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

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Abstract
The coordination of CuII 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 CuII 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.