Sub- <i>m</i> -benziporphyrin: a subcarbaporphyrinoid and its B ^III complex with an unprecedented planar tridentate 14π-aromatic network

Sub-<i>m</i>-benziporphyrins were synthesized by Pd-catalyzed cross-coupling of <i>α</i>,<i>α</i>'-diboryl-<i>m</i>-benzitripyrrane with 9,10-bis(1,1-dibromomethylenyl)anthracene. Reaction of sub-<i>m</i>-benziporphyrin with PhBCl₂ and triethylamine gave its B-phenyl complex as a tetracoordinate nonaromatic B^III complex. In contrast, the reaction with BBr₃ and triethylamine furnished a neutral B^III porphyrinoid with a planar and triangular coordination as the first example, in which the <i>m</i>-phenylene unit was partially reduced, allowing for the global 14π-aromatic circuit. This aromatic B^III complex is stable and inert towards nucleophiles such as pyridine, 4-dimethylaminopyridine, and fluoride anions but undergoes an oxygen-insertion reaction upon refluxing in the air. In addition, this B^III complex displays structured vibronic Q-bands, slow S₁-state decay, and fluorescence (<i>Φ</i> _F = 0.30 and <i>τ</i> _F = 9.7 ns), in line with its aromatic nature, while the nonaromatic B^III complexes show ill-defined absorption spectra and very fast S₁-state decays.