Ayuk M. Ako, Oliver Waldmann, Valeriu Mereacre, Frederik Klöwer, Ian J. Hewitt, Christopher E. Anson, Hans U. Güdel and Annie K. Powell
Odd-Numbered Fe^III Complexes: Synthesis, Molecular Structure, Reactivity, and Magnetic Properties
Inorg. Chem. 46, 756-766 (2007)                             

Abstract: A field-dependent inelastic neutron scattering (INS) experiment on an array of oriented single crystals of Mn12-acetate is reported. The experimental results provide clear evidence that the minority species observed in the INS spectra is the fast-relaxing species observed in, for instance, AC-susceptibility measurements. Three isostructural disklike heptanuclear Fe^III compounds of the general formula [Fe^III₇(m₃-O)₃(L)₃(m-O₂CCMe₃)₆(h¹-O₂CCMe₃)₃(H₂O)₃], where L represents a di- or triethanolamine moiety, display a three-blade propeller topology, with the central Fe atom representing the axle or axis of the propeller. This motif corresponds to the theoretical model of a frustrated Heisenberg star, which is one of the very few solvable models in the area of frustrated quantum-spin systems and can, furthermore, be converted to an octanuclear cage for the case where L is triethanolamine to give [Fe^III₈(m₄O)₃(m₄-tea)(teaH)₃(O₂CCMe₃)₆(N₃)₃]·¹/₂MeCN·¹/₂H₂O or [Fe^III₈(m₄O)₃(m₄-tea)(teaH)₃(O₂CCMe₃)₆(SCN)₃]·2MeCN when treated with excess NaN₃ or NH₄SCN, respectively. The core structure is formally derived from that of the heptanuclear compounds by the replacement of the three aqua ligands by an {Fe(tea)} moiety, so that the 3-fold axis of the propeller is now defined by two central Fe^III atoms. Magnetic studies on two of the heptanulcear compounds established unequivocally S = ⁵/₂ spin ground state for these complexes, consistent with overall antiferromagnetic interactions between the constituent Fe^III ions.