Not only heterometallic systems but also heterovalent complexes have given us access to interesting research topics. For some time we hav been collaborating with Professor Larsen in Aarhus. He is undertaking systematic x-ray studies of the electronic and nuclear structures of polynuclear oxo-bridged metal cores. Particular attention is given to intramolecular electron transfer processes in mixed valence trinuclear oxo-bridged complexes. In general the temperature dependence of the ET varies considerably from compound to compound. Several characteristic structural features, such as changes in the dynamics of solvent or ligand disorder have been put forward as explanations for phase transitions and the temperature dependence of the ET, but structural data at different temperatures are rarely reported. Substituting carboxylate bridges with steric demanding residues by formate resulted in the formation of the trinuclear compound [Fe3O(HCOO)6L3], in which problems with disorder in the substituent R are significant diminished, thus representing a very well suited object to be studied.
Structures of [Fe2IIIFeIIO(tBuCOO)6(pic)3] and [Fe2IIIFeIIO(HCOO)6(Î³-pic)3]
The results of a x-ray diffraction study of the mixed-valence compound [Fe2IIIFeIIO(HCOO)6(Î³-pic)3] . 1.3(Î³-pic) at a number of temperatures above and below the onset of ET were reported together with the results of variable-temperature Mössbauer studies by the group of Professor Tuchagues in Toulouse and magnetic susceptibility measurements performed by us. Whereas X-ray studies and Mössbauer data clearly show the ET being a function of temperature, the magnetization data interestingly gives no indication of any valence delocalization, reflecting the different time scale of the various methods employed.
One of the most interesting mixed-valent iron compound we obtained by the same synthetic route as the trinuclear species [Fe2IIIFeIIO(HCOO)6(Î³-pic)3] by simply varying the oxidation equivalents. In dark blue crystals [Fe2IIIFe2II(HCOO)5(Î³-pic)3] was isolated. The heterovalent complex forms a 3d-lattice, shown below, where p-stacking of the aromatic picoline rings causes a magnetic ordering of the compound at 20 K. This ordering is observed by magnetic measurements, Moessbauer spectroscopy, and in collaboration with F.K. Larsen and J. Overgaard even with very low temperature X-ray crystallography at 16 K.
Ï-stacking of the picoline rings connecting the 2d-layers of iron formate [Fe2IIIFe2II(HCOO)5(Î³-pic)3]
Moessbauer spectra of [Fe2IIIFe2II(HCOO)5(Î³-pic)3] without external magnetic field above and below the ordering temperature.
Developing new synthetic methods for the well-directed preparation of hetero metallic complexes, as well as additional magneto-structural correlations will be future tasks for our group.