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. B−N fusion induced extended conjugation and tetracoordinated boron caused twisted structure in βC−B−N fused Ni(II) porphyrins (1‐trans, 1‐cis, 2‐trans, and 2‐cis) exhibit red shifted absorption than non‐fused compounds. The fused porphyrins are easier to reduce and easier to oxidize compared to compounds without B−N fusion.