A Solomon link through an interwoven molecular grid

Abstract

A molecular Solomon link was synthesized through the assembly of an interwoven molecular grid consisting of four bis(benzimidazolepyridyl)benzthiazolo[5,4-d]thiazole ligands and four zinc(II), iron(II), or cobalt(II) cations, followed by ring-closing olefin metathesis. NMR spectroscopy, mass spectrometry, and X-ray crystallography confirmed the doubly interlocked topology, and subsequent demetalation afforded the wholly organic Solomon link. The synthesis, in which each metal ion defines the crossing point of two ligand strands, suggests that interwoven molecular grids should be useful scaffolds for the rational construction of other topologically complex structures.

Keywords: catenanes; coordination chemistry; molecular grids; self-assembly; supramolecular chemistry.

Figure 1

Metal-template strategies for the rational synthesis of molecular Solomon links: a) via a linear double helicate,, b) via a one-pot circular double helicate assembly, and c) via an interwoven molecular grid (this work).

Scheme 1

Synthesis of interwoven grid [Zn₄1₄](BF₄)₈ and Solomon links [Zn₄2](BF₄)₈ and 2.

Figure 2

Partial ¹H NMR spectra (2:1 CDCl₃/CD₃CN, 600 MHz, 298 K except for (d)) of a) ligand 1, b) grid complex [Zn₄1₄](BF₄)₈, c) Solomon link [Zn₄2](BF₄)₈, and d) demetalated Solomon link 2 (325 K). Proton assignments refer to the labeling in Scheme 1.

Figure 3

X-Ray crystal structure of Solomon link [Zn₄2](BF₄)₈. Isopentyl groups, hydrogen atoms, solvent molecules, and counterions are omitted for clarity. One macrocycle is colored turquoise. Color code for the other macrocycle: C, grey; N, purple; O, red; S, gold; Zn, yellow. a) Viewed from above the plane of the zinc ions. b) Viewed from the side.

Figure 4

Partial ESI-MS spectra of a) demetalated Solomon link 2 and b) tandem MS fragmentation of the [2⋅H]⁺ ion to the constituent macrocycle. See the Supporting Information (Figure S6–S8) for further information.