The first one-dimensional iron phosphite-phosphate, [Fe^III(2,2'-bipyridine)(HPO₃)(H₂PO₄)]: synthesis, structure, and magnetic properties

Abstract

A hydrothermal reaction of a mixture containing iron powder, phosphorus acid, 2,2'-bipyridine, and water at 125 °C for 7 days gave colorless rods of a new hybrid iron phosphite-phosphate, [Fe^III(2,2'-bipyridne)(HPO₃)(H₂PO₄)], I. The structure consists of an edge-shared four-membered ring formed by the connectivity between FeO₄N₂ octahedra, pseudo-pyramidal HPO₃ units. 2,2'-Bipyridine and PO₂(OH)₂ tetrahedra are grafted onto the Fe center and hang into the interchain spaces. This is the first observation, to our knowledge, of a one-dimensional edge-shared chain structure in iron phosphite/phosphate. The structure appears to have close similarity to an iron arsenate-oxalate, [C₄N₂H₁₂][Fe(OH)(HAsO₄)(C₂O₄)]·H₂O, and a cadmium phosphate, [Cd(2,2'-bpy)(H₂PO₄)₂. Magnetic investigations reveal that the iron is present in the +3 oxidation state and shows antiferromagnetic interactions with typical low-dimensional behavior. The magnetic interactions between the Fe³⁺ ions are modeled using classical uniform chain solution with a single coupling constant J, formulated by Fisher involving nearest and next-nearest neighbor exchange interactions, based on the S = 5/2 Heisenberg antiferromagentic chain.