Ruthenium mediated C-H activation of benzaldehyde thiosemicarbazones: Synthesis, structure and, spectral and electrochemical properties of the resulting complexes

Article abstract of DOI:10.1016/j.ica.2011.02.008

Reaction of five 4R-benzaldehyde thiosemicarbazones (R = OCH₃, CH₃, H, Cl and NO₂) with [Ru(PPh₃) ₃(CO)(H)Cl] in refluxing methanol in the presence of a base (NEt ₃) affords complexes of tw

Full text of DOI:10.1016/j.ica.2011.02.008

Inorganica Chimica Acta 372 (2011) 183–190
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Ruthenium mediated C–H activation of benzaldehyde thiosemicarbazones:
Synthesis, structure and, spectral and electrochemical properties
of the resulting complexes
Nabanita Saha Chowdhury a, Dipravath Kumar Seth a, Michael G.B. Drew b, Samaresh Bhattacharya a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
b Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 4 March 2011
Reaction of five 4R-benzaldehyde thiosemicarbazones (R = OCH3, CH3, H, Cl and NO2) with [Ru(PPh3)3(-
CO)(H)Cl] in refluxing methanol in the presence of a base (NEt3) affords complexes of two different types,
viz. 1-R and 2-R. In the 1-R complexes the thiosemicarbazone is coordinated to ruthenium as a dianionic
tridentate C,N,S-donor via C–H bond activation. Two triphenylphosphines and a carbonyl are also coor-
dinated to ruthenium. The tricoordinated thiosemicarbazone ligand is sharing the same equatorial plane
with ruthenium and the carbonyl, and the PPh3 ligands are mutually trans. In the 2-R complexes the thi-
osemicarbazone ligand is coordinated to ruthenium as a monoanionic bidentate N,S-donor forming a
four-membered chelate ring with a bite angle of 63.91(11)°. Two triphenylphosphines, a carbonyl and
a hydride are also coordinated to ruthenium. The coordinated thiosemicarbazone ligand, carbonyl and
hydride constitute one equatorial plane with the metal at the center, where the carbonyl is trans to
the coordinated nitrogen of the thiosemicarbazone and the hydride is trans to the sulfur. The two triphe-
nylphosphines are trans. Structures of the 1-CH3 and 2-CH3 complexes have been determined by X-ray
crystallography. All the complexes show intense transitions in the visible region, which are assigned,
based on DFT calculations, to transitions within orbitals of the thiosemicarbazone ligand. Cyclic voltam-
metry on the complexes shows two oxidations of the coordinated thiosemicarbazone on the positive side
of SCE and a reduction of the same ligand on the negative side.
Dedicated to S.S. Krishnamurthy.
Keywords:
Benzaldehyde thiosemicarbazones
C–H activation
Ruthenium complexes
Coordination modes
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
thiosemicarbazones have displayed five different modes of binding
(I–V). They have been observed to bind to ruthenium, osmium and
The chemistry of thiosemicarbazone complexes of the transi-
tion metal ions has been receiving significant current attention,
largely because of the bioinorganic relevance of these complexes
[1]. A large majority of the thiosemicarbazone complexes have
found wide medicinal applications owing to their potentially ben-
eficial biological (viz. antibacterial, antimalarial, antiviral and anti-
tumor) activities [2]. Studies on the binding of thiosemicarbazones
to different transition metal ions are of considerable importance in
this respect. However, we have been exploring the chemistry of
platinum metal complexes of the thiosemicarbazones [3], mainly
to understand the variable binding mode of these ligands displayed
in their complexes, and the present work has emerged out of this
exploration. For the present study we have selected a group of five
4-R-benzaldehyde thiosemicarbazones (L-R) as the principal li-
gand and ruthenium as the metal. We have observed that in their
reactions with the platinum group of metals the benzaldehyde
iridium, via dissociation of the acidic proton, as monoanionic
bidentate N,S-donors forming a rather unusual four-membered
chelate ring (I) [3a,b,i–k]. Five-membered chelate ring (II) forma-
tion, that involves conformational change around the C@N bond,
has been observed only for palladium and platinum [4]. Besides,
binding in the dianionic C,N,S-mode (III), via C–H bond activation,
has also been observed for rhodium, iridium and platinum [3a,f].
Such C–H bond activation is of significant contemporary impor-
tance, with particular reference to metal mediated chemical trans-
formations of organic molecules [5]. Upon reaction with rhodium
these thiosemicarbazones have also been found to undergo oxida-
tion at the sulfur center whereby they are converted into sulfone,
and the transformed thiosemicarbazones are coordinated as dian-
ionic tridentate C,N,S donors (IV) [3f]. Another interesting mode of
binding (V) has recently been witnessed in a diruthenium complex,
where the thiosemicarbazone is coordinated to one ruthenium as
dianionic tridentate C,N,S-donor, and at the same time the sulfur
is also bonded to the second ruthenium [3b]. However, an uncom-
plicated C,N,S-mode of coordination (III) of the benzaldehyde thio-
⇑
Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.008

184
N.S. Chowdhury et al. / Inorganica Chimica Acta 372 (2011) 183–190
R
R = OCH3, CH3,
R
H, Cl, NO2
M
H
C
N
NH2
S
C
N
S
C
N
H
NH2
H
N
C
N C
H
N
C
S
NH2
M
R
L-R
I
II
H2N
S
N
H
R
N
C
R
R
Ru
Ru
M
Rh
O
C
N
S
R
C
N
S
C
N
S
O
H
N C
H
N
C
H
N C
NH2
NH2
NH2
III
IV
V
semicarbazones (L-R) to ruthenium has remained as a challenge
for long. With the aim of achieving this specific target, reaction
of the chosen benzaldehyde thiosemicarbazones (L-R) has been
carried out with [Ru(PPh3)3(CO)(H)Cl]. This particular ruthenium
complex has been chosen as the starting material as it contains a
metal-hydride bond, which is known to bring about activation of
proximal C–H bond [6]. Reaction of [Ru(PPh3)3(CO)(H)Cl] with
the benzaldehyde thiosemicarbazones has indeed afforded a group
of organoruthenium complexes, in addition to complexes of an-
other type. Herein we report the chemistry of all these complexes,
with particular reference to their formation, structure and, spectral
and electrochemical properties.
1-OCH3 and 2-OCH3: To a solution of L-OCH3 (22 mg, 0.10 mmol)
in methanol (30 mL) was added triethylamine (10 mg, 0.10 mmol)
followed by [Ru(PPh3)3(CO)(H)Cl] (100 mg, 0.10 mmol) and the
mixture was refluxed for 5 h to afford a reddish-orange solution.
The solvent was then evaporated under reduced pressure and the
solid mass, thus obtained, was subjected to purification by thin layer
chromatography on a silica plate using 1:10 acetonitrile–benzene as
the eluant. An orange band separated first followed by a yellow
band, which were extracted with acetonitrile. Evaporation of the
acetonitrile extracts gave 1-OCH3 (Yield: 60%) and 2-OCH3 (Yield:
15%) as orange and yellow microcrystalline solids respectively.
Anal. Calc. for 1-OCH3: C, 64.11; H, 4.53; N, 4.88. Found: C,
64.52; H, 4.51; N, 4.89%. 1H NMR1: 3.46 (s, OCH3); 5.26 (s, NH2);
6.14 (d, 1H, JH–H = 8.3); 6.41 (s, 1H); 6.87 (d, 1H, JH–H = 8.0); 6.99
(s, azomethine); 7.20-7.70 (2PPh3)⁄; Anal. Calc. for 1-CH3: C, 65.33;
H, 4.62; N, 4.97. Found: C, 65.32; H, 4.64; N, 4.96%. 1H NMR: 1.85
(s, CH3); 5.34 (s, NH2); 6.35 (d, 1H, JH–H = 7.2); 6.66 (s, 1H); 6.75
(d, 1H, JH–H = 7.4); 6.96 (s, azomethine); 7.20–7.70 (2PPh3)⁄; Anal.
Calc. for 1-H: C, 64.98; H, 4.45; N, 5.05. Found: C, 65.47; H, 4.46;
N, 5.03%. 1H NMR: 5.33 (s, NH2); 6.40 (t, 1H, JH–H = 7.3); 6.57 (t,
1H, JH–H = 7.3); 6.85 (d, 1H, JH–H = 7.5); 6.97 (s, azomethine); 7.04
(d, 1H, JH–H = 7.2); 7.20–7.70 (2PPh3)⁄; Anal. Calc. for 1-Cl: C,
62.32; H, 4.15; N, 4.85. Found: C, 62.44; H, 4.16; N, 4.89%. 1H
NMR: 5.25 (s, NH2); 6.35 (d, 1H, JH–H = 7.2); 6.66 (s, 1H); 6.75 (d,
1H, JH–H = 7.4); 7.22–7.70 (azomethine + 2PPh3)⁄; Anal. Calc. for 1-
NO2: C, 61.64; H, 4.11; N, 6.39. Found: C, 61.73; H, 4.10; N, 6.37%.
1H NMR: 5.29 (s, NH2); 7.03 (d, 1H, JH–H = 8.2); 7.54 (d, 1H, JH–
H = 8.3); 6.99 (s, azomethine); 7.22–7.89 (1H + 2PPh3)⁄.
2. Experimental
2.1. Materials
Commercial ruthenium trichloride, purchased from Arora Mat-
they, Kolkata, India, was converted to RuCl3Á3H2O by repeated
evaporation with concentrated hydrochloric acid. Triphenylphos-
phine was purchased from Loba Chemie, Mumbai, India.
[Ru(PPh3)3(CO)(H)Cl] was synthesized following a reported proce-
dure [7]. Benzaldehyde, para-substituted benzaldehydes and thio-
semicarbazide were obtained from S.D. Fine-Chem, Mumbai, India.
Tetrabutylammonium hexaflurophosphate (TBHP), obtained from
Sigma–Aldrich, was used for electrochemical work. The thiosemi-
carbazone ligands were prepared by reacting equimolar amounts
of thiosemicarbazide and the respective para-substituted benzal-
dehyde in hot ethanol. All other chemicals and solvents were re-
agent grade commercial materials and were used as received.
Purification of dichloromethane and acetonitrile was performed
as reported in the literature [8].
Anal. Calc. for 2-OCH3: C, 64.04; H, 4.76; N, 4.87. Found: C,
64.35; H, 4.77; N, 4.86%. 1H NMR: À12.17 (t, hydride, JP–H = 19.5);
3.78 (s, OCH3); 4.93 (s, NH2); 6.69–7.73 (4H + azome-
thine + 2PPh3)⁄. Anal. Calc. for 2-CH3: C, 65.24; H, 4.85; N, 4.96.
Found: C, 65.25; H, 4.79; N, 4.94%. 1H NMR: À12.20 (t, hydride,
JP–H = 19.5); 2.38 (s, CH3); 4.91(s, NH2); 6.95–7.82 (4H + azome-
thine + 2PPh3)⁄. Anal. Calc. for 2-H: C, 64.90; H, 4.69; N, 5.05.
2.2. Synthesis of 1-R and 2-R
Each 1-R complex, and the corresponding 2-R complex, were
obtained together by following a general procedure. Specific details
are given below for such a pair of complexes.
1
Chemical shifts are given in ppm and multiplicity of the signals along with the
associated coupling constants (J in Hz) are given in parentheses. Overlapping signals
are marked with an asterisk.

N.S. Chowdhury et al. / Inorganica Chimica Acta 372 (2011) 183–190
185
Found: C, 65.11; H, 4.70; N, 5.04%. 1H NMR: À12.20 (t, hydride, JP–
Table 1
H = 19.5); 4.96(s, NH2); 7.04–7.67 (5H + azomethine + 2PPh3)⁄.
Anal. Calc. for 2-Cl: C, 62.32; H, 4.39; N, 4.85. Found: C, 62.53; H,
4.40; N, 4.84%. 1H NMR: À12.24 (t, hydride, JP–H = 19.5); 4.82(s,
NH2); 6.96-8.05 (4H + azomethine + 2PPh3)⁄. Anal. Calc. for 2-
NO2: C, 61.57; H, 4.45; N, 6.39. Found: C, 61.58; H, 4.43; N,
6.41%. 1H NMR: À12.30 (t, hydride, JP–H = 19.5); 5.05 (s, NH2);
6.70-8.03 (4H + azomethine + 2PPh3)⁄.
Crystallographic data for the 1-CH3 and 2-CH3 complexes.
1-CH3 2-CH3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
2(C46H39N3OP2SRu)ÁC6H6 C46H41N3OP2SRuÁCH3CN
1767.87
triclinic
P1
887.93
triclinic
ꢀ
ꢀ
P1
12.0543(15)
17.6753(18)
21.333 (2)
75.628(9)
82.727(9)
80.755(9)
4345.8 (8)
2
12.4546(5)
13.0401(5)
14.8264(9)
102.853(3)
98.787(3)
108.083(2)
2166.80(19)
2
b (Å)
c (Å)
2.3. Physical measurements
a
(°)
b (°)
c
(°)
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. IR spectra were obtained on a Per-
kin–Elmer Spectrum RX1 spectrometer with samples prepared as
KBr pellets. Electronic spectra were recorded on a JASCO V-570
spectrophotometer. 1H NMR spectra were recorded in CDCl3 solu-
tion on a Bruker Avance DPX 300 NMR spectrometer using TMS as
the internal standard. Cyclic voltammetric studies were done on a
CH Instruments model 600A electrochemical analyzer using a plat-
inum disk working electrode, a platinum wire auxiliary electrode
and an aqueous saturated calomel reference electrode (SCE). All
Cyclic voltammetric experiments were performed under a dinitro-
gen atmosphere, and the data were collected at 298 K and are
uncorrected for junction potentials. Single point calculation of
ground state structures and energy for the complexes were carried
out by density functional theory (DFT) method using the GAUSSIAN
03 (B3LYP/SDD-6-31G) package [9].2
V (Å3)
Z
k (Å)
0.71073
0.71073
0.22 Â 0.20 Â 0.36
293
Crystal size (mm3)
0.03 Â 0.03 Â 0.22
T (K)
150
l
R1
wR2
(mmÀ1
)
0.522
0.524
a
0.0793
0.0503
b
0.1198
0.1468
Goodness-of-fit (GOF)c 0.67
1.04
P
P
a
b
c
R1
=
||Fo| À |Fc||/ |Fo|.
P
P
wR2 = [ {w(F2o À F2c )2}/ {w(Fo2)}]1/2
.
P
GOF = [ (w(F2o À Fc2)2)/(M À N)]1/2, where M is the number of reflections and N
is the number of parameters refined.
minary characterizations on the 1-R complexes indicate the pres-
ence of two triphenylphosphines,
a thiosemicarbazone and a
carbonyl in the coordination sphere. In order to ascertain the stereo-
chemistry of the 1-R complexes, as well as the binding mode of the
thiosemicarbazone in them, structure of a selected member of this
family, viz. 1-CH3, has been determined by X-ray crystallography.
The structure is shown in Fig. 1 and relevant bond parameters are gi-
ven in Table 2. The structure shows that the thiosemicarbazone is in-
deed coordinated to ruthenium in the expected tridentate C,N,S-
mode (III), via loss of two protons, viz. the hydrazinic proton and
the phenyl proton at one ortho position with respect to the imine
function. Two triphenylphosphines and a carbonyl are also coordi-
nated to ruthenium. In this complex ruthenium is thus nested in a
C2NSP2 coordination sphere, which is distorted octahedral in nature
as reflected in the bond parameters around the metal center. The tri-
coordinated thiosemicarbazone ligand is sharing the same equatorial
plane with ruthenium and the carbonyl, and the PPh3 ligands have
taken up the remaining two axial positions and hence they are
mutually trans. The Ru–P and Ru–C(carbonyl) lengths are normal,
as observed in structurally characterized complexes containing these
bonds [3b]. Within the Ru–CNS chelate, the Ru–C, Ru–N and Ru–S
distances are usual [3b], and the other bond distances are consistent
with the bond descriptions shown in III. As all the 1-R complexes
have been synthesized similarly, and they show similar properties
(vide infra), the other four 1-R complexes (R – CH3) are assumed
to have similar structures as the 1-CH3 complex.
2.4. Crystallography
Single crystals of the 1-CH3 and 2-CH3 complexes were ob-
tained by slow evaporation of the solvent from acetonitrile solu-
tions of the respective complexes. Selected crystal data and data
collection parameters are given in Table 1. Data on the crystal of
the 1-CH3 complex were collected on an Oxford Diffaction X-Cali-
bur System and data on the crystal of the 2-CH3 complex were col-
lected on a BRUKER AXS SMART 2 diffractometer. X-ray data
reduction and, structure solution and refinement were done using
SHELXS-97 and SHELXL-97 programs [10]. Hydrogen atoms were
placed geometrically and positional parameters were refined by
using a riding model. The hydride in the 2-CH3 complex was lo-
cated from difference syntheses and refined isotropically. The
structures were solved by direct methods.
3. Results and discussion
3.1. Syntheses and crystal structures
The primary objective of the present study has been to scruti-
nize the interaction of the selected 4-R-benzaldehyde thiosemicar-
bazones (L-R) with [Ru(PPh3)3(CO)(H)Cl] and see whether they
bind to the metal center in the targeted C,N,S-fashion (III). The five
different substituents R (R = OCH3, CH3, H, Cl and NO2), with differ-
ent electron withdrawing properties, have been chosen to study
their influence, if any, on the redox properties of resulting com-
plexes. Reactions of the selected thiosemicarbazones (L-R) with
[Ru(PPh3)3(CO)(H)Cl] proceed smoothly in refluxing methanol in
the presence of triethylamine and from each of these reactions
two types of complexes, orange (1-R) and yellow (2-R) in color,3
have been obtained in about 60% and 15% yield respectively. Preli-
Besides presence of a coordinated hydride, as indicated by 1H
NMR studies, preliminary characterization data on the 2-R com-
plexes point to a composition similar to that of the 1-R complexes.
For an unambiguous characterization of the 2-R complexes, with
particular reference to their stereochemistry and coordination
mode of the thiosemicarbazone in them, crystal structure of the
2-CH3 complex has been solved. This particular complex has been
chosen for crystallographic characterization to do a proper compar-
ison of structural features with the corresponding 1-CH3 complex.
The structure of 2-CH3 (Fig. 2) shows that in this complex the thio-
semicarbazone ligand is coordinated to ruthenium as a monoanion-
ic bidentate N,S-donor forming a four-membered chelate ring (I)
with a bite angle of 63.91(11)°. Formation of such four-membered
chelate ring by the benzaldehyde thiosemicarbazones has been ob-
served before [3a,b,i–k]. Two triphenylphosphines, a carbonyl and a
2
In the DFT calculations, crystal structures of complexes 1-CH3 and 2-CH3 have
been used as the starting model and phenyl rings of the PPh3 ligands have been
replaced by hydrogens.
3
Colors for the R = NO2 complexes are different. The 1-NO2 complex is violet and
the 2-NO2 complex is orange.

186
N.S. Chowdhury et al. / Inorganica Chimica Acta 372 (2011) 183–190
Fig. 1. View of the 1-CH3 complex.
hydride constitute one equatorial plane of the octahedron with
the metal at the center, where the carbonyl is trans to the coordi-
nated nitrogen of the thiosemicarbazone and the hydride is trans
to the sulfur. As before, the two triphenylphosphines are mutually
trans. It must be mentioned here that the same 2-R complexes were
obtained before from another set of reactions reported earlier and
crystal structure of the 2-NO2 complex was also determined [3b].
Formation of the two types of complexes (1-R and 2-R) from the
same synthetic reaction was quite intriguing. Though the exact
mechanism of their formation is not completely clear, some spec-
ulated sequences shown in Scheme 1 seem probable. In solution
the benzaldehyde thiosemicarbazones are known to exist in two
tautomeric forms, viz. thione form and thiol form. In methanol
the thiol tautomer is likely to be the dominant species, which
seems to react with [Ru(PPh3)3(CO)(H)Cl] and forms, via elimina-
tion of PPh3 and HCl, two isomeric species A-R and 2-R, which dif-
fer only in the binding mode of the thiosemicarbazone ligand. In
intermediate A-R the thiosemicarbazone is coordinated as mono-
anionic N,S-donor forming a five-membered ring, while in 2-R
the same ligand is bound as monoanionic N,S-donor forming a
four-membered ring. In the intermediate A-R, the ortho C–H bond
of the pendent phenyl ring of the N,S-coordinated thiosemicarba-
zone ligand comes in close proximity of the Ru–H fragment, result-
ing in a hydrogen-bonding interaction between them, which, in
turn, leads to an ortho C–H activation and affords the cyclometal-
lated 1-R complex via elimination of molecular hydrogen.4 In 2-R,
Table 2
Selected bond lengths (Å) and bond angles (°) for 1-CH3 and 2-CH3 complexes.
1-CH3
Bond lengths (Å)
Ru1–P1
Ru1–P2
Ru1–C7A
Ru1–N4A
Ru1–S1A
Ru1–C1
2.366(2)
2.350(2)
2.077(7)
2.109(6)
2.4614(19)
1.796(8)
C6A–C5A
C5A–N4A
N4A–N3A
N3A–C2A
C2A–S1A
C1–O1
1.424(10)
1.291(9)
1.388(8)
1.347(9)
1.731(7)
1.200(10)
Bond angles (°)
P1–Ru1–P2
C7A–Ru1–S1A
C1–Ru1–N4A
2-CH3
173.41(7)
156.22(19)
172.5(3)
C7A–Ru1–N4A
N4A–Ru1–S1A
Ru1–C1–O1
78.9(2)
77.35(16)
175.4(7)
Bond lengths (Å)
Ru1–H88
Ru1–P1
Ru1–P2
Ru1–N2
1.56(5)
C1–C8
C8–N1
N1–N2
N2–C9
C9–S1
C10–O1
1.458(7)
1.271(6)
1.391(5)
1.315(6)
1.709(5)
1.156(5)
2.3427(10)
2.3524(10)
2.147(3)
2.5782(13)
1.839(4)
Ru1–S1
Ru1–C10
Bond angles (°)
P1–Ru1–P2
C10–Ru1–N2
H88–Ru1–S1
178.17(4)
177.21(16)
159.7(15)
N2–Ru1–S1
Ru1–C10–O1
63.91(11)
177.6(4)
hydride are also coordinated to the metal center. Ruthenium has a
distorted octahedral HCNSP2 coordination environment in this
complex. The thiosemicarbazone ligand, ruthenium, carbonyl and
4
The evolved hydrogen could not be detected.

N.S. Chowdhury et al. / Inorganica Chimica Acta 372 (2011) 183–190
187
Fig. 2. View of the 2-CH3 complex.
the pendent phenyl ring of the N,S-coordinated thiosemicarbazone
ligand remains far away from the Ru–H fragment, and hence no fur-
ther reaction takes place, and it is obtained as a stable complex. In
principle, a slightly different mechanism (Scheme S1) can also be in-
voked, in which the Ru–H bond of [Ru(PPh3)3(CO)(H)Cl] undergoes
initial reaction with the thiosemicarbazone producing two isomeric
intermediates, followed by reaction of the Ru–Cl bond with the sol-
vent (methanol) and/or with the ortho C–H bond of the phenyl ring
ultimately yielding the same 1-R and 2-R complexes. However, in
view of the facile nature of the observed cyclometallation Scheme
1 seems more likely.
of the 2-R complexes show a hydride signal near À12.0 ppm,
which appears as a triplet due to coupling with the two magneti-
cally equivalent phosphorus nuclei. The other spectral features
are mostly similar to those of the corresponding 1-R complexes.
The infrared and 1H NMR spectral data of all the complexes are
therefore consistent with their composition and stereochemistry.
Electronic spectra of the 1-R and 2-R complexes have been re-
corded in dichloromethane solution. Each complex shows one in-
tense absorption in the visible region and few more in the
ultraviolet region (Table 3).5,6 The absorptions in the ultraviolet re-
gion are attributable to transitions within the ligand orbital. To have
an insight into the nature of the absorptions in the visible region,
DFT calculations have been performed on the two crystallographi-
cally characterized complexes, viz. 1-CH3 and 2-CH3 (Footnote 2).
Electron distributions in the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) for the
1-CH3 complex are shown in Fig. 3 and the same for the 2-CH3 com-
plex are deposited as supporting information (Fig. S1). Composition
of these two orbitals is given in Table 4. In both the complexes the
HOMO and LUMO are mostly concentrated over the thiosemicarba-
zone ligand. Hence the absorption in the visible region is assignable
to a transition within these filled and vacant orbitals of the thiosem-
icarbazone ligand.7
3.2. Spectral properties
Infrared spectra of the 1-R complexes show many vibrations of
different intensities in the 2000–400 cmÀ1 region. A strong band
near 1925 cmÀ1 indicates the presence of coordinated CO in these
complexes. Strong vibrations have also been observed near 746,
695 and 516 cmÀ1, due to the coordinated PPh3 ligands. Compari-
son with the spectrum of [Ru(PPh3)3(CO)(H)Cl] shows the presence
of some distinct new bands (e.g. bands near 1581, 1525, 1480,
1434 and 1090 cmÀ1) in the spectra of the 1-R complexes, which
are attributable to the coordinated thiosemicarbazone ligand.
Infrared spectra of the 2-R complexes are qualitatively very similar
to those of the corresponding 1-R analogs. The only significant dif-
ference being appearance of a Ru–H stretch in the 2-R complexes,
which could not be observed as an isolated signal as it falls in the
3.3. Electrochemical properties
Electrochemical properties of the 1-R complexes have been
studied by cyclic voltammetry in 1:9 dichloromethane–acetonitrile
solution (0.1 M TBHP).8 Voltammetric data are given in Table 3
(Footnote 6). All the five 1-R complexes show two oxidative
same region of the
m
(CO) stretch but it appears as a shoulder (near
1860 cmÀ1) on the intense
m
(CO) band. 1H NMR spectra of the 1-R
complexes show broad signals within 7.20–7.89 ppm due to the
coordinated PPh3 ligands. The azomethine proton signal is ob-
served near 6.99 ppm. Signal for the NH2 fragment of the thiosem-
icarbazone is observed near 5.30 ppm. Most of the expected
aromatic proton signals for the coordinated thiosemicarbazone li-
gand have been clearly observed, while few could not be detected
due to their overlap with other signals. Signals for the hydrogen-
containing substituent in the thiosemicarbazone ligand (R = OCH3
and CH3) are also observed in the expected region. 1H NMR spectra
5
The 1-NO2 complex shows a second absorption close to the borderline of visible
and ultraviolet regions.
6
The electronic spectral and cyclic voltammetric data of the 2-CH3 complex,
though reported earlier [3b], are included in Table 3 for comparison.
7
The absorption displayed in the visible region by the 2-CH3 complex (and the
other four 2-R complexes) was earlier assigned as MLCT transition [3b].
8
A
little dichloromethane was necessary to take the complex into solution.
Addition of large excess of acetonitrile was necessary to record the redox responses in
proper shape.

188
N.S. Chowdhury et al. / Inorganica Chimica Acta 372 (2011) 183–190
R
R
C
N
H
NH2
S
C
N
SH
H
N
C
H
N
C
NH2
[Ru(PPh3)3(CO)(H)Cl]
_
PPh3
_
HCl
P=PPh3
P
P
R
CO
H
N
CO
H
N
Ru
Ru
P
H
C
P
C
C
S
S
N
N
H
C
NH2
2-R
NH2
R
A-R
_
H2
P
R
CO
S
Ru
P
C
N
H
N
C
NH2
1-R
Scheme 1. Probable steps behind formation of the complexes.
Table 3
Electronic spectral and cyclic voltammetric data.
responses on the positive side of SCE and a reductive response on the
negative side. All the redox responses are found to be irreversible. A
representative cyclic voltammogram is deposited as supporting
information (Fig. S2). In view of the composition of the HOMO, the
first oxidative response is assigned to oxidation of coordinated thio-
semicarbazone ligand. Similarly based on the composition of the
LUMO, the reductive response is assigned to reduction of the coordi-
nated thiosemicarbazone. The second oxidative response is tenta-
tively assigned to further oxidation of the thiosemicarbazone
ligand. Potential of the first oxidation in the 1-R complexes has been
found to increase with increasing electron-withdrawing character of
the substituent R in the thiosemicarbazone ligand. The plot of this
Compound Electronic spectral data kmax (nm)
Cyclic voltammetric
datab
E, V vs. SCE
a
(e )
, MÀ1 cmÀ1
1-OCH3
1-CH3
1-H
462(950), 347(5700), 300(10600)c,
272(14400)c
1.13d, 0.46d, À1.26e
1.16d, 0.49d, À1.27e
1.24d, 0.52d, À1.35e
1.19d, 0.54d, À1.41e
467(1600), 342(7300)c, 303(13450)c,
270(20000)c
471(1000), 345(4950), 302(10000)c,
269(14900)c
1-Cl
472(1050), 347(3800), 305(8400)c,
269(12900)c
1-NO2
2-CH3
547(1050), 400(4300), 338(7300)c
466(1700), 338c(7160), 302(13200)
1.33d, 0.67d, À1.00e
1.14d, 0.47(72)f, À1.33e
a
oxidation potential versus
OCH3 = À0.27, CH3 = À0.17, H = 0.00, Cl = 0.23 and NO2 = 0.78] is lin-
ear (Fig. S3) with a slope ( ) of 0.19 V ( = reaction constant [12]).
r [r = Hammett constant of R [11];
In dichloromethane.
b
Solvent, 1:9 dichloromethane–acetonitrile; supporting electrolyte, TBHP; scan
rate 50 mV sÀ1
.
q
q
c
Shoulder.
Epa value.
Epc value.
Potential of the other two redox responses, however, did not show
any systematic variation with the electron-withdrawing character
of the substituent R.
d
e
f
E1/2 = 0.5 (Epa + Epc),
DEp = Epa – Epc in mV.

N.S. Chowdhury et al. / Inorganica Chimica Acta 372 (2011) 183–190
189
Science and Technology, New Delhi, India (Grant No. SR/S1/IC-29/
2009) is gratefully acknowledged. NSC thanks the University
Grants Commission, New Delhi for her fellowship and DKS thanks
the Council of Scientific and Industrial Research, New Delhi, for his
fellowship [Grant No. 9/096(0511)/2006-EMR I]. MGBD thanks
EPSRC (UK) and the University of Reading for funds for the Image
Plate System. We thank Prof. Chittaranjan Sinha for his help in
recording the IR spectra, and Dr. Kajal K. Rajak for his help with
the DFT studies.
Appendix A. Supplementary data
Partial molecular orbital diagrams of 2-CH3 (Fig. S1), cyclic vol-
tammogram of the 1-Cl complex (Fig. S2), least-squares plots of Epa
values of the first oxidation versus
r for the 1-R complexes
(Fig. S3), and an alternative mechanism behind formation of the
1-R and 2-R complexes (Scheme S1) have been deposited. CCDC
798418 and 798419 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.ica.2011.02.008.
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