The elusive cyclotriphosphazene molecule and its Dewar benzene–type valence isomer (P ₃ N ₃ )

Abstract

Although the chemistry of phosphorus and nitrogen has fascinated chemists for more than 350 years, the Hückel aromatic cyclotriphosphazene (P3N3, 2) molecule—a key molecular building block in phosphorus chemistry—has remained elusive. Here, we report a facile, versatile pathway producing cyclotriphosphazene and its Dewar benzene–type isomer (P3N3, 5) in ammonia-phosphine ices at 5 K exposed to ionizing radiation. Both isomers were detected in the gas phase upon sublimation via photoionization reflectron time-of-flight mass spectrometry and discriminated via isomer-selective photochemistry. Our findings provide a fundamental framework to explore the preparation of inorganic, isovalent species of benzene (C6H6) by formally replacing the C─H moieties alternatingly through phosphorus and nitrogen atoms, thus advancing our perception of the chemical bonding of phosphorus systems.

Identification of a prismatic P3N3 molecule formed from electron irradiated phosphine-nitrogen ices

“…Since the IEs of 5 and 16 are close to each other, we performed additional experiments to strengthen our identification of 5 . This is achieved through isomer-selective ultraviolet–visible (UV–vis) irradiation experiments, which only photolyze and/or isomerize 5 20 , but not 16 . The experiments first process the ices to prepare the P 3 N 3 isomers followed by wavelength- and hence isomer- selective photoisomerization.…”

“…The experiments were carried out in an ultra-high vacuum chamber (UHV) pumped to a base pressure of 7 × 10 −11 Torr utilizing turbomolecular pumps (Osaka, TG1300MUCWB and TG420MCAB) backed with oil-free scroll pumps (Edwards GVSP30) 20 , 23 , 39 , 40 . Within the chamber, a fine polished silver wafer is mounted to a copper cold head cooled by a closed-cycle helium refrigerator (Sumitomo Heavy Industries, RDK-415E) capable of achieving temperatures to 5.0 ± 0.1 K. The wafer can be translated vertically and rotated in the horizontal plane with a linear translator (McAllister, BLT106) and a rotational feedthrough (Thermionics Vacuum Products, RNN-600/FA/MCO), respectively.…”

“…However, although organometallic molecules with prismatic hexagermanium (Ge 6 ) and hexastannane (Sn 6 ) cores such as 1,2,3,4,5,6-hexakis[bis(trimethylsilyl)methyl]-1,2,3,4,5,6-hexagerma-tetracyclo[2.2.0.0 2,6 .0 3,5 ]hexane [(Me 3 Si) 2 CHGe] 6 15 and hexasupersilyl-triprismo-hexastannane [( t Bu 3 Si) 6 Sn 6 ] 16 , 17 have been synthesized, the preparation of molecules isovalent to prismane (C 6 H 6 , 2 ), in which all CH groups are replaced by nitrogen and phosphorus atoms, represents a fundamental synthetic challenge due to the repulsive interactions of the lone-pair electrons of phosphorus and nitrogen 18 . Whereas cyclotriphosphazene (P 3 N 3 , 4 ) was prepared in low-temperature krypton matrices 6 , 19 and upon exposing ammonia-phosphine ices to energetic electrons at 5 K 20 , prismatic P 3 N 3 molecules such as 1,2,3-triaza-4,5,6-triphosphatetracyclo [2.2.0.0 2,6 .0 3,5 ]hexane (P 3 N 3 , 5 ) have remained elusive. Consequently, all-phosphorus- and nitrogen-substituted prismanes such as 5 represent one of the least explored classes of inorganic molecules.…”

Taming Phosphorus Mononitride (PN)

“…5 In the absence of a noble gas matrix PN polymerizes to [PN]n and aromatic cyclotriphosphazene (PN)3, which also forms in electron irradiation experiments of ammonia (NH3) and phosphine (PH3) containing ices 6 at cryogenic temperatures. 7 The cyclotriphosphazene molecule undergoes a photochemically induced rearrangement to its Dewar benzene-type valence isomer. Under matrix isolation conditions PN was reported to interact with different metal atoms, including Cu, Ag, Au, Co, Ni and Pd.…”

“…(19). Selected interatomic bond angles (°): C1−P1−C8, 80.15 (7); P1-N1-N2, 115.22 (11). B: Explosive decomposition of N3PA followed by molecular beam mass spectrometry (MBMS).…”