Two new hydrogen-bonded supramolecular dioxo-molybdenum(VI) complexes based on acetyl-hydrazone ligands: Synthesis, crystal structure and DFT studies

Article abstract of DOI:10.1016/j.molstruc.2020.129346

Two new cis-dioxomolybdenum(VI) complexes [MoO₂L¹(H₂O)] (1) and [MoO₂L²(H₂O)] (2) have been synthesized using two different tridentate hydrazone Schiff base ligands, L¹H₂

Full text of DOI:10.1016/j.molstruc.2020.129346

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Two New Hydrogen-bonded Supramolecular Dioxo-molybdenum(VI)
Complexes Based on Acetyl-Hydrazone Ligands: Synthesis, Crystal
Structure and DFT Studies
Monami Maiti Conceptualization; Methodology; Writing- Original draft preparation ,
Santarupa Thakurta Conceptualization; Writing- Reviewing and Editing ,
Guillaume Pilet Software; Data Curation , Antonio Bauz a´ ,
Antonio Frontera Software; Formal analysis; Writing- Reviewing and Editing
PII:
S0022-2860(20)31664-1
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129346
MOLSTR 129346
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Received date:
Revised date:
Accepted date:
6 July 2020
23 September 2020
25 September 2020
Please cite this article as: Monami Maiti Conceptualization; Methodology; Writing- Original draft preparation ,
Santarupa Thakurta Conceptualization; Writing- Reviewing and Editing , Guillaume Pilet Software; Data Curation ,
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New Hydrogen-bonded Supramolecular Dioxo-molybdenum(VI) Complexes Based on Acetyl-
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



Two new Mo(VI) complexes based on acetyl-hydrazone ligands have been
synthesized.
Molecular structures of the complexes have been conclusively accomplished by single
crystal X-ray diffraction.
H-bonding and
–
interactions play an important role in the packing of the
complexes in their solid state.
The relative stability of the supramolecular networks is rationalized by computational
studies at the BP86-D3/def2-TZVP level of theory.
1

Two New Hydrogen-bonded Supramolecular Dioxo-molybdenum(VI)
Complexes Based on Acetyl-Hydrazone Ligands: Synthesis, Crystal
Structure and DFT Studies
a
b
c
d
Monami Maiti* , Santarupa Thakurta , Guillaume Pilet , Antonio Bauzá , Antonio
d
Frontera
a
Department of Chemistry, Narasinha Dutt College, Howrah 711101, West Bengal, India.
b
Department of Chemistry, Prabhu Jagatbandhu College, Howrah 711302, West Bengal,
India.
c
Groupe de Cristallographie et Ingénierie Moléculaire, Laboratoire des Multimatériaux et
Interfaces, UMR 5615 CNRS-Université Claude Bernard Lyon 1, Bât. Chevreul, 43 bd du 11
Novembre 1918, 69622 Villeurbanne Cedex, France.
d
Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5,
0
7122 Palma de Mallorca (Balears), Spain.
*
Corresponding author: Tel.: +91 9433968520 (M. Maiti)
Email address: monami.maiti@gmail.com (M. Maiti)
Abstract
Two new cis-dioxomolybdenum(VI) complexes [MoO
1
2
2
L (H
2
O)] (1) and [MoO
2
L (H
2
O)] (2)
1
have been synthesized using two different tridentate hydrazone Schiff base ligands, L H
2
2
[
(E)-N-(2-hydroxybenzylidene)acetohydrazide]
and
L H
2
[(E)-N-(2-hydroxy-3-
methoxybenzylidene)acetohydrazide], respectively. The ligands and their corresponding Mo
complexes are thoroughly characterized by different physicochemical techniques. Molecular
structures of the complexes have been conclusively accomplished by single crystal X-ray
diffraction. All the experimental findings reveal that both complexes have a distorted
octahedral geometry around the Mo(VI) centre. The tridentate hydrazones coordinate to the
metal centres in their enolic form in both 1 and 2. Hydrogen bonding and – interactions
play an important role in the packing of the complexes in their solid state. Computational
studies are also performed using DFT calculations at BP86-D3/def2-TZVP level of theory to
understand the relative stability of the supramolecular networks.
Key words: Molybdenum(VI) complexes; Hydrazone ligand; Schiff base; X-ray crystal
Structure; Non-covalent interactions; Density functional calculations.
2

1
. Introduction
Supramolecular chemistry deals with weak and reversible non-covalent interactions between
molecules. These forces include hydrogen bonding, metal coordination, hydro-phobic forces,
van der Waals forces, π–π interactions, CH–π interactions, electrostatic effects, and so on.
The self-assembly of multimetallic complexes held together by intermolecular forces
(
hydrogen bonds, van der Waals forces etc.) is greatly dependent on the metal ions [1]. Metal
ions can read the information coded in the organic ligands according to their coordination
algorithm [2] and thereby give rise to metal–organic ligand complex species that are
simultaneously assembled via complementary inter-ligand hydrogen bonding forming
supramolecular multimetallic assemblies.
Hydrazone ligands are a special branch of Schiff bases which can accommodate with
different coordination modes and oxidation states according to the demand of variable
geometries and valences of metal ions in coordination complexes. Moreover, they form
interesting hydrogen bonded self-assembly in metal-free state as well as in complexes,
mainly due to the presence of the N–H functionality adjacent to the azomethine (C=N)
chromophore [3]. Moreover, the electron withdrawing N–H group makes imidic C=N bond
highly acidic, hence apart from hydrogen bonding the imidic C=N bond of the metal-
coordinated Schiff base ligand can also interact with electron rich atoms and develop distinct
supramolecular interaction. Transition metal complexes derived from hydrazone Schiff bases
can serve as potential building blocks either as metalloligands (linkers) or as nodes
(
connectors) in the crystal engineering of polymeric frameworks [4-6] and are attracting the
attention of the researchers due to several potential applications in potential biological,
catalytic, and industrial field [7,8]. Many recent reports show that they are good candidates
for the development of new drugs as well as their usage in other biochemical processes like
enzyme inhibition, or the simulation of antimicrobial, anti-cancer, anti-malarial activities
[
9,10].
In this respect, molybdenum-Schiff base complexes have a unique place in the development
of coordination chemistry due to their potential to construct varieties of supramolecular
architectures [11,12]. Moreover, due to its various stable and accessible oxidation states and
versatile coordination chemistry [13] molybdenum plays significant role in industrial [14]
and biological reactions [15]. In spite of the important role of molybdenum in biological
processes, its high potential as catalyst has made it even more attractive. Oxo–peroxo and
dioxo complexes of Mo(VI) with polydentate nitrogen, sulphur and oxygen ligands are
3

considered valuable models for the active site of several molybdenum enzymes [16-18].
2
+
Mo(VI) is stabilized through formation of MoO
2
[19]. An important characteristic of
2
+
coordination compounds containing MoO
2
is their ability to participate in reversible oxo-
transfer reactions in aqueous aerobic media for nitrogenase, aldehyde oxidase, xanthine
dehydrogenase, nitrate reductase, etc. [20-25]. This property has made them popular as
oxidase or reductase catalyst in industrial and biochemical process. Oxygen transfer reactions
are catalysed by Mo(VI)-cofactor-containing enzymes in many organisms and occur in
DMSO reductase, sulfite oxidase, nitrate reductase, and aldehyde oxidoreductase reactions
[
26-28]. Synthesis and characterization of molybdenum(VI) compounds are important
because they are relevant in industrial and biological research.
Herein we report the syntheses, spectral and structural characterizations of two new
1
2
dioxomolybdenum(VI) complexes, [MoO
2
L (H
2
O)] (1) and [MoO
2
L (H
2
O)] (2) derived from
1
two different tridentate hydrazone Schiff base ligands, L H
2
:
[(E)-N-(2-
2
hydroxybenzylidene)acetohydrazide]
and
L H
2
:
[(E)-N-(2-hydroxy-3-
methoxybenzylidene)acetohydrazide], respectively (Scheme 1). Structural characterization of
each complex reveals that the tridentate ligand binds the central metal atom in the respective
enolic form and molybdenum atom displays distorted octahedral geometry. Both the
complexes exhibit supramolecular architectures through hydrogen bonding and π–π
interactions in their solid state. The binding energies through supramolecular forces and the
individual contributions of each kind of hydrogen-bonding (N–H···O, O–H···O, and C–
H···O) and π–π interactions to the binding energies have been quantitatively estimated by
DFT calculations at the BP86-D3/def2-TZVPD level of theory.
Scheme 1: Synthesis of hydrazone ligands.
4

2
. Experimental
2
.1. Materials and Physical measurements
All chemicals and solvents employed for the syntheses were of analytical grade and used as
received without further purification. Salicylaldehyde, o-vanillin (i.e. 2-hydroxy-3-
methoxybenzaldehyde) and acetic hydrazide were reagent grade and purchased from Sigma-
Aldrich. Glacial acetic acid, methanol, HPLC grade acetonitrile were purchased from LOBA
Chemie. MoO
2
(acaca)
2
was prepared and purified according to reported method [29].
The FT-IR spectra of the compounds were recorded on a Perkin-Elmer RX I FT-IR
–
1
spectrometer with KBr pellets in the range 4000–400 cm . The electronic spectra in HPLC
grade acetonitrile were recorded at 300 K on a Perkin-Elmer Lambda 40 (UV-Vis)
spectrometer in a 1 cm quartz cuvette in the range 200-800 nm. C, H, and N microanalyses
were carried out with a Perkin-Elmer 2400 II elemental analyzer.
2
2
.2. Syntheses
1
2
.2.1. Synthesis of the Hydrazone ligands [L H
2
and L H ]
2
1
The ligand L H
2
[(E)-N-(2-hydroxybenzylidene)acetohydrazide] was prepared by the
condensation of acetic hydrazide (0.74 g, 10 mmol) with salicylaldehyde (1.221 g, 10 mmol)
in presence of a single drop of glacial acetic acid in methanol medium (100 mL). By
refluxing the methanolic solution for 5 h a colourless solution was obtained. The solvent was
removed under reduced pressure, and the white residue was purified by recrystallization to
obtain colourless shiny crystals. Yield 1.55 g (87%). Anal. Calcd. (%) for C
9
H
10
N
2
2
O (M =
1
78.1 g/mol): C, 60.66; H, 5.66; N, 15.72. Found: C, 60.55; H, 5.59; N, 15.63.
2
The ligand L H
2
[(E)-N-(2-hydroxy-3-methoxybenzylidene)acetohydrazide] was prepared by
the condensation of acetichydrazide (10 mmol, 0.74 g) with o-vanillin (10 mmol, 1.525 g) in
presence of few drops glacial acetic acid in methanol medium (200 mL). On refluxing the
methanolic solution for 5 h a pale-yellow colouration was observed. The colour indicated the
formation of the hydrazone ligand. The solvent was removed under reduced pressure and the
residue was purified by recrystallisation to give pale yellow crystals. Yield 1.85 g (89%).
Anal. Calcd. (%) for C10
H
12
N
2
O
3
(M = 208.2 g/mol): C, 57.68; H, 5.81; N, 13.45. Found: C,
5
2
2
7.62; H, 5.76; N, 13.44.
.2.2. Synthesis of the complexes
1
.2.2.1. [MoO
2
L (H
2
O)]
(1)
5

1
A mixture of MoO
2
(acac)
2
(0.164 g, 0.5 mmol) and L H (0.089 g, 0.5 mmol) in methanol
2
(
20mL) was refluxed for 1 hour to give a dark orange solution. After 2 days, yellow needle
shaped suitable single crystals appeared on slow evaporation of solvent. The resulting
product was isolated, and dried in air. Yield 0.114 g (71%). Anal. Calcd. (%) for
C
9
H10Mo
1
N
2
O
5
(M: 322.13 g/mol): C, 33.56; H, 3.13; N, 8.70. Found: C, 33.49; H, 3.19; N,
8
.68.
.2.2.2. [MoO
2
2
2
L (H
2
O)]
(2)
2
Complex 2 was prepared following a similar procedure as for 1, using 0.5 mmol of L H
0.104 g) in methanol. Yellow plate shaped single crystals suitable for X-ray diffraction were
obtained after six days. Crystals were isolated by filtration and were air dried. Yield: 0.132 g
75%). Anal. Calcd. (%) for C10 (M: 352.16 g/mol): C, 34.11; H, 3.43; N, 7.95.
Found: C, 34.18; H, 3.47; N, 8.01.
2
(
(
H12Mo
1
N
2
O
6
2
.3. X-ray data collection and refinement
Intensity data of both 1 and 2 ꢀere collected at ꢁꢂꢃ ꢄ using ꢅo-ꢄꢆ radiation ꢇꢈꢉꢊ.ꢋꢌꢊꢍꢂ ꢎ)
with an Oxford Diffraction Gemini diffractometer. Data collection and reduction were
performed with the CrysAlis program [30]. The structures were solved by direct methods
using the program SIR97 [31] programs and refined by full-matrix least-squares methods
with programs CRYSTALS [32]. All the H atoms were generated geometrically and included
in the refinement in the riding model approximation, except those of the aqua ligands in 1 and
2
detected in the Fourier map and refined. Selected crystallographic data, experimental
conditions, and relevant features of the structural refinements for the compounds are
summarized in Table 1.
Table 1: Crystal structure parameters of the complexes 1 and 2
Parameters
1
2
Empirical formula C
9
H10Mo
1
N
2
O
5
C
10
H12Mo
1
N
2
O
6
Formula weight
Temperature
Crystal system
Space group
a (Å)
322.13 g/mol
293 K
352.16 g/mol
293 K
Orthorhombic
Monoclinic
P 1 21/n 1
8.9157(9)
8.2994(8)
P
bca
13.6159(8)
7.1939(5)
b (Å)
6

c (Å)
22.9578(16)
90
17.2194(18)
90
α ()
β ()
9
9
0
0
102.428(11)
90
γ ()
3
V (Å )
2248.8(2)
8
1244.3(2)
4
Z
-3
Dcalc (Mg m )
1.898
1.175
1280
1.880
-1
µ (mm )
1.079
F(000)
704
θ range (deg)
Total data
Unique data
2.9–29.2
7778
2.7–29.2
6585
2657
2909
Obs data [I>2(I)] 2026
2195
R1[I > 2(I)]
wR2[I > 2(I)]
GoF
0.0297
0.0700
0.0731
1.0332
0.055
0.0514
1.0174
R
int
0.028
-3
Residuals (e.Å )
0.34, –0.33
2.14, –2.28
2
.4. Theoretical Methods
The geometries of the complexes included in this study were computed at the BP86-D3/def2-
TZVP level of theory using the crystallographic coordinates within the TURBOMOLE
program [33]. We have used this methodology (X-ray coordinates) since it is convenient to
evaluate the interactions of several synthons as they stand in the solid state [34-36]. This level
of theory that includes the latest available dispersion correction (D3) is adequate for studying
non-covalent interactions dominated by dispersion effects like -stacking [37]. The basis set
superposition error for the calculation of interaction energies has been corrected using the
counterpoise method [38]. The “atoms-in-molecules” ꢇAIꢅ) analysis [39] of the electron
density has been performed at the same level of theory using the AIMAll program [40].
3
. Results and discussion
7

Most of the works on transition metal complexes derived from hydrazone Schiff bases are
based on aryl-hydrazone ligands. Very few metal complexes of hydrazone-Schiff bases
derived from alkyl hydrazides are reported in literature [5,6,41-43]. The synthesis and
2
spectroscopic studies of L H
2
[(E)-N-(2-hydroxy-3-methoxybenzylidene)acetohydrazide] was
first reported by Mitra et al. [44]. In another paper by the same group, the crystal structure
1
and spectroscopic characterization of L H
2
[(E)-N-(2-hydroxybenzylidene)acetohydrazide]
was discussed [45]. However, in both the previous reports, the ligands chelated in keto form
during complex formation with copper(II) and manganese(II). But in the present case, the
ligands exhibit their respective enol forms to generate the dioxo-Mo(VI) complexes. The high
oxidation state of the central metal ion can be considered as the driving force for the
enolisation of the ligands. To the best of our knowledge, Mo(VI) complexes of acetyl
hydrazones with extensive supramolecular weak interactions has not been published so far.
3
.1. Fourier transform infrared spectra
The FT-IR spectra of the complexes 1 and 2 (Figure S1 and S2: ESI) are analyzed in
1
2
comparison with that of the free ligands, L H
2
and L H
2
. The IR spectra of the ligands show
−
1
intense band at 1625–1640 cm due to C=N stretching frequency, which are shifted towards
−
1
lower energy by 10–30 cm in the complexes, indicating coordination through the
azomethine nitrogen. The hydrazone ligands display the (C=O) and the (N–H) absorption
–
1
–1
band at 1645–1662 cm and 3185–3250 cm , respectively. Both of these bands disappear on
complexation, indicating the enolisation of the [–NH–C=O] moiety [46]. The new stretching
–
1
frequencies appearing at 1090–1120 cm for both complexes are attributed to the
coordinated C–O groups. Each spectra of the complexes exhibit two strong bands in the
–
1
region 900–930 cm corresponding to the symmetric and asymmetric stretching frequencies
of [O=Mo=O]. This IR data is characteristic of the cis-[MoO ] fragment [47]. The broad
2
-1
-1
bands centered at 3325 cm for 1 and 3320 cm for 2 are attributable to str(O–H) vibrations
of the coordinated water molecule [48]. The O–H stretching vibration for water appears
-1
generally above 3400 cm . The shift of this band towards lower wave number and its
broadness confirms the presence of strong hydrogen bonding in each complex.
3
.2. Electronic spectra
The electronic spectrum of each complex (Figure S3 and S4: ESI) (displays absorptions at
about 238 nm and 365 nm corresponding to intra-ligand -* transition and n-* transition,
respectively [49]. In both Mo(VI) complexes, the bands appearing at about 432 nm are due to
8

O(p)-Mo(d) charge transfer (LMCT) transitions. This phenomenon is usually observed in
cis-MoO complex [50].
2
1
2
3
.3. Crystal structure descriptions of [MoO
2
L (H
2
O)] (1) and [MoO
2
L (H O)] (2)
2
The crystallographic independent units of 1 and 2 are shown in Figure 1 and 2, respectively,
and the bonding parameters are listed in Table 2. The asymmetric unit of both the complexes
2
+
are built up of ONO donor set of the respective ligands and the cis-MoO
2
metal-oxo unit.
The dianionic enolate ligands meridionally coordinate from the equatorial plane. The bond
lengths C11–O12 [1.325(3) Å] and C11-N10 [1.288(4) Å] in complex 1 and C13–O14
[
1.303(7) Å] and C13-N12 [1.288(7) Å] in complex 2 indicate with the presence of {=N–

N=C(O )–} moiety. The C-O single bond and C=N double bond character confirming the
enolic form of the ligands in the complexes.
Figure 1: Perspective view of the asymmetric unit of 1 with potential hydrogen-bonding
groups.
9

Figure 2: Perspective view of the asymmetric unit of 2 with potential hydrogen-bonding
groups.
The coordination geometry around the Mo centre in both the complexes can be best described
as a distorted octahedron in which the azomethine–N, enolic–O and phenolic-O of the
dianionic ligand occupies meridional position with two anionic oxygens remaining mutually
trans. One oxido group is located trans to the imine nitrogen in the same plane and the other
oxido group occupies one of the axial positions. The other axial position is coordinated by a
water molecule completing the octahedral geometry of molybdenum centre. In the structures
of both complexes, the tridentate Schiff base ligands form a fused six and five-membered
metalacyclic chelate rings. The dihedral angle between the mean planes of five- and six-
coordination rings is 5.29 and 9.80 for complex 1 and 2, respectively. It is noteworthy that
the six-membered metallacycle in complex 1 is almost planar while the same in complex 2 is
significantly twisted with atom O9 having the largest deviation from coplanarity [0.199 Å].
This structural difference between the complexes can be attributed to the steric hindrance
caused by the methoxy side-arm present in complex 2.
The Mo centre in the complexes 1 and 2 are displaced towards the axial oxido atom by 0.318
and 0.330 Å, respectively from the cavity provided by donor atoms present in the equatorial
plane. The twelve cis angles [71.84(9)–105.9(1)° for 1; 71.4(2)–105.7(3)° for 2] and three
trans angles [151.23(9)–170.3(1)° for 1; 148.3(2)–167.5(2)° for 2] surrounding the central
Mo1 atom show significant deviations from the ideal octahedral bond angles. Due to lone
pair repulsion, the O=Mo=O angles in both 1 and 2 have values greater than 90 (Table 2),
which are in the expected range for cis-dioxomolybdenum(VI) complexes [51]. The two
10

oxido groups (O21, O22) in complexes 1 and 2 exhibit typical Mo-O double bond distances
.700(2) and 1.694(3) Å for 1; and 1.690(6) and 1.703(5) Å for 2 [52,53]. Mo-O single bond
1
lengths vary from 1.909(2) to 2.019(2) Å, and Mo-N distances vary from 2.233(2) to 2.243(5)
Å. The longest Mo-O bonds are Mo-Owater, ranging from 2.320(5) to 2.336(3) Å.
Table 2: Selected bond distances (Å) and angles (deg) for 1 and 2
Complex 1
Bond distances
Complex 2
Mo1–O1
Mo1–O12
1.907(2)
2.019(2)
1.700(2)
2.232(2)
1.694(3)
2.335(2)
Mo1–O9
1.934(3)
2.014(4)
1.703(5)
2.243(5)
1.690(6)
2.321(5)
Mo1–O14
Mo1–O22
Mo1–N11
Mo1–O21
Mo1–O31
Mo1–O21
Mo1–N9
Mo1–O22
Mo1–O31
Bond angles
O1–Mo1–N9
O1–Mo1–O12
82.1(1) O9–Mo1–N11 81.2(2)
151.2 O9–Mo1–O14 148.3(2)
O1–Mo1–O21 103.7(1) O9–Mo1–O21 99.6(2)
O1–Mo1–O22 100.4(1) O9–Mo1–O22 104.4(2)
O1–Mo1–O31 82.56(9) O9–Mo1–O31 79.3(2)
O12–Mo1–N9 71.84(9) O14–Mo1–N11 71.4(2)
O12–Mo1–O21 96.0(1) O14–Mo1–O21 97.9(2)
O12–Mo1–O22 94.0(1) O14–Mo1–O22 96.0(2)
O12–Mo1–O31 79.57(8) O14–Mo1–O31 78.0(2)
O21–Mo1–N9 156.4(1) O21–Mo1–N11 93.4(2)
O21–Mo1–O22 105.9(1) O21–Mo1–O22 105.7(3)
O21–Mo1–O31 82.2(1) O21–Mo1–O31 167.5(2)
O22–Mo1–N9 95.3(1) O22–Mo1–N11 158.6(2)
O22–Mo1–O31 170.3(1) O22–Mo1–O31 86.2(2)
O31–Mo1–N9 75.86(8) O31–Mo1–N11 74.0(2)
11

The extensive ligand-based hydrogen-bonding interactions in the respective mononuclear
complexes have resulted in two different intriguing supramolecular architectures. The
hydrogen-bond parameters are listed in Table 3. A detailed investigation of the crystal
structure reveals that complex 1 forms a self-assembled H-bonded dimer. The coordinated
water molecule of one unit acts as hydrogen bond donor to the acceptor enolic oxygen atom
(
O12) of an adjacent unit through cooperative O31-H311···O12 hydrogen bond interaction
forming a dimer. Furthermore, adjacent dimers are connected by O31-H312···N10 hydrogen
bonds involving the imidic N atom (Figure 3a). As a consequence of the interplay of such
intermolecular hydrogen bonds a 2D supramolecular assembly is developed along the ab
plane (Figure 3b) which is further stabilized by π···π stacking interaction.
Figure 3: Two-dimensional hydrogen bonded network in 1 viewed along the (a) ac and
(
b) bc planes
In 2, the crystal packing is different from 1 due to the presence of a methoxy group in the
ligand moiety which acts as a better hydrogen bond acceptor than the enolic oxygen. The
coordinated water molecule of one unit forms hydrogen bonding interaction with the methoxy
oxygen atom (O2) of an adjacent unit and with the imidic nitrogen atom N12 of a second
adjacent unit along the b axis. As a consequence, an infinite supramolecular 1D chain (Figure
4
) is generated through two H-bonding interactions (O31
r
–H100···O2 and O31–
H101···N12).
12

Figure 4: One-dimensional hydrogen bonded chain in 2 viewed along the (a) ab and (b)
bc planes.
Table 3. Hydrogen bond dimensions for 1 and 2
Complex 1
D-H···A
d(D-H) Å d(H···A) Å d(D···A) Å <(DHA)º
C3-H31···O22
C13-H131···O21
O31-H311···N10
O31-H312···O12
0.929
0.954
0.808
0.801
2.544
2.374
3.262(5)
3.321(5)
2.839(5)
2.819(5)
134.41
171.76
156.64
177.00
2.079
2.019
Complex 2
D-H···A
d(D-H) Å d(H···A) Å d(D···A) Å <(DHA)º
O31-H100···O2
O31-H101···N12
0.807
0.799
2.019
1.974
2.826(9)
2.773(9)
179.8(3)
179.6(3)
3
.4. Theoretical study
In the theoretical study we have analysed the hydrogen bonding interactions involving the
Mo-coordinated water molecule that is a strong H-bond donor due to the enhanced acidity of
the hydrogen atoms of the water upon coordination to the Mo(VI) metal centre. Actually, the
enhanced acidity of the water protons in numerous d-block metal complexes has been
demonstrated by theory and experiment [54]. In addition, we have also analysed the –
stacking interactions in compound 1 (Figure 5A). The absence of such interaction in 2 is due
13

to the crucial role of the methoxy substituent of the arene (Figure 5B). These interactions are
vital for understanding the final 3D architecture of the complexes.
Figure 5. X-ray fragments of compounds 1 (A) and 2 (B). Distances in Å.
In Figure 6 we have shown the theoretical models and the interaction energies of three
different dimers, retrieved from the crystallographic coordinates of complexes 1 and 2. For
complex 1 we have computed the interaction energy of the self–assembled dimer (Figure 6A)
that is large and negative (ꢀE = –22.8 kcal/mol), confirming the strong hydrogen bond donor
1
ability of the coordinated water molecule. These self–assembled dimers are connected to each
other by a combination of the Ow-H···N bonding and – stacking interactions (Figure 5A
and Figure 6B). The binding energy of this dimer (Figure 6B) is also very large and negative
(
(
ꢀE
2
= –24.6 kcal/mol). The sum of the interaction energies of both types of dimeric motifs
+ ꢀE = – 47.4 kcal/mol.
shown in Figure 6A and B) is ꢀE
1
2
As aforementioned, complex 2 forms infinite 1D supramolecular chains in the solid state,
which are stabilized by two Ow–H···O(N) hydrogen bonds involving both hydrogen atoms of
the coordinated water molecule. We have computed the interaction energy of a dimeric
fragment retrieved from this supramolecular 1D chain (Figure 6C), and the result is ꢀE
3
= –
2
5.3 kcal/mol that corresponds to both Ow–H···O and Ow–H···N hydrogen bonding
interactions. Each asymmetric unit of compound 2 in the solid state interacts with two
adjacent molecules as a consequence of the formation of the 1D the infinite chain. Thus, to
compare the 1D infinite chain of compound 1 (self-assembled dimers connected by H-bond
and π-stacking) to the one of compound 2, twice the energy of ꢀE
3
needs to be used (2 × ꢀE
3
=
–50.6 kcal/mol). Therefore, the 1D supramolecular chain is more favourable in 2 than in 1.
14

Remarkably, the presence of the methoxy group in the organic ligand of complex 2 allows it
to form a combination of H-bonds that is more stable than the combination of H-bond and π-
stacking observed in 1. The energetic analysis is thus useful to explain the different behaviour
of both complexes in the solid state.
Figure 6. Theoretical model used to calculate the – and hydrogen bonding
interactions in complexes 1 and 2. Distances in Å.
Finally, we have also computed the AIM distribution of critical points in the H-bonding and
–
stacking assemblies observed in complex 1 in order to corroborate the existence of the
interactions and to evaluate the individual contribution of the H-bonds by using the kinetic
energy (Vr) predictor developed by Espinosa et al [55]. The expression used to estimate the
HB dissociation energy is E = –½V(r) at the bond CP. At this point is should be emphasized
d
that this energy predictor is basically sensitive to the distance of the H-bonds but it is not to
strong electrostatic effects. That is, we have recently demonstrated [56,57] that the
dissociation energies of charge assisted H-bonds, which are very strong, estimated using the
V(r) predictor are identical to those of neutral H-bonds, thus evidencing that the V(r)
predictor has to be used in those cases where the H-bonds are not electrostatically enhanced.
The distribution of critical points and bond paths for the self-assembled H-bond dimer is
shown in Figure 7b. It can be observed that the QTAIM analysis (Table S1: ESI) reveals that
in addition to the conventional O–H···O interactions described above, two additional and
symmetrically equivalent C-H···O are also formed. Each H-bond is characterized by a bond
CP and bond interconnecting the H and O-atoms. The V(r) values in a.u. are also indicated in
Figure 7a and the estimated dissociation energies. They indicate that the C–H···O HB is very
weak (0.6 kcal/mol) and the O–H···O H-bond is moderately strong (4.5 kcal/mol). Therefore,
the total contribution of the H-bonds estimated using the V(r) energy predictor is 10.2
kcal/mol, which is half of the interaction energy computed using the supramolecular
15

approach (Figure 6). This confirms the fact that the V(r) energy predictor is not sensitive to
electrostatic effects that are important in 1. These are the enhanced acidity of the coordinated
water molecule and the anionic nature of the ligand. Figure 7b shows the QTAIM analysis of
the HB/π-π dimer of compound 1. It confirms the existence of the stacking interaction that is
characterized by the presence of two bond critical points (red spheres) that connect two
aromatic carbon atoms of one complex with two aromatic carbon atoms of the other complex.
In addition, the interaction is further characterized by the presence of a bond critical point
that connects one aromatic carbon atom of one molecule with the imidic carbon atom of the
other complex. Therefore, the conjugated C=N bond also participated in the – interaction.
As a consequence of the bond critical points that emerge upon complexation, several ring
critical points (yellow spheres) also characterize the interaction. We have also estimated the
contribution of the H-bond using the V(r) value at the bond CP that connects the H and N
atoms. The V(r) and Ed values are very similar to that observed for the O–H···O interaction
in the H-bonded self-assembled dimer.
Figure 7. Distribution of critical points and bond paths in the self-assembled H-bonded
simer (a) and ꢁ–ꢁ/H-bonded complex (b) observed in the solid state of compound 1.
The values of V(r) and the bond CPs are given in a.u.
4
. Concluding remarks
We have synthesized two new Mo(VI) complexes from two different acetyl-hydrazone
2
+
ligands. Structural characterization reveals that both the complexes consist of a cis-MoO
2
core incorporating coordinated tridentate hydrazone in enolic form. Both the compounds
develop hydrogen‐bonded assemblies of different dimensionalities and architectures. DFT
energetic calculations have demonstrated that the metal coordination of the water enhances
the H-bonding interaction. In complex 1, π–π interactions also contribute significantly to the
overall binding energies of the supramolecular assemblies. The presence of a methoxy group
16

in the ligand backbone of compound 2 provokes an energetically favourable supramolecular
architecture in the solid state which is also rationalized by means of the theoretical study.
Supplementary materials
CCDC 2001209 and 2001210 contain the supplementary crystallographic data for complex 1
and 2, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223-336-033; or E-
mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found
in the online version.
Acknowledgements
The authors graciously remember the expert guidance of (Late) Prof. Samiran Mitra in the
field of coordination chemistry. M.M. thanks Dr. Dipali Sadhukhan for some helpful
discussions. We acknowledge Jadavpur University, Kolkata, India for infrastructural support.
A.F. and A.B. thank financial support from MICIU/AEI from Spain, project numbers
RED2018-102331-T and CTQ2017-85821-R, FEDER funds.
References
[
1] a) D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375–1405; b) D. Braga,
F. Grepioni, Coord. Chem. Rev. 183 (1999) 19–41; c) D. Braga, F. Grepioni, Acc. Chem.
Res. 33 (2000), 601–608; d) N.J. Burke, A.D. Burrows, A.S. Donovan, R.W. Harrington,
M.F. Mahon, C.E. Price, Dalton Trans. 20 (2003) 3840–3849; e) D.M.P. Mingos, M.
Alajarin, A.E. Aliev, A.D. Burrows, K.D.M. Harris, A. Pastor, J.W. Steed, D.R. Turner,
Supramolecular Assembly via Hydrogen Bonds, vol. 1, Springer, Berlin, 2004.
[
2] a) J.-M. Lehn, Supramolecular Chemistry, Concepts and Per- spectives, VCH, New York,
995; b) J.W. Steed, J.L. Atwood, Supramolecular Chemistry, John Wiley & Sons, New
1
York, 2000; c) A.M. Beatty, Coord. Chem. Rev. 246 (2003), 131–143; d) H.W. Roesky, M.
Andruh, Coord. Chem. Rev. 236 (2003), 91–119; e) B.J. Holliday, C.A. Mirkim, Angew.
Chem. Int. Ed. 40 (2001) 2022–2043.
[
3] D. Sadhukhan, A. Ray, G. Pilet, G.M. Rosair, E. Garribba, S. Mitra, Bull. Chem. Soc.
Jpn. 8 (2011) 764–777.
[
4] V. Vrdoljak, B. Prugovecki, D.M. – Calogovic, R. Dreos, P. Siega, C. Tavagnacco, Cryst.
Growth Des. 10 (2010) 1373–1382.
17

[
5] S. Sen, C.R. Choudhury, P. Talukder, S. Mitra, M. Westerhausen, A.N. Kneifel, C.
Desplanches, N. Daro, J.P. Sutter, Polyhedron 25 (2006) 1271–1490.
[
6] D. Sadhukhan, A. Ray, G. Pilet, C. Rizzoli, G.M. Rosair, C.J. Gomez Garcia, S.
Signorella, S. Bellu, S. Mitra, Inorg. Chem. 50 (2011) 8326–8339.
[
7] P. Sathyadevi, P. Krishnamoorthy, R.R. Butorac, A.H. Cowley, N. Dharmaraj,
Metallomics 4 (2012) 498–511.
[
8] S.A. Yasrebi, H. Mobasheri, I. Sheikhshoaie, M. Rahban, Inorg. Chim. Acta 400 (2013)
22–227.
2
[
9] R.P. Bakale, G.N. Naik, C.V. Mangannavar, I.S. Muchchandi, I.N. Shcherbakov, C.
Frampton, K.B. Gudasi, Eur. J. Med. Chem. 73 (2014) 38–45.
[
(
10] S. Mukherjee, S. Chowdhury, A. Ghorai, U. Ghosh, H. Stoeckli-Evans, Polyhedron 51
2013) 228–234.
[
11] W.Q. Zhang, A.J. Atkin, I.J. Fairlamb, A.C. Whitwood, J.M. Lynam, Organometallics
3
0 (2011) 4643–4654.
[
12] V. Vrdoljak, B. Prugovecki, D.M.– Calogovic, J. Pisk, CrystEngComm 13 (2011) 4382–
390.
4
ꢏꢌꢃꢐ ꢄ. Hꢑttinger, C. ꢒꢓrster, T. Bund, ꢔ. Hinderberger, ꢄ. Heinze, Inorg. Chem. ꢕꢌ ꢇꢁꢊꢌꢁ)
180–4192.
4
[
14] H. Li, J. Wu, X. Huang, G. Lu, J. Yang, X. Lu, Q. Xiong, H. Zhang, ACS Nano 7 (2013)
0344–10353.
1
[
15] C. Wang, M. Yu, A. F. Kyle, P. Jakubec, D.J. Dixon, R.R. Schrock, A.H. Hoveyda,
Chem. Eur. J. 19 (2013) 2726–2740.
[
[
16] K.C. MacLeod, P.L. Holland, Nature Chemistry 5 (2013) 559–565.
17] H. Sugimoto, S. Tatemoto, K. Toyota, K. Ashikari, M. Kubo, T. Ogura, S. Itoh, Chem.
Commun. 49 (2013) 4358–4360.
[
18] H. Sugimoto, M. Sato, L. J. Giles, K. Asano, T. Suzuki, M.L. Kirk, S. Itoh, Dalton
Trans. 42 (2013) 15927–15930.
[
(
19] M. Bagherzadeh, L. Tahsini, R. Latifi, A. Ellern, L.K. Woo, Inorg. Chim. Acta 361
2008) 2019–2024.
18

[
20] Z.H. Zhou, Y.F. Deng, Z.X. Cao, R.H. Zhang, Y.L. Chow, Inorg. Chem. 44 (2005)
912–6914.
6
[
(
21] F. Jalilehvand, B.S. Lim, R.H. Holm, B. Hedman, K.O. Hodgson, Inorg. Chem. 42
2003) 5531–5536.
[
[
[
[
[
[
22] M. Takuma, Y. Ohki, K. Tatsumi, Inorg. Chem. 44 (2005) 6034–6043.
23] H. Marom, S. Antonov, Y. Popowski, M. Gozin, J. Org. Chem. 76 (2011) 5240–5246.
24] Z.H. Zhou, S.Y. Hou, Z.X. Cao, H.L. Wan, S.W. Ng, Polyhedron 16 (1997) 75–79.
25] B. Fischer, J.H. Enemark, P. Basu, J. Inorg. Biochem. 72 (1998) 13–21.
26] B.E. Schultz, R. Hille, R.H. Holm, J. Am. Chem. Soc., 117 (1995) 827–828.
27] D. Eierhoff, W.C. Tung, A. Hammerschmidt, B. Krebs, Inorg. Chim. Acta 362 (2009)
9
15–928.
[
[
[
[
28] U. Sandbhor, S. Padhye, E. Sinn, Trans. Met. Chem. 27 (2002) 681–685.
29] G.J.-J. Chen, J.W. McDonald, W.E. Newton, Inorg. Chem. 15 (1976) 2612–2616.
30] CrysAlis, Oxford Diffraction Ltd., Abingdon, UK, 2002.
31] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A. Guagliardi,
A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32 (1999) 115–119.
[
32] P.W. Betteridge, J.R. Carruthers, R.I. Cooper, K. Prout, D.J. Watkin, J. Appl.
Crystallogr. 36 (2003) 1487–1488.
[
33] R. Ahlrichs, M. Bär, M. Hacer, H. Horn, C. Kömel, Chem. Phys. Lett. 162 (1989) 165–
69.
1
[
34] A.V. Rozhkov, A.A. Eliseeva, S.V. Baykov, B. Galmés, A. Frontera, V.Y. Kukushkin,
Cryst. Growth Des. 20 (2020) 5908–5921.
[
35] A.V. Rozhkov, I.V. Ananyev, R.M. Gomila, A. Frontera, V.Y. Kukushkin, Inorg. Chem.
9 (2020) 9308–9314.
5
[
36] ꢖ.C. Belmont-ꢗꢘnchez, ꢅ.ꢙ. ꢚarcꢛa-ꢜubiꢝo, A. ꢒrontera, A. ꢅatilla-Hernꢘndez, A.
Castiꢝeiras, ꢖ. ꢞiclꢟs-ꢚutiꢠrrez, Crystals ꢌꢊ ꢇꢁꢊꢁꢊ) ꢁ–16.
[
[
37] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 132 (2010) 154104–154119.
38] S.B. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553–566.
19

[
39] R.F.W. Bader, Atoms in Molecules, A Quantum Theory, Clarendon, Oxford, 1990.
[
(
40] T.A. Keith, T.K Gristmill Software, Overland Park KS, USA, (aim.tkgristmill.com),
2013).
[
41] A. Ray, S. Banerjee, R.J. Butcher, C. Desplanches, S. Mitra, Polyhedron 27 (2008)
2
409–2415.
[
[
42] S. Pal, S. Pal, Eur. J. Inorg. Chem. 23 (2003) 4244–4252.
43] D. Sadhukhan, M. Maiti, A. Bauzá, A. Frontera, E. Garribba, C. J. Gomez-García, Inorg.
Chim. Acta 484 (2019) 95–103.
44] A. Ray, C. Rizzoli, G. Pilet, C. Desplanches, E. Garribba, E. Rentschler, S. Mitra, Eur.
J. Inorg. Chem. 20 (2009) 2915–2928.
[
[
45] D. Sadhukhan, M. Maiti, ꢚ. ꢡilet, A. Bauzꢘ, A. Frontera, S. Mitra, Eur. J. Inorg. Chem.
1 (2015) 1958–1972.
1
[
46] R. Dinda, P. Sengupta, S. Ghosh, H. Mayer-Figge, W.S. Sheldrick, J. Chem. Soc.,
Dalton Trans. 23 (2002) 4434–4439.
[
47] S.Y. Ebrahimipour, H. Khabazadeh, J. Castro, I. Sheikhshoaie, A. Crochet, K.M.
Fromm, Inorg. Chim. Acta 427 (2015) 52–61.
[
(
48] S. Thakurta, J. Chakraborty, G. Rosair, R.J. Butcher, S. Mitra, Inorg. Chim. Acta 362
2009) 2828–2836.
[
49] S. Banerjee, A. Ray, S. Sen, S. Mitra, D.L. Hughes, R.J. Butcher, S.R. Batten, D.R.
Turner, Inorg. Chim. Acta 9 (2008) 2692–2700.
[
50] L.M.R. Hill, M.K. Taylor, V.W. LinNg, C.G. Young, Inorg. Chem. 47 (2008) 1044–
052.
1
[
[
51] M. Nandy, S. Shit, C. Rizzoli, G. Pilet, S. Mitra, Polyhedron 88 (2015) 63–72.
52] R. Dinda, P. Sengupta, S. Ghosh, W.S. Sheldrick, Eur. J. Inorg. Chem. 2 (2003) 363–
3
69.
[
53] V. Vrdoljak, M. Cindri, D. Matkovic-Calogovi, B. Prugovecki, P. Novak, B. Kamenar,
Z. Anorg Allg. Chem. 631 (2005) 928–936.
[
[
54] R. Gilson, M.C. Durrant, Dalton Trans. 46 (2009) 10223–10230.
55] E. Espinosa, E. Molins, C. Lecomte, Chem. Phys. Lett. 285 (1998) 170–173.
20

[
56] A. García-Raso, A. Terrón, B. Balle, A. López-Zafra, A. Frontera, M. Barceló-Oliver,
J.J. Fiol, New J. Chem. 44 (2020) 12236–12246.
[
57] Y. Roselló, M. Benito, N. Bagués, N. Martínez, A. Moradell, I. Mata, J. Galcerà, M.
Barceló-Oliver, A. Frontera, E. Molins, Cryst. Growth Des. 20 (2020) 2985–2997.
Graphical Abstract
Two New Hydrogen-bonded Supramolecular Dioxo-molybdenum(VI)
Complexes Based on Acetyl-Hydrazone Ligands: Synthesis, Crystal
Structure and DFT Studies
Monami Maiti, Santarupa Thakurta, Guillaume Pilet, Antonio Bauzá, Antonio
Frontera
Two new Mo(VI) complexes from two related acetyl-hydrazone ligands have been
synthesized and characterized. These complexes have very unique extensive supramolecular
interactions. Hydrogen bonding and – interactions play an important role in the packing of
the complexes in their solid state. Computational studies are performed using DFT
calculations at BP86-D3/def2-TZVP level of theory to understand the relative stability of the
supramolecular networks.
21

Credit Author Statement
Monami Maiti: Conceptualization; Methodology; Writing- Original draft preparation
Santarupa Thakurta: Conceptualization; Writing- Reviewing and Editing
Guillaume Pilet: Software, Data Curation
Antonio Bauzá: Software
Antonio Frontera: Software; Formal analysis; Writing- Reviewing and Editing
Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
22