A convenient approach to the synthesis of different types of schiff's bases and their metal complexes

Article abstract of DOI:10.1246/cl.2010.1078

Various metal complexes derived from different Schiff bases are conveniently prepared using solvent-free condensation reactions in one-pot synthesis, resulting in significantly enhanced yields. Initial calorimetric studies, and/or TGA of reaction mixtures, led to better optimization of reaction conditions. The ligands like imines, hydrazones, semicarbazones, and nitrones are also synthesized by this method; thus this process is general and expedient.

Full text of DOI:10.1246/cl.2010.1078

doi:10.1246/cl.2010.1078
1078
Published on the web September 4, 2010
A Convenient Approach to the Synthesis of Different Types
of Schiff’s Bases and Their Metal Complexes
Kiran Pradhan,1 Kaliaperumal Selvaraj,2 and Ashis K. Nanda*1
1Department of Chemistry, University of North Bengal, Darjeeling-734 013, West Bengal, India
2Heterogeneous Catalysis Group, National Chemical Laboratory, Pune-411008, India
(Received June 7, 2010; CL-100529)
Table 1. Comparison of reaction time and percentage yield of metal
complexes synthesized in solvent (a) and under solvent-free condition (b)
Various metal complexes derived from different Schiff bases are
conveniently prepared using solvent-free condensation reactions in
one-pot synthesis, resulting in significantly enhanced yields. Initial
calorimetric studies, and/or TGA of reaction mixtures, led to better
optimization of reaction conditions. The ligands like imines,
hydrazones, semicarbazones, and nitrones are also synthesized by
this method; thus this process is general and expedient.
Reaction time
Entry
Metal complex
Reported yield
/%
(a)
(b)
1
1 h7
20 min
70−90
N
O
N
O
M
COOCH3
O
One-pot multicomponent syntheses have received increas-
ing attention of late, because they not only address fundamental
principles of synthetic efficiency and reaction design, but they
also expand the possibilities for extending the reactions into
combinatorial and solid-phase methods.1 The time has now
come for ecological factors to be considered in the development
of synthetic procedures and for them to play an important role in
the assessment of the quality of any new synthesis. One simple
approach to these issues is to perform reactions in the absence of
solvents.2-6 Thus a one-pot solvent-free condition for the
preparation of different Schiff’s bases and their metal complexes
offers the distinct advantages of reduced reaction time and easier
procedures for workup.
O
M
O
2
3
1 h8
20 min
30 min
75
90
N
R
H
N
M
Cl
Cl
6 h9
N
O
R
O
N
M
N
4
5
4 h10
30 min
30 min
50−60
60−70
O
R
HO
Cl
N
2 h10
Ni Cl
Our approach has been to evaluate one-pot multicomponent
synthesis under solvent-free conditions via a study of well-
established metal complexation reactions of various Schiff bases
like salphen, imines, oximes, semicarbazones, hydrazones, and
nitrones. Salphen metal complexes have been widely used in
applications for everything from catalysts to molecular recog-
nition7 because of their versatility, and amino acid Schiff base
metal complexes are involved in a variety of biological
processes such as catalysis of transamination, racemization,
and carboxylation reactions.8 The synthesis and reactivity of
some of these complexes, like those of nitrones,9 have been
playing an important part in the development of coordination
chemistry. Typically the synthesis employs the use of organic
solvents such as toluene or methanol, refluxing for over an hour,
followed by extensive recrystallization and/or chromatography.
Moderate yields are recorded in such processes.10
N
OH
H2N
H2N
CHO
OH
N
N
O
Grind
MCl2
M
Heat at 70-80°C
O
M= Mn, Fe, Co,
Ni, Cu, Zn
M+ for M=Mn(III) &Fe(III)
Scheme 1. An efficient one-pot synthesis of metal salphen complexes
under solvent-free conditions.
pestle and mortar over a period of ca. 10 min. Heating the
reaction mixture in an oil bath for further 20 min is followed by
formation of the product as a colored solid.
We have found that these complexes can be synthesized via
a three component reaction sequence without using solvents. In
this method, salphen and other Schiff’s base derived metal
complexes have been synthesized in significantly higher yields
in a shorter time (Table 1). The same complexes were also
prepared via solution phase, and comparisons have been listed in
the table.
Synthetic details of salphen complexes are summarized in
Scheme 1. In a typical experiment, the metal imine Schiff base
complex is formed in near quantitative yield by grinding one
molar equivalent each of the diamine and the metal acetate or
chloride and two molar equivalents of the aldehyde using a
Initially for optimization of reaction conditions, thermal
gravimetric analysis (TGA) of a three component reaction
mixture involving 1,2-phenylenediamine, salicylaldehyde, and a
metal ion leading to the salphen metal complex was done. The
thermal analysis was particularly useful in determining the
minimum temperature at which the condensation reaction
occurred as the onset of the reaction is found to be accompanied
by the loss of molecule(s) of water. Thus the optimized time for
the reaction is evaluated. This practice dramatically shortened
the reaction time, and it is observed that the yield is almost
quantitative. Moreover, the TGA13 shows initial formation of
ligand precursors followed by metal complexation, indicating
Chem. Lett. 2010, 39, 1078-1079
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1079
R
O
CHO
OH
N
M
N
Grind
NH2-R
MCl2
O
R
Heat at 80-90°C
R= -NHC6H5,
-CONHNH2
M=Ni, Co, Cu
Scheme 2. An efficient one-pot synthesis of metal Schiff base
complexes under solvent-free conditions.
H
Grind
N
M
MCl2
NHOH
Cl
Cl
N
Heat at 80-90°C
N
CHO
O
(a)
O
COOCH3
O
Figure 1. DSC data of nitrone formation in a reaction mixture.
O
R
Grind
M
NH2CH2COOH
M(CH3COO)2
M = Zn, Cu
O
N
Heat at 80-90°C
OH
200
R
R = H, CH3
Solvent
(b)
Solvent-free
150
100
50
Scheme 3. An efficient one-pot solvent-free synthesis of metal com-
plexes of (a) nitrones and (b) hydroxybenzylideneglycine.
apparently that metal ions do not have any significant effect in
the reaction system in solid phase. To further expand the scope,
the new method stated herein was also applied to the synthesis of
a number of other Schiff base metal complexes, details of which
are summarized in Scheme 2. As a general approach one molar
equivalent of the metal salt, two molar equivalents each of
sodium acetate and the corresponding hydrochloride salt of
hydroxylamine, hydrazine, or the semicarbazide were mixed
together with a mortar and pestle. Aldehyde was then added to
the reaction mixture.
Since only a few complexes of metals with simple nitrones
have been reported, the versatility of this solventless method was
also demonstrated by the facile formation of metal nitrone
complexes and metal complexes derived of hydroxybenzyli-
deneglycine, the synthetic plan of which are summarized in
Scheme 3.
In a separate experiment, the ligand precursors of all the
metal complexes were also synthesized in a convenient way
under solvent-free conditions in lesser time, and quantitative
yield is observed. For optimization of reaction conditions, DSC
of a reaction mixture leading to the formation of a nitrone ligand
precursor was done as shown in Figure 1.
Overall, this approach results in a dramatic improvement in
yield in addition to the reduction in reaction time as compared to
conventional methods involving organic solvents. The results of
conductivity measurements carried in DMSO show that all the
metal complexes are nonelectrolytes.11 A typical cyclic voltam-
mogram of Co(II)-salphen in DMSO was also obtained. The
voltammetric characteristics of Co(II)-salphen and the magnetic
moments of the metal complexes are found to be in good
agreement with the literature data.7,12 All the compounds are
characterized13 using IR, MS, and NMR data. The powder X-ray
diffraction data of a few representative metal complexes have
been done. A perfect match in the XRD data of the compounds
are observed with that synthesized in solvent as shown in
Figure 2.
0
0
10
20
30
/°
40
50
60
2
θ
Figure 2. Comparison of Zn(II)-salphen synthesized in solvent and
under solvent-free condition using powder X-ray diffraction.
useful in mimicking reactions done using high intensity grind-
ings in ball mills and will score over other methods for its
simplicity and easy reaction setup.
References and Notes
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13 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
We envisage the application of this strategy could be
extended to prepare a wide range of Schiff base and related
metal complexes. The method is partly mechanochemical and is
Chem. Lett. 2010, 39, 1078-1079
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/