w H L S I i V I H R

':i|;!l:;':il!li;S/fclili''lii|!l/;lH illilii:lut;!'ii!;W

siry I-IIMIW/) I4.'i IM

^Synthesis, electrochemistry and cornplexation studies of ferrocene crown ethers

Ion Ion "•', Jcan-Claiidc Moiilct "', Angela Popescu "", Brie Sainl-Ainan '''•\ I.aure Toma/eswski '', Isabelle Gaulier-lAmeaii ''

Ijih'iiiiloiir (l'i:in livcliiinii' (>n;iiiii<iiic i-l (!,• I'lioKnliiiiiii' Itrilin. IIMHCNIiS 5()M). diiirr

I'lWICl-

Ijihiiidldiir il'l:iii(lf\ l>\ii(iiiiiiiiic\ fl SliuiHiwIfs lie l,i Si'Urliiilr, IIMKCNItS Mi 16. Ihiin

Misqili I'l

• Jiisq'h l-<

lll'.'i.l. .Ifillll (hriiiihli Ci'ikx').

. lir.l.l.JKll-ll CmhihI.'Ci'ilc.t'J.

hmuiny WH: ry mi

Abstract

VoliainiiiL'liic UIKI IIV spcclioplKitoiiiL'lric iiivesiiyalioiis of alkali ami alkaliiif canli iiiflal cations lijiuling hy the new IcirDccnc inacrocyclic liyaiicis 1-4 have hccn pcii'orincci in acctonilrilc electrolyte. Stahility constants were cleterniincil Iroin UV nieasurenients. All the nictal cations surveyed Ibrniecl 1:1 sioichioinetric coni|)lexes with 2 and 3, with the exception of hariuni which produced a 1:2 interinolecuhir sandwich complex with 2. No coniplexalion occurred with I. lilectrocheinical stmlies have denionstruteil that the hinding of I,i'. Na '. K ', Mg ' ' . C'a" * and Ha"' with 2 resulted in shills of the I'erroceMe oxidation wave to more positive |)otentials. The same features were ohserved w ith 3 in tiie presence of Li '. Na ', K ' and Mg-', and with 4 and Ma"'. In contrast, a different type (iwo-wave behaviour) of positive shift levealed the binding of Ca"' and IJa"' with 3. allowing the ainperometric titration of these metal cations. The magnitude and the type of potential shift could not be related to the radius and charge to radius ratio of the cations, or to tiie strength of the host-guest association. It can be assumed that additional effects such as an effective charge transfer interaction between the catioiiic guest and the redox centre control the eleelrocheniical recognition behaviour of these redox ionophores. .Single crystal X-ray structure of (3-l5a|(CI()|)_, has also been determined to gain better understanding of the barium complexation in 3. '• • ':W7 l-lsevier Science S.A.

Kt'vndiils: .Syiilhcsis; /\lk;ili/;i LMflh Ills; Voha iiinciry; L'iry; ray crys

1. Introduction

The prospect of pri)ducing tiew inalcrinls of relevance to the field of molecular sensois has let! in recent years to considerable itilcrcst in the synthesis of redox-aclivc inacrocyclic tnoiecuies that contain a redox ccnire in close proximity to n cation or anion bindin;^ site [l|. Since Saji dcscriiied the first cviiicnce of potential sliifls in the redox potential of a pentuoxal l3]fcrri)Ci;nophane dtie to the addi­tion of sodium salt [2], there has been increasing interest in the design of i'errocene crown ether and cryptand

lail: Sccrclariiil.l.ci)|ir(" iijt'-groiinlilc.fr

incut of Aiiiilyiical Clicn'isiry JiiiJ In:

• Koiruiiii.i. incniiil Ariiilysis.

•' .Sec liH iimlc

ilcliiiica tj'nivcrsiiy. li

ionophores able to recogni/c alkali and alkaline earth metal cations electrochemically [3].

The magnitude of the shift in the redox couple potential produced by guest binding is a factor of paramount impor­tance for poietuial prototypes of amperoinelric and polen-tionietiic devices, in particular, an added degree of the recognition property is coiilerrecl on potentiometric sy.s-lems by the appearaiu J of a new set of redox waves (two-wave behaviour) associated with the oxidation of the ferroceny! subunit in the host-guest complex, compared with a single gradual shift in the potential of the original ferrocene redox couple. Two-wave behaviour ha.s been observed with various ferrocene crown ethers and fer­rocene cryptands in the presence of sodium [4-6], potas­sium [4], calcium [4], magnesium [7,8], barium [7], and lithium [y] cations, for example. In addition, lor the con­struction of prototypes of selective electrochemical sen-

()(l i2-l)7:s/ ' i7/ ' i . l7.()() <•. I W 7 lilscvicr .Science S.A. All riiihts reser

/ ' / / .S()l)22-1)7.1 S( ' )7)(IOO'J.s-»

/, /(/// <•/ (il./Jiiiinmhil iMrlnmmihliail Chn •l-l(IIIW7l l-lfi /.i

sors, il is of ohvioiis impurUmiv lo clfsijiii ictlox-iiclivc hosts ciipiihk' of c()iiiplexiii}j; sclux'livcly, of a-cogni/irig I'li-ctroulicniically unci avspoiuling lo ii given ion in the prcsL-ncL' of excess iinioinits of otiier guest s|ieeies.

Reeently, we liiive ie|n)itecl pieliniinary results on tlie syntiiesis iuui tlie electroelieniieiil hehuviour of llie new leiToeene eiown etlier ionophoie 3, whieii siiowecl ix'riiiirk-iihle iuici selective coniplexatioii unci ekctroeheniical recognition of Ha"' cations among oilier alkali and alka­line earth metal cations [It)], in tiiis paper we present a more dociiled account of UV-visihIe spectrophotonietrie and voliiunmetric studies of the recognition behaviour of a series ol' I'enocene crown ether macrocycies 1-4, including a mass sjieclronietric study of some calion-hosl complexes and the single crystal X-ray structure of [3-Ha|(CI(),),.

1: n = 1 ; Z

2 ; n = 2 ; Z

3 : n = 3 ; Z

4 ; n = 3 ; zi^

2. Kxperimcntal

2./. Climkcils

Ferrocene crown ethers 1-4 were synthesized according to the same general procedure as descrihed below tor 3 {Scheme I). A mixture of NaOH (24mmol) and 1.2-dihy-Jroxybcn/ene (24mmol) in 5()ml of distilled //-butanol was retluxed under a nitrogen atmospiiere. After dissolu­tion of NaOH, 2-[2-(2-ch!oroethoxy)ethoxy]cthanol (6mmol) was added dropwise with stirring. The mixture was retluxed for 48 h, cooled to room temperature, filtered, and neutralized at pH = 7 with aqueous dilute HCI. /;-

2ciic,njO;,c,ti,oM- r^"'^^,:

Uutaiiol was removed in vacuo, lixtracliun of the ai|ueous phase with ('ilif 'k I'oih wed by evaporation of the solvent afforded I as a crude oil. which was |)urifieil on a silica gel column elated with Cli ,CI,:CII ,011 (98:2), Yield 62Vr. F (3mmoi) in lOOnil of dried toinene and M'-bis-chloro-carhony I ferrocene (3mnioi) [I IJ in 100 ml of dried toluene were added dropwi.se and simultaneously with vigorous stirring, over a 4h perioti, to 100ml of dried toluene containing Iriethylaniine (fimmol). under an inert atmo­sphere, 'I'he mixture was stirred for 24 h at room tempera­ture, filtered, and the .solvent was removed in vacuo. The crude product was extracted with CMiC'l, and the organic phase was washed with il_,0, livaporation of CII,CU gave a deep orange crude product which was purified on a neutral akunina column eluted with CII,CI,, lo yield pure 3 as an orange .solid; yield 25%. i'AB-lviSand 'll NMR spectroscopy were used lo characterize its structure [10].

Ligands I and 2 were prepared in the same way. using 2-chloroethanol and 2-(2-chlor()elhoxy)elhanol respec­tively, instead of 2-[2-(2-chloroeihoxy)etlu)xy]eihanol. 4 was .synthesized like 3. starting from 1,3-dihydroxyhe-nzene instead of 1,2-dihydroxybenz.ene. Selected data for 1.2 and 4:

1. F'AB-MS: lu/z, positive mode. 436. 'll NMR (CDCl,) <S/ppm: 4.38 (-CT/,-0~ph. m. 411). 4.46 (Z/ -Cp-. t. 411), 4.57 ( -CO-0-C/ / , - , m. 4M). 4.70 (W„-Cp-, t. 411). 6.1)7 (//-ph-..s. 411),

2. I'AB-M'' //;/:, positive mode. 524. ' H N M R (CDCl,) (VPpm: 3.79 (-C// ,-0-CW,-. m. 8H), 4,14 (-CA/,-0-ph. m. 4H), 4.40 ( -CO-6-C/ / , - , m. 411). 4.74 ('//;,-Cp-. t. 4H). 4.90 (//„~Cp-. t. 411). 6.90 ( / / -ph-. s. 4H).

4. FAB-MS: m/r . positive mode. 612. 'll NMR (CDCl,) ^/ppm: 3.70(-C//,-O-C//,-C//,-O-CW,-. m. 16H). 4.05 (-C//,-0~ph. m". 411). 4,41 (/ / /rCp-, -CO-0-CW,-. m. 811). 4.84 (//,,-Cp-. t. 411). 6.36 (W-ph-. s. IH). 6.47 (/ /-ph-. m. 2H). 7,11 (/7-ph-, t. IH),

Metal cations as their tetrafluoroborate or perchlorate .salts -vere of the highest purity available (>99'7r) and used without further purification. Addition of metal salts to CI! ,CN solutions was carried out from concentrated stock solutions stored over molecular sieves, Tetra-/(-butylam-monium perchlorate (TBAP) was recrystallized from ethyl acetate + cyclohexane and dried under vacuum at 80°C for 3 days.

2.2. Compk'.M's

Complexesof 2 with Li * and 3 with Na' orBa'" were prepared according the following procedure: the ligand (20nig) was dissolved in CFfXl, (2ml) and a stoichio­metric amount of LiClO^. NaCIO^ or Ba(CIOj), was added to the solution with stirring. After dissolution of the reagents, the complex was precipitated by adding diethyl ether (5 ml) and collected as an orange solid by filtration. P-BaKCIOj), complex was recrystallized from CHXI,

/. Inn vt(il./Joimmlof lih'clidwmlylhvl CliniiiMiyhKKIW/i /•.'.'i-/.i •17

hy using tlii;lhyl cllier viipoin-dilTiisioii icrhniciue lo iilToril L'lyslals siiiiiihio for X-ray iiiiiilysis, Uiiroriiiniilcly, (he olhcr solid complexes were nol suitahic for X-ray clilTnii.'-lioii iiiL'asiircmcnis.

[3-i{al(CI0.,),: I'AB-iVIS: //;/.-;. positive mode: M') (M - CIO., ).

[3--Nal(CIO.,): I'AB-MS: iii/z. positive mode: 633 (M - CIO, ).

l2-l.i](CI0,): l-AB-MS: /;(/: . positive mode: 531 (M - CIO,, ).

2.J. lilcilroiliciiiiitil iiwasHiTiiicnts

The electrochemical ec|uipment has been described pre­viously [II]. Bleclrochemical experiments were conducted in CM ,CN electrolyte in a three-compartment electrochem­ical cell inuier an argon atmosphere and at room tempera­ture. The AgllOmM AgNO,-f-0.1 M TBAP in CH ,CN system was used as a reference electrode. The potential of the usual rerrocene/ferriciniimi couple was 0.07 V under our experimental conditions. The working electrode was a platinum disk (3 mm diameter). 0.1 M TBAP in CH,CN (Rathburn, IIPIX' grade S) was used as tiie electrolytic solution. The peak current variations vs. guest cation con­centration (see Table I and Fig. 3(B) and Fig. 4(B)) were corrected for the dilution factor.

2.7. AssnciiitUnt coitstaiil (Iclcniiiiuilioii

The association constants between 1-4 and tlie metal cations were determined from UV spectrophotometric ex-perimenis. The UV spectra in the differential mode were recorded with 1-4 CH ,CN .solutions (ca. 1.5 niM), free of metal cations, in the reference cell (I cm), and 1-4 CH ,CN

solutions of the same concentration containing increasing iunounts of a given melal cation in the .sample cell. Analy­sis at selected wavelengths of the variations of the differ­ence in the absorbance with and without melal cations as a function of the total metal cation concentration, allowed the determination of the association constant using a com-puleri/ed least sc|uares method. Analysis of the titration curves show that for metal cation/ligand concentration ratios in the range 0 to 10, only 1:1 complexes are formed, except for Ba'' and 2 for which both 2:1 and 1:1 (L:M ') complexes are formed, as shown by the appearance of tm inflection on the titration curve at Ba"' / 2 = 0.5. The selected wavelengths for the determination of the associa­tion constants between 2 and Li ' , Na', K', Mg"', Ca" ^ and Ba-' were respectively 323 nm, 359 nm, 330 nm, 358 nm, 366 nm and 364 nm. At these wavelengths, AK (= ^.-(complex) - «(free ligand)) are equal to 1421 mol ' cnr ', 561 mol ' cm ', - 1591 mol"' c m ' ' , 51 1 mol 'cm ', 284 i mol ' cm ', 4001 mol"' cm ' ' for the 1:1 complex between 2 and Li ' , Na', K', Mg-' , Ca- ' and Ba- ' respectively, and equal to 3601 mol ' cm ' for [22-Ba]-'. The selected wavelengths in the case of the complexes with 3 were 363 nm. 359 nm, 363 nm, 372 nm, 330 nm and 368 nm and at these wave­lengths AA-= 97 I mol ' cm '. l l S l m o r ' c m ', 95 1 mol ' cm ', 1561 mol ' cm ', 299 I mol ~' cm ', 2751mol 'cm ' with Li ' , Na', K \ Mg-' , Ca-' and Ba"' respectively. The association constant between 4 and Ba" * was determined at 366 nm (A A- = 1191 mol"' cm ~ ').

2.5. X-niy (Unci collcclion ami ciy.skil sinictiin' deterinimi-lioii of iHiriiiiii coiiipU'xalion hy 3

riaFeC,„H„,0|„-(CI04)2; molecular weight M = 948.69g mol ', orange block. 0.16 X 0.15 X 0.10 mm\

CDiislanIs' K. iiiaximal hall'-vv,

1 process of 2 and 3 with niclal

shitls A / : , , , and iodic peak current characteristic of the

Mil-

.7- - / A -

l og lA ' /M ' )

A / ' | , , , / i i i V

log( A ' / M ' )

A / i ' | , , / n i V

1.47

2.K.3

4.77 + ().(W

13

().4f)

1.72 ± O.Of.

16

1.04 .05

\.'sy ^).l5

i).%5

m±i).\y

O.Of)

3,0.1

4..syi

3.K) + 0,.14

1,01

4,y4 + 0,36

0,K7''

' I'.\pcrimental conditions: CH ,CN + 0,1 M Tl iAl ' : / -= 0,1 V s ' ,

' No eoniplcxation was observed hetween I and the surveyed metal cations,

higt A: ) - .V27 ± 0,05. A / : , / , - 2 2 n i V . / | , / / | , , „ = 1,01 lor the complexation o r 4 w i l h l i a - ' ,

' 2:1 and 1:1 (l-:M*^) complexes are t'orined. K^/M'-.

A two-wave hehaviour was observed (see the text).

/. Iini CI iil./JvuiiHil <>t ElMi-iHimihliail Chi\ •I-IIIIIW7I 1-15^ Ifi

monoLiiiiic. space group / ' 2 | / c . r( = I3.7K5(3)A, /) =

I3.5H3(3)A. r = - l';.7ri7({,)A. /J == l()l.2'J(2)", uiiil-cell

volimii; V = 3(-)}{){2) A \ iiumbcr of roniuila iiiiils per CL-H

Z ~ 4, L'lik'ulaictl ilensily 1)^ ~ 1.74gem ', linear ahsorp-

lii)ii / i = 17.01 em '. The crystal was moiinled on a Hn-

ral'-Nomus CAI.)4 clilTraetometer usiny a graphite crystal

monoehromator ( A ( M o K « ) - 0.71073 A). 14023 rellee-

lions were collected at 293 K in a range 3 < 2^^ < (>{)". aiiii

were corrected for I.oreni/. and polari/ation elTects. The

striicliire was solved hy using TII.XSAN software (Molecular

Structure Corporation (1993). TI;,X.SAN. .Single Crystal

•Structure Analysis Sol'twiire. Version 1.7. MSC. 3200

Research Forest Drive. The Woodlands. TX 77381, USA),

3024 independent reriections with [•> ?itr(F) were u.sed

in the structural analysis (/•': structure factor; <r: standard

deviation on /•'). Al l non4iydrogen atoms were refined

with anisotropic thcrmid parameters. Mydrogcii atoms were

localized on a difference Fourier map except for the

hydrogen atoms of carbon C9'. CIO', C I 1' and C12' which

were g'.-nerated in idealized positions, riding on the carrier

atoms, with isotropic thermal parameters t/(H) = 1.2L' .,|(C)

(( / ( ID: thermal paranieler of the hydrogen iitonis; ((,,|(C):

thermal parameter of the carrier atoms). Hydrogen atoir.s

were added to the structure factor calculations but their

positions weie not refined. Final cycle refinement, includ­

ing 470 parameters, converged to a veracity factor Ril-') =

0.032 (for 3024 /•'> 3rr(/••)); (J/o"),,,^,, = 0.005, J/j,,,,,,

= 0.4ye A '. J / j „ „ „ = - 0 . 4 0 e A ''iA/a: maximal

shift/error: J p : residual electronic density).

3. Ke.sults and di.scu.s.si<)n

J.I. UVsfic'cir(>Si('i>ic iiircilii'iilions

'I'hen the titration curve increases continuously upon fur-

llier addition of barium (Fig. I). This corresponds to the

subsec|uent formation of a weak 1:1 complex (/C =

."iOM ' ) . Such a 2:1 ligand to metal inlermolecular sand­

wich complex has previously been described [13] for a

potassium:crown-substituted ferrocene complex \^ which

the metal cation is complexed by two ligands, not being

able to accommodate Just one macrocycle ring. No linear

correlation between the strength of the interaction and the

radius /• or the charge to radius c/r ratio of the guest

cation could be found. As a matter of fact, the highest

association constant (1:1 complex) was found with L i '

( ( • / / •= 1.47; log A '= 4.77), while lower K values were

obtained with Na ' (<•//•= 1.03; log 7 = 3.93) and Ca- '

((•//•••= 2.02; l o g A : - 4 . 1 3 ) . However, K values decrease

in the alkali metal cations .series and increase in the

alkaline earth metal cations series.

Ligand 3 forms stronger complexes with alkaline earth

metal cations ( M g - ' , Ca" ' and B a - ' ; log A"> 4) than

with alkali cations ( L i ' , Na ' and K ' : log A* < 3). hi

contrast with 2, the binding constants increase in the order

L i ' < Na ' < K ' < M g - ' < Ca ' ' < Ba - ' . Again, no corre­

lation between ;• or c/r and the association constaiio

could be found. As already reported, the formation of a

very strong 1:1 [ 3 -Ba ] - ' complex was evidenced from

UV, ' H NlVlf< and FAB-MS spectromelric experiments

[10]. The shape of the UV titration curve indicated that the

stability constant of this complex is too high iK>

10 '' M ' ) to be determined accurately under our experi­

mental conditions.

The 1:1 stoichioinetry was confirmed for some com­

plexes from FAB-MS spectrometry experiments. MS spec­

tra of isolated U-LiKCIO^), [3-Na](CI04) and [ 3 -

BaKClOj), complexes (.see Section 2, and Fig. 2 in the

UV spectroscopic titration (see Section 2) was used to

evaluate the complexation behaviour of the different fer­

rocene crown ethers towards various Group I and 2 metal

cations in CH,CN. and to determine association constant

data. First, we noticed that no change in the UV spectrum

of ligand I was observed in the presence of an excess (up

to ten-molar ecjuivalents) of L i ' . N a ' . K *. M g - ' . C a ' '

and B a ' ' cations. Owing to its small cavity 1 cannot

presumably accommodate these guest cations. In contrast,

compounds 2 and 3, ct)ntaining larger crown-eiher-like

inaerocycles, form more-or-less stable complexes with all

the surveyed metal cations. The results are summarized in

Table I. The order of magnitude of the association con­

stants is in agreement with that usually observed for

complexes formed between ferrocene crown derivatives

and Group I or 2 metal cations [5.12]. In the series of

complexes with 2. strong 1:1 ( L : M ' ) complexes {K>

10' M ' ) are formed with L i ' (Fig. I) , Na ' and C a ' ' . It

inust be emphasized thai 2 forms first a very strong 2:1

complex with Ba ' ' ( A ' = I 0 ' M ' ) . The corresponding

titration curve displays an intlection at a 0.5 Ba ' ' / L ratio.

0.3

\k

0.1

0

L

J ' /

/ '/

' 1 1 1

B a 2 +

Li+

L. ,^

M+/2

l-'iy. I. l ivolulion of tlic absorbiiiK-c of 2 ( l . 4n iM witli t.i

will) Ua - ' ) in C f l . C N al V.'inm (W ) or .V)4riin ( B a ' '

tJa-^ conccmralion: / = I cm; M ' / 2 : >""l<-" r'Hi" " t [ L i ' ]

[2]; A 4 = /\ - /VM • - i j : ( • ) cxpcriniL-nlal data; (

ciirvL- calculalircl I'roiii tlic paramclcrs iiCTabic I.

» r [ H a ^ M -) tlicorctic

/. lull CI til./Jiinnicil iif ElirliiiiiiHilyiiail (Iwiiiisliy -NO I I'm) 1-15-1?2

f, ik>m UAL

ig. 2. l-'AH-MS spcclniin (irl3-U;i](CI0,,).

case of the latler complex) display exclusively ihe //;/~ peaks eoiTcsponiling lo [2-Li]' . [3-Na]' and [3-[3a](CI0.,)' respectively.

The binding behaviour of fermceiie crown derivatives is controlled by several factors, such as ionic radius and charge density, and involves forces like solvation interac­tion': [5]. The selectivity of compounds 2 and 3 towards the metal cations surveyed is the result of a combination of these different factors.

J.2. Hleclroclu'iiiiccil rccaf'iiilioii

The electrochemical behaviour of the four ferrocene crown ethers was investigated by cyclic voltammetry (CV) in O.IM TBAP + CH,CN electrolyte. Potentials are re­ferred to an AgllOinM AgNO, + {).\ M TBAP + CH,CN reference electrode. The CV curves in niillimolar solutions of 1, 2, 3 and 4 are characterized by a t|uasi-reversible redox wave at / : , / ,= 0.365 V. 0.535 V. 0.548 V and 0.547 V respectively" (A/:,, clo.se to 0.09 V at /-= 0.1 Vs ' ) . corresponding to the ferrocene/ferricinium re­dox couple (see Fig. 3 for example).

The addition of increasing amounts of Li ' , Na*. K' . Mg- *, Ca- *" or Ba~' in electrolytic solutions of 1 did not cause any change in the CV curve of 1, confirming the UV spectrophotometry experiments, i.e. the absence of com-plexation between 1 and these metal cations.

In contrast, a progressive shift of the 2 /2 ' redox wave to more positive potentials is ob.served in the presence of increasing amounts of all these guest cations (Table I). Maximal potential shifts were obtained in the presence of at least ten-molar equivalents of metal cations. The poten­tial shift is negligible (less than 5mV) with K * and Mg- ' . but reaches 40 mV with Ba"'. In addition, analysis of the anodic peak current shows that /,, decreases continuously in the presence of Li " (Fig. 3) and Na', and to a lesser extent Ca"' or Ba"' (Table I). This decrease parallels the positive potential shift (Fig. 3(B)). For example, in the pre.sence of ten-molar equivalents of Li'', /|, is decreased by ca. 509f. This decrease could be due to a smaller

diffusion coefficient of the complex compared lo that of the free ligand. However, considering the weak anodic peak current vtuiations observed in the case of addition of metal cations in eleclrolytie solution of 3 (Table I), a decrease of the diffusion coefficient following the com-plexation of 2 with Li' and Na' cannot be the sole explanation of the large peak current variation observed. One could assume the formation of complexes with stoi-chiometry different from 1:1 and oxidizable at much higher potentials. The shapes of the UV titration curves and the FAB-MS spectra of isolated complexes rule out this last hypothesis. In addition, the i'AB-MS spectrum of the complex synthesized by reacting 2 with four-molar equiva­lents of Li' displayed only the iit/z peak corresponding to the 1:1 complex. Kinetic complications could be respon­sible for the decrease of the redox peak system of 2 in the presence of Li ' , Na ', Ca"' or Ba"'. in spite the ab.senee of significant A/i|, variations. As a matter of fact. A/: decreases only from SOmV to 60 mV upon addition of ten-molar equivalents of Li' in an electrolytic solution of 2 (Fig. 3(A)).

I'ig. } . (A) Cyc-lii TBAI on a 11 (5

Li+/2

illaiiimogrums of 2 (.14mM) in CH,CN+().IM

rale 0.1 Vs~': /; l l>/ \ r III] a r i V-'HIIII ujiiillCtCr) ClCCtrOClL'. .• .llll uin- \/.t » r> , i- \n

AgllOiiiM AgNO.-KI.IM TBA1' + CH,CN: IJCIOj/2 = 0 (1). (LI (2) 1 h). I..S (4). 2 (5), .• (6). 4 (7). .i (S). 10 molar ci)uivalcnts CJ). (B) A/:'|,-. and anodic pcali currcnl vs. mole ralio of [l,i' ] to [2]; / -- /

= 0).

/. Inn t'l (il./Jdiirmil of Ehrtwwmhthvl Vlwmisliy -IK) IIW7I /-/.i-152

The iiclclilion of Li ' . Na' , K* unci Mg-' to an clcc-imlytic solution of 3 also rcsulls in a weak and uonlinuous positive sliifl of the CV wave, wilhoul significant changes in the peak eurrenls. It should he noted that a negligihie shift (•l-4niV) is obscfved in the presence of Mg-' , in spite of the formation of a strong [3-lVIg]-' complex (log A" = 4.26). In contrast, a new redox peak system grows at a more positive potential in the presence of Ca' ' (/s'l/, = ().67V) and Ba-' (/;,/, = 0.71 V) at the expense of the original redox wave for free 3 (see I'lg. 4(A). in the case of the barium cation). The anodic current associated with the new redox couple increa.ses linearly with the concentration of barium or calcium cations until one equiv­alent of metal salt has been added (Fig. 4(B)). At this point, the original wave disappears and the new redox couple reaches full development. This confirms the forma­tion of a very strong 1:1 complex between 3 and Ca' ' or Ba-'.

For all the complexes studied iiere the shift in potential A/:,/, between compiexed and free ferrocene is observed towards the positive potential. Assuming that no structural

0 0.2 0.4 0.6 0.8 1.0 1.2 Ba2+/3

t'ij:. 4. (A) Cyclic voluinmiogram or 3 (.^.7inM) in CH,CN + ().IM THAI' on ;i Pi (5mm diamclcr) electrode: scan rate O.lVs"': /; vs. AglldmM AyNO, +().l M TBAI' + CH ,CN; IJa(CIOj), /} = 0 {I). 0.28 (2), 0.56 (M 0.84 (4). I molar eiiuivalents (5). (B) Oxidation peak current vs. mole ratio of [Ba- * ] to [3] lor the free (/', ) and coniplexed (/;' ) 3.

effect is involved in the complexation process, this shift is linked to the ratio (K/K ^ ) of the stability constants fi)r the complex in its reduced (neutral). K, and oxidl/.ed (charged) state, K,. according to A/i , / , = (W'/'/ ///•')ln( A'/A'i ) [5]. Repulsive forces between oxidi/ed (charged ferrocene) and metal cations lead to a smaller K ^ than A", and therefore a positive potential shifi. The high positive shift found for 3 /3 ' and Ca"' or Fa"' indicates that 3 acts as an effective redox switch'.ible ligand for Ca-' or Ba"'. In addition. i^E^/i ref!ei!ts the balance of the interactions with the metal cation biuween the oxidized and neutral ligand rather than the strength of the interac­tions between the ligand and the metal cation. As a consequence, no clear correlatior, can be found between A/i"|,, and the association consiants, although the smaller potential siiifts arc observed for the weaker complexes. Maximal shifts are reachetl only in the presence of an excess of metal cations (e.g. Fig. 3(B) for the [2-Li] ' complex), even when ihj complexation constant is high. It must be emphasized that the weaker complex [3-Na]'" (log K= 2.74) leads to a greater potential shift (A/ii/^ = 48mV) than the sironger complex [2-Li]' (log K = 4J7; A£'|/2 = 13 mV). Such a discrepancy between the binding strength and the corresponding potential shift has been noticed previously for a number of ferrocene crown deriva­tives [14. i5]. Other factors than the binding ability of the ligand could also be responsible for the magnitude of the shift [5.6,16,17]. An effective electrical connection be­tween the redox centre and the binding site is required to induce a large potential shift upon complexation, e.g. the proximity oi' the host binding site is a factor of paramounf importance in determining the magnitude of the potential shift. Probably due to a particular structural arrangement of the complex in CH,CN .solution, this requirement is not met in the case of the strong complexes formed between 2 and Li ' , Na ' , Ca-' and Ba-', or between 3 and Mg-'. The charge density on the cation, expressed as the ratio c/r'. also affects the magnitude of the shift when the transfer of the electron density from the compiexed metal cation to the redox centre takes place through space [3,5]. For the complexes formed between 2 or 3 and the metal cations surveyed, no correlation between AE,/ , und the ratio c/r- could be observed. Thus, it seems that confor­mational change in the receptors 2 or 3 accompanies the complexation process [17], leading to an unexpected redox potential shift of the ferrocene unit [18].

On the other hand, it appears that a two-wave behaviour is ob.served only for complexes of 3 with Ba"" and Ca- \ cations which arc characterized by the highest association constants {K> IO''M^'), allowing the effective ampero-metric recognition and titration of the guest cation (Fig. 4(B)). The electrochemical behaviour of redox ionophores in terms of the type (one- or two-wave) of potential shift has been rationalized by Miller ct al. [19]. They established that the two-wave behaviour can be observed when the binding constant of the neutral ligand is large (> 10' M "')

/. /()// CI (il. /.loiiriml (iflMrlivwiitlylircil Clwrnhiiy -NO) IW7) l-IH-1.'^2

in associalion with ii large K/K, ratio (e.g. !()''). Uiuler

tiicse two eondilions, tlie two-wave voitanimetrie he-

liavioiir is incicpencient of tiie experimental time .seale. i.e.

it can be rationaii/ec! witlioiit any l<inetic assumptions

concerning tiie rate of all chemical and electrochemical

equilibria [5]. The transition between a two- to a one-voli •

ammclric wave is linked to a decrease in K/K,. Thus,

when K is large, as lor Ca ' ' or Ba" ' with 3, and K/K,

intermediate (A ' /A ' , = 104 and 5()\ respectively, deter­

mined from the corresponding A / i ' , / , ) i-e. only one of the

above conditions is satisfied, a two-wave behaviour re­

quires slow decomplexation kinetics, i.e. the complexation

equilibrium is frozen during the voltammetry time scale

[17].

The increase in the si/e of the crown ether cavity from

1 to 3 leads to strong modifications in the ability of the.se

ligands to complex and recogni/e guest cations elcctro-

chemically. In summary: 1 does not complex and recog­

nize any metal cations; 2 complexes preferentially L i ' ,

Cir' and Ba- ' and is able to recognize Ca" ' and Ba- '

elcctrochcmically through potential shifts: 3 forms strong

complexes with M g - ' , Ca" ' and B a ' ' , and recognizes

Ca" ' or Ba'' ' amperometrically and to a lesser extent Nu '

or K ' potentiometrically. Thus, the control of the design

of the cavity appears to be of prime importance with the

view of electrochemical recognition. This is confirmed by

the study of ligand 4, which differs from 3 only by the

substitution position on the aromatic ring. From 3 to 4, the

a.ssociation con.stant with Ba" ' (log K = 3.27) is decreased

by at least two orders of magnitude, and the presence of

Ba" ' in an electrolytic solution of 4 causes only a small

positive shift in potential up to 22 mV.

The selectivity of the complexation process monitored

by electrochemical methods exists only in the ca.se of

tv.o-wavc behaviour. Obviously an ordinary small shift in

potential is not sufficient to recognize electrochemically

one cation among the others. Competitive complexation

experiments were then performed considering 3 and Ba" '

or Ca " ' . The voitanimetrie fcatmes of [ 3 -Ba ] - ' or [ 3 -

Ca]" ' did not change upon addition of an excess of Na ' ,

K ' or M g " ' . Further, the addition of increasing amounts

of Ba" ' to a .solution of 3 previously complcxed with

Ca" ' led to a gradual positive shift of the redox wave

along with a slight increase in the current peak and a

decrease in AE^^. to end with the typical reversible wave

lor the [ 3 -Ba ] - ' complex after one equivalent of Ba" '

cations had been added [10]. Further addition of Ca" ' up

to lOO-molar equivalents caused no delectable change in

the CV curve. This indicates that 3 provides a highly

selective ligand for barium against calcium and for cal­

cium or barium against L i ' , N a ' , K ' or M g ' . This

electrochemical behaviour is in agreement with the associ­

ation constant values (Table I) determined by UV-v is

spectrophotometry.

3.3. X-ray stnulumi iiivcsliiicitions mi llic imturc of bcir-

iiiin coinpk'.wtion by 3

The crystal structure of the discrete complex (Fig. 5)

reveals that Ba" ' is bound with ten oxygen atoms and sits

inside the cavity. Eight oxygen atoms are provided by the

macrocyclic imit, six from the crown ether moieties and

two from the carbonyl groups. Two oxygen atoms of

perchloratc countcr-anions complete the coordination

l-i;;. 5. Slriictuii; i i f [ J -BaKClOj ) , . Thermal Uispl

alonis ( 06 and OlO) of IIK- perchloratc counler-ani

L-mcnt ellipsoids are drawn at

s are shown, and hydrogen [\h

M)'7r probability Ic

have been omitted.

• clarity, only the coordinatins; oxygen

I.lo ul./.h •1-11)11W7I 1-15-152

sphere of hiiiium. The Hi i -O hoiicis exhihil ii kirge liiiige

Imm 2,707(5) A ( H i i - O D to 3.254(3) A (Hii-()3). Similiir

dislanees Inive been ohservecl in a complex helvveen B i r '

iiiul an a/a-eiown I'enoeene ciypland [20]. The shoilesl

tlistanees are with the two cariionyl oxygen atom.s. i.e.

2.707(5)A ( H a - O D and 2.738(5)A (Ba-OI ) . Fnrliier-

more, the cyelopentadiciiyl rings are liUed by 2.4". Tlie

distances IVoni the iron atom to the centroids of these

planes are very similar. l.(i44(y)A and \.M2(9)A. An

important ob.servation about this complex is that the Fc-Ba

distance is 5.578(1) A, loo long Cor any direct interaction

through space to be invoked. The remarkable voltammelric

two-wave behaviour obtained upon complexalion of Ihe

barium cation with ligand 3 may. in part, be due to

non-coplanarity of the cyclopentadienyl rings which causes

an important jiosiiive polenlial shift, as in strained bridged

lerrocenephanes [IS]. In addition, it seems more than

likely thai there is an interaction between the barium

cation ami Ihe rerrocenyi group through the carbonyl

groups. 1'al imparls special proi.erties of this redox-aclive

receptor.

4. Conclii.sioii

The rerrocene4)ased macrocycies 1-4 are redox

ionophores and have been evaliuiled with respect to tiie

polentiometric or amperometric electrochemical recogni­

tion of IJ ' . N i l ' . K ' . Mg ' . Ca- ' or Ba" ' . The complexa­

lion ability of these ligands towards Group I and 2 metal

cations depends on the size of the crown ether cavity and

on the charge of the metal cations, biu is not correlated to

their cleclrochemical recognition ability which requires an

elTective electrical connection between the redox centre

and the binding site, in the presence of a guest cation, two

dilTerenl types of behaviour have been obtained: a gradual

potential shil't ol" the redox couple allowing the polentio­

metric titration of the guest, or a two-wave behaviour

allowing the amperometric titration of the guest. The first

type has been observed with 2 and all the metal cations

surveyed, with 3 in the presence of L i ' . N a ' , K ' and

M g ' . and for the [4-Ba]-* complex. Thus 3 appears as

the best candidate for the development of Ba- ' and Ca" '

sensory devices. The modification in the clectroaclivity of

the redox centre is due to the addition of a structural effect

and an electrostatic effect leading to the de-stabilization of

Ihe oxidized complex, l-urther development of this work

wil l deal with Ihe immobilization of Ihese redox-active

receptors on an electrode surface, with a view of consirucl-

ing molecular electrochemical sensors.

Ackii»\vlc(lsement.s

The authors thiuik Dr. A. Deronzier i'or fruitful iliscus-

sions and laboratory facilities and "Region Rhone-Alpes"'

for partial financial support through Ihe TBMPRA pro­

gram.

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