Highly Stable Complexes of Divalent Metal Ions (Mg²⁺, Ca²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺) with a Dota‐Like Ligand Containing a Picolinate Pendant

Abstract

The stability constants of complexes of the macrocyclic ligand do3a‐pic4– (H4do3a‐pic = 2,2′,2″‐{10‐[(6‐carboxypyridin‐2‐yl)methyl]‐1,4,7,10‐tetraazacyclododecane‐1,4,7‐triyl}triacetic acid) with several divalent metal ions (Pb2+, Cd2+, Zn2+, Cu2+, Ca2+, and Mg2+) have been determined by using pH‐potentiometric titrations (I = 0.1 M KCl, 25 °C). The stability of these complexes follows the trend Cu2+ > Cd2+ ≈ Pb2+ ≈ Zn2+ >> Ca2+ >> Mg2+. A particularly high stability constant has been determined for the Cu2+ complex [log KCuL = 23.20(4)]. Analysis of the titration curves indicate the presence of protonated forms of the complexes in solution, with protonation constants of log KM(HxL) = 6.9–2.0 (x = 1, 2, or 3). The structure of the complexes in solution has been investigated by using 1H and 13C NMR spectroscopy and DFT calculations performed in aqueous solution at the TPSSh/6‐31G(d) level. In the case of the Pb2+ and Cd2+ complexes, relativistic effects were considered with the use of relativistic effective core potentials. Calculations show that the complexes with the largest metal ions (Pb2+ and Ca2+) are nine‐coordinate, with their coordination polyhedra being best described as capped twisted square antiprisms. The Cd2+ and Mg2+ complexes are seven‐coordinate, with the metal ions being bound to the four nitrogen atoms of the cyclen unit and the three acetate pendant arms. Finally, in the Cu2+ and Zn2+ complexes, the metal ions are six‐coordinated, with the metal ions being asymmetrically placed inside the macrocyclic cavity of the ligand, and the coordination polyhedra can be described as an octahedron and a trigonal prism, respectively.

Definition of the Labile Capping Bond Effect in Lanthanide Complexes

“…[26] The absorption spectra of the complexes recorded in H2O solution show an absorption band with a maximum at 274 nm typical of the picolinate chromophore (Supporting Information). [20] The emission spectra recorded under excitation through the ligand bands (10 -5 M, pH 7.0) show the 5 D0  7 FJ (J = 0-4) and 5 D4  7 FJ (J = 3-6) transitions characteristic of Eu 3+ and Tb 3+ , respectively (Supporting Information). The emission lifetimes of the Eu 3+ ( 5 D0) and Tb 3+ ( 5 D4) excited states of the complexes with 1,7-Medo2ampa 3-provide hydration numbers of q = 1.1 and 0.9 using the equation of Beeby.…”

“…All rights reserved. 20 The latter compound was demonstrated to adopt a square antiprismatic (SAP) structure in solution by analysis of the Yb 3+ -induced 1 H NMR shifts. Thus, we conclude that the [Yb(1,7-Medo2ampa)] complex adopts a SAP structure in solution as well, which is confirmed by the relative energies of the two isomers obtained using DFT calculations (Supporting Information).…”

“…This was accomplished using as a starting point the H4do3ampa ligand (Scheme 1), [20] which was shown to form nine-coordinate complexes in solution where the oxygen atom of the picolinate group occupies the capping position in a capped square antiprismatic (SAP) coordination polyhedron. We hypothesized that removing one the carboxylate groups in trans position to the picolinate arm to give 1,7-H3Medo2ampa should result in the coordination of a water molecule in one of the square faces of the coordination polyhedron, with the capping position being occupied by the oxygen atom of the picolinate group.…”

Nyíltláncú és makrociklusos aminokarboxilát ligandumok szintézise és fémkomplexeik vizsgálata: koordinációs kémia az orvosi képalkotás szolgálatában

“…19 Hasonló következtetésekre jutottunk a DO3A-PIC komplexképzõ két vegyértékû és Ln 3+ -ionokkal képzõdõ komplexei vizsgálata során is: a képzõdõ komplexek stabilitása meghaladta az DO3A ligandum Ln 3+ -komplexei esetében tapasztaltat, ami a pikolinátcsoport koordinációjára utal. 27,28 A komplexképzõdést a DO3A-és DOTA komplexek esetében már tankönyvi példaként ismert lassú reakciók jellemezték. A Gd 3+ -komplex savkatalizált disszociációjára jellemzõ sebességi állandók pedig a megfelelõ DO3A-és DOTA-komplexek sebességi állandói közé estek.…”

“…A vizsgált két vegyértékû fémionok esetében DFT módszerrel végzett számolásaink szerint a pikolinát csoport koordinációja csak a nagyobb méretû Ca 2+ -, ill. Pb 2+ -ionok esetében következik be, míg a Mg 2+ -, Zn 2+ -, Cu 2+ -, Cd 2+ -ionok esetében az oldalláncok közül csak az acetát karok koordinálódnak. 28 …”