Axial coordination of Imidazoles by meso-nitro substituted Zn-octaethylporphyrins Текст научной статьи по специальности «Химические науки»

Порфирины

Porphyrins

Макрогэтэроцмклы

Статья

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http://macroheterocycles.isuct.ru

DOI: 10.6060/mhc131266m

Axial Coordination of Imidazoles by meso-Nitro Substituted Zn-Octaethylporphyrins

Galina M. Mamardashvili, Olga M. Kulikova, Natalya V. Chizhova, Nugzar Zh. Mamardashvili,@ and Oscar I. Koifman

Dedicated to Academician A. I. Konovalov on the occasion of his 80th birthday

G. A. Krestov Institute of Solution Chemistry of Russian Academy of Sciences, 153045 Ivanovo, Russia @Corresponding author E-mail: [email protected]

The study of complex formation between mono-, di-, tri- and tetranitro meso-substituted Zn-octaethylporphyrins with imidazole and 1-methylimidazole by UV-Vis titration method showed that binding ability of the Zn-porphyrins towards investigated N-containing small organic molecules depends on the number of electron-withdrawing nitro-groups in the macrocycle. Taking into account the fact that binding is accompanied by a clear and easily identifiable response in the UV-Vis spectra of the reaction mixture, tri- and tetranitro substituted Zn-porphyrins could be considered as molecular optical sensing devices for small heterocyclic substrates.

Keywords: Zn-octaethylporphyrin, meso-nitro substituted porphyrins, electron withdrawing groups, axial coordination, binding ability, stability constant.

Аксиальная координация имидазолов мезо-нитрозамещёнными Zn-октаэтилпорфиринами

Г. М. Мамардашвили, О. М. Куликова, Н. В. Чижова,

H. Ж. Мамардашвили, @ О. И. Койфман

Посвящается академику РАН А. И. Коновалову по случаю его 80-летнего юбилея

Институт химии растворов РАН им. Г. А. Крестова, 153045 Иваново, Россия @E-mail: [email protected]

Исследованием комплексообразования моно-, ди-, три- и тетра мезо-замещенных Zn-октаэтилпорфиринов с имидазолом и 1-метилимидазолом методом спектрофотометрического титрования показано, что комплексообразующая способность перечисленных Zn-порфиринов по отношению к исследованным N-содержащим малым органическим молекулам определяется числом электроноакцепторных нитро-групп в макроцикле. Принимая во внимание то обстоятельство, что комплексообразование сопровождается четким и легко идентифицируемым откликом в спектрах поглощения реакционной системы, три- и тетра-нитро замещенные Zn-октаэтилпорфирины могут рассматриваться в качестве молекулярных оптических устройств для распознавания малых гетероциклических субстратов.

Ключевые слова: Zn-октаэтилпорфирин, мезо-нитрозамещенные порфирины, электроноакцепторные группы, аксиальная координация, связывающая способность, константа устойчивости.

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Axial Coordination of Imidazoles by Zn-Porphyrins Introduction

Axial coordination in metalloporphyrins is a main driving force of processes of molecular recognition and self-organization of tetrapyrrolic macrocycles into supramolecular ensembles. According with the literature11-41 and our own experimental data[5-11] it is mainly determined by a structure of porphyrin macrocycle, basicity of an extra-ligand and nature of a solvent. Among the listed factors a structure of tetrapyrrole macrocycles renders the highest influence on the metalloporphyrins ability to additional coordination of a ligand, first of all, due to an electronic influence of the substituents. Besides, a chemical modification porphyrin structure can create steric hindrances for ligands axial coordination, or, on the contrary, form intramolecular cavities favorable for additional ligands binding.[11-15] Stability constants of complexes between the correspondingly modified porphyrins and organic ligands of different nature, in comparison with porphyrins of relatively simple structures, can differ by several orders.

This work is devoted to UV-Vis studying of the influence of step-by-step meso-nitro substitution in Zn-octaethylporphyrin on its binding ability towards imidazole (L1) and 1-methylimidazole (L2) in toluene.

Experimental

Zn-5-nitro-2,3,7,8,12,13,17,18-octaethylporphyrin (2), Zn-5,15-dinitro-2,3,7,8,12,13,17,18-octaethylporphyrin (3), Zn-5,10,15-1rinitro-2,3,7,8,12,13,17,18-octaethylporphyrin (4) and Zn-5,10,15,20-tetranitro-2,3,7,8,12,13,17,18-octaethylporphyrin (5) were obtained according to the known procedures.116171 2,8,12,18-Tetrabutyl-3,7,13,17-tetramethylporphyrin (6), 5,15-di-phenyl-2,8,12,18-tetrabutyl-3,7,13,17-tetramethylporphyrin (7) and 5,10,15,20-tetraphenyl-2,8,12,18-tetrabutyl-3,7,13,17-tetra-methylporphyriyn (8) were obtained according to [18]. Zn-2,3,7,8,12,13,17,18-octaethylporphyrin (1) was obtained by complexation of the corresponding porphyrin ligand (2,3,7,8, 12,13,17,18-octaethylporphyrin) with zinc acetate in boiling dimethylformamide and showed the best correlation with the spectral data presented for this compound according to [19]. 2,3,7,8,12,13,17,18-Octaethylporphyrin, imidazole, 1-methyl-imidazole and toluene from Sigma-Aldrich were used without purification. UV-Vis spectra of the porphyrinates and their evolution upon addition of the ligands were measured on a Carry 100 spectrophotometer.

The UV-visible absorption spectral studies reveal red shifted Soret and visible bands upon addition of the ligands to a solution of the investigated porphyrins confirming that the V-containing entity of the ligands binds to the Zn-cation of the coordination centre of the tetrapyrrolic macrocycle.

The stability constant of the metalloporphyrin complexes with the ligands in ratio of 1:1 (Kassoc) according to the literature™ were calculated based on spectrophotometric data at

two wavelengths (decreasing and increasing) using the following relationship:

r , „, A/4. , AA .

[A]-[B] .. >

^•S] AA , AA. .

where, is the decreasing wavelength, is the increasing wavelength, [A] is the Zn-porphyrin concentration, [5] is the ligand concentration, AA 0 is the maximal change of the optical density at the given wavelength, AA( is the change of the optical density of the solution at a given wavelength at a given concentration.

Results and Discussion

Axial coordination of L1 and L2 on Zn-porphyrins (1-5) is accompanied by characteristic red shift of absorption bands in the UV-Vis spectra of the system porphyrinate-ligand. It should be noted that upon complexation of the porphyrinates with the monodentate ligands, over a wide concentration range of the ligands (CL = 0^3T0"4 M), changes in the UV-Vis spectra of the reaction mixture occur with the formation of one family of spectral curves with one set of isosbestic points. The details of spectrophotometric titration upon investigation of the host-guest interactions are described in our previous works.[7-10] The titration curve has one step, which indicates the formation of a single type of complexes in a ratio of 1:1. The changes in the UV-Vis spectra of the system 1-L1 and corresponding binding isotherms are depicted in Figures 1, 2 as an example.

It was found out that step-by-step meso-substitution of the tetrapyrrolic core by one (2), two (3), three (4) and four (5) nitro groups leads to the essential increasing of the stability constants of 1:1 complexes between the porphyrinates 2-5 and monodentate ligands L1 and L2 as compared with the similar complexes of porphyrinate 1 (Table 1, Figure 3).

Table 1. Stability constants of 1:1 complexes (K , M-1) between

~> r \ assoc' '

porphyrinates 1-5 and monodentate ligands L1, L2 in toluene, CZiiP = 1.1-10-5 M.

№ L1 L2

1 25100 32800

2 41900 67200

3 88700 127000

4 188000 196700

5 294000 329000

6 25700 -

7 26200 -

8 26540 -

The error in determining of the stability constants was 5-7 %.[18]

Rr X4-

Rf

R,"

R2

XÍ

R-

-R

-X2 Ri

Xi=X2=X3=Xt = H ; RpRfCiH, (1)

Xi=N02> X2=X3=X4=H; R,=R2=C2H5 (2)

X1=X3=N02>X2=X4=H; R,=R2=C2H5 (3)

X1=X2=X3=N02,X4 = H;R1=R2=C2H5 (4)

X1=X2=X3=X4 = N02; R^R^CjHS (5)

X1=X2=X3=X4=H; R}—C4H9 R2=CÎÎ3 (6)

XrX3= C5H5 , X2=X4=H; R1=C4H9j R2=CH3 (7)

X|=X2=X3=^4 = CgH^ i RC^Hç R2=CH3 (8)

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N. Zh. Mamardashvili et al.

400 450 500 550 600

Wavelength (nm)

Figure 1. Changes in the UV-Vis spectra of the system 1-L1 in toluene at 20 °C, Cl = 1.0-10-5 M.

A

0 0,00005 0,00010 0,00015 0,00020

[L], M

Figure 2. Binding isotherms of the system 1-L1 on the decreasing (400 nm) and the increasing (426 nm) waves lengths in toluene at 20 °C, C = 1.010-5 M.

Kasso„ M"1 350000

Figure 3. Dependence of the stability constants of porphyrinates 1-5 with the ligands L1 and L2 on the nature of small V-containing organic molecules in toluene, 25 °C.

It should be noted that meso-nitro substitution could have strong influence on a porphyrin n-electronic system both due to an electronic effect of the substituents, and deformation of the tetrapyrrolic macrocycle. Step-by-step introduction into the Zn-octaethylporphyrin of one, two, three and four nitro-groups has accompanied by essential distortion of the porphyrinate molecule. Red shift of absorption bands in UV-Vis spectra in going from 1 to 2-5 testifies in favor of this assumption (Table 2).

Similar dependence is known in the case of step-by-step meso-phenyl substitution in a porphyrin macrocycle. [5] However, unlike meso-phenyl substitution of Zn-octa-alkylporphyrins [in going from 2,8,12,18-tetrabutyl-3,7,13,17-tetramethylporphyrin (6) to 5,15-diphenyl-2,8, 12,18-tetrabutyl-3,7,13,17-tetramethylporphyrin (7) and 5,10,15,20-tetraphenyl-2,8,12,18-tetrabutyl-3,7,13,17-tetra-methylporphyrin (8)] in the case of meso-nitro substituted Zn-porphyrinates 2-5 the chemical modification is accompanied

by essential increasing in binding ability of the porphyrinates towards the ligands L1 and L2 (Table 1).

Table 2. Position of the bands in UV-Vis spectra of Zn-porphyrins 1-5 in dichloromethane.[1617]

№ X, nm

1 331 402 490 532 569

2 350 401 497 534 573

3 352 404 503 539 576

4 264 360 413 510 547 571

5 260 370 426 520 561 580

Probably, the nitro-groups, except macrocycle deformation, have strong electronic influence on n-electronic system of tetrapyrrolic macrocycle. It is known that the higher is an electronic density of delocalized n-electronic

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Axial Coordination of Imidazoles by Zn-Porphyrins

system of porphyrin macrocycle, the stronger are the N-Zn bonds of porphyrinate coordination center, and, as a result, donor-acceptor interactions between porphyrinate zinc cation and nitrogen atom of a ligand (Zn-L) is weaker.[1] The acceptor nitro-groups, decreasing an electronic density of the п-electronic system, simultaneously increase a partial positive charge on the porphyrinate zinc cation and facilitate Zn-L interactions. The more nitro-groups are in a macrocycle, the higher is a corresponding binding constant (Table 1).

Conclusions

Thus, the influence of step-by-step meso-nitro substitution in Zn-octaethylporphyrin on its binding ability towards imidazole and 1-methylimidazole was investigated by the method of UV-Vis titration in toluene. The stability constants of the resulting complexes and concentration intervals of their existence were determined. Taking into account the fact that binding is accompanied by a clear and easily identifiable response in the UV-Vis spectra of the reaction mixture, tri- and tetranitro substituted Zn-porphyrins could be considered as molecular optical sensing devices for small heterocyclic substrates.

Acknowledgements. We are indebt for financial support to the Russian Foundation for Basic Research (Project №№ 14-03-00011_a, 14-03-00009) and Program of the Presidium of RAS №24 «Nanomedicines and their targeted delivery».

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Received 01.12.2013 Accepted 25.12.2013

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