All-boron analogue of planar benzene on an osmium template

The synthesis of the planar hexaborane B₆ ring remains a long-standing and elusive ambition in boron chemistry, defying boron's intrinsic electron deficiency that drives it to favour polyhedral three-dimensional geometries rather than chains and rings. Owing to this domination, earlier attempts to stabilize the planar B₆ ring in a monometallic template encountered no success. Herein, we report the synthesis and structure of bis-<i>nido</i>-[Cp*Os(<i>η</i> ⁶-B₆H₁₁)] (Cp* = <i>η</i> ⁵-C₅Me₅) (1), the first planar [B₆H₁₁] ring stabilized by the monometallic late transition metal (TM) fragment [Cp*Os]. The fluxionality due to rapid exchange between the bridging hydrogens in 1 led to considerable structural intrigue, resembling the dynamic behaviour seen in <i>nido</i>-[B₆H₁₀]. Furthermore, we have successfully isolated the intermediate boron chains, <i>nido-arachno</i>-[Cp*Os(<i>η</i> ⁵-B₅H₁₂) (2) and <i>nido-arachno</i>-[Cp*Os(<i>η</i> ⁴-B₄H₉) (3). The B₅ chain in 2 and the B₄ chain in 3 are isoelectronic with the pentadienyl radical (C₅H₇) and the 1,3-butadiene radical cation (C₄H₆)⁺, respectively. Extensive multicenter bonding interactions are demonstrated to stabilize the unique flat ring, as well as the boron chains within monocapped scaffolds.