Macrocyclic Pyclen-Based Gd3+ Complex with High Relaxivity and pH Response

We report the synthesis and characterization of the macrocyclic ligand 2,2′-((2-(3,9-bis­(carboxymethyl)-3,6,9-triaza-1­(2,6)-pyridinacyclodecaphane-6-yl)­ethyl)­azanediyl)­diacetic acid (H4L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-...

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Published in:Inorganic chemistry 2020-05, Vol.59 (10), p.7306-7317
Main Authors: Frías, Juan C, Soriano, José, Blasco, Salvador, García-España, Enrique, Rodríguez-Rodríguez, Aurora, Esteban-Gómez, David, Carniato, Fabio, Botta, Mauro, Platas-Iglesias, Carlos, Albelda, M. Teresa
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Language:English
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Summary:We report the synthesis and characterization of the macrocyclic ligand 2,2′-((2-(3,9-bis­(carboxymethyl)-3,6,9-triaza-1­(2,6)-pyridinacyclodecaphane-6-yl)­ethyl)­azanediyl)­diacetic acid (H4L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-ray diffraction measurements. Two N atoms of the pyclen moiety in the trans position are protonated in the solid state, together with the exocyclic N atom and one of the carboxylate groups of the ligand. The relaxivity of the Gd3+ complex was found to increase from 6.7 mM–1 s–1 at pH 8.6 to 8.5 mM–1 s–1 below pH ≈ 6.0. Luminescence lifetime measurements recorded from H2O and D2O solutions of the Eu3+ complex evidence the presence of a single complex species in solution at low pH (∼5.0) that contains two inner-sphere water molecules. DFT calculations suggest that the coordination environment of the Ln3+ ion is fulfilled by the four N atoms of the pyclen unit, two oxygen atoms of the macrocyclic acetate groups, and an oxygen atom of an exocyclic carboxylate group. The two inner-sphere water molecules complete coordination number nine around the metal ion. At high pH (∼9.3), the lifetime of the excited 5D0 level of Eu3+ displays a biexponential behavior that can be attributed to the presence of two species in solution with hydration numbers of q = 0 and q = 1. The 1H NMR and DOSY spectra recorded from solutions of the Eu3+ and Y3+ complexes reveal a structural change triggered by pH and the formation of small aggregates at high pH values.
ISSN:0020-1669
1520-510X
DOI:10.1021/acs.inorgchem.0c00690