Anion and Solvent Induced Chirality Inversion in Macrocyclic Lanthanide Complexes

Abstract

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 (L(R) or L(S)) 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 L(RI) and L(RII), respectively). The L(RI) 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(RII) form in methanol or acetonitrile solutions, while this process is not observed for the L(RI) diastereomers of analogous chloride derivatives [LnL(H2O)2]Cl3 (Ln = Tm, Yb, Lu). On the other hand, the L(RI) → L(RII) 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 (1)H NMR data indicate initial fast exchange of axial chloride for axial nitrate ligand, followed by slow chirality inversion of the equatorial macrocyclic ligand.

Tripyrrin-armed isosmaragdyrins: synthesis, heterodinuclear coordination, and protonation-triggered helical inversion

“…Intervening of the associated counter anion in this helical inversion sterically or through metal coordination could not be excluded. 18 The solution of 2A or 2B in CD 3 CN reached an equilibrium state (the molar ratio 2A : 2B ≈ 3 : 2 from the integrals of the signals) by treatment with TFA (Fig. S10 † ).…”

Imine- and Amine-Type Macrocycles Derived from Chiral Diamines and Aromatic Dialdehydes

“…Unexpectedly, amine 18 does not bind lanthanoid(III) ions, unlike its imine counterpart. On the other hand, macrocycle 15 forms lanthanoid(III) complexes, which can undergo solvent [47] or anioninduced [48,49] helicity inversion. Macrocycle 15 and its derivatives, as well as macrocycles 16 and 18, can also be used as chiral solvating agents for the enantiodiscrimination of chiral carboxylic acids, such as ibuprofen.…”

Acceleration and deceleration of chirality inversion speeds in a dynamic helical metallocryptand by alkali metal ion binding

“…1B) would considerably vary depending on the chemical structures of the helix, because the inversion kinetics is mainly governed by the intrinsic flexibility and/or rigidity of the helical backbone itself. For example, helical peptides are generally in a quick P/M interconversion equilibrium (39,40), whereas some lanthanidebased helical complexes undergo a slow P/M inversion with a half-life of hours to days (41)(42)(43).…”