Cis-Dioxido-molybdenum(VI) complexes of tridentate ONO hydrazone Schiff base: Synthesis, characterization, X-ray crystal structure, DFT calculation and catalytic activity

Article abstract of DOI:10.1016/j.ica.2014.11.023

Two new cis-MoO₂ [MoO₂(L)(EtOH)] (1), [MoO₂(L)(Py)] (2) [L: (3-methoxy-2oxidobenzylidene)benzohydrazidato], complexes have been synthesized and fully characterized on the basis of elemental analysis, FT-IR, molar conductiv

Full text of DOI:10.1016/j.ica.2014.11.023

Accepted Manuscript
Cis-dioxido-molybdenum(VI) complexes of tridentate ONO hydrazone Schiff
base: Synthesis, characterization, X-ray crystal structure, DFT calculation and
catalytic activity
S. Yousef Ebrahimipour, Hojatollah Khabazadeh, Jesús Castro, Iran
Sheikhshoaie, Aurelien Crochet, Katharina M. Fromm
PII:
S0020-1693(14)00719-1
http://dx.doi.org/10.1016/j.ica.2014.11.023
ICA 16307
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
11 August 2014
10 November 2014
20 November 2014
Please cite this article as: S. Yousef Ebrahimipour, H. Khabazadeh, J. Castro, I. Sheikhshoaie, A. Crochet, K.M.
Fromm, Cis-dioxido-molybdenum(VI) complexes of tridentate ONO hydrazone Schiff base: Synthesis,
characterization, X-ray crystal structure, DFT calculation and catalytic activity, Inorganica Chimica Acta (2014),
doi: http://dx.doi.org/10.1016/j.ica.2014.11.023
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Cis-dioxido-molybdenum(VI) complexes of tridentate ONO hydrazone
Schiff base: Synthesis, characterization, X-ray crystal structure, DFT
calculation and catalytic activity
a,
a
b
a
S. Yousef Ebrahimipour *, Hojatollah Khabazadeh , Jesús Castro , Iran Sheikhshoaie ,
c,d
d
Aurelien Crochet , Katharina M. Fromm
a
Department of Chemistry, Faculty of Science, Shahid Bahonar University of Kerman, Kerman,
Iran
b
Departamento de Química Inorgánica, Universidade de Vigo, Facultade de Química, Edificio
de Ciencias Experimentais, 36310 Vigo, Galicia, Spain
c
FriMat, University of Fribourg, Chemin du Musée 6, CH-1700 Fribourg, Switzerland
d
Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg,
Switzerland
Corresponding author:
Dr. S. Yousef Ebrahimipour
Department of Chemistry
Shahid Bahonar University of Kerman
76169-14111
Kerman, Iran
Tel/fax: +98 34 3132 2143
E-mails adresses: Ebrahimipour@uk.ac.ir, Ebrahimipour@ymail.com
1

Abstract
Two new cis-MoO
2
[MoO
2
(L)(EtOH)] (1), [MoO (L)(Py)] (2) [L : (3-methoxy-
2
2
oxidobenzylidene)benzohydrazidato], complexes have been synthesized and fully characterized
1
13
on the basis of elemental analysis, FT-IR, Molar conductivity, H-NMR, C-NMR and
electronic spectra. The structure of complexes has been accomplished by single crystal X-ray
diffraction. All experimental results confirmed that both complexes have an octahedral geometry
around the Mo(VI) central atom, which is coordinated by the donor atoms of the dianionic
hydrazone ligand, two oxido groups and oxygen/nitrogen atoms of solvent molecules.
Computational studies were also performed using DFT calculations at B3LYP/DGDZVP level of
theory. Furthermore, their catalytic activities were investigated on the electrophilic reaction of
indole with aldehydes in molten tetrabutyl ammonium bromide (TBAB) to obtain
bis(indolyl)methane derivatives.
Keywords: Hydrazone Schiff base; Cis-MoO (VI) complex; Catalytic activity; X-ray Crystal
2
structure; TD-DFT.
1
. Introduction
During recent years, aryl hydrazone Schiff base ligands, and their transition metal complexes,
have attracted more and more attention [1]. They are widely used in different areas, not only
because of their facile synthesis, but also for potential biological, catalytic, and industrial
applications [2, 3]. 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 [4, 5].
2

Furthermore molybdenum complexes have a unique place in the development of coordination
chemistry [6]. A variety of physicochemical investigation of these compounds provides a clear
explanation for their stereochemical and electronic properties. In spite of the important role of
molybdenum in biological processes, its high potential as catalyst has made it even more
attractive [7].
Molybdenum complexes, especially molybdenum(VI) dioxido-complexes, are widely used as
efficient catalysts in different processes such as the oxidation of olefins and sulphides [8].
Moreover, indole rings are featured in a wide variety of biologically active compounds [9]. Some
of the ring-substituted bis(indolyl)methanes such as 1,1-bis(3-indolyl)-1-(para-substituted
phenyl)methanes are cytotoxic to cancer cells and inhibit tumor growth in vivo. For instance,
bis(indolyl)methanes containing p-trifluoromethyl, tert-butyl and cyano moieties activate the
peroxisome proliferator-activated receptor c (PPARc), whereas the methoxy and unsubstituted
analogs activate the nerve growth factor induced-Ba (Nur77), an orphan nuclear receptor. Both
PPARc-active and Nur77-active bis(indolyl)methanes induce both receptor-dependent and -
independent growth inhibitory and proapoptotic pathways in cancer cells and tumors including
activation of ER stress in both pancreatic and ovarian cancer cell lines [10]. Consequently,
numerous methods have been reported for the synthesis of bis(indolyl)methanes. Of these
methods, the acid-catalysed electrophilic substitution reaction of indoles with aldehydes is one of
the most simple and straightforward approaches for the synthesis of bis(indolyl)methanes [11-
1
3]. However, some of the reported methods have the following drawbacks: for example, use of
expensive reagents, longer reaction time and low yields of products.
In this study, two cis-MoO complexes containing tridentate hydrazine Schiff base, (2-hydroxy-
-methoxybenzylidene)benzohydrazide [HL], were synthesized and fully characterized with
2
3
3

spectroscopic methods. DFT calculations were also performed at B3LYP/DGDZVP level of
theory. Finally the catalytic activity of these complexes was investigated in the synthesis of
bis(indolyl) methane derivatives.
2. Experimental
2
.1. Materials and instrumentation
All chemicals and solvents used were of analytical reagent grade and were used as received.
Micro analyses for C, H, N were determined on a Heracuse CHN rapid analyzer. Melting points
were measured on an Electrothermal-9100 apparatus and uncorrected. FT-IR spectra were
-1
recorded on a FT-IR 8400-Shimadzu as KBr discs in the range of 400-4000 cm . Molar
Conductance measurements were made by means of a Metrohm 712 Conductometer in DMSO.
1
13
H-NMR and CNMR spectra were recorded at 25 ºC on the Bruker AVANCE III 500 and 125
MHz spectrometers respectively. Electronic spectra in ethanolic solutions of the complexes were
recorded with a Cary 50 UV–Vis spectrophotometer. X-Ray diffraction data for (1) were
collected using a Stoe IPDS II diffractometer, equipped with an Oxford cryostat and in case of
(
2), with a Bruker Smart 6000 diffractometer.
2
.2. Synthesis of (2-hydroxy-3-methoxybenzylidene)benzohydrazide [HL]
An ethanolic solution (6 mL) containing 1 mmol of 3-methoxy-2-hydroxy benzaldehyde (0.152
g) was added dropwise to an ethanolic solution of benzohydrazide (1 mmol, 0.136 g) with
constant stirring. The mixtures were refluxed for 15 min. and the resulting precipitates were
filtered off, washed with cold ethanol and dried in desiccator over silica gel.
-1
Yield: 0.224 g, 83%. m.p.: 116 °C. Anal. Calc. for C15
H
14
N
2
O (270.28 g mol ): C, 66.66; H,
3
-1
5
.22; N, 10.36. Found: C, 66.18; H, 5.30; N, 10.53%. FT-IR (KBr), cm : ν(NH) 3571, ν(OH)
4

3
1
8
d
1
367, ν(CHar) 2839-3062, ν(C=O) 1647, ν(C=N) 1610, ν(C=Cring) 1470, ν(N-N) 1149, ν(C-O)
1
ᵒ
296. HNMR (500 MHz, DMSO-d
.67 (s, 1H; CH=N), 6.85-7.96 (m, 8H, rings), 3.82 (s, 3H, OCH
, 25 ºC, ppm): 163.7, 149.1, 148.8, 148.1, 133.7, 132.8, 129.4, 128.5, 121.7, 119.9, 119.7,
6
, 25 C, ppm): δ = 12.10 (s, 1H; NH), 11.03 (s, 1H; OH),
1
3
3
). CNMR (125 MHz, DMSO-
6
-
1
-1
14.7, 56.6. UV/Vis (EtOH) λmax, nm (logε, L mol cm ): 225 (4.53), 305 (4.60), 343 (4.07).
2
.3.
Synthesis
of
ethanol
(L)(EtOH)] (1):
An ethanolic solution (4 ml) of MoO (acac) (0.1 mmol, 0.03 g) was added to a solution of HL
0.1 mmol, 0.03 g) in ethanol (3 ml) and the resulting orange mixture was refluxed for 30 min.
(3-methoxy-2-oxidobenzylidene)benzohydrazonato
dioxidomolybdenum(VI) [MoO
2
2
2
(
After cooling, the solution was filtered and left to stand overnight. Orange crystals suitable for
crystallography separated after two days and were dried in a vacuum desiccator over silica gel.
-3
-1
2
-1
Yield: 0.027g, 62%. m.p.: 233 °C. Molar conductance (10 M, DMSO): 27 ohm cm mol .
-1
Anal. Calc. for C17
H
18MoN
2
O (442.29 g mol ): C, 46.16; H, 4.10; N, 6.33. Found: C, 46.12; H,
6
-
1
4
1
.41; N, 6.21%. IR (KBr) cm : ν(OH) 3298, ν(CHar) 2815-2981, ν(C=N) 1602, , ν(C=Cring)
1
450, ν(N-N) 1126, ν(C-O) 1257, νsy(cis- MoO
2
) 941, νasy(cis- MoO ) 910. HNMR (500 MHz,
2
ᵒ
DMSO-d
6
, 25 C, ppm): δ = 8.93 (s, 1H; CN), 7.01-8.01 (m, 8H; rings), 4.36 (S, 1H, OH), 3.80
13
ᵒ
(
s, 3H; OCH
3
), 3.43 (s, 3H; CH
3
), 1.07 (s, 2H, CH
2
). CNMR (125 MHz, DMSO-d , 25 C,
6
ppm): 168.7, 156.0, 149.1, 148.4, 132.0, 129.9, 128.8, 127.9, 125.3, 121.5, 120.5, 117.1, 56.0,
-1
-1
5
4
5.8, 18.5. UV/Vis (EtOH) λmax, nm (logε, L mol cm ): 222(4.71), 303(4.57), 340(4.25),
29(3.40).
5

2
.4. Synthesis of pyridine ethanol (3-methoxy-2-oxidobenzylidene)benzohydrazonato
dioxidomolybdenum(VI) [MoO (L)(py)] (2):
MoO (acac) (0.1 mmol, 0.03 g) and HL (0.1 mmol, 0.03 g) were added to 6 ml ethanol/pyridine
3:3) solution and the mixture stirred for ca 1 h. to give a dark orange solution. After 2 days,
2
2
2
(
suitable single crystals appeared on slow evaporation of solvent. The resulting product was
isolated, and dried in vacuum desiccator over silica gel.
-3
-1
2
-1
Yield: 0.036g, 67%. m.p.: 246 °C. Molar conductance (10 M, DMSO): 31 ohm cm mol .
-1
Anal. Calc. for C20
H
17 Mo N
3
O
5
· 0.5(C
5
H N) (515.86 g mol ): C, 51.27; H, 3.72; N, 10.87.
5
-1
Found: C, 50.83; H, 3.17; N, 10.46%. IR (KBr) cm : ν(CHar) 2835-3059, ν(C=N) 1604,
1
ν(C=Cring) 1446, ν(N-N) 1130, ν(C-O) 1254, νsy(cis- MoO
500 MHz, DMSO-d
rings), 3.80 (s, 3H; OCH
49.2, 148.5, 136.2, 132.0, 130.0, 128.8, 127.9, 125.3, 123.9, 121.5, 120.5, 117.1. UV/Vis
2
) 928, νasy(cis- MoO
2
) 906. HNMR
(
6
, 25 ºC, ppm): δ = 8.94 (s, 1H; CN), 8.57(s, 2H, 2CH), 7.03-8.01 (m, 11H;
1
3
ᵒ
3
). CNMR (125 MHz, DMSO-d , 25 C, ppm): 168.8, 156.1, 149.6,
6
1
-1
-1
(
EtOH) λmax, nm (logε, L mol cm ): 223(4.78), 303(4.60), 339(4.30), 439(3.45).
2
.5 General procedure for the Mo-catalyzed reaction of indole with aldehydes
A mixture of indole (2 mmol), aldehyde (1 mmol) and Mo-complex (0.1 mmol) in
o
tetrabutylammonium bromide (2 mmol) was stirred at 110 C for the appropriate time (See
Section 3.6). The progress of the reaction was monitored by TLC. When the reaction was
completed the reation mixture was dissolved in ethanol and poured into water. The resulting
precipitate was filtered and purified by silica gel column chromatography using chloroform as
eluent, to afford the desired compound in pure form.
6

2
2
3
2
.6 Spectral data of selected compounds
-1
.6.1 Benzyledenebis(indolyl)methane (A): FT-IR (KBr) (νmax, cm ): 1492, 1599, 1616, 2960,
1
054, 3411. HNMR (500 MHz, CDCl
3
) δ: 5.94 (1H, s, CH aliphatic), 6.68 (d, J = 1.6 Hz, 2H,
1
3
CH), 7.06 (t, J = 7.2 Hz, 2H, Ar-H), 7.21-7.46 (m, 8H, Ar-H), 7.89 (s, br, 2H, 2NH). CNMR
125 MHz, CDCl ) δ: 40.7, 111.4, 119.8, 120.3, 122.3, 123.8, 126.3, 127.5, 127.7, 128.4, 128.6,
37.1, 144.4.
(
3
1
2
3
7
1
-1
.6.2 2-Chlorobenzyledenebis(indolyl)methane (C): FT-IR (KBr) (νmax, cm ):1549, 1617,
1
054, 3411. HNMR (500 MHz, CDCl
3
) δ: 6.40 (s, 1H, CH aliphatic), 6.65 (t, 2H, J = 0.8 Hz),
1
3
.06-7.49 (m, 12H, Ar-H), 7.88 (s, 2H, br, NH). CNMR (125 MHz, CDCl ) δ: 37.0, 111.5,
3
18.8, 119.7, 120.3, 122.5, 124.2, 127.1, 127.4, 127.9, 129.9, 130.8, 134.4, 137.1, 141.8.
2
.7. Crystal structure determination
In the case of [MoO (L)(EtOH)] (1), data collection by using Mo-Kα radiation (λ = 0.71073 Å)
2
was performed at 200K with a Stoe IPDS II diffractometer, equipped with an Oxford cryostat.
Absorption correction was partially integrated in the data reduction procedure. The structure was
2
solved and refined using full-matrix least-squares on F with the SHELX-97 package [14].
Crystallographic data for (2) were collected at room temperature using a Bruker Smart 6000
CCD detector and Cu-Kα radiation (λ = 1.54178 Å) generated by a Incoatec microfocus source
equipped with Incoatec Quazar MX optics. The software APEX2 was used for collecting frames
of data, indexing reflections, and the determination of lattice parameters, SAINT for integration
of intensity of reflections, and SADABS for scaling and empirical absorption correction [15].
The crystallographic treatment of (2) was performed with the Oscail program [16], the structure
2
was solved by direct methods and refined by a full-matrix least-squares based on F [17]. Non-
hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were
7

included in idealized positions and refined with isotropic displacement parameters. The
asymmetric unit of (2) contains a molybdenum complex molecule and a half of a solvent
pyridine molecule. Three atoms are found in the density map for this one, two carbon atoms and
the other one was interpreted as half nitrogen and half carbon, although refined as nitrogen.
Details of crystal data and structural refinement are given in Table 1.
2
.8. Computational
Full geometry optimization of the complexes and proposed intermediates was carried out using
DFT-B3LYP [18] with DGDZVP basis set [19] using the G03 program [20]. Starting geometries
for calculation were taken from the X-ray crystal structure of the related complexes. To validate
the optimization of the structures, frequency calculations were performed and the results showed
no negative (imaginary) frequency.
UV–Vis spectra, electronic transitions, absorbance and oscillator strengths were computed with
the time-dependent DFT (TD-DFT) method at the DGDZVP level. Solvent (EtOH) was
considered as a uniform dielectric constant 24.55 and Polarized Continuum Model (PCM).
GAUSSSUM 3.0 with FWHM 0.3 eV have been used for analyzing the contribution percentage
of groups and atoms to the molecular orbitals [21].
To have a better understanding of the feasibility of the title reactions, the relative electronic
energies for all of the intermediates arising from the reaction of indole with benzaldehyde in the
presence of the complex 1 were calculated at the B3LYP/GGDZVP level of theory (ZPE
energies and solvent effects are not included).
3
. Results and discussion
8

The reaction of MoO
2
(acac)
2
with tridentate hydrazine Schiff base ligand (HL) in different
complexes in good yield. Schiff base ligand (HL)
solvents has been led to formation of cis-MoO
2
contributes in all prepared complexes in its enolic form that the schematic diagram for synthesis
of complexes are shown in Fig. 1. The obtained complexes are stable in room temperature and
soluble in DMSO, DMF, ethanol and less soluble in other common solvents like
dichloromethane, acetonitrile and insoluble in benzene, n-hexane and CCl . the low molar
4
solution conductance of the complexes in DMSO, indicating the complexes have non electrolyte
behavior.
3
3
.1 Spectral characterizations
.1.1. FT-IR
-1
Assignments of selected prominent IR bands in the 400–4000 cm region for HL and its Mo(VI)
complexes are listed in the Experimental section. In the FT-IR spectrum of the ligand, the band
-1
with high intensity at 3571 cm corresponds to the NH vibration. The stretching vibrations of
-1
carbonyl group (C=O) appear at 1647 cm [22]. The absence of these bands in the IR spectra of
the Mo(VI) complexes indicates that the free HL exists in its keto form while in its complexes, it
contributes in its enolic form. The characteristic strong band of the CN moiety in the ligand is
-1
-
assigned to 1610 cm while in the complexes, this band shifts to lower wave numbers ( 1602 cm
1
-1
(
1), 1604 cm (2)), supporting the coordination of the azomethine nitrogen to the central atom
in complexes [23]. After complexation, the red shift of the CO vibration toward lower wave
number indicates that coordination also takes place through deprotonated phenolic and aliphatic
-1
oxygen atoms [24]. The symmetric vibrations of cis-MoO are disclosed at 941 and 928 cm for
2
9

-
1
-1
(
(
1) and (2) respectively [25]. Furthermore the strong bands at 910 cm for (1) and 906 cm for
2) are assignable to the asymmetric vibration of the cis-MoO group [26, 27].
2
3
.1.2. NMR spectral studies
1
13
The H-NMR and C-NMR data for the compounds in DMSO are listed in the experimental
section and presented in Figs S1-S6. In the spectrum of HL singlet signal at 12.10 ppm and 11.03
ppm are attributed to OH and NH respectively [28]. Disappearance of these signals in these
complexes confirms the coordination occurs through the deprotonated hydroxyl groups. The
methine proton is revealed at 8.67 ppm in HL, which upon coordination, in the complexes,
shows significant up-field shifts proving the coordination of the azomethine nitrogen to the metal
center [23]. The singlet signal at about 3.8 ppm in the compounds corresponds to protons of the
methoxy group. The protons of the rings are found between 6.85 and 8.01 ppm although in
1
10
6
compound 2, the C H and C H protons of pyridine were disclosed at 8.57 ppm as a singlet
signal. In case of compound (1), Hydroxyl protons of ethanol are disclosed at 4.36 ppm and the
other signals at 3.43 and 1.07 ppm correspond to CH
3
and CH groups of coordinated ethanol
2
respectively.
1
3
In the C-NMR spectrum of HL, the chemical shift positions of carbonyl, phenolic and methine
carbon atoms are observed at 163.7, 148.8 and 148.1 ppm, respectively [22]. Upon coordination,
these signals shift to lower field showing that the donor atoms (ONO) of [HL] are now connected
to the molybdenum central atom.
3
.2. X-ray structures description
10

Compounds [MoO
2
(L)(EtOH)] (1) and [MoO (L)(py)] (2) consist of a molybdenum(VI) atom
2
coordinated by an dianionic tridentate N-[ (3-methoxy) 2-(oxido)benzylidene]
benzenecarbohydrazonato ligand, binding via two O-atoms and one N-atom. A sixth ligand
completes the coordination sphere, i.e. an EtOH (1) or a pyridine (2) ligand. It is worth noting
that in all cases, these ligands are weakly bonded (see below).
The calculated bond valence by PLATON (version 271113) [29] for Mo atom is 6.12 for (1) and
6
.07 for (2) and confirm the oxidation state +6 for molybdenum atom.
An ORTEP view of the asymmetric unit of the complexes (1) and (2) can be found in Fig. 2..
The coordination geometry around the molybdenum(VI) atom shows a distorted octahedral
environment with an NO
5
[for (1) ] or N
2
4
O [for (2)] chromophore. Bond distances and angles
are set out in Table 2.
The aroylhydrazone ligand occupies three positions in the meridional plane of the octahedron.
One oxido group (O5) is located trans to the imine nitrogen in the same plane. The other oxido
group (O4) occupies one of the axial positions, the second being taken by the coordinated EtOH
ligand (1) or the pyridine ligand (2). The two terminal oxido groups O(4) and O(5) are hence cis
to each other and exhibit typical Mo-O double bond distances 1.686(2) and 1.715(1) Å, for (1);
and 1.697(1) and 1.697(2) Å for (2) [30, 31]. Metal ligand bond lengths are, for neutral ligands,
EtOH in (1) and pyridine in (2) rather long. For (1), the Mo-O bond distance resulted to be
2
.358(2) Å, quasi the same value to that found [2.360(5) Å] in a similar complex of formula
[
MoO L(C OH)] (L = N-[1-(2-hydroxynaphthyl)ethylidene]-2-hydroxy benzohydrazide). For
2
2
H
5
this compound this interaction between the EtOH molecule and the molybdenum atom was
demonstrated to be a formal bond [32]. In the case of (2), the pyridine donor nitrogen atom
[
N(1)] is situated at 2.448(2) Å. This value is longer than the sum of covalent radii (2.25 Å) [33]
11

but is was found as a typical bond distance for pyridine coordinated to MoO
a search in the CSD database [34] (version 5.35, Nov. 2013 updated) of MoO
environment MoO , where one of the nitrogen atoms comes from a pyridine ring (excluding 2-
2
centers [30] In fact
2
complexes with
4
N
2
substitued pyridine such chelating ligands derived of 2-amine-pyridine or 2,2’-bipyride) gives 16
compounds and the mean values is 2.461 Å, longer than that found in (2) . The dihedral angle
between the complex equatorial plane and that of the coordinated pyridine ring is 88.58(6)°,
hence almost perpendicular, and allowing a π,π-stacking interaction (vide infra). The trans
angles between the donor atom of the monodentated ligand (EtOH or py) and the oxo atom O(5)
are, respectively, 172.38(6)° and 169.64(7)°, indicating considerable distortion of the
coordination octahedron around the Mo(VI) center. Such distortion was found in similar pyridine
or ethanol MoO complexes [28, 33]. Chelate angles of 81.43(6)° or 80.84(6)° (six-membered
2
ring) and 71.88(6) or 71.52(5)° (five-membered ring) are also an important source of distortion
around metal atom. The five- and six-membered chelate rings form dihedral angles of 6.49(8)º
and 6.43(7)º, respectively, for (1) and of 12.55(8)° and 7.91(9)° for (2), respectively. The five
membered metallacycle ring is thus rather planar, but the six-membered metallacycle ring is
clearly distorted, between envelope or screw-boat distortion with distortion parameters of Q =
0
1
.2657(15) Å, θ = 58.6(4)° and φ= 20.9(5)º for (1) or Q = 0.3756(14) Å, θ = 119.7(3)° and φ=
98.7(3)° for (2).
The supramolecular arrangement of (1) and (2) are driven by some classical and non-classical
hydrogen bonds. Parameters of those interactions are shown in Table 3 One of the classical
hydrogen bonds in (1) produces dimeric units as is displayed in Fig. S7, and they are connected
with their neighbours by further interactions. Another interactions in (1) is the non-classical
contact between a C-H moiety and the oxo group of a neighbour entity, generating the
12

arrangement along the c axis as shown in Fig. S8. A π,π-stacking interaction, shown in Fig. 3.,
marks the arrangement along the b axis, (symm. op. = -x, -1-y, 1-z), with a distance between the
centroids of 3.612(1) Å and a slippage parameter of 1.001 Å. Fig. S9 shows the unit cell for (1).
In the case of compound (2), the crystal structure contains the metal complex and also one half
solvent pyridine molecule, not shown in Fig. 2. This free pyridine molecule is situated on a
special position. Its centroid, with positional parameters 0.0, 0.3895(2), 0.25, belongs to a wall of
the unit cell, and there are no important interactions with the complex molecule [35], but it is
occupying free channels in the cell. In the supplementary material can be found figures showing
the unit cell (Figs S10 and S11) and, for comparison, the unit cell without the free pyridine
molecule. The channels are, as can be seen in such figure, free to receive any molecule of
appropriate size.
The supramolecular arrangement of (2) is driven by some interactions, e.g. a π-π-stacking
interaction between two coordinated pyridine molecules (symm. op. = -x, 2-y, -z), with centroid
distances of 3.661(1) Å and a slippage parameter of 1.409 Å. Dimeric units are generated by
these interaction as is shown in Fig. 4.
Some hydrogen bonds are also found those e.g. between the atoms labeled as C(12) and O(2),
resulting in dimeric units, and those between C(8) and O(2) forming a chain parallel to b axis.
Fig. 5 shows both C-H···O interactions. There are also some C-H··· π interactions (Fig. S12).
Table 4 set out the parameters of these hydrogen bond interactions. Another C-H···π interaction
c
takes place between C(5)-H(5)···Ct4 , (Symm. op= x, 2-y, 0.5+z). This interaction generates an
arrangement along the c axis (Fig. S13).
3
.3. Geometry optimization
13

.
Geometrical calculated parameters of (1) and (2) are gathered in Table 2. Starting geometries
for calculation were taken from the X-ray crystal structure of the related complexes. As can be
seen in Table 2, there is a good agreement between structural parameters from DFT calculation
and experimental results. A little difference between theoretical and experimental values may
originate from the fact that the experimental data was obtained in the solid state, while the
calculated values describe single molecules in the gaseous state without considering lattice
interactions [36].
3
.4. Analysis of frontier molecular orbitals
Frontier Molecular Orbitals (FMO) have an effective role for providing an insight into the
chemical reactivities and some of the physical properties of the molecule [22]. Fig. 6 shows the
molecular orbital energies with isodensity surface plots of the highest occupied molecular orbital
(
HOMO) and the lowest unoccupied molecular orbital (LUMO) of the Mo(VI) complexes of this
study.
Details of the frontier molecular orbitals in (1) and (2) are shown in Table S1. The HOMO acts
as electron donor while the electron accepting ability of a compound is correlated to the LUMO.
The molecular stability of the compound can be correlated to its HOMO-LUMO energy gap,
more stable structures have higher energy gap. The calculated energy gap for compounds 1 and 2
are found to be 3.27 and 3.22 respectively, therefore, 1 is likely to have higher chemical activity
in comparision 2 The HOMO orbitals in both complexes are mainly localized on non-metal
atoms. For both complexes the value of contribution of deprotonated Schiff base in
HOMO/HOMO-2 is equal to 96-100% and its role is reduced to 17-38% for LUMO/LUMO-2. In
The lowest unoccupied molecular orbital, the molybdenum center has 63% electron population
14

for both title complexes. Based on these data, the first transition state from HOMO to LUMO in
both complexes can be predicted as an admixture of ILCT and LMCT.
3
.5. Electronic spectra
The electronic spectra of HL and the title molybdenum(VI) complexes were recorded in EtOH
solutions. In the UV-Vis spectra of HL, the band at 225 nm is correlated to π→π* transitions of
the aromatic rings. Two bands at 305 and 343 nm can be assigned to π → π* and n→ π* of the
azomethine moiety and the hydrazone ligand [36]. For both complexes, intra-ligand transitions
show a shift in comparison with uncomplexed ONO ligand, indicating the enolization followed
by deprotonation of the ligand. In all title Mo(VI) complexes, the bands appearing at about 430
nm are due to ligand to metal charge transfer (LMCT) (O(p) →Mo(d)) transitions.
For more investigation, the observed and computed electronic transitions based on TD-DFT
calculation for (1) and (2) are listed in Tables 5 and 6 respectively. As shown in these tables, the
computed values of λmax are in good agreement with the experimental ones. The transitions
between 303-429 nm for 1 and 303-439 nm for 2 can be assigned to an admixture of LMCT and
ILCT charge transfer. In case of 1 these bands mainly arise from HOMO→LUMO (λmax=422.6
nm), HOMO-1→LUMO (λmax=341.6 nm) and HOMO→LUMO+2 (λmax=297.6 nm)
respectively. Moreover, the similar transition for 2 is observed and related contributions are as
follows: HOMO→LUMO (λmax=448.2 nm), HOMO→LUMO+1 (λmax=387.6 nm), HOMO-
1
→LUMO (λmax=360.9 nm), HOMO→LUMO+2 (λmax=332.8 nm) and HOMO-1→LUMO+3
(
λmax=319.4 nm). Other calculated absorption bands in electronic spectra of complexes can be
described as ILCT charge transfer that mainly due to transitions from lower MOs to higher
15

LUMOs that four bands at 282.9 and 229.6 nm for compound 1 and a band at 232.3 nm for
compound 2 is correlated to this.
3
.6. Catalytic activity
It was of interest to us to investigate the catalytic effect of the title cis-MoO
electrophilic reaction of indole with aldehydes in molten tetrabutyl ammonium bromide (TBAB)
Fig. 7). In initial studies the optimization of the reaction conditions for the synthesis of
bis(indolyl)methanes was investigated. Benzaldehyde (1 mmol) and indole (2 mmol) were
2
complexes on the
(
chosen as model substrates. The reaction in the presence of 10 mol% [MoO (L)(py)] (2) at 110
2
o
C, in molten tetrabutyl ammonium bromide as a solvent, afforded the corresponding product in
9
1% yield (Table 7). With the optimized reaction conditions, we next studied the reaction of a
series of aldehydes with indole. In order to show the general applicability of the method, various
aldehydes were efficiently reacted with two equivalents of indole in the same conditions. As
shown in Table 8, yields are good in most cases. The nature and position of the substituents on
the aromatic ring have minor effect on the reaction times and the yields. Aldehydes with electron
withdrawing groups such as nitro react faster than aldehydes with electron donating groups such
as methoxy. On the other hand ortho substituents decrease the reaction rate to some extent.
Encouraged by these successful results, we used [and [MoO
2
(L)(EtOH)] (2) as a catalyst in the
(L)(py)] (2).
same conditions. [MoO (L)(EtOH)] (1) gave better results compared to [MoO
2
2
In order to assess the efficiency of the Mo(VI) title complexes in comparison with the reported
catalysts for the preparation of bis(indolyl)methane derivatives, the results of the present method
were compared with the reported methods (Table 9). As it is clear from Table 3, the present
method is more efficient when all terms including yields, reaction times, conditions and catalyst
16

are taken into account. Comparison between cis-MoO
efficient than another complex. For example FeCl .6H
MoO (L)(py)] gave this product in 20 min. Another factor that can be compared is the solvent.
2
complexes shows that (1) is more
3
2
O gave the desired in 90 min while
[
2
The reported method [42] used a complicated ionic liquid with high cost while our method uses a
simple quaternary ammonium salt as solvent.
A tentative mechanistic interpretation to explain the formation of the observed
bis(indolyl)methanes might reasonably assume a reaction path that is shown in Fig. 8. The
changes of energies in the case of (1) for all of the reactions in the proposed mechanism were
also calculated using B3LYP/DGDZVP method and shown in Fig. 8. It seems that in the first
step; the reaction proceeds via the substituent of the aldehyde with the coordinated solvent (I).
Further investigation of the potential energy surface of title reaction is needed to understand the
mechanism of this substitution process which is beyond the scopes of the present work. The
aldehyde molecule may be coordinated to the Mo centre of the complex and the coordinated
solvent leave simultaneously the complex from the opposite direction. The computed internal
-1
energy change for this step is +0.63 kcal mol which indicates this substitution reaction is
thermodynamically feasible. In the second step, an indole molecule is added to the compound I
-1
to form the adduct II which is +4.48 kcal mol more unstable than compound I + indole. Then,
the adduct II undergoes a rearrangement reaction to produce an azafulvene adduct (III), H O and
2
-1
the initial complex. This step which requires energy of 19.6 kcal mol is the critical step from
thermodynamic stand point. In the next step, Mo-complex activates the produced azafulvene
molecules (IV) to form an adduct V. The formation of adduct V is an exothermic process and is
predicted to be thermodynamically feasible. Finally, the adduct V rearranges to form
bis(indolyl)methane (VI) and the initial complex.
17

4
. Conclusions
Two new cis-MoO
base, (2-hydroxy-3-methoxybenzylidene)benzohydrazide [HL], and MoO
2
complexes were synthesized from the reaction of ONO tridentate Schiff
2
(acac)
2
in different
1
solvents. All prepared compounds were characterized by elemental analysis, FT-IR, HNMR,
1
3
CNMR and electronic spectra. The structures of the (1) and (2) were also determined by single
crystal X-ray diffraction. All title complexes have octahedral geometry with positions around the
central atom being occupied with donor atoms of HL, oxido groups and O/N donor atom of
coordinated solvent. DFT calculations were done at B3LYP/DGDZVP level of theory to predict
the structural geometry and interpret the electronic spectra. These complexes were used as
efficient catalysts in the synthesis of bis(indolyl)methane derivatives. Results of catalytic
investigations of these complexes show that (1) has more catalytic activity in compared to (2)
5
. Supplementary materials
Crystallographic data for the structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre, CCDC Nos. 1004203 (1) and 1004204 (2). Copies of
this information may be obtained from the Director, CCDC, 12 Union Road, Cambridge, CB2
1
EZ,
UK
(fax:
+44-1223-336033;
e-mail:
deposit@ccdc.cam.ac.uk
or
http://www.ccdc.cam.ac.uk).
Acknowledgements
18

This work was supported by Shahid Bahonar University of Kerman. We would like to thank the
Universidade de Vigo (Spain) as well as University of Fribourg for the support in
crystallography.
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Figure captions:
Fig. 1. Schematic diagram for complexation processes.
Fig. 2. ORTEP diagram of [MoO
2
(L)(EtOH)] (1). (50% probability level) and of
[
MoO (L)(py)] (2) (30% probability level).
2
Fig. 3: H-bonds and π,π-Stacking interaction between dimeric units (a= -x, -1-y, 1-z) in (1).
Fig. 4. π-π-Stacking interaction (i= -x, 2-y, -z) in (2).
Fig. 5. Left, chain growth due C-H···O interactions. Right, the dimeric units generated by C-
H···O interactions for (2).
Fig. 6. Frontier Molecular Orbital (FMO) plots of (1) and (2) complexes.
Fig. 7. Synthesis of bis(indolyl) methanes using cis-MoO
2
complexes
Fig. 8. The proposed mechanism for Mo-complex catalyzed reaction of indole with aldehydes
22

Fig. 1.
23

Fig.. 2
24

Fig. 3.
25

Fig. 4.
26

Fig. 5.
27

Fig. 6.
28

Ar
Mo-complex
TBAB
+
ArCHO
N
H
N
H
N
H
Fig. 7.
29