Conformational preferences of the axial ligands in some metalloporphyrins. A theoretical study

Abstract

The conformational preferences of the axial ligands have been determined for several metalloporphyrins MPL and MPLL′ (M = Mo, Fe; P = porphine dianion; L and L′ being the axial ligands). For MoP(C2H2) a qualitative analysis indicates that the conformation with the acetylenic bond eclipsing two Mo-N bonds will be favored. Ab initio SCF calculations indicate that: (i) iron porphyrins with an axial imidazole ligand show a flat potential energy curve for the rotation of the imidazole ligand; (ii) iron porphyrins with a dioxygen ligand prefer the staggered conformation with the O-O bond projecting along the bisectors of the Fe-N bonds; (iii) in the cis-dinitrosyl molybdenum porphyrin, the nitrosyl ligands should be eclipsed with respect to the Mo-Npyr bonds.