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 hav e been connected to the tris(2-(N-methyl)aminoethyl)amine tripod to give th e podand L-10 that exists as a statistical mixture of four conformers in so lution. In aqueous acidic medium, the protonated apical nitrogen atom of th e tripod (pK(a)([L-10+H](+)) = 4.66(2)) adopts an endo conformation compati ble with the formation of bi- and trifurcated hydrogen bonds with the oxyge n atoms of the proximal carboxamide groups, thus producing a clipped confor mation preorganized for the complexation of lanthanide metal ions. Reaction s of L-10 and [L-10+H](+) with Ln(ClO4)(3) (Ln = La-Lu) in acetonitrile pro vide stable nine-coordinate podates [Ln(L-10)](3+) and [Ln(L-10+H)](4+). Th ermodynamic investigations indicate that the increased electrostatic repuls ion associated with the complexation of the protonated podand is compensate d 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, EuC 46H62.5N10.5O15F15PS3, 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 io n. 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 trig onal prismatic cavity. High-resolution emission spectroscopy demonstrates t hat Eu(III) is efficiently protected within the pedate whose resistance tow ard hydrolysis is significantly improved compared to related nonclipped tri ple-helical complexes. The implications of covalent tripod for the design o f nine-coordinate lanthanide building blocks with predetermined structural, thermodynamic, and electronic properties is discussed.