Inorganic Chemistry Communications 44 (2014) 70–78

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Inorganic Chemistry Communications

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Feature article

Synthesis, spectroscopic characterization, photochemical andphotophysical properties of mono- and tetranuclear Ru(II) and Mn(I)complexes with 4,4′-bipyridine ligand

Inara de Aguiar a, Simone D. Inglez b, Rose M. Carlos a,⁎a Departamento de Química, Universidade Federal de São Carlos, CP 676, 13565-905 São Carlos, SP, Brazilb Departamento de Engenharia Química, Universidade Tecnologica Federal do Paraná, Campus Ponta Grossa, 84016-210 Ponta Grossa, PR, Brazil

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.inoche.2014.02.0481387-7003/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 September 2013Accepted 27 February 2014Available online 12 March 2014

Keywords:RutheniumManganeseElectron transferEnergy transfer

The synthesis, characterization and the photoinduced electron transfer reactions of the tetranuclear complex cis,fac-[(phen)2Ru(4,4′-bpy)2Mn(CO)3(ImH)]2+6 (III) and the mononuclear moieties cis-[Ru(phen)2(4,4′-bpy)2]2+

(II) and fac-[Mn(CO)3(4,4′-bpy)2(ImH)]+ (I) in the presence of MV2+ are reported. The intramolecularenergy/electron transfer process that occurs between the Ru(II) and the Mn(I) upon MLCT (Ru → phen) lightexcitation is evaluated by time-resolved absorption experiments. Complex II was not able to reduce MV2+,even in the ns scale. Otherwise, complex III showed an ability to generateMV+•when exposed to flash photolysisirradiation.

© 2014 Elsevier B.V. All rights reserved.

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Photochemical experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Photophysical experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Theoretical calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

fac-[Mn(CO)3(4,4′-bpy)2(ImH)]+ (I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72cis-[Ru(phen)2(4,4′-bpy)2]

2+ (II) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72cis,fac-[Ru(phen)2(4,4′-bpy)2-Mn(CO)3(ImH)]2(PF6)4(SO3CF3)2 (III) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Molecular structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Absorption and emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75MV2+ to MV+• photo-induced electron transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Introduction

The electrochemical and photochemical properties of manganesecomplexes have received considerable attention over the past severaldecades [1]. One goal has been to examine systematically complexes

of different metal and ligand combinations with interest in photo-induced electron transfer reactions [2]. For this purpose variousmono- and tetranuclear manganese complexes have been extensivelystudied [3]. Our interest in this area started when we observed thatthe fac-[Mn(CO)3(phen)(ImH)]+ ion complex in CH3CN produces along lived radical, which can be efficiently trapped by electron acceptormolecules as MV2+ [4]. From a photocatalytic point of view, ruthenium

71I. de Aguiar et al. / Inorganic Chemistry Communications 44 (2014) 70–78

polypyridine complexes are most attractive for efficient visible light ab-sorption and energy/electron transfer process because of their largemolar absorption coefficient and long emission lifetime [5].

Thus to improve both the visible absorption and stability of thesystem, we have carried out the synthesis of tetranuclear compoundcis,fac-[(phen)2Ru-(4,4bpy)2-Mn(CO)3(ImH)]26+ and its photochemicaland photophysical properties were examined. The correspondingmononuclear subunits cis-[Ru(phen)2(4,4bpy)2]2+ and fac-[Mn(4,4′-bpy)2(CO)3(ImH)]+ were also synthesized for comparison purposes.

Experimental

General

All synthesis and preparations for electrochemical and spectroscopicexperiments were carried out under purified N2 atmosphere, usingSchlenk techniques, RuCl3∙xH2O, 1,10-phenanthroline (phen), 4,4′-bipyridine (4,4′-bpy), imidazole (ImH) and lithium chloride fromAldrich; tetrabutylammonium phosphate (TBAPF6) and bromidepentacarbonyl manganese from Strem. HPLC grade acetonitrile anddichloromethanewere distilled prior to use. The solutionswere careful-ly handled in the dark before the experiments were performed. Thecomplexes cis-[RuCl2(phen)2]∙2H2O [6], cis-[Ru(phen)2CO3]∙2H2O [7]and cis-[Ru(phen)2(OH2)2](PF6)2 [8], were prepared by literatureprocedures. Solutions were deoxygenated with a stream of N2 andmaintained under a positive pressure of N2 during the measurements.

The elemental analysis (C, H, N) was performed using a Carlo ErbaEA 1110 CHNS-O Instrument in the Microanalytical Laboratory atUniversidade Federal de São Carlos (UFSCar). FTIR spectra weremeasured in CaF2 windows in CH2Cl2 solution on a Bomem-Michelson102 spectrometer in the 4000–1000 cm−1 region. Optical spectra wererecorded on an Agilent 8453A UV–Visible spectrophotometer. 1H NMRspectra were measured in a CD3CN solution using a BRUKER DRX-400spectrometer. All chemical shifts (δ) are given in ppm units with refer-ence to the hydrogen signal of the methyl group of tetramethylsilane(TMS) as internal standard and the coupling constants (J) are in Hz. Elec-trochemical measurements were recorded using a μAutolab Type IIIpotentiostat. Solutions typically contained 1.0 × 10−3 mol L−1 of the

I II

III

Fig. 1. Representative structures of compounds I, II and III.

complex. A platinum disk served as the working electrode (d =0.2 mm) and a counter electrode (d = 0.5 mm) and an Ag/AgCl wirewere used as the reference electrode. Solutions contained 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the supporting elec-trolyte. The best results were obtained at scan rates of 100 mV/s.

Photochemical experiments

The photochemical reactivity of complexes was investigated using aphotochemical reactor Rayonet RMR-600 and RMR-3500 Å lamps. Adeoxygenated sample of known volume in a 1 cm path length fourside quartz cells with magnetic stirred bar was irradiated for definedtime periods. The UV–vis or 1H NMR spectrum of the sample wasrecorded after each irradiated period, and this procedure was thenrepeated until approximately 30% of the reaction has been completed.Time-resolved optical spectra were obtained using a laser flash-photolysis apparatus containing a Continuum Q-switched Nd:YAGlaser (Continuum, Santa Clara, CA) with excitation provided by thethird harmonic at λ = 355 nm. The pulse length was 8 ns, the beamdiameter incident on sample was 6 mm, and the repetition rate was10Hz. The laser pulsewas set up to 8mJ per pulse in the photobleachingstudies measured using a Field Master power meter with L-30 V head.The growth-decay kinetics were measured at a single wavelengthusing a monochromator (M300 from Bentham) and a photomultiplier(Hamamatsu, model R928P). Transient decays were averaged using aTektronix TDS 340A digital oscilloscope. The digitized kinetics datawere transferred to a personal computer (PC) for the analysis withsoftware supplied by Edinburgh Instruments.

Photophysical experiments

The steady state emission spectra were obtained on a Shimadzu RF-5301PC fluorescence spectrophotometer. Solutions (10−5 mol L−1;absorbance at the maximum absorption = 0.3) of the complexes inthe appropriated solvent were used. The solutions were deaerated bybubbling with high-purity nitrogen for at least 40 min. Time-correlatedsingle-photon counting (TCSP) method was used to obtain fluorescenceemission decay curves [9]. The excitation source was a Tsunami 3950Spectra Physics titanium–sapphire laser, pumped by a solid stateMilleniaX Spectra Physics laser. The repetition rate of the 5 ps pulses was set to800 kHz using the pulse picker Spectra Physics 3980. The laser wastuned to give an output at 945 nm and a second harmonic generatorLBO crystal (GWN-23PL Spectra Physics) gave the 472 nm excitationpulses that were directed to an Edinburgh FL900 spectrometer, wherethe L-format configuration has allowed the detection of the emission atright angle from the excitation. The emission wavelength was selectedby a monochromator, and the emitted photons were detected by a

2200 2150 2100 2050 2000 1950 1900 1850 1800

A

B

Wavenumber (cm-1)

Fig. 2. FTIR spectra of complexes I (A) and III (B) in CH3CN.

Fig. 3. Optimized structure, gas phase, of complex III (above) and numbered structure(below).

72 I. de Aguiar et al. / Inorganic Chemistry Communications 44 (2014) 70–78

refrigerated Hamamatsu R3809U microchannel plate photomultiplier.The FWHM of the instrument response function was typically 2.20 ns,and measurements were made using time resolution of 0.245 ns perchannel. The software provided by Edinburgh Instruments was used toanalyze the decay curves, and the adequacy of the multi-exponentialdecay fitting was judged by inspection of the plots of weighted residualsand by statistical parameters such as a reduced chi-square.

Theoretical calculations

DFT calculations were performed with the Gaussian 09 (G09)programs packages [10] employing the DFT method with Becke'sthree parameter hybrid functional [11] and Lee-Yang-Parr's gradientcorrected correlation functional (B3LYP) [12]. The LanL2DZ basis set[13] and effective core potential were used for the Ru atom. Theground-state geometries of the complexes were optimized in gas phase.The solvent acetonitrile was included to the calculations through PCM.SCF convergence criteria were used for all optimizations. The electronicanalysis was obtained using TD-DFT calculations with 120 excited states.

Table 1DFT calculated bond lengths (Å) and angles for complex III.

Bonds, Å Angles, °

Ru-N3 (4,4'-bpy) 2.15 N3-Ru-N10 91.94Ru-N10 (4,4'-bpy) 2.15 N4-Mn-N5 91.45Mn-N5 (4,4'-bpy) 2.17 C1-C2-C3-C4 22.30Mn-N4 (4,4'-bpy) 2.14 C5-C6-C7-C8 30.90Ru-N1(phen) 2.13 C9-C10-C11-C12 31.90Ru-N2(phen) 2.10 C13-C14-C15-C16 25.90Mn-N(im1) 2.10Mn-N(im2) 2.12

Synthesis

fac-[Mn(CO)3(4,4′-bpy)2(ImH)]+ (I)Mn(CO)5Br (200 mg, 0.73 mmol) was dissolved in deoxygenated

CH2Cl2 (50 mL), and 4,4′-bpy ligand (1.45 mmol) was added. Thesolution was stirred under dark for 12 h at room temperature. Theyellow precipitate formed was filtrated and dried under vacuum.Upon cooling to room temperature a saturated solution of silver triflatewas added to the solution to precipitate the AgBr whichwas isolated byfiltration. The fac-Mn(CO)3(4,4′-bpy)2(CF3SO3) (0.100 g, 0.16 mmol)was dissolved in deoxygenated CH2Cl2 (40 mL), and imidazole (0.012g, 0.16 mmol) was added. It was stirred in the dark for 12 h. At theend of the reaction the solvent was evaporated under reduced pressureto 10mL and 40mL of hexanewas added to precipitate the solution. Theyellow precipitate formed was dried in vacuum. Yield = 80%. 1H NMR(CD3CN):11.00 (1H, bs, N–H), 9.13 (2H, d), 8.74 (2H,d), 8.70 (2H, d),8.52 (1H, d), 7.78 (3H, t), 7,70 (1H, d), 7.67 (2H, d), 7.51 (1H, s), 7.24(1H, s), 7.18 (3H, m), 6.68 (1H,s). Anal. Calcd for MnC27H20N6O6S3F3:C, 48.51; H, 3.02; N, 12.57%. Found: C, 48.15; H, 3.11; N, 12.49%.

cis-[Ru(phen)2(4,4′-bpy)2]2+ (II)

cis-[Ru(phen)2(OH2)2]∙2H2O (0.200 mg, 0.38 mmol) and 4,4′-bpy(0.075 mg, 0.80 mmol) were refluxed in a 1:1 EtOH/H2O solution for 6h. Upon cooling to room temperature a saturated solution of ammoni-um hexafluorophosphate was added to the solution to precipitate anorange product, which was isolated by filtration. This product waswashed with water and diethylether, and dried in vacuum. 1H NMR(CD3CN): 9.31 (2H, dd), 8.80 (2H, dd), 8.47 (2H, d), 8.45 (4H, m), 8.44(4H, m), 8.40 (8H, m), 8.22 (2H, d), 8.19 (2H, m), 8.11 (2H, d), 7.94(2H, dd), 7.53 (2H, m). Anal. Calcd. for RuC44H32N8P2F12: C, 49.67; H,3.03; N, 10.53%. Found: C, 49.83H, 3.23; N, 10.77%.

cis,fac-[Ru(phen)2(4,4′-bpy)2-Mn(CO)3(ImH)]2(PF6)4(SO3CF3)2 (III)Mn(CO)5Br (74 mg, 0.27 mmol) was dissolved in deoxygenated

CH2Cl2 (50 mL), and the complex cis-[Ru(phen)2(4,4′-bpy)2]2+

(250 mg,0.27 mmol) was added. It was stirred under dark for 12 h atRT. The orange precipitate formedwas filtrated and dried under vacuum.The imidazole ligand, 3 mg (0.044 mmol), was added to a solution of the[Ru(phen)2(4,4′-bpy)2-Mn(CO)3(SO3CF3)]24+ 60 mg (0.044 mmol) inacetone (20 mL) and the resulting red solution was left under stirring inthe dark for 12 h under N2 atmosphere. Acetone was removed undervacuum to 10 mL, then 40 mL of the hexane fresh distilled was addedto precipitate the compound which was filtered off and dried undervacuum. Yield = 80%. 1H NMR (CD3CN): 11.00 (1H, bs, N–H), 10.80(1H, bs, N–H), 9.45 (4H, d), 8.76 (4H, d), 8.57 (8H, m), 8.56 (8H, s),8.44 (8H, d), 8.21 (4H, d), 8.10 (4H, d), 8.04 (4H, d), 7,68 (1H, s), 7.60(1H, s), 7.53 (20H, m), 7.18 (1H, s), 7.17 (1H, s), 6.90 (1H, s), 6.79(1H, s). Anal. Calcd for Ru2Mn2C100H72N20O6P6F36: C, 43.14; H, 2.55; N,9.86%. Found: C, 44.20; H, 2.89; N, 9.90.

The complexes have been identified as follows: fac-[Mn(CO)3(4,4′-bpy)2(ImH)]+ (I); cis-[Ru(phen)2(4,4′-bpy)2]2+ (II); cis,fac-[Ru(phen)2(4,4′-bpy)2-Mn(CO)3(ImH)]26+ (III). A perspectiveview of the structures of compounds with their identificationnumbers are shown in Fig. 1.

Results and discussion

Molecular structure

Complex IIIwas synthesized by reacting Mn(CO)5Br with an equiv-alent amount of cis-[Ru(phen)2(4,4′-bpy)2]2+ followed by substitutionof bromide to triflate. The triflate complex was then substituted byimidazole. Based on related supramolecular structures of 4,4′-bpybridged ruthenium complexes [14] and considering the linear geometryof the individuals components, the formation of a square molecularcomplex is expected. However, all attempts to grow suitable crystals

Fig. 4. Orbital energy diagram and contour surfaces of frontier molecular orbital of complexes I, II and III, in CH3CN.

73I. de Aguiar et al. / Inorganic Chemistry Communications 44 (2014) 70–78

for X-ray characterization so far have failed. Thus, complex III wascharacterized by spectroscopic techniques (1H NMR, FTIR and UV–vis)and theoretical (DFT) studies.

Table 2One-electron energies and percentage composition of selected highest occupied andlowest unoccupied molecular orbitals of complexes I, II and III in CH3CN.

Orbital Energy, eV Orbital composition, %

Complex ILUMO + 6 −1.31 55% 4,4′-bpy, 31% COLUMO + 3 −1.49 55% CO, 28% MnLUMO + 2 −1.58 84% 4,4′-bpyLUMO −2.58 97% 4,4′-bpyHOMO −6.97 61% Mn, 16%CO, 13% imHOMO-2 −7.19 74% Mn, 20% COHOMO-3 −7.40 99% 4,4′-bpyHOMO-5 −7.43 83% im

Complex IILUMO + 7 −1.47 82% phenLUMO + 6 −1.52 99% 4,4′-bpyLUMO + 5 −2.46 27% phen, 67% 4,4′-bpyLUMO + 2 −2.61 93% phenLUMO −2.77 72% phen, 26% 4,4′-bpyHOMO −6.20 76% Ru, 18% phenHOMO-1 −6.30 80% Ru, 13% phenHOMO-2 −6.31 79% Ru, 14% phenHOMO-3 −7.39 91% phenHOMO-5 −7.42 96% 4,4′-bpy

Complex IIILUMO + 7 −2.78 79% phenLUMO + 6 −2.80 82% phenLUMO + 1 −3.12 95% 4,4′-bpyLUMO −3.14 84% 4,4′-bpyHOMO −6.37 74% Ru, 19% phenHOMO-2 −6.49 80% Ru, 13% phenHOMO-4 −6.49 81% Ru, 13% COHOMO-5 −6.5 81% Ru, 13% COHOMO-6 −7.12 69% Mn, 17% COHOMO-7 −7.15 64% Mn, 16% CO, 19% im

The 1H NMR spectrum of complex II in CD3CN solution shows eightproton signals set due to the ring protons of phenanthroline and foursignals set of the 4,4′-bpy protons which is expected for a symmetriccis-octahedral structure. On coordination of Ru(II) complex to Mn(I)moiety, the chemical shifts of the H1,1′ (−0.1); H2,2′ (−0.44); H3,3′(−0.27) and H4,4′ (−0.2) attributed to 4,4′-bpy were shifted tolower frequency, which is expected due to bond formation betweenMn and 4,4′-bpy. The signals corresponding to the phen protonresonances, H2,2′ (+0.30), H4,4′ (+0.14), and H6,6′ (+0.43) werebroadened and showed a shift to higher frequency due to the hinderedrotation and the displacement of electrondensity on the 4,4′-bpy ligand.Further, the phen and 4,4′-bpy integration ratio (1:1) indicates asymmetric tetranuclear complex. No shifts occurred in the remainingparts of the spectrum. The 1H NMR spectrum of I reproduces theresonances of the 4,4′-bpy protons between 8.0 and 9.5 ppm and theimidazole coordinated ligand at 6.0 and 7.0 ppm. The N–H (imidazole)proton is found at 10.8 ppm. The equivalence of both phen and 4,4′-bpy proton signals indicates that the symmetry found in the mononu-clear complexes is kept on coordination of Ru(II) to Mn(I) center.

Consistent with these data, the FTIR spectra of III and I show strongcarbonyl stretching bands corresponding to fac-CO configuration, Fig. 2[15].

The optimized structure of complex III is depicted in Fig. 3 andconfirms the square molecular configuration. The N(bpy)–Ru–N(bpy)and N(bpy)–Mn–N(bpy) angles shown in Table 1 are comparable withthose found for square molecular complexes determined by X-raycrystallography [3]. The distances between the Ru–Ru and Ru–Mnmetal centers are 16.5 and 11.5 Å, respectively, and the distancesbetween the 4,4′-bpy adjacent ligands of 10.0 Å discardπ–π interactionsamong themwhich should increase the electronic coupling between themetal centers [16].

Fig. 4 displays the energies and Mulliken electron density distribu-tion of the HOMO and LUMO orbitals and, Table 2 exhibits the orbitalpopulation calculated for complexes I, II and III. The HOMO in the com-plexes II and III are rutheniumdπ orbitals. The LUMO ismainly centeredon the phen ligand in II but it is located on the bridged 4,4′-bpy in I and

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

-5

0

5

10

15

20

Cur

rent

(μA

)C

urre

nt (

μA)

Cur

rent

(μA

)C

urre

nt (

μA)

Potential (V)

A

0.0 0.4 0.8 1.2 1.6

-10

0

10

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40

50

60

Potential (V)

B

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

-4

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12

Potential (V)

C

0.6 0.8 1.0 1.2 1.4 1.6 1.8-10

-5

0

5

10

15

20 D

Potential (V)

Fig. 5. Cyclic voltammograms in CH3CN solution (vs Ag/AgCl) of complexes a) II, b) I and c, d) III.

300 400 500 6000

10000

20000

ε, m

ol-1

L cm

-1

Wavelength (nm)

I II III

A

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Nor

mal

ized

Wavelength (nm)

0.0

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0.2

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0.4

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0.6

Osc

illat

or S

tren

gth

B

500 550 600 650 700

500

1000

1500

2000

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C

Em

issi

on In

tens

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.u.

500 550 600 650 700 750 8000

20000

40000

60000

80000

100000

120000

D

Em

issi

on In

tens

ity (

a. u

.)

Wavelength (nm)Wavelength, nm

Fig. 6. a) UV–vis electronic absorption spectrum of complexes I, II and III in CH3CN at room temperature; b) simulated and experimental electronic absorption spectrum of complex III inCH3CN. The oscillator strengths are represented by vertical lines; c, d) emission spectrum in CH3CN of complexes II and III.

74 I. de Aguiar et al. / Inorganic Chemistry Communications 44 (2014) 70–78

Table 3Selected TD-DFT calculated lowest-lying singlet excitation energies for complexes II andIII with oscillator strength (O.S.) larger than 0.01.

TD/DFT Experimental

Energy, eV Composition O.S. Energy, eV ε, mol−1 L cm−1

Complex II2.81 HOMO → L + 2 (40%) 0.032 2.81 93002.88 HOMO → LUMO (30%) 0.139

HOMO → L + 2 (38%)3.06 H-1 → L + 3 (50%) 0.094 3.09 10,7003.13 H-1 → L + 5 (38%) 0.289

Complex III2.91 H-4 → L + 7 (29%) 0.300 2.81 10,3002.87 H-5 → L + 6 (33%) 0.2122.80 H-3 → L + 2 (17%) 0.213

H-2 → L + 1 (19%)3.03 HOMO → L + 11 (33%) 0.088 3.09 11,900

H-2 → L + 8 (24%)3.14 H-3 → L + 11 (51%) 0.0463.43 H-7 → LUMO (26%) 0.049 3.58 70003.49 H-9 → L + 1 (25%) 0.0711

H-6 → L + 1 (13%)3.56 H-6 → L + 2 (14%), 0.0194

H-6 → L + 3 (21%)

Table 4Excited state lifetimes (τi, ns) obtained from bi-exponential fit decay profilesa.

Solvent Complex τ1 τ2 %τ1 %τ2 χ2

CH3CN (II) 0.99 ± 0.0023 6.42 ± 0.3021 95.55 4.45 1.13(III) 1.25 ± 0.0014 36.16 ± 0.0024 70.16 29.84 1.06

CH2Cl2 (II) 0.71 ± 0.002 8.21 ± 0.27 91.38 8.62 1.16(III) 0.96 ± 0.0165 12.4 ± 0.293 70.43 29.57 1.39

a I(t) = ∑iαie-t/τI (normalized pre-exponential factor, αi); excitation wavelength was

420 nm.

75I. de Aguiar et al. / Inorganic Chemistry Communications 44 (2014) 70–78

III. Note that the presence of Mn moiety in III leads to a decrease in theenergy of the 4,4′-bpy bridged ligand. As a consequence, the HOMO–LUMO energy gap in III (3.22 eV) is shorter than those obtained for II(3.43 eV) and I (4.35 eV).

In order to support this observation the cyclic voltammograms ofcomplexeswere investigated, Fig. 5. Complex II, displays a redox coupleat E1/2(1)= 1.39 Vwhich is, as expected, more positive than that foundfor [Ru(phen)3]2+ and [RuCl(4,4-bpy)(bpy)2]+.[17] For complex III, it is

0 5 10 15 20 25 30 35 40

CH2Cl

2

Tempo, ns

A

CH3CN

CH2Cl

2

Time (ns)

B

CH3CN

101

102

103

Cou

nts

101

102

103

Cou

nts

0 5 10 15 20 25 30 35 40

Fig. 7. Luminescence time decay of complexes II (A) and III (B) at 295 K in CH3CN andCH2Cl2. Excitation wavelength 420 nm. The decay curves were fitted by bi-exponentialfunction (data in Table 2).

observed a redox couple at E1/2(1) = 0.93 V attributed to Mn(I/II) andone additional irreversible oxidation of Mn(II/III) at + 1.26 V. Thereversible redox couple of Ru(II/III) can be observed at E1/2(2) =1.52 V. In comparisonwith complexes I and II, the cyclic voltammogramof complex III shows a small decrease (by −90 mV) in the oxidationpotential of the MnII unity and an increase in the redox couple for theRuII one (by + 130 mV). This suggests that the Mn moiety exerts anelectron-withdrawing effect, making electron removal from RuII moredifficult, which is in accordance with the 1H NMR shift observed forthe complex III in comparisonwith complexes I and II. Also, the electronwithdrawing properties of 4,4′-bpy suggest that this ligand is the first tobe reduced in complexes II and III. Indeed, the reductive response atnearly −0.96 V observed on the first scan is close to that found for4,4′-bbpy reduction (1−/0) [18].

Absorption and emission spectra

The absorption and emission spectra of the three complexes inacetonitrile solution are shown in Fig. 6A, Table 3. The UV–vis absorptionbands between 200 and 300 nm are mostly π–π* in origin.

For complex I the broad absorptionwithmaximumat 380nmcan beassigned to MLCT (Mn → phen) as revealed by comparison with

300 400 500 600 700-0.12

-0.09

-0.06

-0.03

0.00

0.03

Wavelength (nm)

Wavelength (nm)

A

300 400 500 600 700

-0.10

-0.05

0.00

0.05

0.10

ΔAbs

orba

nce

ΔAbs

orba

nce

B

Fig. 8. Transient absorption spectra of complexes II (a) and III (b) in CH3CN solution uponpulsed laser excitation at 355 nm.

0 500 1000 1500 2000

-0.016

-0.008

0.000

0.008

0.016

a

b

Δ A

bsor

banc

e

Time (s)

Fig. 9. Kinetics decays of complex III in CH3CN (7.0 × 10−5 mol L−1) in the presence ofMV2+ (1.0 × 10−2 mol L−1) recorded at 420 nm (a) and 610 nm (b).

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

A

0.0

0.3

0.6

0.9

1.2

1.5

Abs

orba

nce

B

0.0

0.5

1.0

1.5

2.0

Abs

orba

nce

C

Wavelength (nm)

300 400 500 600 700 800

Wavelength (nm)

300 400 500 600 700 800

Wavelength (nm)

Fig. 10. UV–vis spectral changes of the thermal reaction of a solution containing (a)complex fac-[Mn(CO)3(phen)(ImH)]+, (b) fac-Mn(CO)3(phen)(SO3CF3) and (c) complexI, in presence of MV2+ in pure water after 30 s of continuous irradiation at 355 nm light.

200 300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Abs

orba

nce

Wavelength (nm)

Wavelength (nm)

A

200 300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

B

Fig. 11. Spectral changes during irradiation at 420 nm, in acetonitrile solution ofcomplexes II (a) and III (b).

12 11 10 9 8 7 6

no irraditation

t = 60 min

t = 30 min

A

12 11 10 9 8 7 6

no

irradiated

B

t = 30 min

t = 60 min

δ (ppm)

δ (ppm)

Fig. 12. 1H NMR spectral changes during photolysis at 420 nm of complexes II (a) and III(b) in CD3CN de 420 nm.

76 I. de Aguiar et al. / Inorganic Chemistry Communications 44 (2014) 70–78

77I. de Aguiar et al. / Inorganic Chemistry Communications 44 (2014) 70–78

analogous systems [4]. For complexes II and III the broad and structuredabsorption that appears around 300–500 nm is indicative of overlap-ping MLCT transitions. In fact, the TD-DFT calculated absorption spec-trum of III predicts three distinct but very close in energy MLCTabsorptions at 2.91 eV attributed to Ru → phen, (H-4 81% → L + 779%), 2.88 eV, Ru → 4,4′-bpy, (H-5 81% → L + 6 82%) and 2.80 eV (H-2 80% → L + 1 95%). In addition, MLCT (Mn → 4,4′-bpy) transitionsappear in the region between 3.37 (H-6 69% → L + 1 95%) and3.43 eV (H-7 64% → LUMO 84%). For complex II the major bandsfound in the region of 3.0 eV refers to MLCT, Ru to phen (H-1 → L + 5,3.13 eV) and Ru to 4′4-bpy (HOMO → L + 2, 2.87 eV). As shown inFig. 6B, the calculated and experimental spectra are in good agreement.

The emission spectra of II and III are similar in both spectralstructure and position suggesting a 3MLCT emitting state. Complex I isnot luminescent. The emission decay profile of complexes II and III inCH2Cl2 and CH3CN required a bi-exponential fit suggesting the initialdistribution of two emitting processes, Fig. 7, Table 4. The luminescenceintensity and lifetime of III are slightly intensified compared to complexII.

The shorter emission lifetimes are comparable to ligand fluores-cence whereas the 36 ns emission lifetime, observed in CH3CN, couldbe assigned to either an MLCT excited state or LLCT emissive excitedstate.

In addition, the TD/DFT calculations indicate that excitation in thevisible region is characterized by overlapping charge transitionsMLCT (Ru → phen/4,4′-bpy). The solvent dependence and thechanges in the emission lifetime values and contributions of thetwo components for complex II going to complex III show that anenergy/electron transfer process is operating between the metalcenters.

One possibility is the quenching of the {Ru3+-phen•−} excited stateby a fast energy transfer process from the phen•− to 4,4′-bpy during theemission lifetime.

The electron transfer process was confirmed by nanosecond time-resolved difference absorption spectra. Fig. 8A shows a bleaching ofthe ground state band at 420 nm for complex II and a very weaktransient absorption in the region of 500–600 nm typical of Ru-bpycomplexes. For complex III the bleach at 420 nm is accompanied bytwo apparent maxima occurred at 500 and 640 nm. These absorptionsresemble the anion radical phen•− and the 4,4′-bpy•− [19] suggestingthat the ligand contributions are significant for the emissive excitedstates of complex III.

Overall, the results indicate that theMLCT excitation of II, Ru→ phen,is partially quenched in III by energy transfer and/or electron transferdirect from Ru to 4,4′-bpy π* orbitals or from phen•− to 4,4′-bpy. Thepresence of Mn ion stabilize the charge on the 4,4′-bpy bridged ligand,which leads to an increase in the lifetime of III.

MV2+ to MV+• photo-induced electron transfer

Fig. 9 shows the kinetic profiles of absorption changes at 420 and610 nm. The 610 nm corresponds to the absorptions of the MV•+ and420 nm to the depletion of the ground state MLCT absorption [20]. Inthe case of complex II the excited states are not affected by the presenceof MV2+

, probable due to its short lifetime.The intermolecular electron transfer experiments were also carried

out using a continuous light irradiation, with a 300 W Xe/Hg lamp and420 nm light filter. In these experiments 0.0128 g of MV2+ was addedto a deoxygenated CH3CN solution of complex III (C = 10−4 mol L−1)and the solution was irradiated for 10 s. For complex I a broad absorp-tion band withmaxima at 605 and 394 nm appeared just after stoppingirradiation. These new absorptions match the methyl viologen radical(MV•+) absorption spectrum [20a] (blue solution). The isosbesticconversion at 415 nm and 426 nm attests to the formation of asingle species and probable as ion pairs. This photogenerated specieundergoes a slow thermal back reaction.

For complexes II and III no spectral changes indicating the formationof MV•+ were observed even after exhaustive continuous photolysis. Inthe case of complex III the Mn fragment stabilizes the Ru(II) center andthe electron transfer reaction in the excited states is facilitated on thetime scale of ns.

The reactivity of complex III on the ns time scale shows strongevidence of the reducing ability of photogenerated Mn2+. Thisconclusion is supported by the formation of MV•+ upon irradiationof the fac-[Mn(CO)3(phen)(ImH)]+ and fac-Mn(CO)3(phen)(SO3CF3)complexes in the presence of MV2+, Fig. 10. For the triflate complex,the irradiation results in a permanent chemical change since the MV2+

undergoes an irreversible reduction probably due to the lability ofthe triflate ion. Thus, in comparison to the imidazole complex a fastback reaction (MV•+ → MV2+) is observed, concomitant with theregeneration of the Mn(I) complex. In the case of complex I, the decayof the ion-pair Mn2+–MV+ is apparently a slow process, probably dueto the π → π* interaction between the 4,4′-bpy•− formed, and themethylviologen.

In spite of the photoinduced electron transfer the three complexesundergo photosolvolysis after 30 min continuous photolysis (I0 =1.27 × 10−8 einstein s−1).

Figs. 11 and 12 show the changes in the absorption and 1H NMRspectrum of complexes II and III during continuous light irradiation at420 nm for 60 min. The changes are consistent with 4,4′-bpy releaseand the formation of the photoproduct [21] [Ru(phen)(4,4′-bpy)(CH3CN)]2+.

Our results show that the manganese (I) complexes are goodsystems to participate as electron donor reactants in photoinducedelectron transfer reactions. Although, the presence of Ru(II) has redshifted the absorption, leading to an increase in the absorption proper-ties of manganese complex, and allowing the tetranuclear system toparticipate of intermolecular electron transfer reactions, it did notpreclude the photosolvolysis of the 4,4′-bpy coordinated ligand.

Acknowledgement

The authorswould like to acknowledge FAPESP (Proc. 09/08218-9) forfinancial assistance and, Prof. A. B. P. Lever and Dr. Elaine Dodsworth-Lever for the help with DFT calculation.

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