Hetero-bis(σ-aryl)dirhodium(III) caprolactamates. Electronic communication between aryl groups through dirhodium(III)

Abstract

Fourteen hetero-bis(σ-aryl)dirhodium(III) caprolactamates that differ by the two aryl groups at the axial positions of dirhodium have been synthesized in good yield and characterized. Copper(II) catalyzed oxidation of dirhodium(II) caprolactamate at room temperature in the presence of two arylboronic acids results in a mixture of a hetero-bis(σ-aryl)dirhodium(III) caprolactamate and two homo-bis(σ-aryl)dirhodium(III) caprolactamates for each arylboronic acid combination. The UV-visλ_max values for hetero-bisaryldirhodium(III) caprolactamates fall in between those for the corresponding homo-bisaryldirhodium(III) caprolactamates; electronic interaction between the two aryl groups occurs through dirhodium, but this transmission is probably indirect through the caprolactamate ligands rather than directly between rhodiums. The chemical shift for the carbon bound to Rh shows very limited dependence on the substituent from the aryl group on the adjacent Rh. Bisaryldirhodium(III) caprolactamates with electron-withdrawing substitutions have higher oxidation potentials than those with electron-donating substitutions. A plot of oxidation potentials versus the corresponding UV-visible absorption maxima for the bisaryldirhodium(III) caprolactamates shows a relationship between oxidation potentials and λ_max values. The electronic/electrochemical information obtained for hetero-bis(σ-aryl)dirhodium(III) caprolactamates suggests that communication between aryl substituents occurs.