Stergios Piligkos, Gopalan Rajaraman, Monica Soler, Nadeschda Kirchner, Joris van Slageren, Roland Bircher, Simon Parsons, Hans-Ulrich Güdel, Jens Kortus, Wolfgang Wernsdorfer, George Christou and Euan K. Brechin
Studies of an Enneanuclear Manganese Single-Molecule Magnet
J. Am. Chem. Soc. 127, 5572-5580 (2005)                       

Abstract: The reaction of [Mn₃O(O₂CMe)₆(py)₃] with the tripodal ligand H₃thme (1,1,1-tris(hydroxymethyl)ethane) affords the enneanuclear complex [Mn₉O₇(O₂CCH₃)₁₁(thme)(py)₃(H₂O)₂] 1·1MeCN·1Et₂O. The metallic skeleton of complex 1 comprises a series of 10 edge-sharing triangles that describes part of an idealized icosahedron. Variable temperature direct current (dc) magnetic susceptibility data collected in the 1.8-300 K temperature range and in fields up to 5.5 T were fitted to give a spin ground state of S = ¹⁷/₂ with an axial zero-field splitting parameter D = -0.29 cm^–1. Ac susceptibility studies indicate frequency-dependent out-of-phase signals below 4 K and an effective barrier for the relaxation of the magnetization of U_eff = 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. Inelastic neutron scattering (INS) studies on complex 1 confirm the S = ¹⁷/₂ ground state and analysis of the INS transitions within the zero-field split ground state leads to determination of the axial anisotropy, D = -0.249 cm^–1, and the crystal field parameter, B₄⁰ = 7(4) × 10^–6 cm^–1. Frequency domain magnetic resonance spectroscopy (FDMRS) determined the same parameters as D = -0.247 cm^–1 and B₄⁰ = 4.6 × 10^–6 cm^–1. DFT calculations are fully consistent with the experimental findings of two Mn(II) and four Mn(III) ions "spin up" and three Mn(IV) ions "spin down" resulting in the S = ¹⁷/₂ spin ground state of the molecule, with D = -0.23 cm^–1 and U = 26.2 K.