Modeling the Active Site of the Purple Acid Phosphatase Enzyme with Hetero-Dinuclear Mixed Valence M(II)−Fe(III) [M = Zn, Ni, Co, and Cu] Complexes Supported over a [N₆O] Unsymmetrical Ligand

The active site of the purple acid phosphatase enzyme has been successfully modeled by a series of heterodinuclear M(II)−Fe(III) [M = Zn, Ni, Co, and Cu] type complexes of an unsymmetrical [N₆O] ligand that contained a bridging phenoxide moiety and one imidazoyl and three pyridyl moieties as the terminal N-binding sites. In particular, the hetero-dinuclear complexes, {L[M^II(μ-OAc)₂Fe^III]}^-(ClO₄)₂ [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)], were obtained directly from the phenoxy-bridged ligand (HL), namely 2-{[bis(2-methylpyridyl)amino]methyl}-6-{[((1-meth-ylimidazol-2-yl)methyl)(2-pyridylmethyl)amino]methyl}-4-t-butylphenol (2), upon sequential addition of Fe(ClO₄)₃·XH₂O and M(ClO₄)₂·6H₂O (M = Zn and Ni) or M(OAc)₂·XH₂O (M = Co and Cu), in a low-to-moderate (ca. 32−53%) yield. The temperature-dependent magnetic susceptibility measurements indicated weak antiferromagnetic coupling interactions occurring between the two metal centers in their high-spin states. All of the 3(a−b) and 4(a−b) complexes successfully carried out the hydrolysis of the bis(2,4-dinitrophenyl)phosphate (2,4-BDNPP) substrate in a mixed CH₃CN/H₂O (v/v 1:1) medium in the pH range of 5.5−10.5 at room temperature, thereby mimicking the functional activity of the native enzyme. The spectrophotometric titration suggested a monoaquated and dihydroxo species of the type {L[(H₂O)M^II(μ-OH)Fe^III(OH)]}²⁺ to be the catalytically active species for the phosphodiester hydrolysis reaction within the pH range of ca. 5.80−7.15. Last, the kinetic studies on the hydrolysis of the model substrate, 2,4-BDNPP, divulge a Michaelis−Menten-type behavior for all complexes.