Di- and tricobalt Dawson sandwich complexes:: Synthesis, spectroscopic characterization, and electrochemical behavior of Na18[(NaOH2)2Co2(P2W15O56)2] and Na17[(NaOH2)Co3(H2O)(P2W15O56)2]

The reaction of the trivacant Dawson anion alpha-[P2W15O56](12-) and the divalent cations Co2+ is known to form the tetracobalt sandwich complex [CO4(H2O)(2)(P2W15O56)(2)](16-) (Co4P4W30). Two new complexes, with different Co/P2W15 stoichiometry, [(NaOH2)(2)Co-2(P2W15O56)(2)](18-) (Na2Co2P4W30) and [(NaOH2)Co-3(H2O)(P2W15O56)(2)](17-) (NaCo3P4W30), have been synthesized as aqueous-soluble sodium salts, by a slight modification of the reaction conditions, Both compounds were characterized by IR, elemental analysis, and P-31 solution NMR spectroscopy. These species are "lacunary" sandwich complexes, which add Co2+ cations according to Na2Co2P4W30 + Co2+ --> NaCo3P4W30 + Na+ followed by NaCo3P4W30 + Co2+ --> Co4P4W30 + Na+. A Li+/Na+ exchange in the cavity was evidenced by P-31 dynamic NMR spectroscopy. The electrochemical behaviors of the sandwich complexes [(NaOH2)CO3(H2O)(P2W15O56)(2)](17-) and [(NaOH2)(2)Co-2(P2W15O56)(2)](18-) were investigated in aqueous solutions and compared with that of [Co-4(H2O)(2)(P2W15O56)(2)](16-). These complexes showed an electrocatalytic effect on nitrite reduction.