Synthesis, characterization, and catalytic activity of metal schiff base complexes derived from pyrrole-2-carboxaldehyde

Article abstract of DOI:10.1081/SIM-120027314

Several ruthenium(II) Schiff base complexes derived from bis(pyrrole-2-carboxaldehyde)-3,4-toluenediimine, bis(pyrrole-2-carboxaldehyde)- 1,2-cyclohexanediimine, bis(pyrrole-2-carboxaldehyde)-1,8-naphthalindiimine, bis(pyrrole-2-carboxaldehyde)-1,3-propyl

Full text of DOI:10.1081/SIM-120027314

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Synthesis, Characterization, and Catalytic Activity
of Metal Schiff Base Complexes Derived from
Pyrrole‐2‐carboxaldehyde
Minu G. Bhowon a , H. Li Kam Wah a , A. Dosieah a , M. Ridana a , O. Ramalingum a & D.
Lacour a
a Department of Chemistry, Faculty of Science, University of Mauritius, Réduit, Mauritius
Version of record first published: 16 Nov 2010.
To cite this article: Minu G. Bhowon, H. Li Kam Wah, A. Dosieah, M. Ridana, O. Ramalingum & D. Lacour (2004):
Synthesis, Characterization, and Catalytic Activity of Metal Schiff Base Complexes Derived from Pyrrole‐2‐carboxaldehyde,
Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry, 34:1, 1-16
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 34, No. 1, pp. 1–16, 2004
Synthesis, Characterization, and Catalytic
Activity of Metal Schiff Base Complexes
Derived from Pyrrole-2-carboxaldehyde
*
Minu G. Bhowon, H. Li Kam Wah, A. Dosieah,
M. Ridana, O. Ramalingum, and D. Lacour
Department of Chemistry, Faculty of Science,
´
University of Mauritius, Reduit, Mauritius
ABSTRACT
Several ruthenium(II) Schiff base complexes derived from bis(pyrrole-
2-carboxaldehyde)-3,4-toluenediimine, bis(pyrrole-2-carboxaldehyde)-1,
2-cyclohexanediimine, bis(pyrrole-2-carboxaldehyde)-1,8-naphthalindi-
imine, bis(pyrrole-2-carboxaldehyde)-1,3-propylenediimine, bis(pyrrole-
2-carboxaldehyde)-1,2-phenylenediimine and bis(pyrrole-2-carboxalde-
hyde)-ethylenediimine, and lanthanide(III) Schiff base complexes
derived from N-(2-pyrrolylmethylene)-2-aminophenol were synthesized.
All the complexes were characterized by analytical and spectroscopic
methods. The ruthenium Schiff base complexes were found to be effective
*
Correspondence: Minu G. Bhowon, Department of Chemistry, Faculty of Science,
´
University of Mauritius, Reduit, Mauritius; E-mail: mbhowon@uom.ac.mu.
1
DOI: 10.1081/SIM-120027314
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Bhowon et al.
catalysts for the oxidation of primary alcohols in the presence of
N-methylmorpholine-N-oxide as oxidant.
Key Words: Pyrrole-2-carboxaldehyde; Schiff base complexes;
Synthesis; Characterization.
INTRODUCTION
A wide range of transition and lanthanide metal complexes derived from
Schiff base ligands have been isolated.[1–6] These compounds have played a
major role in the development of modern inorganic coordination chemistry
providing examples of macrocyclic ligand systems[7] and the effects of steric
interactions on coordination geometries. Ruthenium Schiff base com-
plexes have also been found to be effective catalysts in the oxidation of
alcohols and alkenes using N-methylmorpholine-N-oxide (NMO) as
oxidant.[8,9]
We report herein the preparation, characterization and catalytic behaviour
of some ruthenium and lanthanide Schiff base complexes derived from
pyrrole-2-carboxaldehyde and 3,4-diaminotoluene, 1,2-diaminocyclohexane,
1,8-diaminonaphthalin, 1,3-diaminopropane, 1,2-phenylenediamine, 1,2-
diaminoethane and aminophenol. The structures of the Schiff bases are
shown in Fig. 1.
RESULTS AND DISCUSSION
The Schiff bases L1, H2L2, H2L3, L4–L7 were synthesised by the con-
densation of pyrrole-2-carboxaldehyde with 2-aminophenol, 3,4-diaminoto-
luene, 1,2-diaminocyclohexane, 1,8-diaminonaphthalin, 1,3-diaminopropane,
1,2-phenylenediamine and 1,2-diaminoethane in ethanol, respectively.
The compounds H2L2, H2L3 and L4–L7 are bis-condensed Schiff bases.
Table 1 shows the analytical and spectroscopic data of all the Schiff bases
prepared.
1
The lanthanide(III) complexes, [Ln(L )2(NO3)2]NO3 3H2O (1)–(6)
.
(Ln ¼ Nd, Sm, Gd, Dy, Er and La) were prepared by reacting their respective
nitrate salts with the Schiff base L1 in the molar ratio of 1:2 in ethanol at room
temperature according to the general equation:
Ln(NO3)3ꢀnH2O þ 2L1 ꢁ! [Ln(L1)2(NO3)2]NO3ꢀ3H2Oþ(n-3)H2O
(n ¼ 5 or 6; Ln ¼ Nd, Sm, Gd, Dy, Er and La)

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3
Figure 1. Structures of the Schiff base ligands L1, H2L2, H2L3, L4–L7.

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Bhowon et al.
l

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Complexes Derived from Pyrrole-2-carboxaldehyde
5
The ruthenium complexes (7)–(12) were isolated when RuCl2(PPh3)3 and
the Schiff bases H2L2, H2L3 and L4–L7 were refluxed in THF in the molar
ratio of 1:1.
RuCl2(PPh3)3 þ H2X þ H2O ꢁ! [Ru(H2X)Cl2(PPh3)(H2O)] þ 2PPh3
(X ¼ L2 and L3)
RuCl2(PPh3)3 þ Y þ (n þ 1)H2O ꢁ! [Ru(Y)Cl2(PPh3)(H2O)]
ꢂ nH2O þ 2PPh3
(Y ¼ L4 –L7) (n ¼ 0, 2 or 3)
On the other hand, the ruthenium complexes [Ru(L2)(PPh3)2] (13) and
[Ru(L3)(PPh3)2] (14) were obtained after RuCl2(PPh3)3 and the Schiff bases
H2L2 and H2L3 were reacted, respectively, in the presence of triethylamine.
RuCl2(PPh3)3 þ H2X þ 2NEt3 ꢁ! [Ru(X)(PPh3)2] þ PPh3
þ 2Et3NHþClꢁ(X ¼ L2 and L3)
Table 2 shows the analytical and spectroscopic data of all the metal
complexes.
Infrared Spectra
The infrared spectrum of the Schiff base L1 showed a strong band at
1626 cm21 attributable to the n(C55N) stretching frequency vibration.
On complexation with the lanthanide nitrates, this band shifted to the
1649–1657 cm21 region indicating that the nitrogen of the azomethine group
has coordinated to the lanthanide ion.[10] The presence of a band in the
3265–3321 cm21 region suggested that the nitrogen of the pyrrole group is not
involved in bonding or the presence of water molecules. The complexes showed
six absorption bands near 1502, 1307, 1047, 809, 754 and 700 cm21 which are
assigned, respectively, to n4, n1, n2, n6, n3 and n5 due to the coordinated nitrate
groups. The magnitude of n4–n1 is about 195 cm21, indicating that the nitrate
groups are coordinated in a bidentate fashion.[11] Furthermore, a band appearing
at 1384 cm21 suggested the presence of ionic nitrate in the complexes.[12]
The infrared spectra of the ruthenium complexes [Ru(H2L3)Cl2(PPh3)
5
6
.
(H2O)] (8), [Ru(L )Cl2(PPh3)(H2O)] 3H2O (10), [Ru(L )Cl2(PPh3)(H2O)]
(11), [Ru(L7)Cl2(PPh3)(H2O)] (12), [Ru(L2)(PPh3)2] (13) and [Ru(L3)
(PPh3)2] (14) showed the n(C55N) stretching frequency in the 1581–
1639 cm21 region which is at lower frequency compared to the precursor
Schiff bases (1596–1643 cm21) indicating the coordination of the metal ion to

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Bhowon et al.
l

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Complexes Derived from Pyrrole-2-carboxaldehyde
7

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Bhowon et al.
the azomethine nitrogen. On the other hand, [Ru(H2L2)Cl2(PPh3)(H2O)] (7)
4
.
and [Ru(L )Cl2(PPh3)(H2O)] 2H2O (9) exhibited the n(C55N) stretching
frequency at 1635 and 1639 cm21, which is higher when compared to the
Schiff bases H2L2 and L4 (1619 and 1596 cm21), respectively. It is noteworthy
that this is a common feature as reported in the literature.[13] The presence of
the triphenylphosphine ligand in the ruthenium complexes was confirmed by
characteristic bands[14] in the 500–1480 cm21 region.
1H NMR Spectra
The 1H NMR spectrum of the lanthanum complex [La(L1)2(NO3)2]
.
NO3 3H2O (6) in DMSO-d6 indicated a sharp peak corresponding to 2H at
d 8.56 ppm (downfield compared to the Schiff base L1) corresponding to
azomethine protons. The downfield shift is due to the deshielding effect upon
donation of electrons from the azomethine nitrogen to the metal. The presence
of signals due to O–H protons at d 8.56 ppm and N–H protons at d 11.79 ppm
indicated that neither the pyrrole nitrogens nor the hydroxyl oxygens were
deprotonated. No deprotonation of the hydroxyl group has also been observed
in the literature for lanthanide Schiff base complexes derived from 2,3-
dihydroxybenzaldehyde and 1,2-diaminoethane[2] and from o-vanillin and
naphthylamine.[10]
1
The H NMR spectra of [Ru(H2L2)Cl2(PPh3)(H2O)] (7), [Ru(H2L3)Cl2
4
5
(PPh3)(H2O)] (8), [Ru(L )Cl2(PPh3)(H2O)] 2H2O (9), [Ru(L )Cl2(PPh3)
.
6
7
.
(H2O)] 3H2O (10), [Ru(L )Cl2(PPh3)(H2O)] (11) and [Ru(L )Cl2(PPh3)(H2O)]
(12) in DMSO-d6 showed signals in the range d 6.15–7.84 ppm attributed to
the protons of triphenylphosphine, benzene rings and pyrrole ligands.
The peaks due to azomethine protons occurred at d 8.00–8.45 ppm, downfield
compared to the corresponding Schiff bases H2L2, H2L3 and L4–L7 (d 7.92–
8.18 ppm), indicating the coordination of the azomethine nitrogen to the metal
ion in the complexes. The presence of a signal at d12.06–12.68 ppm
confirmed the non-deprotonation and non-coordination of the pyrrole
nitrogen. On the other hand, the H NMR spectra of [Ru(L2)(PPh3)2] (13)
and [Ru(L3)(PPh3)2] (14) showed the absence of a peak at d 12 ppm indicating
bonding through the pyrrole nitrogen after deprotonation. The four aliphatic
1
5
7
.
protons in [Ru(L )Cl2(PPh3)(H2O)] 3H2O (10) and [Ru(L )Cl2(PPh3)(H2O)]
(12) were not observed due to overlapping with the DMSO signal.
UV-Visible Spectra
The UV-Visible spectra of the lanthanide and ruthenium complexes in
methanol or DMSO showed bands in the range 209–275 nm corresponding to

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Complexes Derived from Pyrrole-2-carboxaldehyde
9
p–pꢃ transitions of the aromatic ring. Another band at 290–324 nm was
assigned to the n–pꢃ transition of the non-bonding electrons present on the
nitrogen of the azomethine group of the Schiff base. A band in the region
417–588 nm is attributed to a charge transfer transition of ligand (p) to metal
(d). Another weak peak around 870 nm for the ruthenium complexes could be
due to d–d transitions.
Conductivity Measurements
The conductance values of the lanthanide complexes (1023 M in
methanol) (1)–(6) are found to be 108, 123, 102, 124 and 123 and
80 ohm21 cm2 mole21, respectively. These results indicated that all the
lanthanide complexes exhibited 1:1 electrolytic behaviour[11] in methanol at
278C and they have most probably the structure [Ln(L1)2(NO3)2]NO3 3H2O.
.
On the other hand, all the ruthenium complexes were found to be non-
electrolytic in DMSO.
Magnetic Measurements
All the lanthanide complexes except that of lanthanum were found to be
paramagnetic. The values of the magnetic moment (meff) are 3.68, 1.63, 8.41,
10.92, 10.06 B.M. for (1)–(5), respectively. The magnetic moment values
obtained for the complexes compare favourably with those observed in the
literature indicating that the 4f electrons did not take part in bonding.[15] On
the other hand, all the ruthenium(II) complexes were found to be diamagnetic
in accord with a t2g6 low-spin configuration.
Based on analytical, spectroscopic, magnetic and conductivity data, the
proposed structures of the complexes (1)–(14) are shown in Fig. 2.
Catalytic Activity
All the synthesized ruthenium complexes (7)–(14) have been used
to catalyse the oxidation of alcohols in the presence of N-methylmorpho-
line-N-oxide (NMO) as oxidant and the results are summarised in Table 3.
The oxidations were carried out at room temperature in CHCl3
solutions, from which water was removed using molecular sieves. There
was no detectable oxidation of alcohols when a blank consisting of NMO
and the substrate in chloroform in the absence of the ruthenium complex
was carried out.

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Bhowon et al.
Figure 2. Proposed structures of the metal complexes (1)–(14).

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Complexes Derived from Pyrrole-2-carboxaldehyde
11

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Bhowon et al.
All the ruthenium complexes were found to oxidize primary alcohols such
as cinnamyl, p-methoxybenzyl and piperonyl alcohol to their respective
aldehydes. All the ruthenium complexes were found to be selective towards
oxidation of alcohols and were efficient catalysts as evidenced by the
turnovers. Also, the catalysts in the presence of NMO were effective towards
oxidation of unsaturated alcohols e.g., cinnamyl alcohol without competing
with double bond attack.[9] The synthesised ruthenium Schiff base complexes
were found to be better catalysts than RuCl2(PPh3)3 for the oxidation of
cinnamyl and p-methoxybenzyl alcohol with higher yield and turnover. For
the oxidation of cinnamyl alcohol, complex (9) proved to be the best catalyst
compared to the other complexes with a turnover of 85, while for
p-methoxybenzyl alcohol, complex (11) gave the highest yield (82.9%) and
turnover (82). The percentage yield of piperonal and turnover were found to be
low except for complex (7). The oxidation effected by the catalysts is likely to
occur via ruthenium high oxidation states (Ru(IV) or Ru(V)), which should be
accessible through chemical oxidation with the oxidant.[16,17]
EXPERIMENTAL
General
All chemicals were purchased from either Aldrich or BDH Chemicals and
were used without purification. IR spectra (KBr pellets) were recorded on a
Mattson 1000 FT IR spectrometer in the 4000–400 cm21 range. UV-Visible
spectra were recorded in methanol and DMSO solutions on a PU 8700 series
1
UV-Visible spectrometer. H and 13C NMR spectra were obtained using a
Bruker Spectrospin spectrometer at 250 MHz. The melting points of all
samples were determined using a Stuart Scientific electric melting point
apparatus. Magnetic susceptibility data for the metal complexes were recorded
on a Sherwood Scientific magnetic balance. Carbon, hydrogen and nitrogen
[18]
contents were obtained using a Leco 932 CHNS analyser. RuCl2(PPh3)3
and the Schiff bases derived from pyrrole-2-carboxaldehyde with 2-
aminophenol,[19,20] 1,2-diaminoethane.[21,22] 1,2-phenylenediamine[21,22] and
1,3-diaminopropane[22] were prepared by previously reported methods.
Synthesis of Schiff Bases H2L2, H2L3, and L4
Pyrrole-2-carboxaldehyde (0.95 g, 10 mmol) and diamine (5 mmol) were
refluxed in ethanol (5–10 mL) for 2–6 hours and the resulting solution was
then allowed to stand at room temperature for 1 hour. The obtained solid was

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Complexes Derived from Pyrrole-2-carboxaldehyde
13
filtered, washed with ethanol and dried in vacuo. The reaction with 1,8-
diaminonaphthalin was carried out at room temperature by stirring overnight.
The obtained violet crystals were filtered, washed with ethanol (5 mL) and
diethyl ether (5 mL), and recrystallized with ethanol before drying in vacuo.
Synthesis of Lanthanide Metal Complexes
1
[Ln(L )(NO3)2]NO3 3H2O (1)–(6) (Ln 5 La, Nd,
.
Sm, Gd, Dy, and Er)
A hot ethanolic (5 mL) solution of the Schiff base L1 (0.2 g, 1.07 mmol)
was added with stirring to a solution of hydrated lanthanide(III) nitrate
(0.5 mmol) in ethanol (5 mL). The reaction mixture was stirred at room
temperature. The product, formed after 3–15 hours, was collected by
filtration, washed thoroughly with ethanol and diethyl ether and dried in
vacuo.
Synthesis of Ruthenium Complexes
[Ru(H2L2)Cl2(PPh3)(H2O)] (7), [Ru(H2L3)Cl2(PPh3)(H2O)]
4
(8), [Ru(L )Cl2(PPh3)(H2O)] 2H2O (9),
.
5
6
.
[Ru(L )Cl2(PPh3)(H2O)] 3H2O (10), [Ru(L )Cl2(PPh3)(H2O)]
(11), and [Ru(L7)Cl2(PPh3)(H2O)] (12)
RuCl2(PPh3)3 (0.95 g, 1 mmol) was added to the solution of the Schiff
base (1 mmol) in THF (30 mL). The resulting mixture was refluxed under an
inert atmosphere (nitrogen) for 5 hours. After allowing to stand at room
temperature for 1 hour, the obtained solid was filtered, washed with THF, hot
ethanol, diethyl ether, petroleum ether and dried in vacuo. For the synthesis of
(7), (10) and (11), the dark brown solutions obtained were filtered and
concentrated to 1/10th of their initial volume on a rotary evaporator. The
products were precipitated on addition of petroleum ether (60–80 8C). Black
solids were obtained which were thoroughly washed with THF, ethanol,
petroleum ether and diethyl ether (60–80 8C) and finally dried in vacuo.
Synthesis of [Ru(L2)(PPh3)2] (13) and [Ru(L3)(PPh3)2] (14)
RuCl2(PPh3)3 (0.95 g, 1 mmol) was added to the solution of Schiff base
H2L2 or H2L3 (1 mmol) in THF (30 mL), followed by the addition of
triethylamine (0.30 g, 3 mmol). The resulting solution was refluxed under an
inert atmosphere for 3 hours. The resulting solution was filtered and the red or

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Bhowon et al.
green solid was precipitated on addition of petroleum ether (60–80 8C). The
product was filtered, washed with THF, ethanol, petroleum ether (60–80 8C)
and diethyl ether and then dried in vacuo.
Catalytic Oxidation of Alcohols
To a solution of cinnamyl, p-methoxybenzyl or piperonyl alcohol
(1 mmol) in chloroform (10 mL), NMO (0.30 g, 3 mmol) was added followed
by the ruthenium Schiff base complex (1.00–2.00 ꢂ 1022 mmol). The
˚
solution was stirred for 3 hours in the presence of 4 A molecular sieves (0.5 g).
The mixture was filtered and evaporated to dryness and extracted with diethyl
ether (2 ꢂ 25 mL). The combined ether extracts were filtered and evaporated
to give the corresponding aldehydes. Piperonal, cinnamaldehyde and
p-methoxybenzaldehyde were then quantified as their respective 2,4-
dinitrophenylhydrazones.
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Received June 21, 2002
Accepted August 13, 2003
Referee I: D. N. Blauch
Referee II: G. N. Holder

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