Structural and magnetic studies of six-coordinated Schiff base Dy(iii) complexes

Abstract

With the aim to tune the magnetic anisotropies of six-coordinated Dy(III) complexes, four bis-Schiff bases bearing different spacers and one mono-Schiff base were designed, namely bis(2-hydroxylnaphthalenylmethylene)hydrazine (H₂L¹), bis(2-hydroxylnaphthylmethylene)ethylenediamine (H₂L²), bis(2-hydroxylnaphthylmethylene)-propylenediamine (H₂L³), bis(2-hydroxylnaphthylmethylene)-2-methylpropylenediamine (H₂L⁴) and 1-(3-pyridylmethyliminomethyl)-2-naphthalenol (HL⁵). Their reactions with DyCl₃·6H₂O gave five Dy(III) complexes, namely [Dy(H₂L¹)₂(H₂O)Cl₃] (1), [Dy(H₂L²)_3/2Cl₃]_n (2), [Dy₂(H₂L³)₂Cl₆]·2CH₃CN (3), [Dy(H₂L⁴)₂Cl₂]Cl·3MeOH (4) and [Dy(HL⁵)(H₂L⁵)Cl₄] (5). 1, 4 and 5 are mononuclear metal complexes. 2 exhibits a two-dimensional skeleton, however, with its Dy(III) ions well separated. A dinuclear structure was found for 3, in which the two Dy(III) ions are doubly bridged through Cl⁻ linkers. The Dy(III) ions in all metal complexes show octahedral geometries. 1–4 bear bis-Schiff base ligands with different spacers (from no spacer to –CH₂CH₂–, –CH₂CH₂CH₂–, and –CH₂CH(CH₃)CH₂–) between the two N atoms, which tune the coordination spheres into DyO₃Cl₃ for 1 and 2, DyO₂Cl₄ for 3, and DyO₄Cl₂ for 4. The DyO₂Cl₄ coordination environment can also be achieved in 5 through a mono-Schiff base. It is revealed from the magnetic measurements that 1, 4 and 5 are field-induced single-molecule magnets, and 3 is a typical zero-field single-molecule magnet. The different SMM performances of 1–5 revealed that the different sets of coordination environments (DyO_xCl_6−x) of these octahedral Dy(III) complexes achieved through the adjustment of the spacers in the Schiff base ligands can effectively tune their magnetic properties via affecting the magnetic easy axes and magnetic anisotropies of the Dy(III) ions. Ab initio calculations, which include the quantum tunneling rates of magnetization, g-tensors, crystal field parameters and magnetic easy axes, were carried out for interpreting the observed magnetic properties.