Lanthanide complexation with 1,3,5-triamino 2,4,6-trihydroxycyclohexane

The protonation constants of 1,3,5-triamino-2,4,6-trihydroxycyclohexane (taci), at 25 degreesC in I = 1.00 M (NaClO4) were determined to be: pKa(1), 5.57 (0.08); pKa(2), 7.45 (0.02); pKa(3), 9.05 (0.04). The log of the stability constants, log beta(302), at 25degreesC in I = 1.00 M (NaClO4) for formation of [Ln(H(-3)taci)(2)(3+)] were measured by potentiometry to be: Nd(III), 25.33 (0.09); Eu(III), 26.42 (0.06): Tm(III), 30.07 (0.10); Lu(III), 33.68 (0.07); Y(III), 28.59 (0.07). H-1 NMR spectra were consistent with formation of a single complex from pcH 6 to 10. Laser fluorescence measurements of the F-7(o)-D-5(o) transition of Eu(III) complexed by taci indicated a single complexed species. The shift in this peak relative to that of Eu3+(aq) was significantly greater than the values reported for the complexes of other organic ligands with Eu(III). Luminescence lifetime measurements indicated two water molecules bound to each of the Eu(III) cations in the taci complex. (C) 2004 Published by Elsevier B.V.