Complexation of Trivalent Lanthanides with Planar Tridentate Aromatic Ligands Tuned by Counteranions and Steric Constraints

Abstract

Among the plethora of parameters controlling the stability and structures of lanthanide coordination complexes, it is often difficult to decipher their relative importance in the global complexation processes. The combination of the bond valence method (for analyzing solid state structures) with the thermodynamic site binding model (for unravelling complexation reactions occurring in solution) appears to be an efficient tool for specifically addressing interligand effects, which affect the output of the coordination process. When applied to the reaction of the tridentate aromatic scaffolds 2,2':6',2''-terpyridine (L1) and 2,6-bis(benzimidazol-2-yl)pyridine (L2) with trivalent lanthanides, Ln(III), we demonstrate that the successive fixation of ligands, eventually leading to the triple-helical complexes [Ln(Lk)3]3+, is anticooperative both in the solid state and in solution, with a special sensitivity to the nature of the counteranion and to the peripheral substitution for L2. Consequently, in addition to the classical entropic driving forces resulting from the use of specific metal/ligand ratio, the stoichiometry of the final complex can be tuned by a judicious choice of interligand interactions, as exemplified by the unusual isolation of stable complexes with Ln/L = 2:3 ratios.

Lanthanides: Supramolecular Chemistry

“…Nevertheless, by careful design, highly stable complexes can also be formed from acyclic ligands, which at the same time enable the use of more than one ligand to fulfill the high coordination of the lanthanides. Consequently, this also opens up the avenue of developing self-assembly complexes with precise geometry and function, where at the 14 The formation of self-assemblies using lanthanidedirected synthesis has been elegantly explored by many researchers in recent times; a research endeavor has been pioneered in part by researchers such as Bünzli, Piguet, Raymond, Pecoraro, and others. Much of their work within this area has been dealt with in the reviews mentioned earlier.…”

Solid-state sensors based on Eu3+-containing supramolecular polymers with luminescence colour switching capability

“…17 However, Mebip itself does not fluoresce in the visible range. To shift the fluorescence of this motif into the visible range, an extension of the conjugated π-system was pursued.…”

“…17 UV-Vis spectroscopy was used to follow the formation of the coordination complexes with a 1 : 2 metal : ligand stoichiometry ( Fig. 2a and b).…”

“…Thus, the absorption spectra corroborate that the π-extended ligand derivative 1 and the macromonomer 2 coordinate metal-ions in the same way as the parent Mebip derivative. 17,19,20 Similarly, solutions of 1 (c = 20 µmol L −1 ) and 2 (c = 10 µmol L −1 ) in a CHCl 3 /MeCN mixture (9 : 1) were titrated with Eu(ClO 4 ) 3 and the complex formation was again monitored by UV-Vis spectroscopy. The acquired spectra reveal that the band associated with the ligand absorption at 332 nm shows a more moderate decrease upon addition of Eu 3+ ions than the corresponding titration with Zn(NTf ) 2 and instead of a peak at 388 nm, a broad shoulder centred at 362 nm appears ( ), the dynamic equilibrium of the complex formation was shifted and the levelling off was already observed at a 1 : 1.5 metal : ligand ratio (Fig.…”

“…Hence, due to the lower binding constant of the Eu 3+ coordination complex vis-à-vis the Zn 2+ complex, notably in CHCl 3 , 17 unbound ligand remains present in dilute solutions of 1 and 2 at a stoichiometric 1 : 3 metal-toligand ratio. Indeed, the spectrophotometric titrations in MeCN follow the previously reported behaviour of the parent Mebip ligand, 17 corroborating the successive formation of the 1 : 3, 1 : 2, and 1 : 1 complexes between the Eu 3+ -ions and 1. ; m ≈ 0.36, n ≈ 0.64, p ≈ 55). complex (388 nm) against the europium-salt-to-ligand ratio show a levelling off at the stoichiometric 1 : 1 ratio.…”