Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)

The synthesis of a triphosphenium bromide salt is described and its use as a P⁺ transfer agent is outlined by reactions with an N-heterocyclic carbene and an anionic bisphosphine, yielding an NHC-stabilized P(I) cation and a P(I) containing zwitterion, respectively.

Transcript

The overall goal of this experiment is to prepare the bromide salt of a stable phosphorus(I)cation which can serve as a convenient synthetic source of phosphorus(I)plus ions. The precursors prepared using this method have been used for the preparation of many types of phosphorus-containing molecules with applications in inorganic chemistry, organic chemistry, and catalysis. The main advantages of this technique are that the synthesis is convenient and high yielding from commercially available reagents and the resultant precursor molecule is air and moisture stable.

Demonstrating the synthesis P(I)bromide will be Max, an undergrad student from my laboratory. Demonstrating the synthesis of the Biscarbene Phosphorus(I)Tetraphenylborate will be Justin, a PhD student from my laboratory. Demonstrating the synthesis of a multifunctional zwitterionic phosphenotriphosphineo molecule will be Steph, a PhD student from my laboratory.

To begin this procedure, dissolve 1.00 grams of dppe in DCM under inert gas. Prepare reagents as outlined in the text protocol, then add 40 milliliters of anhydrous degassed dichloromethane to a 250 milliliter Schlenk flask equipped with a rubber septum. Using a syringe, add 2.53 milliliters of dry degassed cyclohexane, then add 0.26 milliliters of phosphorus tribromide drop wise with vigorous stirring.

The solution should immediately turn pale yellow with white precipitate forming after 20 minutes. Allow the reaction to stir overnight. After stirring is complete, filter the reaction mixture through a fritted flask topped with a one inch thick silica plug to remove the precipitate.

Remove the DCM from the resulting solution under vacuum on a flank line, then add 75 milliliters of tetrahydrofuran while stirring vigorously to precipitate the product as a white solid. Immediately after the precipitate forms, collect it via filtration with a fritted flask under an inert atmosphere. In a Schlenk flask, dissolve 0.5 grams of P(I)bromide salt in five milliliters of DCM.

Stir the solution under nitrogen gas, then add 0.35 grams of sodium tetraphenylborate in five milliliters of THF. Stir the reaction overnight. After stirring is complete, centrifuge the suspension to remove the resulting precipitate.

Check that the precipitate has separated and collect the solution by canula transferring to a Schlenk flask. Then remove the solvent under reduced pressure yielding dppe P(I)tetraphenylborate as a white powder. Load a 100 milliliter Schlenk flask with 0.200 grams of methyl-carbene and a stir bar under an inert atmosphere, then load a 100 milliliter round bottom flask with 0.603 grams of dppe P(I)tetraphenylborate under an inert atmosphere.

Canula transfer 20 milliliters of THF to the Schlenk flask. Begin magnetic stirring. Next canula transfer 20 milliliters of THF to the round bottomed flask until the dppe P(I)tetraphenylborate has dissolved.

Then canula transfer the solution of dppe P(I)tetraphenylborate and THF to the Schlenk flask containing the carbine solution. Allow the solution to stir for one hour. After stirring is complete, concentrate the yellow solution to one-third of its original volume under reduced pressure.

Add 40 milliliters of dimethyl ether to the Schlenk flask. Allow the suspension to stir for 20 minutes, then place a sealed frit filter attached to a 100 milliliter flank flask on the Schlenk line. Evacuate the flask using vacuum for five minutes, then refill with nitrogen from the Schlenk line three times to create an inert atmosphere.

Next canula transfer the diethyl ether suspension into the apparatus and filter the precipitate. Wash the precipitate with 10 milliliters of diethyl ether three times, then dry the precipitate under reduced pressure for two hours. Begin by adding 0.484 grams of P(I)bromide to a 150 milliliter Schlenk flask in an inert atmosphere, then add 40 milliliters of dry degassed THF.

Place the flask in a dry ice acetone bath to cool the suspension to minus 78 degree celsius. While the suspension is cooling, prepare a solution of 0.625 grams of potassium 124 trisdiphenylphosphino cyclopentadiene and THF as outlined in the text protocol. Canula transfer the solution while stirring to the Schlenk flask.

After the transfer is complete, remove the Schlenk flask from the bath and allow it to warm to room temperature. Continue stirring for two hours and a white precipitate will form, then remove the precipitate by transferring the suspension to an air-free fitted filter equipped with a 115 milliliter Schlenk flask. Collect the yellow solution, next remove the THF under vacuum.

Add an 80 to 20 mixture of diethyl ether to pentane to the resulting oil, stir for 20 minutes. Then filter the suspension through an air free fitted flask to collect the yellow precipitate. In this study a stable salt containing a low valent phosphorus(I)source is synthesized by the addition of phosphorus tribromide to diphenylphosphino ethane in the presence of excess cyclohexane.

In the samples, the bromide ion can be exchanged for a less reactive anion such as tetraphenylborate. Addition of dppe P(I)tetraphenylborate to a solution containing two equivalents of the methyl-carbine generates the yellow phospho methane diamine dye biscarbine phosphorus(I)tetraphenylborate. The reaction of potassium-1, 2, 4 trisdiphenylphosphino cyclopentadiene in a one to one stoichiometric ratio dppe P(I)bromide yields phosphino triphosphiniums zwitterion.

The representative results of the P-31 NMR spectra for these compounds are seen here. Dppe P(I)bromide shows the presence of a triplet signal that is significantly shielded at minus 220 ppm and a doublet signal at 50 ppm. The P-31 NMR spectrum for dppe P(I)tetraphenylborate is seen to be nearly identical.

The P-31 NMR spectrum for biscarbine phosphorus(I)tetraphenylborate features a singlet signal at minus 113 ppm that is significantly shielded relative to typical phospha alkenes which is consistent with the phosphorus(I)assignment. The spectrum for phosphino trisphosphinium zwitterion reveals the presence of a triplet signal at minus 174 ppm, this is attributable to the dicordinate phosphorus(I)center. The singlet at minus 16.9 ppm is the diphenylphosphino fragment on the backbone and the doublet signal is attributable to the two diphenylphosphino groups that culate the phosphorus(I)center.

Once mastered the high yield syntheses of the phosphous(I)bromide reagent and the compounds derived from it can be done within a 24 hour period. While attempting this procedure, it's important to remember that some of the starting materials are air and moisture sensitive, so all the reactions should be done using air and moisture free techniques.

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Journal (Chemistry)

Journal (Chemistry)

Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)

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