Boryl Radicals and Anions, Borylenes, Diboranes, Neutral and Dianionic Diborenes: Diverse Reduction Chemistry of cAAC-Supported Hydroboron Halides

First designed by Bertrand and co-workers in 2005 cyclic alkyl(amino)carbenes (cAACs)1 have since enabled the isolation of a wide range of hitherto inaccessible low-valent and radical main group species.2 In this... [ view full abstract ]

First designed by Bertrand and co-workers in 2005 cyclic alkyl(amino)carbenes (cAACs)¹ have since enabled the isolation of a wide range of hitherto inaccessible low-valent and radical main group species.² In this fast-advancing field their prevalence over traditional N-heterocyclic carbene (NHC) ligands is owed to their enhanced σ-donor and π-acceptor abilities, the latter preventing the build-up of charge on low oxidation state p-block elements. With regards to boron, cAACs have been successfully employed to stabilize a highly reactive diboracumulene species,³ boryl radicals⁴ and a number of two- and three-coordinate borylenes.⁵

In this contribution we present the surprising variety of low-valent boron species selectively accessible by controlled reduction of cAAC-supported hydroboron halides of the formula [(cAAC)BX_nH_3-n], ranging from novel boryl radicals to easily accessible mixed ligand borylenes via diboranes and highly reactive boryl anions, as well as a new, facile synthetic route to the versatile diborene [(cAAC)BH]₂ and its dianion.

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