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Abstract Chemical reduction of a [4]cumulene with cesium metal was explored, and the structural changes stemming from electron acquisition are detailed using X‐ray crystallography. It is found that the [4]cumulene undergoes dramatic geometric changes upon stepwise reduction, including bending of the cumulenic core and twisting of the endgroups from orthogonal to planar. The structural deformation is consistent with early theoretical reports that suggest that the twisting should occur upon reduction of both even and odd [n]cumulenes. The current results, on the other hand, are inconsistent with a previous experimental study of a [3]cumulene in which the predicted twisting is not observed upon reduction. DFT calculations reveal that the barrier to deformation is an order of magnitude lower in a [3]cumulene than a [4]cumulene, allowing the barrier to be overcome in the solid‐state. 

Abstract π-Conjugated macrocycles behave differently from analogous linear chains because their electronic wavefunctions resemble a quantum particle on a ring, leading to aromaticity or anti-aromaticity. [18]Annulene, (CH)₁₈, is the archetypal non-benzenoid aromatic hydrocarbon. Molecules with circuits of 4n + 2 π electrons, such as [18]annulene (n = 4), are aromatic, with enhanced stability and diatropic ring currents (magnetic shielding inside the ring), whereas those with 4nπ electrons, such as the dianion of [18]annulene, are expected to be anti-aromatic and exhibit the opposite behaviour. Here we use¹H NMR spectroscopy to re-evaluate the structure of the [18]annulene dianion. We also show that it can be reduced further to an aromatic tetraanion, which has the same shape as the dianion. The crystal structure of the tetraanion lithium salt confirms its geometry and reveals a metallocene-like sandwich, with five Li⁺cations intercalated between two [18]annulene tetraanions. We also report a heteroleptic sandwich, with [18]annulene and corannulene tetraanion decks. 

“Redox Properties of Nanographenes”, Y. Zhu, M. A. Petrukhina. Chapter 20 in “Molecular Nanographenes: Synthesis, Properties and Applications”, Edited by N. Martín and C. Nuckols, Wiley-VCH, 2025, 449-482 

An overview of structural responses of helicenes with increasing dimensions and complexity to stepwise electron addition reveals charge- and topology-dependent outcomes ranging from reversible to irreversible core transformations and site-specific reactivity. 

Anionic forms of a macrocyclic cyclophane were crystallized by treating the neutral hydrocarbon with alkali metals (Li, Na and K). The di-anions show decreased bond-length alternation, consistent with global aromaticity, whereas the anti-aromatic tetra-anion has a low-symmetry geometry. 

Stepwise deprotonation of truxene with alkali metals affords truxenyl anions with different charges, exhibiting core curvature dependence on charge and metal binding. UV-vis and PL studies reveal charge-dependent optical properties, which is further supported by DFT calculations. 

Chemical reduction of highly-twisted 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (C74H46, 1) was investigated using Li and Cs metals as the reducing agents. The Cs-induced reduction of 1 in the presence of 18-crown-6 ether enabled the isolation of a solvent-separated ion pair (SSIP) with a “naked” monoanion. Upon reduction with Li metal, a double reductive dehydrogenative annulation of 1 was observed to afford a new C74H422– dianion. The latter was shown to undergo a further reduction to C74H424– without additional core transformation. All products were characterized by single-crystal X-ray diffraction and spectroscopic methods. Subsequent in-depth theoretical analysis of one vs. two and four electron uptake by 1 provided insights into how the changes of geometry, aromaticity and charge facilitated the core transformation of twistacene observed upon two-fold reduction. These experimental and theoretical results pave the way to understanding of the reduction-induced core transformations of highly twisted and strained π-systems.