Conformational Flexibility of Hoveyda-Type and Grubbs-Type Complexes Bearing Acyclic Carbenes and Its Impact on Their Catalytic Properties

In this contributione density functional theory was used to gain insight into the conformational flexibility of acyclic diaminocarbenes and their corresponding Hoveyda- and Grubbs-class ruthenium complexes. Remarkably, all known crystal structures of acyclic carbenes and their Hoveyda-type Ru complexes show only one specific conformer. We demonstrate that rotation about the C–N bonds in the free acyclic diaminocarbenes is thermally accessible (∼20 kcal/mol), but is relatively restricted upon coordination, primarily due to steric constraints. As a consequence, the capacity of the ADC ligands to sample multiple conformations is reduced following coordination. In addition, we show that the alkylidene proton has little contribution to the stabilization of the precatalysts, but has a non-negligible impact on the stabilization and energy barriers of both the transition states and products of the initiation phase of the metathesis.