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• W4280621342 abstract "Molecular recognition is at the heart of the noncovalent synthesis of supramolecular assemblies and, at higher length scales, supramolecular materials. In a recent publication in Nature, Stoddart and co-workers demonstrate that the formation of host-guest complexes can be catalyzed by one of the simplest possible catalysts: the electron. Molecular recognition is at the heart of the noncovalent synthesis of supramolecular assemblies and, at higher length scales, supramolecular materials. In a recent publication in Nature, Stoddart and co-workers demonstrate that the formation of host-guest complexes can be catalyzed by one of the simplest possible catalysts: the electron. Electrochemistry is increasingly able to compete with traditional synthetic chemistry in forming covalent chemical bonds to produce both simple and complex organic molecules.1Horn E.J. Rosen B.R. Baran P.S. Synthetic organic electrochemistry: An enabling and innately sustainable method.ACS Cent. Sci. 2016; 2: 302-308https://doi.org/10.1021/acscentsci.6b00091Crossref PubMed Scopus (634) Google Scholar The advantages of electrochemical transformations include the abundant availability of the “substrate” (the electron), the ability to quantify electric current at various potentials, and the limited waste production, which simplifies workup and purification procedures, resulting in higher sustainability levels. Most of these reactions (e.g., electrochemical cross-coupling2Yuan Y. Yang J. Lei A. Recent advances in electrochemical oxidative cross-coupling with hydrogen evolution involving radicals.Chem. Soc. Rev. 2021; 50: 10058-10086https://doi.org/10.1039/D1CS00150GCrossref PubMed Google Scholar) involve reduction or oxidation of the substrate, which means that >1 equiv of electrons needs to be added or removed to completely convert the substrate. Recently, increasingly more attention has been devoted to electrocatalytic reactions, whereby electrons lower the activation energy of otherwise kinetically forbidden reactions, acting as catalysts for covalent-bond formation by temporarily reducing the substrate within a reaction cycle.3Novaes L.F.T. Liu J. Shen Y. Lu L. Meinhardt J.M. Lin S. Electrocatalysis as an enabling technology for organic synthesis.Chem. Soc. Rev. 2021; 50: 7941-8002https://doi.org/10.1039/D1CS00223FCrossref PubMed Google Scholar On the other hand, supramolecular chemistry focuses on noncovalent interactions between small-molecule building blocks, which are assembled into well-defined, higher-order entities. Although the thermodynamics of these processes are relatively well understood, more effort is required for better understanding the kinetic control of recognition pathways in both biological and artificial systems.4Korevaar P.A. George S.J. Markvoort A.J. Smulders M.M.J. Hilbers P.A.J. Schenning A.P.H.J. De Greef T.F.A. Meijer E.W. Pathway complexity in supramolecular polymerization.Nature. 2012; 481: 492-496https://doi.org/10.1038/nature10720Crossref PubMed Scopus (667) Google Scholar Recent studies have shown that catalysis can serve as an important tool to control supramolecular processes.5Wang Y. Lin H.-X. Chen L. Ding S.-Y. Lei Z.-C. Liu D.-Y. Cao X.-Y. Liang H.-J. Jiang Y.-B. Tian Z.-Q. What molecular assembly can learn from catalytic chemistry.Chem. Soc. Rev. 2014; 43: 399-411https://doi.org/10.1039/C3CS60212ECrossref PubMed Google Scholar This control applies both to the time domain within which equilibrium is reached and to the outcome of the overall process, whereby one pathway can be prioritized over several others. Similar to electrocatalytic covalent-bond formation, molecular recognition processes typically proceed without changes in the reactants’ oxidation state. However, the use of electrocatalysis in controlling the formation of noncovalent assemblies has remained elusive. Now, Stoddart and co-workers report that the formation of supramolecular host-guest complexes can also benefit from the advantages of electrocatalysis mentioned above. Recently in Nature,6Jiao Y. Qiu Y. Zhang L. Liu W.-G. Mao H. Chen H. Feng Y. Cai K. Shen D. Song B. et al.Electron-catalysed molecular recognition.Nature. 2022; 603: 265-270https://doi.org/10.1038/s41586-021-04377-3Crossref PubMed Scopus (24) Google Scholar they report that electrons can initiate radical chain reactions that effectively control the rate of molecular recognition (Figure 1). Specifically, the researchers utilized catalytic reduction by electrons to facilitate radical pairing (i.e., attractive interactions between two radical-cations, shown in magenta in Figure 1). They could introduce the electrons to the system by adding a reducing agent or using an electric current. The authors’ design is based on an earlier system, whereby a dumbbell-shaped guest molecule bearing a bipyridinium radical-cation unit binds to the interior of an aromatic macrocycle comprising two such radical-cation motifs.7Trabolsi A. Khashab N. Fahrenbach A.C. Friedman D.C. Colvin M.T. Cotí K.K. Benítez D. Tkatchouk E. Olsen J.-C. Belowich M.E. et al.Radically enhanced molecular recognition.Nat. Chem. 2010; 2: 42-49https://doi.org/10.1038/nchem.479Crossref PubMed Scopus (247) Google Scholar Although the attractive interactions between the radical-cations make the overall reaction thermodynamically favored, the Coulomb repulsion between the 2-fold positively charged host and the 2-fold positively charged thread (where the second positive charge originates from a moderately bulky pyridinium gatekeeper; Figure 1A) poses a significant kinetic barrier. Therefore, the threading of the host onto the guest is hardly observed. The authors foresaw that reducing either of the two radical species would lower the kinetic barrier to some degree, allowing the recognition process to take place. The addition of a reductant transforms a bipyridinium radical-cation (within either the host or the guest; Figure 1A or 1B, respectively) into a neutral quinoidal species (denoted in red in Figure 1), thus lowering the activation energy for threading from ∼15 to ∼9 kcal/mol. Then, the resulting complex (biradical, 3-fold positive) passes the electron to an uncomplexed thread (the “key step” in Figure 1B) or ring, initiating the second reaction cycle. In this way, substoichiometric amounts of reducing agents or electrons allow a thermodynamic equilibrium state to be reached. To demonstrate the versatility of their concept, the researchers worked with various one-electron reductants (such as cobaltocene and various metals in the zero oxidation state) in both divided and undivided electrochemical cells. In an undivided cell, radical reaction cascades are initiated by the cathodic reduction of one of the two radical-cation species. The main termination reaction is the comproportionation of the reduced components with anodically formed oxidized species. Therefore, the reaction kinetics in an undivided electrochemical cell depend not only on the applied voltage but also on other parameters, which allowed the authors to adjust (“freeze”) the equilibrium between the uncomplexed building blocks and the assembled state by controlling, e.g., the stirring rate. A similar competition was observed in the chemically triggered system, whereby minute amounts of oxygen terminated the chain reaction initiated by the addition of a reducing agent. A significant drawback of the current system is the high reactivity of the recognition motifs. The radical character of the host and the guest requires working under rigorously inert conditions, which not only hampers the collection of experimental data but also limits the scope of the conditions (solvents and additives) and hampers the possibility of integrating the building blocks into more complex systems. Even under strictly inert conditions, the turnover numbers (TONs) were on the order of 10 (i.e., a single electron injected into the system induced the formation of approximately ten host-guest complexes; on the one hand, this is a remarkable finding, but on the other, this number is still far below the TONs typical of many catalytic covalent-bond-formation reactions). Future research should aim to design and synthesize systems based on other recognition moieties that could be operated under more ambient conditions, which will also expand the instrumental toolbox that can be used for analyzing the assembly process. An appealing approach to decreasing fatigue and therefore increasing the effectiveness of electron catalysis could be to preorganize one of the components. Let us imagine multiple copies of the thread confined on the surface of a gold nanoparticle, where the ring components move freely in the surrounding solution. A one-electron reduction of either component would trigger a recognition event, and the resulting host-guest complex would find itself in close proximity to free threads attached to the same nanoparticle, thus transferring the electron in a highly efficient fashion. The chain of electron transfer could, in principle, continue until all the nanoparticle-bound threads are complexed with the rings, thus significantly increasing the TON and the turnover frequency of the process. An extra benefit of working with gold nanoparticles is that no external reductant would need to be added: such particles can eject electrons from their inorganic (Au) core when irradiated with visible light8Cai J. Zhang W. Xu L. Hao C. Ma W. Sun M. Wu X. Qin X. Colombari F.M. de Moura A.F. et al.Polarization-sensitive optoionic membranes from chiral plasmonic nanoparticles.Nat. Nanotechnol. 2022; 17: 408-416https://doi.org/10.1038/s41565-022-01079-3Crossref PubMed Scopus (27) Google Scholar—a stimulus that, unlike reducing agents, can be delivered with high spatiotemporal control.9Weißenfels M. Gemen J. Klajn R. Dissipative self-assembly: Fueling with chemicals versus light.Chem. 2021; 7: 23-37https://doi.org/10.1016/j.chempr.2020.11.025Abstract Full Text Full Text PDF Scopus (60) Google Scholar Another interesting direction would be to trigger electron-catalyzed supramolecular recognition with light through the use of small-molecule photoredox catalysts. Indeed, dyes such as methylene blue and various transition-metal complexes have been shown to initiate catalytic redox processes of other building blocks common in supramolecular chemistry, enabling processes with quantum efficiencies higher than 1 (i.e., a single photon effectively transforms more than one molecule on average, analogous to a TON > 1).10Goulet-Hanssens A. Rietze C. Titov E. Abdullahu L. Grubert L. Saalfrank P. Hecht S. Hole catalysis as a general mechanism for efficient and wavelength-independent Z → E azobenzene isomerization.Chem. 2018; 4: 1740-1755https://doi.org/10.1016/j.chempr.2018.06.002Abstract Full Text Full Text PDF Scopus (49) Google Scholar In the context of systems chemistry, the ability to catalyze molecular recognition processes and control the time scales of supramolecular self-assembly will facilitate the development of new chemical reaction networks.9Weißenfels M. Gemen J. Klajn R. Dissipative self-assembly: Fueling with chemicals versus light.Chem. 2021; 7: 23-37https://doi.org/10.1016/j.chempr.2020.11.025Abstract Full Text Full Text PDF Scopus (60) Google Scholar We gratefully acknowledge the support from the Minerva Foundation with funding from the Federal German Ministry for Education and Research. We thank Prof. Hongliang Chen (Zhejiang University) for providing the graphical representations used in Figure 1B. R.K. is a member of Chem’s advisory board." @default.
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• W4280621342 title "Electron catalysis expands the supramolecular chemist’s toolbox" @default.
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