Rhodamine-based field-induced single molecule magnets in Yb(iii) and Dy(iii) series

Abstract

The reaction between the rhodamine-6G-2-(hydrozinomethyl) quinolin-8-ol (HQR1) ligand and Ln(tta)₃·2H₂O precursors (tta = 2-thenoyltrifluoroacetone) leads to the formation of a series of mononuclear complexes with formula [Ln(QR1)(tta)₂]·(CH₃OH)_x(H₂O)_y (3–6) for (Ln = Yb, x = 1, y = 0 for 3; Ln = Dy, x = 1, y = 0.5 for 4; Ln = Tb, x = 2, y = 0 for 5; Ln = Ho, x = 2, y = 0 for 6) together with [Dy(QR1)₂][NO₃]·(CH₃OH)(H₂O) (2) and the reported [Yb(QR1)₂][NO₃]·(CH₃OH)(H₂O)_0.5 (1), for the purpose of magnetic comparison. Their X-ray structures revealed that the coordination environment of each Ln(III) center is filled by two tta carboxylate groups and a tetrachelate N₂O₂ binding site coming from the deprotonated HQR1 ligand. The Yb and Dy complexes showed the field-induced slow relaxation of magnetization. Both Yb(III)-containing compounds were characterized by X-band EPR and magnetism studies, which revealed the different effective g values and slow paramagnetic relaxation. Comparison between two Yb(III) complexes 1 and 3 shows that the magnetoanisotropy and the barrier height of the magnetic relaxation are sensitive to the subtle change in the coordination environment of the central metal ion. However, in Dy³⁺ series, QTM is difficult to overcome even under the dc field and the subtle variation of the coordination environment leads to a tiny change in the energy barrier of slow magnetization relaxation. These results show that ligand-donating ability while maintaining molecular symmetry can be controlled to design single molecule magnets with enhanced relaxation barriers.