Syntheses, structures, spectroscopic, electrochemical properties and DFT calculation of Ru(II)-thioarylazoimidazole complexes

Article abstract of DOI:10.1016/j.jorganchem.2009.06.021

The reaction of 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazoles (SRaaiNR) (2a/2b) with Ru(II) has synthesized [Ru(SRaaiNR)₂](ClO₄)₂ (3a/3b) in 2-methoxyethanol. The reaction in methanol, however, has synthesized [Ru(SRaaiNR)(

Full text of DOI:10.1016/j.jorganchem.2009.06.021

Journal of Organometallic Chemistry 694 (2009) 3518–3525
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Syntheses, structures, spectroscopic, electrochemical properties and DFT
calculation of Ru(II)–thioarylazoimidazole complexes
T.K. Mondal a, J.-S. Wu b, T.-H. Lu b, Sk. Jasimuddin c, C. Sinha a,
*
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b National Tsing Hua University, Hsinchu, Taiwan 300, ROC
c Assam University, Silchar, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazoles (SRaaiNR) (2a/2b) with Ru(II) has synthe-
sized [Ru(SRaaiNR)2](ClO4)2 (3a/3b) in 2-methoxyethanol. The reaction in methanol, however, has
synthesized [Ru(SRaaiNR)(SRaaiNR)Cl](ClO4) (4a/4b). The solid phase reaction of SRaaiNR and RuCl3 on
silica gel surface upon microwave irradiation has synthesized [Ru(SRaaiNR)(SaaiNR)](PF6) (5a/5b)
[SRaaiNR represents tridentate N,N0,S-chelator; SRaaiNR is N,N0-bidentate chelator where S does not coor-
dinate and SaaiNR refers N,N0,S-chelator where S refers to thiolato binding]. The structural characteriza-
tion of [Ru(SEtaaiNEt)(SEtaaiNEt)Cl](ClO4) (4b) and [Ru(SEtaaiNEt)(SaaiNEt)](PF6) (5b) has been
confirmed by single crystal X-ray diffraction study. The IR, UV–Vis, and 1H NMR spectral data also support
the stereochemistry of the complexes. The complexes show metal oxidation, Ru(III)/Ru(II), and ligand
reductions (azo/azoꢀ, azoꢀ/azo@). The molecular orbital diagram has been drawn by density functional
theory (DFT) calculation. Normal mode of analysis has been performed to correlate calculated and exper-
imental frequencies of representative complexes. The electronic movement and assignment of electronic
spectra have been carried out by TDDFT calculation both in gas and acetonitrile phase.
Received 16 February 2009
Received in revised form 15 June 2009
Accepted 18 June 2009
Available online 24 June 2009
Keywords:
Ruthenium(II)–azoimidazoles
X-ray structure
Electrochemistry and DFT calculation
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
der microwave irradiation. Metal mediated cleavage of carbon–sul-
fur bonds into carbon–hydrogen or carbon–carbon bonds under
The coordination complexes of transition metals with azo-li-
gands are of current attraction due to the interesting physical,
chemical, photophysical and photochemical, catalytic, biological
homogeneous or heterogeneous condition have diverse application
in synthetic chemistry, bioinorganic chemistry and petroleum
industry [20–22]. The reaction condition has influenced the effi-
ciency, rate and nature of the products. The electronic structures
of the precursor and product have been calculated by density func-
tional theory (DFT). The spin-allowed singlet–singlet electronic
transitions of the studied compounds have been calculated with
the time dependent DFT method (TDDFT method), and a good
agreement with the experimental spectra has been observed.
and different material properties. The p-acidity and metal binding
ability of azo nitrogen have drawn attention to the exploration of
the chemistry of metal complexes incorporating azo-ligands [1–
8]. Notable examples of these ligands are arylazobenzene [9],
arylazooxime [10], arylazophenol [11], arylazopyridine [7,8,12],
arylazoimidazole [13], arylazopyrimidine [14], arylazoaniline [15],
azoantipyrene [16] and related ligands. We are engaged for last
decade in the designing of azo-conjugated ligands and their metal
complexes [13,14]. The synthesis of ligands in the framework of dii-
mine (–N@C–C@N–) [17,18] and azoimine functions (–N@N–C@N–)
[13] are of interest in the recent years of chemical research. Re-
cently we have synthesized thioarylazoimidazoles (1 and 2 in
Scheme 1) [19], a tridentate N(imidazole), N(azo) and S(thioether)
chelating molecule. In this work we report ruthenium(II) com-
plexes of 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazoles (SRaaiNR)
and their reactions. We observe also ruthenium mediated selective
C–S bond cleavage when reaction is carried out on silica surface un-
2. Results and discussion
2.1. Synthesis and formulation
Thioarylazoimidazole belongs to tridentate N,N0,S donor system.
They are synthesized (Scheme 1) by coupling o-(thioalkyl)phen-
yldiazonium ions with imidazole in aqueous sodium carbonate
and purified by solvent extraction and chromatographic process
[19]. The alkylation is carried out by adding alkyliodide (MeI, EtI)
in dry THF solution to the corresponding 2-{o-(thioalkyl)phenyl-
azo}imidazole (2) in the presence of sodium hydride. They are
abbreviated as SRaaiNR. The donor centres are N(imidazole), (N),
N(azo) (N0) and thioether (S).
* Corresponding author. Fax: +91 033 2413 7121.
E-mail addresses: tkmondal_ju@yahoo.com (T.K. Mondal), c_r_sinha@yahoo.com
(C. Sinha).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.06.021

T.K. Mondal et al. / Journal of Organometallic Chemistry 694 (2009) 3518–3525
3519
5
4
1
N
/
+
-
N
N
N
N
N
H
3
N
2 Cl
R
HN
N
R-I/NaH
N
S
N
N
N
R
in dry THF
in Na2CO3
S
S
11
10
S
R
R
R
8
9
(1)
2-{o-(thioalkyl)phenylazo}imidazole (SRaaiNR)
(R = Me, (2a) and Et, (2b))
2-Methoxyethenol
reflux
N
Silica gel
+ RuCl3
2
Gummy mass
Brown red gummy mass
N
Evaporated in
rotaevaporator
S
Microwave, CH2Cl2
extract
Me/Et
Eluted by acetonitrile
solution of NH4PF6
Aq. NaClO4
Dissoved in
MeOH
MeOH reflux
Aq. NaClO4
N
N
N
N
Ru
S
N'
N
N'
N
N'
Ru
(PF6)
S
Ru
(ClO4)
(ClO4)2
Cl
S
S
N'
S
Me/Et
N'
N'
Me/Et
S
Me/Et
Me/Et
Me/Et
(3a/3b)
(5a/5b)
(4a/4b)
[Ru(SRaaiNR)2](ClO4)2 : R = Me (3a), Et (3b); [Ru(SRaaiNR) (SRaaiNR)Cl](ClO4) : R = Me
(4a), Et (4b); [Ru(SRaaiNR) (SaaiNR)](PF6) : R = Me (5a), Et (5b)
Scheme 1.
[Ru(SRaaiNR)2](ClO4)2 (3a/3b) are prepared by refluxing RuCl3
[19] which supports efficient back donation, d
p
(Ru(II)) ?
p
*(azo).
and SRaaiNR (2 equivalent of ruthenium) in 2-methoxyethanol at
N2 atmosphere and precipitation by NaClO4 (vide Section 3). The
reaction of RuCl3 and SRaaiNR in 1:2 mole proportion in methanol
and precipitation of the products by NaClO4 give the compound
[Ru(SRaaiNR)(SRaaiNR)Cl](ClO4) (4a/4b) [SRaaiNR represents
tridentate N,N0,S-chelator and SRaaiNR as N,N0-bidentate chelator
where S does not coordinate]. The molar conductance measure-
The normal mode analysis of DFT computed optimized structure
has calculated stretching frequencies. In general the computed fre-
quencies of functional groups appeared at lower frequency region;
for example calculated
m
(N@N) appears at ꢁ1360 cmꢀ1 while ob-
served frequency is ꢁ1380 cmꢀ1 and theoretical
1560 cmꢀ1 and experimental value is ꢁ1585 cmꢀ1. The
m
(C@N) is
m
(ClO4) ap-
pears at 1090–1094 cmꢀ1 and a weak band at 625–630 cmꢀ1 for 3
ments of the compounds
3
and
4
in MeCN support 1:2
and 4; m
(PF6) appears at 840–842 cmꢀ1 for 5.
(KM = 150–160
X
ꢀ1 molꢀ1 cmꢀ1) and 1:1 (KM = 80–90
X
ꢀ1 molꢀ1
The 1H NMR spectra of the complexes are recorded in CDCl3
(proton numbering pattern is shown in Scheme 1) and protons
are assigned on the basis of spin–spin interaction. Data are set
out in Supplementary Table S1. The alkylation of imidazole is sup-
ported by disappearance of d(N–H) at ꢁ10.30 ppm (of 1) and the
appearance of alkyl signal at 1.5–4.5 ppm for 2: The N(1)–Me
(2a) appears as singlet at ca. 4.04 ppm; N–CH2–CH3 (2b) shows a
quartet for –CH2– at ca. 4.46 (9.0 Hz) and a triplet at ca. 1.48
(8.0 Hz) ppm. Imidazoles 4-H and 5-H appear as broad singlet at
7.25–7.29 and 7.07–7.14 ppm, respectively. Thiomethyl group
(–S–Me) also exhibits a singlet at 2.42 ppm in SMeaaiNMe (2a).
The SEtaaiNEt (2b) shows two quartets [4.46 (9.0 Hz) and 2.99
(8.0 Hz) ppm for –CH2– protons of N–CH2–(CH3) and S–CH2–
(CH3), respectively] and two triplets [at 1.48 (8.0 Hz) and 1.31
(8.0 Hz) ppm for –(N–CH2)CH3 and –(S–CH2)CH3, respectively].
Thiophenyl protons (8-H to 11-H) have been influenced by S–R
substituents. The 11-H and 8-H appear as a doublet at ca. 7.80–
7.86 (8.0 Hz) and 7.18–7.22 (8.0 Hz) ppm, respectively. The closer
proximity of electron withdrawing –N@N– group may be the rea-
son for higher d of 11-H.
cmꢀ1) electrolyte, respectively. Besides, microanalytical data and
spectral data support the composition. The structural confirmation
in case of 4b has been carried out by single crystal X-ray diffraction
measurement.
RuCl3 and SRaaiNR (1:2 mole ratio) in methanol is absorbed on
activated silica gel surface (60–120 mesh) and dried in air. Dry
mass is then irradiated (frequency 450 W) with microwave. CH2Cl2
extract is purified by chromatography using MeCN solution of
NH4PF6 as eluent. The compound is then characterized by different
analytical techniques and structure is confirmed in one case by sin-
gle crystal X-ray diffraction study. There are two tris-chelated li-
gands about Ru(II) and one of them shows elimination of R group
from S–R motif. Thus the solid surface activated reaction shows
C–S cleavage which is denoted SaaiNR and the compound is abbre-
viated as [Ru(SRaaiNR)(SaaiNR)](PF6) (5a/5b).
2.2. Infrared and 1H NMR spectra
Infrared spectra of the complexes exhibit m(N@N) and m(C@N) at
1376–1386 and 1584–1592 cmꢀ1, respectively (see Section 3.3).
In the complexes the aryl protons (8-11 H) suffer downfield
shifting by 0.2–0.4 ppm while imidazole 4-H and 5-H are shifted
to downfield by 0.4–0.7 ppm relative to free ligand values. The
The azo (–N@N–) stretching is significantly shifted to lower fre-
quency region compared to free ligand value (1400–1410 cmꢀ1
)

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T.K. Mondal et al. / Journal of Organometallic Chemistry 694 (2009) 3518–3525
Table 1
downfield shift of aromatic protons is in support of electron den-
sity shifting away from the ligand which could possible by coordi-
nation of the ligands to Ru(II). The R groups in –S–R show very
interesting signal modulation. The R groups of coordinated S (3a/
3b) show 0.2 ppm downfield shifting. The presence of uncoordi-
nated –SR groups in 4a/4b are also supported by appearance of
two signals. Thus 1H NMR signal pattern supports retention of
structures in solution-phase also.
Selected bond distances (Å) and bond angles (°) of [Ru(SEtaaiEt)2Cl]ClO4 (4b) and
[Ru(SEtaaiEt)(SaaiEt)]PF6ꢂ(OH2)2 (5b) on X-ray crystallography and singlet ground
state geometries.
(4b)
(5b)
X-ray data
Theo.
X-ray data
Theo.
Ru–N(1)
Ru–N(3)
Ru–N(5)
Ru–N(7)
Ru–S(1)
Ru–Cl(1)
N(1)–N(2)
N(5)–N(6)
N(1)–Ru–N(3)
N(5)–Ru–N(7)
S(1)–Ru–N(5)
2.007(2)
2.064(3)
1.976(3)
2.037(3)
2.3412(10)
2.3731(9)
1.278(3)
1.286(3)
75.68(10)
77.40(11)
84.39(8)
2.011
2.048
2.042
2.074
2.414
2.424
1.325
1.320
78.10
76.39
91.96
Ru–N(1)
Ru–N(4)
Ru–N(5)
Ru–N(8)
Ru–S(1)
Ru–S(2)
N(3)–N(4)
N(7)–N(8)
N(1)–Ru–N(4)
N(5)–Ru–N(8)
S(1)–Ru–N(4)
S(2)–Ru–N(8)
2.009(6)
1.920(6)
2.079(6)
1.959(5)
2.3484(19)
2.357(2)
1.344(6)
1.304(6)
76.5(2)
2.038
2.978
2.104
2.029
2.382
2.465
1.333
1.328
77.58
76.83
83.48
83.87
2.3. Molecular structure
2.3.1. [Ru(SEtaaiNEt)(SEtaaiNEt)Cl](ClO4) (4b)
The molecular structure of [Ru(SEtaaiNEt)(SEtaaiNEt)Cl](ClO4)
(4b) is given in Fig. 1. The coordination sphere around Ru is dis-
torted octahedral as revealed from the metric parameters (Table
1). Out of two chelating ligands one acts as tridendate N,N0,S (imid-
azole-N (N(7)), azo-N (N(5)), thioether-S (S(1)) and other is
N,N0-bidentate chelator (N(1), N(3)). Sixth coordination position
is occupied by Cl. ClOꢀ4 balances charge of the compound. The N–
Ru–N0 chelate angles are \N(5)–Ru–N(7), 77.40(11)° and \N(1)–
Ru–N(2), 75.68(10)° for two different ligands. Remaining chelate
angle is \N(5)–Ru–S(1), 84.39(8)°. The distortion from octahedral
geometry is certainly due to the acute chelate bite angle (ꢁ76°).
The planes constituted by tridentate N,N0,S ligand is deviated sig-
nificantly from planarity (>0.09 Å) than the chelate plane of biden-
tate N,N0-ligand. Each chelate ring is in good plane and no atom
deviating by more than 0.07 Å. The stereochemical arrangement
is cis-N(imidazole) (\N(3)–Ru–N(7), 97.22(11)°) and cis-N(azo)
(\N(1)–Ru–N(5), 99.22(10)°). The coordinated S and Cl are also
in cis-configuration (\Cl(1)–Ru–S(1), 92.46(3)°). Atom S2 is disor-
dered over two sites (S2A and S2B).
76.5(2)
83.62(19)
83.41(17)
ing of dp(Ru) electrons to p
*(azo) of azoimine ligand [23,24]. The
N@N distances are N(1)–N(2), 1.278(3); N(5)–N(6), 1.286(3) Å
which are elongated slightly from that of free ligand value [25].
The coordination can lead to decrease in the N–N bond order due
to both
character having more pronounced effect. The elongation in the
N–N distance is the indication of the existence of considerable
bonding with major involvement of the azo group. The Ru–Cl(1)
distance 2.3865(13) Å appears in the reported range [24].
r-donor and p-acceptor characters of the ligand – the later
p-
2.3.2. [Ru(SEtaaiNEt)(SaaiNEt)](PF6) (5b)
The molecular structure is shown in Fig. 2 and selected bond
parameters are set out in Table 1. Ru is in distorted octahedral po-
sition, which is revealed from the metric parameters. Both the li-
gands serve as tridentate N,N0,S-chelator. Significant difference
between two ligands is the cleavageꢀof one S–Et group and forma-
tion of thiolato (Sꢀ) donor centre. PF6 balances charge of the com-
pound. The donor groups are associated with cis-N(imidazole),
trans-N(azo) and cis-S. The N–Ru–N0 chelate angles are \N(1)–
Ru–N(4), 76.5(2)° and \N(5)–Ru–N(8), 76.50(2)° for two different
ligands. Other chelate angles are \N(4)–Ru–S(1), 83.62(19)° and
\N(8)–Ru–S(2), 83.41(17)°.
The Ru–N(imidazole) distances are different for the two coordi-
nated chelating ligands (Ru–N(3), 2.064(3) and Ru–N(7),
2.037(3) Å). The shorter Ru–N distance for tridentate chelating li-
gand is due to the presence of Ru–S(1) bond trans to Ru–N(7) bond,
enhancing Ru(dp) ? N(pp) back donation. Similarly, the Ru–
N(azo) distances: Ru–N(5) (1.976(3) Å) is shorter than Ru–N(1)
(2.007(2) Å) because of the presence of Cl(1) trans to N(1). It is
interesting to note that Ru–N(azo) distances are shorter than Ru–
N(imidazole) which is mainly due to involvement of p-backbond-
The Ru–N(imidazole) distances are Ru–N(1), 2.009(6) and
Ru–N(5), 2.079(6)
Å and Ru–N(azo) distances are Ru–N(4),
Fig. 1. ORTEP plot of 4b+, H atoms have been omitted.
Fig. 2. ORTEP plot of 5b+, H atoms have been omitted.

T.K. Mondal et al. / Journal of Organometallic Chemistry 694 (2009) 3518–3525
3521
1.920(6) and Ru–N(8), 1.959(5) Å). The Ru–N distances in
Ru(SEtaaiNEt) chelate is longer than Ru(SaaiNEt) chelate system.
The Ru–N(azo) distances are smaller than Ru–N(imidazole) dis-
tances. The N@N distances are N(3)–N(4), 1.344(6); N(7)–N(8),
1.304(6) Å those are longer than azo distance of 4b. The presence
In 4b the HOMO, HOMOꢀ1 and HOMOꢀ3 have 29–43% Ru and
50–53% ligand along with small contribution from Cl. The
HOMOꢀ2 and HOMOꢀ4 to HOMOꢀ10 are constructed by the
azo-ligand (70–95%). The LUMO has 87% ligand (N@N, 53) charac-
ter and the HOMOꢀLUMO energy gap is 2.54 eV. The other low en-
of thiolato Ru–S bond may be the reason for better
p–back bond-
ergy LUMOs (LUMO+1 to LUMO+5) predominantly have ligand(p*
)
ing. Thioether coordination (Ru–S(1), 2.3484(19) Å) does not differ
significantly to Ru–S(thiolato) (Ru–S(2), 2.357(2) Å) distance.
character. The HOMOs and LUMOs for 5b follow the similar char-
acter as 3b and 4b with HOMOꢀLUMO gap of 1.96 eV. The results
in acetonitrile phase are more or less following same sequence of
gas phase calculation. The results of calculation are comparable
with reported data [26].
2.4. DFT calculation
The molecular structures of 3b, 4b and 5b are optimized by
using B3LYP/DFT calculations of G03 programme. The computed
structures well reproduce the experimental structures for 4b and
5b (Table S2). Theoretical Ru–N bond lengths are about 0.01–
0.07 Å longer than that of observed one. The experimental Ru–Cl
distances are shorter by 0.051 Å than theoretical data for 4b. The
Ru–S distances are also elongated in calculated structures by
0.03–0.07 Å.
The orbital energies along with contributions from the ligands
and metal are given in Supplementary Table S3 and Fig. 3 depicts
the features of some selected occupied and unoccupied frontier
orbitals. Energy level correlations along with contribution of
molecular orbitals are given in Fig. 4. The electronic structures of
the complexes are characterized by high degree of mixing between
2.5. TDDFT calculations and spectral analysis
The electronic absorption spectra are measured at room tem-
perature in acetonitrile, and the experimental absorption bands
are assigned using the singlet-excited states calculated with
TDDFT/CPCM method. Absorption spectra of the complexes in ace-
tonitrile are shown in Fig. 5 and data are collected in Table 2. Some
of the calculated excitation wavelength and their assignment are
given in Table 3 and full detail is given in Table S4 in Supplemen-
tary material. As seen, TDDFT calculations well reproduce the
absorption spectrum of the complexes measured in acetonitrile.
A high intense transition band, at <400 nm originates predomi-
nately in the HOMOꢀ4/HOMOꢀ5 ? LUMO/LUMO+1 transition,
and mainly intra or inter-ligand charge transfer transitions
Ru(dp) and ligand-p/pp orbitals. The three low energy occupied
molecular orbitals (HOMO to HOMO-2) of 3b have 24–27% Ru
and 73–76% ligand character. The HOMOꢀ3 is mainly contributed
by ligand while HOMOꢀ4 and HOMOꢀ6 have 32–43% Ru along
with 57–68% ligand character. The orbitals, HOMOꢀ7 to
HOMOꢀ10 are mostly contributed by ligand group of orbitals.
The LUMO has 90% ligand character and mainly concentrated on
azo-p* orbitals (48%) and an energy gap with HOMO of 2.40 eV in
gas phase calculation. The next higher energy unoccupied molecu-
lar orbitals (HOMOꢀ1, HOMOꢀ3 to HOMOꢀ5) also have 79–91% li-
gand contribution whereas in HOMOꢀ2 has an increased amount
of Ru character (42%).
(n ? p* and
p ? p
*). Visible region spectra of the complexes also
exhibit broad intense transitions at 450–470 nm and 510–
560 nm. These bands arise due to HOMOꢀ2/HOMOꢀ1 ? LUMO/
LUMO+1 transitions. In addition to these intense bands the com-
plexes 3b and 5b show low energy weak transition at 730–740
and 925–935 nm, respectively. The TDDFT/CPCM result predicts
that it originates in the HOMO ? LUMO/LUMO+1 transition. Since,
HOMO, HOMOꢀ1 and HOMOꢀ2 are metal and ligand characteris-
tics, and the LUMO/LUMO+1 completely concentrated on ligand;
the low energy transitions can be assign as mixed MLCT and ILCT
Fig. 3. HOMO, HOMOꢀ1 and LUMO of 3b, 4b and 5b in gas phase (isosurface cutoff value = 0.03).

3522
T.K. Mondal et al. / Journal of Organometallic Chemistry 694 (2009) 3518–3525
Fig. 4. Energy level co-relation diagram in gas phase (left) and in acetonitrile (right).
with scan rate (50–250 mV sꢀ1). One electron stoichiometry of
the couple has been confirmed by the current height measurement
by DPV and on comparing with couple of [FeðCNÞ46ꢀ]/[FeðCNÞ63ꢀ].
The DFT calculations show that HOMO of the complexes has Ru
contribution and ligand contributes to constitute LUMO (major
participation from azo group) (Table S3, Supplementary material).
Thus the oxidation may be assigned to the involvement of metal
orbitals and the reduction is azo function of the ligands. Ru(III)/
Ru(II) redox couples in 5a–5b are found lowest than that of 3a–
3b and 4a–4b which reflect higher electron influx on metal centre
in 5a/5b compare to 3a/3b and 4a/4b (Fig. 6). The calculated Mul-
liken charge on Ru is considerably lower than the formal charge +2
and is found 0.540, 0.439 and 0.366 in complex 3b, 4b and 5b,
respectively (Supplementary Table S5). Data in Supplementary Table
S3 show that the stability order of HOMOs of the complexes is
3b(ꢀ11.13 eV) > 4b(ꢀ8.28 eV) > 5b(ꢀ7.61 eV) and so the metal
oxidation follows the reverse order. Thiolato-S is certainly better
Fig. 5. Absorption spectra of (3b)(-.-.-), (4b) (—) and (5b) (———) in acetonitrile.
r
-donor than thioether-S and could be responsible for lowest po-
tential in the complexes 5. Azo reductions are also equally affected
by number of Ru–S bonds, thioether-S or thiolato-S i.e., E(azo/azo–)
and E(azo–/azo@) data are anodically shifted as 5 ? 4 ? 3.
origin or a delocalized MLLCT (metal–ligand to ligand charge trans-
fer) description can be used.
The intensity of these transitions has been assessed from oscil-
lator strength (f). In both gas and acetonitrile phases the longest
wavelength band appears in NIR region, >700 nm. This transition
is weak as evident from very low f value. Transition of high f is ob-
served at 380–420 nm (f > 0.1) and have been assigned
transition.
2.7. Conclusion
1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazoles are used to syn-
thesise ruthenium(II) complexes. Reaction phase and solvents
influence the chemical characteristic of the products. In high boil-
ing solvent, 2-methoxyethanol, the reaction separates the product
where ligand behaves as tridentate N,N0,S-chelator while in meth-
anol (relatively low boiling solvent) one of the two chelating li-
gands acts as bidentate N,N0-chelator. The reaction in silica
surface at solid state upon microwave irradiation has isolated a
product that shows C–S cleavage. The complexes show intense
MLCT transition along with ILCT and affect the metal redox prop-
erty. The DFT and TDDFT computation have been carried out to
p–
p*
2.6. Electrochemistry
The redox data of the complexes are summarised in Table 2. The
complexes show Ru(III)/Ru(II) couple (Fig. 6) when scanned in the
potential range 0.0–2.0 V. The response is quasi reversible in nat-
ure as it is evident from their peak-to-peak separation
(DEp > 100 mV). The nature of voltammogram does not change
Table 2
UV–Vis spectraa and cyclic voltammetricb data
Complex
kmax (nm)a (10ꢀ3 2 Mꢀ1 cmꢀ1
)
Cyclic voltammetric datab
E
V (
D
Ep, mV)
Ligand reduction, V (DEp, mV)
Ru(III)/Ru(II),
(3a)
(3b)
(4a)
(4b)
(5a)
(5b)
730(1.82), 510(6.28), 400(14.54)
740(1.88), 515(5.87), 395(14.22)
558 (5.50), 466 (5.97), 399(11.87)
556 (5.05), 468 (5.24), 399 (10.06)
922 (1.34), 546 (5.04), 456 (7.97), 390 (13.74)
932 (1.26), 550 (4.75), 462 (6.25), 388 (12.12)
1.48 (110)
1.49 (110)
1.30(110)
1.32(110)
1.21 (110)
1.22 (110)
ꢀ0.52(80), ꢀ0.84(90), ꢀ1.42(130)
ꢀ0.48(80), ꢀ0.88(90), ꢀ1.44(140)
ꢀ0.44(70), ꢀ0.80(95), ꢀ1.36(150)
ꢀ0.45(80), ꢀ0.81(90), ꢀ1.38(145)
ꢀ0.78(80), ꢀ0.86(90), ꢀ1.38(140)
ꢀ0.79(70), ꢀ0.97(90), ꢀ1.40(160)
a
In MeCN.
b
Solvent: MeCN, Pt-disk working electrode, SCE reference and Pt–wire auxiliary electrode, [n-Bu4N](ClO4) supporting electrolyte, scan rate 50 mV sꢀ1, E = 0.5 (Epa + Epc) V,
D
Ep = (EpaꢀEpc), mV, Epa = anodic peak potential, Epc = cathodic peak potential.

T.K. Mondal et al. / Journal of Organometallic Chemistry 694 (2009) 3518–3525
3523
Table 3
Selected list of calculated (TDDFT) excitation energies for [Ru(SEtaaiEt)2] (ClO4)2(3b), [Ru(SEtaaiEt)(SEtaaiEt)Cl]ClO4(4b) and [Ru(SEtaaiEt)(SaaiEt)]PF6 (5b) in acetonitrile
Excited state
E (eV)
k, nm(f ꢃ 103)
Major contribution
Charactera
3b
1
2
5
12
14
1.61
1.65
2.48
2.99
3.12
770.6(18.4)
750.8(20.0)
499.4(122.1)
413.4(321.9)
397.4(492.3)
(82%)HOMO ? LUMO
(81%)HOMO ? L+1
(32%)Hꢀ2 ? L+1, (26%)Hꢀ1 ? LUMO
(33%)Hꢀ3 ? L+1, (21%)Hꢀ3 ? LUMO
(37%)Hꢀ5 ? L+1, (18%)Hꢀ5 ? LUMO
MLCT, ILCT
MLCT, ILCT
ILCT
ILCT
MLCT, ILCT
4b
3
6
7
8
2.02
2.15
2.39
2.71
2.98
3.04
612.4(9.4)
575.6(8.5)
518.5(129.2)
457.6(107.8)
415.1(200.6)
406.9(151.3)
(35%)Hꢀ2 ? L+1, (24%)Hꢀ1 ? L+1
(39%)Hꢀ3 ? LUMO, (21%)Hꢀ2 ? L+1
(52%)Hꢀ2 ? LUMO
MLCT, ILCT
MLCT, ILCT
MLCT, ILCT
MLCT, ILCT
ILCT
(51%)Hꢀ3 ? L+1
10
11
(62%)Hꢀ4 ? LUMO
(36%)Hꢀ4 ? L+1
ILCT
5b
2
4
5
7
1.32
2.18
2.28
2.61
3.08
3.16
937.4(23.2)
569.0(16.5)
543.8(23.3)
473.8(213.6)
401.7(225.0)
392.3(555.2)
(84%)HOMO ? L+1
MLCT, ILCT
MLCT, ILCT
MLCT, ILCT
MLCT, ILCT
ILCT
(37%)Hꢀ2 ? LUMO, (35%)Hꢀ1 ? L+1
(35%)Hꢀ3 ? LUMO, (18%)Hꢀ1 ? LUMO
(35%)Hꢀ2 ? L+1, (26%)Hꢀ3 ? LUMO
(64%)Hꢀ4 ? L+1
12
13
(48%)Hꢀ5 ? LUMO
ILCT
a
ILCT = intra/inter-ligand charge transfer; MLCT = metal to ligand charge transfer.
kin–Elmer; model Lambda 25. IR spectra (KBr disk, 4000–
450 cmꢀ1), Perkin–Elmer; model RX-1; 1H NMR spectra, Bruker
(AC) 300 MHz FTNMR spectrometer. Electrochemical measure-
ments were performed using computer-controlled PAR model
250 VersaStat electrochemical instruments with Pt-disk electrodes.
All measurements were carried out under nitrogen environment at
298 K with reference to SCE in acetonitrile using [n-Bu4N][ClO4] as
supporting electrolyte. The reported potentials are uncorrected for
junction potential.
3.3. Synthesis of complexes
3.3.1. Synthesis of [Ru(SMeaaiNMe)2](ClO4)2 (3a) and [Ru(SMeaai-
NMe)2(Cl)] (ClO4) (4a)
Fig. 6. Cyclic voltammogram of 3b (- - - -), 4b (—) and 5b (ꢂ ꢂ ꢂ ꢂ ꢂ) in acetonitrile.
RuCl3ꢂ3H2O (0.27 g, 1.02 mmol) was dissolved in 20 ml 2-
methoxyethanol and refluxed for an hour under N2 atmosphere.
To the resulting green solution 10 ml 2-methoxyethanolic solution
of 1-methyl-2-[o-(thiomethyl)phenylazo]imidazole (SMeaaiNMe)
(0.5 g, 2.15 mmol) was added and reflux for another 4 h. The sol-
vent was completely removed using rotaevaporator. Brown gum-
my mass so left was dissolved in minimum volume of methanol
and aqueous solution of NaClO4 was added to precipitate the prod-
uct. The precipitate was filtered and washed with cold water, and
finally dried in vacuo over P4O10. The dry mass was then dissolved
in minimum volume of CH2Cl2 and subjected to chromatography
on a silica gel column (60–120 mesh). A red band was eluted with
C6H6–CH3CN (5:1, v/v). This was collected and evaporated slowly
in air. The yield was 65%.
explain the electronic structure and vibrational, electronic spectra
and redox properties of the complexes.
3. Experimental
3.1. Materials
RuCl3ꢂ3H2O was purchased from Arrora Matthey, Kolkata, India.
Imidazole and all other organic chemicals and inorganic salts were
available from Sisco Research Lab, Mumbai, India. The purification
of acetonitrile and preparation of n-tetra butylammonium perchlo-
rate [n-Bu4N][ClO4] for electrochemical work were done as before
[13]. Dinitrogen was purified by bubbling through an alkaline
pyrogallol solution. Solvents were distilled over appropriate drying
agents under appropriate condition as per literature under N2 envi-
ronment [27]. All other chemicals and solvents were of reagent
grade and were used without further purification. Commercially
available SRL silica gel (60–120 mesh) was used for column chro-
matography. The syntheses of the ligands were carried out follow-
ing the common procedure [19].
Anal. Calc. for C22H24N8O8S2Cl2Ru (3a): C, 35.29; H, 3.21; N,
14.97. Found: C, 35.14; H, 3.18; N, 14.92%. IRexp (KBr, cmꢀ1):
m
(C@N) = 1586, m(N@N) = 1380, m
(ClOꢀ4 ) = 1090, 626. 1H NMR
(CDCl3): d 8.44 (H11, d, J = 7.5), 7.94 (H4, s), 7.74 (H9,10, m), 7.56
(H8, d, J = 7.0), 7.44 (H5, s), 4.38 (N–CH3, s), 2.60 (S–CH3, s). Anal.
Calc. for C26H32N8O8S2Cl2Ru (3b): C, 38.81; H, 3.98; N, 13.93.
Found: C, 38.76; H, 3.97; N, 13.90%. IRexp (KBr, cmꢀ1):
m
(C@N) = 1584,
m
(N@N) = 1382,
m
(ClO4ꢀ) = 1092, 627. IRtheo
(cmꢀ1):
m(C@N) = 1560, 1565,
m
(N@N) = 1361. 1H NMR (CDCl3): d
3.2. Physical measurements
8.26 (H11, d, J = 7.5), 7.89 (H4, s), 7.64 (H9,10, m), 7.47 (H8, d,
J = 7.0), 7.36 (H5, s), 4.74 (N–CH2–CH3, q, J = 8.0), 1.74 (N–CH2–
CH3, t, J = 7.5), 2.96 (S–CH2-CH3, q, J = 7.5), 1.22 (S–CH2–CH3, t,
J = 7.0).
Microanalytical data (C, H, N) were collected on Perkin–Elmer
2400 CHNS/O elemental analyzer. Spectroscopic data were
obtained using the following instruments: UV–Vis spectra, Per-

3524
T.K. Mondal et al. / Journal of Organometallic Chemistry 694 (2009) 3518–3525
The synthesis procedure of 4 is similar as mentioned above for 3
but the reaction was carried out in dry methanol. The yield was
60–70%.
corded using the
Lorentz polarization effects and for linear decay. Semi-empirical
absorption corrections based on -scans were applied. The struc-
x-scan technique. Data were corrected for
w
Anal. Calc. for C22H24N8O4S2Cl2Ru (4a): C, 38.15; H, 3.47; N,
ture was solved by direct method for all these compounds using
SHELXS-97 and successive difference Fourier syntheses. All non-
hydrogen atoms were refined anisotropically. The hydrogen atoms
were fixed geometrically and refined using the riding model. All
calculations were carried out using SHELXL-97 [28], ORTEP-32 [29]
and PLATON-99 [30] programs.
16.18. Found: C, 38.26; H, 3.49; N, 16.24%. IRexp (KBr, cmꢀ1):
m
(C@N) = 1585, m(N@N) = 1378, m(Ru–Cl) = 342, m
(ClO4ꢀ) = 1091,
625. 1H NMR (CDCl3): d 8.62 (H11, d, J = 8.0), 7.88 (H4, s), 7.78
(H9,10, m), 7.61 (H8, d, J = 7.5), 7.30 (H5, s), 4.30 (N–CH3, s), 2.57,
2.53 (S–CH3, s). Anal. Calc. for C26H32N8O4S2Cl2Ru (4b): C, 41.71;
H, 4.28; N, 14.97. Found: C, 41.60; H, 4.25; N, 14.92%. IRexp (KBr,
cmꢀ1):
1094, 630. IRtheo (cmꢀ1):
(Ru–Cl) = 332. 1H NMR (CDCl3): d 8.32 (H11, d, J = 8.0), 7.82 (H4,
m
(C@N) = 1590,
m
(N@N) = 1386,
m
(Ru–Cl) = 340,
m
(ClO4ꢀ) =
3.5. Computational methods
m
(C@N) = 1557, 1562,
m(N@N) = 1359,
m
All computations were performed using the GAUSSIAN03 (G03)
[31] software package running under Windows. The
Becke’s three-parameter hybrid exchange functional and the
Lee–Yang–Parr nonlocal correlation functional (B3LYP) [32] was
used throughout. Elements except ruthenium were assigned a 6-
31G* basis set in our calculations. For ruthenium the Los Alamos
effective core potential plus double zeta (LanL2DZ) [33] basis set
was employed. Gas and solution-phase optimization was carried
out from the geometry obtained from the crystal structure without
any symmetry constraints. In all cases, vibrational frequencies
were calculated to ensure that optimized geometries represented
local minima. The excitation energies were calculated by the
TDDFT approach. To check the effect of solvation on the calculated
optical absorption spectra, we performed TDDFT calculations of the
low lying excitation at the singlet optimized geometry, including
solvation effect by means of the nonequilibrium implementation
of the polarizable continuum model [34]; as in the experimental
conditions, the chosen solvent is acetonitrile. GaussSum [35] was
used to calculate the fractional contributions of various groups to
each molecular orbital. This is done using Mulliken population
analysis.
s), 7.65 (H9,10, m), 7.56 (H8, d, J = 7.5), 7.26 (H5, s), 4.58 (N–CH2–
CH3, q, J = 8.0), 1.60 (N–CH2–CH3, t, J = 7.5), 3.39, 3.36 (S–CH2–
CH3, q, J = 8.0), 1.29, 1.26 (S–CH2–CH3, t, J = 7.0).
3.3.2. Synthesis of [Ru(SMeaaiNMe)(SaaiNMe)](PF6) (5a)
RuCl3ꢂ3H2O (0.25 g, 0.95 mmol) was dissolved in super dry
MeOH (20 ml) and refluxed under dry N2 gas. The solution color
turned to deep green (suggesting reduction of Ru(III) to Ru(II)).
The ligand, SMeaaiNMe (0.48 g, 2.07 mmol) was added into the
solution and then activated silica gel (60–120 mesh) (15 g) was
added in portion wise with stirring continuously to make a paste.
The whole mass was then transferred into a silica crucible and
dried by passing N2 gas. Crucible was then placed in the microwave
oven and irradiated at 450 W for 5 min ꢃ 5 with 10 min interval at
each step. Solid surface was then turned into black. It was then
cooled and extracted with CH2Cl2. Brown–red solution was then
chromatographed over alumina column prepared in benzene. Light
yellow portion was eluted first by benzene. A dark band was then
eluted by MeCN solution of NH4PF6 (0.05 g per 50 ml). The eluent
was evaporated slowly in air. The crystals were separated. The
crystals were isolated by filtration and washed with cold water.
It was then dried over CaCl2. Yield: 0.48 g, 72%.
Acknowledgments
Anal. Calc. for For C21H21N8F6PS2Ru (5a): C, 36.26; H, 3.02; N,
16.11. Found: C, 36.08; H, 3.01; N, 16.08%. IRexp (KBr, cmꢀ1):
Financial support from the Department of Science and Technol-
ogy and University Grants Commission-CAS programme, New
Delhi are gratefully acknowledged.
m
(C@N) = 1592, m(N@N) = 1378, m
(PFꢀ6 ) = 840. 1H NMR (CDCl3): d
8.51 (H11, d, J = 8.0), 7.68 (H4, s), 7.58 (H9,10, m), 7.48 (H8, d,
J = 7.0), 7.40 (H5, s), 4.33 (N–CH3, s), 2.56 (S–CH3, s). Anal. Calc.
for C24H27N8F6PS2Ru (5b): C, 39.08; H, 3.66; N, 15.20. Found: C,
38.95; H, 3.65; N, 15.16%. IRexp (KBr, cmꢀ1):
m
m
m
(C@N) = 1590;
(C@N) = 1557, 1562;
(N@N) = 1352. 1H NMR (CDCl3): d 8.28 (H11, d, J = 7.5), 7.60 (H4,
Appendix A. Supplementary material
(N@N) = 1376; m m
(PFꢀ6 = 842. IRtheo (cmꢀ1):
CCDC 601286 and 673712 contain the supplementary crystallo-
s), 7.62 (H9,10, m), 7.51 (H8, d, J = 7.0), 7.38 (H5, s), 4.71 (N–CH2–
CH3, q, J = 7.5), 1.72 (N–CH2–CH3, t, J = 7.5), 3.00 (S–CH2–CH3, q,
J = 7.5), 1.20 (S–CH2–CH3, t, J = 7.0).
graphic data the structures [Ru(SEtaaiNEt)(SEtaaiNEt)Cl](ClO4)
(4b) and [Ru(SEtaaiNEt)(SaaiNEt)](PF6) (5b), respectively. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.06.021.
3.4. X-ray crystal structure analysis
The X-ray quality crystals were grown by slow diffusion of
dichloromethane solution into hexane. Details of crystal analyses,
data collection and structure refinement data are given in Supple-
mentary Table S6. Crystal mounting was done on glass fibers with
epoxy cement. Single crystal data collection were performed with
Siemens SMART CCD diffractometer using fine focus sealed graph-
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