Unprecedented chemical transformation of semicarbazones mediated by Wilkinson's catalyst

Article abstract of DOI:10.1021/ic034247j

para-Nitrobenzaldehyde semicarbazone undergoes an unusual chemical transformation upon reaction with [Rh(PPh₃)₃Cl] in the presence of trialkyl and dialkylamines (NR₂R′; R = Et, ⁱPr, ⁿBu; R′ = H or R′

Full text of DOI:10.1021/ic034247j

Inorg. Chem. 2003, 42, 4338−4345
Unprecedented Chemical Transformation of Semicarbazones Mediated
by Wilkinson’s Catalyst
Indrani Pal,1a Swati Dutta,1a Falguni Basuli,1a Savitha Goverdhan,1b Shie-Ming Peng,1c
,1a
Gene-Hsiang Lee,1c and Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity,
Kolkata 700 032, India, Department of Inorganic Chemistry, Indian Institute of Technology,
Kanpur 208016, India, and Department of Chemistry, National Taiwan UniVersity,
Taipei, Taiwan, ROC
Received March 7, 2003
para-Nitrobenzaldehyde semicarbazone undergoes an unusual chemical transformation upon reaction with
i
n
[Rh(PPh3)3Cl] in the presence of trialkyl and dialkylamines (NR R′; R ) Et, Pr, Bu; R′ ) H or R′ ) R) via
2
dissociation of the C−NH bond and formation of a new C−NR bond (where the NR fragment is provided by the
2
2
2
amine). The transformed semicarbazone ligand binds to rhodium as a dianionic C,N,O-donor to afford complexes
of type [Rh(PPh3)2(CNO−NR )Cl] (CNO−NR ) the coordinated semicarbazone ligand). Another group of
2
2
semicarbazones (viz. salicylaldehyde semicarbazone, 2-hydroxyacetophenone semicarbazone, and 2-hydroxynaph-
thaldehyde semicarbazone) has also been observed to undergo a similar chemical transformation upon reaction
with [Rh(PPh3)3Cl] under similar experimental conditions as before, and these transformed semicarbazones bind to
n
n
rhodium as dianionic O,N,O-donors affording complexes of the type [Rh(PPh3)2(ONO −NR )Cl] (ONO −NR ) the
2
2
coordinated semicarbazone ligand; n ) 1−3). The structure of the [Rh(PPh3)2(CNO−NEt2)Cl] and [Rh(PPh3)2-
2
(ONO −NR )Cl] complexes has been determined. All the complexes show characteristic 1H NMR signals. They
2
also show intense absorptions in the visible and ultraviolet region. Cyclic voltammetry on the complexes shows an
oxidative response within 0.52−0.97 V versus SCE and a reductive response within −1.00 to −1.27 V versus SCE,
where both the responses are believed to be centered on the semicarbazone ligand.
Introduction
ligands,4 and the present report has emerged from this
continuous study. For this work, a group of five semicar-
bazones of para-substituted benzaldehydes (1) have been
chosen as the principal ligand, and rhodium has been selected
as the metal.
There has been considerable interest in the chemistry of
transition metal complexes of the semicarbazone and thio-
semicarbazone ligands, primarily because of their bioinor-
ganic relevance2 and particularly because of their potentially
beneficial biological (viz. antibacterial, antimalarial, antiviral,
and antitumor) activities.3 However, we have been studying
chemistry of the platinum metal complexes of the semicar-
bazone and thiosemicarbazone ligands with special reference
to the variable coordination modes displayed by these
* To whom correspondence should be addressed. E-mail:
samaresh_b@hotmail.com.
(1) (a) Jadavpur University. (b) Indian Institute of Technology. (c) National
Taiwan University.
(2) (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.
Our recent studies on the ruthenium complexes of these
ligands have shown that they can bind to the metal as a
bidentate N,O-donor and, depending on the experimental
4338 Inorganic Chemistry, Vol. 42, No. 14, 2003
10.1021/ic034247j CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/11/2003

Using [Rh(PPh3)3Cl] to Transform Semicarbazones
conditions, afford either four- or five-membered chelate
rings.4c The para-nitrobenzaldehyde semicarbazone (1, X )
NO2) can also coordinate to ruthenium as a tridentate C,N,O-
donor affording organometallic complexes.4c Similar C,N,O-
tricoordination by such ligands was also observed by others.5
The facile formation of organometallic complexes by the
benzaldehyde semicarbazones has encouraged us to explore
the possibility of synthesizing organorhodium complexes of
these ligands, and the Wilkinson’s catalyst, viz. [Rh(PPh3)3-
Cl], has been chosen as the rhodium starting material for
this purpose because of its ability, as we have already
experienced, to form organometallic complexes with C,N,O-
donor ligands.6 Reaction of [Rh(PPh3)3Cl] has been carried
out with all five benzaldehyde semicarbazones (1) in the
transformation, a second group of three potentially triden-
tate semicarbazones (3; viz. salicylaldehyde semicarbazone,
2-hydroxyacetophenone semicarbazone, and 2-hydroxynaph-
thaldehyde semicarbazone) have been allowed to react with
[Rh(PPh3)3Cl] under similar experimental conditions. These
new semicarbazones have also been observed to undergo
similar chemical transformation as before and bind to rho-
dium(III) as a dianionic tridentate O,N,O-donor (4) affording
complexes of type [Rh(PPh3)2(ONOn-NR2)Cl] (where
ONOn-NR2 stands for the coordinated ligand in 4). The
i
n
presence of a base (NR2R′, R ) Et, Pr, Bu; R′ ) R, H).
However, only the reaction with para-nitrobenzaldehyde
semicarbazone has afforded a family of interesting organo-
rhodium complexes,7 where the semicarbazone ligand has
been found to undergo an unusual chemical transformation
via dissociation of the C-NH2 bond and formation of a new
C-NR2 bond (where the NR2 fragment is provided by the
base). The transformed ligand remains bound to rhodium-
(III) as a dianionic tridentate C,N,O-donor (2) affording or-
ganorhodium complexes of type [Rh(PPh3)2(CNO-NR2)Cl]
(where CNO-NR2 refers to the coordinated ligand in 2).
In order to check the generality, if any, of the described
chemistry of the [Rh(PPh3)2(CNO-NR2)Cl] and [Rh(PPh3)2-
(ONOn-NR2)Cl] complexes is reported in this paper with
special reference to their formation, structure, and electro-
chemical properties.
(3) (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,
G.; Sassi, M.; Tarasconi, P.; Cornia, M.; Capacchi, S.; Albertini, R.;
Pinelli, S. J. Inorg. Biochem. 2002, 90, 113. (c) Cowly, A. R.;
Dilworth, J. R.; Donnely, P. S.; Labisbal, E.; Sousa, A. J. Am. Chem.
Soc. 2002, 124, 5270. (d) Maurer, R. I.; Blower, P. J.; Dilworth, J.
R.; Reynolds, C. A.; Zheng, Y.; Mullen, G. E. D. J. Med. Chem. 2002,
45, 1420. (e) Patole, J.; Dutta, S.; Padhye, S.; Sinn, E. Inorg. Chim.
Acta 2001, 318, 207. (f) Iakovidou, Z.; Papageorgiou, A.; Demertzis,
M. A.; Mioglou, E.; Mourelatos, D.; Kotsis, A.; Yadav, P. N.; Kovala-
Demertzi, D. Anti-Cancer Drugs. 2001, 12, 65. (g) Kovala-Demertzi,
D.; Miller, J. R.; Kourkoumelis, N.; Hadjikakou, S. K.; Demertzis,
M. A. Polyhedron 1999, 18, 1005. (h) Miller, M. C., III; Stineman,
C. N.; Vance, J. R.; West, D. X.; Hall, I. H. Anticancer Res. 1998,
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. (k) Kovala-Demertzi, D.; Domopoulou, A.; Demertzis,
M. A.; Raptopoulou, C. P.; Terzis, A. Polyhedron 1994, 13, 1917. (l)
Liberta, A. E.; West, D. X. Biometals 1992, 5, 121. (m) Kraker, A.;
Krezoski, S.; Schneider, J.; Mingel, D.; Petering, D. H. J. Biol. Chem.
1985, 260, 13710. (n) Scovill, J. P.; Klayman, D. L.; Franchino, C. F.
J. Med. Chem. 1982, 25, 1261. (o) Agrawal, K. C.; Sartorelli, A. C.
Prog. Med. Chem. 1978, 15, 321. (p) Sartorellic, A. C.; Agrawal, K.
C.; Tsiftsoglou, A. S.; Moore, A. C. AdV. Enzyme Regul. 1977, 15,
117. (q) French, F. A.; Blanz, E., Jr. J. Med. Chem. 1970, 13, 1117.
(4) (a) Gupta, P.; Basuli, F.; Peng, S. M.; Lee, G. H.; Bhattacharya, S.
Inorg, Chem. 2003, 42, 2069. (b) Dutta, S.; Basuli, F.; Peng, S. M.;
Lee, G. H.; Bhattacharya, S. New. J. Chem. 2002, 26, 1607. (c) Basuli,
F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2001, 40, 1126. (d)
Pal, I.; Basuli, F.; Mak, T. C. W.; Bhattacharya, S. Angew. Chem.,
Int. Ed. 2001, 40, 2923. (e) Basuli, F.; Peng, S. M.; Bhattacharya, S.
Inorg. Chem. 2000, 39, 1120. (f) Basuli, F.; Ruf, M.; Pierpont, C. G.;
Bhattacharya, S. Inorg. Chem. 1998, 39, 6113. (g) Basuli, F.; Peng,
S. M.; Bhattacharya, S. Inorg. Chem. 1997, 36, 5645.
Experimental Section
Materials. Rhodium trichloride was obtained from Arora Mat-
they, Kolkata, India. Triethylamine, diethylamine, tri-n-butylamine,
and diisopropylamine were purchased from Aldrich. All other
chemicals and solvents were reagent grade commercial materials
and were used as received. [Rh(PPh3)3Cl] was synthesized by
following a reported procedure.8 The semicarbazone ligands were
prepared by reacting equimolar amounts of semicarbazide hydro-
chloride, sodium acetate, and the respective aldehyde or ketone in
1:1 ethanol-water mixture. Purification of dichloromethane and
acetonitrile and preparation of tetrabutylammonium perchlorate
(TBAP) for electrochemical work were performed as before.9
Preparation of Complexes. [Rh(PPh3)2(CNO-NEt2)Cl]. para-
Nitrobenzaldehyde semicarbazone (23 mg, 0.11 mmol) was taken
in toluene (40 mL) and to it was added triethylamine (70 mg, 0.70
mmol).10 The flask was purged with a stream of nitrogen for 10
min. Then, [Rh(PPh3)3Cl] (100 mg, 0.11 mmol) was added, and
the mixture was heated at reflux under a nitrogen atmosphere for
6 h to yield an orange solution. Evaporation of this solution gave
a yellowish-orange solid, which was subjected to purification by
thin-layer chromatography on a silica plate. With benzene as the
eluant (CAUTION! Benzene is carcinogenic.), an orange band
separated, which was extracted with acetonitrile, and evaporation
of this extract gave [Rh(PPh3)2(CNO-NEt2)Cl] as a crystalline
orange solid. Yield: 75%.
(5) Villa, J. M.; Pereira, M. T.; Ortiguiera, J. M.; Lopez-Torres, M.;
Castineiras, A.; Fernandez, J. J.; Fernandez, A.; Lata, D. J. Organomet.
Chem. 1998, 556, 21.
(6) Dutta, S.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans.
2000, 4623.
(8) Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1967, 10, 67.
(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.
(7) Ligands containing other substituents (1, X * NO2) afforded a dirty
(10) Addition of an equivalent quantity of diethylamine (NHEt2) also gave
the same product.
looking unidentifiable material.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4339

Pal et al.
i
n
Table 1. Crystallographic Data for [Rh(PPh3)2(CNO-NEt2)Cl] and
[Rh(PPh3)2(ONO2-NEt2)Cl
The other two [Rh(PPh3)2(CNO-NR2)Cl] (R ) Pr and Bu)
complexes were prepared by following the same procedure using,
respectively, diisopropylamine and tri-n-butylamine instead of
empirical formula
fw
C50H47ClN5O3P2Rh
965.41
C134H124Cl4N6O4P6Rh3
2518.74
i
triethylamine. The yield was 72% for the R ) Pr complex and
n
space group
a, Å
b, Å
monoclinic, I2/a
24.288(5)
17.010(5)
25.114(5)
90.00(5)
114.370(5)
90.00(5)
9451(4)
4
triclinic, P1h
12.1627(1)
16.4398(2)
16.5077(2)
112.3859(5)
105.4037(4)
90.9540(5)
2916.56(6)
1
78% for the R ) Bu complex.
[Rh(PPh3)2(ONO1-NEt2)Cl]. Salicylaldehyde semicarbazone
(30 mg, 0.17 mmol) was taken in toluene (40 mL) and to it was
added triethylamine (90 mg, 0.90 mmol).10 The flask was purged
with a stream of nitrogen for 10 min. Then, [Rh(PPh3)3Cl] (100
mg, 0.11 mmol) was added, and the mixture was heated at reflux
under a nitrogen atmosphere for 8 h to yield an orange solution.
Evaporation of this solution gave a yellowish-orange solid, which
was subjected to purification by thin-layer chromatography on a
silica plate. Using 10:1 benzene-acetonitrile as the eluant, a
yellowish-orange band separated, which was extracted with 1:1
dichloromethane-acetonitrile. Upon evaporation of this extract,
beautiful yellowish-orange crystals were obtained. As these crystals
were found to contain the [Rh(PPh3)2Cl2] complex in addition to
[Rh(PPh3)2(ONO1-NEt2)Cl] (vide infra), they were again dissolved
in dichloromethane and purified for the second time by thin-layer
chromatography on a silica plate. Using only benzene as the eluant,
a minor yellow band moved first followed by a major orange band.
The orange band was extracted with acetonitrile, and evaporation
of this extract gave [Rh(PPh3)2(ONO1-NEt2)Cl] as an orange solid.
Yield: 52%.
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
λ, Å
0.71069
0.20 × 0.20 × 0.20
293
0.532
0.0860
0.71073
crystal size, mm3
T, K
0.10 × 0.10 × 0.07
150
µ, mm-1
R1a
0.652
0.0530
0.1268
1.040
wR2b
GOFc
0.1666
1.086
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2]]1/2
.
2
2
2
2
c GOF ) [∑[w(Fo - Fc )2]/(M - N)]1/2, where M is the number of
reflections and N is the number of parameters refined.
tion of the 1:1 dichloromethane-acetonitrile extract obtained from
the first chromatographic purification of the complex (see synthetic
procedure). Selected crystal data and data collection parameters are
given in Table 1. Data were collected on a Nonius Kappa CCD
diffractometer using graphite monochromated Mo KR radiation (λ
) 0.71073 Å) by φ and ω scans (1.75° < θ < 27.50°). X-ray data
reduction and structure solution and refinement were done as al-
ready stated. These crystals were found to contain a square planar
[Rh(PPh3)2Cl2] complex molecule per two molecules of the
[Rh(PPh3)2(ONO2-NEt2)Cl] complex. As the observed structural
parameters for the [Rh(PPh3)2Cl2] fragment compare well with those
reported earlier,12 the structural discussion in the text includes only
the [Rh(PPh3)2(ONO2-NEt2)Cl] complex.
The other [Rh(PPh3)2(ONOn-NR2)Cl] complexes were prepared
by following the same procedure using the appropriate semicar-
bazone and amine (NR2R′) instead of salicylaldehyde semicarbazone
and triethylamine, respectively. Each complex was purified by
chromatography twice as described. Yields varied in the range
50-55%.
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 either a Bruker DRX 500 or
a Bruker 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 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.
Results and Discussion
Reaction of [Rh(PPh3)3Cl] was carried out with all the
semicarbazones (1) in refluxing toluene in the presence of
NEt3. However, only the reaction with para-nitrobenzalde-
hyde semicarbazone proceeded smoothly to afford an orange
complex in decent yield.7 Preliminary characterizations
(microanalysis, IR, and NMR) on this complex failed to
indicate any unambiguous formulation for it. Identity of the
complex has been revealed by its structure determination by
X-ray crystallography. The structure is displayed in Figure
1, and selected bond parameters are listed in Table 2. The
structure shows that during the course of the synthetic
reaction the para-nitrobenzaldehyde semicarbazone has
undergone an interesting chemical transformation, viz. the
NH2 group of the semicarbazone has been replaced by a
NEt2 group provided by the base (NEt3). The modified
semicarbazone is coordinated to rhodium, via dissociation
of two protons, as a dianionic C,N,O-donor ligand. Ortho-
metalation has taken place from the phenyl ring of the
modified semicarbazone. Two triphenylphosphines and a
chloride are also bound to the metal center. This complex is
therefore formulated as [Rh(PPh3)2(CNO-NEt2)Cl], where
CNO-NEt2 represents the coordinated semicarbazone. The
Crystallography of [Rh(PPh3)2(CNO-NEt2)Cl]. Single crystals
of [Rh(PPh3)2(CNO-NEt2)Cl] were obtained by slow evaporation
of an acetonitrile solution of the complex. Selected crystal data
and data collection parameters are given in Table 1. Data were
collected on an Enraf Nonius CAD-4 diffractometer using graphite
monochromated Mo KR radiation (λ ) 0.71069 Å) by θ-2θ scans
(1.78° < θ < 22.50°). X-ray data reduction and structure solution
and refinement were done using the SHELXS-97 and SHELXL-
97 programs.11 The structure was solved by direct methods.
Crystallography of [Rh(PPh3)2(ONO2-NEt2)Cl]. Single crys-
tals of [Rh(PPh3)2(ONO2-NEt2)Cl] were obtained by slow evapora-
(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) Ogle, C. A.; Masterman, T. C.; Hubbard, J. L. J. Chem. Soc., Chem.
Commun. 1990, 1733.
4340 Inorganic Chemistry, Vol. 42, No. 14, 2003

Using [Rh(PPh3)3Cl] to Transform Semicarbazones
Scheme 1. Probable Steps in the Formation of the [Rh(PPh3)2(CNO-
NR2)Cl] Complexes
Figure 1. View of the [Rh(PPh3)2(CNO-NEt2)Cl] complex.
Table 2. Selected Bond Distances and Bond Angles for
[Rh(PPh3)2(CNO-NEt2)Cl] and [Rh(PPh3)2(ONO2-NEt2)Cl]
bond. The observed intermediacy in bond orders in the
O(1)-C(8)-N(3) fragment is attributable to the possible
charge delocalization over this fragment.
[Rh(PPh3)2(CNO-NEt2)Cl]
Bond Distances (Å)
Rh-C(1)
Rh-P(1)
1.989(12)
2.371(3)
2.381(4)
1.328(15)
1.35(2)
Rh-O(1)
2.189(9)
2.360(3)
1.968(10)
1.351(14)
1.26(2)
The observed chemical transformation of para-nitroben-
zaldehyde semicarbazone is not only unusual but also, to
our knowledge, unprecedented. Encouraged by this result,
reaction of the para-nitrobenzaldehyde semicarbazone with
[Rh(PPh3)3Cl] was also carried out in the presence of another
trialkylamine (NnBu3) and two dialkylamines (NHR2, R )
Rh-P(2)
Rh-Cl
Rh-N(2)
N(2)-N(3)
C(8)-O(1)
C(9)-N(4)
C(7)-N(2)
C(8)-N(3)
C(8)-N(4)
C(11)-N(4)
1.384(19)
1.43(2)
1.47(2)
Bond Angles (deg)
i
P(2)-Rh-P(1)
N(2)-Rh-Cl(1)
C(1)-Rh-O(1)
177.61(14)
177.2(2)
159.4(5)
N(2)-Rh-C(1)
N(2)-Rh-O(1)
83.1(5)
76.3(5)
Et, Pr) in order to check whether the semicarbazone reacts
with these amines and undergoes a similar transformation.
Each of these reactions indeed gave the expected complex
[Rh(PPh3)2(ONO2-NEt2)Cl]
i
n
of type [Rh(PPh3)2(CNO-NR2)Cl] (R ) Et, Pr, Bu). It is
interesting to note here that the same [Rh(PPh3)2(CNO-
NEt2)Cl] complex is obtained using diethylamine or triethyl-
amine. Elemental (C, H, N) analytical data of all these
complexes agree well with their formulations (Table S1).
As all the [Rh(PPh3)2(CNO-NR2)Cl] complexes have been
prepared by following similar procedures and as they show
similar properties (vide infra), the other two [Rh(PPh3)2-
Bond Distances (Å)
Rh(1)-N(1)
Rh(1)-O(2)
Rh(1)-P(2)
O(1)-C(1)
N(1)-N(2)
C(9)-O(2)
C(10)-N(3)
1.974(3)
2.034(2)
2.3786(10)
1.324(4)
1.398(4)
1.303(5)
1.364(6)
Rh(1)-O(1)
Rh(1)-P(1)
Rh(1)-Cl(1)
C(7)-N(1)
C(9)-N(2)
C(9)-N(3)
C(12)-N(3)
1.996(2)
2.3833(11)
2.3795(10)
1.306(5)
1.340(5)
1.358(5)
1.460(6)
Bond Angles (deg)
i
n
(CNO-NR2)Cl] (R ) Pr, Bu) complexes are assumed to
have a similar structure as [Rh(PPh3)2(CNO-NEt2)Cl].
The mechanism of the interesting chemical transformation
of para-nitrobenzaldehyde semicarbazone is not completely
clear to us. However, that the observed transformation has
been mediated by the metal center is apparent from the fact
that treatment of the para-nitrobenzaldehyde semicarbazone
with the base (NR2R′) alone in refluxing toluene (for 6 h)
failed to bring about any such transformation. The sequences
illustrated in Scheme 1 thus seem probable. In the initial
step, the semicarbazone coordinates to trivalent rhodium as
a monoanionic C,N,O-donor via loss of one phenyl proton.
This particular coordination mode of the para-nitrobenzal-
dehyde semicarbazone was observed in its ruthenium
complex.4c However, isolation of this intermediate has not
been possible in the present case. Nucleophilic attack by the
amine (NR2R′) at the carbonyl carbon then takes place, which
is believed to be the crucial step. The carbonyl carbon in
the uncoordinated ligand (1) is similar to that in urea, and
hence, it does not appear to be electropositive enough so as
P(1)-Rh(1)-P(2)
N(1)-Rh(1)-Cl(1)
O(1)-Rh(1)-O(2)
177.79(4)
173.37(10)
175.41(10)
N(1)-Rh(1)-O(1)
N(1)-Rh(1)-O(2)
95.18(12)
80.78(12)
CNOP2Cl coordination sphere around rhodium is a distorted
octahedron, as reflected in all the bond parameters around
the metal center. The tricoordinated semicarbazone, rhodium,
and chloride have constituted one equatorial plane, and the
two PPh3 ligands have taken up the axial positions and,
hence, they are mutually trans. The Rh-C, Rh-N, Rh-P,
and Rh-Cl distances are all quite normal, as usually
observed in complexes of rhodium(III) containing these
bonds.6,13 However, the Rh-O(1) length is a bit longer than
usual.13b,14 The observed C(8)-O(1) distance is intermediate
between CdO and CsO. The C(8)-N(3) distance similarly
indicates a bond order which is between a single and double
(13) (a) Garcia, M. P.; Jimenez, M. V.; Lahoz, F. J.; Lopez, J. A.; Oro, L.
A. J. Chem. Soc., Dalton Trans. 1998, 421. (b) Bennett, M. J.;
Donaldson, P. B. Inorg. Chem. 1977, 16, 1581.
(14) Das, A.; Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2002,
41, 440.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4341

Pal et al.
Table 3. 1H NMR Data of the Complexes
chemical shift in ppm
semicarbazone
azomethine proton
compd
PPh3
aromatic protona
R
[Rh(PPh3)2(CNO-NEt2)Cl]
[Rh(PPh3)2(CNO-NiPr2)Cl]
[Rh(PPh3)2(CNO-NnBu2)Cl]
[Rh(PPh3)2(ONO1-NEt2)Cl]
[Rh(PPh3)2(ONO1-NiPr2)Cl]
[Rh(PPh3)2(ONO1-NnBu2)Cl]
[Rh(PPh3)2(ONO2-NEt2)Cl]
[Rh(PPh3)2(ONO2-NiPr2)Cl]
[Rh(PPh3)2(ONO2-NnBu2)Cl]
[Rh(PPh3)2(ONO3-NEt2)Cl]
[Rh(PPh3)2(ONO3-NiPr2)Cl]
[Rh(PPh3)2(ONO3-NnBu2)Cl]
7.17-7.56
7.21-7.55
7.22-7.54
7.21-7.67
7.21-7.64
7.20-7.73
7.20-7.64
7.19-7.70
7.20-7.75
7.13-7.70
7.07-7.63
7.13-7.73
6.60(d, 8), 7.46(d, 8), 7.91(s)
6.59(d, 8), 7.83(s)
6.57(d, 8), 7.91(s)
6.56
6.50
6.54
6.52
6.50
6.48
1.81b
1.80b
1.79b
0.76, 2.99
0.87
0.87, 1.08, 2.76, 2.94
0.60, 2.91
0.90, 1.08
0.81, 0.97, 2.83
0.61, 2.93
0.87, 1.06
0.85, 0.93, 2.84
0.67, 2.92
0.79
0.85, 1.06, 2.86
6.03(t, 7), 6.09(d, 7), 6.66(d, 8), 6.74(t, 7)
6.03(t, 6), 6.11(d, 7), 6.66(d, 9), 6.73(t, 8)
6.04(t, 6), 6.10(d, 6), 6.66(d, 8), 6.74(t, 7)
6.09(qn, 4), 6.56(d, 8), 6.70(d, 4)
6.09(qn, 4), 6.56(d, 8), 6.70(d, 4)
6.09(qn, 4), 6.57(d, 8), 6.70(d, 3)
6.84(d, 9), 7.01(t, 7), 7.40(t, 8)
6.84(d, 9), 7.01(t, 7), 7.40(t, 8)
6.84(d, 9), 7.02(t, 7), 7.40(t, 8)
a Multiplicity (s ) singlet, d ) doublet, t ) triplet, qn ) quintet) of the signal and the coupling constant (in Hz) are given in parentheses. b Methyl signal.
to initiate an attack by the amine (NR2R′). Coordination of
the semicarbazone ligand to trivalent rhodium as a monoan-
ionic C,N,O-donor in the first step must have enhanced the
electrophilicity of the carbonyl carbon to such an extent that
it undergoes facile attack by the nucleophile (NR2R′). The
nitro group in the phenyl ring also appears to play a vital
role in this reaction. Being a strong electron-withdrawing
group, it enhances dissociability of the phenyl protons and
thus favors formation of the Rh-C bond, which is also
responsible, to some extent, for increasing the electropositive
nature of the carbonyl carbon. Indirect evidence of the
influence of the nitro group comes from the fact that reactions
of [Rh(PPh3)3Cl] with semicarbazones of other para-
substituted benzaldehydes (1, X * NO2), where the sub-
stituent is less electron-withdrawing than the nitro group (e.g.,
OCH3, CH3, H, and Cl), do not afford a similar product.7
Thus, the observed transformation of the para-nitrobenzal-
dehyde semicarbazone appears to be directed by the com-
bined influence of the nitro group in it and its coordination
to rhodium(III) in the initial step.
to the coordinated PPh3 ligands.4a,6,15 Strong bands are also
observed near 1434, 1411, 1329, 1306, 1280, 1227, 1190,
and 1094 cm-1 in all these complexes, which are absent in
the infrared spectrum of [Rh(PPh3)3Cl], and hence, these
bands are attributed to the coordinated semicarbazone ligand.
Electronic spectra of the [Rh(PPh3)2(CNO-NR2)Cl] com-
plexes have been recorded in dichloromethane solution. Each
complex uniformly shows three intense absorptions, two in
the visible region and one in the ultraviolet region (Table
4). The absorption in the ultraviolet region is believed to be
due to a transition within the ligand orbitals. Among the two
absorptions in the visible region, the higher energy transition
(near 380 nm) is relatively less intense, while the lower
energy transition (near 490 nm) is much more intense. To
have an insight into the nature of transitions in the visible
region, qualitative EHMO calculations have been performed16
on computer generated models of the [Rh(PPh3)2(CNO-
NR2)Cl] complexes, where phenyl rings of the triphenyl-
phosphines were replaced by hydrogen. The results of these
calculations have been found to be similar for all the com-
plexes. A partial MO diagram for a representative complex
is shown in Figure 2, and the composition of some selected
molecular orbitals of it is given in Table 5. The highest
occupied molecular orbital (HOMO) is concentrated almost
entirely (g90%) on the NR2 fragment of the semicarbazone
ligand. The next couple of filled orbitals (HOMO - 1,
HOMO - 2, etc.), however, have significant (g45%)
contribution from the rhodium t2-orbitals. The lowest unoc-
cupied molecular orbital (LUMO) has a major (g95%)
contribution from the semicarbazone ligand and is localized
mostly (g55%) on the nitro group. The next couple of vacant
orbitals (LUMO + 1, LUMO + 2, etc.) also have major
contributions from the semicarbazone ligand. The lower
energy absorption (near 490 nm) may therefore be assigned
to a transition from the HOMO concentrated on one fragment
(NR2) of the semicarbazone ligand to the LUMO localized
on another fragment (NO2) of the same ligand, which is an
allowed transition and thus accounts for its high intensity.
The second absorption near 380 nm, which is of much less
All the [Rh(PPh3)2(CNO-NR2)Cl] complexes are dia-
magnetic, which corresponds to the trivalent state of rhodium
1
(low-spin d6, S ) 0) in these complexes. H NMR spec-
tra of these complexes show all the expected signals (Table
3). The signals from the phenyl protons of the PPh3 ligands
are observed as broad signals within 7.1-7.6 ppm. The
azomethine proton signal is observed near 6.5 ppm. Among
the three expected aromatic proton signals (one singlet and
two doublets) from the nitro-phenyl ring of the CNO-NR2
ligand, two (one singlet and one doublet) are clearly ob-
served within 6.57-7.91 ppm in all the three complexes,
while the third (doublet) signal is observed in only one
complex. It could not be detected in the other two com-
plexes because of its overlap with the PPh3 signals. Signals
for the alkyl groups of the NR2 fragment of the CNO-NR2
ligands are also observed as broad peaks in the expected
region. The 1H NMR spectral data of the [Rh(PPh3)2(CNO-
NR2)Cl] complexes are therefore consistent with their
compositions.
Infrared spectra of the [Rh(PPh3)2(CNO-NR2)Cl] com-
(15) Das, A. K.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans.
2000, 181.
plexes show sharp bands near 745, 695, and 520 cm-1 due
4342 Inorganic Chemistry, Vol. 42, No. 14, 2003

Using [Rh(PPh3)3Cl] to Transform Semicarbazones
Table 4. Electronic Spectral and Cyclic Voltammetric Data
cyclic voltammetric datab
E1/2, V (∆Ep, mV) Epc, V
compd
electronic spectral dataa λmax, nm (ꢀ, M-1 cm-1
)
[Rh(PPh3)2(CNO-NEt2)Cl]
[Rh(PPh3)2(CNO-NiPr2)Cl]
[Rh(PPh3)2(CNO-NnBu2)Cl]
[Rh(PPh3)2(ONO1-NEt2)Cl]
[Rh(PPh3)2(ONO1-NiPr2)Cl]
[Rh(PPh3)2(ONO1-NnBu2)Cl]
[Rh(PPh3)2(ONO2-NEt2)Cl]
[Rh(PPh3)2(ONO2-NiPr2)Cl]
[Rh(PPh3)2(ONO2-NnBu2)Cl]
[Rh(PPh3)2(ONO3-NEt2)Cl]
[Rh(PPh3)2(ONO3-NiPr2)Cl]
[Rh(PPh3)2(ONO3-NnBu2)Cl]
488(12000), 380c(5400), 300(32900)
494(14600), 374c(6100), 298(38800)
490(11100), 380c(5700), 300(31200)
0.97(80)
0.95(80)
0.96(70)
0.73(70)
0.71(80)
0.74(68)
0.69(70)
0.69(73)
0.72(64)
0.58(70)
0.52(80)
0.55(80)
-1.27
-1.11
-1.00
-1.18
-1.26
-1.02
-1.08
-1.14
-1.24
-1.15
-1.04
-1.13
404(9300), 388c(8900), 324(27300), 314(27000), 296(26000)
410(9200), 388(6500), 326(21900), 316(21700), 294(22400)
410(10500), 390(9700), 326(33000), 316(32900), 296(32900)
406c(8300), 388(9200), 326c(27900), 312(30400), 294c(28900)
408(7300), 388(9300), 326c(21600), 314(26600), 296(24300)
412(6300), 390(6800), 326c(21800), 314(23400), 296(23400)
442(8400), 418(7700), 326(20000), 292(32000), 264(39300)
444c(8000), 422(9100), 328(21000), 294(27600), 266(34700)
450(7900), 426(7200), 328c(22000), 294(33400), 266(32000)
a In dichloromethane solution. b Solvent, 1:9 dichloromethane-acetonitrile; supporting electrolyte, TBAP; reference electrode, SCE; E1/2 ) 0.5(Epa
Epc), where Epa and Epc are anodic and cathodic peak potentials; ∆Ep ) Epa - Epc; scan rate, 50 mV s-1 c Shoulder.
+
.
their current heights (ipa for oxidative response and ipc for
the reductive response) with those of the standard ferrocene-
ferrocenium couple under identical experimental conditions.
The observed chemical transformation of the para-
nitrobenzaldehyde semicarbazone, which is believed to be
initiated by its coordination to trivalent rhodium in a tri-
dentate fashion via loss of one proton (Scheme 1), prompted
us to explore other semicarbazone ligands with similar
abilities for more of such reactions. A new group of po-
tentially tridentate semicarbazone ligands (3, viz. salicyl-
aldehyde semicarbazone, 2-hydroxyacetophenone semicar-
bazone, and 2-hydroxynaphthaldehyde semicrbazone) have
been selected for this exploration. These semicarbazones
are known to serve as O,N,O-donor ligands via loss of
two protons.2,17 During their reaction with [Rh(PPh3)3Cl] in
the presence of a base (NR2R′; R ) Et, iPr, nBu and R′ ) R
or H), these ligands are also found to undergo similar
chemical transformation as before and afford another group
of complexes of type [Rh(PPh3)2(ONOn-NR2)Cl], where
ONOn-NR2 refers to the transformed semicarbazones bound
to the metal ion as tridentate O,N,O-donor ligands (4).
Formation of these complexes has been authenticated by
structure determination of one member of this family, viz.
[Rh(PPh3)2(ONO2-NEt2)Cl], by X-ray crystallography. The
structure is shown in Figure 4, and relevant bond parameters
are given in Table 2.
The structure shows that the NH2 group of the semicar-
bazone ligand has been replaced by a NEt2 group and the
modified semicarbazone ligand is bound to rhodium as a
tridentate O,N,O-donor. The coordinated chloride is sharing
the same equatorial plane with the semicarbazone and
rhodium, and the PPh3 ligands have occupied the remaining
two axial positions. The NO2P2Cl coordination sphere around
rhodium is a distorted octahedron. The Rh-P and Rh-Cl
distances compare well with those in the previous structure.
The Rh-O(1) and C(1)-O(1) distances are quite normal,
as observed in structurally characterized complexes of
rhodium(III) bound to phenolate oxygen.13b,18 However,
Figure 2. Partial molecular orbital diagram of [Rh(PPh3)2(CNO-
NEt2)Cl].
intensity, may be attributed to a transition taking place from
the HOMO - 1 to one of the vacant orbitals and hence may
be regarded as a metal-to-ligand charge-transfer (MLCT)
transition.
Electrochemical properties of the [Rh(PPh3)2(CNO-
NR2)Cl] complexes have been studied by cyclic voltammetry
in 1:9 dichloromethane-acetonitrile (0.1 M TBAP) solution.
Each complex shows one oxidative response on the positive
side of SCE and one reductive response on the negative side.
Voltammetric data are given in Table 4, and a selected vol-
tammogram is shown in Figure 3. Keeping the results of
the EHMO calculations (vide supra) in view, both the oxida-
tive and reductive responses are assigned to oxidation and
reduction of the coordinated semicarbazone ligand. The oxi-
dation is reversible in nature, with a peak-to-peak separation
(∆Ep) of 70-80 mV, and the anodic peak-current (ipa) is
almost equal to the cathodic peak-current (ipc). The reductive
response is irreversible in nature. One-electron stoichiometry
of both the redox couples has been established by comparing
(16) (a) Mealli, C.; Proserpio, D. M. CACAO, version 4.0; Italy, 1994. (b)
Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
(17) Thangadurai, T. D.; Natarajan, K. Transition Met. Chem. 2001, 26,
717.
(18) Dutta, S.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2000, 39, 2231.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4343

Pal et al.
Table 5. Composition of Selected Molecular Orbitals
% contributions of fragments
contributing
compd
fragments
HOMO
HOMO - 1
HOMO - 2
LUMO
2%
LUMO + 1
LUMO + 2
[Rh(PPh3)2(CNO-NEt2)Cl]
Rh
Cl
78%
5%
49%
2%
2%
3%
CNO-NEt2
100%
100%
100%
100%
17%
83%
5%
12%
81%
4%
49%
45%
2%
53%
50%
2%
48%
75%
4%
98%
4%
98%
100%
100%
100%
97%
100%
100%
100%
[Rh(PPh3)2(ONO1-NEt2)Cl]
[Rh(PPh3)2(ONO2-NEt2)Cl]
[Rh(PPh3)2(ONO3-NEt2)Cl]
Rh
Cl
ONO1-NEt2
96%
3%
Rh
Cl
ONO2-NEt2
15%
42%
97%
3%
Rh
Cl
ONO3-NEt2
58%
21%
97%
difference in ligand nature and size of the chelate rings. The
observed chemical transformation of these semicarbazone
ligands (3) is believed to be initiated by their coordination
of rhodium(III) as a monoanionic O,N,O-donor via dissocia-
tion of only the phenolic proton (5). Such a coordination
mode of these semicarbazone ligands (3) is well documented
in the literature.3e Attack of the amine (NR2R′) at the carbonyl
carbon then takes place, which is followed by other steps as
shown before in Scheme 1. As all the [Rh(PPh3)2(ONOn-
NR2)Cl] complexes have been synthesized similarly and they
show similar properties (vide infra), the other [Rh(PPh3)2-
(ONOn-NR2)Cl] complexes are assumed to have similar
structure as [Rh(PPh3)2(ONO2-NEt2)Cl].
Figure 3. Cyclic voltammograms of (a) [Rh(PPh3)2(CNO-NEt2)Cl] and
(b) [Rh(PPh3)2(ONO1-NnBu2)Cl] in 1:9 dichloromethane-acetonitrile
solution (0.1 M TBAP) at a scan rate of 50 mV s-1
.
The microanalytical data of these [Rh(PPh3)2(ONOn-
NR2)Cl] complexes are in good agreement with their
1
compositions (Table S1). H NMR spectra of these com-
plexes (Table 3) are also consistent with their composi-
tions. In addition to broad signals for the triphenylphosphines
(7.0-7.8 ppm), some of the expected signals from the
coordinated semicarbazones have been clearly located while
others could not be observed due to their overlap with the
triphenylphosphine signals. The signal for the azomethine
proton is observed in the [Rh(PPh3)2(ONO1-NR2)Cl] com-
plexes around 6.5 ppm, but the same could not be detected
in the [Rh(PPh3)2(ONO3-NR2)Cl] complexes. In the
[Rh(PPh3)2(ONO2-NR2)Cl] complexes, a distinct methyl
signal is observed near 1.8 ppm. Signals for the NR2 fragment
were observed in all these complexes in the expected region
as before.
Infrared spectra of the [Rh(PPh3)2(ONOn-NR2)Cl] com-
plexes are mostly similar to those of the [Rh(PPh3)2(CNO-
NR2)Cl] complexes. Electronic spectra of these complexes,
recorded in dichloromethane solution, show several intense
absorptions in the visible and ultraviolet region (Table 4).
The absorptions in the ultraviolet region are assignable to
transitions within the ligand orbitals. To understand the origin
of the absorptions in the visible region, EHMO calculations
Figure 4. View of the [Rh(PPh3)2(ONO2-NEt2)Cl] complex.
lengths of the other bonds in the Rh(ONO2-NEt2) fragment
are observed to be significantly different from those of the
corresponding bonds in the Rh(CNO-NEt2) fragment of the
previous structure, and this difference is attributable to the
4344 Inorganic Chemistry, Vol. 42, No. 14, 2003

Using [Rh(PPh3)3Cl] to Transform Semicarbazones
The cyclic voltammetric behavior of the [Rh(PPh3)2-
(ONOn-NR2)Cl] complexes, studied in 1:9 dichloromethane-
acetonitrile (0.1 M TBAP) solution, is qualitatively similar
to that of the [Rh(PPh3)2(CNO-NR2)Cl] complexes. Each
[Rh(PPh3)2(ONOn-NR2)Cl] complex shows one reversible
oxidative response on the positive side of SCE and one
irreversible reductive response on the negative side (Table
4, Figure 3), and in view of the results of the EHMO cal-
culations, both the responses are assigned to ligand(semi-
carbazone)-centered redox reactions.
Conclusions
The present study reveals that reactivity of semicarbazone
ligands gets modified considerably upon its coordination to
an electropositive metal center and the coordinated semi-
carbazone ligand undergoes unusual chemical transformation
via initial attack of nucleophiles to the carbonyl carbon. This
study also shows that ligands of this type are likely to display
interesting metal mediated chemical transformations upon
reaction with nucleophiles of different types, and such
possibilities are currently under exploration.
Figure 5. Partial molecular orbital diagram of [Rh(PPh3)2(ONO1-
NEt2)Cl].
Acknowledgment. Financial assistance received from the
Council of Scientific and Industrial Research, New Delhi
[Grant 01(1675)/00/EMR-II], is gratefully acknowledged.
The authors thank Professor B. C. Ranu and Professor S.
Lahiri of the Department of Organic Chemistry, Indian Asso-
ciation for the Cultivation of Science, Kolkata, for their help.
Thanks are also due to the RSIC at Central Drug Research
Institute, Lucknow, India, for the C,H,N analysis data and
the Bose Institute, Kolkata 700054, for NMR spectral mea-
surements. The authors thank the reviewers for their critical
comments and constructive suggestions, which have been
helpful in preparing the revised version.
have been performed on these complexes, which have
afforded qualitatively similar results for all the complexes.
The composition of some selected molecular orbitals of three
representative complexes is given in Table 5, and a partial
MO diagram for one complex is shown in Figure 5. The
HOMO is primarily concentrated on the NR2 fragment of
the semicarbazone ligand, while the HOMO - 1 and HOMO
- 2 have significant (42-83%) contributions from the metal
d-orbitals. The LUMO is delocalized over the arylimine
fragment, and the next few vacant orbitals (LUMO + 1,
LUMO + 2, etc.) are localized on different parts of the
semicarbazone ligand. The lowest energy absorption in the
visible region may therefore be attributed to a transition from
the HOMO concentrated on one part of the semicarbazone
ligand to the LUMO localized on another part of the same
ligand. The higher energy absorptions in the visible region
may be assigned to transitions from the lower filled orbitals
to the higher vacant orbitals.
Supporting Information Available: Table containing mi-
croanalytical data of the [Rh(PPh3)2(CNO-NR2)Cl] and [Rh(PPh3)2-
(ONOn-NR2)Cl] 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.
IC034247J
Inorganic Chemistry, Vol. 42, No. 14, 2003 4345