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Macrocyclic LigandsKristin Bowman-JamesUniversity of Kansas, Lawrence, KS, USA

1 Introduction 12 Classication of Ligands 13 Synthesis 54 Thermodynamics and Structural Aspects 95 Applications 176 Related Articles 187 References 18

Glossary

Bis-macrocycles: two macrocycles joined together Calixarenes: basket-shaped macrocycles with phenyl backbonesCatenands: two interlocked macrocyclesCompartmental ligands: macrocycles with compartmentsfor housing more than one substrateCrown ethers: polyoxa macrocyclesCryptands: bicyclic macrocycles with aza bridgeheadsCyclidenes: lacunar tetraaza macrocycles Expanded porphyrins: macrocycles based on pyrrolicframeworks Lariat ethers: crown ethers with pendant chainsSepulchrates: bicyclic caged macrocycles

Spherands: macrocycles with phenyl backbones

Abbreviations

Polyaza macrocycles: [ n ]aneN m : n = Number of ring atoms; Nm = Number of nitrogen atoms; Polyoxa macrocycles: n-crown- m : n = Number of ring atoms; m = Number of oxygenatoms; TRI = Tribenzo[ b ,f ,j ][1,5,9]triazacyclododecine;TAAB = Tetrabenzo[ b ,f ,j ,n][1,5,9,13]tetraazacyclohexa-decine.

1 INTRODUCTION

Macrocyclic ligands are dened as cyclic moleculesgenerallyconsistingof organic frames intowhichheteroatoms,capable of binding to substrates, have been interspersed.Some reports of synthetic macrocycles (as opposed to thenaturally occurring species such as porphyrins, corrins, and

chlorins) appeared as early as 1936, when the rst synthesis of 1,4,8,11-tetraazacyclotetradecane was reported. 1 Nonetheless,the eld only began to blossom in the early 1960s with the pioneering work of Busch 2 andCurtis discoveryof thenickel-mediated condensation of [Ni(en) 3]2+ with acetone. 3 The

early macrocycles were synthesized with an eye to mimicking biologically occurring macrocycles such as the porphyrins,corrins, chlorins, and, more recently, the corphins.

Another area of macrocyclic development began in the late1960s and initial applications were focused toward modeling biological processes such as ion transport. These macrocyclesinitially included the oxygen-based crown ethers of Pedersen, 4

and the mixed oxygennitrogen bicyclic cryptands of Lehn, 5

both of which exhibit high selectivity toward alkali and alkaline earth metal ions. Several years later, the conceptof preorganized cavities resulted in the synthesis of thecavitands by Cram. 6

Since its birth, the development of macrocyclic chemistryhas proceeded along two lines:

1. as models of the naturally occurring macrocyclic systems,containing predominantly nitrogen donor atoms; and

2. as receptors designed for recognition and supramolecular chemistry, with a variety of donor atoms and recognitioncapabilities.

Macrocyclic chemistry has expanded phenomenally sincethe 1960s to provide exciting and novel chemistry. The award of the 1987 Nobel Prize in Chemistry to Pedersen, Lehn,and Cram is testimony to the importance of this rapidlyexpanding eld.

In such a large subject, this article can only focus oncertain aspects, namely those that involve complexationwith inorganic substrates. We only consider the syntheticmacrocycles, with emphasis on transition metal complexation.Aza, oxa, and, to a lesser extent, thia and phosphamacrocycles are also covered. The naturally occurring porphyrins, corrins, corphins, chlorins, and phthalocyanins, 7

as well as the cyclodextrins, 8 are not included. Because of thegeneral complexity of macrocyclic systems and the resultingcomplicated systematic names, commonly used abbreviationsor simplied names will be employed. This review willencompass the synthesis, thermodynamics, structure, and applications of macrocyclic ligands.

2 CLASSIFICATION OF LIGANDS

Two major areas of complexation have developed over the years with regard to synthetic macrocycles. Those withnitrogen, sulfur, phosphorus, and arsenic tend predominantlyto form traditional covalent coordination complexes withtransition metal ions. A notable exception to this tendency,however, is the rapidly expanding chemistry of the

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2 MACROCYCLIC LIGANDS

polyammonium macrocycles, which are capable of forminga variety of complexes with anionic substrates. Oxygen-derived macrocycles are noted for complexation with alkaliand alkaline earth metal ions, as well as with organic cationsand molecular substrates. In this latter situation, associationstend to be electrostatic in nature, and in many instanceshydrogen-bonding interactions are vital to complex formation.

Macrocyclic ligands will be classied, for the purposes of this article, as rings with at least nine members and three or more donor atoms. In a number of cases of unique structuralunits, elegant descriptive names have developed, which moreappropriately describe the macrocyclic shape. Macrocycleswill be classied as to donor types and, within the donor types, specic classications of macrocycles will be noted where applicable.

2.1 Polyaza Macrocycles (1)(10) 918

2.1.1 Simple Polyaza Macrocycles

Until recently, the tetraaza macrocycles, such as ( 1)(cyclam) and related ligands with extensive varieties of modications including differing degrees of saturation and ring size ( 2), had been the most studied, primarily becauseof the relationship of these molecules to naturally occurringtetraaza macrocycles, such as the porphyrins and corrins.Currently, with interest in metalmetal interactions, increased activity has occurred in the area of larger macrocyclescapable of incorporating more than one metal ion, such as(3) ([24]aneN 8).18 Interest in the smaller triaza macrocycles,such as (4) ([9]aneN 3) and its variations, has also accelerated in recentyears. 14 Added to thesimplepolyaza macrocycles has

been the effort to achieve functionalized macrocycles in order to expand the chemistry of these ligands by combining therigid structural aspects of the macrocyclic ring with the moreexible and kinetically labile properties of pendant chains, asin (5).11

(3)

NH

NH

HN

HN

(1)

N

N N

N

Me

Me

(2)

NH HN

HNNH

HN

HNNH

NH

2.1.2 Cyclidenes

Cyclidenes ( 6) are a subset of the polyaza macrocyclesand are the lacunar ligands rst synthesized and extensively

N

N

N

N

(5)

NH HN

HN

(4)

CO 2H

CO 2HHO 2C

HO 2C

studied by Busch. 19 They coordinate a single metal ion and maintain a persistent void which allows access to smallmolecules within the vaulted cavity.

N

NN

N

N

N

R HN

HNNH

NH

NH

HN

R

R

(6) (7)

2.1.3 Sepulchrates

Sepulchrates ( 7) are polyaza cage macrocycles. They arenotedfor theirexceptionally strong holdon encapsulated metal

ions. 20

2.1.4 Expanded Porphyrins

Expandedporphyrins are macrocycles basedon the pyrrolic backbone of porphyrins, but are expanded in size to achieve alarger cavity ( 8)21 or binucleating capabilities ( 9).22

NN

NHN

NNH NH

HNHN

N

N

N

N

(8) (9)

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2.1.5 Bis-Macrocycles

Bis-macrocycles ( 10 ) provide another mechanism for achieving complexation of more than one metal ion. Theyare joined by a bridge linking two simple macrocycles. 13,23

N NN

Me

NNN

Me

(CH 2)2

(10 )

2.2 Polythia, Polyphospha, and Polyarsa Macrocycles

Polythia macrocycles ( 11 ), the thioether analogs of thecrown ethers, have been known since the 1930s. 24 These arethe most extensively studied macrocycles in line after the polyoxa and polyaza macrocycles.

S

S

S

S

P

P

P

P

(11 ) (12 )

The pure polyphospha macrocycles ( 12 ) (as opposed tothe mixed donor phospha macrocycles) were rst reported in1975. 25 These macrocycles have been found to complex avariety of transition metals, but have not received the sameattention as the more readily accessible polyaza and polyoxamacrocycles.

The polyarsa macrocycles ( 13 ) comprise one of the leastcommon type of macrocycles. 26

MACROCYCLIC LIGANDS 3

As

As

AsAs

As

As

(13 )

2.3 Mixed Donor Macrocycles

2.3.1 Simple Mixed Donor Macrocycles

The simple mixed donor macrocycles ( 14 ) at one time werethe major source of study of the inuence of the incorporationof soft phosphorus and arsenic donors into macrocycles. 27

Mixed oxygennitrogen macrocycles have been studied quiteextensively, since they serve as bridges for examining thecoordination tendencies of the aza macrocycles and the oxacrown ethers. 13

O

N

O

N

O

HN

HN

HNO

P

P

O

(14 ) (15 )

O

O O

2.3.2 Cryptands

Cryptands ( 15 ) are bicyclic macrocycles which can containa variety of donor atoms with bridgehead nitrogen atoms. 5

They are highly selective for alkali and alkaline earthmetal ions.

2.3.3 Compartmental Ligands

Compartmental ligands ( 16 ) are macrocyclic ligands (aswell as nonmacrocyclic ligands) which contain compart-ments for housing more than one metal ion. 28 Only themacrocyclic counterparts will be treated here.

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O

O

O

OMe

MeOOMe

Me

MeMe

(22 )

2.4.4 Calixarenes

Calixarenes ( 23 ) are the macrocyclic result of condensa-tions between phenols and formaldehyde 37 and have beenreferred to as the most easily accessible molecular basket. 38

t -Bu

t -But -Bu

OH

OH HO

t -Bu

OH

(23 )


3 SYNTHESIS

3.1 Polyaza Macrocycles

3.1.1 Conventional (Nontemplate) Syntheses

Reviews of synthetic procedures canbe found for tridentateand pentadentate macrocyclic ligands with nitrogen donors,mixed nitrogen donors, and sulfur donor macrocycles, 39 thetechniques of which can be expanded to other ring sizes. Thegeneral procedures will be summarized below.

Cyclic secondaryamines, [ n ]aneN m , aregenerallyprepared by macrocyclization reactions known as the Richman Atkins procedure. 40 These reactions involve ring closure bycondensation of two precursor fragments of the cyclicmolecule. In general, one fragment consists of a saltof a sulfonamide, while the other contains two terminalleaving groups, which can vary in identity and include

chloride, bromide, hydroxide, or, more often, a sulfonate ester (Scheme 1).Thereaction is performedin polaraprotic solventsand may involve high dilution techniques. Simplied routesto tri-, tetra-, and pentaaza systems have been described. 41 Ahandy synthetic technique for the smaller triaza ring has beendescribed by Alder, where the macrocycle is built by usinga single carbon as template. 42 Treatises on the synthesis of pyridine-containing macrocycles 43 and imidazole-containingmacrocycles 44 have also been reported.

Functionalized macrocycles ( 5) with additional ligatingcomponents attached as pendant arms have been an areaof focus in efforts to expand the chemistry of macrocyclicreceptors by incorporating additional recognition sites.Synthetic techniques for N-functionalized, C-functionalized,

NTs

N

TsN

OMs MsO

HN

HNNH

NH

NH

NH

NTs

TsNNTs

TsN NH 2

HNNH

H2N

H2N

N

NH 2

HN

TsH

Scheme 1

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+

Cl ClPhAs

AsPhPhAs

PhAs As(Ph)Li

As(Ph)Li

Scheme 7

Cryptands are usually synthesized via sequential con-densations between a diamine and an acid chloride, whichyields a diamide, followed by reduction with LiAlH 4to give the macromonocycle. Condensation with another

equivalent of acyl chloride will yield the bicyclic precur-sor, which can be reduced by B 2H6 to give the bicycle(Scheme 8). 5

Compartmental ligands ( 16 ) are derived from diketonatesand triketonates and are usually synthesized from Schiff base

reactions of the ketone with a diamine.28

Catenands ( 17 ) are also made using the metal ion templateeffect. A bis-complex is formed from an , -disubstituted o-phenanthroline. Then the initial product is treated with adiiodoalkane to accomplish the ring closure. 29

3.4 Polyoxa Macrocycles

Polyethers were originally synthesized by templateassistance from an oligo(ethylene glycol), monoglyme, and potassium t -butoxide (Scheme 9). 4

H2N O O NH 2

Cl O O Cl

O O

O O

HN

O O

NHO O

O O

HN

O O

NH

O O

HN

O O

NH

Cl O O Cl

O O+

LAH

B2H6

O O

N

O

N

O

O O

O

O

O O

N

O

N

O

O O

Scheme 8

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O

O

OO

O

O

HO O O OH

Cl O O Cl

+

Scheme 9

Spherands andhemispherands ( 21 )and( 22 ) are synthesized by ring closure reactions of aryllithium with Fe(acac) 3, oftenusing high dilution techniques. 36

Calixarenes ( 23 ) are obtained from base-catalyzed conden-sations of p -substituted phenols with formaldehyde. 37

4 THERMODYNAMICS AND STRUCTURALASPECTS

4.1 Introduction

4.1.1 The Macrocyclic Effect

This term refers to the amazing stability of macrocyclicligands. It was initially described in studies of tetraazamacrocycles with copper(II). 63 For polyaza macrocycles thiseffect has been attributed to both entropic and enthalpicconsiderations and considerable controversy raged for anumber of years as to which was the predominant factor. 64,65

The conicting reports are now realized to be extremelydependent on the experimental methods used for thedetermination of the thermodynamic parameters. Two maintypes of technique have been employed, each of which has itsstrengths and weaknesses: the calorimetric titration method and the use of the temperature variation of the stabilityconstants. The controversy has been largely settled by morerecent studies. 66,67 Important contributions to the enthalpicterm are now attributed to a number of factors, includingsolvation and conformation changes upon bond formation.Likewise, the entropic considerations include the number of species present and particularly solvation effects. Detailed discussions of the historical development can be found. 13,17

Related to the macrocyclic effect are the decreased ratesof dissociation observed for macrocyclic complexes. Buschand co-workers have coined a term to describe these long-term stabilities incurred by synthetic macrocycles: multiple juxtapositional xedness. The premise is that straight-chainligands can undergo dissociative displacements in consecutivesteps starting at one end of the ligand and nishing with theopposite end. This is not the case for macrocyclic ligands, for which each dissociated donor is still held in proximity to themetal ion by the rest of the ligand framework. 68


The macrocyclic effect has been observed for polyaza, polythia, and polyoxa, as well as mixed donor atom,macrocycles. 69

4.1.2 Selectivity

The selectivity of a macrocycle for either a metal ion or another substrate is critically dependent on the structure of the macrocycle and electronic effects, i.e. the types of donor atoms. Some of the important aspects are described below.

1. The number of binding sites is perhaps one of themost crucial inuences on the binding properties of thesubstrate. Electronic effects of the binding of macrocycleswith substrates are charge, polarity, and polarizabilityof the binding sites. For metal ion binding, this meansion pair interactions for negatively charged ligands,iondipole and ioninduced dipole interactions for neutral ligands, and the hard soft acid base criteria.

Nitrogen, phosphorus, and sulfur donors are noted for their complexation of transition metal ions. Oxygen ismore likely to complex alkali or alkaline earth metal ions.

2. The arrangement of the binding sites should be such as tomaximize the potential ligand metal ion interactions. Inthis regard the selection of spacers between donor atomsto allow for the formation of ve- and six-member chelaterings has been the most utilized.

3. The preferred conformations of the macrocycles dictateits propensity to bind a metal ion internally or externallyto the cavity. The propensity of the lone pair to point inor out of the cavity is also a deciding factor. Hence, it isnot always a foregone conclusion that the metal ion will be bound within the macrocyclic cavity.

4. The identity of the macrocyclic framework also playsa major role in structure. For example, saturated hydrocarbon chains provide considerably more exibilitythan incorporated aromatic units. Likewise the presenceof other functional groups, such as amides or esters, serveto stiffen the macrocyclic framework. Decreasing theexibility of the macrocycle by adding selected shapinggroups is the theory behind preorganization, so importantin the cavitands. Another method of creating rigidity is toincrease the dimensionality of the macrocycle, inherent incryptand selectivities.

5. The size of the macrocyclic cavity alsoplays a large role ingoverning the exibility of the ligand, and its propensity

for metal ion binding.Since thefocusof this article is primarilyon transition metal

chemistry, the structural aspects related to complexation of transition metals will be emphasized, and other aspects of complexation will only be briey treated.

In addition to the traditional measurement of thermochem-ical properties, molecular mechanics calculations are nowavailable to supplement and correlate with experimental nd-ings. An extensive review which links the large data base of

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thermodynamic and kinetic data with items such as ring size,number and arrangement of ligand binding sites, and solventeffects, for all types of donor atoms including coronands,cryptands, spherands, and nitrogen donors can be found. 69 Amore recent series of molecular mechanics calculations have

added to this base of thermochemical data and point to struc-tural factors affecting complex stabilities from the viewpointof steric strain. 70

4.2 Polyaza Macrocycles

An extensive review of the thermodynamic aspects of polyaza macrocycles has been reported. 17 Other reviewsinclude the chemistry of tridentate and pentadentate azamacrocycles, 16 1,4,7-triazacyclononaneand derivatives, 14 and polyaza macrocycles with pendant chains. 11 In general, the polyaza macrocycles form extremely stable complexes withtransition metalsof thelater transition series, but show reduced afnity for alkali and alkaline earth metal ions compared tothe oxa macrocycles.

4.2.1 Triaza Macrocycles

Oneof thesimplestand smallest of thepolyaza macrocyclesaccording to denition is 1,4,7-triazacyclononane ( 4). Thegeometrical constraints of the triaza macrocycles are suchthat they do not allow for the incorporation of the metal ionwithin the macrocyclic ring. Hence, these macrocycles arefacially coordinated in either a mono- or bis-ligand complexwith a variety of metal ions. 14 The macrocyclic effect isobserved, and the stability constants of the complexes follow

the IrvingWilliams Series .17

Both microcalorimetric71

and stability constant determinations at different temperatures 72

indicate that the effect is most probably enthalpic in origin.The triaza macrocycles also form extremely stable complexeswith the heavier main group metals (such as Ga III , InIII ,TlI , and Tl III ) as well as transition metals. The chemistryof this macrocycle and its derivatives is wide in scope and is treated extensively in a review which includes the basecompound and its N-functionalized derivatives. 14 Dependingon the appendages employed in N-functionalization, threemore coordination sites are potentially available, rounding outthe coordination to pseudooctahedral.

The formation of dinuclear and higher nuclearity speciesis common for the mono-coordinated triazacyclononane, with bridging acetates, hydroxides, and oxides being very common.Extensive studies of the chemistry of the variety of bridged species have been made using the relatively substitutionallyinert chromium(III) ion. Different dimers and trimers have been isolated and structurally characterized, as in ( 29 ) and (30 ).73,74 Higher nuclear clusters such as an octanuclear ironsystem are relevant as a model for the iron storage proteinferritin ( see Iron Proteins for Storage & Transport & their Synthetic Analogs ).75

CrN O

O

O

CrN

N

O

HON

NN

H

(29 )

CrN

Cr

CrN

N

O

HO

NN

NH

NN

N

(30 )

OH

A more preorganized ligand system is derived from theself-condensation of o-aminobenzaldehyde. 51 The tridentateform of the ligand ( 25 ) (TRI) imparts considerableinertness toward substitution. For example, the salts of the [Ni(TRI)(H 2O)3]2+ ion can be resolved into opticalisomers. 76 A copper(II) complex of the methyl-substituted tetradentatemacrocycle Me 4TAAB, in whichbis-coordinationoccurs, displays a dynamic JahnTeller distortion based oncrystallographic evidence. 77

4.2.2 Tetraaza Macrocycles

Because of the potential relationship to the naturallyoccurring porphyrins and porphyrin-analog macrocycles, thetetraaza macrocycles have been the focus of much attention.

Tetraazamacrocyclesoften,but notalways, form a coplanar arrangement of the four nitrogen donors. Empirical force eld calculations of free macrocycles from 12- to 16-membered rings indicate that cyclam ( 1) exhibits the least strain with the best planarity. A straightforwardassessment of the relationshipof hole size to selectivity is complicated by the conformationalexibility of the ligands. Results of studies for the tetraaza

macrocycles show that hole size does not appear to be the predominant factor in metal ion discrimination. Rather, theselectivity of these macrocycles is governed by the relativestability of the conformers of the macrocycle which havedifferentmetalion sizepreferences. An interestingobservationregarding the relationship of the tetraaza macrocycles withregard to hole size and metal ion selectivity can be found for the most studied of the simple tetraaza macrocycles,cyclam. Cyclam is proposed to have ve congurationalisomers, based on the orientation of the amine hydrogens.From molecular mechanics calculations, where the best MNdistance is calculated as that giving the minimum energy,the trans-III analog of [12]aneN 4 (31 ) is found to have anextremely high strain energy of 19.7kcalmol 1 with a best-tMN distance of 1.81 A, compared to the trans-I form ( 32 )(10.8 kcal mol 1 and 2.11 A, respectively). 70,78 In general, thelarger, more exible, planar coordination is provided by thetrans-I conformer, and often if a metal is too large for themacrocyclic cavity, it will coordinate lying out of the plane of the donor atoms. When the metal ion is not incorporated intothe macrocyclic plane, the factors inuencing stability are thesame as for the acyclic aza analogs, namely that for larger metal ions, as the size of the chelate ring increases from ve

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to six, the complex stability decreases. A detailed discussionof the thermodynamics of changing chelate sizes for tetraazamacrocycles can be found. 17,78

(31 ) (32 )

MNN

H H

N

H

N

H MNN

H

N

H

NH

H

An elegant example of the importance of conformationalchanges in tetradentate macrocycles is the blue to yellowconversion observed for nickel(II) complexes. The yellowform is the low-spin square planar complex NiL 2+ , while the blue form is high-spin pseudooctahedral [NiL(H 2O)2]2+ . Inthe blue to yellow conversion the NiN bonds contract, whichcompensates for the breaking of the axial NiOH 2 bonds. Thereaction is controlled by entropy, and the addition of an inertsalt is such as to favor the dissociation of the water molecules.At equilibrium in aqueous solution, both [12]aneN 4 (cyclam)(1) and the 15-membered analog, [15]aneN 4, have 99% of the high-spin form present, while the 13- and 14-membered macrocycles exist in predominantly the low-spin square planar form (87 and 71%, respectively). 79

For the nonplanar octahedral cis -coordinated macrocycles[n]aneN 4 , changes in the ligand eld correlate well with theanalogous ligand eld strengths for nonmacrocyclic analogs,specically as related to the chelate ring size. A general ruleof thumb is that increasing the chelate ring size from veto six increases the stability of complexes of smaller metals

compared to larger metal ions. The origin of this effect can beattributed to increases in ring strain energy when metal ionslarger than tetrahedral carbon are part of the ring. 78 For the planar-coordinated macrocycles, the equatorial ligand eld,as anticipated, is dependent on the ring size. These ndingshave been related to the calculation of the optimum hole size permitting the macrocycle to adopt its most preferable endoconguration. Thus, it has been found that the macrocyclichole size increases by 1015 pm for each increment in n for [n]aneN 4 .79

In order to introduce a greater rigidity into theexible polyaza macrocycles and to implement greater hole size metal ion size match correlations, reinforced macrocycles such as ( 33 ) have been created, which containfused diaza rings. 80 Crystallographic results for the nickel(II)complex indicate that the NiN bonds are shortened fromthe strain-free value of 1.91 A for diamagnetic nickel to1.86 A. Ligand eld strength is also found to increase, and this has been suggested as being due to the compressionof the bond lengths 80 as well as the presence of tertiarynitrogen donors. 78 In a more recent comparative study of nickel macrocycles with two fused1,3-diazacyclohexane rings(35 ) compared to two fused 1,3-diazacyclopentane rings ( 34 ),


structural results revealed weaker ligand eld strengths for the1,3-diazacyclohexane compared to 1,3-diazacyclopentane. 54

(35 )(34 )

NH

N

HN

N

(33 )

NH

N

N

HN

N

N

NH

N

N

HN

N

N

Attempts to achieve macrocycles that are capable of stabilizing highly oxidized transition metal complexes hasled to the design of noninnocent ligands. 81 The structures of high-valent chromium(V) oxo species with the two tetraamido N ligands ( 36 )and( 37 ) were determined. Both structures were

found to contain distinctly nonplanar amide groups, and in(36 ) all four amides are nonplanar.

NH

NH

HN

HN

O

OO

NH

NH

HN

HNO O

Cl Cl

(36 ) (37 )

Polyaza macrocycles with pendant arms have been studied extensively, in particular with respect to protonation and complexation as well as to the kinetics of metal complexformation. These aspects are treated in a review by Kaden. 11

Of particular interest is the fact that metal complex formationconstants of macrocycles with pendant carboxylates can be103 to 104 times higher than for the unsubstituted analogs.

4.2.3 Higher Polyaza Macrocycles

Transition metal complexes of the larger polyazamacrocyclic ligands have been less extensively studied thanfor the smaller ring systems. For the pentaaza macrocycles,[15]aneN 5 with ethylene bridges appears to form the moststable complexes with most metal ions. 17 Structural datafor a variety of pentaaza macrocyclic complexes have been reviewed. 16 The NH bonds as well as the differentsized chelate rings must be considered in calculating the

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number of possible isomers. For each of the complexes,three congurations of the in-plane NH bonds are possible:(38 ), (39 ), and (40 ). Crystallographic data indicate thatmost of the complexes with pentadentate macrocycles have pseudooctahedral geometries. Pentadentate macrocycles also

tend to stabilize unusual oxidation states. For example, thenickel(II) complexes of [15]aneN 5 (41 ), [16]aneN 5 (42 ), and one of the isomers of [17]aneN 5 (43 ), are readily oxidized to the Ni III analogs. Also, there is little dependence of E 1/ 2values on the macrocyclic ring size, which has been attributed to the absence of in-plane ring size effects. 16

MN

N N

N

MN

N N

NM

N

N N

N

N N N

(38 ) mesosyn (39 ) mesoanti (40 ) racemic

H

H

H

H

H

H

NH

NH

HN

NH HN

NH

NH

HN

NH HN

NH

NH

HN

NH HN

(41 ) (42 ) (43 )

The hexaaza [18]aneN 6 forms complexes with transitionmetal ions and with certain alkali and alkaline earth and lanthanide ions. 82 For the higher aza macrocycles withseven or more donor atoms, dinuclear complexes become possible. A systematic investigation of both the structuraland thermodynamic aspects of copper complexes formed withthe larger polyaza macrocycles from heptaaza to dodecaazahas been published. 18 All of the macrocycles were found toform hydroxo species as well as polynuclear complexes. Anumber of structures have been determined for the higher polyaza macrocycles, both in complexed and noncomplexed forms, and structures range from highly boat shaped to nearly planar. 18,50,83,84

A review of macrocycles possessing subheterocyclic ringshas appeared,whichincludes pyridine, furan, and thiophene. 85

In a study of formation constants for transition metal ionswith pyridine- and furan-containing macrocycles, ( 44 ) and (45 ), it was found that the pyridine macrocycles follow theIrvingWilliams series and bind even more effectively thantheir saturated analogs (i.e. [18]aneN 6 and [18]aneN 4O2). Thefuran analogs showed little tendency to bind, which has beenattributed to the increased rigidity of the furan ring. 86

N

NH

NH

N

HN

HN NH

NH

O

O

HN

HN

(44) (45)

4.2.4 Anion Coordination

While the initial interest in polyaza macrocycles involved metal ion coordination, the nding in 1968 by Simmons 87

that diaza bicyclic catapinands can incorporate halide ionsinto their cavity opened the door on a vast new area of chemistry, that of anion complexation. The thermodynamicsof anion binding can be divided into several differentareas: that of simple inorganic anions; more complexcarboxylate and polycarboxylates; corresponding phosphates, polyphosphates, and nucleotides; and culminating in anionicmetal complexes. 17,8890 Binding is accomplished via bothelectrostatic and hydrogen-bonding interactions between the protonated macrocyclic amines and the anionic substrates.The general trend appears to be that the increased exibilityof larger polyammonium macrocycles tends to facilitatecomplexation of more complex anionic substrates.

The results of studies for complexes formed between polyammonium macrocycles and transition metal complex

anions indicate that cationanion electrostatic attraction is acrucial factor in complexation reactions and serves to regulatethe stoichiometry of the complexes formed. Hydrogen- bonding, size, and conformational factors also play major roles. 89 Anions can be incorporated in or out of the ring.Two illustrative examples are metal ion complexes with theoctaprotonatedmacrocycleH 8[30]aneN 10 (46 ). In thecomplexwith Co(CN) 63 , the anion lies outside the macrocycle. ThePdCl 42 complex is a true inclusion situation, however, inwhich the PdCl 42 is situated along the minor axis of themacrocyclic cavity, and the Cl atoms are out of the frame,forming strong hydrogen bonds with the polyammoniumsites. 90

4.2.5 Cyclidenes

Crystallographic results for the cyclidenes ( 6) show thata wide variety of structural ranges can result from designed modications of the lacunar cavity (or void). 91 The afnityof the cobalt(II) complexes of the cyclidenes for molecular oxygen was found to be very dependent on the identity of theoverhead bridge and was found to increase with increasing bridge length. Further design has also allowed for expanding

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NHNH

HN

NHNH

NH

NH

HN

HN

HN

(46 )

the capability of these macrocycles beyond simple oxygen binding to oxygenase activity observed for the cytochromeP-450s. This has been achieved by adding piperazine risersto increase the cavity size (9 A high) as well as increasing thehydrophobicity of the molecules, and by adding anthraceneand durene roofs. The crystal structure of the anthracene- bridged derivative shows that the macrocycle is indeed capableof hosting an acetonitrile molecule.

4.2.6 Sepulchrates

Sepulchrates ( 7) are the most noted of the caged macrocyclic ligands and are the nitrogen analogs of thecryptands. The CoN distances are 1.99 A for Co III and 2.16 Afor Co II from crystallographic data, and do not vary greatlyfrom other cobalt amines. 20

4.2.7 Expanded Porphyrins

A review of expanded porphyrin ligands can be found. 92

The texaphyrins ( 8) can be considered as 22- -electron benzannulene systems with an 18- -electron delocalization path, based on crystal structure data as well as NMR.The cadmium complex of the macrocycle is found to be planar with pentadentate coordination of the macrocycle tocadmium, which becomes seven-coordinate as a result of axial coordination to two pyridine molecules. The cavity isnearly circular with a center-to-nitrogen distance of 2.39 A.Because of the larger size of this macrocycle, metal ion

coordination is generally seen with the larger transition metalsand lanthanides.

A more exible expanded porphyrin is the accordion porphyrin ( 9).22 The structural aspects of this macrocycleillustrate the importance of exibility in achieving unantici- pated structures. The free-base macrocycle is elliptical withthe inclusion of two water molecules ( 47 ), while the dicop- per(II) complex is highly distorted by means of exo and endoorientations of the imine groups ( 48 ).


NH NH

N

N

N

N

HNHN

Ph

Ph

OH H

OHH

(47 )

N

N N

CuN

N3

N N

NCu

N N3

(48 )

4.3 Polythia and Polyphospha Macrocycles

4.3.1 Polythia Macrocycles

The coordination chemistry of thioether macrocycles hasexpanded greatly only since the mid-1980s, as seen by a number of reviews. 55,9395 The macrocyclic effect is also noted for

thioethers, but to a lesser extent than some of the other macro-cyclic ligands. This is due primarily to the reorganizationalenergy requirements, since a number of the free-ligand thiamacrocycles have a tendency to adopt exodentate conforma-tions in the uncomplexed form, where the sulfurs are pointed out of the macrocycle ( 49 ). Macrocyclic thioethers must thenundergo a reorganization of their exo lone pairs in order toincorporate metal ions within thecavity. It wasfoundin a studyof thecomplexation of a numberof open-chain thia ligands and thia macrocycles that the enthalpy changes were essentiallyidentical for both macrocyclic and nonmacrocyclic ligands.Hence, the favorable macrocyclic effect is more attributable tothe entropy changes in the sulfur macrocycles. 96 The smaller trithia analog of the extensively studied nitrogen donor triaza-cyclononane does not require such organization and, as such,has been extensively studied itself. 55 Because of the prefer-ence for exodentate sulfurs, metal ion coordination in manycases is external to the cavity ( 50 ).97 A comprehensive reviewof the structural aspects of thia macrocycles can be found. 55

Considerable effort has been made with regard toconformation analysis of crown thioethers. It has been found that ligand strain is most evident in torsion angles, wherebyan examination of the deviations from the optimum values of

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S

S

S

S

(49 ) (50 )

S

S

S

SHg

Cl

Cl

HgCl

Cl

60 for gauche or 180 for anti congurations can lead to anassessment of the overall strain in the molecules. 93

Examination of the inuence of ring size has beenreported for the 12- to 16-membered tetrathia systems withcopper(II). 96,98 The results indicate a marked interrelationship between ring size and stability. The stability peaks at 14-membered rings, and the rings are large enough to incorporatethe copper only for the 14- to 16-membered systems.

Results from the correlation of stability constants in

conjunction with redox data have led to insights regarding thecoordination chemistry of thia macrocycles. For example, theelectrochemical behavior of a number of copper(II)/(I) redoxcouples has been investigated, 99 and redox potentials as wellas protonation and stability constants of Cu I species weredetermined for a number of tetradentate and pentadentate thia-derived macrocycles with thia- and mixed thiaaza rings withthe basic backbones ( 51 ) and (52 ). Results of the examinationof the stability constants in conjunction with the Cu II/ I redox potentials indicate that the stability constants for the Cu I

oxidation state are relatively constant regardless of the mixingin of nitrogen donor atoms. Hence, the dramatic increase inthe Cu II/ I redox potential which is observed in the presence of the sulfur macrocycles can be attributed to a destabilization of

the Cu II state rather than stabilization of the Cu I state, contraryto popular belief from the hard soft acidbase system.

S S

SS

(51 ) (52 )

S

S

S S

S

In order to force binding of trithia structural units intoan endodentate conformation, one strategy has been to add rigid xylyl groups into the ring to limit the exibility ( 53 ).100

While the conformation of the free ligands is exodentate,a number of transition metal complexes of this ligand have been found to exhibit endodentate coordination, including Mo,Cu, Ag, Pd, and Rh. Results for the bis-macrocyclic silver complex with a variety of noncoordinating anions, indicatethat the conformational interconversions of the ligand are lowin energy.

S

SS

(53 )

S

SS

S

S

(54 )

Thiophene units have also been incorporated into the thiacrowns ( 54 ).101

4.3.2 Polyphospha Macrocycles

Phosphorus macrocycles can exist in a variety of conformations, a number of which are stable. The barrier for inversion of phosphate is 146.4 kJ mol 1.102 Hence thereare ve conformations possible for the tetraphosphorusmacrocycle ( 12 ). Two are preferred: the one in which themacrocyclic benzo groups are trans (55 ) and that in whichthey are cis (56 ).60,103

P

P

P

P

(55 ) (56 )

P

P

P

P

4.4 Mixed Donor Macrocycles

4.4.1 Simple Mixed Donors

Much of the work in this area has been reported by Lindoyand co-workers, who have performed extensive studies on therole of hole size in complex stability and rates of complexformation. 13,27,104 Bradshaw, Krakowiak, and Izatt have published an extensive text on the synthesis of aza crowns. 105

A review of tri- and pentadentate macrocyclic ligands alsoincludes mixed donor results as well as the inuence of pendant arms. 16 Due to the numerous ramications of thisarea, a few key ndings will be cited for the simplestsystems.

A major focus in the study of mixed metal ion systemshas been to examine metal ion discrimination. In partic-ular, two specic mechanisms can be attributed to metalion discrimination: macrocyclic hole size and what Lindoyhas termed as a dislocation mechanism. The key to this

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NN

OOCu

N N

O O

Cu

O

O

S

O

S

O

Ba

(63 )

occursfor thepotassiumion, which hasa radiusof 1.38 A, thuscorrelating well with the cavity radius. However, 18-crown-6forms extremely stable complexes with all of the alkali and alkaline earth metal ions. Hence, Gokel argues that the dataindicate that the hole size concept is inapplicable, since the binding constants for sodium, potassium, ammonium, and calcium ions are the largest for the 18-crown-6 compared toalmost all of the other simple crown ethers. 119 Hancock has proposed that chelate ring size is the critical factor, and that the

high stabilities observed for the crown ethers with large metalions is a result of the presence of ve-membered chelate rings.Thus the high afnity of these macrocycles for the potassiumion is explained by the fact that potassium is the right size for the ve-membered chelate rings of the crown ethers. 78

O

O

OO

O

O

(65 )

O

O

OO

O

O

O

OO

O

OO

(66 )(64 )

A number of reviews of the structural aspects of crownethers can be found. 115117 These structures vary considerablyincomplexity. Anexample of theexibility of thecrownetherscan be seen in the variation in the structures as a result of ringsize of three different benzo crowns. When the cavity of the

crown matches the radius of the metal ion, the metal ion can bereadily incorporatedin thecavity, such as in thestructure of therubidium thiocyanate complex with the dibenzo-18-crown-6(67 ). In cases where the cavity of the crown is too large tosurround the metal ion snugly, a folded structure can result,

as with the dibenzo-30-crown-10 ( 68 ) and the potassium ion.For very large metal ions incapable of tting into smaller macrocyclic cavities, sandwich-type structures can occur, asin the benzo-15-crown-5 ( 69 ) with the potassium ion. 115

O

O

O

O

O

O

(67 )

O

O

O O O

O O O

O

O

(68 )

OO

O O

O

(69 )

Molecularmechanicsstudies indicate that the lowestenergyconformer of the uncomplexed ligand is not necessarilythat required for complexation, i.e. oxygen donors may beexodentate as in the thia macrocycles.This means that in order for complex formation to occur, the ligand must undergo bothreorganization as well as desolvation. A general rule of thumbwith respect to size, however, is that the larger macrocyclesare more exible and subject to adaptability, while the smaller macrocycles are more rigid and, in that sense, preorganized.Cram has provided an excellent treatise on preorganization. 118

His principle of preorganization is that the more highlyhosts and guests are organized for binding and low solvation prior to their complexation, the more stable will be their complexes. 118 G valuesfor a variety of macrocyclic oxygendonors indicate that the prearranged ligands in general bind

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their guests more strongly and are, in sequence, the spherands> cryptaspherands cryptands > hemispherands > crownethers. 116

A useful correlationof enthalpy entropy considerationsfor complexation has been shown by Inoue, Liu, and Hakushi. 113

The treatment reects enthalpy entropy relationships for given types of ligands. The general concept is that as theenthalpic contributions become strong, a higher level of organization is obtained, which will result in unfavorableentropy changes. For a given type of system with similar entropic versus enthalpic considerations, the T S and H values determined for a series of ligands should thus exhibita linear relationship. This is found for the macrocyclic crownethers, the cryptands, lariat ethers, and bis-crown ethers, aswell as the acyclic polyethers known as podands. The slopesare all positive with high correlation coefcients. Gokel hassuggested that these slopes can be used to assess the ligand exibility: glymes and podands (0.86) > crown ethers (0.76)> cryptands (0.51). 119

4.5.2 Lariat Ethers

The lariat ethers ( 19 ) and (20 ) known to date consistof macrocycles with many different types of podand groups, and much of their complexation chemistry involveselectrostatic binding of guests. Reviews of both structural and thermodynamic aspects of the lariat ethers canbe found. 120122

The trends are noted to be relatively similar for both thecarbon-pivot and nitrogen-pivot types of lariat ethers. Bindingstrengths and selectivities are dependent on ring size and ingeneral increase as ligand size increases. Strong selectivities

are noted for the potassium ion, as in the crown ethers.

4.5.3 Spherands and Hemispherands

The spherands ( 21 ) were specically designed using theconcept of preorganization wherein the oxygen donors arearranged in an enforced spherical cavity. Totally prearranged (spherand) and partially arranged (hemispherand, ( 22 ))complexes are possible. 118 Due to the structural restraintsimposed by the rigidly joined phenyl rings, the spherands areconsidered to be highly preorganized binding sites. In thesemacrocycles the lone pair of electrons will always be pointed toward the center of the macrocyclic cavity.

4.5.4 Calixarenes

The calixarenes ( 23 ) are also highly preorganized molecules which are capable of forming differentconformational isomers. The conformational exibility isdetermined by the size of the ring, with the preferred confor-mation becoming more planar as the ring size increases. 37


5 APPLICATIONS

As macrocyclic chemistry has developed, the variety and scope of the applications of these molecules have continued to multiply. This concluding section is an attempt to provide

an overview of only three of the applications of syntheticmacrocycles. A particularly insightful treatment can befound in the Nobel Lecture of Jean-Marie Lehn, 123 whichdescribes the concept of supramolecular chemistry fromsimple recognition, to cation and anion receptors, multiplerecognition, catalysis, transport, and molecular devices.

5.1 Ion Transport

Ion transport, especially cation transport, was one of theearly focal points in macrocyclic chemistry, revolving primar-ily around the crown ethers and cryptands. Later efforts have been to provide switches to control the rates of cation trans-

port. Two examples of the types of switches that have beendeveloped include photo switches using cryptands, 124 and electrochemical switches using anthraquinone-derived lariatethers. 125

Related to transport capabilities is the use of syntheticmacrocycles in analytical chemistry. Because of their selec-tive complexation of a variety of cations, the crown ethers and related macrocycles have been widely used for separationsand analyses. 126

While transport efforts have largely involved metalcations, more recent developments have led to the use of macrocycles for transport of more complex molecules such asnucleosides. 127

5.2 Catalysis

Catalysis can be broken down into a number of areas,depending on the substrate and the catalytic reaction. One of the prime areas of the initial effort in catalysis has been smallmolecule activation, such as oxygen with a number of transi-tion metal ion macrocycles 128,129 andcarbondioxide, the latter particularly with cobalt(I) and nickel(I) macrocycles. 130,131

Once the polyammonium macrocycles were found to be ableto recognize substrates other than metal ions, other catalysisapplications evolved. For example, phosphoryl transfer catal-ysis with simple polyammonium macrocycles has becomequite accessible. 132

5.3 Magnetic Resonance Imaging

Macrocyclic complexes have gained recognition inmagnetic resonance imaging. 133,134 In order to be effectiveimaging agents, complexes must provide a signicantenhancement in the proton relaxation rates of water,as well as be nontoxic, and thermodynamically stable.Hence, macrocyclic ligands with pendant carboxylates, such

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as (5), have been examined primarily because of their thermodynamic stability.

6 RELATED ARTICLES

Ammonia & N-donor Ligands; Mixed Donor LigandsP-donorLigands;S-donor Ligands;Water & O-donorLigands.

7 REFERENCES

1. J. Van Alphen, Recl. Trav. Chim. Pays-Bas , 1936, 55 , 835.

2. M. C. Thompson and D. H. Busch, Chem. Eng. News , 1962,57.

3. N. F. Curtis, J. Chem. Soc. , 1960, 4409.

4. C. J. Pedersen, J. Am. Chem. Soc. , 1967, 89 , 7017.

5. B. Dietrich, J.-M. Lehn, and J.-P. Sauvage, Tetrahedron Lett. ,1969, 2889.

6. J. Almy, D. C. Garwood, and D. J. Cram, J. Am. Chem. Soc. ,1973, 95 , 2961.

7. T. Mashiko and D. Dolphin, in Comprehensive CoordinationChemistry, eds. G. Wilkinson, R. D. Gillard, and J. A. McCleverty, Pergamon, Oxford, 1982, Vol. 2, p. 813.

8. V. T. Souza and M. L. Bender, Acc. Chem. Res. , 1987, 20 ,146.

9. G. A. Melson ed., Coordination Chemistry of MacrocyclicCompounds, Plenum, New York, 1979.

10. D. H. Busch, Acc. Chem. Res. , 1978, 11 , 393.11. T. A. Kaden, Top. Curr. Chem. , 1984, 121 , 157.

12. E. Kimura, Top. Curr. Chem. , 1985, 128 , 113.

13. L. F. Lindoy, The Chemistry of Macrocyclic Ligand Complexes, Cambridge University Press, Cambridge, MA,1989.

14. P. Chaudhuri and K. Wieghardt, Prog. Inorg. Chem. , 1987,35 , 329.

15. D. Parker, Chem. Soc. Rev. , 1990, 19 , 271.

16. R. Bhula, P. Osvath, and D. C. Weatherburn, Coord. Chem. Rev., 1988, 91 , 89.

17. A. Bianchi,M. Micheloni, and P. Paoletti, Coord.Chem. Rev. ,1991, 110 , 17.

18. A. Bianchi, M. Micheloni, and P. Paoletti, Pure Appl. Chem. ,1988, 60 , 525.

19. D. H. Busch and C. Cairns, in Progress in MacrocyclicChemistry, eds. R. M. Izatt and J. J. Christensen,Wiley, NewYork, 1987, Vol. 3, Chap. 1, p. 1.

20. A. M. Sargeson, Pure Appl. Chem. , 1984, 56 , 1603.

21. J. L. Sessler, T. Murai, and V. Lynch, Inorg. Chem. , 1989, 28 ,1333.

22. F. V. Acholla, F. Takusagawa,and K. B. Mertes, J. Am. Chem.Soc. , 1985, 107 , 6902.

23. I. Murase, K. Hamada, and S. Kida, Inorg. Chim. Acta , 1981,54 , L171.

24. N. B. Tucker and E. E. Reid, J. Am. Chem. Soc. , 1933, 55 ,

775.25. L. Horner, H. Kunz, and P. Walach, Phosphorus Relat. Group

B Elem. , 1975, 6, 63.

26. J. Ennen and T. Kauffmann, Angew. Chem., Int. Ed. Engl. ,1981, 28 , 118.

27. L. Lindoy, in Cation Binding by Macrocycles, eds. Y. Inoueand G. W. Gokel, Dekker, New York, 1990, Chap. 16, p. 599.

28. D. E. Fenton,U. Casellato, P. A. Vigato, and M. Vidali, Inorg.Chim. Acta , 1982, 62 , 57.

29. C. O. Dietrich-Buchecker, J.-M. Kern, and J.-P. Sauvage, J. Am. Chem. Soc. , 1984, 106 , 3043.

30. Y. Inoue and G. W. Gokel eds, Cation Binding byMacrocycles, Dekker, New York, 1990.

31. I. Tabushi, Y. Kobuki, and T. Nishiya, Tetrahedron Lett. ,1979, 20 , 3515.

32. A. Shanzer, J. Libman, and F. Frolow, J. Am. Chem. Soc. ,1981, 103 , 7339.

33. E. Schwartz and A. Shanzer, J. Chem. Soc., Chem. Commun. ,1981, 634.

34. D. J. Cram, S. Karbach, Y. H. Kim, L. Baczynskyj, and W. Kalleymeyn, J. Am. Chem. Soc. , 1985, 107 , 2575.

35. G. W. Gokel, D. M. Dishong, and C. J. Diamond, J. Chem.Soc., Chem. Commun. , 1980, 1053.

36. D. J. Cram, Angew. Chem., Int. Ed. Engl. , 1986, 25 , 1039.

37. C. D. Gutsche, in Calixarenes, ed. G. F. Stoddart, RoyalSociety Chemistry, Cambridge, MA, 1989.

38. C. D. Gutsche, I. Alam, M. Iqbal, T. Mangiaco, K. C. Nam,J. Rogers, and K. A. See, J. Inclusion Phenom. , 1989 7, 61.

39. R. Bhula, P. Osvath, and D. C. Weatherburn, Coord. Chem. Rev., 1988, 91 , 89.

40. J. E. Richman and T. J. Atkins, J. Am. Chem. Soc. , 1974, 96 ,2268.

41. F. Chavez and A. D. Sherry, J. Org. Chem. , 1989, 54 , 2990.

42. R. W. Alder, R. W. Mowlam, D. J. Vachon, and G. R.Weisman, J. Chem. Soc., Chem. Commun. , 1992, 507.

43. M. W. Hosseini, J. Comarmond, and J.-M. Lehn, Helv. Chim. Acta , 1989, 72 , 1066.

44. Y.-D. Choi and J. P. Street, J. Org. Chem. , 1992, 57 , 1258.

45. L. Qian, Z. Sun, M. P. Mertes, and K. B. Mertes, J. Org.Chem. , 1991, 56 , 4904.

46. E. K. Bareeld, D. Chueng, D. vanDerveer, and F. Wagner, J.Chem. Soc., Chem. Commun. , 1981, 302.

47. M. C. Thompson and D. H. Busch, J. Am. Chem. Soc. , 1964,86 , 3651.

7/28/2019 ligands

19/20

48. N. F. Curtis, in Comprehensive Coordination Chemistry,eds. G. Wilkinson, R. D. Gillard, and J. A. McCleverty,Pergamon, Oxford, 1982, Vol. 2, Chap. 21.1, p. 899.

49. S. M. Nelson, Pure Appl. Chem. , 1980, 52 , 461.

50. K. Krakowiak,J. S. Bradshaw, W. Jiang,N. K. Dalley, G. Wu,

and R. M. Izatt, J. Org. Chem. , 1991, 56 , 2675.51. G. A. Melson and D. H. Busch, J. Am. Chem. Soc. , 1964, 86 ,

4834.

52. A. Hohn, R. J. Geue, and A. M. Sargeson, J. Chem. Soc.,Chem. Commun. , 1990, 1473.

53. A. Bencini, A. Bianchi, A. Borselli, M. Ciampolini,P. Dapporto, E. Garcia-Espa na, M. Micheloni, P. Paoli,J. A. Ramirez, and B. Valtancoli, J. Chem. Soc., Perkin Trans.2, 1989, 1131.

54. M. P. Suh, S.-G. Kang, V. L. Goedken, and S.-J. Park, Inorg.Chem. , 1991, 30 , 365.

55. A. J. Blake and M. Schr oder, Adv. Inorg. Chem. , 1990, 35 , 1.

56. D. P. Riley and J. D. Oliver, Inorg. Chem. , 1983, 22 , 3361.

57. R. E. Wolf, J. R. Hartman, J. M. E. Storey, B. M. Foxman,and S. R. Cooper, J. Am. Chem. Soc. , 1987, 109 ,4328.

58. P. Osvath, A. M. Sargeson, B. W. Skelton, and A. H. White, J. Chem. Soc., Chem. Commun. , 1991, 1036.

59. T. A. DelDonno and W. Rosen, J. Am. Chem. Soc. , 1977, 99 ,8051.

60. C. A. McAuliffe, in Comprehensive Coordination Chem-istry, eds. G. Wilkinson,R. D. Gillard, and J. A. McCleverty,Pergamon, Oxford, 1982, Vol. 2, Chap. 14, p. 989.

61. T. L. Jones, A. C. Willis, and S. B. Wild, Inorg. Chem. , 1992,31 , 1411.

62. F. Gonce, A.-M. Caminade, F. Boutonnet, and J.-P. Majoral, J. Org. Chem. , 1992, 57 , 970.

63. D. K. Cabbiness and D. W. Margerum, J. Am. Chem. Soc. ,1969, 91 , 6540.

64. M. Kodama and E. J. Kimura, J. Chem. Soc., Dalton Trans. ,1976, 2341.

65. F. P. Hinz and D. W. Margerum, Inorg. Chem. , 1974, 13 ,2941.

66. A. Bianchi, L. Bologni, P. Dapporto, M. Micheloni, and P. Paoletti, Inorg. Chem. , 1984, 23 , 1201.

67. M. Micheloni, P. Paoletti, and A. Sabatini, J. Chem. Soc., Dalton Trans. , 1985, 1169.

68. D. H. Busch, K. Farmery, V. Goedken, V. Katovic, A. C.

Melnyk, C. R. Sperati, and N. Tokel, Adv. Chem. Ser. , 1971,100 , 44.

69. R. M. Izatt, J. S. Bradshaw, S. A. Nielsen, J. D. Lamb,J. J. Christensen, and D. Sen, Chem. Rev. , 1985, 85 ,271.

70. R. D. Hancock and A. E. Martell, Chem. Rev. , 1989, 89 ,1875.

71. M. Nonoyama and K. Nonoyama, Inorg. Chim. Acta , 1979,35 , 231.


72. L. Fabrizzi and L. J. Zompa, Inorg. Nucl. Chem. Lett. , 1977,13 , 28.

73. K. Wieghardt, W. Schmidt, R. van Eldik, B. Nuber, and J. Weiss, Inorg. Chem. , 1980, 19 , 2922.

74. K. Wieghardt, W. Schmidt, H. Endres, and C. R. Wolfe,

Chem. Ber. , 1979, 112 , 2837.75. K. Wieghardt, K. Pohl, I. Jibril, and G. Huttner, Angew.

Chem., Int. Ed. Engl. , 1984, 23 , 77.

76. L. T. Taylor and D. H. Busch, J. Am. Chem. Soc. , 1967, 89 ,5372.

77. R. I. Sheldon, A. J. Jircitano, M. A. Beno, J. M. Williams, and K. B. Mertes, J. Am. Chem. Soc. , 1983, 105 , 3028.

78. R. D. Hancock, Acc. Chem. Res. , 1990, 23 , 257.

79. L. Fabrizzi,M. Micheloni, andP. Paoletti, Inorg. Chem. , 1982,19 , 535.

80. K. P. Wainwright and A. J. Ramasubbu, J. Chem. Soc., Chem.Commun. , 1982, 277.

81. T. J. Collins, C. Slebodnick, and E. S. Uffelman, Inorg.Chem. , 1990, 29 , 3433.

82. M. Kodama and E. Kimura, J. Chem. Soc., Dalton Trans. ,1978, 1081.

83. L. Qian, Z. Sun, J. Gao, B. Movassagh, L. Morales, and K. B. Mertes, J. Coord. Chem. , 1991, 23 , 155.

84. A. Bencini, A. Bianchi, E. Garcia-Espa na, E. C. Scott,L. Morales, B. Wang, M. P. Mertes, and K. B. Mertes, Bioorg.Chem. , 1992, 20 , 8.

85. G. R. Newkome, J. D. Sauer, J. M. Roper, and D. C. Hager,Chem. Rev. , 1977, 77 , 513.

86. G. L. Rothermel,L. Miao, A. L. Hill, and S. C. Jackels, Inorg.Chem. , 1992, 31 , 4854.

87. E. Simmons and C. H. Park, J. Am. Chem. Soc. , 1968, 90 ,2428.

88. K. B. Mertes and J.-M. Lehn, in ComprehensiveCoordination Chemistry, eds. G. Wilkinson, R. D. Gillard,and J. A. McCleverty, Pergamon, Oxford, 1982, Vol. 2,Chap. 21.3, p. 915.

89. A. Bencini, A. Bianchi, P. Dapporto, E. Garcia-Espa na,M. Micheloni, J. A. Ramirez, P. Paoletti, and P. Paoli, Inorg.Chem. , 1992, 31 , 1902.

90. A.Bencini, A.Bianchi, M. Micheloni,P. Paoletti,P. Dapporto,P. Paoli, and E. Garcia-Espa na, J. Inclusion Phenom. , 1992,12 , 291.

91. D. H. Busch, Acc. Chem. Res. , 1978, 11 , 392.92. J. L. Sessler and A. K. Burrell, Top. Curr. Chem. , 1991, 161 ,

177.

93. S. R. Cooper and S. C. Rawle, Struct. Bonding , 1990, 72 , 1.

94. S. R. Cooper, Acc. Chem. Res. , 1988, 21 , 141.

95. M. Schr oder, Pure Appl. Chem. , 1988, 60 , 517.

96. S. Lucia, W. L. Sokol, L. A. Ochrymowycz, and D. B.Rorabacher, Inorg. Chem. , 1981, 20 , 3189.

7/28/2019 ligands

20/20


97. N. W. Alcock, H. Heron, and P. Moore, J. Chem. Soc., Chem.Commun. , 1976, 886.

98. V. B. Pett, L. L. Diaddario, E. R. Dockal, P. W. Coreld,C. Ceccarelli, and M. D. Glick, Inorg. Chem. , 1983, 22 ,661.

99. M. M. Bernardo, M. J. Heeg, R. R. Schroeder, L. A.Ochrymowycz, and D. B. Rorabacher, Inorg. Chem. , 1992,31 , 191.

100. B. de Groot, H. A. Jenkins, and S. J. Loeb, Inorg. Chem. ,1992, 31 , 203.

101. C. M. Lucas, L. Shuang, M. J. Newlands, J.-P. Charland, and E. J. Gabe, Can. J. Chem. , 1989, 66 , 639.

102. K. Mislow and R. Baechler, J. Am. Chem. Soc. , 1970, 92 ,3090.

103. E. P. Kyba, R. E. Davis, C. W. Hudson, A. M. John, S. B.Brown, M. J. McPhaul, L. K. Liu, and A. C. Glover, J. Am.Chem. Soc. , 1981, 103 , 3868.

104. L. F. Lindoy, in Progress in Macrocyclic Chemistry, eds.

R. M. Izatt and J. J. Christensen, Wiley, New York, 1987,Vol. 3, Chap. 2, p. 53.

105. J. S. Bradshaw, K. Krakowiak, and R. M. Izatt, Aza CrownMacrocycles, Wiley, New York, 1993.

106. F. Arnaud-Neu, M. J. Schwing-Weill, R. Louis, and R. Weiss, Inorg. Chem. , 1979, 18 , 2956.

107. F. Arnaud-Neu, B. Spiess, and M. J. Schwing-Weill, Helv.Chim. Acta , 1977, 60 , 2633.

108. J. Comarmond, B. Dietrich, J.-M. Lehn, and D. Parker, J.Chem. Soc., Chem. Commun. , 1985, 75.

109. A. Bencini, A. Bianchi, E. Garcia-Espa na, M. Micheloni, and P. Paoletti, Inorg. Chem. , 1987, 26 , 1243.

110. J. Comarmond, P. Plumer e, J.-M. Lehn, Y. Agnus, R. Louis,R. Weiss, O. Kahn, and I. Morgenstern-Badaru, J. Am. Chem.Soc. , 1982, 104 , 6330.

111. F. C. J. M. van Veggel, M. Bos, S. Harkema, H. van deBovenkamp, W. Verboom, J. Reedijk, and D. N. Reinhoudt, J. Org. Chem. , 1991, 56 , 225.

112. A. I. Popov and J.-M. Lehn, in Coordination Chemistry of Macrocyclic Compounds, ed. G. A. Melson, Plenum, NewYork, 1979.

113. Y. Inoue andT. Hakushi, in CationBinding by Macrocycles,eds. Y. Inoue and G. W. Gokel, Dekker, New York, 1990,Chap. 1, p. 1.

114. R. L. Brueining, R. M. Izatt, and J. S. Bradshaw, in CationBinding by Macrocycles, eds. Y. Inoue and G. W. Gokel,Dekker, New York, 1990, Chap. 2, p. 111.

115. M. R. Truter, Struct. Bonding , 1973, 16 , 71.

116. I. Goldberg, in The Chemistry of Functional Groups, ed.S. Patai, Wiley, New York, 1980, p. 175.

117. R. Hilgenfeld and W. Saenger, Top. Curr. Chem. , 1982, 101 ,1.

118. D. J. Cram, Science , 1988, 242 , 760.119. G. W. Gokel and J. E. Trafton, in Cation Binding by

Macrocycles, eds. Y. Inoue and G. W. Gokel, Dekker, NewYork, 1990, Chap. 6, p. 253.

120. F. R. Fronczek and R. D. Gandour, in Cation Binding byMacrocycles, eds. Y. Inoue and G. W. Gokel, Dekker, NewYork, 1990, Chap. 7, p. 311.

121. J. Tsukube, J. Coord. Chem. , 1987, 16 , 101.

122. K. E. Krakowiak, J. S. Bradshaw, and D.-J. Zamecka-Krakowiak, Chem. Rev. , 1989, 89 , 929.

123. J.-M. Lehn, Angew. Chem., Int. Ed. Engl. , 1988, 27 , 89.

124. S. Shinkai and O. Manabe, Host Guest Com- plex Chemistry III, Springer-Verlag, Berlin, 1984, p. 67.

125. L. E. Echegoyen, H. K. Yoo, V. J. Gatto, G. W. Gokel,and L. Echegoyen, J. Am. Chem. Soc. , 1989, 111 ,2440.

126. K. Kimura and T. Shono,in Cation Binding by Macrocycles,eds. Y. Inoue and G. W. Gokel, Dekker, New York, 1990,Chap. 10, p. 429.

127. H. Furuta, K. Furuta, and J. L. Sessler, J. Am. Chem. Soc. ,1991, 113 , 4706.

128. C. J. Burrows, in Inclusion Phenomena and Molecular Recognition, ed. J. L. Atwood, Plenum, New York, 1990, p. 199.

129. L. D. Margerum, K. I. Liao, and J. S. Valentine, in MetalClusters in Proteins, ed. L. Que Jr, American ChemicalSociety, Washington, DC, 1988, Chap. 6, p. 105.

130. M. H. Schmidt, G. M. Miskelly, and N. S. Lewis, J. Am.Chem. Soc. , 1990, 112 , 3420.

131. E. Fujita, C. Creutz, N. Sutin, and D. J. Szalda, J. Am. Chem.Soc. , 1991, 113 , 343.

132. M. P. Mertes and K. B. Mertes, Acc. Chem. Res. , 1990, 23 ,413.

133. D. Parker, Chem. Soc. Rev. , 1990, 19 , 271.

134. J. F. Carvalho, S.-H. Kim, and C. A. Chang, Inorg. Chem. ,1992, 31 , 4065.

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