Detection of 1H-Triphosphirene (c-HP₃) and 2-Triphosphenylidene (HP₃): The Isovalent Counterparts of 1H-Triazirine (c-HN₃) and Hydrazoic Acid (HN₃)

Abstract

The hitherto elusive 1H-triphosphirene (c-HP3) and 2-triphosphenylidene (HP3) molecules were prepared in low-temperature matrices and detected isomer selectively through photoionization coupled with reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Our results reveal a thermodynamically preferred cyclic isomer (c-HP3) compared to the acyclic structure (HP3) in contrast to the isovalent HN3 system favoring hydrazoic acid (HN3) compared to 1H-triazirine (c-HN3). Theoretical computations suggest a ring strain energy of 1H-triphosphirene (c-HP3) of only 35 kJ mol–1, which is significantly lower than the tetrahedral phosphorus molecule (P4) of 74 kJ mol–1. This work provides a fundamental benchmark to understand the electronic structure and chemical bonding of cyclic molecules and offers an unconventional approach to preparing highly strained, still elusive molecules such as 1H-triazirine and tetrahedral tetranitrogen (N4) in the near future involving progressive nonequilibrium chemistries.

Diazophosphane HPN2

“…This is consistent with the theoretical results that cyc -HPN 2 (Figure ) is less stable by 12.1 kcal mol –1 with an activation barrier of 26.6 kcal mol –1 (Figure S5), which is larger than the estimated barrier (15.1 kcal mol –1 , Figure S6) for the dissociation of HPN 2 to PH and N 2 . The energy difference (Δ E ) between the cyclic and acyclic HPN 2 isomers is right between those for HN 3 (Δ E = 39.2 kcal mol –1 ) and HP 3 (Δ E = −30.2 kcal mol –1 ) . Therefore, the stronger pyramidalization of phosphorus than nitrogen atom can stabilize the cyclic isomer by reducing the associated strain energy, as also evident from the triatomic molecule OPN, for which the cyclic isomer (Δ E = 22.4 kcal mol –1 ) is detectable, whereas cyc -N 2 O (Δ E = 64.8 kcal mol –1 ) remains yet putative.…”

“…Similar to the molecular structures of HPCO ( ∠ HPC = 86.8°) and HP 3 ( ∠ HPP = 89.7°), HPN 2 is bent as the ∠ HPN angle is 88.7°, which is much smaller than the ∠ HNN angle in HN 3 (109(5)°). The ∠ PNN angle (173.8°) is comparable to the ∠ NNN (172.8­(2)°) and ∠ PPP (169.7°) in HN 3 and HP 3 , respectively. The P–H bond (1.415 Å) is shorter than those in HPCO (1.427 Å) and HP 3 (1.427 Å).…”

“…The energy difference (ΔE) between the cyclic and acyclic HPN 2 isomers is right between those for HN 3 (ΔE = 39.2 kcal mol −1 ) and HP 3 (ΔE = −30.2 kcal mol −1 ). 12 Therefore, the stronger pyramidalization of phosphorus than nitrogen atom can stabilize the cyclic isomer by reducing the associated strain energy, as also evident from the triatomic molecule OPN, 26 for which the cyclic isomer (ΔE = 22.4 kcal mol −1 ) 27 is detectable, whereas cyc-N 2 O (ΔE = 64.8 kcal mol −1 ) remains yet putative. The phosphaazide HNNP is less stable than HPN 2 by 13.3 kcal mol −1 , and the barrier for the isomerization is formidable (81.0 kcal mol −1 , Figure S5).…”

“…In sharp contrast to the rich chemistry of HN 3 , knowledge about its monophosphorus-containing analogue HPN 2 is scarce, although the polymorph of phosphorus nitride imide β-PN­(NH) has been prepared through ammonolysis of P 3 N 5 . Recently, the triphosphorus-containing analogue HP 3 has been detected among the photolysis products of a phosphine (PH 3 )–nitrogen (N 2 ) mixture at 5 K upon irradiation with energetic electrons . By analogy, P 3 N 3 , H 5 CPO 3 , and PH 2 C­(O)­H have also been generated in cryogenic PH 3 -doped astronomical ices.…”

“…11 Recently, the triphosphorus-containing analogue HP 3 has been detected among the photolysis products of a phosphine (PH 3 )−nitrogen (N 2 ) mixture at 5 K upon irradiation with energetic electrons. 12 By analogy, P 3 N 3 , 13 H 5 CPO 3 , 14 and PH 2 C(O)H 15 have also been generated in cryogenic PH 3 -doped astronomical ices. Herein, we report the low-temperature synthesis of HPN 2 (Scheme 2) from the photoreactions of N 2 with PH 3 and HPCO.…”