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)](12-) 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)](16-) [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)](17-) and dinuclear [(NaOH(2))(2)Co(2)(P(2)W(15)O(56))(2)](18-) 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)](16-) [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.
Laboratoire de Chimie Physique (UMR CNRS 8000), Bâtiment 349, Université Paris-Sud 11, 91405 Orsay Cedex, France. lruhlmann@unistra.fr
Laurent
L
Ruhlmann
Delphine
D
Schaming
Iftikhar
I
Ahmed
Aimeric
A
Courville
Jacqueline
J
Canny
René
R
Thouvenot
D000838:Anions / Q000737:chemistry*; D002412:Cations; D003035:Cobalt / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D055664:Electrochemical Techniques; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D012492:Salts; D012964:Sodium; D013056:Spectrophotometry, Ultraviolet; D017973:Tungsten Compounds / Q000737:chemistry*; D014867:Water
D000838:Anions; D002412:Cations; D056831:Coordination Complexes; D012492:Salts; D017973:Tungsten Compounds; D014867:Water; D003035:Cobalt; D012964:Sodium