Sn(IV) Complexes of 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids: A Promising Platform for Evaluating the 20π-Electron Antiaromaticity

Porphyrinoids in the 20π-electron state have been extensively studied from the fundamental viewpoint of investigating their structure–antiaromaticity relationships. However, most of the 20π porphyrinoids are highly distorted and unstable in air, which hinder the comprehensive analysis of paratropic ring-current effects derived from planar π-electron systems. Herein, we present the first examples of antiaromatic Sn(IV) complexes of 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (SnX₂TADAPs), prepared by the complexation of the corresponding freebases with Sn(II) chloride under aerobic conditions and subsequent metathesis of the axial ligands, that show paratropic ring-current effects. Notably, both neutral 20π-electron derivatives and 19π-electron radical cations of SnX₂TADAP are extremely stable in air owing to the intrinsic electronic effects of the central Sn(IV) unit and meso nitrogen atoms. NMR spectroscopy, cyclic voltammetry, and density functional theory calculations were performed to assess the ring-current effects and orbital energies of a series of the 20π-electron SnX₂TADAPs, and the results reveal that the paratropic ring-current effects arising from the planar 20π-electron DAP ring increases with a decrease in the HOMO–LUMO energy gap.