Synthesis, structure and theoretical investigation into a homoleptic tris(dithiolene) tungsten

Article abstract of DOI:10.1016/j.saa.2013.09.069

A new homoleptic dithiolene tungsten complex, tris-{1,2-bis(3,5- dimethoxyphenyl)-1,2-ethylenodithiolene-S,S′}tungsten, was successfully synthesized via a reaction of the thiophosphate ester and sodium tungstate. The thiophosphate ester was prepared from

Full text of DOI:10.1016/j.saa.2013.09.069

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, structure and theoretical investigation into a homoleptic
tris(dithiolene) tungsten
Khuzaimah Arifin a, Lorna Jeffery Minggu a, Wan Ramli Wan Daud a,b, Bohari M. Yamin c, Rusli Daik c,
Mohammad B. Kassim a,c,
⇑
a Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
b Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, University Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
c School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
A new homoleptic dithiolene
tungsten complex was successfully
synthesized.
ꢀ
Spectroscopic analysis and X-ray
single crystal study confirm the
structure and geometry.
ꢀ
ꢀ
HOMO and LUMO comes from
separate moieties of the complex.
The optical properties showed a slight
blue shift in several low dielectric
solvents.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new homoleptic dithiolene tungsten complex, tris-{1,2-bis(3,5-dimethoxyphenyl)-1,2-ethylenodithio-
lene-S,S0}tungsten, was successfully synthesized via a reaction of the thiophosphate ester and sodium
tungstate. The thiophosphate ester was prepared from 3,5-dimethoxybenzaldehyde via benzoin conden-
sation to produce the intermediate 1,2-bis-(3,5-dimethoxyphenyl)-2-hydroxy-ethanone compound, fol-
lowed by a reaction of the intermediate with phosphorus pentasulfide. FTIR, UV–Vis spectroscopy, 1H
NMR and 13C NMR and elemental analysis confirmed the product as tris{1,2-bis-(3,5-dimethoxy-
phenyl)-1,2-ethylenodithiolene-S,S0}tungsten with the molecular formula of C54H54O12S6W. Crystals of
Received 17 May 2013
Received in revised form 19 September
2013
Accepted 26 September 2013
Available online 16 October 2013
Keywords:
Dithiolene
Non-innocent
Tungsten
Homoleptic
DFT
the product adopted
a monoclinic system with space group of P2(1)/n, where a = 12.756(2) Å,
b = 21.560(3) Å, c = 24.980(4) Å and b = 103.998(3)°. Three thioester ligands were attached to the tung-
sten as bidentate chelates to form a distorted octahedral geometry. Density functional theory calculations
were performed to investigate the molecular properties in a generalized-gradient approximation frame-
work system using Perdew–Burke–Ernzerhof functions and a double numeric plus polarization basis set.
The HOMO was concentrated on the phenyl ligands, while the LUMO was found along the W(S2C2)3 rings.
The theoretical optical properties showed a slight blue shift in several low dielectric solvents. The solva-
tochromism effect was insignificant for high polar solvents.
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
⇑
Since the early 1960s, investigation into the synthesis, structure
and molecular properties of metal complexes with dithiolene li-
gands has attracted a number of researchers [1–8]. The dithiolene
Corresponding author at: School of Chemical Sciences and Food Technology,
Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi,
Selangor, Malaysia. Tel.: +60 3 89213980; fax: +60 3 89212410.
E-mail address: mbkassim@ukm.my (M.B. Kassim).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.09.069

K. Arifin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
209
ligand can bond easily to various metals such as nickel (Ni) [7],
tungsten (W) [8,9], molybdenum (Mo) [10,11], cobalt (Co) [1,12]
and platinum (Pt) [13], to form complex molecules with unique
optical, magnetic and conductive properties. The complexes have
been considered for a wide variety of applications, such as enzyme
cofactors [8,14], opto-electronic devices [15,16], building block
materials [17], dye sensitizers for solar cells [18] and photocata-
lysts for water splitting [19–22].
solvatochromism of MTDT were studied by time-dependent density
functional theory (TDDFT) on the optimized molecular structure.
Furthermore, the effects of ligand substitution on the structure,
spectra and molecular orbital (MO) energy properties were also
investigated. For the purpose of comparison, all OCH3 groups were
removed from the phenyl rings of the original X-ray crystal struc-
ture, and the structure was then optimized using the same calcula-
tion parameters.
Dithiolene is known as a bidentate ligand. The metal–ligand
complex forms a 1,2-dithiolene chelate which has the possibility
to become a resonance structure [23]. Electron delocalization
occurring on the dithiolene ring involves a thienyl radical monoan-
ion as shown in Fig. 1. The oxidation state of the metal and the li-
gands in dithiolene complexes are highly variable and difficult to
define. In addition, the dithiolene is often a non-innocent ligand
that depends on the system [24,25]. The majority of non-innocent
ligands are redox-active and are capable of acting as donors or
acceptors depending on the redox state [26].
The bidentate dithiolene ligand can form mono-, bis- or tris-
(1,2-dithiolenes) metal complexes. Molecular geometry of a dithio-
lene complex depends on the variation and number of bidentate li-
gands attached to the metal center. Dithiolene complexes are
classified into three categories: (i) homoleptic dithiolene com-
plexes, in which only 1,2-dithiolene is coordinated to the metal
center forming bis(dithiolene) complexes with a square-planar
geometry or tris-(dithiolene) complexes with either trigonal pris-
matic or octahedral geometries; (ii) heteroleptic dithiolene com-
plexes, in which a dithiolene and other inorganic ligands are
attached to the metal center, such as [(P^P)M(dithiolene)]
(M = Pt, P^P = diphosphine) and [oxo-M(dithiolene)2] (M = Mo,
W) complexes with square-planar geometries; and (iii) organome-
tallic dithiolene complexes, in which a dithiolene and organic li-
gands with metal–carbon bonds are incorporated [27].
Experiments
Materials and instruments
Chemicals: 3,5-dimethoxybenzaldehyde, triethylbenzylammo-
nium chloride, tetrabuthylammonium iodide, potassium hydrox-
ide, phosphorus pentasulfide and all organic solvents were
obtained from Merck. Hydrochloric acid and hydrazine monohy-
drate were obtained from Aldrich. IR spectra were recorded on a
Spectrum One (Perkin Elmer) in the form of KBr disk for solid
material and ATR for liquid. 1H and 13C NMR spectrum were ob-
tained using a Varian JEOL ECP 400 MHz in CDCl3 or deuterated
acetone. Electronic absorption spectra were recorded using a
Lambda 35 UV/Vis spectrophotometer. Elemental analyzes were
performed on a Fisons EA 1108 Elemental Analyzer. Electrochemi-
cal experiments were performed on a Voltalab PGZ402 Radiometer.
The crystal structure was determined using a Bruker AXS Smart
APEX with a CCD detector and a SHELXTL suite program.
Preparation of MTDT
Benzoin condensation (I)
3,5-Dimethoxybenzaldehyde (3.32 g) was dissolved in 12.5 ml
DMF followed by the addition of tetrabutylammonium iodide
(0.63 g) and KCN (0.251 g). The resulting mixture was stirred for
18 h under nitrogen. The reaction was quenched by adding 75 ml
of cold water. A white precipitate was formed, filtered and recrys-
tallized from hot ethanol to give a white flaky benzoin (I). Yield
60%; melting point: 102–103 °C; 1H NMR d(ppm) 3.80; 3.81;
2.86; 6.04; 6.37; 6.62 7.16; 4.93; 13C NMR d(ppm) 100.36;
162.16; 107.61; 142.89; 77.09; 200.10; 137.28; 106.56; 161.90;
The most common homoleptic dithiolene complex is bis-(1,2-
diphenyl-1,2-dithiolene) nickel (II), which is made from the reac-
tion of NiS2 with diphenylacetylene [28]. Other homoleptic bis-
or tris-dithiolenes complexes are generally synthesized from the
reaction of metal halides and dithiolate [29], and also via oxo-
dithiolenes from the photoreaction of metal carbonyls with dithiol-
ate [30]. An alternative route is through thiophosphate esters by
the reaction of
a-hydroxyketones with phosphorus pentasulfide,
106.22; 55.68; 56.00; IR spectrum (m
/cmꢁ1) 3843 (OH); 3098
which is mostly used for the preparation of dithiolene complexes
(CAH); 850 and 1672 (C@O); 1165 (CAOAC); UV/Vis spectrum
(kmax/nm) (acetone solution) 342. Elemental CHNSO analysis (%):
C 65.06, H 6.02, O 29.92, and the theoretical values were C:
63.48; H: 5.87; O: 30.65.
in large scale [28].
In this work, a new tris(dithiolene) compound called tris{1,2-
bis(3,5-dimethoxyphenyl)-1,2-ethylenodithiolenic-S,S0}tungsten
abbreviated MTDT was synthesized from 3,5-dimethoxybenzalde-
hyde in three steps: (i) benzoin condensation to produce an interme-
diate 1,2-bis-(3,5-dimethoxyphenyl)-2-hydroxy-ethanone; (ii)
thioester formation to give 1,2-dithiophosphate; and (iii) reaction
of 1,2-dithiophosphate with a tungsten salt to produce MTDT
[10,11]. The non-innocent behavior of the dithiolene ligands was
examined using structural geometry and density functional theory
(DFT) calculations. The DFT calculations were performed on the
molecular configuration found in the X-ray crystal structure which
was then optimized until no imaginary frequencies to be found.
Frontier molecular orbitals, optical characteristics and
Thioester synthesis (II)
Benzoin I (30 g) and phosphorus pentasulfide (45 g) were dis-
solved in 250 ml of dry dioxane. The mixture was refluxed for 3–
4 h in an inert atmosphere. Hydrogen sulfide produced from the
reaction was trapped using lead acetate. The mixture was cooled
to room temperature before filtering. The filtrate was concentrated
in vacuo to give a dark red oil, yield 37.5%. 1H NMR d(ppm) 3.56;
6.37; 7.27 and 13C NMR d(ppm) 56.44; 66.92; 77.25; 107.56;
160.56. IR spectrum (m
/cmꢁ1) 1254 (P@O), 1081 and 958 (PAO
Fig. 1. Several oxidation states of a dithiolene ligand.

210
K. Arifin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
and PAOAP), 870 (P@S). UV spectrum (kmax/nm) (acetone solution)
300.
utilized the framework of a generalized-gradient approximation
(GGA) system [33]. The starting atomic coordinates were taken
from the final X-ray refinement cycle. Geometry of the molecule
was fully optimized in a double numeric plus polarization (DNP)
basis set using Perdew–Burke–Ernzerhof (PBE) functions [34].
The geometrical optimization was an iterative procedure in
which the coordinates of the atoms were adjusted until the energy
of the structure was brought to a stationary value. The stationary
value of the minimum energy was determined by analysis of the
vibrational frequencies at the ground state geometry of the com-
plex. The absence of imaginary eigenvalues confirmed the validity
of the molecular geometry, from which further properties can be
calculated [35]. The self-consistent field (SCF) method was used
for calculating the electronic structure with the core treatment to
DFT Semi-core Pseudopots (DSPP) [36] and spin-unrestricted Har-
tree-Folk (URHF) mode. TDDFT calculations were conducted on the
ground state in spin-restricted. The effect of a solvent when calcu-
lating optical properties was determined using the conductor-like
screening model (COSMO) approach.
MTDT synthesis
Thioester II (5 g) was added to a mixture of sodium tungstate
(9 g) in 40 ml HCl 1 M. The mixture was refluxed for 2 h under
nitrogen. A dark-green solution was obtained, cooled and then con-
centrated by evaporation. The product was extracted with benzene
(250 ml). The dithiolene complex was extracted by hydrazine 95%
(80 ml), washed and neutralized by HCl 1 M. Yield 39%. Melting
point: 220.4–221.6 °C, 1H NMR d(ppm) 2.84; 3.67; 6.47; 13C
d(ppm) 55.85; 101.90; 108.98; 144.70; 162.62; 171.71. IR spec-
trum (m
/cmꢁ1) 2933 (CAH); 1678 (CAC); 1592 and 1155 (CAS);
1298 (CAOAC). UV/Vis spectrum (kmax/nm) as acetone solution
330; 412; 659. Elemental CHNS Analysis (%): C 51.02 (52.15); H
4.25 (4.84), S 15.12 (13.77).
Cyclic voltammetry
Voltammograms of the samples were recorded on a computer-
controlled Voltalab PGZ402 potentiostat. The cell consisted of a
platinum disk as working electrode, a platinum wire as an auxiliary
electrode and an Ag/AgCl reference electrode. The redox potentials
of the dithiolene complex (0.1 mM) were determined in CH2Cl2
containing 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6) electrolyte under nitrogen.
Results and discussion
Synthesis
The synthetic route proposed by Schrauzer et al. was modified
and used to synthesize the tungsten complex of tris(dithiolene)
starting from 3,5-dimethoxybenzaldehyde in three steps, as shown
in Fig. 2 [37,38]. First, an intermediate 1,2-bis-(3,5-dimethoxy-
phenyl)-2-hydroxy-ethanone was synthesized from 3,5-dime-
thoxybenzaldehyde using a similar method to traditional benzoin
condensation reaction, but at room temperature. The synthesis
used tertiarybutylammonium iodide as an electrolyte and DMF
as the solvent. The desired product was obtained as white crystals
with the melting point of 102–103 °C.
Single crystal X-ray crystallography
Single crystals of MTDT were obtained by recrystallization using
vapor diffusion of diethyl ether into an acetone solution of MTDT.
The crystallographic data of the complex given in Table 1 was col-
lected using a SMART diffractometer [31]. Cell refinement and data
reduction were carried out with SAINT [31]. Data analysis was per-
formed with the SHELXS97 program [32].
The thiolation of the benzoin was exothermic and was accom-
panied by the evolution of H2S. An excessive amount of P2S10
was used and the unreacted P2S10 was recrystallized and elimi-
nated upon cooling. The resulting solution contained thioesters
of the corresponding dithiols and was stable during storage.
Preparation of the metal complex involved mixing the metal
salt solution in HCl (1 M) with the ligand and the resulting mixture
was refluxed. In addition to acting as a solvent, the HCl also hydro-
lyzed the thioester into the corresponding thiols. The unreacted
thioester was quite difficult to separate from the desired complex.
Purification was carried out by addition of hydrazine, and the
resulting compound was purified by silica gel column chromatog-
raphy using benzene as the elutant. The melting point of the dark
green crystals was 220.4–221.6 °C, and the reported melting point
of a similar compound was 250 °C [21]. Micro elemental analysis
for CHNS confirmed the empirical formula of MTDT as C54H54O12S6-
W. The IR spectrum for this complex showed stretching frequen-
Computational details
The isolated-molecule DFT calculations were carried out using
the DMol3 code in Materials Studio 5.5 from Accelrys, which
Table 1
Crystal and structure refinement parameters for MTDT.
Identification code
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system, space group
Unit cell dimensions
C54H54O12S6W
1271.18
298(2) K
0.71073 Å
Monoclinic, P2(1)/n
a = 12.756(2) Å
b = 21.560(3) Å
cies of C@S and CAOAC at 1592 cmꢁ1 and 1297 cmꢁ1
,
c = 24.980(4) Å
b = 103.998(3)°
respectively. The 13C NMR spectrum exhibited six signals corre-
sponding to the four carbon atoms in the benzene ring at d
101.90 ppm, 108.98 ppm, 144.70 ppm and 163.62 ppm, and the
two carbon atoms of the ethylene moiety at d 171.71 ppm. The sig-
nal for the methoxy carbon was recorded at d 55.07 ppm. The 1H
spectrum using the CDCl3 solution exhibited the OCH3 protons as
a singlet at d 2.84 ppm and two aromatic proton signals at around
d 6.46 ppm.
Volume
6666.0(18) Å3
Z, calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
Reflections collected/unique
Completeness to theta = 25.00
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
4, 1.583 Mg/m3
2.463 mmꢁ1
3220
0.49 ꢂ 0.24 ꢂ 0.21 mm
1.26–25.00°
ꢁ13 6 h 6 15, ꢁ25 6 k 6 23, ꢁ29 6 l 6 29
34,059/11,716 [R(int) = 0.0741]
99.8%
Full-matrix least-squares on F2
11,716/0/655
1.051
Crystal structure
R1 = 0.0687, wR2 = 0.1905
R1 = 0.1287, wR2 = 0.2230
The crystallographic data showed the compound adopting a
monoclinic system with space group of P21/n, where
Largest diff. peak and hole
2.443 and ꢁ1.147 (e Åꢁ3
)

K. Arifin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
211
Fig. 2. A schematic diagram showing the synthetic route of MTDT.
a = 12.756(2) Å, b = 21.560(3) Å, c = 24.980(4) Å, b = 103.998(3)°,
Z = 4 with a density of 1.583 Mg/m3 and V = 6666.0(18) Å3. The
crystal system and atomic displacement parameters are shown in
Table 1.
bonding of S atoms confirmed the central W atom to be in that oxi-
dation state.
Optimized geometry
Fig. 3 shows the X-ray structure of MTDT with the numbering
scheme. Three thioester ligands were chelated to the central tung-
sten W1 atom to form a distorted octahedral geometry. The angles
of the W1 atoms were between 80.15° and 137.02°. The WAS bond
lengths were between 2.352(3) and 2.367(3) Å, which are consid-
ered to be in normal range [39]. The CAS bond lengths were
1.710(10) and 1.745(10) Å. The chelated thioesters formed three
5-membered (WS2C2) rings. The presence of OCH3 groups in each
phenyl ring forced the ligands to be twisted around the thiolate
back-bone resulting in a non-planar arrangement. Some of OCH3
groups were found to be disordered due to the position of the
OCH3 on some phenyl rings which were not in the appropriate ori-
entation. However, the disorder had no effect on the distorted octa-
hedron of the tungsten dithiolate backbone. The tungsten atom
remained in oxidation state of VI in the neutral complex molecule.
The WAS bond lengths which were formed by coordinated ion
The selected bond lengths and angles of the optimized structure
of MTDT calculated by PBE on DNP basis set, listed in Table 2, are in
good agreement with the experimental values. The calculation was
run after the disordered carbon atoms from the X-ray data were re-
solved. The total energy changes during the optimization have
been calculated, which are between ꢁ20669.16 Hartrees and
ꢁ20670.26 Hartrees.
Fig. 4 shows a comparison of the X-ray structure and the opti-
mized structure from the top-view. Both the X-ray structure and
the theoretical calculation resulted in a similar geometry, which
was between a distorted octahedron and a trigonal prism. The
changes were observed from simultaneous rotation of OCH3 groups
in some phenyl rings twisted angularly around the thiolate ligands.
Most of the calculated bond lengths were found to be longer than
the experimental data, and only a few were shorter. However, the
calculated results were considered to be in the normal range [39].
The S–W–S angles in two five-member (WS2C2) rings calculated
were found to be narrower than for the X-ray crystal data. Never-
theless, not all of the S–W–S angles between the rings became lar-
ger. Some of them became narrower than the X-ray single crystal.
Some main torsion angles became larger, while some others be-
came smaller. Overall, the calculations resulted in a molecular
structure with more uniform bond lengths and angles compared
to the crystal data parameters.
In the neutral complex molecule, the tungsten atom would be
expected to have an octahedral geometry and had an oxidation
state of VI. However, because dithiolene ligands often exhibit
non-innocent behavior, the VI oxidation state cannot be assured.
The CAS and CAC bond lengths are two parameters that can be
used to determine the complex oxidation state. Significant depar-
ture from the normal bond length of CAS and CAC would be an
indication of ligand reduction or oxidation. X-ray crystallography
and DFT data can be used to determine the non-innocent behavior
of the dithiolene ligands in this system. The CAC bond lengths in
the WS2C2 rings in the crystal structure are typical for CAC double
bond lengths, and all CAS bond lengths are similar and within the
normal (1.733–1750 Å) [39]. Furthermore, the bond lengths did
not change during the optimized structure calculation process,
suggesting the ligands adopted a dithiolate state. This assumption
is supported by cyclic voltammetry (CV) results, as shown in Fig. 5.
The complex shows a reversible redox couple, with oxidation and
reduction peaks at ꢁ0.106 and ꢁ0.197 volts, respectively. The peak
Fig. 3. X-ray crystallographic structure of MTDT with 50% probability ellipsoid.
Hydrogen atoms were omitted for clarity.

212
K. Arifin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
Table 2
Selected bond lengths (A°), bond angles (°) and torsion angles of MTDT from X-ray single crystal analyzes and optimized calculated.
Parameter
MTDT X-ray
MTDT optimized PBE/DNP basis set
MTDT w/o methoxy optimized PBE/DNP basis set
Bond length (A°)
W1AS1
W1AS2
W1AS3
W1AS4
W1AS5
W1AS6
S1AC9
S2AC10
S3AC19
S4AC28
S5AC37
S6AC46
C9AC10
C19AC28
C37AC46
2.352 (3)
2.363 (3)
2.366 (3)
2.360 (3)
2.362 (3)
2.354 (3)
1.737 (11)
1.732 (11)
1.707 (10)
1.727 (11)
1.737 (10)
1.747 (10)
1.367 (14)
1.350 (13)
1.362 (15)
2.388
2.387
2.393
2.393
2.382
2.392
1.737
1.732
1.734
1.734
1.734
1.733
1.392
1.391
1.391
2.394
2.355
2.331
2.447
2.466
2.394
1.736
1.735
1.735
1.735
1.734
1.735
1.392
1.392
1.391
Bond angle (°)
S1AW1AS2
S4AW1AS3
S6AW1AS5
S1AW1AS6
S1AW1AS3
S1AW1AS4
S1AW1AS5
S2AW1AS4
S2AW1AS5
S2AW1AS6
S2AW1AS3
S5AW1AS3
S6AW1AS3
S6AW1AS4
S4AW1AS5
81.41 (10)
81.46 (9)
81.01
81.12
81.06
81.49
82.96
136.33
133.28
81.47
83.098
137.10
133.98
133.06
81.691
135.15
83.31
80.92
81.41
80.79
81.91
83.37
135.65
134.04
81.31
81.25 (10)
80.13 (10)
81.78 (10)
135.45 (11)
134.16 (10)
81.80 (10)
83.21 (10)
135.33 (10)
135.05 (10)
135.65 (10)
81.68 (10)
137.04 (10)
83.85 (9)
82.45
135.62
135.33
135.18
82.21
136.07
82.78
Fig. 4. Comparison of X-ray crystallographic structure (A) and calculated optimized structure of MTDT (B) from a top-view.
separation of the oxidation–reduction wave for the complex was
92 mV, which is a normal value for a process involving one electron
in polar solvent. The characteristics of the CV fit the profile of being
a formal reduction of W(VI) to W(V) [40,41].
concentrated along the WS2C2 rings. The fact that the LUMO comes
from WS2C2 rings are supported by the CV of the compound, which
showed that the formal reduction potential of the complex comes
from W metal center (Fig. 6) with no contribution of the ligands.
The HOMO and LUMO orbital energies were ꢁ0.1970 and
ꢁ0.1606 Hartrees, which were equivalent to ꢁ5.362 eV and
ꢁ4.370 eV, respectively. The band gap energy of the molecule is
therefore, 0.992 eV. The next highest energy level, HOMOꢁ1 and
HOMOꢁ2 as well as LUMO + 1 and LUMO + 2 were also examined.
HOMOꢁ1 and HOMOꢁ2 were located on 3,5-dimethoxyphenyl
rings of the ligands, which were corresponding to ꢁ0.1995 and
ꢁ0.1998 Hartrees, respectively. These orbitals were located on
the other two 3,5-dimethoxyphenyl groups of the ligands.
Meanwhile, the LUMOꢁ1 to LUMO + 2 were located along the
WS2C2 rings, with energy levels of ꢁ0.1184 and ꢁ0.1162 Hartrees,
Orbital configuration
The HOMO/LUMO configurations and frontier molecular orbital
energies of MTDT are depicted in Fig. 6. According to the calcula-
tions, there are 343 molecular orbitals of MTDT complex with
644 total electrons, in which there are 322 occupied and 21 unoc-
cupied molecular orbitals. Fig. 6 shows that the HOMO and the
LUMO involved a separate component on the molecule. The HOMO
was concentrated on the 3,5-dimethoxyphenyl groups of one of the
three bidentate dithiolene ligands, while the LUMO was

K. Arifin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
213
Optical properties
The nature of absorption spectra were investigated via TD-DFT
calculations using PBE on a DNP basis set. Conductor-like screening
model (COSMO) continuum solvation models were used to evalu-
ate the influence of solvent polarity on the absorption spectra
[42]. The calculated absorption spectra of MTDT in vacuo and in
various solvents are shown in Fig. 7, and were compared with
the experimental spectrum (Fig. 8). The solvatochromism effects
were calculated for different dielectric solvents such as benzene
(Fig. 7B), acetone (Fig. 7C), methanol (Fig. 7D) and dimethylsulfox-
ide (Fig. 7E). All of the optical calculations were performed on the
optimized geometry structure of MTDT. The spectra are slightly
different with the solvent effect shifting.
Fig. 5. The CV of MTDT collected in DCM/TBA(PF6) with scan rate 10 mV sꢁ1 at 20 °C
versus Fc+/Fc internal standard.
Fig. 6. The HOMO and LUMO orbital energies of MTDT.
respectively. The molecular orbital configuration clearly shows
that the main charge transfer band in the MTDT complex is due
to a charge transfer from the 3,5-dimethoxyphenyl group to the
WS2C2 ring, which is corresponding to the ligand-to-metal charge
transfer (LMCT) and ligand to ligand charge transfer (LLCT).
Fig. 7. Optical spectra of MTDT, (A) calculated in vacuo, (B) calculated in benzene,
(C) calculated in acetone, (D) calculated in methanol and (E) calculated in
dimethylsulfoxide.

214
K. Arifin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
Table 3
Experimental electronic absorption data of MTDT and similar molecules in acetone–
water (70:30) mixed solvent.
Metal
(M)
R0
R
L ? M kmax (nm)/
(Mꢁ1 cmꢁ1
e
L ? M kmax (nm)/
e
)
(Mꢁ1 cmꢁ1
)
W
o,o-
o,o-
660 (25,000)
685 (18,500)
412 (12,600)
413 (10,250)
CH3OPh CH3OPh
Ph
Wa
p-
CH3OPh
Ph
p-
Wa
Wa
Ph
p-
654 (28,100)
679 (16,600)
416 (22,500)
405 (7000)
CH3OPh CH3OPh
a
Ref. [43]
Fig. 8. Experimental spectrum of MTDT in acetone.
In vacuo (Fig. 7A), there were several absorption peaks in the
visible region with the highest oscillator strength peak was at
approximately 800 nm corresponding to the transition of
HOMOꢁ1 to LUMO. This represents ligand-to-metal charge trans-
fer (LMCT). The lowest energy absorption peak appeared at
1050 nm and was attributed to the transition of an electron from
HOMO to LUMO. The small absorption peak at approximately
650–700 nm are supposed to be d–d transition in visible spectrum.
The solvatochromism calculation of MTDT in benzene with
found on the frontier orbital molecule configuration and the optical
properties only.
Fig. 9 shows the HOMO and LUMO of MTDT without OCH3
groups. The LUMO was found along the WS2C2 rings and was un-
changed relative to the original crystal structure. The HOMO con-
figuration was different from the HOMO of the original MTDT
and was found at the WS2C2 rings and a part of phenyl rings of
the ligand. The calculations showed that the OCH3 groups in the
MTDT acted as electron donors. The effect on the theoretical optical
spectrum of MTDT without OCH3 groups is shown in Fig. 10. The
maximum absorption was at approximately 800 nm, narrower
than the MTDT spectrum area.
dielectric constant, K = 2.284, resulted in
a slightly different
absorption spectrum compared to in vacuo. The absorption spectra
in benzene gave a broad peak of approximately 800–900 nm, due
to the similar intensity of three peaks in overlapping positions. A
new peak appeared at 950 nm, and the lowest energy absorption
peak was found at 1100 nm. All peaks in benzene were shifted
approximately 50 nm compared with in vacuo. Furthermore, the
calculated spectrum in acetone (K = 20.7) showed the lowest en-
ergy absorption peak, which was also at 1100 nm. However, pat-
tern of the absorption between 800 and 900 nm was slightly
different when calculated in vacuo and in benzene, where the
broad peak was split into two peaks. Moreover, the calculations
in methanol (K = 32.63) and dimethylsulfoxide (K = 46.7) also gave
similar spectra. These spectra were similar with the spectrum in
acetone with blue shifts at approximately 100 nm. The highest
oscillator strength peak appeared at 920 nm, while the lowest en-
ergy absorption peak was at 1200 nm. The COSMO optical calcula-
tions of MTDT in various solvents showed that the absorption
spectra gradually blue shifted with the increase of the solvent
dielectric constant. The solvation effect was only found in solvents
with lower dielectric constants than methanol, and no absorption
changes were found in solvents with dielectric constants higher
than methanol.
Conclusion
A
tris{1,2-bis(3,5-dimethoxylphenyl)-1,2-ethylenodithiolenic-
S,S0}tungsten complex (MTDT) was successfully synthesized and
Fig. 9. The HOMO and LUMO orbital energies of MTDT without OCH3 groups.
Table 3 is the experimental absorption of MTDT and similar
molecules from the literature [43]. The strong LMCT peaks appear
at 600 nm and are shifted approximately 100 nm to higher energy
compared to the calculated result. The lowest energy absorption
indicated by the calculation at approximately 1100 nm could not
be observed due to instrument limitations.
Ligand substitution effect
To study the effect of ligand substitution on the structure, spec-
tra and molecular orbital (MO) energy properties of MTDT, the
OCH3 groups on the phenyl rings of the ligands were removed,
and the structure was optimized using the same calculation
parameters. Removal of the OCH3 groups did not affect the original
structure geometry. Only a small effect on the phenyl ring orienta-
tion was observed, as seen from the torsion angles. The effect was
Fig. 10. Optical spectrum calculated of MTDT without OCH3 groups in vacuo.

K. Arifin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 208–215
215
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DFT calculations of the optimized structure showed the molecular
geometry of MTDT to be in between distorted octahedral and trigo-
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The authors would like to acknowledge the Malaysian Ministry
of Science, Technology and Innovation for sponsoring this project
under IRPA project 02-02-02-0006 PR0023/11-11 and UKM-GUP-
BTT-07-30-190.
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