title: [Tl(III)(dota)](-): An Extraordinarily Robust Macrocyclic Complex.
abstract: The X-ray structure of {C(NH2)3}[Tl(dota)]·H2O shows that the Tl(3+) 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 Tl(3+). A multinuclear (1)H, (13)C, and (205)Tl 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. (205)Tl 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 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 10(9) h range at neutral pH. The solution dynamics of [Tl(dota)](-) were investigated using (13)C NMR spectroscopy and DFT calculations. The (13)C 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.
authors.forename: Tamás
authors.initials: T
authors.lastname: Fodor
authors.forename: István
authors.initials: I
authors.lastname: Bányai
authors.forename: Attila
authors.initials: A
authors.lastname: Bényei
authors.affiliation: ∥Departamento de Química Fundamental, Facultade de Ciencias, Universidade da Coruña, Campus da Zapateira-Rúa da Fraga 10, 15008 A Coruña, Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Platas-Iglesias
authors.affiliation: ⊥MTA-DE Homogeneous Catalysis and Reaction Mechanisms Research Group, H-4032 Debrecen, Egyetem tér 1, Hungary.
authors.forename: Mihály
authors.initials: M
authors.lastname: Purgel
authors.affiliation: #Institute of Innate Immunity, University Hospital, University of Bonn, 53127 Bonn, Germany.
authors.forename: Gábor L
authors.initials: GL
authors.lastname: Horváth
authors.forename: László
authors.initials: L
authors.lastname: Zékány
authors.forename: Gyula
authors.initials: G
authors.lastname: Tircsó
authors.forename: Imre
authors.initials: I
authors.lastname: Tóth