Kinetics and mechanism of the ligand exchange reaction between tetradentate schiff base N,N'-ethylen-bis (salicylaldimine) and Ni(N,N'-propylen-bis(salicylaldimine))

Article abstract of DOI:10.17159/0379-4350/2015/v68a4

Visible spectrophotometry is used to study the kinetics of ligand exchange in the system Ni(salpn)/H2salen with or without triethylamine (NEt3) and H2O in dimethylformamide (DMF) solvent at 25 ± 0.1 °C and 0.01MNaNO3 [H2salen and H2salpn are N,N'-ethylen-bis(salicylaldimine) and N,N'-propylen-bis(salicylaldimine), respectively]. It was found that the reaction rate is of the first-order with respect to Ni(salpn). In addition, the effect of NEt3 andH2Oon the rate of the reaction was examined. The rate of the ligand exchange reaction was accelerated by adding NEt3 to the reaction mixture. However, the ligand exchange rate was not changed by adding H2O to the mixture reaction. The effects of NEt3 and H2O on the ligand exchange rate show that deprotonation/protonation of the H2salen ligand and anionic form of H2salen is important. On the basis of these results, the reaction mechanism is discussed.

Full text of DOI:10.17159/0379-4350/2015/v68a4

RESEARCH ARTICLE
R. Vafazadeh and M. Bagheri,
S. Afr. J. Chem., 2015, 68, 21–26,
<http://journals.sabinet.co.za/sajchem/>.
21
Kinetics and Mechanism of the Ligand Exchange Reaction
Between Tetradentate Schiff Base N,N’-ethylen-bis
(salicylaldimine) and Ni(N,N’-propylen-bis(salicylaldimine))
Rasoul Vafazadeh* and Maryam Bagheri
Department of Chemistry, Yazd University, Yazd, Iran.
Received 14 October 2014, revised 17 December 2014, accepted 18 December 2014.
ABSTRACT
Visible spectrophotometry is used to study the kinetics of ligand exchange in the system Ni(salpn)/H2salen with or without
triethylamine (NEt3) and H2O in dimethylformamide (DMF) solvent at 25 0.1 °C and 0.01 M NaNO3 [H2salen and H2salpn are
N,N’-ethylen-bis(salicylaldimine) and N,N’-propylen-bis(salicylaldimine), respectively]. It was found that the reaction rate
is of the first-order with respect to Ni(salpn). In addition, the effect of NEt3 and H2O on the rate of the reaction was examined. The
rate of the ligand exchange reaction was accelerated by adding NEt3 to the reaction mixture. However, the ligand exchange rate
was not changed by adding H2O to the mixture reaction. The effects of NEt3 and H2O on the ligand exchange rate show that
deprotonation/protonation of the H2salen ligand and anionic form of H2salen is important. On the basis of these results, the
reaction mechanism is discussed.
KEYWORDS
Kinetic, mechanism, Schiff base, Ni complexes, ligand exchange.
1. Introduction
the kinetics of ligand exchange, we carried out a kinetic study for
Ligand substitution reaction on planar four-coordinate metal the ligand exchange reaction salpn with the Ni(salen) complex,
complexes with central metal ions of d8 electronic configuration, Ni(N,N’-propylen-bis(salicylaldimine)), reaction 4).
such as Ni(II), have been generally studied using replaceable
Cu(salpn) + H2salen ® Cu(salen) + H2salpn
Cu(salbn) + H2salen ® Cu(salen) + H2salbn
Cu(salbn) + H2salpn ® Cu(salpn) + H2salbn
Ni(salpn) + H2salen ® Ni(salen) + H2salpn
(1)
(2)
(3)
(4)
monodentate ligands.1,2 The results of these studies have almost
universally suggested an associative mechanism with a five-
coordinate intermediate.1,3
Polydentate ligand exchange reactions are very interesting
from a mechanistic point of view due to their importance in
coordination chemistry.4,5 However, the kinetics of polydentate
ligands exchange reactions have not been studied in the same
detail as monodentate ligands. In general, such systems proceed
through intermediates, where the incoming polydentate ligand
is partially coordinated and the leaving ligand is partially disso-
ciated.5–7 The driving force for ligand exchange reactions in
metal-complexes with polydentate ligands is the formation of a
more stable complex.8–10
The structures of the Ni(II) and Cu(II) complexes of multiden-
tate ligands such as salen Schiff base ligands and its derivatives
(Scheme 1) have been studied by various groups.11–17 These stud-
ies have indicated that the increase in the number of CH2 groups
allows sufficient flexibility to the structure of complexes by
changing their structure from a planar to distorted tetrahedral
form or by forming five- or six-coordinate species in the presence
of additional donors.17–21 In addition, the metal complex stability
decreases as the alkyl chain length (number of CH2 groups)
increases.22,23
2. Experimental
2.1. Materials
The Schiff bases H2salen and H2salpn were prepared from
a general method,9,10 namely the condensation reaction between
two equivalents of salicylaldehyde and one equivalent of the
appropriate diamine (1,2-ethylendiamine and 1,3-propan-
diamine), in ethanol. The yellow products were obtained in
yields typically 70 % or better. Purity of products was verified by
comparing with literature melting points (m.p.), 124 and 53 °C
for H2salen and H2salpn, respectively.19,20,23
H2salen: Yield = 80 %, IR (cm–1): n(O-H) 3000–3500, n(C=N)
1633, n(C–O) 1281, n(C=C) 1578. Electronic spectra in DMF sol-
vent: lmax (log e, M–1 cm–1) = 403 (2.23), 316 (3.49), 256 (4.20) nm.
H2salpn: Yield = 70 %, IR (cm–1): n(O-H) 3000–3500, n(C=N)
1629, n(C–O) 1276, n(C=C) 1580. Electronic spectra in DMF
solvent:lmax (log e, M–1 cm–1)=404(2.36), 313(4.02), 253(4.37)nm.
The nickel complexes were prepared by a general method,
using the reaction solution of nickel(II) acetate with the Schiff
base ligand (1:1 molar ratio). 24,25
Ni(salen): Red needle-shaped crystals, m.p. >250, Yield =
61 %, IR (cm–1): n(C=N) 1619, n(C–O) 1345, n(C=C) 1530. Elec-
tronic spectra in DMF solvent: lmax (log e, M–1 cm–1) = 540 (2.18),
345 (3.98), 270 (4.79) nm. Found: C, 59.13; H, 4.34; N, 8.62 %. Calc.
for C16H14N2NiO2 (324.99); C, 58.99; H, 4.27; N, 8.75 %.
In our previous work, we reported the kinetics of ligand
exchange in the systems Cu(salpn)/H2salen, Cu(salbn)/H2salen
and Cu(salbn)/H2salpn, where H2salen, H2salpn and H2salbn are
N,N’-ethylen-bis(salicylaldimine), N,N’-propylen-bis(salicyl-
aldimine) and N,N’-butlen-bis(salicylaldimine), respectively
(reactions 1–3).9,10 In order to investigate the effect of the metal on
* To whom correspondence should be addressed. E-mail: rvafazadeh@yazd.ac.ir /
rvafazadeh@gmail.com
ISSN ISSN 0379-4350 Online / ©2015 South African Chemical Institute / http://saci.co.za/journal
DOI: http://dx.doi.org/10.17159 /0379-4350/2015/v68a4

RESEARCH ARTICLE
R. Vafazadeh and M. Bagheri,
S. Afr. J. Chem., 2015, 68, 21–26,
<http://journals.sabinet.co.za/sajchem/>.
22
Scheme 1
A structural representation of the Schiff base ligands and complexes.
Ni(salpn): Brown needle-shaped crystals, m.p. >250, Yield =
70 %, IR (cm–1): n(C=N) 1606, n(C–O) 1355,n(C=C) 1540. Elec-
tronic spectra in DMF solvent: lmax (log e, M–1 cm–1) = 650 (1.86),
350 (3.81), 275 (4.81) nm. Found: C, 60.01; H, 4.66; N, 8.38 %. Calc.
for C17H16N2NiO2 (339.02); C, 60.23; H, 4.76; N, 8.26 %.
2.2. Kinetics Measurements
To measure the ligand exchange reaction rate, the absorbance
changes of reaction mixtures were recorded as a function of time
using a GBC UV-Visible Cintra 101 spectrophotometer. The
greatest change in molar absorbance between reactants
and products was observed at 530 nm. Therefore, all kinetic
measurements of absorbance versus time were made at this
wavelength. Reaction mixtures were made in dimethylfor-
mamide, DMF, an aprotic polar solvent, ionic strength 0.1 M
(adjusted with NaNO3) and the Ni(salpn) complex concentra-
tion was held constant at 5.0 × 10–3 M, while concentration
of H2salen ligand (with and without triethylamine, NEt3 and/or
H2O) was varied from 5.0 × 10–3 to 1.0 × 10–1 M at 25 0.1 °C.
Under these conditions, pseudo-first-order kinetic behaviour
was observed for reaction 4.
Figure 1 Visible spectra of Ni(salpn) (5.0 × 10–3 M) and Ni(salen) (5.0 ×
10–3 M) complexes in DMF (1 cm quartz cell).
To initiate the reaction, equal volumes of Ni(salpn) and H2salen adding H2salen ligand. The spectrum of Ni(salen) in DMF
were mixed and the absorbance of reaction product at 530 nm (Fig. 1), was similar to the last spectrum shown in Fig. 2. This
was followed as a function of time. At least three runs at each similarity confirms the conversion of the Ni(salpn) to the
concentration were recorded. The absorbance A versus time t Ni(salen) complex (reaction 4).
data were computer-fitted with the sigmaplot 12.0 software
The rate constants for the ligand exchange reaction were
using Equation 5 (irreversible first-order reaction), 6 (biphasic obtained by measuring the absorbance changes at 530 nm under
reaction), and 7 (triphasic reaction) to find the best fit and kobs:
pseudo-first-order conditions, [H2salen] >> [Ni(salpn)] (Figs. 3
and 4). The absorbance at 530 nm was found to increase with
time, due to formation of Ni(salen) complex (Fig. 3). Under these
conditions, the best fit for the absorbance versus time data was
obtained utilizing Equation 6 (biphasic reaction). Therefore, two
rate constants kobs(1) and kobs(2) were obtained. The plots of the
two observed constants, kobs vs. [H2salen], are shown in Fig. 4.
The figure indicates that the first reaction step was fast
and dependent on the [H2salen], while the second reaction step
was slow and independent of the concentration of H2salen
ligand. These results can be described by Equations 8 and 9.
At = a1exp[-kobs(1)t] + A¥
At = a1exp[-kobs(1)t] + a2exp[-kobs(2)t] + A¥
(5)
(6)
At = a1exp[-kobs(1)t] + a2exp[-kobs(2)t] +
a3exp[-kobs(3)t] + A¥
(7)
where a1 and a2 comprise rate constants and molar absorptivities,
respectively:1
εB[A]0k1 εC[A]0k2
a1 = εA [A]0 +
+
k2 − k1
k1 − k2
kobs(1) = k1[H2salen]
kobs(2) = k2
(8)
(9)
k1[A]0(εB − εC)
k1 − k2
a2
=
3. Results and Discussion
The k1 (second-order rate constant) value was obtained from
the slope of line kobs(1) vs. [H2salen], k1 = 5.80 0.28 × 10–1 M–1 s–1
and k2 = 2.56
0.89 × 10–3 s–1 was determined as the mean
3.1. Kinetics study
The visible absorption spectra of Ni(salen) and Ni(salpn)
complexes in DMF solvent show a maximum absorption due to
d-d transition at 540 and 600 nm, respectively (Fig. 1). The UV-Vis
absorption spectra recorded after mixing Ni(salpn) and H2salen
in DMF (Fig. 2) shows a consecutive series of spectra recorded
in DMF for the Ni(salpn)/H2salen system, indicating that the
Ni(salpn) complex is converted to the Ni(salen) complex by
of the experimentally obtained data for kobs(2) (Fig. 4), which was
independent of [H2salen]. The rate constant for the second step
of the ligand exchange reaction of the Ni(salpn)/H2salen system
is similar to the analogous reactions of the Cu(salpn)/ H2salen
system (3.11 1.06 × 10–3 s–1).9
The kinetic study of the ligand exchange reaction was investi-
gated in the presence of H2O (protic solvent) and/or NEt3.

RESEARCH ARTICLE
R. Vafazadeh and M. Bagheri,
S. Afr. J. Chem., 2015, 68, 21–26,
<http://journals.sabinet.co.za/sajchem/>.
23
Figure 2 Spectral changes recorded in DMF solvent for the reaction of Ni(salpn) (5.0 × 10–3 M)/H2salen (5.0 × 10–3 M) system within 20 min; (A): visible
region; (B): UV region (1 cm quartz cell).
Figure 3 Plot of absorbance vs. time for reaction of Ni(salpn) (5.0 ×
10–3 M), (—): H2salen (5.0 × 10–3 M) and (----): H2salen (5.0 × 10–2 M).
Figure 4 Plot of the rate constants kobs(1), p and kobs(2), vs.[H salen]
¡
2
([Ni(salpn)] = 5.0 × 10–3 M, I = 0.1 M NaNO3 and 25 0.1 °C).
3.2. Effect of Triethylamine
The rate of the ligand exchange reaction was observed to be
accelerated by adding NEt3 to the reaction mixture. Consequently,
the effect of [NEt3] on the rate of the reaction was examined. The
absorbance at 530 nm was found to increase with time, due to
formation of Ni(salpn) complex (Fig. 3). The rate of the ligand
exchange reaction at different NEt3 concentrations indicated
that the rate of reaction strongly depends on [NEt3]. The plots
of k1 and k2 values vs. [NEt3] are shown in Fig. 5. As shown in
Fig. 5, the value of k1 initially increased by increasing the NEt3
concentration, while the k2 value was independent of the con-
centration of NEt3 amine. However, as shown in Fig. 5, for the
plot of k1 vs. [NEt3] there is an obvious break between the
0.1–0.3 M concentrations of NEt3 – this could be related to a
change of the reaction species at these concentrations of NEt3.
This effect on the reaction rate is also found in the salen-type
copper(II) complexes.9,10
3.3. Effect of H2O
Like the analogous reaction of copper(II) complexes,9,10 the
ligand exchange rate of H2salen for salpn in Ni(salpn) complex
does not significantly change with the addition of H2O to the
mixture reaction. The values of k1 and k2 in the presence and the
Figure 5 Plots of kobs vs. [NEt3] for ligand exchange reaction [Ni(salpn)] =
5.0 × 10–3 M and [H2salen] = 7.5 × 10–2 M, kobs(1), M–1 s–1 l and kobs(2),
s–1 n.

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R. Vafazadeh and M. Bagheri,
S. Afr. J. Chem., 2015, 68, 21–26,
<http://journals.sabinet.co.za/sajchem/>.
24
Table 1 Rate constants data for the reaction of Ni(salpn) with H2salen ligand in the absence and in the presence of NEt3 and H2Oa. (k1 = second-order
rate constant).
[NEt3] (M)
[H2O] (M)
0
0
0
0.05
0.1
0
0.1
0.4
0.2
0
0.2
0.4
0.2
0.05
k1×10 (M–1 s–1) 5.80 0.28
5.90 0.27
7.07 0.34
5.88 0.30
6.04 0.22
5.81 0.18
6.13 0.39
aSolvent DMF, [Ni(salpn)] = 5.0 × 10–3 M, [H2salen] = 5.0 × 10–3 – 1.0 × 10–1 M at 25 0.1 °C and I = 0.1 M NaNO3
kobs(1)
absence of H2O were of the same order (Table 1). This result al-
Ni(salpn) + H2salen
[Ni(salpn)]·H2salen
[Ni(salen)] + H2salen
(10)
(11)
lowed us to propose that reactive species were similar in both the
kobs(2)
[Ni(salpn)] H2salen
reaction conditions (vide infra).
It is clear from Equation 10 that rate constant kobs(1) should
increase linearly with [H2salen], according to Equation 8. The
formation of the adduct [Ni(salpn] H2salen in Equation 10 is in
3.4. Reaction in Presence of H2O and NEt3
.
The rate of the exchange reaction Ni(salpn) with H2salen in
presence of NEt3 and H2O was studied for three different condi-
tions: 1) [NEt3] < 0.1 M (before break in plot in Fig. 5); 2) [NEt3] >
0.3 M (after break in plot in Fig. 4); 3) 0.1 < [NEt3] < 0.3 M (break
limits in plot in Fig. 5). In the first and second condition, the
ligand exchange reaction rate was considerably decreased
by adding H2O (0.4 M). In the last condition, 0.1 < [NEt3] < 0.3 M,
the rate of reaction was decreased by adding H2O (0.4 M) to the
reaction mixture (Table 1). In spite of this, the reaction rate in this
condition (0.1 < [NEt3] < 0.3 M) was increased by adding of H2O
up to 0.05 M. The effects of NEt3 and H2O show the importance
of protonation/deprotonation on the rate of the ligand exchange
reaction (see proposed mechanism).
agreement with a number of studies on the polydentate ligand
exchange reaction.5–7 The lone pairs electron on the phenolic
oxygen atoms of H2salen provide a reasonable basis to propose
.
the formation of adduct [Ni(salpn] H2salen as the first step in
the reaction.
The second reaction step is first-order and independent
of [H2salen] (Equation 11) with rate constant k2. As indicated in
Scheme 2, substitution of salpn from the Ni(II) complexes
involves initial coordination of phenolic oxygen atoms of
H2salen to the nickel centre in the Ni(salpn) complex followed by
protons-transfer from H2salen to salpn, with bond cleavage
of two-ends of salpn. The ligand exchange reaction is completed
by substituting salpn with salen.
As shown in Fig. 5, the reaction rate increases by adding NEt3.
The effect of NEt3 could be due to its interaction with either
Ni(salpn) or with Schiff base ligand. The UV-Vis spectrum of the
Ni(salpn) complex in DMF solvent does not change with the
addition of NEt3 to the solvent, therefore, the formation of the
adduct Ni(salpn) and NEt3 is not observed. In our previous
study,9,10 we have explained that interaction between H2salen
and NEt3 could lead to the formation of Hsalen– and salen2– ions
(dependent on [NEt3]) by the deprotonation phenolic group(s)
of H2salen (reactions 12 and 13).
3.5. Proposed Mechanism
The ligand exchange reaction Ni(salpn) with H2salen was
found to be a two-step process (biphasic reaction) as described
by the experimental rate constants kobs(1) and kobs(2). The kobs(1),
is depended on the concentration of the H2salen ligand, accord-
ing to Equation 8. The kobs(2) is H2salen concentration independ-
ent (Fig. 4).
A similar effect on reaction rate was also found for the
salen-type ligands copper(II) complexes, suggesting a common
mechanism for the ligand exchange reactions (vide infra).9,10
As observed from the experimental data, the ligand exchange
rate in the absence of NEt3 did not change with addition of H2O
(protic solvent) to the reaction mixture, it is reasonable to assume
that Hsalen– and salen– ion concentrations are negligible, so that
H2salen can be treated as the major reactive species. The acidity
of H2salen and its family of ligands enable us to assume that
under these reaction conditions neutral H2salen ligand was the
original species.26
+
H2salen + NEt3 ® Hsalen– + HNEt3
(12)
(13)
+
Hsalen– + NEt3 ® salen2– + HNEt3
It is clear that in the presence of NEt3, the major reactive species
are Hsalen– and salen2– ions. At relatively low [NEt3], the Hsalen–
ion and at high [NEt3] the salen2– ion is the original reactive
species. This suggests that a plausible mechanism in the
presence of NEt3 is that presented in Scheme 3. In the first step,
NEt3 quickly produced labile Hsalen– and salen2– ions. Then,
Hsalen– ion (path 1) or salen2– ion (path 2) coordinated to a nickel
centre in Ni(salpn) complex by lone pairs electron on the pheno-
These observations lead us to suggest a possible mechanism
for the reaction when a Ni(salpn) complex reacts with H2salen
ligand (Equations 10 and 11):
Scheme 2
The proposed mechanism for ligand exchange reaction in the system Ni(salpn)/H2salen.

RESEARCH ARTICLE
R. Vafazadeh and M. Bagheri,
S. Afr. J. Chem., 2015, 68, 21–26,
<http://journals.sabinet.co.za/sajchem/>.
25
Scheme 3
The proposed mechanism for ligand exchange reaction in the system Ni(salpn)/H2salen in the presence of NEt3.
lic oxygen atoms. This is followed by intramolecular proton
transfer from the coordinated Hsalen– to one of the phenolic
oxygen atoms coordinated to the salpn ligand with bond-
cleavage of one end of salpn (path 1).
H2O (³0.4 M) due to complete protonation of salen2–
and Hsalen– ions and formation of neutral H2salen.
4. Conclusion
As shown in Fig. 5, the slope of the plot at high [NEt3] (0.97
The distortion of Ni(II) in the Ni(salpn) complex due to the
formation the larger chelate ring cause a less stable complex than
the Ni(salen) complex. This is the driving force for replacing
salen with salpn in Ni(salpn) complex. In the presence of NEt3,
the ligand exchange reaction rate will be increased due to the
formation of Hsalen– ion. However, at high concentration
of NEt3, the reaction rate partially decreases in comparison with
low [NEt3], due to the formation of salen2– ion. These observa-
tions confirm that, as with similar reactions of salen-type Cu(II)
complexes, the deporotonation/protonation H2salen ligand
and the anionic form of H2salen are important for the ligand
exchange reaction.
0.07 M–1.s–1, salen2– is the major reactive species, Equation 13)
is less than with low [NEt3] (1.18
0.13 M–1.s–1, Hsalen– is the
major reactive species, Equation 12). Therefore, we can assume
that the Hsalen– ion is a more active species than the salen2– ion in
the ligand exchange reaction.9,10 At high concentration of [NEt3]
(> 0.3 M), although the salen2– ion is similar to the Hsalen– ion
and can be quickly coordinated to a Ni(II) centre in the Ni(salpn)
complex, the salen2– ion has no available proton for intra-
molecular proton transfer and thus cannot quickly undergo
bond cleavage of salpn in the Ni(salpn) complex. Therefore, the
rate of the reaction increases less under these conditions. At
concentration 0.1<[ NEt3]<0.3 M there is equilibrium between
Hsalen– and salen2– ions.
To ensure the validity of the mechanism shown in Scheme 3,
the reactions have been performed in the presence of H2O
and NEt3. These experiments are studied with three different
concentrations of NEt3:
1. In the first case, [NEt3] < 0.1 M (before the break in the plot in
Fig. 5), the ligand exchange rate was decreased by adding
H2O. The decrease of the reaction rate could be due to
protonated NEt3 and Hsalen– ion. The protonated Hsalen–
ion gave rise to the formation of neutral H2salen ligand
and the rate of reaction decreased.
2. In the second case, [NEt3] > 0.3 M (after break in plot in
Fig. 5), the reaction rate was decreased by adding H2O
(0.4 M). In this condition, salen2– ion was protonated by add-
ingH2O, H2salenwasformedandthereactionratedeceased.
3. In 0.1 <[NEt3] < 0.3 M (break limits in plot in Fig. 5), the effect
of H2O on the rate of reaction was dependent on the concen-
tration of H2O. The reaction rate of ligand exchange was
increased by increasing the concentration of H2O up to
0.05 M. The increase of the reaction rate at low [H2O] could be
due to the formation of Hsalen– ion by protonated salen2– ion.
However, the reaction rate was decreased by adding more
Acknowledgements
The authors are grateful to the Yazd University for partial
support of this work.
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