Copper–oxygen coordination in CuII-heteropolyanion compounds: electron paramagnetic resonance studies and CNDO/2 calculations

Abstract

The coordination of Cu^II ions in Anderson and Keggin type heteropolyanion compounds in their polycrystalline and corresponding magnetically diluted forms {(NH₄)₄[Zn(OH)₆Mo₆O₁₈]·nH₂O/Cu²⁺; ((CH₃)₄N)₆[ZnO₄W₁₂O₃₆]·nH₂O/Cu²⁺} have been investigated by means of chemical, spectroscopic (EPR, IR), diffraction (X-ray) and quantum-mechanical methods. Simple model compounds with known Cu—O coordination have been used for comparison. In both the Anderson ion [Cu(OH)₆Mo₆O₁₈]^4– and the Keggin ion [SiO₄W₁₁O₃₀CuO₅(OH₂)]^6– the copper coordination polyhedra consist of six oxygen atoms, forming a distorted octahedron. As expected, the magnetically diluted Anderson compound exhibits a pronounced dynamic Jahn–Teller effect.

No arguments for the existence of a Keggin species with Cu^II ions on the central tetrahedral site [CuO₄W₁₂O₃₆]^6– could be derived from the experimental and theoretical investigations. The Cu²⁺ ions incorporated into the products resulting from different attempts to synthesize the Keggin structure with a central copper heteroatom appear to be [Cu(OH₂)₆]²⁺ units. Therefore, the calculations of selected molecular subunits for the assumed Keggin compounds did not yield the experimental EPR parameters.

The g tensor calculations in the CNDO/2 approximation are consistent with the EPR spectra of the Cu Anderson compound.