Electron-transfer kinetics and equilibria of copper(II/I) complexes with 1,4,7-trithiacyclononane. A square scheme mechanism involving ligand addition

The electron-transfer kinetics of copper(II/I) complexes formed with the macrocyclic terdentate ligand 1,4,7-trithiacyclononane ([9]aneS₃ = TTCN = L) have been investigated under a variety of conditions. The relevant equilibrium constants, complex formation and dissociation rate constants, and redox potentials in both water and acetonitrile have also been determined. The predominant oxidized species in both solvents is Cu^IIL₂, although the 1:1 complex, Cu^IIL(H₂O)₃, can become dominant in water at high Cu(II) concentrations. The predominant reduced species is the 1:1 complex, Cu^IL (i.e., Cu^IL(H₂O) or Cu^IL(CH₃CN)), as confirmed by electrospray mass spectrometry, pulsed square-wave voltammetry, cyclic voltammetry and the ligand dependence of the oxidation kinetics. Electron transfer occurs almost exclusively through the bis redox couple, Cu^II/IL₂, even for solutions containing predominantly Cu^IIL(H₂O)₃. In the latter case, reduction involves a three-step sequence in which (i) Cu^IIL(H₂O)₃ reacts with L to produce Cu^IIL₂, (ii) electron transfer occurs and (iii) L dissociates again to yield Cu^IL(H₂O). The sluggishness of direct electron transfer in the 1:1 complex is attributed to the unfavorable energetics of forming or dissociating strong copper-solvent bonds combined with the accompanying re-structuring of the surrounding solvent.