Polyhedron 27 (2008) 1983–1988

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Ruthenium monoterpyridine complexes with 2,6-bis(benzimidazol-2-yl)pyridine:Synthesis, spectral properties and structure

Amardeep Singh a, Bolin Chetia a, Saikh M. Mobin b, Gopal Das a, Parameswar K. Iyer a, Biplab Mondal a,*

a Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781 039, Indiab Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra 400 076, India

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

Article history:Received 29 November 2007Accepted 10 March 2008Available online 21 April 2008

Keywords:Ruthenium2,6-Bis(benzimidazol-2-yl)pyridineSpectraElectrochemistryStructure

0277-5387/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.poly.2008.03.008

* Corresponding author. Tel.: +91 361 258 2317; faE-mail address: biplab@iitg.ernet.in (B. Mondal).

Ruthenium monoterpyridine complexes with the tridentate 2,6-bis(benzimidazol-2-yl)pyridine (LH2),[Ru(trpy)(LH2)]2+, [1]2+ and [Ru(trpy)(L2�)], 2 (trpy = 2,20:60,200-terpyridine) have been synthesized. Thecomplexes have been authenticated by elemental analyses, UV–Vis, FT-IR, 1H NMR spectra and their sin-gle crystal X-ray structures. Complexes [1]2+ and 2 exhibit strong MLCT band near 475 and 509 nm,respectively, and are found to be very much dependent on solution pH. The successive pH dependent dis-sociations of the N–H protons of benzimidazole moiety of LH2 in [1]2+ lead to the formation of 2. The pro-ton induced inter-convertibility of [1]2+ and 2 has been monitored via UV–Vis spectroscopy and redoxfeatures. The two pKa values, 5.75 and 7.70, for complex [1]2+ have been determined spectroscopically.

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1. Introduction

Protons play an important role in biological processes. Protontransfer and its gradients through biomembranes are the subjectof fundamental importance in bioenergetics [1]. In photosyntheticprocesses, the electron flow can be directly coupled to the protonflow through the membrane to maintain the proton gradients.Thus, the combination of proton and electron transfer processeshas drawn a considerable attention not only for developing the bio-mimic catalysts, but also in designing molecular electronics basedon proton movements [2–4]. Several examples of proton coupledelectron transfer (PCET) processes have been reported so far indi-cating that in transition metal complexes, the ligand based proton-ation/deprotonation processes influence the metal redox potentialto a great extent [5–9]. A number of studies have been made usingvarious imidazole based ligands which can provide one or moredissociable protons [5]. However, the examples of mononuclearcomplexes with ruthenium monoterpyridine frameworks are notknown.

In the present study, we report ruthenium monoterpyridinecomplexes with the well documented benzimidazole based triden-tate ligand (LH2, Scheme 1) which can offer a combination of mod-erate p-acceptor pyridine nitrogen and p-donor imidazole nitrogen[10–13] and their proton-induced tuning of redox as well as chem-ical properties. To the best of our knowledge this work demon-strates the first example where LH2 is present in di-protonated,

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mono-protonated and deprotonated form and the single crystalX-ray structures of the complexes with protonated and deproto-nated LH2.

2. Experimental

2.1. Materials

The starting complex Ru(trpy)Cl3 was prepared according to thereported procedure [14]. 2,20:60,200-Terpyridine, 2,6-pyridinedicar-boxylic were obtained from Aldrich, USA. 1,2-Phenylenediamminewas purchased from Loba Chemie, India. Other chemicals and sol-vents were reagent grade and used as received. For spectroscopicand electrochemical studies HPLC grade solvents were used. Com-mercial tetraethyl ammonium bromide was converted into puretetraethyl ammonium perchlorate by following available proce-dure [15].

2.2. Physical measurements

UV–Vis spectra were recorded with a Perkin–Elmer Lambda-25spectrophotometer. FT-IR spectra were taken on a Perkin–Elmerspectrophotometer with samples prepared as KBr pellets. Solutionelectrical conductivity was checked using a Systronic 305 conduc-tivity bridge. 1H NMR spectra were obtained with a 400 MHz Var-ian FT spectrometer. Cyclic voltammetric, differential pulsevoltammetric and coulometric measurements were carried outusing a PAR model 273A electrochemistry system. Glassy carbonworking, platinum auxiliary electrodes and an aqueous saturated

NN

NH

NH

N N

N NNNH

N N

NN

N

LH2

(Diprotonated)LH-

(Mono-protonated)L2-

(Fully deprotonated)

-H+ -H+

Scheme 1.

1984 A. Singh et al. / Polyhedron 27 (2008) 1983–1988

calomel reference electrode (SCE) were used in a three electrodeconfiguration. The supporting electrolyte was [NEt4]ClO4 and thesolute concentration was �10�3 M. The half-wave potential E0

298

was set equal to 0.5(Epa + Epc), where Epa and Epc are anodic andcathodic cyclic voltammetric peak potentials, respectively. Allexperiments were carried out under a dinitrogen atmosphere andwere uncorrected for junction potentials. The elemental analyseswere carried out with Perkin–Elmer elemental analyser.

2.3. Crystallography

Single crystals were grown by slow diffusion followed by slowevaporation technique. The intensity data were collected using aBruker SMART APEX-II CCD diffractometer, equipped with a finefocus 1.75 kW sealed tube Mo Ka radiation (k = 0.71073 Å) at273(3) K, with increasing x (width of 0.3� per frame) at a scanspeed of 3 s/frame. The SMART software was used for data acquisi-tion. Data integration and reduction were undertaken with SAINT

and XPREP [16] software. Multi-scan empirical absorption correc-tions were applied to the data using the program SADABS [17]. Struc-tures were solved by direct methods using SHELXS-97 and refinedwith full-matrix least squares on F2 using SHELXL-97 [18]. All non-hydrogen atoms were refined anisotropically. The hydrogen atomswere located from the difference Fourier maps and refined. Struc-tural illustrations have been drawn with ORTEP-3 for Windows [19].

2.4. Synthesis of the ligand, LH2

The tridentate ligand, LH2 has been synthesized in a modifiedreported route [20]. A mixture pyridine 2,6-dicarboxylic acid(835 mg, 5 mmol) and ortho-phyenlenediamine (1080 mg,10 mmol) in 10 ml orthophosphoric acid was refluxed for 8 h. Thencool down to room temperature and poured into 200 ml pre-cooledwater with constant stirring which gives a light blue precipitate.On washing with 200 ml with 20% sodium carbonate solution,pinkish precipitate of the ligand LH2 was obtained. It was thenrecrystallized from hot methanolic solution (Yield = 40%). Anal.Calc.: C, 73.30; H, 4.21; N, 22.49. Found: C, 73.21; H, 4.23; N,22.51%.

2.5. Synthesis of the complexes [1] (ClO4)2 and 2

RuIII(trpy)Cl3 (200 mg; 0.44 mmol) and ligand (142 mg;0.46 mmol) was dissolved in 20 ml ethanol and NEt3 (0.7 mmol)was added dropwise in the mixture. It was then heated to refluxwith constant stirring for 4 h. Then the volume of mixture was re-duced to 5 ml under reduced pressure and a precooled saturatedaqueous NaClO4 solution was added. The mixture was kept inrefrigerator for overnight. The dark precipitate was then filteredoff and washed with ice cold distilled water. The product was driedin vacuo over P4O10. It was then passed through the silica columnwith CH2Cl2/CH3CN. The reddish brown color complex, [1](ClO4)2

was separated first with 4:1 CH2Cl2:CH3CN followed by complex2. Yield: complex [1](ClO4)2, 30% and 2, 40%. Anal. Calc. for[1](ClO4)2 � 2H2O: C, 46.05; H, 3.84; N, 12.64. Found: C, 46.11; H,

3.86; N, 12.61%. Anal. Calc. for 2 � 2H2O: C, 59.38; H, 4.98; N,16.29. Found: C, 59.35; H, 4.96; N, 16.25%.

2.6. Crystal data

2.6.1. Complex [1](ClO4)2 � CH3OHCCDC No. 658689, C35H28Cl2N8O9Ru, M = 876.62, triclinic,

a = 8.9369(5), b = 12.9441(6), c = 16.6331(8) Å, a = 79.843(3),b = 76.981(2), c = 81.413(3), V = 1833.20(16) Å3, space group P�1,Z = 2, T = 298(2) K, l (Mo Ka) = 0.637 mm�1, F(000) = 805,GOF = 1.051; final R indices: R1 = 0.0495 [I > 2r(I)], wR2 = 0.2087;R indices (all data): R1 = 0.0713, wR2 = 0.02325.

2.6.2. Complex 2 � 2H2OCCDC No. 658688, C34H26N8O2Ru, M = 679.67, monoclinic,

a = 14.8580(4), b = 13.2833(3), c = 18.4269(5) Å, b = 106.779(2),V = 3481.96(15) Å3, space group P2(1)/n, Z = 4, T = 298(2) K, l (MoKa) = 0.493 mm�1, F(000) = 1500, GOF = 1.009; final R indices:R1 = 0.0757 [I > 2r(I)], wR2 = 0.2084; R indices (all data):R1 = 0.0931, wR2 = 0.2274.

3. Results and discussion

The ligand has been synthesized following a reported procedure[20]. The reaction of LH2 with the ruthenium precursor complexRu(trpy)Cl3, in ethanol afforded a dark solution, from which a darkcolored solid mass has been isolated on addition of excess satu-rated aqueous NaClO4 solution (Scheme 2). Chromatographic puri-fication of the crude product on a silica gel column usingCH2Cl2:CH3CN mixture (4:1) as eluent results in initially the red-dish complex [1](ClO4)2 (112 mg, yield, 30%) (this complex hasbeen synthesized by other group in a different process [21]) fol-lowed by the pink colored neutral complex 2 (114 mg, yield, 40%).

The complexes exhibit satisfactory elemental analyses {seeexperimental section} and [1](ClO4)2 shows 1:2 conductivity[KM(X�1 cm2 mol�1), 248] in acetonitrile solution whereas 2 isneutral. The formulation of the complexes has been further sup-ported by the FT-IR and 1H NMR spectral features (see later).[1]2+, in presence of one equivalent of base affords [1a]+ whichon successive addition of another equivalent of base gives 2. Theformations of [1](ClO4)2 and 2 have been authenticated by theirsingle crystal X-ray structures.

3.1. X-ray crystallographic studies

Single crystals of [1](ClO4)2 were grown by slow diffusion of anacetonitrile solution of it in benzene followed by slow evaporationand for 2, a dichloromethane solution of it was allowed to diffuseslowly in hexane followed by slow evaporation. The crystal struc-tures of [1](ClO4)2 and 2 are shown in Fig. 1.

The single crystal of [1](ClO4)2 contains one methanol and 2contains water as solvent of crystallization in the ratio of 2:H2O = 1:2. The RuN6 chromophores in [1](ClO4)2 and 2 are in dis-torted octahedral arrangement as can be seen from the angles sub-tended at the metal ions in Table 1. It is found that the ligands LH2

Ru

Cl

N

Cl

ClN

NRu

N

N

N

NN

NRu

N

N

N

NN

N

NN

NN

N

H H

NN

NN

N

2+

+

[1]2+ 2

i) Ethanolii) NEt3iii) Ligand (LH2)

iv) reflux for 5 hrs.

N N N = terpyridine

'

''

"

""

= N NN =" ""N NN' ' '

Scheme 2.

Fig. 1. ORTEP plot with atom numbering scheme of complexes (I) [1](ClO4)2 and (II) 2. Hydrogen atoms are omitted for clarity.

Table 1Selected bond distances (Å) and bond angles (�) of complexes [1](ClO4)2 and 2

Complex [1](ClO4)2 Complex 2

Ru1–N1 2.071(7) Ru1–N1 2.051(8)Ru1–N2 1.964(7) Ru1–N2 1.951(7)Ru1–N3 2.072(7) Ru1–N3 2.050(8)Ru1–N4 2.067(7) Ru1–N4 2.064(8)Ru1–N5 2.017(7) Ru1–N5 2.029(8)Ru1–N6 2.070(7) Ru1–N6 2.075(7)

N2–Ru1–N5 177.8(3) N2–Ru1–N5 176.1(3)N2–Ru1–N4 103.9(3) N2–Ru1–N1 79.9(3)N5–Ru1–N4 78.1(3) N5–Ru1–N1 96.7(3)N2–Ru1–N6 100.1(3) N2–Ru1–N3 79.6(3)N5–Ru1–N6 77.9(3) N5–Ru1–N3 103.8(3)N4 Ru1–N6 156.0(3) N1–Ru1–N3 159.5(3)N2–Ru1–N1 79.5(3) N2–Ru1–N6 99.9(3)N5–Ru1–N1 99.5(3) N5–Ru1–N6 78.2(3)N4–Ru1–N1 90.8(3) N1–Ru1–N6 91.4(3)N6–Ru1–N1 93.1(2) N3 Ru1–N6 92.4(3)N2–Ru1–N3 79.1(3) N2–Ru1–N4 103.3(3)N5–Ru1–N3 101.8(3) N5–Ru1–N4 78.6(3)N4–Ru1–N3 94.3(3) N1–Ru1–N4 91.8(3)N6–Ru1–N3 90.6(3) N3–Ru1–N4 92.5(3)N1–Ru1–N3 158.6(3) N6–Ru1–N4 156.8(3)

A. Singh et al. / Polyhedron 27 (2008) 1983–1988 1985

and L2� in [1]2+ and 2 are coordinated in the usual meridional fash-ion and terpyridine and LH2 or L2� in [1]2+ or 2, respectively, are inplanes perpendicular to each other. The pyridine nitrogen of LH2 orL2� in [1]2+ or 2 is in trans orientation with respect to the centralpyridine nitrogen of terpyridine. Bond distances and angles associ-ated with [1]2+ and 2 match well with the reported analogousruthenium-terpy [22–26] complexes. The geometrical constrainsimposed on the meridional configurations of terpyridine and LH2

in [1]2+ and terpyridine and L2� in 2 are reflected in the respectivetrans angles: N(1)–Ru(1)–N(3), 158.6(3)�/N(4)–Ru(1)–N(6),156.0(3)� and N(1)–Ru(1)–N(3), 159.5(3)�/N(4)–Ru(1)–N(6),156.8(3)�. Consequently, Ru(1)–N(2) (central pyridine nitrogen ofterpyridine) distances in [1]2+ and 2, 1.964(7) and 1.951(7) Å,respectively, and the distance between ruthenium center and thecentral pyridine nitrogen (Np) of the ancillary ligand (LH2 andL2�) in [1]2+ and 2 are 2.017(7) and 2.029(8) Å, respectively, whichare approximately 0.05 Å shorter than the corresponding terminalRu–N bonds (Table 1). The average bite angles involving terpyri-dine/LH2 in [1]2+ and terpyridine/L2� in 2 are 79.30�/78.00� and79.75�/78.40�, respectively. It might be expected that an anionic li-gand would be a better donor compared to the neutral and hencethat will lead to shorter bonds. For complex [1](ClO4)2, the crystal

Fig. 3. Tetrameric water clusters inside hydrophilic pocket in complex 2.

0

0.5

1

350 450 550 650 750 850

Wavelength (nm)

Abs

orba

nce

Fig. 4. UV–Vis spectra of [1]2+ (—), [1a]+ (---) and 2 (- �- �) in acetonitrile:water (1:4v/v) at pH: 1.25, 6.50 and 12, respectively.

1986 A. Singh et al. / Polyhedron 27 (2008) 1983–1988

quality was not very good. We have tried several times to growcrystals from different solvent systems and at different conditions(e.g. at room temperature, at low temperature, etc.), also. However,we got the same results. The insignificant difference in the Ru–Nbond distances in the two complexes might be because of this.

A poorly refined single crystal structure of complex [1](ClO4)2

has been published previously along with spectroscopic and elec-trochemical measurements by Chattopadhyay et al. [21].

In the solid-state there is no p-stacking interactions due to theorthogonal orientation of the ligands in both the complexes. How-ever, in complex [1](ClO4)2, imidazole nitrogens are hydrogenbonded with perchlorate and methanol oxygen atoms. In additionto that they forms several weak C–HO and C–Hp type hydrogenbond interactions. It is interesting to note that in solid-state itforms alternate hydrophobic and hydrophilic layers in the bc plane(Fig. 2a). On the other hand, complex 2 forms square boxes alongab plane in the solid-state with average dimension of the boxesis �6 Å (Fig. 2b). Each box is filled with four water molecules ofcrystallization (Fig. 3).

One water molecule (O1) is double hydrogen bond donor whileother one (O2) is as double hydrogen bond donor and single hydro-gen bond acceptor. They form cyclic tetrameric water cluster withalternate arrangement of two symmetrically independent watermolecules. This water cluster is highly stabilized by strong hydro-gen bond between ligands.

3.2. UV–Vis spectroscopy

Complex [1]2+ exhibits metal-to-ligand charge transfer (MLCT)transition at 475 nm and intra-ligand p ? p* transitions appear at347 and 314 nm at pH �5 in acetonitrile/water (1:4 v/v) mixture.The MLCT band position is observed to change with the solutionpH. It appears at 475 nm at a pH 6 5; with increasing pH, 5–7, itis red shifted to 498 nm and finally, above pH 7, the band shifts fur-ther to 509 nm (Fig. 4). It is believed that in the acidic pH range of65, the di-protonated form of LH2 exists in [1]2+ in solution, whichwith increasing pH gets transformed to [1a]+ containing the mono-protonated form of LH� and the doubly-deprotonated form L2� sta-bilizes in 2 in the alkaline pH range of 7–13 (Scheme 3, Fig. 4). Thetwo pKa values, 5.75 and 7.70 are determined from the spectralchanges with the pH of the solution (Supporting Information).The similar spectral changes were reported for [Ru(dtbimpy)(bim-pyH2)]2+ and [Ru(dmbimpy)(bimpyH2)]2+ in acetonitrile/buffer(1:1 v/v) with the pKa values of 5.70, 7.33 and 6.35, 7.87, respec-tively [27]. Bond and Haga have reported similar pKa values for[Os(bpy)2(BiBzImH2)]2+ and [Ru(bpy)2(BiBzImH2)]2+. The pKa val-

Fig. 2. Packing diagram of complexes (a) [1](ClO4) 2 v

ues are also comparable to those of [Ru(L18)(bimpyH2)]2+

(L18 = 2,6-bis(1-octadecylbenzimidazol-2-yl)pyridine) in a micellesolution (10 mM Triton/0.1 M Na2SO4) (pKa = 5.35 and 6.94) [28].

3.3. IR spectroscopy

The FT-IR spectra of the complexes were recorded in KBr pellets(Figs. S1 and S2; Supporting Information). The strong, sharp per-

iewed along a-axis and (b) 2 viewed along c-axis.

NN

NN

N

H H

NN

NN

N

Ru

N

N

N

NN

NRu

N

N

N

NN

N

Ru

N

N

N

NN

N

NN

NN

N

H

1 eq. H+ (HCl)

1 eq. NaOH

+

[1a]+

1 eq. NaOH

1 eq. H+ (HCl)

N N N = terpyridine

=N NN =" ""N NN' ' '

2+

[1]2+

'

'' '

'

2

"

""

=N NN' '

Scheme 3.

A. Singh et al. / Polyhedron 27 (2008) 1983–1988 1987

chlorate vibrations at �1100 and �625 cm�1 of complex [1]2+ werefound to be absent in 2; indicating the dianionic nature of the li-gand LH2 in complex 2.

3.4. 1H NMR spectroscopy

The 1H NMR spectra of complexes [1]2+ and 2 in (CD3)2SO (Sup-porting Information, Fig. S3) show the calculated number of 24 and22 aromatic protons, respectively (overlapping between 5.5–9.2 ppm); eleven from the terpyridine group in each case; thirteenfrom the ancillary ligand LH2 in case of complex [1]2+ and elevenfrom L2� in 2. The NH protons of LH2 in [1]2+ are observed at8.8 ppm, which have, as expected, disappeared on D2O exchange[29]. The observed signals have been assigned to individual aro-matic hydrogens (Supporting Information) with the aid of 1HNMR correlation spectroscopic (COSY) experiments.

3.5. Electrochemical studies

Electrochemical studies have been performed in 1:4 acetoni-trile–water solvent mixture since the complexes are not fullysoluble in water. The RuII/RuIII couple for 2 with fully deprotonatedL2� has been observed at 0.45 V versus SCE (I in Fig. 5) whereas thesame for [1]2+ with doubly-protonated LH2 appears at 0.99 V (IIin Fig. 5). It is interesting to note that RuII/RuIII couple for [Ru(tr-py)2]2+ was reported to appear at 1.30 V [30]. Thus a decrease inpotential of 0.30 V in case of [1]2+ and 0.85 V in case of 2 has been

Fig. 5. pH (1.02–12.5) dependent cyclic voltammograms in acetonitrile:water (1:4v/v).

observed depending on the protonated (LH2) or deprotonated (L2�)form of the ancillary ligand. This can be attributed by the better r-donor characteristic of the benzimidazol as compared to pyridinegroups in trpy [31,32]. With the decrease in pH the redox wave(I in Fig. 5) of the fully deprotonated complex 2 is found to dimin-ish with a gradual growth of a new redox wave at a higher anodicpotential (II in Fig. 5, pH � 5) corresponding to [1]2+ with doubly-protonated LH2 and no more significant change is observed withfurther decrease in pH. Thus RuII/RuIII couple remains pH indepen-dent in the range of <5 and >8.5. The major Ru(II) species in theselimits of pH are [Ru(trpy)(LH2)]2+ [1]2+ and [Ru(trpy)(L2�)] 2,respectively, and the electrode processes can be defined as

½RuIIðtrpyÞðLH2Þ�2þ� ½RuIIIðtrpyÞðLH2Þ þ e� ðpH < 5Þ ðiÞ½RuIIðtrpyÞðL2�Þ�� ½RuIIIðtrpyÞðL2�Þ�a þ e� ðpH < 8:5Þ ðiiÞ

Although pH dependent UV–Vis spectral changes clearly indi-cate the formation of singly-protonated intermediate [1a]+

(Fig. 4, Scheme 2), the voltammogram involving the singly-proton-ated species [1a]+ is not apparently (Fig. 5) involved. Bond andHaga have also reported same observation with [Ru(bpy)(BiB-zImH2)]2+ [7]. Significantly, the shift in redox potential of theRuII/RuIII couple with the change in pH is reversible in nature.

The plot of RuII/RuIII potential as a function of pH is shown inFig. 6. The redox process at 0.99 V going from region A to D in-volves only the protonated species, [1]2+ and the process at0.45 V going from region C to F involves only the deprotonated spe-cies, 2 (Eqs. (i) and (ii)).

Fig. 6. The half-wave potential, E1/2 vs. pH diagram for [1]2+ in acetonitrile:water(1:4 v/v).

1988 A. Singh et al. / Polyhedron 27 (2008) 1983–1988

The slope of the lines indicate the pH dependent processes inwhich proton transfer process is coupled with electron transfer(PCET). According to Nernst equation for a reversible electrode pro-cess, the slope of DE1/2/DpH = (m/n)59 mV/pH-unit where m and nrepresent the number of protons and electrons involved in the pro-cess. From the plot (Fig. 6), it is evident that three distinct PCETprocesses are involved in the present case. The slope of 53 mV/pH in the region A–E implies one-electron/one-proton transfer pro-cess as follows (Eq. iii):

½RuIIðtrpyÞðLH2Þ�2þ� ½RuIIIðtrpyÞðLHÞ�2þ þ Hþ þ e� ðiiiÞ

The change in slope to 114 mV/pH in the region A–F correlateswith the expected two-proton/one-electron transfer process (Eq. iv):

½RuIIðtrpyÞðLH2Þ�2þ� ½RuIIIðtrpyÞðLÞ�þ þ 2Hþ þ e� ðivÞ

Finally, the electrode reaction in the region B–F comes back to aone-proton/one-electron process with a slope 51 mV/pH (Eq. v):

½RuIIðtrpyÞðLHÞ�þ� ½RuIIIðtrpyÞðLÞ�þ þHþ þ e� ðvÞ

Similar observations were reported by Ayers et al. [33] andBond and Haga [7] with [Ru(bpy-bzimH)2]2+ and [(bpy)2Ru(bib-zimH2)]2+ (where bpy-bzimH = 6-(2-benzimidazole)-2,20-bipyri-dyl; bpy = 2,20-bipyridine and bibzimH = 2,20-bibenzimidazole).

4. Conclusion

In conclusion, the present work demonstrates a unique exampleof a set of ruthenium monoterpyridine complexes, [1]2+ and 2, withboth the fully-protonated and deprotonated ancillary ligand. Boththe complexes have been isolated as solid crystals. The complexeswere characterized completely using various spectroscopic tech-niques and single crystal structures. The complex [1]2+, in solution,behaves as a dibasic acid. It, on successive addition of two equiva-lent of OH�, results into complex 2. The conversion of [1]2+ to 2 isfound to be completely reversible in nature. The proton (or pH)dependency of the spectral and redox properties of the complexeshave been studied in detail.

Acknowledgements

We would like to thank the Department of Chemistry, IndianInstitute of Technology Guwahati, India for the support and DST In-dia for FIST project. We would also like acknowledge that a poorlyrefined single crystal structure of complex [1](ClO4)2 has been pub-lished previously along with spectroscopic and electrochemicalmeasurements by Chattopadhyay et al. in Inorg. Chim. Acta 360(2007) 2231.

Appendix A. Supplementary data

CCDC 658689 and 658688 contain the supplementary crystal-lographic data for 1(ClO4)2 and 2, respectively. These data can beobtained free of charge via http://www.ccdc.cam.ac.uk/conts/

retrieving.html, or from the Cambridge Crystallographic DataCentre, 12 Union Roda, Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. The FT-IR, 1HNMR spectra, absorbance versus pH plot are given as SupportingInformation. Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.poly. 2008.03.008.

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