Substituent Effects on the Electrochemistry and Electronic Coupling of Terphenyl-Bridged Cyclometalated Ruthenium–Amine Conjugated Complexes

Abstract

Six terphenyl-bridged cyclometalated ruthenium–amine conjugated complexes 4(PF₆)–9(PF₆) were synthesized and studied. Three different substituents, methoxy, methyl, and chloro, were used to vary the electronic nature of the amine unit, and two terminal ligands 2,2′:6′,2″-terpyridine (tpy) and trimethyl-4,4′,4″-tricarboxylate-2,2′:6′,2″-terpyridine (Me₃tctpy) were used to tune the electronic nature of the ruthenium component. All complexes, except 7(PF₆) with the methoxy substituent and Me₃tctpy ligand, display two well-separated redox waves in the potential range of +0.5 to +1.1 V versus Ag/AgCl. The regular electrochemical changes of these complexes help to establish the oxidation order of ruthenium and amine and hence of the direction of the electron transfer in odd-electron state. The degree of electronic coupling was estimated by analyzing the donor-to-acceptor charge transfer band in the near-infrared region obtained by oxidative spectroelectrochemical measurements. Electron paramagnetic resonance analyses and density functional theory calculations were performed on the one-electron oxidized forms to obtain information on the spin distribution of these complexes.

Introduction

Since the pioneering work of Creutz and Taube,¹ the ground-state electron transfer (ET) between redox-active donors and acceptors has been extensively examined in the form of mixed-valence (MV) compounds obtained by the one-electron oxidation or reduction of symmetric precursors.² These studies are of importance in providing information on the fundamental ET processes by electrochemical, spectroelectrochemical, electron paramagnetic resonance (EPR), and theoretical analysis.³ The presence of multistep redox processes of MV compounds, accompanied by distinct changes of their electronic and optical properties, make them useful for a number of optoelectronic applications such as electrochromism,⁴ molecular switches,⁵ and molecular electronics.⁶

Bridged bisruthenium complexes⁷ and bis(triarylamines)⁸ are representative compounds used in MV chemistry. To date, a large number of such symmetric molecular materials have been synthesized and examined for the fundamental ET studies.^7,8 In these compounds, there is no free energy change (ΔG⁰) between the initial and final states of the ET reaction. In contrast, MV systems with different terminal ligands or substituents on the termini lead to redox asymmetry and free energy change.⁹ The great structure diversity of redox-asymmetric systems allows us to further modulate the degree of charge delocalization and the energy and shape of intervalence charge transfer (IVCT).^9,10

The ET properties of MV compounds are normally analyzed by the classical Marcus–Hush or generalized Mulliken–Hush theory.¹¹ This method could be further expanded for the analysis of asymmetric MV compounds and systems with completely different chemical structures of the donor and acceptor, if certain degree of electronic coupling is present between them and similar charge transfer transitions could be observed in the odd-electron state.¹² The charge transfer transitions in these systems are better to be called donor-to-acceptor charge transfer (DACT) bands, instead of IVCT bands. We recently reported that the combination of cyclometalated ruthenium and triarylamine gives rise to appealing materials with strong charge delocalization and multistep redox processes.¹³ The presence of the Ru–C bond in these compounds ensures strong coupling between cyclometalated ruthenium and triarylamine. As a result, intense DACT bands are observed from these compounds in odd-electron states both in the solution and thin films.¹⁴ One interesting question remains in these systems is that of the direction of ET. Because cyclometalated ruthenium and triarylamine have very similar redox potential and often strong coupling between them, it is sometimes difficult to determine the direction of ET in these compounds. For compounds with a short linker, these compounds display a large degree of charge delocalization and it is meaningless to discuss the direction of ET. We present herein the synthesis, characterization, and ET studies of a series of para-terphenyl-bridged cyclometalated ruthenium–amine conjugated complexes, 4(PF₆)–9(PF₆), with different substituents on the amine group and different terminal ligands on the ruthenium component (Figure 1). The moderate length of the para-terphenyl bridge allows us to observe distinctly different electrochemical and spectroscopic properties by substituent effects, providing reliable information for the determination of the ET direction of these compounds.

Figure 1.

(a) Synthesis of complexes 4(PF₆)–9(PF₆). (b) Chemical structures of the model complexes 10(PF₆) and 11(PF₆).

Results and Discussion

Complexes 4(PF₆)–9(PF₆) were synthesized as outlined in Figure 1. Methoxy, methyl, and chloro groups were used to tune the electronic properties of the amine unit, whereas 2,2′:6′,2″-terpyridine (tpy) and trimethyl-4,4′,4″-tricarboxylate-2,2′:6′,2″-terpyridine (Me₃tctpy) were used as the terminal ligands for the ruthenium components. Ligands 1–3 with different amine substituents were obtained in moderate yield by the palladium-catalyzed Suzuki coupling of 3,5-di(pyrid-2-yl)phenylboronic acid with N,N-di(4-methoxyphenyl)-p-(bromophen-4-yl)aniline, N,N-di(4-methylphenyl)-p-(bromophen-4-yl)aniline, and N,N-di(4-chlorophenyl)-p-(bromophen-4-yl)aniline, respectively. According to the previous procedures,^13,15 the reaction of 1, 2, or 3 with [Ru(tpy)Cl₃] or [Ru(Me₃tctpy)Cl₃] with the aid of AgOTf, followed by anion exchange using KPF₆, gave 4(PF₆)–9(PF₆). The yields of these reactions are moderate, ranging from 49 to 74%. The synthesis and characterization of ligand 1 and compound 4(PF₆) have been reported by us recently.^13b Other complexes have been fully characterized by different analytical techniques, as given in the Experimental Section. In addition, two cyclometalated ruthenium complexes, 10(PF₆) and 11(PF₆), without the amine site were used in the current study for the purpose of comparison (Figure 1).^13a

The electronic properties of the previously mentioned materials were first examined by electrochemical analysis (Figure 2 and Table 1). In the anodic cyclic voltammetry (CV) scan, ligand 1–3 all show one redox wave at +0.73, +0.89, and +1.10 V versus Ag/AgCl, respectively, attributing to the amine oxidation process (the N^•+/0 process). The redox potential of the ligand shifts to the more positive region as the substituent becomes more electron-deficient from OMe to Me and Cl (Figure 2a). The Ru^III/II potential of the Me₃tctpy-containing model complex 11(PF₆) (+0.74 V) is comparable to the N^•+/0 potential of 1. The Ru^III/II process of the tpy-containing model complex 10(PF₆) has the least positive potential (+0.56 V) among these five compounds. This trend is also evident from the differential pulse voltammograms (DPVs) as displayed in Figure 2d.

Figure 2.

(a–c) CV scans at 100 mV/s and (d–f) DPVs of 1–11(PF₆) in the anodic scan in 0.1 M ⁿBuNClO₄/CH₃CN vs Ag/AgCl.

Table 1. Electrochemical Data.

compound	E1/2/V (ΔEp/mV)a	ΔE/mVb	Kcc
1	+0.73 (320)
2	+0.89 (290)
3	+1.10 (300)
4(PF6)2	+0.58 (190), +0.72 (200)	140	236
5(PF6)	+0.61 (260), +0.88 (250)	270	3.8 × 104
6(PF6)	+0.61 (210), +1.10 (180)	490	2.0 × 108
7(PF6)	+0.73 (140)
8(PF6)	+0.70 (100), +0.86 (90)	160	515
9(PF6)	+0.70 (120), +1.01 (130)	310	1.8 × 105
10(PF6)	+0.56 (70)
11(PF6)	+0.74 (80)

^aData in CH₃CN. The electrochemical potentials are reported as the E_1/2 value vs Ag/AgCl. Potentials vs ferrocene^+/0 can be estimated by subtracting 0.45 V. The ΔE_p value in the parenthesis stands for the peak-to-peak potential separation of each redox couple. The relatively large ΔE_p values are possibly caused by slow ET kinetics.

^bThe potential splitting of two waves in the anodic region.

^cK_c = 10^ΔE(mV)/59.

Complexes 4(PF₆)–6(PF₆) possess the same tpy ligand on the ruthenium component. They differ only in the substituents of the amine site. Two well-separated redox waves are observed for these compounds in the range of +0.5 to +1.1 V (Figure 2b,e). The first, less positive, redox wave of 4(PF₆)–6(PF₆) did not vary significantly, locating at +0.58, +0.61, and +0.61 V, respectively. The potential of this wave is comparable, yet slightly more positive, with respect to the Ru^III/II potential of the tpy-containing model ruthenium complex 10(PF₆) (+0.56 V). In contrast, their second wave gradually shifted from +0.72 V for 4(PF₆) to +0.88 and +1.10 V, for 5(PF₆) and 6(PF₆), respectively. This positive-shift trend is consistent with that of the N^•+/0 potential change of ligand 1–3. Indeed, the potential of the second wave of 4(PF₆)–6(PF₆) is almost the same as the N^•+/0 potential of ligand 1–3, respectively. Based on these data, we can safely infer that the first redox wave of these compounds is largely associated with the Ru^III/II process, whereas the second one with the N^•+/0 process.

Complex 7(PF₆) displays an unresolved two-electron redox peak at +0.73 V. This is reasonable considering the corresponding ligand 1 and the model ruthenium complex 11(PF₆) possess a very similar redox peak at almost the same potential. In contrast, complexes 8(PF₆) and 9(PF₆) show the presence of two well-separated waves (Figure 2c,f). This series of complexes share the same electron-deficient Me₃tctpy auxiliary ligand on the ruthenium component and differ in the substituent on the amine site. Again, the potential of their first redox wave is comparable to the Ru^III/II potential of the Me₃tctpy-containing model ruthenium complex 11(PF₆) (+0.74 V) and their second wave change in an ascending order from 7(PF₆) (the second wave overlaps with the first one) to 9(PF₆). This leads to the same conclusion that the oxidation of the ruthenium component of this series [particularly for 8(PF₆) and 9(PF₆)] occurs before the amine oxidation.

The potential splitting ΔE between the two anodic redox waves of 4(PF₆)–9(PF₆) varies in the range of 140–490 mV. Note that redox asymmetry, or the ET free energy change ΔG⁰, is largely responsible for the potential splitting of these weakly-coupled complexes. Compound 6(PF₆) has the largest redox asymmetry, and it also has the largest potential splitting. The comproportionation constants, K_c, of these complexes were determined by K_c = 10^ΔE(mV)/59, which reflects the relative thermodynamic stability in the one-electron-oxidized state.

To probe the degree of electronic coupling of the previously mentioned complexes, they were examined by stepwise oxidative spectroelectrochemistry. Figure 3 shows the visible (vis) and near-infrared (NIR) absorption spectral changes of ligands 1–3 on oxidation. Intensive absorption bands between 600 and 1000 nm, assigned to the ammonium radical cation (N^•+) localized transitions,^8,16 appeared on amine oxidation. The main absorption band slightly shifts to the low-energy region when the amine becomes more electron-deficient from 1 to 3.

Figure 3.

Vis/NIR absorption spectral changes of ligand (a) 1, (b) 2, and (c) 3 on stepwise oxidative electrolysis in 0.1 M ⁿBu₄NClO₄/CH₃CN. The applied potential was referenced vs Ag/AgCl.

On stepwise oxidations, two distinct vis/NIR spectral changes were observed for 4(PF₆)–9(PF₆) (Figures 4 and 5). For instance, when the applied potential was increased stepwise from +0.50 to +0.75 V versus Ag/AgCl in the single-oxidation step, a broad NIR band with an absorption maximum (λ_max) at around 1300 nm appeared (Figure 4a). In accordance with the previously mentioned assignment of ruthenium oxidation in the first step, the metal-to-ligand charge transfer transition at around 500 nm significantly decreased. When we further increased the potential to +1.0 V in the double-oxidation step, the NIR absorptions decreased gradually. Meanwhile, a new intense absorption band between 600 and 1000 nm appeared (Figure 4b). This experiment suggests that the broad NIR absorption in the odd-electron state belongs to the DACT band. The composition of the intense band observed in the double-oxidation step could be complicated, including the ligand-to-metal charge transfer, N^•+-localized transitions, and the bridge-involved charge transfer transitions. Similar two-step vis/NIR spectral changes were observed for 5(PF₆) (Figure 4c,d) and 8(PF₆) (Figure 5c,d) with the appearance of a broad DACT band in the NIR region.

Figure 4.

Vis/NIR absorption spectral changes of complexes (a,b) 4(PF₆), (c,d) 5(PF₆), and (e,f) 6(PF₆) on stepwise oxidative electrolysis in 0.1 M ⁿBu₄NClO₄/CH₃CN. (a,c,e) Single oxidation. (b,d,f) Double oxidation. The applied potential was referenced vs Ag/AgCl.

Figure 5.

Vis/NIR absorption spectral changes of complexes (a,b) 7(PF₆), (c,d) 8(PF₆), and (e,f) 9(PF₆) on stepwise oxidative electrolysis in 0.1 M ⁿBu₄NClO₄/CH₃CN. (a,c,e) Single oxidation. (b,d,f) Double oxidation. The applied potential was referenced vs Ag/AgCl. *: Artifacts.

For complex 6(PF₆), an absorption band with a λ_max at around 1000 nm appeared in the single oxidation step (Figure 4e). However, during the double-oxidation step, this band did not decrease. Instead, intense absorptions with λ_max at around 820 and 1110 nm showed up at exactly the same wavelength region (Figure 4f). We have no solid evidence at this stage to assign the absorption band at 1000 nm of 6²⁺ to the DACT transition. For a complex with a large redox asymmetry and weak coupling, bridge-involved charge transfer transitions could also be observed in the odd-electron state. Complex 9(PF₆) has the same situation (Figure 5e,f). The absorption band at 1000 nm observed for 9²⁺ could not be assigned to the DACT transition at this stage.

Although the oxidations of the ruthenium component and the amine unit of 7(PF₆) occur at a very similar potential, a weak and broad DACT band in the NIR region can still be observed during the stepwise oxidation, which increases at the initial oxidation stage and decreases at the second oxidation stage (Figure 5a,b). Considering the similar oxidation potential of the two redox termini, we did not intend to assign the direction of ET of this complex.

The previously mentioned spectroelectrochemical measurements showed that a distinct DACT band was observed for 4²⁺, 5²⁺, 7²⁺, and 8²⁺. This band was further fitted by Gaussian functions as demonstrated in Figure 6. Corresponding DACT parameters are summarized in Table 2. The classical Hush formula, V_ab = 0.0206 × (ε_maxυ̃_maxΔυ̃_1/2)^1/2/(R_ab),¹¹ was used to estimate the degree of electronic coupling V_ab. In this equation, ε_max, υ̃_max, and Δυ̃_1/2 stand for the molar absorption coefficient, wavenumbers of the absorption maximum in cm^–1, and the full width at half height in cm^–1, respectively. The ET distance R_ab was taken to be 14.83 Å for all complexes from the calculated Ru–N geometrical distance. As can be seen from Table 2, these complexes have similar degree of electronic coupling. The V_ab value is around a few hundreds of wavenumbers. The relatively smaller value of 7²⁺ could be partially caused by the underestimation of ε_max recorded during the spectroelectrochemical measurements. The observed DACT band possibly did not reach its maximum because of the overlapping N^•+/0 and Ru^III/II potentials in this complex. As the DACT band of 6²⁺ and 9²⁺ has not been established in the current study, the degree of coupling of these two complexes is not further discussed.

Figure 6.

Deconvolution of the NIR absorption spectra of (a) 4²⁺, (b) 5²⁺, (c) 7²⁺, and (d) 8²⁺ by Gaussian functions. The blue curves are the fitted DACT bands.

Table 2. Parameters for DACT Transitions^a.

	λ/nm	υ̃max/cm–1	εmax/M–1 cm–1	υ̃1/2/cm–1	Vab/cm–1b
42+	1310	7650	2800	3840	400
52+	1240	8040	3240	4150	450
72+	1220	8230	1400	4340	310
82+	1160	8590	2780	3910	420

^aBased on the spectroelectrochemical results.

^bV_ab values calculated by V_ab = 0.0206 × (ε_maxυ̃_maxΔυ̃_1/2)^1/2/(R_ab).

The above experimental data suggested that the ruthenium component of the previously mentioned complexes, except 7(PF₆), was oxidized before the amine oxidation. We intended to perform the EPR analysis to get some information on the spin distribution of the one-electron oxidized samples. These samples were obtained by oxidation of 4(PF₆)–9(PF₆) with 0.5 equiv of cerium ammonium nitrate (CAN) in CH₃CN. Surprisingly, only 4²⁺ and 7²⁺ show well-defined EPR signals (Figure 7). No distinct EPR signals were observed from the other four samples under the same measurement conditions. Complex 4²⁺ displays a broad EPR signal with the g value of 2.175. The previous interpretation of this signal as an aminum radical cation could be inappropriate.^13b This peak is more likely attributed to a ruthenium(III)-biased species. In contrast, we observed a sharp EPR peak with the g value of 2.00 from 7²⁺. The assignment of this sharp peak to an aminum radical cation is reasonable. Because of the overlapping N^•+/0 and Ru^III/II potentials in this complex, the ruthenium and amine oxidations are expected to occur simultaneously with the addition of CAN and the obtained sample 7²⁺ contains both aminum radical cation and ruthenium(III) species.

Figure 7.

EPR spectra of 4²⁺ and 7²⁺ at frozen CH₃CN at 77 K. The samples were obtained by oxidation of 4(PF₆) and 7(PF₆) with 0.5 equiv CAN.

In many MV and related studies, density functional theory (DFT) calculations have been proven useful in predicting the spin distributions of molecular materials in different redox states.¹⁷ In the beginning, we performed the DFT calculations of the one-electron oxidized forms 4²⁺–9²⁺ with the commonly used UB3LYP hybrid functional.¹⁸ The spin density distributions are displayed in the left column of Figure 8. Complexes 4²⁺, 7²⁺, and 8²⁺ show triarylamine-localized spin density distributions and those of 5²⁺ and 9²⁺ are delocalized across the molecule. Only complex 6²⁺ is predicted to have ruthenium-localized spin density distribution. These results have some discrepancy with the previously mentioned experimental data, which suggested that the spin of all complexes, except 7²⁺, should be localized on the ruthenium component.

Figure 8.

DFT-calculated spin density distribution of 4²⁺–9²⁺ using UB3LYP (left column) or CAM-UB3LYP (right column) functional.

To improve the calculation results, we tried to perform the DFT calculations with the long-range-corrected CAM-UB3LYP functional.¹⁹ The spin density distributions obtained with CAM-UB3LYP are displayed in the right column of Figure 8, which shows some improvements with respect to those with UB3LYP. The tpy-series complexes 4²⁺–6²⁺ are all predicted to have ruthenium-localized spin density distribution, in agreement with the experimental data. However, the Me₃tctpy-containing complexes 7²⁺–9²⁺ are still not satisfactory. In particular, complex 8²⁺ still shows wrong information with triarylamine-localized spin density distribution, reflecting the limitation of DFT calculations in predicting the electronic structures of these complexes.

Conclusions

In conclusion, six terphenyl-bridged cyclometalated ruthenium–amine conjugated complexes were synthesized. The variation of the electronic nature of the amine substituents and the terminal ligand of the ruthenium component led to regular changes of the redox potentials and potential splitting. This result, together with the spectroelectrochemical analysis, helps to establish the oxidation order of the ruthenium component and amine unit in individual complexes. This in turn determines the direction of the ET in the odd-electron state of these complexes. EPR data and DFT calculations provide some complementary information on the electronic structures of these complexes. However, care needs to be taken to interpret these data. Work is under way to search for more reliable calculation methods and functionals to predict the electronic structures of these organic–inorganic conjugated complexes.

Experimental Section

Synthesis

Compounds 1 and 4(PF₆) were prepared according to known procedures.^13b The details for the NMR and mass spectra and microanalysis are provided in our previous publications.²⁰

Synthesis of 2

To a suspension of N,N-di(4-methylphenyl)-p-(bromophen-4-yl)aniline (66 mg, 0.24 mmol), Pd(PPh₃)₄ (12 mg, 0.05 mmol), and K₂CO₃ (138 mg, 1.0 mmol) in a degassed mixed solvent of tetrahydrofuran (THF)/H₂O (4.5 mL/0.5 mL) was added 3,5-di(pyrid-2-yl)phenylboronic acid (86 mg, 0.20 mmol). The mixture was stirred for 12 h at 90 °C. The solvent was evaporated in vacuum and the crude product was purified by column chromatography on silica gel (eluent: petroleum ether/ethyl acetate 10/5) to give 65 mg of 2 in 56% yield as a pale yellow solid. ¹H NMR (400 MHz, CDCl₃): δ 8.76 (d, J = 4.0 Hz, 2H), 8.62 (s, 1H), 8.36 (d, J = 1.6 Hz, 2H), 7.91 (d, J = 8.0 Hz, 2H), 7.85–7.77 (m, 4H), 7.69 (d, J = 8.3 Hz, 2H), 7.52 (d, J = 8.6 Hz, 2H), 7.29–7.27 (m, 2H), 7.13–7.04 (m, 10H), and 2.34 (s, 6H). ¹³C NMR (75 MHz, CDCl₃): δ 157.31, 149.78, 147.88, 145.36, 142.09, 140.52, 140.11, 139.19, 137.05, 133.66, 132.79, 130.06, 128.79, 128.04, 127.42, 127.00, 126.40, 124.88, 122.80, 122.56, 121.08, and 20.98. MS (m/z): 579 for C₄₂H₃₃N₃ (M⁺).

Synthesis of 3

To a suspension of N,N-di(4-chlorophenyl)-p-(bromophen-4-yl)aniline (469 mg, 1.0 mmol), Pd(PPh₃)₄ (58 mg, 0.050 mmol), and K₂CO₃ (691 mg, 6.0 mmol) in a degassed mixed solvent of THF/H₂O (10 mL/2 mL) was added 3,5-di(pyrid-2-yl)phenylboronic acid (331 mg, 1.2 mmol). The mixture was stirred for 12 h at 90 °C. The solvent was evaporated in vacuum, and the crude product was purified by column chromatography on silica gel (eluent: petroleum ether/EtOAc, 10/6) to give 360 mg of 3 in 58% yield as a white solid. ¹H NMR (400 MHz, CDCl₃): δ 8.80 (d, J = 4.1 Hz, 2H), 8.66 (s, 1H), 8.41 (d, J = 1.5 Hz, 2H), 7.95 (d, J = 8.0 Hz, 2H), 7.88 (d, J = 8.3 Hz, 2H), 7.83 (td, J = 7.8, 1.7 Hz, 2H), 7.72 (d, J = 8.3 Hz, 2H), 7.60 (d, J = 8.6 Hz, 2H), 7.33–7.27 (m, 6H), 7.16 (d, J = 8.6 Hz, 2H), and 7.10–7.07 (m, 4H). ¹³C NMR (100 MHz, CDCl₃): δ 157.31, 149.88, 146.59, 146.08, 141.89, 140.65, 139.70, 139.63, 136.91, 135.69, 129.62, 128.30, 128.10, 127.91, 127.13, 126.33, 125.44, 124.66, 124.42, 122.52, and 120.96. MS (m/z): 619 for C₄₀H₂₇Cl₂N₃ (M⁺).

Synthesis of 5(PF₆)

To 30 mL of dry acetone were added [Ru(tpy)Cl₃] (0.10 mmol, 44 mg) and AgOTf (0.30 mmol, 78 mg). The mixture was refluxed under a N₂ atmosphere for 3 h. The mixture was filtered to afford a purple black solution. The filtrate was concentrated to dryness. To the residue were added ligand 2 (0.10 mmol, 58 mg), 3.0 mL of dry dimethylformamide, and 3 mL of ^tBuOH. The resulting mixture was refluxed under microwave heating for 30 min at a power of 200 W and then another 30 min at a power of 375 W. After cooling to room temperature, an excess of aqueous KPF₆ was added. The resulting precipitate was collected by filtering and washing with water and Et₂O. The crude solid was purified through flash column chromatography on silica gel, followed by anion exchange using KPF₆ to give 65 mg of complex 5(PF₆) in 60% yield as a red solid. ¹H NMR (400 MHz, d₆-acetone): δ 9.04 (d, J = 8.4 Hz, 2H), 8.77 (s, 2H), 8.70 (d, J = 8.0 Hz, 2H), 8.51 (d, J = 8.0 Hz, 2H), 8.41 (t, J = 8.0 Hz, 1H), 8.16 (d, J = 8.4 Hz, 2H), 7.88 (d, J = 8.0 Hz, 2H), 7.84 (td, J = 5.2, 1.6 Hz, 2H), 7.80–7.65 (m, 4H), 7.31 (d, J = 4.8 Hz, 2H), 7.22 (d, J = 4.8 Hz, 2H), 7.20–7.10 (m, 8H), 7.05 (d, J = 8.4 Hz, 4H), 6.78 (t, J = 6.0 Hz, 2H), and 2.34 (s, 6H). ¹³C NMR (75 MHz, CDCl₃): δ 168.51, 159.04, 154.60, 152.65, 151.79, 151.63, 147.86, 145.30, 142.50, 142.40, 140.88, 140.73, 139.28, 139.22, 135.30, 135.03, 133.68, 133.64, 132.86, 127.59, 127.52, 127.27, 126.15, 124.90, 123.60, 122.85, 122.58, 121.92, 119.55, and 21.00. MALDI-MS (m/z): 913.0 for C₅₇H₄₃N₆Ru [M – PF₆]⁺. Anal. Calcd for C₅₇H₄₃F₆N₆O₆PRu·H₂O: C, 63.62; H, 4.22; N, 7.81. Found: C, 64.01; H, 4.22; N, 7.77.

Synthesis of 6(PF₆)

Using the similar procedure as for the synthesis of 5(PF₆), complex 6(PF₆) (65 mg) was prepared from [Ru(tpy)Cl₃] (0.10 mmol, 44 mg) and ligand 3 (0.10 mmol, 62 mg) in 63% yield as a red solid. ¹H NMR (400 MHz, d₆-acetone): δ 9.04 (d, J = 8.0 Hz, 2H), 8.78 (s, 2H), 8.71 (d, J = 8.0 Hz, 2H), 8.52 (d, J = 8.0 Hz, 2H), 8.42 (t, J = 8.4 Hz, 1H), 8.19 (d, J = 8.4 Hz, 2H), 7.92 (d, J = 8.4 Hz, 2H), 7.90–7.70 (m, 4H), 7.73 (td, J = 6.8, 1.6 Hz, 2H), 7.40–7.35 (m, 4H), 7.31 (d, J = 4.8 Hz, 2H), 7.25–7.20 (m, 4H), 7.20–7.10 (m, 6H), and 6.79 (td, J = 6.4, 1.2 Hz, 2H). MALDI-MS (m/z): 952.9 for C₅₅H₃₇Cl₂N₆Ru [M – PF₆]⁺. Anal. Calcd for C₅₅H₃₇Cl₂F₆N₆PRu: C, 60.12; H, 3.39; N, 7.65. Found: C, 59.93; H, 3.47; N, 7.77.

Synthesis of 7(PF₆)

Using the similar procedure as for the synthesis of 5(PF₆), complex 7(PF₆) (31 mg) was obtained from the reaction of [Ru(Me₃tctpy)Cl₃] (0.10 mmol, 61 mg) and ligand 1 (0.10 mmol, 61 mg) in 49% yield as a black solid. ¹H NMR (300 MHz, d₆-acetone): δ 9.67 (s, 2H), 9.27 (s, 2H), 8.83 (d, J = 7.0 Hz, 2H), 8.55 (d, J = 8.2 Hz, 2H), 8.22 (d, J = 8.4 Hz, 2H), 8.16 (d, J = 8.4 Hz, 2H), 7.89 (d, J = 8.4 Hz, 2H), 7.78 (d, J = 8.4 Hz, 2H), 7.82–7.62 (m, 8H), 7.20–7.08 (m, 4H), 7.02–6.95 (m, 4H), 6.74 (t, J = 6.6 Hz, 2H), 4.20 (s, 3H), 3.93 (s, 6H), and 3.83 (s, 6H). ¹³C NMR (100 MHz, d₆-acetone): δ 169.55, 169.49, 164.80, 160.76, 157.43, 155.90, 154.08, 153.13, 142.91, 142.80, 141.61, 137.48, 136.97, 136.93, 132.89, 129.55, 128.33, 128.21, 128.14, 128.05, 127.77, 127.51, 126.74, 123.80, 123.67, 123.61, 123.06, 123.01, 121.38, 121.12, 115.76, 55.82, 53.59, and 53.43. MALDI-MS (m/z): 1119.3 for C₆₃H₄₉N₆O₈Ru [M – PF₆]⁺. Anal. Calcd for C₆₃H₄₉F₆N₆O₈PRu·3H₂O: C, 57.40; H, 4.21; N, 6.38. Found: C, 57.37; H, 4.17; N, 6.19.

Synthesis of 8(PF₆)

Using the similar procedure as for the synthesis of 5(PF₆), complex 8(PF₆) (52 mg) was obtained from the reaction of [Ru(Me₃tctpy)Cl₃] (0.10 mmol, 61 mg) and ligand 2 (0.10 mmol, 60 mg) in 84% yield as a black solid. ¹H NMR (300 MHz, d₆-acetone): δ 9.67 (s, 2H), 9.27 (s, 2H), 8.82 (s, 2H), 8.55 (d, J = 8.1 Hz, 2H), 8.27–8.10 (m, 2H), 7.93 (dd, J = 13.1, 8.4 Hz, 2H), 7.84–7.59 (m, 9H), 7.23–6.96 (m, 11H), 6.75 (t, J = 6.5 Hz, 2H), 4.20 (s, 3H), 3.93 (s, 6H), and 2.34 (s, 6H). ¹³C NMR (100 MHz, d₆-acetone): δ 169.57, 169.50, 164.82, 160.74, 166.01, 155.93, 154.11, 153.13, 146.22, 142.93, 142.86, 137.49, 136.98, 133.72, 132.91, 130.92, 129.57, 128.35, 128.23, 128.21, 127.69, 126.75, 125.67, 123.85, 123.81, 123.69, 123.64, 123.42, 123.06, 123.02, 121.37, 54.96, 53.60, and 53.43. MALDI-MS (m/z): 1087.4 for C₆₃H₄₉N₆O₆Ru [M – PF₆]⁺. Anal. Calcd for C₆₃H₄₉F₆N₆O₆PRu·3H₂O: C, 58.83; H, 4.31; N, 6.53. Found: C, 58.74; H, 4.18; N, 6.53.

Synthesis of 9(PF₆)

Using the similar procedure as for the synthesis of 5(PF₆), complex 9(PF₆) (44 mg) was obtained from the reaction of [Ru(Me₃tctpy)Cl₃] (0.10 mmol, 61 mg) and ligand 3 (0.10 mmol, 62 mg) in 69% yield as a black solid. ¹H NMR (300 MHz, d₆-acetone): δ 9.68 (s, 2H), 9.27 (s, 2H), 8.84 (s, 2H), 8.56 (d, J = 7.9 Hz, 2H), 8.21 (d, J = 8.4 Hz, 2H), 7.94 (d, J = 8.3 Hz, 2H), 7.87–7.72 (m, 4H), 7.68 (s, 4H), 7.45–7.29 (m, 4H), 7.24 (d, J = 8.5 Hz, 2H), 7.21–7.08 (m, 6H), 6.77–6.72 (m, 2H), 4.20 (s, 3H), and 3.93 (s, 6H). ¹³C NMR (75 MHz, d₆-acetone): δ 169.50, 165.98, 164.79, 160.71, 155.90, 154.06, 153.13, 147.40, 147.19, 142.86, 141.58, 139.31, 137.62, 136.94, 136.41, 135.37, 133.32, 130.43, 128.70, 128.21, 127.89, 126.72, 126.40, 125.98, 125.50, 123.90, 123.80, 123.52, 123.02, 121.45, 54.96, 53.58, and 53.42. MALDI-MS (m/z): 1127.1 for C₆₁H₄₃Cl₂N₆O₆Ru [M – PF₆]⁺. Anal. Calcd for C₆₁H₄₃Cl₂F₆N₆O₆PRu: C, 57.56; H, 3.40; N, 6.60. Found: C, 57.83; H, 3.64; N, 6.70.

Electrochemistry

The details for the electrochemical measurements are given in our previous publications.^20b,20c

Spectroelectrochemical Measurements

The details for the spectroelectrochemical measurements are given in our previous publications.^20a

EPR Measurements

EPR measurements were performed on a Bruker ELEXSYS E500-10/12 spectrometer at 77 K in frozen CH₃CN. The spectrometer frequency is 9.7 × 10⁹ Hz.

Computational Methods

The details for the computational methods are available in our previous publications.^20b,20c

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03058.

• NMR and mass spectra of new complexes (PDF)

Author Contributions

^§ Z.-J.L. and J.-J.S. contributed equally to this work.

The authors declare no competing financial interest.