matrix isolation conditions. Interestingly, NPO was not observed in this experiment. In later experiments, namely gas-phase IR laser absorption spectroscopy of NO/P4/O2/noble-gas mixtures and a microwave spectroscopic study of a dc glow discharge of NO/H2 over red phosphorus, there was also no experimental evidence for NPO. This is surprising given a recent high-level electronic structure focal point analysis suggesting that NPO is energetically preferred by 1.87 kcal mol -1 over PNO. The Lewis structures of NPO and PNO are best described with formal charges rather than the neutral N≡P=O form (Figure ). Related NPS isomers can be prepared in a similar fashion from thiophosphoryl azide (S=P(N3)3) via photolysis or high-vacuum flash pyrolysis (HVFP). In the HVFP experiments the neutral five-membered sulfur-pnictogen(III) ring SN2P2 was also identified under the pyrolysis products, suggesting head-to-tail dimerization of SNP and subsequent elimination of a sulfur atom. In the solid state, a material of the composition NPO is known to exist in both a β-cristobalite and a slightly thermodynamically less stable amorphous form. Furthermore, isomers of NPO and NPS are also considered potential interstellar molecules, given the presence of PN in interstellar media. Dibenzo-7λ 3 -phosphanorbornadiene derivatives have served as suitable precursors to phosphorous bearing small molecules of interstellar interest. We reported recently the synthesis of an anthracene-based azido phosphine (N3PA) that releases molecular PN in solution and shown to transfer PN to an iron complex under mild conditions. Here we report the oxidation of N3PA to anthracene-based phosphoryl azide (N3P(O)A, Figure ). Given the poor thermal stability of N3PA at room temperature (t1/2 = 29.1 ± 1.6 min) we selected 2,4,6trimethylbenzonitrile N-oxide (MesCNO) as a fast and effective oxygen atom transfer (OAT) reagent. Inside the glovebox N3PA and MesCNO were dissolved in a minimal amount of diethyl ether and tetrahydrofuran (4:1 ratio) and stirred under foil in the dark. After half an hour hexanes was added to the reaction mixture and the slurry was cooled to -20 °C to precipitate out the product. After half an hour the colorless precipitate was collected by vacuum filtration, washed with a minimal amount of pentane and dried under the nitrogen atmosphere of the glovebox providing up to 74% clean product. Single crystals of N3P(O)A grown from diethyl ether at -20 °C were characterized in a single crystal X-ray diffraction experiment and the molecular structure is depicted in Figure (see also Table ). The structure is in line with strong infrared bands for the azide group at 2154 and 2141 cm -1 (Figure ) as well as a single resonance in the P NMR spectrum at δ 75.9 ppm (t, 2 JPH = 11.1 Hz; Figure ). In solution N3P(O)A decomposes upon either thermal or photochemical activation. After 1 h irradiation of a quartz NMR tube containing a solution of N3P(O)A in benzene-d6 at λ = 254 nm only resonances of A could be detected in the 1 H NMR spectrum and no resonances were observed in the 31 P spectrum. Heating N3P(O)A at 80 °C in benzene-d6 for 30 min results in the observation of signals of A in the 1 H NMR spectrum as well as one major resonance at δ 112.7 ppm in the 31 P{ H} spectrum together with some other minor resonances (Figure ). However, none of these resonances could be unambiguously assigned. We followed the thermal decay of N3P(O)A in benzene-d6 by H NMR spectroscopy (Figure -S22, Table -S2) and found that the azide decomposes at 52.5 °C with a first-order kinetics half-life of around half an hour (t1/2 = 25.5 ± 0.4 min). Further kinetic measurements on N3P(O)A decomposition were performed over the temperature range of 52.5-70.0 °C. An Eyring analysis revealed activation parameters of ΔH ⧧ = 27.5 ± 0.3 kcal mol -1 and ΔS ⧧ = 10.6 ± 0.3 cal mol -1 K -1 (Figure ). The first-order behavior is indicative of a unimolecular rate-determining step, consistent with fragmentation of N3P(O)A into A and presumably a O=PN3 fragment (vide infra). We computed the most essential part of the potential energy surface around N3P(O)A at PBE0-D3(BJ)/cc-pVTZ + Gibbs free energy correction with augmented DLPNO-CCSD(T)/cc-pVTZ single point energies (Figure ). Similar as in our previous study of N3PA, we located two minima for N3P(O)A, and the energetically preferred conformer is in line with our crystal structure depicted in Figure . In the higher energy conformer, the P-N single bond is rotated by 180° and the azide group takes a parallel position to a terminal aromatic ring that results in an energy rise of 2.1 kcal mol -1 . Both conformers are connected by a low lying transition state TS3 (3.5 kcal mol -1 ). For the fragmentation of N3P(O)A, we initially considered cleavage of dinitrogen from the azide group. For each of the two N3P(O)A conformers, we located, in contrast to N3PA, an energetically high lying transition state (TS4 (44.6 kcal mol -1 ) and TS6 (39.5 kcal mol -1 )) that is associated with dinitrogen loss and a ring expansion to form tricyclic APNO in a highly exothermic (-43.2 kcal mol -1 ) and cyclic NPO attached to anthracene (c-APNO) in a highly endothermic (25.9 kcal mol -1 ) reaction, respectively. The dissociation reactions are completed with anthracene loss to form linear NPO via TS5 (43.7 kcal mol -1 barrier) and cyclic NPO via TS7 (17.3 kcal mol -1 barrier). However, in general these high reaction barriers cannot be overcome by simple heating at 80 °C. Therefore, we investigated a second dissociation pathway that is initiated by the cleavage of anthracene. We located a concerted transition state TS2 that is associated with a reaction barrier of 25.2 kcal mol -1 , leading to the fragmentation of N3P(O)A into A and OPN3. These calculations also suggest that the latter fragment eliminates N2 via TS1 (26.2 kcal mol -1 ) to form linear NPO. Based on the computed free energy values involving the elimination of A in the first step is energetically favored. The pathway via TS2 with a total barrier of 25.1 kcal mol -1 is in good agreement with the experimental value of our Eyring analysis (ΔG ⧧ = ΔH ⧧ -TΔS ⧧ = 24.4 ± 0.1 kcal mol -1 at 298.15 K). However, the fate of the OPN3 fragment remains unclear. The experimentally observed resonance at δ 112.7 ppm in the 31 P{ 1 H} after thermolysis cannot be assigned to the OPN3 fragment or the NPO molecule. Schulz and co-workers reported on the Staudinger reactivity of S=P(N3)3 with triphenylphosphine (PPh3), which led to the single (S=P(N3)2NPPh3) and double (S=P(N3)(NPPh3)2) but not the triple ((S=P(NPPh3)3)) Staudinger products. Inspired by this work, we treated N3P(O)A with PPh3 at room temperature; however, no reaction was observed. This reaction was repeated with the more electron-rich tricyclohexylphosphine (PCy3) in diethyl ether and immediate gas evolution and precipitation of the Staudinger reaction product Cy3P=NP(O)A was observed (Figure ). The product was isolated by vacuum filtration in 82% yield (Figure ). Cy3P=NP(O)A exhibits two doublets at δ 82.9 and δ 32.3 ( 2 JPP = 21.4 Hz) in the 31 P{ 1 H} NMR spectrum (Figure ). Additionally, Cy3P=NP(O)A was characterized in a single crystal X-ray diffraction experiment and the molecular structure is depicted in Figure .