Peri-Fused Six-Membered Cyclic Nitrenium Ions (PCNIs): Electronic Structure and Aromaticity

Cyclic nitrenium ions (CNIs) with five and seven-membered rings are well known, but the CNIs with six-membered rings are rare. Only a few are known as peri-fused six-membered cyclic nitrenium ions (PCNIs) and they are reported to be nitrogen Lewis acids. Preliminary electronic structure analysis indicated that the electronic properties of PCNIs somewhat deviate from the other CNIs (five or seven-membered). Further, these ring systems proved to be non-aromatic (in the triazine ring), whereas the five and seven-membered analogs are aromatic. In this work, an electronic structure analysis was carried out on the newly designed PCNIs to understand their characteristic features. Several analogs of these species were designed to identify PCNIs with improved electronic stability. The PCNIs are electrophilic because of the empty p orbital and the formal positive charge at the central nitrogen atom. The Lewis acidic character of known and designed PCNIs has been evaluated in terms of hydride ion affinity (HIA), fluoride ion affinity (FIA), ΔH_PMe3 and ΔH_NHC values. This work helped identify a set of new PCNIs whose electronic stability and Lewis acidity parameters are significantly high. Further, a few of these systems were found to be aromatic in all three rings with notable negative NICS(1) values.