Breaking the Hoff/Le Bel rule by an electron-compensation strategy: the global energy minimum of NGaS.

In tetracoordinate chemistry, there is an attractive scientific problem of how to make the planar configuration more stable than the tetrahedral configuration. For tetracoordinate nitrogen, the abundant studies indicate that the planar tetracoordinate nitrogen (ptN) is far less stable than the tetrahedral tetracoordinate nitrogen (ttN). Herein, we introduced four S atoms to the unstable ptN-NGa and stable ttN-NGa by following an electron-compensation strategy. Surprisingly, ptN-NGaS is more stable than ttN-NGaS. Thermodynamically, ptN-NGaS is the global energy minimum, which is 46.7 kcal mol lower in energy than ttN-NGaS. Dynamically, the BOMD simulations indicated that ptN-NGaS has excellent dynamic stability at 4, 298, 500 and 1000 K, but the ttN-NGaS is isomerized at 1000 K. Electronically, the HOMO-LUMO gap of ptN-NGaS (6.91 eV) is much wider than that of ttN-NGaS (5.25 eV). Moreover, AdNDP analyses showed that the eight 2c-2e Ga-S σ-bonds eliminated the 4s lone pair/4s lone pair repulsion between the four Ga atoms and provided a strong spatial protection for ptN-NGaS; and that the four 3c-2e Ga-S-Ga π back-bonds could compensate electrons for Ga, weakening the electron-deficiency of Ga. Simultaneously, the double 6σ/2π aromaticity further enhanced the stability of ptN-NGaS. Thus, as the dynamically stable global energy minimum displaying double aromaticity, ptN-NGaS will be more promising than ttN-NGaS in gas phase generation.