Synthesis and characterisation of some Ru(II) complexes of 2-carbamoylpyridine derivatives

Article abstract of DOI:10.1016/S0277-5387(99)00103-5

p-Substituted N-phenyl derivatives of 2-carbamoylpyridine (L) have been prepared by the reaction of pyridine-2-carboxylic acid with p-substituted aniline. Five complexes of the type [Ru(L)(DMSO)₂Cl₂] have been synthesized by the reaction of [Ru(DMSO)₄Cl₂] with L. The amide ligands have been characterized by elemental analysis, infra red and ¹H NMR spectral studies. The complexes are diamagnetic and show intense absorptions due to metal to ligand charge transfer (MLCT) transitions in the UV-visible spectra. The IR spectra of the complexes show that the amide ligands coordinate to the ruthenium (II) ion as a bidentate ligand coordinating from pyridyl nitrogen and from the carbonyl oxygen of the amide group. The complexes undergo a reversible ruthenium (II)-ruthenium (III) oxidation near 0.55 V in acetonitrile solution. The ruthenium (II)-ruthenium (III) oxidation potentials of the complexes are found to be sensitive to the nature of the substituent on the ligand. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00103-5

Polyhedron 18 (1999) 2157–2162
Synthesis and characterisation of some Ru(II) complexes of
2-carbamoylpyridine derivatives
*
Sujit Dutta, Sarbani Pal, Pabitra K. Bhattacharya
Department of Chemistry, Faculty of Science, M.S. University of Baroda, Vadodara 390 002, India
Received 24 November 1998; accepted 12 April 1999
Abstract
p-Substituted N-phenyl derivatives of 2-carbamoylpyridine (L) have been prepared by the reaction of pyridine-2-carboxylic acid with
p-substituted aniline. Five complexes of the type [Ru(L)(DMSO)2Cl2] have been synthesized by the reaction of [Ru(DMSO)4Cl2] with L.
1
The amide ligands have been characterized by elemental analysis, infra red and H NMR spectral studies. The complexes are diamagnetic
and show intense absorptions due to metal to ligand charge transfer (MLCT) transitions in the UV–visible spectra. The IR spectra of the
complexes show that the amide ligands coordinate to the ruthenium (II) ion as a bidentate ligand coordinating from pyridyl nitrogen and
from the carbonyl oxygen of the amide group. The complexes undergo a reversible ruthenium (II)–ruthenium (III) oxidation near 0.55 V
in acetonitrile solution. The ruthenium (II)–ruthenium (III) oxidation potentials of the complexes are found to be sensitive to the nature of
the substituent on the ligand.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Ruthenium complexes; 2-Carbamoylpyridine derivatives; Amide complexes; Electrochemistry; Geometrical isomers
1. Introduction
The chemistry of ruthenium receives ample attention
due to the fascinating properties of ruthenium complexes
[1,4]. These include the availability of its wide range of
oxidation states of ruthenium in its complexes [5,6],
occurrence of a ligand mediated intermetallic interaction in
binuclear ruthenium complexes [7–9], reactivity of the
complexes without changing the coordination sphere as
well as reactivity of the complexes causing changes in the
coordination sphere [1–3], enhanced catalytic activity
towards organic substrates [10–15], and occurrence of
some less common reactions [16–21]. The chemistry of
the amide complexes of ruthenium also receives consider-
able attention in the context of metal-peptide chemistry
[22–31]. However, these complexes are mostly limited to
pentaammineruthenium amido complexes. Here we are
The presence of a pyridyl nitrogen at the adjacent position
reporting the synthesis and characterization of the aromatic
amide of pyridine-2-carboxylic acid and their complexes
with ruthenium. The general structure of the ligand can be
shown as L.
of the amide linkage helps the ligand (L) to bind to a metal
ion in a bidentate fashion forming a five membered chelate
ring either through N-bonding 1. or through O-bonding 2.
from the amide linkage. Both the linkage isomers, O-
bonded and N-bonded,are known for glycinamide chelated
to tetraammineruthenium(III) center [30,31].
*Corresponding author. Tel.: 191-265-795-552.
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00103-5
1999 Elsevier Science Ltd. All rights reserved.

2158
S. Dutta et al. / Polyhedron 18 (1999) 2157–2162
2. Experimental
white solid was filtered, washed thoroughly with distilled
water and crystallized from aqueous methanol as a color-
less crystalline solid. Yield: 3.9 g (80%), mp 798C. Found
C, 72.36; H, 4.9; N, 13.93; calc. for C12H10N2O, C, 72.73;
H, 5.05; N, 14.14%.
2.1. Physical measurements
Micro analyses (C, H, N) were performed using a
Perkin-Elmer 240C elemental analyzer. IR spectra were
obtained on a Parkin-Elmer 783 spectrophotometer. Elec-
tronic spectra were recorded on a Shimadzu 240-UV–
visible spectrophotometer. Magnetic susceptibilities were
measured with the help of a PAR 155 vibrating sample
magnetometer. The 1H and 13C NMR spectra were ob-
tained with a Varian Fourier-transform spectrometer, using
TMS as an internal standard. The electrospray mass spectra
were recorded on a MICROMASS QUATTRO II triple
quadruple mass spectrometer. Electrochemical measure-
ments were made using the 174A polarographic analyser, a
universal programmer, an X–Y recorder, a platinum work-
ing electrode, a platinum wire auxiliary electrode and
Ag/AgNO3 reference electrode. All electrochemical mea-
surements were performed under dinitrogen atmosphere.
Ferrocene was used as an internal standard; all potentials
are quoted vs. the ferrocene–ferrocenium couple. Support-
ing electrolyte used was Tetraethylammonium perchlorate
(TEAP)
L2 (X5CH3), yield 79%; mp 1088C. Found C, 73.85;
H, 5.88; N, 12.87; calc. for C13H12N2O, C, 73.58; H, 5.66;
N, 13.2%.
L3 (X5OCH3), yield 87%; mp 958C. Found C, 68.73;
H, 5.42; N, 11.9; calc. for C13H12N2O2, C, 68.42; H, 5.26;
N, 12.28%.
L4(X5Cl), yield 68%; mp. 1398C. Found C, 62.3; H,
3.83; N, 12.37; calc. for C12H9N2OCl, C, 61.94; H, 4.18;
N, 12.04%
L5(X5NO2), From the hot reaction mixture the product
was precipitated directly, isolated by filtration, washed
thoroughly with methanol, dried over fused calcium chlo-
ride, crystallized from large volume of methanol. Yield
78%; mp 2338C. Found C, 59.63; H, 3.89; N, 17.24; calc.
for C12H9N3O3, C, 59.26; H, 3.87; N, 17.28%.
2.4. Syntheses of the complexes
Syntheses of the complexes of the type
[Ru(L)(DMSO)2Cl2] were achieved using same general
method. Specific details are given for representative cases:
Dicholoro[2 - (N - phenylcarbamoyl)pyridine]bis(di-
2.2. Materials
RuCl3?xH2O (Loba, India) was converted to RuCl3?
3H2O by repeated evaporation to dryness with concen-
trated hydrochloric acid. Pyridine-2-carboxylic acid (E-
merk, Germany) was used as received. Aromatic amines
(aniline and p-substituted aniline, p-X-C6H4NH2, X52
CH3, 2OCH3, 2NO2 and 2Cl) were either distilled over
KOH or recrystallized before use. Acetonitrile was dis-
tilled over CaH2 before performing electrochemical experi-
ments. Tetraethylammonium perchlorate (TEAP) was pre-
pared using a reported procedure [32]. Methanol was dried
over fused CaO and [Ru(DMSO)4Cl2] (DMSO5
dimethylsulfoxide) was prepared as reported before [33].
methylsulfoxide)ruthenium(II) I:
A
suspension of
[Ru(DMSO)4Cl2] (0.242 g, 0.5 mmol) and 2-(N-
Phenylcarbamoyl)pyridine (L1) (0.099 g, 0.5 mmol) in 15
ml dry methanol was refluxed for 1 h. Ten millilitres of
solvent was distilled out from the reaction mixture. During
this time an orange colored microcrystalline product
separated. The solid was isolated by filtration, washed with
minimum volume of dry methanol and then by (1033) ml
diethylether, dried over fused calcium chloride, yield 0.22
g (83%); Found C, 36.67; H, 4.02; N, 5.77; calc. for
RuC16H22N2O3Cl2S2; C, 36.5; H, 4.18; N, 5.32%.
For complexes II–V the microcrystalline solid was
precipitated directly using L2, L3, L4, L5 instead of L1,
respectively
2.3. Syntheses of ligands
II, yield 68%; Found: C, 37.41; H, 4.31; N, 5.56; calc.
for RuC17H24N2O3S2Cl2; C, 37.77; H, 4.44; N, 5.18%.
III, yield 73%; Found: C, 36.36; H, 4.32; N, 5.21; calc.
for RuC17H24N2O4S2Cl2; C, 36.69; H, 4.32; N, 5.04%.
IV, yield 76%; Found: C, 34.55; H, 3.71; N, 4.78; calc.
for RuC16H21N2O3S2Cl3; C, 34.25; H, 3.75; N, 4.99%.
V, yield 89%; Found: C, 33.82; H, 3.82; N, 7.13; calc.
for RuC16H21N3O5S2Cl2; C, 33.62; H, 3.68; N, 7.35%.
Dichlorobipyridine[2 - (N - phenylcarbamoyl)pyridine-
ruthenium(II) VI: A solid mixture of I, 0.1052 g (0.2
mmol) and bipyridine (bpy), 0.0312 g (0.2 mmol) was
refluxed in dry toluene for 24 h. The solid thus obtained
was isolated by filtration, dissolved in minimum volume of
dichloromethane and was purified by column chromatog-
raphy using silica gel (60–120 mesh). The desired product
All the amide ligands were prepared using the same
general method. Specific details are given for representa-
tive cases.
2-(N-Phenylcarbamoyl)pyridine (L1) (X5H): Into a
solution of pyridine-2-carboxilic acid (3.08 g, 25 mmol) in
10 ml pyridine, aniline (2.28 ml, 25 mmol) was added and
was warmed under stirring conditions for 15 min. Into the
resulting solution 6.6 ml (25 mmol) triphenylphosphite
was added and the mixture was stirred at 1108C for 4 h.
The cold reaction mixture was washed with 100 ml
distilled water and the resulting white paste was taken in
40 ml dichloromethane and extracted in 100 ml 1:1 (v/v)
aqueous hydrochloric acid. The acidic aqueous extract was
neutralized by solid sodium bicarbonate. The resulting

S. Dutta et al. / Polyhedron 18 (1999) 2157–2162
2159
was isolated by eluting the column by 4:1 (v/v) benzene–
acetonitrile mixture. Evaporation of the solvent from the
eluate yielded dark red colored microcrystalline solid.
Yield555 mg (53%); Found C, 50.08; H, 3.53; N, 10.26
calc. for RuC22H18N4Cl2 C, 50.18; H, 3.42; N, 10.64.
one bidentate N,O-donor ligand to two co-ordinated
DMSO ligands. In the aromatic region of the 1H NMR
spectrum of L2 six resonances are found and they are
found to integrate to eight protons. The singlet at d 10.54
is assigned to the NH proton of the amide linkage and the
singlet (3H) at d 2.29 is assigned to the methyl protons.
The signals in the aromatic regions are assigned as: d 7.18
(2H,d,H39), d 7.67 (1H,td,H5), d 7.79 (2H,d,H29), d 8.07
(1H,td,H4), d 8.16 (1H,d,H3) and d 8.74 (1H,d,H6). In the
1H NMR spectrum of II all the signals due to the ligand
protons are retained but are found to be shifted from its
position compared to the same in the free ligand. The
signal due to the NH proton of the amide linkage is found
to appear at d 11.74 as a singlet and the signal due to the
methyl protons appears at d 2.36 (3H,s). Signals due to the
aromatic protons are assigned as: d 7.33 (2H,d,H39), d
7.52 (2H,d,H29), d 7.94 (1H,t,H5), d 8.30 (1H,td,H4), d
8.72 (1H,d,H3) and d 10.33 (1H,d,H6). The methyl
protons of the coordinated DMSO molecules appear as two
singlets at d 3.35 and d 3.26, each of which integrates to
six protons. This indicates that two coordinated DMSO
molecules are nonequivalent.
3. Result and discussion
The synthetic methodology for the ligands has been
adapted from Barnes et al. [34] with a modification for the
isolation step. The reaction between pyridine-2-carboxylic
acid and p-substituted anilines in the presence of tri-
phenylphosphite proceeds smoothly in pyridine, resulting
in a pale yellow oil (for L4 the product precipitated
directly). The product was isolated from the reaction
mixture by acidification followed by neutralization and
finally was crystallized from aqueous methanol. Mi-
croanalytical data (experimental section) agree well with
the empirical formulae of the compounds. In the IR spectra
of the ligands the nNH and nCO of the amide linkage are
observed at ca. 3342 cm21 and ca. 1677 cm21, respective-
ly.
In the proton decoupled 13C NMR spectrum of L2, the
signal due to the carbon atom of the amide linkage appears
at d 162.2 while the methyl carbon signal appears at d
20.5. The spectrum contains nine more signals for 11
aromatic carbon atoms in the region d 150 to d 120,
assignment of these signals has not been attempted. In the
proton decoupled 13C NMR spectrum of II the signal of
the carbon atom of the amide linkage of the bidentate
N,O2 ligand appears at d 169.3, while the signal of the
methyl carbon atom appears at d 20.6. The signals due to
the aromatic carbon atoms appear in the region d 155 to d
122, assignment of the individual signals has not been
attempted.
The reactions of [Ru(DMSO)4Cl2] with an equimolar
quantity of amide ligands in dry methanol proceed smooth-
ly to precipitate microcrystalline [Ru(L)(DMSO)2Cl2]
complexes (Eq. (1)) in reasonably good yields. The
reaction can be shown as follows.
[Ru(DMSO)4Cl2] 1 L → [Ru(L)(DMSO)2Cl2] 1 2DMSO
(1)
Microanalytical data (C, H, N) correspond to the expected
composition of these complexes. Magnetic susceptibility
measurements show that these complexes are diamagnetic
as expected for complexes of ruthenium (II) (low spin d6,
S50). The infrared spectra of these complexes contain
many sharp bands of different intensities due to vibrations
arising from the coordinated DMSO, L and Cl2 ligands
and are therefore complex in nature. No attempt has been
made to assign the individual bands. However, comparison
of the IR spectra of the free ligands, with that of the
complexes, shows a negative shift in the nCO of the amide
linkage in the complex, indicating [35] that the oxygen
atom of the amide linkage is coordinated to the
ruthenium(II) center. In the IR spectra of all the complexes
a sharp band is observed at ca. 1095 cm21. This has been
assigned to nSO. The same band in free DMSO appears at
1055 cm21 [36]. The positive shift in nSO indicates that the
coordination of the DMSO molecules to the ruthenium(II)
center occurs from the sulfur atom [36]. The sharp bands at
306 cm21 (medium intensity) and 274 cm21 are assigned
to nRu–Cl [37] and nRu–N(pyridine) [38] stretching mode.
The 1H and 13C NMR spectra of one representative
complex, II and the corresponding ligand, L2 (X5CH3)
were recorded in (CD3)2SO solvent to confirm the ratio of
3.1. Geometry of the ruthenium(II) center
The ruthenium(II) complexes may in principle, exist in
the following four geometrical isomeric forms:
A single nRu–Cl band is expected for a linear grouping of
the trans-RuCl2 moiety (as in a), whereas, two nRu–Cl
bands of equal intensity are expected for a cis-RuCl2
moiety (as in b, c and d) [39]. Appearance of a single
nRu–Cl band excludes isomers b, c and d, hence the
arrangement of the donor atoms around the ruthenium(II)

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S. Dutta et al. / Polyhedron 18 (1999) 2157–2162
1
center is as depicted in a(Y5DMSO). Further, in the H
NMR spectrum of II, the appearance of two different
signals for the coordinated DMSO molecules indicates that
they are non-equivalent. This is expected when the coordi-
nated DMSO molecules are in cis-geometry, as is sug-
gested in a.
Electronic spectra of these complexes have been re-
corded in mixed solvent, dichloromethane–acetonitrile 4:1
v/v mixture. Spectral data are collected in Table 1 and
representative spectra are shown in Fig. 1. All the com-
plexes display two intense absorptions, one of which is in
the visible region and the second one extends to the UV
region. These two intense bands are assigned to the metal
to ligand charge transfer transitions. The corresponding
parent ligands display an intense absorption in the UV
region, assigned to the intraligand transition. The absorp-
tion due to the intraligand transition appears as a shoulder
in the electronic spectra of the complexes I, II and IV with
a minor shift in the position along with the second
absorption in the UV region, whereas for III the intraligand
transition appears as a separate band at 273 nm (Fig. 1)
and for V the same probably merges with the metal to
ligand charge transfer band.
The electro-spray mass spectra of two representative
complexes, II and IV were recorded. The peaks observed
at m/z 542.9 for II and at m/z 559.9 for IV corresponds to
the molecular ion of the corresponding complexes II and
IV, (calculated molecular weight 540.5 for II and 560.6 for
IV).
Thus the mass spectral data along with 1H and 13C
NMR spectroscopic result, microanalytical, magnetic mo-
ment and IR data collectively establish the suggested
composition and stereochemistry of the complexes.
Fig. 1. Electronic spectra of II (———) and III (–
dichloromethane–methanol 4:1 (v/v) mixture.
–
–
–) in
[Ru(L)(DMSO)2Cl2]á [Ru(L)(DMSO)2Cl2]1 1 e2
(2)
3.2. Electrochemical properties
The DEp values of these couples lie in the range 60–70
mV, which do not change with change in the scan rate and
the ipa /ipc (ipa 5anodic peak current and ipc 5cathodic
peak current) ratio is close to 1.0, as expected for
reversible couples. To establish the one electron stoi-
chiometry of this couple, constant potential coulometric
oxidation of one representative complex, I was carried out.
The electrochemical properties of the complexes were
studied in acetonitrile (0.1 M in TEAP) by cyclic vol-
tammetry. The voltammetric data are presented in Table 1.
Each complex shows a reversible response, due to
ruthenium(II)/ruthenium(III) couple (Eq. (2)) in the po-
tential range 0.54–0.60 V.
Table 1
Cyclic-voltammetric and optical spectral data
Complexes
Electronic spectraa lmax (nm)
(´, M21 cm21
Cyclic-voltammetric datab
)
E2o98, V (DEp, mV) RuII / III
I
II
III
IV
V
425(2360); 293(11437); 277(10302)c
419(2431); 305(10713); 275(8134)c
404(2559); 317(9293); 273(6497)
414(2183); 297(11312); 275(9764)c
446(2239); 309(15830)
0.55 (70)
0.55 (70)
0.54 (70)
0.57 (60)
0.60 (70)
0.83 (63)
VI
516(1928); 376(5351); 296(16930)
a For I–V solvent used is dichloromethane–methanol 4:1 (v/v) mixture and for VI dichloromethane.
b Supporting electrolyte, TEAP (0.1 M); reference electrode Ag/Ag1; E2o98 50.5(Epa 1Epc ) where Epa and Epc are anodic and cathodic peak potential;
DEp 5Epa 2Epc; scan rate 100 mV s21; solvent: acetonitrile.
c Shoulder.

S. Dutta et al. / Polyhedron 18 (1999) 2157–2162
2161
The observed Coulomb count corresponds to one electron
transfer (n50.98; n 5 Q/Q9, where Q9 is the calculated
Coulomb count for one electron transfer and Q is that for
exhaustive electrolysis).
response in the anodic side of the voltammogram (initial
scan anodic), presumably due to ruthenium(II)/
ruthenium(III) couple, Eq. (4), occurs at 0.83 V.
[Ru(L1)(bpy)Cl2] → [Ru(L1)(bpy)Cl2]1 1 e2
(4)
Though stable in the cyclic voltammetric time scale
attempts for the isolation of [Ru(L)(DMSO)2Cl2]1 by
chemical oxidation were not successful. The chemical
oxidation was tried in 1:1 (v/v) aqueous acetonitrile by
ammonium ceric sulphate. The color of the reaction
mixture changed from orange to green after stirring the
reaction mixture for 24 h, but no product could be isolated
from the reaction mixture by the addition of the negative
counter ion ClO24 . This may be attributed to the degra-
dation of the complex.
The DEp value of this couple is 63 mV and it does not
change with the change in scan rate and the ratio Ipa /Ipc is
close to one as expected for a reversible couple. Thus the
ruthenium (II)/ruthenium (III) couple for VI is shifted to a
more positive potential (by 0.28 V) compared to I,
indicating substitution of the DMSO groups by bpy
stabilizes the oxidation state, (II) of the ruthenium center
in the complex.
For the five complexes the potential of the
ruthenium(II)–ruthenium(III) couple were found to be
sensitive to the nature of the substituents, X, on the ligand.
In case of V with X5electron withdrawing NO2, higher
oxidation potential value (0.60 V) of the ruthenium(II)–
ruthenium(III) couple is observed, whereas lower
ruthenium(II)–ruthenium(III) oxidation potential (0.54 V)
is observed for III (X5OCH3), reflects the electron
donating nature of the substituent (Fig. 2).
Acknowledgements
The authors gratefully acknowledge the financial assis-
tance provided by DST, New Delhi, Govt. of India.
Acknowledgments are made to the Regional Sophisticated
Instrumentation Centre (RSIC), Indian Institute of Tech-
nology, Bombay, for providing NMR and IR facility,
RSIC, Central Drug Research Institute, Lucknow, for
providing the electrospray mass spectra and RSIC, Roor-
kee University for room temparature magnetic moment
determination. We are thankful to Dr. Samaresh Bhattach-
arya of Jadavpur University for helpful discussion, Dr.
Amitava Das CSMCRI, Bhavnagar for providing the
facility of constant potential coulometric experiment and to
the Head, Department of Chemistry, for providing the
laboratory facilities. Refrees’ suggestion at the revision
stage were very helpful.
3.3. Substitution reaction
To explore the reactivity of two cis-DMSO ligands,
reaction of complex I, with the bidentate chelating ligand
2,29-bipyridine (bpy) was carried out. Reaction between I
and bpy in dry toluene lead to the formation of VI
according to Eq. (3)
[Ru(DMSO)4(L1)Cl2] 1 bpy → [Ru(L1)(bpy)Cl2] 1
2DMSO.
(3)
Composition of VI was confirmed by microanalytical data
(experimental section). In the UV spectrum of VI three
intense absorption bands are found. Two intense bands at
516 nm and 376 nm are assigned to metal to ligand charge
transfer transitions while the band at 296 nm is assigned to
intraligand transition. The cyclic voltammogram of VI was
recorded on acetonitrile (0.1 M in TEAP). The reversible
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