Reactivity Of Cyano- And Isothiocyanatoborylenes: Metal Coordination, One-Electron Oxidation And Boron-Centred Bronsted Basicity

Doubly base-stabilised cyano- and isothiocyanatoborylenes of the form LL ' BY (L = CAAC = cyclic alkyl(amino)carbene; L ' = NHC = N-heterocyclic carbene; Y = CN, NCS) coordinate to group 6 carbonyl complexes via the terminal donor of the pseudohalide substituent and undergo facile and fully reversible one-electron oxidation to the corresponding boryl radical cations [LL ' BY](+). Furthermore, calculations show that the borylenes have very similar proton affinities, both to each other and to NHC superbases. However, while the protonation of LL ' B(CN) with PhSH yielding [LL ' BH(CN)(+)][PhS-] is fully reversible, that of LL ' B(NCS) is rendered irreversible by a subsequent B-to-C-CAAC hydrogen shift and nucleophilic attack of PhS- at boron.