Reactivity Differences in the Syntheses of Chelating N-Heterocyclic Carbene Complexes of Rhodium Are Ascribed to Ligand Anisotropy

Chelating bis(imidazolium) salts having (CH2)n chains of different lengths (n = 1−4) linking
the diazole rings show very large reactivity differences on metalation with [(cod)RhCl]2. Long
linkers favor a square-planar Rh(I) product, while short linkers favor octahedral Rh(III).
We ascribe the origin of the effect to the restricted rotation of the highly sterically anisotropic
diazole rings and the different preferred orientations of these rings as n changes. Defining
the x and y axes as the Rh−carbene bond directions, we find that with short linkers the
diazole rings tend to be oriented close to the xy plane. This tends to favor Rh(III) because
these complexes, [Rh(bis-carbene)I2(OAc)], have the lowest steric hindrance in the xy plane.
With long linkers, the diazole rings tend to be aligned face to face along the ±z axis. This
tends to favor Rh(I) because these complexes, [(cod)Rh(bis-carbene)]PF6, have the lowest
steric hindrance along the ±z axis. Crystallographic studies are reported. Electrospray MS
data provide evidence for strong metal−carbene binding.