Chiral Metal Organic Cages And Helicates: Design, Syntheses, And Properties

Helicates with special helical configurations and metal-organic cages with remarkable host-guest properties have been widely studied in recent years. A great deal of complicated and novel structures of metal-organic cages and helicates have been reported. Recent examples include the use of metal-organic cages and helicates for gas sequestration, catalysis, molecular recognition, photoreactions, nonlinear optical materials and magnetic materials. This thesis is aimed at controlled self-assembly of a seris of specific number of nuclear molecule metal-organic cages and helicates through designing and clipping of chiral Schiff-base ligands containing imidazole group. And further study on self-sorting of helicates and molecular recongnition, single crystal to single crystal（SCSC） transformation, and spin-crossover（SCO） of metal-organic cages. This thesis includes three parts:（1） Self-assembly of dinuclear helicates 1-4 and self-sorting: Four homochiral dinuclear complexes, namely, [Ag₂（LC₄）₂]（OTf）₂（1）, [Ag₂（LC₄）₂]（OTf）₂（2）, [Ag₂（LC₅）₂]（OTf）₂（3）, [Ag₂（LC₅）₂]（PF₆）₂（4） have been successfully synthesized from subcomponent self-assembly of 1,4-di（imidazole-2-carboxaldehyde）butane or 1,5-di（imidazole-2-carboxaldehyde）pentane, R-phenylethylamine and silver salt. LC4 and LC5 were flexible ligands, containing the same imidazole Schiff-bases coordination motif and varying in alkyl chain lengths with four and five carbon atom, respectively. Single crystal structures revealed that the silver（Ι） center in 1-4 coordinated with 4N donor atoms from two symmetrical chiral ligands assuming Δ configuration with a tetrahedral coordination environment. [Ag2（LCn）2]2+ components all presented double helical structures exhibiting only P-handedness with Ag–Ag, π–π, and C-H?π intramolecular interactions. Combination of imidazole-2-carboxaldehyde derivatives, R-phenylethylamine with AgOTf lead to the observation of two sets of signals in the 1H NMR and ESI-MS, corresponding to self-sorted homocomplexes 1 and 3. Highly selective narcissistic self-sorting behavior can be observed during the self-assembly of 1 and 3.（2） Self-assembly of tetrahedral cages 5-6 and metal-center exchange: Two new tetrahedral cages, [Fe₄L₆]（BF₄）₈（5） and [Ni₄L₆]（BF₄）₈（6） with different metal centers were synthesized through subcomponent self-assembly of metal（II） ions, 1,8-di（imidazole-2-carboxaldehyde）octane, and（R）-1-phenylethylamine. Although 5 and 6 both present inherent tetrahedral cavity, the dense crystal packing in 5 hinders the adsorption of metal ions, while 6 with 1D open channels is more beneficial to recongnition of small molecular（I2, TCNQ）. Effective single crystal to single crystal transformation from tetrahedral Ni cage to FeNi cage was demonstrated. Fortunately, the single crystals of the FeNi cage retain good crystallinity and the originally dodecahedral shape making the structure of FeNi-3 determination possible. The iron（II） centers of FeNi cage can be induced to display spin-crossover behaviors with increasing amount of Fe（II） ions.（3） Self-assembly of tetrahedral cages 7-11 and spin-crossover: By changing the length of edges（ligands） in tetrahedral cages, which affected by the briging alkyl chain length between two imidazole-2-carboxaldehyde, and a series of different cavity size tetrahedral metal-organic cages designed. Magnetic measurements of those cages have been carried out, and we will explore the effect of different cavity size of cages on SCO properties. Five new SCO tetrahedral cages 7-11,([Fe₄L₆]（X）8·solvent, were synthesized through subcomponent self-assembly of iron（II） ions, 1,n-di（imidazole-2-carboxaldehyde）alkane, and（R）-1-phenylethylamine derivative. Single crystal structures revealed that the cavity size of tetrahedral cages become larger with the increasing of the briging alkyl chain length, and stereoselective self-assembly occurred in the cages 7-11 formation process. That is to say, the R-phenylethylamine derivatives induced exclusively fac-Λ form for all the Fe（II） centers, whereas S-phenylethylamine derivatives induced mer-Δ chirality. Magnetic measurements indicated that 7-11 exhibit gradual and incomplete spin-crossover transition behaviors, and display different spin-crossover transition temperatures, which are affected by different inherent cavity size of cages.