Aromatic hexazine [N6]4− anion featured in the complex structure of the high-pressure potassium nitrogen compound K9N56

The recent high-pressure synthesis of pentazolates and the subsequent stabilization of the aromatic [N5]− anion at atmospheric pressure have had an immense impact on nitrogen chemistry. Other aromatic nitrogen species have also been actively sought, including the hexaazabenzene N6 ring. Although a variety of configurations and geometries have been proposed based on ab initio calculations, one that stands out as a likely candidate is the aromatic hexazine anion [N6]4−. Here we present the synthesis of this species, realized in the high-pressure potassium nitrogen compound K9N56 formed at high pressures (46 and 61 GPa) and high temperature (estimated to be above 2,000 K) by direct reaction between nitrogen and KN3 in a laser-heated diamond anvil cell. The complex structure of K9N56—composed of 520 atoms per unit cell—was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. The observed hexazine anion [N6]4− is planar and proposed to be aromatic. Aromatic polynitrogen units can display both high stability and high energy content. A hexazine anion has now been identified in a complex compound, K9N56, which is formed at high pressures and temperatures under laser-heating in a diamond anvil cell. The [N6]4− ring is planar and proposed to be aromatic.