Crystalline Boron-Centered Analogues of Müller's Hydrocarbon

Abstract

Müller's hydrocarbon is a well-known open-shell singlet diradicaloid, yet its structural determination and redox property remain elusive due to its extremely high reactivity. Herein, we report the successful synthesis and full characterizations of the first neutral boron-centered analogue (4) of Müller's hydrocarbon, along with the first dianionic boron-centered analogue (52−) featuring three isolable redox states. In the presence of two equivalents of N-heterocyclic carbene (NHC), reduction of 4,4”-bis(triisopropylphenylbromoboryl)terphenyl 3 with potassium graphite afforded NHC-stabilized boryl diradicaloid 4 with a near-pure diradical character (y0 = 0.93). Stepwise reductions of 4,4”-bis(dimesitylboryl)terphenyl 5 in THF yielded the isolable monoradical anion 5·− and diradical dianion 52−, respectively, accompanied by a decreasing aromaticity within the conjugated spacer. Experimental studies and theoretical analyses revealed that both 4 and 52− exhibit large spin distributions at boron atoms, open-shell singlet ground states, and small singlet-triplet energy gaps.