Jean-François Lemonnier

Optimizing sensitization processes in dinuclear luminescent lanthanide oligomers: selection of rigid aromatic spacers.

This work illustrates a simple approach for optimizing the lanthanide luminescence in molecular dinuclear lanthanide complexes and identifies a particular multidentate europium complex as the best candidate for further incorporation into polymeric materials. The central phenyl ring in the bis-tridentate model ligands L3–L5, which are substituted with neutral (X = H, L3), electron-withdrawing (X = F, L4), or electron-donating (X = OCH3, L5) groups, separates the 2,6-bis(benzimidazol-2-yl)pyridine binding units of linear oligomeric multi-tridentate ligand strands that are designed for the complexation of luminescent trivalent lanthanides, Ln(III). Reactions of L3–L5 with [Ln(hfac)3(diglyme)] (hfac– is the hexafluoroacetylacetonate anion) produce saturated single-stranded dumbbell-shaped complexes [Ln2(Lk)(hfac)6] (k = 3–5), in which the lanthanide ions of the two nine-coordinate neutral [N3Ln(hfac)3] units are separated by 12–14 Å. The thermodynamic affinities of [Ln(hfac)3] for the tridentate binding sites in L3–L5 are average (6.6 ≤ log(β(2,1)(Y,Lk)) ≤ 8.4) but still result in 15–30% dissociation at millimolar concentrations in acetonitrile. In addition to the empirical solubility trend found in organic solvents (L4 > L3 >> L5), which suggests that the 1,4-difluorophenyl spacer in L4 is preferable, we have developed a novel tool for deciphering the photophysical sensitization processes operating in [Eu2(Lk)(hfac)6]. A simple interpretation of the complete set of rate constants characterizing the energy migration mechanisms provides straightforward objective criteria for the selection of [Eu2(L4)(hfac)6] as the most promising building block.

Lanthanide Template Synthesis of a Molecular Trefoil Knot

We report on a complex featuring three 2,6-pyridinedicarboxamide ligands entwined around a lanthanide (Ln(3+)) ion. The ligand strands can be cyclized by ring-closing olefin metathesis to form a molecular trefoil knot in 58% yield. Demetalation with tetraethylammonium fluoride quantitatively generates the wholly organic 81-atom-loop trefoil knot.

N-Heterocyclic Tridentate Aromatic Ligands Bound to [Ln(hexafluoroacetylacetonate)3] Units: Thermodynamic, Structural, and Luminescent Properties

Herein, we discuss how, why, and when cascade complexation reactions produce stable, mononuclear, luminescent ternary complexes, by considering the binding of hexafluoroacetylacetonate anions (hfac(-)) and neutral, semi-rigid, tridentate 2,6-bis(benzimidazol-2-yl)pyridine ligands (Lk) to trivalent lanthanide atoms (Ln(III)). The solid-state structures of [Ln(Lk)(hfac)(3)] (Ln=La, Eu, Lu) showed that [Ln(hfac)(3)] behaved as a neutral six-coordinate lanthanide carrier with remarkable properties: 1) the strong cohesion between the trivalent cation and the didentate hfac anions prevented salt dissociation; 2) the electron-withdrawing trifluoromethyl substituents limited charge-neutralization and favored cascade complexation with Lk; 3) nine-coordination was preserved for [Ln(Lk)(hfac)(3)] for the complete lanthanide series, whilst a counterintuitive trend showed that the complexes formed with the smaller lanthanide elements were destabilized. Thermodynamic and NMR spectroscopic studies in solution confirmed that these characteristics were retained for solvated molecules, but the operation of concerted anion/ligand transfers with the larger cations induced subtle structural variations. Combined with the strong red photoluminescence of [Eu(Lk)(hfac)(3)], the ternary system Ln(III)/hfac(-)/Lk is a promising candidate for the planned metal-loading of preformed multi-tridentate polymers.

Planned failures from the principle of maximum site occupancy in lanthanide helicates.

Despite the recent emergence of a toolbox fitted with microscopic thermodynamic descriptors for predicting the stabilities and speciations of polynuclear complexes in solution, the discovery of novel or unusual type of metal-ligand assemblies in metallosupramolecular chemistry still often relies on serendipity. In order to highlight the novel perspectives offered by a rational exploitation of these thermodynamic parameters, the segmental bis-tridentate ligands L7 and L8 have been designed for providing effective molarities upon reaction with trivalent lanthanides, Ln(III), so small that the saturated binuclear triple-stranded helicates [Ln(2)(Lk)(3)](6+), which obey the well-respected principle of maximum site occupancy, cannot be detected in solution because of their deliberately planned instabilities. The hierarchical evolution of the effective molarities with an increasing number of ligand strands in these complexes indeed favors the formation of the alternative unsaturated single-stranded [Ln(2)(Lk)](6+) and double-stranded [Ln(2)(Lk)(2)](6+) complexes, whose relative speciations in solution depend on the nature of the binding sites introduced into the segmental ligand.

A Simple Chemical Tuning of the Effective Concentration: Selection of Single-, Double-, and Triple-Stranded Binuclear Lanthanide Helicates

The replacement of terminal 2-benzimidazol-6-carboxypyridine (two internal rotational degrees of freedom) with 2-benzimidazol-8-hydroxyquinoline (one internal rotational degree of freedom) into segmental bis-tridentate ligands in going from L2 and [L3-2 H](2-) to [L12 b-2 H](2-) does not significantly affect the structures of the resulting binuclear lanthanide triple-stranded helical complexes [Ln(2)(L2)(3)](6+), [Ln(2)(L3-2 H)(3)], and [Ln(2)(L12 b-2 H)(3)] (palindromic helices, intermetallic contact distance approximately 9 A, helical pitch approximately 1.4 nm per turn). However, their thermodynamic assemblies are completely different in solution, as evidenced by the spectacular decrease of the effective concentrations by two orders of magnitude for [L12 b-2 H](2-). This key parameter in the [Ln(2)(L12 b-2 H)(n)] (n=2, 3) complexes is further abruptly modulated along the lanthanide series (Ln=La to Lu), which provides an unprecedented tool for 1) tuning the number of ligand strands in the final helicates, 2) selectively coordinating lanthanides in the various complexes, and 3) controlling the ratio of lanthanide-containing polymers over discrete assemblies.

Perfluorinated Aromatic Spacers for Sensitizing Europium(III) Centers in Dinuclear Oligomers: Better than the Best by Chemical Design?

Transfer news: the use of a simple method allows the various sensitization steps in Eu(III) -containing complexes to be deciphered. Incorporation of an increasing number of electron-withdrawing fluorine atoms on the rigid and electronically tunable phenyl spacer between two tridentate binding units (see picture, red O, dark blue N) affects the quantum yield, intersystem crossing, and energy-transfer processes in a rational way.

Silver baits for the "miraculous draught" of amphiphilic lanthanide helicates.

The axial connection of flexible thioalkyls chains of variable length (n=1-12) within the segmental bis-tridentate 2-benzimidazole-8-hydroxyquinoline ligands [L12(Cn) -2 H](2-) provides amphiphilic receptors designed for the synthesis of neutral dinuclear lanthanides helicates. However, the stoichiometric mixing of metals and ligands in basic media only yields intricate mixtures of poorly soluble aggregates. The addition of Ag(I) in solution restores classical helicate architectures for n=3, with the quantitative formation of the discrete D(3) -symmetrical [Ln(2) Ag2(L12(C3) -2 H)(3) ](2+) complexes at millimolar concentration (Ln=La, Eu, Lu). The X-ray crystal structure supports the formation of [La(2) Ag(2) (L12(C3) -2 H)(3) ][OTf](2) , which exists in the solid state as infinite linear polymers bridged by S-Ag-S bonds. In contrast, molecular dynamics (MD) simulations in the gas phase and in solution confirm the experimental diffusion measurements, which imply the formation of discrete molecular entities in these media, in which the sulfur atoms of each lipophilic ligand are rapidly exchanged within the Ag(I) coordination sphere. Turned as a predictive tool, MD suggests that this Ag(I) templating effect is efficient only for n=1-3, while for n>3 very loose interactions occur between Ag(I) and the thioalkyl residues. The subsequent experimental demonstration that only 25 % of the total ligand speciation contributes to the formation of [Ln(2) Ag(2) (L12(C12) -2 H)(3) ](2+) in solution puts the bases for a rational approach for the design of amphiphilic helical complexes with predetermined molecular interfaces.

Tuning the thermodynamic stability of oxothiomolybdenum wheels: crystal structures, studies in solution and DFT calculations

The complexes [Mo12O12S12(OH)12(Muco)]2- (Muco2- = muconate, C6H4O4(2-)) and [Mo12,O12S12(OH)12(TMT)]2 (TMT2- = tetramethylterephthalate, C12H12O4(2-)) have been obtained from the condensation of the [Mo2O2S2]2+ building block in the presence of Muco2- and TMT2-, respectively. Both compounds were structurally characterized, revealing host-guest architectures with one or two encapsulated water molecules. 1H NMR spectra in DMSO and D2O showed that both complexes had an average symmetry higher than that in the solid state, due to changes in the distribution of encapsulated water molecules. The relative stabilities in water of the seven complexes encapsulating various di- or tricarboxylate guests, either rigid or non-rigid, have been determined. The stability scale obtained for the dianionic complexes is interpreted in relation with the rigidity or flexibility of the guests. A DFT study demonstrates that additional stabilization arises from the presence of inner hydrogen bonds involving 1, 2 or 3 water molecules, which even permit the extension of the H-bonds network to the first solvation sphere of the anion. DFT calculations were carried out on all investigated complexes as isolated or solvated anions and provide the sequence of the bond energies between the host and the guests, which is compared to the experimental data.