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 (L14) is prepared according to a multistep strategy using the flexible tris(3-(N-methylamino)propyl)amine (Me-TRPN) covalent tripod. L14 exists as a statistical mixture of four conformers in solution whose distribution is slightly affected by protonation of the apical nitrogen atom in [L14 + H]+. The pKa 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 [L13 + H]+ (ethylene spacer) to Me-TRPN in [L14 + H]+ (trimethylene spacer). Reactions of L14 and [L14 + H]+ with Ln(ClO4)3 (Ln = La–Lu) produce flexible and poorly stable 1∶1 podates [Ln(L14)]3+ and [Ln(L14 + H)]4+ in which the terdentate chelating binding units exhibit partial dynamic on–off complexation equilibria. Comparisons of structural and thermodynamic data for [Ln(Ln)]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.