Crown Ether-Lanthanide Complexes Studied by CPL and TL. 2. Solution Species in (2R,3R, 11R, 12R)-2,3,11,12-Tetramethyl-18-crown-6-Europium Nitrate and Perchlorate Systems in MeCN, TFE, and HFiP
The solution behavior of complexes of (2R,3R,11R,12R)-2,3,l l,12-tetramethyl-1,4,7,10,13,16-hexaoxacyclooctadecane ((all-R)-2,3,11,12-tetramethyl-18-crown-6, I) with europium(III) nitrate and perchlorate in the solvents acetonitrile (MeCN), tri-fluoroethanol (TFE), and hexafluoroisopropyl alcohol (HFiP) has been investigated by using the techniques of circularly polarized luminescence (CPL) and total luminescence (TL) spectroscopy, as well as conductivity measurements. The nitrate—crown complex is determined to be [I-Eu(NO3)2]+, with the europium ten-coordinate, to the hexadentate 18-crown-6 ligand and to the two opposing bidentate nitrate anions. The perchlorate complex, while not as structurally robust as the nitrate, appears to have a similar structure, but with monodentate perchlorate anions. Ligand field splittings of the emission transitions are interpreted in terms of D2 effective electronic symmetry in the nitrate complex and effective axial symmetry in the perchlorate. Observed changes in the CPL and TL spectra during titrations of Eu(NO3)3 with I in TFE and MeCN indicate the presence of poly(nitrato) anionic species from the nitrate generated by the formation of the crown complex and the excess Eu(NO3)3 present in solution. In MeCN, the formation of the stable [Eu(NO3)5]2- species limits the formation of additional crown complex, whereas in TFE, the crown complexation goes to completion. In MeCN, the difference in the intensities of the hypersensitive 7F0→ 5D2 transition used for excitation results in preferential excitation of the pentakis(nitrato) species relative to the crown complex. The data indicate that europium exchange between these two species is slow compared to the lifetime of the europium emission. © 1986, American Chemical Society. All rights reserved.
Metcalf, DH; Carter, RC; Ghirardelli, RG; Palmer, RA
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