Synthesis of Bis{meso-Tetrakis(4-N-alkylpyridiniumyl)porphyrinato}cerium and Its Redox Switching Behavior †

A novel double-decker porphyrin complex, bis{meso-tetrakis(4-N-alkylpyridiniumyl)porphyrinato}cerium, was prepared. Electrochemical measurements revealed that this complex exhibited reversible redox waves corresponding to a 1e– redox reaction of the cerium center. Treating the complex alternately with an oxidant and a reductant resulted in the reversible redox switching between the oxidized and reduced states in an organic solvent.

Since the physical properties of the double-decker porphyrinoid complexes are drastically modulated by the combination of the central metal ion and porphyrinoids, systematic investigations are necessary to clarify the relationship between their optical, redox, and electronic properties and their chemical structures. Herein, we report the synthesis and electrochemical properties of a novel bis(porphyrinato)cerium complex (Figure 1b), bis{mesotetrakis(4-N-alkylpyridiniumyl)porphyrinato}cerium (1). Tetrapyridiniumporphyrins are a series of the most common water-soluble tetracationic porphyrins. However, there are only a few reports on the synthesis or detailed redox properties of double-decker complexes using tetrapyridiniumporphyrins as ligands [33][34][35][36][37][38][39]. We speculated that the introduction of long-chain alkyl moieties to the pyridinium unit would improve the solubility in various organic solvents. Improving the solubility will make this complex more tractable and enable its detailed characterization. Moreover, since the eight peripheral pyridinium units are expected to affect the physical properties of the bis(porphyrinato)cerium core, it will be interesting to investigate the detailed redox and optical properties of 1. eral pyridinium units are expected to affect the physical properties of the bis(porphyrinato)cerium core, it will be interesting to investigate the detailed redox and optical properties of 1.

Results and Discussion
We first prepared bis(tetra(4-pyridyl)porphyriato)cerium(IV) (3) by refluxing a solution of Ce(acac)3·nH2O with tetra(4-pyridyl)porphyrin in 1,2,4-trichlorobenzene (isolated yield: 29%) as shown in Scheme 1. Cerium was determined to be tetravalent in 3 based on its sharp 1 H NMR signals ( Figure S1), suggesting that the cerium ion was oxidized by air during purification. Similar oxidation of the cerium center in double decker complexes has been reported previously by some groups [34,35]. Complex 3 was then functionalized with alkyliodide (4), followed by counter anion exchange to give the desired double decker complex with octapyridinium pendants, 1red, in 67% isolated yield. Interestingly, compared to the 1 H NMR signals of 2, the 1 H NMR signals of isolated 1red were significantly shifted and broadened, especially those corresponding to the aromatic groups of the porphyrin moieties ( Figure 2a). This suggested the reduction of the cerium center during the introduction of the alkyl chains. We speculated that the cerium center in 1 was reduced by the I − ion generated during the nucleophilic attack of 3 on iodoalkane 4.
Cyclic voltammograms of 1red in a CH3CN solution containing 100 mM nBu4N + PF6 − was compared with that of the monomeric reference, 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin·4PF6 − (5) (Figure 3). The large redox waves of 1red at −1.11 and −1.28 V vs. Fc + /Fc are assignable to the peripheral pyridinium ions, based on the similar redox peaks observed for 5. Considering that the redox potential of the porphyrin center in 5 was higher than 1.0 V, a reversible redox wave at 0.11 V could be assigned to the Ce 4+ /Ce 3+ conversion in 1. This Ce 4+ /Ce 3+ redox wave for 1 showed a significant positive shift with respect to that for bis(tetraphenylporphyrinato)cerium (E°(Ce 4+ /Ce 3+ ) = −0.36 V vs. Fc + /Fc) [40]. This is presumably due to the effect of the positively charged pyridinium moieties on the ligands. The fact that the rest potential of the solution of 1red subjected to cyclic voltammetry is around −0.26 V, which is much more negative than the E°(Ce 4+ /Ce 3+ ) value of 1 (0.11 V), clearly indicates that the cerium center in 1 is in the Ce(III) state.
Considering that the 1e -oxidation of the cerium center in 1red is reversible, we next attempted the redox switching of the cerium center in 1. Addition of an increasing amount of NO + ·PF6 − , a strong oxidizing reagent, in a CD3CN solution decreased the intensities of the signals corresponding to 1red, while the intensities of the sharp signals corresponding to the oxidized form of 1 (1ox) increased ( Figure 2). After the addition of one equivalent of NO + ·PF6 − , the signals corresponding to 1red completely disappeared, and 1ox was quantitatively generated. We washed the CHCl3 solution of 1ox with an aqueous solution contain-

Results and Discussion
We first prepared bis(tetra(4-pyridyl)porphyriato)cerium(IV) (3) by refluxing a solution of Ce(acac) 3 ·nH 2 O with tetra(4-pyridyl)porphyrin in 1,2,4-trichlorobenzene (isolated yield: 29%) as shown in Scheme 1. Cerium was determined to be tetravalent in 3 based on its sharp 1 H NMR signals ( Figure S1), suggesting that the cerium ion was oxidized by air during purification. Similar oxidation of the cerium center in double decker complexes has been reported previously by some groups [34,35]. Complex 3 was then functionalized with alkyliodide (4), followed by counter anion exchange to give the desired double decker complex with octapyridinium pendants, 1 red , in 67% isolated yield. Interestingly, compared to the 1 H NMR signals of 2, the 1 H NMR signals of isolated 1 red were significantly shifted and broadened, especially those corresponding to the aromatic groups of the porphyrin moieties ( Figure 2a). This suggested the reduction of the cerium center during the introduction of the alkyl chains. We speculated that the cerium center in 1 was reduced by the I − ion generated during the nucleophilic attack of 3 on iodoalkane 4.
ing excess KI, and found that 1red was quantitatively obtained as described in the experimental section. Thus, reversible switching of the redox state, i.e., reversible conversion between 1red and 1ox, was achieved by treating the sample alternately with NO + ·PF6 − and KI.  Cyclic voltammograms of 1 red in a CH 3 CN solution containing 100 mM nBu 4 N + PF 6 − was compared with that of the monomeric reference, 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin·4PF 6 − (5) (Figure 3). The large redox waves of 1 red at −1.11 and −1.28 V vs. Fc + /Fc are assignable to the peripheral pyridinium ions, based on the similar redox peaks observed for 5. Considering that the redox potential of the porphyrin center in 5 was higher than 1.0 V, a reversible redox wave at 0.11 V could be assigned to the Ce 4+ /Ce 3+ conversion in 1. This Ce 4+ /Ce 3+ redox wave for 1 showed a significant positive shift with respect to that for bis(tetraphenylporphyrinato)cerium (E • (Ce 4+ /Ce 3+ ) = −0.36 V vs. Fc + /Fc) [40]. This is presumably due to the effect of the positively charged pyridinium moieties on the ligands. The fact that the rest potential of the solution of 1 red subjected to cyclic voltammetry is around −0.26 V, which is much more negative than the E • (Ce 4+ /Ce 3+ ) value of 1 (0.11 V), clearly indicates that the cerium center in 1 is in the Ce(III) state.
Considering that the 1eoxidation of the cerium center in 1 red is reversible, we next attempted the redox switching of the cerium center in 1. Addition of an increasing amount of NO + ·PF 6 − , a strong oxidizing reagent, in a CD 3 CN solution decreased the intensities of the signals corresponding to 1 red , while the intensities of the sharp signals corresponding to the oxidized form of 1 (1 ox ) increased ( Figure 2). After the addition of one equivalent of NO + ·PF 6 − , the signals corresponding to 1 red completely disappeared, and 1 ox was quantitatively generated. We washed the CHCl 3 solution of 1 ox with an aqueous solution containing excess KI, and found that 1 red was quantitatively obtained as described in the experimental section. Thus, reversible switching of the redox state, i.e., reversible conversion between 1 red and 1 ox , was achieved by treating the sample alternately with NO + ·PF 6 − and KI.  The UV-VIS spectra of 1ox and 1red in CH3CN solution are shown in Figure 4. Complex 1ox showed a characteristic Soret band at 401 nm and Q bands at 497, 545, 588, and 656 nm, which were almost identical to those of bis(tetrapyridylporphyrinato)cerium(IV) (Soret band: 398 nm, Q-bands: 490, 543, 588, and 645) [36]. On the other hand, 1red showed the Soret band at 432 nm and the Q bands at 502, 570, 623, and 680 nm. Thus, there were significant changes in the absorption spectra due to the change in the redox state of the cerium center. This behavior was similar to that observed for the redox change of bis[tetrakis(4-N-methylpyridiniumyl)porphyrinato]cerium(III) heptachloride ([Ce III (TM4PyP)2]Cl7) in an aqueous solution, as reported by Nawra et al. [36].  The UV-VIS spectra of 1 ox and 1 red in CH 3 CN solution are shown in Figure 4. Complex 1 ox showed a characteristic Soret band at 401 nm and Q bands at 497, 545, 588, and 656 nm, which were almost identical to those of bis(tetrapyridylporphyrinato)cerium(IV) (Soret band: 398 nm, Q-bands: 490, 543, 588, and 645) [36]. On the other hand, 1 red showed the Soret band at 432 nm and the Q bands at 502, 570, 623, and 680 nm. Thus, there were significant changes in the absorption spectra due to the change in the redox state of the cerium center. This behavior was similar to that observed for the redox change of bis[tetrakis(4-N-methylpyridiniumyl)porphyrinato]cerium(III) heptachloride ([Ce III (TM 4 PyP) 2 ]Cl 7 ) in an aqueous solution, as reported by Nawra et al. [36].

General Information
The synthetic procedures were carried out under an anhydrous nitrogen atmosphere, unless otherwise specified. All the reagents and solvents were purchased at the highest

General Information
The synthetic procedures were carried out under an anhydrous nitrogen atmosphere, unless otherwise specified. All the reagents and solvents were purchased at the highest commercial quality available and used as received without further purification, unless otherwise stated. The 1 H, and 13 C NMR spectra were recorded using a JEOL JNM-ECS400 (400 MHz for 1 H; 100 MHz for 13 C) or a JEOL JNM-ECA600 (600 MHz for 1 H; 150 MHz for 13 C) spectrometer at a constant temperature of 298 K. Tetramethylsilane (TMS) was used as the internal standard for the 1 H and 13 C NMR measurements in CDCl 3 , CD 3 CN, and DMSO-d 6 . The silica gel column chromatographies and thin-layer chromatography (TLC) were performed using Merck silica gel 60 and Merck silica gel 60 (F254) TLC plates, respectively. The ESI mass spectrometry was performed using a Waters LCT-Premier XE spectrometer, controlled using the Masslynx software. The absorption spectra were recorded using a Hitachi U-4100 spectrophotometer in CHCl 3 solutions at 20 ± 0.1 • C in quartz cells (10 mm optical path length). Cyclic voltammetry measurements were performed with a BAS Electrochemical Analyzer Model 750Ds at room temperature, in acetonitrile solutions containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) in a standard one-component cell under an N 2 atmosphere equipped with a 3 mm-O.D. glassy carbon disk working electrode, and a platinum wire counter electrode as Ag/Ag + reference electrode. All solutions were deoxygenated by N 2 bubbling for at least 10 min. Obtained E 0 vs. Ag/Ag + were converted to those vs. Fc + /Fc based on measured redox potential of ferrocene.

Conclusions
We prepared a novel bis(porphyrinato)cerium(III) complex containing eight pyridinium moieties, 1 red , based on the peralkylation of a bis(tetrapyridylporphyrinato)cerium(IV) complex via the reaction with iodoalkane. The obtained double-decker complex, 1 red , included a Ce(III) core, implying that the cerium center underwent 1ereduction upon the reaction with I − during the introduction of the peripheral alkyl chains, as confirmed by cyclic voltammetry and 1 H NMR and UV-VIS spectroscopies. We demonstrated that the addition of an equimolar amount of an oxidant, NO + ·PF 6 − , converted 1 red into its oxidized form 1 ox quantitatively. This suggested that the alternating addition of NO + ·PF 6 − and KI enabled reversible switching of the redox states of the cerium center in double-decker complex 1 in an organic solvent. As far as we know, this is the first example of a reversible redox switching of a bis(tetrapyridylporphyrinato)cerium complex in an organic solution. Reversible redox of the cerium center with apparent color change should be suitable for a colorimetric indicator of oxidants, such as halogen and nitrosonium ions. This would also suggest potential applications in redox switchable molecular devices.
Author Contributions: Conceptualization, T.N., Y.Y. and K.T.; synthesis and measurements, T.N. and A.Y.; writing-review and editing, T.N., Y.Y. and K.T. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.