title: The fate of NHC-stabilized dicarbon.
abstract: 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).
authors.affiliation: Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, 3086 (Australia).
authors.forename: Dayne C
authors.initials: DC
authors.lastname: Georgiou
authors.forename: Bradley D
authors.initials: BD
authors.lastname: Stringer
authors.forename: Conor F
authors.initials: CF
authors.lastname: Hogan
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Barnard
authors.forename: David J D
authors.initials: DJ
authors.lastname: Wilson
authors.forename: Nicole
authors.initials: N
authors.lastname: Holzmann
authors.forename: Gernot
authors.initials: G
authors.lastname: Frenking
authors.forename: Jason L
authors.initials: JL
authors.lastname: Dutton
keywords: carbenes; density functional calculations; donor-acceptor systems; ligands; synthetic methods