Chemical control on the coordination mode of benzaldehyde semicarbazone ligands. Synthesis, structure, and redox properties of ruthenium complexes

Article abstract of DOI:10.1021/ic000915e

Reaction of benzaldehyde semicarbazone (HL-R, where H is a dissociable proton and R is a substituent (R = OMe, Me, H, Cl, NO₂) at the para position of the phenyl ring) with [Ru(PPh₃)₃Cl₂] and [Ru(PPh₃)₂(CO)₂Cl₂] has afforded complexes of different types. When HL-NO₂ and [Ru(PPh₃)₃Cl₂] react in solution at ambient temperature, trans-[Ru(PPh₃)₂(L-NO₂)Cl] is obtained. Its structure determination by X-ray crystallography shows that L-NO₂ is coordinated as a tridentate C,N,O-donor ligand. When reaction between HL-NO₂ and [Ru(PPh₃)₃Cl₂] is carried out in refluxing ethanol, a more stable cis isomer of [Ru(PPh₃)₂(L-NO₂)Cl] is obtained. The trans isomer can be converted to the cis isomer simply by providing appropriate thermal energy. Slow reaction of HL-R with [Ru- (PPh₃)₂(CO)₂Cl₂] in solution at ambient temperature yields 5-[Ru(PPh₃)₂(L-R)(CO)Cl] complexes. A structure determination of 5-[Ru(PPh₃)₂(L-NO₂)(CO)Cl] shows that the semicarbazone ligand is coordinated as a bidentate N,O-donor, forming a five-membered chelate ring. When reaction between HL-R and [Ru(PPh₃)₂(CO)₂Cl₂] is carried out in refluxing ethanol, the 4-[Ru(PPh₃)₂(L-R)(CO)Cl] complexes are obtained. A structure determination of 4-[Ru(PPh₃)₂(L-NO₂)(CO)Cl] shows that a semicarbazone ligand is bound to ruthenium as a bidentate N,O- donor, forming a four-membered chelate ring. All the complexes are diamagnetic (low-spin d⁶, S = 0). The trans- and cis-[Ru(PPh₃)₂(L-NO₂)Cl] complexes undergo chemical transformation in solution. The 5- and 4-[Ru(PPh₃)₂(L-R)(CO)Cl] complexes show sharp NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry of the 5-[Ru(PPh₃)₂(L-R)(CO)Cl] and 4-[Ru(PPh₃)₂(L-R)(CO)Cl] complexes show the Ru-(II)-Ru(III) oxidation to be within 0.66-1.07 V. This oxidation potential is found to linearly correlate with the Hammett constant of the substituent R.

Full text of DOI:10.1021/ic000915e

1126
Inorg. Chem. 2001, 40, 1126-1133
Chemical Control on the Coordination Mode of Benzaldehyde Semicarbazone Ligands.
Synthesis, Structure, and Redox Properties of Ruthenium Complexes
Falguni Basuli,1a Shie-Ming Peng,1b and Samaresh Bhattacharya*,1a
Departments of Chemistry, Inorganic Chemistry Section, Jadavpur University, Calcutta 700 032, India,
and National Taiwan University, Taipei, Taiwan, Republic of China
ReceiVed August 9, 2000
Reaction of benzaldehyde semicarbazone (HL-R, where H is a dissociable proton and R is a substituent (R )
OMe, Me, H, Cl, NO2) at the para position of the phenyl ring) with [Ru(PPh3)3Cl2] and [Ru(PPh3)2(CO)2Cl2] has
afforded complexes of different types. When HL-NO2 and [Ru(PPh3)3Cl2] react in solution at ambient temperature,
trans-[Ru(PPh3)2(L-NO2)Cl] is obtained. Its structure determination by X-ray crystallography shows that L-NO2
is coordinated as a tridentate C,N,O-donor ligand. When reaction between HL-NO2 and [Ru(PPh3)3Cl2] is carried
out in refluxing ethanol, a more stable cis isomer of [Ru(PPh3)2(L-NO2)Cl] is obtained. The trans isomer can be
converted to the cis isomer simply by providing appropriate thermal energy. Slow reaction of HL-R with [Ru-
(PPh3)2(CO)2Cl2] in solution at ambient temperature yields 5-[Ru(PPh3)2(L-R)(CO)Cl] complexes. A structure
determination of 5-[Ru(PPh3)2(L-NO2)(CO)Cl] shows that the semicarbazone ligand is coordinated as a bidentate
N,O-donor, forming a five-membered chelate ring. When reaction between HL-R and [Ru(PPh3)2(CO)2Cl2] is
carried out in refluxing ethanol, the 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes are obtained. A structure determination
of 4-[Ru(PPh3)2(L-NO2)(CO)Cl] shows that a semicarbazone ligand is bound to ruthenium as a bidentate N,O-
donor, forming a four-membered chelate ring. All the complexes are diamagnetic (low-spin d6, S ) 0). The
trans- and cis-[Ru(PPh3)2(L-NO2)Cl] complexes undergo chemical transformation in solution. The 5- and 4-[Ru-
(PPh3)2(L-R)(CO)Cl] complexes show sharp NMR signals and intense MLCT transitions in the visible region.
Cyclic voltammetry of the 5-[Ru(PPh3)2(L-R)(CO)Cl] and 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes show the Ru-
(II)-Ru(III) oxidation to be within 0.66-1.07 V. This oxidation potential is found to linearly correlate with the
Hammett constant of the substituent R.
Introduction
We have recently observed a very unusual coordination mode
of benzaldehyde thiosemicarbazone (1) in a series of ruthenium
and osmium complexes.2 This has led us to explore the
coordination chemistry of the benzaldehyde semicarbazones 2,
particularly with reference to the different possible coordination
modes of these ligands. However, the present study has been
restricted only to ruthenium complexes of the benzaldehyde
semicarbazones. It is interesting to note here that though
transition-metal complexes of the semicarbazone ligands have
received some attention,3 the ruthenium chemistry of the
benzaldehyde semicarbazones appears to have remained com-
pletely unexplored. The primary objective of this study has been
to synthesize ruthenium complexes of benzaldehyde semicar-
bazones where the semicarbazone ligands are coordinated in
different fashions and where the coordination modes can be
controlled chemically. During the course of our studies we have
found that the coordination mode of the benzaldehyde semi-
carbazones is very sensitive toward minor variations in the
experimental conditions, the nature of the substituent on the
benzaldehyde fragment of the ligand, and the ruthenium starting
material. In the four types of complexes synthesized for the
present study, we have observed three different coordination
(1) (a) Jadavpur University. (b) National Taiwan University.
(2) (a) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 1997, 36,
5645. (b) Basuli, F.; Ruf, M.; Pierpont, C. G.; Bhattacharya, S. Inorg.
Chem. 1998, 37, 6113. (c) Basuli, F.; Peng, S. M.; Bhattacharya, S.
Inorg. Chem. 2000, 39, 1120.
modes of the benzaldehyde semicarbazones: (i) C,N,O-tricoor-
dination (3), (ii) N,O-coordination forming a stable five-
membered chelate (4), and (iii) an unusual four-membered
(3) Padhye, S.; Kauffman, G. B. Coord. Chem. ReV. 1985, 63, 127.
10.1021/ic000915e CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/14/2001

Ru Complexes of Benzaldehyde Semicarbazone Ligands
Inorganic Chemistry, Vol. 40, No. 6, 2001 1127
Table 1. Crystallographic Data for trans-[Ru(PPh3)2(L-NO2)Cl],
5-[Ru(PPh3)2(L-NO2)(CO)Cl], and 4-[Ru(PPh3)2(L-NO2)(CO)Cl]
Complexes
chelate (5) formation as an N,O-donor. An account of the
chemistry of all these complexes is described in this paper with
special reference to synthesis, structure, and electrochemical
properties.
4-[Ru(PPh3)2
[Ru(PPh3)2
(L-NO2)Cl]
5-[Ru(PPh3)2
(L-NO2)(CO)Cl] 0.5CH2Cl2‚0.75C6H6
(L-NO2)(CO)Cl]‚
Experimental Section
empirical C44H37N4O3-
C45H37ClN4-
O4P2Ru
896.25
triclinic,
P1h
10.212(2)
13.011(2)
17.218(3)
111.17(2)
91.12(2)
106.337(13)
2028.3(6)
2
C50H42.50Cl2N4-
O4P2Ru
997.29
triclinic,
P1h
16.9298(2)
17.6296(2)
17.9285(1)
74.937(1)
67.473(1)
80.209(1)
4758.00(8)
4
Materials. Commercial ruthenium trichloride was purchased from
Arora Matthey, Calcutta, India, and was converted to RuCl3‚3H2O by
repeated evaporation with concentrated hydrochloric acid. Semicarba-
zide hydrochloride, benzaldehyde, and para-substituted benzaldehydes
were purchased from Merck, India. Triphenylphosphine and formal-
dehyde were purchased from Spectrochem, India. All other chemicals
and solvents were reagent grade commercial materials and were used
as received. [Ru(PPh3)3Cl2] and [Ru(PPh3)2(CO)2Cl2] were prepared
by following a reported procedure.4 The semicarbazone ligands were
prepared by reacting equimolar amounts of semicarbazide hydrochlo-
ride, sodium acetate, and the respective para-substituted benzaldehyde
in a 1:1 ethanol-water mixture. The purification of acetonitrile and
dichloromethane and the preparation of tetrabutylammonium perchlorate
(TBAP) for electrochemical work were performed as before.5
Preparations of Complexes. trans-[Ru(PPh3)2(L-NO2)Cl]. A solu-
tion of 4-nitrobenzaldehyde semicarbazone (25 mg, 0.12 mmol) in
ethanol (60 mL) was carefully layered over a solution of [Ru(PPh3)3-
Cl2] (100 mg, 0.10 mmol) in dichloromethane (20 mL). The solutions
were allowed to diffuse slowly (∼25 days) to afford single crystals of
trans-[Ru(PPh3)2(L-NO2)Cl]. Yield: 70%.
cis-[Ru(PPh3)2(L-NO2)Cl]. Method A. A finely powdered solid
sample of trans-[Ru(PPh3)2(L-NO2)Cl] (100 mg, 0.15 mmol) was placed
in a three-necked flask. Dry nitrogen gas was passed through the flask
for ∼5 min. The solid was then heated at 155 °C (sand bath) under an
atmosphere of nitrogen for ∼15 min with occasional stirring to afford
cis-[Ru(PPh3)2(L-NO2)Cl]. This solid was then cooled to room tem-
perature under nitrogen. The yield was quantitative.
Method B. To a suspension of [Ru(PPh3)3Cl2] (100 mg, 0.10 mmol)
in ethanol (40 mL) was added 4-nitrobenzaldehyde semicarbazone (25
mg, 0.12 mmol). The resulting mixture was heated at reflux for 3 h.
cis-[Ru(PPh3)2(L-NO2)Cl] started to separate out as a dark microcrys-
talline solid during the reflux. After the solution was cooled to room
temperature, the precipitate was collected by filtration, washed thor-
oughly with ethanol, and dried in air. Yield: 60%.
formula
P2ClRu
868.26
monoclinic,
P21/c
fw
space
group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
9.7178(10)
17.4695(23)
23.109(3)
90
91.864(12)
90
3921.0(8)
4
λ, Å
0.710 73
0.710 73
0.710 73
0.22 × 0.16 ×
0.08
cryst size, 0.50 × 0.45 × 0.50 × 0.20 ×
mm
0.40
295
0.20
293(2)
5.80
T, K
295(2)
5.57
µ, cm-1
5.847
R indices
RF ) 0.036a
Rw ) 0.040b
1.26e
R1 ) 0.0297c
wR2 ) 0.0794d
1.032f
R1 ) 0.0647c
wR2 ) 0.1538d
1.048f
GOF
a RF ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w(Fo)2]1/2
.
.
c R1 ) ∑||Fo| - |Fc||/∑|Fo|. d wR2 ) [∑{w(Fo2 - Fc2)2}/∑{w(Fo )}]1/2
2
e GOF ) [∑(w(|Fo| - |Fc|)2)/(M - N)]1/ 2, where M is the number of
reflections and N is the number of parameters refined. f GOF )
[∑(w(Fo - Fc2)2)/(M - N)]1/ 2, where M is the number of reflections
2
and N is the number of parameters refined.
pellets. Electronic spectra were recorded on a JASCO V-570 spectro-
photometer. Magnetic susceptibilities were measured using a PAR 155
vibrating-sample magnetometer fitted with a Walker Scientific L75FBAL
1
magnet. H NMR spectra were obtained on a Bruker drx500 NMR
spectrometer using TMS as the internal standard. The FAB mass spectra
were recorded on a JEOL SX 102/DA-6000 mass spectrometer/data
system using argon/xenon (6 kV, 10 mA) as the FAB gas. Solid-state
thermal investigations were carried out with the help of a Shimadzu
DT-30 thermal analyzer. Electrochemical measurements were made
using a PAR Model 273 potentiostat. 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. Dinitrogen gas was purified by successively bubbling it
through alkaline dithionite and concentrated sulfuric acid. All electro-
chemical experiments were performed under a dinitrogen atmosphere.
All electrochemical data were collected at 298 K and are uncorrected
for junction potentials. An RE 0089 X-Y recorder was used to trace
the voltammograms.
Crystallography of trans-[Ru(PPh3)2(L-NO2)Cl]. Single crystals
of trans-[Ru(PPh3)2(L-NO2)Cl] were obtained by slow diffusion of an
ethanol solution of HL-NO2 into a dichloromethane solution of [Ru-
(PPh3)3Cl2]. Selected crystal data and data collection parameters are
given in Table 1. The unit cell dimensions were determined by a least-
squares fit of 25 centered reflections (19.00 e θ e 29.60°). Data were
collected on an Enraf-Nonius CAD-4 diffractometer using graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å) by θ-2θ scans
with a maximum 2θ ) 50°. Three standard reflections, used to check
the crystal stability toward X-ray exposure, showed no significant
intensity variation over the course of data collection. X-ray data
reduction, structure solution, and refinement were done using the
NRCVAX package. The structure was solved by direct methods.
Crystallography of 5-[Ru(PPh3)2(L-NO2)(CO)Cl]. Single crystals
of 5-[Ru(PPh3)2(L-NO2)(CO)Cl] were obtained by slow diffusion of
an ethanol solution of HL-NO2 into a dichloromethane solution of [Ru-
(PPh3)2(CO)2Cl2]. Selected crystal data and data collection parameters
are given in Table 1. Data were collected on a Siemens Smart CCD
diffractometer using graphite-monochromated Mo KR radiation (λ )
0.710 73 Å) by ω scans within the angular range 1.74 < θ < 24.98°.
5-[Ru(PPh3)2(L-R)(CO)Cl]. These complexes were all prepared by
following a general procedure. Specific details are given below for a
particular complex.
5-[Ru(PPh3)2(L-NO2)(CO)Cl]. A solution of 4-nitrobenzaldehyde
semicarbazone (30 mg, 0.14 mmol) in ethanol (60 mL) was carefully
layered over a solution of [Ru(PPh3)2(CO)2Cl2] (100 mg, 0.13 mmol)
in dichloromethane (20 mL). The solutions were allowed to diffuse
slowly (∼30 days) to afford single crystals of 5-[Ru(PPh3)2(L-NO2)-
(CO)Cl]. Yield: 70%.
4-[Ru(PPh3)2(L-R)(CO)Cl]. These complexes were all prepared by
following a general procedure. Specific details are given below for a
particular complex.
4-[Ru(PPh3)2(L-NO2)(CO)Cl]. To a hot solution of 4-nitrobenzal-
dehyde semicarbazone (30 mg, 0.14 mmol) in ethanol (40 mL) was
added [Ru(PPh3)2(CO)2Cl2] (100 mg, 0.13 mmol). The mixture was
refluxed for 24 h to produce an orange solution. On evaporation of the
solvent, a yellow residue was obtained, which was purified by
chromatography through a silica gel column. Using 1:10 acetonitrile-
toluene as the eluent, a yellow band was eluted, which was collected,
and evaporation of the eluate gave 4-[Ru(PPh3)2(L-NO2)(CO)Cl] as a
crystalline yellow solid. Yield: 65%.
Physical Measurements. Microanalyses (C, H, N) were performed
using a Perkin-Elmer 240C elemental analyzer. IR spectra were obtained
on a Perkin-Elmer 783 spectrometer with samples prepared as KBr
(4) (a) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1966, 28,
945. (b) Ahmad, A.; Robinson, S. D.; Uttley, M. F. J. Chem. Soc.,
Dalton Trans, 1972, 843.
(5) (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.

1128 Inorganic Chemistry, Vol. 40, No. 6, 2001
Table 2. Microanalytical Data for the Complexes
Basuli et al.
microanal. dataa (%)
compd
C
H
N
trans-[Ru(PPh3)2(L-NO2)Cl]
60.87
(60.86)
60.64
4.30
(4.26)
4.23
6.43
(6.45)
6.42
cis-[Ru(PPh3)2(L-NO2)Cl]
5-[Ru(PPh3)2(L-OMe)(CO)Cl]
5-[Ru(PPh3)2(L-Me)(CO)Cl]
5-[Ru(PPh3)2(L-H)(CO)Cl]
5-[Ru(PPh3)2(L-Cl)(CO)Cl]
5-[Ru(PPh3)2(L-NO2)(CO)Cl]
4-[Ru(PPh3)2(L-OMe)(CO)Cl]
4-[Ru(PPh3)2(L-Me)(CO)Cl]
4-[Ru(PPh3)2(L-H)(CO)Cl]
4-[Ru(PPh3)2(L-Cl)(CO)Cl]
4-[Ru(PPh3)2(L-NO2)(CO)Cl]
(60.86)
62.73
(62.69)
63.88
(63.85)
63.51
(63.49)
61.07
(61.02)
60.32
(60.30)
62.73
(62.69)
63.87
(63.85)
63.52
(4.26)
4.53
(4.54)
4.64
(4.63)
4.49
(4.47)
4.19
(4.18)
4.35
(4.32)
4.55
(4.54)
4.65
(4.63)
4.50
(6.45)
4.79
(4.77)
4.89
(4.86)
4.97
(4.94)
4.78
(4.74)
6.29
(6.25)
4.79
(4.77)
4.83
(4.86)
4.95
(63.49)
61.03
(61.02)
60.32
(4.47)
4.20
(4.18)
4.31
(4.94)
4.77
(4.74)
6.24
(60.30)
(4.32)
(6.25)
a Calculated values are given in parentheses.
Figure 1. Structure of trans-[Ru(PPh3)2(L-NO2)Cl].
X-ray data reduction, structure solution, and refinement were done using
the SHELXTL-PLUS package. The structure was solved by direct
methods.
metrical isomeric forms 6 and 7, which will be henceforth
referred to as the trans and cis isomers, respectively, with regard
to the mutual disposition of the two PPh3 ligands. To find out
Crystallography of 4-[Ru(PPh3)2(L-NO2)(CO)Cl]‚0.5CH2Cl2‚
0.75C6H6. Single crystals of 4-[Ru(PPh3)2(L-NO2)(CO)Cl] were grown
by slow diffusion of benzene into a dichloromethane solution of the
complex. Selected crystal data and data collection parameters are
given in Table 1. Data were collected on a Siemens Smart CCD
diffractometer using graphite-monochromated Mo KR radiation (λ )
0.710 73 Å) by ω scans within the angular range 1.20 < θ < 26.37°.
X-ray data reduction, structure solution, and refinement were done using
the SHELXTL-PLUS package. The structure was solved by direct
methods.
Results and Discussion
Synthesis and Characterization. Semicarbazones of five
para-substituted benzaldehydes (2) have been used in the present
study. The ligands are abbreviated in general as HL-R, where
H stands for any dissociable proton and R for the para
substituent in the benzaldehyde fragment. The reaction of all
five ligands has been first carried out with [Ru(PPh3)3Cl2] in
two different ways. However, it has been observed that only
one semicarbazone with a strong electron-withdrawing sub-
stituent (viz. HL-NO2) has afforded a characterizable product
from these reactions. This may be attributed to the enhanced
acidity of this ligand, which appears to be the primary
requirement for such reactions. When an ethanol solution of
the semicarbazone ligand (HL-NO2) is allowed to diffuse slowly
into a solution of [Ru(PPh3)3Cl2] in dichloromethane at ambient
temperature, a reddish brown crystalline product is obtained.
Elemental C,H,N analytical data (Table 2) indicate that this
complex has the [Ru(PPh3)2(L-NO2)Cl] composition. Magnetic
susceptibility measurements show that it is diamagnetic, which
corresponds to the +2 oxidation state of ruthenium (low-spin
d6, S ) 0) in this complex. As ruthenium(II) usually remains
hexacoordinated in its complexes, the semicarbazone ligand
appears to serve as a monoanionic tridentate ligand in this
complex, coordinating to ruthenium as shown in 3. Thus, the
[Ru(PPh3)2(L-NO2)Cl] complex may exist in the two geo-
the actual coordination mode of the semicarbazone ligand in
this complex and the stereochemistry of this complex as well,
its structure has been determined by X-ray crystallography. The
structure is shown in Figure 1, and selected bond parameters
are listed in Table 3. The semicarbazone ligand is indeed
coordinated to ruthenium as a tridentate C,N,O-donor, forming
two five-membered chelate rings with bite angles of 79.88(14)°
(C-Ru-N) and 75.98(12)° (N-Ru-O). Metalation of the
phenyl ring has taken place from the ortho carbon. The
semicarbazone ligand, ruthenium, and the chloride are pseudo-
planar with the coordinated chloride trans to the semicarbazone
nitrogen. The two PPh3 ligands have occupied mutually trans
positions. Therefore, this complex is the trans isomer of [Ru-
(PPh3)2(L-NO2)Cl] (6). The RuCNOP2Cl core is distorted
octahedral in nature, as reflected in the bond parameters around
ruthenium. Bond distances within the semicarbazone ligand,
particularly the C-O bond order (1.228(5) Å), together with

Ru Complexes of Benzaldehyde Semicarbazone Ligands
Inorganic Chemistry, Vol. 40, No. 6, 2001 1129
Table 3. Selected Bond Distances and Bond Angles for
trans-[Ru(PPh3)2(L-NO2)Cl], 5-[Ru(PPh3)2(L-NO2)(CO)Cl], and
4-[Ru(PPh3)2(L-NO2)(CO)Cl] Complexes
trans-[Ru(PPh3)2(L-NO2)Cl]
Bond Distances (Å)
Ru-P(1)
Ru-P(2)
Ru-Cl
Ru-O(1)
Ru-N(3)
Ru-C(4)
2.3592(11)
2.3762(11)
2.4526(10)
2.310(3)
O(1)-C(1)
N(1)-C(1)
N(2)-N(3)
N(2)-C(1)
N(3)-C(2)
C(2)-C(3)
C(3)-C(4)
1.228(5)
1.336(5)
1.391(4)
1.372(6)
1.287(5)
1.445(6)
1.425(5)
1.962(3)
2.009(4)
Bond Angles (deg)
P(1)-Ru-P(2)
Cl-Ru-N(3)
O(1)-Ru-C(4)
175.68(4)
176.26(10)
155.83(12)
Figure 2. Thermogram of trans-[Ru(PPh3)2(L-NO2)Cl].
O(1)-Ru-N(3)
N(3)-Ru-C(4)
P(1)-Ru-Cl
P(1)-Ru-O(1)
P(1)-Ru-N(3)
P(1)-Ru-C(4)
75.98(12)
79.88(14)
91.36(4)
90.46(8)
91.60(10)
89.71(11)
P(2)-Ru-Cl
P(2)-Ru-O(1)
P(2)-Ru-N(3)
P(2)-Ru-C(4)
Cl-Ru-O(1)
Cl-Ru-C(4)
86.77(4)
93.73(8)
90.42(10)
87.44(11)
101.72(7)
102.45(10)
ruthenium.7 The Ru-O length is noticeably large, indicating
the expected weak bonding between the carbonyl oxygen and
ruthenium. It may be noted here that such cyclometalation of
benzaldehyde semicarbazone appears to be unprecedented in
the literature.
5-[Ru(PPh3)2(L-NO2)(CO)Cl]
Bond Distances (Å)
In complexes of ruthenium(II) containing the Ru(PPh3)2
fragment, the PPh3 ligands usually take up mutually cis positions
for favorable dπ(Ru)-dπ(P) interactions.8 The trans-[Ru(PPh3)2-
(L-NO2)Cl] complex thus appears to be the kinetically controlled
product under the rather mild reaction conditions followed for
its synthesis. This also points to the possible existence of a
thermodynamically stable cis isomer of this complex. To test
this hypothesis, the trans-[Ru(PPh3)2(L-NO2)Cl] complex has
been subjected to thermogravimetric analysis. An exothermic
phase transition has indeed been observed for this complex at
155 °C (Figure 2). This indicates that a thermally induced
stereochemical change, which is likely to be a trans to cis
isomerization with regard to the relative disposition of the two
PPh3 ligands, must be taking place. Encouraged by this result,
the trans-[Ru(PPh3)2(L-NO2)Cl] complex was simply heated at
155 °C to afford a dark solid. This new complex is also found
to be diamagnetic, and its microanalytical data agree well with
the expected composition, viz. [Ru(PPh3)2(L-NO2)Cl]. Mass
spectra of the complex also support this formulation. The
molecular ion (M) is observed at m/z 868, and some other peaks
at m/z 833 (M - Cl), 606 (M - PPh3), and 571 (M - PPh3 -
Cl) are also observed. The infrared spectrum of this complex is
qualitatively similar to that of the trans-[Ru(PPh3)2(L-NO2)Cl]
complex. Apart from strong vibrations near 520, 695, and 745
cm-1 due to the Ru(PPh3)2 fragment,9 each complex shows a
ν(N-H) stretch near 3200 cm-1, a ν(CdO) stretch near 1650
cm-1, and a ν(Ru-Cl) stretch near 330 cm-1. Structural
characterization of this complex has not been possible because
its single crystals could not be grown, even after many attempts.
However, from the characterization data it is clear that this
complex is the cis isomer of [Ru(PPh3)2(L-NO2)Cl] (7). To
explore the possibility of obtaining the cis-[Ru(PPh3)2(L-NO2)-
Cl] complex directly from the reaction between the reactants, a
reaction of the semicarbazone (HL-NO2) with [Ru(PPh3)3Cl2]
has been carried out in refluxing ethanol. The reaction proceeds
Ru-C(1)
Ru-O(2)
Ru-N(1)
Ru-P(1)
Ru-P(2)
Ru-Cl(1)
1.879(3)
2.067(2)
2.115(2)
2.3872(8)
2.3896(8)
2.4015(9)
N(1)-C(3)
1.290(3)
1.382(3)
1.328(4)
1.361(4)
1.270(3)
1.077(3)
N1-N2
N(2)-C(2)
N(3)-C(2)
C(2)-O(2)
C(1)-O(1)
Bond Angles (deg)
C(1)-Ru-N(1)
P(1)-Ru-P(2)
O(2)-Ru-Cl(1)
170.16(10)
178.17(3)
168.76(5)
O(2)-Ru-N(1)
C(1)-Ru-O(2)
O(2)-Ru-P(1)
C(1)-Ru-P(2)
N(1)-Ru-P(2)
C(1)-Ru-Cl(1)
76.43(18)
93.73(10)
89.74(5)
89.92(8)
90.19(6)
97.50(9)
N(1)-Ru-Cl(1)
P(2)-Ru-Cl(1)
C(1)-Ru-P(1)
N(1)-Ru-P(1)
O(2)-Ru-P(2)
P(1)-Ru-Cl(1)
92.34(6)
90.52(3)
88.68(8)
91.00(6)
89.18(5)
90.83(3)
4-[Ru(PPh3)2(L-NO2)(CO)Cl]‚0.5CH2Cl2‚0.75C6H6
Bond Distances (Å)
1.811(5)
2.258(3)
2.4029(13)
2.077(4)
2.4038(13)
2.4235(12)
Ru(1)-C(1)
Ru(1)-O(2)
Ru(1)-P(1)
Ru(1)-N(2)
Ru(1)-P(2)
Ru(1)-Cl(1)
O(2)-C(2)
N(1)-C(2)
N(2)-C(2)
N(2)-N(3)
N(3)-C(3)
C(3)-C(4)
1.297(5)
1.327(6)
1.333(6)
1.358(5)
1.289(6)
1.443(7)
Bond Angles (deg)
C(1)-Ru(1)-O(2) 164.1(2)
173.27(5)
P(1)-Ru(1)-P(2)
N(2)-Ru(1)-Cl(1) 163.56(11)
C(1)-Ru(1)-N(2) 104.2(2)
O(2)-Ru(1)-Cl(1) 103.23(9)
N(2)-Ru(1)-O(2)
N(2)-Ru(1)-P(1)
C(1)-Ru(1)-P(2)
O(2)-Ru(1)-P(2)
C(1)-Ru(1)-Cl(1)
60.33(13) P(2)-Ru(1)-Cl(1)
88.23(11) C(1)-Ru(1)-P(1)
92.6(2)
83.84(9)
92.3(2)
89.53(4)
93.9(2)
89.43(9)
88.67(11)
91.77(4)
O(2)-Ru(1)-P(1)
N(2)-Ru(1)-P(2)
P(1)-Ru(1)-Cl(1)
the presence of the N-H proton clearly indicate that the
semicarbazone ligand is bound to ruthenium in the keto form
(3). While the Ru-C, Ru-P, and Ru-Cl bond distances are
quite normal,6 the Ru-N bond is a bit shorter, which points to
the strong π-interaction between the imine nitrogen and
(7) Das, A. K.; Rueda, A.; Falvello, L. R.; Peng, S. M.; Bhattacharya, S.
Inorg. Chem. 1999, 38, 4365.
(8) (a) Pramanik, A.; Bag, N.; Lahiri, G. K.; Chakravorty, A. J. Chem.
Soc., Dalton Trans. 1990, 3823. (b) Menon, M.; Bag, N.; Chakravorty,
A. J. Chem. Soc., Dalton Trans. 1995, 1417.
(6) (a) McGuiggan, M. F.; Pignolet, L. H. Inorg. Chem. 1982, 21, 2523.
(b) Fox, G. A.; Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1991,
30, 2895.
(9) (a) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1991, 30, 1511.
(b) Sinha, P. K.; Falvello, L. R.; Peng, S. M.; Bhattacharya, S.
Polyhedron, in press.

1130 Inorganic Chemistry, Vol. 40, No. 6, 2001
Basuli et al.
Figure 3. Structure of 5-[Ru(PPh3)2(L-NO2)(CO)Cl].
smoothly to afford a dark crystalline solid. Characterizations
(elemental analysis, magnetic susceptibility measurement, mass
spectrum, and IR spectrum) of this complex show that it is
identical with the cis-[Ru(PPh3)2(L-NO2)Cl] complex obtained
from the solid-state isomerization reaction. Therefore, the cis-
[Ru(PPh3)2(L-NO2)Cl] complex can be obtained directly from
the reaction of HL-NO2 with [Ru(PPh3)3Cl2] or via the formation
of the trans isomer.
is shown in Figure 3, and selected bond parameters are listed
in Table 3. The semicarbazone ligand is indeed coordinated to
ruthenium as a bidentate N,O-donor, via dissociation of the
N-H proton, forming a five-membered chelate ring as shown
in 4 with a bite angle of 76.43(18)°. The CO and chloride are
respectively trans to the nitrogen and oxygen of the semicar-
bazone. The phosphine ligands have taken up mutually trans
positions. As mentioned earlier, in complexes of ruthenium(II)
having the Ru(PPh3)2 moiety, the PPh3 ligands always prefer
to occupy mutually cis positions for better π-interaction.
However, in these complexes the presence of CO, which is a
stronger π-acid ligand, has probably forced the bulky PPh3
ligands to take up mutually trans positions for steric reasons.
The CNOP2Cl coordination sphere around ruthenium is distorted
octahedral in nature, as reflected in the bond parameters around
ruthenium. The Ru-C and C-O bond lengths in the Ru(CO)
fragment are quite normal, as observed in structurally character-
ized carbonyl complexes of ruthenium(II).11 The Ru-N, Ru-
O, Ru-P, and Ru-Cl distances are all quite normal.6,12 Bond
lengths within the five-membered chelate ring indicates that the
negative charge, formed upon dissociation of the N-H proton,
is delocalized over the O(2)-C(2)-N(2)-N(1) backbone. It is
interesting to note here that during the formation of the five-
membered chelate ring (4), the semicarbazone ligand goes
through a conformational change, relative to the free ligand
structure (2), around the CdN bond. As all the [Ru(PPh3)2(L-
R)(CO)Cl] complexes have been synthesized similarly and they
show similar properties (vide infra), the other four [Ru(PPh3)2-
(L-R)(CO)Cl] complexes are assumed to have a similar struc-
ture. This group of complexes will be henceforth referred to as
5-[Ru(PPh3)2(L-R)(CO)Cl] (where 5 stands for the size of the
chelate ring) to distinguish them from the other family of linkage
isomers of these complexes, which are discussed below.
In the above reactions with [Ru(PPh3)3Cl2], the semicarbazone
ligand (HL-NO2) has served as a tridentate C,N,O-donor and
proton loss has taken place from the phenyl ring, while the N-H
proton has remained intact. This has prompted us to look for a
different ruthenium starting material which will undergo com-
plexation by the benzaldehyde semicarbazone but cyclometa-
lation of the phenyl ring of benzaldehyde semicarbazones will
not take place and the ligands will be able to function as
bidentate N,O-donors via loss of the N-H proton. With this
strategy in mind, we planned to try [Ru(PPh3)2(CO)2Cl2] as the
new starting material because its reactivity pattern is known to
be different from that of [Ru(PPh3)3Cl2].10 When an ethanol
solution of the semicarbazone ligand has been allowed to slowly
diffuse into a solution of [Ru(PPh3)2(CO)2Cl2] in dichloro-
methane, crystalline complexes of the type [Ru(PPh3)2(L-R)-
(CO)Cl] have been obtained. It may be mentioned here that
yields of the R * NO2 complexes are significantly lower (20-
30%) than that of the R ) NO2 complex. Elemental analytical
data of the complexes (Table 2) agree well with this composi-
tion. The complexes were found to be diamagnetic, indicating
the bivalent state of ruthenium. Hence, the semicarbazone
ligands must serve as monoanionic bidentate ligands. The
structure of a representative complex, viz. [Ru(PPh3)2(L-NO2)-
(CO)Cl], has been determined by X-ray crystallography in order
to find out the coordination mode of the semicarbazone ligand
as well as the stereochemistry of theese complexes. The structure
(11) (a) Yamazaki, H.; Aoki, K. J. Organomet. Chem. 1976, 122, C54. (b)
Clark, G. R. J. Organomet. Chem. 1977, 134, 51.
(12) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1994, 33, 6038.
(10) Seddon, E. A.; Seddon, K. R. The Chemistry of Ruthenium; Elsevier:
Amsterdam, The Netherlands, 1984.

Ru Complexes of Benzaldehyde Semicarbazone Ligands
Inorganic Chemistry, Vol. 40, No. 6, 2001 1131
Figure 4. Structure of 4-[Ru(PPh3)2(L-NO2)(CO)Cl].
Formation of a five-membered chelate ring by the benzal-
dehyde semicarbazones is particularly interesting, because
similar ring formation by the corresponding thiosemicarbazones
has not yet been observed.2 This has prompted us to modify
the synthetic route so that a four-membered ring, analogous to
that formed by the benzaldehyde thiosemicarbazone ligands (1),
can be formed. When [Ru(PPh3)2(CO)2Cl2] is reacted with the
semicarbazone ligands in refluxing ethanol, complexes of
composition identical to that above, viz. [Ru(PPh3)2(L-R)(CO)-
Cl], are obtained. However, properties displayed by these
complexes (vide infra) are found to be different from those
displayed by the 5-[Ru(PPh3)2(L-R)(CO)Cl] complexes. The
composition of these complexes has been established by their
microanalytical data (Table 2). These complexes are also
diamagnetic, which corresponds to the +2 state of ruthenium
in these complexes. Besides a few minor differences, the infrared
spectra of these complexes are qualitatively similar to those of
their respective 5-[Ru(PPh3)2(L-R)(CO)Cl] analogues. All these
preliminary characterization data indicate that these complexes
are isomers of the previous group of 5-[Ru(PPh3)2(L-R)(CO)-
Cl] complexes. To find out the coordination mode of the
semicarbazones in these complexes, the structure of [Ru(PPh3)2-
(L-NO2)(CO)Cl] has been determined by X-ray crystallography.
This particular complex was chosen for structural characteriza-
tion because of the proper comparison of structural data with
the previous structure. The structure is shown in Figure 4, and
selected bond distances and angles are presented in Table 3.
The semicarbazone ligand is coordinated to ruthenium as a
bidentate N,O-donor, forming a four-membered chelate ring (as
shown in 5) with a bite angle of 60.33(13)°. The two PPh3
ligands are again trans. The semicarbazone, ruthenium, CO, and
chloride constitute the equatorial plane as before. However, in
this complex the CO is trans to the semicarbazone oxygen and
the Cl is trans to the semicarbazone nitrogen. While the Ru-P
and Ru-Cl distances are not very different from those in the
previous structure, the Ru-C, Ru-N, and Ru-O distances are
clearly different. In particular, the Ru-O bond is noticeably
longer. The difference in bond lengths within the two chelate
rings is attributable to the difference in ring sizes. As all the
[Ru(PPh3)2(L-R)(CO)Cl] complexes belonging to this family
have been prepared similarly and they display similar properties
(vide infra), the other four complexes are assumed to have a
structure similar to that of [Ru(PPh3)2(L-NO2)(CO)Cl], and this
second family of [Ru(PPh3)2(L-NO2)(CO)Cl] complexes will
henceforth be referred to as 4-[Ru(PPh3)2(L-R)(CO)Cl] to
indicate the difference in chelate ring size of the coordinated
semicarbazone.
In view of the structure of uncoordinated benzaldehyde
semicarbazones (2),13 the four-membered chelate ring formation
(5) appears quite usual because it does not involve any
conformational change in the ligand frame, while the five-
membered chelate ring formation (4) seems to be less favorable,
as it involves restricted rotation around a CdN bond. However,
the reaction conditions used for the synthesis of the 5-[Ru-
(PPh3)2(L-R)(CO)Cl] complexes are rather mild compared to
those used for the preparation of the 4-[Ru(PPh3)2(L-R)(CO)-
Cl] complexes. The observed ease of formation of the thermo-
dynamically less favorable five-membered chelate ring is
probably due to kinetic reasons.
Solution Properties. The trans- and cis-[Ru(PPh3)2(L-NO2)-
Cl] complexes have been observed to undergo chemical
transformation in solution, characterized by a gradual change
in solution color, and the exact nature of these transformations
is not yet clear. This has vitiated their study in solution. The
5-[Ru(PPh3)2(L-R)(CO)Cl] and 4-[Ru(PPh3)2(L-R)(CO)Cl] com-
plexes are stable in solution. 1H NMR spectra of these
complexes, recorded in CDCl3 solution, show all the expected
signals. For example, in 5-[Ru(PPh3)2(L-NO2)(CO)Cl] four
signals are expected from the coordinated semicarbazone ligand
(one singlet (2H) for the NH2, one singlet (1H) for the
(13) Naik, D. V.; Palenik, G. J. Acta Crystallogr., Sect. B 1974, 30, 2396.

1132 Inorganic Chemistry, Vol. 40, No. 6, 2001
Basuli et al.
Table 4. Electronic Spectral and Cyclic Voltammetric Data for the 5-[Ru(PPh3)2(L-R)(CO)Cl] and 4-[[Ru(PPh3)2(L-R)(CO)Cl] Complexes
electronic dataa
cyclic voltammetric datab
compd
λ
max, nm (ꢀ, M-1 cm-1
)
E1/2, V (∆Ep, mV)
5-[Ru(PPh3)2(L-OMe)(CO)Cl]
5-[Ru(PPh3)2(L-Me)(CO)Cl]
5-[Ru(PPh3)2(L-H)(CO)Cl]
5-[Ru(PPh3)2(L-Cl)(CO)Cl]
5-[Ru(PPh3)2(L-NO2)(CO)Cl]
4-[Ru(PPh3)2(L-OMe)(CO)Cl]
4-[Ru(PPh3)2(L-Me)(CO)Cl]
4-[Ru(PPh3)2(L-H)(CO)Cl]
4-[Ru(PPh3)2(L-Cl)(CO)Cl]
4-[Ru(PPh3)2(L-NO2)(CO)Cl]
380 (2400), 308c (7900), 272c (10 700), 234 (20 300)
380 (1500), 304c (7200), 270c (10 000), 224 (22 600)
382 (13 100), 306c (13 200), 270c (15 100), 236 (29 100)
378 (7900), 304c (15 200), 272c (19 000), 234 (39 800)
411 (6600), 266c (20 500), 229 (32 500)
0.82 (80)
0.85 (70)
0.89 (60)
0.92 (80)
1.07 (60)
0.66 (70)
0.70 (80)
0.73 (60)
0.75 (70)
0.86 (60)
346 (11 400), 271 (23 500),232 (36 100)
352c (9100), 341c (9000), 265c (17 100), 212 (52 200)
352c (10 200), 269c (24 100), 232 (46 600)
361 (8600), 271c (25 100), 229 (50 500)
448 (11 900), 280c (25 200), 229 (53 700)
a In dichloromethane solution. b Conditions and definitions: solvent, acetonitrile; supporting electrolyte, TBAP; working electrode, platinum;
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.
Scheme 1

Ru Complexes of Benzaldehyde Semicarbazone Ligands
Inorganic Chemistry, Vol. 40, No. 6, 2001 1133
azomethine proton, and two doublets (2H each) for the two
phenyl protons), and all of them are observed at 3.71, 6.78,
7.81, and 8.08 ppm, respectively. Similarly, in 4-[Ru(PPh3)2-
(L-NO2)(CO)Cl] the same four signals are again expected from
the coordinated semicarbazone ligand, which are clearly ob-
served at 4.58, 6.34, 7.11, and 8.05 ppm, respectively. The
triphenylphosphine protons are observed as overlapping signals
at 7.15-7.60 ppm in 5-[Ru(PPh3)2(L-R)(CO)Cl] and at 7.20-
7.85 ppm in 4-[Ru(PPh3)2(L-R)(CO)Cl]. The 5-[Ru(PPh3)2-
(L-R)(CO)Cl] and 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes are
soluble in common organic solvents such as dichloromethane,
chloroform, acetonitrile, etc., producing yellow solutions, except
for the R ) NO2 complexes. The solution of the 5-[Ru(PPh3)2-
(L-NO2)(CO)Cl] complex is reddish orange, and that of the
4-[Ru(PPh3)2(L-NO2)(CO)Cl] complex is orange. Electronic
spectra of both the 5-[Ru(PPh3)2(L-R)(CO)Cl] and 4-[Ru(PPh3)2-
(L-R)(CO)Cl] complexes have been recorded in dichloro-
methane solution. All the complexes show intense absorptions
in the ultraviolet and visible regions (Table 4). The absorptions
in the ultraviolet region are assignable to transitions within the
ligand orbitals. The absorptions in the visible region are probably
due to metal-to-ligand charge-transfer transitions.
The electrochemical properties of the 5-[Ru(PPh3)2(L-R)(CO)-
Cl] and 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes were studied in
acetonitrile solution (0.1 M TEAP) by cyclic voltammetry. Each
complex shows one metal-centered oxidative response on the
positive side of the SCE (Table 4). The oxidative response,
observed in the range of 0.66-1.07 V (all potentials are
referenced to SCE), is assigned to the ruthenium(II)-ruthenium-
(III) oxidation. This oxidation is reversible, with a peak-to-peak
separation of 60-80 mV, and the anodic peak current (ipa) is
almost equal to the cathodic peak current (ipc). The one-electron
nature of this oxidation has been established by comparing its
current height with that of the standard ferrocene/ferrocenium
couple under identical experimental conditions. Potentials of
the ruthenium(II)-ruthenium(III) oxidation in both the 5-[Ru-
(PPh3)2(L-R)(CO)Cl] and 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes
are found to be sensitive to the nature of the substituent R in
the semicarbazone ligands. The potentials increase with increas-
ing electron-withdrawing character of R, and the plots of the
oxidation potentials vs Hammett substituent constants (σ) of
R14 (σ values of the substituents: OMe, -0.27; Me, -0.17; H,
0.00; Cl, 0.23; NO2, 0.78) are linear. The slopes of these lines,
which are known as the reaction constants (F)15 and are a
measure of the sensitivity of the oxidation potentials to R, are
respectively 0.23 for the 5-[Ru(PPh3)2(L-R)(CO)Cl] complexes
and 0.18 for the 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes. It is
interesting to note here that the substituent has a greater
influence on the oxidation potential in the 5-[Ru(PPh3)2(L-R)-
(CO)Cl] complexes, where it is seven bonds away from
ruthenium, than in the 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes,
where it is eight bonds away from ruthenium. It is also
interesting to note that a single substituent which is seven or
eight bonds away from the electroactive metal center can still
influence the metal-centered redox potentials in a predictable
manner.
Conclusions
This present study shows that the coordination mode of
benzaldehyde semicarbazone and the stereochemistry of its
complexes are noticeably sensitive toward several experimental
parameters. This has been reflected in the reaction of the
benzaldehyde semicarbazones with [Ru(PPh3)3Cl2] and [Ru-
(PPh3)2(CO)2Cl2] under different experimental conditions, which
is summarized in Scheme 1.
Acknowledgment. Financial assistance received from the
Department of Science and Technology, New Delhi, India
(Grant No. SP/S1/F33/98), is gratefully acknowledged. We thank
the Third World Academy of Sciences for financial support for
the purchase of an electrochemical cell system. F.B. thanks the
University Grants Commission, New Delhi, India, for her
fellowship.
Supporting Information Available: Figures giving the mass spec-
trum of cis-[Ru(PPh3)2(L-NO2)Cl], electronic spectra of 5-[Ru(PPh3)2-
(L-Me)(CO)Cl] and 4-[Ru(PPh3)2(L-Me)(CO)Cl], cyclic voltammo-
grams of 5-[Ru(PPh3)2(L-NO2)(CO)Cl] and 4-[Ru(PPh3)2(L-NO2)(CO)Cl],
and a least-squares plot of Ru(II)-Ru(III) potentials vs σ for 5-[Ru-
(PPh3)2(L-R)(CO)Cl] and 4-[Ru(PPh3)2(L-R)(CO)Cl] and tables con-
taining crystal data and details of structure determination, atomic
coordinates, anisotropic thermal parameters, and bond distances and
angles for trans-[Ru(PPh3)2(L-NO2)Cl], 5-[Ru(PPh3)2(L-NO2)(CO)Cl],
and 4-[Ru(PPh3)2(L-NO2)(CO)Cl]‚0.5CH2Cl2‚0.75H2O. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC000915E
(14) Hammett, L. P. Physical Organic Chemistry, 2nd ed.; McGraw-Hill:
New York, 1970.
(15) Mukherjee, R. N.; Rajan, O. A.; Chakravorty, A. Inorg. Chem. 1982,
21, 785.