Synthesis, characterization, photophysical, and anion-binding studies of luminescent heteroleptic bis-tridentate ruthenium(II) complexes based on 2,6-bis(benzimidazole-2-yl)pyridine and 4′-substituted 2,2′:6′,2″ Terpyridine derivatives

Article abstract of DOI:10.1021/ic100138s

A series of heteroleptic tridentate ruthenium(II) complexes of composition [(H₂pbbzim)Ru(tpy-X)](PF₆)₂ (1-7), where H ₂pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-X = 4′-substituted terpyridine ligands

Full text of DOI:10.1021/ic100138s

Inorg. Chem. 2010, 49, 5049–5062 5049
DOI: 10.1021/ic100138s
Synthesis, Characterization, Photophysical, and Anion-Binding Studies of Luminescent
Heteroleptic Bis-Tridentate Ruthenium(II) Complexes Based on
2,6-Bis(Benzimidazole-2-yl)Pyridine and 40-Substituted 2,20:60,200 Terpyridine Derivatives
Chanchal Bhaumik,† Shyamal Das,† Debasish Saha,† Supriya Dutta,‡ and Sujoy Baitalik*,†
†Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India, and
‡Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
Received January 22, 2010
A series of heteroleptic tridentate ruthenium(II) complexes of composition [(H2pbbzim)Ru(tpy-X)](PF6)2 (1-7), where
H2pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-X = 40-substituted terpyridine ligands with X = H, p-methyl phenyl
(PhCH3),p-bromomethylphenyl (PhCH2Br),p-dibromomethylphenyl (PhCHBr2),p-cyanomethylphenyl (PhCH2CN),p-triphenyl-
phosphonium methylphenyl bromide (PhCH2PPh3Br), and 40-phenylformyl (PhCHO) groups, has been synthesized and charac-
terized by using standard analytical and spectroscopic techniques. These compounds were designed to increase the excited-state
lifetime of ruthenium(II) bisterpyridine-type complexes. The X-ray crystal structure of a representative compound 2, which crys-
tallized with monoclinic space group P2(1)/c, has been determined. The absorption spectra, redox behavior, and luminescence
properties of the ruthenium(II) complexes have been thoroughly investigated. All of the complexes display moderately strong lumi-
nescence at room temperature with lifetimes in the range of 10-58 ns. Correlations have been obtained for the Hammett σp para-
meter with their MLCT emission energies, lifetimes, redox potentials, proton NMR chemical shifts, etc. The anion binding properties
of all the complexes as well as the parent ligand H2pbbzim have been studied in acetonitrile using absorption, emission, and 1H
NMR spectral studies, and it has been observed that the metalloreceptors act as sensors for F-, AcO-, and to some extent
H2PO4-. At a relatively lower concentration of anions, a 1:1 H-bonded adduct is formed; however, in the presence of an excess of
anions, stepwise deprotonation of the two benzimidazole N-H fragments occurs, an event which is signaled by the development
of vivid colors visible with the naked eye. The receptor-anion binding constants have been evaluated. Cyclic voltammetric (CV)
measurements carried out in acetonitrile-dimethylformamide (9:1) provided evidence in favor of anion (F-,AcO-) concentration
dependent electrochemical responses, enabling 1 - 7 to act as suitable electrochemical sensors for F- and AcO- ions.
Introduction
toward the designing of receptors that can selectively recognize
anions and act as sensors. A number of compounds containing
urea, thiourea, amide, pyrrole, or imidazole subunits that are
capable of providing hydrogen bond forming sites have been
reported to exhibit strong affinity and selectivity toward certain
anions.1-12 When the metalloreceptor is designed to function
Supramolecular chemistry of anion recognition and binding
is a subject of considerable contemporary research interest.1-10
Anions being ubiquitous, quite a few of them play important
roles in biological, aquatic, environmental, and industrial pro-
cesses.1-10 Consequently, a lot of efforts have been directed
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*To whom correspondence should be addressed: E-mail: sbaitalik@
hotmail.com.
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Society of Chemistry: Cambridge, U.K., 2006.
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2010 American Chemical Society
Published on Web 05/14/2010
pubs.acs.org/IC

5050 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bhaumik et al.
as a sensor, metal fragments can be used as reporter units
for modulating a signal, usually color, fluorescence, or
electrochemical potentials, as a result of host-guest inte-
raction.3,4,6 In recent years, the development of multi-
channel sensors has emerged as a topic of intensive
studies.2-8 Inasmuch as ruthenium(II) polypyridine com-
plexes are known to display unique photophysical and
redox properties,13-17 a number of ruthenium(II) poly-
pyridine-based receptors have been designed as ion
sensors,18-23 although such sensors for anions are rela-
tively few. In contrast to tris(bidentate) complexes, struc-
turally more appealing [Ru(tpy)2]2þ type complexes give
rod-like assemblies when substituted at the 40 position
of the tpy ligands.24-27 However, usually such complexes
are practically non-luminescent at room temperature and
their excited state lifetime (τ = 0.25 ns)28 is also very short,
and therefore these are the major deterrents for using them
to act as photosensitizers. Consequently, much effort
has been devoted to designing and synthesizing triden-
tate polypyridine ligands that can produce ruthenium(II)
complexes with enhanced emission quantum yields and
excited-state lifetimes. Most of the approaches aim to
increase the energy gap between the radiative 3MLCT
3
3
and quenching MC states. Stabilization of the MLCT
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Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5051
Chart 1
terpyridine]bromide (tpy -PhCH PPh3Br), 40-formyl-2,20:60,200-
2
terpyridine (tpy -PhCHO), 43 and 2,6-bis(benzimidazole-2-yl)-
pyridine (H 2pbbzim)44 were synthesized according to the litera-
ture procedures. [Ru(H2pbbzim)Cl3] was prepared by the reaction
of RuCl3 3H 2O with H2pbbzim in refluxing ethanol.
3
Synthesis of the Ligands. 40-(p-Tolyl)-2,20:60,200-terpyridine
(tpy-PhCH3). The synthesis of tpy-PhCH3 was undertaken by
following a method previously described.41,42 A mixture 2-acetyl-
pyridine (12.5 g, 0.10 mol), p-tolualdehyde (6.2 g, 0.05 mol),
acetamide (92.0 g, 1.6 mol), and ammonium acetate (59.0 g, 0.76
mol) was stirred at reflux for 2 h. The reaction mixture was cooled to
120 ꢀC; then a solution of NaOH (10 g) in water (100 mL) was
added, and refluxing was continued for a further 2 h. At room
temperature, the black, rubber-like solid that formed was filtered off
and washed with water and then with ethanol. The raw product was
then recrystallized from ethanol. Further purification was per-
formed through basic activated alumina column chromatography
with a mixture of hexane and ethyl acetate (8:1) as the eluent. At this
H2pbbzim and terpyridine derivatives have been repor-
ted,38-40 a systematic study dealing with the receptor beha-
vior of these complexes for various anions is lacking. To this
end, we report herein a new series of luminescent heteroleptic
bis-tridentate ruthenium(II) complexes by using H2pbbzim
and different 40-substituted terpyridine derivatives for multi-
channel recognition of ions such as F-, AcO-, and to some
extent H2PO4-. To allow fine-tuning of the electronic proper-
ties, several electron withdrawing substituents have been in-
troduced in the 40 position of each tpy ligand, resulting in seven
complexes as shown in Chart 1. As will be seen, the most
striking feature of these ruthenium(II) complexes is their fairly
strong room temperature luminescence and appreciably long
excited state lifetimes.
I
stage, H NMR revealed the presence of two compounds. The
needles thus obtained (6.0 g) were dissolved in an ethanol-
dichloromethane mixture (1:1, 200 mL), and an ethanol solution
of ferrous perchlorate (2.72 g in 50 mL) was added to give imme-
diately a violet crystalline compound. The precipitate was collected
and washed with toluene (3 ꢀ100 mL). Characterization of the iso-
lated precipitate (elemental analysis, IH NMR, cyclic voltammetry)
confirms the formula [Fe(tpy-PhCH3)2(ClO4)2]. tpy-PhCH3 was
liberated from the complex by oxidation of the Fe(II) complex
by H2O2 in an alkaline medium following the procedure described
by Constable et al.45 This afforded 4.0 g of tpy-PhCH3, 24% yield.
1H NMR (CDCl3): δ (ppm) 2.42 (s, 3H), 7.31 (d, 2H, J=8.0 Hz),
7.36 (tt, 2H, J=5.2 Hz, J=0.8 Hz), 7.82 (d, 2H, J=8.0 Hz),
7.87 (tt, 2H J=5.6 Hz, J= 1.6 Hz), 8.66 (d, 2H, J=8.0 Hz),
8.71 (d, 2H J=8.0 Hz), 8.73 (s, 2H). 13C NMR (CDCl3): δ (ppm)
21.21, 118.64, 118.70, 121.34, 121.59, 123.80, 127.08, 127.24, 129.52,
129.73, 135.71, 136.85, 139.04, 149.08, 149.13, 150.21, 155.90,
156.45.
40-(p-Bromomethylphenyl)-2,20:60,200-terpyridine (tpy-PhCH2-
Br). A mixture of 40-(p-tolyl)-2,20:60,200-terpyridine (tpy-Ph-
CH3; 1.00 g, 3.09 mmol), N-bromosuccinamide (0.56 g, 3.09
mmol), and benzoyl peroxide (0.025 g, 0.10 mmol) in CCl4
(20 mL) was heated at reflux for 8 h under N2 protection and
then cooled down to room temperature. The solid that formed
was filtered and washed with dichloromethane (20 mL). The
filtrate along with the washings was rotary evaporated, and the
solid residue obtained was recrystallized from ethanol-acetone
(3:1) mixture. The resulting solid was collected and washed with
cold ethanol. Yield: 0.48 g (39%). 1H NMR (CDCl3): δ (ppm)
4.49 (s, 2H, CH2Br), 7.27-7.30 (m, 2H), 7.46 (d, 2H, J = 8.0
Hz), 7.79-7.83 (m, 4H), 8.60 (d, 2H, J = 8.0 Hz), 8.70-8.72
(m, 4H). 13C NMR (CDCl3): δ (ppm) 33.24, 119.08, 121.69,
124.19, 128.00, 129.88, 137.34, 138.67, 138.86, 149.22, 149.73,
156.02, 156.12
40-(p-Dibromomethylphenyl)-2,20:60,200-terpyridine (tpy-PhCH-
Br2). A mixture of 40-(p-tolyl)-2,20:60,200-terpyridine (tpy-Ph-
CH3; 1.00 g, 3.09 mmol), N-bromosuccinamide (1.23 g, 6.18
mmol), and benzoyl peroxide (0.049 g, 0.20 mmol) in CCl4
(20 mL) was heated at reflux for 24 h under N2 protection.
After removal of CCl4, the residue obtained was recrystallized
by dissolving it in 50 mL of an ethanol-acetone (3:1) mixture and
then keeping it in a refrigerator overnight. The solid was
collected and washed with cold ethanol. Yield: 1.2 g (80%).
1H NMR (CDCl3): δ (ppm) 6.73 (s, 1H, CHBr2), 7.37-7.40 (m,
2H), 7.73(d, 2H, J = 8.8 Hz), 7.72-7.94 (m, 4H), 8.69 (d, 2H,
J = 8.0 Hz), 8.73-8.77 (m, 4H). 13C NMR (CDCl3): δ(ppm)
40.58, 119.37, 121.78, 124.31, 127.41, 127.91, 137.49, 140.13,
142.79, 149.18, 149.33, 155.96, 156.04.
Experimental Section
Materials. Reagent grade chemicals obtained from commer-
cial sources were used as received. Solvents were purified and dried
according to standard methods. 2,20:60,200-Terpyridine (tpy), 2,6-pyri-
dinedicarboxylic acid, 1,2-phenylenediamine, and the tetrabutylam-
monium (TBA) salt of the anions were purchased from Sigma-
Aldrich. 40-(p-Methylphenyl)-2,20:60,200-terpyridine (tpy-PhCH3),
41,42
0
40-(p-bromomethylphenyl)-2,2 :60,200-terpyridine (tpy-PhCH2Br),
40-(p-dibromomethylphenyl)-2,20:60,200-terpyridine(tpy -PhC-
HBr ), 40-( p- cyanomethylphenyl)-2,20:60,200-terpyridine (tpy-Ph
2
CH2CN), [40-( p -triphenylphosphoniummethylphenyl)-2,20:60,200-
ꢀ~
(38) (a) Padilla-Tosta, M. E.; Lloris, J. M.; Martınez-Manez, R.; Pardo,
T.; Soto, J.; Benito, A.; Marcos, M. D. Inorg. Chem. Commun. 2000, 3, 45–48.
ꢀ~
ꢀ
(b) Padilla-Tosta, M. E.; Lloris, J. M.; Martínez-Manez, R.; Pardo, T.; Sancenon,
F.; Soto, J.; Marcos, M. D. Eur. J. Inorg. Chem. 2001, 1221–1226. (c) Licini, M.;
Williams, J. A. G. Chem. Commun. 1999, 1943–1944. (d) Hamilton, A. D.;
Linton, B. Chem. Rev. 1997, 97, 1669–1680. (e) Goodman, M. S.; Jubian, V.;
Linton, B.; Hamilton, A. D. J. Am. Chem. Soc. 1995, 117, 11610–11611.
(39) (a) Yam, V. W. W. Acc. Chem. Res. 2002, 35, 555–563. (b) Wong,
K. M. C.; Tang, W. S.; Lu, X. X.; Zhu, N.; Yam, V. W. W. Inorg. Chem. 2005, 44,
1492–1498. (c) Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Lu, X. X.; Cheung,
K. K.; Zhu, N. Chem.;Eur. J. 2002, 8, 4066–4076. (d) Yam, V. W. W.; Wong,
K. M. C.; Zhu, N. Angew. Chem., Int. Ed. 2003, 42, 1400–1403.
(40) (a) Xiaoming, X.; Haga, M.; Inoue, T. M.; Ru, Y.; Addison, A. W.;
Kano, K. J. Chem. Soc., Dalton Trans. 1993, 2477–2484. (b) Haga, M.;
Takasugi, T.; Tomie, A.; Ishizuya, M.; Yamada, T.; Hossain, M. D.; Inoue, M.
Dalton Trans. 2003, 2069–2079. (c) Singh, A.; Chetia, B.; Mobin, S. M.; Das, G.;
Iyer, P. K.; Mondal, B. Polyhedron 2007, 27, 1983–1988. (d) Mishra, D.;
Barbieri, A.; Sabatini, C.; Drew, M. G. B.; Figgie, H. M.; Sheldrick, W. S.;
Chattopadhyay, S. K. Inorg. Chim. Acta 2007, 360, 2231–2244. (e) Yang, H.;
Chen, W.-T.; Qiu, D.-F.; Bao, X.-Y.; Xing, W.-R.; Liu, S.-S. Huaxue Xuebao
2007, 65, 2959–2964.
(41) Case, F. H.; Kaspen, T. J. J. Am. Chem. Soc. 1956, 78, 5842–5844.
(42) Spahni, W.; Calzaferri, G. Helv. Chim. Acta 1984, 67, 450–454.
(43) Pott, K. T.; Usifer, D. A.; Abruna, H. D. J. Am. Chem. Soc. 1987,
109, 3961–3967.
(44) (a) Addison, A. W.; Burke, P. J. Heterocycl. Chem. 1981, 18, 803–
805. (b) Addison, A. W.; Rao, T. N.; Wahlgren, C. G. J. Heterocycl. Chem. 1983,
20, 1481–1484.
(45) Constable, E. C.; Ward, M. D.; Corr, S. Inorg. Chim. Acta 1988, 141,
201–203.

5052 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bhaumik et al.
[40-(p-Triphenylphosphoniummethylphenyl)-2,20:60,200-terpyri-
dine] Bromide (tpy-PhCH2PPh3Br). A mixture of 40-(p-bromo-
methylphenyl)-2,20:60,200-terpyridine (tpy-PhCH2Br; 0.372 g, 0.93
mmol) and triphenylphosphine (0.606 g, 2.31 mmol) in toluene
(25 mL) was heated at reflux for 10 h and then cooled down to room
temperature. The white precipitate that was obtained was filtered
off and washed thoroughly with toluene and then dried under a
vacuum. This afforded the phosphonium salt (tpy-PhCH2PPh3Br)
H, 2.67; N, 11.85. 1H NMR data {300 MHz, DMSO-d6, SiMe4,
δ (ppm)}: 13.81 (s, 2H, NH (imidazole)), 9.15 (d, 2H, J = 8.1 Hz,
H3), 8.73 (d, 4H, J = 8.2 Hz, 2H30 þ 2H11), 8.66 (t, 1H, J = 8.1
Hz, H10), 8.60 (t, 1H, J = 7.5 Hz, H40), 7.89 (t, 2H, J = 7.8 Hz,
H4), 7.63 (d, 2H, J = 8.2 Hz, H6), 7.46 (d, 2H, J = 5.4 Hz, H12),
7.22 (m, 4H, 2H5 þ 2H13), 6.97 (t, 2H, J = 7.7 Hz, H14), 5.89 (d,
2H, J = 8.2 Hz, H15). ESI-MS (positive, CH3CN): m/z 323.15
(100%) [(H2pbbzim)Ru(tpy)]2þ; 645.29 (42%) [(Hpbbzim)Ru-
(tpy)]þ. UV-vis [CH3CN; λmax, nm (ε, M-1 cm-1)]: 478
(15 700), 405 (9200), 348 (47 350), 313 (68 700), 271 (41 800).
[(H2pbbzim)Ru(tpy-PhCH3)](PF6)2 (2). Yield: 65%. Anal.
calcd for C41H30N8P2F12Ru: C, 48.01; H, 2.94; N, 10.92. Found:
C, 47.92; H, 3.01; N, 10.85. 1H NMR data {300 MHz, DMSO-
d6, SiMe4, δ (ppm)}: 14.50 (s, 2H, NH (imidazole)), 9.54 (s, 2H,
H30), 9.0 (d, 2H, J = 8.1 Hz, H3), 8.78 (d, 2H, J = 7.9 Hz, H11),
8.6 (t, 1H, J = 7.7 Hz, H10), 8.48 (d, 2H, J=7.8 Hz, H8), 7.92 (t,
2H, J = 7.9 Hz, H4), 7.67- 7.59 (m, 4H, 2H6 þ 2H7), 7.47 (d, 2H,
J = 5.5 Hz, H12), 7.29-7.14 (m, 4H, 2H5 þ 2H13), 6.99 (t, 2H,
J=7.7 Hz, H14), 6.04 (d, 2H, J = 8.3 Hz, H15), 2.53 (s, 3H, H9).
ESI-MS (positive, CH3CN): m/z 367.95 (100%) [(H2pbbzim)-
Ru(tpy-PhCH3)]2þ; 734.87 (40%) [(Hpbbzim)Ru(tpy-PhCH3)]þ.
UV-vis [CH3CN; λmax, nm (ε, M-1 cm-1)]: 487 (18 000),
405 (5100), 348 (34 500), 313 (56 300), 285 (48 300).
1
as a white solid (0.53 g, 86% yield). HNMR (CDCl3): δ (ppm)
5.55 (d, 2H, J=14.4 Hz, -CH2PPh3), 7.20 (d, 2H, J = 8.0 Hz,),
7.26 (t, 2H, J=6.4 Hz), 7.51 (d, 2H, J=8.0 Hz), 7.50-7.80 (m,
17H), 8.45 (s, 2H), 8.51(d, 2H, J=8.0 Hz), 8.59 (d, 2H, J=4.4 Hz).
13C NMR (CDCl3): δ(ppm) 117.60, 118.46, 118.92, 121.66, 124.15,
127.79, 128.43, 130.63, 132.39, 134.73, 134.63, 135.22, 137.33,
149.17, 155.99, 158.34, 159.29.
40-(p-Cyanomethylphenyl)-2,20:60,200-terpyridine (tpy-PhCH2-
CN). A mixture of 40-(p-bromomethylphenyl)-2,20:60,200 terpyr-
idine (tpy-PhCH2Br; 0.402 g, 1.0 mmol), NaCN (0.049 g,
1.0 mmol), and 20 mL of DMF was stirred magnetically for
10 h at room temperature. The resulting mixture was added to
100 mL of water and then extracted with CH2Cl2 (2 ꢀ 30 mL).
After removing solvent, the crude product was purified by
recrystallization from a dichloromethane-hexane (1:10) mix-
ture. This gave the product tpy-PhCH2CN (0.29 g, 83% yield).
1H NMR (CDCl3): δ (ppm) 3.83 (s, 2H, -CH2CN), 7.32-7.35
(m, 2H), 7.3-7.36 (m, 2H), 7.47 (d, 2H, J=8.0 Hz), 7.87 (td, 2H,
J=8.0 Hz, J = 2.0 Hz), 7.90 (d, 2H, J = 7.6 Hz), 8.66 (d, 2H,
J = 8.0 Hz), 8.70-8.72 (m, 2H). 13C NMR (CDCl3): δ (ppm)
23.68, 117.88 (CN-), 118.98, 121.63, 124.19, 128.30, 128.73,
130.87, 137.21, 138.62, 149.35, 149.52, 156.25.
[(H2pbbzim)Ru(tpy-PhCH2Br)](PF6)2 (3). Yield: 41%. Anal.
calcd for C41H29N8BrP2F12Ru: C, 44.58; H, 2.64; N, 10.14.
Found: C, 44.49; H, 2.70; N, 10.02. 1H NMR data {300 MHz,
DMSO-d6, SiMe4, δ (ppm)}: 14.75 (s, 2H, NH (imidazole)), 9.55
(s, 2H, H30), 8.98 (d, 2H, J=8.0 Hz, H3), 8.73 (d, 2H, J=7.8 Hz,
H8), 8.61-8.53 (m, 3H, 1H10 þ 2H11), 7.92 (t, 2H, J=7.8 Hz,
H4), 7.74 (d, 2H, J=8.0 Hz, H7), 7.62(d, 2H, J = 8.1 Hz, H6),
7.46 (d, 2H, J=5.3 Hz, H12), 7.22 (m, 4H, nr, 2H5 þ 2H13), 6.96
(t, 2H, J=7.7 Hz, H14), 6.02 (d, 2H, J = 8.2 Hz, H15), 4.31 (s,
2H, H9). ESI-MS (positive, CH3CN): m/z 407.31 (100%)
[(H2pbbzim)Ru(tpy-PhCH2Br)]2þ; 813.71 (25%) [(Hpbbzim)-
Ru(tpy-PhCH2Br)]þ. UV-vis [CH3CN; λmax, nm (ε, M-1
cm-1)]: 488 (13 100), 405 (5300), 348 (28 400), 313 (40 500), 285
(28 500).
40-(p-Formylphenyl)-2,20:60,200-terpyridine (tpy-PhCHO). A
suspension of 40-(p-dibromomethylphenyl)-2,20:60,200-terpyri-
dine (tpy-PhCHBr2) (0.45 g, 0.93 mmol) and CaCO3 (1.00 g,
10 mmol) in a mixture of 1,4-dioxane-water (4:1, 50 mL) was
heated at reflux for 30 h. The resulting mixture was then filtered.
The filtrate was extracted with CH2Cl2 (2 ꢀ 50 mL). The extracts
were combined and washed successively with a 0.1 M aqueous
EDTA solution (50 mL), 0.1 M aqueous HCl (50 mL), and then
with water (2 ꢀ 50 mL). Solvent was removed on a rotary eva-
porator after drying over anhydrous sodium sulfate. This
[(H2pbbzim)Ru(tpy-PhCH2CN)](PF6)2 (4). Yield: 40%. Anal.
calcd for C42H29N9P2F12Ru: C, 48.00; H, 2.78; N, 12.00. Found:
C, 47.71; H, 2.89; N, 11.48. 1H NMR data {300 MHz, DMSO-
d6, SiMe4, δ (ppm)}: 14.70 (s, 2H, NH (imidazole)), 9.55 (s, 2H,
H30), 8.98 (d, 2H, nr, H3), 8.69 (d, 2H, J = 7.9 Hz, H8),
8.60-8.52 (m, 3H, 1H10 þ 2H11), 7.91 (t, 2H, J = 7.1 Hz,
H4), 7.77 (d, 2H, J = 8.2 Hz, H7), 7.60 (d, 2H, J = 8.1 Hz, H6),
7.45 (d, 2H, nr, H12), 7.23-7.13 (m, 4H, 2H5 þ 2H13), 6.89 (t,
2H, J=7.4 Hz, H14), 5.97 (d, 2H, J=8.2 Hz, H15), 4. 72 (s, 2H,
H9). ESI-MS (positive, CH3CN): m/z 380.54 (100%) [(H2pbb-
zim)Ru(tpy-PhCH2CN)]2þ; 760.13 (10%) [(Hpbbzim)Ru (tpy-
PhCH2CN)]þ. UV-vis [CH3CN; λmax, nm (ε, M-1 cm-1)]: 486
(19 900), 404 (6200), 348 (39 300), 314 (55 400), 285 (55 500).
[(H2pbbzim)Ru(tpy-PhCHBr2)](PF6)2 (5). Yield: 45%. Anal.
calcd for C41H28N8Br2P2F12Ru: C, 41.60; H, 2.38; N, 9.46.
1
furnished product tpy-PhCHO (0.26 g, 77% yield). H NMR
(CDCl3): δ (ppm) 7.32-7.35 (m, 2H), 7.86 (t, 2H, J=7.6 Hz,),
7.96-8.03 (q, 4H, J = 8.8 Hz,), 8.64 (d, 2H, J=8.0 Hz,), 8.68 (d,
2H, J=7.6 Hz,), 8.73 (s, 2H), 10.05 (s, 1H). 13C NMR (CDCl3): δ
(ppm) 119.15, 121.58, 124.27, 128.21, 130.49, 136.61, 137.17,
144.59, 148.97, 149.37, 156.02, 156.37, 192.07.
Synthesis of the Metal Complexes. The complexes were pre-
pared under oxygen and moisture-free dinitrogen using stan-
dard Schlenk techniques.
General Procedure for Preparation of Heteroleptic Ruthenium(II)
Complexes [(H2pbbzim)Ru(tpy-X)](PF6)2 (1-7). [(H2pbbzim)-
RuCl3] (100 mg, 0.19 mmol) was suspended into ethylene glycol
(30 mL) and heated at 100 ꢀC with continuous stirring. To the
suspension was added 0.20 mmol of different 40-substituted
2,20:60,200-terpyridines (tpy-X), and the reaction mixture was
heated at 200 ꢀC under nitrogen protection for 24 h. The
resulting solution was cooled, and the hexafluorophosphate salt
of the complexes was precipitated by pouring the solution into
an aqueous solution of NH4PF6 (0.50 g in 10 mL of water). The
precipitate obtained was filtered and washed with water and
then ether and dried under a vacuum. The compound was then
purified by silica gel column chromatography using a mixture of
CH3CN and 10% aqueous KNO3 (10:1) as the eluent, followed
by a subsequent anion exchange reaction with NH4PF6 to give
the desired compound. The compound was finally recrystallized
from an acetonitrile-methanol (1:1) mixture in the presence of a
few drops of aqueous 10-4 M hexfluorophosphoric acid.
[(H2pbbzim)Ru(tpy)](PF6)2 (1). Yield: 61%. Anal. calcd for
C34H24N8P2F12Ru: C, 43.65; H, 2.58; N, 11.97. Found: C, 43.51;
1
Found: C, 41.59; H, 2.47; N, 9.41. H NMR data {300 MHz,
DMSO-d6, SiMe4, δ (ppm)}: 14.90 (s, 2H, NH (imidazole)), 9.53
(s, 2H, H30), 8.97 (d, 2H, J = 8.0 Hz, H3), 8.60 (m, 4H, 2H8 þ
2H11), 8.47 (t, 1H, J=7.9 Hz, H10), 7.89 (t, 2H, J=7.8 Hz, H4),
7.83 (d, 2H, J=7.2 Hz, H7), 7.53 (d, 2H, J = 8.0 Hz, H6), 7.44
(d, 2H, J=5.3 Hz, H12), 7.21 (t, 2H, J = 6.4 Hz, H5), 7.05 (t, 2H,
J=6.5 Hz, H13), 6.79 (t, 2H, J = 7.7 Hz, H14), 5.93 (s, 1H, H9),
5.91 (d, 2H, J=8.1 Hz, H15). ESI-MS (positive, CH3CN): m/z
446.75 (100%) [(H2pbbzim)Ru(tpy-PhCHBr2)]2þ; 892.57 (40%)
[(Hpbbzim)Ru(tpy-PhCHBr2)]þ. UV-vis [CH3CN; λmax, nm
(ε, M-1 cm-1)]: 490 (23 500), 406 (7800), 348 (49 500), 314
(63 000), 286 (63 700).
[(H2pbbzim)Ru(tpy-PhCH2PPh3Br)](PF6)2 (6). Yield: 48%.
Anal. calcd for C59H44N8BrP3F12Ru: C, 51.84; H, 3.24; N, 8.20.
Found: C, 51.74; H, 3.28; N, 8.12. H NMR data {300 MHz,
1
DMSO-d6, SiMe4, δ (ppm)}: 14.80 (s, 2H, NH (imidazole)), 9.51
(s, 2H, H30), 8.93 (d, 2H, J = 8.1 Hz, H3), 8.70 (m, 2H, H8), 8.56

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5053
(t, 1H, nr, H10), 8.50 (d, 2H, J = 8.1 Hz, H11), 7.99-7.77 (m,
17H, 2H4 þ 15H(PPh3)), 7.59 (d, 2H, J = 8.3 Hz, H7), 7.45 (d,
2H, J = 5.3 Hz, H12), 7.36 (d, 2H, J = 6.4 Hz, H6), 7.22 (t, 2H,
J = 6.5 Hz, H5), 7.16 (t, 2H, J = 7.3 Hz, H13), 6.88 (t, 2H, J =
7.6 Hz, H14), 5.94 (d, 2H, J = 8.2 Hz, H15), 5.41 (s, 1H, H9),
5.36 (s, 1H, H9). ESI-MS (positive, CH3CN): m/z 497.47
(100%) [(H2pbbzim)Ru(tpy-PhCHPPh3)]2þ. UV-vis [CH3-
CN; λmax, nm (ε, M-1 cm-1)]: 488 (22 600), 406 (7000), 348
(44 700), 313 (62 600), 286 (62 900).
Table 1. Crystallographic Data for [(H2pbbzim)Ru(tpy-PhCH3)]2þ [2]2þ
2
formula
fw
T (K)
C41H30N8Ru
735.80
296(2)
cryst syst
space group
a (A)
b (A)
c (A)
monoclinic
P2(1)/c
12.770(4)
16.303(5)
18.976(5)
90.00
104.121(4)
90.00
3831.4(19)
1.276
[(H2pbbzim)Ru(tpy-PhCHO)](PF6)2 (7). Yield: 51%. Anal.
calcd for C41H28N8OP2F12Ru: C, 47.36; H, 2.71; N, 10.77. Found:
C, 47.02; H, 2.80; N, 10.45. 1H NMR data {300 MHz, DMSO-d6,
SiMe4, δ (ppm)}: 15.02 (s, 2H, NH (imidazole)), 10.26 (s, 1H, H9),
9.66 (s, 2H, H30), 9.03 (d, 2H, J = 8.0 Hz, H3), 8.81 (d, 2H, J = 8.0
Hz, H8), 8.78 (d, J = 8.0 Hz, 2H, H11), 8.64 (t, 1H, J = 8.0 Hz,
H10), 8.33 (d, 2H, J = 8.5 Hz, H7), 7.97 (t, 2H, J = 5.9 Hz, H4),
7.66 (d, 2H, J = 8.0 Hz, H6), 7.50 (d, 2H, J = 5.0 Hz, H12), 7.26 (t,
2H, J= 8.5 Hz, H13), 7.25 (t, 2H, J= 7.0 Hz, H5), 7.00 (t, 2H, J=
8.0 Hz, H14), 6.05 (d, 2H, J = 8.0 Hz, H15). ESI-MS (positive,
R (deg)
β (deg)
γ (deg)
3
ꢀ
V (A )
Dc (g cm-3
)
Z
4
0.448
1504
μ (mm-1
F(000)
)
θ range (deg)
data/restraints/params
1.6 - 25.0
6738/0/451
1.065
0.0689
0.2358
1.35/-0.51
CH3CN): m/z 374.65 (100%) [(H2pbbzim)Ru(tpy-PhCHO)]2þ
748.32(22%) [(Hpbbzim)Ru(tpy-PhCHO)]þ. UV-vis [CH3CN;
max, nm (ε, M-1 cm-1)]: 489 (18 100), 405 (6000), 347 (38 000),
;
GOF on F2
R1 [I >2σ(I)]a
wR2b (all data)
ΔFmax/ΔFmin (e A)
λ
315 (50 000), 287 (52 000).
P
P
P
P
Physical Measurements. Elemental (C, H, and N) analyses
were performed on a Perkin-Elmer 2400II analyzer. Electro-
spray ionization mass spectra (ESI-MS) were obtained on a
Micromass Qtof YA 263 mass spectrometer. 1H and {1H-1H}
COSY spectra were obtained on a Bruker Avance DPX 300
spectrometer using DMSO-d6 solutions. For a typical titration
experiment, 3 μL aliquots of a tetrabutylammonium (TBA) salt
of the anion (0.2 M in DMSO-d6) were added to a DMSO-d6
solution of the complexes (2.5 ꢀ 10-3 M).
a R1(F) = [ ||F0| - |FC||/ |F0]. b wR2 (F2) = [ w(F02 - FC2)2/
w(F02)2]1/2
.
working electrode, Pt auxiliary electrode, and an aqueous Ag/AgCl
reference electrode were used. The cyclic voltammetric (CV) and
square wave voltammetric (SWV) measurements were carried out
at 25 ꢀC in an acetonitrile solution of the complex (ca. 1 mM), and
the concentration of the supporting electrolyte (TEAP) was main-
tained at 0.1 M. All of the potentials reported in this study were
referenced against the Ag/AgCl electrode, which, under the given
experimental conditions, gave a value of 0.36 V for the ferrocene/
ferrocenium couple. For electrochemical titrations, 25 μL aliquots
of TBA salts of the anions (2.0 ꢀ 10-2 M in acetonitrile) were added
to a 5 mL (1.0 ꢀ 10-3 M) solution of sensors 1-7 in a acetoni-
trile-dimethylformamide (9:1) mixture.
Electronic absorption spectra were obtained with a Shimadzu
UV 1800 spectrophotometer at room temperature. For a typical
titration experiment, 2 μL aliquots of TBA salts of F-, Cl-, Br-,
I-, NO3-, and AcO- and H2PO4- (4.0 ꢀ 10-3 M in acetonitrile)
were added to a 2.5 mL solution of the complexes (2.0 ꢀ 10-5
absorbance data using eq 1.46
M
in acetonitrile). The binding constants were evaluated from the
Experimental uncertainties were as follows: absorption max-
ima, ( 2 nm; molar absorption coefficients, 10%; emission
maxima, ( 5 nm; excited-state lifetimes, 10%; luminescence
quantum yields, 20%; redox potentials, (10 mV.
Aobs ¼ ðA0 þ A¥K½GꢁTÞ=ð1 þ K½GꢁTÞ
ð1Þ
where Aobs is the observed absorbance, A0 is the absorbance of
the free receptor, A¥ is the maximum absorbance induced by the
presence of a given anionic guest, [G]T is the total concentration
of the guest, and K is the binding constant of the host-guest
entity. Binding constants were performed in duplicate, and the
average value is reported.
Emission spectra were recorded on Perkin-Elmer LS55
fluorescence spectrophotometer. The room temperature spectra
were obtained in acetonitrile solutions, while the spectra at 77 K
were recorded in 4:1 ethanol-methanol glass. Photolumines-
cence titrations were carried out with the same sets of solutions
as were made with spectrophotometry. Quantum yields were
determined by a relative method using [Ru(bpy)3]2þ in the same
solvent as the standard.
Time-correlated single-photon-counting (TCSPC) measure-
ments were carried out in acetonitrile for the luminescence decay
of complexes 1-7. For TCSPC measurement, the photoexcitation
was made at 440 nm using a picosecond diode laser (IBH
Nanoled-07) in an IBH Fluorocube apparatus. The fluorescence
decay data were collected on a Hamamatsu MCP photomultiplier
(R3809) and were analyzed by using IBH DAS6 software.
The electrochemical measurements were carried out with a
BAS 100B electrochemistry system. A three-electrode assembly
comprising a Pt (for oxidation) or glassy carbon (for reduction)
X-Ray Crystal Structure Determination. Crystals suitable for
structure determination were obtained by diffusing toluene to a
solution of 2 in acetonitrile-dichloromethane (1:4). X-ray
diffraction data for the crystal of 2 mounted on a glass fiber
and coated with perfluoropolyether oil was collected at 296 K on
a Bruker AXS SMART APEX II diffractometer equipped with
a CCD detector using graphite-monochromated Mo KR radia-
tion (λ=0.71073 A). Crystallographic data and details of struc-
ture determination are summarized in Table 1. The data were
processed with SAINT,47 and absorption corrections were
made with SADABS.47 The structure was solved by direct and
Fourier methods and refined by full-matrix least-squares on the
basis of F2 using the WINGX software, which utilizes SHELX-
97.48 For the structure solution and refinement, the SHELXTL
software package49 was used. The nonhydrogen atoms were
refined anisotropically, while the hydrogen atoms were placed
with fixed thermal parameters at idealized positions. In the
structure of 2, both the anionic perchlorates are severely dis-
ordered, and the structure was finally solved by removing the
perchlorates by running the program SQUEEZE. The electron
density map also showed the presence of some unassignable
(47) SAINT, version 6.02; SADABS, version 2.03; Bruker AXS Inc.:
Madison, WI, 2002.
€
€
(48) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen,
Germany, 1997.
(46) Schneider, H.-J.; Yatsimirsky, A. Principles and Methods in Supra-
molecular Chemistry; John Wiley & Sons: England, 2000; p 142.
(49) SHELXTL, version 6.10; Bruker AXS Inc.: Madison, WI, 2002.

5054 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bhaumik et al.
Figure 1. ORTEP representation of [(H2pbbzim)Ru(tpy-PhCH3)]2þ (22þ) showing 20% probability of thermal ellipsoids.
peaks, which were removed by running the program SQUEEZE.50
The CCDC reference number is 761796 for 2.
Table 2. Selected Bond Distances (A) and Angles (deg) for [(H2pbbzim)Ru(tpy-
PhCH3)]2þ [2]2þ
2
Results and Discussion
Synthesis. The ruthenium(II) mixed chelates (1-7)
have been straightforwardly prepared in fairly good
yields (40-65%) by reacting [(H2pbbzim)RuCl3] with
the 40-substituted terpyridine derivatives in ethylene gly-
col in the temperature range 190-200 ꢀC under nitrogen
protection and then precipitating the complexes either as
hexafluorophosphate or perchlorate salts. Temperature
was found to be a key factor for the synthesis of the metal
complexes. The compound was then subjected to silica gel
column chromatography using acetonitrile and 10%
aqueous KNO3 (10:1) as the eluent, followed by a sub-
sequent anion exchange reaction either with NH4PF6 or
with NaClO4. The compounds were finally recrystallized
from an acetonitrile-methanol (1:1) mixture under
mildly acidic conditions to keep the benzimidazole NH
protons intact. All the compounds have been character-
ized by their elemental (C, H, and N) analyses, ESI-MS,
UV-vis, and 1H NMR spectroscopic measurements, and
the results are given in the Experimental Section.
Description of the Crystal Structure of [(H2pbbzim)-
Ru(tpy-PhCH3)](ClO4)2 (2). ORTEP representation of
22þ along with the atom labels are shown in Figure 1, and
selected bond distances and angles are given in Table 2. The
structure displays the expected geometry, with both ligands
coordinated in tridentate, meridional fashion to the ruthe-
nium(II) center. The peripheral rings move toward the metal
center, as evidenced by a small decrease of the N6-C24-
C25 and N8-C30-C29 (for tpy-PhCH3) andN2-C7-C8
and N5-C13-C12 (for H2pbbzim) angles compared to the
ideal angle of 120ꢀ. It is to be noted that the interligand angle
of the central nitrogen atoms N3-Ru-N7 is 174.49ꢀ, whe-
reas the other two angles, N2-Ru-N5 and N6-Ru-N8,
are 154.84 and 158.29ꢀ, respectively, for this heteroleptic
Ru-N(2)
2.086(5)
2.015(6)
2.088(8)
2.069(5)
1.935(5)
2.058(5)
77.9(2)
154.8(3)
92.5(2)
101.2(2)
92.9(2)
77.2(3)
106.4(2)
174.49(18)
95.3(2)
91.1(2)
103.9(3)
92.9(2)
79.00(19)
158.3(2)
79.29(18)
Ru-N(3)
Ru-N(5)
Ru-N(6)
Ru-N(7)
Ru-N(8)
N(2)-Ru-N(3)
N(2)-Ru-N(5)
N(2)-Ru-N(6)
N(2)-Ru-N(7)
N(2)-Ru-N(8)
N(3)-Ru-N(5)
N(3)-Ru-N(6)
N(3)-Ru-N(7)
N(3)-Ru-N(8)
N(5)-Ru-N(6)
N(5)-Ru-N(7)
N(5)-Ru-N(8)
N(6)-Ru-N(7)
N(6)-Ru-N(8)
N(7)-Ru-N(8)
complex. The corresponding angles for the parent [Ru-
(tpy)2]2þ complex are 178.8ꢀ, 158.4ꢀ, and 159.1ꢀ.51 The
ruthenium-nitrogen bond lengths in tpy-PhCH3 are within
1.936-2.069 A, whereas in H2pbbzim they are relatively
longer, 2.015-2.088 A. It is interesting to note that, although
the peripheral nitrogen-to-ruthenium bond is similar in the
other four instances, the central nitrogen-to-ruthenium
bond 1.935 A (tpy-PhCH3) and 2.015 A (H2pbbzim) become
shortened. Overall, only small differences in the coordina-
tion geometry have been detected when a tpy-X ligand is
replaced by H2pbbzim.
(51) (a) Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, O.; Kennard, O.;
Macrae, C. F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J. M.; Watson, D. G.
J. Chem. Inf. Comput. Sci. 1991, 31, 187–204. (b) Chartrand, D.; Theobald, I.;
Hanan, G. S. Acta Crystallogr., Sect. E: Struct. Rep. Online 2007, 63, 1561.
€
(c) Schulze, B.; Friebe, C.; Hager, M. D.; Winter, A.; Hoogenboom, R.; Gorls, H.;
(50) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13.
Schubert, U. S. Dalton. Trans. 2009, 787–794.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5055
from the literature are given in Table 3. The absorption
spectra of all the complexes are similar, showing two very
intense bands (ε = 28 000-70 000 M-1 cm-1) in the UV
region and one moderately intense band (ε = 13 000-
23 500 M-1 cm-1) in the visible region. The lowest energy
band observed between 478 nm (for 1) and 490 nm (for 5)
can be attributed to metal (dπ)-π*(tpy-X) MLCT tran-
sition. The next higher energy band located around 348
nm probably originates from a π-π* transition localized
at H2pbbzim. The absorption peaks between 270 and 315
nm are due to π-π* ligand centered transitions. It is
interesting to note that the peak position of the MLCT
band in the complexes undergoes a red-shift, albeit to a
small extent with the increase in electron-withdrawing
ability of the substituent X.
Luminescence Spectra. The emission spectral behavior
of complexes 1-7 has been studied at room temperature and
77 K using CH3CN solutions and methanol-ethanol (1:4)
glass, respectively. Table 3 also summarizes the emission
maxima, quantum yield (Φ), and lifetime (τ) of the com-
plexes. All the complexes on excitation at their MLCT
absorption maximum exhibit one broad luminescent band,
which lies between 662 (1) and 700 nm (7) at 300 K and
between 638 (1) and 675 nm (7) at 77 K (Figure 3) depending
upon the substituents in the terpyridine ligand. The bands
have the characteristics of emission from the 3MLCT excited
state, which corresponds to a spin-forbidden Ru(dπ) f tpy-
X(π*) transition.13-16 In solution, at room temperature, all of
the complexes are weak emitters, with their quantum yields
ranging from 9.5 ꢀ 10-4 to 7.2 ꢀ 10-3. Excitation spectra
showed the measured emission signals to be due to the
complexes under evaluation. The most striking feature of this
class of compounds is that they are luminescent at room tem-
perature in fluid solutions, though the parent [Ru(tpy/tpy-
PhCH3)2]2þ 28 or [Ru(H2pbbzim)2]2þ 40a,b are non-lumi-
nescent. The room-temperature lifetimes of the complexes
which lie between 9 (1) and 58 ns (7) are significantly greater
than that of parent Ru(tpy)2 (0.25 ns) and increase with
electron withdrawing substitution in the phenyl group of the
tpy-X moiety. In fact a linear correlation can be obtained by
plotting the lifetime (τ) vs the Hammett σp parameters (Figure
4a). A linear correlation is also obtained between the energies
of the emission band maxima (both at room temperature and
at 77 K) vs Hammett σp plot (Figure 4b). It should be noted
that the enhanced luminescence properties of the complexes
have been achieved without lowering the excited-state energy
significantly. On going from fluid solution to frozen glass, the
emission maxima gets blue-shifted with a significant increase
of emission intensity and quantum yield (Φ=0.15-0.20).
The blue shift of the emission band on passing from fluid
solution to a rigid matrix is typical of MLCT emitters and is
mainly due to the inability of frozen solvent to reorganize
around the excited molecules. The enhancement of intensity
on lowering of the temperature results from slowing down the
radiationless transitions, including the thermally activated
population of short-lived, upper-lying excited states.13-16
The free ligand, H2pbbzim, on excitation at 300 nm fluoresces
strongly (Φ = 0.5) at 382 nm in acetonitrile.
Figure 2. 1H NMR spectra of 1-5 in (CD3)2SO. Atom numbering is
shown in Scheme 1.
Scheme 1
Proton NMR Spectra. 1H NMR spectra of complexes
1-7 have been recorded in (CD3)2SO, and their chemical
shift values are given in the Experimental Section. The
assignments made for the observed chemical shifts, accord-
ing to the numbering scheme (shown in Scheme 1), are listed
in Table S1 (Supporting Information). The spectral assign-
ments of the complexes have been made with the help of their
{1H-1H} COSY spectra (Figure S2, Supporting Informa-
tion), relative areas of the peaks, and taking into considera-
tion the usual ranges of J values for H2pbbzim and tpy-X.
Figure 2 shows the 1H NMR spectra of the complexes 1-5.
It is interesting to note that the chemical shift of the
CH2-X proton (H9) of tpy-X in 2-6 increases as the
substituent, X, is changed in the order CH3, CH2Br, CH2-
CN, and CHBr2. Indeed, a linear correlation is obta-
ined (Figure S3, Supporting Information) by plotting
δ(CH2-X) vs Hammett σp parameters (σp of PhCH3,
-0.17; PhCH2Br:, 0.14; PhCH2CN, 0.18; PhCHBr2, 0.32;
and PhCHO, 0.42).52
To understand the effect of the 2,6-bis(benzimidazole-
2-yl)pyridine (H2pbbzim) ligand along with the different
electron-withdrawing substituents in the 40 position of the
p-tolylterpyridine moiety, it is useful to recall that the
excited-state lifetimes of Ru(II) polypyridine complexes
Absorption Spectra. The UV-vis spectral data of the com-
plexes (1-7) in acetonitrile and the relevant complexes
(52) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165–195.

5056 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bhaumik et al.
Table 3. Spectroscopic and Photophysical Data in acetonitrile Solutions for 1-7
luminescence, 298a
luminescence, 77 Kb
absorption λmax nm
compd
(ε, M-1 cm-1
)
λmax nm
τ, ns
Φ (ꢀ 10-3
0.94
)
kr, s-1 (ꢀ 105)
1.0
knr, s-1 (ꢀ 106)
109.5
λmax nm
Φ
1c
478(15700)
348(47300)
313(68700)
271(41800)
487(18000)
348(34500)
313(56300)
285(48300)
488(13100)
348(28400)
313(40500)
285(28500
486(19900)
348(39300)
314(55400)
285(55500)
490(23500)
348(49500)
314(63000)
286(63700)
488(22600)
348(44700)
313(62600)
286(62900)
489(18100)
347(38000)
315(50000)
287(52000)
474(10400)
490(28000)
475(17400)
662
9.1
638
650
661
664
671
669
675
598
0.15
2
3
4
5
6
7
675
688
690
696
691
700
14.8
34.0
36.9
50.0
42.5
57.5
1.91
2.10
2.82
5.04
3.70
7.21
e0.05
1.3
0.6
0.8
1.0
0.9
1.3
0.04
67.4
29.4
27.0
19.9
23.4
17.3
90.9
0.16
0.17
0.17
0.18
0.19
0.20
8d
629
640
0.25
<5.0
9e
628, 681 (shoulder)
10f
a In CH3CN. b In MeOH-EtOH (1:4) glass. c ref 40c, 40d. d [Ru(tpy)2]2þ, ref 29. e [Ru(tpy-Ph-CH3)2]2þ, ref 15. f [Ru(H2pbbzim)2]2þ, ref 40a.
are governed by the nonradiative decay rate constant knr,
given by eq 213-15,30
0
0
knr ¼ knr þ knr
ð2Þ
The overall radiationless decay is the sum of two terms. The
first one, knr0, is directly related to the energy transfer from the
MLCT state to the ground state, whereas the second term,
0
knr , is related to the thermally activated process that takes into
account a surface-crossing from the lowest-lying MLCT state
to a closely lying metal-centered (MC) level and therefore
depends upon the energy gap ΔE between MLCT and
MC states.13-15,30 For Ru(II) complexes with tridentate
ligands, the second term normally dominates the equation.
The small ΔE between MLCT and MC states in Ru(II) tri-
dentate polypyridine complexes, a consequence of the reduced
ligand field strength experienced by the metal center compared
to Ru(II) bidentate polypyridine ligands, is due to an ill-fitted
octahedral arrangement, which in turn is responsible for the
poor room-temperature luminescence properties of Ru(tpy)2-
type complexes.13-15,30
On plotting ln knr versus Eemmax, a linear relationship
with a positive slope is obtained (Figure 5).30,53 This
finding probably confirms that the dominant term for
knr in this series of complexes is due to the second term of
eq 2.30,53 Further, the energy of the MC level being considered
Figure 3. Photoluminescence spectra of the sensors 1-7 at room
temperature in acetonitrile (a) and at 77 K in ethanol-methanol (4:1)
glass (b).
(53) (a) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am.
Chem. Soc. 1982, 104, 630–632. (b) Chen, P.; Meyer, T. J. Chem. Rev. 1998, 98,
1439–1478.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5057
Table 4. Electrochemical Dataa for the Complexes 1-7 in Acetonitrile
oxidationb
compd
E1/2(ox), V
reductionc E1/2(red), V
1
2
3
4
5
6
7
0.94
1.01
1.07
1.08
1.12
1.09
1.15
-1.49, -1.90, -2.07
-1.48, -1.86, -2.02
-1.48, -1.82, -2.01
-1.48, -1.82, -2.01
-1.47, -1.79, -2.05
-1.47, -1.82, -2.03
-1.47, -1.78, -1.99
a All the potentials are referenced against the Ag/AgCl electrode with
E1/2=0.36 V for the Fc/Fcþ couple. b Reversible electron transfer process
with a Pt working electrode. E1/2 values obtained from square wave
voltammetric (SWV) using glassy carbon electrode.
c
Figure 4. Plot of lifetime (a) and energy of emission maxima (b) vs
Hammett σp parameters with linear least-squares fit to the data.
Figure 6. (a) Cyclic voltammograms of 1-7 in acetonitrile showing
oxidation up to 1.5 V. Inset shows the plot of E1/2 vs Hammett σp
parameters. (b) Plot of ΔE1/2 vs energy of emission maxima.
max
Figure 5. Plot of ln knr vs Eem
temperature.
for the complexes 1-7 at room
temperature-dependent photophysical measurements will
be required. Unfortunately, due to the lack of such a needed
facility, we are unable to address this issue presently.
to be constant within the series, the MLCT emitting
level is decreased in energy by modulating the electron-
withdrawing influence of the substituent in the 40 position
of the p-tolylterpyridine moiety. Therefore, this linear rela-
tionship probably denotes the reduced efficiency of the
MLCT-to-MC surface-crossing pathway as the MLCT
excited-state energy is decreased. However, the larger energy
gap between MLCT and MC states may not be the only
reason to fully justify the relatively long luminescence life-
times of the complexes. The complete kinetic schemes of the
excited state deactivation of these complexes could be much
more complicated, and for a better understanding, detailed
Redox Properties. The redox activities of complexes
1-7 have been studied in acetonitrile solution, and the
relevant electrochemical data are given in Table 4. In all the
cases, the metal-centered oxidations take place reversibly. A
comparison of the electrochemical data reveals that the E1/2
values of the complexes increase with an increase in the
electron-withdrawing ability of the substituent X, and a
linear correlation is obtained by plotting the E1/2 values of
the RuII/RuIII couple against the Hammett σp parameters
(Figure 6a). The complexes are also found to undergo three
successive one-electron reversible reductions in the potential

5058 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bhaumik et al.
Figure 7. Colorchanges thatoccur when the acetonitrile solutions ofthe
receptors 4 (a) and 6 (b) are treated with various anions as their
tetrabutylammonium (TBA) salts.
window of 0 to -2.2 V. As the oxidation process is involved
in the dπ (t2g) metal orbital and the first reduction process is
involved in the terpyridine π* orbital, a linear correlation is
also obtained by plotting the lowest energy MLCT emission
maxima and the difference between the oxidation and the
first reduction (ΔE1/2) potentials (Figure 6b). The general
trend indeed shows an increase in emission energies with
increasing ΔE1/2
.
Anion Sensing Studies of the Metalloreceptors. Colori-
metric Signaling. The anion sensing ability of receptors
1-7 has been studied on a qualitative basis by visual exa-
mination of the anion-induced color changes in acetonitrile
solutions (5 ꢀ 10-5 M) before and after the addition of an
anion. TBA salts of F-, Cl-, Br-, I-, NO3-, ClO4-, AcO-,
and H2PO4- ions have been used as substrates for the recep-
tors. The photograph in Figure 7 shows the dramatic color
changes of 4 and 6 in the presence of F-, AcO-, and to a
lesser extent H2PO4-, while in contrast the anions Cl-, Br-,
Figure 8. Changes in UV-vis (a) and luminescence (b) spectra of recep-
tor 3 in acetonitrile upon the addition of different anions as TBA salts.
-
I-, NO3-, and ClO4 induce almost no change in color.
Evidently, there is strong interaction (see later) between the
-
receptors with the substrate anions F-, AcO-, and H2PO4
and very weak or no interaction with the other anions.
Absorption Signaling. Sensing of the anions by the
metalloreceptors has been monitored by observing the spec-
tral changes that occur in acetonitrile solutions. As shown in
Figure 8a, the MLCT peak at 488 nm remains practically
unchanged upon the addition of 5 equiv of Cl-, Br-, I-,
NO3-, and ClO4- ions to 2.5 ꢀ 10-5 M solutions of 3. On
the other hand, following the addition of 5 equiv of F- and
AcO-, the said MLCT band gets red-shifted to 525 nm,
indicating that strong interactions occur between the
receptors and anions. On the addition of H2PO4- though,
the MLCT bands of 3 initially shifted to 505 nm, but the
-
addition of excess H2PO4 leads to the precipitation of
the resulting complex. These observations are in conso-
nance with the visual changes already noted in Figure 7.
The red shift of the MLCT bands can be attributed to the
second-sphere donor-acceptor interactions between me-
tal coordinated H2pbbzim and the anions.54 Such inter-
actions (hydrogen bonding or proton transfer, vide infra)
increase the electron density at the metal center, leading to
lowering of the MLCT band energies.
In order to get quantitative insight into sensor-anion
interaction, spectrophotometric titrations have been carried
out with F-, AcO-, and H2PO4- ions. Typical absorption
spectral changes for [(H2pbbzim)Ru(tpy-PhCH2Br)](ClO4)2
Figure 9. Changesin UV-visspectraofsensor 3 in acetonitrileuponthe
addition of AcO- (a-c) and F- (d-f). The inset shows the fit of the
experimental absorbance data to a 1:1 binding profile.
(3) overthe0-8 equiv range of anions are shown in Figure 9.
On close inspection of the changes in the spectral profiles
with the incremental addition of anions, the occurrence of
(54) Balzani, V.; Sabbatini, N.; Scandola, F. Chem. Rev. 1986, 86, 319–337.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5059
Table 5. Equilibrium/Binding Constantsa,b (K, M-1) for 1-7 and H2pbbzim towards Various Anions in acetonitrile at 298 K
from absorption spectra
anions
1
2
3
4
K1
K2
K1
K2
K1
K2
K1
K2
F-
1.58 ꢀ 106
1.01 ꢀ 106
1.13 ꢀ 106
9.14 ꢀ 105
8.10 ꢀ 105
NAd
1.80 ꢀ 106
1.99 ꢀ 106
1.21 ꢀ 106
8.02 ꢀ 105
9.32 ꢀ 105
NA
1.75 ꢀ 106
1.71 ꢀ 106
1.49 ꢀ 106
9.01 ꢀ 105
8.14 ꢀ 105
NA
1.75 ꢀ 106
1.81 ꢀ 106
1.35 ꢀ 106
8.13 ꢀ 105
8.13 ꢀ 105
NA
AcO-
H2PO4
-
5
6
7
H2pbbzimc
K1
K2
K1
K2
K1
K2
K1
K2
F-
1.12 ꢀ 106
2.08 ꢀ 106
7.08 ꢀ 105
8.12 ꢀ 105
8.78 ꢀ 105
NA
2.09 ꢀ 106
1.44 ꢀ 106
1.48 ꢀ 106
8.87 ꢀ 105
2.24 ꢀ 106
1.08 ꢀ 106
8.85 ꢀ 105
NA
7.00 ꢀ 104
4.50 ꢀ 104
NA
NA
NA
NA
AcO-
H2PO4
7.98 ꢀ 105
2.35 ꢀ 106
1.74 ꢀ 106
-
NA
from emission spectra
1
2
3
4
K1
K2
K1
K2
K1
K2
K1
K2
F-
1.28 ꢀ 106
1.12 ꢀ 106
9.23 ꢀ 105
9.24 ꢀ 105
8.50 ꢀ 105
NA
1.77 ꢀ 106
1.95 ꢀ 106
1.20 ꢀ 106
7.82 ꢀ 105
9.39 ꢀ 105
NA
2.26 ꢀ 106
1.70 ꢀ 106
1.25 ꢀ 106
7.74 ꢀ 105
7.38 ꢀ 105
NA
1.68 ꢀ 106
1.89 ꢀ 106
1.25 ꢀ 106
7.92 ꢀ 105
8.53 ꢀ 105
NA
AcO-
H2PO4
-
5
6
7
H2pbbzimc
K1
K2
K1
K2
K1
K2
K1
K2
F-
1.29 ꢀ 106
2.34 ꢀ 106
7.16 ꢀ 105
7.82 ꢀ 105
8.12 ꢀ 105
NA
2.36 ꢀ 106
1.65 ꢀ 106
1.46 ꢀ 106
8.48 ꢀ 105
8.11 ꢀ 105
NA
1.94 ꢀ 106
1.60 ꢀ 106
1.11 ꢀ 106
8.54 ꢀ 105
NA
7.12 ꢀ 104
4.23 ꢀ 104
NA
NA
NA
NA
AcO-
H2PO4
2.48 ꢀ 106
-
a t-Butyl salts of the respective anions were used for the studies. b Estimated errors were <15%. c Estimate, clean binding profiles were not observed.
d Not applicable.
two successive reaction equilibria becomes evident. In the
first case, spectral saturation occurs with the addition of 1
equiv of either F- or AcO- (shown in inset of Figure 9),
suggesting a 1:1 receptor-anion interaction, while for the
attainment of the second equilibrium process, an excess of
anions is required. The spectral patterns of 3 toward the
H2PO4- ion are almost the same as those of either F- or
AcO- up to 1 equiv with two sharp isosbestic points at
496 and 433 (Figure S4, Supporting Information). On
further addition of H2PO4- beyond 1 equiv, no noticeable
changes in the spectral profile occur, and finally the
precipitation of the resulting complex occurs. The ab-
sorption spectral profile of all the complexes is basically
similar except the position and intensity of the band
maxima and follows the same trends with the increasing
concentration of the anions (Figure S5-S6, Supporting
Information). By using eq 1, the equilibrium constant K
for receptor-anion interaction has been evaluated, and
the values are given in Table 5.55 It may be noted that the
values of K for the receptors 1-7 with F-, AcO-, and
H2PO4- are grossly over 6 orders of magnitudes.
It was reported previously that suitably substituted
H-bond donor receptor functionality undergoes depro-
tonation in the presence of excess anions, leading to
classical Brønsted acid-base chemistry.56,57 To examine
such a possibility, spectrophotometric titrations of the
receptors were also carried out with a solution of TBAOH
(Figure S7, Supporting Information). The spectral pat-
terns for the complexes have close resemblance to the
spectra of these receptors in the presence of F- and AcO-
ions.
It would be of interest to compare the binding ability of
the free H2pbbzim ligand with its Ru(II) complexes 1-7.
In order to do so, spectrophotometric titration experi-
ments were carried out in an acetonitrile solution of
H2pbbzim with various anions (Figure 10). It has been
observed that the interactions of the ligand occur only
with F- and AcO- but not with other anions. Titration of
H2pbbzim was also carried out with the OH- ion
(Figure 10). The titration profile indicates that, with F-
and AcO-, only one step occurs, while with OH-, two
successive N-H deprotonation steps occur. It may be
noted that the binding/equilibrium constants of the metal
complexes with the anions are substantially enhanced
(55) Attempts to determine the binding constants of the complexes for
anions with 1:2 stoichiometry did not result in any satisfactory fit. The
binding constants were obtained by fitting the data to two successive 1:1
equilibrium processes using eq 1.
(56) (a) Kang, S. O.; Powell, D.; Day, V. W.; Bowman-James, K. Angew.
Chem., Int. Ed. 2006, 45, 7882–7894. (b) Gunnlaugasson, T.; Kruger, P. E.;
Jensen, P.; Tierney, J; Ali, H. D. P.; Hussey, G. M. J. Org. Chem. 2005, 70,
10875–10878. (c) Gunnlaugasson, T.; Kruger, P. E.; Jensen, P.; Pfeffer, F. M.;
Hussey, G. M. Tetrahedron Lett. 2003, 44, 8909–8913. (d) Gunnlaugasson, T.;
Kruger, P. E.; Lee, T. C.; Parkesh, R; Pfeffer, F. M.; Hussey, G. M. Tetrahedron
Lett. 2003, 44, 6575–6578. (e) dos Santos, C. M. G.; Gunnlaugsson, T. Dalton
Trans. 2009, 4712–4721. (f) Peng, X.; Wu, Y.; Fan, J.; Tian, M.; Han, K. J. Org.
Chem. 2005, 70, 10524–10531.
(57) (a) Gomez, D. E.; Fabbrizzi, L.; Liccheli, M. J. Org. Chem. 2005, 70,
5717–5720. (b) Boiocchi, M.; Boca, L. D.; Gomez, D. E.; Fabbrizzi, L; Licchelli,
M.; Monzani, E. Chem.;Eur. J. 2005, 11, 3097–3104. (c) Amendola, V.;
Boiocchi, M; Fabbrizzi, L; Palchetti, A. Chem.;Eur. J. 2005, 11, 5648–5660.
(d) Amendola, V.; Boiocchi, D.; Colasson, B.; Fabbrizzi, L. Inorg. Chem. 2006,
45, 6138–6147.

5060 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bhaumik et al.
Figure 10. Changes in UV-vis (a-c) and photoluminescence (d-f)
spectra of H2pbbzim in acetonitrile upon the addition of F- and OH-
ions. The inset shows the absorbance change as a function of the
equivalent of anions added.
Figure 11. (a) Changes in photoluminescence intensity of the sensor 6 in
acetonitrile upon the addition of AcO- (a-c) and F- (d-f) ions. Inset
shows the fit of the experimental emission data to a 1:1 binding profile.
(2 orders of magnitude) relative to the ligand, H2pbbzim,
itself. We believe this to be caused by the Ru(II) center,
which makes the bis-benzimidazole ligand electron-defi-
cient, thereby rendering the imidazole NH’s more avail-
able for hydrogen bonding to the anions.
new emission peak of high intensity at 402 nm and then
again gradual quenching of the band.
1H NMR Signaling. 1H NMR spectra of the complexes
(1-7) in the presence of 5 equiv of anions were also
recorded (Figure S9, Supporting Information), and the
chemical shift values are collected in Table S2 (Supporting
Information). It may be noted that the protons associated
with the 2,6-bis(benzimidazole-2-yl)pyridine (H2pbbzim)
ligand in the complexes are all up-field shifted by 0.10-
0.54 ppm relative to their precursors in the absence of
anions. The pronounced shielding effect probably is a
consequence of the increase in electron density of the ben-
zimidazole moiety due to the deprotonation of the imidazole
N-H protons. As expected, the chemical shifts of the
aromatic protons due to tpy-X ligands differ only to a small
extent in the complexes.
Fluorescence Signaling. Figure 8b shows that the emis-
sion intensity of the band at 690 nm undergoes nominal
change with the addition of Cl-, Br-, I-, NO3-, and
ClO4- ions. On the other hand, with the 5-fold addition of
the anions F-, AcO-, and H2PO4-, the emission intensity
gets significantly quenched with a consequent red shift of
the emission maximum from 690 to 740 nm, except with
H2PO4-. Thus, the absorbance and luminescence beha-
vior of the anions show close parlance. Though, in the
presence of a large excess of F- and AcO-, the room
temperature emission of all the complexes gets completely
quenched, there is still some residual emission when the
measurements are carried out at 77 K (Figure S8, Sup-
porting Information). Photoluminescence titrations of
the receptors with various anions have been carried out
in the same way as already described for spectrophoto-
metric measurements. In Figure 11, the effect of incre-
mental addition of F- and AcO- to 1 is shown, and the
inset shows that quenching of luminescence intensity vs
the equivalents of anions added. Table 5 also summarizes
all of the emission data-derived equilibrium constants
measured for the receptors toward different anions.
Spectrofluorometric titrations of the complexes were also
carried out with TBAOH. The spectral patterns have a
close resemblance to the spectra of the receptors in the
presence of anions. The emission spectral profile of
H2pbbzim as a function of F- and AcO- and OH- has al-
ready been shown in Figure 10. The initial emission peak
at 382 nm decreased in intensity gradually and completely
quenched after the addition of around 16 equiv of OH-.
Further addition of OH- leads to an abrupt evolution of a
Figure 12 shows that, with the incremental addition of
TBAAcO, the singlet at δ 15.02 ppm due to two N-H
groups of H2pbbzim get broadened, get down-field shifted,
and finally vanish when almost 1 equiv of AcO- was added
to a DMSO-d6 solution of 7, while the chemical shifts of
C-H protons of H2pbbzim (H10, H11, H12, H13, H14 and H15)
get progressively up-field shifted. Figure S10 (Supporting
Information) shows that the change of chemical shifts for
the above benzimidazole protons as a function of the
equivalents of AcO- added, and the chemical shift values
of different protons with varying amount of AcO- are
summarized in Table S3 (Supporting Information). A
similar behavior was also found with F- ions. But with
H2PO4-, the same titration was not successful due to the
precipitation of the complex on addition of the anion.
Electrochemical Signaling. Because of the solubility
limitation of the receptors in pure acetonitrile, the elec-
trochemical measurements of the receptors with anions
were carried out in acetonitrile-dimethylformamide (9:1)

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5061
Figure 12. 1H NMR titration of sensor 7 in DMSO-d6 solution (5.0 ꢀ 10-3 M) upon the addition of AcO- ions (0-3 equiv).
-
solutions. As shown in Figure 13, with the progressive
addition of F- to 7, the current height of the redox couple
observed at 0.96 V gradually diminishes, and at its expense, a
new couple that appears at 0.41 V grows in current heights.
As the F- ion concentration reaches 3 equiv, the couple at
0.96 V is completely replaced by the 0.41 V couple. The
electrochemical behavior observed with AcO- as the guest
anion is almost identical to that of F- for all the receptors
(Figure S11, Supporting Information). A closer examination
of the current versus equivalent of anions plot shows that, in
the intermediate range, a steady decrease or increase of
current occurs in two steps with a plateau around 1 stoichio-
metric equiv of anion, indicating the presence of two succes-
sive equilibrium processes. Thus, the progressive addition of
anions to the solution of the receptors resulted in a negative
shift of the oxidation potential, demonstrating a strong
interaction between the receptors and anions. It should be
noted that the redox potentials of the complexes in pure
acetonitrile, as reported earlier, differ from those obtained in
an acetonitrile-dimethylformamide (9:1) medium due to a
solvent effect. With H2PO4-, the electrochemical behavior
became complicated due to adsorption and poisoning of the
electrode surface. In contrast to F- and AcO-, no noticeable
changes occur in electrochemical responses for the receptors
in the presence of a large excess of Cl- and NO3-. The results
presented here provide a hint that the complexes 1-7 could
prove useful in the fabrication of electrochemical sensors.
Nature of Receptor-Anion Interaction. The observa-
tions made above from 1H NMR, photometric, lumines-
cence, and electrochemical measurements unequivocally
suggest that F-, AcO-, and to some extent H2PO4 ions
interact strongly with the metalloreceptors 1-7 in 1:1 stoi-
chiometry, albeit such interaction is either very weak or
absent for other halides (Cl-, Br-, I-) and oxyanions (NO3
-
and ClO4-). The close resemblance of both absorption and
emission spectral patterns of the receptors in the presence of
OH- to those in the presence of F- and AcO- ions suggest
that metal-coordinated H2pbbzim in 1-7 gets successively
deprotonated to Hpbbzim-1 and finally to pbbzim-2 in the
presence of an excess of anions. It would appear at first that
the role of the anions F- and AcO- is simply to abstract the
N-H protons according to eq 3:
- Hþ,K1
2þ
½ðtpy-XÞRuIIðH2pbbzimÞꢁ
- Hþ,K2
½ðtpy-XÞRuIIðHpbbzimÞꢁþ
½ðtpy-XÞRuIIðpbbzimÞꢁ
ð3Þ
However, there is growing evidence suggesting that the
picture could be far from simple.1,2,5,48 It appears that
hydrogen bonding, supramolecular interaction of a sec-
ond coordination sphere, and a proton transfer reaction
probably come into play. In the present case, we envisage
that the imidazole N-H protons of the receptors interact
with selective anions X- (F-, AcO-, and to some extent
H2PO4-) to form an incipient N-H X- hydrogen
3 3 3
bond, and the presence of an excess of X- induces further
stretching of the N-H bond, which eventually splits
through a proton transfer reaction as shown in Scheme 2.

5062 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bhaumik et al.
Scheme 2
-
halides and AcO- and, to some extent, H2PO4 among
oxoanions in solution. To allow fine-tuning of the electronic
properties, several electron withdrawing groups have been
introduced in the 40 position of the terpyridine ligand, resulting
in seven complexes in total. The most striking feature of this
class of compounds is that they are luminescent at room tem-
perature in fluid solutions, and their room temperature life-
times lie in the range of 10-58 ns. The sub-nanosecond excited-
state lifetime of normal tpy complexes is widely accepted as
being due to the small energy gap between the emitting 3MLCT
state and the deactivating 3MC level. The important outcome
of this study has been to increase the energy level separation of
3
the two states by modulating the MLCT energy level of the
complexes by varying the electronic influence of the substituent
X in the 40 position of the tpy unit, while keeping the energy of
the 3MC state essentially unchanged. Another point of interest
is that the emitting properties of the compounds are determined
by the protonation state of the benzimidazole rings. This opens
the possibility of the application of such compounds as proton
driven molecular switches. Correlations between the Hammett
σp parameter and MLCT emission energies, room temperature
lifetime, the potentials of the oxidation and reduction pro-
cesses, 1H NMR chemical shifts, etc. show that the greater the
electron accepting ability of the substituents, X, the greater the
stabilization of the LUMO π* ligand centered orbital of tpy-X
than the HOMO π(t2g) metal orbital. The binding properties
are also confirmed by absorption,emission, and 1H NMR spec-
troscopic and cyclic voltammetric techniques. From binding
studies, it has been concluded that, at a relatively lower con-
centration, 1:1 H-bonded adducts between the metallorecep-
tors and the anions are formed, while in the presence of excess
anions, stepwise deprotonation of the two benzimidazole N-H
fragments occurs, an event which is signaled by the develop-
ment of intense and beautiful colors visible with the naked eye.
Cyclic voltammetry (CV) studies carried out in acetonitrile-
dimethylformamide (9:1) have provided evidence of anion-
dependent electrochemical responses with F- and AcO-,
rendering complexes 1-7 as suitable electrochemical sensors.
Figure 13. SWVs (a) and CVs (b) of receptor 7 obtained upon incre-
mental addition of TBAF to an acetonitrile-dimethylformamide (9:1)
solution (1.0 ꢀ 10-3 M). The changes in the current intensities as a
function of equivalents of F- are shown in c.
It has been argued that the higher stability of a poly-
nuclear aggregate, such as HF2-, further facilitates the
deprotonation of the receptor unit. It is interesting to note
that the stepwise deprotonation process is fully reversible,
as indicated by the fact that, on the progressive addition
of water, the violet color of the acetonitrile solutions of
the receptors first turns orange red and then the original
yellow-orange. Finally, we note that, although the recep-
tors 1-7 exhibit a strong response toward sensing F-,
AcO-, and to a lesser extent H2PO4-, they lack selectivity
to differentiate these anions explicitly.
Acknowledgment. Financial assistance received from
the Department of Science and Technology, New Delhi
[Grant No. SR/S1/IC-06/2006] and the Council of Scien-
tific and Industrial Research, New Delhi, India [Grant
No. 01(2084)/06/EMR-II] is gratefully acknowledged.
Thanks are due to the Department of Inorganic Chemis-
try of the Indian Association for the Cultivation of
Science, Kolkata, India, for single crystal X-ray data. C.
B., S.D., and D.S. thank CSIR for their fellowship.
Conclusion
In conclusion, we have developed a new series of heteroleptic
bis-tridentate ruthenium(II) complexes by using the tridentate
ligand 2,6-bis(benzimidazole-2-yl)pyridine (H2pbbzim) and
different 40-substituted terpyridine derivatives (tpy-X), which
can act as multi-channel sensors of anions such as F- among
Supporting Information Available: X-ray crystallographic file
in CIF format for compound 2, Tables S1-S3, and Figures
S1-S12. This material is available free of charge via the Internet
at http://pubs.acs.org.