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

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-F-2-C6H3 < 2,4,5-F-3-C6H2 < 2,6-F-2-C6H3 < 2,4,6-F-3-C6H2. This order of reactivity is consistent with a number of simple NMR measures of the electronic properties of these systems, including (1)J(CH) of the NHCHBF4 salts, delta(Se-77) of the NHC selenides and (1)J(Rh-C) and delta(C-13(carbene)) of the [Rh(cod)Cl(NHC)] complexes.