A Neutral and Aromatic Boron‐Rich Inorganic Benzene

Abstract

A neutral 1,4‐diaza‐2,3,5,6‐tetraborinine derivative (3) featuring a 6π ring system has been synthesized and structurally characterized. The prospective aromatic nature of 3 was assessed by single‐crystal X‐ray analysis, NMR, and UV/Vis absorption spectroscopy as well as computational studies. Furthermore, preliminary reactivity investigations showed that 3 does not only react with diphenyl acetylene but also readily captures CO2 in a [4+2] cycloaddition under ambient conditions.

Inorganic Benzene Valence Isomers

“…[9,14] In 2020, the Kinjo group reported a BN derivative I-BN3 containing four boron and two nitrogen atoms. [15] Neutral B 4 N 2 ring with the six π-electron system was prepared by the reduction of the tetrachloro-substituted precursor with potassium graphite (KC 8 ) in the presence of trimethyl phosphine (PMe 3 ) (Scheme 2). An X-ray diffraction study shows that I-BN3 adopts C 2 symmetry and the B 4 N 2 six-membered ring is completely planar.…”

“…[49] I-BN3 shows an absorption band with a maximum at a wavelength of 607 nm, which is consistent with the calculated HOMO (π)-LUMO (π*) gap (2.45 eV). [15]…”

“…The reported NICS values of inorganic benzenes (real or model molecule) and benzene are summarized in Table 4. [15,35,51] Model compounds (the substituents are replaced with H atoms) are named as I-E' (E represents the elements involved in). The NICS(0) and NICS(1) values of benzene are À 8.2 and À 10.4, respectively.…”

“…The boron-rich system I-BN3 indicates a moderate NICS(1) value (À 4.59), which is probably due to the less polarized nature of the B 4 N 2 ring system of I-BN3 in comparison to I-BN1. [15] Interestingly, I-BN3 can be viewed as a benzene dication isostere supported by two Lewis bases, which system was proposed by Hoffmann et al [52] I-BP' is substantially aromatic according to the NICS(1) value (À 6.9). [51a] On the other hand, I-AlN' shows a small NICS(1) value (À 0.6), indicating a weak aromatic character.…”

Aromatic nature of neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives

“…21 The extant theoretical study on the aromaticity of inorganic benzenes mainly focuses on the above-mentioned (XY) 3 and E 6 systems, because those types of analogues have been isolated to date. Recently, we have reported the synthesis of a neutral B 4 N 2based benzene analogue, namely 1,4-diaza-2,3,5,6-tetraborinine derivative 3, 27 being the rst examples of the Lewis base-stabilized X 4 Y 2 system. 6 Preliminary theoretical investigations including NICS, natural bond orbital (NBO), and anisotropy of the currentinduced density (ACID) analysis suggested the pronounced aromatic nature of 3.…”