tuning of redox potential and visible absorption band of ruthenium(ii) complexes of (benzimidazolyl)...

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Inorganica Chimica Acta 360 (2007) 2231–2244

Tuning of redox potential and visible absorption band ofruthenium(II) complexes of (benzimidazolyl) derivatives:

Synthesis, characterization, spectroscopic and redox properties,X-ray structures and DFT calculations

Dipankar Mishra a,1, Andrea Barbieri b,*, Cristiana Sabatini b, Michael G.B. Drew c,*,Hake M. Figgie d, William S. Sheldrick d, Shyamal Kumar Chattopadhyay a,*

a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, Indiab Istituto per la Sintesi Organica e la Fotoreattivita, Consiglio Nazionale delle Ricerche (ISOF-CNR), Via P. Gobetti 101, 40129 Bologna BO, Italy

c School of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UKd Lehrstuhl fur Analytische Chemie, Ruhr-Universitat Bochum, D-44780 Bochum, Germany

Received 11 August 2006; received in revised form 25 October 2006; accepted 4 November 2006Available online 18 November 2006

Abstract

Six ruthenium(II) complexes have been prepared using the tridentate ligands 2,6-bis(benzimidazolyl) pyridine and bis(2-benzimidaz-olyl methyl) amine and having 2,2 0-bipyridine, 2,2 0:6 0,200-terpyridine, PPh3, MeCN and chloride as coligands. The crystal structures ofthree of the complexes trans-[Ru(bbpH2)(PPh3)2(CH3CN)](ClO4)2 Æ 2H2O (2), [Ru(bbpH2)(bpy)Cl]ClO4 (3) and [Ru(bbpH2)(terpy)]-(ClO4)2 (4) are also reported. The complexes show visible region absorption at 402–517 nm, indicating that it is possible to tune the vis-ible region absorption by varying the ancillary ligand. Luminescence behavior of the complexes has been studied both at RT and at liquidnitrogen temperature (LNT). Luminescence of the complexes is found to be insensitive to the presence of dioxygen. Two of the complexes[Ru(bbpH2)(bpy)Cl]ClO4 (3) and [Ru(bbpH2)(terpy)](ClO4)2 (4) show RT emission in the NIR region, having lifetime, quantum yieldand radiative constant values suitable for their application as NIR emitter in the solid state devices. The DFT calculations on thesetwo complexes indicate that the metal t2g electrons are appreciably delocalized over the ligand backbone.� 2006 Elsevier B.V. All rights reserved.

Keywords: Ruthenium(II) complexes; Benzimidazolyl derivatives; Luminescence spectra redox properties; X-ray structures; DFT calculations

1. Introduction

The study of spectroscopic, electrochemical and lumi-nescence properties of Ru(II) and its congener platinummetals is an ongoing and active area of research [1–18], pri-marily because of the potential applications of these com-pounds in energy research and as sensors. In most ofthese studies, the ligands are heterocyclic imines involving

0020-1693/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2006.11.009

* Corresponding authors. Fax: +91 33 2668 2916 (S.K. Chattopadhyay).E-mail address: [email protected] (S.K. Chattopadhyay).

1 Present address: Tamralipta Mahavidyalaya, Tamluk, Purba Medini-pur, West Bengal, India.

pyridine, pyrazine, pyrimidine and quinoline rings [1–13].Diimine ligands containing imidazole rings are relativelyless studied [14–18]. 2-(2-pyridyl)imidazole and its deriva-tives are attractive as diimine ligands because they offerthe combination of a moderate p-acceptor [19] pyridinenitrogen and a moderate p-donor [16,20,21] imidazolenitrogen. Thus a suitable combination of these two hetero-cyclic rings should in principle afford complexes with tun-able spectroscopic and redox properties. This may alsohelp us in obtaining NIR emitting compounds suitablefor various applications [22,23]. Moreover, the pyrrole typeN–H present in the imidazole ring can participate in hydro-gen bond formation with solvent in solution phase to

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HH

N N

N

N

N

N

HNN

H H

NN

Scheme 1.

2232 D. Mishra et al. / Inorganica Chimica Acta 360 (2007) 2231–2244

influence the spectroscopic and electron transfer propertiesof the molecules. In the solid state also, the N–H hydrogencan interact with neighboring H-bond acceptors to buildup a supramolecular network [24–26]. Finally, there is thepossibility of ‘molecular switching’ by N–H proton transferas observed earlier [27].

Ruthenium complexes of biimidazole, bibenzimidazole,2-pyridyl benzimidazole, 2,2-bis(2-pyridyl) bibenzimidaz-ole and 2,6-bis(benzimidazolyl)pyridine as well as2,6-bis(imidazolyl)pyridine have been explored [14,16–18,28]. Gratzel et al. have shown [29] that Ru(II) complexesof 2,6-bis-(1-methylbenzimidazol-2-yl)pyridine and 2,6-bis-(1-hexylbenzimidazol-2-yl)pyridine containing 4,4 0-dicarb-oxy 2,2 0-bipyridine or 4,4 0-dicarboxy 2,2 0-biquinoline asco-ligand can behave as efficient charge transfer sensitizers,when anchored to nanocrystalline TiO2 films. Haga et al.studied [30] proton induced tuning of electrochemical andphotophysical properties of mononuclear and dinuclearRu(II) complexes of 2,2 0-bis(benzimidazol-2-yl)-4,4 0-bipyridine and 2,2 0-bis(2-pyridyl)bibenzimidazole. Hagaet al. also studied [14] the spectroscopic and electrochemi-cal properties of ITO immobilized Ru(II) complexesinvolving 2,6-bis(1-(4-diphosphonyl)butylbenzimidazol-2-yl)pyridine as terminal ligand and 2,2 0,6,6 0-tetra(benzimidazol-2-yl)-4,4 0-bipyridine as the bridging ligand,and observed that the immobilized Ru(II) complex was sta-ble to repeated oxidation and this electrode has potentialfor electrochromic applications. Ruthenium(II) imidazole/histidine complexes have also been used [31] to modifymetal centers of enzymes to better understand their elec-tronic properties and electron transfer reactions.

In this paper, we report the synthesis, X-ray crystalstructure and spectroscopic characterization of severalruthenium(II) complexes containing the tridentate ligands2,6-bis(benzimidazolyl) pyridine (bbpH2) and bis(2-benz-imidazolyl methyl) amine (bbaH3). Our aim was to estab-lish as to how on using these two bis(benzimidazolyl)ligands, varying of the coligands can be used to tune thespectroscopic, electrochemical and luminescence propertiesof the resultant Ru(II) complexes.

2. Experimental

2.1. Physical measurements

Elemental analyses were performed on a Perkin–Elmer240 C,H,N analyzer. UV–Vis spectra were recorded usinga JASCO7850 spectrophotometer. Infrared spectra wererecorded as KBr pellets on a JASCO FTIR-460 spectropho-tometer. 1H NMR spectra were recorded on BrukerAVANCE DPX 300 MHz spectrometer using Si(CH3)4 asinternal standard. FAB mass spectra were recorded on aJEOL JMS 600 mass spectrometer. Solution conductancewas measured on a Systronics direct reading conductivitymeter (Model 304). Cyclic voltammetric experiments werecarried out using a PAR Versastat-II instrument drivenby E-chem software (PAR). A three-electrode configuration

with Pt working and auxiliary electrodes, Ag/AgCl refer-ence electrode and TEAP as supporting electrolyte wasused. The potentials were calibrated against ferrocene/ferr-ocenium couple (0.44 V versus Ag/AgCl reference).

2.2. Synthesis of ligands

The ligands 2,6-bis(benzimidazolyl) pyridine [32] andbis(2-benzimidazolyl methyl) amine [33] (Scheme 1) wereprepared according to the procedures described in theliterature.

2.3. Synthesis of the complexes

The ruthenium precursors used in this work, e.g.Ru(PPh3)3Cl2 [34], ‘Ru(bpy)Cl3’ [35], and Ru(terpy)Cl3[36], were prepared by the published procedures. Acetoni-trile used for spectroscopic and electrochemical studieswas purified according to reported methods [37]. Tetraethyl ammonium perchlorate (TEAP) used for the electro-chemical work was prepared as reported in the literature[38]. All solvents were of A.R. grade and used as receivedfor synthetic work.

Caution! Perchlorate salts of organic and metallo-organic species are potentially explosive, and though wehave not encountered any problem in our work, neverthe-less they should be handled with care.

2.3.1. trans-[Ru(bbpH2)(PPh3)2Cl]Cl (1)

The compound was prepared following the proceduredescribed by us earlier [24]. Elemental Anal. Calc. for 1:C, 58.71; H, 4.01; N, 6.11. Found: C, 58.63; H, 4.09; N,6.08%. KM (X�1 cm2 mol�1 in acetonitrile solution): 116–121; 1H NMR, (CD3)2SO, 300 MHz, d (ppm): 14.96 (bs,1H), 14.37 (bs, 1H), 8.42 (d, 2H, J = 7.9 Hz), 8.35 (d, 2H,J = 7.9 Hz), 8.07 (d, 2H, J = 7.5 Hz), 7.99–7.94 (m, 2H),7.67 (d, 2H, J = 8.0 Hz), 7.62–7.48 (m, 4H), 7.45–7.33 (m,4H), 7.25–7.18 (m, 5H), 7.14–7.07 (m, 8H), 6.99 (t, 4H,J = 7.4 Hz), 6.91 (t, 4H, J = 7.4 Hz), 6.81–6.75 (m, 2H).

2.3.2. trans-[Ru(bbpH2)(PPh3)2(CH3CN)] (ClO4) Æ2H2O (2)

To a 25 ml red brown acetonitrile solution of trans-[Ru-(bbpH2)(PPh3)2Cl]Cl (252 mg, 0.25 mmol), solid AgNO3

(85 mg, 0.5 mmol) was added and refluxed for 1 h, AgCl soprecipitated was filtered off. The yellow solution thusobtained was concentrated to about 10 ml in a rotary evap-

D. Mishra et al. / Inorganica Chimica Acta 360 (2007) 2231–2244 2233

orator and an aqueous saturated solution of NaClO4 wasadded until complete precipitation. The yellow precipitatewas collected by filtration, washed with water and dried overfused CaCl2. The product was recrystallized from dichloro-methane–hexane (1:1) mixture to give trans-[Ru(bbpH2)(PPh3)2(CH3CN)](ClO4)2 Æ 2H2O. Yield: 81%.Elemental Anal. Calc. for 2: C, 56.43; H, 4.12; N, 6.93.Found: C, 56.39; H, 4.17; N, 6.89%. KM (X�1 cm2 mol�1 inacetonitrile solution): 235–240; 1H NMR, (CD3)2SO,300 MHz, d (ppm): 14.28 (bs, 2H), 8.23 (t, 2H, J = 7.9 Hz),8.12–8.02 (m, 2H), 7.94 (d, 2H, J = 7.8 Hz), 7.80 (d, 2H,J = 7.9 Hz), 7.74 (d, 2H, J = 7.8 Hz), 7.62–7.49 (m, 5H),7.42 (t, 4H, J = 7.4 Hz), 7.37–7.28 (m, 2H), 7.24 (t, 4H,J = 7.4 Hz), 7.09–6.98 (m, 8H), 6.89–6.80 (m, 8H), 2.86 (s,3H).

2.3.3. [Ru(bbpH2)(bpy)Cl]ClO4 (3)

To a 40 ml methanolic solution of 2,6-bis(benzimidaz-olyl) pyridine (156 mg, 0.5 mmol), solid ‘Ru(bpy)Cl3’(0.5 mmol, 181.75 mg) was added and the reaction mixturewas heated to reflux for 6 h; 2 ml saturated methanolicsolution of NaClO4 was then added to the pink red reac-tion mixture. The solution was cooled and filtered. The pre-cipitate was washed successively with water, a smallamount of MeOH and then ether and collected; a secondlot of the same product can be isolated from the filtrateafter concentration. The product was dried over fusedCaCl2. Recrystallization was carried out by the diffusionof the methanolic solution of the compound to ether to give[Ru(bbpH2)(bpy)Cl]ClO4 Æ 1/2C2H5-O-C2H5. Yield: 59%.Elemental Anal. Calc. for 3: C, 49.50; H, 2.98; N, 13.94.Found: C, 49.43; H, 3.03; N, 13.98%. KM (X�1 cm2 mol�1

in acetonitrile solution): 125–140; 1H NMR, (CD3)2SO,300 MHz, d (ppm): 15.33 (bs, 2H), 8.99 (s, 2H), 8.70(s, 2H), 8.47 (d, 2H, J = 7.1 Hz), 8.38–8.32 (m, 2H),8.19–8.12 (m, 2H), 7.82 (s, 2H), 7.67–7.42 (m, 2H), 7.27–7.16 (m, 2H), 7.00–6.89 (m, 2H), 5.42 (s, 1H).

2.3.4. [Ru(bbpH2)(terpy)](ClO4)2 (4)

The procedure is same as that for [Ru(bbpH2)(bpy)Cl]-ClO4 except that Ru(terpy)Cl3 was used instead of Ru-(bpy)Cl3. The product was recrystallized from acetonitrileto obtain a brown red compound. Yield: 48%. ElementalAnal. Calc. for 4: C, 48.34; H, 2.84; N, 13.27. Found: C,48.29; H, 2.87; N, 13.26%. KM (X�1 cm2 mol�1 in acetoni-trile solution): 230–240; 1H NMR, (CD3)2SO, 300 MHz, d(ppm): 15.05 (bs, 2H), 9.18 (d, 2H, J = 8.1 Hz), 8.78 (t, 2H,J = 8.7 Hz), 8.71–8.60 (m, 4H), 7.91 (t, 2H, J = 7.8 Hz),7.67 (d, 2H, J = 8.2 Hz), 7.49 (d, 2H, J = 5.2 Hz), 7.30(d, 2H, J = 7.4 Hz), 7.26–7.20 (m, 2H), 7.02 (t, 2H,J = 7.7 Hz), 5.92 (d, 2H, J = 8.1 Hz). FAB MS, m/z: 412([Ru(bbpH2)]+), 645 ([Ru(terpy)(bbpH2)]+).

2.3.5. [Ru(bbaH3)(bpy)Cl]ClO4 (5)To a 30 ml methanolic solution of bis(2-benzimidazolyl

methyl)amine (139 mg, 0.5 mmol), solid ‘Ru(bpy)Cl3’(0.5 mmol, 181.75 mg) was added and the reaction mixture

was heated to reflux for 4 h. The solution was cooled, 2 mlsaturated methanolic solution of NaClO4 was then addedto the brown red reaction mixture and filtered. The filtratewas evaporated to dryness. The entire crude product waspurified on a preparative TLC plate of silica gel. An intensepink band was eluted with an acetonitrile–methanol mix-ture (9:1). On evaporation of the solvent and recrystalliza-tion from acetonitrile–methanol mixture, the product wasobtained. Yield: 39%. Elemental Anal. Calc. for 5: C,46.63; H, 3.43; N, 14.64. Found: C, 46.59; H, 3.45; N,14.66%. KM (X�1 cm2 mol�1 in acetonitrile solution):125–140; 1H NMR, (CD3)2SO, 300 MHz, d (ppm): 13.51(bs, 1H), 13.19 (bs, 1H), 9.96 (bs, 1H), 8.79–8.60 (m,2H), 8.28–8.21(m, 2H), 8.11–8.02 (m, 2H), 7.87–7.74 (m,2H), 7.54–7.41 (m, 4H), 7.32–7.17 (m, 4H), 4.79 (s, 2H),4.25 (s, 2H). FAB MS, m/z: 375 ([Ru(bbaH3-3H)]+), 531([Ru(bpy)(bbaH3-3H)]+).

2.3.6. [Ru(bbaH3)(terpy)](ClO4)2 (6)

The procedure is the same as that adopted for [Ru-(bbaH3)(bpy)Cl]ClO4, except that Ru(terpy)Cl3 was usedinstead of Ru(bpy)Cl3. The crude product was recrystal-lized from the acetonitrile–methanol (1:1) solution to givea violet red compound. Yield: 48%. Elemental Anal. Calc.for 6: C, 45.92; H, 3.20; N, 13.82. Found: C, 45.89; H, 3.22;N, 13.83%. KM (X�1 cm2 mol�1 in acetonitrile solution):230–240; 1H NMR, (CD3)2SO, 300 MHz, d (ppm): 12.22(bs, 1H), 11.55 (bs, 1H), 9.96 (bs, 1H), 8.90–8.79 (m,4H), 8.74–8.60 (m, 4H), 8.15–7.94 (m, 2H), 7.58–7.51(m, 2H), 7.49–7.33 (m, 2H), 7.36–7.20 (m, 3H), 6.98–6.93(m, 2H), 4.80 (s, 2H), 4.26 (s, 2H); FAB MS, m/z: 375([Ru(bbaH3-3H)]+), 608 ([Ru(terpy)(bbaH3-3H)]+).

2.4. X-ray crystallography

The single crystals of 2 and 4 were grown by the slowevaporation of dichloromethane–hexane solution and ace-tonitrile solution of the complexes, respectively. The singlecrystals of 3 were obtained by diffusion of ether to themethanolic solution of the complex. Data for 2 were col-lected on a Siemens P4 4-circle-diffractometer while thosefor 3 and 4 were collected on MAR research Image PlateSystem all at 293(2) K, using graphite-monochromatedMo Ka (k = 0.71073 A) radiation. The structures weresolved by direct methods and refined by least squares onF2 using SHELX-97 [39]. The non-hydrogen atoms wererefined with anisotropic displacement parameters. Allhydrogen atoms were placed at calculated positions andrefined as riding atoms using isotropic displacementparameters. The refinements converged with residuals sum-marized in Table 1. The relatively high R value for 3 is dueto poor quality of the crystals.

2.5. Computational methods

Calculations were carried out using the ADF program[40]. Starting models were taken from the crystal

Table 1Crystal data and structure refinement for 2 and 3

Complex 2 3

Empirical formula C57H46N6O10Cl2P2Ru C31H26Cl2N7O4.5RuFormula weight 1208.91 740.56Crystal system orthorhombic monoclinicSpace group Pna21 P21/nUnit cell dimensions

a (A) 23.777(5) 10.352(11)b (A) 15.292(3) 20.278(22)c (A) 15.271(3) 15.080(17)a (�) 90 90b (�) 90 94.68(1)c (�) 90 90

V (A3) 5546.6(17) 3155Z 4 4Dcalc (g cm�3) 1.448 1.559Absorption coefficient

(mm�1)0.501 0.717

Reflections collected/unique

5002/5002 19476/6072

Data/restraints/parameters

5002/1/622 6072/0/407

Goodness-of-fit on F2 1.039 1.255Final R indices [I > 2r(I)] R1 = 0.0532,

wR2 = 0.1337R1 = 0.0895,wR2 = 0.1797

R indices (all data) R1 = 0.0733,wR2 = 0.1476

R1 = 0.1019,wR2 = 0.1851

Largest difference in peakand hole (e A�3)

0.594 and �0.478 1.154 and �1.140


structures. For all elements, the ZORA approximation wasused together with the default TZP basis sets using a smallcore. In addition, Vosko, Wilk and Nusair’s local exchangecorrelation potential was used [41] together with Becke’snon-local exchange [42] and Perdew’s correlation correc-tions [43,44]. Structures were optimized till convergence,using the default options. Excitation energies were calcu-lated using Time-dependent density functional theory usingthe iterative Davidson procedure [45,46].

2.6. Optical spectroscopy

Absorption spectra of dilute acetonitrile solutions(10�3–10�5 M) were obtained with a JASCO 7850 spectro-photometer. Luminescence spectra were recorded on aSpex Fluorolog II spectrofluorimeter, equipped with aHamamatsu R928 phototube. Air-equilibrated samplesolutions were excited at the indicated wavelength, anddilution was adjusted to obtain absorbance values below0.15. Although uncorrected luminescence band maximaare used throughout the text, corrected spectra wereemployed for the determination of the luminescence quan-tum yields. The correction procedure is based on the use ofsoftware that takes care of the wavelength-dependent pho-totube response. From the wavelength-integrated area, A,of the corrected luminescence spectra, we obtained lumi-nescence quantum yields for the samples with referenceto [Ru(bpy)3]Cl2 (/r = 0.028 in air-equilibrated water)[47] and by using the following equation [48]:

//r

¼ ODr � n2 � AOD � n2

r � Ar

ð1Þ

where OD and n are absorbance values and refractive indexof the solvent, respectively. Band maxima and relativeluminescence intensities were affected by uncertainties of2 nm and 20%, respectively. Luminescence lifetimes wereobtained by using an IBH 5000F time correlated single-photon counting spectrometer, equipped with entry andexit monochromators. Excitations were performed by using375 and 465 nm Nano-LED sources, and observationswere made in the correspondence of the emission peak. Sin-gle-exponential decays were found in all cases. The uncer-tainty in the lifetime values was within 8%.

3. Results and discussion

3.1. Synthesis and characterization

Six ruthenium complexes have been synthesized usingtwo tridentate N, N, N donor ligands 2,6-bis(benzimidaz-olyl) pyridine (bbpH2) and bis(2-benzimidazolyl methyl)amine (bbaH3), by reacting various ruthenium precursorcompounds with the ligands. Detailed synthetic methodshave been detailed above. Elemental analyses and FAB-MS data are consistent with their formulae.

A broad band in the IR spectra at 1080–1120 cm�1 forcomplexes 2, 3, 4, 5 and 6 confirms the presence of perchlo-rate. The molar conductivities (KM) of compounds 1, 3 and5 are in the range 116–140 X�1 cm2 mol�1 in CH3CN, con-sistent with their 1:1 electrolyte natures and those of 2, 4

and 6 are in the range 212–240 X�1 cm2 mol�1 in the samesolvent indicating that they are 1:2 electrolytes. The dia-magnetic behavior of all compounds indicates that ruthe-nium is in +2 oxidation state with a low spin d6 system.

3.2. Description of crystal structures

3.2.1. trans-[Ru(bbpH2)(PPh3)2(CH3CN)] (ClO4)2 Æ2H2O (2)

The molecular structure of trans-[Ru(bbpH2)(PPh3)2-(CH3CN)](ClO4)2 Æ 2H2O (2) is quite similar to that of 1[24] involving trans-PPh3 groups and the equatorial planebeing made up of the bbpH2 ligand together with acetoni-trile (Fig. 1a). The important bond distances and bondangles are listed in Table 2. The Ru–N distances involvingthe bbpH2 ligand along with the corresponding angles arevery similar to those in 1, which are consistent with aslightly distorted octahedral environment for the metal.The Ru–N (MeCN) distance (2.033(9) A) is shorter thanthe Ru–N (imidazole) distances (average 2.119 A), but itis longer than the Ru–N (pyridine) distance (2.000(7) A).The average Ru–P distance is 2.397 A, which is slightlyshorter than that in 1 (2.411 A). There are hydrogen bondsbetween the N–H hydrogens of imidazole moieties and thelattice water molecules, the water molecules in turn act asH-bond donor towards the perchlorate ions. These H-

Fig. 1a. X-ray structure of the trans-[Ru(bbpH2)(PPh3)2(CH3CN)]2+

cation in 2.

Table 2Bond distances (A) and bond angles (�) for 2

Ru(1)–N(2) 2.000(7)Ru(1)–N(7) 2.033(9)Ru(1)–N(1) 2.137(8)Ru(1)–N(5) 2.101(8)Ru(1)–P(2) 2.400(2)Ru(1)–P(1) 2.394(2)N(7)–C(71) 1.158(14)C(71)–C(72) 1.442(15)

N(2)–Ru(1)–N(7) 177.7(3)N(2)–Ru(1)–N(1) 77.3(3)N(7)–Ru(1)–N(1) 104.8(3)N(2)–Ru(1)–N(5) 78.6(3)N(7)–Ru(1)–N(5) 99.3(3)N(1)–Ru(1)–N(5) 155.8(3)N(2)–Ru(1)–P(2) 89.7(2)N(7)–Ru(1)–P(2) 89.2(2)N(1)–Ru(1)–P(2) 88.7(2)N(5)–Ru(1)–P(2) 92.6(2)N(2)–Ru(1)–P(1) 91.2(2)N(7)–Ru(1)–P(1) 90.0(2)N(1)–Ru(1)–P(1) 88.3(2)N(5)–Ru(1)–P(1) 90.8(2)P(2)–Ru(1)–P(1) 176.62(8)C(71)–N(7)–Ru(1) 176.2(9)N(7)–C(71)–C(72) 177.2(12)


bonds connect the ruthenium complexes into an infinitedouble chain parallel to ‘a’ axis, where an infinite chainof water and perchlorate ions running parallel to ‘a’ axisholds the columns of ruthenium molecules, situated oneither side of it (Fig. 1b). The situation is similar to thatobserved in complex 1.

3.2.2. [Ru(bbpH2)(bpy)Cl]ClO4 (3)

The X-ray crystal structure of 3 Æ 1/2C2H5-O-C2H5

shows the Ru(II) metal centre to have a distorted octahe-dral environment (Fig. 2a). Selected bond distances andbond angles are summarized in Table 3. The trans angles

Fig. 1b. H-bonded double columnar p

vary in between 157� and 175�, whereas the cis angles varyin between 78� and 102�. The Ru–N distances involving thebbpH2 ligand are smaller in this complex compared tothose of 1 and as observed earlier, the Ru–Npy distance issmaller than the Ru–NimzH distances. The average Ru–Npy distances for the bpy ligand are larger than the Ru–Npy distance of the bbpH2 ligand, but it is shorter thanthe average Ru–Npy distances [49] in [Ru(bpy)3]2+.

Intermolecular H-bonds between the N(33)–H hydrogenof the imidazole for one molecule with Cl bound to Ru(II)of an adjacent molecule leads to formation of H-bonded

acking of 2 viewed along ‘c’ axis.

Fig. 2a. X-ray crystal structure of the [Ru(bbpH2)(bpy)Cl]+ cation in 3.

Table 3Bond distances (A) and bond angles (�) for 3

Ru1–N21 1.988(6)Ru1–N41 2.012(6)Ru1–N31 2.054(6)Ru1–N51 2.064(6)Ru1–N11 2.084(6)Ru1–Cl1 2.421(2)

N21–Ru1–N41 97.3(2)N21–Ru1–N31 78.5(2)N41–Ru1–N31 90.1(2)N21–Ru1–N51 175.4(3)N41–Ru1–N51 78.2(2)N31–Ru1–N51 102.6(2)N21–Ru1–N11 78.8(2)N41–Ru1–N11 91.9(2)N31–Ru1–N11 157.2(2)N51–Ru1–N11 100.0(2)N21–Ru1–Cl1 89.7(2)N41–Ru1–Cl1 172.9(2)N31–Ru1–Cl1 90.5(2)N51–Ru1–Cl1 94.8(2)N11–Ru1–Cl1 90.3(2)


dimers in the solid state (Fig. 2b). The other imidazoleN(18)–H hydrogen is involved in H-bonding with the per-chlorate oxygen.

3.2.3. [Ru(bbpH2)(terpy)](ClO4)2 (4)

The molecular structure of [Ru(bbpH2)(terpy)](ClO4)2

(4) is shown in Fig. 3. The lack of good crystal quality of4 resulted in poor refinement of the structure to an R1 valueof 0.2023 so that no detailed comment on the dimensionscan be made. [Crystal data for 4: empirical formula:C34H24Cl2N8O8Ru, formula weight: 844.58, crystal sys-tem/space group: triclinic/P�1, a (A)/a (�): 8.914(10)/

80.26(1), b (A)/b (�): 13.005(15)/76.37(1), c (A)/c (�):16.471(17)/82.13(1), volume/Z: 1819.5(35) A3/4, Dcalc/absorption coefficient: 1.512 g cm�3/0.636 mm�1.] How-ever, the atoms in the structure could be located wellenough to show that the ruthenium centre is surroundedby six N donor atoms from the two tridentate ligands witha distorted octahedral geometry. (Bond distances and bondangles are given in Supplementary Table 1.)

3.3. Computational chemistry

The structures of 3 and 4 were geometry optimized usingthe ADF program. To reduce the computational require-ments, Cs and C2v symmetry was imposed on 3 and 4,respectively, symmetry which is consistent with the crystalstructures. However, we have also done calculation for 4using Cs symmetry and the results are very similar to thatof using the C2v symmetry.

In 3 the Ru–N bond lengths were 2.02 A to the bpyligand and 2.03, 1.96 A to the outer and inner nitrogensof the bbpH2 ligand and the Ru–Cl distance was 2.34 A.The experimentally observed Ru–N and Ru–Cl distancesare, respectively, �0.03 A and 0.08 A longer than the calcu-lated values.

For 4, the Ru–N bond lengths were 2.03, 1.95 A for theouter and inner nitrogens in the terpy ligand (observed 2.06and 1.928 A, respectively) and 1.99, 2.04 A for the bbpH2

ligand (observed 2.09 and 2.26 A, respectively), dimensionsin agreement with those found in the crystal structure.

Details of the frontier molecular orbitals in the twostructures are given in Table 4. As expected in both casesthe LUMOs are constructed from the p* orbitals of the aro-matic ligands. For 3, the LUMO, LUMO + 3 andLUMO + 4 consist of p* orbitals from the bpy ligand,while LUMO + 1, LUMO + 2 are from the bbpH2 ligand.By contrast in 4, the p* orbitals of terpy are responsible for

Fig. 2b. The packing of [Ru(bbpH2)(bpy)Cl]ClO4 in the solid state showing the formation of a centrosymmetric dimer via N–H� � �Cl hydrogen bonds.

Table 4Orbital energies and atomic orbital contributions for the frontiermolecular orbitals of the cations in 3 and 4

MO Energy (eV) Symmetry %Ru %bipy %bbpH2 %Cl


LUMO, LUMO + 2, LUMO + 3 and LUMO + 4, whilethe p* orbitals of bbpH2 are responsible only forLUMO + 1. For the HOMOs, the three highest energyorbitals are the Ru dxy, dyz and dxz orbitals with the highestenergy being the d orbital that is not in the plane of eitherpolydentate ligand, e.g. dxz for 3 and dxy for 4. In bothcases, these three HOMOs contain a sizeable contributionfrom other atoms, in the case of 3 from the chloride in par-ticular, while in 4, the two ligands make significant contri-butions as is apparent from Table 6. The next two

Fig. 3. X-ray crystal structure of the [Ru(bbpH2)(terpy)]2+ cation in 4.

HOMOs, namely HOMO � 3 and HOMO � 4, are purep from bbpH2 in 4, but in 3 HOMO � 3 shows equivalentcontributions from this ligand and the chloride ion whileHOMO � 4 is pure p from bbpH2 as in 4.

For 3

LUMO + 4 �4.71 p*(bpy) 3 97 0 0LUMO + 3 �4.95 p*(bpy) 3 97 0 0LUMO + 2 �5.57 p*(bbpH2) 7 0 93 0LUMO + 1 �5.61 p*(bbpH2) 14 0 84 2LUMO �5.67 p*(bpy) 7 88 5 0HOMO �7.01 Rudxz,dyz,

Clpz

50 16 10 24

HOMO � 1 �7.36 Rudxy,Clpy 43 0 23 34HOMO � 2 �7.77 Rudyz 70 29 0 1HOMO � 3 �8.29 Cl pz,

bbpH2(px)4 0 55 41

HOMO � 4 �8.58 bbpH2(px) 2 0 92 6

bbpH2 in the yz-plane, bpy in the xy-plane

MO Energy (eV) Symmetry %Ru %terpy %bbpH2

For 4

LUMO + 4 �7.36 p*(terpy) 0 100 0LUMO + 3 �8.22 p*(terpy) 6 68 26LUMO + 2 �8.30 p*(terpy) 0 93 7LUMO + 1 �8.34 p*(bbpH2) 12 0 88LUMO �8.37 p*(terpy) 12 86 2HOMO �10.23 dxy, 52 16 32HOMO � 1 �10.59 dxz 58 3 39HOMO � 2 �10.64 dyz 65 29 6HOMO � 3 �11.04 p(bbpH2) 0 0 100HOMO � 4 �11.17 p(bbpH2) 1 0 99

bbpH2 in the yz-plane, terpy in the xz-plane


These orbital descriptions are similar to those observedby Gorelsky et al., for bis(bipyridine)ruthenium derivativesof redox active quinonoid complexes [50] though directcomparisons are difficult because a different program wasused.

One particular point of interest, emphasized by Gorelskyet al. is the amount of metal participation in the LUMOswhich can be correlated with p-back donation from themetal d orbitals. They found that the LUMO had over20% contribution from the metal, but that LUMO + 1and LUMO + 2 had little contribution. In the present case,by contrast in 3, there is 7% contribution to LUMO which ispredominantly a p* bpy orbital and 14% contribution toLUMO + 1 which is p*(bbpH2) orbital. In 4 there is 12%contribution to both LUMO and LUMO + 1 which are,respectively, p* orbitals for terpy and bbpH2.

3.4. 1H NMR spectra

1H NMR spectra of complexes 1, 2, 3, 4, 5 and 6 wererecorded in (CD3)2SO at room temperature. All the spectraare asymmetric in nature as evident from the number ofresonances.

In complex 1, the two N–H protons appear at d 14.96and 14.37. All other aromatic protons appear in the regiond 8.42–6.75. In complex 2 the two N–H protons of thebbpH2 ligand resonate at d 14.28, slightly upfield region,compared to 1. Three protons of coordinated CH3CNappear at d 2.86 as singlet. All aromatic protons alsoappear at relatively upfield region compared to 1.

In complexes 3 and 4, the two N–H protons of eachligand are shifted to a more downfield region with d15.33 and 15.05, respectively, as broad singlets. Aromaticprotons of 3 appear upfield compared to those of 4.

The signals at d 13.51, 13.19 and 9.96, which appear asbroad singlets for complex 5 containing the bbaH3 ligand,may be assigned as the three N–H protons of the ligand.The same signals appear slightly upfield at d 12.22, 11.55and 9.96 for complex 6. The four-methylene protons of eachcomplex resonate in two different positions at d 4.79, 4.25for 5 and d 4.80, 4.26 for 6. Other protons are aromaticand appear at d 8.90–6.93 as overlapping signals.

Table 5UV–Vis spectral data and cyclic voltametric dataa

Compounds k/nm (e/L mol�1 cm�1)

1 430 (3800), 351 (26900), 339 (23300), 317 (19500), 300b (2 402 (5200), 352 (26600), 336 (23900), 320 (20800), 262b (3 510 (8400), 352 (35000), 337 (29300), 314 (23400), 295 (44 475 (13000), 402b (4500), 347 (30700), 329 (29000), 318 (5 505 (3300), 449b (3100), 370b (4400), 318b (7500), 292 (246 517 (6200), 475b (4950), 313 (24700), 274 (19500), 236 (26

a All spectra were taken in acetonitrile solution.b Indicates shoulder.c E 0 values are referenced against Ag/AgCl electrode.

3.5. Absorption spectra

The absorption spectra of the six ruthenium(II) com-plexes with two different ligands each bearing the bis-(benzimidazolyl) moiety, namely 2,6-bis(benzimidazolyl)pyridine (bbpH2) and bis(2-benzimidazolyl methyl) amine(bbaH3), were studied in acetonitrile solution and data aretabulated in Table 5. The spectra of 1 and 2 are very similarin shape and intensity and show three well-resolved LC tran-sitions between 320 nm and 350 nm (e � 20000 M�1 cm�1)(see Supplementary figure).

In addition to the above mentioned bbpH2 based LCtransitions, the absorption spectra of 3 and 4 exhibit twomore poly-pyridine based LC transitions in the UV regionof the spectrum, at 245–295 nm and 235–275 nm (e �40000–50000 M�1 cm�1), respectively.

In all of the above complexes, a relatively weak (com-pared to the LC transitions) MLCT transition can bedetected in the 400–500 nm regions. The assignment ofthe transitions as MLCT is supported by the TD-DFT cal-culations (Tables 6 and 7) on two representative complexes3 and 4. For 3 the calculated spectrum indicates four tran-sitions in the 550–660 nm regions. The experimentallyobtained spectrum shows a broad absorption in the visibleregion centred on 510 nm (Fig. 4). As the calculated spec-trum refers to the gas phase and the halide containing com-plexes are known to be appreciably solvated in polarsolvents, so the agreement between experimental and calcu-lated spectrum may be considered good.

Similarly for 4, the calculated spectrum indicates fourtransitions in the visible region between 471 nm and621 nm, whereas the experimentally measured spectrum(Fig. 5) shows a peak at 475 nm and a shoulder at402 nm, again showing good agreement between theobserved and calculated spectrum.

It may be noted that imposition of symmetry may haveled to some differences between the observed and calculatedspectra in both the cases.

The corresponding bbaH3 derivatives 5 and 6 also showpoly-pyridine based LC transition in the range 240–310 nm, along with relatively weak MLCT transitionsaround 500–520 nm.

E1/2/Vc (DEp/mV)

13000), 258b (22300), 0.84 (107)24000) 1.41 (119)2600), 245 (31900) 1.18 (92)45400), 276b (20400), 269 (31300), 235 (48000) 1.11 (64)600), 283 (25200), 243 (45700) 1.08 (82)300) 0.79 (83)

Table 6Calculated spectrum for [Ru(bpy)(bbpH2)Cl]ClO4 (3)

Wavelength (nm) Oscillator strength Assignment (% of major transitions contributing to the band)

Symmetry A 0

1 876 0.74 · 10�2 HOMO(Ru,Cl)! LUMO(bpy p*) (98%)2 779 0.58 · 10�2 HOMO(Ru,Cl)! LUMO + 2(bbpH2 p*) (86%)3 583 0.63 · 10�2 HOMO(Ru,Cl)! LUMO + 3(bpy p*) (93%)4 565 0.20 · 10�1 HOMO � 1(Ru,Cl)! LUMO + 1(bbpH2 p*) (48%)

HOMO � 2(Ru)! LUMO + 2(bbpH2 p*) (32%)5 553 0.35 · 10�1 HOMO � 2(Ru)! LUMO + 2(bbpH2 p*) (36%)

HOMO � 2(Ru)! LUMO(bpy p*) (28%)HOMO � 1(Ru,Cl)! LUMO + 1(bbpH2 p*) (26%)

6 533 0.33 · 10�2 HOMO(Ru,Cl)! LUMO + 4(bpy p*) (44%)HOMO � 2(Ru)! LUMO(bpy p*) (27%)HOMO � 2(Ru)! LUMO + 2(bbpH2 p*) (24%)

Symmetry A00

1 843 0.20 · 10�2 HOMO(Ru,Cl)! LUMO + 1(bbpH2 p*) (97%)2 723 0.17 · 10�2 HOMO � 1(Ru,Cl)! LUMO(bpy p*) (99%)3 659 0.24 · 10�1 HOMO � 1(Ru,Cl)! LUMO + 2(bbpH2 p*) (96%)4 556 0.36 · 10�1 HOMO � 2(Ru)! LUMO + 1(bbpH2 p*) (97%)

Table 7Calculated spectrum for [Ru(terpy)(bbpH2)](ClO4)2 (4)

Wavelength (nm) Oscillator strength Assignment (% of major transitions contributing to the band)

Symmetry A1

1 621 0.15 · 10�1 HOMO(Ru)! LUMO + 2(terpy p*) (91%)2 577 0.82 · 10�2 HOMO(Ru)! LUMO + 3(terpy p*) (81%)3 471 0.65 · 10�1 HOMO � 2(Ru)! LUMO(terpy p*) (65%)

Symmetry B1

1 642 0.74 · 10�2 HOMO(Ru)! LUMO (terpy p*) (98%)2 520 0.61 · 10�2 HOMO � 2(Ru)! LUMO + 2(terpy p*) (74%)3 498 0.14 · 10�1 HOMO � 2(Ru)! LUMO + 3(terpy p*) (73%)4 464 0.29 · 10�3 HOMO � 3(bbpH2p)! LUMO + 3(terpy p*) (99%)

Symmetry B2

1 630 0.31 · 10�3 HOMO(Ru)! LUMO + 1(bbpH2 p*) (98%)2 537 0.28 · 10�2 HOMO � 1(Ru)! LUMO + 2(terpy p*) (94%)3 505 0.19 · 10�1 HOMO � 1(Ru)! LUMO + 3(terpy p*) (88%)

Fig. 4. Experimentally measured absorption spectrum for 3 in the visibleregion.

Fig. 5. Experimentally measured absorption spectrum for 4 in the visibleregion.


Fig. 7. Emission spectra of 1, 2, 3 and 4 at 77 K in CH3CN.


Thus the UV–Vis spectra show that using the bis benz-imidazoles as the backbone ligand and various pyridyl/phosphine/chloride as ancillary ligands, it is possible totune the MLCT transition over a range of 100 nm.

3.6. Luminescence spectra and lifetime measurement

The photophysical spectral properties of all the com-plexes 1–6 were studied and data are collected in Table 8.The luminescence spectra of the ruthenium(II) complexesunder investigation are collected in Fig. 6 (room tempera-ture) and Fig. 7 (77 K). No emission has been detectedfor the bbpH2-triphenylphosphine complexes 1 and 2 andfor the bbaH3-polypyridyl complexes 5 and 6 at roomtemperature.

The spectral profiles at room temperature of the bbpH2-polypyridyl complexes (Fig. 6) exhibit some kind of vib-ronic resolution, witnessing for a partial 3LC character ofthe emission, and unusual maxima in the NIR region ofthe spectrum, at 926 nm for 3 and at 812 nm for 4, witha marked Stokes shift (8800 cm�1 and 7600 cm�1, respec-tively). Quantum yields close to 10�3 combined with life-times of the triplet state of 24 ns and 14 ns yield fairlyhigh radiative constant values (kr � 104 s�1). All these fea-tures make the poly-pyridyl derivatives, particularly the

Table 8Luminescence and photophysical propertiesa

Complex 298 K 77 K

kmax (nm) / s (ns) kr (s�1) b kmax (nm) s (ls)

1 n.d. n.d. n.d. n.d. 600 3.82 n.d. n.d. n.d. n.d. 602 3.83 926 8.7 · 10�4 23.9 3.8 · 104 733 0.64 812 7.8 · 10�4 13.9 6.1 · 104 713 1.25 n.d. n.d. n.d. n.d. 630 n.d.6 n.d. n.d. n.d. n.d. n.d. n.d.

a In air-equilibrated CH3CN solutions at 298 and 77 K; kexc = 460 nm.b From kr = //s. n.d. means not determined.

Wavelength (nm)

Inte

nsity

(a.

u.)

Fig. 6. Emission spectra of complexes 3 (full line) and 4 (dotted line) at298 K in CH3CN.

bbpH2-terpyridyl complex 4, attractive as candidate forNIR emitter in solid-state devices.

The luminescent complexes were found to be insensitiveto the presence of oxygen; in fact from quantum yield andlifetime they do not significantly change even after carefulexclusion of oxygen with the freeze–pump–thaw technique.

All ruthenium(II) complexes investigated but the terpyderivative 6 are luminescent in frozen solutions at 77 K(Table 8) with an hypsochromic shift of the band maxima,according to the 3MLCT nature of the emitting state(Fig. 7). On the basis of the features observed at room tem-perature and at 77 K, the luminescence emissions areassigned to excited states of mixed 3MLCT (mainly) and3LC nature. This assignment is also supported by the anal-ysis of the frontier orbitals of the corresponding complexesshowing a partial contribution of bbpH2 nature.

3.7. Electrochemistry

The RuIII/RuII potentials for complexes are collected inTable 5. In an earlier paper [24], we discussed in detail theredox properties of 1, and compared them to those foundin a variety of related compounds. We have demonstratedthat the ligand field strength of terpy is higher than that ofbbpH2, resulting in greater stabilization of the dp orbitalsof ruthenium(II). It is well known [49] that imidazole is abetter r-donor and much poorer p-acceptor than pyridine.Both these factors result in higher energy of the HOMO(dp-manifold) for ruthenium(II) complexes of polyimidaz-ole type ligands compared to similar polypyridine com-plexes. This is amply reflected in the redox potential datagiven in Table 5. As expected replacement of p-donor chlo-ride by a good p-acceptor MeCN results in the large posi-tive shift of E0 value. E0 value for the complex[Ru(terpy)(bbpH2)]2+ is �0.13 V higher than the averagevalue of the bis complexes [Ru(terpy)2]2+ and[Ru(bbpH2)2]2+. This may be due to a combination oftwo factors:

(i) The E0 value of [Ru(bbpH2)2]2+ is unusually low,because of large steric distortion and hence weakcrystal field and less stabilization of Ru dp orbitals.


(ii) In [Ru(terpy)(bbpH2)]2+ there is a synergic interac-tion between p-donation from the imidazole frag-ments and p-acceptance by the pyridyl fragments(of terpy in 4 and bpy in 3), which is absent in[Ru(terpy)2]2+ and [Ru(bbpH2)2]2+. This is alsoreflected in the lower Ru–Npy distances in the presentset of complexes compared to that in [Ru(bpy)3]2+.

The E0 of the two complexes containing the bbaH3

ligand is as expected much lower than those found in thecorresponding bbpH2 complex.

4. Conclusion

In this paper, we have shown that using two tridentateN, N, N donor ligands each having two terminal benzimi-dazolyl moieties and with diimine/triimine/phosphine/halide as ancillary ligands, it is possible to tune theRu(III)/Ru(II) potential by 0.62 V (0.79–1.41 V) andthe visible MLCT absorption by 102 nm (402–517 nm).The electrochemical and X-ray structural data indicate thatthere is synergistic interaction between the p-donor imidaz-ole and the p-acceptor pyridine fragments. Though all thecompounds are luminescent, two of them show emissionin NIR region with good quantum yield and excited statelifetime, making them attractive candidates for solid stateapplications. The DFT calculations on these two com-plexes indicate that the metal t2g electrons are appreciablydelocalized over the ligand backbone.

Acknowledgements

D.M. acknowledges CSIR, India, for his fellowship.S.K.C. acknowledges AICTE and UGC, New Delhi, forfinancial support. We also acknowledge the use of IRand UV–Vis spectrophotometers purchased from a DST(India) – FIST Grant. We thank EPSRC (UK) and theUniversity of Reading for funds for the Image Plate Sys-tem. This work was also partly supported by the CNR Pro-ject PM-P03-ISTM-C4/PM-P03-ISOF-M5.

Appendix A. Supplementary material

CCDC 266301 and 266302 contain the supplementarycrystallographic data for 2 and 3. These data can be obtainedfree of charge via http://www.ccdc.cam.ac.uk/conts/retriev-ing.html, or from the Cambridge Crystallographic DataCentre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: [email protected] for 4 were not of sufficient quality to be deposited. Sup-plementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2006.11.009.

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tuning of redox potential and visible absorption band of ruthenium(ii) complexes of (benzimidazolyl)...

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