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Tightening the Hydrophobic Belt:  Effects of Backbone and Donor Group Variation on Podand Ligand Complexes of the Lanthanides

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journal contribution
posted on 19.03.1998, 00:00 by Mark P. Lowe, P. Caravan, Steven J. Rettig, Chris Orvig
The N4O3 tripodal aminomethylene phosphinato ligand tris(4-phenylphosphinato-3-methyl-3-azabutyl)amine (H3ppma) forms mono- and bis(ligand) complexes with lanthanide(III) metal ions Ln when Ln = Sm−Lu. The formation constants of the Lu (log β1 = 1.79, log β2 = 4.40) and the Yb (log β1 = 2.25, log β2 = 4.42) complexes were determined at pH = 1.5 using an unusual 31P NMR spectroscopic method. The molecular structure of the lutetium complex [Lu(H3ppma)2](NO3)3·3H2O (C60H96LuN11O24P6) was solved by X-ray methods; it crystallizes in the trigonal space group Rc, with a = 19.060(1) Å, c = 36.395(3) Å, and Z = 6. The structure was solved by Patterson methods and was refined by full-matrix least-squares procedures to R = 0.024 (Rw = 0.025) for 2061 reflections with I > 3(I). The ligand coordinates in a tridentate manner through the three phosphinate oxygens, resulting in a bicapped octahedral structure of exact S6 symmetry, which is preserved in solution as shown by 1H and 31P NMR spectroscopies (CD3OD, DMSO-d6). The two N3O3 tripodal amine phenol ligands 1,1,1-tris(((2-hydroxy-5-sulfobenzyl)amino)methyl)ethane (H6tams) and 1,2,3-tris((2-hydroxy-5-sulfobenzyl)amino)propane (H6taps) showed a binding modality different from that with H3ppma, forming N3O3 encapsulated complexes whereby all six donor atoms bind to the lanthanide. This complexation was investigated potentiometrically at 25 °C and 0.16 M NaCl. log K values of [M(tams)]3- ([M(Htams)]2-) for M:  La, 9.17; Nd, 11.19; Gd, 11.86 (18.41); Ho, 12.71 (19.40); Yb, 13.78 (20.11). log K values of [M(taps)]3- ([M(Htaps)]2-) for M:  La, 11.33 (18.47); Nd, 13.59 (20.13); Gd, 14.50 (20.88); Ho, 14.71 (21.15); Yb, 15.15 (21.54). The formation constants show an increasing affinity for the heavier lanthanides, with H6taps forming the more stable complexes. 17O NMR spectroscopy of [Dy(tams)]3- and [Dy(taps)]3- indicated the presence of three inner-sphere water molecules, implying a 9-coordinate Dy in each complex. The factors governing the coordination geometries in the solid and solution states and chelation of these and related metal ion complexes are discussed with reference to the hydrophobic belt.