Structural and photoluminescent studies of lanthanide complexes with tripodal triRNTB (N-substituted tris(benzimidazol-2-ylmethyl)amine): ligand substituent, anionic and secondary ligand effects

Abstract

Six N-substituted NTB ligands (NTB = (tris(benzimidazol-2-ylmethyl)amine), namely, triMeNTB (L1), triEtNTB (L2), triPrNTB (L3), triBuNTB (L4), triAlNTB (L5) and triBzNTB (L6), have been synthesized through replacing the H atoms on the NH groups by methyl, ethyl, ⁿpropyl, ⁿbutyl, allyl, and benzyl groups, respectively. Their corresponding Sm³⁺, Eu³⁺ and Tb³⁺ complexes consisting of different anions or secondary ligands have been prepared and characterized by elemental analyses, IR spectra and single-crystal X-ray diffraction measurements. Three types of structural models have been obtained: ML₂ where two triTNTB ligands wrap around the Ln³⁺ ion with anions (ClO₄⁻ and CF₃SO₃⁻) free of coordination, MLA₃ where one triRNTB ligand holds the Ln³⁺ ion with anions (Cl⁻, NO₃⁻ and picrate) participating in coordination, and MLL′₃ where one triNTB ligand and three secondary antipyrine ligands coordinate with the Ln³⁺ ion. The Ln³⁺ ions in ML₂ type complexes are 8-coordinating and display a slightly distorted cubic (LnN₈) coordination geometry. The Ln³⁺ ions in MLA₃ and MLL′₃ types complexes show two different coordination geometries: one is 7-coordinating (MLA₃ with Cl⁻ anion and MLL′₃), which can be described as a severely distorted cube short of one corner (LnN₄Cl₃ or LnN₄O₃), and the other is 10-coordinating (MLA₃ with NO₃⁻ and picrate anions) which can be regarded as a severely distorted cube with one corner substituted by a three-atom cap (LnN₄O₆). The photoluminescent study reveals that the complexes with different Ln³⁺ ions show a general luminescent intensity tendency of Tb > Eu > Sm. The modification of the NTB molecule by altering R groups tunes the triplet states of the ligands and results in differentiable photophysical properties of the complexes. The anions or secondary ligand also have remarkable effects on the luminescent properties of the complexes.