Mendeleev Communications Electronic Version, Issue 5, 2001 1 Outer-sphere association of hexacyanoferrate and nitrogen betaine anions Vitalii Yu. Kotov,*a Yuliya G. Gorbunova,a Sof¡�ya A. Kostina,a Gul¡�nara K. Kadorkina,b Vasilii R. Kostyanovskyb and Remir G. Kostyanovskyb a N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.Fax: +7 095 954 1279; e-mail: tsir@elch.chem.msu.ru b N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russian Federation. Fax: +7 095 938 2156; e-mail: kost@center.chph.ras.ru 10.1070/MC2001v011n05ABEH001464 The electronic spectrum of an aqueous solution of an equimolar mixture of nitrogen betaine (pK1 = 1.72, pK2 = 3.62) and hexacyanoferrate anions exhibited a charge-transfer band at 26100 cm.1, which is indicative of the outer-sphere association of these ions.The cations of aromatic nitrogen-containing heterocycles [such as methyl viologen (MV2+) and pyridinium salts] are used as test materials in the studies of electron-transfer processes. Because the electron affinity of these cations is high, ion pairs with the participation of these cations exhibit absorption in the visible region of the electronic spectrum:1,2 It is well known that charge-transfer bands are characteristic of not only cation.anion associates but also anion.anion systems.3,4 In these latter, the complex ions [Fe(CN)6]3., [FeNO(CN)5]2.or [Co(edta)]. serve as electron acceptors. The contact between anions in these systems takes place by the cooperative interaction: We examined the association of hexacyanoferrate(II) ions and the anions of a nitrogen betaine, 2-N-pyridiniumhydrosuccinate- 1 1.Compound 1 was prepared according to the published procedure5 by the reaction of pyridine with maleic acid in an aqueous solution (10 days at 20 ¡ÆC).¢Ó The dissociation constants of protonated (1 + H)+ and neutral 1 forms of the nitrogen betaine were found by potentiometric titration¢Ô (pK1 = 1.72¡¾0.05 and pK2 = 3.62¡¾0.05).Betaine anion (1 . H). is resistant to an excess of an alkali (pH 11) at 20 ¡ÆC. This fact allowed us to examine its properties in aqueous solutions of compound 1 containing an excess of K2CO3. Note that the potassium salt is more readily soluble in water than compound 1, as evidenced by the 1H NMR spectrum (in D2O) of the residue after evaporation of a solution of the potassium salt (cf.ref. 5). The electronic absorption spectrum¡× of (1 . H). exhibited a long-wavelength absorption band at 38300 cm.1 (e = 3750 dm3 mol.1 cm.1) and no absorption in the visible and UV regions of the spectrum. Thus, the concentration can be varied over a wide range in the course of spectrophotometric measurements.The electronic absorption spectrum of an aqueous solution containing compound 1, K4[Fe(CN)6]¡�3H2O (0.06 mol dm.3 each) and an excess of K2CO3 (CK+ = 2 mol dm.3) exhibited a broad band at 26100¡¾100 cm.1 (n1/2 = 4700¡¾100 cm.1), which was absent from the spectra of the initial components of the mixture. The 1:1 composition of the resulting complex was determined by the isomolar series method (Figure 1).The stability constant (0.54¡¾0.06 mol dm.3 at CK+ = 2.34 mol dm.3) and the molar extinction coefficient at a band maximum (115¡¾15 dm3 mol.1 cm.1) were calculated by the Benesi.Hildebrand method6 (Figure 2). The molar extinction coefficient is close to the typical values of 150.200 dm3 mol.1 cm.1 found for ion pairs of the hexacyanoferrate ion with N-heterocyclic cations.1 The stability constant of the complex formed is close to 0.05.0.3 dm3 mol.1, which is characteristic of outer-sphere anion.anion associates.3,4 Thus, the ¢Ó 1: yield 85%, mp 214 ¡ÆC. 1HNMR (D2O) d: 3.43 (m, 2H, CH2, ABX3 spectrum, .n 68.0 Hz, 3JAX 9.9 Hz, 3JBX 4.4 Hz, 2JAB .18.0 Hz), 5.64 (dd, 1H, CH), 8.02 (dd, 2H, 2¥â-H, 3J 6.1 Hz, 3J 7.8 Hz), 8.52 (t, 1H, ¥ã-H, 3J 7.8 Hz) 8.88 (d, 2H, 2¥á-H, 3J 6.1 Hz).Found (%): N, 7.10, 7.20. Calc. for C9H9O4N (%): N, 7.18. The 1H NMR spectra were measured on a Bruker WM-400 spectrometer. K4[Fe(CN)6]¡�3H2O and other chemicals were of reagent grade. ¢Ô The potentiometric titration of a 0.1 M solution of compound 1 and its mixture with 0.1 M HCl was performed using a 0.1 M NaOH solution and a Mettler Delta 340 pH-meter with a combined pH electrode.¡× The electronic absorption spectra were measured on a Cary 100 spectrophotometer (Varian) in the frequency range 20000.50000 cm.1 at 25 ¡ÆC using quartz cuvettes with an optical path length of 1 cm. The absorption bands were approximated by Gaussian functions. MV2+ + [Fe(CN)6]4.= MV2+,[Fe(CN)6]4. MV2+,[Fe(CN)6]4. + hv ¢ç MV+,[Fe(CN)6]3. [Fe(CN)6]3. + nK+ + [Fe(CN)6]4. = [Fe(CN)6]3.,nK+,[Fe(CN)6]4. [Fe(CN)6]3.,nK+,[Fe(CN)6]4. + hv ¢ç [Fe(CN)6]4.,nK+,[Fe(CN)6]3. + 1 &22+ &+&22+ + + 1 &22 &+&22+ + 1 &22 &+&22 ¡¾ + ¡¾ + ¡¾ + 0.21 0.18 0.15 0.12 0.09 0.06 0.03 0.00 0.0 0.2 0.4 0.6 0.8 1.0 A Mole fraction Figure 1 Absorbance of the anion.anion complex (1 .H).,nK+,[Fe(CN)6]4. at n = 25000 cm.1 as a function of the mole fraction of (1 . H).. C(1-H) + + CFe(CN)6 = 0.12 mol dm.3, CK = 2 mol dm.3. 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 C0lAmax .1 /mol dm.3 cm C.1/dm3 mol.1 Figure 2 Absorbance of the anion.anion complex (1 .H).,nK+,[Fe(CN)6]4. as a function of the concentration of (1 . H). in the Benesi.Hildebrand equation coordinates. CFe(CN)6 = 0.12 mol dm.3, CK = 2.34 mol dm.3.Mendeleev Communications Electronic Version, Issue 5, 2001 2 observed absorption band can be reliably attributed to the outersphere charge transfer between the anions: The position of the charge-transfer band maximum in the test associate at the boundary between the visible and UV regions of the spectrum indicates that the electron affinity of nitrogen betaine anion (1 – H)– is lower than that of N-heterocyclic cations, which were studied previously.1 The absorption bands of the ion pairs of these N-heterocyclic cations with [Fe(CN)6]4– lie in the visible region of the spectrum, and the electron affinity is 2.8–3.5 eV, as estimated according to ref. 7. An analogous estimation gave a value of 2.5 eV for (1 – H)–. This study was supported by the Russian Foundation for Basic Research (grant nos. 00-03-40104 and 00-03-81187) and INTAS (grant no. 99-0157). References 1 H. E. Toma, Can. J. Chem., 1979, 57, 2079. 2 J. C. Curtis, B. P. Sullivan and T. J. Meyer, Inorg. Chem., 1980, 19, 3833. 3 R. Billing and D. E. Khoshtariya, Inorg. Chem., 1994, 33, 4038. 4 A. B. Nikol’skii and V. Yu. Kotov, Mendeleev Commun., 1995, 139. 5 R. G. Kostyanovsky, V. R. Kostyanovsky, G. K. Kadorkina and V. Yu. Torbeev, Mendeleev Commun., 2000, 83. 6 H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703. 7 S. I. Gorelsky, V. Yu. Kotov and A. B. P. Lever, Inorg. Chem., 1998, 37, 4584. (1 – H)– + nK+ + [Fe(CN)6]4– = (1 – H)–,nK+,[Fe(CN)6]4– (1 – H)–,nK+,[Fe(CN)6]4– + hv ® (1 – H)2–,nK+,[Fe(CN)6]3– Received: 20th April 2001; Com.