A theoretical study of the properties of ninety‐two “aromatic” six‐membered rings including benzene, azines, phosphinines and azaphosphinines

Abstract

Ninety‐two unsaturated hexagonal compounds (from benzene to hexaphosphinine, including azines and azaphosphinines) have been theoretically studied at DFT level, B3LYP/6‐311++G(d,p). Lone pair/lone pair repulsion between adjacent N atoms has been deduced using an empirical partition of the energy. Aromaticity based on magnetic criteria and on Bader analysis was studied finding that both criteria were almost orthogonal. The structure that are nonplanar or broken have been analyzed in function of their geometry or the fragments, respectively.

Azaphosphinines and their derivatives

“…53 In 2018, Elguero et al published a computational study that calculated the NICS values for a large number of P-N heterocycles including several theoretical species such as the parent 1,2l 3 , 1,3l 3 , and 1,4l 3 -azaphosphinines (Table 13, right). 131 The small differences in NICS values between the two studies are likely due to different levels of theory and different scanning distances used above the theoretical ring (0.5 vs. 1.0 Å). Regardless, both data sets follow the same basic trend of aromaticity in the azaphosphinine derivatives, with the later work by Elguero et al suggesting that 1,4l 3 -azaphosphinine would be the most aromatic, rivaling that of phosphinine.…”

The elusive cyclotriphosphazene molecule and its Dewar benzene–type valence isomer (P 3 N 3 )

“…The distinct Pauling electronegativities of nitrogen (3.0) and phosphorus (2.2) result in a less efficient delocalization of the  electrons and hence diminished aromaticity compared to benzene (C 6 H 6 , 4). This is supported by nucleus-independent chemical shifts (NICSs) of −3.9 parts per million (ppm) (h = 1.0 Å) and −10.2 ppm (h = 1.0 Å) for 2 and benzene (C 6 H 6 , 4), respectively (6,7), along with the charge population analysis suggesting positive and negative partial charges on the phosphorus and nitrogen atoms, respectively (5). Similarities in the isovalent 4-2 system are evident from the molecular structures of the thermodynamically less stable bicycle [2.2.0]hexa-2,5diene ("Dewar benzene") (6; +354 kJ mol −1 ) (11) and 1,3,5-triphosha-2,4,6-triazabicyclo [2.2.0]hexa-2,5-diene (5; +155 kJ mol −1 ) valence isomers ( Fig.…”

“…Ever since the pioneering discovery of the p-block element phosphorus by the alchemist Hennig Brand 350 years ago ( 1 ), phosphorus allotropes such as white, red, and black phosphorus along with phosphorous heterocycles such as the diphosphatriazolate anion (P 2 N 3 − , 1 ) ( 2 ) and cyclotriphosphazene (P 3 N 3 , 2 ; Fig. 1 ) ( 3 , 4 ) have captivated the interest of the theoretical, physical (in)organic, preparative, and organometallic chemistry communities ( 1 ) from the fundamental viewpoints of electronic structure ( 5 , 6 ) and chemical bonding ( 7 , 8 ), with both heterocycles representing benchmarks of a class of exotic inorganic (4 n + 2)π aromatic molecules. Whereas 1 is isovalent to the cyclopentadienyl anion ( 3 ), 2 can be linked to the prototype 6π-Hückel aromatic ( 9 ) benzene molecule (C 6 H 6 , 4 ) by formally replacing the C─H moieties alternatingly by isovalent phosphorus (P) and nitrogen (N) atoms.…”

On-Surface Synthesis and Real-Space Visualization of Aromatic P3N3

“…18 The negative NICSs value of -3.9 parts per million (ppm) at 1.0 Å above the plane of the ring for P3N3 1 and -10.2 ppm for C6H6 I indicates that 1 is aromatic but much less than benzene. [19][20] Even though a homodesmotic evaluation also indicates that 1 captures only about one third of the aromatic stabilization of benzene, 1 has been computed as the most favorable structure among the many isomers that can be envisaged for P3N3. 21 The pursuit of cyclotriphosphazene 1 has lasted for 45 years but still remains a great challenge in terms of both controlled synthesis and structural elucidation due to its high reactivity and fleeting existence.…”