Porphyrins Fused to N-Heterocyclic Carbenes (NHCs): Modulation of the Electronic and Catalytic Properties of NHCs by the Central Metal of the Porphyrin

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N-heterocyclic carbene ligands 1M. Rhodium(I) complexes 6M-9M were prepared by using 1M ligands with different metal cations in the inner core of the porphyrin (M=Ni-II, Zn-II, Mn-III, Al-III, 2H). The electronic properties of the corresponding N-heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [(1M)Rh(cod)Cl] and [(1M)Rh(CO)(2)Cl] complexes (cod=1,5-cyclooctadiene). Porphyrin-NHC ligands 1M with a trivalent metal cation such as Mn-III and Al-III are overall poorer electron donors than porphyrin-NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni-II and Zn-II. Imidazolium salts 3M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring-opening polymerization of L-lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene.