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Ground-state Baird aromaticity in a stable reduced carbon macrocycle

This study reports the first crystallographic characterization of a stable, planarized C18²⁻ macrocycle that adopts a triplet ground state with Baird aromaticity, thereby avoiding the typical loss of aromatic stabilization upon reduction to a 4n π-electron system.

Original authors: Marcus Korb, Seyed Ghazvini, Ingrid Dell, Yikun Zhu, Zheng Wei, Marina Petrukhina, Muhammad Mehboob, Minu Sheeja, Bartosz Szymański, Cina Foroutan-Nejad

Published 2026-07-08

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CC BY 4.0

Original authors: Marcus Korb, Seyed Ghazvini, Ingrid Dell, Yikun Zhu, Zheng Wei, Marina Petrukhina, Muhammad Mehboob, Minu Sheeja, Bartosz Szymański, Cina Foroutan-Nejad

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Flipping the Rules of Stability

Imagine a group of dancers (electrons) moving in a circle. In the world of chemistry, there's a famous rule called Hückel's Rule. It says that if a ring of dancers has a specific number of steps (specifically 4, 8, 12, etc., or "4n" electrons), the dance floor becomes chaotic and unstable. The dancers trip over each other, and the molecule becomes "antiaromatic"—essentially, it's a mess that wants to fall apart.

However, there is a second rule, discovered by a scientist named Baird, which applies when the dancers are in a different "mood" (an excited state). In this mood, the rules flip: those same chaotic numbers (4n) actually make the dance floor stable and harmonious. This is called Baird aromaticity.

Usually, you can only see this stable "Baird" state when the molecule is excited by light. But this paper asks a big question: Can we trick a molecule into having this stable "Baird" state while it's sitting still in its normal, ground state?

The Experiment: Building a Stressed Ring

The researchers built a giant, circular molecule made of carbon. Think of it like a hula hoop made of stiff, springy metal.

The Shape: It's a large ring (18 carbons) decorated with four smaller, square-shaped "knots" (cyclobutene units).
The Problem: Because of the way it's built, the ring is under a lot of tension. It's like a rubber band stretched tight; it wants to snap or twist to relieve the pressure.
The Expectation: Normally, if you add two extra electrons to this ring (a process called "reduction"), it would become a "4n" system. According to the old rules, this should make it incredibly unstable and antiaromatic.

The Surprise: A Triplet Dance

The team added two electrons to the ring using rubidium metal. They expected the ring to become unstable. Instead, something magical happened:

The Spin Flip: Instead of the electrons pairing up neatly (which would make the ring unstable), they decided to spin in the same direction, creating a Triplet State.
The Magic Trick: By spinning in this specific way, the molecule followed Baird's rule instead of Hückel's. The "4n" electron count, which usually causes chaos, suddenly became the source of stability.
The Result: The molecule didn't fall apart. It became a stable, aromatic ring in its ground state. It's the first time a large, uncoordinated carbon ring has been caught doing this "triplet dance" while sitting still.

The Detective Work: Two Different Looks

To prove this, the scientists created two versions of the reduced ring to see how the electrons behaved:

Version A (The "Naked" Ring): They used a molecular cage (cryptand) to trap the metal ions far away from the ring. This let them see the ring's true shape without any interference.
- What they saw: The ring flattened out, like a hula hoop that finally relaxed. The bonds between the atoms became more equal, showing that the electrons were sharing the load evenly around the whole circle.
Version B (The "Hugging" Ring): They used a different tool (crown ether) that let the metal ions sit right next to the ring.
- What they saw: The metal ions pulled on the ring, twisting it slightly. This proved that the ring's shape was flexible and responsive, but the core stability remained.

The Evidence: A Magnetic Current

How do you know the electrons are actually dancing in a circle? You measure the magnetic field they create.

The Analogy: Imagine the electrons are cars driving around a racetrack. If they drive in a coordinated loop, they create a magnetic field.
The Finding: The neutral ring had a small magnetic current (like a few cars driving). But the reduced ring had a massive magnetic current—more than double the original. This confirmed that the electrons were delocalized (shared) all around the ring, creating a strong, stable aromatic system.

The Conclusion

The paper claims to have found a way to stabilize a highly strained carbon ring by forcing it into a "Triplet" state. By doing this, the molecule bypasses the usual instability of having 4n electrons and instead enjoys the stability of Baird aromaticity.

It's like taking a wobbly, unstable chair, adding two specific screws (electrons), and suddenly finding that the chair is now the most stable seat in the room, provided the screws are spinning in a specific direction. This discovery opens a new door for understanding how carbon rings can behave when they are stressed and reduced, proving that the rules of chemistry can be bent if you know the right "mood" (spin state) to put the molecule in.

Technical Summary: Ground-state Baird aromaticity in a stable reduced carbon macrocycle

Problem and Context
Aromaticity is a foundational concept in chemistry, traditionally governed by Hückel's rule, which dictates that planar monocyclic systems with $[4n+2]$ $\pi$ -electrons are aromatic in their electronic ground state, while $4n$ $\pi$ -electron systems are antiaromatic. Consequently, the two-electron reduction of Hückel aromatic molecules typically yields antiaromatic, closed-shell $4n$ systems. While Baird's rule (1972) established that this pattern reverses in the lowest $\pi\pi^*$ triplet ( $T$ ) state—making $4n$ systems aromatic and $[4n+2]$ systems antiaromatic—experimental evidence of ground-state Baird aromaticity in uncoordinated species is rare. Most known examples, such as cyclobutadiene derivatives, escape unfavorable antiaromatic ground states through geometric distortion or stabilization via transition metal complexes. The challenge remains to synthesize a stable, reduced carbon macrocycle that adopts a triplet ground state with Baird aromatic character without relying on metal coordination for stabilization.

Methodology
The authors synthesized a strained, butafulvene-based dehydro[18]annulene macrocycle (compound 3a) featuring a $C_{18}$ backbone and four cyclobutene motifs. The synthesis involved Sonogashira cross-coupling of 1,2-dibromo cyclobutenes with trimethylsilyl acetylene, followed by deprotection and Cu(OAc) $_2$ -mediated oxidative homocoupling.

To investigate the electronic properties of the reduced species, the macrocycle was chemically reduced using rubidium (Rb) metal in anhydrous THF. Two distinct crystalline products were isolated by varying the capping agent:

Solvent-Separated Ion Pair (SSIP): Using [2.2.2]cryptand, yielding $[Rb(cryptand)]_2[3a]$ . This configuration isolates the "naked" dianion $[3a]^{2-}$ from direct counterion coordination.
Contact Ion Pair (CIP): Using 18-crown-6 ether, yielding $[\{Rb(18\text{-crown-6)}\}_2(3a)]$ , where Rb $^+$ ions coordinate directly to the dianion.

Characterization techniques included single-crystal X-ray diffraction (SC-XRD) to analyze structural planarity and bond lengths, UV-vis spectroscopy, cyclic voltammetry (on a monomeric model), and variable-temperature electron paramagnetic resonance (VT-EPR) to determine spin states. Density Functional Theory (DFT) and high-level FIC-NEVPT2 calculations were employed to evaluate singlet-triplet energy gaps, magnetic current densities, and multicenter indices (MCI).

Key Results

Structural Transformation: Upon reduction to the dianion state, the macrocycle undergoes significant planarization. In the SSIP structure, the $C_{18}$ motif and the four-membered rings become flatter, with reduced dihedral angles and bond-length alternation (BLA) within the butadiyne units, indicating enhanced $\pi$ -conjugation. In contrast, the CIP structure shows reduced planarity due to direct Rb $^+$ coordination, which distorts the core.
Electronic Ground State: The "naked" dianion $[3a]^{2-}$ exists in an equilibrium between singlet ( $S$ ) and triplet ( $T$ ) states. VT-EPR measurements on the SSIP crystal revealed a positive singlet-triplet energy gap ( $\Delta E_{S-T}$ ) of 0.67 kcal/mol, indicating a triplet ground state. This was corroborated by DFT and FIC-NEVPT2 calculations, which predicted a triplet ground state with energy gaps ranging from -0.67 to -3.76 kcal/mol (negative values indicating the triplet is lower in energy).
Aromaticity Evidence:
- Magnetic Criteria: Calculations of magnetically induced current densities showed that the neutral singlet species possesses a diatropic current of ~9 nA/T. Upon reduction to the triplet dianion, the current intensity in the central 18-membered ring more than doubles to 20.1 nA/T, a value characteristic of aromatic systems (comparable to or exceeding benzene).
- Spin Density: The spin density in the triplet dianion is extensively delocalized over the entire 18-membered framework, consistent with global $\pi$ -delocalization.
- Multicenter Index (MCI): The MCI for the 18-membered ring remains high (0.54) in both neutral and reduced states, confirming the preservation of global aromatic character.
Stability: The study demonstrates that the strained butafulvene-based macrocycle successfully avoids the formation of a Hückel antiaromatic ground state upon reduction. Instead, it stabilizes a Baird aromatic triplet ground state.

Significance and Claims
The paper claims to present the first crystallographically characterized macrocyclic 20 $\pi$ Baird-aromatic dianion in its electronic ground state. The work demonstrates that a specific structural design—utilizing a strained butafulvene backbone—can stabilize a triplet ground state in a reduced carbon macrocycle, effectively bypassing the typical antiaromatic instability associated with $4n$ $\pi$ -electron systems in the ground state.

The authors emphasize that this system represents a rare instance of ground-state Baird aromaticity in an uncoordinated species, stabilized by approximately 0.67 kcal/mol (experimentally) and up to 3.76 kcal/mol (computationally). The ability to separate the intrinsic structure of the reduced carbon framework from counterion effects (via the SSIP) provides clear evidence that the aromatic stabilization is an intrinsic property of the dianion. This finding expands the understanding of redox-active carbon architectures and offers a new paradigm for designing stable, electronically tunable macrocycles that leverage excited-state aromaticity rules in their ground state.