Synthesis of iron(II)‘C2-capped strapped’ porphyrin complexes and their reaction with dioxygen

Abstract

The synthesis of potentially quinquedentate ligands based on a C₂-capped porphyrinate ligand, the ‘C₂-capped strapped’ ligands, is described where the ‘strapping’ groups contain a pyridine function. The iron(II) complexes of the ligands undergo reversible oxygenation at room temperature in toluene solution, and show good stability to autoxidation. Interestingly the products of decomposition are not µ-oxo dimers but are iron(III) species which can be directly reduced back to the active iron(II) form. The affinities of the complexes for dioxygen have been determined. The relatively low affinities of these new complexes are ascribed to inhibition of the motion of the iron(II), which must occur on the binding of O₂. This is due to the locking of the complex in a ‘domed’ configuration, enhanced by the strapping group, and to unfavourable steric interactions of the strapping group with the parent porphyrin.