Spectroscopic and electrochemical study of the interconversion and decomplexation of cobalt(II) sandwich polyoxometalates based on a Dawson-type anion.

The reaction of the trivacant Dawson polyoxometalate α-P(2)W(15)O(56) and the divalent cations Co(2+) is known to form a symmetrically derived sandwich complex of formula ββ-Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2) [symbolized as ββ-Co(4)(P(2)W(15))(2)] at low pH (ca. pH 3). We have shown previously that, by a slight modification of the reaction conditions, trinuclear αβ-(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2) and dinuclear (NaOH(2))(2)Co(2)(P(2)W(15)O(56))(2) complexes [symbolized as αβ-NaCo(3)(P(2)W(15))(2) and Na(2)Co(2)(P(2)W(15))(2), respectively] can be synthesized as aqueous-soluble sodium salts. αβ-NaCo(3)(P(2)W(15))(2) is a "lacunary" sandwich complex that can add a Co(2+) cation to form nearly quantitatively an unsymmetrical Dawson tetracobalt sandwich polyoxometalate, αβ-Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2) [symbolized as αβ-Co(4)(P(2)W(15))(2)]. Thus, for Co(4)(P(2)W(15))(2), the junctions between the trivacant {P(2)W(15)} subunits and the central tetrameric unit can be either both β type or β and α types. The interconversion between αβ-Co(4)(P(2)W(15))(2) and ββ-Co(4)(P(2)W(15))(2) and the decomplexation process at low pH, leading to the formation of αβ-NaCo(3)(P(2)W(15))(2) and/or Na(2)Co(2)(P(2)W(15))(2), have been followed in aqueous solution at various pH values by electrochemistry, UV-visible absorption spectroscopy, and (31)P NMR spectroscopy.