Completing the series of boron-nucleophilic cyanoborates: boryl anions of type NHC–B(CN)2−

Abstract

Since the first seminal report of boron-centred nucleophiles, the area of boryl anions has developed only sporadically and requires further systematisation. The boryl anions of type NHC–B(CN)₂⁻ (NHC = N-heterocyclic carbene) described herein complete a consistent series with the known anions cAAC–B(CN)₂⁻ [cAAC = cyclic(alkyl)(amino)carbene] and B(CN)₃²⁻. A novel approach towards NHC-stabilised cyanoboranes based on alkylthio-cyano exchange at boron is presented, and in contrast to other methods affords the products in better purity and yield. Reduction of suitable NHC–dicyanoboranes gave two unprecedented examples of NHC–B(CN)₂⁻ boryl anions. The latter were shown to react as boron-centred nucleophiles with facile formation of B–E bonds, where E = C, Si, Sn, P, Au. Bonding analysis by DFT calculations suggests a systematic variation of the energy of the boron-centred HOMO depending on the carbene, which in turn can control the nucleophilic character.