Photochemical and photophysical properties of ruthenium(II) bis-bipyridine bis-nitrile complexes: Photolability

Inorganica Chimica Acta 363 (2010) 2496–2505

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Photochemical and photophysical properties of ruthenium(II) bis-bipyridine bis-nitrile complexes: Photolability A.J. Cruz a, R. Kirgan a, K. Siam b, P. Heiland a, D.P. Rillema a,* a b

Department of Chemistry, Wichita State University, Wichita, KS 67260, USA Department of Chemistry, Pittsburg State University, Pittsburg, KS 66762, USA

a r t i c l e

i n f o

Article history: Received 22 January 2009 Received in revised form 30 March 2010 Accepted 8 April 2010 Available online 11 May 2010 Keywords: Ruthenium(II) bis(bipyridine) complexes Photosubstitution Density functional theory Electronic spectra Synthesis bis-Nitrile complexes

a b s t r a c t The electrochemical and photophysical properties of two bis-nitrilo ruthenium(II) complexes formulated as [Ru(bpy)2(L)2](PF6)2, where bpy is 2,20 -bipyridine and L is AN = CH3CN and sn = NC–CH2CH2–CN, have been investigated. Electrochemical data are typical of Ru-bpy complexes with two reversible reduction peaks located near 1.3 and 1.6 V assigned to each bipyridine ligand and one RuII/RuIII oxidation wave centered at approximately +1.5 V. The sn derivative is both IR and Raman active with its coordinated CN stretch appearing at 2277 cm1 and 2273 cm1, respectively. The UV/Vis absorption spectrum of the sn derivative is dominated by an intense (emax  58700 M1 cm1) absorption band at 287 nm assigned as a LC (p ? p*) transition. The peak observed at 418 nm (e  10 400 M1 cm1) is an MLCT band while the one at 244 nm (e  23 600 M1 cm1) is of LMLCT character. The AN derivative behaves similarly. Both complexes show low-temperature emission at around 537 nm with a lifetime near 10.0 ls. 1H and 13C assignments are consistent with the formulation of the complexes. The complexes undergo photosubstitution of solvent with quantum efficiencies near one. Calculated and experimental results support replacement of the nitrile ligands by solvent. Based on DFT calculations, the electron density of the HOMO lies on the metal center, the bipyridine ligands and the nitrile ligands and electron density of the LUMO resides primarily on the bipyridine ligands. The electronic spectra obtained from TDDFT calculations closely match the experimental ones. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Many reports have been written about ruthenium(II) tris-bipyridine complexes and derivatives due to interest in their photophysical properties related to solar energy conversion, catalysis and photoinduced chemical reactions [1–3]. The primary driving force in these studies has been the exploitation of excited state redox processes, although interest in photochemical and thermal chelate exchange reactions involving Ru(bpy)32+, where bpy is 2,20 -bipyridine, and derivatives have been reported where one moiety is expelled from the primary coordination sphere and replaced by another [4–9]. The extent of these photosubstitution reactions depends on the lability of these ligands under light conditions. For example, ruthenium(II) complexes involving diimine chelates have been known to undergo photosubstitution reactions and have been investigated using electrospray ionization mass spectrometry [10]. Brown has reported thermal and light-induced decomposition of Ru(bpy)2(N3)2+ in CH3CN [11]. Bonneson et al. demonstrated light-driven nitrile substitution in complexes involving 1,10-phenanthroline (phen) chelating ligands [12]. Baranoff et al. have also * Corresponding author. E-mail address: [email protected] (D.P. Rillema). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.04.014

reported ruthenium complexes with photolabile ligands based on bipyridine derivatives [9]. In this paper, we report an investigation of the photolability of [Ru(bpy)2(AN)2]2+ and [Ru(bpy)2(sn)2]2+, where AN = CH3CN and sn = NC–CH2–CH2–CN, in various solvents. [Ru(bpy)2(AN)2]2+ has been known as an intermediate in coordination synthesis but to our knowledge has not been investigated regarding its photolability [1,4,11–20]. This study gives new insight into solvent photosubstitution reactions in bis-nitrilo complexes of ruthenium(II) which can be useful as intermediates in coordination synthesis as well as in the growing field of ‘‘molecular machines” [5–8]. 2. Experimental 2.1. Materials Ruthenium(II) bis-2,20 -bipyridine dichloride hydrate, [Ru (bpy)2Cl2H2O], ruthenium(II) bis-2,20 -bipyridine carbonate, [Ru(bpy)2CO3] [21–24], and ruthenium(II) bis-2,20 -bipyridine bisacetonitrile hexafluorophosphate, [Ru(bpy)2(CH3CN)2](PF6)2, were synthesized using published procedures [18]. The succinonitrile ligand was purchased from Aldrich and used as received. Diethyl ether, HPLC grade methanol and optima grade acetonitrile were

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505

purchased from Fisher Scientific. Ammonium hexafluorophosphate (NH4PF6), IR grade potassium bromide, deuterated dimethyl sulfoxide (DMSO) and trifluoromethanesulfonic acid (99%) in a vacuum sealed glass container were obtained from Aldrich. Sulfur powder was provided by Thermo Nicolet. Electrochemical grade tetrabutylammonium perchlorate was purchased from Southwestern Analytical. Ferrocene standard and dry acetonitrile in a seal tight bottle were purchased from Aldrich. Butyronitrile used for emission and lifetime studies was obtained from Acros; it was fractionally distilled prior to usage. Potassium ferrioxalate used in chemical actinometry was synthesized using published procedures [25–30]. Deuterated methanol used in photolysis studies were purchased from Cambridge Isotope Laboratories, Inc.

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tolysis consisted of a 100 W xenon lamp (Oriel) and a monochrometer. The light was directed into the cavity of the HP8452A Diode Array spectrophotometer. The solution containing the compound of interest was irradiated in a 1 cm cuvette at 405 nm, magnetically stirred, and its absorbance was observed at right angles to the irradiating light. The intensity of the light source at the wavelength of excitation was determined using potassium ferrioxalate as the actinometer following published procedures [25]. Photolysis progress was monitored using UV–Vis spectroscopy and the absolute quantum yields of photolysis were determined in various solvents. Photosubstitution of methanol (CD3OD) was monitored using UV–Vis and 1H NMR spectroscopy. 2.3. Calculations

2.2. Measurements Infrared spectra were obtained using Perkin–Elmer Model 1600 FT-IR and Nicolet Avatar Model FT-IR Spectrophotometers. All samples were prepared as potassium bromide pellets. Raman spectra were obtained using a Thermo Nicolet Nexus Spectrophotometer with a FT-Raman Module which was calibrated with sulfur. The Nicolet instruments were accompanied by Omni software programs. Proton 1H-, 13C NMR spectra and HETCOR spectra were obtained using Varian Mercury 300 MHz and Varian Inova 400 MHz Fourier Transform-NMR spectrometers (internal standard TMS). Ultraviolet spectra were obtained using a Hewlett–Packard Model 8452A diode array spectrophotometer interfaced with an OLIS software program. Elemental (C, H, & N) analysis was performed by MHW Laboratories. An EG&G PAR Model 263A potentiostat/galvanostat was used to obtain the cyclic voltammograms. All CV measurements were carried out in a typical H-electrochemical cell using a platinum disk working electrode, polished every run, and a platinum wire counter electrode. A Ag/AgNO3 electrode freshly made from AgNO3 in dry CH3CN served as the reference for this study. The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate. Ferrocene was added as an internal reference. The sample preparation for emission studies involved dissolving a small amount of sample in distilled butyronitrile and then measuring the absorbance of the solution. The concentration of the solution was altered in order to achieve an approximate absorbance of 0.10 at 418 nm of the sample. The sample solution was placed in a fluorescent quartz tube and was degassed (minimum of five times) prior to actual measurements. Emission quantum yields were calculated using Eq. (1) [31], where uf is the emission quantum yield, If is the emission intensity and A is the absorbance of the sample at the excitation frequency.

"

logðuf Þ ¼ 0:919  log

If ð1  10ÞA

#

 7:934

ð1Þ

All samples were degassed using the freeze-pump-thaw method a minimum of five times prior to measurements. Sample tubes for 77 K measurements were immersed in a quartz finger Dewar containing liquid N2. The Dewar was mounted in the center of the sample compartment of a SPEX 212 Fluorolog Fluorometer which was modified to allow dry N2 gas to flow over the finger of the Dewar in order to minimize condensation of moisture from the air. The sample was held in place by a series of rubber rings located near the top and middle of the sample tube. This ensured that the tube was centered in all directions. Accurate results were obtained with the sample centered in the excitation beam and with a frost-free finger. Great efforts were made to ensure the maximum emission intensity was obtained from the sample. The excited state lifetimes were determined by exciting the sample at the third harmonic of a Continuum Surlite Nd:YAG laser run at 20 mJ/10 ns pulse. Instrumentation for steady state pho-

GAUSSIAN ’03 (Rev. B.03) software [32] for UNIX was used for calculations. The molecules were optimized using Becke’s threeparameter hybrid functional B3LYP [33] with the local term of Vosko, Wilk, and Nassiar. The basis set SDD [34] was chosen for all atoms and the geometry optimizations were all ran in the gas phase. TDDFT [35] calculations were employed to produce a number of singlet excited states in the gas phase based on the optimized geometry. All oscillator values and singlet and triplet excited state values are presented in the supporting information. All vibrational analyses revealed no negative frequencies and were run in the gas phase only.

2.4. Preparation of [Ru(bpy)2CO3] Ru(bpy)2Cl2H2O (2.00 g, 3.70 mmol) was suspended in 150.0 ml of deaerated water in a 100-mL round-bottom flask. It was heated at reflux under Ar for 15 min. After reflux, excess amount of sodium carbonate monohydrate (7.8 g, 63 mmol) was added into solution and the mixture was allowed to reflux for another 2 h. The reaction was completed; the solution was allowed to cool down in an ice bath. The black solid was vacuum filtered and oven-dried at 60 °C overnight. Color: Black. Yield 74%. IR (KBr pellet, cm1): 3411 br m, 1558 s, 1643m, 1462m, 1425m, 1255 w, 766m 1H NMR (d6-DMSO): d ppm 7.150 (t, 2H), 7.478 (d, 2H), 7.749 (t, 2H), 7.881 (t, 2H), 8.151 (t, 2H), 8.627 (d, 2H), 8.789 (d, 2H), 9.198 (d, 2H). 2.5. Preparation of [Ru(bpy)2(AN)2](PF6)2 Ru(bpy)2CO3 (102.00 mg, 0.237 mmol) was dissolved in 20.0 mL of dried CH3CN and was placed in a 100-mL round-bottom flask. The mixture was allowed to stir at room temperature for about 15 min under Ar conditions. Then, 0.10 mL of trifluoromethanesulfonic acid was added into the mixture. Evolution of gas was observed and the color of the solution immediately changed into a yellow-orange tint. The reaction was set to reflux overnight and the solvent was allowed to evaporate to about 2 mL. The remaining solution was then added dropwise into a saturated aqueous solution of ammonium hexafluorophosphate (NH4PF6). The orange precipitate was collected by vacuum filtration and was dried inside the vacuum oven (40°) for 1 day. Color: Orange. Yield 85%. UV/Vis: kmax = 420 nm. IR (KBr pellet, cm1): 1605 w, 1467 w, 1448 w, 1266 s, 1224m, 1147m, 1029m, 763m, 729 w, 637m, 572 w, 517 w. 1H NMR (d6-DMSO): 2.95 (s, 4H), 7.30 (t, 2H), 7.59 (d, 2H), 7.93 (t, 2H), 8.02 (t, 2H), 8.38 (t, 2H), 8.64 (d, 2H), 8.82 (d, 2H), 9.34 (d, 2H). 2.6. Preparation of [Ru(bpy)2(sn)2](PF6)2 Ru(bpy)2CO3 (105.00 mg, 0.230 mmol) was placed in a 100-mL round-bottom flask and dissolved in 20.0 mL of methanol. The pur-

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ple solution was allowed to stir under argon at room temperature for another 30 min. Then 0.10 mL of trifluoromethanesulfonic acid was added. The solution turned into a burnt-orange color and evolution of carbon dioxide gas was observed. While the solution was allowed to stir for 3 h, the reaction was monitored using UV–Vis spectroscopy until no spectral shift was observed. After the reaction was complete, succinonitrile (60.2 mg, 0.750 mmol) was added and the solution was allowed to stir under reflux overnight. The solution was evaporated to approximately 2 mL and the desired complex was allowed to precipitate from a saturated aqueous ammonium hexafluorophosphate (NH4PF6) solution. It was vacuum filtered to isolate the compound. The orange complex was washed with diethyl ether to remove excess succinonitrile. The complex was placed inside the vacuum oven (45 °C) and was allowed to dry overnight. Color: Yellow-orange. Yield: 90%. Anal. Calc. for RuC24H20N6PF6: C, 38.92; H, 2.80; N, 12.97. Found: C, 39.13; H, 2.61; N, 12.75%. IR (KBr pellet, cm1): 3088m, 2967m, 2277m, 2252m, 1737m, 1606 s, 1468 s, 1448 s, 1426 s, 1315m, 1262 br s, 1160 s, 1031 s, 965 w, 836 br s, 762 s, 730 s, 638 s, 557 s, 517 w, 421 w. Raman (Solid, cm1): 3091 w, 2948 w, 2273 s, 1603 m, 1561 w, 1489 w, 1313 w, 765 w, 717 w, 648 w, 450 w. 1H NMR (d6-DMSO): d ppm 2.84 (t, 4H, J = 5.7 Hz), 3.21 (t, 4H, J = 6.0 Hz), 7.37 (ddd, 2H, J = 0.90 Hz), 7.58 (d, 2H, J = 4.5 Hz), 7.90 (ddd, 2H, J = 1.5 Hz), 8.04 (ddd, 2H, J = 1.5 Hz), 8.35 (ddd, 2H, J = 1.0 Hz), 8.68 (d, 2H, J = 7.8 Hz), 8.82 (d, 2H, J = 7.8 Hz), 9.35 (d, 2H, J = 5.1) 13C NMR (d6-DMSO): d ppm 14.4, 16.7, 117.7, 124.0, 124.5, 126.8, 127.4, 128.2, 138.6, 139.1, 152.5, 153.8, 157.4, 158.3.

3.3. Calculations (DFT/TDDFT) Fig. 3 shows pictures of the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of [Ru(bpy)2(sn)2](PF6)2. The HOMO is more metallic in character compared to the LUMO which has more ligand-centered character. As listed in Table 2, the HOMO contains 75% metal, 14% bpy and 10% sn character while the LUMO contains 2% metal, 98% bpy and 1% sn character. The electron density of the HOMO is concentrated primarily on Ru with a small amount on bpy and CN while the electron density on the LUMO dominantly resides on the two bipyridine ligands. The HOMO-LUMO transition occurs from the metal to the ligand (bpy). The molecular orbital energy diagrams for the singlet and triplet states of [Ru(bpy)2(sn)2]2+ are shown in Fig. 4 and energies are listed in the supplementary material sections S2a and S2b. Singlet-energy calculations results showed that the low-lying unoccupied energy states are more of bipyridyl type. The succinonitrile orbitals lie at much higher energies. HOMO, HOMO-1and HOMO-2 energy levels have most of the electron density lying on the ruthenium. Lower HOMOs, on the other hand, have electron density concentrated on the bipyridyl and succinonitrile ligands. DFT calculations of the triplet state reveal that the low-lying triplet states are of 3MLLCT and 3LC character (Fig. 4).

3.4. Vibrational spectroscopy 3. Results and discussion 3.1. Synthesis The reaction scheme for the preparation of [Ru(bpy)2(sn)2](PF6)2 is shown in Fig. 1. The first step involved preparation of [Ru(bpy)2CO3] from Ru(bpy)2Cl2H2O. The carbonato complex was reacted with equivalent amounts of trifluoromethanesulfonic (triflic) acid to form the bis-triflate complex and carbon dioxide. Step three involved the replacement of the triflate ligands with succinonitrile in the dark.

3.2. Structure The optimized, calculated structure of [Ru(bpy)2(sn)2](PF6)2 is shown in Fig. 2 and the Cartesian coordinates are located in the Supplementary materials section, S1a. The atoms in blue are nitrogen atoms, six of which are coordinated to the metal center. The calculated Ru–N bond lengths in [Ru(bpy)2(sn)2](PF6)2 are listed in Table 1 and are compared to experimental and calculated bond lengths determined for [Ru(bpy)2(AN)2](PF6)2. Both of the calculated Ru–N (–N„C–) bond distances were the same with values of 2.045 Å and both bipyridine rings are equivalent in the optimized structure with average calculated Ru–N (bpy) bond distances of 2.078 Å and 2.098 Å. The shorter Ru–N (bpy) bond distance, 2.078 Å, was from Ru to the pyridine unit cis to the nitrile ligand; the long Ru–N (bpy) bond distance, 2.098 Å, was from Ru to the pyridine unit trans to the nitrile ligand. Shorter bond distances were found between the metal center and nitrile groups compared to the bond distance between the metal and the bipyridyl groups. The optimized structure showed slightly twisted bipyridyl rings, 1°, with one nitrogen atom closer to the ruthenium center than the other. Experimental bond distances [36] in the acetonitrile adduct were shorter than the ones calculated as reported for most optimized structures which are evaluated in the gas phase.

Both infrared and Raman data were obtained where interest was focused on the CN group. Data are listed in Table 3 and the experimental spectra are shown in Fig. 5. The CN vibration of the free succinonitrile ligand in the infrared region was observed at 2254 cm1 whereas two CN stretching vibrations were observed for the sn complex. None were found for [Ru(bpy)2(AN)2]2+. The vibrational frequency found at 2277 cm1 for the sn complex was assigned to the metal-bound nitrile group, while the one observed at 2252 cm1 was attributed to the uncoordinated nitrile group by comparison to the free ligand. The vibrational frequency of the CN group in the Raman spectrum for [Ru(bpy)2(sn)2]2+ was observed at 2273 cm1 and not resolved into two components; none was found for [Ru(bpy)2(AN)2]2+ due to decomposition of the complex in the laser beam. The number of scans was lowered due to sample decomposition upon prolonged exposure to laser source. As a result, the observed intensity of Raman scattering was weak and the aperture was opened completely giving rise to a less resolved peak. DFT calculations were used to simulate the infrared and Raman spectra of both the sn and AN derivatives. The simulated spectra for the sn derivative are shown in Fig. 6. The nitrile stretches are doublets of doublets, and although the splittings of each doublet are small, the two sn ligands occupy two different sites in the molecule which may account for the origin of the doublet. These splittings were not observed in the experimental spectrum, perhaps due to line broadening in the solid state. The high energy absorption at 2311 cm1 and the low energy one located at 2296 cm1 are assigned to bound and unbound portion of the nitrile groups, respectively, as found experimentally. Two very weak infrared vibrations located at 2325 and 2318 cm1 were calculated for the AN complex and the calculated Raman vibration was located at the same frequency as found for the sn derivative. The weak vibrations of the AN complex are consistent with the lack of observing a vibration experimentally. The phase calculations are 20–30 cm1 higher in energy than found experimentally for both the infrared and Raman results. Calculations in the gas phase may account for this observation.

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N N

Ru

N

Cl

.

H2O

Cl

N

O

N

Na2CO3, MeOH reflux (overnight), Ar

Ru

+

O

N

2 NaCl (aq)

O

N

N

(I)

(II)

N

N

O

N

Ru N

triflic acid, MeOH O

O

N

Ru

reflux (3 hrs), Ar

SO3CF3 +

CO2(

)

+ H2O

SO3CF3

N

N

N

(II)

(III)

N N

Ru

SO3CF3

2 ( NC

N

CN )

SO3CF3 reflux (overnight), Ar

N

NC

N

Ru

N

N

NC

CN +2

CF3SO3- (aq)

CN

N

(III)

(IV) Fig. 1. Preparation of [Ru(bpy)2(sn)2](PF6)2 from [Ru(bpy)2CO3].

Table 1 Selected bond lengths (Å) in [Ru(bpy)2(sn)2](PF6)2. Bond Ru–N Ru–N Ru–N Ru–N Ru–N Ru–N a b c

Fig. 2. Optimized structure of [Ru(bpy)2(sn)2]2+.

3.5. NMR behavior The 1H NMR spectrum of the metal complex showed all eight chemically distinct bipyridyl protons compared to four for the free bipyridyl ligands. The chemical shift assignments are tabulated in Table 4 according to the proton designations given in Fig. 7. All chemical shift assignments were based on experiments (coupling

(bpy)b (bpy)c (bpy)c (bpy)b (–N„C–) (–N„C–)

Calculated (sn)

Calculated (parent)

Experimental (parent)a

2.098 2.079 2.078 2.098 2.045 2.045

2.095 2.078 2.077 2.095 2.049 2.049

2.061 2.052 2.051 2.059 2.040 2.045

Ref. [36], parent = [Ru(bpy)2(AN)2]2+. Ru–N trans to nitrile. Ru–N cis to nitrile.

constants and multiplicity), chemical environment. Results show that the two coordinated bipyridyl ligands have distinct pyridine rings on each ligand. While Ha and Hg experience downfield chemical shifts due to ring current effects on the bipyridyl rings, Ha undergoes more deshielding compared to Hg since it resides perpendicular to the CN triple bond which has its magnetic current parallel to the bonding axis. Thus, the chemical shift observed furthest downfield at around d = 9.35 ppm was assigned to the Ha (Ha0 ) protons while the one observed furthest upfield at around d = 7.37 ppm was assigned to Hg (Hg0 ) protons. The succinonitrile methylene proton signals (triplet) were observed at around d = 3.81 before and at 2.81 ppm after coordination. Heteronuclear chemical shift correlation spectroscopy was performed to support the carbon nuclei chemical shift assignments listed in Table 4 following the numbering system in Fig. 7. The 13 C peak for the uncoordinated CN peak was located at

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Fig. 3. Molecular orbital picture of [Ru(bpy)2(sn)2]2+: (a) highest occupied molecular orbital (HOMO) and (b) lowest-unoccupied molecular orbital (LUMO).

Table 2 Percent contributions of each group in HOMO and LUMO in [Ru(bpy)2(sn)2]2+. Energy level

Ru

bpy1

bpy2

sn

HOMO LUMO

75 2

7 51

7 47

10 1

d = 117 ppm while peak for the metal-bound CN group was found at 126 ppm. Two peaks (C5/C6) observed furthest downfield were assigned to the quaternary carbons of the bipyridyl rings. Upon coordination, the two methylene carbon atoms of the succinonitrile ligand are now distinct with two C13 peaks located at d = 14 and 16 ppm. 3.6. Cyclic voltammograms The CV electrochemical data are listed in Table 5 and the cyclic voltammograms is shown in the supplementary information sec-40 -45

tion S3. The CV profile is typical of ruthenium(II) bis-bipyridine complexes. Two reversible peaks were observed at negative potentials for the succinonitrile derivative. The first peak was observed at half-wave potential (E1/2) of 1.33 V while the second one was located at 1.53 V. One reversible peak was prominent at positive potentials. It was observed at half-wave potential (E1/2) of +1.54 V. Chattopadhyay et al. studied the electrochemical behavior of [Ru(bpy)2(AN)2]2+ using cyclic and differential pulse voltammetry [36]. Their studies showed that the each bipyridine ligands underwent one-electron reduction in an irreversible manner at 1.45 V and 1.6 V while two oxidation processes were observed, one irreversible process at 1.23 V and a quasi-reversible wave at +1.45 V. The CV data of [Ru(bpy)2(CH3CN)2]2+ in our lab showed two reversible bipyridine ligand reductions at E1/2 = 1.36 and 1.57 V and a reversible oxidation of the [RuIII/RuII] couple at +1.45 V consistent with a previous report [11]. Compared to [Ru(bpy)2(CH3CN)2]2+, the ligand reductions of [Ru(bpy)2(sn)2]2+ 32.5

a

(+8) Sn (+6) Sn

-50

ILCT

(+4) Bpy

LMLCT

25.0 3

3

-60 (+1) Bpy

(L) Bpy

-65

Gap = 28.9

-70

(-2) Ru

-95

(H) Ru

(-1) Ru

-100 (-3) Bpy

(-4) Bpy

-110

dd

3

dd MLLCT

22.5

3

3

ILCT

20.0 4 3 2

-105 (-6) Sn

LMLCT

3

-1

(+5) Bpy (+2) Bpy

Wavenumbers (x10 cm )

-1

(+3) Bpy

3

3

27.5

-55

Wavenumbers (cm )

b

30.0

(+7) Sn

(-5) Sn (-8) Sn

(-7) Sn

1 1

G.S.

0

-115

Fig. 4. Molecular Orbital Energy Diagram in [Ru(bpy)2(sn)2](PF6)2: (a) singlet states and (b) triplet states.

Table 3 Vibrational frequencies of metal-bound CN groups (cm1). Compound

[Ru(bpy)2(sn)2]2+ [Ru(bpy)2(AN)2]2+ a b c

KBr pellet. Solid in tube. Solution (CCl4).

Infrared

Raman

Calculated

Experimental

Experimental free ligand

Calculated

Experimental

Experimental free ligand

2311 2296 2325 2318

2277a (Ru–NC) 2252 (C–CN) NA

2254

2311 2296 2325 2318

2273b (Ru–NC)

2189c

Decomposition

NA

2200

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a

1.0

b

100

Raman Intensity

% Transmittance

80

60

40

0.5

20

0 2500

2400

2300

2200

2100

0.0 2500

2000

Wavenumbers (cm-1)

20 00 -1

Raman Shift (cm )

Fig. 5. Vibrational spectra of [Ru(bpy)2(sn)2](PF6)2: (a) IR and (b) Raman.

a

b

0.0

0.2

Ru-NC Stretch 2311 2303

C-CN Stretch 2296 2296

1.0

0.8 Ru-NC Stretch 2311 2303

0.4

0.6

0.6

0.4

0.8

0.2

1.0 3000

2500

2000

Wavenumbers

0.0 3000

C-CN Stretch 2296 2296

2500

2000

Wavenumbers

Fig. 6. Calculated DFT (a) infrared spectrum and (b) Raman spectrum of [Ru(bpy)2(sn)2](PF6)2.

occurred at similar potentials, but its oxidation was more positive by 0.09 V. 3.7. Electronic spectra Fig. 8 shows an overlay of the experimental and calculated UV/ Vis absorption spectrum of [Ru(bpy)2(sn)2]2+ in acetonitrile. UV/Vis absorption spectra were weakly solvent dependent with the low energy maximum shifting as follows: acetonitrile, 418 nm; butyronitrile, 418 nm; methanol, 422 nm; i-propanol; 424 nm; DMF, 427 nm and DMSO, 427 nm. The absorption coefficients of the transitions for the experimental spectrum obtained in acetonitrile were determined from Beer’s Law studies using at least five dilution points and are listed in Table 6. The probable assignments of the experimental bands were based on computational assignments of the singlet excited states and related reports of similar type of complexes [37–39]. These assignments are listed in Table 6. There is a close match between the electronic absorption of the [Ru(bpy)2(sn)2]2+ with the one for [Ru(bpy)2(AN)2]2+. Both have a MLCT transition located near 420 nm and a LC transition near 288 nm. The absorption coefficients at 410 nm differ with a slightly lower e value of 9200 M1 cm1 for [Ru(bpy)2(sn)2]2+ compared to 10,400 M1 cm1 for [Ru(bpy)2(AN)2]2+.

Calculated singlet energy state transitions for [Ru(bpy)2(sn)2](PF6)2 using TDDFT calculations are tabulated in Table 7 and the calculated absorption spectrum is shown in Fig. 9 along with the major orbitals involved in the transitions and their percentage contributions to the transition. Calculations gave rise to four prominent bands in the absorption spectrum centered at 402, 309, 273 and 242 nm. The 402 nm band is a combination of H-2 ? L (405 nm, f = 0.112) and H-2 ? L +1 (390 nm, f = 0.044) that are primarily MLCT (Ru ? bpy) in nature. The band found at 309 nm is a combination of contributions from the calculated 310 and 308 nm bands which have oscillator strengths of 0.066 and 0.043, respectively. The 310 and 308 nm bands consist of several transitions primarily MLCT in character, although one of the contributors at 308 nm is a dd transition. The 276 nm band is composed of three components which have oscillator strengths of 0.202 while the one at 243 nm contains only one component which has oscillator strength of 0.069. These transitions mainly involve the bipyridine ligands corresponding to intraligand p ? p* transitions (ILCT) although one of the contributors at 276 nm is a dd transition. The succinonitrile ligands make little contribution to these transitions. Experimentally, only three major bands were observed. The band at 244 nm (41.0  103 cm1) was given an ILCT assignment.

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Table 4 Experimental 1H NMR chemical shifts (ppm) of bpy and sn ligands in [Ru(bpy)2(sn)2]2+.a

a

Proton type

Chemical shift (experimental)

Carbon type

Chemical shift (experimental)

Ha Ha0 Hb Hb0 Hc Hc0 Hd Hd0 He He0 Hf Hf0 Hg Hg0 Hh Hh0 Hi Hi0 Hj Hj0

9.35 (2H)

C1/C10

153.82

7.90 (2H)

C2

128.19

8.35 (2H)

C3

138.69

8.82 (2H)

C4

124.44

8.68 (2H)

C5/C6

157.35

8.04 (2H)

C6/C5

158.35

7.37 (2H)

C7

124.10

7.58 (2H)

C8

139.10

3.21 (4H)

C9

127.38

2.80 (4H)

C10/C1

152.47

C11 C12 C13 C14

126.82 16.70 14.35 117.65

Table 5 Electrochemical data of the [Ru(bpy)2(sn)2]2+ complex. Compound 2+

[Ru(bpy)2(sn)2] [Ru(bpy)2(AN)2]2+

E1/2 (V)oxa

E1/2 (V)red

1.54 1.45

1.33 1.36

1.53 1.57

a Electrode potentials in volts vs. Ag/AgNO3, corrected with the ferrocene/ferrocinium couple. Solvent: dried acetonitrile.

1.0

Absorbance

200

400

500

600

Fig. 8. An overlay of the experimental and calculated UV/Vis absorption spectrum of [Ru(bpy)2(sn)2]2+.

C3

Hd C4

Hb C2 Hi

C7 C6

C9

N

Hi

C1

C5

C8

N

N

C12

Ha

14

C13 Hj

3.8. Emission spectra

Hj

C10

Ru

Hh' C10' C9' C8'

N C6'

N C5'

C7'

Hj'

N Ha'

C12' Hi'

He'

C4' Hd'

Hj' C13'

C11'

C1'

Hi'

C 14'

N

C2' C3'

Hb'

Hc' Fig. 7. Scheme for 1H and

13

and the LUMO is ligand based. The majority of HOMO-2 excited state electron density lies on the dz2 orbital.

C

C11 N

Hh

Hf'

300

Wavelength (nm)

He

Hg'

0.4

0.0

Hc

Hg

0.6

0.2

Solvent: DMSO.

Hf

Rusn calculated Rusn experimental

0.8

C assignments.

Calculations showed that the 244 nm transition observed experimentally occurs upon exciting an electron from the HOMO-4 to LUMO+2 (Fig. 9). This ILCT transition is predominantly ligand-centered (LC) with some metal orbitals involved in the LUMO. The ligand character is centered on the bipyridine ligands with minor contributions from the succinonitrile ligand. The intense band found experimentally at 287 nm (34.8  103 cm1) is primarily ILCT derived from the calculated 273 nm bands with a contribution from the calculated 309 nm band which appears in the simulated spectra as a shoulder on the ligand-centered transition. The band located at 418 nm (23.9  103 cm1) was assigned as a MLCT band which occurs from the HOMO-2 to LUMO, HOMO to LUMO+5 and HOMO-1 to LUMO+3. These HOMO’s are primarily metal based

The emission properties and excited state lifetimes (sem) of [Ru(bpy)2(sn)2]2+ were determined at 77 K in freshly distilled butyronitrile. The value of the emission lifetime was determined by curve-fitting analysis. The low-temperature (77 K) emission data are summarized in Table 8 and the spectrum is shown in Fig. 10a. The emission band of the complex had its peak at 537 nm with vibronic peaks located at 576 nm and 625 nm, respectively. The emission lifetime of the compound was 10 ls at 77 K. The excitation spectrum was determined from the emission maximum located at 537 nm and is shown as an inset in Fig. 10b. It mirrors the MLCT assignments obtained by DFT and TDDFT calculations. DFT calculations of the triplet excited states relative to the ground state were performed. Fig. 4 shows the energy diagram of the first seven low-lying triplet states of the complex. The energy difference between the singlet ground state (1G.S.) and the triplet

Table 6 Experimental electronic transitions and calculated excited-states of [Ru(bpy)2(sn)2]2+ and [Ru(AN)2]2+. Compound

Eexp (nm) (k, 103 cm1)a

e

[Ru(bpy)2(sn)2]2+

244 (41.0) 287 (34.8) 418 (23.9) 242 283 420

23565 58709 10420 18900 58700 9200

[Ru(bpy)2(AN)2]2+

a

Solvent: acetonitrile.

Ecalc Assignments (M1 cm1) (103 cm1) 41.3 36.6 24.9 41.5 36.6 24.4

LC (p ? p*) LC (p ? p*) MLCT LC (p ? p*) LC (p ? p*) MLCT

2503

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505 Table 7 Calculated singlet energy state transitions for [Ru(bpy)2(sn)2]2+. Wavelength k (nm)

W0 ? WE

Contribution (%)

Type

Osc. Strength f

Nature of transition

405 390 310

H-2 ? L H-2 ? L+1 H ? L+5 H-1 ? L+3 H-2 ? L+13 H ? L+5 H-1 ? L+3 H-1 ? L+14 H-2 ? L+13 H ? L+14 H-1 ? L+13 H-4 ? L+1 H-4 ? L+2

80 70 32 15 13 14 27 21 11 22 28 4 73

MLCT MLCT MLCT MLCT MMLCT MLCT MLCT dd MMLCT dd ILCT ILCT ILCT

0.112 0.044 0.066 ‘‘ ‘‘ 0.043 ‘‘ ‘‘ ‘‘ 0.202 ‘‘ ‘‘ 0.069

Ru(80) ? bpy(100) Ru(80) ? bpy(100) Ru(80) ? bpy(100) Ru(80) ? bpy(100) Ru(76) ? Ru(54) bpy(31) sn(15) Ru(75) ? bpy(100) Ru(80) ? bpy(100) Ru(80) ? Ru(70) Ru(76) ? Ru(54) bpy(31) sn(15) Ru(75) ? Ru(70) bpy(100) ? bpy(100) bpy(100) ? bpy(100) bpy(100) ? bpy(98)

308

276

243

L

H-2

H

L+5

A MLCT

B MLCT

L+3

H-1

A nm (3.08 eV) A==402402 nm (3.0 B nm (4.01 eV) B==309309 nm (4.0 C

Extinction Coefficient

60000

B MLCT

C nm (4.54 eV) C==273273 nm (4.5 D nm (5.12 eV) D==242242 nm (5.1

50000 H-4

L+1

40000 C p ? p*

30000 20000

D

B

A

0 200

L+2

H-4

10000

D 250

300

350

400

450

LMLCT

500

Wavelength (nm) Fig. 9. Calculated UV/Vis spectrum and MO pictures involved in electronic transitions in [Ru(bpy)2(sn)2](PF6)2.

metal-to-ligand charge-transfer excited state (3MLCT) is about 2.63 eV (475 nm). The experimentally observed maximum is at 538 nm, Fig. 10a – a difference of 63 nm. The calculation most likely is inaccurate since the role of solvent has not been taken into account. The 77 K lifetime of the luminescence was observed at around 10 ls which is almost twice as that of Ru(bpy)32+ counterpart [40–43]. The emission quantum yield of the [Ru(bpy)2(sn)2](PF6)2 at 77 K was 0.0158.

The quantum yields of photolysis Up in the two solvents are listed in Table 9. The value in methanol was 0.778 and in N,N-dimethyl formamide it was 0.829. Shown in Fig. 12 is the calculated stepwise change in the absorption spectrum of [Ru(bpy)2(sn)2]2+ with incorporation of methanol into the coordination sphere with loss of succinonitrile. The calculated result suggests that the complexes undergo stepwise solvent substitution of the nitrile ligands as given by Eqs.

3.9. Photolysis studies

Table 8 Low-temperature (77 K) emission spectral data of [Ru(bpy)2(sn)2]2+.

[Ru(bpy)2(sn)2](PF6)2 and [Ru(bpy)2(AN)2](PF6)2 were dissolved in methanol and DMF and irradiated with monochromatic radiation at 405 nm. Fig. 11 shows the changes in the UV/Vis spectra of the complex as a function of time. In all cases, the MLCT band at 420 nm in methanol and at 428 nm in DMF decreased in absorbance while a new band appeared at 450 nm in methanol and at 460 nm in DMF. Three isosbestic points located at 320, 365 and 434 nm were found in methanol. In N,N-dimethyl formamide, three isosbestic points were observed centered at 322, 375 and 443 nm.

Eexp (nm) (k, 103 cm1)

Compound

Ecalc

sem (ls)

Uemission

21.3

10.0

0.0158

9.9

0.0148

2+

[Ru(bpy)2(sn)2]

537 (18.62) 576 (17.36) 625 (16.00) [Ru(bpy)2(AN)2]2+ 538 579 631

2504

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505

a

b

1000000

1800000 1600000 1400000

Intensity (cps)

Intensity (cps)

800000

600000

400000

1200000 1000000 800000 600000 400000

200000

200000 0

0 500

600

700

-200000 250

300

350

400

450

500

Wavelength (nm)

Wavelength (nm)

Fig. 10. (a) Low-temperature (77 K) emission spectrum of [Ru(bpy)2(sn)2]2+ at excitation wavelength, kex = 418 nm. (b) Excitation spectrum of [Ru(bpy)2(sn)2]2+ at emission wavelength, kem = 537 nm.

a

0.5

0.0 300

400

500

600

b

1.0

Absorbance

Absorbance

1.0

0.5

0.0 300

400

500

600

Wavelength (nm)

Wavelength (nm)

Fig. 11. UV/Vis spectrum of the MLCT band in [Ru(bpy)2(sn)2]2+ during photolysis for t = 20 min in (a) methanol and (b) N,N-dimethyl formamide.

Table 9 Quantum yields of photolysis in [Ru(bpy)2(sn)2](PF6)2 (I0 = 1.79  1011 quanta/s (Es)). Compound

Eex (nm) (k)

Uphotolysis (solvent)

60000

405

0.778 (methanol) 0.829 (DMF)

50000

405

0.819 (methanol) 0.859 (DMF) 0.908 (methanol)

Ru(bpy)2(sn)2 Ru(bpy)2(sn)(sol) Ru(bpy)2(sol)2

[Ru(bpy)2(sn)2]2+

402 272 428 273 454 275

[Ru(bpy)2(AN)2]

305

(2) and (3). As shown in Fig. 11 in methanol and DMF, only one step is discerned in the photosubstitution process. Additional insight into the photosubstitution processes of [Ru(bpy)2(sn)2]2+ in methanol was obtained using 1H NMR (supplementary information) spectroscopy. Complete photolysis leads to loss of both succinonitrile ligands. Hence step two must be fast compared to step one to account for the observed isosbestic points in Fig. 11. Further, the calculated spectrum and the observed spectrum of [Ru(bpy)2(MeOH)2]2+ have similar absorption maxima near 450 nm. Thus, the NMR and visible spectral results corroborate one another.

Extinction Coefficient

2+

40000

Black: [Ru(bpy)2(sn)2]2+ red: [Ru(bpy)2(sn)(MeOH)]2+ blue: [Ru(bpy)2(MeOH)2]2+

30000

20000

10000

0 200

300

400

500

Wavelength (nm) Fig. 12. Calculated absorption changes of [Ru(bpy)2(sn)2]2+ in methanol. Black: [Ru(bpy)2(sn)2]2+; red: [Ru(bpy)2(sn)(MeOH)]2+; blue: [Ru(bpy)2(MeOH)2]2+. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505 S¼solv ent

½RuðbpyÞ2 ðR—CNÞ2 2þ þ S !½RuðbpyÞ2 ðR—CNÞðSÞ2þ þ R—CN ð2Þ ½RuðbpyÞ2 ðSÞ2þ þ S ! ½RuðbpyÞ2 ðSÞ2 2þ þ R—CN

ð3Þ

Labialization of the coordination sphere is well documented for ruthenium(II) diimine complexes in the literature and is due to thermal population of the 3dd by intersystem crossing from the 3 MLCT state [44,45]. However, the magnitude of the effect is much greater for solvent substitution in these bis-nitrile complexes than found for other systems [1–9,11,13–19]. Acknowledgements We thank the support from the Wichita State University High Performance Computing Center, the Wichita State University Office of Research Administration, and the Department of Energy. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2010.04.014. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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