Binuclear complexes of macrocyclic ligands. Electrochemical and spectral properties of homobinuclear CuIICuII, CuIICuI, and CuICuI species including an estimated intramolecular electron transfer rate

Abstract

Electrochemical techniques were employed to study electron transfer properties of a binuclear copper-macrocyclic ligand complex and to prepare several reduced derivatives. The complex, Cu"Cu"L(C104)2-2H20, was prepared by condensing 1,3-diaminopropane with 5-methyl-2-hydroxyisophthalaldehyde and CuíClCLL-óLLO. The Cu"Cu" complex was reduced in successive, quasi-reversible, one-electron steps at -0.52 and -0.91 V vs. NHE. Constant-potential electrolysis was employed to prepare the reduced species, CuI,CuIL(C104) and Cu'Cu'L, both of which were isolated and fully characterized. The mixed-valence species, Cu"Cu'L+, is stable in oxygen-free solutions (conproportionation constant: 3.97 X 106) and reacts with carbon monoxide to form an adduct, Cu"Cu'L(CO)C1C>4. The diamagnetic Cu'Cu'L species was obtained as essentially insoluble black needles which, however, formed a soluble dicarbonyl adduct, Cu'Cu'L(CO)2, on exposure to CO. The latter complex was not isolated. At the ambient temperature Cu"Cu'L+ exhibits an isotropic seven-line solution EPR spectrum while an anisotropic pattern was observed in frozen solutions (77 K), with four lines for g|| and unresolved g±. Variable-tempera ture experiments indicated coalescence at about 200 K suggesting an intramolecular electron transfer rate of about 1.7 X 10'°s_l at 298 K. Electronic absorption spectral measurements revealed at least two absorptions for Cu"Cu'L(C104)2 at 1700 and 1200 nm (CH2CI2), which are not present in Cu"Cu"L(C1C>4)2-2H20, Cu'Cu'L, or the carbonyl derivatives. The new spectral bands may be attributable to intramolecular electron transfer processes.

An electrochemical investigation on mononuclear and dinuclear copper(I) aminocarbonyl complexes

“…2e). As it can be seen this last step occurs at potentials more positive than the reduction to [Cu'L]', in agreement with the well known behaviour of other copper(I1) derivatives (4,6,7,13 (6), to compute the equilibrium constant for the reaction:…”

“…The presence of some inflexions on the rising part of the cathodic response prevents any graphical analysis of the relevant process. The clear detection of two subsequent oxidation processes in cyclic voltammetry suggests the mixed-valence Cul'Cu' species (7) to exist only on the short-time scale. The available CV data do not allow us to distinguish the oxidation process of copper(1) carbonylated centre from that of copper (1) solvated centre.…”

“…Finally it has to be underlined that the redox potentials of the copper(I)/copper(II) couples measured for I, 11, and I11 are high enough to approach those of the copper sites in common cuproproteins (3,7).…”

“…Increasing attention is paid to the electrochemistry of copper(I)/copper(II) redox couples of biological relevance (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). However, the electrochemical behaviour of copper (1) derivatives is seldom directly dealt (9,11,13).…”

Copper (II) complexes of a macrocyclic binucleating ligand which exhibit two-electron oxidation and two-electron reduction. Structure of [Cu2(C24H34N4O2)(CH3OH)2](ClO4)2, a macrocyclic dicopper(II) complex involving coordinated methanol

“…Binuclear copper(I1) complexes of the unsaturated macrocyclic ligand H2L3 and its derivatives have been the focus of extensive studies (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13) but the half saturated macrocycle, H2L2 (14,15), and the fully saturated macrocycle, H2L1 (16), have received much less attention (Fig. 1).…”