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.