Table Of ContentDefinition and Emergence of Supramolecular
∗
Chemistry
Jonathan W. Steed1, Jerry L. Atwood2, and Philip A. Gale3
1Durham University, Durham, UK
2University of Missouri, Columbia, MO, USA
3University of Southampton, Southampton, UK
be a monatomic cation, a simple inorganic anion, an ion
1 Introduction 1 pair, or a more sophisticated molecule such as a hormone,
pheromone, or neurotransmitter. More formally, the host is
2 Emergence 2
definedasthemolecularentitypossessingconvergent bind-
3 Conclusion 5
ing sites (e.g., Lewis basic donor atoms, hydrogen-bond
References 5
donors, etc.). The guest possesses divergent binding sites
(e.g., a spherical, Lewis acidic metal cation, or hydrogen-
bond-accepting halide anion). In turn, a binding site is
defined as a region of the host or guest capable of taking
1 INTRODUCTION part in a noncovalent interaction. The host–guest relation-
ship has been defined by Donald Cram2 as follows:
Althoughtheword“supramolecular”madeanearlyappear-
Complexesarecomposedoftwoormoremoleculesorions
ance in Webster’s Dictionary in 1903, “Supramolecular held together in unique structural relationships by electro-
chemistry”initsmodernsensewasintroducedonlyin1978 static forces other than those of full covalent bonds . . .
by Lehn, who defined it as the “. . .chemistry of molecu- molecular complexes are usually held together by hydro-
lar assemblies and of the intermolecular bond.”1 Classic gen bonding, by ion pairing, by π-acid to π-base interac-
tions,bymetal-to-ligandbinding,byvanderWaalsattrac-
explanations of supramolecular chemistry describe it as
tive forces, by solvent reorganising, and by partially made
“chemistry beyond the molecule,” “the chemistry of the andbrokencovalentbonds(transitionstates). . .Highstruc-
noncovalentbond,”and“nonmolecularchemistry,”oreven turalorganisationisusuallyproducedonlythroughmultiple
“Legochemistry.”Theearlyworkinthefieldconcernedthe binding sites. . .A highly structured molecular complex is
composedofatleastonehostandoneguestcomponent. . .
formationofsupermoleculescomprisingtwocomponents,a
Ahost–guestrelationshipinvolvesacomplementarystereo-
host and a guest, which interact with one another in a non-
electronicarrangementofbindingsitesinhostandguest. . .
covalentmanner(Figure 1).Thehostisalargemoleculeor The host component is defined as an organic molecule or
aggregatesuchasanenzymeorsynthetic cycliccompound ion whose binding sites converge in the complex. . . The
possessingasizeable,centralhole,orcavity.Theguestmay guestcomponentasanymoleculeorionwhosebindingsites
divergeinthecomplex. . .
∗AdaptedinpartfromSupramolecularChemistry,J.W.Steedand This description might well be generalized to remove
J.L.Atwood,Wiley:Chichester,2ndEd.,2009.
the word “organic,” since more recent work has revealed
a wealth of inorganic hosts, such as zeolites3 and
polyoxometallates,4 or mixed metal–organic coordination
compounds, such as metal–organic frameworks (MOFs)
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2 Concepts
Molecular chemistry Supramolecular chemistry
Host
Molecular
precursors Specific characteristic,
function or properties:
+ Recognition
Catalysis
+ + Transport
Supermolecule (complex):
Degree of order
Covalent molecule:
Chemical nature Guest Interactions between subunits
Symmetry of packing
Shape
Intermolecular interactions
Redox properties
HOMO–LUMO gap
Polarity
Vibration and rotation
Magnetism
Chirality
Figure 1 Definition of traditional supramolecular “host–guest” chemistry according to Lehn.5
(see Zeolitelike Metal–Organic Frameworks (ZMOFs): the supramolecular synthon approach of Desiraju in 1995.
Design, Structure, and Properties, Supramolecular Mate- The years 1989 and 1995 mark milestones in the design
rials Chemistry), which perform similar functions and may and synthesis of coordination polymer systems that have
be thought of under the same umbrella. broughtabouttheexplosionofporousMOFchemistryover
the past decade.
Biological receptor–substrate supramolecular chemistry
2 EMERGENCE and, by generalization, the whole of modern host–guest
chemistry has its roots in three core concepts:
The original supramolecular host–guest complexes
1. TherecognitionbyPaulEhrlichin1906thatmolecules
involve a host molecule that possesses an intrinsic molecu-
do not act if they do not bind, Corporanonaguntnisi
larcavityintowhichtheguestfits;hence,theyare,inprin-
fixata; in this way, Ehrlich introduced the concept of a
ciple, stable in all forms of matter (solid, liquid/solution,
biological receptor.
and the gas phase). The host–guest concept is much older
2. The recognition in 1894 by Emil Fischer that bind-
than the work by Pedersen6 on hosts for alkali metal ions
ing must be selective, as part of the study of recep-
in the late 1960s that gave birth to modern supramolecu-
tor–substrate binding by enzymes. He described this
lar chemistry and can be dated back to the extensive body
byalock-and-key imageofstericfitinwhichtheguest
of clathrate or solid-state inclusion chemistry. This field
begins with the twin descriptions of zeolites or “boiling
stones”discoveredbyAxelCronstedtin1756andclathrate
Substrate
hydrates or “anomalous ice” prepared by Joseph Priestley
in 1778. The evolution of this area is elucidated later in
+ Lock and key
this work by Bishop (see Synthetic Clathrate Systems,
Supramolecular Materials Chemistry) and forms much of
the early part of our subjective timeline of supramolecular (a) Enzyme
chemistry (Table 1).
Interspersed among these milestones is the parallel birth
of self-assembly as in the formation of self-assembled Complex
monolayers first observed as the spreading of oil on water +
by Benjamin Franklin in 1774, and the birth of nanochem-
Induced fit
istry (the 1818 recognition of the particle size-dependent
(b)
color of colloidal gold). We can also see the evolution of
crystal engineering from the early topochemical postulate Figure 2 (a) Rigid lock and key and (b) induced fit models of
and molecular engineering of von Hippel in the 1960s to enzyme–substrate (and hence host–guest) binding.
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Definition and emergence of supramolecular chemistry 3
Table 1 An illustrative timeline charting the development of supramolecular chemistry from its roots in solid-state inclusion
compounds, through the birth of macrocyclic host–guest chemistry in the 1960s to its modern incarnation in self-assembled materials
and nanoscale chemistry.
1756 — AxelCronstedt:descriptionof“boilingstone”(zeolite)
1774 — BenjaminFranklin:spreadingofoilonwater
1778 — JosephPriestly:“anomalous ice”
1810 — SirHumphreyDavy:discoveryofchlorinehydrate
1818 — JeremiasBenjaminRichters:particlesizeexplanationforthecolorof“drinkablegold”;colloidalgoldknownsinceantiquity
(e.g.,Lycurgus cup,fourthcenturyAD)
1823 — MichaelFaraday:formulaofchlorinehydrate
1841 — C.Schafha¨utl:studyofgraphiteintercalates
1849 — F.Wo¨hler:β-quinolH Sclathrate
2
1891 — VilliersandHebd:cyclodextrin inclusioncompounds
1891 — AgnesPockles:thefirstsurfacebalance,leadingtothedevelopment oftheLangmuirtroughandtheLangmuir–Blodgett
technique
1893 — AlfredWerner:coordinationchemistry
1894 — EmilFischer:lock-and-key concept
1906 — PaulEhrlich:introductionoftheconcept ofareceptor
1937 — K.L.Wolf:thetermU¨bermoleku¨le iscoinedtodescribeorganizedentities arisingfromtheassociationofcoordinatively
saturatedspecies(e.g.,theaceticaciddimer)
1939 — LinusPauling:hydrogenbondsareincluded inthegroundbreaking bookTheNatureoftheChemicalBond
1940 — M.F.Bengen:ureachannelinclusioncompounds
1945 — H.M.Powell:X-raycrystalstructuresofβ-quinolinclusioncompounds;theterm“clathrate” isintroducedtodescribe
compounds whereonecomponent isenclosedwithintheframeworkofanother
1949 — BrownandFarthing:synthesisof[2.2]paracyclophane
1953 — WatsonandCrick:structureofDNA
1956 — DorothyCrowfootHodgkin:X-raycrystalstructureofvitaminB
12
1958 — DanielKoshland:induced fitmodel
1959 — DonaldCram:attempted synthesisofcyclophane charge-transfercomplexes with(NC) C=C(CN)
2 2
1961 — N.F.Curtis:firstSchiff’sbasemacrocycle fromacetoneandethylenediamine
1964 — BuschandJa¨ger:Schiff’sbasemacrocycles
1965 — OlgaKennard andJ.D.Bernal:TheCambridgeStructuralDatabase
1962 — vonHippel:birthofcrystalengineering
1967 — CharlesPedersen:crownethers
1968 — ParkandSimmons:Katapinand anionhosts
1968 — F.Toda:“wheel andaxel” inclusioncompoundhosts
1969 — Jean-MarieLehn:synthesisofthefirstcryptands
1969 — JerryAtwood:liquidclathratesfromalkylaluminumsalts
1969 — RonBreslow:catalysisbycyclodextrins
1971 — G.M.J.Schmidt:topochemistry
1973 — DonaldCram:spherandhostsproduced totesttheimportanceofpreorganization
1978 — Jean-MarieLehn:introductionoftheterm“supramolecularchemistry,”definedasthe“chemistryofmolecularassembliesand
oftheintermolecular bond”
1976 — Deliberateclathratedesignstrategies;“hexahosts”D.D.MacNicol andlaterin1982“coordinatoclathrates” E.Weber
1979 — GokelandOkahara: developmentofthelariatethersasasubclassofhost
1981 — Vo¨gtleandWeber: podandhostsanddevelopmentofnomenclature
1986 — A.P.deSilva:fluorescentsensingofalkalimetalionsbycrownetherderivatives
1987 — AwardoftheNobelprizeforChemistrytoDonaldJ.Cram,Jean-MarieLehn,andCharlesJ.Pedersenfortheirworkin
supramolecularchemistry
1989 — G.M.Whitesides:self-assembledthiolmonolayers ongold
1989 — R.Robson:3Dcoordination polymersbasedonrod-likelinkers
1991 — G.M.Whitesides:achemicalstrategyforthesynthesisofnanostructures
1994 — M.Brust:synthesisofthiol-stabilizedgoldnanoparticles
1995 — O.M.Yaghi:firstMOF;keycoordinationpolymerpapersbyM.J.ZaworotkoandJ.S.Moore
1995 — G.Desiraju:supramolecularsynthonapproachtocrystalengineering
1996 — Atwood,Davies,MacNicol,andVo¨gtle:publicationofComprehensiveSupramolecularChemistry containing contributions
frommanykeygroupsandsummarizingthedevelopmentandstateoftheart
1996 — J.K.M.Sanders:thefirstexample ofadynamiccombinatorial chemistrysystem
1998 — RowanandNolte:helicalsupramolecularpolymersfromself-assembly
1999 — J.F.Stoddart:molecularelectronics basedoninterlockedmolecules
2004 — J.F.Stoddart:thefirstdiscreteBorromean-linkedmolecule,alandmarkintopological synthesis
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4 Concepts
has a geometric size or shape complementarity to the Receptor–substrate chemistry underwent a huge
receptororhost(Figure 2a).Thisconceptlaidthebasis paradigm shift in 1958 with Koshland’s “induced fit”
for molecularrecognition, the discrimination by a host model (Figure 2b), and these concepts have since per-
between a number of different guests. meated throughout biological and abiotic supramolecular
3. The fact that selective binding must involve attraction chemistry.
or mutual affinity between the host and guest. This Supramolecular chemistry as we understand it today has
is, in effect, a generalization of Alfred Werner’s 1893 evolved to encompass not just host and guest chemistry
theory of coordination chemistry, in which metal ions but also all aspects of self-assembly. It includes the design
are coordinated by a regular polyhedron of ligands and function of molecular devices and molecular assem-
binding by dative bonds. blies, noncovalent polymers, and soft materials such as
Larger molecule
(Host)
Crystallization
Smaller molecule
(Guest)
(a) Lattice inclusion host–guest complex or clathrate
(Solid-state only)
Covalent
synthesis
Small molecular
‘‘guest’’
Small molecules
Large ‘‘host’’ molecule Host–guest complex
(b)
Covalent Spontaneous
synthesis
Larger molecule
Small molecules
(c) Self-assembled
aggregate
Figure 3 Key paradigms in supramolecular chemistry. (a) Solid-state clathrate paradigm, (b) molecular host–guest paradigm, and
(c) self-assembly paradigm.
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Definition and emergence of supramolecular chemistry 5
liquidcrystals,informednanoscalechemistry,and“bottom- devices,self-assemblyandself-organization,softmaterials,
up” nanotechnology. In 2002, Lehn added a functional nanochemistry and nanotechnology, complex matter, and
definition: “Supramolecular Chemistry aims at developing biological chemistry. Dario Braga has summed up the
highly complex chemical systems from components inter- impact of supramolecular concepts in the following way9:
acting by noncovalent intermolecular forces.”7 Hence, the
The supramolecular perception of chemistry generated a
current emphasis is on increasing complexity and hence
true“paradigmshift”:fromtheonefocusedonatomsand
increasingly sophisticated functionality and on the infor-
bonds between atoms to the one focused on molecules and
mation stored in molecular components that allows this bonds between molecules. In its burgeoning expansion the
complexity to be achieved. supramolecular idea abated, logically, all traditional bar-
Modern supramolecular systems are beginning to dis- riers between chemical subdivisions (organic, inorganic,
organometallic, biological) calling attention to the collec-
play complex emergent properties based on the nonlinear
tivepropertiesgeneratedbytheassemblyofmoleculesand
interactions between the molecular component parts. It is
to the relationship between such collective properties and
clear that there are certain properties and features that
thoseoftheindividualcomponent.
emerge according to the length scale on which a system
assembles, and indeed on which it is studied. Thus, the
way in which ostensibly easily understood molecular-level
3 CONCLUSION
supramolecular interactions scale up into the nanoworld
is not always predictable and represents the frontiers and
It is clear that the molecular-level approach to understand-
future of supramolecular science. As direct microscopic
ing binding phenomena that gave rise to supramolecular
imaging and manipulation on the multinanometer scale
chemistry has found application in a vast array of phenom-
become increasingly technologically feasible, it is increas-
enaandistoagreatextentfuelingtheconceptsandgrowth
ingly possible to study the fascinating consequences of
ofavastswatheofchemicallyrelatedscience.Forexample,
chemical emergence—the “arising of novel and coherent
future applications of supramolecular chemistry in biologi-
structures, patterns, and properties during the process of
cal systems may include new treatments for disease by the
self-organization in complex systems.”8
inhibition of protein–protein interactions or by the pertur-
Fundamentally, supramolecular chemistry concerns the
bation via synthetic channels or carriers of chemical and
mutual interaction of molecules or molecular entities with
potential gradients within cancer cells triggering apoptosis.
discrete properties. This interaction is usually of a nonco-
From molecules to supramolecular assemblies, to nano-
valent type (an “intermolecular bond” such as a hydrogen
materials and complex molecular biosystems, the ensuing
bond, dipolar interaction, or π-stacking). Key to many def-
chapters in these volumes capture in detail the backdrop
initions of supramolecular chemistry is a sense of modu-
and current state of the art in all of these fields that are
larity. Supermolecules, in the broad sense, are aggregates
driven or informed by supramolecular concepts.
in which a number of components (of one or more type)
cometogether,eitherspontaneouslyorbydesign,toforma
larger entity with properties derived from those of its com-
ponents. These aggregates can be of the host–guest type REFERENCES
in which one molecule encapsulates the other or they can
involve mutually complementary, or self-complementary,
1. J.-M. Lehn, Angew.Chem.Int.Ed.Engl., 1988, 27, 89.
components of similar size in which there is no host or
2. D. J. Cram, Angew.Chem.Int.Ed.Engl., 1986, 25, 1039.
guest. We can thus trace the evolution of supramolecular
3. R. Szostak, Molecular Sieves, Van Nostrand Reinhold, New
chemistryfromtheoriginalsolid-state“clathrate”paradigm
York, 1989.
(Figure 3a), through the molecular host–guest paradigm
4. A. Muller, E. Krickemeyer, J. Meyer, etal., Angew. Chem.
(Figure 3b) to the self-assembly paradigm (Figure 3c).
Int.Ed.Engl., 1995, 34, 2122.
As it is currently practiced, supramolecular chemistry,
5. J.-M. Lehn, Supramolecular Chemistry, 1st edn, Wiley-VCH
with its emphasis on the interactions between molecules,
Verlag GmbH, Weinheim, 1995.
underpins a very wide variety of chemistry and materials
6. R. M. Izatt, Chem.Soc.Rev., 2007, 36, 143–147.
science impinging on molecular host–guest chemistry,
7. J.-M. Lehn, Proc.Nal.Acad.Sci.U.S.A., 2002, 99, 4763.
solid-state host–guest chemistry, crystal engineering and
the understanding and control of the molecular solid state 8. J. Goldstein, Emergence:Complex.Organ., 1999, 1, 49.
(including crystal structure calculation), supramolecular 9. D. Braga, Chem.Commun., 2003, 2751.
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Supramolecular Interactions
Dushyant B. Varshey, John R. G. Sander, Tomislav Frisˇcˇic´,
and Leonard R. MacGillivray
University of Iowa, Iowa City, IA, USA
construction of complex molecules, otherwise unavailable
1 Introduction 9 via traditional approaches. An early transition toward this
approach was realized when Emil Fischer, in 1894, pro-
2 Supramolecular Chemistry 9
posed the “lock-and-key” model for enzyme–substrate
3 Supramolecular Interactions 10
interactions.3Theelegantmechanismsofenzymesprovided
4 Construction of Supramolecular Compounds 16
basic principles for the new subject, namely, “Supramolec-
5 Host–Guest Chemistry 16
ularChemistry,”fromwhichprinciplesofmolecularrecog-
6 Molecular Recognition 16 nition and supramolecular function evolved.4,5
7 Self-Assembly 16
8 Supramolecular Structures via Molecular
Recognition and Self-Assembly 17
2 SUPRAMOLECULAR CHEMISTRY
9 Conclusions 21
References 21 The term supramolecular chemistry was coined by Jean-
Marie Lehn in 1969. Lehn defined supramolecular chem-
istry as “the chemistry of molecular assemblies and inter-
molecularbonds,”whichismorecommonlyreferredtothe
1 INTRODUCTION “chemistry beyond the molecule.”6 The Nobel Prize was
awarded to Lehn, Charles Pedersen, and Donald Cram in
1987 for pioneering contributions to supramolecular chem-
To achieve the impeccable ability of nature to construct
istry.7 As molecules are built by connecting atoms by
molecules (e.g., proteins), chemists have traditionally
covalent bonds, supramolecular compounds are built by
employed approaches at the molecular level. Molecu-
linking molecules with intermolecular forces (Figure 1).8
lar chemistry, since the synthesis of urea by Wo¨hler in
1828,1 has relied on building molecules via stepwise for- Thus,inmolecularchemistry,precursormoleculesundergo
covalent-bond making or breaking to produce a target
mation and breakage of covalent bonds. Molecular tech-
molecule A. In contrast, in supramolecular chemistry
niquesofchemistshaveculminatedintothetotalsyntheses
of sophisticated molecules (e.g., vitamin B12).2 How- molecule A can act as a host that interacts with a guest
via noncovalent forces (e.g., hydrogen bonds) to form a
ever, nature routinely utilizes noncovalent interactions to
supermolecule B.
organize molecules to form aggregates that perform spe-
cific functions. Chemists now recognize advantages of
the synthesis paradigm of biology that can facilitate the
2.1 Development of supramolecular chemistry
The concepts and roots of supramolecular chemistry can
be traced to the discovery of chloride hydrate by Sir
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2 Concepts
Molecular chemistry Pedersen(1967),14 whichwerecomparedtonaturalmacro-
cycles(e.g.,ionophores,heme).Seminalcontributionswere
then made by Pedersen and Lehn on crown ethers and
cryptands (1960s),7 respectively, and Cram on spherands
(1970s) (Figure 2).15 Moreover, the development of syn-
Molecular precursors A
thetic receptors introduced molecular recognition as an
area that blossomed into supramolecular chemistry by
Supramolecular chemistry
collaborative concepts that stemmed from biology and
physics. Supramolecular chemistry is now a major inter-
disciplinary field that embodies the expertise of synthetic
organic chemists, inorganic, and solid-state chemists, the-
orists, physicists, and biologists that strive to develop
new molecules and materials with unique properties and
applications.
A Guest
Host
B
Figure 1 An illustration of molecular versus supramolecular 3 SUPRAMOLECULAR INTERACTIONS
chemistry.
Supramolecular compounds are formed by additive and
Humphrey Davy in 1810.8,9 Development was initiated cooperative noncovalent interactions. The noncovalent
through the understanding of selective binding of alkali- interactions include a wide range of attractive and repul-
metal cations by natural,10 as well as synthetic, macro- sive forces. The most common noncovalent interactions,
cyclic, and macropolycyclic ligands11–17 as described by along with approximate energies, are listed in Table 1.8 A
Curtis (1961),11 Busch (1964),12 Ja¨ger (1964),13 and detailed understanding of the origins and scopes, as well
H C
3
O N H3C CH3
O O O O O H3C O CH3
O O
H3C CH3
O O O O O O O
O
O N H3C CH3
H C CH
3 3
(a) (b) (c) CH3
Figure 2 Early developments in supramolecular chemistry: (a) crown ether (Pedersen) (b) cryptand (Lehn) (c) spherands (Cram).
Table 1 Common supramolecular interactions.8
Supramolecularinteractions Directionality Bondenergies (kJmol−1) Examples
Ion–ion Nondirectional 100–350 NaCl
vanderWaals Nondirectional <5 Inclusioncompounds
Closed-shellmetal–metal Nondirectional 5–60 Argentophilic(Ag ···Ag)
Ion–dipole Slightlydirectional 50–200 Na+ crownethercomplex
Dipole–dipole Slightlydirectional 5–50 –C≡Ngroups
Coordinationbonds Directional 100–300 M-pyridine
Hydrogenbonds Directional 4–120 Carboxylicaciddimer
Halogenbonds Directional 10–50 Sulfur–iodinecomplex
π–π interactions Directional 2–50 Benzene(edge-to-face)DNA(face-to-face)
Cation–π andanion–π interactions Directional 5–80 +N(CH ) ·(toluene)
3 4
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Supramolecular interactions 3
as the interplay, of such interactions is a major goal of
researchers worldwide.
K
3.1 Directional versus nondirectional
Supramolecular interactions are used to construct assem-
blies of molecules and/or ions that exhibit specific proper-
tiesandfunctions.Directionalforcesareparticularlyuseful Figure 4 X-ray crystal structure of K+ encapsulated by
since geometric and spatial control of interacting species dibenzo-18-crown-6 (anion omitted for clarity).26
can be optimized.8,18–20 Nondirectional forces, however,
can be important in determining relative distances of inter- 3.3 Ion–dipole and dipole–dipole interactions
acting partners and, when integrated with covalent bonds,
can influence the orientations of partners in an assembly Ion–dipole and dipole–dipole interactions result from
process.8,18–20 an electrostatic attraction between an ion and a neutral
molecule with a dipole or two molecules with dipoles,
respectively. The interactions are intermediate to weak in
3.2 Ion–ion and van der Waals strength. The solvation of metal cations (e.g., hydration)
is dictated by ion–dipole interactions, while interactions
Ion–ion and van der Waals forces lie at extremes in between highly polar molecules (e.g., nitriles) are dic-
termsofstrength,withbothbeingnondirectional.Protypical tated by dipole–dipole interactions (Figure 4).26 Although
examples of materials sustained by ion–ion interactions the interactions are predominantly based on electrostat-
are the crystal structures of simple inorganic salts. The ics, a degree of directionality arises from the anisotropic
electrostaticattractionbetweenthecationandanionisoften nature of the polar molecules. Dipole–dipole interactions
manifestedinanisotropiclattice(e.g.,NaCl),reflectingthe can influence bulk physical properties (e.g., boiling point)
nondirectionalnatureoftheionicforces.Theintegrationof and, owing to relative weakness, will often give rise to
multiplespatialarrangementsandgeometriesofinteracting
formalpositiveandnegativechargesintomolecularspecies
molecules.
(e.g.,macrocycle),however,canachievedirectionalcontrol
of supramolecular association via ion–ion interactions
(e.g., host–guest systems) (Figure 3).21–23 van der Waals
3.4 Closed-shell metal–metal interactions
interactionsarisefromthepolarizationofanelectroncloud
by the proximity of an adjacent nucleus.24 Molecules
Interactionsbetweenclosed-shellmetalcationsofd8-d10-s2
considered “soft” exhibit the most pronounced van der
systems(e.g.,Ag(I))aresignificantcontributorstostabiliz-
Waals interactions, which will particularly be important in
ing the assembly of inorganic, organometallic, and metal-
a condensed phase (e.g., solid state) where solvent effects
are eliminated.18 organic subunits.27 The interactions are weaker in strength
than ionic and covalent bonds yet comparable to hydrogen
bonds.27 Prominent examples include interactions of Ag(I)
and Au(I) ions in the form of agentophilic and aurophilic
forces respectively (Figure 5).28,29 The interactions can be
Fe present in ligand-supported and ligand-unsupported envi-
ronments.vanderWaalsinteractionsinvolvingligandscan,
thus, impart directionality of an assembly process based on
closed-shell forces.
3.5 Coordination bonds
While the structural manifestations of coordination bonds
find deep roots as discussed in the the work of Werner30,31
Figure 3 X-ray crystal structure example of ion–ion inter- involving transition—metal—ion complexes, coordina-
actions demonstrated by the interaction of ammonium with tion bonds have found found widespread application in
Fe(CN)63−.25 supramolecular chemistry.32–34 Coordination bonds are of
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4 Concepts
Ag
Au
F
(a) (b)
Figure 5 Agentophilic and aurophilic interactions in crystalline: (a) dinuclear complex [Ag (trans-1-(4-pyridyl)-2-
2
(phenyl)ethylene) ][CO CF ] and (b) [2]rotaxane with a Au(P(CH ) ) + rod.28,29
4 2 3 2 3 3 2
intermediate strength and, similar to hydrogen bonds, H
Backbone
reversible. The selection of an inert or labile coordination N H O
Backbone
bond is crucial to the assembly of supramolecular struc- N N
O H O
tures as the dynamic nature of the latter allows the com- R R N N H N H
ponents of a supramolecular structure to undergo a series O H O
H N O H N H
of “error corrections” until the thermodynamically favored
Carboxylic acid Guanine H
productisachieved.Onceastablestructureisobtained,the
(a) (b) Cytosine
labile coordination bonds are capable of showing remark-
able cooperativity to impart enhanced ligand stability.35 Figure 6 Hydrogen bonding: (a) carboxylic acid dimer and
Coordination bonds also offer the panoply of tools sup- (b) DNA base pairing.
plied by the field of inorganic chemistry (e.g., coordination
number, chelation) to import function into supramolecu-
lar structures. Consequently, coordination bonds offer a nature, dictating the recognition of substrates by enzymes
unique and attractive means to modify the properties of and being responsible for maintaining the double-helical
supramolecular structures and materials via both metal structure of DNA (Figure 6b). The International Union
ions and ligands (e.g., magnetism, optical).36–41 Geomet- of Pure and Applied Chemistry (IUPAC) has recently
ric parameters that define coordination bonds as derived established the hydrogen bond as “an attractive interaction
from the field of inorganic chemistry are well docu- between a hydrogen atom from a molecule or a molecular
mented.42 fragment X–H in which X is more electronegative than
H, and an atom or a group of atoms in the same or a
different molecule, in which there is evidence of bond
3.6 Hydrogen bonds formation.”46
Hydrogen bonds can have different lengths, strengths,
and geometries (Table 2).8,16,43,47 A strong hydrogen bond
Thehydrogenbond,owingtoarelativelystrongandhighly
can resemble a covalent bond with respect to the energy
directional nature, is often described as the “master-key”
anisotropicinteraction.43Thefirstdescriptionofahydrogen required to break the interaction, while the energy of a
bond was provided by Linus Pauling in 193144 in a paper weak hydrogen bond will be closer to a van der Waals
force. Desiraju has described three extremes of hydrogen
describing the nature of the chemical bond. The term
addressed the incorrect assignment of a hydrogen bond bonds in the solid state. In the A–H···B hydrogen bond,
within the [H : F : H]− ion. Pauling defined the hydrogen when both A and B are quite electronegative; for example,
bond as a bond formed when the electronegativity of A N–H···O, the hydrogen bond is considered to be “strong”
relative to H in an A–H covalent bond is such that it can or “conventional” (20–40kJmol−1).16,47 On the contrary,
withdrawelectronsandleaveprotonspartiallyunshielded.45 if both A and B are moderate to weakly electronegative,
To interact with the donor A–H bond, the acceptor B for example, C–H···O, the hydrogen bond is “weak” or
must possess a lone pair of electrons or polarizable π “nonconventional” (2–20kJmol−1).
electrons. In general, a hydrogen bond can be represented The strength of a hydrogen bond can reach a mostly
as A–H···B, where a hydrogen atom acts as a bridge covalent level as in the HF2− ion (170kJmol−1). Hydro-
between two atoms A and B. A prototypical example that gen bonds involving interactions of N–H, O–H, and
alsoillustratesstructuralconsequencesofhydrogenbondsis C–H groups with double and triple bonds in the form
the carboxylic acid dimer, which is sustained by two O–H of C=C and C≡C bonds, respectively, as well as aro-
···O forces (Figure 6a). Hydrogen bonds are ubiquitous in matics, have also become important for understanding
SupramolecularChemistry:FromMoleculestoNanomaterials,Online2012JohnWiley&Sons,Ltd.
Thisarticleis2012JohnWiley&Sons,Ltd.
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DOI:10.1002/9780470661345.smc003
Supramolecular interactions 5
Table 2 Properties of hydrogen bonds.8,16,43
Strong Moderate Weak
A–H···B Partiallycovalent Mostlyelectrostatic Electrostatic
Energy 60–120kJmol−1 16–60kJmol−1 <12kJmol−1
Lengths(A˚) A–H∼H···B A–H<H···B A–H(cid:3)H···B
H···B 1.2–1.5 1.5–2.2 2.2–3.2
A···B 2.2–2.5 2.5–3.2 3.2–4.0
◦
Angles( ) 175–180 130–180 90–150
Examples Strongacids/bases; Acids;alcohols; Minorcomponents of
protonsponge;HF biologicalmolecules bifurcatedbonds;C–H
complexes ···O,O–H···π
the stabilization of crystals, host–guest interactions, and
biomolecules.48
3.7 Halogen bonds
The concept of halogen bonding was largely pioneered
by Resnati and Metrangolo in the 1990s and 2000s.49–52
The simplest definition of a halogen bond is an attrac-
tive interaction between an electron-deficient halogen atom F I
(i.e., the donor) and an electron-rich atom (i.e., the accep-
tor).49,53 Experimental and theoretical studies demonstrate
that the angle defined by the covalent bond and halo-
gen bond interaction involving the donor atom exhibits
a strong tendency toward linearity.54 The formation of Figure 7 Crystal packing view of α,ω-diiodoperfluoroalkanes
encapsulated through interactions between the host’s I− and the
a halogen bond can be accompanied with the elongation
guests iodine.
of the covalent bond that links the halogen atom to the
rest of the donor molecule.50 Halogen atoms attached to
electron-withdrawing substituents, such as perfluorinated approach of acceptor atoms, leading to the pronounced
or unsaturated (e.g., acetylenic, aromatic) groups, act as linear nature of the halogen-bonding interaction. Although
better halogen bond donors compared to parent hydrocar- computational and gas-phase studies have suggested that
bon residues (Figure 7).52 Less electronegative atoms and halogen bonds can adopt a considerable range of strengths
anionsmakebetterhalogen-bondacceptors.TheN-atomis, between10and200kJmol−1,50theforcestypicallyencoun-
thus, a significantly more efficient halogen-bond acceptor tered in solids lie between 10 and 50kJmol−1.57
than the O-atom, and the I-atom provides a better acceptor While there are very few studies of halogen bond
than very strong N-bases (e.g., dimethylaminopyridine).55 complexation in solution, there are numerous examples
The covalent bond elongation and linearity of halo- in the solid state.49,50,58,59 Early cases of halogen bonds
gen bond interaction are explained by the “σ-hole” con- involved simple molecules such as molecular iodine or
cept.49,56 The σ-hole represents the deformation of the iodoform.60 An example is the the cocrystal of molecular
electron density on the donor halogen atom, resulting in sulfur(S8)andiodoformwhereinthehalogenbondinvolves
theformationofanareaofdiminishedelectrondensitythat an unusually short separation of 3.52A˚ between iodine
roughly coincides with the σ*-antibonding orbital of the and sulfur atoms (Figure 8a).61 A series of isostructural
covalent bond between the halogen and remaining donor cocrystals has more recently been described by Cincˇic´ and
molecule. The area of positive potential is responsible for coworkerswhereinI-andBr-atomsresultedinisostructural
halogen bond formation through the attractive interaction solids when used as donors for halogen bond formation
with the electron-rich acceptor. Strong halogen bonding (Figure 8b).55 Auffinger and coworkers have proposed the
can lead to partial donation of the acceptor electron den- importance of halogen bonds in biology.62 A survey of
sity into the σ*-antibonding orbital, hence leading to elon- protein and nucleic acid structures revealed halogen bond
gation of the covalent bond. The σ-hole is encircled by contacts with possible stabilizing roles in protein folding
a ring of negative potential, which actively hinders the and ligand binding.63
SupramolecularChemistry:FromMoleculestoNanomaterials,Online2012JohnWiley&Sons,Ltd.
Thisarticleis2012JohnWiley&Sons,Ltd.
ThisarticlewaspublishedintheSupramolecularChemistry:FromMoleculestoNanomaterials in2012byJohnWiley&Sons,Ltd.
DOI:10.1002/9780470661345.smc003
