DOSY study on dynamic catenation: self-assembly of a [3]catenane as a meta-stable compound from twelve simple components.
TL;DR: All the results indicate that the [2]catenane is thermodynamically the most stable structure, while the [3]catanane is a meta-stable self-assembly.
Abstract: Synthesis of [2]catenane 6 has been successfully achieved by the combination of Pd complex 1 and pyridines 2 and 3 at a molar ratio of 2:1:1 in D20. A mixture of square molecule 4 (prepared from 1 and 2) and macrocycle 5 (obtained from 1 and 3), in which the final ratio of 1, 2, and 3 was kept 2:1:1 reorganizes in D2O/CD3OD (1:1) to form 6 within one day. However, the same mixture in D2O shows the formation of novel [3]catenane 7 along with the [2]catenane. In order to make 7, the theoretical ratio of components 1, 2, and 3 should be 3:1:2. Thus, deliberately maintaining such ratio of the above-mentioned molecules, a higher proportion of the [3]catenane is observed in D2O as found from 1H NMR spectra of the system. Reorganization of the twelve components to form [3]catenane is supported by studies with the DOSY method. This method is a first attempt to separate, from a mixture, either catenanes or any other supramolecular self-assembly structures. CSI-MS studies further support the assigned catenane super structures 6 and 7. All the results indicate that the [2]catenane is thermodynamically the most stable structure, while the [3]catenane is a meta-stable self-assembly.
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TL;DR: The diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions are outlined and the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates are detailed.
Abstract: More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.
547 citations
TL;DR: The diverse strategies that exist for synthesizing catenanes in the 21st century are outlined and their emerging applications and the challenges that still exist for the synthesis of more complex topologies are examined.
Abstract: Half a century after Schill and Luttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.
394 citations
TL;DR: The first example of a [2]catenane structure to be synthesized using anion templation is described, and a dramatic catenation effect on anion selectivity properties as compared to a noncatenated acyclic receptor is revealed.
Abstract: The first example of a [2]catenane structure to be synthesized using anion templation is described. The nature of the anion template is demonstrated to be crucial to the assembly process, with only chloride anion producing the [2]catenane in acceptable yield. Anion binding studies reveal a dramatic catenation effect on anion selectivity properties as compared to a noncatenated acyclic receptor.
177 citations
TL;DR: In this article, the authors describe self-assemblies formed by the combination of a variety of palladium(II) components and ligands ranging from bi-to polydentate.
Abstract: Palladium(II) has four coordination sites and forms square planar complexes. Discrete self-assemblies are generated by the combination of a variety of palladium(II) components and ligands ranging from bi- to polydentate. The Pd(II) components used are generally of two varieties: cis -protected Pd(II) and unprotected Pd(II). Most common cis -protecting units (X-X) such as ethylenediamine, 2,2′-bipyridine and 1,3-bis(diphenylphosphino)propane and a few other related chelating systems have been exploited for the complexation reactions. The self-assemblies formed are generally represented as [{ cis -Pd(X-X)} x ( L ) y ](monoanion) 2 x and [Pd m ( L ) n ](monoanion) 2 m when generated from the complexation of a suitable ligand ( L ) with cis -protected Pd(II) and simple Pd(II) units, respectively. When Pd(solvent) 2 Cl 2 is complexed with ligands, the solvent molecules are replaced with the incoming ligands, leading to complexes in which the trans positions are occupied by the chloride anions.
172 citations
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TL;DR: In this article, a derivation of the effect of a time-dependent magnetic field gradient on the spin-echo experiment, particularly in the presence of spin diffusion, is given.
Abstract: A derivation is given of the effect of a time‐dependent magnetic field gradient on the spin‐echo experiment, particularly in the presence of spin diffusion. There are several reasons for preferring certain kinds of time‐dependent magnetic field gradients to the more usual steady gradient. If the gradient is reduced during the rf pulses, H1 need not be particularly large; if the gradient is small at the time of the echo, the echo will be broad and its amplitude easy to measure. Both of these relaxations of restrictions on the measurement of diffusion coefficients by the spin‐echo technique serve to extend its range of applicability. Furthermore, a pulsed gradient can be recommended when it is critical to define the precise time period over which diffusion is being measured.The theoretical expression derived has been verified experimentally for several choices of time dependent magnetic field gradient. An apparatus is described suitable for the production of pulsed gradients with amplitudes as large as 100 ...
7,781 citations
TL;DR: The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives.
Abstract: The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.
2,099 citations
TL;DR: In this article, the authors propose a method for the identification of the most likely candidate species of a given species from a set of known species: a.k.a. a. nomenclature.
Abstract: Nomenclature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
1,466 citations
TL;DR: In this article, a study of bipolar pulse pairs in PFGNMR (pulsed field gradient NMR) is presented, which shows a drastic reduction in eddy currents for short (≃ 1 millisecond), closely spaced (<1 ms) gradient pulses.
Abstract: We report a study of BPPs (bipolar pulse pairs) in PFGNMR (pulsed field gradient NMR) that shows a drastic reduction in eddy currents for short (≃1 millisecond), closely spaced (<1 ms) gradient pulses. Improved resolution is obtained in LED-DOSY with reduced settling times (T e ) so that samplzs with shorter values of T 1 can be studied. The three gradient prepulses can be eliminated so that temperature control is improved and convection currents are reduced. Finaly, the 180° RF pulses in combination with the alternating gradients serve to restrict the observable volume so that only regions having a homogeneous B 1 field and a constant gradient are observed.
1,319 citations