Flexibility of N-Heterocyclic Carbene Ligands in Ruthenium Complexes Relevant to Olefin Metathesis and Their Impact in the First Coordination Sphere of the Metal

We present a detailed static and dynamics characterization of 11 N-heterocyclic carbene (NHC) ligands in Ru complexes of the general formula (NHC)Cl2Ru=CH2. Analysis of the dynamic trajectories indicates that the nature of the N substituent can result in extremely different flexibilities of the Ru complexes. In almost all the cases the N substituent trans to the Ru ylidene bond is severely folded so that it protects the vacant coordination position at the Ru center. Limited flexibility is instead associated with the N substituent on the side of the Ru ylidene bond. NHCs with a single ortho substituent, either a simple Me or a bulkier i-Pr group, have a preferential folding that bends the unsubstituted side of the ring toward the halide Ru halide plane. Analysis of the dynamics trajectories in terms of buried volume indicates that the real bulkiness of these systems can be somewhat modulated, and this flexibility is a key feature that allows NHCs to modulate their encumbrance around the metal in order to make room for bulky substrates. Analysis of the buried volume in terms of steric maps showed that NHCs with mesityl or 2,6-diisopropylphenyl N substituents have quite different reactive pockets: rather flat with constant pressure on the halide Ru halide plane in the former and vault-shaped with higher pressure on the sides in the latter. Regarding the NHCs with an ortho tolyl or i-Pr group on the N substituent, the steric maps quantify the higher impact of the unsubstituted side of the ligand in the first coordination sphere of the metal and evidence the overall C-s- and C-2-symmetric reactive pockets of the corresponding complexes. We believe that a detailed characterization of the differently shaped reactive pockets is a further conceptual tool that can be used to rationalize the experimentally different performances of catalysts bearing these ligands or to devise new applications.