Proton exchange and base hydrolysis of syn,anti-cis-dichloro(1,4,7,10-tetra-azacyclododecane)cobalt(III) cations

Abstract

Studies of the exchange of amine protons between syn,anti-cis-[Co(cyclen)Cl₂]⁺(cyclen = 1,4,7,10-tetra-azacyclododecane) and D₂O in deuteriated buffer solutions show that extensive exchange takes place between the solvent and the protons on the nitrogen trans to the co-ordinated chlorides, within the lifetime of the unreacted complex. In the ¹H n.m.r. Spectrum of the complex ion in [²H₆]dimethyl sulphoxide, the chemical shift of one of the protons on a nitrogen trans to chlorine is very sensitive to the presence of added Cl^– presumably because of ion association and hydrogen bonding. The second-order rate constant for the exchange of this proton, 6.9 × 10⁷ dm³ mol^–1 s^–1, is only ca. 3 times greater than that for the exchange of the other, 2.5 × 10⁷ dm³ mol^–1 s^–1. There is no exchange of the protons on the mutually trans nitrogens during the lifetime of the complex. The synchronous deprotonation–chloride dissociation pathway proposed by other workers for this reaction is unlikely.