Trinuclear cis-dioxidomolybdenum(VI) complexes of compartmental C3 symmetric ligands: Synthesis, characterization, DFT study and catalytic application for hydropyridines (Hps) via the Hantzsch reaction

Article abstract of DOI:10.1016/j.poly.2020.114617

Trinuclear cis-dioxidomolybdenum(VI) complexes of the type [{Mo^VIO₂(MeOH)}₃L^1-7] (1–7) have been synthesized using tris(H₂ONO) donor ligands [H₆L^1-7 (I–VII)] assembled from benzene-1,3,5-tricarbohydrazide (bthz) and the corresponding salicylaldehyde (sal). All the ligands and the complexes were characterized by numerous techniques, such as FT-IR, UV–visible, NMR (¹H and ¹³C) spectroscopy, electrochemical study, elemental analysis, thermogravimetric study and single crystal X-ray diffraction of the ligand III and complexes 1 and 5. In the presence of H₂O₂ as an oxidant, these complexes show excellent catalytic potential towards the one-pot three-components [ethyl acetoacetate, benzaldehyde (or its derivatives) and ammonium acetate] dynamic covalent assembly in the Hantzsch reaction. Under solvent free conditions, as high as 98% conversion along with 100% selectivity towards diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate (1,4-DHP) has been achieved in 1 h. Although solvents do not improve the conversion, they do influence the selectivity of the products. With the elapse of time, the conversion of dihydropyridine to the diethyl 2,6-dimethyl-4-phenylpyridine-3,5-dicarboxylate derivative occurs and completes in ca. 10 h with a distinct color change, showing the importance of the catalysts. Efforts have been made to provide suitable reaction pathways for the catalytic reaction based on spectroscopic and density functional theory studies.

Full text of DOI:10.1016/j.poly.2020.114617

Journal Pre-proofs
Trinuclear cis-dioxidomolybdenum(VI) complexes of compartmental C3 sym‐
metric ligands: Synthesis, characterization, DFT study and catalytic applica‐
tion for hydropyridines (Hps) via the Hantzsch reaction
Mannar R. Maurya, Reshu Tomar, Puneet Gupta, Fernando Avecilla
PII:
S0277-5387(20)30274-6
DOI:
Reference:
https://doi.org/10.1016/j.poly.2020.114617
POLY 114617
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Polyhedron
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19 May 2020
Please cite this article as: M.R. Maurya, R. Tomar, P. Gupta, F. Avecilla, Trinuclear cis-dioxidomolybdenum(VI)
complexes of compartmental C3 symmetric ligands: Synthesis, characterization, DFT study and catalytic
application for hydropyridines (Hps) via the Hantzsch reaction, Polyhedron (2020), doi: https://doi.org/10.1016/
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Trinuclear cis-dioxidomolybdenum(VI) complexes of compartmental C3 symmetric ligands:
Synthesis, characterization, DFT study and catalytic application for hydropyridines (Hps)
via the Hantzsch reaction
Mannar R. Maurya*,a, Reshu Tomara, Puneet Guptaa, Fernando Avecillab
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee–247667, India. E–
mail: m.maurya@cy.iitr.ac.in; Fax: +91 1332 273560; Tel: +91 1332 285327.
b Grupo Xenomar, Centro de Investigacións Científicas Avanzadas (CICA), Departamento de
Química, Facultade de Ciencias, Universidade da Coruña, Campus de A Coruña, 15071 A
Coruña, Spain.
Keywords:
Dioxidomolybdenum(VI) complexes
Structure elucidation
Homogeneous catalysis
Hantzsch reaction
DFT studies.
Highlights
1. Trinuclear dioxidomolybdenum(VI) complexes are reported.
2. The optimized structures deduced planar geometries of the complexes.
3. These complexes show excellent catalytic potential towards the one-pot three-components
[ethyl acetoacetate, benzaldehyde (or its derivatives) and ammonium acetate] dynamic
covalent assembly in the Hantzsch reaction.
4. With the elapse of time, the conversion of dihydropyridine to the diethyl 2,6-dimethyl-4-
phenylpyridine-3,5-dicarboxylate derivative occurs and completes in ca. 10 h with a
distinct color change, showing the importance of the catalysts.
_________________________________________
*Correspondence: Mannar R Maurya, E–mail: m.maurya@cy.iitr.ac.in; Fax: +91 1332 273560;
Tel: +91 1332 285327
1

ABSTRACT
Trinuclear cis-dioxidomolybdenum(VI) complexes of the type [{MoVIO2(H2O)}3L1-7] (1–7) have
been synthesized using tris(H2ONO) donor ligands [H6L1-7 (I–VII)] assembled from benzene-
1,3,5-tricarbohydrazide (bthz) and the corresponding salicylaldehyde (sal). All the ligands and the
complexes were characterized by numerous techniques, such as FT-IR, UV-visible, NMR (1H and
13C) spectroscopy, electrochemical study, elemental analysis, thermogravimetric study and single
crystal X-ray diffraction of the ligand III and complexes 1 and 5. In the presence of H2O2 as an
oxidant, these complexes show excellent catalytic potential towards the one-pot three-components
[ethyl acetoacetate, benzaldehyde (or its derivatives) and ammonium acetate] dynamic covalent
assembly in the Hantzsch reaction. Under solvent free conditions, as high as 98% conversion along
with 100% selectivity towards diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-
dicarboxylate (1,4-DHP) has been achieved in 1 h. Although solvents do not improve the
conversion, they do influence the selectivity of the products. With the elapse of time, the
conversion of dihydropyridine to the diethyl 2,6-dimethyl-4-phenylpyridine-3,5-dicarboxylate
derivative occurs and completes in ca. 10 h with a distinct color change, showing the importance
of the catalysts. Efforts have been made to provide suitable reaction pathways for the catalytic
reaction based on spectroscopic and density functional theory studies.
1. Introduction
High-valent dioxidomolybdenum(VI) complexes with a cis-[MoVIO2] core have had
intense research focus in recent years due to their increased catalytic applications, e.g. oxygen
atom transfer (OAT), olefin epoxidation, oxidation/ sulfoxidation, oxidative bromination [1] and
some bio mimetic reactions [1b]. However, dinuclear and trinuclear dioxidomolybdenum(VI)
complexes [2,3] with an open system, in which all metal centers are easily accessible to the
substrate, may enhance their catalytic activity [3]. Such an idea inspired us to synthesize trinuclear
dioxidomolybdenum(VI) complexes of tritopic phloroglucinol based ligands which have already
been seen to show excellent catalytic activity in the oxidation of olefins [4].
Experimenting with the catalytic activity of such trinuclear molybdenum complexes for the
aromatization of 1,4-dihydropyridine (1,4-DHP) via the one pot multicomponent Hantzsch
reaction is an important task to take up as there are several new molecules having 1,4-DHP as
2

parent nuclei, which are clinically important drugs and used worldwide. Examples include
Lercanidipine, Aranidipine, Cilnidipine, Efonidipine hydrochloride ethanol, Barnidipine
hydrochloride, Lacidipine, Amlodipine besylate, Nisoldipine, Felodipine, Nimodipine etc. [5].
Apart from these, 1,4-DHPs are also used as a synthetic tool for selective –C=C reduction of α,
β-unsaturated double bonds [6], endocyclic double bonds in coumarins [7], pyrazoles [8], –C=N
reduction in aldimines [9], quinolones [10], reductive cyclisation of allylic and benzylic bromides
[11]. Further, the involvement of aromatized DHPs in the metabolic process during enzymatic
oxidation also inspired researchers to synthesize their pyridine derivatives. Therefore, the synthesis
of DHPs via the Hantzsch reaction and their aromatization have been carried out by numerous
catalysts [12]. These catalysts require expensive reagents and challenging reaction conditions, but
still end up with a low selectivity of the corresponding pyridine compound. In addition, the
formation of the corresponding pyridine derivatives requires other oxidizing agents in the reaction,
like NaNO2 [13], metallic nitrates [14], CrO2 [15], Yb(OTf)3 [16],nitric acid [17], HZSM-5 zeolite
[18], DDQ [19], bentonite clay [20], KMnO4 [21], hyper valent iodine reagent [22],
electrochemical oxidation [23], microwave [24] and simple air with a DMSO [25] supported
catalyst with an ionic liquid layer (SCILL) [26] etc. Thus, researchers have moved towards metal
complexes like Schiff based complexes of molybdenum [27,3e], vanadium [28] and manganese
[29] and metal salts like SbCl5 [30], MoOCl4 [31], FeF3 [32], RuCl3/O2 [33], Co(OAc)2/H2O2 [34]
etc. Due to the interest in synthesizing various DHPs and their corresponding pyridines, we have
explored the catalytic activity of mononuclear and binuclear dioxidomolybdenum(VI) complexes
towards the three component dynamic covalent assembly via the Hantzsch reaction [3e]. We have
now successfully prepared a new series of trinuclear dioxidomolybdenum(VI) complexes with
compartmental C3 symmetric ligands based on benzene-1,3,5-tricarbohydrazide, (see Scheme 1).
After complete characterization of these complexes, we have tested their catalytic activity for the
aromatization of 1,4-dihydropyridine (1,4-DHP) via a one pot multicomponent Hantzsch reaction.
We have optimized the reaction conditions to improve the catalytic activity of the synthesized
complexes and to tune selectivity towards the pyridine derivative. We have also provided probable
intermediates formed during the catalytic process using density functional theory computations.
<<Scheme 1>>
3

2. Experimental section
2.1. Materials, instrumentation and characterization procedures
All analytical reagent grade chemicals and solvents were obtained from standard sources and were
used without further purification. The precursors benzene-1,3,5-tricarbohydrazide [35] and
[MoVIO2(acac)2] [36] were prepared using standard reported procedures.
Elemental analyses (C, H and N) of the ligands and the complexes were performed with
an Elementar model Vario-EL-III. IR Spectra were recorded in KBr discs on a Nicolet NEXUS
Aligent 1100 FT-IR spectrometer. Electronic spectra of the ligands in DMSO and the complexes
1
13
in MeOH were recorded using a Shimadzu 2450 UV-Vis spectrophotometer. H and C NMR
spectra were obtained in DMSO-d6 with a JEOL (400 MHz) spectrometer using the common
parameter settings. Thermogravimetric analyses of the complexes were acquired using a TG
Stanton Redcroft STA 780. The voltammograms of the ligands and the metal complexes were
recorded in DMF between –1.8 and +1.8 V vs. SCE using 0.1 M TBAPF6 as the supporting
electrolyte with a 100 mV/s scan rate at room temperature. The catalytic reactions were monitored
by a Shimadzu 2010 plus gas chromatograph fitted with a Rtx-1 capillary column (30 m × 0.25
mm × 0.25 µm) and an FID detector. The product formation was confirmed by a GC–MS model
Perkin-Elmer, Clarus 500 by comparing the fragments of each product with the library available.
2.2. Preparations
2.2.1. Ligands H6L1–7
The ligands H6L1–7 were prepared by similar methods. A representative method for I is
presented here. A solution of salicylaldehyde (1.46 g, 12 mmol) in MeOH (30 mL) was added to
a methanolic suspension of benzene-1,3,5-tricarbohydrazide (0.504 g, 2 mmol). The reaction
mixture was refluxed on an oil bath. During this period, the benzene-1,3,5-tricarbohydrazide
slowly dissolved and simultaneously a white solid precipitated. After 24 h of reflux, the reaction
mixture was cooled at room temperature for an hour, the precipitated solid was filtered, washed
with MeOH (2 × 5 mL) and dried in a desiccator over silica gel.
Data for H6L1 (I): Yield 0.954 g (85%). Anal. Calc. for C30H24N6O6 (564): C, 63.83; H,
4.29; N, 14.89. Found: C, 65.31; H, 4.82; N, 13.83%. IR data (cm–1): 3440 (O–H), 3018 (N–H),
1659 (C=O), 1563 (C=N).
4

Data for H6L2 (II): Yield 0.920 g (51%). Anal. Calc. for C54H72N6O6 (901): C, 71.97; H,
8.05; N, 9.33. Found: C, 71.20; H, 8.23; N, 8.83%. IR data (cm–1): 3270 (O–H), 2951 (N–H), 1660
(C=O), 1560 (C=N).
Data for H6L3 (III): Yield 0.980 g (75%). Anal. Calc. for C33H30N6O9 (654): C, 60.55; H,
4.62; N, 12.84. Found: C, 60.10; H, 4.41; N, 12.63%. IR data (cm–1): 3436 (O–H), 3012 (N–H),
1642 (C=O), 1556 (C=N). Single crystals of III suitable for X-ray analysis were grown from DMF.
The crystal of III is now formulated as H6L3·3DMF (IIIa).
Data for H6L4 (IV): Yield 1.00 g (82%). Anal. Calc. for C30H24N6O9 (612): C, 58.82; H,
3.95; N, 13.72. Found: C, 59.15; H, 4.20; N, 13.63%. IR data (cm–1): 3438 (O–H), 2980 (N–H),
1678 (C=O), 1552 (C=N).
Data for H6L5 (V): Yield 0.846 g (63%). Anal. Calc. for C30H21Cl3N6O6 (667): C, 53.95;
H, 3.17; N, 12.58. Found: C, 53.60; H, 3.32; N, 12.63%. IR data (cm–1): 3422 (O–H), 3010 (N–
H), 1661 (C=O), 1560 (C=N).
Data for H6L6 (VI): Yield 0.918 g (76%). Anal. Calc. for C33H30N6O6 (606): C, 65.34; H,
4.98; N, 13.85. Found: C, 65.10; H, 4.60; N, 13.48%. IR data (cm–1): 3434 (O–H), 2989 (N–H),
1671 (C=O), 1567 (C=N).
Data for H6L7 (VII): Yield 0.992 g (64%). Anal. Calc. for C42H51N9O6 (777): C, 64.85; H,
6.61; N, 16.20. Found: C, 64.45; H, 6.33; N, 16.61%. IR data (cm–1): 3442 (O–H), 3015 (N–H),
1653 (C=O), 1558 (C=N).
2.2.2. [{MoVIO2(MeOH)}3L1] (1)
To a suspension of I (0.564 g, 1 mmol) in MeOH (20 mL) was added a filtered solution of
[MoVIO2(acac)2] (1.14 g, 3.5 mmol) in dry/absolute MeOH (20 mL) with stirring, whereupon the
ligand dissolved within a few minutes. The orange solution was refluxed on a water bath for 6 h.
The yellow solid that separated after cooling the reaction mixture to room temperature for 1 h was
collected by filtration, washed with cold MeOH followed by petroleum ether and dried in a
desiccator over silica gel. Yield 0.920 g (89%). Anal. Calc. for C33H30Mo3N6O15 (1038.51): C,
38.17; H, 2.91; N, 8.09. Found: C, 38.12; H, 2.94; N, 8.07%. IR data (cm–1): 3450 (O–H), 1550
(C=N), 1248 (C–O), 923 (O=Mo=O)asym, 901 (O=Mo=O)sym. Single crystals of 1 suitable for X-
ray analysis were grown from DMSO. The crystal of complex 1 is now formulated as
[{MoO2(DMSO)}3L1] (1a).
5

2.2.3. [{MoVIO2(MeOH)}3L2] (2)
Complex 2 was prepared from II (0.901 g, 1 mmol) in MeOH (20 mL) and a filtered
solution of [MoVIO2(acac)2] (1.14 g, 3.5 mmol) in dry/absolute MeOH (20 mL) following the
procedure outlined for 1. Yield 1.010 g (74%). Anal. Calc. for C57H78Mo3N6O15 (1375.15): C,
49.79; H, 5.72; N, 6.11. Found: C, 49.72; H, 5.70; N, 6.13%. IR data (cm–1): 3430 (O–H), 1508
(C=N), 1256 (C–O), 921 (O=Mo=O)asym, 905 (O=Mo=O)sym
.
2.2.4. [{MoVIO2(MeOH)}3L3] (3)
This complex was prepared similarly as reported for 1 from [MoVIO2(acac)2] (1.14 g, 3.5
mmol) and III (0.654 g, 1 mmol) in dry/absolute MeOH. Yield 0.820 g (73%). Anal. Calc. for
C36H36Mo3N6O18 (1128.58): C, 38.31; H, 3.22; N, 7.45. Found: C, 38.32; H, 3.24; N, 7.42%. IR
data (cm–1): 3450 (O–H), 1540 (C=N), 1262 (C–O), 932 (O=Mo=O)asym, 904 (O=Mo=O)sym
.
2.2.5. [{MoVIO2(MeOH)}3L4] (4)
Complex 4 was prepared in dry/absolute MeOH using [MoVIO2(acac)2] (1.14 g, 3.5 mmol)
and IV (0.612 g, 1 mmol) adopting the method reported for 1. Yield 0.934 g (86%). Anal. Calc.
for C33H30Mo3N6O18 (1086.50): C, 36.48; H, 2.78; N, 7.74. Found: C, 36.42; H, 2.74; N 7.75%.
IR data (cm–1): 3470 (O–H), 1541 (C=N), 1264 (C–O), 933 (O=Mo=O)asym, 909 (O=Mo=O)sym
.
2.2.6. [{MoVIO2(MeOH)}3L5] (5)
Complex 5 was prepared in dry/absolute MeOH using [MoVIO2(acac)2] (1.14 g, 3.5 mmol)
and V (0.667 g, 1 mmol) adopting the method reported for 1. Yield 0.900 g (79%). Anal. Calc. for
C33H27Mo3Cl3N6O15 (1141.83): C, 34.71; H, 2.38; N, 7.36. Found: C, 34.72; H, 2.33; N, 7.32%.
IR data (cm–1): 3445 (O–H), 1542 (C=N), 1254 (C–O), 940 (O=Mo=O)asym, 910 (O=Mo=O)sym
Single crystals of 5 suitable for X-ray analysis were grown from DMSO-DMF (1:9). The crystal
of complex is now formulated as
.
5
[{MoO2(0.5H2O·0.5MeOH)}{MoO2(DMF)}{MoO2(DMSO)}L2]·DMF·0.5DMSO (5a).
6

2.2.7. [{MoVIO2(MeOH)}3L6] (6)
Complex 6 was prepared in dry/absolute MeOH using [MoVIO2(acac)2] (1.14 g, 3.5 mmol)
and VI (0.606 g, 1 mmol) adopting the method reported for 1. Yield 0.860 g (80%). Anal. Calc.
for C36H36Mo3N6O15 (1080.59): C, 40.01; H, 3.36; N 7.78. Found: C, 40.02; H, 3.33; N, 7.72%.
IR data (cm–1): 3448 (O–H), 1547 (C=N), 1265 (C–O), 943 (O=Mo=O)asym, 912 (O=Mo=O)sym
.
2.2.8. [{MoVIO2(MeOH)}3L7] (7)
Complex 7 was prepared in dry/absolute MeOH using [MoVIO2(acac)2] (1.14 g, 3.5 mmol)
and VII (0.777 g, 1 mmol) following the method reported for 1. Yield 0.860 g (69%). Anal. Calc.
for C45H57Mo3N9O15 (1251.87): C, 43.17; H, 4.59; N, 10.07. Found: C, 43.12; H, 4.53; N, 10.02%.
IR data (cm–1): 3451 (O–H), 1538 (C=N), 1257 (C–O), 934 (O=Mo=O)asym, 908 (O=Mo=O)sym
.
2.3. X-ray crystal structure determination
Three-dimensional X-ray data were collected on a Bruker Kappa Apex CCD diffractometer
at low temperature for compounds III, 1 and 5 by the - scan method. Reflections were measured
from a hemisphere of data collected from frames, each of them covering 0.3º in . A total of 82697
for III, 41831 for 1 and 89803 for 5 measured reflections were corrected for Lorentz and
polarization effects and for absorption by multi-scan methods based on symmetry-equivalent and
repeated reflections. Of the total, 6848 for III, 3373 for 1 and 6809 for 5 independent reflections
exceeded the significance level (F/F) > 4.0. After data collection, in each case a multi-scan
absorption correction (SADABS) [37] was applied, and the structures were solved by direct
methods and refined by full matrix least-squares on F2 data using the SHELX suite of programs
[38]. Hydrogen atoms were included in calculated positions and refined in the riding mode in 1
and 5. In 5, some hydrogen atoms of solvent molecules were not located due to disorder. In III,
hydrogen atoms were located in the difference Fourier map and left to refine freely, except for
C(1D), O(2), C(2D), C(4D), C(5D), C(6D), C(7D), O(8), C(8D), C(15), C(24) and C(33), which
were included in calculated positions and refined in the riding mode. Refinements were done with
allowance for thermal anisotropy of all non-hydrogen atoms, except for O(3DB), due to the
disorder of this DMF molecule.
The DMSO molecule coordinated to the Mo atom in 1 presents an important disorder. This
disorder has been refined and two atomic sites have been observed and refined with anisotropic
7

atomic displacement parameters for the methyl and sulfur atoms of DMSO. More specifically this
disorder was refined using 49 restraints (SADI, SIMU and DELU restraints were used). The site
occupancy factor was 0.62165 for S(1A)-C(1MA)-C(2MA). In 5, a water or methanol molecule
occupies a coordination position and the disorder was refined. The site occupancy factor was
0.72545 for O(1WA). A final difference Fourier map of III showed no residual density outside
0.834 and -0.267 e Å–3.
For 1, a final difference Fourier map showed an important residual density, 4.248 and –
1.013 e Å–3, next to the DMSO molecule coordinated to the Mo atom, due to the slip plane that
contains a symmetrical spatial group, such as R3, and the impossibility of locating a third disorder
position for the S(1), C(1M) and C(2M) atoms of DMSO. For 5, the final difference Fourier map
showed high values due to the disorder of the solvent molecules, which could be refined: 1.706
2
and –1.229 e Å–3. Weighting schemes w =1/[σ2(Fo ) + (0.096600 P)2 + 0.103000 P] for III,
2
2
1/[σ2(Fo ) + (0.106900 P)2 + 160.176315 P] for 1 and 1/[σ2(Fo ) + (0.177300 P)2 + 0.000000 P] for
5, where P = (|Fo|2 + 2|Fc|2)/3, were used in the latter stages of refinement. Further details of the
crystal structure determinations are given in Table 1.
<<Table 1>>
2.4. Computational details
All calculations were performed using the ORCA quantum chemical program package
[39]. Geometries were optimized with the hybrid-GGA (generalized gradient approximation)
density functional B3LYP [40] in conjunction with def2-TZVP(-f) basis sets [41], without f
polarization functions. To accelerate the overall calculations, the RIJCOSX [42] (resolution-of-
the-identity for Coulomb (RIJ) with a chain of sphere (COS) algorithm for the Hartree-Fock (HF)
exchange (X) part) approximation was applied for the expensive integral calculations. Non-
covalent interactions were accounted for by using atom-pairwise dispersion corrections with
Becke-Johnson (D3BJ) damping [43]. Subsequent numerical frequency calculations were
undertaken for the optimized geometries to confirm that they correspond to stationary points
featuring no imaginary frequencies. The zero-point vibrational energies, thermal corrections and
entropy terms were obtained from the frequency calculations.
8

2.5. Catalytic activity study: One-pot three-component dynamic covalent assembly via the
Hantzsch reaction
All the catalytic reactions were carried out following a general procedure. In a typical
reaction, benzaldehyde (0.53 g, 0.0050 mol), ethyl acetoacetate (1.1 g, 0.010 mol) and ammonium
acetate (0.38 g, 0.0050 mol) were mixed together in a 50 mL round-bottom two necked glass
reaction flask assembled with a water cooled reflux condenser. The appropriate catalyst (0.0010
g) and aqueous 30% H2O2 (0.005, 0.010 or 0.015 mol) under solvent free conditions were added
and the reaction mixture was stirred at a fixed temperature using an oil bath. The progress of the
reaction was examined by taking out small aliquots of the reaction mixture, extracting with n-
hexane and injecting into the GC column. The identities of the products were confirmed by GC-
MS, HRMS and 1H NMR spectroscopy.
3. Results and discussions
Reaction of benzene-1,3,5-tricarbohydrazide [36] with different derivatives of
salicylaldehyde in a 1:3 molar ratio in a MeOH suspension resulted in the formation of the
analytically pure ligands I–VII in good yields (Scheme 1). A suspension of these ligands in MeOH
react with [MoVIO2(acac)2] [37] in a 1:3 molar ratio under reflux conditions to give the
corresponding trinuclear dioxidomolybdenum(VI) complexes [{MoVIO2(MeOH)}3L1] (1),
[{MoVIO2(MeOH)3}L2] (2), [{MoVIO2(MeOH)3}L3] (3), [{MoVIO2(MeOH)3}L4] (4),
[{MoVIO2(MeOH)3}L5] (5), [{MoVIO2(MeOH)3}L6] (6) and [{MoVIO2(MeOH)3}L7] (7) of high
C3-symmetry. Scheme 2 presents the proposed structure of the complexes, which are based on
various evidences whose descriptions are presented below.
<<Scheme 2>>
3.1. Description of the molecular structures of the compounds
ORTEP diagrams for the compounds IIIa, 1a and 5a (IIIa, 1a and 5a are the crystallised
forms of III, 1 and 5, respectively) are shown in Figs. 1, 2 and 3, respectively. Details of the crystal
structure determinations are given in the supporting information. Selected bond distances and
angles of these structures are given in Table 2.
9

Compound IIIa crystallizes with three DMF molecules. In the crystal packing (see Fig.
S1) the principal interactions between the molecules are hydrogen bonds (see Table 3) and C-O···π
interactions. The distance between the O(5H) atom and the next phenyl ring of another molecule,
dO(5H)-c5 [c5, C(18I)-C(19I)-C(20I)-C(21I)-C(22I)-C(23I)], is 3.500(3) Å and the distance between
the O(4H) atom and the centroid situated in the middle of hydrazone group, [c6, N(3H)-N(4H)],
is 3.402 Å. Each arm of the ligand is essentially planar [mean deviation from planarity of the atoms
C(7), C(8), C(9), C(10), C(11), C(12), C(13), C(14), N(1), N(2), O(1) and O(2), plane 1,
0.0242(25) Å; C(16), C(17), C(18), C(19), C(20), C(21), C(22), C(23), N(3), N(4), O(4) and O(5),
plane 2, 0.0513(22) Å; C(25), C(26), C(27), C(28), C(29), C(30), C(31), C(32), N(3), N(4), O(7)
and O(8), plane 3, 0.1407(21) Å], but the three arms are not coplanar. The dihedral angles between
the arms are, 13.54(9)º between planes 1 and 2, 16.98(7)º between planes 2 and 3, and 3.45(8)º
between planes 3 and 1.
<<Figure 1>>
The molecular structures of compounds 1a and 5a present trinuclear complexes which are
best described as having a six-coordinated distorted octahedral geometry around each Mo center.
These structures present a high C3-symmetry, with a phenyl group in the center and three arms
where the Mo atoms appear coordinated. Each Mo atom is coordinated by two oxide atoms of the
dioxido cis-[MoVIO2] moiety [Mo(1)-O(1), 1.710(4) Å and Mo(1)-O(2), 1.694(5) Å in 1a and
Mo(1)-O(1), 1.697(7)Å and Mo(1)-O(2), 1.686(7) Å in 5a], like in other similar compounds. All
the Mo=O bonds are similar to each other in regard to their bond lengths and have the characteristic
oxido-type O atoms. The equatorial planes are occupied by one nitrogen atom and three oxygen
atoms, one of which is a Mo=O bond [Mo-O(1)]. The other three atoms belong to the ligand around
each Mo center. The last axial position is occupied by one oxygen atom of a DMSO molecule in
1a and of H2O or MeOH, DMF and DMSO in 5a. Again, each arm of the ligand is essentially
planar [mean deviation from planarity of the atoms C(3), C(4), C(5), C(6), C(7), C(8), C(9), C(10),
N(1), N(2), O(3) and O(4), 0.0428(50) Å in 1a and C(7), C(8), C(9), C(10), C(11), C(12), C(13),
C(14), N(1), N(2), O(3) and O(4), 0.0655(75) Å, C(15), C(16), C(17), C(18), C(19), C(20), C(21),
C(22), N(3), N(4), O(7) and O(8), 0.0344(6) Å and C(23), C(24), C(25), C(26), C(27), C(28),
C(29), C(30), N(5), N(6), O(11) and O(12), 0.0839 Å in 5a], but the three arms are not coplanar.
The dihedral angles between the planes defined for the arms were measured and identical angles
10

of 26.9(3)º were found due to the high symmetry. These positions of the arms and the coordination
of the DMSO molecule, which is situated in the holes between the complexes, limit the interaction
between them in the crystal packing in 1a (see Fig. S2). In 5a, two of the arms [C(15)-C(22), N(3),
N(4), O(7), O(8) and C(23)-C(30), N(5), N(6), O(11), O(12)] are approximately co-planar with a
dihedral angle of 2.67(5)o between them, and the third arm [C(7)-C(14), N(1), N(2), O(3), O(4)]
makes angles of 11.73(4) and 14.3(6)o with the first two arms, respectively. Additionally, arms in
adjacent molecules show π-π stacking interactions between them (see Fig. S3).
<<Figures 2 and 3>> <<Tables 2 and 3>>
3.2. Thermal study
In general, the thermal analyses of the complexes show their stability up to 100 °C and
then their mass reduces exothermically between 100 and 167 °C, which is equivalent to three
coordinated MeOH molecules, indicating their coordination to the molybdenum atoms. Upon a
further increase in temperature, the anhydrous complexes decompose exothermically in two to
three overlapping steps and form MoO3 at ca. 530 °C. Fig. 4 provides the TGA profiles of
complexes 2 and 4 with the mass lost at the two major stages.
<<Figure 4>>
3.3. FT-IR spectroscopic studies
In order to confirm the coordinating functionalities, the IR spectra of the free ligands were
compared with the corresponding metal complexes and the results are listed in Table S1. The IR
spectra of the ligands register two broad bands at 3018-3167 and 1617-1648 cm–1 due to ν(NH)
and ν(C=O) (of the hydrazide moiety) stretches, respectively. The absence of these bands and the
appearance of a new band at ca. 1250 cm–1 in the metal complexes are in line with the enolization
of the ketonic groups of the hydrazide moieties followed by coordination of the enolate oxygen
atoms. The coordination of phenolic oxygen atom cannot be revealed unequivocally due to the
presence of a methanol molecule [ν(O-H) at 3424-3450 cm–1] associated with the complexes.
However, the coordination of the phenolic oxygen atom after deprotonation has been confirmed
in 1a and 5a by the single crystal X-ray study (vide supra). All these complexes exhibit two
prominent IR bands at 921-943 and 901-912 cm–1, which are assigned to νasym(O=Mo=O) and
νym(O=Mo=O) modes, respectively due to the cis-[MoO2] structure [44]. Further, only two sharp
11

peaks in all the complexes for these modes suggest that the Mo centers in the tris[cis-MoO2]-
complexes are arranged in a symmetrical fashion.
3.4. UV/Vis spectroscopic studies
The electronic absorption spectra of the ligands and their tris[cis-MoO2]-complexes were
recorded in DMSO due to their poor solubility in other solvents. Their absorption maxima and
extinction coefficients are listed in Table S2. All the ligands exhibit three bands at 290-299, 302-
309 and 330-346 nm (Fig. S4). First two bands seem to be split bands of a π → π* transition as
they have almost the same extinction coefficient, while last band is assignable to a n → π*
transition. In most cases, the n → π* transition shifts to a lower wavenumber upon coordination
of the ligands to the metal ion due to a structural rearrangement within the ligand, while the other
two bands collapse and appear as a single band at a lower wavelength. In addition, a new band is
generated at 420–488 nm in all the complexes due to charge transfer of electrons from the filled
p-orbitals of the ligand to the vacant d-orbitals of the metal ion (L → M) in the proper orientation.
3.5. 1H and 13C NMR spectroscopic studies
The coordination behavior of the ligands towards the molybdenum ion was ascertained by
1
1
comparing their H NMR spectra with those of the corresponding complexes. The H NMR
spectral data of the ligands and the complexes, acquired in DMSO-d6, are collected in Table 4
along with the J values for the aromatic protons and representative spectra (e.g. of ligand I and
complex 1) of the series are reproduced in Fig. S5. A sharp signal at δ 12.22-12.92 ppm in the
ligands’ spectra is indicative of their existence in the ketonic form. The absence of this signal in
the spectra of all the complexes is in accordance with the enolization of the ketonic group of the
ligands followed by coordination of the enolate O atoms to the Mo atoms. Similarly, the absence
of a phenolic proton in the complexes, that appears at δ 11.16-12.61 ppm in the ligands, suggests
the coordination of all three phenolate oxygen atoms to the Mo atoms. The protons of the
azomethine group and benzene ring of the hydrazide moiety resonate at ca. δ 8.73 ppm in the
ligands, but in the complexes, the azomethine protons’ signals shift considerably downfield while
the signals for the aromatic protons remain almost unchanged. The downfield shift of the
azomethine proton signifies the coordination of azomethine nitrogen atom. Other aromatic protons
of the ligands and the complexes appear within the expected regions, with only slight variations.
12

Thus, the binding behavior of each unit of ligands is ONO dibasic tridentate in all the metal
complexes.
<<Table 4>>
The coordination behavior of the ligands towards the tris[cis-MoO2] complexes was further
confirmed by comparing the 13C NMR data of all the ligands with their complexes. The 13C NMR
spectra of the representative ligand I and complex 1 are given in Fig. S6 and the complete data are
collected in Table S3. The coordination of the O, N and O atoms of each pocket of the ligands to
the metal ion has direct influence on the carbon atoms present in the vicinity of these coordination
functionalities due to adjustment of the electronic environment. Thus significant coordination-
induced 13C NMR chemical shifts [Δδ = δ(complex) − δ(ligand)] for the carbon atoms associated
with the phenolic oxygen atoms (C6/C6’/C6’’) (Δδ = 2.3 – 7.67 ppm), the azomethine nitrogen
atoms (C4/C4’/C4’’) (Δδ = 2.93 – 8.59 ppm) and enolic oxygen atoms (C3/C3’/C3’’) (Δδ = 2 –
12.71 ppm) have been observed (Table 3) due to coordination of these atoms. Other carbon atoms,
i.e. C9/C9’/C9’’, C7/C7’/C7’’, C10/C10’/C10’’, C8/C8’/C8’’, C2/C2’/C2’’, C1/C1’/C1’’,
C5/C5’/C5’’ are also affected, but only slightly, due to electronic rearrangements. In complexes 2,
3, 6 and 7, a new signal also appears at δ 47.87, 20.43, 20.50 and 56.32 ppm due to the presence
of CH2-CH3, CH3 and OMe groups, respectively.
3.6. Electrochemical properties
The cyclic voltammetry (CV) and differential pulse voltametry (DPV) of ligands I-VII and
complexes 1-7 were studied in DMF solution to explore their redox behavior in the solution state
(Table 5). Representative cyclic voltammograms and differential pulse voltammograms of I and 1
are presented in Fig. 5. The CV trace of complex 1 displays two cathodic peaks at –1.02 and –
0.732 V due to the Mo(VI)→ Mo(V) reduction and the redox behavior of the ligand, respectively.
The nature of the later peak was confirmed by analyzing the CV of ligand I recorded in the same
range, which displays an oxidative peak at 0.613 V and a reductive peak at –0.715 V. The first
cathodic peak is well within the range reported for dioxidomolybdenum(VI) complexes of dibasic
tetradentate salen type ligands, but it is a little lower than the range observed for trinuclear
dioxidomolybdenum(VI) complexes of tritopic 2,4,6-triacetylphloroglucinol based ligands [45].
The reductive peaks of the complex are irreversible in nature in polar aprotic solvents,
corresponding to the Mo(VI)/Mo(V) reduction [46]. The oxidative peak due to metal-centered
13

oxidation of the molybdenum complex is not observed during the anodic scan, probably due to the
fast decomposition of the reduced species [45, 47].
In order to support the above data, the DPV was also recorded immediately after the CV trace
at the same scan rate. It was witnessed that the reduction peaks at –1.13 and –0.715 V and oxidation
peak at 0.613 V of the ligand I in the CV do not match exactly with the corresponding peaks at –
0.878 and 0.486 V in the DPV. Similarly, peaks at –1.02, and –0.732 V in the CV and at –0.892,
–0.112 and 0.599 V in the DPV of complex 1 do not match. This information confirms that both
the systems (i.e. the ligand as well as the complex) are irreversible in nature in DMF solution. The
peaks are unsymmetrical in both cases, which again concludes the irreversible nature of the system.
The CV and DPV of all other complexes display comparable patterns (Figs. S7-S12). A metal
centered oxidation peak is not observed either in the CV traces or in the DPV traces.
<<Table 5>> <<Figure 5>>
3.7. Catalytic activity study – the Hantzsch reaction
An efficient one pot Hantzsch reaction of three components involving ethyl acetoacetate,
benzaldehyde (or its derivatives) and ammonium acetate has been explored using complexes 1-7
as catalysts under solvent free conditions in the presence of a green oxidant, i.e. 30% H2O2.
Literature, in general, reports the formation of as many as four products (Scheme 3) [48], however,
the mononuclear [MoVIO2]2+/[MoVIO(O2)]2+ and [VVO(OMe)]2+ complexes catalyzed the
Hantzsch reaction under the above conditions to give only two different products: (a) diethyl 2,6-
dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate (1,4-DHP) and (b) diethyl 2,6-
dimethyl-4-phenylpyridine-3,5-dicarboxylate (Py). An initial screening using complex 1 as a
representative catalyst also resulted in the formation of the same two major products.
<<Scheme 3>>
The experimental reaction conditions were optimized considering various parameters, like
amounts of oxidant, solvent, catalyst and temperature. Thus, for a mixture of 0.005 mol (0.53 g)
of benzaldehyde, 0.010 mol (1.30 g) of ethyl acetoacetate and 0.005 mol (0.38 g) of ammonium
acetate in a reaction flask, three different amounts of catalyst 1 [0.001g (1×10–3 mmol), 0.002 g
(2×10–3 mmol) and 0.003 g (3×10–3 mmol)] and aqueous 30% H2O2 (0.005, 0.010 and 0.015 mol)
were added and the reaction was run at four different temperatures (30, 40, 50 and 60 °C). Details
14

for all these conditions, the conversion and selectivity of the different products under particular
conditions are summarized in Table 6 (Fig. S13).
<<Table 6>>
It is clear from the data in Table 6 that the optimized reaction conditions to achieve the
maximum conversion (with respect to 1,4-DHP) in 1 h of reaction time are (entry 6): catalyst 1
(0.001 g, 1×10–3 mmol) and H2O2 (0.560 g, 0.005 mol) at 30 °C, where a maximum 88%
conversion along with 100% selectivity of dihydropyridine was achieved under solvent free
reaction conditions. These products were also identified by HRMS (Figs. S14-S16), as well as by
1H NMR spectroscopy (after separating them by column chromatography) (Fig. S17). Increasing
the temperature to 40 °C increases the conversion, but the selectivity of product (a) drops slightly
(entry 7). The conversion as well as the selectivity drop on further increasing the temperature.
Increasing the oxidant has the least effect on the conversion. The effect of different solvents, like
MeOH, EtOH, CH2Cl2, CHCl3, MeCN, DMSO and hexane, have also been studied under the
optimized reaction conditions (i.e. entry 6 of Table 6) and the conversion with respect to the
solvents follows the order: MeOH (90%) > EtOH (87%) > MeCN (85%) > CH2Cl2 (82%) > CHCl3
(78%) > hexane (72% > DMSO (23%) (entries 9 to 16 of Table 6). The blank runs (i.e. without
catalyst) with solvents are given in Table S4 (see also entries 17 to 22 of Table 6), which conclude
that there is no significant role of the solvent to drive the reaction. Thus, the reactions in solvents,
except MeOH, have no added benefit in terms of conversion. However, if importance is given to
the aromatized product, the reactions in solvents do result in an improved selectivity of the
aromatized product (b) and a decrease in the selectivity of product (a). Thus, within the range of
solvents used, non-polar solvents show better selectivity for product (b).
3.8. Scope of the reaction
We have further explored the scope of this reaction using all the complexes as catalysts
while taking different derivatives of benzaldehyde and examined the yield of the obtained reaction
products along with conversion after different time intervals under the optimized reaction
conditions. It was observed that (i) introduction of an electron withdrawing group (–NO2, –Cl or
–F groups) yielded the maximum conversion 94-98 % while an electron donating group (–CH3 or
–OMe) gave a lower conversion 82-87% (Table S5). Thus, after the first 1 h of reaction, the overall
15

conversion for different substituents of benzaldehyde followed the order: NO2 > F > Cl > H > CH3
> OMe, (ii) complex 3 is the best amongst all complexes, though all of them show very good
catalytic activity, (iii) the time analysis suggests that all the complexes result in the formation of
the dihydropyridine derivative (a) selectively at 30 °C in the first 1 h of the reaction and then the
conversion of dihydropyridine to the pyridine derivatives starts and complete conversion occurs
in ca. 10 h (Fig. 6) and (iv) a change in color of the reaction product can also be visualized (Fig.
6(A)).
<<Figure 6>>
For a systematic comparison of the catalytic activity of the trinuclear
dioxidomolybdenum(VI) complexes with the analogous mono- and dinuclear-
dioxidomolybdenum(VI) complexes, we have conducted the catalytic experiment considering the
same molybdenum content of each catalyst as considered for the trinuclear system. It was found
that the mononuclear complex H4ptk-bhz (the Schiff base obtained from equimolar amounts of 2,
4, 6-trihydroxybenzene-1, 3, 5-tricarbaldehyde and benzoylhydrazide) gives 22% conversion with
8% selectivity of (a), while the dinuclear analogue (H4dar-inh = Schiff base obtained from one
equivalent of 4, 6-dihydroxyisophthalaldehyde and two equivalents of isonicotinoylhydrazide)
gives 34% conversion with 12% selectivity of (a) within 1 h of reaction time. Interestingly, the
complete conversion of the reactants into product and the reversal of the selectivity of the product
after about 10 h of reaction time increase the scope of the complexes considered here as catalysts
for these multicomponent reactions
3.9. Possible reaction mechanism for the Hantzsch reaction
3.9.1. UV-visible spectroscopic study
Aiming towards understanding the possible mechanistic pathways for the multi component
Hantzsch reaction we have conducted a series of experiments, whose descriptions are given below.
(i) As a representative example, a methanolic solution of complex 1 was treated with H2O2 and the
resulting changes were monitored by UV-visible spectroscopy. Thus, treatment of 25 ml of 1 (4
× 10–5 M ) in MeOH with one drop portions of 30 % H2O2 dissolved in 10 mL of MeOH (the final
concentration of the H2O2 solution is ca. 9.0 × 10–2 M) causes the splitting of a broad band
appearing at ca. 325 nm into two with an increase in their intensity. The intensity of the first band
at 220 nm slowly increases, while the second band further splits into two (288 and 331 nm) along
16

with an increase in intensity. Based on their intensity, these bands are n → π* and the split bands
are π – π*, respectively [Fig. S18(a)]. Simultaneously, the intensity of the LMCT band also reduces
and finally it becomes almost flat. The other complexes show very similar spectral changes upon
addition of H2O2, except 7 (Figs. S19-S24).
(ii) We have also carried out the above reaction in DMSO and the spectral changes are presented
in Fig. S18(b) and Figs. S25-S30. In DMSO, the addition of H2O2 causes a reduction in the
intensity of the LMCT band at 412 nm, followed by its flattening, while the band at 321 nm usually
splits into two bands (in complexes 1, 5 and 6), one at higher wavelength and other one at a lower
wavelength along with a slight decrease in intensity, as observed in MeOH. In complexes 2, 3 and
7, this band shifts towards the higher wavelength, along with a slight decrease in intensity. Only
the band at ca. 260 nm gains in intensity. These spectral changes finally led to the generation of at
least one isosbestic point at ca. 360 nm, or in some cases two (ca. 360 and ca. 300 nm). These
observations hint towards the interaction of the peroxide ion with the complexes and the generation
of the corresponding oxidoperoxidomolybdenum(VI) species in solution. In the case of 7, three
isobestic points were observed at 409, 371 and 312 nm.
(iii) The systematic addition of one drop portions of benzaldehyde in 10 mL of MeOH (the final
concentration of the benzaldehyde solution is 9.4 × 10–2 M) to the final reaction mixture obtained
after adding H2O2 to 1 in MeOH (see Fig. S18(c)) causes a considerable increase in the intensity
of the 280 and 333 nm bands without altering their positions, while the original band at ca. 410
nm, which disappeared during the peroxido species generation, starts gaining intensity with a slight
shift towards a higher wavelength. Simultaneously, the other two bands i.e. the 225 and 360 nm
bands, gain considerable and marginal increments in intensity, respectively. Successive addition
of one drop portions of an ethyl acetoacetate (7.7 × 10–4 M) solution in MeOH to the above solution
also indicates its interaction with the intermediate species formed [Fig. S18(d)] as the n → π* and
split π – π* bands show further visible changes in their intensity.
(iv) A solution of 25 ml of 1 (4 × 10–5 M) in MeOH was titrated with the successive addition of
one drop portions of benzaldehyde (9.4 ×10–2 M) in MeOH and the resulting spectroscopic changes
are depicted in Fig. S31(a). Thus, the intensities of the UV band at 318 nm and LMCT band at 420
nm improve with no change in their positions, while the 257 nm band shifts to a higher wavelength
with a considerable increase in intensity. Additionally, a new band at 280 nm starts appearing,
suggesting the interaction of benzaldehyde with the metal center. The intensities of all the bands
17

(except that at 420 nm) further increase upon addition of a methanolic solution of H2O2 (9 × 10–2
M) to the above solution of complex 1, while the LMCT band shows the opposite trend, i.e.
decreases with the addition of H2O2 [Fig. S31(b)]. Interestingly, a weak band at ca. 310 nm is also
generated along with the appearance of an isosbestic point at 375 nm. These observations are in
line with the interaction of H2O2 with the benzaldehyde interacted complex 1. The intensity of the
420 nm band remains constant while all the other bands show a positive change in intensity. which
further show its positive effect on the surroundings of the metal center [Fig. S31(c)].
3.9.2. A DFT investigation
Density functional theory (DFT) computations were performed to explore the possible
intermediates present in the reaction. In order to do that, we constructed mononuclear and
trinuclear based computational model systems. The optimized structures deduced planar
geometries of the complexes (see Fig. 7). So as to reduce the computational time, the mononuclear
computational model is preferred.
We have studied two pathways (I and II) commencing from 1(A) (a mononuclear unit of
1) which lead to the intermediate 1(D). The two pathways differ in their order of addition of
benzaldehyde and H2O2. In path-I, benzaldehyde is added first whereas in path-II, H2O2 is mixed
with 1(A) in the first step.
<<Figure 7>>
In path-I, the reaction starts by the addition of benzaldehyde, which replaces the water
molecule present at the axial position of the molybdenum center. This addition leads to the
formation of the complex intermediate 1(B). Interestingly, we see a stabilization energy of 1.5 kcal
mol-1 after the binding of benzaldehyde at the molybdenum center (Fig. 8). This stabilization
energy is due to the π-π stacking, as seen in the figure. Following path-1, in the second step, H2O2
is added to intermediate 1(B), which however destabilizes intermediate 1(BD) as the benzaldehyde
leaves the axial position, and thus the π-π stacking is lost. Intermediate 1(BD) is significantly
stabilized by ~9 kcal mol–1 to yield the intermediate 1(D) by eliminating a water molecule; in this
process benzaldehyde recombines at the axial position to build the π-π stacking once again.
However, no covalent bonding exists between the molybdenum center and the oxygen center of
the benzaldehyde (d(Mo—O) = 2.5 Å), See Fig. 8. Hence, the DFT investigation of path-I clearly
shows that the addition of benzaldehyde builds the π-π stacking interaction in the initial and the
18

final steps of the pathway. Such an interaction helps in stabilizing the reaction intermediates 1(B)
and 1(D).
In path-II, H2O2 is added in the first step and benzaldehyde is mixed in the last. Clearly,
after the addition of H2O2 to 1(A) we did not observe any stabilization as the intermediate 1(AC)
is destabilized by 6 kcal mol–1 relative to 1(A). However, the intermediate 1(AC) is significantly
stabilized by ~20 kcal mol–1 after the elimination of a water molecule. In the final step,
benzaldehyde is reacted with intermediate 1(AC), but to our surprise it was found that intermediate
1(C) is still more stable than intermediate 1(D). Based on our DFT study, we therefore propose
that the reaction mixture will have both intermediate 1(C) and intermediate 1(D), but with a slight
dominance of intermediate 1(C).
We have also studied the addition of methyl acetoacetate (MAA) with 1(D) as well as with
1(B) (see Fig. 8). In case of 1(D) reacting with MAA, the intermediate 1(DC) forms showing the
formation of a C-C bond. The intermediate 1(DC) is relatively more stable than 1(D) by 11.3 kcal
mol–1, which justifies the formation of 1(DC) via 1(D). Reaction of 1(B) with MAA gives rise to
1(BA), in which C-C bond formation takes place as seen in 1(DC). The intermediate 1(DC) is
found to be ~7 kcal mol–1 more stable than 1(BA). So, the relative stability of 1(BA) as compared
to 1(DC) suggests that the later one is preferred to yield Int1 but note the route through 1(BA)
cannot be ignored. Now, Int1 and Int2 undergo a cyclization reaction to yield P1, which reacts
with 1(C) and forms the aromatic product P2 with the complex 1(CA). The complex 1(CA), which
is the highest stable intermediate in the catalytic cycle, gives rise to the starting complex 1(A) by
the removal of a water molecule.
<<Figure 8>>
Since the actual ligand I has three ligating units at the 1, 3 and 5 positions on the benzene
ring, the possibility of the existence of mono-, di- and trinuclear cis-[MoO2]2+ complexes also
exists. Therefore, we have also tried to see through a computational study if these mono-, di- and
trinuclear cis-[MoO2]2+ complexes are stable and produce the corresponding stable peroxido
complexes as well. In fact, for the mononuclear case, the peroxido species 9 is stabilized by ~8.0
kcal mol–1 compared to mononuclear complex 8. Similarly, the dinuclear peroxido system 11 is
~15.0 kcal mol–1 more stable than the dinuclear dioxido complex 10, whilst the trinuclear peroxido
complex (12) is stabilized by ~23.0 kcal mol–1 relative to the trinuclear dioxide complex 1. These
19

studies clearly show that as we move from mono- to di- and then to tri-nuclear complexes, the
relative stability of the corresponding peroxido complex increases with respect to its corresponding
dioxido complex (Figs. S32–34).
Based on the DFT studies, a suitable reaction mechanism has been proposed, as shown
within Fig. 8, where the cis-[MoO2]2+ complex interacts first with H2O2 to form its peroxido
intermediate followed by its interaction with benzaldehyde, which in turn reacts with ethyl
acetoacetate to provide intermediate - I. Formation of intermediate - II by the reaction of
ammonium acetate and ethyl acetoacetate is obvious. Finally, both intermediates react together to
give DHP. This DHP undergoes aromatization in the presence of the peroxido species to give the
corresponding pyridine and original dioxido complex.
4. Conclusions
We have prepared tris[cis-MoO2]2+-complexes, [{MoVIO2(MeOH)}3L1–7] (1–7), with
trinucleating ligands derived from benzene-1,3,5-tricarbohydrazide and salicylaldehyde
derivatives. A single crystal X-ray study confirms the six-coordinated distorted octahedral
geometry around each Mo center and the overall structure adopts a high C3-symmetry with a
phenyl group in the center and three arms. DFT calculations endorse the stabilization of their
peroxido derivatives (e.g. [{MoVIO(O2)(H2O)}3L1] (12)) as well. The trinuclear complex 12 has
∆H = −23.4 kcal mol–1, while the binuclear and mononuclear oxidoperoxido molybdenum(VI)
complexes have relatively lower ∆H values. Cyclic voltammetry data, in support of differential
pulse voltammetry data, are in conformity with the irreversible redox behaviour in DMF. The high
catalytic efficiency of these complexes towards the one pot Hantzsch reaction of three components
involving ethyl acetoacetate, benzaldehyde (or its derivatives) and ammonium acetate in solvent
and solvent free systems has shown very interesting results in that the initial 1 h of the catalytic
reaction produces dihydropyridine selectivity along with a high TOF value, but with the elapse of
time the dihydropyridine derivative slowly changes to the pyridine derivative and finally the
reaction becomes selective for the pyridine derivative after about 10 h. A DFT study provides all
the probable intermediates present in the catalytic reaction. This method provides excellent
selectivity between the products, involving easy workup with excellent yields. This method can be
extended for the synthesis of important hetero-nuclei compounds to develop pharmaceutical drugs.
20

Appendix A. Supplementary data
CCDC 1952306 for complex 1a, 1952307 for 5a and 1952308 for ligand IIIa contain the
supplementary crystallographic data for the structures. 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.
Acknowledgements
MRM thanks the Science and Engineering Research Board (SERB), Government of India,
New Delhi, for financial support of the work (grant number CRG/2018/000182). PG acknowledges
the Faculty Initiation Grant (FIG–100810) and computational facilities provided by IIT Roorkee.
RT is thankful to the Indian Institute of Technology (IIT) Roorkee, India, for the Institute
fellowship
Conflicts of Interest
Authors declare no conflict of interest.
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24

Schemes
Scheme 1. Trinucleating ligands, designated as I-VII, used in this work.
25

Scheme 2. Proposed structure of the dioxidomolybdenum(VI) complexes.
26

Scheme 3. Four expected products of the Hantzsch reaction using reactants benzaldehyde, ethyl
acetoacetate and ammonium acetate: (a) diethyl 2,6-dimethyl-4-phenyl-1,4-hydropyridine-3,5-
dicarboxylate, (b) diethyl 2,6-dimethyl-4-phenylpyridine-3,5-dicarboxylate, (c) diethyl 2,4-
dimethyl-6-phenyl-1,4-dihydropyridine-3,5-dicarboxylate and (d) diethyl 2,4-dimethyl-6-
phenylpyridine-3,5-dicarboxylate.
27

Tables
Table 1
Crystal data and structure refinement for the compounds [{MoO2(DMSO)}3bthz(sal)3] (1a),
[{MoO2(0.5H2O·0.5MeOH)}{MoO2(DMF)}{MoO2(DMSO)}bthz(Cl-sal)3]·DMF·0.5DMSO
(5a) and H6bthz(MeO-sal)3·3DMF (IIIa).
1a
5a
IIIa
Formula
C36H38Mo3N6O18S3 C39.50H40Cl3Mo3N8O16.50S1.50 C42H50N9O12
Formula weight
1176.71
100(2)
1333.06
100(2)
872.91
T, K
100(2)
Wavelength, Å
0.71073
Hexagonal
R3
0.71073
Triclinic
P1
0.71073
Orthorhombic
P212121
4.9394(2)
25.3279(10)
34.4133(14)
90
Crystal system
Space group
a, Å
20.4234(14)
20.4234(14)
21.1588(15)
90
10.3331(8)
15.2717(12)
18.3593(14)
96.946(3)
90.455(3)
99.565(3)
2834.8(4)
2
b, Å
c, Å
, º
β, º
90
90
γ, º
120
90
V, Å3
7643.2(9)
6
4305.3(3)
4
Z
F000
3528
1332
1844
Dcalc, g cm-3
, mm-1
1.534
1.562
1.347
0.914
0.917
0.100
2.89 to 27.15
0.0380
1.65 to 26.42
0.1009
2.37 to 26.42
0.0639
, º
Rint
Crystal size, mm3
Goodness-of-fit on F2
R1[I>2(I)] a
wR2 (all data) b
0.26 × 0.24 × 0.19
1.099
0.04 × 0.15 × 0.22
1.076
0.29 × 0.18 × 0.17
1.066
0.0662
0.0827
0.0538
0.2029
0.2894
0.1569
Largest differences peak and hole, e Å–3 4.248 and –1.013
1.706 and –1.229
0.834 and –0.267
/2
2 2
aR1 = Fo - Fc/Fo. bwR2 = [w(Fo2 -Fc2)2]/[w(Fo ) ]1
28

Table 2
Bond lengths [Å] and angles [°] for the compounds [{MoO2(DMSO)}3bthz(sal)3] (1a),
[{MoO2(0.5H2O·0.5MeOH)}{MoO2(DMF)}{MoO2(DMSO)}bthz(Cl-sal)3]·DMF·0.5DMSO
(5a) and H6bthz(MeO-sal)3·3DMF (IIIa).
Bond lengths
Mo(1)-O(1)
1a
5a
IIIa
1.710(4)
1.694(5)
2.000(4)
1.930(4)
2.247(5)
2.220(6)
1.403(6)
1.697(7)
Mo(1)-O(2)
1.686(7)
Mo(1)-O(3)
1.931(7)
Mo(1)-O(4)
2.003(6)
Mo(1)-N(1)
2.251(8)
Mo(1)-O(1M)
N(1)-N(2)
Mo(1)-O(1WA) 2.298(9)
1.407(9)
1.384(3)
1.370(3)
1.369(3)
N(3)-N(4)
N(5)-N(6)
O(3)-C(3)
1.315(7)
O(3)-C(14) 1.348(12)
O(1)-C(7)
1.231(3)
1.223(3)
1.224(3)
IIIa
O(4)-C(16)
O(7)-C(25)
Angles
1a
5a
O(2)-Mo(1)-O(1)
O(2)-Mo(1)-O(3)
O(1)-Mo(1)-O(3)
O(2)-Mo(1)-O(4)
O(1)-Mo(1)-O(4)
O(3)-Mo(1)-O(4)
O(2)-Mo(1)-N(1)
O(1)-Mo(1)-N(1)
O(3)-Mo(1)-N(1)
O(4)-Mo(1)-N(1)
O(2)-Mo(1)-O(1M)
104.0(3)
97.5(3)
103.8(4)
99.0(3)
101.7(3)
95.2(3)
98.6(3)
151.7(3)
94.8(3)
160.0(4)
82.2(3)
72.2(3)
97.96(19)
97.5(3)
104.1(2)
149.57(19)
89.48(18)
164.38(19)
71.78(13)
81.86(17)
170.7(2)
29