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.

The thermodynamic stability of Fe(III) complexes with a new hexadentate tripodal ligand (O-TRENSOX) incorporating three 8-hydroxyquinoline ("oxine") subunits, linked to a tetraamine ("TREN") via an amide connection, has been investigated by the use of UV-vis spectrophotometry and potentiometric methods. O-TRENSOX has been found to form, at pH < 1, a protonated complex FeLH 5 2+ (orange color) which deprotonates, over the pH range 1-2, to a green complex FeLH 2through a four-proton process. The first protonation constant of ferric O-TRENSOX has been determined to be 5.60. The stability constant log β 110 has been determined to be 30.9. A pFe (pFe ) -log [Fe 3+ ]) value of 29.5 has been calculated at pH ) 7.4, [ligand] tot ) 10 µΜ, and [Fe 3+ ] tot ) 1 µM, indicating that O-TRENSOX is one of the most powerful among the iron synthetic chelators. Cyclic voltammetry experiments have shown that the system Fe III -O-TRENSOX/Fe II -O-TRENSOX is quasi reversible, with a redox potential of 0.087 V vs NHE. This value is related to the high complexing ability of O-TRENSOX for both the ferric and ferrous iron redox states, making it relevant for biological uses. The kinetics of formation and acid hydrolysis of the ferric O-TRENSOX complex have been investigated in acidic medium using the diode array stopped-flow spectrophotometry technique in 2.0 M NaClO 4 /HClO 4 at 25 °C. The determining step for the complex formation involves the reaction of FeOH 2+ with the LH 7 + ligand species, with a rate constant of 789 ( 17 M -1 s -1 . The acid hydrolysis of the FeLH 2complex in 0.02-1.0 M HClO 4 and ionic strength 2.0 M NaClO 4 /HClO 4 leads to the FeLH 5 2+ complex, indicating that O-TRENSOX is a very strong chelating agent for Fe(III) in acidic medium. The kinetic data have been interpreted by a stepwise mechanism related to the successive protonation of four binding sites. The spectroscopic change is consistent with removal of one arm of the ligand followed by a shift from a bis(oxinate) to a bis(salicylate) mode of coordination.