[Tl<sup>III</sup>(dota)]<sup>−</sup>: An Extraordinarily
Robust Macrocyclic Complex
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Abstract
The X-ray structure of {C(NH<sub>2</sub>)<sub>3</sub>}[Tl(dota)]·H<sub>2</sub>O shows that the
Tl<sup>3+</sup> ion is deeply buried in the macrocyclic cavity of
the dota<sup>4–</sup> 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<sup>3+</sup>.
A multinuclear <sup>1</sup>H, <sup>13</sup>C, and <sup>205</sup>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. <sup>205</sup>Tl NMR spectroscopy allowed the protonation
constant associated with the protonation of the complex according
to [Tl(dota)]<sup>−</sup> + H<sup>+</sup> ⇆ [Tl(Hdota)]
to be determined, which turned out to be p<i>K</i><sup>H</sup><sub>Tl(dota)</sub> = 1.4 ± 0.1. [Tl(dota)]<sup>−</sup> does not react with Br<sup>–</sup>, even when using an excess
of the anion, but it forms a weak mixed complex with cyanide, [Tl(dota)]<sup>−</sup> + CN<sup>–</sup> ⇆ [Tl(dota)(CN)]<sup>2–</sup>, with an equilibrium constant of <i>K</i><sub>mix</sub> = 6.0 ± 0.8. The dissociation of the [Tl(dota)]<sup>−</sup> complex was determined by UV–vis spectrophotometry
under acidic conditions using a large excess of Br<sup>–</sup>, and it was found to follow proton-assisted kinetics and to take
place very slowly (∼10 days), even in 1 M HClO<sub>4</sub>,
with the estimated half-life of the process being in the 10<sup>9</sup> h range at neutral pH. The solution dynamics of [Tl(dota)]<sup>−</sup> were investigated using <sup>13</sup>C NMR spectroscopy and DFT
calculations. The <sup>13</sup>C NMR spectra recorded at low temperature
(272 K) point to <i>C</i><sub>4</sub> symmetry of the complex
in solution, which averages to <i>C</i><sub>4<i>v</i></sub> 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