Dimensionality controlled by light exposure: 1D versus 3D oxalate-bridged [CuFe] coordination polymers based on an [Fe(C2O4)3]3− metallotecton

Abstract

Heterodimetallic [Cu^IIFe^III] one-dimensional (1D) coordination polymers {NH₄[{Cu(bpy)}₂(C₂O₄)Fe(C₂O₄)₃]·H₂O}_n (1) and {K[{Cu(bpy)}₂(C₂O₄)Fe(C₂O₄)₃]·H₂O}_n (2) (bpy = 2,2′-bipyridine) were obtained using a building block approach by a layering technique, from the reaction of an aqueous solution of [Fe(C₂O₄)₃]³⁻ and methanol solutions containing Cu²⁺ and bpy. In a test tube without the presence of light, partial decomposition of the tris(oxalato)ferrate(III) anion occurred yielding oxalate-bridged dinuclear [Cu(bpy)(C₂O₄)Cu(bpy)]²⁺ units. These cationic species are mutually connected through oxalate groups from [Fe(C₂O₄)₃]³⁻, thus forming ladder-like topologies in compounds 1 and 2. When the same reaction mixture was exposed to daylight, initial building block [Fe(C₂O₄)₃]³⁻ undergoes photoreduction producing [Cu^IIFe^II] three-dimensional (3D) coordination polymer {[Cu(bpy)₃][Fe₂(C₂O₄)₃]·H₂O}_n (3a). In addition, under hydrothermal conditions, the same reduction occurs giving compound {[Cu(bpy)₃][Fe₂(C₂O₄)₃]}_n (3b) that crystallizes without the water molecule. The molecular structure of 3a and 3b consists of a 3D anionic network {[Fe₂(C₂O₄)₃]}_n²ⁿ⁻ and tris-chelated cations [Cu(bpy)₃]²⁺ occupying the vacancies in the framework. Very strong antiferromagnetic coupling between two copper(II) ions from [Cu(bpy)(C₂O₄)Cu(bpy)]²⁺ species transferred through the oxalate bridge was determined in 1 and 2 from the magnetization measurements. In 3a and 3b, strong exchange interaction is present in the Fe-oxalate network, {[Fe₂(C₂O₄)₃]}_n²ⁿ⁻, and the ground state of the compounds tends to zero-magnetization at the lowest temperature.