Macrocyclic complexes of lanthanides in identical ligand frameworks Part 2. Synthesis of lanthanide(III) and yttrium(III) complexes of an 18-membered hexaaza macrocycle

Abstract

Lanthanide(III) and yttrium(III) complexes of the 18-membered hexaaza Schiff base macrocycle 2,11,17,26,31,32-hexaazapentacyclo-[26.2.1.1 12.16,0 4.9,0 19.24]dotriaconta-1(31),2,4,6,8,10,12,14,16(32),17,19,21,23,25,27,29-hexadecaene (L 6) have been synthesized by the metal template condensation of phthalic dicarboxaldehyde and 2,6-diaminopyridine. Discrete mononuclear complexes [La(L 6)(NO 3) 2(H 2O)]NO 3·2H 2O, [Dy(L 6)(NO 3) 2(H 2O) 2]NO 3·H 2O, [Ln(L 6)(NO 3)(OH)]NO 3· nH 2O (Ln = Pr 3+, Nd 3+, Eu 3+; n = 2 for Pr 3+, Nd 3+, 3 for Eu 3+), [Ln(L 6)(NO 3) 2]NO 3·4H 2O (Ln = Tb 3+, Y 3+) and [Ln(L 6)(NO 3)(H 2O)(OH)]NO 3· nH 2O (Ln = Sm 3+, Gd 3+; n = 2 for Sm 3+, 3 for Gd 3+) have been synthesized by using the respective hydrated metal nitrate as the template. The lanthanum(III) complex [La(L 6)Cl 3(C 2H 5OH)]·H 2O has been synthesized by using LaCl 3·7H 2O. The metal ion is bound to the macrocycle, to one or two bidentate chelating nitrates, and to one or two water molecules and/or a hydroxide ion. Only the lighter lanthanide(III) cations (LaDy, except Ce and Pm) serve as effective templates in the assembly of L 6. The lanthanide(III) complexes of L 6 are quite stable in the solid state and are inert to the release of metal ions in DMF. The coordinated exocyclic ligands are labile and undergo anion metathesis. The complexes are thermally stable and the macrocyclic entity remains intact with the metal ions up to 388 °C. The template potential of the lanthanide(III) cations of various sizes in the assembly of L 6 is due to the flexibility of the macrocycle to adapt to the geometrical requirements of the metal ions and to the steric demands of the exocyclic ligands. The essential requisite to synthesize lanthanide(III) complexes in the same macrocyclic ligand framework is the ability of the macrocycle to coordinate in a conformation favorable for the central metal ion and for the coordination of exocyclic ligands.