A tris-phenylphenol N₈O₃ Schiff-base cryptand: Structural characterization and fluorescence titration for lanthanide coordination

Lanthanide [lanthanum(III), 2; europium(III), 3; and gadolinium(III), 4] cryptates of a trisphenylphenol N₈O₃ Schiff-base cryptand (H₃L) were synthesized by transmetallation with [Na(H₃L)]ClO₄·3H₂O (1) and characterized by spectroscopic and crystal structure analyses. Both 3 and 4 are isomophous and isostructural. The complexes crystallize in the triclinic space group P-1 with cell parameters for 3 of a = 14.9833(2), 6=15.42670(10), c = 16.3770(3) Å, α = 75.3760(10), β = 68.2640(10), γ = 77.0540(10)° and Z = 2 and for 4 of a = 14.9976(3), b = 15.4417(3), c = 16.35350(10) Å, α = 75.4980(10), β = 68.3180(10), γ = 76.8790(10)° and Z = 2. The structures reveal that one lanthanide ion is unsymmetrically encapsulated in the cryptand cavity with a second ligand (solvent DMF). Solution NMR and solid structural studies demonstrated that there are intracavity and intermolecular proton transfer processes during lanthanide complexation towards the cryptand. A fluorimetric titration in acetonitrile for Eu(III) ion to 1 afforded a novel fluorescence intensity (I_F)-equivalents of Eu(III) ion (x) plot signaling a quenching (0 < × < 0.2)-enhancement (0.2 < × < 1) change with formation of the kinetically stable 1:1 cryptate. An energy-transfer mechanism is discussed.