A Stable Doubly Hydrogen-Bridged Butterfly-Shaped Diborane(4) Compound

Abstract

In contrast to the common multiple bonding between carbon atoms, multiply bonded boron compounds have still been a synthetic challenge due to the electron deficiency of boron. We now report that a stable doubly hydrogen-bridged diborane(4), EindB(μ-H)₂BEind, is produced by the two-electron oxidation of a hydrogen-substituted diborane(4) dianion [Li⁺(thf)]₂[Eind(H)BB(H)Eind]²⁻, where Eind denotes the 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl. The X-ray crystallography reveals a short B−B distance of 1.4879(7) Å in comparison with the normal B−B single bond length (1.72 Å), the presence of two hydrogen atoms bridged perpendicular to the B−B bond with a butterfly shape having a dihedral angle of the two BHB triangles of 113(1)°, and a linear geometry around the B−B bond with a C−B−B bond angle of 178.92(4)°. These structural data, experimental electron density analysis, and computational studies confirm the 3-fold bonding (a σ and two π-like bonds) between the two boron atoms incorporating the two bridging hydrogen atoms.