Lanthanide Complexes of the Heterochiral Nonaaza Macrocycle: Switching the Orientation of the Helix Axis

The La(III), Ce(III), Pr(III), Nd(III), Sm(III), and Eu(III) complexes of the racemic heterochiral nonaaza macrocyclic amine L have been synthesized and characterized by spectroscopic methods. The X-ray crystal structures of the [PrL][Pr(NO3)6]·CH3OH and the isomorphic [NdL][Nd(NO3)6]·CH3OH complexes show that all nine nitrogen atoms of the macrocycle coordinate to the Ln3+ ion, completing its coordination sphere. The macrocycle wraps tightly around the metal ion in double-helical fashion. The structures reveal the RRRRSS/SSSSRR configuration at the stereogenic carbon atoms of the three cyclohexane rings, confirming the heterochiral nature of the parent 3 + 3 macrocycle obtained in the condensation of racemic trans-1,2-diaminocyclohexane and 2,6-diformylpyridine. The NMR spectra of the isolated complexes indicate the presence of low C1 symmetry [LnL]3+ complexes. The same symmetry is indicated by the X-ray crystal structures of Pr(III) and Nd(III) complexes, which show that for the RRRRSS enantiomer of the macrocycle L, the helix axis passes through the cyclohexane ring of RR chirality and the opposite pyridine ring. The NMR studies of complex formation in solution by the paramagnetic Pr3+ and Eu3+ ions indicate that the initially formed [LnL]3+ complexes are of C2 symmetry. For the RRRRSS enantiomer of the macrocycle L in the C2-symmetric species, the helix axis passes through the cyclohexane ring of SS chirality and the opposite pyridine ring. The C1-symmetric and C2-symmetric forms of the [LnL]3+ complexes constitute a new kind of isomers and the conversion of the kinetic complexation product of C2 symmetry into the thermodynamic product of C1 symmetry corresponds to an unprecedented switching of the orientation of the helix axis within the macrocycle framework.