Isolable Borane-Based Diradical and Triradical Fused by a Diamagnetic Transition Metal Ion

Complex Fe­(bpyB)3 (1, bpyB = 5,5′-bis­(dimesitylboranyl)-2,2′-bipyridine) and its reduced species [(18-c-6)­K­(THF)2]·[Fe­(bpyB)3] (2) were synthesized. Their solid state and electronic structures were investigated by single crystal X-ray crystallography, electron paramagnetic resonance and UV–vis spectroscopy, and SQUID measurements. In 1 two bpyB radical anions are fused by a diamagnetic FeII ion, whereas in 2 all three bpyB ligands are in the radical state. Complex 1 possesses an open-shell singlet ground state with a singlet–triplet gap of 0.18 kcal mol–1 and 2 features an open-shell doublet ground state with a doublet–quartet gap of 1.4 kcal mol–1, as determined by SQUID measurements. The unpaired-electrons in 1 and 2 mainly delocalize over the boron atoms and the bipyrdine moieties with negligible spin density at the iron center. Complex 2 represents the first isolable example of boron-based triradicals.