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Syntheses and electrochemical properties of ruthenium(II) complexes with 4,4′-bipyrimidine and 4,4′-bipyrimidinium ligands
Inorganica Chimica Acta 357 (2004) 1205–1212 www.elsevier.com/locate/ica
Syntheses and electrochemical properties of ruthenium(II) complexes with 4,40-bipyrimidine and 4,40-bipyrimidinium ligands Tetsuaki Fujihara, Tohru Wada, Koji Tanaka
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Coordination Chemistry Laboratories, Institute for Molecular Science and CREST, JAPAN Science and Technology Agency (JST), 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan Received 2 September 2003; accepted 25 October 2003
Abstract The syntheses and electrochemical properties of novel ruthenium(II) polypyridyl complexes with 4,40 -bipyrimidine, [Ru(tr 0 py)(bpm)Cl](X) ([1](X; X ¼ PF 6 , BF4 )) and with a quaternized 4,4 -bipyrimidinium ligand, [Ru(trpy)(Me2 bpm)Cl](BF4 )3 0 0 00 0 ([2](BF4 )3 ) (trpy ¼ 2,2 :6 ,2 -terpyridine, bpm ¼ 4,4 -bipyrimidine, Me2 bpm ¼ 1,10 -dimethyl-4,40 -bipyrimidinium) are presented. The bpm complex [1]þ was prepared by the reaction of Ru(trpy)Cl3 with 4,40 -bipyrimidine in EtOH/H2 O. The structural characterization of [1]þ revealed, that the bpm ligand coordinated to the ruthenium atom with the bidentate fashion. Diquaternization of the noncoordinating nitrogen atoms on bpm of [1]þ by (CH3 )3 OBF4 in CH3 CN gave [2](BF4 )3 . The electrochemical and spectroelectrochemical properties of the complexes are described. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Ruthenium(II) complex; 4,40 -Bipyrimidine; 4,40 -Bipyrimidinium; Electrochemistry; X-ray structures
1. Introduction Much attention has been paid to the syntheses of transition metal complexes with redox active ligands, since those metal complexes often exhibit unique photochemical and electrochemical properties. Among numerous redox active ligands, metal complexes with dioxolene ligands have been intensively studied [1]. For instance, ruthenium–dioxolene complexes are featured by close energy levels between 3d orbitals of ruthenium and p orbitals of the ligand, resulting in delocalization of electron between two orbitals, that is, charge distribution [2,3]. We have also reported, a series of ruthenium(II) complexes having redox active ligands such as dioxolene [4–8], dithiolene [9] and 1,10-phenanthroline-5,6-dione [10]. In particular, acid–base equi-
*
Corresponding author. Tel.: +81-564-55-7241; fax: +81-564-555245. E-mail address: [email protected] (K. Tanaka). 0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.10.015
librium of an aqua ligand of a ruthenium–dioxolene complex produces a ruthenium–semiquinone oxyl radical complex, [Ru(trpy)(Bu2 SQ)(O )] (Bu2 SQ ¼ 3,5-ditert-butylbenzosemiquinonate) [7,8]. Such unique reversible conversion between aqua and oxyl radical ligands on ruthenium is accomplished by, utilization of low lying LUMO of ruthenium–dioxolene moiety. From the viewpoint of redox active ligand, N ; N 0 dialkylated (diquaternized) 4,40 -bipyrimidine (bpm) is specially interesting, because it has the redox functionality of 1,10 -dialkyl-4,40 -bipyridinium, as well as, the strong chelating ability of 2,20 -bipyridine (Scheme 1) and the strong electron acceptor character of the ligand, has been suggested in ruthenium(II) and rhenium(I) complexes [11,12]. We now pay attention to the redox functionality of 4,40 -bipyrimidinium ligand on the ruthenium(II)–terpyridine unit. In this paper, we will report the syntheses and electrochemical properties of [Ru (trpy)(bpm)Cl](X) [1](X; X ¼ PF 6 or BF4 ), and with a quaternized bipyrimidinium ligand, [Ru(trpy)(Me2 bpm) Cl](BF4 )3 [2](BF4 )3 (Fig. 1).
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Scheme 1.
Fig. 1. Ru(II)–terpyridine complexes bearing 4,40 -bipyrimidine (bpm) and 1,10 -dimethyl-4,40 -bipyrimidinium (Me2 bpm2þ ).
2. Experimental 2.1. Materials 2,20 :60 ,600 -Terpyridine (trpy) was purchased from Aldrich. Pyrimidine and trimethyloxonium tetrafluoroborate ((CH3 )3 OBF4 ) were purchased from TCI. Other agents and solvents were purchased from Wako and used as received. Alumina, A, Super I (ICN biochemicals GmbH) was used for column chromatography. 2.2. Syntheses 2.2.1. Syntheses of precursors Ru(trpy)Cl3 was synthesized by the previous report [13]. 4,40 -bipyrimidine (bpm) [14] and 1,10 -dimethyl-4,40 bipyrimidinium tetrafluoroborate (Me2 bpm)(BF4 )2 [11,12] were also prepared according to previous reports. The subsequent exchange of the counter anion from BF 4 to PF6 gave (bpmMe2 )(PF6 )2 , quantitatively. Found: C, 25.53; H, 2.71; N, 11.95. Anal. Calc. for C10 H12 N4 P2 F12 : C,25.12; H, 2.53; N, 11.71%. 2.2.2. [Ru(trpy)(bpm)Cl](PF6 ) ([1](PF6 )) Ru(trpy)Cl3 (132 mg, 0.3 mmol) and bpm (48 mg, 0.3 mmol) were gently refluxed for 5 h in 24 cm3 of a mixed
solvent of ethanol–H2 O (3:1, v/v) containing LiCl (42 mg) and triethylamine (0.1 cm3 ) as a reductant. The hot reaction mixture was filtered off and then the filtrate was evaporated to dryness. The residue was dissolved in a small amount of EtOH and NH4 PF6 saturated in EtOH was added. The resulting precipitate was collected by filtration, washed with small amounts of EtOH and diethyl ether. The crude product was dissolved in CH3 CN and the solution was loaded on an alumina column. The second purple band eluted with 2% MeOH–CH3 CN was collected and the solvent was removed by using a rotary evaporator. The purple residue was recrystallized from CH3 CN by addition of diethyl ether. The purple crystals thus formed were collected by filtration, washed with Et2 O and dried under reduced pressure. Yield: 14 mg (7%). Found: C, 41.14; H, 2.91; N, 15.03. Anal. Calc. for C23 H17 N7 PF6 ClRu: C, 41.05; H, 2.55; N, 14.57%. 1 H NMR: dH (500 MHz; CD3 CN; standard SiMe4 ) 10.80 (1H, s, bpm), 9.26 (1H, d, J (HH) 5.5 Hz, bpm), 8.71 (1H, d, J (HH) 5.5 Hz, bpm), 8.63 (1H, d, J (HH) 5.0 Hz, bpm), 8.52 (2H, d, J (HH) 8.0 Hz, trpy), 8.42 (1H, d, J (HH) 5.0 Hz, bpm), 8.39 (2H, t, J (HH) 9.0 Hz, trpy), 8.16 (1H, t, J (HH) 9.0 Hz, trpy), 8.15 (1H, s, bpm), 7.92 (2H, t, J (HH) 8.0 Hz, trpy), 7.68 (2H, d, J (HH) 5.0 Hz, trpy) and 7.27 (2H, t, J (HH) 6.5 Hz, trpy). UV–Vis: kmax /nm (CH3 CN) 567 (e/dm3 mol1 cm1 8500), 360 (7200), 315 (30,500), 278 (32,900) and 236 (26,800). ESI MS: m=z (CH3 CN) 528 ([M–PF6 ]þ ). 2.2.3. [Ru(trpy)(bpm)Cl](BF4 ) ([1](BF4 )) The complex was prepared by the similar method of [1](PF6 ) by using NH4 BF4 instead of NH4 PF6 . Yield: 13 mg (7%). Found: C, 45.18; H, 3.52; N, 16.05%. C23 H17 N7 BF4 ClRu requires for C, 44.94; H, 2.79; N, 15.95%. ESI MS: m=z (CH3 CN) 528 ([M–BF4 ]þ ). 2.2.4. [Ru(trpy)(Me2 bpm)Cl](BF4 )3 ([2](BF4 )3 ) To an acetonitrile solution (10 cm3 ) of [1](BF4 ) (12 mg, 0.02 mmol), which was deoxygenated by bubbling of N2 gas for 30 min, was added (CH3 )3 OBF4 (60 mg, 0.4 mmol) under N2 . The reaction mixture was heated gently at 65 °C for 1 h under N2 . After the reaction mixture was cooled to room temperature, the solvent was removed under N2 flow. To the residue THF (20 cm3 ) was added and the resultant precipitates were filtered off, washed with THF and dried under reduced pressure. Yield: 14 mg (86%). Found: C, 36.24; H, 2.86; N, 11.65. Anal. Calc. for C25 H23 N7 B3 F12 ClRu: C, 36.69; H, 2.83; N, 11.98%. 1 H NMR: dH (500 MHz; CD3 CN; standard SiMe4 ) 11.12 (1H, s, Me2 bpm), 9.26 (1H, d, J (HH) 5.5 Hz, Me2 bpm), 9.32 (2H, d+d, Me2 bpm), 9.03 (1H, d, J (HH) 6.5 Hz, Me2 bpm), 8.78 (2H, s+d, Me2 bpm), 8.61 (2H, d, J (HH) 8.0 Hz, trpy), 8.43 (2H, d, J (HH) 8.0 Hz, trpy), 8.38 (1H, t, J (HH) 8.0 Hz, trpy), 8.01 (2H, t, J (HH) 8.0 Hz, trpy), 7.49 (2H, d, J (HH) 5.5 Hz, trpy), 7.28 (2H, t, J (HH) 5.5 Hz, trpy), 4.69 (3H, s,
T. Fujihara et al. / Inorganica Chimica Acta 357 (2004) 1205–1212
Me2 bpm) and 4.09 (3H, s, Me2 bpm). UV–Vis: kmax /nm (CH3 CN) 836 (e/dm3 mol1 cm1 4000), 427 (8900), 305 (32,900), 282 (26,800) and 235 (27,000). 2.3. Physical measurements 1
H NMR and H–H COSY experiments were performed with a JEOL GX-500 spectrometer. UV–Vis spectra were recorded on a Shimadzu UV-3100PC UV–Vis–NIR scanning spectrophotometer. ESI MS spectra were measured with a Shimadzu LCMS-2010 liquid chromatograph mass spectrometer. Elemental analyses were carried out at Research Center for Molecular-scale Nanoscience, Institute for Molecular Science. Cyclic voltammetry was performed with an ALS/ Chi model 660 electrochemical analyzer. Cyclic voltammograms were recorded at a scan rate of 100 mV s1 at room temperature. The working and the counter electrodes were a glassy carbon and a Pt wire, respectively. An Ag/AgNO3 (0.01 mol cm3 ) was used as a reference electrode in the present study. To compare the redox potentials of the complexes with those of analogs reported so far, the potentials were converted to SCE (ESCE ¼ EAg=Agþ þ 0:33 V). The sample solutions in CH3 CN containing n-Bu4 NPF6 or n-Bu4 NBF4 (0.1 mol dm3 ) were deoxygenated by N2 stream. Spectroelectrochemical UV–Vis measurements were performed with a thin-layer electrode cell with a platinum minigrid working electrode sandwiched between two glass side of an optical cell (path length 0.5 mm) by using a Hokuto Denko HA-501 potentiostat and a Shimadzu UV-3100PC UV–Vis–NIR scanning spectrophotometer.
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2.4. X-ray crystallography A summary of the crystal structure refinements of (Me2 bpm)(BF4 )2 and [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) was given in Table 1. The single crystals of (Me2 bpm)(BF4 )2 suitable for X-ray analysis were obtained by recrystallization of the compound with the CH3 CN/toluene solution. The single crystals of [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) were obtained by slow diffusion of Et2 O into an acetonitrile solution of [1](PF6 ). Data were collected on a Rigaku/MSC Mercury CCD diffractometer using graphite-monochro at 173 K, and mated Mo Ka radiation (k ¼ 0:71070 A) processed using Crystal Clear [15]. The structure of (Me2 bpm)(BF4 )2 was solved by a direct method (SIR92) [16]. The structure of [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) was solved by a heavy-atom Patterson method (PATTY) [17] and expanded using Fourier Techniques (DIRDIF94) [18]. These structures were refined by fullmatrix least-square refinement on F 2 . All non hydrogen atoms without those of crystal solvents were refined anisotopically. All hydrogen atoms were located on the calculated positions and not refined. All calculations were performed using the teXsan crystallographic software package [19].
3. Results and discussion 3.1. Syntheses and properties of bpm and Me2 bpm2þ 4,4-Bipyrimidine (bpm) was prepared by the coupling reaction of pyrimidine in the presence of a strong base
Table 1 Crystallographic data of (Me2 bpm)(BF4 )2 and [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) (Me2 bpm)(BF4 )2
[1](PF6 ) 1/2(CH3 CN) 1/2(H2 O)
Empirical formula C10 H12 N4 B2 F8 C24 H19:5 N7:5 O0:5 PF6 ClRu Formula weight 361.84 702.46 Temperature (K) 173 173 Crystal system monoclinic triclinic Space group P 21 =n (#14) P 1 (#2) Unit cell dimension a (A) 5.503(8) 8.719(7) b (A) 10.69(1) 9.672(7) c (A) 12.37(2) 16.74(1) a (°) 90 97.910(8) b (°) 92.71(2) 92.035(8) c (°) 90 109.63(1) 3 ) V (A 726(1) 1311(1) Z 2 2 qc (g cm3 ) 1.66 1.78 Unique reflections 1639 5779 Observed reflections 1009, I > 3rðIÞ 4271, I > 2rðIÞ R1 , wR2 a 0.066, 0.197b 0.060, 0.127c P P P P a R1 ¼ ½jFo j jFc j= jFo j; wR2 ¼ f ½wðFo Fc2 Þ2 = ½wðFo2 Þ2 g1=2 ; w ¼ fr2 ðFo2 Þ þ ½pðmaxðFo2 ; 0Þ þ 2Fc2 Þ=32 g1 . b p ¼ 0:140. c p ¼ 0:075.
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[14]. The quaternized bpm compound, (Me2 bpm)(BF4 )2 , was synthesized by the reaction of bpm with (CH3 )3 OBF4 in 1,2-dichloroethane [11,12]. The molecular structure of (Me2 bpm)(BF4 )2 (Fig. 2) revealed that quaternization takes place exclusively on the nitrogen atoms of para position with respect to the connected carbon atom. There found two counter anions (BF 4 ) in the Me2 bpm unit. The central carbon–carbon distance is somewhat shorter than a single bond, (1.490(5) A) indicating a small contribution of double bond character. The planar structure of the two cycles, therefore, is ascribed to the conjugation of the carbon–carbon bond. The C–C and C–N bond lengths in the pyrimidine ring are essentially similar to those of 4,40 -bipyrimidine [20]. Fig. 3 depicts the CV of (Me2 bpm)(PF6 )2 in CH3 CN together with the UV–Vis spectral changes caused by the reduction of (Me2 bpm)2þ . The CV of (Me2 bpm)2þ showed two reversible (Me2 bpm)2þ /(Me2 bpm)þ and (Me2 bpm)þ /(Me2 bpm)0 couples at E1=2 ¼ þ0:08 and )0.42 V vs. SCE, respectively, (Fig. 3(a)) [11]. The dicationic Me2 bpm2þ has no absorption band in the visible region, and one-electron reduction of Me2 bpm2þ under the electrolysis of the solution at )0.1 V (vs. SCE), brought about an appearance of two new absorption bands at 528 and 382 nm (Fig. 3(b)). The spectrum was consistent with that of (Me2 bpm)þ produced by chemical reductions [11] and the appearance of the strong two absorption bands (528 and 382 nm) were close to that of a cation radical of methyl viologen (603 and 395 nm) [11]. The 528 and 382 nm bands disappeared upon two-electron reduction of (Me2 bpm)2þ under the electrolysis at )0.7 V (Fig. 3(c)). 3.2. Syntheses and characterization of ruthenium(II) complexes The 4,40 -bipyrimidine (bpm) complex, [Ru(trpy) (bpm)Cl]þ ([1]þ ), was synthesized by the reaction of Ru(trpy)Cl3 with bpm in EtOH/H2 O by reference to the synthetic procedure of [Ru(trpy)(bpy)Cl]þ [21]. The ESI MS spectrum of the reaction mixture, using CH3 CN as a solvent, showed several peaks corresponding to
Fig. 2. ORTEP drawing of (Me2 bpm)2þ . Thermal ellipsoids are drawn at the 50% probability level. The central carbon–carbon and N– respectively. C(methyl) distances were 1.490(5) and 1.488(5) A,
Fig. 3. Cyclic voltammogram of (Me2 bpm)(PF6 )2 (a) and UV–Vis spectral changes in the electrochemical reduction of (Me2 bpm)2þ in CH3 CN containing n-Bu4 NPF6 (0.1 mol dm3 ). The first and the second reductions were conducted at )0.1 V (b) and )0.7 V (vs. SCE) (c), respectively. The arrows in the figures indicate the direction of the changes of the spectra during the reduction of (Me2 bpm)2þ .
the molecular weights of [Ru(trpy)(bpm)2 (CH3 CN)]2þ (m=z 346), [Ru(trpy)(bpm)2 ]2þ (326), [1]þ (528), [Ru(trpy) (bpm)(CH3 CN)Cl]þ (569) and [Ru(trpy) (bpm)2 Cl]þ (686), indicating formation of a mixture of Ru complexes with monodentate and bidentate bpm ligands. We successfully separated [1]þ from the reaction mixtures by column chromatography, and the purity of [1](X) was confirmed by the ESI MS spectrum (m=z 528) with the isotope distribution pattern of a ruthenium nuclei, and elemental analysis. Fig. 4 showed the molecular structure of [1]þ determined by X-ray diffraction study. Selected bond lengths and angles around the ruthenium atom are listed in Table 2. The molecular structure of [1]þ clearly showed that bpm coordinated to the ruthenium atom with a bidentate manner. The coordination environment around the ruthenium atom is a distorted octahedral with one chloride atom, three nitrogen atoms of trpy, and two nitrogen atoms of bpm. The distance of Ru–Cl is comparable to those found for other (2.422(2) A)
T. Fujihara et al. / Inorganica Chimica Acta 357 (2004) 1205–1212
Fig. 4. ORTEP drawing of [1]þ . Thermal ellipsoids are drawn at the 50% probability level.
Table 2 and angles (°) for [1]þ Selected bond lengths (A) Bond lengths Ru(1)–N(1) Ru(1)–N(3) Ru(1)–N(6)
2.075(5) 2.086(5) 2.088(5)
Ru(1)–N(2) Ru(1)–N(4) Ru(1)–Cl(1)
1.963(5) 2.049(4) 2.422(2)
Bond angles N(1)–Ru(1)–N(3) N(2)–Ru(1)–N(3) N(2)–Ru(1)–N(6) Cl(1)–Ru(1)–N(1) Cl(1)–Ru(1)–N(3) Cl(1)–Ru(1)–N(6)
158.7(2) 79.0(2) 175.6(2) 90.9(1) 89.8(1) 93.3(1)
N(1)–Ru(1)–N(2) N(2)–Ru(1)–N(4) N(4)–Ru(1)–N(6) Cl(1)–Ru(1)–N(2) Cl(1)–Ru(1)–N(4)
79.7(2) 97.0(2) 78.6(2) 91.1(1) 171.9(1)
Ru(II)–terpyridine-chloride complexes with bidentate N–N ligands [10,22]. The bond lengths of three Ru– respecN(trpy) are 2.075(5), 1.963(5), and 2.086(5) A, tively. The shortening of the Ru–N(2) distance is a typical feature observed in the structures of other ruthenium(II)–terpyridine complexes [22]. The bond lengths of Ru–N(bpm) were 2.049(4) and 2.088(5) A, respectively. The Ru–N bond length trans to the nitrogen atom of the central pyridyl ring for trpy is slightly longer than that trans to chloride atom. The C–N and C–C bond lengths in the pyrimidyl ring were essentially same as those found in [Ru(bpm)3 ](PF6 )2 [23]. 1 H NMR spectra of [1](PF6 ) in CD3 CN displayed 6 signals from trpy and 6 signals from bpm as described in the Section 2. The assignments of all signals were performed by H–H COSY experiments. The pattern of the signals was consistent with the crystal structure of [1]þ . Thus, the molecular structure of [1]þ in the solid state was stably retained in solutions. In CH3 CN, [1]þ showed a strong absorption band at 567 nm assignable to the metal-to-ligand charge transfer (MLCT) band from the dp orbital of Ru to the p orbital of the bpm ligand (vide infra). There are two preparative pathways for introduction of Me2 bpm (Me2 bpm ¼ 1,10 -dimethyl-4,40 -bipyrimidi-
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nium) to the Ru(trpy) framework; one is the reactions of Me2 bpm2þ with Ru–terpyridine species, and the other is quaternization on the bpm ligand of [1]þ . Attempts to prepare the desired complex by the reaction of Me2 bpm2þ with several Ru–trpy complexes were not successful. On the other hand, quaternization of noncoordinated nitrogen of bpm in [1](BF4 ) with (CH3 )3 OBF4 gave [2](BF4 )3 in a good yield [11,12]. Elemental analysis of [2](BF4 )3 was consistent with the composition. Fig. 5 shows the 1 H NMR spectrum of [2](BF4 )3 together with the schematic structure of the assignment determined by H–H COSY experiments. The spectrum remained unchanged for several weeks. Thus, the quaternized complex [2]3þ was stable in solutions. The complex exhibited six signals from the trpy ligand and six from Me2 bpm, and the methyl protons of the quaternized pyrimidyl rings were observed at 4.69 and 4.09 ppm vs. Me4 Si. The chemical shifts of ring protons of the trpy and Me2 bpm ligands of [2]3þ slightly differed from those of [1]þ , but the splitting pattern of the signals was essentially similar to that of [1]þ . The molecular structure of [2]3þ in solution, therefore, is considered to be very close to that of [1]þ . 3.3. Spectroscopic and electrochemical properties of [1]þ The cyclic voltammogram of [1](PF6 ) in the presence of 0.1 mol dm3 n-Bu4 NPF6 in CH3 CN displayed the [1]þ /[1]2þ , [1]þ /[1]0 and [1]0 /[1] redox couples at E1=2 ¼ þ0:97, )0.83 and )1.46 V vs. SCE, respectively, in a potential range between )1.6 and +1.4 V (Fig. 6(a)). One-electron reduction of [1]þ under the electrolysis of the CH3 CN solution at )1.1 V (vs. SCE) resulted in an appearance of a broad absorption band around 700 nm with a slight decrease of the absorption band at 567 nm (Fig. 6(b)). At the same time, two new absorption bands emerged at 499 and 473 nm, indicating that [1]0 exhibits four absorption bands at 473, 499, 567 and 700 nm in the visible region. Re-oxidation of the resultant solution at 0 V completely disappeared three (473, 499 and 700 nm) of the four bands and fully recovered the spectrum of [1]þ . It is worthy of note that the pattern of two (499
Fig. 5. 1 H NMR spectrum of [2](BF4 )3 in CD3 CN.
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Fig. 6. Cyclic voltammogram of [1](PF6 ) (a) and UV–Vis spectral changes in the electrochemical reduction of [1]þ at )1.1 V (vs. SCE) (b) in CH3 CN containing n-Bu4 NPF6 (0.1 mol dm3 ). The arrows in the figures indicate the direction of the changes of the spectra during the reduction of [1]þ .
and 473 nm) of the four bands of [1]0 is quite close to those of the bands at 530 and 496 nm of the characteristic absorption bands of metal free anion radical of bpm in DMF [26]. The redox couple of [1]þ /[1]0 (E1=2 ¼ 0:83 V), therefore, was assigned to the bpmbased reduction. We also tried to detect the absorption spectra of [1] under the electrolysis of [1]0 in CH3 CN. The 499 and 473 nm absorption bands of [1]0 faded out under the electrolysis at )1.7 V, but the re-oxidation of the solution at 0 V did not recover the visible spectra of [1]þ . Thus, [1]0 represented by [Ru(trpy)(bpm )Cl]0 was stable in CH3 CN, and one-electron reduction of [1]0 caused fragmentations of the complex. Based on the redox behavior of analogous Ru(II)–bpm and Ru–trpy complexes [24,25], [1]0 /[1] redox couples is concluded to take place in trpy-based molecular orbitals. The absorption band of [1](PF6 ) at 567 nm completely disappeared in the electrochemical oxidation of the complex at +1.1 V in CH3 CN, and the spectrum of [1]þ fully recovered by the reduction of the resultant solution at 0 V. The metal-centered (RuII /RuIII ) redox couple, therefore, is responsible for such reversible cycle of the appearance and disappearance of the MLCT band (from dp orbital of Ru(II) to the LUMO of bpm) in the [1]þ /[1]2þ redox reaction. The redox potentials of the [Ru(N–N)3 ]2þ=3þ and [Ru(bpy)2 (N–N)]2þ=3þ couples (N–N ¼ bpy, bpz, bpym and bpm, bpz ¼ 2,20 -bipyrazine; bpym ¼ 2,20 -bipyrimidine) reflect the r-donating and p-acceptor characters of those bidentate ligands [24,25,27]. The redox potential
Fig. 7. Cyclic voltammogram of [2](BF4 )3 (a) and UV–Vis spectral changes in the electrochemical reduction of [2]3þ in CH3 CN containing n-Bu4 NPF6 (0.1 mol dm3 ). The first and the second reductions were conducted at +0.2 V (b) and )0.25 V (vs. SCE) (c), respectively. The arrows in the figures indicate the direction of the changes of the spectra during the reduction of [2]3þ .
of the RuII /RuIII couple of [1]þ (E1=2 ¼ þ1:00 V vs. Ag/ AgCl) 1 is in a range of those of the related complexes such as [Ru(trpy)(bpy)Cl]þ (E1=2 ¼ þ0:83, vs. Ag/AgCl) [28], [Ru(trpy)(bpz)Cl]þ (E1=2 ¼ þ1:09 V) 1 and [Ru(trpy) (bpym)Cl]þ (E1=2 ¼ þ1:01 V) [29] (Table 3). The red shift of the MLCT band of [1](PF6 ) (kmax ¼ 567 nm) compared with those of [Ru(trpy)(bpy)Cl]þ (504 nm) [28], [Ru(trpy)(bpz)Cl]þ (513 nm) 1 and [Ru(trpy)(bpym)Cl]þ (516 nm) [28], therefore, results from the lower energy gap between dp orbital of Ru(II) and LUMO of bpm among those of [Ru(trpy)(N–N)Cl]þ , [Ru(N–N)3 ]2þ and [Ru(bpy)2 (N–N)]2þ 1 [24,25,28,29].
1
Electrode potentials based on SCE and Ag/AgCl reference electrodes are correlated with the following equation; EAg=AgCl ¼ ESCE þ 0:03 V.
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Table 3 Data for [Ru(trpy)(N–N)Cl]þ complexes in CH3 CN Complex [Ru(trpy)(bpm)Cl](PF6 )[1](PF6 )b [Ru(trpy)(bpy)Cl](PF6 )c [Ru(trpy)(bpz)Cl](PF6 )c [Ru(trpy)(bpym)Cl](PF6 )b
Potential (V)a
kmax (nm)
RuII /RuIII
MLCT
+1.00 +0.83 +1.09 +1.01
567 504 513 516
Ref. d
[21,28] [28] [29]
a
Potentials referenced to Ag/AgCl. Supporting electrolyte was 0.1 mol dm3 n-Bu4 NPF6 . c Supporting electrolyte was 0.1 mol dm3 n-Bu4 NClO4 . d This work. b
Table 4 Redox potentials of quaternized bpm compounds in CH3 CN Complex
Potential (V) vs. SCE Ligand
[Ru(trpy)(Me2 bpm)Cl](BF4 )3 [2] (BF4 )3 a (Me2 bpm)(PF6 )2 b (Me2 bpm)(BF4 )2 c [Ru(bpy)2 (Me2 bpm)](PF6 )4 c [Re(Me2 bpm)(CO)3 Cl](PF6 )3 c
+0.37, +0.08, +0.03, +0.54, +0.19,
Ref. II
III
Ru /Ru
)0.05 )0.42 )0.46 +0.08 )0.31 (Epc )d
+1.54 (Epc ) – – +1.85 (Epc ) –
e e
[11] [11] [12]
a
Supporting electrolyte was 0.1 mol dm3 n-Bu4 NBF4 . Supporting electrolyte was 0.1 mol dm3 n-Bu4 NPF6 . c Supporting electrolyte was 0.1 mol dm3 n-Bu4 NClO4 . d Potential referred at Fc/Fcþ . e This work. b
3.4. Spectroscopic and electrochemical properties of [2]3þ Fig. 7 shows the CV of [2](BF4 )3 in CH3 CN and the redox potentials of [2](BF4 )3 together with the related compounds are listed in Table 4. Two reversible [2]3þ / [2]2þ and [2]2þ /[2]þ redox couples at E1=2 ¼ þ0:37 and )0.05 V vs. SCE (Fig. 7(a)) are ascribed to the ligandbased redox reactions of Me2 bpm on the basis of the redox behavior of [1]þ and (Me2 bpm)2þ . In addition, an irreversible anodic peak assigned to metal-centered redox process appeared at Epa ¼ þ1:54 V (not shown in Fig. 7). The controlled potential electrolysis of [2](BF4 )3 at +0.2 V (vs. SCE) resulted in a decrease of the absorption band at 836 nm in intensity with time, and very broad absorption bands appeared around 860 and 650 nm (Fig. 7(b)). Taking into account that the 836 nm band was assigned to the MLCT from Ru(II) to Me2 bpm, the shift of the band upon one electron reduction is reasonably explained by the ligand-based redox reaction. One-electron reduction of [2]3þ , therefore, produces [Ru(trpy)(Me2 bpmþ )Cl]2þ , though the pattern of the absorption bands around 650 nm was too broad to compare those of Me2 bpm2þ (kmax ¼ 528 nm). The 860 nm band of [2]2þ disappeared under the electrolysis of the resultant solution at )0.15 V (Fig. 7(c)) due to the occupation of two electrons into the LUMO of the Me2 bpm ligand. The re-oxidation of the CH3 CN solu-
tion of [2]2þ and [2]þ at +0.6 V completely regenerated the UV–Vis spectrum of [2]3þ , indicating that both [2]2þ and [2]þ were stable in solutions. The feature of bpm as a redox active ligand is the significant anodic shift of the redox potential induced by diquaternization, since the redox potentials of bpm and Me2 bpm2þ were )1.34 V (in DMF vs. SCE) [24,26] and +0.03 V (in CH3 CN vs. SCE) [11], respectively. Appearance of the ligand-localized redox reactions at )0.83 V and +0.37 V of [1]þ and [2]3þ , respectively, demonstrated that anodic shifts of the redox potentials upon methylation was +1200 mV. A similar anodic shift by methylation of bpm was also observed in the system between [Ru(bpy)2 (bpm)](PF6 )2 and [Ru(bpy)2 (Me2 bpm)](PF6 )4 [11]. It is worthy of note, that the difference in the redox potential between bpm and Me2 bpm2þ is essentially unchanged even after the coordination of these ligands on Ru(II).
4. Conclusion Novel ruthenium(II) complexes with 4,40 -bipyrimi dine, [Ru(trpy)(bpm)Cl](X) [1](X; X ¼ PF 6 , BF4 ) and 0 0 with a 1,1 -dimethyl-4,4 -bipyrimidinium ligand, [Ru (trpy)(Me2 bpm)Cl](BF4 )3 [2](BF4 )3 were prepared and characterized. Quaternization on the nitrogen atoms of the bpm ligand caused not only anodic shift by 1200 mV
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of the ligand-based redox potential from the [1]þ /[1]0 couple at )0.83 V (vs. SCE) to the [2]3þ /[2]2þ one at +0.37 V, but also red shift of the MLCT band from [1]þ at 567 nm to [2]3þ at 836 nm. Thus, an electron acceptor ability of the bpm ligand of Ru(II)-trpy-bpm was remarkably enhanced by the quaternization of the noncoordinated nitrogen atoms of bpm.
5. Supplementary material Crystallographic data for the structures have been deposited with the Cambridge Center, CCDC Nos. CCDC-212910 and 212911 for structures (Me2 bpm) (BF4 )2 and [1] (PF6 ) 1/2(CH3 CN) 1/2(H2 O), respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (internet)/+44-1223-336033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).
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[9] (a) H. Sugimoto, H. Wada, Y. Wakatsuki, T. Wada, K. Tanaka, Chem. Lett. (2002) 634; (b) H. Sugimoto, K. Tsuge, K. Tanaka, J. Chem. Soc., Dalton Trans. (2001) 57. [10] T. Fujihara, R. Okamura, T. Wada, K. Tanaka, Dalton Trans. (2003) 3221. [11] W. Kaim, W. Matheis, Chem. Ber. 123 (1990) 1323. [12] (a) W. Matheis, J. Poppe, W. Kaim, S. Zalis, J. Chem. Soc., Perkin Trans. 2 (1994) 1923; (b) W. Matheis, W. Kaim, J. Chem. Soc., Faraday Trans. 86 (1990) 3337. [13] B.P. Sullivan, J.M. Calvert, T.J. Meyer, Inorg. Chem. 19 (1980) 1404. [14] N. Ple, A. Turck, K. Couture, G. Queguiner, J. Org. Chem. 60 (1995) 3781. [15] CrystalClear: Software Package, Rigaku and Molecular Structure Corporation (1999). [16] SIR92: A. Altomare, M.C. Burla, M. Camalli, M. Cascarano, C. Giacovazzo, A. Guagliardi, G. Polidori, J. Appl. Cryst. 27 (1994) 435. [17] PATTY: P.T. Beurskens, G. Admiraal, G. Beurkens, W.P. Bosman, R. de Gelder, R. Israel, J.M.M. Smith, The D I R D I F -94 program system; Technical Report of the Crystallography Laboratory, University of Nijmengen, The Netherland, 1994. [18] DIRDIF94: P.T. Beurskens, G. Admiraal, G. Beurkens, W.P. Bosman, R. de Gelder, R. Israel, J.M.M. Smith, The D I R D I F -94 program system; Technical Report of the Crystallography Laboratory, University of Nijmengen, The Netherland, 1994. [19] teXsan: Crystal Structure Analysis Package; Molecular Structure Corporation (1992). [20] G. Tapolsky, F. Robert, J.P. Launay, New J. Chem. 12 (1988) 761. [21] (a) K.J. Takeuchi, M.S. Thompson, D.W. Pipes, T.J. Meyer, Inorg. Chem. 23 (1984) 1845; (b) M.J. Root, E. Deutsch, Inorg. Chem. 24 (1985) 1464. [22] (a) M.-H.V. Huynh, J. Smyth, M. Wetzler, B. Mort, P.K. Gong, L.M. Witham, D.L. Jameson, D.K. Geiger, J.M. Lasker, M. Charepoo, M. Gornikiewicz, J.M. Cintron, G. Imahori, R.R. Sanchez, A.C. Marschilok, L.M. Krajkowski, D.G. Churchill, M.R. Churchill, K.J. Takeuchi, Angew. Chem. Int. Ed. 40 (2001) 4469; (b) C.M. Hartshorn, K.A. Maxwell, P.S. White, J.M. DeSimone, T.J. Meyer, Inorg. Chem. 40 (2001) 601; (c) C. Bonnefous, A. Chouai, R.P. Thummel, Inorg. Chem. 40 (2001) 5851; (d) B. Mondal, M.G. Walawalkar, G.K. Lahiri, J. Chem. Soc., Dalton Trans. (2000) 4209; € (e) V.J. Catalano, R.A. Heck, C.E. Immoos, A. Ohma, M.G. Hill, Inorg. Chem. 37 (1998) 2150; (f) L.F. Szczepura, S.M. Maricich, R.F. See, M.R. Churchill, K.J. Takeuchi, Inorg. Chem. 34 (1995) 4198; (g) A.L. Spek, A. Gerli, J. Reedijk, Acta Cryst., Sect. C 50 (1994) 394. [23] J. Breu, P. Belser, H. Yersin, Acta Cryst., Sect. C 52 (1996) 858. [24] (a) W. Kaim, S. Ernst, V. Kasack, J. Am. Chem. Soc. 112 (1990) 173; (b) S.D. Ernst, W. Kaim, Inorg. Chem. 28 (1989) 1520; (c) S. Ernst, W. Kaim, Angew. Chem. Int. Ed. Engl. 24 (1985) 430. [25] S. Zalis, M. Krejcik, V. Drchal, A.A. Vlcek, Inorg. Chem. 34 (1995) 6008. [26] M. Krejcik, S. Zalis, M. Ladwig, W. Matheis, W. Kaim, J. Chem. Soc., Perkin Trans. 2 (1992) 2007. [27] A.A. Vlcek, Coord. Chem. Rev. 200-202 (2000) 979. [28] A. Gerli, J. Reedijk, M.T. Lakin, A.L. Spek, Inorg. Chem. 34 (1995) 1836. [29] S. Swavey, Z. Fang, K.J. Brewer, Inorg. Chem. 41 (2002) 2598.
Syntheses and electrochemical properties of ruthenium(II) complexes with 4,40-bipyrimidine and 4,40-bipyrimidinium ligands Tetsuaki Fujihara, Tohru Wada, Koji Tanaka
*
Coordination Chemistry Laboratories, Institute for Molecular Science and CREST, JAPAN Science and Technology Agency (JST), 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan Received 2 September 2003; accepted 25 October 2003
Abstract The syntheses and electrochemical properties of novel ruthenium(II) polypyridyl complexes with 4,40 -bipyrimidine, [Ru(tr 0 py)(bpm)Cl](X) ([1](X; X ¼ PF 6 , BF4 )) and with a quaternized 4,4 -bipyrimidinium ligand, [Ru(trpy)(Me2 bpm)Cl](BF4 )3 0 0 00 0 ([2](BF4 )3 ) (trpy ¼ 2,2 :6 ,2 -terpyridine, bpm ¼ 4,4 -bipyrimidine, Me2 bpm ¼ 1,10 -dimethyl-4,40 -bipyrimidinium) are presented. The bpm complex [1]þ was prepared by the reaction of Ru(trpy)Cl3 with 4,40 -bipyrimidine in EtOH/H2 O. The structural characterization of [1]þ revealed, that the bpm ligand coordinated to the ruthenium atom with the bidentate fashion. Diquaternization of the noncoordinating nitrogen atoms on bpm of [1]þ by (CH3 )3 OBF4 in CH3 CN gave [2](BF4 )3 . The electrochemical and spectroelectrochemical properties of the complexes are described. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Ruthenium(II) complex; 4,40 -Bipyrimidine; 4,40 -Bipyrimidinium; Electrochemistry; X-ray structures
1. Introduction Much attention has been paid to the syntheses of transition metal complexes with redox active ligands, since those metal complexes often exhibit unique photochemical and electrochemical properties. Among numerous redox active ligands, metal complexes with dioxolene ligands have been intensively studied [1]. For instance, ruthenium–dioxolene complexes are featured by close energy levels between 3d orbitals of ruthenium and p orbitals of the ligand, resulting in delocalization of electron between two orbitals, that is, charge distribution [2,3]. We have also reported, a series of ruthenium(II) complexes having redox active ligands such as dioxolene [4–8], dithiolene [9] and 1,10-phenanthroline-5,6-dione [10]. In particular, acid–base equi-
*
Corresponding author. Tel.: +81-564-55-7241; fax: +81-564-555245. E-mail address: [email protected] (K. Tanaka). 0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.10.015
librium of an aqua ligand of a ruthenium–dioxolene complex produces a ruthenium–semiquinone oxyl radical complex, [Ru(trpy)(Bu2 SQ)(O )] (Bu2 SQ ¼ 3,5-ditert-butylbenzosemiquinonate) [7,8]. Such unique reversible conversion between aqua and oxyl radical ligands on ruthenium is accomplished by, utilization of low lying LUMO of ruthenium–dioxolene moiety. From the viewpoint of redox active ligand, N ; N 0 dialkylated (diquaternized) 4,40 -bipyrimidine (bpm) is specially interesting, because it has the redox functionality of 1,10 -dialkyl-4,40 -bipyridinium, as well as, the strong chelating ability of 2,20 -bipyridine (Scheme 1) and the strong electron acceptor character of the ligand, has been suggested in ruthenium(II) and rhenium(I) complexes [11,12]. We now pay attention to the redox functionality of 4,40 -bipyrimidinium ligand on the ruthenium(II)–terpyridine unit. In this paper, we will report the syntheses and electrochemical properties of [Ru (trpy)(bpm)Cl](X) [1](X; X ¼ PF 6 or BF4 ), and with a quaternized bipyrimidinium ligand, [Ru(trpy)(Me2 bpm) Cl](BF4 )3 [2](BF4 )3 (Fig. 1).
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Scheme 1.
Fig. 1. Ru(II)–terpyridine complexes bearing 4,40 -bipyrimidine (bpm) and 1,10 -dimethyl-4,40 -bipyrimidinium (Me2 bpm2þ ).
2. Experimental 2.1. Materials 2,20 :60 ,600 -Terpyridine (trpy) was purchased from Aldrich. Pyrimidine and trimethyloxonium tetrafluoroborate ((CH3 )3 OBF4 ) were purchased from TCI. Other agents and solvents were purchased from Wako and used as received. Alumina, A, Super I (ICN biochemicals GmbH) was used for column chromatography. 2.2. Syntheses 2.2.1. Syntheses of precursors Ru(trpy)Cl3 was synthesized by the previous report [13]. 4,40 -bipyrimidine (bpm) [14] and 1,10 -dimethyl-4,40 bipyrimidinium tetrafluoroborate (Me2 bpm)(BF4 )2 [11,12] were also prepared according to previous reports. The subsequent exchange of the counter anion from BF 4 to PF6 gave (bpmMe2 )(PF6 )2 , quantitatively. Found: C, 25.53; H, 2.71; N, 11.95. Anal. Calc. for C10 H12 N4 P2 F12 : C,25.12; H, 2.53; N, 11.71%. 2.2.2. [Ru(trpy)(bpm)Cl](PF6 ) ([1](PF6 )) Ru(trpy)Cl3 (132 mg, 0.3 mmol) and bpm (48 mg, 0.3 mmol) were gently refluxed for 5 h in 24 cm3 of a mixed
solvent of ethanol–H2 O (3:1, v/v) containing LiCl (42 mg) and triethylamine (0.1 cm3 ) as a reductant. The hot reaction mixture was filtered off and then the filtrate was evaporated to dryness. The residue was dissolved in a small amount of EtOH and NH4 PF6 saturated in EtOH was added. The resulting precipitate was collected by filtration, washed with small amounts of EtOH and diethyl ether. The crude product was dissolved in CH3 CN and the solution was loaded on an alumina column. The second purple band eluted with 2% MeOH–CH3 CN was collected and the solvent was removed by using a rotary evaporator. The purple residue was recrystallized from CH3 CN by addition of diethyl ether. The purple crystals thus formed were collected by filtration, washed with Et2 O and dried under reduced pressure. Yield: 14 mg (7%). Found: C, 41.14; H, 2.91; N, 15.03. Anal. Calc. for C23 H17 N7 PF6 ClRu: C, 41.05; H, 2.55; N, 14.57%. 1 H NMR: dH (500 MHz; CD3 CN; standard SiMe4 ) 10.80 (1H, s, bpm), 9.26 (1H, d, J (HH) 5.5 Hz, bpm), 8.71 (1H, d, J (HH) 5.5 Hz, bpm), 8.63 (1H, d, J (HH) 5.0 Hz, bpm), 8.52 (2H, d, J (HH) 8.0 Hz, trpy), 8.42 (1H, d, J (HH) 5.0 Hz, bpm), 8.39 (2H, t, J (HH) 9.0 Hz, trpy), 8.16 (1H, t, J (HH) 9.0 Hz, trpy), 8.15 (1H, s, bpm), 7.92 (2H, t, J (HH) 8.0 Hz, trpy), 7.68 (2H, d, J (HH) 5.0 Hz, trpy) and 7.27 (2H, t, J (HH) 6.5 Hz, trpy). UV–Vis: kmax /nm (CH3 CN) 567 (e/dm3 mol1 cm1 8500), 360 (7200), 315 (30,500), 278 (32,900) and 236 (26,800). ESI MS: m=z (CH3 CN) 528 ([M–PF6 ]þ ). 2.2.3. [Ru(trpy)(bpm)Cl](BF4 ) ([1](BF4 )) The complex was prepared by the similar method of [1](PF6 ) by using NH4 BF4 instead of NH4 PF6 . Yield: 13 mg (7%). Found: C, 45.18; H, 3.52; N, 16.05%. C23 H17 N7 BF4 ClRu requires for C, 44.94; H, 2.79; N, 15.95%. ESI MS: m=z (CH3 CN) 528 ([M–BF4 ]þ ). 2.2.4. [Ru(trpy)(Me2 bpm)Cl](BF4 )3 ([2](BF4 )3 ) To an acetonitrile solution (10 cm3 ) of [1](BF4 ) (12 mg, 0.02 mmol), which was deoxygenated by bubbling of N2 gas for 30 min, was added (CH3 )3 OBF4 (60 mg, 0.4 mmol) under N2 . The reaction mixture was heated gently at 65 °C for 1 h under N2 . After the reaction mixture was cooled to room temperature, the solvent was removed under N2 flow. To the residue THF (20 cm3 ) was added and the resultant precipitates were filtered off, washed with THF and dried under reduced pressure. Yield: 14 mg (86%). Found: C, 36.24; H, 2.86; N, 11.65. Anal. Calc. for C25 H23 N7 B3 F12 ClRu: C, 36.69; H, 2.83; N, 11.98%. 1 H NMR: dH (500 MHz; CD3 CN; standard SiMe4 ) 11.12 (1H, s, Me2 bpm), 9.26 (1H, d, J (HH) 5.5 Hz, Me2 bpm), 9.32 (2H, d+d, Me2 bpm), 9.03 (1H, d, J (HH) 6.5 Hz, Me2 bpm), 8.78 (2H, s+d, Me2 bpm), 8.61 (2H, d, J (HH) 8.0 Hz, trpy), 8.43 (2H, d, J (HH) 8.0 Hz, trpy), 8.38 (1H, t, J (HH) 8.0 Hz, trpy), 8.01 (2H, t, J (HH) 8.0 Hz, trpy), 7.49 (2H, d, J (HH) 5.5 Hz, trpy), 7.28 (2H, t, J (HH) 5.5 Hz, trpy), 4.69 (3H, s,
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Me2 bpm) and 4.09 (3H, s, Me2 bpm). UV–Vis: kmax /nm (CH3 CN) 836 (e/dm3 mol1 cm1 4000), 427 (8900), 305 (32,900), 282 (26,800) and 235 (27,000). 2.3. Physical measurements 1
H NMR and H–H COSY experiments were performed with a JEOL GX-500 spectrometer. UV–Vis spectra were recorded on a Shimadzu UV-3100PC UV–Vis–NIR scanning spectrophotometer. ESI MS spectra were measured with a Shimadzu LCMS-2010 liquid chromatograph mass spectrometer. Elemental analyses were carried out at Research Center for Molecular-scale Nanoscience, Institute for Molecular Science. Cyclic voltammetry was performed with an ALS/ Chi model 660 electrochemical analyzer. Cyclic voltammograms were recorded at a scan rate of 100 mV s1 at room temperature. The working and the counter electrodes were a glassy carbon and a Pt wire, respectively. An Ag/AgNO3 (0.01 mol cm3 ) was used as a reference electrode in the present study. To compare the redox potentials of the complexes with those of analogs reported so far, the potentials were converted to SCE (ESCE ¼ EAg=Agþ þ 0:33 V). The sample solutions in CH3 CN containing n-Bu4 NPF6 or n-Bu4 NBF4 (0.1 mol dm3 ) were deoxygenated by N2 stream. Spectroelectrochemical UV–Vis measurements were performed with a thin-layer electrode cell with a platinum minigrid working electrode sandwiched between two glass side of an optical cell (path length 0.5 mm) by using a Hokuto Denko HA-501 potentiostat and a Shimadzu UV-3100PC UV–Vis–NIR scanning spectrophotometer.
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2.4. X-ray crystallography A summary of the crystal structure refinements of (Me2 bpm)(BF4 )2 and [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) was given in Table 1. The single crystals of (Me2 bpm)(BF4 )2 suitable for X-ray analysis were obtained by recrystallization of the compound with the CH3 CN/toluene solution. The single crystals of [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) were obtained by slow diffusion of Et2 O into an acetonitrile solution of [1](PF6 ). Data were collected on a Rigaku/MSC Mercury CCD diffractometer using graphite-monochro at 173 K, and mated Mo Ka radiation (k ¼ 0:71070 A) processed using Crystal Clear [15]. The structure of (Me2 bpm)(BF4 )2 was solved by a direct method (SIR92) [16]. The structure of [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) was solved by a heavy-atom Patterson method (PATTY) [17] and expanded using Fourier Techniques (DIRDIF94) [18]. These structures were refined by fullmatrix least-square refinement on F 2 . All non hydrogen atoms without those of crystal solvents were refined anisotopically. All hydrogen atoms were located on the calculated positions and not refined. All calculations were performed using the teXsan crystallographic software package [19].
3. Results and discussion 3.1. Syntheses and properties of bpm and Me2 bpm2þ 4,4-Bipyrimidine (bpm) was prepared by the coupling reaction of pyrimidine in the presence of a strong base
Table 1 Crystallographic data of (Me2 bpm)(BF4 )2 and [1](PF6 ) 1/2(CH3 CN) 1/2(H2 O) (Me2 bpm)(BF4 )2
[1](PF6 ) 1/2(CH3 CN) 1/2(H2 O)
Empirical formula C10 H12 N4 B2 F8 C24 H19:5 N7:5 O0:5 PF6 ClRu Formula weight 361.84 702.46 Temperature (K) 173 173 Crystal system monoclinic triclinic Space group P 21 =n (#14) P 1 (#2) Unit cell dimension a (A) 5.503(8) 8.719(7) b (A) 10.69(1) 9.672(7) c (A) 12.37(2) 16.74(1) a (°) 90 97.910(8) b (°) 92.71(2) 92.035(8) c (°) 90 109.63(1) 3 ) V (A 726(1) 1311(1) Z 2 2 qc (g cm3 ) 1.66 1.78 Unique reflections 1639 5779 Observed reflections 1009, I > 3rðIÞ 4271, I > 2rðIÞ R1 , wR2 a 0.066, 0.197b 0.060, 0.127c P P P P a R1 ¼ ½jFo j jFc j= jFo j; wR2 ¼ f ½wðFo Fc2 Þ2 = ½wðFo2 Þ2 g1=2 ; w ¼ fr2 ðFo2 Þ þ ½pðmaxðFo2 ; 0Þ þ 2Fc2 Þ=32 g1 . b p ¼ 0:140. c p ¼ 0:075.
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[14]. The quaternized bpm compound, (Me2 bpm)(BF4 )2 , was synthesized by the reaction of bpm with (CH3 )3 OBF4 in 1,2-dichloroethane [11,12]. The molecular structure of (Me2 bpm)(BF4 )2 (Fig. 2) revealed that quaternization takes place exclusively on the nitrogen atoms of para position with respect to the connected carbon atom. There found two counter anions (BF 4 ) in the Me2 bpm unit. The central carbon–carbon distance is somewhat shorter than a single bond, (1.490(5) A) indicating a small contribution of double bond character. The planar structure of the two cycles, therefore, is ascribed to the conjugation of the carbon–carbon bond. The C–C and C–N bond lengths in the pyrimidine ring are essentially similar to those of 4,40 -bipyrimidine [20]. Fig. 3 depicts the CV of (Me2 bpm)(PF6 )2 in CH3 CN together with the UV–Vis spectral changes caused by the reduction of (Me2 bpm)2þ . The CV of (Me2 bpm)2þ showed two reversible (Me2 bpm)2þ /(Me2 bpm)þ and (Me2 bpm)þ /(Me2 bpm)0 couples at E1=2 ¼ þ0:08 and )0.42 V vs. SCE, respectively, (Fig. 3(a)) [11]. The dicationic Me2 bpm2þ has no absorption band in the visible region, and one-electron reduction of Me2 bpm2þ under the electrolysis of the solution at )0.1 V (vs. SCE), brought about an appearance of two new absorption bands at 528 and 382 nm (Fig. 3(b)). The spectrum was consistent with that of (Me2 bpm)þ produced by chemical reductions [11] and the appearance of the strong two absorption bands (528 and 382 nm) were close to that of a cation radical of methyl viologen (603 and 395 nm) [11]. The 528 and 382 nm bands disappeared upon two-electron reduction of (Me2 bpm)2þ under the electrolysis at )0.7 V (Fig. 3(c)). 3.2. Syntheses and characterization of ruthenium(II) complexes The 4,40 -bipyrimidine (bpm) complex, [Ru(trpy) (bpm)Cl]þ ([1]þ ), was synthesized by the reaction of Ru(trpy)Cl3 with bpm in EtOH/H2 O by reference to the synthetic procedure of [Ru(trpy)(bpy)Cl]þ [21]. The ESI MS spectrum of the reaction mixture, using CH3 CN as a solvent, showed several peaks corresponding to
Fig. 2. ORTEP drawing of (Me2 bpm)2þ . Thermal ellipsoids are drawn at the 50% probability level. The central carbon–carbon and N– respectively. C(methyl) distances were 1.490(5) and 1.488(5) A,
Fig. 3. Cyclic voltammogram of (Me2 bpm)(PF6 )2 (a) and UV–Vis spectral changes in the electrochemical reduction of (Me2 bpm)2þ in CH3 CN containing n-Bu4 NPF6 (0.1 mol dm3 ). The first and the second reductions were conducted at )0.1 V (b) and )0.7 V (vs. SCE) (c), respectively. The arrows in the figures indicate the direction of the changes of the spectra during the reduction of (Me2 bpm)2þ .
the molecular weights of [Ru(trpy)(bpm)2 (CH3 CN)]2þ (m=z 346), [Ru(trpy)(bpm)2 ]2þ (326), [1]þ (528), [Ru(trpy) (bpm)(CH3 CN)Cl]þ (569) and [Ru(trpy) (bpm)2 Cl]þ (686), indicating formation of a mixture of Ru complexes with monodentate and bidentate bpm ligands. We successfully separated [1]þ from the reaction mixtures by column chromatography, and the purity of [1](X) was confirmed by the ESI MS spectrum (m=z 528) with the isotope distribution pattern of a ruthenium nuclei, and elemental analysis. Fig. 4 showed the molecular structure of [1]þ determined by X-ray diffraction study. Selected bond lengths and angles around the ruthenium atom are listed in Table 2. The molecular structure of [1]þ clearly showed that bpm coordinated to the ruthenium atom with a bidentate manner. The coordination environment around the ruthenium atom is a distorted octahedral with one chloride atom, three nitrogen atoms of trpy, and two nitrogen atoms of bpm. The distance of Ru–Cl is comparable to those found for other (2.422(2) A)
T. Fujihara et al. / Inorganica Chimica Acta 357 (2004) 1205–1212
Fig. 4. ORTEP drawing of [1]þ . Thermal ellipsoids are drawn at the 50% probability level.
Table 2 and angles (°) for [1]þ Selected bond lengths (A) Bond lengths Ru(1)–N(1) Ru(1)–N(3) Ru(1)–N(6)
2.075(5) 2.086(5) 2.088(5)
Ru(1)–N(2) Ru(1)–N(4) Ru(1)–Cl(1)
1.963(5) 2.049(4) 2.422(2)
Bond angles N(1)–Ru(1)–N(3) N(2)–Ru(1)–N(3) N(2)–Ru(1)–N(6) Cl(1)–Ru(1)–N(1) Cl(1)–Ru(1)–N(3) Cl(1)–Ru(1)–N(6)
158.7(2) 79.0(2) 175.6(2) 90.9(1) 89.8(1) 93.3(1)
N(1)–Ru(1)–N(2) N(2)–Ru(1)–N(4) N(4)–Ru(1)–N(6) Cl(1)–Ru(1)–N(2) Cl(1)–Ru(1)–N(4)
79.7(2) 97.0(2) 78.6(2) 91.1(1) 171.9(1)
Ru(II)–terpyridine-chloride complexes with bidentate N–N ligands [10,22]. The bond lengths of three Ru– respecN(trpy) are 2.075(5), 1.963(5), and 2.086(5) A, tively. The shortening of the Ru–N(2) distance is a typical feature observed in the structures of other ruthenium(II)–terpyridine complexes [22]. The bond lengths of Ru–N(bpm) were 2.049(4) and 2.088(5) A, respectively. The Ru–N bond length trans to the nitrogen atom of the central pyridyl ring for trpy is slightly longer than that trans to chloride atom. The C–N and C–C bond lengths in the pyrimidyl ring were essentially same as those found in [Ru(bpm)3 ](PF6 )2 [23]. 1 H NMR spectra of [1](PF6 ) in CD3 CN displayed 6 signals from trpy and 6 signals from bpm as described in the Section 2. The assignments of all signals were performed by H–H COSY experiments. The pattern of the signals was consistent with the crystal structure of [1]þ . Thus, the molecular structure of [1]þ in the solid state was stably retained in solutions. In CH3 CN, [1]þ showed a strong absorption band at 567 nm assignable to the metal-to-ligand charge transfer (MLCT) band from the dp orbital of Ru to the p orbital of the bpm ligand (vide infra). There are two preparative pathways for introduction of Me2 bpm (Me2 bpm ¼ 1,10 -dimethyl-4,40 -bipyrimidi-
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nium) to the Ru(trpy) framework; one is the reactions of Me2 bpm2þ with Ru–terpyridine species, and the other is quaternization on the bpm ligand of [1]þ . Attempts to prepare the desired complex by the reaction of Me2 bpm2þ with several Ru–trpy complexes were not successful. On the other hand, quaternization of noncoordinated nitrogen of bpm in [1](BF4 ) with (CH3 )3 OBF4 gave [2](BF4 )3 in a good yield [11,12]. Elemental analysis of [2](BF4 )3 was consistent with the composition. Fig. 5 shows the 1 H NMR spectrum of [2](BF4 )3 together with the schematic structure of the assignment determined by H–H COSY experiments. The spectrum remained unchanged for several weeks. Thus, the quaternized complex [2]3þ was stable in solutions. The complex exhibited six signals from the trpy ligand and six from Me2 bpm, and the methyl protons of the quaternized pyrimidyl rings were observed at 4.69 and 4.09 ppm vs. Me4 Si. The chemical shifts of ring protons of the trpy and Me2 bpm ligands of [2]3þ slightly differed from those of [1]þ , but the splitting pattern of the signals was essentially similar to that of [1]þ . The molecular structure of [2]3þ in solution, therefore, is considered to be very close to that of [1]þ . 3.3. Spectroscopic and electrochemical properties of [1]þ The cyclic voltammogram of [1](PF6 ) in the presence of 0.1 mol dm3 n-Bu4 NPF6 in CH3 CN displayed the [1]þ /[1]2þ , [1]þ /[1]0 and [1]0 /[1] redox couples at E1=2 ¼ þ0:97, )0.83 and )1.46 V vs. SCE, respectively, in a potential range between )1.6 and +1.4 V (Fig. 6(a)). One-electron reduction of [1]þ under the electrolysis of the CH3 CN solution at )1.1 V (vs. SCE) resulted in an appearance of a broad absorption band around 700 nm with a slight decrease of the absorption band at 567 nm (Fig. 6(b)). At the same time, two new absorption bands emerged at 499 and 473 nm, indicating that [1]0 exhibits four absorption bands at 473, 499, 567 and 700 nm in the visible region. Re-oxidation of the resultant solution at 0 V completely disappeared three (473, 499 and 700 nm) of the four bands and fully recovered the spectrum of [1]þ . It is worthy of note that the pattern of two (499
Fig. 5. 1 H NMR spectrum of [2](BF4 )3 in CD3 CN.
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Fig. 6. Cyclic voltammogram of [1](PF6 ) (a) and UV–Vis spectral changes in the electrochemical reduction of [1]þ at )1.1 V (vs. SCE) (b) in CH3 CN containing n-Bu4 NPF6 (0.1 mol dm3 ). The arrows in the figures indicate the direction of the changes of the spectra during the reduction of [1]þ .
and 473 nm) of the four bands of [1]0 is quite close to those of the bands at 530 and 496 nm of the characteristic absorption bands of metal free anion radical of bpm in DMF [26]. The redox couple of [1]þ /[1]0 (E1=2 ¼ 0:83 V), therefore, was assigned to the bpmbased reduction. We also tried to detect the absorption spectra of [1] under the electrolysis of [1]0 in CH3 CN. The 499 and 473 nm absorption bands of [1]0 faded out under the electrolysis at )1.7 V, but the re-oxidation of the solution at 0 V did not recover the visible spectra of [1]þ . Thus, [1]0 represented by [Ru(trpy)(bpm )Cl]0 was stable in CH3 CN, and one-electron reduction of [1]0 caused fragmentations of the complex. Based on the redox behavior of analogous Ru(II)–bpm and Ru–trpy complexes [24,25], [1]0 /[1] redox couples is concluded to take place in trpy-based molecular orbitals. The absorption band of [1](PF6 ) at 567 nm completely disappeared in the electrochemical oxidation of the complex at +1.1 V in CH3 CN, and the spectrum of [1]þ fully recovered by the reduction of the resultant solution at 0 V. The metal-centered (RuII /RuIII ) redox couple, therefore, is responsible for such reversible cycle of the appearance and disappearance of the MLCT band (from dp orbital of Ru(II) to the LUMO of bpm) in the [1]þ /[1]2þ redox reaction. The redox potentials of the [Ru(N–N)3 ]2þ=3þ and [Ru(bpy)2 (N–N)]2þ=3þ couples (N–N ¼ bpy, bpz, bpym and bpm, bpz ¼ 2,20 -bipyrazine; bpym ¼ 2,20 -bipyrimidine) reflect the r-donating and p-acceptor characters of those bidentate ligands [24,25,27]. The redox potential
Fig. 7. Cyclic voltammogram of [2](BF4 )3 (a) and UV–Vis spectral changes in the electrochemical reduction of [2]3þ in CH3 CN containing n-Bu4 NPF6 (0.1 mol dm3 ). The first and the second reductions were conducted at +0.2 V (b) and )0.25 V (vs. SCE) (c), respectively. The arrows in the figures indicate the direction of the changes of the spectra during the reduction of [2]3þ .
of the RuII /RuIII couple of [1]þ (E1=2 ¼ þ1:00 V vs. Ag/ AgCl) 1 is in a range of those of the related complexes such as [Ru(trpy)(bpy)Cl]þ (E1=2 ¼ þ0:83, vs. Ag/AgCl) [28], [Ru(trpy)(bpz)Cl]þ (E1=2 ¼ þ1:09 V) 1 and [Ru(trpy) (bpym)Cl]þ (E1=2 ¼ þ1:01 V) [29] (Table 3). The red shift of the MLCT band of [1](PF6 ) (kmax ¼ 567 nm) compared with those of [Ru(trpy)(bpy)Cl]þ (504 nm) [28], [Ru(trpy)(bpz)Cl]þ (513 nm) 1 and [Ru(trpy)(bpym)Cl]þ (516 nm) [28], therefore, results from the lower energy gap between dp orbital of Ru(II) and LUMO of bpm among those of [Ru(trpy)(N–N)Cl]þ , [Ru(N–N)3 ]2þ and [Ru(bpy)2 (N–N)]2þ 1 [24,25,28,29].
1
Electrode potentials based on SCE and Ag/AgCl reference electrodes are correlated with the following equation; EAg=AgCl ¼ ESCE þ 0:03 V.
T. Fujihara et al. / Inorganica Chimica Acta 357 (2004) 1205–1212
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Table 3 Data for [Ru(trpy)(N–N)Cl]þ complexes in CH3 CN Complex [Ru(trpy)(bpm)Cl](PF6 )[1](PF6 )b [Ru(trpy)(bpy)Cl](PF6 )c [Ru(trpy)(bpz)Cl](PF6 )c [Ru(trpy)(bpym)Cl](PF6 )b
Potential (V)a
kmax (nm)
RuII /RuIII
MLCT
+1.00 +0.83 +1.09 +1.01
567 504 513 516
Ref. d
[21,28] [28] [29]
a
Potentials referenced to Ag/AgCl. Supporting electrolyte was 0.1 mol dm3 n-Bu4 NPF6 . c Supporting electrolyte was 0.1 mol dm3 n-Bu4 NClO4 . d This work. b
Table 4 Redox potentials of quaternized bpm compounds in CH3 CN Complex
Potential (V) vs. SCE Ligand
[Ru(trpy)(Me2 bpm)Cl](BF4 )3 [2] (BF4 )3 a (Me2 bpm)(PF6 )2 b (Me2 bpm)(BF4 )2 c [Ru(bpy)2 (Me2 bpm)](PF6 )4 c [Re(Me2 bpm)(CO)3 Cl](PF6 )3 c
+0.37, +0.08, +0.03, +0.54, +0.19,
Ref. II
III
Ru /Ru
)0.05 )0.42 )0.46 +0.08 )0.31 (Epc )d
+1.54 (Epc ) – – +1.85 (Epc ) –
e e
[11] [11] [12]
a
Supporting electrolyte was 0.1 mol dm3 n-Bu4 NBF4 . Supporting electrolyte was 0.1 mol dm3 n-Bu4 NPF6 . c Supporting electrolyte was 0.1 mol dm3 n-Bu4 NClO4 . d Potential referred at Fc/Fcþ . e This work. b
3.4. Spectroscopic and electrochemical properties of [2]3þ Fig. 7 shows the CV of [2](BF4 )3 in CH3 CN and the redox potentials of [2](BF4 )3 together with the related compounds are listed in Table 4. Two reversible [2]3þ / [2]2þ and [2]2þ /[2]þ redox couples at E1=2 ¼ þ0:37 and )0.05 V vs. SCE (Fig. 7(a)) are ascribed to the ligandbased redox reactions of Me2 bpm on the basis of the redox behavior of [1]þ and (Me2 bpm)2þ . In addition, an irreversible anodic peak assigned to metal-centered redox process appeared at Epa ¼ þ1:54 V (not shown in Fig. 7). The controlled potential electrolysis of [2](BF4 )3 at +0.2 V (vs. SCE) resulted in a decrease of the absorption band at 836 nm in intensity with time, and very broad absorption bands appeared around 860 and 650 nm (Fig. 7(b)). Taking into account that the 836 nm band was assigned to the MLCT from Ru(II) to Me2 bpm, the shift of the band upon one electron reduction is reasonably explained by the ligand-based redox reaction. One-electron reduction of [2]3þ , therefore, produces [Ru(trpy)(Me2 bpmþ )Cl]2þ , though the pattern of the absorption bands around 650 nm was too broad to compare those of Me2 bpm2þ (kmax ¼ 528 nm). The 860 nm band of [2]2þ disappeared under the electrolysis of the resultant solution at )0.15 V (Fig. 7(c)) due to the occupation of two electrons into the LUMO of the Me2 bpm ligand. The re-oxidation of the CH3 CN solu-
tion of [2]2þ and [2]þ at +0.6 V completely regenerated the UV–Vis spectrum of [2]3þ , indicating that both [2]2þ and [2]þ were stable in solutions. The feature of bpm as a redox active ligand is the significant anodic shift of the redox potential induced by diquaternization, since the redox potentials of bpm and Me2 bpm2þ were )1.34 V (in DMF vs. SCE) [24,26] and +0.03 V (in CH3 CN vs. SCE) [11], respectively. Appearance of the ligand-localized redox reactions at )0.83 V and +0.37 V of [1]þ and [2]3þ , respectively, demonstrated that anodic shifts of the redox potentials upon methylation was +1200 mV. A similar anodic shift by methylation of bpm was also observed in the system between [Ru(bpy)2 (bpm)](PF6 )2 and [Ru(bpy)2 (Me2 bpm)](PF6 )4 [11]. It is worthy of note, that the difference in the redox potential between bpm and Me2 bpm2þ is essentially unchanged even after the coordination of these ligands on Ru(II).
4. Conclusion Novel ruthenium(II) complexes with 4,40 -bipyrimi dine, [Ru(trpy)(bpm)Cl](X) [1](X; X ¼ PF 6 , BF4 ) and 0 0 with a 1,1 -dimethyl-4,4 -bipyrimidinium ligand, [Ru (trpy)(Me2 bpm)Cl](BF4 )3 [2](BF4 )3 were prepared and characterized. Quaternization on the nitrogen atoms of the bpm ligand caused not only anodic shift by 1200 mV
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of the ligand-based redox potential from the [1]þ /[1]0 couple at )0.83 V (vs. SCE) to the [2]3þ /[2]2þ one at +0.37 V, but also red shift of the MLCT band from [1]þ at 567 nm to [2]3þ at 836 nm. Thus, an electron acceptor ability of the bpm ligand of Ru(II)-trpy-bpm was remarkably enhanced by the quaternization of the noncoordinated nitrogen atoms of bpm.
5. Supplementary material Crystallographic data for the structures have been deposited with the Cambridge Center, CCDC Nos. CCDC-212910 and 212911 for structures (Me2 bpm) (BF4 )2 and [1] (PF6 ) 1/2(CH3 CN) 1/2(H2 O), respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (internet)/+44-1223-336033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).
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