Iridium assisted S-H and C-H activation of benzaldehyde thiosemicarbazones. Synthesis, structure and electrochemical properties of the resulting complexes

Article abstract of DOI:10.1016/j.ica.2010.04.009

Reaction of five 4-R-benzaldehyde thiosemicarbazones (R = OCH₃, CH₃, H, Cl and NO₂) with [Ir(PPh₃) ₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords complexes of three diff

Full text of DOI:10.1016/j.ica.2010.04.009

Inorganica Chimica Acta 363 (2010) 2848–2856
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Iridium assisted S–H and C–H activation of benzaldehyde thiosemicarbazones.
Synthesis, structure and electrochemical properties of the resulting complexes
Semanti Basu a, Rama Acharyya a, Falguni Basuli a, Shie-Ming Peng b, Gene-Hsiang Lee b, Golam Mostafa c,
Samaresh Bhattacharya a,
*
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
c Department of Physics, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of five 4-R-benzaldehyde thiosemicarbazones (R = OCH3, CH3, H, Cl and NO2) with [Ir(PPh3)3Cl]
in refluxing ethanol in the presence of a base (NEt3) affords complexes of three different types, viz. 1-R, 2-
R and 3-R. In the 1-R complexes the thiosemicarbazone is coordinated to iridium as a monoanionic
bidentate N,S-donor forming a four-membered chelate ring. Two triphenylphosphines, a hydride and a
chloride are also coordinated to the metal center. The 2-R complexes are very similar in composition
and stereochemistry to the corresponding 1-R complexes, except that a second hydride is bound to irid-
ium instead of the chloride. In the 3-R complexes, the thiosemicarbazones are coordinated to iridium as
dianionic tridentate C,N,S-donors forming two adjacent five-membered chelate rings. Two triphenyl-
phosphines and a hydride are also coordinated to the metal center. Structures of the 1-NO2, 2-NO2 and
3-NO2 complexes have been determined by X-ray crystallography. Reaction of the same 4-R-benzalde-
hyde thiosemicarbazones with [Ir(PPh3)3Cl] in refluxing toluene in the presence of NEt3 affords com-
plexes of two types, viz. 3-R and 4-R. The 4-R complexes are very similar in composition and
stereochemistry to the corresponding 3-R complexes, except that a chloride is bound to iridium instead
of the hydride. Structure of the 4-CH3 complex has been determined by X-ray crystallography. In all the
complexes the two PPh3 ligands are trans. All the complexes show intense MLCT transitions in the visible
region. Cyclic voltammetry on the complexes shows an Ir(III)–Ir(IV) oxidation on the positive side of SCE
followed by an oxidation of the coordinated thiosemicarbazone. A reduction of the coordinated thiosem-
icarbazone is also observed on the negative side of SCE.
Received 22 January 2010
Received in revised form 20 March 2010
Accepted 3 April 2010
Available online 10 April 2010
Dedicated to Prof. Animesh Chakravorty
Keywords:
Benzaldehyde thiosemicarbazones
S–H and C–H activation
Iridium complexes
Coordination modes
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
carbazones (I) with iridium. It may be relevant to mention here that
though the chemistry of thiosemicarbazone complexes of many
The chemistry of thiosemicarbazone complexes of the transition
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 beneficial biolog-
ical (viz. antibacterial, antimalarial, antiviral and antitumor) activi-
ties [2]. Systematic 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 plati-
num metal complexes of the thiosemicarbazones [3], with the pri-
mary objective of gaining a chemical control over the variable
binding mode of these ligands, and the present work has emerged
out of this exploration. The aim of the present study has been to
scrutinize the interaction of a group of 4-R-benzaldehyde thiosemi-
transition metals has been extensively studied [1], that of iridium
appears to have remained practically unexplored [3d]. Upon their
reaction with ruthenium and osmium the benzaldehyde thiosemi-
carbazones (I) have been observed to bind to the metal centers,
via dissociation of the acidic proton, as monoanionic bidentate
N,S-donors forming a rather unusual four-membered chelate ring
(II) [3a,h–j]. Our investigations have revealed that in view of the
geometry of these ligands (I) across the C@N bond, formation of
the four-membered chelate ring (II) is most favorable and that of
the five-membered chelate ring (III), which appears to be very likely
for the thiosemicarbazones in general, is not possible because of the
steric hindrance that develops between the phenyl ring of the benz-
aldehyde thiosemicarbazone and the metal center. It is worth noting
here that we have not been able to find a single example of a struc-
turally characterized complex of the benzaldehyde thiosemicarba-
zones (I), where the thiosemicarbazone is coordinated as in III.
Five-membered chelate ring (IV) formation by the benzaldehyde
* Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.04.009

S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
2849
thiosemicarbazones (I) has been observed only in very few cases,
which are associated with conformational change around the C@N
bond [4]. Though five-membered chelate ring (III) formation by
the benzaldehyde thiosemicarbazones (I) without any conforma-
tional change has been realized to be impossible, closeness of the
phenyl ring to the metal center in III points to the possibility of its
orthometallation (V) via C–H bond activation. Such C–H bond acti-
vation is of significant contemporary importance, with particular
reference to metal mediated chemical transformations of organic
molecules [5]. With the intention of inducing coordination mode
V in the benzaldehyde thiosemicarbazone ligands (I), their reaction
a dinitrogen atmosphere for 24 h, whereby a red solution was ob-
tained. Evaporation of this solution afforded a dark solid, which
was subjected to purification by thin layer chromatography on a
silica plate. With benzene as the eluant, two distinct red bands sep-
arated, which were extracted separately with acetonitrile. Evapo-
ration of the first red fraction gave a 1:1 mixture (in the form of
co-crystals) of 1-NO2 and 2-NO2 (Yield: 42%) and that of the second
red fraction afforded 3-NO2 (Yield: 25%).
Anal. Calc. for 1-NO2 and 2-NO2: C, 55.11; H, 4.01; N, 5.84.
Found: C, 55.43; H, 4.00; N, 5.88%. 1H NMR [8]: ꢀ23.32 (t, hydride,
JP–H = 13.5); ꢀ20.88 (d of t, hydrides, JP–H = 16.0, JH–H = 7.0); ꢀ20.82
(d of t, hydrides, JP–H = 16.0, JH–H = 7.0); 4.63 (s, NH2); 4.99 (s, NH2);
7.05 (s, 1H); 7.19 (d, 2H, JH–H = 8.8); 7.21–7.84 (2H + 4PPh3*); 7.99
(d, 2H, JH–H = 8.8); 8.04 (d, 2H, JH–H = 8.8); 8.09 (s, 1H). Anal. Calc.
for 3-NO2: C, 56.22; H, 3.94; N, 5.96. Found: C, 56.35; H, 3.92; N,
5.99%. 1H NMR: ꢀ14.26 (t, hydride, JP–H = 16.7); 4.46 (s, NH2);
6.74 (d, 1H, JH–H = 8.3); 7.20–7.62 (1H + 2PPh3*); 7.32 (d, 1H, JH–
H = 8.1); 7.49 (s, 1H).
has been carried out with
a reactive iridium complex, viz.
[Ir(PPh3)3Cl]. This particular iridium complex has been chosen as
the starting material because of its demonstrated ability to accom-
modate a tridentate ligand via oxidative addition and, more impor-
tantly, its proven efficiency to successfully mediate C–H bond
activation of organic molecules [3d,6]. This simple strategy has in-
deed worked nicely, affording a group of organoiridium complexes,
in addition to complexes of other types. This paper deals with the
chemistry of all these complexes, with special reference to their for-
mation, structure and electrochemical properties.
2.2.2. 2-R and 3-R (R – NO2)
All the 2-R and 3-R (R–NO2) complexes were synthesized by
following the same above procedure using appropriate para-
substituted benzaldehyde thiosemicarbazone (I, R–NO2) instead
of para-nitrobenzaldehyde thiosemicarbazone. A yellow solution
was obtained from the synthetic reaction, which afforded a yellow
solid upon evaporation. Purification was achieved by thin layer
chromatography on a silica plate. With benzene as the eluant,
two distinct yellow bands separated, which were extracted sepa-
rately with acetonitrile. Evaporation of the first yellow fraction
gave 2-R (Yield: ꢁ40%) and that of the second yellow fraction affor-
ded 3-R (Yield: ꢁ25%).
I
II
III
Anal. Calc. for 2-OCH3: C, 58.30; H, 4.53; N, 4.53. Found: C,
58.71; H, 4.47; N, 4.59%. 1H NMR: ꢀ20.25 (d of t, hydride, JP–
H = 16.0, JH–H = 6.5); ꢀ15.80 (d of t, hydride, JP–H = 16.0, JH–
H = 6.5); 3.76 (s, OCH3); 4.48 (s, NH2); 7.21–7.29 (4H*); 7.30–7.65
(azomethine + 2PPh3*). Anal. Calc. for 3-OCH3: C, 58.43; H, 4.33;
N, 4.54. Found: C, 58.91; H, 4.37; N, 4.56%. 1H NMR: ꢀ14.50 (t, hy-
dride, JP–H = 16.5); 3.25 (s, OCH3); 4.25 (s, NH2); 6.12 (s, 1H); 6.09
(d, 1H, JH–H = 7.4); 6.63 (d, 1H, JH–H = 7.5); 6.81 (s, azomethine);
7.13–7.75 (2PPh3*).
IV
V
Anal. Calc. for 2-CH3: C, 59.33; H, 4.61; N, 4.61. Found: C, 59.82;
H, 4.53; N, 4.67%. 1H NMR: ꢀ19.33 (d of t, hydride, JP–H = 16.5, JH–
H = 6.0); ꢀ15.27 (d of t, hydride, JP–H = 16.5, JH–H = 6.0); 1.99 (s,
CH3); 4.49 (s, NH2); 7.05–7.15 (4H*); 7.29–7.66 (azome-
thine + 2PPh3*). Anal. Calc. for 3-CH3: C, 59.46; H, 4.40; N, 4.62.
Found: C, 60.02; H, 4.44; N, 4.67%. 1H NMR: ꢀ14.21 (t, hydride,
JP–H = 16.5); 1.78 (s, CH3); 4.61 (s, NH2); 6.52 (s, 1H); 6.65 (d, 1H,
JH–H = 7.4); 6.96 (s, azomethine); 7.02 (d, 1H, JH–H = 8.2); 7.12–
7.58 (2PPh3*).
Anal. Calc. for 2-H: C, 58.92; H, 4.46; N, 4.69. Found: C, 59.37; H,
4.41; N, 4.62%. 1H NMR: ꢀ19.31 (d of t, hydride, JP–H = 18.0, JH–
H = 6.0); ꢀ15.28 (d of t, hydride, JP–H = 16.5; JH–H = 6.0); 4.55 (s,
NH2); 7.07–7.11 (3H*); 7.13–7.16 (2H*); 7.18–7.68 (azome-
thine + 2PPh3*). Anal. Calc. for 3-H: C, 59.05; H, 4.25; N, 4.70.
Found: C, 59.39; H, 4.23; N, 4.69%. 1H NMR: ꢀ14.48 (t, hydride,
JP–H = 18.0); 4.26 (s, NH2); 6.05 (t, 1H, JH–H = 7.0); 6.43 (t, 1H, JH–
H = 7.3); 6.59 (d, 1H, JH–H = 7.3); 6.64 (d, 1H, JH–H = 7.3); 6.71 (s, azo-
methine); 7.05–7.69 (2PPh3*).
2. Experimental
2.1. Materials
Iridium trichloride was obtained from Arora Matthey, Kolkata,
India. [Ir(PPh3)3Cl] was prepared as before [6b]. Thiosemicarbazide
and the five para-substituted benzaldehydes were purchased from
SRL, Mumbai, India. The thiosemicarbazone ligands were prepared
by reacting equimolar amounts of thiosemicarbazide and the
respective para-substituted benzaldehyde in 1:1 ethanol–water
mixture. Purification of acetonitrile and preparation of tetrabutyl-
ammonium perchlorate (TBAP) for electrochemical work were per-
formed as reported in the literature [7]. All other chemicals and
solvents were reagent grade commercial materials and were used
as received.
2.2. Synthesis
Anal. Calc. for 2-Cl: C, 56.73; H, 4.19; N, 4.51. Found: C, 57.11; H,
4.22; N, 4.55%. 1H NMR: ꢀ19.27 (d of t, hydride, JP–H = 16.5, JH–
H = 6.0); ꢀ15.34 (d of t, hydride, JP–H = 18.0; JH–H = 6.0); 4.58 (s,
NH2); 7.05 (d, 2H, JH–H = 8.0); 7.16 (d, 2H, JH–H = 8.5); 7.20–7.67
(azomethine + 2PPh3*). Anal. Calc. for 3-Cl: C, 56.85; H, 3.98; N,
4.52. Found: C, 57.29; H, 3.92; N, 4.56%. 1H NMR: ꢀ14.49 (t, hy-
dride, JP–H = 18.0); 4.30 (s, NH2); 6.46 (d, 1H, JH–H = 8.0); 6.51 (s,
2.2.1. 1-NO2, 2-NO2 and 3-NO2
para-Nitrobenzaldehyde thiosemicarbazone (24 mg, 0.10
mmol) was dissolved in ethanol (50 mL) and triethylamine
(20 mg, 0.20 mmol) was added to it. The solution was then purged
with a stream of dinitrogen for 10 min and to it was added
[Ir(PPh3)3Cl] (100 mg, 0.10 mmol). The mixture was refluxed under

2850
S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
1H); 6.63 (d, 1H, JH–H = 8.0); 6.90 (s, azomethine); 7.15–7.68
kin–Elmer 783 spectrometer with samples prepared as KBr pellets.
Electronic spectra were recorded on a JASCO V-570 spectropho-
tometer. 1H NMR spectra were recorded in CDCl3 solution on a Bru-
ker Avance DPX 300 NMR spectrometer using TMS as the internal
standard. Electrochemical measurements were made using a CH
Instruments model 600A electrochemical analyzer. A platinum
disk working electrode, a platinum wire auxiliary electrode and
an aqueous saturated calomel reference electrode (SCE) were used
in the cyclic voltammetry experiments. All electrochemical exper-
iments were performed under a dinitrogen atmosphere. All elec-
trochemical data were collected at 298 K and are uncorrected for
junction potentials.
(2PPh3*).
2.2.3. 4-R
All the 4-R complexes were synthesized by following a general
procedure. Specific details for a particular complex, viz. 4-OCH3,
are given below.
para-Methoxybenzaldehyde
thiosemicarbazone
(21 mg,
0.10 mmol) was dissolved in toluene (50 mL) and triethylamine
(20 mg, 0.20 mmol) was added to it. The solution was then purged
with a stream of dinitrogen for 10 min and to it was added
[Ir(PPh3)3Cl] (100 mg, 0.10 mmol). The mixture was refluxed under
a dinitrogen atmosphere for 24 h, whereby a yellow solution was
obtained. Evaporation of this solution afforded a dark solid, which
was purified by thin layer chromatography on a silica plate. With
benzene as the eluant, two yellow bands separated, which were
extracted separately with acetonitrile. Slow evaporation of the first
yellow extract afforded the 4-OCH3 complex (Yield: 45%) and that
of the second yellow extract afforded the 3-OCH3 complex (Yield:
30%).
Anal. Calc. for 4-OCH3: C, 56.33; H, 4.07; N, 4.38. Found: C,
56.59; H, 4.10; N, 4.41%. 1H NMR: 3.43 (OCH3); 4.32 (s, NH2);
6.40 (d, 1H, JH–H = 8.5); 6.59 (s, 1H); 6.69 (s, azomethine); 6.89
(d, 1H, JH–H = 8.1); 7.15–7.6 (2PPh3*). Anal. Calc. for 4-CH3: C,
57.28; H, 4.14; N, 4.45. Found: C, 57.54; H, 4.09; N, 4.47%. 1H
NMR: 2.00 (CH3); 4.60 (s, NH2); 6.57 (d, 1H, JH–H = 7.6); 6.80 (s,
1H); 6.84 (d, 1H, JH–H = 7.9); 6.90 (s, azomethine); 7.18–7.55
(2PPh3*). Anal. Calc. for 4-H: C, 56.85; H, 3.98; N, 4.52. Found: C,
57.04; H, 3.99; N, 4.55%. 1H NMR: 4.31 (s, NH2); 6.51 (t, 1H, JH–
H = 7.3); 6.67 (s, azomethine); 6.74 (t, 1H, JH–H = 7.2); 6.90 (d, 1H,
JH–H = 7.4); 7.14–7.70 (1H + 2PPh3*). Anal. Calc. for 4-Cl: C, 54.82;
H, 3.74; N, 4.36. Found: C, 55.13; H, 3.80; N, 4.39%. 1H NMR: 4.51
(s, NH2); 6.71 (d, 1H, JH–H = 8.2); 6.84 (d, 1H, JH–H = 7.9); 7.06 (s,
1H); 7.19 (s, azomethine); 7.20–7.56 (2PPh3*). Anal. Calc. for 4-
NO2: C, 54.22; H, 3.69; N, 5.75. Found: C, 54.68; H, 3.73; N,
5.76%. 1H NMR: 4.48 (s, NH2); 6.91 (s, azomethine); 6.97 (d, 1H,
JH–H = 7.9); 7.18–7.86 (1H + 2PPh3*); 8.02 (s, 1H).
2.4. Crystallography
Single crystals of the 1-NO2 + 2-NO2, and 3-NO2 complexes
were obtained by slow evaporation of 1:1 dichloromethane-aceto-
nitrile solutions of the respective complexes. Single crystals of the
4-CH3 complex were obtained by slow diffusion of toluene into an
acetonitrile solution of the complex. Selected crystal data and data
collection parameters are given in Table 1. Data on the crystals of
the 1-NO2 + 2-NO2, and 4-CH3 complexes were collected on a Bru-
ker Smart CCD diffractometer, while those on the crystal of the 3-
NO2 complex were collected on a Nonius CAD4 diffractometer. X-
ray data reduction and, structure solution and refinement were
done using SHELXS-97 and SHELXL-97 programs [9]. The structures
were solved by direct methods.
3. Results and discussion
3.1. Syntheses and crystal structures
Five 4-R-benzaldehyde thiosemicarbazones (I) have been used
in the present study, differing in the inductive effect of the substi-
tuent R (R = OCH3, CH3, H, Cl, and NO2), in order to observe their
influence, if any, on the redox potentials of the complexes. Reac-
tions of the selected thiosemicarbazones (I) with [Ir(PPh3)3Cl] pro-
ceed smoothly in refluxing ethanol in the presence of
triethylamine and each of these reactions affords two products.
From the reaction with the para-nitrobenzaldehyde thiosemicarba-
zone two red products are obtained, whereas reaction with each of
2.3. Physical measurements
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. IR spectra were obtained on a Per-
Table 1
Crystallographic data for the 1-NO2, 2-NO2, 3-NO2 and 4-CH3 complexes.
1
1-NO2 + 2-NO2ꢂCH3CN
3-NO2ꢂ /2H2O
4-CH3ꢂC6H5CH3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C45H40N4.50O2P2SCl0.50Ir
C44H38N4O2.50P2SIr
948.98
2(C45H39N3P2SClIr), C7H8
1979.08
979.74
orthorombic
Pnma
21.5655(4)
14.9787
27.5334(5)
90
90
90
8893.9(3)
8
triclinic
triclinic
ꢀ
ꢀ
P1
P1
12.293(3)
19.760(4)
20.014(4)
90.02(3)
98.60(3)
106.97(3)
4592.7(16)
4
12.219(8)
17.699(11)
21.642(14)
74.577(14)
87.688(14)
80.750(14)
4453(5)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
k (Å)
0.71073
0.33 ꢃ 0.25 ꢃ 0.10
295(2)
3.192
0.0646
0.1592
1.002
0.71073
0.50 ꢃ 0.40 ꢃ 0.30
295(2)
3.061
0.0419
0.71073
0.32 ꢃ 0.24 ꢃ 0.12
100(2)
3.213
0.0641
Crystal size (mm3)
T (K)
l
R1
(mmꢀ1
)
a
b
wR2
0.1312
1.091
0.1349
1.00
GOFc
a
b
c
R1
wR2 = [
GOF = [
=
R
||Fo| ꢀ |Fc||/
R|Fo|.
{wðF2o ꢀ F2c Þ }/
R
{wðF2o Þ}]1/2
.
2
R
(wðFo2 ꢀ Fc2Þ )/(M ꢀ N)]1/2, where M is the number of reflections and N is the number of parameters refined.
2
R

S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
2851
Table 2
the other four thiosemicarbazones (with R–NO2) yields two yellow
products. Structures of the two red products obtained from the
reaction with the para-nitrobenzaldehyde thiosemicarbazone have
been determined by X-ray crystallography. The structure of the
first red product [10] reveals it to be a 1:1 mixture of two com-
plexes, henceforth referred to as complexes 1-NO2 and 2-NO2.
Structures of the individual 1-NO2 and 2-NO2 complexes are dis-
played, respectively in Figs. 1 and 2, and some relevant bond
parameters are listed in Table 2. In the 1-NO2 complex (Fig. 1)
the thiosemicarbazone is coordinated to iridium, via loss of the
acidic proton, as a bidentate N,S-donor forming a four-membered
chelate ring (II). Two triphenylphosphines, a hydride and a chlo-
ride are also coordinated to the metal center. Iridium is thus sitting
in a HNP2SCl coordination sphere, which is distorted considerably
from ideal octahedral geometry. The coordinated thiosemicarba-
zone, hydride and chloride have constituted one equatorial plane
with the metal at the center, where the chloride is trans to the sul-
fur and the hydride is trans to the nitrogen. The PPh3 ligands have
taken up the remaining two axial positions and hence they are
mutually trans. The Ir–N distance is a bit longer than usual, prob-
Selected bond lengths (Å) and bond angles (°) for 1-NO2, 2-NO2, 3-NO2 and 4-CH3
complexes.
1-NO2
Bond lengths (Å)
Ir(1)–H(1)
Ir(1)–N(2)
Ir(1)–P(1)
Ir(1)–P(1A)
Ir(1)–S(1)
1.497(10)
2.206(9)
2.334(2)
2.334(2)
2.379(3)
Ir(1)–Cl(1)
C(1)–N(2)
C(2)–N(3)
N(2)–N(3)
C(1)–S(1)
2.409(3)
1.278(15)
1.256(14)
1.393(13)
1.732(13)
Bond angles (°)
H(1)–Ir(1)–N(2) 177(4)
P(1)–Ir(1)–P(1A) 173.80(9)
S(1)–Ir(1)–Cl(1) 165.10(12)
N(2)–Ir(1)–S(1) 65.5(3)
2-NO2
Bond lengths (Å)
Ir(2)–H(2)
Ir(2)–H(3)
Ir(2)–N(6)
Ir(2)–P(2)
Ir(2)–P(2A)
1.504(10)
Ir(2)–S(2)
C(27)–N(6)
C(28)–N(7)
N(6)–N(7)
C(27)–S(2)
2.521(3)
1.496(10)
2.142(10)
2.281(3)
2.281(3)
1.326(17)
1.268(17)
1.337(14)
1.693(15)
Bond angles (°)
H(2)–Ir(2)–N(6) 164(5)
H(3)–Ir(2)–S(2) 143(5)
P(2)–Ir(2)–P(2A) 171.37(10)
N(6)–Ir(2)–S(2) 64.7(3)
3-NO2
Bond lengths (Å)
Ir(1)–H(1)
Ir(1)–C(8)
Ir(1)–N(3)
Ir(1)–P(1)
Ir(1)–P(2)
1.41(2)
Ir(1)–S(1)
C(1)–N(2)
C(2)–N(3)
N(2)–N(3)
C(1)–S(1)
2.424(2)
2.042(8)
2.073(7)
2.302(2)
2.305(2)
1.290(11)
1.301(11)
1.351(9)
1.738(9)
Bond angles (°)
H(1)–Ir(1)–N(3) 175(4)
C(8)–Ir(1)–S(1) 159.7(2)
P(1)–Ir(1)–P(2) 165.43(8)
C(8)–Ir(1)–N(3) 80.3(3)
N(3)–Ir(1)–S(1) 79.5(2)
4-CH3
Bond lengths (Å)
Ir(1)–N(1)
Ir(1)–S(1)
Ir(1)–C(1)
Ir(1)–P(1)
Ir(1)–P(2)
2.006(7)
Ir(1)–Cl(1)
N(1)–N(2)
C(7)–N(1)
C(8)–N(2)
C(8)-S(1)
2.389(3)
2.433(3)
2.013(9)
2.346(3)
2.367(3)
1.462(10)
1.302(11)
1.317(11)
1.745(9)
Bond angles (°)
C(1)–Ir(1)–S(1) 160.4(2)
N(1)–Ir(1)–Cl(1) 173.7(2)
P(1)–Ir(1)–P(2) 169.53(8)
C(1)–Ir(1)–N(1) 79.4(3)
N(1)–Ir(1)–S(1) 81.1(2)
Fig. 1. View of the 1-NO2 complex.
ably due to the strong trans influence of the hydride. The other
bond distances around the metal center and within the chelated
thiosemicarbazone are all quite normal [6,11]. Structure of the 2-
NO2 complex (Fig. 2) is very similar to that of the 1-NO2 complex,
except that a second hydride is coordinated to iridium instead of
the chloride. The Ir-S bond is significantly longer than that in the
1-NO2 complex, and the observed elongation may be attributed
again to the trans influence of the hydride. The other bond param-
eters are comparable to those in the 1-NO2.
The second red product [10] has been found to be a single com-
plex, henceforth referred to as complex 3-NO2, and its structure is
shown in Fig. 3. In this complex the thiosemicarbazone is coordi-
nated to iridium as a tridentate C,N,S-donor (V) via loss of two pro-
tons. Two triphenylphosphines and a hydride are also coordinated
to iridium. The tricoordinated thiosemicarbazone ligand is sharing
the same equatorial plane with iridium and the hydride, and the
PPh3 ligands are trans as before. The Ir–C distance is normal [6],
and the other bond distances (Table 2) within the iridium-bound
thiosemicarbazone fragment are consistent with the bond descrip-
tions shown in V.
Fig. 2. View of the 2-NO2 complex.

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S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
Fig. 4. View of (a) the 4-CH3 complex and (b) the equatorial plane in it.
very similar to the corresponding 3-R complexes except that a
chloride is bound to iridium instead of the hydride. Further
authentication has been done by structure determination of a rep-
resentative complex from this family, viz. 4-CH3, which shows
(Fig. 4, Table 2) that the benzaldehyde thiosemicarbazone is coor-
dinated to iridium as a tridentate C,N,S-donor (V) and, two triphe-
nylphosphines and a chloride are also coordinated to the metal
center. The Ir–Cl bond length is normal and the other structural
features compare well with those of the 3-NO2 complex. As all
the five 4-R complexes have been synthesized similarly and they
show similar properties (vide infra), the other four members of this
group (with R–CH3) are assumed to have a similar structure as the
4-CH3 complex.
The synthetic reactions carried out in two different solvents
show that the nature of products obtained depends on the nature
of solvent used. When ethanol is used as the solvent, complexes
of three types, viz. 1-R (R = NO2), 2-R and 3-R, are obtained. The
mechanism behind formation of these three types of complexes
is not completely clear to us. However, the sequences shown in
Scheme 1 seem probable. The synthetic reaction seems to have
proceeded by following two different kinetic routes. In one route,
oxidative insertion of iridium into the S-H bond (in the thiolate
tautomer) takes place in the initial step associated with simulta-
neous dissociation of one PPh3, and thus the thiosemicarbazone
binds to the metal center as a bidentate N,S-donor forming a
four-membered chelate ring (II) and affords the 1-R complex. Pre-
cedence of such S–H activation is available in the literature [13].
This particular complex could be trapped only for the R = NO2 case.
The 1-R complex then reacts with ethanol in the presence of NEt3,
whereby the coordinated chloride got displaced by a hydride, and
thus the corresponding dihydride complex 2-R is produced. Forma-
tion of such iridium hydride from a chloride precursor is well
Fig. 3. View of (a) the 3-NO2 complex and (b) the equatorial plane in it.
The first group of yellow complexes [12], obtained from reac-
tions with the four thiosemicarbazones with R–NO2, are found to
have composition similar to that of the 2-NO2 complex, and they
show similar spectral and electrochemical properties as the 2-
NO2 complex (vide infra). Hence these yellow complexes are as-
sumed to have similar structures as the 2-NO2 complex. These four
yellow complexes, along with the 2-NO2 complex, are in general
referred to as the 2-R complexes. The second group of yellow com-
plexes [12] are found to be similar, in composition and properties,
to the 3-NO2 complex, and so they are assumed to have similar
structures as the 3-NO2 complex. These second group of yellow
complexes, along with the 3-NO2 complex, are in general referred
to as the 3-R complexes.
The presence of hydride in all the above complexes has been
quite intriguing. Two sources of the hydride seem probable. Oxida-
tive addition of iridium(I) to the thiosemicarbazone may generate
a hydride out of the acidic proton, or the solvent (ethanol) may
serve as a source of the hydride. To check this, reactions of the 4-
R-benzaldehyde thiosemicarbazones (I) with [Ir(PPh3)3Cl] have
also been carried out in a non-alcoholic solvent, viz. toluene, in
the presence of triethylamine. Each of these reactions has afforded
complexes of two types, one of which has been found to be the
same 3-R complex obtained from the ethanol reaction. This clearly
indicates that ethanol has not been the only source of hydride in
these 3-R complexes. Preliminary characterizations (viz. micro-
analysis, IR and 1H NMR) on the complexes of the second type,
henceforth referred to as the 4-R complexes, show that they are

S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
2853
R
R
C
N
H
NH2
S
C
N
SH
H
N
C
H
N C
NH2
[Ir(PPh3)3Cl]
- PPh3
R
P = PPh3
Cl
P
R
H
C
N
NH2
H
N
Ir
H
C
Ir
Ph3P
S
S
C
N
P
H
N
C
PPh3
Cl
NH2
1-R
A-R
R
- HCl
EtOH
EtOH
P
R
H
H
N
C
N
NH2
H
N
C
Ir
P
Ph3P
S
S
C
NH2
Ir
H
C
H
N
PPh3
H
P
R
H
2-R
B-R
- H2
Ir
P
S
C
N
N
H
C
NH2
3-R
Scheme 1. Probable steps of the synthetic reaction in ethanol.
documented in the literature [14]. The 1-R complexes for R–NO2
could not be isolated, probably because they undergo rapid irre-
versible conversion into the corresponding 2-R (R–NO2) com-
plexes in refluxing ethanol. In the other route, S–H bond
activation takes place in the first step, as before, affording a
mono-hydride intermediate A-R, where the thiosemicarbazone is
coordinated to the metal center as a bidentate N,S-donor forming
five-membered chelate ring (III). Hence each A-R intermediate is
linkage isomer of the corresponding 1-R complex. In alcoholic
medium these A-R intermediates may convert into the correspond-
ing dihydride species B-R, which are linkage isomers of the respec-
tive 2-R complexes. In the final step, C–H activation at the ortho
position of the pendant phenyl ring of the coordinated thiosemi-
carbazone (in B-R) takes place via molecular hydrogen leading to
the formation of the 3-R complexes [15]. The same 3-R complexes
may also be obtained from A-R via elimination of HCl. The hydride
in 1-R, as well as one of the two hydrides in 2-R, is therefore gen-
erated from S-H activation by iridium, while source of the second
hydride in 2-R is ethanol. As isolated yield of the complex 2-R
(for R–NO2; or combined yield of 1-NO2 and 2-NO2 complexes)
is found to be ꢁ40% and that of complex 3-R to be ꢁ25%, it may
be assumed that the 1-R and A-R species, which are precursors
of 2-R and 3-R complexes, respectively, are also formed in similar
relative ratio irrespective of the nature of thiosemicarbazone li-
gands or, to be precise, with the nature of substituent R in the thi-
osemicarbazone ligands.
From the synthetic reactions carried out in toluene, organoiridi-
um complexes of two types, viz. 3-R and 4-R, are obtained and the
probable steps behind their formation are illustrated in Scheme 2.
As before, oxidative insertion of iridium into the S–H bond of benz-
aldehyde thiosemicarbazone seems to take place in the initial step
affording the intermediate A-R, as well as its geometric isomer C–R.
It may be noted here that formation of 1-R, the possible linkage
isomer of A-R, has not been favored in refluxing toluene. Isomeri-

2854
S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
R
R
C
N
H
NH2
S
C
N
SH
H
H
N
C
C
N
NH2
[Ir(PPh3)3Cl]
- PPh3
P = PPh3
P
P
R
R
H
Cl
H
Cl
Ir
P
Ir
S
C
N
S
C
N
P
H
N
C
H
N
C
NH2
NH2
A-R
C-R
- HCl
- H2
P
P
R
R
H
S
Cl
Ir
P
Ir
P
C
N
S
C
N
N
H
N
C
H
C
NH2
NH2
3-R
4-R
Scheme 2. Probable steps of the synthetic reaction in toluene.
zation of A-R into C-R in refluxing toluene, which did not take place
in refluxing ethanol, is believed to be thermally driven. The inter-
mediate A-R undergoes cyclometallation, via elimination of HCl,
affording the 3-R complex. Similarly complex 4-R is obtained from
the intermediate C-R through elimination of molecular hydrogen
[15]. The speculated intermediates of these reactions could not
be isolated, probably because they undergo rapid irreversible con-
version into the corresponding cyclometallated species. In view of
the observed yields of the 3-R and 4-R complexes, it appears that
transformation of A-R into 3-R and C-R takes place at comparable
rates.
similar to that of the corresponding 3-R complex. 1H NMR spectra
of the 2-R complexes show broad signals within 7.18–7.68 ppm
due to the coordinated PPh3 ligands. In each complex the two hy-
drides show two distinct signals near ꢀ15.0 and ꢀ20.0 ppm, which
appear as doublet of triplets due to coupling with the two magnet-
ically equivalent phosphorus nuclei as well as coupling between
the two hydrides. Signal for the NH2 fragment of the thiosemicar-
bazone is observed near 4.5 ppm. Most of the expected aromatic
proton signals for the coordinated thiosemicarbazone ligand have
been clearly observed, while few signals could not be detected
due to their overlap with other signals. Signals for the hydrogen-
containing substituents in the thiosemicarbazone ligand
(R = OCH3 and CH3) are also observed in the expected region. 1H
NMR spectra of the 3-R complexes show a hydride signal near
ꢀ14.0 ppm, while the other spectral features are mostly similar
to those of the 2-R complexes. Besides absence of the hydride sig-
nal, 1H NMR spectrum of any 4-R complex is similar to that of the
corresponding 3-R complex. The infrared and 1H NMR spectral data
of all the complexes are therefore consistent with their composi-
tion and stereochemistry.
3.2. Spectral properties
Infrared spectra of the 2-R (R–NO2) and 3-R complexes, respec-
tively show two and one strong Ir–H stretch/stretches within
2097–2151 cmꢀ1
. Three strong bands near 520, 695 and
745 cmꢀ1 are observed in all these complexes due to the coordi-
nated PPh3 ligands. Several prominent bands are also displayed
by these complexes within 1600–1000 cmꢀ1, which are absent in
the spectrum of [Ir(PPh3)3Cl], and hence these are attributable to
the coordinated thiosemicarbazone ligands. Besides absence of
the Ir-H stretch, infrared spectrum of each 4-R complex is very
Electronic spectra of all the complexes have been recorded in
acetonitrile solution. Each complex shows several intense absorp-
tions in the visible and ultraviolet region (Table 3). The absorptions

S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
2855
Table 3
Electronic spectral and cyclic voltammetric data.
Compound Electronic spectral data kmax, nm (
e
,
Cyclic voltammetric datab
E, V vs. SCE
a
Mꢀ1 cmꢀ1
)
2-OCH3
2-CH3
2-H
384(18000), 302(16600)c,
254(28000)c, 234(45000)c
386(14000), 302(14000)c,
254(25900)c, 234(45400)c
388(15700), 306(18200)c,
256(31200)c, 234(54100)c
392(9700), 306(12300)c, 254
(22800)c, 234(37400)c
1.33d, 0.94d, ꢀ1.26e
1.51d, 0.94d, ꢀ1.32e
1.55d, 0.95d, ꢀ1.35e
1.57d, 1.01d, ꢀ1.37e
2-Cl
1-NO2 + 2- 438(11100), 322(7500)c,
NO2
3-OCH3
1.45d, 1.15d, 0.71d,
ꢀ1.10e, ꢀ1.50e
270(16900)c, 236(35800)c
438(8100)c, 276(40700),
236(75300)c
0.92d, 0.43(77)f, ꢀ1.28e
3-CH3
3-H
436(3100), 264(27400)c,
236(53200)c
1.14d, 0.47(72)f, ꢀ1.33e
1.16d, 0.53(70)f, ꢀ1.47e
1.26d, 0.61(65)f, ꢀ1.37e
438(6750), 274(32000)c,
236(68500)c
3-Cl
451(2400), 268(24000)c,
232(49400)c
3-NO2
4-OCH3
4-CH3
4-H
490(5200), 346(10100),
260(20300)
1.31d, 0.78(70)f, ꢀ0.74e,
ꢀ1.27e
450(4200), 342(3900)c,
310(11800)c, 274(29200)
454(3500), 340(6300)c,
310(11300)c, 274(32000)
454(1800), 342(4500)c,
312(7100)c, 272(20700)
460(3400), 342(5400)c,
312(11200)c, 272(30500)
1.09d, 0.64(68)f, ꢀ1.09e
1.24d, 0.66(76)f, ꢀ1.06e
Fig. 5. Partial molecular orbital diagram of the 3-H complex.
1.29d, 0.73(61)f, ꢀ1.04e
1.07d, 0.79(70)f, ꢀ1.05e
4-Cl
plexes. Compositions of selected molecular orbitals are given in Ta-
ble 4 and partial MO diagram of a selected complex is shown in
Fig. 5. Partial MO diagrams of the other complexes are deposited
as supporting information (Figs. S1–S3). The highest occupied
molecular orbital (HOMO) and the next two filled orbitals
(HOMO-1 and HOMO-2) have major contributions from the irid-
ium dxy, dyz and dzx orbitals, and hence these three occupied orbi-
tals may be regarded as the iridium t2 orbitals. The lowest
unoccupied molecular orbital (LUMO) has >85% contribution from
the thiosemicarbazone ligand and is concentrated mostly on the
imine (C@N) fragment. The LUMO+1 is localized on other parts of
the thiosemicarbazone ligand. The lowest energy absorption in
the visible region is therefore assignable to an allowed charge-
4-NO2
532(3700), 382(6600)c, 340(8100), 1.29d, 0.92(73)f, ꢀ1.0e, -
270(26700)
1.36e
a
b
c
d
e
f
In acetonitrile.
Solvent, acetonitrile; supporting electrolyte, TBAP; scan rate 50 mV sꢀ1
.
Shoulder.
Epa value.
Epc value.
E1/2 value (
D
Ep value), where E1/2 = 0.5 ðEpa þ EpcÞ and
D
Ep = (Epa ꢀ Epc).
in the ultraviolet region are attributable to transitions within the
ligand orbitals. To have an insight into the nature of the absorp-
tions in the visible region, qualitative EHMO calculations have
been performed [16] on computer generated models of four repre-
sentative members of the four types of complexes, where phenyl
rings of the triphenylphosphines have been replaced by hydrogens.
The results are found to be qualitatively similar for all the com-
transfer transition from the filled iridium t2-orbital (HOMO) to
*
the vacant
p (imine)-orbital of the thiosemicarbazone ligand
(LUMO). The other intense absorptions in the visible region may
be assigned to charge-transfer transitions from the iridium t2-orbi-
tals to the higher energy vacant orbitals.
3.3. Electrochemical properties
Table 4
Composition of selected molecular orbitals.
Electrochemical properties of all the complexes have been stud-
ied by cyclic voltammetry in acetonitrile solution (0.1 M TBAP).
Voltammetric data are given in Table 3. All the complexes show
two oxidative responses on the positive side of SCE and a reductive
response on the negative side. A representative cyclic voltammo-
gram is deposited as supporting information (Fig. S4). In view of
the composition of the HOMO, the first oxidative response is as-
signed to Ir(III)–Ir(IV) oxidation. The second oxidative response is
tentatively assigned to oxidation of the coordinated thiosemicar-
bazone. In view of the composition of the LUMO, the reductive
response is assigned to reduction of the coordinated thiosemicar-
bazone. One-electron stoichiometry of all the responses has been
established by comparing their current heights with those of stan-
dard ferrocene/ferrocenium couple under identical experimental
conditions. Except the Ir(III)–Ir(IV) oxidation in the 3-R and 4-R
complexes, all the other redox responses are found to be irrevers-
ible. Potential of the irreversible redox responses did not show any
systematic variation with the electron-withdrawing character of
the substituent R in the thiosemicarbazone ligand. However,
Compound Contributing % contribution of fragments to
fragmentsa
HOMO HOMO- HOMO- LUMO LUMO+1
1
2
1-NO2
2-H
Ir
85
10
78
17
77
17
0
98
(NO2,
67)
0
0
98
tsc-NO2
Ir
tsc-H
82
13
79
17
79
16
0
100
99
(C@N,
61)
9
3-H
Ir
tsc-H
72
25
44
50
63
31
0
91
86
(C@N,
39)
10
64
(C@N,
39)
4-H
Ir
tsc-H
75
21
44
49
63
31
0
94
a
tsc-R means thiosemicarbazone ligand, where R is the substituent.

2856
S. Basu et al. / Inorganica Chimica Acta 363 (2010) 2848–2856
(g) R.I. Maurer, P.J. Blower, J.R. Dilworth, C.A. Reynolds, Y. Zheng, G.E.D. Mullen,
J. Med. Chem. 45 (2002) 1420;
potential of the reversible Ir(III)–Ir(IV) oxidation in the 3-R and 4-
R complexes has been found to be sensitive to the nature of the
substituent R. The potential increases with increasing electron-
withdrawing character of the substituent R. The plots of oxidation
(h) J. Patole, S. Dutta, S.B. Padhye, E. Sinn, Inorg. Chim. Acta 318 (2001) 207;
(i) Z. Iakovidou, A. Papageorgiou, M.A. Demertzis, E. Mioglou, D. Mourelatos, A.
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potential versus
r
[
r
= Hammett constant of
R
[17]];
OCH3 = ꢀ0.27, CH3 = ꢀ0.17, H = 0.00, Cl = 0.23 and NO2 = 0.78] are
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linear for both the [Ir(PPh3)2(CNS-R)(H)] and [Ir(PPh3)2(CNS-R)Cl]
complexes (Figs. S5 and S6) with slopes (
q) of 0.33 V and 0.27 V,
respectively ( = reaction constant of this couple [18]). This shows
q
that the para-substituent (R) on the thiosemicarbazone ligand,
which is four-bonds away from the metal center, can still influence
the metal-centered oxidation potential in a predictable manner.
(e) R. Acharyya, S. Dutta, F. Basuli, S.M. Peng, G.H. Lee, Larry R. Falvello, S.
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6113;
4. Conclusions
The present study shows that [Ir(PPh3)3Cl] can successfully
mediate both S-H and C-H activation of the 4-R-benzaldehyde
thiosemicarbazones (I). However, the nature of complexes formed
depends largely on the experimental factors. The present study
also indicates that such bond activation of some other organic mol-
ecules, that have structural similarity with the benzaldehyde thio-
semicarbazone, may be brought about by their reaction with
[Ir(PPh3)3Cl], and such possibilities are currently under
exploration.
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Acknowledgements
The authors thank the reviewers for their constructive com-
ments, which have been helpful in preparing the revised manu-
script. Financial assistance received from the UGC-CAS Program
of the Department of Chemistry, Jadavpur University, is gratefully
acknowledged. The authors thank the RSIC at Central Drug Re-
search Institute, Lucknow, India, for the C, H, N analysis data, and
the Department of Chemistry, Indian Institute of Technology, Kan-
pur, India, for the X-ray diffraction data of the 4-CH3 complex.
(i) T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037;
(j) H.Y. Jang, M.J. Krische, Acc. Chem. Res. 37 (2004) 653.
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(2005) 167;
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43 (2004) 704.
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Wiley, New York, 1974;
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[8] 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.
[9] G.M. Sheldrick, SHELXS-97 and SHELXL-97, Fortran programs for crystal structure
solution and refinement, University of Gottingen, 1997.
[10] The first red product refers to the product obtained from evaporation of the
first red fraction from the chromatography. Similarly the product obtained
from evaporation of the second red fraction is referred to as the second red
product.
Appendix A. Supplementary data
CCDC 761128, 761129 and 761130 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.ac.uk/data_request/cif. Supplementary data
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