Dinuclear Zinc(II)−Iron(III) and Iron(II)−Iron(III) Complexes as Models for Purple Acid Phosphatases

The heterodinuclear ZnIIFeIII complex 1 and the isostructural FeIIFeIII complex 2 with the dinucleating ligand from 2,6-bis[{bis(2-pyridylmethyl)amino}methyl]-4-methoxyphenol (HBPMOP, 3) were prepared and characterized by X-ray crystallography. Solution studies (UV/Vis spectroscopy; electrochemistry) are described. A pH-induced change in the coordination spheres of the metal centers is seen. These complexes serve as models for the mixed-valence oxidation state in purple acid phosphatases. The cleavage acceleration of the activated phosphodiester 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) was investigated in acetonitrile/water (1:1) in the presence of complexes of the ligand BPMOP and its methyl analogue BPMP with regards to its dependence on the pH value. At the optimum pH value (8.5 ± 0.2), the ZnIIFeIII complex from BPMOP shows a 2-fold higher rate acceleration compared with that of the complex containing BPMP. The diiron complex from BPMOP is 4-fold more reactive than the homologous complex from BPMP. The heterodinuclear ZnIIFeIII catalysts are at least 10-fold more reactive than the homonuclear FeIIFeIII catalysts.