Effect of pH on formation of metalloporphyrins Текст научной статьи по специальности «Химические науки»

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Статья Paper

Effect of pH on Formation of Metalloporphyrins

Vladimir B. Sheinin,a,@ Olga R. Simonova,a and Ekaterina L. Ratkovab

aInstitute of Solution Chemistry of Russian Academy of Science, Ivanovo, 153045, Russia. bIvanovo State University of Chemistry and Technology, Ivanovo, 153000, Russia. @Corresponding author E-mail: [email protected]

Although complexation reactions of porphyrins with metal salts in organic solvents have a long research history, the influence of pH on these processes was not studied. The main reason is complications connected with the pH measurements in nonaqueous solutions. To solve this problem the special instrumentation was created for application of the spectropotentiometric method (spectroscopy + pH-metry with glass electrode) in nonaqueous solutions. Using this method it became possible to investigate complexation reactions at different pH values using electronic absorption spectroscopy. The pH control of reaction systems allows us to explain the peculiarities of metalloporphyrins formation without invoking the Fleischer's idea of SAT-complexes.

Introduction

Substituted derivatives of 21^,23^-porphine (H2P) are named porphyrins (H2L). Due to the presence in the coordination cavity of the porphyrin macrocycle of two types of activity centers (acidic imino-groups and basic aza-atoms) porphyrins are amphoteric and can produce in the protolytic reactions four types of ionic species L2-, HL-, H3L+ h H4L2+ according to Equilibria (1)-(4) which are correct for polar solvents (S):

k„

H2L — HL-

HL- + H+

-> L2- + H

H3L+ ® H4Lz

H2L + H+ < к " TTT +

H3L+ + H+ < K ч ^ т2+

(1) (2)

(3)

(4)

Due to the double positive charge and presence of four endocyclic NH-groups which are hydrogen bonds donors, the dication H4L2+ in contrast to H2L and H3L+ exhibits properties of the anion-molecular receptor.[1'2'3] In solutions H4L2+ exists only in the form of supramolecular homogeneous and heterogeneous complexes with solvent molecules and background anions - H4L+S2, HL2+S(X-) and H4L2+(X-)2.

Equilibria (1)-(4) cover a wide pH range. Deuteroporphyrin IX dimethyl ester (H2DP) is the only porphyrin for which both acid and base ionization constants have been reliably determined in one solvent (DMSO). The difference between Ka1 and Kb1 exceeds 24 orders of magnitude. Constants of the Equilibria (1)-(4) for H2DP and some other porphyrins are presented in Table 1.

Substitution of two endocyclic hydrogen atoms in porphyrins by a metal cation leads to metalloporphyrins. In these complexes, e.g. MnL, porphyrins can be considered as dianionic tetradentate ligands. The reactivity of the ionic forms of porphyrins in complexation with metal cations decreases in the order L2- > HL- > H2L > H3L+ > H4L2+ > H4L2+S(HaP) > H4L2+(Hal)2. Therefore acid-base properties of porphyrins and pH of the medium should have a strong influence on the mechanism of metalloporphyrin formation.

Complexes of Cd2+ with "acidic" porphyrazine, H2PA, and its derivatives are formed in System (5) as a result of interaction between the metal ion and the most reactive form of ligand (L2-) according to Equilibrium (6):[5].

Cd(Ac)2 - H2L - HCl04 - DMSO (5)

L2- + Cd2+ < Kst ) CdL (6)

H2P

о

O \

H2DP (R= H) H2MP(R= C2H5) H2PP(R= CH=CH2)

CHN

\^NH N

\

-N NH'

N

н2ра

N

MIIP

Table 1. Constants of Equilibria (1)-(4) measured by spectropotentiometric method at 298 K.

Solvent

DN

Ki

H2PA

H2P

H2DP

H2MP

Dimethyl-

sulfoxide 46.68 29.8 (DMSO)

pK„i pKa2

lgKbi

lgKb2

11.94 ± 0.04 [4'5] 13.45 ± 0.08 [4'5]

22.35 ± 0.02

[6]

25.30 [9-10]

0.87 ± 0.03 [8] 0.04 ± 0.03 [8]

1.48 ± 0.03 [8] 0.65 ± 0.03 [8]

Acetonitrile (AN)

36.02 14.1

lgKb1 lgKb2

9.15 ± 0.15 [7] 6.20 ± 0.15 [7]

9.17 ± 0.03 [8] 5.80 ± 0.03 [8]

11.95 ± 0.05 [1] 7.51 ± 0.05 [1]

The H2PA molecules are ionized partly even in pure DMSO (pH » 10) - the quota of [HPA-] and [PA2-] is 1.14 and 4-10"4 %, respectively. As a result Equilibrium (6) is completely displaced to the right side and the complexes of porphyrazines are formed instantaneously upon mixing of reagents. Equilibrium (6) can be observed only in acidified solutions of DMSO when only H2L and CdL are seen in the electronic absorption spectra of System (5) and Equilibrium (7) can be studied. The stability constant Kst can be derived from Equation (8).

H2L + Cd2+ -

Ke

CdL + 2H+

(7)

(8)

C,%

100

90 80 70 60 50 40 30 20 1

Figure 1. Equilibrium composition observed for H2PA in System (5) at 298 K.

Figure 1 shows equilibrium composition for System (5) in the case of H2PA. In the point of half-conversion the equilibrium concentration of [PA2-] is only 6.38-10-25 M and its quota is 6.38-10-18 %. Nevertheless, Equilibrium (7) in System (5) is achieved very quickly and has the rates comparable with that usual for protolytic processes. The constants of true and formal kinetic Equations (9) and (10) describing formation of CdL are connected by Equation (11).

V = k [M2+][L2-]

V = k' [M2+][H2L]

k'= kKaKa2/[H+]2

(9)

(10) (11)

Common porphyrins (H2P and its derivatives obtained by substitution of exocyclic hydrogen atoms), have much weaker acidic properties than porphyrazines - the difference

in the Ka1 values is more that 11 orders of magnitude. In this case the neutral form H2L was postulated as the reactive particle in the complex formation.[11-13] In neutral DMSO at overall concentration of H2L 110-5 M the equilibrium concentration of [L2-] is not exceeded 10-33 M. In addition, the mechanisms and conditions of formation and dissociation of common porphyrin complexes are different.[14] Because of that complexation reaction of common porphyrins (12) proceeds slowly and irreversibly:

H2L + M2

K

® ML + 2H+

(12)

It is assumed that positively charged protonated forms of common porphyrins H3L+ and H4L2+ can not be coordinated by metal cations.[14-17] It should be noted that information about structure and reactivity of solvato-complexes of metal ions in nonaqueous solutions at different acidity is absent.

Though Reaction (12) is p^-dependent, the influence of the solution acidity on the complexation kinetics of porphyrins has never been previously studied. The main reason is complication of pH measurements in nonaqueous solutions. To solve this problem we have elaborated the equipment for spectropotentiometric titration (spectroscopy + p^-metre with glass electrode)[1819] in nonaqueous solutions. This enables the investigation of the p^ influence on complexation equilibrium using electronic absorption spectroscopy. Results of the spectropotentiometric investti-gation of the influence of solution acidity on Reaction (12) between the neutral form of porphyrins and metal ions in Systems (13) and (14) are reported in this paper.

H2MP - Cu(N03)2-3H20 - DMSO (13)

H2MP - Cu(N03)2-3H20 - HCl04 - DMSO (14)

Experimental

Reagents

Mesoporphyrin IX dimethyl ether (H2MP) was prepared according to the known method[20] and purified by column chromatography on alumina (II degree of activity by Brockmann, eluent - chloroform). Purity of the product was controlled by electronic absorption spectra.

Chemically pure DMSO was kept over NaOH for 24 hours and distilled in vacuum (2-3 mmHg).[21] Residual amount of water was 0.2267%. Chemically pure HCl04-3.5H20 was used without additional purification. Analytically pure Cu(N03)2-3H20 was prepared as described elsewhere.[22] Pure Et4NCl was recrystallized twice from dry acetonitrile and dried during 24 hours under vacuum (0.01 mmHg) at room temperature. Et4NClO4

e

Ke = K,tKa1Ka2

was prepared by precipitation as result of mixing chemically pure HCI04 • 3.5H20 with purified Et4NCl. After that it was recrystallized from glacial distilled water and dried 24 hours under vacuum (0.01 mmHg) at room temperature.

was kept in water. It was washed with DMSO and drained with filter paper before each measurement. The temperature of the solution was maintained with accuracy ± 0.1°C using liquid thermostate.

Measurements

Measurements were pursued with the specially designed spectropotentiometric cell (Figure 2).

Figure 2. Spectropotentiometric cell (1 - rabble; 2 - mercury thermometer; 3 - microsyringe with titrant; 4 - gas feed capillary; 5 - thermostat; 6 - optical cell; 7 - work solution; 8 - reference electrode; 9 - glass electrode; 10 - electrolytic bridge)

Graduation of the Element for pH Measurements in DMSO.

Graduation of the glass electrode was carried out with buffer solutions in DMSO (Table 2.).

Table 2. The pH values of buffer solutions in DMSO in the temperature range 298 - 318 K.[18,23]

Composition (1:1) C, M pH

picric acid + it's lithium salt 0.05 1.10 - 0.005(7 - 298)

salicylic acid + it's sodium salt 0.05 6.05 - 0.010(7 - 298)

2- nitrobenzoic acid + it's sodium salt 0.05 7.16 - 0.001(7 - 298)

benzoic acid + it's sodium salt 0.05 9.60 - 0.003(7 - 298)

Kinetic Experiment

Reaction (12) was investigated in systems I, II, III, IV, V and VI (Table 3). To 75 ml of the pH-neutral or acidified H2MP solution 3 ml of the concentrated solution of Cu(NO3)2 • 3H20 was added and following changes of pH and electronic absorption spectra were registered.

Calculations

Values of [CuMP] and [H2MP] were calculated using Equations (16)-(18).

Electronic absorption spectra were measured with spectrophotometer Agilent 8453. Potentiometric measurements (accuracy 1 mV) were carried out with Element (15), using pH-meter OP 211, glass electrode EGL-43-07 (GE) and silver chloride reference electrode filled with Et4NCl in DMSO.

(15)

saturated 0,01 M System

AgCl solution solution

of Et4NCl Et4NClO4 (13) or (14) GE

in DMSO in DMSO

To separate the investigated solution from chloride ions the reference electrode was supplied by the electrolytic bridge filled with 0.01 M solution of Et4NCl in DMSO. The glass electrode

CH2Mp= [CuMP] + [H2MP] + [H3MP +] + [H4MP 2+(DMSO)2] (16)

-[CuMP])-1

At = [CuMP]- 6(cuMP)l + (CH2MP

1 + Kb1aH + + Kb1Kb2a

2

eCuMP + Kb1aH+e

H+ H3MP

+ K b1K b2a

b1Jvb2"H+

2

b1Jvb2"H+

-e

[H2 MP] =

Ch2mp - [CuMP]

1 + K b1aH+ + K b1K b2 aH+

H4MP2+ (DMSO)2

(17)

(18)

where CH2MP - general concentration of porphyrin, M; AT -current value of optical density ; ei -molar absorption coefficient at analytical wave-length; aH+=10- pH; Kb1 and Kb2 - constants of Reactions (3) and (4); l - thickness of absorbent layer, cm.

Table 3. Initial conditions of kinetic experiments for reaction systems I-VI at 318K.

System (13) (14)

I II III IV V VI

pHc 9.99 9.99 9.99 3.01 1.31 0.61

C a CH2MP 3.38-10-5 2.78-10-5 2.98-10-5 2.98-10-5 3.01-10-5 3.04-10-5

r a CCu(NO3)2 -3H20 2.90-10-2 1.45-10-2 2.90-10-3 2.98-10-3 2.98-10-3 2.98-10-3

a - analytical concentration in M at 298K

x

X

Results and Discussions

Solvolysis of Cu(N03)2-3H20 in DMSO

Addition of Cu(N03)2-3H20 in p^-neutral DMSO leads to drastic decrease of p^ (Figure 3).

Figure 3. Changes of pH in systems I-VI at 318 K.

Figure 4 shows the pH dependence from concentrations of Cu(N03)2-3H20 and HCl04 which was used to determine acidity of Cu(N03)2-3H20 in DMSO. Both dependencies belong to one straight line which obeys Equation (19) with the correlation factor 0.9997.

pH = - 1.241-lgC + 2.91; N=56

(19)

Under such experimental conditions the Cu(N03)2-3H20 is strong one-basic acid. The complex Cu2+(OH-)(DMSO)5 is a product of the salt solvolysis.[24] Moreover, in the case of non-hydrated copper(II) salt the complex Cu2+(OH )(DMSO)5 might be a product of interactions with residual water.

Figure 4. Dependence of pH from the concentrations of Cu(N03)r3H20 (a) and HClO4 (b) in DMSO at 298 K.

Protonation of H2MP in DMSO

Protonation of H2MP according to Equilibria (3) and (4) was investigated in System (20) at 298-318 K.[8]

H2MP-HClO4- DMSO

(20)

As it has been shown, the porphyrinium dication H4MP2+(DMSO)2 exhibits properties of the anion molecular receptors.[1-3] In the system (20) the equilibrium (21) is completely displaces to the right side while formation of complexes with H2O, ClO4- and NO3" are suppressed with excess of solvent.

H4MP2+ + 2DMSO —

H4MP2+(DMSO)2

(21)

It has been shown[8] that increase of stability of the porphyrin solvatocomplex leads to levelling of Kb1 and Kb2 values. The value of lg(KM/Kb2) is equal to 0.83 for H2MP in DMSO and to 4.44 in acetonitrile under the same conditions (at 298 K). Generally, both Equilibria (3) and (4) are overlapped at lg(Kb1/Kb2) < 4.[25] The constants of acid dissociation of H2MP in DMSO are unknown. Value of pKa1 for H2MP in DMSO should be higher than that for H2DP (pKa1=25.30 in DMSO at 298 K[910]) in which two ethyl radicals are absent. Equilibrium structure of the System (17) was calculated using Equations (22)-(25) at the p^ range from - 2.5 to 4.0 (Figure 5) ignoring the equilibrium concentrations of HMP- and MP2-.

Figure 5. Dependences of equilibrium concentrations from pH in System (20)

CH2MP = [H2MP] + [H3MP+] + [H4MP 2+(DMSO)2] (22) 100%

[H2MP] =--2

1 + Kb1 • a„ + + KM • Kb 2 • a2

H

[H3MP + ] = Kb! • a +• [H2MP]

[H4MP2+ (DMSO)2] = KM • Kb2 • a2H + • [H2MP]

(23)

(24)

(25)

In Figure 5 it is possible to allocate areas corresponding individual step of Equilibrim (3) at pH > 1.7 and Equilibrim (4) at pH < - 1.5, while at 1.7 > pH > - 1.5 both Equilibria (3) and (4) are overlapped. In the electronic

absorption spectra (Figure 6) the equilibria between pairs of light-absorbing species H2MP; H3MP+ and H3MP+; H4MP2+(DMSO)2 correspond to individual series of isosbestic points at 462; 512; 612 and 537; 558; 609 nm, respectively. The isobestic points are not observed in the area of the triple equilibrium between H2MP, H3MP+ and H4MP2+(DMSO)2.

460 480 500 520 540 560 580 600 620 640 l, nm

Figure 6. Changes of electronic absorption spectra in System (20) in the pH range from -2.5 to 4.0 at 318 K; (- -) H2MP; (-.-) H4MP2+(DMSO)2.

Kinetics of Complex Formation in Reaction Systems I - VI

Solvolysis of Cu(N03)2-3H2Ü in DMSO leads to acidification of solution (26):

Cu(N03)2-3H20 + 5DMSO ® Cu2+(0H)(DMSO)5 + 2H2O + 2N03- + H+

(26)

Parameters of the investigated kinetic systems with allowance made for solvolysis are presented in Table 4.

Self acidification observed in systems I-III does not lead to protonation of H2MP, which occurs only at lower values of pH. Therefore the porphyrin exists only in the molecular form and all changes of the electronic absorption spectra are caused only by changes of concentrations of H2MP and CuMP (Figure 7). One series of isobestic points at 510 and 576 nm is in agreement with them. In these isobestic points Equation (27) is fulfilled.

Figure 7. Changes of electronic absorption spectra in Systems I-IV (Table 4): (- -) H2MP at pH 9.99; (-.-) CuMP at pH 4.00 - 2.71.

The presence of the isobestic points indicates the absence of other light-absorbing forms of porphyrin, such as sitting-atop (SAT) complexes.[13'26] The formation of CuMP was studied at the constant concentration of H2MP and various excesses of Cu(N03)2-3H20 in systems I-IV. The reaction obeys the linear kinetic Equation (28) and is characterized by the pseudo-first order rate law in H2MP.

ln([H2MP]0 /[H2MP]) = f

(28)

The parameters of the kinetic dependence (28) are presented in Table 4 and Figure 8.

5-,

4-

£ 3"

2-

1 -

III, IV V

0 2000 4000 6000 8000 10000 12000

A = e1-l-([H2MP] + [CuMP])

(27)

Figure 8. Kinetic dependences of Reaction (12) in systems I - VI.

Table 4. Parameters of reaction systems I - VI with regard to solvolysis of Cu(N03)2-3H20 and protonation of H2MP

System (13) (14)

I II III IV V VI

Range of pH 9.99- 3.87 9.99- 3.87 9.99- 4.00 3.01- 2.71 1.31- 1.23 0.61- 0.60

[H2MP]ca 3.3810-5 2.78-10-5 2.98-10-5 2.8940-5 1.9310-5 0.69-10-5

( % of CH2MP) (100) (100) (100) (98.91) (64.01) (22.75)

CCu(DMSO)5 OH + 2.90-10-2 1.45-10-2 2.9040-3 2.98-10-3 2.9840-3 2.98^10-3

R (Figure 5) 0,999 0,999 0,999 0,999 0,999 0,997

lg kef (318 K) -3.12 -3.36 -3.86 -3.86 -4.01 -4.17

lg kv (318 K) 2.95+0.04

a equilibrium concentration, M (at 298 K). 76

0

t. c

The system IV was investigated at initial pH = 3 (beginning of protonation of H2MP) when total equilibrium content of H3MP+ and H4MP2+(DMSO)2 is 1% and the results of kinetic measurements can not be influenced. This value of pH was only one unit less then that at the end of Cu(N03)2- 3H20 solvolysis. The acidification sharply narrows the interval of pH change to 0.3 units (Figure 5). As has been shown above the salt Cu2+H20(DMS0) 5 is a strong one-basic acid in DMSO. This conclusion was proved by full coincidence of kinetic dependences for systems III and IV.

Spectral changes in the system VI are shown in Figure 9. The equilibrium mixtures of H2MP, H3MP+ and H4MP2+(DMS0)2 (Figure 5) were investigated under initial conditions (Table 4). The pH changes in systems V and VI are very small.

son eoo x

Figure 9. Changes of electronic absorption spectra of system VI (Table 4). (—) H2MP at pH 9.99; (-o_) initial equilibrium mixture H2MP, H3MP+, H4MP2+(DMS0)2 at pH 0.61; (-—) CuMP at pH 0.60.

At the constant value of pH the ration of components in the equilibrium mixture H2MP, H3MP+ and H4MP2+(DMS0)2 is constant (Figure 5). The electronic absorption spectra of such mixtures can be similar to that of individual substances. The formation of CuMP is characterized by the same isobestic points at 510 and 576 nm (Figure 9) in systems I - VI. Thus the only reactive form of the ligand is H2MP. In systems I - III the formation of CuMP was studied at various excesses of salt and constant H2MP concentration. In systems IV, V and VI it was studied at constant concentration of Cu(N03)2- 3H20 taken in excess and various concentrations of the porphyrin ligand.

In all these systems the reaction obeys linear kinetic Equation (28) and it is characterized by pseudo-first order rate law in H2MP. The dependences of effective constant of Reaction (29) from the concentration of the salt Cu(N03)2- 3H20 (in Systems I-IV) and on the concentration of H2MP (in Systems IV-VI) were obtained. In the case of the first order in salt, effective kef and true kv constants can be connected by linear Equation (30). It was experimentally confirmed that this equation is correct for all investigated Systems I-VI.

Cu2+(0H)(DMS0)5 + H2MP ®

® CuMP + H20 + H+ + 5DMS0 (29)

kef = kv [H2MP][Cu2+(0H-)(DMS0)s] (30)

The experimental dependence of kef from [H2MP][Cu2+(0H-)(DMS0)s] (Equation (31)) have the correlation coefficient R = 0.9998 (Figure 10).

kef = (884.66 ± 9.47)[H2MP][Cu2+(0H)(DMS0)s] (31)

The value of kv in Equation (30) is equal 884.66 ± 9.47 at 318 K (lg kv = 2.95+0.04).

[H2MP]-[CU(NO3)2-3H2O]-107, M2

Figure 10. Correlation between kef and [H2MP][Cu2+(0H")(DMS0)5] for Reaction (29) in Systems I-VI at 318 K.

Fleischer's SAT-complex

The porphyrinium dications H4L2+ have properties of pH-dependent anion-molecular receptors. They are formed due to sequential protonation coordination cavity of porphyrin macrocycle by two proton. The second proton activates porphyrinium receptor H4L2+ and starts up self-assembling of anion-molecular complexes H4L2+S2, H4L2+SB h H4L2+B2 (B - molecular or anion substrate). The composition and stability of these complexes are determined by each reaction system.

H4L2+S2 + B < KK > H4L2+SB + S (32)

H4L2+SB + B < Kk , H4L2+B2 + S (33)

Generally, values of Kb1, Kb2, Kd1, Kd2 exhibit antibathic dependencies from basicity (DN) and polarity (e) of solvents In highly basic and polar DMSO (reaction System (14)) the dication H4MP2+ exists as molecular complex H4MP2+(DMSO)2. Because of small values of Kb1 and Kb2 (Table 1), large excess of acid is necessary for formation of H4MP2+ in DMSO. The values of KM and K2 are increased by ten and seven of orders magnitude in going from DMSO to moderately basic and polar acetonitrile (AN). The dication H4MP2+ is characterized by high selectivity to halide-ions. As it can be seen (Table 5), the complexes H4MP2+(AN)(Hal") and H2MP2+(Hal")2 are very stable. Due to the formation of H4MP2+(Cl")2 in acetonitrile the energy of stabilization of H4MP2+ achieves 54.4 kJ/mol.

Table 5. Constants of Equilibriums (32) and (33) in system H2MP - НС1О4 - B -acetonitrile at 298 К

lgKi Substrate (B)

ClO4- [3] a H2O [27] Г [27] Br- [27] Cl- [27]

№1 <<1 0.93±0.08 4.27±0.02 5.13±0.01 5.24±0.02

lgKd2 <<1 0.52±0.05 2.83±0.01 3.31±0.03 4.29±0.03

a SCN-, NO3-, HSO4-, IO3-, CH3COO- [28]

Complexes H4MP2+(AN)(Hal-), H4MP2+(Hal-)2 and H4MP2+(AN)2 have very similar electronic absorption spectra.

Solvents with weak polarity (for example chloroform) are characterized by smaller DN and e values in comparison with acetonitrile. Obviously, the stability of the complexes of porphyrin dications with halogens will be higher in such solvents. This fact allows us to interpret the data obtained in System (34) by Fleischer [26] in a new way.

H2PP - FeCl3 - CHCl3

(34)

In this system instead of the Fe111 complex ClFeinPP a stable species is formed which was named the SAT-complex. It is characterized by electronic absorption spectrum which strongly resembles the spectrum of H4PP2+. Addition of such bases as pyridine and ethanol destroys the SAT-complex with appearance of the molecular form H2PP. This SAT-complex was considered as a mixed complex FeCl3-H2PP. This conclusion did not take into account self-acidification in System (34). Hydrogen chloride which is responsible for acidification appears due to hydrolysis of FeCl3 by residual water and oxidation of chloroform by air oxygen. Therefore Fleischer's SAT-complexes are in fact the chloride complexes of protoporphyrinium dication H4PP2+(Cl)2 and not sitting-a-top FeC^-HPP species. These conclusions may be extended to other analogous systems in solvents of low polarity.

Conclusions

Porphyrins are amphoteric compounds. That is why reactions of metalloporphyrins complex formation are pH-dependent. Depending from the pH of solutions they form four types of ionic species HL-, L2-, H3L+ and H4L2+ which differ in their reactivity in metal complex formation. The porphyrins acid-basic properties are changed widely and determined the mechanism of metalloporphyrins formation. In the case of porphyrin anions metallocomplexes formed at the moment of reagent mixing, while H2L reacts with measurable rate. Porphyrinium cations H3L+ and H4L2+ can not be coordinated by metal ions. The dication H4L2+ obtains additional stability in the presence of halogen anions (Hal) due to formation of complexes H4L2+(Hal) and H4L2+(Hal)2. Products of metal salts hydrolysis and solvents themselves may be hidden sources of hydrogen donors and Hal- anions in nonaqueous solutions. The pH control of reaction systems allows to explain

metalloporphyrins formation peculiarities without idea of Fleischer's SAT-complex formation.

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Received 26.03.2008 Accepted 25.06.2008