Phosphorus is a critical ingredient in fertilizers, pesticides, detergents and various industrial and household chemicals but once phosphorus is mined from rocks, getting it into products is hazardous and expensive, so chemists have been trying to streamline the process for decades.

Phosphorus deposits come from fossilized animal skeletons, abundant in dried-up seabeds. Phosphorus deposits exist as phosphate rock, which usually includes impurities such as calcium and other metals that must be removed.   Purifying the rock produces white phosphorus, a molecule containing four phosphorus atoms. White phosphorous is tetrahedral, meaning it resembles a four-cornered pyramid in which each corner atom is bound to the other three. Known as P4, white phosphorus is the most stable form of molecular phosphorus.(1) 

For most industrial uses, phosphorus has to be attached one atom at a time, so single atoms must be detached from the P4 molecule. This is usually done in two steps. First, three of the atoms in P4 are replaced with chlorine, resulting in PCl3 — a phosphorus atom bound to three chlorine atoms.

Those chlorine atoms are then displaced by organic (carbon-containing) molecules, creating a wide variety of organophosphorus compounds such as those found in pesticides. However, this procedure is both wasteful and dangerous — chlorine gas was used as a chemical weapon during World War I — so chemists have been trying to find new ways to bind phosphorus to organic compounds without using chlorine.

A group at MIT says they have developed a new way to attach phosphorus to organic compounds by first splitting the phosphorus with ultraviolet light, which eliminates the chlorine, a health risk for workers handling the chemicals.

The new reaction can't produce the mass quantities needed for large-scale production of phosphorus compounds but it opens the door.

MIT chemistry professor Christopher Cummins says he has long been fascinated with phosphorus, in part because of its unusual tetrahedral P4 formation. Phosphorus is in the same column of the periodic table as nitrogen, whose most stable form is N2, so chemists expected that phosphorus might form a stable P2 structure. However, that is not the case.

For the past few years, Cummins' research group has been looking for ways to break P4 into P2 in hopes of attaching the smaller phosphorus molecule to organic compounds. In the new study, Cummins drew inspiration from a long overlooked paper, published in 1937, which demonstrated that P4 could be broken into two molecules of P2 with ultraviolet light. In that older study, P2 then polymerized into red posphorus.

Cummins decided to see what would happen if he broke apart P4 with UV light in the presence of organic molecules that have an unsaturated carbon-carbon bond (meaning those carbon atoms are able to grab onto other atoms and form new bonds). After 12 hours of UV exposure, he found that a compound called a tetra-organo diphosphane had formed, which includes two atoms of phosphorus attached to two molecules of the organic compound.

This suggests, but does not conclusively prove, that P2 forms and then immediately bonds to the organic molecule. In future studies, Cummins hopes to directly observe the P2 molecule, if it is indeed present.

Cummins also plans to investigate what other organophosphorus compounds can be synthesized with ultraviolet light, including metallic compounds. He has already created a nickel-containing organophosphorus molecule, which could have applications in electronics.

Citation: Daniel Tofan, Christopher C. Cummins, 'Photochemical Incorporation of Diphosphorus Units into Organic Molecules', Angewandte Chemie International Edition Article first published online: 26 AUG 2010 DOI: 10.1002/anie.201004385

NOTE:

(1) So say the folks at MIT anyway.   There are also several polymeric forms, the most common of which are black and red phosphorus, which consist of long chains of broken phosphorus tetrahedrons.  But white phosphorous being the most stable is new.