Synthesis, magnetism and spectral studies of six defective dicubane tetranuclear {M4O6} (M = NiII, CoII, ZnII) and three trinuclear CdII complexes with polydentate Schiff base ligands

Abstract

A series of Ni^II, Co^II, Zn^II and Cd^II complexes 1–9 with polytopic Schiff base ligands have been synthesized. The single-crystal X-ray crystallography results show that tetranuclear complexes 1–6 have common face-shared defective dicubane cores, whereas trinuclear Cd^II complexes 7–9 are almost linear entities. Synthesis methods (solvent evaporation and hydrothermal synthesis), reaction conditions (pH, solvents and dosage) and coligands (azide, methanol, chloride and acetate) play vital roles in determining the final structure of the complexes and therefore their magnetic properties. In complexes 1–6, the terminal and central M²⁺ ions are connected through mixed bridges, μ-phenoxido/μ_1,1,1-X and μ-Oalphatic/μ_1,1,1-X, while central two M²⁺ ions are linked by double bridges, μ_1,1,1-X (X = azido and methoxido groups for 1 and 2–6 respectively). For complex 1, two central Ni^II ions are connected through two μ_1,1,1-N₃⁻ which is relatively less reported. For complexes 7–9, there are two kinds of Cd^II, the centre Cd^II ions are eight-coordinated with triangle dodecahedral geometries, while the two side Cd^II ions are six-coordinated with trigonal prism geometries using chlorides or acetates as terminal ligands. Magnetic susceptibility measurements (χ_M) for compounds 1–6 have been performed, and they reveal predominant ferromagnetic exchange interactions in Co^II and Ni^II tetramers. The photoluminescence studies show that the Zn^II complex 6 and three Cd^II complexes 7–9 have strong fluorescence, and the lifetimes are measured to be in the 10² nanosecond timescale.