Stereochemistry and structural phase transition of Cu(II) complexes containing orthophosphate ions and pyridine/picoline

Copper complexes of the type Cu(L)₄(H₂PO₄)₂ designated as complex (I), and Cu(L)₄ HPO₄ designated as complex (II), (whereL = pyridine (py) or λ-picoline (pic)) have been synthesised, characterised by chemical analyses, IR, and electronic and magnetic susceptibility data. From ESR studies it is concluded that complexes (I) have elongatedtrans octahedral stereochemistry. The temperature dependence of the ESR spectrum of Cu(py)₄(H₂PO₄)₂ suggests a fluxional behaviour in the immediate coordination of Cu(II), whereas very little variation of the ESR spectrum of the Cu(pic)₄(H₂PO₄)₂ complex indicates that the stereochemistry of this complex is essentially static in nature. The differential scanning calorimetric studies in the case of Cu(py)₄(H₂PO₄)₂ complex have given clear evidence for the occurrence of a structural phase transition at 147 K. The absence of any abrupt changes in the ESR spectrum at that temperature excludes the possibility of any changes in the immediate environment of Cu(II).