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

The reaction of triethanolamine (teaH(3)) with [(Fe3O)-O-III(O2CCH3)(6)(H2O)3]Cl center dot 6H(2)O and Ln(NO3)(3)center dot 6H(2)O in acetonitrile yields [Fe(16)Ln(4)(tea)(8)(teaH)(12)(mu-O2CCH3)8](NO3)(4)center dot 16H(2)O center dot 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 Fe-2 and Fe-6 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 Fe-57 Mossbauer 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.