or disulfur

Inorganica Chimica Acta 373 (2011) 142–149

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Preparation and electrochemical properties of novel cyclic dinuclear acetylacetonato ruthenium complexes doubly bridged with sulfur and/or disulfur Akira Endo ⇑, Hitomi Tsuboya, Natsumi Fujita, Yasuyuki Ito, Takeshi Hashimoto, Takashi Hayashita Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan

a r t i c l e

i n f o

Article history: Received 17 January 2011 Received in revised form 11 March 2011 Accepted 4 April 2011 Available online 9 April 2011 Keywords: Cyclic dinuclear complexes Acetylacetonato ruthenium complexes Sulfur bridged dinuclear complexes Electrochemistry Mixed-valence state

a b s t r a c t A novel cyclic dinuclear acetylacetonato ruthenium complex doubly bridged with sulfur and/or disulfur at the c-position of acetylacetonato ligand has been obtained by two different synthetic methods. The molecular structure of the dinuclear complex has been determined by single crystal X-ray diffraction study. Other two cyclic dinuclear b-diketonato ruthenium complexes were also prepared in good yields by the reaction of single bridged dinuclear complexes as starting materials with disulfur dichloride. The cyclic voltammograms of all the dinuclear complexes exhibit two one-electron reduction and oxidation waves in acetonitrile (AN) and dichloromethane (DM). The comproportionation constants (Kc) for mixedvalence state of both RuII/RuIII and RuIII/RuIV were evaluated in both solvents at 25 or 30 °C. The values of both Kc (RuII/RuIII) and log10 Kc (RuIII/RuIV) for double bridged complex are large compared to those of corresponding single bridged complexes. This fact was rationally explained by the double bridging effect caused by the spread of electronic communication and also demonstrated the usefulness of the double bridged dinuclear complexes. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction The ruthenium complexes are very suitable for the purpose of the research of electronic communication between two central metal ions in dinuclear complexes, because of thermodynamic stabilities and fast kinetics of RuII, RuIII, and RuIV oxidation states. As we have described in our recent review [1], the ruthenium dinuclear complexes with various bridging ligands, such as azole [2], azine [3], pyridine [4], dicyannamidobenzene [5], bis(b-diketones) [6], quinine [7], alkoxide [8], oxalate [9], tetraimino-3,6-diketocyclohexane [10], dicyanide [11], and diazabutadiene [12], have been synthesized and actively investigated since the Creutz–Taube complex was synthesized [13]. The mixed-valence states of the dinuclear ruthenium complexes are expected to show useful functions such as electronic and optoelectronic devices [14]. Therefore, the mixed-valence states of the dinuclear ruthenium complexes have attracted attentions and there was excellent review by Kaim and Lahiri [15]. The dinuclear (b-diketonato)ruthenium complexes have a great advantage capable of observing two kinds of mixed-valence states, i.e., Ru(II)/Ru(III) and Ru(III)/Ru(IV), although the bipyridine ruthenium dinuclear complexes such as [{Ru(bpy)2}(tae)](PF6)2 (bpy = 2,20 -bipyridine and H2tae = 1,1,2,2-tetraacetylethane) [6f] and [{Ru(bpy)2}(pzdc)](ClO4) (H3pzdc = pyrazole-3,5-dicarboxylic acid) [2a] show only Ru(II)/Ru(III) mixed-valence states. Hence, ⇑ Corresponding author. Tel.: +81 3 3238 3371; fax: +81 3 3238 3361. E-mail address: [email protected] (A. Endo). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.04.003

we have synthesized some (b-diketonato) ruthenium dinuclear complexes and evaluated the stability of the mixed-valence state by means of electrochemical methods [6e–g]. In the previous paper [6d], we have reported the syntheses of sulfur or disulfur bridged tris(acetylacetonato)ruthenium dinuclear complexes by the direct reaction of tris(acetylacetonato)ruthenium(III) with SCl2 or S2Cl2. Sulfur dichloride (SCl2) or disulfur dichloride (S2Cl2) are known to react with the hydrogen atom bound to the c-carbon of b-diketone to form sulfur bridged bis(b-dik), (b-dik-Sn-b-dik) (n = 1–3) [16]. Similarly, the hydrogen atoms at the c-carbon of the coordinated b-diketone can be reacted with SCl2 or S2Cl2 to form sulfur bridged dinuclear ruthenium complexes. However, in the case of the reaction of tris(acetylacetonato)ruthenium complex with SCl2 or S2Cl2, many types of the sulfur bridged complexes are expected to form as shown in Fig. 1, because one complex molecule has three reaction sites; i.e., dinuclear complex (single bridge (a) and cyclic type (b)), trinuclear complex (linear (c) and cyclic type (d)), tetranuclear complex (linear (e), cyclic (f), and star burst (g) type), etc., and polynuclear complex (linear (h) and dendrimer types). Until now, there are no reports about the synthesis of cyclic or star burst type polynuclear complexes, although single bridged dinuclear complexes [6d] or linear type polynuclear [17] complexes have been synthesized. Among many types of polynuclear complexes, cyclic types of polynuclear complexes are of special interest, because of the following reasons: (1) These complexes are expected to show strong mixed valence states stability

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b

a S-S

S-S

S-S

c

S

S

d S-S

S

S

S-S

e

S S

S-S

S-S

S-S S S

f

S S

g S S

S S S

S

S

S

S S

h S-S

: Ru

S-S

n

: β-diketonato ligand

Fig. 1. The examples of the polynuclear ruthenium complexes predicted by the direct reaction of tris(b-diketonato)ruthenium complexes with S2Cl2.

different from those of the linear type complexes, (2) cyclic type complexes have a potential for a ring constituent of the supramolecular like rotaxane or catenan [18], and (3) these complexes will make special inner space surrounded by the central ruthenium metal and bridging sulfur atoms. This cavity may be feasible to recognize certain ions and molecules having affinity with ruthenium or sulfur atom. Furthermore, the environment of the cavity may be changed by the redox of the ruthenium complexes. This means that the recognition of ions or molecules is easily controlled by the redox of the ruthenium of the complexes. We have already reported the syntheses of two kinds of dinuclear ruthenium b-diketonato complexes bridged by 3,30 thiobis(acetylacetonate) (mtba2) or 3,30 -dithiobis(acetylacetonate) (dtba2) (complex 1 and 2 in Fig. 2) [6d]. In the course of the synthesis of complex 1, several kinds of other products were obtained with lower yields including linear type trinuclear complex and polymer complex as in the case of tris(acetylacetonato)chromium(III) or tris(acetylacetonato)cobalt(III) [17]. Among them, we isolated a novel cyclic dinuclear complex 4, [{Ru(acac)2}2(dtba)(mtba)] (Fig. 2), which was doubly bridged with sulfur and disulfur. This is the first example of a doubly bridged cyclic dinuclear complex by sulfur atoms at c-carbon of acetylacetonato ligands. This complex was also obtained by the reaction of [{Ru(acac)2}2(dtba)] (Hacac = 2,4-pentandione and H2dtba = 3,30 -dithiobis(acetylacetone) and SCl2 with better yield. Similarly, the reaction of [{Ru(acac)(phpa)}2(dtba)] (Hphpa = 2,2,6,6-tetramethyl-3,5-heptandione) with SCl2 or S2Cl2 resulted in the formation of doubly bridged cyclic dinuclear complex 6 and 7 (Fig. 2),

respectively. However, in the reaction of [{Ru(acac)2}2(dtba)] and S2Cl2, we could not isolate cyclic dinuclear complex 5. In this study, we report the syntheses and electrochemical behavior of novel cyclic sulfur bridged dinuclear tris(bdiketonato)ruthenium(III) complexes. Although cyclic types of complexes may be possible to synthesize by using 3,30 -thiobis(acetylacetone) or 3,30 -dithiobis(acetylacetone). In fact, thiobis(b-diketonato) bridged ruthenium(II) and ruthenium(III) complexes containing triphenylphosphine or triphenylarsine have been reported [19,20]. However, in the case of ruthenium acetylacetonato complexes, our earlier efforts in the synthesis of sulfur bridged dinuclear complexes by the reaction of Hacac-Sn-acacH and [Ru (acac)2-(CH3CN)2] resulted in the formation of novel mono- and dinuclear complexes in which a new mode of binding of a bdiketone has been established [6e].

2. Experimental 2.1. Equipment 1

H NMR spectra were recorded in CDCl3 with a JEOL GX-270 Spectrophotometer. UV–Vis spectra were obtained with a Hitachi Model U-3210 spectrometer. Mass spectra of the complexes were taken using JEOL JMS-D3QO or JMS-SX102A. The voltammetric measurements were made by means of a BAS 100B/W electrochemical workstation attached to a personal computer from Bioanalytical Systems (BAS). All the measurements

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Fig. 2. Single and double bridged dinuclear b-diketonato ruthenium complexes.

were carried out at 25.0 °C except measurements at low temperature (40 °C). A platinum disk of diameter 1.6 mm from BAS was used as the test electrode and a spiral platinum wire was used as the counter electrode. All of the potentials were measured against an aqueous Ag|AgCl (3 mol dm3 NaCl aqueous solution) reference electrode from BAS. The reference electrode was connected to the test solution through a salt bridge with Vycor glass plug filled with supporting electrolyte solution. The potentials were determined against the mid-potential of the Fc/Fc+ couple as an internal standard. Tetrabutylammonium perchlorate (TBAP) with 0.1 mol dm3 concentration was used as supporting electrolyte in all the solvents. Single crystals suitable for X-ray studies were obtained from putting a small bottle containing chloroform solution of 4 into a large bottle containing hexane at room temperature. All measurements were made on a Rigaku Mercury CCD diffractometer using graphite monochromated Mo Ka (k = 0.71070 Å) radiation. Diffraction data were collected at 293 K to a maximum 2r values of 55.0°. The space group was determined from systematic absences. The structure was solved by direct methods (SIR-92) and refined by full-matrix least-squares fitting based on peak intensities (F2). The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. All calculations were performed using the Crystal structure crystallographic software package (Crystal Structure 3.5.1: Crystal Structure Analysis Package by Rigaku and Rigaku/MSC).

2.2. Materials S2Cl2 and SC12 were purchased from Wako Pure Chemicals Industries, Ltd. and from Aldrich, and used without further purification. For synthetic experiments, commercially available reagent grade solvents were used and dehydrated by molecular sieves before use. For electrochemical measurements, dehydrated solvents for organic synthesis purchased from Kanto Chemical Co., Inc. were used. Tetrabutylammonium perchlorate (TBAP) (special polarographic grade) was purchased from Nakarai Chemicals, Ltd. 2.3. Synthesis of the complexes 2.3.1. [{Ru(acac)}2 (dtba) (mtba)] (4) This complex was synthesized by following two methods. Method A: A dichloromethane solution of SCl2 (0.25 mmol) was added dropwise to a solution of [Ru(acac)3] (200 mg, 0.5 mmol) in dichloromethane with stirring at room temperature for 1 h. The solvent was then evaporated off and the residue was chromatographed on a silica gel column using a mixture of benzene/acetonitrile (15:1, v/v%) as eluent. The second purple band was collected and the solvent was evaporated to yield complex 4 (yield: 0.7% based on Ru). Method B: A 9.3 cm3 of dichloromethane solution of SCl2 (18.6 lmol) was added dropwise to a solution of [{Ru(acac)2}2(dtba)] (2) (16.34 mg, 19.0 lmol) in 40 cm3 dichloromethane with

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stirring at room temperature for 23 h. The solvent was then evaporated off and the residue was chromatographed on a silica gel column using a mixture of ethylene chloride/acetonitrile (30:1, v/v%) as eluent. The second purple band was collected and the solvent was evaporated to yield complex 4 (yield: 14.5% based on Ru). FAB+ Mass. m/z 889 (M+), 857 (M+S), 825 (M+2S). Anal. Calc. for C30H38O12S3Ru2: C, 40.53; H, 4.31. Found: C, 40.88; H, 4.62%. Electronic spectra in AN: k/nm (log10(e/mol1 dm3 cm1)) = 262 (4.41), 352 (4.18), 539 (3.50), 656 (3.07). 2.3.2. [Ru(acac)2(phpa)] [Ru(acac)2(CH3CN)2] was prepared by method reported previously [21]. [Ru(acac)2(CH3CN)2] (0.51 g, 1.34 mmol) was dissolved into mixture of 90 cm3 of ethanol and 10 cm3 of H2O. Hphpa (0.30 g, 1.63 mmol) and NaHCO3 (0.16 g, 1.90 mmol) were added to the solution and refluxed for 1 h. The solvent was evaporated off and the residue was chromatographed on a silica gel column using a mixture of benzene/acetonitrile (15:1, v/v%) as eluent. The yield was 52% based on Ru. FAB+-Mass. m/z 482 (M+). 2.3.3. [{Ru(acac)(phpa)}2(dtba)] (3) A dichloromethane solution of S2Cl2 (0.35 mmol) was added dropwise to a solution of [Ru (acac)2(phpa)] (330 mg, 0.70 mmol) in of dichloromethane (100 cm3) under argon gas with stirring at room temperature for 1.5 h and then the stirring was continued another 1.5 h. The solvent was evaporated off and the residue was chromatographed on a silica gel column using a mixture of benzene/acetonitrile (15:1, v/v%) as eluent. The second red purple band was collected. The yield was 28.1% based on Ru. ESI TOF MS: m/z 1052 (M+). 2.3.4. [{Ru(phpa)}2(dtba)(mtba)] (6) A 20 cm3 of dichloromethane solution of SCl2 (0.10 mmol) was added dropwise to a solution of [{Ru(acac)(phpa)}2(dtba)] (100 mg, 0.10 mmol) in dichloromethane (100 cm3) under argon gas with stirring at room temperature for 1.5 h and then the stirring was continued another 1.5 h. The solvent was evaporated off and the residue was chromatographed on a silica gel column using a mixture of dichloromethane/acetonitrile (20:1, v/v%) as eluent. The second purple band was main product. The yield was 51.2% based on Ru. ESI TOF MS: m/z 1113 (M+). Anal. Calc. for C42H62O12 S3Ru2: C, 47.28; H, 6.11. Found: C, 47.71; H, 5.91%. 2.3.5. [{Ru(phpa)}2(dtba)2] (7) A 50 cm3 of dichloromethane solution of S2Cl2 (0.10 mmol) was added dropwise to a solution of [{Ru(acac)(phpa)}2(dtba)] (100 mg, 0.10 mmol) in dichloromethane (100 cm3) under argon gas with stirring at room temperature for 1.5 h and then the stirring was continued another 1.5 h. The solvent was evaporated off and the residue was chromatographed on a silica gel column using a mixture of benzene/acetonitrile (15:1, v/v%) as eluent. The second purple band was collected. The yield was based on Ru. ESI TOF MS: m/z 1081 (M+). Anal. Calc. for C42H62O12S4Ru2: C, 46.82; H, 5.86. Found: C, 46.31; H, 5.74%. Electronic spectra in DM: k/nm (log10(e/mol1 dm3 cm1)) = 271 (4.16), 344 (3.85), 540 (3.21).

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than that (0.7%) of Method A. Major product of Method A were single bridged dinuclear complex 1 and 2, and single bridged trinuclear complex of [{Ru(acac)2(dtba)2}2Ru(acac)]. In the case of Method B, seven spots appeared on the TLC for the crude product. Second spot corresponded to cyclic dinuclear complex 4 and the first spot may correspond to the cyclic trinuclear complex (d in Fig. 1) judging from the peak of m/z = 1401 (M++Na+) of ESI TOF MS. Other spots could not be identified because of close Rf values and low yields. The formation of complex 4 was also confirmed by the reaction of [{Ru(acac)2}2(dtba)] with SCl2. When the S2Cl2 was reacted with [{Ru(acac)2}2(dtba)], over 10 spots appeared on TLC(silica gel/benzene:acetonitrile = 13:1). Although each spot could not be separated clearly because of close Rf values, the main product of sixth band showed intense peak corresponding to the cyclic tetranuclear complex (f in Fig. 1) at m/z = 1846 (M++Na+) of ESI TOF MS. We expected the formation of cyclic dinuclear complex bridge by two disulfur atoms, and its signal was found for ESITOF-MS using crude product. However, the yield was very low and the isolation was not successful. For the formation of the cyclic complex 4 by Method B, c-positions of external acetylacetonato ligands in complex 2 must be close to each other. Therefore, the formation of complex 4 by Method B indicates that the complex 2 should maintain the cis form even in the dichloromethane solution as in the case of the single crystal state [6d]. Moreover, we could not confirm the formation of single bridged trinuclear complex by Method B, but by Method A. This fact also supports the above conclusion. To obtain more efficient synthetic route of double bridged dinuclear complexes by Method B, we used another single bridged dinuclear complex, [{Ru(acac)(phpa)}2(dtba)] (3) as starting material. The reason is to reduce the reaction site with SCl2 or S2Cl2 because the hydrogen atom bound to the c-carbon of phpa do not react with SCl2 or S2Cl2 due to a steric effect in the synthetic condition.1 As expected, double bridged dinuclear complexes, 6 and 7, were successfully obtained in good yields (51.2% for 6 and 49.4%for 7). 3.2. X-ray crystal structure of [{Ru(acac)}2(mtba)(dtba)] (4) The structure of complex 4 was determined by single crystal X-ray diffraction.2 The molecular structure is depicted in Fig. 3. The crystal data and the structure determination parameters are summarized in Table 1, and the selected bond lengths and angles in Table 2. The bond lengths and bond angles in the acetylacetonate rings in 4 are almost the same as those observed for [Ru(acac)]3 [22]. The bond lengths of S1–S2 and S1–C2 (or S2– C17) were almost the same as those of complex 2 [6d]. The bond angles of the C–S2–S1–C moiety were also similar to those observed in complex 2. Dihedral angle (62.5°) was slightly narrow compared with that in complex 2 (65°). These facts indicate that the structure of complex 4 is not so different from that of complex 2. 3.3. Electrochemical properties of single and double bridged dinuclear complexes

3.1. Formation of cyclic dinuclear complexes

The cyclic voltammograms of complex 4 and single bridged dinuclear complexes 1 and 2 for the comparison were shown in Fig. 4. Complex 4 exhibited two reduction peaks and two oxidation peaks in both AN and DM. Although the second oxidation peak was ill-defined because anodic potential window was very near to the

Cyclic dinuclear complex 4 was prepared by two ways. However, direct synthesis from [Ru(acaca)3] (Method A) was not suitable because of quite low yield and low reproducibility. Alternatively, the synthesis starting from single bridged dinuclear complex [{Ru(acac)2}2(dtba)] (Method B) gave better yield (14.5%)

1 The reaction of [Ru(phpa)3] with concentrated SCl2DM solution gave three kinds of products. Two products were Cl or SH substitution products at c-position of phpa. third product could not be identified. 2 The data of complex 4 has been deposited with the CCDC (Supplementary No. CCDC 280723).

3. Results and discussion

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[{Ru(acac)2}2(mtba)(dtba)] (4) (c = 0.5 mmol dm-3)

5 μA

[{Ru(acac)2}2(mtba)] (1) (c = 1.0 mmol dm-3)

[{Ru(acac)2}2(dtba)] (2) (c = 1.0 mmol dm-3)

Fig. 3. Molecular structure of [{Ru(acac)2}2(mtba)(dtba)] (4). Anisotropic displacement ellipsoids are shown at the 50% probability level.

Table 1 The crystal data and structural determination parameters of complex 4CHCl3.

Scan rate = 0.1 V/s

[{Ru(acac)2}2(dtba)(mtba)] Empirical formula Crystal size Molecular weight Crystal system Space group unit cell dimension a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z q (g cm3) F(0 0 0) l(Mo Ka) (cm1) T (°C) Ra wRb Absorption correction Refinement method Function minimized Least squares weight Goodness-of-fit indicator R indices [I > 2r(I)] Dqmax, Dqmin (e/Å3) a b

Ru2C31H39O12S3Cl3 0.25  0.27  0.13 mm 1008.32 triclinic  P 1(#2) 11.331(1) 12.152(1) 17.595(1) 71.335(5) 80.381(6) 60.768(5) 2002.91(4) 2 1.672 1016.00 11.66 25 0.107 0.281 Lorentz-polarization Full-matrix least squares on F2

RwðF 2o  F 2c Þ2 1=½0:0010F 2o þ 3:0000rðF 2o Þ=ð4F 2o Þ 3.073 R1 = 0.107, wR2 = 0.281 4.32, 2.49

R = R||Fo|  |Fc||/R|Fo. wR = [Rw(|Fo|  |Fc|)2/Rw|Fo|2]1/2.

Table 2 Major interatomic distances and angles of complex 4. Bond lengths (Å) Ru1–Ru2 Ru1–O1 Ru1–O2 S1–S2 S1–C2 S2–C17 S3–C7 S3–C22

6.26 1.993(7) 1.989(8) 2.064(8) 1.75(1) 1.73(1) 1.836(9) 1.78(1)

Bond angles (Å) C2–S1–S2 C17–S2–S1 C7–S3–C22

104.5(5) 101.9(7) 102.6(5)

C2–S1–S2–C17

62.5(5)

peak, other peaks were corresponding to Nernstian one-electron transfer processes, judging from the peak separations and linear relationship through origin between scan rates and peak currents. The reversible half-wave potential, E1=2 r , was determined by midpotential calculations (Emid = (Epa + Epc)/2; Epa and Epc were the potentials of anodic and cathodic peak potential of CV, respectively). These E1=2 r values against the E1=2 r of Fc|Fc+ couple were

-0.8

-0.4

0

0.4

0.8

1.2

E /V vs. Ag|AgCl(3 mol dm-3 NaCl aq.) Fig. 4. Cyclic voltammograms of the dinuclear ruthenium complexes of 1, 2, and 4 in 0.1 mol dm3 TBAP –DM at platinum disk electrode at 25 °C.

shown in Table 2. Because of ill-defined behavior of the second oxidation of complex 4, E1=2 r values for oxidation were obtained from cyclic voltammograms by using an ultramicroelectrode (u = 25 lm) at a high potential scan rate (v = 1.0 V/s). The voltammetric behaviors of complex 4 were similar to those of complex 1 with sulfur bridge, because the complex 2 with disulfur bridge showed irreversible second oxidation wave at 25 °C (v = 0.1 V/s). However, complex 2 also showed reversible two one-electron Nernstian reversible oxidation waves at low temperature (40 °C) in AN. There was no solvent effect between AN and DM except the values of E1=2 r . To obtain the IVCT spectra of mixed-valence state and to confirm the reduction and oxidation sites of complex 4, a spectroelectrochemical measurement was carried out by means of self-made OTTLE [23]. Fig. 5 showed the UV–Vis (400–1100 nm) and NIR (1350–2600 nm) spectral changes by the reduction and oxidation of complex 4 in 0.3 mol dm3 TBAP-DM at 25 °C. When the potential jumped to 0.6 V (corresponding to the first reduction) from 0 V, the band around 500 nm was increased (Fig. 4a). However, there was no change in NIR region (Fig. 4b). The second reduction jumped to 0.9 V from 0.6 V resulted in the further increase of the intense of the band around 500 nm (Fig. 4c) without any changes in NIR region (Fig. 4d). The spectral changes for the reduction were resembled to those of [Ru(acac)3] [23]. However, the spectral changes for the oxidation were not like to those of the oxidation of [Ru(acac)3] (Fig. 4e) [23]. This may be attributed to the following chemical reaction after the oxidation resulting from the long time scale of spectroelctrochemical measurements. Eventually, we could not obtain IVCT band in NIR region for both the reduction and the oxidation. However, there were many cases of no appearance of IVCT band even in the case of large comproportionation constant [15]. Therefore, it is evident that the redox sites of complex 4 for both oxidation and reduction are central ruthenium ions, as in the case of single bridged dinuclear complexes 1 and 2 [6d]. Almost all the E1=2 r values in DM for both oxidation and reduction were more negative than those in AN. These tendencies can easily be understood by the ease of reduction and the difficulty

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Fig. 5. The UV–Vis (400–1100 nm) and NIR (1350–2600 nm) spectral changes by the reduction and oxidation of complex 4 in 0.3 mol dm3 TBAP–DM by using OTTLE at 25 °C. (a and b) Potential step reduction from 0 to 0.6 V, (c and d) potential step reduction from 0.6 to 0.9 V.

of the oxidation on basis of variation in the electron donor properties of the solvents (Donor No. AN14.1, DM  0) [24]. Kadish et al. also reported the same solvent effect for the redox of the dinuclear complexes bridged by anilinopyridinate anion [25]. The cyclic voltammograms of the other two double bridged (complex 6 and 7) and single bridged complex 3 were depicted in Fig. 6. All complexes exhibited two reduction peaks and two oxidation peaks, but all second oxidation waves were observed as irreversible process at 25 °C by the substitution of the substituents on external b-diketonato ring from methyl to t-butyl. The first reduction waves corresponded to Nernstian one-electron process. On the contrary, the second reduction waves showed quasireversible characteristics, as indicated by the peak separations of 119 mV (v = 0.3 V/s) for complex 6, 83 mV (v = 0.1 V/s) for complex 7, and 80 mV (v = 0.1 V/s) for complex 3, respectively. At low temperature (at 30 °C), all the second oxidation waves showed reversible properties, although the second reduction waves became more irreversible. The E1=2 r values against the E1=2 r of Fc|Fc+ couple were also shown in Table 2. All of the E1=2 r values of complex 6 (sulfur and disulfur bridge) were more negative than those of complex 7 (two disulfur bridge). In the case of single bridged complexes, similar tendency was observed between complex 1 (sulfur bridge) and 2 (disulfur bridge). This may be caused by more effective stabilization of RuIII–RuIII state by sulfur bridge than that by disulfur bridge.

3.4. Stability of the mixed-valence oxidation states Two mixed-valence oxidation states of Ru (RuIII/RuIV and RuII/ Ru ) were observed and their stability was estimated by the comproportionation constant (Kc). The Kc are defined as follows: III

K c ðRuII =RuIII Þ ¼ ½RuII =RuIII 2=½RuII =RuII ½RuIII =RuIII  ¼ exp½E1=2 r ðred1Þ  E1=2 r ðred2ÞF=RT K c ðRuIII =RuIV Þ ¼ ½RuIII =RuIV 2=½RuIII =RuIII ½RuIV =RuIV  ¼ exp½E1=2 r ðox2Þ  E1=2 r ðox1ÞF=RT The values of log10 Kc calculated were shown in Table 3. The values of log10 Kc (RuII/RuIII) and log10 Kc (RuIII/RuIV) lied between 1.35 and 4.56. These values are not so large and hence these complexes may be classified as class II in the Robin and Day classification [26].

Fig. 6. Cyclic voltammograms of the dinuclear ruthenium complexes of 3, 6, and 7 in 0.1 mol dm3 TBAP–DM at platinum disk electrode at 25 and 30 °C. Concentration of the complexes = 1 mmol dm3.

The values of both Kc (RuII/RuIII) and log10 Kc (RuIII/RuIV) for double bridged complex 4 were equal or larger compared to those of corresponding single bridged complexes 1 and 2. This fact can be explained by the double bridging effect caused by the spread of

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Table 3 Reversible half-wave potentials (E1=2 r )a and the comproportionation constants (Kc) of single and double bridged dinuclear complexes at 25 and 30 °C. Type

Single

Complex

a

c

DE(red)/V Red1

AN DM AN DM

1.28 1.43 1.14 1.32

1.13 1.22 1.06 1.17

3

DM

1.34 (1.33)c

4

AN DM DM DM

1.13 1.30 1.36 (1.37)c 1.25 (1.22)c

1

6 7 b

E1=2 r (red)/V Red2

2

Double bridge

Solvent

E1=2 r (ox) (V)

DE(ox) (V)

log10 Kc RuII/RuIII

RuIII/RuIV

0.17 0.20 – –

2.54 3.55 1.35 2.54

2.86 3.38 – –

irrb (0.49)c

(0.08)c

1.69 (2.07)c

(1.66)c

0.91 0.89 irrb (0.71)c irrb (0.73)c

0.21 0.25 (0.19)c (0.12)c

2.54 3.89 3.55 (4.56)c 3.04 (3.53)c

3.55 4.22 (3.94)c (2.49)c

Ox1

Ox2

0.15 0.21 0.08 0.15

0.58 0.56 irrb 0.62

0.75 0.76 irrb irrb

1.24 (1.23)c

0.10 (0.10)c

0.50 (0.57)c

0.98 1.07 1.15 (1.15)c 1.08 (1.05)c

0.15 0.23 0.21 (0.22)c 0.18 (0.17)c

0.70 0.64 0.53 (0.52)c 0.62 (0.61)c

vs. Fc/Fc+. Irreversible at scan rate of 100 mV/s. At 30 °C.

electronic communication. By the double bridging, the interaction between two ruthenium ions became stronger than in the case of single bridging. In fact, the S–S bond length of complex 4 was shorter than that of disulfur bridged complex 2. Similarly, the values of both Kc (RuII/RuIII) and log10 Kc (RuIII/RuIV) for double bridged complex 6 and 7 were also larger compared to those of corresponding single bridged complexes 3. The larger value of Kc (RuII/RuIII) of complex 6 than that of complex 7 can be explained by the stronger stabilization of sulfur bridge, as emerging in the comparison with that of single bridged complexes 1 and 2. With regard to the medium, the Kc values were apparently affected by the nature of the solvent as well as E1=2 r . In general, the solvent effect of mixedvalence state is explained by the dielectric constant, refractive index of the solvent, and Donor or Accepter Number [15,27,28]. In fact, we demonstrated that the plot of log10 Kc against Donor Number showed a decrease in log10 Kc with increase in donor number in the case of single bridged dinuclear complexes [6e]. The double bridged complex 4 was also showed a similar relationship.

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