Electrochemistry of Bis(pyridine)cobalt (Nitrophenyl)corroles in

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

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Electrochemistry of Bis(pyridine)cobalt (Nitrophenyl)corroles in Nonaqueous Media Xiaoqin Jiang,† Mario L. Naitana,‡ Nicolas Desbois,‡ Valentin Quesneau,‡ Stéphane Brandès,‡ Yoann Rousselin,‡ Wenqian Shan,† W. Ryan Osterloh,† Virginie Blondeau-Patissier,§ Claude P. Gros,*,‡ and Karl M. Kadish*,† †

Department of Chemistry, University of Houston, Houston, Texas 77204, United States Université de Bourgogne Franche-Comté, ICMUB (UMR CNRS 6302), 9 Avenue Alain Savary, BP 47870, 21078 Dijon, Cedex, France § Department of Time-Frequency, Université de Bourgogne Franche-Comté, Institut FEMTO-ST (UMR CNRS 6174), 26 Chemin de l’épitaphe, 25030 Besançon Cedex, France ‡

S Supporting Information *

ABSTRACT: A series of bis(pyridine)cobalt corroles with one or three nitrophenyl groups on the meso positions of the corrole macrocycle were synthesized and characterized as to their electrochemical and spectroscopic properties in dichloromethane, benzonitrile, and pyridine. The potentials for each electrode reaction were measured by cyclic voltammetry and the electron-transfer mechanisms evaluated by analysis of the electrochemical data combined with UV−visible spectra of the neutral, electroreduced, and electroxidized forms of the corroles. The proposed electronic configurations of the initial compounds and the prevailing redox reactions involving the electroactive central cobalt ion, the electroactive conjugated macrocycle, and the electroactive meso-nitrophenyl groups are all discussed in terms of solvent binding and the number of the nitrophenyl groups and other substituents on the meso-nitrophenyl rings of the compounds.

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active metal center and a redox-active π-ring system but also redox-active nitrophenyl groups, a fact that has been not previously recognized in a number of publications containing electrochemical studies on meso-nitrophenyl-substituted macrocycles. The currently studied compounds differ from our previously investigated cobalt triarylcorroles in that the meso-nitrophenyl substituents on the macrocycle carry electron-donating (OCH3) or electron-withdrawing (F) groups and the central cobalt ion has two axially ligated pyridine molecules, which can remain coordinated or dissociate depending on the concentration of the corrole and the solution conditions. Thus, the aim of this study is not only to examine the electrochemistry of the redox-active meso-nitrophenyl groups themselves compared to the previously characterized (nitrophenyl)corroles24 but also to see how changes in the number of nitrophenyl groups on the macrocycle, the position of the NO2 group on the meso-phenyl rings, or the presence of other electron-donating or electronwithdrawing substituents on this ring might affect redox reactions of the nitrophenyl groups themselves, the cobalt central metal ion, and the corrole macrocycle. The third point is to see how changes in the solvent, oxidation state, or axial

INTRODUCTION Metallocorroles are now an often-studied group of macrocycles because of their unique spectral and electrochemical properties, as well as their potential applications as catalysts for a variety of reactions.1−11 A rich redox reactivity can be observed for transition-metal corroles, where both metal- and ring-centered reactions have been characterized under a variety of solution conditions. The most-often-studied derivatives have contained cobalt,3,11−29 iron,30−42 or manganese19,32,37,43−56 central metal ions, which have been characterized in both high and low oxidation states. A conversion between the different oxidation states of the corrole is easy to achieve using electrochemical techniques and has been best demonstrated in the case of cobalt derivatives, where the potentials for reduction or oxidation will systematically vary with changes in the number and types of electron-donating or electron-withdrawing substituents on the macrocycle.7,11,12,21,24,26,57,58 As part of our continuing studies into elucidating the electrochemistry of metallocorroles,2,3,11 a new series of cobalt (nitrophenyl)corroles containing two pyridine ligands were synthesized and electrochemically characterized in dichloromethane, benzonitrile, and pyridine. The structures of these compounds are shown in Chart 1 along with a structurally related four-coordinate triarylcorrole, as a reference compound. These cobalt (nitrophenyl)corroles possess not only a redox© XXXX American Chemical Society

Received: October 16, 2017

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DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Chart 1. Structures of the Investigated Six-Coordinate Bis(pyridine)cobalt (Nitrophenyl)corroles and Structurally Related FourCoordinate Triarylcorrole, Mes3CorCo

Scheme 1. Synthesis of Cobalt (Nitrophenyl)corroles 1−4

electrolyte, was obtained from Sigma Chemical Co. The “reference” corrole Mes3CorCo was synthesized as described in the literature.21 Silica gels 60 (70−230 and 230−400 mesh, Sigma-Aldrich) were used for column chromatography. Reactions were monitored by thinlayer chromatography, UV−visible spectroscopy, and mass spectrometry. Deuterated tetrahydrofuran (THF-d8), chloroform (CDCl3), and pyridine (pyridine-d5) were used as solvents for 1H NMR spectra, which were recorded on a Bruker Avance III Nanobay spectrometer (300 MHz). The measurements were made at the PACSMUB-WPCM technological platform, which relies on the “Institut de Chimie Moléculaire de l’Université de Bourgogne” and Welience “TM”, a Burgundy University private subsidiary. The NMR data were processed with TopSpin. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI/TOF MS) spectra were recorded on a Bruker Ultraflex Extreme MALDI Tandem TOF mass spectrometer using dithranol as the matrix or on a LTQ Orbitrap XL (THERMO) instrument in the electrospray ionization (ESI) mode [for the high-resolution MS (HRMS) spectra]. UV−visible spectra 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 (PAR) model 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 from the bulk of the solution by a fritted glass bridge of low porosity, which contained the solvent/supporting electrolyte mixture. Thinlayer UV−visible spectroelectrochemical experiments were performed

coordination of the central metal ion might influence both the redox potentials and the site of electron transfer in these newly synthesized compounds. Thus, the focus of our current study is to evaluate not only the prevailing redox reactions but also any electronic communication that might occur between the three redoxactive sites on the corrole, i.e., the cobalt metal ion, the corrole π-ring system, and the meso-nitrophenyl groups. To accomplish this, the electrochemical behavior of each corrole shown in Chart 1 is examined by cyclic voltammetry in several solvents, and in each solvent, the UV−visible spectra of the neutral, electroreduced, and electrooxidized forms of the corroles are obtained. Each redox reaction is then discussed in terms of the oxidation state of the central cobalt ion, the degree of axial ligand coordination, the properties of the solvent, and the number and types of meso-nitrophenyl substituents on the corrole macrocycle.

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EXPERIMENTAL PART

Materials and Instrumentation. All chemicals and solvents were of the highest grade available and were used without further purification. Dichloromethane (CH2Cl2) and pyridine (Py) were purchased from Sigma-Aldrich Co. and used as received. Benzonitrile (PhCN) was distilled under reduced pressure from P2O5 prior to use. Tetra-n-butylammonium perchlorate (TBAP), used as the supporting B

DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Hz, Ph), 7.28 (s, 4H, Ph overlapped with a CHCl3 deuterated solvent residual signal), 4.20 (s, 3H, OCH3), 2.61 (s, 6H, p-CH3), 1.92 (s, 12H, o-CH3). MS (MALDI/TOF). Calcd for C44H39N5O3 ([M]•+): m/z 685.31. Found: m/z 685.11. 10-(4-Fluoro-3-nitrophenyl)-5,15-bis(4-nitrophenyl)corrole (3H3) was prepared starting from 4-fluoro-5-nitrobenzaldehyde. The reaction mixture was chromatographed on silica gel using CHCl3/ toluene (7:1) as the eluent, and the desired corrole was obtained as pure crystals in 28.5%. UV−visible [THF; λmax, nm (ε × 10−3, L mol−1 cm−1)]: 446 (65.9). 1H NMR (300 MHz, THF-d8): δ 9.08 (d, 2H, 3 JH−H = 4.2 Hz, β-pyrrolic), 8.90 (d, 2H, 3JH−H = 4.8 Hz, β-pyrrolic), 8.84 (dd, 1H, 4JH−H = 2.1 Hz, 4JF−H = 7.0 Hz, Ph), 8.71 (d, 4H, 3JH−H = 8.7 Hz, Ph), 8.60 (m, 8H, β-pyrrolic, Ph), 8.49 (ddd, 1H, 4JH−H = 2.1 Hz, 4JF−H = 11.1 Hz, 3JH−H = 8.7 Hz, Ph), 7.86 (dd, 1H, 3JH−H = 8.7 Hz, 4J F−H = 11.1 Hz, Ph). MS (MALDI/TOF). Calcd for C37H22FN7O6 ([M]•+): m/z 679.16. Found: m/z 678.77. 10-(4-Methoxy-3-nitrophenyl)- 5,15-bis(4-nitrophenyl)corrole (4H3) was prepared from 4-methoxy-5-nitrobenzaldehyde. The reaction mixture was chromatographed on silica gel using CHCl3/ toluene (7:1) as the eluent, and the desired corrole was obtained as a pure crystal in 30.3%. UV−visible [THF; λ, nm (ε × 10−3, L mol−1 cm−1)]: 447 (67.2). 1H NMR (300 MHz, THF-d8): δ 9.01 (d, 2H, 3 JH−H = 4.6 Hz, β-pyrrolic), 8.80 (d, 2H, 3JH−H = 5.1 Hz, β-pyrrolic), 8.64 (m, 6H, β-pyrrolic, Ph), 8.56 (m, 5H, Ph), 8.48 (d, 2H, 3JH−H = 5.0 Hz, β-pyrrolic), 8.31 (dd, 1H, 3JH−H = 8.4 Hz, 4JH−H = 2.1 Hz, Ph), 7.57 (d, 1H, 3JH−H = 8.4 Hz, Ph), 4.16 (s, 3H, OCH3). MS (MALDI/ TOF). Calcd for C38H25N7O7 ([M]•+): m/z 691.18. Found: m/z 690.94. The cobalt corroles 1−4 were prepared from the free-base precursors, as shown in Scheme 1. A similar procedure was used for all four corroles. In a round-bottomed flask, a solution of 1H3−4H3 (0.074 mmol) in 15 mL of pyridine was heated to 100 °C, and Co(OAc)2·4H2O (0.355 mmol) was added. The solution was stirred for 20 min and pyridine then removed under vacuum. The residue was dissolved in chloroform, a few drops of pyridine added, and the mixture chromatographed on a silica gel column with CHCl3/pyridine (98:2, v/v) as the eluent. Fractions containing the corrole were crystallized in 3:1 CH2Cl2/CH3OH while a few drops of pyridine was added to the solution. Bis(pyridine)cobalt 10-(4-fluoro-3-nitrophenyl)-5,15-dimesitylcorrole (1) was obtained as pure crystals in 92.0% yield. UV−visible [CH2Cl2 + 1% pyridine; λmax, nm (ε × 10−3, L mol−1 cm−1)]: 438 (76.9), 453 (79.1), 583 (20.4), 621 (29.7). 1H NMR (300 MHz, THFd8): δ 8.89 (d, 2H, 3JH−H = 4.7 Hz, β-pyrrolic), 8.73 (dd, 1H, 4JF−H = 7.2 Hz, 4JH−H = 2.1 Hz, Ph), 8.42 (d, 2H, 3JH−H = 5.2 Hz, β-pyrrolic), 8.37 (br s, 4H, β-pyrrolic), 8.19 (m, 1H, Ph), 7.67 (dd, 1H, 3JF−H = 10.5 Hz, 3JH−H = 8.5 Hz 1H, Ph), 7.19 (s, 4H, Ph), 6.33 (br s, 2H, ppyridine), 5.48 (br s, 4H, m-pyridine), 2.79 (br s, overlapped with a H2O solvent signal, 4H, o-pyridine), 2.54 (s, 6H, p-CH3), 1.82 (s, 12H, o-CH3). MS (MALDI/TOF). Calcd for C43H33CoFN5O2 ([M − 2py]•+): m/z 729.20. Found: m/z 728.82. HRMS (ESI). Calcd for C43H33CoFN5O2 ([M − 2py]+): m/z 729.1945. Found: m/z 729.1980. Bis(pyridine)cobalt 10-(4-methoxy-3-nitrophenyl)-5,15-dimesitylcorrole (2) was obtained as pure crystals in 88.5% yield. UV− visible [CH2Cl2 + 1% pyridine; λmax, nm (ε × 10−3, L mol−1 cm−1)]: 436 (92.1), 452 (95.6), 583 (29.0), 619 (41.0). 1H NMR (500 MHz, pyridine-d5): δ 9.33 (d, 2H, 3JH−H = 4.0 Hz, β-pyrrolic), 9.08 (d, 2H, 3 JH−H = 4.5 Hz, β-pyrrolic), 9.02 (br s, 2H, β-pyrrolic), 8.87 (br s, 2H, β-pyrrolic), 8.80 (br s, 2H, Ph overlapped with a deuterated solvent residual signal), 8.38 (d, 1H, 3JH−H = 7.5 Hz, Ph), 7.39 (s, 4H, Ph), 3.97 (s, 3H, OCH3), 2.62 (s, 6H, p-CH3), 2.09 (s, 12H, o-CH3). MS (MALDI/TOF). Calcd for C44H36CoN5O3 ([M − 2py]•+): m/z 741.00. Found: m/z 741.21. HRMS (ESI). Calcd for C44H36CoN5O3 ([M − 2py]+): m/z 741.2145. Found: m/z 741.2170. Bis(pyridine)cobalt 10-(4-fluoro-3-nitrophenyl)-5,15-bis(4nitrophenyl)corrole (3) was obtained as pure crystals in 80.3% yield. UV−visible [CH2Cl2 + 1% pyridine; λmax, nm (ε × 10−3, L mol−1 cm−1)]: 453 (46.6), 652 (33.5). 1H NMR (300 MHz, THF-d8): δ 9.25 (d, 2H, 3JH−H = 4.8 Hz, β-pyrrolic), 9.05 (d, 2H, 3JH−H = 5.4 Hz, βpyrrolic), 8.83 (d, 2H, 3JH−H = 4.4 Hz, β-pyrrolic), 8.78 (m, 3H, β-

with a home-built thin-layer cell that has a light-transparent platinumnet working electrode.59,60 Potentials were applied and monitored with an EG&G PAR model 173 potentiostat. High-purity nitrogen from Trigas was used to deoxygenate the solution, and a stream of nitrogen was kept over the solution during each electrochemical and spectroelectrochemical experiment. Crystal Structure Determination. A single crystal of bis(pyridine)cobalt 10-(4-fluoro-3-nitrophenyl)-5,15-dimesitylcorrole (1) was recrystallized from a mixture of cyclohexane and THF by slow evaporation. A suitable crystal was selected and mounted onto a MITIGEN holder oil on a Bruker D8 Venture diffractometer. The crystal was kept at 100(1) K during data collection. Using Olex2, the structure was solved with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package using least-squares minimization. Additional crystallographic information is available in Figure S1 and Tables S1−S7. Crystal data for C53H43CoFN7O2·2.19C6H12·0.81C4H8O (M = 1130.58 g mol−1): monoclinic, space group P21/n (No. 14), a = 13.4928(6) Å, b = 14.9338(7) Å, c = 29.3778(14) Å, β = 96.285(2)°, V = 5884.0(5) Å3, Z = 4, T = 100 K, μ(Cu Kα) = 2.736 mm−1, Dcalcd = 1.276 g cm−3, 81064 reflections measured (6.648° ⩽ 2Θ ⩽ 133.546°), and 10400 unique (Rint = 0.0383 and Rσ = 0.0203), which were used in all calculations. The final R1 was 0.0460 [I > 2σ(I)] and wR2 was 0.1248 (all data). Synthesis of Cobalt (Nitrophenyl)corroles. The precursor freebase corroles 1H3−4H3 were prepared according to a modified procedure of Koszarna and Gryko61 and then metalated with Co(OAc)2·4H2O to give the corresponding cobalt derivatives, as shown in Scheme 1. The reactants 5-(4-nitrophenyl)dipyrromethane and 5-mesityldipyrromethane were synthesized as described in the literature.62 The two arylaldehydes were purchased from SigmaAldrich. The following general procedure was utilized for the synthesis of the free-base derivatives. In a round-bottomed flask, dipyrromethane (1.5 mmol) and arylaldehyde (0.75 mmol) were dissolved in 150 mL of CH3OH, after which 3.8 mL of HClaq (36 wt %) in 75 mL of water was added, and the solution was stirred at room temperature for 2 h. The mixture was extracted with 80 mL of CHCl3. The organic phase was then washed two times with 80 mL of water, dried with sodium sulfate, filtered, and diluted with 250 mL of CHCl3. p-Chloranil (1.5 mmol) was added, and the mixture was stirred overnight at room temperature. The reaction mixture was evaporated to dryness and chromatographed using the conditions described below for each corrole. Fractions containing the pure free-base (nitrophenyl)corrole were combined, evaporated, and crystallized from CH2Cl2/heptane (1:4, v/v), affording 1H3−4H3 in pure form. 10-(4-Fluoro-3-nitrophenyl)-5,15-dimesitylcorrole (1H3) was prepared as described above starting from 4-fluoro-5-nitrobenzaldehyde. The reaction mixture was chromatographed on silica gel using CHCl3/heptane (2:1) as the eluent, and the desired corrole was obtained as pure crystals in 26% yield. UV−visible [CH2Cl2; λmax, nm (ε × 10−3, L mol−1 cm−1)]: 408 (112.5), 426 (92.2), 568, 602, 634. 1H NMR (300 MHz, CDCl3): δ 8.90 (d, 2H, 3JH−H = 4.7 Hz, 2H, βpyrrolic), 8.82 (dd, 1H, 4JF−H = 7.2 Hz, 4JH−H = 2.1 Hz, Ph), 8.53 (d, 2H, 3JH−H = 5.2 Hz, β-pyrrolic), 8.40 (ddd, 1H, 4JF−H = 10.5 Hz, 4JH−H = 2.1 Hz, 3JH−H = 8.5 Hz, Ph), 8.35 (d, 2H, 3JH−H = 5.2 Hz, βpyrrolic), 8.33 (d, 2H, 3JH−H = 4.7 Hz, β-pyrrolic), 7.65 (dd, 1H, 3JF−H = 10.5 Hz, 3JH−H = 8.5 Hz, Ph), 7.28 (s, 4H, Ph overlapped with a CHCl3 deuterated solvent residual signal), 2.61 (s, 6H, p-CH3), 1.92 (s, 12H, o-CH3). MS (MALDI/TOF). Calcd for C43H36FN5O2 ([M]•+): m/z 673.29. Found: m/z 673.00. 10-(4-Methoxy-3-nitrophenyl)-5,15-dimesitylcorrole (2H3) was prepared from 3-nitro-4-methoxybenzaldehyde. The reaction mixture was chromatographed on silica gel using CHCl3/heptane (3:1) as the eluent, and the desired corrole was obtained as pure crystals in 40.2% yield. UV−visible [CH2Cl2; λmax, nm (ε × 10−3, L mol−1 cm−1)]: 408 (119.0), 427 (110.5), 567, 603, 631. 1H NMR (300 MHz, CDCl3): δ 8.88 (dd, 2H, 3JH−H = 4.6 Hz, β-pyrrolic), 8.62 (d, 1H, 4JH−H = 2.4 Hz, Ph), 8.51 (d, 2H, 3JH−H = 5.2 Hz, β-pyrrolic), 8.41 (d, 2H, 3JH−H = 5.2 Hz, β-pyrrolic), 8.33 (m, 3H, β-pyrrolic, Ph), 7.44 (d, 1H, 3JH−H = 8.4 C

DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. UV−visible spectra of Mes3CorCo and 1−4 at (a) 10−5 M in CH2Cl2, (b) 10−3 M in CH2Cl2, and (c) 10−5 M in pyridine. pyrrolic, Ph), 8.65 (d, 4H, 3JH−H = 8.7 Hz, Ph), 8.53 (d, 4H, 3JH−H = 8.7 Hz, Ph), 8.38 (m, 1H, Ph), 7.75 (ddd, 1H, 4JH−H = 2.1 Hz, 4JF−H = 11.1 Hz, 3JH−H = 8.7 Hz, Ph), 6.28 (br s, 2H, p-pyridine), 5.41 (br s, 4H, m-pyridine), 2.45 (br s, overlapped with a H2O solvent signal, 4H, o-pyridine). MS (MALDI/TOF). Calcd for C37H19CoFN7O6 ([M − 2py]•+): m/z 735.07. Found: m/z 734.82. HRMS (ESI). Calcd for C37H19CoFN7O6 ([M − 2py]+): m/z 735.0707. Found: m/z 735.0737. Bis(pyridine)cobalt 10-(4-methoxy-4-nitrophenyl)-5,15-bis(4nitrophenyl)corrole (4) was obtained as pure crystals in 90.8% yield. UV−visible [CH2Cl2 + 1% pyridine; λmax, nm (ε × 10−3, L mol−1 cm−1)]: 453 (46.3), 656 (35.3). 1H NMR (300 MHz, THF-d8): δ 9.17 (d, 2H, 3JH−H = 4.9 Hz, β-pyrrolic), 8.93 (d, 2H, 3JH−H = 5.3 Hz, βpyrrolic), 8.63 (m, 8H, 4H, β-pyrrolic, Ph), 8.48 (m, 5H, Ph), 8.26 (m, 1H, Ph), 7.57 (d, 1H, 3JH−H = 8.6 Hz, Ph), 6.24 (br s, 2H, p-pyridine), 5.37 (br s, 4H, m-pyridine), 4.17 (s, 3H, OCH3), 2.24 (br s, overlapped with a H2O solvent signal, 4H, o-pyridine). MS (MALDI/ TOF). Calcd for C38H22CoN7O7 ([M − 2py]•+): m/z 747.09. Found: m/z 747.03. HRMS (ESI). Calcd for C38H22CoN7O7 ([M − 2py]+): m/z 747.0907. Found: m/z 747.0924.

coordinate) in the utilized electrochemical solvents. As will be discussed below, this is illustrated first by comparing the UV− visible spectra of Mes3CorCo and 1−4 in binding and nonbinding solvents, followed by a comparison of the UV− visible spectra of each corrole at different concentrations and third by titration of Mes3CorCo and 1−4 in CH2Cl2 with pyridine. The UV−visible spectra of Mes3CorCo and 1−4 in CH2Cl2 and pyridine are given in Figure 1, while spectra for the same series of corroles in PhCN are given in Figure S6. The spectra in CH2Cl2 are presented at high and low concentrations of the corrole (10−3 and 10−5 M), while the spectra in PhCN and pyridine are given only at a low concentration of 10−5 M. The higher concentration (10−3 M) corresponds to conditions of the electrochemical and electron paramagnetic resonance (EPR) measurements. Similar spectral envelopes are seen for each of the five corroles in CH2Cl2 or PhCN at a concentration of 10−5 M, but there are slight differences in the wavelengths of the absorption bands between the two solvents. For example, Mes3CorCo at 10−5 M in CH2Cl2 is characterized by bands at 388 and 543 nm (Figure 1) but by bands at 386 and 554 nm in PhCN (Figure S6). Likewise, compound 3 is characterized in CH2Cl2 by bands

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RESULTS AND DISCUSSION UV−Visible Spectra of 1−4. In order to correctly analyze the prevailing redox reactions of cobalt (nitrophenyl)corroles 1−4 under different solution conditions, it was first necessary to evaluate the exact form of the corrole (i.e., four-, five-, or sixD

DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry at 380, 432, 565, and 643 nm (Figure 1), but these bands are at 382, 429, 570, and 644 nm in PhCN (Figure S6). However, more significant changes in the number and positions of the absorption bands occur for 1−4 when the concentration of the corrole in CH2Cl2 is increased from 10−5 M (Figure 1a) to 10−3 M (Figure 1b). New absorption bands are seen in the 10−3 M solutions, which can be associated with the bis(pyridine) adduct, a form of the corrole that is predominant in neat pyridine. For example, compound 1 at a concentration of 10−3 M in CH2Cl2 has new bands at 433, 451, and 619 nm (Figure 1b) that are not seen at a concentration of 10−5 M in CH2Cl2 (Figure 1a) but are observed in pyridine at a corrole concentration of 10−5 M. Moreover, these bands are more intense in this solvent (Figure 1c). This suggests an equilibrium between different axially coordinated forms of the corrole at a concentration of 10−3 M in CH2Cl2 The bands of compound 1 at 433, 451, and 619 nm are assigned to the six-coordinate bis(pyridine) derivative, while those at 380, 424, and 550 nm are assigned to the fivecoordinate mono(pyridine) adduct. Bands associated with the bis(pyridine) adduct are also seen for compounds 2−4 in CH2Cl2 at a concentration of 10−3 M. The UV−visible spectra of the five- and six-coordinate species of 1−4 are also similar to the spectra of the mono- and bis(pyridine) adducts of other (Ar)3CorCo derivatives reported in the literature,63 where the bis(pyridine) adduct has split Soret bands that are separated by about 15 nm and have equal intensity absorptions. As will be shown in the following pages, the dissociation of one pyridine ligand from the cobalt center in 1−4 occurs upon dissolution in CH2Cl2. This reaction is given by eq 1, where (Ar)3Cor represents the corrole macrocycle of 1−4. (Ar)3 CorCo(Py)2 ⇌ (Ar)3 CorCo(Py) + Py

Figure 2. UV−visible spectral changes in CH2Cl2 upon an increase in the concentration of the corrole for Mes3CorCo, 1, and 3.

(1)

showed a concentration dependence of the spectra that was interpreted in terms of the association and dissociation of pyridine.64 The equilibrium given by eq 1 is further examined by monitoring the spectral changes during titration of 1−4 with pyridine in CH2Cl2. A similar titration was also carried out for the structurally related four-coordinate Mes3CorCo reference compound. The titration of Mes3CorCo with pyridine involves a twostep process, as shown in Figure 3. The first step is assigned as a conversion of Mes3CorCo to Mes3CorCo(Py), as confirmed by the diagnostic log−log plot in the figure inset. The second step results in the formation of Mes3CorCo(Py)2 at a higher pyridine concentration, with the corresponding binding constants being calculated as log K1 = 6.5 and log K2 = 2.6. The spectrum of the mono(pyridine) adduct Mes3CorCo(Py) is spectroscopically similar to the initial four-coordinate corrole in CH2Cl2 but possesses a slightly blue-shifted and decreased intensity Soret band, along with a weak Q band, which has shifted from 543 nm for Mes3CorCo to 551 nm for Mes3CorCo(Py) (see Figure 3a). The mono(pyridine) derivative also possesses a shoulder at 416 nm that is not present in the initial four-coordinate corrole and is thus assigned to the five-coordinate species. As will be discussed later in the manuscript, a shoulder is also observed at 424−432 nm for compounds 1, 3, and 4 at a concentration of 10−5 M in CH2Cl2 or PhCN (Figures 1 and S6), which is associated with the five-coordinate mono(pyridine) adduct of each corrole. The UV−visible spectra of Mes3CorCo and Mes3CorCo(Py) in CH2Cl2 (Figure 3a) differ significantly from that of the

A dissociation of pyridine does not occur for 1−4 in pure pyridine where the bis(pyridine) adduct is present at all concentrations of the corrole. Moreover, the equilibrium in eq 1 is shifted to the left at high concentrations of the corrole in CH2Cl2, and the spectra of 1−4 under these conditions are then characterized by bands associated with a mixture of (Ar)3CorCo(Py) and (Ar)3CorCo(Py)2 in solution. As expected, a concentration-dependent equilibrium does not occur for Mes3CorCo, which lacks an axial pyridine ligand in its synthesized form, and the UV−visible spectra in CH2Cl2 are virtually identical at all concentrations between 10−3 and 10−5 M. Additional evidence for the concentration dependence of the UV−visible spectra of the (nitrophenyl)corroles is shown in Figure 2 for compounds 1 and 3, with a comparison made with Mes3CorCo, where the cell path length (b) times the concentration (C) was kept constant for corrole concentrations of 10−5 M (b = 1.0 cm), 10−4 M (b = 0.1 cm), and 10−3 M (b = 0.01 cm). A clear transition between the two different ligated forms of the corrole is observed in CH2Cl2, with a larger amount of the bis(pyridine) adduct being obtained at a concentration of 10−3 M of the corrole for compound 3 compared to compound 1. Evidence for formation of the bis(pyridine) adduct is given by the decrease in the intensity of the original Soret band at 380 nm with increasing concentration and the concomitant gain in the intensity of the Q band at 618 or 650 nm. This latter absorption band has also been wellcharacterized in the literature18,22 and is diagnostic of the bis(pyridine) adduct of (Ar)3CorCo(Py)2. A similar study on related cobalt corroles with coordinated pyridine ligands also E

DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. UV−visible spectral changes during the (a) first and (b) second pyridine ligand binding to Mes3CorCo at a concentration of 10−5 M in CH2Cl2. The inset shows the Hill plot analysis of the data for determining the binding constants.

Table 1. UV−Visible Spectral Data of Mes3CorCo and 1−4 at 10−5 M in CH2Cl2, PhCN, and Pyridinea λmax, nm (ε × 10−4, M−1 cm−1) solvent

compd

CH2Cl2

Mes3CorCo 1 2 3 4 Mes3CorCo 1 2 3 4 Mes3CorCo 1 2 3 4

PhCN

pyridine

a

Soret region 388 380 385 380 380 386 387 389 382 384

(6.1) (6.4) (7.2) (5.9) (6.6) (5.3) (5.5) (6.1) (5.9) (6.2)

402 403 414 414

(2.1) (2.5) (5.1) (5.1)

visible region

424 (2.2) 432 (3.4) 432 (3.7) 429 (1.9) 429 429 437 439 439 439 435

(3.4) (3.7) (5.5) (5.2) (5.9) (5.6) (5.4)

455 455 455 454 453

(5.5) (5.4) (6.3) (5.8) (5.5)

543 550 551 565 567 554 552 553 570 573 543 542 542

(0.7) (0.7) (0.8) (1.0) (1.1) (0.7) (0.6) (0.6) (1.1) (1.1) (0.7) (0.6) (1.0)

585 (1.1) 584 (1.0) 584 (1.2)

616 614 643 645

(0.3) (0.3) (0.4) (0.4)

617 620 644 650 623 620 621 654 656

(0.2) (0.3) (0.5) (0.4) (3.2) (2.3) (3.3) (3.9) (4.1)

The listed bands are associated with the spectroscopically pure corroles at 10−5 M in the three solvents.

Figure 4. UV−visible spectral changes during the second pyridine ligand binding to 1−4 at a concentration of 10−5 M in CH2Cl2. The inset shows the Hill plot analysis of the data for determining the binding constants.

bis(pyridine) adduct Mes3CorCo(Py)2, formed in the second ligand addition of pyridine titration (Figure 3b and Table 1).

The latter spectrum is characterized by split Soret bands at 434 and 453 nm, in addition to an intense Q band at 621 nm and a F

DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry weaker Q band at 585 nm. Virtually the same UV−visible spectrum is obtained for Mes3CorCo at 10−5 M in pyridine (Figure 1c) or 10−3 M in pyridine containing 0.1 M TBAP (spectrum not shown). These results, when combined with the similar spectra of Mes3CorCo(Py) and 1−4 in CH2Cl2 or PhCN at a concentration of 10−5 M (Figures 1a and S6), strongly suggest the dissociation of one pyridine ligand from the cobalt center of 1−4 when they are dissolved in these two solvents (see eq 1). The spectral changes that occur during the titration of 1−4 with pyridine in CH2Cl2 are shown in Figure 4. Unlike the reference compound Mes3CorCo, only a single pyridine molecule is added to 1−4 in CH2Cl2. The inserted Hill plot gives a slope of 1.0, indicating the addition of one pyridine ligand to each of the corroles, and the final spectrum in each case is characterized by an intense Q band at 619−656 nm, suggesting the formation of a bis(pyridine) adduct. Several isosbestic points are obtained in each titration, and the prevailing ligand binding reaction can be characterized as a conversion of (Ar)3CorCo(Py) to (Ar)3CorCo(Py)2 (the reverse of eq 1). Thus, the data in Figure 4 are consistent with the spectroscopic data in Figures 1a and 2, which show that the initial synthesized six-coordinate bis(pyridine)cobalt (nitrophenyl)corroles are converted to their five-coordinate forms when dissolved in CH2Cl2 or PhCN at a concentration of 10−5 M. The measured formation constants for the addition of a second pyridine molecule to the five-coordinate mono(pyridine) derivatives of 1−4 range from log K2 = 3.0 to 3.9 in CH2Cl2. These log K2 values are summarized in Table 2, along with the log K1 and log K2 values for Mes3CorCo under the same solution conditions.

Figure 5. ORTEP view of 1. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and the disordered solvent are omitted for clarity.

nitrogen atoms from the corrole and two nitrogen atoms from the two axial pyridine ligands. The cobalt ion is located at the center of a perfect octahedron with a typical Oh symmetry, as shown by the N−Co−N angles, which are close to 90°. The two pyridine molecules are almost perpendicular with a dihedral angle of 98.325(2)°, and their positions are imposed by the crystal packing and the existence of π−π interactions between pyridine in two adjacent asymmetric units. Details on the structure are given in the Supporting Information. Electrochemistry of Compounds 1−4. Cyclic voltammograms for 1−4 are illustrated in Figures 6 (CH2Cl2), 7 (pyridine), and S7 (PhCN), and a summary of the measured redox potentials under different solution conditions is given in Table 3. Each investigated corrole contains multiple redox-active sites on the molecule, and this is reflected by the large number of oxidation or reduction processes shown by the cyclic voltammograms in the three different solvents. Two or three one-electron oxidations are seen for 1−4, each of which can involve the conjugated macrocycle or the metal center. Compounds 1−4 also undergo three to four reductions. These processes can also involve the conjugated macrocycle or the metal center as well as the redox-active meso-nitrophenyl substituents. Earlier electrochemical studies of cobalt corroles have often assigned the oxidation states and sites of electron transfer based on what was known for the related cobalt porphyrins, namely, cobalt(III)/cobalt(II), cobalt(II)/cobalt(I), or cobalt(III)/ cobalt(IV) in the case of the metal-centered reactions and πcation or π-anion radicals when the electrode reaction involves the conjugated macrocycle.16 However, the site of electron transfer is not so clear in the case of transition-metal corroles, which have been characterized as possessing a noninnocent macrocycle, the most-often-cited examples of which have been given for corrole derivatives containing copper or iron ions.6,7,30,35,37,65−72 Five-coordinate cobalt triarylcorroles were also proposed as containing a noninnocent macrocycle.7,63,73 Thus, the possible noninnocent character of the corrole macrocycle should be considered during analysis of the redox reactions of the currently investigated compounds. The presence of the highly electron-withdrawing and redox-active meso-nitrophenyl groups on the macrocycles of 1−4 as well as the utilized solvents that may strongly bind to the central cobalt ion should also be taken into account when elucidating both the initial oxidation state

Table 2. Pyridine Binding Constants of Cobalt Corroles Mes3CorCo and 1−4 in CH2Cl2 binding constant compd

log K1

log K2

Mes3CorCo 1 2 3 4

6.5

2.6 3.3 3.0 3.9 3.8

Of the five corroles in Table 2, compounds 3 and 4 have the largest log K2 values and Mes3CorCo has the smallest, while the log K2 values for compounds 1 and 2 are in between. This is consistent with the presence of the electron-withdrawing nitrophenyl groups on the molecule, which leads to an enhanced positive charge on the central metal ion and thus an increased pyridine binding ability. Finally, it should be pointed out that two different envelopes of UV−visible spectra are seen for the bis(pyridine) adducts of compounds 1−4 in Figures 1c and 3. One type is for corroles 1 and 2, which possess one meso-nitrophenyl group on the macrocycle, and is characterized by a well-defined split Soret band and a sharp Q band. The other is for the bis(pyridine) complexes 3 and 4, which possess three meso-nitrophenyl groups and have spectra with substantially broadened Soret and Q bands. Single-Crystal X-ray Diffraction of 1. Single crystals of compound 1 were obtained from a mixture of cyclohexane/ THF solution and gave the structure shown in Figure 5. The cobalt center of the resulting corrole is six-coordinated by four G

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Figure 6. Cyclic voltammograms of 1−4 in CH2Cl2 containing 0.1 M TBAP. The anodic peaks indicated by asterisks are reactions of products formed from the irreversible reduction of nitrophenyl groups located at Ep = −1.73 to −1.82 V.

Figure 7. Cyclic voltammograms of 1−4 in pyridine containing 0.1 M TBAP. The anodic peaks indicated by asterisks are reactions of products formed from the irreversible reduction of nitrophenyl groups located at Ep = −1.81 to −1.86 V.

and site of electron transfer for reduction and oxidation of the investigated cobalt (nitrophenyl)corroles. To address the above points, our description of the redox reactions for 1−4 is divided into three sections. The first is that of the redox-active nitrophenyl groups, followed by the electrochemical behavior of Mes3CorCo under different solution conditions and then the metal- or ring-centered redox reactions of 1−4. The reductions of 1−4 at potentials more negative than −1.5 V are irreversible and involve coupled chemical reactions of either the electroreduced NO2Ph groups, the corrole macrocycle, or a combination of both. These processes are shown by dashed lines in Figures 6, 7, and S7 and were not investigated in further detail. One-Electron Reduction of the meso-Nitrophenyl Group(s). Nitrobenzene is reversibly reduced by one electron at E1/2 = −1.08 V in CH2Cl240 or PhCN (this work) and −1.12 V in pyridine (this work). This reduction produces a relatively stable anion radical that can be further reduced at Ep = −2.00 V via an irreversible process in all solvents. A reversible oneelectron reduction was also earlier reported for nitrophenyl groups on the meso positions of macrocycles with a tetraphenylporphyrin or triarylcorrole skeleton containing iron, cobalt, manganese, copper, palladium, or zinc central metal ions.24,40,43,51,74,75 The reported redox potentials for the first one-electron reduction of the NO2Ph groups varied between −1.05 and −1.12 V, independent of either the central metal ion, the number of meso-nitrophenyl groups, or the presence of other electron-donating or electron-withdrawing substituents on the macrocycle.24,33,40,43,44,51,74−97 The meso-

nitrophenyl groups of 1−4 also exhibit similar electrochemical reduction behavior; i.e., they undergo an initial reversible oneelectron reduction process located at E1/2 = −1.10 to −1.14 V or −1.25 to −1.31 V, depending on another substituent on the nitrophenyl ring, followed by a second irreversible reduction step at Ep = −1.73 to −1.86 V, as shown in Figures 6, 7, and S7 and Table 3. The invariant values of E1/2 for the reversible one-electron reduction of meso-nitrophenyl groups on earlier studied metallomacrocycles in CH2Cl2, PhCN, or pyridine suggest the lack of an interaction between these redox-active units on the meso positions of the different compounds, be it a porphyrin, a corrole, or another related conjugated system.24,33,40,43,44,51,74−97 An electronic interaction between the three nitrophenyl groups across the macrocycle is also not electrochemically observed in the currently studied compounds 3 and 4, which both contain three nitrophenyl groups. As shown in Table 3 and Figure 6, the meso-(4-F-3-NO2)Ph group on compound 1 is reversibly reduced by one electron at −1.10 to −1.14 V in the three solvents, and essentially the same value of E1/2 is seen in the cyclic voltammogram of compound 3, which contains a single (4-F-3-NO2)Ph group at the 10 position of the macrocycle and two p-NO2Ph groups at the 5 and 15 positions (see Chart 1). The two different types of nitrophenyl groups are reduced at virtually the same half-wave potential; this is because the sums of the substituent constants for all of the substituents on these two types of meso-phenyl rings are almost the same, i.e., 0.772 for a p-F substituent (σ = H

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Table 3. Half-Wave or Peak Potentials (E1/2 or Ep, V vs SCE) for Reductions and the First Two Oxidations of Cobalt Corroles in CH2Cl2, PhCN, or Pyridine Containing 0.1 M TBAP reduction first

oxidation (E1/2) solvent

compd

second

first

Epc

Epa

CH2Cl2

Mes3CorCo + 2 equiv of Py 1 2 3 4 1 2 3 4 Mes3CorCo 1 2 3 4

0.98 1.02 1.00 1.07 1.05 1.01 0.98 1.05 1.02

0.15 0.27 0.27 0.43 0.41 0.35 0.33 0.47 0.44 0.26 0.34 0.31 0.49 0.46

−0.70 −0.48 −0.48 −0.28 −0.27 −0.41 −0.45 −0.26 −0.25 −1.16 −1.20 −1.18 −0.94 −0.98

−0.24 −0.18 −0.20 −0.14 −0.13 −0.15 −0.17 −0.04 −0.05 −0.70 −0.42 −0.56 −0.42 −0.42

PhCN

pyridine

a

0.90 0.88 0.91 0.87

NO2Ph group(s) (#e) −1.10 −1.25 −1.12 −1.12 −1.12 −1.30 −1.13 −1.14

(1) (1) (3) (2), −1.29 (1) (1) (1) (3) (2), −1.30 (1)

−1.14 −1.31 −1.12 −1.14

(1) (1) (3) (2), −1.31 (1)

further red. −1.78a −1.76a −1.73a −1.77a −1.82a −1.73a, −1.78 −1.75a, −1.84a −1.80a −1.79a −1.80 −1.81a −1.82a −1.82a −1.86a

Irreversible peak potential at a scan rate of 0.1 V s−1.

0.062) and a m-NO2 substituent (σ = 0.71) on one and 0.778 for a single p-NO2 substituent (σ = 0.778)98 on another. Each phenyl ring of these two types of meso-nitrophenyl groups experiences an overall almost identical substituent effect, and therefore the same reduction potential of these two groups is observed. However, this is not the case for compound 4, where separate reduction potentials are observed for the two different nitrophenyl groups on the molecule. The easier reduction (more positive potential) is for the two meso-p-nitrophenyl groups, while the harder reduction (more negative potential) is for the meso-(4-OMe-3-NO2)Ph group. This is because the electron-donating −OMe substituent shifts the reduction of this nitrophenyl group in a negative direction. The reversible half-wave potentials for the easiest-to-reduce nitrophenyl groups in compounds 1, 3, and 4 range from −1.10 to −1.14 V in the three solvents, while E1/2 values for the hardest-toreduce rings ranges from −1.25 to −1.31 V for compounds 2 and 4 (Table 3). Thus, a second electron-donating or electronwithdrawing substituent on the nitrophenyl ring can have a direct electronic effect on the reduction of this redox-active group. Although the half-wave potentials for reduction of the nitrophenyl groups on different metallomacrocycles do not depend upon the specific macrocycle or the specific central metal ion, these substituents do exert a moderate-to-strong electron-withdrawing effect on other redox reactions of the compounds, shifting their potentials in a positive direction by 45−90 mV per NO2Ph group in the case of porphyrins74 and by 35−100 mV in the case of corroles.24,40,75 The first oxidation of the currently investigated compounds 1−4 exhibits a 76 mV potential shift per NO2Ph group in pyridine and a 80 mV shift per NO2Ph group in CH2Cl2, as shown in Figure 8. Electrochemistry of Mes3CorCo. In order to better understand the electrochemical behavior of the currently investigated bis(pyridine)cobalt (nitrophenyl)corroles in different solution conditions and the effect of ligand binding on the redox reactions, the electrochemical reactions of the fourcoordinate Mes3CorCo were monitored in CH2Cl2 (Figure 9a), CH2Cl2 containing 2 equiv of pyridine (Figure 9b), and neat pyridine (Figure 9c). As shown in Figure 9a, the four-

Figure 8. Plots of the half-wave potentials for the first oxidation versus number of meso-NO2Ph groups for Mes3CorCo, 1, and 3 in pyridine and CH2Cl2. The measured E1/2 values are given in Table 1.

coordinate corrole in CH2Cl2 undergoes an initial reversible one-electron reduction located at E1/2 = −0.19 V, consistent with an earlier reported potential of E1/2 = −0.18 V in CH2Cl2.21 Surprisingly, an identical E1/2 value of −0.19 V was also observed for the reversible one-electron reduction of (TPC)Cu under the same solution conditions.67 This might be perceived as a coincidence were it not for the fact that similar reduction potentials have also been reported for the oneelectron reduction of cobalt and copper corroles with other types of macrocycles. Best examples are given for the octaethylcorroles (OEC)Cu (E1/2 = −0.34 V) and (OEC)Co (E1/2 = −0.30 V) in PhCN,53 the pentafluorophenylcorroles (TPFPC)Cu70 (E1/2 = 0.21 V) and (TPFPC)Co21 (E1/2 = 0.24 V) in CH2Cl2, and the triferrocenylcorroles (TFcC)Cu (E1/2 = −0.33 V) and (TFcC)Co (E1/2 = −0.30 V) in CH2Cl2.99 Copper invariably engenders a noninnocent MII-corrole•2− electronic structure, a fact that has been described by a number of laboratories.7,35,53,65,66,68−71,91,100−110 This electronic configuration of a metal(II) ion and a corrole cation radical is also assigned to the currently investigated four-coordinate Mes3CorCo in neat CH2Cl2. Studies on (TPFPC)Co and other four-coordinate cobalt corroles indicate that these derivatives are paramagnetic.111 Moreover, theoretical calculations of the electronic structure carried out specifically for I

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Figure 9. Cyclic voltammograms of Mes3CorCo in (a) CH2Cl2, (b) CH2Cl2 + 2.0 equiv of pyridine, and (c) pyridine containing 0.1 M TBAP.

abstraction from the cobalt center, leading to formation of the cobalt(III) π-cation radical (see eq 3), as supported by EPR and theoretical analysis of the singly oxidized species of a structurally similar compound, (F5Ph)Mes2CorCo, which is unambiguously assigned as forming a [(F5Ph)Mes2Cor•CoIII]+ cation radical after the one-electron abstraction.21 The UV− visible spectrum of electrogenerated [Mes3Cor•Co]+ is characterized in CH2Cl2 by a decrease in the intensity of Soret and Q bands at 383 and 545 nm along with a broad nearIR band at around 670 nm (see a later section of the manuscript). This π-cation-radical-type spectrum of singly oxidized Mes3CorCo in CH2Cl2 also resembles UV−visible spectra obtained for [(F 5 Ph)Mes 2 Cor • Co I I I ] + and [Me4Ph5Cor•CoIII]+, which were both assigned as cobalt(III) corrole cation radicals.15,21

(TPC)Co and (TPC)Co(Py) strongly suggest a cobalt(II) radical ground state.7 A noninnocent corrole ligand was also assigned for related five-coordinate (Ar)3CorCo(Py) derivatives in a recent study by Ghosh and co-workers.63 Thus, in the current study, the five-coordinate mono(pyridine) adduct of Mes3CorCo(Py) formed in a mixture of CH2Cl2/Py and (Ar)3CorCo(Py) of 1−4 when they were dissolved in a CH2Cl2 solution is assigned as containing a CoIICor• electronic structure. This assignment is also rationalized by our EPR data (see the Supporting Information) and gives clear evidence for paramagnetic character of the examined compounds in nonbonding solvents where the fivecoordinate mono(pyridine) adduct is largely present (see the spectroscopic data in Figure 4 and eq 2). However, the bis(pyridine) adduct of compounds 1−4 in neat pyridine is assigned as a cobalt(III) corrole, as suggested by Ghosh based on EPR, NMR, etc.7 This assignment is also supported by the single-crystal data in the current study for compound 1, which exhibits a short equatorial Co−N distance of ∼1.89 Å, indicating an optimum fit with the corrole ligand. It also exhibits Co−Npy distances of ∼1.99 Å. These geometry parameters are all indicative of a classic low-spin cobalt(III) description for these complexes. Although the one-electron reduction of most neutral cobalt corroles has almost always been proposed as involving a cobalt(III) to cobalt(II) process, the fact that identical reduction potentials are observed in CH2Cl2 for a number of structurally related copper and cobalt triarylcorroles is totally inconsistent with a metal(III)/metal(II) transition. Moreover, the reported half-wave potentials for the copper(II)/copper(III) and cobalt(II)/cobalt(III) processes of several structurally related porphyrins have been shown to differ by up to 1.5 V in potential.112 However, this does not occur for the cobalt and copper triarylcorroles, thus strongly suggesting the same site of electron transfer for these two compounds. This site is proposed as the noninnocent corrole macrocyclic ligand. A cobalt(II) central metal ion is therefore present in the neutral species, and the redox reaction would occur as shown in eq 2. (Ar)3 Cor•CoII + e ⇌ [(Ar)3 CorCoII]−

(Ar)3 Cor•CoII ⇌ [(Ar)3 Cor•CoIII]+ + e

(3)

The second and third oxidations of Mes3CorCo at 0.89 and 1.48 V in CH2Cl2 are both proposed to involve one-electron abstraction from the corrole macrocycle, based on spectroelectrochemical data obtained during these two oxidation processes (see Figure S8). In the solution of CH2Cl2 containing 2 equiv of pyridine (Figure 9b), the redox-active species is assigned as Mes3Cor•CoII(Py), as described earlier in this paper. In contrast with the four-coordinate complex that is reversibly reduced by one electron at −0.19 V in CH2Cl2, the fivecoordinate Mes3Cor•CoII(Py) undergoes an irreversible reduction at Epc = −0.70 V, owing to dissociation of the Py ligand. This irreversible reduction is coupled with an oxidation process at Epa = −0.24 V, which corresponds to oxidation of the oneelectron-reduced species [Mes3CorCoII]−. The reduction behavior of the mono(pyridine) adduct of the neutral corrole can be explained via a classical electrochemical EC mechanism (Electron transfer followed by a Chemical reaction), as shown in the top right of Scheme 2. This chemical reaction is a dissociation of one pyridine ligand from [Mes3CorCo(Py)]− in the CH2Cl2/Py mixture and occurs on the forward negative potential scan. A reassociation of one pyridine ligand then occurs on the reverse positive scan, giving back the original corrole via another EC mechanism upon oxidation. The reoxidation peak potential of −0.24 V corresponds to the conversion of four-coordinate [Mes3CorCo]− to [Mes3CorCo],

(2)

Mes3CorCo also undergoes three reversible one-electron oxidations at E1/2 = 0.54, 0.89, and 1.48 V in CH2Cl2 containing 0.1 M TBAP. The first oxidation is assigned as a one-electron J

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manuscript, the redox-active species under these solution conditions, Mes 3 Cor • Co I I (Py), is first oxidized to [Mes3Cor•CoIII(Py)]+ and then coordinates one free pyridine ligand in the solution, forming a bis(pyridine)cobalt(III) cation radical, [Mes3Cor•CoIII(Py)2]+, as shown in the middle left of Scheme 2. The redox-active form of this corrole in neat pyridine is assigned as Mes3CorCoIII(Py)2 based on analysis of the pyridine titration data (Figure 3) and an earlier assignment of the cobalt oxidation state. As shown in Figure 9c, the first irreversible reduction of Mes3CorCo(Py)2 occurs at −1.16 V, a value negatively shifted by 460 mV compared to reduction of the five-coordinate Mes3CorCoIII(Py). Reoxidation of the reduction product is located at −0.70 V, a value consistent with the reduction potential of the five-coordinate mono(pyridine)cobalt corrole in Figure 9b. Thus, reduction of the six-coordinate bis(pyridine)cobalt corrole is explained by another box mechanism shown at the bottom of Scheme 2. The starting (Ar)3CorCoIII(Py)2 undergoes an initial oneelectron reduction, followed by dissociation of one pyridine ligand to form [(Ar)3CorCoII(Py)]−. This five-coordinate species is first reoxidized to (Ar)3CorCoIII(Py) and then reassociated with a Py ligand, yielding six-coordinate (Ar)3CorCoIII(Py)2.

Scheme 2. Reaction Mechanism of Mes3CorCo in (a) CH2Cl2, (b) CH2Cl2 + 2.0 equiv of Pyridine, and (c) Pyridine Containing 0.1 M TBAPa

a

The listed potentials are given for Mes3CorCo and were taken from Figure 9.

as confirmed by the reversible reduction potential of −0.19 V for Mes3CorCo in CH2Cl2 shown in Figure 9a. By way of comparison, the five-coordinate Ph3CorCo(PPh3) has been shown to undergo an irreversible reduction and reoxidation at Epc = −0.73 V and Epa = −0.10 V under the same solution conditions,24 and this was explained by an “electrochemical EC box mechanism” similar to that described above. As seen in Figure 9b, only two oxidations (E1/2 = 0.15 and 0.98 V) can be observed for Mes3CorCo in CH2Cl2 containing 2 equiv of pyridine. As will be further described later in the

Figure 10. Spectral changes during the first reduction and first oxidation of compound 1 in CH2Cl2, PhCN, and pyridine containing 0.1 M TBAP. K

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Table 4. UV−Visible Spectral Data of Neutral, Electrooxidized, and Electroreduced Forms of Mes3CorCo, 1, and 3 in Different Solvents Containing 0.1 M TBAP λmax, nm (ε × 10−4, M−1 cm−1) compound

solvent

Mes3CorCo

CH2Cl2 PhCN CH2Cl2 PhCN Py

1

3

CH2Cl2 PhCN Py

neutral 388 386 383 387 400 542 383 382 414

(6.1), (5.3), (6.4), (5.5), (2.1), (0.6), (6.4), (5.9), (5.1),

543 560 435 429 440 578 424 438 452

(0.7) (0.2) (2.6), (1.9), (5.2), (0.9), (5.7), (4.0), (5.5),

546 546 454 620 568 570 654

singly reduced

(0.9), (0.6), (5.4), (2.2) (0.1), (1.4), (4.0)

616 (0.7) 618 (0.3)

652 (0.2) 650 (1.1)

420 425 420 426 423

572 578 562 576 579

(1.1), (0.8), (0.9), (0.9), (1.1),

412 (6.8), 634 (2.6) 414 (5.2), 605 (2.1) 414 (6.9), 654 (2.8)

The first one-electron oxidation of Mes3CorCoIII(Py)2 is also reversible and located at E1/2 = 0.26 V in pyridine containing 0.1 M TBAP. This singly oxidized corrole is assigned as the bis(pyridine) adduct [Mes3Cor•CoIII(Py)2]+. This is because the one-electron-oxidized species of [TPFPC]CoIII(Py)2, structurally similar to Mes3CorCoIII(Py)2, is unambiguously assigned as [(TPFP)Cor•CoIII(Py)2]+ with corrole cationradical character, as supported by the electron-spin resonance (ESR) data.28 A second oxidation, which is not shown in the figure, is quasi-reversible and located close to the positive potential limit of the solvent. Metal- and Ring-Centered Reactions of 1−4. The redox behavior of compounds 1−4 in CH2Cl2 resembles in part that of Mes3CorCo in a CH2Cl2/pyridine mixture because the redox-active species for each of these corroles is the mono(pyridine) adduct, based on the UV−visible data described earlier in the manuscript. However, large differences do exist, as can be seen in the cyclic voltammograms in Figures 6 and 9b, where the potential separation between Epa and Epc for the first reduction varies with the number of mesonitrophenyl groups on the corrole, with the separation being 460 mV for Mes3CorCo(Py), 280−300 mV for compounds 1 and 2, and 140 mV for compounds 3 and 4. The cyclic voltammetric data for 1 and 2 suggest that these two [( Ar) 3 C o r • C o I I ( P y ) ] d e r i v a t i v e s a r e r e d u c e d t o [(Ar)3CorCoII(Py)]− and then lose one pyridine axial ligand to give the four-coordinate complex on the forward sweep [as shown in Scheme 2b for Mes3CorCo(Py)]. The reverse potential sweep then involves reoxidation of [(Ar)3CorCoII]− to (Ar)3Cor•CoII, followed by reassociation of one pyridine ligand to yield the starting compound (again, as shown in Scheme 2b). This seems to be not the case for 3 and 4, which contain three meso-nitrophenyl substituents. As seen in Figure 6, the first one-electron addition of these two corroles is almost reversible, with a peak-to-peak potential separation of 140 mV, strongly suggesting that the one-electron reduction product, [(Ar)3CorCoII(Py)]−, does not lose its pyridine ligand in CH2Cl2 on the cyclic voltammetric time scale. Thus, the reactants and products in the first reductions of 3 and 4 would be (Ar)3Cor•CoII(Py) and [(Ar)3CorCoII(Py)]− on the time scale of the cyclic voltammetric measurement, as shown in eq 4. (Ar)3 Cor•CoII(Py) + e ⇌ [(Ar)3 CorCoII(Py)]−

(4.4), (5.0), (4.4), (5.3), (5.6),

620 622 622 621 620

singly oxidized (0.7) (0.4) (0.7) (0.6) (0.8)

383 409 409 410 412

(4.3), (4.8), (5.5), (4.8), (4.6),

662 419 431 433 434

(0.6) (4.8), (5.1), (4.5), (4.6),

704 657 643 638

(0.05) (0.7), 691 (0.7) (0.2), 695 (0.4) (0.4), 694 (0.4)

437 (8.6), 660 (0.7), 708 (0.6) 438 (7.2), 650 (0.8), 713 (0.9) 443 (7.0), 654 (2.6)

oxidizing potential. Examples of these spectra are shown in Figure 10 for compound 1 in CH2Cl2, PhCN, and pyridine containing 0.1 M TBAP. In all three solvents, the first oneelectron reduction generates an anionic cobalt(II) corrole that possesses a Soret band at 420 nm in CH2Cl2, 427 nm in PhCN, and 423 nm in pyridine. Singly reduced Mes3CorCo in CH2Cl2 also has a Soret band at 420 nm along with two Q bands at 572 and 620 nm, and similar Q-band absorptions are seen for compound 1 in CH2Cl2 (562 and 622 nm) or PhCN (578 and 620 nm). Furthermore, almost the same UV−visible spectrum is obtained for compound 1 in pyridine, where the singly reduced corrole has a Soret band at 423 nm and Q bands at 579 and 621 nm. The singly oxidized form of compound 1 has the same UV− visible spectra in all three solvents. As seen in Table 4, this singly oxidized corrole has a split Soret band at 409 and 431 nm in CH2Cl2, 410 and 433 nm in PhCN, and 412 and 434 nm in pyridine. All three spectra suggest the oxidation of compound 1 to [(Cor)CoIII(Py)2]+. As shown in Scheme 2, in the case of a CH2Cl2 solution, a second pyridine ligand is added to the cobalt center of the singly oxidized mono(pyridine) adduct [CorCoIII(Py)]+, while in pyridine, two pyridine ligands are coordinated before and after electrooxidation. The singly reduced form of compound 3 also exhibits similar spectra in these three solvents, which suggests that the same species is generated after the initial one-electron addition on the spectroelectrochemical time scale. This is also the case for the singly oxidized species. A summary of the wavelengths and molar absorptivities of neutral, electrooxidized, and electroreduced forms of the corrole derivatives having zero, one, and three meso-nitrophenyl groups is given in Table 4.

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CONCLUSION A new series of bis(pyridine)cobalt (nitrophenyl)corroles have been synthesized and characterized by spectroscopic and electrochemical methods in three nonaqueous solvents. One of the corroles (compound 1) was also structurally characterized. The bis(pyridine)cobalt (nitrophenyl)corroles are stable when dissolved in pyridine, but one axial ligand dissociates in CH2Cl2 or PhCN, giving the five-coordinate corrole, independent of the number of nitrophenyl groups on the macrocycle and the substitution pattern of each nitrophenyl group. This dissociation depends on the concentration and is only complete in a dilute solution of 10−5 M corrole. The electronic configuration of the neutral bis(pyridine)cobalt (nitrophenyl)corroles in solution, i.e., (Ar)3CorCoIII(Py)2 or (Ar)3Cor•CoII(Py), depends on the number of pyridines axially

(4)

Evidence for the above oxidation state and coordination number assignments in both Mes3CorCo and compounds 1−4 is given in part by UV−visible spectra for the singly oxidized and singly reduced corroles, which were generated in a thinlayer cell under the application of a controlled reducing or L

DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry coordinated to the cobalt center but not on the number of the NO2Ph substituents on the macrocycle. Compounds 1−4 can be reduced in three to five electrontransfer steps. The first reduction of the investigated compounds gives the singly reduced four-coordinate corrole [CorCoII]− for compounds 1 and 2, which possess one nitrophenyl group in CH2Cl2 and PhCN, and the fivecoordinate [CorCoII(Py)]− for 3 and 4, which have three nitrophenyl groups in each investigated solvent. For all four corroles, the first reduction at the macrocycle is followed by a reversible one-electron addition at the mesonitrophenyl substituents, with this process being located at E1/2 = −1.10 to −1.14 V or −1.25 to −1.31 V, depending on the type of second substituent on the nitrophenyl ring. Irreversible processes beyond −1.5 V are also observed for all four corroles but were not investigated in this study. Each derivative also undergoes several oxidation processes within the anodic potential limit of the solvent, and for each compound, the first electron abstraction is proposed to yield a π-cation radical and a cobalt(III) metal center with two axially bound pyridine ligands. The redox-active meso-nitrophenyl groups show no evidence of interaction with each other through the conjugated macrocycle, which is probably because the NO2Ph ring is perpendicular to the corrole macrocycle and thus not in direct conjugation with the corrole π-ring system. However, the electron-withdrawing NO2Ph groups affect the UV−visible spectra as well as the metal- and ring-centered redox reactions, shifting the potential by ∼80 mV in a positive direction per each NO2Ph group for the first oxidation in pyridine and CH2Cl2.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Robert A. Welch Foundation (Grant E-680 to K.M.K.). Support was provided by the CNRS (UMR UB-CNRS 6302), the “Université de Bourgogne Franche-Comté”, the FEDER-FSE Bourgogne 2014/2020 (European Regional Development Fund), and the “Conseil Régional de Bourgogne” through the PARI II CDEA project and the JCE program. We are thankful to Sandrine Pacquelet for technical assistance and to Dr. Eric Van Caemelbecke for helpful discussion. The ANR is thanked for financial support (CO3SENS).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02655. UV−visible spectra, cyclic voltammograms, MALDI/ TOF MS, 1H NMR, and ESR spectra, and selected bond distances and angles (PDF) Accession Codes

CCDC 1578032 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +33 (0)3 80 39 61 12. *E-mail: [email protected]. Phone: (+1) 713-743-2740. ORCID

Stéphane Brandès: 0000-0001-6923-1630 W. Ryan Osterloh: 0000-0001-9127-2519 Claude P. Gros: 0000-0002-6966-947X Karl M. Kadish: 0000-0003-4586-6732 Notes

The authors declare no competing financial interest. M

DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02655 Inorg. Chem. XXXX, XXX, XXX−XXX