The coordination chemistry of a family of tripodal tris-(pyrazolyl) ligands with trivalent lanthanides was investigated to determine their potential application as extractants and sensors of An(III) from alkaline aqueous media. Pyrazole ring substitution was found to affect the ligand ability to complex Ln(III). Fluorescence titration experiments provide evidence for the complexation of Ln(III) by ((2,4,6-triethylbenzene-1,3,5-triyl)tris(methylene))tris(1H-pyrazole) (1) and ((2,4,6-triethylbenzene-1,3,5-triyl)tris(methylene))tris(3,5-dimethyl-1H-pyrazole) (2). ((2,4,6-triethylbenzene-1,3,5-triyl)tris(methylene))tris(3,5-diphenyl-1H-pyrazole) (3) did not bind Ln(III), presumably due to inability to accommodate the sterically demanding nitrate counteranions within the binding cavity. The difference in pyrazole ring substitution results in 1 and 2 being different types of fluorescence sensors: Upon addition of Ln(III), 1 shows fluorescence quenching and 2 generally shows fluorescence enhancement. Analysis of fluorescence titration experiments of 1 and 2 with Sm(III) using nonlinear regression analysis of the 1:1 binding isotherm gave K11 = 1.5 × 104 M-1 and K11 = 3.6 × 104 M-1 for 1 and 2 respectively. NMR spectroscopy provided additional evidence of complexation. Theoretical DFT calculations determined the likely structure of the complexes formed with Sm(III): 1:1 complexes of the type [L.Sm(NO3)3] were found to be thermodynamically favored.

Complexation and sensing of Ln(III) by tripodal tris-(pyrazolyl) ligands: Surprising effects by variation of pyrazole substitution

ACS Spring 2022

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