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

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posted on 01.06.2015, 00:00 by Tamás Fodor, István Bányai, Attila Bényei, Carlos Platas-Iglesias, Mihály Purgel, Gábor L. Horváth, László Zékány, Gyula Tircsó, Imre Tóth
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 pKHTl(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 Kmix = 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 C4 symmetry of the complex in solution, which averages to C4v 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.