Synthesis and Structure of an o ‐Carboranyl‐Substituted Three‐Coordinate Borane Radical Anion

Bis(1‐(4‐tolyl)‐carboran‐2‐yl)‐(4‐tolyl)‐borane [(1‐(4‐MeC6H4)‐closo‐1,2‐C2B10H10‐2‐)2(4‐MeC6H4)B] (1), a new bis(o‐carboranyl)‐(R)‐borane was synthesised by lithiation of the o‐carboranyl precursor and subsequent salt metathesis reaction with (4‐tolyl)BBr2. Cyclic voltammetry experiments on 1 show multiple distinct reduction events with a one‐electron first reduction. In a selective reduction experiment the corresponding paramagnetic radical anion 1.− was isolated and characterized. Single‐crystal structure analyses allow an in‐depth comparison of 1, 1.−, their calculated geometries, and the S1 excited state of 1. Photophysical studies of 1 show a charge transfer (CT) emission with low quantum yield in solution but a strong increase in the solid state. TD‐DFT calculations were used to identify transition‐relevant orbitals.
Stabilization via three‐coordinate boron allows the isolation of a paramagnetic radical anion of ortho‐carborane. A photophysical, electrochemical and crystallographic study backed by DFT calculations shines light on closo‐nido intermediate structures, charge transfer processes and electron accepting ability.