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Aquocyanocobalt(III)-hepta(2-phenylethyl)-cobyrinate as a cationic carrier for nitrite-selective liquid-membrane electrodes
Analytica Chimica Acta, 182 (1986) 83-90 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
AQUOCYANOCOBALT(III)-HEPTA(2-PHENYLETHYL)-COBYRINATE AS A CATIONIC CARRIER FOR NITRITE-SELECTIVE LIQUID-MEMBRANE ELECTRODES
RENB STEPANEK, BERNHARD KRAUTLER*, PETER SCHULTHESS, BERNHARD LINDEMANN, DANIEL AMMANN and WILHELM SIMON* Department of Organic Chemistry, Swiss Fedeml Institute of Technology CH-8092 Ziirich (Switzerland)
(ETH),
(Received 20th August 1985)
SUMMARY The synthesis of aquocyanocobalt(III)-hepta( 2-phenylethyl)cobyrinate is described. It is a more lipophilic substitute for a previously reported nitrite-carrier. Membranes based on this more readily accessible cobvrinate exhibit selectivitv coefficients for nitrite (Knot) over nitrate and chloride of 10-4*1 and 104v6, respectively. Near-theoretical slopes were found for 1044.5-10-1.0 M nitrite. After contact of the sensor with flowing tap water for 35 days, no deterioration of the electrode characteristics was observed.
It has been shown recently [l, 21 that certain lipophilic derivatives of vitamin Blz (1, Fig. 1) exhibit considerable selectivity for nitrite when incorporated into solvent polymeric membranes. Such compounds can be used in analytically relevant sensors for nitrite [2]. Unfortunately, the nitrite carrier 1 is not easily synthesized [ 11. In addition, it is desirable to have available an ion-selective component similar to 1 but with a higher lipophilicity. Because of the pronounced dependence of the electrode response on the carrier concentration in the membrane phase (see Fig. 6 [2] ), a high partition coefficient of the ion-selective component between the sample and the membrane phase is essential for the type of carrier discussed, to ensure increased sensor lifetime. The lifetime is expected to be limited by a reduction in the slope of the electrode response at high activities of the primary ion (see Fig. 6 [23 ). Here a convenient synthesis is reported for a considerably more lipophilic vitamin BE derivative than 1 and its use in solvent polymeric membrane electrodes is described. EXPERIMENTAL Membranes
and electrode
system
The solvent polymeric membranes were prepared as described earlier [3] from 1% (w/w) carrier, 66% (w/w) of either bis(l-butylpentyl)adipate (BBPA; purum, 02150, Fluka, 9470 Buchs, Switzerland) or decane-l,lO-diyl0003-2670/86/$03.50
o 1986 Elsevier Science Publishers B.V.
CO&H, HSOti;Co2CH3
-lb
-la
COZR , ,W
CO,R
Ro2jg’ CH, COZR
-2a
,C02R
Ro*~g;::
’
3 CO,R
R
-CH,-CH,-C&,
CO,R
C02R
-2b
Fig. 1. Structural formulae of the charged carriers used. The two coordination isomers (la/lb) of aquocyano-Co(III)-a,b,d,e,f,g-hexamethyl-c-octadecyl cobyrinate perchlorate are available only as the mixture 1. Similarly, the two isomeric forms (2a/2b) of aquocyano-Co(III)-hepta(2-phenylethyl)cobyrinate perchlorate constitute the mixture 2.
diglutarate-bis-( l-butylpentylester) (ETH 469; p.a., 30585, Fluka) and 33% (w/w) poly(viny1 chloride) (PVC; high molecular weight, Fluka). Cell assemblies of the following type were used: Hg; Hg&, KC1 (satd.)ll M LiOAc I sample solution 11membrane II0.01 M NaCl, 0.01 M NaN02, AgCl; Ag. The external reference electrode with a free-diffusion, free-flowing liquid junction which was used will be described elsewhere. The liquid membranes were mounted in Philips IS-561 electrode bodies (N.V. Philips’ Gloeilampenfabrieken, Eindhoven, The Netherlands). Procedures E.m.fi measurements. For all e.m.f. measurements, double-distilled water (quartz equipment) and chemicals of puriss. or p.a. grade were used. The measuring equipment has been described with some comments on the necessary computations [ 21.
85
Separate solution method (as. m, ) [4, 51. The selectivity coefficients, were obtained in 0.1 M solutions of the corresponding sodium log qx, salts. The solutions were buffered with 0.01 M Tris and adjusted to pH 7.35 + 0.05 with concentrated sulfuric acid. Fixed interference method (fi i.m.) [4, 51. The selectivity coefficients were graphically evaluated from the electrode functions of unbuffered sodium nitrite solutions containing 1 M concentrations of sodium chloride and sodium nitrate. Practical response time [4, 51. The electrode was conditioned in 40 ml of 3 X 10m3M sodium nitrite for 3-5 min. The magnetic stirrer was then turned on (for about 2 s) and 1 ml of 0.3 M sodium nitrite was injected immediately (resulting in 1.024 X lo-* M nitrite). E.m.f. values were measured at 160-ms intervals. Drift [4, 5]. E.m.f. drift was examined by using a closed glass vessel with 0.1 M sodium nitrite at 24.5 f 1°C. E.m.f. readings were taken at 15-min intervals. From a 70-h experiment, the second half yielded the results presented in this paper. Lifetime. After the recording of a nitrite-electrode function, membranes based on 1 or 2 mounted in Philips IS561 electrode bodies were subjected to a steady flow of regular Zurich tap water. After 35 days, the nitrite electrode function was re-examined. Syntheses Reagents. 2-Phenylethanol (Fluka, purum) was distilled. Potassium cyanide (Merck, p.a.) was used. p-Toluene sulfonic acid monohydrate (Fluka, puriss. p.a.) was dried by azeotropic removal of water from a benzene solution. Benzene, diethyl ether, hexane and chloroform were all from Fluka (puriss. p.a.). The chloroform was filtered through aluminum oxide (Woelm B, Akt. I). Thin-layer chromatography (t.1.c.) was done on plates coated with silica gel (Merck, Art. 5721). Trifluoroacetic acid (Fluka, purum) was redistilled. Methanol (Fluka, puriss.) was of u.v.-spectroscopic grade. Ethyl acetate, dichloromethane and acetone were of reagent grade. Instruments. Perkin-Elmer PE-555 or Uvikon 810 u.v.-visible spectrophotometers were used; the data given below are X,,, in nanometer (log e) and S indicates shoulder. For circular dichroism (CD) a Jobin-Yvon Mark III instrument was used; the wavelength of the extreme h, and of the zero passages ho are given in nm (molar decadic circular dichroism, Ae). A Perkin-Elmer PE-125 infrared spectrometer was used with chloroform; relative intensities s, m and w denote strong, medium, and weak, respectively. A ‘H-n.m.r. Bruker WM-300 spectrometer was used (CDC13, 300.14 MHz, TMS internal reference, chemical shifts in ppm with 6 (TMS) = 0, coupling constants J in Hz). The ‘3C-n.m.r. spectrometer was a Bruker WM-300 model (CDC13, 75.47 MHz, TMS internal reference, chemical shifts in ppm with S(TMS) = 0, multiplicities from the DEPT spectrum). For fast-atom-bombardment mass spectrometry (f.a.b.m.s.), a Kratos AEI MS-50 was fitted with the M-scan FAB-system (argon bombardment at S-10 kV).
66
Dicyanocobalt(III)-hepta(2-phenylethyl)cobyrinate
(3).
acetate/hexane/methanol + 1% HCN, 200:100:3; Rf = 0.43), was then dissolved in diethyl ether and precipitated by addition of hexane. The product 3 was obtained as 71 mg of a violet powder (ca. 41.2 E.tmol;95% yield) after drying (room temperature, ca. 10T3Torr, ca. 16 h). The following spectral data were obtained. Ultraviolet-visible (methanol + 0.05% HCN; c = 2.698 X 10M5mol 1-i): 251 S (3.91), 257 S (3.87), 265 S (3.89), 268 S (3.91), 277 (4.04), 310 (3.99), 353 S (4.18), 369 (4.50), 419 (3.40), 512 S (3.75), 544 (3.95), 583 (4.04). Circular dichroism (methanol + 0.05% HCN; c = 2.698 X lo-’ mol 1-l): 246 S (-12.8), 253 (-15.8), 266 S (-3.52), 277 (-3.15), 309 (-13.0), 325 (-7.97), 347 (-12.8), 367 (-8.90), 396 (22.1), 427 (13.2), 494 (-0.59), 536 (-0.39), 577 (0.46), 582 (-1.22), 605 (0.45), A0 at 375, 467, 546, 595 nm. Infrared (4%): 3080 w, 3060 w, 3010 m, 2980 m, 2120 w, 1727 s, 1602 w, 1579 s, 1498 s, 1468 m, 1454 m, 1400 m, 1391 m, 1365 m, 1351 m, 1104 m, 1087 m, 1048 m, 1030 m, 1010 m, 1000 m. ‘H-n.m.r.: 1.13, 1.17, 1.18, 1.32, 1.39, 1.44 (6s, 6 CHB), 1.53 (s, H,O), 2.08, 2.16, [2 s, 2 CHB, C(5)-CHJ, C(15)-CH,] superimposed by 1.0-2.8 (m, total ca. 51 H); 2.80-3.04 (m, ca. 16 H); 3.38 [m, 1 H, HC(8)]; 3.68-3.80 [m, 2 H, HC(19) and HC(3)]; 4.15-4.52 (m, 14 H); 5.52 [s, 1 H, HC(lO)] ; 7.05-7.36 (m, including CHCIJ ca. 50 H). Fast-atombombardment m.s.: 1721(32, M’ + l), 1720 (39, M+), 1719 (36), 1718 (14); 1696 (29), 1695 (57, M+ + l--CN), 1694 (100, M+-CN), 1693 (82), 1692 (29); 1669 (23, M’ + l-2CN), 1668 (21), 1667 (25); 1655 (17); 1641 (15); etc. 13C-n.m.r. (CDCIB): 15.1, 15.7, 16.6, 18.4, 19.1, 19.8, 21.9 (7 q, 7 CHB); 24.9, 25.8, 26.4, 30.0 (4 t, 4 C); 30.9 (q, CH,); 31.1, 31.4, 31.9, 32.7, 33.9, 34.8, 34.96 (triple intensity), 35.03 (triple intensity) (8 t, 12 C); 39.2
87
[d, C(18)]; 41.2, 42.3 [2 t, C(2’), C(7’)]; 45.6, 47.0, 48.6 [3 s, C(2), C(7), C(12)]; 53.6, 54.0, 56.6 [3 d, C(3), C(8), C(13)] ; 58.2 (s, C(17)] ; 64.8, 64.9, 65.0 (double intensity), 65.05, 65.11, 65.5 (6 t, 7 C); 74.7 [d, C(19)] ; 82.4 [s, C(l)]; 91.2 [d, C(lO)]; 102.1, 103.4 [2 s, C(5), C(15)] ; 126.4 (double intensity), 126.5 (double intensity), 126.6 (double intensity), 126.7 (4 d, 7 C); 128.37 (sixfold intensity), 128.45 (eightfold intensity), 128.5 (fourfold intensity), 128.8 (tenfold intensity) (4 d, 28 C); 130.1, 130.4 (2 s, 2 CN); 137.3 (double intensity), 137.5, 137.6 (double intensity), 137.7 (double intensity) (4 s, 7 C); 163.37, 163.42 [2 s, C(6), C(14)]; 170.3, 171.0, 171.2, 171.3, 172.0, 172.2, 172.7, 173.0, 175.0, 175.5, 176.0 (11 s, 11 C). Aquocyanocobalt(III~hepta(2-phenylethyl)cobyrinate perchlorate (2). A solution of 3 (36.6 mg, 21.3 E.tmol)in 3 ml of chloroform under nitrogen was treated with 5 ~1 of trifluoroacetic acid and the reaction mixture was stirred at room temperature for 10 min. The volume of the red solution of the product was reduced by half by evaporation (room temperature, 10e3 Torr) and then diluted to its original volume with fresh chloroform. The volume of the solution was again reduced by evaporation of solvent as before, then diluted with 50 ml of dichloromethane and shaken vigorously with ca. 50 ml of aqueous 1 M sodium phosphate buffer (pH 3.1), containing ca. 800 mg of sodium perchlorate. A colour change from red to orange was observed. The organic phase was filtered through a plug of cotton wool and the solvent was stripped off (rotary evaporator at room temperature). The red residue was dissolved in dichloromethane and was precipitated by addition of hexane. After drying the precipitate (10T3 Torr, room temperature, ca. 16 h), compound 2 was obtained as a brick-red powder (37.6 mg, 98%). In t.1.c. (2:l ethyl acetate/hexane) Rf was 0.59 (on NaCIOd-impregnated plates). The following spectral data were obtained. Ultraviolet-visible (methanol, c = 3.004 X lo-’ mol 1-l): 248 S (4.24), 252 S (4.21), 254 S (4.18), 258 S (4.15), 264 S (4.10), 268 S (4.08), 274 (4.08), 321 S (4.10), 353 (4.40), 385 S (3.75), 402 (3.73), 459 S (3.82), 490 (3.93), 522 (3.89), 586 S (2.78). Infrared (4%): 3080 w, 3060 w, 3020 m, 2955 m, 2133 w, 1727 s, 1601 w, 1578 s, 1495 s, 1469 m, 1452 m, 1415 w, 1390 m, 1348 m, 1302 m, 1284 m, 1144 s, 1085 m, 1048 m, 1017 m, 997 m, 901 w. ‘H-n.m.r.: 1.13, 1.23, 1.25, 1.27, 1.31, 1.41, 1.47, 1.54, 1.64, 1.65 (b, superimposed by signal from H,O), 2.197, 2.20, 2.27, 2.30 (14 s) superimposed by 1.1-2.8 m, in total ca. 52 H; 2.803.06 (m, 15 H, 7 CH,-phenyl and 1 H?); 3.22-3.38 (m, 1 H); 3.65-3.80 (m, 1 H); 3.99 (d, J = 10, ca. 1 H); 4.07 (d, J= 11, ca. 1 H): 4.14-4.56 (m, 14 H, 7 CH,-OCO and 2 H) 6.31, 6.38 (2 s, 1 H); 7.08-7.38 (m,including CHCIB, 37 H) (the spectrum indicates a ca. 2:l mixture of the stereoisomeric forms 2a/2b).
RESULTS
AND DISCUSSION
Synthesis of compound 2 The lipophilic cationic carrier aquocyanocobalt(III)-hepta(2-phenylethyl) cobyrinate perchlorate, 2, is available as a (ca. 2:l) mixture of the stereoisomers 2a/2b. It was synthesized via the dicyariocobalt( III)-hepta(2-phenylethyl)cobyrinate, 3: p-toluene sulfonic acid catalyzed esterification of the dicyanocobalt( III)-cobyrinate, 4 [ 6-81 (used as its heptapotassium salt) with 2-phenylethanol and work up with addition of potassium cyanide gave 3 in ca. 95% yield. Treatment of a solution of 3 in chloroform with trifluoroacetic acid converted 3 to a mixture of the stereoisomeric aquocyanocobalt(III)-cobyrinates 2a/2b in practically quantitative yield. Compound 4 is readily produced by acid- [6, 71 or base-catalyzed [8] hydrolysis of the ester functions of dicyanocobalt( III)-heptamethylcobyrinate 5 (“cobester” [ 9, lo] ), which in turn is available in crystalline form by methanolysis of vitamin Bn [9, lo]. The seemingly simpler synthetic approaches to 2 by direct acid-catalyzed esterification of vitamin B 12 [9, lo] or by acid-catalyzed transesterification of 5 with 2-phenylethanol proved unsuitable, because there the formation of 3 was accompanied by sizable fractions of epimers [ 71. Characteristics of the membranes A comparison of the selectivities of membranes based on the new lipophilic carrier 2 with the previously described ionophore 1 is shown in Fig. 2. As expected, the selectivities are almost identical The very high selectivities for nitrite over nitrate and chloride are further illustrated in Fig. 3. As a consequence of these high selectivities, the detection limit for nitrite is at about 10d.‘j M even in the presence of a background of 1 M sodium chloride (Fig. 3). The observed slopes of the electrode response were -56.9 f 1.3 mV (10-4*s-10-1*o M, n = 7) and -57.3 + 1.7 mV (lO~*s-lO-l~o M, n = 7) (theoretical: -58.2 mV) for pure sodium nitrite solutions (Fig. 4, right-hand side). With a background of 1 M sodium chloride, the slope was reduced to -53.0 f 1.8 mV (10-45-10 -l*’ M, n = 6) and for 1 M sodium nitrate to -54.2 f 2.0 mV (1044”-10-’ M, n = 5) (see Fig. 3). The response times observed for a stepwise change in the nitrite concentration from 3 X 10” M to 10” M were 0.9 f 0.3 s (n = 5) and 3.9 f 1.3 s (n = 5) for t90Q and tlmV, respectively. Membrane resistances obtained were in the order of 50 MS2, which corresponds to specific resistances of about 1.2 Gfi cm (20°C). In 10e2 M sodium nitrite, an e.m.f. drift of the cell assembly of <5 PV h-l was observed over a period of 35 h at 24.5 + 1°C. The standard deviation of a single determination was +40 PV (n = 140). Figure 4 corroborates the increased lifetime of sensors based on compound 2 compared to those containing compound 1. For a further improvement of the sensor lifetime, the more lipophilic plasticizer ETH 469 was
89
log Kpo’
--NO; (flml
N; i-
,-SCN-
--
--NOT (flln
_ SCN ---N;
--NO;
EMF
[mvl
NO; J1 M NaCl
i -
Cl 0
,-
I-
F
LIGAND 1 BBPA
LIGAND 2
PVC
PVC
0
ETH 469
I
-6
1
-7
I
I
I
I
-6
-5
-4
-3
q
-2
I
-1 lo9 aNO;
Fig. 2. Selectivity coefficients, log KgtX, for a solvent polymeric membrane based on ligand 1 with BBPA are compared to those of a membrane based on 2 with ETH 469 as plasticizer (both in PVC). Except where specified, the separate solutions method was used [4,5]. Fig. 3. E.m.f. response of a liquid-membrane electrode cell based on carrier 2 (ETH 469, PVC) to varying activities of nitrite in the presence of 1 M NaNO, or 1 M NaCl (unbuffered solutions). The arrows indicate the detection limit for the primary ion in the presence of the interfering ion. The selectivity factors obtained are log K~~2,,, = -4.2 and D log Kr& = -4.6.
used. After 35 days of continuous contact of the membranes with flowing Zurich tap water, the slope of the electrode response of sensors with 2 remained unchanged at high sample activities. Membranes with 1,however, exhibited a significant reduction in slope.
90 EMF
bvl
I
160 mV
-5
I
B
60 mV
-4
I
-3
I
-2
I
-1
I
log =f.q
-5
I
-4
I
-3
I
-2
I
-1
I
log at+,-
Fig. 4. E.m.f. response of liquid membrane electrodes: (A) ligand 1, BBPA, PVC; (B) ligand 2, ETH 469, PVC. (0) Shortly after preparation; (0) after a 35-day immersion in a steady flow of tap water. At high nitrite activities, the electrode based on 1 shows a significant decrease in the slope of the electrode response whereas the slope obtained by the electrode based on 2 remains virtually unchanged.
We thank Prof. Dr. A. Eschenmoser for support of this work, S. Blume for help with the preparative work, and Dr. J. Meili for measuring the f.a.b. mass spectrum. We acknowledge financial support from the Swiss National Science Foundation, from Coming Ltd., Sudbury (to P. S.), and from ETH-Z (to B. K. and D. A.). REFERENCES 1 P. Schulthess, D. Ammann, W. Simon, C. Caderas, R. Stepanek and B. Kriiutler, Helv. Chim. Acta, 67 (1984) 1026. 2 P. Schulthess, D. Ammann, B. Kriiutler, C. Caderas, R. Stepanek and W. Simon, Anal. Chem., 57 (1985) 1397. 3 P. Anker, E. Wieland, D. Amman, R. E. Dohner, R. Asper and W. Simon, Anal. Chem., 53 (1981) 1970. 4 G. G. Guilbault, R. A. Durst, M. S. Frant, H. Freiser, E. H. Hansen, T. S. Light, E. Pungor, G. Rechnitz, N. M. Rice, T. J. Rohm, W. Simon and J. D. R. Thomas, Pure Appl. Chem., 48 (1976) 127. 5 G. G. Guilbault, R. A. Durst, M. S. Frant, H. Freiser, E. H. Hansen, T. S. Light, G. J. Moody, E. Pungor, G. Rechnitz, N. M. Rice, T. J. Rohm, J. RBZiEka, W. Simon and J. D. R. Thomas, IUPAC Inf. Bull., (1978) 70. 6 G. Bartels and A. Eschenmoser, unpublished work. 7 R. Stepanek, dissertation, ETH Zurich, 1986. 8 C. Nussbaumer, Dissertation No. 7623, ETH Ziirich, 1984. 9 R. Keese, L. Werthemann and A. Eschenmoser, unpublished work. 10 L. Werthemann, Dissertation No. 4097, ETH Ziirich, 1968.
AQUOCYANOCOBALT(III)-HEPTA(2-PHENYLETHYL)-COBYRINATE AS A CATIONIC CARRIER FOR NITRITE-SELECTIVE LIQUID-MEMBRANE ELECTRODES
RENB STEPANEK, BERNHARD KRAUTLER*, PETER SCHULTHESS, BERNHARD LINDEMANN, DANIEL AMMANN and WILHELM SIMON* Department of Organic Chemistry, Swiss Fedeml Institute of Technology CH-8092 Ziirich (Switzerland)
(ETH),
(Received 20th August 1985)
SUMMARY The synthesis of aquocyanocobalt(III)-hepta( 2-phenylethyl)cobyrinate is described. It is a more lipophilic substitute for a previously reported nitrite-carrier. Membranes based on this more readily accessible cobvrinate exhibit selectivitv coefficients for nitrite (Knot) over nitrate and chloride of 10-4*1 and 104v6, respectively. Near-theoretical slopes were found for 1044.5-10-1.0 M nitrite. After contact of the sensor with flowing tap water for 35 days, no deterioration of the electrode characteristics was observed.
It has been shown recently [l, 21 that certain lipophilic derivatives of vitamin Blz (1, Fig. 1) exhibit considerable selectivity for nitrite when incorporated into solvent polymeric membranes. Such compounds can be used in analytically relevant sensors for nitrite [2]. Unfortunately, the nitrite carrier 1 is not easily synthesized [ 11. In addition, it is desirable to have available an ion-selective component similar to 1 but with a higher lipophilicity. Because of the pronounced dependence of the electrode response on the carrier concentration in the membrane phase (see Fig. 6 [2] ), a high partition coefficient of the ion-selective component between the sample and the membrane phase is essential for the type of carrier discussed, to ensure increased sensor lifetime. The lifetime is expected to be limited by a reduction in the slope of the electrode response at high activities of the primary ion (see Fig. 6 [23 ). Here a convenient synthesis is reported for a considerably more lipophilic vitamin BE derivative than 1 and its use in solvent polymeric membrane electrodes is described. EXPERIMENTAL Membranes
and electrode
system
The solvent polymeric membranes were prepared as described earlier [3] from 1% (w/w) carrier, 66% (w/w) of either bis(l-butylpentyl)adipate (BBPA; purum, 02150, Fluka, 9470 Buchs, Switzerland) or decane-l,lO-diyl0003-2670/86/$03.50
o 1986 Elsevier Science Publishers B.V.
CO&H, HSOti;Co2CH3
-lb
-la
COZR , ,W
CO,R
Ro2jg’ CH, COZR
-2a
,C02R
Ro*~g;::
’
3 CO,R
R
-CH,-CH,-C&,
CO,R
C02R
-2b
Fig. 1. Structural formulae of the charged carriers used. The two coordination isomers (la/lb) of aquocyano-Co(III)-a,b,d,e,f,g-hexamethyl-c-octadecyl cobyrinate perchlorate are available only as the mixture 1. Similarly, the two isomeric forms (2a/2b) of aquocyano-Co(III)-hepta(2-phenylethyl)cobyrinate perchlorate constitute the mixture 2.
diglutarate-bis-( l-butylpentylester) (ETH 469; p.a., 30585, Fluka) and 33% (w/w) poly(viny1 chloride) (PVC; high molecular weight, Fluka). Cell assemblies of the following type were used: Hg; Hg&, KC1 (satd.)ll M LiOAc I sample solution 11membrane II0.01 M NaCl, 0.01 M NaN02, AgCl; Ag. The external reference electrode with a free-diffusion, free-flowing liquid junction which was used will be described elsewhere. The liquid membranes were mounted in Philips IS-561 electrode bodies (N.V. Philips’ Gloeilampenfabrieken, Eindhoven, The Netherlands). Procedures E.m.fi measurements. For all e.m.f. measurements, double-distilled water (quartz equipment) and chemicals of puriss. or p.a. grade were used. The measuring equipment has been described with some comments on the necessary computations [ 21.
85
Separate solution method (as. m, ) [4, 51. The selectivity coefficients, were obtained in 0.1 M solutions of the corresponding sodium log qx, salts. The solutions were buffered with 0.01 M Tris and adjusted to pH 7.35 + 0.05 with concentrated sulfuric acid. Fixed interference method (fi i.m.) [4, 51. The selectivity coefficients were graphically evaluated from the electrode functions of unbuffered sodium nitrite solutions containing 1 M concentrations of sodium chloride and sodium nitrate. Practical response time [4, 51. The electrode was conditioned in 40 ml of 3 X 10m3M sodium nitrite for 3-5 min. The magnetic stirrer was then turned on (for about 2 s) and 1 ml of 0.3 M sodium nitrite was injected immediately (resulting in 1.024 X lo-* M nitrite). E.m.f. values were measured at 160-ms intervals. Drift [4, 5]. E.m.f. drift was examined by using a closed glass vessel with 0.1 M sodium nitrite at 24.5 f 1°C. E.m.f. readings were taken at 15-min intervals. From a 70-h experiment, the second half yielded the results presented in this paper. Lifetime. After the recording of a nitrite-electrode function, membranes based on 1 or 2 mounted in Philips IS561 electrode bodies were subjected to a steady flow of regular Zurich tap water. After 35 days, the nitrite electrode function was re-examined. Syntheses Reagents. 2-Phenylethanol (Fluka, purum) was distilled. Potassium cyanide (Merck, p.a.) was used. p-Toluene sulfonic acid monohydrate (Fluka, puriss. p.a.) was dried by azeotropic removal of water from a benzene solution. Benzene, diethyl ether, hexane and chloroform were all from Fluka (puriss. p.a.). The chloroform was filtered through aluminum oxide (Woelm B, Akt. I). Thin-layer chromatography (t.1.c.) was done on plates coated with silica gel (Merck, Art. 5721). Trifluoroacetic acid (Fluka, purum) was redistilled. Methanol (Fluka, puriss.) was of u.v.-spectroscopic grade. Ethyl acetate, dichloromethane and acetone were of reagent grade. Instruments. Perkin-Elmer PE-555 or Uvikon 810 u.v.-visible spectrophotometers were used; the data given below are X,,, in nanometer (log e) and S indicates shoulder. For circular dichroism (CD) a Jobin-Yvon Mark III instrument was used; the wavelength of the extreme h, and of the zero passages ho are given in nm (molar decadic circular dichroism, Ae). A Perkin-Elmer PE-125 infrared spectrometer was used with chloroform; relative intensities s, m and w denote strong, medium, and weak, respectively. A ‘H-n.m.r. Bruker WM-300 spectrometer was used (CDC13, 300.14 MHz, TMS internal reference, chemical shifts in ppm with 6 (TMS) = 0, coupling constants J in Hz). The ‘3C-n.m.r. spectrometer was a Bruker WM-300 model (CDC13, 75.47 MHz, TMS internal reference, chemical shifts in ppm with S(TMS) = 0, multiplicities from the DEPT spectrum). For fast-atom-bombardment mass spectrometry (f.a.b.m.s.), a Kratos AEI MS-50 was fitted with the M-scan FAB-system (argon bombardment at S-10 kV).
66
Dicyanocobalt(III)-hepta(2-phenylethyl)cobyrinate
(3).
acetate/hexane/methanol + 1% HCN, 200:100:3; Rf = 0.43), was then dissolved in diethyl ether and precipitated by addition of hexane. The product 3 was obtained as 71 mg of a violet powder (ca. 41.2 E.tmol;95% yield) after drying (room temperature, ca. 10T3Torr, ca. 16 h). The following spectral data were obtained. Ultraviolet-visible (methanol + 0.05% HCN; c = 2.698 X 10M5mol 1-i): 251 S (3.91), 257 S (3.87), 265 S (3.89), 268 S (3.91), 277 (4.04), 310 (3.99), 353 S (4.18), 369 (4.50), 419 (3.40), 512 S (3.75), 544 (3.95), 583 (4.04). Circular dichroism (methanol + 0.05% HCN; c = 2.698 X lo-’ mol 1-l): 246 S (-12.8), 253 (-15.8), 266 S (-3.52), 277 (-3.15), 309 (-13.0), 325 (-7.97), 347 (-12.8), 367 (-8.90), 396 (22.1), 427 (13.2), 494 (-0.59), 536 (-0.39), 577 (0.46), 582 (-1.22), 605 (0.45), A0 at 375, 467, 546, 595 nm. Infrared (4%): 3080 w, 3060 w, 3010 m, 2980 m, 2120 w, 1727 s, 1602 w, 1579 s, 1498 s, 1468 m, 1454 m, 1400 m, 1391 m, 1365 m, 1351 m, 1104 m, 1087 m, 1048 m, 1030 m, 1010 m, 1000 m. ‘H-n.m.r.: 1.13, 1.17, 1.18, 1.32, 1.39, 1.44 (6s, 6 CHB), 1.53 (s, H,O), 2.08, 2.16, [2 s, 2 CHB, C(5)-CHJ, C(15)-CH,] superimposed by 1.0-2.8 (m, total ca. 51 H); 2.80-3.04 (m, ca. 16 H); 3.38 [m, 1 H, HC(8)]; 3.68-3.80 [m, 2 H, HC(19) and HC(3)]; 4.15-4.52 (m, 14 H); 5.52 [s, 1 H, HC(lO)] ; 7.05-7.36 (m, including CHCIJ ca. 50 H). Fast-atombombardment m.s.: 1721(32, M’ + l), 1720 (39, M+), 1719 (36), 1718 (14); 1696 (29), 1695 (57, M+ + l--CN), 1694 (100, M+-CN), 1693 (82), 1692 (29); 1669 (23, M’ + l-2CN), 1668 (21), 1667 (25); 1655 (17); 1641 (15); etc. 13C-n.m.r. (CDCIB): 15.1, 15.7, 16.6, 18.4, 19.1, 19.8, 21.9 (7 q, 7 CHB); 24.9, 25.8, 26.4, 30.0 (4 t, 4 C); 30.9 (q, CH,); 31.1, 31.4, 31.9, 32.7, 33.9, 34.8, 34.96 (triple intensity), 35.03 (triple intensity) (8 t, 12 C); 39.2
87
[d, C(18)]; 41.2, 42.3 [2 t, C(2’), C(7’)]; 45.6, 47.0, 48.6 [3 s, C(2), C(7), C(12)]; 53.6, 54.0, 56.6 [3 d, C(3), C(8), C(13)] ; 58.2 (s, C(17)] ; 64.8, 64.9, 65.0 (double intensity), 65.05, 65.11, 65.5 (6 t, 7 C); 74.7 [d, C(19)] ; 82.4 [s, C(l)]; 91.2 [d, C(lO)]; 102.1, 103.4 [2 s, C(5), C(15)] ; 126.4 (double intensity), 126.5 (double intensity), 126.6 (double intensity), 126.7 (4 d, 7 C); 128.37 (sixfold intensity), 128.45 (eightfold intensity), 128.5 (fourfold intensity), 128.8 (tenfold intensity) (4 d, 28 C); 130.1, 130.4 (2 s, 2 CN); 137.3 (double intensity), 137.5, 137.6 (double intensity), 137.7 (double intensity) (4 s, 7 C); 163.37, 163.42 [2 s, C(6), C(14)]; 170.3, 171.0, 171.2, 171.3, 172.0, 172.2, 172.7, 173.0, 175.0, 175.5, 176.0 (11 s, 11 C). Aquocyanocobalt(III~hepta(2-phenylethyl)cobyrinate perchlorate (2). A solution of 3 (36.6 mg, 21.3 E.tmol)in 3 ml of chloroform under nitrogen was treated with 5 ~1 of trifluoroacetic acid and the reaction mixture was stirred at room temperature for 10 min. The volume of the red solution of the product was reduced by half by evaporation (room temperature, 10e3 Torr) and then diluted to its original volume with fresh chloroform. The volume of the solution was again reduced by evaporation of solvent as before, then diluted with 50 ml of dichloromethane and shaken vigorously with ca. 50 ml of aqueous 1 M sodium phosphate buffer (pH 3.1), containing ca. 800 mg of sodium perchlorate. A colour change from red to orange was observed. The organic phase was filtered through a plug of cotton wool and the solvent was stripped off (rotary evaporator at room temperature). The red residue was dissolved in dichloromethane and was precipitated by addition of hexane. After drying the precipitate (10T3 Torr, room temperature, ca. 16 h), compound 2 was obtained as a brick-red powder (37.6 mg, 98%). In t.1.c. (2:l ethyl acetate/hexane) Rf was 0.59 (on NaCIOd-impregnated plates). The following spectral data were obtained. Ultraviolet-visible (methanol, c = 3.004 X lo-’ mol 1-l): 248 S (4.24), 252 S (4.21), 254 S (4.18), 258 S (4.15), 264 S (4.10), 268 S (4.08), 274 (4.08), 321 S (4.10), 353 (4.40), 385 S (3.75), 402 (3.73), 459 S (3.82), 490 (3.93), 522 (3.89), 586 S (2.78). Infrared (4%): 3080 w, 3060 w, 3020 m, 2955 m, 2133 w, 1727 s, 1601 w, 1578 s, 1495 s, 1469 m, 1452 m, 1415 w, 1390 m, 1348 m, 1302 m, 1284 m, 1144 s, 1085 m, 1048 m, 1017 m, 997 m, 901 w. ‘H-n.m.r.: 1.13, 1.23, 1.25, 1.27, 1.31, 1.41, 1.47, 1.54, 1.64, 1.65 (b, superimposed by signal from H,O), 2.197, 2.20, 2.27, 2.30 (14 s) superimposed by 1.1-2.8 m, in total ca. 52 H; 2.803.06 (m, 15 H, 7 CH,-phenyl and 1 H?); 3.22-3.38 (m, 1 H); 3.65-3.80 (m, 1 H); 3.99 (d, J = 10, ca. 1 H); 4.07 (d, J= 11, ca. 1 H): 4.14-4.56 (m, 14 H, 7 CH,-OCO and 2 H) 6.31, 6.38 (2 s, 1 H); 7.08-7.38 (m,including CHCIB, 37 H) (the spectrum indicates a ca. 2:l mixture of the stereoisomeric forms 2a/2b).
RESULTS
AND DISCUSSION
Synthesis of compound 2 The lipophilic cationic carrier aquocyanocobalt(III)-hepta(2-phenylethyl) cobyrinate perchlorate, 2, is available as a (ca. 2:l) mixture of the stereoisomers 2a/2b. It was synthesized via the dicyariocobalt( III)-hepta(2-phenylethyl)cobyrinate, 3: p-toluene sulfonic acid catalyzed esterification of the dicyanocobalt( III)-cobyrinate, 4 [ 6-81 (used as its heptapotassium salt) with 2-phenylethanol and work up with addition of potassium cyanide gave 3 in ca. 95% yield. Treatment of a solution of 3 in chloroform with trifluoroacetic acid converted 3 to a mixture of the stereoisomeric aquocyanocobalt(III)-cobyrinates 2a/2b in practically quantitative yield. Compound 4 is readily produced by acid- [6, 71 or base-catalyzed [8] hydrolysis of the ester functions of dicyanocobalt( III)-heptamethylcobyrinate 5 (“cobester” [ 9, lo] ), which in turn is available in crystalline form by methanolysis of vitamin Bn [9, lo]. The seemingly simpler synthetic approaches to 2 by direct acid-catalyzed esterification of vitamin B 12 [9, lo] or by acid-catalyzed transesterification of 5 with 2-phenylethanol proved unsuitable, because there the formation of 3 was accompanied by sizable fractions of epimers [ 71. Characteristics of the membranes A comparison of the selectivities of membranes based on the new lipophilic carrier 2 with the previously described ionophore 1 is shown in Fig. 2. As expected, the selectivities are almost identical The very high selectivities for nitrite over nitrate and chloride are further illustrated in Fig. 3. As a consequence of these high selectivities, the detection limit for nitrite is at about 10d.‘j M even in the presence of a background of 1 M sodium chloride (Fig. 3). The observed slopes of the electrode response were -56.9 f 1.3 mV (10-4*s-10-1*o M, n = 7) and -57.3 + 1.7 mV (lO~*s-lO-l~o M, n = 7) (theoretical: -58.2 mV) for pure sodium nitrite solutions (Fig. 4, right-hand side). With a background of 1 M sodium chloride, the slope was reduced to -53.0 f 1.8 mV (10-45-10 -l*’ M, n = 6) and for 1 M sodium nitrate to -54.2 f 2.0 mV (1044”-10-’ M, n = 5) (see Fig. 3). The response times observed for a stepwise change in the nitrite concentration from 3 X 10” M to 10” M were 0.9 f 0.3 s (n = 5) and 3.9 f 1.3 s (n = 5) for t90Q and tlmV, respectively. Membrane resistances obtained were in the order of 50 MS2, which corresponds to specific resistances of about 1.2 Gfi cm (20°C). In 10e2 M sodium nitrite, an e.m.f. drift of the cell assembly of <5 PV h-l was observed over a period of 35 h at 24.5 + 1°C. The standard deviation of a single determination was +40 PV (n = 140). Figure 4 corroborates the increased lifetime of sensors based on compound 2 compared to those containing compound 1. For a further improvement of the sensor lifetime, the more lipophilic plasticizer ETH 469 was
89
log Kpo’
--NO; (flml
N; i-
,-SCN-
--
--NOT (flln
_ SCN ---N;
--NO;
EMF
[mvl
NO; J1 M NaCl
i -
Cl 0
,-
I-
F
LIGAND 1 BBPA
LIGAND 2
PVC
PVC
0
ETH 469
I
-6
1
-7
I
I
I
I
-6
-5
-4
-3
q
-2
I
-1 lo9 aNO;
Fig. 2. Selectivity coefficients, log KgtX, for a solvent polymeric membrane based on ligand 1 with BBPA are compared to those of a membrane based on 2 with ETH 469 as plasticizer (both in PVC). Except where specified, the separate solutions method was used [4,5]. Fig. 3. E.m.f. response of a liquid-membrane electrode cell based on carrier 2 (ETH 469, PVC) to varying activities of nitrite in the presence of 1 M NaNO, or 1 M NaCl (unbuffered solutions). The arrows indicate the detection limit for the primary ion in the presence of the interfering ion. The selectivity factors obtained are log K~~2,,, = -4.2 and D log Kr& = -4.6.
used. After 35 days of continuous contact of the membranes with flowing Zurich tap water, the slope of the electrode response of sensors with 2 remained unchanged at high sample activities. Membranes with 1,however, exhibited a significant reduction in slope.
90 EMF
bvl
I
160 mV
-5
I
B
60 mV
-4
I
-3
I
-2
I
-1
I
log =f.q
-5
I
-4
I
-3
I
-2
I
-1
I
log at+,-
Fig. 4. E.m.f. response of liquid membrane electrodes: (A) ligand 1, BBPA, PVC; (B) ligand 2, ETH 469, PVC. (0) Shortly after preparation; (0) after a 35-day immersion in a steady flow of tap water. At high nitrite activities, the electrode based on 1 shows a significant decrease in the slope of the electrode response whereas the slope obtained by the electrode based on 2 remains virtually unchanged.
We thank Prof. Dr. A. Eschenmoser for support of this work, S. Blume for help with the preparative work, and Dr. J. Meili for measuring the f.a.b. mass spectrum. We acknowledge financial support from the Swiss National Science Foundation, from Coming Ltd., Sudbury (to P. S.), and from ETH-Z (to B. K. and D. A.). REFERENCES 1 P. Schulthess, D. Ammann, W. Simon, C. Caderas, R. Stepanek and B. Kriiutler, Helv. Chim. Acta, 67 (1984) 1026. 2 P. Schulthess, D. Ammann, B. Kriiutler, C. Caderas, R. Stepanek and W. Simon, Anal. Chem., 57 (1985) 1397. 3 P. Anker, E. Wieland, D. Amman, R. E. Dohner, R. Asper and W. Simon, Anal. Chem., 53 (1981) 1970. 4 G. G. Guilbault, R. A. Durst, M. S. Frant, H. Freiser, E. H. Hansen, T. S. Light, E. Pungor, G. Rechnitz, N. M. Rice, T. J. Rohm, W. Simon and J. D. R. Thomas, Pure Appl. Chem., 48 (1976) 127. 5 G. G. Guilbault, R. A. Durst, M. S. Frant, H. Freiser, E. H. Hansen, T. S. Light, G. J. Moody, E. Pungor, G. Rechnitz, N. M. Rice, T. J. Rohm, J. RBZiEka, W. Simon and J. D. R. Thomas, IUPAC Inf. Bull., (1978) 70. 6 G. Bartels and A. Eschenmoser, unpublished work. 7 R. Stepanek, dissertation, ETH Zurich, 1986. 8 C. Nussbaumer, Dissertation No. 7623, ETH Ziirich, 1984. 9 R. Keese, L. Werthemann and A. Eschenmoser, unpublished work. 10 L. Werthemann, Dissertation No. 4097, ETH Ziirich, 1968.