Lanthanide complexes of chiral 3 + 3 macrocycles derived from (1R,2R)-1,2-diaminocyclohexane and 2,6-diformyl-4-methylphenol

Abstract

The enantiopure amine macrocycle H₃L, as well as the parent macrocyclic Schiff base H₃L1, the 3 + 3 condensation product of (1R,2R)-1,2-diaminocyclohexane and 2,6-diformyl-4-methylphenol, are able to form mononuclear complexes with lanthanide(III) ions. The lanthanide(III) complexes of H₃L have been studied in solution using NMR spectroscopy and electrospray mass spectrometry. The NMR spectra indicate the presence of complexes of low C₁ and C₂ symmetry. The ¹H and ¹³C NMR signals of the Lu(III) complex obtained from H₃L have been assigned on the basis of COSY, TOCSY, NOESY, ROESY and HMQC spectra. The NMR data reveal unsymmetrical binding of lanthanide(III) ion and the presence of a dynamic process corresponding to rotation of Lu(III) within the macrocycle. The [Ln(H₄L)(NO₃)₂](NO₃)₂ (Ln = Sm(III), Eu(III), Dy(III), Yb(III) and Lu(III)) complexes of the cationic ligand H₄L⁺ have been isolated in pure form. The X-ray analysis of the [Eu(H₄L)(NO₃)₂](NO₃)₂ complex confirms the coordination mode of the macrocycle determined on the basis of NMR results. In this complex the europium(III) ion is bound to three phenolate oxygen atoms and two amine nitrogen atoms of the monoprotonated macrocycle H₄L⁺, as well as to two axial bidendate nitrate anions. In the presence of a base, mononuclear La(III), Ce(III) and Pr(III) complexes of the deprotonated form of the ligand L³⁻ can be obtained. When 2 equivalents of Pr(III) are used in this synthesis Na₃[Pr₂L(NO₃)₂(OH)₂]₂NO₃·5H₂O is obtained. The NMR, ES MS and an X-ray crystal model of this complex show coordination of two Pr(III) ions by the macrocycle L. The X-ray crystal structure of the free macrocycle H₃L1 has also been determined. In contrast to macrocyclic amine H₃L, the Schiff base H₃L1 adopts a cone-type conformation resembling calixarenes.