Photochemical Properties and Reactivity of a Ru Compound

Feb 5, 2016 - Institute for Cell-Material Sciences, Kyoto University, ACT-Kyoto #507, ... Graduate School of Science and Engineering, University of To...
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Photochemical Properties and Reactivity of a Ru Compound Containing an NAD/NADH-Functionalized 1,10-Phenanthroline Ligand Katsuaki Kobayashi,*,† Hideki Ohtsu,‡ Koichi Nozaki,‡ Susumu Kitagawa,§ and Koji Tanaka*,† †

Institute for Cell-Material Sciences, Kyoto University, ACT-Kyoto #507, Jibucho 105, Fushimi-ku, Kyoto 612-8374, Japan Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan § Institute for Cell-Material Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ‡

S Supporting Information *

ABSTRACT: An NAD/NADH-functionalized ligand, benzo[b]pyrido[3,2-f ][1,7]-phenanthroline (bpp), was newly synthesized. A Ru compound containing the bpp ligand, [Ru(bpp)(bpy)2]2+, underwent 2e− and 2H+ reduction, generating the NADH form of the compound, [Ru(bppHH)(bpy)2]2+, in response to visible light irradiation in CH3CN/ TEA/H2O (8/1/1). The UV−vis and fluorescent spectra of both [Ru(bpp)(bpy)2]2+ and [Ru(bppHH)(bpy)2]2+ resembled the spectra of [Ru(bpy)3]2+. Both complexes exhibited strong emission, with quantum yields of 0.086 and 0.031, respectively; values that are much higher than those obtained from the NAD/NADH-functionalized complexes [Ru(pbn)(bpy)2]2+ and [Ru(pbnHH)(bpy)2]2+ (pbn = (2-(2-pyridyl)benzo[b]-1.5-naphthyridine, pbnHH = hydrogenated form of pbn). The reduction potential of the bpp ligand in [Ru(bpp)(bpy)2]2+ (−1.28 V vs SCE) is much more negative than that of the pbn ligand in [Ru(pbn)(bpy)2]2+ (−0.74 V), although the oxidation potentials of bppHH and pbnHH are essentially equal (0.95 V). These results indicate that the electrochemical oxidation of the dihydropyridine moiety in the NADH-type ligand was independent of the π system, including the Ru polypyridyl framework. [Ru(bppHH)(bpy)2]2+ allowed the photoreduction of oxygen, generating H2O2 in 92% yield based on [Ru(bppHH)(bpy)2]2+. H2O2 production took place via singlet oxygen generated by the energy transfer from excited [Ru(bppHH)(bpy)2]2+ to triplet oxygen.

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The difficulty in the NAD+/NADH recycling of NADH-type molecules from NAD+ analogues via photo- or electrochemical reduction results from the radical coupling of one electron reduced NAD+ species, which inhibits two electron reduction to form NADH. To address this problem, we previously synthesized a series of polypyridyl ligands incorporating an NAD/NADH functionality, and developed their Ru complexes.16−28 The electro- and photochemical one electron reductions of [Ru(pbn)(bpy)2]2+ (Ru-pbn, pbn =2-(2-pyridyl)benzo[b]-1.5-naphthyridine) (Figure 1) in the presence of a proton donor are accompanied by protonation of the

INTRODUCTION The nicotine amide framework functions as an energy storage system in natural biological systems through the redox couple NAD+ and NADH (eq 1).1−4 The participation of one proton

in the two electron reduction of NAD+ forms NADH, which returns to NAD+ by releasing a hydride ion to another molecule. The NAD+/NADH reversible redox system is ubiquitous in biological processes that supply and transfer reducing power,3,4 since NADH possesses the highest reducing ability of any biological reaction component.1,4−6 The Hantzsch esters are known as catalysts used for the hydrogenation of organic molecules among the hydride donors intended for the functionalization of NADH as a nonmetal reductant.7−15 However, these compounds are not readily regenerated from their oxidized forms by photo- or electrochemical methods. Thus, to allow their use in catalytic reactions, Hantzsch esters are often recovered by hydrogen reduction in the presence of noble metal catalysts.13−15 © XXXX American Chemical Society

Figure 1. Photochemical reduction of a Ru complex bearing an NAD/ NADH-functionalized polypyridyl ligand. Received: October 19, 2015

A

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In the present work, we synthesized a new Ru complex having a novel NAD/NADH-functionalized phenanthroline ligand, [Ru(bpp)(bpy)2]2+ (1, bpp = benzo[b]pyrido[3,2f ][1,7]-phenanthroline) (Figure 3a), and also obtained the

nonbonded nitrogen atom of the pbn ligand to produce the [Ru(pbnH•)(bpy)2]2+ (Ru-pbnH•) scaffold. The resultant neutral radical pbnH• ligand subsequently undergoes intermolecular association to form a dimeric π-complex. Electron coupled proton transfer from one Ru-pbnH• scaffold to the other inside the π-dimer results in disproportionation to produce [Ru(pbn)(bpy)2]2+ and [Ru(pbnHH)(bpy)2]2+ (RupbnHH) as an NADH model complex (Figure 1).16−19,29,30 Although the hydride donor ability of the Ru-pbnHH scaffold is normally minimal, treatment of the complex with the strong base PhCOO− in CH3CN greatly enhances the hydricity, such that [Ru(pbnHH)(bpy)2]2+ reacts with CO2 to produce HCOO−, with regeneration of [Ru(pbn)(bpy)2]2+ (kH/kd = ca. 4.0).30,31 This reaction was the first example of an organic hydride transfer to CO2 using a renewable hydride source. However, a strong base is required to confer hydride donor ability on the [Ru(pbnHH)(bpy)2]2+. We therefore attempted to enhance the hydride donor ability of NADH model ligands in Ru complexes without using strong bases. The compound 1,10-phenanthroline, bearing an N-hetero ring expanded at the f-position, can function as a DNA sensor in luminescent Ru-polypyridyl complexes,32−46 such that this type of molecule can detect mismatches in DNA and RNA duplexes.47−49 With regard to the photo- and electrochemical properties of these compounds, the number of N atoms in the expanded hetero rings determines the emission wavelength and redox potential of the Ru complex.50−53 The redox potentials, emission wavelengths, and emission quantum yields of several Ru complexes bearing f-expanded phenanthroline ligands, specifically [Ru(dppz)(bpy)2]2+ (dppz = dipyrido[3,2-a:2′,3′c]phenazine) and [Ru(dppp2)(bpy)2]2+ (dppp2 = pyrido[2′,3′:5,6]pyrazino[2,3-f ][1,10]phenanthroline) (Figure 2),

Figure 3. Molecular structures of [Ru(bpp)(bpy)2]2+ (1) (a) and [Ru(bppHH)(bpy)2]2+ (1·HH) (b).

NADH form of this complex, [Ru(bppHH)(bpy)2]2+ (1·HH) (Figure 3b), by the photochemical reduction of 1. The redox and photochemical properties of 1 and 1·HH were subsequently investigated. Synthesis and spectral properties of the Ru-bpp complex. The one-pot reaction of o-amino-benzyl alcohol and 5-amino-1,10-phenanthroline in 1.0 M HCl (aq) at 100 °C afforded crude bpp. Pure bpp was obtained by chromatographic purification (silica gel column, elution with CHCl3), giving a 32% yield. Complex 1 was synthesized by the reaction of bpp with [Ru(bpy)2Cl2] in 2-methoxy ethanol following the removal of the Cl ligands by treatment with AgPF6. Further purification of the complex was performed by silica gel column chromatography, using CH3CN/CHCl3 as the eluent. Red crystals of the PF6 salt of 1 were obtained by recrystallization from CH3CN/Et2O. The electronic absorption spectrum of 1 in CH3CN (Figure 4) showed a strong band at 454 nm along with a shoulder at

Figure 2. Ru polypyridyl complexes bearing various f-expanded 1,10phenanthroline ligands.

are summarized in Table 1. The introduction of an N-hetero quinoline ring results in an additional negative shift of the ligand redox potential, a blue shift of the emission peak and an increase in the emission quantum yield of the Ru polypyridyl system. Moreover, the pyridine ring of an expanded quinoline moiety is expected to act as an NAD/NADH functionality.

Figure 4. Absorption and emission spectra of [Ru(bpp)(bpy)2]2+ (1) in CH3CN.

Table 1. Photo- and Electrochemical Properties of Various Ru Complexes Relative to [Ru(bpp)(bpy)2]2+ (1) and [Ru(bppHH)(bpy)2]2+ (1·HH) Complex 2+

[Ru(bpp)(bpy)2] (1) [Ru(bppHH)(bpy)2]2+ (1·HH) [Ru(pbn)(bpy)2]2+ [Ru(pbnHH)(bpy)2]2+ [Ru(bpy)3]2+ [Ru(dppz)(bpy)2]2+ [Ru(dppp2)(bpy)2]2+ a

λabs/nma

λem/nma

Φema

422sh, 454 425, 450sh 530 417, 460sh 452 445 440

610 608 775 610 619 631 752

0.086 0.031 0.00015 0.0041 0.062 0.083 0.002

E1/2/V vs SCE (solvent) 1.23, 1.23, 1.30, 1.31, 1.30, 1.33, 1.36,

−1.28, −1.43, −1.63 (DMF) 0.95,b −1.35, −1.55 (DMF) −0.74, −1.36, −1.60 (DMF) 0.94,b −1.33, −1.53 (DMF) −1.31, −1.50 (CH3CN) −0.97, −1.39 (CH3CN) −0.74, −1.26 (CH3CN)

In CH3CN. bIrreversible anodic peak. B

DOI: 10.1021/acs.inorgchem.5b02390 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 420 nm. Both [Ru(dppz)(bpy)2]2+ and [Ru(dppp2)(bpy)2]2+ also generated strong absorption bands at approximately 440 nm (Table 1).50−53 These are assigned to the metal-to-ligand charge transfer (MLCT), since analogous Ru complexes bearing f-extended phenanthroline groups exhibit MLCT bands in this general region. In contrast, the fluorescence spectra of these complexes were quite different. Complex 1 generated an intense band at 610 nm with a relatively good quantum yield (Φ = 0.086) in its fluorescence spectrum, while [Ru(dppz)(bpy)2]2+ and [Ru(dppp2)(bpy)2]2+ exhibit bands at 631 and 752 nm, respectively (Table 1).50−53 Thus, an increase in the number of N atoms in the expanded ring results in a red shift of the emission peak. The characteristic spectral behavior of Ru dppz-type complexes stems from two MLCT states. One of these two states originates from the charge transfer between Ru and the bpy framework (3MLCTprox), while the other is between Ru and the expanded π system (3MLCTdis).51−57 The former is emissive and the latter is inert. The relative energy of the two MLCT states depends on the level of the extended π system and the solvent environment. The energy level of the 3 MLCTprox of [Ru(dppz)(bpy)2]2+ is lower than that of the 3 MLCTdis in dry CH3CN, which leads to emission characteristics similar to those of [Ru(bpy)3]2+. In contrast, the energy level of the 3MLCTprox of [Ru(dppp2)(bpy)2]2+ is higher than that of the 3MLCTdis, such that the emission of the complex shifts to longer wavelengths and there is an extreme decrease in the emission quantum yield.51−53 The spectral features of 1 are very similar to those of [Ru(bpy)3]2+, although the quantum yield of 1 is greater than that of [Ru(bpy)3]2+. This result indicates that the energy level of the emissive 3MLCTprox state is lower than that of the inert 3MLCTdis state in the Ru-bpp framework. Electrochemical properties of the Ru-bpp complex. The cyclic voltammogram of 1 exhibited four reversible redox couples at 1.23, −1.28, −1.43, and −1.63 V vs SCE in DMF (Figure 5a). The redox couple at 1.23 V apparently originates from Ru(II)/Ru(III). Upon the addition of acetic acid to the

solution as a proton source, the reversible redox couple at −1.28 V transitioned to an irreversible one, and a new anodic peak appeared at 0.8 V. The other two couples at more negative potentials (Figure 5b) remained almost unchanged after addition of the acid. These two can be safely assigned to the redox couples of the two bpy ligands. The irreversible +0.8 V anodic wave did not appear in the initial anodic potential sweep, but rather emerged after the potential sweep was returned at −1.4 V. The large separation between the cathodic and anodic peak potentials of the bpp localized redox reaction in the presence of a proton donor clearly results from protonation of the nonbonded nitrogen atom of the reduced bpp ligand. Similar redox behavior was observed in the cyclic voltammogram of [Ru(pbn)(bpy)2]2+. This complex generated ligandbased reversible couples at −0.74, −1.36, and −1.60 V in DMF, the first couple due to pbn and the latter two resulting from the bpy ligands (Figure 5c). The addition of acetic acid to the solution of [Ru(pbn)(bpy)2]2+ resulted in the transition of the reversible pbn/pbn•− couple at −0.74 V to an irreversible couple and the appearance of a new anodic peak at +0.3 V coupled with the irreversible wave (Figure 5d). The irreversible cathodic peak is attributed to the hydrogenation of the pbn ligand to form pbnHH through a two electron and two proton process (eq 2), while the anodic peak is assigned to the oxidation of pbnHH (eq 3). The large peak separation of the pbn/pbnHH couple results from the significant structural change between pbn and pbnHH. [Ru(pbn)(bpy)2 ]2 + + 2H+ + 2e− → [Ru(pbnHH)(bpy)2 ]2 +

E1/2 = −0.74 V (2)

[Ru(pbnHH)(bpy)2 ]2 + → [Ru(pbn)(bpy)2 ]2 + + 2H+ + 2e−

E1/2 = +0.30 V

(3)

According to the assignment of the redox behavior of the Rupbn system, the irreversible cathodic wave of 1 at −1.26 V in the presence of a proton source results from the electrochemical hydrogenation of bpp to form bppHH (eq 4), and the coupled cathodic peak corresponds to the oxidation of bppHH (eq 5). [Ru(bpp)(bpy)2 ]2 + + 2H+ + 2e− → [Ru(bppHH)(bpy)2 ]2 +

E1/2 = −1.26 V (4)

[Ru(bppHH)(bpy)2 ]2 + → [Ru(bpp)(bpy)2 ]2 + + 2H+ + 2e−

E1/2 = +0.80 V

(5) 2+

Attempts to isolate [Ru(bppHH)(bpy)2] (1·HH) as the NADH form by the chemical reduction of 1 were not successful because the treatment of 1 with NaBH4 and NaBHEt3 in alcohol/H2O resulted in fragmentation of the complex. It was also found that Na2S2O4 was not sufficiently powerful to reduce 1. Taking into account that the Ru-pbn complexes were readily reduced to the corresponding Ru-pbnHH analogues under similar reaction conditions, it is evident that the reactivity of the Ru-bpp complexes is different from that of the Ru-pbn. Photochemical reduction of the Ru-bpp complex. Rupbn complexes are quantitatively reduced to Ru-pbnHH by both chemical and photochemical reduction in CH3CN in the

Figure 5. Cyclic voltammograms of [Ru(bpp)(bpy)2]2+ in the absence (a) and presence (b) of 10 equiv of AcOH in DMF. CVs of [Ru(pbn)(bpy)2]2+ in the absence (c) and presence (d) of 10 equiv of AcOH. dV/dt = 0.1 V/s. C

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1/1) also afforded a dicationic Ru complex at m/z = 350.1 (Figure S1c), corresponding to 1·DD, in agreement with the NMR results. Despite the expectation that the photoreduction of 1 in CH3CN/TEA/D2O would produce 1·HD via the attachment of one deuterium atom to the C atom at the para position of the noncoordinated pyridine ring (C13 as depicted in the X-ray structure, Figure 8), 1·DD having two deuterium

presence of a sacrificial electron donor. The photochemical reduction of 1 under irradiation by a Xe lamp (>400 nm) in CH3CN/TEOA (4/1 v/v) shifted its absorption maximum from 454 to 425 nm without any obvious associated isosbestic points during the photoreaction. NMR spectra of the resulting solutions revealed a mixture of reduction products. In contrast, the photoreaction of 1 in CH3CN/TEA/H2O (8/1/1) also showed a shift of the 454 nm band to 425 nm but with clear isosbestic points at 362, 452, and 472 nm (Figure 6). In

Figure 6. Electronic spectral changes of 1 in CH3CN/TEA/H2O (8/ 1/1) during visible light irradiation. (>400 nm, Xe lamp).

addition, the NMR spectrum of the resulting photoreduced 1 displayed a characteristic singlet signal at 4.6 ppm (Figure 7b,

Figure 8. Crystal structures of 1 (a) and 1·HH (b) with 50% thermal ellipsoids. Hydrogen atoms, except for those on the central pyridine, are omitted for clarity.

atoms bonded to C13 was instead formed during the reduction. The conversion from 1·HD to 1·DD during the photoreduction apparently took place through abstraction of a H atom from the C13 of the [Ru(bppHD)(bpy)2]2+ scaffold by photogenerated Et3N+. Subsequent dimerization of [Ru(bppD•)(bpy)2]2+ followed by disproportionation produced 1·DD while regenerating 1. The similar incorporation of two deuterium atoms into a pbn ligand serving as an NAD model was also observed in the photoreduction of [ReCl(pbn) (CO)3].58 Both electrolytic and chemical reduction of 1 resulted in degradation of the complex, whereas photoreduction of 1 produced 1·HH in a stable manner with a quantum yield of Φ = 0.27 in CH3CN/TEA/H2O (8/1/1). It should be noted that both the electrochemical and chemical reduction of 1 naturally produce a mixture of [Ru(bpp•−)(bpy)2]+ (the one electron reduction product) and [Ru(bpp•−)(bpy•−)(bpy)]0 (the two electron reduction product) due to the very close redox potentials of E01/2 = −1.26 and −1.39 V for the ligand localized redox potentials of the (bpp/bpp•−) and(bpy/bpy•−) couples. Thus, the number of electrons transferred to 1 via an external electron source cannot be controlled during the reduction process. Conversely, the photochemical reduction of 1 generates only [Ru(bpp•−)(bpy)2]+ without any accompanying [Ru(bpp•−)(bpy•−)(bpy)]0, since 1 in the photoexcited state acts as a one electron oxidant to oxidize the sacrificial reductant (TEA) through a one electron process. These results can be explained by considering the lability of the two electron reduction complex, [Ru(bpp•−)(bpy•−)(bpy)]0, which is likely thermally unstable and thus decomposes subsequent to the chemical or electrochemical reduction of 1.

Figure 7. NMR spectra of 1 (a), the photoreduction product of 1 in CD3CN/TEA/H2O (8/1/1) (b), and that in CD3CN/TEA/D2O (8/ 1/1) (c) following visible light irradiation. (>400 nm, Xe lamp).

indicated by an asterisk), corresponding to the methylene signal of the NADH framework in [Ru(bppHH)(bpy)2]2+ (1·HH). Photoreaction using D2O rather than H2O resulted in the disappearance of the singlet signal at 4.6 ppm, indicating the deuteration of the two methylene protons (Figure 7c). The ESI-MS spectrum of the photoreduction product of 1 showed a parent peak with a dicationic Ru compound at m/z = 348.6 (Figure S1b). Considering that the dicationic fragment of the original complex 1 was observed at m/z = 347.6 (Figure S1a), a mass peak at 348.6 corresponds to the hydrogenated form of 1. The photoreduction of 1 in CH3CN/TEA/D2O (8/ D

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The coordination structures of 1 and 1·HH in the vicinity of the Ru center are almost equal, and correspond to a [Ru(bpy)3]2+-type octahedral configuration. The planar structure of the f-expanded ligand is conserved in both 1 and 1·HH, suggesting that hydrogenation of bpp did not distort the planarity of the expanded π system. The structural perturbation resulting from the hydrogenation of the bpp ligand occurred at the pyridine group on the expanded ring. The hydrogenation of the noncoordinated pyridine group induced a change in orbital hybridization of the C atom located at the para position relative to the N atom from sp2 in bpp to sp3 in bppHH. Thus, the C12−C13−C14 bond angle of 1·HH (117.4°) was smaller than that of 1 (120.1°) due to the increased sp3 character of C13. As the bond angle at the C13 decreased, the C1−C12 and C14− C19 bond lengths were shortened because of the associated increase in the double bond characters of both C−C bonds. In contrast, the C12−C13 and C13−C14 bonds were elongated. These structural changes were previously observed in the Rupbn system, indicating the formation of a dihydro pyridine framework through hydrogenation of the NAD/NADH-type ligand. Spectral and electrochemical properties of the RubppHH complex. Figure 9 shows the absorption and emission

As such, the photochemical reaction is essentially a one electron process since there is no chance of obtaining [Ru(bpp•−)(bpy•−)(bpy)]0 in the solution because of its instability. Therefore, a reasonable mechanism will include the formation of 1·HH from 1 by a 2e−2H+ process. It has been reported that a similar NAD/NADH ligand system, [Ru(pbn)(bpy)2]2+, can be reduced to [Ru(pbnHH)(bpy)2]2+ by photoreduction in the presence of a sacrificial reagent, and an associated reduction mechanism has been proposed. The first step of the photoreduction of [Ru(pbn)(bpy)2]2+ is the protonation of the nonbonded nitrogen in the bpp ligand of photochemically generated [Ru(pbn•−)(bpy)2]2+. This is followed by the dimerization of [Ru(bpy)2(pbnH•)]2+ through π−π stacking of the neutral pbnH• moieties. Subsequently, intramolecular proton-coupled electron transfer from one pbnH• to another within the {[Ru(pbnH•)(bpy)2]2+}2 leads to disproportionation to produce [Ru(pbnHH)(bpy)2]2+ together with regeneration of [Ru(pbn)(bpy)2]2+. The photoreductions of 1 in CH3CN/TEOA and CH3CN/TEA/H2O afford several byproducts and solely 1·HH, respectively. These results strongly suggest that the protonation of the nonbonded nitrogen atom of the bpp in [Ru(bpp•−)(bpy)2]+ plays a key role in the selective formation of 1·HH. Thus, the photochemical reduction mechanism of [Ru(bpp)(bpy)2]2+ is expected to be very close to that of [Ru(pbn)(bpy)2]2+. Crystal structures of the Ru-bpp and Ru-bppHH complexes. The synthesis of 1·HH was accomplished by the photoreduction of 1 in CH3CN/TEA/H2O (8/1/1) under N2. Evaporation of the reaction solvent afforded 1·HH as the sole product and red plates of 1·HH were obtained by recrystallization from CH3CN/Et2O. We initially attempted to compare the molecular structure of 1·HH and 1 by X-ray structural analysis. Unfortunately, the red crystal of [1](PF6)2, assigned to space group R3̅c, afforded poor-quality X-ray crystallographic data. Good crystallographic data were obtained from the BF4 salt of 1 obtained by recrystallization from acetone/Et2O, and Figure 8 presents the X-ray structures of 1 (a) and 1·HH (b). Selected bond lengths and angles of 1 and 1·HH are summarized in Table 2.

Figure 9. Absorption and emission spectra of [Ru(bppHH)(bpy)2]2+ (1·HH) in CH3CN.

spectra of 1·HH. The absorption spectrum of 1·HH contains a strong band at 425 nm with a shoulder at approximately 450 nm, consistent with the spectrum of 1 following photoreduction in CH3CN/TEA/H2O (8/1/1). 1·HH also generates an emission band at 608 nm. This is similar to the emission properties of [Ru(bpy)3]2+, indicating that the emission of 1· HH originates from the 3MLCTprox. Although the quantum yield of 1·HH decreased to almost one-third of the yield of 1, its emission value was still much larger than those of [Ru(pbn)(bpy)2]2+ and [Ru(pbnHH)(bpy)2]2+. Thus, the photochemical properties of a Ru-bpp complex with NAD/ NADH-functionalized ligands are dramatically improved compared to the Ru-pbn complexes reported to date. The cyclic voltammogram of isolated 1·HH in dry DMF exhibited two pseudoreversible redox couples at −1.35 and −1.55 V due to bpy/bpy•−, along with an irreversible anodic peak and a reversible redox wave at 0.95 and 1.23 V, respectively (Figure 10a). The reversible redox couple at 1.23 V corresponds to Ru(II)/(III), in a manner completely identical to that of 1. The irreversible peak can be assigned to the oxidation of bppHH shown in eq 5, which is positively shifted under aprotic conditions. The cyclic voltammogram of [Ru(pbnHH)(bpy)2]2+ in DMF (Figure 10b) showed three

Table 2. Selected Bond Lengths (Å) and Angles (deg) of 1 and 1·HH

Ru−N2 Ru−N3 Ru−N4 Ru−N5 Ru−N6 Ru−N7 N1−C1 N1−C19 C1−C12 C12−C13 C13−C14 C14−C19 C1−N1−C19 N1−C1−C12 N1−C19−C14 C1−C12−C13 C12−C13−C14 C13−C14−C19

1

1·HH

2.065(5) 2.066(4) 2.055(4) 2.061(4) 2.056(4) 2.065(4) 1.333(7) 1.359(7) 1.434(7) 1.381(7) 1.399(8) 1.424(8) 118.6(4) 123.2(4) 121.9(5) 117.7(4) 120.1(5) 118.4(5)

2.066(3) 2.064(3) 2.056(3) 2.055(3) 2.059(3) 2.061(3) 1.425(5) 1.440(6) 1.379(6) 1.430(5) 1.451(6) 1.396(7) 118.2(4) 121.3(4) 120.7(4) 121.7(4) 117.4(4) 120.6(4) E

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the electronic absorption spectrum of 1·HH in CH3CN smoothly changed to that of 1, with the appearance of isosbestic points at 362, 453, and 467 nm (Figure S2) under visible light irradiation in air. The NMR spectrum of the solution also demonstrated that 1·HH was selectively oxidized to 1 without producing any detectable amounts of degradation products during the photoirradiation (Figure S3). Furthermore, H2O2 was confirmed to have been generated in the reaction solution in 92% yield (as measured using the TiPyP method59−65). It is worthy of note that the photoirradiation of air-stable 1·HH triggered the reaction shown in eq 6. The quantum yield of the reaction was calculated to be Φ = 0.017 based on the conversion from 1·HH to 1. 1·HH + O2 → 1 + HOOH

(6)

Thus, 1·HH demonstrated the ability to reduce O2 while regenerating 1 under photoirradiation. However, photoirradiation of 1 in CH3CN/TEA/H2O (8/1/1) under air produced neither H2O2 nor 1·HH. These results indicate the participation of singlet oxygen in the oxidization of 1·HH during the generation of H2O2 (Scheme 1), since [Ru(bpy)3]2+66,67 and [Ru(dppz)(bpy)2]2+51,68 generate singlet oxygen under visible light irradiation. We further examined the emission behavior of 1 and 1·HH under air. Photoexcitation of 1·HH in CH3CN under aerobic conditions generated a weak fluorescent band at 1270 nm due to the emission of singlet oxygen (Figure S4).67−71 Photoirradiation of 1 also produced the same emission band although with a slightly higher intensity compared to 1·HH, since the larger quantum yield associated with the emission of 1 would produce a greater amount of singlet oxygen. The most reasonable mechanism for eq 6 is shown in Scheme 1. Here, the quenching of photoexcited [1·HH]* by 3O2 and the subsequent reaction of the resultant 1·HH and 1O2 regenerates 1 while producing H2O2. In the absence of O2, the excitation energy of 1 effectively accumulates in the formation of 1·HH, but is preferentially consumed by singlet oxygen generation in the presence of O2. Thus, the photogeneration of 1·HH is practically inhibited in air due to quenching of the photoexcited state of 1 by 3O2.

Figure 10. Cyclic voltammograms of 1·HH (a) and [Ru(pbnHH)(bpy)2]2+ (b) in DMF. dV/dt = 0.1 V/s.

reversible waves at −1.53, −1.33, and 1.31 V and one irreversible wave at 0.94 V, which is quite similar to the results obtained from 1·HH. Since 1·HH and [Ru(pbnHH)(bpy)2]2+ have the same Ru(bpy)3-type coordination core, the three reversible waves derived from the Ru ion and bpy ligands are virtually identical to one another. The most remarkable aspect of the redox properties of 1·HH and [Ru(pbnHH)(bpy)2]2+ is that the oxidation potential of the NADH-form ligands remains constant, even though localized reduction potentials of the NAD-form ligands bpp and pbn are completely different, both in the absence and presence of a proton source (Figure 5). In general, the ligand localized reduction potential originates from the ligand structure, which was reflected in the reduction of pbn and bpp but not in the oxidation of pbnHH and bppHH. Thus, it can be concluded that the oxidation of the dihydroquinoline moiety of the NADH-type ligand is independent of the π system, including the Ru(bpy)3 core. Photochemical oxygen reduction by the Ru-bppHH complex. Complex 1·HH did not undergo any degradation reactions in CH3CN under air over the course of several days as long as the solution was not exposed to light. In the absence of air, 1·HH was also stable even when exposed to light. Indeed,

Scheme 1. Schematic of the Photoreduction of 1 and Oxidation of 1·HH

F

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precipitate was filtered off and bpp (50 mg, 0.178 mmol) was added to the filtrate followed by further reflux for 15 h. The removal of the solvent by evaporation afforded a dark orange solid. The dark orange residue was dissolved in a minimal amount of MeOH and aqueous NH4PF6. A dark orange precipitate appeared during slow evaporation and was collected by filtration. This residue was loaded on a silica gel column (Wako gel C-200) and eluted with acetonitrile:CHCl3 (1:1). The first red band was collected and evaporated under reduced pressure and the resulting orange solid was dried under vacuum. Yield 50 mg (28%). Further purification was performed by recrystallization from acetnitrile-Et2O. ESI-MS: m/z 347.64 {M-2PF6}2+. Anal. Calcd for C39H27N7F12P2Ru•1.5H2O: C 46.30, H 2.99, N 9.69. Found C 46.18, H 2.99, N 9.64. 1H NMR (CD3CN, ppm): 9.81 (1H, s), 9.76 (1H, dd, J 9.6, 1.4), 9.28 (1H, dd, J 9.6, 1.4), 8.56−8.50 (4H, m), 8.41 (1H, d, J 8.2), 8.35 (1H, d, J 8.2), 8.13−7.98 (7H, m), 7.89−7.80 (5H, m), 7.71 (2H, dd, J 10.8, 6.1), 7.46 (2H, t, J 6.6), 7.25 (2H, dd, J 13.8, 6.6). Preparation of [Ru(bppHH)(bpy)2](PF6)2 (1·HH). [Ru(bpp)(bpy)2](PF6)2 (100 mg, 0.101 mmol) was dissolved in a CH3CN/ H2O/TEA (8/1/1) mixture (10 mL) under a N2 atmosphere. The solution was irradiated with a Hg lamp (150 W) through a 0.5 M NaNO2 solution acting as a cutoff filter (>400 nm). After 12 h irradiation, the solvent was removed and the resulting orange solid was washed with Et2O and dried under vacuum. Yield quant. ESI-MS: m/z 348.63 {M-2PF6}2+. Anal. Calcd for C39H29N7F12P2Ru•H2O: C 46.62, H 3.11, N 9.76. Found C 46.63, H 3.05, N 9.51. 1H NMR (CD3CN, ppm): 8.83 (1H, d, J 8.7), 8.52 (2H, d, J 9.2), 8.48 (2H, d, J 8.2), 8.42 (1H, d, J 9.6), 8.12−8.04 (4H, m), 7.99 (2H, t, J 7.9), 7.85−7.80 (3H, m), 7.76 (1H, dd, J 8.5, 5.3), 7.66 (1H, dd, J 8.5, 5.3), 7.54 (2H, d, J 6.0), 7.44 (2H, tt, J 6.6, 1.6), 7.31 (1H, d, J 7.8), 7.26−7.17 (3H, m), 7.05 (1H, dt, J 7.2, 1.5), 4.60 (2H, s). Instruments. UV−vis spectra were recorded on an Agilent 8453 spectrometer and fluorescent spectra were acquired using a JASCO FP-6600. NMR spectroscopy was conducted with a JEOL ECS-400. ESI-MS spectra were obtained using a Waters Micromass LCT and elemental analyses were conducted with a Flash EA 1112 (Thermo Fisher Scientific Inc.). Electrochemistry. Electrochemical measurements were conducted with an ALS/chi Model 660A electrochemical analyzer. Electrochemical experiments were performed using solutions of the complexes (1.0 mM) in CH3CN containing 0.1 M nBu4NPF6. A glassy carbon electrode, a Ag/AgNO3 (0.01 M) reference electrode and a platinum wire were employed as the working, reference and counter electrode, respectively. Photochemical reaction of 1 and 1·HH. The photochemical reduction of 1 in Ar-purged CH3CN/TEA/H2O (8/1/1) was monitored by UV−vis and NMR spectroscopy, employing concentrations of 40 μM and 11 mM, respectively. A 150 mW Xe lamp equipped with a cutoff filter (TOSHIBA Y-42) and a 150 W Hg lamp filtered through 0.5 M NaNO2 were employed as light sources for UV−vis and NMR trials, respectively. The photochemical oxidation of 1·HH was conducted in air-bubbled CH3CN under the same irradiation conditions as applied during the photoreduction of 1. Quantum yield determination. Quantum yields for the photoluminescence of 1 and 1·HH were determined by comparison to [Ru(bpy)3]2+ in CH3CN (Φ = 0.062).73,74 The rates of photoreduction of 1 and oxidation of 1·HH were monitored by UV−vis spectroscopy during 532 nm laser irradiation (Crtsta Laser). A photodigital counter (USBI Photodiode: PD300, Ohir Optronics Solutions Ltd.) was used as an actinometer to estimate quantum yields during both photoconversions. The absorbance values at 532 nm of 1 and 1·HH were adjusted to ca. 0.1 and 0.15 in deaerated CH3CN/ H2O/TEA (8/1/1) and air-bubbled CH3CN, respectively, employing concentrations of approximately 0.1 mM. Quantum yields were calculated upon 5% photoconversion. X-ray crystallographic analysis. Each crystal was mounted in a loop and transferred into a cold N2 stream held at 123 K. X-ray diffraction data were collected on a Rigaku Saturn CCD area detector using graphite monochromatic Mo Kα radiation (λ = 0.71070 A), and processed using the Crystal Clear software package (Rigaku). The

CONCLUSIONS The photochemical properties of an NAD/NADH-functionalized Ru polypyridyl complex were dramatically improved by the introduction of an NAD/NADH function at the f-position of the 1,10-phenanthroline ligand. Not only Ru-bpp (1, the NAD form) but also Ru-bppHH (1·HH, the NADH form) exhibited significant light emission compared with Ru compounds bearing a different NAD/NADH-functionalized ligand (the Ru-pbn series). The emission properties of both 1 and 1·HH were quite similar to that of [Ru(bpy)3], and are believed to originate from the MLCT states of the Ru(bpy)3type core of 1 and 1·HH. Photo energy was stored as reducing energy in the quinoline moiety of the bpp via an NAD/NADH-type structural conversion, and the reduction mechanism of the bpp quinoline ring was determined to be similar to that in the Ru-pbn complex. The reduction potential of bpp linked to Ru is shifted to more highly negative values compared to the values observed for any other Ru-pbn series. In contrast, the oxidation potential of the dihydrogenated bpp (bppHH) was almost the same as that of pbnHH ligands. Typically, molecules having different structures exhibit different redox potentials, although the electrochemical oxidation potentials of the dehydrogenated quinoline moieties of bppHH and pbnHH were almost identical. This fact suggests that the dihydrogenated quinoline moiety is independent of the π system of the adjacent polypyridyl ligand. These photo- and electrochemical features enable the Rubpp system to store photoreducing energy and to promote O2 reduction using this stored energy. The significant emission properties of 1·HH allow this compound to generate singlet oxygen by energy transfer from the excited [Ru(bpy)3]2+ framework to oxygen. The findings of the present study suggest a new concept for the molecular design of renewable hydride catalysts and should expand the potential applications of NAD/NADH-functionalized complexes.

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EXPERIMENTAL SECTION

Materials. The 5-amino-1,10-phenanthroline, 1-aminobenzyl alcohol, and RuCl3·3H2O reagents were purchased from Sigma-Aldrich Co., LLC., Tokyo Chemical Industry Co., Ltd. and Furuya Metal Co., Ltd., respectively, and were used as received. nBu4NPF6 was purchased from the Tokyo Chemical Industry Co., Ltd. and recrystallized form EtOH before use. [Ru(bpy)2Cl2] was prepared according to a literature procedure.72 Preparation of bpp (benzo[b]pyrido[3,2-f ][1,7]-phenanthroline). Quantities of 5-amino-1,10-phenanthroline (100 mg, 0.512 mmol) and 2-aminobenzyl alcohol (63 mg, 0.512 mmol) were dissolved in 20 mL of 6 N HCl and heated at 100 °C for 2 days. During this period, the color of the solution changed from red to orange. The reaction mixture was neutralized with aqueous NaOH and the resulting pale yellow precipitate was collected by filtration. The precipitate was dissolved in 200 mL CHCl3 and washed three times with 50 mL water. The organic layer was dried over Na2SO4 and the solvent evaporated under reduced pressure. The residue was loaded onto a silica gel column (Wako gel C-200) and eluted with CHCl3. The first yellow band was collected and the solvent was removed by evaporation. The resulting pale yellow powder was dried under vacuum. Yield 46 mg (32%). 1H NMR (CDCl3, ppm): 9.58 (1H, d, J 7.8), 9.17 (1H, dd, J 4.1, 1.4), 9.12 (1H, dd, J 4.1, 1.4), 9.08 (1H, s), 8.81 (1H, d, J 8.2), 8.24 (1H, d, J 8.2), 7.99 (1H, d, J 8.2), 7.83 (1H, t, J 7.3), 7.75−7.55 (3H, m). Preparation of [Ru(bpp)(bpy)2](PF6)2 (1). [Ru(bpy)2Cl2] (86 mg, 0.178 mmol) and AgPF6 (90 mg, 0.356 mmol) were dissolved in 10 mL 2-methoxy ethanol and heated at reflux for 90 min. The AgCl G

DOI: 10.1021/acs.inorgchem.5b02390 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry structures were solved by the direct method, using SIR200475 and Il Milione76 for [1](BF4)2 and [1·HH](PF6)2, respectively, and expanded using Fourier techniques. Non-hydrogen atoms were refined anisotropically by the full-matrix least-squares method on F2 using SHELXL-97.77 All hydrogen atoms were located in their idealized positions, with a methyl C−H bond length of 0.98 Å and an aromatic C−H bond length 0.95 Å, and included in the structure calculations without further refinement of the parameters. Crystallographic data are summarized in Table 3.

26410017). K.T. acknowledges the Ministry of Education, Science, Sports, and Culture for a Grant-in-Aid for Scientific Research (No. 26288024).

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(1) Sunil, B.; Talla, S. K.; Aswani, V.; Raghavendra, A. S. Photosynth. Res. 2013, 117, 61. (2) Lassen, L. M.; Nielsen, A. Z.; Ziersen, B.; Gnanasekaran, T.; Møller, B. L.; Jensen, P. E. ACS Synth. Biol. 2014, 3, 1. (3) Silva, C. S.; Seider, W. D.; Lior, N. Chem. Eng. Sci. 2015, 130, 151. (4) Bilan, D. S.; Shokhina, A. G.; Lukyanov, S. A.; Belousov, V. V. Russ. J. Bioorg. Chem. 2015, 41, 341. (5) Dutton, P. L.; Wilson, D. F. Biochim. Biophys. Acta, Rev. Bioenerg. 1974, 346, 165. (6) Walsh, C. Acc. Chem. Res. 1980, 13, 148. (7) Stout, D. M.; Meyers, A. I. Chem. Rev. 1982, 82, 223. (8) You, S.-L. Chem. - Asian J. 2007, 2, 820. (9) Ouellet, S. G.; Walji, A. M.; MacMillan, D. W. C. Acc. Chem. Res. 2007, 40, 1327. (10) Hamasaka, G.; Tsuji, H.; Uozumi, Y. Synlett 2015, 26, 2037. (11) Kim, K.-H.; Lee, C.-Y.; Cheon, C.-H. J. Org. Chem. 2015, 80, 6367. (12) Foubelo, F.; Yus, M. Chem. Rec. 2015, 15, 907. (13) Zheng, C.; You, S.-L. Chem. Soc. Rev. 2012, 41, 2498. (14) Du, W.; Yu, Z. Synlett 2012, 23, 1300. (15) Shi, F.; Gong, L.-Z. Angew. Chem., Int. Ed. 2012, 51, 11423. (16) Koizumi, T.-a.; Tanaka, K. Angew. Chem., Int. Ed. 2005, 44, 5891. (17) Polyansky, D.; Cabelli, D.; Muckerman, J. T.; Fujita, E.; Koizumi, T.-a.; Fukushima, T.; Wada, T.; Tanaka, K. Angew. Chem., Int. Ed. 2007, 46, 4169. (18) Fukushima, T.; Fujita, E.; Muckerman, J. T.; Polyansky, D. E.; Wada, T.; Tanaka, K. Inorg. Chem. 2009, 48, 11510. (19) Fukushima, T.; Wada, T.; Ohtsu, H.; Tanaka, K. Dalton Trans. 2010, 39, 11526. (20) Kimura, M.; Tanaka, K. Angew. Chem., Int. Ed. 2008, 47, 9768. (21) Tannai, H.; Koizumi, T.-a.; Wada, T.; Tanaka, K. Angew. Chem., Int. Ed. 2007, 46, 7112. (22) Padhi, S. K.; Kobayashi, K.; Masuno, S.; Tanaka, K. Inorg. Chem. 2011, 2011, 5321. (23) Padhi, S. K.; Tanaka, K. Inorg. Chem. 2011, 50, 10718. (24) Padhi, S. K.; Fukuda, R.; Ehara, M.; Tanaka, K. Inorg. Chem. 2012, 51, 8091. (25) Padhi, S. K.; Fukuda, R.; Ehara, M.; Tanaka, K. Inorg. Chem. 2012, 51, 5386. (26) Fukushima, T.; Fukuda, R.; Kobayashi, K.; Caramori, G. F.; Frenking, G.; Ehara, M.; Tanaka, K. Chem. - Eur. J. 2015, 21, 106. (27) Cohen, B. W.; Polyansky, D. E.; Zong, R.; Zhou, H.; Ouk, T.; Cabelli, D. E.; Thummel, R. P.; Fujita, E. Inorg. Chem. 2010, 49, 8034. (28) Cohen, B. W.; Polyansky, D. E.; Achord, P.; Cabelli, D.; Muckerman, J. T.; Tanaka, K.; Thummel, R. P.; Zong, R.; Fujita, E. Faraday Discuss. 2012, 155, 129. (29) Polyansky, D. E.; Cabelli, D.; Muckerman, J. T.; Fukushima, T.; Tanaka, K.; Fujita, E. Inorg. Chem. 2008, 47, 3958. (30) Kobayashi, K.; Tanaka, K. Phys. Chem. Chem. Phys. 2014, 16, 2240. (31) Ohtsu, H.; Tanaka, K. Angew. Chem., Int. Ed. 2012, 51, 9792. (32) Friedman, A. E.; Chambron, J.-C.; Sauvage, J.-P.; Turro, N. J.; Barton, J. K. J. Am. Chem. Soc. 1990, 112, 4960. (33) Hartshorn, R. M.; Barton, J. K. J. Am. Chem. Soc. 1992, 114, 5919. (34) Holmlin, R. E.; Stemp, E. D. A.; Barton, J. K. Inorg. Chem. 1998, 37, 29. (35) Metcalfe, C.; Thomas, J. A. Chem. Soc. Rev. 2003, 32, 215. (36) Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. Rev. 1999, 99, 2777. (37) Moucheron, C.; Mesmaeker, A. K.-D.; Choua, S. Inorg. Chem. 1997, 36, 584.

Table 3. Crystallographic Data for [1](BF4)2·2(CH3)2CO and [1·HH](PF6)2·2CH3CN Formula Formula weight Color Crystal size/mm Crystal system Space group a/Å b/Å c/Ǻ α/(deg) β/(deg) γ/(deg) V/Å 3 Z T/K ρc/g cm−3 M(Mo Kα)/mm−1 F(000) Total reflections Unique reflections (Rint) Refln./Param. ratio R1a (I > 2.00σ(I)) wR2b (all data) GOF on F2 a

[1](BF4)2·2(CH3)2CO

[1·HH](PF6)2·2CH3CN

C45H39B2F8N7O2Ru 984.52 Red 0.13 × 0.07 × 0.04 Triclinic P1̅ 8.1539(12) 11.7941(18) 21.890(3) 99.438(2) 90.210(2) 104.926(3) 2092.7(5) 2 123 1.562 0.460 1000 27577 7311 (0.0271) 11.62 0.0683 0.1853 1.086

C43H35F12N9P2Ru 1068.81 Red 0.13 × 0.10 × 0.07 Triclinic P1̅ 11.492(2) 13.097(3) 15.101(3) 99.556(2) 98.400(3) 100.187(3) 2157.8(7) 2 123 1.645 0.460 1076 27771 7508 (0.0232) 11.21 0.0484 0.1263 1.066

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑{w(F02 − Fc2)2}/∑w(Fo2)2]1/2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02390. ESI mass spectra, UV−vis absorption, and 1H NMR spectral changes of 1 during photoirradiation, NIR emission spectra of 1 and 1·HH, and 1H NMR spectra of bpp, 1, 1·HH, and 1·DD (PDF) Crystallographic data of [1](BF4)2·2(CH3)2CO (CIF) Crystallographic data of [1·HH](PF6)2·2CH3CN (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Katsuaki Kobayashi E-mail: [email protected]. *Koji Tanaka E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.K. acknowledges the Ministry of Education, Science, Sports, and Culture for a Grant-in-Aid for Scientific Research (No. H

DOI: 10.1021/acs.inorgchem.5b02390 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02390 Inorg. Chem. XXXX, XXX, XXX−XXX