Facile Generation of A2B Corrole Radical Using Fe(III) Salts and Its Spectroscopic Properties.

A carboxyphenyl-substituted corrole, 5,15-dimesityl-10-(4'-carboxyphenyl)corrole (1), has been synthesized and characterized by UV-vis, fluorescence, 1H NMR spectroscopy, and electrospray ionization (ESI)-mass spectrometry (MS) techniques. An air-stable corrole radical (1•) was obtained with the addition of the Fe(III) salt to 1 in dimethyl sulfoxide (DMSO) and characterized by UV-vis, fluorescence, electron paramagnetic resonance (EPR), ESI-MS techniques, and density functional theory studies. The neutral corrole radical (1•) exhibited a sharp EPR signal at g = 2.006 in DMSO. The reduced bipyrrolic (N-C-C-N) dihedral angle (χ) of 1 from 19.11 to 7.07° leads to the release of angle strain, which is the driving force for the generation of 1•. Notably, trans-dimesityl groups prevent the dimerization or aggregation of the corrole radical. Further, 1• was converted to 1 by excess addition of Fe(II) salts in DMSO at 298 K.

Introduction

In recent years, studies on tetrapyrrolic macrocycles have been given ardent interest to explore their structural, photophysical, and photochemical properties. Corrole is a structural counterpart of porphyrin, and it emanates from the corrin ring. Recently, corroles have emerged as an independent area of research[1−4] due to their useful applications in catalysis, sensors, molecular electronics, solar cells, nonlinear optics, and photodynamic therapy.[5,6] A number of free base corroles and their metal complexes have been synthesized and characterized over the last two decades after the invention of facile synthetic methods. Notably, β-functionalization such as halogenation, nitration, chlorosulfonation, and hydroformylation of the corrole periphery leads to the formation of novel and more sophisticated corroles.[7] Different strategies are available to prepare symmetrical and unsymmetrical corroles in good yields.[8]

Redox chemistry of electron-rich corroles is of great interest due to the formation of radical and cationic/anionic corrole species. Recently, Kadish et al. have generalized the redox mechanism of meso-substituted free base corroles using cyclic voltammetry and spectroelectrochemical studies.[9a]

For example, the oxidation of free base trimesitylcorrole (Cor)H3 yielded [(Cor)H3]•+, which is a strong acid and rapidly transforms into (•Cor)H2 and H+. Further, H+ reacts with another free base corrole and produces [(Cor)H4]+ ions. The electroreduction of (•Cor)H2 gives (Cor)H2–. The mixing of (Cor)H4+ and (Cor)H2– produces a neutral corrole, (Cor)H3, by an acid–base proton-transfer mechanism, as shown in the above scheme. Recently, the 3,17-dichloro-meso-trimesitylcorrole radical was reported by Bröring et al., which was further used to prepare the first-ever Zn(II) corrole radical.[9b] The mesityl groups prevent the aggregation or dimerization of radical species and stabilize the radical corrole in its monomeric form. On the other hand, Anand et al. have synthesized a stable and neutral 25π-pentathiophene radical species, which readily converts into aromatic and antiaromatic electronic circuits on adding appropriate chemical agents.[9c] Kobayashi et al. have reported an extremely air-stable 19π-electron azaporphyrin radical, which was prepared from the reduction of a cationic P(V) azaporphyrin.[9d] The central phosphorus(V) atom and the peripheral bulky tert-butyl groups stabilize the reduced state.

Herein, we report the generation of stable corrole radical species (1•) of 5,15-dimesityl-10-(4′-carboxyphenyl)corrole (1) as well as 5,15-dimesityl-10-(4′-methoxycarbonylphenyl)corrole (MEC) by a chemical oxidation process in quantitative yield using FeIII salts in dimethyl sulfoxide (DMSO) at 298 K. The corrole radical was characterized by UV–vis, fluorescence, electrospray ionization (ESI)-mass spectrometry (MS), electron paramagnetic resonance (EPR) spectroscopic techniques and density functional theory (DFT) studies. Notably, 1 was regenerated by the excess addition of FeII salt to 1• in DMSO at 298 K.

Results and Discussion

Synthesis and Characterization

The synthesis of 1 was carried out in two steps; the first step involves the synthesis of MEC by acid (HCl)-catalyzed condensation of 5-mesityldipyrromethane and methyl 4-formylbenzoate in an H2O/MeOH (1:2, v/v) mixture,[8a] followed by alkaline hydrolysis of MEC in 95% ethanol to afford 1 (45% yield) in the second step. The synthesized free base corrole, 1, was characterized by UV–vis, fluorescence, and NMR spectroscopic techniques and MS (Scheme ).[8b]

Scheme 1

Synthetic Route for 5,15-Dimesityl-10-(4′-carboxyphenyl)corrole (1)

The free base corrole (1) exhibited a Soret band at 410 nm with a shoulder at 417 nm and three Q bands in the visible region located at 567, 602, and 638 nm in CH2Cl2 as shown in Figure S1.[8b]Figure S2 represents the 1H NMR spectra of MEC in CDCl3. Figures S3 and S4 illustrate the 1H NMR spectra of 1 in CDCl3 and DMSO-d6, respectively. The ESI mass spectrum of 1 in CH3CN is shown in Figure S5. To probe the influence of the carboxyphenyl group at the meso-position of the corrole ring, we carried out the cyclic voltammetric studies of 1 using a saturated calomel electrode as the reference electrode in CH2Cl2 at 298 K. 1 exhibited one-electron reversible oxidation and reduction as illustrated in Figure S6. The first oxidation potential of 1 was observed at 0.73 V, while the reduction potential was observed at −0.78 V at 298 K.

Spectroscopic Studies

We used electronic absorption spectral studies to demonstrate the formation of 1•, which is produced from the A2B corrole (1) with the addition of 1 equiv of Fe(III) salt in DMSO at 298 K, as shown in Figure . Further, we have tested with MEC and 5,10,15-tritolylcorrole (TTC) by the addition of 1 equiv of Fe(III) salt in DMSO at 298 K. The spectral changes of MEC are comparable to those of 1 while adding Fe(III) salt with a prominent peak at 390 nm (Figure S8a), indicating the formation of a radical corrole, which is further confirmed by electron spin resonance studies (vide infra). However, TTC exhibited only marginal spectral changes, with a shoulder at 390 nm (Figure S8b), indicating the partial formation of a radical corrole. Herein, we have a brief discussion about the interaction of corrole 1 with FeIII ions in DMSO at 298 K. The observed UV–vis spectral pattern of 1 matches with that of the reported corrole radical by Kadish et al.[9a] and Bröring et al.,[9b] whereas no changes were observed with other metal ions (Figure S7). The appropriate redox reagents such as a one-electron oxidizing agent [Et3O+SbCl6–] and a one-electron reducing agent (KO2) were used to confirm the radical nature of 1. The addition of [Et3O+SbCl6–] to 1 in DMSO leads to the similar color and UV–vis spectral changes as FeIII (Figure S9a). The anionic species of corrole 1 was obtained with the successive addition of FeIII and KO2 (one-electron reducing agent) in DMSO, as shown in Figure S9b. We have also tested the protonation version of 1 with TFA in DMSO, as shown in Figure S10. In general, protonated corroles, [CorH4]+, show a characteristic intense band between 632 and 694 nm. The lack of spectral signatures of the protonated corrole after addition of the Fe(III) salt indicates that the protonated corrole is not formed during the chemical oxidation. The spectral features of 1 are similar to those of the pentathiophene radical reported by Anand et al.[9c] Hence, the conversion of 1 to the neutral corrole radical (1) may follow a radical mechanism (homolytic cleavage of N–H) as reported by Bröring et al.[9b]

Figure 1

UV–vis spectral titration of 1 (18 μM) with FeIII (1.6–20 μM) in DMSO at 298 K.

The driving force for the forward reaction in Scheme is possibly due to the redox reaction between the electronically rich corrole (1) (having two mesityl groups) and the FeIII ion, which behaves as an oxidizing agent. The formed radical species is stabilized in the presence of electron-withdrawing carboxylic acid and carbomethoxy groups (-R effect) and a high polarity of DMSO. The ESI mass spectrum of 1• was found to be 1 mass unit less than the exact mass of 1, which confirms the formation of radical species (Figure S11). The simulated isotopic pattern of the corrole radical is matching with the experimentally observed one (Figure S11).

Scheme 2

Schematic Representation of the Formation of 1 in the Presence of FeIII or [Et3O+SbCl6–] and Its Reversibility While Adding Excess of FeII in DMSO

The UV–vis spectrum of 1 exhibited a Soret band at 414 nm with a shoulder at 431 nm and Q bands at 574, 604, and 638 nm. The UV–vis spectral titration of 1 with Fe(III) perchlorate is depicted in Figure . An incremental addition of FeIII ions into the solution of 1 leads to a concomitant increase in the absorbance at 394 nm with a decrease in absorbance of Soret (414, 431sh) and Q bands (574, 604, and 638 nm) accompanied by a color change from purple to pale brown.

Two isosbestic points were observed at 404 and 516 nm upon addition of FeIII ions into corrole 1, as shown in Figure . Fluorescence quenching was observed at 658 nm (excitation at 574 nm) while addition of FeIII ions to 1, as shown in Figure .

Figure 2

Fluorescence spectral titration of 1 (18 μM) with FeIII (1.6–20 μM) in DMSO at 298 K.

The fluorescence of 1 is completely quenched with 1 equiv of Fe(III) salt in DMSO at 298 K. The 1:1 stoichiometry of 1 with FeIII was calculated by a Job plot, as shown in Figure S12. The addition of excess FeCl2 or Fe(ClO4)2 to 1 in DMSO leads to a reverse absorption profile and also a color change from pale brown to purple due to one-electron reduction, which shows reversible redox behavior of 1, as depicted in Scheme and Figures and S13.

Figure 3

Electronic absorption spectral changes of 1 while adding FeCl3 and excess of FeCl2 in DMSO at 298 K.

EPR Spectral Studies of the Corrole Radical (1)

The formation of the corrole radical (1) was characterized by EPR spectroscopy. We have carried out EPR experiments using the X-band EPR spectrometer in DMSO at 298 K as well as at lower temperatures.

The EPR spectrum of 1 (1.5 mmol) in the presence of FeIII at different temperatures showed a sharp signal at g = 2.006 (∼ organic radical species), which corresponds to 1, as depicted in Figures a and S14b. Similar results were obtained in the solid-state EPR spectrum of 1 + FeIII at 100 K, as shown in Figure b. The EPR spectrum of Fe(ClO4)3 in the solid state at 100 K (Figure S15) is quite different from that of 1. These results clearly indicate the existence of a paramagnetic species, 1. Similar EPR spectral changes were observed with the addition of FeIII to MEC at variable temperatures under similar experimental conditions (Figure S16).

Figure 4

EPR spectra of 1 (a) in DMSO at 298 K and (b) in the solid state at 100 K.

Logic Gate Application

The convenient use of logic gates fascinated the attention of researchers in the construction of molecular switches and molecular keypad devices. Figure shows the truth table and the logic gate operation using UV–vis spectral changes of 1 at a Q-band of 574 nm (a peak at 574 nm: output = 1 and absence of a peak at 574 nm: output = 0) in the presence or absence of FeIII and FeII. We have used four input combinations: (0, 0) (0, 1) (1, 0), and (1, 1). An output signal was observed at the Q-band at 574 nm in the presence or absence of FeIII and FeII (input FeIII and FeII = 0 or 1). The absorbance at 574 nm was diminished (output = 0) in the presence of FeIII (input FeIII = 1 and FeII = 0), whereas its intensity was regained in the presence of excess Fe(II) salt in DMSO at 298 K.

Figure 5

Logic functions and truth table based on UV–vis spectral changes of 1 in the presence of FeIII and FeII ions in DMSO.

DFT Calculations

We optimized the geometry of 1 and 1 using B3LYP functional and the 6-31G basis set in the gas phase. The bond angle (red) and bond lengths (black) in the optimized structure of 1 and 1 are shown in Figure . One pyrrole ring of 1 is deviated from the 23-atom core, whereas the same attains planarity in the case of 1 with the removal of one proton from the corrole core. In addition, the bipyrrolic (N–C–C–N) dihedral angle (χ) in 1 was found to be 19.11°, which is reduced to 7.07° in 1. The reduced N–C–C–N dihedral angle (χ) leads to the release of strain present in the smaller corrole core due to the presence of three NH. The decrement in χ is the driving force for the generation of corrole radical 1.

Figure 6

Optimized geometries of 1 and 1 using B3LYP functional and the 6-31G basis set in the gas phase. The bond angles and bond lengths are represented in red and black colors, respectively.

Further, time-dependent (TD)-DFT calculations were performed to examine the absorption properties of 1. The simulated electronic absorption spectral features using TD-DFT studies matched with the experimentally observed absorption profile (Figure S17).

Conclusions

We have synthesized a carboxyphenyl-substituted corrole (1) and characterized by various spectroscopic techniques. 1 leads to a stable neutral radical species, 1, in the presence of FeIII in DMSO as well as in the solid state. The observed UV–vis spectral features of 1 are in accordance with the reported literature. Further, complete fluorescence quenching of 1 was observed with 1 equiv of FeIII, and 1:1 stoichiometry was confirmed by the Job plot. The formation of radical species was supported by fluorescence quenching, decrement in lifetime, ESI-MS, and by using appropriate one-electron redox reagents. Further, 1 was reconverted to its precursor 1 by the addition of excess Fe(II) salt and to the corresponding protonated corrole, [(Cor)H4]+, by the addition of a trace of TFA in DMSO. 1 shows a sharp EPR signal at g = 2.006 in the presence of Fe(III) in DMSO as well as in the solid state. The angle strain in 1 reduced, and DFT studies suggest that the reduced bipyrrolic (N–C–C–N) dihedral angle (χ) of 1 from 19.11 to 7.07° in 1 is the driving force for the generation of the neutral corrole radical. The simulated absorption spectral features using TD-DFT calculations match with the experimental one. Overall, the spectroscopic and theoretical studies suggest the formation of 1 while adding Fe(III) salt to 1 in DMSO.

Experimental Section

Chemicals

Methyl 4-formylbenzoate and pyrrole were purchased from Alfa Aesar and used as received. CH2Cl2 was dried and distilled over P2O5. Iron perchlorate salts (M(ClO4)·xH2O, M = FeII and FeIII) were used as received from Alfa Aesar. 5-Mesityldipyrromethane was synthesized using the literature method.[8a]

Instrumentation

UV–vis and fluorescence spectra were recorded using a Cary 100 spectrometer and Hitachi F-4600 spectrofluorometer, respectively. All 1H NMR measurements were performed using a Bruker AVANCE 500 MHz spectrometer in CDCl3 and DMSO-d6.The ESI mass spectra were recorded using Bruker Daltanics-microTOF in the positive ion mode in CH3CN. EPR spectra were recorded using a Bruker X-band EPR instrument in DMSO. Electrochemical measurements were carried out with a CHI 620E electrochemical workstation. A three-electrode system was used that consisted of a GC working electrode, an Ag/AgCl reference electrode, and a Pt-wire counter electrode. The concentrations of all of the corroles used were approximately 1 mM. UV–vis and fluorescence titrations of 1 with FeIII ions were carried out in DMSO. Fluorescence lifetime measurements in the nanosecond time domain were recorded in DMSO using a Horiba Jobin Yvon “fluorocube fluorescence lifetime system” equipped with a NanoLED (635 nm) source. Theoretical calculations were carried out using B3LYP functional and the 6-31G basis set in the gas phase.

Synthesis of MEC

5-Mesityldipyrromethane (1 mmol, 264 mg) and methyl 4-formylbenzoate (0.5 mmol, 82 mg) were dissolved in MeOH (100 mL). To this, HClaq (36%, 5 mL) and H2O (50 mL) were added and stirred for 2 h at room temperature. Then, the mixture was extracted with CHCl3, and the organic layer was washed twice with water, dried over anhydrous Na2SO4, filtered, and diluted with 250 mL of CHCl3. p-Chloranil (369 mg, 1.5 mmol) was added and stirred overnight at room temperature. The reaction mixture was concentrated and purified on a silica column using CHCl3 as the eluent. Yield was found to be 8% (0.030 g, 0.04 mmol).

UV–vis (CH2Cl2) λmax (nm): 408, 426 (sh), 568, 602, 639; 1H NMR in CDCl3 (500 MHz): δ (ppm) 8.88 (d, 3JHH = 4.0 Hz, 2H, β-pyrrole-H), 8.50 (d, 3JHH = 4.5 Hz, 2H, β-pyrrole-H), 8.44 (d, 3JHH = 4.5 Hz, 2H, β-pyrrole-H), 8.39 (d, 3JHH = 8 Hz, 2H, esterphenyl-H), 8.33 (d, 3JHH = 4.5 Hz, 2H, β-pyrrole-H), 8.25 (d, 3JHH = 8.0 Hz, 2H, esterphenyl-H), 6.27 (s, 4H, m-mesityl-H), 4.08 (s, 3H, COOCH3), 2.60 (s, 6H, p-mesityl-CH3-H), 1.92 (s, 12H, o-mesityl-CH3); ESI-MS found 669.46 [M + H]+ calcd 669.32; Anal. Calcd for C45H40N4O2: C, 80.81; H, 6.03; N, 8.38; O, 4.78%. Found: C, 80.53; H, 6.34; N, 8.62; O, 4.51%.

Synthesis of 10-(4-Carboxyphenyl)-5,15-dimesitylcorrole (1)

In 30 mL of 95% ethanol, 30 mg of MEC was dissolved. To this, 0.075 g of NaOH in 1 mL of water was added and stirred at 70 °C for 2 h. Then, the reaction mixture was neutralized with 2N HCl, extracted with CHCl3, washed with water, and dried over Na2SO4. The crude product was recrystallized from MeOH/CHCl3 (2:98, v/v) and dried under vacuum. The yield was found to be 45% (0.012 g, 18.3 μmol). The observed spectral data of 1 are consistent with the literature values.[8b]

UV–vis (CH2Cl2) λmax (nm): 410, 417 (sh), 567, 602, 638; 1H NMR in CDCl3 (500 MHz): δ (ppm) 9.90 (s, 1H, COOH), 8.89 (d, 3JHH = 4.0 Hz, 2H, β-pyrrole-H), 8.46 (t, 4H, 3JHH = 8 Hz, carboxyphenyl-H and 3JHH = 4 Hz, β-pyrrole-H), 8.33 (d, 3JHH = 4 Hz, 2H, β-pyrrole-H), 8.29 (d, 3JHH = 8 Hz, 2H, carboxyphenyl-H), 7.27 (s, 4H, m-mesityl-H), 2.60 (s, 6H, p-mesityl-CH3-H), 1.93 (s, 12H, o-mesityl-CH3); ESI-MS found 655.30 [M + H]+ calcd 655.31. Anal. Calcd for C44H38N4O2: C, 80.71; H, 5.85; N, 8.56; O, 4.89%. Found: C, 80.31; H, 5.95; N, 8.25; O, 4.69%.

Synthesis of 10-(4-Carboxyphenyl)-5,15-dimesitylcorrole Radical (1)

1 was generated by mixing equimolar solutions of free base corrole (1) and Fe(III) salt in DMSO. An immediate color change was observed from purple to pale brown.

EPR (DMSO, 9.5 GHz, giso = 2.006); UV–vis (DMSO) λmax (nm): 393 (sh), 417, 549, 574, 603; ESI-MS found 653.289 [M•] calcd 653.292.