Helical Lanthanide(III) Complexes with Chiral Nonaaza Macrocycle

Abstract

The chiral nonaazamacrocyclic amine L, which is a reduction product of the 3+3 Schiff base macrocycle, wraps around the lanthanide(III) ions to form enantiopure helical complexes. These Ce(III), Pr(III), Nd(III), Eu(III), Gd(III), Tb(III), Er(III), Yb(III) and Lu(III) complexes have been isolated in enantiopure form and have been characterized by spectroscopic methods. X-ray crystal structures of the Ln(III) complexes with L show that the thermodynamic product of the complexation of the RRRRRR-isomer of the macrocycle is the (M)-helical complex in the case of Ce(III), Pr(III), Nd(III) and Eu(III). In contrast, the (P)-helical complex is the thermodynamic product in the case of Yb(III) and Lu(III). The NMR and CD spectra show that the (M)-helicity for the kinetic complexation product of the RRRRRR-isomer of the macrocycle is preferred for all investigated lanthanide(III) ions, while the preferred helicity of the thermodynamic product is (M) for the early lanthanide(III) ions and (P) for the late lanthanide(III) ions. In the case of the late lanthanide(III) ions, a slow inversion of helicity between the kinetic (M)-helical product and the thermodynamic (P)-helical product is observed in solution. For Er(III), Yb(III) and Lu(III) both forms have been isolated in pure form and characterized by NMR and CD. The analysis of 2D NMR spectra of the Lu(III) complex reveals the NOE correlations that prove that the helical structure is retained in solution. The NMR spectra also reveal large isotopic effect on the 1H NMR shifts of paramagnetic Ln(III) complexes, related to NH/ND exchange. Photophysical measurements show that LRRRRRR appears to favour an efficient 3ππ*-to-Ln energy transfer process taking place for Eu(III) and Tb(III), but these Eu(III)- and Tb(III)-containing complexes with LRRRRRR lead to small luminescent quantum yields due to an incomplete intersystem crossing (isc) transfer, a weak efficiency of the luminescence sensitization by the ligand, and/or efficient non-radiative deactivation processes. Circularly polarized luminescence on the MeOH solutions of Eu(III) and Tb(III) complexes confirms the presence of stable chiral emitting species and the observation of almost perfect mirror-image CPL spectra for these compounds with both enantiomeric forms of L.

Conformational Control in Metallofoldamers: Design, Synthesis and Structural Properties

“…Following the idea that enantiomerically pure ligands will lead to metallofoldamers with single handed helical structure, Muller and Lisowski have reported a chiral nonaazamacrocycle amine, which coordinates Ln 3+ ions to form enantiopure helical complexes (Figure 7). [25] Moreover, helix inversion between the kinetic and thermodynamic binding products in the Yb 3+ complexes was also demonstrated. The nonaaza macrocycle L was prepared by the condensation of 2,6-diformylpyridine and trans-1,2-diaminocyclohexane, [23a] leading to a 3+3 macrocyclic Schiff base, which was then easily converted into the corresponding macrocyclic amine.…”

Formation of Chiral Heteronuclear LnIII Assemblies by Ion Pair Formation

“…However, this process is not important in the time scale of the CD measurements as indicated by the lack of characteristic changes in the NMR and CD spectra accompanying the (M) to (P) transition. [7,8] Closer inspection of the solutions used for the CD measurements indicated cloudiness for the (M) 3-interacting with some kind of decomposition product. As the signals at δ = 15.2 and 13.9 ppm do not depend on the Ln III present in the cationic complex, the decomposition product has to be a demetalated derivative of macrocycle L, most likely its protonated form.…”

“…The latter complexes are chiral, with L wrapped around the Ln III ion in a helical fashion. [7,8] These assemblies can be isolated as four pure stereoisomers corresponding to the two enantioScheme 1. The cationic complex [LnL] 3+ and the anionic complex [LnЈ(dpa) 3 ] 3-. complexes are of D 3 symmetry and exist in solution as a labile racemic mixture of Λ and Δ enantiomers.…”

Chiroptical Properties of Nonanuclear Tb(III) Clusters with Chiral Champhor Derivative Ligands

“…This is based on the differential emission intensity of left-and righthanded circular polarized light according to the chirality * This paper was presented at the 15th Chitose International Forum of the emitter, and is often used as an indicator of chirality in a metal ion center [16][17][18]. The CPL property is expected to be useful for applications such as future threedimensional displays, security paints, and optical telecommunications.…”