Coordination-driven self-assembly of a molecular figure-eight knot and other topologically complex architectures

Abstract

Over the past decades, molecular knots and links have captivated the chemical community due to their promising mimicry properties in molecular machines and biomolecules and are being realized with increasing frequency with small molecules. Herein, we describe how to utilize stacking interactions and hydrogen-bonding patterns to form trefoil knots, figure-eight knots and [2]catenanes. A transformation can occur between the unique trefoil knot and its isomeric boat-shaped tetranuclear macrocycle by the complementary concentration effect. Remarkably, the realization and authentication of the molecular figure-eight knot with four crossings fills the blank about 4₁ knot in knot tables. The [2]catenane topology is obtained because the selective naphthalenediimide (NDI)-based ligand, which can engender favorable aromatic donor-acceptor π interactions due to its planar, electron-deficient aromatic surface. The stacking interactions and hydrogen-bond interactions play important roles in these self-assembly processes. The advantages provide an avenue for the generation of structurally and topologically complex supramolecular architectures.

Fig. 1

The macrocycles, molecular knots and links prepared in this study with their trivial names and descriptors using the Alexander–Briggs notation. a 0₁ Unknot, (b) 3₁ Trefoil knot (c) 4₁ Figure-eight knot and (d) [2]Catenane

Fig. 2

Synthesis of tetranuclear macrocycle 2a and trefoil knot 2b. Schematic representation of the synthesis of E2 and the stick model of E2; Schematic representation and stick model of L1; Increasing the concentration of 2a can result in gradual transformation of 2a into 2b

Fig. 3

¹H NMR spectrum (400 MHz, CD₃OD) of 2b (a), 2a + 2b (b), and 2a (c). DOSY spectrum (500 MHz, CD₃OD) of 2a + 2b (d), (The peaks at 3.50 and 1.18 ppm belong to diethyl ether)

Fig. 4

Single-crystal X-ray structure of 2a. Top view (a), side view (b), and front view (c). Counteranions and hydrogen atoms are omitted for clarity (N, blue; O, red; C, gray; Rh, purple)

Fig. 5

Single-crystal X-ray structure of 2b. Top view (a) and side view (b). Counteranions and hydrogen atoms are omitted for clarity

Fig. 6

Synthesis of octanuclear figure-eight knot 3. Schematic representation of the synthesis of 3; The stick model (top) and schematic representation (below) of E2 and L2; The stick model (top) and simplified image of 3

Fig. 7

Single-crystal X-ray structure of 3. The reduced representation with four crossings (a) and the four-fold symmetry representation (c) of 3, and simplified structures of the reduced representation with four crossings (b) and the four-fold symmetry representation (d) of 3 in which sticks connect the rhodium centers

Fig. 8

Synthesis of [2]catenane complex 5. Schematic representation of the synthesis of E4 (top) and stick model of 5 (below)

Fig. 9

X-ray structure of 5 ([2]catenane). a,b depictions of the short and long arms (N, blue; O, red; C, gray; Rh, purple) (c) ball-and-stick representations. Counteranions are omitted for clarity