Mixed-ligand tris chelated complexes of Mo(IV) and W(IV): A comparative study

Article abstract of DOI:10.1016/j.ica.2009.03.036

Two hitherto unknown mixed-ligand tris chelated complexes containing 2-aminothiophenolate, [Et₄N]₂[M^IV(NH-(C₆H₄)-S)(mnt)₂] (M = Mo, 1a; W, 2a) and two mixed-ligand tris chelate complex cont

Full text of DOI:10.1016/j.ica.2009.03.036

Inorganica Chimica Acta 362 (2009) 3493–3501
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Mixed-ligand tris chelated complexes of Mo(IV) and W(IV): A comparative study
*
Amit Majumdar, Sabyasachi Sarkar
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, UP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two hitherto unknown mixed-ligand tris chelated complexes containing 2-aminothiophenolate, [Et4N]2
[MIV(NH–(C6H4)–S)(mnt)2] (M = Mo, 1a; W, 2a) and two mixed-ligand tris chelate complex containing
N,N-diethyldithiocarbamate, [Et4N]2[MIV(Et2NS2)(mnt)2] (M = Mo, 1b; W, 2b) have been synthesized
and characterized structurally. Although these complexes are supposed to be quite similar to the well-
known symmetric tris chelate complexes of maleonitriledithiolate (mnt), [Et4N]2[MIV(mnt)3] (M = Mo,
1c; W, 2c), but display both trigonal prismatic and distorted trigonal prismatic geometry in their crystal
structure indicating the possibility of an equilibrium between these two structural possibilities in solu-
tion. Unlike extreme stability of 1b, 2b, 1c and 2c, both 1a and 2a are highly unstable in solution. In con-
trast to one reversible reduction in case of 1b and 2b, 1a and 2a exhibited no possible reduction up to
À1.2 V and two sequential oxidation steps which have been further investigated with EPR study. Differ-
ences in stability and electrochemical behavior of 1a, 1b, 2a and 2b have been correlated with theoretical
calculations at DFT level in comparison with long known 1c and 2c.
Received 1 December 2008
Received in revised form 16 March 2009
Accepted 24 March 2009
Available online 1 April 2009
Keywords:
Mixed-ligand tris chelate
MoIV
WIV
Tris
N,N-diethyldithiocarbamate
2-Aminothiophenolate
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
ligands the intermediate species {MIV(OH)(X)} can be converted to
a diverse class of complexes exhibiting interesting model enzy-
Six coordinate tris chelated complexes have attracted much
attention due to their interesting structural properties. These com-
plexes are known to adopt a trigonal prismatic structure instead of
the usual octahedral geometry [1–3]. The relation between elec-
tronic state of the central metal atom and the structure of the tris
chelated complex has been well established [4–7]. Electronic con-
trol of the ‘‘Bailer Twist” in formally d0–d2 molybdenum tris
(dithiolene) complexes has recently been elegantly described [8].
M(IV) (M = Mo/W) tris(dithiolene) complexes are frequently
been found as an unwanted by-product during the synthesis of
model active sites of mononuclear molybdenum and tungsten en-
zymes [9], as these are thermodynamically most stable complexes
in molybdenum(IV) and tungsten(IV) dithiolene chemistry. The re-
dox series like [Mo(S2C2Me2]0,1À,2À is described earlier in connec-
tion with metal dithiolene series [10]. Tris chelated Mo(V)/W(V)
complexes are studied with a special emphasis to their interesting
EPR spectroscopic properties [11–16] in relevance to mononuclear
Mo/W enzymes.
matic reactions [22,23]. Use of bidentate ligand at this stage lead
to the isolation of mixed-ligand tris chelated complexes 1a, 2a,
1b and 2b (Chart 1). Complex 1b has been reported earlier as a tet-
rabutylammonium salt, mixed with [Bu4N]2[Mo(mnt)3] resulting
from a tedious synthetic procedure [24] although its tungsten ana-
logue 2b has not been structurally characterized by X-ray. Com-
plexes 1a and 2a are found to be unstable both in the solid and
in solution, compared with the stable symmetric tris chelated com-
plexes [25–34] of maleonitrile dithiolate (mnt) [35–38], 1c and 2c
(Chart 1). On the other hand, 1b and 2b were found to be stable
both in the solid as well as in solution state. The differing stability
patterns of these mixed-ligand tris chelated complexes along with
their interesting electrochemical properties are correlated with
theoretical calculation at DFT level. The reversible oxidations
exhibited by 1a and 2a have been exploited further to investigate
EPR spectroscopic signatures of their one electron oxidized species.
2. Experimental
In our endeavor in the field of structural–functional modeling
for the oxidoreductase class of molybdenum and tungsten en-
zymes [17–20,22,23] we observed that the {M@O} moiety in the
complex [MIVO(mnt)2]2À (M = Mo/W) can be opened up [20–23]
with the addition of an acid (HX, where X is a non-coordinating
or weekly coordinating anion) and utilizing different monodentate
2.1. Materials and physical methods
All reactions and manipulations were performed under a pure
argon atmosphere using modified Schlenk technique. 2-Amino-
thiophenol and CH3SO3H was obtained from S.D. Fine Chemicals
Ltd., India. Solvents were distilled and dried by standard procedure
before used. [Ph4P]2[MoIVO(mnt)2] and [PNP]2[MoIVO(mnt)2] were
prepared following the similar procedure for the preparation of
* Corresponding author. Tel./fax: +91 512 2597265.
E-mail address: abya@iitk.ac.in (S. Sarkar).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.03.036

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A. Majumdar, S. Sarkar / Inorganica Chimica Acta 362 (2009) 3493–3501
spectrum (acetonitrile) kmax
(
e
M): 356 (12 960), 376 (13 060), 524
2190 (CN), 3048 (aromatic CH stretch-
ing), 766 (aromatic CH bending) cmÀ1
Complexes
Designations
(4380) nm. IR (KBr pellet):
m
[Et4N]2[MoIV(NH-(C6H4)-S)(mnt)2]
1a
.
[Ph4P][MoIV(Et2NS2)(mnt)2] (1b): To a solution of 4.28 g (4 mmol)
of [Ph4P]2[MoIVO(mnt)2] and 3.32 g (20 mmol) of [NH4][Et2NS2] in
30 ml dichloromethane in cold was added 1 ml methanesulfonic acid
dropwise with constant stirring. Petroleum ether (60–80 °C) was
added into the resulting purple solution to precipitate a purple mass.
This was dissolved in acetonitrile followed by addition of isopropanol
and diethyl ether. The resulting solution upon keeping overnight at
4 °C yielded block shaped deep purple diffraction quality crystals.
Yield: 4.33 g (90%). Anal. Calc. for MoC37H30N5PS6: C, 51.44; H, 3.50;
N, 8.10; S, 22.27. Found: C, 51.39; H, 3.46; N, 8.06; S, 22.22%. Absorp-
[Et4N]2[WIV(NH-(C6H4)-S)(mnt)2]
[Et4N]2[MoIV((Et2NS2)(mnt)2]
[Et4N]2[WIV((Et2NS2)(mnt)2]
[Et4N]2[MoIV(mnt)3]
2a
1b
2b
1c [5, 24)]
[Et4N]2[WIV(mnt)3]
2c [5]
mnt
Maleonitriledithiolate [34-37]
tion spectrum (acetonitrile) kmax
(eM): 365 (7540), 486 (7140), 530
2190 (CN) cmÀ1
.
Chart 1. Designations and abbreviations.
(6420) nm. IR (KBr pellet):
m
[PNP][WIV(Et2NS2)(mnt)2] (2b): To a solution of 4.08 g (4 mmol)
of [PNP]2[WIVO(mnt)2] and 3.32 g (20 mmol) of [NH4][Et2NS2] in
30 ml dichloromethane in cold was added 1 ml methanesulfonic
acid dropwise with constant stirring. Petroleum ether (60–80 °C)
was added into the resulting red solution to precipitate a red mass.
This was dissolved in acetonitrile followed by addition of isopropa-
nol and diethyl ether. The resulting solution upon keeping over-
night at 4 °C yielded block shaped red diffraction quality crystals.
Yield: 4.14 g (90%). Anal. Calc. for WC49H40N6P2S6: C, 51.13; H,
3.50; N, 7.30; S, 16.71. Found: C, 51.09; H, 3.46; N, 7.26; S,
[Et4N]2[MoIVO(mnt)2] [17,39] by using [Ph4P][Br] and [PNP][Cl] in-
stead of [Et4N][Br]. X-ray data were collected on a Bruker-AXS
Smart APEX CCD diffractometer. Infrared spectra were recorded
on a Bruker Vertex 70, FT-IR Spectrophotometer as pressed KBr
disks. Elemental analyses for carbon, hydrogen, nitrogen and sulfur
were carried out with Perkin–Elmer 2400 microanalyser. Elec-
tronic spectra were recorded using a USB 2000 (Ocean Optics
Inc.) UV–Vis spectrophotometer equipped with fiber optics. Cyclic
voltammetric measurements were made with BASi Epsilon-EC Bio-
analytical Systems Inc. Cyclic voltammograms of 10À3 M solution
of the compounds (1a, 2a, 1b and 2b) were recorded (scan
rate = 100 mV/s) using glassy carbon electrode as working elec-
trode, 0.2 M [Bu4N][ClO4] as supporting electrolyte, Ag/AgCl elec-
trode as reference electrode and platinum wire as an auxiliary
electrode. Sample solutions were prepared in acetonitrile. Differen-
tial pulse polarography was performed with scan rate = 20 mV/s,
pulse width = 50 ms, pulse period = 200 ms, pulse ampli-
tude = 50 mV. All electrochemical experiments were done under
argon atmosphere at 298 K. Potentials are referenced against inter-
nal ferrocene (Fc) and are reported relative to the Ag/AgCl elec-
trode (E1/2(Fc+/Fc) = 0.459 V versus Ag/AgCl electrode).
16.67%. Absorption spectrum (acetonitrile) kmax
(e
M): 350 (6620),
415 (7180), 480 (5590) nm. IR (KBr pellet):
m
2190 (CN) cmÀ1
.
2.3. X-ray crystallography
Suitable diffraction quality single crystals were obtained from
the crystallization procedures described in each synthesis. The
crystals used in the analyses were glued to glass fibers and
mounted on BRUKER SMART APEX diffractometer. The instrument
was equipped with CCD area detector and data were collected
using graphite-monochromated Mo Ka radiation (k = 0.71069 Å)
at low temperature (100 K). Cell constants were obtained from
the least-squares refinement of three-dimensional centroids
through the use of CCD recording of narrow
x rotation frames,
2.2. Synthesis
completing almost all-reciprocal space in the stated h range. All
data were collected with SMART 5.628 (BRUKER, 2003), and were
integrated with the BRUKER SAINT [40] program. The structure was
solved using SIR97 [41] and refined using SHELXL-97 [42]. Crystal struc-
tures were viewed using ORTEP [43] and DIAMOND 3.1e. The empirical
absorption correction was applied using SADABS, as described by
Blessing [44]. The space group of the compounds was determined
based on the lack of systematic absence and intensity statistics.
Full-matrix least squares/difference Fourier cycles were performed
which located the remaining non-hydrogen atoms. All non-hydro-
gen atoms were refined with anisotropic displacement parameters.
Complex 1a possesses two anions in its asymmetric unit of which
one shows trigonal prismatic and the other one shows distorted
trigonal prismatic geometry. Complex 1b possesses two anions in
its asymmetric unit both exhibiting distorted trigonal prismatic
geometry. In one anion, the ethyl groups of diethyldithiocarbamate
ligand are in eclipsed conformation and in the other anion they are
in gauche conformation. Complex 2a contains one anion in its
asymmetric unit and shows a distorted trigonal prismatic structure
whereas 2b contains two anions in its asymmetric unit, both
exhibiting trigonal prismatic geometry. Due to these reasons addi-
tional symmetry could not have been assigned.
[Et4N]2[MoIV(NH–(C6H4)–S)(mnt)2] (1a): To a solution of 2.62 g
(4 mmol) of [Et4N]2[MoIVO(mnt)2] and 2.14 ml (20 mmol) of 2-
aminothiophenol in 30 ml dichloromethane in cold was added
1 ml methanesulfonic acid dropwise with constant stirring. Petro-
leum ether (60–80 °C) was added into the resulting blue solution to
precipitate a blue mass. This was dissolved in acetonitrile contain-
ing 2-aminothiophenol followed by addition of isopropanol and
diethyl ether. The resulting solution upon keeping overnight at
4 °C yielded needle shaped dark blue diffraction quality crystals.
Yield: 2.73 g (90%). Anal. Calc. for MoC30H45N7S5: C, 47.41; H,
5.96; N, 12.90; S, 21.09. Found: C, 47.38; H, 5.92; N, 12.86; S,
21.02%. Absorption spectrum (acetonitrile) kmax
(10 120), 558 (8000) nm. IR (KBr pellet): 2195 (CN), 3050 (aro-
matic CH stretching), 740 (aromatic CH bending) cmÀ1
(eM): 390
m
.
[Et4N]2[WIV(NH–(C6H4)–S)(mnt)2] (2a): To a solution of 2.96 g
(4 mmol) of [Et4N]2[WIVO(mnt)2] and 2.14 ml (20 mmol) of 2-ami-
nothiophenol in 30 ml dichloromethane in cold was added 1 ml
methanesulfonic acid dropwise with constant stirring. Petroleum
ether (60–80 °C) was added into the resulting red solution to pre-
cipitate a red mass. This was dissolved in acetonitrile containing
2-aminothiophenol followed by addition of isopropanol and diethyl
ether. The resulting solution upon keeping overnight at 4 °C yielded
needle shaped red colored diffraction quality crystals. Yield: 3.04 g
(90%). Anal. Calc. for WC30H45N7S5: C, 42.49; H, 5.35; N, 11.56; S,
18.91. Found: C, 42.44; H, 5.31; N, 11.52; S, 18.86%. Absorption
2.4. Computational details
All calculations were performed using the GAUSSIAN 03 (Revision
B.04) package [45] on an IBM PC platform. Molecular orbitals were

A. Majumdar, S. Sarkar / Inorganica Chimica Acta 362 (2009) 3493–3501
3495
visualized using ‘Gauss View’ and geometry optimized molecular
structures were shown with DIAMOND 3.1e. Gas phase geometry
optimization and population analysis of the molecular orbitals
were carried out at DFT level. The method used was Becke’s
three-parameter hybrid-exchange functional [46], the non-local
correlation provided by the Lee et al. expression [47], and Vosko,
Wilk, and Nuair 1980 local correlation functional (III) (B3LYP)
spectroscopic signatures of these complexes are shown in Fig. 1,
which are dominated by ligand (2-aminothiophenolate/N,N-dieth-
yldithiocarbamate) to metal charge transfer bands. Desoxo
bis(dithiolene) complexes have earlier been shown by us to under-
go axial ligand exchange reactions leading to interconversion of
synthetic complexes [20,23]. It has also been shown that in solu-
tion these desoxo complexes slowly converted to the thermody-
namically stable tris chelated complexes [20,23]. However,
complexes 1a, 2a and 1b, 2b have been found to be unable to un-
dergo interconversion among them (Scheme 1). Unlike the extreme
stability of 1b and 2b, complexes 1a and 2a are unstable and
decompose readily both in solid and in solution state and among
them 2a is more unstable than 1a. This unusual instability of the
mixed-ligand tris chelated complexes (1a and 2a) may be a mani-
festation of hard-soft ligand donor effect. Nitrogen being a hard do-
nor cannot stabilize the soft Mo(IV) like sulfur (a soft donor).
Eventually complexes 1a and 2a can only be stabilized in solution
in presence of excess 2-aminothiophenol. Degradation of 1a and 2a
in solution are monitored by absorption spectroscopic measure-
ments which are shown in Fig. 2. The intensity of the initial absorp-
tion band at 558 (1a) and 524 nm (2a) decrease with time and the
spectroscopic nature of the final product indicates decomposition
of the complexes involving ligand loss rather than the formation
*
[46,47]. 6–31G + basis set [48] was used for C, N, and H atoms
*
and 6–311G + basis set [49,50] was used for S atom. The LANL2DZ
basis set [51] and LANL2 pseudopotentials of Hay and Wadt [52,53]
were used for Mo and W atoms. The optimized minima were char-
acterized by harmonic-vibration-frequency calculation with the
same method and basis set in which the minimum has no imagi-
nary frequency.
3. Results and discussion
3.1. Synthesis and reactivity
The reactions [17–23] of complex, [MIVO(mnt)2]2À (M = Mo/W)
[17,39] under protic conditions suggest that the {MIV@O} moiety
behaves like the carbonyl group. Thus {M@O} bond can easily be
protonated with the addition of an acid (HX) which may yield an
intermediate species, {MIV(OH)(X)} (M = Mo/W) and when the
counter anion, X is a non coordinating anion like methanesulfo-
nate, the species responds to hydrolysis to yield the stable tris
[5,25–34] species, [MIV(mnt)3]2À (M = Mo, 1c; M = W, 2c) in 67%
yield (based on molybdenum/tungsten). Its formation can be near
quantitative upon addition of one equivalent of free mnt [35–38]
anion into the reaction medium [18]. Addition of 2-aminothiophe-
nol and ammonium salt of N,N-diethyldithiocarbamate into this
reaction medium yielded the complexes, [Et4N]2[MIV(NH–(C6H4)–
S)(mnt)2] (M = Mo, 1a; M = W, 2a), [Ph4P][MoIV(Et2NS2)(mnt)2]
(1b) and [PNP][WIV(Et2NS2)(mnt)2] (2b) (Scheme 1). Absorption
of tris(dithiolene) species or the starting material, [MIVO(mnt)2]2À
.
Infact, complexes 1a and 2a cannot be converted to the corre-
sponding tris (dithiolene) complexes 1c and 2c. Even in the pres-
ence of excess of mnt [35–38] anion into a solution of 1a or 2a
such conversion did not occur. On the other hand, tris chelated
complexes, 1c and 2c cannot be reverted back to either, 1a, 2a or
to 1b, 2b (Scheme 1).
3.2. X-ray structure description
All the four complexes (1a, 1b, 2a and 2b) are characterized by
X-ray structure determinations. Important structural parameters
are summarized in Table 1 and important bond lengths are shown
in Table 2. Structures (ORTEP view) of the four complexes are shown
in Fig. 3. While trigonal prismatic co-ordination remains a distinct
possibility in symmetric tris chelated complexes, it is somewhat
more likely that mixed-ligand tris chelated complexes may adopt
structures between octahedral and trigonal prismatic geometry.
In order to complete the comparative study, we have synthesized
and characterized complex 1b by a new protonation method
Scheme 1. Schematic diagram for the synthesis and no possible interconversion of
Fig. 1. Absorption spectroscopic signature of the complexes 1a, 1b, 2a and 2b in
the synthesized complexes.
acetonitrile.

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A. Majumdar, S. Sarkar / Inorganica Chimica Acta 362 (2009) 3493–3501
Fig. 2. Degradation of 1a (a) and 2a (b) in acetonitrile. Concentration of 1a = 2 Â 10À4 M, total time = 6 h, scan rate = 4 min/scan; concentration of 2a = 2 Â 10À4 M, total
time = 10 min, scan rate = 4 s/scan.
Table 1
Crystallographic dataa for complexes 1a, 1b, 2a and 2b.
Complexes
1a
1b
2a
2b
Formula
Formula weight
Crystal system
Space group
T (K)
C60H90Mo2N14S10
1520.04
triclinic
P1
C37H30MoN5PS6
863.99
monoclinic
P21
C30H45WN7S5
847.92
C49H40WN6P2S6
1151.07
triclinic
P1
triclinic
ꢀ
P1
100
100
100
100
Z
1
4
2
2
a (Å)
b (Å)
c (Å)
7.215(5)
13.772(5)
18.063(5)
87.984(5)
84.861(5)
83.872(5)
1776.8(15)
1.421
7.780(5)
21.712(5)
23.196(5)
90.000(5)
90.532(5)
90.000(5)
3918(3)
1.465
10.214(5)
13.065(5)
14.085(5)
83.563(5)
72.306(5)
81.564(5)
1766.7(13)
1.594
12.062(5)
13.764(5)
15.598(5)
79.611(5)
74.082(5)
84.193(5)
2445.8(16)
1.563
a
(°)
b (°)
c
(°)
V (Å3)
Dcalc (g/cm3)
l
(mmÀ1
)
0.695
0.729
3.597
2.725
h range (°)
2.26–28.36
1.056
0.0953 (0.1865)
1.99–28.35
1.056
0.0811 (0.1739)
2.11–28.37
1.144
0.0770 (0.1610)
2.20–28.30
1.185
0.0557 (0.1276)
Goodness of fit (GOF) (F2)
b
c
R1 (wR2
)
a
Mo K
a
radiation.
P
P
b
c
R1
=
||Fo| À |Fc||/ |Fo|.
P
P
wR2 = { [w(F2o À F2c )2]/ [w(F2o)2]}1/2
.
Table 2
Important bond distances for complexes 1a, 1b, 2a and 2b.
Complexes
M–Naminothiophenol (Å)
M–Saminothiophenol (Å)
M–Sdiethyldithiocarbmate (Å)
[Et4N]2[MoIV(NH–(C6H4)–S)(mnt)2] (1a)
[Et4N]2[WIV(NH–(C6H4)–S)(mnt)2] (2a)
[Ph4P][MoIV((Et2NS2)(mnt)2] (1b)
[PNP][WIV((Et2NS2)(mnt)2] (2b)
2.030
2.044
2.378
2.375
2.435, 2.461
2.478, 2.458
developed recently in dithiolene chemistry [20–23]. Its X-ray
structure shows that complex 1b adopts a distorted trigonal pris-
matic geometry in contrast to the trigonal prismatic geometry
reported earlier [24]. Interestingly, complex 1a possesses two an-
ions in its asymmetric unit of which one shows trigonal prismatic
and the other one shows distorted trigonal prismatic geometry.
Complex 2a contains one anion in its asymmetric unit and shows
a distorted trigonal prismatic structure where as 2b contains two
anions in its asymmetric unit, both exhibiting almost trigonal pris-
matic geometry (deviation = 1.05°). Different structural geometry
adapted by complexes 1a, 2a, 1b and 2b are shown in Supplemen-
tary material. Adaptation of either distorted trigonal prismatic or
trigonal prismatic geometry by these mixed-ligand tris chelated
complexes (1b, 2a, 2b) and the presence of both trigonal prismatic
and distorted trigonal prismatic geometries in the crystal structure
of complex 1a strongly indicate that there may be equilibrium be-
tween these two structural entities in solution.
3.3. Electrochemistry
The electrochemical properties of these complexes are investi-
gated by cyclic voltammetry and by differential pulse polarography
(Figs. 4 and 5). Both 1a and 2a exhibited two sequential one elec-
tron oxidation steps (Fig. 4). Complex 1a exhibited two sequential
reversible oxidations at E1/2 = 0.157 V (DEpp = 66 mV, Ipa/Ipc = 1.01)
and at E1/2 = 0.660 V (
D
Epp = 76 mV, Ipa/Ipc = 1.06). The first oxida-
tion in such low potential is due to MoIV/MoV couple whereas the
second one may be due to the oxidation of coordinated 2-amino-
thiophenolate ligand. In contrast, 2a exhibited one reversible
oxidation at E1/2 = 0.034 V (DEpp = 52 mV, Ipa/Ipc = 1.12) due to

A. Majumdar, S. Sarkar / Inorganica Chimica Acta 362 (2009) 3493–3501
3497
Fig. 3. Structure (ORTEP view) of anions of 1a (a), 2a (b), 1b (c) and 2b (d), showing 50% thermal probability ellipsoids with partial atom labeling scheme. Hydrogen atoms have
been omitted for clarity.
Fig. 5. Cyclic voltammograms (solid line) (scan rate = 100 mV/s) and differential
pulse polarographs (dotted line) (scan rate = 20 mV/s, pulse width = 50 ms, pulse
period = 200 ms, pulse amplitude = 50 mV) for the complexes 1b (a) and 2b (b) in
acetonitrile.
Fig. 4. Cyclic voltammograms (solid line) (scan rate = 100 mV/s) and differential
pulse polarographs (dotted line) (scan rate = 20 mV/s, pulse width = 50 ms, pulse
period = 200 ms, pulse amplitude = 50 mV) for the complexes 1a (a) and 2a (b) in
acetonitrile.
WIV/WV followed by one irreversible oxidation at Epa = 0.560 V due
to coordinated 2-aminothiophenolate ligand. Complexes 1a and 2a
did not show any reductive response up to À1.2 V (not shown in
the figure). On the other hand, 1b exhibited one reversible reduc-
tion at E1/2 = À0.870 V (
versible oxidation at Epa = 0.994 V (Fig. 5a). Complex 2b exhibited
one reversible reduction at E1/2 = À1.14 V Epp = 80 mV, Ipa
Ipc = 0.91) and two irreversible oxidations at Epa = 0.837 V and
DEpp = 70 mV, Ipa/Ipc = 1.02) and one irre-
(D
/

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Epa = 1.14 V (Fig. 5b). These two oxidations at Epa = 0.837 V and
Epa = 1.14 V can be attributed to the oxidation of WIV center in 2b
and the coordinated mnt ligand, respectively. In the case of 1b
these two oxidations (oxidation of MoIV center and oxidation of
coordinated mnt ligand) are almost merged as indicated from cor-
responding differential pulse polarography presented in Fig. 5a.
to W is not well resolved (see Supplementary material). The resul-
tant solutions although remained stable for few minutes at room
temperature, but all attempts to isolate the oxidized species ended
up with uncharacterized products.
3.5. Theoretical calculations and correlations
with experimental results
3.4. EPR spectroscopic investigation
The different electrochemical behavior of these closely related
desoxo complexes can be rationalized in the light of relative energy
levels of their molecular orbitals. At least in a qualitative sense a
gas phase DFT calculation has been found to be successful to inter-
pret the observed electrochemical data and their stability pattern.
DFT calculations were carried out by employing a B3LYP hybrid
functional using the GAUSSIAN 03 program [45] (see Section 2.4).
Geometry of all the synthesized complexes were optimized (see
Supplementary material) using the coordinates obtained initially
from the X-ray crystallographic structure determination, followed
The reversible oxidations exhibited by 1a and 2a at such low
potential (Fig. 4) prompted us to check the possibility of their
one electron chemical oxidation. Complex 1a was treated with
sub equivalent (ꢀ0.25 equivalent) amount of iodine in acetonitrile
solution and the resultant solution was found to exhibit an EPR sig-
nal centered at g = 1.995 with well resolved Mo95,97 hyperfine
splitting (A/G = 36.2 0.5) at room temperature (see Supplemen-
tary material). Complex 2a, under similar treatment, showed an
EPR signal centered at g = 1.975, although hyperfine splitting due
Fig. 6. Relative energy level diagram for the selected molecular orbitals of complex 1a (a) and 2a (b). Isosurface cut off value = 0.04. Color code for atoms: pink, molybdenum;
sky blue, tungsten; yellow, sulfur; blue, nitrogen; grey, carbon; green, hydrogen. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)

A. Majumdar, S. Sarkar / Inorganica Chimica Acta 362 (2009) 3493–3501
3499
by single point energy calculation (Figs. 6–8). The assignment of
the type of each MO was made on the basis of its composition
and by visual inspection of its localized orbital. The coordinate
frames for both the compounds were assigned by the visual inspec-
tion of the dz2 and dxy orbital.
Different stability pattern of the synthesized complexes can be
rationalized with the aid of relative energy level diagrams of their
selected molecular orbitals. These are shown in Figs. 6–8 and the
metal contribution in the HOMOs and LUMOs of the complexes
are summarized in Supplementary material. One interesting fea-
ture observed is that the HOMO and LUMO are essentially inter-
changed in the case of molybdenum and tungsten complexes
with both 2-aminothiophenolato ligand (1a and 2a, Fig. 6) and
N,N-diethyldithiocarbamate ligand (1b and 2b, Fig. 7). Fig. 6 shows
that the HOMO in 1a is predominantly ligand centered whereas in
2a, it is predominantly metal centered (mainly metal dz2 ). Same
situation prevails in case of 1b and 2b. Instability of the synthe-
sized mixed-ligand tris chelated complexes, 1a and 2a, in compar-
ison to the extreme stability of the symmetric tris chelated
complexes of mnt (1c and 2c) are in agreement with relatively
higher energy HOMOs (Fig. 6) in case of 1a (11.29 kcal) and 2a
(11.92 kcal) compared to relatively deeply buried HOMOs (see
Supplementary material) in case of 1c (À5.64 kcal) and 2c
From X-ray structure determination, 2a has been found to adopt
a distorted trigonal prismatic geometry whereas 1a contains two
anions in its asymmetric unit, of which one is trigonal prismatic
and the other one is distorted trigonal prismatic in geometry. Upon
geometry optimization both 1a and 2a adopted a distorted trigonal
prismatic geometry. On the other hand, X-ray structure shows that
1b adopts a distorted trigonal prismatic geometry and 2b adopts a
trigonal prismatic geometry, although upon geometry optimiza-
tion, geometry in both the cases are found to be trigonal prismatic.
Due to these reasons, single point calculation has also been per-
formed with 1a (using trigonal prismatic geometry as of 1a) and
with 1b (using distorted trigonal prismatic geometry of 1b) and
the selected molecular orbitals are shown in Fig. 8.
Fig. 7. Relative energy level diagram for the selected molecular orbitals of complex 1b (a) and 2b (b). Isosurface cut off value = 0.04. Color code for atoms: pink, molybdenum;
sky blue, tungsten; yellow, sulfur; blue, nitrogen; grey, carbon; green, hydrogen. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)

3500
A. Majumdar, S. Sarkar / Inorganica Chimica Acta 362 (2009) 3493–3501
(Fig. 7). A deeply buried HOMO (E = À72.79 kcal/mol) with large li-
gand contribution (Fig. 7) can well account for an irreversible oxi-
dation at Epa = 0.994 V in the case of 1b. Irreversible oxidation at
somewhat lower potential (Epa = 0.837) in the case of 2b is in
agreement with its relatively less buried HOMO (Fig. 7) although
a predominantly metal centered HOMO (mainly metal dz2 ) in this
case, does not account for the irreversible character of the oxida-
tion step. Rather it should support an expected reversible oxidation
step. Sequential reversible oxidation steps in the case of complexes
1a and 2a cannot be directly rationalized in the light of their rela-
tive energy level diagram (Fig. 6). A monomeric system with pre-
dominantly ligand centered HOMO may not account for the
observed two sequential reversible oxidation steps as seen in the
case of 1a. This can only be possible because of the presence of
2-aminothiophenolate ligand which may be responsible for the
second oxidation step. However in the case of 2a, the presence of
a metal centered HOMO (mainly metal dz2 ) can account for the
reversible oxidation at a much lower potential (E1/2 = 0.034 V) fol-
lowed by an irreversible oxidation which may be correlated with
the possible structural change that might have happened during
the second oxidation centered at 2-aminothiophenolate ligand.
The observed absence of any reduction steps up to À1.2 V is a man-
ifestation of the predominantly ligand centered LUMOs (centered
mainly in mnt) in the case of 2a and any possible reduction would
have destabilized the dithiolene bonding to tungsten center fol-
lowed by dissociation of mnt ligand. In the case of 1a, although
the LUMO is metal centered (mainly metal dx2 ), but it is engaged
Ày2
with ligand orbitals along with significant ligand character as
shown in Fig. 6a. Apart from this fact, Fig. 8a shows that the LUMO
of 1a comes out to be predominantly ligand centered (me-
tal = 4.26%) when the trigonal prismatic geometry of 1a is used
for single point calculation involving population analysis. Presence
of this significant ligand character in the LUMO of 1a can account
for the observed absence of any reduction steps up to À1.2 V.
4. Conclusions
Four hitherto unknown mixed-ligand tris chelate complexes of
molybdenum(IV) and tungsten(IV) containing two maleonitriledi-
thiolate and one 2-aminothiophenolate or N,N-diethyldithiocarba-
mate ligand are synthesized by a simple protonation method and
analyzed in comparison with symmetric tris chelate complexes,
[MIV(mnt)3]2À (M = Mo/W). Adaptation of either distorted trigonal
prismatic or trigonal prismatic geometry by these mixed-ligand
tris chelated complexes (1b, 2a, 2b) and the presence of both trigo-
nal prismatic and distorted trigonal prismatic geometries in the
crystal structure of complex 1a strongly indicate the possibility
of equilibrium between these two types of geometries in solution.
In spite of structural similarity and expected chelate effect stability
in all these complexes, 2-aminothiophenolate ligated complexes
have been found to be extremely unstable in solution towards
decomposition. Difference in electrochemical behavior in these
seemingly similar synthesized complexes is correlated with popu-
lation analysis of the respective redox molecular orbitals obtained
from theoretical calculations at DFT level. Sequential reversible
oxidation steps in 2-aminothiophenolato complexes have been fur-
ther investigated by EPR spectroscopy with their one electron oxi-
dized species, which may have importance in the context of Mo(V)/
W(V) state generated during the catalytic cycle of oxidoreductase
class of molybdenum and tungsten enzymes.
Fig. 8. Selected molecular orbitals of complexes 1a ((a), geometry is trigonal
prismatic) and 1b ((b), geometry is distorted trigonal prismatic). Isosurface cut off
value = 0.04. Color code for atoms: pink, molybdenum; yellow, sulfur; blue,
nitrogen; grey, carbon; green, hydrogen. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
(À6.27 kcal). Eventually complexes 1a and 2a can only be stabi-
lized in solution in presence of excess 2-aminothiophenol. Com-
plexes 1b and 2b are highly stable in solution as well as in solid
state and this are exhibited by their deeply buried HOMOs and LU-
MOs (Fig. 7). Infact, the relative energy level diagram indicates 1b
and 2b to be more stable compared to even 1c and 2c.
Comparing Figs. 6a and 8a, it becomes evident that the nature of
the LUMO changes upon using two different possible geometries of
1a, namely, distorted trigonal prismatic (Fig. 6a, metal centered)
and trigonal prismatic (Fig. 8a, ligand centered) although the char-
acter of molecular orbital remains almost same in the case of 1b by
using these two different possible geometries (Figs. 7a and 8b).
Electrochemical behavior of the synthesized complexes can be
explained in the light of these relative energy level diagrams. LU-
MOs of both the complexes 1b and 2b are predominantly metal
Acknowledgments
centered (mainly metal dx2 ). A reversible reduction at E1/2
=
Ày2
A.M. acknowledges research assistantship from DST, India, and
S.S. thanks DST, India, for funding the project.
À0.870 V and E1/2 = À1.14 V in case of 1b and 2b, respectively,
can be attributed to their low lying metal centered LUMOs

A. Majumdar, S. Sarkar / Inorganica Chimica Acta 362 (2009) 3493–3501
3501
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