New pillared dioxomolybdenum(VI) complexes with ONS donor ligands and 4,4′-azopyridine spacer: 3D metal-organic supramolecular architectures and DFT calculations

Article abstract of DOI:10.1016/j.ica.2016.07.004

Two new pillared binuclear dioxomolybdenum(VI) complexes [(MoO₂L¹)₂(4,4′-azpy)] (1) and [(MoO₂L²)₂(4,4′-azpy)] (2) have been synthesized by the reaction of MoO₂(acac)₂

Full text of DOI:10.1016/j.ica.2016.07.004

Accepted Manuscript
Research paper
New pillared dioxomolybdenum(VI) complexes with ONS donor ligands and
4,4′-azopyridine spacer: 3D metal-organic supramolecular architectures and
DFT calculations
Debanjana Biswal, Nikhil Ranjan Pramanik, Syamal Chakrabarti, Michael G.B.
Drew
PII:
S0020-1693(16)30360-7
http://dx.doi.org/10.1016/j.ica.2016.07.004
ICA 17139
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
25 March 2016
13 June 2016
1 July 2016
Please cite this article as: D. Biswal, N.R. Pramanik, S. Chakrabarti, M.G.B. Drew, New pillared
dioxomolybdenum(VI) complexes with ONS donor ligands and 4,4′-azopyridine spacer: 3D metal-organic
supramolecular architectures and DFT calculations, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/
j.ica.2016.07.004
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New pillared dioxomolybdenum(VI) complexes with ONS donor ligands and
4,4′-azopyridine spacer: 3D metal-organic supramolecular architectures and
DFT calculations
Debanjana Biswala, Nikhil Ranjan Pramanik*b, Syamal Chakrabarti*a, Michael G.B Drewc
aDepartment of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road,
Kolkata:700009, West Bengal, India.
bDepartment of Chemistry, Bidhannagar College, EB-2,Sector-1, Salt Lake, Kolkata: 700064,
India.
cDepartment of Chemistry, The University of Reading, Whiteknights, Reading RG66AD, UK.
ABSTRACT
Two new pillared binuclear dioxomolybdenum(VI) complexes [(MoO2L1)2(4,4′-azpy)] (1) and
[(MoO2L2)2(4,4′-azpy)] (2) have been synthesized by the reaction of MoO2(acac)2 with Schiff
base ligands (H2L1 and H2L2) derived from 2-hydroxyacetophenone and S-benzyl/S-methyl
dithiocarbazates respectively and 4,4′-azopyridine as a spacer. Crystal and molecular structures
of the investigated binuclear complexes 1 and 2 were determined by single crystal X-ray
diffraction. The two complexes have similar centrosymmetric dimeric structures in which each
molybdenum atom occupies distorted octahedral six-coordinate environments being bonded to a
dianionic tridentate ONS donor ligand via phenolate oxygen, imine nitrogen and thioenolate
structure as well as two terminal oxygen atoms and a nitrogen atom of the bridging 4,4'-azpy
ligand. Complexes 1 and 2 give rise to 3D metal-organic supramolecular frameworks having
rectangular cavities formed via hydrogen bonding and π-π stacking interactions. The complexes
1

1
were further characterized by elemental analysis, spectroscopic methods (IR, H NMR and UV-
Vis), electrochemical study and thermogravimetric analysis. DFT calculations were used to study
the electronic properties of the complexes.
Keywords: Dioxomolybdenum(VI), Schiff base ligand, 4,4′-azopyridine, Crystal structure,
Supramolecular interactions, DFT calculations
---------------------------------------------------------------------------------------------------------------------
*Corresponding authors. Tel: +91- 033-2350-8386; fax: +91-033-2351-9755
Tel: +91- 033-2337-4389; fax: +91-033-2337-4782
E-mail address: schakrabarti2014@gmail.com
nr_pramanik@yahoo.co.in
1. Introduction
The molybdenum complexes of S-benzyl and S-methyl dithiocarbazate Schiff base ligands
derived from ortho-hydroxyl aldehydes and ketones have been widely studied [1-5]. Interest in
the metal complexes of these ligands is stimulated by their interesting physicochemical
properties and significant biological activities [6,7]. Schiff base ligands are valued as important
sources of biologically active components. Moreover, complexes containing pyridine ring
moieties are of great interest due to their extended applicability in biology and pharmacology as
antimicrobial, antitubercular, antitumor and analgesic agents [8]. Metal-organic supramolecular
frameworks have drawn much attention in recent years not only due to their intriguing structural
diversity [9] but also to their wide range of prospective applications in host-guest chemistry [10],
catalysis [11], ion-exchange [12], gas-adsorption [13] etc. Molecular self-assembly based on
crystal engineering has proved to be efficient for the formation of these architectures. The design
and synthesis of bridging ligands [pyrazine, 4,4′-bipyridine, 1,2-bis(4-pyridyl)ethane etc.]
containing suitable coordination sites connected by spacers with particular orientations are
2

especially important for the construction of desirable frameworks. Supramolecular frameworks
are constructed mostly via hydrogen bonding and π···π contacts. The hydrogen bond is the most
familiar organizing force in supramolecular assemblies due to its moderately directional
intermolecular interaction that may control short-range packing [14]. Formation of the inner
cavities mostly surrounded by organic components is interesting since the shape, size and
function of the cavity become designable. The cavities of the framework are usually occupied by
solvent molecules because the crystals have a general tendency to maximize their packing
density.
Although molybdenum complexes are of great importance, few articles portray molybdenum
complexes as supramolecular architectures [15,16]. The 4,4′-azpy ligand is well-suited for the
construction of supramolecular assemblies based on aromatic interactions because of its
molecular planarity provided by the π-conjugated system and π orbitals. To realize the pivotal
role of non-covalent interactions in supramolecular aggregates of 4,4′-azpy ligand, we describe
herein the synthesis, spectroscopic characterization, thermal analyses and electrochemical
behavior of two new supramolecular binuclear dioxomolybdenum(VI) complexes
[(MoO2L1)2(4,4′-azpy)] (1) and [(MoO2L2)2(4,4′-azpy)] (2) with primary ONS donor Schiff base
ligands together with 4,4′-azpy as the spacer. We also report the crystal structures and structural
design of complexes 1 and 2 which exhibit 3D metal-organic supramolecular pillared
framework. Single-crystal X-ray analysis gives information about topology and packing of the
molecules in the solid state. In these complexes hydrogen bonding plays an important role in the
construction of the supramolecular network. Aside from hydrogen bonds, the π···π interaction is
another useful organizing force in supramolecular assemblies [17]. Intermolecular interactions
have been investigated not only in the context of different crystal packing [18] but also
3

considering their role in supramolecular architecture formation. In the complexes, the cavities
formed by the interaction of the molecules are capable of hosting guest solvent molecules.
Calculations regarding structure and energy of the compounds were carried out using DFT.
2. Experimental
2.1. Materials
Reagent grade solvents were dried and distilled prior to use. 4,4′-azobis-(pyridine) (4,4′-
azpy) was prepared as an orange solid by the procedure reported previously [19]. All other
chemicals used for preparative work were of reagent grade, available commercially and used
without further purification.
2.2. Synthesis
2.2.1. Synthesis of the ligands
The Schiff base ligands H2L1 and H2L2 were prepared by condensing 2-hydroxyacetophenone
and S-benzyl/ S-methyl dithiocarbazate in ethanol by methods reported previously [1] (Scheme
1). The ligands were satisfactorily characterized by elemental analyses, IR and 1H NMR.
S-benzyl-β-N-(2-hydroxyphenylethylidine)dithiocarbazate (H2L1). Yield ~ 80%, Anal. Calc. for
C16H16S2N2O (%): C, 60.76; H, 5.06; N, 8.86, Found: C, 60.22; H, 4.80; N, 8.79, IR (KBr Pellet),
1
cm-1: ν
3435(m), ν(N-H) 3170(m), ν(C=N) 1600(s), ν
1311(s); H NMR(CDCl3): (-S-CH2-)
(O-H)
(C=S)
4.41 s (2H), (aromatic protons) 6.90-7.45 m (9H), (H3C-C=N) 2.47 s (3H), (-NH-) 11.25 s (1H),
(aromatic -OH) 11.26 s (1H).
S-methyl-β-N-(2-hydroxyphenylethylidine)dithiocarbazate (H2L2). Yield ~ 75%, Anal. Calc. for
C10H12S2N2O (%): C, 50.00; H, 5.00; N, 11.66, Found: C, 49.71; H, 4.89; N, 11.43, IR (KBr
4

Pellet), cm-1: ν(O-H) 3432(m), ν(N-H) 3172(m), ν(C=N) 1600(s), ν(C=S) 1342(s); 1H NMR(CDCl3): (-S-
CH3) 2.53 s (3H), (aromatic protons) 6.87-7.60 m (4H), (H3C-C=N) 2.46 s (3H), (-NH-) 11.42 s
(1H), (aromatic -OH) 12.68 s (1H).
*
OH
OH
S
dry EtOH
*
H
N
S
+
C S R
H2N
H
N
*
stir, r.t., 2h
C N
C S R
C O
CH3
CH3
Scheme 1 Reaction diagram for isolation of the ligands
2.2.2. Synthesis of the complexes
The two mononuclear complexes MoO2L1 and MoO2L2 were prepared by refluxing
MoO2(acac)2 and the respective ligands in a 1:1 molar ratio in chloroform for 2h. The complexes
were satisfactorily characterized by elemental analyses, IR and 1H NMR [1].
[(MoO2L1)2(4,4′-azpy)].CHCl3 (1). A mixture of 0.884 g (2 mmol) of MoO2L1 and 0.184 g
(1mmol) of 4,4′-azopyridine in CHCl3 (20 mL) was refluxed for 3 h. The solution was filtered.
Shiny orange-red single crystals suitable for X-ray diffraction analysis were obtained after one
day. Yield ~70%. Anal. Calc. for C44H38S4N8O6Cl6Mo2 (%): C, 40.37; H, 2.90; N, 8.56; Mo,
14.68, Found: C, 40.28; H, 2.66; N, 8.60; Mo, 14.92, IR (KBr Pellet), cm-1: ν(C=N) 1596 (vs),
ν
(N=N) 1533(s), ν
1230 (s), ν(Mo=O) 931(vs), 902(s), ν(Mo-N) 642(s), ν
436(m); UV-
[Mo-N(azo)]
(Mo-S)
5

1
Vis (CH2Cl2) [λ
/nm (ε/dm3mol-1cm-1)]: 292 (9830), 380 (3030); H NMR(CDCl3): (H3C-
max
C=N) 2.93 s (6H), (-S-CH2-Ph) 4.40 s (4H), (aromatic protons) 7.13-7.91 m (26H).
Complex 2 was prepared similarly by refluxing the corresponding parent MoO2L
complex with 4, 4′-azopyridine in a 2:1 molar ratio in CHCl3 for 3 h. (Scheme 2). The solution
was filtered and a dark orange compound was obtained by evaporation of the filtrate at room
temperature. The compound was recrystallized from ethanol. Shiny orange-red single crystals
suitable for X-ray diffraction analysis were obtained after one day. Yield ~ 70-75 %.
[(MoO2L2)2(4,4′-azpy)].0.5 EtOH, 0.5 H2O (2). Anal. Calc. for C31H32S4N8O7Mo2 (%): C, 39.21 ;
H, 3.37 ; N, 11.80; Mo, 20.24, Found: C, 38.95; H, 3.25; N, 11.18; Mo, 19.98, IR (KBr Pellet),
cm-1: ν(C=N) 1593 (vs), ν(N=N) 1535(s), ν
1231(s), ν(Mo=O) 926(vs), 899(s), ν(Mo-N) 640(s),
[Mo-N(azo)]
1
ν
440(m); UV-Vis (CH2Cl2) [λ
/nm (ε/dm3mol-1cm-1)]: 298 (6480), 347 (3070); H
(Mo-S)
max
NMR(CDCl3): (-S-CH3) 2.62 s (6H), (H3C-C=N) 2.99 s (6H), (aromatic protons) 7.17-7.95 m
(16H).
6

Step 1
*
O
*
OH
*
S
O
O
MoO2(acac)2
+
Mo
CHCl3
*
C N
H
C N N C S R
*
*
S
Reflux
2h
H3C
N
CH3
C
H2L
S
R
MoO2L
R
Ligand
-CH2C6H5
-CH3
H2L1
H2L2
Step 2
*
O
O
O
Mo
2
*
C N
N
N
+
*
S
N
N
H3C
N
C
S
R
Reflux
3h
CHCl3
N
S
O
N
O
O
Mo
N
N
Mo
O
N
N
O
O
S
Scheme 2 Reaction diagram for the synthesis of [(MoO2L)2(4,4′-azpy)] complexes
7

2.3. Physical Measurements
Elemental analyses were performed on a Perkin-Elmer 240 C, H, N analyzer. Molybdenum was
estimated gravimetrically by using oxine as the reagent following standard method. NMR spectra
were recorded on a Bruker 300 L NMR spectrometer operating at 300 MHz with TMS as internal
standard. IR spectra were recorded as KBr pellets on a Perkin-Elmer model 883 infrared
spectrophotometer. Electronic spectra were recorded using a HITACHI U-3501 UV-Vis
recording spectrophotometer. Magnetic susceptibility was measured with a PAR model 155
vibrating sample magnetometer with Hg [Co(SCN)4] as calibrant. Electrochemical data were
collected on a Sycopel model AEW2 1820 F/S instrument at 298 K using a Pt working electrode,
Pt auxiliary electrode and SCE reference electrode. Cyclic voltammograms were recorded in
DMF containing 0.1 M TBAP (tetrabutylammonium perchlorate) as supporting electrolyte.
Thermal analyses were carried out in a NETZSCH STA 449 F3 Jupiter thermal Analyzer in a
dynamic atmosphere of dinitrogen (flow rate= 30 cm3min-1).
2.4. Crystallographic Measurements
The crystallographic data for complex 1 was collected on an Oxford Diffraction X-Calibur
System and for 2 was collected on a Bruker APEX-II diffractometer with CCD-area detector.
Both data sets were obtained with graphite-monochromated Mo-Kα (λ=0.71073Å) radiation.
Unit-cell dimensions and intensity data for 1 and 2 were measured at 150(2) and 296(2) K
respectively. Data analysis for 1 was carried out with the CrysAlis program [20] and for 2 with
SAINT [21]. The crystal structures were solved by direct methods using SHELXS-97 [22] and
refined by full-matrix least-squares based on F2 with anisotropic displacement parameters of
non-hydrogen atoms using SHELXL-97 [22]. The hydrogen atoms were included in calculated
8

positions. The structure of complex 2 contains superimposed solvent ethanol and water
molecules both refined with 25% occupation. Absorption corrections for 1 were carried out with
ABSPACK [23] and for 2 with SADABS [24]. The MERCURY [25] and DIAMOND [26]
programs were used for presentation of the structures.
3. Computational details
The Gaussian 03 program [27] was used for all calculations. Starting models were taken from the
crystal structures but with hydrogen atoms given theoretical positions. Methodology involved the
B3LYP density functional together with basis sets LANL2DZ for Mo, 6-31+G* for S and 6-31G
for the remaining atoms. Both single point calculations and geometry optimizations were carried
out. Geometry optimized structures showed no negative frequencies and were therefore
considered as true minima.
4. Results and discussion
4.1. Synthesis
The Schiff base ligands H2L1 and H2L2 were prepared by condensing 2-hydroxyacetophenone
and S-benzyl/ S-methyl dithiocarbazate in ethanol. The ligands were satisfactorily characterized
1
by elemental analyses, IR and H NMR. Both the binuclear Mo(VI) complexes (1-2) of general
formula [(MoO2L)2(4,4′-azpy)] were prepared by refluxing the corresponding parent MoO2L
complex with 4,4'-azpy in 2:1 molar ratios in chloroform.
The orange-red binuclear dioxomolybdenum(VI) complexes (1-2) are air stable in the
solid state. The compounds are readily soluble in alcohol, CH2Cl2, CH3CN, DMF and DMSO but
are insoluble in water. The complexes are diamagnetic at room temperature as is expected for d0
Mo(VI) centers [28]. Molar conductivity data in 10-3M CH2Cl2 solution indicate that they are
9

non-electrolytes. All the binuclear Mo(VI) complexes are satisfactorily characterized by
1
elemental analyses, IR, H NMR, electronic spectra, cyclic voltammetry and thermogravimetric
analysis. The complexes 1-2 have been structurally characterized by single crystal X-ray
diffraction.
4.2. IR and 1H NMR Spectra
Characteristic IR bands and assignments for the binuclear dioxomolybdenum(VI) complexes,
[(MoO2L)2(4,4′-azpy)], are given in the experimental section. A strong ν
band [29,30] was
(C=N)
observed around 1593-1596 cm-1 region for the complexes. Bands at 640-642 cm-1 for the
complexes are assigned to ν stretching [31]. The complexes exhibit a medium intensity
(Mo-N)
band around 436-440 cm-1 assigned to ν
stretching [32]. Like most cis-
(Mo-S)
dioxomolybdenum(VI) complexes, two IR bands are observed in the 931-899 cm-1 region, the
higher and lower frequency bands originating from the anti-symmetric and the symmetric
stretching modes of the cis-[MoO2]2+ moiety [33-35]. It is revealed that the coordination of the
neutral bidentate N-N donor ligand, 4,4′-azopyridine, has the effect of lowering the ν
(Mo=O)
stretching frequencies [1]. The complexes exhibit strong bands around 1230-1231 cm-1
characteristic of ν
stretching [36] which confirm the presence of coordinated 4,4′-
[Mo-N(azo)]
azopyridine, as an ambidentate ligand. In the region expected for the N=N stretch vibration,
complexes 1 and 2 exhibit one single, sharp and strong band at 1533 and 1535 cm-1 respectively
attributable to the presence of the 4,4′-azopyridine moiety in the structures [37].
1H NMR spectral data and their assignments for the ligands and complexes 1 and 2 in CDCl3 are
given in the experimental section. The signals in the spectra corresponding to N-H and O-H at δ
11.25, δ 11.26 ppm of H2L1 and at δ 11.42, δ 12.68 ppm of H2L2 disappear in 1 and 2 indicating
10

their deprotonation upon coordination of the ligand to molybdenum. In the complexes, the
involvement of azomethine nitrogen is indicated by the shift of the –CH3 proton signal from δ
2.47 to δ 2.93 ppm and δ 2.46 to δ 2.99 ppm respectively. This feature is indicative of a decrease
in the electron density caused by the electron withdrawal by the molybdenum. The methylene
protons of the ligand H2L1 at δ 4.41 ppm remain practically unaffected in 1 indicating the non-
involvement of the S-benzyl sulphur in coordination. The methyl protons of the ligand H2L2 and
complex 2 appear at nearly the same position. In complex 1, 26 aromatic protons appear as
multiplets within the range δ 7.13-7.91 ppm. Similar observation for aromatic protons is noted in
case of H2L2 and its corresponding complex 2.
Both the IR and 1H NMR spectral data are in good agreement with the donor sites of the ligands
1
that coordinate to the [MoO2]2+ centers. The H NMR analysis also confirmed chemical
structures for both the complexes in solution.
4.3. Electronic spectra
The electronic absorption spectra of the [(MoO2L)2(4,4′-azpy)] complexes 1 and 2 were recorded
in dry CH2Cl2 and the spectral data are given in the experimental section. Both the complexes
display a broad lowest energy absorption maximum in the 380-347 nm range which is assigned
[38] to S(pπ)→Mo(dπ) LMCT transition involving the promotion of an electron from the filled
HOMO of the ligand to the empty LUMO of molybdenum. The other LMCT bands in the 298-
292 nm range [38-41] for both the complexes may be assigned to nitrogen to molybdenum and
oxygen to molybdenum charge transfer transitions. The absence of a d-d transition absorption
band in the visible region confirms the 4d0 electronic configuration of Mo(VI).
11

4.4. Electrochemical properties
The electron transfer behavior of the two binuclear [(MoVIO2L)2(4,4'-azpy)] complexes 1 and 2
have been studied by cyclic voltammetry. Cyclic voltammograms of the complexes at a Pt
electrode were recorded in dry degassed DMF containing 0.1 (M) TBAP as the supporting
electrolyte over a potential range of 0.00 to -1.50 V. Results of cyclic voltammetric studies of the
complexes are presented in Table 1 and cyclic voltammograms are shown in Figures S1 and S2
The complexes 1 and 2 show two irreversible one electron two step reductions in the potential
range -0.41 to -0.42 V and -0.82 to -0.85 V corresponding to MoVI-MoVI / MoV-MoV and MoV-
MoV / MoIV-MoIV couples [42,43]. On scan reversal, two irreversible one electron two step
oxidative responses are located in the potential range +0.43 to +0.46 V and +0.73 to +0.74 V
which correspond to MoIV-MoIV / MoV-MoV and MoV-MoV / MoVI-MoVI couples. Thus, from
the results it can be concluded that in aprotic solvent the reduction of these binuclear
dioxomolybdenum complexes is generally irreversible.
Table 1
Cyclic voltammetric resultsa (V vs SCE) for the [(MoVIO2L)2(4,4'-azpy)] complexes at 298 K
Complexes
Epc (V)
Epa (V)
[(MoO2L1)2(4,4′-azpy)] (1)
-0.41, -0.85
+0.43, +0.73
[(MoO2L2)2(4,4′-azpy)] (2)
-0.42, -0.82
+0.46, +0.74
_____________________________________________________________________________
aSolvent: DMF (dry, degassed) ; supporting electrolyte: 0.1M TBAP ; solution strength: 10-3M ;
working electrode: platinum ; reference electrode: SCE; scan rate: 100 mVs-1.
4.5. Thermogravimetric analysis and framework stability
Thermogravimetric analyses of complexes 1 and 2 were conducted in the temperature range 30-
700˚C with a 10˚C/min interval in nitrogen atmosphere to assess their framework stabilities
12

(Figures S3 and S4). Above 175˚C, the TG/DTA curve exhibits three steps of weight loss.
Complex 1 shows a weight loss of ~10% supported by an exothermic peak at 175˚C, which
corresponds to the loss of one guest solvent CHCl3 molecule (calc. 9.13%) and the framework is
quite stable up to 350˚C. In the temperature range 350-370˚C, a ~35% weight loss was observed
suggesting the concomitant loss of the second solvent CHCl3 molecule and 4,4′-azpy spacer
(calc. 32.33%). In the third step above 600˚C, a ~47.0% weight loss suggesting the removal of
two molecules of –CH2Ph ligand fragments (calc. 46.25%). Upon further heating the compound
decomposes to unidentified product. Similar decomposition profile was observed in case of
complex 2.
4.6. Oxo-transfer from Mo(VI) to substrate
Both the binuclear complexes 1 and 2 exhibit oxo-transfer reaction in dry degassed CH3CN
medium to the substrate PPh3 forming the corresponding MoIVOL complexes. The reaction may
be represented as:
[(MoVIO2L)2(4,4'-azpy)] + PPh3
CH3CN
2 MoIVOL + OPPh3 + 4,4'-azpy
These complexes are non-electrolytes and diamagnetic in solid state at room temperature
consistent with a d2 Mo(IV) centre [43,44]. Like binuclear complexes 1 and 2, the IR spectra of
these Mo(IV) oxo complexes reveal that the main ligand framework with all its donor points
remain unaltered. These complexes exhibit a single, sharp, strong band in the 970-965 cm-1
region, representing the υ(Mo=O)t mode [45]. It is likely that MoIVOL complexes are polymeric in
nature via Mo=O···Mo linkages, a supposition which is supported by the presence of a medium
intensity band around 815 cm-1 [46].
13

The electronic spectra in dry CH2Cl2 of the MoIVOL complexes display several absorption bands
in the 298-470 nm range including a new band in the low energy 470 nm region, which is a
characteristic feature of the [MoO]2+ core. A representative electronic spectrum is given in Fig.
S5.
Electrochemical studies further support the formations of MoIVOL complexes (Fig. S6). Both the
complexes exhibit oxidation and reduction waves. An initial positive scan at a rate of 100 mV s-1
gives only a single irreversible oxidative response around +0.73 V, which is assigned to a
Mo(IV)/Mo(VI) couple. On scan reversal, this Mo(VI) species exhibits two one-electron
reductive responses around -0.45 V and -1.11 V corresponding to Mo(VI)/Mo(V) and
Mo(V)/Mo(IV) processes [33], respectively. A representative cyclic voltammogram is shown in
Fig. S6.
4.7. Description of the crystal structures of complexes 1 and 2
The orange-red binuclear complex 1 crystallized in the triclinic space group P-1 and complex 2
in the monoclinic space group C2/c. The molecular structure and atom numbering scheme of the
complexes 1 and 2 are shown in Figures 1 and 2. Crystallographic data and refinement, selected
bond lengths, bond angles and geometry of hydrogen bonding interactions are given in Tables 2-
5, S1 and S2.
14

Fig. 1 Molecular structure of complex [(MoO2L1)2(4,4'-azpy)] (1).
Fig. 2 Molecular structure of complex [(MoO2L2)2(4,4'-azpy)] (2).
15

Table 2
Crystal data and details of refinement for complexes 1 and 2
Complex 1
Complex 2
Chemical formula
Formula weight (M)
Crystal system
Space group
a (Å)
C44H38S4N8O6Cl6Mo2
1307.64
C31H32S4N8O7Mo2
948.77
monoclinic
C2/c
triclinic
P-1
7.5367(3)
12.6164(5)
14.6256(6)
96.928(3)
100.339(3)
104.358(4)
1305.33(10)
1
38.22(3)
7.036(6)
17.466(14)
90.00
b (Å)
c (Å)
α(o)
β (o)
93.997(9)
90.00
γ(o)
V (Å3)
4685(7)
4
Z
Crystal size
Temperature (K)
DC (g cm-3)
µ (Mo Kα) (mm-1)
F (000)
0.13 x 0.03 x 0.03
150(2)
0.20 x 0.18 x 0.16
296(2)
1.663
1.345
1.002
0.759
656
1912
Goodness of fit on F2
1.011
1.074
R1, wR2 [I > 2σ(1)]
R1 = 0.0504
wR2 = 0.1300
R1 = 0.0447
wR2 = 0.1363
_____________________________________________________________________________
R = ∑ ΙΙF0Ι – ΙFcΙΙ / ∑ ΙF0Ι; wR(F2) = [∑w(ΙF0Ι2 – ΙFcΙ2)2 / ∑w ΙF0Ι4]1/2
16

Table 3
Experimental and calculated bond distances (Å) in 1 and 2
_______________________________________________________
Complex 1 Complex 2
_______________________________________________________________
Bond Distances(Å) obs calc obs calc
_______________________________________________________________
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(1)
Mo(1)-N(1)
Mo(1)-S(1)
Mo(1)-N(3)
1.698(3)
1.711(2)
1.938(2)
2.279(3)
2.437(1)
2.443(3)
1.737
1.746
1.971
2.340
2.474
2.503
1.688(3)
1.705(3)
1.934(3)
2.287(4)
2.446(2)
2.461(4)
1.737
1.746
1.971
2.341
2.476
2.501
________________________________________________________________
17

Table 4
Experimental and calculated bond angles (deg) in 1 and 2
__________________________________________________________
Complex 1 Complex 2
___________________________________________________________________
Bond Angles(deg) obs calc obs calc
___________________________________________________________________
O(2)-Mo(1)-O(3)
O(2)-Mo(1)-O(1)
O(3)-Mo(1)-O(1)
O(2)-Mo(1)-N(1)
O(3)-Mo(1)-N(1)
O(1)-Mo(1)-N(1)
O(2)-Mo(1)-S(1)
O(3)-Mo(1)-S(1)
O(1)-Mo(1)-S(1)
N(1)-Mo(1)-S(1)
O(2)-Mo(1)-N(3)
O(3)-Mo(1)-N(3)
O(1)-Mo(1)-N(3)
N(1)-Mo(1)-N(3)
S(1)-Mo(1)-N(3)
C(9)-S(1)-Mo(1)
C(1)-O(1)-Mo(1)
C(15)-N(3)-Mo(1)
C(11)-N(3)-Mo(1)
C(7)-N(1)-Mo(1)
N(2)-N(1)-Mo(1)
106.30(13)
98.28(13)
106.13(12)
90.89(12)
160.64(11)
79.44(10)
102.43(11)
90.75(9)
106.20
106.02(17)
106.21
97.93
98.00(14)
97.89
108.46
90.49
107.26(16) 108.39
94.16(15) 90.49
157.53(13) 160.56
160.54
78.26
78.90(14)
98.69(12)
91.03(13)
78.30
101.54
90.03
101.68
89.96
148.36(7)
76.60(7)
148.07
76.64
150.66(10) 148.23
75.97(10) 76.65
171.21(14) 171.65
170.08(12)
83.21(11)
75.92(9)
171.72
81.55
82.73(15)
78.32(13)
77.35(13)
81.61(10)
99.24(17)
121.9(3)
123.8(3)
119.3(3)
125.5(3)
120.7(3)
81.65
76.61
76.48
80.20(9)
82.43
82.29
79.87(7)
80.93
80.93
98.90(11)
125.0(2)
99.92
99.81
128.28
122.75
118.11
125.04
120.15
128.36
122.73
118.12
124.97
120.07
122.0(2)
119.6(2)
124.3(2)
121.4(2)
___________________________________________________________________
18

Table 5
Geometry of hydrogen-bonding interactions in the crystal structures of the complexes 1 and 2
Symmetry
code
D-H
A
H···A (Å)
D···A (Å)
D-H···A (˚)
Complex 1
C(23)-H(23)
C(16)-H(16A)
C(22)-H(22)
Complex 2
S(2)
O(2)
πa
3.14
2.50
2.72
3.998(6)
3.438(5)
3.599
147
162
158
x, y, 1+z
1-x, 1-y, 1-z
1-x, 1-y, 1-z
C(5)-H(5)
O(2)
2.53
2.97
3.417(6)
3.808
160
146
½-x, ½-y, 1-z
½-x, ½-y, 1-z
C(8)-H(8C)
πa
a
centroid C(1)C(2)C(3)C(4)C(5)C(6)
Complexes 1 and 2 both contain crystallographic centres of inversion at the centre of the N=N
bond of the 4,4′-azpy molecule. Analysis of bond lengths and angles revealed only small
difference in the crystal structures of the complexes. The molybdenum atom exhibits a distorted
octahedral coordination sphere comprised of two oxo-oxygen atoms O(2) and O(3), three donor
atoms O(1), N(1), S(1) (phenolate oxygen, azomethine nitrogen and thioenolate sulphur) from
the ligand and a nitrogen atom N(3) of the 4,4′-azpy molecule. Two asymmetric MoO2L moieties
are linked together via the 4,4′-azopyridine to form a binuclear dioxomolybdenum(VI) complex.
The tridentate, dianionic ligand coordinated to the [MoO2]2+ core forming five and six membered
chelate rings with bite angles of N(1)-Mo(1)-S(1), 76.60(6)˚, 75.97(10)˚ and N(1)-Mo(1)-N(3),
80.20(9)˚, 77.35(14)˚ respectively. Three donor atoms O(1), N(1), S(1) of the ligand and one
terminal oxygen O(3) occupy the equatorial plane [38]. The oxo-oxygen O(2) and one terminal
nitrogen N(3) of one half of the 4,4′-azpy occupy the axial position. The molybdenum(VI)
19

centres in the complexes 1 and 2 are displaced by 0.280 and 0.288 Å respectively from the mean
plane N(3), S(1), O(1), O(2). In complexes 1 and 2 the non-bonding interatomic Mo···Mo
separations are 13.863 and 13.881 Å respectively. The Mo(1)-N(3) bond lengths [2.443(3) and
2.462(4) Å] are longer than the other Mo(1)-N(1) bonds [2.279(3) and 2.287(4) Å] due to trans
influence of O(2) which indicate the 4,4′-azpy nitrogen N(3) is comparatively weakly bonded to
the [MoO2]2+ centre. The C(9)-S(1), C(9)-N(2) and N(1)-N(2) bond distances (Table 3) are
comparable to those observed in similar binuclear dioxomolybdenum(VI) complexes
[15b,16,47]. Intermolecular non-covalent interactions have been investigated to study the
different crystal packing [48] and supramolecular framework formations. Both the Schiff base
ligands provide potential supramolecular recognition sites for π···π stacking, C-H···π interaction
and also act as hydrogen bond donors and acceptors to assemble supramolecular frameworks
(Table 5).
As indicated in Fig. 3, complex 1 has C(16)-H(16A)···O(2) hydrogen bonding interactions
operating between one of the oxo-oxygen O(2) (trans to 4,4′-azpy) of molybdenum and hydrogen
atom of the S-benzyl moiety. The other oxo-oxygen O(3) does not participate in hydrogen
bonding.
20

Fig. 3 Packing diagram of complex 1 showing various hydrogen bonding interactions
H(23)···S(2) (blue dotted line), H(16A)···O(2) (green dotted line), H(22)··· π (black dotted
line) and π···π stacking interaction (red dotted line), non-bonding hydrogen atoms are
omitted for clarity.
Fig. 4 Packing diagram of complex 1 along the c-axis. Stacking interactions of aromatic rings are
shown as orange dotted lines between centroids in blue.
21

Fig. 5 (a) Formation of non-interpenetrated three-dimensional framework pillared by
4,4'-azopyridine spacer in complex 2, the solvent molecules occupy the
rectangular cavities; the azpy pillars are shown in green and the MoO2L2
layer is shown in red.
(b) Magnified view showing the hydrogen bonding interactions H(5)···O(2) (blue dotted
line) and H(8C)···π (red dotted line), non-bonding hydrogen atoms are omitted for
clarity.
In complex 1, the π···π interaction [49] is another useful organizing force. The neighboring
binuclear molecules are associated through face-to-face π···π stacking interactions (Fig. 4) with
centroid-centroid distance 3.748 Å between the parallel aromatic phenyl rings of the
acetophenone moiety thus, forming an interwoven 3D supramolecular network. The stability of
the structure is further consolidated by the C-H···π interactions between the aromatic phenyl ring
of the acetophenone moiety and H(22) of the phenyl ring of the S-benzyl moiety.
In complex 2 (Fig. 5b) also only one of the oxo-oxygens O(2) is involved in hydrogen bonding
with H(5) of the acetophenone moiety. The C-H···π interactions help to extend the pillared
22

layers to a three dimensional architecture. Both the H(5)···O(2) and H(8C)···π hydrogen
bonding interactions in complex 2 generate symbiotic donor-acceptor properties (Fig. 5b). In
complex 2 although the adjacent layers of complexes are parallel, no π···π interaction is found
because the layers direct at different orientations (Fig. 6).
Fig. 6 Packing diagram of complex 2 along the c-axis.
The most intriguing structural feature of the supramolecular architectures is the formation of
rectangular cavities. Since 4,4′-azpy (a rigid bridging ligand, of length ~8.99 Å) is employed as
the organic spacer, each 3D supramolecular framework houses rectangular cavities [13.863 Å x
7.537 Å (1), 13.881 Å x 11.185 Å (2)] which facilitate the clathration of the solvent molecules.
In complex 1, the cavities host the solvent CHCl3 molecules (Fig. 7) via H(23)···S(2) weak
hydrogen bonding interactions. Here, the non-coordinated sulfur, S(2), of the S-benzyl moiety
acts as the hydrogen bond acceptor [50].
23

It is interesting to note that despite the fact that CHCl3 was used in the synthesis, it was not found
in the cavities of 2. Instead the cavity was occupied by a solvent ethanol molecule, used in the
recrystallization and a water molecule, both refined with 25% occupancy (Fig. 5a).
The cavities are basically sustained by hydrogen bonding and aromatic-aromatic interactions,
thus, giving rise to host-guest type of architectures.
Fig. 7 Formation of non-interpenetrated three-dimensional framework pillared by
4,4'-azopyridine spacer in complex 1 along the a-axis, with solvent chloroform molecules
occupying the rectangular cavities; the azpy pillars are shown in violet and the MoO2L1
layer is shown in yellow.
With subtle changes in the ligand substituent (i.e. S-benzyl/ S-methyl), supramolecular
architectures having varied cavity size are formed. Compared to H2L2, H2L1 incorporates
additional steric blockage due to the –CH2Ph bulky group, thereby leading to smaller sized
cavities in complex 1 as compared to complex 2 (Fig. 8). Thus, the steric demands of the Schiff
base ligands play a significant role in defining the cavity size of the crystalline frameworks.
24

Fig. 8 Diagram illustrating the cavity formation in complex 1 (a) and complex 2 (b).
Similar binuclear dioxomolybdenum(VI) complexes with different spacers have been reported
previously by our group [15b]. The spacers used therein [4,4'-bipyridine, 1,2-bis(4-
pyridyl)ethene and 1,2-bis(4-pyridyl)ethane] did not give rise to pillared structures. Also cavity
formation was observed only in one of the complexes. The cavity formation was very much
different from those reported in the present work. Six binuclear moieties along with six floating
methanol molecules together form a cylindrical cavity. The solvent molecules help in formation
of cavities and are not entrapped in the cavities. On the other hand, in the presently reported
compounds, only two binuclear moieties are capable of forming rhombus shaped cavities which
entrap the solvent chloroform molecules. Hence, the use of 4,4'-azpy as a spacer in the present
work gave rise to unique supramolecular frameworks.
4.8. DFT calculations of complexes 1 and 2
Both single point calculations and geometry optimizations of the complexes 1 and 2 have been
carried out using DFT/ B3LYP methodology. The calculated bond distance and bond angles are
given in Tables 3 and 4 and are in good agreement with the experimental data.
25

DFT calculations were also carried out to assess the importance of the intermolecular contacts
listed in Table 5. In complex 1 a model was built consisting of one molecule of the complex
together with two chloroform molecules attached via the C-H···S(2) interactions listed in Table
5. The energy of E(model) – E(complex)-2*E(chloroform) was -2.01 kcalmol-1 showing only a
very weak interaction. A second model was then built of the dimer (x, y, z; 1-x, 1-y, 1-z) which
contains the two other interactions shown in Table 5. Now the energy of interaction E(dimer)-
2*E(monomer) was -7.76 kcalmol-1.
Both interactions shown for complex 2 in Table 5 involve molecules x, y, z and ½-x, ½-y, 1-z.
The dimer shown in Fig. 4b was therefore built. The energy of interaction E(dimer)-
2*E(monomer) was calculated as -10.42 kcalmol-1
Analysis of the five HOMOs and LUMOs was also carried out for 1 and 2 after geometry
optimization and show some notable features. The frontier orbitals are very similar in the two
structures and therefore only those for 1 are reported here. Coefficients greater than 0.20 are
noted in Table 6. There is no participation of the Mo atom orbitals in the five HOMOs. Instead
the major contributions are only from N2, S1 and S2 and their symmetry equivalents. The
coefficients for the LUMOs are very different. The two bridging nitrogen atoms N4, N4$1 of the
bipyridine ligand play the major role in the LUMO which is of significant lower energy than the
remaining four orbitals LUMO+1 to LUMO+4 for which the Mo orbitals play the largest role
though with minor contributions from S1. LUMO+1, LUMO, HOMO, HOMO-1 are plotted in
Fig. 9.
26

(a)
(b)
(c)
(d)
Fig. 9 Frontier orbitals in complex 1, (a) LUMO+1 (b) LUMO (c) HOMO (d) HOMO-1
27

Table 6
Orbital contributions in complex 1
____________________________________________________________________________________
Orbital
Eigen value
Major contributions (> 0.20) to the molecular orbitals
______________________________________________________________________________
LUMO-4
LUMO -3
-0.09075
-0.09733
Mo(1) 7d0 0.525, 7d+1 0.247, 7d-1 -0.270, 7d+2 0.400, S1 3py 0.208, 4py 0.234
Mo(1)$1 7d0 0.527, 7d+1 0.248, 7d-1 -0.267, 7d+2 0.395 S(1)$1 3py -0.208,
4py -0.232
LUMO -2
LUMO -1
LUMO
-0.09895
-0.10607
-0.13370
-0.22503
-0.23051
-0.24193
-0.24358
-0.24733
Mo(1) 7d+2 -0.306, 7d-2 0.555
Mo(1)$1 7d+2 -0.317, 7d-2 0.564
N(4) 2py 0.314, 3py 0.283, N(4)$1 2px -0.308, 3px -0.278
N(2) 2px 0.281, S(1) 3px -0.272, S(2)$1 3px-0.267
N(2)$1 2px 0.282, S(2) 3px -0.267, S(1)$1 3px-0.272
S(1) 3px 0.490, 4px 0.270 S(2)$1 3px -0.358, 4px -0.200
S(2)$1 3px 0.340 4px 0.202
HOMO
HOMO -1
HOMO -2
HOMO -3
HOMO -4
S(2) 3px -0.405 4px -0.228 S(1)$1 3px 0.497 4px 0.274
____________________________________________________________________________________________
$1represents symmetry element 1-x, -y, -z
5. Conclusions
Two new 3D pillared binuclear dioxomolybdenum(VI) complexes 1 and 2 have been synthesized
by using ONS donor ligands and 4,4′-azopyridine as a linker. Both the complexes were
characterized by single crystal X-ray crystallography and various physicochemical techniques.
Crystal structures reveal that in each of the complexes, the centrosymmetric structures consist of
two MoO2L moieties bridged by the two pyridine nitrogens of 4,4′-azpy which completes a
distorted octahedral geometry. The azo (N=N) moiety does not contribute to the coordination
properties of this ligand and no N=N bond cleavage takes place. TGA analysis proves well the
framework stability of the two complexes. The electron transfer behavior shows that reduction of
28

the complexes in aprotic solvent is generally irreversible. In this work, the use of mixed ligands
provides the possibility of generating multiple binding forces, such as coordination bonds,
hydrogen bonding and other supramolecular interactions. This study clearly shows that in crystal
structure of complex 1, there is a strong tendency to form π···π stacking synthon between
adjacent aromatic rings whereas no such interaction is found in complex 2. By closely
controlling the steric demands of the Schiff base, a remarkable class of supramolecular
architectures containing diverse cavity size is formed. The cavities are capable of hosting guest
solvent molecules. These results highlight the complexities of the self-assembly process and the
need to understand the relative importance of the interactions between the molybdenum centers
and ligands, as well as the solvent used during the crystallization process. The DFT calculations
provide an energetic study of the non-covalent interactions in the complexes which are
responsible for the supramolecular assemblies in the solid state. The calculations were also used
to identify the compositions along with the energies of the relevant HOMOs and LUMOs.
Acknowledgements
D. Biswal acknowledges UGC, New Delhi for the Senior Research Fellowship. The authors
thank the University of Reading and the EPSRC (U.K) for funds for the Oxford Diffraction
diffractometer and Dr. Pramit Chowdhury (IIT Delhi) for the thermal analysis. We also thank
Prof. Sudhanshu Sekhar Mandal and Dr. Tapas Kumar Raychaudhuri for fruitful discussions.
Appendix A. Supplementary data
Supplementary figures S1-S6 contain cyclic voltammograms, TG-DT curves, and electronic
spectra of the complexes. CCDC 1416352 and 1416353 contains the supplementary
crystallographic data for complexes 1 and 2 respectively. These data can be obtained free of
29