Magnetic, ESR, electrochemical, and potentiometric titration studies of the imidazolate-bridged dicopper(II) ion in a binucleating macrocycle

Abstract

Physical studies of the imidazolate-bridged dicopper(II) ion coordinated to the ligands l, 4,7,13,16,19-hexaaza-10,22-dioxacyclotetracosane (A) and 1,4,7,13,16,19-hexaazacyclotetracosane (A') were carried out to explore the influence of these binucleating macrocycles on the stability of the Cu2(im)3+ core (imH = imidazole). Temperature-dependent magnetic susceptibility studies of solid [Cu2(im)(imH)2CA](C104)3 and [Cu2(im)CA'](C104)3-H20 revealed antiferromagnetic behavior with J = -27.60 (7) and -29.40 (3) cm"1, respectively. These values demonstrate that the magnetic interaction between copper(II) centers in the Cu2(im)3+ ion is unaltered by its coordination to these macrocyclic ligands. The solution properties differed markedly, however. From pH-dependent electron spin resonance and potentiometric titration data it was determined that the imidazolate bridge in [Cu2(im)CA']3+ is much more stable than ih related complexes such as [Cu2(TMDT)2(im)]3+, where TMDT = 1,1,7,7-tetramethyldiethylenetriamine. Equilibrium constants as well as AH and AS values for the bridge-splitting reactions were determined. Cyclic voltammograms of [Cu2(im)CA']3+ in acetonitrile under various conditions showed only irreversible behavior, ascribed to the instability of the cuprous derivatives of the binucleating macrocycle.

Imidazolate-bridged dinuclear copper(II) complex with new macrocyclic ligand bearing two 1H-imidazol-4-yl-pendants

“…7.0-9.3. When the solution pH is adjusted below 5.6, spectral changes (spectra D-B) were observed, namely the signal at about 3600 G, which is typical signal for imidazolate-bridged Cu(II) complex, decreased and the one at about 3340 G, which is characteristic signal of dinuclear Cu(II) complex, increased indicating that the imidazolate bridge is partially disrupted and probably forms the dinuclear Cu(II) complex 2 (Scheme 2), which are in accordance with the reported results [5]. When the solution pH is proceeded to 3.2 (spectrum A), the signals of free Cu(II) ions appeared, indicating the macrocylic ligand is protonated, and the Cu(II) ions are released.…”

“…There is antiferromagnetic interaction with g = 2.08, − 2 J = 66.5 cm − 1 between the two imidazolate-bridged Cu(II) atoms. The complex displays moderate SOD activity with IC 50 of 1.42 μM.Copper(II)-zinc(II) superoxide dismutase (Cu 2 Zn 2 -SOD) is a wellknown metalloenzyme that catalyzes the dismutation of highly toxic superoxide radicals so as to protect cells from the oxidative damage [1][2][3][4][5]. The active site of the enzyme contains an imidazolate-bridged Cu(II)-Zn(II) heterodinuclear metal center, and the reported studies found that the Cu(II) is essential for the SOD activity, while the Zn(II) can be replaced by Cu(II) to give Cu 2 Cu 2 -SOD, which has similar SOD activity as that of Cu 2 Zn 2 -SOD [6,7].…”

“…The active site of the enzyme contains an imidazolate-bridged Cu(II)-Zn(II) heterodinuclear metal center, and the reported studies found that the Cu(II) is essential for the SOD activity, while the Zn(II) can be replaced by Cu(II) to give Cu 2 Cu 2 -SOD, which has similar SOD activity as that of Cu 2 Zn 2 -SOD [6,7]. Therefore, the rational design and synthesis of imidazolate-bridged homo/heterodinuclear complexes to mimic the active site of Cu 2 Zn 2 -SOD have attracted great attention, and the reported studies have provided valuable insights into the structure and activity of the Cu 2 Zn 2 -SOD [5][6][7][8][9]. Among the reported model compounds, macrocyclic ligands, especially those bearing flexible pendants, have been extensively used to construct the imidazolatebridged dinuclear complexes for their enhanced stability, and the obtained complexes have been proved to be successful in mimicking the active site of the enzyme [9][10][11].…”

“…The χ M vs. T curve shows that when the temperature decreases, χ M increases continuously till a maximum at about 80 K, and then decreases to a minimum at 20 K, where the abrupt rise of χ M below 20 K is due to a small amount of paramagnetic impurities. Such facts are characteristics of antiferromagnetic exchange interaction between the Cu(II) atoms in 1 [5]. The magnetic susceptibility data were interpreted by a modified Bleaney-Bowers equation, where −2 J is the magnetic exchange parameter, g is the landé factor and p is the molar fraction of paramagnetic impurity [10].…”

Electrochemical studies of a dinuclear copper complex with imidazole derivative ligand H3bphenim

“…1(a)), similar to that present in reported model compounds [19][20][21][22]. Evidence of the bridge cleavage in 50% aqueous CH 3 CN solution and the key steps of two reactions coupled to the electrochemical processes are presented and their relation with the catalytic activity in the 3,5-di-tertbutylcatechol oxidation shown by the complex is discussed.…”