Unprecedented chemical transformation of benzaldehyde semicarbazone mediated by osmium

Article abstract of DOI:10.1021/ic026154s

para-Nitrobenzaldehyde semicarbazone (O₂N(para) - C₆H₄C(H) = N - NH - CO - NH₂) undergoes unprecedented chemical transformation during its reaction with [Os(PPh₃)₂(CO)₂(HCOO)₂] in different alcoholic (R′OH, R′ = CH₂CH₂OCH₃, CH₂CH₃, CH₂CH₂CH₃, and CH₂CH₂CH₂CH₃) solvents whereby the NH₂ group of the semicarbazone ligand is displaced by a OR′ group provided by the solvents. The transformed semicarbazone ligand binds to osmium as a bidentate N,O-donor forming five-membered chelate ring to afford complexes of type [Os(PPh₃)₂-(CO)(H)(L-OR′)], where L-OR′ refers to the transformed semicarbazone ligand. Structure of the [Os(PPh₃)₂(CO)-(H)(L-OCH₂ CH₂OCH₃)] complex has been determined by X-ray crystallography. All the [Os(PPh₃)₂(CO)(H)(L-OR′)] complexes are diamagnetic and show characteristic ¹H NMR signals. They also show intense absorptions in the visible and ultraviolet region. Cyclic voltammetry on the complexes shows an irreversible oxidative response within 0.69-0.88 V versus SCE.

Full text of DOI:10.1021/ic026154s

Inorg. Chem. 2003, 42, 2069−2074
Unprecedented Chemical Transformation of Benzaldehyde
Semicarbazone Mediated by Osmium
,1a
Parna Gupta,1a Falguni Basuli,1a Shie-Ming Peng,1b Gene-Hsiang Lee,1b and Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity,
Kolkata 700 032, India, and Department of Chemistry, National Taiwan UniVersity,
Taipei, Taiwan, ROC
Received November 2, 2002
para-Nitrobenzaldehyde semicarbazone (O N(para)sC H C(H)dNsNHsCOsNH ) undergoes unprecedented
2
6
4
2
chemical transformation during its reaction with [Os(PPh ) (CO)2(HCOO)2] in different alcoholic (R′OH, R′ )
3 2
CH CH OCH , CH CH , CH CH CH , and CH CH CH CH ) solvents whereby the NH group of the semicarbazone
2
2
3
2
3
2
2
3
2
2
2
3
2
ligand is displaced by a OR′ group provided by the solvents. The transformed semicarbazone ligand binds to
osmium as a bidentate N,O-donor forming five-membered chelate ring to afford complexes of type [Os(PPh3)2-
(CO)(H)(L-OR′)], where L-OR′ refers to the transformed semicarbazone ligand. Structure of the [Os(PPh3)2(CO)-
(H)(L-OCH CH OCH )] complex has been determined by X-ray crystallography. All the [Os(PPh3)2(CO)(H)(L-OR′)]
2
2
3
complexes are diamagnetic and show characteristic 1H NMR signals. They also show intense absorptions in the
visible and ultraviolet region. Cyclic voltammetry on the complexes shows an irreversible oxidative response within
0.69−0.88 V versus SCE.
Introduction
Though the chemistry of transition metal complexes of
the semicarbazone and thiosemicarbazone ligands has been
receiving considerable attention primarily because of their
bioinorganic relevance,2 we have been studying the chemistry
of the platinum metal complexes of these ligands with special
reference to the variable coordination modes displayed by
these ligands in their complexes.3 During our recent explora-
tion of the coordination chemistry of the benzaldehyde
semicarbazones (1), we have observed that in their reaction
* To whom correspondence should be addressed. E-mail: samaresh_b@
hotmail.com.
(1) (a) Jadavpur University. (b) National Taiwan University.
(2) (a) 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. (b) 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. (c) West, D. X.; Padhye, S. B.;
Sonawane, P. B. Struct. Bonding (Berlin) 1992, 76, 1. (d) Haiduc, I.;
Silvestru, C. Coord. Chem. ReV. 1990, 99, 253. (e) Padhye, S. B.;
Kauffman, G. B. Coord. Chem. ReV. 1985, 63, 127. (f) Campbell, M.
J. M. Coord. Chem. ReV. 1975, 15, 279.
(3) (a) Dutta, S.; Basuli, F.; Peng, S. M.; Lee, G. H.; Bhattacharya, S.
New. J. Chem. 2002, 26, 1607. (b) Basuli, F.; Peng, S. M.;
Bhattacharya, S. Inorg. Chem. 2001, 40, 1126. (c) Pal, I.; Basuli, F.;
Mak, T. C. W.; Bhattacharya, S. Angew. Chem., Int. Ed. 2001, 40,
2923. (d) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2000,
39, 1120. (e) Basuli, F.; Ruf, M.; Pierpont, C. G.; Bhattacharya, S.
Inorg. Chem. 1998, 39, 6113. (f) Basuli, F.; Peng, S. M.; Bhattacharya,
S. Inorg. Chem. 1997, 36, 5645.
with ruthenium these semicarbazone ligands display three
different coordination modes (2, 3, and 4) depending on the
nature of the ruthenium starting materials and the experi-
mental conditions as well.3b This has encouraged us to
explore the coordination chemistry of these semicarbazone
ligands with the next congener of ruthenium, viz. osmium.
It is interesting to note here that though transition metal
10.1021/ic026154s CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/15/2003
Inorganic Chemistry, Vol. 42, No. 6, 2003 2069

Gupta et al.
Table 1. Crystallographic Data for
[Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)]
complexes of the semicarbazone ligands have been studied
well,2e the osmium chemistry of the benzaldehyde semicar-
bazones appears to have remained unexplored. For the
present study, we have chosen the [Os(PPh3)2(CO)2(HCOO)2]
complex as the osmium starting material. This particular
complex has recently been synthesized in our laboratory, and
it has already demonstrated its ability as an efficient synthon,
which is believed to be due mainly to the labile nature of
the formate ligands.4 Reaction of each of the five semicar-
bazone ligands (1) has been carried out with [Os(PPh3)2-
(CO)2(HCOO)2] in several alcoholic (R′OH) solvents. How-
ever, except the para-nitrobenzaldehyde semicarbazone, the
other semicarbazone ligands have not afforded any charac-
terizable complexes. During its reaction with the [Os(PPh3)2-
empirical formula
fw
space group
a, Å
b, Å
C50H46N3O6OsP2
1051.05
orthorhombic, P212121
14.6729(9)
16.0198(10)
19.3962(12)
4559.2(5)
4
c, Å
V, Å3
Z
λ, Å
0.71073
cryst size, mm3
T, K
0.35 × 0.15 × 0.15
150(1)
µ, mm-1
R1a
2.921
0.0282
0.0660
1.006
wR2b
GOFc
a R1 ) ∑||Fo| - |Fc|/∑|Fo|. b wR2 ) [∑{w(Fo2 - Fc )2}/∑{w(Fo )}]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.
an orange solution. Evaporation of this solution gave an orange
solid, which was subjected to thin-layer chromatography on a silica
plate. With toluene as the eluant, an orange band separated out.
The orange band was extracted with acetonitrile, and slow evapora-
tion of the acetonitrile extract gave [Os(PPh3)2(CO)(H)(L-OCH2-
CH2OCH3)] as an orange crystalline solid. Yield: 60%.
Physical Measurements. Microanalyses (C, H, N) were per-
formed using a Heraeus Carlo Erba 1108 elemental analyzer. IR
spectra were obtained on a Perkin-Elmer 783 spectrometer with
samples prepared as KBr pellets. Electronic spectra were recorded
on a JASCO V-570 spectrophotometer. 1H NMR spectra were
obtained on 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. Electrochemical
experiments were performed under a dinitrogen atmosphere. All
electrochemical data were collected at 298 K and are uncorrected
for junction potentials.
(CO)2(HCOO)2] complex, the para-nitrobenzaldehyde semi-
carbazone ligand has been found to undergo an interesting
chemical transformation via reaction with the alcoholic
(R′OH) solvent and bind to osmium as a bidentate N,O-donor
(5) affording complexes of type [Os(PPh3)2(CO)(H)(L-OR′)],
where L-OR′ refers to the transformed semicarbazone ligand
in 5. An account of the chemistry of these [Os(PPh3)2(CO)-
(H)(L-OR′)] complexes is described in this paper with special
reference to their formation, structure, and spectroscopic and
electrochemical properties.
Experimental Section
Crystallography of [Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)].
Single crystals of [Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)] were
obtained by slow evaporation of acetonitrile solution of the complex.
Selected crystal data and data collection parameters are given in
Table 1. Data were collected on a Bruker Smart CCD diffractometer
using graphite monochromated Mo KR radiation (λ ) 0.71073 Å)
by φ and ω scans within the θ range 1.65-27.50°. X-ray data
reduction, structure solution, and refinement were done using
SHELXS-97 and SHELXL-97 programs.8 The structure was solved
by the direct methods.
Materials. Commercial osmium tetroxide was purchased from
Arora Matthey, Kolkata, India, and was converted to [NH4]2[OsBr6]
by reduction with hydrobromic acid.5 [Os(PPh3)3Br2] was synthe-
sized, starting from [NH4]2[OsBr6], by following a reported
procedure.6 [Os(PPh3)2(CO)(HCOO)2] was prepared as before.4
Purification of dichloromethane and preparation of tetrabutyl-
ammonium perchlorate (TBAP) for electrochemical work were
performed as reported in the literature.7 All other chemicals and
solvents were reagent grade commercial materials and were used
as received.
Preparations of Complexes. All the [Os(PPh3)2(CO)(H)(L-OR′)]
(R′ ) CH2CH2OCH3, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3)
complexes were prepared in 55-65% yield by following a general
procedure. Specific details are given below for a particular complex.
[Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)]. To a hot solution of
4-nitrobenzaldehyde semicarbazone (24 mg, 0.12 mmol) in 2-meth-
oxyethanol (40 mL) was added [Os(PPh3)2(CO)(HCOO)2] (100 mg,
0.12 mmol). The mixture was heated at reflux for 24 h to produce
Results and Discussion
Reaction of the para-nitrobenzaldehyde semicarbazone
with [Os(PPh3)2(CO)2(HCOO)2] was first carried out in
refluxing 2-methoxyethanol, which afforded a crystalline
orange complex in decent yield. Preliminary characterization
(elemental analysis, IR, 1H NMR, etc.) on this complex (vide
infra) could not point to any definite composition. Identity
of the complex has finally been revealed by its structure
determination by X-ray crystallography. The structure is
(4) Gupta, P.; Basuli, F.; Peng, S. M.; Lee, G. H.; Bhattacharya, S.
Submitted for publication.
(5) Dwyer, F. P.; Hogarth, J. W. Inorg. Synth. 1957, 5, 204.
(6) Hoffman, P. R.; Caulton, K. G. J. Am. Chem. Soc. 1975, 97, 4221.
(7) (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.
(8) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Fortran programs for
crystal structure solution and refinement; University of Gottingen:
Gottingen, Germany, 1997.
2070 Inorganic Chemistry, Vol. 42, No. 6, 2003

Osmium Transformation of Benzaldehyde Semicarbazone
a hydride (H-) are also coordinated to osmium. The expelled
formate ligand or the solvent probably has served as the
source of the hydride. This complex is therefore represented
as [Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)], where (L-OCH2-
CH2OCH3) stands for the transformed semicarbazone ligand
coordinated to osmium. The coordinated semicarbazone, CO,
and hydride ligands constitute the equatorial plane of the
octahedron with the metal ion at the center. The CO and the
hydride are trans respectively to the oxygen and nitrogen of
the semicarbazone ligand. The two PPh3 ligands have
occupied the remaining two axial positions, and hence, they
are mutually trans. The Os-H, Os-C, Os-N, and Os-O
distances are all quite normal, as observed in structurally
characterized complexes of osmium(II) containing these
bonds.10 However, the Os-P distances are a bit shorter than
usually observed in complexes of osmium(II) having the
trans-Os(PPh3)2 fragment.10c-g,i Comparison of the bond
lengths in the coordinated semicarbazone ligand with those
in the free semicarbazone ligand9 shows that upon coordina-
tion the C-O bond has elongated while the adjacent C-N
bond (which is part of the chelate ring) has contracted. These
changes in bond lengths are attributable to stabilization of
the iminolate form of the semicarbazone ligand upon
complexation via loss of the hydrazinic proton.
The observed transformation of the semicarbazone ligand
not only is unusual, but also appears to be unprecedented.
Treatment of the semicarbazone ligand with the refluxing
solvent (2-methoxyethanol) alone (for 24 h) failed to bring
about any chemical change in the semicarbazone, which
indicates that the observed transformation has been mediated
by the metal center. The mechanism of this interesting
chemical transformation, as well as the unusual conforma-
tional change of the semicarbazone ligand, is not completely
clear to us. However, the sequences shown in Scheme 1 seem
probable. In the initial step, the semicarbazone coordinates
to osmium through the imine nitrogen, which favors dis-
sociation of the hydrazinic proton. The negative charge, thus
generated, gets delocalized over the para-nitrophenyl ring
causing a temporary decrease in the imine bond order. This
is followed by the conformational change, driven probably
by steric interaction between the aryl ring and osmium.3d
The conformational change seems to be associated with
simultaneous coordination from the oxygen, which enhances
the electrophilicity of the next carbon atom. Subsequent
nucleophilic attack by the solvent (R′OH) at this amide-like
carbon finally leads to the observed transformation via
elimination of NH3. The other benzaldehyde semicarbazones
Figure 1. View of (a) the [Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)]
complex and (b) the Os(CO)(H)(L-OCH2CH2OCH3) fragment.
Table 2. Selected Bond Distances and Bond Angles for
[Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3]
Bond Distances (Å)
Os-H(1)
Os-C(1)
Os-N(1)
Os-O(2)
Os-P(1)
Os-P(2)
C(1)-O(1)
C(6)-N(1)
1.60(4)
N(1)-N(2)
N(2)-C(2)
C(2)-O(2)
C(2)-O(3)
O(3)-C(3)
C(3)-C(4)
C(4)-O(4)
O(4)-C(5)
1.379(4)
1.316(5)
1.282(5)
1.337(5)
1.447(6)
1.648(9)
1.309(10)
1.664(13)
1.838(5)
2.139(3)
2.125(3)
2.3478(10)
2.3469(10)
1.158(5)
1.303(5)
Bond Angles (deg)
P(1)-Os-P(2)
H(1)-Os-N(1)
171.40(4)
166.3(14)
C(1)-Os-O(2)
N(1)-Os-O(2)
173.43(16)
74.52(11)
shown in Figure 1, and selected bond parameters are listed
in Table 2. The structure shows that during the course of
the synthetic reaction, the semicarbazone ligand has under-
gone an unusual chemical transformation. The NH2 fragment
of the semicarbazone ligand has been replaced by a OCH2-
CH2OCH3 fragment provided by the solvent. The modified
semicarbazone ligand is coordinated to osmium as a bidentate
N,O-donor forming a five-membered chelate ring (5). This
particular coordination mode of the semicarbazone ligand
is also quite unusual as it involves, with reference to the
structure of the uncoordinated ligand (1),9 an interesting
conformational change across the imine (CdN) bond.
Besides the semicarbazone, two PPh3 ligands, one CO, and
(10) (a) Barrio, P.; Esteruelas, M. A.; O˜ nate, E. Organometallics 2002,
21, 2491. (b) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.;
Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1996, 35, 6775. (c)
Robinson, P. D.; Hinckley, C. C.; Ikuo, A. Acta Crystallogr. 1988,
C44, 1491. (d) Pramanik, K.; Shivakumar, M.; Ghosh, P.; Chakravorty,
A. Inorg. Chem. 2000, 39, 195. (e) Das, A.; Basuli, F.; Peng, S. M.;
Bhattacharya, S. Polyhedron 1999, 18, 2729. (f) Rickard, C. E. F.;
Roper, W. R.; Woodgate, S. D.; Wright, L. J. J. Organomet. Chem.
2002, 643-644, 168. (g) Rickard, C. E. F.; Roper, W. R.; Williamson,
A.; Wright, L. J. Organometallics 2002, 21, 1714. (h) Yandulov, D.
V.; Huffman, J. C.; Caulton, K. G. Organometallics 2002, 21, 4030.
(i) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J.
Organometallics 2002, 21, 4862. (j) Esteruelas, M. A.; Gonzalez, A.
I.; Lopez, A. M.; O˜ nate, E. Organometallics 2003, 22, 414.
(9) Naik, D. V.; Palenik, G. J. Acta Crystallogr., Sect. B 1974, 30, 2396.
Inorganic Chemistry, Vol. 42, No. 6, 2003 2071

Gupta et al.
Scheme 1. Probable Steps in the Formation of the
[Os(PPh3)2(CO)(H)(L-OR′)] Complexes
Figure 2. 1H NMR spectrum of [Os(PPh3)2(CO)(H)(L-OCH2CH2CH2-
CH3)] in CDCl3 solution.
CH2CH3) has been obtained. Elemental analytical data of
all the four [Os(PPh3)2(CO)(H)(L-OR′)] complexes are in
good agreement with their composition (Table S1). As all
the [Os(PPh3)2(CO)(H)(L-OR′)] complexes have been syn-
thesized similarly and as they exhibit similar properties (vide
infra), the latter three [Os(PPh3)2(CO)(H)(L-OR′)] (where R′
) CH2CH3, CH2CH2CH3 and CH2CH2CH2CH3) complexes
are assumed to have the same structure as [Os(PPh3)2(CO)-
(H)(L-OCH2CH2OCH3)].
(1, R * NO2) could not undergo similar reaction probably
because the substituent R was not enough electron-withdraw-
ing so as to favor delocalization of the negative charge over
the aryl ring, which seems to play a key role in the observed
transformation.
Encouraged by the interesting reaction of 2-methoxy-
ethanol with para-nitrobenzaldehyde semicarbazone medi-
ated by osmium, similar reactions were also carried out in
few other alcoholic solvents (viz. ethanol, n-propanol, and
n-butanol) in order to check whether similar transformation
also takes place in these solvents. From each of these
reactions, an orange complex of type [Os(PPh3)2(CO)(H)-
(L-OR′)] (where R′ ) CH2CH3, CH2CH2CH3, and CH2CH2-
Magnetic susceptibility measurements show that the [Os-
(PPh3)2(CO)(H)(L-OR′)] complexes are diamagnetic, which
corresponds to the +2 state of osmium (low-spin d6 S ) 0)
1
in these complexes. H NMR spectra of all the complexes
have been recorded in CDCl3 solution. Spectral data are given
in Table 3, and a selected spectrum is shown in Figure 2.
Each complex shows broad signals for the phenyl protons
of the PPh3 ligands within 7.2-7.6 ppm. In each complex,
the hydride signal is observed as a distinct triplet, due to
Table 3. 1H NMR Data of the Complexes
chemical shift in ppm
compd
PPh3
Ha(JH-H, Hz), Hb(JH-H, Hz)
Hc
Hd(JP-H, Hz)
R′
[Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)]
[Os(PPh3)2(CO)(H)(L-OCH2CH3)]
[Os(PPh3)2(CO)(H)(L-OCH2CH2CH3)]
[Os(PPh3)2(CO)(H)(L-OCH2CH2CH2CH3)]
7.2-7.6
7.2-7.6
7.2-7.6
7.2-7.6
8.05(9.0), 7.69(9.0)
8.05(8.9), 7.66(8.9)
8.05(8.8), 7.67(8.8)
8.05(8.8), 7.66(8.8)
6.35
6.44
6.37
6.38
-11.20(18.6)
-11.30(18.3)
-11.21(18.3)
-11.21(18.6)
3.10,a 3.19,b 3.47a
0.86,a 3.48c
0.75,d 1.14,d 3.33a
0.79,a 1.15,d 3.33a
a Triplet. b Singlet. c Quartet. d Multiplet.
2072 Inorganic Chemistry, Vol. 42, No. 6, 2003

Osmium Transformation of Benzaldehyde Semicarbazone
Table 4. Electronic Spectral and Cyclic Voltammetric Data
electronic spectral dataa
max, nm (ꢀ, M-1 cm-1
cyclic voltammetric dataa,b
compd
λ
)
E, V vs SCE
[Os(PPh3)2(CO)(H)(L-OCH2CH2OCH3)]
[Os(PPh3)2(CO)(H)(L-OCH2CH3)]
[Os(PPh3)2(CO)(H)(L-OCH2CH2CH3)]
[Os(PPh3)2(CO)(H)(L-OCH2CH2CH2CH3)]
436(1630), 306(1300),c 274(12700),c 268(13700)c
420(1200), 334(2400),c 276(11500)c 268(14461)c
446(4900), 326(6700),c 276(13400),c 268(16200)c
436(4200), 310(5800),c 276(12700)c 268(14800)c
0.77d
0.69d
0.88d
0.78d
a In dichloromethane. b Supporting electrolyte, TBAP; scan rate 50 mV s-1 c Shoulder. d Anodic peak potential (Epa).
.
coupling with the two phosphorus nuclei, near -11.2 ppm.
All the expected signals of the coordinated semicarbazone
ligand have been clearly observed in all the complexes. The
azomethine proton signal is observed as a singlet near 6.4
ppm, and signals for the two phenyl protons are observed
as doublets near 7.7 and 8.0 ppm. The expected signals for
the R′ groups in the semicarbazone ligand are observed
1
within 0.75-3.48 ppm. The H NMR spectral data of the
[Os(PPh3)2(CO)(H)(L-OR′)] complexes are therefore con-
sistent with their compositions.
Infrared spectra (4000-600 cm-1) of the [Os(PPh3)2(CO)-
(H)(L-OR′)] complexes show many bands of different
intensities (Table S1). Assignment of individual bands to
specific vibrations has not been attempted. However, a strong
band displayed near 1900 cm-1 by all the complexes
indicates the presence of coordinated CO.10f,g,i,11 Another
sharp band, observed near 2040 cm-1 in each complex, is
assigned to the ν(Os-H) stretch.10f,12 Strong bands are also
observed near 520, 695, and 750 nm in all the complexes
due to the coordinated PPh3 ligands.3a,b,10e,13 Comparison of
the infrared spectrum of each [Os(PPh3)2(CO)(H)(L-OR′)]
complex with that of [Os(PPh3)2(CO)2(HCOO)2] shows that
some additional bands (e.g., strong bands near 1600, 1516,
1330, and 1025 cm-1) are present in the spectra of the [Os-
(PPh3)2(CO)(H)(L-OR′)] complexes, which are attributable
to the coordinated semicarbazone ligand. The [Os(PPh3)2-
(CO)(H)(L-OR′)] complexes are soluble in dichloromethane,
chloroform, acetone, acetonitrile, etc., producing intense
orange solutions. Electronic spectra of the complexes have
been recorded in dichloromethane solution. Spectral data are
presented in Table 4, and a selected spectrum is shown in
Figure 3. Each complex shows four intense absorptions, one
in the visible region and three in the ultraviolet region. To
have an idea about the nature of transitions responsible for
these intense absorptions, qualitative EHMO calculations
have been performed14 on computer generated models of all
the four complex molecules, in which the phenyl rings of
the PPh3 ligands have been replaced by hydrogens. A partial
MO diagram is displayed in Figure 4. The molecular orbitals
are found to be similar in composition for all the [Os(PPh3)2-
(CO)(H)(L-OR′)] complexes (Table S2), which indicates that
the R′ group does not have any appreciable influence on these
molecular orbitals. In all the [Os(PPh3)2(CO)(H)(L-OR′)]
Figure 3. Electronic spectrum of [Os(PPh3)2(CO)(H)(L-OCH2CH2CH3)]
in dichloromethane solution.
Figure 4. Partial molecular orbital diagram of [Os(PPh3)2(CO)(H)(L-
OCH2CH3)]: (a) interaction diagram and (b) highest occupied and lowest
unoccupied molecular orbitals.
complexes, the highest occupied molecular orbital (HOMO)
has a major (g75%) contribution from the coordinated
semicarbazone ligand, and a relatively much smaller con-
tribution comes from the CO and osmium. A scrutiny of
atomic contribution shows that the HOMO is localized
mostly on the sNsNdC(H)s fragment of the semicarba-
(11) Diluzio, J. W.; Vaska, L. J. Am. Chem. Soc. 1961, 83, 1262.
(12) Laing, K. R.; Roper, W. R. J. Chem. Soc. A 1969, 1889.
(13) (a) Das, A. K.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton
Trans. 2000, 181. (b) Dutta, S.; Peng, S. M.; Bhattacharya, S. Inorg.
Chem. 2000, 39, 2231. (c) Dutta, S.; Peng, S. M.; Bhattacharya, S. J.
Chem. Soc., Dalton Trans. 2000, 4623.
(14) (a) Mealli, C.; Proserpio, D. M. CACAO, version 4.0; Italy, 1994. (b)
Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
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Gupta et al.
zone ligand and part of the phenyl ring linked to it. The lower
filled orbitals [e.g., HOMO - 1 and HOMO - 2] have larger
contributions from the metal center. The lowest unoccupied
molecular orbital (LUMO) has more than 80% contribution
from the semicarbazone ligand and is concentrated largely
on the NO2 fragment of the semicarbazone ligand. The higher
vacant orbitals (e.g., LUMO + 1 and LUMO + 2) are also
localized on different parts of the semicarbazone ligand. The
absorption in the visible region may therefore be assigned
to the transition from the filled π-orbital (HOMO) delocalized
over one part of the semicarbazone ligand to the vacant π*-
orbital (LUMO) concentrated on another part of the same
ligand. In complexes of osmium(II) containing ligands with
recognized chromophoric groups, usually metal(t2)-to-ligand-
(π*) charge-transfer transitions are observed in the visible
region.15 The present [Os(PPh3)2(CO)(H)(L-OR′)] complexes
thus represent an interesting family where such MLCT
transitions do not occur in the visible region.
Electrochemical properties of the [Os(PPh3)2(CO)(H)(L-
OR′)] complexes have been studied by cyclic voltammetry
in dichloromethane solution (0.1 M TBAP). Voltammetric
data are given in Table 4. Each complex shows a one-electron
oxidative response on the positive side of SCE, and keeping
the results of the EHMO calculations in view, this is assumed
to be a ligand(semicarbazone)-centered oxidation. This
oxidation is irreversible for all the complexes, which indicates
that the oxidized [Os(PPh3)2(CO)(H)(L-OR′)]+ species,
formed during the anodic scan, is unstable and undergoes
rapid decomposition. The one-electron nature of this oxida-
tion has been established by comparing its current height
(anodic peak-current) with that of the standard ferrocene/
ferrocenium couple under identical experimental conditions.
Conclusions
The present study shows that benzaldehyde semicarbazone
may undergo interesting metal-mediated transformation via
nucleophilic attack by suitable species at the amide-like
carbon center. Such possibilities are currently under explora-
tion.
Acknowledgment. Financial assistance received from the
University Grants Commission [Grant F.12-56/2001(SR-I)]
is gratefully acknowledged. Thanks are also due to the
Council of Scientific and Industrial Research, New Delhi
[Grant 01(1675)/00/EMR-II], and the Department of Science
and Technology, New Delhi [Grant No. SP/S1/F33/98], for
financial assistance enabling the purchase of the electro-
chemical analyzer and the spectrophotometer used in this
work. The authors thank the RSIC at Central Drug Research
Institute, Lucknow, India, for the C,H,N analysis data. P.G.
thanks CSIR for her fellowship.
Supporting Information Available: 1H NMR spectra and tables
containing yield and microanalytical and infrared spectral data
(Table S1) and composition of selected molecular orbitals (Table
S2) of the [Os(PPh3)2(CO)(H)(L-OR′)] complexes. X-ray crystal-
lographic data in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
(15) (a) Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron 1999, 18,
391. (b) Kober, E. M.; Casper, J. V.; Sullivan, B. P.; Meyer, T. J.
Inorg. Chem. 1988, 27, 4587.
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