Substituent Effect of the Bridging Ligand in the Trinuclear Ru

Article Cite This: Inorg. Chem. 2017, 56, 12996-13006

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Substituent Effect of the Bridging Ligand in the Trinuclear Ru Complexes on Photocatalytic Oxygenation of a Sulfide and Alkenes Siwas Phungsripheng,† Munetaka Akita,‡ and Akiko Inagaki*,† †

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Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192-0397, Japan ‡ Laboratory of Chemistry and Life Science, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta Midori-ku, Yokohama 226-8503, Japan S Supporting Information *

ABSTRACT: To examine the effect of tuning the electron transfer ability between photosensitizers and reaction centers on the activity of visible-light-driven catalysts, a series of trinuclear Ru photocatalysts containing two Ru(II) photosensitizers and a Ru(II) reaction center bridged by a substituted bipyrimidine was prepared. The introduction of electron-donating or electron-withdrawing groups on the ligands was found to affect the photophysical and electrochemical properties of these catalysts. Density functional theory calculations of the mononuclear Ru complexes with nonsubstituted and substituted bipyrimidine ligands, which correspond to the reaction center of the trinuclear complexes, revealed how the electronic effect of the substituents affect the frontier orbital energies. For photocatalytic oxygenation of a sulfide and terminal alkenes, the catalyst with a 5,5′-dibromo-2,2′bipyrimidine ligand showed higher activity than the catalyst with 5,5′-dimethyl-2,2′-bipyrimidine and nonsubstituted bipyrimidine ligands, as expected according to the electronic effects of these ligands. In contrast, the catalyst with the nonsubstituted bipyrimidine ligand showed the highest activity toward inner alkenes, which may be due to steric effects.

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INTRODUCTION

oxidation potential results in a larger diving force compared with that of the nonsubstituted complex.1d,2,3 The photoredox process of a photosensitizer has been shown to act as an oxidant in the presence of an electron acceptor. Although EWG substituents on the ligand improve the catalytic oxidation activity, in the presence of very strong EWG, such as NO2, dramatic decrease in reactivity is observed. This phenomenon may be caused by high electron deficiency of the metal center, which results in the oxidation potential of the catalyst being higher than or close to the redox potential of the photosensitizer, thus inhibiting the oxidation reaction.3b,c Our group has concentrated on utilizing visible light for catalytic molecular transformations using intramolecular photosensitizing systems, in which catalysts consist of a chromophore and a reaction center linked by a bridging ligand. 4,5 Investigation of the substituent effect on the photocatalytic dimerization of styrene demonstrated that EWG substituents on the bridging ligand (2,2′-bipyrimidine (bpm)) lower the lowest unoccupied molecular orbital (LUMO) energy level, whereas introduction of EDGs onto 2,2′-bipyridyl (bpy) ligands causes an increase of the LUMO+1 energy level. A combination of these two substituent effects significantly

The introduction of electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) on ligands can elaborately tune the electronic properties of the molecular catalysts. The influence of substituent effects on various catalytic reactions has been widely investigated in the variety of catalysis by many research groups.1 In catalysis involving a redox process of the metal center, the ligands substituted with EDGs or EWGs affect the redox potential of the metal center, which has a concomitant influence on catalytic activity. As a result, substitution of an EWG on a ligand shifts the redox potential of a metal center toward a higher potential, whereas the reverse result can be achieved by introducing an EDG on a ligand. For catalytic water oxidation, a clear substituent effect has been demonstrated by Bernhard and Collins’s group, who showed that a high-valent Fe-oxo species is formed by a protoncoulpled electron-transfer (PCET) process using an iron(III)tetraamido macrocyclic ligand (FeIII-TAML) catalyst.2 On the one hand, in the presence of EWGs on the TAMLs, the reactivity is much higher than that of the nonsubstituted catalyst; on the other hand, the introduction of EDGs has the opposite effect. These differences in reactivity may be due to changes in the redox potential of the metal center. For an electron-deficient metal center with EWG substituents, a higher © 2017 American Chemical Society

Received: July 13, 2017 Published: October 23, 2017 12996

DOI: 10.1021/acs.inorgchem.7b01764 Inorg. Chem. 2017, 56, 12996−13006

Article

Inorganic Chemistry Scheme 1. Synthesis of Trinuclear Complexes with Substituted bpm Ligands

Figure 1. 1H NMR (500 MHz, CH3NO2, rt) of 3Me-OH2 (top) and 3Br-OH2 (bottom) in the low-field region (δ7−10 ppm).

accelerates metal-to-ligand charge transfer (MLCT) toward the bridging ligand, which affects the electronic state of the Pd center in the 3T state and results in the drastic acceleration of photocatalytic olefin dimerization.6 These results indicated that photocatalytic reactions can be tuned by varying the substituents on the ligands. Continuous work in our group has concentrated on the photocatalytic oxygenation of organic compounds by the trinuclear Ru catalysts that possess two Ru chromophores and a Ru reaction center.7 Previously, we demonstrated that the trinuclear Ru catalysts showed much higher reactivity on oxygenation of a sulfide and alkenes compared to that of the intermolecular system that contains chromophores and a catalyst separately in the solution. In this study, to tune the electron transfer ability between the photosensitizer and the

reaction center, trinuclear ruthenium complexes with 5,5′dimethyl- and 5,5′-dibromo-2,2′-bipyrimidine bridging ligands were introduced in the catalysts. The photophysical and electrochemical properties of these new trinuclear complexes were compared with those of the nonsubstituted complex. Photocatalytic oxygenation of various organic compounds by the trinuclear catalysts was tested to elucidate the substituent effect. In addition, the quantum yield measurements were conducted to compare the photocatalytic oxygenation performance of the trinuclear catalysts.

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RESULTS AND DISCUSSION Synthesis. The 5,5′-dimethyl-2,2′-bipyrimidine (bpmMe)6b,8 and 5,5′-dibromo-2,2′-bipyrimidine (bpmBr)9 ligands were 12997


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Inorganic Chemistry prepared according to the published procedures. The metal complexes [(bpy)2Ru(bpmMe)]2+ and [(bpy)2Ru(bpmBr)]2+ were synthesized according to a procedure similar to that for the synthesis of [(bpy)2Ru(bpm)]2+.10 To modify the purification procedures and yields of the trinuclear Ru complexes, a new dinuclear Ru intermediate complex 2 was synthesized. Reaction of [(bpy)2Ru(pybpm)]2+ (1) with [Ru(cod)Cl2]n (cod = 1,5-cyclooctadiene) yielded complex 2 ([(bpy)2Ru(pybpm)RuCl2]Cl2; abbreviated as [Ru− Ru−Cl2]2+) in 87% yield. Characterization by 1H NMR spectroscopy showed sharp peaks corresponding to a diamagnetic complex of RuII−RuII−Cl2 (Figure S1a). Electrospray ionization (ESI) mass spectral data matched well with the simulated isotope pattern of [(bpy)2Ru(pybpm)RuCl 2 ]+ (Figure S1b). Two characteristic sets of doublet peaks (J = 5.75 and 5.50 Hz) were observed in the low-field region around 9.5 ppm, which were assigned to proton signals of the pybpm ligand, indicating the presence of isomers. The trinuclear Ru complexes with substituted bpm ligand were synthesized by reaction of dinuclear complex 2 with a Ru chromophore, [Ru(bpy)2(bpmR)]2+ (R = Me, Br, bpmR = 5,5′R2-2,2′-bipyrimidine; Scheme 1). The reactions of 2 with [(bpy)2Ru(bpmMe)]2+ and [(bpy)2Ru(bpmBr)]2+ yielded the trinuclear complexes 3Me-Cl (81%) and 3Br-Cl (75%), respectively. The chloride ligand of the trinuclear complexes was removed using AgBF4 to give the corresponding aqua complexes in high yields (3Me-OH2 (98%), 3Br-OH2 (83%)). The synthetic procedures are described in detail in the Experimental Section. Ligand exchange from chloride to aqua ligands was confirmed by 1H NMR spectroscopy as shown in Figures S2−S5. Similar to the spectrum of the nonsubstituted trinuclear complex 3H-OH2, the presence of isomers for the complexes 3Me-Cl and 3Br-Cl was indicated by multiple signals in the lowfield region of the 1H NMR spectra. Ligand exchange did not affect the number of isomers, and similar spectral evidence for isomers was observed in the 1H NMR spectra of the aqua complexes. The 1H NMR spectra of the trinuclear complexes with different substituent groups on the bpm bridging ligand are shown in Figure 1. Characteristic peaks are observed for complex 3Br-OH2 in the low-field region around 10−11 ppm, whereas those for 3Me-OH2 are shifted slightly toward the higher field around 9.8−9.9 ppm. Photophysical and Electrochemical Properties. UV− Visible Absorption Spectra. The UV−vis absorption spectra of the trinuclear complexes with electron-donating and electronwithdrawing substituents on the bpm ligand were compared with that of the corresponding nonsubstituted trinuclear complex 3H-OH2. The room-temperature (rt) spectra for dimethylformamide (DMF) solution of the trinuclear complexes (10 μM, 3H-Cl, 3H-OH2, 3Me-Cl, 3Me-OH2, 3Br-Cl, and 3Br-OH2) are shown in Figure 2, and the spectral data are summarized in Table 1. All the trinuclear complexes possess broad and wide MLCT absorption band up to 800 nm. The identity of the terminal ligand (chloride or aqua) did not have a significant effect on the spectra of the trinuclear complexes. The presence of substituents on the bpm ligand resulted in decreased molar absorptivity (ε) of the MLCT (dπRu→π*) absorption band at ∼620 nm for 3Br-X and 430 nm for 3Me-X, in comparison with the parent nonsubstituted complex 3H-X. Introduction of the substituents on the bpm ligand resulted in the shifting of various MLCT transitions, including dπRu→πbpm* transition. Thus, resultant absorptivity changes arouse the

Figure 2. UV−vis absorption spectra of the trinuclear Ru complexes in DMF at rt.

isosbestic point at 549 nm. Related changes in the frontier orbital energies will be discussed in the next section. Electrochemistry. The cyclic voltammograms (CVs) of 3MeCl, 3Me-OH2, 3Br-Cl, and 3Br-OH2 in CH3CN solutions under a N2 atmosphere at ambient temperature obtained using glassy carbon electrodes are compared with the nonsubstituted trinuclear complex 3H-Cl and 3H-OH2, as displayed in Figures 3 and 4. The potentials of these complexes are summarized in Table 2. The corresponding differential pulse voltammograms (DPVs) in CH3CN are shown in Figures S6 and S7. Similar to complex 3H-X (X = Cl or H2O), each of the substituted complexes underwent two or three oxidation processes. The first oxidation process at 1.0−1.2 V can be assigned to oxidation of the Ru reaction center (RucatII/RucatIII). The corresponding redox peaks of 3Me-X and 3Br-X (X = OH2 or Cl) were slightly shifted to lower and higher potentials, respectively, compared with that of 3H-X. The second oxidation process can be assigned to oxidation of the Ru chromophore (RuphotoII/III). Complex 3Me-Cl showed two redox waves at 1.22 and 1.35 V for the two different chromophore units (RuphotobpmMeII/III and RuphotopybpmII/III; also observed in the DPV in Figure S6) owing to the different electronic character of the two photosensitizers. However, these two peaks overlapped in the voltammogram of the aqua complex 3Me-OH2 and also in the voltammograms of 3H-Cl and 3Br-Cl. The overlapped peaks were confirmed to be two-electron processes based on the comparison of the ipa values. The change in the electron density at the Ru reaction center was caused by the shift in the redox potential of the ligands, which is consistent with data reported in the literature.1c,d,3c,6 Among the trinuclear complexes, the highest oxidation potential was observed for 3Br-Cl and 3BrOH2 owing to the effect of EWGs introduced on the bpm ligand. The similar trends were also observed for the reduction processes. All the complexes exhibited two redox waves owing to reduction of the bridging ligands (BL, BL′ = pybpm or bpm), whereas many overlapping waves were observed at lower potentials from −1.5 to −2.5 V, which can be attributed to reduction processes of the four bpy ligands in the chromophore units ([Ru(bpy)2(L)]2+). In a previous study, we assigned the first reduction process ((BL)/(BL)•−) to the reduction of pybpm and the second to reduction of bpm by comparison with the redox processes of the dinuclear complexes with 12998


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Inorganic Chemistry Table 1. UV−Vis Absorption Spectral Data for the Trinuclear Ru Complexes in DMF at Room Temperature complex Br

3 -OH2 3H-OH2 3Me-OH2

λmax [nm] 431.5 529.5 426.5 597.0 434.0 643.5

ε [1 × 105·M−1·cm−1]

complexes Br

0.35 0.20 0.33 0.12 0.28 0.09

3 -Cl 3H-Cl 3Me-Cl

λmax [nm]

ε [1 × 105·M−1·cm−1]

431.0 539.5 459.0 659.5 432.0 663.5

0.35 0.17 0.30 0.09 0.28 0.08

Figure 3. CVs of 3Me-Cl (top), 3H-Cl (middle), and 3Br-Cl (bottom) in CH3CN at rt. ([complex] = 1 mM, [nBuNBF4] = 0.1 M, 100 mV/s, potential vs Fc/Fc+).

Figure 4. CVs of 3Me-OH2 (top), 3H-OH2 (middle), and 3Br-OH2 (bottom) in CH3CN at rt. ([complex] = 1 mM, [nBuNBF4] = 0.1 M, 100 mV/s, potential vs Fc/Fc+).

A comparison of the redox potentials of 3H-X and the other complexes clearly shows that the potential of (BL′)/(BL′)•− is

pybpm and bpm bridging ligands ([(bpy)2Ru(pybpm)Ru(OH2)(bpy)]4+ and [(bpy)2Ru(bpm)Ru(OH2)(pybpy)]4+).4c 12999


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Inorganic Chemistry Table 2. Electrochemical Data for the Trinuclear Complexes in CH3CNa E1/2 [V] complexes Br

(BL)/

RucatII/

RuphotoII/

(BL′)/(BL′)·−

(BL)·−

RucatIII

RuphotoIII

−0.98 −0.98 −1.12 −0.90 −0.95 −1.09

−0.76 −0.82 −0.83 −0.70 −0.80 −0.83

1.13 1.01 0.98 1.19 1.18 1.14

1.39 1.27 1.22, 1.35 1.35 1.25 1.23

Br

[Ru(pybpm)Ru(Cl)(bpm )Ru](BF4)5 (3 -Cl) [Ru(pybpm)Ru(Cl)(bpmH)Ru](BF4)5 (3H-Cl) [Ru(pybpm)Ru(Cl)(bpmMe)Ru](BF4)5 (3Me-Cl) [Ru(pybpm)Ru(OH2)(bpmBr)Ru](BF4)5 (3Br-OH2) [Ru(pybpm)Ru(OH2)(bpmH)Ru](BF4)6 (3H-OH2) [Ru(pybpm)Ru(OH2)(bpmMe)Ru](BF4)5 (3Me-OH2) a

[complex] = 1 mM, [nBu4NBF4] = 0.1 M, 100 mV/s, E vs Fc/Fc+. Ru = [(bpy)2Ru], Rucat = Ru reaction center, Ruphoto = Ru photosensitizer.

the two [Ru(bpy)2]2+ moieties of the trinuclear complexes. The optimized structures of these complexes are shown in Figure S8, and the frontier orbitals of complex 4H are shown in Figure 5. The highest occupied molecular orbital (HOMO), HOMO− 1, and HOMO−2 possess mainly ruthenium d-orbital character, whereas the LUMO, LUMO+1, and LUMO+2 possess ligand π-orbital character. A comparison of corresponding orbitals shows that the orbital characters of the HOMO, HOMO−1, HOMO−2 of 4H, 4Br, and 4Me are very similar. On the contrary, clear differences are observed in the characters of the LUMO, LUMO+1, and LUMO+2 (Figure 6). The LUMO of 4H possesses both pybpm and bpm character equally delocalized on both ligands, whereas the LUMO of 4Br and 4Me possessed only bpm and pybpm character, respectively. However, the LUMO+1 of both these complexes possesses pybpm character. Energy diagram of the molecular orbitals energetically close to the HOMO and the LUMO are compared in Figure 7 for 4H, 4Br, and 4Me. As shown, the LUMO and its neighboring orbitals are stabilized in 4Br, but destabilized in 4Me, whereas only small differences are observed in the HOMO energy level. These changes are parallel to the redox potential shifts observed in the electrochemical studies. In oxygenation reactions of alkenes and sulfides catalyzed by 3R (R = H, Br, Me), the 4R unit will act as the reaction center. On the basis of our previous studies, a high-valent Ru(V)O species is generated as the key intermediate for efficient

similar for 3H-X and 3Br-X, whereas the potential of (BL)/ (BL)•− is similar for 3H-X and 3Me-X. Redox processes that have similar redox potentials to those of 3H-X should correspond to the nonsubstituted ligand in 3Me-X and 3Br-X. Thus, the first reduction process of 3Br-X and 3Me-X involves bpm and pybpm, respectively. More details on the characteristics of the frontier orbitals will be discussed in the next section. Theoretical Studies. To assess the inductive electronic effect of the methyl and bromo substituents, we performed density functional theory (DFT) calculations for cationic mononuclear ruthenium complexes 4H, 4Br, and 4Me (Chart 1), which, to reduce computational complexity, do not contain Chart 1. Mononuclear Ruthenium(II) Complexes for DFT Calculation

Figure 5. Calculated molecular orbitals (HOMO−2, HOMO−1, HOMO, LUMO, LUMO+1, and LUMO+2) of 4H. 13000


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Figure 6. LUMO, LUMO+1, and LUMO+2 orbitals of 4Br, 4H, and 4Me.

increase of the electrophilicity of the oxo-ligand to result in the higher reactivities. Photocatalytic Oxygenation. Photocatalytic oxygenation of a sulfide and alkenes was investigated under the conditions described in the Experimental Section. The products were quantified by gas chromatography (GC) mass and GC and assigned by using corresponding standard chemicals, as summarized in Table 3. Consistent with the results obtained with catalyst 3H-OH2, methyl phenyl sulfoxide was formed as a single product in the photocatalytic reactions of thioanisole with 3Me-OH2 and 3BrOH2 (Scheme S1). Trinuclear catalyst 3Br-OH2 exhibited the highest turnover number (TON) of 383, whereas catalyst 3MeOH2 showed the lowest TON of 305. These results suggested that EWGs on the bpm bridging ligand accelerate the photocatalytic oxygenation of thioanisole, which is similar to the previous results.1b,d,3−5,6a Catalytic oxygenation of alkenes was also investigated using the trinuclear catalysts (Table 3). The reactions of styrene and α-methylstyrene resulted in the formation of benzaldehyde and acetophenone, respectively, as main products via an epoxide and a diol with CC bond cleavage.7 Complexes 3Me-OH2 and 3Br-OH2 were also active for the photocatalytic oxygenation of styrenes, with the highest TONs observed with complex 3BrOH2. In the reaction of cis-stilbene, isomerized trans-stilbene was obtained as the main product, together with stilbene oxide, 1,2-diphenylethanedione, benzophenone, and benzaldehyde. The observation of these minor products indicates that benzaldehyde was formed by stepwise oxygenation via the epoxide and dione. However, for this reaction, there was only a small difference among the TONs of the complexes, which may result from steric repulsion between the substituents and the internal alkene suppressing the increased reactivity of 3Br-OH2. For the reaction of cis-2-decene, 3Me-OH2 and 3Br-OH2 showed lower activity than that of 3H-OH2. Although the conversion was low for all the catalysts, interestingly, C7 and C6 terminal aldehydes were formed in addition to the C8 aldehyde. These results suggest that isomerization of the double bond at the 3 or 4 positions occurs during the reaction.

Figure 7. Molecular orbital energy diagram for optimized mononuclear complexes 4Br, 4H, and 4Me.

catalysis.7 The HOMO energy levels of the three complexes (4R) are similar, but the energy levels of the LUMO and the neighboring orbitals are considerably stabilized for 4Br and destabilized for 4Me. This stabilization should contribute to retarding back electron transfer from the [Ru(bpy)2]2+ moiety to electronically neutralize the high-valent Ru(V)O species. Additionally, Mulliken charge value of the ruthenium center for 4Br (1.0901) was much higher than those for 4Me (1.0385), and 4H (1.0420). The increase of the charge will contribute to the 13001


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Inorganic Chemistry Table 3. Photocatalytic Oxygenation of a Sulfide and Various Alkenes by the Trinuclear Catalystsa

a

TON: turn over number for the obtained monooxygenated products (sulfoxide, aldehydes, and ketones).

Scheme 2. Quantum Yield Measurement for the Oxidation of Methylphenyl Sulfide in Phosphate-Buffered Water (pH 6.8)

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case of 3Me-OH2, the quantum yield was the lowest (9.2%) among the trinuclear catalysts but higher than those of the dinuclear complex [(bpy)2Ru(pybpm)Ru(OH2)(bpy)](BF4)4 and the mononuclear complex [(tpy)Ru(OH2)(bpy)]2+ with 2 equiv of photosensitizer ([(bpy)3Ru]2+). Interestingly, in the reaction catalyzed by 3Br-OH2 and 3Me-OH2, the induction period during the initial stage of the catalysis was much shorter than that with 3H-OH2. This acceleration would shorten the time required for catalysis. We think that the induction period corresponds to the time for the active RuVO species to be generated in the reaction solution. According to our proposed mechanism,7 there are multiple steps (a) to (e) to generate the RuVO species (3VO), and introduction of the substituents may accelerate these steps (Scheme S1). For example, introduction of EDG in 3Me should accelerate step (b) owing to the lower oxidation potential of Rucat, whereas introduction of EWG in 3Br should accelerate step (d) as a result of the change of the redox potentials of the bridging ligands. These results confirmed that increased activity was exhibited by the trinuclear catalyst with EWGs on the bridging ligand, which is consistent with the results for the photocatalytic reactions presented in the previous section. It was not possible to compare these results with data from other groups owing to the use of different substrates and conditions.9 However, it is clear that the quantum yield of 3Br-OH2 is one of the highest values among the reported data.11−13

Quantum yield measurements were conducted for the photocatalytic oxygenation of thioanisole by trinuclear complexes 3Me-OH2 and 3Br-OH2, for comparison with 3HOH2 (Scheme 2). The quantum yields were calculated from the slope of the plot of amount of methyl phenyl sulfoxide formed (mmol) versus absorbed photon (1 × 10−3 einstein), in which the value can be calculated from the slope (Figure 8 and Table 4).

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Figure 8. Plot of amount of oxygenated product (sulfoxide) formed vs absorbed photons for quantum yield determination.

SUMMARY Trinuclear catalysts were synthesized with substituent groups on the bpm bridging ligands. The introduction of EWGs or

The highest quantum yield (19.8%) among the three trinuclear catalysts was observed for catalyst 3Br-OH2. In the

Table 4. Quantum Yield (Φ) for Photocatalytic Sulfoxidation by Trinuclear (3Br-OH2, 3H-OH2, 3Me-OH2), Dinuclear, and Mononuclear Catalysts Compared with Literature Results

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evaporation of the solvent, [CoCl(NH3)5]Cl2 (15 mg, 60 μmol) and 3 mL of 0.067 M sodium phosphate buffer (pH 6.8) were added. The resulting solution was degassed by the freeze−pump−thaw method, and the organic substrate (30 μmol) was then added under a N2 atmosphere. The system was sealed, kept in a water bath at 35 °C, and then exposed to the light produced by a 150 W Xe lamp through a cut filter (λ > 420 nm). The solution was stirred during the reaction. The reaction was followed by 1H NMR and GC-MS at appropriate time intervals. Computational Details. DFT calculations were performed using the Gaussian-16 Revision A.03 quantum chemistry program package16 at the B3LYP/LanL2DZ level. 17,18 We used the LanL2DZ pseudopotential for Ru, 6-31G(d)19 split-valence basis set for N and Cl, and 3-21G20 for C and H. The orbital energies were determined by using minimized singlet geometries to approximate the ground state. Synthesis of [(bpy)2Ru(pybpm)RuCl2]Cl2 (2). A mixture of [Ru(cod)Cl2]n (0.073 g, 0.259 mmol) and LiCl (0.061 g, 1.448 mmol) was stirred in degassed 2-methoxyethanol until all the LiCl was dissolved. Then, Ru complex 1 ([(bpy)2Ru(pybpm)]2+, 0.101 g, 0.123 mmol) was added, and the solution was refluxed at 150 °C under N2 for 30 min. A dark brown powder was precipitated by adding Et2O. The obtained solid was dissolved in CH3NO2, and the remaining salt was washed with H2O/CH3NO2 and dried over Na2SO4. Subsequent recrystallization from CH3NO2/Et2O yielded dinuclear [(bpy)2Ru(pybpm)RuCl2]Cl2 (2) in 87.2% yield (0.096 g, 0.108 mmol). Two isomers were obtained owing to Δ- and Λ-[Ru(bpy)(pybpm)]2+ units, as indicated by the two sets of doublet signals around 9.4−9.6 ppm in the 1H NMR spectra showed (Figure S1a). 1H NMR (500 MHz, CD3NO2, rt, δ/ppm): 9.53−9.52 (two doublets representing two isomers, 1 H), 9.42−9.40 (two doublets representing two isomers, 1 H), 8.61−8.58 (m, 5 H), 8.31−8.30 (2 S, 1 H), 8.17−8.08 (m, 7H), 8.04−8.00 (m, 3 H), 7.82−7.79 (2 t, 2 H), 7.74−7.73 (1 d, 1 H), 7.49−7.47 (4 t, 4 H). ESI-MS (m/z): 855.9 [M-Cl]+ (calcd: 856.12). Anal. Calcd for C33H25Cl4N9Ru2·0.5CH3NO2·3H2O: C, 41.22; H, 3.36; N, 13.63. Found: C, 41.24; H, 3.31; N, 13.66%. Synthesis of [(bpy)2Ru(pybpm)RuCl(bpmMe)Ru(bpy)2](BF4)5 (3Me-Cl). A mixture of 2 (0.035 g, 0.040 mmol) and [(bpy)2Ru(bpmMe)](BF4)2 (0.032 g, 0.042 mmol) were refluxed with LiCl (0.015 g, 0.342 mmol) in distilled EtOH (7 mL) for 12 h. After it cooled to room temperature, the dark brown solution was vigorously stirred with a solution of NH4BF4 in EtOH (0.05M, 30 mL) to precipitate trinuclear Ru complex 3Me-Cl ([(bpy)2Ru(pybpm)RuCl(bpmMe)Ru(bpy)2](BF4)5) as a brown solid. Excess salt was removed by diatomaceous earth filtration. The obtained brown solid was recrystallized from CH3NO2/Et2O three times to yield 81.2% (0.058 g, 0.032 mmol). Characteristic peaks in the 1H NMR spectrum indicated the presence of 16 isomers 1H NMR (500 MHz, CD3NO2, rt, δ/ppm): 9.99−9.90 (16 s representing 16 isomers, 1 H), 8.93−8.39 (m, 14 H), 8.31−8.11 (m, 10 H), 8.10−7.86 (m, 6 H), 7.85−7.63 (m, 5 H), 7.61− 7.37 (m, 9 H), 2.60−2.56 (4 s representing 16 isomers, 3 H), 1.90− 1.79 (4 s representing 16 isomers, 3 H). ESI-MS (m/z): 1731.88 [M − BF4]+ (calcd: 1732.09). Anal. Calcd for C63H51B5ClF20N17Ru3· CH3NO2: C, 40.89; H, 2.90; N, 13.41. Found: C, 40.72; H, 2.93; N, 13.12%. Synthesis of [(bpy)2Ru(pybpm)RuOH2(bpmMe)Ru(bpy)2](BF4)6 (3Me-OH2). A mixture of 3Me-Cl (0.048 g, 0.026 mmol) and AgBF4 (0.015 g, 0.079 mmol) were refluxed in H2O (5 mL) for 4 h. After it cooled to room temperature, the solvent (water) was removed under reduced pressure, and then the obtained solid was dissolved in CH3NO2. After diatomaceous earth filtration to remove AgCl salt, the filtrate was concentrated and recrystallized from CH3NO2/Et2O to yield trinuclear aqua complex 3 Me -OH 2 , [(bpy) 2 Ru(pybpm)RuOH2(bpmMe)Ru(bpy)2](BF4)6 as a dark-brown solid (0.049 g, 0.026 mmol, 97.9% yield). 1H NMR (500 MHz, CD3NO2, rt, δ/ppm): 9.84−9.74 (16 s representing 16 isomers, 1 H), 8.78−8.30 (m, 16 H), 8.28−7.86 (m, 15 H), 7.82−7.35 (m, 13 H), 2.63−2.58 (4 s representing 16 isomers, 3 H), 1.92−1.79 (4 s representing 16 isomers, 3 H). ESI-MS (m/z): 1715.01 [M − 2BF4]2+ (calcd: 1714.66). Anal. Calcd for C63H53B6F24N17ORu3·3.5H2O·CH3NO2: C, 38.20; H, 3.16; N, 12.53. Found: C, 37.82; H, 2.77; N, 12.21%.

EDGs on the bpm ligand affected the photophysical properties of the trinuclear catalysts, as clearly observed on the UV−vis spectroscopic data. Moreover, the EWGs or EDGs on the bpm ligands clearly shifted the redox potentials of Rucat, Ruphoto, pybpm, and bpm, owing to the electronic effects of the substituents. On the one hand, in the case of EWGs, the redox waves in the CVs were shifted to higher potentials owing to the electron-deficient Ru center. On the other hand, the EDGs shifted the redox waves to lower potentials. These results corresponded well with the theoretical data. The highest activity for photocatalytic oxygenation of thioanisole was observed for the trinuclear catalyst with EWGs, and the same trend was found for the reactions of terminal alkenes (styrene and α-methylstyrene). However, this catalyst showed lower catalytic activity for the reactions of internal alkenes (cis-stilbene and cis-2-decene), likely owing to the steric effects resulting from incorporation of substrates onto the catalyst reaction center. Quantum yield measurements for the photocatalytic oxygenation of thioanisole indicated that the trinuclear catalyst with EWG substituents on the bpm ligand had the highest quantum yield of ∼20%. In contrast, the lower quantum yield of the trinuclear catalyst with EDG substituents was comparable to that of the dinuclear catalyst. Furthermore, the trinuclear complexes had much higher quantum yields than the corresponding mononuclear catalysts.

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EXPERIMENTAL SECTION

General Method. The metal complexes, [Ru(cod)Cl2]n,14 cis(Cl)fac(S)-RuCl2(dmso) 4, 15 [(bpy) 2Ru(bpm)] 2+,10 cis-Ru(bpy)2 Cl 2· 2H2O,10 [(bpy)2Ru(pybpm)](BF4)2 (1),4c [(bpy)2Ru(bpmMe)]2+,6b and [(bpy)2Ru(bpmBr)]2+,6b were prepared according to the published procedures. 1H NMR spectra were recorded on a Bruker AVANCE500, JEOL ECS-400, and JEOL EX-270 spectrometers. NMR solvents were dried over molecular sieves, degassed, and stored under N2. ESI mass spectra were recorded on a Bruker MicroTOF II mass spectrometer. Elemental analysis was performed using a CE-440 elemental analyzer (Exeter Analytical, Inc.). Physical Measurements. UV−vis absorption spectra were recorded on a JASCO V-670 spectrometer for DMF solutions in quartz cells with a path length of 10 mm. CVs and DPVs were recorded for 1 mM of each Ru complex in acetonitrile using an electrochemical analyzer (model 620B, CH Instruments, Inc.) with a glassy carbon working electrode (diameter 1.0 mm), a Ag/Ag+ reference electrode (0.01 M AgNO3, 0.1 M tetrabutylammonium perchlorate in electrolyte solution), and a Pt wire counter electrode. Acetonitrile was degassed by N2 bubbling for 30 min. The solution of 0.1 M tetrabutylammonium tetrafluoroborate (nBu4NBF4) in degassed acetonitrile was prepared in a Schlenk cell. Measurements were performed under an inert gas flow (Ar + N2) to prevent contamination by oxygen. A scan rate of 100 mV/s was selected. After each measurement, ferrocene was added to the solution as an internal standard (E(Fc/Fc+) = 0 V vs normal hydrogen electrode (NHE)). Quantum Yield Measurements. A reaction solution containing Ru catalyst (20 μM) in 5 mL of phosphate-buffered water (0.067 M, pH 6.8) with thioanisole (10 mM) and [Co(NH3)5Cl]Cl2 (20 mM) as a sacrificial electron acceptor was prepared. Before light irradiation, the solution was degassed by the freeze−pump−thaw method in a 20 mL transparent Schlenk tube. Monochromatic light-emitting diode (LED) light (λ = 435 nm) was used as the light source through an optical lens to maintain the incident light intensity. The number of photons absorbed by the solution was calculated by using a light power meter (StarLite, OPHIR Photonics Solutions, Ltd.) at λ = 435 nm. The slope of the plot of the product formed (methyl phenyl sulfoxide product, mmol) versus number of photons (mmol) gave the quantum yield. General Photocatalytic Reactions. A 0.1 mM solution of the catalyst in CH3NO2 (0.6 mL) was added to the Schlenk tube. After 13004


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Inorganic Chemistry Synthesis of [(bpy)2Ru(pybpm)RuCl(bpmBr)Ru(bpy)2](BF4)5 (3Br-Cl). A mixture of 2 (0.022 g, 0.025 mmol) and [(bpy)2Ru(bpmBr)](BF4)2 (0.023 g, 0.025 mmol) were refluxed with LiCl (0.010 g, 0.23 mmol) in distilled EtOH (2 mL) for 12 h. After it cooled to room temperature, the dark green solution was vigorously stirred with a solution of NH4BF4 in EtOH (0.05 M, 32 mL) to precipitate trinuclear Ru complex 3Br-Cl ([(bpy)2Ru(pybpm)RuCl(bpmBr)Ru(bpy)2](BF4)5) as a black solid. Residual NH4BF4 salt was removed by diatomaceous earth filtration. The obtained brown solid was recrystallized from CH3NO2/Et2O three times to yield 75.1% (0.036 g, 0.018 mmol). 1H NMR signals at ∼10 ppm showed 16 singlet peaks, which correspond to presence of 16 isomers. 1H NMR (500 MHz, CD3NO2, rt, δ/ppm): 10.18−10.09 (16 s representing 16 isomers, 1 H), 8.90−8.35 (m, 15 H), 8.32−8.07 (m, 12 H), 8.06−7.87 (m, 4 H), 7.83−7.65 (m, 4 H), 7.64−7.38 (m, 9 H). ESI-MS (m/z): 843.02 [M − 2BF4]2+ (calcd: 844.12). Anal. Calcd for C61H45B5Br2ClF20N17Ru3: C, 37.60; H, 2.33; N, 12.22. Found: C, 37.56; H, 2.26; N, 12.31%. Synthesis of [(bpy)2Ru(pybpm)RuOH2(bpmBr)Ru(bpy)2](BF4)6 (3Br-OH2). A mixture of 3Br-Cl (0.020 g, 0.011 mmol) and AgBF4 (0.007 mg, 0.038 mmol) was refluxed in H2O 2.5 mL for 5 h. After it cooled to room temperature, the solvent (water) was removed, and the obtained solid was dissolved in CH3NO2. After diatomaceous earth filtration to remove AgCl salt, the filtrate was concentrated and recrystallized from CH3NO2/Et2O to yield 3Br-OH2 as a black solid of (0.018 g, 0.009 mmol, 82.8% yield). 1H NMR (500 MHz, CD3NO2, rt, δ/ppm): 10.02−9.92 (16 s representing 16 isomers, 1 H), 8.77−8.37 (m, 17 H), 8.34−8.01 (m, 15 H), 7.96−7.62 (m, 6 H), 7.58−7.37 (m, 6 H). Anal. Calcd for C61H47B6Br2F24N17ORu3·1.5CH3NO2·Et2O: C, 36.58; H, 2.84; N, 11.87. Found: C, 36.93; H, 2.55; N, 12.19%.

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2415−2417. (b) Inagaki, A.; Akita, M. Visible-Light Promoted Bimetallic Catalysis. Coord. Chem. Rev. 2010, 254, 1220−1239. (c) Neudeck, S.; Maji, S.; López, I.; Dechert, S.; Benet-Buchholz, J.; Llobet, A.; Meyer, F. Establishing the Family of Diruthenium Water Oxidation Catalysts Based on the Bis(bipyridyl)pyrazolate Ligand System. Inorg. Chem. 2016, 55, 2508−2521. (d) Wasylenko, D. J.; Ganesamoorthy, C.; Koivisto, B. D.; Henderson, M. A.; Berlinguette, C. P. Insight into Water Oxidation by Mononuclear Polypyridyl Ru Catalysts. Inorg. Chem. 2010, 49, 2202−2209. (2) Ellis, W. C.; McDaniel, N. D.; Bernhard, S.; Collins, T. J. Fast Water Oxidation Using Iron. J. Am. Chem. Soc. 2010, 132, 10990− 10991. (3) (a) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. Cyclometalated Iridium(III) Aquo Complexes: Efficient and Tunable Catalysts for the Homogeneous Oxidation of Water. J. Am. Chem. Soc. 2008, 130, 210−217. (b) Panda, C.; Debgupta, J.; Díaz Díaz, D.; Singh, K. K.; Sen Gupta, S.; Dhar, B. B. Homogeneous Photochemical Water Oxidation by Biuret-Modified Fe-TAML: Evidence of FeV(O) Intermediate. J. Am. Chem. Soc. 2014, 136, 12273−12282. (c) Roeser, S.; Bozoglian, F.; Richmond, C. J.; League, A. B.; Ertem, M. Z.; Francàs, L.; Miró, P.; Benet-Buchholz, J.; Cramer, C. J.; Llobet, A. Water Oxidation Catalysis with Ligand Substituted Ru-bpp Type Complexes. Catal. Sci. Technol. 2016, 6, 5088−5101. (4) (a) Inagaki, A.; Edure, S.; Yatsuda, S.; Akita, M. Highly Selective Photo-Catalytic Dimerization of α-Methylstyrene by a Novel Palladium Complex with Photosensitizing Ruthenium(II) Polypyridyl Moiety. Chem. Commun. 2005, 5468−5470. (b) Murata, K.; Inagaki, A.; Akita, M.; Halet, J.-F.; Costuas, K. Revelation of the Photoactive Species in the Photocatalytic Dimerization of α-Methylstyrene by a Dinuclear Ruthenium−Palladium Complex. Inorg. Chem. 2013, 52, 8030−8039. (c) Kozawa, K.; Inagaki, A.; Akita, M. Synthesis of Highly Conjugated Dinuclear Ru Complexes Bridged by a Novel N2−N3 Ligand and Their Application in Photocatalytic Oxygenation of Sulfides. Chem. Lett. 2014, 43, 290−292. (5) (a) Murata, K.; Araki, M.; Inagaki, A.; Akita, M. Syntheses, Photophysical Properties, and Reactivities of Novel Bichromophoric Pd Complexes Composed of Ru(II)−Polypyridyl and Naphthyl Moieties. Dalton Trans. 2013, 42, 6989−7001. (b) Murata, K.; Saito, K.; Kikuchi, S.; Akita, M.; Inagaki, A. Visible-Light-Controlled Homoand Copolymerization of Styrenes by a Bichromophoric Ir−Pd Catalyst. Chem. Commun. 2015, 51, 5717−5720. (6) (a) Nitadori, H.; Takahashi, T.; Inagaki, A.; Akita, M. Enhanced Photocatalytic Activity of α-Methylstyrene Oligomerization through Effective Metal-to-Ligand Charge-Transfer Localization on the Bridging Ligand. Inorg. Chem. 2012, 51, 51−62. (b) Inagaki, A.; Yatsuda, S.; Edure, S.; Suzuki, A.; Takahashi, T.; Akita, M. Synthesis of Pd Complexes Combined with Photosensitizing of a Ruthenium(II) Polypyridyl Moiety through a Series of Substituted Bipyrimidine Bridges. Substituent Effect of the Bridging Ligand on the Photocatalytic Dimerization of α-Methylstyrene. Inorg. Chem. 2007, 46, 2432−2445. (7) Phungsripheng, S.; Kozawa, K.; Akita, M.; Inagaki, A. Photocatalytic Oxygenation of Sulfide and Alkenes by Trinuclear Ruthenium Clusters. Inorg. Chem. 2016, 55, 3750−3758. (8) Semmelhack, M. F.; Helquist, P. M.; Jones, L. D. Synthesis with Zerovalent Nickel. Coupling of Aryl Halides with Bis(1,5cyclooctadiene)nickel(0). J. Am. Chem. Soc. 1971, 93, 5908−5910. (9) Schwab, P. F. H.; Fleischer, F.; Michl, J. Preparation of 5Brominated and 5,5′-Dibrominated 2,2′-Bipyridines and 2,2′-Bipyrimidines. J. Org. Chem. 2002, 67, 443−449. (10) Ji, Z.; Huang, S. D.; Guadalupe, A. R. Synthesis, X-ray Structures, Spectroscopic and Electrochemical Properties of Ruthenium(II) Complexes Containing 2,2′-Bipyrimidine. Inorg. Chim. Acta 2000, 305, 127−134. (11) Hamelin, O.; Guillo, P.; Loiseau, F.; Boissonnet, M.-F.; Ménage, S. A Dyad as Photocatalyst for Light-Driven Sulfide Oxygenation with Water as the Unique Oxygen Atom Source. Inorg. Chem. 2011, 50, 7952−7954.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01764. 1 H NMR spectra of complexes 2, 3Br-Cl, 3Br-OH2, 3MeCl, and 3Me-OH2; ESI mass spectra of 2; DPV of 3Me-Cl, 3 H -Cl, 3 Br -Cl, 3 Me -OH 2 , 3 H -OH 2 , and 3 Br -OH 2 ; optimized structures of 4H, 4Br, and 4Br (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Munetaka Akita: 0000-0001-7007-9621 Akiko Inagaki: 0000-0001-9113-5602 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” and JSPS KAKENHI (B) Grant No. 16H0412100. A part of this work was supported by a JSPS Grant-in-Aid for Scientific Research on Innovative Areas (“3D Active-Site Science”: Grant No. 26105003) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. S.P. acknowledges the Tokyo Metropolitan Government (Asian Human Resources Fund) for a predoctoral fellowship.

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REFERENCES

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Substituent Effect of the Bridging Ligand in the Trinuclear Ru

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