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Supramolecular schiff base coordination chemistry : blueprints for self-assembling metallocavitands and nanotubes Frischmann, Peter David

Abstract

Heptametallic zinc(II) and cadmium(II) clusters have been isolated after reacting the metal-acetate salts with large diameter [3+3] Schiff base macrocycles. Two tetrazinc complexes have been characterized and identified as intermediates in the formation of the heptazinc complexes. The heptametallic complexes are, in fact, templated by the Schiff base macrocycles, a process that has been investigated with ¹H NMR spectroscopy and single-crystal X-ray diffraction. In the solid-state the heptametallic complexes have a bowl-shaped geometry, reminiscent of organic cavitands, leading to them being called metallocavitands. Solid-state investigation of the heptazinc and heptacadmium metallocavitands showed they organize into capsules with a cavity volume of 150 and 215 ų, respectively. Solution dimerization was also observed in aromatic solvents and N,N-dimethylformamide (DMF). The thermodynamics of dimerization have been quantified by van’t Hoff analyses of association constants measured with variabletemperature, variable-concentration ¹H NMR spectroscopy. Both metallocavitands exhibit entropy-driven dimerization in all solvents in which dimerization occurs. Unusual for dimerization of cavitands, this entropy-driven process can be attributed to the expulsion of solvent from the monomeric cavity upon dimerization. Inside the cavity of heptacadmium metallocavitands is a μ₃-OH ligand where the proton is located at the base of the cavity and is capable of hydrogen bonding with guest molecules. The μ₃-OH proton resonance is observable in low temperature 1H NMR spectra and exhibits two-bond J-coupling with three cadmium ions. Within capsules of the heptacadmium metallocavitands there are eight Lewis-acidic sites accessible to guest molecules, six unsaturated cadmium(II) centers and two μ₃-OH ligands. Solid-state analysis shows that two DMF molecules are encapsulated in the heptacadmium capsule where they each simultaneously exhibit a host-guest hydrogen-bond and a dative metalligand interaction. New methodology has been developed that facilitates synthesis of polydentate [2+2] Schiff base macrocycles with unsymmetrical salphen pockets. Also a [3+3] macrocycle with triptycenyl substituents has been synthesized to prohibit alkali-metal induced solution aggregation. The one-pot twelve component head-to-tail self-assembly of Pt₄ rings directed by chelating imine-pyridyl donors has been demonstrated. These supramolecules exhibit extensive columnar organization in both solution and the solid-state, a phenomenon that imparts liquid crystalline properties on the macrocycles.

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