Structure and optical properties of lanthanide complexes with scorpionate-like bis-carbacylamidophosphate ligand tetramethyl[pyridine-2,6-diyldi(iminocarbonyl)]diamidophosphate

Abstract

The solvent free lanthanide complexes [Ln(HL)₂NO₃] (Ln = Eu, Gd, Tb) with scorpionate type bis-carbacylamidophosphate ligand tetramethyl[pyridine-2,6-diyldi(iminocarbonyl)]diamidophosphate (H₂ L) were synthesized. IR, diffuse reflectance, and luminescence spectroscopy measurements were carried out for their characterization. The crystal structure of the gadolinium complex was solved. It has been established that the ligands are coordinated to the lanthanide ion in a scorpionate way with formation of six-membered and twelve-membered metal cycles. H₂L, [HL]⁻ and NaHL were theoretically studied by DFT calculations. The analysis of the HOMO-LUMO gap has shown that the anion [HL]⁻is less stable than the molecular form of the ligand, however, the sodium cation in NaHL crystal structure stabilizes the deprotonated form of the ligand. The DFT quantum chemical calculations of the vibrational wavenumbers were performed for the H₂L and NaHL. The europium and terbium complexes exhibit sensitized f-f luminescence with decay times at room temperature equal to 1.63 and 1.96 ms, respectively. Based on luminescence excitation spectra, the ligand-to-metal energy transfer in the europium complex was found to be strongly temperature dependent.