A Stable Neutral Diborene Containing a BB Double Bond

The syntheses and molecular structures of four compounds are reported:  1 (RBBr3), 2 (R(H)2B−B(H)2R), 3 (R(H)BB(H)R), and 4 (RBH3) (R = :C{N(2,6-Pri2C6H3)CH}2). These compounds were characterized by single-crystal X-ray diffraction, 1H and 11B NMR, and elemental analyses. Compounds 2 and 3 were prepared by the KC8 reduction of 1 in Et2O. Compound 3 is the first structurally characterized neutral diborene (mean BB:  1.560(18) Å). The nature of the BB double bond in 3 was delineated by DFT computations.