Synthesis, crystal structure and properties of dinuclear iron(III) complexes containing terminally coordinated phenolate/H2O/OH− groups as models for purple acid phosphatases: efficient hydrolytic DNA cleavage

We present here the syntheses of two dinuclear iron(III) complexes with the polydentate N,O-donor ligand H2BPClNOL (N -(2-hydroxybenzyl)-N -(2-pyridylmethyl)[(3-chloro)(2- hydroxy)]propylamine). The reaction between FeIII(ClO4)3 · 9H2O, the title ligand and two equivalents of NaOAc · 3H2O resulted in the complex View the MathML source[Fe2III(BPClNOL)2(OAc)]ClO4(1). When the synthesis was performed with a lesser amount of NaOAc · 3H2O (half equivalent), the complex without bridging acetate, View the MathML source[Fe2III(BPClNOL)2(H2O)2](ClO4)2·4H2O(2) was obtained. The complexes were characterized by X-ray structural analysis, magnetochemistry, Mössbauer and UV–Vis spectroscopies, and electrochemistry. Complex 2 was also characterized in solution through potentiometric titration. Both complexes crystallize in the monoclinic system. Complex 2 has one water molecule coordinated to each iron centre. Their pKa values are 5.00 and 7.03 for the first protonation/deprotonation equilibrium of each coordinated water molecule. The UV–Vis and electrochemical techniques showed that the absence of an acetate bridge in 2 results in a significant difference in the Lewis acidity of both iron centres, when compared with 1. The lack of an acetate bridge in 2 also results in changes in the anti-ferromagnetic coupling as revealed by magnetic measurements. Complex 2 is an interesting structural model for the active site of iron containing PAPs, since it has an Fe(-alkoxo)2Fe core with an FeIII⋯FeIII distance of 3.122(1) Å, containing phenolate and water molecules coordinated to the iron centres and is soluble in aqueous solutions. Furthermore, the UV–Vis properties of 2 are similar to those of PAPs, since the complex absorbs at 580 nm in the oxidized form (550–570 nm for PAPs) and at 499 nm in the mixed-valence form (505–510 nm for PAPs) as revealed through spectroelectrochemical studies. Finally, complex 2 successfully promoted the hydrolytic cleavage of plasmid DNA under aerobic and anaerobic conditions, producing single and double DNA strand breaks at biological pH values.