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

The N 4 O 3 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 β 1 = 1.79, log β 2 = 4.40) and the Yb (log β 1 = 2.25, log β 2 = 4.42) complexes were determined at pH = 1.5 using an unusual 31 P NMR spectroscopic method. The molecular structure of the lutetium complex [Lu(H 3 ppma) 2 ](NO 3 ) 3 ·3H 2 O (C 60 H 96 LuN 11 O 24 P 6 ) was solved by X-ray methods; it crystallizes in the trigonal space group R 3̄ c, 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 ( R w = 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 S 6 symmetry, which is preserved in solution as shown by 1 H and 31 P NMR spectroscopies (CD 3 OD, DMSO- d 6 ). The two N 3 O 3 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 N 3 O 3 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 H 6 taps forming the more stable complexes. 17 O 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.