The influence of electronic modifications on rotational barriers of bis-NHC-complexes as observed by dynamic NMR spectroscopy.

There has been much debate about the σ-donor and π-acceptor properties of N-heterocyclic carbenes (NHCs). While a lot of synthetic modifications have been performed with the goal of optimizing properties of the catalyst to tune reactivity in various transformations (e.g. metathesis), direct methods to characterize σ-donor and π-acceptor properties are still few. We believe that dynamic NMR spectroscopy can improve understanding of this aspect. Thus, we investigated the intramolecular dynamics of metathesis precatalysts bearing two NHCs. We chose four systems with one identical NHC ligand (N,N'-Bis(2,4,6-trimethylphenyl)-imidazolinylidene (SIMes) in all four cases) and NHC(ewg) ligands bearing four different electron-withdrawing groups (ewg). Both rotational barriers of the respective Ru-NHC-bonds change significantly when the electron density of one of the NHCs (NHC(ewg)) is modified. Although it is certainly not possible to fully dissect σ-donor and π-acceptor portions of the bonding situations in the respective Ru-NHC-bond via dynamic NMR spectroscopy, our studies nevertheless show that the analysis of the rotation around the Ru-SIMes-bond can be used as a spectroscopic parameter complementary to cyclic voltammetry. Surprisingly, we observed that the rotation around the Ru-NHC(ewg)-bond shows the same trend as the initiation rate of a ring-closing metathesis of the four investigated bis-NHC-complexes.