Article pubs.acs.org/JPCA

Visible-Light Activation of the Bimetallic Chromophore−Catalyst Dyad: Analysis of Transient Intermediates and Reactivity toward Organic Sulfides Krishnan Senthil Murugan,† Thangamuthu Rajendran,*,†,‡ Gopalakrishnan Balakrishnan,† Muniyandi Ganesan,† Veluchamy Kamaraj Sivasubramanian,† Jeyaraman Sankar,§ Andivelu Ilangovan,∥ Perumal Ramamurthy,⊥ and Seenivasan Rajagopal*,# †

Post Graduate and Research Department of Chemistry, Vivekananda College, Tiruvedakam West, Madurai 625 234, India Department of Chemistry, PSNA College of Engineering & Technology, Dindugul 624 622, India § Department of Chemistry, Indian Institute of Science Education and Research, Bhopal-462 023, India ∥ School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India ⊥ National Centre for Ultrafast Processes, University of Madras, Chennai 600 113, India # School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India ‡

S Supporting Information *

ABSTRACT: In order to develop a new photocatalytic system, we designed a new redox-active module (5) to hold both a photosensitizer part, [RuII(terpy)(bpy)X]n+ (where terpy = 2,2′:6′,2′′-terpyridine and bpy = 2,2′-bipyridine), and a popular Jacobsen catalytic part, salen−Mn(III), covalently linked through a pyridine-based electron-relay moiety. On the basis of nanosecond laser flash photolysis studies, an intramolecular electron transfer mechanism from salen−MnIII to photooxidized RuIII chromophore yielding the catalytically active high-valent salen−MnIV species was proposed. To examine the reactivity of such photogenerated salen−MnIV, we employed organic sulfide as substrate. Detection of the formation of a MnIII−phenoxyl radical and a sulfur radical cation during the course of reaction using time-resolved transient absorption spectroscopy confirms the electron transfer nature of the reaction. This is the first report for the electron transfer reaction of organic sulfide with the photochemically generated salen−MnIV catalytic center.

1. INTRODUCTION Chemical transformations of organic molecules driven by solar light did not receive due recognition from chemists in the 20th century, though Ciamician1 commented on its importance in the beginning of the century. Photocatalytic organic transformations using sunlight generally involve mild conditions for substrate activation, ideally light alone; this has attracted researchers to devise new photocatalytic systems in recent years.2−7 Though Deronzier et al. developed pioneering photocatalytic systems involving [Ru(bpy)3]2+ (bpy = 2,2′bipyridine) for organic transformations in the 1980s, little attention has been paid to such processes in the past three decades.8,9 However, the recent past has seen a surge of enormous interest in visible light-induced organic transformations particularly using ruthenium(II)−polypyridyl complexes as solar light absorbers.10−14 The advantage with Ru(II) complexes is that they have strong absorption in the visible region and it is possible to tune the photophysical properties by changing the structure of the polypyridyl ligand.15,16 Currently, photochemists have developed many ruthenium(II)−polypyr© XXXX American Chemical Society

idyl−catalyst binuclear dyads where both the photosensitizer and the catalyst fragments are part of the same molecule to perform light-induced redox processes.17−24 In these supramolecular photocatalytic systems the catalytic part is activated by either electron transfer19−23 or energy transfer processes.17,18,24 It is important to mention here that the photosensitizer linked to electron acceptor/donor-mimicking photosystem II (PS II) is an attractive candidate for conversion of solar energy into chemical energy.25−30 In this connection, we recently reported binuclear supramolecular models consisting of a ruthenium(II)− or rhenium(I)−polypyridyl moiety linked covalently with different metal ions including manganese to study the light-induced photophysical processes.31−35 In order to develop systems similar to the PS II, we synthesized a Ru(II)−polypyridyl photosensitizer covalently linked with a methionine donor where a methionine radical Received: January 30, 2014 Revised: May 31, 2014

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Scheme 1. Synthetic Pathway for Formation of Ru−salen−Mn (5)a

a

Conditions: (i) EtOH, 12 h, rt, 83% yield; (ii) MeOH, [Ru(terpy)(bpy)Cl]PF6, 52% yield; (iii) MeOH-AcN, Mn(OAc)2, air, NaCl, 70% yield.

transient intermediates we propose a SET mechanism for reaction of organic sulfides toward the photochemically generated Mn(IV)−salen ion.

could be formed by light-induced intramolecular electron transfer reaction.36 During the past decade, Hammarström, Sun, Åkermark, Styring, Aukauloo, and others have made great efforts to link Ru(II)−polypyridyl (the phototosensitizer part) covalently with Mn(II)/Mn(III) ion (donor part) through bidentate ligands to mimic the donor side of PS II in green plants.21,28,37−42 In the present study we have chosen the [Ru(terpy)(bpy)(X)]n+ moiety as precursor for the photosensitizer part owing to its interesting photophysical properties.43−45 A literature survey shows that there is no report available for a ruthenium−manganese binuclear dyad incorporating [Ru(terpy)(bpy)(py)]2+ (where py = pyridine) as the photosensitizer unit. The manganese−salen system was chosen as the catalytic part of our system due to its known chemical reactivity and novel intermediates formed when activated by chemical, electrochemical, and photochemical methods.46−56 To the best of our knowledge, very few reports analogous with this system have been published.41,42,57 In 2005, Sun et al.41 reported the synthesis and photophysics of a chiral salen Mn(III) complex covalently linked to a Ru(II) tris(bipyridyl) photosensitizer. Thereafter, during the start of this work, Herrero et al. reported the photochemical generation of highvalent salen−MnIV species (active form) by visible light absorption of the photosensitizer component (ruthenium polypyridyls) and subsequent intramolecular electron transfer from the manganese(III)−salen moiety.42 Very recently, Hamelin et al. reported the binuclear photosensitizer−catalyst model (RuII−L−RuII) for oxidation of organic sulfides with water as an oxygen source.57 Though several chemical oxidants are available for oxidation of the organic sulfides58−61 an ecoaware photocatalytic system for sulfide oxidation is necessary from a green chemistry point of view.57,62−67 To the best of our knowledge, there is no report available for sulfide oxidation using photogenerated high-valent manganese−salen ion as the oxidant. Herein we report a simple approach to couple the Mn(III)−salen (catalytic fragment) to the ruthenium(II) chromophore without dramatically perturbing their referential physical properties by introducing a single carbon−carbon covalent bond between the two fragments. We also demonstrate the visible light activation of the catalytic fragment by intramolecular single electron transfer (SET) from Mn(III) to photooxidized Ru(III). On the basis of the investigation of

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All chemicals were purchased from Aldrich and used as received. Some of them, Na2CO3, Na2SO4, and NaCl, were from Merck. All solvents were purified before use as per the standard procedures.68 [RuII(terpy)(bpy)Cl]PF6 was prepared by the previously published report.69 The synthesis and photophysical properties of the precursor complex [Ru(terpy)(bpy)(py)]2+ (6) have already been reported.43−45 NMR studies were performed on a Bruker 400 or 500 MHz instrument by dissolving compounds in deuterated CHCl3 or DMSO or containing 0.03% TMS as an internal standard. Mass spectra were recorded on a High Resolution ESI Mass Spectrometer (micrOTOF-Q II 10330) by direct injection of the samples dissolved in a mixture of CH2Cl2 or MeOH or CH3CN. Electronic absorption and emission spectra were measured with a JASCO V-530 UV−vis spectrophotometer and JASCO FP-6200 spectrofluorimeter, respectively, in acetonitrile medium. All sample solutions used for emission measurements were deaereated for about 30 min by dry nitrogen gas purging by keeping solutions in cold water to ensure that there is no change in the volume of the solution. Transient absorption measurements in the time range from 100 ns to 180 μs were made using nanosecond laser flash photolysis. For laser excitation using 532 nm harmonics output from a Quanta Ray GCR-2 (Spectra Physics, USA) Nd:YAG laser with a pulse width of 8 ns was used in right-angled geometry and with a 1 cm path length cell. Signals were detected using a 250W pulsed xenon lamp, a Czerny−Turner monochromator, and R-928 PMT. Signals were captured in a Hewlett-Packard 54201A digital storage oscilloscope. Acetonitrile solutions with a final concentration of the electron acceptor (NBD) was ∼40−45 mM, and the Ru−Salen−Mn complex was typically ∼0.32 mM. The sample vessel was sealed and then purged with argon for 30 min before the experiment by keeping the solution in cold water to ensure that there is no change in the volume of the solution. Argon purging was kept on during the data collection period. The change in the absorbance of the sample on laser irradiation was used to B

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MHz, CDCl3): δ (ppm) 1.203 (s, 9H, tert-butyl), 1.399 (s, 9H, tert-butyl), 1.454 (s, 9H, tert-butyl), 1.777−2.039 (m, 8H, cyclohexane), 3.323−3.399 (m, 2H, (R,R)−CH-N), 6.957 (s, 1H, Ar−H), 7.262 (s, 1H, Ar−H), 7.30(s, 1H, Ar−H), 7.359 (d, 4 Hz, 2H, Py-H), 7.525 (s, 1H, Ar−H), 8.282 (s, 1H, CH N), 8.354 (s, 1H, CHN), 8.569 (d, 4 Hz, 2H, Py−H), 13.669 (s, 1H, OH), 14.240(s, 1H, OH). 13C NMR (500 MHz, CDCl3): δ (ppm) 166.07, 165.336, 150.011, 128.278, 127.782, 126.906, 125.952, 121.143, 120.926, 117.740, 72.415, 72.229, 34.984, 33.190, 32.979, 31.381, 29.707, 29.371, 29.300, 24.316. FT-IR: 3421, 2954, 1629, 1594, 1467, 1442, 1384, 1361, 1274, 1253, 1139, 1097, 1048, 991, 937, 879, 821, 711, 605, 512 cm−1. MS (HR-ESI) calcd for C37H50N3O2 (M + H)+568.3898; found 568.3905, 336.2065. Anal. Calcd for C37H49N3O2: C, 78.27; H, 8.70; N, 7.40. Found: C, 78.17; H, 8.63; N, 7.31. 2.2.3. Synthesis of RuII−salen (4). [Ru(terpy)(bpy)Cl]PF6 (150 mg, 0.224 mmol) and AgNO3 (38 mg, 0.224 mmol, 1 equiv) were dissolved in 10 mL of methanol and stirred for 1 h at room temperature. The resulting AgCl precipitate was filtered off, and the resulting methanolic solution was allowed to react with asymmetric salen 3 (127 mg, 0.224 mmol, 1 equiv) overnight at 55 °C under nitrogen. The solvent was then removed by reduced pressure, and the resulting product was dissolved in a minimum amount of methanol to which a few drops of a saturated aqueous solution of KPF6 were added. The precipitate was filtered off and collected to yield 140 mg (52% yield). MS (HR-ESI) calcd for C62H69N8O2Ru (M + H)+ 1059.46; found 1059.35. Anal. Calcd for C62H68F12N8O2P2Ru: C, 55.23; H, 5.08; N, 8.31. Found: C, 55.17; H, 5.01; N, 8.19. 2.2.4. Synthesis of RuII−salen−MnIII (5). A 100 mg amount of compound 4 (0.074 mmol) dissolved in 5 mL of acetonitrile and 38 mg of MnII(OAc)2 (0.223 mmol, 3 equiv) dissolved in 5 mL of MeOH were mixed in a two-neck flask and heated to 70 °C for 5 h under nitrogen. While keeping the reaction at this temperature air was bubbled in the solution for 1 h. The solution was then allowed to go back to room temperature, 10 mL of a saturated aqueous solution of NaCl was added, and the solution was stirred for an additional hour. The precipitate formed was filtered off, the solution was extracted carefully by adding additional dichloromethane and water, and the organic phase was dried over anhydrous sodium sulfate. The resulting product was redissolved with a minimum amount of methanol. The product was precipitated by addition of 3 mL of a saturated KPF6 aqueous solution. Yield: 70%. MS (HR-ESI) calcd C62H69ClMnN8O2Ru (M + 3H)+ 1149.34; found 1172.1 [M + Na + 3H]+ ,620.3247 [M + HCOO− + 2H]2+. Anal. Calcd for C62H66ClF12MnN8O2P2Ru: C, 51.83; H, 4.63; N, 7.80. Found: C, 51.75; H, 4.52; N, 7.68.

record the time-resolved absorption transient spectrum. The change in the absorbance on flash photolysis was calculated using the expression ΔA = log I0/(I0 − ΔI ) ΔI = (I − It )

where ΔA is the change in the absorbance at time t and I0, I, and It are the pretrigger voltage, the voltage after flash, and the voltage at a particular time, respectively. The time-resolved transient absorption spectrum was recorded by plotting the change in absorbance at a particular time vs wavelength. Cyclic voltammetry and differential pulse voltammetry were performed at room temperature with a potentiostat-galvanostat (CH Instruments Electrochemical Analyzer) using a 3 mm2 surface glassy carbon electrode as the working electrode, a platinum wire as the counter electrode, and a Ag/Ag+ electrode in acetonitrile as the reference electrode. The acetonitrile solution containing 1 mM complex along with 0.10 M for tetrabutylammonium hexafluorophosphate was purged with nitrogen before the experiment. 2.2. Synthesis and Structural Characterization. The synthetic pathway leading to the target compound 5 is summarized in Scheme 1. 3-tert-Butyl-5-(4-pyridyl)-2-hydroxybenzaldehyde 1 was reacted with the chiral half unit, the monoimino synthon 2, to form the pyridine-modified chiral salen ligand 3. Synthesis of 4 (named RuII−Salen) was performed by coordination of [Ru(terpy)(bpy)Cl]PF6 to the pyridine end of 3. Eventually, synthesis of the target compound 5 (named RuII−Salen−MnIII) was achieved by insertion of manganese(II) acetate in the salen cavity. MnII was oxidized to its MnIII state by air oxidation and treated to anion exchange with NaCl.70 2.2.1. Preparation of 3-tert-Butyl-5-(4-pyridyl)-2-hydroxybenzaldehyde (1). The compound 3-tert-butyl-5-(4-pyridyl)2-hydroxybenzaldehyde (1) was prepared by a modified literature procedure.71 A mixture of 5-bromo-3-tert-butyl-2hydroxybenzaldehyde 2 (2.5 g, 9.72 mmol), 4-pyridinylboronic acid (1.31 g, 10.69 mmol), Na2CO3 (1.55 g, 14.58 mmol), and Pd(PPh3)4 (1.13 g, 0.98 mmol) was refluxed in 1,4dioxane:H2O (3:1 v/v, 60 mL, degassed with N2) at 102 °C for 45−60 min. After cooling to room temperature, the reaction contents were poured into water (50 mL). The aqueous mixture was extracted with CH2Cl2 (50 mL), and the combined extracts were dried over Na2SO4 before being concentrated under reduced pressure. The crude aldehyde was purified by chromatography (silica gel, hexane/ethyl acetate). A pale yellow solid was obtained. Yield: 71%. 1H NMR (400 MHz, CDCl3): δ (ppm) 11.955 (s, 1H, OH), 9.985 (s, 1H, CHO), 8.674 (d, 5.6 Hz, 2H, py−Ar−H), 7.806 (d, 2.0 Hz, 1H, Ar−H), 7.529 (d, 5.6 Hz, 2H, py−Ar−H), 7.707 (d, 2.0 Hz, 1H, Ar−H), 1.477 (s, 9H, tert-butyl). IR (KBr, cm−1): 3421(H-bonded OH), 1650(CO). MS (FAB) calcd for C16H18NO2 (M+H)+ 256.1259; found 256.67. 2.2.2. Preparation of Pyridine-Modified salen Ligand (3). A solution of monoimine synthon 272 (205 mg; 0.621 mmol) in dry ethanol (5 mL) was added in dropwise to a solution of 3tert-butyl-5-(4-pyridyl)-2-hydroxybenzaldehyde 1 (158 mg; 0.621 mmol) in dry ethanol (5 mL) and stirred for 12 h at room temperature. The reaction mixture was concentrated to dryness in vacuo. A yellow solid was obtained. The crude product was purified by chromatography (silica gel, hexane/ ethyl acetate) to yield a yellow solid. Yield: 83%. 1H NMR (500

3. RESULTS AND DISCUSSION 3.1. Electronic Absorption Spectra. The synthesis and photophysical properties of the precursor complex [Ru(terpy)(bpy)(py)]2+ (6) have already been reported.43−45 Complex 6 (Figure 1) shows ligand-centered transitions at 237, 284, 309, and 415 nm and the MLCT absorption maximum at 465 nm, and the values are collected in Table 1. The weak shoulder at 415 nm was assigned as RuII → py MLCT. Attachment of the salen moiety covalently with the ruthenium(II)−polypyridyl chromophore yields RuII−salen 4 (Figure 1), which shows π → π* and n → π* ligand-centered transitions at 282 and 312 nm, respectively, and the broad MLCT band at 462 nm. The blue shift of 4 nm observed for RuII−salen 4 could be ascribed to attachment of the salen moiety with the pyridine ligand, C

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Figure 2. Emission spectra of RuII−salen (4), RuII−salen−MnIII (5), and [Ru(terpy)(bpy)(py)]2+ (6) in acetonitrile at 298 K.

Figure 1. Electronic absorption spectra of RuII−salen (4), RuII− salen−MnIII (5), and [Ru(terpy)(bpy)(py)]2+ (6) in acetonitrile at 298 K.

increase in the excited state lifetime may be attributed to the presence of the salen moiety in the pyridine monodentate ligand with the higher extent of conjugation. However, when the MnIII ion is inserted in the salen cavity, resulting in formation of RuII−salen−MnIII 5, this bimetallic complex also shows the same emission maximum at 604 nm with a doubleexponential emission decay with the lifetime of τ1 = 153 ± 5 ns and τ2 = 5 ± 1 ns in acetonitrile when λex = 460 nm. The slow component (τ1 = 153 ± 5) in 5 matches the 3MLCT emission of the ruthenium(II)−polypyridyl excited state. According to Scaiano et al.55 Mn(III)−salen (7) shows dual emission peaks at 420 and 530 nm in hexane when λex = 340 nm. The emitting excited-state lifetime is too short (S•+),75,77−82 respectively, generated by the electron transfer from the MPS to the photochemically generated MnIV. Identification of the transient intermediates I and II was made on the basis of the similarity with the transient absorption of the other sulfide radical cations.75−82 Control experiments showed that in the absence of light, catalyst 5, or electron acceptor no absorption peak in the region of sulfur radical cation was observed. Thus, the absorption peaks formed at 480 and 550 nm support formation of a sulfur radical cation during oxidation of organic sulfides with complex 5 in the presence of light. 3.5. Kinetics of Oxidation of Para-Substituted Phenyl Alkyl Sulfides. The rates of three electron transfer processes taking place in this photosensitizer−catalyst model system are followed by the transient absorption kinetics at their characteristic transient absorption maxima of different species. The rate of oxidative quenching of excited state [RuIII−L•−]* by NBD has been followed by fluorescence quenching of complex 5 with different concentrations of NBD (Figure S3, Supporting Information). The quenching rate constant, kq, value is determined from the modified Stern−Volmer plot83 using the eqs 1 and 2

Figure 5. Transient absorption spectra of 0.32 mM RuII−salen−MnIII (5) with 0.5 M MPS in aqueous acetonitrile (8:2 v/v CH3CN−H2O) in the presence of an irreversible electron acceptor (∼45 mM NBD) following a single pulse at 532 nm, shown at different time scale.

strong absorption near 850 nm. It is well established that the strong absorption near 850 nm is due to generation of MnIII− phenoxyl radical species49,50,54 by one-electron transfer from MPS to the photochemically generated manganese(IV). To support our argument it is appropriate to quote the work of Kunkely and Vogler,54 who observed a similar and strong absorption at 820 nm for an electron transfer from phenolate to MnIV in the MnIV−phenolate leading to formation of the corresponding MnIII−phenoxyl radical. However, in our case, formation of MnIII−phenoxyl radical appears highly red shifted to near 850 nm due to the higher extent of conjugation possible in our system. In addition, we would like to quote the work of Fujii et al., who reported the appearance of a peak at 905 nm

F0 1 1 = + F0 − F fK[Q ] f

(1)

K = kqτ0

(2)

Here F0 and F are the luminescence intensities of complex 5 in the absence and presence of quencher, K, the Stern−Volmer constant, kq, the quenching rate constant, and τ0 the luminescence lifetime of complex 5 in the absence of quencher. The plot of F0/(F0 − Fi) vs 1/[NBD] yields f−1 as the intercept on the y axis and (f K)−1 as the slope. The quenching rate constant, kq, is calculated from the obtained K value. The rate

Scheme 2. Reaction Scheme Proposed for Light-Induced Electron Transfer Events Following Excitation of Binuclear Complex 5 at 532 nm in the Presence of an External Irreversible Electron Acceptor, NBDa

a

Excited state quenching produces photooxidized Ru(III) and a reduced acceptor (step 1). In step 2 an electron is transferred from MnIII to RuIII chromophore leading to formation of a high-valent MnIV state. In step 3 organic sulfide is oxidized to form a sulfur radical cation by the photochemically generated MnIV catalytic center. F

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constant value for the intramolecular electron transfer reaction from MnIII to RuIII is determined from the slope of the linear plot of log (ΔA) vs time followed at 700 nm (data given in Scheme 2). The rate constant value for the single-electron transfer (SET) from different organic sulfides to the high-valent manganese(IV) is followed at 530−560 nm, the characteristic absorption peak of a monomeric sulfur radical cation. The firstorder rate constant, k1, values are obtained from the slope of the linear plot of log (ΔA) vs time. The second-order rate constant, k2, values are obtained by dividing k1 values by the concentration of sulfide. The second-order rate constant, k2, values for formation of sulfur radical cation for five phenyl alkyl sulfides are presented in Table 2. The change of rate constant with the change of structure of sulfide is in accordance with the oxidation potential of these organic sulfides. The lesser is the oxidation potential of organic sulfide; the greater is the rate constant for the one-electron transfer from alkyl aryl sulfides to the high-valent manganese−salen complexes.84 Further, the linear plot observed when log k2 values are plotted against oxidation potentials of sulfides (Figure 6) also supports an

Figure 7. Normalized kinetic data, overlaid for comparison, are shown for recovery of the MnIII−phenoxyl radical species at 850 nm (squares) and formation of monomeric MPS radical cation at 560 nm (triangles) during addition of 0.5 M MPS to the 0.32 mM RuII−salen− MnIII (5) in aqueous acetonitrile (8:2 v/v CH3CN−H2O) in the presence of an irreversible electron acceptor (∼45 mM NBD) following a single pulse at 532 nm.

Figure 6. Plot of log k2 vs oxidation potential of para-substituted phenyl alkyl sulfides.

electron transfer mechanism.74,84 The rate of recovery of the MnIII−phenoxyl radical species and formation of monomeric MPS radical cation from the intermolecular SET from organic sulfide to the photogenerated MnIV ion was found to take place at the same rate (Figure 7). 3.6. Effect of Substituents. The Hammett plot of log k2 vs σp (Figure 8) helps us to understand the effect of substituents at the para position in the phenyl ring of aryl alkyl sulfides on their reactivity toward the photochemically generated Mn(IV) ion. The slope of this plot gives the reaction constant, ρ, value which is dependent on the nature of the reaction and on the conditions. It is a measure of the sensitivity of a given reaction to the polar effects of the ring substituents, i.e., to changes in the σ value of the meta and para substituent.85 In the present study the electron-releasing groups increase the rate, whereas electron-withdrawing groups decrease the rate giving the negative ρ value85 (ρ = −2.1 ± 0.1). These results may be explained by the fact that the reactivity of Ar−S−R, a nucleophile, is expected to be greater than the electron density on the sulfur atom. The 4-methyl group in phenyl methyl sulfide increases the electron density on the sulfur atom, thereby enhancing the electron-donating ability of the sulfide to the photochemically geneared electron-deficient Mn(IV) center

Figure 8. Hammett plot for oxidation of para-substituted phenyl methyl sulfides by photochemically generated RuII−salen−MnIV ion.

and so facilitate the SET reaction, whereas electron-withdrawing groups such as p-bromo and p-nitro substituents have the reverse trend. The negative ρ value indicates development of positive charge on the sulfur center of PhSMe in the transition state. Our group extensively reported on the [MnIII− salen]-catalyzed PhIO and H2O2 oxidation of organic sulfides and organic sulfoxides.84,86,87 It will be interesting to compare the ρ value observed in the present study with the ρ value obtained from [MnIII−salen]-catalyzed oxidation of organic sulfides. Since the observed ρ value (−2.1 ± 0.1) is very close to the ρ value (−1.9 ± 0.2) found for the SET reaction of parasubstituted phenyl methyl sulfides with oxo(salen)manganese(V) complexes,84 one may anticipate the SET mechanism for the system under study. 3.7. Mechanism of Electron Transfer Reaction from Organic Sulfides to the Photochemically Generated RuII−salen−MnIV Ion. To understand the key role of highvalent metallic species in the synthetic as well as biological oxidation reactions, great efforts have been made to examine G

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Scheme 3. Mechanistic Pathway for Formation of Sulfoxide and Sulfone from Sulfur Radical Cation

photogenerated [RuIII−L•−]* excited state by the irreversible electron acceptor, NBD, forming [RuIII−L],42 which occurs at a rate of (4.8 ± 0.3) × 108 M−1 s−1. The second step is the intramolecular electron transfer from Mn(III) to Ru(III) which occurs at a rate of (2.5 ± 0.2) × 106 s−1. Hammastrom et al.93,94 reported a rate constant of about 1.8 × 105 s−1 for the similar intramolecular reaction from MnII to RuIII in the ruthenium− manganese binuclear photosensitizer−quencher model. The faster rate observed in the present study is attributed to the shorter distance between the Ru(III) center and the quencher Mn(III) compared with the previously published data.93,94 The third step is a SET from phenyl alkyl sulfides to the electrondeficient Mn(IV) which occurs at a rate in the range from (2.0 ± 0.2) × 104 to (2.0 ± 0.1) × 106 M−1 s−1. It is important to mention here that a second-order rate constant from 2.4 × 102 to 4.0 × 104 M−1 s−1 was observed for a SET reaction between various para-substituted phenyl methyl sulfides and different oxo(salen)manganese(V) species.84 Generally the outer-sphere oxidant ([RuII−salen−MnIV] ion) undergoes reaction with electron donors like organic sulfides by second-order kinetics with rate-limiting electron transfer to produce organic sulfur radical cation (I), which has been identified in the wavelength range 530−570 nm from the transient absorption spectral studies. It has been proved earlier that the monomeric sulfur radical cation (>S•+) formed from organic sulfides can be stabilized by the way of dimerization with unoxidized sulfide to form the intermolecular two-center-three-electron-bonded radical cation (II), which is identified by an absorption

the active oxidant species formed during manganese−salencatalyzed oxidation reactions.46−56 We also performed a systematic study on the nature of active oxidant species formed during reaction of [MnIII−salen]-catalyzed PhIO and H2O2 oxidation of organic sulfides and organic sulfoxides.84,86,87 Furthermore, there are also reports available on the intermediates generated when Mn(III)−salen was irradiated with UV or visible light.42,54,55 However, there is still no report available for the sulfide oxidation by such photogenerated highvalent metal−salen species. For oxidation of organic substrates by manganese(III)−salen complexes, there are two wellestablished mechanisms proposed, an electron transfer mechanism48−50,84,88−90 and a direct oxygen-transfer mechanism.47,87,91,92 The negative reaction constant, ρ, value obtained from the Hammett plot of log k2 vs σp indicates an accumulation of positive charge at the sulfur center, while the magnitude of the ρ (−2.1 ± 0.1) value indicates the extent of development of positive charge on the sulfur atom in the transition state of the rate-determining step.84 The close ρ value observed in this work with our previous report84 also supports the SET mechanism for the system under study as shown in Scheme 2. The plot of log k2 with oxidation potential of sulfides is also linear. The linearity of this plot also supports an electron transfer mechanism.74,84 To account for the transient intermediates identified from the transient absorption experiments we propose the oxidation of sulfides by the photogenerated Mn(IV) proceeds according to Schemes 2 and 3. The first step shown in Scheme 2 is oxidative quenching of the H

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maximum at 480 nm observed in the nanosecond flash photolysis. The long lifetime of the sulfur radical cation on the scale of microseconds diminishes the driving force for back electron transfer to the active oxidant species. The initially formed sulfur radical cation (I) further reacts with the water molecule present in the solvent medium, resulting in formation of hydroxyl sulfuranyl radical (III), which is characterized by absorption at a wavelength around 340 nm.95 This reactive radical again donates its electron to the active oxidant species remaining in solution to produce sulfur cation (IV), and finally, deprotonation of IV leading to formation of sulfoxide. HPLC analysis shows sulfoxide as the minimum product and sulfone as the major product (94%) from the light-induced oxidation of sulfide by complex 5, which indicates that the initially formed sulfoxide further reacts with the active oxidant species [RuII− salen−MnIV] to form sulfone similar to the mechanism reported for photooxidized [Ru(NN)3]3+ oxidation of organic sulfoxides96 as demonstrated in Scheme 3. 3.8. Driving Force Dependence of Single Electron Transfer (SET) Reaction of RuII−salen−MnIV with Organic Sulfides. After establishing the electron transfer mechanism (Schemes 2 and 3) we applied the Marcus classical theory of electron transfer97,98 to examine the driving force dependence of the electron transfer from sulfide to MnIV. The rate of electron transfer from a donor to an acceptor molecule in a solvent is controlled by the free energy change (ΔG°), reorganization energy (λ), and electron transfer distance between the donor and the acceptor. The rate of electron transfer is calculated using eqs 3 and 497,98 ket = kBT /h exp[−ΔG⧧/kBT ]

Figure 9. Free-energy dependence of the second-order rate constants of the SET reaction from sulfide to MnIV.

a large deviation from the experimental value. This deviation can be accounted for with the role of this nitro compound as the electron acceptor for the excited state [(Ru)(terpy)(bpy)]2+ component.100−103 The transient derived from the nitro compound after accepting an electron from the excited state Ru(II) complex has absorption in the wavelength range 500− 550 nm.100,102 Thus, the species, the nitro aromatic anion radical and sulfide cation radical, both contribute to the concentration of the species with the absorption maxiumum at 500−550 nm. Thus, the larger experimental rate constant obtained with methyl p-nitrophenyl sulfide is due to formation of two different transients with absorption in the similar wavelength. Similar results have been observed by us and others previously when the quencher contains both electron donor and acceptor parts.104,105 Thus, the Marcus classical theory of electron transfer reproduces the experimental results favorably, confirming the success of the theory of electron transfer and operation of the electron transfer mechanism of the reaction.73,99 3.9. Product Analysis. To 3 mL of an aqueous acetonitrile (CH3CN−H2O 8:2 v/v) solution of the RuII−salen−MnIII complex 5 (10 mg, 0.007 mmol), NBD (82 mg, 0.35 mmol) and methyl p-tolyl sulfide (25 mg, 0.18 mmol) were added and purged with nitrogen in order to prevent photochemical formation of 1O2 as potential oxidant. The resulting solution was then photolyzed using the 532 nm output of a pulsed Nd:YAG laser with a pulse width of 8 ns. Thereafter the resulting products were extracted with dichloromethane and washed with water twice and finally with brine solution. The organic layer was dried over anhydrous Na2SO4 and then concentrated under reduced pressure. The resulting colorless solid was then purified by column chromatography on silica gel with ethyl acetate−hexane as eluent to remove the unreacted reactant. HPLC analysis (Daicel chiralpak AD-H and 9:1 v/v hexane/i-Pr−OH) of the product indicated formation of sulfone (Figure S16, Supporting Information) as the major product (94%) with a small amount of sulfoxide (6%) under the present experimental conditions. The product was also analyzed by 1H NMR spectroscopy. 1H NMR (400 MHz, CDCl3): δ (ppm) 2.457 (s, 3H), 3.034 (s, 3H), 7.367 (d, 8.0 Hz, 2H), 7.829 (d, 8.0 Hz, 2H).

(3)

where kBT/h = 6.21 × 10 at T = 298 K 12

ΔG⧧ = λ /4[1 + ΔG°/λ]2

(4)

where ΔG° is the standard free energy change of the reaction and λ is a “reorganization energy” composed of solvational (λo) and vibrational (λi) components (i.e., λ = λo + λi). The terms kB and h are Boltzmann’s and Planck’s constant, respectively. The solvent reorganization energy99 λo can be calculated using eq 5 λo = e 2 /4πε0[1/2rD + 1/2rA − 1/d][1/Dop − 1/Ds]

(5)

where e is the transferred electronic charge, ε0 is the permittivity of free space, and Dop and Ds are the optical and static dielectric constants, respectively. The terms rD and rA are the radii of the donor and acceptor, respectively, and d is the sum of the radii, rD + rA. The values of rD and rA are estimated by MM2 molecular model, and the λi value can be obtained using eq 6 λi = 1/2 ∑ kj(Q r j − Q p j)2 j

(6)

where − Q j are equilibrium values for the jth normal mode coordinate Q and kj is a reduced force constant 2krjkpj/(krj + kpj) associated with it. The superscripts r and p refer to reactants and products. The λ values estimated for this reaction are in the range from 0.56 to 0.68 eV. Since ΔG° and λ values are known, the value of the rate constant for electron transfer from sulfide to MnIV is calculated. The calculated rate constant, ket, values are shown in Table 2 and plotted against ΔG° values in Figure 9. The data given in Figure 9 show close agreement between the experimental data (squares) and the calculated values (triangles). However, methyl p-nitrophenyl sulfide shows Qrj

p

I

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laser flash photolysis studies. K.S.M sincerely acknowledges the CSIR HRDG, New Delhi, for the fellowship.

4. CONCLUSION The results presented here demonstrate that the active species that are transiently generated by the intramolecular electron transfer from the chiral manganese(III)−salen moiety to the photooxidized ruthenium(III)−polypyridyl chromophore generated by visible light absorption of the photosensitizer component results in photochemical oxidation of organic sulfide to sulfoxide and sulfone using H2O as an oxygen source. One of the most interesting findings in the present study is the shorter distance (separated by only one carbon−carbon bond) between the Ru(III) chromophore and the quencher, Mn(III), compared with the previously published reports,93,94 resulting in the enhanced rate in the intramolecular electron transfer from Mn(III) to the Ru(III) center which occurs at a rate of (2.5 ± 0.2) × 106 s−1. Detection of Mn(III)−phenoxyl radical from the laser flash photolysis study demonstrates the reactivity of such photochemically activated species [RuII−salen−MnIV] toward electron donors, like organic sulfides through a oneelectron transfer mechanism. To the best of our knowledge, this is the first report for the photochemical generation of a MnIV species which is used as the oxidant for oxygenation of organic sulfides using H2O as the oxygen source.

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ASSOCIATED CONTENT

* Supporting Information S

1

H NMR, 13C NMR, and/or ESI-MS spectra of compounds 1, 3, 4, and 5. HPLC analysis of the oxidized product of methyl ptolyl sulfide; fluorescence quenching of the 3MLCT excited state of RuII−salen−MnIII by NBD and its Stern−Volmer plot; transient absorption spectra of complex 5 with the other organic sulfides in the presence of an irreversible electron acceptor; fluorescence decay curve of RuII−salen−MnIII (5) in acetonitrile; kinetic data of RuII−polypyridyl recovery and generation of MnIV−phenolate in the absence of sulfide; kinetic data of decay of MnIV−phenolate and generation of sulfur radical cation in the presence of sulfide; transient absorption spectra of [Ru(terpy)(bpy)(py)]2+ (6) with 0.5 M MPS in the presence of an irreversible electron acceptor. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: +91 4543 258234. Fax: +91 4543 258358. E-mail: [email protected]. *Phone: +91 452 2548246. Fax: +91 452 245139. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.R gratefully acknowledges the CSIR HRDG, New Delhi, India, for financial support in the form of a project [Scheme No. 01(2354)/09/EMR-II]. We thank the Principal, the Management, and the Head of the Department of Chemistry, Vivekananda College, Tiruvedakam West, Madurai, India, for providing all facilities to carry out this research work. We thank Dr. R. I. Kureshy and Dr. N. H. Khan, CSMCRI, Bhavnagar, India, for their useful discussions during the synthetic part of this work. We thank Dr. C. Selvaraju, Assistant Professor, National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai, India, for his kind help in doing J

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