The Fate of NHC-Stabilized Dicarbon

The attempted synthesis of NHC‐stabilized dicarbon (NHCCCNHC) through deprotonation of a doubly protonated precursor ([NHCCHCHNHC]2+) is reported. Rather than deprotonation, a clean reduction to NHCCHCHNHC is observed with a variety of bases. The apparent resistance towards deprotonation to the target compound led to a reinvestigation of the electronic structure of NHC→CC←NHC, which showed that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gap is likely too small to allow for isolation of this species. This is in contrast to the recent isolation of the cyclic alkylaminocarbene analogue (cAACCCcAAC), which has a large HOMO–LUMO gap. A detailed theoretical study illuminates the differences in electronic structures between these molecules, highlighting another case of the potential advantages of using cAAC rather than NHC as a ligand. The bonding analysis suggests that the dicarbon compounds are well represented in terms of donor–acceptor interactions L→C2←L (L=NHC, cAAC). Theory of fate: Attempts to synthesize the N‐heterocyclic carbene (NHC)‐stabilized dicarbon, NHCCCNHC, through deprotonation of [NHCCHCHNHC]2+, yielded NHCCHCHNHC through a reduction process rather than the targeted deprotonation (see figure). A detailed theoretical study indicates why cAACCCcAAC (cAAC=cyclic alkylaminocarbene) is isolable whereas NHCCCNHC is not.