Polyoxometalates (POMs) are early transition metal (M = V, Nb, Ta, Mo and W in their highest oxidation state) oxygen anion clusters. Several classes of POMs have been reported to have potent anti-tumor, anti-viral and anti-bacterial properties, resulting in a substantial interest in the potential medicinal application of POMs. Several studies were recently made on the hydrolysis of DNA- and RNA-model substrates by using the isopolyoxometalates [Mo₇O₂₄^6-]¹ as well as a Zr(IV)-substituted Wells-Dawson type POM² as artificial phosphatases. Phosphodiester bonds form the backbone of DNA and RNA macromolecules and are extremely resistant towards hydrolysis. The half-life for spontaneous phosphoester bond hydrolysis has been estimated to be higher than 100 000 years for DNA and 4 years for RNA at neutral pH and 25 °C. Despite the extreme stability of the phosphoester bond, its efficient cleavage is a required procedure in biochemical fields. In nature phosphatases are responsible for hydrolyzing phosphodiester bonds with impressive rate enhancements. In our quest to understand the biological role of POM anions on a molecular level, the phosphoesterase activity of a bimetal-substituted [α-PW₁₁O₃₉]^7- Keggin type POM is examined.

In this study we report the first example of phosphoester bond hydrolysis promoted by a binuclear Zr(IV)-substituted Keggin POM, [{α-PW₁₁O₃₉Zr(µ-OH)(H₂O)}₂]^8- (ZrK 2:2), as a new artificial phosphatase. The speciation of ZrK 2:2, which is highly dependent on the pD, concentration, and temperature, was fully determined by mean of ³¹P NMR spectroscopy. The hydrolytic activity of ZrK 2:2 towards the phosphoester bond in bis-4-nitrophenyl phosphate (BNPP), a commonly used DNA model substrate, was examined by means of ¹H and ³¹P NMR spectroscopy. The mechanism of BNPP hydrolysis was established by ¹H, ³¹P, DOSY, NOESY NMR spectroscopy.