Anion and Solvent Induced Chirality Inversion in Macrocyclic Lanthanide Complexes

A series
of the lanthanide­(III) or yttrium­(III) complexes of the type [LnL­(NO3)­(H2O)2]­(NO3)2, [LnL­(NO3)­(H2O)]­(NO3)2, [LnL­(H2O)2]­(NO3)3,
and [LnLCl­(H2O)2]­Cl2 where L is an
all-R or all-S enantiomer (LR or LS) of the
chiral hexaaza macrocycle, 2­(R),7­(R),18­(R),23­(R)- or 2­(S),7­(S),18­(S),23­(S)-1,8,15,17,24,31-hexaazatricyclo­[25.3.1.1.0.0]-dotriaconta-10,12,14,26,28,30-hexaene,
and Ln­(III) = Sm­(III), Tb­(III), Ho­(III), Er­(III), Tm­(III), Yb­(III),
Lu­(III), or Y­(III), have been synthesized and structurally characterized.
The crystal structure of the free macrocycle shows a highly twisted
molecule, preorganized for the formation of helical complexes. The
crystal structures of the lanthanide­(III) complexes show two different
diastereomeric forms of the macrocycle with different configurations
at the stereogenic amine nitrogen atoms: (RRRR) or
(RSRS) (denoted as LRI and LRII, respectively). The LRI diastereomeric form of the nitrate derivatives
[LnL­(NO3)­(H2O)]­(NO3)2 (Ln
= Ho, Er) and [LnL­(H2O)2]­(NO3)3 (Ln = Tm, Yb, Lu) convert slowly to the LRII form in methanol or acetonitrile solutions, while this process
is not observed for the LRI diastereomers
of analogous chloride derivatives [LnL­(H2O)2]­Cl3 (Ln = Tm, Yb, Lu). On the other hand, the LRI → LRII conversion
for these Tm­(III), Yb­(III), and Lu­(III) chloride derivatives can be
triggered by the addition of external nitrate anions. The circular
dichroism (CD) and 1H NMR data indicate initial fast exchange
of axial chloride for axial nitrate ligand, followed by slow chirality
inversion of the equatorial macrocyclic ligand.