m-Benzisubporphyrins: The First Subcarbaporphyrin and Its BIII Complex with An Unprecedented Planar Tridentate 14 π-Aromatic Network

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