Akamptisomerism Beyond Porphyrins: Bond Angle Reflection and Stereochemical Divergences in Corrole- and Porphyrin-Anchored B–O–B Bridges

Main-group coordination-driven porphyrins (MGCPs) are an emerging class of complexes featuring central main-group elements. Bis-boron(III) B₂OF₂-porphyrinoids, including porphyrins and corroles, exhibit a rich coordination chemistry with a (F)B-O-B(F) bridge bound to the N₄ core. Porphyrins adopt a <i>transoid</i> bridge, while corroles favor a <i>cisoid</i> arrangement. These systems enable investigation of akamptisomerism ─ an underexplored conformational isomerism driven by restricted bond angle reflection (BAR) at the oxygen bridge. Density functional theory was used to investigate <i>cisoid</i>/<i>transoid</i> preference, the BAR mechanism, and stabilization of the <i>cisoid</i> isomer via noncovalent interactions. Geometric and natural bond orbital analyses revealed that <i>transoid</i> porphyrins, though more distorted, are stabilized by strong N → B interactions, whereas corroles favor <i>cisoid</i> geometry due to reduced distortion. Calculated <i>transoid</i>-BAR barriers were accessible for porphyrin (Δ<i>G</i>^‡ ≈ 25 kcal mol^-1) but prohibitive for corrole (Δ<i>G</i>^‡ > 40 kcal mol^-1), consistent across different theoretical levels. Activation strain and energy decomposition analyses showed that macrocycle-bridge interaction destabilization dominates barrier heights, with orbital interactions aiding BAR in porphyrins but hindering it in corroles. Substituent effects (Y in B₂OY₂ bridges) further stabilized <i>cisoid</i> geometries, though insufficient to allow <i>cisoid</i>-BAR. These findings offer insights into the coordination chemistry of B₂OF₂-porphyrinoids and a framework for rationalizing akamptisomerism in main-group macrocycles.