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Chirality

F. Richard Keene James Cook University, Townsville, Queensland, Australia

1 Introduction 1

2 Practical Aspects 23 Metallosupramolecular Chemistry 34 Helicates 85 Catenanes and Molecular Knots 106 Chiral Polyhedra 127 Conclusions 16References 17Further Reading 19

1 INTRODUCTION

Chirality (or handedness) is the property of an objectbeing not superimposable on its mirror image form. Sucha phenomenon arises when the object does not possess aplane, center, or inversion axis of symmetryit may stillpossess chirality if it has proper axes of symmetry, in whichcase it is referred to as dissymmetric ; if it has no symmetry,it is referred to as asymmetric .

Despite a continuing debate about the origins of molec-ular chirality, its presence in living systems is ubiquitous.The efciency of biological processes serves as a contin-ual challenge to chemists in terms of the design of articialsystems that possess a similar capacity in terms of stereose-lectivity and catalytic efciency, and the aspect of chiralityremains a key feature for such articial systems.

The development of articial supramolecular assem-blieswhere smaller component molecular species form

nite aggregates through noncovalent interactionsand thefactors involved in their formation have been addressed inother contributions of this series. The additional feature of supramolecular chirality arises when the assembly itself isasymmetric or dissymmetric. This situation can arise in oneof four ways:

1. the components of the assembly are chiral;2. the components are achiral but the aggregate is asym-

metric/dissymmetric, in which case a racemic mixtureof chiral forms results;

3. the creation of the structure involves a chiral compo-nent, and subsequent elaboration with achiral compo-nents preserves the memory of the initial chirality; and

4. a chiral capsule aggregate is formed around a chiralguest.

The phenomenon in (4) is to be distinguished from chiralrecognition , which relates to the interaction of a chiralguest with an existing chiral cavity in a supramolecularaggregate. Clearly, this alternative aspect has signicantrelevance in the design of supramolecular assemblies asthere are implications for the recognition of chiral targets.The creation of chiral cavities in supramolecular aggregatesmay profoundly inuence chiral catalytic processes thattake place within them, which has signicance becausesuch processes have a fundamental importance in synthetic

chemistry.The nature of the noncovalent interactions between

molecular components, which give rise to supramolecularassembly, can include hydrogen bonding (which is com-mon in natural systems), stacking, ion/dipole inter-actions, and dispersion forces. Somewhat controversially,coordination bond (metalligand) associations may also be

Supramolecular Chemistry: From Molecules to Nanomaterials , Online 2012 John Wiley & Sons, Ltd.This article is 2012 John Wiley & Sons, Ltd.This article was published in the Supramolecular Chemistry: From Molecules to Nanomaterials in 2012 by John Wiley & Sons, Ltd.DOI: 10.1002/9780470661345.smc015

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2 Concepts

included (giving the so called metallosupramolecular aggre-gates). For the case of the weak intermolecular noncovalentassociation that does not involve metal ligand interaction,there are a large number of examples, and these caseshave been well surveyed in a recent book on Supramolec-ular Chirality, to which the reader is referred. 1 Thepresent overview of chirality in supramolecular systemswill concentrate on aggregates formed using metalligandinteractions.

2 PRACTICAL ASPECTS

2.1 Separation of enantiomers (resolution)

As the vast majority of physical and chemical character-istics of chiral (enantiomeric) forms of a molecule areidentical (bond lengths, bond angles, polarities, meltingpoints, boiling points, spectra, etc.), the properties of enan-tiomers will also be identical unless the environment ischiral. The separation of a mixture of chiral isomers (race-mate) reects this situation. There are two general ways inwhich chiral forms may be separatedresolution or chro-matographyor they may be obtained by enantioselectivesynthesis.

1. Resolution of enantiomers can be divided into two cat-egories. The rst is the relatively rare phenomenon of spontaneous resolution, which requires that a racemiccompound crystallizes in a chiral unit cell. When theresultant crystals are of reasonable size, the enan-tiomeric forms have different (mirror image) geome-tries and may be separated visuallythe case of theseparation of the two enantiomeric forms of sodiumammonium tartrate tetrahydrate crystals by Louis Pas-teur being a classic example. However, more com-monly, resolution is achieved by the use of chiralauxiliaries: the two enantiomers of a racemic mixtureform compounds with an auxiliary of one xed chiral-ity, resulting in two compounds that are diastereoiso-mers ( 2 chiral centers) and since diastereoisomershave different physical properties, separation may beachieved by fractional crystallization, chromatography,and so on. For charged chiral ions, the use of achiral counterion will give rise to diastereoisomericsalts that can be separated. In both cases, the indi-

vidual enantiomers can be isolated from the separateddiastereoisomers.

2. Chromatography can also be used to separate chiralforms of molecules. Given the necessity for a chi-ral environment to differentiate enantiomeric forms,resolution can only be achieved where the chromato-graphic medium is chiral and/or the eluent is chiral. As

an alternative general procedure, since the separationof diastereoisomers is not subject to the condition of the chiral environment, diastereoisomers can be chro-matographically separated on an achiral support usingan achiral eluent, so that the conversion of a racemicmixture to a diastereoisomeric mixture using a chiralauxiliary (as in point 1) prior to chromatography maybe efcacious.

The chiral forms of molecules may in some cases beobtained by enantioselective synthesis. Such a proceduremay be achieved as a consequence of stereoselectivity , inwhich case a precursor is chiral and that congurationalfeature is conserved during the synthetic process, or whena chiral catalyst is used. In other cases, stereospecicitymay be utilized where a chiral feature in the precursorinduces a stereochemical preference in the other regionsof the product.

2.2 Identication and quantication of chirality

The measurement of chirality may be achieved in one of three ways X-ray diffraction, NMR spectroscopy, andchiro-optical methods.

1. Single crystal X-ray diffraction determinations may beused, and are aided by the presence of a chiral marker(such as provided by being in a diastereoisomericsalt). In the absence of such a reference probe, thedetermination of the conguration of an enantiomermay prove difcult, but in many cases can be achievedby an analysis that takes into account the phase of the diffused wave this procedure is aided whenthere are heavier atoms present so that it works moresatisfactorily for metal-containing species.

2. NMR is widely used in the determination of struc-ture and enantiomeric excess, but it cannot distin-guish between enantiomers per se . Accordingly, theenantiomers need to be converted to diastereoiso-mers which may be differentiated and thereforethe enantiomeric conguration derived. This can beachieved in a number of ways:

(a) In organic chemistry, enantiomers may be deriva-tized with a chiral substituent. In inorganicchemistry where ions are involved, the use of anenantiomerically pure counterion will give rise

to diastereoisomeric salts that can allow differ-entiation of the chiral forms under conditions of strong ion association.

(b) Chiral shift reagents usually chiral lanthanidecomplexesare added to the mixture of enan-tiomers, and in cases where they associate dif-ferently with the two enantiomers, it leads to


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Chirality 3

different shifts in some NMR resonances for thetwo enantiomers.

(c) NMR studies in the presence of a chiral sol-vent or in chiral liquid crystals can give rise todiastereoisomeric associations, which allow dif-ferentiation of the enantiomers.

3. The phenomenon of optical rotation, where two enan-tiomeric forms of a molecule will rotate the planeof polarized light equally in opposite directions, is afundamental property of chiral molecules, and is theorigin of the use of the rather strange term opticalisomers. Linearly polarized light has two compo-nents that are circularly polarized (helical) in oppo-site directions. These two components will travelthrough a chiral medium at different speeds, thusmodifying the polarization plane by an angle ,which is dependent on the concentration, solvent, andtemperature.

(a) For a chiral molecule, the specic rotation [ ]T may be quoted at a single wavelength. However,as does depend on , the [ ] is often recordedas a function of called the optical rotatorydispersion (ORD). In cases where the particularcompound absorbs, the refractive index goesthrough a maximum and minimum with a pointof inection, and as a consequence the ORD willshow a point of inection near that absorption,crossing the x -axis. The resultant inection pointis called a Cotton effect and can be negative orpositive depending on its direction. A substancemay give rise to a number of Cotton effects in

its ORD, but the curves will be strictly equal andopposite for the two enantiomers.(b) As the absorption coefcients for the two cir-

cularly polarized components will be differ-ent when the electromagnetic radiation passesthrough a chiral medium, a plot of against gives rise to circular dichroism (CD). As forORD, a Cotton effect will be observed in theregion of an absorption, and in some cases thesign of the Cotton effect(s) in the CD may beused to identify absolute conguration of anenantiomer.

(c) CD may also be observed in the infrared region(vibrational circular dichroism , VCD).

This discussion is intended to provide only a summaryof the practical aspects of the separation and identicationof enantiomers. More detailed descriptions and variationsare given in other excellent texts, to which the reader isreferred (in Further Reading at the end of this chapter) foradditional information.

3 METALLOSUPRAMOLECULARCHEMISTRY

One of the signicant features of metallosupramolecularchemistry is the potential for the assemblies to include a

multitude of structural motifs, arising from the variety of coordination geometries offered by the component metalcenters. In cases where there is planned complementarityin a geometric and/or electronic sense, the phenomenon of the self-assembly of very specic structures is possible. 2

The seminal work of Caulder and Raymond 3 and Stanget al .46 on high-symmetry architectures, and the deliberatedesign of coordination cages by Fujita et al .7 to probenonclassical molecular interactions are but some examples(among many) of this signicant area of current researchin coordination chemistry. In a similar way, the work of Dietrich-Buchecker and Sauvage 8 utilizing metal centers astemplates in the formation of intricate knots, catenanes, and

rotaxanes, and the development of the molecular machineconcept by the research groups of Stoddart, 9 Balzaniet al .,10 Sauvage et al .11 have shown the importance of the geometric versatility of the coordination environmentin supramolecular design.

Coordinated metal centers may also provide the addednuance of stereoisomerism: the component center(s) maypossess chirality (i.e., be asymmetric or dissymmetric sothat they are nonsuperimposable on their mirror image),or in cases where the coordination geometry of a cen-ter involves more than one relative orientation of lig-andsfor example, square planar (where two coordinationpositions can be disposed at 90 or 180 ), trigonal bipyra-midal (90 , 120 , or 180 ), or octahedral (90 or 180 )geometric isomerism/diastereoisomerism can give rise toalternate forms. Geometrical isomerism also arises inbis(bidentate) or tris(bidentate) centers where the bidentateligands are unsymmetrical. 12

Signicantly, while the development of molecular assem-blies in organic chemistry was built on a prior understand-ing of the nature of the geometry of the carbon atom(tetrahedral/trigonal/linear), the present recent advances inmetallosupramolecular chemistry have taken place not onlywith an additional complexity of the possible variationsin component geometries but also with less establishedmethodologies to control the stereochemistries of the com-ponent metal centers. Notwithstanding those vagaries, an

appreciation of the stereochemical issues has developedrapidly and there are many elegant examples of the grow-ing mastery of the area. There have been a substantialnumber of reviews and books which have probed aspectsof the issue of stereochemistry in metallosupramolecularassemblies, and the reader is referred to some of theseto gain an appreciation of the rapid progress. 1,13,14 This


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4 Concepts

chapter is designed to emphasize some of the guiding prin-ciples of chirality in terms of the construction of suchassemblies.

3.1 Building polymetallic assemblies with chiralcomponents

3.1.1 Stereochemistry in polymetallic assemblies

Stereoisomerism is possible in systems containing octa-hedral metal centers with bidentate ligands. 11 When theligands are symmetrical ( C 2 symmetry), chiral forms (and enantiomers) exist for the tris(bidentate) species(Figure 1), whereas for a bis(bidentate) complex involv-ing two monodentate ligands there are geometric isomers(cis/trans ) as well as enantiomers of the cis form. Whenthe bidentate ligands are nonsymmetrical, additional geo-metrical isomerism occurs. Within the class of mononuclearcomplexes containing bidentate ligands, the most detailedstudies were originally undertaken by Dwyer, Sargeson andtheir coworkers, predominantly using cobalt complexes of 1,2-diaminoethane (ethylenediamine) and its analogs. 1517

While there have been many subsequent studies that havebeen concerned with the stereochemistry of mononuclearspecies of bidentate ligands, the same basic principles aremaintained.

In polynuclear species, the stereoisomeric possibilitiesincrease exponentially with the number of metal centers.In such cases, the samples obtained under normal syntheticconditions will be a mixture of stereoisomers in an uncertainratio.

M M

Figure 1 Chiral (enantiomeric) forms of an octahedral tris(bidentate) metal complex.

H2N

H2N NH2

NH2N

N O

O

N

N N

N

N

N

N

N Ru(phen) 2

2 (phen) 2Ru

(phen) 2Ru

2+

4+

+

Figure 2 Stereoselective synthesis of diastereoisomers of [ {Ru(phen )2}2{ -dppz (11 11 )dppz }]4+ .19

3.1.2 Examples of stereoselective synthesisThere are several examples of stereoretentive reactions thatform oligonuclear species of predetermined stereochem-istry. A number of these have involved the condensation of appropriate amines with the coordinated 1,10-phenanthro-line-5,6-dione ligand in chiral precursor complexes of thetype [Ru(pp) 2(phen-5,6-dione)] 2+ (pp = 2,2 -bipyridine or1,10-phenanthroline), resolved by diastereoisomer forma-tion with the arsenyl-( + )-tartrate anion. 18 Lincoln andNord en19 have used this methodology to produce -and -[{Ru(phen) 2}2{dppz (11 11 )dppz }]

4+ (Figure 2),and using a similar procedure, Lehn et al .20 have reportedthe complex shown in Figure 3, in which the stereo-

chemistries of the two ruthenium centers are predeter-mined ( and ) by the use of the same - or -[Ru(phen) 2(1,10-phenanthroline-5,6-dione)] 2+ precursor.

In a related system, MacDonnell and Bodige haveused resolved precursors to form , , and diastereoisomers of the tpphz-bridged dimer (Figure 4). 21

The same research group used a similar method to obtaina D 3-symmetric tetranuclear species (Figure 5). 22

However, these examples are system specic and analternative and more generally applicable strategy wassought in which an enantiomerically pure chiral build-ing block would be used to produce mono- and dinuclear

N

N N

N

N

N N

N

N

N N

N

N

N N

N

(phen) 2Ru Ru(phen) 2

6+

Pt

Figure 3 Stereoselective synthesis of diastereoisomers of a trimetallic species. 20


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Chirality 5

N

N O

O

(phen) 2Ru

(phen) 2Ru

Ru(phen) 2

Ru(phen) 2

2+

+N

N

H2N

H2N

2+

N

N N

N N

N

4+

Figure 4 Stereoselective synthesis of diastereoisomers of [ {Ru(phen 2}2{ -tpphz }]

4+ .21

species of predetermined stereochemistry. The complexes[Ru(pp) 2(py) 2]2+ 21 {pp are bidentate polypyridyl ligandssuch as 2,2 -bipyridine (bpy) and 1,10-phenanthroline(phen); and their derivatives }, and [Ru(pp)(pp )(CO) 2]2+

(pp = pp )22 conveniently resolved by conventionaldiastereoisomer formation using the chiral forms of theO ,O -dibenzoyltartrate and antimonyl-tartrate anions, res-pectivelywere found to undergo substitution of the twomonodentate pyridine or carbonyl ligands (the latter in

a decarbonylation procedure) with complete retention of stereochemical integrity under conditions where the lengthof the reaction and the temperature were controlled. 23,24

These chiral precursors were used to synthesize a widerange of mononuclear and ligand-bridged dinuclear andtrinuclear species with predetermined stereochemistry. 2329

Additionally, Kane-Maguire and coworkers 30 have reportedthe resolution of cis -[Ru(phen) 2(CH 3CN) 2]2+ , which maybe used as a chiral precursor for further synthesis,including chiral neutral species such as cis -[Ru(phen) 2X2](X = CN , Cl ) which are difcult to obtain by othermeans.

Tzalis and Tor 31 reported the use of the bis(pyridine)precursor to produce chiral complexes of functionalizedphen ligands, which were subsequently linked to formdimers with predetermined , , or stereochem-istry (Figure 6).

In this work, these researchers also reported the synthesisof the and diastereoisomers of the trinuclearspecies, derived from the reaction of the -[Ru(bpy) 2(3,8-diethynyl-1,10-phenanthroline)] 2+ with two moles of - or

-[Ru(bpy) 2(3-bromo-1,10-phenanthroline)] 2+ (Figure 7).Yin and Eisenbaumer 32 have reported the use of the same

bis(pyridine) precursor to synthesize homochiral dinuclearand tetranuclear complexes of the bridging ligands 2,2 -bibenzimidizole and bis(2,2 -bibenzimidizole), respectively(Figure 8).

OO

NN

N N N

N

N

N N

NN

NN

NN

NN

NN

Ru

NN N

N

NN

N

NN

N RuRu

N

NN

N

NN N

N N

NNN

N

O

O

OO

Ru

Ru

Ru

2+

2+

+ 3 equivalents

H2N

H2N

3

8+

Figure 5 Synthesis of a stereochemically controlled D 3-symmetric tetranuclear complex. 22


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6 Concepts

N

N

(bpy) 2Ru Ru(bpy) 2

Ru(bpy) 2 Ru(bpy) 2

2+

+

N

N

2+

Br

N N N N

4 +

Figure 6 Stereoselective synthesis of diastereoisomers of alkyne-bridged dinuclear species. 31

N

N

2 (bpy) 2Ru

2+

+

N

N

Ru(bpy) 2

2+

Br

N N N N

Ru(bpy) 2Ru(bpy) 2

6+

N N

Ru(bpy) 2

Figure 7 Stereoselective synthesis of diastereoisomers of alkyne-bridged trinuclear species. 31

3.1.3 Heterochiral systemscombiningstereoselective synthesis and chromatography

In the examples reported on the use of the stereoreten-tive methods (including those cited above), it is importantto note that the products are almost invariably homochi-ral. In many cases, access to the heterochiral analogsrequires the additional use of chromatographic techniques.Keene et al .3335 have provided a cation-exchange strategyin which the eluent counteranion has been chosen so that itdifferentially associates with the stereoisomers to be sepa-

rated, which are predominantly cationic. The technique hasallowed the routine separation of diastereoisomers, the res-olution of enantiomers, the separation of geometric isomers,and the separation of chiral helical forms. Importantly,the combination of the stereoselective and chromatographictechniques has been used for the separation of stereoisomersof dinuclear and trinuclear species.

By way of examples, for the trinuclear complexes involv-ing the ligand 1,4,5,8,9,12-hexaazatriphenylene (HAT),the stereoselective synthesis using the chirally resolvedforms of [Ru(pp) 2(CO) 2]2+ gave rise to the enantiomersof the homochiral forms of the species [ {Ru(pp) 2}3( -HAT)] 6+ (pp = bpy or phen). 27 However, following thesynthesis of a mixture of the homochiral and heterochi-ral forms (Figure 9) using the racemic [Ru(pp) 2(CO) 2]2+

precursor, these diastereoisomers were separated chromato-graphically, and the enantiomers of the homochiral formsubsequently resolved by chromatographic methods. 27 Theenantiomers of the heterochiral trinuclear species wereobtained by reaction of the chromatographically resolvedforms of the rac diastereoisomer of the dinuclear com-plex [ {Ru(pp) 2}2( -HAT)] 4+ ( and ) with racemic[Ru(pp) 2Cl2] and then chromatographically separating theresultant 3/ 2 (or 3/ 2 ) product mixtures. Thisprocedure was extended to the isolation of the ,

, , , , and forms of the

homoleptic heteronuclear species [ {Ru(bpy) 2}2{Os(bpy) 2}( -HAT)] 6+ (where the asterisk refers to the Os cen-ter), and the isolation of the eight stereoisomeric forms( b p m / b p m , b p m / b p m , b p m / b p

m , b p m / b p m) of the heteroleptic homometal-lic trinuclear complex [ {Ru(bpy) 2}{Ru(phen) 2}{Ru((CH 3)2bpy) 2}( -HAT)] 6+ {the subscripts b, bpy; p, phen; m,(CH 3)2bpy (4,4 -dimethyl-2,2 -bipyridine) }.28

3.1.4 Stereospecic synthesischiral buildingblocks and the chiragens

The underlying principle of stereospecicity is that a chiralprecursor metal center will impose stereochemical identityon the other metal centers in oligomers derived from it. Inan imaginative approach, von Zelewsky and his coworkersachieved the chiralization of the bidentate bpy ligand byfunctionalization with the naturally occurring chiral species( )-myrtenal, 36,37 giving rise to the chiral [4,5]-pineno-2,2 -bipyridine ligand shown in Figure 10(a). These ligandsundergo a regioselective deprotonation whereby two suchmoieties may be linked by a spacer to give the Chiragenseries of tetradentate ligands, an example of which isillustrated in Figure 10(b).

The linkages used have included an alkyl chain {denotedCG[ n ]; n = 0, 3, 47},3639 bpy {denoted CG[bpy] },39

or o-, m-, or p -xylendiyl {denoted CG[ o/m/p -xyl] }.4042

The Chiragens CG[ n ] where n > 4 show stereospeciccoordination to the octahedral metal centers ruthenium andosmium, 37 and the same ligands have been used to controlthe stereochemistry of the general species of the type[Ru(CG[ n ])Cl 2] and [Os(CG[ n ])X 2]m+ (X = Cl , m = 0;X = DMSO, m = 2), 42,43 which may be used as precursorsin the syntheses of higher nuclearity assemblies.


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Chirality 7

N

HN

N

HN

2 -[Ru(bpy) 2(py) 2]2+ +

4 -[Ru(bpy) 2(py) 2]2+ +

N

N

N

N

Ru(bpy) 2

Ru(bpy) 2

Ru(bpy) 2

Ru(bpy) 2

Ru(bpy) 2

Ru(bpy) 2

2+

( )

N

HN

N

HN

N

HN

N

HN

N

N

N

N N

N

N

N

4+

( 4-Ru)

Figure 8 Stereoselective synthesis of homochiral tetranuclear complex incorporating the bridge bis(2,2 -bibenzimidizole). 32

(a) (b)

Figure 9 Chem 3D representation of diastereoisomeric forms of [ {Ru(bpy) 2}3 ( -HAT )]6+ : (a) heterochiral { }; (b) homochiral

{ ( ) }.

N

N N

N

[X]

(CH 2)n

N NH2C CH 2

CH2

CH 2X =

N N

(a) (b)

Figure 10 Chiragen ligands: (a) [4,5]-pineno-2,2 -bipyridine, one of the chiral building blocks of the Chiragen ligands; and (b) anexample of a tetradentate Chiragen ligand.


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8 Concepts

N N

N

N

N

N

(a) (b)

Figure 11 [4,5]-dipineno-2,2 -bipyridine (a), and the superchi-ragen[0] ligand (b).

The ligands CG[ n ] (n = 0, 3) and the CG[bpy] do notcoordinate as tetradentate ligands: however, they coordinateto a metal center in a bidentate manner and therefore arepotential bridges. They have been incorporated in di- or

trinuclear species: in such systems stereospecicity wasobserved, induced by the chirality of the metal centersinvolved. 43

A Chiragen based on the dipineno precursor(Figure 11a) has also been developed {designated super-chiragen[0] or SGS[0] } (Figure 11b) and shows similarbehavior to its CG[0] analog. 39,43

The tetradentate Chiragen ligands have made a signif-icant contribution to the transfer of chirality in metallo-supramolecular synthesis. This is discussed below with alimited number of illustrative examples, and it has beensubstantially reviewed. 14,44

4 HELICATES

The helix is a motif prevalent in chemistry and biol-ogythe Crick and Watson canonical double helicalstructure being but one of many well-known examples. Thename helicate was proposed to specically refer to helicesformed using interactions between metal centers and appro-priate ligands. In all examples, a helix is associated with asense of the screw around the helical axisin which caseit is chiral. The chirality is dened by whether the screwsense is described by the right hand (the ngers indicate thedirection of the screw progressing the helix in the directionof the extended thumb) or the left handdesignated plus

(P ) or minus ( M ), respectively, and the pitch is the distancebetween two turns (Figure 12).

The observed chirality in helicates can arise from theinherent asymmetric spiral arrangement of the molecularcomponents, even if the building blocks are achiral; oralternatively, it may result from the use of chiral bridgingagents so that the chirality is transferred to the coordinated

Pitch

M P

Figure 12 The P and M helical forms, showing the pitch of

a helix.

metal center which ultimately controls the supramoleculararchitecture.

As shown in the schematic diagram in Figure 13, theconnection of two or more bidentate coordinating units(often 2,2 -bipyridine or 1,10-phenanthrolinesuch as theligand shown below in Figure 14) with appropriate spacerswill produce extended chain ligands that allow the forma-tion of double-stranded helices when the metal centers aretetrahedral (Figure 13a), but triple-stranded helices whenthe metal centers are octahedral (Figure 13b). In a similarmanner, the connection of two (or more) tridentate units

(e.g., terpyridine; such as the ligand shown in Figure 15)by appropriate spacers leads to extended chains, which areideal for the formation of double-stranded helicates with

(a) (b)

Figure 13 (a) A schematic representation of a trimetallicdouble-stranded helix (formed using a tri-bidentate strand withtetrahedral metal centers, or a tri-tridentate strand with octahedralcenters); and (b) a dimetallic triple-stranded helix (formed usinga di-bidentate strand with octahedral centers).


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Chirality 9

N

N

O

N

N

O

N

N N

N N

N

N

N

N

N

O O

O O

N

N

N

N

23 Cu(I)

Cu +

Cu +

Cu +

Figure 14 A trimetallic double-stranded helix Cu 3L2 (formedusing a tri-bidentate strand) with the tetrahedral Cu(I) metalcenters. 47

N

N N

N

NN

Fe

N

N N

N

NN

Fe

4+

Figure 15 A dinuclear double-stranded Fe(II) helix. 47,48

metal centers that prefer an octahedral geometry (for whichFigure 13a is appropriate). In general, where nonchiral lig-and chains are used, helicates will usually be formed asa racemic mixture of the two homochiral enantiomers: inrare circumstances, it is possible to obtain only one of the helical isomers by spontaneous separation during crys-tallization. 45 However, by incorporating chiral substituentson the ligand chain, a particular twist may be induced onthe resultant helicate. There are many examples of double-

stranded and triple-stranded helicates reported which willadhere to the above principles, although some additionalcases exist where particular structural features of the spac-ers between the coordinating motifs, or specic effects suchas stacking within the chains, have given rise to addi-tional features: this has been extensively reviewed 14,46 andonly some illustrative examples are given here.

In terms of double-stranded helices, the original exampleswere reported by Lehn et al . and involved chains of oligo-bipyridine ligands linked by the tetrahedral coordinationcenter Cu(I): an example is shown in Figure 14 of the prod-uct formed in a self-assembly process. 47 Other analogouslonger (and shorter) double helices have been reported. 14

Subsequent studies by numerous research groups haveproduced many other analogous examples, where oligo(catechol), oligo(1,10-phenanthroline), oligo(pyridine/ imine), and a series of chain ligands containing variouscombinations of pyrimidine, imidizole, benzimidazole, andother heterocyclic donors, have been used. 46 In virtuallyall examples where crystal structures have been performed,homochiral helices have been observed, but obtained asracemates given the achiral nature of the components.Accordingly, positive cooperativity is thought to occur torealize the homochiral helix.

Where octahedral centers are involved, double-strandedhelical structures may be formed using chains with repeat-

ing tridentate units, such as an oligoterpyrine (Figure 13a).A simple example is the di-iron double helicate formedby self-assembly upon reacting Fe(II) and the appropriateligand in equimolar proportions 48,49 (Figure 15).

Similar to the bis(bidentate)copper(I) centers in the ear-lier examples, the octahedral bis(tridentate)iron(II) com-ponent centers are congurationally achiral, the moleculeforms as a racemic mixture of the two chiral helical forms,and there are many examples of double-stranded helices of this genre. 46 In this particular instance above, the two enan-tiomeric forms were separated by cation-exchange chro-matography, using an eluent with a chiral anion. 49

There have been a signicant number of examples of triple-stranded helicate structures, generally formed by self-assembly, using octahedral metal centers and ligand strandcontaining bidentate coordinating groups (Figure 13b). 46

Again, in cases where the ligand is congurationallynonchiral, the product will be a racemate but almostinvariably each helix will be homochiral.

Circular-stranded helicates have also been reported. 14,46

These involve a cyclic arrangement of the metal centers,with bridging ligands that loop around the metals andconnect them: in the case of the di- to hexanuclear species,the trinuclear and hexanuclear species (see Figure 16)have been involved in cases where the chirality has beenaddressed.

In the case of the ligand shown in Figure 17(a)in

which a carbon atom of each of the 1,3-oxazoline ringsis asymmetrican enantiomerically pure trinuclear circu-lar helicate was formed with Ag(I) in which the metalcenters are di-coordinated. 50 In a case involving the Chira-gen ligand shown in Figure 17(b), a hexanuclear circularhelicate formed 51,52 (again with Ag(I), but tetrahedrallycoordinated) is shown in Figure 17(c).


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10 Concepts

(a) (b)

Figure 16 Schematic representations of a trinuclear circularhelicate (a) and an hexanuclear circular helicate (b).

N

N

N

O

OPh

Ph

N

N

N

N

(a) (b)

(c)

Figure 17 (a) Ligand used in the formation of trinuclear cir-cular helicate Ag I3L3

50 ; (b) [5,6]-Chiragen ligand used in theformation of hexanuclear helicate M 6L6 {M = Ag (I) or Cu(I) },with complete enantioselectivity 51,52 ; (c) crystal structure viewsof hexanuclear helicate [Ag 6(L) 6]6 .51 (Figure 17(c) reproducedfrom Ref. 51. Wiley-VCH, 1998.)

The control of the chirality in the formation of helicatesby the use of chiral ligands is demonstrated clearly by thesetetradentate Chiragen ligands, which has been substantially

reviewed. 14,44 As a particularly illustrative example, usingthe [4,5]-Chiragen bearing an m-xylyl spacer (Figure 18),with Cd(II), Zn(II), and Fe(II) the major species formedare triple-stranded helicate species, which have an M 2L3stoichiometry with enantiomerically pure structures. 53

However, the related [5,6]-Chiragens (Figures 17b and18) are somewhat more sterically demanding, and do not

N

N

N

N

N

N

N

N

[Bridge][Bridge]

[4,5]-Chiragen [5,6]-Chiragen

BRIDGES:

p -Xylyl m -Xylyl

Figure 18 [4,5]- and [5,6]-Chiragens.

form well-dened assemblies with octahedral metal centers,and for this ligand with a p -xylyl spacer with tetrahedralions such as Ag(I) or Cu(I), the single-stranded circularhexanuclear helicate results, with complete enantioselectiv-ity. 44,51,54

5 CATENANES AND MOLECULARKNOTS

5.1 Catenanes

One of the signicant developments in the eld of supramolecular chemistry in the last two decades has beenthe formation of catenanes (interlocked rings) and molec-ular knots. The concepts were initially developed largelythrough the work of Dietrich-Buchecker and Sauvage, 55

although other groups have since expanded on the theme.The dominant strategies have involved the templating actionof a tetrahedral Cu(I) center to two bidentate phen-type lig-ating moietiesincorporated either in two open chains thatcould subsequently be end-joined (generally using a di-iododerivative of pentaethylene glycol) to give the interlockedrings, or with one incorporated in a previously formed ringthrough which an open ring was threaded followed by sub-sequent closure. The resultant metal-coordinated species(called a catenate ) was subsequently demetallated to formthe catenane with two interlocking macrocyclic rings. Theformer strategy is shown in Figure 19 for the simplestexample of a [2]catenane.

For the simple case with the symmetrical phen-containing

threads, such as shown in Figure 20(a), the resultant[2]catenane is nonchiral. However, in the case wherenonsymmetrically substituted phens are utilized (such asin Figure 20b), the resultant catenane is chiral, and in thiscase the catenate formed (shown on the left of Figure 20c)was shown to be chiral using 1H NMR in the presence of a chiral reagent. 57


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Chirality 11

2+ Cyclization

Figure 19 Template synthesis of a [2]catenane. The U-shaped fragment contains a bidentate coordinating moiety, and the black dotrepresents a tetrahedrally coordinating metal ion. (Reproduced from Ref. 56. Springer, 1993.)

OO

O

O

O

O

O

O

O

O

O

N

NN

N

O

Cu +

N

N

OH

OH

N

N

OH

OH

(a)

(c)

(b)

N

N

O

OO

O

O

O O

O

O

O

N

N

O

O

Figure 20 (a) A symmetrical bidentate chelating ligand used in the formation of catanates/catenanes; (b) the nonsymmetricallysubstituted phen-containing ligand used in the formation of the chiral catenate (c; on the left) and its catenane (right). 57 (Reproducedfrom Ref. 56. Springer, 1993.)

5.2 Topological chirality

The nature of the chirality in the case of this catenaneis notable as it raises the concept of topological chirality .The normal (Euchlidian or geometric) denition of chiral-ity is based on the nonsuperimposability of rigid molecularobjects: in symmetry terms this means that these forms donot possess a center, plane, or improper axis of symme-try (as discussed earlier in Section 1). Such chiral formsmay be theoretically interconverted by pathways in whichbonds are bent, compressed, or stretched rather than bro-

kenthe concept of the and forms of tris(bidentate)metal complexes being theoretically interconverted by thetrigonal (Bailar) twist through an achiral ( D 3h ) trigonaltrismatic transition state is a simple example. However, ininstances where the molecule is chiral albeit being subjectto deformation in 3D space, the concept of topological chi-rality results in which the mirror image forms only by a

chiral pathway, and racemization cannot be achieved with-out the breaking and reforming of chemical bonds. In auseful analogy, Chambron et al .58 have characterized suchmolecules as molecular rubber gloves, in the sense thatremoving a right-handed glove from the right hand by peel-ing it off while turning it inside out results in an objectwhich is superimposable on a left-handed glovebut atno point did the glove ever attain an achiral conformation.Clearly in the case, the catenane has topological chirality.

This concept will not be further dealt with in this chapter,but has been discussed in the literature, and the reader is

directed to those reviews. 56,58

5.3 Higher catenanes and molecular knots

When the strategy shown in Figure 19 is extended tohigher catenane analogs and to molecular knots, additional


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12 Concepts

P R

SQ

PR + QS

PQ + RS

PS + QR

2

2

2

Figure 21 The three possible connections between the four ends of a dimetallic double-stranded helicate. (Reproduced from Ref. 56. Springer, 1993.)

chirality considerations arise. For example, in the formationof the simplest knotcalled a trefoil knot the strategyinvolves the use of threads containing two bidentate coor-dinating groups. As shown in Figure 21, the methodologyrequires the formation of a molecular thread, which is heli-cal and therefore inherently chiral and which can be linkedas described above for the [2]catenane. There are three pos-sible outcomes, and only the one resulting from the P-Rand Q-S end-closures the trefoil knotis (topologically)chiral (the two chiral forms are shown in Figure 22).

Two of the di-bidentate chains used in this methodologyare shown in Figure 23. Using an m-phenylene (a) {ratherthan an n-butyl (b) } linkage between the phenanthrolinemoieties, the resulting di-copper knot was resolved usingdiastereoisomer formation with a chiral anion, and the topo-logical chirality of the demetallated species was conrmedby NMR studies using a chiral shift reagent. 59,60

A related chiral di-bidentate Chiragen chain has also beenexploited to form stereoselectively determined molecularknots. 61 The molecular thread I (shown at the left inFigure 24) incorporates two bidentate chelating units: two

Figure 22 Topologically chiral forms of a trefoil knot.

N

N N

N

OH OH

N

NN

N

OH

OH

(a)

(b)

Figure 23 Di-bidentate ligands used in the formation of double-stranded helicates with tetrahedrally coordinating metal centers.

are joined by the copper(I) centers giving only one chiralform of the double-stranded helix (II). The appropriateconnection affords the trefoil knot enantioselectively.

6 CHIRAL POLYHEDRA

A number of supramolecular polyhedra are chiral, some-times because of the dissymmetric property of the shapeitself (e.g., a tetrahedron has T symmetry), or as a conse-quence of the coordination mode the ligands adopt relative


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Chirality 13

NN N

N

O

O

O

O

O

O

O

O

Figure 24 Formation of a chiral trefoil knot using a Chiragenligand. 61 (Reproduced from Ref. 61. Wiley-VCH, 2004.)

to the metal centers that form the polyhedron. This areahas been well documented in two recent book reviews 62,63 :

this chapter will draw attention to the principles involvedwith a few illustrative examples, rather than attempting tobe comprehensive.

6.1 Tetrahedra

Much of the seminal work in this area has been done bySaalfrank and coworkers, and by Raymond and coworkers.In these rst examples, four octahedrally coordinating metalcenters were connected by six di-bidentate ligand strands(M 4L6; Figure 25a).

In an example using the ligand shown in Figure 25(b(i)),developed in situ during the reaction, X-ray structuralstudies showed that a tetrahedral complex Co 4L6 involvingfour Co(II) centers was formed as a racemic mixtureof homochiral and enantiomers, whichpossess T point group symmetry. 64 Interestingly, in acomplex of the same M 4L6 formulation involving therelated ligand shown in Figure 25(b(ii)) and Fe(III) as themetal center, X-ray crystal studies revealed the formation of a tetrahedral metal core with an achiral meso conguration

.65

(a) (b) (c)

(d) (e)

O

O

CO 2R

OR

O

O

CO 2R

OR

(i)O

O

CO 2CH 3

OCH 3

O

O

CO 2CH 3

OCH 3

(ii)

O

HNOHHO

HN OHOH

O

(iii)(ii)(i)

N N N N

N

NN N

N

N

Figure 25 A schematic representation of the tetrahedral supramolecular assembly M 4L6 with four octahedrally coordinating centerslinked by di-bidentate ligands (a): di-bidentate ligands involving acac-type bidentate coordinating entities (b) 64,65 ; di-bidentate ligandsbased on catecholate bidentate coordinating entities (c) 66 ; di-bidentate ligands containing the pyrazolyl-pyridine coordinating group,with a series of linkers (d) 67,68 ; a quaterpyridine ligand (e). 69


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14 Concepts

N N N

N

N

(ii)(i) (iii)

NHHO

HO

HO

OH

OH

OH

OH

OH

HN

NH

HO

NN

B

N

O

O

O

O

O

O

BuO

BuO 2C

OBu

OBu

CO 2Bu

CO 2Bu

(a) (b)

N

Figure 26 A schematic representation of the tetrahedral supramolecular assembly M 4L4 with four octahedrally coordinating centerslinked by tripodal ligands containing three di-bidentate entities (a) examples of the tripodal ligands, (b)(i), 72 (ii), 73 and (iii). 74

Ward and coworkers reported a series of Co 4L6 com-plexes involving the series of di-bidentate ligands con-taining the pyrazolyl-pyridine coordinating group shownin Figure 25(d), in which simple anions were incorporatedin the cavity. 67,70 For the quaterpyridine ligand shown inFigure 25(e), reaction with Fe(II) forms an Fe 4L6 tetra-hedron, again incorporating simple anions. 68 In all theseexamples, the assembly is formed as a racemic mixtureof homochiral and enantiomers. Interest-ingly, for the quaterpyridine ligand, reaction with RuCl 3under reducing conditions formed a [Ru 2L3]4+ helix (of the type shown in Figure 13b), for which the enantiomeric

and forms were separated using a cation-exchangetechnique described earlier using a chiral eluent containingthe anion ( )-O ,O -dibenzoyl- l-tartrate (Section 2.1.2). 69

Raymond et al . reported the self-assembly of a series of di-bidentate catecholate ligands (Figure 25c) with a numberof trivalent and tetravalent metal centers to produce chiralM4L6 tetrahedra. Each of the metal centers has octahedraltris(bidentate) coordination, with exclusive formation of aracemic mixture of the homochiral arrangements. However,the structures possess a cavity, and this research group wasable to resolve the racemic mixture of Ga 4L6 , where L isthe ligand shown in Figure 25(c(i)), using the chiral ( )-(S )-methylnicotinium cation as a guest, where the enantiomer preferentially precipitated as the least solublediastereoisomeric form. 66 The congurational stability of this species, investigated by 1H NMR, was found to beremarkable. 71

There are also a series of tetrahedral assemblies withthe formulation M 4L4 . In this case, the three octahedrally

coordinating metal centers on any face of the tetrahedronof metal centers are jointed by trigonally symmetric tri-bidentate tripod ligands, shown schematically for onesuch ligand in Figure 26(a). Three examples of such ligandsare shown in Figure 26(b): Ward et al . reported M 4L4assemblies with tris(pyrazylpyridine)borate (i), 72 Raymondet al . using tri-catecholate tripod (ii), 73 and Raymond,

N

N

N

N

N

N

N

N

(a) (b)

N

Figure 27 (a) Tridentate ligand used by Fujita et al .76 to formthe chiral octahedral cage shown in (b).

Saalfrank et al . with tri- -diketonate species (iii). 74 It isnoted that the metal centers at the vertices of the tetrahedrawere homochiral, and when the ligands were manipulatedto enlarge the cavity in the tetrahedron cage that cationicguests could be encapsulated. 75

6.2 Trigonal (anti)prisms and octahedra

There are limited examples of chiral trigonal prismaticand antiprismatic assemblies: in general, they are isolatedexamples and the homochirality arises either from theasymmetric orientation of the ligands in the framework,or where the achiral framework is rendered dissymmetricby the encapsulation of a guest. 62

An interesting example of a chiral octahedral cage M 6L4was reported by Fujita et al . involving the tridentate ligandshown in Figure 27(a). 76 The six metal centers involvethe Pd(L 2) moiety, where L 2 is a bidentate ligand bpyor en. The ligand adopts a facial coordination to theoctahedron, but as shown in the schematic representation inFigure 27(b), while one pair of ligands share one metal eachfor coordination of their central pyridine groups (shown in


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Chirality 15

NH N

HNNH

NHHN

HN

N

NHN

HNNH

= Zn 2+N

N

N

S

S S

(b) (c)(a)

Figure 28 A (a) cuboctahedron; (b) the tripod ligand; and (c) trithiocyanurate anion (TCA 3 ) linker used in the formation of thechiral cuboctahedron. 77

AcO

AcO Pt OAcOAc

AcO

AcO Pt OAcOAc

Pt PtEt3P PEt 3 Et 3P PEt 3

PEt 3

PEt 3

PEt 3

PEt 3

R

R

(a)

(b)

Figure 29 (a) 2,2 -diacetyl-1,1 -binaphthyl-6,6 -bis(ethyne); and (b) the n = 4 chiral polygon derived from it using the trans -Pt(Et 3)2linker. 78,79

red in the diagram), the other two ligands have their centralpyridine groups coordinated to trans -disposed metal centers(shown in purple), with the three particular metal centershaving a meridional disposition. The resultant octahedralstructure has C2 point group symmetry and is thereforedissymmetric.

6.3 Cuboctahedra

In the nal example of the polyhedra, an example of distorted octahedron has been reported. 77 The cuboctahe-dronshown in Figure 28(a) has eight triangular and sixsquare faces. In the structure, four complexes involvingthree Zn(II)-containing cyclen macrocycles (Figure 28b)attached to a phenyl ring, and four trithiocyanurate anions

(TCA 3 ; Figure 28c), are linked so that each TCA 3 com-bines with three Zn-cyclen moieties from separate com-plexes, forming a triangular face of the cuboctahedron.There is a rotational offset imposed and this twist (eitherclockwise or anticlockwise) renders the assembly chiral.Chiral induction was achieved by encapsulation of a chiralguest molecule.

6.4 Chiral molecular polygons

Lin and coworkers have reported a detailed study usingchiral bridging ligands to generate chiral polygons. 78,79

These rigid atropisomeric ligands such as shown inFigure 29(a)were reacted in enantiomerically pure formwith trans -[Pt(PEt 3)2Cl2] to produce a range of chiral


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16 Concepts

N

N

RO

RO

N

N

RO

RO

RO

RO

N

N

Cl

Cl

Figure 30 Angular di-pyridine ligands based on the 1,1 -binaphthyl framework.

CH3O

CH3O

N

NO

O

CH3O

CH3O

N

N

OCH 3

OCH 3

N

N

M

M

NH2H2N

NH2H2N

4+

(a) (b)

Figure 31 (a) Angular di-pyridine ligands with chiral backbones; and (b) a chiral metallocycle derived from one of the ligands in(a) with M(en) as a linker {M(II) = Pd or Pt }.83,84

metallo-macrocyles of different sizes ( n = 38; the product

with n = 4 is shown in Figure 29b).This strategy was subsequently extended to produce a

series of chiral mesoscopic macrocycles of controllablesize. 80,81

A similar methodology was also employed with therelated series of di-pyridine ligands shown in Figure 30. 82

The further incorporation of a chiral backbone into thesedi-pyridine ligands (Figure 31a) was also used to preparechiral supramolecular species, an example of which isshown in Figure 31(b). 83,84

Stang et al . proposed a number of strategies to cre-ate chiral supramolecular assemblies in self-assembly pro-cesses. 85 By way of examples, the chiral complex shown in

Figure 32(a) {M = Pd, Pt; OTf

= triuoromethanesulfo-nic acid anion (triate) } can be treated with the bridg-ing ligand bis[4-(4 -pyridyl)phenyl] iodonium triate toproduce the chiral square shown in Figure 32(b). 84 Arelated reaction involving the alternative bridge such as2,6-diazaanthracene realized a single diastereoisomer of thesquare shown in Figure 32(c). 86,87

7 CONCLUSIONS

This chapter has attempted to provide a basic insight intosome of the principles involved in chirality in metallo-supramolecular chemistry, without the distraction of beingdispassionately inclusive of the myriad of examples thatsupport those principles, or have minor variations fromthem. These examples have been well covered in excel-lent reviews and other books which are cited in the textand/or are listed in the Further Reading that concludesthis chapter.

One of the pervasive outcomes of the development of supramolecular chemistry is that the assemblies possesscavities, the size and shapeand natureof which can be

controlled by appropriate manipulation of the components.Procavities are sometimes used by species, which act astemplates in the actual initial construction of the assembly,and the cavities of formed assemblies can clearly be used ashost centers. The enticing logical prospect is that in a chiralassembly the cavities have inherent chiral characteristics, sothat for guest molecules there are consequences associated


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Chirality 17

P

PM

PhPh

PhPh

OTf

OTf

N N I+

N

N

MNNI+N

N

M

PHPh 2P

P

Ph 2P

Ph 2

Ph 2

6+

N

N

N

N

N

N

N

N

Ph 2P PPh 2

PPh2

Ph2P

M

M

P

P P

PM M

8+

(a) (b)

(c)

Figure 32 (a) Chiral metal building unit [ M( R-(+ )-BINAP(OTf) 2] {M(II) = Pd , Pt} used by Stang and coworkers to produce a chiralsupramolecular squares (b) 86 and (c). 86,87

with chiral host environment. It follows that there arepotential applications in areas such as asymmetric sensing,asymmetric catalysis, surface interactions, and nanoscienceapplications such as Molecular Machinesjust to name

a few. Some of these concepts are discussed in articlesincluded below in the Further Reading, but they will alsobe expanded within this present series. The possibilitiesin terms of the design of metallosupramolecular speciesare limited only by the imagination, and so the furtherincorporation and exploitation of the chiral environment inmetallosupramolecular chemistry promises to be one of the

most exciting aspects of nanoscience in the immediate andintermediate future.

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FURTHER READING

M. Albrecht, Lets Twist Againdouble-stranded, triple-stranded, and circular helicates. Chem. Rev. , 2001, 101 ,34573497.

H. Amouri and M. Gruselle, Chirality in Transition Metal Chem-istry: Molecules, Supramolecular Assemblies and Materials ,John Wiley & Sons, Inc., Chichester, 2008.

J.-C. Chambron, C. O. Dietrich-Buchecker, and J. P. Sauvage,From Classical Chirality to Topologically Chiral Catenandsand Knots , Topics in Current Chemistry , Springer Verlag,Heidelberg, 1993, vol. 165.

(a) M. Crego-Calama and D. N. Reinhoudt, eds. Supramolecu-lar Chirality , Topics in Current Chemistry , John Wiley &

Sons, Inc., Chichester, 2006, vol. 265; (b) with particularrelevance to metallosupramolecular species, see G. Seeber,B. E. F. Tiedemann, and K. N. Raymond, Supramolecular Chi-rality in Coordination Chemistry, pp. 147183, within thatvolume.

J.-P. Sauvage, ed. Transition Metals in Supramolecular Chem-istry: Perspectives in Supramolecular Chemistry , John Wiley& Sons, Inc., Chichester, 1999, vol. 5.

Supramolecular Chemistry: From Molecules to Nanomaterials , Online 2012 John Wiley & Sons, Ltd.This article is 2012 John Wiley & Sons, Ltd.This article was published in the Supramolecular Chemistry: From Molecules to Nanomaterials in 2012 by John Wiley & Sons, Ltd.DOI: 10 1002/9780470661345 smc015

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