Structure-Property Relationships in Unsymmetric Bis(antiaromatics): Who Wins the Battle between Pentalene and Benzocyclobutadiene?†

Abstract

According to the currently accepted structure-property relationships, aceno-pentalenes with an angular shape (fused to the 1,2-bond of the acene) exhibit higher antiaromaticity than those with a linear shape (fused to the 2,3-bond of the acene). To explore and expand the current view, we designed and synthesized molecules where two isomeric, yet, different, 8π antiaromatic subunits, a benzocyclobutadiene (BCB) and a pentalene, are combined into, respectively, an angular and a linear topology via an unsaturated six-membered ring. The antiaromatic character of the molecules is supported experimentally by ¹H NMR, UV-vis, and cyclic voltammetry measurements and X-ray crystallography. The experimental results are further confirmed by theoretical studies including the calculation of several aromaticity indices (NICS, ACID, HOMA, FLU, MCI). In the case of the angular molecule, double bond-localization within the connecting six-membered ring resulted in reduced antiaromaticity of both the BCB and pentalene subunits, while the linear structure provided a competitive situation for the two unequal [4n]π subunits. We found that in the latter case the BCB unit alleviated its unfavorable antiaromaticity more efficiently, leaving the pentalene with strong antiaromaticity. Thus, a reversed structure-antiaromaticity relationship when compared to aceno-pentalenes was achieved.

Figure 1

General strategies to tune the antiaromaticity of pentalenes. (a) Fusion of aryl/heteroaryl rings; (b) introduction of donor/acceptor substituents either on the pentalene core or on the fused aryl ring; and (c) variation of the fusion pattern around the pentalene core in aceno-pentalene derivatives.

Figure 2

Role of bond order in the topology-dependent antiaromaticity of fused pentalenes. (a) Naphtho-pentalenes; (b) biphenyleno-pentalenes.

Figure 3

Combination of benzocyclobutadiene (BCB) and pentalene with different topologies.

Scheme 1. Synthesis of Biphenyleno-pentalene 9 with an Angular Topology

Scheme 2. Synthesis of Biphenyleno-pentalene 14 with a Linear Topology

Figure 4

(a) Partial ¹H NMR spectra of compounds 9 and 14 (CDCl₃, 500 MHz, room room temperature (rt)). Protons were assigned by the one-dimensional-nuclear over-Hauser effect spectroscopy (1D-NOESY) technique. (b) Reported monoaryl pentalene structures for comparison of the chemical shifts of the pentalene proton (all measured in CDCl₃).

Figure 5

(a) X-ray structure of compound 9 and the corresponding bond lengths; ORTEP representation of 9 is drawn at the 50% probability level; (b) layered structure of compound 9 in the crystalline state presented from the view of the crystallographic b-axis. Blue lines represent intermolecular short contacts (≤ sum of van der Waals radii + 0.1 Å). (c) Calculated bond lengths of compounds 9 and 14.

Figure 6

(a) UV–vis spectra of compounds 9 and 14 (in CHCl₃); (b) UV–vis spectra of naphtho-pentalenes 15 and 16 (in CHCl₃).

Figure 7

Calculated HOMO and LUMO orbitals of biphenyleno-pentalenes 9 and 14 and naphtho-pentalenes 15 and 16.

Figure 8

Calculated HOMO and LUMO orbitals of BCB, pentalene, exo-BCB, and exo-pentalene.

Figure 9

(a) NICS-XY scans of biphenyleno-pentalenes 9′ and 14′ in the S₀ (solid line) and T₁ (dashed line) states. (b) NICS-XY scans of naphtho-pentalenes 15′ and 16′ in the S₀ (solid line) and T₁ (dashed line) states. (c) ACID plots of biphenyleno-pentalenes 9′ and 14′ in the S₀ and T₁ states. (d) ACID plots of naphtho-pentalenes 15′ and 16′ in the S₀ and T₁ states. (For higher-resolution images, see Section S4.2.1, Supporting Information.) Blue and red arrows correspond to diatropicity and paratropicity, respectively. The width of the arrow denotes the strength of the ring current.