Rotational Isomerism of a Phoban-Derived First-Generation Grubbs Catalyst

Abstract

A ruthenium carbene complex bearing a bicyclic phosphine ligand instead of tricyclohexylphosphine of the otherwise identical first-generation Grubbs catalyst exhibits only a very broad singlet in the 31P NMR at room temperature instead of the expected sharp signal. For this complex as well as for similar derivatives, low-temperature NMR experiments and DFT calculations proved the existence of rotational isomers with the employed unsymmetrical bicyclic phosphine ligand. Such rotational isomers cannot be observed for the first- and second-generation Grubbs catalysts, due to the symmetrical nature of their ligands.