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 N′,N′,N-tris­(2-pyridylmethyl)-N-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,3-propanediamin-2-ol
(L1) and N′,N′-bis­(2-pyridylmethyl)-N,N-(2-hydroxybenzyl)­(2-hydroxy-3,5-di-tert-butylbenzyl)-1,3-propanediamin-2-ol
(L2). The structures of the complexes [Cu2(L1)­(μ-OAc)]­(ClO4)2·(CH3)2CHOH (1) and [Cu2(L2)­(μ-OAc)]­(ClO4)·H2O·(CH3)2CHOH (2) were determined by X-ray
crystallography. The complex [Cu2(L3)­(μ-OAc)]2+ [3; L3 = N-(2-hydroxybenzyl)-N′,N′,N-tris­(2-pyridylmethyl)-1,3-propanediamin-2-ol]
was included in this study for comparison purposes only (Neves et
al. Inorg. Chim. Acta 2005, 358, 1807–1822). Magnetic data show that the CuII centers in 1 and 2 are antiferromagnetically
coupled and that the difference in the exchange coupling J found for these complexes (J = −4.3 cm–1 for 1 and J = −40.0
cm–1 for 2) is a function of the Cu–O–Cu
bridging angle. In addition, 1 and 2 were
tested as catalysts in the oxidation of the model substrate 3,5-di-tert-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 CuII-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 CuII center that contains the phenol group with tert-butyl substituents and hydrogen bonding of the protonated phenol
with the phosphate O atom are proposed to increase the hydrolase activity
(Kass. and kcat.) of 1 and 2 in comparison with that found
for complex 3. In fact, complexes 1 and 2 show both oxidoreductase and hydrolase/nuclease activities
and can thus be regarded as man-made models for studying catalytic
promiscuity.