Syntheses, characterization, and crystal structures of few dioxomolybdenum(VI) complexes incorporating tridentate hydrazones

Article abstract of DOI:10.1016/j.poly.2014.12.017

Three new cis-dioxomolybdenum(VI) complexes, [MoO₂(L¹)(CH₃OH)] (1), [MoO₂(L²)(CH₃OH)] (2), and [MoO₂(L³)(CH₃OH)] (3) have been synthesized using three diffe

Full text of DOI:10.1016/j.poly.2014.12.017

Polyhedron 88 (2015) 63–72
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses, characterization, and crystal structures of few
dioxomolybdenum(VI) complexes incorporating tridentate hydrazones
Madhusudan Nandy a, Shyamapada Shit b, Corrado Rizzoli c, Guillaume Pilet d, Samiran Mitra a,
⇑
a Department of Chemistry, Jadavpur University, Raja S.C. Mullick Road, Kolkata 700032, West Bengal, India
b Department of Chemistry, Jalpaiguri Government Engineering College, Jalpaiguri 735102, West Bengal, India
c
´
Dipartimento di Chimica, Universita degli Studi di Parma, Parco Area delle Scienze, 17/A, I-43124 Parma, Italy
d Université de Lyon, Université Claude Bernard Lyon 1, Laboratoire des Multimatériaux et Interfaces (LMI), UMR 5615 CNRS-UCBL, Groupe de Cristallographie et Ingénierie
Moléculaire, Bât. Chevreul, 43 Bd du 11 Novembre, 1918, 69622 Villeurbanne Cedex, France
a r t i c l e i n f o
a b s t r a c t
Three new cis-dioxomolybdenum(VI) complexes, [MoO2(L1)(CH3OH)] (1), [MoO2(L2)(CH3OH)] (2),
and [MoO2(L3)(CH3OH)] (3) have been synthesized using three different tridentate hydrazone Schiff
bases, (E)-N0-(4-oxopentan-2-ylidene)isonicotinohydrazide (H2L1), (E)-N0-(4-oxo-4-phenylbutan-2-
ylidene)isonicotinohydrazide) (H2L2), and (E)-N0-(2,3-dihydroxybenzylidene)benzohydrazide (H2L3),
respectively. The ligands and their metal complexes were characterized by different physicochemical
techniques. The molecular structures of the complexes were conclusively established by single crystal
X-ray diffraction which reveals that six coordinated molybdenum(VI) atom adopts distorted octahedral
geometry. The tridentate hydrazone coordinates to the metal centers in their di-anionic form. The crystal
packing of the complexes also reveals that classical hydrogen bonding between adjacent units result in
stair-like 1D structure for 1 while an interlocked dimer for 2 and 3, respectively. Crystal packing also
reveals that further non-classical interactions lead to 2D (for 1 and 3) and 3D (for 2) supramolecular
structures in their solid state. Hirshfeld surface analyses were carried out to get insight to the intermo-
lecular interactions. Thermal stabilities and electrochemical behavior of the complexes have also been
investigated.
Article history:
Received 26 August 2014
Accepted 17 December 2014
Available online 24 December 2014
Keywords:
Dioxomolybdenum(VI) complexes
Crystal structure
Hirshfeld surface analysis
Electrochemistry
Thermogravimetric analysis
Ó 2015 Published by Elsevier Ltd.
1. Introduction
blocks either as metalloligands (linkers) or as nodes (connectors)
in the crystal engineering of polymeric frameworks [15].
Metal complexes involving supramolecular interactions are
attracting the interest of the researchers due to their wide variety
of properties, such as magnetic [1], catalytic [2], redox [3], electri-
cal [4], sensing [5] or non-liner optical [6] and therefore appear as
very attracting materials in connection with several possible appli-
cations [7]. They can self-assemble as different suitable building
blocks either in solution or in solid states or in both [8]. Therefore
metal complexes with coordinatively unsaturated metal atoms or
those with labile coordination sites achieved special interest [9].
In this context, hydrazone Schiff bases acquired considerable
attention due to their versatile coordination modes [10] and
potential for leading different kinds of supramolecular interactions
[11]. Numerous versatile complexes of different geometries and
nuclearities [12] are generated by critically designing the aryl-
hydrazone Schiff base ligands [13], exhibiting different weak inter-
actions [14]. These complexes also serve as potential building
In this respect, molybdenum-Schiff base complexes also
gained interest due to their potential to construct varieties of
supramolecular architectures [16]. Moreover, due to its various
stable and accessible oxidation states and versatile coordination
chemistry [17] molybdenum plays a significant role in industrial
[18] and biological reactions [19]. Particularly, molybdenum(VI)
complexes containing a cis-[MoO2]2+ core became a fascinating
class of compounds and extensively studied due to their easy
preparation, structural flexibilities, and high stabilities [20,21].
Moreover, molybdenum(VI) complexes have attracted consider-
able interest due to their relevance to the active sites of the
majority of molybdo-enzymes [22,23]. Oxo-peroxo and dioxo
complexes of molybdenum(VI) with polydentate nitrogen, sulfur
and oxygen donor ligands are considered valuable models for
the active site of several molybdenum enzymes [24–26]. They
have been shown to be very effective for oxygen transfer reac-
tions of a wide class of substrates, including olefin epoxidation,
oxidation of saturated hydrocarbons, oxidation of alcohol, and
oxidation of sulfides [27–30].
⇑
Corresponding author. Tel.: +91 33 24146666x2779; fax: +91 33 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
http://dx.doi.org/10.1016/j.poly.2014.12.017
0277-5387/Ó 2015 Published by Elsevier Ltd.

64
M. Nandy et al. / Polyhedron 88 (2015) 63–72
Herein we report the syntheses, spectral and structural
subsequent evaporation of this eluent yielded the ligand in solid
form, which was dried and stored in vacuo over fused CaCl2 for sub-
sequent use. Yield: 0.935 g (80%). Anal. Calc. for C11H13N3O2 (MW:
219.24): C, 60.26; H, 5.88; N, 19.17. Found: C, 60.20; H, 5.80; N,
19.14%. 1H NMR (CDCl3, 300 MHz). d 1.95 (s, 3H), 2.03 (s, 3H),
2.84 (d, J = 18.5 Hz, 1H), 3.08 (d, J = 18.5 Hz, 1H), 7.64–7.69 (m,
2H), 8.72 (brs, 2H) ppm (Scheme 2; see also Fig. S1 (ESI)). ESI-
MS, m/z: [M+H]+ = 220.1132 (Fig. S7 (ESI)). IR bands (KBr, cmꢀ1):
3392–3226 (N–H str.), 2924–2840 (C–H str.), 1620 (C@N). UV–
characterizations of three new dioxomolybdenum(VI) complexes,
[MoO2(L1)(CH3OH)] (1), [MoO2(L2)(CH3OH)] (2) and [MoO2(L3)
(CH3OH)] (3) derived from three different ONO-donor hydrazone
ligands; H2L1 ((E)-N0-(4-oxopentan-2-ylidene)isonicotinohydrazide),
H2L2 ((E)-N0-(4-oxo-4-phenylbutan-2-ylidene)isonicotinohydrazide),
and H2L3 ((E)-N0-(2,3-dihydroxybenzylidene) benzohydrazide)
respectively (Scheme 1). Structural characterization reveals that
molybdenum atom displays octahedral geometry associated with
different degrees of distortion. The tridentate ligands bind the metal
atoms as di-negative ion in these complexes. Crystal packing
supported by Hirshfeld surface analyses reveals that intermolecular
interactions play a dominant role on the solid state structure of the
complexes. Thermal stabilities and redox behavior of the complexes
are also investigated.
Vis bands (k/nm): 274 (p ?
p⁄), 330 and 412 (n ? p⁄).
2.2.2. Synthesis of (E)-N0-(4-oxo-4-phenylbutan-2-ylidene)
isonicotinohydrazide (H2L2)
Solid benzoylacetone (0.811 g, 5 mmol) was added to a 10 mL
methanolic solution of isonicotinic hydrazide (0.686 g, 5 mmol)
and refluxed for 2 h. The resulting mixture turned to orange color
indicating the formation of the ligand. The mixture was cooled to
room temperature. The bright yellow precipitate was filtered and
washed with cold methanol and dried in air. Yield: 1.180 g (79%).
Anal. Calc. for C16H15N3O2 (MW: 281.31): C, 68.31; H, 5.37; N,
14.94. Found: C, 68.28; H, 5.33; N, 14.95%. 1H NMR (CDCl3,
300 MHz): d 2.09 (s, 3H), 3.01 (d, J = 18.7 Hz, 1H), 3.34 (d,
J = 18.7 Hz, 1H), 7.28–7.36 (m, 1H), 7.39–7.41 (m, 2H), 7.45 (d,
J = 7.1 Hz, 2H), 7.76–7.78 (m, 2H), 8.73–8.74 (m, 2H) ppm
(Scheme 2; see also Fig. S2 (ESI)). ESI–MS, m/z:
[M+H]+ = 282.1317 (Fig. S8 (ESI)). IR bands (KBr, cmꢀ1): 3477(N–
H), 3224–3060 (C–H str.), 1650 (C@N). UV–Vis bands (k/nm): 272
2. Experimental
2.1. Materials
Acetylacetone, benzoylacetone, 2,3-dihydroxybenzaldehyde,
isonicotinic hydrazide, and benzhydrazide were purchased from
sigma Aldrich Co. MoO2(acac)2 was synthesized following the liter-
ature procedure [31]. All other chemicals and solvent used for the
syntheses were of AR grade and used without further purification.
(p ?
p⁄), 332 and 410 (n ? p⁄).
2.2. Syntheses of ligands and complexes
2.2.1. Synthesis of (E)-N0-(4-oxopentan-2-
2.2.3. Synthesis of (E)-N0-(2,3-dihydroxybenzylidene)benzohydrazide
(H2L3)
ylidene)isonicotinohydrazide (H2L1)
A 10 mL methanolic isonicotinic hydrazide (0.686 g, 5 mmol)
was added to 10 mL methanolic acetylacetone (0.501 g, 5 mmol).
The mixture was refluxed with vigorous stirring for 2 h and the col-
orless solution changed to yellow, indicating the formation of the
Schiff base ligand. The resulting solution was cooled and analyzed
by TLC, which revealed the presence of some starting materials
along with the Schiff base. The Schiff base ligand was isolated by
column chromatography over silica gel (SRL) 60–120 mesh size,
using a mixture of light petroleum and ethyl acetate (v/v, 2:3);
A 10 mL methanolic solution of isonicotinic hydrazide (0.686 g,
5 mmol) was added to a 10 mL methanolic solution of 2,3-dihy-
droxybenzaldehyde (0.691 g, 5 mmol). The resulting solution was
refluxed with vigorous stirring for 2 h and consequently the mix-
ture changed its color from pale yellow to bright yellow. The solu-
tion was then allowed to cool to room temperature. The pale
yellow crystals were separated by filtration and washed with cold
methanol and dried in air. Yield: 1.020 g (75%). Anal. Calc. for
C14H12N2O3, (MW: 256.257): C, 65.62; H, 4.72; N, 10.93. Found:
Scheme 1. Synthetic scheme of the hydrazone ligands (H2L1, H2L2, and H2L3).

M. Nandy et al. / Polyhedron 88 (2015) 63–72
65
Scheme 2. Proton numbering scheme of H2L1, H2L2, and H2L3.
C, 65.60; H, 4.68; N, 10.90%. 1H NMR (DMSO-d6, 300 MHz): d 6.74
(d, J = 7.7 Hz, 1H), 6.84 (d, J = 6.8 Hz, 1H), 6.95 (d, J = 6.9 Hz, 1H),
7.54 (d, J = 7.54 Hz, 1H), 7.59 (d, J = 7.0, 1H), 7.93 (d, J = 7.5 Hz,
2H), 8.58 (s,1H), 9.20 (s, 1H), 11.14 (s, 1H), 12.09 (s, 1H) ppm
(Scheme 2; see also Fig. S3 (ESI)). ESI-MS, m/z:
[M+H]+ = 257.1000, [M+Na]+ = 279.0808 (Fig. S9 (ESI)). IR bands
(KBr, cmꢀ1): 3411 (O–H), 3193–2921 (C–H str.), 1680 (C@O),
(s, 1H), 7.42–7.48 (m, 3H), 7.79–7.82 (m, 4H), 8.59 (brs, 2H) ppm
(see also Fig. S5 (ESI)). ESI-MS, m/z: [MoO2(C16H15N3O2)+H]+ =
410.0175 (Fig. S11 (ESI)). IR bands (KBr, cmꢀ1): 3454 (O–H), 1617
(C@N), 944 and 898 (cis O@Mo@O). UV–Vis bands (k/nm): 230
and 272 (p ?
p⁄), 330 (n ? p⁄), 445 (LMCT).
2.2.6. Synthesis of [MoO2(L3)(CH3OH)] (3)
mixture of MoO2(acac)2 (0.164 g, 0.5 mmol) and H2L3
1623(C@N). UV–Vis bands (k/nm): 280 (
p ?
p⁄), 327 and 390
A
(n ? p⁄).
(0.124 g, 0.5 mmol) in acetonitrile (20 mL) was refluxed for 2 h. A
small amount of precipitate was formed during reflux. The precip-
itate was filtered off and the resulting filtrate was kept undisturbed
at 10 °C for slow evaporation of the solvent. X-ray diffraction qual-
ity orange colored plate shaped single crystals of 3 were obtained
after twenty days. The crystals were filtered, washed with diethyl
ether and dried in air. Yield: 0.155 g (75% with respect to metal
substrate). Anal. Calc. for C15H14MoN2O6 (MW: 414.23): C, 43.49;
H, 3.41; N, 6.76. Found: C, 43.45; H, 3.39; N, 6.74%. 1H NMR
(DMSO-d6, 300 MHz): d 6.90 (t, 1H), 7.09 (d, 1H), 7.19(d, 1H),
7.49–7.59 (m, 2H), 7.99 (t, 2H), 8.94 (s, 1H), 9.5 (s, 1H) ppm (see
also Fig. S6 (ESI)). ESI-MS (m/z): [MoO2(C14H10N2O3)+H]+ =
384.9858 (Fig. S12 (ESI)). IR bands (KBr, cmꢀ1): 3341 (O–H), 1613
(C@N), 921 and 907 (cis-O@Mo@O). UV–Vis bands (k/nm): 225
2.2.4. Synthesis of [MoO2(L1)(CH3OH)] (1)
A
mixture of MoO2(acac)2 (0.164 g, 0.5 mmol) and H2L1
(0.110 g, 0.5 mmol) in methanol (20 mL) was refluxed for 3 h. A
small amount of precipitate was formed during reflux which was
separated by filtration. The filtrate was kept undisturbed at room
temperature for slow evaporation of the solvent. X-ray diffraction
quality dark red colored prism shaped single crystals of 1 were
obtained after fifteen days. The crystals were filtered, washed with
diethyl ether and dried in air. Yield: 0.161 g (85% with respect to
metal substrate). Anal. Calc. for C12H15MoN3O5 (MW: 377.21): C,
38.21; H, 4.01; N, 11.14. Found: C, 38.19; H, 3.98; N, 11.12%. 1H
NMR (CDCl3, 300 MHz). d 2.13 (s, 3H), 2.42 (s, 3H), 3.49 (s, 3H),
5.53 (s, 1H), 7.73 (d, J = 4.8 Hz, 2H), 8.52 (d, J = 4.6 Hz, 2H) ppm
(see also Fig. S4 (ESI)). ESI-MS, m/z: [MoO2(C11H11N3O2)+H]+ =
348.0026 (Fig. S10 (ESI)). IR bands (KBr, cmꢀ1): 3410 (O–H),
1601(C@N), 943 and 905 (cis O@Mo@O). UV–Vis bands (k/nm):
and 295(p ?
p⁄), 362(n ? p⁄).
2.3. Physical measurements
232 and 271(p ?
p⁄), 313 (n ? p⁄), 416 (LMCT).
Elemental analyses (carbon, hydrogen, and nitrogen) were car-
ried out using a Perkin-Elmer 2400 II elemental analyzer. The Fou-
rier transform infrared spectra of the ligands and complexes were
recorded (4000–400 cmꢀ1) on a Perkin-Elmer RX I FT-IR spectro-
photometer with KBr disc. The electronic spectra were recorded
on a Perkin-Elmer Lambda 40 UV–Vis spectrophotometer using
spectroscopic grade methanol. 1H NMR spectra of H2L1, H2L2, 1
and 2 were recorded on a Bruker 300 MHz FT-NMR spectrometer
using tetramethylsilane as internal standard in CDCl3 while 1H
NMR spectra of H2L3 and 3 were recorded on the same instrument
with the same internal standard in DMSO-d6. The positive ion ESI-
MS were performed in a QTOF micro mass spectrometer using
spectroscopic grade methanol. Electrochemical measurements
were performed using a PAR VersaStat-potentiostat/Galvanostat
2.2.5. Synthesis of [MoO2(L2)(CH3OH)] (2)
mixture of MoO2(acac)2 (0.164 g, 0.5 mmol) and H2L2
A
(0.141 g, 0.5 mmol) in methanol (20 mL) was stirred for 1 h at
50 °C. A small amount of red precipitate was formed during stirring
which was filtered off and the resulting filtrate was kept undis-
turbed at room temperature for slow evaporation of the solvent.
X-ray diffraction quality dark red colored prism shaped single crys-
tals of 2 were obtained after twelve days. The crystals were filtered,
washed with diethyl ether and dried in air. Yield: 0.154 g (70% with
respect to metal substrate). Anal. Calc. for C17H19MoN3O5 (MW:
439.28): C, 46.24; H, 4.34; N, 9.52. Found: C, 46.22; H, 4.30; N,
9.48%. 1H NMR (CDCl3, 300 MHz) d 2.56 (s, 3H), 3.49 (s, 3H), 6.28

66
M. Nandy et al. / Polyhedron 88 (2015) 63–72
II electrochemical analysis system under dry argon using conven-
tional three electrode configurations in acetonitrile with tetrabu-
tylammonium perchlorate as the supporting electrolyte.
Platinized platinum millielectrode and saturated calomel electrode
(SCE) were used as working and reference electrodes, respectively,
along with a platinum counter electrode in cyclic voltammetry
performed at a scan rate of
m
= 150 mV sꢀ1. Thermogravimetric
analyses were carried out at a heating rate of 10 °C/min with a
Mettler-Toledo Star TGA/SDTA-851e thermal analyzer system in a
dynamic atmosphere of N2 (flow rate: 40 mL minꢀ1), the sample was
in an alumina crucible and the temperature range was 25–800 °C.
2.4. X-ray crystallography
Diffraction quality prism shaped red crystals of 1 and 2 were
mounted on Bruker APEX-II CCD diffractometer, equipped with
Fig. 1. An ORTEP view of
probability level.
1
with displacement ellipsoids drawn at the 50%
graphite monochromated Mo K
a radiation (k = 0.71073 Å) from a
fine focus sealed tube radiation source while a plate shaped orange
crystal of 3 was mounted on an Oxford Diffraction Gemini diffrac-
tometer equipped with graphite monochromated Mo Ka radiation
(k = 0.71069 Å) from a rotating anode radiation source. Intensity
data for 1, 2 and 3 were collected at room temperature using the
x
scan technique. Multi-scan absorption corrections were applied
empirically to the intensity values (Tmax = 0.938 and Tmin = 0.855
for 1, Tmax = 0.940 and Tmin = 0.878 for 2) using SADABS [32] while
for 3 analytical absorption correction was applied to the intensity
values (Tmax = 0.940 and Tmin = 0.878) [33]. Data reductions for 1
and 2 were performed using program SAINT [34] while for 3 the
CRYSALIS [34] program was used. All the structures were solved by
direct methods using the program SIR-97 [35] and refined with
full-matrix least-squares based on F2 using program SHELXL-97
[36] (for 1 and 2) or CRYSTALS (for 3) [34]. All non-hydrogen atoms
Fig. 2. An ORTEP view of
probability level.
2 with displacement ellipsoids drawn at the 50%
Table 1
Crystal data and structure refinement parameters for 1, 2 and 3.
Parameters for
1
2
3
Empirical formula
Formula weight
Crystal dimension (mm3)
Crystal system
Space group
a (Å)
C
12H15MoN3O5
C17H17MoN3O5
439.28
C15H14MoN2O6
414.23
377.21
0.07 ꢁ 0.13 ꢁ 0.26
monoclinic
Cc (No. 9)
14.2744(8)
7.6972(4)
14.1195(8)
90
112.3488(7)
90
1434.82(14)
4
0.09 ꢁ 0.12 ꢁ 0.22
monoclinic
P21/n (No. 14)
14.3684(17)
8.4149(10)
15.990(2)
90
113.6104(17)
90
1771.5(4)
4
0.12 ꢁ 0.23 ꢁ 0.29
triclinic
ꢀ
P1 (No. 2)
7.7643(6)
10.9440(8)
10.9479(8)
97.405(6)
107.998(7)
107.916(7)
815.66(10)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
T (K)
293
294
293
kMo
(Å)
a
0.71073
1.746
0.939
0.71073
1.647
0.774
0.71073
1.687
0.837
K
Dcalc (g cmꢀ3
(mmꢀ1
F(000)
)
l
)
760
888
416
h range (°)
Total data
Unique data
3.1–30.7
11093
4366
1.6–26.0
20075
3484
2.9–29.1
4433
3166
Observed data [I > 2
r(I)]
3748
3156
2692
Nref; Npar
4366, 197
0.0213
0.0557b
0.00(3)
0.020
1.053
0.35
ꢀ0.29
3484, 241
0.0243
2460, 217
0.0325
Ra
wR
0.0722b
0.0762c
Flack parameterd
Rint
0.023
1.069
0.63
ꢀ0.30
0.018
1.130
0.60
ꢀ0.46
Goodness-of-fit, S
D
D
qmax (e Åꢀ3
)
)
qmin (e Åꢀ3
P
P
a
b
c
R = (|Fo ꢀ Fc|)/ |Fo|.
P
P
wR = { [w(Fo ꢀ Fc2)2]/ [w(Fo2)2]}
.
2
½
P
P
wR = { [w(|Fo ꢀ Fc|)2]/ [w|Fo|2]}
.
½
d
2147 Friedel pairs.

M. Nandy et al. / Polyhedron 88 (2015) 63–72
67
Table 2
were refined anisotropically. The hydroxy hydrogen atoms in 1 and
2 and the amine hydrogen atoms in 3 were located in a difference
Fourier map and refined isotropically. All other hydrogen atoms in
1, 2, and 3 were first located in the Fourier difference map, then
positioned geometrically and allowed to ride on their respective
parent atoms. The molecular graphics and crystallographic illustra-
tions for 1, 2 and 3 were prepared using the SCHAKAL99 [37], ORTEP
[38], and CAMERON [39] programs. Relevant crystallographic data
and structure refinement parameters of the complexes are summa-
rized in Table 1.
Selected bond lengths (Å) and angles (°) for 1, 2 and 3.
1
Mo1–O1
Mo1–O2
Mo1–O3
C6–O2
O1–Mo1–O2
O1–Mo1–O3
O1–Mo1–O4
O1–Mo1–O5
O1–Mo1–N1
O2–Mo1–O3
O2–Mo1–O4
1.9589(17)
2.0159(13)
1.7047(15)
1.3190(2)
151.91(6)
101.97(8)
98.14(9)
81.53(7)
82.21(6)
96.33(7)
97.29(8)
Mo1–O4
Mo1–O5
Mo1–N1
C2–O1
O2–Mo1–O5
O3–Mo1–O4
O3–Mo1–O5
O3–Mo1–N1
O4–Mo1–O5
O4–Mo1–N1
–
1.6839(17)
2.3514(15)
2.2194(16)
1.3360(3)
80.39(5)
105.39(8)
81.30(7)
155.51(7)
173.18(7)
97.76(8)
–
3. Results and discussion
2
Mo1–O1
Mo1–O2
Mo1–O3
C7–O1
O1–Mo1–O2
O1–Mo1–O3
O1–Mo1–O4
O1–Mo1–O5
O1–Mo1–N1
O2–Mo1–O3
O2–Mo1–O4
O2–Mo1–O5
1.9493(14)
1.9964(14)
1.7017(16)
1.334(2)
150.13(6)
100.71(7)
99.34(8)
80.53(6)
81.39(6)
98.33(7)
97.61(8)
78.97(6)
Mo1–O4
Mo1–O5
Mo1–N1
C11–O2
O2–Mo1–N1
O3–Mo1–O4
O3–Mo1–O5
O3–Mo1–N1
O4–Mo1–O5
O4–Mo1–N1
O5–Mo1–N1
–
1.6910(19)
2.3674(18)
2.2450(17)
1.329(3)
72.52(6)
105.19(8)
83.54(7)
158.02(7)
171.07(7)
95.94(7)
75.18(6)
–
3.1. Fourier transform infrared spectra
The FTIR spectra of the complexes 1–3 were analyzed in com-
parison with those of the free ligands, H2L1–H2L3 (Fig. S13–S18
(ESI)), respectively. In the IR spectra of hydrazone ligands (H2L1,
H2L2 and H2L3), the imine (
mC@N) stretching bands appeared at
1622, 1646, 1629 cmꢀ1 whereas they are shifted to lower stretch-
ing frequencies in the spectra 1, 2, and 3, and observed at 1610,
1611, and 1608 cmꢀ1, respectively, indicating the coordination of
the imine nitrogen to the metal center [40]. A sharp doublet
observed at 905 and 943 cmꢀ1, 898 and 944 cmꢀ1, 907 and
921 cmꢀ1 for 1, 2, and 3, respectively, which is related to the sym-
metric and anti-symmetric stretching vibrations of cis-MoO22+ moi-
ety which appeared at 897 and 943 cmꢀ1 in the spectra of the
precursor complex MoO2(acac)2. This indicates that symmetric
and anti-symmetric stretching vibrations of MoO22+ are indepen-
dent on either electron donating or accepting capacity of the
ligands [41,42]. This data also support the existence of cis-MoO22+
moiety in these complexes since trans-MoO22+ moiety would
normally show only one mMo=O asymmetric stretching vibration
[42]. Bands in the range 2923–3057 cmꢀ1 may be assigned to
3
Mo1–O1
Mo1–O2
Mo1–O3
C8–O2
O1–Mo1–O2
O1–Mo1–O3
O1–Mo1–O4
O1–Mo1–O5
O1–Mo1–N1
O2–Mo1–O3
O2–Mo1–O4
O2–Mo1–O5
1.921(2)
2.007(2)
1.702(2)
1.309(4)
148.95(9)
102.57(10)
98.45(13)
81.59(9)
81.17(8)
98.29(10)
97.51(11)
78.49(9)
Mo1–O4
Mo1–O5
Mo1–N1
–
O2–Mo1–N1
O3–Mo1–O4
O3–Mo1–O5
O3–Mo1–N1
O4–Mo1–O5
O4–Mo1–N1
O5–Mo1–N1
–
1.683(3)
2.359(3)
2.245(2)
–
71.16(8)
106.14(14)
82.79(12)
158.05(13)
170.75(9)
94.52(10)
76.32(8)
–
Fig. 3. Partial crystal packing of 1 showing the formation of a chain parallel to the [101] direction via O–HꢂꢂꢂN hydrogen bonds (dashed lines).

68
M. Nandy et al. / Polyhedron 88 (2015) 63–72
the –CH2– stretching frequency of methyl/methylene moiety of the
free/coordinated ligands.
and Figs. S4–S6 (ESI), respectively. 1H NMR spectroscopy has been
used to extract information regarding the formation of the ligands
and their mode of coordination to the MoO22+ moiety in the com-
plexes. In the 1H NMR spectra of H2L1and H2L2 two geminal-cou-
pled doublets for the –CH2– group (H4 for H2L1 and H6 for H2L2)
3.2. Electronic spectra
(d = 2.84–3.08 ppm) substantiated the presence of two p-acceptor
UV–Vis spectra of the free ligands (Fig. S19) and their com-
plexes (Fig. S20) were recorded at room temperature using spec-
troscopic grade methanol. The absorption bands of the free
ligands can be classified into three regions. Band in the range
271–280 nm in the spectra of ligands is characteristic for the
carbonyl groups. The peaks between d = 7.64–8.74 ppm correspond
to the aromatic protons of H2L1 (H6, H7) and H2L2 (H1, H2, H3, H8,
H9). In the 1H NMR spectra of 1 and 2, two doublets for the protons
of the methylene group are absent at d = 3.7–3.9 ppm, thus indicat-
ing deprotonation of the methylene group [45,46]. This also con-
firms that both ligands coordinate to the molybdenum center in
the enolic form. Moreover, it is clearly observed that the signals
of aromatic hydrogen are broadened compared to the free ligands
(H2L1 and H2L2) possibly due to spin–lattice relaxation caused by
paramagnetic impurities in the complexes. The 1H NMR spectra
of 1 and 2 contain one additional singlet at d = 3.49, tentatively
assigned to the coordinated CH3OH molecule.
p
?
p⁄ transition of the aromatic rings of the ligands. Other two
bands observed between 320 and 450 nm are probably due to
the n ? p⁄ transition of the AC@NA and AC@OA moieties of the
ligands [43]. In the spectra of complexes the bands of the azome-
thine chromophore (n ? p⁄) transition are shifted to lower fre-
quencies indicating that the imine nitrogen atom is involved in
coordination to the metal ion. In addition 1 and 2 also show a
broad band at ca. 443 and 423 nm, which is assignable to a ligand
to metal charge transfer (LMCT) due to the promotion of an elec-
tron from the highest occupied molecular orbital (HOMO) of the
ligand to the lowest unoccupied molecular orbital (LUMO) of
molybdenum atom [44]. Complex 3 does not show any LMCT band.
The 1H NMR spectrum of H2L3 displays two signals at d = 8.58
(H7) and 9.20 (H6) which can be attributed to azomethylene and
ANH@NA protons confirming the formation of the hydrazone.
The aromatic proton signals (H3, H4, H5, H8, H9 and H10) were
observed in the range d = 6.74–7.93 ppm. Moreover, peaks at
d = 11.14 and 12.09 ppm can be attributed to two phenonolic
hydrogen atoms. The 1H NMR spectrum of 3 resembled to that of
H2L3 very much except that the signal of the phenolic-OH proton
was absent which supported its deprotonation during complexa-
tion. The results of the 1H NMR spectra are in good agreement with
the structure of the complexes.
3.3. 1H NMR spectra of the ligands and complexes
The proton numbering scheme of the ligands are represented in
Scheme 2 while 1H NMR spectra of these ligands (H2L1, H2L2, and
H2L3) and their complexes are represented in Figs. S1–S3 (ESI)
Table 3
Hydrogen bonding parameters (Å, °) in 1.
3.4. Structure description of the complexes 1, 2, and 3
D–HꢂꢂꢂA
O5–H5OꢂꢂꢂN3*
C11–H11ꢂꢂꢂO3#
d(D–H)
d(HꢂꢂꢂA)
1.878(18)
2.46
d(DꢂꢂꢂA)
2.686(2)
3.283(3)
\(DHA)
Complexes 1 and 2 crystallize in the monoclinic space group Cc
and P21/n, respectively. An ORTEP view of the asymmetric unit of 1
and 2 with atom labels are shown in Figs. 1 and 2 and relevant
bonding parameters are listed in Table 2. The asymmetric units
0.815(18)
0.93
171(2)
148
Symmetry code: * ꢀ1/2 + x, ꢀ1/2 ꢀ y, ꢀ1/2 + z; # x, ꢀ1 + y, z.
Fig. 4. Crystal packing of 2 showing the formation of centrosymmetric dimers via pairs of O–HꢂꢂꢂN hydrogen bonds (dashed lines).

M. Nandy et al. / Polyhedron 88 (2015) 63–72
69
Table 4
of both complexes consist of one molybdenum(VI) atom, one di-
anionic tridentate hydrazone Schiff base (L1)2ꢀ (for 1) or (L2)2ꢀ
(for 2), two oxo ligands, and one coordinated methanol. The coor-
dination geometry around the Mo center in these complexes can be
described as distorted octahedron in which the tridentate ligand
coordinate through the anionic alkoxo oxygen (O1), the imine
nitrogen (N1), and the anionic carboxamido oxygen (O2) and adopt
mer-arrangement with two anionic oxygen atoms mutually trans
while they remain as cis with respect to the two oxygen atoms
(O3 and O4) of cis-dioxo group (Figs. 1 and 2). The octahedral
geometry of the molybdenum center is completed by the oxygen
atom of the coordinated methanol (O5). The metal atom is dis-
placed from the best equatorial plane formed by the O1, N1, O2,
O3 atoms (maximum deviation from the planarity of 0.068(2) Å
for O1 in 1 and 0.025(2) Å for N1 in 2) toward the O4 apical atom
by 0.3350(5) and 0.3330(2) Å in 1 and 2, respectively.
Hydrogen bonding parameters (Å, °) in 2.
D–HꢂꢂꢂA
d(D–H)
d(HꢂꢂꢂA)
1.92(4)
2.60
d(DꢂꢂꢂA)
2.706(3)
3.475(3)
3.140(3)
\(DHA)
O5–H5OꢂꢂꢂN3*
C4–H4ꢂꢂꢂO5#
C14–H14ꢂꢂꢂO4$
0.79(4)
0.93
0.93
171(3)
157
119
2.58
Symmetry code: *1 ꢀ x, 1 ꢀ y, 1 ꢀ z; # ꢀx, ꢀy, 1 ꢀ z; $ 1/2 ꢀ x, 1/2 + y, 1/2 ꢀ z.
The two transoid basal angles (O3–Mo1–N1 and O1–Mo1–O2)
deviate significantly from the ideal value (180°) while the axial
trans angle (O4-Mo1-O5) is very close to the ideal value. The C2–
O1, C6–O2 and C7–O1, C11–O2 bond lengths (Table 2) in 1 and 2
are comparable to other enolato C–O bond lengths found in the
complexes reported earlier [47,48], thus indicating that carbonyl
functionality coordinates in enolic form in both cases.
A detailed investigation of the crystal structure of 1 reveals that
the alkoxo oxygen of the coordinated methanol molecule acts as
hydrogen donor to the acceptor pyridyl nitrogen (N3) of an adja-
cent complex molecule resulting in the formation of a one dimen-
sional stair-like chain structure propagating along the
crystallographic [101] direction (Fig. 3). Furthermore these stair-
like chains are interlinked by non classical C–HꢂꢂꢂO hydrogen bond
(Table 3) leading to a 2D supramolecular structure.
Fig. 5. An ORTEP view of
probability level.
3 with displacement ellipsoids drawn at the 50%
Table 5
Hydrogen bonding parameters (Å, °) in 3.
D–HꢂꢂꢂA
d(D–H)
d(HꢂꢂꢂA)
d(DꢂꢂꢂA)
\(DHA)
In the crystal packing of 2 (Fig. 4) molecules related by center of
inversion interlocked into dimer through pairs of O5–H5OꢂꢂꢂN3
hydrogen bonds (Table 4), forming a ring of R22(18) graph-set motif
according to Etter’s notation [49]. A comparison of the crystal
O6–H1ꢂꢂꢂO3*
0.85
0.96
0.96
1.90
2.60
2.57
2.753(4)
3.314(4)
3.425(4)
180
132
149
C7–H41ꢂꢂꢂO4$
C12–H101ꢂꢂꢂO6**
Symmetry code: * 1 ꢀ x, 1 ꢀ y, 1 ꢀ z; ꢀz; $ ꢀx, 1 ꢀ y, ꢀz; ** x, ꢀ1 + y, ꢀ1 + z.
Fig. 6. Crystal packing of 3 showing the formation of centrosymmetric dimers via pairs of O–HꢂꢂꢂO hydrogen bonds (dashed lines).

70
M. Nandy et al. / Polyhedron 88 (2015) 63–72
structures of complexes 1 and 2 indicates that the replacement of a
methyl group by a phenyl group leads to a drastic change in the
hydrogen bonding pattern probably due to steric effect of the bulky
phenyl ring. In addition, the dimers in 2 are interwoven by non-
classical C–HꢂꢂꢂO hydrogen bond (Table 4) to form a 3D network.
ꢀ
Complex 3 crystallized in triclinic space group P1. An ORTEP
view of 3 is shown in Fig. 5 and relevant bonding parameters are
summarized in Table 2. Alike 1 and 2, the asymmetric unit of 3 also
consists of a molybdenum(VI) atom, one di-negative tridentate
hydrazone Schiff base (L3)2ꢀ, two oxo ligands and a methanol.
The coordination polyhedron around Mo1 is best described as dis-
torted octahedron where the best equatorial plane is defined by
O1, N1, O2 and O3 atoms leaving O4 and O5 at the two apexes.
All donor atoms in the mean equatorial plane are almost coplanar
(maximum displacement 0.022(3) Å for atom O2 and N1) with the
Mo1 atom protruding by 0.3264(4) Å towards the apical O4 oxygen
atom. The Mo@O bond lengths and the O@Mo@O angles are in the
expected range for cis dioxomolybdenum(VI) complexes. The enolic
nature of the C8–O2 bond is evident from its similarities to other
enol C–O bond distances observed in the reported complexes
[47,48].
Fig. 7. Hirshfeld surface mapped with dnorm (a), shape index (b), and curvedness (c)
for 1, 2 and 3.
Fig. 8. Fingerprint plots of 1, 2 and 3.

M. Nandy et al. / Polyhedron 88 (2015) 63–72
71
The conformation of complex 3 is stabilized by (Table 5) pairs of
cooperative O–HꢂꢂꢂO interactions operating between the phenolic-
OH (O6–H1) and the oxo (O3) groups of centrosymmetrically-
related molecules leads to a dimeric structure (Fig. 6) forming a
ring of R22(12) graph-set motif according to Etter’s notation [49].
The supramolecular dimers are further connected by non-classical
C–HꢂꢂꢂO hydrogen bonds as well as
p. . .p stacking interactions
(centroid-to-centroid distances of 3.679(3) Å) to form 2D layer par-
allel to the [101] plane.
3.5. Hirshfeld surface analyses
Intermolecular interactions are conveniently viewed and quan-
tified by mapping a Hirshfeld surface onto a fragment of each com-
plex using the program CrystalExplorer3.1 [50] and then
generating the corresponding 2D fingerprint plot. The Hirshfeld
surfaces of the title complexes are depicted in Fig. 7, showing sur-
faces that have been mapped over dnorm (ꢀ0.5 to 1.5 Å), shape
index and curvedness. dnorm surfaces of 1 and 2 with the large cir-
cular gloominess visible on the surfaces are indicative of strong O–
HꢂꢂꢂN interactions between the oxygen atom of the coordinated
methanol and the nitrogen atom of the pyridine ring. Intense red
gloominess on dnorm surface of complex 3 can be assigned to strong
O–HꢂꢂꢂO interactions among phenoxo and hydroxyl groups, metha-
nolic hydrogen. Other visible spots in the surfaces are because of
HꢂꢂꢂH contacts. The OꢂꢂꢂH/HꢂꢂꢂO intermolecular interactions appear
as distinct spikes in the 2D fingerprint plot (Fig. 8). Complementary
regions are visible in the fingerprint plots where one molecule act
as donor (de > di) and the other as an acceptor (de < di). The finger-
print plots can be decomposed to highlight particular atoms pair
close contacts [51]. The proportions of OꢂꢂꢂH/HꢂꢂꢂO interaction are
29.9, 25.6, and 35.1% of the Hirshfeld surfaces for each molecule
of 1, 2, and 3, respectively. The OꢂꢂꢂH interactions are represented
by a spike (di = 1.3, de = 1.3 Å in 1, di = 1.1, de = 1.4 Å in 2 and
di = 0.7, de = 1.1 Å in 3) in the bottom left (donor) area of the finger-
Fig. 9. Cyclic voltammograms of complexes 1, 2 and 3, recorded at a scan rate
150 mV sꢀ1
.
print plot (Fig. 8). No significant C–Hꢂꢂꢂ
p interaction is observed in
these complexes while the CꢂꢂꢂH close contacts are 11.6%, 18.7%,
and 12.6% in 1, 2, and 3, respectively. Percentage contribution of
a variety of contacts in the molybdenum complexes are listed
Table 6.
Fig. 10. TGA curves of complexes 1, 2 and 3.
versible reduction peak at ꢀ1.149, ꢀ0.913, and ꢀ1.121 V were
observed for 1, 2, and 3, respectively (Fig. 9) corresponds to
Mo(VI)/Mo(V) redox couple [8,52]. For the complexes, no oxidation
peak is observed during anodic scan on cyclic voltammetry.
The inspections on the Hirshfeld surface of 1 and 2 do not show
adjacent red and blue triangles on the shape-index surfaces (Fig. 7).
Moreover no flat region toward the bottom of both sides of ben-
zene and pyridine moiety is evident on the curvedness surface
(Fig. 7). However 3 shows adjacent red and blue triangles on
shape-index surface as well as flat region toward the bottom of
both sides of benzene and pyridine moieties on curvedness surface.
3.7. Thermogravimetric analyses
Thermogravimetric analysis curves (Fig. 10) for complexes 1, 2,
and 3 were recorded in the temperature range 25–800 °C with a
heating rate of 10 °C minꢀ1 in dynamic nitrogen atmosphere which
reveal their thermal stabilities. The samples were purged by a
stream of dry nitrogen flowing at 40 mL minꢀ1. All three complexes
undergo decomposition in two well defined stages. The loss of
weakly coordinated methanol molecule occurred in between 170,
195, 130 °C for 1, 2, and 3, respectively [53]. The second stage of
decomposition took place in the temperature range of 180–434,
300–597, and 240–750 °C for 1, 2, and 3 corresponding to loss of
the coordinated hydrazone ligands. The final residue in the decom-
position of each complex was identified as MoO2 by qualitative
analysis.
These clearly indicate that 1 and 2 have no
p. . .p stacking in the
solid state but 3 possess . . . stacking interaction.
p
p
3.6. Electrochemical studies
Electrochemical properties of the complexes were studied at
room temperature in HPLC grade acetonitrile using cyclic voltam-
metry at a platinum electrode with tetrabutylammonium perchlo-
rate as supporting electrolyte at a scan rate of 150 mV sꢀ1 within
the potential range 0 to ꢀ1.75 V. During the cathodic scan an irre-
Table 6
Summary of the various contacts percentage contributions to the Hirshfeld surface
area in 1, 2 and 3.
Complexes O–HꢂꢂꢂH–
N–HꢂꢂꢂH–
C–HꢂꢂꢂH–
HꢂꢂꢂH CꢂꢂꢂC C–OꢂꢂꢂO–
4. Conclusions
O
N
C
C
1
2
3
29.9
25.6
30.6
8.4
7.5
1.9
11.6
18.7
12.6
42.4
40.9
41.2
3.0
4.8
7.9
2.2
0.1
0.7
In this paper we have reported the syntheses and spectral char-
acterization of three new cis-dioxomolybdenum(VI) complexes
incorporating different tridentate hydrazone ligands. Structural

72
M. Nandy et al. / Polyhedron 88 (2015) 63–72
characterization reveals that all the complexes consist of a MoO22+
core incorporating a coordinated tridentate hydrazone in di-nega-
tive form. In all the complexes the molybdenum(VI) atom displays
distorted octahedron geometry. Several weak interactions are
present in the complexes and play an important role in their crys-
tal packing which is supported by Hirshfeld surface analyses. An
electrochemical study shows an irreversible redox behavior of
the complexes. The relatively long molybdenum-oxygen (metha-
nolic) bond distances, thermal analyses and ESI-MS analyses reveal
that the coordinated methanol is loosely held to the MoO22+ core of
the complexes suggesting their potential use as catalysts [27–30].
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M. Nandy gratefully acknowledges the Council of Scientific and
Industrial Research (CSIR), New Delhi, Government of India, for
awarding Senior Research Fellowship (SRF) to him [CSIR sanction
No. 09/096(0598)/2009-EMR-I]. S. Shit gratefully acknowledges
the University Grants Commission, New Delhi, India for financial
assistance (Minor Research Project No. F. PSW-65/12-13 (ERO))].
Thanks are also extended to the University Grants Commission,
New Delhi, for financial assistance to S. Mitra as Emeritus Fellow
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