Two unprecedented decanuclear heterometallic [MnMnLn] (Ln = Dy, Tb) complexes displaying relaxation of magnetization

We report the syntheses, crystal structures, and magnetic properties of two 3d-4f heterometallic compounds; namely, [Mn₈Ln₂O₂(OH)₂{(py)₂CO₂}₄(teaH)₄(CH₃COO)₆]·6CH₃CN·2H₂O (Ln^III = Dy (1), Tb (2); (py)₂CO₂H₂ = the gem-diol form of di-2-pyridyl ketone, teaH₃ = triethanolamine). Both compounds were prepared by the reaction of Mn(OAc)₂·4H₂O, Ln(NO₃)₃·5H₂O (Ln = Dy and Tb) with the ligands di-2-pyridyl ketone and triethanolamine in MeCN, and they crystallize in the monoclinic space group C2/c. [Mn₈Ln₂] complexes have not been reported before, and the metallic cores of both complexes were unprecedented. In these cores, two Dy or Tb and two Mn ions comprised a well-known butterfly topology, with three of the remaining six Mn atoms each being situated on either side of the butterfly, linked through two μ₃-O^2- ions. Six Mn^III and two Mn^II were in six-coordinated distorted octahedrons and two Ln^III ions were in nine-coordinated distorted muffins. Interestingly, the coordination sites of Ln^III ions are occupied by six O and two N atoms from two teaH^2- ligands and one μ₃-O^2- atom, without the presence of coordinated solvent molecules such as H₂O and small anions such as NO₃^- ions, which is rare in 3d-4f complexes. Remarkably, alternating current (ac) magnetic susceptibility measurements revealed that both complexes displayed dynamic anisotropic magnetic behaviour. The effective energy barrier (U_eff) of complex 2 was estimated to be 18.97 K through high frequency (111-9111 Hz) ac susceptibility measurements. The low symmetry of the coordination configuration of Ln³⁺ in 1 and 2 may be responsible for the small energy barriers of these two compounds.