Single─Not Double─3D-Aromaticity in an Oxidized Closo Icosahedral Dodecaiodo-Dodecaborate Cluster

Abstract

3D-aromatic molecules with (distorted) tetrahedral, octahedral, or spherical structures are much less common than typical 2D-aromatic species or even 2D-aromatic-in-3D systems. Closo boranes, [B_nH_n]^2- (5 ≤ n ≤ 14) and carboranes are examples of compounds that are singly 3D-aromatic, and we now explore if there are species that are doubly 3D-aromatic. The most widely known example of a species with double 2D-aromaticity is the hexaiodobenzene dication, [C₆I₆]²⁺. This species shows π-aromaticity in the benzene ring and σ-aromaticity in the outer ring formed by the iodine substituents. Inspired by the hexaiodobenzene dication example, in this work, we explore the potential for double 3D-aromaticity in [B₁₂I₁₂]^0/2+. Our results based on magnetic and electronic descriptors of aromaticity together with ¹¹B{¹H} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B₁₂I₁₂] and [B₁₂I₁₂]²⁺, behave as doubly 3D-aromatic compounds. However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I₁₂ shell, in contrast to the 10σ-electron I₆²⁺ ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system. Therefore, the [B₁₂I₁₂]^0/2+ species cannot be considered as doubly 3D-aromatic.

Scheme 1. (a) Schematic Illustration of the Circular σ-Delocalization in Dicationic Hexahalo- or Hexachalco-Substituted Benzene Leading to Hückel σ-Aromaticity; (b) Schematic Representation of the Circular σ-Delocalization in Tricationic Hexabromotropylium Species in Its Lowest-Lying Triplet State Resulting in Baird σ-Aromaticity; (c) Double Aromaticity in C₆E₆²⁺ Requires the Opening of an Electronic Hole by Double Oxidation to Generate σ-Delocalization

Figure 1

Schematic representation of the ¹¹B NMR spectra from samples in acetone-d₆ solutions with the peak assignments for unsubstituted o-carborane (a) and some iodinated derivatives: 9,12-I₂-o-carborane (b), 8,9,10,12-I₄-o-carborane (c), 4,5,7,8,9,10,11,12-I₈-o-carborane (d), and 3,4,5,6,7,8,9,10,11,12-I₁₀-o-carborane (e). The peak assignment was unambiguously done by means of a two-dimensional ¹¹B{¹H}–¹¹B{¹H} COSY NMR spectrum. On the right is shown the mean ¹¹B{¹H} NMR chemical shift for each compound. Figure drawn using the data of ref (124).

Figure 2

B–B and I···I bond distances range (in Å) of closed-shell singlet [B₁₂I₁₂]^2-/0/2+ and triplet ³[B₁₂I₁₂].

Figure 3

NICS scan (ppm) from the center of the boron cluster to the middle of the closest I₃ (or H₃) three-membered ring for [B₁₂I₁₂]^2– (singlet), [B₁₂I₁₂] (singlet and triplet), [B₁₂I₁₂]⁺ (quartet), and [B₁₂I₁₂]²⁺ (singlet) clusters. Comparison to [B₁₂H₁₂]^2– (singlet) is included. Distances in Å.

Figure 4

Current-density susceptibility in the magnified area (black rectangle) of the ¹[B₁₂I₁₂]^2–, singlet and triplet [B₁₂I₁₂], and 1[B₁₂I₁₂]²⁺ computed in a plane at 0.8 Å (see Figure S13). The color scale corresponds to the strength of the modulus of the current-density susceptibility in the range 0.0001 (dark red) to 0.4 (white) nA/(TŲ).

Figure 5

Isosurfaces (isocontour 0.007 e) of the electron density of delocalized bonds (EDDB) for [B₁₂I₁₂]^2–, [B₁₂I₁₂] singlet and triplet states, and [B₁₂I₁₂]²⁺, as well as [B₁₂H₁₂]^2–, C₆I₆, and [C₆I₆]²⁺ for comparison purposes. Numerical results correspond to the EDDB_G population of the whole system (black), boron/carbon atoms (gray), and iodine (purple) atoms separately.