Fodor Tamás, Bányai István, Bényei Attila, Platas-Iglesias Carlos, Purgel Mihály, Horváth Gábor L, Zékány László, Tircsó Gyula, Tóth Imre
∥Departamento de Química Fundamental, Facultade de Ciencias, Universidade da Coruña, Campus da Zapateira-Rúa da Fraga 10, 15008 A Coruña, Spain.
⊥MTA-DE Homogeneous Catalysis and Reaction Mechanisms Research Group, H-4032 Debrecen, Egyetem tér 1, Hungary.
Inorg Chem. 2015 Jun 1;54(11):5426-37. doi: 10.1021/acs.inorgchem.5b00458. Epub 2015 May 15.
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), 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.
{C(NH₂)₃}[Tl(dota)]·H₂O的X射线结构表明，Tl(3+)离子深埋于dota(4-)配体（1,4,7,10-四氮杂环十二烷-1,4,7,10-四乙酸）的大环腔内，Tl-N和Tl-O的平均距离分别为2.464 Å和2.365 Å。金属离子直接与配体的八个供体原子配位，导致Tl(3+)周围形成扭曲的四方反棱柱（TSAP'）配位结构。一项多核(¹H)、(¹³C)和(²⁰⁵)Tl NMR研究结合DFT计算，证实了该配合物在水溶液中的TSAP'结构，其以Λ(λλλλ)/Δ(δδδδ)对映体对形式存在。(²⁰⁵)Tl NMR光谱法可测定与配合物质子化相关的质子化常数，即根据[Tl(dota)]⁻ + H⁺ ⇆ [Tl(Hdota)]，结果为pK(H)Tl(dota) = 1.4 ± 0.1。即使使用过量的阴离子，[Tl(dota)]⁻也不与Br⁻反应，但它与氰化物形成弱混合配合物，[Tl(dota)]⁻ + CN⁻ ⇆ [Tl(dota)(CN)]²⁻，平衡常数Kmix = 6.0 ± 0.8。在酸性条件下，使用大量过量的Br⁻，通过紫外可见分光光度法测定了[Tl(dota)]⁻配合物的解离情况，发现其遵循质子辅助动力学，且即使在1 M HClO₄中也进行得非常缓慢（约10天），该过程在中性pH下的估计半衰期在10⁹ h范围内。使用(¹³C) NMR光谱法和DFT计算研究了[Tl(dota)]⁻的溶液动力学。在低温（272 K）下记录的(¹³C) NMR光谱表明，该配合物在溶液中具有C4对称性，随着温度升高平均为C4v对称性。这种动态行为归因于Λ(λλλλ) ↔ Δ(δδδδ)对映异构化过程，该过程涉及大环单元的翻转和侧链的旋转。根据我们的计算，侧链旋转过程限制了Λ(λλλλ) ↔ Δ(δδδδ)的相互转化。
