Including and Declaring Structural Fluctuations in the Study of Lanthanide(III) Coordination Chemistry in Solution

Abstract

The physicochemical properties of lanthanide-(III) ions are directly linked to the structure of the surrounding ligands. Rapid ligand exchange prohibits direct structure−property relationships from being formed for simple complexes in solution because the property measured will be an average over several structures. For kinetically inert lanthanide(III) complexes, the simpler speciation may alleviate the problem, yet the archetypical complexes formed by ligands derived from cyclen are known to have at least four different forms in solutioneach with a variation in the crystal field that gives rise to significantly different properties. Slow interchange between forms has been engineered, so that a single complex geometry can be studied, but fast or intermediate interchange between forms is much more commonly observed. The rapid structural fluctuation can report on the changing chemical environment and can be disregarded if a specific property of a lanthanide(III) complex is exploited in an application. However, if we are to understand the chemistry of the lanthanide(III) ions in solution, we must include the structural fluctuation that takes place even in kinetically inert lanthanide(III) complexes in our studies. Here, we have scrutinized the processes that determine the speciation of lanthanide(III) complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA)-like ligands, in particular the processes that enable exchange between forms that have different physicochemical properties, exemplified by the exchange between the diastereomeric capped square-antiprismatic (cSAP) and capped twisted-square-antiprismatic (cTSAP) forms of DOTA-like lanthanide(III) complexes. In the characterization of a kinetically inert f-element complex, understanding the structural fluctuation in the system is critical because a single observed property can arise from a weighted average, from all forms present, or from a single form with a dominating contribution. Further, the experimental condition will influence both the distribution of lanthanide(III) species in solution and the rates of the processes that change the coordination sphere of the lanthanide(III) ions. This is highlighted using data from a series of cyclen-derived ligands with different pendant arms and different denticity. The data were obtained in experiments that take place on different time scales to show that the rate of the process that results in a structural change must be considered against the time of the experiment. We conclude that the structural fluctuations must be taken into account and that they cannot be predicted from the ligand structure. Thus, an estimate of the exchange rates between forms, the relative concentrations of the specific forms, and the effect of the specific structure of each form of the complex must be included in the description of the solution properties of f-element chelates.

HOCl Responsive Lanthanide Complexes Using Hydroquinone Caging Units

“…In Tb.L 2 and Tb.L 3 the terbium excited state lifetime is reduced by ligand specific quenching pathways that are absent in Tb.L 1 . Similarly, the luminescent lifetime of Eu.L 1 also resembles those observed from other DO3A complexes [53], while those observed in Eu.L 2 and Eu.L 3 are significantly reduced. This suggest additional quenching pathways operate in Ln.L 2 and Ln.L 3 , both redox mediated charge transfer and back energy transfer to the antenna centred triplet state are viable explanations.…”

“…The 1 H NMR spectra indicate a DOTA like structure of all complexes, with added conformational freedom in Eu.L 2 . This is confirmed by the luminescence emission spectra, where the emission spectrum of Eu.L 2 is more similar to that of DO3A than DOTA [53]. In contrast, the emission spectra of Eu.L 1 and Eu.L 3 are both structured, and the band at 590 nm corresponding to the 5 D 0 → 7 F 1 transition shows the different symmetry of the two complexes.…”

“…Eu.L 3 shows a spectrum very similar to the C4 symmetric DOTA complex. So does Eu.L 2 , but here the symmetry is broken in a fraction of the population, possibly by association to a capping ligand [53]. Please note that all complexes in Figure 4 are predominately in the SAP conformation.…”

“…Time-gating did not provide any benefits in the model system and only steady-state spectra are used in this study. broken in a fraction of the population, possibly by association to a capping ligand [53]. Please note that all complexes in Figure 4 are predominately in the SAP conformation.…”

Versatile Binding Modes of Chiral Macrocyclic Amine towards Rare Earth Ions

“…This effective symmetry, higher than that observed in the solid, likely results from signal averaging and the equilibria related to the exchange of axial ligands (nitrate anions, solvent molecules) as well as to conformational changes of the macrocycle. Due to the fluxional character of rare earth(III) ions such equilibria were previously observed for other macrocyclic complexes . The NMR spectra of 1 also show the presence of considerable amount of free ligand.…”