Phospha- and arsa-bridged cyclononatetraenides: novel zwitterionic 10π aromatic hemispheres
The structural/geometrical features, multinuclear GIAO-NMR characteristics, tropicity, nucleus independent chemical shift NICS(1) and NICS(1)zz values, and NPA charge densities for phosphatriquinacene, its P-methylated phosphonium cation, the zwitterionic P-methylphosphaacepentalenide, and its conjugate acid have been modeled using DFT at the B3LYP/6-311++G(d,p) level of theory and are compared explicitly with computed data for the previously described aza-analogs at the same level of theory. By all investigated criteria, the phosphatriquinacene-based zwitterion is clearly aromatic but somewhat less so than the aza-analog. To probe the variations in anisotropic shielding effects, a number of other +P-R derivatives (R = H, F, CF3, iso-Pr, n-Pr, sec-Bu) were computed and compared with models. Extension of the study to the As-counterpart showed that by the same set of criteria, the arsa-bridged zwitterion is also aromatic. The relative aromaticity order in the main group family is N > P > As, which correlates with deviation from planarity of the annulene ring, as measured by the angle between the ring planes, i.e. (dr) N: 144.7°; P: 123.4°; As: 120.4°. Extension of the study to the dicationic group VI oxa- and thia-bridged analogs led to the same observation, i.e. relative aromaticity O > S. By contrast, an inverse correlation between planarity and aromaticity was found when comparing analogs from the same row of the periodic table, with relative aromaticity orders O > N and S > P. Including a solvation model (IEFPCM method) did not produce significant changes, although increasing solvent polarity led to slightly less negative NICS values and weaker anisotropic shielding.
New Journal of Chemistry
Digital Object Identifier (DOI)
Borosky, Gabriela L.; Laali, Kenneth K.; and Mascal, Mark, "Phospha- and arsa-bridged cyclononatetraenides: novel zwitterionic 10π aromatic hemispheres" (2019). UNF Faculty Publications. 1002.