Synthesis and isolation of dinuclear N,C-chelate organoboron compounds bridged by neutral, anionic, and dianionic 4,4′-bipyridine via reductive coupling of pyridines

Abstract

The reaction of Bppy(Mes)2 (BN1; ppy = 2-phenylpyridine) and BCH₂ppy(Mes)₂ (BN3) with the reducing reagent KC₈ resulted in C–C bond formation via intermolecular radical coupling to generate the 4,4′-bipyridyl ligand compounds BN2 and BN4. Adding 1 equivalent of KC₈ to a THF solution of BN2 and BN4 generated the 4,4′-bipyridyl radical anions BN2K and BN4K. The dianion species BN2K₂ and BN4K₂ could be obtained by adding 2 equivalents of KC₈ to the THF solution of BN2 and BN4. In the presence of 2,2,2-cryptand or 18-crown-6, the radical anion salt BN2K(crypt) and the dianion salt BN2K₂(18c6)₂ were isolated for single-crystal X-ray diffraction analysis. Structural, spectroscopic, and computational studies were performed on the three species of BN2 derivatives (neutral, radical anion, and dianion species). BN2 and BN4 were stable and did not undergo photoisomerization or photoelimination under UV light irradiation.