Neutral Bishomoaromatic Semibullvalenes

Abstract

Isolobal substitution of CH units by boron carbonyl groups (BCO) at C2,6 and C2,8,4,6 in semibullvalene favors the delocalized neutral bishomoaromatic systems substantially. The homoaromaticity is documented by the computed diatropic nucleus independent chemical shifts (NICS). In addition, BCO substitution can result in low-lying triplet and open-shell singlet states. In contrast, substitution at C1,5 more than doubles the related Cope rearrangement barrier. The C2,6 and C2,8,4,6 BCO-substituted barbaralanes, barbaralones, and bullvalenes have substantially reduced barriers.

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

“…Over the last few decades, researchers have discovered and explored a wealth of structures based on 1 ,3, 4 the intriguing fundamental goal being the experimental identification of a semibullvalene having a delocalized bishomoaromatic ground state. Strategies towards this goal have mainly been twofold: (a) introduction of substituents5–15 or coordinating entities16, 17 aiming at electronic stabilization of 1b relative to 1a and (b) annelation of small rings7, 18–24 which by strain destabilizes 1a relative to 1b . Both strategies have been pursued by experimental and computational means, the latter proving to be a valuable tool in the search for homoaromatic structures 5–8, 11–17, 19–21, 23.…”

Carbon boronyls: Species with higher viable possibility than boron carbonyls at the density functional theory

“…The binding energies of other (CBO) n species relative to different combinations of the quadruplet CBO (1) and singlet (CBO) 2 (2) 9,10 and their charged ions are tabulated in Table 1. It should also be mentioned that all carbon boronyls have higher lowest vibrational frequencies (25) 88 (58) 420 (37) 439 (137) 713 (13) 1242 (99) 2067 (151) than the corresponding boron carbonyls.…”

“…For example, in D 5h (CBO) 5 Ϫ , C, B, and O atoms carry the natural charges of Ϫ0.45, ϩ1.06, and Ϫ0.80͉e͉, respectively, corresponding to the electron configurations of C 2 to form the three in-plane bonds and contributes one unpaired electron in p z orbital to form the delocalized molecular orbitals (MOs). Figure 2 shows the pictures of the delocalized MOs of the carbon boronyl series.…”

“…The infrared resonance (IR) active vibrational frequencies of D nh (CBO) n are summarized in Table 2 to facilitate future spectroscopic characterization of these isomers. Concerning the stability of these high symmetry D nh structures, D 6h (CBO) 6 has the binding energies of 3.68 eV (84.9 kcal/mol) with respect to the reaction of 3(CBO) 2 ϭ (CBO) 6 and D 5h (CBO) 5 Ϫ has a much higher binding energy of 11.75 eV (271.0 kcal/mol) with respect to the process of 2(CBO) 2 …”

STRUCTURE AND STABILITY OF MONOCYCLIC $({\rm CH})_{4-n} ({\rm BL})_{n}^{2-}$(L = CO, N2, CS) DIANIONS AND THEIR DILITHIUM COMPLEXES

“…24 In contrast to CH, BCO has the possibility for back donation of electrons into the anti bonding orbital of CO resulting in the stabilization of the delocalized structures. 20 On this basis, we are interested in the stabilizing effect of the BCO group in the substituted (CH) 4−n (BCO) 2− n (n = 1-4) systems, which are isolobal with (CH) 2− 4 . Due to the isolobal property with CO, we are also interested in the structure and stability of N 2 and CS substituted (CH) 4−n (BN 2 ) 2− n and (CH) 4−n (BCS) 2− n (n = 1-4).…”