Effects of 5,5-substitution on dipyrrolylmethane ligand isomerization.

The difference in steric profiles for eta(5) and eta(1)-pyrrolyls can be used to modify barriers for pyrrolyl exchange. The method investigated incorporated 1,3-diaxial interactions in a cyclohexyl backbone of the dipyrrolylmethane. Some of the enthalpic barrier created with the 1,3-steric interactions appears to be mediated by increased entropy in the ground state, but noticeable changes are produced in the pyrrolyl exchange rates by (1)H NMR without steric crowding at the metal center.