[TlIII(dota)]−: An Extraordinarily Robust Macrocyclic Complex

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 dota4– 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 pK 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 K 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 C 4 symmetry of the complex in solution, which averages to C 4v 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.