P hosphorus-based ligands play a prominent role in the field of organometallic chemistry and homogeneous catalysis. 1,2 Most organophosphorus compounds produced on an industrial scale are derived from white phosphorus, P 4 , which is obtained via reduction of inorganic phosphates. P 4 is then converted to organophosphorus compounds via oxidation to PCl 3 with chlorine gas, followed by further derivatization. 3 Interest in P 4 activation stems from the desire to develop methods to directly functionalize P 4 without going through the PCl 3 intermediate. 4 The activation of P 4 by both early and late transition metals has resulted in a variety of fascinating structures ranging from P 4 units in kite, butterfly, and square geometries to cleavage into terminal phosphides, cyclo-P 3 units, and P 2 fragments. [5][6][7][8][9][10] However, the ability for main group and transition metal chemists to explore the rich reactivity of P 4 is limited by the inability to obtain P 4 from commercial vendors owing to welljustified regulations surrounding its production and transportation. P 4 was first synthesized by alchemist Henning Brand in 1669 via distillation of concentrated urine: a less-thandesirable procedure to replicate in the lab. 11,12 A procedure for the generation of white phosphorus from amorphous red phosphorus for class demonstration purposes was reported in 1960, 13 but this procedure lacks appropriate detail and defies modern safety protocols. The need for a reliable preparatory scale procedure with which to obtain pure white phosphorus from its readily available and inexpensive red phosphorus allotrope led to the publication of two preparative routes in Inorganic Syntheses in 2018. 14 The first involves heating a dried sample of red phosphorus to 400 °C under vacuum in an addition funnel and condensing the generated P 4 in an attached Schlenk flask, while the second procedure calls for heating red phosphorus under vacuum in a sublimation apparatus using a Bunsen burner resulting in deposition of P 4 on the water-cooled coldfinger. While both procedures are reliable if performed by capable hands, a laboratory manager might be skeptical about directing a mentee or less experienced researcher to perform either of these procedures on large scale in the laboratory. P 4 itself is highly toxic and ignites spontaneously upon exposure to air, releasing a dynamic mixture of polyphosphoric acids. 15 Red phosphorus is airstable but flammable, and both reported procedures require heating it above its ignition temperature of 240 °C. Thus, one could envision the catastrophic impact of a small error such as a leaky connection or accidental overheating/melting of the glass vessels involved. Herein, we report a safer convenient method for the preparation of white phosphorus from red phosphorus on a gram scale in a sealed quartz tube using a tube furnace. This process allows the entire generation, collection, purification and handling of the as-generated P 4 to occur in an air-free environment.

In the solid-state, red phosphorus is more thermodynamically stable at standard conditions than white phosphorus by ∼12 kJ/mol. 16 Converting red phosphorus into the metastable white phosphorus solid relies on two key points. First, upon sublimation at temperatures below 550 °C, the equilibrium vapor species of both polymorphs is molecular P 4 . 17 Second, when P 4 vapor condenses, the kinetic rate of interconverting white phosphorus to red phosphorus is incredibly slow, especially at temperatures below 250 °C. 18 Therefore, subliming red phosphorus at a high temperature, and having the vapor transport and condense at a low enough temperature (e.g., room temperature) will favor the formation of white phosphorus, P 4 .

To synthesize pure white phosphorus, red phosphorus was loaded into a quartz tube inside an argon-filled glovebox and sealed under vacuum (∼65 mTorr Ar). The tube was loaded into a one-zone tube furnace with one end positioned outside of the furnace at room temperature, which is where P 4 condenses (Figure 1). The furnace was ramped over a 24 h period at a temperature to allow the hot side of the tube to reach ∼ 480 °C, at which point very little red/black residue remained and crude P 4 was formed as a waxy yellow to yelloworange solid at the cold end of the tube (Figure 1, left). Following the heating protocol, the quartz tube was transferred into a glovebox and cracked open, at which point the P 4 was scraped and extracted from the walls of the quartz using toluene. A 31 P NMR spectrum of the crude toluene extracts revealed a single singlet resonance at -521 ppm (Figure S4). Recrystallization from toluene at -35 °C afforded pure white phosphorus as a white and easily weighable powder (Figure 1, right) in 80% yield.

This procedure has proven reproducible in our hands, affording five batches of spectroscopically pure white phosphorus in ∼1 g quantities. The procedure limits safety hazards aside from precautions that need to be taken when cleaning and disposing of quartz and glassware contaminated with P 4 residue upon removal from the glovebox. The convenience of this protocol promises to make white phosphorus, P 4 , more accessible to organometallic and main group chemists seeking to investigate its activation and functionalization.