Heterometallic 20-membered {Fe16Ln4} (Ln = Sm, Eu, Gd, Tb, Dy, Ho) metallo-ring aggregates

Abstract

The reaction of triethanolamine (teaH₃) with [Fe^III₃O(O₂CCH₃)₆(H₂O)₃]Cl·6H₂O and Ln(NO₃)₃·6H₂O in acetonitrile yields [Fe₁₆Ln₄(tea)₈(teaH)₁₂(μ-O₂CCH₃)₈](NO₃)₄·16H₂O·xMeCN (Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6); x = 10 or 11). These 20-membered metallo-ring complexes are the largest such single-stranded oxygen-bridged rings so far reported. The structure is stabilised by two of the acetate ligands, which form anti,anti-bridges across the centre of the ring, pinching the ring and giving it rigidity. The magnetic properties are dominated by the antiferromagnetic couplings between the Fe^III centres. Although the Fe₂ and Fe₆ sub-chains within the ring are fully spin-compensated at low temperatures with S_subchain = 0, coupling between the Gd^III cations and the Fe^III centres at the ends of the sub-chains (in 3) results in a pinning of the lanthanide spins. The ⁵⁷Fe Mössbauer spectra of 3 and 5 obtained at low temperatures are consistent with the presence of Fe^III intracluster strong antiferromagnetic coupling. The applied field spectrum for 3 reveals no magnetic hyperfine interaction apart from that of the nucleus with the applied field, while the one for 5 is a superposition of three subspectra which show contributions from each of the peripheral as well as from the central iron sites.