Pentameric Circular Iron(II) Double Helicates and a Molecular Pentafoil Knot

Abstract

We report on the synthesis of 11 pentameric cyclic helicates formed by imine condensation of alkyl monoamines with a common bis(formylpyridine)bipyridyl-derived building block and iron(II) and chloride ions. The cyclic double-stranded helicates were characterized by NMR spectroscopy, mass spectrometry, and in the case of a 2,4-dimethoxybenzylamine-derived pentameric cyclic helicate, X-ray crystallography. The factors influencing the assembly process (reactant stoichiometry, concentration, solvent, nature and amount of anion) were studied in detail: the role of chloride in the assembly process appears not to be limited to that of a simple template, and larger circular helicates observed with related tris(bipyridine) ligands with different iron salts are not produced with the imine ligands. Using certain chiral amines, pentameric cyclic helices of single handedness could be isolated and the stereochemistry of the helix determined by circular dichroism. By employing a particular diamine, a closed-loop molecular pentafoil knot was prepared. The pentafoil knot was characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography, confirming the topology and providing insights into the reasons for its formation.

The Self‐Sorting Behavior of Circular Helicates and Molecular Knots and Links

“…[2] The use of orthogonal recognition elements is a convenient way to achieve sorting in artificial systems, [1,3] but other methods, [4] including subtle differences in ligand design, [5][6][7] can also be remarkably effective. A beautiful example is the classic experiment by Lehn and co-workers [5] in which a mixture of ligand strands containing two to five 2,2'-bipyridine groups spontaneously self-sort into linear double helicates, each containing two ligands with equal numbers of binding sites, in the presence of Cu I ions.We recently described the synthesis of a molecular Solomon link [8] (a doubly entwined [2]catenane [9] ) and a molecular pentafoil knot, [10] each formed through a combination of metal-ligand coordination, an anion template, and geometric restrictions. These closely related structures are derived from tetra- [8] and pentameric [10] circular helicate scaffolds, respectively, and are assembled from up to 20 common, or similar, components.…”

“…Here we investigate the self-sorting behavior of both the closed molecular topologies and the open circular helicate scaffolds on which they are based (Figure 1). The study provides insights into the self-assembly processes of the individual species and reveals a subtle interplay between the driving forces and kinetic traps involved in their assembly.Despite their structural similarities (a difference of just two oxygen atoms in length), dialdehydes 1 and 2 react individually with a suitable monoamine and FeCl 2 to generate different-sized circular helicates: tetrameric [8] with 1 and pentameric [10] with 2. To investigate the self-sorting potential of the ligands, a 1:1 mixture of aldehydes 1 and 2 was allowed to react with FeCl 2 and n-hexylamine (3) in [D 6 ]DMSO at 60 8C for 18 h, followed by anion exchange through the addition of an aqueous solution of potassium hexafluorophosphate (Scheme 1).…”

“…Despite their structural similarities (a difference of just two oxygen atoms in length), dialdehydes 1 and 2 react individually with a suitable monoamine and FeCl 2 to generate different-sized circular helicates: tetrameric [8] with 1 and pentameric [10] with 2. To investigate the self-sorting potential of the ligands, a 1:1 mixture of aldehydes 1 and 2 was allowed to react with FeCl 2 and n-hexylamine (3) in [D 6 ]DMSO at 60 8C for 18 h, followed by anion exchange through the addition of an aqueous solution of potassium hexafluorophosphate (Scheme 1).…”

“…We recently described the synthesis of a molecular Solomon link [8] (a doubly entwined [2]catenane [9] ) and a molecular pentafoil knot, [10] each formed through a combination of metal-ligand coordination, an anion template, and geometric restrictions. These closely related structures are derived from tetra- [8] and pentameric [10] circular helicate scaffolds, respectively, and are assembled from up to 20 common, or similar, components.…”

“…These closely related structures are derived from tetra- [8] and pentameric [10] circular helicate scaffolds, respectively, and are assembled from up to 20 common, or similar, components. Here we investigate the self-sorting behavior of both the closed molecular topologies and the open circular helicate scaffolds on which they are based (Figure 1).…”