Anionic N 18 Macrocycles and a Polynitrogen Double Helix in Novel Yttrium Polynitrides YN 6 and Y 2 N 11 at 100 GPa.

Two novel yttrium nitrides, YN 6 and Y 2 N 11 , were synthesized by direct reaction between yttrium and nitrogen at 100 GPa and 3000 K in a laser-heated diamond anvil cell. High-pressure synchrotron single-crystal X-ray diffraction revealed that the crystal structures of YN 6 and Y 2 N 11 feature a unique organization of nitrogen atoms-a previously unknown anionic N 18 macrocycle and a polynitrogen double helix, respectively. Density functional theory calculations, confirming the dynamical stability of the YN 6 and Y 2 N 11 compounds, show an anion-driven metallicity, explaining the unusual bond orders in the polynitrogen units. As the charge state of the polynitrogen double helix in Y 2 N 11 is different from that previously found in Hf 2 N 11 and because N 18 macrocycles have never been predicted or observed, their discovery significantly extends the chemistry of polynitrides.