Metal-ligand bonding properties of double-sided porphyrin complexes: Influence of bulky ester groups

Metal-ligand bonding properties of double-sided porphyrinato-iron(II) and -cobalt(II) complexes have been characterized by ESR, IR, and Mössbauer spectroscopy. The smaller A_N value for a 1-methyl-imidazole (mim) adduct of 5,10,15,20-tetra(2,6-dipivaloyloxyphenyl)porphyrinatocobalt(II) compared to that of 5,10,15,20-tetra(2,6-di-tert-butylacetoxyphenyl)porphyrinatocobalt(II) suggested that the cobalt-imidazole bond is weak. The ESR spectrum of the dioxygenated double-sided series in fluid toluene indicated that an electrostatic interaction between the bound dioxygen and the ester fences was rarely found. The relaxation of steric strain on the rear side of the ring plane for axial imidazole bonding resulted in a lowering of the bound CO and O₂ stretching frequencies of iron(II) complexes. This indicates that the π-back donation from the d_π orbital of the iron to the π* orbital of the bound gaseous ligand could be controlled by the strength of the iron-imidazole bonding, which is regulated by the structure of the rear pocket on the macrocycle. The co-ordination structure of various ligands in double-sided porphyrinatoiron complexes is also discussed by means of Mössbauer parameters.