Inorganica Chimica Acta 363 (2010) 2496–2505

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Inorganica Chimica Acta

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

A.J. Cruz a, R. Kirgan a, K. Siam b, P. Heiland a, D.P. Rillema a,*

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

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 January 2009Received in revised form 30 March 2010Accepted 8 April 2010Available online 11 May 2010

Keywords:Ruthenium(II) bis(bipyridine) complexesPhotosubstitutionDensity functional theoryElectronic spectraSynthesisbis-Nitrile complexes

0020-1693/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.ica.2010.04.014

* Corresponding author.E-mail address: paul.rillema@wichita.edu (D.P. Ril

The electrochemical and photophysical properties of two bis-nitrilo ruthenium(II) complexes formulatedas [Ru(bpy)2(L)2](PF6)2, where bpy is 2,20-bipyridine and L is AN = CH3CN and sn = NC–CH2CH2–CN, havebeen investigated. Electrochemical data are typical of Ru-bpy complexes with two reversible reductionpeaks located near �1.3 and �1.6 V assigned to each bipyridine ligand and one RuII/RuIII oxidation wavecentered at approximately +1.5 V. The sn derivative is both IR and Raman active with its coordinated CNstretch appearing at 2277 cm�1 and 2273 cm�1, respectively. The UV/Vis absorption spectrum of the snderivative is dominated by an intense (emax � 58700 M�1 cm�1) absorption band at 287 nm assigned asa LC (p ? p*) transition. The peak observed at 418 nm (e � 10 400 M�1 cm�1) is an MLCT band whilethe one at 244 nm (e � 23 600 M�1 cm�1) 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. 1Hand 13C assignments are consistent with the formulation of the complexes. The complexes undergophotosubstitution of solvent with quantum efficiencies near one. Calculated and experimental resultssupport replacement of the nitrile ligands by solvent. Based on DFT calculations, the electron densityof the HOMO lies on the metal center, the bipyridine ligands and the nitrile ligands and electron densityof the LUMO resides primarily on the bipyridine ligands. The electronic spectra obtained from TDDFT cal-culations closely match the experimental ones.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Many reports have been written about ruthenium(II) tris-bipyr-idine complexes and derivatives due to interest in their photophys-ical properties related to solar energy conversion, catalysis andphotoinduced chemical reactions [1–3]. The primary driving forcein these studies has been the exploitation of excited state redoxprocesses, although interest in photochemical and thermal chelateexchange reactions involving Ru(bpy)3

2+, where bpy is 2,20-bipyri-dine, and derivatives have been reported where one moiety is ex-pelled from the primary coordination sphere and replaced byanother [4–9]. The extent of these photosubstitution reactions de-pends on the lability of these ligands under light conditions. Forexample, ruthenium(II) complexes involving diimine chelates havebeen known to undergo photosubstitution reactions and have beeninvestigated using electrospray ionization mass spectrometry [10].Brown has reported thermal and light-induced decomposition ofRu(bpy)2(N3)2

+ in CH3CN [11]. Bonneson et al. demonstratedlight-driven nitrile substitution in complexes involving 1,10-phe-nanthroline (phen) chelating ligands [12]. Baranoff et al. have also

ll rights reserved.

lema).

reported ruthenium complexes with photolabile ligands based onbipyridine 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 andsn = NC–CH2–CH2–CN, in various solvents. [Ru(bpy)2(AN)2]2+ hasbeen known as an intermediate in coordination synthesis but toour knowledge has not been investigated regarding its photolabil-ity [1,4,11–20]. This study gives new insight into solvent photosub-stitution reactions in bis-nitrilo complexes of ruthenium(II) whichcan be useful as intermediates in coordination synthesis as well asin the growing field of ‘‘molecular machines” [5–8].

2. Experimental

2.1. Materials

Ruthenium(II) bis-2,20-bipyridine dichloride hydrate, [Ru(bpy)2Cl2�H2O], ruthenium(II) bis-2,20-bipyridine carbonate,[Ru(bpy)2CO3] [21–24], and ruthenium(II) bis-2,20-bipyridine bis-acetonitrile hexafluorophosphate, [Ru(bpy)2(CH3CN)2](PF6)2, weresynthesized using published procedures [18]. The succinonitrileligand was purchased from Aldrich and used as received. Diethylether, HPLC grade methanol and optima grade acetonitrile were

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505 2497

purchased from Fisher Scientific. Ammonium hexafluorophosphate(NH4PF6), IR grade potassium bromide, deuterated dimethyl sulf-oxide (DMSO) and trifluoromethanesulfonic acid (99%) in a vac-uum sealed glass container were obtained from Aldrich. Sulfurpowder was provided by Thermo Nicolet. Electrochemical gradetetrabutylammonium perchlorate was purchased from Southwest-ern Analytical. Ferrocene standard and dry acetonitrile in a sealtight bottle were purchased from Aldrich. Butyronitrile used foremission and lifetime studies was obtained from Acros; it was frac-tionally distilled prior to usage. Potassium ferrioxalate used inchemical actinometry was synthesized using published procedures[25–30]. Deuterated methanol used in photolysis studies were pur-chased from Cambridge Isotope Laboratories, Inc.

2.2. Measurements

Infrared spectra were obtained using Perkin–Elmer Model 1600FT-IR and Nicolet Avatar Model FT-IR Spectrophotometers. All sam-ples were prepared as potassium bromide pellets. Raman spectrawere obtained using a Thermo Nicolet Nexus Spectrophotometerwith a FT-Raman Module which was calibrated with sulfur. TheNicolet instruments were accompanied by Omni software pro-grams. Proton 1H-, 13C NMR spectra and HETCOR spectra were ob-tained using Varian Mercury 300 MHz and Varian Inova 400 MHzFourier Transform-NMR spectrometers (internal standard TMS).Ultraviolet spectra were obtained using a Hewlett–Packard Model8452A diode array spectrophotometer interfaced with an OLIS soft-ware program. Elemental (C, H, & N) analysis was performed byMHW Laboratories. An EG&G PAR Model 263A potentiostat/galva-nostat was used to obtain the cyclic voltammograms. All CV mea-surements were carried out in a typical H-electrochemical cellusing a platinum disk working electrode, polished every run, anda platinum wire counter electrode. A Ag/AgNO3 electrode freshlymade from AgNO3 in dry CH3CN served as the reference for thisstudy. The supporting electrolyte was 0.1 M tetrabutylammoniumperchlorate. Ferrocene was added as an internal reference. Thesample preparation for emission studies involved dissolving asmall amount of sample in distilled butyronitrile and then measur-ing the absorbance of the solution. The concentration of the solu-tion was altered in order to achieve an approximate absorbanceof 0.10 at 418 nm of the sample. The sample solution was placedin a fluorescent quartz tube and was degassed (minimum of fivetimes) prior to actual measurements. Emission quantum yieldswere calculated using Eq. (1) [31], where uf is the emission quan-tum yield, If is the emission intensity and A is the absorbance of thesample at the excitation frequency.

logðuf Þ ¼ 0:919 � logIf

ð1� 10Þ�A

" #� 7:934 ð1Þ

All samples were degassed using the freeze-pump-thawmethod a minimum of five times prior to measurements. Sampletubes for 77 K measurements were immersed in a quartz fingerDewar containing liquid N2. The Dewar was mounted in the cen-ter of the sample compartment of a SPEX 212 Fluorolog Fluo-rometer which was modified to allow dry N2 gas to flow overthe finger of the Dewar in order to minimize condensation ofmoisture from the air. The sample was held in place by a seriesof rubber rings located near the top and middle of the sampletube. This ensured that the tube was centered in all directions.Accurate results were obtained with the sample centered inthe excitation beam and with a frost-free finger. Great effortswere made to ensure the maximum emission intensity was ob-tained from the sample.

The excited state lifetimes were determined by exciting thesample at the third harmonic of a Continuum Surlite Nd:YAG laserrun at �20 mJ/10 ns pulse. Instrumentation for steady state pho-

tolysis consisted of a 100 W xenon lamp (Oriel) and a monochrom-eter. The light was directed into the cavity of the HP8452A DiodeArray spectrophotometer. The solution containing the compoundof interest was irradiated in a 1 cm cuvette at 405 nm, magneti-cally stirred, and its absorbance was observed at right angles tothe irradiating light. The intensity of the light source at the wave-length of excitation was determined using potassium ferrioxalateas the actinometer following published procedures [25]. Photolysisprogress was monitored using UV–Vis spectroscopy and the abso-lute quantum yields of photolysis were determined in various sol-vents. Photosubstitution of methanol (CD3OD) was monitoredusing UV–Vis and 1H NMR spectroscopy.

2.3. Calculations

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

2.4. Preparation of [Ru(bpy)2CO3]

Ru(bpy)2Cl2�H2O (2.00 g, 3.70 mmol) was suspended in150.0 ml of deaerated water in a 100-mL round-bottom flask. Itwas heated at reflux under Ar for 15 min. After reflux, excessamount of sodium carbonate monohydrate (7.8 g, 63 mmol) wasadded into solution and the mixture was allowed to reflux for an-other 2 h. The reaction was completed; the solution was allowed tocool down in an ice bath. The black solid was vacuum filtered andoven-dried at 60 �C overnight. Color: Black. Yield 74%. IR (KBr pel-let, cm�1): 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 mLof dried CH3CN and was placed in a 100-mL round-bottom flask.The mixture was allowed to stir at room temperature for about15 min under Ar conditions. Then, 0.10 mL of trifluoromethanesul-fonic acid was added into the mixture. Evolution of gas was ob-served and the color of the solution immediately changed into ayellow-orange tint. The reaction was set to reflux overnight andthe solvent was allowed to evaporate to about 2 mL. The remainingsolution was then added dropwise into a saturated aqueous solu-tion of ammonium hexafluorophosphate (NH4PF6). The orange pre-cipitate was collected by vacuum filtration and was dried insidethe vacuum oven (40�) for 1 day.

Color: Orange. Yield 85%. UV/Vis: kmax = 420 nm. IR (KBr pellet,cm�1): 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-mLround-bottom flask and dissolved in 20.0 mL of methanol. The pur-

2498 A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505

ple solution was allowed to stir under argon at room temperaturefor another 30 min. Then 0.10 mL of trifluoromethanesulfonic acidwas added. The solution turned into a burnt-orange color and evo-lution of carbon dioxide gas was observed. While the solution wasallowed to stir for 3 h, the reaction was monitored using UV–Visspectroscopy until no spectral shift was observed. After the reac-tion was complete, succinonitrile (60.2 mg, 0.750 mmol) wasadded and the solution was allowed to stir under reflux overnight.The solution was evaporated to approximately 2 mL and the de-sired complex was allowed to precipitate from a saturated aqueousammonium hexafluorophosphate (NH4PF6) solution. It was vac-uum filtered to isolate the compound. The orange complex waswashed with diethyl ether to remove excess succinonitrile. Thecomplex was placed inside the vacuum oven (45 �C) and was al-lowed 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, cm�1): 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, cm�1): 3091 w, 2948 w, 2273s, 1603 m, 1561 w, 1489 w, 1313 w, 765 w, 717 w, 648 w, 450w. 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. 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 preparationof [Ru(bpy)2CO3] from Ru(bpy)2Cl2�H2O. The carbonato complexwas 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 withsuccinonitrile in the dark.

3.2. Structure

The optimized, calculated structure of [Ru(bpy)2(sn)2](PF6)2 isshown in Fig. 2 and the Cartesian coordinates are located in theSupplementary materials section, S1a.

The atoms in blue are nitrogen atoms, six of which are coordi-nated 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 toexperimental and calculated bond lengths determined for [Ru(b-py)2(AN)2](PF6)2. Both of the calculated Ru–N (–N„C–) bond dis-tances were the same with values of 2.045 Å and both bipyridinerings are equivalent in the optimized structure with average calcu-lated Ru–N (bpy) bond distances of 2.078 Å and 2.098 Å. The short-er Ru–N (bpy) bond distance, 2.078 Å, was from Ru to the pyridineunit 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 andnitrile groups compared to the bond distance between the metaland the bipyridyl groups. The optimized structure showed slightlytwisted bipyridyl rings, 1�, with one nitrogen atom closer to theruthenium center than the other. Experimental bond distances[36] in the acetonitrile adduct were shorter than the ones calcu-lated as reported for most optimized structures which are evalu-ated in the gas phase.

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 charactercompared to the LUMO which has more ligand-centered character.As listed in Table 2, the HOMO contains 75% metal, 14% bpy and10% sn character while the LUMO contains 2% metal, 98% bpyand 1% sn character. The electron density of the HOMO is concen-trated primarily on Ru with a small amount on bpy and CN whilethe electron density on the LUMO dominantly resides on the twobipyridine ligands. The HOMO-LUMO transition occurs from themetal to the ligand (bpy).

The molecular orbital energy diagrams for the singlet and trip-let states of [Ru(bpy)2(sn)2]2+ are shown in Fig. 4 and energies arelisted in the supplementary material sections S2a and S2b. Sin-glet-energy calculations results showed that the low-lying unoc-cupied energy states are more of bipyridyl type. Thesuccinonitrile orbitals lie at much higher energies. HOMO,HOMO-1and HOMO-2 energy levels have most of the electrondensity lying on the ruthenium. Lower HOMOs, on the otherhand, have electron density concentrated on the bipyridyl andsuccinonitrile ligands. DFT calculations of the triplet state revealthat the low-lying triplet states are of 3MLLCT and 3LC character(Fig. 4).

3.4. Vibrational spectroscopy

Both infrared and Raman data were obtained where interestwas focused on the CN group. Data are listed in Table 3 and theexperimental spectra are shown in Fig. 5. The CN vibration of thefree succinonitrile ligand in the infrared region was observed at2254 cm�1 whereas two CN stretching vibrations were observedfor the sn complex. None were found for [Ru(bpy)2(AN)2]2+. Thevibrational frequency found at 2277 cm�1 for the sn complex wasassigned to the metal-bound nitrile group, while the one observedat 2252 cm�1 was attributed to the uncoordinated nitrile group bycomparison to the free ligand. The vibrational frequency of the CNgroup in the Raman spectrum for [Ru(bpy)2(sn)2]2+ was observedat 2273 cm�1 and not resolved into two components; none wasfound for [Ru(bpy)2(AN)2]2+ due to decomposition of the complexin the laser beam. The number of scans was lowered due to sampledecomposition upon prolonged exposure to laser source. As a re-sult, the observed intensity of Raman scattering was weak andthe aperture was opened completely giving rise to a less resolvedpeak.

DFT calculations were used to simulate the infrared and Ramanspectra of both the sn and AN derivatives. The simulated spectrafor the sn derivative are shown in Fig. 6. The nitrile stretches aredoublets of doublets, and although the splittings of each doubletare small, the two sn ligands occupy two different sites in the mol-ecule which may account for the origin of the doublet. These split-tings were not observed in the experimental spectrum, perhapsdue to line broadening in the solid state. The high energy absorp-tion at 2311 cm�1 and the low energy one located at 2296 cm�1

are assigned to bound and unbound portion of the nitrile groups,respectively, as found experimentally. Two very weak infraredvibrations located at 2325 and 2318 cm�1 were calculated for theAN complex and the calculated Raman vibration was located atthe same frequency as found for the sn derivative. The weak vibra-tions of the AN complex are consistent with the lack of observing avibration experimentally. The phase calculations are 20–30 cm�1

higher in energy than found experimentally for both the infraredand Raman results. Calculations in the gas phase may account forthis observation.

N

N

N

N

RuNC

CN

NCCN

N

N

N

N

Ru

O

O

O

N

N

N

N

RuSO3CF3

SO3CF3

CO2 H2O

N

N

N

N

RuSO3CF3

SO3CF3

NCCN

2 CF3SO3-

(aq)

N

N

N

N

RuCl

Cl

H2O

N

N

N

N

Ru

O

O

O 2 NaCl (aq)

(II)

(III)(II)

(I)

(IV)(III)

triflic acid, MeOH

reflux (3 hrs), Ar + ( ) +

reflux (overnight), Ar

2 ()

+

reflux (overnight), Ar.

+Na2CO3, MeOH

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

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

Table 1Selected bond lengths (Å) in [Ru(bpy)2(sn)2](PF6)2.

Bond Calculated(sn)

Calculated(parent)

Experimental(parent)a

Ru–N (bpy)b 2.098 2.095 2.061Ru–N (bpy)c 2.079 2.078 2.052Ru–N (bpy)c 2.078 2.077 2.051Ru–N (bpy)b 2.098 2.095 2.059Ru–N (–N„C–) 2.045 2.049 2.040Ru–N (–N„C–) 2.045 2.049 2.045

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

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505 2499

3.5. NMR behavior

The 1H NMR spectrum of the metal complex showed all eightchemically distinct bipyridyl protons compared to four for the freebipyridyl ligands. The chemical shift assignments are tabulated inTable 4 according to the proton designations given in Fig. 7. Allchemical shift assignments were based on experiments (coupling

constants and multiplicity), chemical environment. Results showthat the two coordinated bipyridyl ligands have distinct pyridinerings on each ligand. While Ha and Hg experience downfield chem-ical shifts due to ring current effects on the bipyridyl rings, Ha

undergoes more deshielding compared to Hg since it resides per-pendicular to the CN triple bond which has its magnetic currentparallel to the bonding axis. Thus, the chemical shift observed fur-thest downfield at around d = 9.35 ppm was assigned to the Ha

(Ha0) protons while the one observed furthest upfield at aroundd = 7.37 ppm was assigned to Hg (Hg0) protons. The succinonitrilemethylene proton signals (triplet) were observed at aroundd = 3.81 before and at 2.81 ppm after coordination.

Heteronuclear chemical shift correlation spectroscopy was per-formed to support the carbon nuclei chemical shift assignmentslisted in Table 4 following the numbering system in Fig. 7. The13C peak for the uncoordinated CN peak was located at

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

Energy level Ru bpy1 bpy2 sn

HOMO 75 7 7 10LUMO 2 51 47 1

2500 A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505

d = 117 ppm while peak for the metal-bound CN group was foundat 126 ppm. Two peaks (C5/C6) observed furthest downfield wereassigned to the quaternary carbons of the bipyridyl rings. Uponcoordination, the two methylene carbon atoms of the succinonitri-le ligand are now distinct with two C13 peaks located at d = 14 and16 ppm.

3.6. Cyclic voltammograms

The CV electrochemical data are listed in Table 5 and the cyclicvoltammograms is shown in the supplementary information sec-

-115

-110

-105

-100

-95

-70

-65

-60

-55

-50

-45

-40

(+8) Sn (+7) Sn(+6) Sn

(+5) Bpy (+4) Bpy(+3) Bpy(+2) Bpy

(+1) Bpy(L) Bpy

(H) Ru(-1) Ru(-2) Ru

(-3) Bpy(-4) Bpy

(-5) Sn(-6) Sn (-7) Sn (-8) Sn

Gap = 28.9

Wav

enum

bers

(cm

-1)

a

Fig. 4. Molecular Orbital Energy Diagram in [Ru(bpy)2(

Table 3Vibrational frequencies of metal-bound CN groups (cm�1).

Compound Infrared

Calculated Experimental Experimental free

[Ru(bpy)2(sn)2]2+ 23112296

2277a (Ru–NC)2252 (C–CN)

2254

[Ru(bpy)2(AN)2]2+ 23252318

NA 2200

a KBr pellet.b Solid in tube.c Solution (CCl4).

tion S3. The CV profile is typical of ruthenium(II) bis-bipyridinecomplexes. Two reversible peaks were observed at negative poten-tials for the succinonitrile derivative. The first peak was observedat half-wave potential (E1/2) of �1.33 V while the second one waslocated at �1.53 V. One reversible peak was prominent at positivepotentials. 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 under-went one-electron reduction in an irreversible manner at �1.45 Vand �1.6 V while two oxidation processes were observed, one irre-versible 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 tworeversible 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+

0

1

2

3

4

20.0

22.5

25.0

27.5

30.0

32.5

1G.S.

3MLLCT3ILCT

3dd3dd

3LMLCT

3ILCT3 LMLCT

Wav

enum

bers

(x1

03 c

m-1)

b

sn)2](PF6)2: (a) singlet states and (b) triplet states.

Raman

ligand Calculated Experimental Experimental free ligand

23112296

2273b (Ru–NC) 2189c

23252318

Decomposition NA

000200520.0

0.5

1.0

Ram

an In

tens

ity

Raman Shift (cm -1)

2500 2400 2300 2200 2100 20000

20

40

60

80

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

ba

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

0002005200030.0

0.2

0.4

0.6

0.8

1.0

C-CN Stretch22962296

Ru-NC Stretch23112303

Wavenumbers000200520003

1.0

0.8

0.6

0.4

0.2

0.0

C-CN Stretch22962296

Ru-NC Stretch23112303

Wavenumbers

a b

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

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505 2501

occurred at similar potentials, but its oxidation was more positiveby 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/Visabsorption spectra were weakly solvent dependent with the lowenergy maximum shifting as follows: acetonitrile, 418 nm; butyro-nitrile, 418 nm; methanol, 422 nm; i-propanol; 424 nm; DMF,427 nm and DMSO, 427 nm. The absorption coefficients of thetransitions for the experimental spectrum obtained in acetonitrilewere determined from Beer’s Law studies using at least five dilu-tion points and are listed in Table 6. The probable assignments ofthe experimental bands were based on computational assignmentsof the singlet excited states and related reports of similar type ofcomplexes [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 aMLCT transition located near 420 nm and a LC transition near288 nm. The absorption coefficients at �410 nm differ with aslightly lower e value of 9200 M�1 cm�1 for [Ru(bpy)2(sn)2]2+ com-pared to 10,400 M�1 cm�1 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 7and the calculated absorption spectrum is shown in Fig. 9 alongwith the major orbitals involved in the transitions and their per-centage contributions to the transition. Calculations gave rise tofour prominent bands in the absorption spectrum centered at402, 309, 273 and 242 nm. The 402 nm band is a combination ofH-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 at309 nm is a combination of contributions from the calculated310 and 308 nm bands which have oscillator strengths of 0.066and 0.043, respectively. The 310 and 308 nm bands consist of sev-eral transitions primarily MLCT in character, although one of thecontributors at 308 nm is a dd transition. The 276 nm band is com-posed of three components which have oscillator strengths of0.202 while the one at 243 nm contains only one component whichhas oscillator strength of 0.069. These transitions mainly involvethe bipyridine ligands corresponding to intraligand p ? p* transi-tions (ILCT) although one of the contributors at 276 nm is a ddtransition. The succinonitrile ligands make little contribution tothese transitions.

Experimentally, only three major bands were observed. Theband at 244 nm (41.0 � 103 cm�1) was given an ILCT assignment.

C4

C5N

C1

C2

C3

C6

C7C8

C9C10

N

Ru

Hc

Hb

Ha

HdHe

Hf

Hg

Hh

C4'

C5'

NC1'

C2'C3'

C6'C7'

C8'

C9'

C10'N

Hc'

Hb'

Ha'

Hd'

He'

Hf'

Hg'

Hh'

C11

C12C13

CN

N

C11'C12'

C13'C

N

N

Hj

Hi

Hi'

Hj' Hj'

Hi'

Hj

Hi

14'

14

Fig. 7. Scheme for 1H and 13C assignments.

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

Compound E1/2 (V)oxa E1/2 (V)red

[Ru(bpy)2(sn)2]2+ 1.54 �1.33 �1.53[Ru(bpy)2(AN)2]2+ 1.45 �1.36 �1.57

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

200 300 400 500 600

0.0

0.2

0.4

0.6

0.8

1.0

Rusn calculatedRusn experimental

Abso

rban

ce

Wavelength (nm)

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

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

Proton type Chemical shift(experimental)

Carbontype

Chemical shift(experimental)

Ha

Ha0

9.35 (2H) C1/C10 153.82

Hb

Hb0

7.90 (2H) C2 128.19

Hc

Hc0

8.35 (2H) C3 138.69

Hd

Hd0

8.82 (2H) C4 124.44

He

He0

8.68 (2H) C5/C6 157.35

Hf

Hf0

8.04 (2H) C6/C5 158.35

Hg

Hg0

7.37 (2H) C7 124.10

Hh

Hh0

7.58 (2H) C8 139.10

Hi

Hi0

3.21 (4H) C9 127.38

Hj

Hj0

2.80 (4H) C10/C1 152.47

C11 126.82C12 16.70C13 14.35C14 117.65

a Solvent: DMSO.

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

and [Ru(AN)2]2+.

Compound Eexp (nm) (k,�103 cm�1)a

e(M�1 cm�1)

Ecalc

(�103 cm�1)Assignments

[Ru(bpy)2(sn)2]2+ 244 (41.0) 23565 41.3 LC (p ? p*)287 (34.8) 58709 36.6 LC (p ? p*)418 (23.9) 10420 24.9 MLCT

[Ru(bpy)2(AN)2]2+ 242 18900 41.5 LC (p ? p*)283 58700 36.6 LC (p ? p*)420 9200 24.4 MLCT

a Solvent: acetonitrile.

2502 A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505

Calculations showed that the 244 nm transition observed experi-mentally occurs upon exciting an electron from the HOMO-4 toLUMO+2 (Fig. 9). This ILCT transition is predominantly ligand-cen-tered (LC) with some metal orbitals involved in the LUMO. Theligand character is centered on the bipyridine ligands with minorcontributions from the succinonitrile ligand. The intense bandfound experimentally at 287 nm (34.8 � 103 cm�1) is primarilyILCT derived from the calculated 273 nm bands with a contributionfrom the calculated 309 nm band which appears in the simulatedspectra as a shoulder on the ligand-centered transition. The bandlocated at 418 nm (23.9 � 103 cm�1) was assigned as a MLCT bandwhich occurs from the HOMO-2 to LUMO, HOMO to LUMO+5and HOMO-1 to LUMO+3. These HOMO’s are primarily metal based

and the LUMO is ligand based. The majority of HOMO-2 excitedstate electron density lies on the dz

2 orbital.

3.8. Emission spectra

The emission properties and excited state lifetimes (sem) of[Ru(bpy)2(sn)2]2+ were determined at 77 K in freshly distilledbutyronitrile. The value of the emission lifetime was determinedby curve-fitting analysis. The low-temperature (77 K) emissiondata are summarized in Table 8 and the spectrum is shown inFig. 10a. The emission band of the complex had its peak at537 nm with vibronic peaks located at 576 nm and 625 nm,respectively. The emission lifetime of the compound was 10 ls at77 K. The excitation spectrum was determined from the emissionmaximum located at 537 nm and is shown as an inset inFig. 10b. It mirrors the MLCT assignments obtained by DFT andTDDFT calculations.

DFT calculations of the triplet excited states relative to theground state were performed. Fig. 4 shows the energy diagram ofthe first seven low-lying triplet states of the complex. The energydifference between the singlet ground state (1G.S.) and the triplet

Table 7Calculated 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 H-2 ? L 80 MLCT 0.112 Ru(80) ? bpy(100)390 H-2 ? L+1 70 MLCT 0.044 Ru(80) ? bpy(100)310 H ? L+5

H-1 ? L+3H-2 ? L+13

321513

MLCTMLCTMMLCT

0.066‘‘‘‘

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

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

14272111

MLCTMLCTddMMLCT

0.043‘‘‘‘‘‘

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

276 H ? L+14H-1 ? L+13H-4 ? L+1

22284

ddILCTILCT

0.202‘‘‘‘

Ru(75) ? Ru(70)bpy(100) ? bpy(100)bpy(100) ? bpy(100)

243 H-4 ? L+2 73 ILCT 0.069 bpy(100) ? bpy(98)

200 250 300 350 400 450 5000

10000

20000

30000

40000

50000

60000

D

C

BA

Ext

inct

ion

Co

effi

cien

t

Wavelength (nm)

A = 402 nm (3.0B = 309 nm (4.0C = 273 nm (4.5D = 242 nm (5.1

A = 402 nm (3.08 eV)

B = 309 nm (4.01 eV)

C = 273 nm (4.54 eV)

D = 242 nm (5.12 eV)

H-2 L

L+3H-1

L+5H

H-4 L+1

H-4 L+2

AB

B

C

D

MLCTMLCT

MLCT

LMLCT

p ? p*

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

A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505 2503

metal-to-ligand charge-transfer excited state (3MLCT) is about2.63 eV (475 nm). The experimentally observed maximum is at538 nm, Fig. 10a – a difference of �63 nm. The calculation mostlikely is inaccurate since the role of solvent has not been taken intoaccount. The 77 K lifetime of the luminescence was observed ataround 10 ls which is almost twice as that of Ru(bpy)3

2+ counter-part [40–43]. The emission quantum yield of the [Ru(b-py)2(sn)2](PF6)2 at 77 K was 0.0158.

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

Compound Eexp (nm) (k, �103 cm�1) Ecalc sem (ls) Uemission

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

537 (18.62) 21.3 10.0 0.0158576 (17.36)625 (16.00)

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

538 9.9 0.0148579631

3.9. Photolysis studies

[Ru(bpy)2(sn)2](PF6)2 and [Ru(bpy)2(AN)2](PF6)2 were dissolvedin methanol and DMF and irradiated with monochromatic radiationat 405 nm. Fig. 11 shows the changes in the UV/Vis spectra of thecomplex as a function of time. In all cases, the MLCT band at420 nm in methanol and at 428 nm in DMF decreased in absor-bance while a new band appeared at 450 nm in methanol and at460 nm in DMF. Three isosbestic points located at 320, 365 and434 nm were found in methanol. In N,N-dimethyl formamide, threeisosbestic points were observed centered at 322, 375 and 443 nm.

The quantum yields of photolysis Up in the two solvents arelisted in Table 9. The value in methanol was 0.778 and in N,N-di-methyl formamide it was 0.829.

Shown in Fig. 12 is the calculated stepwise change in theabsorption spectrum of [Ru(bpy)2(sn)2]2+ with incorporation ofmethanol into the coordination sphere with loss of succinonitrile.The calculated result suggests that the complexes undergo step-wise solvent substitution of the nitrile ligands as given by Eqs.

250 300 350 400 450 500-200000

0

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

Inte

nsity

(cps

)

Wavelength (nm)007006005

0

200000

400000

600000

800000

1000000

Inte

nsity

(cps

)

Wavelength (nm)

a b

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 emissionwavelength, kem = 537 nm.

300 400 500 6000.0

0.5

1.0

Abso

rban

ce

Wavelength (nm)300 400 500 600

0.0

0.5

1.0Ab

sorb

ance

Wavelength (nm)

ab

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

Compound Eex (nm) (k) Uphotolysis (solvent)

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

405 0.778 (methanol)0.829 (DMF)

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

405 0.819 (methanol)0.859 (DMF)

305 0.908 (methanol) Black: [Ru(bpy)2(sn)2]2+

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

blue: [Ru(bpy)2(MeOH)2]2+

200 300 400 5000

10000

20000

30000

40000

50000

60000

Ext

inct

ion

Coe

ffici

ent

Wavelength (nm)

Ru(bpy)2(sn)

2402 272

Ru(bpy)2(sn)(sol) 428 273

Ru(bpy)2(sol)

2454 275

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+. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

2504 A.J. Cruz et al. / Inorganica Chimica Acta 363 (2010) 2496–2505

(2) and (3). As shown in Fig. 11 in methanol and DMF, only one stepis discerned in the photosubstitution process. Additional insightinto the photosubstitution processes of [Ru(bpy)2(sn)2]2+ in meth-anol was obtained using 1H NMR (supplementary information)spectroscopy. Complete photolysis leads to loss of both succinonit-rile ligands. Hence step two must be fast compared to step one toaccount for the observed isosbestic points in Fig. 11. Further, thecalculated 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.

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

½RuðbpyÞ2ðR—CNÞ2�2þ þ S ����!S¼solvent½RuðbpyÞ2ðR—CNÞðSÞ�2þ þ R—CN

ð2Þ

½RuðbpyÞ2ðSÞ�2þ þ S! ½RuðbpyÞ2ðSÞ2�

2þ þ R—CN ð3Þ

Labialization of the coordination sphere is well documented forruthenium(II) diimine complexes in the literature and is due tothermal population of the 3dd by intersystem crossing from the3MLCT state [44,45]. However, the magnitude of the effect is muchgreater for solvent substitution in these bis-nitrile complexes thanfound for other systems [1–9,11,13–19].

Acknowledgements

We thank the support from the Wichita State University HighPerformance Computing Center, the Wichita State University Officeof Research Administration, and the Department of Energy.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2010.04.014.

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