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

Abstract

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