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Статья Paper
Thermodynamics of Supramolecule Formation between Metal Porphyrin and Pyridine Substituted N-Methylpyrrolidinyl-[60] fullerene
Tatyana N. Lomova,a@ Michael E. Malov,b Michael V. Klyuev,b and Pavel A. Troshinc
aInstitute of the Solution Chemistry of Russian Academy of Sciences, Ivanovo, 153045, Russia bIvanovo State University, Ivanovo, 153025, Russia
cInstitute of Problems of Chemical Physics of Russian Academy of Sciences, Chernogolovka, 142432, Russia Corresponding author E-mail: [email protected]
Thermodynamic characteristics, mechanism, and spectral manifestations of consecutive reactions between (5,10,15,20-tetraphenylporphyrinato)(chloro)indium(III) ((Cl)InTPP) and organic base 2'-(4-pyridyl)-1'-methyl-pyrrolidinyl-[60]fullerene (PyF) in chloroform were studied using spectrophotometric titration andphysicochemical analysis of intermediate forms. The consecutive coordination of two PyF molecules (K1 = 6.9 . 104, K2 = 9.3 . 103 l/mol at 298 K) with the formation of (Cl)(PyF)InTPP and (Cl)(PyF)2 InTPP, and addition of three other PyF molecules on complex periphery producing (Cl)(PyF)2 InTPP. 3(PyF) (K3 = 2.3 l3/mol3) were observed at the first, second, and third steps. The third reaction stage most likely corresponds to n - n complex formation equilibrium between the aromatic complex and PyF with a 1:3 stoichiometry. Perspectives for use of PyF - metalloporphyrin complex as a photovoltaic material are considered.
Keywords: Metal porphyrin, pyridine substituted pyrrolidinyl-[60]fullerene, supramolecule complex formation.
Introduction
Currently, studies of supramolecular interactions between metalloporphyrins and substituted fullerenes as new complexation ligands have very high importance for understanding the principles of action of reaction center in natural and technical photosynthetic antenna models. The additional coordination of nucleophilic particles (molecules and anions) to metalloporphyrins is the simplest example of
supramolecule formation, which can be studied in a most convenient way in a liquid inert solvent. The majority of such studies during recent years,[1-5] however, were directed on organized or chaotically built layers of complexes of cyclic tetrapyrrole ligands. Studies of solid fullerene-porphyrin/phthalocyanine systems allow to characterize the photoinduced electron transfer and optical spectra. They, however, give no new knowledge about the nature of interactions and the stability of these supramolecules. The
Ломова Татьяна Николаевна, родилась в 1950 г. В 1968 - 1973 гг. обучалась в Ивановском химико-технологическом институте (ИХТИ), занимаясь по индивидуальному плану под руководством Б. Д. Березина. В 1973 - 1976 гг. обучалась в очной аспирантуре ИХТИ под руководством Б. Д. Березина и А. С. Акопова и в 1976 г. защитила кандидатскую диссертацию на тему «Синтез и исследование комплексных соединений полимерных фталоцианинов с мостиковыми группами». С1977 по 1979 гг. и с 1980 по 1991 гг. работала соответственно на кафедре органической химии ИХТИ и в Отделе химки порфиринов ИХР РАН, возглавляемых Б. Д. Березиным. В 1990 г. защитила докторскую диссертацию на тему «Реакции сольвопротолитической диссоцииации и факторы стабилизации металлопорфиринов в растворах». С 1992 г. заведует лабораторией «Синтез и реакционная способность металлопорфиринов в растворах» и занимается исследованием смешанных комплексных соединений с порфириновыми лигандами.
Professor Lomova Tatyana Nikolaevna was born in 1950. In 1968-1973 she studied in Ivanovo Institute of Chemical Technology (IICT) and started her research activity in porphyrin chemistry under the direction of Prof. B. D. Berezin. In 1973-1976 she continued her research in the post-graduate courses under the supervision of Prof. B. D. Berezin and Dr. A. S. Akopov and in 1976 received the Degree "Candidate of Chemical Sciences" (PhD) presenting the dissertation "Synthesis and study of polymeric phthalocyanines with bridge groups". In 1977-1979sheheldresearchpositions in the Department of Organic Chemistry of Ivanovo Institute of Chemical Technology and in 1980-1991 in the Department of Porphyrin Chemistry at Institute of Solution Chemistry RAS (Ivanovo) which were headed by Prof. B. D. Berezin. In 1990 she received the degree "Doctor of Chemical Sciences" for the dissertation "Reactions of solvo-protolityc dissociation and factors stabilizing metalloporphyrins in solutions". Since 1992 she is Head of the Department Synthesis and reactivity of metalloporphyrins in solutions at Institute of Solution Chemistry RAS and her current research interests are focused on stady of mixed complex compounds with porphyrin ligands.
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T. N. Lomova et al.
determination of these characteristics in solutions indicates paths for the optimization of fullerene-porphyrin architectures and opens the possibility of the intentional search of substances for practical applications. As a continuation of studies[6] of additional coordination reactions on porphyrin complexes of multiple-charge metal cations as compounds with enhanced coordination unsaturation of the central atom, we studied the thermodynamic characteristics of the reaction of (5,10,15,20-tetraphenylporphyrinato)(chloro)indium(III) ((Cl)InTPP) with organic N-base 2'-(4-pyridyl)-1'-metyl-pyrrolidinyl-[60]fullerene (PyF) in chloroform.
Experimental
(Cl)InTPP was prepared according as described earlier[7] by the coordination reaction of H2TPP with InCl3-3H2O in a 1:2.5 molar ratio in boiling phenol. After the synthesis, the compound was isolated and purified by multiple washing of the cooled reaction mixture in chloroform with warm water and was doubly purified by column chromatography on Al2O3 column using chloroform as an eluant. Yield 60%. Found: In 15.0 ± 0.35 %. C44H28N4ClIn requires In 15.05 %. UV-vis (CHCl3) Xmax nm (lge ): (4282 (3.30), 599 (3.95), 559 (4.32), 520 (3.59), 420 (4".98). !H NMR (CDCl3) 5H ppm: 9.07 (8H, P-protons), 8.38 and 8.12 (8H, o-protons), 7.80 (8H, w-protons), 7.75 (4H, p-protons).
2'-(4-Pyridyl)-1'-methylpyrrolidinyl-[60]fullerene was prepared according to [8] by the reaction in toluene between C60 and azomethinilide that was synthesized from #-methylglycine and 4-carbonylpyridine.
The thermodynamic characteristics of reactions of (Cl)InTPP with PyF in chloroform were studied by spectrophotometric titration. The optical densities of a series of solutions in freshly distilled chloroform with C(Cl)InTPP = 0.86 . 10-7 mol/l and various concentrations of PyF were measured at the working wavelength 560 nm at 298 K. Solutions were held at a constant temperature in a special chamber mounted in the cell compartment of a spectrophotometer. Temperature was maintained constant within ± 0.1 K. The equilibrium constants of reactions were calculated by the equation derived on the basis of the mass action and Buger-Lambert-Beer (for a mixture of two colored compounds) laws (1), where C°MP, CL are the initial concentrations of metalloporphyrin and PyF, respectively, in chloroform; A , A , A
are the optical densities at the working wavelength for solutions of metalloporphyrin, equilibrium mixture at a certain concentration of PyF, and reaction product, respectively; and n is the number of PyF molecules participating in the reaction.
K =
ap- A
A - A
ap - A) A - A0
1-
C - C
ap - A
A„ - A
(1)
The optimization of n and K values and the determination of the root-mean-square deviations for K were performed by the least squares method using the Microsoft Excel program. The relative error in K values was 20-30%. The electronic absorption spectra (quartz cell 1 ± 0.001 cm thick, 298 K) and NMR spectra (tetramethylsilane as internal reference) were recorded on SF-26 and Agilent 8453 UV-vis and Bruker (200 MHz) instruments, respectively. The elemental analysis data were obtained on AAS1 atomic-absorption spectrometer. Quantum-chemical calculations were performed by the PM3 method with complete geometry optimization. Calculations were continued until a gradient of 0.076 - 0.100 kJ/(mol-A) was attained.
Results and Discussion
According to X-ray structure analysis results,[9] In-N and In-Cl bond lengths and the In-Ct distance (Ct is the center of the coordination cavity of porphyrin) are 2.156, 2.369, and 0.71 A, respectively. The In atom in (Cl)InTPP is displaced from the plane of the coordinating porphyrin cavity toward the Cl-axial ligand. The peculiar structure of the five-coordinate (Cl)InTPP complex is responsible for the special features of reactions with the additional coordination of base molecules.
A study of the electronic absorption spectra of (Cl)InTPP complex in chloroform - PyF mixtures over the whole range of component concentrations and the spectrophotometric titration of (Cl)InTPP solution in chloroform by PyF at the working wavelength are indicative of three-step transformation in (Cl)InTPP - PyF system (Figure 1). All the three reaction stages are reversible. This was substantiated
Figure 1. Dependences lg [(A - A0)/ (A^ - A )] - lgCL for the reaction of (Cl)InTPP with PyF in chloroform at the (1) first, (2) second, and (3) third stages (R2 = 0.967, 0.974, and 0.974p respectively).
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Metal Porphyrin - Fullerene Supramolecule Formation
It was found experimentally that equilibrium at the first stage was attained immediately after the mixing of the reagents. The stoichiometry of the reaction between (Cl) InTPP and PyF at the first stage (1:1) was determined by lg [(Ap-A0)/(A№ -Ap)] - lgCL dependence (Figure 1, straight line 1 (tga = 0.86)). Equilibrium mixtures are stable in time. The stoichiometric ratio between the reagents was 1:1, and the absence in the spectra of noticeable absorption band shifts and a comparatively high equilibrium constant value allowed us to suggest that, at the first stage of the reaction of (Cl)InTPP with PyF the coordination of PyF occurs to the sixth coordination place. According to quantum chemical calculations (Table 1) the base is probably coordinated into the trans-position with respect to Cl (Figure 3), i.e. coordination on the side to which the In atom is displaced from the coordination center is sterically hindered.
Equilibrium at the second stage is also established immediately after the mixing of solutions. As at the first stage, it remains stable in time. The equilibrium constant K2 is 9.62 . 103 l/mol, which is seven times smaller than K1. This is indicative of worse conditions for the coordination of the second PyF molecule by the six-coordinated (Cl)(PyF) InTPP complex. There is no any displacement of Cl- acido ligand from the coordination sphere (Table 1, Figure 3). The stoichiometry of the reaction of (Cl)InTPP with PyF is 1:1 (Figure 1, straight line 2, tga = 1.3). This is in agreement with our suggestion. The formation of the seven-coordinated complex likely results in a shift of the central In atom toward the N4 porphyrin plane (the value of l(Nporph-In) changes from 2.078 to 2.099), but the structure of InN4 fragment remains pyramidal with a more or less regular square in the base, which follows from the absence of a noticeable displacement of the absorption band in the electronic absorption spectrum (Figure 2). It is known, the formation of indium(III) complexes with the coordination number 7 is less probable than the formation of six-coordinated complexes.[11] According to the data on ten thousands of compounds with structures determined by X-ray diffraction, the distribution of Inm complexes with the coordination numbers 5, 6, 7, and 8 is as follows: 19, 16, 5, and 2.
Table 1. Some geometrical and energetic parameters for the optimized (PM3) structures.
Parameter PyF (CI)InTPP (Cl)(PyF)InTPP (trans) (Cl)(PyF)InTPP (cis) (Cl)(PyF)InTPP (trans)a (Cl)(PyF)2InTPP
lIn-Cl 2.524 2.651 4.034 2.651 2.931
lN ,-In porph lNcis-In 2.071 2.078 2.097 2.087 2.078 2.099 2.106
lN -In trans 2.158 2.186
qin -0.223 -0.279 -0.237 -0.279 -0.415
qcl -0.259 -0.393 -0.113 -0.393 4.056
qNcis -0.069 -0.071 -1.007
S™, 0.067 0.067 0.118
Eb d bonds -11531.3 -9269.5 -20731.1 -20716.5 -20641.3 -32227.9
E -196240.6 -153580.4 -349751.3 -349736.7 -349661.6 -545957.5
Q 836.3 247.5 1153.4 1168.0 1243.2 2024.1
AEb d bonds -42.9 94.6 144.4 -102.5
a PyF is coordinated as complementary ligand.
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Figure 2. UV-vis spectra of (Cl)InTPP in chloroform (C(Cl)InTPP = 0.86 . 10-7 mol/l) at various concentration of PyF: (7) 3.96 . 10^, (2) 2.47 . 10-4 and (3) 7.45 . 10-4 mol/l. The spectra correspond to the reaction products at the (1) first, (2) second, and (3) third stages. Absorbing layer thickness 1.000 cm.
by study of electronic absorption spectrum trends during the dilution of solutions with respect to PyF (Figure 2). The composition of the coordination sphere of step reaction products was substantiated by spectrophotometric experiments and PM3 calculations (Table 1).
During the titration process the band intensity (2max 560 nm) in the UV-vis spectra is decreased without noticeable shifts of the band maxima and series of isosbestic points corresponding to the first and at second stages are observed (CPyF range is (1.48 - 8.88) . 10-5 and (1.23 - 4.93) . 10-4 mol/l, respectively). The K1 constant for the first stage is 6.92 . 104 l/mol at 298 K. It is substantially higher than is usually determined for the coordination of bases by metalloporphyrins with a mixed coordination sphere.[10]
T. N. Lomova et al.
(Cl)(PyF)InTPP (trans) Figure 3. Optimized structures of six- and seven-coordinated fullerene-porphyrins.
(Cl)(PyF)?InTPP
At the third reaction stage three PyF molecules participate in equilibrium (Figure 1, line 3, tga = 3.1). These molecules cannot enter into the coordination sphere of In because of its saturation. Taking into account what is known about the presence of solvents in crystal lattices of metalloporphyrins,[12] specific n - n interaction between the electron systems of PyF molecules and the coordinated aromatic macroring can be suggested. At this stage, splitting of the band 560 nm in UV-vis spectrum into two components of 556 and 561 nm is observed for the first time for the reacting system (Figure 2). The equilibrium constant is K3 = 2.34-103 l3/mol3. Changes in the electronic absorption spectrum and in the constant of complex formation can mean the transition to solvate of the (Cl)(PyF)2InTPP with three PyF molecules. The results of quantum chemical calculations (Figure 3) show that even when interaction occurs with the first PyF molecule, two aromatic systems are not indifferent to each other, because, in the isolated (Cl) InTPP + PyF system, the arrangement of the pyridine ring of the coordinated PyF molecule is substantially different from axial.
The data of our work show that the mixed complex of porphyrin with multiple-charge cation and pyridine substituted fullerene is the self-organizing system having high stability in the solutions. It is shown[13,14] that the photoinduced separation of charge takes place in these systems. Thus, we deal with the model of the photosynthetic antenna. In this respect the presence of the intensive absorption band at 420 nm in the electronic spectrum of (Cl)InTPP (see Experimental) seems to be very promisable.
Acknowledgments. This work was financially supported by grants of Presidium of Russian Academy of Sciences, Program no. 18, and Analytic Departmental Special Purpose
Program "The Development of Scientific Potential of the Higher School," project no. 2.2.1.1.2820.
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Received 05.06.2009 Accepted 12.06.2009
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