Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Abstract

Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3D-aromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored hitherto should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. 3D-aromatic molecules have highly symmetric structures (or nearly so), leading to (at least) triply degenerate molecular orbitals, and for tetrahedral or octahedral molecules, an aromatic closed-shell electronic structure with 6n + 2 electrons. Conversely, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel's 4n + 2 rule for each macrocyclic path, yet their π-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., with π-electron counts different from 6n + 2, and they are related to naphthalene. Finally, we identify tetrahedral and cubic π-conjugated molecules that fulfill the 6n + 2 rule and exhibit significant electron delocalization. Yet, their properties resemble those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite the fact that these molecules show substantial π-electron delocalization, they cannot be classified as true 3D-aromatics.

Figure 1

Molecular orbitals of (A) benzene as a 2D-aromatic archetype molecule and (B) the closo-borane B₆H₆²⁻ as a 3D-aromatic archetype molecule. (C) Examples of other forms of aromaticity that earlier also have been labeled as 3D-aromaticity. Of these, C₆₀¹⁰⁺ is spherically aromatic and follows Hirsch’s rule, while the C₂₀H₁₂ cyclophane is face-to-face aromatic and Li(C₅H₅) is aromatic with six interstitial electrons. Two species, C₂₀H₁₂ and Li(C₅H₅), have no triply degenerate molecular orbitals. For molecular orbitals and further discussion, see Discussion A and Figures S3–S5 in the Supporting Information.

Figure 2

Three resonance structures of (A) π-conjugated molecular cage 1 and (B) non-planar dithienothiophene-bridged [34]octaphyrins 2 and 3 in their neutral forms. The total number of π-electrons in the three circuits of 1 is 56 (a 6n + 2 number), while in 2 and 3, they are 42 (a 4n + 2 number) as counted by omitting the sulfurs from the main macrocycle conjugation pathway. The synthetized compounds have R = Mes substitution, yet the computations herein were on the parent compounds (R = H).

Figure 3

(A) General structure of a hydrocarbon that is bicycloaromatic and orbital interactions leading to bicycloaromatic stabilization in longicyclic and laticyclic topologies. (B) One example of a bicycloaromatic (C₇H₇⁺) and a bicycloantiaromatic (C₉H₉⁺) species.

Figure 4

The three circuits of naphthalene. A and B correspond to benzenoid circuits, and C corresponds to the naphthalenic circuit.

Figure 5

(A) Generalized descriptions of the three-linker bicyclic aromatic hydrocarbons labeled as Type A and Type B expanded naphthalenes. (B) Expansion of naphthalene to gradually larger three-dimensional bicyclic structures. (C) Application of the electron count approach for description of macrocycles 1, 1⁶⁺, and 2. The numbers of π-electrons are counted by omitting the sulfurs from the main macrocycle conjugation pathway.

Figure 6

(A) Generalized design of potentially π-conjugated molecules, which can be truly 3D-aromatic. (B) Generalized structures of expanded four-linker (Type C and Type D) polycyclic aromatic hydrocarbons with the number of π-electrons in the arms.

Figure 7

(A) Naphthalene in its planar (4) and puckered (5) structures and the corresponding ACID and EDDB plots. (B) Molecular structure and symmetry of benzoCOT dication (6). nMR = n-membered ring. For full-scale images of the ACID plots for 4–6 as well as naphthalene at other distortion angles, see Figures S7 and S8.

Figure 8

(A) Expanded naphthalenes modeled from 2 and NICS-XY scans for the expanded naphthalenes 7 (blue) and 8 (dark orange), (B) ACID plots with the induced current densities of 7 and 8, and (C) EDDB plots of 7 and 8. For full-scale images of the ACID plots, see Figures S11 and S12. For additional EDDB details regarding 8, see Discussion C, Supporting Information.

Figure 9

(A) Schematic resonance structures of available 4n π-electron circuits in naphthalene dications. Triplet state naphthalene dication in the (B) planar (³4²⁺) and (C) distorted (³5²⁺) structures, both ACID and EDDB plots, and (D) EDDB and ACID plots of 8²⁺. For full-scale images of the ACID plots for ³4²⁺, 8²⁺ and different distortions of ³4²⁺, see Figures S13–S15. For the ACID plot for 7²⁺, see Figure S16.

Chart 1. Dihedral angles (δ) at the Bridgeheads in the Molecular Cages

Figure 10

Stepwise insertion of one more thiopheno ring in each tether of molecular cage 1 leads to species with total π-electron counts of 60 (9, i = i′ = 0), 64 (10, i + i′ = 1), and 68 π-electrons (11, i = i′ = 1) and their hexacations.

Figure 11

EDDB results of (A) 1⁶⁺ and (B) 9⁶⁺, 10⁶⁺ (EDDB plot shown), and 11⁶⁺ in their closed-shell singlet ground states and (C) 1³⁺ and (D) 9³⁺, 10³⁺ (EDDB plot shown), and 11³⁺ in their lowest open-shell quartet states. For the percentage delocalized electrons per cycle, see Table S4 in the SI.

Figure 12

Structures displaying the bond lengths as well as the EDDB plots of C₄(C₈)₆^4– (16), C₄(C₁₂H₁₂)₆^4– (22), C₄(C₁₂H₁₂)₆^– (23) (quartet state), C₈(C₆)₁₂ (18), C₈(C₆)₁₂³⁺ (quartet state, 20), and C₈(C₆H₆)₁₂ (25). Distances are in Å, and angles are in deg. The EDDB results based on all radial π-MOs and on all MOs are both given. Essential molecular orbitals are found in Figures S21 and S22 of the SI.

Figure 13

Highest few occupied molecular orbitals (A) C₄(C₈)₆^4– (16) (T_d symmetric) and (B) C₈(C₆)₁₂ (18) (O_h symmetric) at the optimal B3LYP/6-311G(d,p) geometries. The symmetry of the orbitals is also specified. For full-image orbital plots, see Figures S21 and S22.