An Anionic Mesoionic Carbene and its Transformation to Metallo-Mesoionic Carbenes: Synthesis, Properties and Catalysis

Neutral mesoionic carbenes (MICs) based on a 1,2,3-triazole core have had a strong impact on various branches of chemistry such as homogeneous catalysis, electrocatalysis, and photochemistry/photophysics over the past 15 years. We present here the first general route for the synthesis of anionic mesoionic carbenes (anMICs) based on a 1,2,3-triazole core and a borate backbone. The free anMIC is stable under an inert atmosphere at low temperatures, can be stored for several weeks, and can be coordinated to both main group and transition metal fragments. Analysis of donor properties by a battery of spectroscopic methods shows that these anMICs are extremely strong σ-donors, by-passing the donor properties of related strong donors such as MICs, N-heterocyclic carbenes, anionic N-heterocyclic carbenes and N-heterocyclic olefins. Intriguingly, they also display π-accepting properties. The room temperature conversion of the free anMICs leads to two equally interesting compound classes: an amide coordinated borane based on a MIC-borane backbone, and an amide coordinated metallo-MIC-borane. The metallo-MIC-borane is an interesting precursor for the synthesis of further amide-coordinated MIC-borane compounds, and the first example of a gold(I) complex is reported here. Gold(I) complexes of the anMIC ligands are potent catalysts for the hydroamination of alkynes without the need for any additional reagents. This report showcases a straightforward synthesis of anMIC ligands, and their conversion to two equally interesting compounds. The compounds have both exceptional donor properties, and a high potential in catalysis. We thus introduce three new categories of mesoionic compounds here with a huge potential for different branches of chemistry and beyond.