Pentanuclear 3d–4f Heterometal Complexes of
M<sup>II</sup><sub>3</sub>Ln<sup>III</sup><sub>2</sub> (M = Ni, Cu,
Zn and Ln = Nd, Gd, Tb) Combinations: Syntheses, Structures, Magnetism,
and Photoluminescence Properties
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Abstract
A new family of pentanuclear 3d–4f
heterometal complexes of general composition [Ln<sup>III</sup><sub>2</sub>(M<sup>II</sup>L)<sub>3</sub>(μ<sub>3</sub>-O)<sub>3</sub>H](ClO<sub>4</sub>)·<i>x</i>H<sub>2</sub>O (<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><sup>′</sup>,<i>N</i><sup>′</sup>-diethylethylenediamine (H<sub>2</sub>L). 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<sup>II</sup>L] units in these
complexes are acting as bis-bidentate metalloligands and, together
with μ<sub>3</sub>-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<sup>+</sup> 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<sup>II</sup>–Gd<sup>III</sup> compound <b>4</b>, the simulation of magnetic data
provides very weak Cu–Gd (<i>J</i><sub>1</sub> =
0.57 cm<sup>–1</sup>) and Gd–Gd exchange constants (<i>J</i><sub>2</sub> = 0.14 cm<sup>–1</sup>). 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<sup>–1</sup>) result in a very small barrier
(8 cm<sup>–1</sup>) for this compound. Among the three Nd<sup>III</sup><sub>2</sub>M<sup>II</sup><sub>3</sub> (M = Zn<sup>II</sup>, Ni<sup>II</sup>, and Cu<sup>II</sup>) complexes, only the Zn<sup>II</sup> analogue (<b>1</b>) displays an NIR luminescence due
to the <sup>4</sup>F<sub>3/2</sub> → <sup>4</sup>I<sub>11/2</sub> transition in Nd<sup>III</sup> when excited at 290 nm. The rest
of the compounds do not show such Nd<sup>III</sup>/Tb<sup>III</sup>-based emission. The paramagnetic Cu<sup>II</sup> and Ni<sup>II</sup> ions quench the fluorescence in <b>2</b>–<b>5</b> and thereby lower the population of the triplet state