Identification of low-valent iron-porphyrins: Electronic structure and reactivity towards CO₂ reduction

Abstract

The need to close the carbon cycle becomes urgent as concentration of CO₂ in the earth atmosphere increases exponentially. Electrochemically-mediated conversion of CO₂ into biofuels is a possible solution to this problem, as it enables storage of energy while using the harmful CO₂ feedstock as starting material. Notably, low-valent iron-porphyrins have been established as the best and most selective family of homogeneous catalysts for CO₂ reduction into CO, a first step towards the synthesis of biofuel from CO₂. The purpose of this research project is to correlate the high activity of [Fe(TPP)] (TPP=tetraphenylporphyrin) towards CO₂ reduction with its peculiar electronic structure. With this knowledge in mind, a guideline for the synthesis of efficient and selective catalysts is proposed. To realize this purpose, we investigated the electronic structure of [Fe(TPP)], [Fe(TPP)]^- and [Fe(TPP)]^2- using a combination of theoretical chemistry coupled with experimental spectroscopic techniques, such as 57Fe Mössbauer, magnetometry measurements and X-ray absorption spectroscopy. After unambiguously determining the electronic structure of each of these species, a reactivity study was carried out to establish the correlation between electronic structure and reactivity. <br> Our results indicate that [Fe(TPP)] is a triplet iron(II) ground state with a quite unique almost three-fold orbital degeneracy of the d_xz, d_yz and d_z2 orbitals. This peculiar electronic structure leads to a large, unquenched orbital angular momentum lying on the porphyrin plane, as observed experimentally. Comparing spectroscopic data of [Fe(TPP)]^- and [Fe(TPP)]^2- to that of [Fe(TPP)] led to the conclusion that the oxidation state of iron remains unchanged upon one- and two-electrons reduction of [Fe(TPP)]. In other terms, [Fe(TPP)]^- and [Fe(TPP)]^2- are best described as Fe(II) centers antiferromagnetically coupled with a porphyrin ligand radical and diradical, respectively. In fact, our reactivity study shows how the non-innocent porphyrin ligand in [Fe(TPP)]^2- plays a major role in the high reactivity of the complex towards CO₂ reduction, by acting as an electron reservoir able to transfer electrons to the CO₂ molecule via the metal center. We stress that ligand non-innocence is a common feature in CO₂ reduction catalysts, but its role relative to catalytic activity has not yet received sufficient attention.