A new family of Ru(II) polypyridyl complexes containing open-chain crown ether for Mg2+ and Ca2+ probing

Article abstract of DOI:10.1016/j.saa.2008.07.031

Six polypyridyl bridging ligands BL^1-6 containing open-chain crown ether, where BL^1-3 formed by the condensation of 4,5-diazafluoren-9-hydrazine with 1,7-bis-(4-formylphenyl)-1,4,7-trioxaheptane, 1,10-bis-(4-formylphenyl)-1,4,7,10-te

Full text of DOI:10.1016/j.saa.2008.07.031

Spectrochimica Acta Part A 71 (2009) 1944–1951
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new family of Ru(II) polypyridyl complexes containing open-chain
crown ether for Mg2+ and Ca2+ probing
Feixiang Cheng, Ning Tang∗, Xing Yue
College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Six polypyridyl bridging ligands BL1–6 containing open-chain crown ether, where BL1–3 formed by the con-
densation of 4,5-diazafluoren-9-hydrazine with 1,7-bis-(4-formylphenyl)-1,4,7-trioxaheptane, 1,10-bis-
(4-formylphenyl)-1,4,7,10-tetraoxadecane, and 1,13-bis-(4-formylphenyl)-1,4,7,10,13-pentaoxatridecane,
respectively, BL4–6 formed by the reaction of 9-(4-hydroxy)phenylimino-4,5-diazafluorene with diethy-
lene glycol di-p-tosylate, triethylene glycol di-p-tosylate, and tetraethylene glycol di-p-tosylate,
respectively, have been synthesized. Reaction of Ru(bpy)2Cl2·2H2O with BL1–6, respectively, afforded six
Article history:
Received 28 February 2008
Received in revised form 11 July 2008
Accepted 17 July 2008
Keywords:
Ru(II) complex
Open-chain crown ether
Photophysics
Electrochemistry
Cation-binding
−
bimetallic complexes [(bpy)2RuBL1–6Ru(bpy)2]4+ as PF6 salts. Cyclic voltammetry of these complexes is
consistent with one Ru(II)-centered oxidation around 1.32 V and three ligand-centered reductions. These
complexes show metal-to-ligand charge transfer absorption at 413–444 nm and emission at 570 nm. Bind-
ing behavior of complexes with alkali and alkaline-earth metal ions are investigated by UV–vis absorption,
fluorescence, and cyclic voltammetry. Addition of alkali and alkaline-earth metal ions to the solution of
[(bpy)2RuBL1–6Ru(bpy)2](PF6)4 all result in a progressive quenching of fluorescence, a hyperchromic effect
of UV–vis absorption, and a progressive cathodal shift of Ru(II)-centered E1/2. Ru-BL2 and Ru-BL5 show the
highest binding ability toward Mg2+ among the five cations examined while Ru-BL3 and Ru-BL6 exhibit
good selective recognition ability to Ca2+
.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
complexes have proved to be particularly versatile in sensor appli-
cations, because their emitting state energies and excited state
redox properties are quite sensitive to external stimuli [20–30].
Herein, we report the synthesis and characterization of the six
bridging ligands, their Ru(II) complexes, spectroscopic and electro-
chemical properties of these complexes in response to alkali and
alkaline-earth metal ions binding.
There is currently a considerable effort devoted to the design
and synthesis of ligands with recognition unit attached to a metal
polypyridyl core. Properties of the metal polypyridyl complexes
are modified by molecular level interactions between the recog-
nition center and the substrate [1–4], so they are potentially useful
in the preparation of chemical sensors. Chemosensors have drawn
increasing attention for their use in fields as diverse as biology,
medical analysis, environmental monitoring and so on [5–12]. Par-
ticular interest has been focused on the measurement of Mg2+ and
Ca2+ in various systems, especially in biological materials, because
alkaline-earth metal ion is the most abundant divalent cation in
living cells, and plays vital roles in many cellular processes, for
example, as an enzyme cofactor, stabilization of DNA conforma-
tion, ion transport through the membrane, maintenance of cell
shape, and signal transduction. As a result, a great deal of effort has
been devoted to the design and synthesis of sensitive and selective
fluorescent sensors for Mg2+ and Ca2+ [13–19]. Ru(II) polypyridyl
2. Experimental
2.1. Materials
Solvents and raw materials were of analytical grade and
were used as received. An exception was CH3CN, which was
filtered over activated alumina and distilled from P2O5 immedi-
ately prior to use. Tetrabutylammonium perchlorate (TBAP) [31],
4,5-diazafluoren-9-one (dafo) [32], 4,5-diazafluoren-9-hydrazine
[33], diethylene glycol di-p-tosylate [34], triethylene glycol
di-p-tosylate [34], tetraethylene glycol di-p-tosylate [34], 1,7-
bis-(4-formylphenyl)-1,4,7-trioxaheptane [35], 1,10-bis-(4-formyl-
phenyl)-1,4,7,10-tetraoxadecane [35], 1,13-bis-(4-formylphenyl)-
1,4,7,10,13-pentaoxatridecane [35] and Ru(bpy)2Cl2·2H2O [36]
were synthesized according to the literature procedures.
∗
Corresponding author. Tel.: +86 931 8911218; fax: +86 931 8912582.
E-mail address: tangn@lzu.edu.cn (N. Tang).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.07.031

F. Cheng et al. / Spectrochimica Acta Part A 71 (2009) 1944–1951
1945
2.2. Physical measurements
pentaoxatridecane (165 mg, 0.41 mmol) was used instead of
1,7-bis-(4-formylphenyl)-1,4,7-trioxaheptane to react with
4,5-diazafluoren-9-hydrazine (197 mg, 1 mmol). Yield: 245 mg
(78.5%) of a yellow solid. 1H NMR (400 MHz, CDCl3): ı 3.72 (t,
J = 6.0 Hz, 4H), 3.76 (t, J = 6.0 Hz, 4H), 3.92 (t, J = 4.4 Hz, 4H), 4.21
(t, J = 4.4 Hz, 4H), 7.03 (d, J = 8.8 Hz, 4H), 7.29–7.35 (m, 4H), 7.84
(d, J = 8.8 Hz, 4H), 8.16 (d, J = 8.0 Hz, 2H), 8.58 (s, 2H), 8.72 (d,
J = 4.4 Hz, 4H), 8.81 (d, J = 7.2 Hz, 2H). ESI-HRMS Found (Calcd.):
m/z 759.3039 (759.3043) (M+H)+, 781.2978 (781.2863) (M+Na)+.
IR (KBr): 1626.17, 1602.81, 1563.38, 1541.55, 1512.11, 1397.91,
1306.98, 1256.18, 1169.85, 1124.25, 1054.78, 1019.84, 829.94,
Absorption spectra were obtained on
a Varian Cary-100
UV–Visible spectrophotometer. 1H NMR spectra were performed
on a Mercury Plus 400 spectrometer using TMS as an internal stan-
dard. Elemental analyses were taken using a PerkinElmer 240C
analytical instrument. ESI-HRMS was recorded on a Bruker Dal-
tonics APEXII47e mass spectrometer. IR was obtained on a Nicolet
170SX FT-IR spectrophotometer as KBr discs. Electrochemical mea-
surements were carried out at room temperature using a CHI
660B electrochemical workstation. Cyclic voltammetry and differ-
ential pulse voltammetry were obtained in CH3CN and DMF by
using a microcell equipped with a platinum disk working elec-
trode, a platinum auxiliary electrode, and a saturated potassium
chloride calomel with 0.1 M TBAP as supporting electrolyte. All
samples were purged with nitrogen prior to measurement. Emis-
sion spectra were obtained in a 4:1 ethanol:methanol glass matrix
at 77 K with a Hitachi F-4500 spectrophotometer. Emission quan-
752.27 cm−1
.
3.4. 9-(4-Hydroxy)phenylimino-4,5-diazafluorene
A mixture of 4,5-diazafluoren-9-one (277 mg, 1.522 mmol) and
4-aminophenol (192 mg, 1.761 mmol) were refluxed for about 4 h
in 20 mL ethanol containing a catalytic amount of acetic acid. After
refluxing, a suspension formed. The solution was hot filtered, and
then washed with ethanol and diethyl ether affording the desired
product as a red solid. Yield: 311 mg (77.5%). 1H NMR (400 MHz,
DMSO-d6): ı 6.87 (d, J = 6.4 Hz, 2H), 6.92 (d, J = 6.4 Hz, 2H), 7.13 (dd,
J = 8.0, 1.6 Hz, 1H), 7.27 (dd, J = 8.0, 4.8 Hz, 1H), 7.54 (dd, J = 8.0, 4.8 Hz,
1H), 8.28 (dd, J = 8.0, 1.6 Hz, 1H), 8.67 (dd, J = 4.8, 1.6 Hz, 1H), 8.80
(dd, J = 4.8, 1.6 Hz, 1H), 9.54 (s, 1H).
2+
tum yields were calculated relative to Ru(bpy)3 (˚std = 0.376) in
4:1 ethanol:methanol [37].
3. Preparations
3.1. 1,7-Bis-(4-(4,5-diazafluoren-9-ylhydrazinyl)benzylidene)-
1,4,7-trioxaheptane
(BL1)
A
mixture of 1,7-bis-(4-formylphenyl)-1,4,7-trioxaheptane
(64 mg, 0.20 mmol) and 4,5-diazafluoren-9-hydrazine (89 mg,
0.45 mmol) were refluxed for about 10 h in 10 mL ethanol con-
taining a catalytic amount of acetic acid. Suspension formed after
refluxing. The solution was hot filtered, and then washed with
ethanol and diethyl ether affording the desired product as a yellow
solid. Yield: 129 mg (95.6%). 1H NMR (400 MHz, CDCl3): ı 4.02
(t, J = 4.4 Hz, 4H), 4.29 (t, J = 4.4 Hz, 4H), 7.06 (d, J = 8.8 Hz, 4H),
7.31–7.36 (m, 4H), 7.88 (d, J = 8.8 Hz, 4H), 8.19 (d, J = 7.6 Hz, 2H),
8.62 (s, 2H), 8.73 (d, J = 3.6 Hz, 4H), 8.83 (d, J = 6.8 Hz, 2H). ESI-
HRMS Found (Calcd.): m/z 671.2519 (671.2519) (M+H)+, 693.2356
(693.2339) (M+Na)+. IR (KBr): 1624.93, 1603.41, 1563.66, 1539.34,
1512.41, 1399.16, 1311.52, 1263.26, 1167.52, 1132.22, 1047.85,
3.5. 1,7-Bis-(4-(4,5-diazafluoren-9-ylimino)phenyl)-1,4,7-
trioxaheptane
(BL4)
A
mixture of diethylene glycol di-p-tosylate (113 mg,
0.272 mmol), 9-(4-hydroxy)phenylimino-4,5-diazafluorene
(168 mg, 0.615 mmol) and K2CO3 (105 mg, 0.761 mmol) in 10 mL
DMF were heated to 80 ◦C for 24 h under nitrogen atmosphere,
then cooled down and poured the mixture into 200 mL water,
a red precipitate was formed and collected by filtration. The
resulting mixture was chromatographed on silica, eluting first
with CH2Cl2/ethyl acetate 2:1 to remove impurities, then with
CH2Cl2/CH3CH2OH 15:1 affording the desired product as a red
solid. Yield: 99 mg (58.6%). 1H NMR (400 MHz, CDCl3): ı 4.03 (t,
J = 4.8 Hz, 4H), 4.26 (t, J = 4.8 Hz, 4H), 6.93–7.06 (m, 12H), 7.39 (dd,
J = 7.6, 4.8 Hz, 2H), 8.25 (dd, J = 8.0, 1.6 Hz, 2H), 8.65 (dd, J = 4.8,
1.6 Hz, 2H), 8.80 (dd, J = 5.2, 1.6 Hz, 2H). ESI-HRMS Found (Calcd.):
m/z 617.2296 (617.2301) (M+H)+, 639.2115 (639.2121) (M+Na)+.
IR (KBr): 1645.13, 1598.31, 1563.53, 1501.27, 1401.44, 1242.74,
1023.52, 824.96, 750.01 cm−1
.
3.2. 1,10-Bis-(4-(4,5-diazafluoren-9-ylhydrazinyl)benzylidene)-
1,4,7,10-tetraoxadecane
(BL2)
BL2 was prepared by the same procedure as that described
for BL1, except 1,10-bis-(4-formylphenyl)-1,4,7,10-tetraoxadecane
(93 mg, 0.26 mmol) was used instead of 1,7-bis-(4-formylphenyl)-
1,4,7-trioxaheptane to react with 4,5-diazafluoren-9-hydrazine
(121 mg, 0.62 mmol). Yield: 175 mg (94.1%) of a yellow solid. 1H
NMR (400 MHz, CDCl3): ı 3.81 (s, 4H), 3.94 (t, J = 4.4 Hz, 4H), 4.23
(t, J = 4.4 Hz, 4H), 7.04 (d, J = 8.0 Hz, 4H), 7.30–7.35 (m, 4H), 7.85 (d,
J = 8.8 Hz, 4H), 8.17 (d, J = 8.0 Hz, 2H), 8.59 (s, 2H), 8.73 (d, J = 4.4 Hz,
4H), 8.82 (d, J = 8.0 Hz, 2H). ESI-HRMS Found (Calcd.): m/z 715.2776
(715.2781) (M+H)+, 737.2631 (737.2601) (M+Na)+. IR (KBr): 1627.14,
1603.73, 1563.75, 1542.53, 1512.09, 1398.47, 1309.69, 1256.22,
1108.56, 835.92, 755.19 cm−1
.
3.6. 1,10-Bis-(4-(4,5-diazafluoren-9-ylimino)phenyl)-1,4,7,10-
tetraoxadecane
(BL5)
BL5 was prepared by the same procedure as that described for
BL4, except triethylene glycol di-p-tosylate (299 mg, 0.653 mmol)
was used instead of diethylene glycol di-p-tosylate. Yield: 326 mg
(75.3%) of a red solid. 1H NMR (400 MHz, CDCl3): ı 3.84 (s,
4H), 3.95 (t, J = 4.4 Hz, 4H), 4.23 (t, J = 4.4 Hz, 4H), 6.93–7.07 (m,
12H), 7.38 (dd, J = 8.0, 4.8 Hz, 2H), 8.23 (d, J = 7.2 Hz, 2H), 8.65 (d,
J = 4.0 Hz, 2H), 8.79 (d, J = 5.2 Hz, 2H). ESI-HRMS Found (Calcd.):
m/z 661.2555 (661.2563) (M+H)+, 683.2468 (683.2383) (M+Na)+. IR
(KBr): 1648.65, 1597.61, 1562.91, 1501.41, 1400.82, 1243.83, 1108.97,
1169.67, 1114.77, 1021.78, 830.10, 753.10 cm−1
.
3.3. 1,13-Bis-(4-(4,5-diazafluoren-9-ylhydrazinyl)benzylidene)-
1,4,7,10,13-pentaoxatridecane
(BL3)
BL3 was prepared by the same procedure as that
described for BL1, except 1,13-bis-(4-formylphenyl)-1,4,7,10,13-
836.33, 755.43 cm−1
.

1946
F. Cheng et al. / Spectrochimica Acta Part A 71 (2009) 1944–1951
(M–2PF6)2+, 577.1124 (M–3PF6)3+. Elemental anal. Found: C, 46.59;
H, 3.26; N, 10.35. Calcd. for C84H70F24N16 O5P4Ru2: C, 46.73; H,
3.44; N, 10.48. IR (KBr): 1629.14, 1603.12, 1537.16, 1512.68, 1465.44,
1421.85, 1311.91, 1258.45, 1171.65, 1121.62, 1028.08, 841.45, 764.80,
3.7. 1,13-Bis-(4-(4,5-diazafluoren-9-ylimino)phenyl)-1,4,7,10,13-
pentaoxatridecane
(BL6)
BL6 was prepared by the same procedure as that described for
BL4, except tetraethylene glycol di-p-tosylate (325 mg, 0.654 mmol)
was used instead of diethylene glycol di-p-tosylate. Yield: 385 mg
(84.8%) of a red solid. 1H NMR (400 MHz, CDCl3): 3.76 (t, J = 2.8 Hz,
4H), 3.80 (t, J = 2.8 Hz, 4H), 3.93 (t, J = 2.8 Hz, 4H), 4.20 (t, J = 2.8 Hz,
4H), 6.93–7.07 (m, 12H), 7.38 (dd, J = 7.2, 4.8 Hz, 2H), 8.22 (dd,
J = 8.0, 1.6 Hz, 2H), 8.65 (d, J = 4.8 Hz, 2H), 8.79 (d, J = 5.2 Hz, 2H). ESI-
HRMS Found (Calcd.): m/z 705.2815 (705.2825) (M+H)+, 727.2661
(727.2645) (M+Na)+. IR (KBr): 1650.58, 1598.19, 1563.20, 1502.62,
558.32 cm−1
.
3.11. [(bpy)2RuBL4Ru(bpy)2](PF6)4 (Ru-BL4)
A mixture of BL4 (30 mg, 0.049 mmol) and Ru(bpy)2Cl2·2H2O
(58 mg, 0.112 mmol) in 15 mL 2-methoxyethanol was heated
to 115 ◦C for 12 h under nitrogen atmosphere, then solvent
was evaporated under reduced pressure, the residue was puri-
fied twice by column chromatography on alumina eluting first
with CH3CN/ethanol 10:1 to remove impurities, then with
CH3CN/ethanol 5:1 affording complex [(bpy)2RuBL4Ru(bpy)2]Cl4,
which was then dissolved in a minimum amount of water fol-
lowed by dropwise addition of saturated aqueous NH4PF6 until
no more precipitate formed. This precipitate was purified by
recrystallization from an acetonitrile–diethyl ether mixture (vapor
diffusion method) giving a red solid. Yield: 66 mg (66.8%). 1H
NMR (400 MHz, DMSO-d6): ı 3.86 (t, J = 4.4 Hz, 4H), 4.18 (t,
J = 4.4 Hz, 4H), 7.10 (d, J = 8.8 Hz, 4H), 7.17 (d, J = 9.2 Hz, 4H),
7.26 (d, J = 8.0 Hz, 2H), 7.35 (dd, J = 7.6, 5.6 Hz, 2H), 7.49–7.63
(m, 12H), 7.73 (d, J = 5.6 Hz, 2H), 7.81 (d, J = 4.8 Hz, 2H), 7.84
(d, J = 5.2 Hz, 2H), 8.08 (d, J = 5.6 Hz, 2H), 8.11–8.21 (m, 10H),
8.42 (d, J = 7.2 Hz, 2H), 8.79 (d, J = 9.2 Hz, 4H), 8.83 (d, J = 9.2 Hz,
1401.85, 1244.34, 1108.31, 838.15, 755.47 cm−1
.
3.8. [(bpy)2RuBL1Ru(bpy)2](PF6)4 (Ru-BL1)
A mixture of BL1 (52 mg, 0.078 mmol) and Ru(bpy)2Cl2·2H2O
(93 mg, 0.179 mmol) in 8 mL 2-methoxyethanol were heated to
115 ◦C for 12 h under nitrogen atmosphere. After cooling to room
temperature, the solution was precipitated by dropwise addition
of saturated aqueous NH4PF6 until no more precipitate formed.
The solid was collected by filtration and washed with water,
ethanol, followed by diethyl ether affording the crude product.
The crude product was purified thrice by recrystallization from an
acetonitrile–diethyl ether mixture (vapor diffusion method) giving
a deep red solid. Yield: 110 mg (68.6%). 1H NMR (400 MHz, DMSO-
d6): ı 3.90 (s, 4H), 4.29 (s, 4H), 7.19 (d, J = 8.0 Hz, 4H), 7.54–7.60 (m,
8H), 7.62 (d, J = 6.8 Hz, 2H), 7.65 (d, J = 7.6 Hz, 2H), 7.69 (d, J = 5.6 Hz,
2H), 7.73 (d, J = 5.2 Hz, 2H), 7.88 (d, J = 5.6 Hz, 4H), 8.10 (d, J = 8.0 Hz,
4H), 8.16–8.23 (m, 12H), 8.40 (t, J = 6.4 Hz, 2H), 8.82–8.86 (m,
10H), 8.97 (s, 2H). ESI-HRMS: m/z 894.1277 (M–2PF6)2+, 547.7628
(M–3PF6)3+. Elemental anal. Found: C, 46.25; H, 3.01; N, 10.79.
Calcd. for C80H62F24N16 O3P4Ru2: C, 46.38; H, 3.11; N, 10.92. IR (KBr):
1634.60, 1604.25, 1538.23, 1512.85, 1466.27, 1422.59, 1313.52,
4H). ESI-HRMS: m/z 867.0982 (M–2PF6)2+, 529.4225 (M–3PF6)3+
.
Elemental anal. Found: C, 46.30; H, 2.99; N, 9.69. Calcd. for
C78H60F24N14 O3P4Ru2: C, 46.40; H, 3.21; N, 9.82. IR (KBr): 1602.54,
1504.65, 1465.42, 1420.99, 1247.59, 1169.92, 1128.08, 1050.48,
839.98, 765.28, 558.76 cm−1
.
3.12. [(bpy)2RuBL5Ru(bpy)2](PF6)4 (Ru-BL5)
Ru-BL5 was prepared by the same procedure as that described
for Ru-BL4, except BL5 (51 mg, 0.077 mmol) was used instead of
BL4 to react with Ru(bpy)2Cl2·2H2O (92 mg, 0.177 mmol).Yield:
58 mg (36.7%) of a red solid. 1H NMR (400 MHz, DMSO-d6): ı
3.65 (s, 4H), 3.79 (t, J = 4.4 Hz, 4H), 4.16 (t, J = 4.4 Hz, 4H),7.09 (d,
J = 8.4 Hz, 4H), 7.17 (d, J = 9.2 Hz, 4H), 7.27 (d, J = 7.6 Hz, 2H), 7.36
(dd, J = 7.6, 5.6 Hz, 2H), 7.50–7.64 (m, 12H), 7.74 (d, J = 5.6 Hz, 2H),
7.82 (d, J = 6.0 Hz, 2H), 7.86 (d, J = 5.6 Hz, 2H), 8.09 (d, J = 5.6 Hz,
2H), 8.12–8.22 (m, 10H), 8.43 (d, J = 8.0 Hz, 2H), 8.80 (d, J = 8.8 Hz,
1259.99, 1172.67, 1121.64, 1028.81, 840.88, 763.88, 558.14 cm−1
.
3.9. [(bpy)2RuBL2Ru(bpy)2](PF6)4 (Ru-BL2)
Ru-BL2 was prepared by the same procedure as that described
for Ru-BL1, except BL2 (106 mg, 0.148 mmol) was used instead of
BL1 to react with Ru(bpy)2Cl2·2H2O (178 mg, 0.342 mmol). Yield:
149 mg (47.5%) of a deep red solid. 1H NMR (400 MHz, DMSO-d6):
ı 3.63 (s, 4H), 3.79 (s, 4H), 4.22 (s, 4H), 7.14 (d, J = 8.0 Hz, 4H),
7.51–7.58 (m, 8H), 7.60 (d, J = 4.4 Hz, 2H), 7.63 (d, J = 7.2 Hz, 2H),
7.67 (d, J = 5.2 Hz, 2H), 7.70 (d, J = 5.2 Hz, 2H), 7.85 (d, J = 5.2 Hz,
4H), 8.07 (d, J = 8.8 Hz, 4H), 8.13–8.20 (m, 12H), 8.36 (t, J = 6.8 Hz,
2H), 8.79–8.83 (m, 10H), 8.94 (s, 2H). ESI-HRMS m/z 916.1194
(M–2PF6)2+, 562.4444 (M–3PF6)3+. Elemental anal. Found: C, 46.42;
H, 3.14; N, 10.56. Calcd. for C82H66F24N16 O4P4Ru2: C, 46.56; H,
3.32; N, 10.76. IR (KBr): 1631.25, 1603.85, 1538.26, 1512.23, 1465.46,
1422.35, 1312.56, 1258.86, 1172.15, 1121.68, 1028.36, 840.28, 764.16,
4H), 8.84 (d, J = 9.2 Hz, 4H). ESI-HRMS m/z 889.1290 (M–2PF6)2+
,
544.4357 (M–3PF6)3+. Elemental anal. Found: C, 46.48; H, 3.12;
N, 9.48. Calcd. for C80H64F24N14 O4P4Ru2: C, 46.62; H, 3.30; N,
9.72. IR (KBr): 1604.02, 1505.32, 1465.22, 1421.38, 1248.72, 1169.93,
1119.82, 1050.46, 840.95, 764.96, 558.51 cm−1
.
3.13. [(bpy)2RuBL6Ru(bpy)2](PF6)4 (Ru-BL6)
558.24 cm−1
.
Ru-BL6 was prepared by the same procedure as that described
for Ru-BL4, except BL6 (50 mg, 0.071 mmol) was used instead of
BL4 to react with Ru(bpy)2Cl2·2H2O (86 mg, 0.165 mmol).Yield:
93 mg (61.6%) of a red solid. 1H NMR (400 MHz, DMSO-d6): ı
3.58 (t, J = 2.8 Hz, 4H), 3.62 (t, J = 2.8 Hz, 4H), 3.79 (t, J = 4.4 Hz,
4H), 4.16 (t, J = 4.4 Hz, 4H),7.11 (d, J = 8.4 Hz, 4H), 7.19 (d, J = 8.8 Hz,
4H), 7.29 (d, J = 7.6 Hz, 2H), 7.38 (dd, J = 7.6, 6.0 Hz, 2H), 7.51–7.65
(m, 12H), 7.76 (d, J = 5.2 Hz, 2H), 7.83 (d, J = 5.2 Hz, 2H), 7.87
(d, J = 5.6 Hz, 2H), 8.10 (d, J = 5.6 Hz, 2H), 8.14–8.24 (m, 10H),
8.45 (d, J = 8.0 Hz, 2H), 8.82 (d, J = 8.8 Hz, 4H), 8.86 (d, J = 8.8 Hz,
3.10. [(bpy)2RuBL3Ru(bpy)2](PF6)4 (Ru-BL3)
Ru-BL3 was prepared by the same procedure as that described
for Ru-BL1, except BL3 (92 mg, 0.121 mmol) was used instead of
BL1 to react with Ru(bpy)2Cl2·2H2O (143 mg, 0.275 mmol). Yield:
169 mg (64.1%) of a deep red solid. 1H NMR (400 MHz, DMSO-d6): ı
3.56 (s, 4H), 3.60 (s, 4H), 3.78 (s, 4H), 4.22 (s, 4H), 7.15 (d, J = 8.0 Hz,
4H), 7.52–7.58 (m, 8H), 7.60 (d, J = 4.4 Hz, 2H), 7.63 (d, J = 7.2 Hz,
2H), 7.67 (d, J = 4.8 Hz, 2H), 7.70 (d, J = 4.0 Hz, 2H), 7.86 (d, J = 5.6 Hz,
4H), 8.07 (d, J = 8.0 Hz, 4H), 8.14–8.20 (m, 12H), 8.37 (t, J = 7.2 Hz,
2H), 8.79–8.81 (m, 10H), 8.94 (s, 2H). ESI-HRMS m/z 938.1467
4H). ESI-HRMS m/z 911.1359 (M–2PF6)2+, 559.1691 (M–3PF6)3+
.
Elemental anal. Found: C, 46.64; H, 3.25; N, 9.29. Calcd. for
C82H68F24N14 O5P4Ru2: C, 46.48; H, 3.40; N, 9.63. IR (KBr): 1602.29,

F. Cheng et al. / Spectrochimica Acta Part A 71 (2009) 1944–1951
1947
Scheme 1. Synthesis of bridging ligands BL1–6 and corresponding complexes Ru-BL1–6
.

1948
F. Cheng et al. / Spectrochimica Acta Part A 71 (2009) 1944–1951
Table 1
UV–vis absorption data of ligands and corresponding Ru(II) polypyridyl complexesa
ꢀmax (nm) (104ε, M−1 cm−1
BL1
361 (6.10)
)
317 (4.15)
317 (4.27)
317 (4.21)
302 (2.63)
302 (2.75)
302 (2.68)
241(7.01)
242(6.95)
241(6.97)
241(7.78)
241(8.19)
241(7.92)
BL2
361 (6.26)
BL3
BL4
BL5
BL6
361 (6.27)
430 (0.62)
430 (0.64)
430 (0.64)
Ru-BL1
Ru-BL2
Ru-BL3
Ru-BL4
Ru-BL5
Ru-BL6
413 (6.23)
413 (6.10)
413 (6.77)
444 (4.62)
444 (4.21)
444 (4.49)
386 (6.57)
386 (6.30)
386 (7.08)
285 (15.43)
286 (15.11)
286 (16.39)
286 (16.12)
286 (14.68)
286 (15.62)
255 (6.49)
255 (6.58)
255 (6.96)
253 (5.57)
252 (5.17)
252 (5.55)
a
All ligand samples were measured in CHCl3 solution at room temperature, all
complex samples were measured in CH3CN solution at room temperature.
4.2. Absorption spectra
Fig. 1. UV–vis absorption spectra of BL1 (10−5 M, dark cyan, typical for BL1–3
)
and BL4 (10−5 M, blue, typical for BL4–6) in CHCl3 at room temperature, Ru-BL1
Absorption spectra of the six ligands have been studied in CHCl3,
and the corresponding six complexes studied in CH3CN. Working
concentration of all samples is 10−5 M. The spectra are shown in
Fig. 1 with the data summarized in Table 1. Absorption bands of the
(10−5 M, black, typical for Ru-BL1–3
)
and Ru-BL4 (10−5 M, red, typical for Ru-
BL4–6) in CH3CN at room temperature. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of the
article.)
*
ligands can be assigned to ligand-centered intraligand ␲ → ␲ or
*
n → ␲ transitions. Assignments of the absorption bands are made
1504.41, 1465.85, 1421.17, 1248.53, 1169.07, 1115.95, 1059.39,
on the basis of the well-documented optical transitions of other
841.01, 764.92, 558.35 cm−1
.
Ru(II) polypyridyl complexes [39–44]. For complexes Ru-BL1–3
,
absorption bands at around 255 and 286 nm region are attributed to
the intraligand ␲ → ␲* transitions centered on the bipyridine. Peak
observed around 386 nm can be assigned to ligands BL1–3 centered
intraligand ␲ → ␲* transitions. With reference to previous spec-
troscopic studies of Ru(II) complexes with similar diimine ligands
[45–47], ligands BL1–3 have strong ␲ acceptor capacity due to the
large ␲ conjugated framework, which results in a greater effective
nuclear charge on ruthenium. This factor stabilizes the d␲ orbital of
4. Results and discussion
4.1. Synthesis
The outline of the synthesis of the previously unre-
ported six bridging ligands BL1–6 and corresponding Ru(II)
complexes [(bpy)2RuBL1–6Ru(bpy)2](PF6)4 is presented in
Scheme 1. Preparation of the ligands BL1–3 was a Schiff-base
formation from the condensation of 4,5-diazafluoren-9-
hydrazine with 1,7-bis-(4-formylphenyl)-1,4,7-trioxaheptane,
1,10-bis-(4-formylphenyl)-1,4,7,10-tetraoxadecane, and 1,13-bis-
Ru(II). So, the bands at 413 nm are assigned to d␲(Ru) → ␲*(BL1–3
)
transitions.
Absorption spectra of complexes Ru-BL4–6 comprise three dis-
tinct regions. Bands at around 252 and 286 nm are also attributed to
the bipyridine centered intraligand ␲ → ␲* transitions. The strong
band at 444 nm is assigned to admixture of the d␲(Ru) → ␲*(bpy)
(4-formylphenyl)-1,4,7,10,13-pentaoxatridecane,
respectively.
Intermediate 9-(4-hydroxy)phenylimino-4,5-diazafluorene was
prepared by a modified procedure [38]. BL4–6 were prepared
by reaction of 9-(4-hydroxy)phenylimino-4,5-diazafluorene
with diethylene glycol di-p-tosylate, triethylene glycol di-
p-tosylate, and tetraethylene glycol di-p-tosylate, respecti-
vely.
and d␲(Ru) → ␲*(BL4–6). The MLCT absorption maximum is blue-
2+
shifted by 6 nm compared with Ru(bpy)3
[48], which shows
the donor property of BL4–6 is weaker than that of bipyri-
dine.
For each of the ligands, Ru(II) complex was prepared under
nitrogen atmosphere by heating the starting materials in 2-
methoxyethanol. The resulting complexes were characterized by
1H NMR, ESI-HRMS, elemental analysis and IR.
4.3. Electrochemistry
Electrochemical behaviors of complexes have been studied in
CH3CN and in DMF. In CH3CN, the reductions are not well behaved,
Table 2
Electrochemical and fluorescence data of Ru(II) polypyridyl complexes
E1/2 (V) (ꢁEp, mV)a
Emissionb
Complex
Oxidation
Reduction
ꢀmax (nm)
˚
Ru-BL1
Ru-BL2
Ru-BL3
Ru-BL4
Ru-BL5
Ru-BL6
1.30 (122)
1.31 (130)
1.30 (127)
1.35 (146)
1.36 (145)
1.36 (145)
−0.69 (252)
−0.69 (238)
−0.69 (247)
−0.78irr
−1.36 (131)
−1.35 (122)
−1.35 (117)
−1.37 (119)
−1.38 (117)
−1.38 (113)
−1.63 (96)
−1.61 (82)
−1.61 (84)
−1.63 (74)
−1.64 (71)
−1.64 (76)
571
571
570
570
571
571
0.039
0.034
0.028
0.012
0.011
0.010
−0.80irr
−0.79irr
a
Oxidation potentials recorded in 0.1 M TBAP/CH3CN, reduction potentials recorded in 0.1 M TBAP/DMF and potentials were given versus SCE; scan rate = 200 mV/s.
All samples were measured at 77 K in 4:1 ethanol:methanol glassy matrix.
b

F. Cheng et al. / Spectrochimica Acta Part A 71 (2009) 1944–1951
1949
Scheme 2. Reduction processes of bimetallic complex Ru-BL1.
probably due to the adsorption of the reduced species onto the
surface of the platinum electrode. In DMF, the complexes display
three reduction processes, but do not show the oxidative waves due
to the limitation by the solvent. Therefore, the oxidation potentials
recorded in CH3CN, but the reduction potentials recorded in DMF
(Table 2).
Cyclic voltammetry of complex Ru-BL1 exhibits a Ru(II)-based
oxidation at 1.30 V. Compared with [(bpy)2Ru(dafo)](PF6)2 [40],
which can be regarded as parent complex, this oxidation potential
is slightly more negative (by about 90 mV), which indicates that
bridging ligand BL1 is less acidic than dafo. In the current study,
bimetallic complex Ru-BL1 shows a single, unperturbed wave in
cyclic voltammetry and a single peak without broadening in differ-
ential pulse voltammetry. A two-electron process for each couple
of complexes Ru-BL1–6 was confirmed by coulometry. So, this oxi-
dation can be ascribed to a two-electron quasi-reversible process.
Reduction at around −0.69 V is a quasi-reversible process asso-
ciated with the BL1, this is consistent with the addition of electrons
to the low energy LUMO mainly localized on the BL1 giving complex
[(bpy)2RuIIBL2−RuII(bpy)2]2+. Compared with the uncoordinated
ligand BL1 (−1.06 V, a two-electron quasi-reversible process in
DMF), the potential shifts positively about 0.4 V due to the presence
of the positively charged ruthenium centers. The second reduc-
tion at −1.37 V is located on one of the two bpy ligands on each
metallic terminal, adding electrons to the bpy localized LUMO+1
giving the species [(bpy)(bpy•−)RuIIBL2−RuII(bpy•−)(bpy)]. Just as
the oxidation, the reductions of the remote bpy appear at the
same potential, indicating no interaction between the two sites.
The third reduction appearing at −1.75 V is•reversible and yields
the species [(bpy•−)(bpy•−)RuIIBL2−RuII(bpy −)(bpy•−)]2−. Reduc-
tion processes of bimetallic complexes Ru-BL1 are presented in
Scheme 2. Electrochemistry behaviors of complex Ru-BL2–6 are the
same as Ru-BL1 except the first reduction wave of Ru-BL4–6 is irre-
versible.
Fig. 2. Fluorescence spectra of Ru-BL2 (10−5 M) in the presence of Mg2+, Ca2+, Ba2+
,
Li+, and Na+ (10−3 M) at 77 K in 4:1 EtOH:MeOH glassy matrix. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of the article.)
alkali and alkaline-earth metal ions barely perturb the fluores-
cence spectra of Ru-BL1 and Ru-BL4. Emission spectrum of Ru-BL2,
Ru-BL3, Ru-BL5, and Ru-BL6 underwent a progressive decrease in
relative fluorescence intensity that was [M] dependent, but no
shift of emission band maxima was observed. The five cations
quench the fluorescence of Ru-BL2 (Fig. 2) and Ru-BL5 in the
order of Mg2+ > Ca2+ > Ba2+ > Li+ > Na+, quench the fluorescence of
Ru-BL3 and Ru-BL6 in the order of Ca2+ > Mg2+ > Ba2+ > Na+ > Li+
(Table 3). Fluorescence quenching can be ascribed to binding
of the cation to the pseudocyclic polyether cavity, the environ-
ment around the Ru(II) polypyridyl complex chromophore has
been perturbed. Among the complexes Ru-BL1–6, Mg2+ cation-
induced fluorescence quenching of Ru-BL5 and Ca2+ cation-induced
fluorescence quenching of Ru-BL6 are interesting, because the flu-
orescence quenching reached as high as 78% for Ru-BL5 and 63% for
Ru-BL6.
Cation binding studies of Ru-BL1–6 have been pursued further
using UV–vis absorption. It was found that Li+ and Na+ have little
effect on the UV–vis absorption of Ru-BL1–6, and addition of a large
excess of alkali and alkaline-earth metal ions to the CH3CN solution
of complexes barely perturb the UV–vis absorption spectra of Ru-
BL1 and Ru-BL4.
UV–vis absorption spectra tracing upon sequential addition of
Ca2+ to the CH3CN solution of Ru-BL6 is shown in Fig. 3. A sum-
mary of the titration curves monitoring the changes in absorbance
4.4. Fluorescence behaviors
at 444 nm of Ru-BL6 in CH3CN versus the concentration of Mg2+
,
Emission band maxima and emission quantum yields of the
Ru(II) complexes are summarized in Table 2. Upon excitation into
the MLCT band of the complexes, they are non-emissive in CH3CN
at room temperature, but they display vibrational components
Ca2+ and Ba2+ is depicted in Fig. 4. The curves show a gradual
increase in absorbance at 1MLCT band upon increasing the cation
concentration, reaching saturation at higher cation concentrations.
With such absorption information, the binding constants could be
determined with Eq. (1), where A0 is the initial absorbance in the
2+
similar to that of Ru(bpy)3 at 77 K in 4:1 EtOH:MeOH glassy
matrix [49]. Cherry and co-workers [50] have reported crisply
that such mixed imine Ru(II) complexes were poor emitters at
room temperature. Complexes Ru-BL1–6 exhibit their characteris-
tic emission at around 570 nm in 4:1 EtOH:MeOH glassy matrix at
77 K.
Table 3
I/I0 and stability constant Ks of Ru(II) polypyridyl complexes upon the addition of
Li+, Na+, Mg2+, Ca2+ and Ba2+a
b
c
I/I0 (stability constant Ks
)
4.5. Cation binding
Li+
Na+
Mg2+
Ca2+
Ba2+
Ru-BL2
Ru-BL3
Ru-BL5
Ru-BL6
0.90 (−)
0.91(−)
0.70 (−)
0.89 (−)
0.91 (−)
0.88 (−)
0.83 (−)
0.87 (−)
0.79 (852)
0.85 (392)
0.22 (1921)
0.52 (776)
0.86 (473)
0.82 (968)
0.29 (926)
0.37 (1682)
0.87 (262)
0.87 (275)
0.61 (258)
0.72 (398)
It is known that the conformation of noncyclic crown ether
changes drastically from a linear structure to a pseudocyclic struc-
ture upon complex interaction with alkali or alkaline-earth metal
ions. So, the six complexes have been designed to bind adventitious
Li+, Na+, Mg2+, Ca2+ or Ba2+ using the recognition site.
a
[M] = 10−5 M; [Ru-BL2,3,5,6] = 10−3 M.
In 4:1 EtOH:MeOH glassy matrix at 77 K.
In CH3CN at room temperature. (−) Change of UV–vis absorption intensity is too
b
c
When Li+, Na+, Mg2+, Ca2+ and Ba2+ were added to the solu-
tion of Ru-BL1–6, it was found that addition of a large excess of
small to calculate.

1950
F. Cheng et al. / Spectrochimica Acta Part A 71 (2009) 1944–1951
Fig. 5. Cyclic voltammetry of complex Ru-BL5 (5 × 10−4 M) in CH3CN (0.1 M TBAP) in
the absence (blue) and presence (red) of Mg2+. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of the article.)
Fig. 3. UV–vis spectra of Ru-BL6 (10−5 M) in CH3CN solution upon increasing the
ined. Ru-BL2 and Ru-BL5, both including four oxygen atoms in the
open-chain crown ether unit, capture Mg2+, Ca2+, and Ba2+ in the
order: Mg2+ > Ca2+ > Ba2+. Ru-BL3 and Ru-BL6, which contain five
concentration of Ca2+; plotting A0/(A0 − A) at 444 nm versus [Ca2+
]
at 444 nm
−1
(inset). (For interpretation of the references to colour in the artwork, the reader
is referred to the web version of the article.)
oxygen atoms in the open-chain crown ether unit, capture Mg2+
,
Ca2+, and Ba2+ in the order: Ca2+ > Mg2+ > Ba2+. This order is consis-
tent with the sequence of fluorescence quenching.
absence of metal cations, A is the absorbance of the solution mix-
ture at an alkaline-earth metal ion concentration [M2+], εf and εb
are the molar absorption coefficients of the host complexes and the
bound species, respectively, and Ks is the binding constant. Plot-
Oxidation couple of complexes experienced a progressive catho-
dal shift upon addition of alkali and alkaline-earth metal ions to
CH3CN solution. After complete interaction, Ru(II)-centered E1/2
of Ru-BL2 and Ru-BL5 (Fig. 5) exhibit the largest cathodal shift in
the presence of Mg2+, shifting from 1.308 and 1.362 V to 1.274 and
1.302 V respectively. Ru-BL3 and Ru-BL6 show the most cathodal
shift in the presence of Ca2+. Binding of cation introduces positive
charge on the pseudocyclic polyether cavity. The positive charge
may be partly delocalized over the whole bridging ligand frame-
work, which increases the ␴-donor and decreases the ␲-acceptor
capacity of the ligand, resulting in destabilizing the d␲ orbital of
Ru(II). This facilitates the oxidation of Ru(II) by removal of an elec-
tron from d␲ orbital of Ru(II).
ting A0/(A0 − A) versus [M2+
]
−1 gave a satisfactory straight line and
the binding constant can be determined from the ratio of the −y1-
intercept/slope. Plotting of A0/(A0 − A) at 444 nm versus [Ca2+
]
of Ru-BL6 give a satisfactory straight line (Fig. 3), and the binding
constant of complex Ru-BL6 to bind Ca2+ is 1682. Using the same
method, the binding constants of Mg2+ and Ba2+ are calculated to
be 776 and 398, respectively. Ru-BL6 shows the highest binding
ability toward Ca2+ among five cations examined, and the relative
selectivity for Ca2+ is about 2 times of Mg2+, and over four times of
Ba2+
.
According to the studies of UV–vis absorption, fluorescence, and
cyclic voltammetry, Ru-BL2 and Ru-BL5 show the highest binding
ability toward Mg2+ among five cations examined. It is consid-
ered that the ethylenedioxy bis(ethyleneoxy) tether in Ru-BL2
and Ru-BL5 can induce the most favorable conformation for the
size-matched Li+ (ionic diameter 1.36 Å) or Mg2+ (1.32 Å), while
the difference between charge densities (1.47 qÅ−1 for Li+, and
3.03 qÅ−1 for Mg2+) leads to the large affinity only for Mg2+. Based
on the same reason, Ru-BL3 and Ru-BL6 prefer interacting with Ca2+
among the five cations examined.
ꢀ
ꢁꢂ
ꢃ
A0
εf
1
=
1 +
(1)
(A0 − A)
(εf − εb)
Ks[M]
Behaviors of the analogues Ru-BL2, Ru-BL3 and Ru-BL5 are exam-
ined for comparison. Binding constants are collected in Table 3.
Behavior observed for Ru-BL3 is similar to that of Ru-BL6, which
shows the selectivity for Ca2+ among the five cations examined.
Inspection of Table 3 reveals that Ru-BL2 and Ru-BL5 shows the
highest binding ability toward Mg2+ among the five cations exam-
5. Conclusions
Six polypyridyl bridging ligands and corresponding Ru(II) com-
plexes have been prepared. Taking advantage of the character
that some open-chain crown ether can selectively bind alkali and
alkaline-earth metal ions, Li+, Na+, Mg2+, Ca2+ and Ba2+ are added
to the solution of Ru-BL1–6, respectively. Alkali and alkaline-earth
metal ions have no obvious modifying effect on the photophysi-
cal property of Ru-BL1 and Ru-BL4, but addition of metal ions to
the solution of Ru-BL2,3,5,6 all result in a fluorescence quenching,
a hyperchromic effect of the UV–vis absorption, and a progressive
cathodal shift of Ru(II)-centered E1/2. Ru-BL2 and Ru-BL5 show the
highest binding ability toward Mg2+, Ru-BL3 and Ru-BL6 exhibit
selective recognition ability to Ca2+ among five cations examined.
So Ru-BL2 and Ru-BL5 have potential utility as chemical sensor
for probing Mg2+, Ru-BL3 and Ru-BL6 can act as efficient chemical
Fig. 4. UV–vis titration curves of complex Ru-BL6 with Mg2+, Ca2+ and Ba2+ in CH3CN
at room temperature. The absorbance was monitored at 444 nm.
sensor for Ca2+
.

F. Cheng et al. / Spectrochimica Acta Part A 71 (2009) 1944–1951
1951
Synopsis
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We are grateful to the National Natural Science Foundation of
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