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.

The Nature of P(σ2λ3↔σ2λ1) Dualism: 3a,6a-Diaza-1,4-diphosphapentalene as a Form of Stabilized Singlet Phosphinidene.

The current study provides a clear understanding of the chemical properties of annelated 3a,6a-diaza-1,4-diphosphapentalenes (DDPs), which are best viewed as stabilized singlet phosphinidenes. It was found that DDPs undergo reversible oligomerization in solution, which provides 1,2,3-diazaphosphole-substituted cyclotetraphosphines, isolated and characterized by X-ray crystal structure analysis. Transformation of the 10-π-electron heteropentalene system into a stabilized phosphinidene occurs when the P-N bond is lengthened, which is facilitated by weak Lewis acids and bases. DFT calculations show that the lowest unoccupied molecular orbital of DDP has a high localization at the phosphorus atom when the N-P bond distance reaches the value of 2.53 Å. Oligomerization is a concentration-dependent process. Increasing the concentration of the monomer solution promotes tetramer formation, and vice versa: a strong dilution leads to a monomer. Tetramer solutions are photosensitive and yield monomers upon irradiation. The new annelated DDP 2 and its dichloro precursor 4 based on tetralone azine were synthesized. 4 exists in the solid state as a 1,4-dichloro isomer, while in solution it gives an equilibrium mixture of 1,1- and 1,4-isomers. Cyclohexanone-annelated diazadiphosphapentalene 1 forms a weak complex (1:1) with Ph3B, showing an elongated P-B bond (2.114(12) Å), which is noticeably larger than the sum of the covalent radii of the elements.