pH-Dependent solution dynamics of a manganese(ii) polyoxometalate, [Mn4(H2O)2(P2W15O56)2]16−, and [Mn(H2O)6]2+

Abstract

The aqueous reaction dynamics of the manganese(II)-functionalised Wells–Dawson polyoxometalate [Mn₄(H₂O)₂(P₂W₁₅O₅₆)₂]¹⁶⁻ has been determined as a function of pH using variable temperature ¹⁷O NMR, and compared with that of the well-studied monomeric manganese(II) hexa-aqua ion, [Mn(H₂O)₆]²⁺. While the rate of aquo–ligand exchange on the hexa-aqua ion remains independent of pH in the range 3.2–6.0, the rate of water exchange of the polyoxometalate ion varies by a factor of fifteen, from 1.98 × 10⁷ s⁻¹ at pH 3.2 to 1.3 × 10⁶ s⁻¹ at pH 6.0. This decrease in the rate of exchange correlates with the deprotonation of the polyoxometalate framework.