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 hetero-dinuclear 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 (<b>3a</b>), Ni (<b>3b</b>), Co (<b>4a</b>), and Cu (<b>4b</b>)], were obtained directly from the phenoxy-bridged ligand (HL), namely 2-{[bis­(2-methylpyridyl)­amino]­methyl}-6-{[((1-methylimidazol-2-yl)­methyl)­(2-pyridylmethyl)­amino]­methyl}-4-<i>t</i>-butylphenol (<b>2</b>), upon sequential addition of Fe­(ClO₄)₃·<i>X</i>H₂O and M­(ClO₄)₂·6H₂O (M = Zn and Ni) or M­(OAc)₂·<i>X</i>H₂O (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 <b>3</b>(<b>a–b</b>) and <b>4</b>(<b>a–b</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