Synthesis, structure, luminescence, and magnetic properties of a single-ion magnet " mer "-[tris(N-(imidazol-4-yl)-methylidene]-dl- phenylalaninato)terbium(III) and related " fac "-Dl-alaninato derivative

Two Tb^III complexes with the same N₆O₃ donor atoms but different coordination geometries, "fac"-[Tb ^III(HL^dl-ala)₃]·7H₂O (1) and "mer"-[Tb^III(HL^dl-phe)₃] ·7H₂O (2), were synthesized, where H₂L ^dl-ala and H₂L^dl-phe are N-[(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 1 consists of enantiomers "fac"-[Tb ^III(HL^d-ala)₃] and "fac"-[Tb ^III(HL^l-ala)₃], while 2 consists of "mer"-[Tb^III(HL^d-phe)₂(HL ^l-phe)] and "mer"-[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, J = 6, S = 3, L = 3, g_J = 3/2, ⁷F ₆). 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 1 and 2, respectively. Highly efficient luminescences with φ = 0.50 and 0.61 for 1 and 2, 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 1 shows an approximate single-well potential curve, whereas that of 2 shows a double- or quadruple-well potential within the ⁷F₆ multiplets. Complex 2 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, 1 did not show any meaningful out-of-phase ac susceptibility. Pulsed-field magnetizations of 1 and 2 were measured below 1.6 K, and only 2 exhibited magnetic hysteresis. This finding agrees well with the energy diagram pattern from crystal field calculation on 1 and 2. DFT calculation allowed us to estimate the negative charge distribution around the Tb^III ion, giving a rationale to the different magnetic anisotropies of 1 and 2.