Surface-Assisted Selective Air Oxidation of Phosphines Adsorbed on Activated Carbon

Trialkyl- and triarylphosphines readily adsorb onto the surface of porous activated carbon (AC) even in the absence of solvents through van der Waals interactions between the lone electron pair and the AC surface. This process has been proven by solid-state NMR techniques. Subsequently, it is demonstrated that the AC enables the fast and selective oxidation of adsorbed phosphines to phosphine oxides at ambient temperature in air. In solution, trialkylphosphines are oxidized to a variety of P(V) species when exposed to the atmosphere, while neat or dissolved triarylphosphines cannot be oxidized with air. When the trialkyl- and triarylphosphines PnBu3 (1), PEt3, (2), PnOct3 (3), PMetBu2 (4), PCy3 (5), and PPh3 (6) are adsorbed in a mono- or submonolayer on the surface of AC, in the absence of a solvent and at ambient temperature, they are quantitatively oxidized to the adsorbed phosphine oxides, 1ox–6ox, once air is admitted. No formation of any unwanted P(V) side products or water adducts is observed. The phosphine oxides can then be recovered in good yields by washing them off of the AC. The oxidation is likely facilitated by a radical activation of molecular oxygen due to delocalized electrons on the aromatic surface coating of AC, as proven by ESR. This easy and inexpensive oxidation method renders hydrogen peroxide or other oxidizers unnecessary and is broadly applicable to sterically hindered and even to air-stable triarylphosphines. Phosphines adsorbed at lower surface coverages on AC oxidize at a faster rate. All oxidation reactions were monitored by solution- and solid-state NMR spectroscopy.

Table S1.Data for the inversion recovery NMR spectra in Figure S1, used for the fitting in Figure S2.The delay times τ between the 180° and 90° pulses are matched with the intensities of the resulting spectra which were measured using TopSpin software with arbitrary units.The data was normalized with every intensity value divided by the intensity value for a delay time of 0.0001 seconds and multiplied by -1.  . 31P{ 1 H} NMR spectra of P n Bu3 (1) prior to oxidation (A), oxidized as neat substance in air for 30 minutes (B), and oxidized for 30 minutes in air while dissolved in THF (C).The given yields of n Bu3PO were determined by integration.The remaining percentages for each spectrum are accounted for by side products (A: 2%, B: 44%, C: 36%).4.

Figure S13
. 31 P NMR spectra of 3 prior to oxidation (bottom) and after oxidizing the neat phosphine, and 3 dissolved in THF.The top spectrum was recorded after 3 was adsorbed on AC and exposed to the atmosphere for 30 minutes.Yields are reported in Table 4.

Figure S14
. 31 P NMR spectra of 4 prior to oxidation (bottom) and after oxidizing the neat phosphine, and 4 dissolved in THF.The top spectrum was recorded after 4 was adsorbed on AC and exposed to the atmosphere for 30 minutes.Yields are reported in Table 4.

Figure S15
. 31 P NMR spectra of 5 prior to oxidation and after oxidizing neat 5 and 5 dissolved in THF for 30 minutes.Yields are reported in Table 4.

Figure S16
. 31 P NMR spectra after exposing neat 6, its solution in THF, and 6 adsorbed on AC to the atmosphere for 30 minutes.Yields are reported in Table 4.The dot denotes an impurity in the batch of 1 that was present prior to the oxidation (see also Figure S3, spectrum A).
Figure S18. 31P NMR spectra of 6 on AC (98% surface coverage) after exposure to the atmosphere for the indicated times.The yields based on the integration of the signals are reported in Table S1.
Table S3.Oxidation of PPh3 (6) adsorbed on AC with a 98% surface coverage, monitored over time after exposure to the atmosphere.The yields are based on the integration of the NMR signals (Figure S16).The remaining percent of material in the product mixture was residual 6.Table S4.Estimated monolayer surface coverages used to calculate approximate surface coverages.Energy minimized models were created and measured using Avogadro software and the monolayer ratios were calculated from the molecular radii (half of the distance between the atoms furthest apart in the energy minimized model with the P atom at the center) and the surface area of DARCO activated cabron (650 m 2 /g).

Figure S4 .
Figure S4. 31P NMR spectra of AC suspensions after various oxidation experiments using 1 adsorbed on AC (Table 1, entries A-D).Spectrum A: 1 self-adsorbed on AC (150% surface coverage) without solvent overnight and then oxidized in air for 4 hours.Spectrum B: 1 adsorbed on AC (150% surface coverage) from THF solution for 20 minutes and then oxidized in air for 1 hour without removing the solvent.Spectrum C: 1 adsorbed on AC (150% surface coverage) from THF solution for 20 minutes and then oxidized in air for 1 hour after removing THF in vacuo.Spectrum D: 1 adsorbed on AC (500% surface coverage) from THF solution for 20 minutes and then oxidized in air for 18.5 hours after removing THF in vacuo.Variations in chemical shifts of the signal of 1ox are likely due to slightly different concentrations of the phosphine oxide across the different samples, but all values are within the range of reported values (43-49 ppm).

Figure S5 .
Figure S5. 31P NMR of a suspension of AC after the oxidation of 1, adsorbed and oxidized solvent-free (A) and 31 P NMR of the aqueous phase after washing pristine AC with water (B).

Figure S6 .
Figure S6. 31P NMR of AC with adsorbed 1ox after one and three THF washings, demonstrating the complete removal of 1ox.

Figure S7 .
Figure S7. 31P NMR spectra after cycles 2, 3, 4, and 7 (top to bottom) of the oxidation of 1, reusing the same batch of AC.

Figure S8 .
Figure S8.EPR spectra of washed and dried AC prior to any phosphine adsorption and oxidation experiments and the same batch of AC after seven adsorption and oxidation cycles of 1.

Figure S9 .
Figure S9. 13C CP/MAS NMR spectra of AC prior to phosphine adsorption (top) and the same AC batch after seven cycles of phosphine adsorption and oxidation (Table2) (bottom).Asterisks denote rotational sidebands.In the spectrum of the post-oxidation material the peak at about 75 ppm is assigned to the OCH2 carbon of THF.The OCH2CH2 signal of THF is overlapping with the first order rotational sideband of the AC peak.

Figure S10 .
Figure S10. 31P, 13 C, and 1 H NMR spectra of 1ox obtained by surface-assisted oxidation of 1 adsorbed on AC and washed off with THF.The data match those reported in the literature. 36Dots denote peaks of side products (4%) and the label AC in the 31 P NMR spectrum indicates residual phosphate washed off from the AC along with 1ox.

Figure S11 .
Figure S11. 31P NMR spectra of 1ox after oxidizing 1 on AC and washing it off with various solvents.Dots denote peaks corresponding to side products (4%) and the label AC in the 31 P NMR spectrum indicates residual phosphate washed off from the AC along with 1ox.Variations in the chemical shifts of the signal of 1ox are likely due to slightly different concentrations across the different samples, but all values are within the range of reported values in various solvents (43-49 ppm).

Figure S12 .
Figure S12. 31P NMR spectra of 2 prior to oxidation (bottom) and after oxidizing the neat phosphine, and 2 dissolved in THF.The top spectrum was recorded after 2 was adsorbed on AC and exposed to the atmosphere for 30 minutes.Yields are reported in Table4.

Figure S17 .
Figure S17. 31P NMR spectrum of 1 adsorbed and oxidized on the AC brand Norit (81% surface coverage).The dot denotes an impurity in the batch of 1 that was present prior to the oxidation (see also FigureS3, spectrum A).

Figure S19 .
Figure S19. 31P MAS NMR of 6 and 6ox adsorbed on AC for a competition experiment.

Figure S20 .
Figure S20. 31P NMR spectra of 6 adsorbed on AC with a 50% surface coverage, measured after 0.5 h and 24 h of exposing the dry material to the atmosphere.

Figure S21 .
Figure S21. 31P NMR for 6ox, obtained in 92% yield after adsorbing and oxidizing 6 adsorbed on AC in 40% surface coverage for 24 h.

Figure S23 .
Figure S23.EPR spectra of washed and dried AC under air (blue, 3337 G) and under a nitrogen atmosphere (black, 3334 G).

Table S2 .
Parameters and error information for the fit generated from the experimental data in TableS1used to calculate the 31 P T1 time for 6 adsorbed on AC with 98% surface coverage.