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Effect of second-sphere coordination 12. Adduct formation behavior of crown ethers with ammine complexes of ruthenium containing a protic ligand of other type than ammine
Inorganica Chimica Acta 362 (2009) 4862–4866
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Effect of second-sphere coordination 12. Adduct formation behavior of crown ethers with ammine complexes of ruthenium containing a protic ligand of other type than ammine Isao Ando *, Yuzo Takao, Kikujiro Ujimoto, Hirondo Kurihara Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka 814-0180, Japan
a r t i c l e
i n f o
Article history: Received 10 September 2007 Received in revised form 7 April 2009 Accepted 19 July 2009 Available online 24 July 2009 Keywords: Hydrogen bonding site Adduct formation Ruthenium complex
a b s t r a c t Adduct formation of pentaammineruthenium complexes involving a different type of protic ligand, such as imidazole, was investigated for a series of crown ethers with different ring size. Changes in redox potential and in absorption spectra of the complex were measured on addition of crown ether to the complex solution. The magnitude of the change in both properties is dependent on the ring size of crown ethers. 1H-NMR spectra of the complex were measured in the presence of crown ethers in order to elucidate hydrogen bonding sites. The chemical shifts of NH proton of imidazol and ammine protons were measured at various concentrations of crown ethers. Adduct formation was discussed based on the features of dependences of those chemical shifts on crown ether concentration. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction A variety of transition metal complexes interact with organic substrates at the second-sphere of a complex, via weak interactions such as hydrogen bonding, hydrophobic interactions or electrostatic interaction [1–8]. Such second-sphere coordination brings about a perturbation of the electronic state of the complex and modifies the properties of the complex. We have been investigating the second-sphere coordination focusing on the modification of the properties of ruthenium-ammine complexes, especially redox properties [9–16]. Ruthenium–ammine complexes form an adduct with crown ethers through hydrogen bonding at the second-sphere of the complex. The adduct formation gave a novel change in redox potential of the complex [9]. The change in the properties of the complex was affected by following factors: (i) the number of ammine ligands coordinating to ruthenium, (ii) the p-electron acceptability of an ancillary ligand, and (iii) the flexibility of the crown ether ring [11]. The pentaammineruthenium(II) complex, [Ru(NH3)5(py)](PF6)2 {py = pyridine}, formed an adduct with crown ethers through hydrogen bonding between the ammine ligands of the complex and ether oxygens of the crown ether [11]. The adduct formation gave a change in redox potential of the complex. Its magnitude varied depending on the ring size of crown ethers: 12-crown-4 * Corresponding author. E-mail address: [email protected] (I. Ando). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.07.018
ether < 15-crown-5 ether < 18-crown-6 ether. This implies that the flexibility of crown ether ring should become an important factor for favorable hydrogen bonding between the crown ether and the protic ligand of a complex. The 18-crown-6 ether appears to be most favorable for adduct formation with the ammine ligand of the complex. Therefore, selectivity is anticipated in adduct formation between complexes with a protic ligand of different type and crown ethers with different ring size. Adduct formation with aliphatic crown ethers of different ring size was investigated for complexes involving ammine or 4-mercaptopyridine as a protic ligand, [Ru(trpy)(bpy)(L)](PF6)2 {trpy = 2,20 ,200 -terpyridine, bpy = 2,20 -bipyridine, L = NH3 and 4-mercaptopyridine (mpy)} [15]. Formation constants of crown ether adduct are the order of 12-crown-4 ether < 15-crown-5 ether < 18crown-6 ether for the complex containing ammine as a protic ligand. For the complex containing 4-mercaptopyridine as a protic ligand, however, the formation constant for 12-crown-4 ether adduct is larger than that for 18-crown-6 ether adduct. This fact implies that the ammine ligand prefers 18-crown-6 ether to 12-crown-4 and the 4-mercaptopyridine ligand does vice versa, as shown in Scheme 1. Accordingly, for a ruthenium complex containing protic ligands of two types within a complex, it may be anticipated that a certain crown ether should selectively or preferentially interact at the certain ligand site of the complex in the adduct formation of crown ether. In this study, adduct formation with crown ethers was investigated for ruthenium–ammine complexes involving imidazol or 4hydroxypyridine as a protic ligand of another type.
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Scheme 1. Selectivity in adduct formation.
2. Experimental 2.1. Materials Pentaammineruthenium complexes, [Ru(NH3)5(L)](PF6)n {L = imidazole (imH), n = 3, and 4-hydroxypyridine (hpy), n = 2}, were prepared from [Ru(NH3)5(Cl)](Cl)2 according to the literature method and characterized spectrophotometrically by comparison of kmax and emax values [17–19]. 18-Crown-6 ether (abbr. 18C6) was purchased from Wako Pure Chemical Industries and purified by the literature method [20]. 15Crown-5 ether (abbr. 15C5) and 12-crown-4 ether (abbr. 12C4) were purchased from Tokyo Kasei Kogyo and dehydrated by deuterium molecular sieves 3A (Euriso-top). The absence of oxidative impurity was confirmed as mentioned previously [10]. Deuterated nitromethane for NMR measurements was obtained from Aldrich Chemical and used after dehydrated with deuterium molecular sieves 3A. Other chemicals were reagent grade and used without further purification. 2.2. Measurements Absorption spectra and absorbances were measured by a Hitachi 228 spectrophotometer. Measurements of 1H NMR spectra were performed at 400 MHz using a Varian Unity Inova 400 WB NMR spectrometer. The chemical shifts were obtained using tetramethylsilane as an internal standard. A BAS 100W/B electrochemical workstation was used to record the cyclic voltammograms. Voltammograms were observed in 0.10 mol dm3 tetrabutylammonium hexafluorophosphate acetonitrile solution, using a three-electrode assembly, an Ag/AgNO3 reference electrode, a glassy carbon working electrode, and a platinum coil auxiliary electrode. 3. Results and discussion Pentaammineruthenium complexes form an adduct with crown ethers through hydrogen bonding between ammine ligand of the complex and ether oxygen of the crown ether [16]. The complexes, [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2, contain protic
ligands of different types, ammine and imH or hpy, within a complex and the ligands, imH and hpy, are more acidic than the ammine [17,19]. Accordingly, it is possible that these complexes may interact with crown ether to form hydrogen bonds either at protic ligands of both types or preferentially at a more acidic ligand, depending on ring size of crown ether. Adduct formation was spectrophotometrically examined for [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2 with crown ethers of different ring size. [Ru(NH3)5(imH)](PF6)3 shows two LMCT (transition from the imidazole HOMO to unfilled t2g orbital of ruthenium(III)) bands at 304 and 438 nm [17], and [Ru(NH3)5(hpy)](PF6)2 does a MLCT (transition from filled orbital t2g of ruthenium(II) to the 4-hydroxypyridine LUMO) band at 365 nm in acetonitrile [19]. The LMCT bands of [Ru(NH3)5(imH)](PF6)3 exhibited a blue shift and the MLCT band of [Ru(NH3)5(hpy)](PF6)2 did a red shift on adding crown ether. The magnitude of their shifts is summarized in Table 1. Fig. 1 shows the typical spectral change of [Ru(NH3)5(imH)](PF6)3 on adding of 18C6 in acetonitrile. On addition of crown ether, these LMCT bands continuously shifted toward shorter wavelength with increasing crown ether concentration; exhibiting a set of isosbestic points, as shown in Fig. 1. These blue shift of LMCT band indicates qualitatively that the ammine complex forms hydrogen bonds with crown ether at the ammine ligands coordinating to ruthenium(III) [16,21]. When twenty fold excess of crown ether was added to the acetonitrile solution of the complex, the magnitude of their shifts is the order of 18C6 > 15C5 > 12C4. For [Ru(NH3)5(hpy)](PF6)2, on the other hand, a red shift of MLCT band was observed on adding two hundred fold excess of 18C6 but not on addition of 15C5 or 12C4. This red shift of MLCT band also indicates hydrogen bonding between ammine ligand coordinating to ruthenium(II) and ether oxygens of crown ether [16,21]. The magnitude of shifts for these CT band may vary depending the ring size of crown ether. The hydrogen bonding site with crown ether for a ruthenium–ammine complex can be judged from the direction of the shift of these CT band [16,21]. The complexes in the present study is possible to form hydrogen bonds at two different sites because of containing two kinds of site for hydrogen bonding with crown ether; ammine and aromatic ligands such as imH or hpy. Hydrogen bonding with crown ethers gave the shift of CT band of the complex but the direction of the shift becomes reverse on hydrogen bonding at an ammine and at an aromatic ligand [16,21]. The observed shift of the CT band reflects the difference in shifts between hydrogen bonding at ammine site and imH or hpy site. For [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2, thus, it is obscured either to form an adduct through hydrogen bonding at ammine or at both ammine and imH or hpy ligands. and Cyclic voltammograms of [Ru(NH3)5(imH)](PF6)3 [Ru(NH3)5(hpy)](PF6)2 were measured in the absence and presence of crown ethers in acetonitrile solution and shown in Fig. 2. The voltammograms showed a reversible redox couple associated with metal center at 0.150 and 0.051 V versus (Ag+/Ag) for [Ru(N-
Table 1 The values of kmax of CT bands for [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2 in the absence and presence of crown ethers in acetonitrile. Complex
Crown ether
[Ru(NH3)5(imH)](PF6)3
18C6 15C5 12C4 18C6 15C5 12C4
[Ru(NH3)5(hpy)](PF6)2
R = [crown]/[Complex].
kmax/nm (vmax/103 cm1)
Dvmax/cm1
R=0
R = 20
438(22.8) 438(22.8) 438(22.8) 365(27.4) 365(27.4) 365(27.4)
422(23.7) 428(23.4) 438(22.8)
R = 200
428(23.4) 371(27.0) 365(27.4) 365(27.4)
900 600 600 400 0 0
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Fig. 1. Spectral change of [Ru(NH3)5(imH)](PF6)3 on addition of 18C6 in acetonitrile. Concentration of the complex is 1.25 103 mol dm3, and concentrations of 18C6 are 0 (1), 1.25 103 (2), 1.25 102 (3), and 2.5 102 mol dm3.
Fig. 3. Dependences of DE1/2 for (a) [Ru(NH3)5(imH)](PF6)3 and (b) [Ru(NH3)5(hpy)](PF6)2 on crown ether concentration of 12C4(4), 15C5(h), and 18C6(s) in acetonitrile [complex] = 5.0 104 mol dm3.
Table 2 Limiting values of change in redox potential, DE1/2(lim), for the Ru(III)–Ru(II) redox couple in [Ru(NH3)5(L)](PF6)n caused by addition of crown ether in acetonitrile. Crown ether
DE1/2(lim)/mV L = pya
imH
hpy
18C6 15C5 12C4
(129 ± 3) (128 ± 3) >30b
(129 ± 5) (153 ± 12) (58 ± 3)
(130 ± 4) (134 ± 12) (33 ± 7)
a b
Fig. 2. Cyclic voltammograms of (a) [Ru(NH3)5(imH)](PF6)3 and (b) [Ru(NH3)5(hpy)](PF6)2 in the absence and presence of 0.10 mol dm3 crown ether in acetonitrile.
Data in Ref. [16]. The value was DE1/2 on addition of 200-fold excess of crown ether.
voltammetric features were similar to those of the pentaammineruthenium complexes and indicates formation of hydrogen bonds between the ammines coordinating to ruthenium and the ether oxygens of crown ethers [16]. The limiting change in redox potential, DE1/2(lim), was evaluated from the dependences in Fig. 3 by the least square analysis of Eq. (1), [11] and listed in Table 2 together with those of
DE1=2 ¼ DE1=2 ðlimÞ þ A expðB½crown etherÞ H3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2, respectively. The voltammograms shifted continuously toward a negative potential direction depending on crown ethers. The voltammograms were measured at various concentrations of crown ethers. The redox potentials shifted continuously toward negative direction with increasing crown ether concentrations. Fig. 3 shows dependences of change in redox potential on crown ether concentration for [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2. These
ð1Þ
[Ru(NH3)5(py)](PF6)2 (py = pyridine) for comparison. For the typical pentaammineruthenium complex, [Ru(NH3)5(py)](PF6)2, an absolute value of DE1/2(lim) is ca. 130 mV in the case of 18C6 and 15C5 but is much smaller value in the case of 12C4. [Ru(NH3)5 (hpy)](PF6)2 showed comparable value to those of [Ru(NH3)5 (py)](PF6)2. However, [Ru(NH3)5(imH)](PF6)3 gave somewhat larger absolute values of DE1/2(lim) than those of [Ru(NH3)5(py)](PF6)2 by addition of 15C5 and 12C4. This may intimate that [Ru(NH3)5
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Fig. 5. Dependences of chemical shifts of cis-NH3 protons(h), trans-NH3 protons (s), and imidazol N-H proton(4) for [Ru(NH3)5(imH)](PF6)3 on 18C6 concentrations in the range of 0–0.01 mol dm3 [complex] = 5.0 103 mol dm3.
Fig. 4. Dependences of chemical shifts of imidazol N–H proton (a), cis-NH3 protons (b), and trans-NH3 protons (c), for [Ru(NH3)5(imH)](PF6)3 on crown ether concentrations in nitromethane. Symbols are the data for 12C4(4), 15C5(h), and 18C6(s) [complex] = 5.0 103 mol dm3.
(imH)](PF6)3 forms hydrogen bond at imidazole ligand in addition of hydrogen bonding at ammine ligand. Adduct formation was followed in deuterated nitromethane by 1 H NMR spectrometry for [Ru(NH3)5(imH)](PF6)3 – crown ether systems. 1H NMR spectrum of [Ru(NH3)5(imH)](PF6)3 was obtained in fairly low field region by 6000 times accumulation. Relatively broad signals were observed at 124, 233, and 274 ppm with integral ratios of 12.6, 3.0, and 1.4, respectively, and may be assigned to cis-ammine protons, trans-ammine protons, and N–H proton of imidazole according to their integral ratios. The chemical shift of these proton signals were examined in detail at various concentrations of crown ethers.
Fig. 4 shows the dependences of the chemical shift on crown ether concentration. By addition of 12C4 or 15C5, the signals of imidazole N–H proton and cis-ammine protons monotonously shifted toward low field with increasing crown ether concentration to extent depending on ring size of crown ether. However, the signal of trans-ammine protons shifted toward high-field with increasing crown ether concentration. The shift of imidazole N–H proton reached maximum at crown ether concentration of 0.02 mol dm3, nevertheless the signal of cis-ammine protons still shifted toward low field at concentrations more than 0.02 mol dm3. This behavior indicates that [Ru(NH3)5(imH)](PF6)3 forms hydrogen bonds with 12C4 and 15C5 simultaneously at both the coordinated imidazole and the cis-ammines, or successively forms hydrogen bonds at the coordinated imidazole and the cisammines. The high-field shift of the signal of the trans-ammine protons may be due to desolvation caused by crowding around trans-ammine accompanied with hydrogen bond formation at two cis-ammines. On the other hand, the behavior on addition of 18C6 is roughly similar to that on addition of 12C4 or 15C5 but exhibits significant difference in the lower concentration range of crown ether. Fig. 5 shows the enlarged dependences of chemical shift of imidazole N–H proton, cis-ammine protons, and trans-ammine protons on 18C6 concentration at lower range. Although the signal of imidazole N–H proton also exhibited a monotonous dependence on 18C6 concentration, the signals of cis- and trans-ammine protons did intricate dependences. The signal of trans-ammine protons did not shift up to the 18C6 concentration of 2 103 mol dm3 and thereafter shifted toward low field. Moreover, the signal started to shift in reverse direction at the 18C6 concentration more than
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stood in the respects of acidity of coordinating imidazole and interaction site of Ru(III)–ammine complexes with crown ether [10,17]. On the other hand, 18C6 exhibits stepwise adduct formation through hydrogen bonding at imidazole, trans-ammine, and cisammine site of the complex unlike 15C5 and 12C4. This implies that adduct formation is affected by both the acidity of a protic ligand and the steric fitness between a type of protic ligand and crown ethers. References
Scheme 2. Successive adduct formation.
5 103 mol dm3. At that 18C6 concentration, on the other hand, the signal of cis-ammine protons started to shift toward low field although the signal shifted little up to the 18C6 concentration of 5 103 mol dm3. These changes of chemical shift of the ligand proton signals indicate that [Ru(NH3)5(imH)](PF6)3 forms an adduct with 18C6 through successive formation of hydrogen bonds between the protons of the coordinating ligands and the ether oxygen of 18C6 as shown in Scheme 2; 18C6 initially binds to the complex at imidazole N–H proton, moreover binds at trans-ammine protons, and thereafter the hydrogen bonds at trans-ammine protons is gradually replaced by binding at cis-ammine protons with increasing 18C6 concentration. The signal of imidazole N–H proton observed at most low field, 274 ppm, implies that the imidazole N– H proton is most acidic of all protic ligands of the complex. This supports the behavior for the successive adduct formation. In conclusion, adduct formation of [Ru(NH3)5(imH)](PF6)3 with 18C6 exhibits somewhat different behavior from that with 15C5 and 12C4. Although distinct site-selectivity was not observed in adduct formation of [Ru(NH3)5(imH)](PF6)3 with 15C5 and 12C4, the behavior of adduct formation with 15C5 and 12C4 is under-
[1] R.V. Gessner, G.J. Quigley, A.H.-J. Wang, G.A. van der Marel, J.H. van Boom, A. Rich, Biochemistry 24 (1985) 237. [2] S.E. Sherman, S.J. Lippard, Chem. Rev. 87 (1987) 1153. [3] J.-M. Lehn, Angew. Chem., Int. Ed. Engl. 27 (1988) 89. [4] H.M. Colquhoun, J.F. Stoddart, D.J. Williams, Angew. Chem., Int. Ed. Engl. 25 (1986) 487. [5] R. Ballardini, M.T. Gandolfi, L. Prodi, M. Ciano, V. Balzani, F.H. Kohnke, H. Shahriari-Zavareh, N. Spencer, J.F. Stoddart, J. Am. Chem. Soc. 111 (1989) 7072. [6] J.C. Curtis, J.A. Roberts, R.L. Blackbourn, Y. Dong, M. Massum, C.S. Johnson, J.T. Hupp, Inorg. Chem. 30 (1991) 3856. [7] M.D. Todd, Y. Dong, J.T. Hupp, Inorg. Chem. 30 (1991) 4685. [8] M.D. Todd, Y. Dong, J. Horney, D.I. Yoon, J.T. Hupp, Inorg. Chem. 32 (1993) 2001. [9] I. Ando, D. Ishimura, M. Mitsumi, K. Ujimoto, H. Kurihara, Polyhedron 11 (1992) 2335. [10] I. Ando, D. Ishimura, K. Ujimoto, H. Kurihara, Inorg. Chem. 33 (1994) 5010. [11] I. Ando, D. Ishimura, K. Ujimoto, H. Kurihara, Inorg. Chem. 35 (1996) 3504. [12] I. Ando, D. Ishimura, M. Mitsumi, K. Ujimoto, H. Kurihara, Inorg. Chim. Acta 249 (1996) 201. [13] I. Ando, M. Higashi, K. Ujimoto, H. Kurihara, Inorg. Chim. Acta 282 (1998) 247. [14] I. Ando, K. Nishihara, K. Ujimoto, H. Kurihara, Inorg. Chim. Acta 346 (2003) 19. [15] I. Ando, S. Fujiwara, K. Ujimoto, H. Kurihara, Inorg. Chim. Acta 351 (2003) 43. [16] I. Ando, Coord. Chem. Rev. 248 (2004) 185. [17] M.F. Tweedle, H. Taube, Inorg. Chem. 21 (1982) 3361. [18] C. Creutz, H. Taube, J. Am. Chem. Soc. 95 (1973) 1086. [19] I. Ando, T. Sakai, K. Yamaguchi, K. Ujimoto, H. Kurihara, Fukuoka Univ. Sci. Rep. 30 (2000) 103. [20] G.W. Gokel, D.J. Cram, J. Org. Chem. 39 (1974) 2445. [21] I. Ando, H. Fujimoto, K. Nakayama, K. Ujimoto, H. Kurihara, Polyhedron 10 (1991) 1139.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Effect of second-sphere coordination 12. Adduct formation behavior of crown ethers with ammine complexes of ruthenium containing a protic ligand of other type than ammine Isao Ando *, Yuzo Takao, Kikujiro Ujimoto, Hirondo Kurihara Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka 814-0180, Japan
a r t i c l e
i n f o
Article history: Received 10 September 2007 Received in revised form 7 April 2009 Accepted 19 July 2009 Available online 24 July 2009 Keywords: Hydrogen bonding site Adduct formation Ruthenium complex
a b s t r a c t Adduct formation of pentaammineruthenium complexes involving a different type of protic ligand, such as imidazole, was investigated for a series of crown ethers with different ring size. Changes in redox potential and in absorption spectra of the complex were measured on addition of crown ether to the complex solution. The magnitude of the change in both properties is dependent on the ring size of crown ethers. 1H-NMR spectra of the complex were measured in the presence of crown ethers in order to elucidate hydrogen bonding sites. The chemical shifts of NH proton of imidazol and ammine protons were measured at various concentrations of crown ethers. Adduct formation was discussed based on the features of dependences of those chemical shifts on crown ether concentration. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction A variety of transition metal complexes interact with organic substrates at the second-sphere of a complex, via weak interactions such as hydrogen bonding, hydrophobic interactions or electrostatic interaction [1–8]. Such second-sphere coordination brings about a perturbation of the electronic state of the complex and modifies the properties of the complex. We have been investigating the second-sphere coordination focusing on the modification of the properties of ruthenium-ammine complexes, especially redox properties [9–16]. Ruthenium–ammine complexes form an adduct with crown ethers through hydrogen bonding at the second-sphere of the complex. The adduct formation gave a novel change in redox potential of the complex [9]. The change in the properties of the complex was affected by following factors: (i) the number of ammine ligands coordinating to ruthenium, (ii) the p-electron acceptability of an ancillary ligand, and (iii) the flexibility of the crown ether ring [11]. The pentaammineruthenium(II) complex, [Ru(NH3)5(py)](PF6)2 {py = pyridine}, formed an adduct with crown ethers through hydrogen bonding between the ammine ligands of the complex and ether oxygens of the crown ether [11]. The adduct formation gave a change in redox potential of the complex. Its magnitude varied depending on the ring size of crown ethers: 12-crown-4 * Corresponding author. E-mail address: [email protected] (I. Ando). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.07.018
ether < 15-crown-5 ether < 18-crown-6 ether. This implies that the flexibility of crown ether ring should become an important factor for favorable hydrogen bonding between the crown ether and the protic ligand of a complex. The 18-crown-6 ether appears to be most favorable for adduct formation with the ammine ligand of the complex. Therefore, selectivity is anticipated in adduct formation between complexes with a protic ligand of different type and crown ethers with different ring size. Adduct formation with aliphatic crown ethers of different ring size was investigated for complexes involving ammine or 4-mercaptopyridine as a protic ligand, [Ru(trpy)(bpy)(L)](PF6)2 {trpy = 2,20 ,200 -terpyridine, bpy = 2,20 -bipyridine, L = NH3 and 4-mercaptopyridine (mpy)} [15]. Formation constants of crown ether adduct are the order of 12-crown-4 ether < 15-crown-5 ether < 18crown-6 ether for the complex containing ammine as a protic ligand. For the complex containing 4-mercaptopyridine as a protic ligand, however, the formation constant for 12-crown-4 ether adduct is larger than that for 18-crown-6 ether adduct. This fact implies that the ammine ligand prefers 18-crown-6 ether to 12-crown-4 and the 4-mercaptopyridine ligand does vice versa, as shown in Scheme 1. Accordingly, for a ruthenium complex containing protic ligands of two types within a complex, it may be anticipated that a certain crown ether should selectively or preferentially interact at the certain ligand site of the complex in the adduct formation of crown ether. In this study, adduct formation with crown ethers was investigated for ruthenium–ammine complexes involving imidazol or 4hydroxypyridine as a protic ligand of another type.
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Scheme 1. Selectivity in adduct formation.
2. Experimental 2.1. Materials Pentaammineruthenium complexes, [Ru(NH3)5(L)](PF6)n {L = imidazole (imH), n = 3, and 4-hydroxypyridine (hpy), n = 2}, were prepared from [Ru(NH3)5(Cl)](Cl)2 according to the literature method and characterized spectrophotometrically by comparison of kmax and emax values [17–19]. 18-Crown-6 ether (abbr. 18C6) was purchased from Wako Pure Chemical Industries and purified by the literature method [20]. 15Crown-5 ether (abbr. 15C5) and 12-crown-4 ether (abbr. 12C4) were purchased from Tokyo Kasei Kogyo and dehydrated by deuterium molecular sieves 3A (Euriso-top). The absence of oxidative impurity was confirmed as mentioned previously [10]. Deuterated nitromethane for NMR measurements was obtained from Aldrich Chemical and used after dehydrated with deuterium molecular sieves 3A. Other chemicals were reagent grade and used without further purification. 2.2. Measurements Absorption spectra and absorbances were measured by a Hitachi 228 spectrophotometer. Measurements of 1H NMR spectra were performed at 400 MHz using a Varian Unity Inova 400 WB NMR spectrometer. The chemical shifts were obtained using tetramethylsilane as an internal standard. A BAS 100W/B electrochemical workstation was used to record the cyclic voltammograms. Voltammograms were observed in 0.10 mol dm3 tetrabutylammonium hexafluorophosphate acetonitrile solution, using a three-electrode assembly, an Ag/AgNO3 reference electrode, a glassy carbon working electrode, and a platinum coil auxiliary electrode. 3. Results and discussion Pentaammineruthenium complexes form an adduct with crown ethers through hydrogen bonding between ammine ligand of the complex and ether oxygen of the crown ether [16]. The complexes, [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2, contain protic
ligands of different types, ammine and imH or hpy, within a complex and the ligands, imH and hpy, are more acidic than the ammine [17,19]. Accordingly, it is possible that these complexes may interact with crown ether to form hydrogen bonds either at protic ligands of both types or preferentially at a more acidic ligand, depending on ring size of crown ether. Adduct formation was spectrophotometrically examined for [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2 with crown ethers of different ring size. [Ru(NH3)5(imH)](PF6)3 shows two LMCT (transition from the imidazole HOMO to unfilled t2g orbital of ruthenium(III)) bands at 304 and 438 nm [17], and [Ru(NH3)5(hpy)](PF6)2 does a MLCT (transition from filled orbital t2g of ruthenium(II) to the 4-hydroxypyridine LUMO) band at 365 nm in acetonitrile [19]. The LMCT bands of [Ru(NH3)5(imH)](PF6)3 exhibited a blue shift and the MLCT band of [Ru(NH3)5(hpy)](PF6)2 did a red shift on adding crown ether. The magnitude of their shifts is summarized in Table 1. Fig. 1 shows the typical spectral change of [Ru(NH3)5(imH)](PF6)3 on adding of 18C6 in acetonitrile. On addition of crown ether, these LMCT bands continuously shifted toward shorter wavelength with increasing crown ether concentration; exhibiting a set of isosbestic points, as shown in Fig. 1. These blue shift of LMCT band indicates qualitatively that the ammine complex forms hydrogen bonds with crown ether at the ammine ligands coordinating to ruthenium(III) [16,21]. When twenty fold excess of crown ether was added to the acetonitrile solution of the complex, the magnitude of their shifts is the order of 18C6 > 15C5 > 12C4. For [Ru(NH3)5(hpy)](PF6)2, on the other hand, a red shift of MLCT band was observed on adding two hundred fold excess of 18C6 but not on addition of 15C5 or 12C4. This red shift of MLCT band also indicates hydrogen bonding between ammine ligand coordinating to ruthenium(II) and ether oxygens of crown ether [16,21]. The magnitude of shifts for these CT band may vary depending the ring size of crown ether. The hydrogen bonding site with crown ether for a ruthenium–ammine complex can be judged from the direction of the shift of these CT band [16,21]. The complexes in the present study is possible to form hydrogen bonds at two different sites because of containing two kinds of site for hydrogen bonding with crown ether; ammine and aromatic ligands such as imH or hpy. Hydrogen bonding with crown ethers gave the shift of CT band of the complex but the direction of the shift becomes reverse on hydrogen bonding at an ammine and at an aromatic ligand [16,21]. The observed shift of the CT band reflects the difference in shifts between hydrogen bonding at ammine site and imH or hpy site. For [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2, thus, it is obscured either to form an adduct through hydrogen bonding at ammine or at both ammine and imH or hpy ligands. and Cyclic voltammograms of [Ru(NH3)5(imH)](PF6)3 [Ru(NH3)5(hpy)](PF6)2 were measured in the absence and presence of crown ethers in acetonitrile solution and shown in Fig. 2. The voltammograms showed a reversible redox couple associated with metal center at 0.150 and 0.051 V versus (Ag+/Ag) for [Ru(N-
Table 1 The values of kmax of CT bands for [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2 in the absence and presence of crown ethers in acetonitrile. Complex
Crown ether
[Ru(NH3)5(imH)](PF6)3
18C6 15C5 12C4 18C6 15C5 12C4
[Ru(NH3)5(hpy)](PF6)2
R = [crown]/[Complex].
kmax/nm (vmax/103 cm1)
Dvmax/cm1
R=0
R = 20
438(22.8) 438(22.8) 438(22.8) 365(27.4) 365(27.4) 365(27.4)
422(23.7) 428(23.4) 438(22.8)
R = 200
428(23.4) 371(27.0) 365(27.4) 365(27.4)
900 600 600 400 0 0
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Fig. 1. Spectral change of [Ru(NH3)5(imH)](PF6)3 on addition of 18C6 in acetonitrile. Concentration of the complex is 1.25 103 mol dm3, and concentrations of 18C6 are 0 (1), 1.25 103 (2), 1.25 102 (3), and 2.5 102 mol dm3.
Fig. 3. Dependences of DE1/2 for (a) [Ru(NH3)5(imH)](PF6)3 and (b) [Ru(NH3)5(hpy)](PF6)2 on crown ether concentration of 12C4(4), 15C5(h), and 18C6(s) in acetonitrile [complex] = 5.0 104 mol dm3.
Table 2 Limiting values of change in redox potential, DE1/2(lim), for the Ru(III)–Ru(II) redox couple in [Ru(NH3)5(L)](PF6)n caused by addition of crown ether in acetonitrile. Crown ether
DE1/2(lim)/mV L = pya
imH
hpy
18C6 15C5 12C4
(129 ± 3) (128 ± 3) >30b
(129 ± 5) (153 ± 12) (58 ± 3)
(130 ± 4) (134 ± 12) (33 ± 7)
a b
Fig. 2. Cyclic voltammograms of (a) [Ru(NH3)5(imH)](PF6)3 and (b) [Ru(NH3)5(hpy)](PF6)2 in the absence and presence of 0.10 mol dm3 crown ether in acetonitrile.
Data in Ref. [16]. The value was DE1/2 on addition of 200-fold excess of crown ether.
voltammetric features were similar to those of the pentaammineruthenium complexes and indicates formation of hydrogen bonds between the ammines coordinating to ruthenium and the ether oxygens of crown ethers [16]. The limiting change in redox potential, DE1/2(lim), was evaluated from the dependences in Fig. 3 by the least square analysis of Eq. (1), [11] and listed in Table 2 together with those of
DE1=2 ¼ DE1=2 ðlimÞ þ A expðB½crown etherÞ H3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2, respectively. The voltammograms shifted continuously toward a negative potential direction depending on crown ethers. The voltammograms were measured at various concentrations of crown ethers. The redox potentials shifted continuously toward negative direction with increasing crown ether concentrations. Fig. 3 shows dependences of change in redox potential on crown ether concentration for [Ru(NH3)5(imH)](PF6)3 and [Ru(NH3)5(hpy)](PF6)2. These
ð1Þ
[Ru(NH3)5(py)](PF6)2 (py = pyridine) for comparison. For the typical pentaammineruthenium complex, [Ru(NH3)5(py)](PF6)2, an absolute value of DE1/2(lim) is ca. 130 mV in the case of 18C6 and 15C5 but is much smaller value in the case of 12C4. [Ru(NH3)5 (hpy)](PF6)2 showed comparable value to those of [Ru(NH3)5 (py)](PF6)2. However, [Ru(NH3)5(imH)](PF6)3 gave somewhat larger absolute values of DE1/2(lim) than those of [Ru(NH3)5(py)](PF6)2 by addition of 15C5 and 12C4. This may intimate that [Ru(NH3)5
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Fig. 5. Dependences of chemical shifts of cis-NH3 protons(h), trans-NH3 protons (s), and imidazol N-H proton(4) for [Ru(NH3)5(imH)](PF6)3 on 18C6 concentrations in the range of 0–0.01 mol dm3 [complex] = 5.0 103 mol dm3.
Fig. 4. Dependences of chemical shifts of imidazol N–H proton (a), cis-NH3 protons (b), and trans-NH3 protons (c), for [Ru(NH3)5(imH)](PF6)3 on crown ether concentrations in nitromethane. Symbols are the data for 12C4(4), 15C5(h), and 18C6(s) [complex] = 5.0 103 mol dm3.
(imH)](PF6)3 forms hydrogen bond at imidazole ligand in addition of hydrogen bonding at ammine ligand. Adduct formation was followed in deuterated nitromethane by 1 H NMR spectrometry for [Ru(NH3)5(imH)](PF6)3 – crown ether systems. 1H NMR spectrum of [Ru(NH3)5(imH)](PF6)3 was obtained in fairly low field region by 6000 times accumulation. Relatively broad signals were observed at 124, 233, and 274 ppm with integral ratios of 12.6, 3.0, and 1.4, respectively, and may be assigned to cis-ammine protons, trans-ammine protons, and N–H proton of imidazole according to their integral ratios. The chemical shift of these proton signals were examined in detail at various concentrations of crown ethers.
Fig. 4 shows the dependences of the chemical shift on crown ether concentration. By addition of 12C4 or 15C5, the signals of imidazole N–H proton and cis-ammine protons monotonously shifted toward low field with increasing crown ether concentration to extent depending on ring size of crown ether. However, the signal of trans-ammine protons shifted toward high-field with increasing crown ether concentration. The shift of imidazole N–H proton reached maximum at crown ether concentration of 0.02 mol dm3, nevertheless the signal of cis-ammine protons still shifted toward low field at concentrations more than 0.02 mol dm3. This behavior indicates that [Ru(NH3)5(imH)](PF6)3 forms hydrogen bonds with 12C4 and 15C5 simultaneously at both the coordinated imidazole and the cis-ammines, or successively forms hydrogen bonds at the coordinated imidazole and the cisammines. The high-field shift of the signal of the trans-ammine protons may be due to desolvation caused by crowding around trans-ammine accompanied with hydrogen bond formation at two cis-ammines. On the other hand, the behavior on addition of 18C6 is roughly similar to that on addition of 12C4 or 15C5 but exhibits significant difference in the lower concentration range of crown ether. Fig. 5 shows the enlarged dependences of chemical shift of imidazole N–H proton, cis-ammine protons, and trans-ammine protons on 18C6 concentration at lower range. Although the signal of imidazole N–H proton also exhibited a monotonous dependence on 18C6 concentration, the signals of cis- and trans-ammine protons did intricate dependences. The signal of trans-ammine protons did not shift up to the 18C6 concentration of 2 103 mol dm3 and thereafter shifted toward low field. Moreover, the signal started to shift in reverse direction at the 18C6 concentration more than
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stood in the respects of acidity of coordinating imidazole and interaction site of Ru(III)–ammine complexes with crown ether [10,17]. On the other hand, 18C6 exhibits stepwise adduct formation through hydrogen bonding at imidazole, trans-ammine, and cisammine site of the complex unlike 15C5 and 12C4. This implies that adduct formation is affected by both the acidity of a protic ligand and the steric fitness between a type of protic ligand and crown ethers. References
Scheme 2. Successive adduct formation.
5 103 mol dm3. At that 18C6 concentration, on the other hand, the signal of cis-ammine protons started to shift toward low field although the signal shifted little up to the 18C6 concentration of 5 103 mol dm3. These changes of chemical shift of the ligand proton signals indicate that [Ru(NH3)5(imH)](PF6)3 forms an adduct with 18C6 through successive formation of hydrogen bonds between the protons of the coordinating ligands and the ether oxygen of 18C6 as shown in Scheme 2; 18C6 initially binds to the complex at imidazole N–H proton, moreover binds at trans-ammine protons, and thereafter the hydrogen bonds at trans-ammine protons is gradually replaced by binding at cis-ammine protons with increasing 18C6 concentration. The signal of imidazole N–H proton observed at most low field, 274 ppm, implies that the imidazole N– H proton is most acidic of all protic ligands of the complex. This supports the behavior for the successive adduct formation. In conclusion, adduct formation of [Ru(NH3)5(imH)](PF6)3 with 18C6 exhibits somewhat different behavior from that with 15C5 and 12C4. Although distinct site-selectivity was not observed in adduct formation of [Ru(NH3)5(imH)](PF6)3 with 15C5 and 12C4, the behavior of adduct formation with 15C5 and 12C4 is under-
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