Three M(II)-diphosphonate coordination polymers with N-heterocyclic group (M = Ni, Fe, Mg): Synthesis, characterization and magnetic properties

Three new divalent metal organic frameworks (MOFs) derived from diphosphonate coordination polymers, [Ni3(H2L)2(H2O)4]·3H2O 1, [Fe(H3L)(H2O)] 2 and [Mg3(H2L)2(H2O)4]·2H2O 3, (H5L = 1-hydroxy-2-(3-pyridyl)ethylidene-1,1-diphosphonate acid, (H4C5N)CH2C(OH)(PO3H2)2), have been hydrothermally obtained and determined by single-crystal X-ray diffractions, elemental analysis, IR, PXRD, TG–DTA. The single-crystal X-ray diffractions reveal that compound 1 owns a novel 2-D layer structure constructed by 1-D ladder-like inorganic chain with magnetic topology building blocks [Ni4(O–P–O)4]n via pyridine moieties linkage. Compounds 2 and 3 are the first examples of Fe(II) and Mg(II) phosphonates based on ligand H5L, respectively. Compound 2 exhibits a 3-D supramolecular network built from 1-D ladder-like inorganic chain with 4- and 8-membered rings through hydrogen bonds, the organic pyridine ring holding close to both sides of the chain. Compound 3 displays a reticulate 2-D layer structure containing 8- and 16-membered rings built from novel 1-D right-handed helical chain. Moreover the room-temperature fluorescent measurements reveal that all the compounds display two fluorescent emissions centered at 377 nm and 422 nm for 1, 382 nm and 424 nm for 2 as well 383 nm and 425 nm for 3, respectively, caused by intraligand π*–π emission state of organic pyridine ring (λex = 234 nm). Magnetism data indicate that compound 1 exhibits spin canted antiferromagnetic exchange coupling between Ni(II) centers, while compound 2 shows alternating ferromagnetic and antiferromagnetic interactions within 1-D topology Fe(II)-chain, showing promising potential as magnetic materials.