Tetracationic and Tetraanionic Manganese Porphyrins

Jun 29, 2017 - Xiaoqin Jiang†, Claude P. Gros‡ , Yi Chang‡, Nicolas Desbois‡, Lihan Zeng†, Yan Cui†, and Karl M. Kadish†. † Department...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/IC

Tetracationic and Tetraanionic Manganese Porphyrins: Electrochemical and Spectroelectrochemical Characterization Xiaoqin Jiang,† Claude P. Gros,*,‡ Yi Chang,‡ Nicolas Desbois,‡ Lihan Zeng,† Yan Cui,† and Karl M. Kadish*,† †

Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States ICMUB (UMR CNRS 6302), Université de Bourgogne Franche-Comté, F-21000 Dijon, France

‡

S Supporting Information *

ABSTRACT: The electrochemistry and spectroelectrochemistry of four tetrapositively charged and two tetranegatively charged porphyrins were characterized in two nonaqueous solvents (dimethyl sulfoxide and N,N-dimethylformamide) containing 0.1 M tetra-n-butylammonium perchlorate. The tetrapositively charged compounds are represented by the tetrapyridylporphyrins [TRPyPM]4+(X−)4, where R is a methyl or [2-[2-(2-methoxy)ethoxy]ethoxy]ethyl group, M = MnIIII, MnIIICl, CuII, or PdII, and X = I− or Cl−. The tetranegatively charged porphyrins are represented by the tetrasulfonato derivatives [TPPSMn(OAc)]4−(NH4+)4 and [TArPSMn(OAc)]4−(NH4+)4, where Ar = 4-O-[2-[2-(2-methoxy)ethoxy]ethoxy]ethylphenyl. Up to seven electrons can be added to the tetrapyridyl porphyrins in three to five reversible reductions, while up to four electrons can be added to the tetrasulfonato derivatives in four reversible processes. Three types of electrochemical behaviors are observed for reduction of the pyridinium groups on the tetrapyridyl porphyrins. One is for the manganese(II) complexes where the four equivalent pyridinium groups are reduced in a single overlapping four-electron-transfer step. Another is for the free-base porphyrin, where four well-separated one-electron reductions occur, while the third is for copper(II) and palladium(II) derivatives, where reduction of the four pyridinium groups proceeds in two well-separated twoelectron-transfer steps. The electrochemical and spectroelectrochemical properties were also characterized for a 1:1 mixture of the tetrapositively and tetranegatively charged manganese porphyrins to investigate possible interactions between these two species. An interaction between the two porphyrins was indeed observed in both solvents after electroreduction of the four pyridinium groups, which led to a substantial change in the mechanism for reduction of the pyridinium groups from an initial single overlapping four-electron-reduction process to two well-separated two-electron-transfer processes.

■

INTRODUCTION

various ratios to give species with new properties in the solid state29−31 and in solution.32−34 The current manuscript elucidates the electrochemistry of four tetrapositively charged and two tetranegatively charged watersoluble porphyrins in two nonaqueous solvents, dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF). The structures of the investigated compounds 1−6 are shown in Chart 1. The positively charged porphyrins are represented as [TMPyPMn(I)]4+(I−)4 (1), where TMPyP2+ = 5,10,15,20tetrakis-[N-methyl-4-pyridyl]-21H,23H-porphyrin, and [TRPyPM]4+(Cl−)4, where TRPyP2+ = 5,10,15,20-tetrakis[N[2-[2-(2-methoxy)ethoxy]ethoxy]ethyl-4-pyridyl]-21H,23Hporphyrin and M = MnIIICl (2), CuII (5), or PdII (6).35 The negatively charged porphyrins are represented as [TPPSMn(OAc)]4−(NH4+)4 (3) and [TArPSMn(OAc)]4−(NH4+)4 (4), where TPPS 6− = 5,10,15,20-tetrakis(4-sulfonatophenyl)-

Numerous studies have been devoted to the electrochemistry of manganese porphyrins in nonaqueous media, some involving a characterization of derivatives with “simple” neutral meso- or βpyrrole substituted macrocycles such as tetraphenylporphyrin (TPP)1−6 or octaethylporphyrin7 and others with more highly substituted macrocycles such as the fluorinated dodecaphenylporphyrins (DPPF20, DPPF28, and others).8,9 A number of manganese porphyrins with positively or negatively charged macrocycles have also been studied for their electrochemistry in aqueous10,11 and nonaqueous media,12,13 with two examples being given by the positively charged tetramethylpyridyl (TMPyP4+) and negatively charged tetrasulfonato (TPPS4−) derivatives. In our laboratories, we have long been interested in the electrochemistry of cationic and anionic porphyrins,12−20 a group of compounds that are important in the area of medical imaging (magnetic resonance imaging)21−28 and in the formation of new solid-state materials consisting of positively and negatively charged porphyrins, which can be combined in © 2017 American Chemical Society

Received: March 25, 2017 Published: June 29, 2017 8045

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry

to CuII (5), PdII (6),35 or 2H (7) in [TRPyPM]4+(X−)4 might affect the redox properties because quite different electrochemical behaviors were reported earlier for the reduction of [TMPyPM]4+, where M = Cu, Zn, or VO,15 compared to the related cobalt(II)17 or free-base20 derivatives, the latter of which is represented by compound 7 in Chart 1. The tetracationic and tetraanionic porphyrins in Chart 1 can undergo four types of redox processes within the negative potential limits of the two solvents. These are as follows: (i) a MnIII/MnII transition for compounds 1−4, which can be rapid and electrochemically reversible or slow and electrochemically quasi-reversible, depending upon the nature of the studied compound, the solvent and the specific type and number of axial ligands bound to the Mn center in its 3+ and 2+ oxidation states;4,7,9,36 (ii) an overall two-electron reduction of the conjugated macrocycle in compounds 1−7, which can occur in two well-separated one-electron-transfer steps or in a single overlapping two-electron-transfer process; (iii) an overall fourelectron reduction of the four pyridinium groups in compounds 1, 2, and 5−7 in Chart 1, which can occur in individual electrontransfer steps20 or in multiple overlapping steps13,15,17 depending upon the prevailing interaction between the four redox-active pyridinium groups on the macrocycle under the given solution conditions; (iv) a one-electron reduction of the associated NH4+ counterions on 3 and 4. Thus, up to seven electrons can be added to compounds 1 and 2 in the two investigated solvents, while six electrons can be added to compounds 5−7 and up to four electrons to compounds 3 and 4 under similar solution conditions. The exact behavior is described on the following pages.

Chart 1. Structures of Investigated Tetracationic and Tetraanionic Porphyrins, Where M = MnIII, CuII, PdII, and 2Ha

■

a

EXPERIMENTAL SECTION

Materials. N,N′-Dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich Chemical Co. Tetra-n-butylammonium perchlorate (TBAP), used as the supporting electrolyte, was purchased from Sigma-Aldrich or Fluka, recrystallized from ethyl alcohol, and dried under vacuum at 40 °C for at least 1 week prior to use. Instrumentation. 1H and 13C NMR spectra were obtained on a Bruker Avance 500 MHz instrument at 300 K. Chemical shifts are reported in δ (ppm) relative to the residual solvent protons (CDCl3, 7.26 for 1H; DMSO-d6, 2.50 for 1H). UV−vis spectra were recorded in solutions using a Varian Cary 50 spectrophotometer (1-cm-path-length quartz cell). Accurate mass measurements (high-resolution mass spectrometry, HRMS) were carried out using a Bruker micro time-offlight quantum tunnelling of magnetization (TOF-QTM) electrospray ionization (ESI) mass spectrometer. Matrix-assisted laser desorption/ ionization (MALDI)-TOF mass spectrometry was carried out with a Bruker Ultraflex II MALDI-TOF mass spectrometer using dithranol as the matrix. High-performance liquid chromatography (HPLC) measurements were performed on a Dionex Ultimate 3000 system (Thermo Scientific). Conditions #1: C18 Chromolith Speed ROD column (Merck, 2 μm, 50−4.6 mm); flow rate 1.5 mL min−1; injected volume 30 μL; wavelength detection 415 and 465 nm; solvent A, 0.1% 2,2,2-trifluoroacetic acid (TFA) in water (H2O); solvent B, acetonitrile (CH3CN). Conditions #2: C18 Atlantis column (Waters, 3 μm, 50 × 4.6 mm); flow rate 1.0 mL min−1; injected volume 30 μL; wavelength detection 415 and 465 nm; solvent A, 0.1% TFA in H2O; solvent B, 0.1% TFA in CH3CN. UV−vis spectra for spectroelectrochemistry were recorded with a Hewlett-Packard model 8453 diode-array spectrophotometer. Cyclic voltammetry was carried out at 298 K using an EG&G Princeton Applied Research 173 potentiostat/galvanostat. A three-electrode system was used for cyclic voltammetric measurements and consisted of a glassy carbon working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE). The SCE was separated

Data for [TRPyPH2]4+ (7) are described in ref 20.

21H,23H-porphyrin and TArPS6− = 5,10,15,20-tetrakis[3sulfonato-4-O-[2-[2-(2-methoxy)ethoxy]ethoxy]ethylphenyl]21H,23H-porphyrin. One interest in this study was to elucidate how changes in the solvent (DMSO or DMF), anionic axial ligand on MnIII (I−, Cl−, or OAc−), and/or counterions associated with the positively or negatively charged meso-aryl groups of the manganese(III) porphyrins in Chart 1 (I−, Cl−, or NH4+) would affect the redox properties of compounds 1−4 and then to investigate how these redox properties might change for a 1:1 mixture of the tetrapositively and tetranegatively charged manganese porphyrins under the same solution conditions. We also wished to understand how changing the central metal ion from MnIIICl (2) 8046

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry

J = 8.6 Hz, 8H), 7.30 (d, J = 8.6 Hz, 8H), 4.44 (m, 8H), 4.06 (m, 8H), 3.88 (m, 8H), 3.80 (m, 8H), 3.74 (s, 8H), 3.66 (d, 8H), 3.42 (s, 12H), −2.74 (s, 2H). MS (MALDI-TOF). Calcd for C72H86N4O16: m/z 1262.6039. Found: m/z 1262.6045 ([M]+). The spectral and physical properties are in agreement with previously published data.46,47 [TArPSH2]4−(Na+)4. TArP (0.20 g, 0.16 mmol) was dissolved in chloroform (1.5 mL) under dinitrogen, and chlorosulfuric acid (ClSO3H; 0.23 mL, 3.4 mmol) was added dropwise. The reaction mixture was stirred at room temperature for 30 min. The solution was then cooled in an ice/H2O bath, and ice (5.0 g) was added slowly. NaOH (1 M) was then added until the color changed to red. Most of the solvent was evaporated, and the inorganic salts were removed by filtration. The solvent was then totally removed under vacuum. The crude product was chromatographed on a C18 reverse column with CH3CN/H2O as the eluent to get a red solid (180 mg, yield 67%). 1H NMR (DMSO-d6, 300 MHz): δ 8.86 (s, 8H), 8.52 (m, 4H), 8.20 (m, 4H), 7.53 (m, 4H), 4.51 (m, 8H), 3.99 (m, 8H), 3.76 (m, 16H), 3.63 (m, 16H), 3.50 (s, 12H), −2.86 (s, 2H). UV−vis [H2O, one drop of 1 M NaOH; λmax, nm (ε, L mol−1 cm−1)]: 418 (108628), 520 (4881), 559 (4044), 592 (2928), 640 (2649). HRMS (ESI). Calcd for [C72H82N4O28S4]4−: m/z 394.5994. Found: m/z 394.6015 ([M − 4Na]4−). [TArPSMn(OAc)]4−(NH4+)4 (4). [TArPSH2]4−(Na+)4 (0.20 g, 0.12 mmol) and Mn(OAc)2·4H2O (0.11 g, 0.18 mmol) were mixed. The reaction was heated at 90 °C in a mixture of an ammonium acetate buffer solution at pH 6.8 (15 mL) and H2O (5 mL) and monitored by UV−vis spectroscopy. After completion (18 h), the solvent was removed under reduced pressure. The crude product was purified by a reverse C18 column eluted with H2O/CH3CN to get a green solid (110 mg, yield 52%). UV−vis [H2O; λmax, nm (ε, L mol−1 cm−1)]: δ 382 (49091), 402 (50078), 422 (41052), 468 (86991), 517 (5645), 568 (9712), 612 (8686). Purity at 465 nm 98%, tR = 5.74 min (conditions #2). HRMS (ESI). Calcd for [C72H80MnN4O28S4]3−: m/z 543.7735. Found: m/z 543.7767 ([M − 4NH4 − OAc]3−). [TRPyPCu]4+(Cl−)4 (5). 7 (30.0 mg, 19.6 μmol) and CuCl2 (3.20 mg, 23.8 μmol) were dissolved in CH3OH/H2O (6:1, 7.00 mL) and mixed under refluxing conditions. The reaction was monitored by MALDITOF mass spectrometry and UV−vis spectroscopy. After completion of the reaction (3 h), the solvent was removed under reduced pressure. The compound was then purified by filtration on Bio-Beads with DMF. The resulting product was passed through a Dowex 1X8 100-mesh ionexchange column (H2O) to give a red solid (26.0 mg, yield 94%). UV− vis [CH3OH; λmax, nm (ε, L mol−1 cm−1)]: 428 (114181), 549 (11906). HRMS (ESI). Calcd for [C68H84CuN8O12]4+: m/z 316.88707. Found: m/z 316.88692 ([M − Cl]4+).

from the bulk of the solution by a fritted glass bridge of low porosity, which contained a solvent/supporting electrolyte mixture. Thin-layer UV−vis spectroelectrochemical experiments were performed with a homebuilt thin-layer cell that has a light-transparent platinum-net working electrode.37,38 Potentials were applied and monitored with an EG&G PAR model 173 potentiostat. Time-resolved UV−vis spectra were recorded with a Hewlett-Packard model 8453 diode-array spectrophotometer. High-purity N2 from Trigas was used to deoxygenate the solution and kept over the solution during each electrochemical and spectroelectrochemical experiment. Synthesis. [TMPyPMn(I)]4+(I−)4 (1). meso-Tetrakis(4-pyridyl)porphyrin (0.60 g, 0.97 mmol) and Mn(OAc)2·4H2O (0.25 g, 1.02 mmol) were dissolved in DMF (50 mL) under argon. The reaction mixture was heated to reflux and shielded from light. The reaction was monitored by MALDI-TOF mass spectrometry. After completion of the reaction (3 h), the solution was cooled at 45 °C. CH3I (0.6 mL, 9.6 mmol) was then added dropwise, and the reaction mixture was stirred at 45 °C overnight. After filtration, the solid was washed thoroughly with ethyl ether and recrystallized with H2O/acetone (1:2) to obtain a brown solid (1.01 g, yield 80%). UV−vis [H2O; λmax, nm (ε, L mol−1 cm−1)]: δ 377 (38000), 400 (38000), 462 (107000), 562 (12000), 771 (1900). Purity at 415 nm 99%, tR = 4.8 min (see the above HPLC conditions #1). Anal. Calcd for C44H36I5MnN8·5H2O: C, 36.29; H, 3.18; N, 7.69. Found: C, 35.83; H, 3.46; N, 8.03. HRMS (ESI). Calcd for [C44H36MnN8]4+: m/z 182.8105. Found: m/z 182.8099 ([M − 5I]4+). The synthesis of this compound was previously described by a somewhat different method.39 [TRPyPMn(Cl)]4+(Cl−)4 (2). [TRPyPH2]4+(Br−)440 (7; 100 mg, 0.065 mmol) and MnCl2·4H2O (245 mg, 1.24 mmol) were dissolved in methanol (CH3OH)/H2O (6:1, 35 mL) with 1 M sodium hydroxide (NaOH; 200 μL) and mixed under refluxing conditions. The metalation reaction was monitored by UV−vis spectroscopy by the shift of the Soret band from 427 to 462 nm, with complete metalation being observed after 1 h. The compound was then purified, with the addition of NH4PF6 to precipitate the metalloporphyrin, followed by washing with H2O. Counterion exchange to Cl− was then achieved by dissolving the product in acetone, and precipitation of the manganese porphyrin occurs upon the addition of tetrabutylammonium chloride. The final Cl− form of TRPyPMn was then desiccated to dryness during 12 h to get a brown solid (80 mg, yield 86%). UV−vis [H2O; λmax, nm (ε, L mol−1 cm−1)]: δ 379 (36300), 400 (37080), 462 (115970), 564 (10470), 675 (897). Purity at 465 nm 100%, tR = 4.83 min (see the above HPLC conditions #2). HRMS (ESI). Calcd for [C68H84ClMnN8O12]3+: m/z 431.5087. Found: m/z 431.5069 ([M − 4Cl]3+). [TPPSMn(OAc)]4−(NH4+)4 (3). TPPSH241 (500 mg, 0.54 mmol) and Mn(OAc)2·4H2O (316 mg, 1.29 mmol) were mixed. The reaction was allowed to run for 16 h at 90 °C with stirring in an ammonium acetate buffer solution at pH 6.8 (40 mL). The crude product was filtered and washed with acetone. Then it was recrystallized with CH3OH/acetone (1:2) to give a brown solid (350 mg, yield 62%). UV−vis [H2O; λmax, nm (ε, L mol−1 cm−1)]: δ 378 (57000), 400 (58000), 420 (44047), 466 (98000), 514 (7200), 565 (12000), 598 (8600). Purity at 415 nm 95%, tR = 1.63 min (see the above HPLC conditions #1). Anal. Calcd for C46H43MnN8O14S4·6H2O·CH3COCH3: C, 45.93; H, 4.80; N, 8.74; S, 10.1. Found: C, 45.47; H, 4.94; N, 8.80; S, 9.82. HRMS (ESI). Calcd for [C44H24MnN4O12S4]3−: m/z 327.6557. Found: m/z 327.6546 ([M − OAc − 4-NH4]3−). The synthesis of this compound with different counterions or axial ligands was previously described.42−45 TArP. meso-Tetrakis(4-hydroxy)porphyrin (500 mg, 0.74 mmol) was dissolved in CH3OH (30 mL) under dinitrogen, and NaOCH3 (7.0 mL, 1 M) was added. The reaction mixture was stirred at room temperature for 30 min. The solvent was evaporated under reduced pressure. The product was dissolved in DMF (30 mL) under argon, and Br-PEG (1.5 g, 6.6 mmol),40 where PEG = [2-]2-(2-methoxy)ethoxy]ethoxy]ethyl-4pyridyl, was added. The mixture was stirred at 100 °C overnight. The reaction was monitored by MALDI-TOF mass spectrometry. After completion of the reaction, the solvent was removed under vacuum. The crude product was chromatographed on a silica gel column with 1% CH3OH/dichloromethane as the eluent to give a purple solid in quantitative yield. 1H NMR (CDCl3, 300 MHz): δ 8.85 (s, 8H), 8.11 (d,

■

RESULTS AND DISCUSSION Synthesis of PEG Derivatives. A one-step procedure40 was used to prepare the free-base 7 (Scheme 1) starting from the freebase tetrapyridylporphyrin, which was commercially available. This latter compound was reacted with an excess of the BrPEG derivative in DMF to give the free-base tetra-PEG derivative with an efficient 81% yield. The metalation step was then carried out in a CH3OH/H2O mixture using excess MnCl2·4H2O, CuCl2, or PdCl2.48 The reaction progress was monitored by UV−vis spectroscopy, with complete metal insertion observed after a few hours, as indicated by the intensity and position of the Soret band. The PEG tetrasulfonated porphyrin (Scheme 2) was prepared in two steps and in good yield starting from the meso-tetrakis(4hydroxy)porphyrin; the completion of the reaction was monitored by MALDI-TOF mass spectrometry. The four sulfonato substituents were introduced at the ortho positions of the meso-phenyl groups by reaction with ClSO3H and hydrolysis with a 1 M NaOH aqueous solution. The crude product was purified by chromatography on a C18 reverse column with 10% CH3CN/H2O as the eluent. 8047

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry Scheme 1. Synthetic Route for 1 and [TRPyPM]4+(Cl−)4, Where M = MnIIICl (2), CuII (5), or PdII (6)a

a For i, MnCl2·4H2O, CH3OH/H2O, reflux, compound 2 (M = Mn); CuCl2, CH3OH/H2O, reflux, compound 5 (M = Cu); PdCl2, H2O, reflux, compound 6 (M = PdII).

Scheme 2. Synthesis of 4

Figure 1. Cyclic voltammograms of the investigated (a) tetracationic and (b) tetraanionic manganese complexes along with (c) comparison compounds TPPMnCl and NH4Cl, in DMSO containing 0.1 M TBAP. Scan rate = 0.1 V s−1.

(Figure 1c), while compound 3 is also reduced by three reversible-to-quasi-reversible one-electron reductions at Epa = −0.57 V and Epc = −0.01 V for the MnIII/MnII process, E1/2 = −1.33 V for the second, and Epc = −1.78 V for the third under the same solution conditions (Figure 1b). The three one-electron reductions of the tetraanionic compound 4 are slightly more difficult than those of the related compound 3 in both solvents, and this can be accounted for by the electron-donating properties of the four p-OPEG substituents on the molecule; the smallest difference in the potentials between the reductions of the two compounds is seen for the MnIII/MnII process and the largest for the two ringcentered reactions, as seen by the data in Figure 1, S13, and Table 1. Two additional points are noteworthy upon a comparison of the electrochemistry of compounds 3 and 4 to that of TPPMnCl. The first is that the rate of electron transfer for the MnIII/MnII process is slow (quasi-reversible) for TPPMnCl (ΔE1/2 = 120− 140 mV) but even slower for 3 and 4 in DMSO, as evidenced by

Electrochemistry of Manganese Porphyrins. The electrochemistry of compounds 1−4 and TPPMnCl was carried out in DMSO and DMF containing 0.1 M TBAP. Examples of cyclic voltammograms are illustrated in Figures 1 (DMSO) and S14 (DMF), and a summary of the reduction potentials is given in Table 1, which also includes the peak-to-peak separation for the MnIII/MnII processes and the number of electrons transferred for redox reactions involving the conjugated macrocycle and four pyridinium groups. The electrochemistry of compounds 3 and 4 resembles in large part the published redox behavior of TPPMnCl under similar solution conditions.4,49,50 For example, TPPMnCl is characterized by three reversible-to-quasi-reversible one-electron reductions at E1/2 = −0.24, −1.26, and −1.76 V in DMSO 8048

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry

Table 1. Reduction Potentials (E1/2, V vs SCE) of the Investigated Tetracationic and Tetraanionic Manganese Complexes in DMSO and DMF Containing 0.1 M TBAP (Data Also Given for TPPMnCl) reduction solvent

compound

DMSO

4+

DMF

−

[TMPyPMn(I)] (I )4 [TRPyPMn(Cl)]4+(Cl−)4 [TPPSMn(OAc)]4−(NH4+)4 [TArPSMn(OAc)]4−(NH4+)4 TPPMnCl [TMPyPMn(I)]4+(I−)4 [TRPyPMn(Cl)]4+(Cl−)4 [TPPSMn(OAc)]4−(NH4+)4 [TArPSMn(OAc)]4−(NH4+)4 TPPMnCl

MnIII/MnII (ΔEp)a

ring-centered (#e)b

pyridinium (#e)b

−0.01 (0.06) 0.00 (0.06) −0.30 (0.56) −0.33 (0.54) −0.24 (0.14) 0.05 (0.06) 0.09 (0.06) −0.24 (0.22) −0.29 (0.14) −0.18 (0.12)

−0.72 (2e) −0.72 (2e) −1.33 (1e), −1.78c (1e) −1.42 (1e), −1.96c (1e) −1.26 (1e), −1.76 (1e) −0.70 (2e) −0.73 (2e) −1.35 (1e), −1.70c (1e) −1.45 (1e), −1.76c (1e) −1.31 (1e), −1.81 (1e)

−0.94 (4e) −0.94 (4e)

1 2 3 4 1 2 3 4

−0.94 (4e) −0.95 (4e)

ΔEp = potential difference between the cathodic and anionic peaks of the MnIII/MnII process at a scan rate of 0.1 V s−1. b#e = number of electrons transferred. cIrreversible peak potential at a scan rate of 0.1 V s−1. a

Figure 2. UV−vis spectral changes for the MnIII/MnII process of the investigated compounds in DMSO, containing 0.1 M TBAP. A summary of the spectral data is given in Table 2.

not observed at this potential in DMF (Figure S14), and it also not observed for compound 3 in either of the two highly polar solvents, DMF or DMSO. This peak is assigned as involving a reduction of the NH4+ countercations associated with the initial porphyrin added to the solution, as verified by independent measurements of NH4Cl in DMSO, which is reduced at E1/2 = −0.82 V (Figure 1c). The lack of an NH4+ reduction process under the other solution conditions suggests that this cation remains strongly associated with the sulfonate groups under the electrochemical conditions and is thus not reduced in this range of potentials. Quite different redox behavior is observed for compounds 1 and 2 compared to that for 3 and 4. As seen in Figures 1 and S14, the MnIII/MnII reductions of 1 and 2 are reversible in both solvents (ΔE1/2 = 60 mV) and located at E1/2 = −0.01 or 0.00 V in DMSO and 0.05 or 0.09 V in DMF. The 240−250 mV positive potential shift of half-wave potentials for the MnIII/MnII process of 1 and 2, compared to the same metal-centered reaction of

the larger separation between the cathodic and anionic peaks for this process by cyclic voltammetry (ΔE1/2 = 540−560 mV in DMSO and 140−220 mV in DMF). An earlier study of TPPMnX in DMF and DMSO showed reversible behavior, where X was one of several different anionic axial ligands, including N3−, Cl−, ClO4−, and others,4 thus suggesting that the difference in the rate of electron transfer between TPPMnCl and 3 and 4 is not related to a rate-determining dissociation of the OAc− axial ligand from the MnIII center of 3 and 4 after electroreduction. However, an alternate possibility is that compounds 3 and 4 exist as oligomers in DMSO and DMF, as described in the literature51,52 for related TPPSMn compounds, which can form dimers and trimers in these solvents and might thus have slower rates of electron transfer compared to the monomeric porphyrins 1, 2, and TPPMnCl. The second point to note from the cyclic voltammograms in Figure 1b is the presence of a reversible one-electron-reduction process at −0.83 V for compound 4 in DMSO. A redox process is 8049

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry Table 2. UV−Vis Spectral Data λmax (nm) of the Investigated Porphyrins in DMSO and DMF, Containing 0.1 M TBAP MnIII solvent DMSO

DMF

a

compound 4+

−

[TMPyPMn(I)] (I )4 [TRPyPMn(Cl)]4+(Cl−)4 [TPPSMn(OAc)]4−(NH4+)4 [TArPSMn(OAc)]4−(NH4+)4 TPPMn(Cl) [TMPyPMn(I)]4+(I−)4 [TRPyPMn(Cl)]4+(Cl−)4 [TPPSMn(OAc)]4−(NH4+)4 [TArPSMn(OAc)]4−(NH4+)4

Soret band 1 2 3 4 1 2 3 4

MnII Q band

460 460 467 471 465 460 466 468 473

563 565 569 574 567 571 582 569 575

a

Soret band sh

598 598sh 605 613 604 619sh 627sh 606 615

452 457 441 444 440 453 467 438 441

Q band 575 575 573 576 571 576 584 572 576

617 621 612 617 609 621 632 611 616

Shoulder bands can be seen in Figures 2 and S14.

in DMSO. Similar types of spectral changes are also observed upon the reduction of 3 and 4 in DMF. As mentioned earlier in the paper, the electrochemical behavior of compounds 3 and 4 is similar to that of TPPMnCl, and it is also similar to that of other TPP derivatives in that two well-separated ring-centered reductions occur giving a porphyrin π-anion radical and dianion, respectively.1 This behavior is quite different from that of compounds 1 and 2, where the reversible MnIII/MnII reductions are followed at more negative potentials by two reversible multielectron-transfer processes. The first process involves a global two-electron addition to the conjugated macrocycle at −0.70 to −0.73 V and the second an overall fourelectron reduction of the four positively charged N-alkylpyridinium groups, all of which are reduced at the same half-wave potential of −0.94 V in DMSO (Figure 1) or −0.95 V in DMF (Figure S14). A summary of the prevailing reduction mechanisms for compounds 1−4 is given in Scheme 3.

TPPMnCl in DMSO or DMF, can be accounted for by a relatively strong interaction between the four positively charged pyridinium groups and the central metal ion of these porphyrins. The MnIII/MnII process of compounds 1−4 was monitored by UV−vis spectroelectrochemistry in DMSO. Examples of spectral changes that occurred during controlled potential reduction in the thin-layer cell are shown in Figure 2, and a summary of the spectral data for the initial and singly reduced forms of each porphyrin in DMSO and DMF is given in Table 2. The MnIII/ MnII process of TPPMnCl was also monitored by spectroelectrochemistry, and these spectral changes are shown in Figure S14. The UV−vis spectra of compounds 1 and 2 are quite similar to each other in their initial MnIII form and are characterized in DMSO by a Soret band at 460 nm and a Q band at 563 or 565 nm. There is also a shoulder at 598 nm for both compounds in this solvent. During the first one-electron reduction in DMSO, the Soret band at 460 nm shifts to 452 or 457 nm and increases slightly in intensity, while two new Q bands are formed at 575 and 617 or 621 nm. No new absorptions are observed for the singly reduced porphyrin between 700 and 900 nm, consistent with a metal-centered electron transfer1,12 and the conversion of MnIII to MnII. The spectral similarity between compounds 1 and 2 in their neutral or singly reduced forms and the almost identical redox potentials of the MnIII/MnII processes of the two porphyrins in DMSO indicate that the four PEG groups on 2 and the four CH3 groups on 1 have almost the same effect on the spectroscopic and redox properties of these two porphyrins in this solvent. More significant spectral differences between compounds 1 and 2 are seen when the reductions are carried out in DMF. Under these solution conditions, the Soret and Q bands are both red-shifted for compound 2 compared to compound 1, as seen by the data in Table 2. The difference in the spectra between these two structurally related tetracationic porphyrins in DMF might be associated with a difference in axial coordination by the solvent, especially in the case of the manganese(II) porphyrins, where larger differences are seen between the Soret and Q absorption maxima of these two compounds. As seen in Figure 2, the conversion of MnIII to MnII results in quite different spectral changes for compounds 3 and 4 compared to compounds 1 and 2. For example, Soret bands of the manganese(II) porphyrins are blue-shifted by 26−27 nm in DMSO compared to 3−8 nm in the case of compounds 1 and 2 in this solvent. The Soret band absorption is also substantially increased in intensity compared to this band for the initial manganese(III) porphyrin. Moreover, two well-defined Q bands are observed for both the MnIII and MnII forms of the porphyrins

Scheme 3. Proposed Reduction Mechanism of Compounds 1−4 in DMSO (or DMF), Containing 0.1 M TBAPa

a

The listed potentials are those for compounds 1 and 3 in DMSO.

The overlapping two-electron reduction of compounds 1 and 2 at −0.72 V is similar to that of previously the investigated porphyrins with one,19,20 two,20 or four12,13,20 meso-pyridinium groups, all of which undergo a single two-electron addition to the conjugated macrocycle in nonaqueous solvents. However, the overlapping four-electron reduction of the pyridinium groups at E1/2 = −0.94 V for compounds 1 and 2 is quite different from what has been reported in previous electrochemical studies of tetrapyridinium porphyrins where multiple reduction processes are observed. The exact number and potentials of the pyridinium group reductions were previously shown to depend on the solvent, type of central metal ions, and counteranions associated with the pyridinium groups.12,13,15,20 In order to investigate this further, the tetrapositively charged copper and palladium porphyrins, 5 and 6, were examined to better understand factors that might influence the unique redox behavior of compounds 1 and 2 and 8050

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry

−0.72 V in the first step, a difference of 540 mV compared to 240 mV for the metal-centered reduction of the same compound (see Table 1 and Figure 1). An even larger substituent effect of the positively charged pyridinium groups is seen for the ringcentered reduction of the palladium(II) complexes, where the reported E1/2 for the first reduction of TPPPd is −1.34 V,53 while [TRPyPPd]4+ is reduced at E1/2 = −0.60 V in the first step, a difference of 740 mV. The initial two-electron reductions of the four [TRPyPM]4+ porphyrins in Figure 3 were monitored by thin-layer spectroelectrochemistry in DMSO, and the spectral changes during these processes are shown in Figure 4.

also to compare the effect of the specific central metal ion on the UV−vis spectra before and after the reversible two-electron reduction involving the macrocycle. [TRPyPM]4+, Where M = MnII, CuII, PdII, and 2H. As seen in Chart 1, compounds 2, 5, and 6 have the same N−R group and the same counterions (Cl−). The free-base porphyrin 7 also has the same macrocyclic structure as compounds 2, 5, and 6 but contains Br− counteranions instead of Cl−. Earlier electrochemical studies of [TMPyPM]4+, where M = CuII, ZnII, or VOII, showed three well-defined two-electron reductions in DMF, the first of which was assigned as involving the conjugated macrocycle and the latter two as reductions on the four external pyridinium groups of the porphyrins.15 Three well-defined two-electron transfers are also observed for the currently investigated palladium(II) and copper(II) derivatives 5 and 6, as illustrated in Figure 3, which includes cyclic voltammograms of [TRPyPMnCl]4+ and [TRPyPH2]4+, the latter of which is adapted from the literature.20

Figure 3. Cyclic voltammograms illustrating the three types of redox behavior for [TMPyPM]4+, where M is (a) MnIIICl, (b) 2H, and (c) CuII and PdII in DMSO, containing 0.1 M TBAP. Scan rate = 0.1 V s−1. The data for the [TRPyPH2]4+ 7 is adapted from ref 20. Copyright 2016 Wiley.

Almost identical half-wave potentials are observed for reduction of the copper(II) and palladium(II) derivatives in DMF. The first two-electron reduction is located at E1/2 = −0.60 V for copper(II) or palladium(II), and this can be compared to a reversible two-electron reduction at −0.72 V for manganese(II) and −0.51 V for free-base derivatives, respectively. Thus, all four of the tetracationic porphyrins in Figure 3 are characterized by an initial reversible two-electron reduction at a half-wave potential that is substantially easier than that for reduction of the related TPP derivatives under similar solution conditions. For example, the first ring-centered reduction of TPPMnCl is at E1/2 = −1.26 V in DMSO (Table 1), while [TRPyPMn]4+ is reduced at E1/2 =

Figure 4. UV−vis spectral changes during reduction of the [TRPyPM]4+ complexes in DMSO, containing 0.1 M TBAP. The applied reducing potential was set as 200 mV negative of E1/2 for the first ring-centered reduction of 2, 5, and 6. The data for compound 7 are adapted from ref 20. Copyright 2016 Wiley. 8051

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry As seen in this figure, the electroreduction leads in each case to a decrease in the intensity of the initial Soret band and the appearance of a new near-IR band at either 778 nm (compounds 2 and 7) or 830/831 nm (compounds 5 and 6). The doubly reduced porphyrins 5 and 6 also exhibit a new band at 485/484 nm, which is not seen for the doubly reduced 2 or 7 in this region of the spectrum. The spectral changes illustrated in Figure 4, as well as the spectral envelope for the four electrogenerated [TRPyPM]2+ derivatives, are consistent with the previously published spectra for a variety of structurally related tetrapyridylporphyrins in their doubly reduced form, and the near-IR band at 750−850 nm was assigned as being diagnostic of the porphyrin with a reduced πring system.12 However, no obvious trends as a function of the central metal ion can be seen between UV−vis spectra of the different neutral or electroreduced compounds. For example, neutral porphyrins 2 and 5 have a Soret band maximum at 457 and 427 nm, respectively, while the doubly reduced forms of these two porphyrins have near-IR bands at 778 and 830 nm, respectively. Likewise, the Soret bands of compounds 6 and 7 are quite similar to each other in wavelength (420 and 426 nm), but different UV−vis spectra are again seen upon reduction, with the palladium(II) porphyrin spectrum having a near-IR band at 831 nm and the free-base derivative a band at 778 nm. Although two types of similar UV−vis spectra are seen for the four electrogenerated forms of [TRPyPM]2+ in Figure 4, three different types of electrochemical behavior are observed for these compounds, as shown by the cyclic voltammograms in Figure 3 and schematically illustrated in Scheme 4. The first (type I) is for

measured E1/2 for the reduction of compound 2 under the same solution conditions (see Figure 1 and Scheme 4). This suggests that the palladium(II) and copper(II) porphyrins contain two sets of two equivalent pyridinium groups, one set of which is easier and the other harder to reduce by ∼70 mV, compared to the E1/2 value for the global four-electron reduction of compound 2. It might also be pointed out that the midpoint potential between E1/2 for the second and third pyridinium group reductions of compound 7 is −0.89 V, a value not so different from the midpoint potentials for compounds 2, 5, and 6. Electrogeneration and Electrochemistry of “Associated” Manganese Porphyrins. Electrochemical and spectroelectrochemical measurements were also carried out on a 1:1 mixture of compounds 1 and 3 in DMSO and DMF solutions. The aim of these measurements was to investigate possible interactions between the tetrapositively and tetranegatively charged manganese porphyrins, as has been reported in the literature for related compounds both in the solid state29−31 and in solution.32−34 An interaction between the two porphyrins was indeed observed in both solvents, and this led to a substantial change in the mechanism for the global six-electron reduction of the conjugated macrocycle and pyridinium groups on 1. Surprisingly, however, no interaction occurred between the two oppositely charged porphyrins until after electroreduction of the four pyridinium groups on compound 1, as described below and shown by the cyclic voltammograms in Figure 5a for a 1:1 mixture of compounds 1 and 3 in DMSO.

Scheme 4. Types of Electrochemical Behavior for RingCentered and Pyridinium-Based Reductions of Compounds 2 and 5−7 in DMSO, Containing 0.1 M TBAP

the manganese(II) porphyrin 2, where the four equivalent redoxactive pyridinium groups on the compound are reduced in a single overlapping four-electron-transfer step at the same halfwave potential of −0.94 V (Figure 3a) and thus appear to be noninteracting. The second (type II) is for compounds 5 and 6, where reduction of the four pyridinium groups occurs in two separate two-electron-transfer steps located at half-wave potentials of −0.87 V (−0.85) and −1.00 V (−0.97) (Figure 3b), while the third (type III) is for the free-base porphyrin 7, where four well-separated one-electron reductions are observed for this process (Figure 3c), suggesting four structurally equivalent and interacting redox centers in this molecule. It is interesting to note that the two-electron reductions of the pyridinium groups on compounds 5 and 6 are separated by 130 mV, with the midway potential between the E1/2 values of −0.87 and −1.00 V being −0.94 V, a value that is exactly the same as the

Figure 5. Cyclic voltammograms of a 1:1 mixture (mole ratio) of compounds 1 and 3 in DMSO, containing 0.1 M TBAP. Scan rate = 0.1 V s−1. 8052

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry The first two reductions of the 1:1 mixture in Figure 5a are assigned to the MnIII/MnII processes of compounds 1 and 3, respectively, and are located at potentials that are almost identical with the measured E1/2 and Ep values for reduction of the individual porphyrin units when measured separately (see an earlier section of this paper). The next two reductions of the 1:1 mixture involve the global addition of six electrons in two steps, with the potentials for these processes again being the same as those for the reduction of compound 1 in the absence of the negatively charged porphyrin, namely, E1/2 = −0.65 V for the two-electron addition and E1/2 = −0.90 V for the addition of four electrons, as seen in Figure 5a for the initial negative potential scan in the cyclic voltammogram. However, the seven-electron-reduced [TMPyPMnII]2− and the one-electron-reduced [TPPSMnII]4− interact with each other, as shown in Scheme 5, and this then leads to a change

Similar cyclic voltammograms were obtained on scans 2−5, and in each case, the “new” redox behavior for reduction and reoxidation of the 1:1 mixture is then characterized by two MnIII/ MnII reductions followed by three two-electron-transfer processes of equal current height, as seen in the bottom voltammogram of Figure 5a. The overall proposed mechanism for the forward and reverse scans is schematically shown in Scheme 6. The 1:1 mixture of porphyrins 1 and 3 was also investigated by thin-layer spectroelectrochemistry in order to better characterize the site of electron transfer in the “new” two-electron-transfer process at E1/2 = −0.48 V. Spectral measurements were made at selected applied potentials on the forward and reverse scans in the cyclic voltammgram of the 1:1 mixture, and a summary of the spectral changes observed during each controlled potential reduction or reoxidation in the thin-layer cell is given in Figure 6. The first two sets of spectral changes shown in parts a and b are virtually identical with the spectral changes described earlier in the manuscript for reduction of the individual MnIII centers of compounds 1 and 3 when measured separately (Figure 2). The spectral changes in parts c and d are also almost identical with that observed for the two- and four-electron reductions of compound 1, namely, the appearance of a 773 nm band for [TMPyPMnII]2+ upon reduction of the conjugated macrocycle (Figure 4a) and the loss of this near-IR “marker” band upon reduction of the four pyridinium groups and formation of [TMPyPMnII]2− (figure not shown). The spectral changes on the reverse positive potential sweep in the cyclic voltammograms of the 1:1 mixture after generation of the “raft” are illustrated on the right side of Figure 6 and read from bottom to top upon going from an applied potential of −0.80 V to an applied potential of +0.20 V. The spectra of the mixture after reduction at point d in Figure 6 are characterized by a sharp Soret band at 438 nm, a shoulder at 378 nm, and two Q bands at 574 and 612 nm, all of which are associated with the MnII form of the TPPS porphyrin having an unreduced π-ring system (see the spectral data in Figure 2 and Table 2). These bands associated with the [TPPSMnII]4− part of the “raft” remain unchanged upon a switching of the reoxidation potential to −0.80 V (point e in Figure 6), with the only major difference in the final spectrum at this potential being a reappearance of the

Scheme 5. Interaction between [TMPyPMnII]2− and [TPPSMnII]4− To Form the “Raft”

in the redox behavior on the reverse potential sweep such that a global four-electron reoxidation of the four reduced pyridinium groups is no longer observed and is instead replaced by a reversible two-electron oxidation at the same potential as that seen in Figure 5a. This is then followed by two additional twoelectron oxidations, the first at E1/2 = −0.65 V and the second at −0.48 V, the latter of which is a new process that only appears after reduction of the four pyridinium groups as verified by the cyclic voltammogram in Figure 5a, where the scan has been reversed at −1.2 V. After electrogeneration of the proposed “raft” structure schematically shown in Scheme 5, the original redox behavior of compound 1 is no longer observed in the 1:1 mixture of compounds 1 and 3, and in each case, the reduction and reoxidation are illustrated in Figure 5a.

Scheme 6. Proposed Reduction Mechanism for a 1:1 Mixture of Compounds 1 and 3 in DMSO, Containing 0.1 M TBAPa

a

A similar mechanism is proposed to occur in DMF. 8053

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry

Figure 6. UV−vis spectral changes of the 1:1 mixture (molar ratio) of compounds 1 and 3 during the first scan of reductions in DMF, containing 0.1 M TBAP.

773 nm marker band. The final spectrum at −0.80 V (point e) is essentially identical with the spectrum obtained after reduction at −0.85 V (point c in Figure 6), but it must be pointed out that the number of reduced pyridinium groups is different under these

two conditions, 0 under application of an initial reducing potential of −0.85 V (point c) and 2 under application of a reoxidazing potential of −0.80 V (point e in the figure). 8054

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry

association might also occur between the −SO3− groups of compound 3 and the Mn center of compound 1, as was proposed in the literature for related derivatives of TPPSMn.51,52 This latter arrangement is shown in part c of Scheme 7. Although the initial (first scan) cyclic voltammogram of the unreduced 1:1 mixture of 1 and 3 gave no electrochemical evidence for the formation of oligomers in DMSO or DMF containing 0.1 M TBAP, these species may still exist in solution, as evidenced by the fact that quite distinct changes are seen on the reverse potential initial sweep of the cyclic voltammogram and all following cyclic voltammetric scans, as shown in Figure 5. One might then envision a rearrangement of the molecules in the 1:1 mixture after the complete reduction of compound 1, either from an oligomer of the type shown in parts a and b of Scheme 7 to that in part c or, alternatively, a direct electrochemically initiated conversion from porphyrin monomers to the axially coordinated oligomer in Scheme 7c may have occurred. The first two structures in Scheme 7 would be most favorable for oligomer formation before reduction of the four positively charged pyridinium groups on compound 1, but after electrochemical removal of these charges, the −SO3− groups on compound 3 might preferentially bind the MnII center of compound 1, as shown in Scheme 1c, and this oligomer arrangement, once formed, might remain on all further cyclic voltammetric sweeps, as implied by the data in Figure 5. An oligomeric unit similar to that shown in part c of Scheme 7 might also be formed between two or more molecules of the monomeric TTPS derivatives (3 or 4) in DMSO and DMF. If this were to occur, it could be reflected in the irreversible reduction behavior of the MnIII/MnII process for these two compounds, where large differences between the cathodic and anodic peak potentials are observed compared to that seen for compounds 1 and 2 and TPPMnCl (Figure 1). In summary, we have characterized two series of tetrapositively and tetranegatively charged manganese(III) porphyrins as their electrochemistry and spectroscopic properties after reduction in nonaqueous media. We have also examined the electrochemical and spectroscopic properties of related tetrapositively charged copper(II) and palladium(II) derivatives as well as a 1:1 mixture of TMPyP and TPPS derivatives containing manganese(III). Each redox reaction of the 1:1 mixture was monitored by multiscan cyclic voltammetry and thin-layer spectoelectrochemistry, which provided data suggesting an electrochemically initiated conversion from one oligomeric form to another or conversion from monomers to oligomers after the global sevenelectron reduction of [TRPyPMnIII]5+ to [TRPyPMnII]2−. A similar reaction is seen between compounds 1 and 3 in DMF and DMSO, and an overall mechanism for reduction and reoxidation is presented.

The 773 nm band then disappears after the second twoelectron reoxidation at an applied potential of −0.52 V, and the final spectra in part f of the figure is virtually identical with the final spectrum after reduction at −0.60 V (point h). Both spectra are characteristic of manganese(II) porphyrins, but the overall charge on the final porphyrin product in part f is different from that on the final product in part b, and it is only after the third two-electron reoxidation of the 1:1 mixture at part g that all of the reduced macrocycle and pyridinium groups of compound 1 have been returned back to their original MnII form. The lack of spectral changes in part g can be explained by the fact that this reoxidation involves two of the reduced pyridinium groups on compound 1, which have little to no interaction with the conjugated macrocycle. Finally, in part h, both MnII centers are shown as converted to their MnIII forms, but the interaction that occurred between the two oppositely charged porphyrins upon reduction remains unchanged, as indicated by the fact that the second-to-fifth cyclic voltammetric scans of the 1:1 mixture all exhibit three reversible two-electron redox reactions at the conjugated macrocycle and the four pyridinium groups. Similar spectral changes were obtained on the second forward and reverse potential sweeps in the thin-layer cell, as shown in Figure S15. Reversible reduction and reoxidation processes for the three two-electron transfers were recorded as seen in the cyclic voltammogram of this figure, and reversible spectral changes were also recorded upon each controlled reduction and reoxidation, as illustrated in parts (3) and (8) of the figure for the first two-electron processes, parts (4) and (7) for the second, and parts (5) and (6) for the third. Thus, all three two-electrontransfer processes appeared as completely reversible on the cyclic voltammetry and spectroelectrochemistry time scale. In summary, the initial mechanism for reduction of the four pyridinium groups on compound 1 in DMSO has been converted from a process involving a single overlapping fourelectron addition to one involving two well-separated twoelectron additions. The reason for this change in the mechanism may be related to changes in the manganese porphyrin axial coordination and/or the out-of-plane distance of the manganese(II) in compound 1 and/or 3. The formation of a “raft” might also bring the manganese(II) more into the plane of the macrocycle, and the behavior might then be close to that of the copper(II) and palladium(II) derivatives. Prior to electrochemically examining the 1:1 mixture of 1 and 3, we had expected to see an interaction between the four positively charged pyridinium groups on compound 1 and the four negatively charged −SO3− groups on compound 3, which would be reflected by a change in the redox properties of the individual porphyrin components in the mixture.29−34 An interaction between compounds 1 and 3 could result in H- or J-type aggregates, as shown in parts a and b of Scheme 7, or an

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00732. Structural characterization data and spectra of compounds 1−5 (PDF)

Scheme 7. Possible Interaction between Compounds 1 and 3 in DMF or DMSO

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +33 (0)3 80 39 61 12. 8055

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry

[(TMpyP)MIIICl]4+(Cl−)4 in N,N-Dimethylformamide Where M is One of 15 Different Metal Ions. Inorg. Chem. 2005, 44, 3789−3798. (13) Van Caemelbecke, E.; Kutner, W.; Kadish, K. M. Electrochemical and Spectroelectrochemical Characterization of (5,10,15,20-tetrakis(1methyl-4-pyridyl)porphinato)manganese(III) Chloride, [(TMpyP)MnIIICl]4+(Cl−)4, in N,N-dimethylformamide. Inorg. Chem. 1993, 32, 438−444. (14) Kadish, K. M.; Sazou, D.; Araullo, C.; Liu, Y. M.; Saoiabi, A.; Ferhat, M.; Guilard, R. Electrochemistry of Vanadyl Porphyrins in Dimethylformamide. Inorg. Chem. 1988, 27, 2313−2320. (15) Kadish, K. M.; Araullo, C.; Maiya, G. B.; Sazou, D.; Barbe, J. M.; Guilard, R. Electrochemical and Spectral Characterization of Copper, Zinc, and Vanadyl Meso-tetrakis(1-methylpyridinium-4-yl)porphyrin Complexes in Dimethylformamide. Inorg. Chem. 1989, 28, 2528−2533. (16) Kadish, K. M.; Maiya, G. B.; Araullo, C.; Guilard, R. Micellar Effects on the Aggregation of Tetraanionic Porphyrins. Spectroscopic Characterization of Free-base Meso-tetrakis(4-sulfonatophenyl)porphyrin, (TPPS)H2, and (TPPS)M (M = Zinc(II), Copper(II), and Vanadyl) in Aqueous Micellar Media. Inorg. Chem. 1989, 28, 2725− 2731. (17) Araullo-McAdams, C.; Kadish, K. M. Electrochemistry, Spectroscopy, and Reactivity of (Meso-tetrakis(1-methylpyridinium-4-yl)porphinato)cobalt(III,II,I) in Nonaqueous Media. Inorg. Chem. 1990, 29, 2749−2757. (18) Guilard, R.; Senglet, N.; Liu, Y. H.; Sazou, D.; Findsen, E.; Faure, D.; Des Courieres, T.; Kadish, K. M. Studies of Micellar Metalloporphyrins. Synthesis and Spectroscopic Characterization of [(P)H2]+ and [(P)MII]+ where P = the Dianion of 5-(4-N-hexadecylpyridiniumyl10,15,20-triphenylporphyrin Bromide and M = Vanadyl, Nickel or Copper. Inorg. Chem. 1991, 30, 1898−1905. (19) Kadish, K. M.; Liu, Y. H.; Sazou, D.; Senglet, N.; Guilard, R. Structural Effects on Metalloporphyrin Redox Potentials. Electroreduction of Mono-N-hexadecylpyridinium Porphyrins in Nonaqueous Media. Anal. Chim. Acta 1991, 251, 47−52. (20) Cui, Y.; Zeng, L.; Fang, Y.; Zhu, J.; Xu, H.-J.; Desbois, N.; Gros, C. P.; Kadish, K. M. Electrochemical and Spectroelectrochemical Properties of Free-Base Pyridyl- and N-Alkyl-4-Pyridylporphyrins in Nonaqueous Media. ChemElectroChem 2016, 3, 110−121. (21) McCord, J. M.; Fridovich, I. Superoxide Dismutase. Enzymic Function for Erythrocuprein (Hemocuprein). J. Biol. Chem. 1969, 244, 6049−6055. (22) Faulkner, K. M.; Liochev, S. I.; Fridovich, I. Stable Mn(III) Porphyrins Mimic Superoxide Dismutase in Vitro and Substitute for it in Vivo. J. Biol. Chem. 1994, 269, 23471−23476. (23) Piganelli, J. D.; Flores, S. C.; Cruz, C.; Koepp, J.; Batinic-Haberle, I.; Crapo, J.; Day, B.; Kachadourian, R.; Young, R.; Bradley, B.; Haskins, K. A Metalloporphyrin-based Superoxide Dismutase Mimic Inhibits Adoptive Transfer of Autoimmune Diabetes by a Diabetogenic T-cell Clone. Diabetes 2002, 51, 347−355. (24) Spasojevic, I.; Batinic-Haberle, I.; Reboucas, J. S.; Idemori, Y. M.; Fridovich, I. Electrostatic Contribution in the Catalysis of O2.‑ Dismutation by Superoxide Dismutase Mimics. MnIIITE-2-PyP5+ versus MnIIIBr8T-2-PyP. J. Biol. Chem. 2003, 278, 6831−6837. (25) Zhao, Y.; Chaiswing, L.; Oberley, T. D.; Batinic-Haberle, I.; St. Clair, W.; Epstein, C. J.; St. Clair, D. A Mechanism-based Antioxidant Approach for the Reduction of Skin Carcinogenesis. Cancer Res. 2005, 65, 1401−1405. (26) Reboucas, J. S.; DeFreitas-Silva, G.; Spasojevic, I.; Idemori, Y. M.; Benov, L.; Batinic-Haberle, I. Impact of Electrostatics in Redox Modulation of Oxidative Stress by Mn Porphyrins: Protection of SOD-deficient Escherichia Coli via Alternative Mechanism Where Mn Porphyrin Acts as a Mn Carrier. Free Radical Biol. Med. 2008, 45, 201− 210. (27) Rabbani, Z. N.; Spasojevic, I.; Zhang, X.; Moeller, B. J.; Haberle, S.; Vasquez-Vivar, J.; Dewhirst, M. W.; Vujaskovic, Z.; Batinic-Haberle, I. Antiangiogenic Action of Redox-modulating Mn(III) Meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, MnTE-2-PyP5+, via Suppression of Oxidative Stress in a Mouse Model of Breast Tumor. Free Radical Biol. Med. 2009, 47, 992−1004.

*E-mail: [email protected]. Phone: (+1) 713-743-2740. ORCID

Claude P. Gros: 0000-0002-6966-947X Karl M. Kadish: 0000-0003-4586-6732 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Support was provided by the CNRS (UMR 6302), the Université de Bourgogne Franche-Comté, the Conseil Régional de Bourgogne through the 3MIM-integrated project (“Marquage de Molécules par les Métaux pour l’Imagerie Médicale”), the PARI II project “Pharmaco-Imagerie et Théranostiques”, and the Robert A. Welch Foundation (Grant E-680 to K.M.K.). We are very grateful to Dr. Benoit Habermeyer and the PorphyChem Co. for furnishing the meso-tetrakis(4-pyridyl)porphyrin and meso-tetrakis(4-hydroxy)porphyrin precursors.

■

REFERENCES

(1) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. Electrochemistry of Metalloporphyrins in Non-aqueous Media. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 8, pp 1−114. (2) Kadish, K. M. The Electrochemistry of Metalloporphyrins in Nonaqueous Media. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; John Wiley & Sons, Inc.: New York, 1986; Vol. 34, pp 435−605. (3) Kadish, K. M.; Kelly, S. Electron-transfer and Ligand-addition Reactions of (Meso-tetraphenylporphinato)Manganese(II) and Manganese(III) Chloride. Inorg. Chem. 1979, 18, 2968−2971. (4) Kelly, S. L.; Kadish, K. M. Counterion and Solvent Effects on the Electrode Reactions of Manganese Porphyrins. Inorg. Chem. 1982, 21, 3631−3639. (5) Kadish, K. M.; Sweetland, M.; Cheng, J. S. Electron-transfer Kinetics of Chlorotetrakis(p-chlorophenyl)porphinatomanganese(III) in Dimethyl Sulfoxide-imidazole Mixtures. Inorg. Chem. 1978, 17, 2795−2797. (6) Yang, S.; Sun, B.; Ou, Z.; Meng, D.; Lu, G.; Fang, Y.; Kadish, K. M. β-Nitro-substituted Free-base, Iron(III) and Manganese(III) Tetraarylporphyrins: Synthesis, Electrochemistry and Effect of the NO2 Substituent on Spectra and Redox Potentials in Non-aqueous Media. J. Porphyrins Phthalocyanines 2013, 17, 857−869. (7) Kadish, K. M.; Davis, D. G. Electrochemical Studies of Metalloporphyrins. Ann. N. Y. Acad. Sci. 1973, 206, 495−503. (8) Fukuzumi, S.; Nakanishi, I.; Barbe, J.-M.; Guilard, R.; Van Caemelbecke, E.; Guo, N.; Kadish, K. M. Decreased Electron Transfer Rates of Manganese Porphyrins with Conformational Distortion of the Macrocycle. Angew. Chem., Int. Ed. 1999, 38, 964−966. (9) Guilard, R.; Perie, K.; Barbe, J.-M.; Nurco, D. J.; Smith, K. M.; Van Caemelbecke, E.; Kadish, K. M. Electrochemical and Spectroscopic Characterization of Manganese(III) Dodecaphenylporphyrin Derivatives and X-ray Structural Determination of Chloro(5,10,15,20tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18octaphenylporphyrinato)manganese(III). Formation of a Manganese(IV) Species by Ozone and Electrochemical Oxidation. Inorg. Chem. 1998, 37, 973−981. (10) Hambright, P. Chemistry of Water Soluble Porphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 3, pp 129−210. (11) Batinic-Haberle, I.; Spasojevic, I.; Hambright, P.; Benov, L.; Crumbliss, A. L.; Fridovich, I. Relationship among Redox Potentials, Proton Dissociation Constants of Pyrrolic Nitrogens, and in Vivo and in Vitro Superoxide Dismutating Activities of Manganese(III) and Iron(III) Water-Soluble Porphyrins. Inorg. Chem. 1999, 38, 4011−4022. (12) Van Caemelbecke, E.; Derbin, A.; Hambright, P.; Garcia, R.; Doukkali, A.; Saoiabi, A.; Ohkubo, K.; Fukuzumi, S.; Kadish, K. M. Electrochemistry of [(TMpyP)MII]4+(X−)4 (X− = Cl− or BPh4−) and 8056

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057

Article

Inorganic Chemistry (28) Zhang, X.-a.; Lovejoy, K. S.; Jasanoff, A.; Lippard, S. J. Watersoluble Porphyrins as a Dual-function Molecular Imaging Platform for MRI and Fluorescence Zinc Sensing. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 10780−10785. (29) Adinehnia, M.; Mazur, U.; Hipps, K. W. Predicting the Size Distribution in Crystallization of TSPP: TMPyP Binary Porphyrin Nanostructures in a Batch Desupersaturation Experiment. Cryst. Growth Des. 2014, 14, 6599−6606. (30) Eskelsen, J. R.; Qi, Y.; Schneider-Pollack, S.; Schmitt, S.; Hipps, K. W.; Mazur, U. Correlating Elastic Properties and Molecular Organization of an Ionic Organic Nanostructure. Nanoscale 2014, 6, 316−327. (31) Eskelsen, J. R.; Phillips, K. J.; Hipps, K. W.; Mazur, U. Hyperbranched Crystalline Nanostructure Produced from Ionic πconjugated Molecules. Chem. Commun. 2015, 51, 2663−2666. (32) Ruhlmann, L.; Nakamura, A.; Vos, J. G.; Fuhrhop, J.-H. Manganese Porphyrin Heterodimers and -trimers in Aqueous Solution. Inorg. Chem. 1998, 37, 6052−6059. (33) Ruhlmann, L.; Zimmermann, J.; Fudickar, W.; Siggel, U.; Fuhrhop, J. H. Heterodimers and Trimers of Meso-tetra-(isophthalic acid)-porphyrin Octaanions with Meso- and β-tetramethylpyridiniumporphyrin Tetracations and Their Manganese Complexes in Water. Electrochemistry, Spectroelectrochemistry and Fluorescence Quenching. J. Electroanal. Chem. 2001, 503, 1−14. (34) Olaya, A. J.; Schaming, D.; Brevet, P.-F.; Nagatani, H.; Zimmermann, T.; Vanicek, J.; Xu, H.-J.; Gros, C. P.; Barbe, J.-M.; Girault, H. H. Self-Assembled Molecular Rafts at Liquid/Liquid Interfaces for Four-Electron Oxygen Reduction. J. Am. Chem. Soc. 2012, 134, 498−506. (35) Diabate, P. D.; Laguerre, A.; Pirrotta, M.; Desbois, N.; Boudon, J.; Gros, C. P.; Monchaud, D. DNA structure-specific sensitization of a metalloporphyrin leads to an efficient in vitro quadruplex detection molecular tool. New J. Chem. 2016, 40 (7), 5683−5689. (36) Mu, X. H.; Schultz, F. A. Methanol Bonding to and Electrontransfer Reactivity of Chloro(tetraphenylporphinato)manganese(III). Inorg. Chem. 1992, 31, 3351−3357. (37) Lingane, J. J. Interpretation of the Polarographic Waves of Complex Metal Ions. Chem. Rev. 1941, 29, 1−35. (38) Kadish, K. M.; Mu, X. H.; Lin, X. Q. The Construction and Utilization of a Simple Light-transparent FTIR Spectroelectrochemical Cell with Thin-layer Chamber. Electroanalysis 1989, 1, 35−41. (39) Mi, Q.; Cao, Z.; Jia, T.; Zhou, S.; Liu, J.; Song, X.; Li, Z.; Ye, Z. Effect of Photodynamic Therapy with Metalloporphyrin Compound on Human Prostate Cancer PC-3 Cells in Vitro. Zhonghua Nankexue Zazhi 2005, 11, 124−129. (40) Laguerre, A.; Chang, Y.; Pirrotta, M.; Desbois, N.; Gros, C. P.; Lesniewska, E.; Monchaud, D. Surface-promoted Aggregation of Amphiphilic Quadruplex Ligands Drives Their Selectivity for Alternative DNA Structures. Org. Biomol. Chem. 2015, 13, 7034−7039. (41) Dong, Z.; Scammells, P. J. New Methodology for the NDemethylation of Opiate Alkaloids. J. Org. Chem. 2007, 72, 9881−9885. (42) Gaspard, S.; Tran-Thi, T.-H. A New Method for the Synthesis of Zinc Tetrakis(3-methylpyridyloxy)Phthalocyanine and the Physical Properties of Dimers Formed by Complexation of the Cationic Phthalocyanine with Anionic Porphyrins. J. Chem. Soc., Perkin Trans. 2 (1972-1999) 1989, 5, 383. (43) Gaillon, L.; Bedioui, F.; Devynck, J.; Battioni, P. Electrochemical Characterization of Manganese Porphyrins Fixed onto Silica and Layered Dihydroxide Matrixes. J. Electroanal. Chem. 1993, 347 (1−2), 435−42. (44) Crestini, C.; Saladino, R.; Tagliatesta, P.; Boschi, T. Biomimetic Degradation of Lignin and Lignin Model Compounds by Synthetic Anionic and Cationic Water Soluble Manganese and Iron porphyrins. Bioorg. Med. Chem. 1999, 7 (9), 1897−1905. (45) Bhosale, S.; Bhosale, S.; Wang, T.; Kopaczynska, M.; Fuhrhop, J.H. Hydrophobic and Hydrophilic Yoctowells as Receptors in Water. J. Am. Chem. Soc. 2006, 128 (7), 2156−2157.

(46) Lottner, C.; Bart, K.-C.; Bernhardt, G.; Brunner, H. Soluble Tetraarylporphyrin-Platinum Conjugates as Cytotoxic and Phototoxic Antitumor Agents. J. Med. Chem. 2002, 45, 2079−2089. (47) Zhang, J.; Teng, M.; Li, D. Synthesis, Characterization, Photophysical and DNA Photocleavage Properties of Amphiphilic Porphyrins. J. Porphyrins Phthalocyanines 2014, 18, 221−230. (48) Diabate, P. D.; Laguerre, A.; Pirrotta, M.; Desbois, N.; Boudon, J.; Gros, C. P.; Monchaud, D. DNA Structure-specific Sensitization of A Metalloporphyrin Leads to An Efficient in Vitro Quadruplex Detection Molecular Tool. New J. Chem. 2016, 40, 5683−5689. (49) Kadish, K. M.; Morrison, M. M. Investigation of Substituent Effects on the Redox Properties and Oxygen Binding of Several Metalloporphyrins. Bioelectrochem. Bioenerg. 1976, 3, 480−490. (50) Giraudeau, A.; Callot, H. J.; Jordan, J.; Ezhar, I.; Gross, M. Substituent Effects in the Electroreduction of Porphyrins and Metalloporphyrins. J. Am. Chem. Soc. 1979, 101, 3857−3862. (51) Schwarz, U.; Vonderach, M.; Kappes, M.; Kelting, R.; Brendle, K.; Weis, P. Structural Characterization of Metalloporphyrin-Oligomer Multianions by Mass Spectrometry and Ion Mobility SpectrometryObservation of Metastable Species. Int. J. Mass Spectrom. 2013, 339− 340, 24−33. (52) Schwarz, U.; Vonderach, M.; Armbruster, M. K.; Fink, K.; Kappes, M. M.; Weis, P. Cu(II)- and Mn(III)-Porphyrin-Derived Oligomeric Multianions: Structures and Photoelectron Spectra. J. Phys. Chem. A 2014, 118, 369−379. (53) Fang, Y.; Jiang, X.; Ou, Z.; Michelin, C.; Desbois, N.; Gros, C. P.; Kadish, K. M. Redox Properties of Nitrophenylporphyrins and Electrosynthesis of Nitrophenyl-linked Zn Porphyrin Dimers or Arrays. J. Porphyrins Phthalocyanines 2014, 18, 832−841.

8057

DOI: 10.1021/acs.inorgchem.7b00732 Inorg. Chem. 2017, 56, 8045−8057