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

Abstract

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.

Synthesis of 3a,6a-diaza-1,4-diphosphapentalenes and their halogen derivatives. Specific features of the structure and behavior in solutions

“…We associate the lowest potential (-0.42 V for compound 9) with the presence in the solution of 1,1-isomer (9b, see Scheme 8 and Table 3) containing a PCl 2 group, which is confi rmed by the 31 P{ 1 H} NMR spectra (see Table 3). We believe that the reduction peak of dichloride 1 (-0.63 V) may also correspond to the 1,1-isomer, since similar signals were recorded in the 31 P{ 1 H} NMR spectrum in the DMF solution. For other dichlorides, no formation of 1,1-isomers in the DMF solution was recorded.…”

“…Calculated (%): С, 52.63; Н, 3.31; Cl, 19.42; N, 7.67; Р, 16.97. 1 H NMR (THF-d 8 ), δ: 6.10 (m, 2 Н); 7.40-8.00 (m, 10 Н, 2 Ph). 13 C NMR (THF-d 8 ), δ: 152.4 (CN), 132.4, 131.2, 130.3, 130.0, 106.4 (CP).…”

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

“…Toluene and hexane were thoroughly dried and distilled over sodium prior to use. Annelated 3a,6a-diaza-1,4-diphosphapentalenes 1 and 3 were prepared in accordance with the previously described methods. 1,2,4,5-Tetracyanobenzene and 1,4-dicyano-2,3,5,6-tetrachlorobenzene were purchased from Sigma-Aldrich Chemical Co. and used as received.…”

“…Despite the fact that 3a,6a-diaza-1,4-diphosphapentalenes (DDPs) , belong to the class of annelated azaphospholes, , these compounds exhibit unique properties that are based on the dual nature of their electronic structure. DDPs have an aromatic 10-π-electron system that easily transforms into another electronic system resembling a stabilized singlet phosphinidene when the DDP is polarized by the action of weak Lewis acids or bases (Scheme ).…”

“…DDPs have an aromatic 10-π-electron system that easily transforms into another electronic system resembling a stabilized singlet phosphinidene when the DDP is polarized by the action of weak Lewis acids or bases (Scheme 1). 14 Indeed, most of the chemical properties of a DDP can be explained by considering it as stabilized singlet phosphinidene, possessing nucleophilic properties. Oxidative addition to the phosphorus atom 16−20 of DDP proceeds very easily (Scheme 2), whereas for the usual aromatic azaphospholes such reactions are unknown.…”