Generation of Asymmetric Chiral-at-Metal Centers through Supramolecular Coordination Complexes of Chiral Pyridyl Imine Ligands

手性吡啶亞胺配體的超分子配位配合物生成不對稱的手性金屬中心

Student thesis: Doctoral Thesis

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Award date19 Aug 2021

Abstract

Four different ditopic chelating chiral pyridyl imine ligands and one ditopic achiral ligand were synthesized to construct supramolecular coordination complexes with chiral-at-metal centers. First, a general introduction of chirality with some examples of coordination supramolecules, such as triple stranded metallohelicates, tetranuclear metallogrids, metallocages and metallomarcocycles, as well as the way to generate asymmetric complexes with chiral-at-metal centers are presented in Chapter 1. The preparation of three chiral ditopic pyridyl-imine ligands HL1, HL2, HL3 and their Cu(II) complexes, 1, 2, 3 and 4 are presented in Chapter 2. The reaction between the ligands and Cu(II) centers in molar ratio of 1 to 1 generates chiral tetranuclear grids. These complexes have been characterized by ESI-MS, elemental analysis, NMR and X-ray diffraction analysis. In these grids, Cu(II) centers are either in square pyramidal or octahedral geometry. In complexes 1, 2 and 4, they show similar structure arrangement in which the ligands are arranged in parallel pairs above and below the copper pseudo-plane with adopt opposed anti conformation within each pair. The Cu(II) centers show alternative Δ and Λ configurations. However, in complex 3, it shows quite a different structure arrangement when comparing with that of complexes 1, 2 and 4. Two ligands are arranged in a parallel pair between four copper centers with adopt opposed anti conformation. Another two ligands are arranged above and below the copper centers with adopt opposed anti conformation. The opposite Cu(II) centers show pseudo Δ and Λ configurations of the metal centers. In general, the 1H NMR spectra of paramagnetic Cu(II) complexes are difficult to be assigned due to the broadened peaks. However, in these complexes, they show relatively sharp peaks in 1H NMR spectra and they can be assigned with the help of T1 relaxation time, 1H-1H COSY spectra.

In Chapter 3, the use of HL2, HL3 and HL4 (the achiral version of HL3) to construct different coordination supramolecules with different metal ions such as Fe(II), Ni(II) and Mn(II) is reported. The self-assembly of Fe(II) with HL4 and HL3 in a ratio of 2 to 3 gives binuclear mix-valent triple stranded helicates, complexes 5 and 6. The self-assembly of Fe(II) and Mn(II) with HL3 in a ratio of 1 to 1 to 3 gives binuclear mixed metal triple stranded helicates, complex 7. The self-assembly of Ni(II) with HL2 in a ratio of 1 to 1 gives chiral tetranuclear grid, complex 8. The self-assembly of Mn(II) with HL3 in a ratio of 5 to 6 gives pentanuclear cluster, complex 9. These complexes have been successfully characterized by ESI-MS, elemental analysis, NMR (if obtainable) and X-ray diffraction analysis. In the triple-stranded metallohelicates of complexes 5 and 6, both Fe(II) and Fe(III) centers are presented in octahedral geometry with similar structure arrangement. The X-ray analysis shows the two enantiomeric forms of crystal exist in crystal lattice in complex 5, while enantiomeric pure crystal with Δ configuration is observed in complex 6. Therefore, the former is achiral and the latter is chiral. In complex 7, Mn(II) and Fe(II) centers are in octahedral geometry with similar structure arrangement as complexes 5 and 6. Enantiomeric pure crystal with Δ configuration is observed, resulting in chiral triple stranded metallohelicates. In complex 8, two Ni(II) centers are in tetrahedral geometry and another two Ni(II) centers are in octahedral geometry. Interestingly, it shows different structure arrangement when comparing with that of the tetranuclear Cu(II) grids discussed in Chapter 2. Two ligands are arranged in a parallel pair between four Ni(II) centers with adopt opposed anti conformation. Another two ligands are arranged above and below the Ni(II) centers with adopt opposed anti conformation. The opposite centers show pseudo Δ and Λ configurations of the metal centers, resulting in chiral tetranuclear grid. In complex 9, two Mn(II) centers with Λ configuration are in octahedral geometry with three bidentate coordination from three ligands, while the other three Mn(II) centers with Δ configuration are in octahedral geometry with two tridentate coordination from two ligands, resulting in chiral pentanuclear cluster with the formula Mn5L36.

In Chapter 4, Cu(II) metallomacrocycles 10 and 11 with HL5 were prepared previously in Kwong’s group. The self-assembly of Cu(II) ions and HL5 give an octanuclear metallomacrocycle 10 using perchlorate anion and gives a decanuclear metallomacrocycle 11 using triflate anion. They have been characterized by X-ray diffraction analysis and show interconversion in different solvent. The Cu(II) centers are either in square pyramidal or octahedral geometry and show alternative pseudo Δ and Λ configurations. Changing solvent from nitromethane to methanol induces conversion from 10 to 11 and the conversion is reversible when the solvent is changed back to nitromethane as indicated by 1H NMR spectroscopy. However, the full study on the NMR spectra of the complexes is still unexplored. These two complexes, like the paramagnetic grid discussed in Chapter 2, show relatively sharp peaks in 1H NMR spectra, though they have much more metal centers and more complicated structures as shown in X-ray crystal structure. A detailed assignment of the 1H NMR spectra has been carried out with the help of 1H-1H COSY, T1 relaxation time and X-ray structural data.