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

The heterodimetallic [CuIIFeIII] 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 layering technique, from the reaction of aqueous solution of [Fe(C2O4)3]3– and methanol solutions of 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 [CuIIFeII] 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 water molecule. The molecular structure of 3a and 3b consist 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 oxalate bridge was determined in 1 and 2 from the magnetization measurements. In 3a and 3b strong exchange interaction is present in Fe-oxalate network, {[Fe2(C2O4)3]}n2n–, and the ground state of compounds tends to zero-magnetization at the lowest temperature. Additionally, the ability of compound 3a (or 3b) to act as a single-source precursor for the formation of spinel oxide was investigated by thermal analysis (TG and DTA) and powder X-ray diffraction (PXRD).