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[MII(μ-OAc)2FeIII]}­(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-methylimidazol-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)­MII(μ-OH)­FeIII(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.