Structural and Electrochemical Studies of Dicupric Wells–Dawson Sandwich-Type Complexes

A new Wells–Dawson sandwich polyoxometalate, -[(NaOH2)2(CuII)2 (P2W15O56)2]18− (2P), has been obtained in good yield by the dissolution of solid -Na12[P2W15O56]·18H2O in an aqueous solution of Cu(II) and L-glutamic acid at pH 10. The arsenic analogue, -[(NaOH2)2(CuII)2(As2W15O56)2]18− (2As), is likewise prepared by using Na12[As2W15O56]·21H2O instead of Na12[P2W15O56]·18H2O. Diffraction quality crystals of both 2P and 2As were obtained by slow evaporation in air over several days. The X-ray structures of 2P and 2As reveal that two Cu(II) atoms are sandwiched between two -[P2W15O56]12− or two -[As2W15O56]12− ligands, respectively, while the other two positions of the central belt unit are occupied by two Na+ cations. Higher yields of 2As can be obtained by mixing CuCl2·2H2O and [Na12As2W15O56]·21H2O in acetate buffer. The electrochemistry of 2P and 2As is characterized by cyclic voltammograms in which the reduction of the Cu(II) centers is close to the redox pattern of the W-centers. The two Cu(II)-centers are simultaneously reduced to Cu(0); the separate steps could not be resolved for the individual Cu(II) centers. Complexes 2P and 2As constitute the second example, after -[(NaOH2)2(FeIII)2(P2W15O56)2]16− (1P) and -[(NaOH2)2(FeIII)2(As2W15O56)2]16− (1As), of a transition-metal-substituted sandwich polyoxometalate containing two electroactive d-electron metals.