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)(2)B-B(H)(2)R'), with B=B double and B-B single bonds, respectively. These compounds were characterized by single-crystal X-ray diffraction, H-1 and B-11 NMR, and elemental analyses. Neutral diborene 3 exhibits polymorphic planar (3a), twisted (3b), and trans-bent (3c) geometries in the solid state.