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

Abstract

Bis(1-(4-tolyl)-carboran-2-yl)-(4-tolyl)-borane [(1-(4-MeC 6 H 4 )-closo-1,2-C 2 B 10 H 10 -2-) 2 (4-MeC 6 H 4 )B] (1), a new bis(ocarboranyl)-(R)-borane was synthesised by lithiation of the ocarboranyl precursor and subsequent salt metathesis reaction with (4-tolyl)BBr 2 . Cyclic voltammetry experiments on 1 show multiple distinct reduction events with a one-electron first reduction. In a selective reduction experiment the corre-sponding paramagnetic radical anion 1 * À was isolated and characterized. Single-crystal structure analyses allow an indepth comparison of 1, 1 * À , their calculated geometries, and the S 1 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.

Thermodynamic equilibrium between locally excited and charge-transfer states through thermally activated charge transfer in 1-(pyren-2′-yl)-o-carborane

“…This C1-C2 bond is notably longer in S 1 -CT than in S 1 -LE, with a value of 2.34 Å compared to 1.63 Å, respectively (for reference, the ground-state optimized structure shows a C-C bond length of 1.63 Å), in agreement with that in an o-carborane radical anion (2.37/2.39/ 2.311(3) Å). [75][76][77] The bond length is even more stretched when reaching the S 1 -M minimum (right molecular structure in Fig. 8), which has a C-C distance of 2.62 Å.…”

Synthesis of Iminoboryl o-Carboranes by Lewis Base Promoted Aminoborirane-to-Iminoborane Isomerization

“…The synthesis of alkynylated carboranes dates back to 1964, and these species have been widely used as a precursor for carborane B–H functionalization and as basic units in molecular rods, optoelectronic materials, and supramolecular design . In this context, we decided to synthesize the first BN isosteres of alkynylated carboranes, where the carboranyl group should provide kinetic and thermodynamic stabilization …”

NHC induced radical formationviahomolytic cleavage of B–B bonds and its role in organic reactions

“…8 c ,20 With respect to boron-centered radicals, well known anionic species of the type BR 3 ˙ − are generated by occupying the vacant p-orbital of an electron deficient borane with a single electron. 21 For example, our group recently reported a structurally characterized example, in which such a radical anion is stabilized by an ortho -carborane moiety. 21 i In contrast, neutral analogs result from coordination of a Lewis base (L) to the boron center giving an L–BR 2 ˙ adduct.…”