Two-electron homoaromatics with heteroatom bridges

Zwitterionic mono- and bis-homoaromatics 2a–f and 5a comprised of positively charged NMe2, P(C6H5)2 or As(C6H5)2 bridges and anionic three-center two-electron (3c2e) delocalized boron heterocyclic units, were prepared and characterized by NMR as well as by X-ray structure analyses. The boron chemical shifts of the trishomoaromatic dianion 12a with an oxygen bridge compare well with those computed ab initio for model 12b. Analysis of the electronic structure of the bishomoaromatic 5u and its anionic analog 11u gives insight into the origin of the trend of increasing effectiveness of BC2, B2C and B3 3c2e bonds: higher electronegativity of carbon vs boron prevents symmetric delocalization in rings with B2C and especially BC2 centers.