Binuclear dioxomolybdenum(VI) complexes of some tridentate ONS donor ligand containing [MoO2]2+ as the acceptor center: Synthesis, crystal structure, supramolecular architectures via hydrogen bonds, π-π Stacking and DFT calculations

Article abstract of DOI:10.1016/j.poly.2014.08.010

A series of linear diimine and diamine-bridged binuclear dioxomolybdenum(VI) complexes having general formula [(MoO₂L)₂(B-B)] [where, B-B is a bridging linear N,N′-bidentate spacer 4,4′ bipyridine, 1,2 bis (4-pyridyl) ethene and 1,2

Full text of DOI:10.1016/j.poly.2014.08.010

Polyhedron 85 (2015) 196–207
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Binuclear dioxomolybdenum(VI) complexes of some tridentate ONS
donor ligand containing [MoO2]2+ as the acceptor center: Synthesis,
crystal structure, supramolecular architectures via hydrogen bonds,
p-p stacking and DFT calculations
b
c
c,
Nikhil Ranjan Pramanik a, , Manashi Chakraborty , Debanjana Biswal , Sudhanshu Sekhar Mandal
Saktiprosad Ghosh c, Syamal Chakrabarti c, William S. Sheldrick d, Michael G.B. Drew e,
Tapan Kumar Mondal f, Deblina Sarkar f
⇑
⇑
,
a Department of Chemistry, Bidhannagar College, EB-2, Salt Lake, Kolkata 700064, India
b Department of Chemistry, NITMAS, Jhinga, P.O. Amira, Diamond Harbour Road, District 24 Pgs. (S), West Bengal 743368, India
c Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road, Kolkata 700009, India
d Lehrstuhl fur Analytische Chemie, Ruhr-Universitat Bochum, D-44780 Bochum, Germany
e Department of Chemistry, The University of Reading, Whiteknights, Reading RG66AD, UK
f Department of Chemistry (Inorganic Section), Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of linear diimine and diamine-bridged binuclear dioxomolybdenum(VI) complexes having gen-
eral formula [(MoO2L)2(B-B)] [where, B-B is a bridging linear N,N0-bidentate spacer 4,40 bipyridine, 1,2 bis
(4-pyridyl) ethene and 1,2 bis (4-pyridyl) ethane] with the tridentate binegative Schiff base ligands
obtained by the condensation of salicylaldehyde/2-hydroxyacetophenone with S-benzyl/S-methyl dith-
iocarbazate have been synthesized. Elemental analysis, various spectroscopic (IR, UV–Vis, 1H NMR) mea-
surements and electrochemical studies have been used for characterization of these complexes. The
complexes 2, 2a, 9 and 10 were structurally characterized by single crystal X-ray crystallography which
reveal that in these complexes the overall coordination geometry around the Mo(VI) center can be
described as distorted octahedral. Complexes 2, 2a, 9 and 10 give rise to interesting supramolecular
Received 4 June 2014
Accepted 2 August 2014
Available online 12 August 2014
Keywords:
Dioxomolybdenum(VI) complexes
Supramolecular
Hydrogen bond
p-p stacking
architectures via hydrogen bonding and p-p stacking interactions. In addition, extensive hydrogen bond-
DFT calculations
ing interactions in complex 10 lead to the formation of cavities. DFT calculations on the ligand and com-
plexes 2, 2a, 9 and 10 were also carried out.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
and trans-1,2 bis (4-pyridyl) ethane are found to be excellent
bridging ligands, each of which can connect two separate coordina-
The chemistry of Mo(VI) has enticed special attention because
of its relevance to several biological [1,2] and industrial [3–5] pro-
cesses. Dioxomolybdenum complexes exhibit oxo-transfer ability
to some selected substrates and mimic the active centers of some
oxo-transfer molybdoenzymes [6]. Syntheses of binuclear com-
plexes in which two redox active centers can undergo electro-
chemical interaction across a conjugated bridging ligand is of
interest for the study of mixed-valence complexes and the even-
tual development of molecular wires [7]. Linear N,N0-bidentate
spacers 4,40 bipyridine (4,40 bipy), trans-1,2 bis (4-pyridyl) ethene
tion spheres of two coordinatively unsaturated metal centers.
Though many homo and hetero-metallic diimine-bridged com-
plexes of other transition metals are known [8], most of the binu-
clear/polynuclear complexes of molybdenum are oxo-, sulfido- and
halogeno-bridged. A large number of binuclear molybdenum com-
plexes using extended poly pyridyl type bridging ligands have been
reported by Ward and coworkers [9], but the Mo-centers in most of
these complexes are in very low oxidation (I/II) states, contain no
oxomolybdenum center and many of them are not characterized
crystallographically. Zubita and coworkers have synthesized and
structurally characterized a few molybdenum(VI) oxide organo-
diamine compounds [10]. Koo et al. [11,12] have prepared and
structurally characterized a few binuclear dioxo molybdenum(VI)
⇑
Corresponding authors. Tel.: +91 033 2337 4389; fax: +91 033 2337 4782.
E-mail address: nr_pramanik@yahoo.co.in (N.R. Pramanik).
http://dx.doi.org/10.1016/j.poly.2014.08.010
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
197
complexes with tridentate ONS donor Schiff base bridged by 4,40-
2.2.2. Syntheses of complexes
bipyridyl or its derivatives.
All the mononuclear MoO2L complexes were prepared by
refluxing MoO2(acac)2 and the respective ligands in dry methanol
for 2 h. The complexes were satisfactorily characterized by ele-
mental analyses, IR and 1H NMR [14,15].
The field of supramolecular chemistry is one of the most fron-
tier and emergent areas of research because of its structural ele-
gance and enormous potential applications [13a,b]. The design
and construction of suitable ligands with desirable connectivity
for the metal center plays the main role in supramolecular designs.
The crystal structures of these complexes often exhibit various
interesting supramolecular architectures. Extensive hydrogen
2.2.2.1. (MoO2L1)2(4,40 bipy) (1). A mixture of 0.856 g (2 mmol) of
MoO2L and 0.156 g (1 mmol) of 4,40-bipyridine in dry methanol
(20 mL) was refluxed for 3 h. The orange red crystalline compound
was filtered, washed with dry methanol and dried in vacuo over
anhydrous CaCl2. Yield ꢀ70%. Anal. Calc. for C40H32N6S4O6Mo2: C,
47.43; H, 3.16; N, 8.30; Mo, 18.97. Found: C, 46.88; H, 2.84; N,
bonds and p-p stacking interactions play a pivotal role in the self
assembly of the binuclear Mo(VI) complexes.
In this work we have synthesized and structurally characterized
some linear diimine and diamine-bridged binuclear Mo(VI) com-
plexes of the type [(MoO2L)2 (B-B)] [where B-B is a bridging linear
N,N0-bidentate spacer 4,40 bipyridine, 1,2 bis (4-pyridyl) ethene
and 1,2 bis (4-pyridyl) ethane]. In the complexes, two MoO2L units
are joined together by a bridging N,N0-bidentate spacer to form a
binuclear species (where L = H2L1, H2L2, H2L3 and H2L4). The crystal
structures of complexes 2, 2a, 9 and 10 have been determined by
single crystal X-ray diffraction methods. The spectroscopic and elec-
trochemical behavior of these complexes have also been described.
8.34; Mo, 18.72%. IR (KBr Pellet) cmꢁ1
980 (s), 895 (vs), (MoAS) 365 (m), (MoAN) 623 (m); UV–Vis (CH2Cl2)
[kmax/nm (
/dm3 molꢁ1 cmꢁ1)]: 311 (7582), 395 (2907); 1H NMR
: m(C@N) 1594 (vs), m(Mo@O)
m
m
e
(dmso-d6): (ASACH2A) 4.44 (s, 4H), (aromatic protons) 7.10–7.64
(m, 26H), (ACH@N) 8.97 (s, 2H).
2.2.2.2. (MoO2L2)2(4,40 bipy) (2). This complex is isolated when the
same reaction is carried out in CH2Cl2 medium. Anal. Calc. for
C
28H24N6S4O6Mo2: C, 42.56; H, 2.79; N, 9.77; Mo, 22.32. Found:
C, 42.35; H, 2.68; N, 9.72; Mo, 21.98%. IR (KBr Pellet) cmꢁ1
(C@N) 1592 (vs), (Mo@O) 975 (vs), 931 (vs), (MoAS) 363 (m), m(MoAN)
/dm3 molꢁ1 cmꢁ1)]: 241
2. Experimental
:
m
m
m
2.1. Materials
633 (m); UV–Vis (CH2Cl2) [kmax/nm (
e
(18037), 310 (8945), 398 (3306); 1H NMR (dmso-d6): (ASACH3)
2.66 (s, 6H), (aromatic protons) 7.23–7.67 (m, 16H), (ACH@N)
9.08 (s, 2H).
Reagent grade solvents were dried and distilled prior to use. All
other chemicals used for preparative work were of reagent grade,
available commercially and used without further purification.
Complexes 2a-12 were prepared by refluxing the corresponding
parent MoO2L complex with 4,40 bipyridine (4,40 bipy), 1,2 bis
(4-pyridyl) ethene, 1,2 bis (4-pyridyl) ethane in 2:1 M ratios in
dry methanol for 3 h (Scheme 2). The orange red crystalline solids
so formed were filtered, washed with dry methanol and dried in
vacuo over anhydrous CaCl2. Yield ꢀ70–75%.
2.2. Syntheses
2.2.1. Syntheses of the ligands
The Schiff base ligands (H2L1, H2L2, H2L3 and H2L4) were pre-
pared by condensing S-benzyl and S-methyl dithiocarbazates with
salicylaldehyde and 2-hydroxyacetophenone in ethanol [14,15]
(Scheme 1). The ligands were satisfactorily characterized by ele-
mental analyses, IR and 1H NMR.
2.2.2.2a. (MoO2L2)2(4,40 bipy)(CH3OH)2 (2a). Anal. Calc. for C30-
H32Mo2N6O8S4: C, 38.93; H, 3.46; N, 9.08; Mo, 20.76. Found: C,
38.62; H, 3.35; N, 8.95; Mo, 20.58%. IR (KBr Pellet) cmꢁ1
: m(C@N)
1596 (vs), m(Mo@O) 978 (s), 900 (vs), m(MoAS) 365 (m), m(MoAN) 625
(m); UV–Vis (CH2Cl2) [kmax/nm (e
/dm3 molꢁ1 cmꢁ1)]: 317 (9512),
344 (4569); 1H NMR (dmso-d6): (ASACH3) 2.63 (s, 6H), (CH3OH)
3.26 (s, 6H), (CH3OH) 4.53 (s, 2H), (aromatic protons) 7.10–7.59
(m, 16H), (ACH@N) 8.92 (s, 2H).
OH
OH
SH
C
S
C
H
N
C
N
N
SR
C
N
SR
R1
R1
2.2.2.3. (MoO2L3)2(4,40 bipy) (3). Anal. Calc. for C42H36N6S4O6Mo2: C,
48.46; H, 3.66; N, 8.07; Mo, 18.46. Found: C, 48.38; H, 3.35; N, 7.99;
Keto (thione) form
Enol (thiol) form
Mo, 17.86%. IR (KBr Pellet) cmꢁ1
: m(C@N) 1572 (s), m(Mo@O) 998 (vs),
921 (vs),
m
(MoAS) 380 (s),
m
(MoAN) 561 (s); UV–Vis (CH2Cl2) [kmax/nm
(e
/dm3 molꢁ1 cmꢁ1)]: 296 (2042), 378 (7710); 1H NMR (dmso-d6):
OH
OH
dry EtOH
S
H
S
C
H
stir, r.t., 2h
SR
C
N
N
C
SR
C
O
+
H2N
N
R1
R1
R
R1
Ligand
H2L1
H2L2
H2L3
H2L4
CH2C6H5
CH3
H
H
CH2C6H5
CH3
CH3
CH3
Scheme 1. Reaction diagram for isolation of the ligands.

198
N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
(H3CACI@NA) 2.76 (s, 6H), (ArACH@CHAAr) 6.86–6.88 (d, 2H),
(aromatic protons) 7.01–7.88 (m, 16H).
2.2.2.9. (MoO2L1)2 1,2 bis (4-py) ethane (9). Anal. Calc. for
C
42H36N6S4O6Mo2: C, 48.46; H, 3.46; N, 8.07; Mo, 18.46. Found:
C, 48.26; H, 3.41; N, 8.10; Mo, 18.32%. IR (KBr Pellet) cmꢁ1
m(C@N) 1598 (vs), m(Mo@O) 975 (s), 898 (vs), m(MoAS) 368 (m), m(MoAN)
:
632 (m); UV–Vis (CH2Cl2) [kmax/nm (e
/dm3 molꢁ1 cmꢁ1)]: 310
(4473), 393 (1721); 1H NMR (dmso-d6): (ArACH2ACH2AAr) 2.96
(s, 4H), (ASACH2A) 4.40 s (4H), (aromatic protons) 6.90–7.56 (m,
26H), (ACH@N) 8.98 s (2H).
2.2.2.10. (MoO2L2)2 1,2 bis (4-py) ethane (CH3OH) (10). Anal. Calc. for
C32H36N6S4O8Mo2: C, 40.34; H, 3.78; N, 8.82; Mo, 20.17. Found: C,
40.32; H, 3.57; N, 8.62; Mo, 20.36%. IR (KBr Pellet) cmꢁ1
: m(C@N)
1595 (vs), m(Mo@O) 976 (s), 900 (vs), m(MoAS) 364 (m), m(MoAN) 628
(m); UV–Vis (CH2Cl2) [kmax/nm (e
/dm3 molꢁ1 cmꢁ1)]: 277 (7611),
315 (8467), 403 (2643); 1H NMR (dmso-d6): (ASACH3) 2.56 (s,
6H), (ArACH2ACH2AAr) 2.92 (s, 4H), (CH3OH) 3.31 (s, 3H), (CH3OH)
4.46 (s, 2H) (aromatic protons) 6.90–7.55 (m, 16H), (ACH@N) 8.98
(s, 2H).
Scheme 2. Reaction diagram for the isolation of binuclear Mo(VI) complexes.
[B-B = 4,4/bipyridine, 1,2 bis (4-pyridyl) ethene, 1,2 bis (4-pyridyl) ethane].
(H3CACI@NA) 2.79 (s, 6H), (ASACH2A) 4.45 (s, 4H), (aromatic
protons) 6.89–7.90 (m, 26H).
2.2.2.11. (MoO2L3)2 1,2 bis (4-py) ethane (11). Anal. Calc. for
C
44H40N6S4O6Mo2: C, 49.44; H, 3.74; N, 7.86; Mo, 17.98. Found:
C, 49.37; H, 3.64; N, 7.82; Mo, 17.87%. IR (KBr Pellet) cmꢁ1
m(C@N) 1576 (s), m(Mo@O) 997 (vs), 899 (vs), m(MoAS) 375 (m), m(MoAN)
2.2.2.4. (MoO2L4)2(4,40 bipy) (4). Anal. Calc. for C30H28N6S4O6Mo2: C,
:
40.54; H, 3.15; N, 9.46; Mo, 21.62. Found: C, 40.21; H, 3.00; N, 9.52;
Mo, 21.32%. IR (KBr Pellet) cmꢁ1
:
m(C@N) 1580 (s), m(Mo@O) 935 (s),
637 (m); UV–Vis (CH2Cl2) [kmax/nm (e
/dm3 molꢁ1 cmꢁ1)]: 300
900 (vs), m(MoAS) 375 (m), m(MoAN) 620 (m); UV–Vis (CH2Cl2) [kmax
/
(9537), 381 (3669); 1H NMR (dmso-d6): (H3CACI@NA) 2.76 (s,
6H), (ArACH2ACH2AAr) 2.90 (s, 4H), (ASACH2A) 4.42 (s, 4H),
(aromatic protons) 6.86–7.87 (m, 26H).
nm (e
/dm3 molꢁ1 cmꢁ1)]: 241 (13285), 271 (8165), 294 (8369),
370 (3222); 1H NMR (dmso-d6): (ASACH3) 2.58 (s, 6H),
(H3CACI@NA) 2.79 (s, 6H), (aromatic protons) 6.89–7.91 (m, 16H).
2.2.2.12. (MoO2L4)2 1,2 bis (4-py) ethane (12). Anal. Calc. for
2.2.2.5. (MoO2L1)2 1,2 bis (4-py) ethene (5). Anal. Calc. for
C
32H32N6S4O6Mo2: C, 41.92; H, 3.49; N, 9.17; Mo, 20.96. Found:
C, 42.00; H, 3.51; N, 9.13; Mo, 20.85%. IR (KBr Pellet) cmꢁ1
1580 (s), m(Mo@O) 1000 (vs), 900 (vs), m(MoAS) 378 (m), m(MoAN) 634
(m); UV–Vis (CH2Cl2) [kmax/nm (
/dm3 molꢁ1 cmꢁ1)]: 298 (2577),
380 (9233); 1H NMR (dmso-d6): (ASACH3) 2.54
(H3CACI@NA) 2.76 (6H), (ArACH2ACH2AAr) 2.91
C42H34N6S4O6Mo2: C, 48.55; H, 3.27; N, 8.90; Mo, 18.49. Found:
:
m(C@N)
C, 48.43; H, 3.21; N, 8.69; Mo, 18.50%. IR (KBr Pellet) cmꢁ1
:
m(C@N) 1600 (vs), m(Mo@O) 974 (vs), 926 (vs), m(MoAS) 362 (s), m(MoAN)
e
634 (m); UV–Vis (CH2Cl2) [kmax/nm (e
/dm3 molꢁ1 cmꢁ1)]: 285
s
s
(6H),
(4H),
(17200), 407 (3090); 1H NMR (dmso-d6): (ASACH2A) 4.41 (s,
4H), (ArACH@CHAAr) 6.91–6.93 (d, 2H), (aromatic protons)
7.02–7.84 (m, 26H), (ACH@N) 8.99 (s, 2H).
s
(aromatic protons) 6.86–7.87 m (16H).
2.3. Physical measurements
2.2.2.6. (MoO2L2)2 1,2 bis (4-py) ethene (6). Anal. Calc. for
Elemental analyses were performed on a Perkin-Elmer 240 C, H,
N analyzer. 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 voltammo-
grams were recorded in DMF containing 0.1 M TEAP as supporting
electrolyte. Thermal analyses were carried out in a Mettler Toledo
TGA/SDTA 851 thermal Analyzer in a dynamic atmosphere of
dinitrogen (flow rate = 30 cm3 minꢁ1).
C
30H26N6S4O6Mo2: C, 40.63; H, 2.93; N, 9.48; Mo, 21.67. Found:
C, 40.52; H, 2.89; N, 9.42; Mo, 21.51%. IR (KBr Pellet) cmꢁ1
(C@N) 1600 (vs), (Mo@O) 980 (vs), 925 (vs), (MoAS) 365 (m), m(MoAN)
/dm3 molꢁ1 cmꢁ1)]: 298
:
m
m
m
632 (m); UV–Vis (CH2Cl2) [kmax/nm (
e
(14320), 403 (2580); 1H NMR (dmso-d6): (ASACH3) 2.55 (s, 6H),
(ArACH@CHAAr) 6.90–6.93 (d, 2H), (aromatic protons) 7.03–7.75
(m, 16H), (ACH@N) 8.90 (s, 2H).
2.2.2.7. (MoO2L3)2 1,2 bis (4-py) ethene (7). Anal. Calc. for
C
44H38N6S4O6Mo2: C, 49.53; H, 3.56; N, 7.88; Mo, 18.01. Found:
C, 49.36; H, 3.48; N, 7.82; Mo, 17.95%. IR (KBr Pellet) cmꢁ1
(C@N) 1600 (vs), (Mo@O) 927 (vs), 900 (vs), (MoAS) 370 (m), m(MoAN)
/dm3 molꢁ1 cmꢁ1)]: 298
:
m
m
m
642 (m); UV–Vis (CH2Cl2) [kmax/nm (
e
(20990), 382 (4610); 1H NMR (dmso-d6): (H3CACI@NA) 2.76 (s,
6H), (ASACH2A) 4.42 (s, 4H), (ArACH@CHAAr) 6.86–6.88 (d, 2H),
(aromatic protons) 7.01–7.87 (m, 26H).
2.4. Computational details
2.2.2.8. (MoO2L4)2 1,2 bis (4-py) ethene (8). Anal. Calc. for
Full geometry optimization of 2, 2a, 9 and 10 were carried out
using the DFT method at the B3LYP level of theory [16,17]. The
6–31+G(d) basis set was assigned for all elements except molybde-
num. The LanL2DZ basis set with effective core potential was
employed for the molybdenum atom [18–20]. The vibrational fre-
quency calculations were performed to ensure that the optimized
C
32H30N6S4O6Mo2: C, 42.01; H, 3.28; N, 9.19; Mo, 21.00. Found:
C, 42.00; H, 3.19; N, 8.98; Mo, 20.83%. IR (KBr Pellet) cmꢁ1
1600 (vs), m(Mo@O) 925 (vs), 900 (vs), m(MoAS) 368 (m), m(MoAN) 635
(m); UV–Vis (CH2Cl2) [kmax/nm
/dm3 molꢁ1 cmꢁ1)]: 290
(12060), 382 (2460); 1H NMR (dmso-d6): (ASACH3) 2.55 (s, 6H),
:
m(C@N)
(e

N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
199
geometries represent the local minima of potential energy surface
and there are only positive eigen-values. The lowest 50 singlet–sin-
glet vertical electronic excitations based on B3LYP optimized
geometries were computed using the time-dependent density
functional theory (TDDFT) formalism [21–23] in CH2Cl2 using the
conductor-like polarizable continuum model (CPCM) [24–26] with
B3LYP and the above basis sets. All computations were performed
using the GAUSSIAN09 and GAUSSIAN03 programs [27]. GAUSSSUM [28]
was used to calculate the fractional contributions of various groups
to each molecular orbital.
the 1000–895 cmꢁ1 region, the higher frequency band originating
from the anti-symmetric while the lower one from the symmetric
stretching mode of the [MoO2]2+moiety [32,35,36].
1H NMR data for the ligands and the corresponding complexes
(1–12) are summarized in Sections 2.2.1 and 2.2.2. The relative
broad signals at d 10.72 and 10.02 ppm; d 10.24 and 10.09 ppm;
d 11.25 and 11.26 ppm; d 11.42 and 12.68 ppm in the 1H NMR
spectra corresponding to OAH and NAH of the ligands H2L1,
H2L2, H2L3 and H2L4 disappear in the complexes 1–12 as a result
of complexation. The signals at d 8.04 and 8.01 ppm of the ligand
have shifted downfield to d 8.97; 8.92; 8.98; 8.91; 8.99 and
8.91 ppm indicating coordination from the azomethine nitrogen
in the complexes 1, 2, 5, 6, 9 and 10. In the complexes 3, 4, 7, 8,
11 and 12, the involvement of the azomethine nitrogen is indicated
by the shift of the –CH3 proton signal from d 2.47 to 2.79, 2.76 and
d 2.46 to 2.79, 2.76 ppm respectively. The methylene protons of the
ligands and the complexes 1, 3, 5, 7, 9 and 11 appearing at d 4.59,
4.51, 4.44, 4.45, 4.40, 4.42, 4.41 and 4.42 ppm respectively indicate
that the S-benzyl sulfur is not involved in the coordination. The
nine aromatic protons appear as multiplets within the range d
7.24–7.43 (7H), d 6.92–6.98 (2H) and d 7.24–7.431 (7H), d 6.64–
6.90 (2H) ppm for the ligands H2L1 and H2L3 respectively. In com-
plexes 1, 3, 5, 7, 9 and 11 twenty-six aromatic protons appear as
multiplets within the range d 7.10–7.64, d 6.89–7.90, d 6.90–7.56,
d 6.86–7.89, d 7.02–7.84, d 7.01–7.87 ppm respectively.
2.5. Crystallographic measurements
The crystallographic data for complex 2 was collected on an
Oxford Diffaction X-Calibur System, for complexes 2a and 9 on a
Siemens P4 4-circle-diffractometer and for complex 10 was col-
lected on a Bruker APEX-II diffractometer with CCD-area detector.
All data sets were obtained with graphite-monochromated Mo K
a
(k = 0.71073 Å) radiation. Unit-cell dimension and intensity data
for 2, 2a, 9, 10 were measured at 150(2), 294(2), 294(2) and
296(2) K respectively. The crystal structures were solved by direct
methods using SHELXS-97 and refined by full-matrix least-squares
based on F2 with anisotropic displacement parameter of non-
hydrogen atoms using SHELXL-97. The hydrogen atoms were
included in calculated positions. The ligand in 2 was completely
disordered over a pseudo mirror plane containing atoms Mo(1),
O(1) and O(2), each atom occupying two possible sites. The struc-
ture was refined using occupation factors that converged to 0.67(1)
and 0.33(1) respectively.
Similar observations for aromatic protons have been made with
the ligands H2L2 and H2L4 and their corresponding complexes. The
methyl protons of the ligands H2L2 and H2L4 and their complexes
appear at nearly the same positions. Considering IR and 1H NMR
data together, it is clear that results are consistent in describing
the donor sites of the ligands bound to the [MoO2]2+ center.
Electronic spectra of the binuclear molybdenum(VI) complexes
were recorded in dry dichloromethane and the spectral data of
complexes are presented in the Section 2.2.2. Spectra of all the
binuclear [(MoO2L)2 (B-B)] complexes exhibit several bands in
the 407–241 nm range. The absorption maxima are located in the
3. Results and discussion
3.1. Synthesis
The Schiff base ligands (H2L1, H2L2, H2L3 and H2L4) were pre-
pared by condensing S-benzyl and S-methyl dithiocarbazates with
salicylaldehyde and 2-hydroxyacetophenone in ethanol. The
ligands were satisfactorily characterized by elemental analyses,
IR and 1H NMR.
All the binuclear complexes (1–12) of general formula
[(MoO2L)2(B-B)] were prepared by refluxing the corresponding
parent MoO2L complex with 4,40 bipyridine (4,40 bipy), 1,2 bis
(4-pyridyl) ethene and 1,2 bis (4-pyridyl) ethane in 2:1 M ratios
in dry methanol.
400–300 nm range and may be assigned [37] to S(p
LMCT transition caused by the promotion of an electron from the
filled HOMO of the ligand of primarily sulfur p character to the
empty LUMO of molybdenum d character. Other LMCT bands
p)AMo(dp)
p
p
are observed in the region 300–277 nm [31,38–40]. The bands
appearing below 277 nm are due to intra-ligand transitions.
3.3. Electrochemical properties
All the orange yellow Mo(VI) binuclear complexes (1–12) are air
stable in the solid state. The compounds are readily soluble in alco-
hol, CH2Cl2, CH3CN, DMF and DMSO but are insoluble in water. The
complexes are diamagnetic at room temperature as is expected for
d°Mo(VI) centers [29]. Molar conductivity data in 10ꢁ3M CH2Cl2
solution indicate that they are non-electrolytes. All the binuclear
Mo(VI) complexes are satisfactorily characterized by elemental
analyses, IR, 1H NMR, electronic spectra and cyclic voltammetric
data. The complexes 2, 2a, 9 and 10 have been structurally charac-
terized by single crystal X-ray diffraction. DFT calculations on the
ligand and complexes 2, 2a, 9 and 10 were also carried out.
Cyclic voltammograms of the binuclear molybdenum(VI) com-
plexes (1–12) at a platinum electrode were recorded in dry
degassed DMF containing 0.1 M TEAP as the supporting electrolyte
Table 1
Cyclic voltammetric resultsa (V vs. SCE) for the [(MoVIO2L)2(B-B)] complexes at 298 K.
Complexes
Epc (V)
Epa (V)
(MoO2L1)2(4,40 bipy) (1)
ꢁ1.22
ꢁ1.16
ꢁ1.12
ꢁ1.18
ꢁ1.09
ꢁ1.10
ꢁ1.09
ꢁ1.15
ꢁ1.15
ꢁ1.10
ꢁ1.08
ꢁ1.16
ꢁ1.17
+0.68
+0.64
+0.65
+0.68
+0.65
+0.69
+0.67
+0.67
+0.68
+0.90
+0.99
+0.69
+0.73
(MoO2L2)2(4,40 bipy) (2)
(MoO2L2)2(4,40 bipy)(CH3OH)2 (2a)
(MoO2L3)2 (4,40 bipy) (3)
(MoO2L4)2 (4,40 bipy) (4)
(MoO2L1)2 1,2,bis(4-py)ethene (5)
(MoO2L2)2 1,2,bis(4-py)ethene (6)
(MoO2L3)2 1,2,bis(4-py)ethene (7)
(MoO2L4)2 1,2,bis(4-py)ethene (8)
(MoO2L1)21,2,bis(4-py)ethane (9)
(MoO2L2)2 1,2,bis(4-py)ethane (10)
(MoO2L3)2 1,2,bis(4-py)ethane (11)
(MoO2L4)2 1,2,bis(4-py)ethane (12)
3.2. Spectral characteristics
Characteristic IR bands of binuclear molybdenum(VI) com-
plexes are given in Section 2.2.2. A strong m(C@N) band [30,31]
was observed around 1572–1600 cmꢁ1 region for the complexes.
A new band in the 561–642 cmꢁ1 region was observed for com-
a
plexes and is assigned to the
exhibit a medium intensity band around 362–380 cmꢁ1 which
may be assigned to (MoAS) [34]. Two IR bands are observed in
m(MoAN) [32,33]. The complexes
Solvent: DMF (dry, degassed); supporting electrolyte: 0.1 M TEAP; solution
strength: 10ꢁ3 M; working electrode: platinum; reference electrode: SCE:
100 mV sꢁ1
.
m

200
N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
over a potential range of 0.0 to ꢁ1.5 V. Results of cyclic voltammet-
ric studies of all the binuclear Mo(VI) complexes are presented in
Table 1 and the representative diagrams are shown in Fig. S1.
The complexes (1–12) exhibit only one irreversible two electron
one step reduction within the potential window ꢁ1.08 to
ꢁ1.22 V region versus SCE in DMF solution which are assigned to
MoVI–MoVI/MoIV–MoIV process [31,38,40]. A two-electron involve-
ment is established by a comparison of the current height with
authentic two electron species under identical experimental condi-
tion. On scan reversal, one irreversible two-electron oxidative
response is located in the 0.64 to 0.99 V range. It is to be noted that
the reduction of binuclear dioxo Mo(VI) complexes in aprotic sol-
vents is generally irreversible.
When the brown solution of Mo(IV) complexes in CH3CN is
reacted with DMSO or pyridine N-oxide, the brown color of the
solution gradually changes to orange indicating oxo-transfer from
the substrate DMSO or pyridine N-oxide to the mono oxo [MoO]2+
center to regenerate the parent [MoO2]2+ species.
MoOLðB-BÞ þ DMSO CꢁH!3CN MoO2L þ B-B þ Me2S
3.6. Description of the crystal structures of complexes 2, 2a, 9 and 10
The molecular structure and the atom numbering scheme of
complexes 2, 2a, 9 and 10 are shown in Figs. 1–4. Crystallographic
data and selected bond lengths and bond angles are presented in
Tables 2 and 3 respectively.
3.4. Thermal properties of complex 2a
The orange binuclear complex 2 contains a crystallographic
center of inversion. Structural equivalence of the two Mo(VI) cen-
ters is reflected in the simultaneous electrochemical reductions of
the two Mo(VI) centers at the same potential. Each Mo(1) is
bonded to two oxo atoms, O, N and S donor atoms of the ligand
H2L2 and one nitrogen atom of the bridging 4,40 bipyridine mole-
cule in a distorted octahedral coordination environment around
the metal center. Dimensions from the major component of the
disordered ligand will be discussed. Two Mo-atoms (MoAMo dis-
tance = 11.957 Å) are linked by two bridged nitrogen atoms of
4,40-bipyridine. The ligand is active in its enolate form and behaves
in a dianionic tridentate manner. The three donor points O(11),
N(3), S(1) of the ligand and one terminal oxygen O(2) occupies a
meridonial plane [41,42]. The dianionic tridentate ligand forms
one five membered and another six membered chelating ring
The TG/DTA study of complex 2a was conducted in the temper-
ature range of 30–600 °C with a 10 °C/min temperature interval in
nitrogen atmosphere. Above 200 °C, the TG/DTA curve exhibits
three steps of weight losses. The compound starts losing weight
by an exothermic peak around 246 °C in the TG/DTA curve due to
the concomitant removal of two CH3OH molecules and the bridg-
ing 4,40-bipyridyl molecule. Step-1 is reasonable as the 4,40-bipyr-
idyl moiety is attached to the two mononuclear units MoO2CH3OH
through the coordinated methanol and should be removed with
the loss of two CH3OH molecules which hold it through hydrogen
bonding. In the second step, the loss of the CH3ASASACH3 moiety
is suggested by an exothermic peak around 280 °C in the DTA
curve. It can be attributed to the fact that at 280 °C, the stable
CH3ASASACH3 species is eliminated rather than the more reactive
species CH3–S species. The compound finally changes into
stable MoO3 at 570 °C. The observed loss of weights corroborates
the proposed decomposition processes.
around
the
[MoO2]2+
center
with
bite
angles
of
N(3)AMo(1)AS(1),72.6(3)° and O(11)AMo(1)AN(3), 82.2(4)°. The
other oxo-oxygen O(2) occupies an apical position in the coordina-
tion sphere of Mo(1). The Mo(VI) center is slightly displaced by
0.375(3) Å from the equatorial plane in the direction of the termi-
nal oxygen atom O(1). The other apical position is occupied by the
nitrogen atom N(41) of one half of the 4,40 bipy unit. The
Mo(1)AN(41) bond length [2.435(7) Å] is considerably longer than
the other Mo(1)AN(3) bond [2.321(11) Å], no doubt because of the
trans influence of O(1), a value which indicates that the bipyri-
dine nitrogen is comparatively weakly bonded to the [MoO2]2+ core
pointing to the possibility of ligand exchange reaction at this point.
3.5. Study of reactivity
All the binuclear dioxo Mo(VI) complexes exhibit oxo-transfer
to the substrate PPh3, when reacted with PPh3 in 1:1.5 M propor-
tions in dry CH3CN medium. The oxo-transfer reaction may be rep-
resented as
CH3CN
½ðMoO2LÞ2ðB-BÞꢂ þ 2PPh3 ꢁ! 2MoOLðB-BÞ þ 2 OPPh3
Fig. 1. Molecular structure of complex (MoO2L2)2(4,40 bipy) (2).

N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
201
Fig. 2. Molecular structure of complex (MoO2L2)2(4,40 bipy)(CH3OH)2 (2a).
Fig. 3. Molecular structure of complex (MoO2L1)2 1,2 bis (4-py) ethane (9).
Due to coordination in the thiolate form of the ligand, the
C(1)AS(1) bond is expected to assume a single bond character
but in the complex, the length is 1.78(2) Å which lies between
the length of CAS bond and C@S bond and may be attributed to
electron delocalization in the coordinated ligand [43,44]. The adja-
cent C(1)AN(2) bond length is 1.26(2) Å and is closer to the C@N
distance than the normal CAN distance. The N(2)AN(3) bond dis-
tance of 1.38(2)Å reveals its predominant single bond character.
Two types of intermolecular hydrogen bonding interactions
(Fig. S2) are observed between O(1) and H(43) with bond lengths
O(1)ꢃ ꢃ ꢃH(43) 2.57 Å, O(1)ꢃ ꢃ ꢃC(43) 3.230 Å and bond angle
O(1)ꢃ ꢃ ꢃH(43)AC(43) 128° and S(2) and H(43) with bond lengths
S(2)ꢃ ꢃ ꢃH(43) 3.10 Å, S(2)ꢃ ꢃ ꢃC(43) 3.91 Å and bond angle
S(2)ꢃ ꢃ ꢃH(43)AC(43) 145.95°. The Oꢃ ꢃ ꢃH interactions lead to the for-
mation of chain like architecture and the adjacent chains are inter-
connected through the Sꢃ ꢃ ꢃH hydrogen bonds. The packing diagram
for complex 2 is shown in Fig. 5.
In the case of complex 2a, also centrosymmetric, instead of
direct bonding of the two nitrogen atoms of 4,40-bipyridine mole-
cule to the vacant sixth position of Mo(VI) as in complex 2, the
ligand is attached to the bonded methanol molecule through a
strong hydrogen bond [O(3)AH(3)ꢃ ꢃ ꢃꢃN(30)] (Fig. 2) involving the

202
N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
Fig. 4. Molecular structure of complex (MoO2L2)2 1,2 bis (4-py) ethane (10).
Table 2
Crystal data and details of refinement for complexes 2, 2a, 9 and 10.
Complex 2
Complex 2a
Complex 9
Complex 10
Chemical formula
Formula weight (M)
Crystal system
Space group
a (Å)
C
28H24Mo2N6O6S4
C30H32Mo2N6O8S4
924.74
monoclinic
P21/n
7.0612(14)
17.701(4)
14.898(3)
90
100.24(3)
90
1832.5(6)
4
C42H36Mo2N6O6S4
1040.89
triclinic
C32H36Mo2N6O8S4
952.79
trigonal
860.66
monoclinic
P21/n
9.743(2)
12.052(4)
14.434(3)
90
96.53(2)
ꢀ
ꢀ
P1
R3
9.1780(18)
10.450(2)
12.383(3)
79.92(3)
74.63(3)
72.69(3)
1087.3(4)
1
22.9640(17)
22.9640(17)
18.8337(14)
90
90
120
8601.2(11)
9
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
90
1683.9(8)
2
V (Å3)
Z
T (K)
150(2)
1.697
1.043
860
294(2)
1.676
0.968
932
294(2)
1.590
0.823
526
296(2)
1.655
0.931
4338
DC (g cmꢁ3
)
l
(Mo K
F(000)
Goodness of fit (GOF) on F2
R1, wR2 [I > 2 (1)]
a
) (mmꢁ1
)
0.916
1.016
0.937
1.171
r
R1 = 0.0933, wR2 = 0.2205
R1 = 0.0444, wR2 = 0.0779
R1 = 0.0894, wR2 = 0.1632
R1 = 0.1209, wR2 = 0.2454
AOH hydrogen atom of the coordinated methanol, with Oꢃ ꢃ ꢃN
2.646(8), Hꢃ ꢃ ꢃN 1.886 Å and OAHꢃ ꢃ ꢃN angle 171°. Complex 2a is
oxo atoms O(1) and O(2), phenolate oxygen O(11), thioenolate sul-
fur S(1), azomethine nitrogen N(3) and nitrogen atom N(41) of one
half of the 1,2-bis (4-py)ethane unit.
further stabilized via
p-p stacking interactions which are facili-
tated by the electron withdrawing N atom in the bipyridine moiety
which decreases the -electron repulsion by reducing the -elec-
tron density in the ring. Fig. 6 shows the stacking interactions,
centroid–centroid distance 3.499 Å with perforce parallel rings, a
distance comparable to previously reported stacking interac-
tions [45]. The MoAMo distance in complex 2a (14.890 Å) is much
longer than the MoAMo distance in complex 2 (11.958 Å) due to
different types of interaction of the spacer with the Mo-centers
in the two cases. In complex 2a, the Mo(VI) center is slightly dis-
placed by 0.303 Å from the mean equatorial plane described by
O(2),S(1),N(3) and O(11) towards the apical oxo-oxygen O(1).
In complex 9 which is also centrosymmetric the MoAMo dis-
tance is 14.383 Å and the two Mo centers are linked by two bridg-
ing N atoms of 1,2-bis(4-py)ethane moiety. The Mo(VI) center is
present in a distorted donor environment consisting of two cis
The addition of 1,2-bis (4-py)ethane to the vacant position trans
to the oxo atom O(1) generates an O(1)AMo(1)AN(41) angle of
169.9(3)°. The Mo center is slightly displaced by 0.323 Å from the
equatorial plane towards the apical oxo oxygen O(1).
In the crystal packing structure (Fig. S3) of complex 9, an inter-
molecular hydrogen bond was observed between the oxo atom
O(1) and H(25) (1 + x, y, z) (a ring hydrogen atom of the S-benzyl
moiety of the ligand) with dimensions Oꢃ ꢃ ꢃH 2.81 Å, Oꢃ ꢃ ꢃC
3.620 Å and angle Oꢃ ꢃ ꢃHAC 146° (Fig. S4). The hydrogen bonding
pattern thus gives rise to a ladder like architecture along the a-
direction. The hydrogen bond dimensions are consistent with pre-
viously reported values [46].
p
p
p-p
p-
p
Complex 10 is also centrosymmetric. The coordination geome-
try around the Mo(VI) center is distorted octahedral. Here, the
Mo centers are linked by 1,2-bis(4-py)ethane moiety, the MoAMo

N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
203
Table 3
Experimental and calculated dimensions in 2, 2a, 9 and 10 with distances (Å) and angles (deg).
Complex 2
Complex 2a
Complex 9
Complex 10
Bond distances(Å)
obsa
calc
obs
calc
obs
calc
obs
calc
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–S(1)
Mo(1)–O(11)
1.715(6)
1.695(6)
2.548(4)
1.863(8)
2.321(11)
2.435(7)
1.375(18)
1.710
1.720
2.477
1.975
2.337
2.504
1.332
1.707(2)
1.711(2)
2.444(1)
1.956(2)
2.277(3)
–
1.709
1.725
2.476
1.970
2.328
–
1.677(7)
1.724(7)
2.451(3)
1.930(8)
2.286(8)
2.456(8)
1.412(11)
1.711
1.720
2.480
1.973
2.334
2.509
–
1.694(10)
1.711(10)
2.481(5)
2.042(16)
2.278(14)
2.443(13)
1.468(17)
1.711
1.720
2.480
1.977
2.338
2.513
1.383
Mo(1)–N(3)
Mo(1)–N(41)b
N(2)–N(3)
1.423(3)
1.382
Bond angles (°)
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(11)
O(1)–Mo(1)–N(3)
O(1)–Mo(1)–S(1)
O(1)–Mo(1)–N(41)b
O(2)–Mo(1)–O(11)
O(2)–Mo(1)–N(3)
O(2)–Mo(1)–S(1)
O(2)–Mo(1)–N(41)b
O(11)–Mo(1)–N(3)
O(11)–Mo(1)–S(1)
O(11)–Mo(1)–N(41)b
N(3)–Mo(1)–S(1)
N(3)–Mo(1)–N(41)b
S(1)–Mo(1)–N(41)b
105.6(3)
102.5(4)
92.9(3)
99.0(3)
170.5(3)
104.6(4)
158.1(3)
92.6(3)
83.9(3)
82.2(4)
147.5(3)
75.2(3)
72.6(3)
77.7(3)
79.5(2)
106.5
99.7
92.4
101.2
172.6
106.4
158.2
90.6
80.6
80.3
148.0
76.1
74.9
105.3(1)
98.3(1)
92.4(1)
96.1(1)
–
104.6(1)
159.4(1)
91.0(1)
–
82.9(1)
155.1(1)
–
76.4(1)
–
106.4
99.8
92.7
98.6
–
105.4
158.3
88.8
–
81.0
152.1
–
75.1
–
1.677(7)
1.930(8)
92.3(3)
96.8(3)
169.9(3)
102.9(3)
158.9(3)
91.6(3)
83.8(3)
83.3(3)
153.6(2)
78.6(3)
75.9(3)
77.6(3)
81.2(2)
1.771
1.973
92.6
99.6
172.5
104.3
158.6
92.1
81.0
81.0
151.4
77.0
75.0
80.1
–
106.0(5)
96.1(6)
92.5(5)
100.1(4)
169.7(5)
103.8(5)
158.7(5)
90.4(4)
84.1(5)
84.2(5)
154.6(4)
79.5(5)
75.7(4)
77.9(5)
81.2(3)
106.5
99.3
92.2
100.9
172.1
106.6
158.3
90.6
81.1
81.3
151.4
74.9
74.9
81.0
80.3
80.7
80.6
–
–
a
Dimensions from the ligand positions with the highest occupancy.
This atom is N(41) in 2, 2a and 9 but N(1) in 10.
b
Fig. 5. Packing diagram for complex 2 around the b axis, hydrogen atoms are omitted for clarity.
distance being 14.245 Å. Here the metal atom is 0.326 Å away from
the equatorial plane in the direction of the terminal oxygen atom
O(1). Six binuclear moieties along with six floating methanol mol-
ecules together form a cylindrical cavity as shown in Fig. 7. The
cavity is basically formed by three overlapping rings of hydrogen
bonds. The ring in the middle is formed by six hydrogen bonded
methanol molecules. Two types of hydrogen bonding interactions
are responsible for such an interesting supramolecular architec-
ture–C(31)AH(31A)ꢃ ꢃ ꢃO(31) with hydrogen bond length 1.813 Å,
angle 107.10° and C(21)AH(21B)ꢃ ꢃ ꢃ(S2) with hydrogen bond length
3.181 Å, angle 132.43°. Hydrogen bonds involving sulfur atoms are
comparatively longer than those involving N or O because of the
larger size of sulfur and its more diffuse electron cloud [47]. The
top view (along the c-axis) shows that a star shaped cavity
(Fig. S5) is formed which may be used to trap other suitable ions
[48]. In 3-D, the packing structure appears as a sheet of fused
six-membered rings with each ring having a cavity (Fig. S6).
It is to be noted that the structural features of each of the two
units of the binuclear complexes 2, 2a, 9 and 10 are closely similar
to those of their precursor mononuclear complex MoO2L and
remind us of the situation when a substrate binds itself at the
active center of a metalloenzyme. In that case also, the major struc-
tural characteristic of the enzyme remains almost unaltered.
3.7. DFT and TD-DFT calculations of ligands and complexes 2, 2a, 9
and 10
The full geometry optimizations of 2, 2a, 9 and 10 were carried
out using the DFT/B3LYP method. The calculated bond parameters
are compared to experimental values in Table 3 and show good
agreement. For complex 2 the maximum deviation in bond length
is Mo(1)AO(11) 0.112 Å, for 2a N(2)AN(3) 0.041 Å, for
Mo(1)AN(41) 0.053 Å and N(2)AN(3) 0.085 Å for complex 10.
9
In complex 2a, the computed value (1.822 Å) for the hydrogen
bond distance [H(3)ꢃ ꢃ ꢃꢃN(30)] is in good agreement with the exper-
imental value (1.886 Å), the maximum deviation being ꢀ0.064 Å.
The compositions along with the energies of some selected
molecular orbitals of complexes are given in Tables 4–7. Contour

204
N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
Fig. 6. Packing diagram for complex 2a around the a axis showing the hydrogen-bonding (1.886Å) between N(30) and H(3) (the hydrogen bond is represented as a green
dashed line) and the intermolecular
p
-p
stacking interactions (3.499 Å) (the
p-
p
stacking interaction is represented as a black straight line). (Colour online.)
Table 4
Energy and composition of selected molecular orbitals of complex 2.
MO
Energy (eV)
% of composition
Mo
L
bpy
Oxo
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ2.01
ꢁ2.29
ꢁ2.50
ꢁ2.50
ꢁ2.68
ꢁ2.74
ꢁ6.11
ꢁ6.12
ꢁ6.64
ꢁ6.66
ꢁ6.70
ꢁ6.71
ꢁ7.41
ꢁ7.42
ꢁ7.72
ꢁ7.72
ꢁ7.87
24
06
58
57
39
34
01
01
01
01
0
67
15
21
20
37
30
96 (p
95 (p
91 (p
92 (p
98 (p
98 (p
81
80
90
91
23
01
76
0
09
03
21
21
19
19
01
02
05
05
01
0
14
14
02
02
23
01
05
17
02
02
03
01
01
02
03
03
08
07
58
HOMO
p
p
p
p
p
p
S), 27)
HOMOꢁ1
HOMOꢁ2
HOMOꢁ3
HOMOꢁ4
HOMOꢁ5
HOMOꢁ6
HOMOꢁ7
HOMOꢁ8
HOMOꢁ9
HOMOꢁ10
(S), 26)
(S), 66)
(S), 63)
(S), 32)
(S), 39)
0
03
03
0
0
03
Table 5
Energy and composition of selected molecular orbitals of complex 2a.
Fig. 7. Diagram for complex 10 illustrating the formation of a cylindrical cavity as a
result of two types of hydrogen bonding interactions (H(31A)ꢃ ꢃ ꢃO(31) = 1.813 Å (the
hydrogen bond is represented as a blue dashed line) and H(21B)ꢃ ꢃ ꢃ(S2) = 3.181 Å
(the hydrogen bond is represented as a black dashed line)). (Colour online.)
MO
Energy (eV)
% of composition
Mo
L
oxo
MeOH
bpy
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ1.80
ꢁ2.11
ꢁ2.39
ꢁ2.41
ꢁ2.49
ꢁ2.51
ꢁ5.92
ꢁ5.94
ꢁ6.46
ꢁ6.49
ꢁ6.54
ꢁ6.56
ꢁ7.22
ꢁ7.24
ꢁ7.52
ꢁ7.52
ꢁ7.54
25
0
65
0
21
19
35
37
97(p
96(p
92(p
92(p
99(p
98(p
83
83
13
14
90
11
0
0
0
0
0
0
0
01
01
01
01
01
01
01
01
63
63
06
0
99
0
0
0
0
0
01
0
0
0
0
0
0
03
02
01
plots of some selected frontier molecular orbitals of the complexes
are shown in Figs. 8–11. As the molecules are centerosymmetric,
only HOMO, HOMOꢁ2, LUMO and LUMO+2 are given. The higher
energy occupied molecular orbitals (HOMO to HOMOꢁ5) have
59
59
42
42
01
01
01
01
0
01
03
03
01
01
0
21
21
22
22
01
01
06
06
0
ligand (L) contribution with 57–67%
pp(S) contribution in
HOMO
p
p
p
p
p
p
(S), 28)
(S), 27)
(S), 66)
(S), 57)
(S), 34)
(S), 43)
HOMOꢁ2 and HOMOꢁ3. For complexes 2 and 2a, the low energy
HOMOꢁ1
HOMOꢁ2
HOMOꢁ3
HOMOꢁ4
HOMOꢁ5
HOMOꢁ6
HOMOꢁ7
HOMOꢁ8
HOMOꢁ9
HOMOꢁ10
unoccupied molecular orbitals (LUMO to LUMO+3 and LUMO+5)
have d
of p
(O) orbital, while LUMO+4 has p⁄(bpy) character. Similarly
for complexes 9 and 10 the LUMO to LUMO+5 have d
(Mo), p⁄(L)
and p (O) character.
p
(Mo) and p⁄(L) character along with minor contributions
p
0
p
13
13
21
21
03
p
To investigate the electronic spectra of the complexes vertical
electronic excitations have been calculated by the TDDFT/CPCM
method in CH2Cl2 solvent. The calculated excitation wavelength

N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
205
Table 6
Energy and composition of selected molecular orbitals of complex 9.
MO
Energy (eV)
% of composition
Mo
L
Oxo
brid
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ1.91
ꢁ1.92
ꢁ2.50
ꢁ2.50
ꢁ2.65
ꢁ2.65
ꢁ6.02
ꢁ6.03
ꢁ6.56
ꢁ6.56
ꢁ6.63
ꢁ6.64
ꢁ7.00
ꢁ7.01
ꢁ7.13
ꢁ7.13
ꢁ7.42
25
24
57
58
37
38
01
01
01
01
0
64
65
19
19
42
41
96(p
96(p
92(p
92(p
98(p
99(p
97(p
97
10
10
23
23
20
20
01
01
05
04
01
01
01
01
0
01
01
0
0
HOMO
LUMO
HOMO-2
LUMO+2
01
01
02
02
02
02
01
0
01
01
0
0
06
HOMO
p(S), 26)
p (S), 26)
p (S), 65)
p (S), 67)
p (S), 17)
p (S), 19)
p (S), 17)
HOMOꢁ1
HOMOꢁ2
HOMOꢁ3
HOMOꢁ4
HOMOꢁ5
HOMOꢁ6
HOMOꢁ7
HOMOꢁ8
HOMOꢁ9
HOMOꢁ10
0
01
01
0
0
02
100
100
77
0
15
Fig. 9. Contour plots of selected molecular orbitals of complex 2a.
Table 7
Energy and composition of selected molecular orbitals of complex 10.
MO
Energy (eV)
% of composition
Mo
L
Oxo
brid
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ1.95
ꢁ1.95
ꢁ2.43
ꢁ2.43
ꢁ2.62
ꢁ2.62
ꢁ6.05
ꢁ6.05
ꢁ6.58
ꢁ6.58
ꢁ6.64
ꢁ6.64
ꢁ7.34
ꢁ7.35
ꢁ7.63
ꢁ7.64
ꢁ7.73
23
22
58
58
39
38
01
01
01
01
0
61
61
10
10
33
33
96(p
96(p
92(p
91(p
98(p
98(p
80
80
57
70
47
08
08
21
21
22
22
01
01
05
05
0
01
02
0
0
HOMO
LUMO
HOMO-2
LUMO+2
02
02
02
02
02
02
02
02
03
03
41
28
52
HOMO
p
p
p
p
p
p
(S), 27)
(S), 27)
(S), 64)
(S), 63)
(S), 36)
(S), 37)
HOMOꢁ1
HOMOꢁ2
HOMOꢁ3
HOMOꢁ4
HOMOꢁ5
HOMOꢁ6
HOMOꢁ7
HOMOꢁ8
HOMOꢁ9
HOMOꢁ10
0
0
03
03
0
0
0
14
14
02
02
01
Fig. 10. Contour plots of selected molecular orbitals of complex 9.
HOMO
LUMO
HOMO-2
LUMO+2
HOMO
HOMO-2
LUMO
LUMO+2
Fig. 8. Contour plots of selected molecular orbitals of complex 2.
Fig. 11. Contour plots of selected molecular orbitals of complex 10.

206
N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
Table 8
Vertical electronic transitions calculated by TDDFT/CPCM method of complex 2.
Eexcitation (eV)
kexcitation (nm)
Osc. Strength (f)
Key transitions
Character
2.3882
2.3945
3.0869
519.1
517.8
401.7
0.0117
0.0113
0.0246
(74%)HOMO ? LUMO
p
p
p
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
p
p
p
(Mo)
(Mo)
(Mo)
(71%)HOMOꢁ1 ? LUMO
(39%)HOMOꢁ4 ? LUMO
(26%)HOMOꢁ5 ? LUMO
(35%)HOMOꢁ4 ? LUMO+1
(22%)HOMOꢁ5 ? LUMO+1
(43%)HOMOꢁ6 ? LUMO
(20%)HOMOꢁ7 ? LUMO+1
(42%)HOMOꢁ7 ? LUMO+1
(25%)HOMOꢁ6 ? LUMO
(62%)HOMOꢁ3 ? LUMO+6
3.0934
3.4396
3.4796
3.9540
3.9556
400.8
360.5
356.3
313.6
313.4
0.0240
0.1311
0.2077
0.1752
0.1172
p
p
p
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
p(Mo)
p(Mo)
p(Mo)
p
p
p
(S) ? p⁄(L)
(64%)HOMOꢁ2 ? LUMO+6
p
(S) ? p⁄(L)
Table 9
Vertical electronic transitions calculated by TDDFT/CPCM method of complex 2a.
Eexcitation (eV)
kexcitation (nm)
Osc. Strength (f)
Key transitions
Character
2.4393
2.4728
3.0589
508.3
501.4
405.3
0.0101
0.0115
0.0317
(80%)HOMO ? LUMO
p
p
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
p
p
(Mo)
(Mo)
(Mo)
(82%)HOMOꢁ1 ? LUMO
(57%)HOMOꢁ3 ? LUMO
(33%)HOMOꢁ3 ? LUMO+1
(37%)HOMO ? LUMO+5
(34%)HOMOꢁ1?LUMO+5
(67%)HOMOꢁ1 ? LUMO+6
(65%)HOMO ? LUMO+7
(77%)HOMOꢁ3 ? LUMO+6
(76%)HOMOꢁ2 ? LUMO+7
p
p
(S) ? p⁄(L)/d
p
3.2308
383.8
0.0461
p
(L) ? p⁄(L)/d
p
(Mo)
(Mo)
3.5279
3.5346
4.0354
4.0444
351.4
350.8
307.2
306.6
0.2063
0.2290
0.2368
0.1602
p
p
(L) ? p⁄(L)/d
(L) ? dp(Mo)
p
p
p
(S) ? p⁄(L)/d
(S) ? p⁄(L)/d
p
p
(Mo)
(Mo)
p
p
Table 10
Vertical electronic transitions calculated by TDDFT/CPCM method of complex 9.
Eexcitation (eV)
kexcitation (nm)
Osc. Strength (f)
Key transitions
Character
2.3347
2.3405
3.0596
3.0624
3.1582
3.4390
3.4433
4.0096
4.0822
531.1
529.7
405.2
404.9
392.6
360.5
360.1
309.21
303.72
0.0066
0.0079
0.0335
0.0560
0.0332
0.0974
0.1802
0.1327
0.1425
(75%)HOMO ? LUMO
p
p
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
p
p
(Mo)
(Mo)
(Mo)
(Mo)
(78%)HOMOꢁ1 ? LUMO
(66%)HOMOꢁ2 ? LUMO+1
(59%)HOMOꢁ3 ? LUMO
(64%)HOMOꢁ1 ? LUMO+4
(56%)HOMO ? LUMO+7
(55%)HOMOꢁ1 ? LUMO+6
(71%)HOMOꢁ2 ? LUMO+7
(68%)HOMOꢁ3 ? LUMO+6
p
p
(S) ? p⁄(L)/d
(S) ? p⁄(L)/d
p
p
p
p
p
p
p
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
p(Mo)
p(Mo)
p(Mo)
p
p
(S) ? p⁄(L)/d
(S) ? p⁄(L)/d
p
p
(Mo)
(Mo)
p
p
Table 11
Vertical electronic transitions calculated by TDDFT/CPCM method of complex 10.
Eexcitation (eV)
kexcitation (nm)
Osc. Strength (f)
0.0199
Key transitions
Character
2.3881
519.2
(48%)HOMO ? LUMO
p
p
p
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
(L) ? p⁄(L)/d
p
p
p
(Mo)
(Mo)
(Mo)
(47%)HOMOꢁ1 ? LUMO
(35%) HOMOꢁ5 ? LUMO
(28%)HOMOꢁ4 ? LUMO+1
(35%)HOMOꢁ1 ? LUMO+5
(34%)HOMO ? LUMO+4
(39%)HOMOꢁ3 ? LUMO+6
(30%)HOMOꢁ2 ? LUMO+5
(34%)HOMOꢁ2 ? LUMO+5
(29%)HOMOꢁ3 ? LUMO+6
3.0940
3.2051
3.9568
3.9627
400.7
386.8
313.3
312.7
0.0573
0.0407
0.1876
0.1427
p
p
p
(S) ? p⁄(L)/d
p
(Mo)
(Mo)
p
(S) ? p⁄(L)/d
p
and their assignment are summarized in Tables 8–11. In the com-
plexes very weak transitions at > 500 nm corresponding to HOMO/
4. Conclusions
HOMOꢁ1 ? LUMO transitions (
p p(Mo)) have been
(L) ? p⁄(L)/d
The desired linear di-imine and di-amine bridged binuclear
dioxomolybdenum(VI) complexes of the type [(MoO2L)2(B-B)]
[where, B-B is a bidentate spacer 4,40 bipyridine, 1,2 bis (4-pyridyl)
ethene and 1,2 bis (4-pyridyl) ethane] have been synthesized and
characterized by various physico-chemical techniques. Complexes
2, 2a, 9 and 10 were structurally characterized by single crystal X-
ray crystallography. In each of the complexes, the centrosymmetric
found which are not observed in the experimental spectra. How-
ever the moderately intense transitions corresponding to
p
(L) ? p⁄(L)/d
p
(Mo) and
observed. The intense transitions at 300–360 nm having mixed
(L) ? p⁄(L)/d (S) ? p⁄(L)/d
(Mo) and p (Mo) character can be
matched to the experimental bands.
pp p(Mo) have been
(S) ? p⁄(L)/d
p
p
p
p

N.R. Pramanik et al. / Polyhedron 85 (2015) 196–207
[13] (a) S.I. Stupp, L.C. Palmer, Chem. Mater. 26 (2014) 507;
207
structure consists of two MoO2L moieties bridged by two N-atoms
from di-imine/di-amine molecule which completes a distorted
octahedral coordination environment around the Mo(VI) centers.
The complexes also give rise to interesting supramolecular frame-
works. DFT calculation on the ligand and the complexes 2, 2a, 9
and 10 were also carried out and the data were used to identify
the composition of the relevant HOMOs and LUMOs and also to
assign the experimentally observed transitions.
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Acknowledgements
We are grateful to Dr. Ramaprasad Chakraborty, former
Associate Professor, Bidhannagar College and Dr. Tapas Kumar
Raychaudhuri for their constant support and interest in the work.
We thank the University of Reading and the EPSRC (U.K) for funds
for the Oxford Diffraction diffractometer.
Appendix A. Supplementary data
CCDC 979560–979563 contains the supplementary crystallo-
graphic data for 2, 2a, 9 and 10. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2014.08.010.
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