N‑Heterocyclic Carbene-Stabilized Boranthrene as a Metal-Free Platform for the Activation of Small Molecules

The multielectron reduction of small
molecules (e.g., CO2) is a key aspect of fuel synthesis
from renewable electricity. Transition
metals have been researched extensively in this role due to their
intrinsic redox properties and reactivity, but more recently, strategies
that forego transition metal ions for p-block elements
have emerged. In this vein, we report an analogue of boranthrene (9,10-diboraanthracene)
stabilized by N-heterocyclic carbenes and its one- and two-electron
oxidized congeners. This platform exhibits reversible, two-electron
redox chemistry at mild potentials and reacts with O2,
CO2, and ethylene via formal [4+2] cycloaddition to the
central diborabutadiene core. In an area traditionally dominated by
transition metals, these results outline an approach for the redox
activation of small molecules at mild potentials based on conjugated,
light element scaffolds.