Studies of an Fe₉ tridiminished icosahedron

The synthesis and structural characterization of a nonanuclear Fe ^III cage complex is reported. The nine iron centers in [Fe ₉(μ₃-O)₄(O₃PPh)_3- (O₂CCMe₃)₁₃] lie on the vertices of an incomplete icosahedron, with the P atoms of triphenylphosphonate at the other three vertices. The paramagnetic core therefore describes a tridiminished icosahedron. Magnetic studies suggest an S=1/2 ground state for the molecule. Analysis of exchange paths and the susceptibility data point to the interpretation that the cluster can be divided into two nearly decoupled sections: an {Fe₆O₃3) section, with an 5 = 0 ground state, in which three oxo-centered triangles bound a central triangle that is not oxo-centered; and an {Fe₃O} triangle with S = 1/2. The analysis of the susceptibility data leads to a Heisenberg model based on three significant antiferromagnetic exchange inter actions, with values of 173.7 cm^-1 in the {Fe₃O} triangle, and 30.9 and 19.1 cm^-1 within the {Fe₆O₃} section, while the exchange between them is <1 cm^-1. With these assignments, the theoretical low-temperature differential susceptibility is also in very good agreement with measurements up to 50 T. Magnetic measurements in the milli-kelvin range reveal striking hysteresis loops and magnetization reversals associated with a Landau-Zener-Stückelberg (LZS) transition as enhanced by the occurrence of a phonon bottleneck.