Hexagonal Planar [B6H6] within a [B6H12] Borate Complex: Structure and Bonding of [(CpTi)2(μ‐ɳ6 : ɳ6‐B6H6)(μ‐H)6]

Isolation of planar [B6H6] is a long‐awaited goal in boron chemistry. Several attempts in the past to stabilize [B6H6] were unsuccessful due to the domination of polyhedral geometries. Herein, we report the synthesis of a triple‐decker sandwich complex of titanium [(Cp*Ti)2(μ‐η6 : η6‐B6H6)(μ‐H)6] (1), which features the first‐ever experimentally achieved nearly planar six‐membered [B6H6] ring, albeit within a [B6H12] borate. The small deviation from planarity is a direct consequence of the predicted structural pattern of the middle ring in 24 Valence Electron Count (VEC) triple‐decker complexes. The large ring size of [B6H6] in 1 brings the metal–metal distance into the bonding range. However, significant electron delocalization from the M−M bonding orbital to the bridging hydrogen and B−B skeleton in the middle decreases its bond strength. A hexagonal borane [B6H6] ring supported by six B−H−Ti bonds in the coordination sphere of {Cp*Ti} fragments is formed from the reaction of [Cp*TiCl3] with [LiBH4 ⋅ THF] followed by thermolysis in presence of an excess of [BH3 ⋅ THF].