Tetraazaacenes Containing Four-Membered Rings in Different Oxidation States. Are They Aromatic? A Computational Study

Abstract

A symmetrical tetraazaacene incorporating a central cyclobutadiene ring was calculated in different oxidation (hydrogenation) states, displaying different tautomers and conformers. Geometries, thermodynamics, and electronic properties were computed, and the aromaticity of all these species was calculated on a per ring basis by NICS-scans and NICS-X-scans. The results unveil unexpected and fascinating insights into the complex aromaticity of those compounds, including a formally aromatic (!) cyclobutadiene ring.

The Silacyclobutene Ring: An Indicator of Triplet State Baird-Aromaticity

“…Because of the non-planarity of the COT of 3b(T 1 ) a small kink was observed in its NICS scan, in contrast to that of 3a(T 1 ) ( Figure 6 and Figure S1). Discontinuities in NICS-XY scans due to non-planarities were earlier observed by Schaffroth and co-workers for tetraazaacenes [56]. Thus, based on NICS, we have support for our hypothesis that T 1 (anti)aromaticity influences the reaction energies for the ring-openings of 1, 2, and 3.…”

Phase Transition of [2,2]‐Paracyclophane – An End to an Apparently Endless Story

“…Heteroacenes containing cyclobutadiene units are structurally fascinating materials in their own right, to give enlarged but stable azaacenes, potentially interesting for applications in organic electronic devices 6. 12 The condensation route apparently gives excellent results with stable ortho ‐quinones,13 so we set out to condense substituted ortho ‐diamines 7 – 9 ,7 derived from benzene, naphthalene, and anthracene, with 6 8 and investigate biphenylene‐containing azaacenes 10 – 12 with respect to their optical and solid‐state properties, as well as to compute their magnetic behavior, teasing out the influence of the attached benzocyclobutadiene ring upon the aromaticity of these azaacenes. Reaction of 6 with 7 – 9 in a mix of acetic acid and dichloromethane under inert gas gave the desired condensation products 10 – 12 (68–84 %) as yellow to dark green solids (Scheme ).…”

NICS – Past and Present

“…It should be noted that all of the NICS‐based methods work best for planar systems, similarly to most of the other methods that can separate π and σ effects. Nevertheless, qualitative and sometimes even quantitative data can be obtained for non‐planar systems if correctly and carefully applied …”