Hydrogen forms metal-like core in a molecule

Hydrogen, most commonly known as a trace atmospheric gas in its elemental form, has, for the first time, been proven to be able to behave like an alkali metal in the formation of chemical structures by a team writing in Chemical Communications.

Despite being placed in the same group as the alkali metals in the periodic table, hydrogen has not historically been known to exhibit strikingly similar properties to the rest of the elements in its group.  However, hydrogen has been experimentally proven to exhibit metallic physical properties under extremely high pressures. The uncovering of hydrogen-metal similarities is important for a better understanding of fundamental chemical theories, and is also potentially useful to the pharmaceutical industry. Incorporating hydrogen in place of metals within organometallic catalysts can potentially achieve the same highly-valued asymmetric syntheses while eliminating the problems of cost, toxicity and low stability that come with metal-catalysis.

Alkali metal atoms often form 3 or 4 coordinations (interactions or bonds) with neighbouring atoms when present in organic molecules. An equivalent structure with hydrogen would involve the formation of “trifurcated” hydrogen-bonding mode, whereby the hydrogen electrostatically interacts with 3 electron-dense atoms (a highly uncommon occurrence) instead of 1. In one synthetic route, the reaction of triphenolamine with sodium hexamethyldisilazide produces a cubic-shaped (“pseudocubane”) molecule with 4-coordinated sodium atoms taking up several vertex-positions of the cube. The authors modified the synthetic route by using a highly-acidic phenol to give an extra hydrogen atom to the triphenolamine before the reaction – this resulted in the formation of a similar pseudocubane with a 4-coordinated, trifurcated hydrogen at its vertex which effectively replaces the alkali metal atom in the structural motif.

The ability of hydrogen to form bond coordinations alike to those made by the alkali metals is yet another testament to the periodic table’s predictive powers in Chemistry.

Source: D M Cousins, M G Davidson and D García-Vivó, Chem. Commun., 2013, DOI: 101.1039/c3cc47393g