Flexible covalent tripods in nonadentate podands: synthesis of tris[3-(6-diethylcarbamoylpyridine-2-carboxamido)propyl]amine and its complexing properties with trivalent lanthanides

The nonadentate podand tris[3-(6-diethylcarbamoylpyridine-2-carboxamido)propyl]amine (L-14) is prepared according to a multistep strategy using the flexible tris(3-(N-methylamino)propyl)amine (Me-TRPN) covalent tripod. L-14 exists as a statistical mixture of four conformers in solution whose distribution is slightly affected by protonation of the apical nitrogen atom in [L-14 + H](+). The pK(a) value depends on the length of the spacer separating the apical nitrogen and the appended electron-withdrawing tertiary amide groups, and increases by three orders of magnitude when going from the Me-TREN tripod in [L-13 + H](+) (ethylene spacer) to Me-TRPN in [L-14 + H](+) (trimethylene spacer). Reactions of L-14 and [L-14 + H](+) with Ln(ClO4)(3) (Ln = La-Lu) produce flexible and poorly stable 1 : 1 podates [Ln(L-14)](3+) and [Ln(L-14 + H)](4+) in which the terdentate chelating binding units exhibit partial dynamic on-off complexation equilibria. Comparisons of structural and thermodynamic data for [Ln(L-n)](3+) (n = 13 or 14) in solution point to a drastic decrease of the molecular organisation of the podand when the constrained Me-TREN tripod is replaced by the elongated Me-TRPN tripod in nine-co-ordinate lanthanide podates, a crucial limiting factor for the design of supramolecular lanthanide complexes with predetermined properties.