Triphenylene Analogues with B₂N₂C₂ Cores:  Synthesis, Structure, Redox Behavior, and Photophysical Properties

A series of alkyl (1−3), aryl (6), and benzo-annulated (4, 5) heteroaromatic triphenylene analogues with B₂N₂C₂ cores have been synthesized via chelation of pyridazine derivatives using difunctional Lewis acidic diborabiphenyl precursors. In contrast to triphenylene, NICS(1) calculations on 1 suggested high aromaticities for the central (−11.3 ppm) and outer borabenzene rings (−7.7 ppm), along with nonaromatic behavior for the pyridazine ring (−0.7 ppm). Crystal structure analyses supported this analysis. When the a- and c-faces of the pyridazine moiety were free of substitution (1, 3), planar structures resulted, but upon substitution, a twisted B₂N₂C₂ core was observed due to steric repulsion of neighboring hydrogen atoms (e.g., 5). The increase of steric bulk from H (1) to ⁱPr (3) in the planar species was found to result in a dimeric, head-to-tail herringbone packing motif, held together by close intermolecular B···N interactions of 3.39 Å. One-electron reduction by Cp*₂Co was found to afford the radical anions of 3 and 5, which were characterized by broad, featureless singlets in the EPR spectra; [3]^•-[Cp*₂Co]⁺ was characterized by X-ray crystallography. While the planar structures (1−4) were observed to possess weak fluorescence (Φ_F = 0.02−0.08) with either yellow−orange (ca. 555 nm) or green emission (521 nm), the twisted structures (5, 6) were found to be nonfluorescent.