Chiral pyridine-containing ligands for supramolecular chemistry


Student thesis: Doctoral Thesis

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  • Chui Shan TSANG


Awarding Institution
Award date16 Jul 2012


In the study, two types of chiral polydentate pyridine-containing ligands were prepared and reacted with different transition metal ions to afford different supramolecular complexes, and the chemistry of these complexes were explored. In chapter 2, with chiral 2,2′:6′,2:6″,2″:6″′,2″′′-quinquepyridine qqpy1, the diastereoselective synthesis of copper helicates of different nuclearity and valency was carried out. Two new copper helicates, a mixed-valence Cu(I,II) double-stranded helicate [Cu2(qqpy1)2](ClO4)3 (Cu-2) which was afforded from the reaction of qqpy1, Cu(MeCN)4(ClO4) and Cu(ClO4)2∙6H2O in a ratio of 2 to 1 to 1 and a tricopper (I) double-stranded helicate [Cu3(qqpy1)2](ClO4)3 (Cu-3) which was afforded from the reaction of Cu(MeCN)4(ClO4) and qqpy1 in a ratio of 3 to 2, were synthesized. All the complexes were successfully characterized by ESI-MS, elemental analysis and CD spectroscopy. The structures of Cu-2 and Cu-3 were successfully characterized by X-ray diffraction method and a short Cu···Cu distance was observed in the structure of Cu-3. Diastereoselective formation of Cu-3 was investigated using 1H NMR spectroscopy and the ratio of diastereomers was found to be 70:30 which was similar to the previously reported dicopper (I) double-stranded helicate [Cu2(qqpy1)2](ClO4)2 (Cu-1). The conversion of Cu-1 and Cu-3 was established by 1H NMR and CD spectroscopy, indicated that Cu-3 can be formed from Cu-1. For Cu-1 and Cu-2, reversible conversion can be achieved through a one electron transfer redox reaction which is demostrated previously using cyclic voltammetry. In addition, Cu-2 was synthesized from Cu-1 by reacting with Cu(ClO4)2∙6H2O. It was found that there is no change of helical chirality during reaction. Therefore, with this method, the ratio of diastereomers for the formation of Cu-2 was established to be 70:30, same as Cu-1. Because conversions between each helicate were demonstrated and similar ratios of diastereomers were observed, we propose that the formation of mixed-valence Cu-2 and trinuclear Cu-3 is through the formation of binuclear Cu-1. In chapter 3, with chiral tetradentate pyridylthiazole ligand pythz1 and two newly synthesized pyridylthiazole ligand pythz2 and pythz3, dinuclear Ag(I) double-stranded helicates were synthesized from the reaction of Ag(OAc)2∙2H2O and L (L = pythz1-3) in the ratio of 1 to 1. All the complexes were isolated as the PF6− salts and successfully characterized with elemental analysis, ESI-MS and CD spectroscopy. The crystal structures of helicates [Ag2(pythz2)2](PF6)2 and [Ag2(pythz3)2](PF6)2 were successfully characterized by X-ray diffraction method in which short Ag···Ag distance was found in both structures. The ratio of diastereomers of helicates were established using 1H NMR spectroscopy in which the diastereomeric ratio of [Ag2(pythz1)2](PF6)2 was 67:33 and [Ag2(pythz2)2](PF6)2 was greater than 95%. Because of the exchange of diastereomers in [Ag2(pythz3)2](PF6)2, the solution of the complex was freeze to 213 K and the diastereomeric ratio was found to be 83:17. To investigate whether the diastereoselectivity affects by the change of metal ion, the dicopper(I) double-stranded helicates [Cu2(pythz2)2](PF6)2 and [Cu2(pythz3)2](PF6)2 were synthesized and characterized with elemental analysis, ESI-MS and CD spectroscopy. Together with the previously reported dicopper(I) double-stranded helicate [Cu2(pythz1)2](PF6)2, the comparison between the Ag(I) and Cu(I) helicates found that the diastereoselectivity of [Ag2(pythz-1)2](PF6)2 and [Cu2(pythz-1)2](PF6)2 reversed in which the major diastereomer of [Ag2(pythz-1)2](PF6)2 was M-helicate while [Cu2(pythz-1)2](PF6)2 is P-helicate. To address the reverse in the diastereoselectivity upon the change of metal ions, firstly, the simulated structures of diastereomers of [Ag2(pythz1)2](PF6)2, P- and M-helicate were compared and the comparison showed the flattened structures of [Ag2(pythz1)2](PF6)2 diminished the close contact between the pinene C(CH3)2 group and the thiazole ring creating significant steric hindrance at the helical pitch of the P-helicate more than M-helicate, thus M-helicate was more preferred. Finally, the comparison between the simulated structures of M-[Ag2(pythz1)2](PF6)2 and P-[Cu2(pythz1)2](PF6)2 showed, in contrast to P-[Cu2(pythz1)2](PF6)2, the more flattened structure of M-[Ag2(pythz1)2](PF6)2 which was resulted from the short Ag···Ag distance allowing more space for the bulky pinene C(CH3)2 group to place at the head of helicate while the less bulky pinene CH2 group and the thiazole ring at the pitch of helicate did not increase the steric hindrance. As the steric hindrance occurred in P- [Cu2(pythz1)2](PF6)2 and M-[Ag2(pythz1)2](PF6)2 were at different position, opposite diastereoselectivity was observed. In chapter 4, with two new chiral tetradentate pyridyl-thiazole ligands pythz4 and pythz5, the effect of the difference in ligands’ directional angle on the formation of supramolecular structures was explored. Ligand pythz4 was considered as the position isomer of pythz5 because of their identical molecular structure but different in the bridging position of two bidentate pyridyl-thiazole binding units. The complexation of pythz4 and pythz5 with octahedral metal ions resulted in different supramolecular complexes under identical reaction condition, a dinuclear triple-stranded helicate [M2(pythz4)3](ClO4)4 and a tetrahedral cage [M4(pythz5)6](ClO4)8 respectively. All the complexes were successfully characterized with elemental analysis and ESI-MS. Triplestranded helical structure of [Cu2(pythz4)3](ClO4)4 was successfully characterized by Xray diffraction method while no tetrahedral cage structure with pythz5 was characterized. However, cage structures of a similar ligand [Ni4(pythz6)6](ClO4)8 and [Cd4(pythz6)6](ClO4)8 were successfully characterized. The difference in the formation of supramolecular complexes using similar pyridyl-thiazole ligands was explained by the difference in the directional angle of the ligand strands. In the case of M = Mn, complexes of all three ligands (pythz4-6) were active catalysts in the epoxidation of styrene, and [Mn4(pythz5)6](ClO4)8 showed the best result in which the conversion of styrene was 89% with 73% yield of styrene oxide in 3 h.

    Research areas

  • Chirality, Pyridine, Supramolecular chemistry, Ligands (Biochemistry)