Chiral polydentate pyridyl ligands and their helicates : syntheses and characterizations

手性多齒呲啶配體和它們的螺旋體 : 合成和表徵

Student thesis: Master's Thesis

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Author(s)

  • Chui Shan TSANG

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date15 Jul 2008

Abstract

The synthesis of a series of four new chiral 2,2’:6’,2”:6”,2”’:6”’,2””-quinquepyridine (qqpy) ligands L1–3 is reported in Chapter 2. It involves coupling between chiral 6-bromo-2,2’-bipyridine and 2,6-bis(trimethylstannyl)pyridine. The complexation of these ligands with copper(I) ions leads to homovalence bimetallic double-stranded helicates [Cu2(L)2]2+ (L = L1–3). With copper(II) ion, homovalence bimetallic double-stranded helicates [Cu2(L)2]4+ (L = L1–3) are formed. With a mixture of copper(I) and copper(II) ions, complexation leads to mixed-valence double-stranded helicates [Cu2(L)2]3+ (L = L1–3). Circular dichroism (CD) study of the helicates in solution reveal absorption in the region of 324–338 nm for [Cu2(L)2]2+; 332–347 nm for [Cu2(L)2]3+; and 348–384 nm for [Cu2(L)2]4+. Cyclic voltammetric (CV) study of the helicates show reversible redox couples except for dicopper(II) helicates of ligand L2. Because of the diamagnetic properties of copper(I), the diastereoselective formation of dicopper(I) double-stranded helicates can be studied by 1H NMR. Formation of complex [Cu2(L)2]2+ have moderate diastereoselectivity in which the bulkiest ligand shows the highest diastereoselectivity [diastereomeric excess (d.e.) = 44%]. The synthesis of a new C2-symmetric chiral 2,2’-di(pyridi-2-yl)-4,4’-dithiazole L4 is reported in Chapter 3. The synthesis involves a double cyclization reaction between 1,4-dibromo-2,3-butanedione and a chiral pyridylthioamide. The complexations of L4 with [Cu(MeCN)4](PF6) and Re(CO)5Br lead to dinuclear double-stranded helicate [Cu2(L4)2](PF6)2 and dinuclear single-stranded helicate [Re2(L4)(CO)6Br2]. The complexation of L4 with rhenium(I) has been shown to be diastereoselective with diastereomeric excess of 10%. The crystal structure of [Cu2(L4)2](PF6)2 and [Re2(L4)(CO)6Br2] are obtained and solved. In the structure of [Cu2(L4)2](PF6)2, the complex crystallized in the orthorhombic space group P21212 with unit dimensions a = 16.410(3) Å, b = 22.331(4) Å, c = 11.2911(18) Å, θ range = 22.5–26.5°, V = 4137.6(11) Å3, and Z = 2. The two ligand strands are twisted about the bond between the two thiazole rings, interwinded about each other and around the two tetrahedral metal ions in a double-helical fashion, no direct bonding is found between two copper(I) atoms since the Cu1···Cu2 distance is 3.482(2) Å. On the other hand, in the [Re2(L4)(CO)6Br2] structure, the complex crystallized in the triclinic space group P1 with unit dimensions a = 9.1400(9) Å, b = 15.5434(16) Å, c = 16.4405(16) Å, θ range = 22.7–27.5°, V = 2234.7(4) Å3, and Z = 2. The ligand strand is twisted about the bond between the two thiazole rings, coordinate to each rhenium(I) ion in bidentate binding site in single-helical fashion. Each rhenium(I) is octahedrally coordinated by the bidentate binding site of the ligand, a bromide and three carbonyl group coordinated in facial arrangement and no direct bonding is found between the two rhenium(I) atoms since the Re1···Re2 distance is 5.147 (2) Å.

    Research areas

  • Ligands (Biochemistry), Synthesis