Nine-coordinate lanthanide podates with predetermined structural and electronic properties:: Facial organization of unsymmetrical tridentate binding units by a protonated covalent tripod

Three unsymmetrical tridentate pyridine-2,6-dicarboxamide binding units have been connected to the tris(2-(N-methyl)aminoethyl)amine tripod to give the podand L-10 that exists as a statistical mixture of four conformers in solution. In aqueous acidic medium, the protonated apical nitrogen atom of the tripod (pK(a)([L-10+H](+)) = 4.66(2)) adopts an endo conformation compatible with the formation of bi- and trifurcated hydrogen bonds with the oxygen atoms of the proximal carboxamide groups, thus producing a clipped conformation preorganized for the complexation of lanthanide metal ions. Reactions of L-10 and [L-10+H](+) with Ln(ClO4)(3) (Ln = La-Lu) in acetonitrile provide stable nine-coordinate podates [Ln(L-10)](3+) and [Ln(L-10+H)](4+). Thermodynamic investigations indicate that the increased electrostatic repulsion associated with the complexation of the protonated podand is compensated by preorganization leading to only minor effects on the stability of the final podates. A structural characterization in solution using paramagnetic NMR concludes that a weak interaction between Ln(III) and the lone pair of the apical nitrogen atom of the tripod in [Ln(L-10)](3+) is removed in [Ln(L-10+H)](4+) leading to a distortion of the coordination site. The crystal structure of the complex [Eu(L-10+H)](CF3SO3)(3)(PF6)(CH3CN)(0.5) (12, EuC46H62.5N10.5O15F15PS3, trigonal, R (3) over bar, Z = 6) reveals a cationic conical triple-helical podate [Eu(L-10+H)](4+) resulting from the wrapping of the three meridionally tricoordinated chelating units about the metal ion. A remarkable trifurcated hydrogen bond (N-H ...(O=C)(3)) rigidifies the tripod and forces Eu(III) Co lie at the center of the pseudo-tricapped trigonal prismatic cavity. High-resolution emission spectroscopy demonstrates that Eu(III) is efficiently protected within the pedate whose resistance toward hydrolysis is significantly improved compared to related nonclipped triple-helical complexes. The implications of covalent tripod for the design of nine-coordinate lanthanide building blocks with predetermined structural, thermodynamic, and electronic properties is discussed.