Polyyne Rotaxanes: Stabilization by Encapsulation

Abstract

Active metal template Glaser coupling has been used to synthesize a series of rotaxanes consisting of a polyyne, with up to 24 contiguous sp-hybridized carbon atoms, threaded through a variety of macrocycles. Cadiot–Chodkiewicz cross-coupling affords higher yields of rotaxanes than homocoupling. This methodology has been used to prepare [3]rotaxanes with two polyyne chains locked through the same macrocycle. The crystal structure of one of these [3]rotaxanes shows that there is extremely close contact between the central carbon atoms of the threaded hexayne chains (C···C distance 3.29 Å vs 3.4 Å for the sum of van der Waals radii) and that the bond-length-alternation is perturbed in the vicinity of this contact. However, despite the close interaction between the hexayne chains, the [3]rotaxane is remarkably stable under ambient conditions, probably because the two polyynes adopt a crossed geometry. In the solid state, the angle between the two polyyne chains is 74°, and this crossed geometry appears to be dictated by the bulk of the “supertrityl” end groups. Several rotaxanes have been synthesized to explore gem-dibromoethene moieties as “masked” polyynes. However, the reductive Fritsch–Buttenberg–Wiechell rearrangement to form the desired polyyne rotaxanes has not yet been achieved. X-ray crystallographic analysis on six [2]rotaxanes and two [3]rotaxanes provides insight into the noncovalent interactions in these systems. Differential scanning calorimetry (DSC) reveals that the longer polyyne rotaxanes (C16, C18, and C24) decompose at higher temperatures than the corresponding unthreaded polyyne axles. The stability enhancement increases as the polyyne becomes longer, reaching 60 °C in the C24 rotaxane.

The Supramolecular Chemistry of Strained Carbon Nanohoops

“…Taking advantage of an active metal template method earlier used to prepare [2]rotaxanes by Glaser coupling, Anderson and co‐workers described a spectacular [4]catenane, in which three phenanthroline‐based macrocycles are interlocked with a large porphyrin nanoring ( 23 ; Figure ) . Interestingly, the synthesis first featured compound 22 as an intermediate, in which the smaller macrocycle is already mechanically interlocked as a consequence of the bulky meso substituents on the zinc porphyrins.…”

Metal ions in the synthesis of interlocked molecules and materials

“…Thus, as long as the driving force for covalent bond formation is high enough, crowded or otherwise thermodynamically disfavoured products can be formed in AT reactions. However, the nature of the bond forming reaction itself and the macrocycle also play a key role; Anderson and co-workers found that smaller bidentate macrocycles led to a reduced yield in the AT-Glaser reaction, 32 while Crowley and co-workers found that smaller pyridine macrocycles were less efficient than their larger counterparts in the AT-CuAAC reaction. 33 The origin of the conflicting effect of macrocycle size on the yield of the target MIM, which appear to depend on the ligand motif and reaction investigated, is poorly understood and warrants further investigation.…”

Syntheses, Structures, and Spectroscopic Properties of 1,10‐Phenanthroline‐Based Macrocycles Threaded by PtC8Pt, PtC12Pt, and PtC16Pt Axles: Metal‐Capped Rotaxanes as Insulated Molecular Wires

“…Their initial efforts involved the adducts shown in Scheme , with hydrocarbon axles Ar 3 C(C≡C) n CAr 3 with n ranging from 4 (C 8 ) to 12 (C 24 ). These purely organic systems could also be accessed by means of Cadiot–Chodkiewicz cross couplings, and derivatized with metals, but using the phenanthroline moiety of the macrocycle, as shown for 13⋅10 ( n= 6). Crystal structures of the rotaxanes with Ar 3 C(C≡C) 6 CAr 3 and Ar 3 C(C≡C) 8 CAr 3 axles have been reported and extensively analyzed, together with rotaxanes involving related macrocycles…”