Coordination and Spectral Properties of Oxa-Substituted Tetraphenylporphyrin Derivatives

Abstract

Using theoretical conclusions from the Gouterman four-orbital model, a detailed analysis of the formation of absorption and fluorescence spectra of free bases and their zinc complexes has been performed for 5,10,15,20-tetraphenyl-21-oxaporphyrin and 5,10,15,20-tetraphenyl-21,22-dioxaporphyrin. The influence of the symmetry of the molecule on the position and shape of the bands in the electronic absorption and fluorescence spectra is shown. The kinetic parameters of the reactions of complexation of oxasubstituted derivatives of 5,10,15,20-tetraphenylporphine with d-metal salts (Cu(II), Zn(II), and Co(II)) at 288–308 K have been determined for the first time. The kinetic data obtained for oxasubstituted derivatives have been compared with results obtained for their classical analogue, tetraphenylporphine.

INTRODUCTION

The structurally simplest porphyrin, porphine, is a tetrapyrrole macroheterocycle. In the reaction center of porphine there are four nitrogen atoms, which for the most part determine the properties of the molecule [1, 2]. Modification of the architecture of a molecule, as a rule, is carried out in two ways: first, by varying the nature and number of peripheral substituents, and secondly, by changing the macroring itself (hydrogenation, attachment of additional rings, and introduction of other heteroatoms). Substitution of pyrrole nitrogen by atoms of Group VIA leads to the formation of new macroheterocyclic systems, porphyrinoids or heterosubstituted porphyrins with unique little-studied properties that differ significantly from the properties of classical porphyrins [3]. The transformation of the reaction center inevitably affects the electronic structure of the macrocycle. At the same time, both physical and chemical properties of the compound change while maintaining the aromatic character of the π-electron system, stability, and the ability to form complexes with various metal cations. In recent years, the chemistry of heteroporphyrins has been developing rapidly: almost every analog of porphyrin and their derivatives, such as chlorins, corroles, tetrabenzoporphyrins, have been obtained, and the possibilities of their practical use as a more effective replacement for conventional metalloporphyrins have been determined [4].

The formation of complexes is an inalienable property of tetrapyrrole macrocycles; therefore, the study of the patterns of their preparation is an urgent and timely task of modern coordination chemistry.

It is known that tetrapyrrole macroheterocycles are effective converters of the primary analytical signal into the optical response of the sensor. They are characterized by fairly intense spectral-luminescent properties, in particular fluorescent ones, that can be used for these purposes. Tetrapyrrole molecules are intensively studied and, in some cases, are used as a source of an analytical signal, which contributes to solving problems associated with the detection and quantitative determination of the content of various ions in solutions. This area of chemistry has recently been actively developing with the aim of creating ion-sensitive materials [5].

This work presents the results of the study and analysis of the optical and coordination properties of 5,10,15,20-tetraphenyl-21-oxaporphyrin and 5,10,15,20-tetraphenyl-21,22-dioxaporphyrin, as well as their structural analog, 5,10,15,20-tetraphenylporphine.

EXPERIMENTAL

5,10,15,20-Tetraphenylporphine (H₂TPP, I), 5,10,15,20-tetraphenyl-21-oxaporphyrin (HOTPP, II), and 5,10,15,20-tetraphenyl-21,22-dioxaporphyrin (O₂TFP, III) were synthesized and isolated according to known methods. The spectral characteristics of the obtained compounds correspond to those reported [6–8].

5,10,15,20-Tetraphenylporphine (I). ESP (chloroform, λ (logε)): 413 (5.60), 513 (4.26), 546 (3.90), 590 (3.70), 650 (3.73). ¹H NMR (500 MHz, CDCl₃, δ, ppm): 8.30 (m, 8H, phenyl o-H), 7.80 (m, 12H, phenyl m- and p-H), 8.75 (8H, β-C), –3.75 (s, 2H, NH).

Anal. calcd. for C₄₄H₃₀N₄ (%): C, 86.11; H, 4.72; N, 9.16. Found (%): C, 86.12; H, 4.75; N, 9.12.

FAB: m/z = 613 (for C₄₄H₂₉N₄ anal. calcd. m/z = 613.24).

5,10,15,20-Tetraphenyl-21-oxaporphyrine (II). EAS (chloroform, λ (lgε)): 418 (5.41), 514 (4.36), 548 (3.81), 617 (3.48), 678 (3.69); ¹H NMR (500 MHz, CDCl₃, δ_H, ppm): 7.67 (m, m- and p-phenyl), 8.09 (t, o-phenyl), 8.46 (d, pyrrole), 8.52 (d, pyrrole), 8.80 (s, pyrrole), 9.10 (s, furan).

Anal. calcd. for C₄₄H₃₀N₃O (%): C, 85.72; H, 4.88; N, 6.80; O, 2.58. Found (%): C, 85.71; H, 4.87; N, 6.81; O, 2.60.

FAB: m/z = 615 (for C₄₄H₂₉N₃O anal. calcd. m/z = 615.73).

5,10,15,20-Tetraphenyl-21,22-dioxaporphyrine (III). EAS (DMF, λ (lgε)): 417 (4.93), 512 (3.91), 547 (3.72), 587 (3.53), 646 (3.41). ¹H NMR (CDCl₃, δ, ppm): 9.77 and 9.68 (d, 4H, furan); 9.01 and 8.94 (d, pyrrole); 8.16 (m, 8H, o-phenyl); 7.71–7.76 (m, 12H, m- and p-phenyl).

Anal. calcd. for C₄₄H₂₈N₂O₂ (%): C, 85.64; H, 4.55; N, 4.55; O, 5.29. Found (%): C, 85.55; H, 4.55; N, 4.65; O, 5.23.

FAB: m/z = 617.23 (for C₄₄H₂₈N₂O₂ anal. calcd. m/z = 616.71).

The synthesis and purification of zinc complexes with ligands (I–III) was carried out according to known methods [6, 9].

Solvents (acetic acid, DMF) and metal acetates pure for analysis (Russian State Standard) were purified by standard methods [10, 11].

Toluene (Aldrich, water content no more than 0.03%) used to measure fluorescence spectra was used without additional purification.

¹H NMR spectra of solutions of compounds I–III were recorded on a Bruker-500 spectrometer with an operating frequency of 500 MHz in CDCl₃. Tetramethylsilane was used as an internal standard.

The fluorescence spectra of tetrapyrrole compounds and their complexes were measured on a Varian Cary Eclipse fluorometer at a temperature of 298 K.

The EAS of porphyrin solutions and the rate of reactions of the formation of complexes of porphyrins I–III were determined on a Shimadzu UV-1800 and a Hitachi U-2000 spectrophotometers.

The rate of the complexation reaction was measured using thermostatically controlled cuvettes in the temperature range from 288 to 348 K. The temperature fluctuation did not exceed 0.1 K.

The first kinetic order of the reaction of the formation of metalloporphyrins was determined on the basis of the linear dependence log($c_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}^{0}$/${{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}$)–τ, where $c_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}^{0}$ and ${{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}$ are the initial and current concentrations of porphyrin.

The concentration of solutions during the experiment was monitored by the change in optical density. The kinetic experiment was carried out with a ~50–100-fold excess of the salt solution concentration in comparison with the macroheterocycle solution, which made it possible to calculate the effective rate constants (k_eff) of the complexation reaction according to the pseudo-first order equation:

$${{k}_{{{\text{eff}}}}} = {\text{ }}\left( {1{\text{/}}\tau } \right){\text{ln}}\left[ {({{A}_{0}}-{{A}_{\infty }}){\text{/}}(A-{{A}_{\infty }})} \right],$$

(1)

where A₀, A, and A_∞ are the optical density of the porphyrin solution at the initial moment, at the moment of time τ, and at the end of the reaction, respectively. The optical density of solutions was measured for each porphyrin at two wavelengths corresponding to the absorption maxima of the ligand and complex. In this case, the root-mean-square error in determining k_eff did not exceed 3%.

The rate constants of the (n + 1) order were calculated using the equation

$${{k}_{{n\,\, + \,\,1}}} = {{k}_{{{\text{eff}}}}}{\text{/}}c_{{{\text{M}}{{{\left( {{\text{OAc}}} \right)}}_{{\text{2}}}}}}^{n},$$

(2)

where n is the order of reaction (2) with respect to salt M(OAc)₂.

The activation energy (E_a) for the studied temperature range was calculated using the Arrhenius equation:

$${{E}_{{\text{a}}}} = 19.1\left[ {({{T}_{1}}{{T}_{2}}){\text{/}}({{T}_{2}}-{{T}_{1}})} \right]{\text{log}}\left( {{{k}_{2}}{\text{/}}{{k}_{1}}} \right),$$

(3)

where k₂ and k₁ are the effective rate constants of the reaction at T₂ and T₁, respectively, and the entropy of the transition state formation process (ΔS^≠) was calculated according to the equation:

$$\Delta {{S}^{ \ne }} = 19.1{\text{log}}{{k}_{v}} + {{E}_{{\text{a}}}}{\text{/}}T-253.$$

(4)

RESULTS AND DISCUSSION

The replacement of one or two central nitrogen atoms with chalcogen atoms leads to significant changes in the electrical and optical properties of the obtained tetrapyrrole and macroheterocycles in comparison with their classical analogs (Table 1).

Table 1. Electronic absorption spectra of free bases and zinc complexes of porphyrins I–III in toluene

A detailed analysis of the formation of absorption spectra of free bases at 21- and 21,23-heterosubstitution reported [12] allowed us to conclude that there are significant changes in the configurational composition of electronic transitions.

As a result of these changes, the Q_x(0.0) band of the long-wavelength electronic transition of 21-ONTPP in toluene, in comparison with the corresponding band of the H₂TPP molecule, is bathochromically shifted from 650 to 674 nm, and the maximum of the Q_y(0.0) absorption band is hypsochromically shifted to 542 nm. The opposite direction of the spectral shifts of the absorption bands Q_x(0.0) and Q_y(0.0) is explained using the four-orbital Gouterman model [13] (Fig. 1 shows the distribution of the electron density on the molecular orbitals of the macrocycle).

Fig. 1.

LUMO and HOMO of the porphyrin macrocycle: (a) LUMO c₁; (b) LUMO c₂; (c) HOMO b₁; (d) HOMO b₂. The size of the circles is proportional to the atomic orbital coefficients. The positive and negative signs of the coefficients are shown with white and black circles, respectively. Axis x is directed along NH–NH (O–NH or O–O) and axis y is directed along N–N.

When phenyl substituents are introduced into the C_m positions of the macrocycle, the energy of the b₁ orbital noticeably increases, and the one-electron configuration b₁ → c₁ makes the greatest contribution to the configurational composition of the transition. Monooxa-substitution leads to a decrease in the energy of the b₁ and c₁ orbitals due to the lower electron density on the heteroatom, and an increase in the electron density on the C_a and C_b carbon atoms causes a slight increase in the energy of the b₂ and c₂ orbitals. Thus, the Q_x(0.0) band of the long-wavelength electronic transition undergoes a bathochromic shift, while the Q_y(0.0) band is hypsochromically shifted. A similar mechanism was proposed to explain the progressive bathochromic spectral shifts with an increase in the number of heteroatoms in the macrocycle in 21,23-heteroporphyrins [12].

Replacement of neighboring pyrrole rings with furan rings (21,22-substitution) leads to a slightly different character of the spectral shifts (Table 1). This is due to the fact that, first, the symmetry of the 21,22-substituted derivatives is lower than that of the 21,23-substituted derivatives, and, secondly, the different nature of the shifts of the molecular orbitals. Thus, the energies of the b₁ and c₁ orbitals decrease approximately in the same way, which leads to the fact that the energy of the dominant one-electron configuration b₁ → c₁ practically does not change: the maximum of the Q_x(0.0) absorption band is observed at 649 nm. The c₂ orbital tends to decrease in energy due to the lower electron density on the heteroatom, which is due to the combined action of negative inductive and stronger mesomeric effects. However, at the same time, the c₂ orbital tends to increase its energy due to an increase in the electron density on the C_a and C_b carbon atoms. As a result, the position of the c₂ orbital practically does not change. A similar situation is observed for the b₂ orbital. As a result of the totality of these interactions, the electronic absorption spectrum of 21,22-O₂TPP practically does not differ from the absorption spectrum of the initial Н₂TPP (Table 1).

The formation of metal complexes of 21- and 21,22-oxasubstituted TPP molecules with the Zn²⁺ ion is accompanied by significant changes in the electronic absorption spectra; however, they have a more complex structure than the spectra of metal complexes of “classical” porphyrins. Let us consider the formation of the electronic absorption spectrum of the ZnOHTPP molecule (Fig. 2).

Fig. 2.

Absorption spectrum of ZnONTPP in toluene.

The spectrum is characterized by two features: first, three Q-bands are observed in the visible region of the spectrum rather than two, as in “classical” metal complexes; second, the Soret band splits into two bands. The main reason for such spectral manifestations is, in our opinion, in the lower symmetry of the ZnOHTPP molecule in comparison with classical porphyrins. If the latter belong to the point symmetry group D_4h, then for metal complexes of 21-heteroporphyrins the symmetry cannot be higher than C_2v, as well as for their free bases, while the free bases of classical porphyrins have the D_2h symmetry. The transition to a metal complex in classical porphyrins is accompanied by a twofold degeneracy of orbitals, the magnitude of the splitting between which at the free base of porphyrin is ~3000 cm^–1 [14]. Due to the lower symmetry of the metal complexes of 21-heteroporphyrins, the molecular orbitals approach each other, but no degeneration is observed. As a result, the Q_y(0.1) and Q_y(0.0) satellite vibronic bandsof the first and second electronic transitions, respectively, are superimposed. Indeed, the second absorption band has an asymmetric profile with a maximum at 591 nm and a shoulder at 583 nm. If we take the shoulder at 583 nm as the Q_y(0.1) band, and the maximum as the Q_y(0.0) band, then for each of the electronic transitions the vibronic repetition frequency will be 1380 ± 50 cm^–1, and the splitting between electronic states is 1200 cm^–1, which is comparable to the splitting of the monoprotonated form of porphyrins belonging to the same point symmetry group [14]. Accordingly, due to the low symmetry, the Soret band splits into two components: B_x with a maximum at 444.5 nm and B_y with a maximum at 428 nm. Note that no splitting is observed for the 21-ONTPP free base, although the Soret band is noticeably broadened in comparison with the H₂TPP molecule. In our opinion, the differences are caused by the different degree of involvement of the heteroatom in the formation of the conjugated π-system. In the free base, the heteroatom is less “included,” while in metal complexes the heteroatom is directly involved in the formation of the coordination bond with the metal ion and has a greater effect.

The symmetry of the ZnO₂TPP molecule increases (point symmetry group C_2h), as a result of which the Soret band is broadened, but not split into components, as in the ZnOHTPP molecule (Fig. 3). In the visible region of the spectrum, two absorption bands are observed (Table 1), as in the case of “classical” porphyrins.

Fig. 3.

Absorption spectrum of ZnO₂TPP in toluene.

The investigated heteroporphyrins, both in the form of a free base and in the form of a metal complex, exhibit fluorescence. For fluorescence spectra, the frequency rule is satisfied, i.e. the position of the maxima in the fluorescence spectra is approximately mirror-symmetric to the position of the maxima in the absorption spectra (Fig. 4). The value of the Stokes shift for the ZnOHTPP molecule is 220 cm^–1, for the ZnO₂TPP molecule it increases to 340 cm^–1. An increase in the value when passing from the 21-oxa substituted derivative to the 21,22-dioxa-substituted derivative indicates an increase in relaxation processes in the lower excited singlet state. Indeed, according to X-ray diffraction data [15], the ZnOHTPP molecule has a nonplanar molecular macrocycle conformation. On the one hand, this is due to the fact that an axial ligand (salt counterion, acetate) is attached to the zinc ion, which induces the formation of a dome-shaped structure. On the other hand, a slightly smaller radius of the oxygen atom as compared to nitrogen causes the macrocycle to shrink and partially leave the plane of the furan ring with the formation of a corrugated macrocycle. The redistribution of the electron density in the excited state creates the prerequisites for structural relaxation, the value of which increases with an increase in the number of heteroatoms in the nucleus of the macrocycle.

Fig. 4.

Fluorescence spectra of zinc complexes normalized to the maximum: (1) ZnTPP, (2) ZnONTPP, (3) ZnTPP, (4) ZnО₂TPP. Excitation wavelength 500 nm.

The introduction of oxygen atoms into the macrocyclic ring leads not only to a change in the basicity of the compounds [16, 17] and the size of the inner cavity, but also to the absence of one or two protons in the nucleus. These changes significantly affect the characteristic properties of porphyrins such as aromaticity and metal binding capacity.

To date, the well-studied reaction of coordination of porphyrins by doubly charged transition metal cations in non-aqueous solvents occurs in accordance with Eq. (5), which has been convincingly shown in studies [1, 2]:

$$\begin{gathered} {{{\text{H}}}_{{\text{2}}}}{\text{P}} + \left[ {{\text{M}}{{{\text{X}}}_{{\text{2}}}}{{{\left( {{\text{Solv}}} \right)}}_{n}}_{{-2}}} \right] \\ \to {\text{MP}} + {\text{ 2HX}} + (n-2){\text{Solv}}. \\ \end{gathered} $$

(5)

However, there are no data in the literature on the features of the course of coordination processes for heterosubstituted porphyrin analogs. In this work, the rate constants were measured for the first time and the energy parameters of the reaction of formation of zinc, copper, and cobalt complexes of heterosubstituted porphyrins II and III were determined in comparison with tetraphenylporphyrin in DMF (Table 2).

Table 2. Kinetic parameters of the coordination reaction of 5,10,15,20-tetraphenyl-21-oxaporphyrin in comparison with 5,10,15,20-tetraphenylporphine with zinc, cobalt, and copper acetates in DMF^*

It is known that for classical porphyrins, reaction (5) obeys the kinetic equation of the first order with respect to the macrocycle [1, 2]:

$$-d{{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}{\text{/}}d\tau = k{{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}c_{{{\text{M}}{{{\text{X}}}_{{\text{2}}}}}}^{n},$$

(6)

where k is the reaction rate constant, $c_{{{\text{M}}{{{\text{X}}}_{{\text{2}}}}}}^{n}$ is the salt concentration, and ${{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}$ is the porphyrin concentration.

For all studied systems, we have found that the reaction of the formation of metalloporphyrins II, III also has the first kinetic order, which is confirmed by the straightforwardness of the dependences in the coordinates log($c_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}^{0}$/${{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}$)–τ ($c_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}^{0}$ and ${{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}$ are the initial and current concentrations of the ligand) and the presence of clear isobestic points. Typical spectral changes during complexation are shown in Fig. 5.

Fig. 5.

Changes in electronic absorption spectra during the HOTPP coordination reaction with zinc acetate in DMF at 298 K. The inset shows the dependence log($c_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}^{0}$/${{c}_{{{{{\text{H}}}_{{\text{2}}}}{\text{P}}}}}$) on τ for the reaction of ZnOTFP formation at 288, 298, and 308 K (${{c}_{{{\text{Zn(OAC}}{{{\text{)}}}_{{\text{2}}}}}}}$ = 1.3 × 10^–3 mol/L).

As follows from the data listed in Table 2, we failed in measuring the kinetics of coordination of 5,10,15,20-tetraphenyl-21,22-dioxaporphyrin with acetates of the above metals in DMF; however, when acetic acid is used as a solvent, complexation is observed [17].

The replacement of one of the nitrogen atoms of the pyrrole fragment of the macrocyclic compound with an oxygen atom leads to an increase in the basic properties of the ligand by about two orders of magnitude [17]. The growth of a partial negative charge on the atoms of the reaction center promotes the strengthening of bonds with tertiary nitrogen atoms N → M in the transition state and thereby causes an increase in the rate of the complexation reaction in comparison with the classical analog. In the coordinating weakly basic solvent DMF, when passing from HOTFP to H₂TPP, the rate constant of the complexation reaction (5) increases with a simultaneous decrease in the activation energy. This is most clearly observed during the formation of copper complexes: the rate constant increases by ~280 times with a decrease in the activation energy by ~12 kJ/mol.

CONCLUSIONS

A detailed analysis of the formation of electronic absorption and fluorescence spectra of both free bases and zinc complexes of oxa-substituted tetraphenylporphine derivatives was carried out. It is shown that the position and intensity of the bands in the spectra are significantly influenced not only by the introduction of a heteroatom, but also by the geometry (symmetry) of the molecule. The mutually opposite influence of these factors leads to the spectra of 5,10,15,20-tetraphenyl-21,22-dioxaporphyrin, which is close to the classical H₂TPP; large differences are observed for 5,10,15,20-tetraphenyl-21-oxaporphyrin.

For the first time, the kinetics of coordination reactions of transition metal salts with oxasubstituted derivatives was studied in comparison with the classical analog of H₂TPP.

It was shown that the modification of the macrocycle by replacing one of the nitrogen atoms of the pyrrole fragment with an oxygen atom promotes an increase in the reaction rate with a regular decrease in the activation energy and entropy.