Pyrrole ^βC-B-N Fused Porphyrins: Molecular Structures and Opto-Electrochemical Studies

Herein, we report the design, synthesis, structure, and electrochemical study of doubly ^βC-B-N fused Ni(II) porphyrins (1-trans, 1-cis, 2-trans, and 2-cis). These compounds have been synthesized from A₂B₂ type dipyridyl Ni(II) porphyrins (Ar=Ph for 1 a; Ar=C₆F₅ for 2 a) via Lewis base-directed electrophilic aromatic borylation reactions. The solution state structures of these compounds have been established using ¹H NMR, ¹¹B NMR, ¹H-¹H COSY, ¹H-¹³C HSQC, and ¹⁹F-¹³C HSQC NMR techniques. Single crystal X-ray analysis have revealed that 1-trans, 1-cis, and 2-trans adopt ruffled conformations, with alternate meso-carbons on the opposite sides of the mean porphyrin plane. The Soret bands in the absorption spectra of the B-N fused molecules are ~40 nm redshifted compared to unfused Ni(II) porphyrin precursors. The B-N fusion have diminished the redox potential of fused porphyrins. Although 1-trans and 1-cis, show four oxidation processes, 2-trans and 2-cis show only three oxidation processes. DFT studies have revealed that the tetrahedral geometry of the boron has induced a twist in the π-conjugation, which destabilizes the HOMO and stabilizes the LUMO in 1-trans, 1-cis, 2-trans, and 2-cis.