1027. The stability of heteroaromatic ring systems

Abstract

The stability of heteroaromatic ring systems of the type (RXYR'), (analogous to the borazines), where X and Y are members of Groups I11 and V, respectively, is discussed in terms of molecular-orbital theory. The suggestion that p,,-p,, overlap is only important for the first members of these groups is shown to be incorrect. From considerations of resonance integrals, valence-state Coulomb terms, and inner shell-outer shell repulsions, it is suggested that the A1-N system is especially unfavourable for conjugation but that B-P analogues of the borazines may be stable.THE past decade has witnessed a large increase in the number of ring systems containing hereroatoms but still retaining aromatic character; these systems are conveniently referred to as heteroaromatic. In particular, borazine, B,N,H6, which was once called " inorganic benzene " is now known to be just one example of the extensive isoelectronic sequence (C-C to B-N) which includes B-N analogues of the alkanes, alkenes, and numerous arenes.l However, other analogues involving a Group I11 element (X) and a Group V element (Y) might also be expected in view of the above sequence. It is the purpose of this Paper to discuss the stabilities of heteroaromatic systems of the type (RXYR'),, where R and R' are substituents on atoms X and Y , respectively, and to compare the theoretical conclusions with the available experimental evidence.Direct addition compounds of the type R,XYR',, where R and R' are generally CH,,

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“…A more plausible explanation is that one electron from each phosphorus and nitrogen atom is involved in some type of skeletal -bonding which shortens and strengthens the P-N bond. [165][166] In addition, it is assumed that the nitrogen lone-pair electrons can be donated to phosphorus to form a second (in-plane) coordinate ' system, and this exerts a further strengthening influence on the skeleton.…”