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 α -[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)2Co2(P2W15O56)2]18- (Na2Co2P4W30) and [(NaOH2)Co3(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 31P 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 31P dynamic NMR spectroscopy. The electrochemical behaviors of the sandwich complexes [(NaOH2)Co3(H2O)(P2W15O56)2]17- and [(NaOH2)2Co2(P2W15O56)2]18- were investigated in aqueous solutions and compared with that of [Co4(H2O)2(P2W15O56)2]16-. These complexes showed an electrocatalytic effect on nitrite reduction.