Among the iron carboxylate compounds prepared, the trinuclear µ3- oxo bridged unit [Fe3O(RCO2)6L3]0/+ is certainly the most thoroughly studied one. Most of the publications deal with the temperature dependence of electron transfer processes in the mixed-valent FeII/FeIII compounds.
In order to investigate co-operative effects in transition metal complexes of higher nuclearity, we have synthesized a whole series of iron complexes starting from the trinuclear complex [Fe3O(piv)6(H2O)3] · piv, which proved to be a very versatile starting material. We have been successful in isolating several different oxo-, hydroxo-, and peroxo-bridged iron(III) compounds as well as heterometallic complexes via this approach. The formation of the different homometallic compounds is sketched in scheme 1.
All of the compounds shown were crystallographically characterized and the magnetic properties have been studied in detail by magnetic susceptibility measurements. Mössbauer spectra have been recorded and were used to distinguish between five- and six-coordinated iron sites and µ2-hydroxo versus µ4-peroxo bridged trinuclear units as well.
[Fe3O(piv)6(H2O)3] · piv as a versatile starting material.
The same spin topologies: Magneto-structural correlation
Very recently we have, in collaboration with Dr. G. Timco (Academy of Sciences, Moldavia), developed new synthetic pathways to heterometallic carboxylate complexes.
Heating pivalate metal salts in non-coordinating solvents for prolonged times leads to polymeric compounds, which upon addition of [Fe3O(piv)6L3] building blocks react to form polynuclear heterometallic clusters with more than three metal centers, for example [FeNi3(µ4-O)(piv)7(Hpiv)2], or by reacting [Zn(piv)2]n and [Fe3O(piv)6(H2O)3]piv we obtained [Fe2Zn4(µ4-O)2(piv)10]. The crucial step in our synthetic procedure towards large heterometallic systems is the transformation of the heterometal pivalate starting material in a first step into a polymeric compound which is then allowed to react with the trinuclear iron carboxylate compound, thus avoiding the formation of the thermodynamically most stable trinuclear complexes like [Fe2Ni(µ3-O)(piv)6] or [Fe2Zn(µ3-O)(piv)6].
Structure and magnetic measurement of [FeNi3O(piv)7(Hpiv)2]