Synthesis, Magnetostructural Correlation, and Catalytic Promiscuity of Unsymmetric Dinuclear Copper(II) Complexes: Models for Catechol Oxidases and Hydrolases

Herein, we report the synthesis and characterization, through elemental analysis, electronic spectroscopy, electrochemistry, potentiometric titration, electron paramagnetic resonance, and magnetochemistry, of two dinuclear copper­(II) complexes, using the unsymmetrical ligands <i>N</i>′,<i>N</i>′,<i>N</i>-tris­(2-pyridylmethyl)-<i>N</i>-(2-hydroxy-3,5-di-<i>tert</i>-butylbenzyl)-1,3-propanediamin-2-ol (<b>L1</b>) and <i>N</i>′,<i>N</i>′-bis­(2-pyridylmethyl)-<i>N</i>,<i>N</i>-(2-hydroxybenzyl)­(2-hydroxy-3,5-di-<i>tert</i>-butylbenzyl)-1,3-propanediamin-2-ol (<b>L2</b>). The structures of the complexes [Cu₂(<b>L1</b>)­(μ-OAc)]­(ClO₄)₂·(CH₃)₂CHOH (<b>1</b>) and [Cu₂(<b>L2</b>)­(μ-OAc)]­(ClO₄)·H₂O·(CH₃)₂CHOH (<b>2</b>) were determined by X-ray crystallography. The complex [Cu₂(<b>L3</b>)­(μ-OAc)]²⁺ [<b>3</b>; <b>L3</b> = <i>N</i>-(2-hydroxybenzyl)-<i>N</i>′,<i>N</i>′,<i>N</i>-tris­(2-pyridylmethyl)-1,3-propanediamin-2-ol] was included in this study for comparison purposes only (Neves et al. <i>Inorg. Chim. Acta</i> <b>2005</b>, <i>358</i>, 1807–1822). Magnetic data show that the Cu^II centers in <b>1</b> and <b>2</b> are antiferromagnetically coupled and that the difference in the exchange coupling <i>J</i> found for these complexes (<i>J</i> = −4.3 cm^–1 for <b>1</b> and <i>J</i> = −40.0 cm^–1 for <b>2</b>) is a function of the Cu–O–Cu bridging angle. In addition, <b>1</b> and <b>2</b> were tested as catalysts in the oxidation of the model substrate 3,5-di-<i>tert</i>-butylcatechol and can be considered as functional models for catechol oxidase. Because these complexes possess labile sites in their structures and in solution they have a potential nucleophile constituted by a terminal Cu^II-bound hydroxo group, their activity toward hydrolysis of the model substrate 2,4-bis­(dinitrophenyl)­phosphate and DNA was also investigated. Double electrophilic activation of the phosphodiester by monodentate coordination to the Cu^II center that contains the phenol group with <i>tert</i>-butyl substituents and hydrogen bonding of the protonated phenol with the phosphate O atom are proposed to increase the hydrolase activity (<i>K</i>_ass. and <i>k</i>_cat.) of <b>1</b> and <b>2</b> in comparison with that found for complex <b>3</b>. In fact, complexes <b>1</b> and <b>2</b> show both oxidoreductase and hydrolase/nuclease activities and can thus be regarded as man-made models for studying catalytic promiscuity