Connecting Terminal Carboxylate Groups in Nine-Coordinate Lanthanide Podates: Consequences on the Thermodynamic, Structural, Electronic, and Photophysical Properties

The hydrolysis of terminal tbutyl-ester groups provides the novel nonadentate podand tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine} (L13) which exists as a mixture of slowly interconverting conformers in solution.
At pH = 8.0 in water, its deprotonated form [L13 − 3H]3- reacts with Ln(ClO4)3 to give the poorly soluble and
stable podates [Ln(L13 − 3H)] (log(β110) = 6.7−7.0, Ln = La−Lu). The isolated complexes [Ln(L13 − 3H)](H2O)7
(Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(III) to be nine-coordinate in a
pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding
units of L13. The Ln−O(carboxamide) bonds are only marginally longer than the Ln−O(carboxylate) bonds in
[Ln(L13 − 3H)], thus producing a regular triple helix around Ln(III) which reverses its screw direction within the
covalent Me−TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal
carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state
structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with
solvent molecules. The absolute quantum yields obtained for [Eu(L13 − 3H)] (

= 1.8 × 10-3) and [Tb(L13 −
3H)] (

= 8.9 × 10-3) in water remain modest and strongly contrast with that obtained for the lanthanide
luminescence step (ΦEu = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap
between the ligand-centered singlet (1ππ*) and triplet (3ππ*) states which limits the efficiency of the intersystem
crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central
pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The
thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility,
sensitization) brought by the “carboxylate effect” in lanthanide complexes are evaluated for programming predetermined
properties in functional devices.