Octanuclear {Ln(III)8}(Ln = Gd, Tb, Dy, Ho) Macrocyclic Complexes in a Cyclooctadiene-like Conformation: Manifestation of Slow Relaxation of Magnetization in the Dy(III) Derivative

The synthesis of a series of macrocyclic, isostructural octanuclear lanthanide complexes [Gd-8 (LH2)(4) (mu-Piv)(4) (eta(2)-Piv)(4) (mu-OMe)(4)]center dot 6CH(3)OH center dot 2H(2)O (1), [Tb-8 (LH2)(4) (mu-Piv)(4) (eta(2)-Piv)(4) (eta-OMe)(4)](4)CH3OH center dot 4H(2)O (2), [Dys(LH2)(4) (mu-Piv)(4) (eta(2)-Piv)(4) (mu-OMe)(4)]center dot 8CH(3)OH (3), and [Ho-8(LH2)(4)(mu-Piv)(4) (mu-OMe)(4)]center dot(mu-OMe)(4)]center dot CH3OH center dot 4H(2)O (4) have been achieved, using Ln(III) nitrate salts, pivalic acid, and a new multidentate chelating ligand (2E,N'E)-N'-(3-((bis(2- hydroxyethyl)amino)methyl)2-hydroxy-5-methylbenzylidene)-2-(hydroxyimino) propane hydrazide (LH5), containing two unsymmetrically disposed arms; one side of the phenol unit is decorated with a diethanolamine group while the other side is a hydrazone that has been built by the condensation reaction involving 2-hydroxyiminopropanehydrazide. All the compounds, 1-4, are neutral and are held by the four [LH2](3)(-) triply deprotonated chelating ligands. In these complexes all the lanthanide ions are doubly or triply bridged via phenolate, alkoxy, and pivalate oxygens. The metal centers are distributed over the 8 vertices of an octagon, resembling a cyclooctadiene ring core. The details of magnetochemical analysis for complexes 1-4 shows that they exhibit antiferromagnetic interactions between the Ln(3+) ions through the phenoxo, alkoxo, and pivalato bridging groups. None of the compounds exhibits slow relaxation of the magnetization at zero applied direct current (dc) magnetic field, which could be due to the existence of a fast quantum tunneling relaxation of the magnetization (QTM). In the case of 3, the application of a small dc field is enough as to fully or partly suppress the fast and efficient zero-field QTM allowing the observation of slow relaxation above 2 K.