Observation of Iron-valence Detrapping in Mixed-valence Trinuclear Heterometal Pentafluorobenzoate Cluster Complex Cr3+Fe3+Fe2+O(C6F5COO)6(C5H5N)3·0.5C5H5N·CH2Cl2 by 57Fe-Mössbauer Spectroscopy

Abstract

A number of Fe-Mössbauer spectroscopic investigations of mixed-valence trinuclear iron carboxylate cluster complexes with a triangle array of iron cations (Fe, Fe, Fe) have been intensively carried out by many researchers; the general chemical formula is Fe3O(R-COO)6L3· nS, where R-COO is a carboxylate anion, L, a ligand, and S, a crystalline solvated molecule (n = 0, 1, 2, 3...). The cluster molecular structure is depicted in Figure 1. In such cluster complexes, much intensive attention has been paid to the valence-detrapping (valenceaveraging) through the electronic interactions among three iron cations. The first pioneering observation of valence-detrapping by Fe-Mössbauer spectroscopy was reported in 1984 by Hendrickson’s group, who revealed that one set of quadrupole doublet ascribing to the detrapping valence of Fe was measured in Fe3O(CH3COO)6(C5H5N)3·C5H5N at room temperature, while two separate sets of doublet due to Fe and Fe in a valence-trapped state below 100 K. After that, a lot of trinuclear iron carboxylate complexes have been so far studied using Mössbauer techniques and there have been many interesting articles, most of which were about the valence-detrapping phenomena among three iron cations. As a rare case, Sato et al. reported that valence-detrapping between one Fe and one Fe was observed at higher temperature for Fe3O(CH2ClCOO)6(H2O)3·3H2O. This partial valence-detrapping, in which one of two Fe ions participates, was explained from the sites of two Fe ions being not strictly equivalent. In the present research, we measured temperature-dependent Fe-Mössbauer spectra of a novel mixed-valence trinuclear heterometal pentafluorobenzoate cluster complex, CrFeFe O(C6F5COO)6(C5H5N)3·0.5C5H5N·CH2Cl2 (1) synthesized by a modified method of our previous preparation procedure for Fe3 O(C6F5COO)6(C5H5N)3·CH2Cl2 (2). The purpose of this work is to examine whether the valence detrapping occurs or not, and what cations participate in the detrapping if it occurs. 2. Experimental