Synthesis, crystal structures and magnetic properties of a series of chair-like heterometallic [FeLn] (Ln = Gd, Dy, Ho, and Er) complexes with mixed organic ligands.

Four chair-like hexanuclear Fe-Ln complexes containing mixed organic ligands, namely, [FeLn{(py)CO}(pdm)(NO)(HO)Cl]·xCHCN·yHO (Ln = Gd (1, x = 1, y = 0), Dy (2, x = 1, y = 1), Ho (3, x = 0, y = 2), and Er (4, x = 1, y = 3); (py)COH = the gem-diol form of di-2-pyridyl ketone and pdmH = 2,6-pyridinedimethanol) have been obtained by employing di-2-pyridyl ketone and 2,6-pyridinedimethanol reacting with FeCl and Ln(NO) in MeCN. The structures of 1-4 are similar to each other except for the number of lattice solvent molecules. Four Fe and two Ln in these complexes comprise a chair-like core with the "body" constructed by four Fe ions and the "end" constructed by two Ln ions. Among the four compounds, 2 shows field-induced single molecule magnet behavior as revealed by ac magnetic susceptibility studies, with the effective energy barrier and the pre-exponential factor of 22.07 K and 8.44 × 10 s, respectively. Ab initio calculations indicated that, among 2_Dy, 3_Ho and 4_Er fragments, the energy gap between the lowest two spin-orbit states for 2_Dy is the largest, while the tunneling gap for 2 is the smallest. These might be the reasons for complex 2 exhibiting SMM behavior. Additionally, the orientations of the magnetic anisotropy of Dy in 2 were obtained by electrostatic calculations and ab initio calculations, both indicating that the directions of the main magnetic axis of Dy1 ions are almost aligned along Dy1-O5 (O5 from the pdm ligand).