Proton NMR Investigation of the Stoichiometry, Stability, and Exchange Kinetics of Tl+ Complexes with Hexacyclen and Hexamethylhexacyclen in N,N-Dimethylformamide Solutions. Observation of a Three-Site Exchange Process

Abstract

Abstract 1H NMR spectroscopy was used to investigate the stoichiometry, stability, and exchange kinetics and mechanism of ligand interchange for Tl+ complexes with hexacyclen (HCY) and hexamethylhexacyclen (HMHCY) in dimethylformamide (DMF) solution. In both cases, a stable complex with a 1:1 stoichiometry was formed in solution. A complete line-shape analysis was used to evaluate the mean exchange times and exchange rates. The two-site chemical exchange between Tl+ and HCY proceeded via a dissociative ligand-interchange mechanism in the temperature range 250–310 K. However, the 1H NMR spectra of the Tl+–HMHCY complex system in DMF, in about the same temperature range, revealed the predominance of a three-site exchange process in the system. The type of exchange was dependent on the Tl+/HMHCY mole ratio, ρ. At ρ < 1, ligand exchange occurs via a dissociative mechanism, while at ρ > 1, a bimolecular metal exchange is the predominate mechanism. In this case, the main factors controlling the exchange rates are the conformational rearrangement of the ligand during a concerted partial decomplexation of a Tl+ cation and partial complexation of a second one. The activation parameters (Ea, ΔH‡, ΔS‡, and ΔG‡) for the exchange processes (i.e. the ligand exchange, metal exchange, and the conformational change of the complex) were determined and compared with those for a two-site exchange for the Tl+–HCY system.