Synthesis, Magnetostructural
Correlation, and Catalytic
Promiscuity of Unsymmetric Dinuclear Copper(II) Complexes: Models
for Catechol Oxidases and Hydrolases
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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 [Cu<sub>2</sub>(<b>L1</b>)(μ-OAc)](ClO<sub>4</sub>)<sub>2</sub>·(CH<sub>3</sub>)<sub>2</sub>CHOH (<b>1</b>) and [Cu<sub>2</sub>(<b>L2</b>)(μ-OAc)](ClO<sub>4</sub>)·H<sub>2</sub>O·(CH<sub>3</sub>)<sub>2</sub>CHOH (<b>2</b>) were determined by X-ray
crystallography. The complex [Cu<sub>2</sub>(<b>L3</b>)(μ-OAc)]<sup>2+</sup> [<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<sup>II</sup> 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<sup>–1</sup> for <b>1</b> and <i>J</i> = −40.0
cm<sup>–1</sup> 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<sup>II</sup>-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<sup>II</sup> 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><sub>ass.</sub> and <i>k</i><sub>cat.</sub>) 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