Phosphorus mononitride is aninorganic compound with thechemical formulaPN. Containing only phosphorus and nitrogen, this material is classified as abinary nitride. From the Lewis structure perspective, it can be represented with a P-N triple bond with a lone pair on each atom. It isisoelectronic withN2,CO,P2,CS,NO+,CN− andSiO.
The compound is highly unstable in standard conditions, tending to rapidly self polymerize. It can be isolated within argon and krypton matrices at 10 K (−263.1 °C).[1] Due to its instability, documentation of reactions with other molecules is limited.[2] Most of its reactivity has thus far been probed and studied at transition metal centers.[3][4]
Phosphorus mononitride was the first identifiedphosphorus compound in the interstellar medium[5] and is even thought to be an important molecule in the atmospheres of Jupiter and Saturn.[6][7]
The existence of free, gas-phase phosphorus mononitride was confirmed spectroscopically in 1934 by Nobel laureate,Gerhard Herzberg, and coworkers.[8] J. Curry, L. Herzberg, and G. Herzberg made the accidental discovery after observing new bands in the UV region from 2375 to 2992 Å[9] following an electric discharge within an air-filled tube that had been earlier exposed to phosphorus.
In 1987, phosphorus mononitride was detected in theOrion KL Nebula, theW51M nebula inAquila, andSaggitarius B2 simultaneously by Turner, Bally, and Ziurys. Data from radio telescopes allowed for observation ofrotational lines associated with the J = 2-1, 3-2, 5-4, and 6-5 transitions.[5][10]
ALMA data alongside spectroscopic measurements from theRosetta probe have shown PN being carried from the comet67P/Churyumov–Gerasimenko alongside the far more abundant PO.[16] These observations may offer insight to how pre-biotic matter could be transported to planets. In cases where PN and PO are observed in the same region, the latter is more abundant.[17] The consistency of the molecular ratio between these two interstellar molecules across many differentinterstellar clouds is thought to be a sign of a shared formation pathway between the two molecules.[18] PN is mostly detected in hot, turbulent regions, where the shock induced sputtering of dust grain is thought to contribute to its formation. However, it has also been confirmed in massive dense cores which are by comparison "cold and quiescent".[19][20]
In 2022, researchers used data from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) project and reported evidence of phosphorus mononitride in giant molecular clouds within the galaxy,NGC 253. This finding marks phosphorus mononitride as the firstextragalactic phosphorus containing molecule detected as well.[17] In 2023, Ziurys and coworkers showed the existence of PN and PO in WB89-621 (22.6kpc from theGalactic Center) using rotational spectroscopy. Prior, phosphorus was only observed in the inner Milky Way (12kpc). Sincesupernovae do not occur in outer regions of the galaxy, the detection of these phosphorus-bearing molecules in WB89-621 provides evidence of additional alternative sources ofphosphorus formation, such as non-explosive, lower mass asymptotic giant branch stars. The levels were detected at comparable values to that in theSolar System.[21]
Electronic structure, spectral and bonding properties
PN 2D electron density Laplacian contour plot. Calculated analogously to Kupka et al. at the CCSD/aug-cc-pCVQZ level.[22][23][24]N2 2D electron density Laplacian contour plot. Calculated analogously to Kupka et al. at the CCSD/aug-cc-pCVQZ level.[22][23][24]
PN formation from gaseous phosphorus and nitrogen isendothermic.
Early mass spectrometry studies by Gingerich yielded a PNdissociation energy D0 of 146.6 ± 5.0 kcal/mol (613 ± 21 kJ/mol; 6.36 ± 0.22 eV).[26]
It is predicted to have a highproton affinity (PA = 191 kcal/mol (800 kJ/mol)).[27]
Early rotational analysis of 24 of the bands from Herzberg's original study suggested a PN internuclear distance of 1.49 Å, intermediate betweenN2 (1.094 Å) andP2 (1.856 Å). The associated electronic transition,1Π →1Σ, was noted to be similar to that of theisoelectronicCS andSiO molecules.[28] Later rotational spectra studies aligned well with these findings, for example analysis of millimeter wave rotational PN spectra from amicrowave spectrometer yielded a bond distance of 1.49085 (2) Å.[29][30]
Infrared studies of gaseous PN at high temperatures assign its vibrational frequency (ωe) to 1337.24 cm−1 and interatomic separation of 1.4869 Å.[8][31]
Simple comparisons to tabulated experimental and calculated bond lengths match well with a PN triple bond according to Pyykkö's Triple-Bond Covalent Radii.[32]
NBO analyses support a single neutralresonance structure with a PN triple bond and onelone pair on each atom. However, natural population analysis shows nitrogen as significantly negatively charged (-0.82603) and phosphorus as significantly positively charged (0.82603). This is in line with the large dipole moment and partialionic character reflecting the electron density contour plots.[33][34][35][36][37][24]
Monomeric PN in a krypton matrix at 10 K (−263.1 °C) gives rise to a single IR band at 1323 cm−1.[1]
Auer and Neese have produced calculated gas phase31P and15NNMR chemical shifts of 51.61 and -344.71 respectively at the CCSD(T)/p4 level of theory.[38] However, different functionals and basis sets yield dramatically different predictions for chemical shielding and so far experimental NMR shifts for phosphorus mononitride remain elusive.[22]
Molecular beam electric resonance spectroscopy has been used to determine the radio frequency spectrum of phosphorus mononitride generated from P3N5 thermolysis; the experimental results showed an experimental PNdipole moment (μ) of 2.7465 +- 0.0001 D, 2.7380 +-0.001 D, and 2.7293 +-0.0001 D for the first three vibrational levels respectively.[39]
Occupied intrinsic bond orbitals (IBOs) of PN. Example calculated at the PBE0-D3BJ level of theory using the def2-TZVP basis set with ORCA. (Left): Two lone pairs + one P-N sigma bond. (Middle + Right): Degenerate perpendicular pi bond pair.[40][34][35]Valence virtual IBOs of PN. Example calculated at the PBE0-D3BJ level of theory using the def2-TZVP basis set with ORCA. (Top): Degenerate pi antibonding orbitals relevant to pi backbonding (in relation to LUMO).[40][34][35]
Its dipole moment is larger than PO (1.88 D), despite the greaterelectronegativity difference between the constituent P and O atoms and similar bond length (1.476 Å).[41] This a result of the significant differences in bonds and charge distribution within the PN and PO molecules. The large PN dipole moment makes it very favorable with respect to radio-astronomical studies in comparison to N2 - which lacks this property.[41]
In consideration to molecular orbitals of PN, direct analogies can be drawn to the bonding in the N2 molecule. It consists of an P-N σ bonding orbital (HOMO), with two perpendicular degenerate P-N pi bonding orbitals. Likewise, theLUMOs of PN, which consist of a degenerate PN pi-antibonding set, allow it tobackbond with orbitals of appropriate symmetry.
However, in comparison to N2, the HOMO of PN is higher in energy (est. -9.2 eV vs -12.2 eV), and, the LUMOs are lower in energy (-2.3 eV vs -0.6 eV), thus making it both a better σ-donor and pi-acceptor as a ligand.[42]
Evidently, the smaller HOMO-LUMO gap of PN, combined with its polar nature and low dissociation energy contribute to its much greater reactivity than dinitrogen (including at the interstellar level).[43]
The pathways to the formation of PN are still not fully understood, but likely involve competing gaseous phase reactions with otherinterstellar molecules. Important schemes are shown below along with competing exothermic reactions:[44]
PO + N → PN + O
PO + N → P + NO (Competing)
Another important, very exothermic formation reaction:
The abundance of interstellar PN is additionally perturbed by cosmic-ray ionization, visual extinction, and adsorption/desorption from dust grains.[45][46]
Moldenhauer and Dörsam first generated transient PN in 1924 using an electric discharge through N2 andphosphorus vapors, where the characterized product was a notably robust powder containing equal parts phosphorus and nitrogen.[47] This same method led to the actual first observation of PN by Gerhard and coworkers.
PN has also been produced at room temperature using microwave discharges on mixtures of gaseousPCl3 and N2 under moderate vacuum. This preparation was employed to achieve high resolutionFTIR spectra of PN.[48]
Atkins and Timms later generated PN via flashpyrolysis ofP3N5 under high vacuum, allowing the recording of the PN infrared spectrum within a cryogenic kryptonmatrix. Solid triphosphorus pentanitride generates gaseous, free PN when heated to 800–900 °C (1,070–1,170 K) under high vacuum.[1]
P3N5 flash pyrolysis methodology from Atkins and Timms. Depicts intermediate(s) in matrix isolation, and polymerization.[1]
Monomeric PN can only be isolated in krypton or argon matrices at 10 K (−263.1 °C). Upon warming up past 30 K (−243.2 °C), cyclotriphosphazene, which hasD3h symmetry, is formed (up to 50 K (−223.2 °C) before krypton matrix melts). The (PN)3 trimer and is planar and aromatic, with15N-labelling experiments revealing a planar E' mode band at 1141 cm−1.[25] No dimers or other oligomers are even transiently observed.
Without a cryoscopic matrix, these reactions result in the immediate formation of (PN)n polymers.[1]
Thermolysis experiments of dimethyl phosphoramidate have shown PN to form as a major decomposition product along with many other minor components including the·P=O radical and HOP=O. This is contrasting todimethyl methylphosphonate in which said minor components become the major decomposition products, highlighting significantly diverging pathways.[49]
Phosphinoazide pyrolysis to PN and byproducts from Qian, Wende, Schreiner, and Mardyukov. At lower pyrolysis temperatures, a different major product forms.[2]
In 2023, Qian et al. proposed PN to be generated as a major product along with CO and cyclopentadienone byproducts when (o-phenyldioxyl)phosphinoazide is heated to 850 °C (following the loss of N2). However, efforts to observe free PN in argon matrices using this method were unsuccessful due to band overlaps.[2]
Dehalogenation reaction of hexachlorophosphazene using molten silver from Schnöckel et al.[25]
Schnöckel and coworkers later showed an alternative synthesis involving the dehalogenation ofhexachlorophosphazene with molten silver, with concomitant loss of AgCl. In both this route and the P3N5 thermolysis route, only trace P2 and P4 formation is detected even at 1,200 K (930 °C), showing the reaction temperatures occur far from thermodynamic equilibrium.[25]
Anthracene release from dibenzo-7λ3 -phosphanorbornadiene derivatives
Room temperature PN release reagent, N3PA, from Cummins and coworkers.[42]
The aforementioned methods require very high temperatures which are incompatible with standard, homogeneous solution state chemistry.
In 2022,Cummins and coworkers prepared and isolated a molecular PN precursor, N3PA which rapidly decomposes to N2,anthracene, and PN in solution at room temperature (t½ = 30 minutes). With the combination of vacuum and heating to 42 °C, this dissociation is explosive.[42]
Reactions of phosphorus mononitride with other molecules are rare and rather difficult to carry out. The formation of the intermediate (PN)3 trimer (which itself is only isolated in matrices) is highly favorable:
PN generated in both the gaseous phase or in solution that is not subjected to trapping via noble gas matrices or particular metal complexes results in rapid self polymerization even in cases where trapping agents such as dienes or alkynes are present (differentiating its reactivity profile from related molecules such as P2).[50][51]
Light driven reversible PN oxidation to a phosphinonitrene at 10 K from Qian et al.[2]
Phosphorus mononitride's tendency to rapidly polymerize with itself has dominated its reactivity, greatly hindering both the study and diversity of products in its reactions with organic molecules.
In 2023, a rare case of documented reactivity with an organic molecule was reported by Qian and coworkers who demonstrated reversible photoisomerization betweeno-benzoquinone supported phosphinonitrene ando-benzoquinone stabilized phosphorus mononitride at 10 K, which can be isolated in an argon matrix.[2]
Ligation, stabilization, and reactivity at transition metals
The majority of documented well-defined PN reactivity has been carried out at transition metal centers. The electronic and molecular orbital similarities it shares with N2 make it a viable ligating species. While free PN is unstable, phosphorus mononitride has been prepared at metal coordination sites where it can exist as an isolable terminal ligand within a complex.[3][42] In alternative cases, PN ligands can also exist as only as transient, highly reactive intermediates featuring rich chemistry.[4] As a terminal ligand, cases of both preferential P and N bonding modes have been discovered.
[MoPN]− and [MoNP]− preparation and photoisomerization from Smith and coworkers.[3]
Smith and co-workers isolated the first stable M-PN (and M-NP) complexes, using methodology to generate the PN moiety at metal sites. They reacted a tris(amido) Mo(VI) terminal phosphide complex with a tris(carbene)borate Fe(IV) terminal nitride, which undergo reductive coupling to form the corresponding neutral bridging PhB(iPr2Im)3Fe-NP-Mo(N3N) complex. Notably, the Mo-N-P bond angle in the bridging compound is nearly perfectly linear with an N-P bond length of 1.509(6) Å (only slightly elongated from free PN indicating significant multiple bond character).[3] Addition of 3 equivalents of strongly lewis basic tert-butylisocyanide results in the release of the iron adduct as a [PhB(iPr2Im)3Fe-(CNtBu)3]+ cation in the second coordination sphere. The corresponding terminal linear Mo-PN anion can be isolated and converted to its linear Mo-NP isomer by exposure to white light in the solid state. The M-NP isomer of the ligand was determined to be more pi-acidic (N-P = 1.5913(1) Å and P-N = 1.5363(1) Å) and more thermodynamically stable than its isomer.[3]
Diamagnetic iron-NP complex from N3PA precursor.[42]
Cummins and co-workers exploited their N3PA free PN releasing reagent to "trap" and isolate a stable terminal (dppe)(Cp*)Fe-NP complex as aBArF24 salt. The NP bond length in this case was very short at 1.493(2) Å, almost unperturbed from gaseous PN, which is consistent with minimal pi-backbonding from the iron center. Studies confirmed the NP binding mode (as opposed to PN) to be energetically preferred by 36.6 kcal/mol (153 kJ/mol) in this iron complex, creating a significant barrier to isomerization (thought to arise fromPauli repulsion effects).[42] Studies of phosphorus mononitride chemistry at tris(amido) vanadium complexes undertaken by Cummins and coworkers provides the bulk of PN reactivity examples at transition metals to date. In this system, PN is synthetically generated at a vanadium center from respective dibenzo-7λ3 -phosphanorbornadiene derivative precursors. However, it is not stable as a terminal ligand, and instead immediately undergoes trimerization. Notably, a thermodynamic equilibrium exists between this trimer species, along with a dimer and non-observed monomeric intermediate fragment.[4][50]
Synthesis of tris(amido) vanadium PN trimer, dimer, and corresponding monomeric V-NP reactive intermediate from Cummins and coworkers.[4][50][42]
The V-NP fragment undergoes singletphosphinidene reactivity ([2+1] additions) with alkene and alkyne trapping agents, generatingphosphiranes and phospherenes respectively. The products generated from such additions exist in equilibrium (in the case with cis-4-octene andbis-trimethylsilylacetylene), where retention of the cis-4-octene conformer is observed. Upon heating, they reversibly add to generate the V-NP dimer. Such reactivity demonstrates stark contrasts fromP2 as a ligand which instead undergoes formalcycloaddition chemistry.[4]
Reactivity of PN generated at vanadium tris(amido) complexes. Reversible singlet phosphinidene reactivity and equilibrium.[4]
The robust nature of PN reaction products such as (PN)n, could find use in heat resistantceramics or as fire suppressing materials.[52]
There has long been interest in studying PN and its reaction products like (PN)n polymers, noting their relevance to precursors/intermediates in the production offertilizers.[51][53]
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