Pentanuclear 3d–4f Heterometal Complexes of MII 3LnIII 2 (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 [LnIII 2(MIIL)3(μ3-O)3H]­(ClO4)·xH2O (1–5) [Ln = Nd, M = Zn, 1; Nd, Ni, 2; Nd, Cu, 3; Gd, Cu, 4; Tb, Cu, 5] have been synthesized in moderate yields (50–60%) following a self-assembly reaction involving the hexadentate phenol-based ligand, viz., N,N-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)-N ′,N ′-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 [MIIL] 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 (3–5) show a changeover from antiferromagnetic in 3 to ferromagnetic 3d–4f interactions in 4 and 5. For the isotropic CuII–GdIII compound 4, the simulation of magnetic data provides very weak Cu–Gd (J 1 = 0.57 cm–1) and Gd–Gd exchange constants (J 2 = 0.14 cm–1). Compound 4 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 (S = 17/2) and a negative value of D (−0.12 cm–1) result in a very small barrier (8 cm–1) for this compound. Among the three NdIII 2MII 3 (M = ZnII, NiII, and CuII) complexes, only the ZnII analogue (1) displays an NIR luminescence due to the 4F3/2 → 4I11/2 transition in NdIII when excited at 290 nm. The rest of the compounds do not show such NdIII/TbIII-based emission. The paramagnetic CuII and NiII ions quench the fluorescence in 2–5 and thereby lower the population of the triplet state.