Phosphatase-like Activity of Tetranuclear Iron(III) and Zinc(II) Complexes

Three new tetranuclear iron(III) and zinc(II) complexes, [Fe-4(cpdp)(2)(phth)(2)(OH)(2)]center dot 8H(2)O (1), [Fe-4(cpdp)(2)(terephth)(2)(OH)(2)] (2), and [Zn-4(Hcpdp)(2)(suc)]Br-2 center dot 12H(2)O (3), have been synthesized as models for the active site of phosphoester hydrolases by utilizing a polydentate ligand, N,N'-bis[2-carboxybenzomethyl]-N,N'-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol (H(3)cpdp) in combination with exogeneous phthalate (phth), terephthalate (terephth), and succinate (suc). Single crystal X-ray analyses reveal that the metallic core of complex 1 consists of four distorted octahedral iron(III) ions with average intraligand Fe-Fe separation of 3.656(2) angstrom, while the structure 3 represents a tetranuclear metallic core containing four distorted trigonal bipyramidal zinc(II) ions with average intraligand Zn-Zn separation of 3.472(2) angstrom. The molecular structure of complex 2 has been optimized by the DFT method which shows that its core arrangement is similar to that of 1. Complex 1 has a very interesting centrosymmetric structure that includes two crystallographically equivalent [Fe-2(cpdp)](3+) dinuclear units, connected together by a pair of syn-syn bridging phthalates and a pair of bridging hydroxides to generate a dimer of dimers structural motif. In complex 3, a succinate group connects two crystallographically equivalent [Zn-2(Hcpdp)](2+) dinuclear units in a syn-syn bidentate manner forming a dimer of dimers structural design. All three complexes show phosphatase-like activity that has been examined in methanol-water (1:1; v/v) using bis(p-nitrophenyl) phosphate (BNPP) as model substrate by applying the UV-vis spectrophotometric technique. In each case, the kinetic data have been analyzed by the Michaelis-Menten approach. The order of catalytic efficiency for the conversion of substrate to product follows the trend 1 > 2 > 3 with turnover rates (k(cat)) of (2.73 +/- 0.13) X 10(-5) for 1, (1.06 +/- 0.07) X 10(-5) for 2, and (2.33 +/- 0.18) X 10 -6 s(-1) for 3. These k(cat) values are comparable to, albeit slightly lower than, the values reported for similar iron(III)- and zinc(II)-based model complexes in the literature. DFT calculations have been carried out to support the proposed mechanism for phosphatase-like activity.