# Encoding and Expressing the Handedness of a Möbius π System in a Totemic Architecture

Syamasrit Dash, Arnaud Fihey, Ludovic Favereau, Corinne Lagrost, Rajaa Benchouaia, Sébastien Blanchard, Mickaël Ménand, Stéphane Le Gac

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Syamasrit Dash, Arnaud Fihey, Ludovic Favereau, Corinne Lagrost, Rajaa Benchouaia, et al. Encoding and Expressing the Handedness of a Möbius π System in a Totemic Architecture. Journal of the American Chemical Society, 2025, 10.1021/jacs.5c00524. [https://doi.org/10.1021/jacs.5c00524](https://doi.org/10.1021/jacs.5c00524). [hal-05050794](https://wwwHAL.fr/hal-05050794)

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# Encoding and Expressing the Handedness of a Möbius π System in a Totemic Architecture

Syamasrit Dash,ª Arnaud Fihey,ª Ludovic Favereau,ª Corinne Lagrost,ª Rajaa Benchouaia,ª Sébastien Blanchard,ᵇ Mickaël Ménand,\*ᵇ and Stéphane Le Gac\*ª

**ABSTRACT:** Efficient control of the chirality of Möbius π systems remains a challenging task that hinders the development of such molecules into information processing systems. Achieving such control through a redox process would thus open new opportunities. In this context, Redox behaviors of Ni(II) and Pd(II) complexes of a Möbius aromatic [28]hexaphyrin doubly-linked to an α-cyclodextrin have been investigated. This totemic architecture embedding three types of chirality elements generates two pseudo-enantiomers after coordination with either metal. These isomeric pairs possess marked and opposite chiroptical signatures resulting from the *P* and *M* configurations of the Möbius π systems. Chemical oxidation to 26-π systems led to behaviors reminiscent to *The Oak and the Reeds* fable, due to a N3C coordination sphere of Ni(II) being more robust than that of Pd(II). Oxidized Ni(II) complexes (*the Oak*) maintain a Möbius-type conformation at the expense of the π-systems which undergo an interruption due to inevitable water insertion. In contrast, oxidation of Pd(II) complexes (*the Reeds*) converts the Möbius aromatic systems into Hückel (rectangular) aromatic ones that are maintained in the chiral environment provided by the linking pattern with the cyclodextrin. This constitutes an effective chiral instructing site, as reduction back to their original Möbius configuration occurs with high stereoselectivity. Such a reversible shape-shifting process corresponds to a chiral memory phenomenon where the handedness of a cyclic π system is encoded in a scaffold and expressed upon changing an electronic state. For both metals, spectroelectrochemical studies ultimately revealed robust ON-OFF chiroptical switches, which is unprecedented with Möbius π-systems.

## 1. Introduction

Redox-triggered chiroptical switches have received a growing attention over the last few decades, driven by the important development of molecular and optical electronics.¹ In particular, chiral molecules featuring contorted (non-planar) π-conjugated systems such as helicenes,² twistacenes³ or curved graphene derivatives,⁴ have shown a great potential with possible applications in areas such as data storage, optical displays, or spintronics. Whereas Hückel aromaticity is ubiquitous to such materials, the capacity of less usual Möbius aromatic π-systems (inherently chiral) to be involved in chiroptical switching phenomena under redox control has remained virgin of exploration.⁵,⁶,⁷,⁸ In general, the under-investigation of Möbius chirality systems prevented any practical outcome,⁹ and this is mainly due to the following obstacles: (i) lack of general access to Möbius aromatic π-systems; (ii) chemical stability at different redox states and subsequent reversibility; (iii) chiral configuration stability of Möbius rings at different redox states; (iv) marked and sensitive chiroptical responses. Even though polyaromatic hydrocarbons (PAHs) recently afforded Möbius scaffolds with improved efficiency,¹⁰ they do not yet equal the great versatility of porphyrinoids to yield multi-functional Möbius aromatic π-systems in line with rich redox and coordination chemistry.¹¹,¹²

In this context, the regular [28]hexaphyrin scaffold (Scheme 1, Mt = 2H) appears attractive to combine electronic and stereochemical phenomena.¹²d It readily adopts a Möbius conformation which is strongly aromatic, dynamic, and *de facto* inherently chiral.¹²i Therefore, recent studies have shown chiroptical

activity modulation by taking advantage of the dynamic chirality of this scaffold, leading to responsive/adaptive systems¹³ and ultimately to chemically-triggered chiroptical switches.⁸,¹⁴

<figure><img src="image_2.png" /><figcaption>**Scheme 1. Redox-triggered conformational switch between metalated or free base Möbius twisted [28]hexaphyrin and Hückel rectangular [26]hexaphyrin.**</figcaption></figure>

In addition, the regular Möbius [28]hexaphyrin can undergo shape-shifting phenomena, i.e. it rearranges into other conformations upon changes of the environment.¹¹ Notably, oxidation into the [26] state leads generally to a rectangular (planar) Hückel aromatic species (Scheme 1, Mt = 2H).¹²d This phenomenon can be understood as a way to escape from a high-energy (likely unstable) [26]π Möbius antiaromatic state, yet never isolated.¹²j However, this gain in stability comes at the expense of the initial Möbius chirality being cancelled, which at first glance rules out this system for generating redox chiroptical switches.

<footnote>ª Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes)–UMR 6226, Rennes F-35000, France.</footnote>

<footnote>ᵇ Sorbonne Université, CNRS, Institut Parisien de Chimie Moléculaire (IPCM), UMR 8232, 4 place Jussieu, 75005 Paris, France.</footnote>
Herein, we have investigated such an “escape” strategy under the chiral influence of our previously developed hexaphyrin-cyclodextrin hybrid (HCD) and unraveled the very first example of Möbius-type redox chiroptical switch. We will disclose how the original architecture of HCD hybrid provides the specific chiral environment required to control the Möbius twist configuration during redox switching. Moreover, it is worth mentioning that ON/OFF chiroptical switches are challenging and often requires a chiral molecule to be made less chiral.¹⁵ Möbius-planar isomerization is thus an appealing process to realize such type of redox chiroptical switches.

## 2. Results and Discussion

### 2.1 Design

Among different HCD designs,¹⁶ we recently described a member including a Möbius [28]hexaphyrin doubly-linked to an α-cyclodextrin unit (Figure 1b).¹⁶d This unique architecture affords a “totem” of three different chirality elements (Figure 1a) arising from the cyclodextrin (fixed *central chirality*), the bridging pattern (dynamic *planar chirality*), and the hexaphyrin (dynamic *topological chirality*)⁶b and displaying the following features:

*   **Central chirality:** the AD-difunctionalized cyclodextrin unit affords a static source of central chirality with 30 stable stereogenic centres (Figure 1a, bottom).

*   **Planar chirality:** (AB₂)₂₃-type meso-substituted rectangular hexaphyrins allow three possible meso-bridging patterns (Figure 1c, right): a short-side bridging pattern being achiral (5,20-meso bridged hexaphyrin) and two long-side ones affording chiral opposite Rₚ and Sₚ configurations (10,25- and 15,30-meso bridged hexaphyrins respectively – Figure 1a, middle).¹⁷ This planar chirality is dynamic since the diastereomers are in equilibrium through coupled *in-out/out-in* inversions of four pyrroles.¹⁶d

*   **Topological chirality:** introducing a half-twist into the π-conjugated circuit leads to Möbius stereoisomers whose opposite P/M handedness is also in equilibrium (Figure 1a, top). Similarly to the rectangular conformation, planar chirality only arises from the long-side bridging patterns (10,25 and 15,30).

The meso-substitution pattern and Möbius isomerization processes are governed by very different energy barriers, which are roughly 23 kcal.mol⁻¹ and 8 kcal.mol⁻¹, respectively.¹⁶d Combining all types of chirality, this edifice yields a unique chiral network with dual control over chiroptical properties:

(i) Coordination driven stereoselective amplification (Figure 1c, left): chirality transfers are characterized by a planar chirality-to-topological chirality communication in which the meso-substitution pattern determines the twist configuration (i.e. Rₚ → M and Sₚ → P, diastereomeric excess > 95%) combined to a central chirality-to-planar chirality communication in which the cyclodextrin unit discriminates the two meso-substitution patterns (up to 60% d.e.). This stereoselectivity is remotely controlled by coordination of an achiral effector (i.e. Znacac) to the hexaphyrin unit.

(ii) Redox-mediated stereoselective relationship (Figure 1c, right): oxidation to the [26]π state cancels the totem's topological chirality without affecting the two other chiral elements, resulting in a rectangular hexaphyrin with a preserved substitution pattern. Conversely, reduction to the [28]π state restores the Möbius ring to its original P or M configuration. Therefore, the planar chirality element acts as a chiral instructor with respect to the Möbius twist, enabling a redox control of the chiroptical properties. However, a competing long-side to short-side substitution patterns isomerization progressively erases the planar chirality element of the totem and makes the chiral redox process difficult to manage (Figure 1c, right).

<figure><img src="image_3.png" /></figure>
<caption>**Figure 1.** a) Stereogenic elements embedded in the hexaphyrin-cyclodextrin hybrids. b) Representative structures of [28]π Möbius (left) and [26]π planar (right) HCDs.¹⁸ c) Summary of chiral communication events, mediated by coordination (left) or redox (right) processes.</caption>

With these two features as starting points, aiming at efficient redox chiroptical switches, we turned our attention to less dynamic complexes and selected nickel(II) and palladium(II) transition metals to stabilize the different topological/planar configurations of the [28]π and [26]π states of the totemic edifice. Pd(II) forms complexes with both [28]Möbius and [26]rectangular skeletons, and enables them to be interconverted via classical redox reagents (Scheme 1, Mt = Pd).¹⁹ In the case of Ni(II), although interesting behaviors such as tunable metal vs ligand-based redox events have been described with modified hexaphyrins,²⁰ redox processes remain to be explored with regular hexaphyrin Ni(II) complexes.²¹

## 2.2 Synthesis and Characterization

(M)-M[28]1 and (P)-M[28]2 have been synthesized according to our previous work, from an acid catalyzed [1+2] macrocyclisation reaction between bisaldehyde α-CD and tripyrromethane derivatives, followed by DDQ oxidation.¹⁶d Metal insertion was performed with an equilibrated mixture of these isomers (ca.1:1 ratio) using 10 equiv. of either Ni(OAc)₂ or Pd(OAc)₂ salts (Scheme 2). Coordination of Pd(II) proceeded more smoothly than that of Ni(II), requiring longer reaction time at a higher temperature (3 h at 45°C vs 24 h at 70°C, respectively). Remarkably, in both cases, only two Möbius metal complexes were isolated by silica gel column chromatography as turquoise blue isomers, (M)-M[28]1Ni (37.5%) / (P)-M[28]2Ni (39.5%) and (M)-M[28]1Pd (23%) / (P)-M[28]2Pd (20%). HRMS analysis indeed revealed monometallic complexes present in a [28]π oxidation state (see SI.2).²²

<figure><img src="image_4.png" /><figcaption>**Figure 2.** UV/vis/NIR absorption spectra (bottom) and ECD spectra (top) of metal complexes (CHCl₃).</figcaption></figure>

All four complexes displayed typical [28]π Möbius aromatic absorption spectra (Figure 2, bottom) with intense Soret-like bands between 607 and 626 nm and weak Q-like bands in the near infra-red region (700-1100 nm). ¹H NMR spectra of the four complexes were rather similar, and displayed C₁ symmetric signatures characteristic of frozen Möbius conformations, differentiating all the protons with a characteristic distribution resulting from the diatropic ring current (Figure 3). For instance, β-pyrrolic (βπ) protons exhibited the following pattern:
eight deshielded $βπ^{out}$ (ca. 6.5 to 8 ppm, four blue pyrroles), two $βπ^{twisted}$ (ca. 5 to 7 ppm, green pyrrole) and a single $βπ^{in}$ as a strongly shielded singlet (ca. 0 to 1 ppm, red pyrrole). This latter signal is in agreement with a square planar coordination geometry of Ni(II) or Pd(II) (N3C coordination box).²¹ Such a coordination sphere constrains the Möbius conformation and projects a meso-C₆F₅ substituent above the metal ion as displayed in Figure 3. This was further confirmed by ¹⁹F COSY 2D NMR spectra, showing characteristic patterns of through-space couplings with two other meso-C₆F₅ substituents ($^{TS}J_{F,F}$ = 9 to 50 Hz, see SI.3.7).²³ Concerning the cyclodextrin part, the analysis of the HSQC revealed a systematic shielding of the C-4 belonging to A and D glucose units bearing the linkers ($δ[C-4_{A,D}]$ ~ 72 ppm vs $δ[C-4_{B,C,E,F}]$ ~ 80 ppm, $Δδ$ ~ 8 ppm, see SI.3). This shielding, independent from the hexaphyrin field effect, indicates that the glycosidic bonds of the glucose pairs A/F and C/D undergo a strong distortion.²⁴ Conformational restriction due to the coordination is thus likely transferred to the CD torus. Meanwhile, O-6 benzyl rings of glucose units B and E are oriented below the hexaphyrin and experience a shielding up to ~ 5.2 ppm (SI.3). Interestingly, electronic circular dichroism spectra (ECD) of each pair of metal complexes displayed quasi-

mirror signatures at the level of the Soret-like band ($|Δε_{650}|$ ~ 225 M⁻¹·cm⁻¹), with slight difference of intensity for the Q-like bands (Figure 2, top). These observations are in agreement with metal complexes having opposite $S_p/R_p$ configurations of the meso-substitution patterns, leading in turn to opposite P/M Möbius twist configurations [(M)-M][28]1Ni/Pd: negative Cotton effect; (P)-M[28]2Ni/Pd: positive Cotton effect]. In contrast to the related Zn(II) complexes,¹⁶d these two types of chirality are not dynamic anymore in the Ni(II) and Pd(II) complexes.²⁵ Regarding chirality induction phenomena in these edifices, a highly efficient communication occurs between the *planar* and *topological* levels as a single twist was observed for a given meso-substitution pattern. The high P/M stereoselectivity is attributed to a major difference of steric hindrance due to the co-ordination-induced « folding » of a meso-substituent (phenyl-linker or C₆F₅) above or below the N3C coordination box (see SI.2.8). In contrast, no discrimination of the meso-substitution patterns by the cyclodextrin could be observed during the metalation processes, the diastereomeric complexes being formed in roughly equal yields.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) Pyrroles color code (left) and ¹⁹F-¹⁹F through-space couplings (right) for a typical Möbius metal complex ; ¹H NMR spectra (300 K, CDCl₃) of (b) (M)-M[28]1Ni, (c) (P)-M[28]2Ni, (d) (M)-M[28]1Pd and (e) (P)-M[28]2Pd (small colored sticks highlight the position of the most relevant signals; *: solvents, grease or impurities).</figcaption></figure>

We then performed DFT calculation with model compounds of all four complexes (benzyl groups were removed, see SI.8). The optimized geometries were found rather similar and they nicely evidenced highly dissymmetrical architectures with the metal ion slightly off-centered vs the cyclodextrin cavity axis (Figure 4). The twisted Möbius conformation discriminates

both sides of the bridge offering a selective access to the cyclodextrin's cavity and the buried metal ion. This latter is indeed overlaid on the opposite side by an *inward* C₆F₅ group. Ni(II) and Pd(II) exhibit distorted square planar coordination with average metal-N/C bond length slightly shorter for Ni(II) ($\bar{d}_{[N/C-Ni]}$ = 1.91 Å vs $\bar{d}_{[N/C-Pd]}$ = 2.04 Å). As a result, a slightly more constrained Möbius conformation is observed with Ni(II), leading to a shorter distance to the *inward* C₆F₅ moiety ($\bar{d}_{[C₆F₅-Ni]}$ =
3.47 Å vs $\bar{d}_{[C6F5-Pd]}$ = 3.66 Å). TD-DFT calculations were also performed. Calculated absorption and ECD spectra were found qualitatively in good agreement with the experimental ones (see SI.8.7.1 and SI.8.7.2). The spectral features are well described by the interactions of the four frontier molecular orbitals (FMOs, HOMO/HOMO-1 and LUMO/LUMO+1, see SI.8.6), that are mainly localized on the hexaphyrin π system. The distorted Möbius conformation does not favor d-p orbital hybridization, thus weak if any participation of Ni/Pd orbitals account in the FMOs.¹⁹ᵇ

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Optimized structures of (a) (M)-M[28]1Ni, (b) (P)-M[28]2Ni, (c) (M)-M[28]1Pd and (d) (P)-M[28]2Pd (DFT calculations with Bn groups omitted, see details in SI.8).</figcaption></figure>

## 2.3 Redox properties, Chemical Oxidation and Spectroelectro-Chiroptical Switching

The electrochemical behaviors of the four complexes were investigated by cyclic voltammetry. Four monoelectronic oxidation processes were observed, which are well-separated and chemically reversible, at least partially, suggesting that the resulting species are stable at the cyclic voltammetry timescale (Table 1 and SI.4 for details).

A well-defined and chemically reversible reduction peak is also observed for the series of complexes. At more negative potentials, the reduction processes are likely to lead to degradation. By considering the first oxidation and reduction processes, we have estimated from the onset potentials the electrochemical gap, 1.07 eV and 1.14 eV for the Ni and Pd complexes, respectively, in agreement with the optical gaps ($ΔE_{spectro}$ = 1.24 eV for Ni(II) and 1.18 eV for Pd(II)) (see the SI).

<table><caption>**Table 1.** Formal redox potentials (V) of (M)-M[28]1Ni, (P)-M[28]2Ni, (M)-M[28]1Pd and (P)-M[28]2Pd in CH₂Cl₂ with 0.2 M n-Bu₄NPF₆.ª</caption><thead><tr><th>Complexes</th><th>E°<sub>ox1</sub></th><th>E°<sub>ox2</sub></th><th>E°<sub>ox3</sub></th><th>E°<sub>ox4</sub></th><th>E°<sub>red1</sub></th></tr></thead><tbody><tr><td>(M)-M[28]1Ni</td><td>0.12<br>(65)</td><td>0.35<br>(66)</td><td>0.71<br>(75)</td><td>0.92<br>(70)</td><td>-1.29<br>(70)</td></tr><tr><td>(P)-M[28]2Ni</td><td>0.01<br>(60)</td><td>0.24<br>(72)</td><td>0.61<br>(70)</td><td>0.84<br>(66)</td><td>-1.39<br>(70)</td></tr><tr><td>(M)-M[28]1Pd</td><td>0.04<br>(66)</td><td>0.29<br>(60)</td><td>0.68<br>(75)</td><td>0.83<br>(75)</td><td>-1.29<br>(85)</td></tr></tbody></table>

<table><tr><td>(P)-M[28]2Pd</td><td>-0.03<br>(66)</td><td>0.24<br>(70)</td><td>0.63<br>(75)</td><td>0.84<br>(80)</td><td>-1.39<br>(70)</td></tr></table>

ª Potentials were referred vs Fc⁺/Fc redox couple. They were derived from the mid-sum of the corresponding redox peaks potentials. In bracket is given the peak-to-peak separation value (mV).

We thus attempted the stepwise [28]-to-[26] chemical oxidation of the complexes with the single-electron oxidant tris(4-bromophenyl)ammonium hexachloroantimonate (Magic Blue, MB). Our investigations revealed non-trivial oxidation processes leading mainly to non-aromatic species with Ni(II) complexes, whereas straightforward processes occurred with Pd(II) ones leading to Hückel aromatic derivatives (detailed separately hereafter, Figure 5: (M)-M3Ni, (P)-M4Ni and Figure 6: (Rₚ)-R[26]1Pd, (Sₚ)-R[26]2Pd).

### 2.3.1 Oxidation of Ni(II) complexes.
We first monitored the oxidation of the Ni(II) complexes by ¹H NMR titration. Adding progressively 1 equiv. of MB in a CD₂Cl₂ solution of (M)-M[28]1Ni led to the disappearance of the hexaphyrin signals, suggesting a monooxidized radical intermediate with paramagnetic character (SI.5.1.1). The concomitant analysis of the monooxidation by EPR spectroscopy confirmed the formation of a radical with a rhombic signal ($g_x$ = 2.017; $g_y$ = 2.000; $g_z$ = 1.988), being close to that of a free electron ($g_e$ = 2.0023), indicating a ligand-centered process yielding an intermediate [27]π species (SI.5.1.2). This observation is in agreement with DFT calculations performed on monooxidized model compounds which revealed a spin electron density fully delocalized over the Möbius π-system, with little participation of the metal (SI.8.4). Further addition of an excess of MB (2.5 equiv.) led to the slow apparition of a major NMR pattern exhibiting sharp signals (8 h at 300 K). Similar behavior was observed with (P)-M[28]2Ni, and analytical samples of the major oxidized species were obtained by silica gel chromatography purifications (54% and 50% yields respectively, see SI.2.9). In both cases, HRMS analysis showed an ion peak at *m/z* = 3851.1917 (calcd for [M]⁺: [C₂₁₈H₁₈₄N₆O₃₃F₂₀⁵Ni]⁺ = 3851.1933), which is 17 mass units heavier than a 2H atoms abstraction process, suggesting the addition of a hydroxyl group upon oxidation likely resulting from the nucleophilic attack of a residual water molecule.²⁶ NMR analysis revealed that the chemical shifts of the β-π protons are all located between 5.0 and 7.5 ppm attesting the absence of a ring current (see SI.3.5 and SI.3.6). The two oxidized species (M)-M3Ni and (P)-M4Ni (Figure 5a) share however common structural features with their [28]π Möbius aromatic parents: (i) they both display a single βπ<sup>in</sup> singlet (5.44 ppm for (M)-M3Ni / 5.58 ppm for (P)-M4Ni [vs 0.72 and 0.94 ppm for parent [28] derivatives, respectively]) indicating that the N3C coordination box is maintained upon oxidation;²⁷ (ii) their ¹⁹F NMR spectra show characteristic through-space coupling correlations between the inward C₆F₅ located above the metal ion and two other meso-C₆F₅ attesting the preservation of a Möbius-type conformation (Figure 5c,d and SI.3.7.5). Besides, a sharp singlet at 5.47 ppm for (M)-M3Ni (5.68 ppm for (P)-M4Ni) assigned to an OH group suggests the presence of a quaternary carbon responsible for the interruption of the cyclic π-system (see SI.3.6.8). Based on 2D NMR analysis, the most plausible structure corresponds to a meso-insertion of an hydroxyl group into the meso-aryl linker neighboring the C-metalated pyrrole, with an inward orientation relative to the cyclodextrin cavity
(Figure 5b). These structures are supported by related examples such as hydroxy-isoporphyrins originating from the insertion of water onto oxidized metal porphyrins,²⁸ or the bimetallic hexaphyrin gable-shape complexes which undergo the insertion of oxygen atoms at a meso-position upon coordination,²⁹ both situations leading to interrupted π-systems. The UV-vis-NIR absorption spectra of (M)-M3Ni and (P)-M4Ni reflect their non-aromatic character (absence of well-defined Soret-like and Q-like bands) and exhibit absorption bands spanning the near-infrared region, up to 1200 nm (Figure 5e), consistently with the general trend of isoporphyrin derivatives featuring a broken degeneracy of the four FMOs with narrower HOMO-LUMO gap.²⁸,³⁰ Remarkably, the ECD spectra of (M)-M3Ni and (P)-M4Ni are quasi-mirror images (Figure 5e), indicating that neither the meso-substitution pattern (Rₚ/Sₚ) nor the π-system handedness (although interrupted) are affected during the oxidation process.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** a) Oxidation of Ni(II) complexes with magic blue; b) structure of (P)-M4Ni (the Möbius interrupted [26]π-system is highlighted in red); c-d) ¹⁹F NMR spectrum of (P)-M4Ni, dashed lines highlight through-space coupling correlations observed in ¹⁹F NMR (CDCl₃, 470 MHz, 300 K); e) UV/vis/NIR absorption spectra (bottom) and ECD spectra (top) of (M)-M3Ni and (P)-M4Ni in</figcaption></figure>

CHCl₃; f) calculated LOL-π isosurface of (P)-M4Ni (Bn groups removed for the calculation; isovalue = 0.27; contribution of the meso-aryl substituents not displayed for clarity).

DFT calculations on model compounds were performed and support well our interpretation of the experimental analysis (see SI.8). The MO diagrams exhibit a similar trend to that of the hydroxy-isoporphyrins,²⁸b i.e. a destabilization of the FMOs with lifted pseudo-degeneracy (SI.8.3.2). TD-DFT showed a good qualitative agreement of the absorption and ECD calculated spectra with the experimental ones (SI.8.7.3). Moreover, the calculated localized orbital locator isosurface of π orbitals (LOL-π)³¹ clearly highlight (i) the interruption of the [26]π conjugation pathway at the level of the OH insertion, and (ii) the persistent Möbius-type winding of the π-system with opposite M/P twist for the two oxidized complexes (Figure 5f and SI.8.5).

### 2.3.2 Oxidation of Pd(II) complexes.
Similar oxidation experiments were undertaken with the Pd(II) complexes (M)-M[28]1Pd and (P)-M[28]2Pd, revealing straightforward processes using MB in acetonitrile. ¹H NMR titration showed instantaneous formation of species featuring well-defined patterns with both downfield and highfield shifted signals, suggesting a marked diatropic ring current (Hückel aromaticity) (see SI.5.3 and SI.5.4). These species were observed right after addition of 0.25 eq of MB, further suggesting no accumulation of an intermediate mono-oxidized derivative undergoing fast conversion to di-oxidized species.³² Analytical samples of the major products were isolated by silica gel chromatography purification affording (Rₚ)-R[26]1Pd and (Sₚ)-R[26]2Pd as purple solids with respectively 61 % and 70 % yields (Figure 6a,b). In contrast to oxidized Ni(II) complexes, HRMS analysis afforded the expected pattern of the [26]π derivatives, with characteristic shift of 2 mass units relative to the parent [28]π system (see SI.2.13 and SI.2.14). For both samples, NMR characterization revealed rectangular monometallic complexes composed of a mixture of two tautomers, due to the dipyrrin motif that is not involved in coordination (ca. 2:1 ratio, slow exchange on the NMR time scale). Indeed, for each of them, the high-field region (-3 to -4.3 ppm) showed three singlets (2×βπ<sup>in</sup>, NH<sup>in</sup>) while the low-field region evidenced one singlet at ~11.8 ppm (NH<sup>out</sup>) and 8 other signals at ~9 ppm (8×βπ<sup>out</sup>) (see SI.3.8.1 and SI.3.9.1). Such dissymmetric patterns are consistent with a N3C to N2C2 rearrangement of the coordination sphere of Pd(II) along with a [28]π Möbius to [26]π Hückel isomerization of the hexaphyrin scaffold (Figure 6a,b; Scheme 1).¹⁹a

(Rₚ)-R[26]1Pd and (Sₚ)-R[26]2Pd exhibit absorption spectra characteristic of Hückel aromatic hexaphyrins (Figure 6b) with intense Soret-like bands at ca. 570 nm and weak Q-like bands in the NIR region (700-1100 nm). Importantly, their ECD spectra revealed opposite signatures indicating that their mirror meso-substitution patterns (Rₚ/Sₚ) are maintained upon oxidation (Figure 6a). While the Hückel π systems experience this planar chirality,³³ Δε values at the level of the Soret-like transitions are much less intense compared to the parent Möbius systems (Δε₆₅₀ ~30 vs ~210 M⁻¹·cm⁻¹, respectively). It is worth mentioning that Pd(II) coordination further prevents long-side to short-side isomerization, an inevitable process canceling the planar chirality element in the absence of metal (Figure 1c, right).¹⁶d,²⁵ Therefore, such marked evolution of the chiroptical
activities arise from a Möbius-to-Hückel isomerization in a chiral environment that has become resilient to an oxidation process by suitable coordination chemistry.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) Stereoselective and reversible Möbius-rectangular re-
dox rearrangement of HCD Pd(II) complexes; b) UV/vis/NIR ab-
sorption spectra and ECD spectra of (R<sub>p</sub>)-<sup>R</sup>[26]1Pd and (S<sub>p</sub>)-
<sup>R</sup>[26]2Pd in CH<sub>2</sub>Cl<sub>2</sub>; c) <sup>1</sup>H NMR spectra (CDCl<sub>3</sub>, 500 MHz, 300
K, high-field region) of the crude of the reduction of (R<sub>p</sub>)-<sup>R</sup>[26]1Pd
(top), the reference spectra of (M)-<sup>M</sup>[28]1Pd and (P)-<sup>M</sup>[28]2Pd,
and the crude of the reduction of (S<sub>p</sub>)-<sup>R</sup>[26]2Pd (bottom). (*) Anti-
aromatic/radical species are also generated during the reduction
process, see SI.6.</figcaption></figure>

2.3.3 Reduction of Pd(II) complexes. Complexes (R<sub>p</sub>)-<sup>R</sup>[26]1Pd and (S<sub>p</sub>)-<sup>R</sup>[26]2Pd were then treated with an excess of sodium cyanoborohydride in THF.<sup>19a</sup> After usual work-up, TLC and <sup>1</sup>H NMR analysis of the crude mixtures revealed the formation of mainly two species in respectively 1:2 and 1:1 ratio (non-optimized ratio). In both cases, the least polar compound (minor blue spot) fits with the parent Möbius complex (respectively (M)-<sup>M</sup>[28]1Pd and (P)-<sup>M</sup>[28]2Pd), and no trace of the opposite stereoisomer was detected (Figure 6c and SI.6).<sup>34</sup> The more polar compounds (major purple spot) exhibit NMR signatures characteristic of a paratropic ring current, originating possibly from an antiaromatic rectangular 28-π scaffold. However, this latter further converts, during chromatography, to a species featuring a radical character preventing further NMR analysis (see SI.6.1). As a consequence, the 28 → 26 oxidation process thus appears partially reversible, the backward 26 → 28 reduction one following mainly two different pathways.

In contrast to related non-capped hexaphyrin complexes (Scheme 1/Figure 1), the present metalated totemic architectures are C₁ symmetrical (and further exhibit tautomeric forms), which imply that the two *inner* pyrroles are different and generate regioselectivity (Figure 7, blue/red colors). Based on the work of Osuka and coworkers,¹⁹ᵃ an hydride can thus likely attack one or the other C-Pd bonds, but only one of the two *inner* pyrroles can twist to isomerize in a Möbius complex, *i.e.* the one that will bring a C₆F₅ *inward* above the Pd(II) ion (Figure 7, top left). Hydride attack to the opposite C-Pd site (Figure 7, right) likely leads to another reduction pathway, ending with antiaromatic/radical species.³⁵

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Rectangular-to-Möbius isomerization: observed selectivity among the four possible rearrangements concomitant to the reduction of (S<sub>p</sub>)-<sup>R</sup>[26]2Pd (red cross: disfavored compounds due to steric clash).</figcaption></figure>

The reversible pathway is remarkable since the reduction-coupled rearrangement (N2C2→N3C, Figure 6a) proceeds with high stereoselectivity. The R<sub>p</sub>/(S<sub>p</sub>) configuration of the meso-substitution pattern of the rectangular [26]hexaphyrin induces a selective P/(M) Möbius twist configuration, and still behaves as chiral instructing site similarly to the non-metalated hybrids (Figure 1, right).<sup>16d</sup>

The chemical redox behavior of Möbius Ni(II)/Pd(II) com-
plexes is therefore reminiscent to *The Oak and the Reeds* fable’s
message: *better to yield when it is folly to resist.*<sup>36</sup> Indeed, on
the one hand, the Möbius conformation is locked by Ni(II) co-
ordination, leading to a breaking of the π system upon oxidation
(*the Oak behavior*). On the other hand, Pd(II) coordination en-
ables the Möbius conformation to be cancelled and restored in
a stereoselective manner through conformational isomerization
along with the redox process (*the Reeds behavior*). Such a be-
havior defines a new strategy to design redox chiroptical
switches, based on adaptive chemistry combined with a resilient
chiral instructing site. Aiming at practical systems, we next pur-
sued our efforts towards the electrochemical oxido-reduction of
the Möbius metal complexes.

2.3.4 Redox chiroptical switches. To assess these systems as redox chiroptical switches, we further undertook spectroelectrochemical experiments. Solutions of the four metal complexes in DCM, with 0.1 M TBAPF<sub>6</sub> as electrolyte, were subjected to stepwise oxidation by applying increasing potentials (up to ~1 V) and monitored by UV-vis absorption spectroscopy. In the case of (M)-<sup>M</sup>[28]1Ni and (P)-<sup>M</sup>[28]2Ni, absorption spectra
showed the appearance of two consecutive sets of isosbestic points following the sequential formation of the one and two-electron oxidation states (see SI.7.2.1 and SI.7.2.2). The Q-like bands disappear during the first oxidation process, while a global vanishing of the Soret-like band occurs all along both oxidation processes. In the case of $(M)^M[28]1Pd$ and $(P)^M[28]2Pd$, a similar vanishing was observed but, similarly to the NMR titration experiments with MB, the two oxidation processes were found intricate as only one set of isosbestic points was observed in the potential window (Figure SI.7.3.1 and SI.7.3.2). For both type of metal complexes, the backward process was fully reversible upon coming back to the initial potential.

<figure><img src="image_10.png" /><figcaption>**Figure 8.** a,b) Stepwise oxidation of $(M)^M[28]1Ni$, $(P)^M[28]2Ni$, $(M)^M[28]1Pd$ and $(P)^M[28]2Pd$ monitored by ECD spectroelectrochemical experiment. Corresponding chiroptical activity switching monitored at 600 nm for $(M)^M[28]1Ni$ (c) and at 652 nm for $(M)^M[28]1Pd$ and $(P)^M[28]2Pd$ (d) (applied potentials are indicated in inset; detailed conditions described in SI.7).</figcaption></figure>

The same experiments monitored by ECD spectroscopy were carried out for Ni(II) complexes. Stepwise oxidation led to a gradual conversion of the ECD spectra throughout the two oxidation processes, resulting in a broad signature with maximum intensity at 620-630 nm (Figure 8a). Interestingly, $(M)^M$-

$[28]1Ni$ and $(P)^M[28]2Ni$ showed opposite (and reversible) variations indicating that the Möbius twist configuration is fully preserved during two electrons abstraction.³⁷ The two-electron oxidation process showed a gradual change in ECD signatures upon repeated switching between 0 and 0.7 V (Figure SI.7.2.3). Such an observation is in line with the observed instability of the oxidized species generated by MB (*vide supra*). However, switching the applied potential to a narrower range focused on the first oxidation (i.e. 0 and 0.4 V) led to fully reversible and robust ON-OFF behavior (Figure 8c; monitoring at 600 nm, 10 cycles).

For $(M)^M[28]1Pd$ and $(P)^M[28]2Pd$ complexes, the progressive increase in applied potential led to a different chiroptical readout, resulting in a quasi-loss of ECD signal (Figure 8b). This behavior which contrasts with Ni(II) complexes (retaining a chiroptical signature upon two-electron oxidation) is fully compatible with the Möbius-to-rectangular isomerization described above (MB chemical oxidation), leading to a drastic drop of the ECD signature. Importantly, the two-electron oxidation process is fully reversible with no loss of Möbius twist configuration upon coming back to the initial stage. Gratifyingly, switching the applied potential between 0 and 0.8 V led to a robust chiroptical switch with ON-OFF behavior (Figure 8d; monitoring at 652 nm, 20 cycles). The electrochemical oxidation-reduction of the Möbius Pd(II) complexes unraveled the full potential of a chiral instructing site. Indeed, taking advantage of a strong electronic delocalization, the electrochemical reduction prevents undesired reductive pathways inherent to the chemical reduction.

In summary, both Ni(II) and Pd(II) complexes enable redox chiroptical switches, according however to different underlying phenomena: schematically, $[28]\pi$-Möbius ↔ $[27]\pi$-Möbius and $[28]\pi$-Möbius ↔ $[26]\pi$-Hückel, respectively.

### 3. Conclusion

The present contribution has evidenced a new entry in the field of redox chiroptical switches based on a Möbius aromatic $\pi$-system embedded in a unique chiral totem edifice structured by group 10 metal coordination. The difference of behavior between Ni(II) and Pd(II) complexes, reminiscent to *The Oak and the Reeds* fable, illustrates the key role of a metal ion orienting the behavior of a Möbius $\pi$ system to adapt to an electronically unfavorable environment. Key features are:

(i) *a coordination-induced fine restriction of a chiral network:* Ni(II) and Pd(II) coordinations freeze both the Möbius and planar configurations in the $[28]\pi$ state. Upon oxidation, Ni(II) complexes still maintain both configurations, but exhibit an interrupted $[26]\pi$ system due to irreversible water insertion at a meso-position. In contrast, the more labile Pd(II) complexes retain only planar chirality upon oxidation to the $[26]\pi$ state, as they undergo an $N3C \rightarrow N2C2$ coordination rearrangement leading to the cancellation of the Möbius shape in favor of the rectangular conformation;

(ii) *a chiral instructing site resilient to redox processes:* the meso-substitution pattern (planar chirality) acts as an instructing site, enabling not only high chirality induction of the Möbius $[28]\pi$-system, but also the storage of this information in the rectangular $[26]\pi$ state (Pd(II) complexes). Indeed, electrochemical oxidation-reduction process is highly stereoselective, restoring the initial twist configuration dictated by the planar one. Such a reversible shape-shifting process corresponds to a
chiral memory phenomenon where the handedness of a cyclic π system is encoded in a super-edifice and expressed upon changing an electronic state.

These findings open the way to new types of molecular devices for information encoding,³⁸ which is currently under investigation in our laboratories.

## ASSOCIATED CONTENT

**Supporting Information** Full experimental details, materials, methods and spectral data (PDF). Computed structures of metal complexes (xyz). This material is available free of charge via the Internet at http://pubs.acs.org.

## AUTHOR INFORMATION

## Corresponding Author

* stephane.legac@univ-rennes.fr

* mickael.menand@sorbonne-universite.fr

## Funding Sources

We are grateful to the Agence Nationale de la Recherche for financial support (ANR-20-CE07-0026).

## Notes

The authors declare no competing financial interest.

## REFERENCES
Table of Contents

<figure><img src="image_11.png" /></figure>

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¹⁷ For a definition of the $R_p/S_p$ descriptors, see ref. 16b. Moreover, a reviewer has pointed out that the locants should not change when the configuration of the hexaphyrin changes. We have however decided to deviate from the standard convention and use three different locants in order to better distinguish the three types of bridging pattern.

¹⁸ Italic letters between brackets correspond to stereochemical descriptors (topological or planar chirality), and superscripted descriptors (M/R) correspond to Möbius or Rectangular conformations of the hexaphyrin.

¹⁹ a) Inoue, M.; Osuka, A. Redox-Induced Palladium Migrations That Allow Reversible Topological Changes between Palladium(II) Complexes of Möbius Aromatic [28]Hexaphyrin and Hückel Aromatic [26]Hexaphyrin. *Angew. Chem. Int. Ed. Engl.* **2010**, *49* (49), 9488–9491. https://doi.org/10.1002/anie.201005334; b) Kim, J.; Oh, J.; Park, S.; Yoneda, T.; Osuka, A.; Lim, M.; Kim, D. Modulations of a Metal–Ligand Interaction and Photophysical Behaviors by Hückel–Möbius Aromatic Switching. *J. Am. Chem. Soc.* **2022**, *144* (1), 582–589. https://doi.org/10.1021/jacs.1c11705.

²⁰ Blusch, L. K.; Mitevski, O.; Martin-Diaconescu, V.; Präpper, K.; DeBeer, S.; Dechert, S.; Meyer, F. Selective Synthesis and Redox Sequence of a Heterobimetallic Nickel/Copper Complex of the Noninnocent Siamese-Twin Porphyrin. *Inorg. Chem.* **2014**, *53* (15), 7876–7883. https://doi.org/10.1021/ic500460n.

²¹ Mori, S.; Shimizu, S.; Taniguchi, R.; Osuka, A. Group 10 Metal Complexes of Meso-Aryl-Substituted [26]Hexaphyrins with a Metal-Carbon Bond. *Inorg. Chem.* **2005**, *44* (12), 4127–4129. https://doi.org/10.1021/ic050447e.
²² The lower yields for palladium insertion are due to unavoidable formation of bimetallic species, which is not observed with nickel (see: Rath, H.; Tokuji, S.; Aratani, N.; Furukawa, K.; Lim, J. M.; Kim, D.; Shinokubo, H.; Osuka, A. A Stable Organic Radical Delocalized on a Highly Twisted π System Formed Upon Palladium Metalation of a Möbius Aromatic Hexaphyrin. *Angew. Chem. Int. Ed.* **2010**, *49* (8), 1489–1491. https://doi.org/10.1002/anie.200906017.

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²⁴ Jarvis, M. C. Relationship of Chemical Shift to Glycosidic Conformation in the Solid-State ¹³C NMR Spectra of (1 → 4)-Linked Glucose Polymers and Oligomers: Anomeric and Related Effects. *Carbohydr. Res.* **1994**, *259* (2), 311–318. https://doi.org/10.1016/0008-6215(94)84067-9

²⁵ No isomerization occurred upon standing for several days in solution at room temperature.

²⁶ For related nucleophilic attack with oxidized porphyrinoid derivatives, see for instance ref 28d. Addition of an OH group could not be avoided, even in anhydrous conditions, whatever the temperature, solvent or oxidizing agent we tried (see SI.2.9). It is worth mentioning that a faster hydration process occurred in the presence of a base (e.g. lutidine), which is in agreement with a nucleophilic attack by water. The hydration process was still observed using degassed solvents suggesting that dioxygen should not participate in the oxygen insertion. Mechanistic considerations based on the electronic nature of a transient dioxidized species are beyond the scope of this study.

²⁷ These $βπ^in$ signals are shielded to ~ 1 ppm from the chemical shift of a monomeric pyrrole ($δ$ ~ 6.5 ppm). From the DFT-calculated structures of the model compounds, these $βπ^in$ protons point towards the centroid of a facing pyrrole (N-bound to the metal) at a distance of ~ 2.9 Å. We therefore hypothesize that these protons experience the local aromaticity of this facing pyrrole.

²⁸ a) Garcia-Bosch, I.; Sharma, S. K.; Karlin, K. D. A Selective Stepwise Heme Oxygenase Model System: An Iron(IV)-Oxo Porphyrin π-Cation Radical Leads to a Verdoheme-Type Compound via an Isoporphyrin Intermediate. *J. Am. Chem. Soc.* **2013**, *135* (44), 16248–16251. https://doi.org/10.1021/ja405739m; b) Bhuyan, J. Metalloisoporphyrins: From Synthesis to Applications. *Dalton Trans.* **2015**, *44* (36), 15742–15756. https://doi.org/10.1039/C5DT01544H; c) Schweyen, P.; Hoffmann, M.; Krumsieck, J.; Wolfram, B.; Xie, X.; Bröring, M. Metal-Assisted One-Pot Synthesis of Isoporphyrin Complexes. *Angew. Chem. Int. Ed.* **2016**, *55* (34), 10118–10121. https://doi.org/10.1002/anie.201604297; d) Ehudin, M. A.; Senft, L.; Franke, A.; Ivanović-Burmazović, I.; Karlin, K. D. Formation and Reactivity of New Isoporphyrins: Implications for Understanding the Tyr-His Cross-Link Cofactor Biogenesis in Cytochrome c Oxidase. *J. Am. Chem. Soc.* **2019**, *141* (27), 10632–10643. https://doi.org/10.1021/jacs.9b01791.

²⁹ a) Shimizu, S.; Anand, V. G.; Taniguchi, R.; Furukawa, K.; Kato, T.; Yokoyama, T.; Osuka, A. Biscopper Complexes of Meso-Aryl-Substituted Hexaphyrin: Gable Structures and Varying Antiferromagnetic Coupling. *J. Am. Chem. Soc.* **2004**, *126* (39), 12280–12281. https://doi.org/10.1021/ja046102x; b) Mori, S.; Shimizu, S.; Shin, J.-Y.; Osuka, A. Group 12 Metal Complexes of [26]Hexaphyrin(1.1.1.1.1.1). *Inorg. Chem.* **2007**, *46* (11), 4374–4376. https://doi.org/10.1021/ic7004216.

³⁰ Li, C.; Li, Q.; Shao, J.; Tong, Z.; Ishida, M.; Baryshnikov, G.; Ågren, H.; Furuta, H.; Xie, Y. Expanded N-Confused Phlorin: A Platform for a Multiply Fused Polycyclic Ring System via Oxidation within the Macrocycle. *J. Am. Chem. Soc.* **2020**, *142* (40), 17195–17205. https://doi.org/10.1021/jacs.0c09572.

³¹ Lu, T. A Comprehensive Electron Wavefunction Analysis Toolbox for Chemists, Multiwfn. *J. Chem. Phys.* **2024**, *161* (8), 082503. https://doi.org/10.1063/5.0216272

³² An intermediate species with radical character was nonetheless detected by EPR measurements upon addition of 1 eq. of MB to (M)-M[28]1Pd (see SI.5.3.2).

³³ DFT calculated structures of (Rₚ)-R[26]1Pd and (Sₚ)-R[26]2Pd evidence hexaphyrin scaffolds with opposite distortion (see SI.8.2.4).

³⁴ After flash chromatography purification, analytical samples of (M)-M[28]1Pd and (P)-M[28]2Pd were isolated and displayed ECD and NMR spectra identical to those obtained from the metalation of (M)-M[28]1 and (P)-M[28]2 (see SI.6.1 and SI.6.2).

³⁵ The selectivity observed during the reduction process follows the same rational as that described in SI.2.8 for the metalation process. Other reduction pathways cannot be ruled out.

³⁶ It is noteworthy that chemical redox-induced hydroxylation of Ni complexes and isomerization of Pd complexes occur on very different time-scales to those of the CV experiments (hours and minutes vs seconds). Consequently, these induced redox transformations are probably too slow to occur during CV experiments and lead to apparent quasi-reversible redox processes.

³⁷ It is worth to note that the same experiments performed with (M)-M[28]1 and (P)-M[28]2 led to drastic loss of chiroptical activity upon switching the applied potential (see SI.7.1).

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