A computational study of azaphospholes: anions and neutral tautomers

Reactivity of Sodium Pentaphospholide Na[cyclo-P5 ] towards C≡E (E=C, N, P) Triple Bonds

A diglyme solution of Na[cyclo-P5 ] (1) reacts with alkynes and isolobal nitriles and phosphaalkynes to afford the otherwise elusive (aza)phospholide anions 2 a-c, 4 a,b, and 6. The reaction of Na[cyclo-P5 ] with alkynes and nitriles was studied by means of DFT methods, which suggested a concerted mechanism for the formation of 2 a and 4 b. The anions 2 a-c, 4 a,b, and 6 coordinate in an η5 -fashion towards FeII to give the sandwich (aza)phosphametallocenes 3 a-c, 5 a,b and 7 in moderate to good yields. The new compounds were characterized by means of multinuclear NMR spectroscopy, single-crystal X-ray diffraction and cyclic voltammetry.

Synthesis of 1,2,4-azadiphosphole derivatives based on vanadium-catalyzed [2+2+1] cycloaddition reactions of azobenzenes with phosphaalkynes

A new synthetic method is described to construct 1,2,4-azadiphosphole derivatives based on vanadium-catalyzed [2+2+1] cycloaddition reactions. Reactions of azobenzenes as nitrogen sources with phosphaalkynes as phosphorous counterparts in the presence of VCl₂(thf)₂ as a catalyst afford the corresponding 1,2,4-azadiphospholes.

Vanadium-catalyzed [2+2+1] cycloaddition reactions opened a new access to phosphorous-heterocycles.

Stabilization of the Dual-Aromatic cyclo-N5- Anion by Acidic Entrapment

Pentazole anion, the best candidate for full-nitrogen energetic materials, can be isolated only from acidic solution for unclear reasons, which hinders the high-yield realization of a full-nitrogen substance with higher energy density. Herein, we report for the first time the discovery of the dual aromaticity (π and σ) of cyclo-N₅^-, which makes the anion unstable in nature but confers additional stability in acidic surroundings. In addition to the usual π-aromaticity, similar to that of the prototypical benzene, five lone pairs are delocalized in the equatorial plane of cyclo-N₅^-, forming additional σ-aromaticity. It is the compatible coexistence of the inter-lone-pair repulsion and inter-lone-pair attraction within the σ-aromatic system that makes the naked cyclo-N₅^- highly reactive to electrophiles and easily broken. Only in sufficiently acid solution can the cyclo-N₅^- become unsusceptible to the electrophilic attack and gain extra stability through the formation of hydrogen-bonded complex from surrounding electrophiles; otherwise, the cyclo-N₅^- cannot be productively isolated. The dual aromaticity discovered in cyclo-N₅^- is expected to be universal for pnictogen five-membered ring systems.

Generation and reactivity of neutral 1,3-benzazaphosphole and anionic 1,3-benzazaphospholide ytterbium(iii) complexes

Treatment of Cp3Ln (Ln = Yb, Y) with 5-R3-6-R1-2-R2-1H-1,3-benzazaphosphole (HBp) (HBp1 (1a): R1 = H, R2 = 2,4,6-Me3C6H2, R3 = Me; HBp2(1b): R1 = Me, R2 = C6H5, R3 = H; HBp3(1c): R1 = R3 = H, R2 = C6H5) at room temperature gives the crystalline 1 : 1 Lewis acid-base adducts [(η1(p)-HBp)LnCp3] (2a-d) [Ln = Yb: Bp = Bp1 (2a), Bp2 (2b), Bp3 (2c); Ln = Y: Bp2 (2d)] with Ln-P donor bonds in good yields. Heating 2a-c in toluene leads to the liberation of one molecule of CpH to afford the corresponding N-bonded complexes [Cp2YbBp] (Bp = Bp1 (3a), Bp2 (3b), Bp3 (3c)). Interestingly, the P atom of complexes 3a-c can also be further coordinated to another Lewis acid such as Cp3Yb and B(C6F5)3 to give the adducts [Cp2Yb(μ-η1(N):η2(C,C):η1(P)-Bp)YbCp3] (Bp = Bp1 (4a), Bp2 (4b), Bp3 (4c)) and [Cp2Yb(μ-η1(N):η2(C,C):η1(P)-Bp)B(C6F5)3] (Bp = Bp1 (5a), Bp2 (5b), Bp3 (5c)), respectively. The molecular structures of complexes 2a, 4b-4c and 5c are confirmed by X-ray diffraction analysis.