Rhodium assisted C-H activation of benzaldehyde thiosemicarbazones and their oxidation via activation of molecular oxygen

Article abstract of DOI:10.1021/ic050505w

The benzaldehyde thiosemicarbazones are found to undergo oxidation at the sulfur center upon reaction with [Rh(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃). A group of organorhodium complexes are obtained from such reactions, in which the oxidized thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a hydride. From the reaction with para-nitrobenzaldehyde thiosemicarbazone, a second organometallic complex is obtained, in which the thiosemicarbazone is coordinated to rhodium as a tridentate CNS donor, along with two triphenylphosphines and a hydride. Reaction of the benzaldehyde thiosemicarbazones with [Rh(PPh₃)₃Cl] in refluxing ethanol in the absence of NEt₃ affords another group of organorhodium complexes, in which the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a chloride. Structures of representative complexes of each type of complexes have been determined by X-ray crystallography. In all of the complexes, the two PPh₃ ligands are trans. All of the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a Rh(III)-Rh(IV) oxidation on the positive side of SCE. Redox responses of the coordinated thiosemicarbazones are also displayed by all of the complexes.

Full text of DOI:10.1021/ic050505w

Inorg. Chem. 2006, 45, 1252−1259
Rhodium Assisted C
−H Activation of Benzaldehyde Thiosemicarbazones
and Their Oxidation via Activation of Molecular Oxygen
Rama Acharyya,† Swati Dutta,† Falguni Basuli,† Shie-Ming Peng,‡ Gene-Hsiang Lee,‡
Larry R. Falvello,§ and Samaresh Bhattacharya*,†
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity, Kolkata - 700 032,
India, Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, R.O.C., and
Department of Inorganic Chemistry, Faculty of Science, UniVersity of Zaragoza, E-50009
Zaragoza, Spain
Received April 5, 2005
The benzaldehyde thiosemicarbazones are found to undergo oxidation at the sulfur center upon reaction with
[Rh(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3). A group of organorhodium complexes are
obtained from such reactions, in which the oxidized thiosemicarbazones are coordinated to rhodium as tridentate
CNS donors, along with two triphenylphosphines and a hydride. From the reaction with para-nitrobenzaldehyde
thiosemicarbazone, a second organometallic complex is obtained, in which the thiosemicarbazone is coordinated
to rhodium as a tridentate CNS donor, along with two triphenylphosphines and a hydride. Reaction of the benzaldehyde
thiosemicarbazones with [Rh(PPh3)3Cl] in refluxing ethanol in the absence of NEt3 affords another group of
organorhodium complexes, in which the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors,
along with two triphenylphosphines and a chloride. Structures of representative complexes of each type of complexes
have been determined by X-ray crystallography. In all of the complexes, the two PPh3 ligands are trans. All of the
complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a
Rh(III)
−Rh(IV) oxidation on the positive side of SCE. Redox responses of the coordinated thiosemicarbazones are
also displayed by all of the complexes.
Introduction
ligands and rhodium as the metal. It might be relevant to
mention here that, although the chemistry of thiosemicar-
bazone complexes of many transition metals has been
extensively studied,1 that of rhodium appears to have
There is considerable current interest in the chemistry of
transition metal complexes of the thiosemicarbazone ligands,
primarily because of their bioinorganic relevance.1 The
complexes in this class are of particular importance because
of their potentially beneficial biological (viz., antibacterial,
antimalarial, antiviral, and antitumor) activities.2 However,
we have been interested in the chemistry of thiosemicarba-
zone complexes of the platinum metals particularly because
of the variable coordination modes displayed by such ligands
in these complexes.3 For the present study, we have chosen
a group of benzaldehyde thiosemicarbazones (1) as the
* To whom correspondence should be addressed. E-mail: samaresh_b@
hotmail.com.
† Jadavpur University.
‡ National Taiwan University.
§ University of Zaragoza.
(1) (a) West, D. X.; Liberta, A. E.; Padhye, S. B.; Chikate, R. C.;
Sonawane, P. B.; Kumbhar, A. S.; Yerande, R. G. Coord. Chem. ReV.
1993, 123, 49. (b) West, D. X.; Padhye, S. B.; Sonawane, P. B. Struct.
Bonding 1992, 76, 1. (c) Haiduc, I.; Silvestru, C. Coord. Chem. ReV.
1990, 99, 253. (d) Padhye, S. B.; Kaffman, G. B. Coord. Chem. ReV.
1985, 63, 127. (e) Campbell, M. J. M. Coord. Chem. ReV. 1975, 15,
279.
1252 Inorganic Chemistry, Vol. 45, No. 3, 2006
10.1021/ic050505w CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/04/2006

Rh-Assisted C-H ActiWation of Benzaldehyde Thiosemicarbazones
Chart 1
osemicarbazone is coordinated as in 3. Formation of a stable
five-membered ring by the benzaldehyde thiosemicarbazones
can only take place via a conformational change about the
CdN bond in 1, which requires a restricted rotation about a
double bond.5 Being unable to form a five-membered chelate
ring, the benzaldehyde thiosemicarbazones 1 are observed
to bind to the metal center as bidentate N,S donors forming
a rather unusual four-membered chelate ring (4).3c-e Even
though formation of five-membered chelate ring 3 by the
benzaldehyde thiosemicarbazones 1 has been accepted to be
impossible, the proximity of the phenyl ring to the metal
center in 3 points to the possibility of its orthometalation
(5) via C-H activation. Transition-metal-mediated C-H
activation of organic molecules is of significant contemporary
importance, with particular reference to chemical transforma-
tions of organic molecules.6 Such reactions often proceed
via a C-H activation step leading to the formation of
organometallic complexes as reactive intermediates, which
then undergo further reactions to yield the desired product.
With the intention of inducing coordination mode 5 in the
benzaldehyde thiosemicarbazone ligands 1, their reaction has
been carried out with the Wilkinson’s catalyst, viz., [Rh-
(PPh3)3Cl]. We selected this particular rhodium starting
material because of its demonstrated ability to accommodate
a tridentate ligand via oxidative addition3a,7 and, more
importantly, its efficiency in successfully mediating C-H
activation of organic molecules.7a,b This simple strategy has
indeed worked nicely, affording organorhodium complexes
of different types, and the chemistry of these complexes is
reported in this paper with special reference to their synthesis,
structure, and electrochemical properties.
remained almost unexplored.4 Although the thiosemicarba-
zones are usually expected to coordinate metal ions, via
dissociation of the hydrazinic proton, as bidentate N,S donors
forming a five-membered chelate ring (2), we have observed
that formation of such a ring depends primarily on the size
of R1.3c Only if R1 is fairly small (e.g., R1 ) CH3) formation
of a five-membered ring (2) possible, but if R1 is relatively
large (e.g., R1 ) aryl as in ligand 1), such a ring cannot be
formed. The uncoordinated benzaldehyde thiosemicarbazones
1 have the geometry as shown in 1,3c and if such a five-
membered ring has to form keeping the conformation about
the CdN bond in 1 intact, the phenyl ring of the thiosemi-
carbazone ligand comes in contact with the metal center,
and thus a steric hindrance develops between them (3). It
should be mentioned in this context that we were unable to
find a single example of a structurally characterized complex
of the benzaldehyde thiosemicarbazones 1 where the thi-
Experimental Section
Materials. Rhodium trichloride was obtained from Arora Mat-
they, Kolkata, India. All other chemicals and solvents were reagent-
grade commercial materials and were used as received. [Rh(PPh3)3-
Cl] was synthesized following a reported procedure.8 The
benzaldehyde thiosemicarbazones 1 were prepared by reacting
(2) (a) Jouad, E. M.; Thanh, X. D.; Bouet, G.; Bonneau, S.; Khan, M. A.
Anticancer Res. 2002, 22, 1713. (b) Ferrari, M. B.; Bisceglie, F.; Pelosi,
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M. A.; Mioglou, E.; Mourelatos, D.; Kotsis, A.; Yadav, P. N.; Kovala-
Demertzi, D. Anticancer Drugs 2001, 12, 65. (g) Kovala-Demertzi,
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18, 4131. (i) Papageorgiou, A.; Iakovidou, Z.; Mourelatos, D.;
Mioglou, E.; Boutis, L.; Kotsis, A.; Kovala-Demertzi, D.; Domopou-
lou, D. X.; Demertzis, M. A. Anticancer Res. 1997, 17, 247. (j)
Dimmock, J. R.; Puthucode, R. N.; Smith, J. M.; Hetherington, M.;
Quail, J. W.; Pugazhenthi, U.; Lechler, J.; Stables, J. P. J. Med. Chem.
1996, 39, 3984.
(5) (a) Tian, Y. P.; Duan, C. Y.; Lu, Z. L.; You, X. Z.; Fun, H. K.;
Sivakumar, K. Polyhedron 1996, 15, 2263. (b) Sinha, S.; Acharyya,
R.; Basuli, F.; Peng, S. M.; Bhattacharya, S. Jadavpur University,
Kolkata, India, and Taipei, Taiwan, ROC, 2005, unpublished results.
(6) (a) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2003, 3985. (b)
Pamplin, C. B.; Legzdins, P. Acc. Chem. Res. 2003, 36, 223. (c)
Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002, 102, 1731. (d)
Slugovc, C.; Padilla-Martnez, I.; Sirol, S.; Carmona, E. Coord. Chem.
ReV. 2001, 213, 129. (e) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem.
Res. 2001, 34, 633. (f) Tsuji, J. Transition Metal Reagents and
Catalysts; Wiley-VCH: Weinheim, Germany, 2000. (g) Beller, M.;
Bolm, C.; Eds. Transition Metals for Organic Synthesis, 1-2; Wiley-
VCH: Weinheim, Germany, 1998. (h) Hegedus, L. S. Coord. Chem.
ReV. 1998, 168, 49. (i) Cornils, B.; Hermann, W. A.; Eds. Applied
Homogeneous Catalysis with Organometallic Compounds: A Com-
prehensiVe Handbook in Two Volumes; VCH: Weinheim, Germany,
1996. (j) Liebeskind, L. S.; Ed. AdVances in Metal-Organic Chem-
istry; Jai Press: Greenwich, CT, 1996. (k) Abel, E.; Stone, F. G. A.;
Wilkinson, G.; Eds. ComprehensiVe Organometallic Chemistry; Per-
gamon Press: Oxford, U.K., 1995, 12. (l) Hegedus, L. S. Transition
Metals in the Synthesis of Complex Organic Molecules; University
Science Books: Mill Valley, CA, 1994.
(3) (a) Dutta, S.; Basuli, F.; Peng, S. M.; Lee, G. H.; Bhattacharya, S.
New J. Chem. 2002, 26, 1607. (b) Pal, I.; Basuli, F.; Mak, T. C. W.;
Bhattacharya, S. Angew. Chem., Int. Ed. 2001, 40, 2923. (c) Basuli,
F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2000, 39, 1120. (d)
Basuli, F.; Ruf, M.; Pierpont, C. G.; Bhattacharya, S. Inorg. Chem.
1998, 39, 6113. (e) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg.
Chem. 1997, 36, 5645.
(7) (a) Pal, I.; Dutta, S.; Basuli, F.; Goverdhan, S.; Peng, S. M.; Lee, G.
H.; Bhattacharya, S. Inorg. Chem. 2003, 42, 4338. (b) Dutta, S.; Peng,
S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans. 2000, 4623. (c)
Dutta, S.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2000, 39, 2231.
(8) Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1967, 10, 67.
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Popa et al. Lucr. Conf. Nat. Chim. Anal. 3rd 1971, 3, 197.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1253

Acharyya et al.
equimolar amounts of thiosemicarbazide with the respective para-
substituted benzaldehyde in a 1:1 ethanol/water mixture. Purification
of dichloromethane and acetonitrile and preparation of tetrabuty-
lammonium perchlorate (TBAP) for electrochemical work were
performed as before.9
acetonitrile. Evaporation of these extracts yielded the 6-NO2
(Yield: 23%) and 7-NO2 (Yield: 64%) complexes, respectively.
Anal. Calcd for 6-NO2: C, 59.87; H, 4.19; N, 6.35. Found: C,
1
59.95; H, 4.23; N, 6.38. H NMR: -11.75 (t, hydride, J ) 19.5
Hz), 3.58 (NH2), 6.77 (s, 1H), 6.98-7.03 (2H*), 7.07 (s, 1H), 7.12-
7.65 (2PPh3). Anal. Calcd for 7-NO2: C, 62.12; H, 4.35; N, 6.59.
Preparations of Complexes. (i) 6-OCH3. para-Methoxyben-
zaldehyde thiosemicarbazone (25 mg, 0.11 mmol) was dissolved
in ethanol (30 mL), and to it was added triethylamine (25 mg, 0.24
mmol). Then, [Rh(PPh3)3Cl] (100 mg, 0.11 mmol) was added, and
the mixture was heated at reflux for 7 h to yield a deep yellow
solution. Evaporation of this solution gave a brownish-yellow solid,
which was subjected to purification by thin-layer chromatography
on a silica plate. With 1:1 toluene/acetonitrile as the eluant, a yellow
band separated, which was extracted with acetonitrile. Upon
evaporation of the acetonitrile extract, 6-OCH3 was obtained as
crystalline yellow solid. Yield: 53%. Anal. Calcd for 6-OCH3: C,
1
Found: C, 62.31; H, 4.38; N, 6.61. H NMR: -11.15 (t, hydride,
J ) 18.0 Hz), 4.90 (NH2), 6.81 (d, 1H, J ) 6.2 Hz), 6.99 (s, 1H),
7.20-7.61 (2PPh3), 7.90 (s, 1H), 8.01 (d, 1H, J ) 6.2 Hz).
(vi) 8-H. Benzaldehyde thiosemicarbazone (22 mg, 0.12 mmol)
was dissolved in ethanol (40 mL). Then, [Rh(PPh3)3Cl] (100 mg,
0.11 mmol) was added, and the solution was heated at reflux for 7
h to yield a yellow solution. Evaporation of this solution gave a
yellowish-orange solid, which was subjected to purification by thin-
layer chromatography on a silica plate with 10:1 benzene/acetonitrile
as the eluant (Caution! Benzene is carcinogenic.); a yellow band
separated, which was extracted with acetonitrile, and evaporation
of this extract gave 8-H as a crystalline yellow solid. Yield: 60%.
Anal. Calcd for 8-H: C, 62.90; H, 4.41; N, 5.00. Found: C, 63.00;
H, 4.38; N, 5.05. 1H NMR: 4.34 (NH2), 6.53 (t, 1H, J ) 7.6 Hz),
6.63 (s, 1H), 6.80 (t, 1H, J ) 7.2 Hz), 6.96 (d, 1H, J ) 7.4 Hz),
7.13-7.61 (2PPh3).
1
62.29; H, 4.61; N, 4.84. Found: C, 62.33; H, 4.59; N, 4.84. H
NMR:10 -11.45 (t, hydride, J ) 19.5 Hz), 3.35 (OCH3), 4.27 (NH2),
6.64 (s, 1H), 6.76 (d, 1H, J ) 8.0 Hz), 6.82 (s, 1H), 6.92 (d, 1H,
J ) 7.9 Hz), 6.94-7.68 (2PPh3).
(ii) 6-CH3. This complex was prepared by following the same
procedure as described above taking para-methylbenzaldehyde
thiosemicarbazone instead of para-methoxybenzaldehyde thiosemi-
carbazone. Yield: 50%. Anal. Calcd for 6-CH3: C, 63.46; H, 4.70;
(vii) 8-OCH3, 8-CH3, 8-Cl, and 8-NO2. These four complexes
were prepared by following the same procedure as described above
using the respective para-substituted benzaldehyde thiosemicarba-
zone instead of benzaldehyde thiosemicarbazone. Yields varied in
the range of 60-80%.
1
N, 4.94. Found: C, 63.54; H, 4.71; N, 4.96. H NMR: -11.50 (t,
hydride, J ) 19.0 Hz), 1.91 (CH3), 4.69 (NH2), 6.28 (s, 1H), 6.33
(d, 1H, J ) 6.0 Hz), 6.67 (d, 1H, J ) 6.1 Hz), 6.97 (s, 1H), 7.00-
7.79 (2PPh3).
Anal. Calcd for 8-OCH3: C, 62.11; H, 4.49; N, 4.83. Found:
1
C, 62.45; H, 4.53; N, 4.88. H NMR: 3.42 (OCH3), 4.33 (NH2),
(iii) 6-H. This complex was prepared by following the same
procedure as described above taking benzaldehyde thiosemicarba-
zone instead of para-methylbenzaldehyde thiosemicarbazone.
Yield: 52%. Anal. Calcd for 6-H: C, 63.09; H, 4.54; N, 5.02.
Found: C, 63.12; H, 4.52; N, 5.05. 1H NMR: -11.62 (q, hydride,
J ) 19.5 Hz), 3.64 (NH2), 6.25 (t, 1H, J ) 6.00 Hz), 6.57 (t, 1H,
J ) 9.00 Hz), 6.67 (d, 1H, J ) 9.0 Hz), 7.00 (d, 1H, J ) 3.48 Hz),
7.12 (s, 1H), 7.19-7.63 (2PPh3).
6.46 (d, 1H, J ) 8.3 Hz), 6.69 (s, 1H), 6.99 (s, 1H), 7.08 (d, 1H,
J ) 8.4 Hz), 7.15-7.56 (2PPh3). Anal. Calcd for 8-CH3: C, 63.28;
1
H, 4.57; N, 4.92. Found: C, 64.00; H, 4.49; N, 4.92. H NMR:
1.90 (CH3), 4.39 (NH2), 6.59 (d, 1H, J ) 8.5 Hz), 6.61 (s, 1H),
6.86 (d, 1H, J ) 7.5 Hz), 6.93 (s, 1H), 7.14-7.57 (2PPh3). Anal.
Calcd for 8-Cl: C, 60.42; H, 4.12; N, 4.81. Found: C, 61.01; H,
1
4.15; N, 4.79. H NMR: 4.39 (NH2), 6.75 (d, 1H, J ) 8.0 Hz),
6.87 (d, 1H, J ) 8.0 Hz), 7.12 (s, 1H), 7.35 (s, 1H), 7.17-7.57
(2PPh3). Anal. Calcd for 8-NO2: C, 59.70; H, 4.07; N, 6.33.
Found: C, 59.23; H, 3.99; N, 6.37. 1H NMR: 4.87 (NH2), 7.02 (d,
1H, J ) 8.2 Hz), 7.30 (d, 1H, J ) 7.0 Hz), 7.34 (s, 1H), 7.17-
7.61 (2PPh3), 8.08 (s, 1H).
(iv) 6-Cl. This complex was prepared by following the same
procedure as described above taking para-chlorobenzaldehyde
thiosemicarbazone instead of benzaldehyde thiosemicarbazone. The
deep yellow solid obtained after reflux was subjected to purification
by thin-layer chromatography on a silica plate using 10:1 toluene/
acetonitrile as the eluant. A yellow band separated, which was
extracted with acetonitrile. Evaporation of the acetonitrile extract
yielded the 6-Cl complex as a yellow solid. Yield: 52%. Anal.
Calcd for 6-Cl: C, 60.59; H, 4.25; N, 4.82. Found: C, 60.55; H,
Physical Measurements. Microanalyses (C, H, N) were per-
formed using a Heraeus Carlo Erba 1108 elemental analyzer. IR
spectra were obtained on a Shimadzu FTIR-8300 spectrometer with
samples prepared as KBr pellets. Electronic spectra were recorded
on a JASCO V-570 spectrophotometer. Magnetic susceptibilities
were measured using a PAR 155 vibrating sample magnetometer
fitted with a Walker Scientific L75FBAL magnet. 1H NMR spectra
were recorded in CDCl3 solution on a Bruker Avance DPX 300
NMR spectrometer using TMS as the internal standard. Electro-
chemical 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 a three-electrode
configuration. All electrochemical experiments were performed
under a dinitrogen atmosphere. All electrochemical data were
collected at 298 K and are uncorrected for junction potentials.
Crystallography. Single crystals of 6-Cl‚CH3CN‚H2O and
7-NO2‚0.5CH3CN were grown by slow evaporation of acetonitrile
solutions of the respective complexes. Single crystals of 8-Cl were
obtained by slow diffusion of hexane into a dichloromethane
solution of the complex. Selected crystal data and data collection
parameters are reported in Table 1. Data on the crystals of the
1
4.30; N, 4.91. H NMR: -11.42 (t, hydride, J ) 19.3 Hz), 4.75
(NH2), 6.43 (s, 1H), 6.51 (d, 1H, J ) 6.9 Hz), 6.68 (d, 1H, J ) 6.7
Hz), 6.98 (s, 1H), 7.10-7.72 (2PPh3).
(v) 6-NO2 and 7-NO2. para-Nitrobenzaldehyde thiosemicarba-
zone (24 mg, 0.11 mmol) was dissolved in ethanol (30 mL), and
to it was added triethylamine (25 mg, 0.24 mmol). Then, [Rh-
(PPh3)3Cl] (100 mg, 0.11 mmol) was added, and the mixture was
heated at reflux for 7 h to yield a dark red solution. The solid mass
obtained upon evaporation of the red solution was subjected to
purification by thin-layer chromatography on a silica plate using
10:1 toluene/acetonitrile as the eluant. Two prominent bands (a pink
band and a red band) separated, which were extracted with
(9) (a) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry
for Chemists; Wiley: New York, 1974; pp 167-215. (b) Walter, M.;
Ramaley, L. Anal. Chem. 1973, 45, 165.
(10) Chemical shifts are given in ppm, and multiplicity of the signals along
with the associated coupling constants are given in parentheses.
Overlapping signals are marked with an asterisk.
1254 Inorganic Chemistry, Vol. 45, No. 3, 2006

Rh-Assisted C-H ActiWation of Benzaldehyde Thiosemicarbazones
Table 1. Crystallographic Data for 6-Cl, 7-NO2, and 8-Cl
6-Cl‚CH3CN‚H2O
7-NO2‚0.5CH3CN
8-Cl
empirical formula
fw
C46H42ClN4O3P2SRh
931.20
C45H38.50N4.50O2P2SR h
871.21
C44H36Cl2N3P2RhS
874.57
space group
a, Å
b, Å
monoclinic, C2/c
40.8268(9)
10.2807(2)
23.8179(5)
90
120.13(1)
90
8646.5(3)
8
triclinic, P1h
13.611(2)
16.1666(3)
19.4501(18)
94.918(11)
101.705(9)
101.074(18)
4078.1(12)
4
monoclinic, C2/c
23.8312(13)
17.1846(10)
23.2866(12)
90
120.433(2)
90
8222.6(8)
8
c, Å
a, deg
b, deg
g, deg
V, Å3
Z
l, Å
0.71073
0.71073
0.71073
crystal size, mm
T, K
0.25 × 0.20 × 0.10
0.41 × 0.26 × 0.22
0.12 × 0.08 × 0.06
295(2)
299(2)
295(2)
m, mm-1
R1a
0.625
0.0371
0.0860
1.016
0.592
0.0517
0.1124
1.050
0.709
0.709
0.2876
1.016
wR2b
GOFc
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2] ]1/2
.
c GOF ) [∑[w(Fo2 - Fc )2]/(M - N)]1/2 , where M is the number of reflections
2
2
2
and N is the number of parameters refined.
6-Cl‚CH3CN‚H2O and 8-Cl complexes were collected on a
Bruker SMART CCD diffractometer. Data on the crystal of the
7-NO2‚0.5CH3CN complex were collected on an Enraf-Nonius
CAD-4 diffractometer. X-ray data reduction, structure solution, and
refinement were done using the SHELXS-97 and SHELXL-97
packages.11 The structures were solved by direct methods.
course of the synthetic reaction, the thiosemicarbazone
undergoes oxidation at the sulfur center whereby it is
converted into sulfone, and the transformed thiosemicarba-
zone is coordinated to rhodium as a dianionic tridentate
C,N,S donor. Two triphenylphosphines and a hydride are
also coordinated to rhodium. The HCNP2S coordination
sphere around rhodium is distorted octahedral in nature,
which is reflected in the bond parameters (Table 2) around
rhodium. The oxidized thiosemicarbazone, the rhodium, and
the hydride constitute an equatorial plane with the metal at
the center, and the two PPh3 ligands take up the axial
positions and, hence, are mutually trans. The Rh-C, Rh-
N, Rh-P, and Rh-S distances are quite normal.3a,7 However,
the Rh-H bond is rather shorter than usual,13 which can be
attributed to the enhanced electropositive nature of rhodium
due to its link to the sulfone fragment. The C-S, S-O, and
C-N distances in the coordinated thiosemicarbazone are
Results and Discussion
Reaction of the benzaldehyde thiosemicarbazones 1 with
[Rh(PPh3)3Cl] has been first carried out in refluxing ethanol
in the presence of triethylamine. Whereas a yellow complex
is uniformly obtained as the only product from the reactions
with the four thiosemicarbazones 1 with R ) OCH3, CH3,
H, and Cl, reaction with the para-nitrobezaldehyde thiosemi-
carbazone (1, R ) NO2) affords two complexes: a pinkish-
red and a red complex. On the basis of its composition and
properties (vide infra), the pinkish-red complex was deter-
mined to belong to the family of the yellow complexes. The
red complex, however, was found to be different, both in
composition and in properties, than the yellow complexes.
1H NMR spectra of the yellow12 complexes show, in
addition to signals for the coordinated thiosemicarbazone,
broad signals corresponding to two PPh3 ligands and a
distinct triplet near -11.5 ppm indicating the presence of a
coordinated hydride. The triplet nature of the hydride signal
further indicates that the two PPh3 ligands are magnetically
equivalent in these complexes. However, splitting of the
hydride signal due to coupling with the rhodium nucleus (I
1
) /2) was not observed. The preliminary characterization
data (microanalytical and spectroscopic) of these complexes,
although gave some idea of their composition, failed to
indicate any definite formulation for them. The identities of
these complexes were revealed by structure determination
of a representative member of this family by X-ray crystal-
lography. The structure (Figure 1) shows that, during the
(11) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Fortran Programs for
Crystal Structure Solution and Refinement; University of Go¨ttingen,
Go¨ttingen, Germany, 1997.
(12) Pinkish-red for the complex obtained from the reaction with the para-
nitrobezaldehyde thiosemicarbazone (1, R ) NO2).
Figure 1. View of the 6-Cl complex, with thermal ellipsoids drawn at
the 50% probability level.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1255

Acharyya et al.
Table 2. Selected Bond Distances and Bond Angles for 6-Cl, 7-NO2, and 8-Cl
6-Cl‚CH3CN‚H2O
7-NO2‚0.5CH3CN
Bond Distances (Å)
8-Cl
Rh-H(1)
Rh-P(1)
1.437(4)
2.3380(8)
2.3483(8)
2.096(2)
2.039(3)
2.3366(8)
1.301(4)
1.385(3)
1.298(4)
1.326(4)
1.855(3)
1.461(2)
1.465(2)
Rh(1)-H(1H)
Rh(1)-P(1)
Rh(1)-P(2)
Rh(1)-N(2)
Rh(1)-C(39)
Rh(1)-S(1)
C(43)-N(2)
N(2)-N(3)
C(44)-N(3)
C(44)-N(4)
C(44)-S(1)
1.57(6)
Rh-Cl(1)
Rh-P(1)
2.385(3)
2.379(3)
2.371(3)
1.990(10)
2.044(11)
2.450(3)
1.251(14)
1.362(14)
1.337(16)
1.379(15)
1.708(14)
2.3218(16)
2.3041(15)
2.074(4)
2.038(5)
2.4116(15)
1.288(7)
1.370(6)
1.3191(7)
1.3421(7)
1.743(6)
Rh-P(2)
Rh-P(2)
Rh-N(3)
Rh-C(1)
Rh-S(1)
C(7)-N(3)
N(2)-N(3)
C(8)-N(2)
C(8)-N(1)
C(8)-S(1)
S(1)-O(1)
S(1)-O(2)
Rh-N(3)
Rh-C(8)
Rh-S(1)
C(2)-N(3)
N(2)-N(3)
C(1)-N(2)
C(1)-N(1)
C(1)-S(1)
Bond Angles (deg)
H(1)-Rh-N(3)
C(1)-Rh-S(1)
P(1)-Rh-P(2)
C(1)-Rh-N(3)
N(3)-Rh-S(1)
O(1)-S(1)-O(2)
172.61(7)
160.14(9)
165.40(3)
79.73(10)
80.46(7)
H(1H)-Rh(1)-N(2)
C(39)-Rh(1)-S(1)
P(1)-Rh(1)-P(2)
C(39)-Rh(1)-N(2)
N(2)-Rh(1)-S(1)
176(2)
N(3)-Rh-Cl(1)
C(8)-Rh-S(1)
P(1)-Rh-P(2)
C(8)-Rh-N(3)
N(3)-Rh-S(1)
176.1(3)
162.2(3)
179.56(10)
81.1(4)
159.99(17)
169.23(6)
80.2(2)
79.91(12)
81.1(3)
112.74(16)
consistent with its bonding description. As all the five
complexes in this family (6, see Chart 1) were prepared
similarly and display similar spectral and electrochemical
properties (vide infra), the other four 6-R (R * Cl) complexes
are assumed to have a structure similar to that of 6-Cl.
The 1H NMR spectrum of the red complex, obtained as a
second product only from the reaction of para-nitrobenzal-
dehyde thiosemicarbazone with [Rh(PPh3)3Cl], was found
to be quite similar to that of 6-NO2. However, significant
differences were found in the infrared spectra of these two
complexes (vide infra). For an unambiguous characterization
of the red complex, its structure was also determined by
X-ray crystallography. The structure (Figure 2) shows that,
in this complex, the thiosemicarbazone does not undergo any
oxidation and is coordinated to rhodium as a tridentate C,N,S
donor. Two triphenylphosphines and a hydride are also
coordinated to rhodium as before. In this red complex
(7-NO2, Chart 2), the Rh-H length is found to be quite
normal (Table 2).14 The Rh-S distance is significantly longer
in this complex than in 6-Cl, which can be attributed to the
different oxidation states of sulfur in the two complexes.15
The other bond distances in 7-NO2 are comparable to those
observed in 6-Cl.
The mechanism of the observed oxidation at the sulfur
center is not completely clear to us. However, the speculated
sequences shown in Scheme 1 seem probable. In the initial
step, dissociation of the acidic hydrogen from the benzal-
dehyde thiosemicabazones 1 takes place in the presence of
the base. Reaction of the resulting anionic ligands with [Rh-
(PPh3)3Cl] leads to the formation of complexes 7 via
oxidative insertion of rhodium into the phenyl C-H bond.
The presence of the coordinated hydride increases the
electron density on the sulfur center, which allows its direct
oxidation by molecular oxygen to afford the final product
(6). Similar oxidation of coordinated thiolate sulfur to
corresponding sulfinate form by molecular oxygen has
precedent in the literature.16 The intermediate complex 7
could be isolated only in the case of R ) NO2. In ethanolic
solution, 7-NO2 was observed to undergo very slow oxidation
in the presence of air to produce the corresponding sulfone
complex (6-NO2). The strong electron-withdrawing character
of this substituent appears to inhibit further oxidation of
(13) (a) Circu, V.; Fernandez, M. A.; Carlton, L. Inorg. Chem. 2002, 41,
3859. (b) Evans, D. R.; Huang, M. S.; Seganish, W. M.; Chege, E.
W.; Lam, Y. F.; Feltinger, J. C.; Williams, T. L. Inorg. Chem. 2002,
41, 2633. (c) Huang, J.; Stevens, E. D.; Nolan, S. P. Organometallics
2000, 19, 1194. (d) Sjovall, S.; Svensson, P. H.; Anderson, C.
Organometallics 1999, 18, 5412.
(14) (a) Rifat, A.; Patmore, N. J.; Mahon, M. F.; Weller, A. S. Organo-
metallics 2002, 21, 2856. (b) Ball, R. G.; James, B. R.; Mahajan, D.;
Trotter, J. Inorg. Chem. 1981, 20, 254.
(15) Lee, C. M.; Hsieh, C. H.; Dutta, A.; Lee, G. H.; Liaw, W. F. J. Am.
Chem. Soc. 2003, 125, 11492.
(16) (a) Cocker, T. M.; Bachman, R. E. Chem. Commun. 1999, 875. (b)
Mirza, S. A.; Pressler, M. A.; Kumar, M.; Day, R. O.; Maroney, M.
J. Inorg. Chem. 1993, 32, 977. (c) Farmer, P. J.; Solouki, T.; Mills,
D. K.; Soma, T.; Russel, D. H.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 1992, 114, 4601. (d) Kumar, M.; Colpas, G. J.;
Day, R. O.; Maroney, M. J. J. Am. Chem. Soc. 1989, 111, 8323. (e)
Nicholson, T.; Zubieta, J. Inorg. Chem. 1987, 26, 2094.
Figure 2. View of the 7-NO2 complex, with thermal ellipsoids drawn at
the 50% probability level.
1256 Inorganic Chemistry, Vol. 45, No. 3, 2006

Rh-Assisted C-H ActiWation of Benzaldehyde Thiosemicarbazones
Scheme 1. Probable Steps for the Formation of Complexes 6
Figure 3. View of the 8-Cl complex, with thermal ellipsoids drawn at
the 50% probability level.
Scheme 2. Probable Steps for the Formation of Complexes 8
7-NO2. It should be mentioned in this context that activation
of molecular oxygen by transition metal complexes is of
significant importance, with particular reference to biological
and industrial applications of such reactions.17
The above speculation, that rhodium undergoes an oxida-
tive insertion into the acidic phenyl C-H bond affording
the C-RhIII-H fragment, points to the possibility of similar
and more facile activation of the potentially acidic S-H bond
of the thiosemicarbazone (in the tautomeric thiol form),
provided that the S-H bond remains intact prior to its
reaction with the rhodium(I) center in [Rh(PPh3)3Cl]. To
clarify this issue, reactions between [Rh(PPh3)3Cl] and the
benzaldehyde thiosemicarbazones 1 were also carried out as
before, but in the absence of any base. Each of these reactions
afforded a yellow18 complex. Aside from the absence of any
hydride signal, the 1H NMR spectra of these complexes are
qualitatively similar to those of the complexes 6. The
identities of this new group of complexes were unveiled by
structural characterization of a representative member by
X-ray crystallography. The structure (Figure 3) shows that
the thiosemicarbazone is bound to rhodium in the tridentate
C,N,S fashion and that two PPh3 ligands and a chloride are
also coordinated to the metal center. The Rh-Cl distance is
normal,3a,7 and the other structural features are similar to
those observed in 7-NO2 (Table 2). As all five complexes
in this group (8, Chart 1) display similar spectral and
electrochemical properties (vide infra), the other four 8-R
(R * Cl) complexes are assumed to have structures similar
to that of 8-Cl.
(17) (a) AdVances in Catalytic ActiVation of Dioxygen by Metal Complexes;
Simandi, L. I., Ed.; Hungarian Academy of Sciences: Budapest,
Hungary, 2002. (b) Detlef, S.; Helmet, S. Essays Contemp. Chem.
2001, 131. (c) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607.
(d) Fox, S.; Karlin, K. D. In ActiVe Oxygen in Biochemistry; Valentine,
J. S., Foote, C. S., Greenberg, A., Liebman, J. F., Eds.; Blackie
Academic & Professional: London, 1995; pp 188-232. (e) Karlin,
K. D.; Tyeklar, Z. In Models in Inorganic Biochemistry; Eichhorn,
G. L., Marzilli, L. G., Eds.; PTR Prentice Hall: Englewood Cliffs,
NJ, 1994. (f) Klotz, I. M.; Kurtz, D. M. Chem. ReV. 1994, 94, 567.
(g) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of Organic
Compounds; Academic Press: New York, 1981.
The speculated sequences behind the formation of the
complexes 8 are shown in Scheme 2. In the absence of any
base, the S-H bond in the thiolate tautomer seems to play
Inorganic Chemistry, Vol. 45, No. 3, 2006 1257

Acharyya et al.
Table 3. Electronic Spectral and Cyclic Voltammetric Data of the Complexes
electronic spectral dataa
cyclic voltammetric
compd
λ
max, nm (ꢀ, M-1 cm-1
)
datab E, V
6-OCH3
6-CH3
6-H
404 (3700), 288 (18500), 240 (24700)
400 (2700), 286 (14200), 234 (20400)
406 (2500), 288 (14400), 234 (30500)
368 (2800), 292 (11000), 234 (23000)
510 (9100), 360 (14000)
510 (5200), 354 (9100), 286 (17500)
422 (5200), 308 (22800), 274 (25400)
426 (5200), 310 (27300), 274 (31000)
424 (3400), 312 (21700), 272 (24500)
426 (3800), 304 (22800), 272 (21700)
508 (4900), 310 (25400), 272 (23000)
0.64,c 1.07,c -1.18d
0.68,c 1.12,c -1.19d
0.72,c 0.93,c -1.19d
6-Cl
0.77,c 1.32,c -1.14d
6-NO2
7-NO2
8-OCH3
8-CH3
8-H
0.84,c 1.13,c -1.19d
0.88,c 1.13,c -1.3d
0.72e (66),f 1.23,c -1.07d
0.81e (73),f 1.39,c -1.06d
0.86e (76),f 1.42,c -1.23d
0.93e (82),f 1.59,c -1.32d
1.07e (76),f 1.74,c -1.23d
8-Cl
8-NO2
a In dichloromethane solution. b Solvent, 1:9 dichloromethane/acetonitrile; supporting electrolyte, TBAP; reference electrode, SCE; scan rate, 50 mV s-1
.
d
e
c Epa value, where Epa is the anodic peak potential. Epc value, where Epc is the cathodic peak potential. E1/2 ) 0.5(Epa + Epc). f ∆Ep) Epa - Epc.
a very important role. In the initial step, the thiosemicarba-
zones bind to the metal center in [Rh(PPh3)3Cl], via oxidative
insertion of rhodium into this S-H bond and simultaneous
dissociation of one triphenylphospine, generating complexes
9 as the reactive intermediates, which then undergo cyclo-
metalation via elimination of H2 to afford complexes 8.
The infrared spectra of all five 6-R complexes are similar.
Each complex shows strong vibrations near 520, 695, and
745 cm-1, indicating the presence of coordinated PPh3
ligands.3,7 The Rh-H stretch is observed near 1970 cm-1 as
a sharp band.19 Many new vibrations, compared to the
spectrum of [Rh(PPh3)3Cl], are observed in the spectra of
these 6-R complexes (e.g., near 1157, 1387, 1551, 1568, and
1626 cm-1), which are attributable to the coordinated
thiosemicarbazone ligand. Of these new vibrations, the strong
band near 1157 cm-1 is assigned the SdO stretches.15 The
infrared spectrum of 7-NO2 is mostly similar to that of
6-NO2, except that the ν(SdO) vibration was not observed
in the former, as expected. Aside from the absence of the
ν(Rh-H) and ν(SdO) stretches, the infrared spectrum of
each 8-R complex is similar to that of the corresponding
6-R complex.
Electronic spectra of all of the complexes, recorded in di-
chloromethane solution, show intense absorptions in the
visible and ultraviolet regions (Table 3). The absorptions in
the ultraviolet region are attributable to transitions within
the ligand orbitals, and those in the visible region are prob-
ably due to charge-transfer transitions. For a better under-
standing of the nature of the transitions in the visible region,
qualitative EHMO calculations were performed20 on computer-
generated models of all of the complexes in which the phenyl
rings of the triphenylphosphines were replaced by hydrogens.
The results were found to be similar for all of the com-
plexes.21 The compositions of selected molecular orbitals
(Table S1) and partial MO diagrams of the three representa-
tive complexes (Figures S1-S3) are available as Supporting
Information. The highest occupied molecular orbital (HOMO)
and the next two filled orbitals (HOMO - 1 and HOMO -
2) have major contributions from the rhodium dxy, dyz, and
dzx orbitals.21 These three occupied orbitals can therefore be
regarded as the rhodium t2 orbitals. The lowest unoccupied
molecular orbital (LUMO) has >80% contribution from the
thiosemicarbazone ligand and is concentrated mostly (>49%)
on the azomethine imine (sHCdNs) fragment.21 LUMO
+ 1 and LUMO + 2 are localized on other parts of the thio-
semicarbazone ligand. The lowest-energy absorption in the
visible region can therefore be assigned to an allowed charge-
transfer transition from the filled rhodium t2 orbital (HOMO)
to the vacant π*(azomethine imine) orbital of the thiosemi-
carbazone ligand (LUMO).21
The electrochemical properties of all of the complexes
were studied in 1:9 dichloromethane/acetonitrile solution (0.1
M TBAP) by cyclic voltammetry.22 A selected voltammo-
gram (Figure S4) is available as Supporting Information.
Each complex shows two oxidative responses on the positive
side of SCE and a reductive response on the negative side
(Table 3). In view of the composition of the HOMO, the
first oxidative response is assigned to Rh(III)-Rh(IV)
oxidation. The one-electron nature of this oxidation was
verified by comparing its current height with that of the
standard ferrocene-ferrocenium couple under identical
experimental conditions. This oxidation was found to be
irreversible for the 6-R and 7-NO2 complexes. However, for
the 8-R complexes, the same oxidation was observed to be
reversible, characterized by a peak-to-peak separation (∆Ep)
of 60-70 mV and an anodic peak current (ipa) that is equal
to the cathodic peak current (ipc). The second oxidation is
irreversible in nature in all of the complexes and is tentatively
assigned to oxidation of the coordinated thiosemicarbazones.
All of the complexes show an irreversible reductive response,
and with reference to the composition of the LUMO,21 the
reduction is attributed to reduction of the coordinated
thiosemicarbazone. All of the irreversible redox responses
show nonstoichiometric currents. Potentials of the redox
responses do not show any systematic variation with the
nature of the substituent R in the thiosemicarbazone ligand.
(21) In 8-NO2, the HOMO has a major metal contribution, whereas HOMO
- 1 has less metal character. In all of the complexes having R )
NO2, the LUMO has a significant contribution from the NO2 group,
and LUMO + 1 has considerable azomethine imine character.
(22) A little dichloromethane was necessary to take the complex into
solution. Addition of a large excess of acetonitrile was necessary to
record the redox responses in proper shape.
(18) Orange complex obtained from the reaction with the para-nitrobezal-
dehyde thiosemicarbazone (1, R ) NO2).
(19) Fenster, A.; James, B. R.; Cullen, W. R. Inorg. Synth. 1977, 17, 81.
(20) (a) Mealli, C.; Proserpio, D. M. CACAO, version 4.0; Firenze, Italy,
1994. (b) Mealli, C.; Proserpio, D. M. J. Chem. Edu. 1990, 67, 399.
1258 Inorganic Chemistry, Vol. 45, No. 3, 2006

Rh-Assisted C-H ActiWation of Benzaldehyde Thiosemicarbazones
Conclusions
manuscript. Thanks are due to the RSIC at Central Drug Re-
search Institute, Lucknow, India, for the C,H,N analysis data.
R.A. and S.D. thank the Council of Scientific and Industrial
Research, New Delhi, India, for their fellowships [Grants
9/96(438)2004-EMR-I and 9/96(410)/2003-EMR-I, respec-
tively].
The present study shows that the benzaldehyde thiosemi-
carbazones 1 can undergo facile C-H activation at one ortho
position of the phenyl ring, mediated by [Rh(PPh3)3Cl]. The
present study further demonstrates that, in addition to phenyl
C-H activation, S-H activation or oxidation of the sulfur
center can also be brought about by simple manipulation of
the experimental conditions.
Supporting Information Available: Partial molecular orbital
diagrams of 6-Cl (Figure S1), 7-NO2 (Figure S2), and 8-CH3
(Figure S3). Cyclic voltammogram of 8-H (Figure S4). Composition
of selected molecular orbitals for all the complexes (Table S1).
X-ray crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. Financial assistance received from the
Council of Scientific and Industrial Research, New Delhi,
India [Grant 01(1952)/04/EMR-II], is gratefully acknowl-
edged. The authors thank the reviewers for their constructive
suggestions, which were very helpful in preparing the revised
IC050505W
Inorganic Chemistry, Vol. 45, No. 3, 2006 1259