The electron-transfer kinetics of copper(II/I) complexes formed with the macrocyclic terdentate ligand 1,4,7-trithiacyclononane (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 CuIIL2, although the 1 : 1 complex, CuIIL(H2O)3, can become dominant in water at high Cu(II) concentrations. The predominant reduced species is the 1 : 1 complex, CuIL (i.e., CuIL(H 2O) or CuIL(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 CuIIL(H2O)3. In the latter case, reduction involves a three-step sequence in which (i) CuIIL(H 2O)3 reacts with L to produce CuIIL 2, (ii) electron transfer occurs and (iii) L dissociates again to yield CuIL(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.
ASJC Scopus subject areas
- Inorganic Chemistry