Variable temperature 1H NMR studies on Grubbs catalysts

Variable temperature 1H NMR studies were conducted to investigate whether steric congestion is influencing the structural rigidity of (IMesH2)(PCy3)(Cl)2RuCHPh (IMesH2 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) in solution. It was shown that both mesityl ligands rotate at about the same rate around the N-Mesityl bonds in the IMesH2 ligand and that changing the solvent does not significantly alter this rotation. It was found that the increased steric congestion in (IMesH2)(PCy3)(Cl)2RuCHPh compared to (PCy3)2(Cl)2RuCHPh does affect the rates of rotation around the Calkylidene–Ph bonds. Unusual chemical shift positions were also observed in the low temperature 1H NMR spectrum for the aromatic proton signals for (IMesH2)(PCy3)(Cl)2RuCHPh and (PCy3)2(Cl)2RuCHPh.

Graphical abstractVariable temperature 1H NMR studies were conducted on (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene)(PCy3)(Cl)2RuCHPh and (PCy3)2(Cl)2RuCHPh in solution. The findings include that (1) both mesityl goups rotate around the N-Mes bond at approximately the same rate in a range of solvents, (2) the rotation around the Calkylidene–Ph bond was slower for (IMesH2)(PCy3)(Cl)2RuCHPh compared to (PCy3)2(Cl)2RuCHPh.Figure optionsDownload full-size imageDownload as PowerPoint slide