91

CHAPTER III

HYDROGENATION OF BENZYLIDENEANILINE AND ITS PARA

SUBSTITUTED DERIVATIES USING POLYMER-SUPPORTED

PALLADIUM IMIDAZOLE AND PALLADIUM

2-METHYLIMIDAZOLE COMPLEX CATALYSTS

In order to elucidate the potential of the synthesized polymer-

supported palladium-complexes as catalysts, hydrogenation of a few Schiff

bases (C=N), olefins and nitro compounds under ambient conditions has

been undertaken. This chapter is focused on catalytic activity of the

supported complexes towards hydrogenation of Schiff bases like

N-benzylideneaniline and few of its para derivatives under ambient

conditions (Scheme 3.1).

Scheme 3.1 Hydrogenation of benzylideneaniline and its derivatives.

3.1 Introduction

Hydrogenation of imines (C=N) to corresponding amines is an

important reaction which is one among the most fundamental chemical

transformations and a convenient way for synthesizing amines which are

useful pharmaceutical and agrochemicals1-5. Purest form of thallium-201

which is used routinely in nuclear medicine for imaging the myocardium

92

was obtained by liquid extraction of its precursor Pb201using N-benzylaniline

the hydrogenation product of benzylideneaniline by Sodd et.al.6.

Khosla et.al., has also reported the extraction of thallium in the presence of

other metal ions using N-benzylaniline7. Heterogeneous catalytic

hydrogenation of C=N using molecular hydrogen is environmentally clean

and commercially viable8. Earlier researchers have reported the

hydrogenation of Schiff bases like benzylideneaniline using transition metal

complexes as catalysts under high pressure and temperature1,9,10. Herein

the author has used the synthesized polymer-supported catalysts for the

hydrogenation of benzylideneaniline and few of its para substituted

derivatives at ambient conditions. The effect of the presence of substituents

on the rate of hydrogenation of benzylideneaniline has also been

investigated11.

Experimental

3.2 Preparation of Schiff bases

Schiff bases chosen as substrates for hydrogenation reactions;

benzylideneaniline and its para substituted derivatives (Table 3.1) were

prepared according to the procedure described in literature12. The general

condensation reaction is as follows. Eqimolar mixture of corresponding

freshly distilled aldehydes and aniline in ethanol (25 mL) was refluxed in a

round-bottom flask for about 3 hours. The products separated as solids on

pouring the reaction mixture into ice cold water. They were filtered and

recrystallized in ethanol. The prepared Schiff bases were characterized by

their melting point and GC-MS.

93

3.3 Polymer-supported palladium-imidazole complex catalyst for

hydrogenation of substituted benzylideneanilines*

3.3.1 Procedure for hydrogenation of Schiff bases

The reactor consisted of a horizontal water-jacketed, three-necked

glass tube of 75 mL volume connected to a water-jacketed hydrogen burette

and monometer. Water was circulated at a desired temperature from a

thermostat through the outer jacket of the reactor and that of the hydrogen

gas burette. In a typical experiment the catalyst was added to 30 mL of

ethanol in the reaction vessel and allowed to saturate in an atmosphere of

hydrogen for an hour. The system was evacuated and again filled with

hydrogen. A known quantity of the substrate was injected into the system

through the septum cap followed by opening the system to the gas burette

while closing the exit stopcock. The contents inside the reactor were agitated

by fixing it to a shaking machine. The reaction was monitored by the rise in

the level of the liquid in the hydrogen burette as a function of time13,14.

At the end of each reaction, the catalyst was separated by filtration and the

products were identified by IR, HPLC and GC-MS techniques.

3.3.2 Hydrogenation reaction of Schiff bases

Polymer-supported palladium-imidazole complex and its homogeneous

analogue Pd(Imz)2Cl2 was tried as catalysts for hydrogenation of

benzylideneaniline (to N-benzylaniline) in ethanol. The homogeneous

complex was unstable and during the course of the reaction metal got

separated from the solution15.

(*Udayakumar V., Gayathri V., et. al., J. Mol. Catal. A: Chem., 317 (2010) 111-117)

94

Table 3.1 Schiff bases

Benzylideneaniline (Bza)

Melting point ;

Found 49°C,

Expected 48°C

HC

N

p-Chlorobenzylideneaniline (p-ClBza)

Melting point;

Found 63°C,

Expected 64°C

HC

N

Cl

p-Methoxybenzylideneaniline (p-MeOBza)

Melting point;

Found 59°C,

Expected 61°C

p-Hydroxybenzylideneaniline (p-OHBza)

Melting point;

Found 191°C,

Expected 190°C

HC

N

OH

p-Nitrobenzylideneaniline (p-NBza)

Melting point;

Found 91°C,

Expected 90°C

HC

N

NO2

Hence the hydrogenation reactions were not carried out using the

homologous analogue. Whereas the polymer-supported palladium-imidazole

catalyst could be successfully used for the reduction of benzylideneaniline

and its derivatives. It was found that during the reaction metal does not

leach out from the polymer support and was found to be intact even after

recycling for six times (Fig 3.1). All the reactions were conducted in ethanol

at 596 mm Hg of hydrogen pressure12,16. Initial rate of hydrogenation of

various Schiff bases are presented in Fig. 3.2.

96

The percentage conversion of various Schiff bases as determined by HPLC

analysis, relatives rates of hydrogenation and TON (Turnover number) are

presented in Table 3.2.

0

2

4

6

8 p-OHBza

p-NBza

p-MeOBza

p-ClBza

Bza

Substrate

Fig. 3.2 Initial rates of hydrogenation of various Schiff bases using

polymer-supported palladium imidazole catalyst.

3.3.3 Kinetics and reaction mechanism

The kinetics of hydrogenation of the benzylideneanilines was studied

by following the hydrogen uptake at 596 mm Hg hydrogen pressure. The

rate of hydrogenation was calculated from the slope after plotting the

volume of hydrogen uptake by the substrate as a function of time. Initial

rates of hydrogenation of various Schiff bases and their TON have been

determined (Table 3.2). A blank reaction was also carried out in the absence

of the catalyst.

98

The results of hydrogenation reaction carried out using different

solvents indicated that the polar solvents are more favorable than non-polar

solvents and ethanol is most suitable among the solvents studied

(Table 3.3).

Table 3.3 Initial rates of hydrogenation of Schiff bases in various

solvents.

Solvent Initial rate x 10-4

(mol dm-3min-1)

Ethanol 8.77

Methanol 8.48

Toluene 5.25

Tetrahydrofuran 1.10

Benzene 2.11

Acetone 3.52

The dependency of the initial rates of hydrogenation on variables like

catalyst, substrate concentration and temperature was studied for all the

substrates.

a) Effect of catalyst concentration

The influence of catalyst concentration on the rate of hydrogenation

was carried out over a range of 14.8 x 10-4 to 14.8 x 10-3 mol dm-3 Pd at

constant substrate concentration of 33.3 x10-3 mol dm-3 at a temperature of

30oC with 596 mm Hg hydrogen pressure using ethanol as solvent12. Initial

rates are tabulated in Table 3.4 and the plot of log(initial rate) against

log[Catalyst] in the range of 14.8 x 10-4 to 74.2 x 10-4 mol dm-3 Pd is

depicted in Fig. 3.3.

99

Table 3.4 Initial rate of hydrogenation of Schiff bases with different

catalyst concentrations, constant [substrate] = 33.3 x 10-3,

596 mm Hg pressure of hydrogen at 303 K in 30 mL of ethanol.

[Catalyst] x10-4 (mol/dm-3 )

Pd

Initial rate x 10-4 (mol dm-3 min-1)

Bza

Initial rate x10-4 (mol dm-3 min-1)

ClBza

Initial rate x10-4 (mol dm-3 min-1)

OHBza

Initial rate x 10-4 (mol dm-3 min-1 )

MeOBza

14.8 3.08 2.40 3.00 5.20

29.6 5.04 4.61 5.00 5.66

44.4 6.01 5.49 5.20 6.10

59.2 8.77 5.53 6.81 6.64

74.2 7.22 9.46 10.60 6.87

-2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1

-3.7

-3.6

-3.5

-3.4

-3.3

-3.2

-3.1

-3.0

p-MeOBza

p-ClBza

Bza

log[catalyst]

Fig. 3.3 Plot of log (Initial rate) against log [catalyst].

= Benzylideneaniline = p-chlorobenzylideneaniline

-methoxy benzylideneaniline

Order of the reaction determined from the slope of the plot showed

that all the substrates followed fractional order kinetics. However the range

of concentration in which the catalyst follows the fractional order on the rate

is not the same for all the substrates. This may be due to the change in

structure of the substrate molecules. As the rate of hydrogenation of

hydroxy derivative is closer to that of chloro derivative the former has not

100

-2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4

-3.7

-3.6

-3.5

-3.4

-3.3

-3.2

-3.1

p-MeOBza

p-OHBza

p-ClBza

Bza

log[substrate]

been included in the plot. Above 74.2 x 10-4 mol dm-3 Pd concentration,

initial rate was found to be independent of catalyst concentration and it

followed zero order kinetics indicating that for the studied substrate 74.2 x

10-4 mol dm-3 Pd is sufficient to hydrogenate and higher concentration of

catalyst may not be required. Hence the rate becomes independent of

catalyst concentration.

b) Effect of substrate concentration

The influence of substrate concentration on the rate of the reaction

was studied in the range of 3.3 x10-3 to 66.6x10-3 mol dm-3 of substrate at

30oC and at 596 mm Hg hydrogen pressure with a constant catalyst

concentration of 59.2 x 10-4 mol dm-3 Pd for all the substrates in ethanol12.

The plot of log(initial rate) against log[substrate] in the range 3.3 x10-3 to

33.3 x10-3 mol dm-3 of substrate is given in Fig. 3.4. The initial rates of

hydrogenation are tabulated in Table 3.5.

Fig. 3.4 Plot of log (Initial rate) against log [substrate].

= Benzylideneaniline p-hydroxybenzylideneaniline

= p-methoxybenzylideneaniline; -chlorobenzylideneaniline

101

Order of the reaction in this concentration range for all the

substrates was found to be fractional. However the range of concentration

where the substrate follows a fractional order on the rate is not the same for

all the substrates. This may be due to change of substituents in substrate

molecules and coordinating ability of the catalyst towards different

derivatives. Initial rate was independent of substrate concentration above

33.3 x 10-3 to 66.6 x 10-3 mol dm-3 and followed zero order17. This could be

attributed to the fact that as the catalyst concentration is kept constant, all

the catalyst would have been used and no catalyst is available for higher

concentration of the substrate.

Table 3.5 Initial rate of hydrogenation of Schiff bases with different

substrate concentrations, constant [Catalyst] = 59.2 x10-4, 596

mm Hg pressure of hydrogen at 303 K in 30 mL of ethanol.

[Substrate]

x 10-3

(mol/dm-3)

Initial rate x 10-4

(mol dm-3 min-1 )

Bza

Initial rate x 10-4

(mol dm-3 min-1 )

ClBza

Initial rate x 10-4

(mol dm-3 min-1 )

OHBza

Initial rate x 10-4

(mol dm-3 min-1 )

MeOBza

3.3 4.21 2.04 2.40 2.21

8.3 5.29 2.71 4.21 2.76

16.6 6.77 3.69 4.98 4.17

25.0 7.82 3.83 6.40 4.86

33.3 8.77 5.53 6.81 6.64

c) Mechanism of hydrogenation of Schiff bases

Based on the results obtained from the kinetics experiments, a

probable mechanism is proposed for the hydrogenation reaction1 (Scheme

3.2). The polymer bound catalyst binds reversibly with the substrate. In the

102

Pd2+

N

C

R

H RI

Pd2+

N

C

R

H RI

K

+

N

C

R

H RI

N

C

R

H RI

Pd4+

H H

Pd2+

H2

k1

Slow

Pd4+

H

N

C

R

H RI

Pd4+

H H

H

N

C

R

H RI

Fast

Pd2+

N

C

R

H

H

H

RI

Pd4+

H

N

C

R

H

H

RI

Fast+

-complex reacts with hydrogen to form a dihydrido complex

which finally gives the amine.

Catalyst Substrate -Complex (intermediate)

-Complex Dihydrido complex

Dihydrido complex -Complex

- Complex Catalyst Amine

Scheme 3.2 Probable mechanism for hydrogenation of Schiff bases.

The rate law derived for the above mechanism is given in equation 1.

Total Total

1

Unreacted Unreacted

Equation 1

Where K is the equilibrium constant for reversible reaction and k1 is the

velocity constant for the rate determining step. This equation explains that

103

at low concentration, the reaction follows fractional order with respect to

catalyst and substrate concentration. But at higher concentration the rate is

independent of catalyst and substrate concentration.

d) Dependency of the rate on the temperature of the reaction

Rate of hydrogenation reactions for all the substrates were studied

in the range of 30 to 45oC with a constant catalyst concentration of

59.2 x 10-4 mol dm-3 Pd, substrate concentration of 33.3 x 10-3 mol dm-3 in

ethanol at 596 mm Hg pressure of hydrogen. The rate of the reaction was

found to be dependent on temperature (T) of the system (Table 3.6). Values

of activation energy calculated from the slope of Arrhenius plot (Fig. 3.5) and

activation entropy of reactions are tabulated in the Table 3.7. Results

revealed that activation energy decreased after substituting -NO2, -OH, -Cl

or -MeO group on to the para position of aldehyde ring of benzylideneaniline.

The lower activation entropy indicates that the substrate molecules are

bonded to the catalyst surface and they have lost all translational degrees of

freedom and probably gained only three vibrational degrees of freedom.

Further, activation of the substrate with the catalyst may be due to

localization of the substrate on the catalyst surface and this localized

interaction may be necessary for the hydrogenation reaction since a double

bond (C=N) involved is in the middle of the substrate molecule.

104

3.2x10-3

3.2x10-3

3.2x10-3

3.3x10-3

3.3x10-3

-3.2

-3.1

-3.0

-2.9

-2.8

p-MeOBza

p-OHBza

p-ClBza

Bza

1/T

Fig. 3.5 Effect of temperature - Arrhenius plot.

Benzylideneaniline p-hydroxy benzylideneaniline

= p-methoxybenzylideneaniline -chloro benzylideneaniline

Table 3.6 Initial rate of hydrogenation of Schiff bases at different temperatures with [catalyst] = 59.2 x 10-4 mol/dm3 Pd, [substrate] = 33.3 x 10-3 mol/dm3 , 596 mm Hg pressure of hydrogen, in 30 mL of ethanol.

Temperature (K)

Initial rate x 10-4 (mol dm-3 min-1)

Bza

Initial rate x 10-4 (mol dm-3 min-1)

ClBza

Initial rate x 10-4 (mol dm-3 min-1)

OHBza

Initial rate x 10-4 (mol dm-3 min-1 )

MeOBza

303 8.77 5.53 6.81 6.64

308 10.70 6.03 8.08 7.21

313 13.20 6.25 9.86 8.14

318 16.41 6.63 10.48 8.12

Table 3.7 Activation energy, Activation entropy values for hydrogenation of benzylideneaniline and its derivatives using polymer-supported palladium-imidazole catalyst.

Substrate Activation energy

(kJ/mol)

Activation entropy

(J/Kmol)

Benzylideneaniline 34 -200

p-Chlorbenzylideneaniline 9 -286

p-Hydroxybenzylideneaniline 24 -233

p-Methoxybenzylideneaniline 12 -276

p-Nitrobenzylideneaniline 31 -212

105

e) Effect of substituents on the rate of hydrogenation

To determine the effect of substitution at the para position of

benzylideneaniline on hydrogenation, few derivatives with -NO2, -Cl, -OH

and -MeO groups were chosen. Earlier studies have shown that the

introduction of substituents at para position of the amine ring of the

benzylideneaniline has very little effect on the hydrogenation rate18. In our

studies it has been observed that the insertion of substituents on to the

para position of aldehyde ring of benzylideneaniline reduced the activation

energy markedly (Scheme 3.1, Table 3.7). When p-nitrobenzylideneniline

was used as the substrate, both C=N and nitro group were hydrogenated

simultaneously (Fig 3.6) and the catalyst does not show selectivity towards

C=N. Hence the effect of nitro group substitution on the rate could not be

determined. Similar observation was made by Arthur Roe et.al.12.

Effect of substituents on the rate of hydrogenation of

benzylideneaniline was studied by varying the para substituents with

different groups like -OH, -MeO, and Cl16. It was observed that the rate of

hydrogenation was slower upon substitution with these groups. When

log(initial rate) was plotted ag para) it

was a straight line (Fig.3.7) indicating that the substituents do not have

significant effect on the rate of the reaction.

107

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-3.26

-3.24

-3.22

-3.20

-3.18

-3.16

c

b

a

-OH

-Cl

-MeO

Substituent constant

Fig. 3.7 Effect of para substituents on the initial rate of hydrogenation

a: p-Hydroxybenzylideneaniline; b: p-Methoxybenzylideneaniline;

c: p- Chlorbenzylideneaniline.

3.3.4 Recycling efficiency of the catalyst

In order to test metal leaching from the catalyst, metal estimation

was carried out at the end of first cycle and at the end of sixth cycle of the

reaction. Percentage of metal in the catalyst remained almost constant even

after six cycles. (At the end of first cycle 9.9% Pd and at the end of sixth

cycle 9.8% Pd). Metal estimation for the reaction mixture was also carried

out to make sure that there is no loss of metal from the catalyst. From this

analysis it is evident that there is no leaching of metal from the catalyst.

Recycling ability of the catalyst was studied by carrying out reactions over

six cycles at a constant catalyst concentration of 59.2x10-4 mol dm-3 Pd,

substrate concentration of 33.3x10-3 mol dm-3 and temperature 30oC with

596 mm Hg of hydrogen pressure. The initial rate remained almost constant

for over six cycles without any loss in efficiency of the catalyst (Table 3.8).

108

Table 3.8 Recycling efficiency of polymer-supported palladium imidazole

complex catalyst.

Initial rate x 10-4

(mol dm-3 min-1 )

Fresh 8.77

First cycle 8.66

Second cycle 8.30

Third cycle 8.30

Fourth cycle 8.30

Fifth cycle 8.24

Sixth cycle 8.20

3.4 Hydrogenation of benzylideneaniline and its derivatives using

polymer-supported palladium 2-methylimidazole complex

catalyst

Hydrogenation of benzylideneaniline and its para-substituted

derivatives using polymer-supported palladium 2-methylimidazole catalyst

(Scheme 3.1) was carried out under similar conditions as followed for

palladium-imidazole catalyst (Section 3.3.1) and kinetic studies were carried

out as described in section 3.3.3. Initial rates of hydrogenation are depicted

in Fig. 3.7 and tabulated in Table 3.9 along with TON and percentage

conversions as determined by HPLC analysis. When homogeneous analogue

of polymer-supported palladium-2-methylimidazole [Pd(meImz)2Cl2]

(meImz = 2-Methyl imidazole) was used as catalyst, metal got separated

from the solution and hence it could not be used as catalyst.

110

Fig. 3.7 Initial rates of hydrogenation of various Schiff bases using

polymer supported palladium-2-methylimidazole catalyst.

3.4.1 Kinetic studies and reaction mechanism

a) Effect of catalyst concentration

The influence of catalyst concentration on the rate of hydrogenation

was carried out over a range 14.8x10-4 to 11.9 x10-3 mol dm-3 Pd at constant

substrate concentration of 33.3 x10-3 mol dm-3 at a temperature of 30oC with

596 mm Hg hydrogen pressure using ethanol as solvent12 (Table 3.10). The

plot of log(initial rate) against log[Catalyst] in the range of 14.8x10-4 to

74.2x10-4 mol dm-3 Pd is depicted in Fig. 3.8. Order of the reaction

determined from the slope of the plot showed that all the substrates followed

fractional order kinetics. Above 74.2x10-4 mol dm-3 Pd concentration, initial

rate was found to be independent of catalyst concentration and it followed

zero order kinetics.

111

Table 3.10 Initial rate of hydrogenation of Schiff bases with different

catalyst concentrations, constant [substrate] = 33.0 x 10-3,

596 mm Hg pressure of hydrogen at 303 K in 30 mL of ethanol.

[Catalyst] x10-4

(mol/dm3 ) Pd

Initial rate x 10-4

(mol dm-3 min-1)

Bza

Initial rate x 10-4

(mol dm-3 min-1)

ClBza

Initial rate x 10-4

(mol dm-3 min-1)

OHBza

Initial rate x 10-4

(mol dm-3 min-1 )

MeOBza

14.8 5.68 2.31 4.63 4.10

29.6 7.17 3.43 5.38 4.73

44.4 8.31 4.12 5.92 5.15

59.2 9.46 4.84 6.31 5.68

74.2 9.99 5.17 7.04 6.16

-2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1

-3.65

-3.60

-3.55

-3.50

-3.45

-3.40

-3.35

-3.30

-3.25

-3.20

-3.15

-3.10

-3.05

-3.00

Hydroxy

chloro

Methoxy

Bza

log[Catalyst]

Fig. 3.8 Plot of log (Initial rate) against [catalyst].

Benzylideneaniline = p-Chloro benzylideneaniline

= p-Methoxy benzylideneaniline - hydroxy benzylideneaniline

112

b) Effect of substrate concentration

The influence of substrate concentration on the rate of the reaction

was studied in the range of 3.3x10-3 to 66.7x10-3 mol dm-3 of substrate at

30°C and 596 mm Hg hydrogen pressure with a constant catalyst

concentration of 59.2x10-4 mol dm-3 Pd for all the substrates in ethanol12.

The plot of log(initial rate) against log[substrate] in the range 3.3 x 10-3 to

33.3 x 10-3 mol dm-3 is given in Fig. 3.9 and the initial rates of

hydrogenation is tabulated in Table 3.11. Order of the reaction in this

concentration range for all the substrates was found to be fractional. Above

33.3 x 10-3 to 66.7 x 10-3 mol dm-3 initial rate was independent of substrate

concentration and followed zero order13. This could be attributed to the fact

that as the catalyst concentration is kept constant, all the catalyst would

have been used and no catalyst is available for higher concentration of the

substrate.

c) Mechanism of hydrogenation of Schiff bases

The mechanism of hydrogenation of Schiff bases followed by polymer-

supported 2-methylimidazole catalyst and rate equation proposed based on

the kinetic experimental results was observed to be similar as described in

Scheme 3.2 and equation 1.

Total Total

1

Unreacted Unreacted

Equation 1

113

-2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4

-3.75

-3.70

-3.65

-3.60

-3.55

-3.50

-3.45

-3.40

-3.35

-3.30

-3.25

-3.20

-3.15

-3.10

-3.05

log[Substrate]

Table 3.11 Initial rate of hydrogenation of Schiff bases with different

substrate concentrations, constant [Catalyst] = 59.2 x10-4,

596 mm Hg pressure of hydrogen at 303 K in 30 mL of ethanol.

[Substrate] x 10-3

(mol/dm3)

Initial rate x 10-4

(mol dm-3 min-1 )

Bza

Initial rate x 10-4

(mol dm-3 min-1 )

ClBza

Initial rate x 10-4

(mol dm-3 min-1 )

OHBza

Initial rate x 10-4

(mol dm-3 min-1 )

MeOBza

3.3 3.58 1.89 2.63 3.05

8.3 4.42 2.94 3.68 3.68

16.6 5.78 4.21 4.73 4.42

25.0 6.83 4.73 5.68 5.05

33.3 9.46 4.84 6.31 5.68

Fig. 3.9 Plot of log (Initial rate) against [Substrate].

Benzylideneaniline = p-Chlorobenzylideneaniline

= p-Methoxy benzylideneaniline = p-Hydroxybenzylideneaniline

114

d) Dependency of the rate on temperature of the reaction

Rate of hydrogenation for all the substrates were studied in the

range of 30 to 50°C with a constant catalyst concentration of 59.2 x 10-4 mol

dm-3 Pd, substrate concentration of 33.3 x 10-3 mol dm-3 in ethanol at 596

mm Hg pressure of hydrogen. The rate of the reaction was found to be

dependent on temperature (T) of the system (Table 3.12). Values of

activation energy calculated from the slope of Arrhenius plot (Fig. 3.10) and

activation entropy of reactions are tabulated in the Table 3.13. Results

revealed that activation energy decreased after substituting -OH, -Cl or

-MeO group on to the para position of aldehyde ring of benzylideneaniline.

The lower activation entropy indicates that the substrate molecules are

bonded to the catalyst surface and they have lost all translational degrees of

freedom and probably gained only three vibrational degrees of freedom.

Further, activation of the substrate with the catalyst may be due to

localization of the substrate on the catalyst surface and this localized

interaction may be necessary for the hydrogenation reaction since a double

bond (C=N) involved is in the middle of the substrate molecule.

115

Table 3.12 Initial rate of hydrogenation of Schiff bases at different

temperatures with constant [catalyst] = 59.2 x 10-4 mol/dm3

Pd, [substrate] = 33.3 x 10-3 mol/dm3, 596 mm Hg pressure of

hydrogen, in 30 mL of ethanol.

Temperature

(K)

Initial rate x 10-4

(mol dm-3 min-1)

Bza

Initial rate x 10-4

(mol dm-3 min-1)

OHBza

Initial rate x 10-4

(mol dm-3 min-1 )

MeOBza

303 9.46 6.31 5.68

308 11.6 8.17 7.76

313 13.2 10.1 9.36

318 15.0 13.4 11.3

323 16.8 15.2 13.2

3.1x10-3

3.2x10-3

3.2x10-3

3.3x10-3

3.3x10-3

-3.30

-3.25

-3.20

-3.15

-3.10

-3.05

-3.00

-2.95

-2.90

-2.85

-2.80

-2.75

1/T

Fig. 3.10 Effect of temperature - Arrhenius plot. log (Initial rate) ><1/T.

= p-Methoxybenzylideneaniline;

p-Hydroxybenzylideneaniline. Benzylideneaniline.

116

Table 3.13 Activation energy, activation entropy for hydrogenation of

benzylideneaniline and its derivatives using

polymer-supported palladium-2-methylimidazole catalyst.

Substrate Activation energy

(kJ/mol)

Activation entropy (J/Kmol)

Benzylideneaniline 21 -275

p-Hydroxybenzylideneaniline 31 -246

p-Methoxybenzylideneaniline 33 -236

p-Chlorobenzylideneaniline 15 -298

e) Effect of substituents on the rate of hydrogenation of

benzylideneaniline

Effect of substituents on the rate of hydrogenation of

benzylideneaniline was studied by varying the para substituents with

different groups like OH, -MeO, and Cl. Results revealed that the rate of

hydrogenation decreased upon substitution with these groups due to steric

crowding around C=N. The plot of log(Initial rate) against Hammet

para) was a straight line (Fig.3.11). When

p-nitrobenzylideneaniline was used as substrate, both C=N and nitro group

were hydrogenated simultaneously indicating that the catalyst was not

selective towards C=N.

117

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-3.32

-3.30

-3.28

-3.26

-3.24

-3.22

-3.20

C

Cl

-MeO

b

a

-OH

Substituent constant

Fig. 3.11 Effect of para substituents on the initial rate of hydrogenation

Plot of log (Initial rate) >< Hammett nstant.

a: p-Hydroxybenzylideneaniline , b:p-Methoxybenzylideneaniline

c: p- Chlorbenzylideneaniline

3.4.2 Recycling studies

Recycling studies carried out using benzylideneaniline as substrate at

a constant catalyst concentration of 59.2x10-4 mol dm-3 Pd,

benzylideneaniline concentration of 33.3x10-3 mol dm-3 and temperature

30°C with 596 mm Hg of hydrogen pressure revealed that the catalyst have

excellent recycling efficiency over six cycles without leaching of metal from

the polymer-support.

118

Table 3.8 Recycling efficiency of the catalyst

Initial rate x 10-4

(mol dm-3 min-1)

Fresh 9.46

First cycle 9.40

Second cycle 9.32

Third cycle 9.30

Fourth cycle 9.28

Fifth cycle 9.28

Sixth cycle 9.10

3.5 Conclusion

Reactivity of the two synthesized catalysts has been evaluated by

comparing the initial rates of hydrogenation of Schiff bases under identical

conditions. Both catalysts could be used for hydrogenation of C=N group at

ambient temperature and pressure to synthesize corresponding amines. In

general polymer-supported imidazole catalyst showed better catalytic

activity than polymer-supported 2-methylimidazole catalyst for

hydrogenation of substituted benzylideneanilines (Table 3.9). Percentage

conversion, activation energy was relatively higher in case of palladium-

imidazole catalyst when compared to palladium-2-methylimidazole catalyst.

Substrates followed the same reaction mechanism (Scheme 3.2) and similar

rate equation (Equation 1) for both the catalysts. Both the catalysts have an

excellent recycling efficiency over six cycles without leaching of metal from

the polymer-support.

120

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