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

A series of alkyl (<b>1</b>−<b>3</b>), aryl (<b>6</b>), and benzo-annulated (<b>4</b>, <b>5</b>) 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 <b>1</b> 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 <i>a</i>- and <i>c</i>-faces of the pyridazine moiety were free of substitution (<b>1</b>, <b>3</b>), planar structures resulted, but upon substitution, a twisted B₂N₂C₂ core was observed due to steric repulsion of neighboring hydrogen atoms (e.g., <b>5</b>). The increase of steric bulk from H (<b>1</b>) to <i>ⁱ</i>Pr (<b>3</b>) 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 <b>3</b> and <b>5</b>, which were characterized by broad, featureless singlets in the EPR spectra; [<b>3</b>]^•-[Cp*₂Co]⁺ was characterized by X-ray crystallography. While the planar structures (<b>1</b>−<b>4</b>) 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 (<b>5</b>, <b>6</b>) were found to be nonfluorescent.