Breaking the Hoff/Le Bel rule by an electron-compensation strategy: the global energy minimum of NGa4S4+

Abstract

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-NGa₄⁺ and stable ttN-NGa₄⁺ by following an electron-compensation strategy. Surprisingly, ptN-NGa₄S₄⁺ is more stable than ttN-NGa₄S₄⁺. Thermodynamically, ptN-NGa₄S₄⁺ is the global energy minimum, which is 46.7 kcal mol⁻¹ lower in energy than ttN-NGa₄S₄⁺. Dynamically, the BOMD simulations indicated that ptN-NGa₄S₄⁺ has excellent dynamic stability at 4, 298, 500 and 1000 K, but the ttN-NGa₄S₄⁺ is isomerized at 1000 K. Electronically, the HOMO–LUMO gap of ptN-NGa₄S₄⁺ (6.91 eV) is much wider than that of ttN-NGa₄S₄⁺ (5.25 eV). Moreover, AdNDP analyses showed that the eight 2c–2e Ga–S σ-bonds eliminated the 4s² lone pair/4s² lone pair repulsion between the four Ga atoms and provided a strong spatial protection for ptN-NGa₄S₄⁺; 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-NGa₄S₄⁺. Thus, as the dynamically stable global energy minimum displaying double aromaticity, ptN-NGa₄S₄⁺ will be more promising than ttN-NGa₄S₄⁺ in gas phase generation.