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)2Rudouble bond; length as m-dashCHPh (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)2Rudouble bond; length as m-dashCHPh compared to (PCy3)2(Cl)2Rudouble bond; length as m-dashCHPh 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)2Rudouble bond; length as m-dashCHPh and (PCy3)2(Cl)2Rudouble bond; length as m-dashCHPh.