Heteropolynuclear compounds containing [Cu(L)(μ-C2O4)Cu(L)]2+ units bridged by [Cr(C2O4)3]3– or Cr2O72–: a variety of unusual coordination modes

Heteropolynuclear metal complexes have been widely investigated for their ability to form different architectures and topologies, and also due to their interest for applications in catalysis, photoluminescence, gas storage and separation, magnetism and multifunctional molecular materials. In the last few decades, the chemistry of oxalate-containing complexes has become an active area of research – the oxalate moiety, C2O42−, acts as a linker between metal centres with vario us possibilities of bridiging modes . It is particularly interesting due to its ability to mediate electronic effects between paramagnetic metal ions. Stable mononuclear anionic oxalate complexes are often used as ligands toward another metal ion. The use of the [Cr(C2O4)3]3− anion in the reactions with organic and inorganic cations results in different structure types, from discrete polynuclear metal compounds to polymeric one- (1D), two-(2D) or threedimensional (3D) assemblies, featuring a range of distinct magnetic properties. Applying the layering technique i.e. slow liquid diffusion, the turquoise stick-like crystals of compounds {; ; ; ; ; ; [ACrCu2(C2O4)4(bpy)2]·H2O}; ; ; ; ; ; n [A = K+(1) and NH4+(2)] were grown from the reaction of an aqueous solution of A3[Cr(C2O4)3]·3H2O and methanol solution of CuCl2·H2O and 2, 2'-bipyridine (bpy), in the molar ratio of 1 : 1 : 1. Structural analysis has showed that compounds 1 and 2 contain oxalate-bridged [Cu(bpy)(μ-C2O4)Cu(bpy)]2+ units mutually connected through oxalate groups from [Cr(C2O4)3]3–, forming ladder -like 1D chains (Figure 1a). Interestingly, in each of these complexes three different bridging modes of the oxalate ligand are observed. The change of N-donor aromatic ligand from bpy to 1, 10-phenanthroline (phen) in the same reaction mixture influenced greatly the reaction outcome. Unusually, the [Cr(C2O4)3]3– anions present in the starting solution have transformed to the Cr2O72–, accompanied by oxidation of CrIII to CrVI in an almost neutral solution. The obtained compound, yellow polyhedra of formula {; ; ; ; ; ; [Cu2(C2O4)(phen)2(Cr2O7)]}; ; ; ; ; ; n (3), contains ladder-like 1D chains, in which copper(II) ions from [Cu(phen)(μ-C2O4)Cu(phen)]2+ units are bridged through four oxygen atoms of Cr2O72– anions (Figure 1b). This is the first known example of such extraordinary bridging mode of Cr2O72– . In addition to the single crystal X- ray diffraction study, characterization of the new complexes has been accomplished by means of the IR spectroscopy and thermal analysis.