Phosphate Ester Bond Hydrolysis Promoted by Lanthanide-Substituted Keggin-type Polyoxometalates Studied by a Combined Experimental and Density Functional Theory Approach

Hydrolytic cleavage of 4-nitrophenyl phosphate (NPP), a commonly used DNA model substrate, was examined in the presence of series of lanthanide-substituted Keggin-type polyoxometalates (POMs) [Me₂NH₂]₁₁­[Ce^III(PW₁₁O₃₉)₂], [Me₂NH₂]₁₀­[Ce^IV(PW₁₁O₃₉)₂] (abbreviated as (Ce^IV(PW₁₁)₂), and K₄[EuPW₁₁O₃₉] by means of NMR and luminescence spectroscopies and density functional theory (DFT) calculations. Among the examined complexes, the Ce­(IV)-substituted Keggin POM (Ce^IV(PW₁₁)₂) showed the highest reactivity, and its aqueous speciation was fully determined under different conditions of pD, temperature, concentration, and ionic strength by means of ³¹P and ³¹P diffusion-ordered NMR spectroscopy. The cleavage of the phosphoester bond of NPP in the presence of (Ce^IV(PW₁₁)₂) proceeded with an observed rate constant k_obs = (5.31 ± 0.06) × 10^–6 s^–1 at pD 6.4 and 50 °C. The pD dependence of NPP hydrolysis exhibits a bell-shaped profile, with the fastest rate observed at pD 6.4. The formation constant (K_f = 127 M^–1) and catalytic rate constant (k_c = 19.41 × 10^–5 s^–1) for the NPP-Ce­(IV)-Keggin POM complex were calculated, and binding between Ce^IV(PW₁₁)₂ and the phosphate group of NPP was also evidenced by the change of the chemical shift of the ³¹P nucleus in NPP upon addition of the POM complex. DFT calculations revealed that binding of NPP to the parent catalyst Ce^IV(PW₁₁)₂ is thermodynamically unlikely. On the contrary, formation of complexes with the monomeric 1:1 species, Ce^IVPW₁₁, is considered to be more favorable, and the most stable complex, [Ce^IVPW₁₁(H₂O)₂(NPP-κO)₂]^7–, was found to involve two NPP ligands coordinated to the Ce^IVcenter of Ce^IVPW₁₁ in the monodentate fashion. The formation of such species is considered to be responsible for the hydrolytic activity of Ce^IV(PW₁₁)₂ toward phosphomonoesters. On the basis of these findings a principle mechanism for the hydrolysis of NPP by the POM is proposed.