Spectroscopic and Crystal Field Consequences of Fluoride Binding by [Yb⋅DTMA]³⁺ in Aqueous Solution

Abstract

Yb⋅DTMA forms a ternary complex with fluoride in aqueous solution by displacement of a bound solvent molecule from the lanthanide ion. [Yb⋅DTMA⋅F]2+ and [Yb⋅DTMA⋅OH2]3+ are in slow exchange on the relevant NMR timescale (<2000 s−1), and profound differences are observed in their respective NMR and EPR spectra of these species. The observed differences can be explained by drastic modification of the ligand field states due to the fluoride binding. This changes the magnetic anisotropy of the YbIII ground state from easy-axis to easy-plane type, and this change is easily detected in the observed magnetic anisotropy despite thermal population of more than just the ground state. The spectroscopic consequences of such drastic changes to the ligand field represent important new opportunities in developing fluoride-responsive complexes and contrast agents.

Fluoride Binding and Crystal‐Field Analysis of Lanthanide Complexes of Tetrapicolyl‐Appended Cyclen

“…From these results, there is no apparent difference in log K a between the EuL py and YbL py . This contrasts with the tetraamide system, LnDTMA, in which the association constant was found to decrease with decreasing ionic radius of the lanthanide ion …”

“…Figure demonstrates that the europium species are in slow exchange on this timescale: both sets of signals are observed when less than one equivalent of fluoride is added. Once excess fluoride has been added, the original set of peaks is no longer visible, in contrast with complexes of the tetraamides, in which the hydrated species was always present ,…”

“…Analysis and understanding of the crystal‐field effects on the properties of lanthanide complexes has become of great interest, especially in the field of molecular magnetism . We have studied the effects of fluoride binding on the nature of the crystal field in lanthanide complexes tetraamide ligands derived from 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA), such as LnDTMA (Scheme ) . We observed dramatic changes to the spectroscopic properties owing to a switch in magnetic anisotropy from prolate to oblate (or vice versa) upon replacement of an axially coordinated water molecule by fluoride.…”

Triheterometallic Lanthanide Complexes Prepared from Kinetically Inert Lanthanide Building Blocks

“…The choice of trivalent lanthanide ion, the solvent and the presence of coordinating ions will affect the relative population of each form, whereas modifications in the ligand backbone might preclude the formation of one or more forms , , , . Furthermore, the nature of these structures defines the observed behaviour in a wide range of spectroscopic techniques: for open‐shell lanthanide(III) complexes, the overall structure of the lanthanide(III) complex (and the solvation sphere) determines the magnetic anisotropy at the metal atom, which in turn determines the spectroscopic behaviour in NMR, EPR and optical spectroscopy , …”

Lanthanide Complexes that Respond to Changes in Cyanide Concentration in Water

“…Furthermore,t wo mechanisms of energy transfer are possible:aFçrster mechanism involving through-space energy transfer, [10] or aD exter exchange mechanism that involves athrough-bond process. [13] We have shown that fluoride coordination to the metal can have profound consequences for the spectroscopic behavior of these systems,a nd demonstrated that this arises through changes to the magnetic anisotropy at the metal center.T he detection of fluoride has also been accomplished by emissive systems featuring pendant BMes 2 units,a lthough typically with ac ompeting response to cyanide. [12] Furthermore,w ea nd others have explored the coordination of fluoride ions by positively charged lanthanide complexes in solution in water.…”