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

Summary

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.

Abstract

We present herein the optimized synthesis of a triphosphenium bromide salt. Apart from being a versatile metathesis reagent, this unusually stable low-valent-phosphorus-containing compound acts as a useful P⁺ transfer agent. Unlike traditional methods employed to access low-coordinate phosphorus species which usually require pyrophoric phosphorus-containing precursors (white phosphorus, Tris(trimethylsilyl)phosphine, etc.), or harsh reducing agents (alkali metals, potassium graphite, etc.), the current approach does not involve pyrophoric or explosive reagents and can be done on large scales (>20 g) in excellent yields by undergraduates with basic air-free synthetic training. The bromide counter ion is readily exchanged with other anions such as tetraphenyl borate (described herein) using typical salt metathesis reagents to obtain materials with desired properties and reactivities. The versatility of this P⁺ transfer approach is exemplified by the reactions of these triphosphenium precursors with an N-heterocyclic carbene and an anionic bisphosphine, each of which readily displace the neutral bisphosphine to give an NHC-stabilized phosphorus(I) cation and a phosphorus(I) containing zwitterion, respectively.

Introduction

The chemistry of main group elements in unusually low oxidation or valence states has been an area of significant interest over the last two decades.¹ Aside from the fundamental interest arising from their unique bonding and structure, such compounds often display reactivities that are very different from those of their more typical oxidation state counterparts. In this sense, they offer significant potential as reagents to build more complex main group-element containing materials.

A landmark class of low-valent phosphorus containing molecules are "triphosphenium" cations which were first reported by Schmidpeter in the 1....

Protocol

NOTE: Deuterated solvents were dried according to literature procedure when necessary, and all other solvents were dried over a series of Grubbs'-type columns¹² and degassed prior to use. Potassium 1,2,4-tris(diphenylphosphino)cyclopentadiene [K][(Ph₂P)₃C₅H₂] and the N-heterocyclic carbene 1,3,4,5-tetramethylimidazole-2-ylidene (^MeNHC^Me) were synthesized according to literature procedures and the latter was sublimed prior to use.^9,13 Cyclohexene was dried over CaH₂, distilled and degassed prior to use. Acetonitrile-d₃ (CD₃CN) and dichloromethane....

Results

A stable salt ([dppeP][Br]) containing a low valent phosphorus(I) source can be synthesized easily by the addition of PBr₃ to diphenylphosphinoethane (dppe) in the presence of excess cyclohexene (Figure 1)⁶.

Upon work up of the reaction, ³¹P NMR spectra show the presence of a triplet signal that is significantly shielded at -220 ppm, and a doublet signal at 50 ppm. Strong coupling between the nuclei is observed .......

Discussion

Crystal structures and multi-nuclear NMR (³¹P, ¹H, and ¹³C) were obtained for all products reported to confirm connectivity. Electrospray-ionization mass spectrometry was used to confirm presence of the cations and elemental analysis was used to confirm analytical purity of the samples.

It is imperative that all reactions are done in an air-free and dry environment to ensure that no unexpected by-products or decomposition products are formed. Using a slight exc.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
bis(diphenyl)phosphino ethane (dppe)	Strem	1663-45-2	98% Stored in gloved box, used as is.
Anhydrous Dichloromethane (DCM)	Sigma Aldrich	270997	Purified through solvent purification system, or standard methods
Anhydrous Cyclohexene	Sigma Aldrich	29240	Dried over calcium hydride and distilled.
Phosphorus Tribromide (PBr3)	Sigma Aldrich	157783	99% Stored in glove box, used as is. Air sensitive.
Anyhydrous Tetrahydrofuran (THF)	Sigma Aldrich	401757	Purified through solvent purification system, or standard methods
Methylene Chloride-D2 (CD2Cl2)	Sigma Aldrich	DLM-23-25	Dried over phosphorus pentoxide, vacuum transferred or distilled
Acetonitrile	Alfa Aesar	5/8/1975	Stored in glove box, used as is
Sodium Tetraphenylborate	Sigma Aldrich	T25402	Stored in glove box, used as is
Anyhydrous Diethyl Ether	Sigma Aldrich	673811	Purified through solvent purification system, or standard methods
Anhydrous Pentane	Sigma Aldrich	236705	Purified through solvent purification system, or standard methods