Synthesis, structure and electrochemical properties of a family of organoruthenium complexes

Article abstract of DOI:10.1016/j.poly.2007.04.020

Reaction of N-(2′-hydroxyphenyl)benzaldimines (abbreviated in general as H₂L-R, where R stands for the para-substituent in the benzaldehyde fragment and H stands for the dissociable hydrogen atoms) with [Ru(PPh₃)₂(CO)₂Cl₂] affords a family of organoruthenium complexes of the type [Ru(PPh₃)₂(CO)(L-R)] where the N-(2′-hydroxyphenyl)benzaldimine ligand is coordinated to the metal center as tridentate C,N,O-donor. Structure of a representative complex has been determined by X-ray crystallography. All the [Ru(PPh₃)₂(CO)(L-R)] complexes are diamagnetic, and show characteristic ¹H NMR signals and moderately intense MLCT transitions in the visible region. Cyclic voltammetry of the [Ru(PPh₃)₂(CO)(L-R)] complexes shows a reversible Ru(II)-Ru(III) oxidation within 0.38-0.68 V versus SCE, followed by an irreversible oxidation of the coordinated benzaldimine ligand within 1.09-1.27 V versus SCE. An irreversible reduction of the coordinated benzaldimine ligand is also observed near -1.1 V versus SCE. Potential of the Ru(II)-Ru(III) oxidation is observed to be sensitive to the nature of para-substituent R.

Full text of DOI:10.1016/j.poly.2007.04.020

Polyhedron 26 (2007) 3876–3884
www.elsevier.com/locate/poly
Synthesis, structure and electrochemical properties of a family
of organoruthenium complexes
a
a
Chhandasi GuhaRoy , Sakya Singha Sen a,1, Swati Dutta ,
b
a,
*
Golam Mostafa , Samaresh Bhattacharya
a
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b
Department of Physics, Jadavpur University, Kolkata 700 032, India
Received 7 March 2007; accepted 14 April 2007
Available online 30 April 2007
Abstract
Reaction of N-(20-hydroxyphenyl)benzaldimines (abbreviated in general as H2L-R, where R stands for the para-substituent in the
benzaldehyde fragment and H stands for the dissociable hydrogen atoms) with [Ru(PPh3)2(CO)2Cl2] affords a family of organoruthenium
complexes of the type [Ru(PPh3)2(CO)(L-R)] where the N-(20-hydroxyphenyl)benzaldimine ligand is coordinated to the metal center as
tridentate C,N,O-donor. Structure of a representative complex has been determined by X-ray crystallography. All the [Ru(PPh3)2-
(CO)(L-R)] complexes are diamagnetic, and show characteristic 1H NMR signals and moderately intense MLCT transitions in the visible
region. Cyclic voltammetry of the [Ru(PPh3)2(CO)(L-R)] complexes shows a reversible Ru(II)–Ru(III) oxidation within 0.38–0.68 V ver-
sus SCE, followed by an irreversible oxidation of the coordinated benzaldimine ligand within 1.09–1.27 V versus SCE. An irreversible
reduction of the coordinated benzaldimine ligand is also observed near ꢀ1.1 V versus SCE. Potential of the Ru(II)–Ru(III) oxidation is
observed to be sensitive to the nature of para-substituent R.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: C–H activation; Schiff bases; Organoruthenium complexes; Structure; Electrochemical properties
1. Introduction
of the phenolic proton, forming five-membered chelate ring
(2) [3]. Closeness of the pendent phenyl ring to the metal
center in 2 indicates the possibility of its orthometallation
via C–H activation (3), which has been observed to take
place under favorable reaction conditions [4]. The main
objective of the present study was to explore the possibility
of C–H activation in ligand 1 using ruthenium as the medi-
ator. It may be mentioned in this context that transition
metal mediated C–H activation of organic compounds
has been an active arena of research [5], because such reac-
tion often leads to formation of interesting organometallic
complexes. The use of organometallic complexes are
manifold, right from their application in bio-systems [6]
to serving as reactive intermediates in many organic trans-
formations [7]. In order to bring about the targeted C–H
activation of ligand 1, [Ru(PPh3)2(CO)2Cl2] has been
selected as the starting material. This particular compound
has been chosen because of its demonstrated ability to
There has been considerable current interest in the
chemistry of ruthenium complexes, largely due to their fas-
cinating redox, photophysical and photochemical proper-
ties [1]. As all these properties are directed primarily by
the coordination environment around the central metal
ion, complexation of ruthenium by ligands of selected types
is of significant importance, and the present work has orig-
inated from our interest in this area [2]. Herein we have
selected a group of N-(20-hydroxyphenyl)benzaldimines as
the principal ligand (1). Such ligands are known to bind
to metal ions as bidentate N,O-donors, via dissociation
*
Corresponding author. Tel.: +91 33 2414 6223; fax: +91 33 2414 6584.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
Summer project fellow from the Indian Institute of Technology,
1
Kharagpur.
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.04.020

C. GuhaRoy et al. / Polyhedron 26 (2007) 3876–3884
3877
1
accommodate tridentate ligands via displacement of three
pre-coordinated ligands and, more importantly, its proven
efficiency to bring about C–H activation of organic mole-
cules [2b,2c,2i]. Reaction of the N-(20-hydroxyphenyl)benz-
aldimines (1) with [Ru(PPh3)2(CO)2Cl2] has indeed been
observed to afford a family of organoruthenium complexes
containing chelate 3 (M = Ru). An account of the chemis-
try of these organoruthenium complexes is presented here,
with special reference to their formation, structure and,
spectral and electrochemical properties.
Found: C, 69.79; H, 4.64; N, 1.57%. H NMR in CDCl3,
d ppm2: 3.48 (OCH3); 5.72 (t, 1H, J = 7.1); 5.90 (d, H,
J = 6.0); 6.07 (d, 2H)*; 6.37 (s, 1H); 6.46 (t, 1H, J = 7.2);
6.73 (d, 1H, J = 8.2); 7.15–7.49 (2PPh3). IR: 1919 cmꢀ1
(mCO).
[Ru(PPh3)2(CO)(L-CH3)]: Yield: 80%. Anal. Calc. C,
70.99; H, 4.75; N, 1.62. Found: C, 71.01; H, 4.72; N, 1.59%.
1H NMR in CDCl3, d ppm: 1.89 (CH3); 5.70 (t, 1H,
J = 7.1); 5.90 (d, 1H, J = 7.9); 6.08 (d, 1H, J = 8.2); 6.29
(d, 1H, J = 7.4); 6.46 (t, 1H, J = 7.6); 6.69 (2H)*; 7.15–
7.61 (2PPh3). IR: 1920 cmꢀ1 (mCO).
[Ru(PPh3)2(CO)(L-H)]: Yield: 75%. Anal. Calc. C,
70.75; H, 4.59; N, 1.65. Found: C, 70.78; H, 4.57; N,
1.64%. H NMR in CDCl3, d ppm: 5.30 (s, 1H); 5.70 (t,
R = OCH3, CH3, H,
OH
O
O
Cl, NO2
1
M
M
N
C
N
C
N
C
1H, J = 7.3); 5.91 (d, 1H, J = 7.9); 6.03 (d, 1H, J = 8.3);
6.44 (t, 2H); 6.74 (d , 1H, J = 7.2); 7.03 (d, 1H, J = 7.4);
7.13–7.70 (2PPh3). IR: 1917 cmꢀ1 (mCO).
H
H
H
R
R
R
[Ru(PPh3)2(CO)(L-Cl)]: Yield: 74%. Anal. Calc. C,
67.98; H, 4.30; N, 1.58. Found: C, 68.02; H, 4.28; N,
1.55%. H NMR in CDCl3, d ppm: 5.71 (t, 1H, J = 7.4);
1
2
3
1
5.91 (d, 1H, J = 7.8); 6.05 (d, 1H, J = 8.3); 6.46 (t, 1H,
J = 9.4); 6.65 (d, 1H, J = 7.9); 6.83 (s, 1H); 7.17–7.66
(2PPh3). IR: 1921 cmꢀ1 (mCO).
2. Experimental
2.1. Materials
[Ru(PPh3)2(CO)(L-NO2)]: Yield: 78%. Anal. Calc. C,
67.19; H, 4.66; N,1.59. Found: C, 67.21; H, 4.64; N, 1.57%.
1H NMR in CDCl3, d ppm: 5.71 (d, 2H)*; 5.89 (d, 1H,
J = 8.17); 6.11 (t, 1H, J = 10.12); 6.28 (s, 1H); 6.35 (d,
1H, J = 7.57); 6.79 (t, 1H, J = 9.99); 6.91 (s, 1H); 7.16–
7.37 (2PPh3). IR: 1920 cmꢀ1 (mCO).
Commercial ruthenium trichloride was purchased from
Arora Matthey, Kolkata, India. Triphenylphosphine, 2-
aminophenol and the para-substituted benzaldehydes were
obtained from Merck, India. All other chemicals and sol-
vents were reagent grade commercial materials and were
used as received. [Ru(PPh3)2(CO)2Cl2] was prepared follow-
ing a reported procedure [8]. The N-(20-hydroxyphenyl)-
benzaldimines (1) were prepared by reacting equimolar
amounts of respective para-substituted benzaldehyde and
2-aminophenol in ethanol. Purification of acetonitrile and
dichloromethane, and preparation of tetrabutylammo-
nium perchlorate (TBAP) for electrochemical work were
performed as reported in the literature [9].
2.3. Physical measurements
Microanalyses (C, H, N) were performed using a Heraeus
Carlo Erba 1108 elemental analyzer. 1H NMR spectra were
recorded in CDCl3 solution on a Bruker Avance DPX 300
NMR spectrometer using TMS as the internal standard.
IR spectra were obtained on a Shimadzu FTIR-8300 spec-
trometer with samples prepared as KBr pellets. Electronic
spectra were recorded on a JASCO V-570 spectrophotome-
ter. Electrochemical measurements were made using a CH
Instruments model 600A electrochemical analyzer. A plati-
num disc working electrode, a platinum wire auxiliary elec-
trode and an aqueous saturated calomel reference electrode
(SCE) were used in the cyclic voltammetry experiments. All
electrochemical experiments were performed under a dini-
trogen atmosphere. All electrochemical data were collected
at 298 K and are uncorrected for junction potentials.
2.2. Synthesis
The [Ru(PPh3)2(CO)(L-R)] complexes were synthesized
by following a general procedure. Specific details are given
below for a particular complex.
[Ru(PPh3)2(CO)(L-OCH3)]: N-(20-hydroxyphenyl)-
4-methoxybenzaldimine (30 mg, 0.13 mmol) was dissolved
in 2-methoxyethanol (40 mL) and triethylamine (30 mg,
0.30 mmol) was added to it followed by [Ru(PPh3)2-
(CO)2Cl2] (100 mg, 0.13 mmol). The resulting mixture
was then refluxed for 24 h, whereby a violet solution was
obtained. Evaporation of the solution gave a violet solid,
which was subjected to purification by thin layer chroma-
tography on a silica plate. Using benzene as the eluant a
violet band separated, which was extracted with acetoni-
trile. On evaporation of the acetonitrile extract
[Ru(PPh3)2(CO)(L-OCH3)] was obtained as crystalline
solid. Yield: 70%. Anal. Calc. C, 69.70; H, 4.66; N, 1.59.
2.4. X-ray crystallography
Single crystals of [Ru(PPh3)2(CO)(L-H)] were obtained
by slow evaporation of an acetonitrile solution of the com-
plex. Selected crystal data and data collection parameters
2
Chemical shifts are given in ppm and multiplicity of the signals along
with the associated coupling constants (J in Hz) are given in parentheses.
Overlapping signals are marked with an asterisk.

3878
C. GuhaRoy et al. / Polyhedron 26 (2007) 3876–3884
are given in Table S1. Data were collected on a Bruker
SMART APEX CCD area detector system using graphite
monochromated Mo Ka radiation (k = 0.71073 A). X-ray
of the para-substituents (R), in order to observe their influ-
ence, if any, on the redox properties of the complexes.
These ligands are in general abbreviated as H2L-R, where
H2 stands for the two protons, the phenolic proton and
one ortho proton of the phenyl ring in the benzaldehyde
fragment, which are expected to undergo dissociation upon
formation of chelate 3 and R for the para-substituent.
Reactions of the N-(20-hydroxyphenyl)benzaldimines (1)
with [Ru(PPh3)2(CO)2Cl2] proceed smoothly in refluxing
2-methoxyethanol in the presence of triethylamine, afford-
ing a family of violet complexes of type [Ru(PPh3)2(CO)
(L-R)] in good yields. Microanalytical (C, H, N) data of
˚
data reduction, structure solution and refinement were
done using SHELXS-97 and SHELXL-97 programs [10]. The
structure was solved by the direct methods. The absolute
configuration of the [Ru(PPh3)2(CO)(L-H)] complex was
determined by Flack parameter 0.12(4). Expected values
of Flack parameter are 0 for correct and +1 for inverted
structure. Using the option TWIN/BASF, the refined
BASF is found to be identical with the Flack parameter,
which confirmed the correct enantiomer.
Crystal data for C50H39NO2P2Ru, M = 848.83, 0.08 ·
0.11 · 0.18 mm3, monoclinic, space group Cc, a = 23.531
˚
˚
˚
Table 1
(4) A, b = 8.988(2) A, c = 20.553(4) A, b = 115.15(2)ꢁ,
˚
V = 3934.8(15) A , Z = 4, Dcalc = 1.433 Mg mꢀ3, F(000)
3
Selected bond distances (A) and angles (ꢁ) for complex [Ru(PPh3)2(CO)-
(L-H)]
˚
= 1744, k = 0.71073 A, T = 293 K, l = 0.523 mmꢀ1
,
˚
˚
Bond distances (A)
19776 reflections collected, 7677 unique (Rint = 0.019). Final
goodness-of-fit = 1.07, R1 = 0.0543, wR2 = 0.1258, R indi-
ces based on 7024 reflections with I > 2r(I).
Ru(1)–C(52)
Ru(1)–N(1)
Ru(1)–O(2)
Ru(1)–P(1)
Ru(1)–P(2)
Ru(1)–C(39)
2.089(8)
1.992(5)
2.107(6)
2.401(2)
2.354(3)
1.860(6)
C(46)–N(1)
C(40)–N(1)
C(45)–O(2)
C(39)–O(1)
1.336(9)
1.470(10)
1.343(9)
1.057(8)
3. Results and discussion
Bond angles (ꢁ)
C(52)–Ru(1)–O(2)
P(1)–Ru(1)–P(2)
N(1)–Ru(1)–C(39)
3.1. Synthesis and crystal structure
154.7(2)
177.76(7)
172.5(3)
C(52)–Ru(1)–N(1)
N(1)–Ru(1)–O(2)
Ru(1)–C(39)–O(1)
77.2(3)
77.5(2)
168.0(8)
Five N-(20-hydroxyphenyl)benzaldimines (1) have been
used in the present study, differing in the inductive effect
Fig. 1. Structure of the [Ru(PPh3)2(CO)(L-H)] complex. The thermal ellipsoids of several carbon atoms are not of proper shape, probably due to
overestimated empirical absorption correction.

C. GuhaRoy et al. / Polyhedron 26 (2007) 3876–3884
3879
Fig. 2. The hydrogen bonding interactions in the crystal lattice of [Ru(PPh3)2(CO)(L-H)].
the complexes are in well agreement with their composi-
tions. The complexes are diamagnetic, which corresponds
to the bivalent state of ruthenium (low-spin d6, S = 0).
These preliminary characterization data of the complexes
indicate that the benzaldimines are probably serving as
dianionic tridentate ligands in them. In order to authenti-
cate the coordination mode of the benzaldimines in these
complexes, as well as to find out their stereochemistry,
structure of a representative member of this family, viz.
[Ru(PPh3)2(CO)(L-H)], has been determined by X-ray crys-
tallography. The structure is shown in Fig. 1, and selected
bond parameters are listed in Table 1. The structure shows
that the N-(20-hydroxyphenyl)benzaldimine is coordinated
to ruthenium in the tridentate C,N,O-fashion (3,
M = Ru), via loss of the phenolic proton and another pro-
ton from one ortho position of the phenyl ring in the benz-
aldehyde fragment. Thus two adjacent five-membered rings
are formed with O–Ru–N and N–Ru–C bite angles of 77.5ꢁ
and 77.2ꢁ, respectively. Two triphenylphosphines and a
carbonyl are also coordinated to the metal center. Ruthe-
nium is therefore sitting in a C2NOP2 coordination envi-
ronment, which is distorted octahedral in nature as
reflected in all the bond parameters around ruthenium.
The coordinated benzaldimine ligand and the carbonyl
have constituted one equatorial plane with the metal at
the center, while the PPh3 ligands have occupied the
remaining axial positions and hence they are mutually
trans. The Ru–P and Ru–C (carbonyl) and C–O (carbonyl)
distances are all quite normal [2b,2c,2i]. Within the Ru-
(L-H) fragment, the Ru–C, Ru–N and Ru–O distances are
also quite usual [2b,2c,2i,11] and so are the C–O and C–
N distances [4]. However, the Ru–C–O angle (168.0ꢁ) in
the Ru-carbonyl fragment is found to deviate significantly
from linearity. The absence of any solvent of crystallization
in the crystal lattice of [Ru(PPh3)2(CO)(L-H)] indicates
possible existence of non-covalent interaction(s) between
the individual complex molecules. A closer look at the
packing pattern of the crystal reveals that non-covalent
interactions of two types, viz. C–Hꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁp inter-
actions, are active in the lattice, which are illustrated in
Fig. 2. The carbonyl oxygen in each complex molecule is
hydrogen-bonded simultaneously to a phenyl C–H and
the azomethine C–H of the benzaldimine ligand belonging
to an adjacent complex molecule. Several phenyl C–H frag-
ments of the triphenyl phosphines are involved in hydro-
gen-bonding with the p-cloud over the phenyl rings of
the triphenylphosphines of neighboring complex molecules.
Each complex molecule is thus linked with the surrounding
complex molecules through C–Hꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁp interac-
tions, and this extended intermolecular hydrogen-bonding
interactions seem to be responsible for holding the crystal
together. As all the [Ru(PPh3)2(CO)(L-R)] complexes have
been synthesized similarly and they show similar properties
(vide infra), the other four [Ru(PPh3)2(CO)(L-R)] (R ¼ H)
complexes are assumed to have similar structures as the
[Ru(PPh3)2(CO)(L-H)] complex.
Although the exact mechanism behind formation of
the [Ru(PPh3)2(CO)(L-R)] complexes is not completely
clear to us, the sequences shown in Scheme 1 seem prob-
able. In the first step the benzaldimine ligand binds to
ruthenium, via loss of the phenolic proton, as a bidentate
N,O-donor with simultaneous dissociation of a carbon
monoxide and a chloride ion from [Ru(PPh3)2(CO)2Cl2],
thereby producing a reactive intermediate (A). In A the
coordinated chloride and the pendent phenyl ring of
the benzaldimine are appropriately disposed for the C–
H activation to occur at the ortho position of the phenyl
ring and hence it is believed to undergo the expected
cyclometallation via elimination of HCl to produce the
final organoruthenium complex, [Ru(PPh3)2(CO)(L-R)].
Presence of triethylamine seems to have facilitated initial
dissociation of the phenolic proton, as well as elimina-
tion of HCl during orthometallation.3 Attempts to isolate
the intermediate A have failed, probably because it
3
From the same reactions the complexes are obtained in very poor yield
in the absence of triethylamine.

3880
C. GuhaRoy et al. / Polyhedron 26 (2007) 3876–3884
1H NMR spectra of the [Ru(PPh3)2(CO)(L-R)] com-
OH
plexes, recorded in CDCl3 solution, show broad signals
for the coordinated triphenylphosphines within 7.1–
7.6 ppm. The methyl signals of the benzaldimine ligand
in the [Ru(PPh3)2(CO)(L-CH3)] and [Ru(PPh3)2(CO)-
(L-OCH3)] complexes are observed, respectively, at 1.89
and 3.48 ppm. However, signal for the azomethine proton
could not be identified probably due to its overlap with the
triphenylphosphine signals. All other signals expected for
the coordinated benzaldimine ligand are observed clearly
in all the complexes. Thus the infrared and 1H NMR spec-
tral data of the [Ru(PPh3)2(CO)(L-R)] complexes are found
to be consistent with their compositions.
N
C
H
R
1
[RuII(PPh3)2(CO)2Cl2]
-CO, -HCl
x
PPh3
PPh3
O
CO
Cl
O
Cl
II
Ru
II
Ru
CO
N
C
N
C
PPh3
PPh3
All the [Ru(PPh3)2(CO)(L-R)] complexes are readily
soluble in dichloromethane, chloroform, acetone, etc.,
and sparingly soluble in methanol, ethanol, acetonitrile,
etc., producing violet solutions. Electronic spectra of the
complexes have been recorded in dichloromethane solu-
tion. Spectral data are presented in Table 2. Each com-
plex shows several intense absorptions in the visible and
ultraviolet region. The absorptions in the ultraviolet
region are believed to be due to transitions within the
ligand orbitals. To have an insight into the nature of
absorptions in the visible region, semi-empirical EHMO
calculations have been performed on computer-generated
models of all the complexes [12], where phenyl rings of
the triphenylphosphines have been replaced by hydrogens.
The results are found to be qualitatively similar for all the
complexes.4 Compositions of some selected molecular
orbitals are given in Table 3 and partial MO diagram
of a representative complex is shown in Fig. 3. The calcu-
lations show that the highest occupied molecular orbital
(HOMO) and the next two filled molecular orbitals, viz.
HOMO ꢀ 1 and HOMO ꢀ 2, have major contribution
(>47%) from the metal d-orbitals and hence they may
be considered as ruthenium t2 orbitals. The lowest unoc-
cupied molecular orbital (LUMO) is localized almost
entirely on the benzaldimine ligand and is concentrated
mostly (>50%) on the imine fragment. The next two
vacant molecular orbitals, viz. LUMO + 1 and
LUMO + 2, are localized on different parts of the coordi-
nated benzlaldimine ligand. Hence the lowest energy
absorption in the visible region may be assigned to an
electronic transition from the filled ruthenium t2 orbital
(HOMO) to the vacant p*(imine) orbital of the benzaldi-
mine ligand (LUMO). The other absorptions in the visible
region are attributable to transitions occurring from the
ruthenium t2 orbitals to the higher energy vacant orbitals.
H
H
R
R
A
B
-HCl
PPh3
O
N
CO
II
Ru
PPh3
C
R
H
Scheme 1. Probable steps of the formation of the [Ru(PPh3)2(CO)(L-R)]
complexes.
undergoes rapid cyclometallation to afford the
[Ru(PPh3)2(CO)(L-R)] complex. Formation of another
stereoisomer of the intermediate (B), keeping the trans
disposition of the two triphenylphosphines unaltered
and differing only in the relative disposition of carbonyl
and chloride, is, in principle, possible. This stereoisomer
B is unlikely to undergo a similar cyclometallation due
to unfavorable disposition of the chloride. However, this
isomer B has never been obtained from the synthetic
reactions, which suggests that isomer A is the only
intermediate formed under the prevailing reaction
conditions.
3.2. Spectral properties
Infrared spectra of the [Ru(PPh3)2(CO)(L-R)] com-
plexes show many bands of varying intensities within
4000–400 cmꢀ1. Assignment of each individual band to a
specific vibration has not been attempted. However, three
strong bands observed around 518, 695 and 743 cmꢀ1 in
all the complexes are attributable to the coordinated tri-
phenylphosphines, and a very intense band observed near
1920 cmꢀ1 to the coordinated carbonyl. Comparison with
the spectrum of [Ru(PPh3)2(CO)2Cl2] shows presence of
some new bands (viz. 1090, 1314, 1434, 1462, 1566,
1588 cmꢀ1) in the spectra of the [Ru(PPh3)2(CO)(L-R)]
complexes, which are probably due to the coordinated
benzaldimine ligand.
3.3. Electrochemical properties
Electrochemical properties of the [Ru(PPh3)2(CO)(L-R)]
complexes have been studied by cyclic voltammetry in 1:9
4
In the [Ru(PPh3)2(CO)(L-NO2)] complex, the LUMO has significant
contribution (57%) from the NO2 group.

C. GuhaRoy et al. / Polyhedron 26 (2007) 3876–3884
3881
Table 2
Electronic spectral and cyclic voltammetric data
Compound
Electronic spectral dataa
k
max, nm (e, Mꢀ1 cmꢀ1
)
Cyclic voltammetric datab
[Ru(PPh3)2(CO)(L-OCH3)]
[Ru(PPh3)2(CO)(L-CH3)]
[Ru(PPh3)2(CO)(L-H)]
[Ru(PPh3)2(CO)(L-Cl)]
[Ru(PPh3)2(CO)(L-NO2)]
269c(11000), 308c(7000), 380(2500), 551(2000)
267c(22000), 307c(13000), 385(3800), 564(3000)
262c(24000), 302c(14000), 386(6000), 563(4000)
263c(23000), 307c(13000), 388(5000), 567(4000)
0.38d(70)e, 1.09f
0.41d(80)e, 1.17f
0.45d(80)e, 1.27f
0.51d(70)e, 1.27f
0.68d(80)e
ꢀ1.09g
ꢀ1.10g
ꢀ1.07g
ꢀ1.10g
ꢀ1.13g
264c(47000), 323c(24000), 379c(17000), 489(8000), 577(5000)
a
In dichloromethane solution.
b
Solvent, 1:9 dichloromethane–acetonitrile; supporting electrolyte, TBAP; reference electrode, SCE.
Shoulder.
c
d
e
f
E1/2 = 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials, scan rate, 50 mV sꢀ1
.
DEp = Epa ꢀ Epc
.
Epa value.
g
Epc value.
Table 3
Composition of selected molecular orbitals of the complexes
Compounds
Contributing fragments
% Contribution of fragments to
HOMO
HOMO ꢀ 1
57
HOMO ꢀ 2
53
LUMO
LUMO + 1
LUMO + 2
93
[Ru(PPh3)2(CO)(L-OCH3)]
Ru
53
40
3
89
2
96
L-OCH3
27
38
(C@N, 54)
CO
14
[Ru(PPh3)2(CO)(L-CH3)]
[Ru(PPh3)2(CO)(L-H)]
[Ru(PPh3)2(CO)(L-Cl)]
[Ru(PPh3)2(CO)(L-NO2)]
Ru
L-CH3
54
41
56
29
54
39
3
91
2
95
94
94
94
96
(C@N, 53)
CO
13
Ru
L-H
56
41
47
29
54
36
3
91
2
95
(C@N, 53)
CO
13
Ru
L-Cl
48
43
56
28
52
40
3
91
2
96
(C@N, 54)
CO
13
Ru
L-NO2
56
37
56
28
54
37
2
94
2
92
(C@N, 15)
CO
13
dichloromethane–acetonitrile solution (0.1 M TBAP).5 Vol-
tammetric data are presented in Table 2 and a representative
voltammogram is deposited as supplementary data (Fig. S1).
All the complexes show two oxidative responses on the posi-
tive side of SCE6 and a reductive response on the negative
side. The first oxidative response is reversible in nature, char-
acterized by a peak-to-peak separation (DEp) of 70–80 mV,
which remains unchanged upon changing the scan rate and
the anodic peak-current (ipa) is almost equal to the cathodic
peak-current (ipc). The other two responses are irreversible in
nature. In view of the composition of the HOMO, the first
reversible oxidation, observed within 0.3–0.7 V, is assigned
to Ru(II)–Ru(III) oxidation. However, the second irrevers-
ible oxidation, observed within 1.0–1.3 V, is tentatively
attributed to oxidation of the C,N,O-coordinated benzaldi-
mine ligand.6 Based on the composition of the LUMO, the
irreversible reductive response observed around ꢀ1.1 V
may be assigned to reduction of the imine fragment in the
coordinated benzaldimine ligand. One-electron nature of
the two oxidative responses has been verified by comparing
their current heights with that of ferrocene–ferrocenium cou-
ple under identical experimental conditions. However, the
irrerversible reductive response shows non-stoichiometric
current (ipc). Potential of the first oxidation is found to be
sensitive to the nature of the substituent R in the coordinated
benzaldimine ligand. The potential increases with increasing
electron-withdrawing character of the substituent R. The
plot of E1/2 versus r [r = Hammett constant of R [13],
OCH3 = ꢀ0.27, CH3 = ꢀ0.17, H = 0.00, Cl = 0.23 and
NO2 = 0.78] is linear (Fig. S2) with a slope (q) of 0.28 V
5
A little dichloromethane was necessary to take the complex into
solution. Addition of large excess of acetonitrile was necessary to record
the redox responses in proper shape.
6
The second irreversible oxidation has not been observed in the
[Ru(PPh3)2(CO)(L-NO2)] complex.

3882
C. GuhaRoy et al. / Polyhedron 26 (2007) 3876–3884
Fig. 3. Partial molecular orbital diagram of [Ru(PPh3)2(CO)(L-H)] complex.
(q = reaction constant of this couple [14]), which shows that
the substituent (R) on the coordinated benzaldimine ligand,
which is four-bonds away from the metal center, can still
influence the metal-centered oxidation potential in a predict-
able manner.
IC-15/2004] is gratefully acknowledged. The authors
thank Professor Saswati Lahiri of the Department of Or-
ganic Chemistry, Indian Association for the Cultivation
of Science, Kolkata, for her help. Thanks are also due
to the School of Chemistry, University of Hyderabad,
for X-ray diffraction data collection on the crystal. Swati
Dutta thanks the Council of Scientific and Industrial Re-
search, New Delhi, for her fellowship [Grant No. 9/
96(410)/2003-EMR-I] and Chhandasi GuhaRoy thanks
the University Grants Commission, New Delhi, for her
fellowship [Grant No. 10-2(5)/2005(I)-E.U.II].
4. Conclusions
The present study shows that the N-(20-hydroxyphe-
nyl)benzaldimines (1) can undergo facile C–H activation
at one ortho position of the aryl ring in the benzaldimine
fragment, mediated by [Ru(PPh3)2(CO)2Cl2]. This study
also indicates that similar C–H activation of organic mole-
cules having structural similarity with the N-(20-hydroxy-
phenyl)benzaldimines may also be possible upon their
reaction with [Ru(PPh3)2(CO)2Cl2] and other suitable tran-
sition metal complexes, and such possibilities are currently
under exploration.
Appendix A. Supplementary material
CCDC 626175 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.poly.2007.
04.020.
Acknowledgements
Financial assistance received from the Department of
Science and Technology New Delhi, [Grant No. SR/S1/

C. GuhaRoy et al. / Polyhedron 26 (2007) 3876–3884
3883
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