Phosphorus: from the Stars to Land & Sea

Phosphorus: From the Stars to Land & Sea Christopher C. Cummins Abstract: The chemistry of the element phosphorus offers a window into the diverse ½eld of inorganic chemistry. Fundamental investigations into some simple molecules containing phosphorus reveal much about the rami½cations of this element’s position in the periodic table and that of its neighbors. Addition - ally, there are many phosphorus compounds of commercial importance, and the industry surrounding this element resides at a crucial nexus of natural resource stewardship, technology, and modern agriculture. Questions about our sources of phosphorus and the applications for which we deploy it raise the provocative issue of the human role in the ongoing depletion of phosphorus deposits, as well as the transfer of phosphorus from the land into the seas. Inorganic chemistry can be de½ned as “the chemistry of all the elements of the periodic table,”1 but as such, the ½eld is impossibly broad, encompassing everything from organic chemistry to materials science and enzymology. One way to gain insight into and appreciate the rapidly moving and diverse ½eld of inorganic chemistry is to view the science from the perspective of the elements themselves, since they are the basic ingredients for assembling molecules or materials–and indeed, all matter, living or in animate. Although phosphorus may be less cele- brated than carbon or hydrogen, it joins those elements (along with nitrogen, oxygen, and sulfur) to con stitute the six “biogenic elements” (those needed CHRISTOPHER C. CUMMINS,a Fellow of the American Academy in large quantities to make living organisms; see Fig - 2 since 2008, is Professor of Chemis - ure 1). Let us take a look at some of the issues that try at the Massachusetts Institute of arise in inorganic chemistry from the perspective of Technology. His research focuses on phosphorus, illustrating in the process the notion innovating new methods of in or - that each element has its own story to tell. gan ic synthesis, as well as the syn - the sis of new simple substances. His work has recently appeared in Inor - Many phosphorus-containing chemical com- ganic Chemistry, Science, Jour nal of the pounds are commercially valuable and have interest - 3 American Chemical Society, and Chem - ing or important applications. Lithium hexafluoro - ical Science, among other journals. phosphate, for example, is the electrolyte in common © 2014 by Christopher C. Cummins Published under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license doi:10.1162/DAED_a_00301 9 Phosphorus: Figure 1 From the Periodic Table with Nonmetals, Including Biogenic Elements, Above the Stair-Step Line Stars to Land & Sea Biogenic elements are H, C, N, O, P, S in the “nonmetals” region of the periodic table, indicated by the heavy line. Source: Adapted from a graphic found on http://www.openclipart.org. lithium-ion batteries, which are used in phorus if chlorine is not even present in consumer electronics (such as laptops) the products, such as lithium hexafluoro - and automotive applications.4 So how is phosphate, that are the target of synthesis? it made? The synthesis route begins with These industry standard processes suggest the white form of elemental phosphorus, there is room for improvement: if manu- a simple molecular form of the element facturers eliminated the use of chlorine P consisting of tetrahe dral 4 molecules in the synthesis of important phosphorus (Figure 2).5 White phos phorus is com- compounds in which chlorine is absent, bined with elemental chlorine in order to both hazards and waste would be signi½ - bring the phosphorus to the correct oxi- cantly reduced. dation state (+5), and then, in a second Because our research has shown that it is step, chloride is re placed by fluoride. indeed possible to derive organo-phospho - This process is also frequently used to rus compounds directly from white phos- synthesize many organo-phosphorus com - phorus, this is an opportunity for inor- pounds that are important components of ganic chemistry to improve the safety and catalysts used in the chemical industry.6 ef½ciency of the manufacturing process. In these applications, again, white phos- In one advance, we showed that phospho- phorus is ½rst oxidized using chlorine, and rus-carbon bonds can be generated by then the chloride provides the basis for the using white phosphorus together with a formation of carbon-phosphorus bonds.7 source of organic radicals.9 Each of the six But notably, elemental chlorine is hazard - phosphorus-phosphorus bonds present in ous to use and ship, and environmental a molecule of white phosphorus absorbs groups have called for an outright ban on two organic radicals in the process of being 8 it. So why use chlorine to oxidize phos- broken; each P4 tetrahedron is broken 10 Dædalus, the Journal ofthe American Academy of Arts & Sciences Figure 2 Christopher P C. Cummins Tetrahedral Arrangement of Atoms in a 4 (White Phosphorus) Molecule Source: Generated by the author using the platon program. See A. L. Spek, “Single-Crystal Structure Valida- tion with the Program platon,” Journal of Applied Crystallography 36 (2003): 7–13. com pletely apart, and each phosphorus in a wide variety of structural arrange- atom becomes incorporated into a freshly ments, all of which are networks exclusive- formed organo-phosphorus compound. ly based upon phosphorus-phosphorus Our method for developing this new sin gle bonds, three for every phosphorus pro cess was derived from basic inquiries node. The variant known as red phospho- into phos phorus’s relationship to the ele- rus, for example, has cages of phosphorus ments neighboring it on the periodic ta ble. atoms connected into linear tubes (see Fig - Phos phorus is immediately beneath nitro - ure 3),12 which in turn are cross-linked to - gen on the periodic table, suggesting that geth er to form a polymeric net work. these el e ments should have some sim ilar i - Knowing this, we were inspired to ask: ties in their chemical properties. Then why, can we design and synthesize a molecule we won dered, was it the case that, while that would be prone to a fragmentation re - Earth’s atmosphere consists mainly of ac tion wherein one of the fragments pro- N P tri ply-bonded 2 molecules, a similar di - duced would be the diatomic molecule 2? atom ic molecular form of phosphorus is If we could, we would have the opportu- nei ther prevalent nor even particularly sta - nity to study the properties and chemical b l e ? 10 Part of the answer is that nitrogen characteristics of an all-phosphorus mol- is unusual because the stability of its mul- ecule structurally analogous to the main tiple bond far exceeds that of the sum of constituent of Earth’s atmosphere. In our an equivalent number (three) of its single ½rst attempt to produce it, the selected de - bonds. So the only stable form of elemen- sign incorporated a feature patterned after tal nitrogen is the diatomic molecular form the reaction used to inflate an automobile floating innocuously about in the atmos- airbag in the event of a collision, a process phere we breathe; in contrast, phosphorus that rapidly generates nitrogen gas from a (like its diagonal relative, carbon)11 exists solid precursor. Our target molecule em - 143 (4) Fall 2014 11 Phosphorus: Figure 3 From the Arrangement of Atoms in One of the Representative Structural Forms of Red Phosphorus Stars to Land & Sea The box encloses one crystallographic unit cell. Source: Generated by the author using crystallographic coordi- nates from M. Ruck et al., “Fibrous Red Phosphorus,” An gewandte Chemie International Edition 44 (2005), doi:10.1002/ anie.200503017. bed ded a diphosphorus moiety into the generation should not matter. Could there P sta bilizing environment of a niobium be a way to access the 2 molecule by start - com plex (niobium is a transition metal; ing from a stable form of the element, rath - it forms complexes by arranging sets of er than from an exotic niobium complex? molecules or ions–called ligands–around We found the suggestion in a lightly cited itself ), from which it could be released by 1937 paper that the photochemical conver - a stimulus of mild heating.13 Carrying out sion of white phosphorus into the red form the fragmentation reaction in the presence of the element may occur with P2 as the of other molecules permitted the mapping key intermediary, which is initially gener - of the reactivity patterns of diatomic phos - ated and subsequently polymerizes.14 phorus. One important result was the dis - We found by experiment (see Figure 5) that P covery that 2 easily undergoes addition to the addition of methyl isoprene to a solu- unsaturated organic molecules, such as 1,3- tion of white phosphorus during irradia- cyclohexadiene (see Figure 4). tion both inhibits the production of red If diatomic molecular phosphorus is in - phos phorus and yields molecules in the deed capable of direct combination with same class of organo-phosphorus com- organic molecules, then the means of its pounds that we studied earlier in connec- 12 Dædalus, the Journal ofthe American Academy of Arts & Sciences Figure 4 Christopher P C. Cummins A Niobium Complex that Can Act as an “Eliminator” of 2 under Thermal Fragmentation neat neat 65 °C P In the depicted sequence, transient 2 (not observed) combines with two molecules of 1,3-cyclohexadiene resulting in four new P-C single bonds in the stable ½nal product (shown both as a line drawing and in a thermal ellipsoid representation from a single-crystal X-ray diffraction analysis). Abbreviations: tBu is tert-butyl, Ar is aryl (spe - M C H M ci½cally 3,5- e2 6 3), and es* is supermesityl.

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