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Molecules 2000, 5(6), 775-785; https://doi.org/10.3390/50600775

Article
Structure-Stability Relationships of Phthalocyanine Copper Complexes
1
Institute of Non-aqueous Solution Chemistry of Russian Academy of Sciences, 153045, Academicheskaya, 1, Ivanovo, Russian Federation
2
Ivanovo State Academy of Chemistry and Technology, 153000 Engels str., 7, Ivanovo, Russian Federation
*
Author to whom correspondence should be addressed.
Received: 14 March 2000 / Accepted: 11 April 2000 / Published: 12 June 2000

Abstract

:
The influence of NO2, Br, and COOH function substituents in various positions of Cu(II)Pc on the reactivity of the latter concerning the dissociation of the metal - nitrogen bonds in proton donor solvents is discussed.
Keywords:
Phthalocyanine complexes; dissociation; stability

Introduction

The correlation between structure and reactivity of metallophthalocyanines has not been investigated systematically until now. Nevertheless, it is well known [1] that electro-optical, thermodynamic and acid-base properties, as well as reactivity are expressed as functions of the electronic effects arising from metal coordination with phthalocyanines or structural changes in the phthalocyanine macrocycle. Numerous metallophthalocyanine functional derivatives with substituents in the benzene rings have been synthesized and isolated in individual form, and their structure and spectral properties were investigated [2,3,4,5,6].

Results and Discussion

For the present work a series of similar compounds was chosen to study the electronic and steric effects of substituents in the aromatic macrocycles. They are CuPc(R)m (Formula I), where R = 3-NO2; 4-NO2; 3-COOH; 4-COOH; (4-Br, 5-NO2); 3,5-COOH; 4,5-COOH, and m = 4 or 8 and copper(II)-octaethylporphyrin CuOEP (Formula II). The coordination group CuN4 was chosen as the reaction center. The kinetics of dissociation of copper(II)phthalocyanines in hot concentrated sulfuric acid into the solvated metal cation and protonated macrocycle (destroyed under the experimental conditions) was studied (Equations 1, 2).
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The kinetics of Reaction 1 were studied spectrophotometrically. The decrease of concentration of the copper(II)phthalocyanines was measured as the decrease of optical density at certain wavelengths during heating their sulphuric acid solutions at a defined temperature (Fig.1).
To determine the form(s) of the phthalocyanines present in sulphuric acid, the dependence of their UV-vis spectra on the solvent nature was investigated (Table 1). The first absorption band Q(0,0) in the studied copper(II)phthalocyanines spectra practically retains its shape and the maximum position in the sulphuric acid of different concentrations within the Brand region. Only for CuPc(4-NO2)4 and CuPc(3-NO2)4 this band shifts hypsochromically and becomes single when the H2SO4 concentration is decreased up to 12 mole/L. It means that the copper(II)phthalocyanines in the concentrated sulphuric acid are in the same form for all investigated substituents R and for all acid concentrations in the Brand region where their reactivity was studied. A substantial hypsochromic shift of the Q(0,0) band is observed in aprotic solvents compared with sulphuric acid solutions (Table 1). This testifies to protonation of the substituted compounds at outer-cycle N atoms and to a cation form CuPc(R)mH+ in the sulphuric acid solutions, analogous with non-substituted CuPc [1]. Thus in Equation (1) [CuPc(R)m]Solv = CuPc(R)mH+. The rate law (Equation 3) and the rate constants formally obeyed the first order (Table 2, Table 3) and describe Reaction 1 satisfactorily for all the investigated compounds.
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Constants kobs decrease with an increase of the initial acid concentration C H 2 S O 4 0 non-linearly, in a smooth descending curve for all studied phthalocyanine complexes (except CuPc(4-Br)4(5-NO2)4). Also, there is no linear correlation between kobs and H0 values. We tried to correlate the rate constant values with the equilibrium concentrations of proton-donor particles in H2SO4.
In concentrated H2SO4 there are several proton-donor particles in the equilibrium concentrations: non-ionized H2SO4 molecules, H3O+ cations and also HSO4-.H3O+ ion pairs and H2O molecules in trifling amounts [8]. The latter may be left out on account of the considered solvent concentrations ( C H 2 S O 4 0 >15.8 mole/L). Non-ionized H2SO4 molecules and H3O+ cations may be active particles when copper(II)phthalocyanines dissociate because the process demands protonation of donor N-atoms. It is known that the ratio of the activity coefficients of the acid and base forms remains constant in this range of H2SO4 concentrations. This allows us to use for our calculation the equilibrium concentrations instead of activities. The equilibrium concentrations of particles in H2SO4 at 298K, calculated from the Hammet equation [9], were taken from [8]. The temperature dependencies of H0 and p K H 2 S O 4 were taken from [10,11]. As C H 2 S O 4 0 increases the concentration of non-ionized H2SO4 molecules reduces and concentration of H3O+ increases. Thus kobs constants, which reduce in value as C H 2 S O 4 0 increases, correlate with the equilibrium concentration of H3O+ (Figure 2). The dependence is linear (Equation 4) for all investigated compounds at the temperatures shown in Table 2, Table 3 (the correlation coefficient is equal to 0.96÷0.99). The reaction order dependence on the H3O+ concentration (n) which is equal numerically to tangent of the angle of inclination of the lines in Figure 2 is found to be close to 2 for three investigated nitro derivatives CuPc(3-NO2)4, CuPc(4-NO2)4 and CuPc(4-Br)4(5-NO2)4.
Molecules 05 00775 i004
For CuPc(4-Br)4(5-NO2)4 maximums of the kobs vs. C H 2 S O 4 0 dependencies at temperatures 410K and 379K are observed at the same C H 2 S O 4 0 values as maximums of the C H 3 O + vs. C H 2 S O 4 0 dependence [8,12]. The extreme character of kobs vs. C H 2 S O 4 0 dependence for CuPc(4-Br)4(5-NO2)4 can be accounted for by participation of H3O+ cations as active particles in the dissociation processes. For CuPc(3-NO2)4, CuPc(4-NO2)4 the rate constants decrease with increasing solution acidity is considerably larger (Table 2). Perhaps because of this the kobs vs. C H 2 S O 4 0 dependence becomes insensitive to the mentioned extreme of C H 3 O + vs. C H 2 S O 4 0 dependence.
In equation (4) kdis is the real rate constant of Reaction 1. Its values and corresponding values of the activation parameters are shown in Table 4. Data for non-substituted CuPc are taken from [1].
The dissociation reaction of carboxy derivatives of copper(II)phthalocyanine is more complex because the dependence of the order of Reaction 1 on C H 3 O + changes for different carboxy derivatives and temperature conditions (Table 5). It makes it impossible to determine the activation parametres of the dissociation of CuPc(COOH)m.
Taking into account the ability of carboxy groups to act as weak organic acids for protonation in concentrated H2SO4 we suppose that reaction (1) of carboxy substituted copper(II)phthalocyanine is forestalled with the pre-equilibrium depicted in Equation 5 and its stoichiometric mechanism depends on nature of the compound and temperature.
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Therefore the complete kinetic equations 6 and 7 correspond to the dissociation reactions of nitro and carboxy substituted copper(II)phthalocyanines, respectively.
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Thus only nitro derivatives and non-substituted CuPc may be placed in a series of kinetic stability to the hydroxonium cation action as far as they are compounds of the same type and dissociate according to one and the same mechanism. Value of kdis298K increases in series shown in Equation 8:
CuPc(4-NO2)4 ≤ CuPc < CuPc(4-Br)4(5-NO2)4 < CuPc(3-NO2)4
Values of the activation energy of reaction (1) change in the same order (Table 4). This can mean that in the series 8 the compounds are placed in accordance with destabilization of the Cu-N bonds from beginning to end of the series. Taking account of the mixed σπ character of the Cu-N bonds with the π-dative bond direction from Cu to N [1] the order of the compounds in the series (8) can be explained from the point of view of the electron influence of NO2 and Br substituents on the electron density of the bonds. Obviously the negative inductive effect of the substituents has little influence on state of the M-N bonds because of remote location of the substituents from the coordination center (Formula I). In reality, not all the complexes are less stable than CuPc.
The nitro groups in CuPc(4-NO2)4 are para-substituents with regard to the C atom in position 1 and meta-substituents with regard to the C atom in position 2. Inasmuch as the H3O+ attack on the reaction centre CuN4 is a complex nucleophilic-electrophilic process with two types of interaction (O→Cu and H←N), the π-withdrawing effect of the nitro groups can appear from para- and meta-substitution as well. Then 4-NO2 groups can stabilize the copper(II)phthalocyanine due to strengthening of the dative π-bonds Cu→N. This stabilization effect takes place in reality (Series 8). The result of analogous analysis for 3-NO2 substituted copper(II)phthalocyanine in which NO2 groups are simultaneously ortho- and para-substituents with regard to C atoms in positions 2 and 1 is the same. However, 3-NO2 groups are less conjugated with the benzene ring because of close positions of mezo C atoms of the macrocycle. Observed destabilization of CuPc when 3-NO2 is the substitutent (Series 8) shows that the negative Ieffect of the substituents takes place all the same. In the 4-NO2 derivative the substituents are farther from the reaction center and the induction effect is imperceptible.
CuPc(4-Br)4(5-NO2)4 is less stable compared with CuPc due to mutual steric hindrances of adjacent Br- and NO2- groups to conjugation with the macrocycle and their negative induction effect.
Regularities of the change of the activation parameters of the dissociation reaction of copper(II)phthalocyanine with functional substitution (Table 4) are in good correspondence with the considered mechanism of the electronic influence. Larger values of E and ∆S correspond to the more stable complex CuPc(4-NO2)4 as compared with CuPc. An increase in strength of the donor-acceptor bonds Cu-N because of π-electron withdrawing effect of NO2 groups leads to a sharp increase of the activation energy value, which is not compensated by changes of effectiveness of the nucleophilic interaction O→Cu. (∆S increases). For less stable CuPc(3-NO2)4 on the contrary, a decrease of E and ∆S values is observed. For CuPc(4-Br)4(5-NO2)4 the activation parameters are close to those for the non-substituted complex.
An analogous mechanism for the electronic influence of substituents on reactivity in dissociation by dint of influence of the electronic state of the metal - nitrogen bonds was observed also for the earlier investigated metalloporphyrins substituted in β-positions of the macrocycle [13].
The CuOEP studied in the present work is significantly less stable compared with coppertetraphenylporphyrin (CuTPP) (for CuOEP k o b s 298 is equal 1.3.10-5, s-1 in mixed solvent 0.15 M H2SO4 in CH3COOH; for CuTPP k o b s 298 is equal 1.5.10-6, s-1 in 0.5 M H2SO4 in CH3COOH) due to an electron donating action of alkyl groups onto the reverse dative M−N π-bond.
For numerous functional derivatives of metallotetraphenylporphyrins substituted in the benzene rings, the mechanism of the electron influence of the substituents is essentially different: substitution leads basically to changes of state of the n-electron pairs of the N atoms of the metalloporphyrins [14,15]. Our data show that eight alkyl groups introduced into phenyl rings of CuTPP instead of β-positions of the macrocycle decreases the complex stability only within the limits of an order of magnitude ( k o b s 298 are equal 6.96.10-4, s-1 for CuTP(3,4-di-CH3)4P and 1.08.10-4, s-1 for CuTPP in 1 M H2SO4 in C3H7COOH).

Experimental

Functional derivatives of copper(II)phthalocyanin were synthesized by the template method from corresponding derivatives of o-phthalodinitrile and copper acetate and then purified by standard methods [7]. CuOEP was obtained by the coordination reaction of copper acetate with octaethylporphyrin in dimethylformamide and purified by column chromatography (Al2O3, CHCl3). UVvis spectra were recorded on a Specord M400 spectrophotometer.

References and Notes

  1. Berezin, B. D. Coordination Compounds of Porphyrins and Phthalocyanines; Wiley: New York - Toronto, 1981; p. 286. [Google Scholar]
  2. Boston, D. R.; Bailar, J. C. Inorg. Chem. 1972, 11, 1578–1583.
  3. Weber, H.; Busch, D. Inorg. Chem. 1965, 4, 469–471.
  4. Gaspard, S.; Verdquer, M.; Vioig, K. C. R. Acad. Sc. Series C, Paris 1972, 275, 573–579.
  5. Elektronnije spektri phthalocyanina i rodstvennih soedineniy; Lukianetz, E. A. (Ed.) ONIITEChIM: Chercassi, 1989; p. 94.
  6. Phthalocyanines: Properties and Application; Leznoff, C. C. (Ed.) VCH Publishers, Inc., 1996; Vol. IV, p. 515.
  7. Lever, A. B. P. Advances Inorg. and Radiochem. 1965, 7, 28–114.
  8. Majorov, V. D.; Librovich, N. B. Zh. Fiz. Khim. 1973, 47, 2298–2301.
  9. Hammet, L. P.; Deyrup, A. J. A. Series of Simple Basic Indicators. 1. The Acidity Function of mixtures of Sulfuric and Perchloric Acids with Water. J. Amer. Chem. Soc. 1932, 54, 2721–2739. [Google Scholar] [CrossRef]
  10. Vinnik, M. I. Dokladi AN SSSR 1956, 107–108.
  11. Gelbshtein, A. I.; Zheglova, G. G.; Temkin, M. Sh. Zh. Neorgan. Khim. 1956, 1, 506.
  12. Liler, M. Reaction Mechanisms in Sulphuric Acid; Academic Press: London - New York, 1971; p. 280. [Google Scholar]
  13. Klyueva, M. E.; Lomova, T. N.; Berezin, B. D. Russian Journal of General Chemistry 1991, 61, 1131–1136.
  14. Lomova, T. N.; Shormanova, L. P.; Semeykin, A. S.; Voronina, E. V.; Berezin, B. D. Zh. Obshch. Khim. 1988, 58, 661–665.
  15. Lomova, T. N.; Mozhzhukhina, E. G.; Shormanova, L. P.; Berezin, B. D. Russian Journal of General Chemistry 1989, 59, 2077–2084.
  • Samples Availability: Available from the authors.
Figure 1. The electron absorption spectra of CuPc(4-NO2)4 in 16.67 mole/l H2SO4, T = 423K (1-5), τ, s: 1 -1020, 2 - 1260, 3 - 1320, 4 - 1380, 5 - ∞ and in pyridine, T = 298K (6).
Figure 1. The electron absorption spectra of CuPc(4-NO2)4 in 16.67 mole/l H2SO4, T = 423K (1-5), τ, s: 1 -1020, 2 - 1260, 3 - 1320, 4 - 1380, 5 - ∞ and in pyridine, T = 298K (6).
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Figure 2. lgkobs - lgCH3O+ dependence for CuPc(3-NO2)4 (1), CuPc(4-NO2)4 (2) and CuPc(3,5-COOH)8 (3). T,K: 1 - 379, 2,3 - 410.
Figure 2. lgkobs - lgCH3O+ dependence for CuPc(3-NO2)4 (1), CuPc(4-NO2)4 (2) and CuPc(3,5-COOH)8 (3). T,K: 1 - 379, 2,3 - 410.
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Table 1. Position of the low energy maximum of the Q(0,0) band in the UV-vis spectra of copper(II)phthalocyanines (λmax, nm).
Table 1. Position of the low energy maximum of the Q(0,0) band in the UV-vis spectra of copper(II)phthalocyanines (λmax, nm).
CompoundDMFAPyridineSulphuric acid concentration C H 2 S O 4 0 , mole/L
12.0214.0415.8516.6017.0017.68
CuPc(3-NO2)4669669700713719732729733
CuPc(4-NO2)4669-727761758753766761
CuPc(4-Br)4(5-NO2)4666686741745741740745740
CuPc(3-COOH)4665687--718725725723
CuPc(4-COOH)4-685--776776777776
CuPc(3,5-COOH)8----725731730726
CuPc(4,5-COOH)8----745750750750
Table 2. Kinetic parameters of dissociation reactions of nitro derivatives of copper(II)phthalocyanine in sulphuric acid.
Table 2. Kinetic parameters of dissociation reactions of nitro derivatives of copper(II)phthalocyanine in sulphuric acid.
C H 2 S O 4 0 , mole/Lkobs...104,s-1
379К410К423К298К
CuPc(3-NO2)4
15.858±319±239±33.1• 10-5
16.671.9±0.29.5±0.719.8±0.96.4 • 10-3
17.081.5±0.110±119±22.4 • 10-3
17.680.6±0.12.8±0.310.0±0.12.6 • 10-3
CuPc(4-NO2)4
15.8512.2±0.936±258±21.8 • 10-5
16.671.7±0.124±245±310.4 • 10-8
17.081.4±0.220.3±1.529±33.4 • 10-7
17.680.54±0.0410.3±0.923±22.0 • 10-9
CuPc(4-Br)4(5-NO2)4
15.853.7±0.410.2±0.733.2±0.24.2 • 10-6
16.674.0±0.216.1±0.424±26.4 • 10-6
17.081.13±0.0714.0±0.919.8±0.62.3• 10-7
17.680.67±0.035.8±0.716.4±0.98.9• 10-9
Table 3. Kinetic parameters of dissociation reactions of carboxy derivatives of copper(II)phthalocyanine in sulphuric acid.
Table 3. Kinetic parameters of dissociation reactions of carboxy derivatives of copper(II)phthalocyanine in sulphuric acid.
CompoundT,К C H 2 S O 4 0 , mole/Lkobs..104 , s-1
CuPc(3-COOH)437916.851.08±0.08
17.000.08±0.01
17.720.062±0.001
CuPc(4-COOH)437916.035.1±0.1
16.851.7±0.2
17.000.58±0.05
17.720.47±0.04
41015.8533±3
16.6015±1
17.008.8±0.5
17.684.7±0.3
42315.8557±8
16.6040±4
17.0017±2
17.6812±1
CuPc(3,5-COOH)837916.030.40±0.04
16.850.23±0.02
17.000.22±0.04
17.720.18±0.04
41015.852.5±0.2
16.601.1±0.1
17.000.59±0.02
17.680.27±0.02
42315.855.4±0.3
16.602.1±0.3
17.001.4±0.1
17.680.65±0.03
CuPc(4,5-COOH)837916.032.20±0.07
16.850.56±0.09
17.000.25±0.04
17.720.15±0.02
41015.8510±1
16.605.1±0.4
17.002.0±0.1
17.680.95±0.03
42315.8524±1
16.6012±2
17.682.6±0.2
Table 4. The rate constants kdis, the activation energies and entropies of dissociation of nitro derivatives of copper(II)phthalocyanines in sulphuric acid.
Table 4. The rate constants kdis, the activation energies and entropies of dissociation of nitro derivatives of copper(II)phthalocyanines in sulphuric acid.
Compoundkdis .106, s-1E,∆S,
379K410K423K298KkJ/moleJ/(mole.K)
CuPc(3-NO2)40.181.58a)15.82.5•10-515828
CuPc(4-NO2)40.8026.9160b)1.3•10-617279
CuPc(4-Br)4(5-NO2)40.628.13001.05•10-5231224
CuPc [1]c)0.24228016941.4•10-614930
a) k d i s 411 ; b) k d i s 421 ; c) k d i s 373 , k d i s 393 , k d i s 409
Table 5. The rate constants kdis and the reaction orders n of the dissociation of carboxy derivatives of copper(II)phthalocyanines in sulphuric acid.
Table 5. The rate constants kdis and the reaction orders n of the dissociation of carboxy derivatives of copper(II)phthalocyanines in sulphuric acid.
CompoundT.К-lgkdisn
CuPc(3-COOH)437910.26.4
CuPc(4-COOH)43796.93.2
4105.32.5
4233.91.2
CuPc(3,5-COOH)83796.72.2
4107.03.0
4236.42.8
CuPc(4,5-COOH)83797.63.6
4106.83.6
4236.53.6
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