Cationic Lanthanide Complexes of N,N‘-Bis(2-pyridylmethyl)ethylenediamine-N,N‘-diacetic Acid (H2bped)

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Abstract
A series of monocationic lanthanide complexes containing the ligand N,N‘-bis(2-pyridylmethyl)ethylenediamine-N,N‘-diacetate(2−) (bped2-) have been prepared and isolated as either the hexafluorophosphate or perchlorate salts. The complexes have been characterized by 1H and 13C NMR, +LSIMS, IR, and elemental analysis. Complex formation constants have been measured in water at 25 °C (μ = 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 (1H, 13C, 17O) studies, and these indicate only one isomer present in solution; this isomer has 2-fold symmetry and is rigid at 20 °C on the NMR time scale. 17O NMR studies of the paramagnetic lanthanides indicate that the hydration number is 3, [Ln(bped)(H2O)3]+, and that an overall coordination number of 9 is maintained across the lanthanide series.