Synthesis, Structure, Luminescence, and Magnetic Properties of a Single-Ion Magnet “<i>mer</i>”‑[Tris(<i>N</i>‑[(imidazol-4-yl)-methylidene]-dl-phenylalaninato)terbium(III) and Related “<i>fac</i>”-dl-Alaninato Derivative

Abstract

Two Tb^III complexes with the same N₆O₃ donor atoms but different coordination geometries, “<i>fac</i>”-[Tb^III(HL^dl‑ala)₃]·7H₂O (<b>1</b>) and “<i>mer</i>”-[Tb^III(HL^dl‑phe)₃]·7H₂O (<b>2</b>), were synthesized, where H₂L^dl‑ala and H₂L^dl‑phe are <i>N</i>-[(imidazol-4-yl)­methylidene]-dl-alanine and -dl-phenylalanine, respectively. Each Tb^III ion is coordinated by three electronically mononegative NNO tridentate ligands to form a coordination geometry of a tricapped trigonal prism. Compound <b>1</b> consists of enantiomers “<i>fac</i>”-[Tb^III(HL^d‑ala)₃] and “<i>fac</i>”-[Tb^III(HL^l‑ala)₃], while <b>2</b> consists of “<i>mer</i>”-[Tb^III(HL^d‑phe)₂(HL^l‑phe)] and “<i>mer</i>”-[Tb^III(HL^d‑phe)­(HL^l‑phe)₂]. Magnetic data were analyzed by a spin Hamiltonian including the crystal field effect on the Tb^III ion (4f⁸, <i>J</i> = 6, <i>S</i> = 3, <i>L</i> = 3, <i>g</i>_<i>J</i> = 3/2, ⁷F₆). The Stark splitting of the ground state ⁷F₆ was evaluated from magnetic analysis, and the energy diagram pattern indicated easy-plane and easy-axis (Ising type) magnetic anisotropies for <b>1</b> and <b>2</b>, respectively. Highly efficient luminescences with Φ = 0.50 and 0.61 for <b>1</b> and <b>2</b>, respectively, were observed, and the luminescence fine structure due to the ⁵D₄ → ⁷F₆ transition is in good accordance with the energy diagram determined from magnetic analysis. The energy diagram of <b>1</b> shows an approximate single-well potential curve, whereas that of <b>2</b> shows a double- or quadruple-well potential within the ⁷F₆ multiplets. Complex <b>2</b> displayed an onset of the out-of-phase signal in alternating current (ac) susceptibility at a direct current bias field of 1000 Oe on cooling down to 1.9 K. A slight frequency dependence was recorded around 2 K. On the other hand, <b>1</b> did not show any meaningful out-of-phase ac susceptibility. Pulsed-field magnetizations of <b>1</b> and <b>2</b> were measured below 1.6 K, and only <b>2</b> exhibited magnetic hysteresis. This finding agrees well with the energy diagram pattern from crystal field calculation on <b>1</b> and <b>2</b>. DFT calculation allowed us to estimate the negative charge distribution around the Tb^III ion, giving a rationale to the different magnetic anisotropies of <b>1</b> and <b>2</b>