Hexamethylenetetramine Polyoxotungstophosphate Copper Complexes

Abstract

Hitherto undescribed copper(II) hexamethylenetetramine (HMTA) complexes of tungstophosphate metalates Rb₅[PW₁₁O₃₉Cu(H₂O)]‧9H₂O (I), Rb₅[PW₁₁O₃₉Cu(C₆H₁₂N₄)]‧10H₂O (II), Rb₅[PW₁₁O₃₉Zn_0.95Cu_0.05(H₂O)]‧9H₂O (III), and Rb₅[PW₁₁O₃₉Zn_0.95Cu_0.05(C₆H₁₂N₄)]‧10H₂O (IV) were prepared and characterized by IR and electronic spectroscopies, X-ray powder diffraction, and electron paramagnetic resonance (EPR). The copper-ion absorption peak in the spectrum shifts to the longer wavelengths in going from [Cu(H₂O)₆]²⁺ through [PW₁₁O₃₉Cu(H₂O)]^5– [PW₁₁O₃₉Zn_0.95Cu_0.05(H₂O)]^5–, and [PW₁₁O₃₉Cu(C₆H₁₂N₄)]^5–, to [PW₁₁O₃₉Zn_0.95Cu_0.05(C₆H₁₂N₄)]^5– because of the changing ligand-field strength in the inner sphere of the complex. Electron paramagnetic resonance showed a significant difference between the magnitudes of the ligand field around the octahedrally coordinated Cu²⁺ ions in complexes (III) and (IV): the height of the crystal field potential barrier at the location of the Cu²⁺ ion differs more than twofold due to the replacement of a water molecule by an HMTA molecule C₆H₁₂N₄. The results of the studies can be helpful in the preparation and structure determination of other tungstate polyoxometalates with inner-sphere paramagnetic ions.