Phospha- and arsa-bridged cyclononatetraenides: novel zwitterionic 10π aromatic hemispheres

Abstract

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. (d_r) 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.