Cationic Lanthanide Complexes of N,N'-Bis(2-pyridylmethyl)ethylenediamine-N,N'-diacetic Acid (H(2)bped)

A series of monocationic lanthanide complexes containing the ligand N,N'-bis(2-pyridylmethyl)ethylenediamine-N,N'-diacetate(2-) (bped(2)(-)) have been prepared and isolated as either the hexafluorophosphate or perchlorate salts. The complexes have been characterized by (1)H and (13)C NMR, +LSIMS, IR, and elemental analysis. Complex formation constants have been measured in water at 25 degrees C (&mgr; = 0.16 M (NaCl)). log K (Ln(bped)](+)) (log K([Ln(bped)(OH)])): Ln = La, 10.81 (0.06); Ln = Nd, 11.99 (1.45); Ln = Gd, 12.37 (2.10); Ln = Ho, 12.31 (3.00); Ln = Yb, 13.42 (4.43). The stability constants for [M(bped)](+) increase from La(III) to Nd(III), plateau to Ho(III), and increase again to Yb(III), while the formation constants for [Ln(bped)(OH)] increase almost linearly with atomic number. The solution structures of the [Ln(bped)](+) complexes have been probed by multinuclear NMR ((1)H, (13)C, (17)O) studies, and these indicate only one isomer present in solution; this isomer has 2-fold symmetry and is rigid at 20 degrees C on the NMR time scale. (17)O NMR studies of the paramagnetic lanthanides indicate that the hydration number is 3, [Ln(bped)(H(2)O)(3)](+), and that an overall coordination number of 9 is maintained across the lanthanide series.