Modulation of the ring size and nuclearity of metallamacrocycles via the steric effect of ligands: preparation and characterization of 18-membered hexanuclear, 24-membered octanuclear, and 30-membered decanuclear manganese metalladiazamacrocycles with alpha- and beta-branched N-acylsalicylhydrazides

Hexanuclear, octanuclear, and decanuclear manganese metalladiazamacrocycles have been prepared by reacting a series of pentadentate ligands, N-acylsalicylhydrazides (N-(3-methylbutanoyl)salicylhydrazide (H(3)3-mbshz), N-(phenylacetyl)salicylhydrazide (H(3)pashz), N-(3,3-dimethylbutanoyl)salicylhydrazide (H(3)3-dmbshz), N-(2-methylpropanoyl)salicylhydrazide (H(3)2-mpshz), N-((R,S)-2-methylbutanoyl)salicylhydrazide (H(3)RS-2-mbshz), N-((S)-2-methylbutanoyl)salicylhydrazide (H(3)S-2-mbshz), and N-(2,2-dimethylpropanoyl)salicylhydrazide (H(3)2-dmpshz)), with manganese(II) acetate tetrahydrate. The self-assembled, supramolecular complexes assume a nearly planar cyclic structure with an -(Mn-N-N)(n) backbone and measure approximately 2.1, approximately 2.3, and approximately 2.6 nm in outer diameters for n = 6, 8, and 10, respectively. The chiralities of the manganese centers on the metalladiazamacrocycle occur in alternating ...LambdaDeltaLambdaDelta... configurations. While beta-branched N-acylsalicylhydrazides (H(3)3-mbshz, H(3)pashz, H(3)3-dmbshz) with a sterically flexible Calpha methylene group yield 18-membered hexanuclear manganese metalladiazamacrocycles of S(6) point group symmetry, alpha-branched N-acylsalicylhydrazides lead to 24-membered octanuclear manganese metalladiazamacrocycles or 30-membered decanuclear manganese metalladiazamacrocycles depending on the size of the N-acyl substituents. The alpha-branched H(3)2-mpshz ligand with the sterically least demanding isopropyl tail at the N-acyl position yields a 24-membered octanuclear manganese metalladiazamacrocycle of S(8) point group symmetry, but other alpha-branched N-acylsalicylhydrazides such as H(3)RS-2-mbshz, H(3)S-2-mbshz, and H(3)2-dmpshz lead to 30-membered decanuclear manganese metalladiazamacrocycles of S(10) point group symmetry. The magnetic properties of the metalladiazamacrocycles are characterized by a weak antiferromagnetic exchange interaction, with J(eff) = -8.5 to -3.8 K between the Mn(3+) ion spins with S = 2 in the cyclic system.     

Size-selective shell cross-linked interior functionalized siloxane nanocages

A new structure, consisting of a shell cross-linked, 2 nm size siloxane nanocage containing propylamine groups tethered to the interior face of the shell was synthesized, starting with micelles of the surfactant molecule, (triethoxysilyl)propylcetylcarbamate. After hydrolysis of the ethoxysilyl groups and condensation and capping of the silanols to form a cross-linked, one-atom-layer-thick siloxane shell, the carbamate was converted to amine, releasing the cetyl group from the structure and resulting in the desired spherical nanocage. The intermediates in the synthesis process and the final structure were characterized by 1H and 29Si NMR, DLS, TEM, and mass spectroscopy. The amine groups tethered to the interior surface of the shell react readily with ninhydrin but do not interact with the larger ZnTPP, indicating molecular size selectivity by the cross-linked shell. The structure also exhibits confinement effect in the amine-catalyzed decarboxylation of acetoacetic acid, exhibiting higher activity and higher selectivity for acetal than (aminopropyl)triethoxysilane.     

Steric control of a bridging ligand for high-nuclearity metallamacrocycle formation: a highly puckered 60-membered icosanuclear metalladiazamacrocycle

A novel S4-symmetric icosanuclear manganese metalladiazamacrocycle was synthesized using a pentadentate ligand, N-3-phenyl-trans-2-propenoylsalicylhydrazide (H3L), that has a rigid and bulky terminal N-acyl group. A 20 cyclic repeat of an--(Mn-N-N)--linkage resulted in a highly puckered diaza-bridged 60-membered icosanuclear metallamacrocycle. The steric interaction between the ligands in the cyclic system leads to five consecutive Mn(III) centers in a chemically different--(Mn(A)Mn(B)Mn(C)Mn(D)Mn(E))--environment, with the chiralities of the metal centers being in a rather complicated--(LambdaLambdaDeltaLambdaLambda)(DeltaDeltaLambdaDeltaDelta)--sequence.     

High-yield synthesis of multi-walled carbon nanotubes by arc discharge in liquid nitrogen

Synthesis of multi-walled carbon nanotubes (MWNTs) by arc discharge in liquid nitrogen is reported. As liquid nitrogen substituted both vacuum and cooling systems, high-quality MWNTs were produced at a low cost. The content of the MWNTs can be as high as 70% of the reaction product. Auger-spectroscopy analysis revealed that no nitrogen is incorporated in the MWNTs. This method can be an economical route for the mass production of highly crystalline MWNTs. Received: 5 July 2002 / Accepted: 8 July 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +82-54/279-8298, E-mail: ce20047@postech.ac.kr