Carbene-stabilized main group radicals and radical ions

Abstract

Shortly after their discovery at the end of the 80s, stable singlet carbenes have been recognized as excellent ligands for transition metal based catalysts, and as organo-catalysts in their own right. At the end of the 2000s, it has been shown that they can coordinate main group elements in their zero oxidation state, and even activate small molecules. This review covers examples in the literature dealing with the most recent application of stable singlet carbenes, namely their use to stabilize radicals and radical ions that are otherwise unstable.

On the nature of the dative bond: Coordination to metals and beyond. The carbon case

“…Recently, a non-negligible -accepting character of aminocarbenes [75] has been claimed to account for the stabilization of main-group radical or radical ions centered at boron [76], silicon [77], or phosphorus [78]. The same -donating and π-accepting characters as above could also allow the description of tetraaminoethenes as diaminocarbene dimers [79].…”

An overview of N-heterocyclic carbenes

“…Another interesting aspect of NHC coordination to p-block elements that is attracting an increasing amount of research attention is the stabilization of main-group radicals 84 . The p-accepting capability of NHCs once again has an important role in these systems, because delocalization of spin density across the adduct can result in greater stability.…”

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η6-Diborabenzene Ligand

“…We attribute this discrepancy to the significant ligand contribution to the HOMO orbitals in 2-4. The stepwise reduction is evidently favoured by the involvement of the B-CAAC moietiesCAACs are well known for their ability to stabilise radical species [38][39] -and population of the largely CAAC-based LUMO of 2-4 upon reduction would avoid the formation of an unstable 19e -metal centre. Chemical Reduction.…”