title: Lanthanide dota-like complexes containing a picolinate pendant: structural entry for the design of Ln(III)-based luminescent probes.
abstract: In this contribution we present two ligands based on a do3a platform containing a picolinate group attached to the fourth nitrogen atom of the cyclen unit, which are designed for stable lanthanide complexation in aqueous solutions. Potentiometric measurements reveal that the thermodynamic stability of the complexes is very high (log K = 21.2-23.5), being comparable to that of the dota analogues. Luminescence lifetime measurements performed on solutions of the Eu(III) and Tb(III) complexes indicate that the complexes are nine coordinate with no inner-sphere water molecules. A combination of density functional theory (DFT) calculations and NMR measurements shows that for the complexes of the heaviest lanthanides there is a major isomer in solution consisting of the enantiomeric pair Λ(δδδδ) and Δ(λλλλ), which provides square antiprismatic coordination (SAP) around the metal ion. Analysis of the Yb(III)-induced paramagnetic shifts unambiguously confirms that these complexes have SAP coordination in aqueous solution. For the light lanthanide ions however both the SAP and twisted-square antiprismatic (TSAP) isomers are present in solution. Inversion of the cyclen ring appears to be the rate-determining step for the Λ(δδδδ) ↔ Δ(λλλλ) enantiomerization process observed in the Lu(III) complexes. The energy barriers obtained from NMR measurements for this dynamic process are in excellent agreement with those predicted by DFT calculations. The energy barriers calculated for the arm-rotation process are considerably lower than those obtained for the ring-inversion path. Kinetic studies show that replacement of an acetate arm of dota by a picolinate pendant results in a 3-fold increase in the formation rate of the corresponding Eu(III) complexes and a significant increase of the rates of acid-catalyzed dissociation of the complexes. However, these rates are 1-2 orders of magnitude lower than those of do3a analogues, which shows that the complexes reported herein are remarkably inert with respect to metal ion dissociation.
authors: Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira-Rúa da Fraga 10, 15008 A Coruña, Spain.
authors: Martín
authors: M
authors: Regueiro-Figueroa
authors: Bachir
authors: B
authors: Bensenane
authors: Erika
authors: E
authors: Ruscsák
authors: David
authors: D
authors: Esteban-Gómez
authors: Loïc J
authors: LJ
authors: Charbonnière
authors: Gyula
authors: G
authors: Tircsó
authors: Imre
authors: I
authors: Tóth
authors: Andrés
authors: A
authors: de Blas
authors: Teresa
authors: T
authors: Rodríguez-Blas
authors: Carlos
authors: C
authors: Platas-Iglesias
mesh_terms: D000042:Absorption; D002384:Catalysis; D015195:Drug Design; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D007700:Kinetics; D028581:Lanthanoid Series Elements / Q000737:chemistry*; D008024:Ligands; D049408:Luminescent Agents / Q000138:chemical synthesis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010848:Picolinic Acids / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D012996:Solutions
chemical_list: D006573:Heterocyclic Compounds, 1-Ring; D028581:Lanthanoid Series Elements; D008024:Ligands; D049408:Luminescent Agents; D009942:Organometallic Compounds; D010848:Picolinic Acids; D011522:Protons; D012996:Solutions; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; C030614:picolinic acid