Remarkable Electronic Effect on the meso-Tetra(thienyl)porphyrins

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Remarkable Electronic Effect on the meso-Tetra(thienyl)porphyrins Taíse H. O. Leite,† Gregory Grawe,† João Honorato,† Beatriz N. Cunha,*,†,‡ Otaciro R. Nascimento,§ Pamela S. de Vargas,† Carolina Donatoni,† Kleber T. Oliveira,† Jefferson M. S. Lopes,∥ Newton M. Barbosa Neto,∥ Wania C. Moreira,† Luis R. Dinelli,⊥ and Alzir A. Batista*,† †

Departamento de Química, Universidade Federal de São Carlos, CP 676, CEP 13565-905, São Carlos, São Paulo, Brazil Instituto Federal Goiano, Campus Ceres, Rodovia GO-154 KM 03, CP 51, 76300-000, Ceres, Goiás, Brazil § Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, CEP 13560-970, São Carlos, São Paulo, Brazil ∥ Instituto de Ciências Exatas e Naturais, Programa de Pós-graduaçaõ em Física, Universidade Federal do Pará, CEP 66075-110, Belém, Pará, Brazil ⊥ Faculdade de Ciências Integradas do Pontal, Universidade Federal de Uberlândia, Rua Vinte, 1600, CEP 38304-402, Ituiutaba, Minas Gerais, Brazil

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‡

S Supporting Information *

ABSTRACT: Complexes derived from meso-tetra(thienyl)porphyrins (TThP) and meso-tetra(pyridyl)porphyrin (TPyP) containing peripheral ruthenium complexes with general formulas {TPyP[RuCl(dppb)(5,5′-Mebipy)] 4 }(PF 6 ) 4 , {TThP[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4, and {TThP-me-[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4 [5,5′-Mebipy = 5,5′-dimethyl-2,2′-bipyridine and dppb = 1,4-bis(diphenylphosphino)butane] were synthesized and characterized by spectroscopy techniques (1Hand 31P{1H}-NMR, IR, UV/vis, fluorescence, and electron paramagnetic resonance (EPR)), cyclic voltammetry, coulometry, molar conductivity, and elemental analysis. Voltammetry and UV/vis studies demonstrated differentiated electronic properties for ruthenium appended with TThP and TThP-me when compared to ruthenium appended with TPyP. The UV/vis analysis for the ruthenium complex derived from TThP and TThP-me, as well as the Soret and Q bands, characteristics of porphyrins, showed a band at 700 nm referring to the Ru → S electronic transition, and porphyrin TThP-me showed another band at 475 nm from the Ru−N transition. The attribution of these bands was confirmed by spectroelectrochemical analysis. Cyclic voltammetry analysis for the ruthenium complex derived from TPyP exhibited only an electrochemical process with E1/2 = 0.47 V assigned to the Ru(II)/Ru(III) redox pair (Fc/Fc+). On the other hand, two processes were observed for the ruthenium complexes derived from TThP and TThP-me, with E1/2 around 0.17 and 0.47 V, which were attributed to the formation of a mixed valence tetranuclear species containing Ru(II) and Ru(III) ions, showing that the peripheral groups are not oxidized at the same potential. Fluorescence spectroscopic experiments show the existence of a mixed state of emission in the supramolecular porphyrin moieties. The results suggest the formation of Ru(II)−Ru(III) mixed valence complexes when oxidation potential was applied around 0.17 V in the {TThP[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4 and {TThP-me[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4 species.

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INTRODUCTION Porphyrins and their derivatives are materials markedly important in chemistry, materials science, physics, biology, and medicine. These materials are extremely versatile and can be modified in different ways, showing molecules with new chemical, physical, and biological properties in each modification, which can be used for several technical and medicinal applications.1−4 The union of the photophysical and structural properties of porphyrins with biological properties of ruthenium-based compounds is of great interest to the development of new chemotherapeutic agents for cancer treatment.5 Recent studies © XXXX American Chemical Society

have shown unique properties of ruthenium compounds for cancer treatment, while others highlight the possibility using porphyrins in photodynamic therapy, due to their ability to accumulate in tumor tissues.5−7 Some research has focused on joining the properties of porphyrin and ruthenium complexes. Thus, different types of ruthenium complexes and porphyrins have been used to form supramolecular structures with differentiated chemical and physical characteristics, which allow diverse applications, e.g., the meso-tetra(pyridyl)Received: April 15, 2018

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DOI: 10.1021/acs.inorgchem.8b01032 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Proposed structures for the synthesized complexes, 1, 2, 3a, and 3b.

ing phosphine and bipyridine ligands, focusing on studying electronic properties of this new molecular scaffold. For the sake of comparison, the meso-tetra(pyridyl)porphyrin (Figure 1: 2) with the same ruthenium peripheral was synthesized. In addition, the complex with the ruthenium peripheral group containing the phosphine, bipyridine, and the binder thiophene ligand (Figure 1: 1) (substituent of the mesotetra(thienyl)porphyrin macrocycle) was also synthesized in order to study the effect on the presence of the macrocycle and the position of the substituents. Our main interest in this class of macrocycles is to understand its electronic properties and use them as possible nonlinear optical material in the near future.

porphyrins with peripheral ruthenium complexes were used to construct chemically modified electrodes.8,9 This coordination leads to modifications of chemical properties of the rutheniumporphyrin system conferring to them specific functionalities, such as molecular recognition.10−12 In addition to several other studies of Ru-porphyrin compounds, the fluorescence, phosphorescent, and photoelectrochemical properties of these compounds are highlighted, especially as potential photosensitizing chemotherapeutic agents and surface modifiers of semiconductors.13−23 Specifically, for the meso-tetra(thienyl)porphyrins, there are practically no studies in the literature investigating their association with peripheral ruthenium complexes. However, this class of porphyrins has been very prominent over the last decades as they present structural differences, when compared with the TPP and TPyP porphyrins,24−26 enabling applications of electron transfer, electrocatalysis,27 biosensors,28 photovoltaic cells,29 and photoionization,30 as well as photoconductive processes31 and therapeutic application.32 Furthermore, studies have shown that there is a great influence of the thienyl substituent on properties of this class of porphyrins25 and when covalently attached to the macrocycle at the meso positions, their π system strongly interacts with the π system of the macrocycle, which is not observed for TPP and TPyP porphyrins.26 In addition, preliminary studies of mesotetra(thienyl)porphyrins have shown that this class of compounds can be used as sensitizers in photodynamic therapy. Therefore, the structural differences of thienyl porphyrins instigate our interest in their behavior with respect to their coordination to the ruthenium metal in their peripheral thienyl groups. Herein we report a new class of compounds where the meso-tetra(thienyl)porphyrins (Figure 1: 3a and 3b) are coordinated to four peripheral ruthenium complexes contain-

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RESULTS AND DISCUSSION Synthesis and Characterization. The complexes were synthesized by substituting one chlorido ligand from the precursor [RuCl2(dppb)(5,5′-Mebipy)] [5,5′-Mebipy = 5,5′dimethyl-2,2′-bipyridine and dppb = 1,4-bis(diphenylphosphino)butane] by the ligands of interest: thiophene (1), mesotetra(pyridyl)porphyrin (TPyP) (2), meso-tetra(thien-2′-yl)porphyrin (TThP) (3a), and meso-tetra(5′-methyl-thien-2′yl)porphyrin (TThP-me) (3b). The elemental analyses of the complexes are consistent with the proposed structures (Figure 1). The molar conductivity in CH2Cl2 of complex 1 is 41.51 S cm2 mol−1, indicating electrolyte 1:1, which is also consistent with the proposed structure, as shown in Figure 1. For complex 2, the conductivity value is 354 S cm2 mol−1, in acetone, and for complexes 3a and 3b, the values are 124 and 125 S cm2 mol−1, respectively, in CH2Cl2, indicating 1:4 electrolyte, as expected.33 In the IR spectra the main bands of interest are νCN referring to the 5,5′-Mebipy ligands, νP−C referring to B


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Inorganic Chemistry

Figure 2. 31P{1H} NMR spectra for the 1, 2, 3a, and 3b complexes, in CDCl3.

Figure 3. Cyclic voltammograms for the 1, 2, 3a, and 3b complexes. Electrolyte: TBAP 0.1 mol L−1; Solvent: CH2Cl2; Reference: Ag/AgCl; scan rate: 100 mV·s−1.

the dppb ligands, and νP−F from the counterion PF6 (see the Experimental Section).25 Signals from 1H and 13C NMR spectra of 1 were assigned based on one-dimensional (1D) and two-dimensional (2D) correlation spectroscopy (COSY), heteronuclear single quantum coherence (HSQC), and heteronuclear multiple bond coherence (HMBC) experiments. However, there was a great difficulty in assigning all 1H and COSY signals of complexes 2, 3a, and 3b due to the large number of overlaps. Moreover, there are no peaks attributable to either the starting metal complex or free ligand (TPyP, TThP, or TThP-me). The obtained integral values are consistent with the number of protons present in the proposed structure: for complex 2, 186 hydrogens; for complex 3a, 182 hydrogens; and for complex 3b, 190 hydrogens (for complete NMR data, see the Experimental Section).

The 31P{1H} spectra (Figure 2) correspond to the stereochemical arrangement of the ligands around the metal, where there are two doublets showing the nonequivalence of the phosphorus atoms from the dppb ligand.34 Complex 1 has a standard 31P{1H} spectrum of the AX system, while the spectra of complexes 2, 3a, and 3b are AB systems.34 Complex 2 has two closely related doublets, because the phosphorus atoms are trans to similar nitrogen atoms. They undergo the phenomenon of degeneracy of the chemical displacements of 31 1 P{ H}.35 Previous studies conducted in our laboratory have demonstrated that at lower temperatures, solvent variation can promote a greater separation and definition of the doubledeckers.36 The chemical shifts of the 31P{1H} signals and their respective constants of couplings are shown in the Experimental Section. By comparing the 31P(1H} NMR spectra for C


Article

Inorganic Chemistry complexes 2, 3a, and 3b, it can be observed that the differences in the spectra pattern are significant, suggesting a competitive effect between the phosphorus atoms from the diphosphine and the sulfur atoms from the porphyrins. It should be observed that the phosphorus atom trans to the thienyl substituent (41.56 ppm for 3a and 41.52 ppm for 3b) is in a shielded position when compared to the phosphorus atom trans to the thiophene ligand (56.49 ppm), which is more deshielded. This confirms that the electronic richness of the thienyl substituent is related to its attachment to the porphyrin macrocycle. This suggestion gains support in the electrochemical study of the complexes, as will be seen later in the text. All 31P{1H} NMR spectra for the complexes showed a heptet with displacement in the region of the 136−153 ppm reference to the counterion PF6. Electrochemical studies revealed very interesting characteristics for complexes 3a and 3b. As can be seen from Figure 3, complex 1 shows an electrochemical process around 0.47 V, attributed to the Ru(III)/Ru(II) redox pair, which is in accordance with data from the literature corresponding to the substitution of a chlorido ligand (σ and π donor ligand) from the precursor cis-[RuCl2(dppb)(5,5′-Mebipy)], by thiophene, which is a σ donor and π acceptor.37 Furthermore, complex 2, for the same reason, only shows a single electrochemical process for the four ruthenium peripheral complexes, around 0.47 V. Surprisingly, complexes 3a and 3b showed distinct behavior, where two reversible processes in their respective cyclic voltammograms can be observed: the first one is around −0.03 V, and the second one is around 0.47 V. It is known that free thienyl porphyrins exhibit redox processes.38 As shown in the voltammograms of Figures 21S and 22S of the Supporting Information, the TThP and TThP-Me ligands presented predominantly irreversible redox processes. In contrast, complexes 3a and 3b have well-defined redox pairs, suggesting that they are related to the Ru(II)/(III) redox pairs. Thus, our hypothesis was based on the formation of a mixed valence species, where after the first oxidation process, a tetranuclear species containing both Ru(II) and Ru(III) is formed, suggesting that not all the peripheral groups are oxidized at the same potential. The reversibility of the electrochemical processes was studied by the cathodic and anodic current peak ratio (Ipa/Ipc), which was close to unity for all the scan rates evaluated. It indicates that the Ru(III)/Ru(II) redox pair is reversible (for scan rate studies see the Figures 17S−20S of the Supporting Information). Furthermore, a linear trend was observed when the Ipa and the Ipc were graphically plotted as a function of the square root of the scan rate (v1/2), following the Randles-Sevick equation. This fact suggests that the mass transfer of the Ru(III)/Ru(II) redox pair is a diffusioncontrolled process at the Pt surface electrode. Therefore, in order to support this hypothesis, an EPR experiment coupled to electrolysis was carried out. In this analysis, first, the electrolysis was performed for 40 min for complexes 2, 3a, and 3b at a potential of 0.170 V, followed by the EPR experiment. Moreover, the electrolysis analyses were performed at a potential of 0.67 V, for the same 40 min, and then new EPR spectra were measured. The spectra were recorded from a complex solution of 1.0 × 10−3 mol L−1, in CH2Cl2, at 10.5 K, obtained with helium gas. Figure 4 shows the EPR spectra of complexes 2, 3a, and 3b, with the respective values of the g tensors. It can be observed in Figure 4 that after applying both potentials (0.17 and 0.67 V), there are signals concerning the Ru(III) species, presenting three values of g

Figure 4. EPR spectra of the 2, 3a, and 3b complexes in CH2Cl2 at 10.5 K.

tensors, which are typical for this kind of compound with rhombic symmetries,39 showing that the two electrochemical processes are related to the Ru(II)/Ru(III) redox pairs. Hence, knowing that both observed peaks in the cyclic voltammograms of the complexes refer to the Ru(II)/Ru(III) oxidation processes, coulometry experiments with controlled potentials were carried out in order to determine the number of electrons involved in each process. The data from the electrolysis at the potential of 0.17 V shows that the number of electrons involved in this first process for complexes 3a and 3b is approximately one (see Table 1), indicating that only one peripheral ruthenium species is oxidized at this potential. Unfortunately, the exact number of electrons involved in the second electrolysis process of 3a and 3b complexes at 0.67 V could not be determined because the limiting current did not fall exactly as required to obtain the charge value for three D


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Inorganic Chemistry

the following can be observed: 1) a small red-shift of the Soretand Q-band, indicating a decrease of the HOMO−LUMO energy gap; 2) a broadening in the Soret-bandwidth; 3) an increase in the molar extinction coefficient in the region between the Soret- and Q-band, which is attributed to MLCT absorption of the peripheral groups;34 4) the rise of a new band around 315 nm. Similar behavior was previously observed for the other tetraruthenated H2TPyP.34,35 On the other hand, significant changes were observed in the electronic spectra of complexes 3a and 3b, when compared with complex 2 (Figure 5). For these complexes, the band in the region around 310 nm

Table 1. Coulometry Data of the 3a and 3b Complexes at 0.17 V Potential and Complex 2 at 0.67 V Potentiala complex

potential (V)

n

Q (mol·C)

Ne‑

2 3a 3b

0.67 0.17 0.17

5.49 × 10−7 4.75 × 10−7 3.04 × 10−7

2.4 × 10−1 6.0 × 10−2 3.4 × 10−2

4.5 1.3 1.2

a

n = number of moles; Q = charge; Ne‑ = number of electrons.

electrons, but it was close to it. Therefore, it can be seen in Figure 3 that the intensity of the current in the second process is about three times higher than the intensity of the current in the first oxidation process, where only one electron was involved, which is a reasonable approximation. Considering the above results obtained by cyclic voltammetry, EPR, and coulometry studies, it could be suggested that for complexes 3a and 3b the oxidation of all ruthenium peripheral complexes does not occur simultaneously, as for complex 2. This behavior may occur due to the difference in the spatial position of the thienyl and pyridyl substituents. Thus, the five membered meso thienyl rings of TThP display reduced steric hindrance when compared to the larger six membered phenyl ring of TPyP,24,25 which facilitates the electronic communication of the peripherical ruthenium with the macrocyclic, while in the TPyP this interaction occurs with greater difficulty because the pyridyl group is practically perpendicular to the porphyrin core.11 Thus, it is suggested that for complexes 3a and 3b, the first process, at a lower potential, refers only to the oxidation of one of the peripheral ruthenium atoms, while the higher potential is related to the oxidation of the other three peripheral ruthenium atoms. With the first oxidation the other three ruthenium atoms suffer a decrease in their electronic density, due to compensating for the loss of one of the electrons of the system. That is, there is an electronic communication between ruthenium atoms which is made possible by the porphyrin ring, resulting in the formation of a mixed valence complex. Unfortunately, we were not able to detect this species by UV/vis spectra (charge transfer transition, Ru(II) to Ru(III), but the data from the EPR experiment supports this hypothesis as there is an increase in the signal intensity with the application of the second potential (0.67 V). This is expected because with the application of the second potential, three other Ru(II) species will be oxidized to Ru(III). The electronic spectrum in the region of the UV−vis for complex 1 (Figure 1, SI) presents two bands: one in the region of 310 nm (ε = 9.15 × 103 cm−1 mol−1 L) that can be attributed to the transitions of charge π → π * of the 5,5′Mebipy and thiophene ligands and the band in the region of 402 nm (ε = 2.21 × 103 cm−1 mol−1 L) that can be attributed to MLCT Ru(II) dπ → π*40 of the 5,5′-Mebipy and thiophene ligands. Complex 2 presents a characteristic electronic spectrum of porphyrins,41,42 with a Soret band at 424 nm (ε = 3.4 × 105 cm −1 mol−1 L) and four Q bands: 518 nm (ε = 2.8 × 104 cm −1 mol−1 L), 554 nm (ε = 1.2 × 104 cm −1 mol−1 L), 593 nm (ε = 1.2 × 104 cm−1 mol−1 L), and 649 nm (ε = 5.0 × 103 cm −1 mol−1 L), in chloroform solvent. A band at 315 nm (ε = 9.7 × 104 cm −1 mol−1 L) was also observed, which can be attributed to the π → π* transitions of the ligands. When compared with free base tetrapyridyl porphyrin (H2TPyP) without outlying ruthenium groups (Figure 2, SI), complex 2 presents no evidence of suffering a strong influence of the outlying groups on their electronic transitions. Basically,

Figure 5. UV−vis spectra, in CH2Cl2, for complex 2 ({TPyP[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4), complex 3a ({TThP[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4), and complex 3b ({TThP-me-[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4) at (a) the Soret band region and (b) the Qband and MLCT band region.

(ε = 1.5 × 105 cm−1 mol−1 L, for complex 3a and ε = 1.0 × 105 cm−1 mol−1 L for complex 3b) attributed to the π → π* transitions of the 5,5′-Mebipy ligands is considerably enhanced relative to the Soret band [Figure 5(a)]. Additionally, a red-shift of 4 and 10 nm of the Soret band is observed for complexes 3a and 3b, respectively. Finally, different from complex 2, a new sharp and well-defined band appears at 465 nm for complex 3a (ε = 2.7 × 104 cm−1 mol−1 L) and at 471 nm for complex 3b (ε = 4.3 × 104 cm−1 mol−1 L). In the last case, the new band is even more intense than the Soret band. These new bands can be attributed to the transition of type MLCT Ru(II) dπ → π* in the 5,5′-Mebipy and thienyl ligands, and their rise indicates a strong interaction between the porphyrin macrocyclic and the outlying E


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Inorganic Chemistry ruthenium complexes. Concerning the Q-band, the changes in their structuration are huge. For complex 3a, significant changes in its original shape were observed, while for complex 3b the shape of the Q-band was completely substituted by a broad and less energetic band centered at 747 nm (Figure 5b). In order to verify if the above assignments for complexes 3a and 3b are compatible, spectroelectrochemical experiments were also carried out. First, a potential of 0.17 V was applied and the spectral changes registered, and afterward the same procedure was performed at a potential of 0.67 V. For complex 3a (Figure 6), the application of both potentials promoted a

Figure 7. Spectroelectrochemical for 3b complex, in CH2Cl2 (1.0 × 10−3 mol L−1); Electrolyte: TBAP 0.1 mol L−1; Potential application at 0.17 V; Time: 40 min of electrolysis. Insertion: last spectrum obtained after 40 min of electrolysis.

Regarding the systems studied in this work, it was not possible to observe the expected intervalence transition band in the UV/vis spectra of the complexes. This can be justified by the fact that although the porphyrins are excellent conductors, since they are rich systems in π conjugations, the distance between the metallic centers [Ru(II)/Ru(III)] is very large, making this process difficult. In addition, the solvent used in the spectroelectrochemical experiment was CH2Cl2, which has a low dielectric constant and is also not a good conductor, making the electron transfer process even more difficult to be observed. The solvents used in this type of experiment are usually DMF, DMSO, H2O, CH3CN, etc. However, the complexes studied here do not show great stability and solubility in these solvents. Aiming at obtaining more evidence of the transfer processes between the porphyrin ring and outlying ruthenium groups, steady-state fluorescence spectra were acquired for complexes 2, 3a, and 3b, by excitation of the samples at different wavelengths (Figure 8). When the sample is excited at the Soret band (425 nm), both complexes, 2 and 3a, show typical fluorescence spectra observed for porphyrins, without affecting the ruthenium groups, which present the two peaks observed assigned to the vibronic progression of the Qx band43 [Figures 8(a) and (b)]. The only appreciable difference for complex 3a is that the peaks corresponding to the emission from Qx(0,0) and Qx(1,0) are red-shifted of around 15 nm, when compared with complex 2. However, when complex 2 is excited in the region composed by the superposition of intraligand band from the ruthenium groups and at the higher energetic portion of the Soret band (350 nm) and in the region composed by the superposition of MLCT and Q-band (480 nm), a weak, but noticeable new fluorescence feature is observed at around 800 nm [Figure 8(a)]. Since complex 1 does not present emission in this spectral range (only a very weak emission at around 540 nm is observed for this complex (Figure 3, SI), and as this new feature is not present in the H2TPyP moiety, the observed new spectral signature in the fluorescence spectrum can be attributed to transitions involving states of the two moieties (porphyrin ring and ruthenium outlying group).44,45

Figure 6. Spectroelectrochemical for the complex 3a, in CH2Cl2 [1.0 × 10−3 mol L−1]; Electrolyte: TBAP 0.1 mol L−1; Potentials at 0.17 and 0.67 V; Time: 40 min of electrolysis. Insertion amplification of the band regions 496 and 720 nm.

decrease in the absorption of the bands at 465 and 720 nm, confirming our previous attribution, which were bands MLCT Ru(II) dπ → π * of the 5,5′-Mebipy and thienyl, respectively. This decrease in the absorption of the bands occurs due to the oxidation of the Ru(II) to Ru(III), which makes the system deficient in electrons, interrupting the transition of electrons from the metal in a low oxidation state to the empty orbital of the ligand. It was also observed that the Soret and Q bands of porphyrin underwent a nonlinear alteration, indicating that there is an interaction between the ruthenium and porphyrin complexes. For complex 3b (Figure 7), the bands at 474 and at 745 nm decreased in intensity when the potentials were applied, confirming that the bands are also MLCT, Ru(II) dπ → π* of the 5,5′-Mebipy and thienyl ligands, respectively. In addition, it was observed that when the potential at 0.17 V was applied, the charge transfer bands decreased in intensity. Different than 3a, the Soret band increased by about 60% in intensity, which is a strong indication that there is a significant interaction between the porphyrin ring and the peripheral ruthenium complexes. Another interesting fact is concerned with the reversibility of the electrochemical process in the porphyrin complex because when a potential at −0.43 V is applied, after applying the potential of 0.17 V for 40 min, the bands return to the initial intensity. Therefore, considering the application of the potential at 0.67 V, there was nonlinear behavior for the bands at 434 and 474 nm, whereas for the band at 745 nm, there was a decrease in the absorbance. This is a consequence of the lack of total reversibility of the system in this potential. F


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Inorganic Chemistry

ruthenium outlying groups, or 2) thienyl moieties enhance the population of the electronic mixed state. The last observation is in agreement with the electrochemical and EPR data, which show that thienyl groups allow a good and efficient interaction between the outlying complexes and the porphyrin ring. Finally, Figure 8(c) shows the fluorescence spectra acquired for complex 3b. Basically, even when the complex is excited at the Soret band, there is a relaxation process to the bottom of the transfer state formed by the ruthenium peripheral complexes and porphyrin, emitting energy. This observation clearly shows that the presence of methyl thienyl groups enhances the interaction between porphyrin and outlying groups, probably because they are implied in the formation of even more coplanar structures for complex 3b, when compared with complex 3a.

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CONCLUSIONS In the present study, three new supramolecular complexes consisting of meso-tetra(pyridyl)porphyrins (complex 2) and meso-tetra(thienyl)porphyrins (complexes 3a and 3b) with peripheral ruthenium complexes containing phosphinic and bipyridine ligands were synthesized and characterized. [RuCl(dppb)(5,5′-Mebipy)(thiophene)](PF6) (1) was also synthesized and characterized for comparison with the porphyrin complexes. The structures of the complexes were confirmed by the 31P{H} and 1H NMR techniques, elemental analysis, and molar conductance. Absorption spectroscopy studies in the UV/vis region and cyclic voltammetry revealed different electronic behavior for complexes 2, 3a, and 3b, which was possible to be clarified through the experiments of coulometry, spectroelectrochemistry, and EPR coupled to electrolysis. Fluorescence spectroscopic data clearly show the existence of a mixed state of emission in complexes 3a and 3b. The overall experimental results led us to suggest the formation of Ru(II)− Ru(III) mixed valence complexes when oxidation potential was applied around 0.17 V.

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EXPERIMENTAL SECTION

Materials and Reagents. All the solvents used were treated by the method described in the literature,46 in which the main attention focused on removing water from the solvents. All the experimental procedures were performed in an argon atmosphere (White Martins) to avoid the presence of oxygen and moisture. The chemical reagents used were of analytical grade of Sigma-Aldrich origin: RuCl3·3H2O, triphenylphosphine (PPh3), and 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP). The thienyl porphyrins (TThP and TThP-me) were obtained according to the procedure described in the literature.24 The ruthenium complex cis-[RuCl2(dppb)(5,5′-Mebipy)] [5,5′Mebipy = 5,5′-dimethyl-2,2′-bipyridine and dppb = 1,4-bis(diphenylphosphino)butane] was synthesized according to the methodology previously described in the literature.47 Instruments. 1H and 31P{1H} NMR spectra were recorded on a Bruker DRX 400 MHz, in CDCl3. The 31P{1H} chemical shifts are reported in relation to H3PO4, 85%. The IR spectra were recorded on a FT-IR Bomem-Michelson 102 spectrometer in the range 4000−200 cm−1 using CsI pellets. Elemental analysis (C, H, N, S) was performed in a FISONS instrument, CHNS EA-1108. Conductivity data was obtained using 10−3 M solutions of the complex in acetone, at room temperature, by using a Meter Lab CDM2300 instrument. Electrochemical studies were performed using an electrochemical analyzer BAS, model 100B. This experiment was carried out in an electrochemical cell of a compartment, based on three electrodes (Pt-work and auxiliary; Ag/AgCl-reference) in CH2Cl2, using 0.10 M Bu4NClO4 (TBAP) (Fluka Purum) as a support electrolyte. The measurements were taken at room temperature. Under these

Figure 8. Normalized steady-state fluorescence spectra for (a) complex 2 (excitation wavelengths: black line = 350 nm, red line = 425 nm, and blue line = 480 nm); (b) complex 3a (excitation wavelengths: black line = 350 nm, red line = 425 nm, green line = 440 nm, and blue line = 480 nm), and (c) complex 3b (excitation wavelengths: red line = 425 nm, blue line = 480 nm, and green olive line = 715 nm).

Probably, this new fluorescence peak (around 800 nm) is a deactivation path assigned to charge transfer from ruthenium to porphyrin or vice versa. However, when the sample is excited at the Soret band (425 nm), complex 3a shows similar behavior presented by complex (2), and when excited at 350 and 480 nm, more drastic modifications of fluorescence emission can be observed [see Figure 8(b)]. This observation corroborates with two ideas: 1) there is a possible electronic mixed state formed by the superposition of porphyrin and G


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Inorganic Chemistry

dppb); 2.05 (24H, CH3 of 5,5′-Mebipy); −3,35 (2H, internal hydrogen of the porphyrinic macrocycle); Cyclic voltammetry: Epa = 0.52 V. To obtain the tetraruthenated porphyrin {TThP[RuCl(dppb)(5,5′Mebipy)]4}(PF6)4 (3a), 15 mg (0.023 mmol) of the TThP porphyrin was dissolved in 100 mL of previously deaerated chloroform solvent. 59 mg (0.092 mmol) of the cis-[RuCl2(dppb)(5,5′-Mebipy)] complex, at a proportion of 1:4, was added. 73 mg (0.115 mmol) of NH4PF6 was added at a proportion of 1:5. The mixture was left under inert atmosphere and stirred for 36 h at room temperature. Thereafter, the volume was reduced to about 2 mL in the rotoevaporator, and hexane was added to obtain a yellow-green solid. For purification of the product, it was necessary to wash the solid obtained with chloroform, reduce the volume of the solution, and precipitate it again with hexane, as yellow solid, which was filtered off, washed with water and with diethyl ether, and dried under vacuum. Yield: 57 mg (58%). C196H182Cl4F24N12P12Ru4S4·CHCl3. Elemental Analysis: Found (calc.) %: C = 54.99 (54.68); H = 4.33 (4.26); N = 4.00 (3.88); S = 2.61 (2.96). IR (1% CsI) νmax/cm−1 1627 (νN=C); 1053 (νC−P); 840 (νPF6); 796 (νC−S). 31P{1H} NMR (400 MHz, CHCl3, 298 K): δ (ppm) 43.66 (d); 41.56 (d), 2Jp‑p = 36.84 Hz. 1H{1H} NMR (400 MHz, CHCl3, 298 K): δ (ppm) 9.06− 7.76 (aromatic hydrogen of 5,5′-Mebipy and pyrrolic of TThP); 7.63−6.62 (overlapped signals, aromatic hydrogen for dppb and of group thienyl); 4.12, 3.15, 2.35, 2.02 (32H aliphatic of dppb); 1.97 and 2.08 (24H, CH3 of 5,5′-Mebipy); −2,63 (2H, internal hydrogen of the porphyrin macrocycle); Cyclic voltammetry: Epa = −0.01 and 0.44 V. To obtain the tetraruthenated porphyrin [TThP-me[RuCl(dppb)(5,5′-Mebipy)4](PF6)4 (3b) the same procedure for {TThP[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4 was used, but only 15 mg (0.021 mmol) of the TThP-me and 64 mg (0.092 mmol) of the cis[RuCl2(dppb)(5,5-Mebipy)] were used. Yield: 49 mg (54%). C201H196Cl4F24N16P12Ru4·CHCl3. Elemental Analysis: Found (calc.) %: C = 54.56 (55.08); H = 4.35 (4.39); N = 3.94 (3.83); S = 2.64 (2.93). IR (1% CsI) νmax/cm−1 1607 (νN=C); 1092 (νC−P); 843 (νPF6); 741(νC−S). 31P{1H} NMR (400 MHz, CDCl3, 298 K): δ (ppm) 44.04 (d); 41.52 (d), 2Jp‑p = 36.27 Hz. 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) 9.12−7.63 (hydrogen of 5,5′-Mebipy and TThP-me); 7.52− 6,62 (overlapped signals, aromatic hydrogen for dppb, group thienyl, and 5,5′-Mebipy); 3.17, 2.84, 2.60, 2.55 (32H aliphatic of dppb); 1.97 and 2.08 (24H, CH3 of 5,5′-Mebipy); 1.70 (hydrogen of group CH3 reference 5,5′-Mebiy); −2,61 (2H, internal hydrogen of the porphyrin macrocycle); Cyclic voltammetry: Epa = −0.04 and 0.43 V. EPR Coupled to Electrolysis. For the EPR studies, solutions of the complexes were prepared, 1.0 × 10−3 mol L−1 concentration in CH2Cl2 solvent. The potentials applied for the electrolysis were determined in relation to the oxidation potentials of the complexes (0.170 and 0.57 V). After applying the electrolysis for about 40 min, at 0.17 V potential, EPR spectra were measured, after which a new potential, of 0.57 V, was applied for a further 40 min, and then a second measurement of the EPR spectra was performed. The EPR measurements were performed in helium atmosphere, at 10 K.

conditions, the ferrocene (Fc) is oxidized at 0.43 V (Fc+/Fc).48 The EPR measurements were performed in a Bruker model ELEXSYS electron paramagnetic resonance microwave bridge ESP 380-1010. The samples were prepared in solution, in CH2Cl2, at the liquid helium temperature. The fluorescence and electronic spectra of UV/ vis were obtained in CH2Cl2 on a Hewlett-Packard diode array-8452A scanning spectrophotometer. Spectroelectrochemical experiments were performed by coupling the Hewlett Packard UV/Visible Spectrophotometer Model 8452 to a Pine Instrument Company potentiostat (model RDE 4). A specially designed cell was used for this type of experiment. The experiment was carried out using a method containing three electrodes: one platinum electrode as a work electrode, one platinum auxiliary, and one Ag/AgCl as a reference. CH2Cl2 and TBAP (0.1 mol L−1) were used as the support electrode as the solvent. The applied potential was determined in relation to the oxidation processes of each complex. It was prepared for the solution of the complexes at a concentration of 1.0 × 10−3 mol L−1. The fluorescence spectra were acquired exciting the sample with a xenon lamp after passing through a monochromator in order to choose the excitation wavelength. Moreover, the signal was detected by a portable spectrophotometer from Ocean Optics, USB 2000, in a 90° geometrical configuration. Synthesis of the Complex [RuCl(dppb)(5,5′-Mebipy)(thiophene)]PF6. To obtain the complex [RuCl(dppb)(5,5′Mebipy)(thiophene)]PF6 (1), into a 100 mL flask, 25 mL of CH2Cl2 and acetone (in proportion 75:25%) was previously deaerated. Then, 100 mg (0.127 mmol) of the cis-[RuCl2(dppb)(5,5′-Mebipy)] complex and 10.23 μL (0.127 mmol) of thiophene were added. The reaction was kept under stirring at room temperature for 3 h. Afterward, 30 mg (0.50 mmol) of AgPF6 was added, and the reaction was stirred for 1 h longer. After this period, the reaction was filtered off in Celite to remove the AgCl generated during the reaction. The volume of the reaction was reduced to about 1 mL, and deaerated hexane was added for precipitation of a final solid. The precipitate was filtered off, washed with water, and dried in a vacuum. Yield: 84% (110 mg). C44H44ClF6N2P3RuS·2H2O. Elemental Analysis: Found (calc.) %: C = 52.48 (52.20); H = 4.99 (4.78); N = 2.63 (2.77); S = 3.0 (3.2). IR (1% CsI) νmax/cm−1 1638 (νN=C); 1094 (νC−P); 842 (νPF6); 741(νC−S). Molar conductivity in CH2Cl2, 41.5 S cm2 mol−1. 31P{1H} NMR (400 MHz, CDCl3, 298 K): δ (ppm) 56.49 (d); 42.57 (d), 2Jp‑p = 39.59 Hz. 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) 8.82−7.51 (6H aromatic hydrogen of 5,5′Mebipy); 7.40−6.55 (overlapped signals, 24H aromatic hydrogen for dppb and thiophene); 3.98, 3.03, 2.51, 2.38 (8H, aliphatic of dppb); 1.98 and 1,91 (6H, CH3 of 5,5′-Mebipy). 13C NMR (100 MHz, CDCl3, 298 K): δ (ppm) 158.15−151.48 (4C, 5,5′-Mebipy); 138.33− 121.29 (overlapped signals, 34C aromatic hydrogen for dppb and thiophene); 29.60−26.24 (4C aliphatic of dppb) 17.64 and 18.87 (2C, CH3 of 5,5′-Mebipy); Cyclic voltammetry: Epa = 0.47 V. Syntheses of Tetraruthenated Porphyrins Derived from cis[RuCl2(dppb)(5,5′-Mebipy)] Complex. The procedure for obtaining tetraruthenated porphyrin {TPyP[RuCl(dppb)(5,5′-Mebipy)]4}(PF6)4 (2) was based on the literature.11 TPyP (15 mg, 0.024 mmol) was dissolved in 100 mL of chloroform solvent. 76 mg (0.096 mmol) of the cis-[RuCl2(dppb)(5,5′-Mebipy)] complex in a proportion of 1:4 was added. NH4PF6 (94 mg, 0.58 mmol) was solubilized in methanol at a proportion of 1:6. The mixture was left under inert atmosphere and was stirred for 8 h at room temperature. The volume of the solution was reduced to about 2 mL on the rotary evaporator, and ethyl ether was added to give an intense brown solid, which was filtered off, washed with ethyl ether, and dried under vacuum. Yield: 79 mg (78%). C200H186Cl4F24N16P12Ru4. Elemental Analysis: Found (calc.) %: C = 57.44 (57.31); H = 4.66 (4.44); N = 5.28 (5.34). IR (1% CsI) νmax/cm−1 1607 (νN=C); 1434 (νC−H); 1092 (νC−P); 842 (νPF6). Molar conductivity in CH3COCH3, 354.6 S cm2 mol−1. 31 1 P{ H} NMR (400 MHz, CHCl3, 298 K): δ (ppm) 38.32 (d); 37.47 (d), 2Jp‑p = 36.43 Hz. 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) 9.15−8.07 (aromatic hydrogen of 5,5′-Mebipy, pyridic, and pyrrolic of TPyP); 7.80−6.78 (overlapped signals aromatic hydrogen for dppb, 5,5′-Mebipy, and TPyP); 4.32, 3.52, 2.74, 2.45 (32H aliphatic of

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01032. Measurements, tables, and figures of NMR and other performed characterizations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*BNC [email protected]. *AAB [email protected]. H


Article

Inorganic Chemistry

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ORCID

Taíse H. O. Leite: 0000-0001-9348-4232 João Honorato: 0000-0002-1127-6083 Beatriz N. Cunha: 0000-0003-3079-1854 Kleber T. Oliveira: 0000-0002-9131-4800 Newton M. Barbosa Neto: 0000-0001-9082-5433 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financed in part by the Coordenaçaõ de ́ Aperfeiçoamento de Pessoal de Nivel Superior - Brazil (CAPES) - Finance Code 001, CNPq and FAPESP (2015/ 21110-4, 2018/00106-7, 2016/16312-0). The authors are also thankful to the Grupo de Materiais Inorgânicos do Triângulo (GMIT) research group supported by FAPEMIG (APQ00330-14). We are also grateful to Prof. Sanclayton Geraldo Carneiro Moreira from the Programa de Pós-graduaçaõ em ́ Fisica da Universidade Federal do Pará for letting us use his experimental facilities.

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Remarkable Electronic Effect on the meso-Tetra(thienyl)porphyrins

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