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) [Me 2 NH 2 ] 11 [Ce III (PW 11 O 39 ) 2 ], [Me 2 NH 2 ] 10 [Ce IV (PW 11 O 39 ) 2 ] (abbreviated as (Ce IV (PW 11 ) 2 ), and K 4 [EuPW 11 O 39 ] 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 (PW 11 ) 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 31 P and 31 P diffusion-ordered NMR spectroscopy. The cleavage of the phosphoester bond of NPP in the presence of (Ce IV (PW 11 ) 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 (PW 11 ) 2 and the phosphate group of NPP was also evidenced by the change of the chemical shift of the 31 P nucleus in NPP upon addition of the POM complex. DFT calculations revealed that binding of NPP to the parent catalyst Ce IV (PW 11 ) 2 is thermodynamically unlikely. On the contrary, formation of complexes with the monomeric 1:1 species, Ce IV PW 11 , is considered to be more favorable, and the most stable complex, [Ce IV PW 11 (H 2 O) 2 (NPP-κO) 2 ] 7- , was found to involve two NPP ligands coordinated to the Ce IV center of Ce IV PW 11 in the monodentate fashion. The formation of such species is considered to be responsible for the hydrolytic activity of Ce IV (PW 11 ) 2 oward phosphomonoesters. On the basis of these findings a principle mechanism for the hydrolysis of NPP by the POM is proposed.