pH-Controlled Change of the Metal Coordination in a Dicopper(II) Complex of the Ligand H−BPMP:  Crystal Structures, Magnetic Properties, and Catecholase Activity

Abstract

The dinucleating ligand 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol (H-BPMP) has been used to synthesize the three dinuclear Cu(II) complexes [Cu2(BPMP)(OH)][ClO4](2).0.5C4H8O (1), [Cu2(BPMP)(H2O)2](ClO4)(3).4H2O (2), and [Cu2(H-BPMP)][(ClO4)4].2CH3CN (3). X-ray diffraction studies reveal that 1 is a mu-hydroxo, mu-phenoxo complex, 2 a diaqua, mu-phenoxo complex, and 3 a binuclear complex with Cu-Cu distances of 2.96, 4.32, and 6.92 A, respectively. Magnetization measurements reveal that 1 is moderately antiferromagnetically coupled while 2 and 3 are essentially uncoupled. The electronic spectra in acetonitrile or in water solutions give results in accordance with the solid-state structures. 1 is EPR-silent, in agreement with the antiferromagnetic coupling between the two copper atoms. The X-band spectrum of powdered 2 is consistent with a tetragonally elongated square pyramid geometry around the Cu(II) ions, in accordance with the solid-state structure, while the spectrum in frozen solution suggests a change in the coordination geometry. The EPR spectra of 3 corroborate the solid-state and UV-visible studies. The 1H NMR spectra also lead to observations in accordance with the conclusions from other spectroscopies. The electrochemical behavior of 1 and 2 in acetonitrile or in water solutions shows that the first reduction (Cu(II)Cu(II)-Cu(II)Cu(I) redox couple) is reversible and the second (formation of Cu(I)Cu(I) irreversible. In water, 1 and 2 are reversibly interconverted upon acid/base titration (pK 4.95). In basic medium a new species, 4, is reversibly formed (pK 12.0), identified as the bishydroxo complex. Only 1 exhibits catecholase activity (oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone, vmax = 1.1 x 10(-6) M-1 s-1 and KM = 1.49 mM). The results indicate that the pH dependence of the catalytic abilities of the complexes is related to changes in the coordination sphere of the metal centers.

Stopped-flow Fluorescence Studies of Inhibitor Binding to Tyrosinase from Streptomyces antibioticus

“…Such a mechanism was proposed earlier to explain the pH dependence of halide inhibition (22,55,56). A pH-dependent dissociation of Cu 2 bridging hydroxide has been shown to occur for several small dinuclear copper model compounds (57)(58)(59). Therefore, pK a values between 4.0 and 5.5 have been obtained for the dissociation process, comparable with the average pK a of 4.59 Ϯ 0.05 found in the present case.…”

Structural and spectroscopic studies of a model for catechol oxidase

“…Fitting of the experimental magnetic susceptibility data was performed using Octave (http://www.gnu.org/software/octave/) and the results were plotted with gnuplot. [19]. Under these conditions no formation of quinone was observed in the absence of the copper complex.…”

“…2,6-Bis[bis(pyridin-2-ylmethylamino)methyl]-4-methylphenol (H-BPMP), employed previously to model catechol oxidase [19,20], provides a nitrogen-rich coordination set similar to that of the enzyme (three nitrogen donors) and has previously furnished both trigonal bipyramidal and square pyramidal geometries. A hydroxo-bridged dicopper complex with BPMP -has been shown to display catecholase activity [19]. Various complexes with other terminal ligating species have been characterized [19,21,22].…”

“…A hydroxo-bridged dicopper complex with BPMP -has been shown to display catecholase activity [19]. Various complexes with other terminal ligating species have been characterized [19,21,22]. Several structure-activity relationships have been identified from amongst the previously reported catechol oxidase models.…”

Crystal structure of di-μ2-chlorido-dichlorido-bis[(2-(1H-pyrazol-3-yl)- pyridine-κ2N,N’)dicopper(II)], C16H12Cl4Cu2N6

“…Among the reported complexes, Cu(II) containing complexes have attracted considerable interests sinceC u(II)i sa ne ssentialt race elementf or life processes [7]. The penta-coordination of Cu(II) is very commonand usually presents asquare-pyramidal, atrigonal-bipyramidal geometry, or an distorted intermediate geometry.…”