Is the planar hexacoordinate nitrogen molecule NB6− viable?

Abstract

Molecules with hypercoordinate planar centers have continued to receive enthusiastic attention due to their violation of the traditional models of three-dimensional chemical bonding and maximum tetracoordination. These electronic exotic but structurally aesthetic species have been optimistically conceived as building blocks in cluster-assembly for bulky materials. Recently, the planar hexacoordinate nitrogen (phN) unit, NB(6)(-), has been theoretically incorporated into a series of sandwich-like transition-metal compounds. However, the intrinsic stability of NB(6)(-) in both gas-phase and assembly has not been tackled, though it is the key factor for predicting the viability of any molecules. In this paper, at the CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d)+ZPVE level, we investigate for the first time the thermodynamic and kinetic stability of the phN unit, NB(6)(-), in both free and assembled ([NB(6)](2)Fe) forms. The calculated least barrier height of phN towards conversion is 9.2 and 4.4 kcal mol(-1) in free and assembled forms, respectively. Most importantly, the phN structure is thermodynamically rather unstable, by 102.8 and 162.1 kcal mol(-1) higher than the respective lower-lying conversion isomers. Therefore, in view of the combined thermodynamic and kinetic consideration, we propose that isolation of the phN structure of NB(6)(-) in either gas phase or assembly is unlikely. The present results manifest that for predicting any viable molecule with exotic structures, investigation of its "intrinsic stability" is highly necessary. The maintenance of the phN-NB(6)(-) is discussed at the 6-311+G(d)-B3LYP, MP2, CCSD and CCSD(T) optimization levels in comparison with the isoelectronic and milestone phC-CB(6)(2-).

Vier Jahrzehnte Chemie der planar hyperkoordinierten Verbindungen

“…Die Übergangsmetall‐Sandwich‐Komplexe von Li und Mitarbeitern könnten schwierig herzustellen sein, denn die Konfiguration von zwei verschmolzenen B 6 X‐Einheiten (X=C, B, N) ist energetisch viel stabiler als die des Sandwich‐Komplexes. Zum Beispiel ist die Energie der verschmolzenen Isomere von (B 6 N) 2 Fe nach B3LYP/6‐311+G(d)+ZPE‐Rechnungen um 183.8 kcal mol −1 geringer als die der Sandwich‐artigen Form 151. Daher sind die “homo‐decked sandwich”‐Verbindungen [(B 6 X) 2 M] q − (X=C, B, N) thermodynamisch instabil und vermutlich schwierig herzustellen.…”

Geometry, stability, and isomerization of B n N2 (n = 1−6) isomers

“…So far, many reports on small B x N y clusters[] have been investigated experimentally and theoretically. Experimentally, some small boron–nitrogen clusters including BN, B 2 N, BN 2 , B 3 N, BN 3 , and their anions have been investigated by photoelectron spectroscopy, pulsed laser evaporation, and electron spin resonance methods.…”

“…[] Some stable nonclassical geometries of boron‐containing compounds with planar tetracooordinate nitrogen atoms have been computationally designed based on density functional theory by Minkin and coworkers The structure and aromaticity of new BNs were investigated by different theoretical methods . The potential energy surface (PES) of NB 6 – suggested that the planar hexacoordinate nitrogen species NB 6 – is not stable . The structure and property of (BN) n ( n = 2−11) have been investigated theoretically,[] among which researchers found the stability of cyclic B 3 N 3 isomer with D 3h symmetry is ascribed mainly to its aromaticity .…”

“…Boron nitride (BN) materials have wide applications in industry because of their excellent mechanical, thermal, electrical properties, high-temperature stability, corrosion resistance, and oxidation resistance properties. [1][2][3] So far, many reports on small B x N y clusters [4][5][6][7][8][9][10][11][12][13][14][15][16] have been investigated experimentally and theoretically. Experimentally, some small boron-nitrogen clusters including BN, B 2 N, BN 2 , B 3 N, BN 3 , and their anions have been investigated by photoelectron spectroscopy, pulsed laser evaporation, and electron spin resonance methods.…”

“…[15] The potential energy surface (PES) of NB 6 suggested that the planar hexacoordinate nitrogen species NB 6 is not stable. [16] The structure and property of (BN) n (n ¼ 2À11) have been investigated theoretically, [17][18][19][20][21][22][23][24][25] among which researchers found the stability of cyclic B 3 N 3 isomer with D 3h symmetry is ascribed mainly to its aromaticity. [18] The structure and stability of (BN) n (n ¼ 2-4) were investigated by ab initio calculations.…”