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

A new Wells–Dawson sandwich polyoxometalate, $$\alpha\alpha\alpha\alpha$$ -[(NaOH₂)₂(Cu^II)₂ (P₂W₁₅O₅₆)₂]¹⁸⁻ ( 2P), has been obtained in good yield by the dissolution of solid $$\alpha$$ -Na₁₂[P₂W₁₅O₅₆]·18H₂O in an aqueous solution of Cu(II) and L-glutamic acid at pH 10. The arsenic analogue, $$\alpha\alpha\alpha\alpha$$ -[(NaOH₂)₂(Cu^II)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 $$\alpha$$ -[P₂W₁₅O₅₆]¹²⁻ or two $$\alpha$$ -[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 $$\alpha$$ [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 $$\alpha\alpha\alpha\alpha$$ -[(NaOH₂)₂(Fe^III)₂(P₂W₁₅O₅₆)₂]¹⁶⁻ ( 1P) and $$\alpha\alpha\alpha\alpha$$ -[(NaOH₂)₂(Fe^III)₂(As₂W₁₅O₅₆)₂]¹⁶⁻ ( 1As), of a transition-metal-substituted sandwich polyoxometalate containing two electroactive d-electron metals. [ABSTRACT FROM AUTHOR]