Phosphorus mononitride

[6][7] The existence of free, gas-phase phosphorus mononitride was confirmed spectroscopically in 1934 by Nobel laureate, Gerhard Herzberg, and coworkers.

[18] PN is mostly detected in hot, turbulent regions, where the shock induced sputtering of dust grain is thought to contribute to its formation.

Since supernovae 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 of phosphorus formation, such as non-explosive, lower mass asymptotic giant branch stars.

[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.

[32] NBO analyses support a single neutral resonance structure with a PN triple bond and one lone pair on each atom.

[1] Auer and Neese have produced calculated gas phase 31P and 15N NMR chemical shifts of 51.61 and -344.71 respectively at the CCSD(T)/p4 level of theory.

[39] Its dipole moment is larger than PO (1.88 D), despite the greater electronegativity difference between the constituent P and O atoms and similar bond length (1.476 Å).

[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).

[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.

[49] 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).

[2] Schnöckel and coworkers later showed an alternative synthesis involving the dehalogenation of hexachlorophosphazene with molten silver, with concomitant loss of AgCl.

[50] [51] 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.

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.

Smith and co-workers isolated the first stable M-PN (and M-NP) complexes, using methodology to generate the PN moiety at metal sites.

[3] Addition of 3 equivalents of strongly lewis basic tert-butyl isocyanide results in the release of the iron adduct as a [PhB(iPr2Im)3Fe-(CNtBu)3]+ cation in the second coordination sphere.

[3] Cummins and co-workers exploited their N3PA free PN releasing reagent to "trap" and isolate a stable terminal (dppe)(Cp*)Fe-NP complex as a BArF24 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 from Pauli 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.

The V-NP fragment undergoes singlet phosphinidene reactivity ([2+1] additions) with alkene and alkyne trapping agents, generating phosphiranes and phospherenes respectively.

PN 2D electron density Laplacian contour plot. Calculated analogously to Kupka et al. at the CCSD/aug-cc-pCVQZ level. [ 22 ] [ 23 ] [ 24 ]
N 2 2D electron density Laplacian contour plot. Calculated analogously to Kupka et al. at the CCSD/aug-cc-pCVQZ level. [ 22 ] [ 23 ] [ 24 ]
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 ]
P 3 N 5 flash pyrolysis methodology from Atkins and Timms. Depicts intermediate(s) in matrix isolation, and polymerization. [ 1 ]
Phosphinoazide pyrolysis to PN and byproducts from Qian, Wende, Schreiner, and Mardyukov. At lower pyrolysis temperatures, a different major product forms. [ 2 ]
Dehalogenation reaction of hexachlorophosphazene using molten silver from Schnöckel et al. [ 25 ]
Room temperature PN release reagent, N 3 PA, from Cummins and coworkers. [ 42 ]
Light driven reversible PN oxidation to a phosphinonitrene at 10 K from Qian et al. [ 2 ]
[MoPN] − and [MoNP] − preparation and photoisomerization from Smith and coworkers. [ 3 ]
Diamagnetic iron-NP complex from N 3 PA precursor. [ 42 ]
Synthesis of tris(amido) vanadium PN trimer, dimer, and corresponding monomeric V-NP reactive intermediate from Cummins and coworkers. [ 4 ] [ 50 ] [ 42 ]
Reactivity of PN generated at vanadium tris(amido) complexes. Reversible singlet phosphinidene reactivity and equilibrium. [ 4 ]