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.
ABSTRACT
The exploration of planar hypercoordinate carbon (phC) is challenging and significant. It is often puzzling to chemists whether the designed phC species should satisfy the 18-valence-electron rule, an authoritative rule in the phC field. In this study, we introduced a zinc atom into the extremely unstable 16-valence-electron planar tetracoordinate carbon (ptC) species CAl4 and its isoelectronic structure NAl4+ with a planar tetracoordinate nitrogen (ptN), and designed the 18-valence-electron CAl4Zn and NAl4Zn+ possessing a ptC and ptN, respectively. The thermodynamic results indicate that the ptC/N species CAl4Zn and NAl4Zn+ are the global energy minima, and also showed that the 18-valence-electron rule is more appropriate in designing ptC/N species having the CAl4 and NAl4+ skeletons, compared with the 16-valence-electron rule. Simultaneously, the BOMD simulations found that CAl4Zn is dynamically stable. Although NAl4Zn+ was isomerized at 298 and 500 K, it is dynamically viable. The excellent stability may be explained by the perfect electronic structure. First, the HOMO-LUMO gaps became much wider after the introduction of the Zn atom. Second, AdNDP analysis indicated that the introduction of the Zn atom promoted the formation of peripheral Al-Al and Al-Zn covalent bonds, providing a stable and comfortable bonding environment for ptC/N. In addition, the σ and π double aromaticity further stabilized the ptC/N species. Hence, as dynamic global energy minima display σ and π double aromaticity, the ptC/N species CAl4Zn and NAl4Zn+ are promising in gas phase generation.
