Kinetics and mechanism of copper(II) complex formation with tripodal aminopolythiaether and aminopolypyridyl ligands in aqueous solution

The complex formation kinetics of aquated copper(II) ion reacting with 12 r elated tripodal ligands have been studied in aqueous solution at 25 degrees C, mu = 0.10 M (NaClO4). For most of the ligands studied, specific formati on rate constants have been resolved for both the unprotonated and monoprot onated ligand species. All of the tripodal ligands included in this study c ontain a bridgehead amine nitrogen with the three legs consisting of 2-meth ylthioethyl or 2-ethylthioethyl and/or 2-pyridylethyl or 2-pyridylmethyl. S ince the bridgehead nitrogen is too sterically hindered to participate in i nitial coordinate bond formation, the first bond must involve a thiaether s ulfur or a pyridine nitrogen on one of the pendant legs followed by coordin ation to the bridgehead nitrogen to complete the first chelate ring. All ki netic data are interpreted in terms of this presumed sequence in the bond f ormation steps. For the two ligands in which all three pendant legs contain thiaether sulfur donor atoms, the rate-determining step appears to be at t he point of second bond formation (chelate ring closure), although the dist inction is not well defined. For all other unprotonated ligands, the kineti c behavior is consistent with the first-bond formation being rate-determini ng. Upon protonation, the rate-determining step appears to shift to the poi nt of proton loss associated with second-bond formation in several cases. A particularly interesting observation is that the tripodal ligand tris(ethy lthioethyl)amine (TEMEA) exhibits specific Cu(II) complex formation rate co nstants that are virtually identical to those for a closely related macrocy clic ligand, 1,4,8-trithia-11-azacyclotetradecane ([14]aneNS(3)), but the c alculated (CuL)-L-II dissociation rate constants differ by a factor of 1000 . A further comparison of the calculated dissociation rate constants for Cu (II)-tripodal ligand complexes indicates that a Cu(II)-N(pyridine) bond is approximately 10(4) times stronger than a Cu(II)-SR2 bond. This leads to th e conclusion that a 1:1 Cu(II)-SR2 complex would have a predicted stability constant of about 0.04 M-1 in aqueous solution-the first estimate obtained for the strength of a single Cu(II)-S(thiaether) bond.