Synthesis, structural characterization and magnetic behavior of a 2D bi-layer MOF, {[Ni2(C5O5)2(bpe)2]·H2O}n (bpe = 1,2-bis(4-pyridyl)ethylene)

Abstract

Coordination polymers of metal-croconate (C₅O₅²⁻) complex, {[Ni₂(bpe)₂(C₅O₅)₂]·H₂O}_n (1) (bpe = 1,2-bis(-4-pyridyl)ethylene)) with two-dimensional (2D) metal–organic frameworks (MOF), have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction studies. Structural determination reveals that compound 1 contains two crystallographically independent Ni(II) ions which both lie in a distorted octahedral environment bonded to two bpe nitrogen atoms and four oxygen donors from two croconates. The croconate acts as a bridging ligand with two different bridging modes, one presents a bis-bidentate adjacent μ₃-coordination mode and the other a bis-bidentate μ₄-coordination mode. A 2D brick-wall-like layered framework is formed by using a rectangle grid as a basic building unit through the connectivity of Ni^II with hybrid μ₃-, μ₄-croconates and bpe ligands. Two layers are then cross-linked by the other bpe ligands to complete a 2D bi-layered MOF. The TGA associated with in situXRD measurements demonstrated that 1 have quite high thermo-stability that keep their crystalline forms up to 350 °C. Magnetic measurements show the long range magnetic ordering and spin canting behavior in 1 below the critical temperature of 22 K with an approximate canting angle of 0.79°. A large coercivity of 3.5 kOe in the hysteresis loop is observed at 1.8 K, indicating that 1 is a weak hard magnet.