Scientific paper
Kinetics and Mechanism of Ligand Exchange Reaction of Copper(II) Complexes with Tetradentate Schiff
Base Ligands
Rasoul Vafazadeh* and Somayeh Bidaki
Department of Chemistry, Yazd University, Yazd, Iran * Corresponding author: E-mail: rvafazadeh@yazduni.ac.ir
Received: 10-12-2013
Abstract
The kinetics of a ligand exchange in the CuL4/H2Ln system, where H2Ln is the N,N'-alkylen-bis(salicyldimine) tetradentate Schiff base ligand (n = 2 or 3, CH2 groups in the chain length of the amine backbone) was studied spectrophotome-trically in DMF solvent with or without triethylamine (NEt3) and H2O at 25 ± 0.1 °C and an ionic strength of 0.01 M NaNO3. The reaction rate was found to be first-order with respect to CuL4 complex and H2Ln ligand. The rate of the ligand exchange reaction did not change significantly with the addition of H2O to the DMF solvent; however, it increased when NEt3 was added to the reaction mixture. The effect of NEt3 and H2O on the ligand exchange rate shows that the de-protonation/protonation of the H2Ln ligand and anionic form of H2Ln are essential to the reaction. A reaction mechanism is proposed and discussed for the effect of NEt3 and H2O on the ligand exchange rate.
Keyword: Saturation kinetics, kinetic, ligand exchange, UV-Visible, Schiff base.
1. Introduction
The chemistry of the complexes of a salen type Schiff base, H2L2 (scheme 1), and its derivatives have been exhaustively studied to improve the coordination chemistry and because of their applications to modeling bioinor-ganic systems, catalysis, and analytical practice.1-8 Initial focus was on the synthesis and geometry of the salen complexes with bivalent metals. Gradually, research has moved toward synthesis and structural properties of the metal complexes of derivatives of salen with functional groups
and different chain lengths in the amine backbone (number of CH2 groups, Scheme 1).9-12 The increase in the length of the methylene chain of the complexes allows sufficient flexibility in the structure by changing it from a planar to a distorted tetrahedral or forming species with a higher coordination number in the presence of additional donors.13-16
Studies on the structure of the copper(II) complexes of salen and their derivatives have indicated that they are monomers and their flexibility results from an increase in chain length, which changes the geometry from square-planar to distorted tetrahedral.13-17 The increase in the
Scheme 1. A structural representation of the Schiff base ligands and complexes.
number of methylene units (n in the chain of H2Ln, scheme 1) decreases the stability of the copper(II) complexes.18
This decrease in stability could be a driving force for the ligand exchange reaction and formation of a higher stable complex. In our previous study on the ligand exchange reactions,18 we investigated the kinetics and mechanism of the ligand exchange reaction between H2L2 and L3 in the CuL3 complex (reaction 1).
H2L2 + CuL3 ^ CuL2 + H2L3
(1)
The present study investigates the kinetics of the ligand exchange reaction between H2Ln (n = 2, 3) and L4 in the CuL4 complex (reactions 2 and 3).
H2L2 + CuL4 ^ CuL2 + H2L4
H2L3 + CuL4 ^ CuL3 + H2L4
(2) (3)
2. Experimental
2. 1. Reagents
Chemical reagents and all solvents, used in the syntheses and kinetic studies, were purified by standard methods.
The Schiff bases H2Ln were prepared by a general method,18-20 the condensation reaction between 2 equivalents of salicylaldehyde and 1 equivalent of the appropriate diamine (1,2-ethylendiamine, 1,3-propandiamine and 1,4-butanediamine), in ethanol. The yellow products were obtained in yields typically 70% or better. Purity of products were verified by comparing with literature melting points (m.p.), 124, 53 and 90 °C for H2L2, H2L3 and H2L4, respectively.15-17
The copper complexes were prepared by a general
method,1
using the reaction solution of copper(II)
acetate with the Schiff base ligand (1:1 molar ratio).
-	CuL2: Yield 51%. Anal. Calculated for C16H14CuN2O2: C, 58.26; H, 4.28; N, 8.49. Found : C, 58.09; H, 4.23; N, 8.61.
-	CuL3: Yield 50%. Anal. Calculated for C17H16CuN2O2: C, 59.38; H, 4.69; N, 8.15. Found : C, 59.12; H, 4.63; N, 8.21.
-	CuL4: Yield 50%. Anal. Calculated for C18H18CuN2O2, C, 60.41; H, 5.07; N, 7.83. Found : C, 59.98; H, 5.13; N, 7.61.
2. 2. Kinetics Measurements
The reaction of Schiff base H2L2 and H2L3 with a CuL4 complex was studied under first order conditions using a GBC UV-Visible Cintra 101 spectrometer at 570 and 640 nm, respectively. This equals to the greatest change in molar absorptivity between reactants and products. The reaction mixture were made in dimethylformamide (DMF) solvent, I = 0.01 M NaNO3, CuL4 complex (2.5 x
10-3 M) and different concentrations of Schiff base ligand with and without triethylamine, NEt3, and H2O at 25 ± 0.1 °C. Equal volumes of the solution of the CuL4 complex in DMF with the solution containing Schiff base (H2L2 or H2L3) ligand and NaNO3 (with and without NEt3 and H2O) were mixed. The reaction mixture absorbance values were measured at defined times after mixing.
The pseudo-first-order rate constants (kobs) were obtained from the plots of -ln(A^-At) versus time, where At and A^ represent the absorbance of the reaction mixtures at time t and infinity, respectively. At each concentration an average of at least three kinetic runs was carried out and the rate constants (kobs) were statistically averaged. Rate constant k was obtained by fitting the data to kobs versus [H2Ln] using Sigmaplot 12.0.
3. Results and Discussion
3. 1. Absorption Spectra
The visible absorption spectra of the CuL2, CuL3 and CuL4 complexes at equal concentrations in DMF solvent show maximum absorption due to d-d transition at 568, 605 and 642 nm, respectively (Fig. 1). The visible spectrum of the CuLn complexes shows that the visible spectrophotometry easily followed the ligand exchange reaction. Fig. 2 shows a consecutive series of spectra recorded in DMF solvent for the CuL4/ H2L2 system (Fig. 2a) and CuL4/ H2L3 system (Fig. 2b) and shows a decrease in wavelength with respect to the starting CuL4 spectrum. These observations indicate that the CuL4 complex converted to a CuL2 or CuL3 complex (reactions 2, 3) when the H2L2 or H2L3 ligand, respectively, was added.
The spectrum produced by mixing equal amounts of CuL2 or CuL3 complex, H2L4 and NaNO3 in DMF is similar to the last spectrum shown in Fig. 2. This similarity confirms the conversion of CuL4 in the presence of H2L2 or H2L3 to a CuL2 or CuL3 complex, respectively.
Figure 1. Visible spectra of CuLn (2.5 X 10 3 M) complexes in DMF solvent.
a) ^
0.7-
Y		
		
		\
		\
		
		
		
500
600
Wavelength / rrn
700
800
Figure 2. Spectral changes recorded in DMF solvent (a) for the reaction of CuL4 (2.5 X 10-3 M) / H2L2 (2.5 X 10-3 M) (b) CuL4 (2.5 X 10-3 M) / H2L3 (2.5 X 10-3 M).
3. 2. Kinetics Study
The reactions were monitored by following the increase in the 570 nm band for the CuL4/H2L2 system and the decrease in the 640 nm band for the CuL4/H2L3 system (Fig. 3) after mixing equal volumes of CuL4 complex and H2L2 or H2L3 Schiff base ligand in DMF at 25 ± 0.1°C. NaNO3 was used to maintain the ionic strength at 0.01 M. All reactions were followed to at least 95% completion. Sample plots of the absorbance versus time data at 570 and 640 nm are shown in Fig. 3.
The rate constants (kobs) of reaction 2 and 3 were obtained under pseudo-first-order conditions (Table 1). The rate law can be expressed in Eqs. 4 and 5:
cl[CtiL* ] d[CuL'] , r„lA, Rate - - -!——= —= kvbs [CuL"]0
d!
dt
(4)
(5)
The pseudo-first-order rate constants were measured at different ligand concentrations. The order of the reaction rate with respect to Schiff base ligand was determined by plotting kobs as a function of ligand concentration. Fig. 4 shows the variation in kobs versus Schiff base ligand concentration, illustrating the saturation kinetics (see proposed mechanism).
The conversion of CuL4 by adding H2L2 or H2L3 Schiff base ligand to CuL2 and CuL3, respectively, in DMF solvent showed that they were thermodynami-cally more stable than the CuL4 complex. To confirm this, the reverse of reactions 2 and 3, i.e., the conversion of CuL2 or CuL3 when adding H2L4 ligand to the CuL4 does not take place. In this condition, a reverse
Time/s
Time Is
Figure 3. Plot of absorbance vs. time for the typical ligand exchange reaction (A) X = 570 nm for CuL4 (2.5 X 10 3 M) / H2L2 (2.5 X 10 3 M), (B) X = 640 nm for CuL4 (2.5 X 10-3 M) / H2L3 (2.5 X 10-3 M) in DMF solvent.
Table 1 Rate data for the reaction of CuL4 (0.0025 M) with H2Ln (0.025 M) ligand in the presence of variable [NEt3]
	H2L2	H2L3
[NEt3] / M	kobs X 104 / s-1	kobs X 104/ s-1
0	1.49 ± 0.10	1.88 ± 1. 15
0a	1.48 ± 0.12	1.90 ± 1. 13
0.0018	2.67 ± 0.15	3.12 ± 0.14
0.0036	3.19 ± 0.14	3.71 ± 0.15
0.0054	3.35 ± 0.17	5.21 ± 0.11
0.0072	3.56 ± 0.11	6.18 ± 0.17
0.009	3.13 ± 0.14	4.62 ± 0.16
0.011	3.56 ± 0.16	7.76 ± 0.11
0.013	3.89 ± 0.17	8.50 ± 0.13
0.014	3.98 ± 0.19	9.51 ± 0.18
a in the presence of H2O (0.56 M)
Figure 4. Plots of kobs vs. [H2L2] for ligand exchange reaction of L4 in CuL4 by H2L2 in the absence H2O (•) and in the presence H2O (o)
reaction and equilibrium cannot be observed between the two complexes.
2 Schiff base ligand with the ion Cu(II)
formed a square-planar complex only slightly distorted
from planarity.15-17 Increasing the number of methylene units in the chain of the Schiff base ligand (Scheme 1) provided enough flexibility in CuL3 and CuL4 to inverted it from a planar configuration (CuL2) to a distorted tetrahedral (CuL3) or tetrahedral (CuL4) configuration; the dihedral angles are 12.2, 25.4 and 42.6°, respectively.17-18
Distortion about the copper center of the CuL4 complex from the extra methylene group decreased the ligand field strength. In addition, the L4 with Cu(II) forms a larger chelate ring (seven-membered) than the CuL2 and CuL3 complexes (five- and six-membered, respectively). The effect of size of the chelate ring on complex stability has been reported elsewhere.17, 23-25 The complex stability decreased as the chelate ring size increased. The seven-membered chelate ring led to less stable complexes than the six- and five-membered che-late rings.
The decrease in ligand field strength and stability of CuL4 and CuL3 relative to CuL2 are in accord with the trend observed in the ligand exchange reactions (reactions 2 and 3). This is consistent with previous our resulted on kinetics of ligand exchange in the CuL3/H2L2 system (reaction 1).18
The kinetics of the reaction was investigated in the presence of NEt3 and/or H2O to propose a mechanism for this exchange reaction.
3. 3. Effect of Triethylamine
NEt
The rate of the exchange reaction increased when was added to the reaction mixture. The reaction was
The H2L2
carried out at different NEt3 concentrations, which indicates that the ligand exchange strongly depends on NEt3 concentration. The reaction rate increased considerably as the NEt3 concentration increased relative to the reaction in the absence of NEt3 (Fig. 5). There is an obvious break in the plot of kobs versus [NEt3]0 at NEt3 concentrations of ~0.007-0.00i) M in the CuL40H2L2 system (Fig. 5a) and
b)
[*Et3iM0'
,-3
Figure 5. Plots of kobs vs. [NEt3] for ligand exchange reaction (a) the system CuL4 / H2L2 (b) CuL4 / H2L3.
~0.015-0.020 M in the CuL4/H2L3 system (Fig. 5b). This can be attributed to the change in reaction species at these concentrations of NEt,.
3. 4. Reaction in Presence of H2O
The plot of kobs versus [H2Ln] in the presence and absence of H2O is shown in Fig. 4. It indicates that the rate of the ligand exchange reactions (reactions 2 and 3) did not change considerably when H2O (0.56 M) was added to the DMF. The ligand exchange reaction rate in the presence of NEt3, however, considerably decreased when H2O (0.56 M) was added to the reaction solution (Table 1). The effects of NEt3 and H2O show the importance of protonation/deproto-nation in the rate of the ligand exchange reaction.
3. 5. Proposed Mechanism
Fig. 2 shows two isosbestic points at 532 and 622 nm for the CuL4/H2L2 system (Fig. 2a) and one isosbestic point at 598 nm for the CuL4/H2L3 system (Fig. 2b). This indicates that CuL4 converted to CuL2 or CuL3 without the formation of a free Cu2+ ion,26-28 which has a different spectrum from the CuLn complexes. In general, the ligand exchange reaction between polydentate ligands proceeds via intermediates where the incoming ligand is partially coordinated to the metal center and the leaving ligand is partially dissociated.27,28
The acidity of H2Ln and its family Schiff base ligands was low,29 thus, deprotonating the Schiff base ligands did not take place in the absence of a base. This assumption was confirmed by the ligand exchange reaction in the presence of H2O with no changes in reaction rate (Fig. 4 and Table 1).
Fig. 4 shows a representative graph for kobs dependence on the free ligand in the absence and presence of H2O. It shows saturation kinetic dependence on the free li-gand and zero intercepts at the extrapolated zero concentration. The zero intercept confirms the negligible contribution of solvent to the overall rate.
Saturation kinetics indicates that a limiting value of kobs was reached at high [ligand]. The rate increased as the [H2Ln] increased before the limiting value (Fig. 4), which
association complex. At this stage, the interchange of li-gands from outer sphere to inner sphere occurs, i.e., H2Ln attacks the Cu(II) atom to produce an intermediate complex (CuL4 ■ H2Ln, n = 2 and 3).
This suggests production of a formation species from the association between CuL4 and the Schiff base ligand prior to ligand exchange (reaction 6, n = 2, 3).
(6)
The theoretical rate law can be given as Eq. 7:
(7)
where K is the equilibrium constant between the CuL4 complex and H2Ln (n = 2, 3) ligand and k is the rate constant of the ligand exchange reaction.
Displacement of H2Ln from the Cu(II) complexes likely involves the initial coordination of H2Ln oxygen groups to the copper center in the CuL4 complex followed by proton-transfer from H2Ln to the L4 ligand in the CuL4 complex (CuL4 ■ H2Ln), with the bond cleavage of the two-end L4 ligand. The reaction is completed by replacing L4 with H2Ln. (scheme 2).
Fitting Eq. 7 with the experimental data yields k = (9.50 ± 0.28) x 10-3 s-1 (in CuL4/H2L2 system) and k = (2.93 ± 0.16) x 10-2 s-1 (in CuL4/H2L3 system).
The k values are in the same order separately in the presence and absence of H2O at the base Schiff ligands (Fig. 4). This is in agreement with the assumption of H2Ln being a major reaction species. In this case, no species de-protonation or protonic equilibrium was expected for H2Ln under the reaction conditions.
Fig. 6 shows a consecutive series of spectra recorded in DMF solvent for the reaction of CuL4 with H2L2 in the presence of NEt3. The absorption spectra indicate shifted towards a smaller X with respect to the starting CuL4 spectrum, which is similar to Fig. 2. The changes observed in the spectrum indicate that, in the presence of NEt3, the CuL4 complex was converted to a CuL2 complex with the
is probably caused by the formation of the outer sphere addition of the H2L2 ligand.
Scheme 2. The suggest mechanism for ligand exchange reaction in the system CuL4 / H2Ln.
Figure 6. Spectral changes recorded in DMF solvent for the reaction of CuL4 (2.5 x 10-3 M) / H2L2 (2.5 x 10-3 M) and NEt3 (1.8 x 10-3 M).
As shown in Fig. 5 and Table 1, the ligand exchange rate increased when NEt3 was added. The effect of NEt3 could be its interaction with either CuL4 or the H2Ln ligand. The spectrum of the CuL4 complex in the presence or absence of NEt3 in DMF did not change; thus, adduct formation between NEt3 and CuL4 was not observed. The absorption spectrum of the H2Ln ligand changed as NEt3 increased.
The dependence of the reaction rate on the concentration of NEt3 can only be explained by the deprotonated H2Ln ligand.18 As [NEt3] increased, the anionic form of H2Ln ( (HLn)- and (Ln)2-) increased significantly, which was reflected in the rate constant values.
The ligand exchange rate increased when NEt3 was added and the reaction rate decreased when H2O was added to the reaction mixture in the presence of NEt3. There was a break in the plot of kobs versus [in the presence of
NEt3 (Fig. 5) which suggests that (HLn)- ions are major reactive species in the presence of NEt3. Under these reaction conditions, at a relatively low [NEt3], the (HLn)- ion is the main reactive species and, at high [NEt3], the (Ln)2-ion is the main reactive species.18 This suggests that an acceptable mechanism in the presence of NEt3 is provided by scheme 3:
In the first step, NEt3 quickly produced labile (HLn)-and (Ln)2- ions (depending on NEt3 concentration) by the deprotonation of proton(s) from the phenolic group(s) of H2Ln (Eqs. 8 and 9).
H2Ln + NEt3
(HLn) + HNEt3
(8) (9)
The coordination of the oxygen group of the (HLn)-(path 1) or (Ln)2- (path 2) with the copper center in the CuL4 complex was followed intramolecular proton transfer from (HLn)- to L4 (path 1). The bond cleavage of the two-end L4 and ligand exchange was completed, as in the reaction without NEt3.
Using the proposed mechanism the rate law of li-gand exchange can be expressed as:
(10)
At low NEt3 concentrations, the Eq. 8 was the main reaction and the (HLn)- ion was the active species in the ligand exchange reaction. Under this condition, Eq. 10 is assumed to convert to kobs « k1. At high NEt3 concentrations, interaction between the (HLn)- ion and NEt3 produced a (Ln)2- ion (Eq. 9), allowing Eq. 10 to be simplified to kobs « k2. Fig. 5 shows that the slope of plot kobs versus [NEt3] for a high [NEt3] was less than that for a low [NEt3]. It can then be assume that, in Eq. 10, k2 < k1.
Scheme 3. The suggest mechanism for ligand exchange reaction in the system CuL / H2Ln in the present of NEt3.
The rate of the ligand exchange reaction was dependent on the NEt3 concentration such that increasing the NEt3 concentration increased the rate of the reaction because the concentration of the (HLn)- ion increased. The (HLn)- ion quickly coordinates with the Cu center from the oxygen phenolic group, followed by bond cleavage of L4 in the CuL4 complex by intramolecular proton transfer from (HLn)- to L4. At high NEt3 concentrations, the (HLn)-produced from reaction 8 was deprotonated and converted to an (Ln)2- ion (Eq. 9). The (Ln)2- quickly coordinated with the Cu complex, but the (Ln)2- ion cannot quickly rebounded for bond cleavage of L4 in the CuLn complex and the rate of the ligand exchange reaction decreased relative to the reaction at low [NEt3]. This created the observed break in plot kobs versus [NEt3] from the decrease in the concentration of the (HLn)- ion with the increase in [NEt3].
The ligand exchange rate in the presence of NEt3 decreased when H2O was added which is likely the result of the protonation of the (HLn)- and (Ln)2- ions. The proto-neated species of (HLn)- and (Ln)2- ions formed a H2Ln li-gand and the ligand reaction rate decreased. These experimental observations confirm that the deprotonated/proto-nated H2Ln ligand and anonic form of H2Ln was essential to the ligand exchange reaction.
4. Conclusion
Distortion about the Cu(II) in CuLn complex was caused by the formation a larger chelate ring which creates a less stable complex than the CuLn complex. This effect was the driving force for replacing the H2Ln (n = 2 and 3) ligand with L4 in the CuL4 complex. The ligand exchange reaction was investigated for the presence or absence of NEt3 and H2O in the DMF solvent. The reaction rate did not change when H2O was added; however, in the presence of NEt3, the ligand exchange reaction rate increased in response to the deprotonation of the H2Ln li-gand. These observations confirm that the deprotona-tion/protonation of the H2Ln ligand and the anionic form of the ligand are essential to the ligand exchange reaction.
5. Acknowledgments
The authors are grateful to the Yazd University for partial support of this work.
6. Reference
1.	T. P. Camargo, R. A. Peralta, R. Moreira, E. E. Castellano, A. J. Bortoluzzi, A. Neves, Inorg. Chem. Commun. 2013, 37, 34-38.
2.	F. Dehghani, A. R. Sardarian, M. Esmaeilpour, J. Organo-met. Chem. 2013, 43, 87-96.
3.	S. Carradori, C. D. Monte, M. D. Ascenzio, D. Secci, G. Ce-lik, M. Ceruso, D. Vullo, A. Scozzafava, C. T. Supuran, Bi-oorg. Med. Chem. Lett. 2013, 25, 6759-6763.
4.	A. M. Ajlouni, Z. A. Taha, W. A. Momani, A. K. Hijazi, M. Ebqaai, Inorg. Chim. Acta 2012, 388, 120-126.
5.	M. N. Dehkordi, A. K. Bordbar, P. Lincoln, V. Mirkhani, Spectrochim. Acta A. 2012, 90, 50-54.
6.	M. Mariappan, M. Suenaga, A. Mukhopadhyay, B. G. Maiya Inorg. Chim. Acta, 2012, 390, 95-104.
7.	F. J. Fard, Z. M. Khoshkhoo, H. Mirtabatabaei, M. R. Hou-saindokht, R. Jalal, H. E. Hosseini, M. R. Bozorgmehr, A. A. Esmaeili, M. J. Khoshkholgh, Spectrochim. Acta A 2012, 97, 74-82.
8.	Z. A. Taha, A. M. Ajlouni, K. A. Al-Hassan, A. K. Hijazi, A. B. Faiq, Spectrochim. Acta A 2011, 81, 570-577.
9.	N. Saravanan, M. Palaniandavar, Inorg. Chim. Acta 2012, 385, 100-111.
10.	M. Izakovic, J. Sima, Acta Chim. Slov. 2004, 51, 427-436.
11.	M. Asadi, H. Sepehrpour, K. Mohammadi, J. Serb. Chem. Soc. 2011, 76, 63-74.
12.	M. Dieng, I. Thiam, M. Gaye, A.S. Sall, A.H. Barry, Acta Chim. Slov. 2006, 53, 417-423.
13.	R. Vafazadeh, B. Khaledi, A.C. Willis, M. Namazian. Polyhedron 2011, 30, 1815-1819.
14.	R. Vafazadeh, B. Khaledi, A.C. Willis Acta Chim. Slov. 2012, 59, 954-958.
15.	J. Reglinski, S. Morris, D. E. Stevenson, Polyhedron 2002, 21, 2167-2174.
16.	J. Reglinski, S. Morris, D. E. Stevenson, Polyhedron 2002, 21, 2175-2182.
17.	L. C. Nathan, J. E. Koehne, J. M. Gilmore, K. A. Hannibal, W. E. Dewhirst, T. D. Mai, Polyhedron 2003, 22, 887-894.
18.	R. Vafazadeh, S. Bidaki, Acta Chim. Slov. 2010, 57, 310-317.
19.	R. Vafazadeh, M. Hatefi-Ardakani Bull. Korean Chem. Soc. 2006, 27, 1685-1688.
19.	R. Vafazadeh,M. Kashfi, M. Bull. Korean Chem. Soc. 2007, 28, 1227-1230.
20.	R. Vafazadeh, V. Hayeri, A. C. Willis, Polyhedron 2010, 29, 1810-1815.
21.	R. Vafazadeha, R. Esteghamat-Panaha, A. C. Willisc, A. F. Hill, Polyhedron 2012, 48, 51-57.
22.	R. D. Hancock, A. E. Martell, Chem. Rev. 1989, 89, 1875-1914.
23.	R. D. Hancock, Acc. Chem. Res. 1990, 23, 253-257.
24.	B. Wang, C. S. Chung, J. Chem. Soc. Dalton Trans. 1982, 2565-2566.
25.	R. G. Wilkins, Kinetics and Mechanism of Reactions of Transition Metal Complexes, Wiley, New York, 1986, p.p. 156-159
26.	H. Elias, S. Schwartze-Eidam, K. J. Wannowius, Inorg. Chem. 2003, 42, 2878-288.
27.	G.V. Rao, R. Bellam, N. R. Anipindi, Transition Met. Chem. 2012, 37, 189-196.
28.	N. Hirayama, I. Takeuchi, T. Honjo, K. Kubono, H. Koku-sen, Anal. Chem. 1997, 69, 4814-4818.
Povzetek
Kinetika izmenjave ligandov v sistemu CuL4/H2Ln, kjer je H2Ln N,.V-aLkilen-bis(salicildimin) tetradentatni ligand Schiffove baze (n = 2 ali 3; število CH2 skupin v verigi aminskega ogrodja) je bila proučevana spektrofotometrično v DMF v prisotnosti ali brez trietilamina (NEt3) in H2O pri 25 ± 0.1 °C, pri ionski jakosti raztopine 0,01 M NaNO3. Reakcija izmenjave ligandov je prvega reda glede na kompleks CuL4 in H2Ln ligand. Hitrost reakcije izmenjave ligandov se bistveno ne spremeni ob dodatku H2O k topilu DMF, medtem ko se hitrost poveča ob dodatku NEt3 k reakcijski zmesi. Vpliv dodatkov NEt3 in H2O na hitrost izmenjave ligandov kaže, da je proces deprotonacije/protonacije liganda H2Ln in njegove anionske oblike bistvenega pomena za to reakcijo. Predlagan je mehanizem reakcije izmenjave ligandov in obravnavan vpliv NEt3 in H2O na hitrost izmenjave ligandov.