Dual Reactivity of 3a,6a-Diaza-1,4-diphosphapentalene: π‑Donor versus n‑Donor

Herein, we present the synthesis, single-crystal X-ray structures, and spectroscopic properties for the 1:1 donor–acceptor complexes of 1,2,4,5-tetracyanobenzene (TCNB) with annelated 3a,6a-diaza-1,4-diphosphapentalenes (DDPs) based on cyclohexanone azine (2) and tetralone azine (4). These are the first complexes of an organic π-acceptor with donor phosphorus heterocycles. According to the X-ray study, the DDPs and TCNB molecules are alternately stacked with interplanar distances of 3.335 and 3.404 Å for 2 and 4, respectively, which are suitable for intermolecular π···π interactions. The bond lengths and angles in the component molecules agree with values for neutral species, and the infrared spectra indicate a very slight degree of ionicity. The estimated HOMO–LUMO gap from the onset of optical absorption (1.40 eV) is in agreement with the band gap estimated from the density functional theory calculations for 2 (1.47 eV). By contrast, in a reaction with the related electron acceptor, tetrachloroterephthalonitrile, the DDPs proved to be donors of lone electron pairs in a nucleophilic aromatic substitution reaction of chlorine atoms demonstrating the duality of their electronic nature.