Activation of N-heterocyclic carbenes by inorganic molecules

Submission note: Submitted in total fulfilment for the degree of Doctor of Philosophy to the Department of Chemistry and Physics, College of Science, Health and Engineering, La Trobe University, Victoria.

Thesis with publications.

The use of N-Heterocyclic carbenes (NHCs) as stabilising ligands in inorganic chemistry is of great current interest. NHCs have been shown to provide strong complexation with various species as well as the ability to activate small molecules and behave as spectator ligands in catalysis reactions. However, recently several independent studies have observed NHCs undergoing endocyclic ring expansion pathways when interacting with element hydride species. This degradation pathway involves a main group hydride activating the C–N bond, which results in endocyclic ring expansion and subsequent element hydride insertion. This pathway results in the NHC transitioning from the initial five-membered ring to a six-membered ring. Our theoretical studies investigated the insertion pathway using an N-methyl substituted NHC and various isolable molecules, replicating experimental conditions. Definitive trends amongst each of the hydride groups were identified, for which a general degradation pathway was observed, consisting of (1) formation of an adduct between NHC and the hydride, (2) first hydrogen migration from the element hydride to the carbenic carbon of the NHC, (3) elongation of the carbene C–N bond to allow insertion, (4) secondary hydrogen migration to the carbonic carbon to produce a stable six-membered ring with the main group element having inserted adjacent to the carbenic carbon. Our calculations also determined that formation of C-H bonds was the driving force behind this reported insertion. We also investigated the effect of the electronic structure of the NHC, including unsaturated, saturated and benzimidazole NHCs that are typically utilised in synthesis. The effect on reaction thermodynamics and kinetic barriers was able to be rationalised by the aromatic behaviour of the NHCs. Finally, an investigation of the insertion mechanism for groups 12, 13, 14 and 16 element hydrides suggested further possibilities of experimental observation of NHC insertion reactivity.