Ruthenium-bis-terpyridine Complex with Two Redox-Asymmetric

Oct 17, 2014 - Sadig Aghazada , Iwan Zimmermann , Yameng Ren , Peng Wang ... Holm Petzold , Evgenia Dmitrieva , Marco Rosenkranz , Heinrich Lang...
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Ruthenium-bis-terpyridine Complex with Two Redox-Asymmetric Amine Substituents: Potential-Controlled Reversal of the Direction of Charge-Transfer Hai-Jing Nie, Chang-Jiang Yao, Meng-Jia Sun, Yu-Wu Zhong,* and Jiannian Yao* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A ruthenium-bis-terpyridine complex [Ru(NPhtpy)(Ntpy)]2+ (22+) with two redox-asymmetric amine units has been prepared, where NPhtpy is 4′-(di-p-anisylaminophen-4-yl)-2,2′:6′,2″terpyridine and Ntpy is 4′-(di-p-anisylamino)-2,2′:6′,2″-terpyridine. This complex displays two consecutive redox couples at +0.82 and +1.02 V vs Ag/AgCl, which are assigned to the N•+/0 processes of the amine components of the NPhtpy and Ntpy ligands, respectively. The mono-oxidized complex 23+ obtained by oxidative electrolysis shows the presence of the charge transfer from ruthenium(II) to the oxidized aminium radical cation of the NPhtpy ligand (MNNPhtpyCT) around 1000 nm. In the dioxidized form (24+), the MNNPhtpyCT transition decreased distinctly and an opposite charge transfer from ruthenium(II) to the oxidized aminium radical cation of the Ntpy ligand (MNNtpyCT) appeared at 1380 nm. Complexes [Ru(NPhtpy)(tpy)]2+ (tpy is 2,2′:6′,2″-terpyridine), [Ru(Ntpy)(tpy)]2+, and [Ru(NPhtpy)2]2+ have been prepared and studied for the purpose of comparison. TDDFT calculations show that the involvement of the intraligand charge transfer from both NPhtpy and Ntpy ligands is responsible for the enhancement of the visible absorptions of these complexes with respect to [Ru(tpy)2]2+. DFT and TDDFT calculations have been performed on 23+ and 24+ to provide information on the spin distributions and the nature of the near-infrared absorptions. Complex 23+ shows an isotropic EPR signal at room temperature, consistent with an unpaired electron localized on the nitrogen atom.

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bearing sites in mixed-valence systems have been well-known.4,9 We expect that the future hybrid materials of these two components will possess some intriguing properties, in addition to the known properties, such as enhanced light adsorption,10 electrochromism,11 multistage redox processes,12 and intervalence charge-transfer (IVCT),6,13 among others.14 We present herein a combined experimental and theoretical study of complex 2(PF6)2 (Scheme 1), where the Ru(tpy)2 component is used to bridge two redox-active amine units. This complex is designed so that the two amine substituents have different electronic properties and thus different oxidation potentials of individual N•+/0 processes. When these two amine units are stepwise oxidized by potential-controlled electrolysis, the charge transfer of the complex is examined by analyzing the resulting near-infrared (NIR) absorptions. Interestingly, a potential-controlled reversal of the direction of charge transfer has been observed. Complexes 3(PF6)2 through 6(PF6)2 with related structures have been synthesized and studied for the purpose of comparison (Scheme 1). In addition, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed on 22+, 23+, and 24+ with different total charges, which provide complementary and

INTRODUCTION Electron transfer and/or charge transfer are fundamental processes in photosynthesis systems and optoelectronic devices. Since the pioneering experimental1 and theoretical studies2 by Creutz, Taube, Hush, and others, mixed-valence chemistry has become a classical method to examine the charge transfer processes between redox sites.3 To date, a great number of mixed-valent compounds with a general formula of [Man-BLMbn+1] have been reported,4 where BL is an organic bridging ligand and M a and M b are redox-active inorganic or organometallic components. These studies provide very useful information on the effect of the BL on the degree of charge delocalization of the system. Recently, a conceptually reversed system using an inorganic component to bridge two redox-active organic species has attracted some attention.5 For instance, we have used the wellknown polypyridine ruthenium complexes Ru(tpy)2 (tpy = 2,2′:6′,2″-terpyridine) or Ru(bpy)3 (bpy = 2,2′-bipyridine) as the bridging component to mediate the electronic coupling between two redox-active triarylamine units.6 Polypyridine transition-metal complexes are useful optoelectronic materials because of their appealing photophysical and electrochemical properties.7 Triarylamines are versatile organic materials with good electron-donating and hole-transporting abilities.8 The uses of polypyridine complexes or triarylamines as the charge© 2014 American Chemical Society

Received: September 2, 2014 Published: October 17, 2014 6223

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Scheme 1. Synthesis of 2(PF6)2 − 5(PF6)2 and the chemical structure of the known complex 6(PF6)2

insightful information on the compositions of frontier molecular orbitals, spin density populations, and the nature of the electronic absorptions of these complexes.

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RESULTS AND DISCUSSION Synthesis. Complexes 2(PF6)2 through 5(PF6)2 were synthesized as outlined in Scheme 1. Ligand 4′-(di-panisylaminophen-4-yl)-2,2′:6′,2″-terpyridine (1, NPhtpy) was obtained in 70% yield through the Pd-catalyzed C−N coupling between 4′-(p-bromophenyl)-2,2′:6′,2″-terpyridine15 and di(panisyl)amine. The reaction of [Ru(Ntpy)Cl3] (Ntpy =4′-(di-panisylamino)-2,2′:6′,2″-terpyridine)16 with 1 in ethylene glycol under microwave heating, followed by anion exchange using KPF6, gave complex 2(PF6)2 in 54% yield. Complexes 3(PF6)2 and 6(PF6)2 were prepared according to the reported procedures.16 Complex 4(PF6)2 was obtained from the reaction of [Ru(tpy)Cl3] with ligand 1 in acceptable yield. The symmetric complex 5(PF6)2 was obtained from the reaction of ligand 1 with hydrated RuCl3. The details of synthesis and characterization data of these compounds are provided in the Experimental Section. Electrochemical Studies. Figure 1 shows the anodic cyclic voltammograms (CVs) of the above synthesized ligands and ruthenium complexes in CH3CN. The electrochemical data are delineated in Table 1. Ligands NPhtpy and Ntpy display a N•+/0 redox couple at +0.81 and +0.97 V vs Ag/AgCl, respectively. The insertion of a phenyl group between the dianisylamino group and the tpy ligand in NPhtpy decreases the N•+/0 potential by 160 mV with respect to that of Ntpy. When these two ligands are chelated with Ru(tpy) to give complexes 4(PF6)2 and 3(PF6)2, the N•+/0 potentials only vary a little (+0.82 and +0.98 V, respectively). On the basis of these

Figure 1. CVs of ligands Ntpy and 1 and complexes 2(PF6)2 through 6(PF6)2 and DPV of 5(PF6)2 (green curve) at a glassy carbon in 0.1 M Bu4NClO4/CH3CN.

results, the two consecutive redox couples of the asymmetric diamine complex 2(PF6)2 at +0.82 and +1.02 V can be unambiguously assigned to the N•+/0 processes of the amine components of the NPhtpy and Ntpy ligand, respectively. The comproportionation constant Kc for the equation [N1−N2] + [N1•+-N2•+] → 2[N1•+-N2] was determined to be 2450 by Kc = 10ΔE (mV)/59, where ΔE (200 mV) is the potential splitting between two N•+/0 processes and N1 and N2 stand for the amine nitrogen atom of the NPhtpy and Ntpy ligand of 2(PF6)2, respectively. The difference between the anodic and cathodic peak potentials of each N•+/0 process (ΔEp) falls in the range of 80−110 mV, which is somewhat larger than the theoretical value (59 mV) for an electrochemically reversible one-electron redox couple. This may be caused by the relatively slow 6224

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Table 1. Electrochemical Dataa E1/2/V vs Ag/AgCl (ΔEp/mV)a compound 1, NPhtpy Ntpy 2(PF6)2 3(PF6)2 4(PF6)2 5(PF6)2 6(PF6)2 [Ru(tpy)2](PF6)2

anodic +0.81 +0.97 +0.82 +0.98 +0.82 +0.81 +0.96 +1.32

E1/2/V vs Fc+/0 cathodic

(80) (105), +1.41b (90), +1.02 (85), +1.31,b +1.36,b +1.48 (80), +1.35,b +1.44 (88), +1.30,b +1.36 (110), +1.30,b +1.37 (87), +1.23 (95), +1.56,b +1.64

−1.30, −1.28, −1.24, −1.26, −1.34, −1.22,

anodic

cathodic

+0.35 +0.51 +0.36, +0.56 +0.55 +0.37 +0.37 +0.51, +0.78 +0.87

−1.54 −1.52 −1.49 −1.46 −1.58 −1.46

−1.76, −1.71, −1.69, −1.70, −1.79, −1.77,

−2.00 −1.95 −2.04 −1.90 −2.03 −1.91

a Data was recorded in CH3CN. The potential is reported as the E1/2 value vs Ag/AgCl or ferrocene+/0 (Fc+/0). ΔEp refers to the difference between the anodic and cathodic peak potentials. bAnodic peak potential of irreversible processes.

electron transfer kinetics of the N•+/0 process under the measurement conditions. However, each N•+/0 wave has the same current height between the anodic and cathodic peaks, suggestive of chemically reversible processes. The Ntpy-based N•+/0 potential of 2(PF6)2 is 40 mV positively shifted with respect to that of 3(PF6)2. The NPhtpy-based N•+/0 potential of 2(PF6)2 is essentially the same with respect to that of 4(PF6)2. This however does not necessarily mean that an electronic communication is present between two amine units of 2(PF6)2. The potential splitting of consecutive redox processes depends on a variety of different factors apart from the resonance contribution, including the inductive contribution and electrostatic contribution, among others.17 The symmetric complex 5(PF6)2 with two NPhtpy ligands shows a redox couple at +0.81 V vs Ag/AgCl. This wave should be a result of the oxidations of both amine units. The potential separation between two N•+/0 processes is insignificant, which is also supported by its differential pulse voltammogram (DPV). This suggests that little electronic coupling is present between two amine units, if there is any. No IVCT transition has been observed in the following spectroelectrochemical measurements either. In stark contrast, the diamine complex 6(PF6)2 with shorter N−N distance displays two consecutive N•+/0 processes at +0.96 and +1.23 V.16a In the more positive potential region, some redox processes are observed for complexes 2(PF6)2, 4(PF6)2, and 5(PF6)2, which consist of the irreversible oxidation of the aminium radical cations18 and the RuIII/II processes (Figure S1 in the Supporting Information, SI). The RuIII/II potentials of these complexes are slightly positively shifted with respect to that of [Ru(tpy)2](PF6)2 (Table 1).19 This is reasonable considering that the di-p-anisylamino unit was oxidized prior to the RuIII/II processes, and the resulting aminium radical cations behave as electron-withdrawing substituents. In the cathodic scan, these complexes all show two ligand-based reduction waves (Figure S1). Spectroscopic Studies. The UV/vis absorption spectra of 2(PF6)2 through 6(PF6)2 and [Ru(tpy)2](PF6)2 are displayed in Figure 2. When substituted with amine groups, the visible absorptions of 2(PF6)2 through (PF6)2 are distinctly expanded with respect to that of [Ru(tpy)2](PF6)2. The absorption maxima of 2(PF6)2 through 6(PF6)2 locate at 515, 495, 496, 516, and 512 nm, respectively, which are around 20 or 40 nm red-shifted with respect to that of [Ru(tpy)2](PF6)2 (475 nm, Table 2). The following TDDFT results suggest that the visible absorptions of the amine-containing complexes have significant contributions from the intraligand charge transfer (ILCT) transitions, in addition to the metal-to-ligand charge transfer

Figure 2. Electronic absorption spectra of [Ru(tpy)2](PF6)2 and 2(PF6)2 through (PF6)2 in CH3CN.

Table 2. Electronic Absorption Data in CH3CN λabs,max [nm] (ε × 105 M−1 cm−1)

compound 2(PF6)2 3(PF6)2 4(PF6)2 5(PF6)2 6(PF6)2 [Ru(tpy)2](PF6)2

277 274 272 275 302 270

(0.64), (0.65), (0.33), (0.53), (0.94), (0.48),

301 303 308 310 350 307

(0.70), (0.74), (0.50), (0.71), (0.21), (0.78),

515 495 496 404 512 475

(0.35) (0.25) (0.20) (0.18), 516 (0.42) (0.25) (0.17)

(MLCT) transitions. We note that these complexes are essentially nonemissive in fluidic solution at room temperature. The above amine-containing complexes were then subjected to stepwise oxidative electrolysis at a transparent indium−tinoxide (ITO) glass electrode, and the NIR absorption spectral changes were monitored by a UV/vis/NIR spectrometer. When the potential was gradually increased from +0.60 to +0.95 V vs Ag/AgCl to oxidize the amine unit of the NPhtpy ligand of 2(PF6)2, the absorption bands in the visible region significantly decreased (Figure 3a), with the concomitant appearance of the N•+-localized transitions at 750 nm6,9,16,18 and some broad NIR absorptions between 900 and 1700 nm. The NIR absorptions at least contain two components around 1000 and 1300 nm. The former is assigned to the charge transfer from the ruthenium ion to the oxidized aminium radical cation (of the NPhtpy ligand), which is abbreviated as the RuII → N•+ MNNPhtpyCT transition. The latter band has not been observed for complexes 4(PF6)2 and 5(PF6)2 upon one-electron oxidations. The assignment of this band will be discussed below. When the potential was further increased from +0.95 to +1.15 V to oxidize the amine unit of the Ntpy ligand of 2(PF6)2, the MNNPhtpyCT transition at 1000 nm decreased, and an intense absorption band at 1380 nm appeared (Figure 3b). 6225

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Figure 3. Absorption spectral changes of (a,b) 2(PF6)2, (c) 4(PF6)2, and (d) 5(PF6)2 upon stepwise oxidative electrolysis at an ITO glass electrode in CH2Cl2. The applied potentials were referenced versus Ag/AgCl. The inset in (b) shows the enlarged plots in the region between 800 and 1200 nm. The small and weak bands of the black curve in the NIR region of (c) are due to a nonperfect compensation of the background absorption.

The latter band is assigned to the charge transfer from the ruthenium ion to the oxidized aminium radical cation of the Ntpy ligand (MNNtpyCT). This assignment is also supported by the following TDDFT results of 24+. In addition, the MNNtpyCT has been observed at a similar position for complex 3(PF6)2 after one-electron oxidation.16 The decrease of the MNNPhtpyCT transition and the simultaneous appearance of the MNNtpyCT transition indicate that the direction of charge transfer in 24+ is reversed with respect to that in 23+. When the potential was decreased from +1.15 V to +0.60 V, a reverse absorption spectral change can be observed, suggesting the reversibility of the redox reactions. When complexes 4(PF6)2 and 5(PF6)2 were subjected to similar oxidative electrolysis, the visible absorptions decreased significantly (Figure 3c and d). At the same time, the N•+localized transitions at 750 nm and the MNNPhtpyCT transition at 1000 nm appeared. The molar absorptivities of these two transitions of 54+ roughly double with respect to those of 43+. In addition, no IVCT transition can be distinguished for 5(PF6)2 during the spectroelectrochemical measurements. This suggests that no efficient electronic coupling is present between the two amine units of 5(PF6)2, consistent with the above electrochemical studies. Previous studies showed that an IVCT band could be observed for the shorter diamine complex 6(PF6)2 after one-electron oxidation.16a The NIR transitions of 23+ were deconvoluted by Gaussian functions (Figure 4). Two subbands at 9800 and 7500 cm−1 can be distinguished. The more intense band at 9800 cm−1 is assigned to the MNNPhtpyCT transition. The small band at 7500 cm−1 could be a weak IVCT band between two distal amine sites. However, this band is too narrow (1500 cm−1) for a class II compound and two weak (εmax = 700 M−1 cm−1) for a class III compound. In addition, in the transformation of 23+ to 24+ (Figure 3b), the decrease of this weak band has not been observed. Thus, the assignment of this small band as the IVCT transitions is not unambiguous. Another possibility is that this

Figure 4. Gaussian-fitting of the NIR absorption of 23+. The black curve is the experimental data. The red curve is the sum of the deconvoluted subbands.

band is from the charge transfer from the amine site of Ntpy to the ruthenium ion (NNtpyMCT). In this way, the ruthenium ion is involved in the indirect charge transfer from one side of the molecule to the other, as has been proposed in a three-state model.20 Because of the uncertainty in the assignment of this band, no further quantitative analysis has been performed. The above analysis suggests that the presence of the additional phenyl groups within the ligands of 23+ and 53+ with respect to 63+ quenches the electron transfer. Direct electronic interaction is present for complex 63+. For complex 23+ with one additional phenyl group, the degree of direct interaction is decreased or maybe the electron transfer is a twostep mechanism involving the ruthenium ion. The presence of two of those phenyl groups completely prevents the electron transfer for complex 53+. DFT and TDDFT Studies. The DFT calculations of 22+ and 2+ 5 have been performed on the level of theory of B3LYP/ LANL2DZ/6-31G*/CPCM (see details in the Experimental Section). Figure 5 shows the calculated energy diagram and isodensity plots of frontier molecular orbitals of 22+. The highest occupied molecular orbital (HOMO), HOMO−1, and 6226

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Figure 5. DFT-calculated energy diagram of 22+ and the isodensity plots of frontier molecular orbitals.

other hand, the predicted S6 − S8 excitations are mainly associated with the ILCT transitions from the HOMO−1 orbital, namely the amine unit of the Ntpy ligand. The S9 and S10 excitations have major contributions from the HOMO−3 → LUMO and HOMO−3 → LUMO+2 transitions, which are mainly of the MLCT character. These results suggest that the visible absorptions of 22+ have major contributions from the ILCT transitions of both NPhtpy and Ntpy ligands and the MLCT transitions. Similarly, the TDDFT results of complex 52+ show that the visible absorptions have combined contributions from the ILCT transitions of the NPhtpy ligand and the MLCT transitions (see details in Table S1 and Figure S2). Similar situation should apply to complexes 32+ and 42+.

HOMO−2 are dominated by the NPhtpy ligand, the Ntpy ligand, and the ruthenium ion, respectively. These three orbitals are well-separated from each other, with the eigenvalues of −5.32, −5.82, and −6.36 eV, respectively. This result is consistent with the above electrochemical assignment of 2(PF6)2. The lowest unoccupied molecular orbital (LUMO) and LUMO+1 and LUMO+2 are all dominated by the tpy unit of both ligands. DFT calculations have also been performed on the openshell states 23+ and 24+ with the same level of theory. Figure 6

Table 3. TDDFT-Predicted Excitations of 23+ and 24+a complex

No.

λ/nm

f

dominant transitions (percent contribution)

23+

D1 D2 D3 D4 D5 D6 T1 T2 T3 T4 T5 T6

1521.8 922.2 889.0 837.0 749.1 691.8 1092.5 1044.1 1036.1 827.4 758.4 752.1

0.0003 0.1742 0 0.0003 0.2989 0 0.0005 0.0015 0.2723 0.2813 0.2967 0.0068

β-HOSO → β-LUSO (99%) β-HOSO-1 → β-LUSO (96%) β-HOSO-2 → β-LUSO (99%) β-HOSO-3 → β-LUSO (97%) β-HOSO-5 → β-LUSO (97%) β-HOSO-4 → β-LUSO (100%) β-HOSO-1 → β-LUSO (72%) β-HOSO-2 → β-LUSO (96%) β-HOSO → β-LUSO (69%) β-HOSO-4 → β-LUSO (99%) β-HOSO-3 → β-LUSO+1 (96%) β-HOSO-3 → β-LUSO (97%)

Figure 6. DFT-calculated spin density plots of 23+ and 24+. 24+

shows the Mulliken spin density distributions (α − β) of these two complexes. The spin of 23+ is dominated by the NPhtpy ligand. The spins of 24+ are dominated by both NPhtpy and Ntpy ligands. The ruthenium ion makes little contributions to the spin distributions of 23+ and 24+. These results are also consistent with the above electrochemical and spectroscopic studies. However, the overestimation of charge delocalization of the DFT methods should be mentioned here, and these results should be considered with care. To rationalize the origins of the absorption spectra of the above complexes, TDDFT calculations have been carried out on 22+, 52+, 23+, and 24+. The predicted S1 (S = singlet) and S6 − S10 excitations are mainly responsible for the visible absorptions of 22+ (Tables S1). The S1 excitation is dominated by the HOMO → LUMO transition, which is mainly of the ILCT character associated with the NPhtpy ligand. On the

a Computed at the TDDFT/UB3LYP/LANL2DZ/6-31G*/CPCM level of theory. D = doublet; T = triplet; f = oscillator strength; HOSO = highest occupied spin orbital; LUSO = lowest unoccupied spin orbital.

Table 3 and Figure 7 show the TDDFT results of the openshell complexes 23+ and 24+. An IVCT transition with a very weak oscillator strength (f = 0.0003) is predicted by the D1 excitation (λ = 1521.8 nm) of 23+, with the dominant β spin transition from the highest occupied spin orbital (HOSO) to the lowest unoccupied spin orbital (LUSO). The predicted D2 6227

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Figure 7. TDDFT-predicted excitations (red curves) of (a) 23+ and (b) 24+. The observed absorption spectra (black curves) are included for comparison. See details in Table 3

excitation of 23+ of the β-HOSO-1 → β-LUSO character (λ = 922.2 nm, f = 0.1742) is responsible for the MNNPhtpyCT transitions. In addition, the D5 excitation (λ = 749.1 nm, f = 0.2989) is consistent with the N•+-localized transitions. Other transitions are with negligible oscillator strengths. For complex 24+, the predicted T3 (λ = 1036.1 nm, f = 0.2723) excitation associated with the β-HOSO → β-LUSO transition has a major contribution from the MNNtpyCT transitions. This excitation is responsible for the observed absorption band at 1380 nm, although the predicted excitation is 0.3 eV higher in energy with respect to the observed data. The T4 and T5 excitations associated with the β-HOSO-4 → βLUSO and β-HOSO-3 → β-LUSO+1 transitions are consistent with the N•+-localized transitions of both NPhtpy and Ntpy ligands. These TDDFT results are basically in agreement with the previous absorption assignment of these two complexes. Electron Paramagnetic Resonance (EPR) Analysis. Complex 23+, obtained by chemical oxidation using cerium ammonium nitrate, shows a strong isotropic EPR signal at g = 2.006 at room temperature in CH3CN (Figure 8). This indicates that the unpaired electron is localized on the nitrogen atom, instead of the ruthenium ion. Polypyridine ruthenium-

(III) complexes are well-known to be EPR inactive at room temperature due to fast spin relaxation by spin−orbital coupling or display rhombic or axial EPR signals at low temperature.21 Complex 24+ shows a similar but much weaker EPR signal with respect to 23+.

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CONCLUSION

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EXPERIMENTAL SECTION

In summary, a bis-terpyridine ruthenium complex with two redox-asymmetric amine units has been designed and synthesized. Because of the different electronic natures of these two amine units, they can be stepwisely oxidized with a potential separation of 200 mV. This design is reminiscent of the redox-asymmetric mixed-valent systems with either organic or inorganic redox sites.22 In the dioxidized state, the direction of the metal-to-aminium charge transfer is reversed with respect to that in the mono-oxidized state. A weak NIR absorption band at 7500 cm−1 was observed for the mono-oxidized state, which is likely from the intervalence or amine-to-metal charge transfer transition. This supports the presence of a direct or indirect electronic communication between two amine sites of this complex. In addition, mono- and dioxidized states can be distinguished by the significantly different NIR absorption signals, suggesting their potential uses as electrochromic materials and molecular logic gates.11

Synthesis. NMR spectra were recorded in the designated solvent on a Bruker Avance 400 MHz spectrometer. Spectra are reported in ppm values from residual protons of deuterated solvent. Mass data were obtained with a Bruker Daltonics Inc. Apex II FT-ICR or Autoflex III MALDI-TOF mass spectrometer. The matrix for MALDITOF measurement is α-cyano-4-hydroxycinnamic acid. Microanalysis was carried out using a Flash EA 1112 or Carlo Erba 1106 analyzer at the Institute of Chemistry, Chinese Academy of Sciences. 4′-(pBromophenyl)-2,2′:6′,2″-terpyridine,15 [Ru(Ntpy)Cl3], and 3(PF6)2 were prepared according to the previously reported procedures.16 Synthesis of 4′-(Di(p-anisyl)amino-phen-4-yl)-2,2′:6′,2″-terpyridine (1). A suspension of 4′-(p-bromophenyl)-2,2′:6′,2″-terpyr-

Figure 8. EPR spectrum of 23+ in CH3CN at rt. 6228

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reference electrode, respectively. The cell was put into the spectrometer to monitor the spectral change during electrolysis. Electrochemical Measurement. All electrochemical measurements were taken using a CHI 660D potentiostat with onecompartment electrochemical cell under an atmosphere of nitrogen. All measurements were carried out in CH3CN containing 0.1 M n Bu4NClO4 as the supporting electrolyte at a scan rate of 100 mV/s. The working electrode was a glassy carbon with a diameter of 3 mm. The electrode was polished prior to use with 0.05 μm alumina and rinsed thoroughly with water and acetone. A large area platinum wire coil was used as the counter electrode. All potentials are referenced to Ag/AgCl electrode in saturated aqueous NaCl or Fc+/0. Computational Methods. DFT and TDDFT calculations are carried out using the B3LYP,23 exchange correlation functional and implemented in the Gaussian 09 program package.24 The electronic structures of complexes were determined using a general basis set with the Los Alamos effective core potential LanL2DZ basis set for ruthenium, and 6-31G* for other atoms.25 No symmetry constraints were used in the optimization (nosymm keyword was used). Solvent effects (CH2Cl2) are included in all calculations with the conductorlike polarizable continuum model (CPCM).26 Frequency calculations have been performed with the same level of theory to ensure the optimized geometries to be local minima. All orbitals have been computed at an isovalue of 0.02 e/bohr3. The TDDFT-predicted spectra were generated using GaussView 5.0. EPR Measurements. EPR measurements were performed on a Bruker ELEXSYS E500−10/12 spectrometer at room temperature in CH3CN. The spectrometer frequency is 9.7 × 109 Hz. Complexes 23+ and 24+ were obtained by chemical oxidation of 2(PF6)2 with 0.5 or 1.5 equiv cerium ammonium nitrate.

idine (97 mg, 0.25 mmol), di(p-anisyl)amine (69 mg, 0.30 mmol), tris(dibenzylideneacetone)dipalladium (Pd2(dba)3, 11 mg, 0.013 mmol), bis(diphenylphosphino)ferrocene (dppf, 7.0 mg, 0.013 mmol), and NaOtBu (29 mg, 0.30 mmol) in 10 mL dry toluene was heated at 140 °C for 48 h under a N2 atmosphere. The system was cooled to room temperature. The solvent was removed under vacuum and the crude product was purified by chromatography on silica gel (eluting with petroleum ether/ethyl acetate/NH4OH 50/15/1) to yield 94 mg of 1 as a gray solid (70%). 1H NMR (400 MHz, CDCl3): δ 3.82 (s, 6H), 6.87 (d, J = 8.8 Hz, 4H), 7.01 (d, J = 8.8 Hz, 2H), 7.11 (d, J = 9.2 Hz, 4H), 7.35 (t, J = 6.0 Hz, 2H), 7.76 (d, J = 8.8 Hz, 2H), 7.89 (t, J = 7.8 Hz, 2H), 8.67 (d, J = 8.0 Hz, 2H), 8.71 (s, 2H), 8.73 (d, J = 4.0 Hz, 2H). 13C NMR (100 MHz, CDCl3): 156.4, 156.1, 155.7, 149.7, 149.6, 149.0, 140.4, 136.8, 129.5, 127.8, 126.9, 123.6, 121.3, 119.9, 117.9, 114.7, 55.42. EI-HRMS: calcd for C 35 H 28 N 4 O 2 536.22068. Found: 536.22026. Synthesis of Complex 2(PF6)2. To 10 mL ethylene glycol were added ligand 1 (27 mg, 0.050 mmol) and [Ru(Ntpy)Cl3] (33.0 mg, 0.050 mmol). The mixture was refluxed by microwave heating (power =375 W) for 30 min. After cooling to room temperature, the system was treated with an excess of aq. KPF6. The resulting precipitate was collected by filtering and washing with water and Et2O. The obtained solid was subjected to flash column chromatography on silica gel (eluent: saturated aq. KNO3/H2O/CH3CN, 1/10/600) to give 37 mg of 2(PF6)2 as an orange solid in 54% yield. 1H NMR (400 MHz, CD3CN): δ 3.83 (s, 6H), 3.89 (s, 6H), 7.03 (d, J = 8.8 Hz, 4H), 7.04 (m, 4H), 7.15 (d, J = 9.2 Hz, 4H), 7.22 (d, J = 8.8 Hz, 4H), 7.23 (t, J = 6.0 Hz, 2H), 7.31 (d, J = 5.2 Hz, 2H), 7.50 (d, J = 8.8 Hz, 4H), 7.55 (d, J = 5.2 Hz, 2H), 7.74 (t, J = 7.6 Hz, 2H), 7.88 (s, 2H), 7.92 (t, J = 7.5 Hz, 2H), 8.09 (d, J = 8.8 Hz, 2H), 8.09 (d, J = 8.0 Hz, 2H), 8.59 (d, J = 8.0 Hz, 2H), 8.87 (s, 2H). MALDI-MS: 1242.9 for [M − PF6]+, 1098.0 for [M − 2PF6]+. Anal. Calcd for C64H52F12N8O4P2Ru: C, 55.37; H, 3.78; N, 8.07. Found: C, 55.42; H, 3.96; N, 7.70. Synthesis of Complex 4(PF6)2. Using the similar procedure for the synthesis of 2(PF6)2, complex 4(PF6)2 was obtained from the reaction of [Ru(tpy)Cl3] (22.0 mg, 0.050 mmol) and ligand 1 (27 mg, 0.050 mmol) as an orange solid in 48% yield. 1H NMR (400 MHz, CD3CN): δ 3.74 (s, 12H), 6.92 (d, J = 9.2 Hz, 8H), 6.97 (d, J = 8.8 Hz, 4H), 7.12 (d, J = 8.8 Hz, 8H), 7.21 (t, J = 6.0 Hz, 4H), 7.67 (d, J = 5.2 Hz, 4H), 7.95 (t, J = 7.8 Hz, 4H), 8.05 (d, J = 8.8 Hz, 4H), 8.88 (d, J = 8.0 Hz, 4H), 9.19 (s, 4H). 13C NMR (100 MHz, CD3COCD3): 54.6, 114.7, 118.2, 119.8, 123.5, 124.1, 124.2, 126.5, 127.2, 127.3, 127.4, 128.2, 135.6, 137.8, 137.8, 139.3, 148.0, 150.8, 152.1, 152.2, 155.0, 155.3, 156.9, 158.0, 158.3. MALDI-MS: 1318.9 for [M − PF6]+, 1173.9 for [M − 2PF6]+. MALDI-HRMS calcd for C50H38N7O2Ru: 864.2157. Found: 864.2165. Synthesis of Complex 5(PF6)2. Using the similar procedure for the synthesis of 2(PF6)2, complex 5(PF6)2 was obtained from the reaction of ligand 1 (27 mg, 0.050 mmol) and RuCl3·3H2O (7.0 mg, 0.025 mmol) as an orange solid in 65% yield. 1H NMR (400 MHz, CD3CN): δ 3.74 (s, 6H), 6.92 (d, J = 9.2 Hz, 4H), 6.98 (d, J = 8.8 Hz, 2H), 7.13 (d, J = 8.8 Hz, 4H), 7.20 (t, J = 5.6 Hz, 2H), 7.23 (t, J = 5.6 Hz, 2H), 7.58 (d, J = 5.2 Hz, 2H), 7.69 (d, J = 5.2 Hz, 2H), 7.96 (t, J = 7.8 Hz, 4H), 8.04 (d, J = 8.8 Hz, 2H), 8.46 (t, J = 8.2 Hz, 1H), 8.71 (d, J = 8.0 Hz, 2H), 8.89 (d, J = 8.0 Hz, 2H), 8.97 (d, J = 8.0 Hz, 2H), 9.21 (s, 2H). 13C NMR (100 MHz, CD3COCD3): 54.7, 114.8, 118.4, 119.9, 124.3, 126.6, 127.3, 127.4, 128.3, 137.8, 139.4, 147.8, 150.9, 152.2, 155.3, 157.0, 158.5. MALDI-MS: 1015.0 for [M − PF6]+, 871.0 for [M − 2PF6]+. MALDI-HRMS calcd for C70H55N8O4Ru: 1167.3417. Found: 1167.3411. Anal. Calcd for C70H56F12N8O4P2Ru· 3H2O: C, 55.37; H, 4.12; N, 7.38. Found: C, 55.38; H, 3.95; N, 7.63. Spectroscopic Measurement. Absorption spectra were recorded on a PE Lambda 750 UV/vis/NIR spectrophotometer at room temperature. Spectroelectrochemical measurements were performed in a thin layer cell (optical length: 0.2 cm), in which a transparent ITO glass electrode (