On the Electronic Impact of Abnormal C4-Bonding in N-Heterocyclic Carbene Complexes

Graphical Abstract

A world of a difference: While the two diimidazolylidene complexes (see picture) are structurally identical, they feature either abnormal C4-bound (top) or normal C2-bound heterocyclic carbene ligands. This different carbene bonding mode has a marked influence on the stability, reactivity, and catalytic activity of the coordinated metal. The approach presented here allows for an unambiguous differentiation of the electronic impact of abnormal versus normal carbene bonding.

Abstract

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H₂ activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.