Indenylidene Complexes of Ruthenium Bearing NHC Ligands - Structure Elucidation and Performance as Catalysts for Olefin Metathesis

Second-generation catalysts of the general formula Cl2Ru-(SIMes)(L)(3-phenylinden-1-ytidene), 3a (L = PCy3), 3b (L = PPh3), 3c (L = py), and Cl2Ru(SIMe)(L)(3-phenyhnden-1-yl-idene), 4a (L = PCy3), 4b (L = PPh3), 4c (L = py) were found to be of interest in various metathesis transformations. The catalysts containing SIMe ligands showed improved initiation compared to the more robust SIMes substituted catalysts. A strong temperature effect was noted on all of the reactions tested. Interestingly, complex 3a, showing the lowest initiation rate at room temperature, emerged as the most productive of all systems examined at elevated temperature. It is shown that complexes containing the SIMe ligand display higher initiation efficiency than their corresponding SIMes analogues. Since the higher initiation is related to the ease of phosphane dissociation while phosphane dissociation also promotes catalyst decomposition, complexes bearing the SIMe ligand decompose faster. The complete H-1, C-13 and P-31 resonance assignment and the procedure applied to obtain these from a combination of 1D and 2D NMR techniques, is also reported. Combined with the ROESY technique, these enabled to investigate several conformational processes involving rotations around N-phenyl and C-Ru bonds on the millisecond to second timescale. A clear correlation is demonstrated between the bulkiness of the axial ligand (L) and the rotational freedom of the SIMe(s) ligand. A qualitative analysis also suggests that the extra para-methyl of SIMes leads to additional steric interactions with the 3-phenylinden-1-ylidene ligand. The data reported in this paper demonstrates that substitution patterns of the N-aryl have a significant influence on the activity of the second-generation indenylidene catalysts for a given metathesis reaction. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)