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 9H-borabicyclo­[3.3.1]­nonane (9H-BBN) and Piers’
borane ((C6F5)2BH) furnishes the
intramolecularly N→B-coordinated compounds PzBBN and PzBPF 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 PzBBN•– and PzBPF•– 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 [PzBPF]•–[CoCp2]+ allowed further corroboration of the π-delocalized
nature of these radicals.