Search for: All records

Abstract The chemical reduction of a bilayer spironanographene,spiro‐NG(C₁₃₇H₁₂₀), with Na and K metals in the presence of [2.2.2]cryptand to yield [Na⁺(2.2.2‐cryptand)](C₁₃₇H₁₂₁⁻) (1) and [K⁺(2.2.2‐cryptand)](C₁₃₇H₁₂₁⁻) (2), respectively, is reported. X‐ray crystallography reveals the formation of a new “naked” anion (spiro‐NG_H⁻), in which spirocyclic ring cleavage and subsequent hydrogenation have occurred. Density Functional Theory (DFT) calculations suggest that the generation of the radical anion of the parent nanographene (spiro‐NG^•−), upon electron acceptance from Na and K metals, induces the cleavage of the strained spirobifluorene core. The resulting spin density localizes on a particular carbon atom, previously attached to the spiranic sp³carbon atom, facilitating a site‐specific hydrogenation to afford (spiro‐NG_H⁻). The electrostatic potential map of this anion reveals electron density concentrated at the five‐membered ring of the readily formed indenyl fragment, thus enhancing the aromaticity of the system. Furthermore, nuclear magnetic resonance (NMR) and UV–vis absorption spectroscopy experiments allowed to follow the in situ reduction and hydrogenation processes in detail. 

“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 

The stepwise reduction by 1–3 electrons of a π-expanded pyracylene with K and Rb reveals that in-build core asymmetry affects the spin-density distribution in the mono- and trianion radicals, as confirmed by EPR spectroscopy and DFT calculations. 

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. 

The rectangular cyclobutadiene (CBD, C₄H₄) is a unique moiety for building nonbenzenoid polycyclic conjugated hydrocarbons with interesting electron‐accepting properties. Herein, the investigation on chemical reduction of several CBD‐containing polycyclic hydrocarbons with increasing conjugation length is reported: biphenylene (C₁₂H₈), dimethyl[2]naphthalene (C₂₂H₁₆), and tetramethyl‐dibenzo‐[3]phenylene (C₃₀H₂₂). The two‐step sequential reduction is first demonstrated by in situ spectroscopic investigation and then confirmed by the isolation of single crystals of the reduced products. The X‐ray crystallographic analysis reveals the formation of several mono‐ and doubly reduced products in solvent‐separated and complexed forms. The crystal structures for both neutral parents and corresponding reduced products unravel the changes in bond alternation in each ring of the fused systems. Density functional theory (DFT) and nucleus‐independent chemical shift (NICS) scan calculations reveal that the two‐electron addition reduces the aromatic character in the benzenoid rings but has minor influence on the antiaromatic CBD rings. 

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. 

Abstract The two‐fold reduction of tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT, or tetraphenylene,1) with K, Rb, and Cs metals reveals a distinctive core transformation pathway: a newly formed C−C bond converts the central eight‐membered ring into a twisted core with two fused five‐membered rings. This C−C bond of 1.589(3)–1.606(6) Å falls into a single σ‐bond range and generates two perpendicular π‐surfaces with dihedral angles of 110.3(9)°–117.4(1)° in the1_TR²⁻dianions. As a result, the highly contorted1_TR²⁻ligand exhibits a “butterfly” shape and could provide different coordination sites for metal‐ion binding. The K‐induced reduction of1in THF affords a polymeric product with low solubility, namely [{K⁺(THF)}₂(1_TR²⁻)] (K₂‐1_TR²⁻). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb⁺(18‐crown‐6)]₂[1_TR²⁻] (Rb₂‐1_TR²⁻) and [Cs⁺(18‐crown‐6)]₂[1_TR²⁻] (Cs₂‐1_TR²⁻). Both internal and external coordination are observed inK₂‐1_TR²⁻, while the bulky 18‐crown‐6 ligand only allows external metal binding inRb₂‐1_TR²⁻andCs₂‐1_TR²⁻. The reversibility of the two‐fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. Computational analysis shows that the heavier alkali metals enable effective charge transfer from the1_TR²⁻TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected. 

The chemical reduction of a corannulene-based molecular nanographene, C 76 H 64 (1), with Na metal in the presence of 18-crown-6 afforded the doubly-reduced state of 1. This reduction provokes a distortion of the helicene core and has a significant impact on the aromaticity of the system.