Rupturing aromaticity by periphery overcrowding

Aromaticity underpins molecular structure and stability, typically manifesting as cyclic π-electron delocalization with diatropic ring currents and equalized bond lengths. Non-planar aromatic frameworks (helicenes, twistacenes, Möbius aromatics, nanohoops, nanobelts, warped nanographenes) can accommodate substantial geometric distortion while retaining aromatic character. However, the experimental limit at which strain outweighs aromatic stabilization energy (ASE) remains unclear. The tropylium cation, an aromatic seven-membered-ring carbocation with greater conformational flexibility than benzene, is an ideal platform to probe this limit. The central research question is: to what extent can periphery overcrowding distort a π-extended tropylium ring before aromaticity is sacrificed, and what structural/energetic pathways mediate conversion between aromatic tropylium and non-aromatic Dewar tropylium isomers?

Prior studies established that aromatic systems maintain ring currents despite notable distortions, e.g., bent benzene rings in [n]paracyclophanes showing only modest reductions in ring current, and highly strained annulated benzenes exhibiting geometric distortion yet retaining aromaticity. Dewar benzene isomerization pathways and barriers are well characterized, proceeding via conrotatory electrocyclic ring opening through Möbius-like intermediates with high activation barriers. Tropylium chemistry has been leveraged in catalysis, dyes, and PAHs, with its larger ring and smaller substituent angles enabling greater strain-induced deformation than benzene. Nevertheless, precisely tuning strain to balance against ASE and observing the consequences on aromaticity had not been experimentally achieved prior to this work.

• Computational modeling: Density functional theory with B3LYP functional including D3 (GD3BJ) dispersion correction and Becke–Johnson damping, 6-31G(d) basis set, and CH2Cl2 solvent via PCM (IEFPCM). Energetics and geometries for tropylium (TP), Möbius-like tropylium (MT/MT′), and Dewar tropylium (DT) were computed for (CH)7+, and π-extended systems Naph (acenaphthylene-annulated) and Phen (phenanthrene-annulated), as well as designed overcrowded derivatives 1–4. Intrinsic reaction coordinate (IRC) calculations from optimized transition states mapped the TP↔MT↔DT pathways and identified geometric descriptors (H5–C5–C4–H4 torsion; C1–C4 distance). Relaxed potential energy surface scans varied the C4–C2–Cβ–Cβ′ torsion (1° increments) to quantify twisting penalties in fluoranthene, Naph, and pentaphenyl Naph (2). Aromatic stabilization energies (ASEs) were estimated via hyperhomodesmotic reactions (isomerization stabilization energy methods) and cross-checked with M06-2X. Aromaticity indices included NICS(±1) and NICSzz(±1) and ACID current-density visualizations; electron delocalization quantified by EDDB for local (tropylium ring) and global (π-extended framework) paths.
• Target selection: Guided by computed Gibbs energy differences ΔG between TP and DT isomers for substituent patterns, choosing heptaphenyltropylium 1, perphenylated acenaphthyl derivative 2, α,α′-diethyl analogue 3, and phenanthrene-annulated 4 to span TP-favored, borderline, and DT-favored regimes.
• Synthesis: Key precursors 5/6 prepared from diphenylacetylene. 1 obtained via Diels–Alder of 5 with tetracyclone to give cycloheptatriene 7 then oxidation with ICl to 1–ICl2. Analogous annulated cycloheptatrienes and 8 (diethyl) formed using phencyclone, acecyclone, or diethylacecyclone. For 2 and 4, tropones 9/10 (from 6 with acecyclone/phencyclone) underwent PhMgBr addition (introducing phenyl and tertiary alcohol), followed by elimination with Et3O+SbCl6− to give 2-SbCl6 and 4-SbCl6. For 3, stepwise oxidation of 8: mCPBA epoxidation (CHCl3, reflux), isolation of epoxide S1, then BBr3 under inert conditions to give 3 (isolated as 3–BBr4; low yield prevented solution NMR).
• Structural characterization: Single-crystal XRD determined solid-state structures of 1–4 and selected intermediates (with CCDC depositions) and measured torsion and boat angles (θbow, θstern; C4–C2–Cβ–Cβ′). Solution NMR (1H) characterized symmetry and dynamics; variable-temperature NMR and 1H–1H EXSY probed exchange in 4. Lineshape analysis near coalescence furnished exchange rates, and Eyring analysis gave ΔH‡, ΔS‡, and ΔG‡ at 298 K. Hydride trapping with NaBH4 intercepted equilibrating species (yielding bicyclo[3.2.0]heptadienes anti-11/syn-11 and cycloheptatriene 12) to evidence the presence of 4-TP at equilibrium.

• Computational baseline: Tropylium (CH)7+ isomerization mirrors benzene’s Dewar↔benzene pathway but with lower barrier due to seven-membered-ring flexibility. Calculated ASEs for (CH)7+, Naph, and Phen are ≈ −50 kJ mol−1 (about half of benzene’s −98.5 kJ mol−1).
• Twisting energetics: Periphery overcrowding in 2 greatly flattens the twisting energy well; two conformers at φ(C4–C2–Cβ–Cβ′) ≈ 14.3° and 0.4° predicted and observed crystallographically (18.4° and 0.4°). Barriers to access Möbius-like intermediates are drastically reduced by overcrowding: ΔG‡(2→2-MT) ≈ 96 kJ mol−1 versus 259 kJ mol−1 for Naph; for 4, 4-MT and 4-MT′ lie only 67 and 28 kJ mol−1 above 4-TP, compared with 232 and 209 kJ mol−1 for Phen.
• Structural distortions: XRD shows progressive deformation from 1 to 3. In 3–BBr4, an extreme twist φ = 45.2° and boat conformation with θbow = 13.0°, θstern = 29.0° were measured, closely matching DFT (45.4°). Despite distortion, 3 remains an aromatic tropylium in the solid state.
• Thermodynamic tuning: Substituents at α/α′ (abut phenyls) strongly tune TP–DT ΔG: α,α′-dimethyl lowers gap to 42 kJ mol−1; α,α′-diethyl lowers to 15 kJ mol−1. Increasing to phenanthrene-annulation with five proximal phenyls (4) inverts preference to DT by 5.3 kJ mol−1.
• Dewar tropylium isolation and dynamics: 4–SbCl6 crystallizes as Dewar tropylium (first isolation of a non-aromatic valence isomer of a tropylium derivative). Solution 1H NMR reveals loss of C2 symmetry and temperature-dependent broadening due to rapid exchange between degenerate Dewar isomers via aromatic 4-TP. Lineshape/Eyring analysis gives ΔH‡ = 50.0 kJ mol−1, ΔS‡ = −41.5 J K−1 mol−1, ΔG‡(298 K) = 62.4 kJ mol−1. Equilibrium at 298 K is ~90:10 in favor of DT (K ≈ 0.12 for 4-TP), consistent with DFT ΔG = 5.3 kJ mol−1.
• Trapping experiment: NaBH4 reduction of equilibrating 4 affords anti-11:syn-11:12 in 38:58:4, with the cycloheptatriene 12 indicating interception of 4-TP present at equilibrium.
• Aromaticity metrics: ACID plots show diatropic ring currents for 1–3 and 4-TP. NICS(±1) values −14.0 to −18.9 support local aromaticity. EDDB reveals modest reduction in local tropylium aromaticity under distortion: e.g., Naph 3.08 to 3 (2.69; ~13% drop). Overall, 3 and 4-TP retain substantial aromatic character despite large distortions.

The study demonstrates that peripheral overcrowding can tune the delicate balance between strain energy and aromatic stabilization in π-extended tropylium systems. Up to a substantial degree of geometric distortion (helical twist and boat conformations), aromaticity persists with only modest reductions in delocalization, as evidenced by ACID, NICS, and EDDB. However, when steric strain is further increased—particularly through phenanthrene annulation and multiple proximal phenyl groups—the strain surpasses ASE, resulting in rupture of aromaticity and rearrangement to a non-aromatic Dewar tropylium. The ability to isolate 4 as a DT isomer and to observe its rapid equilibrium with an aromatic, highly twisted 4-TP at room temperature provides direct experimental insight into the limit at which aromaticity gives way to strain. The marked reduction in isomerization barriers upon overcrowding (e.g., from ~297 kJ mol−1 in Phen to ~62 kJ mol−1 observed for 4/4-TP exchange) underscores how steric design can kinetically and thermodynamically access high-energy valence isomers. These findings address the central question by mapping the threshold of deformation tolerated by an aromatic carbocycle and establishing a controllable, dynamic aromatic↔non-aromatic equilibrium, relevant to designing non-planar PAHs and functional materials operating at the edge of aromaticity.

Peripheral overcrowding provides a controllable handle to distort π-extended tropyliums, flattening twisting energy surfaces and lowering barriers to Möbius-like and Dewar isomers. Highly twisted, yet still aromatic, tropyliums (e.g., 3 with a 45.2° twist) can be realized, while extreme overcrowding tips the balance to Dewar tropylium (4), establishing a fast, room-temperature equilibrium with an aromatic form (~90:10 DT:TP, ΔG‡ ≈ 62 kJ mol−1). Despite large geometric deformations, aromaticity metrics show only modest reductions for the aromatic isomers, indicating that rearrangement to Dewar is driven chiefly by strain exceeding ASE. This work defines experimental limits of aromaticity under steric deformation and offers design principles for non-planar PAHs and responsive systems based on dynamic valence isomerism. Future directions include expanding substituent patterns and ring topologies to generalize strain–aromaticity tradeoffs, time-resolved studies of isomerization dynamics, and leveraging these equilibria in stimuli-responsive materials and catalysis.

• Structural data are predominantly solid-state; crystal packing and counterion proximity can influence measured geometries, though agreement with solution-phase calculations is good.
• The highly strained cation 3 was obtained in low yield and high reactivity precluded solution-state spectroscopic characterization, limiting dynamic and electronic analysis in solution.
• DFT level (B3LYP-D3BJ/6-31G(d)/PCM) may overestimate aromatic stabilization for larger circuits; alternate functionals (e.g., M06-2X) were consulted for ASE but comprehensive high-level benchmarking remains limited.
• The study focuses on specific annulated frameworks and substituent sets; generality across broader non-benzenoid aromatics and heteroatom-containing systems is inferred but not directly tested.
• Temperature range for dynamic NMR was constrained near coalescence; full kinetic/thermodynamic mapping across solvents and counterions could refine parameters.