Frustrated Lewis Pair Stabilized Phosphoryl Nitride (NPO), a Mono Phosphorus Analogue of Nitrous Oxide (N 2 O)

Phosphoryl nitride (NPO) is a highly reactive intermediate, and its chemistry has only been explored under matrix isolation conditions so far. Here we report the synthesis of an anthracene (A) and phosphoryl azide based molecule (N3P(O)A) that acts as a molecular synthon of NPO. Experimentally, N3P(O)A dissociates thermally with a first-order kinetic half-life that is associated with an activation enthalpy of ΔH⧧ = 27.5 ± 0.3 kcal mol-1 and an activation entropy of ΔS⧧ = 10.6 ± 0.3 cal mol-1 K-1 that are in good agreement with calculated DLPNO-CCSD(T)/cc-pVTZ//PBE0-D3(BJ)/cc-pVTZ energies. In solution N3P(O)A undergoes Staudinger reactivity with tricyclohexylphosphine (PCy3) and subsequent complexation with tris(pentafluorophenyl)borane (B(C6F5)3, BCF) to form Cy3P-NP(A)O-B(C6F5)3. Anthracene is cleaved off photochemically to form the frustrated Lewis pair (FLP) stabilized NPO complex Cy3P⊕-N═P-O-B⊖(C6F5)3. An intrinsic bond orbital (IBO) analysis suggests that the adduct is zwitterionic, with a positive and negative charge localized on the complexing Cy3P and BCF, respectively.


General Remarks
All manipulations were performed in a Vacuum Atmospheres model MO-40M glovebox under an inert atmosphere of purified N2. All solvents were obtained anhydrous and oxygen-free by bubble degassing (N2) and purification through columns of alumina and Q5, 1 and storage over molecular sieves. 2 Literature procedures were followed for the preparation of MgA•3THF, 3 Me2NPA, 4 ClPA, 4 N3PA (A = C14H10, anthracene), 5 and 2,4,6-trimethylbenzonitrile N-oxide (MesCNO). [6][7] Deuterated solvents were purchased from Cambridge Isotope Labs, degassed, and stored over molecular sieves for at least 48 h prior to use. Activated Charcoal Norit CA1 (Aldrich) and Celite 435 (EM Science) were dried by heating above 200 °C under dynamic vacuum for at least 48 h. All glassware was oven dried for at least 3 h at temperatures greater than 150 °C. NMR spectra were obtained on a Bruker Avance-III HD Nanobay spectrometer operating at 400.09 MHz equipped with a 5mm liquid-nitrogen cooled Prodigy broad band observe cryoprobe or on a Bruker Avance Neo spectrometer operating at 500.34 MHz equipped with a 5mm liquid-nitrogen cooled Prodigy broad band observe cryoprobe. 1 H and 13 C NMR spectra were referenced to residual protiated solvent resonances; 8 31 P NMR spectra were referenced externally to 85% aqueous H3PO4 (δ = 0 ppm). Elemental analyses were performed by Midwest Microlab (Indianapolis, IN). High resolution mass spectral (HRMS) data were collected using a Jeol AccuTOF 4G LC-Plus mass spectrometer equipped with an Ion-Sense DART source. Data were calibrated to a sample of PEG-600 and were collected in positive ion. Samples were prepared in THF (ca. 10 μM concentration) and were briefly exposed to air (<5 s) before being placed in front of the DART source.

Synthesis of N3P(O)A
Attention: Solid N3PA is explosive! 5 Inside the glovebox, a 20 mL vial was charged with solid, colorless N3PA (195 mg, 0.78 mmol, 1.0 equiv), 2,4,6-trimethylbenzonitrile N-oxide (MesCNO, 125 mg, 0.78 mmol, 1 equiv) and 4 mL diethyl ether. The solution turned homogeneous after addition of 1 mL THF within a few minutes and was further stirred for 30 min at room temperature under aluminum foil in the dark. After adding 15 mL hexanes, the reaction mixture was placed in the freezer at -20 °C for one hour. A colorless precipitate developed, which was filtered and washed with a minimal amount of pentane and further dried in the glovebox in the dark for 30 min. Yields varied depending on how much anthracene was produced during the course of the reaction, giving up to 154 mg (74%) of clean product. The identity of the product was confirmed by NMR and IR spectroscopy as well as by an X-ray diffraction study performed on a crystal grown from diethyl ether at -20 °C (Figure 2

Synthesis of Cy3P=NP(O)A
Inside the glovebox, a 20 mL vial was charged with solid, colorless N3P(O)A (155 mg, 0.58 mmol, 1.0 equiv), tricyclohexylphosphine (PCy3, 162 mg, 0.78 mmol, 1.0 equiv) and 10 mL diethyl ether. The heterogeneous solution was stirred for 30 min at room temperature. Gas evolution occurred and a colorless precipitate formed that was filtered and washed with a minimal amount of pentane and further dried in the glovebox for 30 min, giving up to 246 mg (82%) clean product. The identity of the product was confirmed by NMR as well as by an X-ray diffraction study performed on a crystal grown from diethyl ether at -20 °C ( Figure 4A). Crystals for elemental analysis were grown from a chloroform/pentane solution.

Synthesis of Cy3P-NP(A)O-B(C6F5)3
Inside the glovebox, a 20 mL vial was charged with solid, colorless Cy3PNP(O)A (50.8 mg, 0.098 mmol, 1.0 equiv), tris(pentafluorophenyl)borane (B(C6F5)3, 50.0 mg, 0.098 mmol, 1.0 equiv) and 5 mL dichloromethane. The solution was stirred for 10 min at room temperature and all volatile materials were removed under reduced pressure, yielding Cy3P-NP(A)O-B(C6F5)3 quantitatively. The identity of the product was confirmed by NMR as well as by an X-ray diffraction study performed on a crystal grown from diethyl ether at -20 °C ( Figure 4B). Crystals for elemental analysis were grown from a chloroform/pentane solution.

Synthesis of Cy3P-NPO-B(C6F5)3
Inside the glovebox, a quartz NMR tube with a J young valve was charged with solid, colorless Cy3P-NP(A)O-B(C6F5)3 (100 mg, 0.192 mmol) and 5 mL benzene or toluene. The homogenous solution was irradiated for 220 min with 254 nm light in a photoreactor outside the glovebox and the reaction followed by NMR spectroscopy. The tube was brought back inside the glovebox and the colorless precipitate that was formed during the irradiation filtered off. The filtrate was layered with pentane and placed in in the freezer (-20 °C) over night, yielding 35.3 mg crystalline Cy3P-NPO-B(C6F5)3 in 42% yield. The identity of the product was confirmed by NMR as well as by an X-ray diffraction study performed on a crystal directly grown from an irradiated benzene/pentane solution at -20 °C ( Figure 4C). The compound did not pass elemental analysis probably due to the poor thermal stability of the molecule. 1

Monitoring the Decay of N3P(O)A by 1 H NMR Spectroscopy
A standard NMR tube was charged with 10 mg of N3P(O)A and 5 mg acenaphthene as an internal standard. The tube was filled with 0.6 mL benzene-d6 and placed into the NMR spectrometer. All kinetic studies were performed by 1 H NMR spectroscopy on a Bruker Avance Neo500 spectrometer ( 1 H 500 MHz). Temperatures were calibrated with an ethylene glycol thermometer. 9 Eight-scan spectra were acquired continuously over a period of 3-5 half-lives depending on the temperature. Integrals of the N3P(O)A bridgehead protons were normalized against the methylene protons of internal standard acenaphthene at 2.99 ppm. The integral of the bridgehead protons (A) at 3.72 and 3.75 ppm was evaluated according to a zero-order (A = A0kt, Figure S20), first-order (ln(A) = ln(A0)kt, Figure   S21) and second-order (A -1 -A0 -1 = kt, Figure S22) kinetics. All experiments were performed three times at a given temperature and averaged. The measurements are summarized in Table S1-S2 and the associated Eyring plot is depicted in Figure S23. All error bars were calculated according to total derivatives.

S17
As only the evaluation according to a first order kinetic profile results in a linear plot, N3P(O)A decays following first-order kinetics. The experiment was repeated three times.
The same set of experiments was repeated at 52.5, 67.5 and 75.0 °C and an Eyring plot ( Figure S23) prepared based on these data.

Molecular Beam Mass Spectrometry (MBMS)
The molecular beam mass spectrometer (MBMS) apparatus has been previously described elsewhere. 4

Computational Studies
Unless otherwise indicated, all calculations were performed with the Gaussian 16 Rev C.01 quantum chemistry package. All geometries were optimized at PBE0-D3(BJ)/cc-pVTZ level of theory. [17][18][19][20] In all cases, computed electronic energies were corrected for thermal energy to obtain the corresponding free energy (all free energies reported at 298.15 K within this SI). To disclose the nature of all stationary points we computed the corresponding frequencies (Nimag=0 for minima and 1 for transition states).