Planar, Twisted, and Trans-Bent:  Conformational Flexibility of Neutral Diborenes

The potassium graphite reduction of R‘BBr₃ (R‘ = :C{N(2,4,6-Me₃C₆H₂)CH}₂) in Et₂O led to the isolation of 3 (R‘(H)BB(H)R‘) and 4 (R‘(H)₂B−B(H)₂R‘), with BB double and B−B single bonds, respectively. These compounds were characterized by single-crystal X-ray diffraction, ¹H and ¹¹B NMR, and elemental analyses. Neutral diborene 3 exhibits polymorphic planar (3a), twisted (3b), and trans-bent (3c) geometries in the solid state.