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

Three new tetranuclear iron(III) and zinc(II) complexes, [Fe₄(cpdp)₂(phth)₂(OH)₂]·8H₂O (1), [Fe₄(cpdp)₂(terephth)₂(OH)₂] (2), and [Zn₄(Hcpdp)₂(suc)]Br₂·12H₂O (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 (H₃cpdp) 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) Å, 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) Å. 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 [Fe₂(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 [Zn₂(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) × 10^-5 for 1, (1.06 ± 0.07) × 10^-5 for 2, and (2.33 ± 0.18) × 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.