# All-boron analogue of planar benzene on an osmium template

## TL;DR

Scientists synthesized a flat boron ring stabilized by a metal complex, resembling benzene's structure.

## Contribution

The first planar hexaborane ring stabilized by a monometallic osmium complex was synthesized and characterized.

## Key findings

- The planar [B6H11] ring was stabilized using a [Cp*Os] fragment.
- Boron chains in the complex showed isoelectronic similarities to organic radicals.
- Dynamic fluxionality in the complex resembled behavior seen in other borane structures.

## Abstract

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-nido-[Cp*Os(η6-B6H11)] (Cp* = η5-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 nido-[B6H10]. Furthermore, we have successfully isolated the intermediate boron chains, nido-arachno-[Cp*Os(η5-B5H12) (2) and nido-arachno-[Cp*Os(η4-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.

The first planar B6 ring stabilized in the coordination sphere of a monometallic complex has been structurally characterized. It exhibits an aromaticity pattern similar to that of a benzene ring stabilized in double-decker sandwich complexes.