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 metal lad iazamacrocycles with alpha- and beta-branched N-acylsalicylhydrazides

Abstract

Hexanuclear, octanuclear, and decanuclear manganese metalladiazamacrocycles have been prepared by reacting a series of pentadentate ligands, N-acylsalicylhydrazides (N-(3-methylbutanoyl)salicylhydrazide (H 33-mbshz), N-(phenylacetyl)salicylhydrazide (H3pashz), N-(3,3-dimethylbutanoyl)salicylhydrazide (H33-dmbshz), N-(2-methyl-propanoyl)salicylhydrazide (H32-mpshz), N-((R,S)-2-methylbutanoyl)salicylhydrazide (H3RS-2-mbshz), N-((S)-2-methylbutanoyl)salicylhydrazide (H3S-2-mbshz), and N-(2,2-dimethylpropanoyl)salicylhydrazide (H32-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 ∼2.1, ∼2.3, and ∼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 ⋯ΛΔΛ Δ⋯ configurations. While β-branched N-acylsalicylhydrazides (H33-mbshz, H3pashz, H33-dmbshz) with a sterically flexible Cα methylene group yield 18-membered hexanuclear manganese metalladiazamacrocycles of S6 point group symmetry, α-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 α-branched H32-mpshz ligand with the sterically least demanding isopropyl tail at the N-acyl position yields a 24-membered octanuclear manganese metalladiazamacrocycle of S8 point group symmetry, but other α-branched N-acylsalicylhydrazides such as H3RS-2-mbshz, H3S-2-mbshz, and H32-dmpshz lead to 30-membered decanuclear manganese metalladiazamacrocycles of S10 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 Mn3+ ion spins with S = 2 in the cyclic system.