Pentanuclear 3d–4f Heterometal Complexes of M^II₃Ln^III₂ (M = Ni, Cu, Zn and Ln = Nd, Gd, Tb) Combinations: Syntheses, Structures, Magnetism, and Photoluminescence Properties

A new family of pentanuclear 3d–4f heterometal complexes of general composition [Ln^III₂(M^IIL)₃(μ₃-O)₃H]­(ClO₄)·<i>x</i>H₂O (<b>1</b>–<b>5</b>) [Ln = Nd, M = Zn, <b>1</b>; Nd, Ni, <b>2</b>; Nd, Cu, <b>3</b>; Gd, Cu, <b>4</b>; Tb, Cu, <b>5</b>] have been synthesized in moderate yields (50–60%) following a self-assembly reaction involving the hexadentate phenol-based ligand, viz., <i>N</i>,<i>N</i>-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)-<i>N</i>^′,<i>N</i>^′-diethylethylenediamine (H₂L). Single-crystal X-ray diffraction analyses have been used to characterize these complexes. The compounds are all isostructural, having a 3-fold axis of symmetry that passes through the 4f metal centers. The [M^IIL] units in these complexes are acting as bis-bidentate metalloligands and, together with μ₃-oxido bridging ligands, complete the slightly distorted monocapped square antiprismatic nine-coordination environment around the 4f metal centers. The cationic complexes also contain a H⁺ ion that occupies the central position at the 3-fold axis. Magnetic properties of the copper­(II) complexes (<b>3</b>–<b>5</b>) show a changeover from antiferromagnetic in <b>3</b> to ferromagnetic 3d–4f interactions in <b>4</b> and <b>5</b>. For the isotropic Cu^II–Gd^III compound <b>4</b>, the simulation of magnetic data provides very weak Cu–Gd (<i>J</i>₁ = 0.57 cm^–1) and Gd–Gd exchange constants (<i>J</i>₂ = 0.14 cm^–1). Compound <b>4</b> is the only member of this triad, showing a tail of an out-of-phase signal in the ac susceptibility measurement. A large-spin ground state (<i>S</i> = 17/2) and a negative value of <i>D</i> (−0.12 cm^–1) result in a very small barrier (8 cm^–1) for this compound. Among the three Nd^III₂M^II₃ (M = Zn^II, Ni^II, and Cu^II) complexes, only the Zn^II analogue (<b>1</b>) displays an NIR luminescence due to the ⁴F_3/2 → ⁴I_11/2 transition in Nd^III when excited at 290 nm. The rest of the compounds do not show such Nd^III/Tb^III-based emission. The paramagnetic Cu^II and Ni^II ions quench the fluorescence in <b>2</b>–<b>5</b> and thereby lower the population of the triplet state