EXAFS, magnetic, spectral, and thermal studies on a new polymeric copper mixed-valence compound with the 2-thioisoorotato bridging ligand

Abstract

From the reaction, in aqueous medium, of 2-thioisoorotic acid and copper carbonate, an orange compound can be isolated. The formula unit contains only one copper and one 2-tioisoorotato anion. This compound has been studied by means of extended X-ray atomic force spectroscopy (EXAFS), magnetic measurements, spectral (infrared, electronic and electron paramagnetic resonance (EPR)) and thermal methods (thermogravimetry, differential scanning calorimetry, evolved gas infrared analysis). EXAFS measurements point out the existence of a first shell around the copper atoms formed by a sulphur atom (Cu–S 0.2209 nm) and two oxygen atoms (Cu–O 0.1947 nm). This coincides with the coordination mode proposed for the ligand from infrared data. The structure may be described as a chain-like structure in which the metal ions are bridged by 2-thioisoorotato anions through both oxygen atoms of the carboxylate group and the sulphur atom, (S2)]–Cu–[(O5,O5′)–2-thioisoorotato–(S2)]–Cu–[(O5,O5′). Also, the weak involvement of an endocyclic nitrogen atom in the four-membered chelate formation with the sulphur atom cannot be rejected except at distances Cu–N beyond 0.23 nm. Whereas EXAFS measurements detect the only presence of Cu(I), magnetic measurements indicate there is a mixture of valence states of copper atoms (10% CuII+90% CuI). On the other hand, the compounds exhibit an axial EPR spectrum which is typical from a magnetically diluted Cu(II) compound with a stronger ligand field in the equatorial plane. Finally, in order to determine the homogeneity of this compound, we have analysed this sample by electron microscopy. It is composed of spheres with radii ranging between 1.3 and 1.7 μm and cylinders whose length ranges between 6 and 10 μm and width between 1.2 and 1.6 μm. Energy dispersive spectroscopic analysis was performed on several points and areas of this polymer. In all analyses, the ratio Cu : S was the expected 1 : 1 without any appreciable difference between spheres and cylinders. These results seem to indicate a similar composition in both morphologies, pointing to the presence of two polymorphic phases. This could be the reason for the unsuccessful attempts to obtain single crystals of this polymer to date. © 1998 Chapman & Hall