Dicopper(II) Complexes of H-BPMP-Type Ligands:  pH-Induced Changes of Redox, Spectroscopic (¹⁹F NMR Studies of Fluorinated Complexes), Structural Properties, and Catecholase Activities

Abstract

Substitution of the methyl group from the H-BPMP (HL(CH)3) ligand (2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol) by electron withdrawing (F or CF(3)) or electron donating (OCH(3)) groups afforded a series of dinucleating ligand (HL(OCH)3, HL(F), HL(CF)3), allowing one to understand the changes in the properties of the corresponding dicopper complexes. Dinuclear Cu(II) complexes have been synthesized and characterized by spectroscopic (UV-vis, EPR, (1)H NMR) as well as electrochemical techniques and, in some cases, by single-crystal X-ray diffraction: [Cu(2)(L(OCH)3)(muOH)][(ClO(4))(2)].C(4)H(8)O, [Cu(2)(L(F))(muOH)][(ClO(4))(2)], [Cu(2)(L(F))(H(2)O)(2)][(ClO(4))(3)].C(3)D(6)O, and [Cu(2)(L(CF)3)(H(2)O)(2)][(ClO(4))(3)].4H(2)O. Significant differences are observed for the Cu-Cu distance in the two mu-hydroxo complexes (2.980 A (R = OCH(3)) and 2.967 A (R = F)) compared to the two bis aqua complexes (4.084 A (R = F) and 4.222 A (R = CF(3))). The mu-hydroxo and bis aqua complexes are reversibly interconverted upon acid/base titration. In basic medium, new species are reversibly formed and identified as the bis hydroxo complexes except for the complex from HL(CF)3 which is irreversibly transformed near pH = 10. pH-driven interconversions have been studied by UV-vis, EPR, and (1)H NMR, and the corresponding pK are determinated. In addition, with the fluorinated complexes, the changes in the coordination sphere around the copper centers and in their redox states are evidenced by the fluorine chemical shift changes ((19)F NMR). For all the complexes described here, investigations of the catechol oxidase activities (oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone) are of interest in modeling the catecholase enzyme active site and in understanding aspects of structure/reactivity. These studies show the pH-dependence for the catalytic abilities of the complexes, related with changes in the coordination sphere of the metal centers: only the mu-hydroxo complexes from HL(CH)3, HL(F), and HL(OCH)3 exhibit a catecholase activity. Modification on R-substituent induces a drastic effect on the catecholase activity: the presence of an electron donating group on the ligand increases this activity; the reverse effect is observed with an electron withdrawing group.

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

“…The K M is rather large, though large K M values seem typical of the faster model complexes, and indeed the enzyme itself. The parameters are quite different from those observed by Belle et al [20] for complexes with the same and similar ligands; however, given the impact that small changes to the methyl group have on the activity of the complexes, this is not unexpected [20,60]. The isostructural bisaqua complex does not display catecholase activity, while the corresponding l-hydroxo complex does (Table 5), and it might be expected that the current complex would be similarly inactive.…”

“…The square plane is defined by N1, N2, N3 and O2 with the copper atom 0.187 Å out of the plane towards the apical O1. The copper-ligand distances in the square plane are very similar, with a mean value of 1.99 Å , typical of equatorial bond lengths in similar complexes [19,20,35,36]. The distance between the copper and the bridging phenoxo in the axial position is the longest of the copperligand bonds, at 2.173 Å .…”

“…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].…”

Coordination characteristics of di-O-picolyl derivative of 1,1′-methylene-bis(2-naphthol): First crystal structure of a monomeric Cu(II) complex of bis{2-[(pyridin-2-yl)methoxy] naphthalen-1-yl}methane

“…The TON value observed in the present case is well within the range observed by a number of mono-and dinuclear copper complexes reported in the literature [8][9][10][11][12][13][14][15][16][17][18][19].…”