Porphyrins fused to N-heterocyclic carbenes (NHCs): modulation of the electronic and catalytic properties of NHCs by the central metal of the porphyrin.

ABSTRACT

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N-heterocyclic carbene ligands 1 M. Rhodium(I) complexes 6 M-9 M were prepared by using 1 M 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 [(1 M)Rh(cod)Cl] and [(1 M)Rh(CO)2 Cl] complexes (cod=1,5-cyclooctadiene). Porphyrin-NHC ligands 1 M 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 3 M (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.