bipyridine- the most widely used ligand. a review of molecules comprising at lesat two 2,2 bpy units

7/29/2019 Bipyridine- The Most Widely Used Ligand. a Review of Molecules Comprising at Lesat Two 2,2 Bpy Units

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Bipyridine: The Most Widely Used Ligand. A Review of Molecules Comprisingat Least Two 2,2-Bipyridine Units

Christian Kaes, Alexander Katz, and Mir Wais Hosseini*

Laboratoire de Chimie de Coordination Organique, Institut Le Bel, Universite Louis Pasteur, F-67000 Strasbourg, France

Received December 7, 1999

Contents

I. Introduction 3553II. Ligands with 2,2-Bipyridine Connected in

Positions 3 and 33554

III. Ligands with 2,2-Bipyridine Connected inPositions 4 and 4

3557

A. Nonmacrocyclic 2,2-Bipyridine MoleculesConnected in Positions 4 and 4

3557

B. 2,2-Bipyridine Macrocycles Connected inPositions 4 and 4

3563

IV. Ligands with 2,2-Bipyridine Connected inPositions 4,4 and 5,5 3564

V. Ligands with 2,2-Bipyridine Connected inPositions 5 and 5

3565


3565


3570

VI. Ligands with 2,2-Bipyridine Connected inPositions 6 and 6

3570


3570


3578

VII. Ligands with 2,2-Bipyridine Connected in MixedPositions 3582

VIII. Polymeric Ligands Containing 2,2-BipyridineUnits

3582

IX. Conclusion 3587X. Acknowledgment 3587

XI. References 3587

I. Introduction

The purpose of the present review is to present thediverse variety of molecules comprising at least two2,2-bipy r idine unit s in t he for m of an ex haus t ive

da t abas e. Rat her t han concent r a t e on a pa r t icularcla s s of a pplica t ion, t he a im her e is t o pr ovide anoverall view on the most explored chelate system incoordination chemistry. The intended audience is, onone side, researchers already active in the area an d,on the other, those who might be newcomers to it .This review will hopefully be useful to readers inverifying wheth er a t argeted ligand h as been previ-

ously prepared and in finding the relevant literatureassociated with its synthesis. Since the 2,2-bipyri-dine unit has been used in a variety of approachesdealing with structura l coordina tion chemistry a nd/or functiona l systems ba sed on 2,2-bipyridine meta lcomplexes, an art icle dealing with the organic syn-

t het ic s t r a t egies employ ed t o pr epar e t he l igandsystem, th e synthetic approaches to form th eir metalcomplexes, a nd phys ical propert ies of meta ls chelatedby 2,2-bipyridine ligand s would ha ve been too broa da theme for a single review. Therefore, in the presentcontribution, we mainly focus on structural as wellas synthetic features. However, when feasible, we doa l s o i n di ca t e t h e f u nct i on a l con t ex t g u id in g t h edesign of th e ligand systems reported.

Since its discovery at the end of nineteenth cen-tury,1 the bipyridine ligand has been used extensivelyin the complexation of metal ions. I t is possible todist inguish (Scheme 1) the symm etrical isomers ofbypridine (2,2, 3,3, and 4,4) from the a symmetrical

ones (2,3, 2,4, and 3,4), and of these, only the 2,3-a n d t h e 3 , 3-bipy r idines a r e found t o be na t ur a l lyabunda nt in certa in va rieties of tobacco.2-4

The 2,2-bipyridine liga nd ha s been extensivelyus ed as a met a l chela t ing l igand due t o i t s r obus tr ed ox s t a b il it y a n d e a s e of f un ct i on a l i za t i on . I ncontra st t o other ligan ds, such as catechol, which isdianionic, and derivatives of the acetylacetonate ion,which are monoan ionic, 2,2-bipyridine is a neutra lligand. It thus forms charged complexes wit h meta lcations, and this property ha s been exploited in t he

* E-mail: [email protected] sbg.fr. Present address: Department of Chemical Engineering; Univer-sity of California at Berkeley, Berkeley, CA 94720-1462.

Scheme 1. Symmetrical and AsymmetricalIsomers of Bipyridine

3553Chem. Rev. 2000, 100, 35533590

10.1021/cr990376z CCC: $35.00 2000 American Chemical SocietyPublished on Web 09/14/2000


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design and synt hesis of metal-bipyridin e complexes.

H er e t he va r ious major cla s s es and s y nt hes es ofmolecules are summarized comprising at least two2,2-bipyridine units (units that cannot be classifiedas higher oligopyridines such a s terpyridines, qua ter-pyridines, q uinquepyridines, etc.), wh ich ar e con-nected in different positions around the aromatic ring(3, 4, 5, and 6) by covalent bonds. Molecules contain-i n g m e t a l-coordinat ion interactions (such as fer-rocene-based ligands conta ining bipyridine) are in-cl u de d h er e on l y i f t h e b ip yr i di n e m oi et i es a r eat ta ched to the molecule following the creat ion of thecoordination bonds that hold the molecule together(polymeric meta l complex netw orks conta ining bipy-ridine5 are therefore not included here).

Ligands cont a ining t w o or mor e 2,2-bipyridineunits can in principle be used a s bridges to int ercon-nect metal centers in a well-defined spatial arrange-ment. The uses of such ligands as precursors forhelical assembly,6 chiral molecular recognition,7,8

luminescent devices,9,10 and ot her applica t ions inphotonics and optoelectronics11,12 a nd electrochemis-t r y 13 have been reviewed elsewhere. The classes of

molecules cont a ining t w o or mor e 2,2-bipyridineunit s can be categorized as (a) molecules hav ing unit s

connected in posit ions 3 a nd 3, (b) nonma crocyclicmolecules ha ving un its connected in positions 4 an d4, (c) ma crocyclic molecules ha ving unit s connectedin posit ions 4 and 4, (d) nonma crocyclic moleculeshaving units connected in positions 4,4 and 5,5, (e)nonma crocyclic molecules ha ving u nits connected inposit ions 5 an d 5 , (f) ma crocyclic molecules h a vingunits connected in positions 5 and 5, (g) nonma cro-cyclic molecules ha ving un its connected in positions6 a n d 6, (h) ma crocyclic molecules ha ving unitsconnected in posit ions 6 and 6, (i) asymm etricallyconnected units , and (j) polymers containing 2,2 -

bipy r idine. Tables cont a ining i l lus t r a t ions of t heligands are labeled according to their correspondingcategory.

II. Ligands with 2,2-Bipyridine Connected inPositions 3 and 3

Ligand 1a14-17 was obtained in three steps from 1,10-phenanthroline, by oxidation of the lat ter withKMnO4 to give 4,5-diaza fluorenone, which subse-quently dimerizes to give 1a (Table 1).16 The subse-quent dehydrogenation of 1a yields 2a,15 w hich hasalso been obtained via a reductive coupling strategy.17

Ligand 5,5-bis(1,10-phenanthroline) 3a h a s b ee n

Christian Kaes was born in Luxembourg in 1970. He graduated inchemistry at the Universite Louis Pasteur, Strasbourg. After completinghis Ph.D. in 1998 under supervision of Professor M. Wais Hosseini atthe Universite Louis Pasteur, Strasbourg, he joined the Goodyear TechnicalCentre in Luxembourg as a staff-engineer.

Alexander Katz was born in Minsk, Republic of Belarus, and raised inMinneapolis, Minnesota. He graduated Cum Laude in chemical engineeringat the University of Minnesota and performed research there with Professor

Michael D. Ward, during which time he developed a piezoelectricmicrorheometer for polymer films. Alexander received a Master of Sciencein Chemical Engineering from the University of Minnesota in 1994 andwas subsequently awarded a Fannie and John Hertz Foundation GraduateFellowship. He pursued doctoral studies in the rational design of catalyticmaterials at Caltech with Professor Mark E. Davis. Upon finishing hisPh.D. in 1998, Alex was awarded a NSF International Awards Fellowshipto investigate the use of supramolecular chemistry in the design andsynthesis of materials, as a postdoctoral researcher in the OrganicCoordination Chemistry Laboratory of Professor M. Wais Hosseini at theInstitute Le Bel, Universite Louis Pasteur, in Strasbourg, France. Currently,he is an Assistant Professor of Chemical Engineering at University ofCalifornia at Berkeley, where his interests are in the design and synthesisof materials for chemical engineering applications.

Mir Wais Hosseini was born in 1955 in Kabul, Afghanistan. In 1972 hemoved to Strasbourg, France. He was mainly educated at the UniversiteLouis Pasteur, Strasbourg, where he obtained his Ph.D. degree in 1983working with Jean-Marie Lehn. In 1981 he joined the French NationalResearch Centre as a permanent member. After a postdoctoral fellowshipspent in 1985 with Kenneth Raymond at Berkeley, he returned toStrasbourg where he continued to work with Jean-Marie Lehn. In 1990he was appointed as a Professor of Organic Chemistry at the UniversiteLouis Pasteur. In 1992 he was nominated for a period of five years atthe Institut Universitaire de France. He was an invited professor at theUniversity of Western Australia at Perth, University of Geneva, The Institutof Materials and Chemical Research, Tsoukuba, Japan, and Universityof Tokyo. His main research interests are in the area of moleculararchitectures in chemistry, biology, and physics and range from molecularreceptors and catalysts to molecular networks and molecular materials.

3554 Chemical Reviews, 2000, Vol. 100, No. 10 Kaes et al.


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Table 1

2,2-Bipyridine Ligands Chemical Reviews, 2000, Vol. 100, No. 10 3555


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synthesized by tw o successive S kraup reactions forthe purpose of forming a compound with chelatingability for Fe(II) and Cu(I).18 Dimers 4a,14,19 5a,20 a nd6a21,22 have been obta ined by sta rt ing from 9-diazo-4,5-dia za fluorene, and compounds 7a20 a nd 8a21 havebeen synthesized by starting from 2-diazo-1,1,3,3-tetramethyl-7,8-diazacyclopenta[1]phenanthrene. Thereduction of 5-nitro-1,10-phenanthroline by NaBH 4drives to the formation of 5,5 -azo-1,10-phenanthro-line 9a (among other byproducts).23 The ligand bis-(1,10-phenanthroline) 10a has been synthesized bysta rtin g from 5-am ino-1,10-phenan thr oline a nd form-

aldehyde, and its interactions with DNA have beeninvestigated.24 Lehn and co-workers prepared a tri-nuclear Ru-P d-Ru complex25 by ut i l iz ing l igand11a, w hich is a lar ger homologue of l igand 12a.Liga nd 11a has been synthesized in the form of itsmononuclear complex w ith Ru(II), by reacting 11,-12-diamino-dipyrido[3,2-a:2 ,3 -c]phenazin e wit h t hecomplex [Ru(phen)2(phen-5,6-dione)]2+.25 Macrocycle12a can be synth esized by tw o different methods: thecondensation of 5,6-dione-1,10-phenanthroline with5,6-diamino-1,10-phenanthroline and the reaction of5,6-dione-1,10-phena nt hroline w ith MeCO 2NH 4. Theluminescence properties and X-ray crystallographicstructure of the dinuclear complex of 12a w i t h R u -

(II) ha ve been investigat ed.26-28 The same ligand ha sbeen used to construct a Ru(II)-based coordinationpolymer. 27 Ligand 13a h a s b ee n s y nt h es iz ed b yfollowing the same type of reaction as for 11a, i.e., acondensation of the complex [Ru(phen or bipy)2(phen-5,6-dione)]2+ with 3,3,4,4-tetraamino-biphenyl, anda large association constant of the binuclear Ru(II)complex of ligand 13a to D NA has been reported.29

Ligands 14a-16a h a v e b een ob t a in ed b y t h econdensa tion of 4,5-dia za fluoren-9-one w ith va riousdiamines. Because of sp2 hybridization of the bridgingnitrogen a tom, protons in the t wo pyridines compris-

ing each bipyridine unit are nonequivalent, and thisis manifested in shifts in their proton NMR behav-ior.30 The binuclear rut henium complexes of ligan ds14a-16a ha ve been reported a nd give rise to metal-to-ligand char ge transfer absorptions and int ra ligandtra nsit ions in the ultra violet spectrum, a s well as toa short-lived emission.31

Ligands 17a-19a represent la rger h omologues of11a a nd 12a. Although attempts to synthesize 17astarting from 5,6-diamino-1,10-phenanthroline andtetra ketopyrene in a ma nner ana logous to 12a w er enot successful due to solubility problems, an alt erna-tive synthetic route was used that involved successivefunctionalization of precursor complexes. Thus reac-

Table 1 (Continued)



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tion of 6-amino-5-nitro-1,10-phenanthroline, whichwas obtained by nucleophilic amination of 5-nitro-1,10-phena nt hroline, wit h Ru(bipy)2C l2H 2O gave thecorresponding amino-nitro complex in high yield,w hich w a s s ubs eq uent ly r educed by hy dr a zine hy -dra te over P d/C t o yield a diam ino-conta ining rut he-nium-bipyridine complex. Condensation of this com-plex with t etra ketopyrene yielded 17a.32 Ligand 17aprovided the first example of supra molecular dim er-

ization of a binuclear metallic complex maintainedonly by -stacking interactions; however, electro-chemical and spectroscopic studies of the binuclearcomplex ha ve s how n t ha t t he va r ious par t s of t h isdimer rema in noninteracting, despite the extendedaromaticity of the complexed bridging ligand dimer.33

Liga nds 18a a nd 19a were prepared as their diru-thenium complexes by coupling benzoyl-substit utedphenazine units with corresponding diamines.34 Theanthraquinone spacer in 19a wa s shown to efficientlyquench emission processes relative to ligand 18a.34

Dimer 20a of 7,8-dia za phencyclone ha s been usedas a s our ce of 1,10-phena nt hr oline, in or der t ostereoselectively at ta ch the la t ter over a r igid mol-

rac-type system.35

In complexing t he phenan throlineof this molrac system by Ru[(bipy)2(CF 3S O3)2]2+,d iads ar e obt a ined w hich can be s t udied for t heirintramolecular charge transfer processes. The heat-ing of 20a to a temperature of 100 C drives isomer-iza t ion t o 21a.35 Ligand 20a and 9-diazo-4,5-diaza-f luor ene ha ve a ls o been us ed a s phenant hr olinesources to synthesize a molrac-type system contain-ing two 1,10-phenanthroline units in ligands 22a,23a, 24a, 25a, a n d 26a.36,37 Alternatively 6,7-diaza-phencyclone can be used a s a phena nt hroline sourceto produce ligands 27a-33a in Diels-Alder-basedreaction involving ring-strained alkenes.37

Liga nds 34a a nd 35a ar e linear oligobipyridines

and were synth esized from 3,3-diamino-2,2 -bipyri-dine for NMR and circular dichroism-based investi-gations of intramolecular hydrogen bonding. 38 Thecoronand 36a has been synthesized by reacting thecesium salt of 3,3-dicarboxy-2,2 -bipyridine with adichloro derivative of poly(ethylene glycol).39 Finallythe disk 37a,40 which belongs to t he sam e family ofl igands a s 11a-12a a nd 17a, ha s been synt hesizedby rea cting 5,6-dione-1,10-phena nt hroline w ith hexa-aminobenzene.

III. Ligands with 2,2-Bipyridine Connected inPositions 4 and 4


Derivatives of 2,2-bipyridine tha t bear functionalgroups in the 4 an d 4 positions have been shown tocomplex met a ls in s q ua r e-plana r , t e t r ahedr a l , andoctahedral coordination geometries. Approximatelytwo decades ago, several investigators observed thatthe lowest energy excited state of several Ru(II) andOs(II) (octahedral coordination) complexes had suf-ficiently long lifetimes in solution to allow for energyt r a n s f er p roce ss es t o occu r . I t w a s f u rt h e rm or ereported that Ru(II) and Os(II) polypyridine com-plexes were suitable reagents for energy and electron

t r a ns fer pr ocess es due t o t heir r ever sible r edoxbeha vior . Since t hen, inves t iga t or s have pr epar edmultimetallic systems in order to further elucidatethe a bove-ment ioned processes. One meth od of syn-thesizing such systems is to covalently link two ormore 2,2-bipyridines in positions 4 and 4 (or simi-larly in positions 5 and 5) by a bridge, wh ich can beeither saturated or unsaturated. The first compound1b (Ta ble 2) prepared a long t hese lines comprised a

dimethylene bridge and was originally synthesizedby Elliott and co-workers with an oxidative additionreaction, by reacting t he monolithium derivat ive of4,4-dimethyl-2,2-bipyridine w ith 1,2-dibromoetha ne,wh ich served as t he oxidant in th e reaction scheme.41

Fur ue and co-w or ker s la t er s y nt hes ized t he s amecompound 1b by a l t er nat ively r eact ing t he mono-lit hium der iva t ive of 4,4-dimethyl-2,2-bipyridinewith 4-bromomethyl-4-methyl-2,2-bipyridine.42 Com-pounds possessing bridges (CH 2)3 2b,42 (CH 2)4 3b,43

(CH 2)5 4b,44 (CH 2)7 5b,45 (CH 2)10 6b46, a n d ( C H 2)127b44 ha ve all been synthesized by reacting 1 equiv oft he monolit hium der iva t ive of 4,4-dimethyl-2,2-bipyridine with the corresponding dibromo deriva-

t ive. Ell iot and co-w or ker s have a ls o s y nt hes izedbis(4-(4-methyl-2,2-bipyridyl)methyl)ether 8b a ndbis(4-(4-methyl-2,2-bipyridyl)methyl)sulfide 9b.47 U s-ing a similar synthetic approach, ligand 10b, 1,3-bis-(4-methyl-2,2 -bipyr idin-4-yl)-2-propan ol, w a s syn -thesized by reacting the monolithium derivative of4,4 -dimethyl-2,2-bipyridine with ethy l forma te wit ha n overa ll yield of 20%.48

By performing a Wittig condensation between (4-(4 -met hy l-2,2- b i p y r i d y l ) m e t h y l ) t r i p h e n y l p h o s -phonium bromide and 4-formyl-4-methyl-2,2-bipy-ridine, Meyer a nd co-workers syn th esized t r a n s -1,2-bis(4-methyl-2,2-bipyridin-4-yl)ethylene 11b.49 AWittig condensat ion w as also used in t he synthesisof 1,4-bis(4-methyl-2,2-bipyridin-4-yl)buta-1,3-diene12b.50 The butadiene ligand was subsequently trans-f or m ed v ia a D i el s-Alder addit ion us ing diet hy lfumara te t o give the diester product 13b.50 Hydroly-s is of t h e l a t t e r w i t h K O H f ol low e d b y a d ou b ledecarb oxylat ion yielded ligan d 1,4-bis(4-methyl-2,2-bipyridin-4-yl)benzene 14b.50 Bot h l igands 14b a nd15b ha ve t he pot ent ia l t o s er ve as r ig id br idgingligands for redox and photoactive applicat ions. Liga nd16b was independently synthesized by the groups ofBeer 51 and Schmehl,52 and bot h r eact ed t he mono-lit hium der iva t ive of 4,4-dimethyl-2,2-bipyridinewith terephtha ldehyde in their synt hetic procedure

to first produce the diol 17b, w hich w a s dehy drat edt o 16b. Ligand 18b was synthesized by reacting themonolithium derivative of 4,4-dimethyl-2,2-bipyri-dine w it h R,R-dibromo-p-xylene,44 a n d i t h a s b e e nused in investigat ing energy t ra nsfer processes be-tw een different meta l centers.53

Elliott investigat ed the photoinduced electronictransfer properties of the different Ru(II) dinuclearcomplexes of var ious bipy r idine l igands t ha t ar econnected by bridges in which the number of con-nections remained fixed but where the steric envi-ronment varied (ligands 19b, 20b, 21b, 22b, 23b,24b).54 The nucleophilic substitut ion of t he mono-lithium d erivat ive of 4,4-dimethyl-2,2-bipyrid ine on



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Table 2



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1,4-cyclohexanedione gives a mixture of ci s 23b a ndt r a n s 24b conformations of 1,4-bis((4-methyl-2,2-bipyridin-4-yl)methyl)cyclohexane-1,4-diol. Dehydra-tion of this diol mixture w ith sulfuric acid in a ceticacid gives a mixture of different dienes containingbis((4-methyl-2,2-bipyridin-4-yl)methyl)cyclohexa-1,4-diene 19b.54 Rearomatization of this diene with

5%P d/C fina lly yields R,R-bis(4

-methyl-2,2

-bipyri-din-4-yl)-p-xylene 20b.54 The ci s 21b a nd t r a n s 22b

derivat ives were obta ined upon hydr ogena tion of thediene mixture.54

By graft ing two fragments of 4-formyl-4-methyl-2,2 -bipyrid ine ont o 3,3-diamino-N-methyl-dipropyl-amine and 1,8-diaminooctane, ligands 25b a nd 26b,r espect ively , w er e s y nt hesized, a nd t he dinuclearRu(II) complexes of t hese ligan ds ha ve been studiedas potentia l candidat es for double intercalat ion w ithDNA.46 Liga nd 27b w as s y nt hes ized by r eact ing 2equiv of the monolithium deriva tive of 4,4-dimethyl-2,2-bipyridine with 1 equiv of bis(hydroxymethyl)-2,2-bipyridine methylsulfonate.55 The unsaturated

homologue 28b,55 which possesses two t r a n s doublebonds, w as synthesized from t he a ddit ion of 2 equivof 4-((diph eny lphosphin oyl)meth yl)-4-methyl-2,2-bi-pyridine (synthesized from the monolithium deriva-t i ve of 4, 4-dimethyl-2,2-bipyridine an d chlorodi-phenylphospha ne) with 1 equiv of 6,6-diformyl-2,2-bipyridine. Similarly structured ligan ds 29b a nd 30b,

comprisin g a 2,2-bipyridin e-ty pe bridg e connected inpositions 4 an d 4, ha ve been synthesized by Beer and

co-workers. 51 U p o n a d d i n g 2 e q u i v o f t h e m o n o -lithium d erivat ive of 4,4-dimethyl-2,2-bipyridine to1 equiv of 4,4-diformyl-2,2-bipyridine, the diol 29bwa s obta ined, which upon dehydra tion yielded ligan d30b. Using the same reaction sequence, compounds31b a nd 32b, possessing bridges of th e coronan d t ype,have also been synthesized, and their Ru(II) com-plexes have been prepared.51

An elegant method of grafting a porphyrin betw eentw o 2,2-bipyridines has been developed by Therien.In t his synt hesis, the monolithium deriva tive of 4,4-dimethyl-2,2 -bipy r idine w a s t r ans for med int o a n

Table 2 (Continued)



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organotin derivative with tr ibutylt in chloride. 56 Acoupling reaction of th is derivat ive w ith 5,15-di-bromo-10,20-diphenylporphyrin wa s performed togive ligan d 33b.56

The dinuclear Ru(II) complex of 34b, possessing aconjugated pentene-type spacer, has been reportedto show an electronic coupling following excitation,des pit e t he r ela t ively la r ge dis t ance bet w een t hemet a ls in t his ca s e . 57 This ligand was synthesized

using a Witt ig rea ction betw een 4-((diphenylphos-phinoyl)methyl)-4-methyl-2,2-bipyridine an d 2,7-diformyl-2,4,6-octatriene.57 The Ru(II), Rh(II), andOs(II) complexes of ligands 35b, 36b, 37b, 38b, a n d39b have been s y nt hes ized, and t he s t abil it y con-stants of the reversible complexes have been mea-s ur ed w it h differ ent ha logen a nions a nd H 2P O4-.58

Thes e cons t a nt s have been s how n t o var y s ignif i-ca n t l y d ep en d in g on t h e n a t u r e of t h e s pa c er .58

B ridges of the ethyne t ype 40b and diethyne type 41bha ve been graft ed between two 2,2-bipyridines.59 Thesynthesis begins from 4-bromo-2,2-bipyridine an dconsists of graftin g the tr imethylsilylacetylene in the

presence of [P d(PP h 3)4] a n d n-PrNH 2. Upon repeatingthe sam e rea ction sequence followin g deprotection oft he TMS moiet y w it h a w eak bas e (K 2C O3 or KF),the compound 40b possessing a n ethy ne-ty pe bridgeis obta ined. Compound 41b, w hich ha s a diet hy ne-t y pe br idge, is pr epar ed by a coupling r eact ionbetween two identical molecules (CuCl, TMEDA, andO2). The reaction of 4-ethynyl-2,2-bipyridine witht r a n s -[Pt(PBu n3)2C l2] us ing a C uI ca t a ly s t a nd H N i -P r 2 a s ba s e leads t o t he or ganomet a l l ic compound42b.60 Beer et al. have grafted 2 equiv of 4-chloro-carbonyl-2,2-bipyridine on a diamine derivative ofcalix-[4]-arene in synthesizing 43b for anion recogni-tion applicat ions.61 Ligand 44b, w h i c h h a s s h o w n

application potential as a molecular switch, requiresthat an azo-type bridge connect two 2,2-bipyridinesand was synthesized by reductive coupling start ingfrom 4-nitro-2,2-bipyridyl-1-oxide using NaBH 4 a ndP d-C .62 Imine 45b h a s b e e n s y n t h e s i z e d b y t h estoichiometr ic add ition of 4,4-diamino-2,2-bipyridineto glyoxal a nd ha s been used a s a model compoundin reference to polymeric ligands prepared via thesam e synthet ic condensat ion sequence.63 Ligand 46bhas been obt a ined by a n ox idat ive addit ion on t heC-Br bond of 47b over [PtMe2(tbu 2bpy)] (tbu 2bpy:4,4 -di(tert-butyl)-2,2-bipyridine); 46b a nd 47b serveas sta rtin g compounds for the step-by-step synt hesis

of organometa llic dendrimers.

64

The silane-containingbridging ligan ds 48b65 a nd 49b66 ha ve been synthe-s ized by our gr oup, by r eact ing t he monoli t hiumderivative of 4,4-dimethyl-2,2-bipyridine with cor-responding silyl derivat ives. The binuclear Ru(II)complex of calix-[4]-arene 50b is an anion bindingelement that possesses selectivity for the H 2P O4-

anion.67 Ligand 51b was synthesized by amide for-ma tion between dimethyldibenzidinium dichloride,which w as synthesized from benzidine via a protec-tion/a lkyla tion/deprotection str a tegy , and 4-methyl-2,2-bipyridine-4-carboxylic a cid using bromotripyr-rolidinophosphonium hexafluorophospha te as thecoupling reagent .68 Dimethyldibenzidinium dichloride

wa s also used to synthesize ligand 52b via couplingwith 4-bromomethyl-4-methyl-2,2-bipyridine usingcesium carbona te, in an a t tempt t o prepare a ligandwith a grea ter redox potentia l relat ive to 51b.68 F orthe same purpose, ligand 53b w as s y nt hes ized bysubstitution of N,N-dimeth yl-benzidine by 4-bro-momethyl-4-methyl-2,2-bipyridine followed by amideformation via coupling with 4-methyl-2,2 -bipyridine-4-carboxylic acid.68 Several compounds have been

s y nt hesized us ing pr ecur s or s cont a ining t w o py -ridines already connected by a bridge and by a ddingbipyridines by continuation to produce ligands 15b,54b, 55b, and 56b.69 A similar approach was followedby Rehber g and Kr onke in t he s y nt hesis of 57b,w hich w as made in s ever a l s t eps via t w o Michaeladdition reactions followed by a double dehydration. 70

Compounds 58b a nd 59b were obtained by an oxida-tive addit ion sta rt ing from the monolithium deriva-tiv e of 4,4-dimethyl-2,2-bipyridine an d a n oxidizingagent s uch as e i t her Br 2, I 2, or 1,2-dibromoeth a ne,an d t heir electronic absorption a nd emission proper-ties ha ve been investigat ed.71

A new fa mily of siderophore, w hich includes com-pounds 60b a nd 61b (Table 3), has been synthesizedby Collet and co-workers by start ing from the tr is-am ine derivat ive of cyclotrivera try lene and 4-formyl-4-methyl-2,2-bipyridine or st ar t ing from t he tr ime-sylate derivat ive of cyclotriveratr ylene and the a cid4-carboxyl-4-methylcarboxylate-2,2-bipyridine.72 Theiriron complexes have been synthesized, and X-raydif fr act ion has been us ed t o s t udy t he s pin s t a t escoexisting in the iron-complexed ligands.73 As withligands 46b a nd 47b, 62b can also be used for theconst ruction of organometa llic dendrim ers.64 Ligand63b was synthesized from 4,4-dimethyl-2,2-bipyri-dine and 1,1,1-tris(4-bromomethylphenyl)ethane us-

ing diisopropylamine/n

-butyllithium as coupling re-agents.74 Its hemicage Ru(II) complex exhibited longerexcited-sta te lifetimes a nd higher emission qua ntu myields than corresponding model complexes, whichw er e a t t r ibut ed t o a decr eas e in t he r a t e of nonr a-diat ive decay via vibrat iona l dea ctivation in the morerigid complexes.74 Ligand 64b compr is es a t e t r a-nuclear Ru(II) complex of calix-[4]-arene which, likeligand 50b, h a s b e e n u s e d a s a n a n i o n b i n d i n gelement that possesses selectivity for the H 2P O4-

anion.67 Ligand 65b wa s synthesized using 4-bipy-r idy lbenzy la ldehy de, w hich w as pr epar ed us ing amodified Hantzsch pyridine synthesis, by reactionwith pyrrole following classic Adler porphyrin syn-

thesis , although reported yields were low with thismethod.75 B y t r e a t i n g 65b w i t h z i n c a ce t a t e i n a nacetic acid-wa ter mixture, 66b wa s obta ined.75 Analy-sis of meta l qu enching of Cu(II)-bound 66b w a s us edto illustra te th at the presence of multiple metal ionb in d in g s it e s c a n b e a s ig n if ica n t f a ct or i n t h econcentration dependence of metal ion quenching.75

Ligand 67b wa s synt hesized star ting from 39,42-bis-(benzyloxy)calix[6]arene, which was converted to thet et r acy ano and t e t r amino analogues by t r ea t mentwith bromoacetonitrile and reduction with diborane,respectively. The tetraamine was quenched with 4equiv of 2,2-bipyridine-4-carbonyl chloride to afford67b in 55%yield.76 Further treatment of ligand 67b



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w it h ci s--[Ru(bpy)2(DMSO)Cl]PF6 led to the forma-tion of a complex retaining sterochemistry a nd show-ing a strong molar circular dichroism spectrum, incomparison with a racemat e-treat ed 67b, which ha dan inactive spectrum.76 The related 68b, w hich is

based on resorcinarene chemistry, w as synthesizedusing simila r coupling chemistr y to 67b. Ligand 68bw as us ed t o bind chlor ide, acet a t e , and benzoat eanions and showed a particular preference for car-boxylate an ions.77

Table 3



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Table 4



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By connecting two 2,2-bipyridines in positions 4a n d 4 by differ ent br idges , macr ocycles havingcoordinat ion sites directed towa rd th e exterior of thema crocycle (exoditopic coordina tion sit es) are formed.Most of these ligands have been synthesized so as tomake exonuclear complexes with transit ion metals(f r om g r ou ps VI I I , I X , a n d X ). C om p le xe s w i t htra nsit ion m etals from group VIII ha ve been inves-

tigat ed in part icular because of their photoinducedelectron transfer behavior, which may be potentiallyuseful for a pplicat ions dealing w ith t he tra nsforma -tion and storage of solar energy.9-12

Du rr et a l. ha ve synt hesized th e heterocyclic coro-n a n d 1c (Table 4), which possesses bridges based onpoly(ethylene glycol), by reacting the dicesium saltof t h e a c id 4, 4-dicarboxylate-2,2-bipyridine withdibromopentaethyleneglycol in DMF with an overallyield of 3%.78 O n a r ela t ed appr oach, B eer a nd co-workers have synthesized macrocycles 2c, 3c, a nd 4c,which serve as novel chromophores for multielec-t r onic phot or edox pr oces s es in t he for m of t heirpolynuclear complexes.51 Macrocycles 2c, 3c, and 4c,

which all have double bonds, result from the dehy-dra tion of the corresponding t etra alcohols with phos-phoric acid. Macrocycles 5c, 6c, a n d 7c have beensynthesized by a one-pot synthesis in four steps, byreacting 1 equiv of lithium diisopropylamide with4,4-dimethyl-2,2-bipyridine, followed by th e add itionof 1/2 equiv of the appropriate aldehyde, 1 equiv ofLDA, and finally an other 1/2 equiv of aldehyde.51 TheSchiff base 8c is obta ined by reacting 4,4-diformyl-2,2-bipy r idine and t r ie t hy lenet r iamine in C H 3C Nand is reduced by the action of NaBH

4in methanol

to give ligand 9c.79 The result ing cyclopha ne ha s beenused to investigate mononucleotide (in its monophos-pha t e for m) complexat ion w it hin t he cavit y of t hecycle.79 Macrocycle 10c, synthesized in the form ofi t s dinuclea r Ru(II) complex , s how s pr onouncedr ecognit ion for chlor ide anion r ela t ive t o H 2P O4-

anion.80 The synt hesis wa s a ccomplished in severalsteps sta rting by condensing 4,4-bis(chlorocarbonyl)-2,2-bipyridine with a tert-butoxycarbonyl monopro-tected ethylenediamine followed by Ru(bipy)2C l22H 2O complexation. The complex thus obtained isdeprotected in acidic media an d condensed w ith t heRu(II) complex of bis(chlorocarbonyl)-2,2-bipyri-

Table 4 (Continued)



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dine.80 Cyclophane 11c wa s synthesized star ting frombis(bromomethyl)-2,2 -bipyridine, wh ich wa s synt he-sized from 4,4-dimethyl-2,2-bipyridine via oxidationwith chromium(VI) oxide in concentra ted H 2S O4,esterificat ion in MeOH a nd H 2S O4, reduction t o thediol w it h Na BH 4 in Et O H , a nd f ina l ly br ominat ionin 48%hydrobromic acid.81 High dilution conditionswere then used to react the bis(bromomethyl)-2,2-bipyridine with 4,4-bipyridine in tw o steps to give

the t arget compound 11c in 25%yield.81

Beer e t a l .synthesized ligands 12c-15c for use in selectiveanion recognition by equimolar addit ion of corre-sponding bis(acid chloride) a nd dia mine in t he pres-ence of t r iet hy lamine bas e . These l igands s how selective complexes with Cl-, H 2P O4-, a n d O A c-

anions depending on ligand topology and receptorcavity size.82 In our laborat ory, we have synt hesizedthe ma crocycles 16c-28c. Many of these exo-ligandsha ve been used as building blocks for the constructionof unidimensional coordinat ion polymers83-87 a ndwere synth esized via the m onolithium derivat ive of4,4-dimethyl-2,2-bipyridine. Ligand 17c w a s s yn -thesized by the dehydrogenation of 16c, and i t s O s -

(II) homobinuclear diastereomeric complexes werepr epa r ed, s epar a t ed, and char act er ized by s ingle-cr y s t a l X-r ay diffr a ct ion, cyclic volt ammet r y , andabsorption spectroscopy, w hich showed the presenceo f w e a k m e t a l-met al int er a ct ions in t his cas e .84

During the course of synt hesizing ligands 16c-24c,we ha ve also been successful in isolat ing the pa rtia llyoxidized compound 25c,84 the tr itopic homologuecompound 26c,85 and t he four 2,2-bipyridine unithomologues of the type 27c85 a nd 28c.86

The first synt hesis of a t ritopic polypyridine ligan dwa s performed by B ossma nn a nd D urr in 1992, usinga templated single-step synthesis from Ru(II) com-plexes of hexacesium-bipyridine-tris-dica rboxylat e

a n d t h e R,-oligo-glycol-dichlorides und er h igh dilu-tion conditions to yield ligands 29c a nd 30c.88 TheRu(II)-corona tes of these ligand s w ere prepa red a nddemonstra ted bimolecular electron tra nsfer and rela-tively high photostabilities.88 The bis[2]cat ena ne 31cincorporating 2,2-bipyridine ligands wa s synt hesizedusing a donor/a cceptor t emplat e-directed sy nt hesisand w as us ed t o make a mechanica l ly int er lockedcoordination polymer bridged with Ag(I) ions.89

C r y pt a nd 32c, possessing coordination sites di-r ect ed t o bot h t he ex t er ior and int er ior , ha s beensynthesized by a (2 + 3) tripodal-type condensa tion,by r ea ct ing 3 eq uiv of 4,4-diformyl-2,2-bipyridinewith 2 equiv of tr is(2-aminoethylamine) in dry ac-

etonitrile w ith a n overa ll yield of 70%.90 The Schiffba s e 32c can be reduced by NaB H 4 to give cryptan d33c.90

IV. Ligands with 2,2-Bipyridine Connected inPositions 4,4 and 5,5

The group of von Zelewsky ha s conceived chi-ragen (chirality generat or)liga nds for the design an dsynthesis of molecular assembly-based structurespossessing a predetermined chirality . This ha s provenespecially useful for complexes wit h tr a nsition meta lshaving an oct a hedr al coor dinat ion geomet r y andcon t a i n in g m or e t h a n on e b ip yr i di n e i n a cisoid

configuration. To synthesize enantiomerically pureRu(II ) complexes, von Zelewsk y a nd co-w orkers ha veprepa red different liga nds, wh ich serve as a ba sis forthe synthesis of polynuclear assemblies. Compound1d (Table 5) was obta ined by a n oxidat ive addit ion,by reacting t he monolithium derivat ive of (-)-[4,5]-pinene-bipyridine with I 2.90 Ligands 2d a nd 3d a re

Table 5



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obtained by reacting the monolithium derivative of(-)-[4,5]-pinene-bipyridine w ith HC OOEt or Me2-SiC l2.90 The CD spectra of ligands 1d, 2d, a n d 3d insolution have allowed conformational information tobe elucidat ed.90 B y preparing t he same monolithiumderivative and by reacting it with 1,3-dibromopro-pane, 1,4-dibromobutane, 1,5-dibromopentane, and1,6-dibromohexane, compounds 4d,92 5d,90 6d,90 a nd7d,93 r es pect ively , w hich have s a t ur a t ed a l iphat ic

chains of di ffer ent lengt h a s s pa cer s, have beens y nt hes ized. Many of t hes e l igands have been r e-ported to form stereoselective complexes with metalsin w hich the h elical chirality of the resulting complexis completely predetermined by the ligand. 92,94 Chi-ragen molecules 8d, 9d, a n d 10d, having xylene-based spacers connected in different positions, havealso been synthesized.95 In a n at tempt to increase thenumber of coordination sites, bis(bromomethyl)-2,2-bipyridine has been used as a bridge that joins twopinene-bipyridine units in 11d, w h i c h a l l o w s , i nprinciple, the synt hesis of a tr inuclear species.92 Thesuperchiragen molecule 12d94 ha s been synthesizedby followin g procedures simila r t o those report ed for

ligand 1d. Liga nd 13d, wh ich is the endoma crocycleequivalent of 10d, for ms cir cular helica t es uponassembly with Ag(I) ions.96 In changing t he s pacerbridge of this endo ligand from phenyl to naphthylin 14d, a polymeric infinite helix (of predeterminedhelicity) wit h Ag(I) ions is obta ined, th us highligh tingthe importa nce of (among others) -sta cking int erac-tions in the supramolecular assembly process.97

V. Ligands with 2,2-Bipyridine Connected inPositions 5 and 5


The dimet hy lene br idge of l igand 1e (Ta ble 6)results from an oxidative addition of the monolithiumderivative of 5,5-dimethyl-2,2-bipyridine. 71 The fluo-rescence emission of 1e, w h i c h s h o w s a n i n t e n s efluorescence in contrast to the corresponding 2,2-bipyridine, was suggested to be favored by the linkingof t he t wo bipyridines in 1e to thus provide for t hepossibility of interaction between one electronicallyexcited part of the molecule with the other part ofthe molecule in the ground sta te. Sim ilar fluorescencebehavior was observed for ligands 58b a nd 59b.71

Compound 2e wa s obta ined by an oxidat ive addit ions t a r t ing fr om 3,3-ethylene dipyridine using a pal-

ladium-carbon catalyst.98

B y heating th e sodium saltof 5-sul fony l-2,2-bipyridine to a tempera tur e of 500C under a nit rogen flow, liga nd 3e is obta ined, whichcan be oxidized by K 2C r 2O7 to obta in compound 4e.99

Liga nd 5e has been synt hesized by reacting 2 equivof 5-bromomet hy l-5-methyl-2,2 -bipy r idine w it h 1equiv of N,N-dimethylethylenediamine in CH 3C Nusing K 2C O3 as t he bas e , a nd 5e has been shown tocoor dinat e chlor ide a nd br omide anionic gues t s ,which has been investigated using NMR spectros-copy.100 The reaction of the monolithium derivativeof 5,5-dimethyl-2,2-bipyridine with diiodometha neleads t o 6e possessing a trimeth ylene bridge, and t hecomplexa tion of this ligan d w ith t wo Fe(II) at oms has

been reported t o lea d to a mesohelica te.101 Ligand bis-(2,2-bipyridin-5-yl)ethyne 7e has been synthesizedby reacting 5-bromo-2,2-bipyridine with 5-ethynyl-2,2-bipyridine in the presence of diisopropylamineand [Pd(PPh 3)4].102 The related bis(2,2-bipyridin-5yl)-but adiy ne 8e ha s been synt hesized by the oxidativeaddition of 5-ethynyl-2,2-bipyridine in the presenceof CuC l as a r igid and preorgan ized ditopic ligan d.102

The r eaction of the monolithium derivative of 5,5-

dimethyl-2,2-bipyridine with 5,5

-bis(bromomethyl)-2,2-bipyridine leads to ligan d 9e, a n d t h e l a t t e r i n

the presence of either Ni(II ) or Fe(II ), depending onthe experimenta l conditions, gives a t riple trinuclearpropeller or a circular pentanuclear helix.103 Like-wise, a similar synthetic coupling procedure was usedin the synt hesis of ligands 10e a nd 11e.65,104 Ligand10e has been reported to form flat tened tetrahedrawith Ag(I) a nd Cu(II) ions.104

Ligand 12e, wh ich is ba sed on a combina tion of thecation complexing [2,2] macrocycle and tw o 2,2-bipyridine units, has been synthesized by the reactionof 5-bromomethyl-2,2-bipyridine on th e crown etherand in t he pr es ence of ba s e ; how ever , i t s how ed

relat ively poor alkali a nd a lkaline eart h meta l cationextraction capability.105 Ligand 13e wa s also synthe-sized by a lkylation using the same reagent. 106 Othermacrocycles containing an amide linkage 14e-17ewere synt hesized by the condensat ion of 2 equiv of5-chlorocarbonyl-2,2-bipyridine a nd the correspond-ing a mine-cont a ining cr ow n et her . Tit r a t ion andelectrochemical da ta from Ru(II) complexes of th eseligands suggest tha t both the crown ether portion andthe bipyridine portion of the meta l-bound ligan d cancooperat ively enha nce the str ength of chloride anioncomplexation.107

Ligands 18e through 20e were syn thesized from

the corresponding dibromina ted a romat ic compound(benzene, na phtha lene, a nd a nthr acene) with 5-ethy-nyl-2,2-bipyridine in n-PrNH 2, w h i c h s e r v e d a s ageneral base and solvent, and [Pd 0( P P h 3)4] a s c a t a -lyst , for energy tra nsfer studies using well-defined,rigid molecular architectures.108 I n a n a t t e m p t t odesign a sy stem for the tr a nsfer of energy or electr onsa long a single preferred direction and over relat ivelylong distan ces, Vogt le et al. synthesized the rigida lky ne 21e.109 This ligand has been synthesized viabis-alkene (E,E) 22e, w h i ch i s o bt a i n ed u s in g aWadsworth-Emm ons reaction. The brominat ion ofthe a lkene gives th e tetra bromo derivative 23e, a n dthe dehydr obromina tion of the lat ter in th e presence

of an excess of KOH gives the desired diyne 21e. TheRu(II) and Os(II) complexes of 22e have been inves-t i ga t e d , a n d e a ch u n it d i sp la y s a n i n dep en d en ta bsorption spectrum a nd electrochemical properties,regard less of the presence of a second meta l at ta chedto the liga nd. I n t he case of the mixed meta l complex,quenching of Ru-based luminescence int ensity byener gy t r a ns fer t o t he O s -ba s ed unit has been r e-ported.110 Adamantane-based ligands 24e a nd 25ewere synthesized by coupling a bipyridine phospho-na t e es t er w it h adamant anedicar box aldehy de andbiadama nt a necar boxaldehy de, r espect ively . C om-plexes containing Ru(II) a nd Os(II) meta l ions w iththese ligands were also prepared. As in the case of



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Table 6



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ligand 22e described above, each un it on t he liganddisplayed its own absorption spectrum and electro-chemical properties, and in the mixed metal unitsconta ining Ru(II) a nd Os(II), electronic energy t ra ns-

fer was reported to occur.111

The subst itut ion of bis-(bromomethyl)-2,2-bipyridine by 5-hydroxymethyl-5-methyl-2,2-bipyridine in th e presence of ba se givesligand 26e.112 The condensa tion of 5-ethyn yl-2,2-bipy r idine w it h 5,5-dibromo-2,2-bipyridine givesligand 27e.113 In constructing molecules that haveanion r ecognit ion ca pabil it y , Beer e t a l . have s y n-t hes ized l iga nds 28e a nd 29e.114 Ligand 28e isobtained by treating 4,4-bipyridine with 2 equiv of5-bromomethyl-2,2-bipyridine. Ligand 29e is ob-t a i n ed b y a g r a f t in g a p pr oa c h i n w h i ch t w o 4, 4-bipy r idines a r e a t t ached t o 5,5-bis(bromomethyl)-2,2-bipyridine followed by t he a ddition of 2 equiv of5-bromomethyl-2,2 -bipyridine.114 Ligand 30e, s y n-

th esized fr om 5-car boxy-2,2-bipyridine, 1,1

-carbon-yldiimidazole, and 1,3-diaminopropane, ha s been

reported t o aut oassemble in th e presence of Fe(II) toa dinuclear tr iple propeller .115 Wi t h t h e g oa l ofconstr ucting molecula r structu res to tran sfer energyvia photochemical processes, Belser et al. have syn-thesized liga nd 31e by condensing the acid chlorideof 2,2-bipy r idine w it h a diamine der iva t ive of an-thracene.116

Wasielewski and co-workers synthesized pseudo-conjugated polymers conta ining chelating ligandsthat were based on model compounds 32e, 33e, a n d34e.117 Liga nd 35e was also synthesized by investi-ga tors in t he field of bipyridine-conta ining polymers

as a model compound for comparing photophysicalcharacterist ics.118 Ligands 36e, 37e, a n d 38e, wh ichhave been synthesized in several steps by solid-phasepeptide synthesis, serve as sensors for the detection

of metallic ions.119

These peptide-based complexingagents have been reported to have special fluorescentproperties th at supposedly result from photoinducedelectronic transfer, which is dependent on the pres-ence or absence of meta ls, with t he ant hra cene groupplaying t he role of a cceptor a nd the dimethoxyben-zene that of donor. In the absence of metallic ionsthere are almost no interactions between the chro-mophores, probably due to the large distance betweenacceptor and donor. In this case, a possible role oft h e m et a l l ic i on m a y b e t o r ed u ce t h e d i st a n c ebetween donor sites a nd t he a cceptors by fixing theconformation of the ligand as a result of complex-ation.119

I n a n a t t e m pt t o s y n t h es iz e a m e t a l lop rot e inmimic, a J apanese group has synthesized a derivativeof gramicidine conta ining t wo unit s of 2,2-bipyridine39e.120 On the ba sis of the structure of gramicidin,the tw o 2,2-bipyridin es in the mim ic are predisposedwith one facing the other, thus allowing a n efficientcomplexat ion of tra nsition meta ls such as C o(II ), Ni-(II), Cu(II), and Zn(II). Ligands 40e a nd 41e wereobtain ed by peptide coupling procedures using dicy-clohexylcarbodiimide as the coupling agent, and Zn-(II) binding t o 40e wa s shown to quench fluorescenceemission.121

Macrocycle 42e (Ta ble 7) ha s been sy nt hesized byreacting 3 equiv of 5-bromomethyl-2,2-bipyridine

Table 6 (Continued)



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with 1,4,7-tria zacyclononane,122 a n d i t h a s beenreported to be a strong chelate for Ru(II).122 A simila rs y nt het ic pr ocedur e w a s us ed t o s y nt hesize 43e,w hich w a s us ed as a n Fe(II) chela t e , a nd a Eu(II I)complex of the sa me ligan d ha s been report ed to showa high luminescence quantum yield.123,124 Anotherl iga nd of t his t y pe, 44e, has been s y nt hes ized byreacting t he monolithium derivative of 5,5-dimethyl-2,2-bipyridine with 1,3,5-tri(bromomethyl)benzene.This ligand w ith its three bipyridine arm s ha s beenreported to complex a metal in an octahedral coor-dinat ion geometry.125 The Ru(II) and Os(II) com-

plexes of tr ipodal ligands 45e-51e have been syn-thesized so t hat their electrochemical and lumines-cence properties could be investigated. 126,127 I n t h i scas e i t w as obs er ved t hat t he ef f iciency of ener gytransfer decreases by increasing the size of the spacerbetween the 2,2-bipyridine units. Another tripodalligand of the bipyridine type, 52e, is reported to gelin t o luene by est a blis hing a net w or k of hy dr ogenbonds (one molecule is supposedly ca pable of m a in-ta ining u p to 8000 molecules of toluene w ithin suchan organogel).128 The other part of this ligand com-plexes F e(II) w it h a n enhanced s t abil it y cons t ant .

Table 7



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However, homologue 53e does not present t his t ol-uene-gelat ion phenomenon.128 Tripodal ligands 54ea nd 55e, containing both soft s ites (comprising bi-pyridine unit s) and har d sites (comprising salicyla-mide units), have been investigated as redox molec-

ular s w it ches .129

Liga nds 56e, 57e, a n d 58e w er esynth esized to investiga te th e effects of bipyridyl an dhydroxama te ion binding groups for the developmentof molecular r edox switches.130 Treatment of ligands56e a nd 57e with Fe(II) a nd ascorbic acid resultedin t he oxidat ion of t he meta l t o Fe(III ), w hich couldbe reversed back to Fe(II) using ammonium persul-fa t e ; in cont r as t , how ever , t he int er molecular iontra nslocation involving 58e and a s imilar t r is -hy -droxamate-containing ligand failed to occur underidentical conditions, suggesting the necessity of anintr a molecular redox process in 56e a nd 57e in thiscase.130

B a sed on ca lix-[4]-a rene-ty pe spacers, poda nds 59e

a nd 60e ha ve been synthesized131

for th e purpose offorming lanthanide complexes, and their lumines-cence behavior has been investigated (Table 8). 132 Thebipyridine N,N-dioxide analogue of 60e ha s a lso beenprepared along with its Eu(III) complex.133 Ligand61e, w hich has been s y nt hes ized by s t ar t ing fr omintermediate 62e, ha s been reported to function as aredox sw itch th at possesses tw o ha rd complexationsites (hydroxamate) and two soft complexation sites(bipyridine).134 In t he presence of a h ar d meta llic ionsuch as Fe(III ), th e tw o sites of th e hydroxama te-typebind the metal ion and the two bipyridine units aredivergent. U pon reducing F e(III ) to Fe(II), the liga ndsupposedly rearr an ges, thus allowing t he complex-

Table 8 Table 9



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at ion of the Fe(II) by the bipyridines wh ile the har dsites remain divergent. In complexing Ru(II) by twobipyridines of the calix-[4]-arene derivative 63e, aluminescent complexing a gent has been reported,w h i ch s h ow s i n t en s it y v a r i a t ion a s a f un ct i on ofpH .135 Depending on whether the phenolic units oft he fr ee ca l ix -[4]-ar ene a r e pr ot onat ed or not , aphotoinduced intr am olecular electron tra nsfer or arestoration of the Ru(II) trisbipyridyl luminescence

u n it s i s r ep or t ed t o b e ob se rv ed i n t h i s ca s e.135

Liga nds 64e a nd 65e were prepared by treat ing 63ewith ethyl 4-bromobutyrat e an d N-(3-bromopropyl)-pyrrole, respectively, and NaH in DMF. Solid lan-tha nide complexes of 64e a nd 65e could not beisolated in this study, a nd this w as explained by thesteric effect of the appended substituents in forcingthe chelating bipyridine units apart . 136 Ligand 66ewa s synt hesized sta rt ing from propoxycalix-[4]-a reneby reaction w ith Na H a nd 5-bromomethyl-5-methyl-2,2-bipyridine, a nd its Eu(III), Tb(III ), a nd G d(III)complexes ha ve been prepar ed.136

B aret et al. synth esized the chiragen-based ligand67e (Table 9) in an at tempt to prepare enant iopure

triple helicates. The key synthetic step in the syn-thesis of 67e relies on a Kron ke-type react ion be-tween (-)-myrtenal and a pyridinium salt interme-dia t e t ha t or ig inat es fr om 2-acet y l-5-alkox y car -bonylpyridine.137 The stereospecific self-assembly ofa binuclear Fe(II) triple helicate using 67e ha s beenreported.138 Ligand 68e, w hich w a s s y nt hes ized bycoupling 5-bromomethyl-2,2-bipyridine a nd 1,4,8,11-tetra aza cyclotetra decan e under basic conditions, wa sused as a sensor in its Ru(II) complexed form. I t w a sused to extract stability constants and rate data formacrocyclic complexation of Cu(II) and Ni(II) viafluorimetric t itra t ion.139 The ma crocyclic polyam ine69e w a s s y n t h e s i z e d i n a m a n n e r s i m i l a r t o 43e,

starting from 5-(bromomethyl)-5-methyl-2,2-bipyri-dine, and i t has been char act er ized by mas s s pec-trometr y, U V spectr oscopy, a nd 1H NMR.123 Ligands70e through 72e possess a -cyclodextr in scaffold forthe organization of six bipyridine moieties. Theseligan ds a re synt hesized in a one-pot procedure (82%yield) based on a phosphinimine a pproach, w hichuses t he rea cta nts hepta kis-[6-a zido]--cyclodextrinand an excess of 5-aminomethylene-5-methyl-2,2 -bypridine in t he presence of triphenyl phosphane. 140

The catenan e 73e, synthesized from amide 74e byVogt le a nd co-w orker s, possess es 2,2-bipyridine groupsbearing sulfonamide functions, which a ct to block thefree rotat ion of one of the two macrocycles. 141 The

bipy r idines have been gr a f t ed on t he ca t enane byreactin g 5-bromomethyl-2,2-bipyridine w ith selec-tively deprotonat ed sulfonamide functions. Fina lly,ligand 75e belongs to the same fam ily as ligan ds 29ca nd 30c, an d it shows high photosta bility of its Ru-(II ) complex.88


Liga nd 1f (Table 10) is in the crypta nd family ofcalix-[4]-barrelan ds, which ha s been so na med be-cause of its arrangement in the form of a barrel. 131

The macrobicycles 2f a nd 3f r es ult f r om a 2 + 3

condensation between the tris(2-amino-ethyl)amineand 5,5-diformyl-2,2-bipyridine.142 The binuclear bis-Cu(I) and tr inuclear-Ag(I) complexes of ligand 2fwere prepared, their 1H NMR behavior was investi-ga ted, a nd t he crysta l structur e of the Ag(I) complexof 2fwas determined.142 The tris(bipyridine) ma cro-bicyclic ligand 3f allows for the endo-complexationof three Ag(I) ions a nd h a s been published jointly bydifferent investigators.142 Macrobicycle 4f has been

obtained by three 2,2-bipyridine a ssemblage unitsconnect ed in pos it ions 5 a nd 5 a n d t w o s pa c er

benzene units substituted in positions 1, 3, and 5.126

This macrobicycle has been reported to bind Fe(II)in the center of the cage with a n extraordina rily highstability constant for a ligand of such large size.126

Other macrobicycles of even greater size such as 5f,6f, 7f, and 8fthat possess a conical cavity have beensynthesized for the complexation of organic mol-ecules.143,144 The calix-[4]-ba rrela nd 9f constitutesan other member of the same fam ily as 1f.131 Finally,the reaction of the meso-tetra(R,R,R,R-o-aminophe-nyl)porphyrin with 5,5-diformyl-2,2-bipyridine lea dsto t he bis-porphyrin 10fwhich after reduction gives

the octa am ine compound 11f.145

I t is int erest ing t onote tha t th e prepara tion of this san dw ich compounddoes not require high dilution reaction conditions,due to the preorga niza tion of the building blocks andtheir intermediates.

VI. Ligands with 2,2-Bipyridine Connected inPositions 6 and 6


Ligand 1g71 (Table 11) as well as its derivativesha ving methyl groups either in posit ions 4 a nd 4 or

in positions 5 a nd 5

and no methyl substituents 2g146

were obta ined by oxidative addit ion sta rting from thecorresponding monolithium derivatives. The productbis(2,2-bipyridyl)ethylene 3g has been reported byperforming a Wittig-Horner reaction.147 Ligand 4gw a s s y nt hesized fr om monomer bipy r idine unit sbearing amine and aldehyde functional groups, andit w as used to form helicat es using Ag(I) and Cu(I)with the potential for displaying self-generation andself-replication processes based on the reversibilityof t he imine bond.148 Liga nds 5g105 through 9g163

cont a ining la t er a l ol igopy r idine a r ms have beensynt hesized by reactin g deriva tives of (bromomethyl)-or (chloromethyl)-bipyridines with crown ethers in

the presence of a base, an d the1

H NMR behavior ofCu(I) complexes with some of th ese liga nds h a s beeninves t iga t ed. Liq uid membr ane t r ans por t exper i-ments have revealed that the double-armed crownethers can exhibit superior cation binding capabilityrelat ive to the corresponding lariat ethers in theseligand systems.105 Ligands 10g, 11g, a n d 12g h a v ebeen s y nt hes ized in a s imilar manner and for medcomplexes w ith Cu(I) in organic solvents,149 a nd 11ga nd 12g w ere shown t o selectively extr a ct Zn(II ) fromsolution.149

Ligands 13g-17g ha ve been reported to providedouble recognition of Ag(I) and Cu(I) cations usingthe combined binding capability of the pseudo-thia-



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Table 10



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Table 11



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Table 11 (Continued)



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cr ow n and bipy r idine.150 I n t h i s s y s t e m , C u ( I ) i sbound to the bipyridine moieties to give the corre-sponding pseudo-thia crow ns, a nd Ag(I) is ta ken intot h e t h i a cr ow n r i ng s el ect i ve ly a n d i n a m a n n e rshowing positive allostery using Cu(I) as an effectorfor certain ligands in this series.150 Ligand 18g w a ssynthesized in 49%yield by treat ing pentaethyleneglycol w ith the corresponding bromide compoundcontaining two bipyridines linked via an oxa bridge,and i t s helica l t e t r ahedr a l a dduct s w it h C u(I) havebeen reported, which do not r acemize at room t em-perature, supposedly due to the greater degree ofcomplexat ion afforded by t he four bipyridine moietiespresent.151 Similar complexation behavior has alsobeen investigated for ligands 19g-21g, which containchiral auxilaries at ta ched to the polyether.152 The C u-(I) complex of 21g has been r epor t ed t o ex is t as amixture of two inert diastereomers a t room temper-at ur e .152

Linking t w o bipy r idines w it h a 1,3-pheny lenespacer results in ligands 22g a nd 23g, w hich spon-taneously self-assemble into double helicates upontreatment with Cu(I), Ag(I), Co(II), and Ni(II) ions.153

The biphenyl homologues of these ligands, 24g a nd25g, were also prepared a nd formed double-helicalstructures with a range of transit ion-metal cationsin w hich the observed intermeta llic separ at ion dis-ta nce within the helix increased relat ive to 22g a nd

23g.154

I t is int er es t ing t o not e t ha t in t he helica t es t r uct ur es obt a ined w it h l iga nds 22g-25g, -stacking interactions do not appear to be of impor-ta nce in th e sta bilizat ion of the structures, which isin contra st to linear oligobipyridine systems w herethese interactions have been shown to be significant(vide supra).154 The disulfide 26g w a s s y nt hes izedfrom 6-(2-aminophenyl)-2,2-bipridine by convertin gt he a mine t o a t hiol , a nd i t s C u(I) and Ag(I) com-plexes were obtained, in which the ligand imposes arelat ively r igid pseudo-tetrahedral geometry aboutthe metal centers.155

In addition to the proven capability of oligopyridinecompounds t o a ct as l iga nds for s of t met a ls , com-

pounds tha t increase their har d meta l binding abilityha ve also been synth esized. Ha rding a nd co-workersha ve incorpora ted significan t steric limitat ions t hatact to control th e assembla ge of binuclea r homochira la nd h eterochira l complexes of th e [Zn2L2]4+ type withthe bis-bipyridine series comprising 27g,156 28g,157

a nd 29g.157 They report ligand 29g to selectively givethe heterochiral complex, ligand 28g t o g i v e a p -proximat ely 60%helicate, a nd liga nd 27g to give 48%helicate with Zn(II) in deuterated chloroform solu-tion.156 These results have been explained by thes t er i c r eq u ir em e nt s of t h e b r id g es , s in ce i n t h ehelica te st ructur es, the bridges a re positioned closert oget her t ha n in t he nonhelica t e s t r uct ur es , a ndcons eq uent ly t his ar r angement is dis favor ed w it hbridges that are sterically demanding.156 The Zn(II)complexes of ligands 28g, 30g,158 a nd 31g158 weresynthesized, and t he effect of the substitut ion pat ternof t he na pht halene s pacer w a s inves t iga t ed. I t w asfound t ha t 28g a nd 31g formed [2 + 2] complexes,a nd 30g formed a [1 + 1] complex in this case.158

Ligands 32g, 33g, a nd 34g containing ester linkageshave been obtained by reacting different acid chlo-rides w ith 6-hydr oxymeth yl-6-methyl-2,2-bipyridine,and they form selective mononuclear complexes inthe presence of Ag(I) a nd Cu(I).159 By introducing achiral bridge betw een the oligopyridines a s in com-pounds 35g-42g, Siegel and co-workers were able

to enant ioselectively synthesize a Cu(I)-conta ininghelicate structure.160 It is interesting to note that t hechiral int egrity reported in th ese helicat e complexess uggest s t ha t s t er eochemical infor mat ion may bet r a ns mit t ed over dis t ances of up t o 1 nm a long amolecula r sca ffold (as observed in th ese structur es).

The sterically constrained ligand 43g w a s s y nt he-s iz ed , a n d i t s C u (I ) a n d C u (I I ) com p le xe s w e r echara cterized by X-ra y diffra ction.161 Amide-contain-ing polyether 44g was synthesized from 6-[[(chloro-carbonyl)oxy]methyl]-6-methyl-2,2-bipyridine an dR-[(piperazinylcarbonyl)oxy]--(piperazinylcarbonyl)-poly(oxytetra methylene) conta ining 28 oxytetra m-e t hy l en e r ep ea t u n it s a n d w a s r ep or t ed t o f or m




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double-stra nded h elices w ith Cu(I) ions.162 By r eact -ing 6-bromomethyl-6-methyl-2,2-bipyridine with 4,4-bipy r idine in C H 3C N, Beer and co-w or ker s ha veisolated the viologen 45g.163 The compound 46gcomprising three 2,2-bipyridine units a nd a n unsa t-ura ted bridge has been synth esized by using Wittig-Horner condensa tion procedures.164 The correspond-ing satura ted bridging ligand 47g ha s been preparedfrom ligand 46g by a four-step rea ction sequence

s t a r t ing fr om commer cia l ly a va i lable compounds .Liga nd 47g has been reported to selectively form atr inuclear d ouble propeller in th e presence Cu(I), and46g has been reported to give a mixture of severalproducts un der th e sam e experimenta l conditions.164

More recently, the Ni(II) and Fe(II) complexes ofligand 48g were obta ined, in w hich th e metal cat ionadopts a distorted octa hedral coordinat ion environ-ment only t o the 5,5-disubstitut ed bipyridine m oietyat the bridge, with t he 6-monosubstitut ed bipyridines idea r ms unbound in t hes e cas es .165 Ligand 49gbelongs t o t he s ame fam ily a s 4g, a nd i t s helica t eswith Ag(I) have also been investigated. 148 The chira ltris(bipyridine) 50g ha s been s y nt hes ized and w a s

reported to give a trinuclear helicate structure.166,167Liga nd 51g forms heterotopic helicates via a processof autoassembly.168 Phenanthroline-bridged 52g w a sprepared by reacting 1 equiv of the dilithium sa lt of2,9-bis(hydr oxymet hyl)-1,10-phena nt hroline wit h 2equiv of 6-bromomethyl-6-methyl-2,2-bypridine.169

Its complexes with Cu(I), Ag(I), an d Zn(II) ca tionswere shown to self-assemble into double-strandedhelica t es , w it h t he Zn(II) helica t e being t he f ir s ttr inuclear double-stra nded zinc helicat e.169 Ligand53g wa s synt hesized in th ree steps from 6-bromo-6-methyl-2,2-bipyridine in the presence of Zn(II) ando-dimethoxybenzene, a nd it assembles int o a chiraland helical metallomacrocyle.170 Ligand 54g incor-porates several functional features that coordinateCu(I) and ha s been reported t o assemble into large(external diameter of 28 and cavity diameter 11) na nocyclic architectures comprising four ligandmolecules and nine Cu(I) ions in nea rly qua ntita t iveyield.171 Ligands 55g, 56g, and 57g were synth esizedfrom 6-(bromom eth yl)-6-methyl-4,4-bis(4-methox-yphenyl)-2,2-bipyridine, which w as obtained by bro-m i n a t i n g 6 ,6-dimethyl-4,4 -bis(4-methoxyphenyl)-2,2-bipyridine with N-bromosuccinimide, and thecorresponding oligoethylene glycol usin g sodium hy-dride in refluxing THF, in a fashion similar to th atreported for ligands 18g-21g.151,172 Similar synthetic

procedures were used in t he synt hesis of ligan ds 58ga nd 59g.172 An interesting observa tion is the report edK+ ion binding selectivity of Cu(I)-bound ligand 57g,which supposedly is a result of both pseudo-crownr ing s ize and elect r ost a t ic r epuls ion bet w een t hebound C u(I) and K+ metal cations.172 The sy nt hesisof ligands 60g,102 61g,59,102,173 62g,59,102 a nd 63g,174

com p ri s in g e t h yn y l , d i et h y n y l, a n d t e t r a e t h y ny lbridges, ha s been described for t he sa me bridges inpositions 4 and 4 an d in posit ions 5 an d 5. Ligands64g175 a nd 65g176 were prepared by the stoichiometricreact ion of 6-(hydr oxymet hyl)-2,2-bipyridine an d6-aminomethyl-2,2 -bipyridine, r espectively, w ith chlo-rocarbonylferrocene, a nd were both chara cterized

us ing cy clic volt am met r y . Ligand 64g h a s b ee nreported to form a mixed tetr anuclear helicat e withCu(I) ions, which has been characterized by X-raydiffraction, 175 a n d l ig a n d 65g has been reported toform a complex with H 2P O4- by utilizing the amideprotons in a hydrogen bonding a rra ngement betweenhost and guest .176 Ligand 66g ha s been reported t ofor m a double pr opeller via s elf-as s embly in t hepresence of potassium, and its complexes with K+,

Cu(II), Gd (II I), and Tl+

ha ve been synthesized.177,178

The combination of coordination and electrostat icinteractions a fforded by 66g in the binding of metalions is s imilar t o t he binding a f for ded by bor oc-r y pt and s y s t ems .179 The oligo-(2,2-bipyridyl)pyra-zines 67g a nd 68g have been prepared through theintermediate compounds 69g a nd 70g, a n d t h e i rcrystal structures w ere studied using single-crystalX-ra y diffraction.180 Ligand 67g r esult s fr om t hereaction of 1,2-diaminoethane with 69g followed byt r eat ment w it h chlor oanil ine, a nd l igand 68g isobta ined by reacting o-phenylenediamine w ith ligand69g. B eer a nd co-workers have synt hesized ligands71g a nd 72g w i t h t h e s pa c er g r ou ps p-toluene-

sulfonamide and N,N,N-tritosyldiethylenetriamine,respectively.163 These tw o ligands form monometa lliccomplexes in t he presence of Ag(I) cat ion. Liga nd 73g,which contains two 2,2-bipyridines connected by adipheny lmet hane-t y pe s pacer , can act e i t her as aN, N-bidentate donor or as a C,N,N -tridentate donor,which leads to a metallocycle. 181 These have beencomplexed with Ru(II), and two coordination modeshave been observed in these complexes. The synthesisof t ris(bipyridine) 74g connected by ethynyl-typebridges in positions 4,4 and 5,5 on 2,2-bipyridinehas been described above (vide supra).102

In the linear oligopyridine series comprising oxy-

(bismethylene) bridges, compound 75g

182

is the small-est ligand. The Cu(I) complex of 75g w a s char a ct er -ized by NMR a nd X-r ay diffr act ion and w a s one ofthe first examples of double-stranded helix formationw it h bipy r idines , in w hich t w o molecular s t r andsw r a p a r ou n d t w o C u (I ) i on s a n d t h us h ol d t h estructure t ogether.183 Ligands 76g,182 77g,182 78g,184

a nd 79g184 s er ve a s int er media t es in t he s y nt hesisof higher homologues (tri-, tetra-, and penta-bipyri-dine subunit-containing ligands). Treatment of 80gwith oxalyl chloride in the prescence of NEt 3 yieldsligand 81g, which was synthesized as an intermedi-at e for t he synth esis of unsymmetrical h igher oligo-bipyridines.185 The s y nt hes is of l igand 82g w a s

performed by reacting 6-(hydroxymethyl)-6-methyl-2,2-bipyridine, which is available via a N-oxidation

r out e , w it h 6-(bromomethyl)-6-(4 -tert-butyloxytet-ramethylenoxymethyl)-2,2 -bipyridine. 185 Ligand 83gwa s synthesized from 82g in 94%yield by treatmentwith 4 N HCl in dioxane under reflux, for the samepurpose of unsymmetrical higher oligobipyridinesynthesis as for 81g.185 Amide and ester functionsw er e int r oduced ont o l igands 84g a nd 85g i n a nat t empt t o or ganize funct iona l gr oups w it hin t hedouble helical structure a lready known from un sub-stituted 75g.166 The polymer 86g is also synthesizedfrom 80g an d ha s been reported to form a dinuclearhelica te complex in th e presence of C u(I) cat ion.



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The tris(bipyridines) 87g a nd 88g are intermedi-at es in t he synt hesis of compounds 39g a nd 41g.160

Liga nd 89g forms helicates in the presence of Cu(I)or Ag(I) ca tions.183,187 Ligand 90g is synth esized bytreating 77g with 6-(4-tert-butyloxytetramethylene-oxymethyl)-6-(hydroxymethyl)-2,2-bipyridine, w hichis also a precursor for th e synth esis of ligan d 82g.185

Mos t of l igands 84g-112g (with the exception of86g,186 87g,160 88g,160 a nd 90g185) were first synthe-sized by Lehn and co-workers by employing varia-tions on Williamson condensation betw een a dibromo-bipyridine-cont a ining compound a nd a correspondingdiol. These ligands provide a basis for demonstra ting

selectivit y, cooperat ivity, a nd self-recognit ion in su-pram olecular helicat e forma tion.166,167,184,187-190 Theiruse in the synth esis and design of helica tes ha s beenreviewed.191

The unsymmetrical ionophore 113g (Ta ble 12)belongs t o t he s ame l igand fa mily as 57g. I ts Cu(I)complex has been studied using NMR for the purposeof investiga ting t he possibility of molecula r chira lityas a function of temperature. 151 Ligand 114g i s a nextension of the concepts introduced in ligands 5g-8g, and i t w a s s how n t o pr efer ent ia l ly bind C u(II)and Zn(II) t o Ba(II) (r ever s e s elect ivi t y t o l igand8g).105 B y h eat ing 6-cyan o-2,2-bipyridine w ith NaH

Table 12



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under a n inert a tmosphere, tr is(2,2-bipyridyl)pyra-zine 115g is obta ined.192 The compound t etra kis(2,2-bipyridyl)pyrazine 116g ha s been prepar ed by heat-ing compound 70g in the presence of an excess ofammonium acet a t e .180 The t ris (6-(2,2-bipyridyl))-phosphine 117g is synthesized by mixing 6-bromo-2,2-bipyridine w ith n-BuLi followed by the additiono f P C l3.193 O t her poly bipy r idine l igands t hat ar eb a s ed on a c y cl ic p ol y a m in e d er i va t i v es s u ch a s

ligands 118g, 119g, a n d 120g or macrocycles sucha s l i g a n d s 121g, 122g, a n d 123g have a ls o beenreported, and their polynuclear metal complexes withRe(II), Fe(II), an d R e(bpy)(CO)3Cl ha ve been synthe-sized.194,195

The calix-[4]-arene 124g (Table 13), obtained bydeprotection of 125g by t r ifluoroacetic a cid, can acta s a m ol ecu la r r ed ox s w it ch w i t h i t s t w o h a r dcomplexation sites (comprising hydroxam at e) andtw o soft complexat ion sites (comprising bipyridine).134

Liga nd 126g has been s y nt hes ized, a nd i t s E u(II I)complex showed a high extinction coefficient an dluminescence quantum yield (15%reported).132 Ther ela t ed l igand 127g has been r epor t ed t o ex t r act

sodium picrate from the solid to a chloroform phaseconta ining the ligand. 196 In ad dition t he Tb(II ) com-plex of 127g w as r epor t ed t o s how int ens e met a lluminescence.196 Int roduction of a chiral substit uent,(S)-2-methylbutyl, close to the bipyridine coordina-tion site in ligand 128g via rea ction of the calixar enewith (+)-(S)-1-bromo-2-methylbutane in the presenceof K 2C O3 resulted in C u(I) helicat es possessing a 30%induced enant iomeric excess.197 The synthesis ofligands 129g-133g a s w e l l a s o f l i g a n d 134g ha sbeen accomplished by the regioselective O-alkylationof calix-[4]-arene, with the product depending on thebase used, and these ligands have been reported tobe Cu(I) and Cu(II) complexing agents.198-200 Ligands

135g-138g w e r e s y nt h e si ze d i n or d er t o ob t a i nluminescent complexes with Eu(II) and Tb(III), byalkylat ing the calixarene with 6-bromomethyl-2,2-bipyridine and R-chloro-N,N-diethyla ceta mide underbasic conditions.196 Ligands 135g a nd 137g w er er epor t ed t o s ens it ize Eu(II) and Tb(II) lumines-cence.196 The cali x-[4]-resorcin a ren e 139g in the formof an octopus has been reported to simultaneouslycomplex eight molecules of C oCl2.201


One of t he first ma crocycles connecting tw o 2,2-

bipyridine units in positions 6 and 6

w a s l igand 1h(Table 14), which was synthesized by using a zinctemplating effect in 1980. According to this proce-dure, 6,6-dichloro-2,2 -bipy r idine w a s mix ed w it h(NH 4)2-ZnCl4, and t he cr ude pr oduct w a s demet a-lated. 202 Several years lat er , the yield for the forma -tion of ma crocycle 1h w a s i m pr ov ed b y r ea c t in g6,6-dichloro-2,2 -bipyridine with 6,6-diamine-2,2-bipyridine in a sealed ampule heat ed at 230 C , thusmaking t he 6,6-dichloro-2,2 -bipyridine react withgaseous ammonia solution under autogenous pres-sure.203 This macrocycle has been reported to existin both t he imine and a mine form, w ith th e equilib-rium between the two forms being sensit ive to the

nature of the solvent environment.202 Ligand 2h ha sserved as a model compound and results from thesuccessive rea ction of NaH a nd n-but yl iodide on cycle1h.203 Macrocycle 3h is synthesized by monoalkylat-ing l igand 1h w i t h n-hexyl bromide and NaOH asbase, and its tautomeric structure was investigatedand shown to be different in methanol and chloroformsolutions.203 Compound 4h was accidently obtainedduring the preparation of N,N-ditosyl-6,6-bis(ami-

nomethyl)-2,2-bipyridine, when 6,6

-dichloro-2,2

-bi-py r idine w a s mix ed w it h t he monos odium s a l t o f

toslya mide in a bsolute etha nol under reflux.204 Thedet os y la t ion of t his l igand in concent r a t ed H 2S O4leads to the macrocyclic diamine 5h,204 w h i ch h a ss er v ed a s a s y nt h e t ic s t a r t i n g p oi n t f o r a l a r g enumber of other macrocycles, simplest among thesebeing 6h, which was synthesized by N-methylationusing CH 2O in HCO2H .205 Macrocycles containing theaza functional group 7h-11h have been studied asa model of ion transport through liquid-phase mem-branes a nd ha ve been compared w ith other types offunctional groups.206 Macrocycle 10h is reported toselectively complex L i+ and is s y nt hes ized in t hr ee

s t eps , s t ar t ing w it h t he t r ea t ment of R-cyanoaceta-mide in DMF w ith NaH and t he subsequent a ddit ionof 6,6-dibromo-2,2-bipyridine. The dinitr ile ma cro-cycle thus obtained is then alkylated by n-butyl iodidein th e presence of NaH, lea ding to th e monoalkyla tedderivative 9h a n d t h e t r a n s a nd ci s derivatives, 7ha nd 8h, respectively, which can be chromatographi-cally separated.207 B y trea ting th e mixture of isomersw it h s ulfur ic acid a t 70 C , compound 10h is ob-ta ined, which like ligand 11h can exist as one of twotautomeric forms.206 Ligand 12h w as s y nt hes izeds t ar t ing fr om 6,6-bis(chloromethyl)-2,2-bipyridineand results from a mixture of glycols generated insitu via a fragm enta tion process.208 The t etra -N-oxide

derivative of ligan d 13h209 as well as t he ma crocycle14h210 have been s y nt hes ized for t he pur pos e ofcomplexing lant han ides and for t he investigation oftheir fluorescence characteristics. Variable temper-ature NMR-based investigations of 12h indica t e ar es t r ict ed r ot a t ion of t he amide bonds a t ambienttemperature and suggest a fixed syn conformationof this molecule in solution, in w hich th e bipyridinesare in a face-to-face arra ngement on the ligan d.211

Ligands 15h,212 16h,213 17h,214 18h,194 19h,213 a nd20h213 have been s y nt hes ized by r eact ing t he ap-propriate halogenated derivatives with macrocycle 5hin t he pr es ence of Na 2C O3 o r N E t 3 (following theaddit ion of t he halogenat ed der iva t ive) in C H 3C N.The ma crocyclic polyam ine 21h, w h i c h h a s b e e nchara cterized by single-crysta l X-ra y diffra ction, ha sbeen synthesized by reacting 6,6-bis(chloromethyl)-2,2-bipyridine with the sodium salt of N,N-ditosyl-et hy lene-diamine in DMF w it h an over a l l y ield of5%.204 Ligand 22h has been synthesized in two stepsby rea cting t he corresponding dia cid chloride of 2,2-bipy r idine w it h cr ow n et her s in t he pr es ence ofNE t 3.215

To provide additional experimental confirmation ofthe strong fluorescence of the lithium complex ofligand 10h, Ogawa and co-workers synthesized mac-rocycle 23h216 by condensing 6,6-diamine-2,2-bipy-



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r i di n e w i t h 6, 6-bis(acid chloride)-2,2-bipyridine.Liga nds 24h a nd 25h r epr esent t w o in a family ofseveral macrocyclic receptors containing bipyridinegr oups l inked t o t w o amino a cids fr om t he s er iescomprising proline a nd va line. Only the va line-basedm a c rocy cl es , w h i ch h a v e t h e t w o p yr i di n e r i ng s

preorganized for binding, act to complex metals .217

The ma crocycles w ere a lso shown to bind phenolichydroxyl groups via hyd rogen bonding to the a midegroups of the bridging cha in.217 Macrocycles 26h a nd27h,218 w hich ha ve been char act er ized by X-r aycry s t a l logr aphy , have been s y nt hesized via four

Table 13



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Table 14



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30/38

concerted Witt ig rea ctions betw een 6,6-diformyl-2,2-bipyridine and m-xylenebis(triphenylphosphonium)bromide. The hydrogenation of intermediate 26h byH 2/P d-C gives ma crocycle 27h. Ma crocycles 28h a nd29h ar e intermediat es tha t were synthesized enrouteto cyclosexipyridine.219 I n a d d it i on t o t h e d im ercompound 30h, which has t wo bridging CH 2-S-C H 2units, New kome a nd co-workers ha ve also isolat ed at r imer der iva t ive, w hich is not r epr esent ed in t hetables.220 Liga nd 31h conta ining thiol groups at thet w o br idges has been s y nt hes ized by r eact ing t he1,3,4-thia diazolium derivative with 3,6-dioxa-1,8-diaminodiy loct a ne in DMF and in t he pr es ence ofNE t 3.221 U s in g a [2 + 2] template condensationbetween 6,6-bis(aminomethyl)-2,2-bipyridine with2,6-diformyl-p-cresol in th e presence of a lan th a nideacetate, Martell and co-workers synthesized ligand32h, whose reduction leads to macrocycle 33h.222

The 2:2 ma crocycle of crow n et hers 34h, 35h, 36h,37h, and 38h have been isolated and separated from

t heir 1:1 homologues , dur ing t he r eact ion of t hecorresponding d isodium sa lt of poly(ethylene glycol)w it h 6,6-dibromo-2,2-bipyridine.223 G i v e n t h a t t h epresence of a linked heteroat om in posit ions 6 a nd6 on the bipyridine hinders metal complexation dueto steric considerations, Newkome and co-workerss y nt hesized macr ocy cles cont a ining a met hy lenegroup betw een the poly(eth ylene glycol) chain a nd th ebipy r idine, by r eact ing 6,6-bis(chloromethyl)-2,2-bipyridine with the corresponding polyglycolates toproduce ligands 39h, 40h, 41h, a n d 42h.208 Macro-cycle 43h has been s y nt hes ized s t ar t ing fr om t hemacrocycle 28h by reaction with hydroxylamine inacetic acid. Other cyclosexipyridines 44h a nd 45h

that contain substituents have been synthesized byfollowing Kroh nke cycliza tion procedures.224 Highy ields in t he s y nt heses of t hes e ma cr ocy cles ar eex pla ined by a ca t ion t emplat ing ef fect o f Na + orNH 4+.224

A la rge number of ma crobicyclic-type crypta ndligands 46h-62h, which are capable of complexingcations such a s Li+, Na +, as well as lantha nides, havebeen synth esized by L ehn a nd co-workers. H ere onlythose cycles conta ining a t least t wo bipyridine unitsare included.225-229 All of these cryptands have beensynthesized by sta rt ing from t he ma crocycle 5h or aderivative thereof. The ma crotricyclic cylindricalcryptand 63h, w hich ha s six bipyridines connected

in positions 6 an d 6, has been obtained as a second-ary product during the synthesis of macrobicycle 47h,by r eact ing 6,6-dibromo-2,2-bipyridine with 6,6-diamino-2,2-bipyridine in a single step.225

Finally, Newkome et al. synthesized the bipyridine-containing crown-ether derivatives 64h-71h by di-rect nucleophilic displacement, a s in t he case of 12h,which is in the same ligand fa mily.208 Complexes wit hCo(II), Cu(II), Zn(II), and Pd(II) have been reportedusing these ligands.208

VII. Ligands with 2,2-Bipyridine Connected in

Mixed Positions

Ligand 1i (Table 15) wa s syn thesized by condens-ing 6-bromomethyl-6-methyl-2,2-bipyridine with themonolithium sa lt of 5,5-dimethyl-2,2-bipyridine ac-cording t o the sa me procedures as for liga nd 9e. I t sCu(I) and Cu(II) double-stranded helical complexesw er e char a ct er ized by X-r ay cr ys t a l logr aphy andw e r e s h ow n t o h a v e a d is t or t ed t e t ra h e d ra l a n dtetra gonal pyra midal geometry, respectively, whichar e significantly different from results obtained usingthe related ligands 9e a nd 1g.230

VIII. Polymeric Ligands Containing 2,2-Bipyridine

Units

There ha ve been fewer reported polymeric ligan dswith 2,2-bipyridine than for the case of the oligobi-py r idine l igands t hat ar e descr ibed above. Ini t ia la t t e m pt s a t p re pa r i n g p ol ym e rs w i t h b ip yr i di n er epea t unit s r e lied more upon pos t s y nt het ic im-mobilization. Tha t is to sa y, methods w ere found t ogr af t bipy r idine-cont a ining funct ionali t y ont o anexisting polymer stra nd. Although t his method canhave practical advantages over the direct copolym-erization of a bipyridine monomer with other mono-mers (such a s overcoming th e synth etic challenge ofpolymerizin g char ged meta l complexes of bipyridine),


Table 15



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it often provides less cont rol over the result ing spa tia ldistribution of bipyridine sites along a single polymers t r and.

O n e o f t h e e a r li es t r ep or t s of u s in g a g r a f t in gapproach to attach bipyridine to an existing polysty-rene polymer strand is ligand 1j (Table 16), whichwas synthesized by Card and Neckers. Ring bromi-nat ion a nd lithiat ion were used to covalently a t ta chb ip yr i di n e s o t h a t l es s t h a n 25% of t h e p h en y l

residues in a polystyrene-2%divinylbenzene poly-mer w ere brominat ed. Conversion from bromina tedphenyl residues to bipyridylphenyl residues occurredin gr ea t er t ha n 50% y ield, r esult ing in poly mer scont a ining one bipyridyl group for each seven to eightpolystyrene phenyl residues.231 Various m etal com-plexes of 1j w er e s y nt hes ized, and t he palladiumacetate-bound 1j produced a n a ctive cata lyst for thehydrogenat ion of olefins at a mbient temperat ure andpressure.231 Polyurea 2j wa s the first r eported polarbipy r idine poly mer , w hich w as pr epar ed fr om t hecopolymeriza tion of a P d(II) complex of 4,4-diamino-2,2-bipyridine by reaction with toluenediisocyana te.Followin g polymer synt hesis, the P d(II ) wa s reducedt o Pd(0) w it h l i t hium a luminum hy dr ide, and t hecatalytic activity of the metal-containing 2j for olefinhydrogenation was investigated.232 Another a pproa chinvolved th e at ta chment of the bipyridine via a midelinkage in polymer 3j t o a n a l r e a d y s y n t h e s i z e dp-aminostyrene/N-vinyl-pyrrolidone copolymer tha twas prepared via free radical polymerization. Thebipy r idine w as a t t ached us ing a mix ed anhy dr idemethod with ethyl chloroforma te a nd 2,2-bipyridine-3,3-dicar boxylic acid to creat e approximat ely 10%molar substitution along the polymer backbone basedon the amount of Ru(II) bound.233 The lu min escenceof the Ru(II)-bound 3j an d quenching of t he excited

s t a t e w it h Fe(II), C o(II), and C u(II) w a s inve

bipyridine- the most widely used ligand. a review of molecules comprising at lesat two 2,2 bpy units

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