Redox-induced reversible [2 + 2] cycloaddition of an etheno-fused diporphyrin†

3,5-Ethenoporphyrin is a π-extended porphyrin containing a fused ethene unit between the meso- and β-positions, exhibiting unique contribution of macrocyclic antiaromaticity. We have recently reported that its analogue, etheno-fused diporphyrin, underwent thermal [2 + 2] cycloaddition to furnish X-shaped cyclobutane-linked tetraporphyrins. Here we demonstrate that the cyclobutane-ring formation is dynamically redox-active. Namely, the tetraporphyrin underwent two-step four-electron oxidation to afford two etheno-fused diporphyrin dications. The reduction of the resulting dication regenerated the cyclobutane-linked tetraporphyrin. The dication was sufficiently stable to allow its isolation under ambient conditions. The structure of the dication has been confirmed by 1H NMR spectroscopy and X-ray diffraction analysis. Importantly, the simultaneous double C–C bond cleavage in the cyclopropane ring in the tetraporphyrin is exceptional among dynamic redox (dyrex) systems to achieve large structural changes, thus offering new insights for the design of novel redox-active functional organic materials for electrochromic dyes, organic batteries, and organic memories.


Introduction
Porphyrins with extended p-conjugation networks exhibit numerous intriguing properties, such as near-infrared absorption, reversible redox activity, characteristic chemical reactivity, and high single-molecule conductance. 1 Such porphyrins have attracted considerable attention in various research elds including organic and supramolecular chemistry as well as materials science. 3,5-Ethenoporphyrin is an extraordinary pextended porphyrin due to the coexistence of 18p-aromaticity and 20p-antiaromaticity in its macrocyclic conjugation ( Fig. 1). 2,3 Consequently, 3,5-ethenoporphyrin exhibits a narrow HOMO-LUMO gap and high reactivity of the fused C-C double bond.
Recently, our research group envisaged the addition of another fused-porphyrin unit to the 3,5-ethenoporphyrin skeleton and attempted the synthesis of etheno-fused diporphyrin 1a via the tandem double-cyclization of b,b-ethynylene-linked dibromodiporphyrin 2a (Fig. 2). 4 Unexpectedly, we discovered the formation of cyclobutane-linked tetraporphyrins 3a and 4a. These two tetraporphyrins were formed through the thermal [2 + 2] cycloaddition reaction of in situ-generated 1a. Due to orbital symmetry, the formation of the cyclobutane in 3a via a [2 + 2] cycloaddition is thermally forbidden. Thus, the formation of 3a and 4a suggests the involvement of a thermally activated triplet state of 1a in the thermal [2 + 2] cycloaddition reaction. Indeed, syn-tetramer 4a isomerizes to anti-tetramer 3a upon heating to 160 C, which implies that the [2 + 2] cycloaddition of 1a is thermally reversible.
We then decided to conduct further investigations into these cyclobutane-linked tetraporphyrins with a focus on their redox properties. Here, we disclose a reversible [2 + 2] cycloaddition through electron-transfer-induced cyclobutane ringopening and -closure. This redox-induced reversible C-Cbond formation constitutes a dynamic redox (dyrex) system; such systems have been actively explored on account of their potential importance as electrochromic dyes, organic batteries, and organic memory devices. 5,6 Importantly, the simultaneous double C-C-bond formation/cleavage observed in the tetraporphyrin system is exceptional among reported dyrex systems.

Results and discussion
Synthesis and characterization of cyclobutane-linked tetraporphyrin b,b-Ethynylene-linked dibromodiporphyrin 2b was prepared according to a slightly modied literature procedure. 4,7 Precursor 2b was subjected to a tandem double cyclization with Ni(cod) 2 , which afforded syn-tetramer 4b in 57% yield without the formation of anti-tetramer 3b (Scheme 1). anti-Tetramer 3b was not obtained even when the reaction temperature was increased to 60 C. The selective formation of 4b is due to the steric effect by bulky mesityl groups at the meso-positions. The structure of syn-tetramer 4b was unambiguously determined by single-crystal X-ray diffraction analysis (Fig. 3).
Dyrex response of cyclobutane-linked tetraporphyrin Cyclic voltammogram. The redox behavior of syn-tetramer 4b was explored. The cyclic voltammogram of 4b was measured in CH 2 Cl 2 with tetrabutylammonium hexauorophosphate as the supporting electrolyte ( Fig. 4 and S20 †). The ferrocene/ ferrocenium couple (Fc/Fc + ) was used as an external reference. In the sweep from À0.60 V to 0.81 V, two peaks were observed at 0.43 and 0.56 V. These values are comparable to the rst oxidation peak of porphyrin Ni(II) complexes (ca. 0.58 V). 8 In the case of porphyrin, the subsequent back-sweep generates a reduction wave at ca. 0.38 V. In contrast to porphyrin, syntetramer 4b displayed reduction peaks at much lower values (0.10 and À0.29 V). The large displacement between the oxidation and reduction peaks implies the occurrence of dynamic structural changes. 5,6 Oxidative titration. To obtain insight into the unique redoxresponse of syn-tetramer 4b, we conducted an oxidative titration with tris(4-bromophenyl)aminium hexachloroantimonate (Magic Blue) in CH 2 Cl 2 while measuring its absorption spectra, which demonstrated two-step spectral changes (Fig. 5). Clear isosbestic points were observed in both cases (Fig. S23 †). The rst change occurred aer the consumption of ca. 2 equiv. of Magic Blue, resulting in the appearance of new peaks at 690 and 1274 nm. The further addition of Magic Blue (ca. 2 equiv.) resulted in the second change, which led to a slight blue shi of the absorption tail to ca. 1250 nm. Similar spectral changes were observed during spectroelectrochemical measurements from 0 to 1.2 V (Fig. S21 †). Furthermore, the subsequent backsweep to À0.5 V recovered the absorption peaks of syntetramer 4b.
Isolation of the dication. The oxidation of syn-tetramer 4b with 4 equiv. of Magic Blue in CH 2 Cl 2 furnished etheno-fused diporphyrin dication 1b[SbCl 6 ] 2 in 89% yield (Scheme 2). The Scheme 1 Synthesis of syn-tetramer 4b.   reduction of 1b[SbCl 6 ] 2 with an excess of cobaltocene recovered 4b in 80% yield. While dication 1b[SbCl 6 ] 2 is sufficiently stable under ambient conditions, repeated recrystallizations were required for its purication, given that 1b[SbCl 6 ] 2 decomposes on silica gel. The 1 H NMR spectrum of 1b[SbCl 6 ] 2 in CDCl 3 exhibited one singlet and six doublets (7.67-9.00 ppm) due to the main skeleton (Fig. 6). The calculated nucleus-independent chemical shi (NICS) 9 values of 1b[SbCl 6 ] 2 nicely coincide with this observation (Fig. S27 †). The presence of a distinct diatropic ring current in 1b[SbCl 6 ] 2 can be rationalized by the removal of two electrons from the antiaromatic contribution of the ethenofused diporphyrin core. Importantly, the signals due to the ortho-methyl groups of the mesityl substituents are observed as two singlet signals. Considering that 1b[SbCl 6 ] 2 contains two magnetically inequivalent mesityl groups, this result indicates that each set of ortho-methyl groups is magnetically equivalent, supporting that 1b[SbCl 6 ] 2 adopts a planar structure. The overall structure of 1b[SbCl 6 ] 2 was determined based on singlecrystal X-ray diffraction analysis, although the crystal data were not of sufficient quality to allow a detailed structural analysis (Fig. 7). Dication 1b[SbCl 6 ] 2 adopts a completely planar structure with a mean plane deviation of 0.07 A. The UV/vis/NIR absorption spectrum of 1b[SbCl 6 ] 2 is in good agreement with that observed aer the electrochemical oxidation of syntetramer 4b (Fig. S21 and S22 †).
Proposed dyrex-mechanism. The absorption spectrum aer the addition of 2.2 equiv. of Magic Blue is clearly different from that of a porphyrin radical cation (Fig. 5b). 10 Considering that the inter-porphyrin interaction in syn-tetramer 4b is essentially negligible due to the non-conjugative nature of the central cyclobutane unit, the initial change in absorption during the titration cannot be explained by the simple oxidation of 4b. Furthermore, this absorption is in good agreement with a theoretical simulation of the radical cation of etheno-fused diporphyrin 1bc (Fig. S25 †).   Based on the results discussed above, we propose the following redox-response process for syn-tetramer 4b (Scheme 3). The rst two-electron oxidation of 4b induces the cleavage of two C-C bonds at the central cyclobutane unit, affording two etheno-fused diporphyrin radical cations 1bc + . The subsequent oxidation of 1bc + furnishes dication 1b 2+ . The reverse reduction process begins with the one-electron reduction of dication 1b 2+ . Our previous DFT calculations have predicted that the HOMO level of the etheno-fused diporphyrin is higher than that of a normal porphyrin due to the extended p-system and potential antiaromaticity. 4 Hence, the reduction potential of 1b 2+ can be expected to be much lower than the oxidation potential of 4b, which would result in a large hysteresis in the cyclic voltammogram. Notably, we have also monitored a similar response during the electrochemical reduction of 4b (Fig. S20 †). However, the identication of the reduced species was unsuccessful owing to the instability of these intermediates.
Importantly, the redox response of 4b, namely the reversible cycloreversion involving the cleavage of two C-C bonds, is exceptional among dyrex systems. 6,7 Quadricyclane, anthracenedimer, and acridizinium dimer undergo double C-C bond cleavage upon electron-transfer, providing norbornadiene, anthracene, and acridizinium, respectively. 11-13 In these cases, however, the reverse C-C bond formation requires lightirradiation. The unique reactivity of the etheno-fused diporphyrin is attributed to the contribution of antiaromaticity in its macrocyclic conjugation. We believe that the current study offers a general insight that antiaromatic molecules 15 are a promising candidate for the design of novel redox-active functional organic materials including electrochromic dyes, organic batteries, and organic memory devices.

Thermal and photo-induced cycloreversion of cyclobutanelinked tetraporphyrin
We also examined the thermal cycloreversion of syn-tetramer 4b, which was monitored using variable-temperature NMR and UV/vis absorption spectroscopy techniques. The 1 H NMR spectrum of 4b in 1,2-dichlorobenzene-d 4 showed slight changes (up to 0.4 ppm) upon increasing the temperature from 20 C to 120 C (Fig. S18 †). However, the absorption spectrum of 4b in 1,2-dichlorobenzene displayed negligible changes upon heating (Fig. S19 †). Consequently, the temperature-dependent change of the 1 H NMR chemical shis can be attributed to the dynamic motion of the meso-aryl groups. Notably, heating the dichlorobenzene solution of 4b to 140 C afforded diketodiporphyrin 5 in 65% yield (Scheme 4). This result suggests the transient generation of etheno-fused diporphyrin 1b, which is instantly oxidized to diketone 5. A similar diketodiporphyrin was formed in our previous study with the corresponding zinc(II) complexes. 4 The excited state of nickel(II) porphyrins generally undergoes a rapid decay through the metal (d,d) state. 14 Consequently, the formation of diketodiporphyrin 5 implies that the thermally activated triplet state of in situ-generated etheno-fused diporphyrin 1b reacted with triplet oxygen.
We also examined the effect of photo-irradiation on cycloreversion. A CH 2 Cl 2 solution of 4b was irradiated by a highpressure mercury lamp equipped with a sharp cut lter (l > 380 nm) (Fig. S24 †). However, no detectable change was observed.

Conclusions
We have prepared X-shaped cyclobutane-linked tetraporphyrin 4b and examined the thermal and redox-mediated cycloreversion of its cyclobutane-ring. Heating 4b in 1,2-dichlorobenzene resulted in negligible changes in the 1 H NMR and UV/ vis absorption spectra. Instead, syn-tetramer 4b undergoes a two-step four-electron oxidation to afford etheno-fused diporphyrin dication 1b 2+ . This redox-mediated cyclobutanering cycloreversion proceeds in a reversible manner and exhibits a large hysteresis in the cyclic voltammogram. Importantly, this process is accompanied by the cleavage of two C-C bonds, which is exceptional among dyrex systems. The current research highlights the unique reactivity of antiaromatic molecules and offers fundamental insights for the design of Scheme 3 Dyrex response of syn-tetramer 4b.
Scheme 4 Thermal conversion of syn-tetramer 4b to 5 under aerobic conditions. novel redox-active functional organic materials including electrochromic dyes, organic batteries, and organic memory devices.

Author contributions
The manuscript was written through contributions of all authors. All authors have approved the nal version of the manuscript. H. S. supervised the project and contributed to conceptualization, project administration, and writing (review & editing) the manuscript. K. M. carried out the synthesis and characterization. I. H. collected the X-ray data of 4b. N. F. wrote the original dra.

Conflicts of interest
There are no conicts to declare.