[Tl^III(dota)]⁻: An Extraordinarily Robust Macrocyclic Complex

Abstract

The X-ray structure of {C­(NH₂)₃}­[Tl­(dota)]·H₂O shows that the Tl³⁺ ion is deeply buried in the macrocyclic cavity of the dota^4– ligand (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) with average Tl–N and Tl–O distances of 2.464 and 2.365 Å, respectively. The metal ion is directly coordinated to the eight donor atoms of the ligand, which results in a twisted square antiprismatic (TSAP′) coordination around Tl³⁺. A multinuclear ¹H, ¹³C, and ²⁰⁵Tl NMR study combined with DFT calculations confirmed the TSAP′ structure of the complex in aqueous solution, which exists as the Λ­(λλλλ)/Δ­(δδδδ) enantiomeric pair. ²⁰⁵Tl NMR spectroscopy allowed the protonation constant associated with the protonation of the complex according to [Tl­(dota)]⁻ + H⁺ ⇆ [Tl­(Hdota)] to be determined, which turned out to be p<i>K</i>^H_Tl(dota) = 1.4 ± 0.1. [Tl­(dota)]⁻ does not react with Br^–, even when using an excess of the anion, but it forms a weak mixed complex with cyanide, [Tl­(dota)]⁻ + CN^– ⇆ [Tl­(dota)­(CN)]^2–, with an equilibrium constant of <i>K</i>_mix = 6.0 ± 0.8. The dissociation of the [Tl­(dota)]⁻ complex was determined by UV–vis spectrophotometry under acidic conditions using a large excess of Br^–, and it was found to follow proton-assisted kinetics and to take place very slowly (∼10 days), even in 1 M HClO₄, with the estimated half-life of the process being in the 10⁹ h range at neutral pH. The solution dynamics of [Tl­(dota)]⁻ were investigated using ¹³C NMR spectroscopy and DFT calculations. The ¹³C NMR spectra recorded at low temperature (272 K) point to <i>C</i>₄ symmetry of the complex in solution, which averages to <i>C</i>_4<i>v</i> as the temperature increases. This dynamic behavior was attributed to the Λ­(λλλλ) ↔ Δ­(δδδδ) enantiomerization process, which involves both the inversion of the macrocyclic unit and the rotation of the pendant arms. According to our calculations, the arm-rotation process limits the Λ­(λλλλ) ↔ Δ­(δδδδ) interconversion