Kinetics and mechanism of the stepwise complex formation of Cu(ii) with tren-centered tris-macrocycles

Abstract

The stepwise complexation kinetics of Cu²⁺ with three tetratopic ligands L¹, L² and L³, tren-centred macrocycles with different bridges connecting the 14-membered macrocycles with the tren unit, have been measured by stopped-flow photodiode array techniques at 25 °C, I = 0.5 M (KNO₃), and pH = 4.96. The reaction between the first Cu²⁺ and the ligand consists of several steps. In a rapid reaction Cu²⁺ first binds to the flexible and more reactive tren-unit. In this intermediate a translocation from the tren unit to the macrocyclic ring, which forms the thermodynamic more stable complex, takes place. This species can react further with a second Cu²⁺ to give a heterotopic dinuclear species with one Cu²⁺ bound by the tren-unit and the other coordinated by the macrocycle. A further translocation occurs to give the homoditopic species with two Cu²⁺ in the macrocycles. Finally a slow rearrangement of the dinuclear complex gives the final species. The rates of the translocation are dependent on the length and rigidity of the bridge, whereas the complexation rates with the tren unit are little affected by it. VIS spectra of the species obtained by fitting the kinetic results, EPR-spectra taken during the reaction, and ES mass spectra of the products confirm the proposed mechanism. The addition of a second, third and fourth equivalent of Cu²⁺ proceeds in an analogous way, but is complicated by the fact that we start and end with a mixture of species. These steps were evaluated in a qualitative way only.