Boraporphyrins: Unlocking Global Aromaticity in Organoboron Macrocycles

We report the synthesis of 5-bora-21,22-dioxaporphyrin <b>5</b> and its main-group and d-block metal complexes <b>6</b> and <b>7</b>, respectively. These macrocyclic boranes constitute the first examples of neutral porphyrins with boron in a <i>meso</i>-position that exhibit global aromaticity. This is evidenced by spectroscopic and structural features as well as calculated nucleus independent chemical shifts (NICS) and anisotropic induced current densities (ACID). The boraporphyrins absorb light strongly in the red spectral region (Q bands) and show enhanced fluorescence with higher quantum yields (<b>5</b>: 29%) compared to conventional porphyrins. DFT calculations reveal that the incorporation of the borane moiety has a distinct impact on the frontier orbital energies, thus leading to altered electrochemical and optical properties.