Switching and tuning processes in the interaction of protons with ferrocenyl amines

Po@hedron Vol. 17, No. 4, pp. 491 495, 1998

~)

Pergamon

:9 t998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277 5387/98 $19.00+0.00

PII:S0277-5387(98)00351-3

Switching and tuning processes in the interaction of protons with ferrocenyl amines Angel Benito, Jose Manuel Lloris, Ram6n Martinez-Mfifiez,* and Juan Soto Departamento de Quimica, Universidad Polit6cnica de Valencia, Camino de Vera s/n, 46071 Valencia, Spain

(Received 24 June 1997 ; accepted 14 August 1997)

Abstract--Some new

ferrocene-functionalized amines have been synthesized and characterized. Electrochemical and potentiometric studies have been carried out in T H F : H20 (60:40 v/v) to determine protonation constants of the oxidized and "reduced" species and the oxidation potential shift between pH acid and basic. Some electrochemical studies have also been performed in CH2C12 : MeOH mixtures. The change in the acidity properties after oxidation and the related redox potential shift after protonation are discussed. © 1998 Elsevier Science Ltd Keywords: ferrocene ; electroactive group ; potentiometry ; electrochemistry.

Ferrocene-functionalized polyamines are new receptors which have been reported to electrochemically recognize different substrates as consequence of the shift of the redox potential of the electroactive group [1-5]. Additionally we have recently shown that some of those receptors can quantitatively determine metal ions and anions in aqueous solutions [6]. It is known that amines can coordinate metal ions and, due to electrostatic forces, the oxidation potential of the redox groups is usually shifted to anodic potentials [1-7]. If the metal-ferrocenyl groups interaction is mainly electrostatic the presence of metal ions with the same charge would in principle produce the same shift of the oxidation potential. However, if the experiment is carried out in aqueous solutions different metal ions form different species in solution and therefore control of the pH can lead to selectivity in the recognition process. Working with amines the easiest coordination study is probably with protons. As a part of our interest in redox-active amines as receptors we have synthesized and studied the interaction of H +, using potentiometric and electrochemical techniques, with some new receptors with the aim of determining (i) the effect that oxidation of the ferrocenyl groups has on the acidity of the ammonium groups and (ii) the effect that the protonation process has on the redox potential of the ferrocenyl groups.

EXPERIMENTAL Solvents and reagents

1,2-phenylenediamine, 1,3-phenylenediamine, 1,4phenylenediamine and ferrocenecarbaldehyde were reagent quality and were used without further purification. Tetrahydrofuran (THF) used was freshly distilled from sodium benzophenone. Carbonate-free potassium hydroxide and hydrochloric acid solutions were used in the potentiometric and electrochemical experiments. Potassium nitrate (0.1 mol dm 3) was used as supporting electrolyte in water. L 3and L 4 w e r e synthesized following literature procedures [8].

Synthesis of L I, L 2 and L 5

* Author to whom correspondence should be addressed.

Ferrocenecarboxaldehyde (856 mg, 4 mmol) was heated with 2 mmol of the corresponding diamine in benzene. After 2 h the solvent was removed under vacuum. The red solid was dissolved in tetrahydrofuran and heated under reflux in the presence of LiA1H4 (10 mmol) for 1 h. The reaction mixture was cooled and small amounts of water were added cautiously in order to eliminate the excess of reductor. The resulting mixture was filtered and the solution evaporated under vacuum. Further addition of water and dichloromethane (30 x 3 cm 3) produced yellow organic phases which were dried with anhydrous mag-

491

492

A. Benito et al.

nesium sulphate. The compounds were recrystallized from dichloromethane-hexane solutions.

N,N'_p_phenylenbis( OCerrocenylmethyl)amine),

L I.

(848 mg, 83%). Anal. F o u n d : C, 67.40; H, 5.50; N, 5.60. Calc. for C28H28N2Fe2: C, 66.70; H, 5.60; N, 5.60%. IR (KBr disk) : 3030w, 1610w, 1500s, 1470w, 1390m, 1280m, 1240w, 1220w, 1095m, 830m, 810s cm -~. N M R (CDCI3) : IH 6 3.92 (s, 4H, CH2), 4.18 (s, 10H, C5H5), 4.13 (t, 4H, C5H4), 4.24 (t, 4H, C5H4), 6.64 (s, 4H, C6H,). ~3C {'H} 6 44.7 (CH2), 67.7 (C5H4), 68.0 (C5H4), 68.4 (C5H5), 86.9 (C5H4), 114.8 (C6H,), 141.0 (C6H4).

N,N'-m-phenylenbis( (ferrocenylmethyl)amine),

L 2.

(859 mg, 86%). Anal. F o u n d : C, 66.20; H, 5.50; N, 5.40. Calc. for C28H28N2Fe2: C, 66.70; H, 5.60; N, 5.60%. IR (KBr disk) : 3060w, 1600s, 1500s, 1400m, 1210m, l150w, ll00m, 810m, 740m cm -~. ~H 6 3.95 (s, 2H, CH2), 4.18 (s, 10H, C5H5), 4.14 (t, 4H, C5H4), 4.25 (t, 4H, C5H4), 5.97 (t, IH, C6H4), 6.10 (dd, 2H, C6H4), 7.04 (s, 1H, C6H4). ~3C {JH} 6 43.3 (CH2), 67.7 (C5H4) , 68.0 (C5H4), 68.4 (C5H5), 86.6 (C5H4), 97.2 (C6Hn) , 103.0 (C6H4), 130.1 (C6H4), 149.6 (C6H4).

N,N'-propilenbis((ferrocenylmethyl)amine),

L 5.

(763 mg, 82%). Anal. F o u n d : C, 63.70; H, 6.20; N, 5.80. Calc. for C2~H30N2Fe2: C, 63.90; H, 6.40; N, 6.00%. IR (KBr disk) : 3095m, 2820m, 1640w, 1425s, 1312m, 1227m, l l l 0 m , ll00s, 1035m, 960w, 822s, 808s cm -1. ~H 6 1.65 (q, 2H, CH2), 2.69 (t, 4H, CH2), 3.50 (s, 4H, CH2), 4.09 (s, 10H, C5H5), 4.11 (s, 4H, CsH,), 4.17 (s, 4H, C5H4). 13C{IH} ~ 30.0 (CH2), 47.9 (CH2), 48.9 (CH2), 67.7 (C5H4), 68.3 (C5H5), 86.8 (C5H4).

Physical measurements N M R spectra were measured on a Bruker AC-200 FT spectrometer operating at 300 K. Chemical shifts for ~H and ~3C {1H} spectra are referenced to TMS and CDC13, respectively. IR spectra were taken on a Perkin-Elmer 1750 spectrophotometer as KBr pellets. Electrochemical data were obtained with a programmable function generator Tacusel IMT-1, connected to a Tacusel PJT 120-1 potentiostat. The working electrode was platinum with a saturated calomel reference electrode separated from the text solution by a salt bridge containing the solvent/supporting electrolyte. The auxiliary electrode was platinum wire. Potentiometric titrations were carried out in T H F : H20 (60 : 40 v/v) using a reaction vessel waterthermostated at 25.0+0.1°C under nitrogen. The titrant was added by a Crison microburete 2031. The potentiometric measurements were made using a Crison 2002 pH-meter and a combined glass electrode. The titration system was automatically controlled by a PC computer using a program that monitored the e.m.f, values and the volume of titrant added. The electrode was calibrated as a hydrogen concentration proved by titration of known amounts of HC1 with CO2-free K O H solution and determining the equi-

valent point by Gran's method [9] which gives the standard potential E '° and the ionic product of water (K~v = [H+][OH ]). The logarithm of K~v for the solvent used was found to be - 15.1 +0.1 (25°C, 0.1 mol dm 3 tetrabutylammonium perchlorate). The computer program S U P E R Q U A D [10] was used to calculate the protonation and stability constants. The titration curves for each system (ca. 200 experimental points corresponding to at least three titration curves, pH=-log[H] range investigated 2.5-10, concentration of the ligand was ca. 1.2 x 10 -3 mol dm -3) were treated either as a single set or as separated entities without significant variations in the values of the stability constants. Finally the sets of data were merged together and treated simultaneously to give the stability constants.

RESULTS AND DISCUSSION

Synthesis and characterization The reaction of ferrocenecarboxaldehyde with the diamines 1,3-, 1,4-phenylendiamine and 1,3-propylendiamine led to the corresponding Schiff base derivatives as red oils. IR spectra revealed that the condensation took place as can be observed by the strong v(C--N) stretching vibration at about 1640 cm 1. The IR also displayed for all the imine derivatives several characteristic ferrocene absorptions. Reduction of the Schiff base derivatives with LiA1H4 in tetrahydrofuran allowed isolation of the corresponding diamines L I, L 2 and L 5. The IR showed that the imino group had been reduced by the absence of the v ( C z N ) imine stretching peak. The lack after hydrogenation in the ~H N M R of any imine protons at 6 about 8.2 and the presence of characteristic CH2 groups attached to ferrocenyl groups (6 about 3.53.9) also supports the proposed formulation. Similar synthetic procedures between ferrocenecarboxaldehyde and 1,2-phenylenediamine did not produce the expected Schiff-base derivatives but compounds L 3 and t 4 as we have reported [8].

Protonation behaviour The protonation behaviour of L j, L 2, L 3, L 4 and L 5 was studied by potentiometric titrations of acidified solutions of the amines in T H F : H20 (60:40 v/v) (0.1 mol dm 3 tetrabutylammonium perchlorate, 25°C). Basicity constants are gathered in Table 1. L j, L 2 and L 3 behave as diprotic bases whereas only one protonation process was observed for the benzimidazol derivatives. As expected the diazaalkane derivative L 5 is more basic than the corresponding aryl amines L j, L 2, L 3 and L 4, with the acidity of the first protonation constant following the order L 2 > L 3 > L 4 > L ~ > L 5. For the sake of comparison the basicity constants of the non-substituted 1,3- and 1,4-phenylenediamine

Interaction of protons with ferrocenyl amines

with a ferrocenyl group in the 2 position increases the basicity of the benzimidazol framework. A similar effect has been found in water when we compare the protonation constant for benzimidazole (log K = 5.47) and 2-methyl-benzimidazole (log K = 6.17) [11].

Fe

+

493

Fe

Fe

Fe

Electrochemical behaviour

L2

L1

Fe H

C(,>w Fe

Fe

were also determined in T H F : H20 (60 : 40 v/v) mixtures, [log K ; 1,4-phenylenediamine : 5.80(1), 1.98(1) ; 1,3-phenylenediamine : 4.37(1), 1.19(2) ; for the processes, L + H ÷ ~ L H + and LH ÷ + H + = LH ] 2, respectively]. It can be observed that the N-functionalization with ferrocenylmethyl groups reduces the basicity of the amine groups for the first protonation process but increases the basicity of the second protonation process of L ~and L 2. Also for the sake of comparison we have determined the protonation constant for benzimidazole in T H F : H 2 0 (60:40 v/v) and have obtained a value of l o g K = 3.09(1) for the process L + H* ~ L H ÷, indicating that the functionalization

Two related effects were studied electrochemically ; the possible modification of the acidity of the receptors by oxidation of the redox-active groups and the redox potential shift of the electro-active groups by addition of protons to the medium. The receptors U - L 5 show an important difference with respect to the starting non-functionalized polyamines, due to the fact that L~-L 5 are molecules whose basicity strength can be switched from "'outside" by oxidation or reduction of the pendant redox-active ferrocenyl groups. That switching process between two different states containing oxidized or " r e d u c e d " ferrocenyl groups can be quantified from the study of the E1/2-pH (half-wave potential vs pH) curves. Figure 1 shows an example of the shift of the oxidation potential of the receptor L s. F r o m the analysis of the E12 pH curves it is possible to evaluate the protonation constants of the amines when the redox groups are oxidized. Table 2 shows the stepwise protonation constants for the oxidized L l, L 2, L 3, L 4 and L s receptors obtained by fitting the EI/2-pH curves with an equation which relates the half-wave potential with the proton concentration and the acidity constants for the "reduced" and the oxidized species [2]. As can be observed, oxidation of the ferrocenyl groups switches the acidity strength of the a m m o n i u m groups and oxidized species behave as stronger acids than the corresponding "reduced" species. Thus first protonation constants are reduced by 6 times for L ~ and by 38 times for L 4 due to the oxidation of the ferrocenyl groups. We and others have recently published that the change in the protonation constants is due mainly to electrostatic forces as a consequence of the existence of positively charged ferrocenyl groups after oxidation [12,13]. Another study on the ferrocene-functionalized

Table 1. Stepwise protonation constants (logK) of U, L:, L :', L 4 and L s determined in T H F : H 2 0 (60:40 v/v) at 298.1 K in 0.1 mol dm -3 tetrabutylammonium perchlorate Reaction L + H + ~.~HL + HL+ +H+.~--H2 L+2

LL

L2

L3

L4

L~

5.59(1) ~ 3.69(4)

3.93(1) 2.84(2)

4.28(2)

4.80(1)

9.17(5) 7.19(3)

Values in parentheses are standard deviations on the last significant figure.

A. Benito et al.

494

Table 2. Stepwise protonation constants (log K) for the oxidized form of L 1, L2, L 3, L 4 and L 5 determined in THF : H20 (60 : 40 v/v) at 298.1 K in 0.1 mol d m -3 tetrabutylammonium perchlorate Reaction °XL+H÷ : ° X L H + °XLH+ + H + ~ °XLH~-2

Lt

L2

4.8(1)" 1.8(1)

3.0(1) 0.8(2)

L3 2.78(6)

L4

L5

3.22(3)

7.9(1) 5.5(1)

"Values in parentheses are standard deviations on the last significant figure.

460 -

11-14398

1 (2x10 6 1051~2211

A E : jn[_ : - I - - eE L -j irji + \

440-

~2

+

/I j i 4 j

(1) 420

400

380

360

340

320

2

l

l

4

6

T

~

l

8

10

12

14

pH

Fig. 1. Plot of the half-wave potential (Etl2) vs pH for compound L5.

amines is how the protonation of the amine groups affects the oxidation potential of the ferrocenyl moieties. For a fixed pH E~/2 is a function of the acidity constants of the oxidized and "reduced" species and a steady shift is usually observed when the pH is changed. We can therefore say that the proton concentration can tune the value of the half-wave potential as can be observed for example in the EI/2-pH curve shown in Fig. 1. On the other hand another interesting parameter is the m a x i m u m shift that can be achieved by changing the pH and can be measured as the difference between E~/2 at basic pH and at pH = 0. That difference, AE, determined in T H F : H20 (60:40 v/v) was 79, 85, 42, 92 and 105 for molecules L 1, L 2, L 3, L 4 and L% respectively. We have recently shown that eqn (1) allows a prediction of the potential shift due to the coordination of H ÷ in redoxfunctionalized polyamines as a function of the n u m b e r of electrons involved in the electrochemical experiment (n) ; the n u m b e r of redox groups (j) ; the distance between the protonation sites (i) and the ferrocenyl groups (rj~) and the permittivity of the medium (e) [12].

Distances can be calculated using molecular modeling programs such as P C M O D E L [14]. The predicted AE values are 89, 92, 68, 165 and 93 mV, for L J, L 2, L 3, L 4 and L s, respectively. Although there is a good agreement between the predicted and the observed oxidation potential shift in L j, L 2 and L 5 there is an important deviation for the benzimidazole derivatives probably due to the different nature of the protonated nitrogens. The study of the change of Ell2 as a function of the H ÷ concentration shows for all the compounds studied in T H F : H20 (60 : 40 v/v) a steady one-wave shift, suggesting that the equilibrium is fast on the electrochemical time-scale. However the use of a different solvent may change the observed behaviour and produce a switching effect in the oxidation potential rather than a tuning process. Thus we have also studied the effect that the addition of H + has on the redox potential of L l, L 2 and L 5 in dichlor o m e t h a n e : m e t h a n o l (10: 1) mixtures. Whereas no well defined voltammograms were observed for L ~, addition of acid (HC1, 0.1 M in dichlor o m e t h a n e : m e t h a n o l 10:1) to L 2 and L 5 results in the appearance of a second wave at higher anodic potentials. After addition of two equivalents of acid the first wave disappears and only the second oxidation process is observed. The switching process is reversible and addition of K O H ( K O H 0.1 M in dichloromethane:methanol 10: 1) regenerates the first oxidation potential [8]. The AE between the first and the second wave is about 65 and 106 mV for L 2 and L 5, respectively. A similar behaviour has recently been observed for the benzimidazol derivatives L 3 a n d L 4.

Acknowledyements--We thank the Direcci6n General de Investigaci6n Cientifica y T6cnica (proyecto PB95-1121C02-02) for support. REFERENCES

1. Beer, P. D., Chen, Z., Drew, M. G. B., Kingston,

Interaction of protons with ferrocenyl amines

2. 3.

4. 5. 6. 7.

J., Ogden, M. and Spencer, P., J. Chem. Soc., Chem. Commun., 1993, 1046; Beer, P. D., Chem. Soc. Rev., 1989, 18, 409. Tendero, M. J. L., Benito, A., Martinez-Mfifiez, R., Soto, J., Payfi, J., Edwards, A. J. and Raithby, P. R., J. Chem. Soc., Dalton Trans., 1996, 343. Tendero, M. J. L., Benito, A., Martinez-Mfifiez, R., Soto, J., Garcia-Espafia, E., Ramirez, J. A., Burguete, M. I. and Luis, S. V., J. Chem. Soc., Dalton Trans., 1996, 2923. Tendero, M. J. L., Benito, A., Martinez-Mfifiez, R. and Soto, J., J. Chem. Soc., Dalton Trans., 1996, 4121. Beer, P. D., Chen, Z., Drew, M. G. D., Johnson, A. O. M., Smith, D. K. and Spencer, P., Inorg. Chim. Acta, 1996, 246, 143. Padilla-Tosta, M. E., Martinez-M~ifiez, R., Pardo, T., Soto, J. and Tendero, M. J. L., Chem. Commun., 1997, 887. Tendero, M. J. L., Benito, A., Cano, J., Lloris, J. M., Martinez-Mfifiez, R., Soto, J., Edwards, A.

8. 9. 10. 11. 12. 13. 14.

495

J., Raithby, P. and Rennie, M. A., J. Chem. So¢., Chem. Commun., 1995, 1643. Benito, A., Martinez-M~ifiez, R., Pay~i, J., Soto, J., Tendero, M. J. L. and Sinn, E., J. Organomet. Chem., 1995, 503, 257. Gran, G., Analyst (London), 1952, 77, 661 ; Rossotti, F. J. and Rossotti, H. J., J. Chem. Educ., 1965, 42, 375. Gans, P., Sabatini, A. and Vacca, A., J. Chem. Soc., Dalton Trans., 1985, 1195. Lane, T. J. and Qinlan, K. P., J. Am. Chem. Sot., 1960, 82, 2994. Benito, A., Martinez-M~ifiez, R., Soto, J. and Tendero, M. J. L., J. Chem. Soc. Faraday Trans., 1997, 93, 2175. Plenio, H., Yang, J., Diodone, R. and Heinze, J., Inorg. Chem., 1994, 33, 4098. PCMODEL, Molecular Modelling for Personal Computers and Workstations. Serene Software, 1988.