Mössbauer Study on the Electron Transfer Rate Depending on the Intermolecular Interaction in Iron (II, III) Mixed-Valence Complex

Abstract

active sites of metalloproteins play important roles in biological systems. The model compounds of these compounds have been studied intensively and characterized by using various methods. One of the model complexes is a family of carboxylato mixed-valence diiron(II,III) complexes. For example, mixed-valence diiron complexes with a heptadentate polypyridine ligand (bpmp), the ligand having imidazol groups (bimp), and the ligand having phenol groups (bbpmp), have been reported previously (scheme 1). In the investigation of mixed-valence complexes with such ligands, knowledge of the mechanism of the electron transfer between the two iron centers is most important. Furthermore, in the field of nanomaterial science, understanding the electron transfer phenomena at single molecule level is also important. The mixed-valence diiron(II,III) bpmp complexes with various carboxylic acid were reported by Maeda et al. previously, and characterized by using Mössbauer spectroscopy. They reported that the electron trapped/detrapped states are affected by the kind of carboxylic acids, being bridging ligands. They also suggested that the rate of the intramolecular electron transfer is influenced by the crystal packings. Therefore it is thought that the intermolecular interactions play an important role in the electron transfer of the mixed-valence state. We focused on the intermolecular interactions in the mixed-valence state, and prepared two types of samples of the mixed-valence diiron(II,III) compound [FeFe(bpmp) (ena)2](ClO4)2, where Hbpmp represents 2,6-bis[bis-(2-pyridylmethyl)-aminomethyl]-4-methylphenol and Hena represents enanthic acid (Figure 1). One is a polycrystalline sample as prepared and the other is a film sample. The film sample was prepared by mixing an acetonitrile solution of poly(methyl methacrylate) (PMMA) and an acetonitrile solution of the complex, and obtained as a transparent film. In the film sample, the complex molecules were dispersed completely at a single molecular level and thus the intermolecular interaction would be very weak. Herein, we report that the rates of the electron transfer for the polycrystalline and film samples are different, depending on the strengths of the intermolecular interactions. Hereafter, the polycrystalline sample of [FeFe(bpmp)(ena)2](ClO4)2 is represented as 1, and the film sample as 2. Mössbauer Study on the Electron Transfer Rate Depending on the Intermolecular Interaction in Iron(II, III) Mixed-Valence Complex