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Inorg. Chem. 1999, 38, 5184-5186
Aminoferrocene Derivatives in Chloride Recognition and Electrochemical Sensing Konstantinos Kavallieratos, Sharon Hwang, and Robert H. Crabtree* Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107 ReceiVed July 8, 1999 Introduction Redox-active anion receptors1-9 giving potential shifts on binding are useful in chemical sensor applications:10-15 cobaltocenium,1-5,16,17 ferrocene,1-7,18-20 and transition-metalbipyridyl8,9 cases are known. Our readily available isophthalamide (1)21 and disulfonamide (2)22 receptors bind Cl- and other anions very strongly in CH2Cl2,21,22 suggesting the neutral diferrocenyl analogues 3 and 4 might act as Cl- sensors because the anion is closely coupled to the ferrocenyl (Fc) group.23 Related aminomethylferrocene diamide analogues have recently been reported by Beer et al.24 Amide receptors with two FcNH groups have not yet been reported since FcNH2 is not commercially available and is difficult to prepare.25-29 (1) Beer, P. D. Chem. Soc. ReV. 1989, 18, 409. (2) Beer, P. D. AdV. Inorg. Chem. 1992, 39, 79. (3) Beer, P. D.; Smith, J. K. Prog. Inorg. Chem. 1997, 46, 1 and references therein. (4) Beer, P. D.; Schnitt, P. Curr. Opin. Chem. Biol. 1997, 1, 475. (5) Beer, P. D. Acc. Chem. Res. 1998, 31, 71. (6) Jagessar, R. C.; Burns, D. H. Chem. Commun. 1997, 1685. (7) Scherer, M.; Sessler, J. L.; Gebauer, A.; Lynch, V. Chem. Commun. 1998, 85. (8) Beer, P. D.; Szemes, F.; Balzani, V.; Sala, C. M.; Drew, M. G. B.; Bent, S. W.; Maestri, M. J. Am. Chem. Soc. 1997, 119, 11864. (9) Szemes, F.; Hesek, D.; Chen, Z.; Dent, S. W.; Drew, M. G. B.; Goulden, A. J.; Graydon, A. R.; Grieve, A.; Mortimer, R. J.; Wear, T.; Weightman, J. S.; Beer, P. D. Inorg. Chem. 1996, 35, 5868. (10) Antonisse, M. M. G.; Reinhoudt, D. N. Chem. Commun. 1998, 443. (11) Antonisse, M. M. G.; Snellink-Ruel, B. H. M.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Chem. Soc., Perkin Trans. 2 1998, 773. (12) Ohyama, T.; Wang, D. Q.; Cowan, J. A. Chem. Commun. 1998, 467. (13) Gale, P. A.; Chen, Z.; Drew, M. G. B.; Heath, J. A.; Beer, P. D. Polyhedron 1998, 17, 405. (14) Mathisson, S.; Bakker, E. Anal. Chem. 1998, 70, 303. (15) Tsagatakis, J. K.; Chaniotakis, J. A.; Jurkschat, K. Quim. Anal. 1997, 16, 105. (16) Beer, P. D.; Hesek, J.; Hodacova, J.; Stokes, S. E. J. Chem. Soc., Chem. Commun. 1992, 270. (17) Atwood, J. L.; Hollman, K. T.; Steed, J. W. Chem. Commun. 1996, 1401. (18) Beer, P. D.; Chen, Z.; Goulden, A. R.; Graydon, S. E.; Stokes, S. E.; Wear, T. J. Chem. Soc., Chem. Commun. 1992, 270. (19) Chen, Z.; Graydon, A. R.; Beer, P. D. J. Chem. Soc., Faraday Trans. 1995, 91, 295; 1996, 92, 97. (20) Beer, P. D.; Szemes, F. Inorg. Chem. 1997, 36, 2112. (21) Kavallieratos, K.; de Gala, S. R.; Austin, D. J.; Crabtree, R. H. J. Am. Chem. Soc. 1997, 119, 2325. (22) Kavallieratos, K; Bertao, C. M.; Crabtree, R. H. J. Org. Chem. 1999, 64, 1675. (23) Beer, P. D.; Drew, M. G. B.; Jagessar, R. J. Chem. Soc., Dalton Trans. 1997, 881. (24) Beer, P. D.; Smith, D. K. J. Chem. Soc., Dalton Trans. 1998, 417. (25) Knox; G. R.; Paulson, P. L.; Willison, D. Organometallics 1990, 9, 301. (26) Nesmeyanov A. N. Dokl. Akad. Nauk SSSR, Ser. Khim. 1955, 102, 535. (27) Arimoto, F.; Haven, A. C. J. Am. Chem. Soc. 1955, 77, 6295. (28) Herberhold, M.; Ellinger, M.; Kremnitz, W. J. Organomet. Chem. 1983, 241, 227.
We have synthesized 3-5, which, being neutral, are very soluble in organic solvents. The chloride-binding properties of 4 are studied. We also report an improved synthesis of FcNH2. Results and Discussion Synthesis of FcNH2 and 3-5. FcNH2, synthesized by a known route,25 but with an improved isolation procedure, gave yields of 21-26% versus 8-14%.25,30 The disulfonamide 4, synthesized in good yield (54%) in one step from FcNH2 and 1,3-benzenesulfonyl dichloride in EtOH,31 was isolated as a yellow powder, soluble in CH2Cl2, and was characterized by 1H NMR, FT-IR, and elemental analysis. Attempts to synthesize receptor 3 in CH2Cl2 gave only 8% of 3 but in EtOH gave a 24% yield of the amide ethyl ester 5; both were fully characterized.
(1)
(2)
Anion Binding by 3-5 (1H NMR). Addition of Bu4NX (X ) Cl-) to CD2Cl2 solutions of 3 and 4 caused large chemical (29) Herberhold, M.; Ellinger, M.; Haumeier, L. Organometallic Syntheses; King, R. B., Eisch, J. J., Eds.; Elsevier: Amsterdam, 1986; Vol. 3, p 81. (30) Details are given in the Experimental Section. (31) Ertas, M.; Ahsen, V.; Gu¨l, A.; Bekaˆroglu, O. J. Organomet. Chem. 1987, 333, 383.
10.1021/ic990813s CCC: $18.00 © 1999 American Chemical Society Published on Web 10/06/1999
Notes
Inorganic Chemistry, Vol. 38, No. 22, 1999 5185
Figure 3. Titration curve of 5 with (n-Bu)4NCl at 19.4 °C in CD2Cl2. Figure 1. 1H NMR spectra of 4 before and after addition of 10 equiv of (n-Bu)4NCl. The resonance corresponding to the N-H (2H) shows a dramatic downfield shift from 6.4 to 9.4 ppm. Similarly the 2-C-H (1H) shows a smaller downfeld shift from 8.2 to 9.5 ppm. The other aromatic protons are affected only slightly by anion addition.
Table 2. FT-IR νN-H Stretching Frequencies in CH2Cl2 Solution and a Thin Film for the Free Receptors and Their Adducts (X ) Cl) FT-IR spectrum of 4 4‚X-
Figure 2. Binding conformation of chloride-receptor complexes based on the NMR data and previous work on similar receptors.21,22 Table 1. Chemical Shift Changes (∆δmax) for N-H and Aromatic 2-C-H Protons in the 1H NMR Spectrum upon Chloride Addition compound
∆δ (N-H) (R‚Cl-) (ppm)
∆δ (2-C-H) (R‚Cl-) (ppm)
3 4 5
3.37 3.13 2.88
1.07 1.03 0.23
shift changes for protons (N-H and 2-C-H) affected by hydrogen bonding (Figure 1 and Table 1). Monoamide 5 gave smaller changes for the same protons. The 2-C-H shift (0.23 ppm) being much smaller than the 4-C-H (0.50 ppm) suggests an anti binding conformation. On the other hand, the large shift changes for the 2-C-H protons of 3 and 4 suggest21,22 a syn-syn binding conformation is adopted (Figure 2). No crystals were obtained for X-ray study of any complex of 3 and 4, but previous studies21,22 with 1 and 2 strongly suggest that all have a syn-syn binding conformation with two convergent N-H‚‚‚Cl hydrogen bonds (Figure 2). Solution and Thin Film FT-IR for Receptors and Adducts. To compare Cl- binding of 4 and 5 in solution and the solid state, we examined the solution and thin film32 FT-IR spectra of the receptors and their adducts (Table 2). Free 4 (10-1 M), gave νN-H bands at 3360 and 3228 cm-1 (w) for self-associated N-H. The thin film spectrum shows only the 3228 cm-1 band. With a slight excess of (n-Bu)4NCl, a new band appears for N-H‚‚‚Cl- at 3260 cm-1 (thin film, 3258 cm-1). (32) The thin film is formed by slow evaporation on a NaCl plate of the same solution used for the solution FT-IR study.
νN-H (solution) (cm-1) 3360 and 3228 (weak) 3360 and 3260
νN-H νN-H (thin FT-IR film) spectrum (solution) (cm-1) of (cm-1) 3228
5 5‚X-
3434 3434
νN-H (thin film) (cm-1) 3263 3176
3258
As before,22 the self-associated sulfonamide band (3228 cm-1) does not appear in the solution or film spectra of 4‚X-. For the monoamide ferrocenyl receptor we were able to observe the free N-H band in solution at 3434 cm-1 and the self-associated N-H band at 3263 cm-1 in a thin film. For 5‚Cl- only a broad N-H‚‚‚Cl- band at 3175 cm-1 appeared in thin film, but the solution spectra did not show many other differences from that of free 5, suggesting a much weaker anion complexation.33 1H NMR Titrations. Association Constants. The association constants, Ka, were determined for 4 and 5 with Cl- in a CD2Cl2 solution, by a standard NMR titration34 monitoring ∆δ(N-H) and ∆δ(2-C-H) in a dilute receptor CD2Cl2 solution of concentration range (0.5-1) × 10-3 M, with the addition of 1.0 × 10-2 to 1.0 × 10-1 M CD2Cl2 solutions of (n-Bu)4NCl in the same receptor concentration. One to one binding curves for the N-H protons (Figure 3) were analyzed using a nonlinear regression method.35 The 1:1 binding isotherm34 of eq 3 gave consistent fits for both 2-C-H and N-H.
∆δ ) ([R]t + [Cl-]t + Ka-1 - ((([R]t + [Cl-]t + Ka-1)2 4[Cl-]t[R]t)1/2))∆δmax/(2[R]t) (3) Comparison of the data for Cl- binding (Ka ) 9500 M-1 ((30%), 4‚Cl-; 30 M-1 ((5%), 5‚Cl-; ∆Gf ) -22.3 kJ/mol, 4‚Cl-; -8.3 kJ/mol, 5‚Cl-) demonstrates the importance of the two convergent hydrogen-bonding groups;36-38 the second N-H increases Ka by >300-fold and ∆Gf by >2-fold. In our previous studies with the organic analogues 1 and 2,21,22 we found similar binding constants for the two receptors, suggesting that the higher acidity of the sulfonamide is a secondary factor. (33) We have found that typically for association constants of
