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-<i>R</i> or all-<i>S</i> enantiomer (L^<i>R</i> or L^<i>S</i>) of the chiral hexaaza macrocycle, 2­(<i>R</i>),7­(<i>R</i>),18­(<i>R</i>),23­(<i>R</i>)- or 2­(<i>S</i>),7­(<i>S</i>),18­(<i>S</i>),23­(<i>S</i>)-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: (<i>RRRR</i>) or (<i>RSRS</i>) (denoted as L^<i>RI</i> and L^<i>RII</i>, respectively). The L^<i>RI</i> 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 L^<i>RII</i> form in methanol or acetonitrile solutions, while this process is not observed for the L^<i>RI</i> diastereomers of analogous chloride derivatives [LnL­(H₂O)₂]­Cl₃ (Ln = Tm, Yb, Lu). On the other hand, the L^<i>RI</i> → L^<i>RII</i> 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