ReB6-: A Metallaboron Analog of Metallabenzenes

Metallabenzenes are a class of molecules in which a CH unit in benzene is replaced by a functionalized transition-metal atom. While all-boron analogues of aromatic and antiaromatic hydrocarbons are well-known, there have not been any metallaboron analogs. We have produced and investigated two metal-doped boron clusters, ReB6- and AlB6-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra have been obtained and compared with the theoretical results. The ReB6- cluster is found to be perfectly planar with a B-centered hexagonal structure (C-2v, (1)A(1)), while AlB6- is known to have a similar structure, but with a slightly out-of-plane distortion (C-s, (1)A'). Chemical bonding analyses show that the closed-shell ReB6- is doubly sigma- and pi-aromatic, while A1B(6)(-) is known to be sigma-aromatic and pi-antiaromatic. The out-of-plane distortion in AlB6- is due to antiaromaticity, akin to the out-of-plane distortion of the prototypical antiaromatic cyclooctatetraene. The pi-bonding in ReB6- is compared with that in both benzene and rhenabenzene [(CO)(4)ReC5H5], and remarkable similarities are found. Hence, ReB6- can be viewed as the first metallaboron analog of metallabenzenes and it may be viable for syntheses with suitable ligands.