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 related tripodal ligands have been studied in aqueous solution at 25 degrees C, mu = 0.10 M (NaClO4). For most of the ligands studied, specific formation rate constants have been resolved for both the unprotonated and monoprotonated ligand species. All of the tripodal ligands included in this study contain a bridgehead amine nitrogen with the three legs consisting of 2-methylthioethyl or 2-ethylthioethyl and/or 2-pyridylethyl or 2-pyridylmethyl. Since the bridgehead nitrogen is too sterically hindered to participate in initial coordinate bond formation, the first bond must involve a thiaether sulfur or a pyridine nitrogen on one of the pendant legs followed by coordination to the bridgehead nitrogen to complete the first chelate ring. All kinetic data are interpreted in terms of this presumed sequence in the bond formation steps. For the two ligands in which all three pendant legs contain thiaether sulfur donor atoms, the rate-determining step appears to be at the point of second bond formation (chelate ring closure), although the distinction is not well defined. For all other unprotonated ligands, the kinetic behavior is consistent with the first-bond formation being rate-determining. Upon protonation, the rate-determining step appears to shift to the point of proton loss associated with second-bond formation in several cases. A particularly interesting observation is that the tripodal ligand tris(ethylthioethyl)amine (TEMEA) exhibits specific Cu(II) complex formation rate constants that are virtually identical to those for a closely related macrocyclic ligand, 1,4,8-trithia-11-azacyclotetradecane (14]aneNS3), but the calculated CuIIL 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 the 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.