≤9
−13≤l≤18
γ (°)
Volume(V) (Å3)
Z
Calculated density (ρ) (Mg m−3
)
)
Absorption coefficient, µ (mm−1
F(000)
Crystal size (mm3)
θ (°)
Limiting indices
Reflections collected/unique reflections 13,465/4503
6445/3486
6001/3397
(Rint)
[Rint =0.0242]
25.24 (99.8%)
[Rint =0.0255]
[Rint =0.0187]
Completeness to θ
25.24 (96.9%)
28.00 (98.4%)
Absorption correction
Maximum and minimum transmission
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I>2σ(I)]
Semi-empirical from equivalents Semi-empirical from equivalents Semi-empirical from equivalents
0.817 and 0.706
4503/66/250
1.066
0.505 and 0.455
3437/3/264
1.020
0.728 and 0.702
3344/0/277
0.946
R1 =0.0567
R1 =0.0282
R1 =0.0252
wR2 =0.1485
wR2 =0.0730
wR2 =0.0684
R indices (all data)
R1 =0.0683
R1 =0.0315
R1 =0.0279
wR2 =0.1589
wR2 =0.0765
wR2 =0.0722
Largest difference peak and hole (e Å−3
)
1.551 and −0.668
1.065 and −1.065
0.404 and −0.453
2
2 2
2 2 1/2
R1 =Σ||F0| − |Fc||/Σ|F0|, wR2 =[Σw(F0 -Fc ) /Σw(F0 ) ]
Table 2 Infrared spectroscopic
assignments (in cm−1) of the
aroylhydrazones and their
dioxidomolybdenum(VI)
complexes
Compound ν(C=O) ν(N–H) ν(O–H) ν(C=N) ν(Caro–O) ν(N–N) ν(C–O) ν(C=N)a ν(Mo=O)
H2L1
1658
–
3051
–
3345
–
1604
1550
1611
1569
1604
1580
1470
1422
1442
1412
1470
1442
1131
1143
1185
1197
1184
1198
–
–
–
Complex 1
1244
–
1533
–
902, 932
–
H2L2
1659
–
1653
–
3045
–
3027
–
3244
–
3399
–
Complex 2
H2L3
Complex 3
1256
–
1263
1532
–
1538
920, 939
–
903, 927
In comparison to the IR spectra of aroylhydrazones, the
spectra of complexes were devoid of the bands correspond-
ing to the C=O, N–H and O–H, demonstrating the enoliza-
tion and deprotonation of the hydrazones prior to coordina-
tion to the MoO22+ core. This was further confirmed by the
shift of azomethine band of acylhydrazone observed in the
1604–1611 cm−1 range to lower frequency in the spectra of
complexes, indicating the coordination of azomethine nitro-
gen to the metal centre [39]. The increase in the ν(N–N)
frequency in complexes, due to the increase in double bond
1 3

Journal of Chemical Crystallography
Table 4 NMR spectral assignments, δ (ppm) for the aroylhydrazone
character is another proof for the coordination of aroyl-
hydrazone through the azomethine nitrogen. The ν(C–O)
bands present in the ligands are shifted to lower frequency in
complexes suggesting the coordination of phenolic oxygen.
Similarly the presence of a new band around 1250 cm−1 in
the complexes is assignable to ν(Cenolic–O) stretch. All the
molybdenum complexes exhibited two strong bands in the
region, 900–950 cm−1 assigned to symmetric and asymmet-
ric vibrations of cis-MoO22+ core [40].
ligand and its dioxidomolybdenum(VI) complexes
Compound
Chemical shifts δ (ppm)
Types of protons
N–NH Phenolic O–H HC=N Aromatic
H2L1
13.32 9.10
12.29 9.09
12.54 9.10
8.79
8.78
8.81
9.15
8.94
8.94
6.96–8.64
6.79–8.78
7.59–8.79
6.96–8.64
6.79–8.77
7.61–8.77
H2L2
H2L3
[MoO2L1(DMSO)]
[MoO2L2(H2O)]⋅DMF
[MoO2L3(DMSO)]
–
–
–
–
Electronic Spectra
The electronic spectra of the hydrazones and their com-
plexes were recorded in DMF and the electronic spectral
assignments are given in Table 3. The electronic spectra of
the hydrazones exhibited three spectral bands in the region
300–410 nm corresponding to the n → π*/π → π* transi-
tions. All these bands appear in the spectra of the complexes
with slight variation. In addition, they also exhibit a low
intensity band above 400 nm due to charge transfer from
phenolate oxygen to an empty orbital of the molybdenum
atom (LMCT) [41]. The absence of d-d transition absorp-
tion bands in the visible region confirms the 4d0 electronic
configuration of molybdenum(VI).
All the aromatic proton signals of the hydrazones and
their complexes were observed in the expected range of
δ=6.79–8.77 ppm.
Molecular and Crystal Structures of Molybdenum
Complexes
Complex 1 got crystallized in monoclinic space group, P21/c
whereas complexes 2 and 3 crystallized into triclinic space
group, P1. The asymmetric units of all the complexes con-
sist of a single molecule. In crystal structure of complex 2,
one dimethylformamide molecule was present without any
coordination to the metal centre.
1H NMR Spectra
The 1H NMR spectra of the hydrazones and
their molybdenum(VI) complexes were recorded in
The coordination geometry around molybdenum(VI) in
complexes revealed a distorted octahedral environment with
an NO5 coordination sphere (Figs. 2, 3, 4) where the dian-
ionic tridentate aroylhydrazone ligands, H2L1–3 coordinate
to the molybdenum ion through phenolate oxygen, O(1),
azomethine nitrogen, N(1) and enolate oxygen, O(2). The
equatorial plane was formed by the aforementioned donor
atoms and one of the terminal oxido oxygen atom, O(4)
(in the case of complexes 1 and 2) or O(5) (in the case of
complex 3). The equatorial plane in all the three complexes
are slightly distorted with respect to the planarity (mean
deviation from planarity: 0.0252(2) Å for O1/N1/O2/O4 in
complex 1, 0.0283(4) Å for O1/N1/O2/O4 in complex 2 and
0.0263(2) Å for O1/N1/O2/O5 in complex 3). Finally the
oxygen atom of a neutral monodentate solvent molecule,
1
DMSO. The H NMR spectra of the free hydrazones dis-
played singlets in the δ = 12.29–13.32 ppm range and
δ = 9.09–9.10 ppm range due to the iminolic proton (HN-
N) and phenolic OH proton (OHphenolic) respectively.
Himine–C=N protons resonate at δ= 8.78–8.81 ppm range
(Table 4).
The OH and NH proton signals disappeared upon com-
plexation with molybdenum, indicating the deprotonation
of the corresponding protons and subsequent coordination
of the respective oxygen atoms to the molybdenum cen-
tre. The participation of the imine nitrogen in complexa-
tion was indicated by an appreciable downfield shift of the
azomethine proton signal in the corresponding complexes.
Table 3 Electronic spectral
assignments, λ (nm) for the
aroylhydrazone ligand and
the dioxidomolybdenum(VI)
complexes
Component
λ (nm) (ε/M−1 cm−1
LMCT
)
n→π*/π→π*
H2L1
–
300 (11,580), 336 (10,110), 403 (4770)
301 (16,410), 335 (12,840)
338 (12,947), 409 (7550)
404 (12,350)
304 (8700), 340 (5820), 407 (2740)
405 (7560)
[MoO2L1(DMSO)]
H2L2
409 (1680)
-
475 (3710)
-
478 (6413)
[MoO2L2(H2O)]⋅DMF
H2L3
[MoO2L3(DMSO)]
1 3

Journal of Chemical Crystallography
Fig. 2 ORTEP plot of
[MoO2L1(DMSO)] (1) along
with atom numbering scheme of
the non-hydrogen atoms. Dis-
placement ellipsoids are drawn
at 50% probability
Fig. 3 ORTEP plot of
[MoO2L2(H2O)]·DMF (2) along
with atom numbering scheme of
the non-hydrogen atoms. Dis-
placement ellipsoids are drawn
at 50% probability
Fig. 4 ORTEP plot of
[MoO2L3(DMF)] (3) along with
atom numbering scheme of the
non-hydrogen atoms. Displace-
ment ellipsoids are drawn at
50% probability
DMSO (in 1) or H2O (in 2) or DMF (in 3) completes the
coordination sphere of the distorted octahedron. The oxygen
atoms of these coordinated solvent molecules and the other
oxido oxygen atom, O(3) (in the case of complexes 1 and 2)
and O(4) (in the case of complex 3) occupies the axial posi-
tions. Moreover the axial positions defined by these donor
1 3

Journal of Chemical Crystallography
atoms forms O(3)–Mo(1)–O(5) bond angle of 169.51(2)°
and 170.95(14)° for complexes 1 and 2 respectively and
O(3)–Mo(1)–O(4) bond angle of 169.8(9)° for complex 3.
The cis configuration acquired by the MoO22+ moiety can
be confirmed from the O(3)–Mo(1)–O(4) bond angle of
105.2(2)° for complex 1, O(4)–Mo(1)–O(3) bond angle of
106.3(2)° for complex 2 and O(4)–Mo(1)–O(5) bond angle
of 105.9(1)° for complexes 3. The tridentate hydrazone
ligands,[L1−3 2−
bind to the molybdenum atom in a meridi-
]
onal fashion forming a five membered and a six membered
metallocycle involving the MoO22+ moiety.
The hydrazone ligands coordinate to the metal centre in
their dianionic iminolate form which is evident from the
C(8)–O(2) bond length values (1.353(7) Å for complex 1,
1.306(4) Å for complex 2 and 1.316(3) Å for complex 3)
which are closer to the C–O single bond distance of 1.311 Å
[42] rather than the reported C=O distance of 1.22(2) for
a similar compound [43]. This fact was further substanti-
ated by the C(8)–N(2) bond length values of the complexes
[1.293(6) Å for 1, 1.303(5) Å for 2 and 1.290(4) Å for 3].
The Mo–Ooxido bond distances were closer to the already
reported Mo=O bond distances in these type of complexes
[44]. The longer Mo=O bond (1.700(4) Å vs 1.692(4) Å for
complex 1, 1.700(3) Å verses 1.683(3) Å for complex 2 and
1.696(2) Å vs 1.681(2) Å for complex 3) are found to be
trans to the neutral monodentate ligands like DMSO, H2O
and DMF in complexes 1, 2 and 3 respectively [45].
Fig. 5 Various types of hydrogen bonding interactions in the mole-
cules of [MoO2L2(H2O)]·DMF (2)
Another significant observation regarding the crys-
tal structures of these complexes is that, similar to other
reported molybdenum complexes [44, 46–49], the oxygen
atom of the respective coordinated neutral monodentate
ligands in these complexes are attached to molybdenum
metal ion at a greater bond length (2.314(3) Å for complex
1, 2.329(3) Å for complex 2 and 2.340(3) Å for complex
3) compared to the other Mo–O bonds such as enolate and
phenolate distances indicating a weak coordination of these
ligands to the metal centre which can act as a potential sub-
strate binding site in catalytic reactions. Among the three
complexes, complex 3 can serve as an appropriate candidate
for this purpose since it has got the weakest (or longest)
Mo–O distance.
Fig. 6 Diagram showing various hydrogen bonding and π–π interac-
tions in the complex [MoO2L1(DMSO)] (1)
The six membered metallocycles (Mo1/O1/C1/C6/C7/
N1) in the complexes are puckered with puckering ampli-
tudes of Q = 0.138(3) Å, 0.314(3) Å and 0.3446(18) Å
and φ = 4(2)°, 23.5(8)° and 30.5(5)° respectively for com-
plexes 1, 2 and 3 respectively. Among the various classi-
cal and weak non-classical hydrogen bonding interactions
present in these complexes, the most significant one is
that exhibited by the molecules of complex 2 (Fig. 5) in
which the oxygen atom of the DMF molecule, O(6) present
outside the coordination sphere is involved in bifurcated
hydrogen bonding interaction with the hydrogen atom of
the coordinated water molecule, H(5B) and the aldehydic
proton, H(7). This hydrogen bonding interactions con-
nect two independent molecules along with the solvent,
DMF. The crystal packing of the complexes were further
stabilized by various types of π-π interactions which are
presented in Figs. 4, 6, 7 and 8. The interaction parameters
for the hydrogen bonding interactions and π-π interactions
are summarized in the Tables 5, 6 and 7.
1 3

Journal of Chemical Crystallography
Fig. 7 π–π interactions present in the molecules of [MoO2L2(H2O)]·DMF (2)
Fig. 8 π–π interactions
present in the molecules of
[MoO2L3(DMF)] (3)
lengths (Fig. 10). Bright red shaded area in the dnorm and in
shape index (Fig. 9) represents the major hydrogen bonding
interactions like C–H⋯O and C–H⋯N in the crystals and
the contributions made by individual molecular contacts to
the 2D-fingerprint plots are as follows. C⋯H (10.9% for 1,
5.5% for 2 and 6.9% for 3) exhibits the maximum and C⋯O
Hirshfeld Surface Analysis
Hirshfeld surfaces 2D fingerprint, 3D dnorm, shape index,
curvedness were plotted using Crystal Explorer Version 17.5
[30–33]. 2D fingerprint plots portray the molecular hydrogen
bonds in the crystals as two distinct spikes of almost equal
1 3

Journal of Chemical Crystallography
Table 5 Hydrogen bonding interactions presenting complexes 1–3
Compound
D–H⋯A
d(D–H) (Å)
d(H⋯A) (Å)
d(D⋯A) (Å)
< (DHA) (°)
[MoO2L1(DMSO)] (1)
C(12)–H(12)⋯O(3)a
C(10)–H(10)⋯N(2)
C(2)–H(2)⋯O(1)
0.93
0.93
0.93
0.93
0.86(4)
0.85(3)
0.93
0.93
0.93
2.57
2.46
2.62
2.58
1.86(4)
2.00(3)
2.62
2.47
2.40
3.186(9)
2.798(7)
3.250(5)
3.477(5)
2.699(5)
2.847(4)
3.310(5)
2.802(4)
2.904(4)
3.500(3)
3.476(18)
124
101
[MoO2L2(H2O)]·DMF (2)
[MoO2L3(DMF)] (3)
125.9
161.7
168(4)
173(4)
131.3
101
114
170
155
C(7)–H(7)⋯O(6)
O(5)–H(5B)⋯O(6)b
O(5)–H(5A)⋯N(2)b
C(12)–H(12)⋯O(3)c
C(10) –H(10)⋯N(2)
C(16)–H(16)⋯O(5)
C(14A)–H(14F)⋯O(1)d
C(15A)–H(15F)⋯O(4)e
0.96
0.96
2.55
2.58
Equivalent position codes: a=1−x, ½ + y, ½−z; b=−x, 2−y, 1−z; c=1−x, 2−y, 1−z; d=−x, −y, 1−z; e=x, −1+y, z
Table 6 Cg⋯Cg (π⋯π)
Compound
π⋯π interactions
Cg(I)⋯Cg(J)
interactions in 1–3
Cg⋯Cg (Å)
α°
β°
γ°
[MoO2L1(DMSO)] (1)
Cg(3)⋯Cg(4)a
Cg(3)⋯Cg(4)b
Cg(3)⋯Cg(3)c
Cg(3)⋯Cg(4)d
3.686(3)
3.691(3)
3.871(3)
3.598(17)
1.4(3)
5.3(2)
0
19.1
25.6
21.9
16.9
17.7
25.6
21.8
16.4
[MoO2L2(H2O)]·DMF (2)
[MoO2L3(DMF)] (3)
11.92(14)
Equivalent position codes: a = 1 − x, ½ + y, ½ − z; b = − x, 2 − y, 1 − z; c = 1 − x, 2 − y, 1 − z; d =
1 + x, − 1 + y, z
α (°) = Dihedral angle between planes I and J
β (°) = Angle between Cg(I)⋯Cg(J) vector and Cg(J) perp
γ (°) = Angle between Cg(I)⋯Cg(J) vector and Cg(I) perp
Cg, centroid of the ring
Cg–Cg = Distance between ring centroids
Cg(3) = N(3)/C(10)/C(9)/C(13)/C(12)/C(11), Cg(4) = C(1)/C(2)/C(3)/C(4)/C(5)/C(6)
Table 7 Non-bonding
O−H⋯π interaction
O−C⋯π interaction
O−H(I)⋯Cg(J)
H⋯Cg (Å)
O–H⋯Cg (Å)
O⋯Cg (Å)
interactions present in complex
O(5)–H(5A)⋯Cg(1)a
O(5)–H(5B)⋯Cg(1)a
O−C(I)⋯Cg(J)
2.74(4)
2.95(4)
C⋯Cg (Å)
3.946(5)
72(3)
58(3)
O–C⋯Cg (Å)
68.8(3)
2.599(3)
2.599(3)
O⋯Cg (Å)
3.685(6)
2
O(6)–C(16)⋯Cg(4)b
Equivalent position codes: a = x, y, z; b = − x, 1 − y, 1 − z
(1.2% for 1, 2.1% for 2 and 2.6% for 3) exhibits the mini-
mum 2D surfaces generated in the carbon and other elements
interactions of a molecule (Fig. 10).
6.7% for 2 and 4.7% for 3) on comparison to all atoms
2D surface of the molecule. These interactions appear as
bow-tie patterns in the shape index (Fig. 9). The Hirshfeld
surface analysis was also helpful in visualizing the promi-
nent hydrogen bonding interaction present in the crystal
structure of complex 2 which is portrayed in Fig. 11.
The asphericity and globularity values obtained for
these complexes (Table 8) indicate that they deviate from
symmetry and spherical surface to some extent.
Similarly the hydrogen and other elements contribution
H⋯H (31.0% for 1, 34.1% for 2 and 27.6% for 3), O⋯H
(23.4% for 1, 25.1% for 2 and 20.4% for 3) and N⋯H (5.7%
for 1, 4.9% for 2 and 5.7% for 3) 2D surfaces are observed.
Among the various interaction between the same elements,
maximum contribution was made by C⋯C (6.8% for 1,
1 3

Journal of Chemical Crystallography
Fig. 9 Hirshfeld surfaces for
1, 2 and 3 mapped with a dnorm
over a fixed color scale of
−0.16 (red) to 1.58 Å (blue)
b shape index and c curved-
ness. ‘Bow-tie’ pattern (marked
by black circles) on the HS’s
indicating the relative percent-
age of π⋯π interactions. (Color
figure online)
Conclusion
Supplementary Materials
Three new dioxidomolybdenum(VI) complexes of three
different aroylhydrazones derived from nicotinic acid
hydrazide were successfully synthesized and spectrally
CCDC numbers 1854447-1854449 contain the supple-
mentary crystallographic data for this paper. These data
can be obtained, free of charge, at http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge Crys-
tallographic Data Centre (CCDC) at 12 Union Road,
Cambridge CB2 1EZ, UK. Fax: +44(0) 1223 336 033 and
e-mail: deposit@ccdc.cam.ac.uk.
1
characterized (FT-IR, UV–Vis, H NMR). The crystal
structures of the synthesized compounds were confirmed
from single crystal X-ray diffraction methods. All the three
complexes hold a labile sixth coordination site that can be
utilized for substrate binding.
1 3

Journal of Chemical Crystallography
Fig. 10 2D finger print plots with de and di ranging from 0.6 to 2.6 Å for 1–3 and their major decomposition plots
Fig. 11 a Classical bifurcated hydrogen bonding interaction present in 2 involving oxygen atom (O6) of DMF molecule and hydrogen atom of
coordinated water (H5B) and aldehydic proton (H7) b viewing the same short contact in Hirshfeld surface with external molecules
1 3

Journal of Chemical Crystallography
18. Dinda R, Sengupta P, Ghosh S, Sheldrick WS (2003) Eur J Inorg
Chem 363
Table 8 Attributes of Hirshfeld surfaces of complexes 1–3
Complex Volume (Å3) Area (Å2) Globularity
Asphericity (Ω)
19. Seena EB, Kurup MRP (2007) Polyhedron 26:3595
20. Purohit S, Koley AP, Prasad LS, Manoharan PT, Ghosh S (1989)
Inorg Chem 283:735
(G)
1
2
3
445.35
482.19
472.73
395.98
442.66
424.41
0.712
0.672
0.691
0.170
0.179
0.133
21. Dinda R, Ghosh S, Falvello LR, Tomas M, Mak TCW (2006)
Polyhedron 25:2375
22. Mathew N, Kurup MRP (2011) Spectrochim Acta Part A 78:1424
23. Sudheer R, Sithambareshan M, Sajitha NR, Manoj E, Kurup MRP
(2005) ActaCryst. E71:702
24. Aravindakshan AA, Joseph B, Kala UL, Kurup MRP (2017) Poly-
hedron 123:206
Acknowledgements The authors thank the Cochin University of Sci-
ence and Technology, Kochi, India for financial assistance and thank-
ful to the Sophisticated Analytical Instrumentation Facility, Cochin
University of Science and Technology, Kochi, India for elemental
ananlysis, 1H NMR and Single crystal X-ray diffraction measurements.
25. SMART and SAINT, Area Detector Software Package and SAX
Area Detector Integration Program, Bruker Analytical X-ray,
Madison, 1997
26. SADABS, Area Detector Absorption Correction Program; Bruker
Analytical Xray; Madison, 1997
27. Sheldrick GM (2015) Acta Crystallogr C 71:3
28. Farrugia LJ (2012) J Appl Crystallogr 45:849
29. Brandenburg K (2011) Diamond version 3.2 g. Crystal Impact
GbRBonn, Germany
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Affiliations
T. M. Asha1 · M. R. P. Kurup1,2
2
T. M. Asha
Department of Chemistry, School of Physical Sciences,
ashatm1989@gmail.com
Central University of Kerala, Tejaswini Hills, Periye,
Kasaragod 671 316, India
1
Department of Applied Chemistry, Cochin University
of Science and Technology, Kochi, Kerala 682 022, India
1 3