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Effect of second-sphere coordination 13. Consideration of factors affecting adduct formation of ruthenium–ammine complexes with crown ethers based on the stability constants
Inorganica Chimica Acta 411 (2014) 56–60
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Effect of second-sphere coordination 13. Consideration of factors affecting adduct formation of ruthenium–ammine complexes with crown ethers based on the stability constants Isao Ando ⇑, Hitoshi Katae, Masashi Okamura 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 16 July 2013 Received in revised form 15 October 2013 Accepted 10 November 2013 Available online 27 November 2013 Keywords: Ruthenium complex Stability of adduct Flexibility of crown ether Strength of hydrogen bond p-Electron acceptability of aromatic ligand
a b s t r a c t Adduct formation was systematically investigated for ruthenium–ammine complex and crown ether systems. This study involved crown ether systems with different flexibility, the complex system with different numbers of ammine ligands, and the pentaammine complexes systems with aromatic ligands with different p-electron acceptability. Stability constants of the crown-ether adduct of the complexes were determined for the above systems by 1H NMR spectroscopy. The factors affecting adduct formation were discussed on basis of the stability constant. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Weak interaction, especially hydrogen bonding, may play an important role in a wide variety of areas. A variety of transition metal complexes with a protic ligand form hydrogen bonds with organic substrates at the second sphere of a complex [1]. Such second-sphere coordination brings about a perturbation in the electronic state of a complex and hence modifies the properties of the complex. Thus, the interaction may become interesting phenomena to design new function of a complex. Crown ether is an excellent second-sphere ligand for forming adducts with transition metal complexes carrying protic ligands in their first coordination sphere. We have been investigating the second sphere coordination of crown ethers to ruthenium–ammine complexes [2]. The second sphere coordination brings about prominent changes in the redox potential of the complex. The magnitude of change in the redox potential was affected by some factors. However, the relationship between this change in the redox potential and the stability of the crown-ether adduct is not yet clear. Hupp and co-workers have investigated the second sphere coordination of large aromatic crown ether to ruthenium complexes [3]. They have evaluated the stability constants of the crown ether adduct from the spectral changes of MMCT or MLCT band on ⇑ Corresponding author. Tel.: +81 928716631. E-mail address: [email protected] (I. Ando). 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.11.006
adduct formation. It is difficult to evaluate the stability constant of crown ether adduct with ruthenium–ammine complexes for systems of crown ether with small ring-size from the spectral change owing to their small change. We found that 1H NMR spectral measurement were required to effectively evaluate the stability constant of the crown-ether adduct [4]. In this study, adduct formation was systematically investigated by 1H NMR spectroscopy for systems with small ring-size crown ethers with and ruthenium–ammine complexes.
2. Experimental 2.1. Materials Ruthenium–ammine complexes were prepared according to literature methods or the analogous methods and characterized by spectrophotometry, comparing of kmax and emax values [5]. The aromatic ligands used were 2,20 ,200 -terpyridine (trpy), 2,20 -bipyridine (bpy), N-methyl-4,40 -bipyridinium (Me-4,40 -bpy), pyridine (py), 2-cyanopyridine (2-cpy), N-methyl-2-cyanopyridinium (Me-2-cpy), 3-cyanopyridine (3-cpy), N-methyl-3-cyanopyridinium (Me-3-cpy), N-methyl-4-cyanopyridinium (Me-4-cpy), pyrazine (pz), N-methylpyrazinium (Mepz), isonicotinamide (isn), benzonitrile (bn), and 4-dimethyaminobenzonitrile (dmabn). 18-Crown-6 ether (18C6), 18-thiacrown-6 ether (18S6), dibenzo-18-crown-6 ether (DB18C6), dibenzo-24-crown-8 ether (DB24C8), and dibenzo-30-crown-10 ether (DB30C10) were puri-
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fied by the literature methods [6]. 12-Crown-4 ether (12C4) and 15-crown-5 ether (15C5) were dehydrated using deuterium molecular sieves 3A (Euriso-top). The absence of oxidative impurities was confirmed for all crown ethers, as previously mentioned [7]. 2.2. Measurements Absorption spectra were recorded on a Shimadzu UV-3600 UV–Vis–near-infrared spectrophotometer. Measurements of 1H NMR spectra were performed at 400 MHz using a Varian Unity Inova 400WB NMR spectrometer and the chemical shifts were obtained using a signal of non-deuterated nitromethane as a standard. Electrochemical measurements were performed by means of a BAS 100B/W electrochemical workstation using a three-electrode assembly, with an Ag/AgNO3 reference electrode, a glassy carbon working electrode, and a platinum auxiliary electrode. Redox potentials were obtained from cyclic voltammograms in 0.10 mol dm3 tetrabutylammonium hexafluorophosphate acetonitrile solution. The p-electron acceptability of an aromatic ligand, PL, was calculated from the redox potentials of the complexes using the equation below [8]
PL ¼ E1=2 f½RuðNH3 Þ5 LðPF6 Þ2 g E1=2 f½RuðNH3 Þ5 ðCH3 CNÞ ðPF6 Þ2 g
ð1Þ
The change in redox potential of the complex caused by adduct formation, |DE1/2(lim)|, was determined from the dependence of E1/2 on crown ether concentrations as previously reported [8]. 2.3. Evaluation of stability constants of crown ether adduct [4] The stability constant was evaluated as follows. The chemical shift of the ammine proton of ruthenium–ammine complexes was obtained in the presence of crown ether with various concentrations in deuterated nitromethane. When a rapid equilibrium of adduct-formation is holding between a ruthenium–ammine complex and crown ether, the apparent chemical shift of the ammine proton, dapp, can be expressed using the stability constant of the adduct, K, as follows
dapp ¼
dML dadd þ dadd K½C þ 1
ð2Þ
where dML, dadd, and C represent the chemical shifts of the ruthenium–ammine complex and the adduct, and the concentration of crown ether, respectively. Stability constants of the adduct were obtained from the dependence of dapp on the concentration of crown ether by the least square analysis of the above equation. 3. Results and discussion 1 H NMR spectra of [Ru(NH3)5py](PF6)2 were measured in deuterated nitromethane in both the absence and presence of 18C6. Signals were observed in the absence of 18C6 at 2.19, 2.62, 7.38, 7.81, and 8.51 ppm, which were attributed to cis-ammine protons, trans-ammine protons, and aromatic protons of the 2,4-,3-, and 1,5-position of pyridine, respectively [4]. The signals of cis- and trans-ammine protons were shifted toward lower fields by the addition of 18C6. The down-field shifts were determined in detailed at various 18C6 concentrations and the results are shown in Fig. 1. It can be seen that the shift of the signal of the trans-ammine protons is greater than that of the cis-ammine protons. This implies that the complex forms adducts with 18C6 through hydrogen bonding mainly at trans-ammine and additionally at cis-ammine. Thus, the stability constant of the adduct was obtained from the dependencies of dapp of both trans- and cis-ammine protons on the 18C6 concentration by the least squares analysis of
Fig. 1. Dependences of chemical shift of trans-ammine protons (solid circles) and cis-ammine protons (open circles) for [Ru(NH3)5py](PF6)2 on 18C6 concentration in CD3NO2. Solid lines represent the regression lines of the equation in Section 2. [complex] = 5.0 104 mol dm3.
Eq. (2) in Section 2. Table 1 summarizes the stability constants and other parameters. The obtained stability constants determined from the trans- and cis-ammine data agreed within experimental uncertainty. Thus, the stability constants were determined from the dependences of dapp of more acidic ammine protons (the signal at lower field). Adduct formation was investigated for [Ru(NH3)5py](PF6)2 and crown ethers with various ring sizes. The stability constants are summarized together with the change in redox potential caused by adduct formation, |DE1/2(lim)| in Table 2. For a homologous series of crown ethers, the stability of the adduct increased with increasing ring size of the crown ether; 12C4 < 18C6, DC18C6 DC24C8, DB18C6 < DB24C8 < DB30C10. The stability constant of the DB30C10 adduct agreed with that presented by Hupp and co-workers [3b]. Furthermore, for crown ethers with the same ring size, the stability of the adduct was depressed by the introduction of phenyl groups; 18C6 DC18C6 > B18C6 > DB18C6. This trend corresponds to the effect on |DE1/2(lim)| and indicates that the flexibility of crown ether ring influences the stability of their adduct [8]. Thiacrown ether could not form an adduct with the ammine-complex due to the large electronegativity of the sulfur atom, in spite of its flexibility. Fig. 2 shows the approximate linear relations of ln K and |DE1/2(lim)| to Dd. The Dd value represents the strength of hydrogen bonding between ether oxygen and the coordinating ammine. The strength of hydrogen bonding influences the stability of the adduct and the change in redox potential, although there is a poor correlation between the stability and the change in redox potential.
Table 1 Stability constants of the18C6 adduct and other parameters for [Ru(NH3)5py](PF6)2 obtained from the dependencies of the chemical shift of trans-ammine and cisammine protons on the 18C6 concentration in CD3NO2.
K (mol1 dm3) dML (ppm) dadd (ppm) Dd (ppm)
trans-Ammine
cis-Ammine
(6.58 ± 1.03) 10 2.60 ± 0.01 2.93 ± 0.02 0.33
(7.58 ± 2.34) 10 2.17 ± 0.00 2.27 ± 0.01 0.10
Dd = dadd dML. Error limits are the standard deviation.
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Table 2 Stability constants of crown ether adducts for [Ru(NH3)5py](PF6)2, other parameters in CD3NO2, and the change in redox potential caused by adduct formation. Crown ether
K (mol1 dm3)
dadd (ppm)
Dd (ppm)
|DE1/2(lim)| (mV)a
12C4 18C6 18S6 DC18C6 DC24C8 B18C6 DB18C6 DB24C8 DB30C10
1.92 ± 6.17 (6.58 ± 1.03) 10 0 (8.36 ± 3.06) 10 (8.30 ± 1.01) 10 (2.22 ± 0.73) 10 0 (6.09 ± 1.40) 10 (6.58 ± 0.48) 102
2.94 ± 0.86 2.93 ± 0.02 2.70 ± 0.11 2.99 ± 0.07 2.91 ± 0.01 2.84 ± 0.04 2.64 ± 0.00 2.73 ± 0.01 3.10 ± 0.01
0.31 0.33 0.07 0.38 0.29 0.21 0 0.10 0.47
>30 129 – 87 115 92 15 29 143
Dd = dadd dML. Error limits are the standard deviation. a Taken from Ref. [8].
Fig. 2. Relations of ln K (open circles) and |DE1/2(lim)| (solid circles) to Dd for a system of [Ru(NH3)5py](PF6)2 and crown ethers with various ring sizes. The values of |DE1/2(lim)| were taken from Ref. [8].
The stability constants of 18C6 adduct were determined for [Ru(NH3)n(L)(L0 )](PF6)2 {n = 1–6, L, L0 = pyridine or polypyridine}.
These results are listed in Table 3 together with the change in redox potential caused by adduct formation, |DE1/2(lim)|. The dependence of the stability constants on Dd, the number of ammine ligands in the complex, and, |DE1/2(lim)| were examined as shown in Fig. 3. The strength of hydrogen bonding may affect both the stability of the adduct and |DE1/2(lim)|. Fig. 3a shows an approximately linear relationship between ln K and Dd, except for [Ru(NH3)4(bpy)](PF6)2, although there is no correlation between |DE1/2(lim)| and Dd. A nonlinear relationship was observed between ln K and the number of ammine ligands in Fig. 3b, although a linear relationship was reported between |DE1/2(lim)| and the number of ammine ligands [8]. The stability constant increases with increasing number of ammine ligands for the monoammineand diammine-complexes but decreases with increasing number of ammine ligands for the complexes with more than two ammine ligands, except for [Ru(NH3)4(bpy)](PF6)2 and trans-[Ru(NH3)4(py)2](PF6)2. This trend is explained as follows: the number of possible hydrogen bonds increases with increasing number of ammine ligands in the complex, while the extent of p-electron acceptability of aromatic ligands in the complex decreases with increasing number of ammine ligands. Thus, the above correlation indicates that the two factors had an opposing influence on the stability of the adduct. For the tetraammine-complexes, the greater p-electron acceptability of 2,20 -bipyridine than that of two molecules of pyridine enhanced the stability of adduct, while the trans-position of pyridines and/or ammines depressed the stability of adduct. The p-electron acceptability of the bpy ligand is significantly reflected
Table 3 Stability constants of 18C6 adduct for [Ru(NH3)xL](PF6)y, other parameters in CD3NO2, and the change in redox potential caused by adduct formation in CH3CN. Complex
K (mol1 dm3)
Dd (ppm)
|DE1/2(lim)| (mV)
[Ru(NH3)(trpy)(bpy)](PF6)2 cis-[Ru(NH3)2(bpy)2](PF6)2 mer-[Ru(NH3)3(trpy)](PF6)2 [Ru(NH3)4(bpy)](PF6)2 cis-[Ru(NH3)4(py)2](PF6)2 trans-[Ru(NH3)4(py)2](PF6)2 [Ru(NH3)5(py)](PF6)2 [Ru(NH3)6](PF6)2 [Ru(NH3)5(pz)](PF6)2 [Ru(NH3)5(Mepz)](PF6)3 [Ru(NH3)5(Me-4,40 -bpy)](PF6)3 [Ru(NH3)5(isn)](PF6)2 [Ru(NH3)5(dmabn)](PF6)2 [Ru(NH3)5(bn)](PF6)2 [Ru(NH3)5(2-cpy)](PF6)2 [Ru(NH3)5(Me-2-cpy)](PF6)3 [Ru(NH3)5(3-cpy)](PF6)2 [Ru(NH3)5(Me-3-cpy)](PF6)3 [Ru(NH3)5(Me-4-cpy)](PF6)3
(6.68 ± 0.60) 10 (8.93 ± 0.42) 10 (8.64 ± 1.84) 10 (2.05 ± 0.26) 102 (7.83 ± 0.83) 10 (2.89 ± 0.40) 10 (6.58 ± 1.03) 10 (3.31 ± 0.72) 10 (8.46 ± 0.38) 10 (3.73 ± 0.31) 102 (1.04 ± 0.05) 102 (7.27 ± 0.42) 10 (1.08 ± 0.06) 102 (1.16 ± 0.09) 102 (1.69 ± 0.08) 102 (1.71 ± 0.17) 102 (1.68 ± 0.08) 102 (1.32 ± 0.09) 102 (1.90 ± 0.10) 102
0.29 0.28 0.35 0.19 0.25 0.08 0.33 0.13 0.44 0.72 0.43 0.36 0.21 0.26 0.26 0.56 0.27 0.40 0.41
32a 57a 70a 122a 99a 91a 129a 169a 131a 105a 131b 133a 173a 183b 171b 110b 189b 134b 129b
Dd = dadd dML. Error limits are the standard deviation. a Taken from Ref. [8]. b Determined in this study.
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I. Ando et al. / Inorganica Chimica Acta 411 (2014) 56–60 Table 4 The values of PL for aromatic ligands of [Ru(NH3)5L](PF6)n (n = 2 or 3).
a
La
PL (mV)
py pz Mepz Me-4,40 -bpy isn dmabn bn 2-cpy Me-2-cpy 3-cpy Me-3-cpy Me-4-cpy
0.113 0.064 0.488 0.016 0.077 0.027 0.590 0.134 0.416 0.104 0.231 0.292
Ref. Ref. Ref. this Ref. Ref. this this this this this this
[8] [8] [8] work [8] [8] work work work work work work
Abbreviations of ligands are described in Section 2.
Fig. 4. Relations of the stability constants of 18C6 adduct to (a) the p-electron acceptability of aromatic ligand, PL, and (b) the strength of hydrogen bonding, Dd, for pentaammineruthenium(II) complexes involving aromatic ligand with various p-electron acceptability. Open and solid circles represent the data of the complexes of cyanopyridine derivative and pyridine derivative groups, respectively.
Fig. 3. Relations of the stability constants of 18C6 adduct to (a) Dd , (b) the number of the ammine ligands of the complex, and (c) |DE1/2(lim)| for ruthenium(II)– ammine complexes containing only pyridine moiety with different number of ammine ligand. The values of |DE1/2(lim)| were taken from Ref. [8].
on the stability of adduct. As shown in Fig. 3c, there is a similar correlation to Fig. 3b between ln K and |DE1/2(lim)|. The stability constants of 18C6 adduct were determined for pentaammineruthenium(II) complexes with ligands of various p-electron acceptability. The results are also listed together with the values of |DE1/2(lim)| in Table 3. The measure of p-electron acceptability of aromatic ligands, PL, was obtained from the redox potential of the complexes using the equation in Section 2.2 and those values are listed in Table 4.
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the complex. The p-electron acceptability and the strength of hydrogen bonding enhance the stability of adduct. The effect on the stability is greater in the complexes with pyridine group than in the complexes with cyanopyridine derivative group. The correlation between |DE1/2(lim)| and PL was previously reported for the complexes with pyridine derivative groups [8], similar behavior was obtained here as shown in Fig. 6. The p-electron acceptability and the strength of hydrogen bonding enhanced the stability of adduct but the p-electron acceptability depresses the change in redox potential caused by the adduct formation. 4. Conclusion
Fig. 5. Correlation between the strength of hydrogen bonding in the 18C6 adduct and the p-electron acceptability of aromatic ligand in the complex for pentaammineruthenium(II) complexes in Table 3. Open and solid circles represent the data of the complexes of cyanopyridine derivative and pyridine derivative groups, respectively.
The stability constant of crown ether adduct is difficult to evaluate for crown ether systems with small ring-sizes, as the change in the specra upon adduct formation is only small. In this study, the stability constants of adduct could be accurately evaluated for crown ethers with a small ring-size and ruthenium–ammine complexes by 1H NMR spectroscopy. The following factors were observed to affect adduct formation of crown ether to ruthenium–ammine complexes from examinations based on the stability constant: (i) the flexibility of the crown ether ring, (ii) the number of possible hydrogen bonds, (iii) the number of ammine ligands in the complex, (iv) the p-electron acceptability of ancillary ligands, (v) the strength of hydrogen bonding. It was confirmed that these factors act to enhance the stability of the adduct but factor (iv) decreased the change in redox potential caused by adduct formation. The effects of PL and Dd on the stability are significantly different depending on the type of ancillary ligand in the complex. The effect of PL on Dd was different for each group. This could be ascribed to coordination of the CN-group with strong electronwithdrawing ability however the reason is not yet clear. References
Fig. 6. Correlation between the change in redox potential caused by adduct formation and the p-electron acceptability of aromatic ligand in the complex for pentaammineruthenium(II) complexes in Table 3. Open and solid circles represent the data of the complexes of cyanopyridine derivative and pyridine derivative groups, respectively.
The effect on the stability of the 18C6 adducts was examined about the strength of hydrogen bonding and the p-electron acceptability of aromatic ligand. The results are shown in Fig. 4. The stability of the 18C6 adducts roughly increases with increasing both the strength of hydrogen bonding in the adduct and the p-electron acceptability of the aromatic ligand in the complex, but the obtained correlations were poor for all the complexes investigated in this study. It is apparent that both factors affect the stability of the 18C6 adduct. The plots of Dd against PL in Fig. 5 show good linearity. This indicates that the complexes are classified into two groups, depending on the ancillary ligand L; pyridine derivative or cyanopyridine derivative groups. The plots of ln K against PL or Dd in Fig. 4a and b, respectively, show a good linear relationship in the individual complex groups. The larger p-acceptability of the aromatic ligand makes hydrogen bonding stronger in the adduct and the magunitude of the effect is somewhat different on
[1] (a) H.M. Colquhoun, J.F. Stoddart, D.J. Williams, Angew. Chem. Int. Ed. Engl. 25 (1986) 487; (b) M.T. Ballardini, L. Gandolfi, M. Prodi, V. Ciano, F.H. Balzani, H. Kohnke, N. Shahriari-Zavareh, J.F. Spencer, Stoddart, J. Am. Chem. Soc. 111 (1989) 7072; (c) J.C. Curtis, J.A. Roberts, R.L. Blackbourn, Y. Dong, M. Massum, C.S. Johnson, J.T. Hupp, Inorg. Chem. 30 (1991) 3856; (d) A. Drljaca, D.C.R. Hockless, B. Moubaraki, K.S. Murray, L. Spicia, Inorg. Chem. 36 (1997) 1988; (e) D.I. Yoon, S. Belanger, J.T. Hupp, C.L. Stern, Acta Crystallogr., Sect. C 54 (1998) 1427; (f) M. Schafer, Angew. Chem. Int. Ed. 42 (2003) 1896. [2] There are references in I. Ando, Coord. Chem. Rev. 248 (2004) 185. [3] (a) M.D. Todd, Y. Dong, J.T. Hupp, Inorg. Chem. 30 (1991) 4685; (b) M.D. Todd, Y. Dong, J. Horney, D.I. Yoon, J.T. Hupp, Inorg. Chem. 32 (1993) 2001; (c) X.L. Zhang, C.R. Kankel, J.T. Hupp, Inorg. Chem. 33 (1994) 4738; (d) X.L. Zhang, D.I. Yoon, J.T. Hupp, Inorg. Chim. Acta 240 (1995) 285. [4] I. Ando, K. Nishihara, K. Ujimoto, H. Kurihara, Inorg. Chim. Acta 346 (2003) 19. [5] (a) R.E. Clarke, P.C. Ford, Inorg. Chem. 9 (1970) 227; (b) R.G. Gaunder, H. Taube, Inorg. Chem. 9 (1970) 2627; (c) G.E. Bryant, J.E. Fergusson, H.K.J. Powell, Aust. J. Chem. 24 (1971) 257; (d) J.E. Fergusson, J.L. Love, Inorg. Synth. 13 (1972) 208; (e) C. Creutz, H. Taube, J. Am. Chem. Soc. 95 (1973) 1086; (f) G.M. Brown, N. Sutin, J. Am. Chem. Soc. 101 (1979) 883; (g) B.P. Sullivan, J.M. Calvert, T.J. Meyer, Inorg. Chem. 19 (1980) 1404; (h) J.C. Curtis, B.P. Sullivan, T.J. Meyer, Inorg. Chem. 22 (1983) 224; (i) Z.N. Da Rocha, E. Tfouni, Polyhedron 11 (1992) 2375. [6] (a) G.W. Gokel, D.J. Cram, J. Org. Chem. 39 (1974) 2445; (b) L.A. Ochrymowycz, C.P. Mak, J.D. Michna, J. Org. Chem. 39 (1974) 2079; (c) C.J. Pedersen, Org. Synth. 52 (1972) 66; (d) C.J. Pedersen, J. Am. Chem. Soc. 89 (1967) 7017; (e) M. Shamsipur, A.I. Popov, J. Am. Chem. Soc. 101 (1979) 4051. [7] I. Ando, D. Ishimura, K. Ujimoto, H. Kurihara, Inorg. Chem. 33 (1994) 5010. [8] I. Ando, D. Ishimura, K. Ujimoto, H. Kurihara, Inorg. Chem. 35 (1996) 3504.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Effect of second-sphere coordination 13. Consideration of factors affecting adduct formation of ruthenium–ammine complexes with crown ethers based on the stability constants Isao Ando ⇑, Hitoshi Katae, Masashi Okamura 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 16 July 2013 Received in revised form 15 October 2013 Accepted 10 November 2013 Available online 27 November 2013 Keywords: Ruthenium complex Stability of adduct Flexibility of crown ether Strength of hydrogen bond p-Electron acceptability of aromatic ligand
a b s t r a c t Adduct formation was systematically investigated for ruthenium–ammine complex and crown ether systems. This study involved crown ether systems with different flexibility, the complex system with different numbers of ammine ligands, and the pentaammine complexes systems with aromatic ligands with different p-electron acceptability. Stability constants of the crown-ether adduct of the complexes were determined for the above systems by 1H NMR spectroscopy. The factors affecting adduct formation were discussed on basis of the stability constant. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Weak interaction, especially hydrogen bonding, may play an important role in a wide variety of areas. A variety of transition metal complexes with a protic ligand form hydrogen bonds with organic substrates at the second sphere of a complex [1]. Such second-sphere coordination brings about a perturbation in the electronic state of a complex and hence modifies the properties of the complex. Thus, the interaction may become interesting phenomena to design new function of a complex. Crown ether is an excellent second-sphere ligand for forming adducts with transition metal complexes carrying protic ligands in their first coordination sphere. We have been investigating the second sphere coordination of crown ethers to ruthenium–ammine complexes [2]. The second sphere coordination brings about prominent changes in the redox potential of the complex. The magnitude of change in the redox potential was affected by some factors. However, the relationship between this change in the redox potential and the stability of the crown-ether adduct is not yet clear. Hupp and co-workers have investigated the second sphere coordination of large aromatic crown ether to ruthenium complexes [3]. They have evaluated the stability constants of the crown ether adduct from the spectral changes of MMCT or MLCT band on ⇑ Corresponding author. Tel.: +81 928716631. E-mail address: [email protected] (I. Ando). 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.11.006
adduct formation. It is difficult to evaluate the stability constant of crown ether adduct with ruthenium–ammine complexes for systems of crown ether with small ring-size from the spectral change owing to their small change. We found that 1H NMR spectral measurement were required to effectively evaluate the stability constant of the crown-ether adduct [4]. In this study, adduct formation was systematically investigated by 1H NMR spectroscopy for systems with small ring-size crown ethers with and ruthenium–ammine complexes.
2. Experimental 2.1. Materials Ruthenium–ammine complexes were prepared according to literature methods or the analogous methods and characterized by spectrophotometry, comparing of kmax and emax values [5]. The aromatic ligands used were 2,20 ,200 -terpyridine (trpy), 2,20 -bipyridine (bpy), N-methyl-4,40 -bipyridinium (Me-4,40 -bpy), pyridine (py), 2-cyanopyridine (2-cpy), N-methyl-2-cyanopyridinium (Me-2-cpy), 3-cyanopyridine (3-cpy), N-methyl-3-cyanopyridinium (Me-3-cpy), N-methyl-4-cyanopyridinium (Me-4-cpy), pyrazine (pz), N-methylpyrazinium (Mepz), isonicotinamide (isn), benzonitrile (bn), and 4-dimethyaminobenzonitrile (dmabn). 18-Crown-6 ether (18C6), 18-thiacrown-6 ether (18S6), dibenzo-18-crown-6 ether (DB18C6), dibenzo-24-crown-8 ether (DB24C8), and dibenzo-30-crown-10 ether (DB30C10) were puri-
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fied by the literature methods [6]. 12-Crown-4 ether (12C4) and 15-crown-5 ether (15C5) were dehydrated using deuterium molecular sieves 3A (Euriso-top). The absence of oxidative impurities was confirmed for all crown ethers, as previously mentioned [7]. 2.2. Measurements Absorption spectra were recorded on a Shimadzu UV-3600 UV–Vis–near-infrared spectrophotometer. Measurements of 1H NMR spectra were performed at 400 MHz using a Varian Unity Inova 400WB NMR spectrometer and the chemical shifts were obtained using a signal of non-deuterated nitromethane as a standard. Electrochemical measurements were performed by means of a BAS 100B/W electrochemical workstation using a three-electrode assembly, with an Ag/AgNO3 reference electrode, a glassy carbon working electrode, and a platinum auxiliary electrode. Redox potentials were obtained from cyclic voltammograms in 0.10 mol dm3 tetrabutylammonium hexafluorophosphate acetonitrile solution. The p-electron acceptability of an aromatic ligand, PL, was calculated from the redox potentials of the complexes using the equation below [8]
PL ¼ E1=2 f½RuðNH3 Þ5 LðPF6 Þ2 g E1=2 f½RuðNH3 Þ5 ðCH3 CNÞ ðPF6 Þ2 g
ð1Þ
The change in redox potential of the complex caused by adduct formation, |DE1/2(lim)|, was determined from the dependence of E1/2 on crown ether concentrations as previously reported [8]. 2.3. Evaluation of stability constants of crown ether adduct [4] The stability constant was evaluated as follows. The chemical shift of the ammine proton of ruthenium–ammine complexes was obtained in the presence of crown ether with various concentrations in deuterated nitromethane. When a rapid equilibrium of adduct-formation is holding between a ruthenium–ammine complex and crown ether, the apparent chemical shift of the ammine proton, dapp, can be expressed using the stability constant of the adduct, K, as follows
dapp ¼
dML dadd þ dadd K½C þ 1
ð2Þ
where dML, dadd, and C represent the chemical shifts of the ruthenium–ammine complex and the adduct, and the concentration of crown ether, respectively. Stability constants of the adduct were obtained from the dependence of dapp on the concentration of crown ether by the least square analysis of the above equation. 3. Results and discussion 1 H NMR spectra of [Ru(NH3)5py](PF6)2 were measured in deuterated nitromethane in both the absence and presence of 18C6. Signals were observed in the absence of 18C6 at 2.19, 2.62, 7.38, 7.81, and 8.51 ppm, which were attributed to cis-ammine protons, trans-ammine protons, and aromatic protons of the 2,4-,3-, and 1,5-position of pyridine, respectively [4]. The signals of cis- and trans-ammine protons were shifted toward lower fields by the addition of 18C6. The down-field shifts were determined in detailed at various 18C6 concentrations and the results are shown in Fig. 1. It can be seen that the shift of the signal of the trans-ammine protons is greater than that of the cis-ammine protons. This implies that the complex forms adducts with 18C6 through hydrogen bonding mainly at trans-ammine and additionally at cis-ammine. Thus, the stability constant of the adduct was obtained from the dependencies of dapp of both trans- and cis-ammine protons on the 18C6 concentration by the least squares analysis of
Fig. 1. Dependences of chemical shift of trans-ammine protons (solid circles) and cis-ammine protons (open circles) for [Ru(NH3)5py](PF6)2 on 18C6 concentration in CD3NO2. Solid lines represent the regression lines of the equation in Section 2. [complex] = 5.0 104 mol dm3.
Eq. (2) in Section 2. Table 1 summarizes the stability constants and other parameters. The obtained stability constants determined from the trans- and cis-ammine data agreed within experimental uncertainty. Thus, the stability constants were determined from the dependences of dapp of more acidic ammine protons (the signal at lower field). Adduct formation was investigated for [Ru(NH3)5py](PF6)2 and crown ethers with various ring sizes. The stability constants are summarized together with the change in redox potential caused by adduct formation, |DE1/2(lim)| in Table 2. For a homologous series of crown ethers, the stability of the adduct increased with increasing ring size of the crown ether; 12C4 < 18C6, DC18C6 DC24C8, DB18C6 < DB24C8 < DB30C10. The stability constant of the DB30C10 adduct agreed with that presented by Hupp and co-workers [3b]. Furthermore, for crown ethers with the same ring size, the stability of the adduct was depressed by the introduction of phenyl groups; 18C6 DC18C6 > B18C6 > DB18C6. This trend corresponds to the effect on |DE1/2(lim)| and indicates that the flexibility of crown ether ring influences the stability of their adduct [8]. Thiacrown ether could not form an adduct with the ammine-complex due to the large electronegativity of the sulfur atom, in spite of its flexibility. Fig. 2 shows the approximate linear relations of ln K and |DE1/2(lim)| to Dd. The Dd value represents the strength of hydrogen bonding between ether oxygen and the coordinating ammine. The strength of hydrogen bonding influences the stability of the adduct and the change in redox potential, although there is a poor correlation between the stability and the change in redox potential.
Table 1 Stability constants of the18C6 adduct and other parameters for [Ru(NH3)5py](PF6)2 obtained from the dependencies of the chemical shift of trans-ammine and cisammine protons on the 18C6 concentration in CD3NO2.
K (mol1 dm3) dML (ppm) dadd (ppm) Dd (ppm)
trans-Ammine
cis-Ammine
(6.58 ± 1.03) 10 2.60 ± 0.01 2.93 ± 0.02 0.33
(7.58 ± 2.34) 10 2.17 ± 0.00 2.27 ± 0.01 0.10
Dd = dadd dML. Error limits are the standard deviation.
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Table 2 Stability constants of crown ether adducts for [Ru(NH3)5py](PF6)2, other parameters in CD3NO2, and the change in redox potential caused by adduct formation. Crown ether
K (mol1 dm3)
dadd (ppm)
Dd (ppm)
|DE1/2(lim)| (mV)a
12C4 18C6 18S6 DC18C6 DC24C8 B18C6 DB18C6 DB24C8 DB30C10
1.92 ± 6.17 (6.58 ± 1.03) 10 0 (8.36 ± 3.06) 10 (8.30 ± 1.01) 10 (2.22 ± 0.73) 10 0 (6.09 ± 1.40) 10 (6.58 ± 0.48) 102
2.94 ± 0.86 2.93 ± 0.02 2.70 ± 0.11 2.99 ± 0.07 2.91 ± 0.01 2.84 ± 0.04 2.64 ± 0.00 2.73 ± 0.01 3.10 ± 0.01
0.31 0.33 0.07 0.38 0.29 0.21 0 0.10 0.47
>30 129 – 87 115 92 15 29 143
Dd = dadd dML. Error limits are the standard deviation. a Taken from Ref. [8].
Fig. 2. Relations of ln K (open circles) and |DE1/2(lim)| (solid circles) to Dd for a system of [Ru(NH3)5py](PF6)2 and crown ethers with various ring sizes. The values of |DE1/2(lim)| were taken from Ref. [8].
The stability constants of 18C6 adduct were determined for [Ru(NH3)n(L)(L0 )](PF6)2 {n = 1–6, L, L0 = pyridine or polypyridine}.
These results are listed in Table 3 together with the change in redox potential caused by adduct formation, |DE1/2(lim)|. The dependence of the stability constants on Dd, the number of ammine ligands in the complex, and, |DE1/2(lim)| were examined as shown in Fig. 3. The strength of hydrogen bonding may affect both the stability of the adduct and |DE1/2(lim)|. Fig. 3a shows an approximately linear relationship between ln K and Dd, except for [Ru(NH3)4(bpy)](PF6)2, although there is no correlation between |DE1/2(lim)| and Dd. A nonlinear relationship was observed between ln K and the number of ammine ligands in Fig. 3b, although a linear relationship was reported between |DE1/2(lim)| and the number of ammine ligands [8]. The stability constant increases with increasing number of ammine ligands for the monoammineand diammine-complexes but decreases with increasing number of ammine ligands for the complexes with more than two ammine ligands, except for [Ru(NH3)4(bpy)](PF6)2 and trans-[Ru(NH3)4(py)2](PF6)2. This trend is explained as follows: the number of possible hydrogen bonds increases with increasing number of ammine ligands in the complex, while the extent of p-electron acceptability of aromatic ligands in the complex decreases with increasing number of ammine ligands. Thus, the above correlation indicates that the two factors had an opposing influence on the stability of the adduct. For the tetraammine-complexes, the greater p-electron acceptability of 2,20 -bipyridine than that of two molecules of pyridine enhanced the stability of adduct, while the trans-position of pyridines and/or ammines depressed the stability of adduct. The p-electron acceptability of the bpy ligand is significantly reflected
Table 3 Stability constants of 18C6 adduct for [Ru(NH3)xL](PF6)y, other parameters in CD3NO2, and the change in redox potential caused by adduct formation in CH3CN. Complex
K (mol1 dm3)
Dd (ppm)
|DE1/2(lim)| (mV)
[Ru(NH3)(trpy)(bpy)](PF6)2 cis-[Ru(NH3)2(bpy)2](PF6)2 mer-[Ru(NH3)3(trpy)](PF6)2 [Ru(NH3)4(bpy)](PF6)2 cis-[Ru(NH3)4(py)2](PF6)2 trans-[Ru(NH3)4(py)2](PF6)2 [Ru(NH3)5(py)](PF6)2 [Ru(NH3)6](PF6)2 [Ru(NH3)5(pz)](PF6)2 [Ru(NH3)5(Mepz)](PF6)3 [Ru(NH3)5(Me-4,40 -bpy)](PF6)3 [Ru(NH3)5(isn)](PF6)2 [Ru(NH3)5(dmabn)](PF6)2 [Ru(NH3)5(bn)](PF6)2 [Ru(NH3)5(2-cpy)](PF6)2 [Ru(NH3)5(Me-2-cpy)](PF6)3 [Ru(NH3)5(3-cpy)](PF6)2 [Ru(NH3)5(Me-3-cpy)](PF6)3 [Ru(NH3)5(Me-4-cpy)](PF6)3
(6.68 ± 0.60) 10 (8.93 ± 0.42) 10 (8.64 ± 1.84) 10 (2.05 ± 0.26) 102 (7.83 ± 0.83) 10 (2.89 ± 0.40) 10 (6.58 ± 1.03) 10 (3.31 ± 0.72) 10 (8.46 ± 0.38) 10 (3.73 ± 0.31) 102 (1.04 ± 0.05) 102 (7.27 ± 0.42) 10 (1.08 ± 0.06) 102 (1.16 ± 0.09) 102 (1.69 ± 0.08) 102 (1.71 ± 0.17) 102 (1.68 ± 0.08) 102 (1.32 ± 0.09) 102 (1.90 ± 0.10) 102
0.29 0.28 0.35 0.19 0.25 0.08 0.33 0.13 0.44 0.72 0.43 0.36 0.21 0.26 0.26 0.56 0.27 0.40 0.41
32a 57a 70a 122a 99a 91a 129a 169a 131a 105a 131b 133a 173a 183b 171b 110b 189b 134b 129b
Dd = dadd dML. Error limits are the standard deviation. a Taken from Ref. [8]. b Determined in this study.
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I. Ando et al. / Inorganica Chimica Acta 411 (2014) 56–60 Table 4 The values of PL for aromatic ligands of [Ru(NH3)5L](PF6)n (n = 2 or 3).
a
La
PL (mV)
py pz Mepz Me-4,40 -bpy isn dmabn bn 2-cpy Me-2-cpy 3-cpy Me-3-cpy Me-4-cpy
0.113 0.064 0.488 0.016 0.077 0.027 0.590 0.134 0.416 0.104 0.231 0.292
Ref. Ref. Ref. this Ref. Ref. this this this this this this
[8] [8] [8] work [8] [8] work work work work work work
Abbreviations of ligands are described in Section 2.
Fig. 4. Relations of the stability constants of 18C6 adduct to (a) the p-electron acceptability of aromatic ligand, PL, and (b) the strength of hydrogen bonding, Dd, for pentaammineruthenium(II) complexes involving aromatic ligand with various p-electron acceptability. Open and solid circles represent the data of the complexes of cyanopyridine derivative and pyridine derivative groups, respectively.
Fig. 3. Relations of the stability constants of 18C6 adduct to (a) Dd , (b) the number of the ammine ligands of the complex, and (c) |DE1/2(lim)| for ruthenium(II)– ammine complexes containing only pyridine moiety with different number of ammine ligand. The values of |DE1/2(lim)| were taken from Ref. [8].
on the stability of adduct. As shown in Fig. 3c, there is a similar correlation to Fig. 3b between ln K and |DE1/2(lim)|. The stability constants of 18C6 adduct were determined for pentaammineruthenium(II) complexes with ligands of various p-electron acceptability. The results are also listed together with the values of |DE1/2(lim)| in Table 3. The measure of p-electron acceptability of aromatic ligands, PL, was obtained from the redox potential of the complexes using the equation in Section 2.2 and those values are listed in Table 4.
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the complex. The p-electron acceptability and the strength of hydrogen bonding enhance the stability of adduct. The effect on the stability is greater in the complexes with pyridine group than in the complexes with cyanopyridine derivative group. The correlation between |DE1/2(lim)| and PL was previously reported for the complexes with pyridine derivative groups [8], similar behavior was obtained here as shown in Fig. 6. The p-electron acceptability and the strength of hydrogen bonding enhanced the stability of adduct but the p-electron acceptability depresses the change in redox potential caused by the adduct formation. 4. Conclusion
Fig. 5. Correlation between the strength of hydrogen bonding in the 18C6 adduct and the p-electron acceptability of aromatic ligand in the complex for pentaammineruthenium(II) complexes in Table 3. Open and solid circles represent the data of the complexes of cyanopyridine derivative and pyridine derivative groups, respectively.
The stability constant of crown ether adduct is difficult to evaluate for crown ether systems with small ring-sizes, as the change in the specra upon adduct formation is only small. In this study, the stability constants of adduct could be accurately evaluated for crown ethers with a small ring-size and ruthenium–ammine complexes by 1H NMR spectroscopy. The following factors were observed to affect adduct formation of crown ether to ruthenium–ammine complexes from examinations based on the stability constant: (i) the flexibility of the crown ether ring, (ii) the number of possible hydrogen bonds, (iii) the number of ammine ligands in the complex, (iv) the p-electron acceptability of ancillary ligands, (v) the strength of hydrogen bonding. It was confirmed that these factors act to enhance the stability of the adduct but factor (iv) decreased the change in redox potential caused by adduct formation. The effects of PL and Dd on the stability are significantly different depending on the type of ancillary ligand in the complex. The effect of PL on Dd was different for each group. This could be ascribed to coordination of the CN-group with strong electronwithdrawing ability however the reason is not yet clear. References
Fig. 6. Correlation between the change in redox potential caused by adduct formation and the p-electron acceptability of aromatic ligand in the complex for pentaammineruthenium(II) complexes in Table 3. Open and solid circles represent the data of the complexes of cyanopyridine derivative and pyridine derivative groups, respectively.
The effect on the stability of the 18C6 adducts was examined about the strength of hydrogen bonding and the p-electron acceptability of aromatic ligand. The results are shown in Fig. 4. The stability of the 18C6 adducts roughly increases with increasing both the strength of hydrogen bonding in the adduct and the p-electron acceptability of the aromatic ligand in the complex, but the obtained correlations were poor for all the complexes investigated in this study. It is apparent that both factors affect the stability of the 18C6 adduct. The plots of Dd against PL in Fig. 5 show good linearity. This indicates that the complexes are classified into two groups, depending on the ancillary ligand L; pyridine derivative or cyanopyridine derivative groups. The plots of ln K against PL or Dd in Fig. 4a and b, respectively, show a good linear relationship in the individual complex groups. The larger p-acceptability of the aromatic ligand makes hydrogen bonding stronger in the adduct and the magunitude of the effect is somewhat different on
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