View source for Chemistry:Mechanically interlocked molecular architectures

{{short description|Molecules connected due to their topology}}
In [[Chemistry:Chemistry|chemistry]], '''mechanically interlocked molecular architectures''' ('''MIMAs''') are [[Physics:Molecule|molecule]]s that are connected as a consequence of their [[Chemistry:Topology|topology]]. This connection of molecules is analogous to keys on a [[Engineering:Keychain|keychain]] loop. The keys are not directly connected to the keychain loop but they cannot be separated without breaking the loop. On the molecular level, the interlocked molecules cannot be separated without the breaking of the [[Chemistry:Covalent bond|covalent bond]]s that comprise the conjoined molecules; this is referred to as a '''mechanical bond'''. Examples of mechanically interlocked molecular architectures include [[Chemistry:Catenane|catenane]]s, [[Chemistry:Rotaxane|rotaxane]]s, [[Chemistry:Molecular knot|molecular knot]]s, and [[Chemistry:Molecular Borromean rings|molecular Borromean rings]]. Work in this area was recognized with the 2016 Nobel Prize in Chemistry to Bernard L. Feringa, [[Biography:Jean-Pierre Sauvage|Jean-Pierre Sauvage]], and J. Fraser Stoddart.<ref>{{cite journal |last1= Browne |first1= Wesley R. |last2= Feringa |first2= Ben L. |year= 2006 |title= Making molecular machines work |url= https://www.rug.nl/research/portal/en/publications/making-molecular-machines-work(d2240246-0144-4bb5-b4c1-42038a5d281c).html |journal= [[Engineering:Nature Nanotechnology|Nature Nanotechnology]] |volume= 1 |issue= 1 |pages= 25–35 |doi= 10.1038/nnano.2006.45 |pmid= 18654138 |bibcode= 2006NatNa...1...25B|s2cid= 29037511 |hdl= 11370/d2240246-0144-4bb5-b4c1-42038a5d281c |hdl-access= free }}</ref><ref>{{cite journal |last1= Stoddart |first1= J. F. |year= 2009 |title= The chemistry of the mechanical bond |journal= Chem. Soc. Rev. |volume= 38 |issue= 6 |pages= 1802–1820 |doi= 10.1039/b819333a |pmid= 19587969}}</ref><ref>{{cite journal |last1= Coskun |first1= A. |last2= Banaszak |first2= M.|last3= Astumian |first3= R. D. |last4= Stoddart |first4= J. F. |last5= Grzybowski |first5= B. A. |year= 2012 |title= Great expectations: can artificial molecular machines deliver on their promise? |journal= Chem. Soc. Rev. |volume= 41 |issue= 1 |pages= 19–30 |doi= 10.1039/C1CS15262A |pmid= 22116531}}</ref><ref>{{cite journal |last1= Durola |first1= Fabien |last2= Heitz |first2= Valerie |last3= Reviriego |first3= Felipe |last4= Roche |first4= Cecile |last5= Sauvage |first5= Jean-Pierre |last6= Sour |first6= Angelique |last7= Trolez |first7= Yann |year= 2014 |title= Cyclic [4]Rotaxanes Containing Two Parallel Porphyrinic Plates: Toward Switchable Molecular Receptors and Compressors |journal= [[Chemistry:Accounts of Chemical Research|Accounts of Chemical Research]] |volume= 47 |issue= 2 |pages= 633–645 |doi= 10.1021/ar4002153 |pmid= 24428574}}</ref>

The synthesis of such entangled architectures has been made efficient by combining [[Chemistry:Supramolecular chemistry|supramolecular chemistry]] with traditional covalent synthesis, however mechanically interlocked molecular architectures have properties that differ from both "[[Chemistry:Supramolecular assembly|supramolecular assemblies]]" and "covalently bonded molecules". The terminology "mechanical bond" has been coined to describe the connection between the components of mechanically interlocked molecular architectures. Although research into mechanically interlocked molecular architectures is primarily focused on artificial compounds, many examples have been found in biological systems including: [[Biology:Cystine knot|cystine knot]]s, [[Biology:Cyclotide|cyclotide]]s or lasso-peptides such as [[Biology:Microcin|microcin]] J25 which are [[Biology:Protein|protein]]s, and a variety of [[Chemistry:Peptide|peptide]]s.

==Residual topology==
'''Residual topology''' <ref>{{cite journal|doi=10.1002/chem.200305203|title=Residual Topological Isomerism of Intertwined Molecules|journal=Chemistry: A European Journal|volume=10|issue=8|pages=1878–1883|year=2004|last1=Lukin|first1=Oleg|last2=Godt|first2=Adelheid|last3=Vögtle|first3=Fritz|pmid=15079826 }}</ref> is a descriptive stereochemical term to classify a number of intertwined and interlocked molecules, which cannot be disentangled in an experiment without breaking of [[Chemistry:Covalent bond|covalent bond]]s, while the strict rules of mathematical [[Topology|topology]] allow such a disentanglement. Examples of such molecules are [[Chemistry:Rotaxane|rotaxane]]s, [[Chemistry:Catenane|catenane]]s with covalently linked rings (so-called pretzelanes), and open knots (pseudoknots) which are abundant in [[Biology:Protein|protein]]s.

The term "residual topology" was suggested on account of a striking similarity of these compounds to the well-established topologically nontrivial species, such as catenanes and knotanes (molecular knots). The idea of residual topological isomerism introduces a handy scheme of modifying the [[Chemistry:Molecular graph|molecular graph]]s and generalizes former efforts of systemization of mechanically bound and bridged molecules.

== History ==
Experimentally the first examples of mechanically interlocked molecular architectures appeared in the 1960s with catenanes being synthesized by Wasserman and Schill and rotaxanes by Harrison and Harrison. The chemistry of MIMAs came of age when Sauvage pioneered their synthesis using templating methods.<ref>{{Cite journal|last1=Mena-Hernando|first1=Sofía|last2=Pérez|first2=Emilio M.|date=2019-09-30|title=Mechanically interlocked materials. Rotaxanes and catenanes beyond the small molecule|url=https://pubs.rsc.org/en/content/articlelanding/2019/cs/c8cs00888d|journal=Chemical Society Reviews|language=en|volume=48|issue=19|pages=5016–5032|doi=10.1039/C8CS00888D|pmid=31418435|s2cid=201020016 |issn=1460-4744}}</ref> In the early 1990s the usefulness and even the existence of MIMAs were challenged. The latter concern was addressed by X ray crystallographer and structural chemist David Williams. Two postdoctoral researchers who took on the challenge of producing [5]catenane (olympiadane) pushed the boundaries of the complexity of MIMAs that could be synthesized their success was confirmed in 1996 by a solid‐state structure analysis conducted by David Williams.<ref>{{Cite journal|last=Stoddart|first=J. Fraser|date=2017|title=Mechanically Interlocked Molecules (MIMs)—Molecular Shuttles, Switches, and Machines (Nobel Lecture)|journal=Angewandte Chemie International Edition|volume=56|issue=37|pages=11094–11125|doi=10.1002/anie.201703216|pmid=28815900|issn=1521-3773|doi-access=free}}</ref>

==Mechanical bonding and chemical reactivity==
The introduction of a mechanical bond alters the chemistry of the sub components of rotaxanes and catenanes. Steric hindrance of reactive functionalities is increased and the strength of [[Chemistry:Non-covalent interactions|non-covalent interactions]] between the components are altered.<ref name=":02">{{Cite journal|last1=Neal|first1=Edward A.|last2=Goldup|first2=Stephen M.|date=2014-04-22|title=Chemical consequences of mechanical bonding in catenanes and rotaxanes: isomerism, modification, catalysis and molecular machines for synthesis|journal=Chemical Communications|language=en|volume=50|issue=40|pages=5128–42|doi=10.1039/c3cc47842d|pmid=24434901|issn=1364-548X|url=https://eprints.soton.ac.uk/370644/1/Chem.%2520Commun.%2520-%25202014%2520-%2520Neal%252C%2520Goldup.pdf|doi-access=free}}</ref>

===Mechanical bonding effects on non-covalent interactions===
The strength of non-covalent interactions in a mechanically interlocked molecular architecture increases as compared to the non-mechanically bonded analogues. This increased strength is demonstrated by the necessity of harsher conditions to remove a metal template ion from catenanes as opposed to their non-mechanically bonded analogues. This effect is referred to as the "catenand effect".<ref>{{Cite journal|last1=Albrecht-Gary|first1=Anne Marie|last2=Saad|first2=Zeinab|last3=Dietrich-Buchecker|first3=Christiane O.|last4=Sauvage|first4=Jean Pierre|date=1985-05-01|title=Interlocked macrocyclic ligands: a kinetic catenand effect in copper(I) complexes|journal=[[Organization:Journal of the American Chemical Society|Journal of the American Chemical Society]]|volume=107|issue=11|pages=3205–3209|doi=10.1021/ja00297a028|issn=0002-7863}}</ref><ref>{{Cite book|title=The Nature of the Mechanical Bond: From Molecules to Machines|last1=Stoddart |last2= Bruns|first1=J. Fraser |first2= Carson J|publisher=Wiley|year=2016|isbn=978-1-119-04400-0|pages=90}}</ref> The augmented non-covalent interactions in interlocked systems compared to non-interlocked systems has found utility in the strong and selective binding of a range of charged species, enabling the development of interlocked systems for the extraction of a range of salts.<ref>{{cite journal |last1=Wilmore |first1=Jamie T. |last2=Beer |first2=Paul D. |title=Exploiting the Mechanical Bond Effect for Enhanced Molecular Recognition and Sensing |journal=Advanced Materials |year=2024 |doi=10.1002/adma.202309098|doi-access=free }}</ref> This increase in strength of non-covalent interactions is attributed to the loss of [[Chemistry:Degrees of freedom (physics and chemistry)|degrees of freedom]] upon the formation of a mechanical bond. The increase in strength of non-covalent interactions is more pronounced on smaller interlocked systems, where more degrees of freedom are lost, as compared to larger mechanically interlocked systems where the change in degrees of freedom is lower. Therefore, if the ring in a rotaxane is made smaller the strength of non-covalent interactions increases, the same effect is observed if the thread is made smaller as well.<ref>{{Cite journal|last1=Lahlali|first1=Hicham|last2=Jobe|first2=Kajally|last3=Watkinson|first3=Michael|last4=Goldup|first4=Stephen M.|date=2011-04-26|title=Macrocycle Size Matters: "Small" Functionalized Rotaxanes in Excellent Yield Using the CuAAC Active Template Approach|journal=Angewandte Chemie International Edition|language=en|volume=50|issue=18|pages=4151–4155|doi=10.1002/anie.201100415|issn=1521-3773|pmid=21462287}}</ref>

===Mechanical bonding effects on chemical reactivity===
The mechanical bond can reduce the kinetic reactivity of the products, this is ascribed to the increased steric hindrance. Because of this effect [[Chemistry:Hydrogenation|hydrogenation]] of an alkene on the thread of a rotaxane is significantly slower as compared to the equivalent non interlocked thread.<ref>{{Cite journal|last1=Parham|first1=Amir Hossain|last2=Windisch|first2=Björna|last3=Vögtle|first3=Fritz|date=1999-05-01|title=Chemical Reactions in the Axle of Rotaxanes – Steric Hindrance by the Wheel|url=http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1099-0690(199905)1999:53.0.CO;2-Q/abstract|journal=[[Chemistry:European Journal of Organic Chemistry|European Journal of Organic Chemistry]]|language=en|volume=1999|issue=5|pages=1233–1238|doi=10.1002/(sici)1099-0690(199905)1999:5<1233::aid-ejoc1233>3.0.co;2-q|issn=1099-0690}}</ref> This effect has allowed for the isolation of otherwise reactive intermediates.

The ability to alter reactivity without altering covalent structure has led to MIMAs being investigated for a number of technological applications.

===Applications of mechanical bonding in controlling chemical reactivity===
The ability for a mechanical bond to reduce reactivity and hence prevent unwanted reactions has been exploited in a number of areas. One of the earliest applications was in the protection of organic dyes from [[Engineering:Industrial dye degradation|environmental degradation]].

==Examples==
*[[Chemistry:Olympiadane|Olympiadane]]
<gallery>
File:Rotaxane cartoon.jpg|[[Chemistry:Rotaxane|Rotaxane]]
File:CatenaneScheme.svg|[[Chemistry:Catenane|Catenane]]
File:Trefoil knot arb.png|[[Chemistry:Molecular knot|Molecular knot]]
File:Molecular Borromean Ring.png|[[Chemistry:Molecular Borromean rings|Molecular Borromean rings]]
</gallery>

==References==
{{Reflist}}

==Further reading==
*{{cite book |last1=Schill |first1=Gottfried |title=Catenanes, Rotaxanes, and Knots |date=1971 |publisher=Academic Press |isbn=9781483275666 |pages=204 |edition=1st |url=https://www.elsevier.com/books/catenanes-rotaxanes-and-knots/schill/978-0-12-625450-1}}
*{{cite journal |author= G. A. Breault, C. A. Hunter and P. C. Mayers |year= 1999 |title= Supramolecular topology |journal= Tetrahedron |volume= 55 |issue= 17 |pages= 5265–5293 |doi= 10.1016/S0040-4020(99)00282-3}}

{{Chemical bonds}}

[[Category:Supramolecular chemistry]]
[[Category:Molecular topology]]

{{Sourceattribution|Mechanically interlocked molecular architectures}}