Synthesis, characterization, crystal structure and density functional theory (DFT) calculations of dioxomolybdenum (VI) complexes of an ONS donor ligand derived from benzoylacetone and S-benzyl dithiocarbazate

Article abstract of DOI:10.1016/j.poly.2012.12.006

A few dioxomolybdenum complexes of the type MoO₂L and MoO ₂L·B of a new diprotic tridentate ONS chelating ligand, H ₂L, have been synthesized with the aim to examine their potential to behave as models for the active site of an oxidoreductase molybdoenzyme like xanthineoxidase. The MoO₂L complex produces MoO₂L·B on treatment with neutral monodentate Lewis bases such as γ-picoline, 2-methylimidazole or 1-allylimidazole, utilizing the vacant sixth coordination site. They have been characterized by spectroscopic and electrochemical techniques. The complexes MoO₂L (1), MoO₂L(γ-pic) (2) and MoO₂L(1-allyl imz) (4) were structurally characterized by single crystal X-ray diffraction. The complex Mo^VIO₂L exhibits oxotransfer to PPh₃ in acetonitrile medium leading to the formation of Mo^IVOL, which is reoxidized to Mo^VIO ₂L on treatment with DMSO or pyridine N-oxide. DFT calculations on the ligand and complexes 1, 2 and 4 were also carried out.

Full text of DOI:10.1016/j.poly.2012.12.006

Polyhedron 50 (2013) 602–611
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization, crystal structure and density functional theory
(DFT) calculations of dioxomolybdenum (VI) complexes of an ONS donor
ligand derived from benzoylacetone and S-benzyl dithiocarbazate
Manashi Chakraborty a, Sathi Roychowdhury b, Nikhil Ranjan Pramanik c, Tapas Kumar Raychaudhuri c,
Tapan Kumar Mondal d, Subhankar Kundu d, Michael G.B. Drew e, Saktiprosad Ghosh c,
Sudhanshu Sekhar Mandal c,
⇑
a Department of Chemistry, NITMAS, Jhinga, P.O. Amira, Diamond Harbour Road, District 24 Pgs. (S), West Bengal 743368, India
b Sinthi Kasturba Kanya Vidyapith, 1, Kalicharan Ghosh Road, Kolkata 700050, India
c Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road, Kolkata 700009, India
d Department of Chemistry (Inorganic Section), Jadavpur University, Kolkata 700032, India
e Department of Chemistry, The University of Reading, Whiteknights, Reading RG66AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2012
Accepted 4 December 2012
Available online 20 December 2012
A few dioxomolybdenum complexes of the type MoO2L and MoO2LꢀB of a new diprotic tridentate ONS
chelating ligand, H2L, have been synthesized with the aim to examine their potential to behave as models
for the active site of an oxidoreductase molybdoenzyme like xanthineoxidase. The MoO2L complex pro-
duces MoO2LꢀB on treatment with neutral monodentate Lewis bases such as
c-picoline, 2-methylimidaz-
ole or 1-allylimidazole, utilizing the vacant sixth coordination site. They have been characterized by
Keywords:
spectroscopic and electrochemical techniques. The complexes MoO2L (1), MoO2L(c-pic) (2) and MoO2-
Dioxomolybdenum (VI) complexes
Chelating ligands
X-ray structures
L(1-allyl imz) (4) were structurally characterized by single crystal X-ray diffraction. The complex MoVIO2L
exhibits oxotransfer to PPh3 in acetonitrile medium leading to the formation of MoIVOL, which is reoxi-
dized to MoVIO2L on treatment with DMSO or pyridine N-oxide. DFT calculations on the ligand and com-
plexes 1, 2 and 4 were also carried out.
DFT calculations
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Lewis base) of a tridentate ONS chelating ligand derived from ben-
zoylacetone and S-benzyldithiocarbazate. Characterization by ele-
Apart from their versatile behavior in relation to numerous oxi-
dation states (ꢁ2 to +6), variable stereochemical and electronic
properties [1], and catalytic activity toward some industrially
important chemical processes [2,3] in the higher oxidation states,
complexes of Mo(V) and Mo(VI) ligated to some NS/ONS donor li-
gands [4–7] have still retained the attention of coordination chem-
ists and bio-inorganic chemists, especially because of their deep
involvement in some important bio-chemical processes occurring
in several forms of plants and some lower order animals [8,9].
Attempts to design and synthesize model complexes that con-
tain characteristic features which hold the possibility of forming
complexes that may mimic the active center of a molybdoenzyme
are still actively being pursued. In this backdrop, we have under-
taken the present work. It may be mentioned that for several years
we have continuously been involved in this area of molybdenum
chemistry [10–15]. In this paper, we report the synthesis of the
complexes MoO2L and MoO2LꢀB (where B is a neutral monodentate
mental analyses and various spectroscopic (IR, UV–Vis and 1H
NMR) methods are also reported. The crystal and molecular struc-
tures of three complexes, MoO2L (1), MoO2L(c-pic) (2) and MoO2-
L(1-allyl imz) (4), have been determined by single crystal X-ray
crystallography. The chemical activity and the electro-chemical
behavior of these complexes have been examined.
2. Experimental
2.1. Materials
Reagent grade solvents were dried and distilled prior to use. All
otherchemicalsusedfor thepreparativework were of reagent grade,
available commercially and used without further purification.
2.2. Synthesis
2.2.1. Synthesis of the ligand (H2L)
⇑
A mixture of 10 ml (0.2 mol) hydrazine hydrate and 11.2 g
(0.2 mol) KOH in 70 ml of 90% ethanol was cooled down to
Corresponding author. Tel.: +91 033 2350 8386; fax: +91 033 2351 9755.
E-mail address: ssmandal2000@gmail.com (S.S. Mandal).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.12.006

M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
603
ꢁ10 °C, then 15 ml (0.25 mol) of CS2 was added dropwise with con-
stant stirring. The yellow oil which formed was separated and dis-
solved in 60 ml of previously cooled 40% ethanol. To this, 23 ml
(0.2 mol) of benzyl chloride was added slowly with vigorous stir-
ring. The white product which formed was collected and after
recrystallization from benzene was dissolved in 50 ml of absolute
ethanol. To this solution, 32.4 g (0.2 mol) of benzoylacetone in
50 ml ethanol was added. The resulting mixture was stirred for
1 h at room temperature. The crystals which settled down were fil-
tered off, washed with ethanol and dried in vacuo (Scheme 1).
Yield: ꢂ80%; M.p.: ꢂ120 °C. Anal. Calcd. for C18H18N2S2O: C,
63.16; H, 5.26; N, 8.19. Found: C, 62.19; H, 5.53; N, 7.89%. IR
1598 (s), m(Mo@O) 905 (vs), 840 (vs), m(MoAN) 583 (m), m(MoAS) 471
(w); UV–Vis (CH2Cl2) [kmax/nm (
/dm3 molꢁ1 cmꢁ1)]: 258 (5550),
342 (4030), 406 (2750); 1H NMR (dmso-d6): (
) 2.49
(s, 3H), (ACH2A) 4.43 (s, 2H), (ACH) 6.42 (s, 1H), (aromatic pro-
tons) 7.27–7.37 (m, 5H), 7.42–7.50 (m, 5H).
e
Compounds 2–4, of the general formula MoO2LꢀB, were
prepared by refluxing the parent complex 1 in CH3OH with the
appropriate monodentate Lewis base (such as
c-picoline,
2-methylimidazole or 1-allylimidazole) in a 1:2 molar ratio for
ꢂ4 h (Scheme 2). Red crystalline solids so formed were filtered,
washed with dry CH3OH and dried in vacuo over anhydrous CaCl2,
in yields of ꢂ70–75%.
(KBr pellet), cmꢁ1
(s), m(C@S) 1315 (s); 1H NMR (dmso-d6): (
:
m(NAH) 3025 (w), m(OAH) 3357 (s), m(C@N) 1627
) 2.46 (s, 3H),
2.2.2.2. MoO2L(
C, 51.33; H, 4.09; N, 7.48; Mo, 17.11. Found: C, 51.05; H, 3.95;
N, 7.48; Mo, 16.97%. IR (KBr pellet) cmꢁ1
m(C@N) 1598 (s), m(Mo@O)
940 (vs), 840 (vs), m(MoAN) 581 (m), (MoAS), 471 (w); UV–Vis (CH2-
Cl2) [kmax/nm (
/dm3 molꢁ1 cmꢁ1)]: 256 (22 000), 341 (12 630),
414 (10 330); 1H NMR (dmso-d6): (
) 2.46 (s, 3H),
c-pic) (2). Anal. Calcd. for C24H23N3S2O3Mo:
(ACH2A) 4.36 (s, 2H), (ACH) 6.48 (s, 1H), (ANHA) 12.1 (s, 1H),
(AOH) 12.26 (s, 1H), (aromatic protons) 7.20–7.33 (m, 10H).
:
m
e
2.2.2. Synthesis of the complexes
2.2.2.1. MoO2L (1). To a filtered solution of 0.33 g (1.0 mmol) of
MoO2(acac)2 in 20 ml of dry CH3OH was added 0.342 g (1.0 mmol)
of the ligand H2L in 15 ml of dry CH3OH. The resulting red solution
was stirred at room temperature for 2 h. The crystalline red com-
pound that separated out was filtered, washed with cold CH3OH
and dried in vacuo over anhydrous CaCl2, in a yield of ꢂ80%. The
compound was recrystallized from dry CH3OH. Anal. Calcd. for C18-
H16N2S2O3Mo: C, 46.15; H, 3.42; N, 5.98; Mo, 20.51. Found: C,
(ACH2A) 4.39 (s, 2H), (ACH) 6.38 (s, 1H), (ACH3) 2.42 (s, 3H), (aro-
matic protons) 7.17–7.33 and 7.38–7.46 (m, 14H).
2.2.2.3. MoO2L(2-meth imz) (3). Anal. Calcd. for C22H22N4S2O3Mo: C,
48.00; H, 4.00; N, 10.18; Mo, 17.12. Found: C, 47.82; H, 3.90; N,
10.02; Mo, 16.85%. IR (KBr pellet) cmꢁ1
: m(C@N) 1521 (vs), m(Mo@O)
967 (s), 904 (s), m(MoAN) 562 (m), m(MoAS) 480 (w); UV–Vis (CH2Cl2)
46.01; H, 3.39; N, 5.25; Mo, 20.02%. IR (KBr pellet), cmꢁ1
:
m(C@N)
[kmax/nm
(e
/dm3 molꢁ1 cmꢁ1)]: 265 (3570), 311 (2090), 416
S
H3C
H3C
S
R
N
O
O
S
dry EtOH
Stir, r.t. 2h
N
H
H2N
R
+
H2C
H2C
S
NH
O
H5C6
H5C6
Scheme 1. Reaction diagram for the isolation of the ligand (R = -CH2C6H5).
O
S
O
Mo
O
S
H3C
H2C
S
N
O
O
B in MeOH
Reflux; 4h
R
MoO2(acac)2, MeOH
r.t., 2h
N
O
Mo
S
N
H
N
O
O
S
O
B
Mo
H5C6
N
O
O
γ
(B = – picoline, 2–methylimidazole or1-allylimidazole)
Scheme 2. Reaction diagram for the isolation of the Mo (VI) complexes.

604
M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
(3730); 1H NMR (dmso-d6): (
) 2.46 (s, 3H), (ACH2A)
3. Results and discussion
4.38 (s, 2H), (ACH) 6.37 (s, 1H), (ACH3) 2.43 (s, 3H), (aromatic pro-
tons) 7.20–7.35 and 7.37–7.46 (m, 12H).
3.1. Synthesis
2.2.2.4. MoO2L(1-allyl imz) (4). Anal. Calcd. for C24H24N4S2O3Mo: C,
The Schiff base ligand was prepared by condensing S-ben-
zyldithiocarbazate with benzoylacetone in ethanol. The ligand
was satisfactorily characterized by elemental analysis, IR and 1H
NMR data.
Dioxomolybdenum complex 1, of the formula MoO2L, was pre-
pared by stirring at room temperature with the appropriate ligand
in a 1:1 molar proportion using methanol as the solvent. Mo(VI)
50.00; H, 4.17; N, 9.72; Mo, 16.67. Found: C, 49.82; H, 4.00; N, 9.53;
Mo, 16.32%. IR (KBr pellet) cmꢁ1
:
m(C@N) 1572 (vs), m(Mo@O) 924
(s), 894 (vs), m(MoAN) 565 (m), m(MoAS) 475 (m); UV–Vis (CH2Cl2)
[kmax/nm
/dm3 molꢁ1 cmꢁ1)]: 263 (5730), 307 (3060), 416
(4810); 1H NMR (dmso-d6): (
) 2.46 (s, 3H), (ACH2A)
(e
4.38 (s, 2H), (ACH) 6.37 (s, 1H), (ACH2ACH@CH2) 5.97 (m, 2H),
4.61 (d, 1H), 5.17 (d, 1H cis), 5.64 (d, 1H trans), (aromatic protons)
7.20–7.32 and 7.37–7.46 (m, 13H).
complexes (2–4) of the general formula MoO2LꢀB (where B =
c-pic-
oline, 2-methylimidazole or l-allylimidazole) were also prepared
by refluxing 1 with the corresponding neutral monodentate Lewis
base in methanol medium. The base (B) is coordinated in the sixth
coordination position. All the complexes are red crystalline solids.
All the complexes have been formulated on the basis of elemental
analyses, are stable in the solid state and are diamagnetic, as ex-
pected for a species with d0 Mo(VI). All the compounds are quite
soluble in polar aprotic solvents, like CH2Cl2, CH3CN, DMF and
DMSO, but are insoluble in water. The molar conductivity data in
10ꢁ3 (M) CH2Cl2 solution indicates their non-electrolytic character.
Electronic spectra of all the complexes were recorded in CH2Cl2
and cyclic voltammograms were recorded in DMF. Complexes 1,
2 and 4 have been structurally characterized by the single crystal
X-ray diffraction technique.
2.3. Physical measurements
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 an internal stan-
dard. IR spectra were recorded as KBr pellets on a Perkin-Elmer
model 883 infrared spectrophotometer. Electronic spectra were re-
corded using a HITACHI U-3501 UV–Vis recording spectrophotom-
eter. Magnetic susceptibility was measured with a PAR model 155
vibrating sample magnetometer with Hg [Co(SCN)4] as the cali-
brant. 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 voltam-
mograms were recorded in DMF containing 0.1 M TBAP as a sup-
porting electrolyte. Thermal analyses were carried out in
a
3.2. Spectral characteristics
Mettler Toledo TGA/SDTA 851 Thermal Analyzer in a dynamic
atmosphere of dinitrogen (flow rate = 30 cm3 minꢁ1).
Characteristic IR bands of the ligand and the Mo(VI) complexes
are given in Section 2.2.2. The ligand exhibits two broad IR bands of
medium intensity in the 3357–3025 cmꢁ1 region, the higher fre-
quency one being due to the m(OAH) mode, while the lower one is
assigned to the m(NAH) vibration [28]. Strong m(C@N) bands around
1627 cmꢁ1 for the free ligand are red shifted to 1521–1598 cmꢁ1
in the corresponding complexes, indicating coordination by the
azomethine [29,30] nitrogen to the metal ion. The m(C@S) band, ob-
served in the 1315 cmꢁ1 region for the ligand, disappeared upon
complex formation. A new band in the 562–583 cmꢁ1 region was
observed in the spectra of the complexes and is assigned to m(MoAN)
[28]. The complexes exhibit a medium intensity band around
471–480 cmꢁ1, which may be assigned to m(MoAS) [31]. Like most
cis-dioxo Mo(VI) complexes, two IR bands are observed in the
967–840 cmꢁ1 region, the higher and the lower frequency bands
originating from the anti-symmetric and the symmetric stretching
frequencies of the [MoO2]2+ moiety [32–34].
2.4. Computational details
Full geometry optimization of different tautomeric forms of the
ligand H2L and complexes 1, 2 and 4 were carried out in the singlet
state using the DFT method at the B3LYP level of theory [16,17]
with a net charge = 0 for the complexes. The 6-31+G(d) basis set
was assigned for all elements except molybdenum. The SDD basis
set with an effective core potential was employed for the molybde-
num atom [18,19]. Vibrational frequency calculations were per-
formed to ensure that the optimized geometries represent the
local minima of the potential energy surface and there are only po-
sitive eigen-values used. The electronic stabilities of the optimized
geometries have been checked. The lowest 40 singlet–singlet ver-
tical electronic excitations based on B3LYP optimized geometries
were computed using the time-dependent density functional the-
ory (TD-DFT) formalism [20–22] in CH2Cl2 with a conductor-like
polarizable continuum model (CPCM) [23–25], using the same
B3LYP level and basis sets. All computations were performed using
the GAUSSIAN03 (G03) program [26]. GaussSum [27] was used to cal-
culate the fractional contributions of various groups to each molec-
ular orbital.
Electronic spectra of the Mo(VI) complexes 1–4 were recorded
in dry CH2Cl2 and the spectral data are presented in Section 2.2.2.
The lowest energy absorption maxima, located in the 307–342 nm
range, may be assigned [35] to an S(p
caused by the promotion of an electron from the full HOMO of
the ligand, of primarily sulfur p character, to the empty LUMO
of molybdenum d character. Other LMCT bands are observed in
p)-Mo(dp) LMCT transition
p
p
2.5. Crystallographic measurements
the region 307–265 nm [36–38]. These bands may be assigned to
nitrogen to molybdenum and oxygen to molybdenum charge
transfer transitions [35] respectively. The bands appearing below
265 nm are due to intra-ligand transitions.
Crystallographic data for complexes 1 and 2 were collected on a
MAR research Image Plate diffractometer and for complex 4 on a
Bruker-Nonius APEX-II diffractometer with a CCD-area detector
1H NMR data for the ligand and the corresponding complexes
are summarized in Section 2.2.2. The signals at d 12.26 and
12.10 ppm in the 1H NMR spectrum of the ligand H2L, correspond-
ing to OAH and NAH, are found to disappear in complexes 1–4 as a
result of complexation. The methylene protons of the ligand and
equipped with graphite-monochromated Mo K
a (k = 0.71013 Å)
radiation. Unit-cell dimensions and intensity data were measured
at 150 and 296 K. The crystal structures were solved by direct
methods using SHELXS-97 and refined by full-matrix least-squares
based on F2 with anisotropic displacement parameters for non-
hydrogen atoms using SHELXL-97. The hydrogen atoms were in-
cluded in calculated positions.
complexes 1–4, appearing at
d 4.36, 4.43, 4.39, 4.38 and
4.38 ppm respectively, indicate that the S-benzyl sulfur is not
involved in coordination. The 10 aromatic protons appear as

M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
605
multiplets within the range d 7.20–7.33 ppm for the ligand. In
complex 1, 10, in 2, 14, in 3, 12 and in 4, 13 aromatic protons
appear as multiplets within the ranges d 7.27–7.37 (5H) and
7.42–7.50 (5H), 7.17–7.33 and 7.38–7.46 (14H), 7.20–7.35 and
7.37–7.46 (12H), and 7.20–7.32 and 7.37–7.46 (13H) ppm respec-
recording the electronic spectra of the Mo(VI) complexes and their
adducts MoIVO2LꢀB in DMF, which exhibit identical spectral fea-
tures. It is to be noted that the reduction of di-oxo Mo(VI) com-
plexes in aprotic solvents are generally irreversible [11,39].
tively. In complexes 2, 3 and 4, the ACH3 group in
c-picoline/the
3.4. Description of the crystal structures of the complexes MoO2L (1),
substituted imidazole moiety showed the usual chemical shift
value in the 1H NMR spectra. Similarly, the ACH2ACH@CH2 group
in complex 4 also exhibited the usual proton signals.
MoO2L(c-pic) (2) and MoO2L(1-allyl imz) (4)
The molecular structures and atom numbering schemes of 1, 2
and 4 are shown in Figs. 2–4. Crystallographic data are given in
Table 2 and selected bond lengths and bond angles are presented
in Tables 3 and 4. In 1, the three donor atoms O(11), N(15) and
S(18) of the ligand and one terminal oxo-atom, O(42), occupy the
meridonial plane [36,40] and N(15) is situated trans to the oxo-
oxygen O(42), the angle O(42)-Mo(1)-N(15) being 155.5(3)°. Along
with the two oxo-oxygens, O(42) and O(41), the three ONS donor
points of the ligand complete the five coordinate environment
around the Mo(VI) acceptor center in complex 1. The sixth coordi-
nation site, trans to the oxo-oxygen O(41), is occupied by an oxo-
oxygen O(41) of the next neighboring complex molecule, and this
pattern is repeated leading to a chain of MoO2L molecules (stacking
diagram: Fig. 2a).
3.3. Electrochemical properties
Cyclic voltammograms of the ligand and complexes 1–4 at a Pt-
electrode were recorded in dry degassed DMF containing 0.1 (M)
TBAP as the supporting electrolyte over the potential range 0.08
to ꢁ1.5 V. The numerical data are listed in Table 1 and representa-
tive voltammograms are shown in Fig. 1. For MoVIO2L, only one
irreversible reductive response could be located in the potential
range 0.0 to ꢁ1.23 V. This response is assigned to a metal center
2eꢁ process [10,11,32,38]. A two-electron involvement is estab-
lished by a comparison of the current height with authentic two-
electron species under identical experimental conditions. On scan
reversal, two irreversible one-electron oxidative responses are lo-
cated in the 0.57–0.82 V range. Cyclic voltammograms of the li-
gand and its Zn-complex did not exhibit any reductive response
within the potential window scan. Cyclic voltammograms of MoVI-
The dianionic tridentate ligand forms one five-membered and
one six-membered metallocycle involving the [MoO2]2+ core, the
N(15)–Mo(1)–S(18) and N(15)–Mo(1)–0(11) bite angles being
76.77(19) and 82.8(3)° respectively.
The overall coordination geometry around the Mo(VI) center
can be described as distorted octahedral, as is evident from the un-
equal bond lengths (Table 3). The Schiff base ligand is bonded to
the [MoO2]2+ core through the enolate oxygen O(11), thioenolate
sulfur S(18) and the azomethine nitrogen N(15). The oxo-oxygen
O(41) occupies one apical position, while the other apex is occu-
pied by an oxo-oxygen O(41) of the nearest neighbor complex mol-
ecule, the O(41)–Mo(1)–O(41) angle being 173.34(11)°. This may
be visualized as the effect of stacking of the complex molecules
along the z-axis. The length of the Mo(1)–O(41) bond (2.415 Å) is
considerably longer than the other Mo–O bond [Mo(1)–
O(11) = 1.950(6) Å] and is indicative of weaker coordination by
the oxo-oxygen O(41), and hence is expected to be the point of
maximum reactivity in the complex. This can be utilized as a sub-
strate binding site leading to a more stable hexacoordinate species.
This expectation is realized in the facile formation of the complex
MoO2LꢀB (where B is a neutral monodentate Lewis base like
O2LꢀB (where B =
c-picoline, 2-methyl imz, l-allyl imz) exhibit little
difference from those of the parent complex 1 and therefore the ac-
tual species in the solution are MoO2L(DMF). This is confirmed by
Table 1
Cyclic voltammetric peak potential dataa (V vs. SCE) in DMF solution at 298 K.
Complexes
Epc (V)
Epa (V)
MoO2L (1)
MoO2L(c-pic) (2)
ꢁ1.23
ꢁ1.07
ꢁ1.24
ꢁ1.12
+0.57, +0.82
+0.53, +0.76
+0.52, +0.87
+0.55, +0.81
MoO2L (2-meth imz) (3)
MoO2L1 (1-allyl imz) (4)
a
Solvent: DMF (dry, degassed); supporting electrolyte: 0.1 M TBAP; solution
strength: 10ꢁ3 M; electrode: platinum; reference electrode: SCE; scan rate:
100 mV sꢁ1
.
c-picoline, 2-methylimidazole or 1-allylimidazole).
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
The O(41)–Mo(1)–O(42) bond angle of 104.4(3)° and the
Mo@O bond lengths Mo(1)–O(42) = 1.704(6) Å and Mo(1)–O(41)
= 1.666(6) Å are unexceptional [41–44]. The azomethine nitrogen
N(15) is trans to the oxo-oxygen O(42) and the Mo(1)–N(15) bond
length of 2.238(7) Å is rather long due to the trans influence of the
oxo-oxygen. The C(17)–N(16) bond length, 1.279(11) Å, is close to
the usual C@N bond length [32,37].
The Mo(1)–S(18) distance of 2.410(2) Å is very near to the val-
ues observed in several molybdenum complexes [36,45,46]. Care-
ful analysis of the bond lengths and bond angles data leads to
the overall conclusion that complex 1 has a highly distorted octa-
hedral coordination around the Mo(VI) center, and the Mo(VI) cen-
ter is slightly displaced by 0.369 Å from the median square plane
described by N(15), O(11), S(18) and O(42) toward the apical
oxo-oxygen O(41).
In complexes 2 and 4, the Mo (VI) center is also present in a dis-
torted octahedral donor environment consisting of two cis-oxo
atoms, O(41) and O(42), phenolate oxygen O(11), thio-enolate sul-
fur S(18), azomethine nitrogen N(15) and the tertiary nitrogen
atom N(51). In fact, the structures of 2 and 4 can be derived from
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
E(V)
the structure of 1, MoO2L, by the addition of
c-picoline and 1-allyl
Fig. 1. Cyclic voltammogram of MoO2L (1) in DMF at 298 K.
imidazole to the vacant axial position trans to the oxo atom O(41),

606
M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
Fig. 2. Molecular structure of the complex MoO2L (1).
Fig. 4. Molecular structure of MoO2L(1-allyl imz) (4).
Table 2
Crystal data and refinement details for complexes 1, 2 and 4.
Fig. 2a. (a) Stacking diagram of complex MoO2L (1).
Complex 1
18H16MoN2O3S2
Complex 2
Complex 4
Chemical
formula
Formula
C
C24H23MoN3O3S2
C24H24MoN4O3S2
468.39
561.51
576.53
weight (M)
Crystal system monoclinic
monoclinic
P21/c
monoclinic
P21/c
Space group
a (Å)
P21/a
7.9463(3)
13.3296(5)
17.4073(8)
91.949(4)
1842.73(13)
4
11.1998(6)
13.6358(12)
15.8342(12)
97.029(7)
2400.0(3)
4
15.3789(15)
10.1043(10)
17.9013(17)
115.270(3)
2515.5(4)
4
b (Å)
c (Å)
b (o)
V (Å3)
Z
Temperature
(K)
150(2)
150(2)
296(2)
Dcalc (g cmꢁ3
)
1.688
0.959
1.554
0.752
1.522
0.720
l
(Mo K
(mmꢁ1
F (0 0 0)
a)
)
944
0.987
1144
0.890
1176
1.073
Goodness-of-
Fig. 3. Molecular structure of complex MoO2L1
(c-pic) (2).
fit on F2
R1, wR2
R1 = 0.0782,
R1 = 0.0402,
R1 = 0.0901,
[I > 2
r
(1)]
wR2 = 0.2108
wR2 = 0.1009
wR2 = 0.2313
generating an O(41)-Mo(1)-N(51) angle of 170.8(8) and 169.4(3)°,
respectively. This points to a considerable distortion of the coordi-
nation octahedron around Mo(VI) centers in complexes 2 and 4,
with the Mo center slightly displaced by 0.331 and 0.311 Å from
the median plane described by N(15), O(11), S(18) and O(42) to-
ward the apical oxo-oxygen O(41) respectively. A remarkable fea-
ture of the structures of both 2 and 4 is that the metric values of
the bond lengths and the bond angles involving the Mo(VI) center
of the precursor complex 1 remain mostly unchanged in 2 and 4.

M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
607
Table 3
Selected bond lengths (Å) for complexes 1, 2 and 4.
Complex 1
Complex 2
Complex 4
Mo(1)–O(42)
Mo(1)–O(41)
Mo(1)–O(11)
Mo(1)–O(41)
Mo(1)–N(15)
Mo(1)–S(18)
C(17)–S(18)
C(17)–N(16)
N(15)–N(16)
C(14)–N(15)
C(13)–C(14)
C(12)–C(13)
C(12)–O(11)
1.704(6)
1.666(6)
1.950(6)
2.415(6)
2.238(7)
2.410(2)
1.739(9)
1.279(11)
1.406(10)
1.322(10)
1.445(11)
1.351(12)
1.338(10)
Mo(1)–O(42)
Mo(1)–O(41)
Mo(1)–O(11)
Mo(1)–N(51)
Mo(1)–N(15)
Mo(1)–S(18)
C(17)–S(18)
C(17)–N(16)
N(15)–N(16)
C(14)–N(15)
C(13)–C(14)
C(12)–C(13)
C(12)–O(11)
1.708(18)
1.704(18)
1.954(17)
2.430(19)
2.279(2)
2.420(7)
1.751(3)
1.279(3)
1.400(3)
1.322(3)
1.424(3)
1.351(3)
1.338(3)
Mo(1)–O(42)
Mo(1)–O(41)
Mo(1)–O(11)
Mo(1)–N(51)
Mo(1)–N(15)
Mo(1)–S(18)
C(17)–S(18)
C(17)–N(16)
N(15)–N(16)
C(14)–N(15)
C(13)–C(14)
C(12)–C(13)
C(12)–O(11)
1.701(6)
1.698(6)
1.950(5)
2.366(6)
2.284(6)
2.427(2)
1.747(8)
1.274(10)
1.408(8)
1.316(9)
1.421(10)
1.359(10)
1.330(8)
Table 4
Selected bond angles (°) for complexes 1, 2 and 4.
Complex 1
Complex 2
Complex 4
O(41)–Mo(1)–O(42)
O(42)–Mo(1)–O(41)
O(42)–Mo(1)–O(11)
O(41)–Mo(1)–O(41)
O(41)–Mo(1)–O(11)
O(11)–Mo(1)–O(41)
S(18)–Mo(1)–O(41)
S(18)–Mo(1)–O(41)
S(18)–Mo(1)–N(15)
N(15)–Mo(1)–O(41)
N(15)–Mo(1)–O(41)
N(15)–Mo(1)–O(11)
O(42)–Mo(1)–N(15)
S(18)–Mo(1)–O(11)
O(42)–Mo(1)–S(18)
104.4(3)
82.2 (2)
104.0(3)
173.34(11)
99.5(3)
77.8(2)
99.9(2)
80.89(15)
76.77(19)
97.6(3)
76.1(2)
82.8(3)
155.5(3)
153.4(2)
88.6(2)
O(41)–Mo(1)–O(42)
O(42)–Mo(1)–N(51)
O(42)–Mo(1)–O(11)
O(41)–Mo(1)–N(51)
O(41)–Mo(1)–O(11)
O(11)–Mo(1)–N(51)
S(18)–Mo(1)–O(41)
S(18)–Mo(1)–N(51)
S(18)–Mo(1)–N(15)
N(15)–Mo(1)–O(41)
N(15)–Mo(1)–N(51)
N(15)–Mo(1)–O(11)
O(42)–Mo(1)–N(15)
S(18)–Mo(1)–O(11)
O(42)–Mo(1)–S(18)
106.16(9)
83.03(8)
106.28(8)
170.8(8)
98.53(9)
77.99(7)
100.14(8)
80.20(5)
76.46(5)
89.87(8)
81.25(7)
82.31(7)
160.09(8)
151.53(5)
88.89(7)
O(41)–Mo(1)–O(42)
O(42)–Mo(1)–N(51)
O(42)–Mo(1)–O(11)
O(41)–Mo(1)–N(51)
O(41)–Mo(1)–O(11)
O(11)–Mo(1)–N(51)
S(18)–Mo(1)–O(41)
S(18)–Mo(1)–N(51)
S(18)–Mo(1)–N(15)
N(15)–Mo(1)–O(41)
N(15)–Mo(1)–N(51)
N(15)–Mo(1)–O(11)
O(42)–Mo(1)–N(15)
S(18)–Mo(1)–O(11)
O(42)–Mo(1)–S(18)
105.9(3)
84.7(3)
102.9(2)
169.4(3)
97.8(3)
78.5(2)
96.0(2)
84.37(16)
76.83(15)
93.7(3)
76.1(2)
82.4(2)
158.6(3)
155.69(15)
92.4(2)
Even the length of metal-oxygen double bond (Mo@O) axial to the
coordinated -picoline/1-allyl imidazole (which is known to be
highly influenced by axial coordination in many oxo metal com-
plexes and is thought to be a measure of the -donor capacity of
c
r
the coordinated ligand) stand practically unchanged [32,41,47].
The lengths of the Mo(1)–N(51) bonds in 2 and 4 are longer
[2.430(19) and 2.366(6) Å] than the other Mo–N(azomethine)
bonds, Mo(1)–N(15) [2.279(2) and 2.284(6) Å], revealing rather
weak attachment of
c-picoline and 1-allylimidazole to the
[MoO2]2+ moiety. This is substantiated from a TGA and DTA study
of 2 and 4 which exhibited easy loss of c-picoline/1-allylimidazole
on controlled heating.
3.5. DFT and TD-DFT calculations of the ligand and the complexes
MoO2L (1), MoO2L( -pic) (2) and MoO2L(1-allyl imz) (4)
c
DFT and TD-DFT calculations of the ligand and the structurally
characterized complexes 1, 2 and 4 were performed. In order to
gain an insight into the tautomeric forms of the ligand H2L in solu-
tion in the absence of a metal ion, geometry optimizations of all the
four possible structures have been performed in CH2Cl2. However,
the major objective was to examine whether the calculated data
and experimental parameters for complexes 1, 2 and 4 are in
agreement with each other. The total energy of the optimized
geometries shows that the diketo form I is more stable and compa-
rable in energy with IV, with an energy gap of 1.91 kcal/mol. So,
structures I and IV may be in equilibrium in solution. However,
the crystal structures of complexes 1, 2 and 4 clearly show the
involvement of the ligand in the di-enol form (II), which appears
to be generated in situ in the presence of the [MoO2]2+ acceptor
center, leading to the formation of the isolated complexes.
Structure
Energy (a.u)
I
-1677.562092
II
-1677.184026
-1677.555227
-1677.559054
III
IV

608
M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
Table 5
Table 7
Theoretical bond length parameters (Å) for complexes 1, 2 and 4 calculated by the
Energy and composition of some selected MOs of complex 1.
DFT/B3LYP method.
MO
Energy (eV)
% of composition
Complex 1
Complex 2
Complex 4
Mo
Oxo
Ligand
Mo(1)–O(42)
Mo(1)–O(41)
Mo(1)–O(11)
Mo(1)–N(15)
Mo(1)–S(18)
C(17)–S(18)
C(17)–N(16)
N(15)–N(16)
C(14)–N(15)
C(13)–C(14)
C(12)–C(13)
C(12)–O(11)
1.716
1.705
1.982
2.291
2.418
1.765
1.293
1.381
1.331
1.423
1.379
1.321
Mo(1)–O(42)
Mo(1)–O(41)
Mo(1)–O(11)
Mo(1)–N(15)
Mo(1)–S(18)
C(17)–S(18)
C(17)–N(16)
N(15)–N(16)
C(14)–N(15)
C(13)–C(14)
C(12)–C(13)
C(12)–O(11)
Mo(1)–N(51)
1.721
1.708
2.001
2.312
2.456
1.760
1.292
1.383
1.326
1.428
1.379
1.319
2.507
Mo(1)–O(42)
Mo(1)–O(41)
Mo(1)–O(11)
Mo(1)–N(15)
Mo(1)–S(18)
C(17)–S(18)
C(17)–N(16)
N(15)–N(16)
C(14)–N(15)
C(13)–C(14)
C(12)–C(13)
C(12)–O(11)
Mo(1)–N(51)
1.720
1.709
2.006
2.315
2.467
1.757
1.291
1.388
1.325
1.427
1.379
1.315
2.491
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ0.87
ꢁ0.93
ꢁ1.77
ꢁ1.94
ꢁ2.54
ꢁ2.94
ꢁ6.24
ꢁ6.90
ꢁ7.04
ꢁ7.27
ꢁ7.38
ꢁ7.56
ꢁ7.64
ꢁ8.03
ꢁ8.31
ꢁ8.46
ꢁ9.00
05
03
54
46
60
37
01
01
02
0
02
01
24
15
24
15
02
02
04
01
01
0
11
12
23
83
30
93
96
22
39
16
48
HOMO
97 (S, 23)
97
94 (S, 72)
99
99 (S, 21)
100
86
87
68 (S, 42)
12
HOMO-1
HOMO-2
HOMO-3
HOMO-4
HOMO-5
HOMO-6
HOMO-7
HOMO-8
HOMO-9
HOMO-10
0
0
03
01
09
05
07
The full geometry optimization of complex 1, which may be
visualized as a penta coordinate monomeric unit leading to the ob-
served polymeric structure, was carried out by the DFT/B3LYP
method. Complexes 2 and 4 were also subjected to similar treat-
ment. The calculated bond length and bond angle parameters are
given in Tables 5 and 6.
63
Table 8
Energy and composition of some selected MOs of complex 2.
The computed structural parameters are in good agreement
with the experimental data for complexes 2 and 4, with maximum
deviations of ꢂ0.08 Å for the bond lengths and ꢂ3.0° for the bond
angles. In the case of complex 1, the maximum deviation in the
Mo(1)–N(15) bond distance is ꢂ0.12 Å. On optimization, the penta
coordinate monomeric structural unit has been considered. A large
deviation in bond angles has been observed compared to the poly-
meric octahedral X-ray structure. The maximum deviations in
bond angles have been found for O(42)–Mo(1)–O(11), 5.1° (calcu-
lated, 109.1°; X-ray, 104.0°); S(18)–Mo(1)–O(41), 8.9° (calculated,
108.8°; X-ray, 99.9°) and S(18)–Mo(1)–O(11), 16.2° (calculated,
137.2°; X-ray, 153.4°).
MO
Energy (eV)
% of composition
Mo
Oxo
Ligand
c-pic
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ0.66
ꢁ1.11
ꢁ1.40
ꢁ1.78
ꢁ2.15
ꢁ2.43
ꢁ5.63
ꢁ6.37
ꢁ6.71
ꢁ6.88
ꢁ7.02
ꢁ7.08
ꢁ7.11
ꢁ7.40
ꢁ7.53
ꢁ7.70
ꢁ7.86
01
08
57
36
60
28
01
01
0
0
02
22
12
22
16
01
03
02
0
04
01
16
48
18
54
95
89
05
04
0
02
01
02
01
0
HOMO
97 (S, 24)
94 (S, 67)
97 (S, 25)
HOMO-1
HOMO-2
HOMO-3
HOMO-4
HOMO-5
HOMO-6
HOMO-7
HOMO-8
HOMO-9
HOMO-10
0
100
0
0
100 (S, 21)
0
01
01
02
0
03
08
08
05
05
01
65
17
90
92
83
04
14
54
01
02
10
95
18
21
The compositions along with the energies of some selected
molecular orbitals of the complexes are listed in Tables 7–9. Con-
tour plots of some selected frontier molecular orbitals of the com-
plexes are shown in Figs. 5–7. The highest energy occupied
molecular orbital (HOMO) of the complexes has p-bonding nature
and is concentrated on the chelated ligand (L). HOMO-2 for 1 and
HOMO-1 for complexes 2 and 4 are composed of non-bonding
complexes were been performed in CH2Cl2. The calculated excita-
tion wavelengths and their assignments are given in Tables
10–12 for the complexes. The low energy bands >400 nm are char-
orbitals of S atoms. Other low-lying occupied orbitals have
character of L with minor contributions from S atoms. The low
energy unoccupied orbitals LUMO to LUMO+3 have Mo(d ) charac-
ter along with minor contributions of L(p⁄) and O(p
) orbitals.
To get better insight into the electronic spectra of the com-
plexes, TD-DFT calculations on the optimized geometries of the
p-bonding
p
acterized as mixed L(
Strong transitions around 375 nm have been observed for the com-
plexes due to S(p ) ? Mo(d ) charge transfer transitions. The high
p) ? Mo(dp) and L(p
) ? L(p⁄) transitions.
p
p
p
Table 6
Theoretical bond angle parameters (°) for complexes 1, 2 and 4 calculated by the DFT/B3LYP method.
Complex 1
Complex 2
Complex 4
O(41)–Mo(1)–O(42)
O(42)–Mo(1)–O(11)
O(41)–Mo(1)–O(11)
S(18)–Mo(1)–O(41)
S(18)–Mo(1)–N(15)
N(15)–Mo(1)–O(41)
N(15)–Mo(1)–O(11)
O(42)–Mo(1)–N(15)
S(18)–Mo(1)–O(11)
O(42)–Mo(1)–S(18)
O(42)–Mo(1)–N(51)
108.7
109.1
97.72
108.8
75.71
98.42
79.63
151.8
137.2
88.44
80.85
O(41)–Mo(1)–O(42)
O(42)–Mo(1)–O(11)
O(41)–Mo(1)–O(11)
S(18)–Mo(1)–O(41)
S(18)–Mo(1)–N(15)
N(15)–Mo(1)–O(41)
N(15)–Mo(1)–O(11)
O(42)–Mo(1)–N(15)
S(18)–Mo(1)–O(11)
O(42)–Mo(1)–S(18)
O(42)–Mo(1)–N(51)
O(41)–Mo(1)–N(51)
O(11)–Mo(1)–N(51)
S(18)–Mo(1)–N(51)
N(15)–Mo(1)–N(51)
106.4
105.4
98.38
102.5
75.94
92.52
81.00
158.6
149.4
89.97
82.09
171.7
75.49
81.19
81.11
O(41)–Mo(1)–O(42)
O(42)–Mo(1)–O(11)
O(41)–Mo(1)–O(11)
S(18)–Mo(1)–O(41)
S(18)–Mo(1)–N(15)
N(15)–Mo(1)–O(41)
N(15)–Mo(1)–O(11)
O(42)–Mo(1)–N(15)
S(18)–Mo(1)–O(11)
O(42)–Mo(1)–S(18)
106.3
104.1
97.41
100.8
76.03
92.76
81.44
159.0
151.5
91.57
O(41)–Mo(1)–N(51)
O(11)–Mo(1)–N(51)
S(18)–Mo(1)–N(51)
N(15)–Mo(1)–N(51)
171.2
77.81
81.10
79.31

M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
609
Table 9
Energy and composition of some selected MOs of complex 4.
MO
Energy (eV)
% of composition
Mo
Oxo
Ligand
100
1-Allyl-imz
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ0.40
ꢁ0.62
ꢁ1.21
ꢁ1.58
ꢁ2.06
ꢁ2.34
ꢁ5.50
ꢁ6.33
ꢁ6.67
ꢁ6.81
ꢁ6.92
ꢁ7.01
ꢁ7.06
ꢁ7.18
ꢁ7.31
ꢁ7.61
ꢁ7.65
0
03
60
38
59
30
01
0
0
01
23
12
23
16
01
04
0
04
06
0
04
02
03
67
20
0
01
01
01
0
01
01
07
94
01
01
0
01
01
04
22
55
95
16
49
18
53
HOMO
97 (S, 23)
89 (S, 78)
06
94 (S, 18)
HOMO-1
HOMO-2
HOMO-3
HOMO-4
HOMO-5
HOMO-6
HOMO-7
HOMO-8
HOMO-9
HOMO-10
0
01
01
0
02
0
01
03
04
92
100
93
97
92
08
21
Fig. 5. Contour plots of some selected molecular orbitals of complex 1.
Fig. 6. Contour plots of some selected molecular orbitals of complex 2.
energy bands around 260 nm are characterized as O(p
transitions.
p
) ? Mo(d
p)
cules. When 1 is treated with neutral monodentate Lewis bases
like -picoline, imidazole and a substituted imidazole (B), com-
c
plexes of the type MoO2LꢀB could be isolated.
3.6. Study of the reactivity of the dioxo Mo(VI) complexes
3.6.1. Substrate binding
The dioxo molybdenum (VI) complex 1 has been prepared and
isolated from methanol, and it is found to be oligomeric in the solid
state due to weak coordination between adjacent complex mole-
3.6.2. Oxo-transfer to substrate
Complex 1, as well as 2, 3 and 4, exhibit oxotransfer to PPh3
when reacted with PPh3 in CH3CN/CH3OH medium. The band
around 342 nm due to an S(p
parent complex is shifted to lower energy, around 420 nm, and a
p)–Mo(dp) LMCT transition in the

610
M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
Fig. 7. Contour plots of some selected molecular orbitals of complex 4.
Table 10
Vertical electronic excitations of complex 1 calculated by the TD-DFT/CPCM method in CH2Cl2.
Eexcitation (eV)
kexcitation (nm)
Osc. strength (f)
Key transition
Character
2.5908
2.9859
478.6
415.2
0.0104
0.0757
(90%)HOMO ? LUMO+1
(50%)HOMO ? LUMO+2
(22%)HOMO-1 ? LUMO
(85%)HOMO-2 ? LUMO+1
(61%)HOMO-3 ? LUMO
(20%)HOMO-1 ? LUMO+1
(56%)HOMO-3 ? LUMO+3
(29%)HOMO-9 ? LUMO+1
(47%)HOMO-7 ? LUMO+2
(32%)HOMO-8 ? LUMO+1
L(
L(
p
p
) ? Mo(d
) ? Mo(d
p
p
)
)/L(p⁄
)
3.2917
3.5217
376.7
352.1
0.3126
0.0734
L(S(pp)) ? Mo(dp)
L(
p
) ? Mo(d
p
)
4.5125
4.8257
274.8
256.9
0.1247
0.2592
L(
O(p
O(p
p
) ? Mo(d
p
)
p
p
p
p
) ? Mo(d
) ? Mo(d
)
)
Table 11
Vertical electronic excitations of complex 2 calculated by the TD-DFT/CPCM method in CH2Cl2.
Eexcitation (eV)
kexcitation (nm)
Osc. strength (f)
Key transition
Character
2.2774
3.0528
544.4
406.1
0.0580
0.0528
(91%)HOMO ? LUMO
L(
L(
L(
L(S(p
L(S(p
p
p
p
) ? Mo(d
) ? Mo(d
) ? Mo(d
p
p
p
)/L(p⁄
)/L(p⁄
)
)
)
(61%)HOMO ? LUMO+2
(28%)HOMO ? LUMO+3
(45%)HOMO-1 ? LUMO+3
(26%)HOMO-1 ? LUMO+2
(46%)HOMO-3 ? LUMO
(22%)HOMO-6 ? LUMO
(81%)HOMO-9 ? LUMO+1
3.3058
3.4813
4.7382
375.1
356.1
261.7
0.3788
0.0890
0.1637
p
p
)) ? Mo(d
)) ? Mo(d
p
p
)
)/L(p⁄
)
L(p
) ? Mo(d
p
)/L(p⁄
)
O(p
p) ? Mo(dp)
Table 12
Vertical electronic excitations of complex 4 calculated by the TD-DFT/CPCM method in CH2Cl2.
Eexcitation (eV)
kexcitation (nm)
Osc. strength (f)
Key transition
Character
2.1758
3.0593
569.8
405.3
0.0291
0.0213
(90%)HOMO ? LUMO
L(
L(
L(
L(S(p
L(S(p
p
p
p
) ? Mo(d
) ? Mo(d
) ? Mo(d
p
p
p
)/L(p⁄
)/L(p⁄
)
)
)
(44%)HOMO ? LUMO+2
(29%)HOMO ? LUMO+3
(48%)HOMO+1 ? LUMO+3
(35%)HOMO+1 ? LUMO+2
(62%)HOMO-9 ? LUMO
(79%)HOMO-9 ? LUMO+1
3.3010
375.6
0.2598
p)) ? Mo(d
p
p
)
p)) ? Mo(d
)/L(p⁄
)
3.9415
4.8204
314.6
257.2
0.1012
0.1247
O(p
O(p
p
p
) ? Mo(d
) ? Mo(d
p
p
)
)
new band appears around ꢂ626 nm. The isolated complex, MoIVOL,
exhibits the same spectral features. The reaction, indicating oxo-
transfer to the substrate, may be represented as:
which may be considered as a direct oxygen atom transfer (OAT)
process. The reverse process exhibiting oxo-transfer from DMSO/
pyridine N-oxide can be represented as:
MoVIOL þ DMSO=C5H5NO CH!3CN MoVIO2L þ Me2S=C5H5N
CH3CN
MoVIO2L þ PPh3 ! MoVIOL þ OPPh3

M. Chakraborty et al. / Polyhedron 50 (2013) 602–611
611
[6] R.J. Greenwood, G.L. Wilson, J.R. Pilbrow, A.G. Weed, J. Am. Chem. Soc. 115
(1993) 5385. and reference therein.
4. Conclusion
[7] (a) E.I. Stiefel, D. Coucouvans, W.E. Newton (Eds.), Molybdenum Enzymes
Cofactor and Model Systems, ACS Symposium Series, 535 American Chemical
Society, Washington, DC, 1993.;
(b) G.N. George, R.C. Kipke, R.A. Sunde, J.H. Enemark, Biochem. J. 256 (1988)
307.
[8] D. Collison, C.D. Garner, J.A. Joule, Chem. Soc. Rev. 25 (1996) 25.
[9] R. Hille, Chem. Rev. 96 (1996) 2757.
[10] N.R. Pramanik, S. Ghosh, T.K. Raychaudhuri, S. Ray, R.J. Butcher, S.S. Mandal,
Polyhedron 23 (2004) 1595.
[11] N.R. Pramanik, S. Ghosh, T.K. Raychaudhuri, S. Chaudhuri, M.G.B. Drew, S.S.
Mandal, J. Coord. Chem. 60 (2007) 2177.
[12] N.R. Pramanik, S. Ghosh, T.K. Raychaudhuri, S.S. Mandal, J. Coord. Chem. 62
(2009) 3845.
[13] N.R. Pramanik, S. Ghosh, T.K. Raychaudhuri, S.S. Mandal, J. Indian Chem. Soc.
86 (2009) 564.
The desired dioxomolybdenum (VI) complexes of the type
MoO2L and MoO2LꢀB have been successfully synthesized and char-
acterized by various physico-chemical techniques, and complexes
1, 2 and 4 were structurally characterized by X-ray crystallographic
analysis. The substrate binding characteristic of model complexes
is demonstrated by the formation of MoO2LꢀB adducts involving a
neutral monodentate Lewis base such as
tuted imidazole.
c-picoline and substi-
The resemblance of these complexes with the active center of
some known oxo-transfer molybdoenzymes containing a ONS do-
nor environment has been demonstrated through oxo-abstraction
of the dioxo Mo(VI) complexes by PPh3 to the corresponding
mono-oxo Mo(IV) species, and re-oxidation of the resultant
mono-oxo Mo(IV) complexes to the parent Mo(VI) species by the
use of DMSO/pyridine N-oxide has been achieved. The one step
two electron reduction of MoO2L is supported by electrochemical
studies. Relevant DFT calculations on the ligand and complexes 1,
2 and 4 were also carried out and the data was used to identify
the composition of the relevant HOMOs and LUMOs and also to as-
sign the experimentally observed transitions.
[14] N.R. Pramanik, S. Ghosh, T.K. Raychaudhuri, R.J. Butcher, S.S. Mandal, J. Coord.
Chem. 64 (2011) 1207.
[15] N.R. Pramanik, S. Ghosh, T.K. Raychaudhuri, M.G.B. Drew, T.K. Mondal, S.S.
Mandal, Inorg. Chim. Acta 383 (2012) 60.
[16] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[17] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[18] D. Andrae, U. Haeussermann, M. Dolg, H. Stoll, H. Preub, Theor. Chim. Acta 77
(1990) 123.
[19] P. Fuentealba, H. Preuss, H. Stoll, L.V. Szentpaly, Chem. Phys. Lett. 89 (1982)
418.
[20] R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 256 (1996) 454.
[21] R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218.
[22] M.E. Casida, C. Jamorski, K.C. Casida, D.R. Salahub, J. Chem. Phys. 108 (1998)
4439.
[23] V. Barone, M. Cossi, J. Phys. Chem. A 102 (1998) 1995.
[24] M. Cossi, V. Barone, J. Chem. Phys. 115 (2001) 4708.
[25] M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 24 (2003) 669.
[26] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox,
H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, GAUSSIAN 03, Revision D.01, Gaussian, Inc., Wallingford, CT, 2004.
[27] N.M. O’Boyle, A.L. Tenderholt, K.M. Langner, J. Comput. Chem. 29 (2008) 839.
[28] Y.-L. Zhai, X.-X. Xu, X. Wang, Polyhedron 11 (1992) 415.
[29] F.A. Cotton, R.M. Wing, Inorg. Chem. 4 (1965) 867.
[30] M. Goodgame, P.J. Hayward, J. Chem. Soc. A (1966) 632.
[31] M. Chaudhury, J. Chem. Soc., Dalton Trans. (1984) 115.
[32] C. Bustos, O. Burckhardt, R. Schrebler, D. Carrillo, A.M. Arif, A.H. Cowley, C.M.
Nunn, Inorg. Chem. 29 (1990) 3996.
Acknowledgments
We are grateful to Dr. Sasankasekhar Mohanta, Department of
Chemistry, Calcutta University and Dr. Ramaprasad Chakraborty,
former Associate Professor, Bidhannagar College, for their constant
support and interest in the work. Thanks are also due to Prof. T.K.
Basu, Director, NITMAS, Jhinga, District: 24 Pgs. (S), West Bengal
743368 for regular encouragement and help.
Appendix A. Supplementary data
CCDC 874432–874434 contain the supplementary crystallo-
graphic data for 1, 2 and 4, respectively. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.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.
[33] R.H. Holm, P. Kennepohl, E.I. Solomon, Chem. Rev. (1996) 2239.
[34] S. Bhattacharyya, S.B. Kumar, S.K. Dutta, E.R.T. Tiekink, M. Chaudhury, Inorg.
Chem. 35 (1996) 1967.
References
[35] M. Chaudhury, Inorg. Chem. 24 (1985) 3011.
[1] (a) C.L. Rollinson, Chromium, molybdenum and tungsten, Chap. 36, in:
Comprehensive Inorganic Chemistry, vol. 3, Pergamon Press, Oxford, 1973, p.
623.;
[36] R. Hahn, U. Kusthardt, W. Scherer, Inorg. Chim. Acta 210 (1993) 177.
[37] R. Mattes, V. Mikloweit, Inorg. Chim. Acta 122 (1986) L19.
[38] A.P. Koley, S. Purohit, S. Ghosh, L.S. Prasad, P.T. Manoharan, J. Chem. Soc.,
Dalton Trans. (1988) 2607.
[39] (a) S. Purohit, A.P. Koley, L.S. Prasad, P.T. Manoharan, S. Ghosh, Inorg. Chem. 28
(1989) 3735;
(b) P. Subramanian, J.T. Spence, R. Ortega, J.H. Enemark, Inorg. Chem. 23
(1984) 2564.
[40] J.A. Craig, E.W. Harlan, B.S. Snyder, M.A. Whitener, R.H. Holm, Inorg. Chem. 28
(1989) 2082.
[41] J.M. Berg, R.H. Holm, J. Am. Chem. Soc. 106 (1984) 3035.
[42] I. Buchanan, M. Minelli, M.T. Ashby, J.T. King, J.H. Enemark, C.D. Garner, Inorg.
Chem. 23 (1984) 495.
[43] S.K. Dutta, D.B. McConville, W.J. Youngs, M. Chaudhury, Inorg. Chem. 36 (1997)
2517.
[44] J. Limatainen, A. Lehtonen, R. Sillanpaa, Polyhedron 19 (2000) 1133.
[45] B.B. Kaul, J.H. Enemark, S.L. Merbs, J.T. Spence, J. Am. Chem. Soc. 170 (1985)
2885.
[46] E.I. Steifel, in: G. Wilkinson (Ed.), Comprehensive Coordination Chemistry, vol.
3, Pergamon, Oxford, 1987, pp. 1375–1420.
(b) F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed., Wiley-
Interscience, NY, 1988. p. 804;
(c) N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press,
Oxford, 1984. p. 1167.
[2] R.K. Grasselli, Catal. Today 49 (1999) 141.
[3] K.J. Ivin, J.C. Mol, Olefin Metathesis Polymerization, Academic Press, London,
1997.
[4] (a) A. Muller, B. Kerbs, Sulfur, its significance for chemistry, for the geo and
cosmosphere and technology, in: Studies in Inorganic Chemistry, vol. 5,
Elsevier Science Publishers, Amsterdam, 1984; (b) S.D. Conradsson, B.K.
Burgess, W.W. Newton, K.O. Hodgson, J.W. McDonald, J.F. Rubinson, S.F.
Gheller, L.E. Mortenson, M.W.W. Adams, P.K. Mascharak, W.A. Armstrong, R.H.
Holm, J. Am. Chem. Soc. 107 (1985) 7935–7940; (c) P.M. Harisson,
Metalloproteins, Part 1, Metalloproteins with Redox Roles, Verlag, Chemie,
Weinheim, FRG, 1985.
[5] (a) R.H. Holm, J.M. Berg, J. Am. Chem. Soc. 107 (1985) 925;
(b) J.H. Enemark, C.G. Young, Adv. Inorg. Chem. 40 (1993) 1;
(c) L.J. Laughlin, C.G. Young, Inorg. Chem. 35 (1996) 1050;
(d) J. Christiansen, R.C. Tittsworth, B.J. Hales, S.P. Cramar, J. Am. Chem. Soc. 117
(1995) 10017.
[47] A. Rana, R. Dinda, P. Sengupta, S. Ghosh, L.R. Falvello, Polyhedron 21 (2002)
1023.