C. R. Acad. Sci. Paris, t. 2, Serie II c, p, 79-83, 1999 Chimie inorganique molCculairelMo/ecu/ar inorganic chemistry

Dendritic effect in the recognition of small inorganic anions using a poly- cationic nona-cobalticinium dendrimer Christine VALgRIO, Jaime RUIZ, Jean-Luc FILLAUT, Didier MTRUC *’

Groupe de chimie supramoleculaire des metaux de transition, LCOO, URA CNRS 5802. universite Bordeaux-I, 35 1, tours de la Liberation, 33405 Valence cedex, France

(Received 22 October 1998. accepted 25 November 1998)

Abstract - A polycationic nona-amido-cobalticinium dendrimer has been shown to be efficient for sensing small inorganic anions (H,POi, HSO; and Cl-) using cyclic voltammetry and ‘H NMR. 0 Academic des sciences/Elsevier, Paris

polycationic dendrimer I recognition of anions I cobalticinium / redox activity I ‘H NMR

Version frarqaise abrCghe - Effet dendritique pour la reconnaissance de petits anions inorga- niques utilisant un dendrimkre polycationique nona-cobalticinium. Les dendrimeres [l] constituent actuellement un domaine de recherche particulierement attractif Ptant don& leurs applications en sciences des materiaux [2] et en biologie [3]. En particulier, les metallodendrimeres [4-81 ont servi de bases molkculaires pour la conception de systemes photoredox et qq antenne 11 (41. Par ailleurs, leurs proprietts catalytiques sont etablies [5], y compris pour la catalyse multielectronique [X-lo]. Des dendrimeres a terminaisons metalloce- niques [7, 81, en particulier cobalticinium [8d-fl, ont kte synthetises recemment, impliquant de remarquables proprietes redox [9]. Depuis, les etudes de pionnier de Lehn et de ses collaborateurs [lo], les recherches sur la reconnaissance anionique se sont trts largement amplifiees [ 111. Les complexes mCtalloceniques ont et& utilises en reconnaissance moleculaires [ 1 l-l 31, notamment par Beer avec des mttallocenes [ 131. Nous avons montre que les dendrimeres ferroceniques manifestent un effet dendritique en reconnaissance anionique, c’est-a-dire qu’ils montrent par voltametrie cyclique des effet plus marques que leurs analogues tripods de meme structure terminale que les tripods dendritiques. Cependant, la reconnaissance n’est gineralement pas possible par RMN car les dendrimeres ferroceniques sont neutres dans leur forme in 18 electrons diamagnetique, et une bonne reconnaissance n’est possible que par synergie entre une liaison hydrogene NH...anion et une attraction elec- trostatique entre l’anion et un centre mttalloctnique cationique. Nous avons done test6 le dendrimere nona- amidocobalticinium Y-Co (srhima I), synthetise a partir de la nona-amine correspondante [8c-e, 141, pour reconnaitre les petits anions inorganiques (H,PO;, HSOi et Cll) en utilisant la titration par le se1 de n-Bu4N+ de l’anion suivie par RMN ‘H grace au d&placement du signal NH amidique dans le cas de l’anion Cl- et par voltametrie cyclique pour les anions H,POi, HSOi et Cll. Les rtsultats de ces cleux techniques (tabkzu r) concordent et montrent que l’anion interagit fortement avec la terminaison amido-cobalticinium, a raison d’l equiv d’anion par branche dendritique. Cette interaction est aussi plus forte qu’avec les tripods a terminaison cobalticinium [ 131. Enfin, le dendrimere l&Co, analogue a ~-CO et comportant un coeur dendritique identique et neuf bi-branches N(CH,CH,CH,NHCO-r15-C,H4Co+~5-CsHsjz, d onne des resultats nettement moins satisfaisants en raison de problemes d’insolubilite et de precipitation lors des titrations (tdblenu r). 11 est remar- quable qu’en voltametric cyclique la premiere vague cathodique reversible (CoCp;/CoCp,) de reduction nona- Clectronique du dendrimere ~-CO impliquant la forme polycationique du dendrimere subisse de grandes varia- tions de potentiel suivant la nature de l’anion ajoute dans la cellule electrochimique, alors que la seconde vague

Communicated by Michel POUCHARD.

* Correspondence and reprints.

’ Institut univcrsitaire dc Fr;mce.

1387-1609/99/000200079 0 AcadCmie des sciences/Elswier, Paris 79

C. Vakrio et al.

nonaelectronique reversible (CoCpJCoCpj) impliquant la forme polyanionique ne subit pratiquement aucune influence. Ce phenomene, illustre dam I’equation (I), ouvre la perspective (de reconnaissance simultanee d’anions et de cations. En conclusion, le dendrimere polycationique nona-amidocobalticinium Y-Co s’avere excellent pour la reconnaissance des petits anions inorganiques par RMN ‘I-I dans le cas de Cl- et par voltametrie cyclique pour H,POi, HSO, et Cl-. 0 Academic des sciences/Elsevier, Paris

dendrimhre polycationique / reconnaissance anionique / cobalticinium / activitC redox / RMN ‘H

-

Dendrimers have received considerable atten- tion [I] given their potentiel applications in material science [ 21 and molecular biology [3]. In particular, metallodendrimers [4-S] have recently been designed to search photoredox and antenna effect (4-B] and specific catalytic properties [5-B] including polyelectronic redox catalysis [B-l 01. Iron- and cobalt-sandwich dendrimers have recently been reported [7, 81. Polycobalticinium dendrimers [Bd] are an attractive goal because of their positive charges. Indeed, the electrostatic repulsion between the

charged metallocene units around the den- drimers insures a better radial distribution of their location, and anionic guests can obviously be attracted by the polycationic dendrimer in its diamagnetic 1 B.-electron form. Moreover, the cobalt sandwich can exist under three oxi- dation states, easily detectable by cyclic voltam- metry [9], which allows the observation of the large variation of the electrostatic interaction between the dendrimer and the anionic guest upon scanning the potential scale. Since the seminal work by Lehn and his group [lo], the

80

Dendritic effect of small anions using cobalticinium dendrimer

Table I. Titration of 9-G and IS-Co by n-BuqNt salts monitored by ‘H NMR an d y I’ c c ~c voltammetry. P = precipitation

upon titration. The apparent association constant Kapp was determined using the EQ NMR program [I?]. For IS-Co,

adsorption and precipitation upon titration prevented precise electrochemical analysis.

n-BqN’

salt

‘H NMR (cl,-DMSC))

A6 NH (ppm) K&,, associarion

9-co 1 SC0 9-co 18-Co

Cyclic voltammetry (Pt)

CD,CN/DMF (2: 15)

AP (CO”IC0”‘) (mV) -

9-co 18-Co

6,, = 8.77 6, = 8.77

H$‘O; P P 270 90

HSC),- I’ P 205 50

Cl-~ 0.495 0.105 64 32 60

research on recognition of anions has devel- opped extensively [ 111. The use of redox cen- tres as sensors attached to an endo-receptor for the recognition of guest ions is well known [11-l 31. In particular, Beer has succesfully used amidoferrocene and amidocobalticinium units in this way to recognize small inorganic anions and shown the synergy of electrostatic and H-bonding interactions [12, 131. So far, no report has appeared on such a strategy with cobalticinium- or other cationic dendrimers. Dendrimers may be viewed as exe-receptors in which the guest-host interactions occur at the outskirt of the host. Large dendrimers have a fractal surface like viruses and cancer cells [3] and, therefore, the study of peripheral host-guest interactions is of great interest.

The nona-cobalticinium dendrimer (~-CO) shown in scheme 1 was synthesized according to the published procedure by reaction bet- ween the corresponding nona-amine dendrimer [&z-e, 14, 151 and [C,H5CoC5H&OCl]‘PF; in MeCN at room temperature in the presence of NEt,. Another way, although slower, consists in using the protected ester [C5HjCoC,Hb CO,N(COCH,),]‘PF, under analogous reac- tion conditions. The electrospray mass spec- trum of ~-CO confirmed its structure, showing the molecular peaks with successive loss of PF, anions [Sd].

Titration of ~-CO by n-Bu*N+ salts of H,PO,, HSO, and Cl- is monitored by cyclic voltammetry in CH,CN/DMF and by ‘H NMR in d,-DMSO. Whereas electrochemistry monitors the difference of host-guest interac- tion [ 161 between the cationic l&electron and neutral l‘+electron forms by the shift of the reversible Co(II/III) nona-electronic wave at -0.87 V versus SCE, ‘H NMR only reflects the recognition by the 1%electron cation. Since the

recognition by the cation is much stronger than that of the neutral species due to the electro- static interaction in the former, the two meth- ods should give comparable results with the Co dendrimer (which is not the case for ferrocene dendrimers [IS]). The NH signal in ‘H NMR (6) and the 18e/19e cathodic wave in the cyclic voltammogram

( E” =: EPc ; EP,)

are progressively shifted upon addition of the n- Bu4N+ salt. The titrations using these two methods indicate that the equivalence point is reached for one anion per dendritic branch. Table / shows the titration results, the data being reported for the shift of 6 and E” observed for one equiv; anion per branch (equivalence point). The values obtained with ~-CO are larger than those observed using a tri- dendate host [I 3a] and much larger than those with a monometallic host. This shows the den- dritic effect in the recognition. The dendrimer ~-CO can be used under excellent conditions in electrochemistry with all these anions and in ‘H with chloride. Precipitation upon addition of the salts of the other anions inhibited sensing these anions by ‘H NMR.. Finally, it is of inter- est to note that the second nona-electronic reduction wave (Co”“) of the Co dendrimer at -1.63 V versus SCE is not significantly shifted upon titration by the anions, in striking con- trast to what is shown in table 1 for the first wave. This is due to the repulsion between the added anion and the polyanionic dendrimer in the 20-electron form of the cobalt sandwich. At this time, the loose interaction in the neutral form, only reflecting the H bonding, domi- nates, but its magnitude is small [equation (l)]. This dichotomy of behavior between the first and second nona-electronic waves opens the

81

C. ValCrio et al.

~~~~~~~~~~-~ (1)

A-

0 A- @ A-

strong electrostatic attraction no electrostatic attraction electrostatic repulsion

good recognition loose recognition no recognition

E” depends on A

possibility of simultaneous recognition of ani- ons and cations. Although we also synthesized, from the same nona-amine, a 18-Co dendrimer containing nine N(CH,CH,CH,NHCOCp Co’Cp), bi-branches analogous to the reported 18-ferrocene dendrimer [8c], sensing of anions was less satisfactory with the latter due to insol- ubility problems (table I).

In conclusion, the polycationic nona-amido- cobalticinium dendrimer Y-Co proved efficient and selective in the recognition of small inor-

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Acknowledgement

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83