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

Substitution of the methyl group from the H-BPMP (HLCH3) ligand (2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol) by electron withdrawing (F or CF3) or electron donating (OCH3) groups afforded a series of dinucleating ligand (HLOCH3, HLF, HLCF3), allowing one to understand the changes in the properties of the corresponding dicopper complexes. Dinuclear CuII complexes have been synthesized and characterized by spectroscopic (UV−vis, EPR, 1H NMR) as well as electrochemical techniques and, in some cases, by single-crystal X-ray diffraction: [Cu2(LOCH3)(μΟΗ)][(ClO4)2]·C4H8O, [Cu2(LF)(μΟΗ)][(ClO4)2], [Cu2(LF)(H2O)2][(ClO4)3]·C3D6O, and [Cu2(LCF3)(H2O)2][(ClO4)3]·4H2O. Significant differences are observed for the Cu−Cu distance in the two μ-hydroxo complexes (2.980 Å (R = OCH3) and 2.967 Å (R = F)) compared to the two bis aqua complexes (4.084 Å (R = F) and 4.222 Å (R = CF3)). The μ-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 HLCF3 which is irreversibly transformed near pH = 10. pH-driven interconversions have been studied by UV−vis, EPR, and 1H 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 (19F 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 μ-hydroxo complexes from HLCH3, HLF, and HLOCH3 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.