Равновесия протонирования порфина, 5,10,15,20-тетрафенилпорфина и 5,10,15,20-тетракис- (4'-сульфонатофенил)порфина в метаноле Текст научной статьи по специальности «Химические науки»

Порфирины Porphyrins

Макрогэтэроцмклы

http://macroheterocycles.isuct.ru

Статья Paper

DOI: 10.6060/mhc2012.120989s

Protonation Equilibriums of Porphin, 5,10,15,20-Tetraphenylporphin, 5,10,15,20-Tetrakis(4'-sulfonatophenyl)porphin in Methanol

Vladimir B. Sheinin,a@ Sergey A. Shabunin,a Elena V. Bobritskayab, Tatiana A. Ageeva,b and Oscar I. Koifmanab

aG.A. Krestov Institute of Solution Chemistry of RAS, 153045 Ivanovo, Russia bIvanovo State University of Chemistry and Technology, 153000 Ivanovo, Russia @Corresponding author E-mail: [email protected]

The comparative investigation of substituent effects and media effects in relation to protonation equilibriums of porphyrinic platform (H2P) in series of porphin, 5,10,15,20-tetraphenylporphin and 5,10,15,20-tetrakis(4'-sulfonatophenyl)-porphin was carried out by spectropotentiometric and computer chemistry (PM3) methods in methanol at 298 K. It was shown, that the receptor Hp++forms methanol solvatocomplexes [Hp++](CH3OH) like "roost" and[H4P++](CH3OH)2 like "double roost", but the equilibriums of the second protonation step ofporphyrins practically are totally shifted to [Hp++](CH3OH). The appropriate step protonation and complexation constants were determined. It was determined, that media effects are the reason of the leveling of the step protonation constants of porphyrins, measured by spectrophotometry method, which "doesn't distinguish" the light-absorbing centers Hp++, [Hp++](CH3OH) and [Hp++] (CH3OH) 2.

Keywords: Tetras(sulfonatophenyl)porphin, protonation, pH-controlled receptor, J-aggregates.

Равновесия протонирования порфина, 5,10,15,20-тетрафенилпорфина и 5,10,15,20-тетракис-(4'-сульфонатофенил)порфина в метаноле

В. Б. Шейнин,а@ С. А. Шабунин,а Е. В. Бобрицкая,3 Т. А. Агеева,13 О. И. КойфманаЬ

Институт химии растворов им. Г.А. Крестова РАН, 153045 Иваново, Россия ьИвановский государственный химико-технологический университет, 153000 Иваново, Россия ®Е-таИ: [email protected]

Методами спектропотенциометрии и компьютерной химии (РМ3) выполнено сравнительное исследование эффектов заместителей и эффектов среды в отношении равновесий протонирования порфириновой платформы (Н2Р) в ряду порфин, 5,10,15,20-тетрафенилпорфин и 5,10,15,20-тетракис(4'-сульфонатофенил)пор-фин в метаноле при 298 К. Показано, что рецептор НР++ образует метанольные сольватокомплексы [НР++] (СН3ОН) типа "насест " и [Н4Р++](СН3ОН)2 типа "двойной насест ", но равновесия второй ступени протонирования практически полностью сдвинуты в сторону [Н4Р++](СНрН)2. Определены соответствующие ступенчатые константы протонирования и комплексообразования. Установлено, что эффекты среды являются причиной нивелирования ступенчатых констант протонирования порфиринов, измеренных спектрофотометрическим методом, который "не различает" поглощающие центры Нр++, [Н4Р++](СН3ОН) и [Нр++](СН3ОН)2.

Ключевые слова: Тетра(сульфонатофенил)порфин, протонирование, рН-управляемый рецептор, /-агрегаты.

Introduction

The formal equations, which are usually used for investigation of reaction of porphyrin protonation (1, 2) and other organic compounds, are equations of proton affinity and don't carry any information about medium effects.

h2p + h+ < b1 > H3P+

H3P+ + H+ < b2 > H4P++

(1) (2)

Previously we have shown, that porphyrinic platform H2P can be diprotonated forming H4P++ receptor, which generates homogeneous 1:G:G and mixed 1:G^G2 complexes like "double roost" (Figure 1) type with solvents hydrogen bond acceptors and anions as guests.[1-5] The investigation of formation processes of solvatocomplexes [H4P++]S and [H4P++]S2 is complicated by the absence of H4P++ optical response. We suppose, that in ionizing solvent the second protonation always starts instantaneous self-assembly of solvatocomplex [H4P++]S2, which is the most stable particle in presence of indifferent acid anion (e.g. perchlorate or triflate).

Figure 1. [H4P

moy5]

These specific interactions, where the solvent plays the role of a reagent, influences on the result of H2P protonation in the solvents of different composition. Previously we have shown, that formation of aquacomplex [H4P++(PhSO3")4](H2O)2 is one of the reasons for the synchronous diprotonation of this compound in water (lg^b1 and lgKb2 equal to 4.85 and 4.71, respectively). [5] But the role of the substituents and other medium effects were not determined. Significance of this investigations

is determined by the fact, that zwitterions H4P++(PhSO3-)4 are tectons for pH-controlled ionic self-assembly of porphyrinic ./-aggregates and nanotubes on their basis, possessing interesting chemical, optical and electronic properties, which can be applied for developing the nanodevices.[6]

The aim of this study was the comparative investigation of substitution effects and solvent effects in relation to protonation equilibrium H2P in series of porphin (H^r), 5,10,15,20-tetraphenylporphin (Н2РPh4) and 5,10,15,20-tetrakis(4'-sulfonatophenyl)porphin in methanol. Methanol was chosen as the suitable solvent for this prophyrins and for potentiometry with glass pH-electrode.[7] Besides, /-aggregates self-assemble from p^+CPhSO^JCCHpH^ sufficiently slowly in methanol, which allows to investigate the protonation equilibriums of Н2Р(PhSO3-)4 with no complications, just like in water.

Experimental

Synthesis. Porphin (H2Por), 5,10,15,20-tetraphenylporphin (H2P(Ph)4) and 5,10,15,20-tetrakis(4'-sulfophenyl)porphin (H2P(PhSO3H)4) in the form of tetrahydrate of tetraammonium salt were obtained by the well-known methods.[8-10]

Spectropotentiometry. The investigation of protonation equilibriums of H2Por, H2P(Ph)4 and H2P(PhSO3-)4 was carried out by spectropotentiometric method at 298 K.[1,5] We add, that we used spectropotentiometric cell in 100 ml volume with optical path legth of 3.5 cm. The glass electrode was graduated in water buffer solutions by Equation 3 with a glance correction for proton activity coefficient in methanol.[7,11]

p^MeOH = pHHOO + 2.34

(3)

The electronic absorption spectra were recorded by spectrophotometer AvaSpec-2048-2 (180-1100 nm).

Calculations. The protonation constants Kb1 and Kb2 were calculated by the method of fitting parameters in Equation 4[12] using program SigmaPlot® software provided by Systat Software Inc. (SSI).

4 =

4CH2P)+ Kb1 -10-pH • A

+ K •10-2pH • A

0(H3P+)+ Kb2 10 A0(H4P++ )

1 + Kb1 •10-pH + Kb2 •10-2pH

(4)

Where A is the current value of solution absorbance on analytic wavelength; A0 , A° and A0 are the component

J ° ' °CH2P)' (H3P+) (HP) r

absorbance, corresponding to the analytical porphyrin concentration (A0 = C0 -6-0.

SO3H

H2Por

H,P(Ph)4

SO3H H2P(PhSO3H)4

SO3H

HO3S

The enthalpies of chemical reactions in the ideal hypothetical gas phase were calculated in terms of Hess law on the basis of PM3-formation enthalpies of reagents.

Results and Discussion

Porphyrin Protonation in Methanol

H2Por. In methanol, only one protolytic Equilibrium 1 between light-absorbing centers H2P and H3P+ (Figure 2а^) was found, which is confirmed by the single assemblage of isosbestic points at 362, 394, 447, 501 and 564 nm and by linear correlation between At of absorption bands in UV-Vis spectra (Figure 2e). The spectropotentiometric titration curve (Figure 2с) is single-step, and the relation lg(C P+/C°H ) = n pH - lgKb1 (Figure 2d) is characterized by the constant of proportionality n (it determines the number of protons, added to H2Por) exactly equal to one. The lgKb1 value for H2Por in methanol is 3.59 ± 0.03 (Table 1).

H2P(Ph)4. Phenyl substituents increase the basicity of the porphyrinic platform and shift the titration curve of

Table 1. Conditional protonation constants of porphyrins in methanol.

Porphyrin lg^b! ± 0°3 lg^b2 ± 0.03 ^b2 - ^b!

H2Por 3.59

H2P2(Ph)4 4.77 2.87 -!.90

H2P(PhSÜ3-)4 5.98 4.37 -1.61

H2P(Ph)4 into the region of greater p^ values (Figure 3d). Due to these facts, both protonation steps for H2P(Ph)4 can be observed in methanol. The first (1) and the second (2) protonation steps correspond to their own families of isosbestic points at 419, 494 and 523 nm (H2P(Ph)4/H3P(Ph)4+) and 367, 427, 447 and 621 nm (H3P(Ph)4+/[H4P(Ph)4++](CH3OH)2) respectively, and to the line sections of correlation dependence A\n = /(At435) (Figure 3o). The measured lgKb1 and lgKb2* values were 4.77± 0.03 and 2.87 ± 0.03, respectively (Table 1). The titration curve is characterized by the small difference lgKb2-lgKb1 = -1.90, therefore it is smooth, and the maximum of CH3P(Ph)4+ is only 83% at pH 3.82 (Figure 3a,b).

a 2,0

1,5-

Я 1,0

0,5

0,0-

388

Ь 1001

300 400 500 600 Wavelength, nm

700

SS 50 О

3 4 5 6 7 pH

1.9 и R=0.ï

1,8-

S. 1,7

fi

S 1,| €

1,5-

1.0

1 2 3 4 5 6 7

pH

1,9-1

1,8

5 1.7-

< 1,6-

1,5-

R=0.99

0,0 0,1 0,2 Absorbance, au (548 nm)

Figure 2. Results of spectropotentiometric titration of H2Por by perchloric acid in methanol at 298 К: H2P (red), H3P+ (blue).

1,0-

§ 0,5

0,0

100 H

400

500 600 Wavelength, nm

700

1,0-

§ 0,5

0,0 0,5 1,0

Absorbance, au (435 nm)

1,0

0,5

R=0.9999

-3 -2-10 1

2 3 4 5 6 7 8 pH

(CH3OH)2 (green).

H2P(PhSO3")4. The protonation results of H2P(PhSO3-)4 (Figure 4 a,c-e) and H2P(Ph)4 are similar, but sulfonate groups shift the titration curve into the region of greater pH values and decrease lgKb2-lgKb1 from -1.90 down to -1.61 (Table 1). As a result, the maximum value of CH ^ decreases to 77% (at pH=5.10). The first and the second protonation steps correspond to their own families of isosbestic points at 421, 484, 527 nm and 374, 430, 452, 620 nm, and to the two line sections on correlation dependence A\n = /(At435) (Figure 4c). Weak absorbance at 489 and 700 nm, which signalizes about the self-assembly of ./-aggregates, appears only at the finish line of titration. We should note, that the more basic water eliminates Kb1 and Kb2 for H2P(PhSO3-)4 more than methanol. The transfer of Reactions 1 and 2 from methanol to water decreases the value lgKb2-Kb1 for H2P(PhSO3-)4 down to 0.14. [5]

Effects of Substituents and Media

association, where the solvent is a reagent, with generating of solvatocomplexes [H4P++]CH3OH, [H4P++] (CH3OH)2, solvated protons CH3OH2 + and dimers (CH3OH)[13]

I protonation step

± H3P+ + 0.5(CH3OH)2

II protonation step

± H4P++ + 0.5(CH3OH)2

H4P++ + 0.5(CH3OH)2 < 3 > [H4P++](CH3OH)

[H4P++KCHPH) + 0.5(CH3OH)2 <= [H4P++](CH3OH)2

H3P+ +CH3OH2+ + 0.5(CH3OH)2 ^

K2 K3K4 y

¿p+kchoh)

(5)

(6)

(7)

(8)

(9)

The Equilibriums of I and II protonation steps of H2P in methanol can be characterized by Equations 5-8 with regarding specific interactions of solvent with H4P++, H+ and its self-

In this case, K., and L, determined from the titration

b1 b2

curve AV = f(pH) by Equation 4, are conditional,[14] because K,, = K /C, K., = KKK C (here C is the concentration

b1 1 s> b2 2 3 4 s v s

Figure 3. Results of spectropotentiometric titration of H.P(Ph). by perchloric acid in methanol at 298 K: H2P (red), H3P+ (blue), [H.P++]

2

32

HP+ + CH OH+ <

32

1,0-1

g 0,5-

0,0-

437

414

b 100

700

400

500 600 Wavelength, nm

700

50-

3 4 5 6 pH

8 9 10

n 0,5

0,0 0,2 0,4 0,6 0,8 1,0 Absorbance, au (437 nm)

S 0,5-

-1 0 1

23456789 10 pH

Figure 4. Results of spectropotentiometric titration of H2P(PhSO3")4 by perchloric acid in methanol at 298 K: H2P (red), H3P+ (blue), [H4P++](CH3OH)2 (green).

(activity) of solvent in solution). The value of K3K4Cs2 is the measure of leveling effect of a solvent in relation to

conditional constants Kb1 and Kb2. The K2,

K3 and K4 values

can be measured only in the form of K2K3K4, and Cs can be calculated by the formula: Cs=1000 p/M, which shows, that the leveling effect Cs2 of the certain solvent is determined by it's density and molecular mass.

In order to differentiate the substituents' effects and media effects in protonation reactions of porphyrins, we used the Equations 10 and 11 at 298 K.

D G0 = D G0 - AG0

s g tr

DG0 = - 1.3639 lg K

(10) (11)

Where AsG° and AgG0 are the standard Gibbs energies of chemical reaction in solution and in absence of media in hypothetical ideal gas phase, respectively, and AtrG0 is the Gibbs energy of chemical reaction transfer into solution, K is the thermodynamic equilibrium constant.

In this case, AgG0 is the measure of absolute (maximal) chemical affinity of the reagents, which depends only on

their molecular structure, and A G0 is the measure of media

' tr

effects, that reduce AgG0. For I and II protonation steps of H2P in methanol, the expressions 10, 11 are transformed to 12 and 13, respectively.

AG,0 = - (PAH2P - PACH3OH) + °.5AgG°(CH3OH)2 - DA0 (12)

D G 0 = - (PAH P+ - PA OH) + 0.5D G0 OH) +

s II v H3P+ CH3OH g (CH3ÜH)2

+D G30 + D G40 - DtrGII0

g 3 g 4 tr II

(13)

Where PAH P, PAH P+ and PACH OH make the proton

\ 3 ^OH ^gG0CCH3OH)2, AgG30 and

AgG40 are contributions of specific interactions, AtrGI0 and

affinity (PA = - DgG°298K), PA,

AtrGII0 are the energies of reactions 5 and 9 transfer from gas

g4

VG

tr II

phase into methanol.

Internal Substituent Effects

The internal[10] (absolute) substituent effects in the absence of medium 80A tG° and 8„A tG° can be calculated

R int b1 R int b2

on the basis of PA„ D and PA„ which are the measure

H2 P H3 r y

of maximal basicity of H2P and H3P+, or on the basis of

Table 2. Internal substituent effects

Basis R AH0 g b1 4 SBA. tG0b1 R int b1 aH0 g b2 4 SrA. tG0 R int b2

kcal/mol % kcal/mol %

H2Por -209.62 -135.61

H2P(Ph)4 -Ph -213.99 -4.37 2.08% -152.84 -17.23 12.71%

H2P(PhSO3-)4 -PhSO3- -351.71 -142.09 67.78% -295.83 -160.22 118.15%

appropriate enthalpies of AgH°b1 and AgH°b2 (the relation Media Effect in Methanol 8A G0 = 8A H is performed in gas phase) by Formulas 14

and 15, like it was done in this work (Table 2).

48RAmtG0b, = - (PA,™. - PA„ „or) =

= A H - A H

g b1(H2PR4) g b1(H2Por)

The integral media effect is made up of PACH OH,

A G0 OH), A G30, A G40, A)rGI0 and AtrGII0.

g (CH3OH)2 g 3 g 4 tr I tr II

(14) Proton Solvation (PAMeOH)

48„A. tG° = - (PA„ - PA„ D +)=

R int b2 v H3P+R4 H3Por+/

= A H0 - A H0

g b2(H3P+R4) g b2(H3Por+)

Meso-phenyls show the polarity effect,[17] which heighten the proton affinity of porphyrinic platform by 2.08% and 12.71% on the first and the second protonation steps, respectively. In H2P(PhSO3-)4, the 5PhAmtG0b1 effect is increased by the negative charge of sulfonate group approximately by 30 times on the first and by 10 times on the second protonation steps. The reason for substituents effects intensification can be the expansion of isoelectronic (18 ne) conjugation major loop from 18 to 20 atoms in series of H2P, H3P+, H4P++ and, consequently, is one of the reasons for reducing the step difference PAH P+ - PAH P in series of RPor, HP(Ph)4, HP(PhSO3-)4.

-N N- „ // "HH' \

Hs

-N N-.

H3P+

The PAMeOH value determines the acidity of solvated (15) proton CH3OH2+ and, consequently, the lower bound of methanol pH scale.[11,18] The differences PA„ - PA„„ „„and

1 H2P CH3OH

PA„ „+ - PA„„ „„ show, that proton solvation (A H° „„ + =

H3P CH3OH 1 g CH3OH2

145.07 kcal/mol) causes the sharp decrease of protonation energy of H2P and H3P+ (Tables 3 and 4). As a result,

the proton transfer from

Œ3Œ2+

to H Por+ becomes

H2P

H4P++

disadvantageous. This fact allows to explain the absence of the second protonation step of H3Por+ in methanol solution.

Formation of Solvatocomplexes [Hp++]CH3OH and [H4P++](CH3OH)2 (AG°(CH3OH)2> AgG0 \Gf)

The [H3P+](CH3OH) complexes were not found in solutions experimentally. The calculations show, that formation of [H3P+](CH3OH) complexes for H2Por, H2P(Ph)4 and H2P(PhSO3-)4 is disadvantageous in gas phase as well, where H3P+, unlike H4P++, can not tear the solvent molecule away from dimer (CH3OH)2. H4P++ form 2 solvatocomplexes [H4P++](CH3OH) and [H4P++](CCH3OH)2 with similar AgG30 and AgG40 values, which is the evidence of weak guest interference effect.[2]

Transfer into Methanol (A G 0and A, G°)

Figure 5. Isoelectronic conjugation major loop of porphyrinic platform and its protonated forms.

The solvation of reagents by transfer of Reactions 5 and 9 into methanol reduces their chemical affinities and,

Table 3. Contributions of solvation effects in kcal/mol.

Basis I step II step

-(PAH2P - PA(CH3OH)) atgi0 tr I - (PAH3P+- pA (CH3OH)) a G30 g 3 aG,1 g 4 atGII0 tr II

H2P -64.55 -59.05 +9.46 -6.06 -6.35

H2P(Ph)4 -68.92 -61.81 -7.77 -5.18 -4.38 -24.79

H2P(PhSO3-)4 -206.64 -197.88 -150.76 -3.08 -2.17 -164.67

Table 4. Contributions of solvation effects in %.

Basis I step II step

PACH3OH AtrGI0 Sum PA CH3OH a G30 g 3 ag40 g 4 atGII0 tr II Sum

H2Por 69.21 28.17 97.38 -106.98 4.47 4.68

H2P(Ph)4 67.79 28.88 96.67 -94.92 4.15 2.86 -10.87 -98.77

H2P(PhSO3")4 41.25 56.35 97.60 -49.04 1.04 0.73 -51.80 -99.06

consequently, the energy of all chemical interactions (Tables 3 and 4). Relatively large values of AtrGI0 and AtrGII0 for H2P(PhSO3-)4 are caused most likely by specific solvation of sulfonate groups. In general, media effects in methanol lower AgGI0 and AgGII0 of porphyrins by 97-99 % up to values of AsGI0 and AsGII0 typical for solutions.

Calculation of K K and K

concentration current value of H3P+, H4P++, [H4P++](CH3OH) and [H4P++](CH3OH)2 on pH of porphyrin solutions in methanol (Figure 6) and compared them with the experimental ones (Figures 1b, 2b, 3b).

C = p

H2P n=5

-•100%

(18)

S Ci

The transfer reduces the differences of protonation step energies of porphyrins. We took notice of the fact, that Relation 16 is constant for H2P(Ph)4 and H2P(PhSO3-)4.

c += kj0ü •kx)o/o

H3P+ n=5

S Ci

(19)

DsGI0 -DsGI0 DgG0 -DgGi0

= 0.12

(16)

It shows that transfer into methanol lowers A G„0 - A G,0

g ii g i

by 88% regardless of the porphyrin molecular structure. Relation 16 was used for calculation of the value K2K3K4 for H2Por. Than, assuming that transfer equally decreases the energy properties of any elemental reaction, we used the Relation 17 to differentiate between K2, K3 and K4 for H2Por, H2P(Ph)4 and H2P(PhSO3-)4 (Table 5).

K1 • K2 ^l0-2pH •100%

H4P++ n=5

C

S Ci i=1

• k2 • K3 • 10"2pH • Cs

[H4P++](CH3OH)

•100%

SCi

C ++ = K1 • K2 • K3f4 •10-2PH • Cs2 •100//

[H4P++](CH3OH)2 n=5

(20)

(21)

(22)

SCi

lg K

D g G

lg(K K3 K4) De G20 +Dg G30 +De G,

(17)

Table 5. Constants of I and II protonation steps of porphyrins in methanol.

Porphyrin lgK1 lgK2 lgK3 lgK4 lgK2-lgK1

H2Por 4.98 1.17 -0.87 -0.91 -3.81

H2P2(Ph)4 6.16 0.72 0.41 0.35 -5.44

H2P(PhSO3-)4 7.37 2.88 0.06 0.04 -4.49

To verify the calculated Kv K2, K3 and K4 values, we made the simulation dependences (Equations 18-23) of

^ Ci = 1 + K •10-pH + Kj • K2 •10-2pH +

i=1

+K • K2 • K3 • 10-2pH • Cs + Kj • K2 • K3 • K4 • 10-2pH • C2 (23)

In the case of H2Por the maximal concentration of H3P+ (Figure 6, H2Por, n=5) reaches only 89% (2.40 pH) compared to 97% in the experiment. The observed divergence can be explained by sharp deviate of the real experimental system properties from model on the edge of methanol acidity. For H2P(Ph)4 and H2P(PhSO3-)4 simulation dependences (Figure 6, H2P(Ph)4 and H2P(PhSO3-)4 at n=5) agree ideally with the experimental ones, showing, that equilibriums of the second protonation step of these porphyrins practically are totally

s

.0

1

H2Por

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

PHmoh

£ o" S

H2P(PhS03)4

-4 -3 -2 -1 012345678 PHkmh

[H4P++](CH3OH)2 (dark green). Points n=5, lines n=3.

^^^ ^ 3

1

i=1

i=1

i=1

i=1

4

shifted to [H4P++](CH3OH)2 because of the big concentration (activity) of the solvent in the solution, just as we supposed in the beginning of this work.

For graphic illustration of the media effects, we made hypothetical dependences (18-20), neglecting the formation of solvatocomplexes [H4P++](CH3OH) and [H4P++](CH3OH)2 (Figure 6, n=3). These dependences (solid lines) show, that without complexing the concentration of H3P+ reaches 100% in all cases and, consequently, the titration curves would be two-step. But in the methanol pH scale we would observe only the first step of protonation of H2P(Ph)4 and the half of the second one for H2P(PhSO3-)4.

Conclusions

The diprotonated porphyrinic platform H4P++ is a molecular receptor and bundles up the methanol molecules, forming the solvatocomplexes [H4P++](CH3OH) and [H4P++] (CH3OH)2. The equilibriums of the second protonation step of H2P composed of 5,10,15,20-tetraphenylporphin and 5,10,15,20-tetrakis(4'-sulfonatophenyl)porphin practically are totally shifted to solvatocomplexes [H4P++](CH3OH)2. This effect is the reason for levelling the step protonation constants of porphyrins, measured by spectrophotometric method, which "doesn't distinguish" the light-absorbing centers H4P++, [H4P++](CH3OH) and [H4P++](CH3OH)2.

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Received 30.09.2012 Accepted 14.10.2012