Anion selective polymeric membrane electrodes based on metallocenes

ANALmcA

CHIMICA ACIA

EISEVIER

Analytica

Chimica Acta 304 (1995) 171-176

Anion selective polymeric membrane electrodes based on metallocenes Hideaki Hisamoto a, Dwi Siswanta a, Hiroshi Nishihara b, Koji Suzuki a3* ’ Department ofApplied Chemistry, Keio Uniuersity, 3-14-l Hiyoshi, Kohoku-ku, Yokohama 223, Japan ’ Department

of Chemistry, Keio University, 3-14-l Hiyoshi, Kohoku-ku,

Received 20 May 1994; revised 7 November

Yokohama 223, Japan

1994; accepted 13 November

1994

Abstract Seven types of metallocenes (his-cyclopentadienyl compounds) were used as anion ionophores for polymeric membrane electrodes and their anion selectivities were examined. The electrodes based on metallocenes exhibited different selectivity behaviour compared with the electrode using a classical anion exchanger such as tetraalkyl ammonium. The selectivity for tetraalkyl ammonium simply obeys the lipophilicity of anions, specifically, hafnocene dichloride exhibited a highly specific response to anions that have r-electrons such as the salicylate ion. This electrode demonstrated a Nernstian response to the salicylate anion ranging from 1 X 10K3 to 1 X 10-l M in a buffered solution (0.05 M Tris-HzSO,) adjusted to a pH of 7.4. Keywords:

Ion selective electrodes;

Metallocenes;

Salicylate

1. Introduction In contrast to the large number of investigations on cation ionophores, only a few effective anion ionophores have been reported in the last decade [l-12]. Recent reports on anion selective electrodes indicate that some ionophores such as 5,10,15,20-tetraphenyl(porphyrinato)tin dichloride (for salicylate) [5], aquocyanocobalt-hepta(2-phenylethyl)-cobyrinate (for nitrite) [6], tetrakis[ &dimethylsulphoxideO:O)]-bis{ p-[hexafluoropentanedionatd2 - )O:O’]}his{ p-[4,5-dimethyl-3,6-bis(octyloxy)-1,2-phenylenelltetramercury (for chloride) [8], N,N’-diocta-

* Corresponding

author

0003-2670/95/$09.50 0 1995 El sevier Science B.V. All rights reserved SSDI 0003.2670(94)00614-8

decyl-1,4diazabicyclo[2.2.2]octane diiodide (for iodide) [9], 15-hexadecyl-1,4,7,10,13-pentaazacyclohexadecane (for nucleotide and dicarboxylate anions) [lo], have specific interactions with the respective anions. These anion ionophores involve a cobalt, tin, or mercury metal ion as the center metal which exhibits an unusual anion response, different from the so-called Hofmeister series, when they are used as an anion sensing component for polymeric membrane electrodes [l-8]. This unusual anion selectivity behaviour is based on the property of the center metal and the chemical environment around the center metal (e.g., a stereospecific configuration which is formed by the organic ligand coordinated to the center metal), so that the anion affinity property is different from the simple lipophilicity of anions, which is the so-called Hofmeister sequence [13].

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H. Hisamoto et al. / Analytica Chimica Acta 304 (1995) 171-l 76

Thus, it can be expected that examination of anion affinities of organometallic compounds can lead to the development of novel anion ionophores. In the present study, we first report on the anion selective properties of seven types of metallocenes involved with Co, Fe, Ni, Hf, Nb, Ti, and Zn which all have two cyclopentadienes as r-electron donors, in which four of them (Hf-, Nb-, Ti- and Zn-based metallocenes) are dichloride adducts. The electrodes based on these metallocenes exhibited an antiHofmeister anion selectivity pattern. Among these, the electrodes based on hafnocene dichloride demonstrated a highly specific response to anions having n-electrons such as salicylate and benzoate ions. It is noteworthy that this metal rr-complex still has rrelectron acceptable properties even though it forms a stable complex with two cyclopentadienes as a rrelectron donor. This metal r-complex is a novel anion sensing ligand for an ion-selective electrode, whose selectivity pattern obviously deviates from the Hofmeister sequence as well as those of electrodes using other lipophilic organometallic compounds reported to date [l-12].

2. Experimental 2.1. Reagents Dicyclopentadienyl cobalt (cobaltocene), dicyclopentadienyl iron (ferrocene), dicyclopentadienyl nickel (nickelocene), bis(cyclopentadieny1) hafnium dichloride (hafnocene dichloride), bis(cyclopentadienyl) niobium dichloride (niobocene dichloride), bis(cyclopentadieny1) titanium dichloride (titanocene dichloride), and bis(cyclopentadieny1) zirconium dichloride (zirconocene dichloride) were purchased from Aldrich (Milwaukee, WI). Bis(l-butylpentyl) adipate (BBPA), bis(2-ethylhexyl) adipate (BEHA), bis(2-ethylhexyl) sebacate (BEHS), bis(2-ethylhexyl) phthalate (BEHP), and 2-nitrophenyl octyl ether (NPOE), which were used as the membrane solvent (plasticizer), were also obtained from Aldrich. Poly(viny1 chloride) (PVC) used as the membrane material was obtained from Sigma (St. Louis, MO) (P3938, high molecular weight type). All other chemicals used were commercially available reagents of the highest grade.

2.2. Electrode

assemblies

and EMF measurements

EMF measurements were made according to the following electrochemical cell system used in a Phillips IS-561 liquid membrane electrode kit as the ion-selective electrode assembly: Ag;AgCl, 3 M KCl]test solution(membrane]0.05 M Ph(OH)COONa, 0.05 M KCl, AgCl;Ag. The apparatus, electrochemical equipment, and measurement techniques were as previously reported[l4]. All plasticized PVC membranes were prepared under argon gas atmosphere and their compositions were l-4 wt% ionophore (metallocene compounds), 66 wt% membrane solvent (plasticizer) and 30-33 wt% PVC. The test solutions were prepared with 0.05 M Tris-H 2SO, (pH 7.4) or 0.05 M MES-NaOH (pH 5.5). The activity of all tested electrolyte solutions was estimated using the equation proposed by Kielland [15]. In this case, the ionic strength of the buffered solutions containing Tris-H ,SO, or MES-NaOH was estimated by the amounts of H,SO, or NaOH and other sodium or organic anion salts presented in the test solutions. The selectivity coefficients were determined by the separate solution method (SSM) according to IUPAC recommendations [16], and were obtained with the respective 0.1 M anion solutions of sodium salts.

3. Results

and discussion

Some lipophilic organometallic compounds and metalloporphyrin derivatives were reported as unusual anion ionophores that have specific anion selective behaviour, which is different from those of the classical anion exchangers, when they were used as the anion sensing component of polymeric membrane electrodes [l-8,11,12]. Their anion selectivities were mainly governed by the specific interaction between the center metal and anions rather than the lipophilicity of the anions or simple opposite charge interactions with anions. Therefore, there are possibilities that additional investigations of the interaction between organometals and anions will lead to the development of novel anion sensing ligands for an ion-selective electrode.

H. Hisamoto Edal. / Analytica Chimica Acta 304 (I 995) 171-I 76

We have chosen seven types of metallocene derivatives as anion ionophores for polymeric membrane electrodes, whose chemical structures are shown in Fig. 1. These are bis-cyclopentadienyl compounds of Co, Fe, Ni, Hf, Nb, Ti, or Zn, and four of them (Hf-, Nb-, Ti- and Zr-complexes) are dichloride adducts. These metal r-complexes were never tested as an ion-sensing component for a potentiometric ion-selective electrode. The potentiometric ion selectivity coefficients of the electrodes based on these metallocenes are summarized in Fig. 1. All of the electrodes based on metallocene compounds showed an anionic response, and their selectivity patterns were different from that of the electrode based on a classical anion exchanger whose selectivity pattern simply obeys the lipophilicity of the anions (Hofmeister sequence). However, the electrodes based on metallocenes, except hafnocene dichloride and zirconocene dichloride,

ohhxex IBPNWC

F-

Ntckdaalc

BBPMVC

BLIP&PVC

Elafnmm dirhkca BBPM’VC

173

showed a poor response to all the anions examined, and their logarithmic selectivity coefficients (log K,r’y’) of these electrodes ranged from 0 to - 2 for all ’ eleven types of anion species tested as shown in Fig. 1. Fig. 1 also presents the potentiometric selectivity coefficients of the electrodes based on trioctylmethylammonium chloride (TOMACI) and the blank membrane without an ionophore. The electrode based on a classical anion exchanger showed a typical Hofmeister selectivity pattern where it does not respond strongly to salicylate and benzoate ions. Compared with the electrode based on TOMACl, the electrode based on hafnocene dichloride apparently showed an anti-Hofmeister selectivity pattern and a high salicylate and benzoate selectivity. Our previous research on an anion selective optode based on hafnocene dichloride and an anionic dye, suggested that the chloride of hafnocene dichlo-

ND_

dichloride BBPAJWC

TiMoxne .3chha

zirmmocrr dalaide

Trimybnmhyl nlsnmim-

BBPMVC

BBP&QVC

BBPkwC

None BBPAIWC

-\m-

0.05 M Tris / H2S04 -4

pH7.4 -Lw

Fig. 1. Ion selectivity coefficients of the electrodes based on metallocenes and quaternary ammonium salt. All values were calculated by SSM based on the IUPAC recommendation [16] using EMF values of 0.1 M sodium salts except benzoate (PhCOO- 1 as an ammonium salt. Membrane composition: 1 wt% organometallic compound, 66 wt% BBPA and 33 wt% PVC.

174

H. Hisamoto et al. /Analytica

Chimica Acta 304 (1995) 171 -I 76

ride can dissociate from the center metal as a chloride ion, and this metal n-complex interacts with anions in a dehalogenated cation form [17]. In fact, a precipitate of AgCl was apparently observed in the water phase, when the AgNO, aqueous solution and hafnocene dichloride in chloroform solution were vigorously mixed. Hafnium originally has strong w electron accepting properties and it can be understood that hafnocene strongly interacts with anions having rr-electrons other than anions without a 7~electron. In this case, hafnocene dichloride releases chloride and accepts an anion having rr-electrons. This anion exchange property is the principal of the EMF response of the electrode showing an unusual anti-Hofmeister selectivity pattern. Zirconocene

CI

BBPAJPVC

BEBA/l’VC

BEHS/PVC

dichloride also has similar properties to those of hafnocene dichloride. Among all four types of metallocene dichlorides, hafnocene has the strongest 7~electron accepting properties and exhibits favourable characteristics in salicylate or benzoate ion. Fig. 2 shows the selectivity coefficients for the hafnocene electrodes, where the electrode membrane solvent was varied with five different plasticizers (BBPA, BEHA, BEHS, BEHP, and NPOE). The first four esters have low polarity (E = 4; E = dielectric constant) whereas the last one (an ether) has a relatively high polarity (E = 4). Regardless of the large differences in their polarities, employing BEHP and NPOE as the membrane solvents for the hafnocene electrodes demonstrated the inability to

Cl

BEHWVC

o-NPol3Pvc -c101-

0

-3

I...,-

,____.____.. _b

______.-___. -,

-I=

_/-=

0.05M Tris / Hzso4 pH7.4

Fig. 2. Ion selectivity coefficients of the electrodes based on hafnocene dichloride with different membrane solvents. All values were calculated by SSM based on the IUPAC recommendation [16] using EMF values of 0.1 M sodium salts except benzoate (PhCOO-) as an ammonium salt. Membrane composition: 1 wt% hafnocene dichloride, 66 wt% plasticizers (membrane solvent), and 33 wt% PVC. Plasticizers: BBPA, BEHA, BEHS, BEHP and NPOE.

H. Hisamoto et al. /Analytica

obtain high selectivities for benzoate and salicylate. This fact is caused by the high electron density of these membrane solvents, because both solvent molecules have a benzene unit. The other three plasticizers (BBPA, BEHA, and BEHS) have similar properties, however, the electrode using BBPA exhibited a higher selectivity for the salicylate ion than the other electrodes using the other two plasticizers. The ionophore concentration in the electrode membrane also affects the selectivity of the electrode. As shown in Fig. 3, increasing the ionophore concentration yielded higher favourable values for the selectivity coefficients for salicylate and benzoate ions. The most favourable EMF response to salicylate ion was observed when a l-2 wt% ionophore concentration (approximately 4 wt% ionophore is the saturation concentration in the membrane) was used to measure a 0.05 M MES-NaOH buffer (pH 5.5) in a sample solution. However, as shown in Fig. 4, the response curve for salicylate samples where they involve MES

contents of hafnocene dichlmide M%j

0

-1

3.25

, 0.5 , 1.0 , 2.0 , 2.0 Ph(OHW30

-\_,-_-\ a=_, -I

=++__ -\---

+

Cl04 \ SCN-

war

-\ \

PICOO-

INO3-

NO2.HFoOAc‘Brcl-

-4

SO+

Fig. 3. Response curves for salicylate obtained with an electrode based on hafnocene dichloride. Membrane composition: 4 wt% organometallic compound, 66 wt% BBPA and 30 wt% PVC.

Chimica Acta 304 (1995) 171-l 76

I

17.5

I

I

I

I

BBPAlPVC

A 59mVldecade

-5

-4

-3 log

-2

-1

a PhtOH)Coo-

Fig. 4. Ion selectivity coefficients of the electrodes hafnocene dichloride with different ionophore contents.

based

0"

buffer showed an over-Nernstian response. Similar super-Nernstian response behaviour types were reported using an electrode based on tin tetraphenylporphyrin as an anion ionophore in which the same buffer solution (0.05 M MES-NaOH (pH 5.5)) was also used [5]. The electrode based on an ion sensing component possessing ionic charge (a charged ionophore) often exhibits an non-ideal super-Nernstian response [14]. This non-ideal response may be due to the hafnocene cation but the exact mechanism is not known. On the other hand, in the case where 0.05 M Tris-HzSO, (pH 7.4) buffer solutions were used as the sample solution, a near Nernstian response (58 mV/activity decade of salicylate ion) was observed with the electrode based on hafnocene dichloride as shown in Fig. 4 in which the linear response range was observed in the concentration ranging from 1 X lo-” to 1 X 10-l M salicylate ion. The electrodes based on organometals often show a slow response to anions; however, the electrode using hafnocene dichloride responded to salicylate ion within 30 s (95% response time) in a concentration in the linear response range. The sensitivity and selectivity of this electrode were almost constant for at least 14 days,

176

H. Hisamoto et al. /Analytica

but lifetime studies of over two weeks have not been done. In conclusion, hafnocene dichloride is one of several unique anion sensitive ionophores which prefer anions with r-electrons such as salicylate and benzoate. Development of novel anion ionophores with unusual anion selective properties may be achieved by other unique lipophilic investigating many organometallic compounds.

Acknowledgements The authors thank the late Prof. W. Simon of ETH-Zurich for his helpful discussions on the present research. This work was partially supported by the Iketani Science and Technology Foundation and a Kurata Research Grant.

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Chimica Acta 304 (1995) 171-l 76 [3] D. Ammann, M. Huser, B. Krautler, B. Rusterholz, P. Schulthess, B. Lindemann, E. Halder and W. Simon, Helv. Chim. Acta, 69 (1986) 8494. [41 N.A. Chaniotakis, A.M. Chasser, M.E. Meyerhoff and J.T. Groves, Anal. Chem., 60 (1988) 185. [51 N.A. Chaniotakis, S.B. Park and M.E. Meyerhoff, Anal. Chem., 61 (1989) 566. 161R. Stepanek, B. Krautler, P. Schulthess, B. Lindemann, D. Ammann and W. Simon, Anal. Chim. Acta, 182 (1986) 83. 171 P. Schulthess, D. Ammann, W. Simon, C. Caderas, R. Stepanek and B. Krautler, Helv. Chim. Acta, 67 (1984) 1026. k31M. Rothmainer and W. Simon, Anal. Chim. Acta, 271 (1993) 135. [91 V.J. Wotring, D.M. Johnson and L.G. Bachas, Anal. Chem., 62 (1990) 1506. [lOI Y. Umezawa, M. Kataoka, W. Takami, E. Kimura, T. Koike and H. Nada, Anal. Chem., 60 (1988) 2392. illI S.S.S. Tan, P.C. Hauser, K. Wang, K. Flurr, K. Seiler, B. Rusterholz, G. Suter, M. Kruttli, U.E. Spichiger and W. Simon, Anal. Chim. Acta., 255 (1991) 35. WI H. Hisamoto, K. Watanabe, H. Oka, E. Nakagawa, U.E. Spichiger and K. Suzuki, Anal. Sci., 10 (1994) 615. [I31 F. Hofmeister, Arch. Exp. Pathol. Pharmakol., 14 (1888) 247. [141 K. Suzuki, K. Tohda, H. Aruga, M. Matsuzoe, H. Inoue and T. Shirai, Anal. Chem., 60 (1988) 1714. 1151J. Killand, J. Am. Chem. Sot., 59 (1937) 1675. [I61IUPAC Recommendation for Nomenclature of Ion-Selective Electrodes, Pure Appl. Chem., 48 (1976) 129. [171 K. Suzuki and W. Simon, The 63th Annu. Cong. of Chem. Sot. of Jpn., Abst. March (1992) 857.