Lanthanide(III) Complexes with Ligands Derived from a Cyclen Framework Containing Pyridinecarboxylate Pendants. The Effect of Steric Hindrance on the Hydration Number

Abstract

Two new macrocyclic ligands, 6,6′-((1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H2DODPA) and 6,6′-((4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H2Me-DODPA), designed for complexation of lanthanide ions in aqueous solution, have been synthesized and studied. The X-ray crystal structure of [Yb(DODPA)](PF6)·H2O shows that the metal ion is directly bound to the eight donor atoms of the ligand, which results in a square-antiprismatic coordination around the metal ion. The hydration numbers (q) obtained from luminescence lifetime measurements in aqueous solution of the Eu(III) and Tb(III) complexes indicate that the DODPA complexes contain one inner-sphere water molecule, while those of the methylated analogue H2Me-DODPA are q = 0. The structure of the complexes in solution has been investigated by 1H and 13C NMR spectroscopy, as well as by theoretical calculations performed at the density functional theory (DFT; mPWB95) level. The minimum energy conformation calculated for the Yb(III) complex [Λ(λλλλ)] is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the Yb(III)-induced paramagnetic 1H shifts. The nuclear magnetic relaxation dispersion (NMRD) profiles recorded for [Gd(Me-DODPA)]+ are typical of a complex with q = 0, where the observed relaxivity can be accounted for by the outer-sphere mechanism. However, [Gd(DODPA)]+ shows NMRD profiles consistent with the presence of both inner- and outer-sphere contributions to relaxivity. A simultaneous fitting of the NMRD profiles and variable temperature 17O NMR chemical shifts and transversal relaxation rates provided the parameters governing the relaxivity in [Gd(DODPA)]+. The results show that this system is endowed with a relatively fast water exchange rate k(ex)(298) = 58 × 10(6) s(–1).

Spectroscopic Properties of a Family of Mono‐ to Trinuclear Lanthanide Complexes

“…The synthesis of the ligands L 1 – L 3 are depicted in Scheme and Scheme . 1,7‐Dipicolinic cylen precursor 1 was obtained according to literature procedures . Alkylation of 1 with methyl iodide afforded a mixture of the monomethylated ( 2 ) and dimethylated ( 3 ) cyclen precursors.…”

“…In previous works,, it has been shown that the macrocyclic ligand H 2 L 1 (Scheme ) forms eight‐coordinate complexes with Ln ions both in the solid state and in solution. The presence of the methyl substituents in positions 1 and 7 of the cyclen moiety introduces a certain degree of steric hindrance on the picolinate pendants that prevents the coordination of water molecules.…”

The confluence of structure and dynamics in lanthanide(iii) chelates: how dynamics help define structure in solution

“…31 A similar effect has been observed in a related system published recently by Platas-Iglesias and co-workers. 21 Thus, the chemist studying chelates that undergo very rapid dissociative water exchange should understand that changes in the dissociative water exchange rate are intrinsically linked to the hydration state of the chelate, both through static structural effects described by Parker and co-workers 33 and through the dynamic effects described herein.…”

Coupling Fast Water Exchange to Slow Molecular Tumbling in Gd3+ Chelates: Why Faster Is Not Always Better

“…36-41 However, our chosen approach is unique in that in addition to a chelate with very fast dissociative water exchange kinetics the same methods can be applied to generate an isomeric chelate with slower dissociative water exchange kinetics. 34,42 This allows us, for the first time, to compare the effects of accelerating inner-sphere dissociative water exchange on relaxivity.…”