Intramolecular N→B Coordination as a Stabilizing Scaffold for π‑Conjugated Radical Anions with Tunable Redox Potentials

In this work we report on a preparative strategy that allows the facile tuning of the electron affinities of π-conjugated organic n-type materials via intramolecular N→B coordination: 2-fold hydroboration of a pyrazine-derived substrate with 9<i>H</i>-borabicyclo­[3.3.1]­nonane (9<i>H</i>-BBN) and Piers’ borane ((C₆F₅)₂BH) furnishes the intramolecularly N→B-coordinated compounds <b>PzBBN</b> and <b>PzBPF</b> in high yield. For each borane racemic mixtures of the respective chiral derivatives and achiral meso compounds have been isolated, and their electrochemical, optical, and structural properties have been investigated and complemented by DFT calculations. The boranes exhibit high electron affinities with electrochemical LUMO energy levels of −3.69 and −4.30 eV, respectively. The corresponding radical anions <b>PzBBN</b>^{<b>•–</b>} and <b>PzBPF</b>^{<b>•–</b>} can be reversibly generated and are stable in solution at ambient temperature. Analysis of the radical species by EPR spectroscopy and single-crystal analysis of [<b>PzBPF]</b>^{<b>•–</b>}<b>[CoCp</b>_<b>2</b><b>]</b>^<b>+</b> allowed further corroboration of the π-delocalized nature of these radicals