The binding of unstable diphosphorus to a single metal ion


The arrangement of the elements on the periodic table is made in such a way as to highlight specific relationships. There are families, groups (the vertical columns) and periods (the horizontal lines). The elements of each of these covenants have certain commonalities.

This diagonal relationship between phosphorus and carbon established the expectation that the diphosphorus molecule should mimic the attributes of the hydrocarbon acetylene. Image credit: Figueroa Lab Group, UC San Diego.

In the periodic table, diagonal relationships are present between two elements located diagonally to each other that display similar chemical compositions. Boron and silicon, lithium and magnesium, and phosphorus and carbon are all examples.

An important diagonal relationship has long been identified between phosphorus and carbon, especially in scenarios where multiple element-element bonds exist, for example, diphosphorus (P2) in which two phosphorus atoms are joined by a weak triple bond.

This diagonal association between carbon and phosphorus led to the anticipation that the diphosphorus molecule should mimic the characteristics of the hydrocarbon acetylene (C2H2). For example, acetylene and diphosphorus react with other organic molecules through their pi bonds, a kind of covalent bond found in molecules with multiple bonds.

A coordination complex contains a central ion or atom that is usually metallic and is flanked by bound ions or molecules, called complexing agents or ligands. Coordination complexes are essential for life on earth and include chlorophyll and hemoglobin. They are also widely used in industrial applications as catalysts.

While acetylene has precise coordination chemistry with unique transition metals, coordination complexes that include diphosphorus bound to a single metal center have remained undefinable.

Recently, scientists from the University of California San Diego, Ohio State University and the University of Rochester were able to bind diphosphorus to a single metal center. Their work appears in the journal Science March 25and publish.

Diphosphorus, unlike acetylene, is very reactive and unstable. When produced in free form, diphosphorus reacts or polymerizes rapidly with substrate molecules. Simply put, diphosphorus does not stay diphosphorus for very long – it is in its nature to join with other molecules and elements. This makes it difficult to analyze or control it.

Many synthetic routes have been implemented to develop multinuclear diphosphorus complexes. The most common technique is to separate the tetrahedral P4 molecule, more commonly called white phosphorus. But white phosphorus is deadly and extremely flammable (it was a key component of many flammable bombs used during World War II).

The work presented here provides a synthetic strategy for accessing mononuclear diphosphorus complexes in the laboratory. We predict that this mode of coordination could further allow the development of selective reactions of transfer of phosphorus atoms to organic molecules.

Professor Joshua Figueroa, Principal Investigator and Study Co-Author, Department of Chemistry and Biochemistry, UC San Diego

When preparing for the experiment, Shuai Wang, a postdoctoral researcher at Figueroa and UC San Diego, used iron as the metal ion because it provided a good coordination basis that allowed small molecules to bind efficiently.

By binding diphosphorus to an iron ion, they could combine the two phosphorus atoms in a way that avoided the free release of diphosphorus, providing the highly preferred stability.

Considering the extreme sensitivity of the free diphosphorus molecule as a short-lived species, it is remarkable how stable it becomes upon coordination with the sterically hindered mononuclear iron center.

Shuai Wang, Study First Author and Postdoctoral Researcher, UC San Diego

Wang led the synthesis work.

The scientists used X-ray crystallography to establish the exact 3D structure of the molecules and Mossbauer spectroscopy to note the variations in the bonding interactions between the diphosphorus and the iron ion. This was an important method because it allowed the scientists to demonstrate that diphosphorus and an acetylene molecule affected the composition of the iron center in quite similar ways.

If diphosphorus can be found in a relatively stable and selectively reactive form, researchers can attach it to substrates in what is called “click” chemistry. Click chemistry does not define a single, specific reaction, but illustrates a method of producing substances by combining small modular units. This could pave the way for new discovery routes in synthetic chemistry for the manufacture of pharmaceutical compounds.

We are excited about this work because it demonstrates the importance of using fundamental concepts learned in first-year chemistry to guide new discoveries.

Professor Joshua Figueroa, Principal Investigator and Study Co-Author, Department of Chemistry and Biochemistry, UC San Diego

This study was supported by the National Science Foundation through grants CHE-1802646 (to JSF) and CHE-1954480 (to MLN), and from the National Institutes of Health through grant R01GM111480 (to MLN).

Journal reference:

Wang, S. et al. (2022) Lateral coordination of diphosphorus at a mononuclear iron center. Science.



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