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Scientific paper
Kinetics of the Ligand Exchange Reaction Between Tetradentate Schiff Base N,N'-ethylen-bis (salicylaldimine) and Cu(N,N'-propylen-bis(salicylaldimine))
Rasoul Vafazadeh and Somayeh Bidaki
Department of Chemistry, Yazd University, yazd, Iran
* Corresponding author: E-mail: rvafazadeh @yazduni.ac.ir Tel: +98 351 8214778; Fax: +98 351 7250110
Received: 04-08-2009
Abstract
Visible spectrophotometry is used to study the kinetic of ligand exchange in the system Cu(salpn)/H2salen with or without triethylamine (NEtj) and H2O in acetonitrile 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 is found that the reaction rate is first-order with respect to Cu(salpn). In addition, the ligand exchange rate increases when NEt3 is added to the reaction mixture, as shown by a break in the kobs vs. [NEtj] plot. The effects of NEtj and H2O on the ligand exchange rate are discussed, and reaction mechanism is proposed.
Keyword: Kinetic, mechanism, Schiff base, Cu complexes, ligand exchange, saturation kinetics
1. Introduction
Schiff base complexes of salen and its derivatives are one of the most exhaustively studied topics in coordination chemistry.1 The magnetic properties,2-5 electronic spectra6' 7 and the structures of copper(II) complexes of salen5, 8' 9 and its derivatives have been studied by various research groups.4' 5 Tetradentate Schiff bases generally react with divalent metal ions by losing of two hydroxyl protons to form neutral and stable complexes. The resulting complexes are monomers that have flexible structures. For example, it has been shown that the increasing the length of methylen chains in Cu(II) complexes of the salen ligand family cause their structures to change from a planar to distorted tetrahedral.2' 3' 10-12 Moreover, in the presence of additional donors, they may form five or six coordinate species.12-16 Electronic spectral data indicate that the ligand field strength of the complexes decreases as the alkyl chain length increases.6' 7
The stability of metal complexes is dependent upon both the metal center environment' and the ligand conformational flexibility. Ligand flexibility could be a driving force that causes the ligand exchange reaction to form a more stable complex. However, there is no report investigation on the relationship between metal complex geometrical structure and the kinetics of ligand exchange reaction of copper complexes of tetradentate
Schiff base. Therefore the present study on the kinetics of the ligand exchange between salen and salpn in Cu(salp) (reaction 1) was performed to investigate the influence of ligand flexibility on the stability of copper complexes.
Cu(salpn) + H2salen ^ Cu(salen) + H2salpn (1)
2. Experimental
2. 1. Syntheses of Ligands
The Schiff base H2salen and H2salpn were prepared by a general method,12' 17 involving the condensation reaction between 2 equivalents of salicylaldehyde and 1 equivalent of the appropriate diamine.
2. 2. Syntheses of Cu(salen) and Cu(salpn) Complexes
The copper complexes (Scheme 1) were prepared by a general method,12, 18, 19 using the reaction between copper acetate and the Schiff base ligand (1:1 ratio) in methanol.
- Cu(salen): Yield 51%. Anal. Calculated for C16H14CuN2O2 : C, 58.26; H, 4.28; N, 8.49. Found : C, 584.09; H, 4.23; N, 8.61.
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■ Cu(salpn): Yield 50%. Anal. Calculated for C17H16CuN2O2 : C, 59.38; H, 4.69; N, 8.15. Found : C, 596.12; H, 4.63; N, 8.21.
Cu(salen)
Scheme 1
2. 3. Chemicals
Chemical reagents and all solvents, used in the syntheses and kinetic studies, were purified by standard methods.
2. 4. Kinetic Measurements
To measure the reaction rates, absorbance changes of reaction mixtures were followed using a GBC UV-Vi-sible Cintra 101 spectrometer at 570 nm, where the greatest change in molar absorptivity between reactants and products occurred. Reaction mixtures were studied in ace-tonitrile, CH3CN solvent (with and without triethylamine, NEt3 and H2O) at an ionic strength of I = 0.01 M in Na-NO3 25 ± 0.1 °C. Both H2salen and sodium nitrate have limited solubility in acetonitrile, and so their concentrations were restricted to 0.05 M and 0.01 M respectively.
To initiate reaction, equal volumes of Cu(salpn) at 2.00 x 10-3 M and H2salen (2.00 x 10-3 - 5.00 x 10-2) were mixed, and absorbance versus time measurements were taken. It was found that the absorbance of the reaction mixture increases with time. Pseudo-first-order rate constants (kobs) given in table 1 are obtained from the plots of -ln(A^-At) vs. time, where At and A^ represent the absor-bance of the reaction mixtures at time t and infinity, respectively. At least three runs were made at each concentration and the average values of rate constants are reported. The rate constants, k, were obtained by fitting data results of kobs vs. [H2salen] using sigmaplot 9.0.
3. Results and Discussions
3. 1. Absorption Spectra
The visible absorption spectra of Cu(salen) and Cu(salpn) complexes in CH3CN solvent show a maximum absorption due to d-d transition at 568 and 605 nm respectively (Fig.1).
Figure 1. Visible spectra of Cu(salpn) (6.0 X 10 3 M) and Cu(salen) (6.0 X 10-3 M) complexes in CH3CN.
It is clear from the visible spectrum of Cu(salen) and Cu(salpn) complexes that, it is practical to follow the li-gand exchange reaction spectrophotometrically. Fig. 2 shows a consecutive series of spectra recorded in CH3CN solvent for Cu(salpn)/ H2salen system, which indicates an increasing hypsochromic shift, with respect to the initial Cu(salpn) spectrum. In fact the changes observed in the spectrum are being caused by changing in the ligand field
0.6
0.5
S OA
a
«
a
© 0.3
A
<
0.2-
0.1
	MF \
	r \
	\
450 500 550 600 650 700
Wavelength/nm
750
Figure 2. Spectral changes recorded in CH3CN solvent for the reaction of Cu(salpn) (2.00 x 10-3 M)/ H2salen (2.00 x 10-3 M) system within 40 min.
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complex. These observations indicate that the Cu(salpn) complex is converted to the Cu(salen) complex (reaction 1) by adding H2salen ligand.
The spectrum produced by mixing corresponding amounts of Cu(salen) complex, H2salpn and NaNO3 in CH3CN is similar to the last spectrum shown in Fig. 2. This similarity confirms the conversion of the Cu(salpn) to the Cu(salen) complex.
3. 2. Kinetic Study
The rate law can be expressed as equation 2:
(2)
where [Cu(salpn)]0 is the concentration of Cu(salpn) at time initial. The pseudo-first-order rate constants were measured at various H2salen ligand concentrations. The order of the reaction rate with respect to H2salen ligand was determined by plotting kobs as a function of the concentration of H2salen. As shown in Fig. 3, the variations of kobs vs. concentration of H2salen showed saturation kinetics (See proposed mechanism).
The conversion of Cu(salpn) into Cu(salen) in CH3CN solvent showed that Cu(salen) is thermodynami-cally more stable than Cu(salpn) complex. To confirm this conclusion, the reverse reaction, i.e. the conversion of Cu(salen) to the Cu(salpn) in the presence or in the absence of NEt3 does not take place. Therefore, the reverse reaction and its equilibrium cannot be observed between two complexes in the studied condition.
The electronic spectra shown in Fig. 1, indicate that the Cu(salen) complex has a stronger ligand field (Amax = 568 nm) than Cu(salpn) complex (Amax = 605 nm).14
Table 1. Rate constants data for the reaction of Cu(salpn) with H2salen ligand in the absence and in the presence at different con-
[NEt3]/M	kx103/s-1	[NEt3]/M	kx103/s-1
0	0.031±0.11	0.90	2.504±1.51
0.18	1.298±0.65	1.08	2.987±0.64
0.36	1.688±0.78	0b	0.031±0.11
0.54	1.383±1.04	1.08b	1.347±0.51
0.72	1.831±1.03		
a Solvent CH3CN, at 25 ± 0.1 °C and I = 0.01 M NaNO3 b in the presence of H2O (0.28 M)
The salen Schiff base forms square planar complex with the ion copper(II). Increasing the number of methylene units in the chain of the Schiff base ligand (Scheme 1) provides enough flexibility in Cu(salpn) in comparison with the Cu(salen), in such a way that it is inverted from a planar configuration to a distorted tetrahedral configura-
Figure 3. Plots of kobs vs. [H2salen] for ligand exchange reaction of salpn in Cu(salpn) by H2salen in the absence H2O (A) and in the presence H2O, [H2O] = 0.28 M (□)
tion.12, 14, 18 It is important to emphasize that the change in the ligand strength of the metal complex correlated with the geometry about the metal indicating that small structural changes cause profound effects in the ligand field strength.
The distortion about the copper center in Cu(salpn) complex causes a decrease in the ligand field strength due to a chelate effect caused by the extra methylene group. The Cu(salpn) is less stable because it forms larger chela-te ring (six-member) than of the Cu(salen) complex (five-member). The size effect of chelate ring on complexes stability has been reported before.12, 20-22 It has been shown that the complex stability decreases along with increasing the chelate ring size. The six-member chelated ring leads to less stable complexes than five-member chelate ring.
The decrease in the ligand field strength and stability of Cu(salpn) relative to the Cu(salen) is in agreement
Figure 4. Plots of kobs vs. [H2salen] for ligand exchange reaction of salpn in Cu(salpn) by H2salen in the presence NEt3, [NEt3] = 0.18 M (A) and [NEt3] = 0.90 M (•)
centration of NEt3
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with the trend observed in the ligand substitution (reaction 1). To propose a mechanism for this exchange reaction, the kinetic study of the reaction is investigated in the presence of non-coordinating amine (triethyl amine with a variable concentration and H2O). The effect of triethyl amine and water on the kinetic behavior of the reaction is explained in the next section.
3. 3. Effect of Triethyl Amine
It is observed that, the rate of the exchange reaction increases when NEt3 is added to the reaction mixture. Carrying out the reaction at different NEt3 concentrations, indicates that the ligand exchange is strongly depends on the NEt3 concentration. As shown in Fig. 4, by increasing NEt3 concentration, the reaction rate become considerably faster than when carried out in the absence of NEt3. However, as shown in Fig. 5 (plot of kobs vs. [NEt3]0), there is an obvious breaking point at -0.23 to 0.38 M concentration of NEt3, dependent of [H2salen]0, which can be related to the change of the reaction species at these concen-
tration of NEt3. As shown in Fig. 5, by increasing H2salen concentration, the break point on the plot of kobs vs. [NE-t3]0 observed in lower NEt3 concentration.
3. 4. Reaction in Presence of H2O
The plot of kobs vs. [H2salen] in the presence and absence of H2O has been shown in Fig. 3 which shows that the rate of the ligand exchange reaction (Eq. 1) is not considerably changed by adding H2O (0.28 M) to CH3CN. However in presence of NEt3, the ligand exchange reaction rate considerably decreases by adding H2O (0.28 M) to solution of the reaction (Fig. 6). The effects of NEt3 and H2O show the importance of protonation/deporotonation on the rate of the ligand exchange reaction.
3. 5. Proposed Mechanism
As shown in Fig 2, there are two isosbestic points at 487 and 646 nm. This means that the Cu(salpn) converts to the Cu(salen) without the formation of free Cu2+ ion,23 which has a different spectrum than Cu(salpn) and Cu(sa-len) complexes. In general, ligand exchange reactions between multidentatate ligands proceed through intermediates in which the incoming ligand is partially coordinated to metal center and the leaving ligand is partially dissocia-ted.23, 24 Results from this study, namely that: (a) the ligand exchange rate did not change by adding H2O to solvent and (b) the plot of kobs vs. [H2salen] showed saturation kinetics indicates that a limiting value of kobs is reached at high [H2salen]. This result implies that there is an association between Cu(salpn) and H2salen prior to ligand exchange. We also know that because of the low acidity of H2salen and its family ligands,25 Hsalen- and salen2- ions concentration were negligible so that H2salen could be treated as the major reactive species under these conditions. This assumption was confirmed by performing the
Fig 5. Plots of kobs vs. [NEt3] for ligand exchange reaction [Cu(salpn)] = 2.00 X 10-3 M and [H2salen] = 2.00 X 10-3 M (D)and [H2salen] = 0.05 M (■)
Figure 6. Plots of kobs vs. [H2salen] for ligand exchange reaction of salpn in Cu(salpn) by H2salen, [NEt3] = 1.018 M in the absence H2O (■) and in the presence, [H2O] = 0.28 M (▲)
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ligand exchange reaction in the presence of H2O with no changes in reaction rate.
In the absence and presence of H2O, the plot of kobs vs. [H2salen] showed saturation kinetics (Fig. 3), implying that a limiting value of kobs is reached at high [H2salen]. This leads us to assume that there is an association between Cu(salpn) and H2salen prior to ligand exchange.
Cu(saipn) + Hisalen ■ k Cu(salen) + I hsalpn
[Cu( salpn J.l-bsal en]
(3)
The theoretical rate law can be given as: kK[H2salen\
Kobs
\+R[H2salen]
(4)
Where K denotes the equilibrium constant between Cu(salpn) complex and H2salen ligand and k is the rate constant of the ligand exchange reaction. Detachment of salpn from the Cu(II) complexes likely involves initial coordination of oxygen groups of the H2salen to copper center in Cu(salpn) complex followed by protons-transfer from H2salen to salpn ligand, with the bond cleavage of two-end of salpn. Finally, the reaction is completed by replacing salpn with salen. (Scheme 2).
Figure 7. Spectral changes recorded in CH3CN solvent for the reaction of Cu(salpn) (2.00 X 10-3 M), H2salen (2.00 X 10-3 M) and NEt3 (0.90 M) system within 15 min.
Cu(salpn) complex is converted to the Cu(salen) complex by adding H2salen ligand, in the presence of NEt3.
As shown in Fig 4, the ligand exchange rate will increase by adding NEt3. The effect of NEt3 could be due to its interaction with either Cu(salpn) or with the H2salen li-gand. The spectrum of Cu(salpn) complex in the presence
Scheme 2
Fitting equation (3) with the experimental data yields K = 13.06 ± 3.12 M-1 and k = (3.13 ± 1.06) x 10-3 s-1 in the presence of H2O, K = 13.79 ± 3.84 M-1 and k = (3.11 ± 1.08) x 10-3 s-1 in the absence of H2O. Similar dependence of kobs on [H2salen], both in presence and absence of H2O (Fig. 3), is in agreement with our assumption that H2salen is the major reaction species. Therefore, no protonic equilibrium is expected for H2salen under the reaction conditions.
Fig. 7 shows a consecutive series of spectra recorded in CH3CN solvent for reaction of Cu(salpn) with H2salen in the presence of NEt3. The absorption spectra indicate a shift towards smaller wavelength with respect to the starting Cu(salpn) spectrum, which is similar to Fig. 2 . In fact the changes observed in the spectrum indicate that the
of NEt3 in CH3CN does not change with respect to its spectrum in the absence of NEt3. Therefore, adduct formation between NEt3 and Cu(salpn) is not observed. But, the absorption spectrum of H2salen ligand changes with increasing of NEt3.
The electronic spectrum of H2salen ligand in CH3CN shows three transitions in UV region, the bands at 212 and 254 nm assigned to the tc^tc* transition are due to transition involving molecular orbitals located on the phenolic chromophore and C = N chromophore to benzene ring, respectively. The last band at 314 nm is assigned to the n^n* transition involving the promotion of one of the lone-pair electrons of the nitrogen atom of C = N to the n* molecular orbital of benzene ring.26, 27 The n^n* transition at 314 nm is absent upon complex formation
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and this indicates that the imine nitrogen atom appears to be coordinated to the copper ion.28 The bands which assigned to the tc^tc* transition have been shifted to the longer wavelength (lower energies) because of the extension of the conjugate system.28
Upon increasing NEt3, the band at 314 nm (n^n* transition) does not change, however, the bands at 212 and 254 nm (n^n* transition) change. The changes of the bands in the presence of NEt3 indicate that there is interaction between H2salen ligand and NEt3. Therefore, the observed dependence of the reaction rate on concentration of NEt3 can only be explained by consideration of the de-protonated of H2salen ligand. With the increase of [NEt3], the amount of anionic form of H2salen (i.e. Hsalen- and salen2-) increases significantly, and this is reflected in the rate constant values.
As shown in Fig. 5, there is strong dependence of the breaking point in the plot of kobs vs. [NEt3]0 with H2sa-len concentration. At higher concentration of H2salen, the breaking point observed at lower NEt3 concentration. The dependence of the breaking point on [H2salen] confirms that the deprotonation of H2salen ligand and formation Hsalen- and salen2- species.
The following observations (a) the increase of li-gand exchange rate by adding NEt3, (b) the decrease of reaction rate in the presence of NEt3 by adding H2O to reaction mixtures (Fig. 6) and (c) the break in the plot of kobs vs. [NEt3], in the presence of NEt3 as shown in Fig. 5, all suggest that Hsalen- and salen2- ions are major reactive species. At relatively low [NEt3], the Hsalen- ion dominates while at high [NEt3], the salen2- ion is major reactive species. A plausible mechanism explaining the NEt3 effect can be given by scheme 3.
In the first step, the NEt3 could quickly produce labile Hsalen- and salen2- ions, by the deprotonation of the
phenolic group(s) (reaction 5 and 6).
H2salen + NEt3
Hsalen" + NEt,
Hsalen" + HNEt3+ (5)

salen2" + HNEt3+
(6)
Then coordination of oxygen group of the Hsalen-(path 1) or salen2- (path 2) to copper center in Cu(salpn) complex, is followed by a intramolecular proton transfer from Hsalen- to salpn (path 1). Finally, the bond cleavage of two-end of salpn and ligand exchange is completed similar to the reaction without NEt3.
Using the proposed mechanism the rate law of li-gand exchange can be expressed as

k,[Hsalen } + k2[sa!e>r \ [ Hsalen m\ + \salen1 '
(7)
At low concentration of NEt3, the reaction 5 dominates with Hsalen- ion being the active species in promoting ligand exchange. Under this condition, [Hsalen ] >> [salen2-] and equation 7 is converted to kobs = k1. On the other hand, at high concentration of NEt3, reaction 6 dominates with [salen2-] >> [Hsalen ], in which case equation 7 simplifies to kobs = k2. As shown Fig. 5, the slop of plot kobs vs. [NEt3] at high [NEt3] is smaller than at low [NEt3], and therefore from equation 7, k2 < k1.
The rate of the reaction 1 is dependent on the NEt3 concentration, in such a way that, by increasing NEt3 concentration, the rate of reaction 1 increases because of increasing Hsalen- concentration. The Hsalen- ion rapidly coordinates to a copper center through its phenolic group, and this step is followed by intramolecular proton transfer from Hsalen- to salpn, causing Cu(salpn) to dissociate.
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However, at the high concentration of NEt3, the Hsalen-produced from reaction 5 is deprotonated and converted to salen2- ion. However, the salen2- quickly coordinates to a Cu complex, it cannot quickly undergo a bond cleavage, and therefore the rate of the ligand exchange reaction shows a small increase. A breaking point is observed in the plot of kobs vs. [NEt3] at concentration of -0.23 to 0.38 M (dependent of [H2salen]0) as a result of a decrease in the concentration of Hsalen- ion. These results suggest that at high NEt3 concentrations, reaction 6 is major reaction.
As shown in Fig. 6, the ligand exchange rate in the presence of NEt3 will decrease by adding H2O with respect to the reaction without H2O. The decrease of ligand exchange rate by adding H2O can be due to protonated of NEt3 or Hsalen- and salen2- ions. The protoneated species give rise H2salen ligand mainly species in ligand exchange reaction, and rate reaction decreases. Experimental observations confirm that the deprotonation/protonation H2salen ligand and anonic form of H2salen is important for the ligand exchange reaction.
4. Conclusion
The ligand exchange reaction was investigated in systems Cu(salpn)/H2salen in the present or the absence of NEt3 and H2O by using visible spectrophotometry in the acetonitrile. The rate of reaction was not changed by adding H2O, but the markedly increased by adding NEt3. These observations show the importance of protona-tion/deprotonation of H2salen ligand in the rate of the li-gand exchange reaction. The conversion of the Cu(salpn) into the Cu(salen) in the Cu(salpn)/H2salen system indicated that Cu(salen) should be more stable than Cu(salpn). The decrease in the stability of the Cu(salpn) complex (distorted tetrahedral configuration) relative to the Cu(salen) complex (square planer) may be due to an extra methylene group in the chain of the Schiff base ligand.
5. Acknowledgments
This study was supported by the Yazd University graduate school. We thank Dr. Hossein Farrokhpour for reading the manuscript and his valuable comments.
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Povzetek
Kinetiko izmenjave ligandov v sistemu Cu(salpn)/H2salen v vodi, acetontrilu in v raztopini 0.01 M NaNO3 ob prisotnosti trietilamina (NEt3) in brez njega smo proučevali z VIS-spektroskopijo pri 25 ± 0.1 °C (H2salen in H2salpn sta oznaki za N,N'-etilen-bis(salicilaldimin) oz. N,N'-propilen-bis(salicilaldimin)). Ugotovili smo, da je izmenjava rekacija prvega reda glede na Cu(salpn) kompleks ter da hitrost narašča z dodatkom NEt3. Opisan je tudi predlagani mehanizem reakcije.
Vafazadeh and Bidaki et al.: Kinetics of the Ligand Exchange Reaction Between H2salen and Cu(salpn)