Tightening the hydrophobic belt: Effects of backbone and donor group variation on podand ligand complexes of the lanthanides

The N4O3 tripodal aminomethylene phosphinato ligand tris(4-phenylphosphinato-3-methyl-3-azabutyl)amine (H-3-ppma) forms mono- and bis(ligand) complexes with lanthanide(III) metal ions Ln when Ln = Sm-Lu. The formation constants of the Lu (log beta(1) = 1.79, log beta(2) = 4.40) and the Yb (log beta(1) = 2.25, log beta(2) = 4.42) complexes were determined at pH = 1.5 using an unusual P-31 NMR spectroscopic method. The molecular structure of the lutetium complex [Lu(H(3)ppma)(2)](NO3)(3) . 3H(2)O (C60H96LuN11O24P6) was solved by X-ray methods; it crystallizes in the trigonal space group R(3) over barc$, with a = 19.060(1) Angstrom, c = 36.395(3) Angstrom, and Z = 6. The structure was solved by Patterson methods and was refined by full-matrix least-squares procedures to R = 0.024 (R-W = 0.025) for 2061 reflections with I > 3(l). The ligand coordinates in a tridentate manner through the three phosphinate oxygens, resulting in a bicapped octahedral structure of exact Sg symmetry, which is preserved in solution as shown by H-1 and P-31 NMR spectroscopies (CD3OD, DMSO-d(6)). The two N3O3 tripodal amine phenol ligands 1,1,1-tris(((2-hydroxy-5-sulfobenzyl)amino)methyl)ethane (H(6)tams) and 1,2,3-tris((2-hydroxy-5-sulfobenzyl)amino)propane (H(6)taps) showed a binding modality different from that with H(3)ppma, forming N3O3 encapsulated complexes whereby all six donor atoms bind to the lanthanide. This complexation was investigated potentiometrically at 25 degrees 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 H(6)taps forming the more stable complexes. O-17 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.