Simple NMR predictors of catalytic hydrogenation activity for [Rh(cod)Cl(NHC)] complexes featuring fluorinated NHC ligands

Abstract

A series of imidazolium salts precursors for N-heterocyclic carbenes (NHCs) featuring fluoroaryl substituents have been prepared along with their selenides and rhodium complexes. Tests of the catalytic activity of the [Rh(cod)Cl(NHC)] complexes in the transfer hydrogenation of acetophenone with iPrOH shows that the rhodium complexes bearing fluorinated NHCs are better than their non-fluorinated counterparts. The order of activity being 4-F-C₆H₄ < 2,4-F₂-C₆H₃ < 2,4,5-F₃-C₆H₂ < 2,6-F₂-C₆H₃ < 2,4,6-F₃-C₆H₂. This order of reactivity is consistent with a number of simple NMR measures of the electronic properties of these systems, including ¹J_CH of the NHC·HBF₄ salts, δ(⁷⁷Se) of the NHC selenides and ¹J_Rh–C and δ(¹³C_carbene) of the [Rh(cod)Cl(NHC)] complexes.