Electron-transfer kinetics of tris(2-(methylthioethyl))aminecopper(II/I).: A tripodal ligand complex exhibiting virtual C3v symmetry

The tripodal ligand TMMEA (tris(2-methylthioethyl)amine) forms a trigonal bipyramidal complex with copper(II) in which the bridgehead nitrogen occupies one axial site, a solvent molecule (or anion) occupies the opposite axial site, and the three thioether sulfurs occupy the three planar sites. Upon reduction to copper(I), the axial solvent molecule (or anion) dissociates to leave a trigonal pyramidal complex with shortened Cu-S bonds and an elongated Cu-N bond. Therefore, both oxidation states maintain virtual C-3v symmetry similar to that found in the type I blue copper protein sites. The electron-transfer cross-reaction rate constants have been determined for the Cu-II/I(TMMEA) system reacting with three reductants and three oxidants. The Marcus cross relation was then utilized to generate apparent values for the Cu(II/I) electron self-exchange rate constant (k(11)) from the kinetic data for each of the six reactions. The median value obtained from the three reduction reactions is log k(11(Red)) -1.5 while the median from the three oxidation reactions is log k(11(Ox)) = +0.9. This difference of 2.4 orders of magnitude is consistent with the dual-pathway square scheme mechanism which we have previously proposed for electron transfer in Cu(II/I) complexes. For this tripodal ligand system, however, the pathway involving a metastable (CuL)-L-II intermediate (pathway B) appears to be preferred over the pathway involving a metastable (CuL)-L-I intermediate (pathway A), which is opposite to the trend we have previously observed for a number of systems involving macrocyclic and acyclic tetrathiaethers. Both pathways exhibit relatively sluggish electron-transfer kinetics which is attributed to the rupture/formation of the strongly bound inner-sphere water molecule and the accompanying solvent reorganization.