Proton-Sensitive Luminescent Ruthenium(II) Complexes with

The red shift of the absorption bands for the Ru(II)–CPC complexes upon protonation is possibly due to the lowering of the energy of the lowest lyin...
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Proton-Sensitive Luminescent Ruthenium(II) Complexes with Pyrazine-Based Pincer-Type N-Heterocyclic Carbene Ligands Chen-Shiang Lee,† Rui Rui Zhuang,† Ju-Chun Wang,‡ Wen-Shu Hwang,*,† and Ivan J. B. Lin† †

Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan, Republic of China Department of Chemistry, Soochow University, Taipei 111, Taiwan, Republic of China

‡

S Supporting Information *

ABSTRACT: New Ru(II) complexes with pyrazine-based pincer-type Nheterocyclic carbene ligands [( R CPC R ) 2 Ru]X 2 ( R CPC R = 2,6-bis(alkylimidazol-2-ylidene)pyrazine; R = Me and n-Bu; X = Cl and PF6) were synthesized and fully characterized. X-ray structure determinations revealed that complexes [( nBu CPC nBu ) 2 Ru]Cl 2 , [( Me CPC Me ) 2 Ru][PF 6 ] 2 , and [(nBuCPCnBu)2Ru][PF6]2 have pseudo-octahedral configurations around the Ru(II) center with the two pincer ligands in the core structure. Protonation and methylation of the noncoordinated pyrazine nitrogen atom of the complexes resulted in dramatic variation of both absorption and emission spectrum.

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the triplet metal-centered state (3MC) by raising the energy level of 3MC.8 Chung and co-workers first reported the photophysical behavior of a Ru(II) complex containing a pincer-type NHC−pyridine−NHC ligand, denoted as [Ru(CNC) 2 ]2+, where CNC is 2,6-bis(1-methylimidazol-2ylidene)pyridine.9 The bromide salt of the complex exhibited long-lifetime emission in water (3100 ns), suitable for practical applications such as artificial photosynthesis. In a related study, functionalization of the central pyridine in the parent complex [Ru(CNC)2]2+ with carboxylic acid strongly influences its electronic structure.10 In light of such an extraordinary photophysical property of [Ru(CNC)2]2+ complexes as well as their tunable electronic structure through ligand modification, we anticipate that the related d6 metal−NHC complexes may have great potential in the development of chemosensing. It is surprising that only a few metal−NHC complexes were applied to sensor systems,11 and to the best of our knowledge, only one case of the utilization of a Ru(II)−NHC complex as proton sensor has been reported very recently.12 In this work, we extend the pioneering work of Chung and co-workers to synthesize a new pincer NHC−pyrazine−NHC ligand, denoted as CPC, in which pyrazine is employed as central linker between two NHCs. The ligand readily binds to Ru(II) to form a complex in which a noncoordinated nitrogen atom in the pyrazine ring behaves as a potential binding site for electrophiles such as protons. Moreover, methylating the [Ru(II)(CPC)2]2+ complex results in a new compound in which the noncoordinated pyrazine nitrogen atom was

INTRODUCTION Molecules that can precisely and reversibly respond to the physicochemical stimulation are considered molecular switches.1 This field has received tremendous interest due to the high potential development in materials science, information, and communication technology.2 Complexes with second- and third-row metal ions (such as Ru(II), Os(II), Ir(III)) that emit in the sub-microsecond range in the visible region under ambient conditions became attractive because they may offer high sensitivity and selectivity of the signal through discrimination from the background fluorescence and are potentially useful in biological and environmental applications.3 In the past decades, many studies on the photophysical and photochemical properties of Ru-polypyridyl complexes have led to a better understanding of charge-transfer transitions and allow designing complexes with desired properties.4 Despite the excellent luminescent performance of [Ru(bpy)3]2+ (2,2′bipyridine), there has also been a growing interest in [Ru(tpy)2]2+ (2,2′:6′,2″-terpyridine) compounds. The achiral Ru(II) complexes of the tpy family present synthetic and structural advantages over the chiral bpy-type complexes, which usually results in a stereochemical problem in polynuclear assemblies.5 However, the low efficiency of the photophysical property6 of the [Ru(tpy)2]2+ type retards its development in practical use. In this regard, considerable attention has been paid to the modification of tpy-type ligands to improve its photophysical properties.7 N-Heterocyclic carbene (NHC) ligands are well known for their excellent σ-donating property in metal−NHC complexes. They may diminish the fast radiationless deactivation of the triplet metal-to-ligand charge transfer state (3MLCT) through © 2012 American Chemical Society

Received: March 20, 2012 Published: July 2, 2012 4980

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structure of A with the atomic numbering scheme is shown in Figure 2. The core structure and the coordination environment

methylated. Dramatic changes in absorption and emission spectra upon protonation and methylation are observed.

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RESULTS AND DISCUSSION Complexes [(RCPCR)2Ru]Cl2 (R = Me and n-Bu) (Figure 1) were synthesized via reaction of the ligand precursor [RCPCR-

Figure 1. Ru(II) complexes containing pyrazine-based pincer-type NHC ligands.

H]Cl2 with RuCl3 carried out in ethylene glycol at 190 °C for 1 day.13 Assignments of the 1H NMR spectra of the products in D2O were made by comparing with other analogous Ru(II) complexes of the CNC ligand.9,10,14 1H NMR signals of the coordinated RCPCR are significantly upfield shifted relative to those of the ligand precursors. 13C NMR spectra show a carbenoic carbon at 188.1 ppm for [(MeCPCMe)2Ru]Cl2 and at 186.9 ppm for [( nBu CPC nBu ) 2 Ru]Cl 2 . The PF 6 − salts, [(MeCPCMe)2Ru][PF6]2 and [(nBuCPCnBu)2Ru][PF6]2, were obtained in moderate yields when their chloride salts were treated with an equal molar amount of NH4PF6 in H2O at ambient conditions. X-ray quality crystals of [( Me CPC M e ) 2 Ru][PF 6 ] 2 , nBu [( CPCnBu)2Ru]Cl2, and [(nBuCPCnBu)2Ru][PF6]2 were obtained by slow diffusion of diethyl ether into acetonitrile solution and all subjected to an X-ray diffractometer for structural analysis. Crystal data and refinement parameters are listed in Table S1 (Supporting Information). Selected bond lengths and angles are summarized in Table 1. In [(MeCPCMe)2Ru][PF6]2, the asymemetric unit displays two independent cations, which were designated as A and B. The

Figure 2. (Top) Molecular structure of one (defined as cation A) of the two independent cations in [(MeCPCMe)2Ru][PF6]2 showing the atom-labeling scheme. Hydrogen atoms and the anion are omitted. (Bottom) Lattice view of the supramolecular network showing the cations interconnected through weak interactions (N14···H−C45 3.370(7) Å).

of A are quite similar to those of B with slight changes in its bond parameters. The two pincer ligands are orthogonally arranged around the core to form a pseudo-octahedral geometry, as reported for [Ru(CNC)2]2+ or [Ru(tpy)2]2+. The major features of the structures of [(nBuCPCnBu)2Ru]Cl2 and [( nBu CPC nBu ) 2 Ru][PF 6 ] 2 are similar to those in [(MeCPCMe)2Ru][PF6]2. Molecular structures of [(nBuCPCnBu)2Ru]Cl2 and [(nBuCPCnBu)2Ru][PF6]2 are depicted in Figures S1 and S2, respectively. The Ru−N bond lengths (central pyrazinyl N atom) in [(MeCPCMe)2Ru][PF6]2 (ca. 2.008 Å), [(nBuCPCnBu)2Ru]Cl2 (ca. 1.998 Å), and [(nBuCPCnBu)2Ru][PF6]2 (ca. 2.003 Å) are slightly longer than those found in [Ru(tpy)2]2+ (1.976 ± 0.018 Å)15 but shorter than those of [Ru(CNC)2]2+ (2.022 ± 0.009 Å).9,12,14 The Ru−CNHC bond distances of 2.048(5)−2.061(5) Å in [( M e CPC M e ) 2 Ru][PF 6 ] 2 , 2.019(10)−2.049(9) Å in [( nBu CPC nBu ) 2 Ru]Cl 2 , and 2.054(5)−2.077(6) Å in [(nBuCPCnBu)2Ru][PF6]2 are comparable with those in other Ru(II) complexes with pincer NHC ligands (2.048(2)− 2.068(7) Å).9,10,16 In addition, the trans-CNHC−Ru−CNHC angles ranging form 153.24(18)° to 153.38(18)° in [( MeCPCMe)2Ru][PF 6]2, 152.27(36)° to 153.50(42)° in [(nBuCPCnBu)2Ru]Cl2, and 153.38(18)° to 153.24(18)° in [(nBuCPCnBu)2Ru][PF6]2 are in the range of those found in the analogous complexes (153.27(28)−154.35(28)°).9,12,14 It is worth mentioning that in [(MeCPCMe)2Ru][PF6]2 a hydrogenbonding interaction between the outward pyrazinyl N and a NHC ring proton is found (N14···H−C45), which extends along the b-axis to give a 1D infinite chain (Figure 2, bottom).

Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) of Complexes [(nBuCPCnBu)2Ru] Cl2, [(MeCPCMe)2Ru][PF6]2, and [(nBuCPCnBu)2Ru][PF6]2

C47−Ru1 C37−Ru1 C33−Ru1 C29−Ru1 N13−Ru1 N16−Ru1 C47−Ru1− C37 N13−Ru1− N16 C33−Ru1− C29

[(nBuCPCnBu)2Ru] [PF6]2

[(nBuCPCnBu)2Ru] [Cl]2

[(MeCPCMe)2Ru] [PF6]2

2.062(7) 2.054(5) 2.056(6) 2.077(6) 2.005(5) 2.000(5) 153.04(21)

2.049(9) 2.063(9) 2.030(11) 2.019(10) 1.998(7) 1.997(7) 152.27(36)

2.048(5) 2.054(6) 2.061(5) 2.052(5) 2.005(4) 2.009(4) 153.38(18)

179.24(19)

178.78(29)

177.30(14)

153.08(20)

153.50(42)

153.24(18)

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are most likely to be spin-allowed 1MLCT transitions, as suggested by the time-dependent density functional theory calculation (Figure S3, Supporting Information),18 while a broad tailing above 500 nm, which had been observed in related Ru(II)−NHCs,19 is tentatively assigned to the spin-forbidden d(Ru)−π*(NHC) 3MLCT transition. The 1MLCT bands are blue-shifted compared to that of the [Ru(tpy)2]2+ complexes but are red-shifted relative to that of [Ru(CNC)2]2+ complexes. The former observation could be due to the higher lying π* orbital of NHC,9 whereas the latter bathochromic shift might be attributed to the better electron-withdrawing character of the pyrazine moiety, which provides a stronger π-acceptor property than the pyridine in the CNC ligand. The cyclic voltammogram of [(nBuCPCnBu)2Ru][PF6]2 in CH3CN exhibits a single quasi-reversible RuII/III redox couple at E1/2 = +1.52 V (Figure 4), which is higher than that for the Ru(II)−CNC complex (E1/2 = +1.38 V) and is consistent with a better π acceptor for the CPC ligand than for the CNC ligand. A similar electronic effect of the pincer-type NHC−pyridine−NHC ligand has been discussed in the literature.10 All the CPC complexes also displayed typical 3MLCT-based luminescence at room temperature. As an example, [(nBuCPCnBu)2Ru][PF6]2 showed a structureless orange emission at λmax = 577 nm (Figure 5) upon excitation at λexc = 400 nm. Electronic transitions of both the [(nBuCPCnBu)2Ru][PF6]2 and [(MeCPCMe)2Ru][PF6]2 complexes are influenced by strong acid. Upon addition of HClO4(aq) into the CH3CN solution of the complexes, significant changes in the absorption as well as emission spectra of the complexes are observed, as shown in Figures 6 and 5, respectively. As shown in Figure 6, upon increasing the amount or HClO4 added, the absorption intensity of the original MLCT band at 407 nm decreases along with an increase of the absorption band at 475 nm. Changes were also observed for other higher energy transitions (λ < 330 nm). There is no further change above ca. 320-fold of H+ being added. During the titration, a set of well-defined isosbestic points at 430, 380, 351, 336, 305, 291, 278, and 263 nm was found, indicating that only two absorbing species exist in the solution. This phenomenon is reversible. Addition of Et3N reassumes the absorption spectra of the original Ru(II)−CPC complexes. Therefore, H+ and Et3N could reversibly change their electronic absorption spectra. Interestingly, the CPC complex responds only toward strong acids, such as HClO4, but is almost inert toward weak acids, such as p-toluenesulfonic acid, benzilic acid, triflic acid, or acetic acid. The red shift of the absorption bands for the Ru(II)−CPC complexes upon protonation is possibly due to the lowering of the energy of the lowest lying π* molecular orbital and, therefore, narrowing the energy gap between HOMOs and LUMOs. The emission property of [(RCPCR)2Ru][PF6]2 complexes also responds toward HClO4 in the CH3CN solution at room temperature. As shown in Figure 5, the intensity of the emission band at 577 nm dramatically diminished upon protonation. The quench of the emission is probably a result of the shift of the 3MLCT state to a lower energy, which leads to a more efficient nonradiative decay. This phenomenon has been interpreted by the so-called “energy-gap law”, which refers to the decrease of the luminescence intensity as the emission energy decreases due to the more efficient nonradiative decay over a smaller gap.20 The possibility of quenching via protonation at CH3CN cannot be ruled out at this stage. On the other hand, given that protonation of the uncoordinated sites causes them to be effective electron acceptors, it is also likely that the quenching

These observations imply the possibility for the complex to act as a proton acceptor in solution (vide inf ra). On the contrary, such a hydrogen-bonding interaction is not observed in the two n-butyl-substituted complexes, [(nBuCPCnBu)2Ru]Cl2 and [(nBuCPC nBu )2 Ru][PF6]2 , presumably due to the steric hindrance of the longer butyl chain. UV−vis spectra of these Ru(II) complexes in CH3CN solution were recorded at 300 K (Figure 3). The data are

Figure 3. Absorption spectra of [MeCPCMe)2Ru]Cl2 (green line), [(nBuCPCnBu)2Ru]Cl2 (blue line), [(MeCPCMe)2Ru][PF6]2 (red line), and [(nBuCPCnBu)2Ru][PF6]2 (black line) in acetonitrile at 300 K.

Table 2. Absorption Data of Ru(II) Complexes complexa Me

Me

[( CPC )2Ru]Cl2 [(MeCPCMe)2Ru][PF6]2 [(nBuCPCnBu)2Ru]Cl2 [(nBuCPCnBu)2Ru][PF6]2 H+[(nBuCPCnBu)2Ru][PF6]2 [(nBuCMePCnBu)2Ru][PF6]2[BF4]2 [Ru(tpy)2]2+b [Ru(CNC)2]2+c

λmax (nm) 344, 345, 347, 347, 383, 361, 474 343,

404 405 406 407 475 486 382

ε/(104 M−1 cm−1) 1.16, 1.80 1.08, 1.44 1.04, 1.34 0.85, 1.12 1.2, 1.3 1.6, 2.1 1.7 1.2, 1.5

Spectra were recorded in CH3CN at 300 K (2.5 × 10 −5 M). bRef 19. c Ref 9. a

tabulated in Table 2. The absorption spectra of all these complexes exhibit intense bands in the UV region with similar spectral patterns, indicating that the N-substituents and the anions of the complexes have minimal impact on the electronic structure. As shown in Figure 3, the Ru(II) complexes containing methyl groups have larger absorbances than those containing n-butyl groups. Previous investigation on the electronic absorption of Ru(II)−NHC complexes suggested that a tiny change in the structure on the chromophores to similar structures may drastically influence the absorption efficiency.10 We tentatively suggested that the intermolecular interactions that connect chromophores may be responsible for the difference in absorbance.17 [(MeCPCMe)2Ru]2+ complexes, which have smaller steric hindrance and weak intermolecular interactions such as hydrogen bonding, may be more feasible to form a “pseudo-polymetallic complex” than [(nBuCPCnBu)2Ru]2+ complexes and are expected to have higher absorption. The absorption bands at λmax ≈ 345 and 406 nm 4982

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Figure 4. Cyclic voltammogram of 1 mM [(nBuCPCnBu)2Ru][PF6]2 (blue line) and 1 mM [(nBuCMePCnBu)2Ru][PF6]2[BF4]2 (red line) in CH3CN solution containing 0.1 M TBAP at a scan rate 1000 mV/s at 25 °C. Electrode potential in mV vs saturated Ag/AgCl electrode.

Figure 5. Variation of emission spectrum of 2.5 × 10−5 M [(nBuCPCnBu)2Ru][PF6]2 in CH3CN with increasing [HClO4] at λexc = 380 nm. The green line represents the spectrum of [(nBuCPCnBu)2Ru][PF6]2 before acid titration, the black line represents the change observed on the titration of acid, and the red line represents the spectrum after acid titration. Inset: Plot of emission intensity at 577 nm vs the total concentration of HClO4.

in the protonated species occurs by rapid electron transfer from the 3MLCT excited state to the protonated pyrazine rings. Similar observations have been reported for methylated pendant pyridyl groups, which can act as electron-transfer quenchers in this manner.21 It is also worth noting that a blue shift of the emission from 577 to 562 nm has been observed. The blue shift of the emission upon protonation of the Ru(II)− NHC complex was also reported in a similar case,12 which is attributed to the relatively higher reduction potential of the NHC-based ligand than a tpy-based ligand (−1.24 V).4e In our case, the distribution of LUMOs may be changed when the pyrazine is protonated. Probably, after protonation the LUMOs may have more NHC character, and hence a blue shift of the emission results. An attempt to isolate the protonated Ru(II)−CPC complex led to an insoluble dark brown powder, which was difficult to

further characterize. In order to investigate the effect of protonation of the Ru(II)−CPC complex on the emissive property, an N-methylated complex from [(nBuCPCnBu)2Ru][PF6]2 was prepared. Treating [(nBuCPCnBu)2Ru][PF6]2 with 2 equivalents of methyl triflate at room temperature gave methylated compound [( nBu CP Me C nBu ) 2 Ru][PF 6 ] 2 [BF 4 ] 2 (Scheme 1). The 1H NMR spectrum of the methylated species is in accord with the formation of a double-methylated product, as evidenced by the presence of a symmetry-related spectrum pattern with an additional methyl singlet at ca. 4.8 ppm (Figure S4, Supporting Information). The assignment of the structure of this complex can be further supported on the basis of the 13C NMR features (Figure S4, Supporting Information), which display an additional signal at 51.09 ppm for the methylated carbons. 4983

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Figure 6. Variation of absorption spectrum of 2.5 × 10−5 M [(nBuCPCnBu)2Ru][PF6]2 in CH3CN with increasing concentrations of [HClO4]. The green line represents the spectrum of [(nBuCPCnBu)2Ru][PF6]2 before acid titration, the black line represents the change observed on the titration of acid, and the red line represents the spectrum after acid titration. Inset: Plot of absorbance at 477 nm vs the total concentration of HClO4.

Scheme 1. Methylation of [(nBuCPCnBu)2Ru][PF6]2

Figure 7. Absorption spectra of [(nBuCPCnBu)2Ru][PF6]2 (dashed line), protonated [(nBuCPCnBu)2Ru][PF6]2 (thin line), and [(nBuCMePCnBu)2Ru][PF6]2[BF4]2 (bold line) in CH3CN at 300 K.

protonated compound H+[(nBuCPCnBu)2Ru][PF6]2. Like the protonated compound H+[(nBuCPCnBu)2Ru][PF6]2, the drastic change of UV absorptions is attributed to the methylation at

As shown in Figure 7, the absorption spectrum of [( CMePCnBu)2Ru][PF6]2[BF4]2 is distinct from that of [(nBuCPCnBu)2Ru][PF6]2 and is comparable to that of the nBu

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1). Anal. Calcd for C12H14Cl2N6·2/3H2O: C 27.08, N 15.79, H 2.65. Found: C 27.08, N 15.89, H 3.16. Synthesis of [nBuCPCnBu-H]Cl2. Mixing 2.70 g (21 mmol) of 1butylimidazole with melted 2,6-dichloropyrazine (1.06 g, 7 mmol) at 70 °C, followed by raising the reaction temperature to 90 °C and stirring for 12 h, gave a brown slurry solution with a white precipitate. The crude product was isolated as a white powder by collecting the precipitate and washing with 20 mL of THF. Analytically pure product can be obtained by further recrystallization from a hot CH3CN solution of the white powder (69%). 1H NMR (D2O): 9.88 (s, 2H, NCHN), 9.24 (s, 2H, py-H), 8.33 (s, 2H, imidazole H), 7.78 (s, 2H, imidazole H), 4.33 (t, 4H, NCH2CH2CH2CH3), 1.91, 0.77, and 0.52 (m, 14H, nBu) ppm. 13C NMR (D2O): 141.7 (NCN), 136.1 and 135.4 (py-C), 124.1 and 119.7 (imidazole C), 50.56 (NCH2CH2CH2CH3) 31.04 (NCH2CH2CH2CH3), 18.79 (NCH2CH2CH2CH3), 12.61 (NCH2CH2CH2CH3) ppm. MS (MALDI): m/z 325.4 (M2+ − 1). Anal. Calcd for C18H26Cl2N6·1/2 CH3CN·1/2 H2O: C 53.46, N 21.33, H 6.73. Found: C 53.56, N 21.84, H 6.60. Synthesis of [(MeCPCMe)2Ru]Cl2. Treating [MeCPCMe-H]Cl2 (0.31 g, 1.0 mmol) with RuCl3·H2O (0.1 g, 0.5 mmol) in ethylene glycol (10 mL) under a N2 atmosphere immediately resulted in a dark brown solution. The reaction mixture was refluxed at 190 °C for 24 h to give a dark solution. The solution was allowed to cool to room temperature, and the ethylene glycol was removed by vacuum distillation to give a deep brown solid (66%). 1H NMR (D2O): 9.01(s, 4H, py-H), 8.08 (d, JH−H = 2.2 Hz, 4H, imidazole H), 6.97 (d, JH−H = 2.2 Hz, 4H, imidazole H), 2.54 (s, 12H,CH3) ppm. 13C NMR (D2O): 188.1 (NCN), 147.3 and 126.1 (py-C), 124.8 and 116.6 (imidazole C), 35.1 (CH3) ppm. MS (MALDI): m/z 580.6 (M2+ − 1). Anal. Calcd for C24H24Cl2N12Ru·H2O: C 42.99, N 25.07, H 3.91. Found: C 43.01, N 25.51, H 4.12. Synthesis of [(MeCPCMe)2Ru][PF6]2. A 100 mL aqueous solution containing [(MeCPCMe)2Ru]Cl2 (0.2 g, 0.3 mmol) was added to a 10 mL aqueous solution of NH4PF6 (0.1 g, 0.6 mmol), and the reaction mixture was stirred for 2 h. The crude product was collected by filtration. The product was purified by column chromatography (CH3CN/H2O:/KNO3, 40:6:1) to give a dark brown power (68%). 1 H NMR (CD3CN): 9.13 (s, 4H, py-H), 8.12 (d, JH−H = 2.3 Hz, 4H, imidazole H), 7.05 (d, JH−H = 2.2 Hz, 4H, imidazole H), 2.58 (s, 12H,CH3) ppm. 13C NMR (CD3CN): 186.9 (NCN), 153.4 and 127.1 (py-C), 125.1 and 118.5 (imidazole C), 35.6 (CH3) ppm. MS (MALDI): m/z 580.6 (M2+ − 1). Anal. Calcd for C24H24F12N12P2Ru: C 33.07, N 19.26, H 2.78. Found: C 33.01, N 19.30, H 3.01. Synthesis of [(nBuCPCnBu)2Ru]Cl2. Treating [nBuCPCnBu-H]Cl2 (0.61 g, 1.0 mmol) with RuCl3·H2O (0.11 g, 0.5 mmol) in ethylene glycol (10 mL) under a N2 atmosphere resulted immediately in a brown solution. The reaction mixture was refluxed at 190 °C for 24 h to give a dark solution. The solution was allowed to cool to room temperature, and the ethylene glycol was removed by vacuum distillation to give a deep brown solid. The solid was washed with CH2Cl2 to give pure product (51%). 1H NMR (D2O): 9.21 (s, 4H, pyH), 8.29 (d, JH−H = 2.4 Hz, 4H, imidazole H), 7.21 (d, JH−H = 2.4 Hz, 4H, imidazole H), 2.77 (t, 4H, NCH2CH2CH2CH3), 0.77 and 0.52 (m, 14H, nBu) ppm. 13C NMR (D2O): 186.98(NCN), 147.46 and 126.35 (py-C), 124.74 and 117.15 (imidazole C), 49.9 ( NCH 2 CH 2 CH 2 CH 3 ) 32.0 (NCH 2 CH 2 C H 2 CH 3 ) , 1 9. 0 7 (NCH 2 CH 2 CH 2 CH 3 ), 12.83 (NCH 2 CH 2 CH 2 CH 3 ) ppm. MS (MALDI): m/z 748.9 (M2+ − 1). Anal. Calcd for C36H48Cl2N12Ru: C 52.68, N 20.48, H 5.89. Found: C 52.57, N 20.45, H 5.67. Synthesis of [(nBuCPCnBu)2Ru][PF6]2. A 100 mL aqueous solution containing [(nBuCPCnBu)2Ru]Cl2 (0.2 g, 0.3 mmol) was added to a 10 mL aqueous solution of NH4PF6 (0.1 g, 0.6 mmol), and the reaction mixture was stirred for 2 h. The crude product was collected by filtration. The product was purified by column chromatography (CH3CN/H2O/KNO3, 40:6:1) to give a dark brown powder (65%). 1 H NMR (CD3CN): 9.21(s, 4H, py-H), 8.21 (d, JH−H = 2.1 Hz, 4H, imidazole H), 7.17 (d, JH−H = 2.1 Hz, 4H, imidazole H), 2.75 (t, 4H, NCH2CH2CH2CH3), 0.79 and 0.62 (m, 14H, nBu) ppm. 13C NMR (CD3CN): 184.32 (NCN), 148.47 and 126.31 (py-C), 122.2, 118.59 (imidazole C), 51.1 (NCH2CH2CH2CH3) 31.3 (NCH2CH2CH2CH3),

the distant nitrogen atom in [(nBuCPCnBu)2Ru][PF6]2 . Methylation may stabilize the LUMOs and shift the lowenergy MLCT band to a lower region by 79 nm. The stabilization of the LUMOs is further supported by the electrochemical measurement of the complexes. As depicted in Figure 4, [(nBuCPCnBu)2Ru][PF6]2 shows two quasi-reversible reduction couples at E1/2 = −1.32 and −1.77 V vs Ag/AgCl, which can be ascribed to the reduction of the pincer ligand. Upon methylation, the charge of the complex changes from 2+ to 4+. Therefore, the reduction potential of the ligand dramatically shifts to a more positive potential (ca. 1 V) and becomes almost irreversible. It is worthy to note that the high positive charge at the [(nBuCMePCnBu)2Ru][PF6]2[BF4]2 makes its oxidation much more difficult. Hence, the quasi-reversible oxidative Ru II/III couple of the [( nBu C Me PC nBu ) 2 Ru][PF6]2[BF4]2 shifts to much more positive than that of [(nBuCPCnBu)2Ru][PF6]2 and is not observable in the CH3CN window. Additionally, the methylated complex [(nBuCMePCnBu)2Ru][PF6]2[BF4]2 is nonemissive in acetonitrile at room temperature, consistent with that of the emission behaviors of protonated complexes.

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CONCLUSION In summary, we report the synthesis and structural characterization of novel Ru(II) complexes with a pyrazine-based pincertype NHC ligand. Unlike the conventional CNC pincer-type NHC ligand, the noncoordinated nitrogen atom on the pyrazinyl ring provides the opportunity for sensing protons in a CH3CN solution. A significant switch of the absorption spectra is observed with well-defined isosbestic points during the addition of acid, indicating that there are only two species transformed with each other in the process. The presence of acid also influences the emission behavior of these compounds; there is a dramatic decrease of the emission intensity upon the addition of acid. Investigation on the details of the sensing mechanism is currently under way.

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EXPERIMENTAL SECTION

General Procedures, Materials, and Physical Measurements. The solvents and reagents were used without further purification. Solvents were purchased from Aldrich Chemical Co. The 1H NMR spectra were recorded on a Bruker DPX-300 spectrometer (1H, 299.96 MHz; 13C, 75.43 MHz) with tetramethylsilane as an internal standard. Elemental microanalyses were performed by the Taiwan Instrumentation Center. UV−vis spectra were recorded on a Shimadzu UV2101PC spectrophotometer. Emission were obtained with an Aminco Bowman AD2 luminescent spectrofluorometer. Electrochemistry was performed on a BAS-100W electrochemical analyzer. Cyclic voltammograms of the complexes were recorded with a stationary platinum microelectrode at room temperature in CH3CN solution containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. Synthesis of [MeCPCMe-H]Cl2. Mixing 1.65 g (20.1 mmol) of 1methylimidazole with 2,6-dichloropyrazine at 70 °C, followed by increasing the reaction temperature to 90 °C and stirring for 12 h, gave a brown slurry solution. After cooling to room temperature, 25 mL of MeOH was added to dissolve all the precipitate. Adding the solution into 200 mL of THF gave a large amount of white precipitation, which was collected by filtration and washed with 10 mL of THF. The collected white powder was then dried under vacuum to give a pure product (78%). 1H NMR (DMSO-d6): 10.65 (s, 2H, NCHN), 9.83 (s, 2H, py-H), 8.81 (d, JH−H = 1.7 Hz, 2H, imidazole H), 8.08 (d, JH−H = 1.7 Hz, 2H, imidazole H), 4.04 (s, 6H,CH3) ppm. 13C NMR (DMSOd6): 141.36 (NCN), 137.55 and 136.26 (py-C), 125.66 and 119.76 (imidazole C), 37.11 (CH3) ppm. MS (MALDI): m/z 240.9 (M2+ − 4985

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19.07 (NCH2CH2CH2CH3), 12.84 (NCH2CH2CH2CH3) ppm. MS (MALDI): m/z 748.9 (M2+ − 1). Anal. Calcd for C36H48F12N12P2Ru: C 41.58, N 16.16, H 4.65. Found: C 41.50, N 16.20, H 4.43. Synthesis of [(nBuCMePCnBu)2Ru][PF6]2[BF4]2. A portion of 0.15 mmol of [(nBuCPCnBu)2Ru][PF6]2 was dissolved in 10 mL of CH2Cl2 under an argon atmosphere with stirring. Methyl triflate (1 mL, 0.3 M in CH2Cl2, 0.3 mmol) was added dropwise to the vigorously stirred solution, which was allowed to stir for an additional 6 h. Evaporation of the solvent afforded a red, oily residue. The oily residue was dissolved with a minimal amount of CH3CN and added to a 10 mL aqueous solution containing ca. 0.5 g of NaBF4. The resulting mixture was extracted with CH2Cl2 until the aqueous phase became colorless. The organic layers were dried over MgSO4, and the solvent was removed under vacuum, affording a crude product. Repeated recrystallizations from acetone/diethyl ether (1:2, v/v) mixtures gave a pure product (23%). 1H NMR (CD3CN): 9.51 (s, 4H, py-H), 8.36 (d, JH−H = 2.1 Hz, 4H, imidazole H), 7.37 (d, JH−H = 2.1 Hz, 4H, imidazole H), 4.81(s, 6H, Me), 2.81 (m, 4H, NCH2CH2CH2CH3), 0.98 and 0.62 (m, 14H, nBu) ppm. 13C NMR(CD3CN): 184.3 (NCN) and 148.5 (py-C), 126.3, 118.6 (imidazole C), 51.1 (Me), 50.7 (NCH 2 CH 2 CH 2 CH 3 ), 31.4 (NCH 2 CH 2 CH 2 CH 3 ), 19.08 (NCH2CH2CH2CH3), 12.8 (NCH2CH2CH2CH3) ppm. Anal. Calcd for C38H54B2F20N12P2Ru·1.5CH3CN·ether: C 39.19, N 13.71, H 5.11. Found: C 39.21, N 13.77, H 5.11. X-ray Crystallography. X-ray quality crystals of [(MeCPCMe)2Ru][PF6]2, [(nBuCPCnBu)2Ru]Cl2, and [(nBuCPCnBu)2Ru][PF6]2 were obtained by slow diffusion of diethyl ether into an acetonitrile solution of the compound. Data were obtained on an APEX II diffractometer, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data reduction was performed with SAINT, which corrects for Lorentz and polarization effects. Multiscan (SADABS was used) for the rest of the compounds was performed. All H atoms were added in idealized positions. Structures were solved by the use of direct methods, and refinement was performed by the least-squares methods on F2 with the SHELXL-97 package.22 Crystal data, details of data collection and refinement, and full geometric information are available in CIF format.

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(3) (a) Tormo, L.; Bustamante, N.; Colmenarejo, G.; Orellana, G. Anal. Chem. 2010, 82, 5195. (b) Gunnlaugsson, T.; Leonard, J. P. Chem. Commun. 2003, 2424. (4) (a) Kalyanasundaram, K.; Grätzel, M. Coord. Chem. Rev. 1998, 177, 347. (b) Baranoff, E.; Collin, J.-P.; Flamigni, L.; Sauvage, J.-P. Chem. Soc. Rev. 2004, 33, 147. (c) McCusker, J. K. Acc. Chem. Res. 2003, 36, 876. (d) Wagenknecht, P. S.; Ford, P. C. Coord. Chem. Rev. 2011, 255, 591. (e) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. Rev. 1994, 94, 993. (f) Barigelletti, F.; Flamigni, L.; Balzani, V.; Collin, J. P.; Sauvage, J. P.; Sour, A.; Constable, E. C.; Thompson, A. J. Am. Chem. Soc. 1994, 116, 7692. (5) (a) Kumar, R. J.; Karlsson, S.; Streich, D.; Rolandini Jensen, A.; Jäger, M.; Becker, H.-C.; Bergquist, J.; Johansson, O.; Hammarström, L. Chem.Eur. J. 2010, 16, 2830. (b) MacDonnell, F. M.; Bodige, S. Inorg. Chem. 1996, 35, 5758. (c) Richard Keene, F. Coord. Chem. Rev. 1997, 166, 121. (6) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (7) Constable, E. C.; Housecroft, C. E.; Thompson, A. C.; Passaniti, P.; Silvi, S.; Maestri, M.; Credi, A. Inorg. Chim. Acta 2007, 360, 1102. (8) Medlycott, E. A.; Hanan, G. S. Chem. Soc. Rev. 2005, 34, 133. (9) Son, S. U.; Park, K. H.; Lee, Y.-S.; Kim, B. Y.; Choi, C. H.; Lah, M. S.; Jang, Y. H.; Jang, D.-J.; Chung, Y. K. Inorg. Chem. 2004, 43, 6896. (10) Park, H.-J.; Kim, K. H.; Choi, S. Y.; Kim, H.-M.; Lee, W. I.; Kang, Y. K.; Chung, Y. K. Inorg. Chem. 2010, 49, 7340. (11) (a) Qin, D.; Zeng, X.; Li, Q.; Xu, F.; Song, H.; Zhang, Z.-Z. Chem. Commun. 2007, 147. (b) Lee, C.-S.; Zhuang, R. R.; Sabiah, S.; Wang, J.-C.; Hwang, W.-S.; Lin, I. J. B. Organometallics 2011, 30, 3897. (c) Lee, C.-S.; Sabiah, S.; Wang, J.-C.; Hwang, W.-S.; Lin, I. J. B. Organometallics 2010, 29, 286. (12) Park, H.-J.; Chung, Y. K. Dalton Trans. 2012, 41, 5678. (13) The mechanism for the formation of Ru(II)−NHC complexes has not been established unambiguously yet. One of the reviewers suggested the formation of Ru(II)−NHC complexes might involve an oxidative addition reaction followed by reductive elimination. In related studies with phosphine-substituted benzimidazoles, a similar mechanism had also been proposed. See: (a) Hahn, F. E.; Naziruddin, A. R.; Hepp, A.; Pape, T. Organometallics 2010, 29, 5283. (b) Naziruddin, A. R.; Hepp, A.; Pape, T.; Hahn, F. E. Organometallics 2011, 30, 5859. (14) Poyatos, M.; Mata, J. A.; Falomir, E.; Crabtree, R. H.; Peris, E. Organometallics 2003, 22, 1110. (15) Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, O.; Kennard, O.; Macrae, C. F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J. M.; Watson, D. G. J. Chem. Inf. Comput. Sci. 1991, 31, 187. (16) (a) Wright, J. A.; Danopoulos, A. A.; Motherwell, W. B.; Carroll, R. J.; Ellwood, S. J. Organomet. Chem. 2006, 691, 5204. (b) Danopoulos, A. A.; Winston, S.; Motherwell, W. B. Chem. Commun. 2002, 1376. (c) Masllorens, E.; Rodríguez, M.; Romero, I.; Roglans, A.; Parella, T.; Benet-Buchholz, J.; Poyatos, M.; Llobet, A. J. Am. Chem. Soc. 2006, 128, 5306. (17) (a) Polson, M. I. J.; Loiseau, F.; Campagna, S.; Hanan, G. S. Chem. Commun. 2006, 1301. (b) Ott, S.; Borgströ m , M.; Hammarström, L.; Johansson, O. Dalton Trans. 2006, 1434. (18) In order to gain insight into the electronic structures of the CPC complexes, time-dependent density functional theory (TDDFT) calculations at the B3LPY/LANL2DZ level15 were employed to identify the composition of the MOs involved in the MLCT transitions. For the sake of simplicity, [MeCPCMeRu]2+ is selected as an example, and the coordinates are directly from its X-ray crystallography. Figure 5 shows the plots of selected TDDFT frontier orbitals for [MeCPCRu][PF6]2. These orbitals were found to be related to the electronic transitions. The results show that the HOMOs are contributed ∼50% by the Ru center; the ligand contribution in the HOMO is slightly increased in comparison with that in the HOMO−1 and HOMO−2. The HOMO−1 and HOMO−2 are energetically close, suggesting that the two orbitals are nearly degenerate. On the other hand, the unoccupied molecular orbitals LUMO and LUMO+1,

ASSOCIATED CONTENT

S Supporting Information *

CIF data and ORTEP drawings of complexes, 1H NMR and 13 C NMR spectra of [( n B u CPC n B u ) 2 Ru](PF 6 ) 2 and [(nBuCMePCnBu)2Ru][PF6]2[BF4]2, and HOMO and LUMO diagrams for [(MeCPCMe)2](PF6)2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +88638633570. Tel: +88638633577. E-mail: hws@mail. ndhu.edu.tw. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Science Council (Taiwan) and National Dong Hwa University for financial support. We would like to thank the three reviewers for constructive criticism of an earlier version of this paper

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REFERENCES

(1) Feringa, B. L.; van Delden, R. A.; ter Wiel, M. K. J. In Molecular Switches; Wiley-VCH Verlag GmbH, 2001. (2) (a) Russew, M.-M.; Hecht, S. Adv. Mater. 2010, 22, 3348. (b) Willner, I. Acc. Chem. Res. 1997, 30, 347. (c) Irie, M. Chem. Rev. 2000, 100, 1683. 4986

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as well as LUMO+2 and LUMO+3, were basically π* orbitals localized on the CPC ligand with a large contribution on the pyrazine ring. LUMO and LUMO+1 are degenerate as a pair and so are the LUMO +2 and LUMO+3 levels. Regarding the substantial contributions of the ligand in both HOMOs and LUMOs, transitions involving these frontier orbitals should be considered as intrinsically ligand-centered transitions perturbed by the Ru center, although the major characters are MLCT. (19) Dinda, J.; Liatard, S.; Chauvin, J.; Jouvenot, D.; Loiseau, F. Dalton Trans. 2011, 40, 3683. (20) (a) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 630. (b) Treadway, J. A.; Loeb, B.; Lopez, R.; Anderson, P. A.; Keene, F. R.; Meyer, T. J. Inorg. Chem. 1996, 35, 2242. (21) (a) Ryu, C. K.; Wang, R.; Schmehl, R. H.; Ferrere, S.; Ludwikow, M.; Merkert, J. W.; Headford, C. E. L.; Elliott, C. M. J. Am. Chem. Soc. 1992, 114, 430. (b) Yonemoto, E. H.; Riley, R. L.; Kim, Y. I.; Atherton, S. J.; Schmehl, R. H.; Mallouk, T. E. J. Am. Chem. Soc. 1992, 114, 8081. (c) Cooley, L. F.; Headford, C. E. L.; Elliott, C. M.; Kelley, D. F. J. Am. Chem. Soc. 1988, 110, 6673. (22) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112.

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