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 H-1 and C-13 NMR, +LSIMS, IR, and elemental analysis. Complex formation constants have been measured in water at 25 degrees C (mu = 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, 13.32 (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 (H-1, C-13, O-17) 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. O-17 NMR studies of the paramagnetic lanthanides indicate that the hydration number is 3, [Ln(bped)(H2O)(2)](+), and that an overall coordination number of 9 is maintained across the lanthanide series.