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Abstract

The bis-bidentate ligand R,S-1,2-diphenyl-N,N'-bis(2-quinolinemethylidene)ethane-1,2-diamine (ligand 4), containing two (iminomethyl)quinoline moieties separated by a cis-1,2-diphenylethylene spacer, forms stable complexes with both Cu^I and Cu^II. With Cu^II, the monomeric 1:1 complex [Cu^II(4)]²⁺ is obtained both in CH₃CN and CH₂Cl₂. With Cu^I and overall 1:1 metal/ligand molar ratio, an equilibrium mixture is obtained in CH₃CN, consisting of [Cu^I(4)₂]⁺, [Cu^I₂(4)₂]²⁺ and [Cu^I₂(4)(CH₃CN)₄]²⁺. The preponderant species is the two-metal one-ligand "open" complex [Cu^I₂(4)(CH₃CN)₄]²⁺, in which each Cu⁺ cation is coordinated in a tetrahedral fashion by one (iminomethyl)quinoline unit and by two CH₃CN molecules. Precipitation from the equilibrium mixture yields only crystals of [Cu^I₂(4)(CH₃CN)₄](ClO₄)₂·2CH₃CN, whose crystal and molecular structures have been determined. On the other hand, in the poorly coordinating CH₂Cl₂ solvent, only the dimeric helical [Cu^I₂(4)₂]²⁺ complex is obtained, when the overall metal/ligand 1:1 molar ratio is chosen. Addition of large quantities of acetonitrile to solutions of [Cu^I₂(4)₂]²⁺ in dichlorometane results in the formation of [Cu^I₂(4)(CH₃CN)₄]²⁺, i.e. in the solvent-driven disassembling of the Cu^I helicate. While electrochemistry in CH₃CN is poorly defined due to the presence of more than one Cu^I species, cyclic voltammetry experiments carried out in CH₂Cl₂ revealed a well defined behavior, with irreversible oxidation of [Cu^I₂(4)₂]²⁺ and irreversible reduction of [Cu^II(4)]²⁺ taking place at separate potentials (∆E ≈ 700 mV). Irreversibility and separation of the redox events are due to the self-assembling and disassembling processes following the reduction and oxidation, respectively.

Keywords: Helical structures; Electrochemistry; Molecular machines; Copper complexes; Imine ligands; Nitriles; Salenes; Quinolines; Chiral ligands; Cyclic voltammetry.

References: 30 live references.