A Persistent Boron Tetraradical with Tetraphenylethene-Linked Boraphenanthrene-Type Units

We report the first paramagnetic boron tetraradical, comprising four boraphenanthrene-type units with boryl radical centers bridged by a central tetraphenylethene (TPE) linker. With strongly π-accepting and sterically demanding cyclic(alkyl)(amino) carbene ligands (3), spin densities localize on the boron-carbene fragments (92%), consistent with a true boron-centered tetraradical. Magnetic measurements of 3 reveal minimal spin-spin coupling, consistent with four noninteracting S = 1/2 centers. In contrast, weaker π-accepting diamino N-heterocyclic carbene ligands delocalize spin density over the TPE core (88%), yielding a boron-containing hydrocarbon biradical (4). Compound 4 exhibits antiferromagnetic interactions (2J = -118 J·mol-1), supporting an open-shell singlet ground state with a thermally accessible triplet excited state.