Stabilizing the EN triple bonds in pnictogen mononitrides

Abstract

The viability of a main-group triple bond depends critically on the strength of its π-manifold. Among the 15 possible diatomic homo- and interpnictogens, EE′, (E, E′ = group 15 element), the NN linkage of dinitrogen stands out as one of the strongest triple bonds that exists, whereas the heavier pnictogens form thermodynamically unstable triple bond motifs that either decompose to single-bonded oligomers or extrude N₂, under standard conditions. Considering the fundamentally simple chemistry of a diatomic molecule, coupled with the enticing synthetic challenge of accessing any other EE′ dipnictogen than N₂, we here survey the chemistry of the mononitride family, EN (E = P, As, Sb, Bi). We describe how these unusual bonding motifs were first observed as transient species in the gas phase, later isolated in cryogenic noble-gas matrix experiments, and recently have become the subject of synthetic studies in solution. We delineate strategies to tame the highly reactive EN motifs by incorporating them into adducts with organic fragments or transition metal nodes, enabling studies of their reaction chemistry under controlled conditions. These efforts have opened fascinating perspectives in pnictogen multiple-bond reactivity, spanning radical and closed-shell transformations, electrophilic as well as nucleophilic reactivity of the EN fragments, oxidative addition, oligomerization, cyclization, inorganic aromaticity, and even EN group transfer. Finally, we identify topics in the triple-bond chemistry of pnictogen mononitrides that remain ambiguous or poorly explored, pointing toward future directions in the field.